Computational Study of the Adsorption of Trichlorophosphate, Dimethyl Methylphosphonate, and Sarin on Amorphous SiO 2

Transcription

1 9314 J. Phys. hem. 2007, 111, omputtionl Study of the Adsorption of Trichlorophosphte, Dimethyl Methylphosphonte, nd Srin on Amorphous SiO 2 V. M. Bermudez* Electronics Science nd Technology DiVision, NVl Reserch Lbortory, Wshington, D ReceiVed: Februry 23, 2007; In Finl Form: April 19, 2007 Ab initio quntum chemicl clcultions hve been performed to study the dsorption of trichlorophosphte, dimethyl methylphosphonte, nd Srin vi hydrogen bonding to Si-OH groups on the morphous SiO 2 (-SiO 2 ) surfce. Two SiO 2 models re used: smll Si 5 O 7 H 8 cgelike cluster nd lrger Si 21 O 56 H 28 structure designed to pproximte the locl environment in -SiO 2. In the ltter cse, regions of different locl OH density re considered s dsorption sites. Adsorption energies, bonding geometries, nd dsorbte vibrtionl modes re obtined, nd nhrmonicity is explicitly included in the tretment of the SiO-H stretching mode. The computed results for the dsorption-induced shift in frequency of the SiO-H stretch nd of the moleculr PdO stretch re compred with the vilble experimentl dt. For ll three species, the most stble dsorption geometry involves hydrogen bonding between two Si-OH groups nd the O tom of the PdO group. 1. Introduction The interction of chemicl wrfre gents (WAs) with mterils is n importnt issue in protection nd remedition. Of specific concern is bsic understnding of the long-term stbility of such species (termed gent fte ) under environmentl conditions nd of the nture of the chemisorption bond t the mteril surfce. These subjects re difficult to ddress experimentlly becuse of the extreme hzrds in working with rel WAs. Reltively benign simulnts re typiclly used insted, but it is often uncertin the extent to which such species mimic the dsorption properties of rel WAs. A direct evlution of prticulr simulnt involves, gin, experimenttion with the corresponding rel gent. An obvious pproch to this problem is to pply quntum chemicl (Q) methods to model the systems of interest. This hs been done in number of cses 1-11 involving minly the interction of WAs with ionic oxides or with moleculr species. For most metl oxides, dsorption generlly involves Lewis cid-bse chemistry for OH-free surfces nd Brønsted cid-bse chemistry for hydroxylted surfces. On the other hnd, for SiO 2 nd relted mterils hydrogen bonding (H bonding) to surfce silnol (Si-OH) groups is involved, nd ccurte theoreticl tretment of these reltively wek surfce H bonds cn be difficult. 12 Recently the dsorption, vi H bonding, of WAs on luminosilicte cly surfces hs been modeled. 3,9 The structures formed re complex nd depend on the extent of hydroxyltion of the surfce. The present work pplies b initio Q methods to the interction of three relted phosphte nd phosphonte molecules on morphous silic (-SiO 2 ). These re (Figure 1) trichlorophosphte[l 3 P)O,TP],dimethylmethylphosphonte[(H 3 O) 2 - (H 3 )P)O, DMMP] nd Srin [(ipro)(h 3 )(F)P)O, where ipr (H 3 ) 2 H-]. TP is used here s reltively simple species * To whom correspondence should be ddressed. E-mil: victor. bermudez@nrl.nvy.mil. Phone: Fx: Figure 1. Schemtic digrm of the moleculr structures of TP, DMMP, nd Srin. for testing the computtionl procedures nd for probing the rectivity of vrious prts of the model clusters. DMMP is importnt s sfe simulnt for the nerve gent Srin. The min focus of this work is in compring theoreticl results for the dsorption of TP nd DMMP on -SiO 2 with experimentl dt, especilly the infrred (IR) vibrtionl spectrum nd in compring results for the dsorption of DMMP versus Srin. The gols re n evlution of the relibility of Q methods in describing the dsorption nd lso side-by-side comprison of the behvior of the simulnt nd the rel gent. 2. omputtionl Methods All clcultions were done using the Gussin03 suite of progrms 13 with stndrd bsis sets built into the pckge. Most of the work ws done with G(2d,2p) bsis sets, but some results were obtined for comprison t the G(d, p) nd G(d, p) levels. It is cler from previous work 14,15 tht hevy-tom diffuse functions, together with polriztion functions both on hevy toms nd on H, re necessry for n /jp071529m : $ Americn hemicl Society Published on Web 06/07/2007

2 Adsorption of TP, DMMP, nd Srin on Amorphous SiO 2 J. Phys. hem., Vol. 111, No. 26, dequte description of H bonding in the present cse. Hence, G(d, p) is considered to be the smllest bsis set tht cn be used relibly. It hs lso been shown 15 in studies of NH 3 dsorption on SiO 2 tht G(2d,2p) bsis sets yield somewht more ccurte dsorption energy ( E ds, see below) thn do G(d, p) but tht further enlrgement, up to G(2d,2p), hs no significnt dditionl effect on E ds. All clcultions were done using density functionl theory (DFT) with the B3LYP functionl. In the pst, questions hve sometimes been rised concerning the pplicbility of DFT to H bonding. However, severl previous studies (e.g., refs 14-20) hve demonstrted the vlidity of DFT with the B3LYP functionl in describing the H bonding of dsorbtes (including phosphontes 14,19 )onsio 2 surfces. Much of the present work involves comprison of observed nd clculted IR vibrtionl spectr s mens of evluting the computtionl methods nd results. Prticulr ttention is given to the shift to lower frequency, ν(o-h), of the SiO-H stretch tht results from dsorption. Previous work 15,16,21 hs shown tht ccurte results for ν(o-h) require tht nhrmonicity be treted explicitly. omputtion of nhrmonic terms for ll 3N-6 modes of n N-tom model (dsorbte plus SiO 2 cluster) is not fesible for systems of the size studied here; therefore, the pproch described previously 22 is followed. The ν(o-h) mode is well seprted in energy from (nd therefore decoupled from) ll other modes. Beginning with the relxed structure, series of single-point clcultions is done in which the H tom is displced incrementlly in direction prllel to the O-H bond. In totl, 28 displcements in the rnge of e δ O-H e Å, reltive to the equilibrium O-H distnce of bout 0.96 Å, were used. The one-dimensionl (1D) energy surfce thus obtined ws lest-squres fitted with polynomil of order 12 e k e 15 in δ OH, nd the resulting potentil function ws then used to solve the 1D Schrödinger eqution numericlly. 23 A high-k polynomil ws necessry to optimize the fit over wide rnge of δ O-H.A similr procedure ws lso used to estimte the effects of nhrmonicity in the PdO stretching mode of TP. The mss used for the oscilltor is the reduced mss, µ XY ) (m X m Y )/(m X + m Y ). From the lowest three eigenvlues (E 0,E 1, nd E 2 ) the quntities of interest re obtined. These re the nhrmonic fundmentl nd first-overtone frequencies ν 01 ) (E 1 - E 0 ) nd ν 02 ) (E 2 - E 0 ) nd the nhrmonicity prmeter ν e x e ) (2ν 01 - ν 02 )/2 where ν e is the hrmonic frequency. The pproch described bove involves n pproximtion tht should be vlid in the cse of ν(o-h). In obtining the 1D potentil energy surfce, the bond stretching is modeled entirely in terms of H-tom displcement; wheres, the true norml mode is more ccurtely described in terms of the individul tomic displcements s δ O-H ) 0.94δ H δ O. Different effects on the totl energy re expected for equivlent displcements ofhndo(δ H nd δ O ), but ny error in the 1D potentil function resulting from the pproximtion δ O-H δ H hs been neglected. The frequencies of other modes re obtined in the hrmonic pproximtion nd scled using fctors given in previous work. The scling ccounts for nhrmonicity in n pproximte mnner, bsed on n verge correction for wide rnge of molecules nd norml modes. For B3LYP/ G(d, p) the fctor 24 is , nd for B3LYP/ G(d, p) the fctor 25 is To our knowledge, no scling fctor hs been reported specificlly for B3LYP/ G(2d,2p); hence the fctor of for B3LYP/ G(d, p) will be used. The uncertinty in these fctors is in the rnge of ( (ref 24). Figure 2. Schemtic digrm of the model -SiO 2 cluster. A model rective region with reltively high OH density is shown in (). The sme rective region is shown in (b) s prt of the totl cluster. Si (O) toms re shown in blue (red) except for the Si toms t the rective region which re in green. H toms re gry. The structure shown in () constitutes the ONIOM model system, nd the structure shown in (b) is the ONIOM rel system. Further detils re given in the text. Other results of interest re E ds nd relted quntities, defined by where the first term in E ds is the totl energy of the relxed dsorption system nd the next two re the relxed energies of the isolted molecule nd cluster. E BSSE is the correction for bsis set superposition error (BSSE), which is obtined vi the usul counterpoise method. In generl, no correction is pplied for zero-point vibrtionl energy (ZPE); however, n estimte of the mgnitude of E ZPE will be given below. A negtive 0 constitutes n exothermic process. H ds E ds ) E(mol + cluster) - E(mol) - E(cluster) E ds ) E ds + E BSSE 0 H ds ) E ds + E ZPE 3. onstruction of the Model 3.1. The -SiO 2 Model. The -SiO 2 lttice hs been modeled by Vn Ginhoven et l. 26 who showed tht the physicl properties of the bulk glss cn be ccurtely described by verging over finite set of three-dimensionlly periodic structures formed from properly constructed smll cells. Ech cell is formed by cooling (in moleculr dynmics (MD) clcultion) gs of 24 Si nd 48 O toms under control of n empiricl potentil energy function. onstrints re imposed only on the tom density nd on the minimum Si-O, O-O, nd Si-Si nerest-neighbor distnces. Becuse the process is rndom, ech 72 tom Si 24 O 48 cell is unique nd represents n rbitrrily chosen smll piece of the bulk glss. In modeling bulk glss, 26 ech of ten different cells synthesized in this mnner is then used to form crystl with periodic boundry conditions nd subjected to further nneling using DFT MD tretment. The verge structurl properties (rdil distribution function, ring size distribution, etc.) of these ten different glsses re in excellent greement with experiment nd with empiricl MD simultions for much lrger smples (493 SiO 2 units). The present work uses one of the 72 tom clusters s the model system (cf. Figures 2 nd 3) from which re cut smll

3 9316 J. Phys. hem., Vol. 111, No. 26, 2007 Bermudez Figure 3. Similr to Figure 2 but showing rective region with reltively low OH density. Digrms () nd (b) show lrge nd smll models for the rective region, respectively. The sterisk in () mrks the OH site used for TP dsorption (see text). Si toms tht re common to ll three digrms re shown in green. In (c), the view is from perspective different from tht in Figure 2b. sections to represent the rective dsorption sites. A similr pproch hs lredy been used 27 to tret the self-trpped exciton in SiO 2. The model is essentilly cluster of finite size tht, to the extent possible, exhibits structurl chrcteristics expected for n -SiO 2 surfce. This includes distribution of different Si-OH configurtions s well s siloxne rings of vrying size nd shpe. Hence, the model permits comprison of the dsorption chrcteristics of severl different types of surfce sites with no ltertion of the model itself. The present pproch differs from tht of other studies in which -SiO 2 is modeled s smll cluster contining only few SiO 4 tetrhedr (sometimes only one) or s crystlline structure with single, well-defined siloxne ring geometry. A similr structure, 28 n OH-free Si 24 O 48 cluster, hs been used to tret the rection of H 2 O with defects on the -SiO 2 surfce. Here, the model ws formed by cutting n Si 24 O 48 cluster from crystlline R-qurtz nd then subjecting it to simulted nneling. In the present cse, ll dngling Si (O) bonds on the periphery of the cluster were sturted with OH (H). Dngling -O-Si(OH) 3 units, which re known 29 to be unstble on the -SiO 2 surfce bove 300, were removed nd replced with -OH. The resulting cluster then hs stoichiometry of Si 21 O 56 H 28. Next, the positions of the O nd H toms in the OH groups were optimized t either the semiempiricl PM3 level 30 or the b initio restricted Hrtree-Fock (RHF)/3-21G level while the rest of the cluster remined fixed. Two rective sites were then selected. One (Figure 2) is site of reltively high OH density nd consists of n isolted (-O-) 3 Si-OH with djcent geminl (-O-) 2 Si(OH) 2 groups. The other (Figure 3,b) is site of reltively low OH density nd consists minly of isolted Si-OH groups. Mny other combintions of isolted nd geminl groups re possible, but these two hve been chosen s representtive of the hydroxylted -SiO 2 surfce (see below). A fully dehydroxylted -SiO 2 surfce, with only (-O-) 3 Si-O-Si(-O-) 3 bridges, does not occur nturlly nd requires vcuum nneling 31,32 t 1000 to form. The OH density on fully hydroxylted -SiO 2 surfce hs been found experimentlly 32 to be 5 OHnm -2. Heting in vcuo to 450 is typicl substrte pretretment in IR studies of TP nd DMMP dsorption (see below). This desorbs ny physisorbed H 2 O nd converts most H-bonded Si-OH groups to Si-O-Si bridges vi H 2 O elimintion, leving coverge of non-h-bonded groups 32 equl to bout 2 OH nm -2. For the model used here, the totl coverge is bout 4.0 OH nm -2, bsed on the surfce re of sphere enclosing the cluster. The cluster consists predominntly of isolted nd geminl Si-OH groups, nd only two pirs of Si-OH groups re sufficiently close to H bond to ech other. The OsH bond length between two H-bonded silnol sites is tken here to be 2.0 Å, bsed on results 18 t the MP2/6-31G(d) level for H 2 O nd H 3 OH intercting with Si(OH) 4. Such sites re not used in the present work becuse the vilble experimentl dt for TP nd DMMP dsorption (see below) ll focus on -SiO 2 surfces with low coverge of H-bonded OH groups. If the H-bonded sites re excluded, the coverge on the model cluster becomes bout 3.4 OH nm -2. Hence, the model corresponds firly closely to the surfces used in experimentl studies, lthough the coverge of non-h-bonded Si-OH groups is 50% higher thn on the rel mteril ONIOM Tretment. In the next step, n ONIOM procedure 33 is pplied in which the smll rective-site subcluster (Figure 2 or 3,b) nd the dsorbte define the model system while the entire cluster plus dsorbte comprises the rel system. During geometry optimiztion, ll toms in the ONIOM model system re llowed to relx without constrint while the rest of the cluster remins fixed in position to prevent distortion of the cluster from its bulk-glss configurtion. The ONIOM energy is then given by E ) E low high rel + E model - E low model. Low refers to low-level tretment which, in the present work, is either semiempiricl (PM3 30 or MNDO 34 ) or b initio (RHF/ 3-21G). High refers to high-level b initio tretment which, in the present work, is minly t the B3LYP/ G(2d,2p) level with n ultrfine integrtion grid nd tight convergence criteri for geometry optimiztion to increse the level of ccurcy in the tretment of wek H-bonding interctions. Thus, in the definition of E ds given bove, E(mol + cluster) is the ONIOM energy with the dsorbte present, nd E(cluster) is the ONIOM energy without the dsorbte. The effects of the choice of high- nd low-level method will be discussed below. Becuse of progrm limittions, E BSSE cnnot be obtined directly for n ONIOM model. Hence, fter optimizing the ONIOM dsorption structure the model system is detched nd H toms used to sturte the dngling Si-O bonds. The counterpoise procedure is then pplied to this isolted structure to get E BSSE. Hydrogen link toms were used in clcultions for the ONIOM model system. Solns-Monfort et l. 35 hve compred ONIOM nd periodiclttice clcultions for the dsorption of NH 3 or H 2 O t cidic OH sites in the luminosilicte mteril chbzite. They concluded tht the two pproches give comprble geometries but tht use of RHF/3-21G, rther thn semiempiricl method, s the ONIOM low-level method is needed to obtin E ds in greement with the periodic-lttice result. Furthermore, it ws found tht good ONIOM results could be obtined by optimizing the geometry using MNDO s the low-level method followed by single-point clcultion with RHF/3-21G s the low-level method. Such single-point clcultions of E ds converged

4 Adsorption of TP, DMMP, nd Srin on Amorphous SiO 2 J. Phys. hem., Vol. 111, No. 26, rpidly to the periodic-lttice result s the size of the ONIOM model system incresed. As low-level method, MNDO ppered to be somewht superior to AM1. It should be noted tht periodic-lttice pproch might not be prcticble for the dsorbtes of interest here. Most WA molecules re even lrger thn Srin (Figure 1), nd very lrge unit cells would be required to prevent interction between nerest-neighbor dsorbtes. onclusions similr to those mentioned bove were reched by Roggero et l. 20 who compred ONIOM results for cgelike cluster model of SiO 2 with those obtined in fully b initio, high-level tretment of the entire cluster. In prticulr, it ws found tht ONIOM results with RHF/3-21G s the lowlevel method were in excellent greement with those for the fully high-level tretment nd tht the ONIOM results converged rpidly s the size of the model system incresed. It ws lso found 20 tht PM3 performs reltively poorly s n ONIOM lowlevel method for SiO 2. However, Michlkov et l. 3 found tht PM3 nd RHF/3-21G give similr results when used s the ONIOM low-level method in modeling dsorption of Srin on the luminosilicte mteril dickite. The difference my be relted to the size of the ONIOM model system, which ws quite lrge in ref 3 versus here nd in ref 20. It is likely tht with sufficiently lrge model system the ccurcy of the lowlevel method becomes reltively unimportnt. A comprison of AM1, PM3, nd MNDO/d in predicting the structures of smll Si-contining molecules hs lso been given by Brtlett et l. 36 In the present work, clcultion for the bre (i.e., dsorbtefree) cluster (Figure 2) with PM3 s the ONIOM low-level method nd B3LYP/ G(2d,2p) for the high-level gve model Mulliken chrge of q Si )+1.28 e for Si toms in the model system. The sme clcultion with MNDO (RHF/3-21G) s the low-level method gve q model Si )+1.83 e (+2.03 e ). The RHF/3-21G result is virtully identicl to the Mulliken chrge of e found in periodic RHF tretment 37 of model bulk R-SiO 2. Hence, q Si is sensitive to the choice of lowlevel method, nd even simple b initio method gives chrge result close to tht obtined for bulk lttice. Further discussion is given below regrding the different results obtined for the three low-level methods. 4. Results nd Discussion 4.1. TP Adsorbed on -SiO 2. The min interest in the present work is in the dsorption of phosphontes on the hydroxylted -SiO 2 surfce. To our knowledge there re few previous theoreticl studies 14,19 of this prticulr problem. The dsorption of TP ws tken s strting point becuse it is smller nd simpler molecule thn either DMMP or Srin; hence, test clcultions cn be done more quickly. Furthermore, the mode of TP dsorption (PdOsH-O bonding) is unmbiguous. 38,39 Thus, it cn be used to probe differences in the rectivity of vrious regions of the model cluster more esily thn cn lrger, polyfunctionl molecule like DMMP. The gol of the TP studies is to ssess the computtionl procedures before proceeding to the lrger nd more complex dsorbtes. Experimentl dt, in the form of IR vibrtionl spectr, hve been reported 38,39 for TP dsorbed on highsurfce-re (HSA) -SiO 2 powders. The substrte preprtion 40 used in these studies (vcuum nneling t ) yields mteril with predominntly non-h-bonded Si-OH groups tht my be either isolted or geminl. Structurl dt, obtined from microwve spectroscopy, 41 re lso vilble for gs-phse TP. These give r(pdo) ) ( Å nd r(p-l) ) ( Å for the bond lengths nd (l-p-l) bond Figure 4. Schemtic digrm showing the structure of l 3PdO dsorbed on the Si 5O 7H 8 cluster. The configurtion shown results from geometry optimiztion t the B3LYP/ G(2d,2p) level. The hevy green line shows the chemisorption bond. ngle of ( 0.2. In the present work, the corresponding vlues computed t the B3LYP/ G(2d,2p) level re nd Å nd Smll-luster Model. As further simplifiction, the present study begn with the Si 5 O 7 H 8 cgelike cluster (Figure 4) described by ivlleri et l. 16 Among the different such clusters considered by this group this ws the smllest tht gve results for dsorption of NH 3, which were resonbly independent of cluster size. Tble 1 shows tht there is little difference mong the E ds vlues for different bsis sets. Both the TP nd the entire cluster were llowed to relx during optimiztion. From the computed vibrtionl modes (t the G(d, p) level) of the dsorbte + cluster, bre cluster, nd free dsorbte, E ZPE )+0.81 kcl/mol ws obtined fter scling the mode frequencies by fctor of (see bove), 0 resulting in H ds )-3.7 kcl/mol. E ZPE is not expected to depend significntly on the bsis set. Thus, the corresponding result for G(2d,2p) ws E ZPE )+0.70 kcl/mol, giving 0 H ds ) -3.6 kcl/mol. Hence, the E ds results remin nerly identicl fter further correction for E ZPE. Figure 4 shows the optimized structure for dsorbed TP. A clcultion ws lso done t the G(2d,2p) level for dsorption by P-lsH-O bonding. The result ws E ds ) kcl/mol nd r(lsh) ) Å, indicting only wek interction. This is consistent with the smller chrge on l versus O, s shown by the tomic polrizbility tensor 42 (APT) results for the free molecule in Tble 2. E ds for this type of bonding corresponds to 10% of the dsorption energy due to PdOsH-O interction. The possibility of dsorption vi n electrosttic interction between the highly chrged P tom (cf. Tble 2) nd the O of the Si-OH group ws lso investigted, but n even smller interction ws found ( E ds kcl/ mol, r(pso) Å). Furthermore, structure of this type would be stericlly hindered for the lrger DMMP nd Srin molecules (cf. Figure 1). Tble 1 shows tht proper tretment of nhrmonicity (specificlly, of the chnge in ν e x e cused by dsorption) is significnt fctor in ν(o-h), s noted in previous studies. 15,16,21,22 The results for the bre surfce, ν 01 (O-H) nd ν e x e, do not depend strongly on the choice of bsis sets, but ν(pd O) nd ν 01 (O-H) re in somewht better greement with experiment for G(2d,2p). The ν 01 (O-H) nd ν e x e results re close to those reported 16 by ivlleri et l. (3762 nd 78 cm -1, respectively) for the Si 5 O 7 H 8 cluster t the B3LYP/ Dunning DZP(6d) level nd with experimentl results 38,39,43 for isolted OH groups on HSA -SiO 2 powders. After dsorption of TP, ν e x e of bout 106 cm -1 is computed for the three bsis sets. In compring observed nd clculted results for ν 01 (O- H), it must be reclled tht the former re obtined t room

5 9318 J. Phys. hem., Vol. 111, No. 26, 2007 Bermudez TABLE 1: DFT/B3LYP Results for TP Adsorption on the Si 5 O 7 H 8 luster for Different Bsis Sets method E b ds r(pdo---h) ν(pdo) c ν s(pl 3) c ν 01(O-H) d ν ex e e ν 01(O-H) f G(d, p) -5.9 (-4.5) (3759) (-168) G(d, p) -6.4 (-4.1) (3786) (-179) G(2d,2p) -4.9 (-4.3) (3769) (-181) Experiment g Experiment h Experiment i E ds is in kcl/mol, r(pdosh) is the H-bond distnce in Å, nd ν, ν ex e, nd ν re in cm -1. b Vlues for E ds (the BSSE-corrected E ds) re in prentheses. c ν(pdo) nd ν s(pl 3) re the frequency shifts (reltive to the gs phse) obtined for purely hrmonic potentils fter scling s described in the text. d ν 01(O-H) is the nhrmonic SiO-H frequency for the bre cluster s described in the text. The vlue in prentheses is the frequency obtined for purely hrmonic potentil fter scling. e Anhrmonicity prmeter for the bre cluster. After dsorption, ν ex e is bout 106 cm -1 for the different bsis sets. f ν 01(O-H) is the chnge in ν 01(O-H) cused by dsorption of TP. Numbers in prentheses re the corresponding results obtined fter scling for purely hrmonic potentil. g Dt for bre -SiO 2 nd for dsorbed TP from ref 38 (recorded t room temperture). h Dt for bre -SiO 2 nd for dsorbed TP from ref 39 (recorded t RT). i Dt for bre -SiO 2 from ref 43 (recorded t RT). ν ex e ) (2ν 01-ν 02)/2 ws obtined from the reported ν 01(O-H) nd ν 02(O-H) frequencies. TABLE 2: APT Atomic hrges in Free Molecules molecule P*dO PdO* R-O* P-X* l 3PdO DMMP Srin All vlues re obtined t the B3LYP/ G(2d,2p) level with n ultrfine integrtion grid. The sterisk mrks the tom being described. R ) H 3- for DMMP nd (H 3) 2H- for Srin. X ) l for TP nd F for Srin. temperture while the lter pertin to T ) 0. It hs been shown experimentlly 44 for NH 3 dsorbed on HSA -SiO 2 powders tht ν(o-h) )-700 cm -1 t room temperture (RT) nd -900 cm -1 t 4 K. With lrge bsis sets, periodic-lttice model nd the inclusion of nhrmonicity, the DFT/B3LYP result 15 for ν 01 (O-H) is -733 cm -1. After vrious corrections re pplied, this becomes -633 cm -1, which is still close to the experimentl RT vlue but underestimtes the T ) 0 vlue. No IR dt re vilble for TP dsorbed on SiO 2 t cryogenic tempertures, but the results in ref 44 suggest tht the pprent good greement between theory nd experiment for ν 01 (O-H) in Tble 1 my be misleding. Nevertheless, the greement is better when nhrmonicity is explicitly included. To determine whether explicit tretment of nhrmonicity would lso improve the greement between the observed nd clculted ν(pdo), similr nlysis ws done for this mode. Exmintion of the norml-mode displcements for TP, either free or dsorbed on the Si 5 O 7 H 8 cluster, shows tht the PdO stretch is essentilly decoupled from other modes; hence, the pproch outlined bove is pplicble. In this cse, the nhrmonicity is much smller thn for the O-H stretch. For free TP, ν e x e ) 7.0 cm -1 for ν(pdo), using G(2d,2p) bsis sets, vs 77 cm -1 (Tble 1) for ν(o-h) of the bre Si-OH group. The pproximtion, noted bove, of describing the ν(pdo) norml mode entirely in terms of displcement of the lighter tom is less vlid here thn in the cse of ν(o-h). Nevertheless, the computed nhrmonic ν 01 (PdO) for free TP, 1268 cm -1, is closer to the experimentl gs-phse vlue 38,39 of 1322 cm -1 thn is the result (ν(pdo) ) 1246 cm -1 )of scling the purely hrmonic frequency. However, the dsorptioninduced shift obtined with explicit inclusion of nhrmonicity, ν 01 (PdO) )-17 cm -1, is negligibly different from the vlue of -20 cm -1 (Tble 1) obtined simply by scling the purely hrmonic frequency shift. Thus the effect of dsorption on ν e x e ppers to be much smller for ν(pdo) thn for ν(o-h). Finlly, Tble 1 shows tht ll three models give resonble results for ν s (Pl 3 ), the shift reltive to the gs phse for the symmetric Pl 3 stretching mode. This mode is doubly degenerte in the gs phse but splits by 3 to 6 cm -1 in the dsorbed stte (depending on the method of clcultion) when the 3V symmetry of the free molecule is eliminted. The shifts given in Tble 1 re bsed on the verge of the two frequencies for the dsorbed species Lrge-luster Model. The smll cluster described bove my pper dequte for describing the dsorption of TP; however, deficiencies in this model will be identified below. In ny cse lrger, polyfunctionl molecules such s DMMP nd Srin require model with multiple OH groups. Even for reltively simple molecule like TP, relistic tretment of dsorption must llow the possibility of interction with djcent sites not directly bonded to the dsorbte. 35 Severl tests of the dsorption of TP on the lrge-cluster model will now be performed. These include comprison of the high-oh-density (Figure 2) nd low-oh-density (Figure 3,b) models, comprison of isolted nd geminl sites, nd n ssessment of the effects of the size of the ONIOM model system. Tble 3 shows results for TP dsorption t the isolted Si-OH group of the model shown in Figure 2. These were obtined fter optimiztion using semiempiricl low-level methods. onsistent results for E ds nd r(pdosh-o) were found only with subsequent single-point clcultion with RHF/ 3-21G for the low-level method. Without the single-point clcultion, MNDO gives shorter bond length but smller E ds thn does PM3. In both cses, the optimized structure (not shown) gve no indiction of interction with sites other thn the dsorption site. For exmple, the nerest djcent OH site is locted such tht the resulting OHsOdP distnce is 3.98 Å, nd ny other relevnt distnces (e.g., OHsl) re even greter. model In view of the results in Tble 3, the q Si vlues noted bove nd the discussions in refs 20 nd 35, ll subsequent ONIOM work will mke use of RHF/3-21G s the low-level method. One djustment ws necessry, nmely, the use of 3-21G(d) bsis set for the P tom. In the molecules of interest here, P is hypervlent, hving one π- nd three σ-bonds. This requires higher degree of vritionl freedom (i.e., d-orbitls) in the P bsis set, without which SF convergence problems were often encountered. In the following (nd in Tble 3), 3-21G(d) for the P tom should be understood whenever reference is mde to 3-21G bsis sets for the low-level method. Tble 4 nd Figure 5 show results for TP dsorption t different sites, which indicte some significnt points. First is the fct tht for dsorption t the isolted Si-OH site of Figure 2, E ds for optimiztion with RHF/3-21G s the low-level method (-9.5 kcl/mol) is close to the single-point result fter optimiztion with MNDO s the low-level method (-10.1 kcl/mol, Tble 3). This is consistent with previous results 35 for different system (NH 3 dsorption on n lumi-

6 Adsorption of TP, DMMP, nd Srin on Amorphous SiO 2 J. Phys. hem., Vol. 111, No. 26, TABLE 3: Results for TP Adsorption on Lrge-luster -SiO 2 Models for Different ONIOM Methods method E b ds r(pdo---h-o) B3LYP/ G(2d,2p):PM (-10.5) B3LYP/ G(2d,2p):PM3//:RHF/3-21G c -2.9 (-2.5) B3LYP/ G(2d,2p):MNDO -5.8 (-5.3) B3LYP/ G(2d,2p):MNDO//:RHF/3-21G (-9.7) Energies re in kcl/mol; bond lengths re in Å. In ll cses, dsorption occurs t the isolted OH of the model shown in Figure 2. b Numbers in prentheses re the BSSE-corrected ( E ds ) vlues. The sme E BSSE (+0.42 kcl/mol) is used in ll cses becuse the high-level method is the sme. c The nottion mens tht the geometry ws first optimized using B3LYP/ G(2d,2p) for the high-level nd PM3 for the low-level method. Then single-point clcultion ws done with RHF/3-21G s the low-level method (with 3-21G(d) for the P tom). TABLE 4: Results for TP Adsorption on Different -SiO 2 Lrge-luster ONIOM Models model b E c ds r(pdo---h-o) (i) isol OH (Figure 2) -9.5 (-9.0) (ii) gem OH (Figure 2) -6.2 (-5.7) (iii) isol OH; lrge model (Figure 3) (-10.4) 2.148, d (iv) isol OH; smll model (Figure 3b) -7.6 (-7.0) All clcultions re done t the B3LYP/ G(2d,2p):RHF/3-21G level (with 3-21G(d) bsis set for the P tom). Energies re in kcl/ mol, nd bond lengths re in Å. b isol ) isolted; gem ) geminl. The structures re lbeled (i), etc., to fcilitte referencing in the text nd in Figure 5. c Vlues in prentheses re E ds for E BSSE ) kcl/mol. d This structure exhibits H bonding between PdO nd two Si-OH groups. Figure 5. Optimized structure for TP dsorbed t the Si-OH sites indicted in Tble 4. The lbels (i), etc., correspond to those in Tble 4. For clrity, only the ONIOM model systems re shown, nd ech is in pproximtely the sme orienttion s in Figure 2 or 3,b. The dditionl H toms used s ONIOM link toms re not shown. The hevy green lines show the dominnt bonding interctions. nosilicte mteril) nd serves s useful test of the present computtionl procedure. In ll cses shown in Tble 4, E ds is lrger thn the vlue of -4.3 kcl/mol found for the smll Si 5 O 7 H 8 cluster (Tble 1). This my be becuse of the rigidity of the Si 5 O 7 H 8 structure, which consists of fused siloxne rings with only three or four Si toms, s opposed to the loose nd open structure of the lrge-cluster model. This permits only limited relxtion in response to dsorption, even though the entire Si 5 O 7 H 8 cluster is unconstrined during optimiztion. In this context, one notes tht of structures (i), (ii), nd (iv) the most stble is structure (i) in which dsorption occurs t the center of the fully relxed subcluster. When only one Si-OH group is involved in H-bonding, geminl- nd isolted-oh sites hve been found 45 to give similr E ds vlues. Hence, the difference in E ds between (i) nd (ii) is not thought to be due to the presence of the second, nonintercting geminl OH group in (ii). The optimized structures in Figure 5 exhibit differing degrees of interction between TP nd multiple OH sites. This emphsizes bsic difficulty in treting disordered system with smll finite-cluster model. For exmple, (iii) shows bidentte structure with H bonding between TP nd two Si-OH sites. This cn be chieved only for locl geometry in which two such sites re present with the correct seprtion nd reltive orienttion. Note tht dsorption t the geminl site (ii) does not led to this energeticlly most fvorble bidentte structure. Other structures suggest possible contribution from O-Hsl-P interction (not shown) involving OH sites outside of the ONIOM model system, lthough this should be reltively wek, s noted bove. One structure, (ii), shows no indiction of such interctions. None of these cn be tken to represent unique nd universl model for TP dsorption on -SiO 2. Tken together, the results in Tbles 1 nd 4 suggest tht there is requirement for sufficiently flexible structure, one with dequte freedom to relx in the vicinity of the dsorption site nd with n ONIOM model system tht includes ll sites tht might be involved in dsorbte bonding. When these conditions re resonbly well-stisfied, there is not gret difference in E ds between structures with quite different geometries (e.g., (i) nd (iii) in Figure 5). A more precise tretment of TP dsorption would focus on rnge of different dsorption sites, ll modeled in ccord with the criteri stted bove. This would presumbly decrese the spred in E ds vlues shown in Tble 4. However, the min interest here is in the dsorption of DMMP nd Srin. Further refinements in the tretment of TP dsorption re therefore not justified but the results obtined bove will be pplied to modeling the dsorbtes of interest. Finlly, the SiO-H stretching frequency ws exmined for the cse of dsorption with the structure in Figure 5(i). For the bre cluster, ν 01 (O-H) ) 3744 nd ν e x e ) 77 cm -1 were obtined which re in good greement with experiment (Tble 1). Adsorption of TP gve ν 01 (O-H) )-267 cm -1. A similr clcultion for structure (ii), which hs the smllest E ds of those shown in Figure 5, gve ν 01 (O-H) ) -260 cm -1. These vlues (which pertin to T ) 0) re slightly lrger thn the RT experimentl result of bout -250 cm -1 (Tble 1). Bering in mind the remrks mde previously regrding the temperture dependence of ν 01 (O-H), the greement with experiment is resonble. It is useful to compre the results for E ds, r(pdosh-o), nd ν 01 (O-H) in Tble 1 with those for structures (i), (ii), nd (iv) given bove nd in Tble 4. In ll cses, r(pdosh-o) nd ν 01 (O-H), which directly reflect the strength of the H-bond interction, re similr, wheres, the

7 9320 J. Phys. hem., Vol. 111, No. 26, 2007 Bermudez E ds vlues differ. For molecules in solution, the Bdger- Buer reltionship 12 predicts strong nd nerly liner increse in ν 01 (O-H) with H-bond strength. The present results suggest tht E ds depends on other fctors in ddition to the H-bond strength, such s relxtion t the dsorption site nd possibly interction of the dsorbte with djcent sites. Thus, the smll Si 5 O 7 H 8 cluster performs firly well in describing the strength of single H bond but not in predicting the dsorption energy DMMP Adsorbed on -SiO 2. The insights gined in the study of TP dsorption will now be pplied to the cse of DMMP. The pproch will be to use the lrge-cluster model to determine E ds, the dsorption geometry nd ν 01 (O-H) nd to use the Si 5 O 7 H 8 cluster in computing the internl vibrtionl modes of dsorbed DMMP. The ltter step is tken for two resons. First, even with the use of nlyticl grdients, complete norml-mode clcultion for the lrge-cluster model is too computtionlly expensive. Second, the prtil geometry optimiztion performed for the lrge-cluster model (see bove) precludes n ccurte clcultion of the lower-energy modes of dsorbed DMMP becuse, unlike the SiO-H stretch, mny of these re coupled to displcements of toms in the cluster. The foregoing discussion of TP indictes tht resonble results re to be expected for the internl modes of n dsorbte bonded to the Si 5 O 7 H 8 cluster. Experimentl nd theoreticl results for the conformtion nd vibrtionl modes of gs-phse DMMP hve been discussed in ref 10. As in ref 10, the gsphse DMMP conformer (Figure 1b) used in the present work is the one identified previously 46 s the lowest in energy Experimentl Results. The interction of DMMP with -SiO 2 HSA powder hs been studied by Knn nd Tripp 38 using IR spectroscopy nd by Henderson et l. 47 using temperture-progrmmed desorption nd Auger electron spectroscopy. The IR results re interpreted in terms of non-dissocitive dsorption vi H-bond formtion between isolted or geminl Si-OH sites nd the two moleculr H 3 O- groups. The PdO group is thought not to be directly involved in dsorption. The ctivtion energy for desorption is estimted 47 to be 16.9 kcl/ mol. The min effects in the IR spectrum tht result from dsorption re the following: () shift of the SiO-H stretch by ν(o-h) )-524 cm -1, reltive to the bre surfce; (b) shift of the SiO-H bend by δ(o-h) +160 cm -1, reltive to the bre surfce; (c) shift of the PdO stretch by ν(pdo) )-19 cm -1, reltive to the gs-phse; (d) little or no chnge in the ν (-H) nd ν s (-H) ntisymmetric nd symmetric stretches of the -OH 3 groups; (e) wekening nd/or brodening of the ν s nd ν symmetric nd ntisymmetric P-O-H 3 stretching modes, reltive to the gs phse, to the point of disppernce. As further point, it is noted tht H bonding between DMMP nd H 2 O is found 48 both theoreticlly nd experimentlly to occur vi PdOsH-O linkge with experimentl vlues of ν(pdo) )-17 cm -1 nd ν(o-h) )-203 cm -1 for the symmetric stretching mode of H 2 O. Studies 14,19 t the B3LYP/ G(d, p) level of the interction between orthosilicic cid, Si(OH) 4, nd the dimethyl phosphonte ion, [(H 3 O) 2 PO 2 ] s, lso indicte n energetic preference for H bonding to the two lone O toms versus to the methoxy O toms. In IR spectroscopic studies of the interction of DMMP with orgnic selfssembled monolyers terminted in OH til groups 49 nd with polymeric siloxne mterils, 50 bonding is suggested to occur Figure 6. Initil nd finl (optimized) structures for dsorbed DMMP. For clrity, only the molecule nd the ONIOM model system (cf. Figure 3) re shown. The dditionl H toms used s ONIOM link toms re not shown. Digrms () nd (c) show the initil structures, nd (b) nd (d) show the optimized structures. The hevy green lines show the dominnt bonding interctions. through PdOsH-O formtion, which leds to shifts in ν(pd O) s lrge s -48 cm -1 reltive to the gs phse. Thus the interprettion proposed by Knn nd Tripp, 38 involving dsorption exclusively vi the DMMP methoxy groups, differs from models put forth in connection with other similr systems. However, strong support for this interprettion comes from IR dt 38 for the dsorption of (H 3 ) x Si(OH 3 ) 4-x (x ) 1, 2, 3). The modes involving -OH 3 groups behve identiclly to those in DMMP, lthough ν(o-h) is only -397 cm -1 versus -524 cm -1 for DMMP. In the cse of DMMP, only one shifted SiO-H stretch is observed, not two s would be expected if dsorption involved both strong bond to H 3 O- nd wek bond to PdO. It therefore ppers tht the surfce chemistry of -SiO 2 might differ from tht of the model systems discussed bove omputed Results. lcultions were done strting with model in which dsorption occurs vi single PdOsH-O bond (Figure 6) or single (H 3 )(P)OsH-O bond (Figure 6c). As before, B3LYP/ G(2d,2p) ws used for the ONIOM high-level method, nd RHF/3-21G [3-21G(d) for P] ws used for the low-level method. The -SiO 2 model in ll cses is tht shown in Figure 3. The results re summrized in Tble 5. In the first cse, rerrngement of the initil structure occurs to give finl optimized structure (Figure 6b) with bonding of the PdO to two Si-OH groups, s did TP (Figure 5(iii)). The results obtined re E ds )-21.6 kcl/ mol ( E ds )-20.0 kcl/mol) nd nd Å for the two H-bond lengths. In the second cse, the optimized structure (Figure 6d) shows short H bond (1.783 Å) between one H 3 O- nd n Si-OH group nd longer bond (2.134 Å) between the other H 3 O- nd second Si-OH. This is essentilly the structure proposed by Knn nd Tripp. 38 The dsorption energy in this cse is E ds )-15.1 kcl/mol ( E ds )-13.6 kcl/mol). These E ds vlues re both lrger thn the corresponding result for TP (-10.4 kcl/mol, Tble 4) which is consistent with the experimentl observtion 38 tht DMMP dsorbs more strongly on -SiO 2 thn does TP. The E ds vlues re lso close to the observed desorption

8 Adsorption of TP, DMMP, nd Srin on Amorphous SiO 2 J. Phys. hem., Vol. 111, No. 26, TABLE 5: Results for DMMP Adsorption on -SiO 2 E b ds r(o---h) b,c ν 01(O-H) d δ(o-h) e ν(pdo) e Figure 6b Figure 6d experiment f Energies re in kcl/mol, bond lengths re in Å, nd frequencies re in cm -1. b Vlues obtined for dsorption on the lrge-cluster model shown in Figure 3. c Two H bonds re formed in either cse (see text). The two H-bond lengths for ech structure re tbulted. d Vlues obtined from n exct tretment of nhrmonicity (see text) for dsorption on the lrge-cluster model shown in Figure 3. Only the stronger (i.e., shorter) of the two H bonds is considered. e Vlues obtined for the smll-cluster Si 5O 7H 8 model. The clculted hrmonic vlues hve been scled by fctor of (see text). f Dt from ref 38. energy 47 of bout 16.9 kcl/mol, nd including smll E ZPE term (<1 kcl/mol, see bove) would improve the greement in the cse of dsorption vi the PdO group. A clcultion ws lso done strting with structure (not shown) in which DMMP dsorbs vi short H bonds ( 1.8 Å) between both H 3 O- groups nd two isolted Si-OH groups. This ws done in n effort to see if more stble form of the structure shown in Figure 6d could be forced by strting closer to the desired finl configurtion. Erly in the geometry optimiztion, the DMMP rerrnged from the initil structure to one exhibiting n H bond to single H 3 O- group nd second bond between nother Si-OH nd the PdO group. The resulting E ds ws kcl/mol, indicting metstble (i.e., locl minimum) configurtion reltive to those shown in Figure 6b,d. In effect, the H bond to either group (PdO or H 3 O-) interferes with optimum bonding to the other. It thus ppers tht the structure shown in Figure 6b is the most stble tht cn be chieved. The relevnt vibrtionl properties cn now be studied (Tble 5). Anhrmonic ν 01 (O-H) vlues for the structures shown in Figure 6b,d were obtined s described bove. In either cse, only the shorter (stronger) H bond ws considered, nd this gve ν 01 (O-H) ) -420 cm -1 (-423 cm -1 ) for the structure in Figure 6b (Figure 6d). These re to be compred with the experimentl vlue of ν 01 (O-H) )-524 cm -1.As is typiclly found, 15,16,21 the clculted vlues (which re nerly identicl for the two structures) underestimte somewht the experimentl result. The internl vibrtionl modes of DMMP were computed for dsorption in either of the two possible configurtions t the isolted Si-OH group of the Si 5 O 7 H 8 cluster (s discussed bove). The clcultion ws done t the B3LYP/ G(2d,2p) level with complete relxtion of both cluster nd dsorbte. The E ds vlues were -9.7 nd -4.8 kcl/mol, respectively, for H bonding to PdO nd to H 3 O-. The H-bond lengths were nd Å, respectively. As ws found for TP dsorption, the smll-cluster energies re significntly smller in mgnitude thn those for the lrger nd more flexible cluster with two H bonds per DMMP. Nevertheless, DMMP dsorbed vi the PdO group is gin found to be the more fvorble of the two structures, which is consistent with the lrge-cluster results given bove. ν(pdo) for H bonding to the PdO group is -29 cm -1, wheres for H bonding to the H 3 O- group it is +7 cm -1. The experimentl result is ν(pdo) )-19 cm -1. Thus, H bonding to the H 3 O- group gives ν(pdo) of the wrong sign. For gs-phse DMMP, the out-of-phse nd in-phse ν(- O) modes re computed (t the B3LYP/ G(2d,2p) level) to lie t 1007 nd 1031 cm -1, respectively, fter scling s described bove. The corresponding experimentl vlues 49 re 1050 nd 1075 cm -1. The bsolute vlues differ somewht from experiment, due to systemtic errors 24 in the frequency clcultion, but the difference between the in-phse nd out-of-phse modes is prcticlly identicl in theory (24 cm -1 ) nd experiment (25 cm -1 ). For DMMP dsorbed vi the PdO group, these modes show little or no shift in frequency ( 3 cm -1 ). For dsorption vi the H 3 O- group, ν(-o) is t 980 cm -1 for the H-bonded H 3 O- nd 1025 cm -1 for the free H 3 O-. This shift of 45 cm -1 is not by itself expected to mke the H-bonded -O stretch undetectble in the IR spectrum. For either type of dsorption, exmintion of the norml mode tomic displcements shows tht the -O stretching modes re coupled to Si-O-Si bond-stretching lttice modes of the cluster nd/or to the Si-O-H bending mode, ll of which occur in bout the sme energy region. The coupling cn be described s strong in the sense tht the -O stretching norml modes lso involve lrge Si-O stretching nd/or Si-O-H bending motions. This my ccount for the brodening seen experimentlly 38 for the -O stretching modes when either DMMP or (H 3 ) x Si(OH 3 ) 4-x is dsorbed on -SiO 2. Thus the brodening of the -O stretching modes is not necessrily n indiction of H bonding to the O tom of the H 3 O- groups in DMMP. Tble 5 lso gives results for the effect of dsorption on δ(o-h), the frequency of the Si-O-H bending mode. An exct vlue for δ(o-h) is difficult to determine in the computtionl results becuse of strong coupling between this mode nd the Si-O nd -O stretching modes, s discussed bove. For the bre cluster, δ(o-h) is found t 832 cm -1. For DMMP dsorbed vi the PdO group, Si-O-H bending contributes significntly to severl modes in the rnge of bout 1002 to 1089 cm -1. The highest of these ppers to be lmost pure Si-O-H bending. Using this vlue results in n estimte of δ(o-h) +257 cm -1. In sense, the mixing of Si-O-H bending with the -O nd Si-O stretching modes is the result of dsorption, which shifts the bending mode into the energy rnge of the stretches. Similr considertions for the cse of dsorption vi the H 3 O- group give δ(o-h) +170 cm -1, bsed on the frequency of the mode tht most closely pproximtes pure Si-O-H bending motion. Although this result is very close to the experimentl vlue ( +160 cm -1 ), the results for E ds nd for ν(pdo) continue to rgue in fvor of H bonding to the PdO group Srin Adsorbed on -SiO 2. There re, to our knowledge, no experimentl studies of Srin dsorbed on -SiO 2. Michlkov et l. 9 hve reported theoreticl results for the interction of Srin with Si(OH) 4. Bonding occurs vi O-HsOdP interction (with n H-bond distnce of Å) ccompnied by n dditionl interction between n H of the -H 3 group nd the O of second Si-OH group (t distnce of Å). H bonding to the O tom of the ipr-o-p group is not found to be importnt. At the B3LYP/6-31G(d) level, E ds ) -7.6 kcl/mol is obtined. Experimentl nd theoreticl results for the conformtion nd vibrtionl modes of free Srin hve been discussed in ref 10. The conformer used for gs-phse Srin, shown in Figure 1c, is identicl to tht used in ref 10. This is the second-lowest-energy conformer, 51,52 which lies <0.2 Kcl/mol bove the bsolute lowest-energy conformer. Different Srin conformers re defined in terms of rottions bout the ipr-o- nd -O-P bonds, nd different enntiomers of the sme conformer, obtined by interchnging the F nd phosphonyl O toms, re isoenergetic. 51

9 9322 J. Phys. hem., Vol. 111, No. 26, 2007 Bermudez TABLE 6: Results for Srin Adsorption on -SiO 2 H-bond site E ds E ds r(h---x) b PdO , ipr-o-p-f (O), (F) P-F , Energies re in kcl/mol, nd bond lengths rein Å. The tom shown in bold is the one involved in H bonding. b XtO orf.forpdo nd P-F, H bonds re formed to two Si-OH groups. For ipr-o-p-f, one H bond forms to the O tom nd nother to the F tom. Figure 7. Similr to Figure 6b but showing the optimized structure for Srin dsorbed vi H bonding to the PdO group. The violet sphere is the F tom. In the present work, three modes of dsorption were considered. These re H bonding to the O tom of the PdO or ipr-o-p group, s for DMMP, nd lso to the F tom. In ech cse, the lrge-cluster model (Figure 3) is used, nd the strting structure involves only one H bond (s in Figure 6,c). The results for the finl optimized structures re summrized in Tble 6. As in the cse of TP nd DMMP, the most stble structure (Figure 7) involves H bonding between the PdO group nd two Si-OH groups. Adsorption vi the F tom lso involves H bonding between the F nd two Si-OH groups but is less stble thn bonding to PdO. The optimized structures were exmined for the possibility of H bonds to other functionl groups; however, ll such intertomic distnces were 3.2 Å or greter. In comprison to the H-bond distnces shown in Tbles 1, 4, nd 6, this is considered to be too lrge for significnt contribution to E ds. The initil structure with single H bond to the O tom of the ipr-o-p group rerrnged during optimiztion to one with this H bond intct nd with second H bond between F nd nother Si-OH. However, this is gin less stble thn two H bonds to the PdO group. The vibrtionl properties, computed for the most stble optimized structure using the procedure described bove, were ν 01 (O-H) )-510 cm -1 (for the shorter of the two H bonds) nd ν(pdo) )-25 cm -1. Given the results for DMMP (Tble 5), the computed ν 01 (O-H) for Srin probbly underestimtes the experimentl vlue, nd the computed ν(pdo) probbly overestimtes the experimentl vlue. 5. Summry Ab initio Q clcultions hve been performed to study the interction of TP, DMMP, nd Srin vi H bonding to Si- OH groups on the hydroxylted -SiO 2 surfce. The results re s follows. (1) A smll, cgelike cluster gives resonble results for the vibrtionl spectrum of dsorbed TP nd DMMP. This includes the dsorption-induced red shift of the silnol ν(o-h) stretching mode when nhrmonicity is treted explicitly. However, the dsorption energy is underestimted becuse of the rigidity of the model, which limits the extent of relxtion t the dsorption site, nd to the omission of interctions between the dsorbte nd more thn one Si-OH. (2) For the dsorbtes studied here, n ONIOM tretment requires the use of n b initio method (here RHF/3-21G) for the low level, rther thn semiempiricl method. Including d orbitls in the low-level bsis set (e.g., 3-21G(d)) is required for the dsorbte P tom. (3) For ll three species studied here, H-bond formtion between the O tom of the PdO group nd two Si-OH groups is the most energeticlly fvorble dsorption mechnism. For DMMP nd Srin, H bonding to lkoxy O toms is less fvorble, s is H bonding to the Srin F tom. (4) The energetics nd geometry of dsorption for DMMP nd Srin on -SiO 2 depend on the locl Si-OH environment. The bidentte PdO(sH-O) 2 structure is the dominnt fctor in dsorption. However, contributions from other, weker interctions cn be expected in rel systems if the locl Si-OH configurtion is such tht these effects re energeticlly fesible. H bonding to more thn one functionl group is seen in the present work for dsorption of DMMP in metstble configurtion with one H bond to the O tom of H 3 -O-P group nd second to the PdO. It is lso seen for Srin dsorption (gin in metstble configurtion) vi the O tom of the ipr-o-p group, where second H bond to the F tom lso forms. Similr effects hve lso been observed in computtionl studies 3,4,9 of Srin on other oxides. (5) The chrcteristics of DMMP nd Srin with respect to dsorption on the -SiO 2 surfce re closely similr. In prticulr, the computed dsorption energies of the two species re virtully identicl. Hence, DMMP is good simulnt for Srin in this regrd. It is not, however, to be concluded tht the sme is necessrily true for ll dsorbent mterils nd under ll conditions. For exmple, the effect of codsorbed H 2 Oon the dsorption of DMMP or Srin hs not yet been nlyzed, either computtionlly or experimentlly. Furthermore, for dsorption on OH-free γ-al 2 O 3, which occurs vi different mechnism, 10 DMMP my interct somewht more strongly thn does Srin. Acknowledgment. We re indebted to R. M. Vn Ginhoven for providing tomic coordintes for the -SiO 2 cluster nd to A. Michlkov for criticl reding of the mnuscript. This work ws supported by the Defense Thret Reduction Agency (DTRA) nd by grnt of computer time from the DOD High- Performnce omputing Moderniztion Progrm t the AS- MSR, Wright-Ptterson Air Force Bse. References nd Notes (1) Wng, J.; Gu, J.; Leszczynski, J. J. Phys. hem. B 2006, 110, (2) Šečkute, J.; Menke, J. L.; Emnett, R. J.; Ptterson, E. V.; rmer,. J. J. Org. hem. 2005, 70, (3) Michlkov, A.; Gorb. L.; Ilchenko, M.; Zhikol, O. A.; Shishkin, O. V.; Leszczynski, J. J. Phys. hem. B 2004, 108, (4) Michlkov, A.; Ilchenko, M.; Gorb, L.; Leszczynski, J. J. Phys. hem. B 2004, 108, (5) Hurley, M. M.; Wright, J. B.; Lushington, G. H.; White, W. E. Theor. hem. Acc. 2003, 109, 160. (6) Zheng, F.; Zhn,.-G.; Ornstein, R. L. J. hem. Soc., Perkin Trns , (7) Glukhovtsev, M. N.; Bch, R. D.; Ngel,. J. J. Phys. hem. A 1998, 102, (8) Ptterson, E. V.; rmer,. J. J. Phys. Org. hem. 1998, 11, 232. (9) Michlkov, A.; Mrtinez, J.; Zhikol, O. A.; Gorb, L.; Shishkin, O. V.; Leszczynsk, D.; Leszczynski, J. J. Phys. hem. B 2006, 110, (10) Bermudez, V. M. J. Phys. hem. 2007, 111, 3719.