Theoretical Study of the Potential Energy Surfaces of the Van Der Waals H 2 O-X 2

Transcription

1 722 J. Phys. Chem. A 2008, 112, Theoreticl Study of the Potentil Energy Surfces of the Vn Der Wls H 2 O-X 2 (X ) Cl or Br) Complexes Thr Ayed,, Rmón Hernández Lmoned,*, nd Kenneth C. Jnd Centro de InVestigciones Químics, UniVersidd Autónom del Estdo de Morelos, CuernVc, Morelos 62210, Mexico, nd Deprtment of Chemistry, UniVersity of Cliforni t IrVine, IrVine, Cliforni ReceiVed: October 8, 2007; In Finl Form: NoVember 1, 2007 An b initio study of the interctions between H 2 O nd Cl 2 nd H 2 O nd Br 2 hs been performed. We present clcultions using both the UMP2 level nd the UCCSD(T) level of correltion with the ug-ccpvtz bsis. The ug-cc-pvqz bsis ws tested for selected geometries nd ws found to yield results similr to the smller bsis. For the H 2 O-Cl 2 ction, C 2V structure hs been identified s the minimum, with D e ) 6500 cm -1 (78 kj/mol). A low-lying excited stte hs D e ) 6000 cm -1 (72 kj/mol). The dibtic nd verticl ioniztion energies of the complex re 10.7 nd 11.0 ev, compred to the experimentl dibtic vlue, 11.5 ev, for free chlorine. For the H 2 O-Br 2 ction, the clcultions re more subtle due to secondorder Jhn-Teller effects nd result in C s structure t the minimum, with D e ) 6300 cm -1 (75 kj/mol), yielding n dibtic ioniztion energy of 9.9 ev compred to the corresponding experimentl vlue, 10.5 ev, for free bromine. The reltively lrge binding energies give rise to strong norml mode couplings such tht the hlogen stretching mode becomes mixed with the wter bending nd other intermoleculr modes, resulting in very lrge frequency shifts. Verticl ioniztion energies nd ion vibrtionl frequencies lso re reported nd used to discuss possible experiments to obtin more precise dt for ech of the complexes. I. Introduction In 1976 it ws discovered tht the dimer of the interhlogen molecule ClF nd the potentil hydrogen bond donor molecule HF is stbilized by electron pir dontion from the HF to the σ* orbitl on the Cl end of the ClF moiety rther thn by the expected hydrogen bond to the fluorine tom. 1 Since then, there hs been rpidly expnding literture regrding electron pir dontion to hlogen molecules. 2,3 Legon nd co-workers propose tht the phenomenon is distinct enough to deserve its own nme: hlogen bonding. 4,5 Microwve spectr hve been reported for both the H 2 O-Cl 2 nd the H 2 O-Br 2 dimers. In ech cse, the O-X bond length is quite short, 2.85 Å, compred to the sum of the vn der Wls rdii, O, 1.52 Å; Cl, 1.75 Å; nd Br, 1.85 Å. Recent clcultions of the well depth for the neutrl dimers using the CCSD(T) correltion method, n ugmented qudruple-ζ bsis set, nd the counterpoise correction for bsis set superposition error yielded 917 cm -1 (11 kj/mol) for H 2 O-Cl 2 nd 1183 cm -1 (14 kj/mol) for H 2 O- Br 2. 6 These well depths re significnt frction of the wter dimer well depth, 1756 cm -1 (21 kj/mol), nd greter thn the 8 kj/mol well depth for the HCl dimer. 7 In ddition, the H 2 O- Cl 2 nd the H 2 O-Br 2 interctions were found to be very sensitive to the X-X distnce. For instnce, modultion of the Br-Br bond by (0.05 Å, pproximtely the clssicl mplitude for the ground vibrtionl stte, modultes the O-X bond strength by over 100 cm -1, 10% of the well depth. These results identify the wter-hlogen dimers s both interesting nd complex with respect to their intermoleculr forces. There is lso recent experimentl dt tht illustrtes tht the interctions between wter molecules nd hlogens re unusully * Author to whom correspondence should be ddressed. E-mil: rmon@buzon.uem.mx. Universidd Autónom del Estdo de Morelos. University of Cliforni t Irvine. complex nd interesting. The pek of the Br 2 visible vlence bnd excittion shifts 1700 cm -1 towrd higher energy in going from the gs phse to queous solution. This blue shift hs been ttributed to the sme neighbor dontion of n oxygen tom lone electron pir from wter molecule to the Br 2 σ* orbitl tht results in the reltively deep dimer well. Vlence excittion of the bromine promotes n electron from the π* orbitl to the σ* orbitl, resulting in O-X repulsion in the excited stte. In contrst, for bromine surrounded by wter forming nerly symmetric cge of clthrte-hydrte crystl, the shift of the visible excittion bnd mximum from the gs phse is reduced to only 360 cm -1. In this cse the lone electron pirs of the wter molecules re ll involved in hydrogen bonding nd thus less vilble to interct with the bromine molecule even though it is in close contct. The unusul properties of neutrl wter-hlogen dimers hve inspired us to investigte the dimer ctions. These species re interesting in their own right nd lso s possible subject for multiphoton ioniztion (MPI) spectroscopy. The ioniztion potentils of H 2 O, Cl 2, nd Br 2 re, respectively, 12.62, 11.50, nd ev. 8,9 Thus the lowest ioniztion chnnel is expected to be removl of n electron from the hlogen π* orbitl. In the isolted hlogen, this results in doubly degenerte electronic stte. Interction with the wter splits this degenercy, resulting in some interesting computtionl chllenges. The H 2 O-Cl 2 nd the H 2 O-Br 2 ctions hve different symmetries t their minim. Below, we explin the resons for this difference. Although the neutrl nd ction dimers hve somewht different structures, they re similr enough tht MPI spectroscopy should yield long-lived ction sttes tht cn be used to extrct the rich vibrtionl structure tht will provide significnt insight into the intermoleculr interctions. Also, MPI spectroscopy will be useful technique for double resonnce spectroscopy to obtin the neutrl dimer vibrtionl spectrum /jp709831q CCC: $ Americn Chemicl Society Published on Web 01/05/2008

2 H 2 O-X 2 (X ) Cl or Br) Complexes J. Phys. Chem. A, Vol. 112, No. 4, Figure 1. Moleculr orbitl digrms of Cl 2,H 2O, nd Br 2. II. Theoreticl Detils The ground potentil energy surfces (PESs) of the ction H 2 O-X 2 (X ) Cl or Br) systems were studied using Møller- Plesset perturbtion theory (MP2) 10,11 with the unrestricted formlism. The geometries of ll of the sttionry points re optimized using the ug-cc-pvtz bsis sets 12,13 nd hrmonic vibrtionl frequency nlysis performed to identify minim nd sddle points. In ddition, we performed some intrinsic rection coordinte (IRC) clcultions to connect trnsition sttes nd minim nd lso to locte other minim tht were difficult to find. The reltive stbilities of severl optimized structures were evluted lso by single-point energy clcultions t the openshell coupled-cluster level tking into ccount the perturbtive contributions of connected triple excittions UCCSD(T). 14 The zero-point energy (ZPE) correction lso ws clculted t the UMP2/ug-cc-pVTZ level. Both ZPE effects nd higher-level correltion tretment re required to obtin relible thermochemicl informtion for these systems. The effect of bsis set sturtion ws tested t the UCCSD- (T) level by using both ug-cc-pvtz nd ug-cc-pvqz. 15 The verticl trnsition energies hve been clculted t this high level of theory. All of our clcultions re full-electron nd were performed with the Gussin 03 progrm. 16 III. Results nd Discussion A. Clcultions for the Monomer Constituents. The strting point for our study ws the clcultion of the ioniztion potentils (IPs) for wter nd the two dihlogen molecules. In Tble 1, we report dibtic ioniztion energy vlues for H 2 O, Cl 2, nd Br 2 clculted t the UMP2 nd UCCSD(T) levels of theory using the ug-cc-pvtz bsis set. (The smll correction for ZPE is not included.) It is encourging tht theory vlues gree well with experiment nd do not depend too strongly on the bsis set or method used. It is interesting tht the moleculr orbitl scheme illustrted in Figure 1 is sufficient for interpreting the ioniztion energies. The highest occupied moleculr orbitl (HOMO) of ech dimer is the ntibonding π* type orbitl of the X 2 frgment. B. Results for H 2 O-Cl 2. Due to the unpired electron, the hlogen ctions hve degenerte ground stte. Attching wter molecule with C 2V symmetry constrint (the symmetry xis is colliner with the dihlogen) leds to electronic sttes of B 1 nd B 2 sptil symmetry becuse the unpired electron occupies the π* gx orbitl of b 1 symmetry or the π* gy orbitl of b 2 symmetry, respectively. When decresing the group of symmetry to C s, the electronic sttes sptil symmetry will be A nd A. The relevnt structures re illustrted in Figures 2 nd 3. For H 2 O-Cl 2, optimiztion t the UMP2 level of theory under the C 2V symmetry constrint revels tht both the B 1 nd the B 2 electronic sttes correspond to locl minim on their potentil energy surfces. The oxygen nd the two chlorine toms form the C 2 xis of symmetry. The C 2V structure is shown in Figure 2, the optimized moleculr prmeters re listed in Tble 2, nd the dissocition energies re given in Tble 3. The B 1 electronic stte, for which the unpired electron occupies the orbitl perpendiculr to the plne contining ll toms, is more stble, by 614 cm -1, thn the B 2 electronic stte in which the unpired electron is in the plne of the molecule. This result is s expected becuse the b 1 orbitl (π* gx ) of the Cl 2 frgment is close in energy to the HOMO of wter (one of the lone pirs) with the sme symmetry. Therefore the two orbitls cn interct strongly. For the B 2 electronic stte, the unpired electron occupies the b 2 orbitl, nd the interction between this orbitl nd the wter orbitl of b 2 symmetry is smll becuse the ltter is bonding orbitl with lower energy. Mulliken popultion nlysis shows tht the chlorine tom ner the oxygen becomes somewht more positive, 0.59 nd 0.58 in the B 1 nd B 2 electronic sttes, respectively, compred to 0.50 for the isolted Cl 2. The Cl-Cl bond distnce increses only slightly due to the interction. In contrst, the clculted Cl-Cl stretching frequency is significntly higher (lmost fctor of 2) for the complex thn tht for the free ction. Most likely the reson for the observed difference is due to the shortening of the O-X distnce in the ction, 2.34 Å compred to 2.77 Å in the neutrl compound, nd the corresponding substntil increse in the binding energy of the complex tht stiffens the Cl 2 stretching

3 724 J. Phys. Chem. A, Vol. 112, No. 4, 2008 Ayed et l. Figure 2. Geometry of the H 2O-Cl 2 complex t the globl minimum. motion within the complex. Furthermore looking t the corresponding norml mode we observe mixing of the chlorine stretching motion with the bending of the wter molecule nd the intermoleculr stretch. Reltively smll shifts re clculted for the norml modes of the wter moiety in the complex. Note tht the chrge polriztion nd smll shifts for wter re lso found in the cse of the neutrl dimer; 6 in tht cse the chlorine stretch showed smll shift from the monomer. Both the B 1 nd the B 2 electronic sttes exhibit one very lowfrequency motion, 14 nd 19 cm -1, respectively. The lowfrequency mode for the B 1 stte hs b 1 symmetry, nd it exhibits bending of the wter molecule out of the HOCl plne, wheres the low-frequency mode of the B 2 stte hs b 2 symmetry nd exhibits bending of the wter in the plne of the complex. An explntion of the low vlues nd symmetries of these norml modes will be given lter. Here we simply note tht ech stte will exhibit wide-mplitude vibrtionl motion. C. Results for H 2 O-Br 2. The minim for the H 2 O-Br 2 ction hve C s symmetry s illustrted in Figure 3. However, we show preliminry nlysis constrining the geometry to C 2V symmetry (using the UMP2/ug-cc-pVTZ level of theory) for comprison to the H 2 O-Cl 2 results. The structurl results re given in Tble 4. The results t this level for the H 2 O- Br 2 ction re similr to those for the H 2 O-Cl 2 ction: short O-Br bond, 2.5 Å, high Br-Br stretching frequency, 825 cm -1, with more modest chnges in the other prmeters reltive to the free molecules. Frequency clcultions for the B 1 nd B 2 electronic sttes of H 2 O-Br 2 yield smll imginry frequencies, 45i nd 46i cm -1, respectively, for the two modes tht hd smll but rel frequencies for H 2 O-Cl 2. Thus these structures represent trnsition sttes between the rel minim, indicting the need to relx the C 2V symmetry constrint. To locte the minimum structure linked with this trnsition stte we employed the IRC technique. This consists of following the imginry frequency descent pth from the trnsition stte down to the minimum structure on the potentil energy surfce. Figure 4 shows the reltive stbility of ech geometry clculted t UMP2 level when geometry constrints re relxed s represented in Figure 3. The C 2V,B 1 trnsition stte optimizes to C s structure with A electronic stte symmetry, 76.4 cm -1 below the trnsition stte. Thus this sddle point seprtes two equivlent minim. Correcting for ZPE rises the reltive energy of the C s structure bck bove tht of the B 1 trnsition stte (Figure 4). So, this level of theory predicts tht the stte will exhibit wide-mplitude motion, resulting in vibrtionl verging to the more symmetric B 1 structure. Note tht significnt frction of the ZPE increse for the C s minimum is due to the incresed Br-Br stretching frequency, which is strongly coupled to the intermoleculr stretch nd bend nd lso the bending mode of the wter moiety. Figure 3. Optimized structures of the H 2O-Br 2 complex clculted t the UMP2 level. Two electronic sttes hve C 2V structure tht is trnsition stte. One relxes to C s globl minimum, nd the second to nother trnsition stte with C s symmetry. This finl trnsition stte relxes to secondry minimum with no symmetry (C 1) s shown. In the light of the UMP2 results, we decided to perform single-point UCCSD(T)/ug-cc-PVTZ clcultion using the UMP2/ug-cc-pVTZ geometries. At this higher level, the stbility of the C s structure reltive to the B 1 trnsition stte increses to cm -1. Adding ZPE corrections clculted t the UMP2 level to the reltive electronic energy clculted t the UCCSD(T) level leves the C s structure more stble but only by 36.7 cm -1. The flt surfce for this bending coordinte indictes tht it my be difficult to completely converge the clcultion while including ZPE effects. Multiphoton ioniztion spectroscopy my be ble to yield vibrtionl frequencies tht will id in definitive ssignment. Further optimiztion of the C 2V,B 2 stte of the H 2 O-Br 2 ction yields even more complicted results. Following the norml mode of the imginry frequency leds to new sddle point structure hving C s symmetry rther thn to the expected minimum. The symmetry of this electronic stte is A, nd ll the toms of the complex lie in the plne of symmetry s shown in Figure 3. This structure will be denoted s C s nd is 81 nd 177 cm -1, respectively, for UMP2/ug-cc-pVTZ nd UCCSD- (T)/ug-cc-pVTZ below the B 2 stte. As for the B 1 stte, dding the UMP2 ZPE corrections to the UMP2 energy vlues reverses the reltive stbilities of the B 2 nd A trnsition sttes, while the ZPE correction to the UCCSD(T) results leves the A stte s the lower minimum by 27 cm -1. Further relxtion of the symmetry yields C 1 minimum energy structure s illustrted in Figure 3. This structure is chrcterized by Br-Br nd Br O distnces equl to nd Å, which re similr to but distinguishble from those of the C s minimum. Zero-point energy corrections leve this s the locl minimum for both UMP2 nd UCCSD(T). For UCCSD(T), the C 1 structure is 523 cm -1 below the C s sddle point nd 551 cm -1 below the B 2 sddle point, including ZPE. Figure 4 summrizes the reltive energies of the H 2 O-Br 2 sttes for different levels of clcultion. The C s structure remins the globl minimum but only by 24 cm -1 if ZPE effects re included. Tble 4 gives the geometric prmeters of both C s nd C 1 structures. We note tht the min differences between

4 H 2 O-X 2 (X ) Cl or Br) Complexes J. Phys. Chem. A, Vol. 112, No. 4, TABLE 1: Theoreticl Vlues of the Ioniztion Energies (ev) Compred to the Experimentl Vlues UMP2/ug-cc-pVTZ UCCSD(T)/ug-cc-pVTZ UCCSD(T)/ug-cc-pVQZ expt. H 2O Cl b Br b Reference 8. b Reference 9. TABLE 2: Min Geometric Prmeters of the H 2 O-Cl 2 Dimer Obtined from the UMP2/ug-cc-pVTZ Level of Clcultions nd Norml Mode Vlues of Wter nd Cl 2 Clculted t the Sme Level monomer H 2O Cl 2 (Cl 2,H 2O) B 1 electronic stte B 2 electronic stte Cl1-Cl Cl2 O Cl1-Cl2 O H1-O-H ν 1 ( 1) b ν 2 ( 1) b ν 3 (b 2) b ν b Cl-Cl q(cl1) c q(cl2) c Bond lengths in re Å, nd bond ngles in degrees. b Wvenumbers re in cm -1. c Atomic chrge clculted t the UMP2 level from Mulliken popultion nlysis. these two minim re the vlue of the Br-Br-O ngle, which is equl to in the former nd in the ltter, nd the Br-Br-O-H dihedrl ngles. D. Comprison of H 2 O-Cl 2 nd H 2 O-Br 2. As illustrted bove, symmetry breking for H 2 O-Br 2 results in significnt differences from the potentil energy surfce of the H 2 O-Cl 2 ction. The stbilities of the C 2V structures for both chlorine nd bromine complexes cn be rtionlized by invoking the well-known second-order Jhn-Teller (SOJT) effects. 18 Looking t the MO digrm of Figure 1, we cn predict tht low-lying excited stte is obtined by promoting the unpired electron from the π* gx or π* gy orbitls to σ* u orbitl. Within the C 2V symmetry, the corresponding electronic stte symmetries re B 1,B 2, nd A 1. Appliction of the SOJT suggests tht for the 2 B 1 electronic stte there will be n importnt contribution to lower the vibrtionl frequency of modes with b 1 symmetry; nlogously for the 2 B 2 electronic stte low-frequency modes of b 2 symmetry re expected. Indeed in the cse of chlorine the lowest frequencies belong to these symmetries for the corresponding, B 1 nd B 2, electronic sttes: 14 nd 19 cm -1.Inthe cse of bromine, the C 2V structures re unstble with respect to SOJT effects, leding to trnsition sttes with imginry frequencies of expected symmetries nd vlues of 45i nd 46i cm -1. The difference in chlorine nd bromine is consistent with the fct tht the energy difference between π* nd σ* is smller in the ltter. Still the vlues for ll of these frequencies re smll enough to estblish chllenge for ccurte predictions using b initio quntum chemistry. The second sddle point found for the bromine complex in C s symmetry (the C s structure in Figure 3) lso cn be rtionlized using SOJT effects, though the rgument involves different excited electronic stte: In this cse the promotion is out of the doubly occupied π* orbitl becuse this will led to n excited stte of the complex of symmetry consistent with n imginry frequency for norml mode of tht sme symmetry s observed in the clcultions. Finlly, we note tht the lrge increse in the dihlogen stretching frequency upon complextion hs no simple correltion with chnge in dihlogen bond length. In the cse of chlorine, the bond length increses, perhps contrry to chemicl intuition. The cse of bromine is more complicted due to the vriety of structures found. For the C 2V structures, nlogous to chlorine, the bond length decreses noticebly ( Å), but in the cse of ll other structures the chnge in bond length is less mrked, nd in prticulr for the globl minimum it is negligible. E. Verticl Ioniztion Clcultions. The highest intensity for nonresonnt MPI spectroscopy will be in the region of verticl ioniztion. Therefore it is useful to both clculte the verticl ioniztion energies nd exmine how high the levels of the ions re over the dibtic minim. Both the H 2 O-Cl 2 nd the H 2 O-Br 2 neutrl dimers hve C s symmetry similr to tht shown in Figure 3 with smll brrier t the C 2V structure. For the purpose of this study, we clculted the verticl ioniztion energy from the globl minimum to ech of the two ction sttes. Tble 3 lists the results using both the UMP2 nd the UCCSD(T) levels of theory with the ug-cc-pvtz bsis. The UMP2 energy vlues corresponding to the 1 A - 2 A nd 1 A - 2 A verticl trnsitions re equl to nd ev for H 2 O-Cl 2 nd to nd ev for H 2 O-Br 2. Thus the stte splittings for the ctions re reltively smll t the neutrl geometry. These results were confirmed by the UCCSD- (T) clcultions. Figure 5 shows the potentil energy curves s function of the O-Br distnce for both the neutrl H 2 O-Br 2 nd the ction complex clculted with UCCSD(T)/ug-cc-pVTZ. The symmetry nd ngles re fixed t the corresponding vlues for ech minimum geometry. Although the equilibrium Br-O bond length for the neutrl compound, 2.8 Å, is significntly longer thn tht for the ction complex, 2.5 Å, the much deeper well for the ction mens tht verticl ioniztion leves the ction well below its dissocition limit. As discussed bove, we expect there to be strong bend stretch coupling for the ction, so there should be strong progressions in severl modes, probbly spreding to the dibtic minimum energy. This prediction hs two importnt implictions for the experiment. First, it should be possible to excite to ner the ground stte of the ction, yielding long-lived stte tht will llow mss selection. This will llow MPI to be used s detection technique to obtin mss-selected infrred spectroscopy of the neutrl dimer. Second, the extensive vibrtionl structure expected for the ction could yield n ccurte determintion of its PES. Given the complexity of the systems, nlysis of the vibrtionl structure will require creful comprisons between the experiment nd the theory. F. Possible Clcultions for Lrger Clusters. Eventully, we hope to extend these studies to lrger clusters with hopes of helping to interpret the condensed phse experiments discussed in the Introduction. In tht regrd, the reltive insensitivity of the well depths nd ioniztion energies to the bsis set nd level of theory is encourging. It ppers tht UMP2/ug-cc-pVTZ clcultions provide relible option to investigte the ddition of more wter molecules to both the neutrl hlogens nd their ctions.

5 726 J. Phys. Chem. A, Vol. 112, No. 4, 2008 Ayed et l. Figure 4. () Reltive energies of ech H 2O-Br 2 structure with reference to the globl minimum clculted t UMP2 level without the ZPE corrections; (b) the corrected UMP2 reltive energy; (c) the UCCSD(T) single-point energy; nd (d) the UCCSD(T) single-point energy corrected with the UMP2 ZPE corrections. TABLE 3: Dissocition Energy nd Verticl Trnsition Vlues of Both Chlorine nd Bromine Complexes Clculted t the UMP2 nd UCCSD(T) Levels of Theory with the ug-cc-pvtz Bsis Set UMP2 level UCCSD(T) level b H 2O-Cl 2 H 2O-Br 2 H 2O-Cl 2 H 2O-Br 2 D e 7113 (0.88) (B 1) 6971 (0.86) 6515 (0.81) (B 1) 6088(0.76) 6498 (0.81) (B 2) (C s,a ) 5989 (0.74) (B 2) (C s,a ) 6770 (0.84) (B 1) b 6322(0.78) 6169 (0.76) (B 2) b (C s,a ) b Verticl Trnsition 1 A - 2 A (10.99) (10.12) (11.03) (10.16) 1 A - 2 A (11.01) (10.20) (11.05) (10.24) Vlues re in cm -1 ; the vlues between prentheses re in ev. b Vlues re clculted with the ug-cc-pvqz bsis set. TABLE 4: Selected Bond Distnces (Å), Angles (deg), nd Frequencies (cm -1 ) for the Trnsition Sttes nd Minim Structures of the H 2 O-Br 2 Dimer Optimized t the UMP2 Level C 2V(TS) C s(br1-br2-o-h1*0) C s C 1 electronic stte Br 2 B 1 B 2 A A A Br1-Br Br2 O Br1-Br2 O Br2-O-H1(2) (127.2) Br1-Br2-O-H1(2) (99.6) ν 1 ( 1) b ( ) 1674( ) 1676() ν 2 ( 1) b ( ) 3778( ) 3777() ν 3 (b 2) b ( ) 3883( ) 3879() ν b Br-Br q(br1) c q(br2) c Bond lengths re in Å, nd bond ngles in degrees. b Wvenumbers re in cm -1. c Atomic chrge clculted t the UMP2 level from Mulliken popultion nlysis. IV. Summry nd Conclusions In the present study, we performed b initio clcultions of the PESs of the dihlogen-wter ction complexes (chlorine nd bromine). The UMP2/ug-cc-pVTZ level of theory ws employed to optimize the geometries of ll sttionry points. The C 2V structures corresponding to the B 1 nd B 2 electronic sttes were identified s minim for H 2 O Cl 2, wheres they re first-order sddle points for the bromine complex. For the ltter, C s symmetry structure, nlogous to the neutrl compound, hs been identified s the globl minimum nd correlting with the B 1 trnsition stte. Following the imginry frequency coordinte chrcterizing the B 2 electronic stte, we

6 H 2 O-X 2 (X ) Cl or Br) Complexes J. Phys. Chem. A, Vol. 112, No. 4, ctions, detiled ssignment of the spectr will still be complex. Finlly, we note the very lrge shifts in the hlogen stretching mode upon complex formtion due to strong mixing with the bending mode of wter nd intermoleculr modes. Acknowledgment. T.A. nd K.C.J. cknowledge support by the U. S. Ntionl Science Foundtion (Grnt No. CHE ). T.A. nd R.H.L. cknowledge finncil support from NSF-CONACYT Bilterl Grnt No. J nd SESIC- FOMES2000 for unlimited time on the IBM p690 supercomputer t UAEM. Figure 5. Potentil energy curves of the bromine complexes (ction complex in dshed line nd neutrl complex in solid line) clculted t the UCCSD(T)/ug-cc-pVTZ level of theory. The x-xis gives the center of mss seprtion, Å, between the Br 2 nd the H 2Otthe geometry of ech dimer minimum. hd found second sddle point (C s ). Upon further optimiztion one reches locl minimum without symmetry nd tht lies only 24 cm -1 bove the globl minimum t the highest level of theory. The min fetures of the PES were rtionlized invoking the SOJT effect. Due to the smll energy brriers nd vriety of isomers, it ws deemed necessry to perform higher-level clcultions tht consisted of single-point UCCSD(T)/ug-ccpVTZ with the UMP2/ug-cc-pVTZ geometries s references. The electronic energy ordering for ech bromine structure remins the sme s tht for the UMP2 clcultions. Finlly, ZPE corrections clculted t the UMP2/ug-cc-pVTZ level were dded to the UCCSD(T) vlues to obtin the most relible estimtes for the stbilities of the different structures. The verticl trnsition energies were clculted for both chlorine nd bromine systems. We find tht the A nd the A electronic sttes corresponding to the sme ction complex re very close in energy nd tht the IP of the bromine complex is smller thn the IP of the chlorine complex. The effect of the bsis sets hs been tested, nd the theoreticl results show smll difference between the dissocition energy vlues clculted t the UCCSD(T) level with the ug-cc-pvtz bsis nd the ones clculted t the sme level nd with the ug-cc-pvqz bsis. The bromine PESs re quite flt. This, long with the ner degenercy of the electronic sttes, mkes detiled chrcteriztion of the surfces chllenging. Also, widemplitude motion nd ZPE effects my yield verge structures somewht different from the minim. Thus, lthough the present results will be vluble for n experimentl serch of these References nd Notes (1) Nowick, S. E.; Jnd, K. C.; Klemperer, W. J. Chem. Phys. 1976, 65, (2) Legon, A. C.; Thumwood, J. M. A.; Wclwik, E. R. Chem.sEur. J. 2002, 8, 940. (3) Dvey, J. B.; Legon, A. C.; Thumwood, J. M. A. J. Chem. Phys. 2001, 114, 61. (4) Nguyen, H. L.; Horton, P. N.; Hursthouse, M. B.; Legon, A. C.; Bruce, D. W. J. Am. Chem. Soc. 2004, 126, 16. (5) Legon, A. C. Angew. Chem., Int. Ed. 1999, 38, (6) Hernández-Lmoned, R.; Uc Ross, V. H.; Bernl-Uruchurtu, M. I.; Hlberstdt, N.; Jnd, K. C. J. Phys. Chem. A, in press. (7) Bunker, P. R.; Ep, V. C.; Jensen, P.; Krpfen, A. J. Mol. Spectrosc. 1991, 146, 200. (8) Potts, A. W.; Prince, W. C. Proc. R. Soc. London., Ser. A. 1972, 326, 181. (9) Yench, A. J.; Hopkirk, A.; Hiry, A.; Donovn, R. J.; Goode, J. G.; Mier, R. R. J.; King, G. C.; Kvrn, A. J. Phys. Chem. 1995, 99, (10) Hed-Gordon, M.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1988, 153, 503. (11) Frisch, M. J.; Hed-Gordon, M.; Pople, J. A. Chem. Phys. Lett. 1990, 166, 275. (12) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, (13) Kendll, R. A.; Dunning, T. H., Jr.; Hrrison, R. J. J. Chem. Phys. 1992, 96, (14) Wtts, J. D.; Guss, J.; Brtlett, R. J. J. Chem. Phys. 1993, 98, (15) Woon, D. E, Dunning, T. H., Jr. J. Chem. Phys. 1993, 98, (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseri, G. E.; Robb, M. A.; Cheesemn, J. R.; Zkrzewski, V. G.; Montgomery, J. A., Jr.; Strtmnn, R. E.; Burnt, J. C.; Dpprich, S.; Millm, J. M.; Dniels, A. D.; Kudin, K. N.; Strin, M. C.; Frks, O.; Tomsi, J.; Brone, V.; Cossi, M.; Cmmi, R.; Mennucci, B.; Pomelli, C.; Admo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayl, P. Y.; Cui, Q.; Morokum, K.; Slvdor, P.; Dnnenberg, J. J.; Mlick, D. K.; Rbuck, A. D.; Rghvchri, K.; Foresmn, J. B.; Cioslowski, J.; Ortiz, J. V.; Bboul, A. G.; Stefnov, B. B.; Liu, G.; Lishenko, A.; Piskorz, P.; Komromi, I.; Gomperts, R.; Mrtin, R. L.; Fox, D. J.; Keith, T.; Al-Lhm, M. A.; Peng, C. Y.; Nnykkr, A.; Chllcombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzlez, C.; Hed-Gordon, M.; Replogle, E. S.; Pople, J. A.Gussin 03, revision C.02; Gussin, Inc.; Wllingford CT, (17) Rmondo, F.; Sodeu, J. R.; Roddis, T. B.; Willims, N. A. Phys. Chem. Chem. Phys. 2000, 2, (18) Simons, J. Energetic Principles of Chemicl Rections; Jones nd Brtlett Publishers: Boston, 1983.