Thermochemical Properties and Structure of Phenol-(H 2 O) 1-6 and Phenoxy-(H 2 O) 1-4 by Density Functional Theory

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1 6062 J. Phys. Chem. A 2000, 104, Thermochemicl Properties nd Structure of Phenol-(H 2 O) 1-6 nd Phenoxy-(H 2 O) 1-4 by Density Functionl Theory R. C. Guedes,, B. J. Cost Cbrl,*,, J. A. Mrtinho Simões,, nd H. P. Diogo Deprtmento de Químic e Bioquímic, Fculdde de Ciêncis d UniVersidde de Lisbo, Lisbo, Portugl, Centro de Físic d Mtéri Condensd d UniVersidde de Lisbo, AV. Professor Gm Pinto 2, Lisbo, Portugl, Centro de Ciêncis Moleculres e Mteriis, Fculdde de Ciêncis d UniVersidde de Lisbo, Lisbo, Portugl, nd Centro de Químic Estruturl, Complexo I, Instituto Superior Técnico, Lisbo, Portugl ReceiVed: Jnury 5, 2000; In Finl Form: Mrch 9, 2000 Structurl, vibrtionl, nd thermochemicl properties of phenol-(h 2 O) 1-6 nd phenoxy rdicl-(h 2 O) 1-4 complexes were clculted by using density functionl theory. The insertion of phenol molecule in wter cluster keeps some similrities with the ddition of wter molecule. The interction of the phenoxy rdicl with wter clusters shows strong dependence on the cluster size. The results re compred with theoreticl nd experimentl dt for the vibrtionl structure of phenol-wter complexes nd with thermochemicl dt for the phenol O-H bond dissocition enthlpy. 1. Introduction The study of hydrogen bonding between wter molecules nd orgnic systems is importnt to understnd the solvtion of these systems nd therefore their chemicl rectivity in solution. Some reltively simple moleculr structures cn be considered s model systems in this type of studies. One of them is the phenol molecule. Severl experimentl nd theoreticl studies on phenol 1-10 nd phenol-wter complexes 5,11-29 hve been reported, focusing on some relevnt spects, including vibrtionl spectrum, structure, nd binding energies. It is known tht the O-H breking mechnism in ionized phenol-wter clusters is crucil for the understnding of protontrnsfer rections One of the most interesting spects of this mechnism is the dependence of the proton-trnsfer rection on the number of wter molecules in the cluster. 31,32 In ddition, the importnce of the phenoxy rdicl is well recognized since this is n intermedite in mny biologicl nd industril processes. The rective nture of the phenoxy rdicl mkes difficult its direct structure determintion nd severl quntum chemicl studies contributed to chrcterize the rdicl structure, bonding, nd vibrtionl spectrum Most of the studies for the phenoxy rdicl deled with the isolted system. However, the chemicl rectivity of this rdicl in solution mkes the study of its interctions with wter clusters of gret interest. It is lso expected tht studies on the complextion of phenol nd phenoxy rdicl with wter clusters will contribute to understnd their solvtion in wter, t moleculr level. In the present work we report dt for the structures, vibrtionl spectr, nd thermochemicl properties of phenolwter [PhOH-(H 2 O) 1-6 ] nd phenoxy rdicl-wter [PhO - (H 2 O) 1-4 ] complexes, bsed on density functionl theory. Our min objectives re to provide ccurte theoreticl dt on these systems nd to estblish closer link with recent experimentl Deprtmento de Químic e Bioquímic. Centro de Físic d Mtéri Condensd d Universidde de Lisbo. Centro de Ciêncis Moleculres e Mteriis. Instituto Superior Técnico. dt on the vibrtionl structure 25,28,29 nd thermochemicl properties of phenol in the condensed phse Computtionl Detils Moleculr energies (E) were clculted by using eq 1 E ) V NN + H CORE + V ee + E X [F] + E C [F] (1) where V NN is the nucler-nucler interction, H CORE is monoelectronic contribution to the totl energy, including electron kinetic nd electron-nucler interction energies, nd V ee is the Coulombic interction between the electrons. E X [F] nd E C [F] re respectively the exchnge nd correltion energies, functionls of the electronic density F. Complete geometry optimiztions hve been crried out with different exchnge nd correltion functionls nd the following combintions were used: Becke representtion for the exchnge 44 nd the grdient-corrected Lee, Yng, nd Prr correltion functionl 45 (BLYP); the Becke s three prmeter hybrid method 46 with the LYP correltion functionl (B3LYP) nd with the Perdew nd Wng 47 nonlocl correltion functionl (B3PW91). Clcultions with different bsis set including 6-31G(d,p), G(d,p), 49 D95V(d,p), 50 nd cc-pvtz 51 hve been crried out. Phenol-wter [PhOH-(H 2 O) 1-6 ], phenoxy-wter [PhO - (H 2 O) 1-4 ], nd wter clusters [(H 2 O) 1-6 ] hve been fully optimized with the B3LYP nd B3PW91 functionls using Dunning s D95V(d,p) 50 bsis set. In our geometry optimiztions we hve selected the most stble conformer for the complexes of phenol nd phenoxy with wter molecules nd lso for the wter clusters. Experimentl 52,53 nd theoreticl works indicte tht the wter trimer, tetrmer, nd pentmer hve cyclic minimum energy structures. The wter hexmer is the first to dopt three-dimensionl cge structure. 53,55 However, in the cse of the wter hexmer, recent MP2 clcultions of Kim nd Kim 56 indicte tht the five most stble conformers of the wter hexmer re isoenergetic within 3 kjmol -1 t 0 K. Thus, we hve directed our geometry optimiztions to the wter [(H 2 O) 3-6 ] /jp CCC: $ Americn Chemicl Society Published on Web 06/06/2000

2 DFT Studies of Phenol- nd Phenoxy-Wter Clusters J. Phys. Chem. A, Vol. 104, No. 25, TABLE 1: Theoreticl Results for the Phenol Molecule (PhOH) nd for the Phenoxy Rdicl (PhO ) E PhOH PhO H D(PhO-H) b,c BLYP/6-31G(d,p) d BLYP/6-311+G(d,p) e B3LYP/6-31G(d,p) d B3LYP/6-311+G(d,p) f B3LYP/cc-pVTZ f B3LYP/D95V(d,p) d B3LYP/cc-pVTZ g B3PW91/6-31G(d,p) d B3PW91/cc-pVTZ h B3PW91/D95V(d,p) d B3PW91/6-311+G(d,p) d Totl energies E corrected for zero point vibrtionl energies in.u.; D(PhO-H) (in kj mol -1 ) is the enthlpy for the rection PhOH(g) f PhO (g) + H (g) t K. b D(PhO-H) hs been estimted s D(PhO-H) ) E + RT, where E E e + E v. E e is the electronic energy difference nd E v is the difference between zero point energies. c The recommended experimentl vlue is ( 2.3 kj mol -1 (ref 42). d Optimized geometry t this level of the theory. e Single-point clcultion with the BLYP/6-31G(d,p) geometry. f Single-point clcultion with the B3LYP/6-31G(d,p) geometry. g Single-point clcultion with the B3LYP/D95V(d,p) geometry. h Single-point clcultion with the B3PW91/6-31G(d,p) geometry. cyclic conformers. Single-point energy clcultions with lrger bsis set (cc-pvtz) with the geometries optimized t the B3LYP/D95V(d,p) level re lso reported. All the clcultions were performed with the Gussin-94/ DFT progrm Results nd Discussion A. Thermochemicl Properties. 1. O-H Bond Dissocition Enthlpy in PhOH. The results obtined for the homolytic O-H bond dissocition enthlpy in phenol (rection 2) by using different functionls nd bsis sets re reported in Tble 1. PhOH(g) f PhO (g) + H (g) (2) In greement with Wu nd Li, 9 D(PhO-H) t K from BLYP/6-31G(d,p) clcultions (331 kj mol -1 ), is much lower thn the recommended experimentl vlue (371 ( 2kJmol -1 ). 42 B3LYP results re in better greement with experiment thn BLYP predictions. D(PhO-H) from B3LYP/cc-pVTZ with the geometry optimized t B3LYP/D95V(d,p) is kj mol -1. The combined functionl B3PW91 lso gives vlues in resonble greement with experiment nd our better prediction, D(PhO-H) ) kj mol -1, is from B3PW91/D95V(d,p) clcultion. However, this vlue is still 6% lower thn the recommended experimentl vlue. 2. Phenol-Wter nd Phenoxy-Wter Clusters. Severl works 24,42,43 including recent review rticle, 42 hve ddressed the energetics of the phenol O-H bond dissocition enthlpy in the gs phse, D(PhO-H), nd in solution, D sln (PhO-H). A route to estimte D sln (PhO-H), strting from gs phse dt, is bsed on eq 3 D(PhO-H) ) D sln (PhO-H) + sln H (PhOH,g) - sln H (PhO,g) - sln H (H,g) (3) where sln H (PhOH,g) nd sln H (PhO,g) represent the enthlpies of solvtion of phenol nd phenoxy rdicl, nd sln H - (H,g) represents the enthlpy of solvtion of the hydrogen tom, which is considered to be similr to the enthlpy of solvtion of H 2, sln H (H 2,g). 58 A vlue proposed by Wilhelm 59 for sln H (H 2,g) in wter (-4 kjmol -1 ) will be dopted. A possible pproch for estimting the enthlpies of solvtion of PhOH nd PhO in wter is to ssume tht these quntities cn be identified with the enthlpies of rections 4 nd 5, respectively, provided tht n (the number of wter molecules in the cluster) is lrge enough. nd PhOH(g) + (H 2 O) n (g) f [PhOH - (H 2 O) n ](g) (4) PhO (g) + (H 2 O) n (g) f [PhO - (H 2 O) n ](g) (5) In fct, we would nticipte tht the enthlpies of these rections would be nerly constnt (within c. 10 kj mol -1 ) for n g 2. A preliminry study involving the sme type of phenolwter clusters with one nd two wter molecules confirmed the obvious ide tht the strongest interctions with one nd two wter molecules would be through pir of hydrogen bonds nd therefore tht the OH group would be sturted with two wter molecules. 43 On the other hnd, the erly results for the phenoxy rdicl indicted firly strong hydrogen bond between the oxygen nd the first wter molecule, nd weker interction between the second wter molecule nd hydrogen from the romtic ring. By exmining lrger clusters we cn now test those ides nd get better understnding bout the influence of the specttor wter molecules (those which do not interct directly with the orgnic solute) on the solvtion energetics. Totl energies for the phenol-wter clusters [PhOH- (H 2 O) 1-6 ], phenoxy-wter clusters [PhO -(H 2 O) 1-4 ], nd wter clusters [(H 2 O) 1-6 ] re reported in Tble 2. These vlues were used to clculte enthlpies of rections 4, 5, nd 6 t T ) K, which re collected in Tble 3. H 2 O(g) + (H 2 O) n (g) f (H 2 O) n+1 (g) (6) Let us first consider the dt obtined for the wter clusters. Bsed on simple considertions, similr to those mde bove, we would expect firly constnt vlue for the enthlpy of rection 6 for n g 3. Indeed, our erly clcultions using semiempiricl method (AM1) 43 led to firly constnt vlue for the enthlpy of rection 6, -38 ( 3kJmol -1 (n ) 3-6). Moreover, the negtive of this vlue is similr to the enthlpy of vporiztion of wter, vp H (H 2 O) ) 44.0 kj mol The dt in Tble 3, clculted from the energies of the cyclic conformers for n ) 3-6, lso indicte tht the enthlpies of rection 6 (in the reverse direction) re in resonble greement with the experimentl vp H (H 2 O), prticulrly those bsed on the B3LYP optimiztion.

3 6064 J. Phys. Chem. A, Vol. 104, No. 25, 2000 Guedes et l. TABLE 2: Totl Energies E (in.u.) for the Phenol-Wter Complexes [PhOH-(H 2 O) 1-6 ], Phenoxy-Wter Complexes [PhO -(H 2 O) 1-4 ], nd Wter Clusters [(H 2 O) n ] B3PW91/D95V(d,p) B3LYP/D95V(d,p) B3LYP/cc-pVTZ b E E E PhOH-(H 2O) PhOH-(H 2O) PhOH-(H 2O) PhOH-(H 2O) PhOH-(H 2O) PhOH-(H 2O) (H 2O) (H 2O) (H 2O) (H 2O) (H 2O) (H 2O) PhO -(H 2O) PhO -(H 2O) PhO -(H 2O) PhO -(H 2O) Totl energies re corrected for zero point vibrtionl energies. b Single-point clcultions with geometries optimized t the B3LYP/D95V(d,p) level. TABLE 3: Theoreticl Rection Enthlpies t K ( r H in kj mol -1 ) for Phenol-Wter Clusters, Phenoxy Rdicl-Wter Clusters, nd Wter Clusters - rh b PhOH + (H 2O) n f PhOH-(H 2O) n n B3LYP/D95V(d,p) B3LYP/cc-pVTZ c B3PW91/D95V(d,p) PhO + (H 2O) n f PhO -(H 2O) n n B3LYP/D95V(d,p) B3LYP/cc-pVTZ c B3PW91/D95V(d,p) (H 2O) n + H 2O f (H 2O) n+1 n B3LYP/D95V(d,p) B3LYP/cc-pVTZ c B3PW91/D95V(d,p) D sln(pho-h) d D(PhO-H) n B3LYP/D95V(d,p) B3LYP/cc-pVTZ c B3PW91/D95V(d,p) D sln(pho-h) (in kj mol -1 ) is the PhO-H bond dissocition enthlpy in the wter clusters. D(PhO-H) (in kj mol -1 ) is the enthlpy for the rection PhOH(g) f PhO (g) + H (g) t K. b rh ) E - RT, where E E e + E v. E e is the electronic energy difference nd E v is the difference between zero point energies. c Single-point clcultions with the geometry optimized t the B3LYP/D95V(d,p) level. d Experimentl dt (ref 42) for D sln(pho-h) in severl solvents re (in kj mol -1 ): (benzene); (isooctne); (cetonitrile); (ethyl cette). The results for the phenol-wter clusters in Tble 3 exhibit the sme generl pttern of the wter clusters. This is mde cler in Figure 1. Here, D[(H 2 O) n -PhOH] nd D[(H 2 O) n -H 2 O] represent, respectively, the dissocition enthlpies of phenol molecule nd wter molecule from [(H 2 O) n ] cluster nd re equl (but hve opposite signs) to the enthlpies of rections 4 nd 6. If the trends for D[(H 2 O) n -PhOH] nd D[(H 2 O) n -H 2 O] were identicl, ll the points would be in the unit slope stright line drwn in Figure 1. While this is not exctly the cse, it is observed tht the line fits the dt within c. 10 kj mol -1.In other words, from thermochemicl point of view, the insertion of one phenol molecule in wter clusters of these sizes is quite similr to the ddition of one wter molecule. As discussed bove, our ttempt to model solvtion energetics by using few solvent molecules must be used with cution. Nevertheless, it is interesting to compre the experimentl vlue for the enthlpy of solvtion of phenol in wter, sln H - (PhOH,g), with the dt shown in Tble 3, for the phenolwter clusters. The well-known sublimtion enthlpy of phenol, sub H (PhOH) ) 68.7 ( 0.5 kj mol -1, 61 nd vlue for the enthlpy of solution of phenol in wter, sln H (PhOH,cr) ) 10.7 ( 0.5 kj mol -1, 62 obtined by rection-solution clorimetry, led to sln H (PhOH,g) )-58.0 ( 0.7 kj mol -1. We observe tht this vlue is firly close to the enthlpy of rection 4 for n ) 3 but significntly more exothermic thn the enthlpy of the sme rection for n ) 4-6. The enthlpy of rection 5 is very dependent on the cluster size (Tble 3) nd D[(H 2 O) n -PhO ] (the dissocition enthlpy of phenoxy rdicl from [(H 2 O) n ] cluster, equl to minus the enthlpy of rection 5) is quite smll for n ) 4, reflecting

4 DFT Studies of Phenol- nd Phenoxy-Wter Clusters J. Phys. Chem. A, Vol. 104, No. 25, Figure 1. Dissocition enthlpy of phenol, D[(H 2O) n-phoh], versus dissocition enthlpy of wter molecule, D[(H 2O) n-h 2O], from [(H 2O) n] cluster, clculted t different theoreticl levels. Figure 2. Dissocition enthlpy of the phenoxy rdicl, D[(H 2O) n- PhO ], versus dissocition enthlpy of wter molecule, D[(H 2O) n- H 2O], from [(H 2O) n] cluster, clculted t different theoreticl levels. wek interction between the phenoxy rdicl nd the wter tetrmer, nd lso the stbility of the wter tetrmer. These conclusions re stressed in Figure 2, which shows plot of D[(H 2 O) n -PhO ] ginst D[(H 2 O) n -H 2 O]. As in the cse of the PhOH, here it is lso observed tht the complextion of the phenoxy rdicl with one wter molecule is more stble thn wter dimeriztion (the point is bove the unit slope stright line in Figure 2), reflecting the importnce of the rdicl s proton cceptor. On the other hnd, in contrst to the cse of phenol (Figure 1), the interction between the rdicl nd the wter clusters with n g 2 is weker thn the interction of one wter molecule with the wter clusters of the sme size. Our predictions for the PhO-H bond dissocition enthlpy in the wter clusters, which we cll D sln (PhO-H), re lso displyed in Tble 3. They were derived from eq 3, by using the theoreticl gs phse D(PhO-H) dt nd solvtion enthlpies from the sme tble. The dependence of D sln (PhO- H) on the number of wter molecules in the cluster reflects the bove discussion for D[(H 2 O) n -PhOH] nd D[(H 2 O)-PhO ] (or the enthlpies of rections 4 nd 5). D sln (PhO-H) D(PhO-H) when n ) 1 nd 2 but D sln (PhO-H) becomes considerbly higher (by kj mol -1 ) thn D(PhO-H), for the lrger clusters. It is interesting to point out tht dt for D sln (PhO-H) in vrious solvents were recently compiled. 42 Some of these vlues re collected in footnote for Tble 3. We cn observe strong solvent dependence of the difference between D sln (PhO-H) nd D(PhO-H). This difference rnges from 5kJmol -1 (benzene) to 20 kj mol -1 (ethyl cette). Our theoreticl predictions for this difference, when n ) 4, re 34.9 kj mol -1 (B3LYP/D95V- (d,p)), 20 kj mol -1 (B3LYP/cc-pVTZ), nd 38.4 kj mol -1 (B3PW91/D95V(d,p)). Although these vlues re in keeping with the observed experimentl trend, we should stress once gin tht the comprison is bsed on the ssumption tht the dominnt effect in the solvtion enthlpy is relted to the interctions of the solute with few wter molecules in the first coordintion shell. B. Structure nd Vibrtionl Spectrum. 1. Structure. The structure of phenol-wter complexes hs been nlyzed by numerous experimentl 22,23,27-29 nd theoreticl 16-18,21,22,26,28,29 works. Theoreticl studies were bsed on b initio Hrtree- Fock (HF) theory 16-18,21,26,28,29 nd Møller-Plesset perturbtion theory. 17,21,26 The ioniztion of phenol-wter nd phenolmmoni complexes hs been recently nlyzed by clcultions using different theoreticl methods, including B3LYP, MP2, nd modified coupled pir MCPF. 31 Very recently, DFT clcultions on the structure of ionized phenol-(h 2 O) 1-4 clusters hve been reported. 32 The structures of neutrl [phenol-(h 2 O) 1-6 ] clusters, s obtined in this work by density functionl theory, re displyed in Figure 3, nd the corresponding Crtesin coordintes re vilble s Supporting Informtion. We hve directed our clcultions to the most stble conformer for ech phenol-wter complex. These structures re in keeping with those found by Wtnbe nd Iwt 26 for the most stble conformers nd some bond distnces from their work re lso shown in Figure 3. Tble 4 reports dt for the phenol O-H bond in the different clusters. We cn observe n increse of the phenol O-H bond length (d O-H ) nd reduction of the H-OH 2 distnce between the hydrogen in the phenol OH group nd the wter oxygen (d H-OH2 ) when n increses from 1 to 5. However, while d H-OH2 increses by bout 3%, d H-OH2 decreses by 14%, reflecting the relevnt role plyed by the wter oxygen s proton cceptor. This feture is reinforced by the collective coupling of the hydrogen bonding interctions mong the wter molecules, which increses with n but seems to stbilize fter n ) 5. Although our results stress the role of the phenol molecule s proton donor in wter, it is interesting to estblish comprison with the sitution in the ionized [PhOH-(H 2 O) 1-4 ] + complexes. Tble 4 lso reports d O-H nd d H-OH2 for the ionized clusters, obtined by Re nd Osmur. 32 From these vlues it is cler tht proton trnsfer only occurs for n g 3 in the ionized complexes. The only similrity with the neutrl complexes concerns the enhncement of the proton cceptor chrcter of the oxygen tom closer to the phenol O-H bond by incresing the number of wter molecules. For the neutrl complexes, our clcultions indicte tht this collective effect levels off when phenol is surrounded by five wter molecules. Figure 4 presents the geometric structures of the phenoxy rdicl nd phenoxy rdicl-wter complexes. Tble 5 reports the C-O bond length (d C-O ) nd the distnce between the phenoxy oxygen nd the wter hydrogen tom (d O-HOH ). The C-O bond length is not significntly modified by the presence of the wter molecules but d O-HOH is reduced by bout 0.1 Å when we move from the complex with one wter molecule to the complex with two wter molecules. A smller chnge is

5 6066 J. Phys. Chem. A, Vol. 104, No. 25, 2000 Guedes et l. Figure 3. Structures of phenol-wter clusters [PhOH-(H 2O) 1-6] showing hydrogen bond distnces (in Å). The vlues in prentheses re from Wtnbe nd Iwt (ref 26). TABLE 4: O-H Bond Lengths in Phenol (d O-H in Å), Distnces Between the Hydrogen of the Phenol O-H Group nd the Wter Oxygen (d H-OH2 in Å), nd Frequencies (ν O-H in cm -1 ) for Phenol nd Phenol-Wter Clusters [PhOH-(H 2 O) 1-6 ] B3LYP/D95V(d,p) exp. b d O-H d H-OH2 ν O-H ν O-H PhOH c PhOH-(H 2O) (1.027) (1.532) PhOH-(H 2O) (1.074) (1.393) PhOH-(H 2O) (1.354) (1.091) PhOH-(H 2O) (1.4) (1.069) PhOH-(H 2O) PhOH-(H 2O) Vlues in prentheses re from Re nd Osmur (ref 32) for the ionized complexes. b Experimentl vlues re from Wtnbe et l. (ref 25). c Density functionl results for ν O-H in isolted phenol re (in cm -1 ): 3658 (BLYP/6-31G(d,p)); 3822 (B3LYP/6-31G(d,p)); 3861 (B3PW91/6-31G(d,p); 3863 (B3PW91/6-311+G(d,p)). observed when third wter molecule is introduced. Complextion of the phenoxy rdicl with four wter molecules increses d O-HOH reltive to the system with three wter molecules nd two different d O-HOH distnces re now observed (see Figure 4). 2. O-H Stretch Vibrtions. The vibrtionl spectrum of phenol-wter clusters 15,19,20,22,25,27,28 is chrcterized by strong red-shift of the phenol O-H vibrtionl frequency (ν O-H ) with incresing number of wter molecules. 22,25,26 This red-shift reflects wekening of the phenol O-H bond strength due to hydrogen bonding with wter molecules. Tble 4 shows experimentl dt 25 nd DFT frequencies ssigned to the phenol O-H bond vibrtions. For isolted phenol, unscled ν O-H from BLYP/6-31G(d,p) (3658 cm -1 ) clcultions re in very good greement with experiment (3657 cm -1 ). 2 B3LYP nd B3PW91 (see Tble 4) results clerly overestimte ν O-H by 200 cm -1. Comprison between ν O-H from DFT clcultions nd experimentl dt for phenol-wter complexes shows much better greement. Recently, Curtiss et l. 63 suggested tht DFT zero-point energies should be scled for comprison with experimentl dt. In prticulr, B3LYP/6-31G(d) zero-point energies should be scled by If such scling is used in our results, our prediction for ν O-H in the phenol molecule improves significntly but the greement between scled frequencies for the phenol-wter complexes worsens. If scling is to be used, in our cse the fctor 0.98 proposed by Buschlicher 64 seems to be more pproprite. Tble 5 reports vibrtionl frequencies represented by ν O H-OH nd ssigned to the O-H stretch in the wter molecule closer to the phenoxy rdicl oxygen. This vibrtionl mode is redshifted by bout 100 cm -1 when the phenoxy rdicl complextes with two wter molecules in comprison with the complex with one wter molecule, indicting tht the role of the phenoxy rdicl s proton cceptor is incresed by the presence of second wter molecule. A similr trend is observed in the complextion of the phenoxy rdicl with three wter molecules. However, ν O H-OH is blue-shifted by bout 100 cm -1 reltive to the complex with one wter molecule, when the phenoxy rdicl is coupled to four wter molecules, reflecting weker interction of the phenoxy rdicl with the wter tetrmer.

6 DFT Studies of Phenol- nd Phenoxy-Wter Clusters J. Phys. Chem. A, Vol. 104, No. 25, Figure 4. Structures of phenoxy rdicl-wter clusters [PhO -(H 2O) 1-4] showing hydrogen bond distnces (in Å). TABLE 5: C-O Bond Length (d C-O in Å), Distnces Between the Phenoxy Oxygen nd the Wter Hydrogen (d O-HOH in Å), nd Wter O-H Frequencies (ν O H-OH in cm -1 ) for the Phenoxy-Wter Complexes [PhO -(H 2 O) 1-4 ] B3LYP/D95V(d,p) B3PW91/D95V(d,p) d C-O d O-HOH ν O H-OH d C-O d O-HOH ν O H-OH PhO PhO -(H 2O) PhO -(H 2O) PhO -(H 2O) PhO -(H 2O) Conclusions This work reports dt for the energetics, structure, nd vibrtionl spectrum of phenol, phenol-wter complexes, phenoxy rdicl, nd phenoxy rdicl-wter complexes, bsed on density functionl theory clcultions. The O-H bond dissocition enthlpy of phenol clculted by density functionl theory show some dependence on the bsis set nd on the exchnge-correltion functionls. We hve verified tht BLYP underestimtes D(PhO-H) nd tht better greement with experiment is observed for clcultions with hybrid functionls B3LYP nd B3PW91. However, our better prediction for D(PhO-H), bsed on density functionl clcultions, is still 6% lower thn the recommended experimentl vlue. In ddition, density functionl clcultions bsed on the B3LYP nd B3PW91 exchnge-correltion functionls overestimte the phenol ν O-H frequency by 200 cm -1. The insertion of phenol molecule in wter cluster hs some similrities, t lest from thermochemicl point of view, with the insertion of wter molecule. A significnt increse of the phenol O-H bond length nd corresponding reduction of the distnce PhOH OH 2 is observed in neutrl phenolwter clusters when the number of wter molecules increses, showing the relevnt role plyed by the phenol molecule s proton donor in wter. The nlysis of the interctions of the phenoxy rdicl with wter clusters evidences the role plyed by this system s proton cceptor in wter. By nlyzing the energetics of phenol-wter, phenoxy rdicl-wter complexes, nd wter clusters, we were ble to estimte the phenol O-H bond dissocition enthlpy in wter clusters, D sln (PhO-H). Our predictions for D sln (PhO-H) in wter clusters re in qulittive greement with literture dt for the solvtion of phenol in other solvents. Acknowledgment. R. C. Guedes grtefully cknowledges the support of the Fundção pr Ciênci e Tecnologi (FCT) through PhD grnt (PRAXIS XXI/BD/15920/98). This work ws prtilly supported by the PRAXIS XXI Progrm (Grnt No. PRAXIS/2/2.1/QUI/51/94), Portugl. R. C. Guedes nd B. J. Cost Cbrl re grteful to Pulo Couto for dvise on the construction of the figures using the Molekel softwre. Supporting Informtion Avilble: The Supporting Informtion includes tbles (1S to 10S) including the identifiction

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