Effect of the Substituent and Hydrogen Bond on the Geometry and

Sep 20, 2010 - Interrelations between intra- and intermolecular interactions were analyzed by using computational modeling of the para-X-substituted ...
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J. Phys. Chem. A 2010, 114, 10885–10890

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Effect of the Substituent and Hydrogen Bond on the Geometry and Electronic Properties of OH and O- Groups in para-Substituted Phenol and Phenolate Derivatives Halina Szatylowicz* Faculty of Chemistry, Warsaw UniVersity of Technology, Noakowskiego 3, 00-664 Warsaw, Poland

Tadeusz M. Krygowski Department of Chemistry, Warsaw UniVersity, Pasteura 1, 02-093 Warsaw, Poland ReceiVed: July 29, 2010; ReVised Manuscript ReceiVed: August 31, 2010

Interrelations between intra- and intermolecular interactions were analyzed by using computational modeling of the para-X-substituted derivatives of phenol and phenolate (where X ) NO, NO2, CHO, COMe, COOH, CONH2, Cl, F, H, Me, OMe, and OH) and their equilibrium H-bonded complexes with HB and B- (where HB ) HF and HCN and B- ) F- and CN-). B3LYP/6-311++G** computation was applied. Both the substituent effect and H-bonding changed the electronic properties of the -O- and -OH groups and geometric parameters of phenol and phenolate derivatives and their H-bonded complexes. C-O bond lengths and aromaticity indices of the ring were found to depend linearly on σp- of the substituents. In the first case the greatest sensitivity on the substituent effect was for 4-X-C6H4OH · · · CN- and 4-X-C6H4O- · · · HF complexes, whereas for 4-X-C6H4O- · · · HCN systems it was comparable with that for phenol derivatives and a little smaller than that for 4-X-C6H4O- derivatives. This means that the strength of H-bonding may considerably change the sensitivity of the C-O bond length to the substituent effect. The greatest sensitivity of the aromaticity indices, both HOMA and NICS(1)zz, to σp- was found for phenolate and then for phenolate H-bonded complexes, followed by phenol complexes, and the lowest sensitivity was observed for phenol derivatives. The interatomic proton-acceptor distance, being a measure of the H-bond strength, was found to depend linearly on σp- of the substituents with a positive slope for O · · · HB (HF or HCN) interactions and a negative slope for OH · · · B- interactions. NBO charges on the oxygen and hydrogen atoms also depend on σp- of the substituents. In the latter case for strong H-bonded complexes (energy less than ∼-20 kcal/mol) the substituent effect works oppositely for 4-X-C6H4OH · · · B- in comparison with the 4-X-C6H4O- · · · HB systems. Moreover, following the Espinoza et al. [J. Chem. Phys. 2002, 117, 5529] and Grabowski et al. [J. Phys. Chem. B 2006, 110, 6444] classifications, the above and q(H) vs proton-acceptor distance relationships suggest a partially covalent character of the hydrogen bond for these complexes and the degree of its covalent nature depending on the substituent. Introduction Substituent effects on the physicochemical properties of substituted systems are still the subject of a vivid discussion, as shown in two recent reviews.1,2 Apart from the purely applicative way of using various Hammett-like equations,3,4 there are many new approaches to the description of the substituent effect by means of various quantum chemistry driven properties and hence new numerical characteristics. Molecular similarity modeling based on parameters in bond critical points obtained by use of the theory of atoms in molecules5 was used to obtain a numerical description of substituent effects.6 Similar modeling was applied by using the molecular electrostatic potential.7 The same was also applied for quantitative assessment of the inductive effect,8 or σ° constants.9 Changes in π-electron delocalization in the ring of exocyclically substituted fulvene and heptafulvene10 served for estimation of σ+ and σ- substituent constants, respectively. Due to quantum chemical modeling a new kind of substituent effect was discovered:11 the angular group induced bond alternation (for a review see ref 12). Another successful application of quantum chemical modeling * To whom correspondence should be addressed. Phone: (+48) 22 234 7755. Fax: (+48) 22 628 2741. E-mail: [email protected].

was a substantial support for the through-space mechanism of inductive substituent effects for substituted bicyclo[2.2.2]octane1-carboxylic acid and bicyclo[1.1.1]pentane-1-carboxylic acid13 and substituted benzoic acids.14 Numerous papers by Otto Exner et al.15 were based on carefully selected homodesmotic reactions to document the contributions of various effects participating in an overall substituent effect. Recently, a new concept has appeared for numerical characteristics of the substituent effect. It relates the substituent constants σ to the sum of charges of the substituted carbon atom C1 and all atoms of the substituent X and is named the charge of the substituent active region, qSAR(X).16 It is known that both the proton-donating and proton-accepting groups change their electronic properties as a result of the resonance-assisted hydrogen bond approach (RAHB)17 and the like.18 More quantitatively, these kinds of changes were observed for the intramolecular H-bond in malonaldehyde and o-hydroxybenzaldehyde.19 It was also found that as a result of H-bond formation the electronegativity of OH/O- in the phenol/ phenolate pair20 and NH-/NH2/NH3+ groups in appropriate derivatives of anilines21 changed in a substantial way. Very recently it was found22 that para-substituted phenol and phenolate derivatives form equilibrium complexes with F-/CN- and HF/

10.1021/jp1071204  2010 American Chemical Society Published on Web 09/20/2010

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SCHEME 1: Schematic Presentation of the Studied H-Bonded Complexesa

HB ) HF and HCN. B- ) F- and CN-. X ) NO, NO2, CHO, COCH3, COOH, CONH2, Cl, F, H, CH3, OCH3, and OH. a

HCN, respectively. At present, we report the results of investigations showing how the geometry and electronic properties of OH and O- groups in the H-bonded complexes depend on the substituent attached in the para position and the strength of the hydrogen bond. Theoretical Calculations The geometries of the para-X-substituted derivatives of phenol and phenolate (where X ) NO, NO2, CHO, COMe, COOH, CONH2, Cl, F, H, Me, OMe, and OH) and their H-bonded complexes with HB and B- (where HB ) HF and HCN and B- ) F- and CN-; for clarity, see Scheme 1) were obtained22 on the basis of density functional theory (DFT).23 Becke’s three-parameter functional24 with correlation energy according to the Lee-Yang-Parr formula25 and 6-311++G (d,p)26 basis set (B3LYP/6-311++G**) were applied. All geometry parameters of the studied systems were fully relaxed by searching for the minimum of energy. The harmonic frequencies were calculated to confirm that the obtained geometry corresponds to a minimum on the potential energy surface. The energy of the intermolecular H-bond, Etot, was calculated as the sum of the interaction and deformation energies,27 taking into account the basis set superposition error (BSSE).28 Calculations were carried out using the Gaussian0329 series of programs. Subsequently, the natural bond orbital method,30 NBO 5.G,31 was applied to study the electronic properties of the OH and O- groups, that is, the atomic charges at the oxygen and hydrogen atoms. Geometry parameters of the ring, that is, CC bond lengths, were used to estimate the aromaticity index HOMA.32 The magnetism-based aromaticity index NICS(1)zz (nucleusindependent chemical shift, the perpendicular component of the NICS(1)33 tensor)34 was calculated35 at the HF/6-31+G* level of theory using the GIAO method. Results and Discussion To shorten the notation of the chemical formula systems under study, in the following part of the paper, Ph will be used in the sense of C6H4. Phenol/phenolate H-bonded complexes with F-/HF and CN-/ HCN differ in a significant way. The first complexes are all of the same type, since HF is a weaker acid in the gas phase (∆acidH ) 371.3 kcal/mol)36,37 than any of the compounds from a series of para-substituted phenol derivatives (∆acidH ) 327.8 ( 2.1 and 350.4 ( 2.1 kcal/mol for 4-nitro- and 4-hydroxyphenol, respectively).36,38 In the case of another series of complexes, i.e., those with CN-/HCN, the acidity of the conjugated acid of CN- (∆acidH ) 350.90 ( 0.20 kcal/mol)36,39 is in some cases greater than that of phenolate, but in others it is lower. Hence,

Figure 1. Dependence of (a) Etot and (b) the C-O bond lengths, dCO, on the hydrogen bond length, dH · · · A, for H-bonded complexes of parasubstituted phenol and phenolate derivatives.

there are two kinds of H-bonded complexes: 4-X-PhO- · · · HCN and 4-X-PhOH · · · CN-, depending on the kind of substituent. Therefore, further discussion will be provided for these two kinds of complexes (Scheme 1): 4-X-PhO- · · · HB and 4-XPhOH · · · B-, where HB is either HF or HCN and B- means CN-. At the beginning let us look at the effects of the strength of the interaction on the structure parameters in the region of interaction. The H-bonding strength is often described by use of the interatomic proton-acceptor distance (H · · · A, where A may be either O- or CN-), dH · · · A. It was shown many times that this characteristic correlates well with the energy of H-bond formation.40,41 In our case correlation coefficient, cc, values for this kind of regression are extremely high, as shown in Figure 1a; for full statistics see Table S1 in the Supporting Information. In the case of the complexes under study a nice measure of the strength of interaction is also the C-O bond length (Figure 1b). Moreover, the relationship presented in Figure 1b allows us to differentiate phenol and phenolate H-bonded complexes. An increase of the H-bond strength (i.e., shortening of the H-bond length, Figure 1a) for 4-X-PhO- · · · HB complexes is associated with a lengthening of the C-O bond, contrary to the case of 4-X-PhOH · · · B- systems, where a shortening is observed, Figure 1b. In the case of fluoride approaching the hydroxyl group in para-substituted phenol, see Scheme 1, at a certain distance dO · · · F, the proton transfers from -OH to fluoride and the equilibrium complex X-PhO- · · · HF is formed,42 the properties of which depend on the nature of X. Thus, there are two sources of variability in equilibrium complexes, H-bonding and the substituent effect, reflected by the electron distribution and geometry of the systems in question.

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Figure 2. Dependence of dCO on σp- for para-substituted derivatives of phenol and phenolate and their H-bonded complexes, as indicated in the inset.

Consider first the changes due to the substituent effects: how do these effects work in the case of free molecules of parasubstituted phenol and phenolate derivatives and then in the case of their H-bonded complexes? The electronic state of the -O- group in the case of the -O- · · · HB part of the complex should be somewhere between those for the -O- and -OH groups. They are both electron-donating groups with substituent constants σ+ ) -2.3 and -0.92, respectively.4 In the context of the substituent effect on the -O- and -OH groups in the systems studied, the C-O bond length, dCO, is a very important geometric characteristic. Figure 2 displays the dependence of dCO on σp- values, whereas Table S2 (Supporting Information) presents statistics indicating good correlations between the data. As can be seen, there is a substantial separation between the dCO values for phenolates and phenols and their H-bonded complexes. Interestingly, the sensitivity to the substituent effect differs, depending on the kind and strength of the interactions. Evidently, strong intermolecular interaction by H-bonding, that is, for 4-X-PhOH · · · CN- and 4-X-PhO- · · · HF complexes (Figure 1a), intensifies the sensitivity of the C-O bond length to the substituent effects. In the case of weaker 4-X-PhO- · · · HCN systems (Figure 1a), the above sensitivity is comparable to that for free phenol derivatives and is slightly smaller than that for 4-X-PhO- derivatives. The relationships presented in Figure 2 suggest that the shortening of dCO along with an increase of the electronaccepting power of the substituent can be associated with an increase of the quinoid structure contribution43 and as a consequence with a decrease of the aromatic character of the ring.44 The decrease of the aromaticity of the ring (decrease of the HOMA index32 and increase of the NICS(1)zz index)34 is associated with the mode of communication between the “reaction site” (OH, OH · · · B-, O- · · · HB, O-) and the sub-

Figure 3. Dependence of HOMA (a) and NICS(1)zz (b) on σp- of the substituents for para-substituted derivatives of phenol and phenolate and their H-bonded complexes.

stituent, which is realized via an increase of the weight of the quinoid structure of the ring (see Scheme 2). Figure 3 presents the dependence of HOMA and NICS(1)zz on σp- of the substituents, and Table S3 (Supporting Information) presents the statistics. The mutual agreement between HOMA and NICS(1)zz is good (see Figure S1, Supporting Information); the correlation coefficient amounts to -0.976. Another very interesting question is the analysis of the slopes of linear regressions HOMA and NICS(1)zz vs σp-, presented in Table S3 (Supporting Information): the lowest value (the absolute value of the slope in the case of HOMA vs σp-) was obtained for phenol derivatives, then for H-bonded complexes of phenols, and next for complexes of phenolate derivatives, and the highest value was that for free phenolates. Moreover, the strength of the interaction between the proton and the oxygen atom decided the C-O bond length and aromaticity of the ring: the 4-X-PhO- · · · HCN complexes are weaker than those with HF (see Figure 1a), and hence, the lowest aromaticity was found for the former and is closer to the values for free phenolates. Finally, it is worth observing how the H-bond strengths depend on the substituent constants. Parts a and b of Figure 4

SCHEME 2: Most Important Resonant Structures for Electron-Attracting Substituted Phenolates (a) and Phenols (b) in H-Bonded Complexes

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Figure 5. Scatter plot of the charge at the oxygen atom, q(O), vs σpfor para-substituted phenol and phenolate derivatives and their Hbonded complexes.

Figure 4. Dependence of the H-bond strength expressed by (a) dH · · · A and (b) Etot on σp- for H-bonded complexes of para-substituted phenol and phenolate derivatives. cc values for X-PhO- · · · HF complexes are equal to 0.988 for (a) and 0.989 for (b).

show the dependence of dH · · · A (a) and Etot (b) on σp-, whereas Table S4 (Supporting Information) contains their statistics. As expected, the equivalency of these two scatter plots is very good. Moreover, interpretation is easy: the more electron accepting the substituent, the weaker the H-bond in 4-X-PhO- · · · HB systems. In other words, the more electron donating properties the substituent has, the stronger the H-bond. A reverse relation takes place for the 4-X-PhOH · · · B- complexes, as shown for substituents with σp- > 0. Apart from the analyses of geometry and energetic parameters of the systems in this study, a new insight comes from application of the NBO approach.30 First, let us consider the changes in electronic properties of the oxygen atom, q(O), of the proton-accepting group, i.e., -O-, and the proton-donating group, i.e., -OH, due to the substituent effect. It is shown in the scatter plot in Figure 5 that the electronic properties of the -O- group in phenolates change: an increase of the electronaccepting property of the substituent X causes a decrease of the negative charge of the group, and the slope of regression displays the greatest sensitivity of these changes to the substituent effect. In the latter case, i.e., the -OH group in phenols, the sensitivity is the lowest. As can be seen, all regressions in Figure 5 are linear with good correlation coefficients; see Table S5 (Supporting Information). As mentioned above, the electron state of the -O- group in the case of the -O- · · · HB part of the complex should be somewhere between those for the -O- and -OH groups for uncomplexed species. A question arises as to which of these two limit cases is closer to the charge of the oxygen atom involved in H-bond formation with HB. There are two characteristics which differentiate 4-X-PhO- from the complexes 4-XPhO- · · · HB. First, when looking at the variability of the charge

at the oxygen atom and the regression lines on σp- for 4-XPhO- and 4-X-PhO- · · · HB, one finds that they are very close to one another, ranging from -0.786 to -0.798 for X ) H (Figure 5; see also the b values in Table S5, Supporting Information) and far away from the line for 4-X-PhOH (b ) -0.679). Second, the slopes of the linear regressions indicate that the sensitivity of O- · · · HB interactions is double that for X-PhOH and half of that for X-PhO- (Table S5). This means that the stronger the interaction between the hydrogen and oxygen atoms, the less sensitive the charge at the oxygen atom, q(O), to the substituent effect. For almost covalent OH interaction (4-XPhOH, 4-X-PhOH · · · B-) the sensitivity is the lowest, whereas for free -O- it is the highest. Finally, it should be noted that for electron-donating substituents, σp < 0, q(O) for free phenolate molecules is more negative than in any of the complexes, whereas for electron-accepting substituents, σp > 0, the charge at the oxygen atom is less negative than that for the -O- · · · HB complexes. The problem will be considered in more detail later. The hydrogen atom involved in the interaction is another very important component of the H-bond. Figure 6 presents the dependence of the charge on this atom on the strength of H-bonding, expressed by the dH · · · A distance, and the substituent effect; Table S6 (Supporting Information) presents the statistics. The observed changes of the charge at the hydrogen atom, which becomes less positive with an increase of the H-bond strength (Figure 6a), are in line with those presented in Figure 1a for the 4-X-PhO- · · · HF and 4-X-PhOH · · · CN- complexes. The observed dependence shows that the decrease of the dH · · · A value, i.e., an increase of the H-bond strength, is associated with a decrease of the charge at the H atom involved in the H-bond. This kind of dependence is in line with the approaches by Espinosa et al.45 and Grabowski et al.46 to classification of H-bonding: with a decrease of the proton-acceptor distance, dH · · · A, the covalent character of the H-bond increases. The dependence of Etot on σp- (Figure 4b) for 4-X-PhO- · · · HB complexes suggests that for the stronger X ) H complex (Table S4b, Supporting Information) we have Etot ) -28.7 kcal/mol (HB ) HF) and for the weaker complex (for the hypothetic complex with HB ) HCN) Etot ) -20.9 kcal/mol. In both cases the H-bond strength increases with a decrease of the electronaccepting power of the substituent. However, the substituent effect on q(H) (Figure 6b) for these two cases is different: the slope for HB ) HF is positive (it amounts to 0.011, Table S6b, Supporting Information), whereas for HB ) HCN it is negative (-0.005). The charges at the hydrogen atoms in free HF and HCN molecules, noted as q(H)HF and q(H)HCN, are 0.55 and

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J. Phys. Chem. A, Vol. 114, No. 40, 2010 10889 linearity (B3LYP/6-311+G**), is presented in Tables S7 and S8 (Supporting Information). Conclusions From the data reported, the relationships between charge at the hydrogen atom involved in H-bonding and the substituent constant for the title systems, it results that for strong H-bonded complexes (that is, where the energy is less than ca. -20 kcal/ mol) the substituent effect works oppositely for 4-X-PhOH · · · Bin comparison with the 4-X-PhO- · · · HB complexes. The observed dependence may remind us of a suggestion that follows the Espinosa et al.45 and Grabowski et al.46 classifications: the charge at the hydrogen atom, participating in H-bonding, can express to some degree the covalent nature of the hydrogen bond. For this purpose a comparison of the charge at the hydrogen atom participating in the interaction with the one not involved in the H-bonding, that is, in the free monomer, seems to be very helpful. Another conclusion is that the changes in the geometric and electronic structure of the -O- and -OH groups in monomers and complexes with HB (HF or HCN) or B- (CN-) depend nicely on the substituent effect. Moreover, both kinds of properties (bond length and charge at the oxygen atom) of the C-O bond in the 4-X-PhO- · · · HB complexes are much closer to those of 4-X-PhO- than the properties of C-O in 4-X-PhOH · · · CN- are to those for phenol derivatives themselves. Additionally, joint activity of the substituent and H-bonding on the π-electron structure of the ring, expressed by aromaticity indices, also follow well the dependence on the substituent constant, σp-.

Figure 6. Dependence of the charge at the hydrogen atom participating in the interaction, q(H), on the (a) H-bond strength, expressed by the hydrogen bond length, dH · · · A, and (b) substituent constant, σp-, for H-bonded complexes of para-substituted phenol and phenolate derivatives (in part b phenol derivatives are included).

0.22, respectively (these data were obtained at the same level of computation as for all systems reported). If these charges are taken as references for their H-bonded complexes with phenolate derivatives, the difference, ∆, between charges in complexes 4-X-PhO- · · · HB and free HB ) HF or HCN changes in opposite directions. For weaker complexes, 4-X-PhO- · · · HCN, the difference ∆ increases with an increase of the H-bond strength. For 4-X-PhO- · · · HF complexes, the stronger ones, the modulo of ∆ increases too, but the ∆ values change in the opposite direction (∆ < 0). In the case of 4-X-PhOH and 4-X-PhOH · · · CN- systems two consequences due to intra- and intermolecular interactions are observed. First, for free phenol derivatives q(H) increases with an increase of the electron-accepting power of the substituent, which is expected. However, when the -OH group is involved in H-bonding, the slope is opposite (in modulo ca. 10 times greater). This means that, with an increase of the electrondonating strength of the substituent, and hence a decrease of the H-bond strength (Figure 4; Table S4, Supporting Information), an increase of the positive charge at the hydrogen atom, q(H), and the difference ∆ (the modulo ∆ value decreases) is observed. Finally, it should be noted that all data presented in this paper are insignificantly different from those which were obtained by computation with the assumption of the linearity of the H-bond. Comparison of the geometry in the hydrogen bond region for [4-X-PhO · · · H · · · B]- complexes, obtained without constraints (B3LYP/6-311++G**) and with the assumption of the H-bond

Acknowledgment. We thank the Interdisciplinary Centre for Mathematical and Computational Modeling (Warsaw, Poland) for computational facilities and the Warsaw University of Technology for financial support. Supporting Information Available: Tables containing parameters of linear regression of the features (geometric and electronic properties, substituent constants) for para-substituted phenol and phenolate derivatives and their H-bonded complexes, the geometry in the H-bond region, and substituent constants and a figure showing the relationship between the aromaticity indices NICS(1)zz and HOMA for the systems under study. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Krygowski, T. M.; Stepien, B. T. Chem. ReV. 2005, 105, 3482– 3512. (2) Exner, O.; Bohm, S. Curr. Org. Chem. 2006, 10, 763–778. (3) (a) Hammett, L. P. Physical Organic Chemistry; McGraw-Hill: New York, London, 1940. (b) Jaffe, H. H. Chem. ReV. 1953, 53, 191–261. (c) Exner, O. In AdVances in Linear Free Energy Relationships; Chapman, N. B., Shorter, J., Eds.; Plenum Press: London, 1972; Chapter 1, p 1. (d) Johnson, C. D. The Hammett Equation; Cambridge University Press: Cambridge, U.K., 1973. (e) Charton, M. Electrical Effect Substituent Constants for Correlation Analysis. Prog. Phys. Org. Chem. 1981, 13, 119– 251. (f) Shorter, J. In Similarity Models in Organic Chemistry, Biochemistry and Related Fields; Zalewski, R. I., Krygowski, T. M., Shorter, J., Eds.; Elsevier: Amsterdam, 1991; Chapter 2, p 77. (g) Exner, O.; Krygowski, T. M. Chem. Soc. ReV. 1996, 25, 71–75. (4) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165–195. (5) Bader, R. W. F. Atoms in Molecules: A Quantum Theory; Oxford University Press: New York, 1990. (6) Popelier, P. L. A. J. Phys. Chem. A 1999, 103, 2883–2890. (7) Sadlej-Sosnowska, N. J. Phys. Chem A 2007, 111, 11134–11140. (8) Suresh, C. H.; Alexander, P.; Vijayalakshumi, K. P.; Sajith, P. K.; Gadre, S. R. Phys. Chem. Chem. Phys. 2008, 10, 6492–6499. (9) Galabov, B.; Ilieva, S.; Schaefer, H. F., III. J. Org. Chem. 2006, 71, 6382–6387.

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