Nature of the Interaction of Pyridines with OCS. A Theoretical Investigation.

Ab initio calculations were carried out to investigate the interaction between para-substituted pyridines (X-C5H4N, X=NH2, CH3, H, CN, NO2) and OCS. Three stable structures of pyridine.OCS complexes were detected at the MP2=full/aug-cc-pVDZ level. The A structure is characterized by N…S chalcogen bonds and has binding energies between -9.58 and -12.24 kJ/mol. The B structure is bonded by N…C tetrel bond and has binding energies between -10.78 and -11.81 kJ/mol. The C structure is characterized by π-interaction and has binding energies between -10.76 and -13.33 kJ/mol. The properties of the systems were analyzed by AIM, NBO, and SAPT calculations. The role of the electrostatic potential of the pyridines on the properties of the systems is outlined. The frequency shift of relevant vibrational modes is analyzed.

Unusual Fluorine Substitution Effect on S···Cl Bonding between Sulfides and Atomic Chlorine.

A theoretical investigation on the interaction of various sulfides and their fluorinated counterparts (H2S, HSF, F2S, CH3SH, CH3SF, CH2FSH, CH2FSF, NH2SH, NH2SF) with atomic chlorine has been carried out using density functional theory (DFT) based LC-BLYP/aug-cc-pVTZ and sophisticated ab initio CCSD(T)/aug-cc-pVQZ methods. The present study is intended to discuss the influence of the substituents implanted at the sulfur atom on the bonding parameters. The optimized geometries reveal that intermolecular S···Cl distances are short and range between 2.423 and 2.561 Å. A strong contraction of the S-F bond is also predicted. Two-center-three-electron S···Cl bonds are formed; the interaction energies are large and range from -33.9 to -70.1 kJ mol-1. Very surprisingly, the interaction energies are greater and the intermolecular distances are shorter for F-substituted sulfides than for unsubstituted ones. This is in complete contrast with the lower proton affinities and less negative electrostatic potentials of fluorinated sulfides. AIM analysis, the charge transfer from the sulfur atom to the Cl atom, and the spin densities on the Cl and S atoms are considered to explain this unusual behavior. The hyperconjugation energies from the LP(F) to the σ*(S-Cl) antibonding orbital can be considered as one of the stabilizing factors for the greater stability of the fluorinated complexes over the nonfluorinated ones.

Theoretical investigation of the halogen bonded complexes between carbonyl bases and molecular chlorine.

The halogen bonded complexes between six carbonyl bases and molecular chlorine are investigated theoretically. The interaction energies calculated at the CCSD(T)/aug-cc-pVTZ level range between -1.61 and -3.50 kcal mol(-1). These energies are related to the ionization potential, proton affinity, and also to the most negative values (V(s,min)) on the electrostatic potential surface of the carbonyl bases. A symmetry adapted perturbation theory decomposition of the energies has been performed. The interaction results in an elongation of the Cl-Cl bond and a contraction of the CF and CH bonds accompanied by a blue shift of the ν(CH) vibrations. The properties of the Cl2 molecules are discussed as a function of the σ*(Cl-Cl) occupation, the hybridization, and the occupation of the Rydberg orbitals of the two chlorine atoms. Our calculations predict a large enhancement of the infrared and Raman intensities of the ν(Cl-Cl) vibration on going from isolated to complexed Cl2.

Strong hyperconjugative interactions in isolated and water complexes of desflurane: a theoretical investigation.

Ab initio MP2/aug-cc-pvDZ and density functional B3LYP calculations with the 6-311++G(d,p) basis set are performed to investigate the conformation of desflurane (CHF2OCHFCF3), its acidity/basicity and its interaction with one water molecule. The calculations include the optimized geometries, the harmonic frequencies of relevant vibrational modes, the binding energies with water, and a detailed natural bond orbital (NBO) analysis Iincluding the NBO charges, the hybridization of the C atoms and the intra- and intermolecular hyperconjugations. The relative energies of the two most stable conformers are discussed as a function of the total hyperconjugative energies resulting from the interaction of lone pairs of the O and F atoms to the different antibonding orbitals of desflurane. The proton affinity is the same for both conformers but the acidity of the CH bond is larger for the less stable conformer. The binding energies of the complexes of two desflurane conformers with one water molecule range from -2.75 to -3.23 kcal mol(-1). Depending on the structure of the complexes, the CH bonds involved in the interaction are contracted or elongated. The σ*(CH) occupation predominates over the hybridization effect in determining the CH bond length. There is an unexpected charge transfer to the external OH bond of the water molecule. This effect is in good agreement with theoretical data on the complexes between fluorinated dimethyl ethers and water and seems to depend on the number of F atoms implanted on the ether molecule.

Theoretical study of the O···Cl interaction in fluorinated dimethyl ethers complexed with a Cl atom: is it through a two-center-three-electron bond?

Theoretical investigations are carried out on the interaction between fluorinated dimethyl ethers (FDME, nF = 0-4) and the Cl atom. Short intermolecular O···Cl distances between 2.401 and 2.938 Å reveal the formation of a new class of complexes. The interaction energies calculated with the G2(MP2) method range between -9.1 (nF = 4) and -26.0 (nF = 0) kJ/mol. The charge transfer occurring from the ethers to atomic Cl is moderate and ranges between 0.012 e (nF = 4) to 0.188 e (nF = 0). The binding energies are linearly related to the proton affinity, to the charge transfer (CT) occurring in the molecular system and inversely proportional to the ionization potential and electron affinity (IP-EA) values. The CT and spin density data indicate substantial two-center-three-electron O···Cl interaction in CH3OCH3···Cl and CH3OCH2F···Cl systems, whereas for highly fluorinated ethers the interaction is predominantly electrostatic in nature. The formation of the complex results in a contraction of the CH bonds, especially in the gauche position. The blue shifts of the C-H stretching vibrations calculated in the partially deuterated isotopomers range between 2 and 54 cm(-1) and are correlated to the variation of the CH distances.

A theoretical investigation of the interaction between substituted carbonyl derivatives and water: open or cyclic complexes?

The structures and binding energies of complexes between substituted carbonyl bases and water are the B3LYP/6-311++G(d,p) computational level. The calculations also include the proton affinity (PA) of the O of the C=O group, the deprotonation enthalpies (DPE) of the CH bonds along a natural bond orbital analysis. The calculations reveal that stable open C=O···H(w) O(w) as well as cyclic CH···O(w)H(w) ···O=C complexes are formed. The binding energies for the open complexes are linearly related to the PAs, whereas the binding energies for the cyclic complexes depend on both the PA and DPE. Different indicators of hydrogen bonds strength such as electron charge density, intramolecular and intermolecular hyperconjugation energy, occupation of orbitals, and charge transfer show significant differences between open and cyclic complexes. The contraction of the CH bond of the formyl group and the corresponding blue shift of the ν(CH) vibration are explained by the classical trans lone pair effect. In contrast, the elongation or contraction of the CH(3) group involved in the interaction with water results from the variation of the orbital interaction energies from the σ(CH) bonding orbital to the σ* and π* antibonding orbitals of the C=O group. The resulting blue or red shifts of the ν(CH(3)) vibrations are calculated in the partially deuterated isotopomers.

Methyl formate and its mono and difluoro derivatives: conformational manifolds, basicity, and interaction with HF theoretical investigation.

The conformational manifolds, scenarios of protonation, and hydrogen bond propensity of methyl formate and its mono and difluoro derivatives, which possess two oxygen atoms with different basicities, are studied at the B3LYP/6-311++G(3df,3pd) computational level. The optimized geometries of the title molecules, their energetics, and relevant harmonic vibrational frequencies, mainly of the ν(CH) mode of the H-C═O group, are of a primary focus. The Natural Bond Orbital analysis is invoked to obtain the second-order intra- or intermolecular hyperconjugation energies, occupations of antibonding orbitals, and hybridization of the carbon atoms. It is demonstrated that the Z conformers (and their rotamers) of the three title molecules are characterized by a higher stability compared to the E ones. The stabilities depend on the intramolecular hyperconjugative interaction and on the attraction or repulsion nonbonded interaction. The proton affinity of the carbonyl oxygen exceeds, by 15-20 kcal·mol(-1), that of the methoxy oxygen. Fluorine substitution causes a moderate lowering of the proton affinity of the oxygens. Protonation on the oxygen atoms yields a contraction of the C-H bond and large concomitant blue shift of the ν(CH) vibration. These changes are mainly determined by a lowering of the occupation of the corresponding σ*(CH) orbitals. The esters under consideration are probed on the interaction with the HF molecule. The complexes that are formed under this interaction on the oxygen of the H-C═O group are stronger than those formed on the oxygen belonging to the methoxy one. It is deduced that the hydrogen bond energies show a linear dependence on the proton affinities of the corresponding oxygen atoms. Hydrogen-bonded complexes of moderate strength are also formed, while HF interacts with the fluorine atoms of the fluorinated esters.

Theoretical investigation of the conformation, acidity, basicity and hydrogen bonding ability of halogenated ethers.

MP2/6-311++G(d,p) calculations have been carried out to investigate the conformation, protonation and the hydrogen bonding interactions with water of several halogenated ethers (CH(3)OCH(2)Cl, CH(2)ClOCH(2)Cl, CH(3)OCHCl(2), CHFClOCHF(2)). The optimized geometries, ν(CH) harmonic vibrational frequencies and the SAPT decomposition of the interaction energies are studied. The interaction with one water molecule gives several stable structures characterized by O(w)H(w)...O and CH...O(w) hydrogen bonds or by O...Cl halogen bonding. The MP2/CBS calculated binding energies of different complexes between the halogenated ethers and water vary between 1.7 and 7.7 kcal mol(-1). The energies of these structures are discussed as a function of the proton affinity of the ethers and the deprotonation enthalpy of the CH bonds. The contraction of the CH bonds and blue shifts of the corresponding stretching vibrations in the O-protonated ethers and their O...H(w)O(w) complexes are compared. A natural bond orbital analysis has revealed that substitution of the H atoms by one or several halogen atoms has a great influence on the hyperconjugative effects from the two non-equivalent O lone pairs to relevant antibonding orbitals, and the subsequent geometry of the hydrogen bonded complexes.

Theoretical investigation of the interaction between fluorinated dimethyl ethers (nF = 1-5) and water: role of the acidity and basicity on the competition between OH...O and CH...O hydrogen bonds.

Theoretical calculations have been carried out using ab initio MP2 and B3LYP density functional methods to investigate the interaction between fluorinated dimethyl ethers (nF = 1-5) and water. Depending on the number of F atoms implanted on the dimethyl ethers, linear structures stabilized by intermolecular O(w)H(w)...O or CH...O(w) hydrogen bonds or closed structures involving both hydrogen bonds are formed. Binding energies of the hydrogen-bonded complexes range between 4 and 12 kJ mol(-1). Blue shifts of the CH stretching vibrations are predicted even in the absence of a direct CH...O interaction. The red shifts of the OH stretching vibrations of water in the open and closed structures are analyzed as well. The natural bond orbital analysis includes the sigma*(O(w)H(w)) and sigma*(CH) occupation, the hybridization of the C atom, the atomic charges, and the intra- and intermolecular hyperconjugation energies. These parameters are discussed as a function of the proton affinity (PA) of the O atom and the deprotonation enthalpy (DPE) of the CH bonds of the fluorinated ethers calculated in a previous work. (16) Our results show that the effective PA in determining the intermolecular O --> sigma*(O(w)H(w)) hyperconjugation energies decreases with increasing acidity of the CH bond. In turn, the effective acidity of the CH bond in determining the intermolecular O(w) --> sigma*(CH) hyperconjugation energies decreases with increasing basicity of the O atom.

Blue shifts of the C-H stretching vibrations in hydrogen-bonded and protonated trimethylamine. Effect of hyperconjugation on bond properties.

The optimized geometry of isolated trimethylamine (TMA), its hydrogen bond complexes with phenol derivatives and protonated TMA is calculated at the B3LYP/6-31++G(d,p) level. A natural bond orbital (NBO) analysis on these systems is carried out at the same level of theory. In isolated TMA, one of the C-H bond in each of the three CH(3) groups is more elongated than the two other ones. As revealed by the NBO data, this results from a hyperconjugative interaction from the N lone pair to the sigma*(C-H) orbitals of the C-H bonds being in a transoid position with respect to the N lone pair. The formation of an intermolecular OH...N hydrogen bond with phenols results in a decrease of the lone pair effect. A linear correlation is found between the decrease in occupation of the sigma*(C-H) orbitals and the decrease in the hyperconjugative interaction energy in the complexes and isolated TMA. Complex formation with phenols results in a blue shift of 55-74 cm(-1) of the C-H stretching vibrations involved in the lone pair effect. Smaller blue shifts between 14 and 23 cm(-1) are predicted for the other C-H bonds. In these complexes, a linear correlation is found between the frequency shifts and the elongation of the C-H bonds. Protonation of TMA results in a nearly equalization of all the C-H distances and a blue shift of 180 cm(-1) of the C-H bonds involved in hyperconjugation with the N lone pair.

Theoretical and experimental studies of enflurane. Infrared spectra in solution, in low-temperature argon matrix and blue shifts resulting from dimerization.

Theoretical studies are performed on enflurane (CHFCl-CF(2)-O-CHF(2)) to investigate the conformational properties and vibrational spectra. Calculations are carried out at the B3LYP/6-31G(d) level along with a natural bond orbital (NBO) analysis. Experimental infrared spectra are investigated in carbon tetrachloride solution at room temperature and in argon matrix at 12 K. In agreement with previously reported data (Pfeiffer, A.; Mack, H.-G.; Oberhammer, H. J. Am. Chem. Soc. 1998, 120, 6384), it is shown that the four most stable conformers possess a trans configuration of the C-C-O-C skeleton and a gauche orientation of the CHF(2) group (with respect to the central C-O bond). These conformations are favored by electrostatic interaction between the H atom of the CHF(2) group and the F atoms of the central CF(2) group. Hyperconjugation effects from the O lone pairs to the antibonding orbitals of the neighboring C-H and C-F bonds also contribute to the stability of the four conformers. The vibrational frequencies, infrared intensities, and potential energy distributions are calculated at the same level of theory for the most stable conformers. On the basis of the theoretical results, these conformers are identified in an argon matrix. The influence of the concentration on the nu(CH) vibrations suggests the formations of higher aggregates in solution. Theoretical calculations are carried out on the enflurane dimer. The results show that the dimer is formed between two enflurane conformers having the largest stability. The dimer has an asymmetric cyclic structure, the two enflurane molecules being held together by two nonequivalent C-H...F hydrogen bonds, the C-H bond of the CHFCl group acting as a proton donor, and one of the F atoms of the CHF(2) groups acting as a proton acceptor. The theory predicts a contraction of 0.0014-0.0025 A of the two CH bonds involved in the interaction along with a blue shift of 30-38 cm(-1) of the corresponding nu(C-H) bands, in good agreement with the blue shifts of 35-39 cm(-1) observed in an argon matrix.

Theoretical and vibrational study of the conformation of 2-methoxy- 1,2-diphenylethanone.

The conformation and vibrational properties of 2-methoxy-1,2-diphenylethanone (MDPE) are investigated in the gas phase and in organic solvents. Ab initio calculations carried out at the B3LYP/6-31G(d) level demonstrate that three stable conformers having cisoid, skewed and transoid structures are present in the gas phase. In the gas phase, the conformers are separated by a low energy barrier and their relative energies do not differ by more than 7.2 kJ mol (-1) Like in crystalline MDPE 'Acta Crystallogr. Sect. C 44 (1988) 894', weak CH...O hydrogen bonds are present in the cisoid conformation. The IR and Raman spectra of solid MDPE are discussed. Several vibrational modes are split in organic solvents. A comparison between the theoretical data and the experimental dipole moments indicates that two conformers are present in solution, the population of the cisoid form increasing with the permittivity of the medium.

Theoretical studies of the interaction between enflurane and water.

Increase of the atmospheric concentration of halogenated organic compounds is partially responsible for a change of the global climate. In this work we have investigated the interaction between halogenated ether and water, which is one of the most important constituent of the atmosphere. The structures of the complexes formed by the two most stable conformers of enflurane (a volatile anaesthetic) with one and two water molecules were calculated by means of the counterpoise CP-corrected gradient optimization at the MP2/6-311++G(d,p) level. In these complexes the CH…O(w) hydrogen bonds are formed, with the H…O(w) distances varying between 2.23 and 2.32 Å. A small contraction of the CH bonds and the blue shifts of the ν(CH) stretching vibrations are predicted. There is also a weak interaction between one of the F atoms and the H atom of water, with the H(w)…F distances between 2.41 and 2.87 Å. The CCSD(T)/CBS calculated stabilization energies in these complexes are between -5.89 and -4.66 kcal mol(-1), while the enthalpies of formation are between -4.35 and -3.22 kcal mol(-1). The Cl halogen bonding between enflurane and water has been found in two complexes. The intermolecular (Cl···O) distance is smaller than the sum of the corresponding van der Waals radii. The CCSD(T)/CBS stabilization energies for these complexes are about -2 kcal mol(-1).

A theoretical investigation on the conformation and the interaction of CHF2OCF2CHF2 (desflurane II) with one water molecule.

The conformation and the interaction of CHF2OCF2CHF2 (desflurane II) with one water molecule is investigated theoretically using the ab initio MP2/aug-cc-pvdz and DFT-based M062X/6-311++G(d,p) methods. The calculations include the optimized geometries, the harmonic frequencies of relevant vibrational modes along with a natural bond orbital (NBO) analysis including the NBO charges, the hybridization of the C atom and the intra- and intermolecular hyperconjugation energies. In the two most stable conformers, the CH bond of the F2HCO- group occupies the gauche position. The hyperconjugation energies are about the same for both conformers and the conformational preference depends on the interaction between the non-bonded F and H atoms. The deprotonation enthalpies of the CH bonds are about the same for both conformers, the proton affinity of the less stable conformer being 3 kcal mol−1 higher. Both conformers of desflurane II interact with water forming cyclic complexes characterized by CH…O and OH…F hydrogen bonds. The binding energies are moderate, ranging from −2.4 to −3.2 kcal mol−1 at the MP2 level. The origin of the blue shifts of the ν(CH) vibrations is analyzed. In three of the complexes, the water molecule acts as an electron donor. Interestingly, in these cases a charge transfer is also directed to the non bonded OH group of the water molecule. This effect seems to be a property of polyfluorinated ethers.

Theoretical study of the interaction between HNZ (Z = O, S) and H(2)XNH(2) (X = B, Al). Conventional and dihydrogen bonds.

Theoretical calculations at the MP2/6-311++G(2d,2p) level are used to analyze the interaction between HNZ (Z = O, S) and H(2)XNH(2) (X = B, Al). In the most stable conformation, the complexes are cyclic, the molecules being held together by conventional NHZ hydrogen bonds and by XHHN dihydrogen bonds. Binding energies including ZPE- and BSSE-corrections lie in the range 6.2-6.9 kJ mol(-1) and there is little sensitivity to the nature of the X and Z atoms. In the XHHN dihydrogen bonds, the NH stretching vibrations are blue-shifted in the HNO complexes and red-shifted in the H(2)AlNH(2)-HNS complex. In the conventional NHZ hydrogen bonds, the NH stretching vibrations are red-shifted. The topological parameters at the bond critical point are in the usual range for hydrogen or dihydrogen bonds. A natural bond orbital analysis including the calculation of the atomic charges, hybridization, occupation of the antibonding orbitals and hyperconjugation energies shows that the shifts of the NH stretching vibrations in the conventional and dihydrogen bonds are mainly determined by the changes in occupation of the sigma*(NH) antibonding orbitals. The mechanism of intramolecular coupling is discussed and appears to be different for the HNO and HNS complexes. The analysis of all the theoretical data reveals that the NHZ bonds are stronger in the H(2)BNH(2) than in the H(2)AlNH(2) systems and that the XHHN dihydrogen bonds are stronger in the H(2)AlNH(2) than in the H(2)BNH(2) complexes.

Anomeric effects in the symmetrical and asymmetrical structures of triethylamine. Blue-shifts of the C-h stretching vibrations in complexed and protonated triethylamine.

Quantum mechanical calculations using density functional theory with the hybrid B3LYP functional and the 6-31++G(d,p) basis set are performed on isolated triethylamine (TEA), its hydrogen-bond complex with phenol, and protonated TEA. The calculations include the optimized geometries and the results of a natural bond orbital (NBO) analysis (occupation of sigma* orbitals, hyperconjugative energies, and atomic charges). The harmonic frequencies of the C-H stretching vibrations of TEA are predicted at the same level of theory. Two stable structures are found for isolated TEA. In the most stable symmetrical structure (TEA-S), the three C-C bond lengths are equal and one of the C-H bond of each of the three CH2 groups is more elongated than the three other ones. In the asymmetrical structure (TEA-AS), one of the C-C bonds and two C-H bonds of two different CH2 groups are more elongated than the other ones. These structures result from the hyperconjugation of the N lone pair to the considered sigma*(C-H) orbitals (TEA-S) or to the sigma*(C-C) and sigma*(C-H) orbitals of the CH2 groups (TEA-AS). The formation of a OH...N hydrogen bond with phenol results in a decrease of the hyperconjugation, a contraction of the C-H bonds, and blue-shifts of 28-33 cm-1 (TEA-S) or 40-48 cm-1 (TEA-AS) of the nus(CH2) vibrations. The nu(CH3) vibrations are found to shift to a lesser extent. Cancellation of the lone pair reorganization in protonated TEA-S and TEA-AS results in large blue-shifts of the nu(CH2) vibrations, between 170 and 190 cm-1. Most importantly, in contrast with the blue-shifting hydrogen bonds involving C-H groups, the blue-shifts occurring at C-H groups not participating in hydrogen bond formation is mainly due to a reduction of the hyperconjugation and the resulting decrease in the occupation of the corresponding sigma*(C-H) orbitals. A linear correlation is established between the C-H distances and the occupation of the corresponding sigma*(C-H) orbitals in the CH2 groups.

Theoretical study of (CH...C)- hydrogen bonds in CH(4-n)X(n) (X = F, Cl; n = 0, 1, 2) systems complexed with their homoconjugate and heteroconjugate carbanions.

This work deals with a theoretical study of the (CH...C)- hydrogen bonds in CH4, CH3X, and CH2X2 (X = F, Cl) complexed with their homoconjugate and heteroconjugate carbanions. The properties of the complexes are calculated with the B3LYP method using the 6-311++G(d,p) or 6-311++G(2df,2p) basis sets. The deprotonation enthalpies (DPE) of the CH bond or the proton affinities of the carbanions (PA(C-) are calculated as well. All the systems with the exception of the CH4...CHCl2(-) one are characterized by a double minimum potential. In some of the complexes, the (CH(b)...C)- hydrogen bond is linear. In other systems, such as CH3F...CH2F- and CH3F...CHF2(-), there is a large departure from linearity, the systems being stabilized by electrostatic interactions between the nonbonded H of the neutral molecule and the F atom of the carbanion. In the transition state, the (CH(b)...C)- bond is linear, and there is a large contraction of the intermolecular C...C distance. The binding energies vary within a large range, from -1.4 to -11.1 kcal mol(-1) for the stable complexes and -8.6 to -44.1 kcal mol(-1) for the metastable complexes. The energy barriers to proton transfer are between 5 and 20 kcal mol(-1) for the heteroconjugate systems and between 3.8 and 8.3 kcal mol(-1) for the homoconjugate systems. The binding energies of the linear complexes depend exponentially on 1.5DPE - PA(C-), showing that the proton donor is more important than the proton acceptor in determining hydrogen bond strength. The NBO analysis indicates an important electronic reorganization in the two partners. The elongations of the CH bond resulting from the interaction with the carbanion depend on the occupation of the sigma*(CH(b)) antibonding orbitals and on the hybridization of the C bonded to H(b). The frequency shifts of the nu(CH)(A1) stretching vibration range between 15 and 1150 cm(-1). They are linearly correlated to the elongation of the CH(b) bond.

Hydrogen bonding to pi-systems of indole and 1-methylindole: is there any OH...phenyl bond?

The weak hydrogen-bonded complexes between proton donors and the pi-cloud of indole and 1-methylindole (MI) are investigated theoretically by three different methods: DFT/B3LYP, MPW1B95, and MP2. This study addresses the question as to whether the 1:1 complex can only form between the proton and the pi-cloud of the pyrrole part of indole or if there also exists a 1:1 complex between the proton and the pi-cloud of the phenyl ring. For the water-indole system, the more elaborate MP2 and MPW1B95 methods yield only one minimum with a hydrogen bond to the pyrrole part and weak secondary interactions to the phenyl ring, in agreement with a recent criticism by Van Mourik (Chem. Phys. 2004, 304, 317-319) that the B3LYP functional is unable to account for the dispersion interaction. However, for the 1:1 complexes between MI and 2-propanol, all three methods indicate that both the five-membered and the six-membered rings of the indole chromophore can form pi-complexes. For the MI-trifluoroethanol (TFE) system, it is shown that the ethanol conformation is specific for the interaction site: for the complex to the five-membered ring, TFE is in the cis-gauche conformation, while for the complex to the six-membered ring site, it is in the trans conformation. These results are discussed as a function of local interactions in the systems.

Theoretical study of the symmetry of the (OH...O)- hydrogen bonds in vinyl alcohol-vinyl alcoholate systems.

The interactions between substituted vinyl alcohols and vinyl alcoholates (X = NH(2), H, F, Cl, CN) are studied at the B3LYP/6-311++G(d,p) level of theory. In a first step, the conformation of the monomers is investigated and the proton affinities (PA(A(-))) of the enolates are calculated. The enols and enolates are held together by strong (OH...O)(-) hydrogen bonds, the hydrogen bond energies ranging from 19.1 to 34.6 kcal mol(-1). The optimized O...O distances are between 2.414 and 2.549 A and the corresponding OH distances from 1.134 and 1.023 A. The other geometry parameters such as C[double bond]C or CO distances also indicate that, in the minimum energy configuration, the hydrogen bonds are characterized by a double well potential. The Mulliken charges on the different atoms of the proton donors and proton acceptors and the frequencies of the nu(OH) stretching vibrations agree with this statement. All the data indicate that the hydrogen bonds are the strongest in the homomolecular complexes. The transition state for hydrogen transfer is located with the transition barrier estimated to be about zero. Upon addition of the zero-point vibration energies to the total potential energy, the barrier vanishes. This is a characteristic feature of low-barrier hydrogen bonds (LBHBs). The hydrogen bond energies are correlated to the difference 1.5 PA(AH) - PA(A(-)). The correlation predicts different energies for homomolecular hydrogen bonds, in agreement with the theoretical calculations. Our results suggest that a PA (or pK(a)) match is not a necessary condition for forming LBHBs in agreement with recent data on the intramolecular hydrogen bond in the enol form of benzoylacetone (J. Am. Chem. Soc. 1998, 120, 12117).