The energetics of intramolecular hydrogen bonds (H-bonds) is a subject of fundamental importance in chemistry and biochemistry. In contrast with intermolecular H-bonds, whose enthalpy can be determined by experiment or accurately evaluated through a supermolecular approach, there is no general accepted procedure to determine the enthalpy of an intramolecular H-bond. In this work, different ways for assessing the energetics of intramolecular H-bonds of selected aromatic systems were applied and compared. They include the widely used conformational analysis approach (cis−trans method), a recently proposed isodesmic reaction method, and a new procedure that we designate as the ortho−para method. Energy calculations were carried out at several theory levels, including a modified complete basis set extrapolation method (CBS-QMPW1), in which the geometries are based on MPW1PW91/aug-cc-pVDZ density functional theory optimizations. The obtained results, together with a simple dipole−dipole interaction model, help to explain why the enthalpies of intramolecular H-bonds are often overestimated by the cis−trans method. The results also show that intramolecular H-bond enthalpies based on the isodesmic reaction method may be unreliable. The ortho−para method, which can be applied when accurate theoretical or experimental standard enthalpies of formation are available, is probably the best way of estimating the enthalpies of intramolecular hydrogen bonds. Finally, our results illustrate the important role played by intramolecular H-bonds in the energetics of homolytic dissociation reactions involving di-substituted benzenes.

The hydration of the hydroxyl OH radical has been investigated by microsolvation modeling and statistical mechanics Monte Carlo simulations. The microsolvation approach was based on density functional theory ͑DFT͒ calculations for OH-(H 2 O) 1-6 and (H 2 O) 1-7 clusters. The results from microsolvation indicate that the binding enthalpies of the OH radical and water molecule to small water clusters are similar. Monte Carlo simulations predict that the hydration enthalpy of the OH radical, ⌬ hyd H(OH,g), is Ϫ39.1 kJ mol Ϫ1. From this value we have estimated that the band gap of liquid water is 6.88 eV, which is in excellent agreement with the result of Coe et al. ͓J. Chem. Phys. 107, 6023 ͑1997͔͒. We have compared the structure of the hydrated OH solution with the structure of pure liquid water. The structural differences between the two systems reflect the strong role played by the OH radical as a proton donor in water. From sequential Monte Carlo/DFT calculations the dipole moment of the OH radical in liquid water is 2.2Ϯ0.1 D, which is ϳ33% above the experimental gas phase value ͑1.66 D͒.

Density functional theory results for the structure and conformational equilibrium of four calix [4]arene conformers are reported. The results are compared with experiment, force field, and semiempirical molecular orbital calculations. The energy difference between the two most stable conformers of calix[4]arene (cone and partial-cone) is 10 kcal mol -1 at the BLYP/6-31G** level with the geometries optimized at BLYP/6-31G*. For the most stable conformer, results for the protonated structure are also reported. Electrostatic potential surfaces for the cone calix[4]arene and the corresponding tetra-O-H-depleted structure have been calculated. It is suggested that their representation may be of relevance to understand the known ability of calix[n]arene systems to form complexes with charged species in host-guest chemistry.

Time-resolved photoacoustic calorimetry (TR-PAC) and quantum chemistry calculations were used to investigate the energetics of sulfur-hydrogen bonds in thiophenol and four para-substituted thiophenols, 4-XC 6 H 4 SH (X ) CH 3 , OCH 3 , Cl, and CF 3 ). The result obtained for the PhS-H gas-phase bond dissociation enthalpy, derived from the PAC experimental results in solution, is 349.4 ( 4.5 kJ mol -1 . This value is significantly higher than recent literature values but agrees with a value suggested some 20 years ago in a widely used review. The PAC result also concurs with the value computed at a high theory level, G3(MP2), 346.8 kJ mol -1 . The data obtained for the substituted thiophenols support the idea that substituent effects are less pronounced on the S-H bond dissociation enthalpy than on the O-H bond dissociation enthalpy of the corresponding phenols. † Part of the special issue "Jack Beauchamp Festschrift".

We are reporting density functional theory results for the binding energies, structures, and vibrational spectra of (H-Cl) 2-6 and (H-F) 2-10 clusters. The performance of different functionals has been investigated. The properties of HF clusters predicted by hybrid functionals are in good agreement with experimental information. The HCl dimer binding energy ⌬E e is underestimated by hybrid functionals. The Perdew and Wang exchange and correlation functional ͑PW91͒ result for ⌬E e is Ϫ9.6 kJ mol Ϫ1 , in very good agreement with experiment (Ϫ9.5 kJ mol Ϫ1). However, PW91 overestimates binding energies of larger clusters. Hydrogen bonding cooperativity depends on the cluster size n but reaches a limit for moderately sized clusters (nϭ8 for HF). The average shift to low frequencies ͑⌬͒ of the X-H (XϭCl,F) stretching vibration relative to the monomer is in good agreement with experimental data for HF clusters in solid neon. However, some discrepancies with experimental results for HCl clusters were observed. The behavior of ⌬ as a function of the cluster size provides an interesting illustration of hydrogen-bond cooperative effects on the vibrational spectrum. The representation of the electronic density difference shows the rearrangement of the electronic density induced by hydrogen bonding in the clusters and supports the view that hydrogen-bond cooperativity is related to electronic sharing and delocalization.

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The hydration of mesityl oxide (MOx) was investigated through a sequential quantum mechanics/ molecular mechanics approach. Emphasis was placed on the analysis of the role played by water in the MOx synanti equilibrium and the electronic absorption spectrum. Results for the structure of the MOx-water solution, free energy of solvation and polarization effects are also reported. Our main conclusion was that in gas-phase and in low-polarity solvents, the MOx exists dominantly in synform and in aqueous solution in anti-form. This conclusion was supported by Gibbs free energy calculations in gas phase and in-water by quantum mechanical calculations with polarizable continuum model and thermodynamic perturbation theory in Monte Carlo simulations using a polarized MOx model. The consideration of the in-water polarization of the MOx is very important to correctly describe the solute-solvent electrostatic interaction. Our best estimate for the shift of the p-p * transition energy of MOx, when it changes from gas-phase to water solvent, shows a red-shift of -2,520 ± 90 cm -1 , which is only 110 cm -1 (0.014 eV) below the experimental extrapolation of -2,410 ± 90 cm -1 . This red-shift of around -2,500 cm -1 can be divided in two distinct and opposite contributions. One contribution is related to the syn ? anti conformational change leading to a blue-shift of *1,700 cm -1 . Other contribution is the solvent effect on the electronic structure of the MOx leading to a red-shift of around -4,200 cm -1 . Additionally, this red-shift caused by the solvent effect on the electronic structure can by composed by approximately 60 % due to the electrostatic bulk effect, 10 % due to the explicit inclusion of the hydrogen-bonded water molecules and 30 % due to the explicit inclusion of the nearest water molecules.

Electronic properties of benzene in water were investigated by a sequential quantum mechanical/molecular dynamics approach. Emphasis was placed on the analysis of the structure, polarization effects, and ionization spectrum. By adopting a polarizable model for both benzene and water the structure of the benzene-water solution is in good agreement with data from first principles molecular dynamics. Further, strong evidence that water molecules acquire enhanced orientational order near the benzene molecule is found. Upon hydration, the quadrupole moment of benzene is not significantly changed in comparison with the gas-phase value. We are also reporting results for the dynamic polarizability of benzene in water. Our results indicate that the low energy behaviour of the dynamic polarizability of gas-phase and hydrated benzene is quite similar. Outer valence Green's function calculations for benzene in liquid water show a splitting of the gas-phase energy levels associated with the 1e(1g)(π), 2e(2g), and 2e(1u) orbitals upon hydration. Lifting of the orbitals degeneracy and redshift of the outer valence bands is related to symmetry breaking of the benzene structure in solution and polarization effects from the surrounding water molecules.