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The optimized geometries, harmonic vibrational frequencies, and energies of the three cyclic structures of the thymine−water complex are computed using density functional theory (B3LYP) combined with the 6-31++G(d,p) basis set. The proton affinity of the oxygen atoms and the deprotonation enthalpy of the NH bonds of thymine are computed at the same level and compared with recent data on uracil. In both uracil and thymine, the deprotonation enthalpies are lower (1391−1449 kJ mol-1) than those of the biological NH donors of the peptide links (1470−1485 kJ mol-1) (Mautner, M. J. Am. Chem. Soc. 1988, 110, 3075). Harmonic vibrational frequencies are also reported for the uracil−water complexes. In both uracil and thymine complexes, the most stable hydrogen bond is formed at the O site characterized by the smallest proton affinity and at the NH site characterized by the highest acidity. The intermolecular distances and the energies of the hydrogen bonds formed at the different sites of thymine and uracil depend on the proton affinity and the deprotonation enthalpy of these sites. New correlations between these parameters are presented and the cooperativity in the closed structures discussed.

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The homolytic C-H bond dissociation enthalpies (BDEs) of toluene and its para- and meta-substituted derivatives have been estimated by using the (RO)B3LYP/6-311++G(2df,2p)//(U)B3LYP/6-311G(d,p) procedure. The performance of two other hybrid functionals of DFT, namely, B3PWP91 and O3LYP, has also been evaluated using the same basis sets and molecules. Our computed results are compared with the available experimental values and are found to be in good agreement. The (RO)B3LYP and (RO)O3LYP procedures are found to produce reliable BDEs for the C-H bonds in toluene and the C-X (X = F, Cl) bond in alpha-substituted toluene (C6H5-CH2X) and their substituted derivatives. The substituent effect on the BDE values has been analyzed in terms of the ground-state effect and the radical effect. The effect of polarization of the C-H bond on the substituent effect is also analyzed. The BDE(C-H) and BDE(C-X) values for alpha-substituted (X = F and Cl) toluenes with a set of para substituents are presented for the first time.

Density functional theory has been used with different combinations of exchange and correlation functionals to study the proton affinities of six organic molecules, namely H2CO, CH3CHO, CH3OH, C2H5OH, HCOOH, and CH3COOH. Complete geometry optimizations have been carried out for both the neutral and protonated species with all combinations of functionals. The proton affinity values are then compared with the corresponding experimental values. It has been observed that combination of Perdew's and Becke's exchanges with Proynov's correlation functional is the most effective in reproducing the proton affinity.

We review in details some recently proposed kinetic models of opinion dynamics. We discuss several variants including a generalised model. We provide mean field estimates for the critical points, which are numerically supported with reasonable accuracy. Using nonequilibrium relaxation techniques, we also investigate the nature of phase transitions observed in these models. We also study the nature of correlations as the critical points are approached.

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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.