Modeling studies on the uptake of hydrogen molecules by graphene.

Detailed ab initio molecular orbital calculations on the interactions of molecular hydrogen, H2, with various poly-aromatic hydrocarbons (PAHs) as a model system for graphene were carried out to accurately describe the physisorption phenomenon. The binding energies corrected for the basis set superposition error, ΔEbind(BSSE), were obtained using the optimized geometries at the MP2 level with a large basis set and were compared with the single point binding energies, denoted as ΔEbind(BSSE-s), using large basis sets on the geometries optimized at the small basis sets, such as SVP and TZVP. The calculations showed that the ΔEbind(BSSE-s) values were similar to those at the MP2 level with the large basis sets.

Comprehensive studies on the tautomerization of glycine: a theoretical study.

The tautomerization process of glycine between the neutral (NE) and zwitterionic (ZW) forms in aqueous solution was explored theoretically using the conductor-like polarizable continuum model (CPCM) by adopting the PAULING cavity model at the B3LYP, MP2 and CCSD levels with the 6-311+G(d,p) basis set. The tautomerization of glycine is unable to be predicted satisfactorily within the equilibrated framework of the CPCM method. Instead, in this study, three plausible non-equilibrated solvation situations were assumed: (S-1) one water molecule attached to the transferring proton in the ZW moves together with the transferring proton; (S-2) one water molecule attached to the transferring proton in the ZW remains motionless at a fixed position near the NH(2) fragment at the TS structure; and (S-3) proton transfer occurs without changing the position of the surrounding water molecules from their initial state, the ZW form, in the eight water clusters.

Reexamination of the π-bond strengths within H2C=XHn systems: a theoretical study.

The accurate determination of π-bond energies, D(π), in doubly-bonded species has been an important issue in theoretical chemistry. The procedure using the divalent state stabilization energy defined by Walsh has been suggested, and the procedure seems to be conceptually reasonable and applicable to all kinds of doubly-bonded species. Therefore, the aim of this study was to examine whether the procedure could be a reliable methodology for estimating the D(π) values for a variety of H(2)C=XH(n) species.

Comparative studies on the reactions of acetyl and thioacetyl halides with NH3 in the gas phase and in aqueous solution: a theoretical study.

The reactions of acetyl halides, CH3C(═ O)X and corresponding sulfur analogues, thioacetyl halides, CH3C(=S)X, where X = F and Cl, with NH3 nucleophile were studied theoretically, at the QCISD level of theory, in the gas phase and in aqueous solution. All reactions occurred via the tetrahedral species, and reactions through neutral intermediates both in the gas phase and in aqueous solution could be ruled out, except for the case of the gas-phase reaction of acetyl fluoride. The tetrahedral structure was a transition state (TS) in the reactions of acetyl chloride, while it was a stable intermediate in reactions of thioacetyl halides.

Effects of basis set superposition error on optimized geometries and complexation energies of organo-alkali metal cation complexes.

Theoretical studies were performed to study the binding of alkali metal cations, X(+) (X = Li, Na, K), to poly(ethylene oxide) (PEO, I), poly(ethylene amine) (PEA, II), and poly(ethylene N-methylamine) (PEMA, III) by the Hartree-Fock (HF) and B3LYP methods using the 6-31G(d) and 6-311+G(d,p) basis sets. Two types of complex were considered in this study: a singly coordinated system (SCS) and a doubly coordinated system (DCS). Complexation energies were calculated both without and with basis set superposition error (BSSE).

Prediction of densities for solid energetic molecules with molecular surface electrostatic potentials.

The densities of high energetic molecules in the solid state were calculated with a simplified scheme based on molecular surface electrostatic potentials (MSEP). The MSEP scheme for density estimation, originally developed by Politzer et al., was further modified to calculate electrostatic potential on a simpler van der Waals surface.

DFT-PCM studies of solvent effects on the cross-interaction constants in benzhydryl cation and anion formation.

The Hammett rho+ and rho- values have been determined by varying substituent Y' for a given Y in the benzhydryl cation and anion formation (YH4C6-CH-C6H4Y' where C is a cationic or an anionic center) at the RHF/3-21G, RHF/6-31G, RHF/6-31+G, and B3LYP/6-31+G levels. The failure of RHF theory in accounting for the stabilization by delocalization leads to the smaller magnitudes of rho+ and rho- with electron-donating and -withdrawing substituents, Y, respectively, than the corresponding DFT values. The effects of solvent (benzene, dichloroethane, and acetonitrile) on the rho values were calculated by applying the conductor polarizable continuum model method to the DFT results.

Unusual pi-donating effects of pi-accepting substituents on the stabilities of benzylic cations: a theoretical study.

The pi-donating effects of pi-accepting X-substituents in substituted benzylic cations, X-C(6)H(5)-CHR(+) where R = CF(3), H and OCH(3), and X = p-NH(2), p-OCH(3), p-CH(3), H, p-F, p-Cl, p-CHO, m-CN, p-CN, m-NO(2) or p-NO(2), have been studied theoretically by using isodesmic hydride transfer reactions at various levels of theory. It might be difficult to determine the pi-donating effects of pi-acceptors using the simple Hammett-type linear equation, because it is not sensitive enough to include small pi-donating effects. Therefore, this effect was estimated using the NBO deletion energy (DeltaE(D)) of the second-order charge-transfer interaction (DeltaE(ct)) between the pi-orbitals (or lone pair orbitals) of the X-substituent and the pi-orbitals of phenyl ring.

Prediction of physicochemical properties of organic molecules using van der Waals surface electrostatic potentials.

The generalized interaction properties function (GIPF) methodology developed by Politzer and coworkers, which calculated molecular surface electrostatic potential (MSESP) on a density envelope surface, was modified by calculating the MSESP on a much simpler van der Waals (vdW) surface of a molecule. In this work, vdW molecular surfaces were obtained from the fully optimized structures confirmed by frequency calculations at B3LYP/6-31G(d) level of theory. Multiple linear regressions for normal boiling point, heats of vaporization, heats of sublimation, heats of fusion, liquid density, and solid density were performed using GIPF variables from vdW model surface.

Transition-state variation in the nucleophilic substitution reactions of aryl bis(4-methoxyphenyl) phosphates with pyridines in acetonitrile.

The kinetics and mechanism of the reactions of Z-aryl bis(4-methoxyphenyl) phosphates, (4-MeOC(6)H(4)O)(2)P(=O)OC(6)H(4)Z, with pyridines (XC(5)H(4)N) are investigated in acetonitrile at 55.0 degrees C. In the case of more basic phenolate leaving groups (Z = 4-Cl, 3-CN), the magnitudes of beta(X) (beta(nuc)) and beta(Z) (beta(lg)) indicate that mechanism changes from a concerted process (beta(X) = 0.

Gas-phase identity nucleophilic substitution reactions of cyclopropenyl halides.

The gas-phase identity nucleophilic substitution reactions of halide anions (X = F, Cl, and Br) with cyclopropenyl halides, X(-) + (CH)(3)X <= => X(CH)(3) + X(-), are investigated theoretically at four levels of theory, B3LYP/6-311+G**, MP2/6-311+G**, G2(+)MP2//MP2/6-311+G**, and G2(+)//MP2/6-311+G**. Four types of reaction paths, the sigma-attack S(N)2, pi-attack S(N)2'-syn, and S(N)2'-anti and sigmatropic 1,2-shift, are possible for all the halides. In the fluoride anion reactions, two types of stable adducts, syn- and anti-1,2-difluorocyclopropyl anions, can exist on the triple-well-type potential energy surface of the identity substitution reactions with rearrangement of double bond (C=C), S(N)2'-syn, and S(N)2'-anti processes.