The intrinsic acidity of dimethylhalonium ions has been determined, both by theoretical methods and by gas-phase reactions of the isolated ions with pyridine bases. The calculated geometry of the dimethylhalonium ions shows a bent structure with the C-X-C angle decreasing in the order Cl > Br > I. Thermochemical calculations for the reaction of the dimethylhalonium ions with pyridine, 2,6-dimethylpyridine, and 2,6-di-tert-butylpyridine indicate that proton transfer, with the formation of the dimethylhalonium ylide is endothermic, whereas methyl transfer, with formation of methylhalide, is exothermic. The endothermicities for proton transfer are, nevertheless, dependent on the steric hindrance of the base. The bulkier the bases, the less endothermic the proton-transfer reaction is. Experimental gas-phase reactions support the calculations, showing that methyl transfer is the major reaction of dimethylchloronium and dimethyliodonium with pyridine, whereas proton transfer, as well as single electron transfer, is observed for the bulkier bases. The calculations also indicate that acidity increases in the order chloronium > bromonium > iodonium. NBO calculations predict that hyperconjugation with the sigma carbon-halogen orbital plays a role in stabilizing the halonium ylide species in the gas phase.

No abstract

No abstract

Systematic investigations of conformational effects of the hydroxyl group on carbon or hydrogen chemical shifts of different types of alcohols reveal that, besides stereoelectronic effects, hyperconjugation with lone pairs may have a strong influence. In view of the growing use of chemical shifts in probing the structure of biologic molecules, we employed DFT/GIAO/NBO calculations at the B3LYP level for conformers obtained from partial optimization of structures resulting from 30°variations of the COCOXOH (XAO, N) dihedral angles to verify if nitrogen responded in a fashion similar to oxygen. Although the hybridization and geometry of hydroxyl and amine groups are distinct, the lone pair on nitrogen reveals hyperconjugation with suitably positioned orbitals on alkyl groups.

Structures of B2H2 n 2+ dications (n = 1−4) were investigated at the QCISD(T)/6-311G** level of theory. Thermodynamic and kinetic stabilities of the dications were also estimated. Although deprotonations of the dications are exothermic (except the dication B2H2 2+), they show considerable kinetic barriers. The charge distributions and Wiberg bond indices were calculated by the NBO method. 11B NMR chemical shifts were also calculated by GIAO-MP2 and GIAO-DFT methods.

In this work, the rate-limiting steps of Δ(3)-carene oxidation by ozone and OH radicals were studied. The thermochemical and kinetic parameters were evaluated using the B3LYP, PBE1PBE and BHandHLYP functionals, coupled cluster methods and the 6-311G(d,p) and 6-311++G(d,p) basis sets. The attack on the double bond may occur in different orientations, leading to different oxidation products. The rate coefficients of each step of the reactions were evaluated using conventional canonical transition-state theory and variational canonical transition-state theory whenever necessary. The theoretical rate coefficient for the ozonolysis mechanism, evaluated at the CCSD(T)/6-31G(d,p)//PBE1PBE/6-311++G(d,p) level, was 2.08 × 10(-17) cm(3) molecule(-1) s(-1). The coefficient for the oxidation initialised by the OH radical, calculated at the BHandHLYP/6-311++G(d,p) level, was 5.06 × 10(-12) cm(3) molecule(-1) s(-1). These values are in reasonable agreement with the experimental results. The importance of these reactions in atmospheric chemistry is discussed.

No abstract

No abstract