The spin-coupled VB method is used to study all the singlet and triplet valence excited states, as well as the n=3,4 singlet and triplet Rydberg states of benzene below the first ionization potential at 9.25 eV. The valence excited states are classified in an obvious physical way into covalent or ionic states, from which it follows at once that covalent states are well described using the approximation of σ/π separation and a frozen σ core, whereas the error in the computed transition energies to the ionic states is much larger and these states require additional σ/π correlation for their proper description. The Rydberg states are very well-described, provided that a suitable σ core, derived from a calculation on the C6H+6 ion, is used. The numerical accuracy of the final results for the transition energies is at least the same as that given by the largest MO-CI- or CASSCF-CI-based methods reported to date. The spin-coupled VB approach has the obvious advantage in providing a compact and clear picture of the various states.

Recent experimental results on positive charged formic acid clusters generated by the impact of (252)Cf fission fragments (FF) on icy formic acid target are examined in this paper by quantum mechanical calculations. Structures for the clusters series, (HCOOH)(n)H(+) and (HCOOH)(n)H(3)O(+), where 2 < or = n < or = 4, are proposed based on ab initio electronic structure methods. Results show that cluster growth does not present a regular pattern of nucleation. A stability analysis was performed considering the commonly defined stability function, where E is the total electronic energy plus the zero point vibrational energy correction, including the BSSE correction. The stability analysis leads to a picture that is compatible with experimental observations, indicating a decay of the stability with the increase of cluster mass. Temporal behavior of the clusters was evaluated by Born-Oppenheimer molecular dynamics to check the mechanism that provides cluster stability. The evaluated temporal profiles indicate the importance of hydrogen atom migration between the formic acid moieties to maintain the stability of the structures.

A combined density functional theory and transition-state theory study of the mechanisms and reaction coefficients of gas-phase ozonolysis of geraniol-trans, 6-methyl-5-hepten-2-one, and 6-hydroxy-4-methyl-4-hexenal is presented. The geometries, energies, and harmonic vibrational frequencies of each stationary point were determined by B3LYP/6-31(d,p), MPW1K/cc-pVDZ, and BH&HLYP/cc-pVDZ methods. According to the calculations, the ozone 6-methyl-5-hepten-2-one reaction is faster than the ozone 6-hydroxy-4-methyl-4-hexenal reaction, but both are slower than the ozone geraniol-trans reaction. By using the BH&HLYP/cc-pVDZ data, a global rate coefficient of 5.9 x 10(-16) cm(3) molecule(-1) s(-1) was calculated, corresponding to the sum of geraniol-trans, 6-methyl-5-hepten-2-one, and 6-hydroxy-4-methyl-4-hexenal reactions with the ozone. These results are in good agreement with the experimental studies.