An extensive quantum mechanical study of the potential energy surface for the isomerization and dissociation
reactions of CHNS is reported. The calculations were performed using density functional theory and correlated
ab initio methods and employing large aug-cc-pVTZ basis set. Nine CHNS isomers and eleven transition
states linking them have been found on the singlet PES. Dissociation energies of the singlet most stable open
chain CHNS isomers have been evaluated. On the triplet PES, eight mostly highly energetic CHNS isomers
and five transition states for their interconversion and dissociation have been also found. Several available
routes for isomerization and dissociation have been identified. A prediction has been made for the possible
mechanism explaining the formation of the singlet HSCN and HSNC during UV photolysis of HNCS/Ar and
HNCS/N2 low-temperature matrixes observed recently by one of us.
Theoretical studies are performed on enflurane (CHFCl-CF 2 -O-CHF 2 ) to investigate the conformational properties and vibrational spectra. Calculations are carried out at the B3LYP/6-31G(d) level along with a natural bond orbital (NBO) analysis. Experimental infrared spectra are investigated in carbon tetrachloride solution at room temperature and in argon matrix at 12 K. In agreement with previously reported data (Pfeiffer, A.; Mack, H.-G.; Oberhammer, H. J. Am. Chem. Soc. 1998, 120, 6384), it is shown that the four most stable conformers possess a trans configuration of the C-C-O-C skeleton and a gauche orientation of the CHF 2 group (with respect to the central C-O bond). These conformations are favored by electrostatic interaction between the H atom of the CHF 2 group and the F atoms of the central CF 2 group. Hyperconjugation effects from the O lone pairs to the antibonding orbitals of the neighboring C-H and C-F bonds also contribute to the stability of the four conformers. The vibrational frequencies, infrared intensities, and potential energy distributions are calculated at the same level of theory for the most stable conformers. On the basis of the theoretical results, these conformers are identified in an argon matrix. The influence of the concentration on the ν(CH) vibrations suggests the formations of higher aggregates in solution. Theoretical calculations are carried out on the enflurane dimer. The results show that the dimer is formed between two enflurane conformers having the largest stability. The dimer has an asymmetric cyclic structure, the two enflurane molecules being held together by two nonequivalent C-H‚‚‚F hydrogen bonds, the C-H bond of the CHFCl group acting as a proton donor, and one of the F atoms of the CHF 2 groups acting as a proton acceptor. The theory predicts a contraction of 0.0014-0.0025 Å of the two CH bonds involved in the interaction along with a blue shift of 30-38 cm -1 of the corresponding ν(C-H) bands, in good agreement with the blue shifts of 35-39 cm -1 observed in an argon matrix.
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