The photodissociation of formic acid has been studied experimentally and theoretically. Ab initio calculations were performed to study the dissociative profiles of five reaction channels on the S 0 , S 1 , and T 1 potential energy surfaces. The vibrationally excited nascent products were detected using a time-resolved Fourier transform infrared spectrometer after laser photolysis at 248 or 193 nm. In the 248 nm photolysis, the HCOOH molecule was first excited to the S 1 state, but it was found that the dissociation takes place on the S 0 surface after internal conversion. The products of the vibrationally excited CO, CO 2 (v 3) and H 2 O(v 1) were detected. During the dissociation process the vibrationally energized molecule is geometrically memorized and dynamically controlled, with the yield preference of CO and H 2 O over that of CO 2 and H 2. The ratio of CO(vу1)/CO 2 (v у1) is estimated as Ͻ7.5. Vibrationally excited CO (v) and CO 2 (v 3) are also found in the 193 nm photolysis but the CO/CO 2 ratio increases to 11. Most of the dissociation is thought to occur on the S 0 state. At this wavelength another dissociation channel which produces OH and HCO radicals on S 1 surface has been identified. The dissociation is unlikely to occur on the T 1 surface, as the energy barriers are fairly high.

Photodissociation of acetic acid in the gas phase was investigated using ab initio molecular orbital methods. The stationary structures on the ground-state potential energy surfaces were mainly optimized at the MP2 level of theory, while those on the excited-state surfaces were determined by complete active space SCF calculations with a correlation-consistent basis set of cc-pVDZ. The reaction pathways leading to different photoproducts are characterized on the basis of the computed potential energy surfaces and surface crossing points. The calculations reproduce the experimental results well and provide additional insight into the mechanism of the ultraviolet photodissociation of acetic acid and related compounds.

Abstract:The mechanisms of the reactions of CH (x211) with NH,, H,O, and H F have been studied by means of ab initio molecular orbital calculations incorporating electron correlation with Mdler-Plesset perturbation theory up to the second order. For each of the three CH reactions, the insertion path has been found in the potential energy surface; in the calculated insertion path there exists an intermediate complex prior to the transition state that has a lower energy than the reactants. Energetic results indicate that insertion paths are favourable channels for these CH reactions, which is in line with proposals based on kinetic experiments.Key words: CH radical, ammonia, water, hydrogen fluoride, reaction mechanism.

The monomer and dimer structures of cyclo[(-L-Phe 1 -D-Ala 2 ) n -] and cyclo[(-L-Phe 1 -D-Me N-Ala 2 ) n -] (n ) 3-6) were studied by using the semiempirical molecular orbital AM1 method and the density functional B3LYP method. The structural characteristics of these molecules were revealed, some of which are not yet confirmed experimentally. The influences of the substituents and ring size on molecular structure and the self-assembly process are discussed in detail. The inherent impetus for these molecules to self-assemble to polypeptide nanotubes is discussed.