Thermal and IR-induced conformer interconversion processes for 2-fluoroethylamine in several low-temperature matrix media and ab initio 4-31G, 4-31G(N*), and 6-31G** calculations

Abstract

IR-induced conformer interconversion processes have been found to occur for 2-fluoroethylamine in solid Ar, Kr, Xe, and N2 matrices but not in solid CH4. Using information obtained from the photoprocesses as well as from thermal processes and ab initio calculations, it was possible to find out the conformers present, to evaluate the conformer energies and torsional barrier heights, and to unravel the occurring processes. The total IR process consists of two parallel first-order C–C rotations, Gg→Tg and Gt→Tt. The influence of the host molecules upon the rates and barrier heights, the efficiency of different spectral regions in initiating the process, and the thermal processes were studied. The latter occur as C–C rotations opposite to the photoprocesses and as conformational equilibria involving rotations around the C–N bond. Extensive ab initio calculations were carried out on the 4-31G, 4-31G(N*), and 6-31G** levels. Geometries of the conformers were fully optimized at these levels, and torsional potential energy barrier heights and vibrational frequencies [at the 4-31G(N*) level] were calculated; the latter confirmed the experimental conformer assignment. IR-induced conformer changes were observed also for 2-chloro- ethylamine in Ar. The mechanism of the photorotamerizations of alcohols and amines is discussed and the importance of C–C torsional barriers is emphasized. A linear relationship between the logarithm of the first-order rate constant and the ab initio 4-31G calculated barrier height of the C–C torsion has been found (including several alcohols, 2-fluoroethylamine, and two aldehydes).