IR-induced photorotamerization of allyl alcohol in low-temperature matrices and <i>ab initio</i> calculations

Abstract

The infrared regions νOH, τOH, and 950–900 cm−1 of allyl alcohol (2-propen-l-ol) were studied in low-temperature Ar, Kr, Xe, N2, and CO matrices. A fast IR-induced process and a similar slow dark process were found. It is concluded that the photoprocess is a rotamerization (obviously Cg→Gg′), since the process was similar in all five matrix media and also for other allyl compounds. The photoprocess was found to obey the first order rate law. Radiation from 9000 to below 2600 cm−1 was found to be active in initiating the process, especially the radiation corresponding to the νOH and methylenic νCH2 regions. The process is very fast in xenon. A possible mechanism for the process is discussed. To aid the interpretation of the results, ab initio calculations were carried out. All five conformers of allyl alcohol were optimized on the 4-31G level, and the corresponding semirigid potential energy curves of the torsional coordinates were constructed. Some calculations were carried out also on the 6-31G**, MP/4-31G, and CID/4-31G levels, using the 4-31G geometries. These calculations indicated that the conformers Gg′ and Cg are the lowest-energy species of quite similar energies.