REINVESTIGATION OF INFRARED-INDUCED CONFORMATIONAL ISOMERIZATIONS OF 1,2-ETHANEDIOL IN LOW-TEMPERATURE AR MATRICES AND REVERSE REACTION IN THE DARK

Infrared-induced conformational isomerizations of 1,2-ethanediol in low-temperature Ar matrices have been reexamined by using better experimental facilities. Three types of reactions (R1, R2, and R3) occur under infrared irradiation with different rates, and a reverse process of R1 takes place in the dark. The reactants of R1, R2, and R3 have been identified as conformers tG+g-, g+G+g-, and g-G+g-, respectively, all of which have the gauche conformation (G) around the CC bond. The products of R1 and R2 are two conformers among tTt, tTg-, and g+Tg- which have the trans conformation (T) around the CC bond. A comparison of the observed frequencies and the normal frequencies obtained from ab initio molecular orbital calculations has suggested that tTt is the R1 product and g+Tg- the R2 product. Although the product of R3 has not been specified, it must be one of the above-mentioned five conformers. Structure parameters of the six conformers have been optimized by ab initio calculations on the 6-31G** level, and total energies have been reevaluated by the MP2 method. The barriers to R1 and R2 are discussed by combining observed data with the results of ab initio calculations.