UV-Induced Rotamerization and Vibrational Spectra of the Conformers of Cyanomethyl Formate:  Matrix Isolation Infrared and ab Initio Studies

Abstract

Ab initio RHF and MP2 calculations with the standard 6-31G* and 6-311G** basis sets were carried out on cyanomethyl formate (HCOOCH2CN). The calculations indicated two stable conformers (Z,ap and Z,sc) for this molecule. However, the energy difference between the conformers varied at different levels of theory. The calculations with the 6-311G** basis set and correlated wave functions indicated equal energy for these conformers, whereas all the other calculations slightly preferred the Z,ap conformer (by 0.2−2.1 kJ mol-1). The existence of the Z,ap and Z,sc conformers of cyanomethyl formate was confirmed by matrix isolation infrared studies in argon. Immediately after deposition the spectrum showed two peaks both in carbonyl and asymmetric C−O−C stretching regions. The behavior of these pairs of infrared peaks upon UV irradiation and the comparison with the results obtained for related esters strongly indicate that these doublets are caused by two different conformers. It was observed that the Z,ap and Z,sc conformers can be surmounted thermally in an argon matrix. This is consistent with the calculated values of 2.6 kJ mol-1 (MP2/6-31G*) or 3.7 kJ mol-1 (RHF/6-31G*) for the potential energy barrier of this process. A conformer interconversion process, proceeding from both the Z,ap and Z,sc to the E,sc conformer, was induced by UV irradiation at 248 nm. The E,sc conformer is the main product of isomerization. This was established by ab initio calculations and by analysis of the spectral changes in the (CO) and (C−O−C) stretching regions. Upon prolonged irradiation at the same wavelength, the E,sc conformer decomposed to stable products including a hydrogen-bonded (1:1) complex of CO and cyanomethanol. (CO2 production was also observed. However, its counterpart could not be identified, presumably due to further decomposition.)

Conformation-Sensitive Infrared Bands and Conformational Characteristics of Stereoregular Poly(methyl methacrylate)s by Variable-Temperature FTIR Spectroscopy

“…The planarity is, in turn, attainable only under a cis or a trans conformation of the OC−O−C moiety. The cis conformation has been known for many years as the major conformer for most esters but evidence about the relative importance of the trans conformation has been obtained only recently. − In particular, according to recent ab initio calculations and experimental results obtained for formic acid esters, the ν a (C−O−C) band frequency is highly sensitive to the conformation of the OC−O−C skeleton. Namely, all the formates studied so far show the peak of the trans conformation at lower wavenumbers than that of the corresponding cis conformation.…”

Gauche-Trans Conformational Changes in Ethyl Formate Chemisorbed on Ni(111): Coverage-Dependent RAIRS Spectra

“…The ν as (C−O−C) and ν(CO) band frequencies have proven very useful for differentiating ( E ) and ( Z ) conformations of free formic acid esters such as cyanomethyl formate, methyl formate, and chloromethyl formate . In each case, the s - trans , trans structure displays a blue-shifted ν(CO) band and a red-shifted ν as (C−O−C) band with respect to the s - cis , trans conformation.…”

Conformational analysis of dichloromethyl formate by the combined use of infrared and matrix-infrared spectroscopy and ab initio calculations

“…E barrier for methyl formate 9 and the Z,ap ! E,sc barrier for cyanomethyl formate, 5 respectively, but significantly larger than the 45 kJ mol À1 calculated for the Z,sc ! E,sc barrier for chloromethyl formate at the HF/4-31G* level.…”

“…Since formic acid esters possess a high energy barrier to internal rotation about the C(=O)-O bond, any conformational equilibrium between the Z and E conformers which may exist in the gas phase can readily be trapped in a low-temperature argon matrix. Furthermore, it has been demonstrated by previous investigations 5 that photorotamerization of formate esters from the Z to E conformers in a low-temperature argon matrix can be effected by UV irradiation. These results thus enable the application of a new and interesting methodology in the conformational analysis of formic acid esters.…”

“…Argon matrix spectra of dichloromethyl formate at different temperatures were also recorded and the sample annealed at 35 K. The results of these investigations also indicated that equal or near equal energy ap and sc conformers were not present, as has been reported previously for ethyl-6,7 and cyanomethyl formates. 5 Since either the Z,ap conformer, belonging to the C s point group, or the Z,sc conformer, belonging to the C 1 point group, represents the most stable conformation of dichloromethyl formate in all phases, it remains only to distinguish between these two.…”