Rotational isomerism, structure and vibrational assignment of methoxyallene: joint IR and Raman investigation and non-empirical MO-SCF and valence force field calculations

Abstract

The rotational isomerism of methoxyallene has been studied both by ab initio calculations and vibrational analysis. The internal potential energy function, calculated at the 6-31G**//3-21G level, indicates the occurrence of two isomers. The more stable one has a planar s- cis structure; the other is predicted at a torsional angle of 140.0° ( skew form), with a relative energy of 2.68 kcal mol −1. The enantiomers corresponding to the latter are indicated in a double-minimum potential well with a central barrier height of only 0.34 kcal mol −1 (119 cm −1). From IR and Raman spectra evidence it was found that methoxyallene exists predominantly in the s- cis form; by measurements of the temperature dependence of the relative band intensities, the enthalpy difference between the skew and the s- cis isomers was found to be 1.96 ± 0.14 kcal mol −1. The vibrational assignment was based on ab initio derivation, at the 3-21G level, of the harmonic force constants. The calculated frequencies were scaled by factors obtained from analogous calculations for the strictly related molecules allene and methylvinylether. The accord between the normal frequencies predicted by this procedure and the observed ones was quite satisfactory, the differences ranging between 0 and 18 cm −1 for twenty-four out of the twenty-seven fundamentals. From the assigned torsional frequency, the derived barrier to internal rotation of the methyl group was 3.1 kcal mol −1 (1048 cm −1).