Analysis of internal rotations in ethyl nitrite using molecular beam Fourier transform microwave spectroscopy

Abstract

The microwave spectrum of ethyl nitrite (C₂H₅ONO) shows the presence of three rotational isomers, corresponding to cis–trans, cis–gauche, and trans–gauche structures. We have performed an investigation of the internal rotation of the methyl group in the cis–trans and cis–gauche rotamers and the motion of the ethyl group in the trans–gauche structure by using molecular beam-Fourier transform microwave (MB-FTMW) spectroscopy. Rotational spectra of these three isomers were recorded in the frequency regions 1–4 GHz and 7–26 GHz with two different spectrometers. For cis–trans ethyl nitrite we observed µ_a- and µ_b-type and for the cis–gauche form we detected µ_a-, µ_b-, and µ_c-type spectra. Due to the internal rotation of the methyl group, some lines were split and the torsional barriers could be determined, V₃(CH₃)=1082(2) cm^-1 for the cis–trans and V₃(CH₃)=918(10) cm^-1 for the cis–gauche form. For the trans–gauche form we found µ_a-, µ_b-, and µ_c-type spectra. The µ_b- and µ_c-type lines showed a doubling due to the interconversion between the two equivalent gauche conformers through rotation about the C–O axis. We determined the difference between the two lowest energy levels of the torsion around the C–O bond axis to be 68(3) kHz. Based on the observed splittings and the rotational constants, the Fourier coefficients of the potential hindering this internal rotation were determined to be V₁=-1322 cm^-1, V₂=-1048 cm^-1, V₃=-412 cm^-1, and V₄=-109 cm^-1 with a low barrier of 183(3) cm^-1 in the trans position and a higher barrier of 1916(100) cm^-1 in the cis position. Rotational, centrifugal distortion, and quadrupole coupling constants and some structural parameters have also been obtained from the analysis of spectra.