Microwave Spectrum of Ethyl Cyanide; r0-Structure, Nitrogen Quadrupole Coupling Constants and Rotation-Torsion-Vibration Interaction

Abstract

An investigation of the microwave spectra of ethyl cyanide CH 3CH 2CN and the isotopes C H 2D C H 2CN, CH 3CD2CN was carried out. The ground vibrational state of CH 3C H 2CN was reexamined under high resolution to give three centrifugal distortion constants D j, D j k and D k. From the rotational constants of the ground vibrational state of CH 3CH 2CN, CH3CD2CN,C H 2D C H 2CN (symmetric) and CH 2D C H 2CN (asymmetric). C H 3C H 2,3CN and CD 3CHDCN a r0-structure is derived. For the isotopes CH 3CD2CN, CH 2D C H 2CN (symmetric) and CH 2D CH 2CN (asymmetric) the diagonal elements yaa, ybb and yCc of the quadrupole tensor with respect to the principal axes of inertia were deduced from the hyperfine structure of the rotational lines. The offdiagonal element yab for CH3CD2CN and C H 2D C H 2CN (symmetric) and the principal elements of the quadrupole coupling tensor were obtained from yaa, ybb of the two molecules and from the principal axis rotation angle about the c-axis produced by isotopic substitution. For the analysis of the rotational spectra in the first excited states of methyl torsion and the lowest frequency in plane bending vibration of normal ethyl cyanide a molecular model with two internaldegrees of freedom is considered. From the rotational line splittings in both states the coefficients Vo and V6 of the Fourier expansion of the potential hindering the internal rotation of the methvl group are determined. © 1974, Walter de Gruyter. All rights reserved.