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 CH3CH2CN and the isotopes CH2DCH2CN, CH3CD2CN was carried out. The ground vibrational state of CH3CH2CN was reexamined under high resolution to give three centrifugal distortion constants DJ , DJK and DK. From the rotational constants of the ground vibrational state of CH3CH2CN, CH3CD2CN, CH2DCH2CN (symmetric) and CH2DCH2CN (asymmetric), CH3CH2 13CN and CD3CHDCN a r0-structure is derived. For the isotopes CH3CD2CN, CH2DCH2CN (symmetric) and CH2DCH2CN (asymmetric) the diagonal elements Χaa , Χbb and Χcc of the quadrupole tensor with respect to the principal axes of inertia were deduced from the hyperfine structure of the rotational lines. The off-diagonal element Χab for CH3CD2CN and CH2DCH2CN (symmetric) and the principal elements Χzz , Χxx of the quadrupole coupling tensor were obtained from Χaa, Χbb 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 internal degrees of freedom is considered. From the rotational line splittings in both states the coefficients V3 and V6 of the Fourier expansion of the potential hindering the internal rotation of the methyl group are determined.