Analysis of the rotational spectrum of the ground and first torsional excited states of monodeuterated ethane, CH3CH2D

Abstract

The pure rotational spectrum of mono-deuterated ethane, CH3CH2D, has been measured up to 1600GHz and spectroscopic constants have been fit to 984 transitions in the ground state and 422 transitions in the first torsional excited state (ν18). Analyses of the ground state data were performed with the programs SPFIT, ERHAM and XIAM and of the first torsional state with SPFIT and ERHAM to extract molecular and spectroscopic constants. A combined fit of both states using ERHAM was used to determine ρ=0.4344026(68), which in the symmetric limit is the ratio Iα/Iz and a measure of the periodicity of the internal rotation energy with K and the energy differences between the A and E torsional substates, ΔE(E–A), of 74.167(18) and −3382.23(34)MHz for the ground and excited states, respectively. Using these energy differences and the overtone transitions Δv=2 from Raman measurements in the literature, the coefficients V3 and V6 of the potential function of the internal rotation in CH3CH2D were determined as V3=1004.56(4)cm−1 and V6=7.09(12)cm−1. This analysis lays the ground work for the assignment of the IR spectrum of CH3CH2D between (680–880cm−1) which will help quantify isotopic ratios by remote sensing missions.