Interpretation of the microwave spectrum of 2-methoxy ethylamine using its ab initio structures

Abstract

The 7 3 ← 6 3 μ a K −1 doublet line series of 2-methoxy ethylamine, previously recorded with the radiofrequency microwave double resonance technique, was interpreted for the T and G conformations of this compound with a number of structural models including the 4-21G optimized geometries. A single model of G can reproduce the doublet series as the vibrational progression of the CO skeletal mode, starting with the υ CO = 0 groundstate at 35 457 MHz, and stepping in approximately 95 MHz intervals up to the vibrational satellite of υ CO = 9, with the υ CO = 5 doublet missing. When model calculations are performed for T with the structural parameters of G which reproduce the 7 3 ← 6 3 K −1 doublet series, except for rotating the NH 2 group by 120° to obtain T, the frequency of its groundstate doublet is found at lower frequencies than that of G. In contrast to this, when the empirically corrected 4-21G parameters of T are used in the analysis, the calculated groundstate of T coincides with the expected υ CO = 6 doublet of G. The 4-21G geometries of G and T are in good agreement with the rotational constants and the conformational energy difference derived from the microwave spectra. Thus, this analysis clearly confirms the assignment of the observed 7 3 ← 6 3 K −1 series as the progressions of the υ CO = 0 to υ CO = 4 states of G, and the υ CO = 0 to υ CO = 3 states of T.