A MICROWAVE STUDY OF METHYL ETHYL FORMAMIDE: ROTATIONAL CONSTANTS, CONFORMATION, ${^{14}}N$ QUADRUPOLE COUPLING, AND METHYL GROUP INTERNAL ROTATION BARRIER

Abstract

Microwave transitions of methyl ethyl formamide have been observed which display resolved ${^{14}}N$ quadrupole splittings and internal rotation splittings from the N-methyl group. The quadrupole splittings were fit (88 components from 18 transitions) to a standard deviation of 6 kHz by the quadrupole coupling constant values: $\chi_{aa} = 1.665(4) MHz, \chi_{bb} = 2.132(5) MHz$ and $X_{cc} = -3.797 MHz$. The hypothetical unsplit line centers (11 A-state and 7 E-state a-type R-branch transitions) were fit using a hamiltonian including rotational constants and a 3-fold barrier to internal rotation for the N-methyl group to a standard deviation of 47 kHz by the values: $A_{r} = 6696.6(10) MHz, B_{r} = 2326.212(7) MHz. C_{r} = 1932.123(5) MHz$, and $V_{3} = 245.2(4) cm^{-1}$. The conformation of the molecule is that shown with all the heavy atoms coplanar except the C atom of the terminal methyl of the ethyl group. NMR spectra reveal the presence of another conformer corresponding to the O on the ethyl side. Rotational transitions from this latter conformer have not yet been identified. The structure and properties of the assigned conformer will be compared to those of its isoelectronic carcinogenic analogue, methyl ethyl nitrosamine.