MOLECULAR-STRUCTURE OF N-METHYLPYRROLE AS DETERMINED BY GAS ELECTRON-DIFFRACTION AND AB-INITIO CALCULATIONS

The molecular structure of N-methylpyrrole has been investigated by gas electron diffraction and ab initio calculations at the MP2/6-31G**//HF/6-31G* level. The rotational constants, B0 and C0, and the potential energy function for internal rotation of the methyl group determined by microwave spectroscopy (W. Arnold, H. Dreizler and H.D. Rudolph, Z. Naturforsch., Teil A, 23 (1968) 301) were used in the structural analysis. Referring to the results of ab initio calculations, it has been assumed that a C H bond in the methyl group is perpendicular to the pyrrole ring in the equilibrium state and that the molecule takes a planar geometry except for methyl hydrogen atoms. Observed structural parameters with the estimated limits of error (3sigma) in parentheses are: r(g)(N-C(ring)) = 1.372(3) angstrom, r(g)(N-C(methyl)) = 1.452(3) angstrom, r(g)(C=C) = 1.386(4) angstrom, [r(g)(C-H)] = 1.102(5) angstrom, angle(alpha)CNC = 109.2(4)-degrees, angle(alpha)NCC = 108.5(5)-degrees, angle(alpha)NCH(ring) = 120.7-degrees, angle(alpha)C=CH = 126.1-degrees and angle(alpha)NCH(methyl) = 108.0(19)-degrees, where [] denotes average values. The difference between r(g)(N-C(ring)) and r(g)(N-C(methyl)), and the values of the NCH(ring) and C=CH bond angles were taken from the 6-31G* calculation.