Microwave spectrum of formamide--water and formamide--methanol complexes

The microwave spectra of the formamide--water and formamide--methanol complexes have been investigated with a pulsed beam Fabry--Perot cavity Fourier transform microwave spectrometer. The observed hyperfine structure due to the 14N nuclear quadrupole interaction was used to assign the rotational transitions for both species. For formamide--water the rotational analysis of ten transitions provides the constants: A = 11 227.931(1) MHz, B = 4586.9628(10) MHz, C = 3258.8278(7) MHz, eQq/sub a//sub a/ = 1.332(3) MHz, and eQq/sub b//sub b/ = 2.037(3) MHz. The formamide--methanol spectrum exhibits an additional splitting from internal rotation of the methyl group. Eighteen observed transitions from the A and E symmetry states have been assigned and fitted with the rotational constants: A = 10 186.594(6) MHz, B = 2090.36(59) MHz, and C = 1762.80(56) MHz with hyperfine constants close to those of formamide--water. By assuming a methyl top moment of inertia I/sub α/ = 3.206 uA2, the barrier to internal rotation V3 = 231.01(17) cm-1 is obtained. This barrier height is about 36% smaller than that of methanol. The structures determined for these complexes agree well with prior ab initio calculations which indicate essentially planar, double hydrogen bonded structures for both species