Dynamical structure of peptide molecules: Fourier transform microwave spectroscopy and ab initio calculations of N-methylformamide

Rotational spectra of both trans and cis forms of the N-methylformamide normal as well as deuterated (HCONDCH(3), referred to as N-D) species were observed by Fourier transform microwave spectroscopy in the frequency region from 5 to 118 GHz. Samples were prepared in the form of a beam by a pulsed Jet valve maintained at 50 degrees C and were introduced in a high-vacuum cavity cell, with either Ne or Ar as a carrier gas at a backing pressure of 100 kPa. The observed spectra were analyzed to yield molecular parameters including rotational constants and barrier, V(3), to CH(3) internal-rotation: 53.9 (6) and 301 (4) cm(-1) for the trans and cis forms of the normal species, respectively, and 41.9 (6) and 309 (4) cm(-1) for the trans and cis forms of the N-D species, respectively. Spectra of four trans isotopologues with (13)C, (15)N, or (18)O singly-substituted in the internal-rotation A state were observed and analyzed to derive the r(s) structure of the trans form. For comparison with the experimental data, ab initio calculations were carried out at MP2/6-31G** level to derive molecular structure, potential barrier to CH(3) internal rotation, and the energy difference between the cis and trans forms. An extensive coupling was found between the CH(3) internal rotation and N-H out-of-plane bending, suggesting that the potential function for the CH(3) internal-rotation deviates considerably from a simple cos(3 alpha) form. The effects of the V(6) term is briefly discussed. (C) 2010 Elsevier Inc. All rights reserved.