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

Abstract

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 °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 13C, 15N, or 18O 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(3α) form. The effects of the V 6 term is briefly discussed.