Spectroscopic and theoretical studies of some 2-substituted N-methoxy-N-methyl-amides

• Published in: Journal of molecular structure Vol. 1031; no. 16; pp. 91 - 103
• Main Authors: Olivato, Paulo R., Gomes, Roberto da Silva, Rodrigues, Alessandro, Domingues, Nelson L.C., Rittner, Roberto, Colle, Maurizio Dal
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 16-01-2013
• Subjects: 2-Substituted N-methoxy-N-methyl amides; Carbon monoxide; Conformational analysis; Correlation; Derivatives; electrostatic interactions; gases; Infrared radiation; Infrared spectroscopy; Mathematical analysis; Mathematical models; Orbitals; Populations; spectroscopy; Theoretical calculations; Infrared spectroscopy; 2-Substituted N-methoxy-N-methyl amides; Theoretical calculations; Conformational analysis
• ISSN: 0022-2860; 1872-8014
• DOI: 10.1016/j.molstruc.2012.07.005

► Conformational analysis study of 2-substituted-N-methoxy-N-methyl-amides (Weinreb amides). ► IR study on the conformational equilibrium in solution. ► Fermi resonance in the carbonyl stretching fundamental region. ► Theoretical calculations and NBO analysis.
B3LYP/6-311++G(3df,3pd) calculations along with the NBO analysis of some 2-substituted N-methoxy-N-methyl-amides Y-C(R)2C(O)N(OMe)Me [Y=Br 1, SEt 2, SePh 4 (for R=H) and SEt 3 (for R=Me)] indicated the existence of three conformers for 1, 2, 4, namely one cis (c) and two gauche (g1, g2), and two gauche (g1, g2) forms for 3. The g1 conformer is the most stable and the least polar for all derivatives, while the c one is the most polar for 1, 2 and 4. As far as the stability is concerned, the g2 conformer is more abundant with respect to the c one for compounds 1 and 2, and slightly less stable than c for 4. For the derivative 3, the g1 conformer is more stable and slightly less polar than the g2 one. Moreover, the computed (νCO) frequencies and the relative populations of the two forms correlate quite well with the IR (νCO) doublet frequency components and their relative intensities in solution.
The occurrence of Fermi resonance in the fundamental νCO region, in solution, precludes the estimative of the relative populations of the c, g1, g2 conformers for 1, 2 and 4. NBO analysis showed that the nN→πCO∗ orbital interaction is the main factor which stabilizes the gauche (g1, g2) conformers into a larger extent relative to the cis (c) one for 1, 2, 4. For these compounds along with the 3 derivative, the same interaction stabilizes more the g1 conformer than the g2 one. The sum of the orbital interactions from NBO analysis and the trend of the electrostatic interactions justifies quite well the populations of the (c) and gauche (g1, g2) conformers for 1, 2 and 4, along with the (g1, g2) conformers populations for 3 found in the gas phase.