Conformational analysis of N-methylformamide in ground So and excited S1 and T1 electronic states

For conformers of the N-methylformamide (HCONHCH3) molecule, calculations of equilibrium geometry parameters, harmonic vibration frequencies, energy differences and potential barriers to conformational transitions were performed in the ground (So) and lowest excited singlet (S-1) and triplet (T-1) electronic states. In the S-0 state, the molecule exists in trans and cis stable conformations (having C-s symmetry). Our calculations show that the electronic excitations T-1 <- S-0 and S-1 <- S-0 cause changes in the structure of conformers: both HCON and HNCC fragments become pyramidal and rotate around the CN bond. As a result, in each excited electronic state under consideration, there are 12 minima forming six pairs of equivalent conformers separated by relatively small potential barriers. One- and two-dimensional potential energy surface sections corresponding to different intramolecular large-amplitude motions were calculated using the MP2/aug-cc-pVTZ (S-0) and CASPT2/cc-pVTZ (S-1 and T-1) methods. Anharmonic vibrational problems for large amplitude motions were solved, and the corresponding frequencies were estimated. (C) 2016 Elsevier Ltd. All rights reserved.