Аннотация:Formamide (HCONH2), acetamide (CH3CONH2) and N-methylformamide
(HCONHCH3) are the first molecules in homological series of amides. Due to
this, they are the simplest objects to investigate conformational properties of
the peptide fragment. In order to discover conformational preferences of
molecules under consideration, one should focus on the so-called large
amplitude motions (LAM): torsion, inversion and in some particular cases
out-of-plane vibrations.
In the ground electronic state (S0) in the simplest amides RCONHR’ (R, R’ = H,
CH3) LAMs are as follows: internal rotation around CcarbN bond, rotations of
methyl tops in acetamide and N-methylformamide, and out-of-plane vibration
of CNHR’ fragment. In concordance with earlier data it was shown that internal
rotation around central CN bond is coupled with pyramide-like deformations
of CNHR’ group. In addition we calculate 2D potential energy surface (PES)
sections by LAM coordinates for molecules mentioned above (ab initio
method used is MP2/aug-cc-pVTZ).
In S1 (1(n,π*)) and Т1 (3(n,π*)) electronic states PES of amide molecules were
computed by means of CASPT2/cc-pVTZ procedure. Electronic excitation
was shown to cause strong pyramide-like deformation of both HNCR’ and
RCON fragments as well as shift of mutual orientation owing to rotation around
CcarbN bond. Our calculation for 1,3(n,π*) states of formamide and
acetamide predicted three different minima to exist while for
N-methylformamide there are six of them.
For all amide conformers in different electronic states geometry parameters,
harmonic vibrational frequencies of conformers and barriers to conformational
transitions were computed. Based on potential energy surface (PES) sections,
LAM frequencies were calculated variationally and found to be close to
available experimental data.