Аннотация:The structure and conformational dynamics of the acetamide (CH3CONH2) molecule in the ground and lowest excited n,π* electronic states were investigated by high level ab initio methods (MP2, CCSD(T), CASSCF, CASPT2, etc.).
The geometric structure of heavy atoms frame of molecules under investigation in the ground electronic state is near-planar, but the torsion motion of the amino group is accompanied by significant pyramidalization of CNH2 fragment. As a result in this molecule the torsion and NH2 out-of-plane vibrations are strongly coupled. The two-dimensional section of potential energy surface was calculated for correct description of this coupling. Acetamide molecule also has a very low (less than 30 cm–1) barrier to methyl group rotation. The barrier height depends on the values of other non-rigid coordinates (internal rotation around the CN bond and non-planar amine group distortion) are considered.
Geometric parameters, conformer energy differences, and one- and many-dimensional potential energy surfaces (PES) sections along the coordinate of large-amplitude motions of acetamide in the lowest n,π* excited electronic states were obtained. Our calculations demonstrate that the electronic excitations cause pyramidal distortions of both the carbonyl and amino fragments and changes of these groups orientation.
An analysis of the potential energy surface and the results for different one- and multidimensional vibrational problems shows that the coupling the internal rotation around the C-N bond and the non-planar vibrations of amides in these excited electronic states is far weaker than in the ground state.
The conjugation in molecules under investigation in different electronic states was compared based on topological analysis of electron density (Bader’s Quantum Theory of Atoms in Molecules, QTAIM) and population analysis of the natural bond orbitals theory (NBO).