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Rotation about the CN bond in 2‐aza‐1,3‐butadiene and the NN bond in 2,3‐diaza‐1,3 butadiene: A molecular orbital study
Author(s) -
Bock Charles W.,
George Philip,
Trachtman Mendel
Publication year - 1984
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.540050418
Subject(s) - chemistry , dihedral angle , crystallography , steric effects , bond length , molecular geometry , basis set , single bond , molecule , computational chemistry , stereochemistry , crystal structure , group (periodic table) , density functional theory , hydrogen bond , organic chemistry
The geometry and energy of 2‐aza‐1,3‐butadiene and 2,3‐diaza‐1,3‐butadiene have been calculated using the 6‐31G* basis set as a function of the CNCC and CNNC dihedral angles, respectively. With the 2‐aza derivative potential minima are located at 0° (trans) and at about 130° for a gauche structure approximately 9.5 kJ mol −1 less stable than the trans. Potential maxima are at about 75° giving a gauche barrier height of approximately 19 kJ mol −1 relative to the trans structure, and at 180° (cis) giving a barrier height of approximately 14.5 kJ mol −1 relative to the 130° gauche structure. With the 2,3‐diaza derivative the gauche barrier has disappeared and there are a series of gauche structures in the region 70°–100° of almost equal energy 12.5‐15 kJ mol −1 less stable than the trans. In addition the cis barrier is much greater, nearly 70 kJ mol −1 relative to the trans structure. Inclusion of electron correlation, accounting for about 50% of the correlation energy, produces no significant changes in the shape of the potential energy curves. There are systematic and progressive changes in almost all the geometrical parameters as the CH groups in butadiene are replaced by N. The outward tilt and compression within the methylene groups show adverse steric interactions to be operative in the cis structures. The values of V nn indicate that gauche structures of both the 2‐aza and the 2,3‐diaza derivatives near the cis structure are more compact (as with butadiene), and gauche structures of the 2‐aza derivative near the trans structure are less compact (as with butadiene). Originating in the changes in bond lengths and bond angles, rotation‐independent nuclear–nuclear interactions again play an important role.