ISSN:
0020-7608
Keywords:
Computational Chemistry and Molecular Modeling
;
Atomic, Molecular and Optical Physics
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The vibronic interaction in an (A)2N chain (e.g., polyacetylene) has been studied within the Hückel framework. A Hückel framework scheme for calculating the parameters of molecular dynamic structure, i.e., the linear and quadratic orbital vibronic constants (OVCs), has been presented. Selection rules for the OVCs in this scheme have been obtained and discussed by using graph theory and group theory, under various boundary conditions. A Hückel noncoupling rule has been concluded and discussed. The dimerization of polyacetylene has then been discussed based on the molecular dynamic structure. It has been shown that for a finite undoped (A)2N chain the occupied orbital energy εn at the unified configuration has a negative slope in direction of Q2n. As a result, there exists a net Hellmann-Feynman force leading to dimerization. When the chain length goes to infinity, the slopes and forces tend to zero. However, a significant negative curvature in potential surface may occur in the direction of Q2n, due to the two-phonon coupling of the π-electrons, which could also induce the dimerization. These can be interpreted as the results of the hidden C4N+2 symmetry and the imaginary degeneracy in an (A)2N chain, according to the graph theory for molecular orbitals. Thus, the dimerization of an (A)2N chain actually destroys its hidden symmetry of C4N+2 and reduces its imaginary degeneracy.
Additional Material:
5 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/qua.560410310
