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Method for the evaluation of normal modes and molecular mechanics with reduced sets of force constants. 1. Principles and reliability test
Author(s) -
Unger Esko,
Lipski Robert J.,
Dreybrodt Wolfgang,
SchweitzerStenner Reinhard
Publication year - 1999
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/(sici)1097-4555(199901)30:1<3::aid-jrs340>3.0.co;2-3
Subject(s) - chemistry , force field (fiction) , molecule , normal mode , ab initio , computational chemistry , molecular physics , atom (system on chip) , molecular vibration , eigenvalues and eigenvectors , local symmetry , symmetry (geometry) , quantum mechanics , vibration , physics , mathematics , geometry , organic chemistry , computer science , embedded system
Abstract A novel method was developed for molecular mechanics calculationsand normal mode analysis. In this approach, the number of freeparameters is strongly reduced compared with other empirical forcefields. and in contrast to them is generally smaller than the numberof available wavenumber values. The molecule is subdivided into localunits, each of which is constituted by a distinct atom and itsnearest neighbors. The vibrational force field is then expressed asthe sum over the contributions from all local units, and each localunit's potential function is assumed to depend solely on theatomic positions within the unit. Local units often exhibit highsymmetry, because each atom forms bonds which are characteristic ofits valencies and hybridization state, and the bonds are thereforearranged in a symmetrical way. This local (pseudo)symmetryimposes group theoretical restrictions that reduce the number ofpossible interaction parameters. As suggested by ab initio results, the internal force constants of each local unit aretransferable to other molecules. It is therefore possible tocalculate the internal force constants of each local unit from smallmolecules and these are then used to calculate the potential of largemolecules such as porphyrins. A series of alkanes, ethene, somehomo‐ and heterocyclic aromatic compounds and porphyrins wereanalyzed. The results for the normal mode wavenumbers and theireigenvectors are comparable to those reported in the literature andto results from DFT calculations[B3‐LYP/6–31G(d)]. The forceconstants were close to those obtained from ab initio calculations using local symmetry coordinates for ethene, ethane andpropane. Moreover, the above procedure reproduces very well thevibrational wavenumbers and mode compositions of aromatic compoundsand porphyrins, as shown by comparison with DFT calculations. Incontrast to general valence force field calculations, the number offree parameters is reduced by 40–80%. Copyright ©1999 John Wiley & Sons, Ltd.