Semiempirical molecular-orbital calculations of dissociation energies of small molecules containing light elements

Abstract

A semiempirical molecular-orbital approach has been developed to calculate the dissociation energies of 57 second-row neutral and charged diatomic molecules. One-electron energies and wave functions were used. The 2s and 2p electrons have separate energies and separate values of Z_2S and Z_2P. Calculations were made with a CNDO/INDO FORTRAN programme modified to a level of approximation that is more similar to extended Hückel calculations than to either CNDO or INDO. The correlation between calculated and experimental dissociation energies was favourable with a relative standard deviation of 15%. Additionally, dipole moments for the molecules were calculated concurrently with the same parameters. The correlation between calculated and experimental dipole moments was favourable for seventeen molecules with a relative standard deviation of 19%. The correlation between calculated and experimental ionisation potentials for fourteen molecules was also favourable with a relative standard deviation of 11%. Also, a repulsion work function was formulated and used in this work between pairs of atoms. For 90% of the molecules studied, the maximum calculated dissociation energy occurred within ±0.1 Å of the experimental bond distance. The semiempirical methods used in the present work could be used for future studies of larger molecules, including organic molecules.

GRAPHICAL ABSTRACT

KEYWORDS:

• Diatomic
• molecule
• dissociation
• energy
• semiempirical

Acknowledgements

This work was a collaboration between the authors. RWS is a retired PhD chemist. SR, also a PhD chemist, is an associate professor in the Chemistry Department of the University of Tennessee in Knoxville, Tennessee 37996, USA.

Authors’ contributions

RWS performed the programming and the computer calculations. RWS wrote the manuscript. SR guided the research and edited the manuscript. Both authors read and approved the final manuscript.

Computer programs

Fortran 77 source code can be freely downloaded from the website, <http://github.com/smithwickrw/fortran-software>.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplemental information

Supplemental information has been included with this article.