Modified MINDO/3 13C chemical shift calculations for simple hydrocarbons

✍ Scribed by D. B. Chesnut; C. Zhang

• Publisher: John Wiley and Sons
• Year: 1988
• Tongue: English
• Weight: 690 KB
• Volume: 9
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/jcc.540090417

✦ Synopsis

Calculations of 13C chemical shifts in some simple hydrocarbons have been carried out using the GIAO approach in the MINDOW semiempirical formalism. In order to achieve reasonable agreement with experiment it is necessary to modify (increase) the vacant orbital energies in the MIND0/3 calculation in order to reduce the magnitude of the paramagnetic contribution, and to also modify this dominant term by generally reducing it as a function of the number of hydrogen and carbon atoms bonded to the resonant nucleus in question. For a set of 34 resonant nuclei of the simpler hydrocarbons, agreement with experiment of the order of 7.8 ppm is attained; however, pathological cases such as cyclopropane and some simple allenes continue to cause problems, increasing the standard deviation of the full set to 12.5 ppm.

Our results indicate that the MINDO/3 approach is as viable for 13C chemical shift calculations as other semiempirical approaches, all of which seem currently to be limited to a standard deviation of the order of 10 ppm.