Molecular mechanics force field for tertiary carbocations

A new force field for tertiary carbocations has been developed, based on the Delft molecular mechanics (DMM) hydrocarbon force field (A. C. T. van Duin, J. M. A. Baas and B. van de Graaf, J. Chem. Soc., Faraday Trans., 1994, 90, 2881). As in this force field, the Mortier method (W. J. Mortier, S. K. Ghosh and S. Shankar, J. Am. Chem. Soc., 1986, 108, 4315) is used to calculate geometry-dependent charges. The force field was optimized using enthalpies of formation, solvolysis-rate data and ab initio vibration and geometry data. The ab initio vibration data were rescaled using experimental spectra from the isoelectronic alkylboranes. To check the influence of electron delocalization on the geometry of tertiary carbocations, the results of the force field were compared with crystal structures. A correlation coefficient between the calculated and the experimental solvolysis rates for alkyl bromides of 0.9881 was obtained, showing that the force field gives a good description of carbocation energies.