Ion chemistry of cyanides and isocyanides. 1. The carbon lone pair as proton acceptor: proton affinities of isocyanides. Alkyl cation nitrogen, oxygen, and carbon lone pair donors

The proton affinities (PAs) of the isocyanides RNC, R = CH/sub 3/, C/sub 2/H/sub 5/, i-C/sub 3/H/sub 7/, t-C/sub 4/H/sub 9/, and C/sub 6/H/sub 5/, range from 199 to 207 kcal/mol. The PAs of all the isocyanides are higher than those of the corresponding cyanides, RCN, by a constant 11.5 +/- 1 kcal/mol, regardless of the identity of R. The isomerization energies of the RNCH/sup +/ ions to RCNH/sup +/ are 5-11 kcal/mol and are smaller by 11-15 kcal/mol than the isomerization energies of the respective neutral cyanides. The bond dissociation energies D/sup 0/ for R/sup +/-NCH, R/sup +/-CNH, R/sup +/-OH/sub 2/, and R/sup +/-NH/sub 3/ show unexpectedly good linear correlations with the stabilities of the R/sup +/ ions as measured by the hydride affinity, i.e., D/sup 0/ (R/sup +/-H/sup -/). Ab initio calculations are used to analyze the trends in proton affinities. The results show that the main structural effects of protonation on RNC are the shortening of the N-C and the lengthening of the R-N bonds. The calculations also suggest that the larger PAs of RNC vs. RCN arise primarily from larger charge transfer and electrostatic interactions in the isocyanides. However, the parallel increase of the PAs ofmore » both RCN and RNC as R gets bigger is due primarily to the increasing polarizabilities of the substituents.« less