A theoretical study of (old and new) non-classical carbocations derived from cyclic saturated hydrocarbons

Nine non-classical carbocations derived from cyclic saturated hydrocarbons were studied by means of ab initio calculation, MP2/6-311G**. The geometry, energetics, orbital interactions within the Natural Bond Orbital formalism, electron density and chemical shieldings were obtained and analyzed. In all cases a similar geometrical distribution ('non-classical moiety,' NCM) was obtained with the carbon atoms located in a triangle with two of their sides of 1.84 Angstrom and the third of 1.40 Angstrom. The energetic stability of the carbocations was rationalized on the basis of the strain in the rings involved. The NBO analysis shows that the charge is largely distributed over the three centers and also the existence of a two-electron, three-center orbital with similar contribution of the three atoms. Electron density maps were obtained and analyzed. In all cases, the chemical shieldings of the CH2 group of the NCM are in the region of saturated hydrocarbons whereas those for the CH group are similar to those found in double-bonded systems. Copyright (C) 2003 John Wiley Sons, Ltd.