Theoretical study of (CH...C)- hydrogen bonds in CH(4-n)X(n) (X = F, Cl; n = 0, 1, 2) systems complexed with their homoconjugate and heteroconjugate carbanions

This work deals with a theoretical study of the (CH...C)- hydrogen bonds in CH4, CH3X, and CH2X2 (X = F, Cl) complexed with their homoconjugate and heteroconjugate carbanions. The properties of the complexes are calculated with the B3LYP method using the 6-311++G(d,p) or 6-311++G(2df,2p) basis sets. The deprotonation enthalpies (DPE) of the CH bond or the proton affinities of the carbanions (PA(C-) are calculated as well. All the systems with the exception of the CH4...CHCl2(-) one are characterized by a double minimum potential. In some of the complexes, the (CH(b)...C)- hydrogen bond is linear. In other systems, such as CH3F...CH2F- and CH3F...CHF2(-), there is a large departure from linearity, the systems being stabilized by electrostatic interactions between the nonbonded H of the neutral molecule and the F atom of the carbanion. In the transition state, the (CH(b)...C)- bond is linear, and there is a large contraction of the intermolecular C...C distance. The binding energies vary within a large range, from -1.4 to -11.1 kcal mol(-1) for the stable complexes and -8.6 to -44.1 kcal mol(-1) for the metastable complexes. The energy barriers to proton transfer are between 5 and 20 kcal mol(-1) for the heteroconjugate systems and between 3.8 and 8.3 kcal mol(-1) for the homoconjugate systems. The binding energies of the linear complexes depend exponentially on 1.5DPE - PA(C-), showing that the proton donor is more important than the proton acceptor in determining hydrogen bond strength. The NBO analysis indicates an important electronic reorganization in the two partners. The elongations of the CH bond resulting from the interaction with the carbanion depend on the occupation of the sigma*(CH(b)) antibonding orbitals and on the hybridization of the C bonded to H(b). The frequency shifts of the nu(CH)(A1) stretching vibration range between 15 and 1150 cm(-1). They are linearly correlated to the elongation of the CH(b) bond.