INTRINSIC BARRIERS AND TRANSITION-STATE STRUCTURES IN THE GAS-PHASE CARBON-TO-CARBON IDENTITY PROTON TRANSFERS FROM NITROMETHANE TO NITROMETHIDE ANION AND FROM PROTONATED NITROMETHANE TO ACI-NITROMETHANE - AN AB-INITIO STUDY

The identity carbon-to-carbon proton transfers between nitromethane an d nitromethide anion and between oxygen protonated nitromethane and ac i-nitromethane have been studied by ab initio methods. Group charges c alculated by Mulliken and NPA methods as well as geometrical parameter s such as pyramidal angles and C-N bond lengths indicate that the tran sition states of these reactions are strongly imbalanced. Further evid ence for the imbalance comes from a consideration of the relative ener gies of the various corners representing hypothetical intermediates on More O'Ferrall-Jencks diagrams. Our results for the CH3NO2/CH2=NO2- s ystem, in conjunction with previous findings on other CH3Y/CH2=Y- syst ems, indicate an increase in the imbalance in the order CN much less t han CH=O less than or equal to CH=CH2 less than or equal to NO2 consis tent with the notion that imbalances increase with pi-acceptor strengt h of Y. However, when comparing the CH3N+O2H/CH2=NO2H system with the CH3NO2/CH2=NO2- system, the results are somewhat ambiguous as to wheth er the stronger pi-acceptor (N+O2H) leads to a stronger imbalance. In contrast to numerous observations in solution reactions, there is no s imple relationship between reaction barrier and imbalances in the gas phase, as becomes apparent when comparing our results with those for t he systems CH3CH=O/CH2=CHO-, CH3CH=O+H/CH2=CHOH, CH3CN/CH2CN-, and CH3 CH=CH2/CH2=CHCH2- reported in the literature. It is shown that the dep endence of the barriers on the pi-acceptor is the result of a complex interplay among resonance/ imbalance effects, inductive/field effects, and electrostatic/hydrogen bonding effects.