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Ab Initio Molecular Orbital Study on a Linear Relationship between Activation Energies of Menshutkin Reactions and Proton Affinities of Nitrogen Bases

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journal contribution
posted on 11.06.1998, 00:00 by Kenzi Hori, J.-L. M. Abboud, C. Lim, M. Fujio, Y. Tsuno
In a previous paper (J. Am. Chem. Soc. 1991, 113, 4738), we reported that there was a linear relationship between the activation free energies ΔG of the Menshutkin reaction of CH3I and the proton affinities for the corresponding nitrogen donor bases (δPAobs, relative to PAobs of ammonia). The ΔG is dependent on the hybridization of the nitrogen, which means that a plot, δPAobs vs ΔG for the N(sp2) donors, yielded a straight line that was different from a corresponding line for the N(sp3) donors. Although N,N-dimethylaniline and its 4-substituted derivatives have formally sp3 nitrogen donors, their δPAobs − ΔG points were observed to be neither on the sp3 nor on the sp2 line. In the present study, ab initio molecular orbital (MO) calculations are adopted to explain why there are different linear relationships between the two energies of the three types of bases. Our calculations show that at the MP2/6-31+G*//RHF/6-31+G* level of theory the calculated δPAcalc values have a linear correlation with the δPAobs. Differences of calculated activation energies (δEa, relative to that of NH3) also show a good linear relationship with the δPAcalc. The C−X distances (X = N or Cl) at the TS structures are also closely correlated with the δEa, i.e., the Menshutkin reaction with a shorter C−N distance in the TS has to overcome a higher activation energy. The δPAcalc values should be reduced when we use the δPAs of the sp2 bases as indices of nucleophilicity in Menshutkin reactions in the gas phase.