jo971707f_si_001.pdf (1.32 MB)

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

journal contribution

posted on 11.06.1998, 00:00 by Kenzi Hori, J.-L. M. Abboud, C. Lim, M. Fujio, Y. TsunoIn a previous paper (

*J.**Am.**Chem.**Soc.***1991**,*113*, 4738), we reported that there was a linear relationship between the activation free energies Δ*G*^{⧧}of the Menshutkin reaction of CH_{3}I and the proton affinities for the corresponding nitrogen donor bases (δPA_{obs}, relative to PA_{obs}of ammonia). The Δ*G*^{⧧}is dependent on the hybridization of the nitrogen, which means that a plot, δPA_{obs}vs Δ*G*^{⧧}for the N(sp^{2}) donors, yielded a straight line that was different from a corresponding line for the N(sp^{3}) donors. Although*N*,*N*-dimethylaniline and its 4-substituted derivatives have formally sp^{3}nitrogen donors, their δPA_{obs}− Δ*G*^{⧧}points were observed to be neither on the sp^{3}nor on the sp^{2}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 δPA_{calc}values have a linear correlation with the δPA_{obs}. Differences of calculated activation energies (δ*E*_{a}, relative to that of NH_{3}) also show a good linear relationship with the δPA_{calc}. The C−X distances (X = N or Cl) at the TS structures are also closely correlated with the δ*E*_{a}, i.e., the Menshutkin reaction with a shorter C−N distance in the TS has to overcome a higher activation energy. The δPA_{calc}values should be reduced when we use the δPAs of the sp^{2}bases as indices of nucleophilicity in Menshutkin reactions in the gas phase.