jp9810825_si_001.pdf (204.4 kB)
High-Level ab Initio Molecular Orbital Calculations of Imine Formation
journal contribution
posted on 1998-05-29, 00:00 authored by Nathan E. Hall, Brian J. SmithAb initio molecular orbital calculations at the
G2(MP2,SVP) level are reported for the reaction
between
methylamine and formaldehyde to form the Schiff-base,
N-methylmethanimine, with loss of water. The
gas-phase barrier to carbinolamine formation through intramolecular proton
transfer is found to be dramatically
reduced when explicit water molecules are employed to facilitate the
proton transfer. The lowest barrier lies
14.7 kJ mol-1 above the complex between
methylamine, formaldehyde, and two water molecules and
passes
through a zwitterionic intermediate. The structural features of
the transition state for this process closely
resemble those predicted experimentally. Although intramolecular
proton transfer through water molecules
to form the imine from the carbinolamine requires significantly less
energy than without the water, the barrier
remains quite large. With two waters the barrier is calculated to
be 111.9 kJ mol-1. A considerably
smaller
barrier is predicted for the protonated carbinolamine, 67.9 kJ
mol-1, and while there is no experimental
evidence
for internal proton transfer, the calculated structure shows
characteristics of the experimentally predicted
transition state. Calculated aqueous-phase dissociation energies
are too small in comparison with the
experimentally observed equilibrium constants.