72 Computational methods for carbenes

Christopher Cramer (cramer@chemsun.chem.umn.edu)
Mon, 12 Jun 1995 19:07:03 -0500 (CDT)

ECTOC participants,

It is mildly daunting to imagine that ANY comment/question can encompass
sufficiently large a span of topics to interest 50+% of the diverse
participants in this conference. In the interest of seeing the hypermail
listing increase, however, I'll indulge, and hope to stimulate those more
circumspect than I to behave similarly . . .

Paper 72 http://neon.che.ilstu.edu/standard/toc.html examines
computationally the interaction of singlet carbenes with several solvents and
considers different modes of complexation (ylides, H-bonds, etc.)

Singlet carbenes and isoelectronic species are typically rather difficult
to accurately treat as they have a very low lying doubly excited state
(the so-called pi^2 state, as opposed to the sigma^2). Accurately handling
this state requires a MCSCF or GVB calculation. I wonder if the authors would
comment on this point. Although I can imagine that the RHF and MP2
calculations presented might do reasonably well on the relative binding
energies of the various ylides (where the doubly excited state probably lies
considerably higher), the absolute binding energies would be expected to be
too negative because the reference methylene is artificially high in energy.

In that same vein, the hydrogen bonded complexes might have reasonable
absolute binding energies, because the non-dynamic correlation will be
very roughly the same for the complex as the reactants, but the comparison to
the ylide binding energies may be less trustworthy.

Finally, the monomer solvents are bulk liquids characterized by moderate
to high dielectric constants. The ylides might be expected to gain
significant stabilization from solvation. Increasingly available continuum
solvent models might provide some interesting data!



Christopher J. Cramer University of Minnesota Department of Chemistry 207 Pleasant St. SE Minneapolis, MN 55455-0431 (612) 624-0859 cramer@maroon.tc.umn.edu http://dionysus.chem.umn.edu/

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