72: reply to C. Cramer

Jean M. Standard (standard@krypton.che.ilstu.edu)
Tue, 13 Jun 95 17:44:08 -0500

In response to Christopher Cramer's comments on Poster 72, we would
like to first point out that the computational results presented in
the poster are preliminary. We are certainly planning to investigate
these carbene complexes at higher levels of theory.

The comment about an MCSCF or GVB type approach being necessary to
adequately treat these carbene systems is a point well taken. We
believe, however, that the preliminary results given in the poster for
the binding energies between the singlet carbenes and the oxygen-
containing compounds are likely to be qualitatively correct. The
reason is that as far as ylides go, these systems are relatively weakly
bound. As discussed with respect to Table 2 in our poster, the electron
transfer from the oxygen-containing compounds to methylene in these
systems is about 0.1 at the HF/6-31G* level, while for N-, P, and
S-containing ylides, the charge transfer is 0.4 - 0.5 at a similar level
of theory (HF/DZP) [R. A. Eades, P. A. Gassman, and D. A. Dixon, J. Am.
Chem. Soc. 103, 1066 (1981)]. At the MP2/6-31G* level of theory, the
binding energies for the ylides CH2NH3, CH2PH3, and CH2SH2 range from
41 - 70 kcal/mol [B. F. Yates, W. J. Bouma, and L. Radom, J. Am. Chem.
Soc. 109, 2250 (1987)], 2 to 4 times the binding energy for the oxonium
ylide CH2-H2O at the same level of theory (19 kcal/mol). What this
suggests is that the water has a much smaller effect on the carbene
than an N-, P-, or S- containing compound. So an oxonium ylide is more
"carbene-like" than the others. Although in our calculations methylene
may be too high in energy due to the neglect of the low-lying doubly
excited state, the oxonium ylide will also be too high in energy for the
same reason. Any overestimate of the binding energy will be much less
for oxonium ylide than for the N-, P-, or S-containing ylides (since the
error in neglecting the doubly excited state would be similar in
methylene and the oxonium ylide). We don't have any quantitative
results comparing MCSCF or GVB calculations to RHF or MP2 level
calculations for oxonium ylides (if anyone knows of any, we would be
interested in the references).

The suggestion to carry out calculations on the oxonium ylides and
other complexes using a continuum solvent model is one that we have
considered. We are planning to work on such calculations for the
oxonium ylide complexes as well as in studies of additions of the
carbenes and the ylides to double bonds in the presence and absence
of solvent.

Finally, in reference 2 for Part 2 of the poster we (DeLuca and
co-workers) describe the directing effect of a substrate methoxy
substituent on the addition of 1:CH2 to a double bond and discuss the
effect in terms of a reversibly formed ylide/complex. We found that
ether (vs pentane) as solvent quenches the directing effect and believe
that this may be due to, as you suggest, an increase in ylide binding
energy with increased dielectric constant of the solvent.

Jean M. Standard (standard@krypton.che.ilstu.edu)
JoAnn P. DeLuca (deluca@xenon.che.ilstu.edu)

Jean M. Standard standard@krypton.che.ilstu.edu
Department of Chemistry
4160 Illinois State University Phone: (309) 438-7700
Normal, IL 61790-4160 FAX: (309) 438-5538

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