Subject: Thermochemistry of radicals, a summary.

From: "Sergio Emanuel Galembeck" <[email-masked]>
Date: Tue, 9 Mar 1999 15:48:54 -0300

Dear Netters,

    Some weeks ago I sent the following question to CCL:

    What is the best method to calculate energies 
of reactions involving radicals? Is compound 
methods like CBS-Q as good choice?

Thank you very much to Steen Hammerum, Paul Soper,
Karl K. Irikura, Denes Szieberth and Theis Soelling.

Their answers:

===================================================

>From Steen Hammerum:

Not an easy question to answer, inasmuch as there are not that many
good numbers to calibrate against.

For simple alkyl radicals, CBS-Q is not particularly successful (it gives
results for the heats of formation that are up to 10 kJ/mol different from
the literature values).  G3 on the other hand provides very good
correspondence with the currently recommended experimental heats of
formation (4 kJ off for methyl, very good for ethyl, propyl, isopropyl,
butyl, secbutyl, isobutyl).

Radom recently described a composite method, CBS-RAD, that is supposed to
be better for radicals, but it is quite demanding in terms of computer
resources.  Petersson's CBS-B3 (already incorporated in Gaussian 98) may
also be well suited for radicals.

 ===================================================

>From Paul Soper:

    I would recommend using isodesmic reactions with heats of
formation for simple radicals taken from the JANAF tables.

 ===================================================

>From Karl K. Irikura:

It depends upon the accuracy you require.  Radom has published a
variant called "CBS-rad", or something like that, designed
specifically for radicals.  Computing thermochemistry for radicals is
the same as for closed-shell molecules, except that spin
contamination is more common.  If you only need accuracy to about 25
kJ/mol, B3LYP is probably good and is far less expensive than CBS.

 ===================================================

>From Denes Szieberth:

You can find a comparison of different model chemistries in:

Ochterski, Petersson, Wiberg : J. Am. Chem. Soc. 1995, 117, 11299-11308

or you can use some not so computationally demanding approaches described
in e.g.:

Sana, Nguyen:  CPL 1992 196(3,4) 390

===================================================

>From Theis Soelling:

Recently, Radom and coworkers recommeded a modified CBS procedure which is
supposed to give excellent DfH of radicals. In addition, I find that the
new G3-method by Curtiss et al. yields good agreement with experiment for
e.g. CH3-dot, allyl-dot.

References:

1) P.M. Mayer, C.J. Parkinson, D.M. Smith, L. Radom, J. Chem. Phys. 108
(1998) 604.

2) L.A. Curtiss, K. Raghavachari, P.C. Redfern, V. Rassolov, J.A. Pople, J.
Chem. Phys. 109 (1998) 7764.

===================================================

        Best wishes,

                    Sergio

==============================================
Sergio Emanuel Galembeck
Assistant Professor in Physical Chemistry
Laboratorio de Modelagem Molecular
Departamento de Quimica
Faculdade de Filosofia, Ciencias e Letras de Ribeirao Preto
Universidade de Sao Paulo

fax: +55-16-633-81-51
e-mail: [email-masked]

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