Subject: Methods for transition states of radical cations (Summary)

From: Herbert Homeier t4720 <[email-masked]>
Date: Fri, 14 May 1999 23:32:49 +0200

Dear CCL members,

some time ago I posted the a question to CCL and received some answers
included below and some requests for a summary. 

I would like to thank all those who answered. I appreciate your efforts
highly.

Best regards

The original question:
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Some colleagues are interested in calculations of radical cations
containing about 10 C, some N and O and about 15 H with extended pi
systems (distonic ions). They have the following questions that I 
would like to forward to the experts of such calculations in practice:

A) What do you consider as state-of-the-art method(s) for calculating
the energy of transitions states of such systems with an accuracy of
about 5 kcal/mol?
(Is this accuracy really realistic to expect?)

B) Do you regard semiempirical methods as reliable for the calculation of
the energy barriers of such systems? If yes, which ones?

Also, pointers to literature on such issues would be appreciated.
-------------------------------------------------

Answer no. 1:
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From: Frank Jensen <[email-masked]>

	5 kcal/mol should be borderline possible with various DFT
methods, try e.g. B3LYP or B3PW91 (available in G98). System of this
size should be no problem with DFT, unless you have very limited
computing facilities.

	AM1/PM3 would probably have a hard time getting 5 kcal/mol
accuracy, but they may be useful as a starting point for getting
geometries roughly OK before turning on the DFT.

	Frank
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Answer no. 2: 
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From: Ernest Chamot <[email-masked]>

I don't have specific information about distonic ions, but (depending on
what reactions you are considering) based on the literature I've seen
(radical additions, abstraction reactions, electrocyclic reactions) I don't
believe you will get the 5 kcal/mol accuracy you want from semiempirical
methods.  PM3 can come close for some activation energies (as opposed to
absolute energies of transition states), but AM1 and MNDO will only be
accurate to within about 10-20 kcal.  See:

B. H. Bessler, W. L. Hase, & K. C. Haas, J. Phys. Chem., 96, 9639-76 (1992).
J. Baker, M. Muir, J. Andzelm, J. Chem. Phys., 102(5), 2063-79 (1995).
M. A. Ries & J. Rodriquez, J. Comp. Chem., 13(7), 860-6 (1992).

This is starting to be a big system for ab initio HF calculations, but
accuracy will be a problem for you here, too.  Even with MP2 correlation
you may only get accuracies within about 10 kcal/mol.  See:

Baker et. al.
A. Pross et. al., J. Phys. Org. Chem., 4, 135-40 (1991).
L. Fan & T. Ziegler, J. Amer. Chem. Soc., 114, 10890-7 (1992).

I think your best bet would be to use DFT, but you will need to use GGA
rather than Local Density Functionals to get within 2-6 kcal/mol.  See:

Baker et. al.
Fan & Ziegler.
Recent work by B. Jursac and by J. Durant (I think these are in press, but
they may have come out in the last year or so.)

I hope this helps.

EC
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Ernest Chamot
Chamot Laboratories, Inc.
530 E. Hillside Rd.
Naperville, IL 60540
(630) 637-1559
[email-masked]
http://www.chamotlabs.com/cl

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