Subject: Re: CCL:G:CALCULATING A RATE COSTANT

From: Qiang Cui <[email-masked]>
Date: Tue, 15 Dec 1998 16:54:33 -0500 (EST)

On Tue, 15 Dec 1998, E. Lewars wrote:

> Tued 1998 Dec 15
> 
> Subject: Calculating rate constant
> 
> Hello,
> Has anyone any suggestions for a fairly easy way to calculate a rate constant
> using Gaussian? I want to avoid the intricacies of RRKM theory and just get
> a k_rate accurate to within a factor of, say, 10. For example, consider
> 
>                       HNC   --->   HCN
> 
> At some temp T  HNC in, say, the gas phase (or nonpolar solvent?) has a
> certain halflife (=ln2/k_r). It's easy to calculate a good number for the
> activation energy, but what's a simple way to estimate a realistic k_r (and
> thus t_1/2)?

	First of all, you will need a reasonably accurate barrier height
and reaction coordinate. I guess u will need to push your method a little
bit to get it right for the particular HNC/HCN system, 'cause you are
breaking the triple bond and electron correlation effect should be
significant. But there should be enough data in the literature, 'cause
it's one of the favorable system for people doing quantum dynamics. Read
some paper from Prof. J. Bowman, who did CCSD(T) level PES and some
complex L2 etc. as well. 

	As to the isomeration rate constant, I guess a decent approach is
VTST with semi-classical tunneling, 'cause tunneling should be significant
for your system. You can also try the approximate instaton approach, which
appear to be rather accurate as well for tunneling splittings and rate
constant etc. Any way, POLYRATE and DOIT are the program packages for the
above two approaches, respectively. Finally, as mentioned above, if you
want to push the limit, do the quantum mechanical calculations. The paper
by Bowman and co-workers should be very useful references. 

	Good luck.

> 
>   Thanks
>         E. Lewars
> ================
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___________________________________________________________________________	
Qiang Cui                   
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