Subject: Reduced moment of inertia

From: "Alexei Khalizov" <[email-masked]>
Date: Wed, 21 Mar 2001 12:42:12 -0500

Dear all,

I need to calculate the reduced moment of inertia for a hindered 
rotation (Ir) in order to correct molecular partition function. Is there 
any program to do it? 

I know that it is very a simple procedure for symmetrical coaxial 
rotors, especially if rotation is free, and it can be done with a pencil 
and paper (S.W. Benson, Thermochemical kinetics). It is still not 
very difficult for unsymmetrical cases, but involves some 
mathematical transformations involving calculation of the moment 
of inertia of each fragment about an axis, which is parallel to the 
bond connecting those fragments and passing through the center 
of gravity of the fragment. 

I have tried GEOM program (UNIMOL package, R.G. Gilbert, S.C. 
Smith, Theory of unimolecular and recombination reactions), but 
there are two problems. First, it calculates moments of inertia of 
the fragments about the bond connecting fragments, not about the 
axis parallel to it and passing through the center of gravity of the 
fragment. Second, it looks that there is a bug in the program, since 
it produces zero moment of inertia for a fragment where it should 
not be zero. For example, in hydrogen peroxide, H2O2, all three 
moments of inertia of OH group about axis passing through O-O 
bond must be non-zero, but program produces:

 PRINCIPAL MOMENTS OF INERTIA AROUND PIVOT
       0.955       0.955       0.000 (AMU ANGSTROM**2)

I have also tried Gaussian98 with undocumented command 
freq=hindrot (described in http://www.gaussian.com/thermo.htm), 
which is implementation of P.Y. Ayala, H.B. Schlegel, 
J.Chem.Phys., v.108, 2314, 1998. It also produces strange and in 
some cases corrupted results.

Sincerely,
Alex

-------------------------
Dr. Alexei Khalizov
Department of Chemistry
McGill University,
801 Sherbrooke St. W.,
Montreal, Quebec, CANADA,
H3A 2K6

Phone: (514) 398-6920
Fax: (514) 398-2382