Subject: Summary: Reduced moment of inertia

From: "Alexei Khalizov" <[email-masked]>
Date: Fri, 30 Mar 2001 18:48:34 -0500

Dear all,

I received two advices on how to calculate the reduced moment of 
inertia for free rotation of chemical bond - thanks a lot! They are 
given in the end of this message. 

Frankly speaking, I am still confused. In order to calculate the 
reduced moment of inertia I(r) one needs the moments of inertia of 
both fragments about SOME axis I(r)=I1*I2/(I1+I2). According to [1] 
the bond connecting fragments should be used as an axis, while 
other sources [2,3] recommend to use separate axes for each 
fragment, each axis parallel to the bond connecting fragments and 
passing through the center of mass of corresponding fragment. 
There is no difference between these aproaches for molecules alike 
ethane, but in general case (e.g. 1,2-dichloroethane) they lead to 
significantly different results. Moreover, HROTOR utility included in 
POLYRATE gives a third different value:

1,2-dichloroethane

------------------------------------------------------------

Method            I(r), amu A**2         Q(fr)

------------------------------------------------------------

Method [1]            50.86                44.32

Method [2,3]         16.44                25.20

HROTOR              87.34                58.06

------------------------------------------------------------

Where is truth?

Best regards,

Alexei

[1] <color><param>0100,0100,0100</param><FontFamily><param>MS Sans Serif</param>	Gilbert, S. C. S. Theory of unimolecular and recombination 
reactions; Blackwell scientific publication: 1990. </color><FontFamily><param>Arial</param>

[2] <color><param>0100,0100,0100</param><FontFamily><param>MS Sans Serif</param>	Benson, S. W. Thermochemical kinetics, 2nd ed.; Wiley-
Interscience: New York, 1976. </color><FontFamily><param>Arial</param>

[3] Herschbach, D.R., Johnston, H.S., Pitzer, K.S., Powell, R.E. 
Theoretical pre-exponential factors for twelve bimolecular reactions. 
J. Chem. Phys., 25(4), 736 (1956)

-----------------------------------------------

GEOM is intended to be used with UNIMOL and hence calculates 
the Ir appropriate for its implementation of variational RRKM theory.

for other cases, I normally use a spreadsheet and calculate

    ir = sum m(x^2+ ...) etc

directly.

for something as simple as H2O2, this is very fast.   :-)

for more complicated molecules use something like Chem3d

(or similar 3-d modelling program) to get the x,y,z coordinates you 
want. Chem3d even will move the molecule so that a chosen bond 
is coincident with a specified axis ... and then put the coordinates 
into a spreadsheet as before.

Kieran

------------------------------------------------------------

  Dr Kieran F Lim             Biol. and Chemical Sciences

  (Lim Pak Kwan)              Deakin University

  ph:  + [61] (3) 5227-2146   Geelong          VIC   3217

  fax: + [61] (3) 5227-1040   AUSTRALIA

  <underline><color><param>0000,8000,0000</param>mailto:[email-masked]</underline></color>    <underline><color><param>0000,8000,0000</param>http://www.deakin.edu.au/~lim</underline></color>  

------------------------------------------------------

You can try the routine implemented in POLYRATE

(<underline><color><param>0000,8000,0000</param>http://comp.chem.umn.edu/polyrate/</underline></color>). It is described in the 
manual of that program (accessible in the same page) and you will 
find also several references to how it is done.

Jordi

-

Jordi Villa i Freixa

<underline><color><param>0000,8000,0000</param>[email-masked]</underline></color>   <underline><color><param>0000,8000,0000</param>http://laetro.usc.edu/wgroup/people/jorgevil

</underline></color>Department of Chemistry, University of Southern California

Los Angeles, CA, USA, 90089-1062

Tlf: 1-(213)-740 7671 Fax: 1-(213)-740 2701

<nofill>
-------------------------
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