Subject: Branched vs. straight chain alkanes

From: "J. Pat Cannady, Ph.D." <[email-masked]>
Date: Wed, 07 Aug 1996 14:50:50 -0700

Matthew Harbowy wrote:                                                     
                                                                            
 >     My results for AM1 and MM+ show (Hyperchem 3.0 calculations)         
 >                                                                          
 >                         am1                mm+               exp         
 >                                                                          
 >     octane             -58.628            (6.448 s.e.)       -49.82      
 >                                                                          
 >     2,2,3,3,TMB        -40.155            (12.36 s.e.)       -53.99      
 >                                                                          
 >     -------------     ----------        --------------      ------------ 
 >                                                                          
 >     oct - tmb          -18.473            -5.912              4.17       
 >                                                                          

To this I should like to add the pm3 results (as suggested earlier) (MOPAC 6):

octane:  	-50.75
2,2,3,3, TMB:  	-49.96

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

oct - tmb:        -.79

Thus the better (but still not quite good enough) accuracy of pm3 for this type of
system over that of am1 is easily demonstrated.  The error relative to experimental
for octane is less than 1 kcal, that for TMB just 4 kcal, much better in each case
than the am1 values.  It has been said elsewhere (check the CCL archive) that when
one has a system with "lots of protons", pm3 should be used over am1.  I think this 
is a good example.

-- 
J. Patrick Cannady
Dow Corning Corporation
[email-masked]

All opinions expressed are those of the author and not
necessarily those of Dow Corning Corporation.