Subject: Re: CCL:Branched vs straight chain alkanes

From: [email-masked] (Joel Polowin)
Date: Wed, 7 Aug 96 18:19:14 -0400

> Date: Wed, 7 Aug 1996 14:51:28 +22305133 (EDT)
> From: Michael Kotelyanskii <[email-masked]>
> 
> Are you sure, that the energy is calculated in the LOWEST
> conformation? 
> I mean global, vs. just a local minimum?
> 
> >      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

I get -58.861 and -40.210 kcal/mol as the heats of formation of the two 
structures.  The former is for the stretched-out conformation of octane, 
which certainly ought to be the lowest-energy conformation, and the latter
is for the symmetrical conformation of 2,2,3,3-tetramethylbutane.  A
slightly skewed conformation of the latter gives me -41.132 kcal/mol;
there might be other still lower-energy conformations.  I'm using AM1, 
lowest state, RHF, converged to an RMS gradient of 0.001 kcal/(A mol).

Joel

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