Subject: MM and diastereomers

From: "E. Lewars" <[email-masked]>
Date: Mon, 25 Apr 1994 17:26:52 -0400

Matthew Stahl asked if MM can be used to calculate the relative energies of
diastreomers.  In principle yes, for diastereomers (unlike enantiomers)
are essentially just ordinary isomers.  The problem is that they often
differ only slightly in energy.  Here are somesample results:
FMeHC-CFMeH,meso diastereomer, C-F/C-Me anti conformer (A) vs
threo diast, C-F/C-Me anti conf (B):
MM2    A, 0.09 kJ    B, 0 kJ (RELATIVE E's)
AM1    A, 0.2  kJ    B, 0 
3-21G  A, -5.59 kJ   B, 0
For Z- and E- (i.e cis and trans) 1,2-diFethene (E/Z isomers are diastereomers
too):
MM2    Z, -5.10 kJ    E, 0
AM1    Z,  0.0  kJ    E, 0.0 kJ (sic)
3-21G  Z,  5.88 kJ    E, 0
6-31G* Z,  1.13 kJ    E, 0
====
As with any calc, the only way to be confident you can trust your results is to
see how well analogous cases match experiment.
===