Subject: verify a far-out MM3 result?

From: Karl Irikura <[email-masked]>
Date: Mon, 23 Oct 2000 15:52:12 -0400

I recently obtained the following enthalpy of formation (298 K) for
bicyclo[2.1.0]pent-2-ene (C5H6; cyclobutene with a cyclopropane ring
fused to the edge opposite the double bond):

MM3	545 kJ/mol	(expt	333 kJ/mol)

It's so far off I wonder if there's some problem with my copy of MM3
(came with Tripos' "Alchemy2000" software).  It reports missing
bond-equivalent parameters for two bonds (the sp2-sp3 bonds)--could
they be worth -106 kJ/mol each? The corresponding bonds in the
bicyclopentane are worth +23 kJ/mol each.

The results look great for these related molecules: 

cyclobutene		MM3 = 158	expt = 157 kJ/mol
cyclopropane		MM3 = 54	expt = 53
bicyclo[2.1.0]pentane	MM3 = 156	expt = 158

I don't think the experimental value is bad:  from G2 I get 320
kJ/mol and from B3LYP//cc-pVTZ//6-31G* I get 331 kJ/mol.

I'd be grateful to learn the MM3 result obtained from other software
(or even from an independent installation of Alchemy2000).

Thanks for any help!

Karl

----------------------------------------------
Dr. Karl K. Irikura
National Institute of Standards and Technology
100 Bureau Drive, Stop 8380
Gaithersburg, MD  20899-8380
voice: 301-975-2510	fax: 301-869-4020
e-mail: [email-masked]
http://www.nist.gov/compchem/
----------------------------------------------