Subject: TS calculation

From: Gary Breton <[email-masked]>
Date: Thu, 12 Dec 2002 11:00:26 -0500

Hello everyone,

I have been running some TS calculations on the decompositions of
diazetines, which are four-membered ring azo compounds.  Diazetines
eliminate N2 to give alkenes at elevated temperatures.  A paper was recently
published (Yamabe, S.; Minato, T. J. Chem. Phys. A 2001, 7281) that
determined the geometry of the TS for the unsubstituted case (i.e., C2H4N2).
They reported  that due to the high biradical character of the TS, HF and
DFT methods failed.  The completely active space method (CASSCF(2,2)) was
found to work well. I similarly found that HF and DFT methods failed to
locate a TS, but CASSCF(2,2) located the TS efficiently.

I am interested in the decomposition of a series of derivatives of this
diazetine (i.e., with substitution).   I made a guess at the TS for the
substituted derivatives using the TS geometry of the unsubstituted
diazetine, and ran the following calculation in Gaussian:

# casscf(2,2)/6-31g(d) opt=(ts,calcFC,noeigentest) Freq

On two derivatives this worked very well.  I obtained a TS with a single
negative frequency, and when animated it followed the expected pathway.  On
the third (and crucial one of course) the calculation failed with the
following error:

CALCULATION TERMINATED

ATTEMPT TO DO ORBITAL ROTATION GREATER THAN 45 DEGREES

YOU HAVE CHOSEN THE WRONG STARTING ORBITALS

********** 

Error termination via Lnk1e in C:\G98W\l510.exe.

Job cpu time: 0 days 9 hours 46 minutes 24.0 seconds.

File lengths (MBytes): RWF= 903 Int= 0 D2E= 0 Chk= 4 Scr= 1

Gaussian was nice enough to tell me I chose the wrong starting orbitals, but
exactly where should I proceed from here.  Also, all of these derivatives
are very close to each other in structure (the failed compound is simply a
saturated derivative of another that worked fine, and the C=C double bond is
remote from the reaction site) and it surprises me that two work well and
the other experiences this problem.

Any sage words of advice would be appreciated.

Gary W. Breton
Chair and Associate Professor
Department of Chemistry
Berry College
PO Box 495016
Mount Berry, GA 30149

"There's a light at the
end of the tunnel, but it
may be an oncoming train"