Subject: TS on a SN2 reaction

From: "Dr. Antonio Entrena" <[email-masked]>
Date: Wed, 04 Sep 1996 11:37:54 -0700

Dear users:

	I need to calculate the TS for a variety of SN2 reactions using 
semiempirical Mopac calculation and I have some problems. I will be very 
greatfull for every help that you can gave me.

	The strategy that I have used is the following:

	I have define a dummy atom in order to fix the geometry of the 
nuclephile and the leaving group as in the following scheme:

	Du..........C
                    |
                    |
        Nu..........C---L

	I have defined the distance between the dummy atom (Du) and the upper 
carbon atom about 10 A and between the nucleophile (Nu) and the lower carbon 
atom about 5 A.

	The Du-C-C-Nu and Du-C-C-L dihedral angles have been defined as 0 and 
180 degrees, respectively.

	I used the symmetry keyword (option 17) in order to varies the C-C-Nu 
bond angle as 180 degrees - C-C-L bond angle during the calculation.

	I believe that with those definitions, the nucleophile, the carbon 
atom and the leaving group (L) must be in straight line when the distances 
Nu-C is decreased at 0.1 or 0.05 A intervale.

	I expect that when the nucleophile is located at a apropiate distance 
the distance C-L begin to increase so that the geometry of higher energy 
could be used to optimized the TS of the reaction, but this does not occur. 
When the nuclephile is about 2.4-2.2 A, the leaving group is inmediately 
ejected to a distance of 3.5 A or more.

	Can anybody with experience in this type of calculations gave me some 
orientations to resolve this problem?.

	Please, e-mail directly to me and I will summarized to the net.

	Sincerely

	A. Entrena