Subject: SINGLET DIRADICAL CALCULATINS__QUESTION

From: "E. Lewars" <[email-masked]>
Date: Sun, 24 Mar 1996 15:33:19 -0500 (EST)

1996 March 24

  Hello, I have a question about calc's involving SINGLET diradicals.

  Consider for example the ring-opening of tetrahedrane, Td (CH)4:
             H                             H           H
     H       |                   H       /             |
      \    / C                    \   / C              C
       C/. /   \                   C . /             / | \
       | /   .   \        --->     | / .      =  H--C  |  C--H
       C  ------- C                C-----C           \ | /
      /             \             /       \            C
    H                 H          H         H           |
            1                                  2       H

                                     singlet diradical: each of the two
                                     tricoordinate C's has one unpaired electron
                                     and the diradical has a multiplicity of
                                     1, not 3, i.e the unpaired electrons
                                     have opposite spins

  Suppose one wants to find  the activation energy and the reaction energy
(i.e E_transition state - E_1, and E_2 - E_1).  Then
  (1)  What is the lowest-level calc that will give meaningful (not necessarily
quantitatively good) results?

  (2)  What level of calc is needed to get quantitatively reliable results?

   In particular, how can one calculate the geometry and energy of a singlet
diradical like 2 (I presume the transition state between 1 and 2 can be
handled by the same technique one applies to 2, provided of course one can
find the TS, and that 1, 2 and the TS must all be treated by the same method
if the relative energies are to be calc)?
  References would be very much appreciated.

E. Lewars