Subject: AM1 vs PM3 vs RHF/6-31G* for isocyanates

From: [email-masked] (Doug Smith)
Date: Fri, 7 Jun 1996 22:21:22 +0000

Dear All:
        
        We are using PM3 to calculate several compounds, including some
which contain one or more isocyanate (R-N=C=O) functional groups.  The
choice of PM3 is dictated by a number of factors which do not bear going
into here. However, the results are not intuitively obvious.  For example,
using H-N=C=O as a test case, the bond angles and distances are as given in
the table.

                      distance                  angle
                N-C             C-O             N-C-O
              -------         -------         ---------

        AM1    1.23 A          1.20 A           166.7

        PM3    1.25 A          1.18 A           166.7

  RHF/6-31G*   1.20 A          1.15 A           174.2 

        Similar deviations from linearity are seen in the angles of
substituted isocyanates. What is the expected N-C-O angle based on
experiment and/or higher levels of theory?  Why do the semiempirical methods
fail to reproduce the expected and observed linearity of the N-C-O bond?  Is
it appropriate to fix the N-C-O angle at a linear or near-linear value for
these semiempirical studies, and if so, at what angle?

        Thanks for the help and advice in advance.

Douglas Smith
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