CALCS ON HCN/HNC ISOMERIZATION

From: "E. Lewars" <elewars (+ at +) alchemy.chem.utoronto.ca>
Subject: CALCS ON HCN/HNC ISOMERIZATION
Date: Fri, 5 Aug 1994 19:05:13 -0400
 ipcake (+ at +) iisc.ernet asked about the HCN/HNC isomerization.  Here are the
 results of some calcs:
         H
       /   \
      N-----C                      H--N--C              H--C--N
        TS                         linear               linear
                           Semiempirical AM1 method
        118.5 kcal/mol            48.8 kcal/mol          31.0 kcal/mol
   ZPE   6.440  "    "     ZPE  10.747  "    "    ZPE  10.950
 "    "
     NC  1.217              NH   0.967             CH   1.069
     NH  1.399              NC   1.178             CN   1.160
     CH  1.298
    HNC 59.0
    NHC 53.5
    NCH 67.6           -1
    imag. freq -1407 cm
                           Ab Initio, HF/3-21G
       -92.246 04 H            -92.339 71 H             -92.354 08 H
   ZPE   6.549 kcal/mol    ZPE  11.022            ZPE    11.531 kcal/mol
     NC  1.183              NH   0.983             CH     1.050
     NH  1.407              NC   1.160             CN     1.137
     CH  1.213
    HNC 55.1
    NHC 53.0
    NCH 71.9           -1
     imag. freq -1216 cm
                           Ab Initio, HF/6-31G*
       -92.791 95 H            -92.855 33              -92.875 20
   ZPE   7.287kcal/mol     ZPE  10.70 kcal/mol    ZPE   11.287 kcal/mol
     NC  1.169              NH   0.985             CH    1.059
     NH  1.454              NC   1.154             CN    1.133
     CH  1.155
    HNC 50.8
    NHC 51.7
    NCH 77.5             -1
      imag. freq -1227 cm
   The geom's of HCN and HNC have probably been measured by microwave spec.
 For an indication of the reliability of these calcs, see Hehre et al, "Ab
 Initio Molecular Orbital Calculations".  For good quantitative estimates of
 relative energies, you probably should do MP2/6-31G* optimizations, and then
 maybe MP4/6-31G* single-points on these, or (best) G1 or G2 single-points.
 The HF geom's are likely not bad, and the energies are in the right order
 anyway.
 Errol Lewars
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