Computational Chemistry

All the normal modes are present in the results of a semiempirical frequency
calculation, as is the case for an ab initio or DFT calculation, and animation of these
will usually give, approximately, the frequencies of these modes. A very extensive
compilation of experimental, MNDO and AM1 frequencies has been given by
Healy and Holder, who conclude that the AM1 error of 10% can be reduced to
6% by an empirical correction, and that entropies and heat capacities are accurately
calculated from the frequencies [ 104 ]. In this regard, Coolidge et al. conclude –
surprisingly, in view of our results for the four molecules in Figs.6.5–6.8– from a
study of 61 molecules that (apart from problems with ring- and heavy atom-stretch
for AM1 and S–H, P–H and O–H stretch for PM3) “both AM1 and PM3 should
provide results that are close to experimental gas phase spectra” [ 105 ].

0

0

4000

AM1

3500 3000 2500 2000

FAEQ_VAL

2063

1438

1338

FREQ_VAL

1500 1000 500

4000 3500 3000 2500 2000 1500 1000 500

77

30

60

90

1.5e+002

2.3e+002

IR_INTENS

IR_INTENS

0

4000 3000 2000

1739

(^13661217)
1636
1000
20
40
60
80
MP2 / 6-31G 1792 1448
1286
1534
1549
Acetone
Experimental
Fig. 6.5 Experimental (gas phase), AM1 and ab initio (MP2(fc)/6–31G) calculated IR spectra of
acetone
6.3 Applications of Semiempirical Methods 425