Computational Chemistry

be unusual. A few IR spectra calculated at routinely practical levels are shown in
Figs.5.33–5.36and compared with gas-phase spectra taken by the author. This
sample, although very limited, gives one an idea of the kind of similarity one can
expect between experimental and ab initio IR spectra. A detailed resemblance cannot
be expected, but the general features of a spectrum are reproduced. Probably the main
utility of calculated ab initio IR spectra is in predicting the IR spectra of unknown
molecules, as an aid to their synthesis, and the levels shown here are evidently
adequate for this.

5.5.4 Properties Arising from Electron Distribution: Dipole

Atoms-in-Molecules (AIM)...................................... Moments, Charges, Bond Orders, Electrostatic Potentials,

Atoms-in-Molecules (AIM)

We have seen three applications of ab initio calculations: finding the shapes
(geometries), the relative energies, and the frequencies of stationary points (usually
minima and transition states) on a potential energy surface:

Acetone

Experimental

80

60

40

20

0

0

0

30

60

90

77

IR_INTENS

IR_INTENS

1.5e+002

2.3e+002

4000

4000

3000

3500 3000

2000

FREQ_VAL

FREQ_VAL

2500 2000

1000

1500 1000 500

4000 3500 3000 2500 2000 1500 1000 500

1739 1366 1217

1636

HF / 6-31G*

MP2 / 6-31G*

1549,

1534

1352

2022

1792 1448

1286

1555

1633, 1614

Fig. 5.33 Experimental (gas phase), HF/6–31G and MP2(fc)/6–31G calculated IR spectra of
acetone

5.5 Applications of the Ab initio Method 337