Computational Chemistry

calculated relative intensities may be expected to be similar too. This is
supported by Figs.7.4through7.7.Let’sexaminetheIRspectraofacetone,
benzene, dichloromethane, and methanol, the same four compounds used in
Chapters 3, 5 and 6 (Figs. 3.14–3.17, 5.33–5.36 and 6.5–6.8) to illustrate spectra
calculated by molecular mechanics, ab initio, and semiempirical methods. The
DFT spectra in Figs.7.4through7.7are compared with experiment (gas-phase
spectra taken by the author) and, for commonality with Chapters 3, 5 and 6, MP2
(fc)/6-31G. B3LYP/6-31G was chosen because, as was justified in retaining it
for geometries (Section7.3.1), it is still the most popular functional. We see that
here B3LYP/6-31G simulates the experimental IRs reasonably well, and is in
this regard very similar to MP2(fc)/6-31G.

80

60

40

20

0

IR_INTENS

0

53

1.1e+002

1.6e+002

4000

4000

3000

3500 3000

2000

2500 2000

1000

1500 1000 500

AcetoneExperimental 1739

1636

1366 1217

1549

1822

1522

1499

(^14141246)
B3LYP / 6-31G
1534
1792 1448
1286
MP2 / 6-31G
IR_INTENS
0
30
60
90
4000 3500 3000 2500 2000 1500 1000 500
FREQ_VAL
FREQ_VAL
Fig. 7.4 Experimental (gas phase), DFT (B3LYP/6-31G) and ab initio (MP2(fc)/6-31G)
calculated IR spectra of acetone
486 7 Density Functional Calculations