Biomedical Engineering Reference
In-Depth Information
Figure 17.6
Vibrational spectra
Table 17.1
Calculated (HF-LCAO) and experimental vibration wavenumbers (in cm
−1
)
Mode
Expt
STO-3G
STO/6-31G
∗
STO/6-311G
∗∗
Symmetric stretch (
g
)
1388.17
1435.2
1518.7
1522.8
Asymmetric stretch (
u
)
2349.16
2535.7
2585.4
2593.5
Degenerate bend (
)
667.40
566.2
745.7
766.8
is not infrared active because the dipole moment does not change during the vibration, and
the doubly degenerate bending vibration has a lower intensity than the asymmetric stretch.
One decision that has to be made with a HF-LCAO calculation is the choice of basis
set, and this is usually a matter of experience. I ran the calculation with three fairly
simple basis sets, and the results are also shown in Table 17.1. Apart from the minimal
basis set calculation, it is a common experience that HF-LCAO calculations overestimate
