Essentials of Computational Chemistry

BIBLIOGRAPHY AND SUGGESTED ADDITIONAL READING 515
Table 14.5 Measured and predicted absorption and emission energies (eV)
for 8-hydroxyquinoline and Alq3
Process Level of theory 8-hydroxyquinoline Alq3
Absorption CIS/3-21+G∗∗ 5.35 4.68
CIS/pVTZ 5.80
CIS/INDO/S (12x12) 3.48
CIS/INDO/S (15x15) 3.83 3.28
TD-B3LYP/3-21+G∗∗ 3.76 3.00
TD-B3LYP/6-31G(d) 3.72 2.90
Experiment 3.84 3.20
Emission CIS/3-21+G∗∗ 3.58
TD-B3LYP/3-21+G∗∗ 2.30
Experiment 2.40
6-31G(d) results again would seem to indicate that the former basis set may be regarded
as adequate.
With respect to emission, the geometry computed for the excited-state Alq3 structure at
the CIS/3-21+G ∗∗ level was found to be significantly different from that for the ground
state. This leads to a Stokes shift of 0.8 eV, which is not particularly well reproduced at
the CIS level but is quite accurately predicted at the TD-B3LYP level.
Having obtained good agreement with experiment for the various spectroscopic data,
Halls and Schlegel go on to analyze the MOs involved in the photoluminescence. They
find that the orbitals involved are highly localized on a single one of the three aryl ligands
in Alq3, and that these orbitals are quite similar to those involved in the S0 → S1 absorption/emission
of 8-hydroxyquinoline itself. They also express the difference between the
excited-state geometry and the ground-state geometry of Alq3 in terms of the normal modes
(this procedure is in essence a multilinear regression involving displacement vectors). They
find that a particular normal mode having high intensity in the vibrational spectrum makes
a significant contribution in this analysis, thereby rationalizing observations of vibrational
structure in the absorption and emission spectra of Alq3 under matrix isolation conditions.
They carry out their vibrational analysis using BLYP/6-31G(d) frequencies, as these were
found to be in very good agreement with the experimental ground-state IR spectrum.
The work of Halls and Schlegel illustrates particularly effectively how different levels
of theory may be used for studying different aspects of a complex chemical problem.
Furthermore, repeated comparisons of theoretical predictions to experimental measurements
in order to validate the chosen levels of theory provides solid support for the quality of
further predictions using those levels.
Bibliography and Suggested Additional Reading
Aguilar, M. A. 2001. ‘Separation of the Electric Polarization into Fast and Slow Components: A
Comparison of Two Partition Schemes’ J. Phys. Chem. A, 105, 10 393.
Cave, R. J., Burke, K., and Castner, E. W., Jr. 2002. ‘Theoretical Investigation of the Ground and
Excited States of Coumarin 151 and Coumarin 120’ J. Phys. Chem. A, 106, 9294.