spectra, and ionization energies (ionization potentials) and electron affinities will be
discussed here.
6.3.5.1 UV Spectra
As pointed out in Section5.5.5, although ultraviolet spectra result from the promotion
of electrons from occupied to unoccupied orbitals, UV spectra cannot be calculated
with reasonable accuracy simply from the HOMO/LUMO gap of the ground elec-
tronic state, since the UV bands represent energy differences between the ground
andexcitedstates. Furthermore the HOMO/LUMO gap does not account for the
presence of theseveralbands often found in UV spectra, and gives no indication of
the intensity of a band. In wavefunction theory, accurate prediction of UV spectra
requires calculation of the energies of excited states. Semiempirical UV spectra are
usually calculated with programs specifically parameterized for this purpose, such as
INDO and ZINDO (intermediate neglect of differential overlap and Zerner’s INDO),
sometimes denoted INDO/S and ZINDO/S, S¼spectra (Section6.2.4)[ 20 ]. These
are in, e.g., HyperChem [ 113 ]; INDO and ZINDO, which appears to have largely
superseded INDO, are included in the primarily ab initio and DFT package Gaussian
[ 55 ]; keyword simply ZINDO, note the use ofþfunctions for the RCIS calculation,
see Section 5.3.3.4). Table6.5compares the UV spectrum of methylenecyclopropene
calculated by ZINDO/S on the AM1 geometry with the ab initio-calculated RCIS
(Table5.16) and the experimental spectra [ 114 ] (calculations with Gaussian 03 [ 55 ];
keyword simply ZINDO, note the use ofþfunctions for the RCIS calculation, see
Section5.3.3.4). The ZINDO/S spectrum resembles the experimental spectrum
considerably better than does the ab initio one (the experimental 242 nm, and
particularly the 309 nm band, are matched better than by the ab initio calculation).
The times for the calculations on a vintage ca. 2002 computer were 0.5 and 1 min
(ZINDO/S and RCIS). Parameterized methods like ZINDO/S are probably the only
way to calculate reasonably accurate UV spectra for large molecules.
6.3.5.2 Ionization Energies and Electron Affinities
The concepts of IE and EA were discussed in Section5.5.5. In Table6.6the results
of some semiempirical calculations are compared with ab initio and experimental
Table 6.5 Calculated and experimental [ 114 ] UV spectra of methylenecyclopropene. The semi-
empirical calculations were done with ZINDO/S in G94W; the ab initio results are from Table5.16
Calculated Experimental
ZINDO/S//AM1 RCIS/6–31+G//HF/6–31G
Wavelength Relative
intensity
Wavelength Relative
intensityWavelength Relative
(nm) (nm) (nm) intensity
288 12 222 15 309 13
224 0.2 209 7 242 0.6
213 100 196 0 206 100
204 1 193 9
193 100
432 6 Semiempirical Calculations