3 Results and Discussion
3.1 Nitromethane
The converged geometries of the two conformers of nitromethane are shown at
Fig. 2. The lowest energy structure is the eclipsed conformer. The computed
electronic energy difference between them equals only 0.019 kcal/mol. Therefore,
the two conformers are almost degenerate and for this reason were given equal
weights in the construction of the convoluted spectra.
Both conformers have C s symmetry and the occupied orbitals are in order of
increasing energy:
(1a′)
2 (2a′)
2 (3a′)
2 (4a′)
2 (5a′)
2 (6a′)
2 (7a′)
2 (8a′)
2 (9a′)
2 (1a′′)
2 (10a′)
2 (11a′)
2 (2a′′)
2
(12a′)
2 (13a′)
2 (3a′′)
2
In Fig. 3 we show the measured photoelectron spectrum of nitromethane [35]
and our SAC-CI/Monopole Approximation results obtained according to the
procedure described in the last section. The experimental spectrum displays a
vibrational structure not included in our calculations. We also make comparisons
with other measurements of I.P’s below [25, 42].
The computed spectra of the two conformers (Fig. 3) are very similar because:
(1) essentially the only geometric difference between them are the distinct orientations of the methyl group and; (2) the ionized spectra is dominated by transitions
involving the atoms in the NO 2 group. The position of the computed peaks without
Fig. 2 The two conformers
of nitromethane converged in
this work
Photoionization Spectra and Ionization Potentials …
151