This discrepancy could be due to limitations either from our calculations or from
the experiment, but we cannot establish its origin.
When examining the SAC-CI character of each ionized state, there are inversions in comparison with the order of the HF molecular orbitals according to the
increasing energies in the spirit of the Koopmans’ theorem. These inversions were
seen before in previous studies of molecules having nitro groups using the SAC-CI/
Monopole Approximation [7–9, 11].
3.2 The Photoionization Spectra of the Four Molecules
In this section, we discuss the common features of calculations of photoionization
spectra of the four energetic molecules (Figs. 1 and 2).
The first ionized states of the four closed-shell molecules are characterized by
single configurations (i.e., one-electron processes) involving single ionization from
orbitals mostly localized in the NO 2 moiety of the molecules. The role of the NO 2
group is also prominent in the electronic excitation processes of energetic molecules
having this moiety, as we showed before [7–11]. In the four molecules of the
present work, the character of the deeper inner-shell ionized states can be a combination of a single excitation and a single ionization—these are satellite or correlation peaks; see an illustration of that in the results of nitromethane (Table 1).
For each molecule, the computed ionization peaks typically appear in sets
dominated by ionizations from orbitals having close energies. For the nitramide,
N,N-dimethylnitramine [8] and nitromethane molecules [7], which have experimental photoionization spectra, there are sets of computed ionization peaks
describing wide bands that may span some electron-volts—see Figs. 3 and 4, the
latter displaying the computed and measured spectra of dimethylnitramine and
nitromethane. For the four molecules we provided the assignments of the measured
ionization bands. Because FOX-7 has two NO 2 moieties thus more available
orbitals for ionizations, it has more sets of computed peaks [11].
We can evaluate the accuracy of our results by comparing them with the
available measured spectra. Overall, when the SAC-CI/Monopole Approximation
ionization energies are compared to available measured values, the agreement is
very good. For the N,N-dimethylnitramine molecule [7], the largest difference is
0.01 eV and for nitramide [8] the worst agreement has a difference of 0.43 eV for
the 7th-ionization potential, while the remaining differences are lower than 0.1 eV.
For nitromethane, as discussed above, the agreement is also good. Although the
1,1-diamino-2,2-dinitroethylene molecule (FOX-7) still does not have photoelectron measurements, we expect the SAC-CI/Monopole Approximation results to be
accurate because of the favorable results for the other three molecules also having
an NO 2 moiety.
The comparison between the SAC-CI/Monopole Aproximation computed photoionization spectra and measured spectra can be seen at Figs. 3 (nitromethane) and
4 (nitramide and N,N-dimethylnitramine). The experimental peak maximum was
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I. Borges Jr. and E. Uhl
the experiment, but we cannot establish its origin.
When examining the SAC-CI character of each ionized state, there are inversions in comparison with the order of the HF molecular orbitals according to the
increasing energies in the spirit of the Koopmans’ theorem. These inversions were
seen before in previous studies of molecules having nitro groups using the SAC-CI/
Monopole Approximation [7–9, 11].
3.2 The Photoionization Spectra of the Four Molecules
In this section, we discuss the common features of calculations of photoionization
spectra of the four energetic molecules (Figs. 1 and 2).
The first ionized states of the four closed-shell molecules are characterized by
single configurations (i.e., one-electron processes) involving single ionization from
orbitals mostly localized in the NO 2 moiety of the molecules. The role of the NO 2
group is also prominent in the electronic excitation processes of energetic molecules
having this moiety, as we showed before [7–11]. In the four molecules of the
present work, the character of the deeper inner-shell ionized states can be a combination of a single excitation and a single ionization—these are satellite or correlation peaks; see an illustration of that in the results of nitromethane (Table 1).
For each molecule, the computed ionization peaks typically appear in sets
dominated by ionizations from orbitals having close energies. For the nitramide,
N,N-dimethylnitramine [8] and nitromethane molecules [7], which have experimental photoionization spectra, there are sets of computed ionization peaks
describing wide bands that may span some electron-volts—see Figs. 3 and 4, the
latter displaying the computed and measured spectra of dimethylnitramine and
nitromethane. For the four molecules we provided the assignments of the measured
ionization bands. Because FOX-7 has two NO 2 moieties thus more available
orbitals for ionizations, it has more sets of computed peaks [11].
We can evaluate the accuracy of our results by comparing them with the
available measured spectra. Overall, when the SAC-CI/Monopole Approximation
ionization energies are compared to available measured values, the agreement is
very good. For the N,N-dimethylnitramine molecule [7], the largest difference is
0.01 eV and for nitramide [8] the worst agreement has a difference of 0.43 eV for
the 7th-ionization potential, while the remaining differences are lower than 0.1 eV.
For nitromethane, as discussed above, the agreement is also good. Although the
1,1-diamino-2,2-dinitroethylene molecule (FOX-7) still does not have photoelectron measurements, we expect the SAC-CI/Monopole Approximation results to be
accurate because of the favorable results for the other three molecules also having
an NO 2 moiety.
The comparison between the SAC-CI/Monopole Aproximation computed photoionization spectra and measured spectra can be seen at Figs. 3 (nitromethane) and
4 (nitramide and N,N-dimethylnitramine). The experimental peak maximum was
154
I. Borges Jr. and E. Uhl