Knowledge of the electronic spectra of energetic molecules supplies an understanding of associated processes and can improve practical applications [1]. It is
experimentally known that detonation initiation of energetic materials is related to
electronic excitation phenomena [1, 4, 18, 27, 36, 44, 45]. Moreover, the component molecules have diffuse electronic spectra and propensity to dissociate and
rearrange [22].
In the detonation initiation of these materials, different photochemical processes
play important roles [5–7, 20, 21, 34, 41]. Furthermore, because nitro compounds
strongly absorb in the UV region, detailed knowledge of the molecular electronic
excited states can also assist the design of detectors of explosives, an important
issue in these days [37].
Despite the importance of excited states in energetic materials research, only few
studies are available [4].
The especial properties of explosive materials result from a combination of
electron—donating and electron—acceptor groups. Nitro explosives are characterized by having NO 2 as the electron acceptor group; different combinations of
electron-donating groups determine the properties of the explosive material [2].
Energetic materials are usually thermally unstable and composed of easily
broken covalent bonds such as N–N, O–C and C–N. These molecules have X–NO 2
bonds (X = N, C), typical of widely used energetic materials. The explosophore
NO 2 group of these molecules, so named for giving the explosive properties of the
material, is easily released.
The study of ionization processes is an important tool to investigate the electronic
structure of atoms, molecules and solids [24]. Recent advances in synchrotron
radiation have led to a myriad of useful spectroscopies and new measurements await
interpretation. From the standpoint of theory, over the decades different theoretical
methods have been developed to address the complicated problem of photoionization. However, most of these methods are limited to atoms or small molecules and in
most cases electron correlation is included very approximately [29].
In spite of the challenges involved in computing absolute photoelectron cross
sections, relative intensities can be simulated in the monopole approximation [28, 39].
When this approximation is combined with an ab initio wave function to accurately
describe electron correlation, accurate and enlightening comparisons with experimental photoelectron spectra can be achieved. In this work, we explore the combination of the monopole approximation with the Symmetry-Adapted-Cluster (SAC)
[33]—Configuration Interaction (CI) wave function [23, 30–32] of Nakatsuji to
compute relative photoionization cross sections. This method can provide accurate
ionization energies and relative intensities for molecules as large as the typical
molecules of biological interest [15, 16].
The SAC-CI wave function belongs to the family of coupled-cluster (CC)
approaches [20], being equivalent to the Equation-of-Motion-CC (EOM-CC) [38]
and the CC-Linear Response Theory (CC-LRT) methods [26], but it was developed
much earlier. The SAC-CI wave function provides an accurate and balanced
description of excited states and good comparison with available experimental data,
as we verified in previous studies [7, 9–11, 13, 38].
148
I. Borges Jr. and E. Uhl
experimentally known that detonation initiation of energetic materials is related to
electronic excitation phenomena [1, 4, 18, 27, 36, 44, 45]. Moreover, the component molecules have diffuse electronic spectra and propensity to dissociate and
rearrange [22].
In the detonation initiation of these materials, different photochemical processes
play important roles [5–7, 20, 21, 34, 41]. Furthermore, because nitro compounds
strongly absorb in the UV region, detailed knowledge of the molecular electronic
excited states can also assist the design of detectors of explosives, an important
issue in these days [37].
Despite the importance of excited states in energetic materials research, only few
studies are available [4].
The especial properties of explosive materials result from a combination of
electron—donating and electron—acceptor groups. Nitro explosives are characterized by having NO 2 as the electron acceptor group; different combinations of
electron-donating groups determine the properties of the explosive material [2].
Energetic materials are usually thermally unstable and composed of easily
broken covalent bonds such as N–N, O–C and C–N. These molecules have X–NO 2
bonds (X = N, C), typical of widely used energetic materials. The explosophore
NO 2 group of these molecules, so named for giving the explosive properties of the
material, is easily released.
The study of ionization processes is an important tool to investigate the electronic
structure of atoms, molecules and solids [24]. Recent advances in synchrotron
radiation have led to a myriad of useful spectroscopies and new measurements await
interpretation. From the standpoint of theory, over the decades different theoretical
methods have been developed to address the complicated problem of photoionization. However, most of these methods are limited to atoms or small molecules and in
most cases electron correlation is included very approximately [29].
In spite of the challenges involved in computing absolute photoelectron cross
sections, relative intensities can be simulated in the monopole approximation [28, 39].
When this approximation is combined with an ab initio wave function to accurately
describe electron correlation, accurate and enlightening comparisons with experimental photoelectron spectra can be achieved. In this work, we explore the combination of the monopole approximation with the Symmetry-Adapted-Cluster (SAC)
[33]—Configuration Interaction (CI) wave function [23, 30–32] of Nakatsuji to
compute relative photoionization cross sections. This method can provide accurate
ionization energies and relative intensities for molecules as large as the typical
molecules of biological interest [15, 16].
The SAC-CI wave function belongs to the family of coupled-cluster (CC)
approaches [20], being equivalent to the Equation-of-Motion-CC (EOM-CC) [38]
and the CC-Linear Response Theory (CC-LRT) methods [26], but it was developed
much earlier. The SAC-CI wave function provides an accurate and balanced
description of excited states and good comparison with available experimental data,
as we verified in previous studies [7, 9–11, 13, 38].
148
I. Borges Jr. and E. Uhl