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A multireference configuration interaction study of the photodynamics of nitroethylene.

Borges I, Aquino AJ, Lischka H - J Phys Chem A (2014)

Bottom Line: The vertical calculations mostly confirm the previous assignment of experimental spectrum and theoretical results using lower-level calculations.Based on the conical intersections found, a photochemical nonradiative deactivation process after a π-π* excitation to the bright S5 state is proposed.In particular, the possibility of NO2 release in the ground state, an important property in nitro explosives, was found to be possible.

View Article: PubMed Central - PubMed

Affiliation: Departamento de Química, Instituto Militar de Engenharia Praça General Tibúrcio , 80, 22290-270 Rio de Janeiro, Brazil.

ABSTRACT
Extended multireference configuration interaction with singles and doubles (MR-CISD) calculations of nitroethylene (H2C═CHNO2) were carried out to investigate the photodynamical deactivation paths to the ground state. The ground (S0) and the first five valence excited electronic states (S1-S5) were investigated. In the first step, vertical excitations and potential energy curves for CH2 and NO2 torsions and CH2 out-of-plane bending starting from the ground state geometry were computed. Afterward, five conical intersections, one between each pair of adjacent states, were located. The vertical calculations mostly confirm the previous assignment of experimental spectrum and theoretical results using lower-level calculations. The conical intersections have as main features the torsion of the CH2 moiety, different distortions of the NO2 group and CC, CN, and NO bond stretchings. In these conical intersections, the NO2 group plays an important role, also seen in excited state investigations of other nitro molecules. Based on the conical intersections found, a photochemical nonradiative deactivation process after a π-π* excitation to the bright S5 state is proposed. In particular, the possibility of NO2 release in the ground state, an important property in nitro explosives, was found to be possible.

No MeSH data available.


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SA-MCSCF active spacenitroethylene molecular orbitals.
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fig1: SA-MCSCF active spacenitroethylene molecular orbitals.

Mentions: The calculations were carried out in two steps. The first one consistedof a state-averaged multiconfiguration self-consistent field (SA-MCSCF)calculation followed by a multireference configuration interactionwith single and doubles excitations (MR-CISD). The state averagingwas performed with equal weights and included six states (SA-6), namely,the ground state S0 (11A′), three A″and two A′ states in the case of vertical excitations. Fordescribing these excitations, a complete active space—CAS(8,6)—includingeight electrons and six orbitals, the four highest occupied orbitals(two a″ and two a′), and two virtual orbitals of a″symmetry, was chosen. The SA-MCSCF character of the orbitals are 2a″[π (NO2)], 15a′ [nσ], 16a′ [nσ(O)],3a″ [π (C=C+ NO2)], 4a″ [π*]and 5a″ [π*] (see Figure 1). The15a′ and 16a′ orbitals are respectively n+ and n– type-orbitals because the former correspondsto symmetric linear combinations of the oxygen lone pair orbital andthe latter, the highest occupied molecular orbital (HOMO), representsthe antisymmetric combination.28 This SA-6-CASSCFcalculation was followed by a MR-CISD with the reference configurationsgenerated from the same CAS (8,6) space and applying the interactingspace restriction.40 This orbital spacehas been chosen to represent the reference space for the computationof the five lowest singlet valence excited states as discussed inour previous work.28 Since the focus ofthis work is laid on the exploration of the photodeactivation mechanismof the valence states, Rydberg states, which appear above those inthe upper range of the states chosen,28 were not included in our calculations.


A multireference configuration interaction study of the photodynamics of nitroethylene.

Borges I, Aquino AJ, Lischka H - J Phys Chem A (2014)

SA-MCSCF active spacenitroethylene molecular orbitals.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4279882&req=5

fig1: SA-MCSCF active spacenitroethylene molecular orbitals.
Mentions: The calculations were carried out in two steps. The first one consistedof a state-averaged multiconfiguration self-consistent field (SA-MCSCF)calculation followed by a multireference configuration interactionwith single and doubles excitations (MR-CISD). The state averagingwas performed with equal weights and included six states (SA-6), namely,the ground state S0 (11A′), three A″and two A′ states in the case of vertical excitations. Fordescribing these excitations, a complete active space—CAS(8,6)—includingeight electrons and six orbitals, the four highest occupied orbitals(two a″ and two a′), and two virtual orbitals of a″symmetry, was chosen. The SA-MCSCF character of the orbitals are 2a″[π (NO2)], 15a′ [nσ], 16a′ [nσ(O)],3a″ [π (C=C+ NO2)], 4a″ [π*]and 5a″ [π*] (see Figure 1). The15a′ and 16a′ orbitals are respectively n+ and n– type-orbitals because the former correspondsto symmetric linear combinations of the oxygen lone pair orbital andthe latter, the highest occupied molecular orbital (HOMO), representsthe antisymmetric combination.28 This SA-6-CASSCFcalculation was followed by a MR-CISD with the reference configurationsgenerated from the same CAS (8,6) space and applying the interactingspace restriction.40 This orbital spacehas been chosen to represent the reference space for the computationof the five lowest singlet valence excited states as discussed inour previous work.28 Since the focus ofthis work is laid on the exploration of the photodeactivation mechanismof the valence states, Rydberg states, which appear above those inthe upper range of the states chosen,28 were not included in our calculations.

Bottom Line: The vertical calculations mostly confirm the previous assignment of experimental spectrum and theoretical results using lower-level calculations.Based on the conical intersections found, a photochemical nonradiative deactivation process after a π-π* excitation to the bright S5 state is proposed.In particular, the possibility of NO2 release in the ground state, an important property in nitro explosives, was found to be possible.

View Article: PubMed Central - PubMed

Affiliation: Departamento de Química, Instituto Militar de Engenharia Praça General Tibúrcio , 80, 22290-270 Rio de Janeiro, Brazil.

ABSTRACT
Extended multireference configuration interaction with singles and doubles (MR-CISD) calculations of nitroethylene (H2C═CHNO2) were carried out to investigate the photodynamical deactivation paths to the ground state. The ground (S0) and the first five valence excited electronic states (S1-S5) were investigated. In the first step, vertical excitations and potential energy curves for CH2 and NO2 torsions and CH2 out-of-plane bending starting from the ground state geometry were computed. Afterward, five conical intersections, one between each pair of adjacent states, were located. The vertical calculations mostly confirm the previous assignment of experimental spectrum and theoretical results using lower-level calculations. The conical intersections have as main features the torsion of the CH2 moiety, different distortions of the NO2 group and CC, CN, and NO bond stretchings. In these conical intersections, the NO2 group plays an important role, also seen in excited state investigations of other nitro molecules. Based on the conical intersections found, a photochemical nonradiative deactivation process after a π-π* excitation to the bright S5 state is proposed. In particular, the possibility of NO2 release in the ground state, an important property in nitro explosives, was found to be possible.

No MeSH data available.


Related in: MedlinePlus