^{1}, Juan C. Otero

^{1}, Daniel Peláez

^{1}and Juan Soto

^{1}

### Abstract

The photodissociation dynamics of nitromethane starting at the excited state has been studied at the complete active space self-consistent field level of theory in conjunction with atomic natural orbital type basis sets. In addition, the energies of all the critical points and the energy profiles connecting them have been recomputed with the multiconfigurational second-order perturbation method. It is found that the key step in the reaction mechanism is a radiationless decay through an conical intersection. The branching space spanned by the gradient difference and nonadiabatic coupling vectors of this crossing point comprises dissociation into excited nitromethane plus singlet atomic oxygen and deactivation, respectively. Furthermore, deactivated nitromethane can decompose in subsequent steps into , where is generated at least in two different electronic states ( and ). It is shown that formation of excited nitric oxide arises from generated in the previous step. In addition, four crossings between singlet and triplet states are localized; however, no evidence is found for a relevant role of such crossings in the photochemistry of initiated at state in the gas phase.

This research was supported by the Spanish Ministerio de Ciencia y Tecnologia (Project No. BQU2003-1453).

I. INTRODUCTION

II. METHODS OF CALCULATION

III. RESULTS

A. Singlet potential energy surfaces and conical intersections

B. Triplet potential energy surfaces and intersystem crossings

IV. DISCUSSION

V. SUMMARY

### Key Topics

- Dissociation
- 23.0
- Surface crossings
- 18.0
- Surface states
- 17.0
- Potential energy surfaces
- 15.0
- Dissociation energies
- 12.0

## Figures

Geometries of the critical points of nitromethane localized on the singlet surfaces at the CAS (14,11)/ANO-L level; conical intersections calculated at the CAS(14,10)/ level. The arrows in the lower structures correspond to imaginary modes (for stationary points) or nonadiabatic coupling (left) and gradient difference (right) vectors (for conical intersections). (a) , staggered minimum; (b) , eclipsed minimum; (c) , first-order saddle point; (d) , transition state for dissociation into ; (e) , minimum; (f) , transition state for dissociation into ; (g) , first-order saddle point; (h) , transition state for dissociation into ; (i) conical intersection; (j) conical intersection; (k) (Ci3) conical intersection; and (l) conical intersection. Structures represented with the MACMOLPLT program (Ref. 42).

Geometries of the critical points of nitromethane localized on the singlet surfaces at the CAS (14,11)/ANO-L level; conical intersections calculated at the CAS(14,10)/ level. The arrows in the lower structures correspond to imaginary modes (for stationary points) or nonadiabatic coupling (left) and gradient difference (right) vectors (for conical intersections). (a) , staggered minimum; (b) , eclipsed minimum; (c) , first-order saddle point; (d) , transition state for dissociation into ; (e) , minimum; (f) , transition state for dissociation into ; (g) , first-order saddle point; (h) , transition state for dissociation into ; (i) conical intersection; (j) conical intersection; (k) (Ci3) conical intersection; and (l) conical intersection. Structures represented with the MACMOLPLT program (Ref. 42).

Singlet MS-CASPT2/ANO-L linear interpolations ( symmetry). The geometry of the initial point corresponds to the minimum of nitromethane, the geometry of the end point corresponds to (a) conical intersection; (b) conical intersection; (c) (Ci3) conical intersection. Two roots have been included in each symmetry block for the MS-CASPT2 calculations.

Singlet MS-CASPT2/ANO-L linear interpolations ( symmetry). The geometry of the initial point corresponds to the minimum of nitromethane, the geometry of the end point corresponds to (a) conical intersection; (b) conical intersection; (c) (Ci3) conical intersection. Two roots have been included in each symmetry block for the MS-CASPT2 calculations.

Singlet MS-CASPT2/ANO-L linear interpolations ( symmetry): (a) from the Franck–Condon point to the conical intersection; (b) from the conical intersection to dissociation into . Four roots have been included in the MS-CASPT2 calculations.

Singlet MS-CASPT2/ANO-L linear interpolations ( symmetry): (a) from the Franck–Condon point to the conical intersection; (b) from the conical intersection to dissociation into . Four roots have been included in the MS-CASPT2 calculations.

Geometries of the critical points of nitromethane localized on the triplet surfaces at the CAS(14,11)/ANO-L level; intersystem crossings calculated at the CAS(14,10)/ level. The arrows in the lower structures correspond to imaginary modes (for stationary points) or gradient difference vectors (for intersystem crossings). (a) , minimum; (b) , transition state for dissociation into ; (c) , first-order saddle point; (d) , transition state for dissociation into ; (e) , minimum; (f) , transition state for dissociation into ; (g) intersystem crossing; (h) intersystem crossing; (i) intersystem crossing; and (j) intersystem crossing. Structures represented with the MACMOLPLT program (Ref. 42).

Geometries of the critical points of nitromethane localized on the triplet surfaces at the CAS(14,11)/ANO-L level; intersystem crossings calculated at the CAS(14,10)/ level. The arrows in the lower structures correspond to imaginary modes (for stationary points) or gradient difference vectors (for intersystem crossings). (a) , minimum; (b) , transition state for dissociation into ; (c) , first-order saddle point; (d) , transition state for dissociation into ; (e) , minimum; (f) , transition state for dissociation into ; (g) intersystem crossing; (h) intersystem crossing; (i) intersystem crossing; and (j) intersystem crossing. Structures represented with the MACMOLPLT program (Ref. 42).

Singlet and triplet MS-CASPT2/ANO-L linear interpolations ( symmetry). The geometry of the initial point corresponds to the minimum of nitromethane, and the geometry of the end point corresponds to (a) intersystem crossing; (b) intersystem crossing; (c) intersystem crossing. Two roots have been included in each symmetry block for the MS-CASPT2 calculations, excepting the state which has been computed using one root. The lower graphics at the right of each principal figure represent the magnitude of the spin–orbit coupling constants along the respective interpolation curve.

Singlet and triplet MS-CASPT2/ANO-L linear interpolations ( symmetry). The geometry of the initial point corresponds to the minimum of nitromethane, and the geometry of the end point corresponds to (a) intersystem crossing; (b) intersystem crossing; (c) intersystem crossing. Two roots have been included in each symmetry block for the MS-CASPT2 calculations, excepting the state which has been computed using one root. The lower graphics at the right of each principal figure represent the magnitude of the spin–orbit coupling constants along the respective interpolation curve.

MS-CASPT2/ANO-L linear interpolations ( symmetry): (a) from the Franck–Condon point to the intersystem crossing; (b) from the intersystem crossing to dissociation into . Four roots have been included in the MS-CASPT2 calculations.

MS-CASPT2/ANO-L linear interpolations ( symmetry): (a) from the Franck–Condon point to the intersystem crossing; (b) from the intersystem crossing to dissociation into . Four roots have been included in the MS-CASPT2 calculations.

Energy-level diagrams of all of the critical points involved in the photochemistry of nitromethane. (a) Single states. (b) Triplet states.

Energy-level diagrams of all of the critical points involved in the photochemistry of nitromethane. (a) Single states. (b) Triplet states.

## Tables

MS-CASPT2 energies of the critical points of nitromethane on the singlet surfaces. Zero point energy correction is included. Absolute reference energy, hartree.

MS-CASPT2 energies of the critical points of nitromethane on the singlet surfaces. Zero point energy correction is included. Absolute reference energy, hartree.

MS-CASPT2 energies of the critical points of nitromethane on the triplet surfaces. Zero point energy correction is included. Absolute reference energy, hartree.

MS-CASPT2 energies of the critical points of nitromethane on the triplet surfaces. Zero point energy correction is included. Absolute reference energy, hartree.

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