^{1,a)}, Juan F. Arenas

^{1}, Juan C. Otero

^{1}and Juan Soto

^{1,b)}

### Abstract

Photodissociation mechanisms of nitrosamine have 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 potential energy curves connecting them have been recomputed with the multiconfigurational second-order perturbation method. Ground state minimum of nitrosamine has a nonplanar structure with the hydrogen atoms of the amino moiety out of the plane defined by the N–N–O bonds. Electronic transitions to the three lowest states are allowed by selection rules: (i) has an oscillator strength of and it is assigned as an transition, (ii) has an oscillator strength of and it is assigned as an transition, and (iii) has an oscillator strength of and it is assigned as an transition. It is found that N–N bond cleavage is the most likely process in all the photochemical relevant states, namely, , , and . While and yield exclusively homolytic dissociation: , on the latter process constitutes the major path, but two additional minor channels are also available: adiabatic homolytic dissociation: , and adiabatic oxygen extrusion: . The excited species experiences a subsequent ultrafast decay to the ground state, the final products in all cases the fragments being in their lowest electronic state. We have not found a unimolecular mechanism connecting excited states with the ground state. In addition, homolytic dissociation in the ground state, tautomerizations to NHNOH and NHNHO, and intersystem crossings to are considered. The most favorable process on this state is the isomerization to NHNOH.

This research has been supported by the Spanish Ministerio de Educación y Ciencia (Project No. BQU2003-1426). One of the authors (D.P.) thanks the Spanish Ministerio de Educación y Ciencia for Grant No. BES-2004-6033. The authors thank Professor Dr. J. Márquez and Dr. F. J. Alonso from the Biochemistry Department for fruitful discussions on the biochemistry of nitrosamines.

I. INTRODUCTION

II. METHODS OF CALCULATION

III. RESULTS AND DISCUSSION

A. Vertical transitions

B. Photodissociation of nitrosamine into and NO on the lowest-lying valence singlet surfaces

C. Homolytic dissociation of nitrosamine on ground state and tautomerizations to hydroxydiimide (NHNOH) and diimide-N-oxide (NHNHO)

D. Triplet state surfaces

IV. SUMMARY

### Key Topics

- Dissociation
- 40.0
- Ground states
- 26.0
- Dissociation energies
- 16.0
- Oscillators
- 15.0
- Photochemistry
- 12.0

## Figures

Relevant critical points of nitrosamine on the singlet surfaces. The arrows in the smaller figures correspond to (i) stationary points: imaginary modes, (ii) surface crossings: gradient difference and nonadiabatic coupling vectors; (a) minimum (Ml); [(b) and (c)] transition states for the NO rotation around the N–N bond (Sd1 and Sd2); (d) first order saddle point (Sd3); (e) first order saddle point (Sd4); (f) transition state for the N–N bond cleavage (Sd5); (g) conical intersection (Ci1); (h) minimum (M2); (i) minimum (M3); (j) first order saddle point (Sd6); (k) conical intersection (Ci2); (l) conical intersection (Ci2b); (m) conical intersection (Ci3); (n) conical intersection (Ci4); (o) intersystem crossing (Isc1).

Relevant critical points of nitrosamine on the singlet surfaces. The arrows in the smaller figures correspond to (i) stationary points: imaginary modes, (ii) surface crossings: gradient difference and nonadiabatic coupling vectors; (a) minimum (Ml); [(b) and (c)] transition states for the NO rotation around the N–N bond (Sd1 and Sd2); (d) first order saddle point (Sd3); (e) first order saddle point (Sd4); (f) transition state for the N–N bond cleavage (Sd5); (g) conical intersection (Ci1); (h) minimum (M2); (i) minimum (M3); (j) first order saddle point (Sd6); (k) conical intersection (Ci2); (l) conical intersection (Ci2b); (m) conical intersection (Ci3); (n) conical intersection (Ci4); (o) intersystem crossing (Isc1).

Energy profiles (MS-CASPT2) of the main dissociation channels of nitrosamine on the relevant low-lying singlet states. Initial and final geometries are represented on top of the vertical axis: (a) minimum (M1) to ; (b) dissociation transition state (Sd5) to ; (c) minimum (M1) to ; (d) minimum (M3) to . The electronic state of the products is presented at the end of the potential energy curves. The character of the states is indicated at the beginning of the curves.

Energy profiles (MS-CASPT2) of the main dissociation channels of nitrosamine on the relevant low-lying singlet states. Initial and final geometries are represented on top of the vertical axis: (a) minimum (M1) to ; (b) dissociation transition state (Sd5) to ; (c) minimum (M1) to ; (d) minimum (M3) to . The electronic state of the products is presented at the end of the potential energy curves. The character of the states is indicated at the beginning of the curves.

Critical points of nitrosamine related to its tautomers. The arrows in the smaller figures correspond to (i) stationary points: imaginary modes, (ii) surface crossings: gradient difference and nonadiabatic coupling vectors; (a) *cis*-hydroxydiimide minimum on (M4); (b) *trans*-hydroxydiimide minimum on (M5); (c) transition state for the 1,3-hydrogen shift (Sd7); (d) conical intersection (Ci5); (e) diimide-N-oxide minimum on (M6); (f) transition state for the 1,2-hydrogen shift (Sd8); (g) first order saddle point (Sd9); (h) diimide-N-oxide minimum on (M7).

Critical points of nitrosamine related to its tautomers. The arrows in the smaller figures correspond to (i) stationary points: imaginary modes, (ii) surface crossings: gradient difference and nonadiabatic coupling vectors; (a) *cis*-hydroxydiimide minimum on (M4); (b) *trans*-hydroxydiimide minimum on (M5); (c) transition state for the 1,3-hydrogen shift (Sd7); (d) conical intersection (Ci5); (e) diimide-N-oxide minimum on (M6); (f) transition state for the 1,2-hydrogen shift (Sd8); (g) first order saddle point (Sd9); (h) diimide-N-oxide minimum on (M7).

MS-CASPT2 potential energy profiles comparison between the adiabatic lowest energy channels on the ground state surface based on CAS-SCF(16,12) reference wave functions. The products are presented at the end of the correspondent curve. Transition states, if applicable, are indicated.

MS-CASPT2 potential energy profiles comparison between the adiabatic lowest energy channels on the ground state surface based on CAS-SCF(16,12) reference wave functions. The products are presented at the end of the correspondent curve. Transition states, if applicable, are indicated.

Relevant critical points of nitrosamine on the first excited triplet state. The arrows in the smaller figures correspond to (i) stationary points: imaginary modes, (ii) surface crossings: gradient difference vectors; (a) minimum (M8); (b) minimum (M9); (c) nitrene minimum (M10); (d) second order saddle point (Sd10); [(e) and (f)] intersystem crossings (Isc2 and Isc3).

Relevant critical points of nitrosamine on the first excited triplet state. The arrows in the smaller figures correspond to (i) stationary points: imaginary modes, (ii) surface crossings: gradient difference vectors; (a) minimum (M8); (b) minimum (M9); (c) nitrene minimum (M10); (d) second order saddle point (Sd10); [(e) and (f)] intersystem crossings (Isc2 and Isc3).

MS-CASPT2 interpolations between M1 and the intersystem crossings: (a) Isc2; (b) Isc3. Initial and final geometries are represented on top of the vertical axis. The electronic state of the fragments is indicated at the end of each of the curves. The character of the states is indicated at the beginning of the curves.

MS-CASPT2 interpolations between M1 and the intersystem crossings: (a) Isc2; (b) Isc3. Initial and final geometries are represented on top of the vertical axis. The electronic state of the fragments is indicated at the end of each of the curves. The character of the states is indicated at the beginning of the curves.

Energy diagram of the relevant critical points and chemical paths of nitrosamine photochemistry. stands for vertical transition, , for minimum, , for saddle point, , for conical intersection, and , for intersystem crossing, is an ordinal. Between parentheses (brackets) is indicated the relative energy value of the critical point (Franck-Condon point) in kcal/mol.The horizontal dotted lines represent the energy level of the correspondent vertical transition. The vertical arrows represent the main electronic absorptions and their wideness displays qualitatively the magnitude of the oscillator strength.

Energy diagram of the relevant critical points and chemical paths of nitrosamine photochemistry. stands for vertical transition, , for minimum, , for saddle point, , for conical intersection, and , for intersystem crossing, is an ordinal. Between parentheses (brackets) is indicated the relative energy value of the critical point (Franck-Condon point) in kcal/mol.The horizontal dotted lines represent the energy level of the correspondent vertical transition. The vertical arrows represent the main electronic absorptions and their wideness displays qualitatively the magnitude of the oscillator strength.

## Tables

MS-CASPT2 energies of the singlet and triplet vertical transitions of nitrosamine.

MS-CASPT2 energies of the singlet and triplet vertical transitions of nitrosamine.

MS-CASPT2 energies of the critical points of nitrosamine relevant to its photodecomposition on singlet states.

MS-CASPT2 energies of the critical points of nitrosamine relevant to its photodecomposition on singlet states.

MS-CASPT2 energies of the critical points and their corresponding vertical transitions of the structures related to nitrosamine tautomers on the lowest relevant potential energy surfaces.

MS-CASPT2 energies of the critical points and their corresponding vertical transitions of the structures related to nitrosamine tautomers on the lowest relevant potential energy surfaces.

MS-CASPT2 energies of the relevant critical points on the first triplet excited state of nitrosamine.

MS-CASPT2 energies of the relevant critical points on the first triplet excited state of nitrosamine.

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