The NCN diradical is an important intermediate of prompt nitric oxide formation in flames. The mechanism of intersystem crossing (ISC) in the NCN molecule formed via pyrolysis or photolysis of NCN3 is of relevance to the interpretation of experiments that utilize NCN3 as a precursor for laboratory studies of NCN kinetics. This mechanism has been investigated by means of multi-reference configuration interaction calculations. From the potential energy surfaces for NCN3 dissociation, it was inferred that both thermal and photo-chemical decomposition initially lead to NCN in its lowest singlet state, $$\tilde{a}^{-1}$$$$Δ_g$$, with a possible contribution from the $$\tilde{b}^{-1}\Sigma_g^+$$ state at low photolysis wavelengths. Direct formation of the triplet ground state $$\tilde{X}^{-3}\Sigma_g^-$$ is also feasible for the photolytic pathway. Ananalysis of surface crossings between $$\tilde{a}$$ or $$\tilde{b}$$ and the triplet ground state $$\tilde{X}^{-3}\Sigma_g^-$$ in the absence and presence of a helium atom revealed an ISC channel 1NCN ($$\tilde{a}$$) → 3NCN($$\tilde{X}$$) via a strongly bent structure. However, its barrier of 38 kcal mol-1 relative to the singlet minimum turned out to be much too high to explain the fast ISC observed in experiments. A rigid-bender model including Renner-Teller interactions was used to examine the occurrence of mixed-multiplicity rovibrational states-so-called gateway states-that could enhance collision-induced ISC. The results of this study indicate that a gateway mechanism is probably not operative in the case of the $$\tilde{a}$$/$$\tilde{X}$$pair of states in NCN.
Pfeifle, Mark, et al. "Theoretical investigation of intersystem crossing in the cyanonitrene molecule, <sup>1</sup>NCN → <sup>3</sup>NCN." Journal of Chemical Physics, vol. 147, no. 8, Aug. 2017. https://doi.org/10.1063/1.4999788
Pfeifle, Mark, Georgievskii, Yuri, Jasper, Ahren W., & Klippenstein, Stephen J. (2017). Theoretical investigation of intersystem crossing in the cyanonitrene molecule, <sup>1</sup>NCN → <sup>3</sup>NCN. Journal of Chemical Physics, 147(8). https://doi.org/10.1063/1.4999788
Pfeifle, Mark, Georgievskii, Yuri, Jasper, Ahren W., et al., "Theoretical investigation of intersystem crossing in the cyanonitrene molecule, <sup>1</sup>NCN → <sup>3</sup>NCN," Journal of Chemical Physics 147, no. 8 (2017), https://doi.org/10.1063/1.4999788
@article{osti_1393554,
author = {Pfeifle, Mark and Georgievskii, Yuri and Jasper, Ahren W. and Klippenstein, Stephen J.},
title = {Theoretical investigation of intersystem crossing in the cyanonitrene molecule, <sup>1</sup>NCN → <sup>3</sup>NCN},
annote = {The NCN diradical is an important intermediate of prompt nitric oxide formation in flames. The mechanism of intersystem crossing (ISC) in the NCN molecule formed via pyrolysis or photolysis of NCN3 is of relevance to the interpretation of experiments that utilize NCN3 as a precursor for laboratory studies of NCN kinetics. This mechanism has been investigated by means of multi-reference configuration interaction calculations. From the potential energy surfaces for NCN3 dissociation, it was inferred that both thermal and photo-chemical decomposition initially lead to NCN in its lowest singlet state, $\tilde{a}^{-1}$$Δ_g$, with a possible contribution from the $\tilde{b}^{-1}\Sigma_g^+$ state at low photolysis wavelengths. Direct formation of the triplet ground state $\tilde{X}^{-3}\Sigma_g^-$ is also feasible for the photolytic pathway. Ananalysis of surface crossings between $\tilde{a}$ or $\tilde{b}$ and the triplet ground state $\tilde{X}^{-3}\Sigma_g^-$ in the absence and presence of a helium atom revealed an ISC channel 1NCN ($\tilde{a}$) → 3NCN($\tilde{X}$) via a strongly bent structure. However, its barrier of 38 kcal mol-1 relative to the singlet minimum turned out to be much too high to explain the fast ISC observed in experiments. A rigid-bender model including Renner-Teller interactions was used to examine the occurrence of mixed-multiplicity rovibrational states-so-called gateway states-that could enhance collision-induced ISC. The results of this study indicate that a gateway mechanism is probably not operative in the case of the $\tilde{a}$/$\tilde{X}$pair of states in NCN.},
doi = {10.1063/1.4999788},
url = {https://www.osti.gov/biblio/1393554},
journal = {Journal of Chemical Physics},
issn = {ISSN 0021-9606},
number = {8},
volume = {147},
place = {United States},
publisher = {American Institute of Physics (AIP)},
year = {2017},
month = {08}}