CHNO - Formylnitrene, Cyanic, Isocyanic, Fulminic, and Isofulminic Acids and their Interrelationships at DFT and CASPT2 Levels of Theory

Didier Bégué; William Lafargue-Dit-Hauret; Alain Dargelos; Curt Wentrup

doi:10.1021/acs.jpca.3c05805

CHNO - Formylnitrene, Cyanic, Isocyanic, Fulminic, and Isofulminic Acids and their Interrelationships at DFT and CASPT2 Levels of Theory

J Phys Chem A. 2023 Nov 2;127(43):9088-9097. doi: 10.1021/acs.jpca.3c05805. Epub 2023 Oct 24.

Authors

Didier Bégué¹, William Lafargue-Dit-Hauret¹, Alain Dargelos¹, Curt Wentrup²

Affiliations

¹ CNRS/Université de Pau et des Pays de l'Adour/E2S UPPA, Institut des Sciences Analytiques et de Physicochimie pour l'Environnement et les Matériaux, UMR5254, 64000, Pau, France.
² School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland 4072, Australia.

PMID: 37875391
DOI: 10.1021/acs.jpca.3c05805

Abstract

Fulminic and cyanic acids played a decisive role in the conception of isomerism 200 years ago. Cyanic (HOCN), isocyanic (HNCO), and fulminic (HCNO) acids have been detected in several interstellar sources, but isofulminic acid (HONC) is little known. Here we examine the interrelationships between the four acids and formylnitrene, HC(O)N, at the CASPT2 and three DFT levels. Formylnitrene has a triplet ground state, T₀, a closed shell singlet (CSS), S₀, and an open-shell singlet (OSS), S₁, lying ∼7 and 27 kcal/mol above T₀, respectively. The CSS is weakly stabilized by a 12 kcal/mol bond between the N and the O atoms. A conical intersection 12 kcal/mol above T₀ permits easy T₀-S₀ interchange. Formyl azide and formylnitrene (T₀ and S₀) are isomerized thermally to HNCO. HOCN is best obtained via dissociation of the nitrene (or of HNCO) to H^• + NCO^• radicals ∼46 kcal/mol above the T₀ nitrene. Isofulminic acid, HONC, isomerizes readily to cyanic acid, HOCN, in thermal and photochemical reactions. Fulminic acid, HCNO, can isomerize to HNCO via CSS formylnitrene. Easy tautomerization prevents the preparation of HOCN in quantity. The barrier to isomerization is strongly reduced in small hydrogen-bonded aggregates so that trace amounts of HOCN can exist in equilibrium with HNCO.