Although the electronic structure according to valence bond theory can be written as H−N=C=O, the vibrational spectrum has a band at 2268.8 cm−1 in the gas phase, which some say indicates a carbon–nitrogen triple bond.[9][10] If so, then the canonical form H−N+≡C−O− is the major resonance structure.
However, classic vibrational analysis would indicate that the 2268.8 cm−1 is the asymmetric N=C=O stretch, as per Colthup et al.,[11] as well as the NIST Chemistry WebBook,[12] which also reports the corresponding symmetric N=C=O stretch (weak in infrared, but strong in Raman) to be 1327 cm−1. Based on these classic assignments, there is no need to invoke a full charged state for the N and O atoms, to explain the vibrational spectral data.
The tautomer, known as cyanic acid, HOCN, in which the oxygen atom is protonated exists in equilibrium with isocyanic acid to the extent of about 3%.[citation needed][dubious – discuss] The vibrational spectrum is indicative of the presence of a triple bond between the nitrogen and carbon atoms.[13]
At high concentrations, isocyanic acid oligomerizes to give the trimercyanuric acid and cyamelide, a polymer. These species usually are easily separated from liquid- or gas-phase reaction products.
Isocyanic acid reacts with amines to give ureas (carbamides):
HNCO + RNH2 → RNHC(O)NH2
This reaction is called carbamylation.
HNCO adds across electron-rich double bonds, such as vinylethers, to give the corresponding isocyanates.
Isocyanic acid, HNCO, is a Lewis acid whose free energy, enthalpy and entropy changes for its 1:1 association with a number of bases in carbon tetrachloride solution at 25 °C have been reported.[15] The acceptor properties of HNCO are compared with other Lewis acid in the ECW model.
Low-temperature photolysis of solids containing HNCO creates the tautomer cyanic acid H−O−C≡N, also called hydrogen cyanate.[16] Pure cyanic acid has not been isolated, and isocyanic acid is the predominant form in all solvents.[14] Sometimes information presented for cyanic acid in reference books is actually for isocyanic acid.[citation needed]
^Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN0-07-049439-8
^ abWilliam R. Martin and David W. Ball (2019): "Small organic fulminates as high energy materials. Fulminates of acetylene, ethylene, and allene". Journal of Energetic Materials, volume 31, issue 7, pages 70-79. doi:10.1080/07370652.2018.1531089
^"Isocyanic acid". National Institute of Standards and Technology (U.S. Department of Commerce). Retrieved 2023-04-20.
^Teles, Joaquim Henrique; Maier, Günther; Andes Hess, B.; Schaad, Lawrence J.; Winnewisser, Manfred; Winnewisser, Brenda P. (1989). "The CHNO Isomers". Chem. Ber. 122 (4): 1099–0682. doi:10.1002/cber.19891220425.
^ abA. S. Narula, K. Ramachandran “Isocyanic Acid” in Encyclopedia of Reagents for Organic Synthesis, 2001, John Wiley & Sons, New York. doi:10.1002/047084289X.ri072m Article Online Posting Date: April 15, 2001.
^Nelson, J. (1970) Hydrogen-bonded complexes of isocyanic acid: Infrared spectra and thermodynamic measurements. Spectrochimica Acta Part A: Molecular Spectroscopy 26,109-120.
^Jacox, M.E.; Milligan, D.E. (1964). "Low-Temperature Infrared Study of Intermediates in the Photolysis of HNCO and DNCO". Journal of Chemical Physics. 40 (9): 2457–2460. Bibcode:1964JChPh..40.2457J. doi:10.1063/1.1725546.
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