Isofulminic Acid, HONC: ab initio Theory and Microwave Spectroscopy
Résumé
Isofulminic acid, HONC, the most energetic stable isomer of isocyanic acid HNCO, higher in energy by 84 kcal/mol, has been detected spectroscopically by rotational spectroscopy supported by coupled cluster electronic structure calculations. The fundamental rotational transitions of the normal, carbon-13, oxygen-18, and deuterium isotopic species have been detected in the centimeter band in a molecular beam by Fourier transform microwave spectroscopy, and rotational constants and nitrogen and deuterium quadrupole coupling constants have been derived. The measured constants agree well with those predicted by ab initio calculations. A number of other electronic and spectroscopic parameters of isofulminic acid, including the dipole moment, vibrational frequencies, infrared intensities, and centrifugal distortion constants have been calculated at a high level of theory. Isofulminic acid is a good candidate for astronomical detection with radio telescopes. Because HOCN and HCNO have recently been identified in space, detection of HONC is not out of the realm of possibility. It is calculated to lie only 14 kcal/mol higher in energy than HCNO and is comparably polar, so it may be found in warm regions where HCNO is abundant. If detected in space, the abundance ratios between HONC and other isomers may provide further insight into the complex gas and surface chemistry which is thought to produce these isomers.