The bending-rotation spectrum of fulminic acid and deuterofulminic acid

Abstract

A quasilinear molecular model is needed to account for the infrared absorption spectrum of HCNO and DCNO in the spectral region from 100 cm −1 to 1000 cm −1. The observed systems of infrared bands arising from the ν 5 vibrational manifold have all been assigned. The rotational structure of the absorption bands at 225 cm −1, 275 cm −1, 315 cm −1, and 317 cm −1 for HCNO has been resolved using a Fourier spectrometer. The rotational constants and the band centers have been determined for the above bands, which represent the transitions (0000 01 1) c ←0000 00 0 (0000 02 2) c,d ←(0000 01 1) c,d both components (0000 03 3) c,d ←(0000 02 2) c,d 0000 02 0←(0000 01 1) c . By means of the Ritz combination principle the infrared transitions could be used to build up the vibrational energy level scheme of the ν 5 vibrational mode for HCNO and DCNO. The data are only reconcilable with a potential function for ν 5 which exhibits a low barrier opposing linearity. Preliminary values of the potential parameters were obtained using different approximate theoretical approaches. A reinterpretation of the r 8 structure parameters of fulminic acid in the light of the quasilinear model leads to an explanation of the extraordinarily short CH internuclear distance of 1.027 Å as the projection of a CH bond length of 1.060(5) Å upon the heavy-atom axis. The isotopic shift upon deuteration observed in the infrared data indicate that the ν 5 fundamental vibration is primarily an HCN bending motion. The ν 4 fundamental vibration (skeletal bending motion) of HCNO is located at 537 cm −1 and does not exhibit any hot band structure which would be indicative of a perturbed potential function.