Gas-phase infrared spectrum of indazole. Scaled quantum mechanical force field and complete spectrum assignment

The gas-phase IR spectrum of indazole has been recorded from 100 to 4000 cm-1, using a multipass cell heated to 120 °C, and completely assigned using theoretical predictions based on the scaled quantum mechanical (SQM) method. The single-crystal IR spectrum of this molecule, previously reported, has been compared with our data and partially reassigned. The harmonic force field of indazole, evaluated at the HF-SCF level using 6-31G** orbitals, is corrected by scaling the force field over a convenient set of internal coordinates. Scaling factors were determined by least-squares fitting of the theoretical to the experimental frequencies of two parent molecules, benzene and pyrazole and their perdeuteriated isotopomers. Our final prediction gives frequencies for indazole which, on average, differ from experiment by 24 cm-1. We confirm the validity of the SQM method as a practical tool for a complete analysis of vibrational spectra, even for molecules of this complexity.