Study of vibration-rotation levels in formamide with high resolution FTIR spectroscopy: The ν12, 2ν12, ν11, and ν9 bands

The far infrared and infrared spectra of formamide (HCONH2) have been recorded at high resolution (0.00125 cm−1) in the region of 90-1060 cm−1. Over 20,000 transitions from the out-of-plane NH2 wagging motion (n12 = 1 ← 0 fundamental, n12 = 2 ← 0 overtone, n12 = 2 ← 1 difference bands), torsion (n11 = 1 ← 0 bands), and out-of-phase NCO/NH2 bend (n9 = 1 ← 0 bands) have been assigned. Molecular parameters have been obtained for the ground state and the unperturbed n12 = 1 state. The least-squares fit calculations were completed with the microwave data available in the literature. The complicated resonance system between the n12 = 2, n11 = 1, and n9 = 1 states has been investigated carefully. Thus, we have been able to verify almost all resonances (avoided crossing) existing in the region J, K investigated. In the coupled Hamiltonian used for the fit, all Watson's reduced parameters, including the octic ones and 16 Coriolis coupling parameters were taken into account. The rms deviation obtained from the fit was 0.000247 cm−1.