Perpendicular bands of cyclopropane in the 3.5 μm region

The spectrum of the cyclopropane molecule was measured in the region 2898.5 to 3157.3 cm^-1 with Doppler-limited resolution using a difference frequency laser spectrometer. Deconvolution was used to further enhance the effective resolution in the main part of the highly perturbed spectrum. The region below ∼3060 cm^-1 contains perpendicular bands, including the ν₈ fundamental of species E′ in close anharmonic resonance with the E′ component of the state ν₇ + ν₉ + ν₁₄ and with other E′ states, and the ν₂ + ν₉ band which borrows its intensity from ν₈ via Fermi resonance. The ν₂ + ν₉ E′ state is also perturbed by anharmonic interactions with additional E′ states. A comprehensive analysis of this complex system of perpendicular bands based on a Hamiltonian model of the main interactions yielded much improved spectroscopic constants for the ν₈ fundamental, for the ν₂ + ν₉ combination, and for the newly observed ν₇ + ν₉ + ν₁₄ state, as well as information on seven tentatively identified perturbing states.