High resolution infrared spectroscopy of [1.1.1]propellane

The infrared spectrum of [1.1.1]propellane has been recorded at high resolution (0.002 cm⁻¹) with individual rovibrational lines resolved for the first time. This initial report presents the ground state constants for this molecule determined from the analysis of five of the eight infrared-allowed fundamentals ν₉(e′), ν₁₀(e′), ν₁₂(e′), , as well as of several combination bands. In nearly all cases it was found that the upper states of the transitions exhibit some degree of perturbation but, by use of the combination difference method, the assigned frequencies provided over 4000 consistent ground state difference values. Analysis of these gave for the parameters of the ground state the following values, in cm⁻¹: B₀ = 0.28755833(14), D_J = 1.1313(5) × 10⁻⁷, D_JK = −1.2633(7) × 10⁻⁷, H_J = 0.72(4) × 10⁻¹³, H_JK = −2.24(13) × 10⁻¹³, and H_KJ = 2.25(15) × 10⁻¹³, where the numbers in parentheses indicate twice the uncertainties in the last quoted digit(s) of the parameters. Gaussian ab initio calculations, especially with the computed anharmonic corrections to some of the spectroscopic parameters, assisted in the assignments of the bands and also provided information on the electron distribution in the bridge-head carbon-carbon bond.