High-resolution infrared spectra of bicyclo[1.1.1]pentane

Infrared spectra of bicyclo[1.1.1]pentane (C5H8) have been recorded at a resolution (0.0015 cm−1) sufficient to resolve for the first time individual rovibrational lines. This initial report presents the ground state constants for this molecule determined from the detailed analysis of three of the ten infrared-allowed bands, ν14(e′) at 540 cm−1, ν17 (a2″) at 1220 cm−1, ν18(a2″) at 832 cm−1, and a partial analysis of the ν11(e′) band at 1237 cm−1. The upper states of transitions involving the lowest frequency mode, ν14(e′), show no evidence of rovibrational perturbations but those for the ν17 and ν18 (a2″) modes give clear indication of Coriolis coupling to nearby e′ levels. Accordingly, ground state constants were determined by use of the combination-difference method for all three bands. The assigned frequencies provided over 3300 consistent ground state difference values, yielding the following constants for the ground state (in units of cm−1): B0 = 0.2399412(2), DJ = 6.024(6) × 10−8, DJK = −1.930(21) × 10−8. For the unperturbed ν14(e′) fundamental, more than 3500 transitions were analyzed and the band origin was found to be at 540.34225(2) cm−1. The numbers in parentheses are the uncertainties (two standard deviations) in the values of the constants. The results are compared with those obtained previously for [1.1.1]propellane and with those computed at the ab initio anharmonic level using the B3LYP density functional method with a cc-pVTZ basis set.