High-resolution infrared spectra of spiropentane, C 5H 8

Abstract

Infrared spectra of spiropentane (C 5H 8) have been recorded at a resolution (0.002 cm −1) sufficient to resolve for the first time individual rovibrational lines. This initial report presents the ground state rotational constants for this molecule determined from the detailed analysis of the ν 16 ( b 2) parallel band at 993 cm −1. In addition, the determination included more than 2000 ground state combination-differences deduced from partial analyses of four other infrared-allowed bands, the ν 24( e) perpendicular band at 780 cm −1 and three ( b 2) parallel bands at 1540 cm −1 ( ν 14), 1568 cm −1 ( ν 5 + ν 16), and 2098 cm −1 ( ν 5 + ν 14). In each of the latter four cases, the spectra show complications; in the case of ν 24, these complications are due to rotational l-type doublings, and in the case of the parallel bands, the spectral complexities are due to Fermi resonance and Coriolis interactions of the upper states with nearby levels. The unraveling of these is underway but the assignment of many of these transitions permit the confident use of the ground state differences in determining the following constants for the ground state (in units of cm −1): B 0 = 0.1394741(1), D J = 2.461(1) × 10 −8, D JK = 8.69(3) × 10 −8. For the unperturbed ν 16 fundamental, more than 3000 transitions were fit and the band origin was found to be at 992.53793(3) cm −1. The numbers in parentheses are the uncertainties (two standard deviations) in the value of the last digit of the constants. Surprisingly, the very accurate B 0 value measured here is lower than the value (0.1418 cm −1) calculated from an electron diffraction structure, instead of being higher, as expected. Where possible, the rovibrational results are compared with those computed at the anharmonic level using the B3LYP density functional method with a cc-pVTZ basis set. These too suggest that the electron diffraction results are in question.