The ν3 infrared bands of C 12H 4 and C 13H 4

Abstract

The infrared spectrum of 92 mole % C 13H 4 was obtained over the region 2870–3130 cm −1 at a resolution of about 0.3 cm −1 and was compared with the spectrum of C 12H 4. The tetrahedral splitting was partly resolved and the splitting constants were determined by applying the theoretical treatments of Herranz and Hecht. With the aid of a partial analysis of the Q branch, and assuming a value for the ground-state rotational constant B 0, the molecular constants were obtained. Significant changes in going from C 12H 4 to C 13H 4 were observed for the following constants: Δ ν 0 = −9.95 cm −1; Δ B eff Q = +0.0027 cm −1; Δ α 3 = −0.0013 cm −1; Δ ζ 3 = −0.0088; as well as for some of the splitting constants. These changes are discussed and are shown to be generally in agreement with theoretical predictions using previously established force constants and harmonic frequencies. Expected vibrational frequency shifts, Δ ν, are tabulated for a number of vibrational modes; it is shown that analyses of C 13H 4 spectra will be useful in making assignments in the overtone region and may lead to more precise force constants for methane.