High-resolution infrared spectrum and analysis of thev₁₁, A2u(B₂) fundamental band of¹²C₆H₆and¹³C¹²C₅H₆

High-resolution interferometric Fourier-transform infrared (instrumental bandwidth 0·0024 cm-1) and diode-laser (0·001 cm-1) spectra have been taken for the natural isotopic mixture of benzene in the range 640–710 cm-1. Accurate rotational-vibrational constants have been obtained for the fundamental v 11 (673·97465 cm-1) of 12C6H6 and for 13C12C5H6 = 673·6092 cm-1). Three hot bands have been analysed for 12C6H6 (v 11 + v 6 - v 6, 673·50873 cm-1; v 11 + v 16 - v 16, 672·87601 cm-1; and 2v 11 - v 11, 674·1021 cm-1). The results are discussed in relation to anharmonic and rotational-vibrational interaction constants and to force-field calculations. An analysis of available data for the ground-state rotational constant B 0 is presented, which shows rather large systematic discrepancies. All high-resolution results are consistent with B 0 = 5689·16 ± 0·3 MHz, where the absolute error estimate is 50 times the standard deviation from the present, most precise determination.