Hot bands in overtone absorption of pyrrole, methyl isocyanide and isobutane: Photoacoustic spectra at 140 C

Abstract

The intracavity laser photoacoustic spectra of the 4νN–H and 4νC–H overtones of pyrrole, the 5νC–H of methyl isocyanide and acetonitrile, the 5νC–H and 6νC–H of isobutane, and the 6νC–H of cyclohexane were measured at temperatures up to 140 C (T140) and compared to the room temperature (Trt) spectra. Hot bands were identified by their temperature dependence in pyrrole, methyl isocyanide, and isobutane. From the temperature dependence of the relative intensities of the hot band compared to the main band, the following average wave numbers of the thermally populated states were deduced: 340, 300, and 208 cm−1 for pyrrole, isobutane, and methyl isocyanide, respectively. These values compare well to the bending motion frequencies in these molecules. Large redshifts of the hot bands from the main peak were observed, as expected for high overtone transitions due to large anharmonicities. The anharmonic constants determined from these shifts are −25 cm−1 for the N–H in pyrrole and −20 cm−1 for the C–H in methyl isocyanide and isobutane. In the case of the N–H absorption the hot band was completely separated from the main transition. In the C–H overtone spectra these hot band absorptions were not separated and result in an apparent increased width of the overtone transition.