Fluorescence-dip infrared spectroscopy of tropolone and tropolone-OD

Abstract

Fluorescence‐dip infrared spectroscopy (FDIRS) is employed to record the infrared spectra of the isolated, jet‐cooled tropolone molecule (TrOH) and its singly deuterated isotopomer TrOD in the O–H and C–H stretch regions. The ability of the method to monitor a single ground‐state level enables the acquisition of spectra out of the lower and upper levels of the zero‐point tunneling doublet free from interference from one another. The high power of the optical parametric oscillator used for infrared generation produces FDIR spectra with good signal‐to‐noise despite the weak intensity of the C–H and O–H stretch transitions in tropolone. The expectation that both spectra will exhibit two OH stretch transitions separated by the OH(v=1) tunneling splitting is only partially verified in the present study. The spectra of TrOH are compared with those from deuterated tropolone (TrOD) to assign transitions due to C–H and O–H, which are in close proximity in TrOH. The appearance of the spectra out of lower (a₁ symmetry) and upper (b₂ symmetry) tunneling levels are surprisingly similar. Two sharp transitions at 3134.9 cm⁻¹ (out of the a₁ tunneling level) and 3133.9 cm⁻¹ (out of the b₂ tunneling level) are separated by the ground‐state tunneling splitting (0.99 cm⁻¹), and thereby terminate in the same upper state tunneling level. Their similar intensities relative to the C–H stretch transitions indicate that the y‐ and z‐polarized transitions are of comparable intensity, as predicted by ab initio calculations. The corresponding transitions to the other member of the upper state tunneling doublet are not clearly assigned by the present study, but the broad absorptions centered about 12 cm⁻¹ below the assigned transitions are suggested as the most likely possibility for the missing transitions.