Rotational spectra of mono-substituted asymmetric C₆H₆-H₂O dimers

Abstract

This paper reports the assignment of the rotational spectra of the m=0 and 1 states of ¹³CC₅H₆-H₂O and C₆H₅D-H₂O dimers. The m=1 progression was not identified or assigned for both ¹³CC₅H₆-H₂O and C₆H₅D-H₂O in the earlier work, though for the symmetric isotopomers (C₆H₆-H₂O/D₂O/H₂₁₈O), they were identified [H.S. Gutowsky, T. Emilsson, E. Arunan, J. Chem. Phys. 99 (1993) 4883]. The m=1 transitions for ¹³CC₅H₆-H₂O and C₆H₅D-H₂O were split into two, unlike that of the parent C₆H₆-H₂O isotopomer. The splitting varied, somewhat randomly, with quantum numbers J and K. The m = 0 lines of ¹³CC₅H₆-H₂O had significant overlap with the m = 1 lines of the parent isotopomer, clouding proper assignment, and leading to an rms deviation of about 200 kHz in the earlier work. The general semi-rigid molecular Hamiltonian coupled to an internal rotor, described recently by Duan et al. [Y.B. Duan, H.M. Zhang, K. Takagi, J. Chem. Phys. 104 (1996) 3914], is used in this work to assign both m=0 and 1 states of ¹³CC₅H₆-H₂O and C₆H₅D-H₂O dimers. Consequently, the m=0 fits for ¹³CC₅H₆-H₂O/D₂O have an rms deviation of only 4/7 kHz, comparable to experimental uncertainties. The fits for m=1 transitions for ¹³CC₅H₆-H₂O and C₆H₅D-H₂O dimers have an rms deviation of about 200 kHz. However, it is of the same order of magnitude as that of the m=1 state of the parent C₆H₆-H₂O dimer. The A rotational constants determined from the m=0 fits for both ¹³CC₅H₆-H₂O and ¹³CC₅H₆-D₂O isotopomers are identical and very close to the C rotational constant for ¹³CC₅H₆. This provides a direct experimental determination for the C rotational constant of ¹³CC₅H₆, which has a negligible dipole moment.