Rotational spectrum of the weakly bonded C₆H₆-H₂S dimer and comparisons to C₆H₆-H₂O dimer

Abstract

Two symmetric-top, ΔJ = 1 progressions were observed for the C₆H₆-H₂S dimer using a pulsed nozzle Fourier transform microwave spectrometer. The ground-state rotational constants for C₆H₆-H₂S are B=1168.53759(5)MHz, D_J= 1.4424(7)kHz and D_JK=13.634(2)kHz. The other state observed has a smaller B of 1140.580(1) MHz but requires a negative D_J=−13.80(5)kHz and higher order (H) terms to fit the data. Rotational spectra for the isotopomers C₆H₆-H₂³⁴S, C₆H₆-H₂³³S, C₆H₆-HDS, C₆H₆-D₂S and ¹³CC₅H₆-H₂S were also obtained. Except for the dimer with HDS, all other isotopomers gave two progressions like the most abundant isotopomer. Analysis of the ground-state data indicates that H₂S is located on the C₆ axis of the C₆H₆ with a c.m. (C₆H₆)-S distance of 3.818 Å. The angle between the a axis of the dimer and the C_2v axis of the H₂S is determined to be 28.5°. The C₆ axis of C₆H₆ is nearly coincident with a axis of the dimer. Stark measurements of the two states led to dipole moments of 1.14(2) D for the ground state and 0.96(6) D for the other state. A third progression was observed for C₆H₆–H₂S which appear to have K ≠ 0 lines split by several MHz, suggesting a nonzero projection of the internal rotation angular momentum of H₂S on the dimer a axis. The observation of three different states suggests that the H₂S is rotating in a nearly spherical potential leading to three internal rotor states, two of which have M_j = 0 and one having M_j = ±1,M_j being the projection of internal rotational angular momentum on to the a axis of the dimer. The nuclear quadrupole hyperfine constant of the ³³S nucleus in the dimer is determined for the two symmetric-top progressions and they are -17.11MHz for the ground state and -8.45MHz for the other state, consistent with the assignment to two different internal-rotor states. The 17O quadrupole coupling constant for the two states of C₆H₆-H₂O were measured for comparison and it turned out to be nearly the same in the ground and excited internal rotor state, −1.89 and −1.99MHz, respectively. The rotational spectrum of the C₆H₆-H₂S complex is very different from that of the C₆H₆-H₂O complex. Model potential calculations predict small barriers of 227, 121, and 356cm⁻¹ for rotation about a, b and c axes of H₂S, respectively, giving quantitative support for the experimental conclusion that H₂S is effectively freely rotating in a nearly spherical potential. For the C₆H₆-H₂O complex, the corresponding barriers are 365, 298 and 590cm⁻¹.