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

Two symmetric-top,delta J = 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, DJ = 1.4424(7) kHz and DJK = 13.634(2) kHz. The other state observed has a smaller B of 1140.580(1) MHz but requires a negative DJ = –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 A. The angle between the a axis of the dimer and the C2v 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 K0 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 Mj = 0 and one having Mj = ±1,Mj 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 –17.11 MHz for the ground state and –8.45 MHz 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 –1.99 MHz, 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 356 cm–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 590 cm–1.