Rotational spectrum of the weakly bonded C 6 H 6 -H 2 S dimer and comparisons to C 6 H 6 -H 2 O dimer

Two symmetric-top, ΔJ = 1 progressions were observed for the C 6 H 6 -H 2 S dimer using a pulsed nozzle Fourier transform microwave spectrometer. The ground-state rotational constants for C 6 H 6 -H 2 S 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 C 6 H 6 -H 2 34 S, C 6 H 6 -H 2 33 S, C 6 H 6 -HDS, C 6 H 6 -D 2 S and 13 CC 5 H 6 -H 2 S 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 H 2 S is located on the C 6 axis of the C 6 H 6 with a c.m. (C 6 H 6 )-S distance of 3.818 A. The angle between the a axis of the dimer and the C 2v axis of the H 2 S is determined to be 28.5°. The C 6 axis of C 6 H 6 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 C 6 H 6 –H 2 S which appear to have K ≠ 0 lines split by several MHz, suggesting a nonzero projection of the internal rotation angular momentum of H 2 S on the dimer a axis. The observation of three different states suggests that the H 2 S 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 33 S 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 C 6 H 6 -H 2 O 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 C 6 H 6 -H 2 S complex is very different from that of the C 6 H 6 -H 2 O complex. Model potential calculations predict small barriers of 227, 121, and 356cm −1 for rotation about a, b and c axes of H 2 S, respectively, giving quantitative support for the experimental conclusion that H 2 S is effectively freely rotating in a nearly spherical potential. For the C 6 H 6 -H 2 O complex, the corresponding barriers are 365, 298 and 590cm −1 .