Journal of Chemical Physics, Vol.117, No.21, 9766-9776, 2002
Rotational spectrum of the weakly bonded C6H6-H2S dimer and comparisons to C6H6-H2O dimer
Two symmetric-top, DeltaJ=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-H-2 34S, C6H6-H-2 33S, C6H6-HDS, C6H6-D2S and (CC5H6)-C-13-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 C-6 axis of the C6H6 with a c.m. (C6H6)-S distance of 3.818 Angstrom. The angle between the a axis of the dimer and the C-2v axis of the H2S is determined to be 28.5degrees. The C-6 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 Knot equal0 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 S-33 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 O-17 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).