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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 Å. 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.

Ab initio calculations were carried out for dimers of acetylene and a polyyne chain with two to five triple bonds at the MP2 level. The structure and energies of several symmetry-constrained structures were found and used to guide refinement of a model potential that could be applied to longer polyyne chains. The slice of the intermolecular potential surface for sliding an acetylene along the polyyne chain in a T-shaped orientation was mapped and showed shallow corrugation over a large region. This amounts to slipperiness in this type of weak bonding.

An interaction potential previously developed for the acetylene dimer was applied to small and large clusters of acetylene molecules. With the inclusion of the effects of vibrational dynamics via q...

A weak interaction model has been used to examine the potential energy surfaces of weakly bound dimers, that include a hydrogen sulfide molecule. The comparison with corresponding water clusters shows certain structural similarities but significantly different energetics. Interconversion barriers are mostly smaller for H 2 S complexes than for the corresponding water complexes. This is attributed in part to the size of sulfur relative to oxygen, leading to greater equilibrium separations from bonding partners and thereby decreased electrical stabilization. Values are reported for rotational constants, dipole moments, and interconversion barriers.

Rotational transitions of the (H2O–HCN)–Ar trimer have been measured at 3–17 GHz with the Balle/Flygare Mark II pulsed nozzle FT microwave spectrometer for the parent, 18O, 13C, 15N, D2O, and HDO isotopic species. The isotopomers exhibit both a‐ and b‐dipole transitions with 14N hyperfine structure and all but the HDO have two sets of transitions assigned to 000 and 101 internal rotational states of the H2O or D2O. Rotational constants were determined by fitting the line centers separately for each isotopic set to the Watson Hamiltonian for an asymmetric top. A molecular mechanics for clusters (MMC) calculation of the potential energy surface and an approximate substitution analysis of the rotational constants give a nearly planar, Δ‐shaped structure which is a somewhat distorted superposition of the H2O–HCN, H2O–Ar, and Ar–HCN dimers. MMC also gives a barrier of ≲25 cm−1 to internal rotation of the H2O. Factors governing the formation of trimers are discussed. The effects on trimer structure of differences in the pair interactions are found to be appreciable while the role of three‐body effects is small.

The previously reported rotational spectrum of Ar–HCN [J. Chem. Phys. 81, 4922 (1984)] has shown the weakly bound dimer to be highly nonrigid. Superficially linear, the dimer has several anomalies, including large centrifugal distortion and an unexpectedly large bending amplitude of the HCN. We here describe high‐resolution rotational spectra which identify another anomaly. The 14N hyperfine interaction constant of the dimer increases linearly with J(J+1) for Ar–HCN, 36Ar–HCN, and Ar–DCN, indicating a decrease in the average HCN bending amplitude (θ). For Ar–HCN this is from 30.97° for J=0 to 30.17° for J=5. At the same time, there is an increase in the average Ar to HCN c.m. separation R from 4.3433 to 4.3496 Ar. The cause of this behavior and of the other anomalies is found to be the shape of the potential function as calculated with a largely classical electrical model employing low‐order moments and multipole polarizabilities. The calculated potential surface exhibits strong coupling between radi...