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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...

The geometry of the Fabry–Perot cavity makes it difficult to use for measuring the Stark effect. A “Stark cage” is described which generates an electric field suitable for this purpose. The cage is used to measure first and second order Stark splittings of several low-J transitions of the benzene-water dimer previously reported [Gutowsky, Emilsson, and Arunan, J. Chem. Phys. 99, 4883 (1993)]. The dipole moment is found to depend somewhat on rotational state, ranging from 1.65 to 2.00 Debye for both ground m=0 and first excited m=1 internal rotation states of the dimer. Additional m=1 transitions are reported, including the previously missing downshifted line of a k=0′ doublet. Its presence and various Stark effects require reassignment of the m=1 spectrum. The results demonstrate that each J→J+1 spectrum consists of three distinct components which arise from the H2O in an unusual way. In addition to the k-doublets, there are two progressions; a set of (J+1) negative k’s running from −J to 0, and a set of ...

Correlated ab initio calculations of the electrical properties of a large set of organic molecules have been carried out and used in the selection of transferable parameters in an additive model of electrical response. The set of parameters of the model consists of anisotropic atomic dipole polarizabilities for H, C, N, O, and F atoms in different bonding environments and bond dipole moments. The model uses a distribution of atomic dipole polarizabilities and bond dipoles according to the bonding and structure of a molecule. The model describes a molecule's long-range electrostatic potential and the change in that potential from polarization by an external electrostatic potential. To very good accuracy, it yields mean molecular dipole polarizabilities and magnitudes of dipole moments, and to good accuracy, it yields dipole polarizability anisotropies and dipole moment orientations. The capability of the model reinforces the idea that to a good extent molecular polarization is tied to electronic structure ...

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.

A comparative study has been made of the rotational properties of C6H6–HCN and Ar3–HCN, observed with the Balle/Flygare pulsed beam, Fourier transform microwave spectrometer. C6H6–HCN is found to be a prolate symmetric top and Ar3–HCN an oblate one, both with the H in the middle. The rotational constants B0, DJ, and DJK of the parent species are 1219.9108(4) MHz, 1.12(3) kHz, and 18.32(8) kHz for C6H6–HCN, and 886.4878(1) MHz, 10.374(2) kHz, and 173.16(1) kHz for Ar3–HCN. Rotational constants are reported for the isotopic species C6H6–H13CN, ‐HC15N, and 13CC5H6–HC15N, and for Ar3–HC15N and ‐DCN. Analysis of the 14N hyperfine interaction χ finds its projection on the figure axis to be −4.223(4) MHz in C6H6–HCN and −1.143(2) in Ar3–HCN. They correspond to average projection angles θ between the HCN and figure axes of 15.2° and 45.3°, respectively. A pseudodiatomic analysis of the rotational constants gives the c.m. to c.m. distance to be 3.96 A in C6H6–HCN and 3.47 A in Ar3–HCN. While the rotational propert...

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 rotational spectrum of Ar2–HCN has been observed between 2.5 and 11.5 GHz with the pulsed nozzle, Fourier transform, Balle/Flygare Mark II microwave spectrometer. Eighteen transitions were found and their 14N quadrupole hyperfine structure analyzed. The line centers were fitted with the Watson Hamiltonian giving ground state rotational constants of 1769.366, 1743.854, and 857.600 MHz. The effective geometry of the cluster is found to be T‐shaped with C2v symmetry and the H end of the HCN closest to the Ar2. However, the rms deviation of the fit is poor (300 kHz), the centrifugal distortion and inertial defect are huge, the Ar to HCN c.m. distance is nearly 0.2 A shorter than in the Ar–HCN dimer, and the average angular displacement of the HCN from the C2 axis is both large (39°) and highly anisotropic (10°). In contrast, the Ar2 subunit exhibits an in‐plane, average angular displacement of only 6°. These anomalies led us to calculate potential surfaces for Ar2–HCN and Ar2–HF using the molecular mechan...

Rotational spectra of several isotopomers of Ar3–H2O and Ar3–H2S tetramers were obtained with a Balle–Flygare Fourier transform microwave spectrometer. Both were found to be symmetric tops, the former being an oblate and the latter a prolate one. The rotational constants B, Dj, and DJK were determined to be 1172.1323(1) MHz, 7.199(1) kHz, and –5.545(2) kHz for the H2O and 819.0385(1) MHz, 3.346(1) kHz, and +3.145(2) kHz for the H2S containing tetramer. Substitution analysis with the rotational constants of various isotopomers led to an Ar–Ar distance of 3.848 (3.865) A and an Ar-c.m.(H2X) distance of 3.675 (4.112) A for H2O (H2S) complexes. The angle between the C2 axis of the H2X and the C3 axis of the tetramer is estimated to be 74° for H2O and 13° for the H2S complex. No evidence for any excited tunneling/internal rotor states was found for either of the tetramer. MMC calculations show that the equilibrium geometry has the H2X positioned above the plane of the Ar3 with both the protons pointing towards one Ar each. The barrier for the "pseudorotation" in which the protons hop between the argons is determined to be about 6 (8) cm–1 only for H2O (H2S) making the H2X moiety very mobile and effectively making both the tetramers symmetric tops. Rigid body diffusion quantum Monte Carlo (RBDQMC) calculations with the MMC potential have been carried out for vibrational analysis.

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...