The structure of a prototype CH···π system, benzene···acetylene, has been determined in the gas phase using Fourier-transform microwave spectroscopy. The spectrum is consistent with an effective C(6v) structure with an H···π distance of 2.4921(1) Å. The HCCH subunit likely tilts by ~5° from the benzene symmetry axis. The dipole moment was determined to be 0.438(11) D from Stark effect measurements. The observed intermolecular distance is longer than in similar benzene···HX complexes and than the distances observed in the benzene···HCCH cocrystal and predicted by many high level ab initio calculations; however, the experimentally estimated binding energy of 7.1(7) kJ mol(-1) is similar to previously studied benzene···HX complexes. Several additional sets of transitions were observed in the rotational spectrum, likely corresponding to excited states arising from low energy intermolecular vibrational modes of the dimer.

Rotational spectra of two different structural forms of the 1:1 weak complex between vinyl fluoride (C2H3F) and carbon dioxide were measured using 480 MHz bandwidth chirped-pulse and resonant cavity Fourier-transform microwave spectroscopy in the 5-17 GHz region. Both structures have the CO2 molecule situated in the plane of the vinyl fluoride, such that the CO2 is interacting either with a CHF side or with a HC═CF edge of the vinyl fluoride subunit. Both observed structures are close to those predicted by ab initio geometry optimizations (corrected for basis set superposition error) at the MP2/6-311++G(2d,2p) level. Dipole moment measurements and structural fits, including determinations of principal axis coordinates for all three carbon atoms, confirm the geometries of the assigned species.

The microwave spectrum of the dimer Ar-1,3-difluorobenzene from 2 to 18 GHz is reported. The spectrum has been observed using a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer that has recently been expanded to include the 2-6 GHz region of the electromagnetic spectrum. Details of this upgraded spectrometer are reported. Eighty-seven transitions were observed for the parent dimer spectrum, which was adequately fit to a semirigid rotational Hamiltonian consisting of A, B, and C as well as four quartic centrifugal distortion constants. Observations of C species in natural abundance were aided by utilizing smaller chirp ranges of 7-9 and 9-11 GHz for 1.9 million and 3.73 million averages, respectively. Assignment ofC isotopologues allowed for determination of the Kraitchman coordinates of the carbon atoms as well as inertial fits of the complex. The quantum-chemical structure predicts an Ar to monomer center of mass distance of 3.48 Å, compared with 3.564(1) Å determined from experimental structural analysis. This new study indicates that in fluorinated benzene-Ar dimers, when the fluorines are separated by more carbon atoms, the Ar-ring center distance decreases.

Rotational spectra for four isotopologues of the 1:1 weakly bound complex between trifluoroethylene (HFC═CF) and carbon dioxide (CO) were recorded using 480 MHz bandwidth chirped-pulse and resonant cavity Fourier transform microwave spectroscopy between 5.0 and 18.5 GHz. Two planar forms are possible: experimental rotational constants, planar moments, and dipole moment components are consistent with the form in which CO is positioned at the CHF end of the TFE subunit and is approximately perpendicular to the C═C bond; the other form, with CO aligned roughly parallel to the C═C bond, is not observed, consistent with ab initio relative energy predictions. Symmetry-adapted perturbation theory (SAPT) calculations provided interaction energies for possible structural forms of this complex, and comparisons are made with this and other members of the series of carbon dioxide complexes with fluorinated ethylenes (vinyl fluoride, 1,1-difluoroethylene, cis- and trans-1,2-difluoroethylene, and trifluoroethylene).

Rotational spectra for the normal isotopic species and for six additional isotopologues of the 1,2-difluorobenzeneacetylene (C6H4F2HCCH) weakly bound dimer have been assigned in the 6-18 GHz region using chirped-pulse Fourier-transform microwave spectroscopy. This is the third complex in a series of fluorinated benzeneacetylene dimers. In 1,2-difluorobenzeneHCCH, the Hπ distance (2.725(28) Å) is longer by about 0.23 Å, and the estimated binding energy (EB = 2.3(6) kJ mol(-1)) is weaker by about 1.8 kJ mol(-1), than in the previously studied fluorobenzeneHCCH complex. In addition, in 1,2-difluorobenzeneacetylene, HCCH tips ∼46(3)° away from perpendicular to the aromatic ring, with the H nearest the ring moving away from the fluorine atoms along the C2 axis of the monomer, while in the fluorobenzene and benzene complexes HCCH is perpendicular (benzeneHCCH) or nearly perpendicular (fluorobenzeneHCCH, ∼7° tilt) to the ring plane. Results from ab initio and DFT calculations will be compared to an experimental structure determined from rotational constants for the DCCD and five unique (13)C substituted isotopologues.