Determination of the "Privileged Structure" of 8-Hydroxyquinoline.

An accurate semi-experimental equilibrium structure of 8-hydroxyquinoline (8-HQ) has been determined combining experiment and theory. The cm-wave rotational spectrum of 8-HQ was recorded in a pulsed supersonic jet using broadband dual-path reflection and narrowband Fabry-Perot-type resonator Fourier-transform microwave spectrometers. Accurate rotational and quartic centrifugal distortion constants and 14 N quadrupole coupling constants are determined. Rotational constants of all 13 C, 18 O and 15 N singly substituted isotopologues in natural abundance and those of a chemically synthesized OD isotopologue were used to obtain geometric parameters for all the heavy atoms and the hydroxyl hydrogen from a number of structure determination models. Theoretical approaches allowed for the determination of a semi-experimental equilibrium structure, reSE in which computed rovibrational and electronic corrections were utilized to convert vibrational ground state constants into equilibrium constants. Despite the molecule having only a horizontal plane of symmetry and possessing 11 individual heavy atoms, microwave spectroscopy has allowed for a reliable and accurate structure determination. A mass dependent, rm2 structure was determined and proved to be equally reliable by comparison with the B3LYP-D3(BJ)/aVTZ equilibrium structure.

XeOCS: relatively straightforward?

We report a benchmark-quality equilibrium-like structure of the XeOCS complex, obtained from microwave spectroscopy. The experiments are supported by a wide array of highly accurate calculations, expanding the analysis to the complexes of He, Ne, Ar, Kr, Xe, and Hg with OCS. We investigate the trends in the structures and binding energies of the complexes. The assumption that the structure of the monomers does not change significantly upon forming a weakly bound complex is also tested. An attempt at reproducing the r structure of the XeOCS complex with correlated wavefunction theory is made, highlighting the importance of relativistic effects, large basis sets, and inclusion of diffuse functions in extrapolation recipes.

Ab initio conformational analysis of 1,2,3,4-tetrahydroquinoline and the high-resolution rotational spectrum of its lowest energy conformer.

The saturated part of the 1,2,3,4-tetrahydroquinoline (THQ) molecule allows for the possibility of multiple conformers' existence. High-resolution microwave spectroscopy, supported by high-level quantum chemistry calculations, was used to determine the precise molecular structures of the conformers of THQ. Via the MP2 calculations, we were able to discriminate four stable conformations, i.e. two pairs of energetically equivalent enantiomorphic conformers. The results of the calculations also indicate that energetically non-equivalent conformers are separated by a low energy barrier (104 cm-1) that allows for conformational cooling to occur. The high resolution rotational spectrum with resolved hyperfine structure in the frequency range of 7-20 GHz was obtained using both the In-phase/quadrature-phase-Modulation Passage-Acquired-Coherence Technique (IMPACT) and the coaxially oriented beam resonator arrangement (COBRA) to perform Fourier transform microwave (FTMW) spectroscopy. The precise values of the rotational constants, 14N nuclear hyperfine coupling parameters and centrifugal distortion parameters were determined from the measured transition frequencies. Based on our experimental results, only the most stable enantiomeric pair of THQ contributes to the rotational spectrum under the conditions of our experiment as the less stable conformers seem to efficiently relax to the lower energy conformers. Thus the experimentally evaluated molecular constants unambiguously define the lowest energy conformer of 1,2,3,4-tetrahydroquinoline.

Inversion of Bicyclic Decanes: Rotational Spectra of the Trans and Double Cis Conformations of 2-Decalone.

The conformational landscape of the bicyclic molecule 2-decalone has been studied in a jet-cooled expansion by using rotational spectroscopy. The investigation covered the frequency region 7-19 GHz using broadband fast-passage IMPACT Fourier-transform microwave techniques. The introduction of the asymmetric carbonyl substituent in the double-chair decalin skeleton originates two distinct inverting conformers with cis ring junction, which were independently identified and characterized in the gas phase. Additionally, a single trans conformer was detected, as expected for the non-inverting equatorial ring junction. Accurate rotational parameters and quartic centrifugal distortion constants have been determined for the three observed species. A population estimation is given for the observed conformations based on relative intensities. Supporting ab initio calculations up to MP2/cc-pVTZ complement the experimental work.

The radio spectra of planar aromatic heterocycles: how to quantify and predict the negative inertial defects.

The simplest tricyclic aromatic nitrogen heterocyclic molecules 5,6-benzoquinoline and 7,8-benzoquinoline are possible candidates for detection of aromatic systems in the interstellar medium. Therefore the pure rotational spectra have been recorded using frequency-scanned Stark modulated, jet-cooled millimetre wave absorption spectroscopy (48-87 GHz) and Fourier Transform Microwave (FT-MW) spectroscopy (2-26 GHz) of a supersonic rotationally cold molecular jet. Guided by theoretical molecular orbital predictions, spectral analysis of mm-wave spectra, and higher resolution FT-MW spectroscopy provided accurate rotational and centrifugal distortion constants together with 14N nuclear quadrupole coupling constants for both species. The tricyclic frames of these species undergo low energy out-of-plane zero-point vibrations resulting in deviations from the moments of inertia that the rigid structure would exhibit. The determined inertial defects, along with those of similar species are used to develop an empirical formula for calculation of inertial defects of aromatic ring systems. The predictive ability of the formula is shown to be excellent in general for planar species with a number of pronounced out-of-plane vibrations. The resultant constants for the benzoquinolines are of sufficient accuracy to be used in astrophysical searches for planar aromatic heterocycles.