Microwave spectra of van der Waals complexes of importance in planetary atmospheres

The Fourier-transform Fabry-Perot pulsed-molecular-beam microwave spectrometer at NIST was used to study the microwave spectra of a number of molecular dimers and trimers that may be present in planetary atmospheres. The weak van der Waals bonds associated with these species usually give rise to rotational-tunneling splittings in the microwave spectra. The microwave spectrum of the water dimer species was used to illustrate the complications that can arise in the study of the rotational spectra of these loosely bound species. In addition to the water dimer species, the microwave spectra of the following hydrogen-bonded and van der Waals complexes were studied: (CO2)2-H2O, CO2-(H2O)2, CO2-H2S, N2-H2O, CO-H2O, SO2-H2O, and O3-H2O

The microwave spectrum, structure and dipole moment of 1,4-pentadiyne

The microwave spectra of 1,4-pentadiyne and 1,4-pentadiyne-1,5-d2 are assigned and rotational and centrifugal distortion constants obtained. A unique structure could not be determined. However, analysis of the moments of inertia indicates that the bond distances in C5H4 are close to typical values found in other related compounds. An interaction involving the acetylene moieties is evidenced by the derived bond angles. The data are consistent with either the central CCC angle being close to the tetrahedral value with the acetylene groups pushed away from linearity by approximately 3-4[deg] or opening of the central CCC angle to about 113[deg] along with linear acetylene groups. A range of structures between these two is also possible. The dipole moment is determined to be 0.516(5) D.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24440/1/0000713.pd

Structures and Aggregation of the Methylamine−Borane Molecules, MenH3−nN·BH3 (n = 1−3), Studied by X-ray Diffraction, Gas-Phase Electron Diffraction, and Quantum Chemical Calculations

The structures of the molecules methylamine-borane, MeH(2)N.BH(3), and dimethylamine-borane, Me(2)HN.BH(3), have been investigated by gas-phase electron diffraction (GED) and quantum chemical calculations. The crystal structures have also been determined for methylamine-, dimethylamine-, and trimethylamine-borane, Me(n)H(3-n)N.BH(3) (n = 1-3); these are noteworthy for what they reveal about the intermolecular interactions and, particularly, the N-H...H-B dihydrogen bonding in the cases where n = 1 or 2. Hence, structures are now known for all the members of the ammonia- and amine-borane series Me(n)H(3-n)N.BH(3) (n = 0-3) in both the gas and solid phases. The structural variations and energetics of formation of the gaseous adducts are discussed in relation to the basicity of the Me(n)H(3-n)N fragment. The relative importance of secondary interactions in the solid adducts with n = 0-3 has been assessed by the semi-classical density sums (SCDS-PIXEL) approach

MICROWAVE SPECTRUM AND DIPOLE MOMENT OF 2,3-DlAZABICYCLO(2.2.1)HEPT-2-ENE

$^{1}$ J. F. Chiang, R. L. Chiang, and M. T. Kratus, J. Mol. Struct., to be published.""Author Institution: Department of Chemistry, Harvard UniversityThe extremely rich microwave spectrum of the title compound has been assigned and analyzed using a combination of conventional Stark modulation spectroscopy and radio frequency-microwave double resonance (RFMDR). The dipole moment has been measured and found to be approximately equally distributed along the a and c principal axes ($\mu_{a} = 2.76 \pm 0.01$ D and $\mu_{c} = 2.12 \pm 0.07$ D and $\mu_{t} = 3.48 \pm 0.06$ D). The microwave spectra of all the singly substituted $^{13}C$ were assigned using RFMDR on a sample containing a mixture of isotopically enriched $^{13}C$ species. This data allowed the determination by the $r_{s}$ method of several pertinent structural parameters associated with the carbon atom ring: $C_{5} - C_{6} = 1.552 \pm 0.002 {\AA}, C_{1} - C_{4} = 2.194 \pm 0.002 {\AA}, C_{1} - C_{7} = 1.533 \pm 0.005 {\AA}, \angle C_{1} - C_{7}- C_{4} = 91.3 \pm 0.5 {\AA}, \angle C_{1} - C_{4} - C_{5} = 78 \pm 1{\AA}$. The corresponding values from a previous electron diffraction study are: $^{1}$ $C_{5} - C_{6} - 1.530 {\AA}, C_{1} - C_{4} - 2.50 {\AA}, \angle C_{1} - C_{7} = 1.547 {\AA}, \angle C_{1} - C_{7} - C_{4} = 108.0^\circ C_{1} - C_{4} - C_{5} - 71.5^\circ$. A small value for the a-coordinate ($\sim$ 0.03 {\AA}) of $C_{1}$ (and $C_{4}$ by symmetry) precludes the determination of some of the parameters, by the $r_{s}$ method, namely the $C_{1}-C_{6}$ bond length and several angles involving $C_{1}$. The determination of these parameters will be discussed as well as the discrepancies between the microwave and electron diffraction results. Qualitative measurements of the relative intensities of the $^{13}C$ species and the small value of the inertial defect for the $7-^{13}C$ species indicate that the molecule possesses $C_{s}$ symmetry

MICROWAVE STUDIES OF POLYCYCLIC HYDROCARBONS: THE BENZVALENEaBENZVALENE^{a}-TRICYCLO(2.2.0.02,6^{2,6}) HEXANE SYSTEM

Author Institution: Department of Chemistry, Harvard UniversityThe microwave spectrum of tricycle (2.2.0.0$^{2,6}$)hexane has been studied in the region of 8--18 GHz using a conventional microwave spectrometer and in the region of 18--40 GHz using a commercial microwave spectrometer. The interest in these studies (aside from molecular structure determination) lies in the metal ion catalyzed rearrangements that these strained molecules undergo. Also, valuable mechanistic information can be obtained from isotopic labeling. For tricyclo(2.2.0.0$^{2,6}$) hexane the microwave spectrum consists of b- and c-type lines with the c-type being the stronger. Rotational constants for the normal species are A = 5954.07 MHz, B = 5256.39 MHz, and C = 4174.39 MHz. A mixture of mono-deuterated isotopic species has been synthesized and three of these species have been assigned. A synthetic prodecure which should yield a mixture of singly substituted $^{13}C$ isotopic species will be $described.^{a}$ TRICYCLO(3.1.0.0$^{2,6})$ HEX-3-EN

MICROWAVE STRUCTURAL STUDIES OF POLYCYCLIC HYDROCARBONS: BENZVALENE

Author Institution: Department of Chemistry, University of KansasMicrowave spectroscopic results will be presented for polycyclic hydrocarbons which have been investigated recently. In particular the complete $r_{S}$ structural determination of benzvalene (the tricyclic benzene isomer) will be described. Some systematics concerning the structural parameters of cyclic hydrocarbons will be presented, including correlations with semiempirical molecular orbital calculations

MILLIMETER WAVE SPECTRUM OF GLYCINE

$^{1}$ R. D. Suenram and F. J. Lovas, J. Mol. Spectrosc. 72, 372 (1978). $^{2}$ R. D. Brown, P. D. Godfrey, J. W. V. Storey and M. -P. Bassez, J. C. S. Chem. Comm., 547, (1978). $^{3}$ H. L. Sellers and L. Schafer, J. Am. Chem. Soc. 100, 7728 (1978). $^{4}$ L. E. Snyder, J. M. Hollis, L. W. Brown, D. Buhl, R. D. Suenram and F. J. Lovas, work in progress.Author Institution:The millimeter wave spectrum of glycine ($NH_{2} CH_{2} COOH$) the simplest amino acid has been observed and $analyzed.^{1}$ The a-type transitions have been fit to a centrifugal distortion model for both the ground and first excited virbrational state. The spectral data are consistent with the compact conformation as the source of the observed spectrum. This conformer involves a hydrogen bond between the lone pair of electrons of the amino group and the hydrogen atom of the hydroxyl group. These results are in agreement with the lower frequency study by Brown et. $al.^{2}$ According to a recent geometry optimized ab initio $calculation^{3}$, the conformer assigned may not be the lowest energy conformer. In order to access the validity of the geometry optimized ab-initio results, isotopic species of glycine are being studied to determine the structure of the assigned conformer. Searches for a second conformer based on spectral predictions from the ab-initio study are also in progress and will be discussed. Finally, the results of a collaborative $effort^{4}$ concerning the initial interstellar searches for the glycine conformer which has been assigned will be presented