The microwave spectrum, structure, and tunneling motion of the sulfur dioxide dimer

The microwave spectrum of (SO2)2 has been reinvestigated using a pulsed beam Fourier‐transform microwave spectrometer. Several new a‐type transitions for the normal species and the a‐type spectra of eight isotopically substituted species were measured. The spectra indicate that the SO2 dimer undergoes a high‐barrier tunneling motion. Based on the analysis used for (H2O)2 by Coudert and Hougen [J. Mol. Spectrosc. 130, 86 (1988)], the internal motion is identified as a geared interconversion motion similar to that displayed by (H2O)2. From the analysis of the moments of inertia of the various isotopic species, an ac plane of symmetry is established for the dimer and the tilt angles of the C2 axes of each subunit relative to the line joining their centers of mass were determined. From Stark effect measurements, μa was redetermined and μc was shown to be nearly zero. Electrostatic calculations using distributed multipoles were carried out to explore the structure of this dimer.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70674/2/JCPSA6-94-11-6956-1.pd

Microwave spectrum of toluene⋅SO2: Structure, barrier to internal rotation, and dipole moment

The microwave spectrum of toluene⋅SO2 was observed with a pulsed beam Fourier‐transform microwave spectrometer. The spectrum displays a‐, b‐, and c‐dipole transitions. The transitions occur as doublets arising from the internal rotation of the methyl group. The transitions were assigned using the principal‐axis method (PAM) internal rotation Hamiltonian with centrifugal distortions. Assuming a threefold symmetry for the internal rotation potential, the barrier height was determined as V3=83.236(2) cm−1. The torsional–rotational spectra of toluene‐CD3⋅SO2 and toluene‐d8⋅SO2 were also assigned. Additional small splittings of the c‐dipole transitions for the normal species and toluene‐CD3⋅SO2 suggest a reorientation tunneling motion of SO2 with respect to the aromatic plane. The moment of inertia data show that the two monomer units are separated by Rcm=3.370(1) Å, with the SO2 located above the aromatic ring. The projection of the C2 axis of SO2 on the aromatic plane makes an angle of τ=47.0(1)° with the C3 axis of toluene. The dipole moment of the complex is μT=1.869(27) D.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69732/2/JCPSA6-98-5-3627-1.pd

Microwave spectrum, dipole moment, structure, and internal rotation of the cyclopropane‐sulfur dioxide van der Waals complex

The rotational spectrum of the cyclopropane‐sulfur dioxide complex was observed by Fourier transform microwave spectroscopy. The spectrum exhibited a‐ and c‐dipole selection rules with the c‐dipole transitions split into doublets of unequal intensity separated by about 150 kHz. The structure has Cs symmetry with the sulfur and carbon atoms all lying in the ac plane; the oxygen and hydrogen atoms straddle the plane. The sulfur dioxide plane is nearly parallel to a C–C bond edge. The distance from the center of mass of the SO2 to the C–C bond center is 3.295 Å. The dipole moment of the complex is 1.681(1) D, with components μa=0.815(1) D and μc=1.470(1) D. The splittings in the spectrum arise from an internal rotation of the cyclopropane subunit about its local C2 axis which lies nearly along the line connecting the centers of mass.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69351/2/JCPSA6-96-3-1784-1.pd

Microwave spectrum, structure, dipole moment, and deuterium nuclear quadrupole coupling constants of the acetylene–sulfur dioxide van der Waals complex

Thirty‐three a‐ and c‐dipole transitions of the acetylene–SO2 van der Waals complex have been observed by Fourier transform microwave spectroscopy and fit to rotational constants A=7176.804(2) MHz, B=2234.962(1) MHz, C=1796.160(1) MHz. The complex has Cs symmetry with the C2H2 and SO2 moieties both straddling an a–c symmetry plane (i.e., only the S atom lies in the plane). The two subunits are separated by a distance Rcm=3.430(1) Å and the C2 axis of the SO2 is tilted 14.1(1)° from perpendicular to the Rcm vector, with the S atom closer to the C2H2. The dipole moment of the complex is 1.683(5) D. The deuterium nuclear quadrupole hyperfine structure was resolved for several transitions in both C2HD⋅SO2 and C2D2⋅SO2. A lower limit for the barrier to internal rotation of the C2H2 was estimated to be 150 cm−1 from the absence of tunneling splittings. The binding energy was estimated by the pseudo‐diatomic model as 2.1 kcal/mol. A distributed multipole analysis was investigated to rationalize the structure and binding of the complex.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69970/2/JCPSA6-94-11-6947-1.pd

The mechanism of the oxidation of propene to acrolein over antimony-tin mixed oxide catalysts

The oxidation of propenes such as 13CH2=CH---CH,3, CH2=CH-CD3, cis---CHD=CD---CH3, and CH2---CH---CH3 was studied over Sb6O13, SnO2, and Sb---Sn mixed oxide catalysts. The results with 13CH2=CH---CH3 and CH2=CH---CD3 were consistent with a [pi]-allyl intermediate. The isotope effect for allylic hydrogen abstraction was 110.55 (kHlkp) over the Sb---Sn oxide catalysts, indicating that this is the slowest step in the formation of acrolein as with other catalyst systems. The oxidation of CHD=CH---CH3 did not exhibit a marked isotope effect for the second hydrogen abstraction. This is inconsistent with a fast [pi]-allyl to [sigma]-allyl equilibration process or the irreversible [pi]-allyl to [sigma]-allyl conversion observed over other metal oxide catalysts. The absence of an isotope effect is similar to oxidations over rhodium. The roles of Sn and Sb ions in the oxidation are also discussedPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28599/1/0000407.pd

Microwave spectrum, structure, barrier to internal rotation, dipole moment, and deuterium quadupole coupling constants of the ethylene–sulfur dioxide complex

The microwave spectra of the complex between ethylene and sulfur dioxide and nine of its isotopic species have been observed in a Fourier transform microwave spectrometer. The spectra exhibit a and c dipole selection rules; transitions of the normal species and several of the isotopically substituted species occur as tunneling doublets. The complex has a stacked structure with Cs symmetry; the C2H4 and SO2 moieties both straddle the mirror plane with the C2 axis of SO2 crossed at 90 ° to the carbon–carbon bond axis (i.e., only the S atom lies in the symmetry plane). The distance between the centers of mass (Rcm) of C2H4 and SO2 is 3.504(1) Å and the deviation of their planes from perpendicular to Rcm is 21(2) ° and 12(2) °, respectively. The tunneling splittings arise from a rotation of the ethylene subunit in its molecular plane. The barrier to internal rotation is 30(2) cm−1. The dipole moment of the complex is 1.650(3)D. The deuterium nuclear quadrupole coupling constants for C2H3D⋅SO2 are χaa=−0.119(1) MHz, χbb=0.010(1) MHz, and χcc=0.109(1) MHz. The binding energy is estimated to be 490 cm−1 from the pseudo‐diatomic approximation. A distributed multipole electrostatic model is explored to rationalize the structure and binding energies.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70892/2/JCPSA6-93-10-7030-1.pd

The microwave spectrum of argon-phosphorus trifluoride

The argon-PF3 complex has been prepared in a supersonic expansion of Ar (98%) and PF3 (2%). A Fourier-transform micro-wave spectrometer employing a Fabry-Perot cavity was used to assign 28 rotational transitions. The rotational constants (MHz) and distortion constants (kHz) were A = 7332.468(10), B = 1023.055(2), C = 952.564(2), DJ = 3.53(1), DJK = 60.4(1) and d1 = -0.240(7). The argon atom is 3.953 A (rc.m.) from the PF3 center of mass and rc.m. makes an angle of 70.3[deg] with the C3 axis of the PF3.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26836/1/0000396.pd

The microwave spectrum and structure of allyl alcohol

The microwave spectrum of the normal species of the gauche, gauche isomer of allyl alcohol has been reassigned and rotational and centrifugal distortion constants have been determined. Rotational constants of nine deuterated species three carbon-13 species and the oxygen-18 species have also been obtained. These data were used to estimate structural parameters by least-squares fitting of the effective moments of inertia. Good agreement with an earlier electron diffraction and ab initio study was obtained. Unassigned transitions from another isomer were observed, presumably the cis, gauche conformer.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26503/1/0000039.pd

The benzene-SO2 and pyridine-SO2 complexes

The benzene-SO2 and pyridine-SO2 complexes have been observed for the first time using Fourier transform microwave spectroscopy. The complexes have different geometries. In benzene-SO2, the two planar species are stacked one above the other. In pyridine-SO2, the pyridine plane rotates by 70[deg], so that it is more nearly perpendicular to the SO2 plane.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27846/1/0000257.pd

The microwave spectrum and structure of krypton--phosphorus trifluoride

The rotational spectrum of the weak complex between krypton and phosphorus trifluoride has been observed using a Fourier transform microwave spectrometer with a pulsed supersonic nozzle molecular beam source. The complex is an asymmetric top. The distance rcm between the krypton atom and the center of mass of the PF3 molecule is 4.0722 A and the angle between the rcm vector and the C3 axis of the PF3 is 68.25[deg]. Based on the centrifugal distortion constant DJ and a pseudodiatomic model, the binding energy of the complex is estimated to be 218 cm-1.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27096/1/0000087.pd