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

Centrifugal distortions in molecules: An ab initio approach with application to phosphine

Our procedure for employing analytic gradients of ab initio potential energy hypersurfaces in the description of centrifugally distorted molecules is applied to a symmetric top, namely phosphine. Quartic centrifugal spectroscopic coefficients are obtained and are in excellent agreement with the coefficients from the Kivelson and Wilson method for J || z. We proposed a Borel form that enables us to fit the stabilization energies up to J = 80 for the vibrational ground state of phosphine. The sextic spectroscopic constant for J || z is obtained. Both single determinantal (HF/6-31G**) and multideterminantal Moller-Plesset (MP2/6-31G*) surfaces were utilized.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27562/1/0000606.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, 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 structures and dipole moments of Ar–PF3 and Kr–PF3

The complexes of PF3 with Ar and Kr, were studied by Fourier transform microwave spectroscopy. The force constants and amplitudes of vibration for the van der Waals modes of the complexes and the average moments of inertia and structural parameters were estimated from the centrifugal distortion constants. The distance (Rc.m. )ave between the rare‐gas atom and the center of mass of PF3 is 3.959 Å for the Ar complex and 4.077 Å for Kr while the angle (θc.m. )ave between the Rc.m. vector and the C3 axis of the PF3 is 69.30° and 67.25°, respectively. The dipole moments of both complexes and of free PF3 were determined. The induced dipole components estimated for the rare gas using electric fields from ab initio calculations of PF3 agree with the experimental values for a conformation with the rare gas over a PF2 face. The PF2 face conformation is also consistent with the observed and ab initio estimates of the 83 Kr nuclear quadrupole coupling constant for the 83 Kr–PF3 species.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70747/2/JCPSA6-90-12-6949-1.pd

Internal rotation and interconversion motions in weakly bound complexes of sulfur dioxide.

The microwave spectra of toluene$\cdot$SO\sb2, benzene$\cdot$SO\sb2 and (SO\sb2)\sb2 have been investigated using a pulsed molecular beam Fourier-transform microwave spectrometer. Evidence of internal motions was observed in the spectra of these three complexes and their isotopic species. In toluene$\cdot$SO\sb2, the internal motion is associated with an internal rotation of the methyl group about its C\sb3 axis. In benzene$\cdot$SO\sb2, the internal motion is identified as an internal rotation of benzene about its C\sb6 axis. In (SO\sb2)\sb2, the two monomers undergo a high barrier interconversion tunneling motion analogous to that displayed by (H\sb2O)\sb2. In the case of toluene$\cdot$SO\sb2 and benzene$\cdot$SO\sb2 the principal-axis method (PAM) internal rotation Hamiltonian was applied to the analysis of their spectra. The barriers to internal rotation are V\sb3 = 84(1) cm\sp{-1} and V\sb6 = 0.28(1) cm\sp{-1} for toluene$\cdot$SO\sb2 and benzene$\cdot$SO\sb2, respectively. An identical Hamiltonian has also been utilized to estimate the internal rotation barrier for ethylene$\cdot$SO\sb2. The internal rotation corresponds to a rotation of ethylene in its molecular plane and the barrier is V\sb2 = 30(2) cm\sp{-1}. The structures of toluene$\cdot$SO\sb2, benzene$\cdot$SO\sb2 and (SO\sb2)\sb2 were also determined from analyses of moments of inertia of their respective isotopic species. In toluene$\cdot$SO\sb2 and benzene$\cdot$SO\sb2, the SO\sb2 moiety sits above the aromatic ring. In (SO\sb2)\sb2, an ac-plane of symmetry was established where one monomer lies in the plane of symmetry while the oxygens of the second monomer straddle it. The electric dipole moments of these three complexes were also derived from Stark effect measurements. Electrostatic calculations using distributed multipoles were carried out to rationalize the internal rotation barriers, the binding energies and the structures of the three complexes. The van der Waals stretching force constants were also estimated for each complex based on the pseudo-diatomic model.Ph.D.ChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105793/1/9208663.pdfDescription of 9208663.pdf : Restricted to UM users only

Microwave spectrum of benzene⋅SO2: Barrier to internal rotation, structure, and dipole moment

The microwave spectrum of the benzene⋅SO2 complex was observed with a pulsed beam Fourier‐transform microwave spectrometer. The spectrum was characteristic of an asymmetric‐top with a‐ and c‐dipole selection rules. In addition to the rigid‐rotor spectrum, many other transitions were observed. The existence of a rich spectrum arose from torsional–rotation interactions from nearly free internal rotation of benzene about its C6 axis. Transitions from torsional states up to m=±5 were observed. The principal‐axis method (PAM) internal rotation Hamiltonian with centrifugal distortion was used to assign the spectrum. Assuming six‐fold symmetry for the internal rotation potential, the barrier height was determined as V6=0.277(2) cm−1. The spectrum of C6D6⋅SO2 was also assigned. Analysis of the moments of inertia indicated that the complex has a stacked structure. The distance Rcm separating the centers of mass of benzene and SO2, as well as the tilt angles of the benzene and SO2 planes relative to Rcm were determined. The values obtained were Rcm=3.485(1) Å, θC6H6=±12(1)° and θSO2=44(6)°. While SO2 is certainly tilted with the sulfur end towards benzene, the sign of the benzene tilt angle could not be unambiguously determined. The dipole moment of C6H6⋅SO2 was determined as μa=1.691(2) D, μc=1.179(2) D, and μT=2.061(2) D.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69623/2/JCPSA6-97-5-2996-1.pd

The microwave spectrum and structure of the neon-phosphorus trifluoride complex

Symmetric top spectra were observed for the 20Ne[middle dot]PF3 and 22Ne[middle dot]PF3 van der Waals dimers using a Fourier-transform microwave spectrometer. The center-of-mass distance between Ne and PF3 is 3.373(3) A. The experimental data, in conjunction with the van der Waals radii of the atoms and with ab initio calculations, are consistent with the neon atom on the symmetry axis over the F3 face of the PF3. Normal mode analysis of the van der Waals vibrations based on the centrifugal distortion constants yields force constants fR = 0.00647 mdyn A-1 and f[Theta] = 0.00464 mdyn A for the 20Ne[middle dot]PF3 isotopic species.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28424/1/0000207.pd

The microwave spectrum and structure of the furan [middle dot] sulfur dioxide complex

The microwave spectrum of furan [middle dot] sulfur dioxide complex has been observed with a pulsed molecular beam Fourier transform microwave spectrometer. Transitions having a-, b-, and c-dipole selection rules were all seen. The rotational constants of the normal isotopic species are A = 3190.7285(11), B = 1119.3596(2), and C = 1015.6841(2) MHz. In addition, the rotational spectra of the C4H4O [middle dot] 34SO2, C4H4O [middle dot] S18O2, C4D4O [middle dot] SO2, and two different C4H4O [middle dot] SO18O isotopic species were assigned. Stark effect measurements gave dipole moment components of [mu]a = 1.79(1) D, [mu]b = 0.50(2) D, [mu]c = 0.73(1) D, and [mu]total = 1.99(2) D. The moment of inertia data show that the two monomers are separated by 3.43(1) A (Rcm), with the SO2 lying above the furan plane. The two C2 axes of the monomer units are skewed by about 65[deg] and the plane of the SO2 is tipped considerably from parallel to the furan plane, with the sulfur atom closest to the furan. While one oxygen in SO2 lies approximately above the oxygen of the furan, the other is located closer to a [beta]-carbon of the furan. Splittings of the c-dipole transitions for various isotopes suggest a tunneling motion between two equivalent forms of the complex.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30353/1/0000755.pd