Rebecca A. Peebles

Rotational spectrum, structure and modeling of the OCS–CS2 van der Waals dimer

The rotational spectrum of a 1 : 1 weakly bound complex between OCS and CS2 has been measured by Fourier-transform microwave spectroscopy, giving ground state rotational constants of A = 2369.6942(9) MHz, B = 994.4467(6) MHz and C = 700.5137(3) MHz for the normal isotopic species. The experimental dipole moment components are μa = 0.2893(4) D and μb = 0.6364(27) D, with μtotal = 0.6991(25) D. The rotational constants and dipole moment components are consistent with a structure of Cs symmetry, in which the CS2 and OCS monomers are aligned almost parallel to one another, with a center of mass separation of 3.8017(2) A. This structure is in good agreement with the lowest energy geometry obtained from an ab initio calculation at the MP2/6-311++G(2d,2p) level which predicts rotational constants of A = 2322 MHz, B = 1036 MHz and C = 716 MHz and dipole moment components of μa = 0.32 D and μb = 0.69 D. Semi-empirical modeling using the ORIENT program gives similarly good agreement, although the predicted rotational constants and dipole moment are a little further from the experimental results (A = 2458 MHz, B = 1027 MHz and C = 725 MHz and μa = 0.34 D and μb = 0.68 D).

Characterization of C-H···π interactions in the structure of the CHClF2-HCCH weakly bound complex

The microwave spectra of four isotopologues of the CHClF(2)-HCCH dimer have been measured and used to determine the structure of the complex. An initial scan over the 7-18 GHz region using the chirped-pulse microwave spectrometer at the University of Virginia provided initial assignments of the (35)Cl and (37)Cl isotopologues, with two additional H(13)C(13)CH species assigned using the resonant cavity Balle-Flygare microwave spectrometer at Eastern Illinois University. For the most abundant isotopologue, the rotational constants and quadrupole coupling constants are: A = 3301.21(4) MHz, B = 1353.4268(19) MHz, C = 1153.7351(18) MHz, χ(aa) = 34.681(12) MHz, χ(bb) = -69.70(3) MHz, χ(cc) = 35.02(2) MHz and χ(ab) = -8.8(3) MHz, in good agreement with ab initio calculations at the MP2/6-311++G(2d,2p) level. The alignment of CHClF(2) with respect to acetylene reveals a C-Hπ interaction, with a secondary C-ClH-C interaction also present between the two monomers. The fitted distance between the CHClF(2) hydrogen atom and the center of the triple bond is 2.730(6) Å, the distance between the chlorine atom and the acetylenic hydrogen is 3.061(38) Å, and the C-Hπ angle is 148.2(6)°. In addition, the centrifugal distortion constants give an estimate of the binding energy for the weak interaction of about 4.9(5) kJ mol(-1), in reasonable agreement with several similar complexes.

Rotational Spectroscopic Studies of C–H···F Interactions in the Vinyl Fluoride···Difluoromethane Complex

Rotational spectra of the normal isotopic species and three (13)C isotopologues of the 1:1 complex between vinyl fluoride (CH2 ═ CHF) and difluoromethane (CH2F2) have been measured using 480 MHz bandwidth chirped-pulse Fourier-transform microwave spectroscopy in the 6.5-20 GHz region. A structure for this dimer has been determined by fitting the moments of inertia of all isotopologues and confirmed by calculation of Kraitchman single isotopic substitution coordinates. The structure is consistent with that determined by ab initio geometry optimization at the MP2/6-311++G(2d,2p) level and has the difluoromethane subunit located on the CHF side of the vinyl fluoride subunit with three C-H · · · F contacts and with the hydrogen atoms of the CH2F2 straddling the vinyl fluoride symmetry plane.

Characterization of two isomers of the vinyl fluoride···carbon dioxide dimer by rotational spectroscopy.

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.

Microwave spectra and barrier to internal rotation in cyclopropylmethylsilane.

Rotational spectra for 3 silicon isotopologues (28Si, 29Si, 30Si) of cyclopropylmethylsilane (c-C3H5SiH2CH3) have been observed in natural abundance using Fourier-transform microwave spectroscopy, and the dipole moment of the most abundant (28Si) isotopologue has been determined using the Stark effect. The observed rotational constants (A = 8800.5997(9) MHz; B = 2238.6011(3) MHz; C = 2001.0579(3) MHz) and dipole moment components (mu(a) = 0.195(2) D, mu(b) = 0.674(11) D, mu(c) = 0.362(19) D, mu(total) = 0.790(13) D) for the 28Si species are consistent with ab initio predictions (MP2/6-311+G(d)) for a gauche conformation about the Si-cyclopropyl bond. All of the observed transitions were split into doublets due to internal rotation of the methyl group, allowing a determination of the V3 barrier to internal rotation of 6.671(9) kJ mol(-1) for the most abundant isotopologue. This barrier will be compared to those for other Si-CH(3) containing compounds and will be related to a partial structure determination from the available microwave and ab initio data.

C-H···π interactions in the CHBrF2···HCCH weakly bound dimer.

The microwave spectra of four isotopologues of the CHBrF(2)···HCCH weakly bound dimer have been measured in the 6-18 GHz region using chirped-pulse and Balle-Flygare Fourier-transform microwave spectroscopy. Spectra of (13)CH(79)BrF(2) and (13)CH(81)BrF(2) monomers have also been measured, and spectroscopic constants are reported. Measurement of spectra for the (79)Br and (81)Br isotopologues of CHBrF(2) complexed with both (12)C(2)H(2) and (13)C(2)H(2) have allowed the determination of a structure with C(s) symmetry for this complex. CHBrF(2) interacts with the triple bond of acetylene via a C-H···π contact (R(H···π) = 2.670(8) Å) with the Br atom lying in the ab plane, located 3.293(40) Å from a hydrogen atom of the HCCH molecule. The structure of CHBrF(2)···HCCH has been compared with recently studied related acetylene complexes, including a comparison with (and further structural analysis of) the CHClF(2)···HCCH complex.

Microwave, infrared and Raman spectra, r0 structural parameters, ab initio calculations and vibrational assignment of 1-fluoro-1-silacyclopentanea)

The microwave spectrum (6500-18 ,500 MHz) of 1-fluoro-1-silacyclopentane, c-C(4)H(8)SiHF has been recorded and 87 transitions for the (28)Si, (29)Si, (30)Si, and (13)C isotopomers have been assigned for a single conformer. Infrared spectra (3050-350 cm(-1)) of the gas and solid and Raman spectrum (3100-40 cm(-1)) of the liquid have also been recorded. The vibrational data indicate the presence of a single conformer with no symmetry which is consistent with the twist form. Ab initio calculations with a variety of basis sets up to MP2(full)/aug-cc-pVTZ predict the envelope-axial and envelope-equatorial conformers to be saddle points with nearly the same energies but much lower energy than the planar conformer. By utilizing the microwave rotational constants for seven isotopomers ((28)Si, (29)Si, (30)Si, and four (13)C) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r(0) structural parameters have been obtained for the twist conformer. The heavy atom distances in Å are: r(0)(SiC(2)) = 1.875(3); r(0)(SiC(3)) = 1.872(3); r(0)(C(2)C(4)) = 1.549(3); r(0)(C(3)C(5)) = 1.547(3); r(0)(C(4)C(5)) = 1.542(3); r(0)(SiF) = 1.598(3) and the angles in degrees are: [angle]CSiC = 96.7(5); [angle]SiC(2)C(4) = 103.6(5); [angle]SiC(3)C(5) = 102.9(5); [angle]C(2)C(4)C(5) = 108.4(5); [angle]C(3)C(5)C(4) = 108.1(5); [angle]F(6)Si(1)C(2) = 110.7(5); [angle]F(6)Si(1)C(3) = 111.6(5). The heavy atom ring parameters are compared to the corresponding r(s) parameters. Normal coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values, and infrared band contours. These experimental and theoretical results are compared to the corresponding quantities of some other five-membered rings.

Alkynes as CH/π Acceptors: Microwave Spectra and Structures of the CH2F2···Propyne and CH2ClF···Propyne Dimers.

Rotational spectra of weakly bound complexes of chlorofluoromethane (CH2ClF) and difluoromethane (CH2F2) with propyne (HCCCH3) have been measured using chirped-pulse and resonant-cavity Fourier-transform microwave spectroscopy, adding to a relatively small body of high resolution spectroscopic data on propyne complexes. Both dimers contain CH/π contacts, as well as secondary contacts between one or both halogen atoms and the methyl group of propyne. A detailed structural determination for CH2F2···propyne has been made by study of the normal, one deuterated and four (13)C substituted isotopologues, with the second lowest energy configuration predicted from ab initio calculations agreeing well with the observed structure. Experimental rotational constants for the most abundant isotopologue of CH2F2···propyne are A00 = 5815.5858(15) MHz, B00 = 1341.1191(5) MHz, C00 = 1099.2040(4) MHz (uncorrected for internal rotation effects), and the dipole moment components, determined by Stark effect measurements, are μa = 1.568(2) D, μb = 0.587(2) D, and μtot = 1.674(3) D. For CH2ClF···propyne, only (35)Cl and (37)Cl isotopologues have been assigned, providing rotational constants and chlorine atom coordinates consistent with the lowest energy structure from a series of ab initio predictions. Rotational constants for the (35)Cl isotopologue are A = 3423.639(7) MHz, B = 1253.7562(20) MHz, and C = 1200.4828(15) MHz and the diagonal and two off diagonal components of the quadrupole coupling tensor have also been determined.

Effect of aromatic ring fluorination on CH⋯π interactions: rotational spectrum and structure of the fluorobenzene⋯acetylene weakly bound dimer

The rotational spectra for the normal isotopic species and for six (13)C singly substituted isotopologues (in natural abundance) of the fluorobenzene···acetylene (C6H5F···HCCH) weakly bound dimer have been measured in the 6.5-18.5 GHz region using chirped-pulse Fourier-transform microwave spectroscopy. The HCCH molecule interacts with the fluorobenzene via a CH···π contact and is determined to lie almost over the center of, and approximately perpendicular to, the aromatic ring, with an H···π distance (perpendicular distance from the H atom to the ring plane) of around 2.492(47) Å; a slight tilt of HCCH towards the para carbon atom of the fluorobenzene is evident. Binding energies of this complex and related benzene and fluorobenzene dimers obtained from the pseudodiatomic approximation are compared and indicate that fluorobenzene···acetylene lies among the more weakly bound of the complexes exhibiting some type of CH···π interaction.

Rotational Spectrum and Structure of the 1,1-Difluoroethylene···Carbon Dioxide Complex.

Rotational spectra for five isotopologues of the 1:1 weak complex between 1,1-difluoroethylene (H2C═CF2) and carbon dioxide (CO2) have been measured using 480 MHz bandwidth chirped-pulse and resonant cavity Fourier-transform microwave spectroscopy between 5.5 and 18.5 GHz. The observed structure of the complex is planar, with the CO2 aligned roughly parallel to the C═C bond, and experimental structural parameters derived from rotational constants are consistent with the most stable geometry predicted by basis set superposition error and zero point energy corrected ab initio geometry optimizations at the MP2/6-311++G(2d,2p) level. Comparisons with the recently characterized vinyl fluoride···carbon dioxide complex reveal slightly longer intermolecular distances in the present complex, but very similar binding energies.