σ-Hole activation and structural changes upon perfluorination of aryl halides: direct evidence from gas phase rotational spectroscopy.

Enhancement of the σ-hole on the halogen atom of aryl halides due to perfluorination of the ring is demonstrated by use of the Extended Townes-Dailey (ETD) model coupled to a Natural Atomic Orbital Bond analysis on two perfluorinated aryl halides (C6F5Cl and C6F5Br) and their hydrogenated counterparts. The ETD analysis, which quantifies the halogen p-orbitals populations, relies on the nuclear quadrupole coupling constants which in this work are accurately determined experimentally from the rotational spectra. The rotational spectra investigated by Fourier-transform microwave spectroscopy performed in supersonic expansion are reported for the parent species of C6F5Cl and C6F5Br and their 13C, 37Cl or 81Br substituted isotopologues observed in natural abundance. The experimentally determined rotational constants combined with theoretical data at the MP2/aug-cc-pVTZ level provide precise structural information from which an elongation of the ring along its symmetry axis due to perfluorination is proved.

Microwave Spectra and Structure of Ar-1,3-Difluorobenzene.

The microwave spectrum of the dimer Ar-1,3-difluorobenzene from 2 to 18 GHz is reported. The spectrum has been observed using a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer that has recently been expanded to include the 2-6 GHz region of the electromagnetic spectrum. Details of this upgraded spectrometer are reported. Eighty-seven transitions were observed for the parent dimer spectrum, which was adequately fit to a semirigid rotational Hamiltonian consisting of A, B, and C as well as four quartic centrifugal distortion constants. Observations of 13C species in natural abundance were aided by utilizing smaller chirp ranges of 7-9 and 9-11 GHz for 1.9 million and 3.73 million averages, respectively. Assignment of 13C isotopologues allowed for determination of the Kraitchman coordinates of the carbon atoms as well as inertial fits of the complex. The quantum-chemical structure predicts an Ar to monomer center of mass distance of 3.48 Å, compared with 3.564(1) Å determined from experimental structural analysis. This new study indicates that in fluorinated benzene-Ar dimers, when the fluorines are separated by more carbon atoms, the Ar-ring center distance decreases.

Effect of aromatic ring fluorination on CHπ interactions: microwave spectrum and structure of the 1,2-difluorobenzeneacetylene dimer.

Rotational spectra for the normal isotopic species and for six additional isotopologues of the 1,2-difluorobenzeneacetylene (C6H4F2HCCH) weakly bound dimer have been assigned in the 6-18 GHz region using chirped-pulse Fourier-transform microwave spectroscopy. This is the third complex in a series of fluorinated benzeneacetylene dimers. In 1,2-difluorobenzeneHCCH, the Hπ distance (2.725(28) Å) is longer by about 0.23 Å, and the estimated binding energy (EB = 2.3(6) kJ mol(-1)) is weaker by about 1.8 kJ mol(-1), than in the previously studied fluorobenzeneHCCH complex. In addition, in 1,2-difluorobenzeneacetylene, HCCH tips ∼46(3)° away from perpendicular to the aromatic ring, with the H nearest the ring moving away from the fluorine atoms along the C2 axis of the monomer, while in the fluorobenzene and benzene complexes HCCH is perpendicular (benzeneHCCH) or nearly perpendicular (fluorobenzeneHCCH, ∼7° tilt) to the ring plane. Results from ab initio and DFT calculations will be compared to an experimental structure determined from rotational constants for the DCCD and five unique (13)C substituted isotopologues.

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.

Rotational Spectrum, Structure, and Interaction Energy of the Trifluoroethylene···Carbon Dioxide Complex.

Rotational spectra for four isotopologues of the 1:1 weakly bound complex between trifluoroethylene (HFC═CF2) and carbon dioxide (CO2) were recorded using 480 MHz bandwidth chirped-pulse and resonant cavity Fourier transform microwave spectroscopy between 5.0 and 18.5 GHz. Two planar forms are possible: experimental rotational constants, planar moments, and dipole moment components are consistent with the form in which CO2 is positioned at the CHF end of the TFE subunit and is approximately perpendicular to the C═C bond; the other form, with CO2 aligned roughly parallel to the C═C bond, is not observed, consistent with ab initio relative energy predictions. Symmetry-adapted perturbation theory (SAPT) calculations provided interaction energies for possible structural forms of this complex, and comparisons are made with this and other members of the series of carbon dioxide complexes with fluorinated ethylenes (vinyl fluoride, 1,1-difluoroethylene, cis- and trans-1,2-difluoroethylene, and trifluoroethylene).

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.

Benzene···acetylene: a structural investigation of the prototypical CH···π interaction.

The structure of a prototype CH···π system, benzene···acetylene, has been determined in the gas phase using Fourier-transform microwave spectroscopy. The spectrum is consistent with an effective C(6v) structure with an H···π distance of 2.4921(1) Å. The HCCH subunit likely tilts by ~5° from the benzene symmetry axis. The dipole moment was determined to be 0.438(11) D from Stark effect measurements. The observed intermolecular distance is longer than in similar benzene···HX complexes and than the distances observed in the benzene···HCCH cocrystal and predicted by many high level ab initio calculations; however, the experimentally estimated binding energy of 7.1(7) kJ mol(-1) is similar to previously studied benzene···HX complexes. Several additional sets of transitions were observed in the rotational spectrum, likely corresponding to excited states arising from low energy intermolecular vibrational modes of the dimer.

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.

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.

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.

Molecular structure of methyldifluoroisocyanato silane: a combined microwave spectral and theoretical study.

The structure of methyldifluoroisocyanato silane, MeF2SiNCO (2), has been studied by molecular rotational spectroscopy. The rotational spectrum has a complicated structure from (14)N nuclear quadrupole coupling and internal rotation of the methyl group. Cavity Fourier-transform microwave spectroscopy measurements were important for providing high spectral resolution to analyze the quadrupole and internal rotation fine structure. Broadband chirped-pulse Fourier-transform microwave spectroscopy was used to achieve high measurement sensitivity making it possible to observe the lower abundance C, N, O, and Si isotopologues in natural abundance for structure determination. Analysis of the microwave spectrum of the most abundant isotopomer of MeF2SiNCO (2) yields the rotational constants: A = 3827.347(7), B = 1264.5067(14), and C = 1240.6182(11) MHz. The spectrum has been analyzed in the I(r) representation for Cs symmetry, with inclusion of the 3-fold rotor (V3 = 446(50) cm(-1)). A partial substitution structure was obtained for the C, Si, N, and O atoms. The analysis was assisted by calculations of the equilibrium structure, using a 6-311++G (3df, 3pd) basis set, with calculations at each of the B3LYP, MP2, and CCSD(T) levels. The calculated and experimental rotational constants are only consistent with a trans-orientation at each of the HCSiN, CSiNC, and SiNCO centers; there is relatively close agreement between the experimental and the theoretical structures, especially at the CCSD(T) level. In addition, the observed low value for the (14)N quadrupole coupling term (χbb - χcc) implies a wide SiNC angle, which is consistent with the calculated values: 165.3° (B3LYP), 157.6° (MP2), and 157.4° (CCSD(T)). The skeletal bending potential is discussed.

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

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.

Observation of a double C-H···π interaction in the CH2ClF···HCCH weakly bound complex.

The structure of the CH(2)ClF···HCCH dimer has been determined using both chirped-pulse and resonant cavity Fourier-transform microwave spectroscopy. The complex has C(s) symmetry and contains both a double C-H···π interaction, in which one π-bond acts as acceptor to two hydrogen atoms from the CH(2)ClF donor, and a weak C-H···Cl interaction, with acetylene as the donor. Analysis of the rotational spectra of four isotopologues (CH(2)(35)ClF···H(12)C(12)CH, CH(2)(37)ClF···H(12)C(12)CH, CH(2)(35)ClF···H(13)C(13)CH, and CH(2)(37)ClF-H(13)C(13)CH) has led to a structure with C-H···π distances of 3.236(6) Å and a C-H···Cl distance of 3.207(22) Å, in good agreement with ab initio calculations at the MP2/6-311++G(2d,2p) level. Both weak contacts are longer than those observed in similar complexes containing a single C-H···π interaction that lies in the C(s) plane; however, this appears to be the first double C-H···π contact to be studied by microwave spectroscopy, so there is little data for direct comparison. The rotational and chlorine nuclear quadrupole coupling constants for the most abundant isotopologue are: A = 5262.899(14) MHz, B = 1546.8074(10) MHz, C = 1205.4349(7) MHz, χ(aa) = 28.497(5) MHz, χ(bb) = -65.618(13) MHz, and χ(cc) = 37.121(8) MHz.

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.

Characterization of C-H...π interactions in the structure of the CHClF(2)-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 spectra and conformational analysis of diethylsilane and diethyldifluorosilane.

The rotational spectra belonging to the (28)Si isotopologue for three conformers of diethyldifluorosilane (Et(2)SiF(2)) and three conformers of diethylsilane (Et(2)SiH(2)) have been measured between 4 and 17 GHz by using pulsed-jet Fourier-transform microwave spectroscopy. Rotational spectra for the gauche-gauche, trans-trans, and trans-gauche conformers were identified for both molecules, while the significantly higher energy gauche-gauche' conformer was not observed. Ab initio calculations at the MP2(Full)/6-311+G(2d,2p) and MP2(Full)/6-311+G(2df,2pd) levels for the fluorine and the hydrogen analogues, respectively, provided rotational constants, dipole moment components, and the relative energies of the conformers to assist in the spectral assignment. In both molecules, the gauche-gauche conformer was identified by the ab initio calculations as the most stable. Agreement of the ab initio rotational constants with those from experiment was at worst 5%, with most rotational constants being predicted to within 2%.

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.

Microwave spectrum, dipole moment, and internal dynamics of the methyl fluoride-carbonyl sulfide weakly bound complex.

Rotational spectra for the normal and four isotopically substituted species of the 1:1 complex between methyl fluoride (H3CF) and carbonyl sulfide (OCS) have been measured using Fourier-transform microwave spectroscopy in the 5-16 GHz frequency region. The observed spectra fit well to a semirigid Watson Hamiltonian, and an analysis of the rotational constants has allowed a structure to be determined for this complex. The dipole moment vectors of the H3CF and OCS monomers are aligned approximately antiparallel with a C...C separation of 3.75(3) A and with an ab plane of symmetry. The values of the Pcc planar moments were found to be considerably different from the expected rigid values for all isotopologues. An estimate of approximately 14.5(50) cm-1 for the internal rotation barrier of the CH3 group with respect to the framework of the complex has been made using the Pcc values for the H3CF-OCS and D3CF-OCS isotopic species. Two structures, very close in energy and approximately related by a 60 degrees rotation about the C3 axis of the methyl fluoride, were identified by ab initio calculations at the MP2/6-311++G(2d,2p) level and provide reasonable agreement with the experimental rotational constants and dipole moment components.

Dimers of fluorinated methanes with carbonyl sulfide: the rotational spectrum and structure of difluoromethane-OCS.

The pure rotational spectra of four isotopologues of the difluoromethane-carbonyl sulfide dimer have been measured in the 5-15 GHz region with use of pulsed-nozzle Fourier-transform microwave spectroscopy. The complex was determined to possess an ab plane of symmetry with a center of mass separation of 3.41(2) A and dipole moment components mu(a) = 1.1386(18) D, mu(b) = 0.4840(63) D, mu(total) = 1.2372(41) D. Experimental planar moments indicate that the two fluorine atoms straddle the symmetry plane while one of the C-H bonds of the difluoromethane monomer is aligned to interact with the oxygen atom of the OCS molecule. The assignment of the rotational spectrum for this dimer completes the experimental studies of the series of dimers involving fluorinated methanes (HCF(3), H(2)CF(2), and H(3)CF) complexed with OCS and makes possible a comparison of properties within this series.

Oxygen-17 hyperfine structures in the pure rotational spectra of SrO, SnO, BaO, HfO and ThO.

Hyperfine structures arising from the couplings of the nuclear spin angular momentum of (17)O (I = 5/2) with the end over end rotation of several metal-containing diatomic monoxides have been observed using a Fourier transform microwave spectrometer. The molecules have been produced by reacting (17)O(2) with laser ablated metal atoms. The oxygen-17 nuclear quadrupole coupling constants have been determined for the title molecules and are interpreted in terms of a simple Townes-Dailey model. Also, the oxygen-17 nuclear spin-rotation constants have been determined and used to calculate the oxygen-17 shieldings for each molecule.