Centrifugal Distortion Constants ΔJ Research Articles

Microwave spectrum and molecular structure calculations for η4-butadiene ruthenium tricarbonyl

The microwave spectrum of η4-butadiene ruthenium tricarbonyl was measured in the 5–15 GHz frequency range using a Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometer. The rotational constants for the 102Ru isotopologue were determined to have the following values: A = 932.20099(42), B = 858.03248(47) and C = 831.35161(37) MHz. The centrifugal distortion constant dJ is 0.0862(29)kHz. 22 a-dipole and 4c-dipole transitions were measured. Extensive high-level G16 calculations were made using DFT and MP2 methods with various basis sets, some including core-potentials (ECP). The best structure was calculated with Gaussian 16 using B3LYP/def2-QZVPP, which includes a core potential (ECP). Extensive all-electron calculations were made based on the best ECP structure to predict 101Ru and 99Ru quadrupole coupling strengths. Quadrupole hyperfine structure splittings were measured for both 101Ru and 99Ru. The hyperfine structure splittings for the 101Ru nuclear quadrupole were measured, yielding the values of 1.5χaa = 98.12(17) MHz and 0.25(χbb-χcc) = 36.059(30). Measured hyperfine structure splittings for 99Ru quadrupole coupling yielded the values of 1.5χaa = 16.99(77) MHz and 0.25(χbb-χcc) = 6.23(32). These values are in reasonable agreement with some of the all-electron calculations.

Determination of a semi-experimental equilibrium structure of 1-phosphapropyne from millimeter-wave spectroscopy of [formula omitted] and [formula omitted

Trideuterated 1-phosphapropyne (CD3CP) has been produced by co-pyrolysis of phosphorus trichloride and esadeuterated ethane. The rotational spectra of CD3CP in the ground and the low-lying vibrational states v8 (CCP bending mode) and 2 v8 have been investigated in the millimeter-wave region. Very accurate values of the quartic centrifugal distortion constants DJ and DJK and of the sextic distortion constants HJK and HKJ have been obtained for the ground state. l-type resonance effects have been taken into account in the analysis of the spectra of the degenerate bending states, so that the energy difference between the v8|l| = 20, and v8|l| = 22 states could be determined, together with a number of spectroscopic constants involved in the l-type resonance. Moreover, for the parent isotopologue CH3CP a new set of excited-state lines (v8 = 1, 2 and v4 = 1) were recorded up to 330 GHz, and a global analysis including all available rotational and rovibrational transitions related to the ground and v8 = 1, 2 states has been performed. A satisfactory fit could only be obtained by making a reassignment of the few transition wavenumbers previously measured for the 2ν80←ν8±1 hot band [M.K. Bane et al., J. Mol. Spectrosc.275 (2012) 9–14].The experimental work has been combined with quantum-chemical computations. Quadratic and cubic force constants of 1-phosphapropyne have been calculated at MP2 level of theory with a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived by correcting experimental ground-state rotational constants by means of theoretical vibration-rotation interaction constants.

Improved ground state and [formula omitted] = 1 state rovibrational constants of 13C2D4

The rovibrational spectrum of the ν12 band of isotopologue ethylene (13C2D4) was recorded using the Fourier transform infrared (FTIR) spectrometer with an unapodized resolution of 0.0019 cm−1 in the 1000–1140 cm−1 region. From the analysis of this A-type band, the v12 = 1 state rovibrational constants up to four quartic terms (ΔJ, ΔK, δJ, δK) were improved from previous work and a sextic term was derived for the first time. A total of 2403 infrared transitions were fitted using the Watson's A-reduced Hamiltonian in the Ir representation with a root-mean-square (rms) deviation of 0.00026 cm−1. The band centre of the ν12 band of 13C2D4 was found to be 1069.969827(13) cm−1. The ground state rotational constants (B, and C) and the centrifugal distortion constants (ΔJ, δJ, δK) were improved with higher accuracy while a sextic term was obtained for the first time through the fitting of 1209 ground state combination differences (GSCDs) derived from the IR transitions of the ν12 band, together with 504 previously reported GSCDs of the ν9 band of 13C2D4. The root-mean-square (rms) deviation value of this fit was 0.00027 cm−1. The uncertainty of the measured infrared lines was estimated to be ±0.0003 cm−1. Additionally, equilibrium state rovibrational constants up to five quartic terms, and rotational constants of the ground and v12 = 1 state were derived from theoretical anharmonic calculations at two different levels of theory, B3LYP and MP2 with the cc-pVQZ basis set, for comparison with the experimental results. These calculated ground state rotational constants of 13C2D4 agreed well within 0.60% of those determined from the GSCD fit in this work. Rotational constants (A, B, and C) of the v12 = 1 state of 13C2D4 were also calculated using B3LYP/cc-pVQZ and MP2/cc-pVQZ levels with an agreement of 0.56% or less to those derived experimentally.

A two force-constant model for complexes B⋯M-X (B is a Lewis base and MX is any diatomic molecule): Intermolecular stretching force constants from centrifugal distortion constants D(J) or Δ(J).

A two force-constant model is proposed for complexes of the type B⋯MX, in which B is a simple Lewis base of at least C2v symmetry and MX is any diatomic molecule lying along a Cn axis (n ≥ 2) of B. The model assumes a rigid subunit B and that force constants beyond quadratic are negligible. It leads to expressions that allow, in principle, the determination of three quadratic force constants F11, F12, and F22 associated with the r(B⋯M) = r2 and r(M-X) = r1 internal coordinates from the equilibrium centrifugal distortion constants DJ (e) or ΔJ (e), the equilibrium principal axis coordinates a1 and a2, and equilibrium principal moments of inertia. The model can be applied generally to complexes containing different types of intermolecular bond. For example, the intermolecular bond of B⋯MX can be a hydrogen bond if MX is a hydrogen halide, a halogen-bond if MX is a dihalogen molecule, or a stronger, coinage-metal bond if MX is a coinage metal halide. The equations were tested for BrCN, for which accurate equilibrium spectroscopic constants and a complete force field are available. In practice, equilibrium values of DJ (e) or ΔJ (e) for B⋯MX are not available and zero-point quantities must be used instead. The effect of doing so has been tested for BrCN. The zero-point centrifugal distortion constants DJ (0) or ΔJ (0) for all B⋯MX investigated so far are of insufficient accuracy to allow F11 and F22 to be determined simultaneously, even under the assumption F12 = 0 which is shown to be reasonable for BrCN. The calculation of F22 at a series of fixed values of F11 reveals, however, that in cases for which F11 is sufficiently larger than F22, a good approximation to F22 is obtained. Plots of F22 versus F11 have been provided for Kr⋯CuCl, Xe⋯CuCl, OC⋯CuCl, and C2H2⋯AgCl as examples. Even in cases where F22 ∼ F11 (e.g., OC⋯CuCl), such plots will yield either F22 or F11 if the other becomes available.

A Study of the Monohydrate and Dihydrate Complexes of Perfluoropropionic Acid Using Chirped-Pulse Fourier Transform Microwave (CP-FTMW) Spectroscopy.

This work reports the first known spectroscopic observation of the monohydrate and dihydrate complexes of perfluoropropionic acid (PFPA). The spectra have been observed using a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer in the 7750 to 14,250 MHz region. The structures of the species have been confirmed with the aid of ab initio quantum chemical calculations. Rotational constants A, B, and C have been determined and reported for both species along with centrifugal distortion constants ΔJ, ΔJK, ΔK, δJ, δK for H2O-PFPA and ΔJ, ΔJK, and δJ for (H2O)2-PFPA. Effects due to large amplitude motions were not observable in these experiments. Structures of the complexes have been determined using a combination of experimental second moment values and ab initio results. The complexation of the -OH of one or two water molecules has been found to occur in the plane of the carboxylic acid group forming a six- or eight-member ring.

Characterisation of the weak halogen bond in N2⋯ICF3 by pure rotational spectroscopy

Rotational spectra of the symmetric-top complexes 14N2⋯ICF3 and 15N2⋯ICF3 were observed and analysed to give rotational constants B0, centrifugal distortion constants DJ and DJK, and nuclear quadrupole coupling constants χaa(X). Significantly different values of χaa(14Ni) and χaa(14No) establish chemical inequivalence of the two 14N nuclei and an average zero-point oscillation angle of 19.8(5)° for the N2 subunit. A distance rN⋯I=3.443(1)Å only slightly shorter than the sum of the N and I van der Waals radii, a small intermolecular stretching force constant kσ=2.94Nm−1 and negligible charge redistribution on complex formation demonstrate the presence of a weak halogen bond.

Fourier transform microwave spectroscopy of HSiBr: Exploring the Si–Br bond through quadrupole hyperfine coupling

The 1(01)-0(00) (9-10 GHz) and 2(02)-1(01) (18-19 GHz) rotational transitions of HSi 79Br and HSi 81Br have been measured in a pulsed discharge jet expansion to an experimental uncertainty of approximately 1 kHz using Fourier transform microwave spectroscopy. The data have yielded an effective rotational constant, the centrifugal distortion constant Dj, the bromine nuclear quadrupole coupling constants, and the bromine nuclear spin-molecular rotation interaction parameter for both isotopomers. The derived parameters have been compared to their values calculated ab initio, and the nuclear quadrupole coupling tensor has been used to investigate the Si-Br bond, giving a sigma bond ionic character of 0.60, a pi bond character of 0.22, and a total Si-Br ionic character of 0.38. These bond characteristics have been compared to trends in other halosilylenes, silanes, and the analogous carbenes.

Observation and rovibrational analysis of the intermolecular HCl libration band ν16 of HCN–HCl, DCN–HCl and H13CN–HCl

The high-resolution far-infrared absorption spectrum of the intermolecular HC1 libration band upsilon6(1) (upsilonB) of the gaseous molecular complex H12CN-HCl and the two isotopically substituted species H13CN-HCl and D12CN-HCl is recorded by means of static gas-phase Fourier transform far-infrared spectroscopy at 205 K using an electron storage ring source. The rotational structure of the upsilon6(1) band has the typical appearance of a perpendicular type band of a linear polyatomic molecule. The structure is analyzed using a standard semi-rigid linear molecule model including l-type doubling to yield the band origin upsilon0, together with values for the upper state rotational constant B', the upper state quartic centrifugal distortion constant D'j and the value for the l-type doubling constant q6. The values for the ground-state spectroscopic constants B" and D"j for D12CN-H35Cl and H13CN-H35Cl are determined for the first time by ground state combination difference analyses. A number of upsilon6(1) + upsilon7(1) - upsilon7(1) and upsilon6(1) + 2upsilon7(2) - 2upsilon7(2) hot bands are observed in the spectra and the sum of the anharmonicity constants X(6,7) + g(6,7) is estimated. The observed decrease of the rotational constant B together with the simultaneous increase of the quartic centrifugal distortion constant Dj upon excitation of the HCl libration mode indicate that the hydrogen bond in the molecular complex is significantly destabilized upon intermolecular vibrational excitation. The calculated harmonic force constants for the intermolecular hydrogen bond stretching vibration upsilon(sigma) for the ground state and the excited HCl libration state indicate that the excitation of the HCl libration mode destabilizes the intermolecular interaction between HCN and HCl by almost 20%. The hydrogen bond is elongated by 0.030 A upon excitation of the upsilon6(1) mode.

The rotational spectrum of thiophene⋯HBr and a comparison of the geometries of the complexes B⋯HX, where B is benzene, furan or thiophene and X is F, Cl or Br

The ground-state rotational spectra of three isotopomers C4H4S⋯H79Br, C4H4S⋯H81Br and C4H4S⋯D79Br of a weakly bound complex formed by thiophene and hydrogen bromide have been observed in the gas phase by means of a pulsed-jet, Fourier-transform instrument. Each spectrum was analysed and fitted to give rotational constants A0, B0 and C0, centrifugal distortion constants ΔJ, ΔJK, ΔK, δJ and δJK and the components χaa, χbb − χcc and χab of the bromine nuclear quadrupole coupling tensor. A detailed analysis of the spectroscopic constants established that the geometry of the complex is of the face-on type. The Br atom of HBr is located close to the perpendicular drawn through the centre of mass of the thiophene ring and the H atom of HBr lies between the Br atom and the ring. The angle αaz made by the HBr internuclear axis z with the a-axis has the two possible values ±9.83°. The preferred structure is that generated when the positive value of the angle is chosen and has the HBr sub-unit pointing in the direction of the S atom of thiophene. The determined geometrical parameters are r(S⋯H) = 2.728(3) A, ϕ = 116.0(2)° and θ = 7.08(4)°, where ϕ is the angle made by the S⋯H internuclear line with the local C2 axis of thiophene and θ is the angular deviation of the S⋯H–Br nuclei from collinearity.

Doppler-free laser polarization and optical–optical double resonance polarization labeling spectroscopies of a large molecule: Naphthalene

Doppler-free laser polarization (DFLP) spectroscopy is successfully applied to a large polyatomic molecule, naphthalene. Rotationally resolved spectra of the 33(b2g)01 vibronic band of the à 1B1u←X̃ 1Ag electronic transition of gas phase naphthalene at room temperature have been measured with the technique of DFLP spectroscopy. The typical width of the observed spectral lines was 15 MHz, and the absolute wave numbers were measured with an accuracy of better than ±0.0002 cm−1. In order to assign the dense and complicated parts of the spectra, the technique of Doppler-free optical–optical double resonance polarization labeling spectroscopy was used. Both V-type and Λ-type double resonances were observed, and these signals were found to be very useful for the unambiguous assignment of the complicated lines of DFLP spectrum. Three rotational constants A, B, and C, three symmetric-top quartic centrifugal distortion constants ΔJ, ΔJK, and ΔK, and two asymmetric-top distortion constants δJ and δK were determined by a least-squares fitting of 4632 spectral lines in the range J=4–154 and Ka=0–40 with a standard deviation of 0.0002 cm−1. No appreciable perturbation was observed.

A hydrogen bonded complex C2H2…HBr isolated and characterized in the gas phase using pulsed-jet, Fourier transform microwave spectroscopy

A complex formed from ethyne and hydrogen bromide has been isolated and characterized by using a fast-mixing nozzle in conjunction with a pulsed-jet, Fourier transform microwave spectrometer. Any possible chemical reaction between the two components when mixed in the usual way was thereby precluded. The rotational constants A, B and C, the quartic centrifugal distortion constants δJ, δJk and δJ and the Br nuclear hyperfine coupling constants Xaa and Xbb—Xcc (nuclear quadrupole) and Mbb (spin-rotation) were determined for each of the five isotopomers C2H2…H79Br, C2H2…H81 Br, C2H2…D79Br, C2H2…D81Br and C2D2…H79Br. Interpretations of the spectroscopic constants show that the complex is planar and T shaped in the equilibrium conformation, with HCCH internuclear axis forming the cross of the T and the HBr internuclear axis lying along the C2 axis of C2H2. The H of HBr is closer to the centre (*) of the π bond of ethyne, and therefore HBr is involved in a hydrogen bond to the π system, in which the distance of H from * is r (*…H) = 2.469(1) A. The intermolecular stretching force constant is estimated as kσ = 5.38(2) Nm−1 for the species involving a hydrogen bond (C2H2…H79Br, C2H2…H81Br and C2D2…H79Br) while this quantity increases to kσ = 5.68(2)Nm−1 for those complexes bound through a deuterium bond (C2H2…D79Br and C2H2…D79Br). The opportunity is taken to consider similarities in the properties of complexes within the two series B…HBr and B…HC1 for a range of Lewis bases B, including B = C2H2. Some family relationships are identified in the two series.

Rotational spectrum, dynamics, and bond energy of the floppy dimethylether⋯neon van der Waals complex

The equilibrium conformation, dynamics, and dissociation energy of the weakly bonded dimethylether⋯Ne complex have been deduced from its pulsed jet millimeter wave spectrum. Each rotational transition is split into two component lines. The corresponding vibrational splitting has been determined to be ΔE=807.2(9) MHz. The inversion barrier between the two equivalent minima with Ne above or below the COC plane has been calculated from this datum to be 16 cm−1. The dissociation energy of the complex is estimated, from the centrifugal distortion constant DJ, to be ca. 1.0 kJ/mol.

The rotational spectra of oxirane⋯xenon (129Xe, Xe131, Xe132 Xe134, Xe136 isotopomers): Bond energy and dynamics of Xe

The pulsed jet millimeter wave spectra of five isotopomers of the weakly bonded oxirane⋯Xe complex have been measured. Information on the equilibrium conformation, dynamics and dissociation energy have been deduced. The equilibrium distance of Xe with respect to the center-of-mass of the molecules is 3.82 Å, with Xe tilted 15° from the perpendicular to the center-of-mass of the ring toward the oxygen atom. The dissociation energy is estimated, from the centrifugal distortion constant DJ, to be ≈3.9 kJ/mol.

Are members of the family of hydrogen-bonded complexes formed by furan with the hydrogen halides isostructural? An answer from the rotational spectrum of furan⋯HBr

The ground-state rotational spectra of the five isotopomers C4H4O⋯H79Br, C4H4O⋯H81Br, C4D4O⋯H79Br, C4H4O⋯D79Br, and C4H4O⋯D81Br of a weakly bound complex formed by furan with hydrogen bromide in the gas phase have been observed with a pulsed-jet, Fourier-transform instrument. Each spectrum was analyzed and fitted to give rotational constants A0, B0, and C0, centrifugal distortion constants ΔJ and ΔJK, and components χaa, χbb-χcc, and χab (or χac in the case of C4D4O⋯H79Br) of the bromine nuclear quadrupole coupling tensor. A detailed analysis of the spectroscopic constants reveals that the observed complex does not have C2v symmetry, with HBr lying along the C2 axis of furan. Instead, the geometry is of the face-on type, with the Br atom of HBr lying close to the perpendicular drawn through the center of the mass of the furan ring. The H atom of HBr lies between the Br atom and the face of the furan ring. The angles αaz made by the HBr internuclear axis z with the a axis has the two possible values ±11.929°. The preferred structure is that generated when the positive value of the angle is chosen and has the HBr subunit pointing in the direction of the O atom of furan. The determined geometrical parameters are r(O⋯H)=2.599(3) Å, φ=112.90(14)°, and θ=6.05(4)°, where φ is the angle made by the O⋯H internuclear line with the local C2 axis of furan and θ is the angular deviation of the O⋯H–Br nuclei from collinearity. Reasons why furan⋯HCl, but not furan⋯HBr, obeys some simple rules for predicting angular geometries are discussed.

Inter- and intramolecular electron transfer on formation of 15N2⋯BrCl determined from halogen nuclear quadrupole coupling in the rotational spectrumElectronic Supplementary Information available. See http://www.rsc.org/suppdata/cp/b1/b108824f/

The ground-state rotational spectra of the four isotopomers 15N2⋯79Br35Cl, 15N2⋯81Br35Cl, 15N2⋯79Br37Cl and 15N2⋯81Br37Cl of a complex formed between 15N2 and bromine monochloride were observed by pulsed-jet, Fourier-transform microwave spectroscopy. Observed spectra were characteristic of a linear or quasi-linear molecule, with Br involved in the weak interaction, and were analysed to give the rotational constant B0, the centrifugal distortion constant DJ, the nuclear quadrupole coupling constants χaa(X) (X=Br or Cl) and the Br spin–rotation coupling constant Mbb(Br) for each isotopomer. Interpretation of χaa(X) values showed that the intermolecular electron transfer δ(Ni→Br)e on complex formation is negligible while its intramolecular counterpart δ(Br→Cl)e=0.012(2) e is also small. The electronic redistribution is consistent with a weak interaction of N2 and BrCl, as confirmed by the intermolecular stretching force constant kσ=4.40(2) N m−1. Values of r(N⋯Br), δ(Ni→Br)e, δ(Br→Cl)e, and kσ for 15N2⋯BrCl are compared with those for HCN⋯BrCl and H3N⋯BrCl, all of which involve weak binding of N to Br.

A non-linear hydrogen bond F⋯H–Br in vinyl fluoride ⋯HBr characterised by rotational spectroscopy

The ground-state rotational spectra of four isotopomers C2H3F⋯H79Br, C2H3F⋯H81Br, C2H3F⋯D79Br and C2H3F⋯D81Br of a complex formed between vinyl fluoride and hydrogen bromide have been observed with a pulsed-nozzle, Fourier-transform microwave spectrometer. Rotational constants (A0, B0, C0), all five quartic centrifugal distortion constants (ΔJ, ΔJK, ΔK, δJ, δK), the Br nuclear quadrupole coupling tensor χαβ(α,β = a,b,c) and the Br spin-rotation tensor M were determined in each case. A detailed interpretation of the χαβ tensor leads to the conclusion that the complex is planar, or nearly so. The direction cosine Φaz associated with the HBr axis z and the principal inertial axis a was determined together with the principal components χzz, χxx, χyy of the Br nuclear quadrupole coupling tensor. The geometry of the complex was obtained by fitting the rotational constants of its isotopomers under the constraint that the resulting geometry must reproduce Φaz. The HBr molecule was found to form a non-linear hydrogen bond F⋯H–Br [deviation from collinearity θ = 20.0(1)°] to the F atom of vinyl fluoride, with r(F⋯H) = 2.205(9) A and the angle C1F⋯H = 123.8(5)°, when the nuclei were assumed coplanar. A comparison of the geometry with that of vinyl fluoride ⋯HCl determined in the same way shows that the angular geometries of the HBr and HCl complexes of vinyl fluoride are isomorphous. This is the case even for the extent of the non-linearity θ of the hydrogen bond, which has the values θ = 20.0(1)° and 18.3(1)° in C2H3F⋯HBr and C2H3F⋯HCl, respectively. A small extent of non-planarity is also consistent with the observations for both complexes but the conclusions are not significantly affected by relaxing the assumption of coplanar nuclei.

Interaction of water and dichlorine in the gas phase: An investigation of H2O⋯Cl2 by rotational spectroscopy and ab initio calculations

Ground-state rotational spectra of the seven isotopomers H2O⋯35Cl2, H2O⋯37Cl35Cl, H2O⋯35Cl37Cl, D2O⋯35Cl2, D2O⋯37Cl35Cl, HDO⋯35Cl2, and HDO⋯37Cl35Cl of the complex formed between water and dichlorine were observed by pulsed-jet, Fourier-transform microwave spectroscopy. Rotational constants B0 and C0, centrifugal distortion constants ΔJ and ΔJK, and Cl nuclear quadrupole coupling constants χaa(Clx) and {χbb(Clx)−χcc(Clx)}, where x=i (inner) or o (outer), were obtained via spectral analyses. The spectroscopic constants were interpreted, on the basis of models for the complex, to give the geometry, the binding strength, and the extent of inter- and intramolecular electron transfer on complex formation. The zero-point geometry is defined in terms of the effective values r(O⋯Cli)=2.8479(3) Å and φ=43.4(3)°, where φ is the angle made by the bisector of the HÔH angle with the O⋯Cl internuclear line. The intermolecular stretching force constant kσ=8.0(1) N m−1. Fractions δi=0.005(5) and δp=0.034(3) of an electron were estimated to be transferred from O to Cl and from Cli to Clo, respectively. The geometrical conclusions are supported by ab initio calculations at the aug-cc-pVDZ/MP2 level of theory, with good agreement for φ and r(O⋯Cli). A comparison of the properties of H2O⋯Cl2 with those of H2O⋯HCl provides evidence in support of a recent proposal of a halogen bond B⋯XY that is the analog of the hydrogen bond B⋯HX, where X and Y are halogen atoms.

A π-electron donor–acceptor complex C2H4···Br2 characterised by its rotational spectrum

A complex formed from ethene and dibromine was isolated and characterised by using a pulsed-jet, Fourier-transform microwave spectrometer that incorporated a fast-mixing nozzle as a means to preclude chemical reaction between the two components. Rotational constants A, B and C, the quartic centrifugal distortion constants ΔJ, ΔJK and δJ , and the Br nuclear quadrupole coupling constants χaa(Brx) and {χbb(Brx) − χcc(Brx)} (where x is either i or o for the inner or outer Br nucleus) were determined for each of the four isotopomers C2H4···79Br79Br, C2H4···81Br79Br, C2H4···79Br81Br and C2H4···81Br81Br. Interpretation of the various spectroscopic constants shows that in the complex the Br2 internuclear axis lies along the C2 axis of C2H4 that is perpendicular to the plane of the C and H nuclei, so that Bri interacts with the centre (*) of the π bond of ethene. The distance of Bri from * is r(*···Bri) = 3.068 (2) Å. It is estimated from the changes in the coupling constants χaa(Bri) and χaa(Bro) relative to those of the free Br2 molecule that 0.026(4)e is transferred from C2H4 to Bri on complex formation while the concomitant polarization of Br2 can be represented by a net transfer of 0.053(3)e from Bri to Bro. The intermolecular stretching force constant is estimated to be kσ = 8.8(3) N m−1. The properties determined for C2H4···Br2 are compared with those similarly established for C2H4···Cl2, C2H4···BrCl and C2H4···ICl and some family relationships are identified.

Rotational spectrum, dynamics and bond energy of 2,5-dihydrofuran—krypton van der Waals complex

The equilibrium conformation, dynamics and dissociation energy of the weakly bonded 2,5-dihydrofuran-Kr complex have been deduced from its free jet millimetre-wave spectrum. The equilibrium distance of Kr with respect to the centre of mass of the molecule is 3.61 Å, with Kr tilted 12.8° from the perpendicular to the centre of mass of the ring towards the oxygen atom. The dissociation energy is estimated from the centrifugal distortion constant DJ to be about 3.5kJ mol−1.

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Spectroscopic constants for 1A1 MgC2 have been calculated at the multireference single- and double-excitation configuration interaction (MR-SDCI) + Davidson correction (Q) level with the correlation-consistent polarized valence quadruple zeta (cc-pVQZ) basis sets to estimate rotational transition frequencies. The MR-SDCI + Q potential energy function predicts the rotational constants A0, B0, and C0 to be 51,794, 11,494, and 9379 MHz and the centrifugal distortion constants ΔJ, ΔJK, ΔK, δJ, and δK to be 0.014, 0.21, -0.023, 0.0027, and 0.14 MHz, respectively. The C-C and Mg-C internuclear distances in the T-shaped equilibrium geometry have been found to be 1.275 and 2.012 A, respectively. MgC2 consists of an Mg+ ion and a (CC)- moiety with dipole moment of 7.9 D. The ν1 (C-C stretching), ν2 (Mg-C2 stretching), and ν3 (rocking) vibrational frequencies have been estimated to be 1704, 595, and 456 cm-1, respectively. These results indicate that MgC2 is a rigid molecule, in contrast to SiC2 which is known to show a large-amplitude motion. Rotational transition frequencies have been estimated based on these spectroscopic constants.