Nuclear Quadrupole Coupling Constants Χaa Research Articles

On the planarity of benzyl cyanide

The microwave spectra of benzyl cyanide, C6H5CH2CN, were recorded using a molecular jet Fourier transform microwave spectrometer operating in the frequency range 2–26.5 GHz. Analyses of the 14N quadrupole hyperfine structures were performed simultaneously with the rotational frequencies. Rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants χaa and χbb − χcc were all determined with very high accuracy in a fit containing 465 lines with standard deviation within the measurement accuracy. Results from quantum chemical calculations at many levels of theory employing the MP2 method indicate a non-planar heavy atom skeleton structure, where the CH2CN group is tilted up to 50° with respect to the phenyl ring. This conflicts with results from many calculations employing density functional theory methods and also with the conclusion of a previous microwave spectroscopic work (X.-Z. Liu, R.K. Bohn, S.A. Sorenson, N.S. True, J. Mol. Struct.243 (1991) 325–339), stating that the CH2CN group and the phenyl ring are in-plane. By comparing the planar defect with those of several planar molecules reported in the literature, and with extensive support from quantum chemistry, we present for the first time evidence suggesting that the heavy atom structure of benzyl cyanide is probably not planar.

Structure and dynamics of cyclic amides: The rotational spectrum of 1,3-dimethyl-2-imidazolidinone

The structure and the internal dynamics of the lactam 1,3-dimethyl-2-imidazolidinone, also known as N,N′-dimethylethyleneurea, have been investigated through the analysis of its free-jet absorption rotational spectrum. One conformer has been assigned. The pure μb-type spectrum, recorded in the 59.6–74.4GHz frequency range entails an inertial defect Δc=−16.39uÅ2, indicating that the molecule has C2 symmetry with a twisted arrangement of the ring. The methyl internal rotation barrier V3=7.181 (3)kJmol−1 and the 14N diagonal nuclear quadrupole coupling constants χaa=2.14 (14) and (χbb-χcc)=7.26 (6)MHz were determined from the analysis of the hyperfine structure. They are in good agreement with the ab initio MP2/6-311++G(d,p) calculations which also estimate the electric dipole moment value as 3.9 D.

Formula omitted]N quadrupole coupling in the microwave spectra of N-vinylformamide

The microwave spectra of two conformers, trans and cis, of the title compound were recorded using two molecular beam Fourier transform microwave spectrometers operating in the frequency range 2–40GHz, and aimed at analyses of their 14N quadrupole hyperfine structures. Rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants χaa and χbb-χcc, were all determined with very high accuracy. Two fits including 176 and 117 hyperfine transitions were performed for the trans and cis conformers, respectively. Standard deviations of both fits are close to the measurement accuracy of 2kHz. Complementary quantum chemical calculations – MP2/6-311++G(d,p) rotational constants, MP2/cc-pVTZ centrifugal distortion constants, and B3PW91/6-311+G(d,p)//MP2/6-311++G(d,p) nuclear quadrupole coupling constants – give spectroscopic parameters in excellent agreement with the experimental parameters.

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.

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.

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.

Inter- and intramolecular electronic transfer on formation of H3P···ICl as determined by rotational spectroscopy

Ground-state rotational spectra of the three isotopomers H3P···I35Cl, H3P···I 37Cl and D3P···I35Cl of a symmetric-top complex formed between phosphine and iodine monochloride were observed by pulsed-nozzle, Fourier transform microwave spectroscopy. The spectroscopic constants B0, DJ, DJK, χaa(I), χaa(Cl), Maa(I) and Mbb(I) were determined in each case. Changes in B0 on isotopic substitution established that the nuclei lie in the order H3P···ICl, with the distance r(P···I)=2.963(1) Å. The changes in the halogen nuclear quadrupole coupling constants χaa(I) and χaa(Cl) from their counterparts in the free ICl molecule led to the conclusion that a fraction δi =0.144(7) of an electron is transferred from P to I on complex formation and that the polarization of ICl by PH3 is equivalent to the intramolecular transfer of a fraction δp(Cl)=0.157(2) from I to Cl. The complex is about as strongly bound as H3N···HCl, when the intermolecular stretching force constant kσ is used as a criterion. It is found that the distance r(P···I) contracts by 0.017 Å between H3P···I35Cl and D3P···I35Cl. The systematic variation of δi within a series of complexes B···ICl is discussed.

Rotational spectroscopy of mixtures of ethyne and iodine monochloride: isolation and characterisation of the π-type complex C2H2···ICl

The ground-state rotational spectrum of a complex formed by ethyne and iodine monochloride was observed by using pulsed-nozzle, Fourier-transform microwave spectroscopy. A fast-mixing nozzle was utilised to avoid chemical reaction of the component gases prior to their supersonic expansion. Rotational constants A0, B0 and C0, quartic centrifugal distortion constants ΔJ, ΔJK and δJ, and halogen nuclear quadrupole coupling constants χaa(X) and {χbb(X)-χcc(X)}, where X=Cl or I, were determined for the three isotopomers C2H2···I35Cl, C2H2···I37Cl and C2D2···I35Cl. Detailed interpretation of the rotational constants established that the equilibrium geometry of the complex has a planar, T-shape of C2v symmetry in which ethyne acts as the bar of the T. This geometry, with the zero-point distance r(*···I)=3.115(1) A between the centre of the π-bond of ethyne and the nearest halogen atom I, establishes that the interaction in this complex is between ethyne as a π electron donor and I of ICl as the electron acceptor. The halogen nuclear quadrupole coupling constants χaa(X) were interpreted on the basis of a simple model to show that, on complex formation, fractions δ1=0.026 and δ2=0.056 of an electronic charge are transferred from the ethyne π bond to I and from I to Cl, respectively, leading to a net decrease of 0.030e at I. The complex is weakly bound, according to the intermolecular stretching force constant kσ=12.2(1) Nm-1 determined for the isotopomers C2H2···I35Cl and C2H2···I37Cl from ΔJ values. The opportunity is taken to compare the properties of C2H2···ICl established here with those similarly determined for the series C2H2···XY, where XY=Cl2, ClF, or BrCl.

Is there significant intermolecular charge transfer in the ground state of the HCN···ICI complex? An answer from rotational spectroscopy

The ground-state rotational spectra of the four isotopomers HC14N···I35Cl, DC14N···I35Cl, HC15N···I35Cl and HC15N···I37Cl of a linear complex of hydrogen cyanide and iodine monochloride were detected by pulsed-jet, Fourier transform microwave spectroscopy and analysed to give rotational constants B0, centrifugal distortion constants DJ, nuclear quadrupole coupling constants χaa(I), χaa(Cl) and χaa(14N), and iodine spin-rotation coupling constants Mbb(I). Detailed interpretations of various spectroscopic constants on the basis of simple models yielded a range of properties of the complex. The order of the nuclei was found to be HCN···ICl with r(N···I)=2.850(1) A. The changes in the nuclear quadrupole coupling constants on complex formation lead to the conclusion that a fraction δi=0.021(4) of an electron is transferred from N to I while the polarisation of the ICl subunit by the HCN subunit, and viceversa, can be expressed in terms of transfers of fractions δp(Cl)=0.075(1) and δp(N)=0.07(2) of electrons within ICl and HCN to Cl and N, respectively. The intermolecular stretching force constant kσ has the value 14.5 N m-1. The properties of HCN···ICl are compared with those of other HCN···XY complexes, where XY=F2, Cl2, BrCl and ClF and some general conclusions are drawn. The intermolecular charge transfer in HCN···ICl is similar in magnitude to that in OC···ICl but smaller than in the other n-pair donor type complexes H2S···ICl and H3N···ICl, a result consistent with the order of the energies required to remove an electron from the n-pair orbitals in the series of Lewis bases HCN, CO, H2S and NH3.

Spectroscopic investigation of weak interactions in the van der Waals trimer NeArCO2

Rotational transitions of the ternary van der Waals complex NeArCO2 were measured with a pulsed molecular beam cavity Fourier transform microwave spectrometer. Six isotopomers, namely 20Ne40Ar12C16O2, 22Ne40Ar12C16O2, 20Ne40Ar17O12C16O, 20Ne40Ar13C16O2, 22Ne40Ar13C16O2 and 20Ne40Ar18O13C16O, were investigated. Both a- and b-type transitions were observed in the frequency range from 4.0GHz to 19.0GHz. Structural information from the rotational constants indicates that the three dimer subunits are only slightly modified upon formation of the trimer system. It was possible to resolve hyperfine structure due to the nuclear electric quadrupole moment of 17O in rotational transitions of 20Ne40Ar17O12C16O. The resulting nuclear quadrupole coupling constants χaa, χbb and χcc were used to obtain information about the extent of intermolecular vibrational motions within the complex. During the spectral search, previously unassigned transitions of minor isotopomers of NeCO2, ArCO2 and Ar2CO2, were found. For Ar2CO2, ь-type rotational transitions of two 2 2 2 2 minor isotopomers with 13C16O2 and 18O13C16O were recorded and analysed, and are reported in this paper.

Is pyridinium hydrochloride a simple hydrogen-bonded complex C5H5N···HCl or an ion pair C5H5NH+···Cl- in the gas phase? An answer from its rotational spectrum

The ground-state rotational spectra of the three isotopomers C5H514N···H35Cl, C5H514N···H37Cl and C5H514N···D35Cl of a complex formed by pyridine with hydrogen chloride have been observed by using a fast-mixing nozzle in combination with a pulsed-nozzle, FT microwave spectrometer. Rotational constants A0, B0, C0, centrifugal distortion constants, ΔJ, ΔJK, δJ, δK and nuclear quadrupole coupling constants χaa(A) and χbb(A) − χcc(A) (where A = 14N or Cl) were determined in each case. A detailed interpretation of the spectroscopic constants led to the conclusion that the observed complex has a planar, C2v geometry, with the HCl subunit forming a hydrogen bond to N and lying along the C2 axis of pyridine. The nitrogen to chlorine distance was determined to be 2.999(2) Å. The magnitudes of the nuclear quadrupole coupling constants χaa(A) and the intermolecular stretching force constants kσ in comparison with those expected in the hydrogen-bond C5H5N···HCl and ion-pair C5H5NH+···Cl− limits show that the extent of proton transfer from HCl to pyridine is small.

Rotational spectrum and molecular properties of the dinitrogen–chlorine monofluoride complex

The ground-state rotational spectra of the three isotopomers 14N2⋯35ClF, 15N2⋯35ClF and 15N2⋯37ClF of a complex formed by dinitrogen and chlorine monofluoride have been observed with a pulsed-nozzle, Fourier-transform microwave spectrometer. The spectroscopic constants B0, DJ, χaa(A)(A =14Ni, 14No or Cl) and Mbb(Cl) are reported. The complex is shown to have a linear (or nearly linear) arrangement NoNi⋯ClF of the nuclei in the equilibrium conformation with r(Ni⋯Cl)= 2.920(2)A. The intermolecular stretching force constant, kσ= 5.00(5) N m–1, is implied by the centrifugal distortion constant DJ. Interpretation of the nuclear quadrupole coupling constants χaa(A) leads to the oscillation angles θav= cos–1〈cos2θ〉1/2= 17.8(5)° and ϕav= cos–1〈cos2θ〉1/2= 10(3)° for the N2 and ClF subunits, respectively. Additionally, the diffence χaa(Ni)–χaa(No) leads, on the basis of a simple model, to the conclusion that the polarisation of N2 attending complex formation is equivalent to the transfer of a fraction δ≈ 0.02 of an electronic charge from No to Ni. A comparison of the properties of four related complexes N2⋯YF and OC⋯YF, where Y = Cl or H, is presented.

Rotational spectrum of the gas-phase dimer OC⋯BrCl

The ground-state rotational spectra of the six isotopomers 16O12C⋯79Br35Cl, 16O12C⋯81Br35Cl, 16O12C⋯79Br37Cl, 16O12C⋯81Br37Cl, 16O13C⋯79Br35Cl, and 16O13C⋯81Br35Cl, of a dimer formed by carbon monoxide and bromine monochloride have been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. The rotational constant B0, the centrifugal distortion constant DJ and the halogen nuclear quadrupole coupling constants χaa(X), where X = Cl or Br, have been interpreted to give the properties of the weakly bound complex. It was found that the molecule is linear and has the arrangement OC⋯BrCl, with the Br atom forming the intermolecular bond to carbon. Values of the rs type for the distances r(C⋯Br) and r(BrCl) have been derived and the limitations imposed on these by the small coordinate aBr are considered. The effect of bond shrinkage resulting from isotopic substitution at Br indicates that the rs value of aBr is underestimated and it is concluded that there is probably only a small increase in r(BrCl) on formation of the complex. An interpretation of the χaa(X) values indicates that the electric charge distribution in BrCl is also only slightly perturbed.

Rotational spectroscopy of a mixture of thiirane and hydrogen bromide: detection and characterization of a short-lived complex (CH2)2SċHBr in a pulsed jet

A hydrogen-bonded dimer formed by thiirane with hydrogen bromide has been detected and characterized in the gas phase by means of its ground-state rotational spectrum. The spectrum was detected within ca. 1 µs of the creation of the nascent dimer by using a fast mixing nozzle in conjunction with a pulsed-nozzle, FT microwave spectrometer. With the former device the reactive components thiirane and hydrogen bromide remain separate until they expand into the Fabry–Pérot cavity of the latter. Rotational constants A0, B0 and C0, centrifugal distortion constants ΔJ, ΔJK and δJ, Br nuclear quadrupole coupling constants χaa, χbb and χab, and Br spin–rotation coupling constants have been determined for each of the isotopomers (CH2)2SċH79Br and (CH2)2Sċ;H81Br. Interpretation of the various spectroscopic constants leads to the conclusion that the dimer has Cs symmetry, with HBr lying in the plane perpendicular to the thiirane heavy-atom plane and with a pyramidal arrangement at S. There is some evidence of a bent hydrogen bond. The angle ϕ≈ 80° made by the SċH line with the thiirane plane is discussed in terms of some simple rules for predicting angular geometries of hydrogen-bonded dimers and a non-bonding electron-pair model for thiirane.

The rotational spectrum and geometry of the heterodimer oxirane ⋅ ⋅ ⋅ hydrogen cyanide in the vibrational ground state and the vβ(0)=1 state

The rotational spectrum of the heterodimer formed by oxirane and hydrogen cyanide has been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. Two sets of rotational transitions of similar intensity have been identified for each of the isotopic species (CH2)2O ⋅⋅⋅ HC14N, (CH2)2O ⋅⋅⋅ HC15N, and (CH2)2O ⋅⋅⋅ DC14N. Each set of rotational transitions was fitted to give rotational constants B and C, centrifugal distortion constants ΔJ, ΔJK, δJ, and HJK, and, where appropriate, 14N–nuclear quadrupole coupling constants χaa and χbb. The rotational constants for each of the two states (labeled A and B) were interpreted in terms of a molecule of Cs symmetry, with a hydrogen bond O ⋅⋅⋅ HCN and a pyramidal geometry at the oxygen atom. For the species (CH2)2O ⋅⋅⋅ HC14N, the angle φ between the bisector of the COC angle and the HCN axis increases from 52.2(3)° in state A to 61.3(3)° in state B, while the distance r(O ⋅⋅⋅C) correspondingly increases from 3.035(4) to 3.130(4)Å. Similar changes are observed in (CH2)2O ⋅⋅⋅ HC15N and (CH2)2O ⋅⋅⋅ DC14N between states A and B. It is concluded that the potential energy barrier to inversion of the configuration at O by means of the low frequency hydrogen bond bending mode νβ(0) is relatively low and that states A and B correspond to the ground state and the state having vβ(0)=1, respectively.