Cl Nuclear Quadrupole Coupling Research Articles

Rotational spectrum of vinyl fluoride···ClF: Are the π-bonding or nonbonding electrons the most nucleophilic region of vinyl fluoride?

The ground-state rotational spectra of C2H3F···35ClF and C2H3F···37ClF have been observed with a pulsed-nozzle, Fourier-transform microwave spectrometer. Rotational constants (A0, B0, C0), centrifugal distortion constants (ΔJ, ΔJK, δJ, δK) and the Cl nuclear quadrupole coupling tensor χαβ (α, β=a, b, c) were determined in each case. A detailed interpretation of the χαβ tensor leads to the conclusion that the complex is nearly planar and that the interaction between the vinyl fluoride and ClF subunits is weak. The direction cosines Φαz (α=a, b or c) associated with the ClF z axis and the principal inertial axes were determined together with the principal components χzz, χxx, χyy of the Cl nuclear quadrupole coupling tensor. The geometry of the complex was obtained by fitting the rotational constants of the two isotopomers under the constraint that the resulting geometry must reproduce the Φαz. The ClF molecule was found to form a nearly linear ‘chlorine’ bond F···Cl–F (deviation from collinearity ϑ=0.6°) to the F atom of vinyl fluoride, with r(F···Cl)=2.719(5) A and the angle C(1)F···Cl=125.7(3)°. A comparison with the geometry of vinyl fluoride···HCl, determined in the same way, revealed the angular geometries of the ClF and HCl complexes of vinyl fluoride to be isomorphous, except for a significant deviation (ϑ=18.3°) of the hydrogen bond in C2H3F···HCl from collinearity.

Rotational spectrum of thiophene⋯ClF and the role of thiophene as a π- or n-electron pair donor in weakly bound complexes

Rotational constants, centrifugal distortion constants and all five independent components χaa, χbb−χcc, χab, χac and χbc of the Cl nuclear quadrupole coupling tensor χαβ have been determined for thiophene⋯35ClF and thiophene⋯37ClF. A method for determining the geometry of this complex of C1 symmetry is presented which uses the direction cosines θaz, θbz and θcz (z is the ClF axis), determined by diagonalisation of the χαβ tensor, in combination with principal moments of inertia. Four geometries of thiophene⋯ClF are consistent with the observed quantities, but each has ClF interacting with the π-electron system on one face of the thiophene ring.

Evidence concerning the relative nucleophilicities of non-bonding and π-bonding electrons in furan from the rotational spectrum of furan···ClF

The rotational spectra of the three isotopomers C4H4O···35ClF, C4H4O···37ClF and C4D4O···35ClF of a complex formed between furan and chlorine monofluoride have been observed by means of a pulsed-nozzle, Fourier-transform microwave spectrometer fitted with a fast-mixing nozzle to preclude any chemical reaction among the component gases. Transitions allowed by all three components µa, µb and µc of the electric dipole moment were observed, those associated with µc exhibiting a small doubling (ca. 20 kHz). Spectral analysis yielded rotational constants, centrifugal distortion constants and (for the isotopomers C4H4O···35ClF and C4H4O···37ClF) all five independent components of the Cl nuclear quadrupole coupling tensor χαβ (α, β = a, b, c). Diagonalisation of the tensor χαβ led to the values of the components χii (i = x, y, z) in the ClF principal axis system and the direction cosines θαz (α = a, b, c) relating the ClF axis (z) to the principal inertial axis system. The geometry obtained by fitting the principal moments of inertia of the three observed isotopomers is consistent with the θαz values and has the ClF subunit approximately perpendicular to one of the C(2)–C(3) bonds of furan. The end δ+Cl of ClF thus appears to interact with the π electron density between these two atoms. The distance of the Cl atom from the point of intersection of the extrapolated ClF axis with the ring is r(···Cl) = 2.726 A. The doubling of the c-type transitions is interpreted in terms of a motion in which the interaction switches from C(2)–C(3) to C(2′)–C(3′). The contrast between the angular geometries of furan···HCl and furan···ClF is discussed in the context of the relative nucleophilicities of the n-pair and the aromatic π-electrons of furan.

Interaction of benzene and halogens in the gas-phase: rotational spectrum of C6H6···ClF

Ground-state rotational spectra of three isotopomers, C6H6···35ClF, C6H6···37ClF and C6D6···35ClF, of a complex formed by benzene with chlorine monofluoride have been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. Chemical reaction between the two components was precluded by use of a fast-mixing nozzle. A vibrational satellite associated with a low lying state (v=1) was also observed for the C6H6···35ClF and C6D6···35ClF isotopomers. The spectra were established to be of the symmetric-top type, but with large centrifugal distortion effects associated both with the frequencies of the unperturbed line centres and with the Cl nuclear quadrupole hyperfine structure. In the ground state, these effects were reflected in a large centrifugal distortion constant DJK, in higher order distortion constants of significant magnitude, and in a strong dependence of the Cl nuclear quadrupole coupling constant on the quantum number K. In the v=1 state, the constant DJK and the centrifugal distortion terms χK and χD that affect the nuclear quadrupole hyperfine structure were a similar magnitude to those of the v=0 state, but of opposite sign. The nuclear quadrupole coupling constants, χR, appropriate to the J=0, K=0 states, and the changes in the rotational constants B0 with isotopic substitution, were interpreted in terms of a model for the complex in which, at equilibrium, the ClF axis is inclined at an angle ϑ≈14° to the C6 axis of benzene, with the two axes intersecting at the ClF centre of mass. The Cl atom lies closer to the benzene ring than does the F atom. The extrapolation of the ClF internuclear axis intersects the plane of the benzene ring at a point (*) that lies at a distance of ca. 0.24 Ainside the ring from the centre of a C–C bond. The conformation with ϑ=0° (ClF axis and benzene C6 axis coincident) corresponds to a potential energy maximum, concentric with which is an approximately circular valley corresponding to ϑ≈14°. This type of potential energy surface is consistent with the symmetric-top nature of the observed spectra, with the relatively small energy separation of the v=0 and v=1 states, and with large, but oppositely signed, centrifugal distortion effects in the two states originating in a Coriolis interaction between them. The distance r(*···Cl)=3.313 Awas also determined.

Rotational spectrum of thiophene···HCl Does thiophene act as an aromatic π-type electron donor or an n-type electron donor in hydrogen-bond formation?

The rotational spectra of the four isotopomers [32S]-thiophene···H35Cl, [32S]-thiophene···H37Cl, [32S]-thiophene···D35Cl and [34S]-thiophene···H35Cl of a complex formed between thiophene and hydrogen chloride have been observed by using a pulsed-nozzle, Fourier-transform microwave spectrometer. Rotational constants, centrifugal distortion constants and Cl-nuclear quadrupole coupling constants χaa, χbb-χcc and χab were determined. Interpretation of the spectroscopic constants led to the conclusion that the observed complex has Cs symmetry, with the Cl atom of HCl lying almost directly above the centre of mass of the thiophene ring but with the H atom of HCl pointing at the π-electron density near to the S atom. The S···H–Cl nuclei are almost collinear [ϑ=0.9(6)°] but the relatively large distance r(S···H)=2.7474(29) A indicates that the S···H interaction is weak. The angle φ between the C2 axis of thiophene and the S···H internuclear line was found to be 64.53(16)°. The distance r(*···Cl)=3.693 A from the centre of mass (*) of the thiophene ring to Cl and the angle (S*Cl)=98.9° are very similar in magnitude to the corresponding quantities in thiophene···Ar. Indeed, a comparison of thiophene···Ar, thiophene···HCl, benzene···Ar and benzene···HCl revealed a strong family relationship between the geometries of these four complexes. It is concluded that the non-bonding electron pair carried by S in thiophene is so weakly nucleophilic that when thiophene forms a hydrogen bond with HCl it does so via the aromatic π-electron system. In this respect, thiophene resembles benzene and is in stark contrast to its oxygen analogue, furan, with which HCl forms a hydrogen-bonded complex of C2v symmetry via the non-bonding electron pair on O.

Configuration at oxygen and deviation of the O…ClF system from linearity in 2,5-dihydrofuran…ClF from rotational spectroscopy

Rotational spectra of two isotopomers of the 2,5-dihydrofuran/ClF complex were observed by using the fast-mixing nozzle/Balle-Flygare spectrometer combination. The complex has C s symmetry and a pyramidal configuration at oxygen. The angle between the 35ClF axis ( z) and the a-axis, α az = 17.95(3)°, was determined from the Cl nuclear quadrupole coupling tensor. The geometry r( O… Cl) = 2.422(5) A ̊ , θ = 127.0(3)° and θ = 1.99(15)° is established when rotational constants are fitted under the constraints of unperturbed monomers and α az = 17.95(3)°. the configurations θ at O and the nonlinearities θ of the B…ClF/B…HCl systems are compared for B = 2,5-dihydrofuran and oxirane.

The rotational spectrum and angular geometry of a pre-reactive complex of allene and chlorine monofluoride

Ground-state rotational constants, centrifugal distortion constants and Cl nuclear quadrupole coupling constants were determined from the rotational spectra of two isotopomers (allene⋯ 35ClF and allene⋯ 37ClF) of a pre-reactive complex of allene and chlorine monofluoride. Chemical reaction between the components was avoided by using a fast-mixing nozzle in a Fourier transform microwave spectrometer. It is concluded that the observed complex is of the weakly bound, Mulliken outer type in which the Cl of ClF interacts with the π-bond of allene to give an L-shaped geometry with the heavy atoms coplanar.

Identification and characterization of a gas-phase complex of methylenecyclopropane and chlorine monofluoride by rotational spectroscopy

The rotational spectrum of a complex formed between methylenecyclopropane (mecp) and chlorine monofluoride was observed by using a fast-mixing nozzle in combination with a Balle–Flygare microwave spectrometer. Rotational constants A0, B0 and C0, quartic centrifugal distortion constants ΔJ, ΔJK, δJ and δK, and the components χaa, χbb − χcc and χab of the Cl nuclear quadrupole coupling tensor were evaluated for each of the isotopomers mecp···35ClF and mecp···37ClF. Interpretations of the various spectroscopic constants demonstrated that the observed species is a Mulliken outer complex which may be described formally as of the bπ.aσ type, i.e. with the electrophilic end δ+Cl of ClF interacting weakly with the π bond of mecp and with only minor electric charge redistribution within the ClF subunit. The geometry of the complex was established to be of Cs symmetry. The ClF molecule lies in the principal inertial plane ab, which is the molecular symmetry plane and is perpendicular to the plane of the four carbon atoms of mecp. The angle ϕ, subtended at the centre (*) of the π bond by the unique carbon atom of the mecp ring and the Cl atom, is 92.5(5)° and the distance r(*···Cl) = 2.675(10) A. The system *···Cl–F deviates from collinearity by 4.9(1)°. These properties of mecp···ClF are discussed in the context of two different series of B···ClF complexes. In the first, the molecules B are the prototype π or pseudo-π electron donors ethene, cyclopropane, allene, mecp and buta-1,3-diene. The molecules B in the second series are the three-membered rings (CH2)2Y, where Y is either one of the n-pair donors O or S or the π donor CCH2.

Identification and geometry of the pre-reactive complex buta-1,3-diene···ClF by rotational spectroscopy

The rotational spectra of the two isotopomers C 4 H 6 ··· 35 ClF and C 4 H 6 ··· 37 ClF of a complex formed between buta-1,3-diene and chlorine monofluoride were observed by using a fast-mixing nozzle in a Balle–Flygare Fourier-transform microwave spectrometer. Rotational constants, quartic centrifugal distortion constants and Cl nuclear–quadrupole and spin–rotation coupling constants were determined in each case. Diagonalisation of the complete 35 Cl nuclear quadrupole coupling tensor (χ aa , χ bb , χ cc , χ ab , χ ac and χ bc ) led to the principal axis values 〈χ ii 〉 (i = x, y and z). A detailed interpretation of the observed spectroscopic constants for both isotopomers led to the conclusion that the Cl end of the ClF molecule interacts with the C 3 C 4 π bond of buta-1,3-diene to give a complex in which the ClF molecule is perpendicular to the plane of buta-1,3-diene. The distance r(*···Cl) of the Cl atom from the centre (*) of the C 3 C 4 π bond is 2.736(4) A, the angle C 3 –*–Cl = 95.0(2)° and the system *···Cl–F is nearly collinear (ϑ = 177.4°). A comparison of the properties of the prototype bπ.aσ complexes ethene···ClF, allene···ClF and buta-1,3-diene···ClF is presented. Several similarities are noted but the weaker binding in buta-1,3-diene···ClF (k σ = 6.2 N m −1 ) is consistent with a weakening of the formal π bonds C 1 C 2 and C 3 C 4 by conjugation in buta-1,3-diene.

Nature and angular geometry of the pre-reactive complex thiirane-chlorine monofluoride from its rotational spectrum

The rotational spectrum of the pre-reactive complex thiirane-ClF was detected with a fast-mixing nozzle in an FT microwave spectrometer. Rotational constants, centrifugal distortion constants, Cl-nuclear quadrupole and spin-rotation coupling constants were determined for the isotopomers (CH 2) 2S… 35ClF and (CH 2) 2S… 37ClF. The complex has C s symmetry, with a nearly collinear arrangement of the S…ClF nuclei ( σ≈3.5°) and the ClF axis making an angle φ=95° with the C 2 axis of thiirane. The Cl-nuclear quadrupole coupling constant χ zz along the ClF internuclear axis suggests a significant contribution of the ionic structure [(CH 2) 2SCl] +…F − to a valence-bond description of the complex.

Molecular geometry of SO 2…ClF from its rotational spectrum: a cis, planar complex with a linear O…ClF bond

The ground-state rotational spectra of SO 2… 35ClF and O 2… 37ClF were observed by using a fast-mixing nozzle/pulsed Fourier transform microwave spectrometer combination. Rotational constants, centrifugal distortion constants and the complete Cl-nuclear quadrupole coupling tensor were determined in each case. Interpretation of the spectroscopic constants shows that the complex has a planar geometry with ClF forming a weak bond to one of the O atoms of SO 2 [ r( O… Cl) = 2.733(11) A ̊ ] in a cis arrangement with respect to the SO double bond [ φ = 228.4(6)°]. The O…ClF nuclei deviate from collinearity by only θ = −0.7(2)°.

Geometry and nature of the binding of the pre-reactive complex C 2H 4…ClF from its rotational spectrum

The encounter complex C 2H 4…ClF was isolated by using a fast-mixing nozzle before chemical reaction could occur between the components and was characterised by Fourier-transform microwave spectroscopy. Rotational constants, centrifugal distortion constants, Cl nuclear quadrupole constants and Cl spin-rotation constants were determined for the isotopomers C 2H 4… 35ClF and C 2H 4… 37ClF. The complex has C 2v symmetry with the ClF subunit perpendicular to the plane of C 2H 4 and oriented so that Cl is closer to C 2H 4. Both the centrifugal distortion constant Δ J and the Cl nuclear quadrupole coupling constants indicate that the complex is relatively weakly bound and it is concluded that the interaction between the subunits is largely electrostatic in origin.

Are B…ClF bonds linear? The angular geometry of a pre-reactive complex of oxirane and chlorine monofluoride from rotational spectroscopy

A pre-reactive complex formed by oxirane and ClF was characterised through the ground-state rotational spectra of the isotopomers (CH 2) 2O… 35ClF and (CH 2) 2O… 37ClF. The complex has C s symmetry, with the O…ClF nuclei confined to the symmetry ( ac) plane. Determination of the complete Cl nuclear quadrupole coupling tensor χ aa , χ bb - χ cc and χ ac led to the equilibrium angle α az between the ClF axis ( z) and the a-axis directions. By fitting the observed rotational constants A 0, B 0 and C 0 under the assumption of unchanged monomer geometries and with α az fixe the geometrical parameters r( O…Cl) = 2.437(2) A ̊ , f = 112.7(1)° and the deviation θ = 2.91(3)° from collinearity of the O…ClF nuclei were established.

Evidence for a pre-reactive intermediate in a gaseous mixture of ethyne and chlorine monofluoride. Rotational spectrum of the Mulliken bπ· aσ complex C2H2⋯ClF

A complex of ethyne and chlorine monofluoride has been identified and characterised in a supersonically expanded mixture of the two gases by means of its ground-state rotational spectrum. A fast-mixing nozzle was used in combination with a pulsed-nozzle, Fourier-transform microwave spectrometer to preclude chemical reaction of the components and to observe the spectrum of each of the isotopomers C2H2⋯35ClF, C2H2⋯37ClF and C2D2⋯35ClF. Analysis of the spectrum led to the rotational constants A0, B0 and C0, the centrifugal distortion constants ΔJ, ΔJK and δJ, the Cl-nuclear quadrupole coupling constants χaa(Cl) and χbb(Cl)–χaa(Cl), and the spin–rotation coupling constant ½[Mbb(Cl)+Mcc(Cl)] in each case. Arguments based on nuclear-spin statistical weight effects and a detailed interpretation of the spectroscpic constants revealed that the detected pre-reactive intermediate has a planar T-shaped geometry, with C2H2 acting as the bar of the T, and ClF as the stem but oriented so that the Cl atom was closer to the ethyne π-bond than F. The distance between the centre (*) of the π-bond and the Cl nucleus is r(*⋯Cl)= 2.783 (8)A. The small intermolecular stretching force constant kσ= 10.0 N m–1 and the minor changes in the Cl nuclear qudrupole coupling constants of ClF on formation of the complex demonstrate that the complex is weakly bound and that the electric charge distribution of ClF suffers only a small perturbation. It is concluded that C2H2⋯ClF is best described formally as a complex of the Mulliken bπ· aσ weak outer type.

Pre-chemical interactions in a pulsed jet: the angular geometry of thiirane ⋯ HCl and the nonlinearity of its hydrogen bond from rotational spectroscopy

Ground-state rotational spectra of three isotopomers, (CH 2 ) 2 32 S ⋯ H 35 Cl, (CH 2 ) 2 32 S ⋯ H 37 Cl, and (CH 2 ) 2 34 S ⋯ H 35 Cl, of a complex formed by hydrogen chloride and the three-membered S-containing ring thiirane were observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. A fast-mixing nozzle was used to keep the components separate until they expanded coaxially into the Fabry-Pérot cavity and to preclude a chemical reaction. Spectral analysis gave rotational constants, centrifugal distortion constants and the complete Cl-nuclear quadrupole coupling tensor ( χ aa , χ bb − χ cc and χ ab ) for each isotopomer. For (CH 2 ) 2 32 S ⋯ H 35 Cl, the components M aa and Tr( M ) of the Cl-spin-rotation tensor were also determined. The off-diagonal nuclear quadrupole coupling constant χ ab led to a good approximation to the equilibrium angle α az between the directions of the HCl axis ( z ) and the a -principal inertial axis. The complex was established to have C s symmetry and then its detailed angular geometry was determined by fitting the moments of inertia and the angle α az . It was found that the angle between the thiirane C 2 axis and the S ⋯ H bond direction was Φ = 85.5 (20)°, the distance r( S ⋯ H ) = 2.329(28) A ̊ and the deviation of the hydrogen bons S ⋯ HCl from collinearity was δ = 21.0(5)°. These results are rationalised in terms of a simple model for predicting angular geometrics in which the primary hydrogen bond is made by HCl with the thiirane S atom, while a secondary hydrogen bond involving the thiirane CH 2 groups and the Cl atom is responsible for the nonlinearity of the primary interaction.

Characterisation of the Intermediate C2H4…︁Cl2 in a Gaseous Mixture of Ethene and Chlorine by Rotational Spectroscopy: A Weak π‐Type Complex

AbstractA complex of ethene and Cl2 has been characterised in the gas phase. Rotational spectra of the isotopomers C2H4…35Cl2, C2H4…35Cl37Cl and C2H4…37Cl35Cl were recorded by using a fast‐mixing nozzle in an FT microwave spectrometer. Rotational constants, centrifugal distortion constants and Cl nuclear quadrupole coupling constants. χgg (Cl) are reported in each case. The complex is of the π‐donor–acceptor type and has a C2v geometry in which Cl2 lies along the C2 axis perpendicular to the plane of the C2H4 nuclei. The binding is weak, and only small changes in the χgg (Cl) attend complex formation. A simple model attributes these changes to a transfer of around 0.02e from the inner to the outer Cl nucleus, thus confirming that the complex is of the Mulliken outer type. Similarities in the properties of C2H4… Cl2 and C2H4… HCl indicate that the angular geometry is in both cases determined mainly by the electrostatic part of the interaction. The distance from the π‐bond midpoint to Cl decreases from C2H4… HCl to C2H4… Cl2; this suggests that Cl2 is “snub‐nosed”.

H2S⋯Cl2characterised in a pre-reactive gas mixture of hydrogen sulfide and chlorine through rotational spectroscopy: the nature of the interaction

The ground-state rotational spectra of five isotopomers (H2S⋯35Cl2, H2S⋯35Cl37Cl, H2S⋯37Cl35Cl, HDS⋯35Cl2 and D2S⋯35Cl2) of a complex formed by hydrogen sulfide and chlorine have been observed with a pulsed-nozzle, Fourier-transform microwave spectrometer. The reaction of H2S and Cl2 was precluded by employing a fast-mixing nozzle. The rotational constant ½(B0+C0), the centrifugal distortion constant DJ and the Cl-nuclear quadrupole coupling constants χaa(Cli) and χaa(Cl0)(i = inner Cl atom, o = outer Cl atom) were determined in each case. The rotational constants were interpreted, under the assumption of unperturbed monomer geometries, to establish that H2S⋯Cl2 has a geometry in which the SClCl nuclei are collinear or almost collinear, with the Cl2 subunit nearly perpendicular to the plane of the H2S nuclei. The observed angular geometry is rationalised in terms of a set of rules previously used to discuss hydrogen-bonded complexes B⋯HX. The strength of the interaction, as measured by the force constant kσ(determined from DJ), and the electric charge redistribution within Cl2 on formation of H2S⋯Cl2[determined from the χaa(Clx)] both indicate that the complex is of the weak, outer type described by Mulliken. A comparison of the properties of H2S⋯Cl2 and H2S⋯HCl reveals that the complexes are similar in several respects and explanations for these similarities are given in terms of the electric charge distributions of Cl2 and HCl. In particular, the reason why the angular geometries of both can be predicted by some electrostatically based rules is discussed.

The bπ.aσ complex C2H2⋯Cl2characterised by rotational spectroscopy as an intermediate in a reactive mixture of ethyne and chlorine

An intermediate C2H2⋯Cl2 belonging to the bπ.aσ class of complex defined by Mulliken has been isolated in a reactive mixture of ethyne and molecular chlorine and characterised by means of its ground-state rotational spectrum, as observed with a fast-mixing nozzle incorporated in a pulsed-nozzle, Fourier-transform microwave spectrometer. Rotational constants A0, B0 and C0, quartic centrifugal distortion constants ΔJ, ΔJK and δJ, and Cl-nuclear quadrupole coupling constants χaa(Clx) and χbb(Clx)–χcc(Clx)(where x= i for inner or o for outer) were determined for the four isotopomers C2H2⋯35Cl2, C2H2⋯37Cl35Cl, C2H2⋯35Cl37Cl and C2D2⋯35Cl2. Detailed analyses of the spectroscopic constants established unambiguously that the observed complex has a planar, T-shaped geometry of C2v symmetry with Cl2 as the stem of the T, that the extent of electric charge redistribution within Cl2 on formation of the complex is equivalent to a transfer of only 0.02e from Cli to Clo, and that the binding strength, as measured by the intermolecular stretching force constant kσ, is very small. The systematic shortening of the B⋯Cl distance from B⋯HCl to B⋯Cl2 identified previously was also observed for B = C2H2 and has been attributed to the ‘snub-nosed’ nature of molecular chlorine.

Can the nonlinearity of hydrogen bonds B…HX be measured precisely? A method from rotational spectroscopy and its application to 2,5-dihydrofuran…HCl

Rotational constants, centrifugal distortion constants and the complete Cl-nuclear quadrupole coupling tensor referring to the principal inertial axis system have been determined for the ground state of 2,5-dihydrofuran…HCl from its rotational spectrum. The complex has C s symmetry, with the hydrogen bond completing a pyramidal arrangement at the oxygen atom of 2,5-dihydrofuran. The coupling tensor allows a good approximation to the equilibrium angle α az between the HCl( z) axis and the a axis, and thence the nonlinearity θ=9.5° of the hydrogen bond, to be determined. The results are rationalised in terms of a refinement of a simple model of the hydrogen bonds.

Properties of the intermediate ethyne…Cl 2 from its rotational spectrum and some generalisations for a series B…Cl 2

A T-shaped planar complex of ethyne and Cl 2 has been isolated in a mixture of the substances using a pulsed-nozzle, FT microwave spectrometer and characterised through its rotational spectrum. Rotational constants ( A 0, B 0, C 0), centrifugal distortion constants (Δ J , Δ JK , δ J ), and Cl-nuclear quadrupole coupling constants χ aa (Cl) and χ bb (Cl)  χ cc (Cl) of both Cl nuclei are reported for the isotopomer (HCCH, 35Cl 2). The angular geometry, electric charge redistribution within the Cl 2 subunit and intermolecular force constant k σ, as derived from the spectroscopic constants, are compared with those of other B…Cl 2 and of the corresponding series B…HCl to give some generalisations.