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.

14N nuclear quadrupole coupling and methyl internal rotation in 3-methylpyrrole investigated by microwave spectroscopy

The molecular structure of 3-methylpyrrole in the gas phase has been determined using a combination of high-resolution spectroscopy and quantum chemical calculations. The rotational spectrum was recorded using a molecular jet Fourier transform microwave spectrometer covering the frequency range from 2.0 to 26.5 GHz. The experimental data were analyzed using the programs XIAM and BELGI-Cs-hyperfine. Because the internal rotor axis accidentally lies along the principal a-axis of inertia, the rho axis system and the principal axis system coincide, enabling a direct comparison of the fits. With the program XIAM, the rotational constants A = 8631.1629(12), B = 3342.19750(43), and C = 2445.73846(42) MHz were obtained. Torsional splittings due to internal rotation of the methyl group were observed, leading to the determination of the V3 potential of 245.92445(31) cm−1. Hyperfine splittings arising from the nuclear quadrupole coupling of the 14N nucleus could be resolved, and the quadrupole coupling constants χaa = 1.4159(49) and χbb − χcc = 4.1622(86) MHz were found.

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.

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.

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.

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 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.

Pure rotational spectra of the van der Waals complexes Ne–CO, Kr–CO, and Xe–CO

The pure rotational spectra of the van der Waals dimers of Ne, Kr, and Xe with CO have been measured using a pulsed jet, cavity microwave Fourier transform spectrometer. All transitions measured were a-type R-branches, obeying selection rules ΔJ=+1, ΔKa=0, and ΔKc=+1. Spectra with Ka=0 were measured for 7 isotopomers of Ne–CO, 13 of Kr–CO, and 17 of Xe–CO. Transitions with Ka=1 were measured for Ne20–C12O16 and Kr-84C12O16. Rotational constants and centrifugal distortion constants have been determined for all species, as well as the O17 quadrupole coupling constants χaa for Kr-184C13O17 and Ne20C13O17. Effective structural parameters have been calculated from the rotational constants. Results derived from the O17 quadrupole coupling constants and centrifugal distortion constants indicate that Ne–CO is considerably more flexible than Ar–CO, Kr–CO, or Xe–CO. Failure to observe hyperfine structure due to the Ne21, Kr83, and Xe131 nuclei is discussed in terms of the weak rare gas–CO bonding. Comparisons have been made to the isoelectronic rare gas–N2 van der Waals complexes.

The14N quadrupole hyperfine structure in the rotational spectrum of laser vaporized urea observed by molecular beam Fourier transform microwave spectroscopy

The microwave spectrum of urea has been recorded in the frequency range 8-19 GHz using a laser ablation molecular beam Fourier transform microwave spectrometer. The analysis of the nuclear quadrupole hyperfine splitting caused by two equivalent 14N nuclei yields the quadrupole coupling constants χaa = 2·205(26) MHz and χ bb = 1·8884(29) MHz. Seven different transitions transitions are reported. Distortion analysis is made with these seven plus 41 transitions previously published. We determine the rotational constants A = 11233·3161(87) MHz, B = 10369·4074(88) MHz, and C = 5416·6392(86) MHz and Watson's A reduced centrifugal distortion constants ΔJ = 10·58(37) kHz, Δ JK = -20·982(53) kHz, Δ K = 10·80(29) kHz, Δ J = 0·2208(43) kHz, and δ K = -14·6439(73) kHz.

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.

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.

The 33S Nuclear Quadrupole Hyperfine Coupling in the Rotational Spectrum of Dimethyl Sulfide and Dimethyl Sulfide-d6

We investigated the rotational spectrum in the vibrational ground state of 33S-dimethyl sulfide and its perdeuterated isotopomer in 33S natural abundance by molecular beam Fourier transform microwave spectroscopy. The 33S nuclear quadrupole coupling was analyzed. In addition, the internal rotation parameters of the two methyl groups were obtained. We derived the rotational constants A = 17598.3002(14) MHz, B = 7622.2643(24) MHz, C = 5694.7067(24) MHz, and the quadrupole coupling constants χaa = −38.4052(25) MHz and χbb − χcc = −57.8733(47) MHz for (CH3)233S. For (CD3)233S we found A = 13434.9569(41) MHz, B = 5792.7537(39) MHz, C = 4507.9773(39) MHz, χaa = −38.3580(95) MHz, and χbb − χcc = −57.67419(26) MHz.

The nuclear quadrupole coupling constants and the structure of the para–para ammonia dimer

Expressions are derived for the nuclear quadrupole splittings in the E3 and E4 (para–para) states of (NH3)2 and it is shown that these can be matched with the standard expressions for rigid rotors with two identical quadrupolar nuclei. The matching is exact only when the off-diagonal Coriolis coupling is neglected. However, the selection rules for rotational transitions are just opposite to those for the rigid rotor. Hyperfine splittings are measured for the J=2←1 transitions in the E3 and E4 states with ‖K‖=1; the quadrupole coupling constants χaa=0.1509(83) MHz and χbb−χcc=2.8365(83) MHz are extracted from these measurements by the use of the above mentioned correspondence with the rigid rotor expressions. The corresponding results are also calculated, with and without the Coriolis coupling, from the six-dimensional vibration–rotation–tunneling (VRT) wave functions of (NH3)2, which were previously obtained by Olthof et al. [E.H.T. Olthof, A. van der Avoird, and P.E.S. Wormer, J. Chem. Phys. 101, 8430 (1994)]. From the comparison of χaa with the measured value it follows that the semiempirical potential and the resulting VRT states of Olthof et al. are very accurate along the interchange (ϑA,ϑB) coordinate. From χbb−χcc it follows that this potential is probably too soft in the dihedral angle γ̄=γA−γB, which causes the torsional amplitude to be larger than derived from the experiment.