Chlorine Nuclei Research Articles

Towards the complete analysis of the rotational spectrum of (CH 3) 3SnCl

The rotational spectrum of the symmetric top trimethyl tin chloride (CH 3) 3SnCl has been studied using a pulsed molecular beam Fourier transform microwave spectrometer in the frequency range from 3 to 24 GHz. The spectrum is exceedingly complicated by the internal rotation motions of the three equivalent methyl tops, the high number of Sn- and Cl-isotopes and the quadrupole hyperfine structure of the chlorine nucleus. In this paper, we present the microwave spectrum, ab initio calculations, permutation inversion (PI) group-theoretical considerations, Stark-effect measurements and finally the K = 0 assignments and fits of the different torsion–rotation species. Based on the Stark-effect measurements, the dipole moment is μ = 3.4980(30) D. Due to Δ K = ± 1 -mixing effects we observe linear Stark-effect behavior and additional quadrupole splitting for some K = 0 torsion–rotation transitions in (CH 3) 3SnCl, which can be group-theoretically explained. The symmetric rotor fit of A 1 and A 2 torsion-rotation states leads to an effective B-constant of 1680.040124(92) MHz for the main isotopologue (CH 3) 3 120Sn 35Cl. A global fit of 182 K = 0 torsion–rotation transitions yields a V 3 torsional barrier of 1.774(6) kJ.

35Cl NQR and pseudorotation in crystalline molecular complexes of antimony trichloride with aromatic hydrocarbons

The pseudorotation of chlorine atoms in molecular complexes of SbCl3 with benzene and its substituted derivatives was used as an example to consider the effect of the thermally activated intramolecular mobility on the quadrupole spin-lattice relaxation of moving nuclei. The data obtained by nuclear quadrupole resonance were analyzed using the model that implies the existence of a temperature dependence of the activation energy for pseudorotation. This model accounts for the great preexponential factors in the expression for the thermal activation component of the relaxation rate of chlorine nuclei found for some complexes and provides the way for rapid estimation of the pseudorotation activation energy in the characteristic temperature points using both relaxation and spectral NQR parameters.

Use of spin labels to study membrane proteins by high-frequency electron nuclear double resonance spectroscopy

The applicability of spin labels to study membrane proteins by high-frequency electron nuclear double resonance spectroscopy is demonstrated. With the use of bacteriorhodopsin embedded in a lipid membrane as an example, the spectra of protons of neighboring amino acids are recorded, electric field gradients at the membrane surface are detected, and the constant of hyperfine interaction with the chlorine nucleus at the site of ion trapping is measured.

Nuclear quadrupole coupling in chloroform and calibration of ab initio calculations

The complex hyperfine structures in the J = 1 ← 0, and J = 2 ← 1 ground state rotational transitions of 35Cl 3CH and 35Cl 2 37ClCH were resolved and measured at conditions of supersonic expansion. Accurate spectroscopic constants for the two isotopomers have been derived from global fits of the hyperfine structure together with hyperfine-free high- J millimetre wave data. The complete inertial and principal quadrupole tensors of the chlorine nuclei have been determined, and the symmetric top treatment for 35Cl 3CH and the asymmetric top treatment for 35Cl 2 37ClCH yield identical results for the principal tensor components of the 35Cl nucleus. The availability of precise experimental splitting constants for many molecules allows benchmarking of ab initio field gradient calculations, and it is found that for the chlorine nucleus optimum predictive performance for molecules of moderate size is obtained at the B3LYP/aug-cc-pVDZ level by using a scaling factor of 1.0619(23).

The Chiral Molecule CHClFI: First Determination of Its Molecular Parameters by Fourier Transform Microwave and Millimeter-Wave Spectroscopies Supplemented by ab Initio Calculations

The rotational spectrum of chlorofluoroiodomethane (CHClFI) has been investigated. Because its rotational spectrum is extremely crowded, extensive ab initio calculations were first performed in order to predict the molecular parameters. The low J transitions were measured using a pulsed-molecular-beam Fourier transform spectrometer, and the millimeter-wave spectrum was measured to determine accurate centrifugal distortion constants. Because of the high resolution of the experimental techniques, the analysis yielded accurate rotational constants, centrifugal distortion corrections, and the complete quadrupole coupling tensors for the iodine and chlorine nuclei, as well as the contribution of iodine to the spin-rotation interaction. These molecular parameters were determined for the two isotopologs CH35ClFI and CH37ClFI. They reproduce the observed transitions within the experimental accuracy. Moreover, the ab initio calculations have provided a precise equilibrium molecular structure. Furthermore, the ab initio molecular parameters are found in good agreement with the corresponding experimental values.

Rotational spectrum, nuclear quadrupole coupling constants, and structure of six isotopomers of the argon-chlorocyclobutane van der Waals complex

The Fourier Transform Pulsed Jet Microwave spectra of the argon-chlorocyclobutane van der Waals complex have been assigned for six isotopomers. These include the 35Cl and 37Cl each with four 12Cs in the ring plus the four isotopomers of the 35Cl with one 13C at each position in the ring. The spectroscopic constants for the Ar- 12C 4H 7 35Cl complex are A=2675.2641(6) MHz, B=977. 8306(3) MHz, C=764.9382(2) MHz; Δ J=1.737(1), Δ JK=7.176(6), Δ K=15.28(2), δ J=0.4148(6), δ K=6.61(3) kHz; and χ aa=28.011(6) MHz, ( χ bb– χ cc)=−80.378(6) MHz, χ ab=−22.8(1) MHz, χ ac=−5.4(7) MHz and χ bc=−26.47(9) MHz. The structure of the ground vibrational state of the complex has an equatorial chlorine as in the non-complexed ring and the argon coordinates are a=1.267 Å, b=2.824 Å and c=2.517 Å in the principal axis system of the monomer ring. The chlorine coordinate in this axis system is a=1.66, b=0.0 and c=0.08 Å. The argon is exo to the ring 2.4 Å above the C–Cl bond but 2.8 Å to the side of the plane of symmetry of the chlorocyclobutane monomer. Argon is within the van der Waals radii of the nearest cross-ring methylene hydrogen and outside the van der Waals radii of chlorine. It is not directly above the ring as has been observed in other four-membered ring complexes. The off-diagonal quadrupole moment of the monomer, chlorocyclobutane, has been measured, χ ac=32.0(6) MHz. The quadrupole tensor for the chlorine in the complex was fit including all three off diagonal elements. The van der Waals bond does not significantly perturb the electronic gradients at the chlorine nucleus of the monomer, since each element of the quadrupole tensor measured for the complex, except one small off-diagonal constant, is within 3% of that projected onto the complex from chlorocyclobutane. The argon position is also consistent with the observation and intensities of a, b, and c-type rotational transitions from the projection of the monomer dipole moment onto the complex principal axis system.

Systematic discrepancy of theoretical predictions of NMR chemical shifts for chlorinated aromatic carbons using the GIAO DFT method

Theoretical calculations of carbon-13 NMR chemical shifts using the gauge including atomic orbitals (GIAO) DFT approach with a moderately large set of basis functions usually yield quite satisfactory results. In the case of chlorinated aromatic carbons, however, abnormally large differences between experimental and calculated values have been noticed. This discrepancy has been proven not to be caused by improper referencing, or the basis set effect, and probably not by neglect of vibrational corrections. One of the possible sources of the chlorine effect could be the impact of relativistic phenomena on electrons moving about the chlorine nucleus. The second, probably more important factor is the influence of electron correlations, ignored in Hartree–Fock SCF and only partially included in DFT calculations. Surprisingly, however, the observed divergence has been significantly larger for DFT than for Hartree–Fock results. In the latter case the observed divergence between theoretical and experimental 13C NMR chemical shifts of chlorine-bonded carbons is systematic but rather small (3.4–4.4ppm).

Phosphorus hyperfine structure in the electronic spectrum of the HPCl free radical.

The 444 nm 2 0 (1) bands of the A 2A'-X 2A" transition of the jet-cooled HP 35Cl and HP 37Cl radicals have been studied at high resolution using the pulsed electric discharge technique with a precursor mixture of PCl3 and H2. Spectra recorded with linewidths of approximately 360 MHz revealed resolved hyperfine structure in both isotopomers arising from the excited state Fermi contact interaction of the unpaired electron with the magnetic moment of the 31P nucleus, with aF'=0.0641(10) cm(-1) and 0.0636(31) cm(-1) for HP 35Cl and HP 37Cl, respectively. No contribution from the ground state, or excited state contributions from the hydrogen or chlorine nuclei were resolved, confirming ab initio predictions that HPCl is a p pi radical in the X state, and an s sigma radical with a substantial contribution from the phosphorus 3s atomic orbital in the A state. The free atom comparison method has been used to estimate that the singly occupied molecular orbital in the excited state has 14% phosphorus 3s character.

A new method for distinguishing between Al 2X 6 (X=Cl, Br) conformers based on ab initio calculated nuclear quadrupole coupling constants

Nuclear quadrupole coupling constants ( χ) of 27Al, 35Cl and 81Br in AlX 3 monomers as well as Al 2X 6 (X=Cl, Br) dimers are calculated at the RHF/6-311G* and B 3LYP/6-311G* levels, using G aussian 98 package. Correlations are made between χ and dihedral angles θ, of Al 2X 6. These θs are produced through the ring puckering motions about the hinge line which joins the two bridge halogens (X b). Nuclear quadrupole coupling constants of 35Cl, 81Br and 27Al are used as probes for monitoring the departure of the symmetry of Al 2X 6 from a high symmetry point group D 2h to a lower one. The χs of chlorine nuclei of AlCl 3 differ significantly from those of Al 2Cl 6. These differences appear negligible for AlBr 3 in comparison to Al 2Br 6. This work demonstrates the considerable sensitivity of nuclear quadrupole resonance in distinguishing between Al 2X 6 conformers. This is in comparison to the usage of energy differences which is customarily employed.

An ab initio study of trans-1-chloro-2-fluoroethylene: Equilibrium structure and molecular properties

The equilibrium structure and molecular properties of trans-1-chloro-2-fluoroethylene have been investigated theoretically at high level of theory. Very accurate results are presented. As far as the equilibrium structure is concerned, coupled-cluster approach with perturbative inclusion of triples and basis sets of triple and quadruple zeta quality have been employed. Core correlation effects and basis set limit extrapolation have been taken into account in order to obtain best estimates of equilibrium geometry. The molecular dipole moment has been calculated at coupled-cluster level using bases of different quality including diffuse functions and performing the extrapolation to the infinite basis set limit. In addition, the complete inertial nuclear quadrupole tensor, evaluated from the electric field gradient at the chlorine nucleus, has been computed at different level of theory: The multiconfiguration self-consistent field, the Møller–Plesset many-body perturbation to second order and the coupled-cluster methods have been employed. Finally, the harmonic force field has been calculated by means of coupled-cluster approach with a basis of triple zeta quality.

Studies of molecular dynamics of thiazides by35Cl NQR spectroscopy and DFT calculation

Molecular dynamics of three derivatives of 1,2,4-benzothiadiazine-1,1-dioxide, hydrochlorothiazide (HCTZ), althiazide (ATZ) and chlorothiazide (CTZ), was studied by35Cl nuclear quadrupole resonance (NQR) spectroscopy. The temperature dependence of the resonance frequency was analyzed within the 6 known standard models. The activation energies estimated from the temperature dependence of the35Cl NQR frequency assuming the Bayer model were 1.07, 2.35 and 2.76 kJ/mol for HCTZ, ATZ and CTZ respectively, which confirms that HCTZ is less rigid than CTZ and ATZ is much more rigid than HCTZ, and suggests that the mechanism of relaxation is based on small amplitude librations. The characteristic temperatures estimated from the Bayer model, with that for CTZ (332.5 K) being much higher than for HCTZ (132.1 K), mean that the intermolecular interactions in CTZ are much stronger than in HCTZ, as suggested by the melting point of CTZ being higher than that for HCTZ. For ATZ the characteristic temperature (288 K) takes an intermediate value, which suggests that the intermolecular interactions in this compound are stronger than in HCTZ and weaker than in CTZ. A significant narrowing of the resonance35Cl NQR line observed for all these compounds at room temperature, relative to that at the liquid nitrogen temperature, suggests an averaging of dipolar interactions as a result of fast rotation of nonquadrupole nuclei in the vicinity of the quadrupole nuclei, when 2πνQτc ≫ 1 (a rotation of the −NH2 group in the direct neighborhood of the chlorine nuclei) or a change in the gradient orientation with its value preserved (which is equivalent to rotation of the quadrupole nucleus Cl). The influence of the rotations of the −NH2 and −CH2SCH2CH=CH2 groups (ATZ) or −CHCl2 group (TCTZ) on the35Cl NQR frequency was modelled by the B3LYP/6-31G* method. The frequencies of the libration vibrations calculated from the temperature dependence of the NQR resonance frequency were compared with experimental ones and those implied by the density functional theory, infrared and Raman spectra. For HCTZ the anomalies in the temperature dependence of the35Cl NQR frequency, the lack of hysteresis and small but notable changes in the slope and the jump in the frequency observed at 253 K which does not exceed 0.05 MHz suggest a second-order phase transition at 253 K.

Nuclear quadrupole resonance in 2,4-dinitrochlorobenzene

A study of the spin lattice relaxation ( T 1) and the nuclear quadrupole resonance frequency ( ν Q) gives an important information about the dynamics of molecular groups in molecular solids. In the present paper, we analyze the contributions of the reorientational motion of nitro groups of 2,4-dinitrochlorobenzene to the NQR parameters of the chlorine nucleus in the molecule. We found two contributions to T 1 and ν Q due to the onset of the reorientation of nitro groups in the molecule; one of these contributions is mostly due to intermolecular effects in the crystal. For the chlorine nuclei, the efficiency of the modulation mechanism is usually provided by the change of the electric field gradient due to the moving molecular group; this gives us a way of how to assign each contribution to T 1 from the ortho and para positions of the NO 2 groups in the molecule. It is observed that there are two different potential barriers depending on the position of the nitro groups in the molecule. The behavior in the temperature dependence of the line width shows a thermal history dependence of the molecular crystal.

Dynamic orientational disorder of the nitro group in 2-chloro-nitrobenzene revealed through35Cl nuclear quadrupole resonance

In the present paper we analyse the contributions of nitro groupmovements in 2-chloro-nitrobenzene to the nuclear quadrupoleresonance (NQR) parameters of the chlorine nucleus in the molecule.We found two contributions to the spin–lattice relaxation time(T1) and the NQRfrequency (νQ)due to the onset of nitro group movements in the molecule. One of thesecontributions is the well-known semirotation of the nitro group around the N–C axis.The other one is attributed to some tilting or tipping of the nitro plane away fromthe benzene ring introducing some dynamic orientational disorder of this group inthe crystal only observed as a contribution from the temperature dependence ofT1 andνQ.Its activation energy is similar to that of the nitro group reorientation(21.9 and 23.6 kJ mol−1 for the two processes) and may arise fromcompeting crystalline and steric chlorine nucleus effects. The presentinvestigation shows that in chloronitrobenzenes the NO2 group dynamicorientational disorder can produce modulation effects on the chlorineT1which are large enough to be observed by means of the NQR.

The Microwave Spectrum and Quadrupole Coupling Constants of cis-2-Chlorophenol

The rotational spectrum of cis-2-chlorophenol was observed from 5 to 12 GHz by molecular beam Fourier transform microwave spectrometer (MB-FTMW). The rotational and quadrupole coupling constants for the 35Cl of the molecule were determined: A=2985.4479(22) MHz, B=1549.8591(3) MHz, C=1020.1915(1) MHz, χaa=−68.2429(83) MHz, χ−=−0.922(20) MHz, χab=−11.4(19) MHz. The quadrupole coupling constants of the chlorine nuclei of cis-2-chlorophenol were nearly equal to that of the chlorobenzenes. We concluded that the electric field gradient of the chlorine atom in cis-2-chlorophenol is similar to that of other chlorobenzenes, although the suggestion of intramolecular hydrogen bonding.

NMR Cross-Relaxation Study of Ultraslow Motion of the DomainWall in Channel Inclusion Compound

We report on an NMR study of ultraslow motions in the channel thiourea-hexachloroethane inclusion compound, [2.95(NH2)2CS] C2Cl6. Temperature dependent 1H NMR relaxation measurements of the powder thiourea-hexachloroethane at different resonance frequencies, from 23 to 55.5 MHz, have been carried out. Significant reduction of the spin-lattice relaxation time at 38 and 47 MHz is caused by the cross-relaxation of protons via the quadrupole chlorine nuclei. We show that the effective 1H-35Cl cross-relaxation observed in a powder sample is due to a slow mode process, when the molecules of the domain wall exhibit a correlated translational and rotational motion over the channel. Such propagation of the domain wall is confirmed by atom-atomic potential calculation. - Pacs: 76.60-k, 76.60.Es, 61.44.Fw, 61.66.Hq

Residual 31P, 35,37Cl Dipolar Coupling in 31P MAS Spectra of Chlorocyclophosphazenes

In 31P MAS NMR spectra of chlorocyclophosphazenes, characteristic splittings have been observed for PCl or PCl2 groups. At different applied magnetic fields, the fine structure and total width of the patterns change in a characteristic way, demonstrating that the splittings are due to indirect spin–spin and residual dipolar interactions with the chlorine nuclei directly bonded to phosphorus. For trans-nongeminal N3P3Cl3(NMe2)3 and N3P3Cl6 as examples, the spectra have been analyzed to obtain information on chlorine nuclear quadrupole coupling constants and 35,37Cl, 31P indirect spin–spin coupling constants. Neglect of these interactions may result in misinterpretations of the multiplicity in 31P MAS spectra of chlorophosphazenes.

Ultraslow motion of domain wall in channel inclusion compound studied by means of NMR cross-relaxation

We report temperature dependent 1H NMR relaxation measurements of the powder thiourea-hexachloroethane inclusion compound, [2.95(NH 2) 2CS]·C 2Cl 6, at different resonance frequencies, from 23 to 55.5 MHz. Significant reduction of spin–lattice relaxation time at 38–47 MHz is caused by the cross-relaxation of protons via the quadrupole chlorine nuclei. We conclude that the effective 1H– 35Cl cross-relaxation observed in powder sample is due to a slow mode process, when the molecules of the domain wall exhibit a correlated translational and rotational motion over the channel. Such a propagation of the domain wall was predicted by atom-atomic potential calculation. We show that the NMR cross-relaxation effect is an effective tool in studying ultraslow motions, in particular in incommensurate structures.

Nuclear Quadrupole Tensors for 35Cl and 37Cl in cis-1-chloro-2-fluoroethylene Obtained by Detection of Perturbation-Allowed ΔJ = 2 and ΔJ = 3 Transitions

The nuclear quadrupole hyperfine structure in rotational transitions of cis-1-chloro-2-fluoroethylene was measured for both 35Cl and 37Cl containing isotopomers in the millimeter-wave region. Near degeneracies of the 93,7 and 102,9 levels of 35ClHCCFH and of the 199,10/11 and 1710,7/8 levels of 37ClHCCFH cause strong perturbations of the hyperfine patterns and give rise to perturbation-allowed ΔJ = 2 and ΔJ = 3 transitions. Lamb-dip measurements of such perturbed hyperfine patterns and of forbidden ΔJ = +2 and perturbation-enhanced ΔJ = 0 transitions of 35ClHCCFH provided an accurate determination of χab, the off-diagonal element of the inertial nuclear quadrupole tensor. For 37ClHCCFH, χab was determined for the first time thanks to the observation of perturbed hyperfine patterns and of forbidden ΔJ = +3 and ΔJ = −2 transitions. Additional measurements of hyperfine structures led to improved values for the diagonal elements of the χ tensor of both 35Cl and 37Cl. Moreover, the complete inertial nuclear quadrupole tensor was evaluated from the field gradient at the chlorine nucleus computed by using the Multi-Configuration Self-Consistent Field (MCSCF) approach and employing basis sets of triple-zeta quality: very good agreement with the experiment was obtained.

H NMR Study of Ionic Motions in High Temperature Solid Phases of (CH3NH3)2ZnCl4

Abstract The reorientation of the tetrahedral complex anion ZnCl4 2- and the self-diffusion of the cation in (CH3NH3)2ZnCl4 were studied by 1H NMR spin-lattice relaxation time (1H T1) experiments. In the second highest-temperature phase, the temperature dependence of 1H T1 observed at 8.5 MHz could be explained by a magnetic dipolar-electric quadrupolar cross relaxation between 1H and chlorine nuclei, and the activation energy of the anion motion was determined to be 105 kJ mol -1 . In the highest-temperature phase, the activation energy of the self-diffusion of the cation was determined to be 58 kJ mol -1 from the temperature and frequency dependence of 1H T1

35Cl NQR and structural-dynamic properties of crystalline CCl3C(O)OC6H2Cl3-2,4,6

The temperature dependences of the resonance frequency and quadrupole spin-lattice relaxation time of the chlorine-35 nuclei in crystalline CCl3C(O)OC6H2Cl3-2,4,6 were studied. Owing to the presence of resonant nuclei in various fragments of the molecules, the character of dynamics of these molecular fragments and the entire molecule was established. It is shown that thermal librations of molecules are quasiharmonic over the whole temperature range from 77 K to the melting point of the crystal. The reorientational motion of the CCl3 group bonded to the three-coordinated carbon atom was revealed. This motion causes an exponential increase in the spin-lattice relaxation time of the chlorine nuclei of this group and subsequent damping of NQR signals (chlorine-35 resonance signals of the aryl radical were observed before melting of the sample). The activation energy of the reorientational motion is found; its value, which is 27.3 kJ/mole, is considered in comparison with the activation energies obtained by the Cl NQR method in solids for CCl3 reorientations in similar molecular structures.