35Cl Nuclear Quadrupole Resonance Spectra Research Articles

Direct Investigation of Covalently Bound Chlorine in Organic Compounds by Solid‐State 35Cl NMR Spectroscopy and Exact Spectral Line‐Shape Simulations

35/37Cl NMR spectroscopy studies of organic systems are very rare, with only a few neat liquids having been studied.1 The lack of chlorine NMR spectroscopy data may be explained by the fact that 35Cl and 37Cl are quadrupolar (spin I=3/2) and low-frequency isotopes. The quadrupole moments of the chlorine nuclei couple with the electric field gradient (EFG) tensor at the nuclei; this phenomenon is known as the quadrupolar interaction (QI). The quadrupolar coupling constant, CQ, and the quadrupolar asymmetry parameter, ηQ, describe the magnitude and asymmetry of the QI. In solution, one of the consequences of the QI is fast relaxation, which means that the 35/37Cl NMR signals for covalently bound chlorines are very broad and are of low intensity.1 For these reasons, chemically distinct chlorine sites are very difficult to distinguish with solution NMR spectroscopy. However, in the solid state, nuclear spin relaxation is typically slower, thus enabling higher quality 35Cl NMR spectra to be collected, at least in principle. Unfortunately, the magnitude of the QI for covalently bound chlorines is very large because of the substantial, anisotropic EFG at the Cl atom, owing mainly to its electronic configuration when it forms a chlorine–carbon bond. Conventional wisdom is that such chlorine sites cannot be studied in powders by solid-state NMR spectroscopy as the central transition (CT; mI=1/2↔−1/2) can span tens of megahertz in typical commercially available magnetic fields. For this reason, only ionic chlorides2 and inorganic chlorides3 have been studied, as the EFG at these chlorides is often an order of magnitude smaller than at covalently bound chlorine atoms in organic molecules. The bonding environments for these types of chlorine atoms are substantially different from the environments in those chloride-containing molecules that have been studied previously.2, 3 A partial 35Cl NMR spectrum for hexachlorophene has been briefly mentioned in the literature.4 On the other hand, most of the interesting chlorine chemistry occurs when Cl is covalently bound to a carbon atom, where the chlorine atom often acts as a leaving group. Chlorine atoms are also important in many organic pharmaceuticals as well as in crystal design applications where they can form halogen bonds.5 Recent studies show that covalently bound chlorine is also important in biological chemistry where, for example, the tryptophan 7-halogenase was found to selectively chlorinate tryptophan moieties.6

The application of frequency swept pulses for the acquisition of nuclear quadrupole resonance spectra

The acquisition of nuclear quadrupole resonance (NQR) spectra with wideband uniform rate and smooth truncation (WURST) pulses is investigated. 75As and 35Cl NQR spectra acquired with the WURST echo sequence are compared to those acquired with standard Hahn-echo sequences and echo sequences which employ composite refocusing pulses. The utility of WURST pulses for locating NQR resonances of unknown frequency is investigated by monitoring the integrated intensity and signal to noise of 35Cl and 75As NQR spectra acquired with transmitter offsets of several hundreds kilohertz from the resonance frequencies. The WURST echo sequence is demonstrated to possess superior excitation bandwidths in comparison to the pulse sequences which employ conventional monochromatic rectangular pulses. The superior excitation bandwidths of the WURST pulses allows for differences in the characteristic impedance of the receiving and excitation circuits of the spectrometer to be detected. Impedance mismatches have previously been reported by Marion and Desvaux [D.J.Y. Marion, H. Desvaux, J. Magn. Reson. (2008) 193(1) 153–157] and Muller et al. [M. Nausner, J. Schlagnitweit, V. Smrecki, X. Yang, A. Jerschow, N. Muller, J. Magn. Reson. (2009) 198(1) 73–79]. In this regard, WURST pulse sequences may afford an efficient new method for experimentally detecting impedance mismatches between receiving and excitation circuits, allowing for the optimization of solids and solution NMR and NQR spectrometer systems. The use of the Carr–Purcell Meiboom–Gill (CPMG) pulse sequence for signal enhancement of NQR spectra acquired with WURST pulses and conventional pulses is also investigated. Finally, the utility of WURST pulses for the acquisition of wideline NQR spectra is demonstrated by acquiring part of the 63/65Cu NQR spectrum of CuCN.

Nuclear quadrupole resonance in two sublattice 2-D antiferromagnetic cuprate Sr 2Cu 3O 4Cl 2

The results of the first nuclear quadrupole resonance (NQR) investigations on the 63,65Cu and the 35Cl nuclei in oxychloride Sr 2Cu 3O 4Cl 2 are presented. The copper spectrum above T N2=40 K is attributed to 63,65Cu Zeeman spectrum of Cu1 nuclei in the local in-plane magnetic field of 93 kOe perturbed by quadrupole splitting in the first order. Below T N2 NQR spectrum is essentially changed and the low-frequency part appears, which can be assigned to the ordering of the Cu2 magnetic subsystem. The 35Cl NQR spectra reveals the existence of the local magnetic field of 210 Oe on Cl site pointing to the ferromagnetic alignment of the Cu1 magnetic moments along the c-axis.

Menshutkin's {(Arene)·nSbX3} Molecular CompoundsExploring the Correlation between Crystal Structures and Nuclear Quadrupole Resonance Spectra

By use of data from the literature to assign 35Cl, 81Br, 121Sb, and 123Sb nuclear quadrupole resonance (NQR) frequencies to particular atoms, an attempt is made to correlate crystal structures of a number of {(arene)·nSbX3} (X = Cl, Br) Menshutkin molecular compounds with their NQR spectra. Comparing NQR spectra of crystalline neat SbX3 (X = Cl, Br) with those of the molecular compounds shows well-established effects due to the perturbations introduced by changes in the detailed structures of the networks of SbX3 moieties and in the interactions of Sb atoms with the arene moieties. NQR spectra are much more sensitive in this regard than the changes in interatomic distances that are among the main quantitative outcomes of crystal structure analysis. Although a qualitative pattern can be discerned that associates the frequency of the 35Cl NQR spectra with the different structural roles of the crystallographically independent chlorines, it has not been possible to identify particular interactions with the differences found in the NQR frequencies. Generalizations based on NQR spectra do not appear currently achievable, and it seems likely that the spectra of the various molecular compounds will require explanation on an individual basis.

Comparison of 35Cl NQR Spectra between the Mixed Crystals K2Sn1-xRexCl6 and the Al3+ Doped Crystals K2SnCl6:Al3+

Abstract 35Cl Nuclear Quadrupole Resonance (NQR) has provided a valuable tool for investigating the local structure in mixed crystals K2S1-xRexCl6 and Al3+ doped crystals K2SnCl6:Al3+ . The measured NQR line shapes and relaxation times of both kinds of impurity containing crystals in the cubic phase of the host K2SnCl6 show markedly different impurity effects; static impurity effects in K2Sn1-x RexCl6 and dynamic effects in K2SnCl6 :Al3+ . The 35Cl NQR spectra of K2SnCl6:Al3+ near the transition temperature (Tc) indicate the presence of pretransition of the local structure in the high temperature cubic phase.

35Cl nuclear quadrupole resonance and crystal structures of small guest molecules in tris(2,4-pentanedionate)M(III) inclusion compounds

Tris(acetylacetonate)M(III) complexes, where M = AI, V, Cr, Mn, Fe, Co and Ga, form inclusion compounds with different solvent molecules. The compounds have a host/guest ratio of 1:2 for the small guest molecules CHCl 3 and CH 2Cl 2 and of 1:1 for the slightly bigger guest molecules trans-CHClCHCl, CH 2ClCH 2Cl and CCl 4. The 35Cl nuclear quadrupole resonance spectra of these clathrates have been measured in the temperature range 4–200 K. The site symmetry, the existence of different crystal phases, the molecular disorder and molecular motions of the guest molecules in the host cavities, and the host-guest interactions, were evaluated from the NQR frequency measurements. The crystal structures of the M(AcAc) 3/ trans-CHClCHCl compounds were measured at room temperature and confirm the NQR results.

Crystal Structure and Nuclear Quadrupole Resonance. The Bond Length C — Cl and 35Cl NQR

AbstractThe intramolecular distances dC—Cl for chlorine bound to sp2 carbon and to sp3 carbon, respectively, and the 35Cl NQR frequencies assigned to the corresponding Cl atoms correlate quite well according to the theory: v(35Cl) ∼ (dC—Cl)−3. The relation is v(35Cl) = (4 ± 6) MHz + (166 ± 31) · 106 · (dC—Cl)−3 · MHz · pm3 for {sp2‐C}‐Cl and v(35Cl) = (21 ± 5) MHz + (101 ± 25) · 106 · (dC—Cl)−3 · MHz · pm3 for {sp3‐C}‐Cl. — The crystal structures and chemical bonds of 1 (4‐methyl‐4‐trichloromethyl‐2,5‐cyclohexadiene‐1‐one), 2 (2‐chloro‐4,5‐dimethyl‐4‐trichloromethyl‐2,5‐cyclohexadiene‐1‐one), 3 (4,5‐dichloro‐1‐phenyl‐pyridazine‐6‐one), and 4 (2,2,4,5‐tetrachloro‐cyclo‐pentene‐1,3‐dione) were studied by single crystal X‐ray diffraction and 35Cl nuclear quadrupole resonance (NQR). 1 crystallizes with the space group D152h‐Pbca, Z = 8. The lattice constants are a = 1206.0(3) pm, b = 1341.3(3) pm, and c = 1178.8(3) pm. 2 crystallizes monoclinic, space group C52h‐P21/c, Z = 4, a = 696.3(3) pm, b = 1404.0(5) pm, c = 1151.4(5) pm, and β = 100.23(1)°. 3 crystallizes in the orthorhombic space group D4h‐P212121, Z = 4, a = 4303(1) pm, b = 586.8(2) pm, and c = 390.5(1) pm. 4 crystallizes cubic, space group T4h‐Fd3, with 48 molecules in the unit cell, a = 2181.5(5) pm. The 35Cl NQR spectra were observed for the two 2,5‐cyclohexadiene‐1‐ones, 1 and 2 in the temperature range 77 < T/K < melting point. For 3, the 35Cl NQR spectra was determined at T = 77 K and T = 295 K. No phase transition has been found for the three compounds in the temperature range in which the 35Cl NQR spectroscopy was performed.

35Cl nuclear quadrupole resonance spectra of paradichlorobenzene as a guest molecule in clathrates formed by the werner complexes M (II)(L 4)X 2

The 35Cl nuclear quadrupole resonance (NQR) spectra of paradichlorobenzene included in the Werner complexes M (II) (L 4)X 2 (L=4-picoline, 4-ethylpyridine, 4-vinylpyridine or 4-acetylpyridine; M (II)=Mn, Fe, Co or Ni; X − =SCN − or SeCN −) are reported. In all these complexes only one 35Cl resonance is exhibited, thus showing that there is only one type of inclusion site and that the symmetry of the site preserves the equivalence of the two chlorine atoms in PDCB. The temperature dependence of the 35Cl NQR frequency provides a measure of the degree of mobility of the guest molecule and shows that this mobility increases in the order Ni> Co> Fe> Mn and in the order 4-vinylpyridine>4-ethylpyridine>4-methylpyridine. In general, a change of anion from SCN − to SeCN − also increases the guest mobility. An independent check on the space available to the guest molecule has been obtained by measuring the unit-cell dimensions for a number of these complexes.

Nuclear quadrupole resonance studies of chelated antimony complexes. Part 6. Tricyclic molecules

The 35Cl Nuclear Quadrupole Resonance spectra of three tricyclic molecules of known structure containing SbCl 3 fragments have been measured. Several other analogous compounds have been shown, by a combination of 35Cl, 121Sb and 123Sb NQR spectroscopy, to have similar structures. The relationship between bond length and 35Cl resonance frequency for these compounds containing SbCl 3 fragments was investigated; the resonance frequency is less sensitive to bond length than related complexes containing the SbCl 4 group.

Nuclear quadrupole resonance studies of chelated antimony complexes. Part 5. Eight-membered rings containing phosphorus or sulphur

The 35Cl Nuclear Quadrupole Resonance spectra of two cyclic phosphorus-containing compounds, ((CH 3) 2PO 2SbCl 4) 2 and (Cl 2PO 2SbCl 4) 2, and five sulphur-containing compounds, (RSO 3SbCl 4) 2, which have been supposed to have similar structures, are reported. The NQR spectra support the hypothesis that the sulphur compounds have similar structures to the phosphorus compounds while showing that for (FSO 3SbCl 4) 2 the symmetry of the eight- membered ring is different from that of the other six compounds.

35Cl NQR study of the anomalous ferroelectric-paraelectric transition in (CH 3NH 3)HgCl 3

A detailed study of 35Cl nuclear quadrupole resonance spectra of (CH 3NH 3)HgCl 3 has been carried out from room temperature up to 80°C. A discontinuous change in the Cl spectra and a large thermal hysteresis were observed at the first-order phase transition temperature T c = 62° C. In the ferroelectric room temperature phase three Cl lines were observed, as predicted from the crystal structure. In the high temperature paraelectric phase the number of Cl lines increases to five, demonstrating a decrease in the point group symmetry. According to our knowledge this is the first time that such an effect has been observed at a paraelectric-ferroelectric phase transition.

35Cl NQR Spectra of a layer compound of graphite with tin(IV) chloride

In a layer compound of graphite, C 54SnCl 6, the existence of an octahedral SnCl 6 2− anion has been established by means of 35Cl nuclear quadrupole resonance spectra.

14N and 35Cl nuclear quadrupole resonance data for nitrogen mustards: Attempted correlations with chemical and biological activities

AbstractThe 14N nuclear quadrupole resonance (NQR) spectra of several nitrogen mustard compounds are reported and analyzed in the framework of the Townes and Dailey theory. For the aniline derivatives, a correlation exists between l–σ (where l is the nitrogen lone pair electron density and σ the average NC sigma‐bond electron density) and the enhanced Hammett sigma constant σ−. An improved correlation is obtained between l–σ and σR−, which emphasizes the importance of resonance effects in determining l–σ. The increase of hydrolysis and alkylation rates with increasing values of l–σ is in agreement with the identification of the aziridinium ion as the intermediate in aromatic nitrogen mustard hydrolysis and alkylation. A possible correlation is noted between the 35Cl NQR spectra for some of the mustards and measures of toxic and antitumor activity.

35-Cl nuclear quadrupole resonance spectra in diazepam and its 1:1 complex with chloral hydrate

The 35-Cl NQR spectra of diazepam and some of its related compounds are reported; they yield information about the hydrogen bonded complex that diazepam forms with chloral hydrate. The results reflect large changes in electron distribution at the CCl 3 group while the diazepam chlorine at the 7 position remains almost unaffected.

Observation of several phase transitions in tetrachloro- o-benzoquinone using 35Cl nuclear quadrupole resonance

The temperature dependence of the 35Cl nuclear quadrupole resonance spectra of tetrachloro- o-benzoquinone (TOB) is reported. An analysis of the change in line multiplicities suggests that TOB undergoes three phase changes in going from 77 to 330 K. The transition temperatures have been determined.

Reaction of 5-substituted 4,6-dichloro-2-aminopyrimidines with oxalyl chloride

The reaction of 5-substituted 4,6-dichloro-2-aminopyrimidines with oxalyl chloride gives 2-isocyanatopyrimidines, the structures of which were proved by chemical means and the IR, PMR, and 35Cl nuclear quadrupole resonance spectra. The intermediate pyrimidinyloxamic acid chlorides were not isolated; this is evidently explained by the low basicities of 2-amino-pyrimidines.

Use of chlorine-35 nuclear quadrupole resonance spectroscopy for structural assignments in chlorocyclotriphosphazatrienes

The 35Cl nuclear quadrupole resonance spectra of a series of aminochlorocyclotriphosphazatrienes N3P3Cl6–nRn(R = NMe2, n= 1–4; R = NC5H10, n= 1–3) have been recorded at 77 and/or 273 K. These data show that positional isomers may be distinguished, but that unambiguous structural assignments to geometrical isomers are not generally possible. Similar conclusions may be drawn for other aminochloro- and phenylchloro-cyclotriphosphazatrienes. Resonance frequencies diagnostic of PCl2, PCl·NR2, and PClPh groups fall in the ranges 26–29, 22–25, and 23–25 MHz respectively, but the last two groups cannot be distinguished by this technique.

35Cl Nuclear quadrupole resonance spectra of some carboranecarboxylic chlorides

A study was made of the35Cl NQR spectra in the series of o-, m-, and p-carboranecarboxylic acids. Some of the highest NQR frequencies of the chlorine atom in the COCl group are observed in the studied compounds.

35Cl nuclear quadrupole resonance spectra of etherates of trichloromethyl derivatives of mercury

Complexes of trichloromethyl derivatives of mercury were investigated. In a series of cases, the intramolecular coordination C-Cl... Hg which exists in the initial compound is disturbed upon complex formation.

Kernquadrupolresonanz(KQR)-Spektroskopie an 35Cl in Komplexverbindungen von Arsentrichlorid, AsCl3, mit einigen aromatischen Kohlenwasserstoffen

By measuring the 35Cl nuclear quadrupole resonance (NQR) spectra it is shown that AsCl3, like SbCl3, is able to form crystalline complex compounds with aromatic hydrocarbons. Investigation of the freezing-point diagrams reveal the existence of the following compounds: 2 AsCl3 · CeH6; 2 AsCl3 · C10H8; 2 AsCl3 · (C6H5) 2CH2; 2 AsCl3 · p-C6H4 (CH3)2; 2 AsCl3· C6H5C2H5 and AsCl3 · C6H5C2H5. The 35Cl NQR spectra of these compounds are given and compared with the NQR spectra of the analogous SbCl3-complexes.