35Cl NQR Research Articles

Non-covalent Cl⋯X interactions in several silver compounds based on [B12Cl12]2− clusters: 35Cl NQR and X-ray diffraction

Silver cluster compounds Ag2[B12Cl12](1), (Ag(CH3CN)4)2[B12Cl12] (2), and (Ag(CH3CN)2)2[B12Cl12] (3) with very different types of silver environment in crystals were synthesized and investigated using X-ray diffraction analysis (XRD), X-ray phase analysis and nuclear quadrupole resonance (35Cl NQR). Non-covalent interactions of chlorines with surrounding atoms were identified and analyzed using 35Cl NQR spectroscopy. The results were compared with those of XRD to show that the “short” contacts between atomic nuclei do not necessarily indicate the presence of an attraction interaction between the corresponding atoms. Numerous weak interhalogen Cl⋯Cl interactions were revealed in 1, while the Ag⋯Cl ineractions seem to be weakened due to high coordination number of silver atom. In 2, no Ag⋯Cl interactions were detected by X-ray diffraction, while non-covalent Cl⋯C, Cl⋯H and Cl⋯Cl interactions were found. As the NQR spectra indicated, interhalogen interactions in 2 appeared to be the strongest of all the others. In 3, interhalogen Cl⋯Cl interactions were the weakest, the largest contribution to the electric field gradient on the Cl atoms being made by silver atoms.

Salts Based on Perchlorinated Closo-Dodecaborate Anion: First 35cl Nqr Studies and Crystal Structure

Salts Based on Perchlorinated Closo-Dodecaborate Anion: First 35cl Nqr Studies and Crystal Structure

Noncovalent interactions in perchlorinated sulfonium and ammonium derivatives of closo-decaborate anion: 35Cl NQR and crystal structure

Three salts of perchlorinated sulfonium and ammonium derivatives of tetrabuthylammonium nonachloro-closo-decaborate, (n-Bu4N)2[2-B10Cl9-cyclo-S(CH2)4S-B10Cl9] (1), (n-Bu4N)[2-B10Cl9-cyclo-S(C2H4)2O] (2) and (n-Bu4N)[1-B10Cl9N(n-Pr3)] (3) are synthesized and characterized using IR, 11B, 1H, 13C NMR, X-Ray and 35Cl NQR-spectroscopy. The noncovalent interactions involving chlorine atoms were identified by 35Cl NQR spectra. The results were discussed in comparison with those of X-ray diffraction analysis which are based on measuring contacts shorter than the sum of van der Waals (vdW) radii of atoms. It has been found that the crystal lattice of 2 exhibits dynamic properties strongly different from those of 1 and 3. Whereas the 35Cl NQR spectra of the latters were extremely wide (up to 1200 kHz), the spectrum of 2 demonstrated a narrowing of almost an order of magnitude (up to 120 kHz) which indicated a significant weakening of the tendency for the formation of intermolecular bonds by chlorine atoms of this cluster.

Structural and functional synthesis of the continuous wave NQR temperature sensor with increased conversion linearity

The paper describes development of the detailed structure and circuit diagrams of the continuous wave NQR temperature sensor with increased conversion linearity. It is experimentally established that at amplitude modulation of 40% and change of input voltage in the range of 20–1000 ​mV, the circuit of a symmetric marginal oscillator with a linear active demodulator provides better linearity of transfer characteristic than the circuits of asymmetric marginal oscillators with JFET or diode detectors. As a thermometric substance of the proposed NQR sensor, copper oxide Cu2O was used, which is characterized by a strong temperature dependence of the resonance frequency of 63Cu NQR. In contrast to 35Cl NQR in KClO3, for cuprous oxide the temperature dependence of 63Сu NQR frequency in the frequency range 26.621–25.658 ​MHz is linear in the temperature range 100–390 ​K. It is experimentally confirmed that the use of a low mass sample (less than 200 ​mg) as a thermometric substance of the proposed NQR sensor is quite sufficient for successfully observation of the resonance line at the SNR equal to 9.1 ​dB.

Study of the polymorphic states of chloral hydrate by 35Cl NQR relaxometry

In this article the results of an experimental study of the molecular crystal of chloral hydrate in α - and β -forms by 35Cl NQR relaxometry method with using of inverse Laplace transformation are presented. The results indicate the presence of intramolecular hydrogen bridge bonds of two chlorine atoms of C-Cl3 groups with hydrogen of hydroxyl groups. An analysis of the spin-lattice relaxation times shows that the mobility of all chlorine atoms increases upon the transition from the unstable β - form of chloral hydrate to the stable α – form.

Noncovalent Interactions in Compounds Based on Perchlorinated Boron Cluster as Monitored by 35Cl NQR (Review)

The article surveys the first 35Cl NQR results on detecting and characterizing noncovalent interactions in systems assembling [B10Cl10]2− clusters via halogen atoms. A number of compounds of decachloro-closo-decaborates with alkali metal or organic cations as well as with silver(I), copper(II) and iron(II) complexes were studied using 35Cl NQR and X-ray diffraction. The main interest of this survey is a capability of chlorine atoms to participate in noncovalent interactions with organic cations, ligand and solvent molecules without touching upon issues of synthesis. Whereas X-ray diffraction establishes the noncovalent interactions relying on the sum of vdW radii which serves as the basic and sometimes the only experimental criterion for identifying such interactions, the 35Cl NQR technique registers subtle perturbations in electron density distribution on the chlorine site caused by secondary interactions Cl···X. They adequately split the 35Cl NQR spectra thus selecting from the entire set of interatomic contacts those actually induced by noncovalent interactions.

35 Cl NQR frequency and spin lattice relaxation study in 3,4-dichloronitrobenzene as a function of temperature and pressure.

35 Cl NQR frequency and spin lattice relaxation time in 3,4-dichloronitrobenzene have been measured as a function of temperature and pressure. Two NQR signals were observed in the temperature range 77 to 300K and pressure up to 5.1kbar at 300K. The contributions to the relaxation from the torsional motion of the molecule and reorientational motion of the nitro group have been analyzed on the basis of the Woessner and Gutowsky model. The temperature dependence of the average torsional lifetimes of the molecules, transition probabilities, and the activation energy for the reorientation of the nitro group was estimated. The pressure dependence of the NQR frequency in 3,4-Dichloronitrobenzene shows a nonlinear increase in NQR frequency with increase in pressure, indicating increased contribution from the static effects at higher pressures. A thermodynamic analysis of the data was carried out to determine the constant-volume temperature coefficients of the NQR frequency. The spin-lattice relaxation was found to be weakly dependent on pressure.

Synthesis, structure, and physicochemical properties of triply-bridged binuclear copper(II) complex [Cu2Phen2(µ-CH3CO2)2(µ-OH)]2[B10Cl10

The reaction between copper(II) acetate and Phen in the presence of [B10Cl10]2− anion affords mononuclear copper(II) complex [CuPhen2(CH3CO2)]2[B10Cl10]·[CuPhen(CH3CO2)2]. Continuous heating of the latter in DMSO results in triply-bridged binuclear copper(II) complex [Cu2Phen2(CH3CO2)2OH]2[B10Cl10] isolated from the reaction solution as a DMSO solvate hydrate [Cu2Phen2(CH3CO2)2OH]2[B10Cl10]·3DMSO·0.5H2O. Both complexes are studied by IR, 11B NMR, and X-ray diffraction; the binuclear complex is studied by 35Cl NQR and EPR spectroscopies as well as X-ray powder diffraction technique. Despite the short Cu−Cu distance revealed by X-ray diffraction analysis in the binuclear cationic complex (3.234 Å), no interactions between copper atoms are detected by EPR spectroscopy. According to 35Cl NQR studies, chlorine atoms bound with equatorial boron vertices of the boron cage participate in non-covalent interactions with H atoms of Phen molecules.

Reoriention of diprotonated DABCO (1,4-Diazabicyclo[2.2.2]octane) cation and proton transfer in organic ferroelectric adduct DABCO-2(2-Chlorobenzoic acid)

Temperature dependences of 1H NMR as well as 35Cl NQR spin-lattice relaxation times T1 were investigated of a ferroelectric molecular adduct with Tc = 323 K, in which 1,4-diazabicyclo[2.2.2]octane (DABCO) is sandwiched between two 2-chlorobenzoic acid (2-ClBA). The NQR frequencies clearly show that proton transfer from 2-ClBA to DABCO is occurred and the molecular adduct consists of diprotonated DABCO cation and two 2-chlorobenzoate anions. The correlation time of reorientational motion of the diprotonated DABCO molecule was determined as a function of temperature. The activation energy Ea of the motion was estimated as 22 kJ mol−1 below Tc. The steep decrease of the NQR T1 with Ea = 50 kJ mol−1, observed above ca. 280 K in the ferroelectric phase, suggests a slow fluctuation of electric field gradient at chlorine nucleus.

Iron(II) Complexes with Boron Cluster Anion [B10Cl10]2–: Intermolecular Interactions according to 35Cl NQR Spectroscopy and X‐ray Diffraction

Secondary interactions, in which decachloro‐closo‐decaborate anions are involved were studied in solvates [Fe(phen)3][B10Cl10]·0.875CH3CN·0.125H2O (1), {[Fe(bipy)3][B10Cl10]}·2Bipy·0.5CH3CN (2), {[Fe(bipy)3]2[B10Cl10]2}·2.25CH3CN (3), and [Fe(bipy)3]3[B10Cl10]3·2CH3CN·H2O (4). Structures 1–4 were determined by X‐ray diffraction analysis, and complexes 1–3 were studied using 35Cl NQR spectroscopy. A number of C–H···Cl and p···π interactions between chlorine atoms and delocalized electron density of phenyl rings or C≡N groups of acetonitrile molecules were identified. The results obtained by the two methods were compared and the relative strength of secondary interactions were estimated.

Structural features of SnCl4–ortho-dimethoxybenzene complexes as derived from ab initio computations

Computations for the possible structures of SnCl4–ortho-dimethoxybenzene complexes and initial components have been performed by MP2/LANL2DZ method. Computation results have been compared with experimental data of 35Cl nuclear quadrupole resonance. cis-Octahedral structure of the complex through the coordination of both oxygen atoms has been confirmed. This complex differs considerably from the majority of studied tetrachlorostannane complexes of cis-octahedral structure in terms of the ratios of axial and equatorial Sn–Cl bond distances, p σ electron density, and 35Cl NQR frequencies for axial and equatorial chlorine atoms.

Secondary interactions as defined by 35Cl NQR spectra in cesium decachloro-closo-decaborates prepared in non-aqueous solutions

Two solvates of cesium decachloro-closo-decaborate, Cs2[B10Cl10]·2CH3CN and Cs2[B10Cl10]·4DMF, are synthesized and characterized by IR-, 11B NMR, and 35Cl NQR-spectroscopies as well as X-ray diffraction and powder diffraction analyses. The weak secondary interactions involving chlorine atoms were selected by the 35Cl NQR spectra of the full set of interatomic contacts characterized by X-ray diffraction. It has been found that the chlorine atoms form numerous secondary interactions with solvate molecules (B−Cl…H and B−Cl…C interactions with methyl groups and p…π with CN groups), whereas no noticeable perturbation of the chlorine electron density distribution, which could result in the splitting of the 35Cl NQR spectra, has been found due to the cesium cations or Cl…Cl interactions.

35Cl NQR study of deuteration effect on proton dynamics in chlorobenzoic acid

35Cl NQR frequencies and spin-lattice relaxation times T1 were measured of some deuterated chlorobenzoic acid, 3,5-dichlorobenzoic acid-d1 (35ClBA-d1), 2,6-dichlorobenzoic acid-d1 (26ClBA-d1), and 4-amino-3,5- dichlorobenzoic acid-dn (4A35ClBA-dn). The slightly higher frequencies were observed for the deuterated analogues. The temperature dependence of the T1 showed a minimum which can be explained by the fluctuation of electric field gradient (EFG) due to the proton transfer motion in the dimeric chlorobenzoic acid molecules. By the deuteration, the 35Cl T1 minimum shifted to the high-temperature side and showed approximately one order of magnitude shorter value. From comparison with the T1 data of the normal compounds, the tunneling contribution to the proton transfer motion in the dimeric molecular unit of chlorobenzoic acid in the crystal is discussed.

Decachloro-closo-decaborate anion in copper(II) complexation reactions with N-donor ligands: 35Cl NQR and X-ray studies

Copper(II) complexation reactions with neutral N-donor ligands (Bipy, Phen, NH3) in the presence of the bulky decachloro-closo-decaborate dianion were studied. Copper(II) complexes can be prepared from copper(II) salts or copper(I) salts by redox reactions proceeding in air. The complexes [Cu(Bipy)3][B10Cl10]·2CH3CN (1·2CH3CN), [Cu(Bipy)3][B10Cl10]·2Bipy (1·2Bipy), [Cu(Bipy)2Cl]2[B10Cl10]·2DMF (2·2DMF), [Cu(Phen)2Cl]2[B10Cl10]·DMSO·1.25H2O (3·DMSO·1.25H2O) and [Cu(NH3)4(CH3CN)2][B10Cl10] (4) were prepared and characterized by X-ray diffraction as well as IR and 11B NMR spectroscopies. Complexes 1·2CH3CN, 2·2DMF, and 4 were also studied using 35Cl NQR spectroscopy. Secondary interactions were selected from all the interatomic contacts found in these crystals by X-ray diffraction. Hydrogen N–H⋯Cl–B, C–H⋯Cl–B and p⋯π (B–Cl⋯C) secondary bonds were identified in all the compounds, the spectroscopic data being generally in accordance with the results of the X-ray diffraction.

Secondary interactions in decachloro-closo-decaborates of alkali metals M2[B10Cl10] (M = K+ and Cs+): 35Cl NQR and X-ray studies

New salts containing bulky decachloro-closo-decaborate dianion of composition M2[B10Cl10] (M=K+ and Cs+) were synthesized and characterized using the IR, 11B NMR and 35Cl NQR spectroscopies, powder and single crystal X-ray diffractions. The potassium salt was obtained as solvate K2[B10Cl10]·3H2O, and cesium salt – as solvates Cs2[B10Cl10]·H2O and Cs2[B10Cl10]·2H2O. Using 35Cl NQR, the interatomic contacts, caused by secondary interactions, were selected of all the totality of interatomic contacts, measured by X-ray diffraction. The B–Cl…H secondary bonds were identified in all the compounds, whereas no contributions to the electric field gradient on the appropriate chlorine atoms were registered from cations M (M=K or Cs). The high coordination numbers of alkali ions screened the effective positive charge on them leading to insignificant perturbation of the chlorine electron density distribution which resulted in negligibly small splitting of the 35Cl NQR spectra due to the alkali cations.

NQR application to the study of hydrogen dynamics in hydrogen-bonded molecular dimers

The temperature dependences of 1H NMR as well as 35Cl NQR spin-lattice relaxation times T1 were investigated in order to study the hydrogen transfer dynamics in carboxylic acid dimers in 3,5-dichloro- and 2,6-dichlorobenzoic acids. The asymmetry energy A/ kB and the activation energy V/ kB for the hydrogen transfer were estimated to be 240 K and 900 K, and 840 K and 2500 K, respectively, for these compounds. In spite of a large asymmetric potential the quantum nature of hydrogen transfer is recognized in the slope of the temperature dependence of T1 on the low-temperature side of the T1 minimum. The NQR T1 measurements was revealed to be a good probe for the hydrogen transfer dynamics.

Secondary interactions in decachloro-closo-decaborates R2[B10Cl10] (R = Et3NH+, Ph4P+, and [Ag(NH3)2]+): 35Cl NQR, PW-DFT, and X-ray studies

Three new salts R2[B10Cl10] (R=Et3NH+, Ph4P+, [Ag(NH3)2]+, 1–3) containing bulky decachloro-closo-decaborate dianion have been synthesized and characterized using the IR, 11B NMR, and 35Cl NQR spectroscopy. Neither IR nor 11B NMR spectra have shown distinctions between compounds 1–3, whereas 35Cl NQR spectra of all the compounds have been split evidencing for crystallographic non-equivalence of chlorine atoms caused by secondary interactions. Crystal structures of 2 and 3 have been studied using X-ray diffraction and periodic DFT calculations, whereas unit cell parameters and space group of 1 have been estimated using powder X-ray diffraction. The 35Cl NQR data supported by analysis of calculated electron density functions within the framework of Bader’s Quantum Theory of Atoms in Molecules (QTAIM) showed that both cation and solvent molecules take part in N–H…Cl and C–H…Cl bonding as strong as 2.5 and 2.1kcal/mol, respectively. The QTAIM study has revealed in 2 a number of p…π interactions between lone pairs of chlorine atoms and delocalized electron density of phenyl rings or CN groups of acetonitrile molecules, and the strongest of them (0.9kcal/mol) has been reflected in the 35Cl NQR spectrum. The results have shown that the halogen-substituted boron clusters can act as tags sensitive to H…Hal and p…π bonding, which can be identified by the NQR spectra even in the absence of structural data.

High-field solid-state 35Cl NMR in selenium(IV) and tellurium(IV) hexachlorides

We report solid-state 35Cl NMR spectra in three hexachlorides, (NH4)2SeCl6, (NH4)2TeCl6 and Rb2TeCl6. The CQ(35Cl) quadrupole coupling constants in the three compounds were found to be 41.4±0.1 MHz, 30.3±0.1 MHz and 30.3±0.1 MHz, respectively, some of the largest CQ(35Cl) quadrupole coupling constants ever measured in polycrystalline powdered solids directly via 35Cl NMR spectroscopy. The 35Cl EFG tensors are axial in all three cases reflecting the C4v point group symmetry of the chlorine sites. 35Cl NMR experiments in these compounds were only made possible by employing the WURST-QCPMG pulse sequence in the ultrahigh magnetic field of 21.1 T. 35Cl NMR results agree with the earlier reported 35Cl NQR values and with the complementary plane-wave DFT calculations. The origin of the very large CQ(35Cl) quadrupole coupling constants in these and other main-group chlorides lies in the covalent-type chlorine bonding. The ionic bonding in the ionic chlorides results in significantly reduced CQ(35Cl) values as illustrated with triphenyltellurium chloride Ph3TeCl. The high sensitivity of 35Cl NMR to the chlorine coordination environment is demonstrated using tetrachlorohydroxotellurate hydrate K[TeCl4(OH)]∙0.5H2O as an example. 125Te MAS NMR experiments were performed for tellurium compounds to support 35Cl NMR findings.

Quantitative Analysis of Hydration Using Nitrogen-14 Nuclear Quadrupole Resonance.

Hydration is a quite common process in pharmaceutical solids. Sometimes it is desirable, as it stabilizes the crystal structure; in other cases it is unwanted, as it changes the physical and chemical properties of drugs. We here use (14)N NQR spectroscopy to quantitatively analyze hydration of a model compound, 5-aminotetrazole. (14)N NQR has some great advantages compared to other routinely used techniques to study hydration, like a very simple spectrum, single point calibration, and no need for special sample preparation, but the method's great disadvantage is a rather small sensitivity. Nevertheless, here we demonstrate that (14)N NQR, although being significantly less sensitive than XRD, NIR, and also (35)Cl NQR, is still capable of providing excellent quantitative accuracies. We can achieve errors <1% of the total amount, provided good temperature stabilization is implemented, which then allows long experimental times. We also present results obtained with a SLSE pulse sequence, which is a less robust approach but allows the use of much shorter measuring times (∼200×) and could be used for quantitative real time monitoring of hydration or dehydration.

Proton dynamics in the hydrogen bonds of 4-amino-3,5-dihalogenobenzoic acid

On the polycrystalline sample of 4-amino-3,5-dihalogenobenzoic acid, 4-NH2-3,5-X2C6H2COOH, which has a symmetric dimer structure in the crystal, the proton tunneling in the hydrogen bonds has been investigated by NQR and NMR spin–lattice relaxation times T1 measurements. Two 35Cl NQR lines of the X=Cl derivative show the existence of two crystallographically inequivalent chlorine atoms in the high-temperature phase, in consistency with the reported crystal structure. Below 138K, each splits into a doublet indicating the symmetry breaking of the benzoic acid dimer. The proton dynamics was analyzed by a coherent and incoherent tunneling models, for the high- and low-temperature phases, respectively. The temperature dependence of the correlation time of proton translation was estimated. As for the X=I derivative, the proton dynamics was discussed similarly by 1H NMR T1 data by assuming occurrence of a phase transition at low-temperature.