35Cl Nuclear Quadrupole Resonance Research Articles

Characterization of solid phases and study of transformation kinetics in m-chlorofluorobenzene by 35Cl nuclear quadrupole resonance

Polymorphism is of widespread occurrence in the world of molecular crystals. In this work we present experimental results showing the existence of four solid phases in m-chlorofluorobenzene. A glass structure is achieved by quenching the liquid phase at 77K. This glassy state crystallizes in a disordered phase at T∼143K, which in turn transforms to the high-temperature stable phase (phase I) at T∼153K. Depending on the thermal history of the sample, a different ordered phase (phase III) can be obtained. The disorder is attributed to a molecular orientational disorder. There is no evidence of molecular reorientation in any phase. A study of the disorder-order phase transformation kinetics, using nuclear quadrupole resonance, is presented. The results are analyzed following Cahn's theory. Nucleation seems to take place at grain boundaries. Growth rates for different temperatures have been determined.

Protonic motion in one-dimensional hydrogen-bonded molecular crystal [4,6-dmpH][Hca] as revealed by 35Cl NQR

A protonic motion in the 1-D hydrogen-bonded zigzag chain in 4,6-dimethylpyrimidinium hydrogen chloranilate, [4,6-dmpH][Hca], was studied from the measurements of 35Cl nuclear quadrupole resonance (NQR) spin-lattice relaxation time (T1). The four NQR frequencies were observed at 36.846, 36.767, 35.945, and 35.905MHz at 77K. The temperature dependences of these frequencies were smooth and continuous in 77–360K. The absence of solid–solid phase transition and the metastable phase was confirmed by heat capacity measurements. Temperature dependence of the NQR T1 suggests an excitation of a motion above ca. 200K with the activation energy of 11.5kJmol−1. In the stable phase too, the excitation of proton transfer is expected to occur as suggested for the metastable phase of as-grown crystals. Such transfer motion may be possible at the kink structure of the 1-D ferroelectric zigzag chain, and the kink can migrate in the chain accordingly.

Stereoelectronic structure and 35Cl NQR parameters of 4-(trichlorgermyl)butan-2-one using ab initio calculations

Abstract The results of ab initio calculations at the RHF/6-31G(d) and MP2/6-31G(d) levels of two stable structures of the 4-(trichlorgermyl)butan-2-one molecule with total optimization of their geometry have been represented. The structure with pentacoordinated Ge atom is energetically more advantageous as compared with that with tetracoordinated one. Using these results, the 35 Cl nuclear quadrupole resonance (NQR) frequencies and asymmetry parameters of the electric field gradient (EFG) at the 35 Cl nuclei in molecule with pentacoordinated Ge atom have been assessed, the frequencies satisfactorily agreeing with experimental data. Calculations at the RHF/6-31G(d) level have been performed also at various Ge⋯O distances. It has been demonstrated that convergence of the Ge and O coordination centers leads to the increase of positive charge at the Ge coordination center and of negative charge at the O coordination center, at that, electron density from the Ge atom shifts mainly to the axial Cl atom and from the C atom of carbonyl group – to its O atom. The electron density transfer from the O to Ge atom does not occur.

Features of the molecular structure of compounds of (Cl3PNAr)2 series (Ar = C6H4Z, Z = H, Cl) by quantum-chemical calculations and the data of 35Cl nuclear quadrupole resonance

A series of dimeric molecules (Cl3PNAr)2 where Ar = C6H4Z, Z = H or Cl in 2–4 positions, was calculated by the RHF/6-31G* method with the full geometry optimization. A specific nature of the intramolecular steric orientation of aryl groups was revealed, therewith in the case Ar = C6H4Cl-2 short nonvalent contacts were founf between the ortho-chlorine atom and PCl3 group manifested in the peculiarity of geometric parameters and NQR spectroscopy data of this compound. The correlation of the 35Cl NQR frequencies of P-Cl bonds with the values of the charges on the chlorine atoms obtained by quantum-chemical calculations of the molecules was analyzed.

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.

Correlation between proton transfer and 35Cl NQR frequency as well as molecular geometry of chloranilic acid in co‐crystals with some organic bases

Proton transfer in hydrogen-bonded organic co-crystals of chloranilic acid with some organic bases was investigated by nuclear quadrupole resonance (NQR) spectroscopy. The (35)Cl NQR frequencies of chloranilic acid molecule as well as (14)N NQR frequencies of the organic base molecule were measured with the conventional pulse methods as well as double-resonance methods, respectively. The extent of proton transfer in the O...H...N hydrogen bond was estimated from Townes-Dailey analysis of the (14)N NQR parameters. The (35)Cl NQR frequency and molecular geometry of chloranilic acid are correlated to the extent of proton transfer in the protonation process of the organic base molecule. It is shown that the hydrogen bond affects the pi-electron system of chloranilic acid. Geometry dependence of the O...H...N hydrogen bond, i.e. the H-N valence bond order versus the hydrogen-bond geometry correlation is also discussed.

Two solid phases of pyrimidin-1-ium hydrogen chloranilate monohydrate determined at 225 and 120 K

The crystal structures of two solid phases of the title compound, C(4)H(5)N(2)(+) x C(6)HCl(2)O(4)(-) x H(2)O, have been determined at 225 and 120 K. In the high-temperature phase, stable above 198 K, the transition temperature of which has been determined by (35)Cl nuclear quadrupole resonance and differential thermal analysis measurements, the three components are held together by O-H...O, N...H...O, C-H...O and C-H...Cl hydrogen bonds, forming a centrosymmetric 2+2+2 aggregate. In the N...H...O hydrogen bond formed between the pyrimidin-1-ium cation and the water molecule, the H atom is disordered over two positions, resulting in two states, viz. pyrimidin-1-ium-water and pyrimidine-oxonium. In the low-temperature phase, the title compound crystallizes in the same monoclinic space group and has a similar molecular packing, but the 2+2+2 aggregate loses the centrosymmetry, resulting in a doubling of the unit cell and two crystallographically independent molecules for each component in the asymmetric unit. The H atom in one N...H...O hydrogen bond between the pyrimidin-1-ium cation and the water molecule is disordered, while the H atom in the other hydrogen bond is found to be ordered at the N-atom site with a long N-H distance [1.10 (3) A].

Quantitative 35Cl Nuclear Quadrupole Resonance in Tablets of the Antidiabetic Medicine Diabinese

Pulsed (35)Cl nuclear quadrupole resonance (NQR) experiments have been performed on 250-mg tablets of the antidiabetic medicine Diabinese to establish the conditions needed for noninvasive quantitative analysis of the medicine in standard bottles. One important condition is the generation of a uniform radio-frequency (RF) field over the sample, which has been achieved by two designs of sample coil: one of variable pitch, and the other a resonator that has been fabricated from a single turn of copper sheet with a longitudinal gap bridged by tuning capacitors. The results from blind tests show that the number of tablets in a bottle could be predicted to within +/-3%.

Measurement of temperature and temperature gradient in millimeter samples by chlorine NQR

A mini-thermometer based on the 35Cl nuclear quadrupole resonance (NQR) frequency temperature dependence in the chlorates KClO3 and NaClO3 was built and successfully tested by measuring temperature and temperature gradient at 77 K and higher in about 100 mm3 active volume of a mini Joule–Thomson refrigerator. In the design of the tank-circuit coil, an array of small coils connected in series enabled us (a) to achieve a suitable ratio of inductance to capacity in the NQR spectrometer input tank circuit, (b) to use a single crystal of KClO3 or NaClO3 (of 1–2 mm3 size) in one coil as a mini-thermometer with a resolution of 0.03 K and (c) to construct a system for measuring temperature gradients when the spatial coordinates of each chlorate single crystal within an individual coil are known.

Electronic structure and biological activity of chosen DDT‐type insecticides studied by 35Cl‐NQR

A correlation between the electronic structure and biological activity of chosen dichlorodiphenyltrichloroethane (DDT)-type insecticides: 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane, 1,1-dichloro-2,2-bis(4-chlorophenyl)ethane, 2,2-bis(p-chlorophenyl)-1,1-dichloroethylene, 2,2-bis(4-chlorophenyl)ethanoic acid and 4,4'-dichlorobenzophenone (used in agriculture) has been analysed on the basis of the (35)Cl-nuclear quadrupole resonance (NQR) spectroscopy. The (35)Cl-NQR resonance frequencies measured at 77 K have been correlated with the lethal dose (LD(50)) parameter that characterises the biological activity of these insecticides.

Thermal stability and molecular dynamics of triclosan in solid state studied by 35Cl–NQR spectroscopy and DFT calculations

The stability of 5-chloro-2-(2′,4′-dichlorophenoxy)-phenol, triclosan (TCS) in solid state has been studied by nuclear quadrupole resonance (NQR) spectroscopy. The NQR spectrum of TCS at 77 K reveals three resonance lines of the frequencies 35.07, 35.534 and 36.189 MHz assigned to chlorine atoms at positions 5, 4′, and 2′ of the biphenyl ring. TCS in solid state is thermally stable, no temperature-induced release of chlorine or decomposition of this compound have been detected, which is consistent with the DTA results. The temperature dependence of the resonance frequency suggested the occurrence of small-angle torsional vibrations of the mean activation energy of 1.62 kJ/mol. B3LYP/6-311++G** method reproduced very well the NQR resonance frequencies and the frequencies of the small-angle torsional vibrations for TCS.

Using the Combined Approach of 35Cl NQR, 14N NQR and DFT Calculations To Study the 14N NQR Spectrum of Diazepam

The electronic structure of diazepam was analyzed by pulsed 14N nuclear quadrupole resonance (NQR) spectroscopy and density functional theory (DFT) calculations using the hybrid method B3PW91 with the 6-31G(d,p) basis set. Before searching for 14N NQR spectra, a 35Cl NQR line in diazepam was recorded to evaluate the degree of disorder in the crystal lattice and thus the chances to find much weaker NQR spectra of nitrogen-14 in this substance. In order to speed up the detection of weak NQR spectra, the spin-locked spin-echo multipulse sequence had to be applied. Despite the expected chemical inequivalence of the two nitrogen atoms in the diazepam molecule, only two resonance lines ν+ and ν− were then found for this compound at 77 K. Possible reasons for this anomaly are discussed. From the experimental frequencies, the quadrupole coupling constant, the asymmetry parameter and the principal components of the electric field gradient tensor were determined. Those parameters were compared with the results of theoretical DFT calculations showing a reasonable accuracy of the DFT model used.

Reactivity of 2,6-Dichloropurine Ribonucleoside Studied by 35Cl NQR Spectroscopy

2,6-Dichloropurine ribonucleoside is a precursor in the potential synthesis of anticancer and antiviral drugs. 35Cl nuclear quadrupole resonance (NQR) studies on electronic structure and the character of C–Cl bonds may provide a valuable information about the difference in reactivity of chlorine atoms present in this compound. In this work 35Cl NQR frequencies, asymmetry parameters and double-bond character for both C–Cl bonds have been determined experimentally. These parameters were then compared with the corresponding theoretical values obtained by means of the density functional theory.

35Cl NQR Study of Cation Polarizability in Metal Salts of Monochloro Acetic Acid

Various metal salts (Na, K, Rb, and NH4) of monochloro acetic acid were prepared and the Cl-35 nuclear quadrupole resonance frequencies were measured at room temperature. A comparative study of nuclear quadrupole resonance frequencies of monochloro acetic acid and its metal salts is carried out. The frequency shifts obtained in the respective metal chloroacetates are used to estimate the changes in the ionicity of C-Cl bond. Further, the changes in the ionicity of C-Cl bond were used to estimate the percentage of intra-molecular charge transfer between respective cation-anion of the metal salts of chloro acetic acid. The nuclear quadrupole resonance frequency is found to decrease with increasing ionicity of the alkali metal ion.

Pressure and temperature dependence of the chlorine NQR in caesium and sodium chlorates

The (35)Cl nuclear quadrupole resonance (NQR) frequencies (nu(Q)) in caesium and sodium chlorates were measured as a function of temperature, from 77 to 300 K at different pressures up to 5.1 kbar, and the data were analysed to estimate the volume dependence of the electric field gradient (EFG), torsional frequency and also the contributions to the NQR frequency from static and dynamic effects. The variation of spin-lattice relaxation time with pressure at different temperatures was studied in the case of sodium chlorate and at room temperature in case of caesium chlorate. The pressure dependence of the spin-lattice relaxation time (T(1)) suggests that the relaxation is mainly due to the torsional motions.

Anomalous H/D isotope effect on 35Cl NQR frequencies in piperidinium p-chlorobenzoate

Anomalous isotope effects were detected in the 35Cl nuclear quadrupole resonance (NQR) frequency of piperidinium p-chlrobenzoate (C5H10NH· ClC6H4COOH) by deuteration of hydrogen atoms. The atoms were determined to form two kinds of N–H···O type H-bonds in the crystal structure. Large frequency shifts of the 35Cl resonance lines reaching 288 kHz at 77 K and 278 kHz at room temperature were caused upon deuteration, in spite of the fact that the Cl atoms in the molecule do not form hydrogen bonds in the crystal. Results of single crystal X-ray diffraction measurements and density-functional-theorem calculations suggest that a dihedral-angle change of 1.8° between benzene and the piperidine ring contributes to 35Cl NQR anomalous frequency shifts.

Dependence of 35Cl NQR on hydrogen bonding and temperature in dichlorophenol–aniline charge transfer complexes

The hydrogen-bonded charge transfer complexes of aniline with pi-acceptors (or proton donors) such as 2,5-, 2,6-, 3,4- and 3,5-dichlorophenol were prepared. The (35)Cl nuclear quadrupole resonance (NQR) frequencies of these charge transfer complexes in the temperature range 77-300 K were measured to ascertain the existence or otherwise of a phase transition upon complex formation. Further, the NQR frequency and asymmetry parameter of the electric field gradient at the site of quadrupole nucleus were used to estimate the chemical bond parameters, namely ionic bond, double bond character of the carbon-chlorine(C--Cl) bond and the percentage charge transfer between the donor-acceptor components in charge transfer complexes. The effect of hydrogen bonding and temperature on the charge transfer process is analysed.

35Cl NQR study of lattice dynamic and magnetic property of a crystalline coordination polymer {CuCA(phz)(H 2O) 2} n

Copper(II) compounds {CuCA(phz)(H 2O) 2} n (H 2CA = chloranilic acid, phz = phenazine) having a layer structure of –CuCA(H 2O) 2– polymer chains and phenazine were studied by 35Cl nuclear quadrupole resonance (NQR). The single NQR line observed at 35.635 MHz at 261.5 K increased to 35.918 MHz at 4.2 K. The degree of reduction of electric field gradient due to lattice vibrations was similar to that of chloranilic acid crystal. Temperature dependence of spin–lattice relaxation time, T 1, of the 35Cl NQR signal below 20 K, between 20 and 210 K, and above 210 K, was explained by (1) a decrease of effective electron-spin density caused by antiferromagnetic interaction, (2) a magnetic interaction between Cl nuclear-spin and electron-spins on paramagnetic Cu(II) ions, and (3) an increasing contribution from reorientation of ligand molecules, respectively. The electron spin-exchange parameter ∣ J∣ between the neighboring Cu(II) electrons was estimated to be 0.33 cm −1 from the T 1 value of the range 20−210 K. Comparing this value with that of J = −1.84 cm −1 estimated from the magnetic susceptibility, it is suggested that the magnetic dipolar coupling with the electron spins on Cu(II) ions must be the principal mechanism for the 35Cl NQR spin–lattice relaxation of {CuCA(phz)(H 2O) 2} n but a delocalization of electron spin over the chloranilate ligand has to be taken into account.

35Cl NQR of an organic ferroelectric phenazine chloranilic acid co-crystal

A ferroelectric transition of phenazine (Phz) chloranilic acid (H 2ca) co-crystal was studied by 35Cl nuclear quadrupole resonance (NQR). The single NQR line observed at 36.689 MHz at 295 K, split into a doublet below the ferroelectric transition temperature T c=253 K in accord with the inversion-symmetry breaking at the H 2ca molecular center in the crystal. The continuous variation of the splitting-width through T c suggests a second-order nature of this phase transition. The NQR frequency is consistent with the crystal structure in which the two acid hydrogen atoms of H 2ca remain as attached to the quinone molecules. The spin-lattice relaxation time T 1 showed a decrease with decreasing temperature in the ferroelectric phase. This suggests the existence of a fast motion of nearby atomic group, which is a cause of electric field gradient fluctuation.

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.