Chlorine Electric Field Gradient Research Articles

Uncovering Halogen Mixing and Octahedral Dynamics in Cs2SnX6 by Multinuclear Magnetic Resonance Spectroscopy

Cs2SnX6 (X = Cl, Br, and I) compounds have emerged as promising lead-free and ambient-stable materials for photovoltaic and optoelectronic applications. To advance these promising materials, it is crucial to determine the correlations between physical properties and their local structure and dynamics. Solid-state NMR spectroscopy of multiple NMR-active nuclei (133Cs, 119Sn, and 35Cl) in these cesium tin(IV) halides has been used to reveal the atomic structure, which plays a key role in the materials’ optical properties. The 119Sn NMR chemical shifts span approximately 4000 ppm, and the 119Sn spin-lattice relaxation times span 3 orders of magnitude when the halogen goes from chlorine to iodine in these diamagnetic compounds. Moreover, ultrawideline 35Cl NMR spectroscopy of Cs2SnCl6 indicates an axially symmetric chlorine electric field gradient tensor with a large quadrupolar coupling constant of ca. 32 MHz, suggesting the presence of a chlorine moiety that is directly attached to Sn(IV) ions. Variable-temperature 119Sn spin-lattice relaxation time measurements support the presence of dynamics of octahedral SnI6 units in Cs2SnI6. We further show that complete mixed-halide solid solutions of Cs2SnClxBr6–x and Cs2SnBrxI6–x (0 ≤ x ≤ 6) form at any halogen compositional ratio. 119Sn and 133Cs NMR spectroscopy resolve the unique local SnClnBr6–n and SnBrnI6–n (n = 0–6) octahedral and CsBrmI12–m (m = 0–12) cuboctahedral environments in the mixed-halide samples. The experimentally observed 119Sn NMR results are consistent with magnetic shielding parameters obtained by density functional theory computations that were obtained to model the random halogen distribution in mixed-halide analogues. Finally, we demonstrate the difference in the local structures and the optical absorption properties of Cs2SnI6 samples prepared by solvent-assisted and solvent-free synthesis routes.

Application of multinuclear magnetic resonance and gauge-including projector-augmented-wave calculations to the study of solid group 13 chlorides

A series of four anhydrous group 13 chloride salts has been studied by (35/37)Cl solid-state NMR spectroscopy and complementary quantum chemical calculations. Due to the large (35/37)Cl quadrupolar interactions in these salts, a high magnetic field (21.1 T) and the variable-offset QCPMG technique was used to obtain full chlorine central transition (m = -1/2 <--> 1/2) NMR spectra. Analyses of the NMR spectra of the synthetically important Lewis acid trichlorides of aluminium, gallium, and indium, as well as gallium dichloride, allowed for characterisation of the chlorine electric field gradient (EFG) tensors and, in three cases, the chlorine chemical shift (CS) tensors. The quadrupolar interaction was found to dominate the central transition chlorine NMR spectrum in all cases, with chlorine-35 quadrupolar coupling constants (C(Q)) ranging in magnitude from 22.45 +/- 2.00 to 40.44 +/- 2.00 MHz, and the spectral breadths ranging from approximately 1.0 to 2.5 MHz. For the trichloride salt of gallium, it was confirmed that the terminal chlorine sites exhibit larger chlorine C(Q) values than do the bridging chlorines. The isotropic chemical shifts range from 150 +/- 100 to 375 +/- 100 ppm while the largest CS tensor span is 500 +/- 200 ppm, for InCl(3). The chlorine chemical shift was found to increase with increasing M-Cl distance in all cases. Quantum chemical calculations of the EFG and magnetic shielding tensors, performed using the gauge-including projector-augmented-wave (GIPAW) method as implemented in the CASTEP program, were found to be in excellent agreement with the experimentally determined values, reproducing C(Q)((35)Cl) to within 7% in all cases. The agreement between experiment and theory substantiates the accuracy of the NMR parameters. Solid-state NMR spectra of the cation species (aluminium-27, gallium-69/71 and indium-113/115) were also collected, and the EFG and CS parameters were determined in some cases. The study demonstrates the utility of multinuclear solid-state magnetic resonance studies of half-integer spin quadrupolar nuclei in ionic systems when the central transition is broadened greatly by the quadrupolar interaction, and in particular contributes to our understanding of the relationship between solid-state structure and chlorine NMR interaction tensors.

Application of Solid-State35Cl NMR to the Structural Characterization of Hydrochloride Pharmaceuticals and their Polymorphs

Solid-state (35)Cl NMR (SSNMR) spectroscopy is shown to be a useful probe of structure and polymorphism in HCl pharmaceuticals, which constitute ca. 50% of known pharmaceutical salts. Chlorine NMR spectra, single-crystal and powder X-ray diffraction data, and complementary ab initio calculations are presented for a series of HCl local anesthetic (LA) pharmaceuticals and some of their polymorphs. (35)Cl MAS SSNMR spectra acquired at 21.1 T and spectra of stationary samples at 9.4 and 21.1 T allow for extraction of chlorine electric field gradient (EFG) and chemical shift (CS) parameters. The sensitivity of the (35)Cl EFG and CS tensors to subtle changes in the chlorine environments is reflected in the (35)Cl SSNMR powder patterns. The (35)Cl SSNMR spectra are shown to serve as a rapid fingerprint for identifying and distinguishing polymorphs, as well as a useful tool for structural interpretation. First principles calculations of (35)Cl EFG and CS tensor parameters are in good agreement with the experimental values. The sensitivity of the chlorine NMR interaction tensor parameters to the chlorine chemical environment and the potential for modeling these sites with ab initio calculations hold much promise for application to polymorph screening for a wide variety of HCl pharmaceuticals.

Alkaline Earth Chloride Hydrates: Chlorine Quadrupolar and Chemical Shift Tensors by Solid‐State NMR Spectroscopy and Plane Wave Pseudopotential Calculations

A series of alkaline earth chloride hydrates has been studied by solid-state (35/37)Cl NMR spectroscopy in order to characterize the chlorine electric field gradient (EFG) and chemical shift (CS) tensors and to relate these observables to the structure around the chloride ions. Chlorine-35/37 NMR spectra of solid powdered samples of pseudopolymorphs (hydrates) of magnesium chloride (MgCl(2).6H(2)O), calcium chloride (CaCl(2).2H(2)O), strontium chloride (SrCl(2), SrCl(2).2H(2)O, and SrCl(2).6H(2)O), and barium chloride (BaCl(2).2H(2)O) have been acquired under stationary and magic-angle spinning conditions in magnetic fields of 11.75 and 21.1 T. Powder X-ray diffraction was used as an additional tool to confirm the purity and identity of the samples. Chlorine-35 quadrupolar coupling constants (C(Q)) range from essentially zero in cubic anhydrous SrCl(2) to 4.26+/-0.03 MHz in calcium chloride dihydrate. CS tensor spans, Omega, are between 40 and 72 ppm, for example, Omega= 45+/-20 ppm for SrCl(2).6H(2)O. Plane wave-pseudopotential density functional theory, as implemented in the CASTEP program, was employed to model the extended solid lattices of these materials for the calculation of their chlorine EFG and nuclear magnetic shielding tensors, and allowed for the assignment of the two-site chlorine NMR spectra of barium chloride dihydrate. This work builds upon our current understanding of the relationship between chlorine NMR interaction tensors and the local molecular and electronic structure, and highlights the particular sensitivity of quadrupolar nucleus solid-state NMR spectroscopy to the differences between various pseudopolymorphic structures in the case of strontium chloride.

A high-field solid-state 35/37Cl NMR and quantum chemical investigation of the chlorine quadrupolar and chemical shift tensors in amino acid hydrochlorides

A series of six L-amino acid hydrochloride salts has been studied by 35/37Cl solid-state NMR spectroscopy (at 11.75 and 21.1 T) and complementary quantum chemical calculations. Analyses of NMR spectra acquired under static and magic-angle-spinning conditions for the six hydrochloride salts, those of aspartic acid, alanine, cysteine, histidine, methionine and threonine, allowed the extraction of information regarding the chlorine electric field gradient (EFG) and chemical shift tensors, including their relative orientation. Both tensors are found to be highly dependent on the local environment, with chlorine-35 quadrupolar coupling constants (CQ) ranging from -7.1 to 4.41 MHz and chemical shift tensor spans ranging from 60 to 100 ppm; the value of CQ for aspartic acid hydrochloride is the largest in magnitude observed to date for an organic hydrochloride salt. Quantum chemical calculations performed on cluster models of the chloride ion environment demonstrated agreement between experiment and theory, reproducing CQ to within 18%. In addition, the accuracy of the calculated values of the NMR parameters as a function of the quality of the input structure was explored. Selected X-ray structures were determined (L-Asp HCl; L-Thr HCl) or re-determined (L-Cys HCl.H2O) to demonstrate the benefits of having accurate crystal structures for calculations. The self-consistent charge field perturbation model was also employed and was found to improve the accuracy of calculated quadrupolar coupling constants, demonstrating the impact of the neighbouring ions on the EFG tensor of the central chloride ion. Taken together, the present work contributes to an improved understanding of the factors influencing 35/37Cl NMR interaction tensors in organic hydrochlorides.

Chlorine-35/37 NMR Spectroscopy of Solid Amino Acid Hydrochlorides: Refinement of Hydrogen-Bonded Proton Positions Using Experiment and Theory

Trends in the chlorine chemical shift (CS) tensors of amino acid hydrochlorides are investigated in the context of new data obtained at 21.1 T and extensive quantum chemical calculations. The analysis of chlorine-35/37 NMR spectra of solid L-tryptophan hydrochloride obtained at two magnetic field strengths yields the chlorine electric field gradient (EFG) and CS tensors, and their relative orientations. The chlorine CS tensor is also determined for the first time for DL-arginine hydrochloride monohydrate. The drastic influence of 1H decoupling at 21.1 T on the spectral features of salts with particularly small 35Cl quadrupolar coupling constants (CQ) is demonstrated. The chlorine CS tensor spans (Omega) of hydrochloride salts of hydrophobic amino acids are found to be larger than those for salts of hydrophilic amino acids. A new combined experimental-theoretical procedure is described in which quantum chemical geometry optimizations of hydrogen-bonded proton positions around the chloride ions in a series of amino acid hydrochlorides are cross-validated against the experimental chlorine EFG and CS tensor data. The conclusion is reached that the relatively computationally inexpensive B3LYP/3-21G* method provides proton positions which are suitable for subsequent higher-level calculations of the chlorine EFG tensors. The computed value of is less sensitive to the proton positions. Following this cross-validation procedure, /CQ(35Cl)/ is generally predicted within 15% of the experimental value for a range of HCl salts. The results suggest the applicability of chlorine NMR interaction tensors in the refinement of proton positions in structurally similar compounds, e.g., chloride ion channels, for which neutron diffraction data are unavailable.

Solid-State35/37Cl NMR Spectroscopy of Hydrochloride Salts of Amino Acids Implicated in Chloride Ion Transport Channel Selectivity: Opportunities at 900 MHz

The results of a detailed systematic chlorine solid-state NMR study of several hydrochloride salts of amino acids implicated in chloride ion transport channel selectivity are reported. (35)Cl and (37)Cl NMR spectra have been obtained for stationary and/or magic-angle spinning powdered samples of the following compounds on 500 and/or 900 MHz spectrometers: DL-arginine HCl monohydrate, L-lysine HCl, L-serine HCl, L-glutamic acid HCl, L-proline HCl, L-isoleucine HCl, L-valine HCl, L-phenylalanine HCl, and glycine HCl. Spectral analyses provide information on the anisotropic properties and relative orientations of the chlorine electric field gradient and chemical shift (CS) tensors, which are intimately related to the local molecular and electronic structure. Data obtained at 900 MHz provide unique examples of the effects of CS anisotropy on the NMR spectrum of a quadrupolar nucleus. The range of chlorine quadrupolar coupling constants (C(Q)) measured, -6.42 to 2.03 MHz, demonstrates the sensitivity of this parameter to the chloride ion environment and suggests the applicability of chlorine solid-state NMR as a novel experimental tool for defining chloride binding environments in larger ion channel systems. Salts of hydrophobic amino acids are observed to tend to exhibit larger values of C(Q) than salts of hydrophilic amino acids. A simple model for rationalizing the observed trend in C(Q) is proposed. For salts for which neutron diffraction structures are available, we identify a quantum chemical method which reproduces experimental values of C(Q) with a root-mean-square deviation of 0.1 MHz and a correlation coefficient of 0.9998. On the basis of this, chlorine NMR tensors are predicted for the Cl(-) binding site in ClC channels.

High-Field Chlorine NMR Spectroscopy of Solid Organic Hydrochloride Salts: A Sensitive Probe of Hydrogen Bonding Environment

A series of organic hydrochloride salts has been investigated using solid-state 35Cl and 37Cl NMR spectroscopy at applied magnetic field strengths of 9.4 and 18.8 T. Magic-angle spinning, static Hahn-echo, and quadrupolar Carr−Purcell Meiboom−Gill (QCPMG) echo experiments have been applied to investigate the chlorine electric field gradient (EFG) and chemical shift (CS) tensors for l-tyrosine hydrochloride, l-cysteine methyl ester hydrochloride, l-cysteine ethyl ester hydrochloride, quinuclidine hydrochloride, and tris sarcosine calcium chloride. Chlorine-35 nuclear quadrupolar coupling constants for these compounds range from 2.23 to 5.25 MHz, and isotropic chemical shifts range from approximately 9 to 53 ppm relative to the chloride ion in aqueous solution. The results demonstrate the feasibility and benefits of high-field 35/37Cl NMR studies of organic chloride salts. A discussion of the data in the context of the known X-ray or neutron diffraction structures for these compounds suggests that the chlor...

Investigation of the polymorphs of dimethyl-3,6-dichloro-2,5-dihydroxyterephthalate by (13)C solid-state NMR spectroscopy.

Two of the three conformational polymorphs of dimethyl-3,6-dichloro-2,5-dihydroxyterephthalate are studied by solid-state NMR techniques. The structural differences between the polymorphs have previously been studied by X-ray. In these two polymorphs named white and yellow due to their color, the major structural difference is the torsional angle between the ester group and the aromatic ring. The yellow form has a dihedral angle of 4 degrees between the plane of the aromatic ring and the plane of the ester group, while the white form has two different molecules per unit cell with dihedral angles of 70 degrees and 85 degrees. This change greatly affects the conjugation in the pi-electronic system. In addition, there are differences in the hydrogen-bonding patterns, with the white form having intermolecular hydrogen bonds and the yellow form having intramolecular hydrogen bonds. In this work, the carbon isotropic chemical shift values and the chlorine electric field gradient (EFG) tensor information are extracted from the (13)C MAS spectra, and the principal values of the chemical shift tensors of the carbons are obtained from 2D FIREMAT experiments. Quantum chemical calculations of the chemical shift tensor data as well as the EFG tensor are performed at the HF and DFT levels of theory on individual molecules and on stacks of three molecules to account for the important intermolecular interactions in the white form. The differences between the spectral data on the two polymorphs are discussed in terms of the known electronic and structural differences.

Quantum chemical study of the HCl molecule and its binary complexes with CO, C 2H 2, C 2H 4, PH 3, H 2S, HCN, H 2O and NH 3: Hydrogen bonding and its effect on the 35Cl nuclear quadrupole coupling constant

The results of ab initio quantum chemical calculations are reported on the HCl molecule and the hydrogen bonded complexes B…HCl, where BCO, C 2H 2, C 2H 4, PH 3, H 2S, HCN, H 2O and NH 3. The main emphasis of this work is the accurate calc of the 35Cl nuclear quadrupole coupling constants, using SCF and averaged coupled pair functional methods, accounting for the electrical and geometrical polarization effects as well as librational averaging. Other quantities of interest that were calculated and compared with experiment include a range of one-electron properties for the HCl molecule such as its dipole and quadrupole moments, polarizability, the chlorine electric field gradient response tensors as well as derivatives of the field gradient. For the complexes the binding energies, dipole moment shifts, changes in the HCl bond length as well as in its stretching frequency and intensity are also reported. The overall agreement between theory and experiment, where comparison is possible, is very good. The hydrogen bonding and its effects on the dipole moment and the Cl field gradient are also analysed into electrostatic, exchange, polarization and charge transfer contributions using the Morokuma method and the constrained spatial orbital variation approach; these calculations convincingly demonstrate the importance of intermolecular exchange, especially with regard to the Cl field gradient.

35Cl pure quadrupole resonance in acetylated glycopyranosyl chlorides. Relation between chlorine nuclear quadrupole coupling constant and molecular conformation

Nuclear pure quadrupole resonance of the chlorine atom bound to carbon atom C1 in several hexo- and pentopyranosyl derivatives has been detected. Observed resonance frequencies are distributed over two groups, centered, respectively, at 35.535 and 35.123 MHz. The two groups can be related to the two possible positions, axial and equatorial, of the chlorine atom bound to the anomeric carbon. The interpretation is supported by general rules of conformational analysis and by conclusions derived from the study of the equilibrium between conformers in solution. An all electron, ``ab initio'' calculation of molecular wavefunction, carried out on a simplified model (2-chlorotetrahydropyran) similar to the molecules under study, yields electronic densities on each atomic orbital, especially those of chlorine, as well as bond indexes for both axial and equatorial conformations. Values of the chlorine electric field gradient, as deduced from the computed populations with the help of the Townes and Dailey theory, are consistent with the variation of the experimental frequencies between axial and equatorial compounds.

Lowest Triplet State of Substituted Benzenes. I. The Optically Detected Magnetic Resonance of para-Dichlorobenzene

The optically detected magnetic resonance (ODMR) of para-dichlorobenzene in its first 3ππ* state is reported. The structures of the three zero-field transitions are interpreted in terms of a Hamiltonian incorporating the electron spin—spin, chlorine nuclear quadrupole, and chlorine hyperfine interactions. It is concluded from an analysis of the spin-Hamiltonian parameters that the triplet spin density in the chlorine out-of-plane orbitals is small, the chlorine electric field gradient is significantly reduced in the 3ππ* state as compared to the value for the ground state, and that the molecule most likely is distorted.