Simulation Of NMR Spectra Research Articles

Synthetic routes to mono- and bi-copper hydrazone complexes uncovered by computer simulation

The chelating properties of two polyfunctional di-2-pyridylketone hydrazone ligands toward the copper ion have been investigated in mono- and bimetallic complex species. The complexation reaction is strongly influenced by the nature of the inorganic salt reagent. Structural characterization of the mono- and bimetallic copper species by powder X-ray diffraction analysis and computer simulation made an important contribution to the discovery of the synthesis conditions. Simulated IR and 1H NMR spectra were also calculated.

Synthesis and Characterization of Thiolate-Thiol Complexes of Ruthenium Nitrosyl Porphyrins and Their Symmetrical and Unsymmetrical Dithiolate-Bridged Bimetallic Derivatives.

Monometallic nitrosyl thiolate-thiol complexes of the form (por)Ru(NO)(S(CH(2))(n)()SH) (por = TPP, n = 2; TTP, n = 2-4) have been prepared in 49-61% isolated yields from the reaction of the (por)Ru(NO)(O-i-C(5)H(11)) alkoxide precursors with HS(CH(2))(n)()SH. The (OEP)Ru(NO)(SCH(2)CH(2)SH) complex was prepared in 71% isolated yield via addition of HSCH(2)CH(2)SNO to (OEP)Ru(CO). These (por)Ru(NO)(S(CH(2))(n)()SH) complexes have been fully characterized by elemental analyses, infrared and (1)H NMR spectroscopy, and by mass spectrometry. The molecular structure of (OEP)Ru(NO)(SCH(2)CH(2)SH) has been determined by single-crystal X-ray crystallography. The Ru-N-O moiety is linear with a bond angle of 170.9(9) degrees. The symmetrical bimetallic [(por)Ru(NO)](2)(&mgr;-S(CH(2))(n)()S-S,S') complexes (por(1) = por(2); i.e., TPP/TPP, TTP/TTP, OEP/OEP) have also been prepared by variations of the procedures used in the preparation of their monometallic derivatives, and they have been fully characterized by elemental analyses, infrared and (1)H NMR spectroscopy, and by mass spectrometry. The novel unsymmetric [(TPP)Ru(NO)](&mgr;-SCH(2)CH(2)S-S,S')[Ru(NO)(OEP)] complex has also been obtained, and the experimental and simulated (1)H NMR spectrum of the protons of the &mgr;-dithiolate bridge indicate that all four protons are inequivalent.

NMR Study of Intrinsic Defects in Congruent Lithium Niobate

The simulations of NMR spectra of 93Nb nuclei for different models of intrinsic defects in single crystal of congruent lithium niobate have been performed. It has been shown that the most probable defects in LiNbO3 are complex (NbLi + 3 VLi) and isolated VLi.

Effect of Milling of V2O5on the Local Environment of Vanadium as Studied by Solid-State51V NMR and Complementary Methods

Milling of V2O5 in a ball mill increased the surface area from 7 to a maximum of 32 m2/g. Simultaneously, XRD line broadening suggested a reduction of the microcrystallites. A color change during milling indicated oxygen loss, and Raman spectroscopy provided evidence for the formation of shear structures such as V6O13. The number of V4+ ions produced was determined by ESR spectroscopy and that of V3+ ions was obtained from magnetic susceptibility measurements. The local environment of the vanadium nucleus in V2O5 after milling in a ball mill was characterized by the combination of static (wide line) and MAS 51V NMR techniques together with theoretical simulations of NMR spectra. Important information on the quadrupole and chemical shielding tensors, including the relative tensor orientation and the distribution of magnetic resonance parameters, is discussed for the vanadium nucleus in V2O5 after milling. Special attention was given to the formation of paramagnetic V3+ and V4+ ions and their influence on 5...

NMR study of intrinsic defects in congruent LiNbO 3. 2. “Overlapping” defects

Results of the computer simulations of NMR spectra of 93Nb nuclei for different models of intrinsic “overlapping” defects in single crystal of congruent lithium niobate have been presented. It has been shown that the model of “overlapping” defects does not change our previous conclusion [1–3], that the most probable intrinsic defects in LiNbO 3 are complex [Nb Li+(2–3)V Li] and isolated (2–1)V Li.

Computer-assisted structural analysis of oligo- and polysaccharides: An extension of CASPER to multibranched structures

The CASPER program which is used for determination of the primary structure of oligo- and polysaccharides has been extended. It can now handle a reduced number of experimental signals from an NMR spectrum in the comparison to the simulated spectra of structures that it generates, an improvement which is of practical importance since all signals in NMR spectra cannot always be identified. Furthermore, the program has been enhanced to simulate NMR spectra of multibranched oligo- and polysaccharides. The new developments were tested on four saccharides of known structure but of different complexity and were shown to predict the correct structures.

NMR study of intrinsic defects in congruent LiNbO 3. 1. “Unoverlapping” defects

The simulations of NMR spectra of 7Li and 93Nb nuclei for different models of intrinsic defects in single crystal of congruent lithium niobate have been performed. It has been shown that the most probable defects in LiNbO 3 are complex (Nb Li + 3V Li) and isolated V Li. The NMR spectra of 7Li have been simulated assuming that the potential surface of Li ion in distorted LiO 6 octahedron has four minima.

Anchoring and droplet deformation in polymer dispersed liquid crystals: NMR study in an electric field

We report a deuterium NMR study of nematic droplets, formed in a polymer dispersed liquid crystal (PDLC). An electric field with variable strength and direction orthogonal to the NMR magnetic field is applied to this liquid crystal, confined to submicrometer quasispherical cavities with planar surface anchoring. A broad transition from predominantly magnetic field aligned droplets to the electric field aligned limit is observed. The Frank-Oseen free energy with an additional surface energy term is used to find nematic director fields, free energies, and finally simulated NMR spectra of structures in different magnetic and electric fields. We are particularly interested in the influence of surface anchoring strength and nonspherical cavity shape on nematic structures and NMR spectra. Cavities are modeled by randomly oriented rotationally symmetric ellipsoids with a Gaussian distribution of elongations measuring droplet deformation. Simulated NMR spectra describe the experimental data very well and yield for the surface anchoring strength $(1\ifmmode\pm\else\textpm\fi{}0.15)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ J/m${}^{2}$ and for the width of the deformation distribution $(15\ifmmode\pm\else\textpm\fi{}1)%$. The agreement with the scanning electron microscopy results indicates that deformations are the main cause of the random orientation of bipolar structures in these PDLC's. Using the known dielectric anisotropy of our nematic liquid crystal we found the electric field in the droplets to be 60% of the undisturbed electric field.

Simulation of many-spin system dynamics via sparse matrix methodology

A sparse-matrix-based numerical method is constructed to simulate NMR spectra of many-spin systems, including the effects of chemical exchange and/or relaxation. The associated computational demands are predicted to scale like O(22n), as the number of spins n increases. This is vastly superior to the inevitable O(26n) scaling of conventional Householder-based methodology. The improved scaling is verified via numerical computations of simple isomerization systems with four to nine spins. The new method is based on splitting the propagator and use of Chebyshev polynomial expansion of the exponential function.

Conformational analysis of 2-(carboxycyclopropyl)glycine agonists of glutamate receptors in aqueous solution using a combination of NMR and molecular modelling experiments and charge calculations

Two classes of glutamate receptors [metabotropic (group-II) and ionotropic (NMDA) subclasses] are characterized by the binding of α-(carboxycyclopropyl)glycine (CCG) isomers, (2S,3S,4S)-CCG (L-CCG-I) and (2S,3R,4S)-CCG (L-CCG-IV) which contain an embedded L-glutamate moiety in a partially restricted conformation [relative to the C(3)–C(4) bond]. The spatial orientation of the perceived functional groups have been elucidated by a conformational analysis in aqueous solution of L-CCG-I and L-CCG-IV using a combination of NMR experimental results, theoretical simulation of NMR spectra, mechanics and dynamics calculations. It was of interest to compare the charge distributions resulting from a number of quantum calculations on the cyclopropane ring. One important conclusion of the study is that the best theoretical model is the MD in solvent. This study shows clearly the preferred ‘t-A’ and ‘g+-B’ conformations of the C(3) aminocarboxymethyl side chain for L-CCG-I and L-CCG-IV, respectively. Weak pH-dependent effects on the structure of the principal L-CCG-I and L-CCG-IV conformers have been established in aqueous solution. The conformations may be grouped by the two backbone torsion angles, χ1 [α-CO2––C(2)–C(3)–C(4)] and χ2 [+NC(2)–C(3)–C(4)–γ-CO2–] and by the two characteristic distances between the potentially active functional groups, α-N+–γ-CO2– (d1) and α-CO2––γ-CO2– (d2). The conformational preferences in solution of L-CCG-I and L-CCG-IV are discussed in the light of the physical features known for specific metabotropic (ACPD) and specific ionotropic (NMDA) agonists, respectively.

Simulation of Slow Variable-Angle Spinning NMR Spectra. Application to13C Bonded to N

Abstract Simulations of NMR spectra for spin- 1 2 nuclei in rigid solids spun at relatively slow speeds and at angles other than the magic angle are presented. Particular emphasis is on the case where13C is bonded to a quadrupolar nucleus such as14N. The feasibility of quantitatively extracting the chemical-shift tensor, bond lengths through the dipolar interaction, and the electric-field gradient of the neighboring nucleus from a single (1-D) spectrum is demonstrated. The sensitivity of the observed changes in the spectra to changes in the physical and experimental parameters is discussed and illustrated using nitrile13C spectra of solid CH3CN. Equations are derived to extend the theory beyond the adiabatic approximation.

SpinA-AT and DNMR-SIM — two new PC-programs for analysis and simulation of NMR spectra

Two novel PC programs for simulation and interpretation of the NMR spectra SpinA-AT and DNMR-SIM, are introduced and demonstrated with examples. SpinA-AT covers the classical literature on explicit solutions for transition frequencies and intensities given for 2–4 spins. DNMR-SIM is a simulation program for dynamic NMR spectra.

Approximations in the Simulation of NMR Spectra; Effects on Line Splittings

Computer simulations of NMR spectra were used to examine several of the approximations that are often explicitly or implicitly made in studies of the effects of relaxation on lineshapes and line positions. It is known that relaxation shifts the positions of lines in NMR spectra, but it is shown that approximations made in applying the theory of relaxation to NMR simulations can also shift the positions of the lines.

Orientation and Motion of Tetrahydrofuran in Graphite Intercalation Compounds: Proton NMR Studies of Cs(THF)1.3C24 and K(THF)2.5C24

The orientation and motion of tetrahydrofuran (THF) in the ternary graphite intercalation compounds Cs(THF){sub 1.3}C{sub 24} and K(THF){sub 2.5}C{sub 24} have been studied by proton NMR. Simulations of the NMR spectra indicate that the THF molecules in Cs(THF){sub 1.3}C{sub 24} have their mean planes oriented parallel to the layers of the host lattice, while the THF molecules in K(THF){sub 2.5}C{sub 24} have their mean planes oriented at an angle between 50{degree} and 75{degree} from the graphite layers. The proton NMR spectra of both compounds show evidence that the THF molecules rotate about the normal to the graphite layers and confirm X-ray diffraction studies showing a degree of orientational disorder in the samples, corresponding to a mosaic spread in the graphite layer orientation. Simulations indicate that the conformation of intercalated THF is different than gas or liquid phase THF, which has been found to have a ring puckering amplitude of 0.38-0.44 A and to undergo nearly free pseudorotation through a series of conformations. Best agreement between simulated and experimental NMR spectra of Cs(THF){sub 1.3}C{sub 24} was obtained with THF interconverting between two conformations of C{sub s} symmetry and a puckering amplitude of 0.30 A. 31 refs., 15 figs.

The 19F NMR spectrum of bis-trifluoromethylmercury Hg(CF 3) 2 in the nematic phase: DAISY — a novel program system for the analysis and simulation of NMR spectra

The 19F NMR spectrum of bis-trifluoromethylmercury in the nematic phase has been analysed using the DAISY program system. The scalar and the dipolar coupling constants have been obtained.

Lithium Insertion Study in Vanadium Oxide Bronze by NMR and X Ray Diffraction

The insertion of lithium in vanadium pentoxide at high temperature is followed both by Xray diffraction and 7 Li NMR studies. A correlation between the lithium sites found by structure determination and the isotropic chemical shifts is established and a mechanism of the lithium insertion proposed. Calculations of the valence of the vanadium atoms and the simulation of NMR spectra allows to localise the V4 + in the highest lithium rate phases

Superoperator Propagators in Simulations of NMR Spectra

Computer simulations of NMR spectra usually are based on computation of the density matrix extant at each step in the sequence of pulses and delays that defines an experiment. This approach can be time consuming, even for those parts of the experiment as elementary as the free induction decay, and may be prohibitive in simulations of experiments that include many pulses, such as when an MLEV mixing sequence is present or when decoupling with a WALTZ sequence. In the present work, an efficient formalism is developed that provides a basis for simulations in these instances and is applicable to the treatment of the effects of relaxation using Liouville space superoperators. The approach leads to appreciable simplification of programs for such simulations and a several orders of magnitude reduction in program run times.

Folic Acid: Solution Structure and NMR Strategy for Conformational Analysis

Abstract The solution structure of folic acid was determined by the combined use of NMR experimental results, mechanics and dynamics calculations, and theoretical simulation of NMR spectra. The solution structure of folic acid showed a different molecular rearrangement to the X-Ray conformation. In particular, we identified a hydrogen bond which stabilizes a folded glutamic acid molecular moiety between the NH(18) and the C(23) carboxylic group. A different spatial conformation of the phenyl ring was also found.

Attempted synthesis of esters of tricarballylic acid and 3,5-dibromosalicylic acid. Structure and properties of tricarballylic acid, its anhydrides and some of their derivatives, and an investigation of the reaction pathways of anhydride formation by1H NMR spectroscopy, MNDO and molecular mechanics methods

Reactions ordinarily leading to the formation of esters of carboxy acids afford, in the case of tricarballylic acid, the cyclic anhydride. A comparison of the experimental and simulated 1H NMR spectra reveals that the product contains a five-membered ring structure. To explain the experimental findings, theoretical calculations were performed at the level of molecular mechanics and MNDO methods, in order to obtain information on the structure and thermodynamic stability of various substrate conformations and reaction products, as well as on the reaction pathways of anhydride formation. Theoretical predictions reveal that the formation of the five-membered ring structure is preferred in the case of tricarballylic acid anhydride.

The Study of Orientational Order in Mixed Plastic Crystal (CH3)2CCl2 - CCl4 by 1H NMR Spectroscopy

Abstract 1H NMR spectra of noncubic plastic single crystals of 2,2-dichloropropane - carbon tetrachloride mixtures have been studied. Orientational order parameters of 2,2-dichloropropane molecules in this phase were determined by computer simulation of NMR spectra.