Analysis of NMR shifts of high-spin cobalt(II) pyrazolylborate complexes

We present a detailed study on the correlation of the nuclear magnetic resonance (NMR) parameters with the results of density functional theory (DFT) calculations performed for paramagnetic high-spin cobalt(II) complexes with trispyrazolylborate ligands. This work is a first attempt to calculate dipolar and contact shifts in high-spin cobalt(II) pyrazolylborate systems. The calculation results show frontier orbitals that may be responsible for the contact shift. The contact shift values are compared with the dipolar shift ones. The latter shift values were both obtained from experimental data and calculated using DFT methods. We attempt to explain the dipolar effect, which may have a great share in NMR chemical shift of paramagnetic compounds.     

Variable temperature Mössbauer spectroscopic studies of oxo-centered trinuclear mixed-valence iron succinate, mesaconate and isophthalate complexes

Three new oxo-centered trinuclear mixed-valence iron complexes, iron succinate, iron mesaconate and iron isophthalate have been prepared. Temperature dependent valence delocalization processes were observed for all the complexes on variable temperature Mössbauer spectroscopic studies. Two clear quadrupole split doublets attributed to high-spin Fe(III) and high-spin Fe(II) states were existed for the complexes at liquid nitrogen temperature. At room temperature a nearly averaged singlet peak was observed for mesaconate complex and averaged doublet peaks were observed for succinate and isophthalate complexes with IS values 0.68 and 0.68 mms^?1 and QS values 0.44 and 0.48 mms^?1 respectively.     

An NMR study of the origin of dioxygen-induced spin-lattice relaxation enhancement and chemical shift perturbation

Due to its depth-dependent solubility, oxygen exerts paramagnetic effects which become progressively greater toward the hydrophobic interior of micelles, and lipid bilayer membranes. This paramagnetic gradient, which is manifested as contact shift perturbations (19F and 13C NMR) and spin-lattice relaxation enhancement (19F and 1H NMR), has been shown to be useful for precisely determining immersion depth, membrane protein secondary structure, and overall topology of membrane proteins. We have investigated the influence of oxygen on 19F and 13C NMR spectra and spin-lattice relaxation rates of a semiperfluorinated detergent, (8,8,8)-trifluoro (3,3,4,4,5,5,6,6,7,7)-difluoro octylmaltoside (TFOM) in a model membrane system, to determine the dominant paramagnetic spin-lattice relaxation and shift-perturbation mechanism. Based on the ratio of paramagnetic spin-lattice relaxation rates of 19F and directly bonded 13C nuclei, we conclude that the dominant relaxation mechanism must be dipolar. Furthermore, the temperature dependence of oxygen-induced chemical shift perturbations in 9F NMR spectra suggests a contact interaction is the dominant shift mechanism. The respective hyperfine coupling constants for 19F and 13C nuclei can then be estimated from the contact shifts <(deltav/v0)19F> and <(deltav/v0)13C>, allowing us to estimate the relative contribution of scalar and dipolar relaxation to 19F and 13C nuclei. We conclude that the contribution to spin-lattice relaxation from the oxygen induced paramagnetic scalar mechanism is negligible.     

Determination of Orientational Anisotropy in Glassy Solids by 2D Dipolar Spectra With Sample Flipping

A method is proposed for the quantitative measurement of orientational anisotropy in glassy solids based on 2D dipolar NMR spectra with sample flipping (dipolar DECODER experiment). Purely dipolar spectra are obtained by chemical shift refocusing by a multiple pulse sequence. The experiment is applied to an investigation of a doubly¹³C-labeled sample of bisphenol-A polycarbonate deformed in a channel-die apparatus. The orientational distribution function is determined by an expansion of the distribution in terms of spherical harmonics up to degree four.     

Mössbauer study of cobalt and iron in the cyanobacterium (blue green alga)

Mössbauer emission and absorption studies have been performed on cobalt and iron in the cyanobacterium (blue-green alga). The Mössbauer spectrum of the cyanobacterium cultivated with⁵⁷Co is decomposed into two doublets. The parameters of the major doublet are in good agreement with those of cyanocobalamin (vitamin B₁₂) labeled with⁵⁷Co. The other minor doublet has parameters close to those of Fe(II) coordinated with six nitrogen atoms. These suggest that cobalt is used for the biosynthesis of vitamin B₁₂ or its analogs in the cyanobacterium. The spectra of the cyanobacterium grown with⁵⁷Fe show that iron is in the high-spin trivalent state and possibly in the form of ferritin, iron storage protein.     

Mononuclear Co(III) and Ni(II) Complexes with Polypyridyl Ligands, [Co(phen)2(taptp)]3+ and [Ni(phen)2(taptp)]2+: Synthesis, Photocleavage and DNA-binding

Two novel, mixed ligand complexes of cobalt(III) and nickel(II), [Co(phen)₂(taptp)]³⁺ (1) and [Ni(phen)₂(taptp)]²⁺ (2) (phen = 1,10-phenanthroline and taptp = 4,5,9,18-tetraazaphenanthreno [9,10-b]triphenylene), were synthesized and characterized by elemental analyses, UV-visible and NMR spectroscopies. The binding interactions of the two complexes with DNA have been investigated using absorption and emission spectroscopy methods and electrophoresis measurement mode. The intrinsic binding constants for these complexes to DNA are in the order of 10⁵. In Tris buffer, the Co(III) complex shows a moderate luminescence which was enhanced after binding to DNA. However for complex Ni(II), no emission was observed in Tris buffer. The [Co(phen)₂(taptp)]³⁺ and [Ni(phen)₂(taptp)]²⁺ can cause the photocleavage of DNA supercoiled pBR322 upon irradiation by 360 nm light. Based on the data, an intercalative mode of DNA binding is suggested for the two complexes.     

Nuclear inelastic scattering studies on a dinuclear iron(II) spin crossover complex

The vibrational properties of the (high-spin)-(high-spin) and the (high-spin)- (low-spin) states of the dinuclear Fe(II) spin crossover complex[{Fe(L-N₄Me₂)}₂(BiBzIm)](ClO₄)₂·2EtCN¹ have been studied by means of nuclear inelastic scattering. At a temperature of 80 K typical low spin marker bands are detected in the region around 400 cm^?1, these bands almost completely disappear after increasing temperature to 190 K. Corresponding density functional theory calculations using the functional B3LYP* and the basis set CEP-31G reproduce the experimental data and thus allow a deeper understanding of the vibrational properties of dinuclear Fe(II) spin crossover complexes.     

Fluorine-19 Solid-State NMR Magic-Angle-Turning Experiments Using Multiple-Pulse Homonuclear Decoupling

For compounds giving “crowded” 1-dimensional magic-angle-spinning spectra, information about the local atomic environment in the form of the chemical shift anisotropy (CSA) is sacrificed for high resolution of the less informative isotropic chemical shift. Magic-angle-turning (MAT) NMR pulse sequences preserve the CSA information by correlating it to the isotropic chemical shift in a 2-dimensional experiment. For low natural abundance nuclei such as ¹³C and ¹⁵N and under ¹H heteronuclear dipolar decoupling conditions, the dominant NMR interaction is the chemical shift. For abundant nuclei such as ¹H, ¹⁹F, and ³¹P, the homonuclear dipolar interaction becomes a significant contribution to the observed linewidth in both F₁ and F₂ dimensions. We incorporate MREV8 homonuclear multiple-pulse decoupling sequences into the MAT experiment to give a multiple-pulse MAT (MP-MAT) experiment in which the homonuclear dipolar interaction is suppressed while maintaining the chemical shift information. Extensive use of computer simulation using GAMMA has guided the pulse sequence development. In particular, we show how the MREV8 pulses can be incorporated into a quadrature-detected sequence such as MAT. The MP-MAT technique is demonstrated for a model two-site system containing a mixture of silver trifluoroacetate and calcium difluoride. The resolution in the isotropic evolution dimension is improved by faster sample spinning, shorter MREV8 cycle times in the evolution dimension, and modifications of the MAT component of the pulse sequence.     

A15N-1H dipolar CSA solid-state NMR study of polymorphous polyglycine (-CO-CD2-15NH-)n

The solid-state¹H MAS (magic-angle spinning),²H static,¹⁵N CP (cross polarization)-MAS and¹⁵N-¹H dipolar CSA (chemical shielding anisotropy) NMR (nuclear magnetic resonance) spectra of two different modifications of C_α-deuterated^l5N-polyglycine, namely PG I and PG II (-CO-CD₂-^l5NH-)_n are measured. The data from these spectra are compared to previous NMR, infrared, Raman and inelastic neutron scattering work. The deuteration of C_α eliminates the largest intramolecular¹H-¹H dipolar coupling. The effect of the remaining (N)H-(N)H interaction (～5 kHz) is not negligible compared to the¹⁵N-¹H coupling (about 10 kHz). Its effect on the dipolar CSA spectra, described as a two-spin system, is analyzed analytically and numerically and it is shown that those parts of the powder spectrum, which correspond to orientations with a strong dipolar¹⁵N-¹H interaction, can be described as an effective two-spin system, permitting the measurement of the strength of the¹⁵N-¹H dipolar interaction and the orientation of the dipolar vector with respect to the¹⁵N CSA frame. While in the PG II system the¹⁵N CSA tensor is collinear with the amide plane, in the PG I system the CSA tensor is tilted ca. 16° with respect to the (δ₁₁δ₂₂) CSA plane.     

Optical and electrochemical characteristics of ethylenediamine complexes of Pt(II) and Ir(III) with metalated 2-phenyl- and 2-naphthylbenzothiazole

The cyclometalated complexes [Pt(С^N)En]PF₆ and [Ir(C^N)₂En]PF₆ ((C^N)^– are deprotonated forms of 2-phenylbenzothiazole or 2-naphthylbenzothiazole and En is ethylenediamine) are studied by ¹Н NMR, IR, electronic absorption, and emission spectroscopy, as well as by voltammetry. Metalation of heterocyclic ligands leads to the formation of five-membered {M(C^N)} cycles in the composition of squareplanar Pt(II) complexes and octahedral Ir(III) complexes of the cis-С,С structure. A bathochromic shift of the metal-to-cyclometalated ligand charge transfer bands and a decrease in the potential difference between the single-electron waves of metal-centered oxidation and ligand-centered reduction of complexes upon substitution of 2-phenylbenzothiazole by 2-naphthylbenzothiazole and of Pt(II) by Ir(II) are shown. The phosphorescence of complexes in the visible region is assigned to the radiative transition from the metal-modified intraligand electronic excited state.     

1H NMR Study of Some Thiosalicylic Acid Complexes

Five thiosalicylic acid (TSA) complexes were prepared. The ¹H NMR of these complexes were recorded and analysed. It was found that molybdenum, arsenic, selenium, silver and tellurium metals coordinates through the sulfur atom. The ¹H NMR chemical shift of ring protons of As, Se and Te shift to low field upon complexation, while it shifts to high field when TSA complex with Ag and Mo     

Effect of the donor–acceptor properties of ligands on the spectroscopic and electrochemical properties of mixed-ligand complexes of Pt(II) and Ir(III) with cyclometalated 2-phenylbenzothiazole

The results of the spectroscopic NMR (¹H, ¹³C, and ¹⁹⁵Pt), infrared, optical, and voltammetric characteristics of the mixed-ligand complexes of Pt(II) and Ir(III) with metalated 2-phenylbenzothiazole and tert-butylisocyanide (tBuNC), acetonitrile (AN), ethylenediamine (En), O-ethyldithiocarbamate (Exn^–), and diethyldithiocarbamate (Dtc–) ions are presented. It is demonstrated that the change in donor–acceptor interaction of ligands tBuNC, AN, En, Exn^–, and Dtc^– with metal leads to an increase in the energy of the highest occupied molecular orbital of the complexes and is accompanied by a shift of the cathode potential of the metal-centered oxidation, a bathochromic shift of the spin-allowed and spin-forbidden metal-tocyclometalated ligand optical charge transfer transitions, and an increase the degree of mixing of the ¹MLCT and triplet intraligand states, responsible for the phosphorescence of the complexes.     

分离稀土诱导位移的双稀土混合方法

稀土顺磁性诱导位移包含接触位移和偶极位移两种成份,如果把z>或D符号相反的两种稀土离子以适当的浓度比混合于同一溶液中。则实验上观测到的顺磁位移仅含一种成份,另外一种位移成份将被抵消。双稀土混合分离诱导位移技术要求底物分子与稀土离子间的的配位过程满足快速交换平衡条件且形成的稀土-底物配合物具有轴对称性或有效轴对称性。与其它分离诱导位移的方法相比,双稀土混合方法具有简单明确的特点。本文将这一方法应用于L-抗坏血酸¹³C稀土诱导位移的分离验证了其可行性。     

Suppression of Diagonal Peaks in TROSY-Type H NMR NOESY Spectra of N-Labeled Proteins

A novel method for suppression of diagonal peaks in the amide region of NOESY NMR spectra of ¹⁵N-labeled proteins is presented. The method is particularly useful for larger proteins at high magnetic fields where interference between dipolar and chemical shift anisotropy relaxation mechanisms results in large TROSY effects, i.e., large differences in ¹H^N linewidths depending on the spin state of attached ¹⁵N nuclei. In this limit the new TROSY NOESY method does not compromise sensitivity. It is demonstrated using a perdeuterated ¹⁵N-labeled protein sample, Neural Cell Adhesion Molecule 213–308 (NCAM) from rat, in H₂O at 800 MHz.     

Optical and electrochemical properties of cyclometalated Pd(II) and Pt(II) complexes with a metal-metal chemical bond

Cyclometalated [M(C^N)(μ-(N-S))]₂ complexes ((M = Pd(II), Pt(II)), (C^N)^? are the deprotonated forms of 2-tolylpyridine and benzo[h]quinoline, and (N-S)^? are pyridine-2-thiolate and benzothiazole-2-thiolate ions) are studied by ¹H NMR, IR, electronic-absorption, and emission spectroscopy, as well as by voltammetry. It is shown that the formation of the metal-metal chemical bond and the σ _dz2 ^* orbital as a HOMO of complexes leads to the long-wavelength spin-allowed (410–512 nm) and spin-forbidden (595–673 nm) optical transitions σ _dz2 ^* ?π _(C^N) ^* in the absorption and phosphorescence spectra, as well as to the two-electron and successive one-electron oxidation with the formation of binuclear Pt(III) and Pd(III) complexes. The substitution of Pt(II) by Pd(II) is characterized by hypso- and bathochromic shifts of the spin-allowed and forbidden σ _dz2 ^* ?π _(C^N) ^* optical transitions in the absorption and phosphorescence spectra of complexes, by phosphorescence quenching of Pd(II) complexes in liquid solutions, and by an anodic shift of the oxidation potential of Pd(II) complexes compared with Pt(II) complexes.     

NMR studies of magnetic properties in Heusler-type Mn 3 Si

In order to microscopically investigate the magnetic properties of both paramagnetic and antiferromagnetic phases in Mn₃Si (T _N?=?23 K), the ⁵⁵Mn NMR has been carried out at temperatures between 2.2 K and 300 K. The temperature dependences of the spectrum, Knight shift (or resonance frequency shift) and spin-lattice relaxation time T ₁ of ⁵⁵Mn NMR have been measured. In the paramagnetic phase, only one resonance spectrum can be obtained. The observed spectrum is identified to be a signal corresponding to the Mn(II) site. In the antiferromagnetic phase, two different spectra corresponding to the Mn(I) and Mn(II) sites are found at the resonance frequencies of 145 and 6 MHz, respectively, by the zero field NMR at 4.2 K. From these results, the internal magnetic fields on the ⁵⁵Mn(I) and ⁵⁵Mn(II) nuclei are found to be 13.6 and 0.6 T, respectively. According to the NMR results, the helical structure in incommensurate Mn spin states is better explained compared with the transverse sinusoidal structure.     

Synthesis,Spectral Characterization,DNA Binding Studies and Antimicrobial Activity of Co(II), Ni(II), Zn(II), Fe(III) and VO(IV) Complexes with 4-Aminoantipyrine Schiff Base of Ortho-Vanillin

A series of transition metal complexes of Co(II), Ni(II), Zn(II), Fe(III) and VO(IV) have been synthesized involving the Schiff base, 2,3-dimethyl-1-phenyl-4-(2-hydroxy-3-methoxy benzylideneamino)-pyrazol-5-one(L), obtained by condensation of 4-aminoantipyrine with 3-methoxy salicylaldehyde. Structural features were obtained from their FT-IR, UV–vis, NMR, ESI Mass, elemental analysis, magnetic moments, molar conductivity and thermal analysis studies. The Schiff base acts as a monovalent bidentate ligand, coordinating through the azomethine nitrogen and phenolic oxygen atom. Based on elemental and spectral studies six coordinated geometry is assigned to Co(II), Ni(II), Fe(III) and VO(IV) complexes and four coordinated geometry is assigned to Zn(II) complex. The interaction of metal complexes with Calf thymus DNA were carried out by UV–VIS titrations, fluorescence spectroscopy and viscosity measurements. The binding constants (K_b) of the complexes were determined as 5?×?10⁵ M^?1 for Co(II) complex, 1.33?×?10⁴ M^?1 for Ni(II) complex, 3.33?×?10⁵ M^?1 for Zn(II) complex, 1.25?×?10⁵ M^?1 for Fe(III) complex and 8?×?10⁵ M^?1 for VO(IV) complex. Quenching studies of the complexes indicate that these complexes strongly bind to DNA. Viscosity measurements indicate the binding mode of complexes with CT DNA by intercalation through groove. The ligand and it’s metal complexes were screened for their antimicrobial activity against bacteria. The results showed the metal complexes to be biologically active, while the ligand to be inactive.     

Electron spin resonance and optical spectroscopic studies of Co-ZSM-5 with nitric oxide

The reaction of nitric oxide with ZSM-5 zeolite ion-exchanged by Co(II) (Co-ZSM-5) has been studied by electron spin resonance (ESR), Fourier transform infrared (FTIR) and diffuse reflectance ultraviolet-visible spectroscopy. When dehydrated Co-ZSM-5 reacts at room temperature with nitric oxide, the ESR spectrum of Co(II) atg = 5.5, which is only observable below 45 K, is greatly reduced and a new⁵⁹Co hyperfine octet forms at g_avg = 2.11. The overall Co(II) ESR intensity decreases by about 50% which suggests formation of some diamagnetic cobalt complex. Mononitrosyl cobalt complexes such as Co(I)-NO⁺ or less probably Co(III)-(NO)⁻ are suggested as possible precursors of a dintrosyl cobalt complex. The octet indicates hyperfine interaction with⁵⁹Co and is associated with a cobalt dinitrosyl complex. FTIR bands at 1813 and 1896 cm⁻¹ confirm a dinitrosyl species and a broad band from 1600–1800 cm⁻¹ is tentatively interpreted as a mononitrosyl species. The visible spectrum for dehydrated Co-ZSM-5 shows a tetrahedral Co(II) band from 500–700 nm with three components which disappears after NO adsorption at room temperature. We suggest that Co(0)-(NO)₂²⁺ forms after NO adsorption onto Co(II)-ZSM-5 zeolite on the basis of ESR and ultraviolet-visible spectroscopy.     

1H–13C hetero-nuclear dipole–dipole couplings of methyl groups in stationary and magic angle spinning solid-state NMR experiments of peptides and proteins

¹³C NMR of isotopically labeled methyl groups has the potential to combine spectroscopic simplicity with ease of labeling for protein NMR studies. However, in most high resolution separated local field experiments, such as polarization inversion spin exchange at the magic angle (PISEMA), that are used to measure ¹H–¹³C hetero-nuclear dipolar couplings, the four-spin system of the methyl group presents complications. In this study, the properties of the ¹H–¹³C hetero-nuclear dipolar interactions of ¹³C-labeled methyl groups are revealed through solid-state NMR experiments on a range of samples, including single crystals, stationary powders, and magic angle spinning of powders, of ¹³C₃ labeled alanine alone and incorporated into a protein. The spectral simplifications resulting from proton detected local field (PDLF) experiments are shown to enhance resolution and simplify the interpretation of results on single crystals, magnetically aligned samples, and powders. The complementarity of stationary sample and magic angle spinning (MAS) measurements of dipolar couplings is demonstrated by applying polarization inversion spin exchange at the magic angle and magic angle spinning (PISEMAMAS) to unoriented samples.     

Solid State C NMR of 30-Crown-10 Ether and 30-Crown-10.4H2O

The ¹³C NMR solution spectra of 30-crown-10 ether and its tetrahydrate show only one resonance at all accessible temperatures. In contrast, the solid state ¹³C NMR spectrum of the 30-crown-10.4H₂O shows two resonances in the ratio of 4:1, separated by 1.2 ppm. In the case of 30-crown-10 itself, six resolvable ¹³C resonances in the ratio of 4:1:1:2:1:1 are observed in the solid with an overall chemical shift dispersion of 5 ppm. The remarkably different spectral behavior of these two systems in the solid state is discussed in terms of the torsional environments of the crystallographically unique carbons and the results of GIAO calculations of isotropic ¹³C shieldings for simpler model compounds. Results of dipolar dephased ¹³C CPMAS spectra indicate that 30-crown-10 does not undergo a large amplitude molecular motion, in contrast to earlier results for 18-crown-6. Only a small amount of residual intensity is found in the dipolar dephased spectrum of 30-crown-10.4H₂O, indicating that it also is relatively rigid in the solid.