三核过渡金属配合物〔M_3~ⅢO(OOCR)_6L_3〕 (M=Cr,Fe,Mn;R=CH_3,C_2H_5,CH_2NH_2;L=C_5H_5N,H_2O)的~1H NMR性质

采用１ＨＮＭＲ谱研究了通式为〔Ｍ３ⅢＯ（ＯＯＣＲ）６Ｌ３〕＋（Ｍ＝Ｃｒ,Ｆｅ,Ｍｎ;Ｒ＝ＣＨ３,Ｃ２Ｈ５,ＣＨ２ＮＨ２;Ｌ＝Ｃ５Ｈ５Ｎ,Ｈ２Ｏ）的一系列氧心三核过渡金属配合物,主要考察其１Ｈ化学位移随金属、配体、温度、溶剂等因素变化而变化的规律。结果表明,骨架金属对化学位移的影响最大,Ｍ３Ｏ中的３个金属离子间存在反铁磁交换相互作用。对Ｍｎ配合物中顺磁中心对化学位移和线宽的影响机制的研究表明,其１Ｈ各向同性位移主要由接触作用贡献     

(67)Zn NMR chemical shifts and electric field gradients in zinc complexes: a quantum chemical investigation

We have used quantum chemical methods to predict 67Zn NMR chemical shifts as well as quadrupole coupling constants (CQ) in a series of biomimetic and inorganic zinc complexes. The 67Zn chemical shifts are predicted with an R2 = 0.975, corresponding to a 24.3 ppm or 6.7% error over the entire 365 ppm 67Zn chemical shift range. The 67Zn CQ values are predicted with an R2 = 0.991, corresponding to a 1.17 MHz or 3.0% error over the entire 38.75 MHz range. The 67Zn NMR shifts in a series of complexes containing N,O ligands are, in general, highly correlated with the number of oxygen ligands. The ability to compute 67Zn NMR shifts as well as CQ values opens up the possibility of using both of these properties in structure determination or refinement in proteins.     

Oxygen-17 NMR, Electronic, and Vibrational Spectroscopy of Transition Metal Peroxo Complexes: Correlation with Reactivity

The (17)O NMR chemical shifts of several previously characterized mono- and diperoxo complexes of vanadium(V), molybdenum(VI), tungsten(VI), and titanium(IV) were measured. Compilation of NMR, electronic, and vibrational spectroscopic data and metric parameters for these and other complexes permits us to draw correlations among (17)O peroxo chemical shift, the electronic charge transfer band, the O-O vibrational frequency, and the length of the oxygen-oxygen bond. Monoperoxo complexes exhibit (17)O chemical shifts of 500-660 ppm, while those of diperoxo complexes fall in the range 350-460 ppm. The correlation of chemical shift with the inverse ligand-to-metal charge transfer energy from electronic spectra is consistent with a formalism developed by Ramsey, despite the variations in the metals, the number of peroxo ligands, and the nature of the remaining ligands in the coordination sphere. Vibrational frequency and length of the oxygen-oxygen bond also correlate with the inverse ligand-to-metal charge transfer energy. Monoperoxo complexes show values of nu(O)(-)(O) above 900 cm(-)(1) and O-O distances in the range 1.43-1.46 ?. Diperoxo complexes have values of nu(O)(-)(O) below 900 cm(-)(1) and O-O distances of 1.46-1.53 ?. The assignment of nu(O)(-)(O) = 910 cm(-)(1) for the infrared spectrum of ammonium aquaoxoperoxo(pyridine-2,6-dicarboxylato)vanadium(V), NH(4)[VO(O(2))(dipic)(H(2)O)], was made by isotopic substitution. The stretching frequency and length of the O-O bond for peroxo complexes are explained in terms of sigma-bonding between a metal d orbital and a peroxo pi orbital. A comparison of the spectroscopic properties of these complexes with their reactivity as oxidizing agents suggests that the strength of the O-O bond is an important factor. The most reactive species exhibit lambda(max) values below 400 nm, stretching frequencies below 900 cm(-)(1), and (17)O chemical shifts below 600 nm. These generalizations may permit the prediction of peroxometal reactivity from spectroscopic information.     

Synthesis, X-ray crystallographic, and NMR characterizations of platinum(II) and platinum(IV) pyrophosphato complexes

A series of mononuclear cis-diamineplatinum(II) pyrophosphato complexes containing ammine (am), trans-1,2-cyclohexanediamine (dach), and 1,2-ethanediamine (en) as the amine ligands were synthesized and characterized by (31)P and (195)Pt NMR spectroscopy. Chemical shifts of (31)P NMR resonances of these completely deprotonated complexes appear at 2.12, 1.78, and 1.93 ppm, indicating a coordination chemical shift of at least 8 ppm. The (195)Pt NMR chemical shifts for the am and dach complexes were observed at -1503 and -1729 ppm. The complexes are highly stable at neutral pH; no aquation due to the release of either phosphate or amine ligands was observed within 48 h. Furthermore, no partial deligation of the pyrophosphate ligand was detected within several days at neutral pH. At lower pH, however, release of a pyrophosphate ion was observed with concomitant formation of a bridged pyrophosphatoplatinum(II) dinuclear complex. The extended crystal structure containing the dach ligand revealed a zigzag chain stacked in a head-to-tail fashion. Moreover, two zigzag chains are juxtaposed in a parallel fashion and supported by additional hydrogen bonds reminiscent of DNA structures where two strands of DNA bases are held by hydrogen bonds. Theoretical calculations support the notion that the two dinuclear units are held together primarily by hydrogen bonds between the amine and phosphate moieties. Platinum(II) pyrophosphato complexes were readily oxidized by hydrogen peroxide to yield cis-diamine-trans-dihydroxopyrophosphatoplatinum(IV) complexes. Two of these complexes, containing am and en, were characterized by X-ray crystallography. Notable structural features include Pt-O (phosphate) bond distances of 2.021-2.086 A and departures from 180 degrees in trans-HO-Pt-OH bond angles, >90 degrees in O-Pt-O, and >90 degrees in cis-N-Pt-N bond angles. The departure in the trans-HO-Pt-OH angle is more pronounced in the 1,2-ethanediamine complex compared to the dach analogue because of the existence of two molecules possessing enantiomeric conformations within the asymmetric unit. (31)P NMR spectra exhibited well-resolved (195)Pt satellites with coupling constants of 15.4 Hz for the ammine and 25.9 Hz for both the 1,2-ethanediamine and trans-1,2-cyclohexanediamine complexes. The (195)Pt NMR spectrum of the ammine complex clearly showed coupling with two equivalent N atoms.     

Quantum‐chemical analyses of aromaticity,UV spectra,and NMR chemical shifts in plumbacyclopentadienylidenes stabilized by Lewis bases

We carried out a series of zeroth‐order regular approximation (ZORA)‐density functional theory (DFT) and ZORA‐time‐dependent (TD)‐DFT calculations for molecular geometries, NMR chemical shifts, nucleus‐independent chemical shifts (NICS), and electronic transition energies of plumbacyclopentadienylidenes stabilized by several Lewis bases, (Ph)₂(^tBuMe₂Si)₂C₄PbL₁L₂ (L₁, L₂ = tetrahydrofuran, Pyridine, N‐heterocyclic carbene), and their model molecules. We mainly discussed the Lewis‐base effect on the aromaticity of these complexes. The NICS was used to examine the aromaticity. The NICS values showed that the aromaticity of these complexes increases when the donation from the Lewis bases to Pb becomes large. This trend seems to be reasonable when the 4n‐Huckel rule is applied to the fractional π‐electron number. The calculated ¹³C‐ and ²⁰⁷Pb‐NMR chemical shifts and the calculated UV transition energies reasonably reproduced the experimental trends. We found a specific relationship between the ¹³C‐NMR chemical shifts and the transition energies. As we expected, the relativistic effect was essential to reproduce a trend not only in the ²⁰⁷Pb‐NMR chemical shifts and J[Pb‐C] but also in the ¹³C‐NMR chemical shifts of carbons adjacent to the lead atom. © 2014 Wiley Periodicals, Inc.     

2-苯基-4-亚甲基-1,3-二氧环戊烷与受电子单体的自动共聚反应

<正> 在极性相反的单体对中,有些无需加自由基引发剂,就能进行所谓自动共聚合反应.带乙烯基的螺环原酸酯、碳酸酯和环缩醛或酮等是给电子单体,可能与接受电子的单体,如马来酸酐(Manh)进行自动共聚反应.至今,这方面研究工作报道甚少。 我们合成的2-苯基-4-亚甲基-1,3-二氧环戊烷(PMDO)能进行自由基开环聚合反应.本文研究它与受电子单体,如Manh、丙烯腈(AN)和甲基丙烯酸甲酯(MMA)的自动共聚反应.     

Solvent effects on 195Pt and 205Tl NMR chemical shifts of the complexes [(NC)5Pt--Tl(CN)n]n- (n=0-3), and [(NC)5Pt--Tl--Pt(CN)5]3- studied by relativistic density functional theory

The 295Pt and 205Tl NMR chemical shifts of the complexes [(NC)5Pt-Tl(CN)n]n- n=0-3, and of the related system [(NC)5Pt--Tl--Pt(CN)5]3- have been computationally investigated. It is demonstrated that based on relativistically optimized geometries, by applying an explicit first solvation shell, an additional implicit solvation model to represent the bulk solvent effects (COSMO model), and a DFT exchange-correlation potential that was specifically designed for the treatment of response properties, that the experimentally observed metal chemical shifts can be calculated with satisfactory accuracy. The metal chemical shifts have been computed by means of a two-component relativistic density functional approach. The effects of electronic spin-orbit coupling were included in all NMR computations. The impact of the choice of the reference, which ideally should not affect the accuracy of the computed chemical shifts, is also demonstrated. Together with recent calculations by us of the Pt and Tl spin-spin coupling constants, all measured metal NMR parameters of these complexes are now computationally determined with sufficient accuracy in order to allow a detailed analysis of the experimental results. In particular, we show that interaction of the complexes with the solvent (water) must be an integral part of such an analysis.     

NMR shielding calculations across the periodic table: diamagnetic uranium compounds. 2. Ligand and metal NMR

In this and a previous article (J. Phys. Chem. A 2000, 104, 8244), the range of application for relativistic density functional theory (DFT) is extended to the calculation of nuclear magnetic resonance (NMR) shieldings and chemical shifts in diamagnetic actinide compounds. Two relativistic DFT methods are used, ZORA ("zeroth-order regular approximation") and the quasirelativistic (QR) method. In the given second paper, NMR shieldings and chemical shifts are calculated and discussed for a wide range of compounds. The molecules studied comprise uranyl complexes, [UO(2)L(n)](+/-)(q); UF(6); inorganic UF(6) derivatives, UF(6-n)Cl(n), n = 0-6; and organometallic UF(6) derivatives, UF(6-n)(OCH(3))(n), n = 0-5. Uranyl complexes include [UO(2)F(4)](2-), [UO(2)Cl(4)](2-), [UO(2)(OH)(4)](2-), [UO(2)(CO(3))(3)](4-), and [UO(2)(H(2)O)(5)](2+). For the ligand NMR, moderate (e.g., (19)F NMR chemical shifts in UF(6-n)Cl(n)) to excellent agreement [e.g., (19)F chemical shift tensor in UF(6) or (1)H NMR in UF(6-n)(OCH(3))(n)] has been found between theory and experiment. The methods have been used to calculate the experimentally unknown (235)U NMR chemical shifts. A large chemical shift range of at least 21,000 ppm has been predicted for the (235)U nucleus. ZORA spin-orbit appears to be the most accurate method for predicting actinide metal chemical shifts. Trends in the (235)U NMR chemical shifts of UF(6-n)L(n) molecules are analyzed and explained in terms of the calculated electronic structure. It is argued that the energy separation and interaction between occupied and virtual orbitals with f-character are the determining factors.     

15N NMR chemical shifts for the identification of dipyrrolic structures

A variety of dipyrromethanes and dipyrromethenes have been prepared, and their 15N NMR chemical shifts have been measured by two-dimensional correlation to 1H NMR signals. The nitrogen atoms in five examples of dipyrromethanes consistently exhibit chemical shifts around -231 ppm, relative to nitromethane. Seven examples of hydrobromide salts of meso-unsubstituted dipyrromethenes consistently display 15N chemical shifts around -210 ppm, while their corresponding zinc(II) complexes exhibit chemical shifts around -170 ppm. The presence of electron-withdrawing substituents on one of the pyrrolic rings of dipyrromethenes affects the chemical shifts of both of the nitrogen nuclei in the molecule. Boron difluoride complexes of meso-unsubstituted dipyrromethenes display 15N chemical shifts around -190 ppm. Two examples of free-base dipyrromethenes bearing substituents at the meso-position exhibit 15N chemical shifts at approximately -156 ppm, and for the zinc complexes of these compounds at -162 ppm. One-bond nitrogen-hydrogen coupling constants, when measurable, were consistently in the range of -96 Hz. Since the measured 15N chemical shifts have such a high regularity correlated to structure, they can be used as diagnostic indications for identifying the structure of dipyrrolic compounds.     

Platinum-modified adenines: unprecedented protonation behavior revealed by NMR spectroscopy and relativistic density-functional theory calculations

Two novel Pt(IV) complexes of aromatic cytokinins with possible antitumor properties were prepared by reaction of selected aminopurines with K(2)PtCl(6). The structures of both complexes, 9-[6-(benzylamino)purine] pentachloroplatinate (IV) and 9-[6-(furfurylamino)purine] pentachloroplatinate (IV), were characterized in detail by using two-dimensional NMR spectroscopy ((1)H, (13)C, (15)N, and (195)Pt) in solution and CP/MAS NMR techniques in the solid state. We report for the first time the X-ray structure of a nucleobase adenine derivative coordinated to Pt(IV) via the N9 atom. The protonation equilibria for the complexes in solution were characterized by using NMR spectroscopy (isotropic chemical shifts and indirect nuclear spin-spin coupling constants) and the structural conclusions drawn from the NMR analysis are supported by relativistic density-functional theory (DFT) calculations. Because of the presence of the Pt atom, hybrid GGA functionals and scalar-relativistic and spin-orbit corrections were employed for both the DFT calculations of the molecular structure and particularly for the NMR chemical shifts. In particular, the populations of the N7-protonated and neutral forms of the complexes in solution were characterized by correlating the experimental and the DFT-calculated NMR chemical shifts. In contrast to the chemical exchange process involving the N7-H group, the hydrogen atom at N3 was determined to be unexpectedly rigid, probably because of the presence of the stabilizing intramolecular interaction N3-H···Cl. The described methodology combining the NMR spectroscopy and relativistic DFT calculations can be employed for characterizing the tautomeric and protonation equilibria in a large family of transition-metal-modified purine bases.     

Structure determination of protein-protein complexes using NMR chemical shifts: case of an endonuclease colicin-immunity protein complex

Nuclear magnetic resonance (NMR) spectroscopy provides a range of powerful techniques for determining the structures and the dynamics of proteins. The high-resolution determination of the structures of protein-protein complexes, however, is still a challenging problem for this approach, since it can normally provide only a limited amount of structural information at protein-protein interfaces. We present here the determination using NMR chemical shifts of the structure (PDB code 2K5X) of the cytotoxic endonuclease domain from bacterial toxin colicin (E9) in complex with its cognate immunity protein (Im9). In order to achieve this result, we introduce the CamDock method, which combines a flexible docking procedure with a refinement that exploits the structural information provided by chemical shifts. The results that we report thus indicate that chemical shifts can be used as structural restraints for the determination of the conformations of protein complexes that are difficult to obtain by more standard NMR approaches.     

Determining valine side-chain rotamer conformations in proteins from methyl 13C chemical shifts: application to the 360 kDa half-proteasome

A method is presented for determining Val side-chain χ(1) rotamer distributions in proteins based exclusively on measured (13)C(γ1) and (13)C(γ2) chemical shifts. The approach selects an ensemble of 20 χ(1) values, calculates average methyl (13)C(γ1,γ2) chemical shifts via theoretical quantum chemical calculations and maximizes the agreement with the experimentally measured shifts using a genetic algorithm. The methodology is validated with an application involving six proteins for which (13)C(γ) chemical shifts and three-bond methyl-backbone scalar couplings are available. The utility of the methodology is demonstrated with an application to the 360 kDa 'half-proteasome' where the χ(1) rotameric distributions of Val residues are calculated on the basis of chemical shifts. For the most part the χ(1) profiles so obtained compare very well with those generated from the high-resolution (2.3 ?) X-ray structure of the proteasome. Both NMR and X-ray distributions are cross-validated by comparing calculated (1)H-(13)C methyl residual dipolar couplings with measured values, and the level of agreement is at least as good for the NMR derived χ(1) values. Notably, as the resolution of the X-ray data improves (rotamer distributions from 3.4 and 2.3 ? X-ray structures are compared with the NMR data), the agreement with the NMR gets significantly better. This emphasizes the importance of NMR approaches for the study of high molecular weight complexes that can be recalcitrant to high resolution X-ray analysis.     

DFT calculations of 1H and 13C NMR chemical shifts in transition metal hydrides

Transition metal hydrides are of great interest in chemistry because of their reactivity and their potential use as catalysts for hydrogenation. Among other available techniques, structural properties in transition metal (TM) complexes are often probed by NMR spectroscopy. In this paper we will show that it is possible to establish a viable methodological strategy in the context of density functional theory, that allows the determination of 1H NMR chemical shifts of hydride ligands attached to transition metal atoms in mononuclear systems and clusters with good accuracy with respect to experiment. 13C chemical shifts have also been considered in some cases. We have studied mononuclear ruthenium complexes such as Ru(L)(H)(dppm)2 with L = H or Cl, cationic complex [Ru(H)(H2O)(dppm)2]+ and Ru(H)2(dppm)(PPh3)2, in which hydride ligands are characterized by a negative 1H NMR chemical shift. For these complexes all calculations are in relatively good agreement compared to experimental data with errors not exceeding 20% except for the hydrogen atom in Ru(H)2(dppm)(PPh3)2. For this last complex, the relative error increases to 30%, probably owing to the necessity to take into account dynamical effects of phenyl groups. Carbonyl ligands are often encountered in coordination chemistry. Specific issues arise when calculating 1H or 13C NMR chemical shifts in TM carbonyl complexes. Indeed, while errors of 10 to 20% with respect to experiment are often considered good in the framework of density functional theory, this difference in the case of mononuclear carbonyl complexes culminates to 80%: results obtained with all-electron calculations are overall in very satisfactory agreement with experiment, the error in this case does not exceed 11% contrary to effective core potentials (ECPs) calculations which yield errors always larger than 20%. We conclude that for carbonyl groups the use of ECPs is not recommended, although their use could save time for very large systems, for instance in cluster chemistry. The reliance of NMR chemical shielding on dynamical effects, such as intramolecular rearrangements or trigonal twists, is also examined for H2Fe(CO)4, K+[HFe(CO)](-), HMn(CO)5 and HRe(CO)5. The accuracy of the theory is also examined for complexes with two dihydrogen ligands (Tp*RuH(H2)2 and [FeH(H2)(DMPE)2]+) and a ruthenium cluster, [H3Ru4(C6H6)4(CO)]+. It is shown that for all complexes studied in this work, the effect of the ligands on the chemical shielding of hydrogen coordinated to metal is suitably calculated, thus yielding a very good correlation between experimental chemical shifts and theoretical chemical shielding.     

NMR and EPR studies of chloroiron(III) tetraphenyl-chlorin and its complexes with imidazoles and pyridines of widely differing basicities

The NMR and EPR spectra of two bisimidazole and three bispyridine complexes of tetraphenylchlorinatoiron(III), [(TPC)Fe(L)2]+ (L = Im-d4, 2-MeHIm, 4-Me2NPy, Py, and 4-CNPy), have been investigated. The full resonance assignments of the [(TPC)Fe(L)2]+ complexes of this study have been made from correlation spectroscopy (COSY) and nuclear Overhauser enhancement spectroscopy (NOESY) experiments and Amsterdam density functional (ADF) calculations. Unlike the [(OEC)Fe(L)2]+ complexes reported previously (Cai, S.; Lichtenberger, D. L.; Walker, F. A. Inorg. Chem. 2005, 44, 1890-1903), the NMR data for the [(TPC)Fe(L)2]+ complexes of this study indicate that the ground state is S = 1/2 for each bisligand complex, whereas a higher spin state was present at NMR temperatures for the Py and 4-CNPy complexes of (OEC)Fe(III). The pyrrole-8,17 and pyrroline-H of all [TPCFe(L)2]+ show large magnitude chemical shifts (hence indicating large spin density on the adjacent carbons that are part of the pi system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from ADF calculations. The magnitude of the chemical shifts decreases with decreasing donor ability of the substituted pyridine ligands, with the nonhindered imidazole ligand having slightly larger magnitude chemical shifts than the most basic pyridine, even though its basicity is significantly lower (4-Me2NPyH+ pKa = 9.7, H2Im+ pKa = 6.65 (adjusted for the statistical factor of 2 protons)). The temperature dependence of the chemical shifts of all but the 4-Me2NPy bisligand complexes studied over the temperature range of the NMR investigations shows that they have mixed (dxy)2(dxz,dyz)3/(dxzdyz)4(dxy)1 electron configurations that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with an S = 3/2 excited state in each case that in most cases lies at more than kT at room temperature above the ground state. The observed pattern of chemical shifts of the 4-CNPy complex and analysis of the temperature dependence indicate that it has a pure (dxzdyz)4(dxy)1 ground state and that it is ruffled, because ruffling mixes the a(2u)(pi)-like orbital of the chlorin into the singly occupied molecular orbital (SOMO). This mixing accounts for the negative chemical shift of the pyrroline-H (-6.5 ppm at -40 degrees C) and thus the negative spin density at the pyrroline-alpha-carbons, but the mixing is not to the same extent as observed for [(TPC)Fe(t-BuNC)2]+, whose pyrroline-H chemical shift is -36 ppm at 25 degrees C (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (TPC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct.     

Understanding sterol-membrane interactions part I: Hartree-Fock versus DFT calculations of 13C and 1H NMR isotropic chemical shifts of sterols in solution and analysis of hydrogen-bonding effects

1H and 13C NMR chemical shifts are exquisitely sensitive probes of the local environment of the corresponding nuclei. Ultimately, direct determination of the chemical shifts of sterols in their membrane environment has the potential to reveal their molecular interactions and dynamics, in particular concerning the hydrogen-bonding partners of their OH groups. However, this strategy requires an accurate and efficient means to quantify the influence of the various interactions on chemical shielding. Herein the validity of Hartree-Fock and DFT calculations of the 13C and 1H NMR chemical shifts of cholesterol and ergosterol are compared with one another and with experimental chemical shifts measured in solution at 500 MHz. A computational strategy (definition of basis set, simpler molecular models for the sterols themselves and their molecular complexes) is proposed and compared with experimental data in solution. It is shown in particular that the effects of hydrogen bonding with various functional groups (water as a hydrogen-bond donor and acceptor, acetone) on NMR chemical shifts in CDCl3 solution can be accurately reproduced with this computational approach.     

A solid state 13C NMR, crystallographic, and quantum chemical investigation of chemical shifts and hydrogen bonding in histidine dipeptides

We report the first solid-state NMR, crystallographic, and quantum chemical investigation of the origins of the 13C NMR chemical shifts of the imidazole group in histidine-containing dipeptides. The chemical shift ranges for Cgamma and Cdelta2 seen in eight crystalline dipeptides were very large (12.7-13.8 ppm); the shifts were highly correlated (R2= 0.90) and were dominated by ring tautomer effects and intermolecular interactions. A similar correlation was found in proteins, but only for buried residues. The imidazole 13C NMR chemical shifts were predicted with an overall rms error of 1.6-1.9 ppm over a 26 ppm range, by using quantum chemical methods. Incorporation of hydrogen bond partner molecules was found to be essential in order to reproduce the chemical shifts seen experimentally. Using AIM (atoms in molecules) theory we found that essentially all interactions were of a closed shell nature and the hydrogen bond critical point properties were highly correlated with the N...H...O (average R2= 0.93) and Nepsilon2...H...N (average R2= 0.98) hydrogen bond lengths. For Cepsilon1, the 13C chemical shifts were also highly correlated with each of these properties (at the Nepsilon2 site), indicating the dominance of intermolecular interactions for Cepsilon1. These results open up the way to analyzing 13C NMR chemical shifts, tautomer states (from Cdelta2, Cepsilon1 shifts), and hydrogen bond properties (from Cepsilon1 shifts) of histidine residue in proteins and should be applicable to imidazole-containing drug molecules bound to proteins, as well.     

Coordination chemistry of Rh(III) porphyrins: complexes with hydrazine, disulfide, and diselenide bridging ligands

Rh(III) porphyrin complexes with bridging hydrazine and substituted hydrazine ligands were characterized in solution by (1)H NMR spectroscopy and in the solid state by X-ray diffraction. Addition of further ligand to these species afforded 1:1 complexes in which methylhydrazine and N,N-dimethylhydrazine preferentially bound to the Rh center through the substituted nitrogen atom, as evidenced by (1)H NMR chemical shifts. An alkylated Rh(III) porphyrin was isolated as a decomposition product of the reaction of N,N-dimethylhydrazine with Rh(III) porphyrin in the presence of light and oxygen. Me(2)Se(2) and Me(2)S(2) formed bridging and nonbridging complexes with Rh(III) porphyrin, analogous to that observed with N,N'-dimethylhydrazine.     

Experimental and quantum-chemical studies of 1H, 13C and 15N NMR coordination shifts in Pd(II) and Pt(II) chloride complexes with quinoline, isoquinoline, and 2,2'-biquinoline

1H, 13C, and 15N NMR studies of platinide(II) (M=Pd, Pt) chloride complexes with quinolines (L=quinoline-quin, or isoquinoline-isoquin; LL=2,2'-biquinoline-bquin), having the general formulae trans-/cis-[ML2Cl2] and [M(LL)Cl2], were performed and the respective chemical shifts (delta1H, delta13C, delta15N) reported. 1H coordination shifts of various signs and magnitudes (Delta1Hcoord=delta1Hcomplex-delta1Hligand) are discussed in relation to the changes of diamagnetic contribution to the relevant 1H shielding constants. The comparison to the literature data for similar complexes containing auxiliary ligands other than chlorides exhibited a large dependence of delta1H parameters on electron density variations and ring-current effects (inductive and anisotropic phenomena). The influence of deviations from planarity, concerning either MN2Cl2 chromophores or azine ring systems, revealed by the known X-ray structures of [Pd(bquin)Cl2] and [Pt(bquin)Cl2], is discussed in respect to 1H NMR spectra. 15N coordination shifts (Delta15Ncoord=delta15Ncomplex-delta15Nligand) of ca. 78-100 ppm (to lower frequency) are attributed mainly to the decrease of the absolute value of paramagnetic contribution in the relevant 15N shielding constants, this phenomenon being noticeably dependent on the type of a platinide metal and coordination sphere geometry. The absolute magnitude of Delta15Ncoord parameter increased by ca 15 ppm upon Pd(II)-->Pt(II) replacement but decreased by ca. 15 ppm following trans-->cis transition. Experimental 1H, 13C, 15N NMR chemical shifts are compared to those quantum-chemically calculated by B3LYP/LanL2DZ+6-31G**//B3LYP/LanL2DZ+6-31G*, both in vacuo and in CHCl3 or DMF solution.     

Simultaneous Estimation of Two Coupled Hydrogen Bond Geometries from Pairs of Entangled NMR Parameters: The Test Case of 4-Hydroxypyridine Anion

The computational method for estimating the geometry of two coupled hydrogen bonds with geometries close to linear using a pair of spectral NMR parameters was proposed. The method was developed based on the quantum-chemical investigation of 61 complexes with two hydrogen bonds formed by oxygen and nitrogen atoms of the 4-hydroxypyridine anion with OH groups of substituted methanols. The main idea of the method is as follows: from the NMR chemical shifts of nuclei of atoms forming the 4-hydroxylpyridine anion, we select such pairs, whose values can be used for simultaneous determination of the geometry of two hydrogen bonds, despite the fact that every NMR parameter is sensitive to the geometry of each of the hydrogen bonds. For these parameters, two-dimensional maps of dependencies of NMR chemical shifts on interatomic distances in two hydrogen bonds were constructed. It is shown that, in addition to chemical shifts of the nitrogen atom and quaternary carbon, which are experimentally difficult to obtain, chemical shifts of the carbons and protons of the CH groups can be used. The performance of the proposed method was evaluated computationally as well on three additional complexes with substituted alcohols. It was found that, for all considered cases, hydrogen bond geometries estimated using two-dimensional correlations differed from those directly calculated by quantum-chemical methods by not more than 0.04 Å.