NMR Timescale (nmr + timescale)

Distribution by Scientific Domains

Selected Abstracts

Spins as probes of different electronic states

Dieter Suter
Abstract Nuclear spins are efficient probes of electronic states. Because most NMR experiments are performed in thermal equilibrium, they probe the electronic ground state,the only state that is significantly populated under ambient conditions. Probing electronically excited states becomes possible, if magnetic resonance techniques are combined with optical (laser) excitation. Depending on the nature of the electronic state, drastic changes of the magnetic resonance parameters may be observed. We discuss the basic principles of this type of investigation. Depending on the lifetime of the electronically excited state, it is possible to measure separate spectra of ground and excited state if the lifetime is long on the NMR timescale, or an averaged spectrum if the lifetime is short. We present examples for both limiting cases using rare earth ions and semiconductor heterostructures. © 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part A 30A: 116,126, 2007. [source]

Self-Assembly, Structure and Solution Dynamics of Tetranuclear Zn2+ Hydrazone [2×2] Grid-Type Complexes

Mihail Barboiu
Abstract We describe the self-assembly processes as well as the structural and physico-chemical properties of [2×2]Zn2+4 grid complexes involving the bis-tridentate ligands 7,12, based on bis(hydrazone)pyrimidine complexation subunits and octahedrally coordinated Zn2+ ions. The NMR spectroscopic data and the X-ray crystal structure results indicate that in solution and in the solid state the complexes 13,18 adopt a very compact arrangement providing stable [2×2] hydrazone-grid arrays. The ,,, stacking between the phenyl ring and the hydrazone units of the perpendicular ligands in the complexes induces a perfect orthogonal arrangement suitable for applications in self-organized metallosupramolecular systems. Zinc complexes provide an opportunity to study the acid,base chemistry without the added effects due to paramagnetism or redox chemistry. The intermediate protonated grids undergo relatively rapid proton exchange on the NMR timescale, the presence of a sharp pyrimidine proton resonance suggesting that there is significant delocalization of the negative charge along the backbone of the ligand. Rotation of the phenyl ring is observed. It involves probably a mechanism in which one of the ligands partially dissociates allowing the initially intercalated phenyl group to rotate, before recoordination of the terminal pyridine. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source]

NMR Study of L-Shaped (Quinoxaline)platinum(II) Complexes , Crystal Structure of [Pt(DMeDPQ)(bipy)](PF6)2

Enrico Rotondo
Abstract A 1H and 13C NMR study of nine PtII complexes of DMeDPQ [6,7-dimethyl-2,3-bis(2-pyridyl)quinoxaline] and BDPQ [2,3-bis(2-pyridyl)benzo[g]quinoxaline], and the crystal structure of one of them, are reported. The results are consistent with Cs symmetry of "L-shaped square-planar complexes". The rigid seven-membered chelated quinoxaline ligand holds the fused aromatic rings nearly perpendicular to the PtII coordination plane, generating the peculiar L-shaped structure. Ancillary ligands in the residual coordination sites are: a) bidentate flexible-planar 2,2,-bipyridine (bipy; complexes 1 and 2); b) bidentate rigid-planar dipyrido[3,2- a:2,3,- c]phenazine (dppz) or benzo[b]dipyrido[3,2- h:2,,3,- j]phenazine (bdppz; complexes 3,6); or c) 3-substituted monodentate pyridines (3-Rpy; complexes 7,9). The L-shaped geometry has been exploited to gain insight into the steric and dynamic features that regulate the noncovalent interactions of these square-planar complexes with DNA. We have shown previously, for [Pt(bipy)(n -Rpy)2]2+, that bipy twisting can be frozen out on the NMR timescale below 260 K. Preservation of the Cs symmetry at low temperature indicates a lack of bipy fluxionality within these L-shaped structures. The static butterfly-like symmetric orientation of the quinoxaline pyridyl rings accounts for the hampered twisting of Pt(bipy), which is otherwise assisted by the synchronous "windscreen wiper" conrotatory rocking of the ancillary pyridine rings. The L-geometry can also be used to monitor the ancillary n -Rpy rotation by NMR spectroscopy. The quasi-vertical quinoxaline pyridyl rings alignment leave room in the coordination plane for the crossing of the opposite pyridine rings, thereby reducing their rotational barriers about the Pt,N bond. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

Orientation- and Temperature-Dependent Rotational Behavior of Imidazole Ligands (L) in ,-[Ru(azpy)2(L)2](PF6)2 Complexes

Aldrik H. Velders
Abstract The synthesis and characterization of the cis bifunctional coordinated ruthenium(II) complexes ,-[Ru(azpy)2(MeIm)2](PF6)2 (,-MeIm) and ,-[Ru(azpy)2(MeBim)2](PF6)2 (,-MeBim) (azpy = 2-phenylazopyridine, MeIm = 1-methylimidazole and MeBim = 1-methylbenzimidazole) is reported. In ,-MeIm the two MeIm ligands can both freely rotate around the Ru,N axes on the NMR timescale. In ,-MeBim the two MeBim ligands appear restricted in their rotation around the Ru,N axes, which becomes slow on the NMR timescale at low temperatures. In contrast to the analogous complexes ,-[Ru(azpy)2(MeBim)2](PF6)2 and cis -[Ru(bpy)2(MeBim)2](PF6)2, only one atropisomer is observed for the two MeBim ligands in ,-MeBim. The orientation of the MeBim ligands appears to correspond to an HT isomer which is similar to the orientation of the MeBim ligands in the most abundant atropisomer found in the related ,-[Ru(azpy)2(MeBim)2](PF6)2. A stacking interaction between the phenyl ring of one azpy and one MeBim ligand is likely to stabilize the observed atropisomer of ,-MeBim, and is such that the rotation of the phenyl ring of one of the two azpy ligands is restricted. At very low temperatures this rotation, or flipping of the phenyl ring between two identical positions, is in the slow-exchange range on the NMR timescale. (© Wiley-VCH Verlag GmbH & Co KGaA, 69451 Weinheim, Germany, 2003) [source]

Off the Back or on the Side: Comparison of meso and 2-Substituted Donor-Acceptor Difluoroborondipyrromethene (Bodipy) Dyads

Andrew C. Benniston
Abstract The preparation of several difluoroborondipyrromethene (Bodipy) dyads is described incorporating covalently attached hydroquinone/quinone groups at the 2-position (BD-SHQ, BD-SQ, BD-SPHQ, BD-SPQ). The compounds, currently under investigation as chemical sensors for reactive oxygen species, show various levels of fluorescence depending on the oxidation state of the appended group. The 19F NMR spectrum for BD-SHQ in CDCl3 at room temperature reveals the two fluorines are inequivalent on the NMR timescale. In contrast, the 19F NMR spectrum for the counterpart quinone compound, BD-SQ, is consistent with two equivalent fluorine atoms. The two results are interpreted as the quinone is free to rotate around the connector bond, whereas this motion is restricted for the hydroquinone group and makes the fluorines chemically inequivalent. Cyclic voltammograms recorded for all derivatives in CH2Cl2 electrolyte solution are consistent with typical Bodipy-based redox chemistry; the potentials of which depend on factors such as presence of the phenylene spacer and oxidation state of the appended group. A comparison of the electrochemical behaviour with the counterpart meso derivatives reveals some interesting trends which are associated with the location of the HOMO/LUMOs. The absorption profiles for the compounds in CH3CN are again consistent with Bodipy-based derivatives, though there are some subtle differences in the band-shapes of the closely-coupled systems. In particular, the absorption spectra for the dyad, BD-SQ, in a wide range of solvents are appreciably broader than for BD-SHQ. Femtosecond transient absorption spectroscopy performed on the hydroquinone derivatives, BD-SHQ and its meso analogue is interpreted as electron transfer occurs from the hydroquinone unit to the first-excited singlet (S1) state of the Bodipy center, followed by ultrafast charge recombination to reinstate the ground state. The coupling of OH vibrations to the return electron transfer process is invoked to explain the lack of clear identification of the charge-separated state in the transient records. [source]

Synthesis and Conformational Analysis of Tetrahydroisoquinoline-Fused 1,3,2-Oxazaphospholidines and 1,2,3-Oxathiazol­idines

Ildikó Schuster
Abstract The cyclizations of tetrahydroisoquinoline 1,2-amino alcohols with phenylphosphonic dichloride, bis(2-chloroethyl)phosphoramidic dichloride, thionyl chloride and sulfuryl chloride were utilized to synthesize 1,5,6,10b-tetrahydro-1,3,2-oxazaphospholo[4,3- a]isoquinolines (2, 3), 1,5,10,10a-tetrahydro-1,3,2-oxazaphospholo[3,4- b]isoquinolines (8, 9), 1,5,6,10b-tetrahydro-1,2,3-oxathiazolo[4,3- a]isoquinolines (4,6) anda 1,5,10,10a-tetrahydro-1,2,3-oxathiazolo[3,4- b]isoquinoline (11), which are the first representatives of these ring systems. NMR spectroscopic analysis revealed the existence of conformational equilibria that are fast on the NMR timescale. Theoretical DFT calculations pointed to the participation of generally two preferred conformers in the conformational equilibria; the positions of the equilibria were indicated by the experimental NMR spectroscopic parameters, and they are in good agreement with the theoretically calculated energy differences of the participating conformers. For two compounds, which could be not isolated (10, 12), both the preferred conformers and the stereochemistry could be concluded from the DFT calculation results. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

Detection of platinum dihydride bisphosphine complexes and studies of their reactivity through para -hydrogen-enhanced NMR methods

Cyril Godard
Abstract In-situ NMR studies on the reactions of Pt{CH2 = CHSi(Me)2}2O)(PCy3) with phosphines, HSiEt3 and - hydrogen or Pt(L)(L,)(Me)2 alone enable the detection of cis -Pt(L)(L,)(H)2 [L = PCy3 and L, = PCy2H, PPh3 or PCy3] which then undergo hydride site interchange and H2 reductive elimination on the NMR timescale. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Helical chirality in hexamethylene triperoxide diamine

Chunlei Guo
Abstract The primary explosive hexamethylenetriperoxide diamine has previously been found to exist in the solid state as a racemic mixture of helically chiral, threefold symmetric enantiomers; another enantiomeric pair of low-energy conformers has been predicted, but has never been observed. We show by solution 2D NMR at 14 T, in achiral solution and by addition of chiral shift reagents, that all four optically isomeric conformers coexist at slow equilibrium on the NMR timescale at room temperature, and can be observed. Calculations of the 1H and 13C NMR chemical shifts using gauge-including atomic orbital methods are in excellent agreement with experiment; thermochemical calculation of the free energies in solution are in somewhat worse agreement, but correctly predict the relative stability of the conformers. Analysis of the effects of chiral shift reagents on the NMR spectra suggests that discrimination between chiral isomers is primarily around the molecular equator, around which the enantiomeric gauche OO linkages are arrayed. Copyright © 2006 John Wiley & Sons, Ltd. [source]

The Application of 199Hg NMR and 199mHg Perturbed Angular Correlation (PAC) Spectroscopy to Define the Biological Chemistry of HgII: A Case Study with Designed Two- and Three-Stranded Coiled Coils

Olga Iranzo Dr.
Abstract The use of de novo designed peptides is a powerful strategy to elucidate HgII,protein interactions and to gain insight into the chemistry of HgII in biological systems. Cysteine derivatives of the designed ,-helical peptides of the TRI family [Ac-G-(LaKbAcLdEeEfKg)4 -G-NH2] bind HgII at high pH values and at peptide/HgII ratios of 3:1 with an unusual trigonal thiolate coordination mode. The resulting HgII complexes are good water-soluble models for HgII binding to the protein MerR. We have carried out a parallel study using 199Hg NMR and 199mHg perturbed angular correlation (PAC) spectroscopy to characterize the distinct species that are generated under different pH conditions and peptide TRI,L9C/HgII ratios. These studies prove for the first time the formation of [Hg{(TRI,L9C)2 -(TRI,L9CH)}], a dithiolate,HgII complex in the hydrophobic interior of the three-stranded coiled coil (TRI,L9C)3. 199Hg NMR and 199mHg PAC data demonstrate that this dithiolate,HgII complex is different from the dithiolate [Hg(TRI,L9C)2], and that the presence of third ,-helix, containing a protonated cysteine, breaks the symmetry of the coordination environment present in the complex [Hg(TRI,L9C)2]. As the pH is raised, the deprotonation of this third cysteine generates the trigonal thiolate,HgII complex Hg(TRI,L9C)3, on a timescale that is slower than the NMR timescale (0.01,10,ms). The formation of the species [Hg{(TRI,L9C)2(TRI,L9CH)}] is the result of a compromise between the high affinity of HgII to form dithiolate complexes and the preference of the peptide to form a three-stranded coiled coil. [source]