Organic ion imaging beyond the limit of static secondary ion mass spectrometry

Secondary ion mass spectra and images were obtained from spikes of choline chloride, acetylcholine chloride, and methylphenylpyridinium iodide deposited onto specimens of porcine brain tissue. Samples were subsequently subjected to a dose of 10-keV Cs⁺ sufficient to suppress secondary ion emission characteristic of the targeted analytes. Following ablation of the samples by massive glycerol clusters generated by electrohydrodynamic emission, secondary ion mass spectra and images could be obtained that reflected the identity and location of the spiked analytes. The absolute intensity of secondary ion emission that followed ablation was found to be between 30 and 100% of the intensity obtained prior to exposure to the high dose of Cs’. Not all chemical noise is removed by ablation, however, so that the signal-to-noise ratios after ablation correspond to between 10 and 85% of their values observed under conditions of low primary ion dose.     

Esr studies of crown complexed semiquinone ion pairs in solution

ESR spectra of alkyl-substituted p-benzosemiquinone anion radicals produced by sodium reduction in the presence of 18-crown-6 in tetrahy drofuran show, remarkable temperature and concentration dependences, not observed in the absence of the crown. The observed spectral changes are interpreted in terms of equilibria between structurally different ion pairs.     

Effect of a metal ion on intramolecular spin exchange in spin-labeled complexes

The characteristic features of intramolecular spin exchange in 14 complexes of Ag^I, Hg^II, Ni^II, Pd^II, Pt^II, Au^III, and Pt^IV with spin-labeled ligands were studied by ESR spectroscopy. The measured values of the exchange integral ‖J‖ and the differences between the enthalpies of the efficient conformations (ΔH) were compared with the electronic polarization (refraction)R _f of the Ni^II, Pd^II, and Pt^II ions and Klopman's rigidity parameters σ_K, which characterize the total polarazibility of the ions and the degree of covalence of the bond between the metal atom and the donor atom of the ligand, respectively. Delocalization of the electron spin density and the efficiency of spin exchange are determined by the relative contributions of the s, p, and d orbitals, which produce the overlap integral of wave functions, ‖J‖, and by the geometric features of the coordination polyhedron, which affect the mutual orientation of the N−O fragments. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2005–2009, October, 1999.     

Relaxation dynamics of the first excited electronic singlet state of azulene in solution

An upper limit to the relaxation time of the first excited electronic singlet state (S₁) of azulene in cyclohexane has been determined for two excitation frequencies. The lifetimes of S₁ excited by single picosecond duration optical pulses of frequency 18910 cm^?1 and 16000 cm^?1 are ? 1 ps and ? 2 ps respectively.     

Density functional perturbational orbital theory of spin polarization in electronic systems. II. Transition metal dimer complexes

We present a theoretical scheme for a semiquantitative analysis of electronic structures of magnetic transition metal dimer complexes within spin density functional theory (DFT). Based on the spin polarization perturbational orbital theory [D.-K. Seo, J. Chem. Phys. 125, 154105 (2006)], explicit spin-dependent expressions of the spin orbital energies and coefficients are derived, which allows to understand how spin orbitals form and change their energies and shapes when two magnetic sites are coupled either ferromagnetically or antiferromagnetically. Upon employment of the concept of magnetic orbitals in the active-electron approximation, a general mathematical formula is obtained for the magnetic coupling constant J from the analytical expression for the electronic energy difference between low-spin broken-symmetry and high-spin states. The origin of the potential exchange and kinetic exchange terms based on the one-electron picture is also elucidated. In addition, we provide a general account of the DFT analysis of the magnetic exchange interactions in compounds for which the active-electron approximation is not appropriate.     

Awakening of a ferrous complex's electronic spin in an aqueous solution induced by a chemical stimulus

A low-spin, macrocyclic iron(II) complex in an aqueous solution responds to the addition of a chemical reactant (dithionite) by transformation into a high-spin complex, detectable by measurement of the longitudinal relaxation time (T(1)) of surrounding water hydrogen nuclear spins. The initial compound does not modify T(1) of pure water at concentrations as high as 4 mM. The response is pH-dependent, and the complex is robust at a variety of conditions.     

Proton spin relaxation in some paramagnetic transition-metal acetylacetonate complexes. Comparison between theory and experiment

Proton spin-lattice (T₁) and spin-spin (T₂) relaxation times have been measured for CH₃ protons in a series of paramagnetic transition-metal acetylacetonate complexes and the results interpreted in terms of current relaxation theory, τ_r, the correlation time for molecular reorientation, was estimated from the ¹³C T₁ in the analogous diamagnetic Co(III) and Pd(II) complexes. Using this approach and treating in detail the effects of fast CH₃ group internal motion good agreement is obtained between theory and experiment. In all cases electron-nuclear dipolar coupling dominates T₁ whereas the hyperfine contribution can be important for T₂.     

Selective metal ion recognition using a fluorescent 1,8-diquinolylnaphthalene-derived sensor in aqueous solution

The use of anti-1,8-bis(2,2′-diisopropyl-4,4′-diquinolyl)naphthalene, 1, for metal ion-selective fluorescence recognition has been investigated. Employing CuCl₂, ZnCl₂, FeCl₂, and FeCl₃ in fluorescence titration experiments of 1 revealed formation of a bluegreen light emitting bimetallic complex. A dramatic red-shift of the fluorescence maximum of 1 and metal ion-selective quenching was observed in the presence of Cu(II), Fe(II), and Fe(III)chlorides in acetonitrile. By contrast, addition of ZnCl₂ was found to result in fluorescence enhancement, whereas Cu(I) did not induce any significant fluorescence change of 1. The sensor was found to undergo highly ion-selective fluorescence quenching in aqueous solution. Screening of main group and transition metal ions showed excellent selectivity for FeCl₃ even in the presence of competing metal ions.     

Permeation of complexed metal ions through a hydrophobic membrane

Selective concentration of a heavy metal complex with acetylacetone (acac) through a hydrophobic polystyrene membrane was carried out under a pressure gradient. A chelate forming heavy metal was selectively concentrated about 2 fold by this method. When using a coating membrane with a high water flux, the permeabilities increased with increasing complex fraction in the aqueous solution, while using a membrane with a low water flux, a bulky complex was not highly concentrated because of steric hindrance. The complex partitioned on the membrane surface was transported and concentrated under a pressure gradient and a linear relationship was found to exist between permeabilities and partition coefficients. It will be possible to concentrate hydrophobic organic solutes by this method, for acac was concentrated when the Cu—acac complex was formed. As the permeabilities increased with decreasing pressure and membrane compaction was strong for a coating membrane, it seems effective to permeate at a low pressure.     

Stochastic Liouville equation treatment of the electron paramagnetic resonance line shape of an S-state ion in solution

The current approaches used for the analysis of electron paramagnetic resonance spectra of Gd3+ complexes suffer from a number of drawbacks. Even the elaborate model of [Rast et al., J. Chem. Phys. 113, 8724 (2000)] where the electron spin relaxation is explained by the modulation of the zero-field splitting (ZFS), by molecular tumbling (the so called static contribution), and deformations (transient contribution), is only readily applicable within the validity range of the Redfield theory [Advances in Magnetic Resonance, edited by J.-S. Waugh (Academic, New York, 1965), Vol. 1, p. 1], that is, when the ZFS is small compared to the Zeeman energy and the rotational and vibrational modulations are fast compared to the relaxation time. Spin labels (nitroxides and transition metal complexes) have been studied for years in systems that violate these conditions. The theoretical framework commonly used in such studies is the stochastic Liouville equation (SLE). The authors shall show how the physical model of Rast et al. can be cast into the SLE formalism, paying special attention to the specific problems introduced by the [Uhlenbeck and Ornstein, Phys. Rev. 36, 823 (1930)] process used to model the transient ZFS. The resulting equations are very general and valid for arbitrary correlation times, magnetic field strength, electron spin S, or symmetry. The authors demonstrate the equivalence of the SLE approach with the Redfield approximation for two well-known Gd3+ complexes.     

Calculation of equilibrium constants of ligand binding by a metal ion in solution using a chemometric procedure

The method of calculating equilibrium constants has been considered in the work using a chemometric procedure based on the factor analysis of systems, in which a ligand added to a central ion forms in solution a number of complexes [MX _n ], n = 1?4. Spectrophotometric titration curves were simulated by means of a matrix model of complex formation. The analysis of the simulated multivariable dependences has shown an opportunity to apply the evolutiing factor analysis to the adequate estimation of complex-formation parameters in the absence of spectral selectivity of separate spectral forms, and also in the absence of concentration regions of full domination of separate components and products of their interaction. The algorithm was developed, which allows true values of matrix model parameters determining the shape of spectrophotometric titration curves to be found without preliminary postulation of a chemical model of the system.     

Practical route to relative diffusion coefficients and electronic relaxation rates of paramagnetic metal complexes in solution by model-independent outer-sphere NMRD. Potentiality for MRI contrast agents

The relaxation of electronic spins S of paramagnetic species is studied by the field-dependence of the longitudinal, transverse, and longitudinal in the rotating frame relaxation rates R1, R2, and R1rho of nuclear spins I carried by dissolved probe solutes. The method rests on the model-independent low-frequency dispersions of the outer-sphere (OS) paramagnetic relaxation enhancement (PRE) of these rates due to the three-dimensional relative diffusion of the complex with respect to the probe solute. We propose simple analytical formulas to calculate these enhancements in terms of the relative diffusion coefficient D, the longitudinal electronic relaxation time T1e, and the time integral of the time correlation function of the I-S dipolar magnetic interaction. In the domain of vanishing magnetic field, these parameters can be derived from the low-frequency dispersion of R1 thanks to sensitivity improvements of fast field-cycling nuclear relaxometers. At medium field, we present various approaches to obtain these parameters by combining the rates R1, R2, and R1rho. The method is illustrated by a careful study of the proton PREs of deuterated water HOD, methanol CH3OD, and tert-butyl alcohol (CH3)3COD in heavy water in the presence of a recently reported nonacoordinate Gd(III) complex. The exceptionally slow electronic relaxation of the Gd(III) spin in this complex is confirmed and used to test the accuracy of the method through the self-consistency of the low- and medium-field results. The study of molecular diffusion at a few nanometer scale and of the electronic spin relaxation of other complexed metal ions is discussed.     

Calculation of a relative fraction of metal ion passed in hydroxy complex depending on pH of a solution

Russian Journal of Applied Chemistry -     

Observation of a photoexcited state of a paramagnetic transition metal complex by time-resolved electron paramagnetic resonance spectroscopy

The first observation of a spin polarized excited state of a paramagnetic metal-complex using time-resolved electron paramagnetic resonance (TREPR) spectroscopy is reported for octaethylporphinatooxovanadium(iv). The TREPR spectra show well resolved orientation dependent hyperfine splitting to the I = 7/2 vanadium nucleus. The reduction of the hyperfine splitting by a factor of 3 compared to the ground state and the observation of a multiplet pattern of spin polarization allow the TREPR spectra to be assigned to the excited quartet state of the complex. The spin polarization patterns evolve with time and it is postulated that this is a result of the equilibration between the lowest excited quartet and doublet states.     

Investigation of uranyl nitrate ion pairs complexed with amide ligands using electrospray ionization ion trap mass spectrometry and density functional theory

Ion populations formed from electrospray of uranyl nitrate solutions containing different amides vary depending on ligand nucleophilicity and steric crowding at the metal center. The most abundant species were ion pair complexes having the general formula [UO(2)(NO(3))(amide)(n=2,3)](+); however, singly charged complexes containing the amide conjugate base and reduced uranyl UO(2)(+) were also formed as were several doubly charged species. The formamide experiment produced the greatest diversity of species resulting from weaker amide binding, leading to dissociation and subsequent solvent coordination or metal reduction. Experiments using methyl formamide, dimethyl formamide, acetamide, and methyl acetamide produced ion pair and doubly charged complexes that were more abundant and less abundant complexes containing solvent or reduced uranyl. This pattern is reversed in the dimethylacetamide experiment, which displayed lower abundance doubly charged complexes, but augmented reduced uranyl complexes. DFT investigations of the tris-amide ion pair complexes showed that interligand repulsion distorts the amide ligands out of the uranyl equatorial plane and that complex stabilities do not increase with increasing amide nucleophilicity. Elimination of an amide ligand largely relieves the interligand repulsion, and the remaining amide ligands become closely aligned with the equatorial plane in the structures of the bis-amide ligands. The studies show that the phenomenological distribution of coordination complexes in a metal-ligand electrospray experiment is a function of both ligand nucleophilicity and interligand repulsion and that the latter factor begins exerting influence even in the case of relatively small ligands like the substituted methyl-formamide and methyl-acetamide ligands.     

Transfer of spin density in molecular complexes of paramagnetic metalloporphyrins

Using the¹H and¹³C NMR method, we have observed delocalization of spin density from the paramagnetic metalloporphyrins to aromatic nitro compounds in chloroform solutions. We have shown that spin transfer occurs directly from the metal atoms. The effectiveness of spin transfer varies in the order Co²⁺ > Mn³⁺ > Fe³⁺ > Cu²⁺, correlating with the accessibility of the d_z2 orbital of the metal containing the unpaired electrons.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1009–1015, May, 1990.We thank A. B. Solov'eva and G. V. Ponomarev for providing the metalloporphyrins.