The AsO radical detected by infrared laser magnetic resonance

The Pauli master equation for intramolecular vibrational relaxation and the heat bath feedback Bloch equations for radiative pumping of polyatomic molecules can be derived by replacing the standard assumption of random matrix element coupling between zero-order vibrational states by an assumption that relaxation is governed by restricted quantum exchange.     

Zero-field optically detected magnetic resonance of model compounds for pheophytins

ZFS parameters and kinetic constants of the lowest triplet state of chlorin and tetraphenylchlorin free base in n-octane have been determined by fluorescence-detected ODMR at 4.2 K. These compounds can be considered as model compounds for pheophytin, a compound of biological interest. For both compounds the middle spin-level is the most active one in the populating and depopulating pathway. In the lowest triplet state the NHNH axis in both chlorins is probably fixed to one orientation not involving the reduced ring, and no evidence was found for the occurrence of two tautomeric forms as in the corresponding porphyrins.     

Theory of covariance nuclear magnetic resonance spectroscopy

Covariance nuclear magnetic resonance (NMR) spectroscopy provides an effective way for establishing nuclear spin connectivities in molecular systems. The method, which identifies correlated spin dynamics in terms of covariances between 1D spectra, benefits from a high spectral resolution along the indirect dimension without requiring apodization and Fourier transformation along this dimension. The theoretical treatment of covariance NMR spectroscopy is given for NOESY and TOCSY experiments. It is shown that for a large class of 2D NMR experiments the covariance spectrum and the 2D Fourier transform spectrum can be related to each other by means of Parseval's theorem. A general procedure is presented for the construction of a symmetric spectrum with improved resolution along the indirect frequency domain as compared to the 2D FT spectrum.     

Optically detected zero-field triplet state magnetic resonance in photosynthetic bacteria

Triplet state transitions of the photosynthetic bacteria Rhodospirillum Rubrum, Rhodopseudomonas Spheroides and Chromatium Vinosum in chemically reduced preparations have been observed by zero-field optical detection of magnetic resonance at 2 K. For each bacterial preparation two sharp, structureless, zero-field EPR transitions were observed as microwave-induced decreases in the fluorescence intensity of the frozen cellular preparations. The depopulating rate constants for the spin sublevels of the triplet states observed in R Rubrum and R Spheroides were also measured. The similarities of the triplet state frequencies, spectral features and intersystem crossing rates suggest a common structure for the reaction centers in the photosynthetic bacteria.     

Optically detected magnetic resonance study of the phosphorescent states of thiouracils

The triplet states of 1-methyl-2-thiouracil (1-Me-s²U), 1-methyl-4-thiouracil (1-Me-s²s⁴U) and 1-methyl-2,4-dithiouracil (1-Me-s² s⁴) have been investigated by optically detected magnetic resonance in zero magnetic field. The zero field splittings (ZFS) and the individual sublevel kinetic parameters are reported. The ZFS (|D|, |E|) values (in cm^?1) are found to increase in the order: 1-Me-s² U (0.2895, 0.0728) < 1-Me-s⁴U, (0.605, 0.0500) < 1-Me-s²s⁴ U (0.870, 0.0458). The triplet state lifetimes decrease in the same order, and both effects are attributed to an internal heavy atom effect of sulfur substitution. The vibronic structure of the phosphorescence emission indicates that the thiouracil phosphorescent states are ³(π, π^*). The low phosphorescence quantum yields of 1-Me-s⁴ U and of 1-Me-s²s⁴U result from radiationless decay of the triplet state rather than from inefficient intersystem crossing from the excited singlet state. The efficient radiationless decay of the triplet state appears to be a feature of the S-substitution at the 4-position of uracil. Phosphorescence polarization measurements of the individuals triplet sublevel emissions at ca. 1.2 K are consistent with 1-Me-s²U and 1-Me-s⁴U being non-planar in the phosphorescent state; the thiouracil phosphorescence from each triplet sublevel is polarized in the average plane of the distorted molecule. In the absence of σπ separability, spin—orbit mixing of ¹(π, π^*) and ³(π, π^*) states is enhanced and the radiative properties of the triplet state may be dominated by this mechanism rather than by the mixing of ¹(n, π^*), ¹(σ, π^*), or ¹(π, σ^*) with ³(π, π^* states which generally is the dominant mechanism for planar aromatic molecules.     

Electron-spin relaxation phenomena in irradiated saccharides detected by pulsed electron paramagnetic resonance spectroscopy

We measured the relaxation times of radicals in saccharides upon γ-irradiation by means of X-band pulsed electron paramagnetic resonance (EPR) spectroscopy. We found that the field-swept signal of irradiated fructose by pulsed EPR showed three to four peaks depending on the dose. The relaxation times (T₁ and T₂) of the side peaks were longer than those of the main peak(s) from each irradiation, indicating that the radicals showing side peaks interact less with the surrounding environment. From relaxation time measurements of several irradiated saccharides, we conclude that T₂ relaxation times decrease with the increasing irradiation dose. In contrast, T₁ relaxation times show no correlation with the irradiation dose.     

Theory and computation of nuclear magnetic resonance parameters

The art of quantum chemical electronic structure calculation has over the last 15 years reached a point where systematic computational studies of magnetic response properties have become a routine procedure for molecular systems. One of their most prominent areas of application are the spectral parameters of nuclear magnetic resonance (NMR) spectroscopy, due to the immense importance of this experimental method in many scientific disciplines. This article attempts to give an overview on the theory and state-of-the-art of the practical computations in the field, in terms of the size of systems that can be treated, the accuracy that can be expected, and the various factors that would influence the agreement of even the most accurate imaginable electronic structure calculation with experiment. These factors include relativistic effects, thermal effects, as well as solvation/environmental influences, where my group has been active. The dependence of the NMR spectra on external magnetic and optical fields is also briefly touched on.     

Solvent effects on optically detected magnetic resonance in triplet spin labels

We have calculated solvent effects on the zero-field splitting (ZFS) constants induced by electron spin–spin coupling (SSC) in the low-lying triplet states of azaaromatic molecules in solutions using multiconfiguration self-consistent-field wave functions and the polarizable continuum model. The second-order spin–orbit coupling (SOC) contribution to the splitting of the ^3* states is found to be almost negligible, and the calculations therefore provide a good estimate of the ZFS parameters and their solvent dependence based only on the electron spin–spin coupling expectation values. The correlation between the shift in the ZFS and the phosphorescence frequency that has been observed in optically detected magnetic resonance experiments in low-temperature glasses is supported by our direct SSC calculations without taking SOC into account. This makes it possible to distinguish between the two theories that earlier were proposed to explain the inhomogeneous broadening of triplet state spectra, and discard the one that is exclusively based on the SOC-induced mixing of the singlet and triplet states.Contribution to the Jacopo Tomasi Honorary IssueAcknowledgments. This work was supported (B. M.) by the Swedish Royal Academy of Science (KVA). This work was also supported by the Norwegian Research Council through a grant of computer time from the Program for Supercomputing. We are grateful to B. Schimmelpfenning for his valuable assistance in the computations.     

Spin dynamics of the radical pair in a low magnetic field studied by the transient absorption detected magnetic field effect on the reaction yield and switched external magnetic field

The spin dynamics of the radical pair generated from the photocleavage reaction of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMDPO) in micellar solutions was studied by the time-resolved magnetic field effect (MFE) on the transient absorption (TA) and by a novel technique, absorption detected switched external magnetic field (AD-SEMF). Thanks to the large hyperfine coupling constant (A = 38 mT), a characteristic negative MFE on the radical yield was observed at a magnetic field lower than 60 mT whereas a positive effect due to the conventional hyperfine (HFM) and relaxation mechanisms (RM) was observed at higher magnetic field. The negative effect can be assigned to the mechanism "so-called" low field effect (LFE) mechanism and has been analyzed thoroughly using a model calculation incorporating a fast spin dephasing process. The time scale of the spin mixing process of LFE studied by AD-SEMF is shorter than the lifetime of the recombination kinetics of the radical pair. These results indicate that the LFE originates from the coherent spin motion. This can be interfered from the fast spin dephasing caused by electron spin interaction fluctuations.     

Probing interactions by means of pulsed field gradient nuclear magnetic resonance spectroscopy

Molecular self-diffusion coefficients (D) of species in solution are related to size and shape and can be used for studying association phenomena. Pulsed field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy has been revealed to be a powerful analytical tool for D measurement in different research fields. The present work briefly illustrates the use of PFG-NMR for assessing the existence of interactions in very different chemical systems: organic and organometallic compounds, colloidal materials and biological aggregates. The application of PFG-NMR is remarkable for understanding the role of anions in homogenous transition metal catalysis and for assessing the aggregation behaviour of biopolymers in material science.     

Theoretical study of spin-one nuclear quadrupole effects in triplet-state optically detected magnetic resonance

Nuclear quadrupole effects on the triplet-state optically detected magnetic resonance (ODMR) of molecules having spin I = 1 nuclei have been investigated by theoretical methods. Beginning with an exact diagonalization of the triplet-state spin Hamiltonian, these methods have involved a systematic variation in quadrupolar parameters, which allows us to distinguish between “allowed” and “forbidden” transitions and also enables us to assign the observed lines in any 2 E ODMR transition.     

Study of molecular motion in polysilastyrene by pulsed 1H nuclear magnetic resonance

Pulsed NMR spectra of protons in polysilastyrene, $ \rlap{--} [{\rm Si(CH}_{\rm 3} {\rm )}_{\rm 2} {\rm  Si(CH}_{\rm 3} )({\rm C}_6 {\rm H}_5 )\rlap{--} ]_n $, with n ≈ 60, have been measured in the temperature range 80–450 K. The linewidth is constant at 7.4 G up to 200 K and narrows considerably above 250 K to a constant value of 0.3 G above 360 K. The motion responsible for this effect has an activation energy of 43.7 kJ/mol and is identified with the large-scale motion occurring in the vicinity of the glass transition temperature. The spin-lattice relaxation time T₁ was measured by the π-t-½π pulse sequence as a function of temperature. Two motional minima in T₁ were observed. The low-temperature motion has an activation energy of 3.7 kJ/mol and is identified with methyl group reorientation. The high-temperature motion has an activation energy of 29.1 kJ/mol and might be due to segmental motion.     

Fluorochrome-functionalized magnetic nanoparticles for high-sensitivity monitoring of the polymerase chain reaction by magnetic resonance

Easy to find: magnetic nanoparticles bearing fluorochromes (red) that intercalate with DNA (green) form microaggregates with DNA generated by the polymerase chain reaction (PCR). These aggregates can be detected at low cycle numbers by magnetic resonance (MR).     

Theory of stochastic dipolar recoupling in solid-state nuclear magnetic resonance

Dipolar recoupling techniques in solid-state nuclear magnetic resonance (NMR) consist of radio frequency (rf) pulse sequences applied in synchrony with magic-angle spinning (MAS) that create nonzero average magnetic dipole-dipole couplings under MAS. Stochastic dipolar recoupling (SDR) is a variant in which randomly chosen rf carrier frequency offsets are introduced to cause random phase modulations of individual pairwise couplings in the dipolar spin Hamiltonian. Several aspects of SDR are investigated through analytical theory and numerical simulations: (1) An analytical expression for the evolution of nuclear spin polarization under SDR in a two-spin system is derived and verified through simulations, which show a continuous evolution from coherent, oscillatory polarization exchange to incoherent, exponential approach to equilibrium as the range of random carrier offsets (controlled by a parameter f(max)) increases; (2) in a many-spin system, polarization transfers under SDR are shown to be described accurately by a rate matrix in the limit of large f(max), with pairwise transfer rates that are proportional to the inverse sixth power of pairwise internuclear distances; (3) quantum mechanical interferences among noncommuting pairwise dipole-dipole couplings, which are a complicating factor in solid-state NMR studies of molecular structures by traditional dipolar recoupling methods, are shown to be absent from SDR data in the limit of large f(max), provided that coupled nuclei have distinct NMR chemical shifts.     

Nuclear magnetic resonance studies of convection in the 1,4-cyclohexanedione-bromate-acid reaction

The manifestation and development of convection during pattern formation in the 1,4-cyclohexanedione-acid-bromate reaction was investigated using pulsed gradient spin-echo nuclear magnetic resonance (PGSE NMR) experiments. An apparatus was devised that enabled convection to be probed inside an NMR spectrometer and prevented hydrodynamic motion arising from extraneous sources, such as poor mixing or temperature gradients imposed by the experimental setup. PGSE experiments were performed concurrently with magnetic resonance imaging (MRI) experiments to show that convection arose spontaneously from inhomogeneities associated with the chemical patterns. Quantitative data on diffusion coefficients and hydrodynamic velocities are reported.     

Molecular exchange through vesicle membranes: a pulsed field gradient nuclear magnetic resonance study

The permeability of block copolymer vesicles is studied using pulsed field gradient nuclear magnetic resonance spectroscopy together with a numerical data analysis procedure. Polyethylene oxide molecules of various molecular masses are used to sample the permeability of the vesicle membrane by observing the trans-membrane exchange process under equilibrium conditions. For shorter polyethylene oxide chains, the analysis yields a nearly linear dependence of the logarithmic trans-membrane exchange rate on the hydrodynamic radius of the sample molecules.     

Self-consistent Perturbation Theory of 13C magnetic resonance parameters in pyridines

The ¹³C chemical shifts for pyridine and 22 of its monosubstituted derivatives, the ¹³C? ¹⁹F couplings for fluoropyridines and the ¹³C? ¹⁵N couplings for pyridine, the pyridinium cation and pyridine-N-oxide have been calculated using the SCF-INDO Finite Perturbation Theory. Experimental ¹³C chemical shifts show only modest correlation with calculated shieldings; trends and magnitudes are, however, reasonably reproduced in some cases. Theory yields a correct account of the magnitudes, signs and trends for the various couplings except for ²J(CF). Addition of an empirical correction of + 33.5 Hz to the Fermi contact term leads also to excellent reproduction of this coupling.     

Static and pulsed field gradient nuclear magnetic resonance studies of water diffusion in protein matrices

Static field gradient and pulsed field gradient NMR are used to study the temperature dependence of water diffusion in myoglobin and lysozyme matrices for low hydration levels of about 0.3?g/g. We show that in order to determine reliable self-diffusion coefficients D in a broad temperature range, it is very important to consider an exchange of magnetization between water and protein protons, often denoted as cross relaxation. Specifically, upon cooling, the observed stimulated-echo decays, which reflect water diffusion near ambient temperature, become more and more governed by cross relaxation. We demonstrate that comparison of experimental results for inhomogeneous and homogeneous magnetic fields enables successful separation of diffusion and relaxation contributions to the stimulated-echo decays. Making use of this possibility, we find that in the temperature range 230-300?K, the temperature-dependent diffusivities D exhibit a Vogel-Fulcher-Tammann behavior, where water diffusion in the studied protein matrices is substantially slower than in the bulk. By comparing present and previous data, we discuss relations between translational and rotational motions and between short-range and long-range water dynamics in protein matrices. In addition, we critically examine the significance of results from previous applications of NMR diffusometry to the temperature-dependent water diffusion in protein matrices.