The nuts and bolts of distance determination and zero- and double-quantum coherence in photoinduced radical pairs

We describe in some detail the new method of distance determination for a photoinduced radical pair. Emphasis is on giving the nuts and bolts of the calculations that result in analytical expressions for in- and out-of-phase electron spin echo (ESE) envelope modulations, pulse flip-angle dependencies, zero- and double-quantum coherences, and the distance between the two radicals. The theoretical results are illustrated by a set of recent experiments on photosynthetic reaction centers.     

溶液中自扩散过程的NMR方法研究

对脉冲梯度场－核磁共振（ＰＦＧ－ＮＭＲ）中测定溶液分子自扩散系数的Ｓｔｉｍｕｌａｔｅｄ ｅｃｈｏ方法进行了改进,把测定自扩散系数的Ｓｔｉｍｕｌａｔｅｄ ｅｃｈｏ脉冲序列与测定自旋－晶格弛豫时间的脉冲序列串接起来,设计了两个新的脉冲序列。     

Doubly compensated multiplicity-edited HSQC experiments utilizing broadband inversion pulses

We propose a family of doubly compensated multiplicity-edited heteronuclear single quantum coherence (HSQC) pulse sequences. The key difference between our proposed sequences and the compensation of refocusing inefficiency with synchronized inversion sweeps (CRISIS)-HSQC experiments they are based on is that the conventional rectangular 180 degrees pulses on the proton channel in the latter have been replaced by the computer-optimized broadband inversion pulses (BIPs) with superior inversion performance as well as much improved tolerance to B(1) field inhomogeneity. Moreover, all adiabatic carbon 180 degrees pulses during the INEPT and reverse-INEPT periods in the CRISIS-HSQC sequences have also been replaced with the much shorter BIPs, while the adiabatic sweeps during the heteronuclear spin echo for multiplicity editing are kept in place in order to maintain the advantage of the CRISIS feature of the original sequences, namely J-independent refocusing of the one-bond (1)H--(13)C coupling constants. These modifications have also been implemented to the preservation of equivalent pathways (PEP)-HSQC experiments. We demonstrate through a detailed comparison that replacing the proton 180 degrees pulses with the BIPs provide additional sensitivity gain that can be mainly attributed to the improved tolerance to B(1) field inhomogeneity of the BIPs. The proposed sequences can be easily adapted for (19)F--(13)C correlations.     

Multiphoton femtosecond phase-coherent two-dimensional electronic spectroscopy

In this paper the authors compare 400 nm one-photon and 800 nm two-photon two-dimensional Fourier transform electronic spectra of the organic laser dye Coumarin 102 in methanol using collinear optical pulse sequences and phase cycling. Results from the two different experiments show differences in the photon echo peak positions and shapes, reflecting differences in the two-photon and one-photon selection rules.     

NMR measurements of diffusion in concentrated samples: avoiding problems with radiation damping

Pulsed field gradient spin echo NMR is generally the method of choice for diffusion measurements on liquid samples. With modern high field instruments, however, severe problems can arise when it is applied to samples with very high proton concentrations because of the presence of radiation damping. The problems may be greatly reduced by a suitable choice of experimental parameters, in particular the use of modified stimulated echo pulse sequences with a reduced flip angle for the first pulse.     

Spin relaxation in the phosphorescent triplet state of several pyridyl carbonyl compounds

Spin relaxation is observed in benzophenone, isomers of benzoylpyridines and dipyridyl ketones. Models are used to describe optically detected echo shapes observed by transient techniques in the phosphorescent triplet state. Framed after the density matrix approach, the echo shapes were found to adequately fit the analytical expressions. Hahn spin echo, Carr-Purcell multiple echo, and rotary echo pulse sequences were used to study the nuclear contribution to the electron spin dephasing and the proximity effect of the nitrogen nuclei to the localized triplet excitation in the carbonyl portion of the molecule.     

Quadrupolar nuclear magnetic resonance spectroscopy in solids using frequency-swept echoing pulses

The acquisition of ideal powder line shapes remains a recurring challenge in solid-state wideline nuclear magnetic resonance (NMR). Certain species, particularly quadrupolar spins in sites associated with large electric field gradients, are difficult to excite uniformly and with good efficiencies. This paper discusses some of the opportunities that arise upon departing from standard spin-echo excitation approaches and switching to echo sequences that use low-power, frequency-swept radio frequency (rf) pulses instead. The reduced powers demanded by such swept rf fields allow one to excite spins in different crystallites efficiently and with orientation-independent pulse angles, while the large bandwidths of interest that are needed by the measurement can be covered, thanks to the use of broadband frequency sweeps. The fact that the spins' evolution and ensuing dephasing starts at the beginning of such rf manipulation calls for the use of spin-echo sequences; a number of alternatives capable of providing the desired line shapes both in the frequency and in the time domains are introduced and experimentally demonstrated. Sensitivity- and lineshape-wise these experiments are competitive vis-a-vis current implementations of wideline quadrupolar NMR based on hard rf pulses; additional opportunities that may derive from these ideas are also briefly discussed.     

Multiple rotary echoes in nitrogen-14 quadrupolar spin-system

This is a study of the influence of multi-pulse sequences consisting of blocks of short-repetition pulses on the nitrogen-14 NQR spin-system. The experiment demonstrated that the application of such sequences generates multiple rotary echo signals in the effective field of the pulse sequence similar to those generated by conventional spin-locking multi-pulse sequences. The detected RE signals were analysed and the obtained results presented, adding to the understanding of the dynamic properties of the quadrupolar spin-system. The experimental results are obtained for polycrystalline NaNO₂.     

J‐compensated PGSE: an improved NMR diffusion experiment with fewer phase distortions

Peak distortion caused by homonuclear ¹H J‐coupling is a major problem in many spin‐echo‐based experiments such as pulsed gradient spin‐echo (PGSE) experiments. Although peak phase distortions can be lessened by the incorporation of anti‐phase purging sequences, the sensitivity is substantially decreased. Techniques for lessening the effect of homonuclear J‐coupling evolution in spin‐echo‐based experiments have been investigated. Two potentially useful candidates include a J‐compensated inversion sequence that is efficient over a wide range of J‐coupling values and a pulse sequence that refocuses homonuclear J‐evolution during the spin‐echo. The latter was found to work superbly on samples containing two spin (AX or AB) systems and still provided significant advantage over the standard method on samples containing more complicated spin systems. Implementation of this J‐refocusing technique into a PGSE‐type experiment (J‐PGSE) leads to dramatic improvement of spectra and easier data analysis. The J‐PGSE sequence should find applications in many diffusion studies where the PGSE‐type method is required and should be a viable alternative to PGSTE especially in dilute samples due to its enhanced sensitivity. Copyright © 2009 John Wiley & Sons, Ltd.     

Electron Spin Echo Study of Molecular Structure and Dynamics: New Approaches Based on Spontaneous Fluctuations of Magnetic Interactions

Modulation phenomena that take place during electron spin echo signal decay have long been used in structural studies of free radicals and their environment. These phenomena are based on coherent dynamic effects, arising from simultaneous excitation (by microwave pulses) of two or more transitions in the EPR spectrum. Recently, a new source of stimulated electron spin echo (ESE) modulation was discovered due to spontaneous changes in the magnetic parameters of radicals during the operation of the pulse sequence. For monoradicals, these changes are caused by intramolecular motions. For radical pairs, additional mechanisms are longitudinal relaxation of spin counterparts and transformations of the paramagnetic partners during chemical reactions. Promising applications of this phenomenon to structural studies of radicals and radical pairs in solids and to investigations of their mobility and chemical transformations are considered.     

Exploring the crystallinity of different powder sugars through solid echo and magic sandwich echo sequences

Time-domain nuclear magnetic resonance techniques are frequently used in polymer, pharmaceutical, and food industries as they offer rapid experimentation and generally do not require any considerable preliminary sample preparation. Detection of solid and liquid fractions in a sample is possible with the free induction decay (FID). However, for the classical FID sequence that consists of a single pulse followed by relaxation decay acquisition, the dead time of the probe (ring out of resonance circuitry) occurs and varies between 5 and 15 μs for standard 10-mm tubes. In such a case, there arises a risk that the signal from the solid fraction cannot be detected correctly. To obtain quantitative measurement on crystalline and more mobile amorphous fractions, alternative sequences to the classical FID in the solid-state nuclear magnetic resonance were developed. Solid echo and magic sandwich echo sequences perform the relaxation decay refocusing somehow excluding the dead time problem and allow detection of the signal from the solid fraction. In this study, knowledge of amorphous/crystal fraction, which is obtained through solid echo and magic sandwich echo, has been explored on powder sugar samples for the purpose of developing a groundwork for a reliable quality control method. Different sugars were examined for the utilization of the sequences. What is important to add and make this study unique is that the method proposed did not involve multiparameter fitting of the “bead” pattern FID signal that normally suffers from ambiguity; just the integration of the fast Fourier transform of the solid echo was needed to calculate the second moment, (M₂).     

Effective Hamiltonians by optimal control: solid-state NMR double-quantum planar and isotropic dipolar recoupling

We report the use of optimal control algorithms for tailoring the effective Hamiltonians in nuclear magnetic resonance (NMR) spectroscopy through sophisticated radio-frequency (rf) pulse irradiation. Specifically, we address dipolar recoupling in solid-state NMR of powder samples for which case pulse sequences offering evolution under planar double-quantum and isotropic mixing dipolar coupling Hamiltonians are designed. The pulse sequences are constructed numerically to cope with a range of experimental conditions such as inhomogeneous rf fields, spread of chemical shifts, the intrinsic orientation dependencies of powder samples, and sample spinning. While the vast majority of previous dipolar recoupling sequences are operating through planar double-or zero-quantum effective Hamiltonians, we present here not only improved variants of such experiments but also for the first time homonuclear isotropic mixing sequences which transfers all I(x), I(y), and I(z) polarizations from one spin to the same operators on another spin simultaneously and with equal efficiency. This property may be exploited to increase the signal-to-noise ratio of two-dimensional experiments by a factor of square root 2 compared to conventional solid-state methods otherwise showing the same efficiency. The sequences are tested numerically and experimentally for a powder of (13)C(alpha),(13)C(beta)-L-alanine and demonstrate substantial sensitivity gains over previous dipolar recoupling experiments.     

NMR experiments for the analysis of mixtures: beyond 1D 1H spectra

State-of-the-art technologies and methodologies in NMR spectroscopy make it possible to obtain very informative and high-quality spectra in much less experimental time than classical methods by making better choices of NMR pulse sequences and acquisition parameters. This review presents some recent NMR methods allowing rapid identification, assignment and structural characterization of the components in mixtures. The relative merits of the different NMR pulse sequences are briefly discussed and recommendations are made for the preferred choice of sequences to obtain rapidly artifact-free data. This review covers diffusion experiments (DOSY), HSQC and HMBC experiments, ultra-resolved 2D spectra exploiting the property of aliasing and NOESY/ROESY experiments. It will be in particular shown that selective 1D NOESY/ROESY sequences can be more informative and reach higher resolution in less experimental time than the corresponding 2D sequences.     

Comparison of 1H–19F two‐dimensional NMR scalar coupling correlation pulse sequences

The effectiveness of hetero‐COSY, HETCOR, HMQC, and HSQC two‐dimensional NMR pulse sequences for detection of ¹⁹F–¹H correlations by scalar coupling was evaluated on monofluorinated and polyfluorinated test compounds. All four of these sequences were effective in observing ¹H–¹⁹F correlations, using either ¹⁹F or ¹H as the observe nucleus. All four sequences were amenable, to some degree, to adjustment to observe larger or smaller couplings preferentially. A 1/2J echo filter was effectively applied to remove artifacts from ²J_FF strong coupling. The HETCOR experiments afforded the best overall combination of sensitivity, resolution and selectivity for J_HF. Copyright © 2014 John Wiley & Sons, Ltd.     

Dynamics of water probed with vibrational echo correlation spectroscopy

Vibrational echo correlation spectroscopy experiments on the OD stretch of dilute HOD in H(2)O are used to probe the structural dynamics of water. A method is demonstrated for combining correlation spectra taken with different infrared pulse bandwidths (pulse durations), making it possible to use data collected from many experiments in which the laser pulse properties are not identical. Accurate measurements of the OD stretch anharmonicity (162 cm(-1)) are presented and used in the data analysis. In addition, the recent accurate determination of the OD vibrational lifetime (1.45 ps) and the time scale for the production of vibrational relaxation induced broken hydrogen bond "photoproducts" ( approximately 2 ps) aid in the data analysis. The data are analyzed using time dependent diagrammatic perturbation theory to obtain the frequency time correlation function (FTCF). The results are an improved FTCF compared to that obtained previously with vibrational echo correlation spectroscopy. The experimental data and the experimentally determined FTCF are compared to calculations that employ a polarizable water model (SPC-FQ) to calculate the FTCF. The SPC-FQ derived FTCF is much closer to the experimental results than previously tested nonpolarizable water models which are also presented for comparison.     

Communication: Phase incremented echo train acquisition in NMR spectroscopy

We present an improved and general approach for implementing echo train acquisition (ETA) in magnetic resonance spectroscopy, particularly where the conventional approach of Carr-Purcell-Meiboom-Gill (CPMG) acquisition would produce numerous artifacts. Generally, adding ETA to any N-dimensional experiment creates an N + 1 dimensional experiment, with an additional dimension associated with the echo count, n, or an evolution time that is an integer multiple of the spacing between echo maxima. Here we present a modified approach, called phase incremented echo train acquisition (PIETA), where the phase of the mixing pulse and every other refocusing pulse, φ(P), is incremented as a single variable, creating an additional phase dimension in what becomes an N + 2 dimensional experiment. A Fourier transform with respect to the PIETA phase, φ(P), converts the φ(P) dimension into a Δp dimension where desired signals can be easily separated from undesired coherence transfer pathway signals, thereby avoiding cumbersome or intractable phase cycling schemes where the receiver phase must follow a master equation. This simple modification eliminates numerous artifacts present in NMR experiments employing CPMG acquisition and allows "single-scan" measurements of transverse relaxation and J-couplings. Additionally, unlike CPMG, we show how PIETA can be appended to experiments with phase modulated signals after the mixing pulse.     

Application of pulse radiolysis to the study of the chemistry of radical anions

The application of pulse radiolysis to the measurement of rates of reaction of aromatic and olefinic radical anions in organic solvents is discussed, emphasis being placed on the problem of reaction of the radical anion with the radiolytically-generated-counter ion. Some previous experiments of the authors that utilised tetrahydridoaluminate salts to scavenge the counter ion are reviewed. Some new data on the rates of electron transfer from aromatic-radical anions to some substituted styrenes are presented and experiments aimed at using organic amides as solvents for these studies are described.     

Multiple quantum correlated spectroscopy revamped by asymmetric z-gradient echo detection signal intensity as a function of the read pulse flip angle as verified by heteronuclear 1H/31P experiments

Heteronuclear multiple quantum (n=+/-0 and n=+/-2) correlated spectroscopy revamped by asymmetric z-gradient echo detection (CRAZED) experiments were performed on the spins 31P and 1H in a H3PO4 solution in order to determine the optimum flip angle for the read pulse. It has been shown that for the negative quantum signals, the maximum signals appear at beta=0, and for the positive quantum signals, the maximum signals appear at beta=pi. The CRAZED signals were compared to the single quantum signals in two-pulse two-gradient experiments. It is found that the CRAZED signals can also be distinguished into gradient echoes and spin echoes. The gradient-echo-type CRAZED signal requires beta=0 and the spin-echo-type CRAZED signal requires beta=pi for maximum echo intensities, in the same way as in single quantum experiments.     

NMR dipolar echoes in liquid crystals

The proton dipolar echo responses to both in-phase and in-quadrature rf pulse sequences in nematic 4,4′-bis(hexyloxy)azoxybenzene are shown to be characteristic of a spin-1/2 pair system. This result is consistent with the chemical structure of this mesogen. The echo technique has therefore been used to measure the interpair second moment M₂(inter) as a function of temperature over the entire nematic range. The total second moment M₂ has been extracted from the FID signal. The intra- and interpair second moments are discussed in relation to the order parameter S and are shown to be averaged in distinctly different ways by the internal motions of the mesogen.