Biomedical subjects

A K Khitrin

Publications and source records attributed to A K Khitrin.

13 recordsLinked to original sources

PubMed2006

Detection of multiple-quantum coherences with projective nuclear magnetic resonance measurement.

It is shown that in nuclear magnetic resonance, multiple-quantum (MQ) coherences can be detected "instantly" by exploiting the principle of quantum-mechanical projective measurement. Therefore, the mixing period, which involves collective multispin dynamics and converts MQ coherences into observable single-quantum coherence (magnetization), is not necessary. The experimental examples are given for two finite clusters: benzene in liquid crystal and liquid crystal 4'-n-pentyl-4-cyanobiphenyl, and for solid adamantane with an infinite network of dipolar couplings.

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PubMed2006

Accurate measurement of 13C-15N distances with solid-state NMR.

Solid-state NMR technique for measuring distances between heteronuclei in static powder samples is described. It is based on a two-dimensional single-echo scheme enhanced with adiabatic cross polarization. As an example, the results for intramolecular distances in alpha-crystalline form of glycine are presented. The measured NMR distances (13)C(alpha)-(15)N and (13)C(')-(15)N are 1.496+/-0.002 and 2.50+/-0.02 A, respectively.

Journal Article↗

PubMed2005

Experimental demonstration of quantum state expansion in a cluster of dipolar-coupled nuclear spins.

It is experimentally demonstrated that an arbitrary quantum state of a single spin 1/2, a| upward arrow+b| downward arrow, can be converted into a superposition of the two ferromagnetic states of a spin cluster: a| upward arrow upward arrow... upward arrow upward arrow+b| downward arrow downward arrow... downward arrow downward arrow. The physical system is a cluster of seven dipolar-coupled nuclear spins of single-labeled 13C-benzene molecules in a liquid-crystalline matrix. In this complex system, the pseudopure ground state and the required controlled unitary transformations have been implemented. The experimental scheme can be considered as an explicit model of quantum measurement.

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PubMed2005

Pseudopure state of a twelve-spin system.

Pseudopure states of a system of twelve interacting spins are experimentally demonstrated. The system is a cluster of dipolar-coupled nuclear spins of fully labeled (13)C(6)-benzene in a liquid crystalline matrix. At present, this is the largest and the most complex composite system where individual quantum states have been addressed.

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PubMed2005

Adiabatic cross-polarization via intermediate dipolar-ordered state.

It is experimentally demonstrated that an adiabatic demagnetization-remagnetization in the laboratory frame, where the Zeeman order of abundant nuclei is first adiabatically converted into the dipolar order, and then, into the Zeeman order of rare nuclei, can significantly increase polarization of rare nuclei compared to the conventional cross-polarization technique.

Journal Article↗

PubMed2004

Quantum amplifier: measurement with entangled spins.

It is experimentally demonstrated that entangled quantum states can be used to amplify perturbations and to increase changes in observable values. The physical system is seven nuclear spins of single-labeled (13)C-benzene in a liquid crystalline matrix. An entangled state of six proton spins was used to monitor interaction with the (13)C spin.

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PubMed2004

Nuclear magnetic resonance study of self-diffusion in liquid crystals.

A simple and accurate pulsed-gradient nuclear magnetic resonance technique for measuring coefficients of self-diffusion in liquid crystals is described. It is based on exciting sharp response signals with long weak pulses. The method uses an extremely weak radio-frequency field, which eliminates the problem of radio-frequency heating of the sample. The temperature dependencies of coefficients of self-diffusion for two liquid crystals, 5CB (4-pentyl-4(')-cyanobiphenyl) and EBBA (N-(4-ethoxybenzylidene)-4-butylaniline), are presented.

Letter↗

PubMed2004

High-resolution NMR imaging of objects with dipolar-broadened spectra.

It is shown that, in some substances with dipolar-broadened conventional NMR spectra, it is possible to use long-lived coherent response signals, excited by long and weak radiofrequency pulses, for producing NMR images with high spatial resolution. Compared to other techniques, the method does not require high field gradients or strong radiofrequency fields, and therefore, can be applied to large objects.

Adamantane↗

PubMed2003

Phase-modulated heteronuclear decoupling in NMR of solids.

A theoretical description of heteronuclear decoupling in solids, which takes into account homonuclear dipole-dipole interactions and spinning of the sample, is presented. Based on this analysis, a simple and efficient decoupling sequence nPPM is introduced. It consists of n/4 TPPM subcycles, followed by another n/4 subcycles with reversed phases.

Journal Article↗

PubMed2002

14N chemical shifts and quadrupole coupling constants of inorganic nitrates.

The isotropic chemical shift and the nuclear quadrupole coupling constant for (14)N were obtained for 14 inorganic nitrates by solid-state MAS NMR measurements at two different field strengths, 9.4 and 11.7 T. The compounds studied were polycrystalline powders of AgNO(3), Al(NO(3))(3), Ba(NO(3))(2), Ca(NO(3))(2), CsNO(3), KNO(3), LiNO(3), Mg(NO(3))(2), NaNO(3), Pb(NO(3))(2), RbNO(3), Sr(NO(3))(2), Th(NO(3))(4)center dot4H(2)O, and UO(2)(NO(3))(2)center dot3H(2)O. Even though the spectra show broadening due to (14)N quadrupole interactions, linewidths of a few hundred hertz and a good signal-to-noise ratio were achieved. From the position of the central peaks at the two fields, the chemical shifts and the nuclear quadrupole coupling constants were calculated. The chemical shifts for all compounds studied range from 282 to 342 ppm with respect to NH(4)Cl. The nuclear quadrupole coupling constants range from 429 kHz for AgNO(3) to 993 kHz for LiNO(3). These data are compared with those available in the literature.

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PubMed2001

Indirect NMR detection in solids with multiple cross-polarization periods.

The use of indirect detection for signal enhancement in solids is much less common than in liquids, but has attracted renewed interest recently. In this work we describe an indirect detection scheme that offers a large signal enhancement for rare spins in solids. The method uses multiple periods of cross polarization, each followed by an evolution period. The latter is increased stepwise in a pseudo 2D experiment, in which the signal of the rare spin is detected as modulation of the abundant spin. As an illustration of this method, the natural abundance deuterium NMR spectrum of a static powder sample of 1,2,4,5-tetramethylbenzene is presented.

Deuterium↗

PubMed2000

An improved broadband decoupling sequence for liquid crystals and solids.

Recently we developed an efficient broadband decoupling sequence called SPARC-16 for liquid crystals ¿J. Magn. Reson. 130, 317 (1998). The sequence is based upon a 16-step phase cycling of the 2-step TPPM decoupling method for solids ¿J. Chem. Phys. 103, 6951 (1995). Since then, we have found that a stepwise variation of the phase angle in the TPPM sequence offers even better results. The application of this new method to a liquid crystalline compound, 4-n-pentyl-4'-cyanobiphenyl, and a solid, L-tyrosine hydrochloride, is reported. The reason for the improvement is explained by an analysis of the problem in the rotating frame.

Crystallization↗

PubMedDate not supplied

Filtering of spinning sidebands in 1D MAS NMR spectra.

Spinning sidebands (SSBs) in the MAS NMR spectrum of a polycrystalline solid are related to the principal values of the chemical shift or quadrupole coupling tensors. At present, 2D methods are widely used to sort out the SSBs for each isotropic peak. Here a simple and efficient method for separating the SSBs in 1D MAS NMR spectra is described. It is based on finding the optimal spinning rate with a mathematical algorithm and subsequently treating the spectra with filtering functions.

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