52 research outputs found
Generating and sustaining long-lived spin states in 15N,15N′-azobenzene
Long-Lived spin States (LLSs) hold a great promise for sustaining non-thermal spin order and investigating various slow processes by Nuclear Magnetic Resonance (NMR) spectroscopy. Of special interest for such application are molecules containing nearly equivalent magnetic nuclei, which possess LLSs even at high magnetic fields. In this work, we report an LLS in trans-15N,15N′-azobenzene. The singlet state of the 15N spin pair exhibits a long-lived character. We solve the challenging problem of generating and detecting this LLS and further increase the LLS population by converting the much higher magnetization of protons into the 15N singlet spin order. As far as the longevity of this spin order is concerned, various schemes have been tested for sustaining the LLS. Lifetimes of 17 minutes have been achieved at 16.4 T, a value about 250 times longer than the longitudinal relaxation time of 15N in this magnetic field. We believe that such extended relaxation times, along with the photochromic properties of azobenzene, which changes conformation upon light irradiation and can be hyperpolarized by using parahydrogen, are promising for designing new experiments with photo-switchable long-lived hyperpolarization
Hyperpolarization of cis-15N,15N'-azobenzene by parahydrogen at ultralow magnetic fields
Development of the methods to exploit nuclear hyperpolarization and search for molecules whose nuclear spins can be efficiently hyperpolarized is an active area in nuclear magnetic resonance. Of particular interest are those molecules that have long nuclear relaxation times, making them to be suitable candidates as contrast agents in magnetic resonance imaging. In this work, we present a detailed study of SABRE SHEATH (Signal Amplification By Reversible Exchange in Shield Enabled Alignment Transfer to Heteronuclei) experiments of 15N,15N' azobenzene. In SABRE SHEATH experiments nuclear spins of the target are hyperpolarized by transfer of spin polarization from parahydrogen at ultralow fields during a reversible chemical process. The studied system is complicated, and we are concerned only about a subset of the data, presenting details for the molecules that experience fast chemical exchange at the catalytic complex and thus are involved in polarizing the free azobenzene. Azobenzene exists in two isomers trans- and cis-. We show that all nuclear spins in cis-azobenzene can be efficiently hyperpolarized by SABRE at suitable magnetic fields. Enhancement factors (relative to 9.4 T) reach several thousands of times for 15N spins and a few tens of times for the 1H spins. There are two approaches to observe either hyperpolarized magnetization of 15N/1H spins or hyperpolarized singlet order of the 15N spin pair. We compare these approaches and present the field dependencies of SABRE experiments for them. No hyperpolarization of trans 15N,15N' azobenzene was observed. The results presented here will be useful for further experiments where hyperpolarized cis-15N,15N' azobenzene is switched by light to trans 15N,15N' azobenzene for storing the produced hyperpolarization in the long-lived spin state of the 15N pair of trans-15N,15N' azobenzene
Analysis of Nutation Patterns in Fourier-Transform NMR of Non-Thermally Polarized Multispin Systems
The complex spin order of hyperpolarized multispin systems giving rise to
anomalous NMR spectral patterns that vary with the RF excitation angle is
analyzed by decomposing its nutation behavior in a superposition of Fourier
harmonics. The product operator formalism is applied to calculating the
spectral contributions of the various mutual alignments of scalar coupled
spins. Two cases are treated, namely systems exhibiting only differences in
population of their spin states and systems showing in addition zero-quantum
coherences between states, a situation often seen at hyperpolarization. After
deriving the general solution a number of representative examples are
discussed in detail. The theoretical treatment is applied to analyzing the
spin order observed in a hyperpolarized two-spin system that is prepared in
the singlet state by para-hydrogen induced polarization
К вопросу о контроле и автоматизации шахтных дегазационных систем
Розглянуті існуючі системи контролю та автоматизації шахтної газопровідної мережі, а
також засобі контролю газодинамічних параметрів метано-повітряної суміші у дільничному
дегазаційному трубопроводі.The existing system of control and automation of mine degassingnetwork, and controls the gasdynamic parameters of methane-air mixture in the precinct line degassing
Surprising absence of strong homonuclear coupling at low magnetic field explored by two-field nuclear magnetic resonance spectroscopy
Strong coupling of nuclear spins, which is achieved when their scalar
coupling 2πJ is greater than or comparable to the difference
Δω in their Larmor precession frequencies in an external magnetic field, gives rise to efficient coherent longitudinal polarization transfer. The strong coupling regime can be achieved when the external magnetic field is sufficiently low, as Δω is reduced proportional to the field strength. In the present work, however, we
demonstrate that in heteronuclear spin systems these simple arguments may
not hold, since heteronuclear spin–spin interactions alter the
Δω value. The experimental method that we use is two-field nuclear magnetic resonance (NMR), exploiting sample shuttling between the high field, at which NMR spectra are acquired, and the low field, where strong couplings are expected and at which NMR pulses can be applied to affect the spin dynamics. By using this technique, we generate zero-quantum
spin coherences by means of a nonadiabatic passage through a level
anticrossing and study their evolution at the low field. Such zero-quantum
coherences mediate the polarization transfer under strong coupling
conditions. Experiments performed with a 13C-labeled amino acid
clearly show that the coherent polarization transfer at the low field is
pronounced in the 13C spin subsystem under proton decoupling. However, in the absence of proton decoupling, polarization transfer by coherent processes is dramatically reduced, demonstrating that heteronuclear spin–spin interactions suppress the strong coupling regime, even when the external field is low. A theoretical model is presented, which can model the reported experimental results.</p
The Influence of Scavenging on Cidnp Field Dependences in Biradicals during the Photolysis of Large-Ring Cycloalkanones
Investigation of the CIDNP field dependences of acyl-alkyl biradicals formed by Norrish type-I reaction during the photolysis of cycloundecanone and cyclododecanone in the presence of the scavenger CBrCl3 has been performed at low (up to 0.08 T) magnetic fields. In addition to the emissive polarization with the main maxima caused by the S-T mechanism we observed an absorptive polarization with maxima at low fields (< 0.01 T) for alpha-CH2 protons of initial ketones. A quantitative analysis of CIDNP amplitude dependences on scavenger concentration allows the estimation of the kinetics of biradical geminate recombination near the emissive maxima. It is demonstrated that the biradical scavenging rate constant can be determined using the kinetic data for high magnetic fields. The scavenging rate constant of CBrCl3 estimated by this method is (2.3 +/- 0.4) X 10(9) M(-1) s(-1). Two competitive channels of singlet-triplet conversion have been revealed and the qualitative picture of their contribution to the kinetics of the low-field CIDNP is presented
Parahydrogen-Induced Hyperpolarization of Unsaturated Phosphoric Acid Derivatives
Parahydrogen-induced nuclear polarization offers a significant increase in the sensitivity of NMR spectroscopy to create new probes for medical diagnostics by magnetic resonance imaging. As precursors of the biocompatible hyperpolarized probes, unsaturated derivatives of phosphoric acid, propargyl and allyl phosphates, are proposed. The polarization transfer to 1H and 31P nuclei of the products of their hydrogenation by parahydrogen under the ALTADENA and PASADENA conditions, and by the PH-ECHO-INEPT+ pulse sequence of NMR spectroscopy, resulted in a very high signal amplification, which is among the largest for parahydrogen-induced nuclear polarization transfer to the 31P nucleus
Exchange interaction in short-lived flavine adenine dinucleotide biradical in aqueous solution revisited by CIDNP (chemically induced dynamic nuclear polarization) and nuclear magnetic relaxation dispersion
Flavin adenine dinucleotide (FAD) is an important
cofactor in many light-sensitive enzymes. The role of the adenine moiety of
FAD in light-induced electron transfer was obscured, because it involves an
adenine radical, which is short-lived with a weak chromophore. However, an
intramolecular electron transfer from adenine to flavin was revealed several
years ago by Robert Kaptein by using chemically induced dynamic nuclear
polarization (CIDNP). The question of whether one or two types of biradicals of
FAD in aqueous solution are formed stays unresolved so far. In the present
work, we revisited the CIDNP study of FAD using a robust mechanical sample
shuttling setup covering a wide magnetic field range with sample
illumination by a light-emitting diode. Also, a cost efficient fast field
cycling apparatus with high spectral resolution detection up to 16.4 T for
nuclear magnetic relaxation dispersion studies was built based on a 700 MHz
NMR spectrometer. Site-specific proton relaxation dispersion data for FAD
show a strong restriction of the relative motion of its isoalloxazine and
adenine rings with coincident correlation times for adenine, flavin, and
their ribityl phosphate linker. This finding is consistent with the
assumption that the molecular structure of FAD is rigid and compact. The
structure with close proximity of the isoalloxazine and purine moieties is
favorable for reversible light-induced intramolecular electron transfer from
adenine to triplet excited flavin with formation of a transient
spin-correlated triplet biradical F⚫−-A⚫+. Spin-selective recombination of the biradical leads to the formation of CIDNP
with a common emissive maximum at 4.0 mT detected for adenine and flavin
protons. Careful correction of the CIDNP data for relaxation losses during
sample shuttling shows that only a single maximum of CIDNP is formed in the
magnetic field range from 0.1 mT to 9 T; thus, only one type of FAD
biradical is detectable. Modeling of the CIDNP field dependence provides
good agreement with the experimental data for a normal distance distribution
between the two radical centers around 0.89 nm and an effective electron
exchange interaction of −2.0 mT.</p
- …