4 research outputs found
analytical solutions for spectral patterns and their field dependence
We have obtained analytical solutions for Para-Hydrogen Induced Polarization
(PHIP) for several types of coupled spin systems, namely, for AB-, ABX-,
AA´A´´- and A2B-systems. Scalar spin-spin interactions were considered the
factor, that determines the PHIP spectral pattern; it is the variation of the
spin coupling regime (from strong coupling at low field to weak coupling at
high field), which is responsible for the PHIP magnetic field dependence. The
field dependence of polarization was considered in detail, general
peculiarities of PHIP were found, PHIP patterns were calculated for the
systems mentioned. Special attention was paid to the effects of field
switching on PHIP
Transfer of Parahydrogen Induced Polarization in Scalar Coupled Systems at Variable Magnetic Field
Para-Hydrogen Induced Polarization (PHIP) experiments were performed in
coupled multispin systems at variable magnetic fields. We studied the magnetic
field dependence of PHIP in styrene, which is the product of hydrogenation of
phenylacetylene. At low magnetic fields where the spins are coupled strongly
by scalar interaction efficient polarization transfer among the interacting
protons takes place. The experimentally observed spectra are in good agreement
with the simulation, which takes into account eight coupled spins. We also
demonstrate effects of nuclear spin level anti-crossings on the PHIP pattern.
It is shown that rapid passage through the level anti-crossing enables highly
efficient polarization transfer between specific spin orders. In addition, we
studied PHIP transfer to 13C and 19F hetero-nuclei. It is shown that hetero-
nuclei can be efficiently polarized in a wide field range; in particular, for
polarizing them it is not necessary to go to ultra-low fields, which provide
their strong coupling to protons. The resulting polarization is of the
multiplet type and gives strong enhancements of the individual NMR lines. In
general, variation of the magnetic field gives the opportunity for
manipulating PHIP patterns and transferring polarization to target spins of
choice
Influence of spin 1/2 hetero-nuclei on spin relaxation and polarization transfer among strongly coupled protons
Effects of spin-spin interactions on the nuclear magnetic relaxation
dispersion (NMRD) of protons were studied in a situation where spin ½ hetero-
nuclei are present in the molecule. As in earlier works [K. L. Ivanov, A. V.
Yurkovskaya, and H.-M. Vieth, J. Chem. Phys.129, 234513
(2008)10.1063/1.3040272;S. E. Korchak, K. L. Ivanov, A. V. Yurkovskaya, and
H.-M. Vieth, J. Chem. Phys.133, 194502 (2010)10.1063/1.3495988], spin-spin
interactions have a pronounced effect on the relaxivity tending to equalize
the longitudinal relaxation times once the spins become strongly coupled at a
sufficiently low magnetic field. In addition, we have found influence of 19F
nuclei on the proton NMRD, although in the whole field range, studied protons
and fluorine spins were only weakly coupled. In particular, pronounced
features in the proton NMRD were found; but each feature was predominantly
observed only for particular spin states of the hetero-nuclei. The features
are explained theoretically; it is shown that hetero-nuclei can affect the
proton NMRD even in the limit of weak coupling when (i) protons are coupled
strongly and (ii) have spin-spin interactions of different strengths with the
hetero-nuclei. We also show that by choosing the proper magnetic field
strength, one can selectively transfer proton spin magnetization between
spectral components of choice
exploiting the advantages of coherent electron spin motion
A theoretical approach is proposed to describe Overhauser-type Dynamic Nuclear
Polarization (DNP) for pulsed EPR pumping by application of a train of short
pulses with a duration on the nanosecond time scale. We obtained an elegant
general expression for the NMR enhancement provided by the DNP effect. The
expression for the enhancement is similar to that known for cw-pumping except
for the saturation factor, which is re-defined as the deviation of the
electron spin magnetization from its equilibrium value averaged over the cycle
of the pulse sequence. It is shown that one can achieve the maximal
theoretically allowed NMR enhancement for pulsed pumping even when the duty
cycle of pumping is low. This becomes possible because coherent motion of the
electron spins in the B1-field is exploited, a key feature of the pulsed DNP
experiment also enabling optimization of the achievable NMR enhancement. The
dependence of the effect on the duty cycle, pulse duration and electron spin
relaxation times has been studied in detail. Once the lines in the EPR
spectrum are inhomogeneously broadened, higher DNP effects are expected in the
pulsed pumping mode than in the cw-mode for the same total power of microwave
irradiation. The theoretical results are in good agreement with experimental
data obtained for the pumping frequencies of 300 MHz and 1.4 GHz