Longitudinal Nuclear Relaxation in an A₂Spin System Initially Polarized through Para-Hydrogen

Abstract

Os 3 (CO) 10 (NCCH 3 ) 2 has been hydrogenated with para-hydrogen to yield H 2 Os 3 (CO) 10 where the two hydrogen nuclei remain magnetically equivalent and therefore in a singlet state. Despite this equivalence, an enhanced longitudinal magnetization is observed to decrease toward thermal equilibrium. It is postulated that this enhanced magnetization is created through a nuclear relaxation interference mechanism (cross-correlation between dipolar interaction and chemical shift anisotropy); decay curves are then successfully analyzed by means of a spin relaxation theory adapted to this situation.

Role of the reaction intermediates in determining PHIP (parahydrogen induced polarization) effect in the hydrogenation of acetylene dicarboxylic acid with the complex [Rh (dppb)]+ (dppb: 1,4-bis(diphenylphosphino)butane)

“…The formation of hyperpolarized longitudinal magnetization derived from parahydrogen on magnetically equivalent protons was already observed 11 and it was demonstrated to be dependent on the intensity of the magnetic field at which the hydrogenation reaction takes place. In that case nuclear relaxation interference mechanism due to cross-correlation between dipolar interaction and chemical shift anisotropy was the reason for the observed hyperpolarized longitudinal magnetization derived from parahydrogen spin order on the A 2 spin system.…”

“…Symmetrical substrates have already been shown to yield 1 H polarized patterns due to reaction intermediates 11 or to long range coupling with other protons of the product. 12 DFT optimization of the reaction intermediates and transition states are carried out in order to obtain mechanistic support to the experimentally observed PHIP effects and, in turn, to elucidate the occurring hydrogenation pathway.…”

Para‐hydrogen enrichment and hyperpolarization

“…In such circumstances, the Hamiltonian is reduced to the J-coupling part and may thus induce more transfers as explained in recent publications (28 -30). Nevertheless, as soon as the sample is placed in the strong magnetic field of the NMR spectrometer, the quantities that will survive are of the longitudinal spin-order type, unless this longitudinal spin order is transformed into magnetization by relaxation processes (20,31). This aspect is beyond the scope of this article.…”

Parahydrogen-based NMR methods as a mechanistic probe in inorganic chemistry

“…This differential enhancement was interpreted in terms of kinetic selectivity, demonstrating that the activation barrier for H 2 addition over the S-Ir-CO axis is significantly lower than that for H 2 addition over the S-Ir-P axis. Aime et al studied the addition of hydrogen to a tri-osmium cluster species, Os 3 (CO) 10 (NCCH 3 ) 2 , using PHIP methods [49]. The authors observed that the cluster reacted with parahydrogen, yielding an unexpectedly enhanced resonance for the chemically and magnetically equivalent bridging hydride ligands of the product Os 3 (CO) 10 (H) 2 .…”