Synthesis
of novel trityl-nitroxyl biradicals and their performance
as polarization agents in DNP-enhanced solid-state MAS NMR spectroscopy
is presented. Signal enhancements in 1H, 1H
→ 13C CP MAS, and 13C MAS experiments
obtained with these radicals dissolved in 1,1,2,2-tetrachloroethane
(TCE) solution are compared with the enhancements obtained from TCE
solutions of binitroxyl radicals. The signal enhancements are correlated
with the distance between the radical centers of the biradicals, as
determined by theoretical structure calculations. Some of the biradical
TCE solutions display direct-channel resonances in 13C
MAS experiments as well as indirect channel resonances induced via
the proton spin reservoir. Differential scanning calorimetry reveals
that only these solutions do not form any solid crystalline phases
upon rapid cooling, suggesting that molecular motions needed for polarization
transfer from radicals to 13C via the proton spin reservoir
remain active at the experimental low temperatures of nominal 120
K. DNP magnetic field sweep enhancement profiles for selected new
biradicals are presented as well. These indicate that the DNP transfer
is dominated by the cross-effect mechanism.
The kinetic resolution of trans-cyclohexane-1,2-diol with a lipophilic oligopeptide catalyst shows extraordinary selectivities. To improve our understanding of the factors governing selectivity, we quantified the Gibbs energies of interactions of the peptide with both enantiomers of trans-cyclohexane-1,2-diol using nuclear magnetic resonance (NMR) spectroscopy. For this we use advanced methods such as transverse relaxation (R2), diffusion measurements, saturation transfer difference (STD), and chemical shift (δ) analysis of peptide-diol mixtures upon varying their composition (NMR titrations). The methods employed give comparable and consistent results: The molecular recognition by the catalyst is approx. 3 kJmol−1 in favor of the preferentially acetylated (R,R)-enantiomer in the temperature range studied. Interestingly, the difference of 3 kJ mol−1 is also confirmed by results from reaction monitoring of the acylation step under catalytic conditions, indicating that this finding is true regardless of whether the investigation is performed on the acetylated species or on the free catalyst. To arrive at these conclusions the self-association of both catalyst and substrate in toluene were found to play an important role and thus need to be taken into account in reaction screening.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.