Transferred Overhauser DNP: A Fast, Efficient Approach for Room Temperature 13 C ODNP at Moderately Low Fields and Natural Abundance.

Overhauser dynamic nuclear polarization (ODNP) is investigated at a moderately low field (1.2 T) for natural abundance 13 C NMR of small molecules in solution state at room temperature. It is shown that ODNP transferred from 1 H to 13 C by NMR coherence transfer is in general significantly more efficient than direct ODNP of 13 C. Compared to direct 13 C ODNP, we demonstrate over 4-fold higher 13 C sensitivity (signal-to-noise ratio, SNR), achieved in one-eighth of the measurement time by transferred ODNP (t-ODNP). Compared to the 13 C signal arising from Boltzmann equilibrium in a fixed measurement time, this is equivalent to about 1500-fold enhancement of 13 C signal by t-ODNP, as against a direct 13 C ODNP signal enhancement of about 45-fold, both at a moderate ESR saturation factor of about 0.25. This owes in part to the short polarization times characteristic of 1 H. Typically, t-ODNP reflects the essentially uniform ODNP enhancements of all protons in a molecule. Although the purpose of this work is to establish the superiority of t-ODNP vis-à-vis direct 13 C ODNP, a comparison is also made of the SNR in t-ODNP experiments with standard high resolution NMR as well. Finally, the potential of t-ODNP experiments for 2D heteronuclear correlation spectroscopy of small molecules is demonstrated in 2D 1 H-13 C HETCOR experiments at natural abundance, with decoupling in both dimensions.