Cross-polarization

Cross-polarization (CP), originally published in 1962 as nuclear double resonance in the rotating frame by Hartmann and Hahn[1] is a solid-state nuclear magnetic resonance (ssNMR) technique used to transfer nuclear magnetization from different types of nuclei via heteronuclear dipolar interactions. The 1H-X cross-polarization dramatically improves the sensitivity of ssNMR experiments of most experiments involving spin-1/2 nuclei, capitalizing on the higher 1H polarization, and shorter T1(1H) relaxation times.

In 1972 CP was crucially adapted to magic angle spinning (MAS) by Michael Gibby, Alexander Pines and John S. Waugh at the Massachusetts Institute of Technology[2][3] who adapted a variant of the Hartmann and Hahn experiment designed by Lurie and Slichter.[4] The technique is now widely known as CPMAS.

When the Hartmann Hahn condition is matched, energy levels align in the RF rotating frame, allowing the magnetization transfer.

In CP, the natural nuclear polarization of an abundant spin (typically 1H) is exploited to increase the polarization of a rare spin (such as 13C, 15N, 31P) by irradiating the sample with radio waves at the frequencies matching the Hartmann–Hahn condition:[1]

γHB1(1H)=γXB1(X)±nωR

where γ are the gyromagnetic ratios, ωR is the spinning rate, and n is an integer. This process is sometimes referred to as "spin-locking". The power of one contact pulse is typically ramped to achieve a more broadband and efficient magnetization transfer.

The evolution of the X NMR signal intensity during the cross polarization is a build-up and decay process whose time axis is usually referred to as the "contact time". At short CP contact times, a build-up of X magnetization occurs, during which the transfer of 1H magnetization from nearby spins (and remote spins through proton spin diffusion) to X occurs. For longer CP contact times, the X magnetization decreases from T1ρ(X) relaxation, i.e. the decay of the magnetization during a spin lock.

## References

1. ↑ 1.0 1.1 Hartmann, S. R.; Hahn, E. L. (1962). "Nuclear Double Resonance in the Rotating Frame". Phys. Rev. 128 (5): 2042–2053. doi:10.1103/PhysRev.128.2042. Bibcode: 1962PhRv..128.2042H. http://astrophysics.fic.uni.lodz.pl/100yrs/pdf/09/065.pdf.
2. ↑ Pines, A.; Gibby, M. G.; Waugh, J. S. (1972-02-15). "Proton Enhanced Nuclear Induction Spectroscopy. A Method for High Resolution NMR of Dilute Spins in Solids" (in en). The Journal of Chemical Physics 56 (4): 1776–1777. doi:10.1063/1.1677439. ISSN 0021-9606. Bibcode: 1972JChPh..56.1776P. http://aip.scitation.org/doi/10.1063/1.1677439.
3. ↑ "Proton-enhanced nuclear induction spectroscopy" US patent 3792346
4. ↑ Lurie, Fred M.; Slichter, Charles P. (1964-02-17). "Spin Temperature in Nuclear Double Resonance". Physical Review 133 (4A): A1108–A1122. doi:10.1103/PhysRev.133.A1108. https://journals.aps.org/pr/abstract/10.1103/PhysRev.133.A1108.

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