Tan, Kong Ooi, Ralph T. Weber, Thach V. Can, and Robert G. Griffin. “Adiabatic Solid Effect.” The Journal of Physical Chemistry Letters, April 20, 2020, 3416–21.
The solid effect (SE) is a two spin dynamic nuclear polarization (DNP) mechanism that enhances the sensitivity in NMR experiments by irradiation of the electron-nuclear spin transitions with continuous wave (CW) microwaves at 𝜔0S ± 𝜔0I, where 𝜔0S and 𝜔0I are electron and nuclear Larmor frequencies, respectively. Using trityl (OX063), dispersed in a 60/40 glycerol/water mixture at 80 K, as a polarizing agent, we show here that application of a chirped microwave pulse, with a bandwidth comparable to the EPR linewidth applied at the SE matching condition, improves the enhancement by a factor of 2.4 over the CW method. Furthermore, the chirped pulse yields an enhancement that is ~20 % larger than obtained with the ramped-amplitude NOVEL (RA-NOVEL), which to date has achieved the largest enhancements in time domain DNP experiments. Numerical simulations suggest that the spins follow an adiabatic trajectory during the polarization transfer; hence, we denote this sequence as an adiabatic solid effect (ASE). We foresee that ASE will be a practical pulsed DNP experiment to be implemented at higher static magnetic fields due to moderate power requirement. In particular, the ASE uses only 13 % of the maximum microwave power required for RA-NOVEL.
Jain, Sheetal K., Guinevere Mathies, and Robert G. Griffin. “Off-Resonance NOVEL.” The Journal of Chemical Physics 147, no. 16 (October 28, 2017): 164201.
Dynamic nuclear polarization (DNP) is theoretically able to enhance the signal in nuclear magnetic resonance (NMR) experiments by a factor gamma_e/gamma_n, where gamma’s are the gyromagnetic ratios of an electron and a nuclear spin. However, DNP enhancements currently achieved in high-field, high-resolution biomolecular magic-angle spinningNMRare well below this limit because the continuous-wave DNP mechanisms employed in these experiments scale as w0^(-n) where n ~ 1–2. In pulsed DNP methods, such as nuclear orientation via electron spin-locking (NOVEL), the DNP efficiency is independent of the strength of the main magnetic field. Hence, these methods represent a viable alternative approach for enhancing nuclear signals. At 0.35 T, the NOVEL scheme was demonstrated to be efficient in samples doped with stable radicals, generating 1H NMR enhancements of 430. However, an impediment in the implementation of NOVEL at high fields is the requirement of sufficient microwave power to fulfill the on-resonance matching condition, omega_0I = omega_1S, where omega_0I and omega_1S are the nuclear Larmor and electron Rabi frequencies, respectively. Here, we exploit a generalized matching condition, which states that the effective Rabi frequency, omega_1Seff, matches omega_0I . By using this generalized off-resonance matching condition, we generate 1H NMR signal enhancement factors of 266 (70% of the onresonanceNOVEL
enhancement) with omega_1S/2pi = 5 MHz.We investigate experimentally the conditions for optimal transfer of polarization from electrons to 1H both for the NOVEL mechanism and the solid-effect mechanism and provide a unified theoretical description for these two historically distinct forms of DNP.
Annabestani, Razieh, Maryam Mirkamali, and Raffi Budakian. “Adiabatic-NOVEL for Nano-Scale Magnetic Resonance Imaging.” ArXiv:1712.09128 [Quant-Ph], December 25, 2017.
We propose a highly efficient dynamic nuclear polarization technique that is robust against field in-homogeneity. This technique is designed to enhance the detection sensitivity in nano-MRI, where large Rabi field gradients are required. The proposed technique consists of an adiabatic half passage pulse followed by an adiabatic linear sweep of the electron Rabi frequency and can be considered as an adiabatic version of nuclear orientation via electron spin locking (adiabatic-NOVEL). We analyze the spin dynamics of an electron-nuclear system that is under microwave irradiation at high static magnetic field and at cryogenic temperature. The result shows that an amplitude modulation of the microwave field makes adiabatic-NOVEL highly efficient and robust against both the static and microwave field in-homogeneity.
Mathies, G., et al., Pulsed Dynamic Nuclear Polarization with Trityl Radicals. The Journal of Physical Chemistry Letters, 2016. 7(1): p. 111-116.
Continuous-wave (CW) dynamic nuclear polarization (DNP) is now established as a method of choice to enhance the sensitivity in a variety of NMR experiments. Nevertheless, there remains a need for the development of more efficient methods to transfer polarization from electrons to nuclei. Of particular interest are pulsed DNP methods because they enable a rapid and efficient polarization transfer that, in contrast with CW DNP methods, is not attenuated at high magnetic fields. Here we report nuclear spin orientation via electron spin-locking (NOVEL) experiments using the polarizing agent trityl OX063 in glycerol/water at a temperature of 80 K and a magnetic field of 0.34 T. (1)H NMR signal enhancements up to 430 are observed, and the buildup of the local polarization occurs in a few hundred nanoseconds. Thus, NOVEL can efficiently dynamically polarize (1)H atoms in a system that is of general interest to the solid-state DNP NMR community. This is a first, important step toward the general application of pulsed DNP at higher fields.
Can, T.V., et al., Ramped-amplitude NOVEL. J. Chem. Phys., 2017. 146(15): p. 154204.
We present a pulsed dynamic nuclear polarization (DNP) study using a ramped-amplitude nuclear orientation via electron spin locking (RA-NOVEL) sequence that utilizes a fast arbitrary waveform generator (AWG) to modulate the microwave pulses together with samples doped with narrow-line radicals such as 1,3-bisdiphenylene-2-phenylallyl (BDPA), sulfonated-BDPA (SA-BDPA), and trityl- OX063. Similar to ramped-amplitude cross polarization in solid-state nuclear magnetic resonance, RA-NOVEL improves the DNP efficiency by a factor of up to 1.6 compared to constant-amplitude NOVEL (CA-NOVEL) but requires a longer mixing time. For example, at mix = 8 s, the DNP efficiency reaches a plateau at a ramp amplitude of 20 MHz for both SA-BDPA and trityl-OX063, regardless of the ramp profile (linear vs. tangent). At shorter mixing times (mix = 0.8 s), we found that the tangent ramp is superior to its linear counterpart and in both cases there exists an optimum ramp size and therefore ramp rate. Our results suggest that RA-NOVEL should be used instead of CA-NOVEL as long as the electronic spin lattice relaxation T1e is sufficiently long and/or the duty cycle of the microwave amplifier is not exceeded. To the best of our knowledge, this is the first example of a time domain DNP experiment that utilizes modulated microwave pulses. Our results also suggest that a precise modulation of the microwave pulses can play an important role in optimizing the efficiency of pulsed DNP experiments and an AWG is an elegant instrumental solution for this purpose.
Can, T.V., et al., Frequency-Swept Integrated Solid Effect. Angew Chem Int Ed Engl, 2017. 56(24): p. 6744-6748.
The efficiency of continuous wave dynamic nuclear polarization (DNP) experiments decreases at the high magnetic fields used in contemporary high-resolution NMR applications. To recover the expected signal enhancements from DNP, we explored time domain experiments such as NOVEL which matches the electron Rabi frequency to the nuclear Larmor frequency to mediate polarization transfer. However, satisfying this matching condition at high frequencies is technically demanding. As an alternative we report here frequency-swept integrated solid effect (FS-ISE) experiments that allow low power sweeps of the exciting microwave frequencies to constructively integrate the negative and positive polarizations of the solid effect, thereby producing a polarization efficiency comparable to (+/-10 % difference) NOVEL. Finally, the microwave frequency modulation results in field profiles that exhibit new features that we coin the “stretched” solid effect.
Can, T.V., et al., Time domain DNP with the NOVEL sequence. J Chem Phys, 2015. 143(5): p. 054201.
We present results of a pulsed dynamic nuclear polarization (DNP) study at 0.35 T (9.7 GHz/14.7 MHz for electron/(1)H Larmor frequency) using a lab frame-rotating frame cross polarization experiment that employs electron spin locking fields that match the (1)H nuclear Larmor frequency, the so called NOVEL (nuclear orientation via electron spin locking) condition. We apply the method to a series of DNP samples including a single crystal of diphenyl nitroxide (DPNO) doped benzophenone (BzP), 1,3-bisdiphenylene-2-phenylallyl (BDPA) doped polystyrene (PS), and sulfonated-BDPA (SA-BDPA) doped glycerol/water glassy matrices. The optimal Hartman-Hahn matching condition is achieved when the nutation frequency of the electron matches the Larmor frequency of the proton, omega1S = omega0I, together with possible higher order matching conditions at lower efficiencies. The magnetization transfer from electron to protons occurs on the time scale of approximately 100 ns, consistent with the electron-proton couplings on the order of 1-10 MHz in these samples. In a fully protonated single crystal DPNO/BzP, at 270 K, we obtained a maximum signal enhancement of epsilon = 165 and the corresponding gain in sensitivity of epsilonT1/TB (1/2)=230 due to the reduction in the buildup time under DNP. In a sample of partially deuterated PS doped with BDPA, we obtained an enhancement of 323 which is a factor of approximately 3.2 higher compared to the protonated version of the same sample and accounts for 49% of the theoretical limit. For the SA-BDPA doped glycerol/water glassy matrix at 80 K, the sample condition used in most applications of DNP in nuclear magnetic resonance, we also observed a significant enhancement. Our findings demonstrate that pulsed DNP via the NOVEL sequence is highly efficient and can potentially surpass continuous wave DNP mechanisms such as the solid effect and cross effect which scale unfavorably with increasing magnetic field. Furthermore, pulsed DNP is also a promising avenue for DNP at high temperature.