Ames Laboratory Accepted Manuscripts Ames Laboratory

6-4-2018 Enhancing the Sensitivity of Solid-State NMR Experiments with Very Low Gyromagnetic Ratio Nuclei with Fast Magic Angle Spinning and Proton Detection Amrit Venkatesh Iowa State University and Ames Laboratory, [email protected]

Matthew .J Ryan Iowa State University, [email protected]

Abhranil Biswas Iowa State University, [email protected]

Kasuni C. Boteju Iowa State University, [email protected]

Aaron D. Sadow Iowa State University and Ames Laboratory, [email protected]

See next page for additional authors Follow this and additional works at: https://lib.dr.iastate.edu/ameslab_manuscripts Part of the Physical Chemistry Commons, and the Radiochemistry Commons

Recommended Citation Venkatesh, Amrit; Ryan, Matthew J.; Biswas, Abhranil; Boteju, Kasuni C.; Sadow, Aaron D.; and Rossini, Aaron J., "Enhancing the Sensitivity of Solid-State NMR Experiments with Very Low Gyromagnetic Ratio Nuclei with Fast Magic Angle Spinning and Proton Detection" (2018). Ames Laboratory Accepted Manuscripts. 194. https://lib.dr.iastate.edu/ameslab_manuscripts/194

This Article is brought to you for free and open access by the Ames Laboratory at Iowa State University Digital Repository. It has been accepted for inclusion in Ames Laboratory Accepted Manuscripts by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Enhancing the Sensitivity of Solid-State NMR Experiments with Very Low Gyromagnetic Ratio Nuclei with Fast Magic Angle Spinning and Proton Detection

Abstract Many transition metals commonly encountered in inorganic materials and organometallic compounds possess NMR-active nuclei with very low gyromagnetic ratios (γ) such as 89Y, 103Rh, 109Ag, and 183W. A low-γ leads to poor NMR sensitivity and other experimental challenges. Consequently, nuclei with low-γ are often impossible to study with conventional solid-state NMR methods. Here, we combine fast magic angle spinning (MAS) and proton detection to enhance the sensitivity of solid-state NMR experiments with very low-γ nuclei by 1–2 orders of magnitude. Coherence transfer between 1H and low-γ nuclei was performed with low-power double quantum (DQ) or zero quantum (ZQ) cross-polarization (CP) or dipolar refocused insensitive nuclei enhanced by polarization transfer (D-RINEPT). Comparison of the absolute sensitivity of CP NMR experiments performed with proton detection with 1.3 mm rotors and direct detection with 4 mm rotors shows that proton detection with a 1.3 mm rotor provides a significant boost in absolute sensitivity, while requiring approximately 1/40th of the material required to fill a 4 mm rotor. Fast MAS and proton detection were applied to obtain 89Y and 103Rh solid-state NMR spectra of organometallic complexes. These results demonstrate that proton detection and fast MAS represents a general approach to enable and accelerate solid-state NMR experiments with very low-γ nuclei.

Disciplines Chemistry | Physical Chemistry | Radiochemistry

Authors Amrit Venkatesh, Matthew J. Ryan, Abhranil Biswas, Kasuni C. Boteju, Aaron D. Sadow, and Aaron J. Rossini

This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/ameslab_manuscripts/194 Subscriber access provided by Iowa State University | Library A: Spectroscopy, Molecular Structure, and Quantum Chemistry Enhancing the Sensitivity of Solid-State NMR Experiments with Very Low-Gyromagnetic Ratio Nuclei with Fast MAS and Proton Detection Amrit Venkatesh, Matthew J. Ryan, Abhranil Biswas, Kasuni C. Boteju, Aaron D. Sadow, and Aaron J Rossini J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05107 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

Just Accepted

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1 2 3 4 5 6 7 Enhancing the Sensitivity of SolidState NMR 8 9 10 11 Experiments with Very LowGyromagnetic Ratio 12 13 14 15 16 Nuclei with Fast MAS and Proton Detection 17 18 19 20 Amrit Venkatesh,1,2 Matthew J. Ryan,1 Abhranil Biswas,1 Kasuni C. Boteju,1 Aaron D. Sadow,1,2 Aaron J. 21 22 Rossini1,2* 23 24 25 1 26 Iowa State University, Department of Chemistry, Ames, IA, USA, 50011 27 28 2US DOE Ames Laboratory, Ames, Iowa, USA, 50011 29 30 31 32 33 34 35 36 37 38 39 AUTHOR INFORMATION 40 41 Corresponding Author 42 43 44 *email: [email protected], phone: 5152948952. 45 46 47 48 49 50 51 52 53 54 55 56 57 58 1 59 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 2 of 27

1 2 3 4 Abstract 5 6 7 8 Many transition metals commonly encountered in inorganic materials and organometallic 9 10 compounds possess NMRactive nuclei with very low gyromagnetic ratios (γ) such as 89Y, 103Rh, 11 12 109Ag and 183W. A lowγ leads to poor NMR sensitivity and other experimental challenges. 13 14 15 Consequently, nuclei with lowγ are often impossible to study with conventional solidstate 16 17 NMR methods. Here, we combine fast magic angle spinning (MAS) and proton detection to 18 19 20 enhance the sensitivity of solidstate NMR experiments with very lowγ nuclei by one to two 21 1 22 orders of magnitude. Coherence transfer between H and lowγ nuclei was performed with low 23 24 power double quantum (DQ) or zero quantum (ZQ) crosspolarization (CP) or dipolar refocused 25 26 27 insensitive nuclei enhanced by polarization transfer (DRINEPT). Comparison of the absolute 28 29 sensitivity of CP NMR experiments performed with proton detection with 1.3 mm rotors and 30 31 direction detection with 4 mm rotors shows that proton detection with a 1.3 mm rotor provides a 32 33 significant boost in absolute sensitivity, while requiring approximately 1/40th of the material 34 35 89 36 required to fill a 4 mm rotor. Fast MAS and proton detection were applied to obtain Y and 37 38 103Rh solidstate NMR spectra of organometallic complexes. These results demonstrate that 39 40 proton detection and fast MAS represents a general approach to enable and accelerate solidstate 41 42 43 NMR experiments with very lowγ nuclei. 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 2 59 60 ACS Paragon Plus Environment Page 3 of 27 The Journal of Physical Chemistry

1 2 3 Introduction 4 5 6 Solidstate NMR spectroscopy is a powerful tool for the study of inorganic materials 7 13 8 because it can potentially be applied to nearly all of the elements in the periodic table. 9 10 However, many elements have unfavorable properties for NMR spectroscopy such as low natural 11 12 abundance, large quadrupole moments (for quadrupolar nuclei with spin I > ½) and low 13 14 15 15 gyromagnetic ratios (γ and Larmor frequency below that of N). For example, the transition 16 17 metals silver, tungsten, yttrium and rhodium all possess highly abundant NMRactive isotopes 18 19 4 20 with very lowγ (Table S1). 21 22 Unfortunately, nuclei with a lowγ are often very difficult to observe and manipulate in 23 24 NMR experiments because γ determines the Larmor frequency (ν0 = γB0), the radiofrequency 25 26 27 (RF) field (ν1 = γB1), the equilibrium nuclear magnetization and the magnitude of the induced 28 29 NMR signal (NMR sensitivity ∝ γ3/2).5 Furthermore, NMR experiments with lowγ nuclei are 30 31 32 generally impeded by additional factors such as long longitudinal relaxation times (T1), acoustic 33 34 ringing, poor coherence transfer efficiency, etc. The sensitivity of solidstate NMR experiments 35 36 with lowγ nuclei has been improved by using cross polarization (CP)6 to transfer the 37 38 19 1 39 polarization from highγ nuclei such as F or H to the lowγ nucleus. For example, Merwin and 40 109 89 183 41 Sebald applied CP to enhance sensitivity and obtain direct detection Ag, Y and W CP 42 43 magic angle spinning (CPMAS) solidstate NMR spectra.79 However, CP experiments with low 44 45 γ nuclei often require extremely long contact times in excess of 30 ms that lead to very high 46 47 9 48 probe duty cycles. Subsequently, Levitt and coworkers demonstrated that the dipolar 49 50 recoupling sequence PRESTO may be used to transfer polarization from 19F to 109Ag in 51 52 10 1 19 AgSbF6. The advantage of PRESTO is that dipolar recoupling is only applied on the H or F 53 54 55 56 57 58 3 59 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 4 of 27

1 2 3 channel greatly reducing the probe duty cycle. However, the sensitivity of CP or PRESTO 4 5 6 experiments with lowγ nuclei are often poor because the lowγ nucleus is directly detected. 7 8 Proton detection can dramatically improve the sensitivity of NMR experiments with low 9 10 γ nuclei. Compared to an NMR spectrum obtained with excitation of the proton spins followed 11 12 13 by CP (or PRESTO) and direct detection of the lowγ nucleus, an NMR spectrum obtained with 14 15 initial excitation and detection of proton spins potentially provides a maximum gain in sensitivity 16 17 3/2 1/2 5, 11 (ξ) of ca. ξ ≈ (γ1H/ γX) ×(WX/W1H) , where W is the full width at half maximum. For 18 19 89 103 20 example, for the lowγ nuclei Y and Rh proton detection can potentially provide a gain in 21 22 sensitivity on the order of 92 to 176, respectively (considering only γ). However, efficient proton 23 24 1 25 detection in solidstate NMR requires narrowing of the H NMR signals (reduction of W1H), 26 11 27 which is most often achieved by using fast MAS frequencies (νrot) exceeding ca. 25 kHz. For 28 29 lowγ nuclei such as 15N it was shown that a sensitivity gain from proton detection (ξ) on the 30 31 1112 32 order of 3 was obtained with νrot = 28 kHz. Fast MAS and proton detection are now routinely 33 34 applied to indirectly detect common spin1/2 nuclei such as 13C, 15N and 29Si and the lowγ spin 35 36 14 1119 37 1 quadrupolar N nucleus. Notably, Carnevale et al. have recently demonstrated that CP 38 39 based pulse sequences can be used to indirectly detect 14N solidstate NMR spectra with 40 41 efficiency comparable to heteronuclear multiple quantum coherence spectroscopy (HMQC).20 42 43 44 We have recently shown that high sensitivity gains can be obtained by employing proton 45 46 detection to record wideline solidstate NMR spectra of heavy spin1/2 nuclei and halfinteger 47 48 quadrupolar nuclei.2122 We have also recently applied fast MAS and proton detection to observe 49 50 109Ag sites in silverbased ionic liquids for gas chromatography; however, the gains in sensitivity 51 52 23 53 from proton detection were not measured.. 54 55 56 57 58 4 59 60 ACS Paragon Plus Environment Page 5 of 27 The Journal of Physical Chemistry

1 2 3 Here we demonstrate that fast MAS and proton detection can routinely provide sensitivity 4 5 6 enhancements of one to two orders of magnitude for solidstate NMR experiments with very 7 89 103 109 183 8 lowγ spin1/2 nuclei such as Y, Rh, Ag and W. Absolute sensitivity comparisons of 9 10 solidstate NMR experiments with 4 mm and 1.3 mm rotors demonstrate that proton detection 11 12 13 with small diameter rotors provides a significant net gain in absolute sensitivity. Coherence 14 15 transfer between 1H and lowγ nuclei was performed with lowpower double quantum (DQ) or 16 17 zero quantum (ZQ) crosspolarization (CP) or dipolar refocused insensitive nuclei enhanced by 18 19 20 polarization transfer (DRINEPT). The limitations and advantages of these different methods are 21 22 demonstrated experimentally and with numerical simulations. Proton detection and fast MAS are 23 24 then applied to obtain 2D HETCOR solidstate NMR spectra of yttrium and rhodium 25 26 27 organometallic complexes. 28 29 30 31 Experimental 32 33 All fast MAS experiments were performed on a double resonance 1.3 mm HX probe, 34 35 1 36 with a Bruker Avance III HD spectrometer and a 9.4 T (ν0( H) = 400 MHz) widebore NMR 37 38 magnet. Shunt capacitors were inserted in parallel with the primary variable tuning capacitor of 39 40 41 Xchannel of the probe to tune to the required very low Larmor frequencies. The capacitor for 42 109 183 43 tuning the 1.3 mm HX probe to Ag/ W was provided courtesy of Mr. Albert Donkoh (Bruker 44 45 Biospin Inc., Billerica, MA). Additional capacitors were purchased from NMR Service Inc. and 46 47 the shunt capacitor inserts for the probe were made by soldering copper strips onto the capacitors 48 49 50 (Figure S1, Table S2). Experiments with large diameter rotors were performed on a 4 mm triple 51 52 resonance HXY probe configured in double resonance mode. The probe was tuned to 183W/109Ag 53 54 by adding capacitor inserts in parallel to the Xchannel variable tuning capacitor. 55 56 57 58 5 59 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 6 of 27

1 2 3 Fast MAS solidstate NMR experiments using the 1.3 mm HX probe were performed at a 4 5 1 6 50 kHz MAS frequency. The H T1 was measured with saturation recovery for all the samples 7 8 and optimum recycle delays of 1.3 × T1 were used for all experiments shown in the main text 9 10 except where noted otherwise. The pulse calibrations were performed directly on each sample 11 12 13 using a π/2 − spinlock pulse sequence (Figure S2) by determining the second order rotary 14 3 24 25 15 resonance recoupling (R ) condition (2 × νrot) and the HORROR (0.5 × νrot) conditions. The 16 17 calibrated 100 kHz RF power was utilized for all 1H π/2 and π pulses. In the 2D HETCOR NMR 18 19 1 20 experiments, unwanted background H magnetization was removed by saturation blocks 21 22 consisting of a π/2 pulse followed by a 300 – 500 s spinlock pulse at the HORROR condition 23 24 26 25 and a 10 – 25 ms dephasing delay (Figure S3). The saturation blocks were generally repeated 26 27 10 to 20 times. The HORROR condition was also applied for heteronuclear decoupling during 28 29 the t1evolution periods in the 2D HETCOR experiments and during acquisition in the direct 30 31 detected 1D experiments.25 Proton detected CPHETCOR NMR spectra were acquired with the 32 33 11 1 34 standard pulse sequence that uses forwards CP, zstorage, H saturation and backCP steps 35 36 (Figure S3A). Alternatively, DRINEPT pulse sequences22, 27 were also used for proton 37 38 detection, where the forwards and backwards CP steps were replaced with DRINEPT blocks 39 40 41 (Figure S3B). 42 1 43 The doublequantum and zeroquantum CP conditions (ν1( H) ± ν1(X) = n × νrot) were 44 45 optimized directly on the samples of interest by observing the intensity of the 1D 1H detected CP 46 47 1 48 signal as a function of H spinlock RF fields with a fixed RF field for the X spinlock pulse. 49 50 Unless specifically indicated, all CP experiments were performed under n = 1 conditions, the 51 52 precise CP conditions are listed in Table S3. All CP experiments employed ramped 1H spinlock 53 54 1 55 pulses with a 95%100% amplitude ramp on the forward ( H→X) CP blocks and a 100%95% 56 57 58 6 59 60 ACS Paragon Plus Environment Page 7 of 27 The Journal of Physical Chemistry

1 2 3 amplitude ramp on the backward (X→1H) CP blocks.28 Figure S4 shows a comparison between 4 5 1 109 6 experimental and SIMPSON simulations of the H Ag CPMAS optimizations. The lengths of 7 1 8 the X π/2 pulses were directly optimized on each sample using the H detected CP experiment 9 10 (Figure S3A). An example of this optimization is provided in Figure S4D and the optimized 11 12 13 pulse lengths and experimental conditions for all the samples are listed in Table S3. Other 14 15 parameters used to acquire and process the spectra are listed in Table S4. In the DRINEPT 16 17 experiments symmetry based SR42 recoupling29 was applied on the 1H channel at the optimized 18 1 19 20 100 kHz RF fields (2 × νrot). The duration of each recoupling block (m × τr in Figure S3, where τr 21 22 corresponds to one rotor period) was directly optimized on each sample to give maximum signal. 23 24 2 25 Proton T1ρ and T2’ during SR41 recoupling were measured using the pulse sequences shown in 26 27 28 Figure S2 and fit in MATLAB to the equation S(τ) = A×exp(–τ /T) where τ was the duration of 29 30 the spinlock pulses or recoupling pulse sequence, T is the T1ρ or T2’ time constants (Table S5) 31 32 1 103 33 and A is the fit signal intensity at τ = 0. The proton detected H Rh DRINEPT Jresolved 34 35 experiment was performed using the pulse sequence shown in Figure S3B. The simultaneous 36 37 application of π pulses on the 1H and 103Rh channels in the Jresolved block causes signal 38 39 103 1 40 dephasing due to evolution of Rh H scalar couplings. The experiment was performed in an 41 42 interleaved manner where for each τJ value, a control experiment was also performed with the 43 44 103 3031 45 application of only the Rh πpulse (Figure S5). A normalized Jdephasing curve free from 46 47 transverse relaxation was then constructed by dividing each Jdephased data point by the 48 49 corresponding control data point. The normalized Jdephasing curve was fit to a simple cosine 50 51 1 103 1 52 function, S(τJ) = cos (π × J × τJ), to determine J( Rh, H). Fitting was performed using the 53 54 curve fitting tool in MATLAB 8.6.0 (R2015b). 55 56 57 58 7 59 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 8 of 27

1 2 3 Solidstate NMR experiments using 4 mm rotors were performed at a MAS rate of 8 kHz. 4 5 1 6 The H pulses were calibrated by recording a nutation curves and determining the 2πpulse. The 7 8 RF on the Xchannel was initially determined by acquiring 109Ag spin echo NMR spectra using a 9 10 solution of 9 M AgNO3 in D2O. After optimization of CP conditions a more accurate pulse 11 12 13 calibration was performed directly using a CP – π/2flipback – π/2read pulse sequence (Table S6). 14 15 1H→183W and 1H→109Ag CPMAS experiments used 183W/109Ag spinlock pulses with ca. 12 16 17 kHz RF field (ca. 80 – 95 W of input power) on the 183W/109Ag channel and the matching 1H 18 19 20 spinlock pulse RF field was optimized for maximum signal (Table S6). Optimum CPMAS 21 22 signal was obtained with 1H spinlock RF fields of 20 kHz and 27 kHz for 183W and 109Ag NMR 23 24 experiments, respectively (Table S7) with a 95%100% amplitude ramp. These CP conditions 25 26 183 27 correspond to zeroquantum CP conditions (ν1,X + n*νrot = ν1,1H) with n = 1 for W and n = 2 28 29 for 109Ag. The n = 2 CP condition likely gave more signal for 109Ag CPMAS experiments on 30 31 silver methanesulfonate because the 1H T increases with higher 1H spinlock RF fields. High 32 1ρ 33 34 quality direct detected CP spectra for sensitivity comparisons were obtained using a CP spin 35 36 echo pulse sequence with a total spin echo 4 rotor cycles in duration to eliminate acoustic 37 38 ringing. All other acquisition and processing parameters were the same as the corresponding fast 39 40 41 MAS direct detected experiments (Table S4). 42 43 1H NMR chemical shifts were referenced to neat tetramethylsilane using adamantane 44 45 1 89 103 109 183 (δiso( H) = 1.82 ppm) as a secondary chemical shift standard. Y, Rh, Ag and W 46 47 48 chemical shifts were indirectly referenced using the previously published relative NMR 49 4 50 frequencies. The average of four signaltonoise ratio (SNR) measurements were considered for 51 52 calculating the sensitivities of all NMR spectra. All NMR spectra were processed in Bruker 53 54 Topspin 3.5 pl7. 55 56 57 58 8 59 60 ACS Paragon Plus Environment Page 9 of 27 The Journal of Physical Chemistry

1 2 3 DFT calculations were performed on compound 1 with planewave pseudopotentials and 4 5 32 6 periodic boundary conditions using CASTEP. The heavyatom positions were fixed from the 7 8 Xray crystal structure and the hydrogen atom positions were optimized prior to performing 9 10 NMR parameter calculations. All calculations used the PerdewBurkeErnzerhof (PBE) 11 12 generalized gradient approximation (PBEGGA) functional33 with the TkatchenkoScheffler (TS) 13 14 34 35 15 dispersion (DFTD) correction scheme and ultrasoft pseudopotentials generated on-the-fly. 16 17 P1 symmetry was imposed on the unit cell prior to performing NMR calculations. Magnetic 18 19 shielding, electric field gradient and Jcoupling tensors were calculated using the GIPAW 20 21 36 37 22 method implemented in CASTEP under the ZeroOrder Relativistic Approximation (ZORA). 23 24 38 Optimizations were converged to a kinetic energy cutoff of 630 eV for the planewave basis 25 26 set. The integrals were calculated over a Brillouin zone with a minimum kpoint spacing of 0.07 27 28 1 3941 29 Å . SIMPSON v4.1.1 was used to perform all numerical simulations. 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 9 59 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 10 of 27

1 2 3 Results and Discussion 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 183 30 Figure 1. W solidstate NMR spectra of ammonium tetrathiotungstate ((NH4)2WS4). 2D 31 1H{183W} HETCOR NMR spectra obtained with (A) DQCP, (B) ZQCP and (C) DRINEPT for 32 coherence transfers. (D) Comparison of 183W solidstate NMR spectra obtained with direct 33 183 34 detection of W using DQCP, ZQCP and DRINEPT methods and the positive projections 35 from the corresponding 2D HETCOR NMR spectra. The absolute sensitivity (S) and the gain in 36 sensitivity from proton detection (ξ) are indicated next to the NMR spectra. The total experiment 37 times are indicated for the 2D HETCOR and the 1D direct detected 183W NMR spectra. 38 183 39 First, W NMR is used to demonstrate the potential of proton detection and fast MAS 40 41 183 (νrot = 50 kHz in all cases) to accelerate solidstate NMR experiments with lowγ nuclei. W 42 43 183 44 has a natural isotopic abundance (NA) of 14.3% and ν0( W) = 16.7 MHz at a magnetic field of 45 1 46 9.4 T (ν0( H) = 400 MHz). Ammonium tetrathiotungstate ((NH4)2WS4) was chosen as a setup 47 48 183 49 compound for W solidstate NMR because its chemical shift and CP characteristics have been 50 8 51 previously reported. The total experiment times, number of scans, signaltonoise ratio (SNR) 52 53 and sensitivity (S = SNR x t–1/2, where t is the total experiment time) for all direct detection and 54 55 proton detection experiments are compared in Table 1. The recycle delays in both proton 56 57 58 10 59 60 ACS Paragon Plus Environment Page 11 of 27 The Journal of Physical Chemistry

1 2 3 detected and direct detected experiments are determined by the proton T1 (T1 = 14.4 s for 4 5 6 (NH4)2WS4). In all experiments the recycle delay was set to 1.3×T1 to maximize sensitivity (the 7 8 recycle delay was 18.7 s for (NH4)2WS4, Table 1). 9 10 11 Prior to discussing the NMR results, we briefly describe the CP conditions accessible 12 13 with fast MAS. Under MAS, the RF fields used for CP obey the modified HartmannHahn match 14 15 conditions.4246 With fast MAS it is possible to use lowpower doublequantum (DQCP) and 16 17 4649 1 18 zeroquantum (ZQCP) conditions. For DQCP, ν1( H) + ν1(X) = n × νrot, while for ZQCP, 19 1 183 20 ν1( H) – ν1(X) = n × νrot (with n = 1 for both conditions). For all of the W CP NMR 21 22 experiments, ν (183W) ≈ 15 kHz and ν = 50 kHz, therefore, ν (1H) ≈ 65 kHz for ZQCP and 23 1 rot 1 24 1 3941 1 109 25 ν1( H) ≈ 35 kHz for DQCP. Numerical simulations using SIMPSON on a test H Ag two 26 27 spin system confirm that both conditions operate at similar efficiencies in the absence of 28 29 1 30 relaxation (Figure S4). With the 1.3 mm double resonance probe used here, the H CP spinlock 31 32 pulse power required input powers of only 0.4 W (35 kHz) and 1.3 W (65 kHz) for DQCP and 33 34 183 ZQCP, respectively. Only 8.0 W of input power was required to obtain a ν1( W) of 15 kHz. 35 36 37 Therefore, both DQCP and ZQCP conditions can be accessed with low input powers and low 38 39 probe duty. The low probe duty enables long CP contact times in excess of 20 ms that are 40 41 required for efficient CP, without risking damage to the probe or preamplifiers. The CP 42 43 conditions used for all the experiments are provided in the SI (Table S3). 44 45 1 46 The choice of whether to use DQCP or ZQCP is dictated by the H longitudinal 47 1 48 relaxation times in the rotating frame (T1ρ). A H spinlock experiment can be used to measure 49 50 1 51 the proton T1ρ at the H spinlock RF fields required for the two CP conditions (Figure S2). 52 1 53 Whichever H spinlock RF field gives the longest T1ρ will generally give the best CP efficiency. 54 55 1 56 The H T1ρ of ((NH4)2WS4) was 200 ms and 275 ms at spinlock RF fields of 35 kHz and 64 57 58 11 59 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 12 of 27

1 2 3 kHz, respectively (Table S3) and accordingly the ZQCP condition was slightly more efficient 4 5 6 than DQCP. The CP contact time was directly optimized on all of the samples. For (NH4)2WS4 7 8 the CP signal continuously increased as the CP contact time for each step was increased up to 30 9 10 ms (Figure S6). However, the CP contact time of the forwards and backwards steps were limited 11 12 to 30 ms (60 ms total contact time) to avoid damaging the probe or preamplifiers. 13 14 15 8 hours of signal averaging (1536 scans) were required to obtain the direct detection 16 1 183 17 H→ W DQCP and ZQCP NMR spectra of (NH4)2WS4 which gave a SNR of only 3.4 and 18 19 4.0, corresponding to a S of 0.2 min–1/2 for both experiments (Figure 1D). The isotropic chemical 20 21 183 8 22 shift of the W resonance is 3648 ppm, in agreement with the previously reported value. On 23 24 the other hand, the 1D proton detected DQCP and ZQCP 1H{183W} NMR spectra gave a SNR 25 26 of 97 and 92 after only 10 minutes of signal averaging (32 scans), corresponding to a S of 31 27 28 1/2 1/2 29 min and 29 min (Table 1, Figure S7). Comparison of the sensitivities of the 1D proton 30 183 31 detected and W detected DQCP NMR spectra shows that the gain in sensitivity due to proton 32 33 detection (ξ) is ca. 155 (31 min1/2/0.2 min–1/2). The experimentally observed ξ of 155 is greater 34 35 183 1 3/2 36 than the expected gain for W and H considering only the γ of each nucleus (ξ ≈ (γ1H/ γ183W) 37 38 = 118). Note that the 1H and 183W linewidths (W) are comparable, therefore, the 1H channel of 39 40 the probe is likely more efficient than the 183W channel of the probe, which typically further 41 42 11 183 43 favors proton detection. In order to obtain a proton detected W solidstate NMR spectrum, 44 45 full 2D 1H{183W} DQCP and ZQCP heteronuclear correlation (HETCOR) NMR spectra were 46 47 obtained (Figure 1A and 1B). The 2D CP HETCOR spectra required a total experiment time of 48 49 183 50 only 2.7 hours each. The W NMR spectrum obtained from the positive projections of the 51 52 indirect dimension of the ZQCP HETCOR spectrum had a SNR of 414 which correspond to S of 53 54 33 min–1/2 and ξ = 165. This large gain in sensitivity corresponds to a factor 27225 (ξ2) 55 56 57 58 12 59 60 ACS Paragon Plus Environment Page 13 of 27 The Journal of Physical Chemistry

1 2 3 reduction in total experiment time as compared to the corresponding direct detected 1H→183W 4 5 6 CPMAS spectra. This result demonstrates the value of the proton detected solidstate NMR 7 8 experiments for very lowγ nuclei. 9 10 11 12 183 13 Table 1. Comparison of sensitivities of proton detected and direct detected W experiments 14 1 15 Method NS Recycle Experiment H Sensitivity X Sensitivity 16 Delay Time (min) SNR SNR•t1/2 SNR SNR•t1/2 17 (s) (min1) (min1/2) 18 Direct Detection 183W NMR Spectra 19 1D DQCP 1536 18.7 478.7 3.4 0.2 20 1D ZQCP 1536 18.7 478.7 4.0 0.2 21 1D DRINEPT 2048 18.7 638.3 3.4 0.1 22 Proton Detected 183W NMR Spectra 23 24 1D DQCP 32 18.7 10.0 97 31 25 1D ZQCP 32 18.7 10.0 92 29 1D DRINEPT 64 18.7 20.0 53 12 26 27 2D DQCP 2×256 18.7 159.6 337 27 414 33 28 2D ZQCP 2×256 18.7 159.6 341 27 409 32 29 2D DRINEPT 2×256 18.7 159.6 134 11 166 13 1 30 H Spin Echo 4 18.1 1.2 4289 3915 31 32 33 The DRINEPT pulse sequence22, 27 (Figure S3) was also utilized to record a 1D 34 35 1H→183W direct detected and 1H{183W} 2D HETCOR NMR spectra as shown in Figure 1D and 36 37 38 1C. The DRINEPT pulse sequence places a very low power demand on the probe and pre 39 40 amplifiers because the 183W channel only requires rotorsynchronized π/2 and π pulses, while 41

42 2 29 1 43 lowpower symmetry based SR41 recoupling is applied to the H channel with an RF field of 44 1 45 two times the MAS frequency (ν1( H) = 2 × νrot) (Figure S3). The sensitivity of the direct 46 47 1 183 1/2 48 detected H→ W DRINEPT experiment was 0.1 min which is about half the sensitivity of 49 50 the direct detected CP experiments (Figure 1, Table 1), suggesting that the efficiency of the D 51 52 RINEPT experiment is about half that of CP. A full 2D HETCOR spectrum was obtained with 53 54 DRINEPT in 2.7 hours (Figure 1C, Table 1), yielding a 183W sensitivity of 13 min1/2 on the 55 56 57 58 13 59 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 14 of 27

1 2 3 positive projection of the indirect 183W dimension. This corresponds to ξ of 130, which is 4 5 6 slightly lower than the ξ of ca. 160 obtained with CP. This reduction in ξ is likely due to low 7 8 intrinsic efficiency of the DRINEPT polarization transfer process which is limited by the short 9

10 1 2 1 11 T2’ of the H spins under SR41 dipolar recoupling (Table S5). In comparison, the H 12 13 T1ρ relaxation times that dictate the efficiency of CP are typically at least an order of magnitude 14 15 16 longer (Figure S2, Table S5). However, DRINEPT provides a larger excitation bandwidth than 17 18 CP because the finite π/2 and π pulses have a much greater bandwidth than the spinlock pulses. 19 20 With the typical RF fields we have used (Table S3), the CP excitation bandwidth is about 8 kHz 21 22 23 while the DRINEPT provides a larger excitation bandwidth of 24 kHz (Figure S8). Many low 24 25 γ transition metal nuclei display large chemical shift ranges of several thousand ppm. Therefore, 26 27 1D proton detected DRINEPT could be useful to quickly scan across the chemical shift range 28 29 30 and locate the signal of the lowγ nucleus, followed by a CPbased experiment to obtain a higher 31 32 quality 2D NMR spectrum. We note an alternative CP condition would be to use ν1(X) = 35 kHz 33 34 1 35 and match this to ν1( H) = 85 kHz. Simulations suggest that this alternative condition offers 36 37 improved CP excitation bandwidth of ca. 16 kHz (Figure S8). 38 39 109Ag (NA = 48.2%) and 89Y (NA = 100%) solidstate NMR experiments were performed 40 41 42 on silver methanesulfonate (Ag(SO3CH3)) and yttrium nitrate hexahydrate (Y(NO)3•6H2O), 43 44 respectively, to demonstrate the general applicability of proton detection and fast MAS for very 45 46 lowγ nuclei. Both of these compounds are used as CP setup standards for these nuclei.7, 9, 50 47 48 49 These results are summarized in Figure 2, Figure S9S10 and Table S8. We have very recently 50 51 applied proton detection and fast MAS to obtain 109Ag solidstate NMR spectra of several silver 52 53 based ionic liquids used as stationary phases in gas chromatography.23 54 55 56 57 58 14 59 60 ACS Paragon Plus Environment Page 15 of 27 The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 109 23 Figure 2. Comparison of proton detected and direct detected (A) Ag solidstate NMR spectra 89 24 of silver methanesulfonate (AgSO3CH3) and (B) Y solidstate NMR spectra of yttrium nitrate 25 hexahydrate (Y(NO3)3•6H2O). The CP condition/pulse sequence used for acquisition, sensitivity 26 (S), total experiment time and gain in sensitivity from proton detection (ξ) are indicated. The 27 proton detected NMR spectra were obtained from the positive projections of the corresponding 28 29 2D HETCOR spectrum. 30 31 32 33 A ξ of ca. 19 to 22 was obtained for proton detected 109Ag solidstate NMR experiments 34 35 36 on Ag(SO3CH3) (Figure 2A). Proton detected DRINEPT performed significantly worse than the 37 38 1 2 89 CP experiments, likely due to a short H T2’ during SR4 recoupling (see Table S5). For Y 39 1 40 41 solidstate NMR experiments on Y(NO)3•6H2O proton detection with CP provided a ξ of 5 to 7. 42 43 1 The relatively small ξ values for Y(NO)3•6H2O likely occur because this sample had a short H 44 45 1 1 46 T1ρ (7 ms and 13 ms for DQCP and ZQCP H RF fields, respectively). The short H T1ρ likely 47 48 occurs due to the dynamics of the coordinated water molecules and/or the very large 1H 49 50 homonuclear dipolar couplings for water. For yttrium, the ZQCP condition was preferred 51 52 1 1 53 because the H T1ρ was slightly longer at the higher power H spinlock RF field used for ZQ 54 55 CP. ZQCP and DQCP required optimal contact times of 16 ms and 10 ms, respectively, which 56 57 58 15 59 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 16 of 27

1 2 3 1 are much shorter than was required for the other samples and reflect the short H T1ρ (Figure S5). 4 5 1 2 6 The H T2’ during SR41 recoupling was also under 1 ms, which causes a rapid loss of 7 8 magnetization during the DRINEPT transfer. Consequently, it was not possible to obtain proton 9 10 1 89 89 11 or direct detected H Y DRINEPT NMR spectra in a reasonable experiment time. The Y 12 13 1 2 NMR experiments highlight that H T1ρ and T2’ during SR41 recoupling are likely to determine 14 15 1 16 the efficiency of proton detection. However, the H T1ρ at different spinlock RF fields and T2’ 17 18 during SR42 recoupling can usually be rapidly measured before attempting any CP or D 19 1 20 21 RINEPT experiments and the spectroscopist can decide which CP condition or pulse sequence 22 23 will likely work the best. 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 Figure 3. Comparison of the absolute sensitivity of proton detection with small diameter rotors 46 183 and direct detection with large diameter rotors for (left) W NMR experiments on NH4WS4 and 47 109 48 (right) Ag NMR experiments on Ag(SO3CH3). Direct detected CPMAS NMR spectra obtained 49 with (A, B) a 4 mm rotor and νrot = 8 kHz or (C, D) a 1.3 mm rotor and νrot = 50 kHz. (E, F) 50 Proton detected spectra obtained from the positive projections of 2D HETCOR NMR spectra. 51 The gain in absolute sensitivity (ξA) is indicated. Experiment times, SNR and sensitivities are 52 summarized in Tables S7 and S8. 53 54 55 56 57 58 16 59 60 ACS Paragon Plus Environment Page 17 of 27 The Journal of Physical Chemistry

1 2 3 Absolute Sensitivity Comparison of Large and Small Diameter Rotors. The results 4 5 6 described above demonstrate that proton detection provides a substantial gain in sensitivity 7 8 compared to direct detection within the 1.3 mm rotor. However, direct detection CPMAS 9 10 experiments would most likely be performed with larger diameter rotors because they can 11 12 potentially provide better absolute sensitivity by virtue of a larger sample volume. Nishiyama has 13 14 15 presented an extensive theoretical and experimental investigation of absolute NMR sensitivity 16 17 for different rotor diameters and shown that absolute sensitivity is approximately proportional to 18 19 d3/2, where d is the rotor diameter.18 For example, comparing a 1.3 mm and 4 mm rotor, the 1.3 20 21 18 22 mm rotor should provide ca. 5fold lower sensitivity because of reduced sample volume. 23 24 However, proton detection can compensate for this sensitivity reduction, and provide a net gain 25 26 in sensitivity. 27 28 29 Figure 3 compares the sensitivities of direct detected CPMAS NMR spectra of NH4WS4 30 31 and Ag(SO3CH3) obtained with 4 mm diameter rotors (ca. 100 L sample volume) with the 32 33 direct detected and proton detected spectra acquired using 1.3 mm diameter rotors (ca. 2.5 L 34 35 36 sample volume). The spectra shown in Figure 3 were acquired with experimental conditions that 37 38 yielded maximum sensitivities for the different rotors (Tables S6 and S7). Comparing the direct 39 40 detected CPMAS spectra shows that higher sensitivities of 4 min1/2 and 41 min1/2 are obtained 41 42 1/2 1/2 43 with the 4 mm rotors compared to the lower sensitivities of 0.2 min and 6 min obtained 44 45 with the 1.3 mm rotors for NH4WS4 and Ag(SO3CH3), respectively. Because of the greater 46 47 amount of material in the 4 mm rotor the direct detection CPMAS experiments with 4 mm rotors 48 49 50 provide 7 to 20 times better sensitivity compared to the corresponding experiments with 1.3 mm 51 1/2 1/2 52 rotors. However, with proton detected experiments sensitivities of 33 min and 116 min 53 54 were obtained for 183W and 109Ag using 1.3 mm rotors. Comparison of proton detected 1.3 mm 55 56 57 58 17 59 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 18 of 27

1 2 3 rotor sensitivity and direct detected 4 mm rotor sensitivity shows that proton detection with a 4 5 183 6 small rotor provides significant gains in absolute sensitivity (ξA) of factors 8 and 3 for W and 7 8 109Ag NMR spectra, respectively. In summary, proton detection with 1.3 mm rotors provides 9 10 superior absolute sensitivity for these very lowγ nuclei and requires only 1/40th of the sample as 11 12 13 compared to the 4 mm rotor. Addtionally, CP experiments with the 1.3 mm diameter rotor 14 15 require much lower powers (ca. 8 to 12 W lowγ spinlock pulse power) as compared to the 4 16 17 18 mm rotor (ca. 80 – 100 W lowγ spinlock pulse power). Therefore, with the 1.3 mm rotor, very 19 20 long CP contact times can be used without risking damage to the probe/spectrometer. 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 1 103 41 Figure 4. H and Rh solidstate NMR spectra of an organometallic rhodium complex (1). 42 Proton detected (A) 2D 1H{103Rh} DQCP spectrum and (B) 2D 1H{103Rh} DRINEPT 43 spectrum. (C) Structure of the organometallic rhodium complex. (D) Plot showing the results of 44 a Jresolved experiment on 1. The fit of the Jresolved curve yields 1J(103Rh,1H) = 19 Hz. (Figure 45 1 46 S5). The hydride directly attached to rhodium appears at 13.5 ppm in the 1D H spin echo and 47 has a low relative signal intensity (marked by asterisk).

48 49 50 Solid-State NMR of Organometallic Compounds. Proton detection was applied to obtain 51 52 the 103Rh solidstate NMR spectrum of a previously reported rhodium complex51 formed in an 53 54 55 PhSiH3 oxidative addition reaction (denoted as compound 1, Figure 4) which is a precursor to a 56 57 58 18 59 60 ACS Paragon Plus Environment Page 19 of 27 The Journal of Physical Chemistry

1 2 3 catalyst for partial deoxygenation of esters.52 Although 103Rh is 100% naturally abundant it has a 4 5 6 very low Larmor frequency of only 12.8 MHz at 9.4 T (Table S1). To the best of our knowledge, 7 103 53 8 there has been only one previous report of Rh solidstate NMR spectra, most likely because 9 10 of the challenges associated with the lowγ of 103Rh. 11 12 1 103 13 Figure 4 shows the H spin echo NMR spectrum and proton detected Rh solidstate 14 1 15 NMR spectra of 1 that were obtained with 2D DQCP and DRINEPT. The H spin echo 16 17 spectrum shows a low intensity signal at –13.5 ppm which is assigned to the hydride and much 18 19 103 20 more intense NMR signals from methyl groups and aromatic protons. The Rh NMR spectrum 21 103 22 obtained with proton detected DRINEPT gave a Rh positive projection spectrum that had a 23 24 SNR of ca. 56 after 3.6 hours of total acquisition. This corresponds to a 103Rh sensitivity of 3.8 25 26 min1/2. The hydride appearing at ca. –13.5 ppm facilitates the polarization transfer between 1H 27 28 103 1 29 and Rh due to a relatively strong heteronuclear dipolar coupling of ca. 930 Hz for a 1.6 Å H 30 31 103Rh distance. The lower sensitivity of the DQCP spectrum in comparison to the DRINEPT 32 33 spectrum is likely due to the short 1H T (Table S5) and because 1H spin diffusion during the 34 1ρ 35 1 1 36 spinlock pulse distributes H signal intensity across the spectrum. The H spin diffusion is 37 38 evidenced by the appearance of a crosspeak in the DQCP spectrum between the distant methyl 39 40 groups and the rhodium signal. With DRINEPT the coherence transfer is more selective because 41 42 1 43 homonuclear H dipolar couplings are suppressed and only a crosspeak between rhodium and 44 45 the hydride 1H is observed. 46 47 The 103Rh NMR spectrum of 1 shows two distinct peaks with 103Rh isotropic chemical 48 49 50 shifts of –8300 ppm and –8375 ppm. Initially, we hypothesized that the peak splitting was due 51 14 52 to scalar and residual dipolar coupling to N. However, SIMPSON simulations suggest that the 53 54 residual dipolar couplings to 14N only results in peak broadening on the order of 2 ppm (Figure 55 56 57 58 19 59 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 20 of 27

1 2 3 S11). Therefore, we believe that the two distinct 103Rh isotropic chemical shifts observed are due 4 5 1 103 6 to different solid forms (polymorphs or solvates). A proton detected H Rh Jresolved 7 8 experiment was performed using the pulse sequence shown in Figure S3B. This measurement 9 10 yielded a rhodiumhydrogen onebond scalar coupling (1J(103Rh,1H)) of 19 Hz (Figure S5). The 11 12 1J(1H103Rh) measured in the solidstate is comparable to the previously reported coupling of 13 14 1 51 15 21.3 Hz measured with solution H NMR. Scalar coupling constants predicted with planewave 16 17 DFT implemented in CASTEP32 are shown in Table S9. 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 Figure 5. 1H and 89Y solidstate NMR spectra of compound 2. (A) Molecular structure of 46 compound 2. (B) 1D 1H spin echo spectrum and proton detected 1D 1H{89Y} DQCP spectrum. 47 1 89 48 Proton detected 2D H{ Y} DQCP spectra with (C) 9 ms forward and backward CP contact 49 times and (D) 9 ms forward and 1 ms backward CP contact times. (E) Comparison of direct 50 detected 1D 1H→89Y DQCP spectrum with positive projections of 89Y from the 2D spectra. 51 89 52 Sensitivities (S) are indicated for the Y spectra shown in (E). The total experiment times are 53 indicated for the direct detected 1H→89Y DQCP spectrum and the proton detected 2D 1H{89Y} 54 DQCP spectra. All 1H89Y experiments were performed with a 280 K temperature setting on the 55 56 variable temperature unit. 57 58 20 59 60 ACS Paragon Plus Environment Page 21 of 27 The Journal of Physical Chemistry

1 2 3 4 5 89 6 Proton detection was also applied to obtain the Y solidstate NMR spectrum of 7 8 Y{N(SiHMe2)tBu}3 (denoted as compound 2). 2 has been used for solutionphase grafting of 9 10 yttrium sites onto silica for heterogeneous hydroamination reactions, and 2 also has potential 11 12 applications in chemical vapor deposition.5455 Notably, highquality 2D proton detected 1H{89Y} 13 14 15 NMR spectra of 2 were obtained in less than 2 hours (Figure 5). Comparing the sensitivities, a 16 17 direct detected 1D DQCP spectrum had a S of 4 min1/2 whereas the positive projection from the 18 19 proton detected 2D HETCOR spectrum had a S of 52 min1/2 corresponding to a ξ of 13 (Figures 20 21 1 22 5C and 5E). As the H T1 was short for this compound (ca. 0.86 s), the proton detected 2D 23 24 experiments were acquired with a longer recycle delay of 1.5 s to reduce the probe duty. Note 25 26 that using a shorter recycle delay would have yielded a higher gain in sensitivity due to proton 27 28 89 1 29 detection. A second 2D spectrum was obtained with a short Y→ H backward CP contact time 30 31 to minimize 1H spin diffusion during the spinlock pulse and to enhance the intensity of 32 33 correlations to more strongly dipolar coupled 1H nuclei (Figure 5D). Although a reasonable 2D 34 35 89 36 spectrum was obtainable with a short back CP contact time, the sensitivity of the Y positive 37 1/2 1/2 38 projections dropped from 52 min to 8 min when the contact time of the back CP step was 39 40 decreased from 9 ms to 1 ms (Figure 5E). The intense correlations between the SiH and yttrium 41 42 43 peaks are consistent with a relatively short internuclear distance of ca. 2.5 Å that results from 44 55 45 secondary bonding interactions between these hydrogen atoms and yttrium. 46 47 48 49 Conclusions 50 51 89 109 183 52 In summary, fast MAS and proton detection were used to accelerate Y, Ag, W and 53 54 103Rh solidstate NMR spectra experiments. In the samples studied here, the gain in sensitivity 55 56 57 58 21 59 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 22 of 27

1 2 3 provided by proton detection (ξ) was found to be on the order of 5 – 165 which correspond to 4 5 6 timesavings of a factor 25 – 27225. The absolute sensitivity of proton detected NMR spectra 7 8 acquired with 1.3 mm rotors was found to be superior to those obtained with direct detection and 9 10 4 mm rotors. For 183W and 109Ag NMR spectra, proton detection with a 1.3 mm rotor was found 11 12 13 to provide factors 8 and 3 times better sensitivity than direct detection with a 4.0 mm rotor that 14 1 15 holds ca. 40 times more sample. Furthermore, the high resolution in the H dimension of 2D 16 17 HETCOR NMR spectra is easily obtained without the use of homonuclear decoupling sequences. 18 19 20 The enormous sensitivity gains and timesavings provided by proton detection should permit the 21 22 routine characterization of materials containing these elements by solidstate NMR spectroscopy, 23 24 even in heterogeneous catalysts or inorganic materials where the elements may be very dilute. 25 26 Dynamic nuclear polarization (DNP) is an alternative approach to enhance the sensitivity of 27 28 5558 29 direct detection solidstate NMR experiments with very lowγ nuclei. With the development 30 31 of fast MAS DNP probes5960 we anticipate that proton detection and fast MAS could be 32 33 combined with DNP to obtain further improvements in the sensitivity of solidstate NMR 34 35 36 experiments with very lowγ nuclei. 37 38 39 40 41 Acknowledgements 42 43 AV and AJR were supported by the U.S. Department of Energy (DOE), Office of Science, Basic 44 Energy Sciences, Materials Science and Engineering Division. KB, AB, and ADS were 45 supported by U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, 46 47 Chemical Sciences, Geosciences, and Biosciences. The Ames Laboratory is operated for the U.S. 48 DOE by Iowa State University under contract # DEAC0207CH11358. AJR thanks Iowa State 49 University and the Ames Laboratory (Royalty Account) for additional support. We are grateful 50 51 to Mr. Albert Donkoh (Bruker Biospin Inc., Billerica, MA) for providing a capacitor for tuning 52 the 1.3 mm probe to 109Ag/183W. 53 54 Supporting Information 55 56 57 58 22 59 60 ACS Paragon Plus Environment Page 23 of 27 The Journal of Physical Chemistry

1 2 3 Additional supplementary tables, figures, and NMR experiment details can be found in the 4 5 supporting information. 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 23 59 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 24 of 27

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