Lixin
Liang
ab,
Yi
Ji
ab,
Zhenchao
Zhao
a,
Caitlin M.
Quinn
c,
Xiuwen
Han
a,
Xinhe
Bao
a,
Tatyana
Polenova
c and
Guangjin
Hou
*a
aState Key Laboratory of Catalysis, National Laboratory for Clean Energy, 2011-Collaborative Innovation Center of Chemistry for Energy Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China. E-mail: ghou@dicp.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
cDepartment of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, USA
First published on 20th July 2021
Heteronuclear dipolar coupling is indispensable in revealing vital information related to the molecular structure and dynamics, as well as intermolecular interactions in various solid materials. Although numerous approaches have been developed to selectively reintroduce heteronuclear dipolar coupling under MAS, most of them lack universality and can only be applied to limited spin systems. Herein, we introduce a new and robust technique dubbed phase modulated rotary resonance (PMRR) for reintroducing heteronuclear dipolar couplings while suppressing all other interactions under a broad range of MAS conditions. The standard PMRR requires the radiofrequency (RF) field strength of only twice the MAS frequency, can efficiently recouple the dipolar couplings with a large scaling factor of 0.50, and is robust to experimental imperfections. Moreover, the adjustable window modification of PMRR, dubbed wPMRR, can improve its performance remarkably, making it well suited for the accurate determination of dipolar couplings in various spin systems. The robust performance of such pulse sequences has been verified theoretically and experimentally via model compounds, at different MAS frequencies. The application of the PMRR technique was demonstrated on the H-ZSM-5 zeolite, where the interaction between the Brønsted acidic hydroxyl groups of H-ZSM-5 and the absorbed trimethylphosphine oxide (TMPO) were probed, revealing the detailed configuration of super acid sites.
Rotary resonance recoupling (R3) was first discovered by Oas and Levitt in 1988, and it occurs when the nutation frequency (ν1) of the applied RF field matches the MAS frequency (νr) at ν1 = nνr, (n = 1 or 2).12,15 Under this condition, the heteronuclear dipolar interaction averaged by the MAS rotation can be restored, whereas the homonuclear dipolar interaction is reintroduced simultaneously for n = 1, or suppressed for n = 2. Although the R3 method is easy to implement experimentally, it suffers from the lack of strict selectivity for recoupling and high sensitivity to rf mismatch or inhomogeneity,12 limiting its practical applications. Rotational echo double resonance (REDOR) developed by Gullion and Schaefer is another widely used technique for measuring heteronuclear dipolar couplings;13,16 therein the utilization of phase-cycled π pulses greatly improved the robustness to experimental imperfections.17–22 However, the homonuclear dipolar interactions are partially reintroduced during REDOR irradiation, which causes the loss of accuracy in heteronuclear dipolar coupling measurements.23 To obtain precise heteronuclear dipolar recoupling with more restrictive selections, the R-symmetry concept has been applied for the design of pulse sequences suited for H–X dipolar measurements in fully protonated systems.14,24–27R-Symmetry sequences can be used for the accurate determination of heteronuclear dipolar couplings in a wide range of RF fields and MAS frequencies. While more versatile than most other approaches, these sequences suffer from the generally low scaling factor, interference from the chemical shift anisotropy (CSA) interaction and sensitivity to RF mismatch.27 Some improvements have been made in recent years to the basic R-symmetry dipolar recoupling, such as the phase alternating R-symmetry (PARS) scheme28 and windowed PARS with composite pulses of ,29 but the complexity of these recoupling sequences limits their widespread applications. Alternatively, the phase cycled R421 symmetry sequence, dubbed SR4,30–32 was introduced for heteronuclear dipolar determination, where interference of CSA and dipolar truncation can be avoided. According to symmetry theory, the homonuclear dipolar and chemical shift interactions are also symmetry-allowed by R421, but suppressed efficiently by the utilization of [R421R4−21] phase cycling. SR4 with relatively low RF requirement of ν1 = 2νr is particularly suitable for fast MAS conditions. However, the low RF requirement hampers its robustness to resonance offset and homonuclear dipolar interactions, which becomes more pronounced under slow-to-moderate MAS conditions.
Here we present a new robust heteronuclear recoupling sequence, dubbed phase-modulated rotary resonance (PMRR). The PMRR recoupling sequence with an adjustable RF field strength of ≥2νr allows for accurate measurement of dipolar coupling in a broad range of spinning frequencies from slow to ultrafast MAS. Benefiting from the original R3 at n = 2, the PMRR method has several advantages: (i) high recoupling efficiency with a scaling factor ≥0.50, (ii) efficient suppression of homonuclear dipolar interactions, and (iii) straightforward set up. More importantly, the introduction of phase modulation significantly improves the robustness to RF mismatch (or inhomogeneity), interference from CSA and resonance offset. With window modification on the PMRR scheme, the scaling factor can be further improved from 0.50 to over 0.60, and the robustness to the undesired interferences can be further boosted. The performance of heteronuclear (H–X or X–Y) dipolar recoupling by PMRR sequences has been evaluated numerically and experimentally in U–13C, 15N–fMLF and U–13C, 15N–histidine·H2O·HCl at different MAS frequencies. An application by proton-detected PMRR is then demonstrated in a practical zeolite sample to measure the inter-atomic distance between the 31P of adsorbed TMPO and 1H of Brønsted acids in the zeolite.
1D 31P NMR spectra were acquired by 1H–31P CP with 5.0 ms contact time at a MAS frequency of 20 kHz. In 2D 31P–1H HETCOR spectra, 128t1 increments in the indirect dimension (F1) were recorded with 16 scans and a recycle delay of 2 s. For the 3D 31P/1H–31P/1H correlation NMR experiment, the 1H 90° pulse length was 2.82 μs, and the first and second 1H–31P CP conditions are the same with experimentally optimized RF fields matching the first-order Hartmann–Hahn conditions. 1H SPINAL decoupling with a RF field strength of 88.7 kHz was applied during the t1 dimension. The RF amplitude of windowed PMRR-REDOR (fw = 0.55) recoupling was 88.7 kHz.
General 2D PMRR-based pulse sequences for heteronuclear dipolar measurements are shown in Fig. 1b and c, corresponding to either the conventional REDOR13 or dipolar and chemical shift (DIPSHIFT)36 schemes, respectively. PMRR-REDOR and PMRR-DIPSHIFT sequences recouple identical zero-quantum dipolar terms.37 The spin echoes produced by the simultaneous π pulses of the PMRR-REDOR scheme in Fig. 1b refocus the chemical shift terms for both I and S spins, making the method insensitive to CSA and resonance offset. We note that both spins are suitable for observation in this experiment.38 In contrast, in the PMRR-DIPSHIFT scheme (Fig. 1c), the chemical shift terms of spin S alone are refocused, making S the only observable channel. Since the zero-quantum CSA terms commute with the zero-quantum dipolar terms, the dipolar evolution generated by PMRR-DIPSHIFT is also insensitive to the CSA interaction, similar to PMRR-REDOR, as illustrated in Fig. S1b.† It should be noted that both PMRR-REDOR and PMRR-DIPSHIFT are affected by T2 relaxation. The influence of T2 decay in the REDOR-type scheme can be taken into account by conducting an additional control experiment (S0) which omits all the pulses on the non-observed channel, and ΔS/S0 is often used to denote the dipolar evolution.13 Alternatively, the influence of T2 decay can be removed by using a constant echo time,39 as shown in PMRR-DIPSHIFT (Fig. 1c). Inevitably, longer constant echo time results in more signal loss.
The small ratio of ν1/νr = 2 makes PMRR suitable for fast-to-ultrafast MAS conditions, and it is expected that better performance would be achieved at a faster MAS frequency, mainly due to the higher applied RF field. We first evaluated the performance of the PMRR sequence at an ultrafast MAS frequency of 60 kHz by numerical simulations, and compared it to that of other commonly used recoupling approaches including R3 REDOR, SR4, and single-quantum RNνn sequences. The robustness of the recoupling sequences was evaluated for different conditions, including RF mismatch, resonance offset and 1H–1H dipolar coupling. The results are shown in Fig. 1d–f, respectively. The simulations clearly indicate that the original R3 sequence is highly sensitive to the RF mismatch and resonance offset. For instance, a slight RF mismatch of 0.5 kHz results in over 50% error (Fig. 1d), and a 6.5 kHz resonance offset leads to an error of over 10% (Fig. 1e). Considering the similar RF field requirement of 2.25νr, R1814 was selected for examining the performance of the single-quantum recoupling scheme at a MAS frequency of 60 kHz. As reported previously,35R1814 with ν1/νr < 2.5 is easily influenced by RF mismatch and resonance offset, but exhibits better tolerance than R3, as shown in Fig. 1d and e. In comparison, the significantly improved robustness can be achieved by the PMRR scheme, and the phase modulation efficiently compensates for the influence from RF mismatch and resonance offset, which is similar to REDOR-xy4 and SR4. Under the given simulation condition, it is noted that PMRR shows slightly better tolerance than SR4 and REDOR (Fig. 1d and e). More specifically, by the PMRR recoupling scheme, even a large RF mismatch of up to ±10% only results in a minor deviation of less than 4%. Assuming that the allowable error of dipolar measurements is 10%, the tolerable resonance offsets are ±40, ±38.5 and ±21.5 kHz for PMRR, REDOR and SR4 schemes, respectively (Fig. 1e).
For practical applications of H–X dipolar coupling measurements, 1H–1H homonuclear dipolar decoupling is another major factor in evaluating the performance of recoupling sequences, except for proton dilute or deuterated systems.23,40,41 Although REDOR is as robust as PMRR in terms of RF mismatch and resonance offset, REDOR suffers from the influence of simultaneously recoupled homonuclear dipolar interactions, as demonstrated in Fig. 1f. In contrast, in the R3 (n = 2) and R1814 sequences, homonuclear dipolar interactions are decoupled at MAS frequencies exceeding 40 kHz, due to the inherent averaging of the first-order homonuclear Hamiltonian terms. Surprisingly, the PMRR scheme exhibits greatly improved homonuclear dipolar decoupling compared to the original R3 (n = 2), effectively suppressing the influence of 1H–1H dipolar couplings in the H–X dipolar coupling measurements. In addition, it should be noted that in SR4 homonuclear dipolar decoupling takes place via a supercycle over the R421 unit, although R421 essentially also recouples the partial first-order homonuclear dipolar Hamiltonian terms.14,26 Similar efficiency of suppressing 1H–1H dipolar coupling can be achieved by SR4 and PMRR schemes, as shown in Fig. 1f.
As demonstrated above, the PMRR scheme exhibits superior performance with respect to heteronuclear dipolar recoupling at the ultrafast MAS frequencies. In order to evaluate the versatility of the PMRR recoupling scheme, we have further investigated the recoupling performance of the PMRR scheme at different MAS frequencies from 20 to 120 kHz, which is demonstrated as the tolerance to RF mismatch, resonance offset and homonuclear dipolar couplings at each MAS frequency, as shown in Fig. 1g–i. Not surprisingly, at a higher spinning speed, higher RF field strength is required, which directly leads to higher tolerance to experimental imperfections. When considering RF mismatch in units of percentage rather than kHz, the PMRR performance depends very little on the MAS frequency, as shown in Fig. S2a,† which indicates the robustness to RF inhomogeneity. The effective recoupling bandwidth can also be improved with the increasing MAS frequency, as shown in Fig. 1h. Assuming an allowable accuracy deviation of 10%, the efficient recoupling bandwidth is ±16 kHz at a MAS frequency of 20 kHz, ±50 kHz at a MAS frequency of 60 kHz, and ±100 kHz at a MAS frequency of 120 kHz. The broadband recoupling is rather beneficial for spin systems containing nuclei with a large chemical shift range, such as 19F and 31P. In addition, the capability of suppressing homonuclear dipolar coupling can be further improved with faster MAS frequencies, as shown in Fig. 1i. As a zero-quantum recoupling sequence, PMRR is non-γ-encoded and theoretically sensitive to the fluctuation of spinning frequency, and it is preferable to be strictly synchronized with rotor spinning.14 Although the increase of the MAS frequency can improve the performance of PMRR, fast MAS also increases the risk of spinning fluctuation that might disrupt the synchronization between pulse irradiation and rotor spinning, leading to performance degradation. However, a fluctuation of ±100 Hz at a MAS rate of 20 kHz yields only an error of less than 0.3% by PMRR, as demonstrated in simulations (Fig. S2b†). For practical MAS NMR experiments, the spinning fluctuation in excess of ±100 Hz should be unusual and attributed to the failure of the MAS control system. Therefore, despite PMRR being non-γ-encoded, the effect of MAS fluctuation is negligible.
Taken together, the above results indicate that the PMRR scheme is a robust heteronuclear recoupling sequence, particularly suited for MAS frequencies of 60 kHz and higher. It is worth noting that, at slow-to-moderate MAS rates, the performance of PMRR might be degraded mostly due to the low applied RF field, although precise determination can be achieved under specific conditions. Therefore, a more general PMRR scheme with boosted performance suitable for a broad range of MAS frequency is highly desirable.
Numerical simulations have been performed to evaluate the robustness of wPMRR at a relatively low MAS frequency (10 kHz), in comparison to the windowless PMRR. As discussed above, the low RF field (ν1 = 2νr) in windowless PMRR at a lower MAS frequency weakens the recoupling robustness, which would become a challenge for practical applications, especially in spin systems containing nuclei with a large chemical shift range, or with strong homonuclear dipolar couplings. As can be seen from the simulations (Fig. 2c to e), with the introduction of window insertion, the tolerance to the experimental imperfections and spin system is profoundly improved. Therefore, a higher RF field is preferred, as long as the probe hardware permits. At a relatively slow MAS rate of 10 kHz, the robustness of the wPMRR scheme with fw = 0.8 is further demonstrated by comparisons with other commonly used recoupling methods, especially in the suppression of homonuclear dipolar couplings (Fig. S3†). In addition, the introduction of window insertion into the SR4 scheme can also improve the recoupling efficiency as well as the performances. The pulse sequence and performance evaluation of windowed SR4 (wSR4) are shown in Fig. S4.†
In practical applications, multiple interferences may simultaneously come into play, resulting in serious degradation of the dipolar recoupling performance. As illustrated in Fig. 3a, for windowless PMRR at a MAS frequency of 10 kHz, even a slight RF mismatch can significantly deteriorate the performance of 1H–1H dipolar decoupling, making the H–X dipolar measurements unreliable. Similarly, RF mismatch also seriously reduces the performance of suppressing CSA interactions by PMRR, as shown in Fig. 3c, although in principle, the recoupled zero-quantum CSA terms commute with heteronuclear dipolar interactions. And vice versa, the tolerance to RF mismatch by windowless PMRR is greatly reduced when the spin system under study contains CSA or/and homonuclear dipolar interactions. However, wPMRR with fw = 0.8 shows significantly reduced dependency on synchronous RF mismatch/1H–1H coupling and RF mismatch/1H CSA, as demonstrated in Fig. 3b and d, respectively. In short, the window modulation makes wPMRR flexible and adaptive to a wide range of spinning frequencies, showing outstanding performance under various conditions. Also, it should be mentioned that the wPMRR scheme makes the experimental setup and optimization greatly simplified, where the only parameter the researcher needs to measure is the π pulse length.
As demonstrated above, the PMRR scheme is suited for the accurate determination of heteronuclear dipolar couplings in a wide range of MAS frequencies, where interference from both the experimental imperfections and undesired interactions can be suppressed efficiently. The simulation results show that the PMRR scheme is much less sensitive to RF mismatch or inhomogeneity as well as the resonance offset than the original RR scheme, thanks to the effective compensation by the XiX phase modulation. Nevertheless, it is essential to experimentally verify tolerance to pulse imperfections and resonance offset that can accompany practical applications. To do so, we have performed a series of 1H–15N/13C PMRR/wPMRR-DIPSHIFT experiments with varying RF mismatches and resonance offsets on fMLF at MAS frequencies of 20 kHz and 60 kHz. The errors of the dipolar coupling measurements as a function of RF mismatch and resonance offset are shown in Fig. 4e and f, respectively. It is clear that PMRR has superior tolerance to RF mismatch and resonance offset at fast MAS frequencies: very small errors are observed, even with resonance offset up to 40 kHz, consistent with the simulations results. On the other hand, the performance of PMRR degrades at slower MAS frequencies, mainly due to the low RF amplitude (2νr) required for the experiments. However, significantly improved performance can be achieved by wPMRR with the RF amplitude of 100 kHz (fw = 0.6) at slower MAS frequencies, which is accessible in most commercial probes, as shown in Fig. 4e and f. More specifically, with wPMRR of fw = 0.6, an RF mismatch of ±8 kHz or resonance offset of up to ±20 kHz results in negligible errors in the measured heteronuclear dipolar constants. Notably, the increasing dipolar scaling factor with increasing RF amplitudes results in faster dipolar oscillation corresponding to broader Fourier transformed lineshapes, as indicated in Fig. S7.† These results demonstrate that the PMRR and wPMRR schemes introduced in this work have excellent tolerance to experimental imperfections and undesired interactions. The robustness to RF mismatch means less stringent requirements for calibration of experimental conditions, the RF homogeneity and stability of NMR probes/amplifiers. Moreover, the good robustness to resonance offset indicates the advantage of broadband recoupling. As mentioned above, in order to maximize the superior robustness for dipolar recoupling, it is recommended to perform PMRR at high MAS frequencies (≥40 kHz) or choose wPMRR with high-power recoupling irradiations for slow-to-moderate MAS frequencies (<40 kHz).
The strong proton network from abundant BASs and methyl groups of TMPO is often problematic for the accurate determination of 1H–31P internuclear distances, because the 1H–1H homonuclear dipolar interaction may interfere with the 1H–31P heteronuclear dipolar interaction when conventional recoupling techniques, such as REDOR, are employed.52,53 As demonstrated above, PMRR and wPMRR sequences can effectively suppress the influence of homonuclear dipolar couplings, and are well suited for heteronuclear dipolar measurements in a broad range of MAS conditions. Considering the detection sensitivity and spectral resolution, a medium-scale 3.2 mm NMR rotor with an accessible moderate MAS frequency was utilized. To achieve the optimum robustness to RF mismatch and resonance offset, we performed wPMRR (fw = 0.55) NMR experiments for 1H–31P dipolar measurements at a MAS frequency of 20 kHz.
1D 31P MAS and 2D 1H–31P HETCOR NMR spectra are shown in Fig. S8,† and as indicated, in addition to multiple TMPO sites (60–90 ppm) adsorbed on acid sites, the physically adsorbed TMPO with the 31P signal at 46 ppm was also detected.48,54 Although the saturated adsorption of TMPO was conducted in the preparation, a small amount of BAS was unoccupied by TMPO due to steric hindrance, as suggested by the signal at 4.2 ppm. The signal overlap of protons in TMPO with the BAS proton degrades the 1H spectral resolution, which obscures the direct observation of the 1H signal for tracking 1H–31P dipolar couplings on different BASs, i.e., 2D proton-detected wPMRR-REDOR (Fig. S9a†). On the other hand, 2D heteronuclear 31P-detected PMRR-DIPSHIFT (Fig. S9b†) suffers from the interference of nine protons of methyl groups in TMPO. Therefore, we used a 3D proton-detected wPMRR-REDOR 31P/1H–31P/1H pulse sequence, illustrated in Fig. 6a, for site resolved 1H–31P dipolar measurements with high accuracy. The two CP contacts serve not only for building up 1H–31P correlations for resolving multiple sites, but also as a filter for selecting the proton signals in close proximity to TMPO. The 2D 1H–31P correlation plane sliced from the resulting 3D spectrum at t2 = 0 is shown in Fig. 6e, and two cross-peaks at (13.1 ppm, 78.1 ppm) and (7.9 ppm, 88.1 ppm) are observed unambiguously. The former is assigned to TMPO adsorbed at bridging hydroxyl groups (Si–OH–Al) acting as a Brønsted acid, consistent with previous reports.48–50,54,55 The latter can be assigned to TMPO adsorbed at BAS with super acidity, i.e. the acid is stronger than pure sulfuric acid, as shown by a 31P chemical shift over 86 ppm which was thought to be the threshold of superacidity.48–51,55
The 1H NMR spectrum in the direct dimension by 3D wPMRR-REDOR is shown in Fig. 6d; a main signal at 13.1 ppm with a downfield shoulder and a signal at 7.9 ppm correspond to the bridging acid and super acid sites, respectively. The dephasing curves for each site were extracted via spectral deconvolution, and one deconvolution example is shown in Fig. 6f. It is noted that the shoulder signal of the main 1H signal can be identified, but the overlap and relatively weak intensity hinder the accurate measurements of the associated dipolar coupling constant. Thus, the data for the shoulder peak are not included. The experimental and simulated 1H–31P dipolar dephasing curves are shown in Fig. 6g. The extracted 1H–31P DCC for the site at (13.1 ppm, 78.1 ppm) is 2.5 ± 0.2 kHz, corresponding to the internuclear distance of 2.69 ± 0.07 Å, which agrees well with the previous theoretical study.54,55 In contrast, the super acid site at (7.9 ppm, 88.1 ppm) shows a smaller 1H–31P DCC of 1.8 ± 0.1 kHz which corresponds to a slightly longer internuclear distance of 3.00 ± 0.05 Å. Such adsorption characteristics of the base probe molecule on the super acid sites of H-ZSM-5 suggest that the formation mechanism of the super acid site should be different from the conventional BASs, which may be related to the nearby configurations.
To further investigate the origin of the super acid sites in H-ZSM-5, the TMPO/H-ZSM-5 sample was exposed in a slight humid environment, following the procedure reported previously.49 As shown in Fig. 7a, the 1H NMR spectrum of the direct dimension in the 3D wPMRR-REDOR experiment indicates that the shoulder peak at 14.8 ppm increases apparently after the rehydration process, with an integral area comparative to the main signal at 13.1 ppm. In the meantime, the 1H signal at 7.9 ppm of the super acid site mostly disappears upon the water adsorption, and the corresponding 31P signal at 88.1 ppm is much reduced as well, as shown in the 31P CP/MAS spectrum (Fig. 7b). As a consequence, the correlation peak at (7.9 ppm, 88.1 ppm) is hardly observed in the 2D 1H–31P correlation plane (Fig. 7c). Several theoretical studies49,52,56,57 have been reported exploring the origin of the super acidity in H-ZSM-5, and one accepted view is that this is the result of the proximity between the Lewis acid site (LAS) and the BAS.49,52,57–59 According to the interaction model, it was deduced that the nearby LAS participated in the adsorption of TMPO on BAS, which leads to the super acid sites. Our results reveal that, instead of interacting solely with the acid proton, TMPO interacts with both BAS and LAS, rendering a longer 1H–31P distance on the super acid sites. Fig. 7d shows the experimental and simulated 1H–31P dipolar dephasing curves of rehydrated TMPO/H-ZSM-5 by wPMRR-REDOR. Interestingly, the signal at 14.8 ppm shared the same dipolar dephasing dynamics as the main signal at 13.1 ppm, and the extracted DCCs are 2.6 ± 0.2 kHz (2.59–2.73 Å), which is almost identical to that of the main site at (13.1 ppm, 78.1 ppm) in the as-prepared TMPO/H-ZSM-5. It can be inferred that the high affinity of LAS to H2O breaks the interaction between TMPO and LAS, which results in the disappearance of the super acid site. The quantitative 1H–31P dipolar measurements by the wPMRR method provide detailed insights into the changes of the TMPO adsorption status on acid sites in zeolites.
The outstanding recoupling performance makes PMRR a promising method for measuring the DCC with accuracy and revealing the local chemical structure and dynamics. In probing the host–guest interaction in TMPO adsorbed H-ZSM-5, a proton-detected 3D wPMRR-REDOR pulse sequence has been utilized for determining 1H–31P distances between BAS and TMPO. The results showed unexpectedly longer 1H–31P distances for the super acid site which, in combination with a rehydration experiment, revealed that the super acid site is the result of the synergy of BAS and LAS. The PMRR and wPMRR schemes introduced in this work show superior recoupling performance, excellent tolerance to experimental conditions and imperfections, and more notably, ease of setup. Moreover, it should be noted that PMRR and wPMRR are also suited for determining heteronuclear dipolar coupling between spin-1/2 and spin>1/2, such as 1H–17O and 31P–27Al, where the rotor-synchronized RF field pulses are applied on spin-1/2. Therefore, it is believed that PMRR/wPMRR can find widespread applications in a variety of spin systems.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1sc03194e |
This journal is © The Royal Society of Chemistry 2021 |