Ibraheem M.
Haies
ab,
James A.
Jarvis
c,
Harry
Bentley
a,
Ivo
Heinmaa
d,
Ilya
Kuprov
a,
Philip T. F.
Williamson
c and
Marina
Carravetta
*a
aSchool of Chemistry, University of Southampton, SO17 1BJ, Southampton, UK. E-mail: m.carravetta@soton.ac.uk
bDepartment of Chemistry, College of Science, University of Mosul, Mosul, Iraq
cSchool of Biological Sciences, University of Southampton, SO17 1BJ, Southampton, UK
dNational Institute of Chemical Physics and Biophysics, Akadeemia tee 23, Tallinn, 12618, Estonia
First published on 30th January 2015
Overtone 14N NMR spectroscopy is a promising route for the direct detection of 14N signals with good spectral resolution. Its application is currently limited, however, by the absence of efficient polarization techniques for overtone signal enhancement and the lack of efficient numerical simulation techniques to aid in both the development of new methods and the analysis and interpretation of experimental data. In this paper we report a novel method for the transfer of polarization from 1H to the 14N overtone using symmetry-based R-sequences that overcome many of the limitations of adiabatic approaches that have worked successfully on static samples. Refinement of these sequences and the analysis of the resulting spectra have been facilitated through the development of an efficient simulation strategy for 14N overtone NMR spectroscopy of spinning samples, using effective Hamiltonians on top of Floquet and Fokker–Planck equations.
An alternative approach to 14N NMR relies on the detection of the so-called overtone transition which corresponds to the change in spin projection quantum number by Δm = 2, formally a double-quantum transition, with direct detection at twice the Larmor frequency.25 Multiple-quantum transitions are in general forbidden, but the presence of a strong quadrupolar interaction mixes Zeeman energy levels which in turn leads to a non-zero probability for the overtone transition.26–28 Overtone transitions are insensitive to the first order quadrupolar interaction and have been observed on both static and spinning samples containing 14N since the 1980's.26–28 Both direct excitation and cross-polarization excitation schemes have been employed, the latter giving enhancements in sensitivity of up to a factor of 5.9 in single crystals.27 O'Dell and co-workers demonstrated that direct excitation of 14N overtone transitions with high resolution is possible under MAS,4,29 proving that good resolution is indeed achievable in 14N NMR. While direct excitation schemes are typically narrow-band, broadband excitation of the 14N overtone transition has been achieved using WURST-based methods,29 but this leads to a further loss in signal intensity. More recently, very fast MAS experiments on 14N overtone demonstrated that the overtone signal can be acquired in the indirect dimension of a 2D experiment using an approach that resembles indirect detection 14N NMR experiments.30–32 Adiabatic CP methods demonstrated for static overtone excitation have not been successfully extended to rotating solids so far. The only successful report to date of 14N overtone based CP methods on rotating solids is from Rossini et al.,30 who used cryogenic MAS dynamic nuclear polarization (DNP) conditions, where ramped CP was applied to transfer polarization to 14N overtone from hyperpolarized 1H nuclei. The 14N overtone signal is significant,30 but the same experiment produced no visible signal in the absence of 1H hyperpolarization.
In part the slow development of 14N-overtone NMR can be attributed to the absence of efficient simulation strategies, which have been employed so successfully elsewhere for the development of solid-state NMR experiments. Simulation of the overtone transitions is demanding because: (i) there is no rotating frame that would remove all time dependence from the Hamiltonian; (ii) overtone transition moment is small, therefore all 14N overtone pulses are very long and cannot be treated as ideal; (iii) the system is time-dependent at both twice the Larmor frequency and at the MAS frequency, and those frequencies are orders of magnitude away from each other; (iv) for powdered samples, three-angle spherical averaging is needed. Currently the simulations are performed by brute force in the laboratory frame and are therefore very time consuming.4
In this work we present a new approach based on the PRESTO (Phase-shifted Recoupling Effects a Smooth Transfer of Order), a symmetry based R-sequence, to establish 1H to 14N overtone correlation and achieve polarization transfer to enhance the 14N overtone transition under MAS.33–35 Complementing earlier indirect methods that employ high spinning speeds (>50 kHz) and high RF field (>250 kHz), the PRESTO sequence provides significant enhancements at moderate spinning speeds (<20 kHz) and moderate RF fields (<70 kHz), which makes it applicable to larger samples. Through the development of computationally efficient methods for the simulation of 14N OT spectra we have been able to simulate the line shapes obtained under both direct excitation and PRESTO, accounting for both finite pulse effects and the polarization transfer process. The algorithm uses effective Hamiltonians on top of Floquet and Fokker–Planck equations. All methods described below are implemented into versions 1.5 and later of the Spinach library.36
The PRESTO approach is demonstrated on two model samples, glycine and N-acetyl-valine (NAV), which represent two different situations in terms of the magnitude of the quadrupolar interaction. Sensitivity increases significantly with PRESTO, with enhancements factors between 2.5 and 3.8 compared to direct excitation, when comparing the experiments by the number of acquisitions. There is no enhancement from the polarization transfer alone if the signal is measured per unit time on these samples, as the relaxation time for proton is longer than for 14N (from literature). We also demonstrate the application of PRESTO technique to the acquisition of 1H–14N overtone correlation spectra.
NMR measurements at 20.0 T were performed using 850 MHz Bruker spectrometer equipped with a 3.2 mm wide-bore triple resonance probe and 3.2 mm zirconium oxide rotors. At this field the 14N overtone transitions are expected to be located at 122.8331 MHz. All 20.0 T spectra were referenced to this frequency.
All experiments were performed with the RF carrier frequency in resonance with the +2 overtone spinning sideband. For direct excitation, the optimal excitation pulse width was found by taking the time of maximum signal intensity with respect to the pulse duration using direct overtone excitation under MAS with a pulse acquire sequence (shown in Fig. 1). Optimal pulse durations for direct detection were found to be 260 μs for both glycine and NAV at 14.1 T and 275 μs for glycine at 20 T.
Fig. 1 Experimental data and the corresponding simulation of the nutation curve for the 14N overtone signal of glycine powder, observed at its +2 spinning sideband under MAS at 9.92 kHz, using an RF pulse with a nominal RF amplitude of 55 kHz, as calibrated on the 17O signal from a water sample. The simulation was performed as described in the theory section using experimental parameters for glycine from literature (Table 1), and probe geometry without any fitting or adjustment, except for the arbitrary overall scaling multiplier. The π/2 and π pulses for the 14N overtone are found to be 260 μs and 520 μs, respectively. |
During acquisition, 72 kHz SPINAL proton decoupling37 was applied at 14.1 T and 89 kHz SPINAL decoupling was used for the experiments at 20.0 T. The effect of decoupling during the pulse was considered at 14.1 T and was found to be very minor for glycine, but improved significantly the excitation performance of the NAV signal. The pulse repetition delay for direct overtone excitation and spin echo were set to 0.5 s for glycine and 0.4 s for NAV (these values are compatible with previous overtone studies4,29). Repetition delays for PRESTO were set to 2.5 s for both glycine and NAV. Due to probe ringing near the overtone frequency, the first 70 μs of the FID were removed by applying the appropriate left shift of the experimental data points followed by phase correction for all direct excitation experiments.
Data were referenced indirectly to 0 ppm (liquid ammonia) using the 14N signal of ammonium chloride at 39.3 ppm.38 The overtone reference frequency was taken to be twice the 14N reference frequency.
(1) |
The experimental optimization of PRESTO was done in stages. First, the signal intensity was optimized by direct 14N overtone detection (described above), then the conditions for the spin echo were optimized. These parameters were used to set up the experiment using PRESTO-II. Then both the π/2 and π 14N overtone pulse lengths were optimized again and the best pulse durations were found to be 360 μs and 720 μs respectively for glycine at 14.1 T, 375 μs and 750 μs respectively for glycine at 20.0 T, 170 μs and 340 μs respectively for NAV. Excitation and reconversion times were kept equal and the optimal duration was found to be 231 μs at 20.0 T and 201 μs at 14.1 T. Optimal spin echo pulse durations within PRESTO-II are not necessarily the same, but were found to be similar to the basic spin echo sequence.
PRESTO-II measurements were conducted using the following experimental conditions: spinning frequency ωr/2π = 9.92 kHz for R1871 (data shown only as 2D) and ωr/2π = 19.84 kHz for R1852. During PRESTO, the basic element R = {180+φR180−φR}0 was employed during the excitation block, and R = {180+φR180−φR}90 during the reconversion block.
The phase of the excitation block was cycled in two steps to remove contributions coming from direct overtone excitation, both in the experiments and in the simulations. The signal contributions from excitation through the spin echo as well as PRESTO could both be retained without this phase cycling step. For PRESTO-II, a pre-acquisition delay of 10 μs prior to detection was found to be sufficient, as for the overtone spin echo experiments.
Proton RF amplitudes of 89.2 was utilized during PRESTO. For the two-dimensional version of the PRESTO-II sequence displayed in Fig. 2B, Frequency-Switched Lee–Goldburg (FSLG) decoupling was used during the t1 time interval to suppress the homonuclear 1H couplings and improve resolution. The proton RF amplitude was set to 89.2 kHz corresponding to a LG frequency offset of 63 kHz.39
The narrow bandwidth of the PRESTO experiments is attributed to the overtone spin echo step in the pulse sequence, which is currently using simple sinusoidal wave 14N overtone pulses.
(2) |
(1) There is no convenient rotating frame. The standard Zeeman rotating frame normally used in high-field MAS NMR44,45 is ineffectual against the double-quantum frequency under the cosine. A double-quantum rotating frame would flip the sign of the Zeeman term, but not eliminate it. Laboratory frame simulations require two points per period of the fastest oscillation, meaning many millions of propagation steps per evolution second in a 14.1 T magnet, taking hours to days on modern workstations.4
(2) The pulses cannot be assumed to be ideal (defined as “short enough for the background Hamiltonian evolution to be negligible during the pulse”) – the mixing caused by the strong quadrupolar interaction means that the transition between the outer energy levels under ŜX is no longer forbidden,46 but the transition moment for the overtone is two orders of magnitude smaller than that of the fundamental frequency. At the limit of the available hardware, the 90 degrees pulse on 14N overtone is around 200 μs long. The RF pulse term in eqn (2) cannot be avoided by starting the simulation from 0 = ŜX as the initial condition and a finite-duration pulse must be properly accounted for.29
(3) The system is spinning with a frequency typically in the range of 10–70 kHz, which is three orders of magnitude away from the frequencies of the spin evolution processes. This makes the Hamiltonian time-dependent in a numerically difficult way. At the matrix exponentiation stage, the difference between the adjacent time slices of the laboratory frame propagation problem are perilously close to the numerical accuracy limits of double-precision arithmetic.
(4) The system is a powder and a spherical average is therefore required. The lab frame simulation with millions of time steps would have to be repeated thousands of times to obtain a three-angle powder pattern.44,45,47 Our simulations indicate that the spherical integration grid ranks required to converge an overtone powder pattern are considerably greater than those needed for the single-quantum transition signal with a comparable number of spinning sidebands.
To overcome these issues we have developed the following simulation method that avoids the time-consuming laboratory frame time propagation. Its stage are:
(1) The time dependence is eliminated from the MAS part of eqn (2) by switching it into Floquet48 or Fokker–Planck49 formalism (both give identical results), in which MAS is treated as a time-independent interaction between spin space and lab space degrees of freedom. In the Fokker–Planck picture49 we get:
(3) |
(4) |
(2) The effective Hamiltonian method,56 using matrix logarithms, is employed to remove the time dependence from the pulse part of eqn (2). The matrix logarithm technique is used because the corresponding Magnus expansion57 does not converge even at the fourth order, due to the above mentioned lack of convenient rotating frame in overtone NMR. We could not find a way around the slow convergence of the Magnus series and the exact effective Hamiltonian expression is therefore used instead:
(5) |
(3) Pulses are applied using propagator squaring. The effective Hamiltonian in eqn (5) is over the RF period (about 10 nanoseconds), but the following relation:55
(6) |
(4) The problem is moved into the frequency domain before the detection stage simulation is run. The overtone spectrum only occupies a few hundred points within a very large laboratory frame frequency range – it is therefore advantageous to replace millions of points that would be needed in the time domain with a few hundred points directly evaluated in the frequency domain. With a known initial condition and a time-independent Hamiltonian (both of which are available as a result of applying stages 1–3) the problem can be moved into the frequency domain analytically:
(7) |
(8) |
Nuclei other than 14N are kept in their corresponding rotating frames for the entirety of the simulation. The simulation procedure described above is implemented in version 1.5 of Spinach library36 available at http://spindynamics.org. Complete annotated source code for the simulations performed in this paper is included with the example set. For the systems described below, the wall clock simulation time depends on the number of spectrum digitization points as well as target accuracy, determined by Wigner D-function rank in the Fokker–Planck formalism49 and spherical integration grid rank in the Floquet formalism,48 but is generally of the order of minutes to hours.
C Q (MHz) | η Q | r NH (Å) | Euler angles, DDa | σ ISO (ppm) | Δσ (ppm) | η | Euler angles, CSAa | Lebedev grid rank | |
---|---|---|---|---|---|---|---|---|---|
a Dipolar and CSA interaction tensor orientations are quoted relative to the eigenframe of the 14N quadrupolar interaction tensor. | |||||||||
Gly9,63 | 1.18 | 0.53 | 1.28 | [0, 0, 0] | 32.4 | — | — | — | 77 |
NAV64–66 | 3.21 | 0.32 | 1.06 | [0, 90°, 0] | 121 | 105 | 0.23 | [−90°, −90°, −17°] | 131 |
For NAV, the signal we observe is very weak, possibly also because of the long dead time of 70 μs. Even though simulations suggest shorter pulses are needed for systems with large quadrupolar interaction,32 a reliable nutation curve as in Fig. 1 was not obtained for NAV as for long pulse widths we burn a hole through the overtone line shape. The NAV signal shows a local maximum near 75 μs but after a small drop it increases again, and the “global” maximum signal intensity is observed close to 260 μs, and therefore this value was used for the direct excitation measurements reported here. The optimal pulse duration for echo measurements on NAV was found to be 170 μs. These cannot be related to well-defined flip angles.
The signals obtained through direct excitation of the 14N overtone signal of glycine and NAV at 14.1 T and 20.0 T, as well as the corresponding simulations, are shown in Fig. 3. All measurements were performed at ωr/2π = 19.84 kHz. While overtone transitions are insensitive to the first order quadrupolar interaction, lines can still be significantly broadened by higher order terms. However, it is still possible to record direct overtone spectra in a matter of minutes for glycine, which has a line width of approximately 0.83 kHz at 14.1 T (Fig. 3B). NAV is more challenging system because its larger quadrupolar interaction broadens the observed overtone signal to about 5 kHz. Combined with the long dead time of 70 μs, this results in an acquisition time of about 17 hours for the data displayed in Fig. 3A.
Fig. 3 14N overtone direct excitation experimental and simulated spectra obtained at the +2 spinning sideband under MAS at 19.84 kHz with a nominal RF amplitude of 55 kHz at 14.1 T and 70 kHz at 20.0 T, as calibrated on a water sample. (A) Experimental data for NAV, 160000 scans at 14.1 T and with an excitation pulse width τp = 260 μs. (B) Experimental data for glycine, 1024 scans at 14.1 T with τp = 260 μs. (C) Experimental data for glycine, 1024 scans at 20.0 T with τp = 275 μs. (D, E) Simulation for NAV and glycine, respectively, at 14.1 T. (F) Simulation for glycine at 20.0 T. The simulation approach is described in the Theoretical methods section, with the input parameters summarized in Table 1, and probe geometry at the magic angle, without any fitting or adjustment, except for an arbitrary overall scaling multiplier. The simulated spectra used Lorentzian line broadening of 2000 Hz for NAV, 300 Hz for glycine. |
As expected, the line shape for glycine is significantly narrower at higher magnetic field (Fig. 3C), but there is no noticeable loss in signal intensity. For all systems, the line shape and peak position can be reproduced by numerical simulations. The simulation conditions and parameters are summarized in Table 1 – the protons are kept in the direct excitation simulation to make sure that identical spin systems are being compared for the two methods. For glycine, we assume that the nitrogen–proton interaction can be replaced by a spin pair with the proton on the symmetry axis to account for the NH3 rotation.61,62 NAV is also more challenging from the simulation point of view, as convergence requires a large number of orientations in the spherical grid. As a consequence, typical simulation times on a contemporary workstation are up to an hour for direct excitation.
Optimal signal intensity was achieved under unusual conditions, with no delay between the overtone pulses. Experimental data at 14.1 T and corresponding simulations were obtained for τ1 = 0, τ2 = 10 μs and are shown in Fig. 4, together with the corresponding simulations. Data and simulations are presented with no phase correction. More “traditional” echo experiments, with a non-zero delay between the overtone pulses, are provided in the ESI† (see Fig. S2), with only minor signal loss. Spin echo lines are visibly narrower than direct excitation, there is a small loss in integrated signal intensity, but the echo signal height increases by a factor of 1.2 and 2.0 respectively for glycine and NAV, without any need for a change in the pulse delay.
Fig. 4 Experimental direct excitation and spin-echo spectra and simulations at 14.1 T and 19.84 kHz on resonance with the +2 spinning sideband of the 14N overtone signal. (A) Experimental direct excitation on NAV with 160000 scans. (B) Experimental direct excitation on glycine with 1024 scans. (C) Experimental spin-echo data on NAV with 40000 scans and durations of 170 μs and 340 μs for the overtone pulses, τ1 = 0 μs and τ2 = 10 μs for the delays. The signal enhancement is 2.0 in terms of peak height, and 0.8 in terms of peak area. (D) Experimental spin-echo spectrum on glycine, 1024 scans and durations of 360 μs and 720 μs for the overtone pulses, τ1 = 0 μs and τ2 = 10 μs. The signal enhancement is 1.2 in terms of peak height, and 0.8 in terms of peak area. (E and F) Numerical simulations of spin-echoes in data sets C and D, using the spin system parameters given in Table 1 and the same signal processing parameters as those used for the experimental data. |
To enhance the sensitivity of the directly detected overtone signal, PRESTO-II was applied to transfer polarization from protons. The resulting spectra for glycine and NAV are shown in Fig. 5 for the symmetry R1852 at ωr/2π = 19.84 kHz and two fields (PRESTO-II data were also obtained on glycine at 9.92 kHz and 14.1 T using R1871, with similar performance). A comparison between the 14N overtone signals as obtained through direct excitation and PRESTO-II, and the corresponding simulations are shown in Fig. 5. There is no enhancement if the signal is measured per unit time using the fully relaxed proton bath as starting condition, as the repetition delay for PRESTO-II is set to 2.5 s, significantly longer than the values used for the 14N overtone measurements. In the PRESTO-II experiments reported here we focus purely on the polarization transfer step from protons in order to address strengths and limitations of this approach, hence we implemented a two-step phase cycling which removes the signal coming from direct excitation through the overtone spin echo within the PRESTO sequence. A comparison between PRESTO and spin echo data acquired with a small delay between overtone pulses at 14.1 T is given in the ESI† (Fig. S2). The directly excited spin echo overtone signal could potentially be retained and lead to a further increase of the PRESTO signal intensity and peak shape. PRESTO data acquisition can be achieved much more time-effectively by pre-saturating the 1H bath and transferring polarization with just 1 s delay, while retaining about 80% of the signal (see Fig. S3 in ESI†).
Fig. 5 Experimental direct excitation and PRESTO-II spectra and PRESTO-II simulations at 19.84 kHz on resonance with the +2 spinning sideband of the 14N overtone signal. (A) Experimental direct excitation on NAV with 160000 scans at 14.1 T (B) Experimental direct excitation on glycine with 1024 scans at 14.1 T. (C) Experimental direct excitation on glycine with 1024 scans at 20.0 T, τ1 = 0 μs and τ2 = 10 μs. (D) Experimental PRESTO-II data on NAV at 14.1 T with 20000 scans and durations of 170 μs and 340 μs for the overtone pulses. The signal enhancement is 3.8 in terms of peak height, and 2.0 in terms of peak area. (E) Experimental PRESTO-II spectrum on glycine at 14.1 T with 1024 scans and durations of 360 μs and 720 μs for the overtone pulses, τ1 = 0 μs and τ2 = 10 μs. The signal enhancement is 2.5 in terms of peak height, and 1.5 in terms of peak area. (F) Experimental PRESTO-II spectrum on glycine at 20.0 T, 1024 scans and durations of 375 μs and 750 μs for the overtone pulses, τ1 = 43 μs and τ2 = 40 μs. The signal enhancement is 3.3 in terms of peak height, and 2.0 in terms of peak area. (G–I) Numerical simulations of PRESTO-II, corresponding to data sets D–F, using the spin system parameters given in Table 1 and the same signal processing parameters as those used for the experimental data. |
When comparing experimental signal intensity for a given number of scans, the PRESTO-II experiment leads to signal enhancement by a factor of 2.5 and 3.8 for glycine and NAV respectively at 14.1 T, and by a factor of 3.3 on glycine at 20.0 T. PRESTO-II gives a narrower signal than direct excitation; hence the signal enhancement is smaller in terms of peak area, with factors of 1.5 and 2.0 respectively when compared to direct excitation on glycine and NAV at 14.1 T, and 2.0 for glycine at 20.0 T. The simulated PRESTO-II overtone peak position and line shape are in good agreement with the experimental data for both samples, and are quite sensitive to the size of the quadrupolar interaction. The signal enhancement with PRESTO is only moderately affected by an increase in magnetic field, which is encouraging for applications to more complex systems, where the added resolution and line narrowing from higher magnetic fields may be important.
There have been reports of polarization transfer to the overtone transitions under MAS using ramped CP, but with unclear evidence of signal enhancement due to the 1H to 14N overtone transfer. Rossini and co-workers have reported significant signal enhancements using DNP,30 but these enhancements come mainly from the hyperpolarization of the 1H bath through DNP rather than the polarization transfer,30 enhancements we expect would be mirrored when applying PRESTO. Transfer from protons is an essential step when using DNP methods and the high-gamma species allow for a more effective spread of the hyperpolarization signal, hence direct excitation of 14N under those conditions would be much more unfavourable, despite its shorter relaxation time.
The experimental enhancement in the signal intensity is moderate on glycine, but is significant on NAV. This is encouraging, as it indicates the potential for applications in biological samples, because NAV is a good model system for the peptide bond. In the past, enhancement factors close to 6 have been reported for static solids, in particular for single crystals, using static adiabatic CP methods.27 PRESTO-II achieves significant enhancement in peak heights, but with the advantages of MAS in terms of partial averaging of other anisotropic interactions. The combination of higher signal enhancement provided by polarization transfer in combination with the narrowing of the overtone transition with PRESTO-II may prove advantageous in more complex systems, but only if narrowing of the lines can be maintained while enhancing the very modest bandwidth of the current PRESTO-II sequence (see Fig. S4 in ESI†). The reduced bandwidth of PRESTO is attributed to the reduced bandwidth of the spin-echo part of the sequence, which we will address in future with the use of broadband instead of continuous-wave pulses.
This approach complements methods for indirect overtone detection via protons under ultra-fast MAS. PRESTO-II operates at moderate spinning frequencies and RF fields easily achievable on common 3.2 mm or 4 mm probes. On the other hand, methods based on detection via1H have been very successfully demonstrated on a very fast spinning probes,31,32 but achieving optimal performance in that case requires a combination of both high RF power and high spinning speed.
Simulation results using the parameters in Table 1 are shown in Fig. 6. While both direct excitation and PRESTO excite the same spectral regions, signal intensity distribution is significantly different and there are many more orientations which do not contribute much to the overall signal intensity when using PRESTO. The jagged intensity distribution pattern obtained for NAV also explains why a large number of spherical grid angles is required to achieve convergence. The narrow line shapes seen in PRESTO spectra are therefore the result of the very long overtone transition pulses only exciting a part of the full powder pattern.
Fig. 6 Simulated signal intensity (A–D) and position (E–H) distributions with respect to {α,β} angles of the spherical averaging grid (two-angle Lebedev sets specified in Table 1, γ angle is averaged over by the Floquet formalism). (A) Signal intensity distribution for direct excitation on glycine. (B) Signal intensity distribution for PRESTO-II acquisition on glycine. (C) Signal intensity distribution for direct excitation on NAV. (D) Signal intensity distribution for PRESTO-II acquisition on NAV. (E) Signal position distribution for direct excitation on glycine. (F) Signal position distribution for PRESTO-II acquisition on glycine. (G) Signal position distribution for direct excitation on NAV. (H) Signal position distribution for PRESTO-II acquisition on NAV. |
Fig. 7 1H–14N overtone correlation experiment obtained with pulse sequence in Fig. 2B. The 2D data set consists of 96 t1 increments, 48 scans per increment using the PRESTO-II sequence with symmetry R1871 at ωr/2π = 9.92 kHz. |
A number of ways are known to enhance the sensitivity of 14N overtone detection – both fast MAS and very strong RF fields have been demonstrated to be beneficial for the efficient excitation of the overtone transition.31,32 Here we have investigated an alternative approach to 14N overtone signal enhancement that relies on the efficient transfer of polarization from the abundant protons to the 14N overtone using the PRESTO-II sequence. We found this method to be effective with moderate RF fields and MAS spinning frequencies. This work also demonstrates alternative methods for obtaining multidimensional correlation experiments involving 14N overtone and describes the computational tools required to continue their development.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cp03994g |
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