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1H and 13C High-Resolution Solid-State NMR ofParamagnetic Compounds Under Very Fast

Magic Angle SpinningYoshitaka Ishii and Nalinda P. Wickramasinghe

Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607, USA

Introduction

More than one-third of the elements in the periodictable exhibit paramagnetism. Paramagnetic metal ionsplay a variety of significant roles in material science[1,2], bioinorganic chemistry [3–5], nanoscience [6–8],and pharmacology [9,10] as complexes with organic lig-ands. On the other hand, development of novel param-agnetic complexes has been often hindered by lack ofefficient characterization methods, in particular, for non-crystalline solids. High-resolution solid-state NMR (SS-NMR) using magic angle spinning (MAS) is a power-ful technique for structural analysis of non-crystallineorganic materials in solids [11–13]. However, the largedispersion of paramagnetic shifts and associated techni-cal difficulties have impeded progress in high-resolutionSSNMR studies of paramagnetic systems [14], which con-trasts with recent tremendous advancement in SSNMR fordiamagnetic systems including biomolecules [12,13,15–19]. 13C and 1H SSNMR have been the most widely usedmethods for organic solid materials. For paramagneticsystems, however, because of the large shift dispersion, in-sufficient 1H or 1H–1H RF decoupling with the limited RFfields available in a conventional probe typically resultsin severe loss of resolution in 1H and 13C SSNMR spec-tra. Also, large paramagnetic shifts mask the diamagneticshifts that are characteristic of chemical groups, makingsignal assignment difficult. Recent studies demonstratedthat 2D labeling eliminates the requirement of 1H RFdecoupling, and selective 13C labeling offers resolutionand reliable assignments in 13C SSNMR for small para-magnetic systems [20–25]. On the other hand, isotopelabeling is costly and often not justified for analysis ofsmall compounds, resulting in applications of SSNMR toonly a handful of paramagnetic systems. Moderate MASaround 10 kHz was reported to provide modest resolu-tion in 1H and 13C SSNMR for unlabeled paramagneticsystems [22,26]. Although a path for SSNMR analysis ofunlabeled paramagnetic systems has been opened by thesestudies, applicability of this approach is limited to systemshaving small 1H–1H dipolar couplings due to large 1H shift

dispersion or motions. Also, procedures for assignmentsin unlabeled paramagnetic systems have not been estab-lished. As a result, 13C and 1H SSNMR of paramagneticsystems have been largely unexplored.

Recently, our group demonstrated that a new approachusing very fast MAS (VFMAS; spinning speed ωR/2π >20 kHz) permits excellent resolution/sensitivity and sig-nal assignments in 13C and 1H SSNMR even for unla-beled systems [27,28]. Although MAS over 50 kHz iscurrently available [29], we define VFMAS as MAS atωR/2π > 20 kHz because spinning over 20 kHz inducescrucial changes in spin dynamics for organic solids byeliminating the majority of 1H–1H and 1H–13C dipolarcouplings. As will be discussed, this change by VFMASforms the foundation of our approach. This review out-lines the principles and recent applications of this VFMASapproach for paramagnetic complexes.

One-Dimensional (1D) 1H SSNMR forParamagnetic Systems

Spinning Speed Dependence and Sensitivityof 1H MAS NMR

Figure 1 shows spinning speed dependence of 1H MASNMR spectra of unlabeled Cu(II)(dl-Ala)2·H2O (a–c) andMn(III)(acac)3 (d–f). Clearly, VFMAS significantly im-proves the sensitivity and resolution. In Figure 1(c andf) at 5 kHz MAS, no signals are identified because ofline broadening due to large 1H–1H couplings and 1Hanisotropic paramagnetic shifts. In contrast, under VF-MAS, excellent sensitivity and resolution are displayedfor Cu(dl-Ala)2 at 24 kHz (a), demonstrating that VF-MAS efficiently removes broadening due to 1H–1H dipo-lar couplings and 1H paramagnetic shifts [28]. Althoughonly weak signals for NH2 are observed at −44 and −78ppm, this is because the NH2 protons are directly coor-dinated to Cu(II) and subject to significant anisotropicparamagnetic shifts [22]. As will be described below, theassignments were obtained by separate two-dimensional

C

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200 0 -200(ppm)

(b)

CH3

CH -200(ppm)

-150

NH2

400 0 -400(ppm)

(d) CH3 CH3CH

50 0(ppm)

CH3

CH3CH(a)

(c)

(e)

(f)

Fig. 1. 1H MAS spectra of Cu(dl-Ala)2·H2O (a–c) and Mn(acac)3 (d–f)at spinning speeds (a) 24 kHz, (b ande) 10 kHz, (c and f) 5 kHz, and (d)27.8 kHz obtained at 1H frequency of400.2 MHz with 1-pulse excitation and arotor synchronous echo (τR–π–τR). Thetwo insets in (a) and (d) are the expandedregions of NH2 signals in (a) and center-lines in (d). The sample amount was 17(a–c) and 14 mg (d–f). Total experimen-tal times were only 18 (a–c) and 12 ms(d–f) with four scans for each spectrum.

(2D) 13C–1H chemical shift correlation NMR under VF-MAS except for NH2, for which we adopted the assign-ment by Liu et al. using 2D SSNMR [22]. For Mn(acac)3,in spite of numerous sidebands remaining even under VF-MAS at 27.8 kHz in Figure 1(d), the center peaks are wellresolved, as shown in the inset. It is worth pointing outthat anisotropic paramagnetic shifts are generally propor-tional to (S + 1)S/R3

I S , where S is an electron spin num-ber, and RI S is the distance between the nuclear spin Iand the electron spin S at a paramagnetic center [14,26].Because the spin numbers S for Mn(III) and Cu(II) are5/2 and 1/2, respectively, it is reasonable that more side-bands were observed for Mn(acac)3. The line widths in (aand d) are comparable to those for diamagnetic systems.Large dispersion of 1H chemical shifts provides excellentresolution, which permits characterization using 1D 1HSSNMR.

The other important feature of 1H SSNMR for param-agnetic systems under VFMAS is its high sensitivity. Forthis reason, 1H VFMAS is usually the first experiment wetypically attempt for systems previously uncharacterizedby SSNMR. Short 1H T1 values of paramagnetic com-pounds (∼ms) allow us to acquire signals rapidly. Be-cause of the excellent sensitivity of 1H VFMAS SSNMR,the 1H spectra in Figure 1(a and d) were obtained in totalexperimental times of only 18 and 12 ms, respectively.

There has been a popular conception that the sensitiv-ity of SSNMR for paramagnetic systems is significantlylower than that for diamagnetic systems because of para-magnetic broadening. However, because the sensitivity inFigure 1(a and d) under VFMAS appears excellent, wetheoretically reexamined this conception. Sensitivity ofFT NMR with a matched window function is generallygiven by [30]

ξ = 〈s(t)2〉1/2(tmax/T )1/2/ρN, (1)

where s(t) is an envelope function of an FID, tmax is anacquisition period of a FID, T is a recycle time or an

interval between two scans, ρN is the r.m.s. noise am-plitude in a unit bandwidth. The factor 〈s2〉 is the aver-age signal power. For simplicity, we assumed that s(t)is given by an exponential decay, as s(t) = exp(−t/T2).When tmax is matched to T2, as tmax = cT2 for a given con-stant c, 〈s2〉 is independent of T2. Thus, the sensitivity,ξ , depends only on the receiver duty factor (tmax/T ). InSSNMR experiments, T is usually adjusted to 3T1, andhence, tmax/T = (c/3)T2/T1. For 1H SSNMR of diamag-netic systems, tmax/T is only about 0.03–0.1% (tmax ∼1 ms and T ∼1–3 s). On the other hand, for paramag-netic systems, tmax/T is as large as 10–30% (tmax ∼ 0.5–1ms and T ∼ 3–5 ms) because of enhanced resolution byVFMAS and short T1 values. Hence, when sidebands aresufficiently suppressed by VFMAS, the theoretical sensi-tivity of 1H SSNMR for paramagnetic systems is greaterthan that for diamagnetic systems by a factor of 10–30with VFMAS.

Microanalysis by 1H SSNMR

This exceptional sensitivity can be utilized in a form of1H SSNMR microanalysis of unlabeled paramagnetic sys-tems. We applied this 1H VFMAS method to two crystalforms of polycrystalline Cu(II)(8-quinolinol)2 [CuQ2] inorder to examine whether polymorphs of paramagneticdrugs or materials can be distinguished in a nanomolescale by 1H SSNMR. CuQ2 is an apoptosis inducer inhuman leukemia cells [31], and its β-form is known tobe thermally more stable [32]. Figure 2 shows the 1HVFMAS spectra of 20 nmol of (a) α- form and (b) β-form CuQ2. The sensitivity is excellent after only 10 minof signal accumulation. Clearly, these spectra are dis-tinguishable on the basis of the line positions and linewidths even without signal assignment. Considering thefact that 1H SSNMR rarely displays sufficient resolutionto distinguish polymorphs for diamagnetic systems and

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* * * * ** *

#

#

(a) α-form

(b) β-form

-100100 0 (ppm)

Fig. 2. 1H MAS spectra of (a) α-form and (b) β-form ofCu(II)(8-quinolinol)2 obtained at 1H frequency of 400.2 MHzwith 1-pulse excitation at ωR/2π = 27.03 kHz. Only 20 nmol(7 µg) of the sample was used for each spectrum. A total of(a) 36,560 and (b) 98,304 scans were recorded with recycle de-lays of (a) 15 ms and (b) 5 ms. The total experimental time was10 min each. Background signals were suppressed by subtract-ing a spectrum obtained for a rotor without the sample. Residualbackground signals and spinning sidebands are marked by # and*, respectively.

that various small paramagnetic metal complexes func-tion as drugs [10,31,33] and materials [8,34], the presentdata suggest unique possibility of distinguishing molecu-lar packing or supramolecular structures for paramagneticcomplexes by 1H SSNMR. As will be discussed, the highresolution in 1H SSNMR attained by VFMAS can be com-bined with 2D NMR with suitable polarization transfermethods.

1D 13C VFMAS SSNMR for Paramagnetic Systems

Spinning Speed Dependence of 13C MAS Spectra

Figure 3(a–c) shows the spinning speed dependence ofthe 13C MAS SSNMR spectra of unlabeled Cu(dl-Ala)2

obtained with high-power CW 1H RF decoupling (100kHz). In Figure 3(a), at ωR/2π = 5 kHz, only one peakthat corresponds to CH3 can be identified. In contrast,Figure 3(c) at ωR/2π = 24 kHz clearly displays threepeaks for CH3 (172 ppm), CO (−191 ppm), and CH (−276ppm) groups in Cu(dl-Ala)2 with improved resolution andsensitivity. Figure 3(d) shows the 13C VFMAS spectrumat ωR/2π = 24 kHz without 1H RF decoupling. The res-olution in (d) is superior to that in (c), in particular, for theCH signal. This is well explained by the fact that VFMASeffectively removes 1H–13C dipolar coupling regardlessof 1H resonance offsets and anisotropic shifts due to largeparamagnetic shifts, while the efficiency of 1H RF decou-pling is strongly affected by these parameters.

Sensitivity Enhancement by Cross-Polarization

Polarization transfer by cross-polarization (CP), one ofthe vital techniques in 13C SSNMR, has been ineffectivefor most paramagnetic systems because of large paramag-netic shift dispersion. In a few successful cases, includingthe initial high-resolution 13C SSNMR for paramagneticsystems by Bryant and coworkers [35], signals within alimited bandwidth (∼200 ppm) were observed at lowerfield (1H frequency ∼200 MHz) [23,35,36]. For systemshaving larger paramagnetic shift dispersion, CP transferefficiency is suppressed because large resonance offsetscause deviations from the Hartmann–Hahn condition withthe limited RF intensities that are available in a conven-tional MAS probe.

We recently demonstrated that further sensitivity en-hancement in 13C SSNMR spectra for paramagnetic sys-tems can be obtained using polarization transfer from1H spins with the strong RF fields available in VFMASprobes [27]. Figure 3(e) shows the 13C CP-VFMAS spec-trum of Cu(dl-Ala)2 obtained with high-power rampedCP. Because of short 1H T1 values, it is possible to ac-quire a larger number of scans within a given experimenttime. Clearly, the sensitivity was significantly enhancedin Figure 3(e), compared with (d). The sensitivity en-hancement factors in Figure 3(e) are 2.2–3.6 and 1.2,compared with the spectra in Figure 3(d) for protonatedand non-protonated 13C signals, respectively. Comparedwith the spectrum in (b) under moderate MAS at 10kHz, which was utilized in previous 13C SSNMR stud-ies for paramagnetic systems [22], the enhancement fac-tors are 4–5. As a result, the excellent sensitivity and res-olution in Figure 3(e) was obtained in only 1 min. Wewill discuss signal assignment of the spectrum in a latersection.

It has long been a problem for synthetic chemists thatsolution NMR of small paramagnetic systems is oftensubject to severe paramagnetic broadening because of along electron spin relaxation time for isolated paramag-netic molecules in solution [37]. Since excellent sensi-tivity was identified in the above 13C SSNMR spectraunder VFMAS, we compared the sensitivity between SS-NMR and solution NMR in order to examine the potentialof 13C SSNMR for analysis of paramagnetic systems asan alternative to 13C solution NMR. Figure 3(f) showspreliminary results of 13C solution NMR spectra of satu-rated Cu(dl-Ala)2 in D2O. To compensate for the lowsolubility of this sample (3.8 mg in 0.7 ml D2O), weacquired the signal for 3 h (102,400 scans) rather than1 min. Apparently, no signals were identified in (f). Al-though further studies are needed to examine applicabil-ity of SSNMR to other paramagnetic systems, it is en-couraging that 13C high-resolution SSNMR spectra wereobtained for the sample, for which solution NMR is noteffective.

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(b)

(c)

(d)(f)

(e)

**

400 0 -400(ppm) 400 0 -400(ppm)

Fig. 3. 13C MAS spectra of Cu(dl-Ala)2·H2O obtained with (a–d) 1-pulse excitation, and (e) adiabatic-CP, together with (f) 13Csolution NMR spectra of a saturated Cu(dl-Ala)2 solution in D2O. 13C NMR frequency was 100.6 MHz for all the spectra. Thespectra in (a–c) were acquired at ωR/2π of (a) 5 kHz, (b) 10 kHz, and (c) 24 kHz with 1H CW RF decoupling (100 kHz) witha recycle delay (τ d) of 0.1 s. The spectra in (d and e) were acquired without 1H RF decoupling and with τ d of 0.1 s, and 50 ms,respectively; (f) was obtained by 1-pulse excitation with WALZ 64 decoupling. For each of spectra (a–e), an experimental time was1 min with (a–d) 614, and (e) 1200 scans, while in (f) an experimental time was 3 h with 102,400 scans. The spectrum in (e) is scaledso that the spectra (a–e) display a common noise level in the figure. τ d for (a–d) was matched to three times of 13C T1 values. τ d in(e) was restricted by an RF duty factor (1%) to prevent a probe arcing. The 13C pulse widths for π/2 and π pulses were 2.5 and 5.0µs, respectively. In the CP experiment, the 13C RF field was swept from 107.5 to 124.5 kHz during a contact time of 0.5 ms, whilethe 1H RF field was kept constant at 92 kHz. The spinning sidebands are indicated by * in the spectra. The sample amount for (a–e)was 15 mg. For (f), 3.8 mg of the sample was dissolved in 0.7 ml D2O.

1H Decoupling Dependence of 13C VFMASSpectra

In Figure 4, we show 1H decoupling dependence of 13CMAS spectra of Cu(dl-Ala)2 (a–d) and Mn(acac)3 (e–h).The spectra were obtained with (a and e) no 1H RF decou-pling (decoupling by VFMAS), (b and f) 1H high-powerCW decoupling, (c and g) 1H TPPM decoupling, and (dand h) XY-8 π-pulse train decoupling [38], with a com-mon number of scans for each sample. For Cu(dl-Ala)2,the spectrum obtained with no RF decoupling in (a) dis-plays the resolution higher than that obtained with TPPMor CW RF decoupling. This is because majority of 1H–13Cdipolar couplings are removed by VFMAS over 20 kHz.We also tested π-pulse train decoupling, which was suc-cessfully applied to 19F decoupling [38]. In the π-pulsetrain, one π-pulse was rotor synchronously applied in onerotation cycle in order to avoid interference between the

RF decoupling and the averaging of 13C–1H dipolar cou-pling by VFMAS. Our result in (d) shows the π-pulsetrain decoupling does not enhance sensitivity comparedwith no decoupling in (a). For Mn(acac)3 in Figure 4(e–h),CW or TPPM decoupling sequences caused loss of signalor substantially reduced resolution. As already discussed,1H anisotropic shifts for this system reach 700 ppm (±140kHz in 1H 400 MHz); hence, it is reasonable that 1H RF de-coupling is ineffective. In contrast, with no RF decouplingin (e) or with a π-pulse train in (h), well-resolved signalswere identified, although sidebands were not completelysuppressed. It is also noteworthy that CW and TPPM de-coupling substantially increases the RF duty factor andlimits the maximum repetition rate. In contrast, droppingRF decoupling permits fast repetition for samples havingshort 13C T1 such as Mn(acac)3.

As recently reported by Ernst [39], averageHamiltonian analysis for an isolated I –K two spin system

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1000 0 -1000(ppm) (ppm)1000 0 -1000

(f) cw(e) no dec

(g) TPPM (h) XY8

(a) no dec (b) cw

400 0 -400

(d) XY8

(ppm) 400 0 -400(ppm)

(c) TPPM

Fig. 4. 1H decoupling dependence of 13C VFMAS spectra of (a–d) Cu(dl-Ala)2·H2O and (e–h) Mn(acac)3 at 13C NMR frequencyof 100.6 MHz. The spectra were obtained by 1-pulse excitation with (a, e) no 1H RF decoupling, (b and f) CW, (c and g) TPPM, and(d and h) XY-8 π -pulse train 1H decoupling. The spinning speed in (a–d) and (e–h) was set to 24.00 and 26.31 kHz, respectively.The spectra for each sample are displayed in a common scale. In CW or TPPM decoupling, a 1H RF field of 100 kHz was applied.In XY-8 decoupling, an XY-8 π -pulse train of a (d) 5-µs or (h) 3.4-µs pulse width was rotor synchronously applied with a π -pulseper rotation cycle. For Cu(dl-Ala)2, each spectra were recorded with common numbers of scans (1024 scans), recycle delays (τ d)of 0.1 s, and total experimental times of 1.8 min. For Mn(acac)3, the experimental times after 40,960 scans were (e, h) 11 and (f, g)31 min with τ d of (e, h) 15 or (f, g) 45 ms. The sample amount was (a–d) 15 and (e–h) 13.5 mg.

(I = 1H and K = 13C) shows that the second-order crossterm between 1H–13C dipolar coupling and 1H anisotropicshifts under high-power 1H CW RF decoupling is given by

H (2)CS−D =

2∑m=−2m =0

ωI K (−m)ωI (m)

ω12Iy K Z , (2)

where we assumed that the nutation frequency due to 1HRF decoupling, ω1/2π, is much larger than the spinning

frequency, ωR/2π (ω1 � ωR). In Equation (2), ωIK(n)and ω I (n) are Fourier coefficients for the frequencyof nωR/2π in time-dependent heteronuclear dipolarcoupling and anisotropic shift for the I spin, respectively.When ω I (m) due to the paramagnetic anisotropic shiftis comparable to ω1, |H (2)

CS−D | becomes comparable to|ωIK(−m)|, where |H (2)

CS−D | denotes the norm of H (2)CS−D .

Hence, 1H RF decoupling reintroduces 1H–13C dipolarcouplings for systems having large anisotropic shift,rather than removes the couplings, as shown in Figure 4(f and g). On the other hand, VFMAS eliminates all

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the higher order terms for the isolated I –K two-spinsystem, regardless of anisotropic shifts or resonanceoffsets. Therefore, we conclude that eliminating 1H RFdecoupling is the best option for small paramagneticsystems at this point. If 1H–13C J couplings need to beremoved, an XY-8 π-pulse train is an alternative.

Signal Assignments and Multi-dimensional NMR

1D Assignment Method

Signal assignments in SSNMR spectra of paramagneticsystems are often problematic because large paramag-netic shifts mask diamagnetic shifts, which are used toassign signals to specific chemical groups. To addressthis issue, we recently proposed a signal editing methodfor paramagnetic systems based on 13C–1H dipolar re-coupling using 1H–13C REDOR pulse sequence shown inFigure 5(a). With the two 1H π pulses (filled squares),this sequence reintroduces 13C–1H dipolar couplings, se-lectively dephasing protonated 13C signals; on the other

(b)

(c)

13CH3

13CO2

13CH-

200 0 -200(ppm)

(a)

1H

13C

CPx

CPx

2 2

Fig. 5. (a) A 1H–13C REDOR pulse sequence for 13C–1H dipolarfilter signal editing. When the two 1H π pulses (filled boxes) areapplied, 13C–1H dipolar couplings are restored under VFMAS.13C CPMAS spectra of Cu(dl-Ala)2·(H2O) obtained (b) withoutand (c) with 13C–1H dipolar dephasing using the pulse sequencein (a) at ωR/2π = 22.989 kHz (τR = 43.5 µs). 1H and 13Cπ-pulse widths were set to 5 µs.

hand, without these π pulses the sequence functions asa simple rotor synchronous echo sequence. In a controlexperiment for l-valine, we observed S/S0 = 90, 47, and8% for CO−

2 , CH3, and CH, respectively, where S andS0 denote the signal intensities obtained with and with-out the 1H π pulses, respectively. It is reasonable thatdephasing for a CH3 group is less than that for a CHgroup because CH3 rotation along the symmetry axis par-tially truncates 13C–1H dipolar couplings. Figure 5(b andc) shows 13C spectra of Cu(dl-Ala)2 obtained (b) with-out and (c) with 1H π pulses using the pulse sequenceshown in (a). S/S0 = 52, 83, and 12% for the peaks at173, −183, and −269 ppm, respectively. The assignmentbased on this result is indicated in Figure 5(b); this assign-ment agreed well with that by Liu et al. using selective 13Cand 2D labeling [22]. Compared with the previous assign-ment methods using selectively labeled samples, signalediting using dipolar recoupling under VFMAS providesreliable signal assignments for small amounts of unla-beled samples.

Assignment by 2D 13C–1H HeteronuclearCorrelation NMR

2D chemical shift correlation SSNMR has been use-ful for characterization of materials and biomolecules[12,30]. However, applications of this fundamental tech-nique to paramagnetic compounds have been limited to13C-labeled materials even for small molecules becauseof limited sensitivity and difficulty in broadband polariza-tion transfer. The VFMAS approach efficiently removes1H–1H and 1H–13C dipolar couplings, permitting high-resolution 1H and 13C SSNMR for paramagnetic com-plexes. Hence, with efficient broadband CP under VF-MAS, it is possible to correlate high-resolution 13C and1H SSNMR for unlabeled paramagnetic compounds in 2D13C–1H correlation SSNMR.

Figure 6(a) and (b) shows 1D 13C CPMAS and 2D13C–1H chemical shift correlation SSNMR spectra ofV(III)(acac)3,(acac =CH3–CO–CH–CO–CH3), respec-tively. In (B), six lines and three lines, which are overlap-ping in the 1D spectrum in (A), are resolved around 13Cshifts of −180 and 100 ppm. With 13C–1H dipolar dephas-ing, the six lines and the three lines were assigned to CH3

and CH groups, respectively [27]. Based on this result,we concluded that it is most likely that V(acac)3 has threemagnetically non-equivalent ligands because of distortionaway from the symmetry of the isolated molecule and thesignals for CO are broadened out by strong hyperfine cou-plings. The result agrees well with recent high-resolutionX-ray crystallography data, in which non-equivalence ofthe three ligand molecules was identified [40]. The 2Dspectrum in Figure 6 also provides the assignments in 1HSSNMR.

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(b)

(a)

150 50 -200

30

1 H s

hift

(ppm

)40

50

13C shift (ppm)

100 0 -100 -20013C shift (ppm)

Fig. 6. (a) 1D 13C CP-VFMAS and (b) 2D 13C–1H correlationVFMAS NMR spectra of V(III)(acac)3 obtained with rampedCP with a contact time of 0.5 ms, together with 1D skyline pro-jections in (b). The spinning speed is 22.99 kHz. In the rampedCP, the 13C RF field was swept from 74 to 82 kHz, while 1H RFfield was kept constant at 101 kHz. The recycle delay was 0.1s after each 2 ms of signal acquisition. In (a), only 1024 scanswere collected within a total experimental time of 1.8 min. In (b),25 t1 complex points were recorded with t1 increments of 43.5 µsand total of 1536 scans were collected for the real or imaginarycomponents of each t1 point. The total 2D experimental time was2.2 h.

Experimental Aspects

All the SSNMR data were recorded at 9.4 T with a VarianInfinityplus 400 spectrometer equipped with a Varian T33.2-mm MAS double-resonance probe and a home-built2.5-mm MAS double-resonance probe. All the SSNMRspectra obtained by 1-pulse excitation or CP were ac-quired with a rotor synchronous echo sequence prior tosignal acquisition. Since the spectral width of the para-magnetic complexes is large (up to 200 kHz at 9.4 T ), it

is difficult to obtain a flat baseline without the echo se-quence. To minimize receiver dead times, short samplingintervals (2–5 µs) were employed. After phase correc-tions, the baselines were corrected by the built-in poly-nomial correction function at Varian Spinsight software.It is well known that paramagnetic isotropic shifts havea 1/T dependence (Curie’s law). Because of this, severeline broadening can be induced by the temperature distri-bution over a sample heated by fast spinning. To suppressthe broadening, cooling air (−10 to 23 ◦C) was used atthe flow rate of 140–160 (ft)2/h using a Varian VT stack.A profile of the temperature distribution can be easilycharacterized by observing the 207Pb NMR line shape ofPb(NO3)2 under spinning [41].

Conclusion

We presented a set of novel fundamental techniques toachieve significant sensitivity and resolution enhance-ment as well as signal assignments using VFMAS in 1Dand 2D 1H and 13C SSNMR for paramagnetic complexes.The presented results in the VFMAS approach offers pos-sibilities of analyzing a variety of unlabeled paramagneticsystems by 1H and 13C high-resolution SSNMR for lim-ited sample volumes in a simple experimental setting. Wealso demonstrated applications of recoupling techniqueson paramagnetic systems, which had not been previouslyattempted. Structural analysis of paramagnetic systemsusing the recoupling techniques will be discussed else-where.

Acknowledgments

The authors are grateful to Prof. Cynthia Jameson at UICfor stimulating discussion. We are also grateful to Prof.Klaus Schmidt-Rohr at Iowa State University for sug-gesting XY-8 decoupling. This study was supported inpart by research grants from the Alzheimer’s Association(NIRG 035123) and the NSF CAREER program (CHE-0449952).

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