Precise measurement of 1H 90° pulse in solid-state NMR spectroscopy for complex and heterogeneous molecular systems

ORIGINAL PAPER

Precise measurement of 1H 90° pulse in solid-stateNMR spectroscopy for complex and heterogeneousmolecular systems

Pellegrino Conte & Alessandro Piccolo

Received: 27 October 2006 /Revised: 23 January 2007 /Accepted: 26 January 2007 / Published online: 17 February 2007# Springer-Verlag 2007

Abstract The 90° pulse calibration is essential in NMRspectroscopy to prevent artefacts in the liquid state or toenhance cross-polarization efficiency in the solid state. Weverified pulse-angle (PA) errors due to circuit impedancesin solid-state NMR and suggested a possible solution toprevent the inconvenience of PA errors. The classic pulsesequences used to calibrate 1H 90° pulse lengths by directdetection of protons or by cross-polarization were modifiedin order to replace single 1H pulses with 1H pulse trains.Pulse trains were found to decrease the effect of PAimperfections in the calibration of basic pulses (i.e. 90°and 180°) for a number of organic substrates. The modifiedsequences are especially important to rapidly obtain pulsecalibration of complex and heterogeneous molecular sys-tems such as humic substances, which usually require along time when using single 1H pulses.

Keywords Cross polarization magic angle spinningNMR . 1H 90° pulse . Pulse calibration . Circuit impedance .

Complex molecular systems

Introduction

Solid-state NMR spectra can be obtained by either directdetection (DD) of selected nuclei or cross-polarization(CP). The DD technique is applied to nuclei with a highrelative natural abundance. Conversely, CPMAS is used fornuclei with a low relative n0atural abundance, whose NMRsensitivity can be enhanced by magnetization transfer fromthe most abundant spin population that belongs to the samemolecular system. For example, in the case of organicsystems, CPMAS 13C-NMR experiments can be conductedby energy exchange from the very abundant proton spinpopulation to the rare 13C nuclear spins [1].

A key factor in obtaining reliable CPMAS 13C-NMRspectra is the correct setup of the NMR instrumentation.This is achieved by adjusting the magic angle with a KBrsample, verifying the 1H resonance frequency and the 1H90° pulse with adamantane or glycine (Gly), and, finally, bysetting the Hartmann–Hahn condition on Gly [2].

The calibration of the 1H 90° pulse in CPMAS 13C-NMRspectroscopy is of special importance since the CP efficiencydepends not only on the precise Hartmann–Hahn setting, butalso on the optimal value of the 1H 90° pulse [2, 3]. In fact,the 13C NMR sensitivity may decrease as the 1H pulse lengthdiffers from the optimal 90° value [2, 3].

The calibration of the 1H 90° pulse can be achieved byinversion-recovery (IR) experiments by either DD of protonsor CP [2]; namely the proton pulse length is increased untilDD and CP signals disappear [2, 4]. The pulse length thatcorresponds to the disappearance of the NMR peaks can be

Anal Bioanal Chem (2007) 387:2903–2909DOI 10.1007/s00216-007-1156-8

P. Conte (*)Dipartimento di Ingegneria e Tecnologie Agro-Forestali (DITAF),Università di Palermo,Viale delle Scienze 13,90128 Palermo, Italye-mail: [email protected]

P. Conte :A. PiccoloCentro di Ricerca Interdipartimentale per la Spettroscopia diRisonanza Magnetica Nucleare (CERMANU), Facoltà di Agraria,Università di Napoli Federico II,Via Università 100,80055 Portici, Italy

A. PiccoloDipartimento di Scienze del Suolo, della Pianta,dell’Ambiente e delle Produzioni Animali (DiSSPAPA),Università di Napoli Federico II,Via Università 100,80055 Portici, Italy

either a 1H 180° or a 1H 360° pulse from which the 1H 90°pulse can be easily obtained by dividing the experimentalresult by 2 or 4, respectively. This procedure commonlyinvolves the use of standards such as Gly and adamantanewhich are highly NMR sensitive and can be measured by CPwith only a few scans [2]. Conversely, owing to the highrelative natural abundance of protons, IR experiments by DDcan be easily conducted not only on Gly and adamantane,but also on different organic systems with a higher degree ofstructural complexity.

The limits of the IR measurements are related to the useof universal standards, such as Gly and adamantane, whichmay have a CP behaviour different from that of morecomplex materials, and to strong 1H–1H dipolar couplingsthat prevent NMR resolution in the DD of proton nuclei.Moreover, both CP and DD measurements may be affectedby pulse-angle (PA) errors that are generated by circuitimpedance. Circuit impedance changes the theoretical pulseshapes from rectangular to trapezoidal, thereby favouringdistortions in 1H 90° pulse calibration.

The PA errors are well known in liquid-state NMRspectroscopy. In fact, NMR manufacturers suggest modify-ing pulse sequences for PA calibration in liquid-state NMRby replacing single pulses with pulse trains (Bruker Biospin,personal communication). Thus, a single pulse sequenceapplied for 1H 90° calibration by IR in liquid-state NMR ischanged to a sequence containing four pulses whose sumcorresponds to a 1H 360° flip-angle pulse (4×90° sequence).The advantage of the use of a 4×90° sequence in liquid-stateNMR over a 2×90° one, which also leads to annulment ofNMR signals, relies on the shorter time delay needed byprotons to recover the ground state, and, thus, on thereduction of the time needed for the experiment.

In the present work we first verified the occurrence ofPA errors in solid-state NMR spectroscopy, and, then,modified DD and CP sequences in order to replace singleproton pulses with 4×90° pulse trains. Our objective was tosuggest a 4×90° sequences for a precise calibration of 1H90° pulses in the solid-state NMR spectroscopy forcomplex and heterogeneous molecular systems.

Materials and methods

Model compounds

Gly, hexamethylbenzene (HMB), carboxymethylcellulosesodium salt (CMC), sodium dodecyl sulphonate (SDS) andpepsin were purchased from Sigma and used without furtherpurification. A humic acid (HA1), a fulvic acid (FA1) and ahumin (UM1) were isolated from an Italian volcanic soil,while other humic acids were obtained from a 90-day-oldcompost (HA2), a lignite from North Dakota (HA3) and a

Nigerian soil (HA4). Further information on extractions andpurification methods is given elsewhere [5].

NMR spectroscopy

CP magic angle spinning (CPMAS) 13C-NMR spectra wereacquired with a Bruker AVANCE™ 300, equipped with a4-mm-wide-bore MAS probe, operating at a 13C resonatingfrequency of 75.475 MHz, and a rotor spin rate of 5,000±1 Hz. Samples were packed in 4-mm zirconia rotors withKel-F caps. In total, 3,782 data points (TD) were collectedover an acquisition time (AQ) of 50 ms, a recycle delay (RD)from 2.0 to 5.0 s, and from 4 to 500 scans (NS). Recycledelays and the number of scans were chosen depending onthe sample. Gly and HMB were analysed with RD=5 s andNS=4 and 20, respectively. CMC and SDS were studiedusing RD=2 s and NS=100. Humic substances and pepsinwere analysed with RD=2 s and NS=500. Contact timesranged from 1 ms (for humic acids, FA1 and UM1) to 5 ms(for Gly). The attenuation level (PL2) applied on the 1Hchannel for the calibration of the 1H 90° pulse was −0.50 dB.A 1H ramp sequence was used to account for possibleinhomogeneity of the Hartmann–Hahn condition [6].

DD of 1H-NMR spectra was also performed. Spectra wereacquired by applying one scan, AQ=38 ms, TD=3,782,RD=5 s and an attenuation level on the proton channel of−0.50 dB. Proton pulse length was varied in the range 2.5–7.5 μs in order to find the optimum 1H 90° pulse length. Arotor spin rate of 13±1 kHz was used to account for thestrong 1H–1H dipolar interactions that broaden peaks anddecrease solid-state 1H-NMR resolution.

The Bruker Topspin 1.3 software was used to collectspectra, while evaluation of the spectra was conducted byMestre-C software, version 4.9.9.9. All the free inductiondecays were transformed by applying, first, an 8 k zerofilling, and, then, an exponential filter function with a linebroadening of 50 Hz. A third-order polynomial with theBernstein algorithm was used for baseline correction of theNMR spectra [7].

Results and discussion

1H pulse calibration by direct detection

The easiest way to calibrate 90° pulses in NMR spectroscopyis to change the pulse length applied on a selected nucleusuntil its resonance disappears (IR measurement) [4]. Oncethis happens, either 180° or 360° pulses are achieved.Finally, an appropriate multiplication by 0.5 (when 180°pulses are measured) or 0.25 (when 360° pulses areobserved) provides a correct value for the 90° pulse lengthof the nucleus under investigation.

2904 Anal Bioanal Chem (2007) 387:2903–2909

The solid-state 1H-NMR spectra of Gly measured byvarying the proton pulse length in the interval 3.5–7.5 μs ata rotor spin rate of 13±1 kHz are reported in Fig. 1a. Thecentral broad peak included all the signals of Gly protons.Its width was due to the strong homonuclear dipolarinteractions that were not effectively reduced even by thefast rotor spin rate that was used for the DD experiments. Afurther resolution enhancement can be only obtained byapplying a special multipulse sequence, such as CRAMPS[8, 9], which requires special hardware settings unavailablefor the instrument used here.

Figure 1a shows that the 1H 180° pulse was included inthe 6.0–6.5-μs time interval. More precise experimentsrevealed that the 1H 180° pulse length was 6.32 μs, fromwhich a 1H 90° pulse of 3.16 μs was calculated. A fastcheck of the correctness of the 3.16-μs pulse length wasconducted by repeating the 1H DD experiment with a pulseof 12.64 μs, corresponding to a 1H 360° pulse. If the 1H 90°pulse of 3.16 μs was correct, the use of the 1H 360° pulseshould have annulled the proton signal. However, owing tothe effect of circuit impedance, a positive signal was stillobserved, as reported in the upper spectrum of Fig. 1b. Thecircuit impedance affects the shape of the pulses, thusfavouring trapezoidal pulse shapes instead of theoreticalrectangular ones. The suggestion of NMR manufacturers tocompensate for PA errors in liquid-state NMR duringcalibration of basic pulses (i.e. 90° and 180°) is to apply asequence where the calibrating pulse is split into severalsmaller pulses (Bruker Biospin, personal communication).On this basis, the pulse sequence for the direct observation

of the proton nuclei in solid-state experiments was changedinto four consecutive 90° pulses on the 1H channel (4×90°DD). The 4×90° DD sequence confirmed that the 1H 90°pulse of 3.16 μs was too long (the middle spectrum inFig. 1b was still positive) and allowed us to rapidly find apulse length of 2.80 μs as the correct one to annul theproton spectrum of Gly (bottom spectrum in Fig. 1b).

An experimental approach similar to that used for Glywas also applied to evaluate the spectra of SDS, CMC,HMB, pepsin and a series of humic materials. Table 1reports the 1H 90° pulses measured for these differentmolecular systems by using either the IR or the 4×90° DDsequences. The IR measurements revealed that the 1H 90°pulses were all in the range 2.96–3.14 μs (Table 1). Infact, the broad proton signals of SDS, CMC, HMB, pepsinand humic substances were annulled by applying 1H 180°pulses in the interval 5.92–6.34 μs. However, afterapplication of 360° pulses (in the range 11.84–12.68 μs),the spectra of all the samples were not cancelled asexpected if the 1H 90° pulses by IR were correct. Therapid application of the 4×90° DD pulse sequence showedthat SDS, CMC and HMB had a similar 1H 90° pulselength of 2.83 μs, whereas all the systems with highermolecular complexity (pepsin and humic materials) had1H pulse lengths ranging between 2.80 and 2.90 μs(Table 1).

Table 1 1H 90° pulse lengths measured by direct detection (DD) withinversion recovery and 4×90° DD sequence

Samples 1H 90° pulse (μs) byDD (inversion-recoverymeasurement)

1H 90° pulse (μs) by4×90° DD sequencea

SDS 3.12 2.83CMC 3.02 2.83HMB 3.06 2.83Pepsin 3.11 2.80HA1 3.17 2.90FA1 3.12 2.88UM1 3.04 2.80HA2 2.96 2.80HA3 2.98 2.84HA4 3.05 2.84

SDS sodium dodecyl sulphonate, CMC carboxymethylcellulosesodium salt, HMB hexamethylbenzene, HA1 a humic acid isolatedfrom an Italian volcanic soil, FA1 a fulvic acid isolated from anItalian volcanic soil, UM1 a humin isolated from an Italian volcanicsoil, HA2 humic acid obtained from a 90-day-old compost, HA3humic acid obtained from a lignite from North Dakota, HA4 humicacid obtained from a Nigerian soil

a The 1 H 90° pulse lengths measured by a 4×90° cross-polarizationsequence were similar to those measured by the 4×90° DD sequence

Fig. 1 1H-NMR spectra of glycine obtained by direct detection.a Inversion-recovery experiments. b Top spectrum acquired with a 1H360° pulse; middle and bottom spectra obtained by application of the4×90° direct detection sequence

Anal Bioanal Chem (2007) 387:2903–2909 2905

1H pulse calibration by cross-polarization

The classic CPMAS 13C-NMR experiment consists in theapplication of a 1H 90° pulse (generally phased along -x) thatproduces a y component of the proton magnetization (1HMy). Afterwards, two different radiofrequency (rf) fields areapplied, with amplitudes matching the Hartman–Hahncondition, and set to the Larmor frequencies of protons andcarbons, respectively. The effect of the rf fields is to spin-lock the 1H My during the contact time and to favour energytransfer from protons to carbons (CP). The net result is a 13Cpolarization that allows the acquisition of 13C-NMR spectra[10].

CP can only be achieved when a y component of theproton magnetization is generated. Moreover, the sign andthe intensity of the 13C-NMR signals also depend on theproton pulse length (Fig. 2a). In fact, when a 1H pulse isable to reverse the proton magnetization towards the xy-plane (1H 90° flip-angle pulse) the maximum intensity of apositive 13C signal is obtained (Fig. 2a). As the 1H pulselength is increased, the carbon signal obtained by CP is firstcancelled (1H 180° pulse), then it assumes the largestnegative value (1H 270° pulse) and afterwards it is cancelledonce again for the application of a 1H 360° pulse (Fig. 2a) [2].

In order to measure a precise value of the 1H 90° pulselength and compare it with that measured by DD, CP IRexperiments were conducted. In these experiments, theproton pulse length was varied, as previously reported, tofind the 1H 180° pulse corresponding to the disappearanceof the 13C-NMR signals [2].

The evaluation of the data reported in Fig. 2b revealedthat the signals generated by the two carbons of Gly (acarboxylic and a methylenic carbon resonating at 176 and43 ppm, respectively) were cancelled with a 1H pulselength ranging between 5.50 and 6.00 μs. More detailed IRmeasurements in the 5.50–6.00-μs interval showed that the1H 180° pulse length was 5.92 μs, from which a 1H 90°pulse length of 2.96 μs was calculated.

To confirm the correctness of this measurement, the CPsequence was modified by replacing the 1H 90° pulse witha 1H 360° pulse. On the assumption that the 1H 90° pulse

Fig. 2 a 13C signal behaviourby cross-polarization as affectedby proton pulse length. b Spec-tra of glycine obtained by astandard cross-polarization (CP)procedure where the pulse onthe protons was varied until aspectrum with a null 13C-NMRsignal was observed (inversion-recovery sequence)

Fig. 3 a–f Spectra of glycine and sodium dodecyl sulphonate (SDS).A CP spectrum of glycine with a proton pulse of 2.96 μs as calibratedwith the inversion-recovery sequence; B as in A but with a protonpulse of 11.84 μs corresponding to a 1H 360° pulse; C CP spectrumof glycine obtained with the 4×90° pulse sequence and a 1H pulse of2.80 μs; D CP spectrum of SDS acquired with a 1H 90° pulse of2.80 μs; E as in D but with a 1H 360° pulse of 11.20 μs; F SDSspectrum acquired with the 4×90° pulse sequence and a 1H 90° pulseof 2.83 μs. G CP spectrum of carboxymethylcellulose sodium salt(CMC) obtained with a proton pulse of 2.80 μs. H CMC spectrumacquired with a 1H 360° pulse of 11.20 μs. I CMC spectrum acquiredwith the 4×90° sequence with a proton pulse of 2.83 μs. J CPspectrum of hexamethylbenzene (HMB) with a 1H pulse of 2.80 μs. KHMB spectrum with a 1H 360° pulse of 11.20 μs. L A 4×90° HMBspectrum with a 1H pulse of 2.83 μs. M CP spectrum of pepsinobtained with a proton pulse of 2.80 μs. N Spectrum of pepsinacquired with the 4×90° pulse sequence and a 2.80-μs proton pulse.The pepsin spectrum acquired with a CP sequence containing a1H 360° pulse of 11.20 μs was similar to that reported in B. O CPspectrum of a humic acid isolated from an Italian volcanic soil (HA1)with a 1H pulse of 2.80 μs. P HA1 spectrum with a 1H 360° pulse of11.20 μs. Q A 4×90° HA1 spectrum with a 1H pulse of 2.90 μs

�



measured by IR was correct, the application of the 360°pulse on protons should have produced a realignment of the1H magnetization along the z-axis, thereby inhibiting CPand cancelling 13C-NMR signals. The CPMAS 13C-NMRspectra of Gly acquired with the 1H 90° pulse of 2.96 μs,and with a 1H 360° pulse of 11.84 μs (i.e. 4×2.96 μs), areshown in Fig. 3, spectra A and B, respectively. The positivesignals in Fig. 3, spectrum B indicated that a residual 1HMy

was still present after application of the 360° pulse onprotons, thereby implying that the 1H 90° pulse measuredby the IR technique was too long.

The incorrect 1H 90° pulse lengths measured by IR wereattributed to the imperfect shape of the 1H pulse appliedduring the experiments owing to circuit impedances. Asalready done in the calibration of the 1H 90° pulse by DD,we attempted to reduce PA errors in CPMAS by applying asequence where the 1H 360° pulse was split into fourconsecutive 1H 90° pulses (4×90° CP sequence). Theapplication of the 4×90° CP sequence on Gly (Fig. 3,spectrum C) revealed that the 13C signals of this modelcompound were successfully cancelled by a 1H 90° pulse of2.80 μs.

Pulse calibration to obtain CPMAS 13C-NMR on differentmolecules and materials is commonly conducted on simpleorganic standards having high NMR sensitivity such as Gly[2]. However, it is possible that the 1H 90° pulse measuredfor Gly may be incorrect when structurally more complexmaterials are analysed. A number of molecular systems ofdifferent complexity and heterogeneity were subjected toeither the 1H 90° pulse measured for Gly or to the alternativepulse sequence in order to achieve the most rapid and preciseproton pulse calibration.

The spectrum of SDS (Fig. 3, spectrum D) revealed fourmain peaks attributed to C1 (79 ppm), C2–C10 (33 ppm),C11 (24 ppm) and C12 (14 ppm). After application of a 1H360° pulse of 11.20 μs (4×2.80 μs), a residual negative signalof C2–C10 was still observed (Fig. 3, spectrum E); hence, the1H 90° pulse as calibrated for Gly was too short to completelycancel the alkyl carbons of SDS. A rapid application of the4×90° CP pulse sequence revealed that the disappearance ofall the SDS signals could be achieved when the 1H 90° pulselength was 2.83 μs (Fig. 3, spectrum F).

Figure 3, spectrum G reports the spectrum of CMC.The signal at 178 ppm was assigned to carboxyl carbons,the resonance at 104 ppm was due to anomeric carbons, thepeak at 74 ppm was attributed to the C2–C5 and theresonance at 62 ppm was generated by the –O-CH2–groups. PA errors due to circuit impedances did not allowthe complete disappearance of the C1–C5 signals onapplication of a 1H 360° pulse of 11.20 μs based on theGly calibration (4×2.80 μs). In fact, negative signals werestill present at 104 and 74 ppm in the spectrum of CMC(Fig. 3, spectrum H); therefore, the 1H 90° pulse calibrated

using Gly was too short for CMC. Conversely, theapplication of the 4×90° CP sequence revealed that thecorrect 1H 90° pulse length was 2.83 μs (Fig. 3, spectrum I).

As for SDS, also the HMB spectrum, containing onlytwo signals at 132 ppm (aromatic carbons) and 17 ppm(methyl groups) (Fig. 3, spectrum J), was not completelycancelled and showed residual negative peaks when the 1H360° pulse of 11.20 μs was applied (Fig. 3, spectrum K).Only the use of the 4×90° CP sequence enabled us tomeasure a pulse length of 2.83 μs that annulled all HMBsignals (Fig. 3, spectrum L).

The spectra of pepsin and only one of the differenthumic substances (HA1) (Fig. 3, spectra M and O,respectively) were largely more complex than those of theaforementioned molecules. The signals may be generallyattributed to carboxyls (172 ppm), aromatic carbons(128 ppm), acetal carbons (from 107 to 92 ppm), O-alkyl/N-alkyl groups (from 92 to 54 ppm) and to alkyl carbons(from 54 to 0 ppm) [11]. The application of a 1H 360° pulseof 11.20 μs when obtaining the CPMAS 13C-NMR spectraof pepsin did not produce residual signals. The same resultwas obtained when the 4×90° CP sequence was applied(Fig. 3, spectrum N), thereby showing no differencebetween the latter pulse and that obtained with calibrationusing Gly.

The application of the same 11.20-μs proton pulse lengthproduced spectra of humic materials (Fig. 3, spectrum Oshows only that of HA1 as an example) with negativesignals in HA1 (Fig. 3, spectrum P), FA1, HA3 and HA4.This result suggested that the 1H 90° pulse calibrated usingGly was too short for these complex molecular systems.The application of the 4×90° CP sequence showed insteadthat the correct 1H 90° pulse was 2.90 μs for HA1 (Fig. 3,spectrum Q), 2.88 μs for FA1 and 2.84 μs for HA3 andHA4. Conversely, the spectra of UM1 and HA2 wereannulled when the 11.20-μs pulse length was applied,thereby indicating that the 1H 90° pulse of 2.80 μs, ascalibrated using Gly, was the correct 90° pulse for UM1and HA2.

In order to confirm previous findings by Taylor [2] andGoetz [3] on the increase in sensitivity of CPMAS 13C-NMR signals when a correct 1H 90° pulse is applied, wemeasured the CP sensitivity achieved when the 90° pulsewas obtained by the 1H 4×90° sequence in comparison tothat obtained by the 1H 360° pulse. The results (spectra notreported) revealed that the improvement in sensitivity of theNMR signals in CPMAS 13C-NMR spectra was in the 2–6% range. The lowest increase in sensitivity was observedin spectra of molecular systems whose 1H 90° pulse did notexcessively differ from that measured for Gly. Conversely,the spectra of HA1 and FA1, showing a much longer 1H90° pulse than that of Gly (Table 1), provided the largestenhancement in sensitivity.


Conclusions

The present work confirmed that PA errors due to circuitimpedances affect the correct measurement of standard pulsesnot only in liquid-state spectroscopy, but also in solid-stateNMR. In order to account for PA errors, calibration of 1H 90°pulses in solid-state NMR spectroscopy was achieved with anew sequence modified from the classic DD and CP ones byapplying four pulses on the proton channel. In fact, in linewith informal applications in liquid-state NMR spectroscopy,the use of pulse trains, instead of single pulses in the IRmethod, reduced the errors in the 1H 90° pulse calibrationalso in solid-state NMR experiments.

This work also showed that 1H 90° pulses may differdepending on the nature of the molecular system underanalysis. However, the prediction of 1H 90° pulse cannot beachieved simply on the basis of the structural complexity ofsamples investigated. In fact, it was found that the 1H 90°pulse measured for Gly was identical to that calibratedusing pepsin, UM1 and HA2 (Table 1). Conversely, thestructurally simple SDS, HMB and CMC revealed a 1H 90°pulse length (2.83 μs) that was only slightly different fromthat measured for the more structurally complex andheterogeneous HA3 and HA4 (2.84 μs). The 1H 90° pulselengths of SDS, HMB, CMC, HA3 and HA4 were, instead,very different from those of HA1 and FA1, which revealedproton pulse lengths of 2.90 and 2.88 μs, respectively(Table 1). Nevertheless, it is important to recognize that acorrect value for the 1H 90° pulse can be rapidly obtainedby applying a 4×90° DD sequence on a number ofmolecular systems without relying on that simply measuredfor Gly. The use of the train pulse described here becomesessential for complex and heterogeneous molecular systems

such as humic matter from the environmental compartment,recycled biomasses and the mining industry, whose 1H 90°pulse calculation by traditional IR may be extremely time-consuming and still subject to PA errors. Moreover, owingto the enhancement in sensitivity from 2 to 6% measuredfor complex systems when the correct 1H 90° pulse wasapplied, a precise measurement of the 1H 90° pulse may bealso recommended to decrease errors in the quantitativeevaluation of CPMAS 13C-NMR spectra of complexmaterials.

Acknowledgements The authors gratefully acknowledge a fruitfuldiscussion with Francesca Benevelli from Bruker Biospin, Italy, andthe help of Daniela Šmejkalová (Università di Napoli Federico II) inevaluation of spectra.

References

1. Duer MJ (ed) (2002) Solid-state NMR spectroscopy: principlesand applications. Blackwell, Oxford

2. Taylor RE (2004) Concepts Magn Reson Part A 22:37–493. Goez M (1992) Magn Reson Chem 30:958–9614. Berger S, Braun S (eds) (2004) 200 and more NMR experiments.

Wiley-VCH, Weinheim5. Piccolo A, Conte P, Spaccini R, Mbagwu JSC (2005) Eur J Soil

Sci 56:343–3526. Conte P, Spaccini R, Piccolo A (2004) Prog Nucl Magn Reson

Spectrosc 44:215–2237. Brown DE (1995) J Magn Reson 114:268–2708. Hafner S, Spiess HW (1997) Solid State Nucl Magn Reson

8:17–249. Lewitt MW, Lowe AJ (1997) Thermochim Acta 294:13–21

10. Hartmann SR, Hahn EI (1962) Phys Rev 128:2042–205311. Wilson MA (1987) NMR techniques and applications in geo-

chemistry and soil chemistry, 1st edn. Pergamon, Oxford


Documents

Precise measurement of 1H 90° pulse in solid-state NMR spectroscopy for complex and heterogeneous molecular systems