Letter - correspondence Received: 12 August 2014
Accepted: 29 September 2014
Published online in Wiley Online Library: 22 October 2014
(wileyonlinelibrary.com) DOI 10.1002/mrc.4171
Acetone-induced polymerization of 3-aminopropyltrimethoxysilane (APTMS) as revealed by NMR spectroscopy – revisited Jan Schraml,a* Stefan Korec,b Martin Krumpb and Jan Čermáka,c Introduction Silane coupling agents, represented for example by 3-aminopropyltriethoxysilane (APTES, H2N–(CH2)3Si(OC2H5)3) or 3-aminopropyltrimethoxysilane (APTMS, H2N–(CH2)3Si(OCH3)3) find a widespread use in various scientific and industrial applications, and so, their chemical reactions are studied intensely, for the leading references, see refs.[1–3] Recently, Mazzei, Fusco and Piccolo published[4] a very detailed and careful NMR study of acetone-induced polymerization of APTMS. Using the full panoply of NMR techniques applied to 1H, 13 C and 29Si nuclei, they proved that acetone reacts with the silane amino group to form an imine ((CH3)2C¼N(CH2)3Si(OCH3)3, IPTMS or N-isopropylidene-3-aminopropyltrimethoxysilane. The released water then hydrolyses the methoxysilane inducing thus formation of a siloxane Si–O–Si bridge. (For the detailed description refer to Figs. 1 and 2 of reference.[4]) We would like to correct one error and comment on some details of these reactions without questioning the fundamental findings of the quoted paper,[4] i.e. formation of imine and siloxane as well as 1H and 13C NMR line assignments. The error concerns the interpretation of 29Si chemical shifts of APTMS and IPTMS. Although the 13C spectrum recorded 0.88 h from the reaction start (Fig. 4[4]) showed approximately 1 : 1 mixture of APTMS and IPTMS, no 29Si line of IPTMS was found in 29Si DEPT spectra (Fig. S4[4]) that exhibited only one line at 41.8 ppm that was assigned to APTMS.
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All the spectral measurements were performed on a Varian/Agilent INOVA-500 spectrometer (operating at 499.9, 125.7 and 99.3 MHz for 1H, 13C and 29Si NMR, respectively) using a 5 mm switchable PFG broadband probe. The standard software (vnmrJ 3.2A) was used. Each 29Si and 13C NMR measurement was preceded by 1H NMR measurement, all performed at 25°C. In 29Si INEPT[5] measurements of the reaction mixture, the delays were not optimized, routine parameters for trimethylsilyl groups[6] provided sufficient S/N. When a liner was employed, 160 transients were accumulated to keep the total measuring time reasonably short (8 min) on the time scale of the reaction; without the liner, the number of transients was reduced to 120. For the measurement of tetramethylsilane (TMS) signal, the delays were optimized [7] to match proton coupling to 12 methyl protons and 2 J(29Si–1H) = 6.5 Hz; the measurement required accumulation of 26 400 transients. The 29Si NMR spectra were measured also by the ring down elimination (RIDE)[8,9] pulse sequence. The read pulse (80° rectangular) was
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preceded by WURST (wideband, uniform rate, smooth truncation)[10] inversion pulse; 2080 transients were accumulated. The 29Si NMR FIDs were acquired using the spectral width of 25 kHz. WALTZ[11] proton decoupling was applied during acquisition (1.0 s) (gated decoupling to suppress NOE which might be negative) and not through the relaxation delays (20.0 s in RIDE and 2.0 s in INEPT experiment). Zero filling to 128 K and a mild line broadening (0.1 Hz in INEPT and 3.0 Hz in RIDE) were used in data processing unless noted otherwise. 13 C NMR spectra were acquired using 35 kHz spectral width, 1.0 s acquisition time and 5.0 s relaxation delay, 80° flip angle, 64 transients provided sufficient quality spectra using 1.0 Hz line broadening and zero filling to 128 K and the same proton decoupling as in INEPT spectra except that it was used for the whole duration of the measurements. The spectra were referenced to the central line of chloroform-d1 at δ = 76.99. 1 H NMR spectra were measured by a simple one-pulse sequence using 70° flip pulse, the spectral width of 7 kHz, acquisition time of 2.0 s and relaxation delay of 5 s. Four transients were accumulated; the FIDs were zero filled to 128 K; no weighting was used. Since the chemical shifts (and also the lock signal) exhibit large solvent and concentration effects in the reaction mixture, the 29Si NMR line of the imine product at δ = -41.750 was used as a secondary reference. Its chemical shift was determined relatively to TMS signal ((CH3)4Si, δ = 0.000) in a separate experiment performed on the reaction mixture 69 h after mixing. The chemical shift differences (Δδ) within one spectrum are reliable at least to 0.01 ppm; 1 H, 13C and 29Si chemical shifts (in δ scale) are reliable less as secondary references had to be used and as the chemical shift of the primary reference (TMS) for all three nuclei measured varied in the reaction mixture studied. APTMS, 3-aminopropyltrimethoxysilane (H2N(CH2)3Si(OCH3)3) was Aldrich 97% product, it was used as delivered. The results reported here were obtained in Aldrich’s 99.8 at% CDCl3 with 0.03% (V/V) of TMS. Acetone of G.R. grade was provided by Lachner.
* Correspondence to: Jan Schraml, Institute of Chemical Process Fundamentals of the ASCR, v.v.i., Rozvojová 135, 165 02 Prague 6, Czech Republic. E-mail: [email protected] a Institute of Chemical Process Fundamentals of the ASCR v. v. i., Rozvojová 135, 165 02, Prague 6, Czech Republic b SChem a.s., Pobřežní 394/12, 186 00, Prague 8, Czech Republic c Department of Chemistry, Faculty of Science, Purkinje University Ústí nad Labem, České mládeže 8, 400 96, Ústí nad Labem, Czech Republic
Copyright © 2014 John Wiley & Sons, Ltd.
Acetone-induced polymerization of 3-aminopropyltrimethoxysilane Deuterochloroform was stored over activated molecular sieves 4A for several days before use. Solvent and reaction components were weighted into a dried glass vial, vigorously mixed for a short period of time (2 min) and then transferred by a dry pipette directly into a NMR tube (Wilmad 528) or the same NMR tube equipped with a PTFE-FEP liner (Wilmad). The tubes were sealed for the long experiments; for shorter ones (24 h), septum stoppers were used; the liner was closed with the stopper provided. The first spectra were measured some 5–12 min after mixing depending how much time was consumed by probe rf. tuning and magnetic field shimming. It should be noted that rf. tuning changed considerably during the course of the reaction. We used various initial concentrations (all concentrations are given in mole fractions) for the reaction mixture; the results reproduced here started from the following initial concentrations: for 29Si measurements in the liner 0.119 APTMS and 0.289 acetone, in the NMR tube 0.102 APTMS and 0.334 acetone; for 13C measurements 0.122 (or 0.071) APTMS and 0.291 (or 0.229) acetone. (The original concentrations were 0.116 APTMS and 0.347 acetone.[4])
Results and discussion The supporting information of the discussed work[4] presents (Fig. S1) a part (δ = 0–4 ppm) of 1H NMR spectrum of APTMS in chloroform where the line due to NH2 protons is missing. The explanation offered by the authors – the signal is very broadened as a consequence of involvement in chemical exchange – raises some doubts as it contradicts our experience. As the 1H NMR spectra in Fig. 1 show, the line due to NH2 protons is close to its predicted position (δ = 1.092 [12]) both in the neat APTMS (δ = 1.098, Fig. 1, bottom) and in its 10% (V/V) solution in dry chloroform-d1 (δ = 1.027, Fig. 1, top). In the reaction mixture, however, the exchangeable protons are hard to see during the initial reaction stages as illustrated in Fig. 2. Then, the broad signal of exchangeable protons becomes noticeable and moves to its final position (δ = 4.6 ± 0.3) as described.[4] It should be mentioned that the NH2 protons exchanged with D from CDCl3, so in the course of time, the line of the residual CHCl3 increases in intensity (accompanied with a
1
H NMR spectra of APTMS. Top, 10% (V/V) solution in chloroform-d1; bottom, neat APTMS sample (spectra referenced to CH3O line at δ = 3.490).
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Figure 1.
decrease in the lock level) while the intensity of NH2 line decreases. This exchange is apparent from the 13C NMR spectrum (Fig. 3) of chloroform carbon as an increase in the intensity of CH carbon signal within the triplet of CD carbon. The 29Si DEPT spectra of the reaction mixture (Fig. S4) exhibited[4] surprisingly large line widths about 10 Hz (partly due to the line broadening of 5 Hz by exponential multiplication of the FIDs). In our experience,[13,14] the 29Si NMR lines have routinely line widths less than 1 Hz. The question therefore arises whether there is some fast exchange process involving Si atoms. Repeating the 29Si part of the work[4] as closely as possible, we obtained more complex picture, refer to Figs. 4 and 5. The lines in these spectra measured in the liner have the line widths around 1 Hz; narrower lines (0.5 Hz) are obtained if the liner is not used. So, what appeared in the discussed paper[4] as one line (δ = 41.8) are in fact two lines (Fig. 5), one at δ = 41.787 and the other at δ = 41.750 (Δδ = 0.037 ppm), and no exchange process is taking place on the silicon atom. Since these two lines are the most intense lines for the first hours of the reaction and since the intensity of the former line decreases while that of the latter increases during the first 10 h (to decrease afterwards), one concludes that the former line is due to the silicon in the APTMS and the other line belongs to the first intermediate product, the imine, ((CH3) 1 13 C NMR 2C¼N(CH2)3Si(OCH3)3, IPTMS) identified through H and spectra.[4] This conclusion is supported by the small 29Si chemical shift difference between this product and the starting APTMS that is in line with the substituent effect expected for a change in gamma position to the silicon.[15] Also, separations of 13C satellites (97.8 Hz in APTMS and 97.3 Hz in IPTMS) matches the separations of 29Si satellites in the assigned 13C NMR spectra (CH2–Si carbons). These separations agree with the values of 1 J(29Si–13C) couplings in similar systems.[16] The other possibilities would lead to much larger change in 29Si chemical shift. Hydrolytic replacement of one OCH3 group by OH is estimated to cause deshielding in chloroform solution (i.e. in the correct direction) of about 0.6 ppm on the basis of literature data.[17–19] Even larger (10 ppm) and in the opposite direction, 29Si shifts (shielding) are known to accompany replacement of OCH3 group by a silyl substituent (for a review refer
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Figure 2. H NMR spectrum in the course of initial reaction stages (0.071 APTMS and 0.229 acetone). The time (in hours) since mixing is indicated for each trace; signals from exchangeable protons are marked by the arrows in the traces with amplified intensities.
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Figure 3. C NMR spectra (chloroform part) of the reaction mixture (0.122 APTMS and 0.291 acetone) in the course of the reaction. Acquisition starting time since mixing is indicated in hours.
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to.[20]) According to this silylation shift, the line around δ = 50.3 was assigned to silicon atoms with only one Si–O–Si linkage[4] and the line around δ = 57.7 to silicon with two such linkages.[4] In polysiloxane terminology,[21] the former line is the terminal M unit (MRR′R″, where R = H2N(CH2)3 or (CH2)3N¼C(CH3)2 and R′ or R″ = OCH3 or OH) while the later is the chain prolonging unit (DRR′). Our measurements (Fig. 4) confirm the reported time dependence of these signals[4] but again show more complex picture. In the M region, there are in fact three lines (M1, M2, and M3) with the chemical shifts δ = 50.157, δ = 50.360, and δ = 50.570 (at 15.7 h since mixing) in an approximate integral ratio 10 : 4 : 1, respectively. While
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the most intense line M1 does not move with the reaction time, M2 and M3 move further away from M1. In a detailed look into the first three spectra, M1 line appears as two (with the line separation Δδ = 0.013 ppm). Since at this reaction time, there are only signals from APTMS and IPTMS besides the discussed pair of M1 lines, the two lines cannot terminate any siloxane chain and so are most probably results of IPTMS–IPTMS and IPTMS–APTMS dimer formation, the latter being converted into the former in the course of reaction. The two remaining lines M2 and M3 are shifted upfield from the dimer by approximately 0.2 and 0.4 ppm, respectively. Since hydrolytic replacement of an alkoxy group by hydroxyl group
Copyright © 2014 John Wiley & Sons, Ltd.
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Figure 4. Si INEPT NMR spectra of the reaction mixture in the liner at different times since mixing. For the experimental conditions, refer to Experimental, 0.1 Hz line broadening was used in all the spectra. The line at δ = 41.750 was used as a secondary reference in all these spectra. Its chemical shift was determined relatively to TMS (δ = 0.00) in a separate measurement 69 h after mixing (under identical experimental conditions except that 26 400 transients were necessary for a good S/N ratio). Different horizontal and vertical scales were used in different parts of the spectra to make the discussed features visible. The traces (from the bottom up) were obtained 0.22, 0.38, 0.57, 0.75, 1.03, 1.28, 1.47, 1.68, 2.10, 2.47, 2.95, 3.72, 4.85, 6.00, 7.23, 9.38, 11.53, 15.68, 19.83, 24.98, 35.55, 41.70, 47.85, 59.83, 67.98, 68.12, 97.82, 105.95, and 119.28 h since mixing. The depicted parts cover (from the left) monomer region, terminal (M) and chain prolonging (D) spectral regions.
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Figure 5. Si NMR INEPT spectra of the reaction mixtures taken 15 min (left) and 28 min (right) since mixing. Spectrum on the left was measured in 5 mm NMR tube; spectrum on the right was measured in the same NMR tube but with the liner employed. Number of transients is 120 and 160, respectively. The top traces were obtained with the same line broadening (5.0 Hz) as in ref.;[4] the middle trace used 1.0 Hz, and the bottom trace 0.5 Hz line broadening. Note that the shoulders are noticeable in the top traces only thanks to the large expansion of the spectra. Bottom left trace still shows small FID truncation effects despite the 0.5 Hz line broadening.
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terminal: prolonging units 2 : 1). On the origin of the weakest M3 line, one can only speculate. The line in the D region becomes noticeable about 1.6 h after mixing; the chemical shift of the weak broad line is δ = 59.17. While the previous two groups of lines
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decreases the shielding (refer to the preceding texts) and because the lines in this region appear only shortly before the chain prolonging unit appears in the spectrum, we suggest that M2 signal comes from terminal units of a linear trimer (with the ratio of
J. Schraml et al. had their chemical shifts in a reasonable agreement with the published results,[4] the D line (assigned to the central Si of Si–O–Si– O–Si moiety) is shifted 1.5 ppm to higher shielding in our measurements. Similarly as the authors,[4] we have not detected any line close to δ = 64.5 that would indicate the presence of silicon with three siloxane linkages (i.e. Si(–O–Si)3 moiety). Besides the discussed 29Si NMR lines, there is also a very weak line (not shown in Fig. 4) downfield from the IPTMS line by Δδ = 1.24 that appears about 0.3 h after mixing to disappear 8 h later. Its origin will be discussed later. 29 Si DEPT experiment after 16.63 h (Fig. S4)[4] had noticeably lower S/N ratio; we noticed similar effect in our INEPT experiments. The decrease is at least in part due to probe getting rf. mistuned through the reaction reducing thus receiver sensitivity and making the polarization transfer 1H → 29Si less efficient in these experiments. In order to evaluate this sensitivity loss, we measured the 29 Si NMR spectra by the RIDE method[9] that does not involve any polarization transfer. Because of low sensitivity, the long RIDE experiments can be applied only when the reaction rate is slowed down. The RIDE spectrum (Fig. 6) reproduces well the lines seen in the INEPT spectrum (Fig. 4) in the first two regions, but in the third region, it shows four additional weak lines (δ = 59.341, 59.396, 59.609, and 59.967) of equal intensity on top of a broad hump. Another broad hump appears centered at δ = 66, i.e. approximately where the signals from the silicon with three siloxane linkages were expected.[4] It is these two humps into which 29Si signals leaked. Apparently, some siloxane polymers consisting of branching (TR, (≡Si0)3SiR)) and chain prolonging (DR,R′, (≡Si0)2SiRR′) units[21] were formed (with R = H2N(CH2)3 or (CH3)2C¼N(CH2)3 and R′ = OCH3 or OH)). It is not clear whether the broad signals are caused by a distribution of structures (and lines) or due to shortening of spin–spin relaxation times T2 by reduced mobility of these species. Possibility that some signal leaked also into a hump around δ = 109, which is otherwise due to silicon in Q4 units (i.e. (≡Si0)4Si units) of NMR tube glass,[9] cannot be excluded on the basis of 29Si NMR experiments (subtraction of the background signal is not precise enough). On chemical grounds, it seems unlikely as
it would require replacing Si–C bond with Si–O–Si linkage. Also, in C NMR spectra, we do not see any new product formed from the organic substituent that was split off, and moreover, some broad humps appear also in the 13C NMR spectra close to the identified sharp lines. It is worth noting that no 29Si signal is seen in the RIDE spectrum in the regions δ = 20…39 and δ = 80…90 where signals from ≡Si–O–Si(NHR)n(OCH3)3-n and (Si–O–)3Si–N moieties, respectively, were identified[2] for R = (CH2)3Si(OCH3)3. Thus, no aminolysis is taking place under the mild reaction conditions used. 13 C NMR spectra (Fig. 7) agree with the above picture. Until 0.6 h since mixing, the spectra show only conversion of APTMS into IPTMS. In agreement with the proposed reaction scheme4, intensities of all 13C NMR lines of the starting APTMS decrease until they all disappear in about 22 h. The intensity of methanol line (δ = 48.83) is increasing for the whole duration of the reaction (it appears as the first sign of hydrolysis at 0.63 h) similarly as the intensities of all APTMS lines decrease. Lines belonging to IPTMS (and its derivatives) show more complex behavior. Initially, their intensities are rising (as APTMS is being converted into IPTMS). When hydrolysis becomes visible, new weak lines (derivative lines) close to the lines of IPTMS began to appear. Some are shifted to a higher frequency from the parent line (e.g. C¼N: Δδ = 1.102 ppm or CH2–Si: Δδ = 0.733, 0.743, 1.869 ppm); their intensities go to maxima around 5 h to disappear around13 h. Other lines continue to increase at the expense of the parent signal (e.g. CH2–Si: Δδ = 1.461 ppm). This is true for all the carbons along the N–CH2–CH2–CH2–Si chain; they are all shifted downfield from the respective parent lines, and their final separation from the parent signal decreases with increasing number of bonds between carbon and silicon atoms. In contrast, the derivative lines from C¼N and CH3–O–Si carbons (the intensity of which also increases in reaction time) are upfield from the respective parent lines (Δδ = 0.069 ppm and Δδ = 0.199 ppm, respectively). In a detailed look on the spectra measured after 10 h, several of the product and derivative lines are found on small humps with a few weaker lines apparent. No derivative lines are seen in the vicinity of APTMS lines. 13
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Figure 6. Si NMR RIDE spectra of the reaction mixture (top and insert) in the liner 48 h after mixing (2080 transients accumulated with relaxation delay of 20 s, line broadening of 3.0 Hz used) and background signal (bottom) from the NMR tube and the liner.
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Figure 7. Time dependence of relevant expansions of C NMR spectra of the reaction mixture. Reaction time since mixing is indicated in hours. For the assignment of intense lines, see ref.[4] The expansions cover the following spectral regions (from the left): (i) imine C¼N carbons, (ii) CH2–N and CH3–O carbons, (iii) CH3–(C) and CH2–(C) carbons and (iv) CH2–Si carbons.
When the reaction is carried out in a flask, changes in the solution are apparent to the naked eye. The clear true colorless solution produced by mixing of the components turns slowly into a fine emulsion some 40 min later (about the time when methanol becomes visible in the spectrum). It starts to clear up about 8–10 h later to become clear true solution in about 16 h since mixing. According to the theory, formation of emulsion with two immiscible phases having different magnetic susceptibility leads to splitting of each line if the compound is present in both phases.[22] In the studied reaction mixture, the two liquids would exercise also different specific solvent effects, so the measured line separation (in ppm) is not the same for all the lines in the spectra. Nevertheless, emulsion formation can explain all those 13C and 29Si NMR lines that appear after 0.6 h, rise in intensity and vanish around 10 h.
Conclusion Resolving 29Si NMR lines of APTMS and IPTMS allows consistent interpretation of the reactions studied. 29Si INEPT (as well as DEPT) spectra do not show 29Si broad lines that are visible when the spectra are measured without polarization transfer. The spectra show that the siloxane polymers containing besides the chain prolonging units also branching units have broad lines; narrow lines are limited to starting monomers (APTMS and IPTMS), dimers, trimers and perhaps some terminal units. These conclusions are supported by 13C NMR spectra that are affected by transitional emulsion formation.
References [1] K. C. Vrancken, L. De Coster, P. Van Der Voort, P. J. Grobet, E. F. Vansant. J. Colloid Interf. Sci. 1995, 170, 71–77. [2] S. Ek, E. I. Iiskola, L. Niinistö, J. Vaittinen, T. T. Pakkanen, A. Root. J. Phys. Chem. B 2004, 108, 11454–11463. [3] Z. Luan, J. A. Fournier, J. B. Wooten, D. E. Miser. Micropor. Mesopor. Mat. 2005, 83, 150–158. [4] P. Mazzei, L. Fusco, A. Piccolo. Magn. Reson. Chem. 2014, 52, 383–388. [5] G. A. Morris, R. Freeman. J. Am. Chem. Soc. 1979, 101, 760–762. [6] J. Schraml. Collect. Czech. Chem. Commun. 1983, 48, 3402–3406. [7] V. Blechta, J. Schraml. J. Magn. Reson. 1986, 69, 293–301. [8] W. Kozminski, K. Jackowski. Magn. Reson. Chem. 2000, 38, 459–463. [9] J. Schraml, P. Sandor, S. Korec, M. Krump. Magn. Reson. Chem. 2013, 51, 403–406. [10] É. Kupče, R. Freeman. J. Magn. Reson. 1995, 115A, 273–276. [11] A. J. Shaka, J. Keeler, R. Freeman. J. Magn. Reson. 1983, 53, 313–340. [12] MNOVA. version 9.0, Mestrelab Research S.L., Spain, 2013. [13] J. Schraml. Prog. NMR Spectrosc. 1990, 22, 289–348. [14] J. Schraml, in The chemistry of organosilicon compounds, (Eds: Z. Rappoport, Y. Apeloig), Wiley, Chichester, 2001, pp. 223–339. [15] J. Schraml, N.-D. Chuy, V. Chvalovský, M. Mägi, E. Lippmaal. Org. Magn. Reson. 1975, 7, 379–385. [16] M. Kurfürst, V. Blechta, J. Schraml. Magn. Reson. Chem. 2011, 49, 492–501. [17] M. J. van Bommel, T. N. M. Bernards, A. H. Boonstra. J. Non-Cryst. Solids 1991, 128, 231–242. [18] C. W. Turner, K. J. Franklin. J. Non-Cryst. Solids 1987, 91, 402–415. [19] Y. Sugahara, S. Okada, S. Sato, K. Kuroda, C. Kato. J. Non-Cryst. Solids 1994, 167, 21–28. [20] R. H. Glaser, G. L. Wilkes, C. E. Bronnimann. J Non-Cryst. Solids 1989, 113, 73–87. [21] R. K. Harris, J. D. Kennedy, W. McFarlane, in NMR and the Periodic Table, (Eds: R. K. Harris, B. E. Mann), Academic Press, London, 1978, pp. 309–378. [22] P. W. Kuchel, B. E. Chapman, W. A. Bubb, P. E. Hansen, C. J. Durrant, M. P. Hertzberg. Concept. Magn. Reson. A 2003, 18, 56–71.
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Copyright © 2014 John Wiley & Sons, Ltd.
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Accepted: 29 September 2014
Published online in Wiley Online Library: 22 October 2014
(wileyonlinelibrary.com) DOI 10.1002/mrc.4171
Acetone-induced polymerization of 3-aminopropyltrimethoxysilane (APTMS) as revealed by NMR spectroscopy – revisited Jan Schraml,a* Stefan Korec,b Martin Krumpb and Jan Čermáka,c Introduction Silane coupling agents, represented for example by 3-aminopropyltriethoxysilane (APTES, H2N–(CH2)3Si(OC2H5)3) or 3-aminopropyltrimethoxysilane (APTMS, H2N–(CH2)3Si(OCH3)3) find a widespread use in various scientific and industrial applications, and so, their chemical reactions are studied intensely, for the leading references, see refs.[1–3] Recently, Mazzei, Fusco and Piccolo published[4] a very detailed and careful NMR study of acetone-induced polymerization of APTMS. Using the full panoply of NMR techniques applied to 1H, 13 C and 29Si nuclei, they proved that acetone reacts with the silane amino group to form an imine ((CH3)2C¼N(CH2)3Si(OCH3)3, IPTMS or N-isopropylidene-3-aminopropyltrimethoxysilane. The released water then hydrolyses the methoxysilane inducing thus formation of a siloxane Si–O–Si bridge. (For the detailed description refer to Figs. 1 and 2 of reference.[4]) We would like to correct one error and comment on some details of these reactions without questioning the fundamental findings of the quoted paper,[4] i.e. formation of imine and siloxane as well as 1H and 13C NMR line assignments. The error concerns the interpretation of 29Si chemical shifts of APTMS and IPTMS. Although the 13C spectrum recorded 0.88 h from the reaction start (Fig. 4[4]) showed approximately 1 : 1 mixture of APTMS and IPTMS, no 29Si line of IPTMS was found in 29Si DEPT spectra (Fig. S4[4]) that exhibited only one line at 41.8 ppm that was assigned to APTMS.
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All the spectral measurements were performed on a Varian/Agilent INOVA-500 spectrometer (operating at 499.9, 125.7 and 99.3 MHz for 1H, 13C and 29Si NMR, respectively) using a 5 mm switchable PFG broadband probe. The standard software (vnmrJ 3.2A) was used. Each 29Si and 13C NMR measurement was preceded by 1H NMR measurement, all performed at 25°C. In 29Si INEPT[5] measurements of the reaction mixture, the delays were not optimized, routine parameters for trimethylsilyl groups[6] provided sufficient S/N. When a liner was employed, 160 transients were accumulated to keep the total measuring time reasonably short (8 min) on the time scale of the reaction; without the liner, the number of transients was reduced to 120. For the measurement of tetramethylsilane (TMS) signal, the delays were optimized [7] to match proton coupling to 12 methyl protons and 2 J(29Si–1H) = 6.5 Hz; the measurement required accumulation of 26 400 transients. The 29Si NMR spectra were measured also by the ring down elimination (RIDE)[8,9] pulse sequence. The read pulse (80° rectangular) was
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preceded by WURST (wideband, uniform rate, smooth truncation)[10] inversion pulse; 2080 transients were accumulated. The 29Si NMR FIDs were acquired using the spectral width of 25 kHz. WALTZ[11] proton decoupling was applied during acquisition (1.0 s) (gated decoupling to suppress NOE which might be negative) and not through the relaxation delays (20.0 s in RIDE and 2.0 s in INEPT experiment). Zero filling to 128 K and a mild line broadening (0.1 Hz in INEPT and 3.0 Hz in RIDE) were used in data processing unless noted otherwise. 13 C NMR spectra were acquired using 35 kHz spectral width, 1.0 s acquisition time and 5.0 s relaxation delay, 80° flip angle, 64 transients provided sufficient quality spectra using 1.0 Hz line broadening and zero filling to 128 K and the same proton decoupling as in INEPT spectra except that it was used for the whole duration of the measurements. The spectra were referenced to the central line of chloroform-d1 at δ = 76.99. 1 H NMR spectra were measured by a simple one-pulse sequence using 70° flip pulse, the spectral width of 7 kHz, acquisition time of 2.0 s and relaxation delay of 5 s. Four transients were accumulated; the FIDs were zero filled to 128 K; no weighting was used. Since the chemical shifts (and also the lock signal) exhibit large solvent and concentration effects in the reaction mixture, the 29Si NMR line of the imine product at δ = -41.750 was used as a secondary reference. Its chemical shift was determined relatively to TMS signal ((CH3)4Si, δ = 0.000) in a separate experiment performed on the reaction mixture 69 h after mixing. The chemical shift differences (Δδ) within one spectrum are reliable at least to 0.01 ppm; 1 H, 13C and 29Si chemical shifts (in δ scale) are reliable less as secondary references had to be used and as the chemical shift of the primary reference (TMS) for all three nuclei measured varied in the reaction mixture studied. APTMS, 3-aminopropyltrimethoxysilane (H2N(CH2)3Si(OCH3)3) was Aldrich 97% product, it was used as delivered. The results reported here were obtained in Aldrich’s 99.8 at% CDCl3 with 0.03% (V/V) of TMS. Acetone of G.R. grade was provided by Lachner.
* Correspondence to: Jan Schraml, Institute of Chemical Process Fundamentals of the ASCR, v.v.i., Rozvojová 135, 165 02 Prague 6, Czech Republic. E-mail: [email protected] a Institute of Chemical Process Fundamentals of the ASCR v. v. i., Rozvojová 135, 165 02, Prague 6, Czech Republic b SChem a.s., Pobřežní 394/12, 186 00, Prague 8, Czech Republic c Department of Chemistry, Faculty of Science, Purkinje University Ústí nad Labem, České mládeže 8, 400 96, Ústí nad Labem, Czech Republic
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Acetone-induced polymerization of 3-aminopropyltrimethoxysilane Deuterochloroform was stored over activated molecular sieves 4A for several days before use. Solvent and reaction components were weighted into a dried glass vial, vigorously mixed for a short period of time (2 min) and then transferred by a dry pipette directly into a NMR tube (Wilmad 528) or the same NMR tube equipped with a PTFE-FEP liner (Wilmad). The tubes were sealed for the long experiments; for shorter ones (24 h), septum stoppers were used; the liner was closed with the stopper provided. The first spectra were measured some 5–12 min after mixing depending how much time was consumed by probe rf. tuning and magnetic field shimming. It should be noted that rf. tuning changed considerably during the course of the reaction. We used various initial concentrations (all concentrations are given in mole fractions) for the reaction mixture; the results reproduced here started from the following initial concentrations: for 29Si measurements in the liner 0.119 APTMS and 0.289 acetone, in the NMR tube 0.102 APTMS and 0.334 acetone; for 13C measurements 0.122 (or 0.071) APTMS and 0.291 (or 0.229) acetone. (The original concentrations were 0.116 APTMS and 0.347 acetone.[4])
Results and discussion The supporting information of the discussed work[4] presents (Fig. S1) a part (δ = 0–4 ppm) of 1H NMR spectrum of APTMS in chloroform where the line due to NH2 protons is missing. The explanation offered by the authors – the signal is very broadened as a consequence of involvement in chemical exchange – raises some doubts as it contradicts our experience. As the 1H NMR spectra in Fig. 1 show, the line due to NH2 protons is close to its predicted position (δ = 1.092 [12]) both in the neat APTMS (δ = 1.098, Fig. 1, bottom) and in its 10% (V/V) solution in dry chloroform-d1 (δ = 1.027, Fig. 1, top). In the reaction mixture, however, the exchangeable protons are hard to see during the initial reaction stages as illustrated in Fig. 2. Then, the broad signal of exchangeable protons becomes noticeable and moves to its final position (δ = 4.6 ± 0.3) as described.[4] It should be mentioned that the NH2 protons exchanged with D from CDCl3, so in the course of time, the line of the residual CHCl3 increases in intensity (accompanied with a
1
H NMR spectra of APTMS. Top, 10% (V/V) solution in chloroform-d1; bottom, neat APTMS sample (spectra referenced to CH3O line at δ = 3.490).
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Figure 1.
decrease in the lock level) while the intensity of NH2 line decreases. This exchange is apparent from the 13C NMR spectrum (Fig. 3) of chloroform carbon as an increase in the intensity of CH carbon signal within the triplet of CD carbon. The 29Si DEPT spectra of the reaction mixture (Fig. S4) exhibited[4] surprisingly large line widths about 10 Hz (partly due to the line broadening of 5 Hz by exponential multiplication of the FIDs). In our experience,[13,14] the 29Si NMR lines have routinely line widths less than 1 Hz. The question therefore arises whether there is some fast exchange process involving Si atoms. Repeating the 29Si part of the work[4] as closely as possible, we obtained more complex picture, refer to Figs. 4 and 5. The lines in these spectra measured in the liner have the line widths around 1 Hz; narrower lines (0.5 Hz) are obtained if the liner is not used. So, what appeared in the discussed paper[4] as one line (δ = 41.8) are in fact two lines (Fig. 5), one at δ = 41.787 and the other at δ = 41.750 (Δδ = 0.037 ppm), and no exchange process is taking place on the silicon atom. Since these two lines are the most intense lines for the first hours of the reaction and since the intensity of the former line decreases while that of the latter increases during the first 10 h (to decrease afterwards), one concludes that the former line is due to the silicon in the APTMS and the other line belongs to the first intermediate product, the imine, ((CH3) 1 13 C NMR 2C¼N(CH2)3Si(OCH3)3, IPTMS) identified through H and spectra.[4] This conclusion is supported by the small 29Si chemical shift difference between this product and the starting APTMS that is in line with the substituent effect expected for a change in gamma position to the silicon.[15] Also, separations of 13C satellites (97.8 Hz in APTMS and 97.3 Hz in IPTMS) matches the separations of 29Si satellites in the assigned 13C NMR spectra (CH2–Si carbons). These separations agree with the values of 1 J(29Si–13C) couplings in similar systems.[16] The other possibilities would lead to much larger change in 29Si chemical shift. Hydrolytic replacement of one OCH3 group by OH is estimated to cause deshielding in chloroform solution (i.e. in the correct direction) of about 0.6 ppm on the basis of literature data.[17–19] Even larger (10 ppm) and in the opposite direction, 29Si shifts (shielding) are known to accompany replacement of OCH3 group by a silyl substituent (for a review refer
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Figure 2. H NMR spectrum in the course of initial reaction stages (0.071 APTMS and 0.229 acetone). The time (in hours) since mixing is indicated for each trace; signals from exchangeable protons are marked by the arrows in the traces with amplified intensities.
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Figure 3. C NMR spectra (chloroform part) of the reaction mixture (0.122 APTMS and 0.291 acetone) in the course of the reaction. Acquisition starting time since mixing is indicated in hours.
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to.[20]) According to this silylation shift, the line around δ = 50.3 was assigned to silicon atoms with only one Si–O–Si linkage[4] and the line around δ = 57.7 to silicon with two such linkages.[4] In polysiloxane terminology,[21] the former line is the terminal M unit (MRR′R″, where R = H2N(CH2)3 or (CH2)3N¼C(CH3)2 and R′ or R″ = OCH3 or OH) while the later is the chain prolonging unit (DRR′). Our measurements (Fig. 4) confirm the reported time dependence of these signals[4] but again show more complex picture. In the M region, there are in fact three lines (M1, M2, and M3) with the chemical shifts δ = 50.157, δ = 50.360, and δ = 50.570 (at 15.7 h since mixing) in an approximate integral ratio 10 : 4 : 1, respectively. While
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the most intense line M1 does not move with the reaction time, M2 and M3 move further away from M1. In a detailed look into the first three spectra, M1 line appears as two (with the line separation Δδ = 0.013 ppm). Since at this reaction time, there are only signals from APTMS and IPTMS besides the discussed pair of M1 lines, the two lines cannot terminate any siloxane chain and so are most probably results of IPTMS–IPTMS and IPTMS–APTMS dimer formation, the latter being converted into the former in the course of reaction. The two remaining lines M2 and M3 are shifted upfield from the dimer by approximately 0.2 and 0.4 ppm, respectively. Since hydrolytic replacement of an alkoxy group by hydroxyl group
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Figure 4. Si INEPT NMR spectra of the reaction mixture in the liner at different times since mixing. For the experimental conditions, refer to Experimental, 0.1 Hz line broadening was used in all the spectra. The line at δ = 41.750 was used as a secondary reference in all these spectra. Its chemical shift was determined relatively to TMS (δ = 0.00) in a separate measurement 69 h after mixing (under identical experimental conditions except that 26 400 transients were necessary for a good S/N ratio). Different horizontal and vertical scales were used in different parts of the spectra to make the discussed features visible. The traces (from the bottom up) were obtained 0.22, 0.38, 0.57, 0.75, 1.03, 1.28, 1.47, 1.68, 2.10, 2.47, 2.95, 3.72, 4.85, 6.00, 7.23, 9.38, 11.53, 15.68, 19.83, 24.98, 35.55, 41.70, 47.85, 59.83, 67.98, 68.12, 97.82, 105.95, and 119.28 h since mixing. The depicted parts cover (from the left) monomer region, terminal (M) and chain prolonging (D) spectral regions.
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Figure 5. Si NMR INEPT spectra of the reaction mixtures taken 15 min (left) and 28 min (right) since mixing. Spectrum on the left was measured in 5 mm NMR tube; spectrum on the right was measured in the same NMR tube but with the liner employed. Number of transients is 120 and 160, respectively. The top traces were obtained with the same line broadening (5.0 Hz) as in ref.;[4] the middle trace used 1.0 Hz, and the bottom trace 0.5 Hz line broadening. Note that the shoulders are noticeable in the top traces only thanks to the large expansion of the spectra. Bottom left trace still shows small FID truncation effects despite the 0.5 Hz line broadening.
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terminal: prolonging units 2 : 1). On the origin of the weakest M3 line, one can only speculate. The line in the D region becomes noticeable about 1.6 h after mixing; the chemical shift of the weak broad line is δ = 59.17. While the previous two groups of lines
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decreases the shielding (refer to the preceding texts) and because the lines in this region appear only shortly before the chain prolonging unit appears in the spectrum, we suggest that M2 signal comes from terminal units of a linear trimer (with the ratio of
J. Schraml et al. had their chemical shifts in a reasonable agreement with the published results,[4] the D line (assigned to the central Si of Si–O–Si– O–Si moiety) is shifted 1.5 ppm to higher shielding in our measurements. Similarly as the authors,[4] we have not detected any line close to δ = 64.5 that would indicate the presence of silicon with three siloxane linkages (i.e. Si(–O–Si)3 moiety). Besides the discussed 29Si NMR lines, there is also a very weak line (not shown in Fig. 4) downfield from the IPTMS line by Δδ = 1.24 that appears about 0.3 h after mixing to disappear 8 h later. Its origin will be discussed later. 29 Si DEPT experiment after 16.63 h (Fig. S4)[4] had noticeably lower S/N ratio; we noticed similar effect in our INEPT experiments. The decrease is at least in part due to probe getting rf. mistuned through the reaction reducing thus receiver sensitivity and making the polarization transfer 1H → 29Si less efficient in these experiments. In order to evaluate this sensitivity loss, we measured the 29 Si NMR spectra by the RIDE method[9] that does not involve any polarization transfer. Because of low sensitivity, the long RIDE experiments can be applied only when the reaction rate is slowed down. The RIDE spectrum (Fig. 6) reproduces well the lines seen in the INEPT spectrum (Fig. 4) in the first two regions, but in the third region, it shows four additional weak lines (δ = 59.341, 59.396, 59.609, and 59.967) of equal intensity on top of a broad hump. Another broad hump appears centered at δ = 66, i.e. approximately where the signals from the silicon with three siloxane linkages were expected.[4] It is these two humps into which 29Si signals leaked. Apparently, some siloxane polymers consisting of branching (TR, (≡Si0)3SiR)) and chain prolonging (DR,R′, (≡Si0)2SiRR′) units[21] were formed (with R = H2N(CH2)3 or (CH3)2C¼N(CH2)3 and R′ = OCH3 or OH)). It is not clear whether the broad signals are caused by a distribution of structures (and lines) or due to shortening of spin–spin relaxation times T2 by reduced mobility of these species. Possibility that some signal leaked also into a hump around δ = 109, which is otherwise due to silicon in Q4 units (i.e. (≡Si0)4Si units) of NMR tube glass,[9] cannot be excluded on the basis of 29Si NMR experiments (subtraction of the background signal is not precise enough). On chemical grounds, it seems unlikely as
it would require replacing Si–C bond with Si–O–Si linkage. Also, in C NMR spectra, we do not see any new product formed from the organic substituent that was split off, and moreover, some broad humps appear also in the 13C NMR spectra close to the identified sharp lines. It is worth noting that no 29Si signal is seen in the RIDE spectrum in the regions δ = 20…39 and δ = 80…90 where signals from ≡Si–O–Si(NHR)n(OCH3)3-n and (Si–O–)3Si–N moieties, respectively, were identified[2] for R = (CH2)3Si(OCH3)3. Thus, no aminolysis is taking place under the mild reaction conditions used. 13 C NMR spectra (Fig. 7) agree with the above picture. Until 0.6 h since mixing, the spectra show only conversion of APTMS into IPTMS. In agreement with the proposed reaction scheme4, intensities of all 13C NMR lines of the starting APTMS decrease until they all disappear in about 22 h. The intensity of methanol line (δ = 48.83) is increasing for the whole duration of the reaction (it appears as the first sign of hydrolysis at 0.63 h) similarly as the intensities of all APTMS lines decrease. Lines belonging to IPTMS (and its derivatives) show more complex behavior. Initially, their intensities are rising (as APTMS is being converted into IPTMS). When hydrolysis becomes visible, new weak lines (derivative lines) close to the lines of IPTMS began to appear. Some are shifted to a higher frequency from the parent line (e.g. C¼N: Δδ = 1.102 ppm or CH2–Si: Δδ = 0.733, 0.743, 1.869 ppm); their intensities go to maxima around 5 h to disappear around13 h. Other lines continue to increase at the expense of the parent signal (e.g. CH2–Si: Δδ = 1.461 ppm). This is true for all the carbons along the N–CH2–CH2–CH2–Si chain; they are all shifted downfield from the respective parent lines, and their final separation from the parent signal decreases with increasing number of bonds between carbon and silicon atoms. In contrast, the derivative lines from C¼N and CH3–O–Si carbons (the intensity of which also increases in reaction time) are upfield from the respective parent lines (Δδ = 0.069 ppm and Δδ = 0.199 ppm, respectively). In a detailed look on the spectra measured after 10 h, several of the product and derivative lines are found on small humps with a few weaker lines apparent. No derivative lines are seen in the vicinity of APTMS lines. 13
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Figure 6. Si NMR RIDE spectra of the reaction mixture (top and insert) in the liner 48 h after mixing (2080 transients accumulated with relaxation delay of 20 s, line broadening of 3.0 Hz used) and background signal (bottom) from the NMR tube and the liner.
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Figure 7. Time dependence of relevant expansions of C NMR spectra of the reaction mixture. Reaction time since mixing is indicated in hours. For the assignment of intense lines, see ref.[4] The expansions cover the following spectral regions (from the left): (i) imine C¼N carbons, (ii) CH2–N and CH3–O carbons, (iii) CH3–(C) and CH2–(C) carbons and (iv) CH2–Si carbons.
When the reaction is carried out in a flask, changes in the solution are apparent to the naked eye. The clear true colorless solution produced by mixing of the components turns slowly into a fine emulsion some 40 min later (about the time when methanol becomes visible in the spectrum). It starts to clear up about 8–10 h later to become clear true solution in about 16 h since mixing. According to the theory, formation of emulsion with two immiscible phases having different magnetic susceptibility leads to splitting of each line if the compound is present in both phases.[22] In the studied reaction mixture, the two liquids would exercise also different specific solvent effects, so the measured line separation (in ppm) is not the same for all the lines in the spectra. Nevertheless, emulsion formation can explain all those 13C and 29Si NMR lines that appear after 0.6 h, rise in intensity and vanish around 10 h.
Conclusion Resolving 29Si NMR lines of APTMS and IPTMS allows consistent interpretation of the reactions studied. 29Si INEPT (as well as DEPT) spectra do not show 29Si broad lines that are visible when the spectra are measured without polarization transfer. The spectra show that the siloxane polymers containing besides the chain prolonging units also branching units have broad lines; narrow lines are limited to starting monomers (APTMS and IPTMS), dimers, trimers and perhaps some terminal units. These conclusions are supported by 13C NMR spectra that are affected by transitional emulsion formation.
References [1] K. C. Vrancken, L. De Coster, P. Van Der Voort, P. J. Grobet, E. F. Vansant. J. Colloid Interf. Sci. 1995, 170, 71–77. [2] S. Ek, E. I. Iiskola, L. Niinistö, J. Vaittinen, T. T. Pakkanen, A. Root. J. Phys. Chem. B 2004, 108, 11454–11463. [3] Z. Luan, J. A. Fournier, J. B. Wooten, D. E. Miser. Micropor. Mesopor. Mat. 2005, 83, 150–158. [4] P. Mazzei, L. Fusco, A. Piccolo. Magn. Reson. Chem. 2014, 52, 383–388. [5] G. A. Morris, R. Freeman. J. Am. Chem. Soc. 1979, 101, 760–762. [6] J. Schraml. Collect. Czech. Chem. Commun. 1983, 48, 3402–3406. [7] V. Blechta, J. Schraml. J. Magn. Reson. 1986, 69, 293–301. [8] W. Kozminski, K. Jackowski. Magn. Reson. Chem. 2000, 38, 459–463. [9] J. Schraml, P. Sandor, S. Korec, M. Krump. Magn. Reson. Chem. 2013, 51, 403–406. [10] É. Kupče, R. Freeman. J. Magn. Reson. 1995, 115A, 273–276. [11] A. J. Shaka, J. Keeler, R. Freeman. J. Magn. Reson. 1983, 53, 313–340. [12] MNOVA. version 9.0, Mestrelab Research S.L., Spain, 2013. [13] J. Schraml. Prog. NMR Spectrosc. 1990, 22, 289–348. [14] J. Schraml, in The chemistry of organosilicon compounds, (Eds: Z. Rappoport, Y. Apeloig), Wiley, Chichester, 2001, pp. 223–339. [15] J. Schraml, N.-D. Chuy, V. Chvalovský, M. Mägi, E. Lippmaal. Org. Magn. Reson. 1975, 7, 379–385. [16] M. Kurfürst, V. Blechta, J. Schraml. Magn. Reson. Chem. 2011, 49, 492–501. [17] M. J. van Bommel, T. N. M. Bernards, A. H. Boonstra. J. Non-Cryst. Solids 1991, 128, 231–242. [18] C. W. Turner, K. J. Franklin. J. Non-Cryst. Solids 1987, 91, 402–415. [19] Y. Sugahara, S. Okada, S. Sato, K. Kuroda, C. Kato. J. Non-Cryst. Solids 1994, 167, 21–28. [20] R. H. Glaser, G. L. Wilkes, C. E. Bronnimann. J Non-Cryst. Solids 1989, 113, 73–87. [21] R. K. Harris, J. D. Kennedy, W. McFarlane, in NMR and the Periodic Table, (Eds: R. K. Harris, B. E. Mann), Academic Press, London, 1978, pp. 309–378. [22] P. W. Kuchel, B. E. Chapman, W. A. Bubb, P. E. Hansen, C. J. Durrant, M. P. Hertzberg. Concept. Magn. Reson. A 2003, 18, 56–71.
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