Anal. Chem. 1986, 58, 2641-2642
2641
Quantitative Protonated Heteroatom Determination by Silicon-29 Nuclear Magnetic Resonance Spectrometry and Polarization Transfer Pulse Sequences. Application to Asphaltene Jean-Marie Dereppe and Bhukandas Parbhoo* Department of Physical Chemistry, Catholic University of Louvain, 1348 Louuain-la-Neuve, Belgium Ouantltatlve analysis of hydroxyl functlons In an asphaltene Is determined for the first time uslng trlmethylsllyi derivatlzation and INEPT and DEPT sequences In silicon-29 nuclear magnetlc resonance spectrometry. Spectra could be processed wlthln only 4 h for an asphaltene contalnlng 7% oxygen atoms dlstrlbuted wlthin various hydroxyl and carbonyl groups. A high degree of preclslon (3%) is obtained and the degree of accuracy is the same as for the classical pulse sequence.
Recently some papers have reported the qualitative and quantitative analyses by 19Fand 29Sinuclear magnetic resonance (NMR) of spectra specifically trifluoroacetylated and trimethylsilylated hydroxyl, thiol, amive, and carboxlic functions (1-4). Silicon-29 NMR spectra have proven to be more reliable and convenient compared to fluorine-19 NMR spectra (4). However, the %i nucleus presents an important disadvantage, its low NMR sensitivity necessitating a long spectral accumulation time (5, 6). At the beginning of this decade multipulse sequences were proposed to boost the sensitivity of nuclei of low magnetogyric ratio: insensitive nuclear enhancement via polarization transfer-refocused (INEPTR) (7-1 1) and distortionless enhancement by polarization transfer (DEPT) (12-1 7).These pulse sequences transfer the proton nuclear spin polarization to the observed silicon-29 nuclear spins via the "-%Si scalar coupling. Furthermore, the transfer depends only on proton relaxation. These two processes-polarization transfer and proton relaxation-significantly enhance 29Si NMR signal intensities. Trimethylsilyl groups are particularly well suited for an analytical study using these sequences for four main reasons. First, the two-bond spin-spin coupling constant 2J(29Si-C-1H) is quite insensitive to silicon bonding to nitrogen, oxygen, or sulfur atoms. The observed range being 6.3-7.7 Hz (18). Second, the maximum enhancement factor ( e ) on proton decoupling for a given value of 2J(29Si-C-1H) is the same for all signals being a function of the number n of protons scalarcoupled to the observed nucleus (19). The theoretical values for are equivalent for INEPTR and DEPT sequences and equal to 9.4 for trimethylsilyl groups. These polarization transfer enhancement factors provide a significant improvement of the signal to noise compared to the maximum nuclear Overhauser effect enhancement factor of -1.52 (5,6). Third, the observed chemical shift range (12 ppm) of Me3Si derivatized molecules allows the choice of a short observation spectral width. The pulse widths can then be supposed to be identical over the whole spectrum. Fourth, proton spinlattice relaxation governing the relaxation delay is of the same order of magnitude for Me3Si groups either attached to various heteroatoms or attached to a specific heteroatom but in different chemical environments. The polarization transfer from 'H to %Si nuclei may then be assumed optimal and identical for all Me3Si signals. 0003-2700/86/0358-2641$0 1.50/0
Quantitative determination is therefore reliable.
EXPERIMENTAL SECTION A Bruker WM250 FT superconductor NMR spectrometer operating at 49.7 MHz (?3i) and 250.0 MHz ('H) is used. Current parameters were assessed using model derivatized compounds and hexamethyldisiloxane in CDC1,. The proton channel pulse width was calibrated following Ad BAX's method (20). The second-order coupling constant was set at 6.7 Hz. Proton and silicon spin-lattice relaxation times of Me3Si groups were found to be 2.6 f 0.25 and 31.0 f 0.5 s, respectively. Addition of Cr(ACAC)3 (0.02 M) shortens 29SiT1to 1.9 & 0.1 s. AU experimental factors have been considered for each sequence when designing the pulse sequence in order to optimize the signal to noise ratio for a total data averaging time of 4 h. Asphaltene is extracted from treated wood in a liquefaction process by hydrogenation under catalytic pressure and temperature conditions (4). Trimethylsilylation is realized as described in ref 4. Three DEPT and three INEPTR spectra are acquired to test the reproducibility. Relaxation reagent is then added and three inverse gated hetercdecoupled (IGHD) spectra are recorded.
RESULTS AND DISCUSSION In a preliminary study we derivatized naphthol, phenol, to ascertain benzoic acid, and (1,l-dimethylethy1)cyclohexanol that polarization transfer techniques can be applied for quantitative analyses. A typical asphaltene was then analyzed. Results are shown in Figure 1. Signal to noise improvements of 8.1 and 6.6 are obtained for optimized INEPTR and DEPT spectra, respectively, compared to optimized IGHD spectrum. The e(INEPTR)/ e(DEPT) ratio enhancement factors for MesSi signals is 1.2 experimentally while it is 1.0 theoretically. This difference is probably due to two reasons. First, signal refocusing is not perfect for both sequences, and second, the DEPT sequence although containing fewer pulses than the INEPTR sequence is more sensitive to spinspin relaxation during the pulse train (21). Nevertheless, these imperfections do not affect the accuracy of the quantitative measurements. Owing to the spin-echo pulse included in the polarization transfer pulse sequence the effects of field inhomogeneity are suppressed. As a result, better resolution is obtained as can be seen in Figure 1. Furthermore peaks of relatively small intensity appear in the range of 22.5-23.5 ppm. The signals are hidden by the noise in the IGHD sequence and could be missed. The expanded INEPTR spectrum of the derivatized asphaltene is shown in Figure 2. Model molecules that might correspond to the observed signals are drawn above the corresponding chemical shifts (2, 3). Signals appearing in the range of 22.5-23.5 ppm are attributed to alicyclic carboxylic functions in various chemical environments. Resonances between 19.0 and 21.0 ppm are assigned to naphthols and polyhydroxybenzene. The broad shoulder extending from 17.0 to 19.0 ppm is due to aromatic and/or nitrogen-containing aromatic hydroxyls. Table I summarizes the relative oxygen content in the observed three main chemical functions. Conclusions are similar with already published results: the most abundant oxygen containing functions in wood asQ 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
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Table I. Distribution of Oxygen (Relative Weight Percentage) between Three Main Types of Chemical Functions Found in Wood Asphaltene'
D
IGHD
il
I
As, ppm
chemical functions
re1 %
17.0-19.0 19.0-21.0 22.5-23.5
phenols naphthols, polyhydroxyphenols alicyclic acids
34 54 12
Internal chemical shift reference, tetramethylsilane.
Table 11. Quantitative Oxygen Analysis by Si NMR with Polarization Transfer Pulse Sequences" % oxygen as
% oxygen as
pulse sequence
aromatic hydroxyls
aromatic carboxylic acids
IGHD DEPT INEPTR
6.9 f 0.5 (8%) 6.7 f 0.2 (3%) 7.0 f 0.2 (3%)
0.9 f 0.5 1.0 f 0.1 0.9 f 0.1
OPercentage in weight (relative error %).
Table I1 for the three pulse sequences. Reliable results are obtained for all three methods. However the precision is appreciably increased for polarization transfer sequences. This underlines another advantage inherent to these sequences.
DEPT
I
I
IGHD
A
I I
L
25
.
.
.
I 20
PPM
/
.
.
.
J 15
1
I
Figure 1. Si NMR spectra of trimethylsilylatedasphaltenes obtained with three different pulse sequences: inverse gated heterodecoupling (IGHD) (A, D), distortionless enhancement by polarization transfer (DEPT) (B), and insensitive nuclei enhancement by polarization transfer refocused (INEPTR) (C). For comparison spectrum A has the same noise level as spectrum B and C and spectrum D has same signal level as spectrum C.
CONCLUSIONS Polarization transfer pulse sequences can be successfully applied to quantitative analysis of hydroxyl and carboxyl functions in complex mixtures of natural molecules through trimethylsilyl derivatization. Me& groups are ideal for these multipulse sequences. The INEPTR sequence appears to give better results than the DEPT sequence. The enhancement in signal to noise ratio and in resolution improves the precision and the accuracy of the analysis. At the same time it shortens drastically the total accumulation time of a spectrum. The analytical detection limit of the acidic heteroatom is also lowered. As a result of these multiphase sequences NMR is shown to be the best technique for a detailed qualitative and quantitative structural analysis of acidic heteroatoms contained in complex molecules. LITERATURE CITED
I
I 25
20
l5
I
PPY
Figure 2. INEPTR spectrum (see Figure 1). Overlaid structures of model compounds correspond to the chemical shifts of their Me& derivatives.
phaltenes are naphthol type hydroxyls. Phenol and/or polyphenol type hydroxyls are abundant although to a lesser extent. Only few alicyclic acid functions are present. There is no evidence of thiol and of primary or .secondary amine functions in this asphaltene. Integrating these NMR signals of Me,Si-derivatized asphaltenes relative to a standard signal, one can calculate the relative weight of oxygen atoms contained in the original asphaltene (4). Results are reported in
Trussel, F. C. Anal. Chem. 1985, 5 7 , 191R. Rose, K. D.; Scouten, C. G. AIPConf. Proc. 1981, No. 70, 82-100. Coleman, W. M., 111; Boyd, A. R. Anal. Chem. 1982, 5 4 , 133. Dereppe, J. M.; Parbhoo, 9. Anal. Chem. 1984, 5 6 , 2740. Williams, E. A.; Cargioli. J . D. Annu. Rep. NMR Spectrosc. 1979, 9 , 221-318. Williams, E. A. Annu. Rep. NMR Specfrosc. 1983, 15, 235-289. Morris, C. A.; Freeman, R. J . Am. Chem. SOC. 1979? 101, 760. Morris, C. A. J . Am. Chem. SOC. 1980, 102, 428. Morris, C. A. J . Magn. Reson. 1980, 4 1 , 185. Burum, D. P.; Ernst, R. R. J . Magn. Reson. 1980, 3 9 , 163. Bolton, P. H. J . Magn. Reson. 1980, 41, 287. Doddrell, D. M.; Pegg, D. T.; Brooks, W. M.; Bendail, M. R. J . Am. Chem. Soc. 1981,-703, 727. Pegg, D. T.; Doddrell, D. M.; Brooks, W. M.; Bendall, M. R. J . Magn. Reson. 1981, 44, 32. Pegg. D. T.; Doddrell, D. M.; Bendall, M. R. J . Magn. Reson. 1981, 4 4 , 238. Pegg, D. T.; Doddrell, D. M.; Bendall, M. R. J . Magn. Reson. 1982, 4 6 , 43. Doddrell, D. M.; Pegg, D. T.; Bendall, M. R. J . Magn. Reson. 1982, 4 8 , 323. Bendall, M. R.; Dcddreii, D. M.; Pegg, D. T.; Hull, W. E. High Resolutlon Murtiphase NMR Specfrum Edltlng and DEPT; Bruker Publication: Karlsruhe, 1982. Schraml, J. Collect. Czech. Chem. Commun. 1983, 48, 3402. Schenker, K. V.; von Philipsborn, W.J . Magn. Res. 1985, 6 1 , 294. Bax, A. J . Magn. Reson. 1983, 52, 76. Pegg, T. D.: Dcddrell, D. M.; Bendail, M. R. J . Chem. Phys. 1982, 7 7 , 2745.
RECEIVED for review November 18,1985. Resubmitted June 10, 1986. Accepted June 24, 1986.
