Anal. Chem. 1988, 58, 1889-1892
aration of parent ions is possible, meaning that isobaric parent ions can not be individually selected for a MS/MS experiment. In addition, the high-resolution measurement can be done on daughter ions formed by either high-energy CAD or low-energy CAD. The ultimate resolution obtainable should be better on the sector/quadrupole geometries than on the quadrupole/sector geometries due to the differences in energy spread in ions formed in an ion source (sector/quadrupole) and by collision-activated dissociation (quadrupole/sedor). However, this is counterbalanced by the fact that the daughter ions have a lower mass and thus fewer empirical formulas within given error limits. Also, since high-resolution measurements are being made on more than one mass in the quadrupole sector geometry, more information may be available to determine structural subunits and to “piece”the parent ion back together. We think that the above arguments suggest that the quadrupole/sector geometry provides more information than the sedor/quadrupole geometry except possibly in cases where isobaric parent ions are present. In these cases, the ability to separate parent ions at high resolution is desirable. Thus, in choosing a hybrid instrument for mixture analysis, the expected nature of the mixtures to be analyzed could be important in deciding which geometry is preferable. Registry No. n-Butylbenzene, 104-51-8;3-methyl-2-butanone, 563-80-4.
LITERATURE CITED (1) Cooks, R. 0.;Glish, 0. L. Chem. Eng. News 1081, 59(48), 40. (2) Tandem Mass Spectrometry; McLafferty, F. W., Ed.; Wlley: New Yo&. 1983. (3) Beynon, J. H.; Cooks, R. G.; Amy, J. W.; Baltlnger, W. E.; Ridley, T. Y. Anal. Chem. 1073, 45. 1023A. (4) Wachs, T.; Bente, P.F. 111; Mclafferty, F. W. I n t . J . Mass Specfrom. Ion Phys. 1072, 9 , 333. (5) Kondrat, R. W.: Cooks, R. G. Anal. Chem. 1078, 5 0 , 81A.
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(6) Yost, R. A.; Enke. C. G. Anal. Chem. 1070, 51, 1251A. (7) Maquestlau. A.; van Haverbeke, Y.; Flammang, R. Avrassart. M.; Finet, D.Bull. SOC. Chlm. Belg. 1078, 87, 765. (8) McLafferty,F. W.; Todd, P.J.; McGilvery, D.C.; Baldwln. M. A. J . Am. Chem. SOC. 1080, 702, 3360. (9) Vrscaj, V.; Kramer, V.; Medved, M.; Kralj, B.; Marsel, J.; Beynon, J. H. Ast, T. Int. J . Mass Spectrom. Ion Phys. 1080, 3 3 , 409. (10) Russell, D. H.; Smith, D. H.; Warmack, R. J.; Bertram, L. K. Int. J . Mass Spectrom. Ion Phys. 1080, 3 5 , 381. (11) Glish, G. L.; McLuckey, S. A.; Rldley, T. Y.; Cooks, R. G. Int. J . Mass Spectrom. Ion phvs. 1082. 4 1 , 157. (12) Green, B. N.; Bateman, R. H.; Tummers, M. H.; Smith, D. C. Fresenulus’ 2.Anal. Chem. 1083. 316, 217. (13) Schoen, A. E.; Amy, J. W.; Ciupek, J. D.;Cooks, R. G.; Dobberstein. P.;Jung, G. Int. J . Mass Spechom. Ion Processes 1085, 65, 125. (14) Hass, J. R.; Green, B. N.; Bott, P. A,; Bateman. R. H. submitted for publication in Int J Mass Specfrom Ion Processes. (15) Bursey, M. M.; Hass, J. R. J . Am. Chem. SOC. 1085, 107. 115. (16) Brulns, A. P.; Jennlngs, K. R.; Evans, S. Int. J . Mass Spectrom. Ion Phys. 1078, 26, 395. (17) Boyd, R. K.; Beynon, J. H. Org. Mass Spectrom. 1077, 12, 163. (18) Biiton, J. N.; KyrlakMis, N.; Walght, E. S. Org. Mass Spectrom. 1978, 13, 489. (19) Cody, R. B.; Freiser, B. S. Anal. Chem. 1082, 5 4 , 1431. (20) Carlin, T. J.; Frelser, B. S. Anal. Chem. 1083, 5 5 , 571. (21) McIver. R. T.; Hunter, R. L.; Bowers, W. D. Int. J . Mass Specfrom. Ion Processes 1085. 6 4 , 67. (22) Gllsh, G. L.; McLuckey, S. A.; McBay, E. H.; Bertram, L. K. Int. J . Mass Specfrom Ion Processes, in press. (23) Cooks, R. G.; Beynon, J. H.; Caprioii, R. M.; Lester, G. R. Metastable Ions; Elsevler: Amsterdam, 1973.
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Gary L. Glish* Scott A. McLuckey Analytical Chemistry Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37831 RECEIVED for review January 21, 1986. Accepted March 18, 1986. This work was supported by the U.S.Department of Energy, Office of Basic Energy Sciences, through Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
Mass Spectrometric Determination of Amines after Formation of a Charged Surface-Active Derivative Sir: Sputtering of organic molecules from liquid solution is a new and very powerful technique for organic secondary ion mass spectrometry (SIMS) (1). It has been demonstrated that when glycerol is used as solvent, sensitivity for this technique is closely related to analyte polarity and surface activity with the strongest signals being obtained for charged surfactants. Conversely, nonpolar and/or uncharged analytes may require derivatization before useful signals can be obtained. The use of “reverse derivatization“ to enhance polarity in relatively nonpolar analytes was first suggested by Cooks et al. (2). This concept has been developed further by Kidwell et al. (3) and DiDonato and Busch (4)who used Girard’s reagent T to introduce a quaternary ammonium function into otherwise uncharged analytes, thereby greatly enhancing sensitivity. Barber et al. (5) have noted the importance of surface activity in analysis of peptides, and Ligon and Dorn (6) have suggested that “reverse derivatization” procedures should be designed to enhance both polarity and surface activity. Ligon (7)has shown that derivatization of small peptides with dodecanal can provide enhanced response. Further, Ligon and Dorn (8-10) have demonstrated the analysis of small inorganic anions through the use of cationic surfactants. In this case, the surfactants serve to bind the 0003-2700/86/0358-188980 1.50/0
anions to the surface of glycerol solutions. Surfactants have also been used to bind organic analytes to the glycerol surface (11). In practice, many classes of organic analytes lack either polarity or surface activity (or both) and can benefit from derivatization before SIMS analysis. For each such class of materials, it is necessary to seek reagents that can confer the required combination of physical properties. In this correspondence, we report the use of 2-dodecen-1-ylsuccinic anhydride as a reagent to confer both a negative charge and surface activity on primary and secondary amines. Busch et al. (12)have described reagents for analysis of amines that impart charge but do not significantly enhance surface activity.
EXPERIMENTAL SECTION The mass spectrometer used in these experiments was a Finnigan-MAT 731. The operating parameters for the SIMS experiment have been described previously (6). Derivatization Procedure. One millimole of each amine (see below) was combined with 1 mmol of 2-dodecen-1-ylsuccinic anhydride in 10 mL of hexane or hexane/diethyl ether as solvent. A small amount of diethyl ether was added to enhance the solubility of the amine in the case of aniline. The reaction mixtures were then heated to 60 O C for 15 min. The solvent was subse0 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
quently removed at room temperature using a nitrogen stream leaving an oily residue. Derivatives of the following amines were prepared in this way: bis(2-ethoxyethyl)amine, 4-methylpiperidine, aniline, 1-pentylamine,1-hexylamine,di-n-butylamine, and diethylamine. The yield of acylated products from this reaction has been found to be 90-93 % by 13CNMR spectroscopy. Traces of unreacted anhydride and diacid are the only side products. Diacid appears because no attempt was made to dry the solvents or the amine before derivatization. Amine attack on this unsymmetrical anhydride occurs at both carbonyls producing two different amic acids. For the case of diethylamine, attack on the less hindered carbonyl predominates by a factor of 2:l as determined by 13C NMR. These two structures have the same mass and for purposes of this paper will be considered a single "derivative". The reaction failed with 2,2,6,6-tetramethylpiperidine. An amic acid derivative of cytidine was obtained in exactly the same manner except that the reaction was performed with dry dimethylformamide (DMF) as solvent. Mild heating was applied to facilitate removal of DMF after completion of the reaction. Analytical solutions (0.001 M) of each derivative in glycerol solvent were prepared as follows: In each case the oily residue obtained as described above was dissolved in 10 mL of methylene chloride to give 0.1 M stock solutions of each derivative. The reader will note that the stock solutions are actually slightly less than 0.1 M because, as noted above, the yields are slightly less than 100%. A 100-pLaliquot of the stock solution was combined with 100 pL of a 0.1 M solution of tetramethylammonium hydroxide and 10 mL of glycerol. Each of these solutions was examined independently by SIMS to evaluate the purity of the amic acid derivatives. In every case only the amic acid and a trace of diacid was observed in the SIMS spectrum. A second stock solution was prepared in methylene chloride containing all of the amic acids at 0.001 M. Aliquots of this mixed stock solution were combined with appropriate quantities of glycerol, tetramethylammonium hydroxide, and (in some cases) hexadecylpyridinium acetate to produce the solutions described in the text. In each case, solvents present other than glycerol were removed by evaporation using a stream of dry nitrogen. About 5 p L of each glycerol solution was loaded onto the target for each SIMS experiment. Imidization Procedure. An aliquot (100 pL) of the mixed stock solution was treated at reflux with acetic anhydride as solvent for 15 min, after which the solvent was removed at room temperature with a stream of dry nitrogen. Analytical solutions in glycerol of the resulting product mixture were prepared as described above. Reagents. All of the amines were obtained from Aldrich Chemical Co. except aniline,which was obtained from J. T. Baker Chemical Co., and 1-hexylamine, which was obtained from Eastman Organic Chemicals. 2-Dodecen-1-ylsuccinicanhydride was obtained from Aldrich and was used as received without purification. It is important to avoid technical grades of this material as they often contain homologues. The preparation of hexadecylpyridinium acetate has been described previously (9).
DISCUSSION Relatively small primary and secondary amines lack both a positive or negative charge and surface activity and therefore fail to provide useful mass spectra when evaporated from glycerol by sputtering. In some cases it has been possible to obtain spectra on such amines by making the solvent (glycerol) acidic thereby generating the protonated amine (11). However, the lack of surface activity has often dictated very low sensitivity even in these cases. It is clear, therefore, that the advantages of this new mode of ionization can be extended to include these materials only if reagents can be found that confer the appropriate combination of properties. It is fairly obvious that any derivatization procedure must be relatively simple to perform and must provide high yields. We have found that the reaction of amines with 2-dodecen-1-ylsuccinic anhydride provides a simple high-yield procedure for producing derivatives that are well suited to organic
I
E I C E
Ln
z r W
z
' 300
MASS (AMUI
I4O0
Figure 1. Negative secondary ion mass spectrum of the amlc acid derivatives of seven amines in glycerol solvent with added tetramethylammonium hydroxide (O.OOO1 M). Each amine is present at the 1 X I O 4 M level. The signals observed are assigned as follows: (A) diethylamine, m l z 338; (B) n-pentylamine, m l z 352; (C) aniline, m l z 3 5 8 (D) 4-methylpiperidine, m l z 364; (E) n-hexylamine, m l z 366; (0) dibutylamine, m / z 394; (H) bis(2-ethoxyethyl)amine, m / z 426.
SIMS analysis. The general form of the reaction is illustrated below, R1 and R2 may be alkyl, aryl, or hydrogen. cH3"H2$40
+
H-N,R.
I
-
C H &CH,I,
R2
As shown, this reaction provides an amic acid. High yields have been observed. Although not shown explicitly in the scheme, the reaction actually produces two acylated products (see Experimental Section) because the anhydride is unsymmetrical. These two products have the same mass and therefore, for purposes of mass spectrometry, constitute a single derivative. It is interesting that the large increase in mass incurred with this reagent is not a disadvantage, since it moves the masses of interest into a "cleaner" part of the mass spectrum where there is generally less "chemical noise". The reaction fails with highly hindered amines such as 2,2,6,6-tetramethylpiperidine. Figure 1shows the negative secondary ion mass spectrum obtained for a glycerol solution of amic acid derivatives where each amic acid is present at 1 x M. Tetramethylammonium hydroxide has been added at the 1 X lo-* M level to make the solution very basic and thereby ensure that the amic acids are fully ionized. It is important to note that without this base treatment small traces of diacid, which are also produced from the reagent by hydrolysis, tend to completely dominate the SIMS spectrum. Presumably, the strong base forms the dianion of the diacid thereby greatly increasing its glycerol solubility. As a result the diacid is almost completely removed from the glycerol surface and, consequently, from the SIMS spectrum. It may be seen in Figure 1that all of the amic acids produce very similar ion currents in spite of the wide variety of structures among the amines themselves. This similar response is important in quantitative applications and is hypothesized to arise because the surface activity of the amic acids is dominated by the 12-carbon chain of the anhydride. Since every derivative has this same chain, every derivative has similar surface activity. These amic acids produce almost exclusively [M - HI- ions with little or no fragmentation. It is interesting that this reagent also provides a mechanism for distinguishing primary and secondary amines. Treatment of the amic acids with acetic anhydride (see Experimental Section) changes the amic acid derived from primary amines into imides. Imides lack the ability to become charged in solution and consequently are effectively transparent in the SIMS spectrum. Figure 2 shows the SIMS spectrum obtained after such an imidization procedure. It may be seen that all of the amic acids derived from primary amines have largely disappeared from the spectrum. Unfortunately, this procedure
ANALYTICAL CHEMISTRY, VOL. 58,NO. 8,JULY 1988
1891
185
320
340
360
380
400
420 440 460 480
MASS (AMW
Flgurs 2. Negative secondary ion mass spectrum of the same amic acid derivatives shown in Figure 1 after treatment wtth acetic anhydride. The letters are used to deslgnate the same signals as in Flgure 1. In addition, letters F and I designate glycerol tetramer and pentamer, respectively. The solution contains 0.001 M tetramethylammonium hydroxide,
F
I I
F
C I
I
H
LL
..*./
,
L
l ' / ' I i I i I ' l ' I i l '
320
340
360
380 400 420
440 460
MASS (AMUI
Figure 3. Negative secondary ion mass spectra from more dilute solutions of the same amic acids shown in Figure 1. The peak assignments are the same as for Figure 1 with the addition of F and I, which represent glycerol tetramer and pentamer, respectively. Every solutkn contains tetramethylammoniumhydroxide at the 0.001 M level: (I) 1 X lo4 M in each component, (11) 1 X lod M In each component with reduced primary ion current, (111) 1 X lom5in each component with 0.001 M hexadecylpyridiniumacetate added, (IV) 1 X lod M in each component with 0.00 1 M hexadecylpyridinium acetate added.
is not completely specific, since it may be noted that the signal for the amic acid of 4-methylpiperidine is also much diminished. This most likely suggests that this particular secondary amic acid has undergone a reverse reaction to give amine and regenerate the anhydride. This effect is not fully understood, and the reader must be aware of this potential ambiguity in the imidization experiment. In an additional series of experiments, we have endeavored to determine a practical limit of detection for these amic acids. Figure 3 shows a series of spectra obtained under a variety of conditions. The top spectrum (I)shows the mixture of amic acids a t a concentration of 1 X 10" M in glycerol with base added (0.001 M) as described above. It is clear that a t this concentration the spectrum is dominated by glycerol clusters and the amic acids are barely detectable. The second spectrum (11) shows the same solution examined a t reduced primary ion current (about 2 x 10" particles cmS2s-l instead of the usual value of 2 X 10l2particles cm-2 8-l). In this case we see that matrix ions (glycerol-derived ions) are reduced in
4 50
500
550
600
MASS (AMU)
Figure 4. (top) Positive secondary ion mass spectrum of a 0.001 M solution of cytidine in glycerol solution. (bottom) Negative secondary ioin mass spectrum of a 0.001 M solution of the amic acid derivative of cytidine in glycerol solution.
intensity relative to the amic acids. This improvement is hypothesized to arise from the reduced rate of surface erosion by the primary beam resulting in greater fractional surface coverage by the analyte. The third spectrum (111)shows the result of adding 1 X 10"' M hexadecylpyridinium acetate to the solution used for spectra I and 11. It is interesting that this reagent (which is transparent in the negative ion SIMS spectrum because of its positive charge) greatly enhances the signal-to-noise ratio. The fourth spectrum (IV) shows the same group of amic acids at 1X lo4 M in glycerol using base and hexadecylpyridinium acetate. Here we finally appear to have reached the practical limits of detectability. The new signals that appear a t m / z 319,333, and 400 arise from very low level glycerol impurities and have not been identified. Since only about 5 p L of glycerol solution was loaded onto the target, this represents only about 1 X mol of sample on the target of which only a very small fraction is used to obtain the spectrum. Most of the glycerol solution is recovered from the target. It is interesting that addition of hexadecylpyridinium acetate has reduced the detection limit from above 1 X M to about 1 X lo4 M. This reagent is hypothesized to act in three ways. First, it eliminates glycerol from the surface thereby removing a potentially competing carrier of negative charge. It may be seen from spectra I11 and IV that glycerol ions are absent or greatly reduced. Second, the surfactant provides a hydrocarbon-like phase on the surface that tends to extract into itself other hydrocarbon-like molecules, such as the amic acids, which are of interest. Third, the positive charge of the hexadecylpyridinium cation requires that negatively charged species migrate to the surface in order to maintain charge neutrality. As we have described earlier (9), the acetate ion originally associated with the surfactant does not tend to migrate to the surface. Therefore, the job of balancing charge falls to other species in solution such as the amic acids. Finally, in order to demonstrate that this new reagent for amine analysis is useful with biologically important molecules, we produced the amic acid derivative of cytidine. Figure 4 compares the negative ion SIMS analysis of this derivative with the positive ion SIMS analysis of underivatized cytidine. It may be seen that the underivatized sample provides no data whatsoever at 0.001 M, whereas the derivatized sample provides excellent data at the same concentration. This spectrum is also interesting because it demonstrates that amic acid derivatives can be formed in the presence of an hydroxyl function even when that function occurs in the same molecule. Only a trace of oxygen acylation is detectable in the spectrum at higher mass.
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Anal. Chem. 1888, 58, 1892-1894
SUMMARY The reaction of primary and secondary amines with 2-dodecen-1-ylsuccinic anhydride provides a simple high-yield route to derivatives that provide useful negative ion SIMS spectra from glycerol solution. Imidization of the amic acid derivative of primary amines provides a method for distinguishing primary and secondary amines. Detection limits for the amic acids lie in the lo4 M range and are improved by the use of cationic surfactants such as hexadecylpyridinium acetate. Addition of a cationic surfactant has the additional advantage that glycerol matrix ions are eliminated from the mass spectrum. ACKNOWLEDGMENT We thank E. A. Williams for assistance with 13C nuclear magnetic resonance spectroscopy. Registry No. A, 102072-09-3;B, 102072-10-6;B (acetimide derivative),102072-16-2;C, 102072-11-7;C (acetimide derivative), 102072-17-3;D, 102072-12-8;E, 102072-13-9; E (acetimide derivative), 102072-18-4; G, 102072-14-0; H, 102072-15-1; hexacytidine amic decylpyridinium acetate, 7439-73-8; cytidine, 65-46-3; acid derivative, 102072-19-5;2-dodecen-1-ylsuccinicanhydride, 19780-11-1; acetic anhydride, 108-24-7. LITERATURE CITED (1) Barber, M.; Bordoli, R. S.; Elliott, G. H.; Sedgewlch. R. D.; Tyler, A. N. Anal. Chem. 1982, 5 4 , 645A.
Busch, K. L.; Unger, S. E.; Vlncze, A.; Cooks, R. G.; Keough, T. J . Am. Chem. SOC. 1982, 104, 1507. KMweii, D. A.; Ross, Mark M.;Chiton, R. J. Blamed. Mass Spectrom 1985, 12, 254. DiDonato, G. C.; Busch, K. L. Biomed. Mass Spectrom. 1985, 12, 364. Clench, M. R.; Garner, G. V.; Gordon, D. B.; Barber, M. Biomed. Mass Spectrom. 1985. 72, 355. L@n. W. V.; Dorn S. B. Int. J . Mass Specworn. Ion Processes 1984. 57, 75. Ligon, W. V. Anal. Chem. 1985, 58, 485-487. Ligon, W. V.; Dorn, S. B. Int. J . Mass Spectrom. Ion Processes 1985, 6 3 , 315. Ligon, W. V.; Dorn, S.B. Anal. Chem. 1985, 5 7 , 1993. Ligon, W. V.; Dorn, S. B. Int. J . Mass Spectrom. Ion Processes 1988, 68, 337. Ligon, W. V.; Dorn, S. B. Int. J . Mass Spectrom. Ion Processes 1904, 67, 113. Busch, K. L.; DMonato, G. C.; Krohe. K. J.; Hittle, L. R. Abstracts, 1985 Pittsburgh Conference & Esposffion on Analytical Chemistry and Applied Spectroscopy, Paper 37 1.
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Woodfin V. Ligon, Jr.* Steven B. Dorn General Electric Company Corporate Research and Development Schenectady, New York 12301 RECE~VED for review January 21, 1986. Accepted March 17, 1986.
Polar Functional Group Analysis of Mixtures by Silicon-29 Nuclear Magnetic Resonance Sir: Trimethylsilylation is a well-established method for isolation purposes, mass spectrometry, gas chromatography, and numerous synthetic procedures (1). In combination with proton NMR, trimethylsilylation enhances the accuracy of quantitation of polar groups (2-5). Proton chemical shifta of the trimethylsilyl group are, however, not sufficiently sensitive to variations of molecular structure, and so differentiation of various polar functions in polyfunctional compounds or in mixtures is usually not possible by 'H NMR spectroscopy. In total contrast, 29SiNMR chemical shifts in such derivatives are highly sensitive to molecular structure (6) and the conventional 29SiNMR signals are very weak (7). Despite the need for large sample amounts and/or long-time signal accumulation, 29SiNMR spectroscopy has been suggested (6) and used for analysis of polyfunctional compounds (8-12) and recently applied to very complicated mixtures that are encountered in coal extracts and oil fractions (13-16). %i Nh4R has been found to be more convenient than 19F NMR of trifluoroacetyl derivatives for qualitative analysis of such complex mixtures (15). In this communication we wish to report on three improvements of the procedures used in '%i NMR analysis of complex mixtures (13-16). The fist one enhances the NMR signal by application of the INEPT technique; the second increases the reproducibility of chemical shift values and reduces the cost of analysis by replacing the expensive solvent (pyridine-d,) by the much less expensive chloroform-d. Finally, we suggest replacing the volatile reference (tetramethylsilane) with hexamethyldisilane, which is much easier to handle. EXPERIMENTAL SECTION NMR Spectra. The spectra were measured on a Varian XL-200 spectrometer operating at 200 MHz for 'H NMR and at 0003-2700/86/03581892$01.50/0
39.7 MHz for NMR. In a 5-mm broad-band probe, 90° pulses were 8.5 and 49 c long for %i and 'H, respectively. A relaxation period of 40 s was employed for measurements with gated decoupling, unless the relaxation reagent, tris(acety1acetonate)chromium(III), was added to the solution. In the latter case, the delay was only 10 s as in the INEPT measurements. The spectra were measured with 4-kHz sweep width; data were acquired for 1s. FIDs were zero-fied to 16k and exponentially weighted with line broadening of 1.0 Hz. The samples for integral evaluation were approximately 0.5 M solutions of the more abundant component in deuteriochloroform placed into 5-mm NMR tubes. Trimethylsilylation. All work was carried out with dry reagents and under dry nitrogen. Approximately 100 mg of a sample was mixed in a vial with 0.4 mL of bis(trimethylsily1)acetamide and 0 . 4 mL of pyridine, depending on solubility. The heterogenous mixture was shaken for 2 h at 60-80 O C . Then the remaining silylating reagent and pyridine were removed by nitrogen stream, followed by l-h heating (up to 80 "C) under reduced pressure.
RESULTS AND DISCUSSION Conventional ?Si FT NMR spectra (13-16) are measured with gated proton decoupling (to suppress the negative nuclear Overhauser effect), and the measuring time is shortened by a relaxation reagent (7). Much larger time savings can be achieved if the silicon signal is enhanced by polarization transfer from protons. Two general polarization transfer techniques are suitable for this purpose: INEPT and DEPT (for comparison of different variants of these methods and references to the original literature see reviews in ref 17 and 18). For measurement of proton decoupled NMR spectra of trimethylailyl groups, the INEPT technique is slightly superior to DEPT (17). In adopting INEPT pulse sequence (90' [H]-~/2-180~[HI, 180° [Si]-~/2-90' [HI, 90° [Si]-A/2-180° [HI, 180' [Si]@ 1986 American Chemical Society
