Improved Analysis of Minor Species in Mixtures Using Carbon-13 Decoupling in One-Dimensional Proton NMR

L A R R Y S. S I M E R A L Albemarle Corporation, Technical Center, P.O. Box 14799, Baton Rouge, Louisiana 70898

Index Headings: Proton NMR; Carbon-13 decoupling; Spectroscopic techniques.

INTRODUCTION

Proton nuclear magnetic resonance (NMR) is a useful tool to identify and to quantitate species in mixtures of

Received 4 August 1994; accepted 12 December 1994. * Author to whom correspondence should be sent.

organic chemicals. However, carbon-13 satellites often interfere with the identification and integration of reso- nances from species at low concentrations (< 2%) in such mixtures. Carbon- 13 satellites arise from proton coupling to carbon-13 (spin 1/2, 1.1% isotope abundance). Each satellite is 0.55% of the area of the proton resonance for protons bound to carbon-12. The interferences become particularly troublesome when the principal component of the mixture has many or overlapping resonances.

Decoupling of magnetically coupled nuclei through ir- radiation of one of the partners in coupled pairs of nuclei is widely practiced in NMR spectroscopy to simplify spec- tra and to aid structure elucidation. Proton decoupling is important in X-nucleus (C- 13, P-31, Si-29, etc.) N MR to eliminate proton coupling to the X-nucleus for spectral simplification? X-nucleus decoupling finds importance in some indirect-detection two-dimensional N MR ex- periments? Routine decoupling of X-nuclei from protons across the entire spectrum in one-dimensional proton NMR has not been practiced. In this discussion we dem- onstrate the general utility of carbon-13 decoupling in one-dimensional proton NMR analysis of chemical mixtures.

< H H H H I C-C C - C~--~ R H R H

• C-13 SATELLITES

R R C-C H H

C - C R H

"7"

6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 ppm

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1

H H R C ~ C - C - R' C -- C - CH 3

6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 ppm

FIG. 2. C-13 decoupled 400-MHz proton NMR spectrum of the same mixture shown in Fig. l. Instrument conditions are identical to those for Fig. l except for approximately t.3 W of decoupling power at 100.6 MHz applied with the use of a WALTZ-16 decoupling sequence.

E X P E R I M E N T A L

Proton N M R spectra were obta ined at 400 M H z on a Bruker /GE Omega 400WB ins t rument with the use of a 5 - m m rpt (reverse polar izat ion transfer) probe and the s tandard b road -band decoupling unit. A 100-MHz band- pass filter (K&L Microwave Industries, Inc.) was placed in line between the p r eam p and the decoupling channel on the probe to reduce noise feedthrough f rom the de- coupling to the pro ton side & t h e probe. The spectra were obtained in a nonspinning mode to el iminate any spin- ning sidebands. Data collection used a s tandard one-pulse sequence. When decoupling was employed, the decoupler

was turned on throughout the experiment . Exper imenta l parameters included 8-kHz sweep width; 32K real data points; 35 ° pulse; 6.1-s recycle t ime (4.1-s acquisit ion time); 128 acquisitions; and a Bessel function audio filter. Spectra with carbon- 13 decoupling used 1.3 W of power (7H2 = 1600 Hz) with a W A L T Z - 16 decoupling sequence at 100.63 M H z (centered at 110 p p m in the C-13 spec- trum). Ambien t - t empera tu re nitrogen gas was fed to the probe through the VT port at 30 SCFH. This nitrogen flow helped reduce any sample heating caused by the decoupling. No addit ional t empera ture control was used.

The sample mixture used here was 1-hexene with small amoun t s of other olefin isomers as 10% (v/v) in deuter- ochloroform.

Fro. 1. Norma l 400-MHz proton N M R spec t rum of an olefin mixture . The olefin region o f the spec t rum is plotted with the y axis expanded 120 x to show the C-13 satellites and the minor components. The vinylidene isomer (4.7 ppm) is about 0.2 wt % of the mixture. The spectrum was obtained in a nonspinning mode in a 5-mm rpt probe, and the chemical shifts are reference to TMS at 0 ppm.

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DISCUSSION

Figure 1 shows a 400-MHz proton NMR spectrum of an olefin mixture. The mixture contains mainly vinyl olefin with small amounts of other isomers. The vinyl C-13 satellites completely hide the trisubstituted olefin (3-alkyl substituted 2-ene) resonance at 5.17 ppm and substantially interfere with the resonance of vinylidene olefin (2-alkyl subst i tuted 1,ene) at 4.7 ppm and 3-substituted 1-enes at 5.7 ppm. Identification and in- tegration of the minor components are ditficult, if not impossible. If the minor species are known, quantitation can be accomplished by subtraction of the appropriate amount of C-13 satellite area from the area of the species of interest plus the satellite. At very low concentrations, this procedure amounts to subtraction of two large num- bers, leading to substantial errors in quantitation.

Figure 2 shows the 400-MHz proton NMR spectrum of the same mixture with the use of C-13 decoupling. The trisubstituted olefin and 3-alkyl substituted 1-ene are clearly resolved and easily identified from the chemical shift and coupling patterns. Integration of all the species becomes straightforward. The vinylidene also becomes better resolved.

Sample heating from the dccoupling is potentially del- eterious to spectral resolution. It is desirable to use the minimum amount of decoupling power necessary and to pay attention to the spectral resolution while decoupling. In the present case, only 1.3 W of power was used, and the decoupling frequency was placed in the center of the C-13 spectral region where decoupling was desired. The magnetic field homogenei ty of the instrument was shimmed under data acquisition conditions with the de- coupler on, to mitigate any effect of heating on spectral resolution. A small degradation of the resolution is re-

flected in the depth of the splitting valleys of the main peaks and on the vinylidene resonance at 4.7 ppm. (Close- ly compare Figs. 1 and 2.) The line shape at the base of the major resonances is not severely affected. The elim- ination of spectral interferences in the present case clearly compensates for the small degradation in line shape and resolution. Decoupling schemes which use significantly less decoupling power for the C-13 decoupling and pro- vide broader decoupling ranges are available. 3,4 Sample cooling or temperature control through the VT unit could also be used.

The approach described here is easily implemented on any NMR instrument having an rpt probe and broad- band decoupling unit. C-13 decoupling finds routine ap- plications anytime C-13 satellites can interfere with the spectral analysis, such as in the determination of enan- tiomeric purity using chiral solvating agents and in proton homonuclear correlation spectroscopy of minor compo- nents in complex mixtures.

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