612 J. Jpn. Oil Chem. Soc. (YUKAGAKU) ORIGINAL Ammonium Sulfate as a Standard for 33S-NMR Spectra Yoshio KOSUGI Department of Applied Chemistry, School of Engineering, Nagoya University (Chikusa ku, Nagoya-shi, 464-01) Ammonium sulfate was found possibly useful as a standard for the chemical shift and line-width of sulfur- 33 NMR. 2 M aqueous solution gave a signal with S/N larger than 4 in 10-43s and, thus showing it useful for probe tuning and matching as well as deciding on optimum pulse conditions in a short time. A line-width relative to that of ammonium sulfate or a reference was essentially constant for all pulse conditions. Thus, by measuring the line-width of signals at any experimental pulse condition linewidth can be expressed for any of the following standard pulse conditions : spectral width of 1500 Hz, data points of 4 K, pulse width 7/2, pulse delay time of 0.1 s, and a line broadening factor of 5 Hz, by which ammonium sulfate of 1 M solution gives a width of 8.5 Hz. The present method is applicable to all other insensitive and broad NMR spectra. Introduction A number of compounds are used in NMR spectroscopy to define not only the chemical shift but also sensitivity, resolution etc. from which the quality of a spectrometer is evaluated". Tetramethylsilane (TMS) is set to be zero ppm on the chemical shift scale for proton and carbon- 13 NMR after the long research history. Owing to the recent advancement of NMR spectrometers, various nuclei are objects of measurements. In most cases, however, approved standards, like TMS have not yet been determined. For example, ammonium nitrate, nitromethane, acetonitrile, ammonia are used for the standard of nitrogen-15 NMR2). The chemical shifts reported using different standards are troublesome in discussion, and accurate comparisons are often impossible. A similar situation was experienced a few decades ago in proton and carbon-13 NMR when they became accessible by any chemists. Only single standard is desirable, but several points should be considered before setting a standard : i) inertness, ii) handling, iii) availability, and iv) spectral features such as sharpness in a short time. A compound for the standard should be chemically and physically stable, not like TMS. TMS in an ampoule is not a favorable compound to handle because of its low boiling point of 26 Ž, nor a cheep trivial compound, either. TMS has the role of an absolute standard, including solvent effects, temperature dependency, etc. Therefore, it is too late to replace TMS with a more suitable standard without causing enormous confusions. A preferable compound would also suffice for the following aspects. The first problem to be considered is the selection of pulse conditions and the optimum tuning and matching of a spectrometer probe which affect seriously the spectral quality of an insensitive nucleus. For these preparatory operations, a strong signal in a short time or with fewer transients is required. The second problem is an evaluation of the spectral line-width which is usually an important information from the spectra of nuclei with spin quantum number more than one half. Sulfur- 33 NMR, a typical insensitive and broad spectrum, because of the low natural abundance of 0.76% and spin quantum number of 3/2, is a pertinent example to discuss on these problems besides a standard of the chemical shift. It is neither early nor late to set one standard for sulfur- 33 NMR be- 52
Vol. 42. No. 8 (1993) 613 cause an appropriate size of data body is available today, and the outline of spectral features has been roughly disclosed. Experimental Ammonium sulfate and sulfolane with natural abundance of sulfur-33 were purchased (Wako Pure Chemical Ind., LTD.) and used without further purification. The spectra were recorded at 25 C on a Bruker AC-250, operated at 19.19 MHz, with an ASPECT 3000 computer. The field lock was attained by the deuterium signal of deuterium oxide. Pulse conditions employed, unless otherwise specified, were as follows : spectral width of 1500 Hz, data points of 4 K, pulse width of n-/2 or 18,a s, 0.1 s pulse delay time, 250 IL s dead time after the pulse, 0.819 s acquisition time, decoupler power of 5 W. Normally, 104-105 transients or scan numbers were accumulated and an exponential broadening factor of 5 Hz was used as the window function. A double tube method was carried out in the following manner. A sample solution was transferred to an usual NMR cylindrical tube of 10 mm o. d. and 180 cm high. A reference solution was made in another slender tube (3 mm o. d., 180 mm high) with a bulb-shaped portion (8 mm o. d., ca. 12 mm high) at the bottom. The bulb Fig. - 1 Double tube method for NMR. portion of the slender tube fitted coaxially inside the NMR sample tube at the bottom. The upper portion of the slender tube was tightened through an NMR tube cap with a hole, and was sealed around the cap with parafilm. The entire double tube was spun at a rate of 20 s-1 Results and Discussion The standard for the chemical shift of sulfur-33 NMR varies with literatures ammonium sulfate, sulfolane, and carbon disulfide are major three compounds often used for the standard. Among these three compounds, carbon disulfide is at higher field by more than 200 ppm3) Sulfolane is often used for samples of organic solvents, but the chemical shift is dependent on solvent 4),5). On the other hand, ammonium sulfate gives a sharp peak and is a suitable standard for water soluble sulfur compounds, such as sulfonates0,7), sulfates8),9). The 2 M deuterium oxide solution shows a signal with S/N of 4 in 10-30 s or with the accumulation of 100 or less transients. As the chemical shift is generally affected by various factors such as counter ions 9), concentrations, ph, solvents, the double tube method is preferred. Prior to the searching a standard for the chemical shift, the double tube method was examined. Double tube method. Ammonium sulfate and sulfolane were used in the examination of the double tube method with various kinds of pulse conditions and concentrations. The first sample was ammonium sulfate with sulfolane in the inner double tube [ (ammonium sulfate/(sulfolane)) the description of this kind is used in this article], and the second group was sulfolane with ammonium sulfate in the inner tube [ Csulfolane/(ammonium sulfate)]]. If any magnetic susceptibility in the double tube method had an effect on the chemical shifts either positively or negatively, the distance of the two peaks should change with the first and the second samples. The result did not show any meaningful change (Table-1). A further examination was carried out using the sample of (ammonium 53
614 J. Jpn. Oil Chem. Soc. (YUKAGAKU) Table- 1 The double tube method An usual NMR cylindrical tube(10 mm o.d., 180 mm high) An inner slender tube(3 mm o.d., 180 mm high) with bulb shape portion (8 mm o.d., ca. 12 mm high) Table- 2 Concentration effects on 33S NMR spectra of ammonium sulfate. sulfate/(ammonium sulfate) of the same concentration. The resolution was increased by employing the spectral width of 500 Hz and data points of 32 K. These pulse conditions are enough to give two peaks of ammonium sulfate (0 ppm) and sodium sulfate (0.06 ppm)8), but any splittings of the peaks were not observed for the present sample. All results given in Table-1 have revealed that no susceptibility correction for the chemical shifts is necessary for the double tube method, contrary to the sensitive proton NMR10). Standard for the chemical shift. The following compounds other than the mentioned above were examined : a) cesium sulfate has been used as a standard by some NMR spectroscopists5),11), but any superiority in handling nor availability to ammonium sulfate were not found, b) sodium sulfate gave a peak8 as sharp as ammonium sulfate, but with less solubility, and c) benzenesulfonate showed a reasonably sharp peak, but the applied ph of the sample solution was limited. After surveying of these data and other literatures, it has been concludes that ammonium sulfate is the best compound for a standard of sulfur-33 NMR. In order to examine a concentration dependency of the chemical shift, the concentration of ammonium sulfate was varied using the double tube method with neat sulfolane in an inner tube, [ (ammonium sulfate/(neat sulfolane)) 1. The result in Table-2 indicates that the chemical shift is independent of, but the line-width is dependent on the concentration. The line-width. A nucleus with a spir quantum number larger than one half ha; quadrupole moments causing the linewidth broad. Usually the line-width at on( half the peak height is used to evaluate the spectral features and is used as an important information without the consideratior on conditions for spectra taken. However, an emphasis should be put on the fact thai the line-width changes not only with the concentration, but with the pulse conditior employed and a window function for the spectral processing to increase the signal tc noise ratio for broad signals. The effect of the concentration on the line-width was exemplified in Table-2. ThE phenomenon is explicable with an increase of the counter ion (NH 4+) which acceleratee the relaxation rate 12) resulting in a broad peak. In the present study, examined were the effect of basic pulse conditions such ae spectral width, data points, pulse delay time, and the line broadening factor or the damping factor. Shown in Fig.-2 is a few examples of the free induction decay (fid) multiplied by an exponential function leading to a damping the spectral noise at the expense of resolution (cf. Fig.-7) by 2-1C hertz unit equivalent to the broadening factor. A sample of (neat sulfolane/(ammonium sulfate)) was used for the examination. In Figs.-3,-7, line-widths of sulfolane, ammonium sulfate and their ratios were plot- 54
Vol. 42. No. 8 (1993) 615 Fig. - 2 Effect of line broadening factor (LB) on free induction decays and spectra of a sample of sulfolane (at lower field) and ammonium sulfate (at 0 Hz). 55
616 J. Jpn. Oil Chem. Soc. (YUKAGAKU) Fig. - 3 Effect of the data point on the line-width of ammonium sulfate (Wa Hz, 0) and sulfolane (Ws Hz,) at one half the peak height, and their ratio (R=Ws/Wa, 0) with constant pulse conditions : spectral width (1500 Hz), pulse delay time (0.1 s besides acquisition time of 1.36 s), pulse angle (i1-/2) and a line broadening factor of 5 Hz. ted against the pulse conditions and the line broadening factors employed. These results have revealed that the linewidth is dependent on the experimental conditions. Therefore, the line-width as a NMR datum should be given with a specification of experimental conditions. But it is not a convenient way in discussion or comparison with other spectra to use the linewidth with different experimental conditions. On the other hand, the relative linewidth of two peaks is essentially constant with exceptions of the case that an improper pulse angle (z/8) and inadequate broadening factors (0,-2 Hz) cause a poor signal to noise ratio. Therefore, if the line-width, 8.5 Hz of ammonium sulfate of 1 M solution at the condition : (spectral width of 1500 Hz, data points of 4 K, pulse width 71-/2, pulse delay time of 0.1 s, line broadening factor of 5 Hz] is accepted as a reference, a relative Fig. - 4 Effect of the spectral width on the line width of ammonium sulfate (Wa Hz, -0) and sulfolane (Ws Hz, 0) at one half the peak height, and their ratio (R=Ws/Wa 0) with constant pulse conditions : pulse delay time (0.1 s besides the acquisition time), pulse angle (r/2), data points (4 K) and a line broadening factor of 5 Hz. line-width can be converted to an absolute value at the standard condition. Thus, the signal of ammonium sulfate can be a standard for line-widths as well as chemical shifts under any experimental conditions applied. The line-width may be affected by other factors such as inhomogeneity of the magnet, strength of magnetic field and temperatures. But it is assumed that the problem of inhomogeneity of the magnet field has been diminished as the effect on chemical shifts has not been subjects any more, owing to the great advancement of manufacturing techniques of the instrument. The 56
Vol. 42. No. 8 (1993) 617 Fig.-5 Effect of the pulse delay time including acquisition time (1.36 s except for the case of blacken symbol or 0.34 s) on the line-width of ammonium sulfate (Wa Hz, o) and sulfolane (Ws Hz, E) at one half the peak height, and their ratio (R= Ws /W., 0 or œ) with constant pulse conditions : spectral width (1500 Hz), pulse angle (z/2), data points (4 K) and a line broadening factor of 5 Hz. Fig.-6Effect of the pulse angle on the linewidth of ammonium sulfate (Wa Hz, o) and sulfolane (Ws Hz, ) at one half the peak height, and their ratio (R=Ws /Wa 0) with constant pulse conditions : spectral width (1500 Hz), pulse delay time (0.1 s besides acquisition time of 1.36 s), data points (4 K) and a line broadening factor of 5 Hz. other two factors may affect a little on the value, 8.5 Hz with a magnet of 5.78 T, at room temperature. And the calibration for the value, if necessary, is accessible easily. We recommend that all sulfur-33 NMR spectra should be taken in the presence of a standard compound, preferably ammonium sulfate, and the data are standardized by the chemical shift and the line width of ammonium sulfate. The resulting data would bring information on the nature of the sulfur compounds. It is also expected for NMR data committees to collect these data throughout the world, and the standard value of the line- width may be refined. The procedure described herein for the determination of a standard for both the chemical shift and the line-width of sulfur-33 NMR can be applied to any other NMR. Acknowledgements Financial support for a part of this research by Natoco Paint Co. Ltd. is greatly acknowledged. Helpful suggestions by Prof. T. Watanabe (Tokyo Univ. of Fisheries) were given. (Received Nov. 13, 1992) References 1) ASTM, for example. 2) G.C. Levy and R.L. Lichter, "Nitrogen-15 Nuclear Magnetic Resonance Spectroscopy", 57
618 J. Jpn. Oil Chem. Soc. (YUKAGAKU) John Wiley, New York (1981). 3) R. Annunziata and G. Barbarella, Org. Magn. Reson., 22, 250 (1984). 4) A.A.M. Ali, R.K. Harris, and P.S. Belton, Magn. Reson. Chem., 28, 318 (1990). 5) P.S. Belton, I.J. Cox. and R.K. Harris, J. Chem. Soc., Faraday Trans., 2, 81, 63(1985). 6) Y. Kosugi, Anal. Sci., 5, 253 (1989). 7) Y. Kosugi, Anal. Sci., 7, 209 (1991). 8) Y. Kosugi and H. Okazaki, Anal. Sci., 7, 849 (1991). 9) P. S. Belton, I. J. Cox, and R. K. Harris, Magn. Reson. Chem., 24, 171 (1986). 10) K. Momoki and Y. Fukazawa, Anal. Chem., 62, 1665 (1990). 11) M. Haller, W. E. Hertler, 0. Lutz and A. Nolle, Solid State Commun., 33, 1051 (1980). 12) J. F. Hinton and D. Shungu, J. Magn. Reson., 54, 309 (1983). Fig.-7Effect of the line broadening factor of exponential type on the line-width of ammonium sulfate (Wa Hz, o) and sulfolane (Ws Hz, ) at one half the peak height, and their ratio (R=Ws/Wa, 0), and on the peak height of ammonium sulfate ( A ) and sulfolane ( A ) with constant pulse conditions : spectral width (1500 Hz), pulse delay time (0.1 s besides acquisition time of 1.36 s), pulse angle (Ĕ/2) and data points (4 K).