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1、NMR Chemical Shifts of Common Laboratory Solvents as Trace ImpuritiesHugo E. Gottlieb,* Vadim Kotlyar, and Abraham Nudelman*Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, IsraelReceived June 27, 1997In the course of the routine use of NMR as an aid for organic chemistry, a day-to-day
2、 problem is the identifica- tion of signals deriving from common contaminants (water, solvents, stabilizers, oils) in less-than-analyti- cally-pure samples. This data may be available in the literature, but the time involved in searching for it may be considerable.Another issue is the concentration
3、dependence of chemical shifts (especially1H); results obtained two or three decades ago usually refer to much more concentrated samples, and run at lower magnetic fields, than todays practice. We therefore decided to collect1H and13C chemical shifts of what are, in our experience, the most popular “
4、extra peaks” in a variety of commonly used NMR solvents, in the hope that this will be of assistance to the practicing chemist.Experimental SectionNMR spectra were taken in a Bruker DPX-300 instrument (300.1 and 75.5 MHz for1H and13C, respectively).Unless otherwise indicated, all were run at room te
5、mperature (24 ( 1 C). For the experiments in the last section of this paper, probe temperatures were measured with a calibrated Eurotherm 840/T digital thermometer, connected to a thermocouple which was introduced into an NMR tube filled with mineral oil to ap- proximately the same level as a typica
6、l sample.At each temperature, the D2O samples were left to equilibrate for at least 10 min before the data were collected. In order to avoid having to obtain hundreds of spectra, we prepared seven stock solutions containing approximately equal amounts of several of our entries, chosen in such a way
7、as to prevent intermolecular interactions and possible ambiguities in assignment. Solution 1: acetone, tert-butyl methyl ether, di- methylformamide, ethanol, toluene. Solution 2: benzene, di- methyl sulfoxide, ethyl acetate, methanol. Solution 3: acetic acid, chloroform, diethyl ether, 2-propanol, t
8、etrahydrofuran. Solution 4: acetonitrile, dichloromethane, dioxane, n-hexane, HMPA. Solution 5: 1,2-dichloroethane, ethyl methyl ketone, n-pentane, pyridine. Solution 6: tert-butyl alcohol, BHT, cyclo- hexane, 1,2-dimethoxyethane, nitromethane, silicone grease, triethylamine. Solution 7: diglyme, di
9、methylacetamide, ethyl- ene glycol, “grease” (engine oil). For D2O. Solution 1: acetone, tert-butyl methyl ether, dimethylformamide, ethanol, 2-propanol. Solution 2: dimethyl sulfoxide, ethyl acetate, ethylene glycol, methanol. Solution 3: acetonitrile, diglyme, dioxane, HMPA, pyridine. Solution 4:
10、1,2-dimethoxyethane, dimethylacetamide, ethyl methyl ketone, triethylamine. Solution 5: acetic acid, tert- butyl alcohol, diethyl ether, tetrahydrofuran.In D2O and CD3OD nitromethane was run separately, as the protons exchanged with deuterium in presence of triethylamine.ResultsProton Spectra (Table
11、 1). A sample of 0.6 mL of the solvent, containing 1 L of TMS,1was first run on its own. From this spectrum we determined the chemical shifts of the solvent residual peak2and the water peak. It should be noted that the latter is quite temperature-dependent (vide infra). Also, any potential hydrogen-
12、 bond acceptor will tend to shift the water signal down- field; this is particularly true for nonpolar solvents. In contrast, in e.g. DMSO the water is already strongly hydrogen-bonded to the solvent, and solutes have only a negligible effect on its chemical shift. This is also true for D2O; the che
13、mical shift of the residual HDO is very temperature-dependent (vide infra) but, maybe counter- intuitively, remarkably solute (and pH) independent. We then added 3 L of one of our stock solutions to the NMR tube. The chemical shifts were read and are presented in Table 1.Except where indicated, the
14、coupling constants, and therefore the peak shapes, are essentially solvent-independent and are presented only once. For D2O as a solvent, the accepted reference peak ( ) 0) is the methyl signal of the sodium salt of 3-(trimeth- ylsilyl)propanesulfonic acid; one crystal of this was added to each NMR
15、tube. This material has several disadvan- tages, however: it is not volatile, so it cannot be readily eliminated if the sample has to be recovered. In addition, unless one purchases it in the relatively expensive deuterated form, it adds three more signals to the spectrum (methylenes 1, 2, and 3 app
16、ear at 2.91, 1.76, and 0.63 ppm, respectively). We suggest that the re- sidual HDO peak be used as a secondary reference; we find that if the effects of temperature are taken into account (vide infra), this is very reproducible. For D2O, we used a different set of stock solutions, since many of the less polar substrates are not significantly water- soluble (see Table 1). We also ran sodium acetate and sodium formate (chemical shifts: 1.90 and 8.44 ppm, respectively). Carbon Spectra (Table 2
