Disolventes Nmr

7512

J. Org. Chem. 1997, 62, 7512-7515

NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities
Hugo E. Gottlieb,* Vadim Kotlyar, and Abraham Nudelman*
Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel
Received June 27, 1997

In the course of the routine use of NMR as an aid for organic chemistry, a day-to-day problem is the identification of signalsderiving from common contaminants (water, solvents, stabilizers, oils) in less-than-analytically-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 dependence of chemical shifts (especially 1H); results obtained two or three decades ago usually refer to much more concentrated samples, and runat lower magnetic fields, than today’s practice. We therefore decided to collect 1H and 13C chemical shifts of what are, in our experience, the most popular “extra peaks” in a variety of commonly used NMR solvents, in the hope that this will be of assistance to the practicing chemist. Experimental Section
NMR spectra were taken in a Bruker DPX-300 instrument (300.1 and 75.5 MHz for 1H and 13C,respectively). Unless otherwise indicated, all were run at room temperature (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 approximately the same level as a typical sample. At each temperature, the D2Osamples 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 as to prevent intermolecular interactions and possible ambiguities in assignment. Solution 1: acetone, tert-butyl methyl ether,dimethylformamide, ethanol, toluene. Solution 2: benzene, dimethyl sulfoxide, ethyl acetate, methanol. Solution 3: acetic acid, chloroform, diethyl ether, 2-propanol, tetrahydrofuran. 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, cyclohexane, 1,2-dimethoxyethane,nitromethane, silicone grease, triethylamine. Solution 7: diglyme, dimethylacetamide, ethylene 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: 1,2-dimethoxyethane, dimethylacetamide, ethylmethyl ketone, triethylamine. Solution 5: acetic acid, tertbutyl alcohol, diethyl ether, tetrahydrofuran. In D2O and CD3OD nitromethane was run separately, as the protons exchanged with deuterium in presence of triethylamine.

Figure 1. Chemical shift of HDO as a function of temperature.

Results Proton Spectra (Table 1). A sample of 0.6 mL of the solvent, containing 1 µL of TMS,1 was firstrun on its own. From this spectrum we determined the chemical shifts of the solvent residual peak2 and the water peak. It should be noted that the latter is quite temperature(1) For recommendations on the publication of NMR data, see: IUPAC Commission on Molecular Structure and Spectroscopy. Pure Appl. Chem. 1972, 29, 627; 1976, 45, 217.

dependent (vide infra). Also, any potential hydrogenbondacceptor will tend to shift the water signal downfield; 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 chemical shift of the residual HDO is very temperature-dependent (vide infra) but, maybe counterintuitively,...