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Friday, March 30, 2012

The Extremely Complicated 1H NMR Spectrum of Ethane

It is often incorrectly assumed that simple compounds yield simple NMR spectra. The 1H NMR spectrum of ethane is such an example. The complexity arises when one takes into account the inequivalence between methyl groups in the mono 13C isotopomer which accounts for 2% of the naturally occurring ethane. In this isotopomer, one methyl group experiences a one-bond 1H - 13C coupling (1JH-C) while the other methyl group experiences a two-bond 1H - 13C coupling (2JH-C). Also, the effects of the three-bond 1H - 1H coupling (3JH-H) are exhibited in the spectrum due to the inequivalence. These couplings have a dramatic effect on the spectrum. Furthermore, there is a very small isotope effect on the 1H chemical shifts of each methyl group due to the presence of 13C vs 12C. This effect however, is very small (~0.002 ppm) and has very little effect on the spectrum. The left panel of the figure below shows a simulation of the 1H NMR spectrum of the 12CH3-12CH3 which accounts for 98% of naturally occurring ethane. As expected, the spectrum is a singlet as both methyl groups are equivalent to one another. The middle panel of the figure shows a simulation of the 1H NMR spectrum of the 13CH3-12CH3 isotopomer which accounts for 2% of naturally occurring ethane. In this case the spectrum is extremely complex due to the 1JH-C , 2JH-C and 3JH-H coupling. The panel on the right shows a simulation of a scaled up representation of what one would expect for naturally occurring ethane.
The parameters for the simulations are as follows: ΔδH between -12CH3 and -13CH3= 0.002 ppm, 1JH-C = 125 Hz, 2JH-C = -4.67 Hz, 3JH-H = 8 Hz and LB = 0.5 Hz.

Thursday, March 29, 2012

The Major Constituents of Natural Gas

The major constituent of natural gas is methane however, other gaseous hydrocarbons are also present. One way to identify other components is to dissolve some natural gas in a solvent and examine the 1H NMR spectrum. The spectrum in the figure below was acquired on a sample prepared by bubbling natural gas through benzene-d6 for several minutes. The spectrum clearly shows the presence of methane, ethane, propane and water. The spectrum also indicated other impurities at much lower levels (not shown in the figure).

Friday, March 16, 2012

Protic Samples in Aprotic Solvents

The appearance of the 1H NMR signals of protic samples in aprotic solvents depends critically on the concentration of the sample. The -OH, -NH2 or -COOH signals can have chemical shift values and line widths over a wide range due to varying extents of hydrogen bonding and chemical exchange. The concentration can also determine whether or not one is able to observe J coupling between and -OH proton and other protons in the sample. An example of this is illustrated below. The figure shows the spectrum of methanol in deuterated acetone. The spectrum on the top is that of concentrated methanol and it consists of two singlets. In this case the methanol molecules are hydrogen bonded to one another and the -OH protons are undergoing fast exhange with one another. The spectrum on the bottom is that of very dilute methanol. It is a second order spectrum with two signals approximating a doublet and a quartet due the J coupling between the methyl protons and -OH proton, respectively. Note that the chemical shift of the -OH proton is much lower for the dilute methanol compared to the concentrated methanol. In this case the methanol molecules are not hydrogen bonded to one another and there is no (or very slow) exchange among the -OH protons between molecules allowing for the observation of the J coupling.

Thursday, March 15, 2012

Double Presaturation

Presaturation is a common method of reducing the water signal in the 1H NMR spectra of aqueous samples. Sometimes, a sample may contain more than one undesirable resonance which a user may want to presaturate. In such a case, one must presaturate at multiple frequencies simultaneously. On a two-channel Bruker spectrometer, two signals can be presaturated. This is accomplished by using both Signal Generation Units (SGUs). One of the undesirable signals is put on-resonance and is presaturated with the signal from SGU1 after which the hard pulse is given (also through SGU1). The second undesirable resonance is presaturated using SGU2. The configuration is as follows:If one has a three-channel system, one can presaturate three resonances using three SGUs. The figure below shows an example of double presaturation on a two-channel system. The sample consisted of phenylalanine dissolved in D2O contaminated with methanol. The one-pulse spectrum in the bottom left panel shows the intense HDO and methanol signals. The double presaturation spectrum in the top left panel is on the same vertical scale as the one on the bottom left. One can see that both solvent signals have been almost completely eliminated. The spectra on the right-hand side are the same data as on the left except the vertical scale has been increased by a factor of 100. You can try this on your Bruker spectrometer using the pulse program "lc1prf2".