Question

A) Predict the number of different chemically shifted 1H peaks and the fine structure of each peak that you would observe for nitroethane, CH3-H2C-NO2 assuming that 2JNH>3JNH>3JHH and that only 1H, 12C, 14N and 16O isotopes are present.

B) for a fixed frequency of the radio frequency field, 1H and 14N will be in resonance at different values of the static magnetic field. Calculate the value of B0 for these nuclei to be in resonance at the frequency of 250 Hz

Answer 1

A) Considering the molecule nitroethane, and the mentioned isotopes being present in the molecule one can deduce following statements for the nmr spectrum:

1) Assuming the ²J coupling being the strongest and largest in magnitude in the given compound, the coupling is not observed.

Also the peaks are variable and broad, appearing over a range from 0.5 to 4ppm

2) The ³J coupling being lesser in magnitude than ²J coupling is also not observed in the compound due to chemical exchange. The coupling here will behave as the protons being attached to the alpha position and may be observed over the range of 2-3ppm

3) Although the protons in the compound will couple, but under given condition where the ³J coupling between protons is weakest, we will observe the decreased coupling behavior between protons and the constant value may get reduced to 4-5 Hz meaning we won't get much refined peaks in the spectrum. The methylene protons attached to nitrogen will have chemical shift in the range of 2.2-2.9 ppm due to electronegative nitrogen atom, quartet peak of lower coupling constant will be observed here.

4) The methyl protons will observe at about 0.7 - 1.3 ppm and a triplet peak will be observed here due to adjacent methyl group of lower coupling constant value.