NMR spectra usually are displayed like the one below with the applied field strength increasing from left to right and the intensity of absorption plotted on the vertical axis. Chloropropanone In order that we get consistent positions for absorptions we add an internal standard, usually TMS (tetramethylsilane, (CH3)4Si). The protons in this compound are all identical and are assigned a chemical shift of zero. It is selected for both 1H and 13C spectra because its single peak occurs upfield of almost all other absorptions. The chemical shift is the name given to the position in the spectrum at which an absorption occurs. The horizontal scale is measured in delta or parts per million (ppm), 1 ppm is 1 part per million of the spectrometer frequency. For example, if we used a 200 MHz spectrometer, we would have each ppm would be equal to a separation of 200 Hz. The reason for this is that there are many different spectrometers available, operating at many different frequencies, and so to eliminate any variability from one instrument to another this system is used. So what causes chemical shift? If you look at the spectrum of chloropropanone above you'll see that there are only two peaks. One for each non-equivalent set of protons. The position of one peak relative to another is dictated by the electron density around the protons. The more electron density near a proton, the more shielding of the external magnetic field that can occur, which causes the peak to appear further upfield (to the right) in the spectrum. If a set of protons is neighboring a highly electronegative element or an electron withdrawing group such as a carbonyl group, the electron density around the protons is diminished and the peak is brought into resonance further downfield. This can be seen for chloropropanone which has one group neighboring a chlorine atom, pulling it downfield to around 4 ppm, and another group neighboring a carbonyl group, appearing at around 2 ppm. The electron dentiy plot of this molecule is shown below. Blue/purple/white colors indicate negative charges; red/yellow indicate positive charges. The range in which NMR absorptions occurs is quite narrow. 1H absorptions occur normally in the range 0 - 12 ppm downfield from TMS and 13C absorptions occur from 0 - 250 ppm from TMS. Tables are provided for both 13C and 1H to the right to help you interpret where a particular group is normally found. In learning how to interpret NMR spectra first look at 13C spectra. They are more complex to produce but are easier to interpret. Once you understand those then concentrate on interpreting 1H spectra. They are more complex.
In order that we get consistent positions for absorptions we add an internal standard, usually TMS (tetramethylsilane, (CH3)4Si). The protons in this compound are all identical and are assigned a chemical shift of zero. It is selected for both 1H and 13C spectra because its single peak occurs upfield of almost all other absorptions. The chemical shift is the name given to the position in the spectrum at which an absorption occurs. The horizontal scale is measured in delta or parts per million (ppm), 1 ppm is 1 part per million of the spectrometer frequency. For example, if we used a 200 MHz spectrometer, we would have each ppm would be equal to a separation of 200 Hz. The reason for this is that there are many different spectrometers available, operating at many different frequencies, and so to eliminate any variability from one instrument to another this system is used.
So what causes chemical shift? If you look at the spectrum of chloropropanone above you'll see that there are only two peaks. One for each non-equivalent set of protons. The position of one peak relative to another is dictated by the electron density around the protons. The more electron density near a proton, the more shielding of the external magnetic field that can occur, which causes the peak to appear further upfield (to the right) in the spectrum. If a set of protons is neighboring a highly electronegative element or an electron withdrawing group such as a carbonyl group, the electron density around the protons is diminished and the peak is brought into resonance further downfield. This can be seen for chloropropanone which has one group neighboring a chlorine atom, pulling it downfield to around 4 ppm, and another group neighboring a carbonyl group, appearing at around 2 ppm. The electron dentiy plot of this molecule is shown below. Blue/purple/white colors indicate negative charges; red/yellow indicate positive charges. The range in which NMR absorptions occurs is quite narrow. 1H absorptions occur normally in the range 0 - 12 ppm downfield from TMS and 13C absorptions occur from 0 - 250 ppm from TMS. Tables are provided for both 13C and 1H to the right to help you interpret where a particular group is normally found. In learning how to interpret NMR spectra first look at 13C spectra. They are more complex to produce but are easier to interpret. Once you understand those then concentrate on interpreting 1H spectra. They are more complex.
The range in which NMR absorptions occurs is quite narrow. 1H absorptions occur normally in the range 0 - 12 ppm downfield from TMS and 13C absorptions occur from 0 - 250 ppm from TMS. Tables are provided for both 13C and 1H to the right to help you interpret where a particular group is normally found.
In learning how to interpret NMR spectra first look at 13C spectra. They are more complex to produce but are easier to interpret. Once you understand those then concentrate on interpreting 1H spectra. They are more complex.
