Abstract on NMR Spectroscopy

The nucleus of an atom contains protons and neutrons. Some nuclei have the property of spin. To
aid in the structure determination of organic molecules we are primarily concerned with two nuclei, which
both possess spin, they are:

1. The hydrogen nucleus which is simply a proton and is symbolized by 1H.
2. An isotope of carbon, symbolized 13C (referred to as Carbon 13).

When the nucleus of an atom (which is positively charged) spins it generates a magnetic field and therefore
possesses a magnetic moment. This essentially means that the nucleus behaves as if it were a tiny bar
magnet. When placed in a magnetic field the nucleus can assume two different orientations with respect to
the field, aligned with or against. These two orientations correspond to two different energy levels (also
called spin states), the orientation aligned with the field being the lower energy level. The difference in
these energy levels depend on two things:
1. The nucleus
2. The magnetic field the nucleus experiences.
When one shines light of just the right energy on a sample containing a spin active nucleus, absorption of
the light occurs. The energy of the light must match exactly the difference in energy of the two spin states
and corresponds to one frequency of light. Measuring the frequency of light at which this absorption
occurs is what the nmr experiment is all about.

The frequency of light which provides
just the right energy to match this
difference is called the resonance
frequency. When this frequency is
matched, absorbance occurs and the spin
flips from one state to the other.

E=hv

No Magnetic Field

External Magnetic Field Applied

The frequency of light absorbed is plotted vs. intensity. The actual plot is not of frequency vs intensity (as
it is in IR spectroscopy). Instead, the absorbances (or peaks) are plotted in units of ppm (called the delta
scale) from TMS (TetraMethylSilane), a reference compound which is given the value of 0.

Two important points about the absorption:
1. 1H and 13C nuclei absorb at much different frequencies when placed in the same magnetic field.
Therefore one observes only one of these nuclei at a time(1H or 13C nmr spectrum)
2. The magnetic field that the nucleus experiences is dependent on the external magnetic field
(which does not change) and its immediate magnetic environment. The immediate magnetic environment
depends on the electron density around the nucleus being observed. The more electron density around the
nucleus the more shielded the nucleus is from the external magnetic field and the more upfield (closer to
TMS) the signal will appear. The less electron density around a nucleus the more deshielded the nucleus is
from the external magnetic field and the more downfield (farther away from TMS) the signal will appear.
The Bottom Line:Each chemically unique 1H and 13C absorb light of a different frequency and the
frequency of absorption depends on the chemical environment.