When we put a sample into our machine a portion of the sample is in the gas phase, and with a buffer gas we send this portion into the cavity of the spectrometer.  Inside the cavity is a standing mode of microwave radiation that we can change by adjusting the separation of the mirrors and the frequency of light input into the chamber.

Figure 1. The Microwave Cavity

Click here to view an image of our spectrometer cavity

    We then detect an 'echo' signal on a computer that informs the scanner whether or not the molecule is absorbing that frequency.  If there is no absorbance we view noise but if there is absorbance we view what is called a free induction decay signal.

Figure 2. A Free Induction Decay.
 
 

    Depending on how many spectral lines are in the region being scanned these FID's can be simple, a singly oscillating (sinusoidal or cosinusoidal) function with a superimposed exponential decay.  However when more than one line is present in the region being scanned we often see two or more different frequencies in the FID pattern (such as the one above).  These frequencies correspond to differences between the natural absorbance frequency of the molecule and the frequency of the light in the cavity.  We can accurately determine the natural frequency of the molecule by doing a Fourier Transform of the FID and adding (or subtracting) the difference frequency from the frequency of the light in the cavity.  The Fourier Transform of this FID is shown below.

Figure 3.  The Fourier Transform of the FID above.

    After we take data on a few lines we move around the frequency spectrum to find more of them in an effort to decipher the pattern and simoultaneously assign the line their respective quanta.  The two lines shown here are part of a quadruplet caused by the I = 3/2 quadrupole nucleus of the Chlorine atom which is in the molecule that was being studied.

Figure 4. A quadrupole quadruplet pattern in the microwave spectrum of chloroferrocene.

    In this case the quadrupole structure is a fine (small scale) part of the spectrum and on a larger scale the spectrum reveals that the molecule is an asymmetric top.  Because asymmetric tops have three unique axes of rotation there are many more rotational transitions available in complarison to a symmetric top molecule.  This is revealed by the presence of K-states, in which K is a unit of angular momentum projected onto the molecular axes (in a symmetric top all K states are equivalent).

Figure 5.  A series of K-state transitions in the J=5--4 region of the microwave spectrum of chloroferrocene.

    The large scale structure of the microwave spectrum reveals that most of the transitions are grouped into regions, each of these regions is a set of different J level transitions.  J is one quanta of angular momentum of the molecule about it's major rotation axes.  Thus a molecule in its' J = 2 state is spinning twice as fast as a molecule in the J = 1 state.

Figure 6. The entire microwave spectrum of chloroferrocene in the 4-11 GHz frequency range.

 

Coming soon, interpretation of spectra and solving molecular structures!