What do I do? 5: Microwave Spectroscopy in France

When people find out that I am doing a PhD in Chemistry, they often ask, somewhat foolishly (by those who aren't really that interested), what it is I do.  I then find myself in a position of trying perform a juggling act to find the balance between a number of things.  How in detail do they want me to go?  Where's the right place between talking over their head, and sounding like a condescending jerk.  Are they actually interested, or are they just being polite.  Based on their background, what parts will they find most interesting?  On top of that, I need to plot a coherent course through my thought process so that if they are interested, they don't get lost in the maze inside my head.

So, I've decided to write it down, and see if I can make a logical series of articles about what I do, starting with the background information that's needed, eventually ending up on what I do day to day. Today I'm going to talk about microwave spectroscopy.

You may remember my general post about spectroscopy, where I talked about how the energy of different regions of the electromagnetic spectrum correspond to the spacing between energy levels for different types of motion in the molecule.  Last time I talked about infrared spectroscopy, where we were looking at vibrational energy ladders.  This week, I want to talk about my main project in France, using a microwave spectrometer.  Microwaves you'll remember correspond to the rotation of a molecule.

Why am I interested in the rotational energy states of a molecule? A major benefit of rotational spectroscopy is that it is 100% dependent on the structure of the molecule.  If you can properly analyze what you are seeing in the spectrum, you can recreate the shape of the molecule.  Vice versa, with the structure of the molecule, you can perfectly predict what the spectrum will look like (that's a  slight lie.  In theory you should be able to...).  And because no two molecules are identical (if they are, we call them the same molecule), this spectrum acts like a fingerprint for the molecule, and can be used to identify the molecule with certainty.  This comes in handy when you want to monitor a reaction in real time, and need to be able to tell multiple molecules apart.  It also comes in handy when trying to identify what molecules we're seeing with our telescopes.  Rotational spectra can also give information like temperature, pressure, and concentration.  In the main experiment I do in California, we can use it to watch how the amount present of a particular molecule changes over time.  There are two challenges with using rotational spectroscopy this way.

The first problem is that going from a spectrum of the molecule to the structure of the molecule is extremely difficult if you know nothing about the molecule.  For anything that is not a straight line of atoms, the spectrum produced is complicated.  Here's an example of a small portion of the spectrum of vinyl cyanide, a relatively small molecule (smallecule) with only 7 atoms.

All of those are part of the fingerprint of vinyl cyanide.  To be fair, this spectrum is transitions that involve vibrational and rotational quantum states, but I have the spectrum for just pure rotation, and it is not any more clear than that.  There is so much data in these measurements, that my monitor doesn't have a fraction of the pixels it would need to display all of it at once.  If you zoom in further, there are even more lines hiding inside of other ones.  Imagine what it would look like if you had more than one molecule in the mixture, instead of just vinyl cyanide.

Ok Zach.  So you can't use rotational spectroscopy on unknown molecules.  But if you know what molecule you have, you can predict what the spectrum will look like.  So using it to monitor your reactions should be easy.

hahahahaha.  *tears of laughter and sadness...

If we could solve the quantum mechanical equations (schrodinger in particular) for the molecules, you would be right.  But there is really only 1 molecule we can do that for.  H₂⁺.  Yep.  Not even H₂ with both it's electrons.  H₂ as long as it only has 1 electron.  The predictions we can make for other molecules range from pretty good to not even close.  In order to figure out the exact shape of a molecule, to the point needed for predicting it's spectrum, you need to fit it to experimental data.

The stage is now set for what I'm doing here in the lab in France.  Our goal here is to select molecules that look interesting, or are relevant, or haven't been studied, and actually measure the rotational spectra, then fit the computational guesses of the structure of those molecules to our data, so predictions in other regions get better.  I jokingly call it stamp collecting, because we're collecting molecular data.

We have two experiments set up to allow us to do this study.  We have one cell for studying stable molecules, and one for producing a plasma discharge, and studying the crazy things that makes.  While I've been here, I've helped build both set ups, and I've used them for studying multiple molecules, and have been working on the analysis of the data.  Work done here then can be compared to telescope data to identify features in them, or used in labs like my California lab for studies of kinetics.

Experimental setup at ISMO.  The cell lit up at the front (with a plasma) is for studying molecules produced in a plasma, while the cell in the back is used for studying stable molecules.

Next week I'm going to change gears, and write about fluid dynamics, particularly the math that I did in designing a rocket nozzle.