## Friday, June 05, 2015

### What does a molecule sound like?

We all learn in high school chemistry or earlier that atoms can bind together to form molecules, and like a "highly sophisticated interlocking brick system", those atoms like to bind in particular geometrical arrangements.  Later we learn that those bonds are dynamic things, with the atoms vibrating and wiggling like masses connected by springs, though here the (nonlinear) spring constants are set by the detailed quantum mechanical arrangement of electrons.  Like any connected set of masses and springs, or like a guitar string or tuning form, molecules have "normal modes" of vibration.  Because the vibrations involve the movement of charge, either altering how positive and negative charge are spatially separated (dipole active modes) or how the charge would be able to respond to an electric field (roughly speaking, Raman active modes), these vibrations can be excited by light.  This is the basis for the whole field of vibrational spectroscopy.  Each molecule has a particular, distinct set of vibrations, like a musical chord.

Because the atoms involved are quite light (one carbon atom has a mass of 2$\times$10-26 kg) and the effective springs are rather stiff, the vibrations are typically at frequencies of around 1013 Hz and higher - that's 10 billion times higher than the frequency of a typical acoustic frequency (1 kHz).  Still, suppose we shifted the frequencies down to the acoustic range, using a conversion of 1 cm-1 (a convenient unit of frequency for molecular spectroscopists) $\rightarrow$ 1 Hz.  What would molecules sound like?  As an example, I looked at the (surface enhanced) Raman spectrum of a small molecule, pMA. The Raman spectrum is from this  paper (Fig. 3a), and I took the three most dominant vibrational modes, added the pitches with the appropriate amplitude, and this is the result (mp3 - embedding audio in blogger is annoying).

I thought I was being clever in doing this, only to realize that, as usual, someone else had this same idea, beat me to it, and implemented it in a very cool way.  You should really check that out.