Energy storage in the internal states of molecules - old and new

A science story first, then a US research ecosystem story later.

When we think about using molecules to store energy, it's usually in the context of food or fuel, so that chemical reactions take place - bonds are broken and remade, and in an exothermic reaction, the products end up with more kinetic energy (center of mass motion, molecular vibrations and rotations) than the initial reactants.  However, there are other ways that molecules can store energy.  I read about a cool example of this last week, but first I want to give tell you an old and very quantum mechanical story that I learned about in grad school when I did very low temperature physics.

Diatomic hydrogen, H2, is the simplest molecule there is, just two electrons and two protons.  Roughly speaking, the \(1s\) orbitals of the H atoms hybridize to form \(\sigma\) bonding and \(\sigma*\) antibonding molecular orbitals.  The lowest electronic state is the two electrons in a spin singlet, \((1/\sqrt{2})(|\uparrow \downarrow\rangle - |\downarrow \uparrow\rangle)\) in the \(\sigma\) molecular orbital.  Remember, the electrons are fermions, so the electronic wavefunction has to be antisymmetric (pick up a minus sign) under exchange of the electrons. The spin singlet is antisymmetric under exchange, the \(\sigma\) orbital is spatially symmetric under exchange, so the full electronic wavefunction (product of the spin and spatial components) is appropriately antisymmetric.  

That's not all there is to it, though, as explained thoroughly here.  The protons (while being made up of quarks and gluons, etc.) are (composite) fermions, so we have to think about the quantum wavefunction that describes them, too.  There are two possibilities.  In the "para" configuration, the proton spins are in a singlet (antisymmetric), meaning that the spatial wavefunction of the protons must be symmetric under exchange.  The spatial state of the bound protons can have some orbital angular momentum \(\mathbf{L}\), and the simplest, lowest energy situation is with quantum numbers \(\ell =0\) and therefore \(m_{\ell} = 0\).  In contrast, in the "ortho" configuration, the proton spins form a triplet state (symmetric under exchange), meaning that the spatial wavefunction must be antisymmetric, \(\ell = 1\).  Approximating the H2 molecule as a rigid barbell-like rotor with some moment of inertia \(I\), then ortho molecule has a rotational energy \(\hbar^2/2I\) larger than the para case.  That works out to about 15 meV of energy per molecule.  So, para-hydrogen is the true ground state.  It turns out that the ortho/para spin isomer energy difference makes liquefying hydrogen a challenge, since the latent heat of vaporization for H2 is only 9.4 meV.  That is, every time an ortho-hydrogen molecule converts to para-hydrogen through some collisional process, it releases enough energy to kick a hydrogen molecule out of the liquid.  I learned about this in my thesis work playing around at ~ 1 mK temperatures - any H2 adsorbed or otherwise stuck in the apparatus could result in detectable long-term heating effect as it slowly converted from ortho to para.  Bottom line:  Energy can be stored in the internal states of molecules.

From Fig. 1 of this paper.

This seems very esoteric, but the idea of storing energy in some internal state of a molecule for later release shows up elsewhere.  Last week, in this article in Science, for example, the authors report a molecule inspired by aspects of DNA that can be put via UV exposure into a distorted form ("Dewar isomer") where it is metastable at room temperature (half-life of 481 days).  It can be induced to pop back into the undistorted isomer by heat, acid exposure, or via a catalyst, and when it does, each molecule releases the stored energy (2.36 eV per molecule!) into vibrations and rotations that heat its surroundings.  The stored energy density in this stuff is about 4% of the releasable energy density of gasoline, which is not too shabby.  The authors propose a system where exposure to sunlight can store energy in the molecules, and this can later be released on demand via catalyst.  They demonstrated that the heat release from enough dissolved molecules can readily boil water.  Very neat stuff.