|Posted on March 25, 2015 at 5:05 PM|
A few weeks ago we published our work on molecular electrides in Chemical Communications. The work has received some additional attention by Chemistry World, the blog from the Royal Society of Chemistry. In this post I will try to summarize the contents of our recently published work.
Ionic compounds are chemical compounds consisting of positive and negative ions held together by electrostatic forces. For instance, in sodium chloride ⎯commonly known as table salt⎯ the anionic part (negative ion) is Cl- and the cationic part (positive ion) is Na+. Electrides are unique ionic compounds where the anionic part is constituted only by isolated electrons. This feature grants electrides many different properties. Thus far, only solid-state electrides have been reported and despite there have been some suggestions of molecular electrides in the literature, their electronic structure has not been confirmed as a true electride one.
James L. Dye is the father of electrides: he postulated its existence in 1960s, synthesized and characterized the first electride in 1980s, and it was the first to produce a room-temperature-stable organic electride in 2005. The first room-temperature-stable electride was inorganic and due to Prof. Hosono, see below).
Barely a handful of electrides have been synthesized and only three of them are stable at room temperature. Electrides show particular magnetic (exalted susceptibilities), chemical (organic synthesis, preparation of nanoscale metal and alloy particles), electric (an ideal electride should be a (Mott) insulator) and optical properties (low optical spectra peaks as compared to alkali anions; large nonlinear optical properties). Large second hyperpolarizabilities make electrides of high interest due to their potential utilization in optical and opto-electronic devices. Indeed, electrides have found a plethora of diverse applications, including the catalysis of the ammonia synthesis, its usage as reversible H2 storage devise, electron emitters and chemical reagents —to mention a few. All these applications are due to the group of Prof. Hosono, who synthesized the last two electrides: [Ca24Al28O68]·4e− and [Ca2N]+·e−.
Unfortunately, electrides are difficult to synthesize and identify because their experimental characterization is only possible by indirect means. The density of a free electron (or a handful of them) is not large enough to be located in the X-ray of a crystal structure. As a consequence, the presence of isolated electrons in electrides always comes from indirect evidences such as the similarity of this structure with analog alkalides, the chemical shift of the corresponding cation (133Cs), EPR studies, magnetic susceptibilities, electrical resistivity or optical reflectance experiments. These evidences merely suggest the presence of an electride; they do not guarantee its existence.
In our paper we provide an unambiguous computational means to distinguish electrides from similar species, proving the existence of some electrides in gas phase. In contrast with solid state, we use the term molecular electrides for the gas-phase species. The molecular electrides studied in this work were previously characterized by frontier molecular orbital analysis and large nonlinear optical properties. Namely, these studies found an occupied orbital with large density values in the vicinity of the position where one would expect the isolated electron of the electride. Some works also included large second hyperpolarizabilities to support the discovery of a molecular electride. However, neither of these criteria are enough to assess the existence of an electride, as we have also proved in this work.
The electron localization function (ELF) and the non-nuclear attractors of the electron density were used to characterize solid-state electrides. Our study proves that these properties are actually necessary conditions for the existence of electrides. However, these features also show in molecules that do not have an electride structure such as acetylene. In our work we show that large nonlinear optical properties can be used in conjunction with the latter techniques to unambiguously characterize electrides.
The electronic structure of electrides shows an important signature: a maximum of the electron density in a non-nuclear position. This rare feature opens a new route towards the design of new electrides. We currently study the possibility to enforce non-nuclear maxima of the electron density (NNA). Since NNA are not a frequent feature of molecular densities, its mere existence increases the probability of having an electride. We believe that learning how to construct molecules with NNAs could pave the way towards the design of new electrides.