Novel Chemistry driven by physical forces

The common chemistry that we learned at ambient condition has implicit boundaries rooted in the atomic shell structure: the inner shell electrons and the outer shell orbits do not play major role in bonding and chemical reactions. Recently, through a series of DFT studies, we showed that these conceptual boundaries are not absolute, especially under extreme conditions like high pressure.

Cs in high oxidation state and as a p-block element



Our chemistry knowledge of alkali and alkaline metals is that they invariably assume +1 and +2 charge state in compounds and chemical reactions, which is the result of their atomic shell structure. Using first principles calculations, we show that the 5p electrons can become reactive under moderate pressure (>20 GPa). As a result, Cs and F form stable CsFn compounds under pressure. These compounds consist of CsFn molecules or molecular ions, in which Cs and F form covalent bonds. The structure and property of CsFn molecules resemble the isoelectronic XeFn molecules, showing that Cs behaves as p-block elements. 

Pressure-stabilized lithium caesides with caesium anions beyond -1 state



Opposite to the previois work that pressure may make the inner shell 5p electrons of Cs reactive, here we show that pressure can cause large electron transfer from light alkali metals such as Li to Cs, causing Cs to become anionic with a formal charge much beyond −1. Although ​Li and ​Cs only form alloys at ambient conditions, we demonstrate that these metals form stable intermetallic LinCs (n=1–5) compounds under pressures higher than 100 GPa. Once formed, these compounds exhibit interesting structural features, including capped cuboids and dimerized icosahedra. Our work reveals that the outer shell orbitals of elements can also become the essecial part of the chemical bonding under high pressure. 

Enclosed space: a chemical entity that explains the formation of high pressure electrides

 

Electrides, in which electrons occupy interstitial regions in the crystal and behave as anions, appear as new phases for many elements (and compounds) under high pressure. We propose a unified theory of high pressure electrides (HPEs) by treating electrons in the interstitial sites as filling the quantized orbitals of the interstitial space enclosed by the surrounding atom cores, generating what we call an interstitial quasi-atom, ISQ. Our model not only explains the formation of HPE but also predicts the general trend of HPE for various elements. Our theory reveals that the enclosed empty space can play improtant roles as real chemiscal species since it features quantized orbitals that can accomodate electrons and form chemical bonds with the neighboring species. 

  • M. S. Miao, J. A. Kurzman, N. Mammen, S. Narasimhan and R. Seshadri, Inorg. Chem. 51, 7569 (2012).

 

Electronic structure of Au compounds and its implication to catalytic activities



There is strong evidence from both density functional calculations and experiments that the catalytic activities of transition metal atoms in various chemical environments are associated to the position of their d-bands. However, there is lack of way to manipulate the d-band position through the engineering of structures such as depositing on different surfaces, alloying, and doping. In this letter, we demonstrate through first principles calculations on a large amount of Au compounds as well as Ag and Pt compounds that the d band center correlates strongly with the d-band filling, namely the larger the d-band charge, the higher the d-band center. And we also show that this correlation is due to the strong on-site Coulomb repulsions of the d band electrons. Thus one can attempt to increase the catalytic activities of Au and other transition metal-based catalysts by increasing their d band filling that can be more directly related to the structural features.