Our research looks at bonding in all types of chemical systems–discrete molecules or extended solids, organic or inorganic. While our methods rest on detailed, reliable calculations, our interests lie more in building qualitative understanding than in producing high-precision computational data. Through our analysis of specific bonding puzzles, we hope to provide a conceptual framework that will aid experimentalists in their attempts to synthesize new compounds with unusual structures and interesting properties.
The Low-Lying Electronic States of Pentacene and Their Roles in Singlet Fission
Tao Zeng, Roald Hoffmann, and Nandini Ananth, J. Amer. Chem. Soc. 136, 5755-5764 (2014).
A detailed study of pentacene monomer and dimer serves to reconcile extant views of its singlet fission. In order to understand the mechanism of singlet fission in pentacene, we use a diabatization scheme to characterize the six low-lying singlet states of a pentacene. The local, single-excitonic diabats are not directly coupled with the important multiexcitonic state but rather mix through their mutual couplings with one of the charge-transfer configurations.
Silicon Monoxide, at 1 atm and Elevated Pressures: Crystalline or Amorphous?
Khalid AlKaabi, Dasari L. V. K. Prasad, Peter Kroll, N. W. Ashcroft, Roald Hoffmann. J. Amer. Chem. Soc., 136, 3410-3423 (2014).
The absence of a crystalline SiO phase under ordinary conditions is an anomaly in the sequence of group 14 monoxides. We explore theoretically ordered ground state and amorphous structures for SiO at P= 1atm, and crystalline phases also at pressures up to 200GPa. Several competitive ground state P= 1atm structures are found (two are shown here), perforce with Si-Si bonds, and possessing Si-O-Si bridges similar to those in silica (SiO2) polymorphs.
High Pressure Electrides: A Predictive Chemical and Physical Theory
Mao-Sheng Miao and Roald Hoffmann, Acc. Chem. Res. 47, 1311-1317 (2014).
Electrides, in which electrons occupy interstitial regions in the crystal, appear as new phases for many elements (and compounds) under high pressure. We propose a 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, an ISQ. With increasing pressure, the energies of the valence orbitals of atoms increase more significantly than the ISQ levels, due to repulsion, exclusion by the atom cores, effectively giving the valence electrons less room in which to move. At a high enough pressure, which depends on the element and its orbitals, the frontier atomic electron may become higher in energy than the ISQ, resulting in electron transfer to the interstitial space and the formation of an HPE.