Peter R. Schreiner

From isolated diamondoids to a van-der-Waals crystal: A theoretical and experimental analysis of a trishomocubane and a diamantane dimer in the gas and solid phase

The electronic properties of sp2/sp3 diamondoids in the crystalline state and in the gas phase are presented. Apparent differences in electronic properties experimentally observed by resonance Raman spectroscopy in the crystalline/gas phase and absorption measurements in the gas phase were investigated by density functional theory computations. Due to a reorganization of the molecular orbitals in the crystalline phase, the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy gaps are lowered significantly by 0.5 eV-1 eV. The π → π* transition is responsible for large absorption in both gas and crystalline phases. It further causes a large increase in the Raman intensity of the C=C stretch vibration when excited resonantly. By resonance Raman spectroscopy we were able to determine the C=C bond length of the trishomocubane dimer to exhibit 1.33 Å in the ground and 1.41 Å in the excited state.

Experimental measurement of the diamond nucleation landscape reveals classical and nonclassical features

Significance Nucleation is the limiting step for thermodynamic phase transitions. While classical models predict that nucleation should be extremely rare, nucleation is surprisingly rapid in the gas-phase synthesis of diamond, silicon, and other industrial materials. We developed an approach for measuring nucleation landscapes using atomically defined precursors and find that diamond critical nuclei contain no bulk atoms, which leads to a nucleation barrier that is four orders of magnitude lower than prior bulk estimations. Our findings suggest that metastable molecular precursors play a key role in lowering nucleation barriers during materials synthesis and provide quantitative support for recent theoretical proposals of multistep nucleation pathways with much lower barriers than the predictions of classical nucleation theory. Nucleation is a core scientific concept that describes the formation of new phases and materials. While classical nucleation theory is applied across wide-ranging fields, nucleation energy landscapes have never been directly measured at the atomic level, and experiments suggest that nucleation rates often greatly exceed the predictions of classical nucleation theory. Multistep nucleation via metastable states could explain unexpectedly rapid nucleation in many contexts, yet experimental energy landscapes supporting such mechanisms are scarce, particularly at nanoscale dimensions. In this work, we measured the nucleation energy landscape of diamond during chemical vapor deposition, using a series of diamondoid molecules as atomically defined protonuclei. We find that 26-carbon atom clusters, which do not contain a single bulk atom, are postcritical nuclei and measure the nucleation barrier to be more than four orders of magnitude smaller than prior bulk estimations. These data support both classical and nonclassical concepts for multistep nucleation and growth during the gas-phase synthesis of diamond and other semiconductors. More broadly, these measurements provide experimental evidence that agrees with recent conceptual proposals of multistep nucleation pathways with metastable molecular precursors in diverse processes, ranging from cloud formation to protein crystallization, and nanoparticle synthesis.

CCDC 1844941: Experimental Crystal Structure Determination

Related Article: Afsaneh Pilevar, Abolfazl Hosseini, Marina Sekutor, Heike Hausmann, Jonathan Becker, Kevin Turke, and Peter R. Schreiner|2018|J.Org.Chem.|||doi:10.1021/acs.joc.8b01392

Palladium-Catalyzed C2−H Arylation of Unprotected (N−H)-Indoles “On Water” Using Primary Diamantyl Phosphine Oxides as a Class of Primary Phosphine Oxide Ligands

We present the Pd‐catalyzed arylation of (N−H)‐indoles with functionalized haloarenes “on water” using hitherto untested primary diamantyl phosphine oxides (PPO) as ligands. Remarkable C2−H arylation selectivity was achieved by employing functionalized iodoarenes and N‐unprotected indoles. We provide evidence that the in situ generated oxide of (9‐hydroxydiamant‐4‐yl)phosphine L1 is key for the reaction efficiency by comparing a set of diamantane‐based compounds structurally related to L1. Our results demonstrate the power of the new PPO ligands for the C−H functionalization of unprotected (N−H)‐heterocycles.