Dissertation Defense: Zhongyu Mou
Candidate Name: Zhongyu Mou
Thesis Advisor: Miklos Kertesz, Ph.D.
Title: THE PANCAKE BONDING IN ORGANIC RADICALS
Interaction between two closed shell molecules are usually van der Waal (vdW) interaction. While the vdW interaction is important and well-known and often very weak comparing to chemical bond. A new intermolecular interaction through multi-electron/multi-center (me/mc) interaction is found between some open-shell radical and multiradicals featuring highly orientated configurations and low-lying triplet/multiplet. This intermolecular interaction is covalent-like, often termed as “pancake bond”. The pancake bond is found at many newly synthesized radical-based materials exhibiting exciting magnetic and electronic properties. Unlike close-shell molecules with vdW, open-shell molecules with pancake bonding poses many challenges to theoretical chemists including the multireference singlet ground states, special dispersion interactions and the shorter than vdW intermolecular distances. In this dissertation, we push the limit of the understanding of pancake bonding by investigating on a series of pancake-bonded systems including with the help of density functional theory (DFT).
Phenalenyl and its derivatives are found to be stable radicals. Using the UM05-2X DFT, we calculated a series of homogeneous phenalenyls including two new triphenal and trimethyl derivatives. We found they can aggregate through π-dimers or σ-dimers based on their substitution group and the calculated interaction energy. We also use calculations to explain an experimentally observable interchange between π-dimers and σ-dimers of trimethyl phenalenyl. Then we calculated hypothetical hetero-phanelnyls and found that electrostatics attraction between the phenalenyls with very different electronegativity can stabilize the hetero-π-dimer two or three times stronger than the corresponding homo- π-dimers.
We extended the singly pancake bonded phanelnyl dimers to the triangulene family. These multiradicals has more than one singly occupied molecular orbitals and can form pancake-bonded π-dimers up to a formal bond order of five. Unlike other covalent bonds, we found a linear relationship between the pancake bond strength and the pancake bond orders.
We finally investigated on a validation of DFTs used on computing pancake bonds. We examined over fifty DFTs including some with dispersion corrections against high-level calculation MR-AQCC results on four different systems. We found no universal DFT applicable for all pancake bonded systems. However, we found some DFTs that worked well for each individual system.
Thursday, December 7, 2017 at 1:00pm to 3:00pm
Reiss Science Building, 238
37th and O St., N.W., Washington