Jinlong Yang

Structure of Graphene Oxide: Thermodynamics versus Kinetics

Graphene oxide (GO) is an important intermediate to prepare graphene, and it is also a versatile material with various applications. However, despite its importance, the detailed structure of GO is still unclear. For example, previous theoretical studies based on energetics have suggested that hydroxyl chain is an important structural motif of GO, which, however, is found to be contrary to nuclear magnetic resonance (NMR) experiments. In this study, by calculating vibrational frequencies, we find that hydroxyl chain structure is also inconsistent with infrared experiment. To resolve this controversy, we check both thermodynamic and kinetic aspects of GO structure. First-principles thermodynamics gives a free-energy based stability ordering similar to that solely based on inner energy, and the hydroxyl chain is indeed thermodynamically very favorable. Therefore, kinetics during GO synthesis is expected to have an important role in GO structure. Transition state calculations predict large energy barriers be...

On the Electronic Structures and Electron Affinities of the m-Benzoquinone (BQ) Diradical and the o-, p-BQ Molecules: A Synergetic Photoelectron Spectroscopic and Theoretical Study

Electron affinity (EA) is an important molecular property relevant to the electronic structure, chemical reactivity, and stability of a molecule. A detailed understanding of the electronic structures and EAs of benzoquinone (BQ) molecules can help rationalize their critical roles in a wide range of applications, from biological photosynthesis to energy conversion processes. In this Article, we report a systematic spectroscopic probe on the electronic structures and EAs of all three isomers-o-, m-, and p-BQ-employing photodetachment photoelectron spectroscopy (PES) and ab initio electronic structure calculations. The PES spectra of the three BQ(●-) radical anions were taken at several photon energies under low-temperature conditions. Similar spectral patterns were observed for both o- and p-BQ(●-), each revealing a broad ground-state feature and a large band gap followed by well-resolved excited states peaks. The EAs of o- and p-BQ were determined to be 1.90 and 1.85 eV with singlet-triplet band gaps of 1.68 and 2.32 eV, respectively. In contrast, the spectrum of m-BQ(●-) is distinctly different from its two congeners with no clear band gap and a much higher EA (2.89 eV). Accompanied theoretical study confirms the experimental EAs and band gaps. The calculations further unravel a triplet ground state for m-BQ in contrast to the singlet ground states for both o- and p-BQ. The diradical nature of m-BQ, which is consistent with its non-Kekulé structure, is primarily responsible for the observed high EA and helps explain its nonexistence in bulk materials.

Electron Affinities and Electronic Structures of o-, m-, and p- Hydroxyphenoxyl Radicals: A Combined Low-Temperature Photoelectron Spectroscopic and Ab initio Calculation Study

Hydroxyl substituted phenoxides, o-, m-, p-HO(C(6)H(4))O(-), and the corresponding neutral radicals are important species; in particular, the p-isomer pair, i.e., p-HO(C(6)H(4))O(-) and p-HO(C(6)H(4))O*, is directly involved in the proton-coupled electron transfer in biological photosynthetic centers. Here we report the first spectroscopic study of these species in the gas phase by means of low-temperature photoelectron spectroscopy (PES) and ab initio calculations. Vibrationally resolved PES spectra were obtained at 70 K and at several photon energies for each anion, directly yielding electron affinity (EA) and electronic structure information for the corresponding hydroxyphenoxyl radical. The EAs are found to vary with OH positions, from 1.990 +/- 0.010 (p) to 2.315 +/- 0.010 (o) and 2.330 +/- 0.010 (m) eV. Theoretical calculations were carried out to identify the optimized molecular structures for both anions and neutral radicals. The electron binding energies and excited state energies were also calculated to compare with experimental data. Excellent agreement is found between calculations and experiments. Molecular orbital analyses indicate a strong OH antibonding interaction with the phenoxide moiety for the o- as well as the p-isomer, whereas such an interaction is largely missing for the m-anion. The variance of EAs among three isomers is interpreted primarily due to the interplay between two competing factors: the OH antibonding interaction and the H-bonding stabilization (existed only in the o-anion).

Nearly Free Electron State in Graphane Nanoribbon Superlattice

Nearly free electron (NFE) state has been widely studied in low dimensional systems. Based on first-principles calculations, we identify two types of NFE states in graphane nanoribbon superlattice, similar to those of graphene nanoribbons and boron nitride nanoribbons. Effect of electron doping on the NFE states in graphane nanoribbon superlattice has been studied, and it is possible to open a vacuum transport channel via electron doping.

STM studies of single molecules: molecular orbital aspects

As a fundamental and frequently referred concept in modern chemistry, the molecular orbital plays a vital role in the science of single molecules, which has become an active field in recent years. For the study of single molecules, scanning tunneling microscopy (STM) has been proven to be a powerful scientific technique. Utilizing specific distribution of the molecular orbitals at spatial, energy, and spin scales, STM can explore many properties of single molecule systems, such as geometrical configuration, electronic structure, magnetic polarization, and so on. Various interactions between the substrate and adsorbed molecules are also understood in terms of the molecular orbitals. Molecular engineering methods, such as mode-selective chemistry based on the molecular orbitals, and resonance tunneling between the molecular orbitals of the molecular sample and STM tip, have stimulated new advances of single molecule science.