Wanrun Jiang

Molecular orbital analysis of the hydrogen bonded water dimer

As an essential interaction in nature, hydrogen bonding plays a crucial role in many material formations and biological processes, requiring deeper understanding. Here, using density functional theory and post-Hartree-Fock methods, we reveal two hydrogen bonding molecular orbitals crossing the hydrogen-bond’s O and H atoms in the water dimer. Energy decomposition analysis also shows a non-negligible contribution of the induction term. Our finding sheds light on the essential understanding of hydrogen bonding in ice, liquid water, functional materials and biological systems.

First-Principles Calculations of Magnetism in Nanoscale Carbon Materials Confining Metal with f Valence Electrons

The f electrons in the unfilled shell of actinide and lanthanide display complex bonding behavior and the hybridized sp electrons in carbon could show spin polarization in finite nanostructures. Correspondingly, materials combining these two features exhibit abundant magnetic properties. In this paper, we outline our first-principles calculations on various nanoscale carbon materials confining U and Gd which are representative actinide and lanthanide, respectively. The complex interaction between f electrons and sp electrons make the induced magnetic property sensitive to metal specie and carbon confinement. Specially, (1) The magnetism could be suppressed by stronger adsorption with vacancy sites on graphene and adjusted by varying the valence state of some endohedral metallofullerenes (EMFs). (2) The magnetic coupling between metal and carbon structures could be promoted by large curvature when confinement site is carbon nanotubes and altered by the adatom defect on fullerene cages. (3) Untrivial magnetic property with large net spin and asymmetric spin distribution is obtained by confining U atom and Gd atom in one fullerene as a heteronuclear EMF. These results contribute to a systematic understanding of the magnetism in nanoscale carbon materials confining metal with f valence electrons.

Slippage in stacking of graphene nanofragments induced by spin polarization

Spin polarization and stacking are interesting effects in complex molecular systems and are both presented in graphene-based materials. Their possible combination may provide a new perspective in understanding the intermolecular force. The nanoscale graphene structures with zigzag edges could possess spin-polarized ground states. However, the mechanical effect of spin polarization in stacking of graphene nanofragments is not clear. Here we demonstrate the displacement between two stacked rhombic graphene nanofragments induced by spin polarization, using first-principles density-functional methods. We found that, in stacking of two rhombic graphene nanofragments, a spin-polarized stacked conformation with zero total spin is energetically more favorable than the closed-shell stacking. The spin-polarized conformation gives a further horizontal interlayer displacement within 1 angstrom compared with the closed-shell structure. This result highlights that, besides the well-known phenomenologically interpreted van der Waals forces, a specific mechanism dependent on the monomeric spin polarization may lead to obvious mechanical effects in some intermolecular interactions.

Water film inside graphene nanosheets: electron transfer reversal between water and graphene via tight nano-confinement

Based on quantum dynamics simulations using the density functional tight-binding (DFTB) method, we provide a detailed geometric and electronic structure characterization of a nano-confined water film within two parallel graphene sheets. We find that, when the distance between the graphene bilayer is reduced to 4.5 A, the O–H bonds of the water molecules become almost parallel to the bilayer; however, further reducing the distance to 4.0 A induces an abnormal phenomenon characterized by several O–H bonds pointing to the graphene surface. Electronic structure analyses revealed that the charge transfers of these nano-confined water molecules are opposite in these two situations. In the former scenario, the electron loss of each water molecule in the confined aqueous monolayer is approximately 0.008 e, with electrons migrating to graphene from the p orbitals of water oxygen atoms; however, in the latter case, the electron transfer is reversed, with the water monolayer gaining electrons from graphene in excess of 0.017 e per water molecule. This reversed behavior arises as a result of the empty 1s orbitals of H atoms, which are disturbed by the delocalized π orbitals formed by the p electrons of the carbon atoms. Our current study highlights the importance of the nano-confinement on the electronic structures of interfacial water, which can be very sensitive to small changes in physical confinement such as a small reduction in the graphene interlayer distance, and may have implications in de novo design of graphene nano-channels with unique water transport properties for nanofluidic applications.

Chirality dependent spin polarization of carbon nanotubes

The spin polarization of carbon nanotubes (CNTs) offers a tunable building block for spintronic devices and is also crucial for realizing carbon-based electronics. However, the effect of chiral CNTs is still unclear. In this paper, we use the density functional theory (DFT) method to investigate the spin polarization of a series of typical finite-length chiral CNTs (9, m). The results show that the spin density of chiral CNTs (9, m) decreases gradually with the increase in m and vanishes altogether when m is larger than or equal to 6. The armchair edge units on both ends of the (9, m) CNTs exhibit a clear inhibition of spin polarization, allowing control of the spin density of (9, m) CNTs by adjusting the number of armchair edge units on the tube end. Furthermore, analysis of the orbitals shows that the spin of the ground state for (9, m) CNTs mainly comes from the contributions of the frontier molecular orbitals (MOs), and the energy gap decreases gradually with the spin density for chiral CNTs. Our work further develops the study of the spin polarization of CNTs and provides a strategy for controlling the spin polarization of functional molecular devices through chiral vector adjustment.

Depolymerization of Free‐Radical Polymers with Spin Migrations

The mechanism of depolymerization is one of the most essential issues in chemical engineering and materials science. In this work, we investigate the depolymerization reactions of three typical free-radical poly(alpha-methylstyrene) tetramers by using first-principles density functional theory. The calculated results show that these reactions all need to overcome the energy barriers in the range of 0.58 to 0.77 eV, and that breaking the C-C bond at the chain end leads to the dissociation of alpha-methylstyrene monomers from the polymers. Electronic-structure analysis indicates that the reactions occur easily at the CR3 unsaturated end, and that the frontier molecular orbitals that participate in the reactions are mainly localized at the unsaturated ends. Meanwhile, spin population analysis presents the unique net spin-transfer process in free-radical depolymerization reactions. We hope the current findings can contribute to understanding the free-radical depolymerization mechanism and help guide future experiments.

Cooperative vibrational properties of hydrogen bonds in Watson–Crick DNA base pairs

A detailed analysis of hydrogen bonding (H-bonding) nuclear motions is presented for adenine–thymine (AT) and guanine–cytosine (GC) Watson–Crick DNA base pairs. Using first-principles calculations, we investigated the infrared (IR) spectroscopy and nuclear vibrating patterns of multiple H-bond interactions, compared with those of the individual H-bonding base pairs. For the first time, we have shown that multiple H-bonds arouse collective nuclei vibrations, and retain “intensifying” and “bounding” effects on symmetric and asymmetric donor stretching, respectively. This gives the H-bonds an unexpectedly amplified effect. On the other hand, H-bonds that donate charge in different directions reinforce each other through enhanced orbital interactions, indicating a correlated fashion of electronic activities. The coordinated nuclei motion and electron transport constitutes a simple form of H-bond cooperation. This study brings a new perspective of H-bond cooperativity and should enhance our knowledge of the control of H-bonds. Due to their important universality, such properties can benefit experimental applications in spectroscopy, material designing, and biological processes for complex H-bonding systems.

Chirality recognition in concerted proton transfer process for prismatic water clusters

Proton transfer and chiral conversion via hydrogen bonds (HBs) are important processes in applications such as chiral recognition, enzymatic catalysis, and drug preparation. Herein, we investigate the chiral conversion and interlayer recognition, via concerted intralayer proton transfer (CIPT) processes, of small prismatic water clusters, in the form of bilayer n−membered water rings (BnWRs, n = 4, 5, 6). Density functional theory (DFT) calculations show that despite the small energy variations between the initial and final states of the clusters of less than 0.3 kcal·mol−1, the vibrational circular dichroism (VCD) spectrum provides clear chiral recognition peaks in the range of 3,000 to 3,500 cm−1. The vibrational modes in this region correspond to stretching of intralayer HBs, which produces strong signals in the infrared (IR) and Raman spectra. The electronic circular dichroism (ECD) spectrum also reveals obvious chiroptical characteristics. The molecular orbitals involved in the interlayer interaction are dominated by O 2p atomic orbitals; the energy of these orbitals increased by up to 0.1 eV as a result of the CIPT processes, indicating corresponding recognition between monolayer water clusters. In addition, isotopic substitution by deuterium in the BnWRs results in characteristic peaks in the VCD spectra that can be used as fingerprints in the identification of the chiral structures. Our findings provide new insights into the mechanism of chiral recognition in small prismatic water clusters at the atomic level as well as incentives for future experimental studies.

Charging-induced asymmetric spin distribution in an asymmetric (9,0) carbon nanotube

Asymmetry in the electronic structure of low-dimensional carbon nanomaterials is important for designing molecular devices for functions such as directional transport and magnetic switching. In this paper, we use density functional theory to achieve an asymmetric spin distribution in a typical (9,0) carbon nanotube (CNT) by capping the CNT with a fullerene hemisphere at one end and saturating the dangling bonds with hydrogen atoms at the other end. The asymmetric structure facilitates obvious asymmetry in the spin distribution along the tube axis direction, with the maximum difference between the ends reaching 1.6 e Å(-1). More interestingly, the heterogeneity of the spin distribution can be controlled by charging the system. Increasing or decreasing the charge by 2e can reduce the maximum difference in the linear spin density along the tube axis to approximately 0.68 e Å(-1) without changing the proportion of the total electron distribution. Further analyses of the electron density difference and the density of states reveal the loss and gain of charge and the participation of atomic orbitals at both ends. Our study characterizes the asymmetric spin distribution in a typical asymmetric carbon system and its correlation with charge at the atomic level. The results provide a strategy for controlling the spin distribution for functional molecular devices through a simple charge adjustment.

Energetics competition in centrally four-coordinated water clusters and Raman spectroscopic signature for hydrogen bonding

Extending the electronic structure of four-coordinated hydrogen bonds (H-bonds) to medium sized water cages (H2O)n (n = 17, 19, 20, 21, 23 and 25), we separate the H-bonded neighbour molecules of their centrally four-coordinated water (C4CW) molecules from other molecules in outer cages and discover these two regions interact competitively with the central molecule, showing complementary interaction energy curves with respect to size changes. Raman spectral analyses clearly reveal the characteristic vibration response of water molecules to different H-bonding environment, where the C4CW structure is relatively sensitive. Our theoretical research advances a new perspective for the study of H-bonding interaction in liquid water.