Chia-Chung Sun

Do single-electron lithium bonds exist? Prediction and characterization of the H3C⋯Li–Y (Y=H, F, OH, CN, NC, and CCH) complexes

A new kind of single-electron lithium bonding complexes H(3)C...LiY (Y=H, F, OH, CN, NC, and CCH) was predicted and characterized in the present paper. Their geometries (C(3v)) with all real harmonic vibrational frequencies were obtained at the MP2/aug-cc-pVTZ level. For each H(3)C...LiY complex, single-electron Li bond is formed between the unpaired electron of CH(3) radical and positively charged Li atom of LiY molecule. Due to the formation of the single-electron Li bond, the C-H bonds of the CH(3) radical bend opposite to the LiY molecule and the Li-Y bond elongates. Abnormally, the three H(3)C...LiY (Y=CN, NC, and CCH) complexes exhibit blueshifted Li-Y stretching frequencies along with the elongated Li-Y bonds. Natural bond orbital analyses suggest ca. 0.02 electron transfer from the methyl radical (CH(3)) to the LiY moiety. In the single occupied molecular orbitals of the H(3)C...LiY complexes, it is also seen that the electron could of the CH(3) radical approaches the Li atom. The single-electron Li bond energies are 5.20-6.94 kcal/mol for the H(3)C...LiY complexes at the CCSD(T)aug-cc-pVDZ+BF (bond functions) level with counterpoise procedure. By comparisons with some related systems, it is concluded that the single-electron Li bonds are stronger than single-electron H bonds, and weaker than conventional Li bonds and pi-Li bonds.

The static polarizability and first hyperpolarizability of the water trimer anion: ab initio study.

This work predicts the extraordinary hyperpolarizability of inorganic clusters: two water trimer anions. The first hyperpolarizabilities of (H2O-)(3) are considerable, beta(0)=1.715 x 10(7) a.u. for configuration A and beta(0)=1.129 x 10(7) a.u. for configuration B at MP2/d-aug-cc-pVDZ+x level. The first hyperpolarizabilities of (H2O-)(3) (configuration A) and related systems [(H2O)(3) and (H2O)(3)F-] are compared at the MP2/d-aug-cc-pVDZ+x level. These results are beta(0)=1.715 x 10(7) a.u. for (H2O-)(3), beta(0)=35 a.u. for (H2O)(3) [the neutral core of (H2O-)(3)], and beta(0)=46 a.u. for (H2O)(3)F-). Comparing the beta(0) values of related systems, we find that the dipole-bound excess electron is the key factor in the extraordinary first hyperpolarizability of (H2O-)(3) species. It will provide a future in the development of some materials with the excess electron (e.g., electrides) that exhibit large nonlinear optical response.

Single-electron hydrogen bonds in the methyl radical complexes H3C⋯HF and H3C⋯HCCH: an ab initio study

Abstract The methyl radical (CH 3 ) complexes with hydrogen fluoride (HF) and ethyne (HCCH) are reported to show the existence of a single-electron hydrogen bond. Their geometrical structures are optimized at the MP2/aug-cc-pVDZ and MP2/aug-cc-pVTZ levels and C 3v stationary structures are obtained for the two complexes. The single-electron hydrogen bond energies of H 3 C⋯HF and H 3 C⋯HCCH are calculated at six levels of theory [SCF, MP2, MP3, MP4, CCSD, and CCSD(T)] and their harmonic vibrational frequencies are calculated at the MP2/aug-cc-pVTZ level.

Alkali metal atom‐aromatic ring: A novel interaction mode realizes large first hyperpolarizabilities of M@AR (M = Li, Na, and K, AR = pyrrole, indole, thiophene, and benzene)

Several new electride compounds M@pyrrole (M = Li, Na, and K), Li@AR (AR = indole, thiophene, and benzene), Li@tryptophan and Li@serotonin were designed and investigated, which exhibit considerably large first hyperpolarizabilities (β0) (6705, 1116, 11399, 5781, 4808, 1536, 8106, and 9389 au, respectively) by comparison with their corresponding sole molecules pyrrole (β0 = 30 au), indole (104 au), thiophene (6 au), benzene (0 au), tryptophan (159 au) and serotonin (151 au), respectively. The computational results revealed that the interaction of the alkali metal atom with π‐conjugated aromatic ring (AR) is one effectively new approach to produce diffuse excess electron to get a large β0 value, which is advantageous for the design of the novel high‐performance NLO materials with π‐conjugated AR: alkali metal atoms doped nanomaterials and biomolecules.

Exceptionally Large Second-Order Nonlinear Optical Response in Donor–Graphene Nanoribbon–Acceptor Systems

Graphene nanoribbon (GNR) has been used, for the first time, as an excellent conjugated bridge in a donor-conjugated bridge-acceptor (D-B-A) framework to design high-performance second-order nonlinear optical materials. Owing to the unique diradical planar conjugated bridge of GNR, D(NH(2))-GNR-A(NO(2)) exhibits exceptionally large static first hyperpolarizability (β(0)) up to 2.5×10(6) a.u. (22000×10(-30) esu) for H(2)N-(7,3)ZGNR-NO(2) (ZGNR=zigzag-edged GNR), which is about 15 times larger than the recorded value of β(0) (1470×10(-30) esu) for the D-A polyene reported by Blanchard-Desce et al. [Chem. Eur. J. 1997, 3, 1091]. Interestingly, we have found that the size effect of GNR plays a key role in increasing β(0) for the H(2)N-GNR-NO(2) system, in which the width effect of GNR perpendicular to the D-A direction is superior to the length effect along the D-A direction.

New acceptor-bridge-donor strategy for enhancing NLO response with long-range excess electron transfer from the NH2...M/M3O donor (M = Li, Na, K) to inside the electron hole cage C20F19 acceptor through the unusual σ chain bridge (CH2)4.

Using the strong electron hole cage C20F19 acceptor, the NH2...M/M3O (M = Li, Na, and K) complicated donors with excess electron, and the unusual σ chain (CH2)4 bridge, we construct a new kind of electride molecular salt e(-)@C20F19-(CH2)4-NH2...M(+)/M3O(+) (M = Li, Na, and K) with excess electron anion inside the hole cage (to be encapsulated excess electron-hole pair) serving as a new A-B-D strategy for enhancing nonlinear optical (NLO) response. An interesting push-pull mechanism of excess electron generation and its long-range transfer is exhibited. The excess electron is pushed out from the (super)alkali atom M/M3O by the lone pair of NH2 in the donor and further pulled inside the hole cage C20F19 acceptor through the efficient long σ chain (CH2)4 bridge. Owing to the long-range electron transfer, the new designed electride molecular salts with the excess electron-hole pair exhibit large NLO response. For the e(-)@C20F19-(CH2)4-NH2...Na(+), its large first hyperpolarizability (β0) reaches up to 9.5 × 10(6) au, which is about 2.4 × 10(4) times the 400 au for the relative e(-)@C20F20...Na(+) without the extended chain (CH2)4-NH2. It is shown that the new strategy is considerably efficient in enhancing the NLO response for the salts. In addition, the effects of different bridges and alkali atomic number on β0 are also exhibited. Further, three modulating factors are found for enhancing NLO response. They are the σ chain bridge, bridge-end group with lone pair, and (super)alkali atom. The new knowledge may be significant for designing new NLO materials and electronic devices with electrons inside the cages. They may also be the basis of establishing potential organic chemistry with electron-hole pair.

The interaction between superalkalis (M3O, M = Na, K) and a C20F20 cage forming superalkali electride salt molecules with excess electrons inside the C20F20 cage: dramatic superalkali effect on the nonlinear optical property

It is well known that electrides are a type of multielectron many-cage solid salt with excess electron anions inside the cages. The main concern regarding these structures is how to construct the organic single-caged electride molecules with an electron inside its cage. Using the perfluorinated fullerene cage C20F20 as the electron hole, the alkali metal atoms (M = Na, K) and superalkali atoms (M3O, M = Na, K) with a smaller vertical detachment energy (VDE) value as the source of the electrons, we can construct new nonlinear optical (NLO) organic single-caged electride salt molecules M+(e@C20F20)− and (M3O)+(e@C20F20)− due to the long-range charge transfer from the (super)alkali to inside the cage, forming an electron-hole pair within the molecule. To measure the nonlinear optical response, static first hyperpolarizabilities (β0) and the superalkali effect on β0 are exhibited for these new molecules. The β0 values are 400 and 600 au for M+(e@C20F20)− which are considerably smaller than 13 000 and 10 000 au for (M3O)+(e@C20F20)−. It is revealed that the superalkali effect on the β0 value is dramatic and the β0 value increases by about 20–30 times. New single-caged superalkali electride salt molecules (M3O)+(e@C20F20)− possess not only a large nonlinear optical property but also higher stability. They hold potential as high-performance nonlinear optical materials.

Theoretical study on photophysical properties of ambipolar spirobifluorene derivatives as efficient blue-light-emitting materials.

The aim of this work is to provide an in-depth interpretation of the optical and electronic properties of a series of spirobifluorene derivatives. These materials show great potential for application in organic light-emitting diodes as efficient blue-light-emitting materials due to the tuning of the optical and electronic properties by the use of different electron donors (D) and electron acceptors (A). The geometric and electronic structures of the molecules in the ground state are studied with density functional theory (DFT) and ab initio HF, whereas the lowest singlet excited states are optimized by ab initio CIS. The energies of the lowest singlet excited states are calculated by employing time-dependent density functional theory (TD-DFT). The results show that the HOMOs, LUMOs, energy gaps, ionization potentials, electron affinities, reorganization energies, and exciton binding energies for these complexes are affected by different D and A moieties. Also, it has obtained that these blue-light-emitting materials have improved charge transport rate and charge transfer balance performance and can be used as efficient ambipolar-transporting materials in organic light-emitting diodes.

Mechanism Insights of Ethane C-H Bond Activations by Bare [Fe-III=O](+): Explicit Electronic Structure Analysis

Alkane C-H bond activation by various catalysts and enzymes has attracted considerable attention recently, but many issues are still unanswered. The conversion of ethane to ethanol and ethene by bare [Fe(III)═O](+) has been explored using density functional theory and coupled-cluster method comprehensively. Two possible reaction mechanisms are available for the entire reaction, the direct H-abstraction mechanism and the concerted mechanism. First, in the direct H-abstraction mechanism, a direct H-abstraction is encountered in the initial step, going through a collinear transition state C···H···O-Fe and then leading to the generation of an intermediate Fe-OH bound to the alkyl radical weakly. The final product of the direct H-abstraction mechanism is ethanol, which is produced by the hydroxyl group back transfer to the carbon radical. Second, in the concerted reaction mechanism, the H-abstraction process is characterized via overcoming four/five-centered transition states (6/4)TSH_c5 or (4)TSH_c4. The second step of the concerted mechanism can lead to either product ethanol or ethene. Moreover, the major product ethene can be obtained through two different pathways, the one-step pathway and the stepwise pathway. It is the first report that the former pathway starting from (6/4)IM_c to the product can be better described as a proton-coupled electron transfer (PCET). It plays an important role in the product ethene generation according to the CCSD(T) results. The spin-orbital coupling (SOC) calculations demonstrate that the title reaction should proceed via a two-state reactivity (TSR) pattern and that the spin-forbidden transition could slightly lower the rate-determining energy barrier height. This thorough theoretical study, especially the explicit electronic structure analysis, may provide important clues for understanding and studying the C-H bond activation promoted by iron-based artificial catalysts.

Structural and Dynamic Basis of Human Cytochrome P450 7B1: A Survey of Substrate Selectivity and Major Active Site Access Channels

Cytochrome P450 (CYP) 7B1 is a steroid cytochrome P450 7α-hydroxylase that has been linked directly with bile salt synthesis and hereditary spastic paraplegia type 5 (SPG5). The enzyme provides the primary metabolic route for neurosteroids dehydroepiandrosterone (DHEA), cholesterol derivatives 25-hydroxycholesterol (25-HOChol), and other steroids such as 5α-androstane-3β,17β-diol (anediol), and 5α-androstene-3β,17β-diol (enediol). A series of investigations including homology modeling, molecular dynamics (MD), and automatic docking, combined with the results of previous experimental site-directed mutagenesis studies and access channels analysis, have identified the structural features relevant to the substrate selectivity of CYP7B1. The results clearly identify the dominant access channels and critical residues responsible for ligand binding. Both binding free energy analysis and total interaction energy analysis are consistent with the experimental conclusion that 25-HOChol is the best substrate. According to 20 ns MD simulations, the Phe cluster residues that lie above the active site, particularly Phe489, are proposed to merge the active site with the adjacent channel to the surface and accommodate substrate binding in a reasonable orientation. The investigation of CYP7B1-substrate binding modes provides detailed insights into the poorly understood structural features of human CYP7B1 at the atomic level, and will be valuable information for drug development and protein engineering.