Zhi-Ru Li

Compounds of superatom clusters: preferred structures and significant nonlinear optical properties of the BLi6-X (X = F, LiF2, BeF3, BF4) motifs.

A new type of superalkali-(super)halogen compound, BLi6-X (X = F, LiF2, BeF3, BF4), is theoretically predicted. The interaction between superalkali BLi6 and different shaped superhalogen X is found to be strong and ionic in nature. Bond energies of these BLi6-X species are in the range of 151.3-220.6 kcal/mol at the CCSD(T)/aug-cc-pVDZ level, which are much larger than the traditional ionic bond energy of 130.1 kcal/mol for LiF. Furthermore, because of their delocalized electron character, these superalkali-(super)halogen species exhibit extraordinarily large first hyperpolarizabilities with beta0 ranging from 5166.5 to 17791.0 au. Besides, the examination of the variation of nonlinear optical (NLO) properties with the size of (BLi6-BeF3)n assemblies shows the dependence of NLO properties on the chain length of (BLi6-BeF3)n. The present investigation gives hints to scientists in extending the research in atomic dimension to direct the interaction between superatoms, and using superatoms as building blocks to synthesize novel meaningful materials with unusual properties such as nonlinear optical properties.

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.

Ab Initio Investigation on a New Class of Binuclear Superalkali Cations M2Li2k+1+ (F2Li3+, O2Li5+, N2Li7+, and C2Li9+)

Superalkalies with low ionization potentials (IPs) can exhibit behaviors reminiscent of alkali atoms and hence be considered as potential building blocks for the assembly of novel nanostructured materials. A new series of binuclear superalkali cations M(2)Li(2k+1)(+) (M = F, O, N, C) has been studied using ab initio methods. The structural features of such cations are found to be related to the central atoms. In the preferred structures of F(2)Li(3)(+), O(2)Li(5)(+), and N(2)Li(7)(+), two central atoms are bridged by lithium atoms. While in the global minima of C(2)Li(9)(+), two central carbon atoms directly link each other and the C-C unit extends to the surface of the whole system. These M(2)Li(2k+1)(+) species exhibit very low vertical electron affinities of 2.74-4.61 eV at the OVGF/6-311+G(3df) level and hence should be classified as superalkali cations.

Low ionization potentials of binuclear superalkali B2Li11

A new type of binuclear superalkali B(2)Li(11) and its corresponding cation B(2)Li(11) (+) were theoretically predicted based on the density functional theory calculations. B(2)Li(11) was found to have six minimum energy structures corresponding to five cation states exhibiting superalkali nature. The global minima of B(2)Li(11) and B(2)Li(11) (+) are similar to each other in structure, where two central boron atoms directly link each other and the whole geometry resembles a capsule with an additional Li atom localized on its side. The vertical electron affinities for the B(2)Li(11) (+) cations at the OVGF/6-311+G(3df) level are in the range of 3.40-3.73 eV, which are lower than the ionization potential (IP) of Cs atom, and even lower than the IP=3.75 eV of the mononuclear superalkali BLi(6). Hence, the studied B(2)Li(11) (+) species should be classified as superalkali cations, and the B(2)Li(11) species can be regarded as superalkalies. Such binuclear superalkalies added candidates to the research on superatoms and offered potential building blocks for the assembly of new materials in which strong electron donors are involved. Note that the electronic shell structure of B(2)Li(11) is not consistent with the prediction of the cluster electronic shell model. It demonstrates that the doped nonmetal atoms make the molecular orbital-level distribution of heteronuclear species much more complex than that of homonuclear metal clusters.

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.

Role of Excess Electrons in Nonlinear Optical Response

The excess electron is a kind of special anion with dispersivity, loosely bounding and with other fascinating features, which plays a pivotal role (promote to about 10(6) times in (H2O)3{e}) in the large first hyperpolarizabilities (β0) of dipole-bound electron clusters. This discovery opens a new perspective on the design of novel nonlinear optical (NLO) molecular materials for electro-optic device application. Significantly, doping alkali metal atoms in suitable complexants was proposed as an effective approach to obtain electride and alkalide molecules with excess electron and large NLO responses. The first hyperpolarizability is related to the characteristics of complexants and the excess electron binding states. Subsequently, a series of new strategies for enhancing NLO response and electronic stability of electride and alkalide molecules are exhibited by using various complexants. These strategies include not only the behaviors of pushed and pulled electron, size, shape, and number of coordination sites of complexants but also the number and spin state of excess electrons in these unusual NLO molecules.

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.

A Dependence on the Petal Number of the Static and Dynamic First Hyperpolarizability for Electride Molecules: Many-Petal-Shaped Li-Doped Cyclic Polyamines

Doping Li atom into higher flexible cyclic polyamines with many amine unit petals (ethyleneimine) forms the n-petal-shaped Li-doped cyclic polyamines (n = 3-5). Three structures, referred to as three-petal-shaped Li-[9]aneN(3), four-petal-shaped Li-[12]aneN(4), and five-petal-shaped Li-[15]aneN(5), with all-real frequencies are obtained at the MP2/6-31+G(d) level. Because the chemical doping with Li and the deformation of the complexant produce more diffuse excess electron, the three molecules with the excess electrons exhibit considerably large static first hyperpolarizabilities (beta(0)) at the MP2 level. Additionally, the beta(0) value increases with increasing the petal number (n) as follows: 52282 (n = 3) < 65505 (n = 4) < 127617 au (n = 5). This shows a new complexant effect on beta(0), that is, a dependence on the petal number (n) of beta(0) owing to the flexibility of the complexants increasing with the petal number. The MP2 frequency-dependent beta values are estimated by using the multiplicative approximation. The frequency dispersion is found to be strong. For the MP2 frequency-dependent beta values, the more pronounced dependence on the petal number (n) of beta (-2omega; omega, omega) and beta (-omega; omega, 0) are shown.

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.