ConspectusOrbital correlation diagrams are central to chemistry. Based on the symmetry compatibility and orbital overlap amplitude, they link the energy-ordered frontier molecular orbitals (MOs) of reactants and products and have long been a powerful and essential tool for understanding chemical interactions (reactions) and molecular properties. The frontier MOs typically include the highest occupied MOs (HOMOs) and the lowest unoccupied MOs (LUMOs), along with a few nearby orbitals of the reactants. However, it is also known that some reactions cannot be well explained with a few frontier MOs. The main drawback of traditional orbital correlation diagrams is that the orbital energies of the reactants shown in the diagram are calculated assuming they are in free, isolated states. But orbital energy levels can be significantly shifted by external fields and the existence of neighboring molecules. In other words, orbital energy levels can be notably reshuffled when we put reactants "physically" (via electrostatic interactions, Pauli repulsion, and van der Waals interactions) together, even without "chemical" interactions (via orbital mixtures or electron transfers).Here, we introduce a novel concept, "in situ" orbital correlation, and demonstrate its applications. This concept is based on our developed block-localized wave function (BLW), which is the simplest variant of ab initio valence bond (VB) theory. The uniqueness of the BLW method lies in its ability to derive orbital energies of a molecule self-consistently in the presence of other species or external fields, as a BLW solution essentially corresponds to a hypothetical diabatic (or resonance) state, a mathematical construct in which all electron transfers between interacting species are "disabled". In such a way, we can correlate orbitals by considering the field (physical) effects from neighboring species even without any orbital (chemical) interactions.This "in situ" orbital correlation concept was first proposed in the study of the activation mechanism of CO by the diboryne compound B2(NHCR)2, where we demonstrated that when CO approaches B2(NHCR)2, there is a HOMO-LUMO swap in B2(NHCR)2 primarily due to the Pauli repulsion from the carbon lone pair of CO, leading to the compatibility of HOMO and HOMO-1 of B2(NHCR)2 with both π* orbitals of CO. Since then, this concept has been adopted in much of our research. For instance, in our most recent study of NCCL- anions (L = N2, CO, CS), which exhibit notable geometric differences, "in situ" orbital correlation diagrams reveal an orbital swap in the fragment NCC- with the approach of the ligand L and subsequently confirm the C(0) theory proposed by the Frenking group. Previously, we explored the "anti-electrostatic" nature of the Al-Mg bond and confirmed that the bond is purely ionic. This contradicts the view from frontier orbitals of Al(I) and Mg compounds, which exhibit a perfect match for a dative covalent bond between them. Now, with the help of the "in situ" orbital correlation diagram, it becomes obvious that the metal-metal bond is a typical ionic bond, because when the Mg compound is brought close, the energy level of the HOMO of Al(I) compound decreases significantly, leading to a reversal of the HOMO-LUMO energy level order and the extension of the HOMO-LUMO band gap and subsequently minimal probability of any electron transfer. We expect that the novel concept of "in situ" orbital correlation will fundamentally enrich our understanding of chemical reactions, electron transfer pathways, and molecular bonding.

Excited-state aromaticity expands the concept of aromaticity to describe additional molecular stability and reactivity upon photoexcitation. While both Hückel and Möbius excited-state aromatic species have been identified, Craig excited-state aromaticity involving [4n+2] electrons in planar metallacycles remains unrecognized. Herein, we report that early transition metal (M = Ti, Sc, Y, La, Ac)-based metallabenzenes exhibit Craig 6π aromaticity in their lowest singlet and triplet ππ* excited states, which is supported by a range of aromaticity indices based on electronic, geometric, energetic, and magnetic properties. Notably, ab initio valence bond theory reveals that the dyz orbital dominates the cyclic electron delocalization in the excited-state wave function, resulting in phase inversion between neighboring atomic orbitals of π-symmetry. In contrast, the dyz orbital is usually doubly occupied in well-identified metallabenzenes with late transition metals which thus display Hückel or Baird (anti)aromaticity via the dxz orbital. Our findings provide the first direct evidence and origin of Craig excited-state aromaticity and establish a unified framework for understanding the electronic structure of metallabenzenes, addressing a significant gap in the exploration of excited-state metalla-aromaticity.

In this work, we proposed an efficient and accurate approach to separate the σ-inductive and π-resonance effects of substituents on modulating the strength of resonance-assisted hydrogen bonding (RAHB) in malonaldehyde. This is achieved using the block-localized wavefunction (BLW) method, which effectively quenches the resonance effect by localizing the π electrons solely on the substituent at the density functional theory (DFT) level. By comparing the BLW-DFT results for substituted systems and the DFT results for unsubstituted malonaldehyde, we were able to isolate the σ-inductive effect. Subsequent analysis of the structural, energetic, and spectral differences between the BLW-DFT and DFT results for substituted systems reveals the π-resonance effect. Our computational results indicate that the σ-inductive effect overwhelms the π-resonance effects on tuning the RAHB in malonaldehyde, suggesting that the σ-inductive effect should not be neglected when analyzing the overall behavior of substituents in conjugated systems.

We propose a novel concept of "in situ" orbital correlation to gain a deep understanding of the nature of the chemical bond. In stark contrast to popular traditional orbital correlations, where the orbital energies are derived from the free and noninteracting states of isolated species, the "in situ" orbital correlations consider the field effects from neighboring species even without any orbital (chemical) interactions. Such field effects may profoundly impact the orbital energies of all of the involved moieties. This is achieved with our block-localized wave function (BLW) method that is the simplest variant of ab initio valence bond (VB) theory and can self-consistently derive a hypothetical diabatic state where the species stay physically together but exclude chemical interactions. Case studies of an exemplary dative bond in H3B-NH3 and an unconventional ionic bond in lithium-aluminum dimetallocenes demonstrate that the novel "in situ" orbital correlation diagram not only provides more insight than the traditional one in general cases but also reshuffles the orbital correlations in cases where the traditional orbital correlation diagram fails.

Understanding the bonding nature between actinides and main-group elements remains a key challenge in actinide chemistry due to the involvement of f orbitals. Herein, we propose a unique "aromaticity-assisted multiconfiguration" (AAM) model to elucidate the bonding nature in actinide nitrides (An2N2, An = Ac, Th, Pa, U). Each planar four-membered An2N2 with equivalent An-N bonds possesses four delocalized pi electrons and four delocalized sigma electrons, forming a new family of double M & ouml;bius aromaticity that contributes to the molecular stability. The unprecedented aromaticity further supports actinide nitrides to exhibit multiconfigurational characters, where the unpaired electrons (2, 4 or 6 in naked Th2N2, Pa2N2 or U2N2, respectively) either are spin-free and localized on metal centres or form metal-ligand bonds. High-level multiconfigurational computations confirm an open-shell singlet ground state for actinide nitrides, with small energy gaps to high spin states. This is consistent with the antiferromagnetic nature observed experimentally in uranium nitrides. The novel AAM bonding model can be authenticated in both experimentally identified compounds containing a U2N2 motif and other theoretically modelled An2N2 clusters and is thus expected to be a general chemical bonding pattern between actinides and main-group elements. Actinide nitride An2N2 (An = U, Pa, Th and Ac) clusters were characterized by a new family of double M & ouml;bius aromaticity, which enables unpaired electrons spin-freely to localize on actinides or form chemical bonds with ligands.

The block-localized wavefunction (BLW) method is the simplest ab initio variant of valence bond (VB) theory. It can derive the self-consistent wavefunction for a strictly electron-localized state, which can be a Lewis or resonance structure in the traditional resonance-theoretic language, or a generalized hypothetical non-charge transferred state based on the specific research question to be addressed. By comparing the computational results from regular MO or DFT methods at the same theoretical level, the BLW method allows the quantification of the energetic, geometrical, and spectral impacts of intra- and intermolecular electron transfer processes and reactions. In this article, we introduced the essences of the BLW method and showcased how this method can be applied to the elucidation of non-covalent interactions with the examples of halogen bonding, and binding and activation mechanisms of small molecules such as CO, N2 and CO2 by main group catalysts.

The Möbius rule predicts that a planar four-membered metallacycle can be aromatic with four mobile electrons, but such a simple ring has escaped recognition because it usually favors Hückel anti-aromaticity. Here, we report that a quasi-square four-membered actinide compound (Pa2B2) is doubly Möbius aromatic. Chemical bonding analyses reveal that this diboron protactinium molecule has four delocalized π electrons in addition to four delocalized σ electrons, satisfying the 4n Möbius rule for both σ and π components. Energetically, the block-localized wavefunction method, which is the simplest variant of ab initio valence bond theory, shows that the delocalization energy for the π and σ electrons reaches up to 65.0 and 72.3 kcal/mol, respectively, while the extra cyclic resonance energy (ECRE) amounts to 45 kcal/mol. The large positive ECRE values strongly confirm the unprecedented double Möbius aromaticity in Pa2B2. We anticipate that this new type of aromatic molecule can enrich the concept of Möbius aromaticity and open a new avenue for actinide compounds.

AbstractA unique thorium‐thorium bond was observed in the crystalline tri‐thorium cluster [{Th(η8‐C8H8)(μ3‐Cl)2}3{K(THF)2}2]∞, though the claim of σ‐aromaticity for Th3bond has been questioned. Herein, a new type of core–shell syngenetic bonding model is proposed to describe the stability of this tri‐thorium cluster. The model involves a 3c–2e bond in the Th3core and a multicentered (ThCl2)3charge‐shift bond with 12 electrons scattering along the outer shell. To differentiate the strengths of the 3c–2e bond and the charge‐shift bond, the block‐localized wavefunction (BLW) method which falls into the ab initio valence bond (VB) theory is employed to construct a strictly core/shell localized state and its contributing covalent resonance structure for the Th3core bond. By comparing with the σ‐aromatic H3+and nonaromatic Li3+, the computed resonance energies and extra cyclic resonance energies confirm that this Th3core bond is truly delocalized and σ‐aromatic.

Recently, metal mediated molecularly imprinted polymers (MMIPs) raise extensive attention due to their special adsorption/desorption mechanism. And the metal ion plays a key role both for MMIPs preparation and molecular recognition. But it is still a big question to select one suitable metal ion. To overcome above problem, one computational approach was proposed by calculating interaction energy (& UDelta;E) of ternary complexes (monomer -M2+-template) with GAMESS software at the PBE0 level. Finally Cu(II) was screened from four doubly charged metal ions with different radii. Then Cu mediated MIP was prepared via surface imprinting technique with Fe3O4 as the core in a water system. Subsequently, the adsorption behavior of the MMIP immobilized Cu(II) was investigated in detail. The results indicated the microspheres own excellent specificity and selectivity towards tetracycline (TC), as well as long-term stability and good reproducibility. The apparent maximum binding ca-pacity and the imprinting factor were as high as 163.5 mg g? 1 and 19, respectively. Coupled with HPLC-PDA, the microspheres was successfully used as a dispersed solid phase extraction material to extract trace TC in chicken liver. At three spiking levels, mean recoveries ranged from 89.3% to 97.1% with relative standard deviations less than 4.0% (n = 3). Limits of detection and quantitation were 0.0073 mg kg? 1 and 0.024 mg kg? 1, respectively, which can meet the strict requirement of the regulations stipulated by EU. Moreover, preliminary studies showed the radius of the metal ion and & UDelta;E of the complexes all affect the adsorption capacity, specificity and mass transfer rate of the MMIP.

This paper combines the valence bond block diabatization approach with the idea of orbital breathing. With highly compact wave functions, the breathing orbital valence bond (BOVB) method is applied to investigate several atomic and molecular properties, including the electron affinity of F, the adiabatic and diabatic potential energy curves and the dipole moment curves of the two lowest-lying 1Σ+ states, the electronic coupling curve and the crossing distance of the two diabatic states, and the spectroscopic constants of the ground states for LiF. The configuration selection scheme proposed in this work is quite general, requiring only the selection of several de-excitation and excitation orbitals in a sense like the restricted active space self-consistent field method. Practically, this is also the first time that BOVB results are extrapolated to complete basis set limit. Armed with the chemical intuition provided by valence bond theory, the classic but challenging covalent-ionic interaction in the title molecule is not only conceptually interpreted but also accurately computed.

The chemical bond between a transition metal and a methyl group (M-CH3) is typically defined as a single covalent bond, which is of fundamental significance and general interest in understanding the structural properties and reactivity of transition metal alkyl compounds. Herein, we demonstrate that the M-CH3 bonding involves varying σ and π components and thus should be best described in terms of the partial double M═CH3 bond. The often-neglected π bonding stems from an occupied π-symmetric orbital of the methyl group comprising all three C-H σ bonds (but one C-H' contributes more than the other two) and a vacant low-lying metal d(π) orbital, and is associated with the intramolecular C-H'···M agostic effect (i.e., an acute M-C-H' angle and a short H'···M distance), whose origin is still controversial. We quantify the geometric and energetic impacts of the π interaction involved in the M-CH3 bond by explicitly computing the intramolecular πCH' → dM interaction with the ab initio valence bond (VB) theory. Our computations of the ligand-free [TiCH3]3+ and a series of metallocene catalysts provide a direct proof for the presence of the π bonding in M-CH3 bonds, which is the cause for the agostic effect. The partial double M═CH3 bonding model is not only validated by a range of bonding analyses including VB self-consistent field (VBSCF)-based energy decomposition and quantum theory of atoms in molecules (QTAIM) but also authenticated by the specific activity of double M═CH3 bonds in the C-H activation and olefin insertion. More importantly, the σ bond gradually switches from a classical covalent bond to a novel charge-shift bond with the π bonding becoming increasingly significant. We anticipate that the recognition of the π interaction between electrophilic metal centers and C-H bonds can benefit the understanding of the nature of metal-carbon bonds in transition metal ethyl, alkyl, and carbene compounds.

It has been generally recognized that the α-agostic interaction (M⋯H-C) in transition metal carbene compounds LnMCHR (R = H, Me etc.) can be interpreted with a double metal-carbon bonding model. This bonding model involves the reorganization of the σ component, which can be illustrated in terms of three-center two-electron (3c-2e) M-H-C covalent bond as in transition metal alkyl compounds. Herein, we propose an alternative partial triple metal-carbon bonding model to elucidate the agostic interaction in LnMCHR. Apart from the well-defined σ and π bonds, there exists a seemingly weak but decisive third force, namely the πCHR→dM bonding between an occupied π-like symmetric CHR orbital and a vacant metal d orbital, which is the true origin of the α-agostic effect. This partial triple bonding model is authenticated on both Fischer- and Schrock-type carbenes by an ab initio valence bond (VB) method or the block-localized wavefunction (BLW) method, which has the capability to quantify this notable π bonding and further demonstrate its geometric, energetic and spectral impacts on agostic transition metal carbene compounds. We also show that ancillary ligands can modulate the πCHR→dM bonding through electronic and steric effects.

To explore alternative approaches to the CO2 reduction to formate and provide an insight into the spin state effect on the CO2 reduction, we theoretically designed a kind of low-valence iron(I) model complex, whose doublet, quartet, and sextet states are denoted as 2Fe(I), 4Fe(I), and 6Fe(I), respectively. This complex is featured with an iron(I) center, which bonds to a 1,2-ethanediamine (en) and a 2-hydroxy-biphenyl group. Reaction mechanisms for the CO2 reduction to formate catalyzed by this iron(I) model complex were explored using density functional theory (DFT) computations. Studies showed that the univalent iron(I) compound can efficiently fix and activate a CO2 molecule, whereas its oxidized forms with trivalent iron(III) or bivalent iron(II) cannot activate CO2. For the iron(I) compound, it was found that the lowest spin state 2Fe(I) is the most favorable for the CO2 reduction as the reactions barriers involving 2Fe(I), 4Fe(I), and 6Fe(I) are 25.6, 37.2, and 35.9 kcal/mol, respectively. Yet, a photosensitizer-free visible-light-mediated high-low spin shift from 4Fe(I) and 6Fe(I) to 2Fe(I) is likely through the reverse intersystem crossing (RIC) because the 4Fe(I) and 6Fe(I) compounds have strong absorption in the visible-light range. Notably, the synergistic interaction between the hydrogen bonding from the auxiliary hydroxyl group in the 2-hydroxy-biphenyl moiety to CO2 and an intermediate five-membered ring promotes the proton transfer, leading to the formation of the -COOH moiety from CO2 and the Fe-O bond. With the addition of H2, one H2 molecule is split by the Fe-O bond and thus serves as H atom sources for both the CO2 reduction and the recovery of the auxiliary hydroxyl group. The present theoretical study provides a novel solution for the challenging CO2 reduction, which calls for further experimental verifications.

The peculiar Au···H hydrogen bonding has garnered significant interests, but its existence and nature remain an open question in gold chemistry. In this paper, we established and authenticated the first intramolecular Au···H-O hydrogen bonding in gold(I) complexes. Our computational results based on an ab initio valence bond (VB) method, namely the block-localized wave function (BLW) method, clarified that this Au···H-O hydrogen bonding is a new type of resonance-assisted hydrogen bond (RAHB). In this RAHB, π conjugation in the hydrogen bond donor has the capability to modulate the Au···H-O hydrogen bonding. In contrast to conventional RAHBs, however, the Au···H-O hydrogen bonding is antielectrostatic and is dominated by the charge transfer interaction including cooperative σ electron donation from the d orbital of the metal center to the antibonding orbital of the hydroxyl group and π conjugation from the hydroxyl group to adjacent π deficient or strong π electron withdrawing groups. This novel theoretical perspective not only confirms the existence and reveals the nature of intramolecular Au···H hydrogen bonding but also provides a promising strategy to rationally design strong gold hydrogen bonds.

Agostic interaction refers to the bonding between a coordinatively unsaturated metal atom and its ligand, and is characterized by the drawing of the ligand towards the metal. The most important type of agostic interaction in organometallic chemistry is the C-H center dot center dot center dot Metal coordination that has the capability of activating the inert C-H bond in transition metal complexes. While the existence of such CH center dot center dot center dot M coordination seems unambiguous and well recognized, there are a few quite different theoretical interpretations on the agostic interaction, leading its origin remaining elusive. Therefore, an improved understanding of the nature for agostic interactions is in need with improved theoretical approaches. So far, significant efforts have been made for the purpose of characterizing and elucidating the agostic interaction by means of a wide range of computational approaches, including the natural orbital bond (NBO) method, the quantum theory of atoms in molecules (QTAIM) approach, various energy decomposition analysis (EDA) schemes and the block-localized wavefunction (BLW) method. To this end, in this review, we discussed the development of the agostic interaction concept in history and outlined the computational tools for characterizing the agostic interaction. More importantly, we summarized the popular theoretical perspective on the origin of the agostic interaction in early-transition metal compounds, especially from the view of valence bond theory, and demonstrated that the nature of the agostic effect is complicated and varies in different compounds. (C) 2020 Elsevier B.V. All rights reserved.

The agostic interaction is a ubiquitous phenomenon in catalytic processes and transition-metal complexes, and hyperconjugation has been well recognized as its origin. Yet, recent studies showed that either short-range London dispersion or structural constraints could be the driving force, although proper evaluation of the role of hyperconjugation therein is needed. Herein, a simple variant of valence bond theory was employed to study a few exemplary Ti complexes with α- or β-agostic interactions and interpret the agostic effect in terms of the steric effect, hyperconjugation, and dispersion. For the complexes [MeTiCl3 (dmpe)] and [MeTiCl3 (dhpe)] with α-agostic interactions, hyperconjugation plays the dominant role with comparable magnitudes in both systems, but dispersion is solely responsible for the stronger agostic interaction in the former compared with the latter. For the complexes [EtTiCl3 (dmpe)] and [EtTiCl3 (dhpe)] with β-agostic interactions, however, hyperconjugation and dispersion play comparable roles, and the weaker steric repulsion leads to a stronger agostic effect in the former than in the latter. Thus, the present study clarifies the variable and sensitive roles of steric, hyperconjugative, and dispersion interactions in the agostic interaction.

The secondary electrostatic interaction (SEI) has been regarded as the fundamental cause for the relative strengths of multiple hydrogen bonds for decades, though recent studies challenged its validation. Here, we used our developed block-localized wave function (BLW) method, which is a variant of ab initio valence bond (VB) theory and can self-consistently derive the wave function for a strictly electron-localized state, to study a series of exemplary multiply hydrogen-bonded complexes and critically examine the role of SEI in the binding. Our computations show that the multiple hydrogen bond in self-assembled complexes is a kind of resonance-assisted hydrogen bond (RAHB) in nature, and the π resonance which moves electron density from the hydrogen bond donor to the acceptor is the true origin of the different hydrogen bond strengths. By quenching the π resonance effect, the hydrogen bond strengths become nearly identical for various neutral doubly, triply, and quadruply hydrogen-bonded dimers where in general the SEI model works. In other words, the SEI plays only a minor role in multiply hydrogen-bonded complexes, and the π resonance, which changes not only electron densities but also molecular polarities (dipole moments), is the major force.

目的 通过观察冠心病患者治疗前后代谢产物的变化,探索血府逐瘀汤加减方治疗冠心病心血瘀阻证患者的作用机制. 方法 采用随机数字表法将40例冠心病心血瘀阻证患者分成治疗组22例和对照组18例,对照组采用西医规范治疗方案,治疗组采用西医规范治疗方案联合血府逐瘀汤加减方口服.运用气相色谱-飞行时间质谱(GC-TOF-MS)联用技术,并采用主成分分析法(PCA)和正交偏最小二乘法-判别分析(OPLS-DA)检测分析2组治疗前后血浆小分子代谢产物的变化,采用单因素方差分析血浆代谢产物的差异性. 结果 治疗组治疗前后代谢产物有明显差异,样本分布在完全不同的区域;治疗组治疗后丙酮酸、磷酸甲酯、2-羟基戊酸、木糖醇、D-半乳糖醛酸含量明显下降,胆固醇明显升高(P＜0.05),对照组治疗前后代谢物差异无显著性(P＞0.05).结论 血府逐瘀汤加减方可能通过调整糖类、脂肪、氨基酸等三大能量产物的代谢,发挥其治疗效果.

Both computations and experiments have confirmed that amides have stronger self-associations than imides. While this intriguing phenomenon is usually explained in the term of secondary electrostatic repulsion from the additional spectator carbonyl groups in imides, recently it was proposed that the π resonance effect from the spectator carbonyl which alters the balance between the acidity of the hydrogen-bond (H-bond) donor and the basicity of the H-bond acceptor is the major cause. In this work, we examined the roles of π resonance and the secondary electrostatic interaction in the formation of amide and imide dimers by deactivating the π conjugation from the spectator carbonyl and flipping the spectator carbonyl using the block-localized wave function method which is the simplest variant of valence bond theory. Energetic, geometrical, and spectral results show that three major forces, namely the σ induction effect (IE), π resonance effect (RE), and secondary electrostatic interaction (SEI), contribute to the different binding energies in the dimers of amides and imides. Whereas IE favors stronger binding among imides, both RE and SEI diminish the self-association of imides. Obviously, the negative force from RE and SEI exceeds the positive force from IE. Relatively, SEI plays a little bigger role than RE.