Nonbonding Electrons Research Articles

Electron Localization Function in Excited States: The Case of the Ultrafast Proton Transfer of the Salicylidene Methylamine.

The physical characterization of the chemical bond in the ground state has been a central theme to theoretical chemistry. Among many techniques, quantum chemical topology (QCT) has emerged as a robust technique to understand the features of the chemical bond and electron organization within molecules. One consolidate tool within QCT is the topological analysis of the electron localization function (ELF). Most research on ELF and chemical bond has focused either on singlet ground states or the first excited triplet. However, most photochemical reactions and photophysical processes occur in excited states with the same spin-symmetry as the ground state. In this work, we develop a proposal on how to compute the ELF in excited states of any symmetry within linear-response time-dependent density functional theory. Then, we study the evolution of the chemical bonds in the ground- and excited-state intramolecular proton transfer (ESIPT) of a prototypal Schiff base (the salicylidene methylamine). We found that the topological analysis of the ELF along reaction paths explains the presence of a barrier for the proton transfer in the ground state and the absence of it in the excited state. Briefly, in the ground state, the cleavage of the O-H bond results in a structure with high electrostatic potential energy due to an excess of electron lone-pairs (3) in the oxygen atom, which explains the barrier. In the excited state, the electronic transition promotes an enhancement of the basicity of nitrogen by allocating three nonbonding electrons in the basin of its lone-pair. This excess of electrons in the N exerts an electrostatic attraction of the proton, which we suggest as the primary driven-force of the barrierless reaction. Because in excited states the molecule can develop more vibrational kinetic energy than in the ground state, we performed an ab initio molecular dynamics of the proton transfer in the excited state and corroborate that our conclusions on the topology of the ELF do not change due to dynamic effects.

Origins of the Electronic Modulations of Bacterio- and Isobacteriodilactone Regioisomers.

Advances in the utilization of porphyrinoids for photomedicine, catalysis, and artificial photosynthesis require a fundamental understanding of the relationships between their molecular connectivity and resulting electronic structures. Herein, we analyze how the replacement of two pyrrolic Cβ═Cβ bonds of a porphyrin by two lactone (O═C-O) moieties modulates the ground-state thermodynamic stability and electronic structure of the resulting five possible pyrrole-modified porphyrin isomers. We made these determinations based on density functional theory (DFT) and time-dependent DFT computations of the optical spectra of all regioisomers. We also analyzed the computed magnetically induced currents of their aromatic π-systems. All regioisomers adopt the tautomeric state that maximizes aromaticity, whether or not transannular steric strains are incurred. In all isomers, the O═Cβ-Oβ bonds were found to support a macrocycle diatropic ring current. We attributed this to the delocalization of nonbonding electrons from the ring oxa- and oxo-atoms into the macrocycle. As a consequence of this delocalization, the dilactone regioisomers are as-or even more-aromatic than their hydroporphyrin congeners. The electronic structures follow different trends for the bacteriochlorin- and isobacteriochlorin-type isomers. The presence of either oxo- or oxa-oxygens conjugated with the macrocyclic π-system was found to be the minimal structural requirement for the regioisomers to exhibit distinct electronic properties. Our computational methods and mechanistic insights provide a basis for the systematic exploration of the physicochemical properties of porphyrinoids as a function of the number, relative orientation, and degree of macrocycle-π-conjugation of β-substituents, in general, and for dilactone-based porphyrinic chromophores, in particular.

Cation substitution induced reactivity variation on the tricalcium silicate polymorphs determined from first-principles calculations

Abstract Alite is one of the most important phases in cement clinker. It is a solid solution of tricalcium silicate (Ca3SiO5, C3S for short, C = CaO, S = SiO2) which has seven polymorphs. M1-C3S and M3-C3S are the most common polymorphs in cement clinker. Much experimental research had identified that cations doping affected the polymorph of alite and the hydration properties of cement clinker. This article systemically investigated the influence of cation doping on crystal structures and electronic structures of M1-C3S and M3-C3S employed by first-principles calculations based on density functional theory. Non-bonding electrons dispersed on the p orbitals of Oi (the isolated oxygen atoms) and Oc (covalent bonded oxygens in nesosilicate tetrahedrons) in C3S were determined and quantified by the integration of partial density of states at specified energy levels. Non-bonding electrons are the most probable electrons that will be transferred when the oxygen atoms suffer from electrophilic attack as minerals contact with water. The results showed that there were more non-bonding electrons in both of Oi and Oc atoms in M1-C3S than that in M3-C3S, which lead to M1-C3S more reactive. The substituted cations with specific valence electrons configurations and weak electronegativity are determined, which will typically increase the amount of non-bonding electrons in the systems generally. These would help to understand how cation doping affected the hydration of alite, and furthermore, improve the quality of cement clinker.

Doubly resonant sum frequency spectroscopy of mixed photochromic isomers on surfaces reveals conformation-specific vibronic effects.

Doubly resonant infrared-visible sum-frequency generation (DR-IVSFG) spectroscopy, encompassing coupled vibrational and electronic transitions, provides a powerful method to gain a deep understanding of nuclear motion in photoresponsive surface adsorbates and interfaces. Here, we use DR-IVSFG to elucidate the role of vibronic coupling in a surface-confined donor-acceptor substituted azobenzene. Our study reveals some unique features of DR-IVSFG that have not been previously reported. In particular, vibronic coupling resulted in prominent SFG signal enhancement of selective stretching modes that reveal electronic properties of coexisting photochromic isomers. Our analysis explores two concepts: (1) In partially isomerized azobenzene at the surface, coupling of the fundamental vibrations to the S0 → S1 transition is more prominent for the cis isomer due to symmetry breaking, whereas coupling to the S0 → S2 transition was dominant in the trans isomer. (2) A strong coupling between the fundamental vibrations and the valence π-electron density, promoted by the initial absorption of an infrared photon, may result in suppression of the intensity of the hot band vibronic transition. This may translate into a suppressed sum-frequency generation signal at sum frequency wavelengths resonant with the S0 → S2 transition of the trans isomer. The weaker coupling of the fundamental vibrations to the non-bonding electron density localized on the azo group can therefore produce detectable sum-frequency generation at the resonance wavelength of the weaker S0 → S1 transition in the cis form. These results are explained in the framework of a linear coupling model, involving both Franck-Condon and Herzberg-Teller coupling terms. Our theoretical analysis reveals the important role played by molecular conformation, orientation, and vibronic interference in DR-SFG spectroscopy.

Non-Covalent Interactions Involving Alkaline-Earth Atoms and Lewis Bases B: An ab Initio Investigation of Beryllium and Magnesium Bonds, B···MR2 (M = Be or Mg, and R = H, F or CH3)

Geometries, equilibrium dissociation energies (De), intermolecular stretching, and quadratic force constants (kσ) determined by ab initio calculations conducted at the CCSD(T)/aug-cc-pVTZ level of theory, with De obtained by using the complete basis set (CBS) extrapolation [CCSD(T)/CBS energy], are presented for the B···BeR2 and B···MgR2 complexes, where B is one of the following Lewis bases: CO, H2S, PH3, HCN, H2O or NH3, and R is H, F or CH3. The BeR2 and MgR2 precursor molecules were shown to be linear and non-dipolar. The non-covalent intermolecular bond in the B···BeR2 complexes is shown to result from the interaction of the electrophilic band around the Be atom of BeR2 (as indicated by the molecular electrostatic potential surface) with non-bonding electron pairs of the base, B, and may be described as a beryllium bond by analogy with complexes such as B···CO2, which contain a tetrel bond. The conclusions for the B···MgR2 series are similar and a magnesium bond can be correspondingly invoked. The geometries established for B···BeR2 and B···MgR2 can be rationalized by a simple rule previously enunciated for tetrel-bonded complexes of the type B···CO2. It is also shown that the dissociation energy, De, is directly proportional to the force constant, kσ, in each B···MR2 series, but with a constant of proportionality different from that established for many hydrogen-bonded B···HX complexes and halogen-bonded B···XY complexes. The values of the electrophilicity, EA, determined from the De for B···BeR2 complexes for the individual Lewis acids, A, reveal the order A = BeF2 > BeH2 > Be(CH3)2—a result that is consistent with the −I and +I effects of F and CH3 relative to H. The conclusions for the MgR2 series are similar but, for a given R, they have smaller electrophilicities than those of the BeR2 series. A definition of alkaline-earth non-covalent bonds is presented.

A chalcogen-bonded complex H3N⋯S=C=S formed by ammonia and carbon disulfide characterised by chirped-pulse, broadband microwave spectroscopy.

Ground-state rotational spectra were observed for ten symmetric-top isotopologues H3N⋯S=C=S, H3N⋯34S=C=S, H3N⋯S=C=34S, H3N⋯S=13C=S, H3 15N⋯S=C=S, H3 15N⋯34S=C=S, H3 15N⋯S=C=34S, H3 15N⋯S=13C=S, H3 15N⋯33S=C=S, and H3 15N⋯S=C=33S, the first five in their natural abundance in a mixture of ammonia and carbon disulphide in argon and the second group with enriched 15NH3. The four asymmetric-rotor isotopomers H2DN⋯S=C=S, H2DN⋯34S=C=S, H2DN⋯S=C=34S, and HD2N⋯S=C=S were investigated by using a sample composed of ND3 mixed with CS2. Rotational constants, centrifugal distortion constants, and 33S nuclear quadrupole coupling constants were determined from spectral analyses and were interpreted with the aid of models of the complex to determine its symmetry, geometry, one measure of the strength of the intermolecular binding, and information about the subunit dynamics. The complex has C3v symmetry, with nuclei in the order H3N⋯S=C=S, thereby establishing that the non-covalent interaction is a chalcogen bond involving the non-bonding electron pair of ammonia as the nucleophile and the axial region near one of the S atoms as the electrophile. The small intermolecular stretching force constant kσ = 3.95(5) N m-1 indicates a weak interaction and suggests the assumption of unperturbed component geometries on complex formation. A simple model used to account for the contribution of the subunit angular oscillations to the zero-point motion leads to the intermolecular bond length r(N⋯S) = 3.338(10) Å.

The carbenoid-type reactivity of simplest nitrile imine from a molecular electron density theory perspective

The [3 + 2] cycloaddition (32CA) reactions of the simplest nitrile imine with ethylene and electrophilic dicyanoethylene have been studied within the Molecular Electron Density Theory (MEDT) with the aim of characterising its reactivity. Topological analysis of the electron localisation function of NI shows that it has a carbenoid structure. The activation energy of the 32CA reaction of the simplest nitrile imine with dicyanoethylene is 5.6 kcal mol−1 lower than that involving ethylene, in agreement with the high polar character of the former reaction. Bonding Evolution Theory accounts for the cb-type reactivity of nitrile imine. Along the more favourable ortho regioisomeric path associated with the 32CA reaction involving dicyanoethylene, which takes place through a non-concerted two-stage one-step mechanism, formation of the first C3-C4 single bond takes place at a C-C distance of 2.02 Å, by donating the non-bonding electron density of the carbenoid center of nitrile imine to the β-conjugated C4 carbon of dicyanoethylene.

Neodymium-decorated graphene oxide as a corrosion barrier layer on Ti6Al4V alloy in acidic medium.

Ti6Al4V alloy is light weight and is used in construction, oil industries and airbus, automobile, and bio implant materials. The native oxide layers of the alloy are not stable at high temperatures and strong mineral acid environments. The conventional epoxy-based layers are porous and the alloy finally fails in the harsh environment in the long term. Therefore, the carbon-based functional materials are being proposed as coating materials to overcome the alloy degradation. In the present contribution, we have used the neodymium-decorated graphene oxide as the corrosion inhibiting barrier for the Ti6Al4V alloy. As a novelty, we found that the few-layer graphene decorated with neodymium acts as a self-cleaning coating. The Nd-decorated graphene oxide were studied by XRD, TEM, FESEM, FTIR, UV, and Raman spectroscopy. The corrosion inhibition efficiency was studied by electrochemical methods.

Pressure induced excellent thermoelectric behavior in skutterudites CoSb3 and IrSb3.

Utilizing the first-principle calculations combined with Boltzmann transport equation (BTE) and semiclassical analysis, we have systematically investigated the electronic structure, lattice thermal conductivity κL, Seebeck coefficient S, and the dimensionless figure of merit zT as a function of hydrostatic pressure P in crystalline skutterudites CoSb3 and IrSb3. Interestingly, as the pressure increases, the band gap and κL show an approximate parabolic trend, which results in extraordinarily high S and excellent thermoelectric properties, and zT even exceeds 1.4(1.09) in IrSb3(CoSb3) at 54(58) GPa. This anomalous behavior arises from the electron distribution and intrinsic scattering processes. Further analyses indicate that (i) nonbonding electron pairs of Sb atoms are gradually transferred to the region between Co(Ir) and Sb atoms as the pressure increases, which leads to the formation of a partial metallic bond and thus the band gap first expands and then shrinks; (ii) the change of the strength of the anharmonic phonon scattering process results in the variation of κL. As a result, these behaviors cause excellent thermoelectric properties. Our results provide insight into the thermal transport properties of skutterudites, meanwhile, forecast potential high pressure applications for thermoelectric materials.

Influence factors and adjusting method of refractive indices of solid media

It is of great significance to explore changing behavior and adjustment method of the refractive index. Expression of the refractive index, which was derived from the electron-cloud conductor model, contains the equivalent volume V of the electron clouds and the molecular density [Formula: see text]. The expression shows that the refractive index increases with V and [Formula: see text]. Based on this expression, main factors influencing the refractive index of solid medium are analyzed and summarized: the Nephelauxetic Effect, number of bonding electrons in the molecule, chemical bond property, number of nonbonding electrons in outermost layer of the molecule, and molecular density of the medium (electromagnetic wave factor is not considered). The relationship between the refractive index and the Nephelauxetic Effect is established. The calculations show that the Nephelauxetic Effect helps to increase the refractive index. Some viewpoints related to the Nephelauxetic Effect are supplemented. Characteristics of the solid medium that has a high refractive index are proposed: (1) Covalent bond component is high, and number of covalent bonds in one molecule is large. (2) Periodic number is as large as possible without increasing the ionic bond component. (3) Molecular density is high. According to these characteristics, method of adjusting the refractive index is discussed, and the results are consistent with the real situations.

Electronic Spectra of ortho-Substituted Phenols: An Experimental and DFT Study

The electronic spectra of phenol, 2-chlorophenol, 2-aminophenol, and 2-nitrophenol have been studied both experimentally and computationally. The effect of the substituents on the solvatochromic behavior of the phenols were investigated in polar protic (methanol) and aprotic (dimethyl sulfoxide (DMSO)) solvents. The spectra of 2-nitrophenol recorded the highest red shift in methanol. The observed spectral changes were investigated computationally by means of density functional theory (DFT) methods. The gas phase compounds were fully optimized using B3LYP functionals with 6-31++G(d,p) bases set. The effects of the substituents on the electron distribution in the σ-bonds as well as the natural charge on the constituent atoms were analyzed by natural bond orbital (NBO) and natural population analysis (NPA). Second-order perturbation analyses also revealed substantial delocalization of nonbonding electrons on the substituents onto the phenyl ring, thereby increasing its electron density. Full interaction map (FIM) also showed regions of varying propensities for hydrogen and halogen bonding interactions on the phenols.

Second-Order Jahn–Teller (SOJT) Structural Distortions in Multiply Bonded Higher Main Group Compounds

The geometrical deviations from planar to folded or from linear to bent structures observed in multiply bonded heavy main group element analogues of ethylene and acetylene are treated from a second-order Jahn–Teller (SOJT) perspective. The structural distortions can be regarded as a consequence of a mixing of the filled highest occupied molecular orbital (HOMO) and an unoccupied orbital (LUMO+1). This mixing stabilizes the HOMO and destabilizes the unoccupied LUMO+1 orbital and produces nonbonding electron density, which causes the geometrical distortion through interelectronic repulsion. This occurs more readily in the heavier species because of their generally weaker bonds, which permit a closer approach and hence greater interaction of the molecular levels. Predicting which geometrical distortion enables the HOMO/LUMO+1 mixing requires knowledge of the symmetries of these frontier orbitals and the symmetry of the distortion. For the undistorted structures the product of the symmetries of the interactin...

An Ab Initio Investigation of the Geometries and Binding Strengths of Tetrel-, Pnictogen-, and Chalcogen-Bonded Complexes of CO₂, N₂O, and CS₂ with Simple Lewis Bases: Some Generalizations.

Geometries, equilibrium dissociation energies (De), and intermolecular stretching, quadratic force constants (kσ) are presented for the complexes B⋯CO2, B⋯N2O, and B⋯CS2, where B is one of the following Lewis bases: CO, HCCH, H2S, HCN, H2O, PH3, and NH3. The geometries and force constants were calculated at the CCSD(T)/aug-cc-pVTZ level of theory, while generation of De employed the CCSD(T)/CBS complete basis-set extrapolation. The non-covalent, intermolecular bond in the B⋯CO2 complexes involves the interaction of the electrophilic region around the C atom of CO2 (as revealed by the molecular electrostatic surface potential (MESP) of CO2) with non-bonding or π-bonding electron pairs of B. The conclusions for the B⋯N2O series are similar, but with small geometrical distortions that can be rationalized in terms of secondary interactions. The B⋯CS2 series exhibits a different type of geometry that can be interpreted in terms of the interaction of the electrophilic region near one of the S atoms and centered on the C∞ axis of CS2 (as revealed by the MESP) with the n-pairs or π-pairs of B. The tetrel, pnictogen, and chalcogen bonds so established in B⋯CO2, B⋯N2O, and B⋯CS2, respectively, are rationalized in terms of some simple, electrostatically based rules previously enunciated for hydrogen- and halogen-bonded complexes, B⋯HX and B⋯XY. It is also shown that the dissociation energy De is directly proportional to the force constant kσ, with a constant of proportionality identical within experimental error to that found previously for many B⋯HX and B⋯XY complexes.

Assembly Study of Melamine and its Derivatives

Melamine is well established nitrogen rich triazine ring containing molecule with three additional nitrogen atoms those can easily protonate and enhance the solubility of polar solvents [1]. Recently, derivatization of melamine gained the significant advanced for variety of purposes including corrosion inhibition. Literature survey revealed that melamine and its several derivatives have been investigated as effective corrosion inhibitors for metals and alloys in aggressive solutions owing to their high protection ability which is in turn attributed to the adsorption of these compounds by their protonizable amino groups and non-bonding electrons of nitrogen atoms and π-electrons of three double (-C=N- ) bonds [2,3]. Melamine is a nontoxic, white, odorless hetero-aromatic substance, which is used as a raw material for various industrial products, mostly as a high molecular weight compound obtained from reaction with formaldehyde. Melamine formaldehyde (MF) resins are obtained by poly-condensation of melamine with formaldehyde in acidic or basic media [4].

Trends in Bond Dissociation Energies for the Homolytic Cleavage of Successive Molecular Bonds

The students in our physical chemistry course pointed out an interesting trend in the bond dissociation energies (BDEs) of successive homolytic cleavages of hydrogens from methane and water. Namely, while there is an increase in BDE from CH4 to CH3, there is a decrease in BDE from H2O to OH. In order to explain this trend, we employed a theoretical approach that could be used in an undergraduate setting. We compare our theoretical results with experimental and highly accurate theoretical values readily available in the literature. Having validated our theoretical approach, we use equilibrium molecular structure as evidence of the change in dominating attraction and repulsion effects of the central atom nucleus and nonbonding electrons and explain the anomaly in BDEs in undergraduate chemistry terms.

Nonbonding Electrons Driven Strong SHG Effect in Hg2GeSe4: Experimental and Theoretical Investigations.

A Hg-based ternary infrared nonlinear optical (NLO) material, Hg2GeSe4, with the defect diamond-like (DL) structure was systematically investigated for the first time. The experimental results show that Hg2GeSe4 exhibits an enhanced second harmonic generation (SHG) response about 2.1 times that of the normal DL selenide AgGaSe2 ( d36 = 33 pm/V) at the particle size of 150-200 μm, as well as good phase-matchable ability. Moreover, theoretical analysis reveals that the nonbonding electrons around Se atoms in the defect DL structure make a dominant contribution to the improvement of the NLO property: d36 = 78.83 pm/V and Δ n = 0.11. This study highlights the promise of electronic engineering strategies and opens new avenues toward the design of new infrared NLO crystals with high performance.

Heavy Carbene Analogues: Donor-Free Bismuthenium and Stibenium Ions.

Kinetically stabilized congeners of carbenes, R2 C, possessing six valence electrons (four bonding electrons and two non-bonding electrons) have been restricted to Group 14 elements, R2 E (E=Si, Ge, Sn, Pb; R=alkyl or aryl) whereas isoelectronic Group 15 cations, divalent species of type [R2 E]+ (E=P, As, Sb, Bi; R=alkyl or aryl), were unknown. Herein, we report the first two examples, namely the bismuthenium ion [(2,6-Mes2 C6 H3 )2 Bi][BArF4 ] (1; Mes=2,4,6-Me3 C6 H2 , ArF =3,5-(CF3 )2 C6 H3 ) and the stibenium ion [(2,6-Mes2 C6 H3 )2 Sb][B(C6 F5 )4 ] (2), which were obtained by using a combination of bulky meta-terphenyl substituents and weakly coordinating anions.

Magnetic engineering in InSe/black-phosphorus heterostructure by transition-metal-atom Sc-Zn doping in the van der Waals gap

Within the framework of the spin-polarized density-functional theory, we have studied the electronic and magnetic properties of InSe/black-phosphorus (BP) heterostructure doped with 3d transition-metal (TM) atoms from Sc to Zn. The calculated binding energies show that TM-atom doping in the van der Waals (vdW) gap of InSe/BP heterostructure is energetically favorable. Our results indicate that magnetic moments are induced in the Sc-, Ti-, V-, Cr-, Mn- and Co-doped InSe/BP heterostructures due to the existence of non-bonding 3d electrons. The Ni-, Cu- and Zn-doped InSe/BP heterostructures still show nonmagnetic semiconductor characteristics. Furthermore, in the Fe-doped InSe/BP heterostructure, the half-metal property is found and a high spin polarization of 100% at the Fermi level is achieved. The Cr-doped InSe/BP has the largest magnetic moment of 4.9 μB. The Sc-, Ti-, V-, Cr- and Mn-doped InSe/BP heterostructures exhibit antiferromagnetic ground state. Moreover, the Fe- and Co-doped systems display a weak ferromagnetic and paramagnetic coupling, respectively. Our studies demonstrate that the TM doping in the vdW gap of InSe/BP heterostructure is an effective way to modify its electronic and magnetic properties.

Interaction between poly(vinyl pyrrolidone) PVP and fullerene C60 at the interface in PVP-C60 nanofluids–A spectroscopic study

Fourier transform infrared and Raman bands shows a discernible enhancement in band intensity of C–H stretching, C=O stretching, C–N stretching, C–H2 bending, and C–H2 in-plane bending in PVP molecules in the presence of C60 molecules. Amplification in intensity is ascribed to microscopic interactions results when a donation of nonbonding electron (n) occurs from a “>N–C=O” entity of PVP into a lowest unoccupied molecular orbital of the C60 molecule in PVP-C60 charge transfer (CT) complex. The C=O stretching band intensity (integrated) Vs C60 content plot exhibits a peak near a critical 13.9 μM C60 value owing to percolation effect. Light emission spectra show that even a small addition of 4.63 μM C60 able to suppress the band intensity by ~23% as a result of an energy loss. The integrated band intensity also decreases through a peak near 13.9 μM when plotted against the C60-content. In correlation to the vibration spectra, the maximum effect observed both in light emission and excitation spectra suggests a percolation effect in the CT complex. Exhibition of percolation threshold in C60-PVP donor-acceptor complex will be helpful in optimizing the photovoltaic properties vital for solar cell applications.

Photoredox catalysis unleashes carbynes’ synthetic power

Methylidyne, a carbon-based gas believed to have helped form the first molecules of life and one of the earliest molecules ever discovered among the stars, belongs to an unusual class of compounds called carbynes. These highly reactive compounds are not to be confused with the contentiously categorized, one-dimensional string of carbons of the same name or with the more familiar carbenes. Carbynes, according to organic chemistry’s definition, are monovalent carbon species with three nonbonded electrons. These species have tantalized researchers with the prospect of forming three new bonds from a single atom, but synthetic chemists have been unable to tame these unruly molecules. A team led by Marcos G. Suero at the Institute of Chemical Research of Catalonia has developed a new method that sidesteps carbynes’ unstable nature by generating their molecular equivalent, a diazomethyl radical species, using photoredox catalysis (Nature 2018, DOI: 10.1038/nature25185). The researchers can install these radical