Non-bonding Molecular Orbitals Research Articles

Designing a Redox Noninnocent Phenalenyl-Based Copper(II) Complex: An Autotandem Catalyst for the Selective Oxidation of Polycyclic Aromatic Hydrocarbons (PAHs).

A square-planar [CuIIL] complex 1, based on the redox-active phenalenyl unit LH2 = 9,9′-(ethane-1,2-diylbis(azanediyl))bis(1H-phenalen-1-one), is prepared and structurally characterized by single-crystal X-ray diffraction analysis. Complex 1 crystallizes at room temperature with the P1 space group. The molecular structure of 1 reveals the presence of intriguing C–H···Cu intermolecular anagostic interactions of the order ∼2.7715 Å. Utilizing the presence of anagostic interactions and the free nonbonding molecular orbitals (NBMOs) of the closed-shell phenalenyl unit in 1, the oxidation reactions of some industrially important polycyclic aromatic hydrocarbons (PAHs) in the presence of the [CuIIL] complex under very mild conditions have been reported. The direct conversion of anthracene-9-carbaldehyde to 9,10-anthraquinone in one step concludes that the catalyst shows dual activity in the chemical transformations. This also includes the first report of a “single-step” catalytic transformation of pyrene-1-carbaldehyde to the synthetically difficult pyren-4-ol, a precursor for the synthesis of several novel fluorescent probes for cell imaging.

Multi-Resonance Deep-Red Emitters with Shallow Potential-Energy Surfaces to Surpass Energy-Gap Law*.

Efficient organic emitters in the deep-red region are rare due to the "energy gap law". Herein, multiple boron (B)- and nitrogen (N)-atoms embedded polycyclic heteroaromatics featuring hybridized π-bonding/ non-bonding molecular orbitals are constructed, providing a way to overcome the above luminescent boundary. The introduction of B-phenyl-B and N-phenyl-N structures enhances the electronic coupling of those para-positioned atoms, forming restricted π-bonds on the phenyl-core for delocalized excited states and thus a narrow energy gap. The mutually ortho-positioned B- and N-atoms also induce a multi-resonance effect on the peripheral skeleton for the non-bonding orbitals, creating shallow potential energy surfaces to eliminate the high-frequency vibrational quenching. The corresponding deep-red emitters with peaks at 662 and 692 nm exhibit narrow full-width at half-maximums of 38 nm, high radiative decay rates of ca. 108 s-1 , ≈100 % photo-luminescence quantum yields and record-high maximum external quantum efficiencies of ca. 28 % in a normal planar organic light-emitting diode structure, simultaneously.

Protonated Ground-State Singlet meta-Pyridynes React from an Excited Triplet State.

The gaseous 2,6-didehydropyridinium cation and its derivatives transfer a proton to reagents for which the reaction for their singlet ground states is too endothermic to be observed. These reactions occur from the lowest-energy excited triplet states, which has not been observed (or reported) for other meta-benzyne analogues. Quantum chemical calculations indicate that the (excited) triplet states are stronger Brønsted acids than their (ground) singlet states, likely due to unfavorable three-center, four-electron interactions in the singlet-state conjugate bases. The cations have substantially smaller (calculated) singlet-triplet (S-T) splittings (ranging from ca. -11 to -17 kcal mol-1) than other related meta-benzyne analogues (e.g., -23.4 kcal mol-1 for the 3,5-isomer). This is rationalized by the destabilization of the singlet states (relative to the triplet states) by reduced (spatial) overlap of the nonbonding molecular orbitals due to the presence of the nitrogen atom between the radical sites (making the ring more rigid). Both the singlet and triplet states are believed to be generated upon formation of these biradicals via energetic collisions due to their small S-T splittings. It appears that once the triplet states are formed, the rate of proton transfer is faster than the rate of intersystem crossing unless the biradicals contain heavy atoms.

Donor-Acceptor Conjugated Macrocycles with Polyradical Character and Global Aromaticity.

SummaryPolyradical character and global aromaticity are fundamental concepts that govern the rational design of cyclic conjugated macromolecules for optoelectronic applications. Here, we report donor-acceptor (D−A) conjugated macromolecules with and without π-spacer derivatives to tune the antiferromagnetic couplings between the unpaired electrons. The macromolecules without π-spacer have a closed-shell electronic configuration and show global nonaromatic character in the singlet and lowest triplet states. However, the derivatives with π-spacer develop a nearly pure open-shell diradical and a very high polyradical character, not reported for D−A type macromolecules. Furthermore, the π-spacer derivatives display global nonaromaticity in the singlet ground state, but global aromaticity in the lowest triplet state, according to Baird's rule. The absorption spectra of the open-shell macromolecules calculated with time-dependent density functional theory indicate intensive light absorption in the near-infrared region and broadening to 2,500 nm, making these materials suitable for numerous optoelectronic applications.

On Relationships of Eigenvalue–Based Topological Molecular Descriptors

Three eigenvalue-based topological molecular descriptors are compared using several datasets of alkanes. Two of them are well-known and frequently employed in various QSPR/QSAR investigations, and third-one is a newly derived whose predictive potential is yet to be proven. The relations among them are found and discussed. Structural parameters that govern these relations are identified and the corresponding formulas based on multiple linear regression have been obtained. It has been shown that all three investigated indices are encoding almost the same structural information of a molecule. They differ only by the extent of the sensitivity on a structural branching of a molecule and on the number of non-bonding molecular orbitals.

Translationally symmetric graphene strips

The character of non-bonding molecular orbitals (and the related spin-density) is addressed in the general circumstance of translationally symmetric graphenic strips. Besides a systematic characterization of such possible strips, couplings are addressed between boundary-localized unpaired spins, including the case when opposite sides of a strip are two different types of (translationally symmetric) boundaries.

Pharmaceutical property interpretation and biological property study on Ethionamide using quantum spectroscopic and computational tools

ABSTRACTIn this work, the molecular spectroscopic technique was used in a nonconventional way to predict the drug mechanism action of the drug compound, Ethionamide. The formatted molecular charge levels, due to the injection of substitutions in suitable positions, were measured. The molecular geometry distortion by the adoption of ligands in the ring system was noted at the nodal points of the core and the change of force constant of the heterogeneous intra-molecular assembly. The activity of the compositional parts of the molecule was purposively monitored from which the entire core and allied parts of the molecule were evaluated by the vibrational characteristics. The molecular reaction mechanism for saturating drug potential was observed from the chemical shift of core and substituted carbons. The fabricated charge transfer complex was identified and recognized from the base and ligand-binding structure. The major part of the exchange of electro-chemical quantized energy in non-bonding molecular orbitals for arranging biological activity in the compound was measured. The rule of five and Lipinski parameters to recognize the drug quality were checked and estimated.

In Situ Study of Molecular Structure of Water and Ice Entrapped in Graphene Nanovessels.

Water is ubiquitous in natural systems, ranging from the vast oceans to the nanocapillaries in the earth crust or cellular organelles. In bulk or in intimate contact with solid surfaces, water molecules arrange themselves according to their hydrogen (H) bonding, which critically affects their short- and long-range molecular structures. Formation of H-bonds among water molecules designates the energy levels of certain nonbonding molecular orbitals of water, which are quantifiable by spectroscopic techniques. While the molecular architecture of water in nanoenclosures is of particular interest to both science and industry, it requires fine spectroscopic probes with nanometer spatial resolution and sub-eV energy sensitivity. Graphene liquid cells (GLCs), which feature opposing closely spaced sheets of hydrophobic graphene, facilitate high-resolution transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) measurements of attoliter water volumes encapsulated tightly in the GLC nanovessels. We perform in situ TEM and EELS analysis of water encased in thin GLCs exposed to room and cryogenic temperatures to examine the nanoscale arrangement of the contained water molecules. Simultaneous quantification of GLC thickness leads to the conclusion that H-bonding strengthens under increased water confinement. The present results demonstrate the feasibility of nanoscale chemical characterization of aqueous fluids trapped in GLC nanovessels and offer insights on water molecule arrangement under high-confinement conditions.

Development of molecular fragment interaction method for designing organic ferromagnets

The design and preparation of organic ferromagnets are important and challenging from both experimental and theoretical viewpoints. We propose a molecular fragment interaction method for predicting the high-spin stability of conjugated hydrocarbon radicals without quantum chemistry calculations, which comprise three steps: (1) division of the system into fragments and numbering of the carbon atoms, (2) construction of a secular equation, and (3) solving of the secular equation. If there are two or more nonbonding molecular orbitals (NBMOs) and the smallest number of bonds between two carbon atoms with unpaired electrons is an even number, then the conjugated hydrocarbon radical has a high-spin ground state. This method can also be used to compare the high-spin stability between different conjugated hydrocarbon radicals. If a conjugated hydrocarbon radical has a high-spin ground state, then its derivative has a high-spin ground state as long as the degeneracy of the NBMOs is kept. This method is useful for the rapid design of organic ferromagnets.

Total π-Electron Energy of Conjugated Molecules with Non-bonding Molecular Orbitals

Abstract Lower and upper bounds for the total π-electron energy are obtained, which are applicable to conjugated π-electron systems with non-bonding molecular orbitals (NBMOs). These improve the earlier estimates, in which the number of NBMOs has not been taken into account.

Unusual Tautomeric Energetics in 4n Azaheterocycles

Abstract Unusual imino preference in 4n azaheterocycles is theoretically analyzed. It is attributed to less topological charge stability of the amino forms due to antisymmetric nonbonding molecular orbitals. The theory is applicable for designing stable imino species. The same analysis is also applicable for hydroxyl/oxo tautomerism, and results in very stable lactams.

Development of minimized mixing molecular orbital method for designing organic ferromagnets.

Predicting the high spin stability of organic radicals correctly for designing organic ferromagnets remains a significant challenge. We have developed a method with an index (L(min)) for predicting the high spin stability of conjugated organic radicals at the restricted open-shell Hartree-Fock level. Unitary transformations were performed for localizing the coefficients of nonbonding molecular orbitals, and subsequently the localized coefficients were used to calculate L(min) that indicates the high spin stability of conjugated organic radicals. This method can be combined with the elongation method to treat huge high spin open-shell systems. Thus, this method is useful for designing organic ferromagnets.

Triradicals

Consistent with the definition of diradicals, triradicals arespecies in which three electrons occupy three (nearly) degenerate orbitals, resulting in close-lying quartet and doublet states. The same concepts and rules that can be used to predict and rationalize the ground state multiplicity of diradicals also apply to triradicals, but the greater number of states in triradicals generally leads to more complex electronic structures. Most experimentally accessible triradicals are based onorganic π-systems; therefore triradicals are classified according to the σ/π symmetry of the nominally nonbonding molecular orbitals (NBMOs). The tridehydrobenzenes are prototypal σσσ triradicals with doublet ground states and significant doublet-quartet energy splittings. In all three isomers, the ordering of electronic states depends critically on the distance between the m-radical centers, which makes computational studies of these systems demanding. The experimental IR spectrum of matrix-isolated 1,2,3-tridehydrobenzene led to a revision of the previous ground state assignment based on computations. This work demonstrates the close interplay between experiment and theory in this realm of reactive intermediate chemistry. 1,3,5-Tridehydrobenzene can be isolated as its trifluoro derivative. The stabilization of dehydrophenyl nitrenes, typical members of the σσπ family of triradicals, also requires ortho-fluorination. Because of their quartet ground states, derivatives of 2-dehydrophenyl nitrene and 4-dehydrophenyl nitrene could be studied using IR or EPR spectroscopy. The zero-field splitting parameters of these systems provide direct evidence for the contribution of carbenoid resonance structures to the resonance hybrid of the high-spin systems. According to computations, the through-bond coupling of the in-plane electrons thermodynamically stabilizes the doublet ground states of m-dehydrophenyl nitrenes. But for the same reasons, these systems are prone to ring-opening reactions, which make them difficult to isolate. Remarkably, the m-phenylene unit leads to strongly antiferromagnetic coupling in σσπ triradicals, while o- or p-coupling results in high-spin systems. The more common all-π systems show the opposite pattern because the latter connectivity naturally results in closed-shell arrangements. Within the family of σππ triradicals, we could characterize 2-dehydro-m-xylylene and 4-dehydro-m-xylylene by EPR spectroscopy, whereas the 5-isomer features a doublet ground state. This observation is readily rationalized considering the nodal characteristics of the NBMOs involved and by simple spin polarization models. 1,3,5-Trimethylenebenzene strongly prefers ferromagnetic coupling and features a robust quartet ground state. We have synthesized this πππ triradical in cryogenic matrices and characterized it by IR and EPR spectroscopy. Interestingly, the triradical is photochemically much more stable than m-xylylene, a diradical that shows fascinating rearrangements upon irradiation in cryogenic matrices.

Spin–orbit effects in the bismuth atom and dimer: tight-binding and density functional theory comparison

We present a simple tight-binding model for the bismuth atom and dimer whose main feature is the inclusion of the spin–orbit coupling in such a way that it allows the study of several electronic properties as a function of the spin–orbit coupling strength. Density functional theory calculations (norm-conserving and full-potential linearized augmented plane wave) are used to obtain the tight-binding parameters and to check the accuracy of the model. The model is then used to show in a straightforward way that, in the case of the bismuth dimer, the inclusion of the spin–orbit coupling produces a set of molecular orbitals that are a mixture of bonding and non-bonding non-relativistic molecular orbitals, thus weakening the molecular bond.

An Analytical Approach to Predict High-spin Stability of Conjugated Hydrocarbon Radical Polymers using Minimized Mixing Nonbonding Molecular Orbitals

A method of analytical prediction (AP) and the corresponding index LAP used for predicting high-spin stability of alternant conjugated hydrocarbon radical polymers were generalized for both non-disjoint closed-type and open-type systems. The general formulas of LAP were derived analytically as the function of the number of unpaired electrons, denoted as N, without performing any localization procedure to attain the minimum mixing between nonbonding molecular orbitals (NBMOs). To examine the reliability of this analytical treatment, the relationship between the LAP and the energy stability in high-spin state was investigated by ab initio method for open-shell systems. It was shown that this approach can be a useful tool for qualitative prediction of ferromagnetism in alternant conjugated hydrocarbon radical polymers. Keywords: Alternant conjugated hydrocarbon, Analytical prediction, Disjoint system, Ferromagnetism, High-spin, Hückel method, LMO, Localization, Molecular orbital method, NBMO method, Non-disjoint system, Polyradical, Quantum chemistry, Radical polymer, ROHF, Unpaired electrons

Electronic structures of WAlOy and WAlOy− (y = 2–4) determined by anion photoelectron spectroscopy and density functional theory calculations

The anion photoelectron spectra of WAlO(y)(-) (y = 2-4) are presented and assigned based on results of density functional theory calculations. The WAlO(2)(-) and WAlO(3)(-) spectra are both broad, with partially resolved vibrational structure. In contrast, the WAlO(4)(-) spectrum features well-resolved vibrational structure with contributions from three modes. There is reasonable agreement between experiment and theory for all oxides, and calculations are in particular validated by the near perfect agreement between the WAlO(4)(-) photoelectron spectrum and a Franck-Condon simulation based on computationally determined spectroscopic parameters. The structures determined from this study suggest strong preferential W-O bond formation, and ionic bonding between Al(+) and WO(y)(-2) for all anions. Neutral species are similarly ionic, with WAlO(2) and WAlO(3) having electronic structure that suggests Al(+) ionically bound to WO(y)(-) and WAlO(4) being described as Al(+2) ionically bound to WO(4)(-2). The doubly-occupied 3sp hybrid orbital localized on the Al center is energetically situated between the bonding O-local molecular orbitals and the anti- or non-bonding W-local molecular orbitals. The structures determined in this study are very similar to structures recently determined for the analogous MoAlO(y)(-)/MoAlO(y) cluster series, with subtle differences found in the electronic structures [S. E. Waller, J. E. Mann, E. Hossain, M. Troyer, and C. C. Jarrold, J. Chem. Phys. 137, 024302 (2012)].

Theoretical investigation of rotationally inelastic collisions of CH2(${\tilde{X}}$X̃) with helium

Following our earlier work on collisions of He with the methylene radical in its excited ã(1)A(1) state [L. Ma, M. H. Alexander, and P. J. Dagdigian, J. Chem. Phys. 134, 154307 (2011)], we investigate here the analogous relaxation of CH(2) in its ground X(3)B(1) electronic state. The molecule is treated as semi-rigid, with fixed bond lengths but a varying bond angle. We use an ab initio potential energy surface (PES) which is averaged over the CH(2) bending angle weighted by the square of the bending wave function. The PES for the interaction of He with CH(2) in the X state is considerably less anisotropic than for interaction with the ã state since the two 2p electrons on the C atom are evenly distributed among the bonding and non-bonding molecular orbitals. We report quantum scattering calculations of state-to-state and total removal cross sections as well as total removal rate constants at room temperature. Because of the less pronounced anisotropy, these cross sections and rate constants are considerably smaller than for collisions of CH(2)(ã) with He. Finally, we investigate the dependence of rotational inelasticity on the bending vibrational quantum number.

Multi-Zinc-Expanded Oligoacenes: An Intriguing Class of Well-Defined Open-Shell Singlet Diradicals

Two classes of multi-Zn-expanded oligoacenes from benzene to pentacene are computationally designed through introducing a Zn array into acene rings in two ways: acene-chain axial versus single-ring quasi-transversal direction. Combined density functional theory, CASSCF, and CCSD calculations predict that all these multi-Zn-expanded oligoacenes have the open-shell singlet diradical ground states, in contrast with the common fact that their parent oligoacenes are closed-shell systems or may have a triplet ground state and only acenes larger than octacene have open-shell singlet diradical ground states. These results offer the first theoretical prediction that the multi-Zn introduction into the acene ring(s), forming the Zn-expanded oligoacenes, can lead them to diradical structures. The diradical character of the ground states of these molecules arises from the Zn-participation-induced disjoint nature of the nonbonding molecular orbitals that are singly occupied in the diradicals. This work provides a strategy to design perfect and stable singlet diradicals from oligoacenes or their derivatives.

High-spin stability of urea dication

High-spin stability of urea dication is analyzed as a trimethylenemethane analogue. MCSCF calculations revealed that the ground-state 3B 2 lies below any other singlet state. The lowest singlet state 1B 1 with a π– n configuration lies 9.74 kcal/mol above the 3B 2 state, and a closed-shell singlet 1A 1 lies far above the 3B 2 state. The high-spin stability is due to pseudo non-bonding molecular orbitals and one-centered exchange integrals on cationic nitrogens. Band structure of polyuret is also presented as a design for novel ferromagnetic polymers.

Communication: A dramatic transition from nonferromagnet to ferromagnet in finite fused-azulene chain

One-dimensional fused-azulene oligomers (n = 2-6) are studied with the effective valence bond as well as density functional theory methods. A nonferromagnetic (closed-shell singlet) to ferromagnetic (triplet) ground state transformation is witnessed with increasing length of oligomers. The computational results are interpreted in terms of spin coupling between the unpaired electrons of two nonbonding molecular orbitals localized, respectively, on the top and bottom chains of the oligomers. The present study provides a theoretical suggestion for understanding the ferromagnetic spin polarizations that has been observed very recently in graphene nanoribbons.