Bond Alternation Research Articles

Molecular Structure of Salicylic Acid and Its Hydrates: A Rotational Spectroscopy Study.

We present a study of salicylic acid and its hydrates, with up to four water molecules, done by employing chirped-pulse Fourier transform microwave spectroscopy. We employed the spectral data set of the parent, 13C, and 2H isotopologues to determine the molecular structure and characterize the intra- and intermolecular interactions of salicylic acid and its monohydrate. Complementary theoretical calculations were done to support the analysis of the experimental results. For the monomer, we analyzed structural properties, such as the angular-group-induced bond alternation (AGIBA) effect. In the microsolvates, we analyzed their main structural features dominated by the interaction of water with the carboxylic acid group. This work contributes to seeding information on how water molecules accumulate around this group. Moreover, we discussed the role of cooperative effects further stabilizing the observed inter- and intramolecular hydrogen bond interactions.

Lithium Salt of 2,5-Bis(trimethylsilyl)stannolyl Anion: Synthesis, Structure, and Nonaromatic Character

The aromatic character of silolyl and germolyl anions markedly depends on the substituents in the 2,5-positions; carbon-substituted derivatives are nonaromatic, whereas silyl-substituted ones tend to exhibit an aromatic character. However, only carbon-substituted derivatives have been reported for stannolyl anions. In this study, we present the synthesis and structure of a 2,5-disilylated stannolyl anion. Transmetalation of a 2,5-disilyl-1-zirconacyclopentadiene with SnCl4 gave a dichlorostannole 1, which reacted with potassium tris(trimethylsilyl)silanide to introduce a bulky silyl group on the tin atom. Reduction of the 1-chloro-1-silylstannole 2 with lithium generated the lithium salt of the desired stannolyl anion 3 that adopts an η1-coordination to the lithium atom. We concluded that the stannolyl anion 3 is nonaromatic based on the pyramidalized tin center and the C–C bond alternation in the five-membered ring as well as the NMR properties.

Theoretical design of durable and strong polycarbonates against photodegradation.

The photodegradation mechanism of polycarbonate (PC) was investigated by quantum chemistry, and a novel antidegradation molecular design using substituents was proposed. It was demonstrated that electron-withdrawing substituents in the phenyl moiety controlled bond alternation, leading to inhibition of the O-C bond cleavage in the carbonate moiety. These results provide a promising alternative for durable PC synthesis.

Vis-NIR-Electrochromic Interconversion of Dithienylmethylium-Type Cyanine Dyes with a σ-Bonded Dimer or Isolable Neutral Radical

Abstract 4-Methoxyphenyl-substituted di(2-thienyl)methyliums were designed and synthesized as a new class of cationic dyes exhibiting absorption in the visible (Vis) and near-infrared (NIR) region. Their cyanine-type π-conjugation was confirmed by alternation of positive charges along the polymethine chain as well as by a significant disappearance of bond alternation. Upon one-electron reduction of these cations, neutral radicals were generated, which could be isolated as stable entities when an anthryl-type substituent was attached to the central methine carbon while the secondary radical without any substituent quantitatively converted to a σ-bonded dimer. Since both the stable radical and the dimer regenerated the starting cyanine dyes upon oxidation, the present redox pairs can serve as electrochromic materials that exhibit a change in absorption in the Vis-NIR region.

Selective control of donor-acceptor Stenhouse adduct populations with non-selective stimuli

Selective control of donor-acceptor Stenhouse adduct populations with non-selective stimuli

Crystal growth and magnetic properties of the coupled alternating S=1 spin chain Sr2Ni(SeO3)3

The structural, magnetic, and thermodynamic properties of a quasi-one-dimensional (1D) $S=1$ alternating spin chain compound ${\mathrm{Sr}}_{2}\mathrm{Ni}{(\mathrm{Se}{\mathrm{O}}_{3})}_{3}$ are investigated by using synchrotron x-ray powder diffraction, magnetic susceptibility $\ensuremath{\chi}(H,T)$, and heat capacity ${C}_{\mathrm{P}}(H,T)$ measurements together with density functional theory (DFT) calculations. The $\ensuremath{\chi}(H,T)$ and ${C}_{\mathrm{P}}(H,T)$ data reveal long-range antiferromagnetic order at ${T}_{\mathrm{N}}=3.4(3)$ K and short-range order at ${T}_{\mathrm{m}}\ensuremath{\approx}7.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The short-range magnetic order together with 95% of spin entropy release above ${T}_{\mathrm{N}}$ signifies the importance of 1D spin correlations persisting to $\ensuremath{\sim}8{T}_{\mathrm{N}}$. Theoretical DFT calculations with generalized gradient approximation determine leading exchange interactions, suggesting that interchain interactions are responsible for the observed long-range magnetic ordering. In addition, the temperature-field phase diagram of ${\mathrm{Sr}}_{2}\mathrm{Ni}{(\mathrm{Se}{\mathrm{O}}_{3})}_{3}$ is determined based on the $\ensuremath{\chi}(T,H)$ and ${C}_{\mathrm{P}}(T,H)$ data. Interestingly, a nonmonotonic phase boundary of ${T}_{\mathrm{m}}$ is found for an external field applied along a hard axis. Our results suggest that the ground state and magnetic behavior of ${\mathrm{Sr}}_{2}\mathrm{Ni}{(\mathrm{Se}{\mathrm{O}}_{3})}_{3}$ rely on the interplay of single-ion anisotropy, bond alternation, and interchain interactions.

Frontier Orbital Views of Stacked Aromaticity.

Recent studies have theoretically and experimentally demonstrated that antiaromatic molecules with 4n π electrons exhibit stacked aromaticity according to π-π stacking when arranged in a face-to-face manner. However, the mechanism of its occurrence has not been clearly studied. In this study, we investigated the mechanism of stacked aromaticity using cyclobutadiene. When the antiaromatic molecules are stacked in a face-to-face manner, the orbital interactions between the degenerate singly occupied molecular orbitals (SOMOs) of the monomer unit cause a larger energy gap between the degenerate highest-occupied molecular orbitals (HOMOs) and the lowest-unoccupied molecular orbitals (LUMOs) of the dimer. However, the antiaromatic molecules are more stable in less symmetric conformations, mainly because of pseudo-Jahn-Teller distortions. In the case of cyclobutadiene, the two SOMOs of the monomer unit split into HOMO and LUMO because of the bond alternation. When the molecules are stacked in a face-to-face manner, the HOMO-LUMO gap of the dimer is smaller than that of the monomer due to the interactions between the HOMOs and LUMOs of the two monomer units. When the monomer units are within a specific distance of each other, the HOMO and LUMO of the dimer, which correspond to antibonding and bonding between the units, respectively, are interchanged. This alternation of molecular orbitals may result in an increase in the bond strength between the monomer units, exhibiting stacked aromaticity. We demonstrated that it is possible to control the distance exhibited by stacked aromaticity by engineering the HOMO-LUMO gap of the monomer units.

Topological Impact of Delocalization on the Stability and Band Gap of Partially Oxidized Graphene.

Strategic perturbations on the graphene framework to inflict a tunable energy band gap promises intelligent electronics that are smaller, faster, flexible, and much more efficient than silicon. Despite different chemical schemes, a clear scalable strategy for micromanaging the band gap is lagging. Since conductivity arises from the delocalized π-electrons, chemical intuition suggests that selective saturation of some sp2 carbons will allow strategic control over the band gap. However, the logical cognition of different 2D π-delocalization topologies is complex. Their impact on the thermodynamic stability and band gap remains unknown. Using partially oxidized graphene with its facile and reversible epoxides, we show that delocalization overwhelmingly influences the nature of the frontier bands. Organic electronic effects like hyperconjugation, conjugation, aromaticity, etc. can be used effectively to understand the impact of delocalization. By keeping a constant C4O stoichiometry, the relative stability of various π-delocalization topologies is directly assessed without resorting to resonance energy concepts. Our results demonstrate that >C=C< and aromatic sextets are the two fundamental blocks resulting in a large band gap in isolation. Extending the delocalization across these units will increase the stability at the expense of the band gap. The band gap is directly related to the extent of bond alternation within the π-framework, with forced single/double bonds causing the large gap. Furthermore, it also establishes the ground rules for the thermodynamic stability associated with the π-delocalization in 2D systems. We anticipate that our findings will provide the heuristic guidance for designing partially saturated graphene with the desired band gap and stability using chemical intuition.

Spin-singlet ground state of the coupled Jeff = 12 alternating chain system Sr2Co(SeO3)3

We report a detailed study of the structural, magnetic, thermodynamic, and electronic properties of a coupled ${J}_{\mathrm{eff}}$ = $\frac{1}{2}$ alternating chain ${\mathrm{Sr}}_{2}\mathrm{Co}{({\mathrm{SeO}}_{3})}_{3}$ compound using magnetic susceptibility $\ensuremath{\chi}(T)$, magnetic specific heat ${C}_{\mathrm{m}}(T)$, magnetization, and neutron diffraction measurements along with first-principles calculations. The first-principles calculations based on the density functional theory suggest that ${\mathrm{Sr}}_{2}\mathrm{Co}{({\mathrm{SeO}}_{3})}_{3}$ forms a quasi-one-dimensional chain with bond alternation and interchain interactions. $\ensuremath{\chi}(T), {C}_{\mathrm{m}}(T),$ and neutron powder diffraction measurements confirm that no long-range magnetic ordering occurs down to 100 mK. Instead, a maximum in $\ensuremath{\chi}(T)$ and ${C}_{\mathrm{m}}(T)$ and an exponential drop of $\ensuremath{\chi}(T)$ and ${C}_{p}(T)$ as $T\ensuremath{\rightarrow}0$ K point to a spin-singlet ground state. The analysis of $\ensuremath{\chi}(T)$ and ${C}_{\mathrm{m}}(T)$ based on a ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ alternating Heisenberg model shows the bond alternation $\ensuremath{\alpha}={J}_{2}/{J}_{1}\ensuremath{\approx}0.7$ and a spin gap of $\mathrm{\ensuremath{\Delta}}\ensuremath{\approx}3$ K. Our work demonstrates that ${\mathrm{Sr}}_{2}\mathrm{Co}{({\mathrm{SeO}}_{3})}_{3}$ is a coupled alternating chain system based on spin-orbit entangled ${J}_{\mathrm{eff}}$ = $\frac{1}{2}$.

Synthesis, Structures, and Properties of Tropone-Fused Coumarin Dyes

Abstract A nonbenzenoid aromatic ring, tropone-fused 7-methoxycoumarin dye (1a) and its chlorinated product (1b) were designed and successfully synthesized via transannulation of a coumarin derivative and dehydrobromination of the dibromo- and tribromoprecursors. X-ray crystallography revealed that 1a and 1b are distorted at the annulated tropone skeleton with bond alternation, unlike typical 3,4-annulated π-extended coumarin dyes. These tropone-fused coumarin dyes exhibited redshifted absorption and emission bands compared with that of planar 3,4-benzo-7-methoxycoumarin. These compounds demonstrated relatively efficient fluorescence emission (ΦF = 2–10%) despite their curved skeletons, and solvent dependency owing to the intramolecular charge transfer in the excited state. A computational study clarified that compound 1 changes to a flat structure including the tropylium unit in the excited state. Furthermore, in the fluorescence spectra of 1a with trifluoroacetic acid, the emission intensity increased dramatically to ΦF = 19% owing to the formation of a charge-separated structure with the tropylium cation.

The competitive mechanism between photoisomerization and excited state intramolecular proton transfer process of 2′-Hydroxychalcone system

The photoisomerization and excited state intramolecular proton transfer (ESIPT) are the most common processes in molecular photoswitches. Their coexistence and competing relationship in molecules usually make the deactivation process of the molecules become complex. In the current work, we study the underlying deactivation mechanism of trans-form 2′-hydroxychalcone (2′HC), which possesses both photoisomerization and ESIPT in the excited state, by combining high-level electronic structure calculations and on-the-fly surface hopping dynamics simulations. Four minimum energy conical intersections (MECIs) are found to be involved in the whole deactivation process. Following excitation to the S3 state of the most stable trans-enol tautomer, a strong coupling from the ππ* onto the nπ* state facilitates the ultrafast S3/S1 internal conversion process through an intricate conical intersection seam among S1, S2 and S3 state. The subsequent S1 to S0 state relaxation pathway is divided into two branches. The first process is initiated by twisting motion of C8─C9 bridging bond, which decays directly to the ground state via enol MECI. The other one is triggered by an ultrafast ESIPT process firstly and followed by twisting motion of the C1─C7 bond, leading the system funnel to the ground state by a keto type MECI, and finally generates the precursor of coloured flavanone. The proposed S3→(S2) S1 → S0 two step decay pattern is consistent with previous experimental observations. At the molecular microscopic level, the CO bond alternation motion plays a vital role in the whole relaxation process. Our results indicate that further practical application of 2′HC as molecular photoswitches can be achieved by modifying the molecular structure to enhance the ESIPT process.

S-Shaped Fused Azacorannulene Dimer: Structural and Redox Properties

S-Shaped Fused Azacorannulene Dimer: Structural and Redox Properties

Hedgehog lattice and field-induced chirality in breathing-pyrochlore Heisenberg antiferromagnets

We theoretically investigate a ${J}_{1}\text{\ensuremath{-}}{J}_{3}$ classical Heisenberg model on the breathing pyrochlore lattice, where the nearest-neighbor (NN) exchange interactions for small and large tetrahedra, ${J}_{1}$ and ${J}_{1}^{\ensuremath{'}}$, take different values due to the breathing bond alternation and ${J}_{3}$ is the third NN antiferromagnetic interaction along the bond direction. It is found by means of Monte Carlo simulations that for large ${J}_{3}$, a hedgehog lattice, a three-dimensional periodic array of magnetic monopoles and antimonopoles, emerges in the form of a quadruple-$\mathbf{Q}$ state characterized by the ordering vector of $\mathbf{Q}=(\ifmmode\pm\else\textpm\fi{}\frac{1}{2},\ifmmode\pm\else\textpm\fi{}\frac{1}{2},\ifmmode\pm\else\textpm\fi{}\frac{1}{2})$, being irrespective of the signs of ${J}_{1}$ and/or ${J}_{1}^{\ensuremath{'}}$ as long as ${J}_{1}\ensuremath{\ne}{J}_{1}^{\ensuremath{'}}$. It is also found that in an applied magnetic field, there appear six quadruple-$\mathbf{Q}$ states depending on the values of ${J}_{1}$ and ${J}_{1}^{\ensuremath{'}}$, among which three phases including the in-field hedgehog-lattice state exhibit nonzero total chirality ${\mathbit{\ensuremath{\chi}}}^{\mathrm{T}}$ associated with the anomalous Hall effect of chirality origin. In the remaining two chiral phases, which are realized in the presence of ferromagnetic ${J}_{1}$ and/or ${J}_{1}^{\ensuremath{'}}$, the spin structure is not topologically nontrivial, in spite of the fact that ${\mathbit{\ensuremath{\chi}}}^{\mathrm{T}}\ensuremath{\ne}0$. The role of the topological objects of the monopoles in ${\mathbit{\ensuremath{\chi}}}^{\mathrm{T}}$ is also discussed.

Controlled modification of polyvinylidene fluoride as a way for carbyne synthesis

In the present work, the possibility of carbyne synthesis by modifying polyvinylidene fluoride (PVDF) with synchrotron radiation was investigated for the first time. The analysis of the kinetic dependences of the CF2 groups' loss in PVDF was done, and the mechanism of dehydrofluorination of PVDF under the action of monochromatic X-ray radiation of various energies was elucidated. As a result, the two-stage model for the process of PVDF carbonization was proposed. According to it, first one fluorine atom and one hydrogen atom are split off and removed in the form of an HF molecule with the formation of a double carbon-carbon bonds (CFCH). Second, the formation of carbyne occurs, in which carbon atoms are connected in a chain by means of either double carbon-carbon bonds (CC) or alternation of carbon single and triple bonds (CC). A joint analysis by high-resolution near-edge X-ray absorption fine structure (NEXAFS) spectra and X-ray photoelectron spectra made it possible to put forward a hypothesis that, by irradiating the polymer with photons of monochromatic X-ray radiation with a given energy, it is possible to achieve the formation of uniform layers of amorphous carbyne of various depths, which offers a challenge of producing a next generation of devices for carbon and 1D nanoelectronics.

Extraordinary mechanical performance in charged carbyne

Carbyne, the linear chain of carbon, promises the strongest and toughest material but possesses a Peierls instability (alternating single-bonds and triple-bonds) that reduces its strength and toughness. Herein, we computationally found that the gravimetric strength, strain-to-failure, and gravimetric toughness can be improved from 74 GPa⋅g−1⋅cm3, 18%, and 9.4 kJ⋅g−1 for pristine carbyne to the highest values of 106 GPa⋅g−1⋅cm3, 26%, and 19.0 kJ⋅g−1 for carbyne upon hole injection of +0.07 e/atom, indicating the charged carbyne with record-breaking mechanical performance. Based on the analyses of the atomic and electronic structures, the underlying mechanism behind the record-breaking mechanical performance was revealed as the suppressed and even eliminated bond alternation of carbyne upon charge injection.

Synthesis, Reactivity, and Properties of Benz[a]azulenes via the [8 + 2] Cycloaddition of 2H-Cyclohepta[b]furan-2-ones with an Enamine.

Starting with the reaction of 2H-cyclohepta[b]furan-2-ones with an enamine, which was prepared from 4-tert-butylcyclohexanone and pyrrolidine, benz[a]azulenes having both formyl and tert-butyl groups were obtained in the three-step sequence. Subsequently, both the formyl and tert-butyl groups were eliminated by heating the benz[a]azulene derivatives in 100% H3PO4 to give benz[a]azulenes without these substituents in high yields. In terms of product yield, this method is the best one ever reported for the synthesis of the parent benz[a]azulene so far. The conversion of the benz[a]azulene derivatives with a formyl group into cyclohept[a]acenaphthylen-3-one derivatives was also investigated via Knoevenagel condensation with dimethyl malonate, followed by Brønsted acid-mediated intramolecular cyclization. The structural features including the bond alternation in the benz[a]azulene derivatives were revealed by NMR studies, NICS calculations, and a single-crystal X-ray structural analysis. The optical and electrochemical properties of a series of benz[a]azulene derivatives were evaluated by UV/Vis, fluorescence spectroscopy, and voltammetry experiments. As a result, we found that some benz[a]azulene derivatives showed remarkable luminescence in acidic media. In addition, the benz[a]azulene derivatives with the electron-withdrawing group and cyclohept[a]acenaphthylen-3-one derivative displayed good reversibility in the spectral changes under the electrochemical redox conditions.

Symmetry analysis of bond-alternating Kitaev spin chains and ladders

In this work, we analyze the nonsymmorphic symmetry-group structures for a variety of generalized Kitaev spin chains and ladders with bond alternations, including Kitaev-Gamma chain, Kitaev-Heisenberg-Gamma chain, beyond-nearest-neighbor interactions, and two-leg spin ladders. The symmetry analysis is applied to determine the symmetry-breaking patterns of several magnetically ordered phases in the bond-alternating Kitaev-Gamma spin chains, as well as the dimerization order parameters for spontaneous dimerizations. Our work is useful in understanding the magnetic phases in related models and may provide guidance for the symmetry classifications of mean field solutions in further investigations.

Emergent skyrmion-based chiral order in zero-field Heisenberg antiferromagnets on the breathing kagome lattice

We show that classical Heisenberg antiferromagnets on the breathing kagome lattice can be a platform to realize a zero-field topological order of the scalar spin chirality which can be viewed as a miniature skyrmion crystal (SkX) of discrete form with a small number of spins in its magnetic unit cell. In the model, a third nearest-neighbor (NN) antiferromagnetic interaction along the bond direction ${J}_{3}$ and the breathing bond alternation characterized by the ratio of the NN interaction for large triangles to that for small ones, ${J}_{1}^{\ensuremath{'}}/{J}_{1}$, are essential. It is found by means of Monte Carlo simulations that a commensurate triple-$\mathbf{Q}$ state appearing for relatively strong ${J}_{3}$ at zero field is the noncoplanar state with the SkX structure in the breathing case of ${J}_{1}^{\ensuremath{'}}/{J}_{1}\ensuremath{\ne}1$, while in the uniform case of ${J}_{1}^{\ensuremath{'}}/{J}_{1}=1$, it is a collinear state favored by thermal fluctuations. The origin of this chiral order and experimental implications of our result are also discussed.

First Order Transitions Between the Gapped Spin-Liquid and Ferrimagnetic Phases in \((1/2,1/2,1)\) Mixed Diamond Chains with Bond Alternation

The ground-state phases of mixed diamond chains with bond alternation $\delta$, and ($S, \tau^{(1)}, \tau^{(2)})=(1/2,1/2,1)$, where $S$ is the magnitude of vertex spins, and $\tau^{(1)}$ and $\tau^{(2)}$ are those of apical spins, are investigated. The two apical spins in each unit cell are connected by an exchange coupling $\lambda$. The exchange couplings between the apical spins and the vertex spins take the values $1+\delta$ and $1-\delta$ alternatingly. This model has an infinite number of local conservation laws. For large $\lambda$ and $\delta \neq 0$, the ground state is equivalent to that of the spin $1/2$ chain with bond alternation. Hence, the ground state is a gapped spin liquid. This energy gap vanishes for $\delta=0$. With the decrease of $\lambda$, the ground state undergoes a transition at $\lambda=\lambda_{\rm c0}(\delta)$ to a series of ferrimagnetic phases with a spontaneous magnetization $m_{\rm sp}=1/p$ per unit cell where $p$ is a positive integer. It is found that this transition is a first order transition for $\delta\neq 0$ with a discontinuous change in $m_{\rm sp}$, while no discontinuity is found for $\delta=0$. The critical behaviors of $m_{\rm sp}$ and $\lambda_{\rm c0}(\delta)$ around the critical point $(\delta,\lambda) =(0, \lambda_{\rm c0}(\delta))$ are also discussed analytically.

Bond Length Alternation and Internal Dynamics in Model Aromatic Substituents of Lignin.

Broadband microwave spectra were recorded over the 2-18 GHz frequency range for a series of four model aromatic components of lignin; namely, guaiacol (ortho-methoxy phenol, G), syringol (2,6-dimethoxy phenol, S), 4-methyl guaiacol (MG), and 4-vinyl guaiacol (VG), under jet-cooled conditions in the gas phase. Using a combination of 13 C isotopic data and electronic structure calculations, distortions of the phenyl ring by the substituents on the ring are identified. In all four molecules, the rC(1)-C(6) bond between the two substituted C-atoms lengthens, leading to clear bond alternation that reflects an increase in the phenyl ring resonance structure with double bonds at rC(1)-C(2) , rC(3)-C(4) and rC(5)-C(6) . Syringol, with its symmetric methoxy substituents, possesses a microwave spectrum with tunneling doublets in the a-type transitions associated with H-atom tunneling. These splittings were fit to determine a barrier to hindered rotation of the OH group of 1975 cm-1 , a value nearly 50 % greater than that in phenol, due to the presence of the intramolecular OH⋅⋅⋅OCH3 H-bonds at the two equivalent planar geometries. In 4-methyl guaiacol, methyl rotor splittings are observed and used to confirm and refine an earlier measurement of the three-fold barrier V3 =67 cm-1 . Finally, 4-vinyl guaiacol shows transitions due to two conformers differing in the relative orientations of the vinyl and OH groups.