Search

(E)-6-Methoxy-1-methyl-3-(2-(3-methyl-4-nitroisoxazol-5-yl)vinyl)quinolin-2(1H)-one dye (MQI) has been synthesized, and its structural and electronic properties have been characterized by employing UV-vis spectroscopy in combination with computational methods. The MQI dye has been assessed as an activated Michael acceptor-type probe toward bisulfite ions. This reaction was kinetically tested in different mild, cationic (cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, and cetylpyridinium bromide), and zwitterionic (N-decyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, and N-hexacyl-N,N-dimethyl-3-ammonio-1-propanesulfonate) micellar solutions at pH ∼ 5.5. Both micellar media remarkably allow the addition reactions, increasing the reactivity of MQI towards bisulfite ions, the biggest effects were found in the presence of cationic micelles. The binding constants of MQI with the micelles and the rate constants were determined from kinetic data, which were interpreted on the basis of the pseudophase kinetic model. The kinetic study and the product analysis allow us to highlight the relevant role of the association between MQI dye and the micellized surfactant, allowing efficient nucleophilic addition of bisulfite ions. The findings of this work will be valuable for the use of micellar solutions as an alternative medium to replace the use of toxic solvents to carry out organic reactions to perform nucleophilic addition reactions of bisulfite. [ABSTRACT FROM AUTHOR]

This study examines systems containing planar tetracoordinate group 13–15 atoms (E) within pentagonal C4H2E rings bridged by Si or Ge atoms. A detailed chemical bonding analysis of eleven candidates shows that true tetracoordination is achieved only in the C4H2NGe2+ system. [ABSTRACT FROM AUTHOR]

In this study, we delved into the structure of B5H5 and questioned some of its accepted assumptions. By exploring the potential energy surface, we found a new three-dimensional structure as the global minimum. This finding is in contrast with the previously hypothesized planar and cage-like models. Our exploration extends to the kinetic stability of various B5H5 isomers, offering insights into the dynamic behavior of these molecules. [ABSTRACT FROM AUTHOR]

Using various exploration strategies, in this study, we investigated the potential energy surfaces (PES) of CBe5H5+ and CnBe3n+2H2n+22+ (n = 2–4) clusters. Previous studies proposed that the planar pentacoordinate carbons (ppCs) were the global minima of these clusters. However, our study identified new putative global minima and competitive isomers, refuting some previous assignments. We employed several methods, including evolutive-inspired stochastic approaches guided by "chemical criteria", and ab initio molecular dynamics simulations at elevated temperatures. Our results showed that the size of the scanned population significantly affected the evolutive method and that constrained or guided procedures showed an advantage in identifying better minima for larger systems. This study demonstrated that using multiple complementary strategies can result in a wider variety of minima in a given energy range. Our findings provide valuable insights into exploring the potential energy surfaces of clusters, mainly medium-sized clusters, which could be the connections between small clusters and nanomaterials. [ABSTRACT FROM AUTHOR]

A systematic exploration of the potential energy surface reveals two global minima with three planar tetra coordinate carbons (ptCs) and two global minima with three quasi-ptCs for E6C15 (E = Si–Pb) combinations. These consist of aromatic polycyclic templates suitable for further design of different materials without hindering the ptC texture. [ABSTRACT FROM AUTHOR]

Though search algorithms are appropriate tools for identifying low-energy isomers, fixing several constraints seems to be a fundamental prerequisite to successfully running any structural search program. This causes some potential setbacks as far as identifying all possible isomers, close to the lowest-energy isomer, for any elemental composition. The number of explored candidates, the choice of method, basis set, and availability of CPU time needed to analyze the various initial test structures become necessary restrictions in resolving the issues of structural isomerism reasonably. While one could arrive at new structures through chemical intuition, reproducing or achieving those exact same structures requires increasing the number of variables in any given program, which causes further constraints in exploring the potential energy surface in a reasonable amount of time. Thus, it is emphasized here that an integrated approach between search algorithms and chemical intuition is necessary by taking the C12O2Mg2 system as an example. Our initial search through the AUTOMATON program yielded 1450 different geometries. However, through chemical intuition, we found eighteen new geometries within 40.0 kcal mol−1 at the PBE0-D3/def2-TZVP level. These results indirectly emphasize that an integrated approach between search algorithms and chemical intuition is necessary to further our knowledge in chemical space for any given elemental composition. [ABSTRACT FROM AUTHOR]

Here, we analyze the possibility of predicting local and global current densities in a series of bicyclic hydrocarbons with 4n and 4n + 2 π-electrons from the nucleus-independent chemical shifts (NICS) computations. Our results show that although it is possible to establish the net current flows through C–C bonds, it is impossible to predict all the local and global weak current density flows. Thus, for polycyclic systems, it is essential to analyze magnetically induced current densities in conjunction with NICS measurements, or with induced magnetic fields, to have an adequate representation of the molecules' magnetic behavior and their relationship with the aromaticity phenomenon. [ABSTRACT FROM AUTHOR]

Previous experimental results on the free radical-scavenging properties of boldine, one of the most potent natural antioxidants, suggested that the benzylic hydrogen, neighboring a nitrogen lone electron pair, might be the key to its protective and antioxidant effect, in contrast to the usually assumed labile phenolic hydrogen atoms. Our computations suggest that both boldine and glaucine can undergo two successive hydrogen transfers under attack by an oxidizing radical (from C-6a-H and C-7-H). The hydroxyl groups of boldine could also participate via the HAT mechanism, although to a lesser extent. In the case of quaternary N-methylated derivatives, the sharing of the nitrogen lone pair makes the transfer of the C-6a hydrogen energetically unfeasible, supporting the earlier proposal based on experimental evidence. The hydroxyl groups of boldine and its derivatives could contribute to antioxidant activity via the sequential proton loss electron transfer mechanism, although this would only be relevant at higher pH values. [ABSTRACT FROM AUTHOR]

The relativistic effects on the aromaticity of a set of benzene analogues, E3M3H3 (E = C–Pb; M = N–Bi) heterocycles, using magnetically induced current density (MICD) and the NICSzz component of the conventional nucleus independent chemical shift (NICS), is hereby examined. The relativistic effects were evaluated by means of four-component relativistic MICD, and two-component NMR relativistic shielding tensor methods. MICD and NICS were also computed in a non-relativistic fashion, to assess the influence of scalar-relativistic and spin–orbit effects. Most of the studied compounds exhibit a net diatropic ring current (aromatic), excluding the nitrogen-containing compounds which are non-aromatic (except for C3N3H3), in agreement with their higher E–N electronegativity difference. Only in the case of bismuth compounds, E3Bi3H3, aromaticity is substantially decreased when relativistic effects are included (mainly due to the spin–orbit contribution). The larger the mass of the system, the larger the magnitude of this change, in line with the expected relativistic effects for heavier elements. The analysis based on the NICSzz computations agrees with that of the MICD, thus supporting both the magnetic behavior and the aromatic character of these compounds. [ABSTRACT FROM AUTHOR]

The aromaticity in porphyrinoids results from the π conjugation through two different annular perimeters: the macrocyclic ring and the local heterocyclic rings appended to it. Analyses, based on aromatic stabilization energies (ASE), indicate that the local circuits (6π) are responsible for the significant aromatic stabilization of these systems. This local aromaticity can be coupled with the one from 4n + 2π macrocyclic circuit. It can either compensate for the destabilization due to a 4n π macrocyclic circuit, or be the only source of aromatic stabilization in porphyrinoids with macrocycles without π-conjugated bonds. This "multifaceted" aromatic character of porphyrinoids makes it challenging to analyze their aromaticity using magnetic descriptors because of the intricate interaction of local versus macro-cyclic circulation. In this contribution, we show that the analysis of the bifurcation of the induced magnetic field, Bind, allows clear identification and quantification of both local, and macrocyclic aromaticity, in a representative group of porphyrinioids. In porphyrin, bifurcation values accurately predict the local and macrocyclic contribution rate to overall aromatic stabilization determined by ASE. [ABSTRACT FROM AUTHOR]

The aromaticity and dynamics of a set of recently proposed neutral 5- and 6-membered heterocycles that are closed by dative (donor–acceptor) or multi-center σ bonds, and have resonance forms with a Hückel number of π-electrons, are examined. The donors and acceptors in the rings include N, O, and F, and B, Be, and Mg, respectively. The planar geometry of the rings, coupled with evidence from different measures of aromaticity, namely the NICSzz, and NICSπzz components of the conventional nucleus independent chemical shifts (NICS), and ring current strengths (RCS), indicate non-trivial degrees of aromaticity in certain cases, including the cyclic C3B2OH6 and C3BOH5 isomers, both with three bonds to the O site in the ring. The former is lower in energy by at least 17.6 kcal mol−1 relative to linear alternatives obtained from molecular dynamics simulations in this work. Some of the other systems examined are best described as non-aromatic. Ring opening, closing, and isomerization are observed in molecular dynamics simulations for some of the systems studied. In a few cases, the ring indeed persists. [ABSTRACT FROM AUTHOR]

A simple and chemically intuitive approach is used to design ptC-containing E–C clusters (E = Si–Pb). This approach consists in replacing three or two consecutive protons from an aromatic hydrocarbon by one E24+ or one E2+ fragment, respectively. In the model, electrons from E are removed from the pz orbitals, emptying them. Si–Pb favors the formation of a 3c-2e (E–C–E) σ-bond, which involves the ptC. Additionally, the π-electronic cloud is delocalized through the E-pz orbitals allowing the E atoms to effectively take part in the electronic delocalization, preserving the 4n + 2 Hückel's rule from the parent hydrocarbon. Two aromatic monocycles and one aromatic bicycle – benzene (C6H6), cyclopentadienyl anion (C5H5−) and pentalene dianion (C8H62−) – have been transformed into C–E systems. After an extensive exploration of their potential energy surfaces, four new global minima with ptC are identified, resulting from the substitution of the protons by Si and Ge cations in C5H5− and C8H62− (E3C5 and E4C8). The analysis of both the chemical bonding and the magnetic response to an external magnetic field confirms the aromatic character of these species. [ABSTRACT FROM AUTHOR]

Isomers of (NaCl)n(H2O)2n clusters, with n = 1, 2 and 4, were investigated using an implementation of the simulated annealing method that imposes a virtual confinement on each isomer to stay within a sphere with an arbitrary radius. By using several confinement radii, we reported 2, 56 and 38 isomers for n = 1, 2 and 4, respectively. For n = 1, the CCSD(T)/aug-cc-pVTZ method was used as a reference by optimizing the corresponding isomers. These results were contrasted with those obtained by the MP2 method and eight exchange–correlation functionals: PBE, PBE0, B3LYP, PBE-D3, PBE0-D3, B3LYP-D3, LC-̉PBE and M06-2X. From this analysis we conclude that the M06-2X method gives reliable results and then, isomers with n = 2 and 4 were analyzed with this exchange–correlation functional. From the analysis of these two systems, the resulting isomers with the lowest energy showed cuboid structures and no breaking bond was present for NaCl molecules. Thus, in the nucleation process of the NaCl dihydrate our results suggest that the Na–Cl bond is present. Besides, bond length and electron density evaluated at bond critical points for the biggest system delivered values close to those found in the crystal structure. [ABSTRACT FROM AUTHOR]

The objective of the current work is to evaluate the effectiveness of the isoelectronic substitution (IS) principle on a series of complexes with the general formula OsCl2(SX3H3)(PH3)2, where X3 represents the moieties CCC, CCB, CCN, CBN, CNB or NCB, formed by substitution of the carbon atoms in CCC by either the isoelectronic B− or N+ separately, or by both. The SX3H3 moiety forms, together with Os, an aromatic five-membered ring (5-MR) called osmathiophene. The preservation of stability and aromaticity in the resulting systems is used to indicate the effectiveness of the IS principle. The aromaticity of the proposed molecules is analyzed according to the magnetic (induced magnetic field (Bind)) and electronic (through the multicenter index (MCI)) criteria. In addition a chemical bonding analysis on selected species is performed by the adaptive natural density partitioning (AdNDP) method. [ABSTRACT FROM AUTHOR]

In this paper, we provide solid evidence to show that among an overwhelming structural diversity, alkaline earth metals (Ca, Sr, Ba) have the ability to form inverted sandwich compounds with C6H6, C7H7+, and C8H82+ of Dnh symmetry and general formula [E–(CnHn)–E]n−6 (n = 6–8) with planar 10-π-electron aromatic cores by virtue of transferring two electrons per metal atom to the ring. However, the origin of the orbital interaction between the metals and the carbon ring is quite different; while [E–(C6H6)–E] complexes are dominated by δ-interactions, both π- and δ-interactions are important in [E–(C7H7)–E]+ and [E–(C8H8)–E]2+ complexes. [ABSTRACT FROM AUTHOR]

The stability of the LinBnH2n (n = 3–6) series was analyzed using quantum chemical calculations, and it was found that cyclic isomers are not energetically favored. This is different to what happens in their organic counterparts (CnH2n), where cyclopentane (C5H10) and cyclohexane (C6H12) are the low-lying isomers. Apparently, aromaticity is a key-stabilizing factor that needs to be considered for designing stable lithium-boron hydride analogues of cyclic organic compounds. This is verified in the Li3B3H3+ system, which has been designed as the smallest aromatic carbocation (C3H3+) analogue. The global minimum structure of Li3B3H3+ contains a triangular B3H32− moiety, which has structural and chemical bonding features similar to its organic counterpart. Besides, this new cluster is classified as aromatic according to both the 4n + 2 Hückel rule and the analysis of the induced magnetic field. This theoretical evidence leads us to propose this cluster as a viable target for experimental detection in the gas phase. [ABSTRACT FROM AUTHOR]

The potential energy surfaces (PESs) of Lin(BH)5n−6 systems (where n = 5, 6, and 7) were explored using the gradient embedded genetic algorithm (GEGA) program, in order to find their global minima conformations. This search predicts that the lowest-energy isomers of Li6(BH)5 and Li7(BH)5+ contain a (BH)56− pentagonal fragment, which is isoelectronic and structurally analogous to the prototypical aromatic hydrocarbon anion C5H5−. Li7(BH)5+, along with Li7C5+, Li7Si5+ and Li7Ge5+, joins a select group of clusters that adopt a seven-peak star-shape geometry, which is favored by aromaticity in the central five-membered ring, and by the preference of Li atoms for bridging positions. The theoretical analysis of chemical bonding, based on magnetic criteria, supports the notion that electronic delocalization is an important stabilization factor in all these star-shaped clusters. [ABSTRACT FROM AUTHOR]

We have explored in silicothe potential energy surfaces of the C5Linn−6(n= 5, 6, and 7) clusters using the Gradient Embedded Genetic Algorithm (GEGA) and other computational strategies. The most stable forms of C5Li5−and C5Li6are two carbon chains linked by two lithium atoms in a persistent seven membered ring capped by two Li atoms. The other Li atoms are arrayed on the edge of the seven membered ring. In contrast, the global minimum structure for C5Li7+is a bicapped star of D5hsymmetry. The molecular orbital analysis and computed magnetic field data suggest that electron delocalization, as well as the saturation of the apical positions of the five-membered carbon ring with lithium atoms in C5Li7+plays a key role in the stabilization of the carbon-lithium star. In fact, the planar star sub-structure for the carbon ring are unstable without the apical caps. This is also what has been found for the Si analogues. The split of the Bindzin its σ- and π-contribution indicates that C5Li7+is a π-aromatic and σ-nonaromatic system. [ABSTRACT FROM AUTHOR]

The B19- anion and other boron species have been dubbed 'Wankel motors' for the almost barrierless rotation of inner and outer concentric rings relative to each other in these compounds. A single substitution in B19- is shown to shut down the well-established fluxionality in the anion. A carbon atom substituted in the structure to give a neutral CB18 species is shown computationally to enforce bond localization. [ABSTRACT FROM AUTHOR]

The effectiveness of our proposed approach to stabilize a planar tetracoordinate carbon (ptC) in cyclic aromatic hydrocarbons, introduced in the title article, is unquestionable as our exhaustive searches on the singlet and triplet potential energy surfaces of the new ptC molecules identified as viable species are reproducible. Besides, the T1 diagnostic value for the Si2C5H2 system reported in the comment seems to be the T1 amplitudes. We recomputed the T1 diagnostic value using different software (Gaussian and ORCA), which gave similar values to that reported in our communication. Additionally, a multiconfigurational (complete active space SCF) calculation fully confirms the mono-configurational character of the questioned Si2C5H2 ptC structure. We accept that the linear isomer for the C7H2 system, in the triplet electronic state, is competitive with the isomer reported in our article, in the singlet electronic state, as mentioned in the title comment. However, this is a minor correction that does not affect the primary goal and main conclusions of our communication. [ABSTRACT FROM AUTHOR]

In this study, we delved into the structure of B 5 H 5 and questioned some of its accepted assumptions. By exploring the potential energy surface, we found a new three-dimensional structure as the global minimum. This finding is in contrast with the previously hypothesized planar and cage-like models. Our exploration extends to the kinetic stability of various B 5 H 5 isomers, offering insights into the dynamic behavior of these molecules.

Using various exploration strategies, in this study, we investigated the potential energy surfaces (PES) of CBe 5 H 5 + and C n Be 3 n +2 H 2 n +2 2+ ( n = 2-4) clusters. Previous studies proposed that the planar pentacoordinate carbons (ppCs) were the global minima of these clusters. However, our study identified new putative global minima and competitive isomers, refuting some previous assignments. We employed several methods, including evolutive-inspired stochastic approaches guided by "chemical criteria", and ab initio molecular dynamics simulations at elevated temperatures. Our results showed that the size of the scanned population significantly affected the evolutive method and that constrained or guided procedures showed an advantage in identifying better minima for larger systems. This study demonstrated that using multiple complementary strategies can result in a wider variety of minima in a given energy range. Our findings provide valuable insights into exploring the potential energy surfaces of clusters, mainly medium-sized clusters, which could be the connections between small clusters and nanomaterials.

A systematic exploration of the potential energy surface reveals two global minima with three planar tetra coordinate carbons (ptCs) and two global minima with three quasi-ptCs for E 6 C 15 (E = Si-Pb) combinations. These consist of aromatic polycyclic templates suitable for further design of different materials without hindering the ptC texture.

The relativistic effects on the aromaticity of a set of benzene analogues, E3M3H3 (E = C-Pb; M = N-Bi) heterocycles, using magnetically induced current density (MICD) and the NICSzz component of the conventional nucleus independent chemical shift (NICS), is hereby examined. The relativistic effects were evaluated by means of four-component relativistic MICD, and two-component NMR relativistic shielding tensor methods. MICD and NICS were also computed in a non-relativistic fashion, to assess the influence of scalar-relativistic and spin-orbit effects. Most of the studied compounds exhibit a net diatropic ring current (aromatic), excluding the nitrogen-containing compounds which are non-aromatic (except for C3N3H3), in agreement with their higher E-N electronegativity difference. Only in the case of bismuth compounds, E3Bi3H3, aromaticity is substantially decreased when relativistic effects are included (mainly due to the spin-orbit contribution). The larger the mass of the system, the larger the magnitude of this change, in line with the expected relativistic effects for heavier elements. The analysis based on the NICSzz computations agrees with that of the MICD, thus supporting both the magnetic behavior and the aromatic character of these compounds.

In this paper, we provide solid evidence to show that among an overwhelming structural diversity, alkaline earth metals (Ca, Sr, Ba) have the ability to form inverted sandwich compounds with C6H6, C7H7(+), and C8H8(2+) of Dnh symmetry and general formula [E-(CnHn)-E](n-6) (n = 6-8) with planar 10-π-electron aromatic cores by virtue of transferring two electrons per metal atom to the ring. However, the origin of the orbital interaction between the metals and the carbon ring is quite different; while [E-(C6H6)-E] complexes are dominated by δ-interactions, both π- and δ-interactions are important in [E-(C7H7)-E](+) and [E-(C8H8)-E](2+) complexes.

The potential energy surfaces (PESs) of Lin(BH)5(n-6) systems (where n = 5, 6, and 7) were explored using the gradient embedded genetic algorithm (GEGA) program, in order to find their global minima conformations. This search predicts that the lowest-energy isomers of Li6(BH)5 and Li7(BH)5(+) contain a (BH)5(6-) pentagonal fragment, which is isoelectronic and structurally analogous to the prototypical aromatic hydrocarbon anion C5H5(-). Li7(BH)5(+), along with Li7C5(+), Li7Si5(+) and Li7Ge5(+), joins a select group of clusters that adopt a seven-peak star-shape geometry, which is favored by aromaticity in the central five-membered ring, and by the preference of Li atoms for bridging positions. The theoretical analysis of chemical bonding, based on magnetic criteria, supports the notion that electronic delocalization is an important stabilization factor in all these star-shaped clusters.

The B19(-) anion and other boron species have been dubbed 'Wankel motors' for the almost barrierless rotation of inner and outer concentric rings relative to each other in these compounds. A single substitution in B19(-) is shown to shut down the well-established fluxionality in the anion. A carbon atom substituted in the structure to give a neutral CB18 species is shown computationally to enforce bond localization.

We explore in detail the potential energy surfaces of the Al(x)Mg(y) (x, y = 1-4) systems as case studies to test the utility and limitations of simple rules based on electron counts and the phenomenological shell model (PSM) for bimetallic clusters. We find that it is feasible to design stable structures that are members of this set of small Al-Mg binary clusters, using simple electron count rules, including the classical 4n + 2 Hückel model, and the most recently proposed PSM. The thermodynamic stability of the title compounds has been evaluated using several different descriptors, including the fragmentation energies and the electronic structure of the systems. Three stable systems emerge from the analysis: the Al(4)Mg, Al(2)Mg(2) and Al(4)Mg(4) clusters. The relative stability of Al(4)Mg is explained by the stability of the Al(4)(2-) subunit to which the Mg atom donates its electrons. Here the Mg(2+) sits above the aromatic 10 π-electron Al(4)(2-) planar ring. The Al(2)Mg(2) and Al(4)Mg(4) clusters present more complicated 3D structures, and their stabilities are rationalized as a consequence of their closed shell nature in the PSM, with 10 and 20 itinerant electrons, respectively.

The potential energy surfaces of the Na(Si(5-n)(BH)(n))(-) and Si(5-n)(BH)(n)(2-) (n = 0-5) systems have been explored in detail. We established that all the global minimum structures of anionic and dianionic systems can be obtained by substitution of one or more silicon atoms of the global minima of NaSi(5)(-) and Si(5)(2-) for B-H units. The conservation of the overall structure upon isoelectronic substitution was shown to be due to the preservation of the chemical bonding pattern. Theoretical VDEs were calculated for all of the sodiated Na(Si(5-n)(BH)(n))(-) (n = 0-5) systems to facilitate their experimental detection.