Viability of möbius topologies in [26]- and [28]hexaphyrins

Recently, hexaphyrins have emerged as a promising class of π-conjugated molecules that display a range of interesting electronic, optical, and conformational properties, including the formation of stable M?bius aromatic systems. Besides the M?bius topology, hexaphyrins can adopt a variety of conformations with Hückel and twisted Hückel topologies, which can be interconverted under certain conditions. To determine the optimum conditions for viable M?bius topologies, the conformational preferences of [26]- and [28]hexaphyrins and the dynamic interconversion between the M?bius and Hückel topologies were investigated by density functional calculations. In the absence of meso?substituents, [26]hexaphyrin prefers a planar dumbbell conformation, strongly aromatic and relatively strain free. The M?bius topology is highly improbable: the most stable tautomer is 33?kcal?mol(-1) higher in energy than the global minimum. On the other hand, the M?bius conformer of [28]hexaphyrin is only 6.5?kcal?mol(-1) higher in energy than the most stable dumbbell conformation. This marked difference is due to aromatic stabilization in the M?bius 4n electron macrocycle as opposed to antiaromatic destabilization in the 4n+2 electron system, as revealed by several energetic, magnetic, structural, and reactivity indices of aromaticity. For [28]hexaphyrins, the computed activation barrier for interconversion between the M?bius aromatic and Hückel antiaromatic conformers ranges from 7.2 to 10.2?kcal?mol(-1) , in very good agreement with the available experimental data. The conformation of the hexaphyrin macrocycle is strongly dependent on oxidation state and solvent, and this feature creates a promising platform for the development of molecular switches.     

Theoretical study on S_1(~1B_(3u)) state electronic structure and absorption spectrum of pyrazine

Making use of a set of quantum chemistry methods, the harmonic potential surfaces of the ground state (S0(1Ag)) and the first (S1(1B3u)) excited state of pyrazine are investigated, and the electronic structures of the two states are characterized. In the present study, the conventional quantum mechanical method, taking account of the Born-Oppenheimer adiabatic approximation, is adopted to simulate the absorp-tion spectrum of S1(1B3u) state of pyrazine. The assignment of main vibronic transitions is made for S1(1B3u) state. It is found that the spectral profile is mainly described by the Franck-Condon progression of totally symmetric mode ν6a. For the five totally symmetric modes, the present calculations show that the frequency differences between the ground and the S1(1B3u) state are small. Therefore the displaced harmonic oscillator approximation along with Franck-Condon transition is used to simulate S1(1B3u) absorption spectra. The distortion effect due to the so-called quadratic coupling is demonstrated to be unimportant for the absorption spectrum, except the coupling mode ν10a. The calculated S1(1B3u) ab-sorption spectrum is in reasonable agreement with the experimental spectra.     

Analysis of Hückel's [4n + 2] rule through electronic delocalization measures

In the present work, we analyze the pi-electronic delocalization in a series of annulenes and their dications and dianions by using electron delocalization indices calculated in the framework of the quantum theory of atoms in molecules. The aim of our study is to discuss the Hückel's 4n + 2 rule from the viewpoint of pi-electronic delocalization. Our results show that there is an important increase of electronic delocalization (of about 1 e) when going from antiaromatic 4n pi systems to aromatic (4n + 2)pi systems. Less clear is the change in pi-electronic delocalization when we move from a (4n + 2)pi-aromatic to a 4n pi-antiaromatic species by adding or removing a pair of electrons.     

Aromatic pathways in mono- and bisphosphorous singly Möbius twisted [28] and [30]hexaphyrins

Magnetically induced current densities and strengths of currents passing through selected bonds have been calculated for monophosphorous [28]hexaphyrin ((PO)[28]hp) and for bisphosphorous [30]hexaphyrin ((PO)(2)[30]hp) at the density functional theory level using our gauge-including magnetically induced current (GIMIC) approach. The current-density calculations yield quantitative information about electron-delocalization pathways and aromatic properties of singly M?bius twisted hexaphyrins. The calculations confirm that (PO)[28]hp sustains a strong diatropic ring current (susceptibility) of 15 nA T(-1) and can be considered aromatic, whereas (PO)(2)[30]hp is antiaromatic as it sustains a paratropic ring current of -10 nA T(-1). Numerical integration of the current density passing through selected bonds shows that the current is generally split at the pyrroles into an outer and an inner pathway. For the pyrrole with the NH moiety pointing outwards, the diatropic ring current of (PO)[28]hp takes the outer route across the NH unit, whereas for (PO)(2)[30]hp, the paratropic ring current passes through the inner C(β)=C(β) double bond. The main diatropic ring current of (PO)[28]hp generally prefers the outer routes at the pyrroles, whereas the paratropic ring current of (PO)(2)[30]hp prefers the inner ones. In some cases, the ring current is rather equally split along the two pathways at the pyrroles. The calculated ring-current pathways do not agree with those deduced from measured (1)H NMR chemical shifts.     

Theoretical study of the geometry and electronic structure of the trinuclear [AunAgm (HNCOH)3] (m + n = 3) complex

The structures and properties of different gold and silver mixed-metal trinuclear complexes, [Au_nAg_m(HNCOH)₃] (m + n = 3), were investigated theoretically. The computed properties were compared with those of the [Au₃(HNCOH)₃] complex. The geometries of all complexes were optimized at the B3LYP level of theory using the GEN basis set. The optimization results revealed that the most stable structures of pure Au and Ag complexes are almost similar. In addition, all complexes are flat and highly symmetric. It was shown that the silver substitution had a significant influence on the electronic properties. The metal–metal distances were in the order of: Au–Au < Au–Ag < Ag–Ag. The ionization potential and hardness were found to be decreased while the electron affinity, HOMO–LUMO gap and chemical potential were found to be increased from the [Au₃(HNCOH)₃] to the [Ag₃(HNCOH)₃] complex. The [Au₃(HNCOH)₃] complex was the least reactive in the studied series with the electronic chemical potential equal to −3.98 eV. On the other hand, the value of the chemical potential characterizing the most reactive complex, [Ag₃(HNCOH)₃], was −3.80 eV.     

Theoretical prediction on vibrational spectra of [Ar…Ar-H]~+

As a special substance between isolated molecules and condensed phase, clusters have drawn more and more attention by theoreticians and experimentalists. The protonated rare gas cluster is one kind of simplest clusters, which can be looked on as the simplest solution composed of the simplest solute, proton, and the simplest solvent, rare gas. The study on such a system can help us to know more about the complex condensed matter. However, the information for the molecular properties of protonat…     

Theoretical investigation on structures, electronic spectra and nonlinear optical properties of gold compounds [X-“Au(PMe3)”2]

The structures, electronic spectra, polarizability and third-order nonlinear optical properties of six gold compounds [X-“Au(PMe₃)”₂] were investigated by density functional theory (DFT) B3LYP and BhandHLYP methods. It was found that the calculation methods and basis set are rational for the object of study, and molecular structures change slightly when PPh₃ is replaced by PMe₃. The spatial effects of the bridging section have a significant influence on the polarizability, but indistinctive to the third-order nonlinear optical (NLO) coefficient. As a result of the conjugated effect in different compounds, the third-order polarizability of molecule 1a is the smallest, while that of molecule 2a is the largest. Au has donor ability in molecule 1a but acceptor ability in molecules 2a–6a by analyzing the electronic spectra and frontier molecular orbitals constitute maximal absorption, which indicates the contribution of Au to NLO properties in the six molecules is different.     

Theoretical study of structural and electronic properties of oligo（thiophene-phenylene）s in comparison with oligothiophenes and oligophenylenes

In this work, a quantum-chemical investigation on the structural and opto-electronic properties of oligo（thiophene-phenylene） （4TP） is carried out. The results are discussed in comparison with the properties of corresponding oligothiophene （8T） and oligophenylene （8P）. As the opto-electronic properties of this type of conducting polymers are governed by their electronic band gap, we shall also present a comparison among HOMO, LUMO and band gap energies of these three materials.     

Theoretical study of the substituent effects on structure and dissociation of RNSi and RSiN (R = H, Li)

The effects of substituent on stability, structural properties and dissociation of HSiN, LiSiN and their isomers have been studied in detail using ab initio MP2, CCSD and CCSD(T) and density functional B3LYP methods. After dissociation of HNSi, LiNSi, HSiN and LiSiN, the fragmented atoms have been considered to be either in their ground state or in their valence excited state in various dissociation channels. Only allowed dissociations of these molecules are considered. Various dissociation channels of HSiN and LiSiN and their isomers have been explored and an interesting trend has been observed for the dissociation of stable isomers HNSi and LiNSi and less stable isomers HSiN and LiSiN. The effect of substituents on dipole moment has been discussed. The potential energy surfaces for the RSiN ↔ RNSi isomerization reactions have been analyzed. The structural properties of these molecules agree well with the theoretical and experimental values wherever available.     

5,20-Bis(α-oligothienyl)-substituted [26]hexaphyrins possessing electronic circuits strongly perturbed by meso-oligothienyl substituents

A series of [26]hexaphyrins(1.1.1.1.1.1) bearing two α-oligothienyl substituents at 5,20-positions have been synthesised and are shown to have a dumbbell hexaphyrin conformation, to which the α-oligothienyl groups are linked with small dihedral angles to form an acyclic helix-like conjugated network. While their distinct diatropic ring currents and four reversible reduction waves characteristic of aromatic [26]hexaphyrins indicate that the [26]hexaphyrin aromatic circuits are viable, the absorption spectra and excited state dynamics are significantly perturbed, which becomes increasingly evident with elongation of the oligothienyl substituents. DFT calculations of these hexaphyrins indicated that the LUMO and LUMO + 1 are localised on the hexaphyrin circuit and the HOMO and HOMO – 1 are spread over the acyclic helix-like conjugation network, which can explain the perturbed absorption spectra.     

Theoretical Studies on Stabilities of C26BN Isomers

The possible stable stractures of substituted fullerene C26BN formed on the initial C28 cage with Td symmetry have been systematically studied on UHF/3-21g level with constrained syrnmetry(Cs or C1, the charge and multiplicity of all the isomers are zero and five, respectively.The geometry optimization and the vibrational frequencies analysis were performed on the same level with constrained symmetry(Cs or C1. The results show that the most stable isomer of C26BN is formed by boron and nitrogen atoms doping at 5,2-sites.     

A double-twist Möbius-aromatic conformation of [14]annulene

[structure: see text] B3LYP and KMLYP/6-31G(d) calculations predict a double-helical and chiral conformation of [14]annulene with the topological properties of a double-twist M?bius band as highly aromatic; its energy with respect to the known Hückel-aromatic conformation is predicted to be stabilized by suitable ring substitution.     

Theoretical study of the substituent effect of hydroxy group on tandem Cope rearrangement and [2 + 2] cycloaddition in cis-1,2-diethynylcyclopropane and its mono-hetero analogues

The tandem Cope rearrangement and [2?+?2] cycloaddition of cis-1,2-diethynyl-1,2-dihydroxycyclopropane and its mono-hetero analogues have been investigated at the B3LYP/6-31+G* level. The presence of the hydroxy group lowers the activation enthalpies for the Cope rearrangement, whereas activation enthalpies for the [2?+?2] cycloaddition are raised as compared to those for their non-hydroxy derivatives. The NBO analysis indicates that in the transition structure involved in the Cope rearrangement, lone pairs of the oxygen atoms of the hydroxy groups are transferred into the σ* C–C bond undergoing migration, as a result of which it is weakened. On the other hand, the lone pairs of the oxygen atoms interact with the π* C=C orbitals of the bis-allenic systems in the intermediate thereby stabilizing it and, thus, suppressing its driving ability for the [2?+?2] cycloaddition. In the products so formed, 6π electrons are delocalized conferring stability on them, which is further augmented by extended conjugation with the hydroxy groups. Due to high stability of these products, activation barrier for the change of enol into ketone is very high.

     

Theoretical Investigation on the Structural and Spectroscopic Properties of “Cross” [Anthraquinone-Diamine/Diimine-Pt] Complexes

Russian Journal of Physical Chemistry A - The Frontier molecular orbitals, UV–Vis absorption spectra, electron excitation properties, and triplet excited states of 1 and 2 were investigated...     

Theoretical study of molecular hydrogen elimination from the undecahydrodecaborate monoanion [B10H11]−. Exopolyhedral substitution intermediates: [B10H9]− monoanion and neutral [B10H10] cluster

The elementary reaction of molecular hydrogen elimination from the [B₁₀H₁₁]^? anion, which is presumably the rate-limiting stage of acid-catalyzed reactions of substitution of exopolyhedral H atoms in the [B₁₀H₁₀]^2? decahydro-closo-decaborate anion, has been calculated by the density functional theory method (in the B3LYP/6-311++G** approximation). Specific transition states of H₂ elimination in which vacancies form near the boron atoms have been localized. It has been demonstrated that regioselectivity of substitution reactions can be related to the significant difference between the activation barriers for the pathways of H₂ elimination from boron atoms with different coordination numbers (CN 6 and 5). The electron density of the [B₁₀H₉]^? anion that forms after hydrogen molecule elimination has a characteristic shape of the lowest unoccupied molecular orbital for the interaction with nucleophilic reagents; in acid-catalyzed reactions, different anions, for example, a carboxylic acid residue, can act as such. The direct reaction of the [B₁₀H₉]^? intermediate with nucleophilic anions is hindered by the Coulomb charge repulsion. To overcome this hindrance, the possibility of [B₁₀H₉]^? protonation to form the neutral [B₁₀H₁₀] system has been considered. It has been shown that the proton affinity of the [B₁₀H₉]^? anion is ～280–290 kcal/mol. For the [B₁₀H₁₀] cluster, the lowest-lying and low-lying isomers have been considered. For all the systems under consideration, the electronic chemical potential and Pearson hardness have been evaluated.     

Theoretical investigation of the complexation,structural, and electronic properties of complexes between oseltamivir drug and cucurbit[n = 6–9]urils

Structural Chemistry - The structural geometries of cucurbit[n]uril CB[n]&nbsp;with n?=?6–9 and their complexes with oseltamivir (OST) drug were obtained using the density...     

Theoretical Study on the Structures and Stability of Isomers and Complex of[Si,C,O,O] Sytem

A detailed singlet potential energy surface(PES) of [Si,C,O,O] system including a van der Waals (vdW) comples SiO……CO2,eight isomers,and twelve transiton states is investigated by MP2 and QCISD(T) (single-point)methods.At the final QCISD(T)/6-311 G(2df)//MP2/6-311G(d) level with zero-point energy included,the complex SiO……CO is found to be thermodynamically and kinetically the most stable species.Although eight ismoers are located as local energy minima,they are rather unstable toward isomerization to the dissociation fragments or comples.For the reaction of silocon atoms with carbon dioxide,two competitive reaction channels are found,and the primary pathway,which leads to the products of SiO and CO fragments,is the direct oxygen-abstraction process from carbon dioxide by silicon atom with a41.16 kJ/mol reaction barrier height.Our predications are in good agreement with previous experimental and theoretical studies.