Search results for "Aromaticity"
showing 10 items of 138 documents
The Interplay of Inverted Redox Potentials and Aromaticity in the Oxidized States of New π-Electron Donors: 9-(1,3-Dithiol-2-ylidene)fluorene and 9-(…
2006
Derivatives of 9-(1,3-dithiol-2-ylidene)fluorene (9) and 9-(1,3-dithiol-2-ylidene)thioxanthene (10) have been synthesised using Horner-Wadsworth-Emmons reactions of (1,3-dithiol-2-yl)phosphonate reagents with fluorenone and thioxanthen-9-one. X-ray crystallography, solution electrochemistry, optical spectroscopy, spectroelectrochemistry and simultaneous electrochemistry and electron paramagnetic resonance (SEEPR), combined with theoretical calculations performed at the B3P86/6-31G** level, elucidate the interplay of the electronic and structural properties in these molecules. These compounds are strong two-electron donors, and the oxidation potentials depend on the electronic structure of t…
From Pentalene to Dicyclopenta[b,g]naphthalene, or the Change towards Delocalized Structures
2005
Lining triples-corrected coupled-cluster methods as well as other high-level theoretical approximations, the optimized parameters and isomerization barriers of the family of compounds cyclopentadiene-(benzene) x -cyclopentadiene (x 0, 1, 2) are computed. In contrast to previous studies, s-indacene presents a localized C 2 h geometry. Also, the localized structure of pentalene is found to be the most stable, but when two benzene rings are intercalated between the five-member rings of pentalene, the resulting molecule preferably adopts a delocalized D 2 h conformation.
Clathration of Five-Membered Aromatic Rings in the Bimetallic Spin Crossover Metal–Organic Framework [Fe(TPT)2/3{MI(CN)2}2]·G (MI = Ag, Au)
2014
Six clathrate compounds of the three-dimensional spin crossover metal−organic framework formulated [Fe(TPT)2/3{MI (CN)2}2]· nG, where TPT is 2,4,6-tris(4-pyridyl)-1,3,5-triazine, MI = Ag or Au and G represent the guest molecules furan, pyrrole and thiophene, were synthesized using slow diffusion techniques. The clathrate compounds were characterized by single-crystal X-ray diffraction at 120 and 300 K, thermogravimetric analysis and thermal dependence of the magnetic susceptibility. All compounds crystallize in the R3̅ m trigonal space group. The FeII defines a unique [FeN6] crystallographic site with the equatorial positions occupied by four dicyanometallate ligands while the axial positio…
Photochemistry of 1,2,4-oxadiazoles. A DFT study on photoinduced competitive rearrangements of 3-amino- and 3-N-methylamino-5-perfluoroalkyl-1,2,4-ox…
2006
The photoinduced competitive rearrangements of 5-perfluoroalkyl-3-amino(N-alkylamino)-1,2,4-oxadia- zoles have been investigated by DFT calculations and UV-vis spectroscopy. The observed product selectivity depends on the number of hydrogen atoms present in the amino moiety and involves two or three possible routes: (i) ring contraction-ring expansion (RCRE), (ii) internal-cyclization isomerization (ICI), or (iii) C(3)-N(2) migration-nucleophilic attack-cyclization (MNAC). UV absorption and fluorescence spectra of the reactants, and vertical excitation energy values, calculated by time dependent DFT, support the involvement of a neutral singlet excited state in the photoexcitation process. …
How nitrogen modifies the nuclear magnetic shielding in tetraazanaphthalenes
2009
Although for planar conjugated hydrocarbons the out-of-plane component of proton magnetic shielding is an unquestionable quantitative aromaticity indicator, the same is not true for tetraazanaphthalenes. As in these compounds the (core + sigma)-currents associated to the nitrogen nuclei diminish the perpendicular component of shielding, abnormal values of (1)H NMR sigma(zz) are obtained. Therefore, a consistent aromaticity measure must be based only on the pi-contribution to the out-of-plane component of proton magnetic shielding. Otherwise, the behavior of these compounds in presence of an external magnetic field parallels that of naphthalene, with the nitrogen nuclei contributing to the r…
Aromaticity of the most stable adenine and purine tautomers in terms of Hückel's 4N+2 principle
2019
Abstract Electron structures of the fused rings in 7H and 9H tautomers of purine and adenine follow the 4N + 2 rule; the values of pEDA, HOMA, NICS and FLU indices document their aromatic character. In the 1H and 3H tautomers, these rings contain five or seven π electrons, hence they do not follow this rule and consequently exhibit lower aromaticity. This also applies to the aromaticity of whole molecules.
Surface-Enhanced Raman Study of the Interactions between Tripodal Cationic Polyamines and Polynucleotides
2011
Raman and surface-enhanced Raman spectra of new DNA/RNA-binding compounds consisting of three imidazole (Im) and three pyridine (Py) rings connected by tripodal polyaminomethylene linkages were obtained by the near-infrared excitation at 1064 nm. Study of interactions of Im and Py polyamines with single-stranded RNA polynucleotides (poly A, poly G, poly C, poly U), double-stranded DNA polynucleotides (poly dAdT-poly dAdT, poly dGdC-poly dGdC) and calf thymus DNA (ct-DNA) by surface-enhanced Raman spectroscopy (SERS) reveals unambiguous enhancement of the Raman scattering from the small molecules as well as appearance of new bands in spectra associated mainly with nucleobases. The SERS exper…
Synthesis and electronic aspects of tetrahydrobenzothienopyrimidine derivatives
2015
Abstract The chemistry of thiophenes, pyrimidines, triazolopyrimidines and benzothiophenes has drawn much attention because of their biological activities. Their interesting properties are connected with their complex π-electron delocalisation effects. Herein the synthesis, crystal and molecular structures at 100 K and DFT calculated structures of three tetrahydrobenzothienopyrimidine derivatives are reported i.e. 4-hydrazino-5,6,7,8-tetrahydro[1]benzothieno[2,3-d]pyrimidine which was a substrate for 2-phenyl-8,9,10,11-tetrahydro[1]benzothieno[3,2-e][1,2,4]triazolo[1,5-c]pyrimidine and 3-methyl-9,10,11,12-tetrahydro-2H-[1]benzothieno[2′,3′:4,5]pyrimido[1,6-b][1,2,4]triazin-2-one. Moreover t…
Mechanistic Studies on the Metal-Free Activation of Dihydrogen by Antiaromatic Pentarylboroles
2012
The perfluoro- and perprotiopentaphenylboroles 1 and 2 react with dihydrogen to effect H-H bond cleavage and formation of boracyclopentene products. The mechanism of this reaction has been studied experimentally through evaluation of the kinetic properties of the slower reaction between 2 and H(2). The reaction is first-order in both [borole] and [H(2)] with activation parameters of ΔH(‡) = 34(8) kJ/mol and ΔS(‡) = -146(25) J mol(-1) K(-1). A minimal kinetic isotope effect of 1.10(5) was observed, suggesting an asynchronous geometry for H-H cleavage in the rate-limiting transition state. To explain the stereochemistry of the observed products, a ring-opening/ring-closing mechanism is propos…
DFT and experimental studies on structure and spectroscopic parameters of 3,6-diiodo-9-ethyl-9H-carbazole
2015
The first report on crystal and molecular structure of 3,6-diiodo-9-ethyl-9H-carbazole is presented. Experimental room-temperature X-ray and 13C chemical shift studies were supported by advanced theoretical calculations using density functional theory (DFT). The 13C nuclear magnetic shieldings were predicted at the non-relativistic and relativistic level of theory using the zeroth-order regular approximation (ZORA). Theoretical relativistic calculations of chemical shifts of carbons C3 and C6, directly bonded to iodine atoms, produced a reasonable agreement with experiment (initial deviation from experiment of 44.3 dropped to 4.25 ppm). The changes in ring aromatic character via simple harm…