A unified orbital model of delocalised and localised currents in monocycles, from annulenes to azabora-heterocycles

Why are some (4n+2)π systems aromatic, and some not? The ipsocentric approach to the calculation of the current density induced in a molecule by an external magnetic field predicts a four‐electron diatropic (aromatic) ring current for (4n+2)π carbocycles and a two‐electron paratropic (antiaromatic) current for (4n)π carbocycles. With the inclusion of an electronegativity parameter, an ipsocentric frontier‐orbital model also predicts the transition from delocalised currents in carbocycles to nitrogen‐localised currents in alternating azabora‐heterocycles, which rationalises the differences in (magnetic) aromaticity between these isoelectronic π‐conjugated systems. Ab initio valence‐bond calculations confirm the localisation predicted by the naïve model, and coupled‐Hartree–Fock calculations give current‐density maps that exhibit the predicted delocalised‐to‐localised/carbocycle–heterocycle transition.     

Evaluating Shielding‐Based Ring‐Current Models by Using the Gauge‐Including Magnetically Induced Current Method

Magnetically induced current densities and integrated ring‐current strength susceptibilities have been calculated at the density functional theory (DFT) level for a test set consisting of 17 ring‐shaped molecules using the gauge‐including magnetically induced current (GIMIC) method. Reliable values for the ring‐current strengths have been obtained by performing numerical integration of the current‐density susceptibility passing a cut plane perpendicularly to the molecular ring. The current densities and ring current strengths were calculated at the DFT level using the B3LYP functional and def2‐TZVP basis sets. Current densities and ring‐current strengths have also been calculated at the Hartree‐Fock self‐consistent field (HF‐SCF) level using Dunning’s aug‐cc‐pVTZ basis sets, which allow a direct comparison with ring‐current strengths that have previously been estimated using ring‐current models based on magnetic shielding calculations. Current density calculations at both levels of theory show that the magnetic shielding based ring‐current models are not a very accurate means to estimate the magnetically induced ring current strengths, whereas they provide qualitatively the correct aromaticity trends for the studied molecules.     

π‐Electron currents in larger fully aromatic benzenoids

Recently, we have reported on calculation of π‐electron ring currents in several smaller fully benzenoid hydrocarbons having up to eight fused benzene rings and five Clar π‐aromatic sextets. In contrast to early HMO ring current calculations and more recent ab initio calculations of π‐electron density, our current calculations are based on a graph theoretical model in which contributions to ring currents comes from currents associated with individual conjugated circuits. In this contribution, we consider several larger fully benzenoid hydrocarbons having from 9 to 13 fused rings and from six or seven π‐aromatic sextets. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

The NICS‐XY‐Scan: Identification of Local and Global Ring Currents in Multi‐Ring Systems

Nucleus‐independent chemical shift (NICS)‐based methods are very popular for the determination of the induced magnetic field under an external magnetic field. These methods are used mostly (but not only) for the determination of the aromaticity and antiaromaticity of molecules and ions, both qualitatively and quantitatively. The ghost atom that serves as the NICS probe senses the induced magnetic field and reports it in the form of an NMR chemical shift. However, the source of the field cannot be determined by NICS. Thus, in a multi‐ring system that may contain more than one induced current circuit (and therefore more than one source of the induced magnetic field) the NICS value may represent the sum of many induced magnetic fields. This may lead to wrong assignments of the aromaticity (and antiaromaticity) of the systems under study. In this paper, we present a NICS‐based method for the determination of local and global ring currents in conjugated multi‐ring systems. The method involves placing the NICS probes along the X axis, and if needed, along the Y axis, at a constant height above the system under study. Following the change in the induced field along these axes allows the identification of global and local induced currents. The best NICS type to use for these scans is NICS_πZZ, but it is shown that at a height of 1.7 Å above the molecular plane, NICS_ZZ provides the same qualitative picture. This method, namely the NICS‐XY‐scan, gives information equivalent to that obtained through current density analysis methods, and in some cases, provides even more details.     

Derivatizing Tribenzothiophene‐Fused Hexa‐peri‐hexabenzocoronenes with Tunable Optoelectronic Properties

A series of trisbenzothieno[1,2:7,8:13,14]hexa‐peri‐hexabenzocoronenes were synthesized and characterized by a combination of NMR, 2D NMR, MALDI‐TOF MS, UV/Vis absorption spectroscopy, and 2D‐WAXS measurement. By structural modulation like decoration of electro‐donating alkoxyl chain, and conversion from an electron‐rich thiophene ring into an electron‐poor thiophene‐S,S‐dioxide moiety, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the hexabenzocoronenes derivatives can be effectively tuned which is further verified by the DFT calculations and cyclic voltammetry.     

3‐Amino‐1,2,4‐triazine

In the crystal structure of 3‐amino‐1,2,4‐triazine, C₃H₄N₄, the mol­ecules form hydrogen‐bonded chains that are almost parallel to the b axis (3.2°), and which are inclined to the a and c axes by ～21 and ～69°, respectively. The distortion of the 1,2,4‐triazine ring in the crystal is compared with gas‐phase ab initio molecular‐orbital calculations.     

2‐Bromo‐6‐(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)pyridine,a new unexpected bifunctional building block for combinatorial chemistry

The first reported structure of a pyridin‐2‐ylboron derivative, viz. the title compound, C₁₁H₁₅BBrNO₂, (I), is compared with its regioisomer 2‐bromo‐5‐(4,4,5,5‐tetra­methyl‐1,3,2‐dioxa­borolan‐2‐yl)­pyridine, (II) [Sopková‐de Oliveira Santos, Lancelot, Bouillon & Rault (2003). Acta Cryst. C59, o111o113 ]. Structural differences are observed, firstly in the orientation of the dioxaborolane ring with respect to the pyridine ring and secondly in the bond angles of the BO₂ group. These differences do not explain the experimentally observed differences in chemical reactivity between (I) and (II) but do confirm the relatively lower stability of (I). However, ab initio calculations of the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), based on the known crystal structures of the two compounds, show different distributions, which correspond to the differences observed during chemical reactions.     

Noncollinear spin density functional theory for spin‐frustrated and spin‐degenerate systems

We have implemented ab initio linear combinations of Gaussian‐type orbital calculations with generalized localized spin density approximation (GLSDA) for a dimer of equilateral H₃ as a model of the noncollinear magnetic clusters. It has been found that the GLSDA solution with the three‐dimensional noncollinear spin structure is, contrary to prior band calculations by other groups, the ground state near the O_h conformation. Further computational results are compared to that of ab initio generalized Hartree–Fock. The difference between them and the influence of the correlation correction were discussed. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Acenaphthylene‐Fused Cyclo[8]pyrroles with Intense Near‐IR‐Region Absorption Bands

An acenaphthylene‐fused cyclo[8]pyrrole was synthesized by using an oxidative coupling reaction of the corresponding 2,2′‐bipyrrole. Two conformational isomers 1 a and 1 b were isolated, and their molecular structures were elucidated by X‐ray crystallographic studies. The less‐polar and lower‐symmetry 1 b isomer can be converted into the 1 a isomer through a thermal ring flip. Application of the perimeter model developed by Michl to magnetic circular dichroism spectroscopic data and theoretical calculations demonstrate that there is a marked redshift of the near‐IR absorption maxima relative to cyclo[8]isoindole because there is a significant stabilization of the LUMO due to the differing effects of a fused ring expansion with acenaphthylene and benzene moieties on the frontier π molecular orbitals.     

3,17‐Dioxoandrost‐4‐en‐4‐yl acetate

The title compound, C₂₁H₂₈O₄, has a 4‐acetoxy substituent positioned on the steroid α face. The six‐membered ring A assumes a conformation intermediate between 1α,2β‐half chair and 1α‐sofa. A long Csp³—Csp³ bond is observed in ring B and reproduced in quantum‐mechanical ab initio calculations of the isolated molecule using a molecular‐orbital Hartree–Fock method. Cohesion of the crystal can be attributed to van der Waals interactions and weak C—H...O hydrogen bonds.     

2,2,4,4‐Tetra‐tert‐butyl‐1,3,5,2,4‐benzotrioxadisilepine‐7‐carbaldehyde

The mol­ecule of the title compound, C₂₃H₄₀O₄Si₂, features an approximate non‐crystallographic C₂ symmetry axis. The aldehyde group is disordered over two positions with similar occupancies. The geometry of the isolated mol­ecule was studied by ab initio quantum mechanical calculations employing a mol­ecular orbital Hartree–Fock method. The calculations reproduce well the equilibrium geometry but slightly overestimate the value of the Si—O bond lengths of the trioxadisilepine ring.     

Metal‐stabilized rare tautomers: N4 metalated cytosine (M = Li+, Na+, K+, Rb+ and Cs+), theoretical views

Ab initio calculations indicate that metalation of the exocyclic amino group of cytosine by the elements of Group IA (Li, Na, K, Rb and Cs) induces protonation of a nucleobase ring nitrogen atom, and hence causes a proton shift from an exocyclic to an endocyclic nitrogen atom. Thus, this metal‐assisted process leads to the generation of rare nucleobase tautomers. The calculations suggest that this kind of metalation increases the protonation energies of the aromatic ring of the nucleobase. The present study reports the quantum chemistry analysis of the metal‐assisted tautomerization. The calculations clearly demonstrate that metalation of the exocyclic amino group of the nucleobase significantly increases the protonation energy of the aromatic rings of the nucleobase. Also, absolute anisotropy shift, molecular orbital and natural bond orbital calculations are compatible with these results. Copyright © 2003 John Wiley & Sons, Ltd.     

Thermochemistry of N‐heterocyclic carbenes with 5‐, 4‐, 3‐, and 2‐membered rings

N‐heterocyclic carbenes (NHCs) based on imidazole‐2‐ylidene ( 1 ) or the saturated imidazolidine‐2‐ylidene ( 2 ) scaffolds are long‐lived singlet carbenes. Both benefit from inductive stabilization of the sigma lone pair on carbon by neighboring N atoms and delocalization of the N pi lone pairs into the nominally vacant p‐pi atomic orbital at the carbene carbon. With thermochemical schemes G4 and CBS‐QB3, we estimate the relative thermodynamic stabilization of smaller ring carbenes and acyclic species which may share the keys to NHC stability. These include four‐membered ring systems incorporating the carbene center, two trivalent N centers, and either a boron or a phosphorus atom to complete the ring. Amino‐substituted cyclopropenylidenes have been reported but three‐membered rings containing the carbene center and two N atoms are not known. Our calculations suggest that amino‐substituted cyclopropenylidenes are comparable in stability to the four‐membered NHCs but that diazacyclopropanylidenes would be substantially less effectively stabilized. Concluding the series are acyclic carbenes with and without neighboring N atoms and a series of “two‐membered ring” azapropadienenylidene cations of form :C?N?W with W = an electron‐withdrawing agent. We have studied W = NO₂, CH₂(+), CF₂(+), and (CN)₂C(+). Although these systems display a degree of stabilization and carbene‐like electronic structure, the stability of the NHCs is unsurpassed. © 2014 Wiley Periodicals, Inc.     

Impact of Mn on the solution enthalpy of hydrogen in austenitic Fe‐Mn alloys: A first‐principles study

Hydrogen interstitials in austenitic Fe‐Mn alloys were studied using density‐functional theory to gain insights into the mechanisms of hydrogen embrittlement in high‐strength Mn steels. The investigations reveal that H atoms at octahedral interstitial sites prefer a local environment containing Mn atoms rather than Fe atoms. This phenomenon is closely examined combining total energy calculations and crystal orbital Hamilton population analysis. Contributions from various electronic phenomena such as elastic, chemical, and magnetic effects are characterized. The primary reason for the environmental preference is a volumetric effect, which causes a linear dependence on the number of nearest‐neighbour Mn atoms. A secondary electronic/magnetic effect explains the deviations from this linearity. © 2014 Wiley Periodicals, Inc.     

3‐Hydro­xy­benz­aldehyde

The title compound, C₇H₆O₂, forms infinite chains where the mol­ecules are hydrogen bonded via the hydroxyl and aldehyde groups, with an O?O distance of 2.719 (3) Å. Interchain interactions are weak. The geometry of the ring differs from the ideal form due to the effect of the substituents. Abinitio (Hartree–Fock self‐consistent field–molecular orbital and density functional theory) calculations for the free mol­ecule reproduce well the observed small distortions of the ring. In the crystal, the geometry deviates from the ideal C_s symmetry of the free mol­ecule, as given by the ab initio calculations. The aldehyde and hydroxyl groups are twisted around the single bonds which join them to the ring as a result of the intermolecular hydrogen‐bond interactions. These are also responsible for an elongation of the hydroxy C—OH bond compared with that calculated for the free mol­ecule.     

N‐Inversion in N‐phenyloxaziridine: substituent and solvent effects via density functional theory

In this work, using density functional theory, the kinetic effects of the substitution of a t‐butyl group and\or the incorporation of an oxygen atom, and both, at the aziridine ring moiety were investigated for N‐inversion in N‐phenylaziridine. Then, for N‐inversion in 3‐t‐butyl‐N‐phenyloxaziridine, the kinetic Hammett substituent effects were studied using the different para‐substituted groups on the N‐phenyl ring moiety. The natural bond orbital (NBO) study was the last case in this work. The calculations were performed in the gas phase and solution (in carbon tetrachloride and dichloromethane). The incorporation of an oxygen atom in the aziridine ring strongly weakens the N‐inversion process. In addition, while both t‐butyl substituent and solvent slightly reinforce the N‐inversion of N‐phenyloxaziridine, in N‐phenylaziridine, they decrease the N‐inversion rate to some extent. In both phases, more pronounced in solution and especially in dichloromethane, and in agreement with the NBO results, the electron‐withdrawing groups on para position of the N‐phenyl ring strongly increase the rate of N‐inversion of 3‐t‐butyl‐N‐phenyloxaziridine molecule.     

1H‐Indazole and 2H‐indazole derivatives of androsta‐5,16‐dien‐3β‐ol

The title compounds, 17‐(1H‐indazol‐1‐yl)androsta‐5,16‐dien‐3β‐ol, (I), and 17‐(2H‐indazol‐2‐yl)androsta‐5,16‐dien‐3β‐ol, (II), both C₂₆H₃₂N₂O, have an indazole substituent at the C17 position. The six‐membered B ring of each compound assumes a half‐chair conformation. A twist of the steroid skeleton is observed and reproduced in quantum‐mechanical ab initio calculations of the isolated molecule using a molecular orbital Hartree–Fock method. In the 1H‐indazole derivative, (I), the molecules are joined in a head‐to‐head fashion via O—H...O hydrogen bonds, forming chains along the a axis. In the 2H‐indazole derivative, (II), the molecules are joined in a head‐to‐tail fashion with one of the N atoms of the indazole ring system acting as the acceptor. The hydrogen‐bond pattern consists of zigzag chains running along the b axis. Substituted steroids have proven to be effective in inhibiting androgen biosynthesis through coordination of the Fe atoms of some enzymes, and this study shows that indazole‐substituted steroids adopt twisted conformations that restrict their intermolecular interactions.     

Comprehensive Analysis of Fragment Orbital Interactions to Build Highly π‐Conjugated Thienylene‐Substituted Phenylene Oligomers

π‐Conjugated thienylene? phenylene oligomers with fluorinated and dialkoxylated phenylene fragments have been designed and prepared to understand the interactions in fragment orbitals, the influence of the substituents (F, OMe) on the HOMO–LUMO gap, and the role of intramolecular non‐covalent cumulative interactions in the construction of π‐conjugated nanostructures. Their strong conjugation was also evidenced in the gas phase by UV photoelectron spectroscopy and theoretical calculations. These results can be explained by the crucial role of the relative energetic positions of the π orbitals of the dimethoxyphenylene, which was used to model the dialkoxyphenylene entity, in determining the π/π^* orbital levels of the fluorinated phenylene entity. Dialkoxyphenylenes raise the HOMO orbitals, whereas fluorinated phenylenes lower the LUMO orbitals in the oligomers. In addition, the presence of S???F and H???F interactions in the fluorinated phenylene? thienylene compounds add to the S???O interactions in the mixed targets and contribute to the full conjugation in the oligomer, inducing weak inter‐ring angles between the involved aromatic cycles. These results, which showed extended conjugation of the π system, were corroborated by a narrow HOMO–LUMO gap (according to DFT calculations) and by a relatively strong maximum wavelength (as obtained by TD‐DFT calculations and experimental UV/Vis measurements). The crystallographic data of two mixed thienylene? (fluorinated and dialkoxylated phenylene) five‐ring oligomers agree with the above results and show the formation of quasi‐planar conformations with non‐covalent S???O, H???F, and S???F interactions. These studies in the solid and gas phases show the relevance of associating dialkoxyphenylene and fluorinated phenylene fragments with thiophene to lead to oligomers with improved electronic delocalization for electronic or optoelectronic devices.     

A π‐Stacked Porphyrin–Fullerene Electron Donor–Acceptor Conjugate That Features a Surprising Frozen Geometry

A “frozen” electron donor–acceptor array that bears porphyrin and fullerene units covalently linked through the ortho position of a phenyl ring and the nitrogen of a pyrrolidine ring, respectively, is reported. Electrochemical and photophysical features suggest that the chosen linkage supports both through‐space and through‐bond interactions. In particular, it has been found that the porphyrin singlet excited state decays within a few picoseconds by means of a photoinduced electron transfer to give the rapid formation of a long‐lived charge‐separated state. Density functional theory (DFT) calculations show HOMO and LUMO to be localized on the electron‐donating porphyrin and the electron‐accepting fullerene moiety, respectively, at this level of theory. More specifically, semiempirical molecular orbital (MO) configuration interaction (CI) and unrestricted natural orbital (UNO)‐CI methods shed light on the nature of the charge‐transfer states and emphasize the importance of the close proximity of donor and acceptor for effective electron transfer.     

DFT‐GIAO1H NMR chemical shifts prediction for the spectral assignment and conformational analysis of the anticholinergic drugs (−)‐scopolamine and (−)‐hyoscyamine

The relatively large chemical shift differences observed in the ¹H NMR spectra of the anticholinergic drugs (?)‐scopolamine 1 and (?)‐hyoscyamine 2 measured in CDCl₃ are explained using a combination of systematic/molecular mechanics force field (MMFF) conformational searches and gas‐phase density functional theory (DFT) single point calculations, geometry optimizations and chemical shift calculations within the gauge including/invariant atomic orbital (GIAO) approximation. These calculations show that both molecules prefer a compact conformation in which the phenyl ring of the tropic ester is positioned under the tropane bicycle, clearly suggesting that the chemical shift differences are produced by the anisotropic effect of the aromatic ring. As the calculations fairly well predict these experimental differences, diastereotopic NMR signal assignments for the two studied molecules are proposed. In addition, a cursory inspection of the published ¹H and ¹³C NMR spectra of different forms of 1 and 2 in solution reveals that most of them show these diastereotopic chemical shift differences, strongly suggesting a preference for the compact conformation quite independent of the organic or aqueous nature of the solvent. Copyright © 2010 John Wiley & Sons, Ltd.