Sulfinfinitenes: infinitenes of fused thiophene rings.

Sulfinfinitenes, analogues of infinitene constructed from thiophene rings and assembled by applying a "cut-twist-restitch" sequence to two sulflowers, are explored through DFT calculations. The sulfinfinitene with 16 thiophene rings is only slightly more strained than the [8]sulflower, which has been synthesized, and can be considered as a promising synthetic target.

Excited-state aromaticity reversals in norcorrole.

Aromaticity reversals between the electronic ground state (S) and the lowest triplet (T) and singlet (S) excited states of Ni norcorrole (NiNc) and norcorrole (HNc) are investigated by comparing the HOMA (harmonic oscillator model of aromaticity) values at the optimized S, T and S geometries, and by analysing the changes in the nucleus-independent chemical shift (NICS) values and in the isotropic magnetic shielding distributions between the S and T states. The results strongly suggest that the antiaromatic features of the S states of the NiNc and HNc molecules, two very similar antiaromatic "internal crosses", undergo aromaticity reversals upon excitation to T or S and merge with the aromatic peripheries to produce Baird-aromatic systems with 24 π electrons each. Somewhat counterintuitively, the geometries of the fully aromatic T and S states of NiNc and HNc turn out to have larger bowl depths and so are more non-planar than the corresponding S geometries at which both molecules display antiaromatic features.

Aromaticity and Antiaromaticity Reversals between the Electronic Ground State and the Two Lowest Triplet States of Thiophene.

It is shown, by examining the variations in off-nucleus isotropic magnetic shielding around a molecule, that thiophene which is aromatic in its electronic ground state (S) becomes antiaromatic in its lowest triplet state (T) and then reverts to being aromatic in T. Geometry relaxation has an opposite effect on the aromaticities of the ππ* vertical T and T: The antiaromaticity of T is reduced whereas the aromaticity of T is enhanced. The shielding picture around T is found to closely resemble those around certain second singlet ππ* excited states (S), for example, those of benzene and cyclooctatetraene, thought to be "strongly aromatic" because of their very negative nucleus-independent chemical shift (NICS) values.

Aromaticity in the Electronic Ground and Lowest Triplet States of Molecules with Fused Thiophene Rings.

Analysis of the variations of the off-nucleus isotropic magnetic shielding, σ(r), around thiophene, thienothiophenes, dithienothiophenes and sulflowers in their electronic ground (S) and lowest triplet (T) states reveals that some of the features of aromaticity and bonding in these molecules do not fit in with predictions based on the popular Hückel's and Baird's rules. Despite having 4n π electrons, the S states of the sulflowers are shown to be aromatic, due to the local aromaticities of the individual thiophene rings. To reduce its T antiaromaticity, the geometry of thiophene changes considerably between S and T: In addition to losing planarity, the carbon-carbon two 'double' and one 'single' bonds in S turn into two 'single' and one 'double' bonds in T.

α-Functionalisation of Cyclic Sulfides Enabled by Lithiation Trapping.

A general and straightforward procedure for the lithiation trapping of cyclic sulfides such as tetrahydrothiophene, tetrahydrothiopyran and a thiomorpholine is described. Trapping with a wide range of electrophiles is demonstrated, leading to more than 50 diverse α-substituted saturated sulfur heterocycles. The methodology provides access to a range of α-substituted cyclic sulfides that are not easily synthesised by the currently available methods.

Reassessing the Composition of Hybrid Orbitals in Contemporary VB Calculations.

Large variations in the ratios between the p and s components of individual hybrid orbitals that have been observed in contemporary ab initio VB calculations are reassessed, and links are established to specific energy terms that drive bond formation. It is demonstrated that the ratios between the p and s components for individual hybrid orbitals are not indicative of the overall hybridization status of the relevant atom, which exhibits only relatively small variations with the level of theory, irrespective of whether or not non-dynamical and dynamical electron correlation effects are accounted for. An alternative orbital representation that turns out to be far more consistent with the overall hybridization of the relevant atom is examined.

Excited-State Aromaticity Reversals in Naphthalene and Anthracene.

Aromaticity reversals between the electronic ground (S) and low-lying singlet (S, S) and triplet (T, T, T) states of naphthalene and anthracene are investigated by calculating the respective off-nucleus isotropic magnetic shielding distributions using complete-active-space self-consistent field (CASSCF) wavefunctions involving gauge-including atomic orbitals (GIAOs). The shielding distributions around the aromatic S, antiaromatic S (L), and aromatic S (L) states in naphthalene are found to resemble the outcomes of fusing together the respective S, S, and S shielding distributions of two benzene rings. In anthracene, L is lower in energy than L, and as a result, the S state becomes aromatic, and the S state becomes antiaromatic; the corresponding shielding distributions are found to resemble extensions by one ring of those around the S and S states in naphthalene.

How Different are the Diamagnetic and Paramagnetic Contributions to Off-Nucleus Shielding in Aromatic and Antiaromatic Rings?

The spatial variations in the diamagnetic and paramagnetic contributions to the off-nucleus isotropic shielding, , and to the zz component of the off-nucleus shielding tensor, , around benzene (C H ) and cyclobutadiene (C H ) are investigated using complete-active-space self-consistent field wavefunctions. Despite the substantial differences between and around the aromatic C H and the antiaromatic C H , the diamagnetic and paramagnetic contributions to these quantities, and , and and , are found to behave similarly in the two molecules, shielding and deshielding, respectively, each ring and its surroundings. The different signs of the most popular aromaticity criterion, the nucleus-independent chemical shift (NICS), in C H and C H are shown to follow from a change in the balance between the respective diamagnetic and paramagnetic contributions.

Magnetic Shielding Analysis of Bonding in [1.1.1]Propellane.

The bonding in [1.1.1]propellane, bicyclo[1.

The Effect of Hydrogenation on the Contest between Aromaticity and Antiaromaticity in Norcorrole.

Magnetic shielding studies demonstrate that successive hydrogenation of Ni norcorrole (NiNc), a stable molecule combining aromatic and antiaromatic features, first weakens and then eliminates the central antiaromatic region, even though the NiNc antiaromatic "core", a 14-membered conjugated cycle with 16 π electrons, is formally preserved throughout the H NiNc-H NiNc series. The differences between aromatic and non-aromatic isotropic shielding distributions and nucleus-independent chemical shift (NICS) values in these hydrogenated porphyrin analogues are highlighted by comparing the results for the members of the H NiNc-H NiNc series to those for the aromatic Ni porphyrin complex. The results strongly support the unexpected and counterintuitive conclusion that H NiNc will be nonaromatic, without even a trace of antiaromaticity.

Aromaticity reversals and their effect on bonding in the low-lying electronic states of cyclooctatetraene.

Aromaticity reversals and their effect on chemical bonding in the low-lying electronic states of cyclooctatetraene (COT) are investigated through a visual approach which examines the variations in isotropic magnetic shielding in the space surrounding the molecule. The ground state (S) of COT is shown to be strongly antiaromatic at the π-bond-shifting transition state (TS), a regular octagon of symmetry; S antiaromaticity decreases at the planar bond-alternating tub-to-tub ring-inversion TS but traces of it are shown to persist even at the tub-shaped local minimum geometry. The lowest triplet (T) and first singlet excited (S) states of COT are found to have very similar geometries and visually indistinguishable shielding distributions closely resembling that in benzene and indicating similarly high levels of aromaticity.

Magnetic shielding paints an accurate and easy-to-visualize portrait of aromaticity.

Chemists are trained to recognize aromaticity semi-intuitively, using pictures of resonance structures and Frost-Musulin diagrams, or simple electron-counting rules such as Hückel's 4 + 2/4 rule. To quantify aromaticity one can use various aromaticity indices, each of which is a number reflecting some experimentally measured or calculated molecular property, or some feature of the molecular wavefunction, which often has no visual interpretation or may not have direct chemical relevance. We show that computed isotropic magnetic shielding isosurfaces and contour plots provide a feature-rich picture of aromaticity and chemical bonding which is both quantitative and easy-to-visualize and interpret.

Role of Dynamical Electron Correlation in the Differences in Bonding between CaAlH and MgAlH.

The most important factor behind the intriguing differences between the geometries of the M'AlH (M' = Mg, Ca) molecules is shown to be dynamical electron correlation and not intramolecular Coulombic interactions, as previously thought. Spin-coupled generalized valence bond (SCGVB) calculations reveal the different bonding situations in the two molecules at their optimal geometries but do not explain why these geometries differ so much; the solution to this conundrum comes instead from detailed analysis of coupled-cluster (CCSD(T)) energies at model and optimal geometries.

Spin-Coupled Generalized Valence Bond Theory: New Perspectves on the Electronic Structure of Molecules and Chemical Bonds.

Spin-Coupled Generalized Valence Bond (SCGVB) theory provides the foundation for a comprehensive theory of the electronic structure of molecules. SCGVB theory offers a compelling orbital description of the electronic structure of molecules as well as an efficient and effective zero-order wave function for calculations striving for quantitative predictions of molecular structures, energetics, and other properties. The orbitals in the SCGVB wave function are usually semilocalized, and for most molecules, they can be interpreted using concepts familiar to all chemists (hybrid orbitals, localized bond pairs, lone pairs, etc.

Detailed Visualization of Aromaticity Using Isotropic Magnetic Shielding.

For many years, Clar's aromatic sextet theory has served as a qualitative method for assessing the aromatic character of polycyclic aromatic hydrocarbons. A new approach, based on the calculation of isotropic magnetic shielding (IMS) contour plots, is shown to provide a feature-rich picture of aromaticity that is both quantitative yet still easily interpreted. Chemists are visual creatures who are adept at discerning reactivity and chemical behavior from molecular structures.

Excited-State Aromaticity Reversals in Möbius Annulenes.

It is suggested that Möbius annulenes follow a rule similar to Baird's rule such that the 4 and 4 + 2 criteria for Möbius electronic ground-state aromaticity and antiaromaticity are reversed in the lowest triplet and first singlet excited electronic states. Support comes from an investigation of aromaticity in the ground (S), lowest triplet (T), and first singlet excited (S) electronic states of the Möbius-aromatic cyclononatetraenyl cation, CH, using isotropic magnetic shielding isosurfaces calculated with state-optimized complete-active-space self-consistent field wave functions constructed from gauge-including atomic orbitals. Examination of these isosurfaces demonstrates that while the S state of CH is aromatic, the T and S states are antiaromatic.

Norcorrole: Aromaticity and Antiaromaticity in Contest.

Magnetic shielding studies demonstrate that nickel norcorrole (NiNc) and norcorrole (HNc) provide unusual examples of stable molecules with high antagonistic levels of antiaromaticity and aromaticity: Both incorporate an antiaromatic "core", a 14-membered cyclic conjugated subsystem with 16 π electrons, surrounded by an aromatic "halo" in the form of a ring of either 14 atoms and 14 π electrons with a new type of homoconjugation (NiNc), or 18 atoms with 18 π electrons (HNc).

Are Multicentre Bond Indices and Related Quantities Reliable Predictors of Excited-State Aromaticity?

Systematic scrutiny is carried out of the ability of multicentre bond indices and the NOEL-based similarity index to serve as excited-state aromaticity criteria. These indices were calculated using state-optimized complete active-space self-consistent field wavefunctions for several low-lying singlet and triplet states of the paradigmatic molecules of benzene and square cyclobutadiene and the inorganic ring SN. The comparison of the excited-state indices with aromaticity trends for individual excited states suggested by the values of magnetic aromaticity criteria show that whereas the indices work well for aromaticity reversals between the ground singlet and first triplet electronic states, addressed by Baird's rule, there are no straightforward parallels between the two sets of data for singlet excited states.

Controlling the S Energy Profile by Tuning Excited-State Aromaticity.

The shape of the lowest singlet excited-state (S) energy profile is of primary importance in photochemistry and related materials science areas. Here we demonstrate a new approach for controlling the shape of the S energy profile which relies on tuning the level of excited-state aromaticity (ESA). In a series of fluorescent π-expanded oxepins, the energy decrease accompanying the bent-to-planar conformational change in S becomes less pronounced with lower ESA levels.

Can Anti-Aufbau DFT Calculations Estimate Singlet Excited State Aromaticity? Correspondence on "Dibenzoarsepins: Planarization of 8π-Electron System in the Lowest Singlet Excited State".

The simple anti-aufbau DFT approach for estimating singlet excited state aromaticity suggested in a recent Communication published in this journal is shown to produce incorrect results because it targets a linear combination of the singlet and triplet configurations involving the HOMO and LUMO rather than the first singlet excited state. If the S state of a molecule is dominated by the HOMO→LUMO excitation, a comparably simple but theoretically consistent and qualitatively correct approximation to the S wavefunction can be achieved by performing a small "two electrons in two orbitals" CASSCF(2,2) calculation which can be followed by the evaluation of magnetic aromaticity criteria such as NICS.

Demonstration of Baird's rule complementarity in the singlet state with implications for excited-state intramolecular proton transfer.

The aromatic character of an arene is proposed to switch from aromatic in the ground state (S0) to antiaromatic in the S1 and T1 excited states. This behavior is known as Baird's rule and has been invoked to explain excited-state properties, primarily in the triplet state, whereas rationalization of antiaromaticity in the singlet state is less developed. This work demonstrates the first application of Baird's rule to rationalize previously unexplained experimental behavior of the singlet state process known as excited-state intramolecular proton transfer (ESIPT).

Statistical Thermodynamics Unveils How Ions Influence an Aqueous Diels-Alder Reaction.

The kinetics of Diels-Alder (DA) reactions in water has been known to be altered by salts for a long time. Yet the question how salts influence the reaction rate, either as rate-enhancing or rate-reducing additives, has so far remained unresolved. Conflicting hypotheses involve (i) indirect salt contributions through the modulation of internal pressure and (ii) making (or breaking) of the so-called "water-structure" by salts that strengthen (or weaken) the hydrophobic effect.

Does the Electronic Structure of Möbius Annulenes Follow Heilbronner's Ideas?

It is shown, on the example of the monocyclic cyclononatetraenyl cation, , that the fully-variational optimization of modern ab initio wavefunctions based on spin-coupled generalized valence bond (SCGVB) theory vindicates, in a surprising level of detail, essential features of Heilbronner's ideas for the electronic structure of Möbius annulenes such as the arrangement of overlapping carbon 2p atomic orbitals along a Möbius strip, leading to a phase inversion between the first and last orbitals. In the SCGVB description, the aromaticity of this Möbius system with eight π electrons follows from the extensive resonance between VB structures.

Magnetic Shielding, Aromaticity, Antiaromaticity and Bonding in the Low-Lying Electronic States of S N.

Aromaticity, antiaromaticity and chemical bonding in the ground (S ), first singlet excited (S ) and lowest triplet (T ) electronic states of disulfur dinitride, S N , were investigated by analysing the isotropic magnetic shielding, σ (r), in the space surrounding the molecule for each electronic state. The σ (r) values were calculated by state-optimized CASSCF/cc-pVTZ wave functions with 22 electrons in 16 orbitals constructed from gauge-including atomic orbitals (GIAOs). The S and T electronic states were confirmed as 1 A and 1 B , respectively, through linear response CC3/aug-cc-pVTZ calculations of the vertical excitation energies for eight singlet (S -S ) and eight triplet (T -T ) electronic states.