Diatropic Currents Research Articles

Magnetically Induced Current Densities in Aromatic, Antiaromatic, Homoaromatic, and Nonaromatic Hydrocarbons

The magnetically induced current densities for ring-shaped hydrocarbons are studied at the density functional theory (DFT) and second-order Møller-Plesset (MP2) levels using gauge-including atomic orbitals. The current densities are calculated using the gauge-including magnetically induced current approach. The calculations show that all studied hydrocarbon rings sustain strong diatropic and paratropic ring currents when exposed to an external magnetic field, regardless whether they are unsaturated or not. For nonaromatic rings, the strength of the paratropic current flowing inside the ring is as large as the diatropic one circling outside it, yielding a vanishing net ring current. For aromatic molecules, the diatropic current on the outside of the ring is much stronger than the paratropic one inside, giving rise to the net diatropic ring current that is typical for aromatic molecules. For antiaromatic molecules, the paratropic ring-current contribution inside the ring dominates. For homoaromatic molecules, the diatropic current circles at the periphery of the ring. The ring current is split at the CH(2) moiety; the main fraction of the current flow passes outside the CH(2) at the hydrogens, and some current flows inside the carbon atom. The diatropic current does not take the through-space short-cut pathway, whereas the paratropic current does take that route. Calculations of the ring-current profile show that the ring current of benzene is not transported by the pi electrons on both sides of the molecular ring. The strongest diatropic ring current flows on the outside of the ring and in the ring plane. A weaker paratropic current circles inside the ring with the largest current density in the ring plane. Due to the ring strain, small unconjugated and saturated hydrocarbon rings sustain a strong ring current which could be called ring-strain current. Nuclear magnetic shieldings calculated for 1,3,5-cycloheptatriene and homotropylium at the DFT and MP2 levels agree well with experimental values.

Origin of Stacked-Ring Aromaticity

Although strong diatropic currents run in the pi-stacked dimers of 4n pi annulenes, they are still antiaromatic with negative topological resonance energies (TREs). The TRE difference between two annulene molecules and the pi-stacked annulene dimer can be used as a practical measure of stacked-ring aromaticity. As predicted by Corminboeuf et al., all pi-stacked 4n pi annulene dimers exhibit marked stacked-ring aromaticity. In contrast, the TRE for the entire conjugated system of a pi-stacked benzene dimer is much smaller than twice the TRE for benzene. The analysis of circuit contributions to energetic and magnetic properties shows that such stacked-ring aromaticity and diatropicity arise from many (4n + 2)-site circuits created by the stacking of two antiaromatic rings. Tetragonal faces formed in the dimers are responsible for the creation of these (4n + 2)-site circuits.

A density functional study of NBO, NICS and 14N NQR parameters of 5-methylcytosine tautomers in the gas phase

The NBO (natural bond orbital analysis), NICS (nucleus independent chemical shift) and 14N NQR parameters of the most stable tautomers of 5-methylcytosine in the gas phase were predicted using density functional theory (DFT) method. The NBO analysis revealed that the resonance interaction between lone pair of the nitrogen atom and empty (σ* or π*) non-Lewis NBO increases with increasing the p character of the nitrogen lone pair. The present investigation indicated the π clouds in the considered heterocyclic ring contained six electrons, and these tautomers have the aromatic character. The NICS study utilizing the gauge-invariant atomic orbital (GIAO) method showed that there are diatropic currents in the heterocyclic rings of the tautomers, so we determined the order of aromaticity of these tautomers. The results of NQR parameter calculations showed three parameters are effective on nuclear quadrupole coupling constant; the p character value of lone pair electrons of nitrogens, and the related occupancies and the portion of lone pair electrons interfering in magnetic currents of ring and whenever, the lone pair electrons of nitrogens participate in the formation of chemical bond and/or π system of the ring, the q zz and consequently its χ decreases.

Stacked-Ring Aromaticity: An Orbital Model

Visualization of induced current density using the ipsocentric CHF/CTOCD-DZ/6-31G** approach gives a direct demonstration of the literature proposal of reversal of [4n]annulene antiaromaticity on stacking cyclooctatetraene (COT) rings into a superphane. Through-space interactions lead to a closed-shell in which paratropicity of planar COT units is quenched, and layered diatropic currents arise from magnetic response of two pairs of frontier orbitals. A general orbital model rationalizes the differences in current between stacked aromatic and antiaromatic rings.

Electric-field perturbations of ring currents in π systems

Ipsocentric distributed-gauge calculations are used to map differential ring currents induced in [4 n+2] and [4 n] π systems by simultaneous application of an axial magnetic and electric field (field gradient). In benzene, electric fields and gradients produce stratified first-order π ring currents, opposing/reinforcing the intrinsic diatropic current, according to the height profile of charge depletion/enhancement induced by the electric perturbation. In (planarised) cyclooctatetraene, action of a uniform electric field is similarly rationalised, but response to the (totally symmetric) field gradient is dominated by a specific orbital effect, closing/widening of the small HOMO–LUMO gap leading to reinforcement/reduction of the intrinsic paratropic current.

Aromaticity of organic heterocyclothiazenes and analogues.

Members of a series of carbon-poor sulfur-nitrogen heterocycles and polycycles are shown by direct ab initio ipsocentric calculation to support diatropic ring currents and hence to be aromatic on the basis of magnetic criteria. They include 7-cycles S(3)N(2)(CH)(2), S(3)N(3)(CH), and S(3)N(4) and 8-cycles S(2)N(4)(CH)(2) and S(2)N(2)(CH)(4), all with 10 pi electrons. The unknown trithiatetrazepine S(3)N(4) is predicted to be at least as aromatic as its known diaza and triaza homologues. Angular-momentum arguments show that the pi-electron-rich nature of (4n + 2) SN heterocycles is the key to their diatropic current. The Woodward dithiatetrazocine parent framework S(2)N(4)(CH)(2) supports a diatropic ring current, as does its analogue in which N and CH groups are formally exchanged. Formal expansion of (4n + 2)-pi carbocyclic systems by insertion of NSN motifs in every CC bond is predicted to lead to structures that support diatropic ring currents: explicit ab initio calculation of magnetic response predicts the 24-center, 30-pi-electron heterocycle S(6)N(12)(CH)(6), formally derived from benzene, to be aromatic on the basis of this criterion.

Ring currents as probes of the aromaticity of inorganic monocycles : P5-, As5-, S2N2, S3N3-, S4N3+, S4N42+, S5N5+, S42+ and Se42+.

Current-density maps were calculated by the ipsocentric CTOCD-DZ/6-311G** (CTOCD-DZ=continuous transformation of origin of current density-diamagnetic zero) approach for three sets of inorganic monocycles: S(4) (2+), Se(4) (2+), S(2)N(2), P(5) (-) and As(5) (-) with 6 pi electrons; S(3)N(3) (-), S(4)N(3) (+) and S(4)N(4) (2+) with 10 pi electrons; and S(5)N(5) (+) with 14 pi electrons. Ipsocentric orbital analysis was used to partition the currents into contributions from small groups of active electrons and to interpret the contributions in terms of symmetry- and energy-based selection rules. All nine systems were found to support diatropic pi currents, reinforced by sigma circulations in P(5) (-), As(5) (-), S(3)N(3) (-), S(4)N(3) (+), S(4)N(4) (2+) and S(5)N(5) (+), but opposed by them in S(4) (2+), Se(4) (2+) and S(2)N(2). The opposition of pi and sigma effects in the four-membered rings is compatible with height profiles of calculated NICS (nucleus-independent chemical shifts).

FRONTIER-ORBITAL RING CURRENTS AND THE ANNULENE ANALOGY

Perimeter and ring currents induced in planar monocycles and polycycles by perpendicular magnetic fields are commonly taken as diagnostic of aromaticity and antiaromaticity in these systems. Diatropic π currents are associated with aromaticity, paratropic with its opposite. The ipsocentric method is an accurate and economical way of calculating ab initio current-density maps. In π systems it provides a natural interpretation of ring currents in terms of nonredundant orbital contributions, governed by simple symmetry rules for π–π* virtual excitations and dominated by the frontier electrons. Thus, in planar [4n + 2] monocycles, the product of π HOMO and π* LUMO symmetries includes that of the in-plane translations and leads to a 4-electron diatropic current. In planar [4n] monocycles, this product includes the symmetry of the in-plane rotation and leads to a 2-electron paratropic current. Perturbation arguments based on the monocycle explain the opposite senses of the π ring currents in naphthalene (diatropic) and pentalene (paratropic) as consequences of their different frontier-orbital symmetry products. In a generalization to heterocycles, ring current maps for benzotriazole (BtH), its conjugate base (Bt−) and the cation (Bt+) are calculated at an ab initio level. The diatropic current of the two 10π systems and the paratropic current of the 8π system are rationalized in terms of the perturbed-annulene orbital model, giving an explanation of the applicability of simple electron counting in these cases: where the frontier-orbital structure remains close to that of the [4n+ 2]/[4n] annulene, so does the current-density map.

Aromaticity, polarisability and ring current

Electric dipole polarisability has been proposed as an alternative to magnetic criteria of aromaticity. The ipsocentric formulation of the molecular response to a magnetic field gives selection rules for diatropic ring current that coincide with those for in-plane polarisability. Thus, induced π current strength might correlate, if at all, with HOMO contributions to the in-plane components of polarisability of diatropic systems. No equivalent correlation is expected a priori for anti-aromatic paratropic systems, or clamped systems in which diatropic current is quenched by interaction with π systems. Ab initio calculations of π current density maps and dipole polarisability contributions confirm the expected correlation for monocycles, but show the limitations of polarisability as a measure of aromaticity.

Perimeter Effects on Ring Currents in Polycyclic Aromatic Hydrocarbons: Circumcoronene and Two Hexabenzocoronenes

AbstractCurrent‐density maps are calculated at an ab initio level for the three symmetrical polycyclic aromatic hydrocarbons, circumcoronene [1 (D6h)], hexabenzo[bc,ef,hi,kl,no,qr]coronene [2 a (D6h) and 2 b (D3d)], and hexabenzo[a,d,g,j,m,p]coronene [3 a (D6h), 3 b (D6) and 3 c (D3d)], all of which can be formally derived by annelation of benzene rings to a coronene core. Whilst 1 is planar, 2 has a non‐planar minimum that is effectively isoenergetic with its planar form, and 3 has a well defined non‐planar structure. The shape of the molecular boundary rather than the planarity of the molecule plays the critical rôle in the character of the predicted currents. Formal deletion of outer hexagons from circumcoronene (1) in two different ways produces either hexabenzocoronene 2 with a prediction of disjoint local benzenoid diatropic currents linked by a global perimeter, or 3 with a giant diatropic perimeter current enclosing a weak paramagnetic circulation on the central hexagon. The current density map of 1 is effectively a superposition of those of 2 and 3. Its strong diatropic perimeter current subsumes the six weaker diatropic benzenoid circulations evident in 2, and bifurcates in the six outer benzenoid rings that form the corners of the giant hexagon; its benzene “hub” sustains a diatropic current, as would be expected from the partial cancellation of the strong diatropic hub current of 2 by the weaker paratropic hub current of 3. The relationship between the three molecules is rationalised by considering orbital contributions to their current density maps.

Nucleus-independent chemical shifts and local aromaticities in large polycyclic aromatic hydrocarbons

Central rings of coronene and kekulene are known to be paratropic with small positive nucleus-indepedent chemical shift (NICS) values at the centers although they are (4 n + 2)-membered rings. However, careful inspection of the bond resonance energy (BRE) pattern in each π-system revealed that all constituent rings are aromatic in nature. Individual rings were found to sustain diatropic currents even if some bond currents are vanishingly small. Thus, NICS values at the ring centers of large polycyclic π-systems, including fullerenes, cannot always be used as indicators of local aromaticity. BRE is better suited for estimating the degree of local aromaticity in polycyclic π-systems.

The aromaticity and antiaromaticity of dehydroannulenes

The molecular structures and the magnetic properties of hexadehydro[12]annulenes and dodecadehydro[18]annulenes have been studied at ab initio and density-functional levels. The calculations show that the unsubstituted dehydro[12]annulene sustains a paratropic ring current in an external magnetic field, while the ring current for the unsubstituted dehydro[18]annulene is diatropic. Since the induced ring current is one measure of the molecular aromaticity, dehydro[18]annulene can be considered aromatic and dehydro[12]annulene antiaromatic. Fused benzene or cyclobutadiene rings destroy both the antiaromaticity and the aromaticity of the dehydroannulenes, while the substitution of the six hydrogens by ethinyl groups affects the aromatic properties of the dehydroannulenes only slightly. The calculations show that the bond alternation is smaller for molecules sustaining large diatropic or paratropic ring currents than for nonaromatic molecules. The ring-current contributions to the 1H NMR chemical shieldings have been estimated for the molecules studied.

13 - The Use of Cytotoxic Chemotherapy in Conjunction with Gynaecological Surgery

The effect of the systematic replacement of a CH group by an N atom in the aromaticity and stability of a benzene is a question in chemistry that has several answers. In this article we will re-examine the aromaticity using the magnetic criteria through ring current strength (RCS) and NICSzz descriptors dissected in their σ and π contributions at DFT level and the relative stability and ΔEgap of azines at the CCSD(T) level. The results are pragmatic and show that, in effect, the replacement of a CH unit by N atom produces a reduction in aromaticity of azines being less aromatic those with more C-N units. This reduction is small so that systems still present large aromatic behaviour. Additionally, it is shown that there is an influence of the local diatropic currents located in the nitrogen atoms therefore the dissected indicators such as RCSπ and NICSπ,zz(1) are those that best describe the aromaticity in azines. Finally, it is observed a trend between RCS values and ΔEgap at the CCSD(T) level which could be explained using the maximum hardness principle. The systemic replacement causes a reduction in the relative stability of azines, and however, there is no direct relationship with aromaticity since it is necessary to take into account the σ effects.