-
[show abstract]
[hide abstract]
ABSTRACT: The effect of fluorine substitution on the aromaticity of polycyclic hydrocarbons (PAH) is investigated. Magnetically induced current densities, current pathways, and current strengths, which can be used to assess molecular aromaticity, are calculated using the gauge-including magnetically induced current method (GIMIC). The degree of aromaticity of the individual rings is compared to those obtained using calculated nucleus-independent chemical shifts at the ring centers (NICS(0) and NICS(0)(zz)). Calculations of explicitly integrated current strengths for selected bonds show that the aromatic character of the investigated polycyclic hydrocarbons is weakened upon fluorination. In contrast, the NICS(0) values for the fluorinated benzenes increase noteworthy upon fluorination, predicting a strong strengthening of the aromatic character of the arene rings. The integrated current strengths also yield explicit current pathways for the studied molecules. The current pathways of the investigated linear polyacenes, pyrene, anthanthrene, coronene, ovalene, and phenanthro-ovalene are not significantly affected by fluorination. NISC(0) and NICS(0)(zz) calculations provide contradictory degrees of aromaticity of the fused individual ring. Obtained NICS values do not correlate with the current strengths circling around the individual rings.
The Journal of Physical Chemistry A 09/2012; 116(41):10257-68. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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 Möbius 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.
Physical Chemistry Chemical Physics 09/2011; 13(46):20659-65. · 3.57 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report calculations of the gauge-independent magnetically induced current densities in [n]cycloparaphenylenes ([n]CP), n = 6-11. In addition to the neutral [n]CPs, the dianion of [6]CP and the current densities of the corresponding metal complexes Li(2)[6]CP and Mg[6]CP are also investigated. By the ring current criterion, the [6]CP with 4n pi electrons has a slight antiaromatic character, while [7]CP has (4n + 2) pi electrons and is weakly aromatic with a ring current susceptibility strength that is about 25% of the ring current of benzene. The larger neutral [n]CPs, n = 8-11, do not sustain any net ring current around the nanohoop and are essentially nonaromatic. The weak paramagnetic ring current susceptibility of [6]CP flows along a 4n pi pathway on either edge of the phenylene rings. For the dianions, the ring current susceptibility strengths are 24-35 nA/T diatropic and thus the addition of two electrons induces an electron delocalization and an aromatic character of the nanohoops. The dilithium complex of [6]CP with (4n + 2) pi electrons is aromatic with a net ring current strength of 28 nA/T or 2.4 times the ring current strength of benzene, involving all 62 pi electrons in the current pathway. The (1)H NMR chemical shieldings and the nucleus-independent chemical shifts correlate with the strengths of the magnetically induced currents. The aromatic [n]cycloparaphenylenes have a quinoid structure, whereas the weakly aromatic or nonaromatic ones are benzoidic.
The Journal of Organic Chemistry 09/2010; 75(17):5867-74. · 4.45 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The aromatic pathways and the degree of aromaticity of expanded porphyrins have been determined by explicit calculations of the routes and strengths of the magnetically induced currents using the gauge-including magnetically induced current (GIMIC) approach. Density functional theory calculations show that the doubly twisted hexaphyrins fulfilling Hückel's (4n + 2) pi-electron rule for aromaticity and those obeying the 4n pi-electron rule for antiaromaticity are aromatic and antiaromatic, respectively. The investigated [26]hexaphyrin (2) and (3) and [30]hexaphyrin (5) isomers are aromatic, and [28]hexaphyrin (4) is antiaromatic. The formally antiaromatic [24]hexaphyrin (1) does not sustain any strong ring current and must be considered nonaromatic. A detailed analysis of the current pathways of the hexaphyrins is presented. It was found that the current pathways of the investigated aromatic hexaphyrins are not always dominated by the flow along the inner route through the non-hydrogenated C-N-C moieties, as previously proposed. The current flow is often split into two branches at the pyrrole rings, but sometimes it takes the outer route via the C=C bond of the pyrrole. The current pathway of the weak paratropic ring current of [24]hexaphyrin is dominated by the outer C=C route. The calculations show that the routes of the current transport cannot be assessed merely by inspection or from nucleus independent chemical shifts; explicit calculations of the current pathways are compulsory. The current-density studies also show that the pyrrole rings do not sustain any strong ring currents of their own.
The Journal of Physical Chemistry A 07/2010; 114(26):7153-61. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The electronic absorption and emission spectra of the [n]cycloparaphenylenes with n = 6,7,...,11 ([n]CP) have been studied at the time-dependent density functional theory level. The calculations show that the optical gap increases with increasing size of the ring due to reduced ring strain in the larger carbon nanohoops, whereas the energy of the first bright state follows the opposite trend for the studied [n]CPs. For the excited-state structures, the C-C bonds between the phenylene groups have a significant double-bond character giving rise to a continuous electron delocalisation pathway around the ring. The torsion angles between the phenylene moieties are much smaller for the excited state than for the ground state suggesting that the excited state has a stronger electron delocalisation around the carbon nanohoop than for the ground state. The double bond character of the phenylene C-C bonds declines and the phenylene torsion angle increases with increasing ring size. The aromatic stabilisation of the excited state due to the continuous electron delocalisation pathway is probably the main reason for the large Stokes shift. The excited state of the larger [n]CPs are less aromatic than the smaller ones explaining why the Stokes shift decreases with increasing size of the ring. For large [n]CPs, the excitation-energy spectrum forms bands making localisation of the excitons feasible. Localisation of the excitons probably leads to the observed ring-size independence of the electronic excitation spectra for large [n]CPs.
Physical Chemistry Chemical Physics 03/2010; 12(11):2751-7. · 3.57 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The ring currents, NMR chemical shifts, topology of the chemical bonding, and UV-vis spectra of bianthraquinodimethane-stabilized [16]annulenes possessing Möbius and Hückel topology are investigated. The aromatic character of the title compounds is discussed on the basis of the magnetically induced current density obtained using the gauge-including magnetically induced current (GIMIC) approach. Numerical integration of the current density circling around the [16]annulene ring shows that both the Hückel and the Möbius isomers are non-aromatic. The [16]annulene ring of both isomers sustains a net ring current whose strength is only 0.3 nA/T. The ring current consists of a diamagnetic flow on the outside of the [16]annulene ring and a paramagnetic current inside it. Since the net ring-current strength of the [16]annulene is less than 5% of the ring current strength for benzene, both isomers must be considered non-aromatic by the ring current criterion. The similar bond length alternation of the [16]annulene rings also points to a similarity in aromatic character of the two isomers. The shape of the ring current of the Möbius isomer shows that the current density is somewhat more outspread than that of the Hückel isomer. Spatially separated diatropic and paratropic currents of equal strength follow the annulene bonds. The atoms-in-molecules (AIM) analysis reveals a cage critical point in the region of the outspread current density of the Möbius isomer. Intramolecular CH...pi and pi-pi interactions identified by AIM analysis, in combination with the outspread current density, stabilizes the Möbius isomer relative to the Hückel one. The molecule is characterized by calculating the (13)C and (1)H NMR chemical shifts and the UV-vis spectrum and comparing these to experimental spectra. The (13)C NMR and (1)H NMR chemical shifts are rather similar for the two isomers. The UV-vis spectra are compared with the excitation energies calculated at the time-dependent density functional theory (TDDFT) level using Becke's three-parameter hybrid functional together with the LYP correlation functional (B3LYP), as well as at the approximate coupled cluster singles (CCS) and at the approximate coupled cluster singles and doubles (CC2) levels of theory. The CC2 calculations yield excitation energies in fairly good agreement with experimental data.
The Journal of Organic Chemistry 10/2009; 74(17):6495-502. · 4.45 Impact Factor
