Is either direct photolysis or photocatalysed H-shift of peroxyl radicals a competitive pathway in the troposphere?

Peroxyl radicals (ROO.) are key intermediates in atmospheric chemistry, with relatively long lifetimes compared to most other radical species. In this study, we use multireference quantum chemical methods to investigate whether photolysis can compete with well-established ROO. sink reactions. We assume that the photolysis channel is always ROO. + h nu => RO + O(P-3). Our results show that the maximal value of the cross-section for this channel is sigma = 1.3 x 10(-18) cm(2) at 240 nm for five atmospherically representative peroxyl radicals: CH3OO., C(O)HCH2OO., CH3CH2OO., HC(O)OO. and CH3C(O)OO.. These values agree with experiments to within a factor of 2. The rate constant of photolysis in the troposphere is around 10(-5) s(-1) for all five ROO.. As the lifetime of peroxyl radicals in the troposphere is typically less than 100 s, photolysis is thus not a competitive process. Furthermore, we investigate whether or not electronic excitation to the first excited state (D-1) by infrared radiation can facilitate various H-shift reactions, leading, for example, in the case of CH3OO. to formation of O.H and CH2O or HOO. and CH2 products. While the activation barriers for H-shifts in the D-1 state may be lower than in the ground state (D-0), we find that H-shifts are unlikely to be competitive with decay back to the D-0 state through internal conversion, as this has a rate of the order of 10(13) s(-1) for all studied systems.Peer reviewe

Closed-shell paramagnetic porphyrinoids

Magnetizabilities and magnetically induced ring-current strength susceptibilities have been calculated at the Hartree-Fock, density functional theory and second order Moller-Plesset levels for a number of antiaromatic closed-shell carbaporphyrins, carbathia-porphyrins and isophlorins. The calculations yield a linear relation between magnetizabilities and ring-current strength susceptibilities. The calculations show that the porphyrinoids with the largest ring-current strength susceptibility are closed-shell paramagnetic molecules with positive magnetizabilities. The closed-shell paramagnetism is due to the large paramagnetic contribution to the magnetizability originating from the strong paratropic ring current in the antiaromatic porphyrinoids.Peer reviewe

Bicycloaromaticity and Baird-type bicycloaromaticity of dithienothiophene-bridged [34]octaphyrins

Aromatic properties of two recently synthesized dithienothiophene-bridged (DTT) [34]octaphyrins have been investigated by calculating magnetically induced current densities and vertical excitation energies. These intriguing molecules have been proposed to be the first synthesized neutral bicycloaromatic compounds. The triplet state of their dications was even suggested to be Baird-type bicycloaromatic rendering them very interesting as a new prototype of molecules possessing simultaneously the two rare types of aromaticity. Here, we investigate computationally the aromatic properties of the neutral as well as the singly and doubly charged DTT-bridged [34]octaphyrins. Our study provides unambiguous information about changes in the aromatic properties of the DTT-bridged [34]octaphyrins upon oxidation. The calculations identify two independent diatropic ring currents in the neutral DTT-bridged [34]octaphyrins, showing that they are indeed bicycloaromatic. The current-density flow of the two independent ring currents of the bicycloaromatic compounds are visualized and individual aromatic pathways are quantified by performing numerical integration. The calculations show that two independent diatropic ring currents can indeed be sustained by molecules consisting of two aromatic rings that share a common set of π electrons. The current density calculations on the singly charged DTT-bridged [34]octaphyrins show that they are weakly antiaromatic, which does not agree with the suggested aromatic character deduced from spectroscopical studies. The triplet state of the two DTT-bridged [34]octaphyrin cations with very similar molecular structures have unexpectedly different aromatic character. One of them is Baird-type bicycloaromatic, whereas the triplet state of the other dication has one aromatic and one nonaromatic ring, which could not be resolved from available spectroscopical data. Calculations of excitation energies reveal that a simple model cannot be employed for interpreting the electronic excitation spectra of the present molecules, because more than 20 excited states contribute to the spectra above 2.5 eV (500 nm) showing the importance of computations. The present work illustrates how detailed information about molecular aromaticity can nowadays be obtained by scrutinizing calculated current densities.Peer reviewe

Optical and magnetic properties of antiaromatic porphyrinoids

Magnetic and spectroscopic properties of a number of formally antiaromatic carbaporphyrins, carbathiaporphyrins and isophlorins with 4n pi electrons have been investigated at density functional theory and ab initio levels of theory. The calculations show that the paratropic contribution to the magnetically induced ring-current strength susceptibility and the magnetic dipole-transition moment between the ground and the lowest excited state are related. The vertical excitation energy (VEE) of the first excited state decreases with increasing ring-current strength susceptibility, whereas the VEE of the studied higher-lying excited states are almost independent of the size of the ring-current strength susceptibility. Strong antiaromatic porphyrinoids, based on the magnitude of the paratropic ring-current strength susceptibility, have small energy gaps between the highest occupied and lowest unoccupied molecular orbitals and a small VEE of the first excited state. The calculations show that only the lowest S-0 -> S-1 transition contributes signficantly to the magnetically induced ring-current strength susceptibility of the antiaromatic porphyrinoids. The decreasing optical gap combined with a large angular momentum contribution to the magnetic transition moment from the first excited state explains why molecules III-VII are antiaromatic with very strong paratropic ring-current strength susceptibilities. The S-0 -> S-1 transition is a magnetic dipole-allowed electronic transition that is typical for antiaromatic porphyrinoids with 4n pi electrons.Peer reviewe

Predicting the degree of aromaticity of novel carbaporphyrinoids

Magnetically induced current densities have been calculated for dioxaporphyrin, dithiaporphyrin, true carbaporphyrins, and N-confused porphyrins using the gauge-including magnetically induced current (GIMIC) method. The current-strength susceptibilities (current strengths) have been obtained by numerically integrating the current flow passing selected chemical bonds. The current strength calculations yield very detailed information about the electron delocalization pathways of the molecules. The strength of the ring-current that circles around the porphyrinoid macroring is used to estimate the degree of molecular aromaticity. The studied porphyrinoid structures have been obtained by replacing the NH and N groups of porphin with formally isoelectronic moieties such as O, S, CH and CH2. Replacing an NH moiety of trans-porphin with isoelectronic O and S does not significantly change the current strengths and pathways, whereas substitution of N with an isoelectronic CH group leads to significant changes in the current pathway and current strengths. CH2 groups cut the flow of diatropic currents, whereas in strongly antiaromatic molecules a significant fraction of the paratropic ring-current is able to pass the sp(3) hybridized inner carbons. N-confused porphyrinoids sustain a ring current whose strength is about half the ring-current strength of porphin with the dominating current flow along the outer pathway via the NH moiety. When no hydrogen is attached to the inner carbon of the inverted pyrrolic ring, the current prefers the inner route at that ring.Peer reviewe

Photophysical properties of the triangular [Au(HN & xe001;COH)](3) complex and its dimer

Rate constants for radiative and non-radiative transitions of the [Au(HN & xe001;COH)](3) complex and its dimer were calculated within the Herzberg-Teller approximation based on quantum mechanical principles. A high triplet quantum yield was estimated for the monomer. Internal conversion (IC) was found to be the major competing process to the intersystem crossing (ISC) from the lowest excited singlet state (S-1) to the lowest triplet state (T-1). ISC and IC from the spin-mixed T & x303;(1) state also dominate the triplet relaxation process resulting in a negligible phosphorescence quantum yield for the monomer. The IC and ISC rate constants of the dimer are considerably smaller due to much lower Franck-Condon factors. For the dimer a triplet quantum yield of 0.71 was estimated using the extended multi-configuration quasi-degenerate second-order perturbation theory (XMCQDPT2) method to calculate the transition energies. Fluorescence is the major competing process to the ISC relaxation of the S-1 state of the dimer. The ISC and IC processes are insignificant for the relaxation of the T-1 state, resulting in unity phosphorescence quantum yield. The high triplet and phosphorescence quantum yields of the [Au(HN & xe001;COH)](3) dimer make it and its higher oligomers potential candidates as dopants for phosphorescent organic light emitting diodes and as down-converters in solid-state lighting systems.Peer reviewe

Theoretical studies as a tool for understanding the aromatic character of porphyrinoid compounds

The present review gives an overview over different theoretical approaches to study the aromaticity of porphyrins and porphyrin based compounds with the focus on magnetically induced current densities including recent developments and applications in our groups. Observed trends as well as new visualization options for current densities are presented. It is shown that new insights can be obtained for porphyrin based compounds by complementing experimental work with computed current densities. A thorough current density analysis can lead to novel viewpoints and detailed interpretations of experimental findings as compared to many other approaches. Current density studies might also serve as inspiration for future experimental works. Merely a visual inspection of the current density might not be sufficient and should therefore always be accompanied by an integration analysis of the current flow, which yields accurate current-density pathways. Recent attempts to link calculated current strength susceptibilities of antiaromatic porphyrinoids to optical properties and magnetizabilities are discussed.Peer reviewe

Computational Studies of Aromatic and Photophysical Properties of Expanded Porphyrins

Magnetically induced current densities and ring-current pathways have been calculated at density functional theory (DFT) and second-order Moller-Plesset perturbation theory (MP2) levels of theory for a set of expanded porphyrins consisting of five or six pyrrolic rings. The studied molecules are sapphyrin, cyclo [6]pyrrole, rubyrin, orangarin, rosarin, and amethyrin. Different functionals have been employed to assess the functional dependence of the ring-current strength susceptibility. Vertical singlet and triplet excitation energies have been calculated at the second-order approximate coupled cluster (CC2), expanded multiconfigurational quasi-degenerate perturbation theory (XMC-DPT2), and time-dependent density functional theory levels. The lowest electronic transition of the antiaromatic molecules was found to be pure magnetic transitions providing an explanation for the large paratropic contribution to the total current density. Rate constants for different nonradiative deactivation channels of the lowest excited states have been calculated yielding lifetimes and quantum yields of the lowest excited singlet and triplet states. The calculations show that the spin-orbit interaction between the lowest singlet (S-0) and triplet (T-1) states of the antiaromatic molecules is strong, whereas for the aromatic molecule the spin-orbit coupling vanishes. The experimentally detected fluorescence from S-2 to S-0 of amethyrin has been explained. The study shows that there are correlations between the aromatic character and optical properties of the investigated expanded porphyrins.Peer reviewe