Intramolecular Nitrofuran Diels-Alder Reactions: Extremely Substituent-Tolerant Cycloadditions via Asynchronous Transition States.

Nitrofurans undergo intramolecular Diels-Alder reactions with tethered electron-poor dienophiles more rapidly and in higher yield than non-nitrated furans. Computational studies indicate that increased stabilization of a partial positive charge on the nitro-substituted carbon in both transition state and product is the driving force for these reactions. Frontier molecular orbital energy differences indicate a switch from normal to inverse electron demand upon nitration. There does not appear to be a contribution from any differences in aromatic stabilization energy between furans and nitrofurans. Calculations show that the nitrofuran reactions proceed via a highly asynchronous transition state allowing easier bond formation between two sterically hindered carbons.

Gelation Landscape Engineering Using a Multi-Reaction Supramolecular Hydrogelator System.

Simultaneous control of the kinetics and thermodynamics of two different types of covalent chemistry allows pathway selectivity in the formation of hydrogelating molecules from a complex reaction network. This can lead to a range of hydrogel materials with vastly different properties, starting from a set of simple starting compounds and reaction conditions. Chemical reaction between a trialdehyde and the tuberculosis drug isoniazid can form one, two, or three hydrazone connectivity products, meaning kinetic gelation pathways can be addressed. Simultaneously, thermodynamics control the formation of either a keto or an enol tautomer of the products, again resulting in vastly different materials. Overall, this shows that careful navigation of a reaction landscape using both kinetic and thermodynamic selectivity can be used to control material selection from a complex reaction network.

Crossed McMurry Coupling Reactions for Porphycenic Macrocycles: Non-Statistical Selectivity and Rationalisation.

Crossed McMurry reactions of bifuran- or bithiophenedicarbaldehydes with bipyrroledicarbaldehydes have been studied for the first time. Only those porphycenic macrocycles derived from homocoupled McMurry products were formed. The results are explained by using both density functional theory and electron propagator computations to model the electron affinity of the dialdehyde starting materials. It was predicted that bifuran\bithiophene cross-coupling would indeed occur, and this was demonstrated by the first synthesis of a novel dioxa,dithio hetero-porphycenoid annulene. This approach will allow the prior identification of viable substrates for related crossed McMurry reactions.

Unusual structure-energy correlations in intramolecular Diels-Alder reaction transition states.

Detailed analysis of calculated data from an experimental/computational study of intramolecular furan Diels-Alder reactions has led to the unusual discovery that the mean contraction of the newly forming C-C σ-bonds from the transition state to the product shows a linear correlation with both reaction Gibbs free energies and reverse energy barriers. There is evidence for a similar correlation in other intramolecular Diels-Alder reactions involving non-aromatic dienes. No such correlation is found for intermolecular Diels-Alder reactions.

Ultrafast photo-induced ligand solvolysis of cis-[Ru(bipyridine)2(nicotinamide)2](2+): experimental and theoretical insight into its photoactivation mechanism.

Mechanistic insight into the photo-induced solvent substitution reaction of cis-[Ru(bipyridine)2(nicotinamide)2](2+) (1) is presented. Complex 1 is a photoactive species, designed to display high cytotoxicity following irradiation, for potential use in photodynamic therapy (photochemotherapy). In Ru(II) complexes of this type, efficient population of a dissociative triplet metal-centred ((3)MC) state is key to generating high quantum yields of a penta-coordinate intermediate (PCI) species, which in turn may form the target species: a mono-aqua photoproduct [Ru(bipyridine)2(nicotinamide)(H2O)](2+) (2). Following irradiation of 1, a thorough kinetic picture is derived from ultrafast UV/Vis transient absorption spectroscopy measurements, using a 'target analysis' approach, and provides both timescales and quantum yields for the key processes involved. We show that photoactivation of 1 to 2 occurs with a quantum yield ≥0.36, all within a timeframe of ~400 ps. Characterization of the excited states involved, particularly the nature of the PCI and how it undergoes a geometry relaxation to accommodate the water ligand, which is a keystone in the efficiency of the photoactivation of 1, is accomplished through state-of-the-art computation including complete active space self-consistent field methods and time-dependent density functional theory. Importantly, the conclusions here provide a detailed understanding of the initial stages involved in this photoactivation and the foundation required for designing more efficacious photochemotherapy drugs of this type.

On the linear and non-linear electronic spectroscopy of chlorophylls: a computational study.

A theoretical analysis of linear and non-linear (two-photon absorption) electronic spectroscopy of all known porphyrinic pigments has been performed using linear and quadratic density functional response theory, with the long-range corrected CAM-B3LYP functional. We found that higher Soret transitions often contain non-Gouterman contributions and that each chlorophyll has the possibility for resonance enhanced TPA in the Soret region, although there is also significant TPA in the Q region.

Halogenation effects in intramolecular furan Diels-Alder reactions: broad scope synthetic and computational studies.

For the first time a comprehensive synthetic and computational study of the effect of halogen substitution on both furan and dienophile for the intramolecular Furan Diels-Alder (IMDAF) reaction has been undertaken. Contrary to our initial expectations, halogen substitution on the dienophile was found to have a significant effect, making the reactions slower and less thermodynamically favourable. However, careful choice of the site of furan halogenation could be used to overcome dienophile halogen substitution, leading to highly functionalised cycloadducts. These reactions are thought to be controlled by the interplay of three factors: positive charge stabilisation in the transition state and product, steric effects and a dipolar interaction term identified by high level calculations. Frontier orbital effects do not appear to make a major contribution in determining the viability of these reactions, which is consistent with our analysis of calculated transition state structural data.

Manipulating dynamics with chemical structure: probing vibrationally-enhanced tunnelling in photoexcited catechol.

Ultrafast time-resolved velocity map ion imaging (TR-VMI) and time-resolved ion-yield (TR-IY) methods are utilised to reveal a comprehensive picture of the electronic state relaxation dynamics in photoexcited catechol (1,2-dihydroxybenzene). After excitation to the S1 ((1)ππ*) state between 280.5 (the S1 origin band, S1(v = 0)) to 243 nm, the population in this state is observed to decay through coupling onto the S2 ((1)πσ*) state, which is dissociative with respect to the non-hydrogen bonded 'free' O-H bond (labelled O(1)-H). This process occurs via tunnelling under an S1/S2 conical intersection (CI) on a timeframe of 5-11 ps, resulting in O(1)-H bond fission along S2. Concomitant formation of ground state catechoxyl radicals (C6H5O2(X)), in coincidence with translationally excited H-atoms, occurs over the same timescale as the S1 state population decays. Between 254-237 nm, direct excitation to the S2 state is also observed, manifesting in the ultrafast (~100 fs) formation of H-atoms with high kinetic energy release. From these measurements we determine that the S1/S2 CI lies ~3700-5500 cm(-1) above the S1(v = 0) level, indicating that the barrier height to tunnelling from S1(v = 0) → S2 is comparable to that observed in the related 'benchmark' species phenol (hydroxybenzene). We discuss how a highly 'vibrationally-enhanced' tunnelling mechanism is responsible for the two orders of magnitude enhancement to the tunnelling rate in catechol, relative to that previously determined in phenol (>1.2 ns), despite similar barrier heights. This phenomenon is a direct consequence of the non-planar S1 excited state minimum structure (C1 symmetry) in catechol, which in turn yields relaxed symmetry constraints for vibronic coupling from S1(v = 0) → S2- a scenario which does not exist for phenol. These findings offer an elegant example of how even simple chemical modifications (ortho-hydroxy substitution) to a fundamental, biologically relevant, UV chromophore, such as phenol, can have profound effects on the ensuing excited state dynamics.

Photoracemization and excited state relaxation through non-adiabatic pathways in chromium (III) oxalate ions.

Computational studies on the photochemistry of the open-shell chromium oxalate [Cr(C(2)O(4))(3)](3-) ion, including its non-adiabatic relaxation pathways, have been performed. The presence of the peaked conical intersection of a quasi-Jahn-Teller type, connecting the (4)T state with (4)A(2) ground state, accounts for the observed photoinduced racemization. This involves the rupture of one of the Cr-O bonds and the complex forms an unstable trigonal bipyramid form that connects both ground state stereoisomers with the excited quartet manifold. Intersystem crossing seams have been located between the (4)T and lower lying (2)E state which can quench the quartet reaction and lead to (2)E → (4)A(2) emission.

Photostereochemistry and photoaquation reactions of [Cr(tn)3]3+: theoretical studies show the importance of reduced coordination conical intersection geometries.

We have performed TD-DFT and CASSCF calculations to understand the spectroscopy and reactive photochemistry of the [Cr(tn)(3)](3+) complex. Our results show that, after population of a quartet ligand field excited state, the system relaxes by dissociation of a Cr-N bond to reach a quasi-trigonal bipyramid five-coordinate species that is a conical intersection connecting the excited and ground quartet manifolds. Nonadiabatic relaxation through these leads to square pyramidal structures that can coordinate water and account for the observed monoaquated photoproducts. Such features are also present on the potential energy surfaces of these photoproducts and account for the range of experimentally observed photostereoisomers of the photoaquation reactions.

Theoretical study of the pseudo-Jahn-Teller effect in the edge-sharing bioctahedral complex Mo(2)(DXylF)(2)(O(2)CCH(3))(2)(mu(2)-O)(2).

A study of the D(2h) to C(2h) pseudo-Jahn-Teller distortion in the edge-sharing bioctahedral complex Mo(2)(DXylF)(2)(O(2)CCH(3))(2)(mu(2)-O)(2) is presented. We have performed extensive density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations. For both the full target complex and a model derived by replacing xylyl and methyl groups with hydrogens we observe that the central Mo(2)(mu(2)-O)(2) motif displays C(2h) rather than D(2h) symmetry. Analytical CASSCF frequency calculations prove that the rhomboidal distortion of the complex from D(2h) to C(2h) is due to a vibronic mixing of the ground electronic state and a low-lying pidelta* excited state.