On the photorelease of nitric oxide by nitrobenzene derivatives; a CASPT2//CASSCF model

Nitroaromatics compounds can photorelease NO after UV absorption. The efficiency of the photoreaction depends on the molecular structure and two features have been pointed out as particularly important for the yield of the process: the presence of methyl groups at the ortho position with respect to the nitro group and the degree of conjugation of the molecule. In the present contribution we provide a theoretical characterization at the CASPT2//CASSCF level of theory of the photorelease of NO for four molecules derived from nitrobenzene through the addition of ortho methyl groups and/or the elongation of the conjugation. Our previously described mechanism obtained for the photorelease of NO in nitrobenzene has been adopted as a model for the process. According to this model, the process proceeds through a reactive singlet-triplet crossing (STC) region that the system can reach from the triplet 3 (π O π*) minimum. The energy barrier that must be surmounted in order to populate the reactive STC can be associated with the efficiency of the photoreaction. The here obtained results display clear differences for the efficiency of the photoreaction in the studied systems, and can be correlated with experimental results. The model thus proves its ability to highlight differences in the photoreaction efficiency for the nitroaromatic compounds studied here

Similar chemical structures, dissimilar triplet quantum yields: a CASPT2 model rationalizing the trend of triplet quantum yields in nitroaromatic systems

The photophysics of nitroaromatics compounds stand out for being characterized by an ultrafast decay into the triplet manifold and by a significant value of the triplet quantum yield. The latter quantity can change dramatically from one system to another, as proven for the molecules 2- nitronaphthalene, 1-nitronaphthalene, and 2methyl-1nitronaphthale, whose triplet quantum yield have been previously measured to be 0.93 ± 0.15, 0.64 ± 0.12, and 0.33 ± 0.05, respectively (J. Phys. Chem. A 2013, 117, 14100). In the present contribution we rationalize the reported trend for the triplet quantum yield on the basis of the different ability that the excited S 1 state has in the three molecules to reach a non-previously characterized conical intersection with the ground state. Such a path is in competition with the one leading to triplet states population, which, on the basis of the present static description, appear to be equally favorable in the three systems. Performing high-level ab-initio computations, the energy barrier from the S 1 CASPT2//CASSCF minimum to a CASPT2 minimum-energy-crossing-point of the mentioned S 1 /S 0 conical intersection have been computed to follow the same trend than the values of triplet quantum yield in the three nitroaromatics system here under analysis. The CASPT2 minimum-energy-crossing-point have been obtained using the projected constrained optimization method as recently implemented in the Molcas code. The same path has been characterized also for nitrobenzene, obtaining a value for the mentioned energy barrier that nicely fit in the model derived for the three nitro-naphthalene systems, and in agreement with its high value of the triplet quantum yield (greater than 0.8). The ability of the present model to not only rationalize the experimental data of a single molecule but to reproduce a trend for four slightly different systems speaks in favor of its reliability

How important is roaming in the photodegradation of nitrobenzene?

At low excitation energies nitrobenzene photoreleases NO with low translational and rotational energy, while at higher excitation energies NO is photoreleased with both low and high translational and rotational energy. The fast products are formed through a singlet-triplet crossing (STC) region featuring an oxaziridine ring, while a ground state roaming mechanism was suggested to produce the slow molecules. Computing translational and rotational energies performing CASSCF classical dynamics, we here prove how the same oxaziridine STC can account for both fast and slow photoproducts, depending on the region of the seam through which the ground state is populated. A roaming-type STC/CI has also been characterized, from which slow NO molecules can also be formed through a roaming photodegradation mechanism, here in the excited state. The higher accessibility of the oxaziridine STC mechanism, 1.53 eV lower in energy than the roaming path, questions the contribution of roaming in nitrobenzene NO photoproduction

On the photorelease of nitric oxide by nitrobenzene derivatives: A CASPT2//CASSCF model

Nitroaromatic compounds can photorelease nitric oxide after UV absorption. The efficiency of the photoreaction depends on the molecular structure, and two features have been pointed out as particularly important for the yield of the process: the presence of methyl groups at the ortho position with respect to the nitro group and the degree of conjugation of the molecule. In this paper, we provide a theoretical characterization at the CASPT2//CASSCF (complete active space second-order perturbation theory//complete active space self-consistent field) level of theory of the photorelease of NO for four molecules derived from nitrobenzene through the addition of ortho methyl groups and/or the elongation of the conjugation. Our previously described mechanism obtained for the photorelease of NO in nitrobenzene has been adopted as a model for the process. According to this model, the process proceeds through a reactive singlet-triplet crossing (STC) region that the system can reach from the triplet 3(πOπ*) minimum. The energy barrier that must be surmounted in order to populate the reactive STC can be associated with the efficiency of the photoreaction. Here, the obtained results display clear differences in the efficiency of the photoreaction in the studied systems and can be correlated with experimental results. Thus, the model proves its ability to highlight the differences in the photoreaction efficiency for the nitroaromatic compounds studied here

Generating symmetry-adapted bases for non-Abelian point groups to be used in vibronic coupling Hamiltonians

AbstractThe vibronic coupling Hamiltonian is a standard model used to describe the potential energy surfaces of systems in which non-adiabatic coupling is a key feature. This includes Jahn–Teller and Renner–Teller systems. The model approximates diabatic potential energy functions as polynomials expanded about a point of high symmetry. One must ensure the model Hamiltonian belongs to the totally symmetric irreducible representation of this point group. Here, a simple approach is presented to generate functions that form a basis for totally symmetric irreducible representations of non-Abelian groups and apply it to D∞h (2D) and O (3D). For the O group, the use of a well known basis-generating operator is also required. The functions generated for D∞h are then used to construct a ten state, four coordinate model of acetylene. The calculated absorption spectrum is compared to the experimental spectrum to serve as a validation of the approach

Field modified spin-orbit potential curves of IBr. Preliminary dynamical results

In a seminal work the photodissociation of IBr has been controlled using a strong non-resonant IR pulse [Sussman et al., Science, 2006, 314, 274], changing the branching ratio of products in different final states via the relative timing of pump and control pulses. In this paper, we revisit the control of this molecule. Potential surfaces for the complete spin-orbit manifold of IBr states dissociating into the ground and first excited states of the constituent atoms have been calculated at the multi-reference configuration interaction (MRCI) level of theory as a function of applied field. Both the strength and direction of field have been taken into account and it is seen how the avoided crossing between the states thought to be key in the control mechanism shift as a function of field strength. These surfaces will enable full calculations of the molecule in the pump-control field. Preliminary dynamics calculations with the field placed along the molecular axis show that a Hamiltonian including all 36-states agrees with earlier results and is able to model the basic features of the control. However, just like earlier results, this restricted model is not able to reproduce the timescale of the contro

Quantum Interference Paves the Way for Long-Lived Electronic Coherences

The creation and dynamical fate of a coherent superposition of electronic states generated in a polyatomic molecule by broadband ionization with extreme ultraviolet pulses is studied using the multiconfiguration time-dependent Hartree method together with an ionization continuum model Hamiltonian. The electronic coherence between the hole states usually lasts until the nuclear dynamics leads to decoherence. A key goal of attosecond science is to control the electronic motion and design laser control schemes to retain this coherence for longer timescales. Here, we investigate this possibility using time-delayed pulses and show how this opens up the prospect of coherent control of charge migration phenomenon

Do we need delocalised wavefunctions for the excited state dynamics of 1,1-difluoroethylene?

In this work we set up a model Hamiltonian to study the excited state quantum dynamics of 1,1-difluoroethylene, a molecule that has equivalent atoms exchanged by a torsional symmetry operation leading to equivalent minima on the potential energy surface. In systems with many degrees of freedom where the minimum energy geometry is not unique, the ground state wavefunction will be delocalised among multiple minima. In this small test system, we probe the excited state dynamics considering localised (in a single minimum) and delocalised (spread over among multiple minima) wavefunctions and check whether this choice would influence the final outcome of the quantum dynamics calculations. Our molecular Hamiltonian comprises seven electronic states, including valence and Rydberg states, computed with the MS-CASPT2 method and projected onto the vibrational coordinates of the twelve normal modes of 1,1-difluoroethylene in its vibrational ground state. This Hamiltonian has been symmetrised along the torsional degree of freedom to make both minima completely equivalent and the model is supported by the excellent agreement with the experimental absorption spectrum. Quantum dynamics results show that the different initial conditions studied do not appreciably affect the excited state populations or the absorption spectrum when the dynamics is simulated assuming a delta pulse excitation.Comment: This is a post-peer-review, pre-copyedit version of an article accepted in the Canadian Journal of Chemistry. The final authenticated version will be available online at their websit

Effectiveness and safety of orally administered immunotherapy for food allergies: a systematic review and meta-analysis

The aim of using oral and sublingual immunotherapy with food allergies is to enable the safe consumption of foods containing these aller- gens in patients with food allergies. In the present study, a systematic review of intervention studies was undertaken; this involved the searching of eleven international databases for controlled clinical trials. We identified 1152 potentially relevant papers, from which we selected twenty-two reports of twenty-one eligible trials (i.e. eighteen randomised controlled trials and three controlled clinical trials). The meta-analysis revealed a substantially lower risk of reactions to the relevant food allergen in those receiving orally administered immu- notherapy (risk ratios (RR) 0·21, 95 % CI 0·12, 0·38). The meta-analysis of immunological data demonstrated that skin prick test responses to the relevant food allergen significantly decreased with immunotherapy (mean difference 2 2·96 mm, 95 % CI 2 4·48, 2 1·45), while aller- gen-specific IgG4 levels increased by an average of 19·9 (95 % CI 17·1, 22·6) m g/ml. Sensitivity analyses excluding studies at the highest risk of bias and subgroup analyses in relation to specific food allergens and treatment approaches generated comparable summary estimates of effectiveness and immunological changes. Pooling of the safety data revealed an increased risk of local (i.e. minor oropharyngeal/gastro- intestinal) adverse reactions with immunotherapy (RR 1·47, 95 % CI 1·11, 1·95); there was a non-significant increased average risk of systemic adverse reactions with immunotherapy (RR 1·08, 95 % CI 0·97, 1·19). There is strong evidence that orally administered immu- notherapy can induce immunomodulatory changes and thereby promote desensitisation to a range of foods. However, given the paucity of evidence on longer-term safety, effectiveness and cost-effectiveness, orally administered immunotherapy should not be used outside experimental conditions presently