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Anthocyanins are the natural plant pigments responsible for most of the red, blue and purple colors of flowers and fruit. One method of stabilization of the color of anthocyanins in nature is intramolecular copigmentation, in which a copigment molecule covalently attached to one of the sugar residues complexes with the anthocyanin cation chromophore. In the present work, two quantum chemical methodologies, time-dependent density functional theory (TD-DFT) and second.order algebraic diagrammatic construction (ADC(2)), were employed to predict the absorption spectra in vacuum and conductor-like screening model (COSMO) water of a natural anthocyanin containing an ester of coumaric acid (copigment) bound to the sugar residue of a cyanidin chromophore. ADC(2) in water adequately reproduces the experimental spectra with and without intramolecular copigmentation, pointing to this theoretical technique as a promising approach for predicting the spectroscopic properties of natural (and nature-inspired) dyes and pigments.

Density functional theory calculations, using the B3LYP parameterisation, were performed to determine structures, vibrational frequencies, and binding energies for complexation of Ni+ and Ni2+ cations with benzene and naphthalene molecules and clusters. The calculations employed the Stuttgart basis set with ECP pseudo potentials for the Ni cations and basis sets of at least triple ζ plus polarisation, and diffuse quality for C and H. The effect of electron correlation on non-bonded interactions was accounted for by the Grimme GD3 dispersion correction. Counterpoise computations were made for BSSE. Comparison between experiment and theory provide fascinating new insight into the bonding for these prototypical organometallic (OM) complexes. These structures have a sandwich topology, indicating major structural reorganisations occuring when benzene or naphthalene interact with Ni cations. Adiabatic electron affinities and ionisation potentials agree well with experiment when available. Binding energies were also determined, providing insight into the stability of the complexes. The results presented here provide important information for future studies to address additional investigations of both problems of the electronic structure properties of these complexes, as well as the role of the polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM) and soot formation in combustion. The Ni+/Ni2+ + aromatic organometallic bonding is of the same order of stability as an aromatic C–H bond. Such bonding modifies the IR spectrum of the complexed aromatic molecules by enhancing the 3.3 μm feature and decreasing the C–H bands in the 11–12 μm range (γ C–H). Organometallic complexation reactions may contribute significantly to metal depletion in the ISM.

WOS:000350840800038; International audience; The absorption and fluorescence spectra of poly(p-phenylenevinylene) (PPV) oligomers with up to seven repeat units were theoretically investigated using the algebraic diagrammatic construction method to second order, ADC(2), combined with the resolution-of-the-identity (RI) approach. The ground and first excited state geometries of the oligomers were fully optimized. Vertical excitation energies and oscillator strengths of the first four transitions were computed. The vibrational broadening of the absorption and fluorescence spectra was studied using a semiclassical nuclear ensemble method. After correcting for basis set and solvent effects, we achieved a balanced description of the absorption and fluorescence spectra by means of the ADC(2) approach. This fact is documented by the computed Stokes shift along the PPV series, which is in good agreement with the experimental values. The experimentally observed band width of the UV absorption and fluorescence spectra is well reproduced by the present simulations showing that the nuclear ensemble generated should be well suitable for consecutive surface hopping dynamics simulations.

The excited-state intramolecular proton transfer (ESIPT) is reviewed for several benchmark systems [o-hydroxybenzaldehyde (OHBA), salicylic acid and 2-(2′-hydroxyphenyl)-benzothiazole (HBT)] in order to verify the applicability of the time-dependent density functional theory (TDDFT) and the resolution-of-the-identity approximate second-order coupled cluster (RI-CC2) methods. It was found that these approaches are very well suited for the description of ESIPT processes. A comparative investigation of previous and new excited-state dynamics simulations is performed for HBT, 10-hydroxybenzo[h]quinoline (HBQ), and [2,2′-bipyridyl]-3,3′-diol (BP(OH)2). The time scale for the ESIPT process in these systems ranges in the time interval of 30−40 fs for HBT and HBQ and amounts to about 10 fs for the first proton transfer step in BP(OH)2. The dynamics simulations also show that the proton transfer in HBT is strongly supported by skeletal modes and the proton plays a rather passive role, whereas in HBQ a semipassive mechanism is found due to its increased rigidity in comparison to HBT. The special role of the double proton transfer in BP(OH)2 is discussed as well.

Ab initio second-order algebraic diagrammatic construction (ADC(2)) calculations using the resolution of the identity (RI) method have been performed on poly-(p-phenylenevinylene) (PPV) oligomers with chain lengths up to eight phenyl rings. Vertical excitation energies for the four lowest π-π* excitations and geometry relaxation effects for the lowest excited state (S1) are reported. Extrapolation to infinite chain length shows good agreement with analogous data derived from experiment. Analysis of the bond length alternation (BLA) based on the optimized S1 geometry provides conclusive evidence for the localization of the defect in the center of the oligomer chain. Torsional potentials have been computed for the four excited states investigated and the transition densities divided into fragment contributions have been used to identify excitonic interactions. The present investigation provides benchmark results, which can be used (i) as reference for lower level methods and (ii) give the possibility to parametrize an effective Frenkel exciton Hamiltonian for quantum dynamical simulations of ultrafast exciton transfer dynamics in PPV type systems.

This dissertation will examine the electronic properties of several materials used in state-of-the art technology and ongoing scientific investigation by means of high-level ab initio quantum chemical calculations. In this way, the application of such calculations to graphene reactivity and defects, organic photovoltaic processes, and excited states and biradical character of polycyclic aromatic hydrocarbons (PAH) is presented. Graphene reactivity and defect characterization is currently the source of intense scrutiny. The reactivity of pristine graphene and single and double carbon vacancies toward a hydrogen radical was assessed using complete active space self-consistent field theory (CASSCF) and multireference configuration interaction singles and doubles (MR-CISD) calculations using a pyrene model. While all carbon centers formed stable, novel C-H bonds, the single carbon defect was found to be most stabilizing thanks to the dangling bond at the six-membered ring. Bonding at this carbon center shows interaction with several closely-spaced electronic excited states. A rigid scan was also used to show the dynamic landscape of the potential energy surfaces of these structures. Double carbon vacancy pyrene and circumpyrene were used to investigate the structure and electronic states of an intrinsic silicon impurity in graphene by density functional theory (DFT) methods. The ground state was found to be low spin, non-planar and D2 symmetric with a nearby low spin, planar state with D2h symmetry. Characterization with natural bond orbital (NBO) analyses showed that the hybridization in the non-planar structure is sp3 and sp2d in the planar structure. Charge transfer from the silicon dopant to the surrounding carbon atoms is demonstrated via analysis with natural charges and molecular electrostatic potential (MEP) plots. Experiment shows drastically enhanced power conversion efficiency (PCE) in a bulk heterojunction (BHJ) device consisting of poly(3-hexylthiophene-2, 5-diyl) (P3HT), terthiophene (T3), and fullerene (C60). This BHJ scheme was investigated using DFT and several environmental models. The state-specific (SS) environment was found to best reproduce the experimental findings showing that initial photoexcitation of a local, bright exciton at P3HT was able to decay through several charge transfer (CT) bands of P3HT→C60 and T3→C60 character explaining the increased PCE. The spectra calculated for the isolated system and linear response (LR) environment were very similar to each other and showed the initial photoexcitation at P3HT decayed instead into bands of local C60 excitonic states quenching the CT states. Crucial benchmark calculations of the structure and electronic excited states missing from the current literature were performed for oligomers of poly(p-phenylenevinylene) (PPV) dimers using high-level scaled opposite-spin (SOS) second-order Møller-Plesset (SOS-MP2) and second-order algebraic diagrammatic construction (SOS-ADC(2)) calculations. Comparisons were also made to DFT methods. The dimer structures were studied in two conformations: sandwich-stacked and displaced; the latter exhibited greater stacking interactions. Good agreement was found between the cc-pVQZ and aug-cc-pVQZ basis sets and the cc-pVTZ basis. Comparisons of the interaction energies were made to the complete basis set limit. Basis set superposition error was also accounted for. The excited state spectra presented a varied and dynamic picture that was analyzed using transition density matrices and natural transition orbitals (NTOs). The assessment of the biradical character of several trans-diindenoacenes, cis-diindenoacenes, and trans-indenoindenodi(acenothiophenes) was accomplished utilizing the multireference averaged quadratic coupled-cluster (MR-AQCC) method. These systems have been previously identified as highly reactive in experiments, often subject to unplanned side-reactions. The biradical character was characterized using the singlet-triplet splitting energy (∆ES-T), effective number of unpaired electrons (NU), and unpaired electron density. For all structures, low spin ground states were found. In addition, increasing the number of core benzene rings decreased the ∆ES-T, increased the NU, and increased the unpaired electron density. Good agreement to experimental values for the ∆ES-T is also demonstrated.

Ion-molecule interactions and reactions are studied with chemical dynamics simulations, statistical theories, and semiclassical initial value representation methods to investigate non-statistical reaction dynamics and microsolvation in unimolecular and bimolecular reactions and anharmonicity effects in RRKM theory and ion-molecule spectroscopy. The microsolvated SN2 reaction CH3I + OH-(H2O)2 was studied with direct dynamics simulations at the B97-1/ECP/d level of electronic structure theory. Non-statistical dynamics is significant in the post-transition state dynamics of this bimolecular reaction. Non-statistical dynamics in a unimolecular reaction was studied for benzene dimer decomposition and originates from weak couplings between the intermolecular and intramolecular vibrational modes. In addition, non-statistical unimolecular decomposition of the rotationally excited F----HCH2I ion-molecule complex results from weak vibrational and rotational couplings. When the complex is excited with vibrational energy, its unimolecular dynamics is statistical and in accord with RRKM. However, with a large rotational energy, the decomposition is non-statistical as a result of weak mixing of the K rotational levels. Anharmonicity and anharmonic vibrational frequencies in ion-molecule interactions were studied by calculating the anharmonic vibrational density of states and anharmonic frequencies. The anharmonic density of states of the vibrationally excited complexes I-(H2O), (H2O)2, and I-(H2O)2 are ~2, ~3 and ~260 times larger than the harmonic oscillator prediction, respectively, at their dissociation thresholds. Anharmonic Cl----CH3Cl power spectrum calculated with semiclassical initial value representation (SC-IVR) method gives an accurate anharmonic zero-point energy (ZPE) of 8435 cm-1 compared with the harmonic ZPE of 8513 cm-1. Weak couplings between intermolecular and intramolecular modes are also analyzed in the Cl----CH3Cl semiclassical dynamics.

Within the past few decades there has been a societal push towards cleaner energy and, more specifically, efficient and safe hydrogen fuel technologies. In order to progress in this avenue of research it is vital to have a clear understanding and characterization of how materials of interest interact with hydrogen. Due to the inherently quantum mechanical nature of hydrogen, any theoretical studies of these systems must be treated quantum dynamically. Many Kubas complexes have been studied both experimentally and theoretically since their discovery in 1984. This study examines the behavior of the hydrogen ligand in the Fe(H)2(H2)(PEtPh2)3 Kubas complex, allowing two rotational degrees of freedom and quantum dynamic behavior. Our density functional theory (DFT) electronic structure calculations utilizing the B3LYP functional of the “gas-phase” molecule allows better insight into possible q–f correlation and a comprehensive look into librational ground state tunneling splittings. The success of this in-depth study indicates similar calculations can be done on similarly complex systems.

Classical trajectories are propagated using the energy gradient calculated from quantum mechanical methods, through which atomic level chemical dynamics are used to investigate the properties of unimolecular and bimolecular reactions. The program development work involves the improvement of the interface between the classical trajectory simulation program (VENUS) and quantum chemical programs (NWChem and GAMESS) by adding trajectory restart capability and alternate initial SCF guesses. Intramolecular and intermolecular energy was followed along the trajectories to investigate energy partitioning and transferring, e.g., the partitioning between relative translational energy and internal (rotational and vibrational) energy to compare with molecular beam experiments, or the energy transfer between solvent and solute to investigate the solvent affect in chemical reaction. The simulations were performed on the chemical system of dissociation of 1,2-Dioxetian, F- + CH3I SN2 reaction, formation and dissociation of 1,5-Dinitrobiuret and spin-orbital coupling affect of HBr+ + CO2 reaction.

The structural, energetic, and infrared spectral properties of water clusters interacting with model perfluorosulfonate (Nafion) ionomer segments have been investigated by means of quantum chemical calculations. Density functional theory (DFT) calculations were conducted in order to model interfacial water condensed in pores and channels of Nafion and help shed light on the structural and vibrational properties of hydrated Nafion membranes. The computational models examine the orientations of the water molecules in the vicinity of the two phases of the ionomer, and were applied to understand the appearance of O-H stretching bands that deviated from the typical bulk water values, as well as the appearance of multiple free O-H stretching bands. The computational models consist of a sodium counter ion, the Nafion ionomer, and a cluster of water molecules positioned in three different regions of the membrane. In region 1 the waters are placed close to the SO3- group and the counter ion. In region 2 the waters are located between the SO3- group and the fluorinated segment and in region 3 the waters are interacting with the fluorinated groups. The results reveal insight into the structure, orientation, and energetics of Nafion and water clusters in and around the membrane interface; the results show that hydrogen atoms at the water-ionomer interface form strong hydrogen bonds to the hydrophilic sulfonate group as well as weak hydrogen bond interactions with the hydrophobic fluorinated backbone. They also suggest that hydrogen bonding stabilizes the types of water-ionomer environments that can lead to multiple free O-H stretching vibrational features in experimental spectra.

Manganese oxides (Mn-oxides) are ubiquitous in soils and sediments and have been shown to be effective adsorbents of heavy metals from solution. The adsorption of heavy metals by Mn-oxides provides an important sink for heavy metals, thus contributing to the control of contaminant mobility in the environment. An understanding of the surface reactivity of Mn-oxides is important to assess the potential use of Mn-oxides for contaminant remediation. This study focused on pyrolusite (β-MnO2), the most stable polymorph of MnO2. Two commercial pyrolusite samples were studied, Spectrum Chemical and Sigma-Aldrich. Extensive characterization of the pyrolusite samples was completed using SEM and TEM imaging, XRD, TGA and BET surface area measurements. The SEM and TEM images showed large crystals with a blocky morphology and the XRD analysis confirmed that the samples were pure, crystalline pyrolusite. The TGA analysis was performed from 25 to above 500 °C, with a small weight loss of approximately 2%. The surface area of both samples was low, < 1 m2 g-1; therefore, the Sigma-Aldrich pyrolusite sample was crushed, increasing the surface area to 16.4 m2 g-1. The surface reactivity of pyrolusite was studied by potentiometric titration. The potentiometric titrations were performed from pH 3-10 at 25° C, in NaCl media at ionic strengths of 0.03, 0.06 and 0.30 m. A sample of the final titration solution was withdrawn and the Mn concentration was determined by ICP-AES. The surface charging behavior of pyrolusite was highly pH-dependent, but there was a limited ionic strength effect. The pHznpc of pyrolusite was determined by the intersection of the three ionic strength solutions (pH 5.8) for the Spectrum Chemical pyrolusite. A similar pHznpc value of 6.1 was determined by the zero proton intersection point for the crushed Sigma-Aldrich pyrolusite sample. Dissolution of pyrolusite began at approximately pH 5.3 and increased with decreasing pH. The crushed Sigma-Aldrich pyrolusite sample had a higher dissolved Mn concentration which is attributed to the smaller particle size. The ionic strength of the solutions did not affect the dissolved Mn concentration. The dissolution of pyrolusite and the apparent pHznpc narrows the range of environmental conditions underwhich pyrolusite would be effective for remediation. Pyrolusite would adsorb heavy metals in systems with a pH ≥ 6.0. Anions would be adsorbed by pyrolusite in the pH range 5-6, below the pHznpc and above the pH where dissolution becomes significant.