Simulation of optical spectra is essential to molecular characterization and, in many cases, crit... more Simulation of optical spectra is essential to molecular characterization and, in many cases, critical for interpreting experimental spectra. The most common method for simulating vibronic absorption spectra relies on the geometry optimization and computation of normal modes for ground and excited states. In this report, we show that utilization of such a procedure within an adiabatic linear response theory framework may lead to state mixings and a breakdown of the Born-Oppenheimer approximation, resulting in a poor description of absorption spectra. In contrast, computing excited states via a self-consistent eld method in conjunction with a maximum overlap model produces states that are not subject to such mixings. We show that this latter method produces vibronic spectra much more aligned with vertical excitation procedures, such as those computed from a vertical gradient or molecular dynamics trajectory-based approach. For the methylene blue chromophore, we compare vibronic absorp...
The calculation of photoionization cross-sections can play a key role in spectral assignments usi... more The calculation of photoionization cross-sections can play a key role in spectral assignments using modeling and simulation. In this work, we provide formal relationships between pole strengths, which are proportional to the photoionization cross-section, and terms related to the Natural Ionization Orbital model for Delta-SCF calculations. A set of numerical calculations using the developed models is carried out. Pole strength values computed using the two approaches developed for Delta-SCF calculations demonstrate excellent agreement with an electron propagator theory model.
Cerium suboxide clusters have been shown to react with water to yield H2. Since reactions of meta... more Cerium suboxide clusters have been shown to react with water to yield H2. Since reactions of metal oxides with water are of great fundamental and practical importance in energy science, detailed study of this reaction can provide new key knowledge for future catalyst design. Using density function theory calculations, we identified and report complete reaction pathways for two spin-states of Ce2O– reacting with H2O. Additionally, examining the molecular orbitals of initial and final cerium oxide clusters, we found that metal d electrons facilitate the reduction of water. This work provides new understanding of this reaction and insights to the reactivity enhancements for cerium-doped surfaces.
Lanthanide hydroxides are key species in a variety of catalytic processes and in the preparation ... more Lanthanide hydroxides are key species in a variety of catalytic processes and in the preparation of corresponding oxides. This work explores the fundamental structure and bonding of the simplest lanthanide hydroxide, LnOH (Ln=La-Lu), using density functional theory calculations. Interestingly, the calculations predict that all structures of this series will be linear. Furthermore, these results indicate a valence electron configuration featuring an occupied sigma orbital and two occupied pi orbitals for all LnOH compounds, suggesting that the lanthanide-hydroxide bond is best characterized as a covalent triple bond.
This work evaluates the quality of exchange coupling constant and spin crossover gap calculations... more This work evaluates the quality of exchange coupling constant and spin crossover gap calculations using density functional theory corrected by the Approximate Projection model. Results show that improvements using the Approximate Projection model range from modest to significant. This study demonstrates that, at least for the class of systems examined here, spin-projection generally improves the quality of density functional theory calculations of J-coupling constants and spin crossover gaps. Furthermore, it is shown that spin-projection can be important for both geometry optimization and energy evaluations. The Approximate Project model provides an affordable and practical approach for effectively correcting spin-contamination errors in molecular exchange coupling constant and spin crossover gap calculations.
Readily prepared cis-β-(α’,α’-dimethyl)-4’-methindolylstyrenes undergo Brønsted acid-catalyzed in... more Readily prepared cis-β-(α’,α’-dimethyl)-4’-methindolylstyrenes undergo Brønsted acid-catalyzed intramolecular hydroarylation to afford a variety of 3-aryl-5,5-dimethyl-1,3,4,5-tetrahydrobenzo[cd]indoles. Our experimental and computational investigations suggest that dispersive interactions between the indole and styrene preorganize substrates such that 6-membered ring formation occurs via a concerted hydroarylation step. When dispersability is attenuated (by a substituent or heteroatom), regioselectivity erodes and competing oligomerization predominates for cis substrates; all trans-configured substrates that we evaluated failed to cyclize efficiently.
A compact orbital representation of ionization processes is described utilizing the difference of... more A compact orbital representation of ionization processes is described utilizing the difference of calculated one-particle density matrices. Natural orbital analysis involving this difference density matrix simplifies interpretation of electronic detachment processes and allows differentiation between one-electron transitions and shake-up/shake-off transitions, in which one-electron processes are accompanied by excitation of a second electron into the virtual orbital space.
Angewandte Chemie (International ed. in English), Jan 14, 2015
The strongest carborane acid, H(CHB11 F11 ), protonates CO2 while traditional mixed Lewis/Brønste... more The strongest carborane acid, H(CHB11 F11 ), protonates CO2 while traditional mixed Lewis/Brønsted superacids do not. The product is deduced from IR spectroscopy and calculation to be the proton disolvate, H(CO2 )2 (+) . The carborane acid H(CHB11 F11 ) is therefore the strongest known acid. The failure of traditional mixed superacids to protonate weak bases such as CO2 can be traced to a competition between the proton and the Lewis acid for the added base. The high protic acidity promised by large absolute values of the Hammett acidity function (H0 ) is not realized in practice because the basicity of an added base is suppressed by Lewis acid/base adduct formation.
Simulation of optical spectra is essential to molecular characterization and, in many cases, crit... more Simulation of optical spectra is essential to molecular characterization and, in many cases, critical for interpreting experimental spectra. The most common method for simulating vibronic absorption spectra relies on the geometry optimization and computation of normal modes for ground and excited states. In this report, we show that utilization of such a procedure within an adiabatic linear response theory framework may lead to state mixings and a breakdown of the Born-Oppenheimer approximation, resulting in a poor description of absorption spectra. In contrast, computing excited states via a self-consistent eld method in conjunction with a maximum overlap model produces states that are not subject to such mixings. We show that this latter method produces vibronic spectra much more aligned with vertical excitation procedures, such as those computed from a vertical gradient or molecular dynamics trajectory-based approach. For the methylene blue chromophore, we compare vibronic absorp...
The calculation of photoionization cross-sections can play a key role in spectral assignments usi... more The calculation of photoionization cross-sections can play a key role in spectral assignments using modeling and simulation. In this work, we provide formal relationships between pole strengths, which are proportional to the photoionization cross-section, and terms related to the Natural Ionization Orbital model for Delta-SCF calculations. A set of numerical calculations using the developed models is carried out. Pole strength values computed using the two approaches developed for Delta-SCF calculations demonstrate excellent agreement with an electron propagator theory model.
Cerium suboxide clusters have been shown to react with water to yield H2. Since reactions of meta... more Cerium suboxide clusters have been shown to react with water to yield H2. Since reactions of metal oxides with water are of great fundamental and practical importance in energy science, detailed study of this reaction can provide new key knowledge for future catalyst design. Using density function theory calculations, we identified and report complete reaction pathways for two spin-states of Ce2O– reacting with H2O. Additionally, examining the molecular orbitals of initial and final cerium oxide clusters, we found that metal d electrons facilitate the reduction of water. This work provides new understanding of this reaction and insights to the reactivity enhancements for cerium-doped surfaces.
Lanthanide hydroxides are key species in a variety of catalytic processes and in the preparation ... more Lanthanide hydroxides are key species in a variety of catalytic processes and in the preparation of corresponding oxides. This work explores the fundamental structure and bonding of the simplest lanthanide hydroxide, LnOH (Ln=La-Lu), using density functional theory calculations. Interestingly, the calculations predict that all structures of this series will be linear. Furthermore, these results indicate a valence electron configuration featuring an occupied sigma orbital and two occupied pi orbitals for all LnOH compounds, suggesting that the lanthanide-hydroxide bond is best characterized as a covalent triple bond.
This work evaluates the quality of exchange coupling constant and spin crossover gap calculations... more This work evaluates the quality of exchange coupling constant and spin crossover gap calculations using density functional theory corrected by the Approximate Projection model. Results show that improvements using the Approximate Projection model range from modest to significant. This study demonstrates that, at least for the class of systems examined here, spin-projection generally improves the quality of density functional theory calculations of J-coupling constants and spin crossover gaps. Furthermore, it is shown that spin-projection can be important for both geometry optimization and energy evaluations. The Approximate Project model provides an affordable and practical approach for effectively correcting spin-contamination errors in molecular exchange coupling constant and spin crossover gap calculations.
Readily prepared cis-β-(α’,α’-dimethyl)-4’-methindolylstyrenes undergo Brønsted acid-catalyzed in... more Readily prepared cis-β-(α’,α’-dimethyl)-4’-methindolylstyrenes undergo Brønsted acid-catalyzed intramolecular hydroarylation to afford a variety of 3-aryl-5,5-dimethyl-1,3,4,5-tetrahydrobenzo[cd]indoles. Our experimental and computational investigations suggest that dispersive interactions between the indole and styrene preorganize substrates such that 6-membered ring formation occurs via a concerted hydroarylation step. When dispersability is attenuated (by a substituent or heteroatom), regioselectivity erodes and competing oligomerization predominates for cis substrates; all trans-configured substrates that we evaluated failed to cyclize efficiently.
A compact orbital representation of ionization processes is described utilizing the difference of... more A compact orbital representation of ionization processes is described utilizing the difference of calculated one-particle density matrices. Natural orbital analysis involving this difference density matrix simplifies interpretation of electronic detachment processes and allows differentiation between one-electron transitions and shake-up/shake-off transitions, in which one-electron processes are accompanied by excitation of a second electron into the virtual orbital space.
Angewandte Chemie (International ed. in English), Jan 14, 2015
The strongest carborane acid, H(CHB11 F11 ), protonates CO2 while traditional mixed Lewis/Brønste... more The strongest carborane acid, H(CHB11 F11 ), protonates CO2 while traditional mixed Lewis/Brønsted superacids do not. The product is deduced from IR spectroscopy and calculation to be the proton disolvate, H(CO2 )2 (+) . The carborane acid H(CHB11 F11 ) is therefore the strongest known acid. The failure of traditional mixed superacids to protonate weak bases such as CO2 can be traced to a competition between the proton and the Lewis acid for the added base. The high protic acidity promised by large absolute values of the Hammett acidity function (H0 ) is not realized in practice because the basicity of an added base is suppressed by Lewis acid/base adduct formation.
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Papers by Hrant Hratchian