Quantum-chemistry simulations based on potential energy surfaces of molecules provide invaluable ... more Quantum-chemistry simulations based on potential energy surfaces of molecules provide invaluable insight into the physicochemical processes at the atomistic level and yield such important observables as reaction rates and spectra....
A recent experimental study with time-resolved velocity map imaging demonstrated that the total k... more A recent experimental study with time-resolved velocity map imaging demonstrated that the total kinetic energy release spectra obtained from photodissociation of ortho and meta fluorophenols have distinct features after excitation into the origin of the S1 state. A peak at 6000 cm-1 was observed for both molecules, while another at 13000 cm-1 dominates the spectrum of ortho-fluorophenol. The peak at 6000 cm‑1 was assigned to H tunneling. Nevertheless, the 13000 cm-1 feature remains unassigned. In this work, we performed a theoretical analysis, investigating two hypotheses for explaining the 13000 cm‑1 signal. The first hypothesis is that it is due to one-photon absorption followed by ionization through resonant multiphoton dissociation. The second hypothesis is that the signal is due to two-photon absorption into a superexcited state, which dissociates yielding an H atom. We discuss the pros and cons of each hypothesis, laying the groundwork for future experiments.
Despite the continuous development of theoretical methodologies for describing nonadiabatic dynam... more Despite the continuous development of theoretical methodologies for describing nonadiabatic dynamics of molecular systems, there is a lack of approaches for processes where the norm of the wave function is not conserved, i.e., when an imaginary potential accounts for some irreversible decaying mechanism. Current approaches rely on building potential energy surfaces of reduced dimensionality, which is not optimal for more involving and realistic multidimensional problems. Here, we present a novel methodology for describing the dynamics of complex-valued molecular Hamiltonians, which is a generalisation of the trajectory surface hopping method. As a first application, the complex surface fewest switches surface hopping (CS-FSSH) method was employed to survey the relaxation mechanisms of the shape resonant anions of iodoethene. We have provided the first detailed and dynamical picture of the p*/s* mechanism of dissociative electron attachment in halogenated unsaturated compounds, which...
The analysis of the photoabsorption spectra of molecules shows that the band maximum is usually r... more The analysis of the photoabsorption spectra of molecules shows that the band maximum is usually redshifted in comparison to the vertical excitation. We conducted a throughout analysis of this shift based on low-dimensional analytical and numerical model systems, showing that its origin is rooted in the frequency change between the ground and the excited states in multidimensional systems. Moreover, we deliver a benchmark of ab initio results for the shift based on a comparison of vertical excitations and band maxima calculated with the nuclear ensemble approach for the 28 organic molecules in the Mülheim molecular dataset. The mean value of the shift calculated over 60 transitions is 0.11 ± 0.08 eV. The mean value of the band width is 0.32 ± 0.14 eV.
Functionalization of the sugar group can be used to control the triplet decay rate of thionucleos... more Functionalization of the sugar group can be used to control the triplet decay rate of thionucleosides.
Journal of chemical theory and computation, Jan 14, 2017
Photosensitized singlet oxygen generation occurring in a PS-O2 complex, where PS is a photosensit... more Photosensitized singlet oxygen generation occurring in a PS-O2 complex, where PS is a photosensitizer chromophore, is a weakly coupled intermolecular energy-transfer process, a still challenging problem for theoretical chemistry. To investigate the reaction rate directly from quantum-chemical calculations, we built a semiclassical kinetic model that minimizes the computational effort for the calculation of diabatic couplings, activation energies, and reorganization energies, which are the components of the rate. The model splits the system into sets of orthogonal coordinates, which are then explored to compute the reaction rate. This model offers an effective way to evaluate the reaction probability of singlet oxygen generation along different directions and intramolecular distances of the PS-O2 complex. The model can also be applied to other similar intermolecular energy-transfer problems, to connect the reaction kinetics and quantum-chemical calculations.
The journal of physical chemistry letters, Jan 2, 2017
First-principles quantum-chemical description of photosensitized singlet oxygen generation kineti... more First-principles quantum-chemical description of photosensitized singlet oxygen generation kinetics is challenging because of the intrinsic complexity of the underlying triplet fusion process in a floppy molecular complex with open-shell character. With a quantum-chemical kinetic model specifically tailored to deal with this problem, the reaction rates are investigated as a function of intermolecular incidence direction, orientation, and distance between O2 and the photosensitizer. The adopted photosensitizer, 6-azo-2-thiothymine, combines practical interest and prototypical variability. The study quantitatively determined maximum singlet oxygen generation rates for 15 incidence/orientation directions, showing that they span 5 orders of magnitude between the largest and the smallest rate. Such information may provide a hands-on guideline for the experimental molecular design of new photosensitizers as well as further higher-level theoretical research.
Physical chemistry chemical physics : PCCP, Jan 12, 2017
A three-state model for the photo-Fries rearrangement (PFR) is proposed based on multiconfigurati... more A three-state model for the photo-Fries rearrangement (PFR) is proposed based on multiconfigurational calculations. It provides a comprehensive mechanistic picture of all steps of the reaction, from the photoabsorption to the final tautomerization. The three states participating in the PFR are an aromatic (1)ππ*, which absorbs the radiation; a pre-dissociative (1)nπ*, which transfers the energy to the dissociative region; and a (1)πσ*, along which dissociation occurs. The transfer from (1)ππ* to (1)nπ* involves pyramidalization of the carbonyl carbon, while transfer from (1)nπ* to (1)πσ* takes place through CO stretching. Different products are available after a conical intersection with the ground state. Among them is a recombined radical intermediate, which can yield ortho-PFR products after an intramolecular 1,3-H tunneling. The three-state model is developed for phenyl acetate, the basic prototype for the PFR, and it reconciles the theory with a series of observations from time-...
Physical chemistry chemical physics : PCCP, Jan 15, 2017
Small molecular systems exhibiting Excited State Intramolecular Proton Transfer (ESIPT) attract c... more Small molecular systems exhibiting Excited State Intramolecular Proton Transfer (ESIPT) attract considerable attention due to their possible role as ultrafast, efficient, and photostable molecular photoswitches. Here, by means of static potential energy profile scan and on-the-fly non-adiabatic dynamics simulations we study the photodeactivation process of a minimal-chromophore aromatic Schiff base, salicylidene methylamine (SMA), and its two derivatives 6-cyano-salicylidene methylamine (6-CN-SMA) and 3-hydroxy-salicylidene methylamine (3-OH-SMA). We show that the dominant character of the lowest excited singlet state - ππ* vs. nπ* - plays a crucial role in the system's photophysics and controls the ESIPT efficiency. We also show that the relative alignment of the ππ* and nπ* states may be controlled through chemical substitutions made to the aromatic ring of the Schiff-base molecule. We believe that our findings will improve the rational-design strategies employed for the ESIPT...
In the present study, we examined the UV excitations of a newly introduced molecular set, Halons-... more In the present study, we examined the UV excitations of a newly introduced molecular set, Halons-9, composed of nine gaseous halon molecules. The performance of the density functional-based multi-reference configuration interaction method (DFT/MRCI) and time-dependent density functional theory with CAM-B3LYP functional (TD-CAM-B3LYP) in the computation of singlet and triplet excited states of this set was evaluated against coupled-cluster with singles and doubles (CCSD). Excited states up to the corresponding ionization limits, including both localized and delocalized excitations, have been benchmarked. TD-CAM-B3LYP significantly underestimates excitation energies of the higher mixed valence-Rydberg and Rydberg states, with computed mean absolute deviations from the equation of motion (EOM)-CCSD results 1.06 and 0.76 eV, respectively. DFT/MRCI gives a significantly better description of higher excited states, albeit still poor, compared to the TD-CAM-B3LYP. The mean absolute deviati...
Quantum-chemistry simulations based on potential energy surfaces of molecules provide invaluable ... more Quantum-chemistry simulations based on potential energy surfaces of molecules provide invaluable insight into the physicochemical processes at the atomistic level and yield such important observables as reaction rates and spectra....
A recent experimental study with time-resolved velocity map imaging demonstrated that the total k... more A recent experimental study with time-resolved velocity map imaging demonstrated that the total kinetic energy release spectra obtained from photodissociation of ortho and meta fluorophenols have distinct features after excitation into the origin of the S1 state. A peak at 6000 cm-1 was observed for both molecules, while another at 13000 cm-1 dominates the spectrum of ortho-fluorophenol. The peak at 6000 cm‑1 was assigned to H tunneling. Nevertheless, the 13000 cm-1 feature remains unassigned. In this work, we performed a theoretical analysis, investigating two hypotheses for explaining the 13000 cm‑1 signal. The first hypothesis is that it is due to one-photon absorption followed by ionization through resonant multiphoton dissociation. The second hypothesis is that the signal is due to two-photon absorption into a superexcited state, which dissociates yielding an H atom. We discuss the pros and cons of each hypothesis, laying the groundwork for future experiments.
Despite the continuous development of theoretical methodologies for describing nonadiabatic dynam... more Despite the continuous development of theoretical methodologies for describing nonadiabatic dynamics of molecular systems, there is a lack of approaches for processes where the norm of the wave function is not conserved, i.e., when an imaginary potential accounts for some irreversible decaying mechanism. Current approaches rely on building potential energy surfaces of reduced dimensionality, which is not optimal for more involving and realistic multidimensional problems. Here, we present a novel methodology for describing the dynamics of complex-valued molecular Hamiltonians, which is a generalisation of the trajectory surface hopping method. As a first application, the complex surface fewest switches surface hopping (CS-FSSH) method was employed to survey the relaxation mechanisms of the shape resonant anions of iodoethene. We have provided the first detailed and dynamical picture of the p*/s* mechanism of dissociative electron attachment in halogenated unsaturated compounds, which...
The analysis of the photoabsorption spectra of molecules shows that the band maximum is usually r... more The analysis of the photoabsorption spectra of molecules shows that the band maximum is usually redshifted in comparison to the vertical excitation. We conducted a throughout analysis of this shift based on low-dimensional analytical and numerical model systems, showing that its origin is rooted in the frequency change between the ground and the excited states in multidimensional systems. Moreover, we deliver a benchmark of ab initio results for the shift based on a comparison of vertical excitations and band maxima calculated with the nuclear ensemble approach for the 28 organic molecules in the Mülheim molecular dataset. The mean value of the shift calculated over 60 transitions is 0.11 ± 0.08 eV. The mean value of the band width is 0.32 ± 0.14 eV.
Functionalization of the sugar group can be used to control the triplet decay rate of thionucleos... more Functionalization of the sugar group can be used to control the triplet decay rate of thionucleosides.
Journal of chemical theory and computation, Jan 14, 2017
Photosensitized singlet oxygen generation occurring in a PS-O2 complex, where PS is a photosensit... more Photosensitized singlet oxygen generation occurring in a PS-O2 complex, where PS is a photosensitizer chromophore, is a weakly coupled intermolecular energy-transfer process, a still challenging problem for theoretical chemistry. To investigate the reaction rate directly from quantum-chemical calculations, we built a semiclassical kinetic model that minimizes the computational effort for the calculation of diabatic couplings, activation energies, and reorganization energies, which are the components of the rate. The model splits the system into sets of orthogonal coordinates, which are then explored to compute the reaction rate. This model offers an effective way to evaluate the reaction probability of singlet oxygen generation along different directions and intramolecular distances of the PS-O2 complex. The model can also be applied to other similar intermolecular energy-transfer problems, to connect the reaction kinetics and quantum-chemical calculations.
The journal of physical chemistry letters, Jan 2, 2017
First-principles quantum-chemical description of photosensitized singlet oxygen generation kineti... more First-principles quantum-chemical description of photosensitized singlet oxygen generation kinetics is challenging because of the intrinsic complexity of the underlying triplet fusion process in a floppy molecular complex with open-shell character. With a quantum-chemical kinetic model specifically tailored to deal with this problem, the reaction rates are investigated as a function of intermolecular incidence direction, orientation, and distance between O2 and the photosensitizer. The adopted photosensitizer, 6-azo-2-thiothymine, combines practical interest and prototypical variability. The study quantitatively determined maximum singlet oxygen generation rates for 15 incidence/orientation directions, showing that they span 5 orders of magnitude between the largest and the smallest rate. Such information may provide a hands-on guideline for the experimental molecular design of new photosensitizers as well as further higher-level theoretical research.
Physical chemistry chemical physics : PCCP, Jan 12, 2017
A three-state model for the photo-Fries rearrangement (PFR) is proposed based on multiconfigurati... more A three-state model for the photo-Fries rearrangement (PFR) is proposed based on multiconfigurational calculations. It provides a comprehensive mechanistic picture of all steps of the reaction, from the photoabsorption to the final tautomerization. The three states participating in the PFR are an aromatic (1)ππ*, which absorbs the radiation; a pre-dissociative (1)nπ*, which transfers the energy to the dissociative region; and a (1)πσ*, along which dissociation occurs. The transfer from (1)ππ* to (1)nπ* involves pyramidalization of the carbonyl carbon, while transfer from (1)nπ* to (1)πσ* takes place through CO stretching. Different products are available after a conical intersection with the ground state. Among them is a recombined radical intermediate, which can yield ortho-PFR products after an intramolecular 1,3-H tunneling. The three-state model is developed for phenyl acetate, the basic prototype for the PFR, and it reconciles the theory with a series of observations from time-...
Physical chemistry chemical physics : PCCP, Jan 15, 2017
Small molecular systems exhibiting Excited State Intramolecular Proton Transfer (ESIPT) attract c... more Small molecular systems exhibiting Excited State Intramolecular Proton Transfer (ESIPT) attract considerable attention due to their possible role as ultrafast, efficient, and photostable molecular photoswitches. Here, by means of static potential energy profile scan and on-the-fly non-adiabatic dynamics simulations we study the photodeactivation process of a minimal-chromophore aromatic Schiff base, salicylidene methylamine (SMA), and its two derivatives 6-cyano-salicylidene methylamine (6-CN-SMA) and 3-hydroxy-salicylidene methylamine (3-OH-SMA). We show that the dominant character of the lowest excited singlet state - ππ* vs. nπ* - plays a crucial role in the system's photophysics and controls the ESIPT efficiency. We also show that the relative alignment of the ππ* and nπ* states may be controlled through chemical substitutions made to the aromatic ring of the Schiff-base molecule. We believe that our findings will improve the rational-design strategies employed for the ESIPT...
In the present study, we examined the UV excitations of a newly introduced molecular set, Halons-... more In the present study, we examined the UV excitations of a newly introduced molecular set, Halons-9, composed of nine gaseous halon molecules. The performance of the density functional-based multi-reference configuration interaction method (DFT/MRCI) and time-dependent density functional theory with CAM-B3LYP functional (TD-CAM-B3LYP) in the computation of singlet and triplet excited states of this set was evaluated against coupled-cluster with singles and doubles (CCSD). Excited states up to the corresponding ionization limits, including both localized and delocalized excitations, have been benchmarked. TD-CAM-B3LYP significantly underestimates excitation energies of the higher mixed valence-Rydberg and Rydberg states, with computed mean absolute deviations from the equation of motion (EOM)-CCSD results 1.06 and 0.76 eV, respectively. DFT/MRCI gives a significantly better description of higher excited states, albeit still poor, compared to the TD-CAM-B3LYP. The mean absolute deviati...
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