Ground State Minimum Research Articles

A predictive screening tool to evaluate the efficiency of Z/E photoisomerizable molecular switches

The evaluation of the Z/E photoisomerization efficiency is an essential task to design photoactive molecular devices. Nevertheless, this photoreactivity can be correctly described only by applying extensive and expensive computational methods. In this study, a predictive tool to screen the photoinduced Z/E isomerization efficiency of molecular switches is presented, based on three key properties: the structure of the ground state minimum, the nature of the electronic transition populating the optically bright state, and the presence of crossings between the optically bright state and the one lower in energy. Our methodology allows evaluating these properties by few and affordable calculations, potentially enabling the screening of large sets of photoswitches. After presenting the formal aspects, the tool is applied to model systems of paradigmatic classes of photoswitches (retinal, green fluorescent protein, hemithioindigo, chiroptical, and stilbene compounds) including derivatives. A comparison with the available experimental data is performed to validate our approach.

Variational approach to learning photonic unitary operators

Structured light, light tailored in its internal degrees of freedom, has become topical in numerous quantum and classical information processing protocols. In this work, we harness the high dimensional nature of structured light modulated in the transverse spatial degree of freedom to realize an adaptable scheme for learning unitary operations. Our approach borrows from concepts in variational quantum computing, where a search or optimization problem is mapped onto the task of finding a minimum ground state energy for a given energy/goal function. We achieve this by a pseudo-random walk procedure over the parameter space of the unitary operation, implemented with optical matrix-vector multiplication enacted on arrays of Gaussian modes by exploiting the partial Fourier transforming capabilities of a cylindrical lens in the transverse degree of freedom for the measurement. We outline the concept theoretically, and experimentally demonstrate that we are able to learn optical unitary matrices for dimensions d = 2, 4, 8, and 16 with average fidelities of >90%. Our work advances high dimensional information processing and can be adapted to both process and quantum state tomography of unknown states and channels.

First-principles predictions of structure, half-metallic antiferromagnetism, optoelectronic, and elastic properties of double perovskites A2TaNiO6 (A = Ca, Sr, and Ba) for energy harvesting

In this article, the electronic, half-metallic antiferromagnetism, and optical characteristics of A2TaNiO6 (A = Ca, Sr, and Ba) double perovskites have been investigated by using WIEN2k code with the GGA + PBEsol approximation. The cubic phase structures were optimized with minimum ground state energy. The electronic attributes showed a semiconductor nature for spin-up channels and a metallic nature for spin-down channels, indicating half-metallicity. Magnetic properties have demonstrated the magnetic moments and antiferromagnetic nature of the studied perovskites. The optical response was assessed by examining dielectric function, absorption coefficient, reflectivity, and energy loss function. Higher values of these parameters in the energy range of 2–4 eV suggest their potential for optoelectronics. Mechanical properties have been also investigated which demonstrated that these compounds have outstanding mechanical stability, anisotropy, mechanical strength, and ductility. The collective results reveal the importance of the analyzed compounds as promising candidates for spintronics, photovoltaics in space, UV-photodetectors, and other UV-based optoelectronic applications.

Adsorption of Water on Vacancy Defective h-BN Bilayer at B and N Sites

<>Two dimensional (2D) materials are used to manufacture the various gadgets. Their properties are therefore appealing. In the present work, we have studied the structural, electronic, and magnetic properties of 2D bilayer hexagonal-Boron Nitride (h-BN), single B vacancy defect on h-BN (B-hBN), single N vacancy defect on h-BN (N-hBN), water adsorption; on B-hBN (w-B-hBN) and on N-hBN (w-N-hBN), materials by spin-polarized density functional theory (DFT) methods employing Quantum ESPRESSO (QE) computational tools. The structural stability of the h-BN, B-hBN, N-hBN, w-B-hBN, & w-N-hBN materials has been investigated by determining their minimum ground state energy and binding energy, and found that they are stable materials. The pristine h-BN bilayer supercell structure is also found to be more compact than other defective configurations. By analyzing the band structures and density of states (DoS) plots of these materials, we have examined their electronic properties, and found that pristine h-BN has a broad bandgap, whereas B-hBN, N-hBN, w-B-hBN, & w-N-hBN exhibit semiconducting nature. The DoS and partial density of states (PDoS) computations of the considered materials are used to study their magnetic properties. It is found that pristine h-BN is a non-magnetic, and B-hBN, N-hBN, w-B-hBN & w-N-BN are magnetic materials. Therefore, vacancy defects on pristine h-BN, and water adsorption on vacancy defected h-BN materials cause it to change from non-magnetic to magnetic. They could find use in the domain of device applications.

Unraveling the potential of backbone modifications in iron(II) NHC complexes for olefin aziridination and imination

A model catalysis reaction for a C2 + N1 type aziridination and imination of p-tolyl azide with cis-cyclooctene in toluene is reported. Applying 10 equivalents related to the azide, cis-cyclooctene is consumed within 24 h. A reaction temperature of 90 °C is chosen for solubility and activity reasons. A variety of Fe(II) N-heterocyclic carbene (NHC) complexes (1–5) are tested as additives. All show promising formation of aziridine and its imine isomer in the benchmark reaction. However, similar activities are also achieved by a metal-free control reaction together with a similar product distribution of A (aziridine), B (imine) and C (aziridine). The metal compounds cannot be considered as catalysts, but they do indeed influence the isomeric distribution of the reaction. To contextualize the conceivable product isomers, density functional theory calculations were performed (B3LYP/6–31G(d,p)) to determine the minimum ground state energies. As expected, these are nearly identical. However, the backbone modification of the open-chain NHC ligated iron(II) complex (5) allows switching of the predominant reaction, favoring imine (B) as main product.

A theoretical study of ferrocene based on combined configuration interaction singles (CIS) and time-dependent density functional theory (TDDFT) approach

The state-of-the art density functional theory (DFT) is used to clearly resolve the two parallel cyclopentadienyl rings of ferrocene, which are either staggered (D5d symmetry) or eclipsed (D5h symmetry), in their ground-state conformation. Present result revealed that the eclipsed conformer with D5h point group represents the true minimum ground state structure of ferrocene. Natural population analysis is used to determine how atomic charge is distributed across different atoms of ferrocene D5h conformer and also the distribution of electrons in the core, valence, and Rydberg sub-shells. It is further investigated in potential energy scan that the rotation of the dihedral angle δ from 0° to 3π/5 will reproduce three times D5h or D5d conformers periodically as the period of 2π/5 due to the pentagonal structure of the CP ring. Further to examine optical spectra in the ultraviolet-visible (UV–vis) range, configuration interaction single (CIS) and time-dependent density functional theory (TDDFT) have conducted which help in locating the significant electronic shifts between different energy levels. Absorption spectra for high spin states were also generated in order to comprehend the characteristics of low-lying spin excitation. According to our estimates, the greatest absorption intensity is restricted to an energy range of 4–6 eV. Knowledge of ferrocene conformers will improve the research on other metallocenes and their derivatives, which have applications in biotechnology, nanotechnology, and solar technology.

Photodissociation and formation of an ion-pair in CH2 FCl (HCFC-31).

Although CH2 FCl (HCFC-31) recently became of great atmospheric importance, studies concerning its excited states are almost nonexistent. Several excited singlet states were studied (valence nσ* and Rydberg n3s, n3p, σ3s, and σ3p) through highly correlated multireference configuration interaction with singles and doubles, including extensivity correction. Comparison with the states of CH3 Cl indicates a strong influence of the F atom. Potential energy curves suggest formation of an electrostatically bound complex that relaxes to a hydrogen-bonded contact ion-pair (HBCIP) which can decay yielding CH2 F + Cl or to the ground state minimum of CH2 FCl. The HBCIP has a dipole moment of 9.57 D, a CI wavefunction described as 0.65ionic + 0.20biradical and it is strongly bonded by 4.72 eV. Its H bond has characteristics of moderate and strong H bonds. The simulated absorption spectrum confirms the nσ* assignment for the first and suggests the n3s + n3pσ assignment for the second band.

Ab initio calculations of structural, electronic, optical, and magnetic properties of delafossite SMoO2 (S = Na, K, Rb, Cs) for spintronics

Half metallic ferromagnetic (HMF) are key materials for quantum computing information storage devices and green energy. So, the structural, electronic, optical, and magnetic properties of SMoO2 (S = Na, K, Rb, Cs) are studied by quantum simulations based on density functional theory (DFT) within WIEN2K code. The Perdew − Burke − Ernzerhof (PBE + GGA) was used for the exchange correlation potential to study the said compounds. The volume optimization properties indicated that these compounds have minimum ground state energy in monoclinic crystal structure with space group 12 C2/m. Moreover, electronic band gap properties and density of states (DOS) graphs depicted the metallic nature for spin up a channel and semiconductor nature for spin down channel. This confirms the half-metallic ferromagnetism (HMF), and Heisenberg's classical model confirms the 100% spin polarization. Various frequency dependent optical parameters explain the optical properties. The double exchange process, crystal field energy ∆Ecry, direct Δx(d), and indirect exchange energies Δx(pd), is demonstrated by the quantum ferromagnetic behavior. The negative value of Δx(pd) and magnetic exchange constants confirm the ferromagnetism due to the quantum exchange mechanism of electrons. In addition to the above, it has been noted from the density of states that Mo d-states are responsible for the response of half metallic nature of these compounds. The magnetic parameter values of SMoO2 (S = Na, K, Rb, and Cs) show higher magnetic moment and major contribution coming from the Mo atom. Hence the studied results show that these compounds are valuable materials for spintronic applications.

DFT and TD-DFT study of hydrogen bonded complexes of aspartic acid and n water (n = 1 and 2)

Hydrogen bonds (HB) influence the conformational preferences of biomolecules and their optical and electronic properties. The directional interaction of molecules of water can be a prototype to understand the effects of HBs on biomolecules. Among the neurotransmitters (NT), L-aspartic acid (ASP) stands out due to its importance in health and as a precursor of several biomolecules. As it presents different functional groups and readily forms inter- and intramolecular HBs, ASP can be considered a prototype for understanding the behavior of NTs when interacting by HB with other substances. Although several theoretical studies have been performed in the past on isolated ASP and its formed complexes with water, both in gas and liquid phases, using DFT and TD-DFT formalisms, these works did not perform large basis set calculations or study electronic transitions of ASP-water complexes. We investigated the HB interactions in complexes of ASP and water molecules. The results show that the interactions between the carboxylic groups of ASP with water molecules, forming cyclic structures with two HBs, lead to more stable and less polar complexes than other conformers formed between water and the NH2 group. It was observed that there is a relationship between the deviation in the UV-Vis absorption band of the ASP and the interactions of water with the HOMO and LUMO orbitals with the stabilization/destabilization of the S1 state to the S0 of the complexes. However, in some cases, such as 1:1 complex ASP-W2, this analysis may be inaccurate due to small changes in ΔE. We studied the landscapes of the ground state surface of different conformers of isolated L-ASP and the L-ASP-(H2O)n complexes (n = 1 and 2) using the DFT formalism, with the B3LYP functional, and six different basis sets: 6-31 + + G(d,p), 6-311 + + G(d,p), D95 + + (d,p), D95V + + (d,p), cc-pVDZ, and, cc-pVTZ basis sets. The cc-pVTZ basis set provides the minimum energy of all conformers, and therefore, we performed the analysis with this basis set. We evaluated the stabilization of the ASP and complexes using the minimum ground state energy, corrected by the zero point energy and the interaction energy between the ASP and the water molecules. We also calculated the vertical electronic transitions S1 ← S0, and their properties using the TD-DFT formalism at B3LYP/cc-pVTZ level with the optimized geometries for S0 state with the same basis set. For the analysis of the vertical transitions of isolated ASP and the ASP-(H2O)n complexes, we calculated the electrostatic energy in the S0 and S1 states. We performed the calculations with the Gaussian 09 software package. We used the VMD software package to visualize the geometries and shapes of the molecule and complexes.

Effect of substitution on the superconducting phase of transition metal dichalcogenide Nb(Se_{x}S_{1-x})_{2} van der Waals layered structure

By means of first-principles cluster expansion, anisotropic superconductivity in the transition metal dichalcogenide Nb(Se_{x}S_{1-x})_{2} forming a van der Waals (vdW) layered structure is observed theoretically. We show that the Nb(Se_{0.5}S_{0.5})_{2} vdW-layered structure exhibits minimum ground-state energy. The Pnnm structure is more thermodynamically stable when compared to the 2H–NbSe_{2} and 2H–NbS_{2} structures. The characteristics of its phonon dispersions confirm its dynamical stability. According to electronic properties, i.e., electronic band structure, density of states, and Fermi surface indicate metallicity of Nb(Se_{0.5}S_{0.5})_{2}. The corresponding superconductivity is then investigated through the Eliashberg spectral function, which gives rise to a superconducting transition temperature of 14.5 K. This proposes a remarkable improvement of superconductivity in this transition metal dichalcogenide.

Substitution effects on novel bicyclo[2.2.1]hepta-7-silylenes by DFT.

Substitution effects on stability (ΔΕs-t) of novel singlet and triplet forms of bicyclo[2.2.1]hepta-7-silylenes are compared and contrasted, at B3LYP/6-311++G** level of theory. All species appear as ground state minima on their energy surface, for showing no negative force constant. Singlets (1s-24s) are ground state and more stable than their corresponding triplets (1t-24t). The most stable scrutinized silylenes appear to be 2,3-disilabicyclo[2.2.1]hepta-7-silylene (9) for showing the highest value of ΔEs-t. This stability can be related to our imposed topology and β-silicon effect. The band gaps (ΔΕHOMO-LUMO) show the same trend as ΔEs-t and the lowest unoccupied molecular orbital energies. Also, the electrophilicity appears inverse correlation with our results of ΔΕs-t. The purpose of the present work was to assess the influence of 1 to 6 silicon substitutions on the stability, band gaps, nucleophilicity, electrophilicity, and proton affinity. Finally, our investigation introduces novel silylenes with possible applications in chemistry such as semiconductors, cumulated multidentate ligands, etc. Synopsis Substitution effects on stability (ΔΕs-t) of novel singlet (s) and triplet (t) forms of bicyclo[2.2.1]hepta-7-silylenes are compared and contrasted, at B3LYP/6-311++G** level of theory. All species appear as ground state minima on their energy surface, for showing no negative force constant. Singlets (1s-24s) are ground state and more stable than their corresponding triplets (1t-24t). The most stable scrutinized silylenes appear to be 2,3-disilabicyclo[2.2.1]hepta-7-silylene (9) for showing the highest value of ΔEs-t. This stability can be related to our imposed topology and β-silicon effect. The purpose of the present work was to assess the influence of 1 to 6 silicon substitutions on stability (ΔΕs-t), band gaps (ΔΕHOMO-LUMO), nucleophilicity (N), electrophilicity (ω), and proton affinity (ΔΕPA). Finally, this new generation has the intrinsic potential to form accumulated multidentate ligands.

Electronic, magnetic, optical and transport properties of wurtzite-GaN doped with rare earth (RE= Pm, Sm, and Eu): First principles approach

A density functional theory (DFT) and semiclassical Boltzmann transport equations (BTE) has been used to explore the electronic, magnetic, optical and transport properties of Ga1−xRExN (RE=Pm, Sm, and Eu). We have studied the impact of doping of rare-earth (RE) elements on GaN wurtzite structure. In all the doped systems, the stability is ferromagnetic state with minimum ground state energy. Our theoretical calculations revealed that all the doped compounds exhibit half-metallic ferromagnet (HMF) behavior with 100% spin polarization at the Fermi level witin GGA, mBJ and Spin-orbit coupling (SOC). However, within GGA+U, Ga1−xRExN exhibits dilute magnetic semiconducting behaviour with exact integer value of total magnetic moment. A large gap was established with the concentrations of 6.25%, 12.5%. The shift of fermi energy towards the bottom of the conduction band, make these systems an n-type. The mechanism of ferromagnetism in this system has been proposed according to this positioning. The majority-spin density of states (DOS) mainly comprises of Eu-4f states appears in the energy gap. Yet, in the case of the Pm, and Sm-doped GaN, some part of the impurity states of 4f lies below Fermi energy (Ef). The optical absorption for RE-doped GaN is significantly enhanced, in the ultraviolet region. We have noticed the improvement of electrical conductivity as compared to the pristine one for both the concentrations. These features, make RE-doped GaN a transparent material for optoelectronic tools.

Switching resonance character within merocyanine stacks and its impact on excited-state dynamics

Summary In this study, the optical properties and excited-state dynamics of the unique self-assembled donor-acceptor (DA) merocyanine dye stacks from dimer up to octamer, prepared via dipole-dipole interactions, are reported in terms of coherent exciton dynamics and formation of an excimer-like state. Our findings are based on the steady-state absorption/emission, time-resolved fluorescence, and transient absorption (anisotropy) measurements, including wavepacket analysis and quantum mechanical calculations. Coherent exciton of torsional motions-restricted dye stacks rapidly localizes into the weakly emissive excimer-like state, by shortening the inter-moiety distance and changing the bond-length alternation pattern. The inner merocyanine moiety, having two neighboring units, has a reversed resonance character (non-polar (N) Z) in the ground state. This difference has led to two conclusions: (1) tetramers and octamers exhibit different features of excimer-like state than the dimer, and (2) octamers exhibit slower localization dynamics due to the enhanced homogeneity (six inner-moieties) compared with tetramers (two inner moieties).

Novel azaborastannylenes by DFT

We have reached novel borastannylenes with an unprecedented common frame work that can accommodate up to five nitrogen heteroatoms. All species appear as ground state minima on their energy surface, for showing no negative force constant. Singlets 7-boratricyclo[1.1.1.01,7.07,3.07,5]hexa-2-stannylenes (1s-20s) are ground states and more stable than their corresponding triplets (1t-20t). They are compared and contrasted with respect to their geometrical parameters, stability (ΔΕs-t), heat of hydrogenation (ΔEH), nucleophilicity (N), electrophilicity (ω), and isodesmic reactions at B3LYP/AUG-cc-pVTZ//B3LYP/6–311++G** level of theory. The highest stability belong to the singlet 1,3-diaza-7-boratricyclo[1.1.1.01,7.07,3.07,5]hexa-2-stannylene (5) which shows the highest value of ΔEs-t. Boracyclics singlet 1,4,6-triaza-7-boratricyclo[1.1.1.01,7.07,3.07,5]hexa-2-stannylene (14s) with coordinate covalent bond between nitrogen and stannylene center and 1,4,5,6-teraaza-7-boratricyclo[1.1.1.01,7.07,3.07,5]hexa-2-stannylene (18s) with high LPN → LP*Sn interactions have low ω. Also, the highest band gap (ΔΕHOMO–LUMO) belong to theirs. Our investigation introduces novel stannylene with possible applications in chemistry such as semiconductors, cumulated multi-dentate legends, etc.

Effects of nitrogen atoms on the stability and reactivity of tricyclic boracarbenes by DFT

Following our quest for novel carbenes, effects of substituting 1 to 5 nitrogen atoms on the stability and reactivity of singlet (s) and triplet (t) forms of 7-boratricyclo[1,1,1,01,7,07,3,07,5]hexa-2-carbylenes (1–20) are compared and contrasted, at B3LYP/aug-cc-pvtz level of theory. All species appear as ground state minima on their energy surface, for showing no negative force constant. Singlets (1s–20s) are ground states and more stable than their corresponding triplets (1t–20t). Reactivity of the species (1s–20s vs. 1t–20t) is discussed in terms of isodesmic reactions, considering nucleophilicity (N), electrophilicity (ω), and heat of hydrogenation. As well as, the addition of nitrogen atoms decreased nucleophilicity (N), while increasing electrophilicity (ω). Despite the enormous steric strain involved in their cubic structures, the most stable scrutinized carbenes appear to be singlet 1,4,5-triaza-7-boratricyclo[1,1,1,01,7,07,3,07,5]hexa-2-carbylene (13) for showing the highest value of ΔEs–t. Such higher stabilization is attributed to a coordinate covalent bond observed between the carbenic center and the boron atom. This study offers new insights into the chemistry of these exotic tricyclic shaped carbenes.

Ultrafast Dynamics of the Two Lowest Bright Excited States of Cytosine and 1-Methylcytosine: A Quantum Dynamical Study.

The nonadiabatic quantum dynamics (QD) of cytosine and 1-methylcytosine in the gas phase is simulated for 250 fs after a photoexcitation to one of the first two bright states. The nuclear wavepacket is propagated on the coupled diabatic potential energy surfaces of the lowest seven excited states, including ππ*, nπ*, and Rydberg states along all the vibrational degrees of freedom. We focus in particular on the interplay between the bright and the dark nπ* states, not considering the decay to the ground electronic state. To run these simulations, we implemented an automatic general procedure to parametrize linear vibronic coupling (LVC) models with time-dependent density functional theory (DFT) computations and interfaced it with Gaussian package. The wavepacket was propagated with the multilayer version of the multiconfigurational time dependent Hartree method. Two different density functionals, PBE0 and CAM-B3LYP, which provide a different description of the relative stability of the lowest energy dark states, were used to parametrize the LVC Hamiltonian. Part of the photoexcited population on lowest HOMO-LUMO transition (πHπL*) decays within less than 100 fs to a nπ* state which mainly involves a promotion of an electron from the oxygen lone pair to the LUMO (nOπL*). The population of the second ππ* state decays almost completely, in <100 fs, not only to πHπL* and to nOπL* states but also to another nπL* state involving the nitrogen lone pair. The efficiency of the adopted protocol allowed us to check the accuracy of the predictions by repeating the QD simulations with different LVC Hamiltonians parametrized either at the ground-state minimum or at stationary structures of different relevant excited states.

Conformational Behavior and Optical Properties of a Fluorophore Dimer as a Model of Luminescent Centers in Carbon Dots

The explanation of the origin of the fluorescence properties of carbon dots (CDs) represents an important task because of the great interest in the promising capabilities of these nanomaterials. 5-Oxo-1,2,3,5-tetrahydroimidazo-[1,2-α]-pyridine-7-carboxylic acid (IPCA), a molecular fluorophore, which is being created during the synthesis of CDs from citric acid and ethylenediamine, has been identified as an origin for the fluorescence of CDs. Using a combination of computational methods, we analyzed the UV absorption and fluorescence properties of the IPCA monomer and stacked IPCA dimers as basic models for the fluorescent centers in CDs. Density functional theory (DFT) for the ground state and time-dependent DFT calculations for excited states have been performed for the gas phase and for aqueous solution using a polarized continuum model. Classical molecular dynamics (MD) simulations of the dimer in the ground state have been carried out as well to investigate spontaneous association processes of IPCA and to analyze the ground state dynamics. Due to the complex charge distribution of the monomer and various possibilities of forming hydrogen bonds, in total, seven dimer structures have been identified at the DFT level as ground state minima with similar energies. Stabilities have been confirmed by domain-based local pair natural orbital (DLPNO) coupled cluster with singles and doubles and perturbative triples CCSD(T) calculations. The MD simulations confirm this picture, showing rotational flexibility processes of the two monomers with respect to each other. The lowest excited states have been characterized in terms of their orbital excitations and excitonic splitting. These calculations demonstrate the dominance of monomer π → π* transitions for the explanation of the observed UV spectra. Optimization of the dimer in the excited state did not lead to well-defined single structures but shows the picture of either intersection pathways to S1/S0 crossings which would quench fluorescence or stacked dimers with red-shifted fluorescence in comparison to the UV absorption. In the gas phase, both types of processes have been observed, whereas in solution only the stacked structures were found without any non-adiabatic radiationless deactivation processes.

Study of magnetism in Fe doped CoCr2O4 using magnetic Compton scattering and first-principles computations

Temperature dependent spin momentum densities of polycrystalline CoCr2-xFexO4 (x = 0.4 and 1) with inverse spinel structure have been measured using magnetic Compton spectrometer. Estimated site specific and absolute spin moments are compared with the magnetization measurements to deduce the orbital magnetic moments. To supplement the present magnetic Compton scattering data, spin-polarized relativistic band structure calculations employing Korringa-Kohn-Rostoker Green’s-function formalism have also been performed. Ferri-magnetic nature of CoCr2-xFexO4 (x = 0.4 and 1) systems have been explained on the basis of distribution of Fe and Co atoms between tetrahedral and octahedral sites. The suggested mixed spinel structure arrangement is supported by experimentally observed spin magnetic moments and minimum ground state energies which shows the potential of magnetic Compton spectroscopy in resolving the structural-cum-magnetic issues of such oxides.

Characterisation of gas phase halide-acetone complexes with photoelectron spectroscopy and ab initio calculations

Anion photoelectron spectra of gas phase halide-acetone complexes (X−⋯CH3COCH3 where X = Cl, Br, I) are presented. Electron binding energies, corresponding to the electron affinities of the neutral species, are determined from experiment. Additionally, two ground state minima are predicted from MP2 calculations (with corresponding energies from CCSD(T) calculations) for each set of complexes: one Cs geometry for the anion complexes with the halide located midway between two hydrogens from each methyl group of acetone, and a C1 neutral structure with the halogen appended to the carbonyl oxygen.

Computing the Ultrafast and Radiationless Electronic Excited State Decay of Cytosine and 5‐methyl‐cytosine Cations: Uncovering the Role of Dynamic Electron Correlation

AbstractPhotoionisation in DNA, i. e. the process of photoinduced electron removal from the chromophoric species – the nucleobases – leading to their cationic form, has been scarcely studied despite being considered to be responsible for significant damaging instances in our genetic material. In this contribution we theoretically characterise the electronic ground and excited state decay pathways of cationic DNA nucleobase cytosine+ and its epigenetic derivative 5‐methyl‐cytosine+, including the effects of dynamic electron correlation on energies and geometries of minima and conical intersections. We do this by comparing the results of XMS‐CASPT2 calculations with CASSCF estimates and we find some significant differences between the results of these two methods. In particular, including dynamic electron correlation is found to significantly reduce the barrier to access the conical intersection. We find notable similarities in both cytosine and 5‐methyl‐cytosine cations, and accessible conical intersections in the vicinity of the Franck‐Condon region are found. This points towards an ultrafast depopulation of their electronic excited states. Moreover, the shape of the ground state potential energy surface strongly directs the decaying excited state population towards the cationic ground state minimum on ultrafast timescales, preventing photo‐fragmentation and thus explaining their photostability. To better compare our calculations with the available experimental data we compute the UV (ground and excited state) and IR absorptions.