In a recent publication, Lobsiger et al. [Phys. Chem. Chem. Phys. 12, 5032 (2010)] presented infrared and electronic absorption spectra of supersonic jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), the enol form of deoxythymine. In addition, they reported on the fast nonradiative decay of the S1 population to a dark state. In the present paper, we have investigated the mechanism and rate constants of this nonradiative decay by means of quantum chemical multi-configuration methods. To this end, minima of the lowest excited singlet and triplet states as well as the minimum-energy crossing point of singlet and triplet potential energy hypersurfaces (PEHs) have been determined employing a numerical DFT/MRCI gradient where DFT/MRCI stands for a combination of density functional theory (DFT) and a semi-empirical multi-reference configuration interaction (MRCI) approach. Rate constants have been calculated in the Condon approximation using a time-dependent approach based on harmonic oscillator functions and electronic spin–orbit coupling matrix elements evaluated at the DFT/MRCI level. It is shown that the first excited triplet state possesses 3(n?→?π*) character in the gas phase. Fast intersystem crossing is mediated by the low-lying 3(π?→?π*) state whose PEH crosses both, the S11(n?→?π*) and T13(n?→?π*) PEHs. 相似文献
Abstract Indoline‐2‐thione (BC), benzimidazole‐2‐thione (BN), benzoxazole‐2‐thione (BO), and benzothiazole‐2‐thione (BS) define an interesting series of aromatic compounds containing a NCS synthonic unit in a heterocyclic ring of five centers, substituted by atomic centers of the type C, N, O, or S, where the main electronic absorption bands are localized in the spectral range of ultraviolet A or B. The first two singlet electronic transitions of this series, 1S0→1S1(n,π*) and 1S0 → 1S2(π,π*), determine the main spectroscopic characteristic of these compounds in order to be used as potential photochemical actinometers of solar ultraviolet radiation. Furthermore, the second electronic transition, localized in the 270–360 nm ultraviolet spectral range, presents a hipsochromic spectral shift as function of the electronic nature of the heteroatomic centers in the heterocyclic ring. In order to determine a spectroscopic assignment of the main absorption bands in aqueous solution and analyze the effect of the substituent on the electronic charge distributions in the ground and the first two singlet excited electronic states, we have used a semiempirical molecular orbital calculation in the INDO/S‐CIS approach. On the other hand, we have carried out a molecular orbital calculation in the AM1 framework, in order to determine the energetic stability of the thiones with respect to the thiol compounds. 相似文献
We have investigated the electronic transition, chiroptical properties, and the second-order nonlinear optical (NLO) properties of eight novel chiral diborate compounds and elucidated structure–property relationships from the micromechanism. These compounds show calculated first hyperpolarizabilities (β) ranging from 2738.52 to 83976.45?×?10?33?esu, which means that subtle structural modifications can substantially enhance the first hyperpolarizability. The cooperativity of intramolecular charge transfer and an effective way to enhance the NLO response were also systemically investigated. The linear correlation between the first hyperpolarizability and the inverse of the electronic transition energy suggests that the electronic transition energy plays a key role in determining the NLO response. These compounds have the potential to be excellent second-order NLO materials from the standpoint of the large β values, high transparency and the intrinsic non-centrosymmetry. The electronic transition and chiroptical properties have been assigned and analysed. The main UV–visible absorption features are best described as π?→?π* transitions. Moreover, the effects of different functionals and basis sets on the first hyperpolarizability were investigated. 相似文献
Molecular electronic structure calculations, employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methodologies, have been carried out to improve the performance of the synthesised dye YD2-o-C8 which is characterised by 11.9%–12.7% efficiencies. We aimed to narrow the band gap of YD2-o-C8 to extend the light-harvesting region to near-infrared (NIR). This was done by incorporating Cd instead of Zn onto the porphyrin ring and elongating the length of π-conjugation by adding ethynylene link and anthracene unit, so that the performances of the suggested cells could be expected to exceed the 11.9%–12.7% efficiencies with TiO2, ZnO2, and WO3 oxide electrodes. The effects of modifying the central metal and elongating the length of π-conjugation on cell performance are confirmed in terms of frontier molecular orbital (FMO) energy gaps, density of states (DOS), molecular electrostatic potentials (MEPs), non-linear optical (NLO) properties, ultraviolet–visible (UV–vis) electronic absorption, and 1H nuclear magnetic resonance chemical shifts. Increasing the length of π-conjugation of the D–π–A dyes leads to increasing the DOS near Fermi levels, more active NLO performance, strong response to the external electric field, delocalisation of the negative charges near the anchoring groups, deep electron injection, suppressing macrocycle aggregation, active dye regeneration, and inhibited dye recombination. The calculated band gap/eV of the present DMP-Zn is correlated with the experimental (E1/2(oxidation)–E1/2(reduction)/V) potentials of the identical YD2-o-C8. A co-sensitiser is suggested for NIR sensitisation (550–950 nm) to increase the power-to-conversion efficiency beyond 14%. 相似文献
ABSTRACTMetal-organic super container (MOSC) molecules are ideal candidates for photocatalysis due to their construction with transition metal centres and tuneable cavity sizes that could house catalytic sites. The basic electronic structure for a model of extremely large size (more than 2000 ions) is explored by single point calculation using unrestricted density functional theory, and Perdue–Burke–Ernzerhof functional in Vienna abinitio simulation package software. The information obtained through these calculations (such as density of states, absorbance spectra, and charge density) will allow for analysis of a MOSC's catalytic ability. Electronic characteristics of the nanostructures (MOSCs and their building blocks) in the ground and photoexcited electronic configurations are examined. We explore if the presence of transition metal ions with open shells in such close proximity to one another may result in high spin configurations and show any arrangement into ferromagnetic ordering. Spin-unrestricted computation was applied to evaluate how optical properties could be affected by d–d transitions. A scan of a spin-polarisation parameter allows one to resolve spin configuration and obtain a connection between theory and experiment. Analysis of Kohn–Sham orbitals of interest provides insight into charge transfer mechanisms, which were found to contribute to multiple low-energy charge transfer states to the electronic structure. 相似文献
Anticancer character of gold cluster has been indicated through its free radical scavenging properties. This is in contrast to its free radical promoting ability suggested by other workers. Here, we address this controversy by probing the stabilizing effects of Au3 cluster on RO? vs its impacts on RO–H bond dissociation enthalpy, at B3LYP/ LACVP+* level (R═H, methyl, ethyl, n‐propyl, i‐propyl, n‐butyl, t‐butyl, and phenyl). In the presence of Au3 cluster, bond dissociation enthalpy of O–H bond and the spin density at the RO? oxygen are reduced dramatically. These are clear evidences for both the Au3 facilitation of the RO–H bond breakage and its scavenging of RO? radical. Since O–Au anchoring bond is responsible for the interaction of Au3 cluster and ROH (or RO?), its nature was interpreted by means of the quantum theory of atoms in molecules and the natural bond orbital. The results indicate that O–Au bond is stronger and has more covalent character in RO?–Au3 than in ROH–Au3. The interaction of Au3 cluster with RO? is 1.5 to 3 times more than that with ROH. As a result, gold cluster scavenging property appears more prominent than its free radical initiation activity. 相似文献
A triplet diphenylcarbene, bis[3-bromo-5-(trifluoromethyl)[1,1'-biphenyl]-4-yl]methylidene (B3B), with exceptional stability was discovered by chemists from Japan's Mie University. To investigate its different quantum chemical features, a theoretical analysis was predicated on Density Functional Theory (DFT) and Time Dependent-DFT (TD-DFT) based technique. According to the findings, the singlet–triplet energy gap (ES-T), as well as HOMO–LUMO energy bandgap (EH-L), was found to be diminished when nucleophilicity (N) rose. We looked at the geometrical dimensions, molecular orbitals (MOs), electronic spectra, electrostatic potential, molecular surfaces, reactivity characteristics, and thermodynamics features of the title carbene (B3B). Its electronic spectra in different solvents were calculated using TD-DFT and Polarizable Continuum Model (PCM) framework. The estimated absorption maxima of B3B were seen between 327 and 340 nm, relying on the solvents, and were attributed to the S0?→?S1 transition. Estimated fluorescence spectral peaks were found around 389 and 407 nm with the S1 and S0 transitions being identified. Its fluorescence/absorption intensities revealed a blue shift change when the solvent polarity was increased. The least exciting state has been discovered to be the π?→?π* charge-transfer (CT) phase. According to the Natural Bonding Orbital (NBO) exploration, ICT offers a significant role in chemical system destabilization. Furthermore, several hybrid features were used to determine the NLO (nonlinear optical) features (polarizability, first-order hyperpolarizability, and dipole moment). The calculated values suggest that B3B is a promising candidate for further research into nonlinear optical properties.
Density functional theory (DFT) calculations introduced triplet ground states for [6]n SiC‐cyclacenes and ‐acenes with alternate silabenzene rings including silicon atoms in 2 opposite edges (n = 6, 8, 10, 12). The singlet‐triplet energy gap (ΔE(S‐T)), binding energy per atom (BE/n), and NBO calculation with very small band gap (ΔELUMO‐HOMO) confirmed the triplet ground states. In contrast to polyacenes, the singlet [6]n SiC‐cyclacenes displayed more stability improvement than triplets, through n increasing. This may open the way for synthesis of larger stable [6]n SiC‐cyclacenes. The ΔE(S‐T), BE/n, and the strain energy through homodesmic equations indicated more stability for larger [6]n SiC‐cyclacenes, which was more noticeable in singlet states. Cyclacenes and acenes with high conductivity and full point charge were introduced as suitable candidates for hydrogen storage. 相似文献
ABSTRACTThe structural, electronic, intramolecular charge transfer (ICT) and nonlinear optical (NLO) properties of the donor-π-acceptor (D-π-A) azo linked dyes bearing coumarin thiophene bridge with different acceptors were inspected by Z-scan and DFT methods. The dye 3a exhibits bathochromic absorption maxima (649 and 650?nm) in the near IR region in DMF and DMSO. The dye 3a holds low HOMO–LUMO gap elucidated by CV and DFT indicating strong ICT character. The thermal stability is high for 3a and it shows enhanced NLO property by Z-scan and DFT methods as predicted in both global and range-separated hybrid functionals. The molecular geometry was optimised using B3LYP/6-311?+?g(d,p). The ICT characteristics are correlated with NLO properties obtained by Z-scan and DFT techniques. 相似文献
2?+?1 resonance-enhanced multiphoton ionization (REMPI) spectra of allene at 7.0–10.5?eV have been observed. The excited vibronic symmetry has been determined from polarization-ratio measurements. Based on the vibronic energies and peak intensities calculated using ab initio MO and time-dependent density functional theory, the very congested REMPI spectra have been assigned as due to π*?←?π, 3p?←?π, 4s?←?π, 4p?←?π, and 4d?←?π transitions. Vibrational progressions related to the CH2 twisting (ν4 ~770?cm?1) have been observed for several excited electronic states. Calculated Franck–Condon factors also confirm that CH2 twisting is the most active mode in the vibronic spectra of allene. In this study, theoretical calculations of two-photon intensities and polarization ratios have been made through the ab initio computed one-photon transition dipole moments to various electronic states as intermediates. As a starting point to interpret the complicated vibronic spectrum of allene, the theoretical approach, without vibronic couplings, has been applied to predict the peak positions, spectral intensities, and polarization ratios of Rydberg states, and qualitatively shows a considerable agreement with experimental observations. 相似文献
The Skyrme model is generalized for a noncommutative spacetime with the Weyl‐operators of SU(2) matrices and the corresponding star‐product. The unitary condition and the topological current can be extended to star‐exponential matrices. The Wess‐Zumino term which breaks unphysical symmetries of the Skyrme action is gauged with the U*(1) group to allow for electromagnetic processes in a noncommutative spacetime. Apart from corrections to the anomalous decay γ→π0π+π– in commuting spacetime, the additional anomalous process γ→π0π0π0 is found in the U*(1) gauged Wess‐Zumino action for a noncommutative spacetime. 相似文献