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An Investigation on the Unexplored Photochemistry of 5,5-Dimethyl-1-Pyrroline 1-Oxide (DMPO)
摘要： A comparative study of 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) and its 2-methyl-substituted analogue (2-Me-DMPO) has revealed their contrasting reaction pathways of oxaziridine and lactam (pyrrolidone) formation. The initial photo-excitation populates the second excited singlet states (S2) in both the systems with S0-S2 transition moment value of 3 Debye (oscillator strength 0.4); this subsequently undergoes (S0/S1) conical intersection through a structure having a CNO-kink and situated around 35-40 kcal / mol below the vertically excited geometry of the first excited singlet state (S1). This conical intersection is found to be responsible for the formation of the oxaziridine photoproduct in these systems. In DMPO, this oxaziridine eventually forms the corresponding lactam compound through a [1,2] H shift after overcoming a barrier of 35 kcal / mol and following the imaginary frequency of 1517i cm-1. The reverse thermal process of parent nitrone formation proceeds through a transition state situated at 60 kcal / mol above the oxaziridine geometry and the corresponding imaginary frequency is 1514i cm-1. On the other hand, in 2-Me-DMPO, the oxaziridine formed is more stable and lactam formation does not happen from it in a similar manner.
关键词： Quantum Chemistry,Oxaziridine,Lactam,DMPO,Photochemistry,Spectroscopy,Molecular Structure
更新于2025-09-10 09:29:36
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Gas phase detection and rotational spectroscopy of ethynethiol, HCCSH
摘要： We report the gas-phase detection and spectroscopic characterisation of ethynethiol (HCCSH), a metastable isomer of thioketene (H2C2S) using a combination of Fourier-transform microwave and submillimetre-wave spectroscopies. Several a-type transitions of the normal species were initially detected below 40 GHz using a supersonic expansion-electrical discharge source, and subsequent measurement of higher-frequency, b-type lines using double resonance provided accurate predictions in the submillimetre region. With these, searches using a millimetre-wave absorption spectrometer equipped with a radio frequency discharge source were conducted in the range 280–660 GHz, ultimately yielding nearly 100 transitions up to rR0(36) and rQ0(68). From the combined data set, all three rotational constants and centrifugal distortion terms up to the sextic order were determined to high accuracy, providing a reliable set of frequency predictions to the lower end of the THz band. Isotopic substitution has enabled both a determination of the molecular structure of HCCSH and, by inference, its formation pathway in our nozzle discharge source via the bimolecular radical-radical recombination reaction SH + C2H, which is calculated to be highly exothermic (?477 kJ/mol) using the HEAT345(Q) thermochemical scheme.
关键词： Rotational spectroscopy,astrochemistry,molecular structure,quantum chemistry
更新于2025-09-09 09:28:46
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<i>Ab Initio</i> Prediction of Fluorescence Lifetimes Involving Solvent Environments by Means of COSMO and Vibrational Broadening
摘要： The fluorescence lifetime is a key property of fluorophores that can be utilized for microenvironment probing, analyte sensing, and multiplexing as well as barcoding applications. For the rational design of lifetime probes and barcodes, theoretical methods have been developed to enable the ab initio prediction of this parameter, which depends strongly on interactions with solvent molecules and other chemical species in the emitter′s immediate environment. In this work, we investigate how a conductor-like screening model (COSMO) can account for variations in fluorescence lifetimes that are caused by such fluorophore-solvent interactions. Therefore, we calculate vibrationally broadened fluorescence spectra using the nuclear ensemble method to obtain distorted molecular geometries to sample the electronic transitions with time-dependent density functional theory (TDDFT). The influence of the solvent on fluorescence lifetimes is accounted for with COSMO. For an example 4-hydroxythiazole fluorophore containing different heteroatoms and acidic and basic moieties in aprotic and protic solvents of varying polarity, this approach was compared to experimentally determined lifetimes in the same solvents. Our results demonstrate a good correlation between theoretically predicted and experimentally measured fluorescence lifetimes except for the polar solvents ethanol and acetonitrile that can specifically interact with the heteroatoms and the carboxylic acid of the thiazole derivative.
关键词： Fluorescence Lifetimes,Quantum Chemistry,Molecular Structure,TDDFT,Spectroscopy,Vibrational Broadening,COSMO
更新于2025-09-09 09:28:46
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Low-Energy Photoelectron Spectrum and Dissociative Photoionization of the Smallest Amides: Formamide and Acetamide
摘要： The threshold photoelectron spectrum and low-energy dissociative photoionization processes of formamide and acetamide were studied using photoelectron photoion coincidence spectroscopy and vacuum ultraviolet synchrotron radiation. Ab initio calculations and Franck–Condon simulations helped us assign the main vibrational progressions in the spectra and enabled the first conclusive assignment of the first electronically excited states. The adiabatic ionization energies to the ???+ and ???+ states of formamide (10.236 ± 0.004 eV and 10.643 ± 0.015 eV) and acetamide (9.734 ± 0.008 and 10.282 ± 0.020 eV) have been re-evaluated and spectroscopic transitions were assigned using a Franck–Condon approach. The cationic potential energy surface was explored to rationalize the observed fragmentation patterns and to construct a statistical model, which was fitted to the experimental breakdown diagram. Thermochemical thresholds were measured and calculated for H, CO, and NH2 loss from HCONH2+ as well as for CH3, NH2, CO, HCCO, and NH3 loss from CH3CONH2+. We present the first comprehensive, experimental and theoretical treatise of these fragmentation processes. The statistical model confirms fast internal conversion between the ???+ and ???+ states in formamide, as H-transfer in CO loss is shown to take place on the excited state surface. It also explains the five almost simultaneously opening dissociation channels in the acetamide cation quantitatively. The derived 0 K appearance energies have been confirmed by ab initio calculations and by comparison with state-of-the-art thermochemical data, and revise some of the previous results by more than ten times their stated uncertainty.
关键词： Dissociative Photoionization,Photoelectron Spectrum,Acetamide,Formamide,Quantum Chemistry,Spectroscopy,Molecular Structure
更新于2025-09-09 09:28:46
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Two-dimensional ultraviolet spectroscopy of proteins
摘要： Two-dimensional ultraviolet (2DUV) spectroscopy is a novel technology for probing molecular structure. We have developed a generalized quantum mechanics/molecular mechanics (QM/MM) approach to simulate the electronic transitions of protein backbones and aromatic amino acids in aqueous solution. These transitions, which occur in the ultraviolet (UV) region, provide a sensitive probe of molecular structure. The features of 2DUV spectra are accurately characterized and enable us to trace small variations in the structure and dynamics as well as evolution propensity with high accuracy. Various structures and dynamic phenomena are investigated to construct a systematic framework for 2DUV simulation mechanisms, so as to explore further applications of this technique. In this feature article, we summarize the theory and applications of 2DUV spectroscopy we have engaged in recently, present the important roles of 2DUV spectroscopy, and outline directions for future development. We hope this article can offer a platform for more scientists in different research fields to gain a clear overview of 2DUVand further attract more people to explore this promising field.
关键词： two-dimensional ultraviolet spectroscopy (2DUV),quantum mechanics/molecular dynamics (QM/MM),molecular structure and dynamics,protein
更新于2025-09-04 15:30:14
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Impact of molecular structure on secondary organic aerosol formation from aromatic hydrocarbon photooxidation under low NO<sub><i>x</i></sub> conditions
摘要： The molecular structure of volatile organic compounds (VOC) determines their oxidation pathway, directly impacting secondary organic aerosol (SOA) formation. This study comprehensively investigates the impact of molecular structure on SOA formation from the photooxidation of twelve different eight to nine carbon aromatic hydrocarbons under low NOx conditions. The effects of the alkyl substitute number, location, carbon chain length and branching structure on the photooxidation of aromatic hydrocarbons are demonstrated by analyzing SOA yield, chemical composition and physical properties. Aromatic hydrocarbons, categorized into five groups, show a yield order of ortho (o-xylene and o-ethyltoluene) > one substitute (ethylbenzene, propylbenzene and isopropylbenzene) > meta (m-xylene and m-ethyltoluene) > three substitute (trimethylbenzenes) > para (p-xylene and p-ethyltoluene). SOA yields of aromatic hydrocarbon photooxidation do not monotonically decrease when increasing alkyl substitute number. The ortho position promotes SOA formation while the para position suppresses aromatic oxidation and SOA formation. Observed SOA chemical composition and volatility confirm that higher yield is associated with further oxidation. SOA chemical composition also suggests that aromatic oxidation increases with increasing alkyl substitute chain length and branching structure. Further, carbon dilution theory developed by Li et al. (2015a) is extended in this study to serve as a standard method to determine the extent of oxidation of an alkyl substituted aromatic hydrocarbon.
关键词： photooxidation,low NOx conditions,molecular structure,secondary organic aerosol,aromatic hydrocarbons
更新于2025-09-04 15:30:14
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Stability and Reactivity of Silicon Magic Numbers Doped with Aluminum and Phosphorus Atoms
摘要： The progressive scaling down of the silicon-based electronics has allowed to develop devices at nanometer scale, requiring new engineering techniques guided by fundamental chemical and physical concepts. Particularly, the nanostructured cluster systems are promising materials since their physical-chemical properties are sensitive to its shape, size and chemical components, such that completely different materials can be produced by the simple addition or removal of a single atom. These size-tunable properties can open a new area in materials science and engineering. In the present work, quantum chemical methods were used to study the chemical substitution effects caused by subvalent (aluminum) and supervalent (phosphorus) atoms in the physical-chemical properties of some small silicon clusters which demonstrate high stability, called magic numbers. The changes in the electronic structure and chemical acceptance to the dopants were evaluated with respect to: ionization potential, electronic excitation energy, stability and reactivity parameters. Taken together, these results enable to identify the most stable silicon-doped clusters. Regarding electrophilic reactions, Si10P is the most favorable system, while for nucleophilic reactions, none of the doped clusters resulted in higher stability.
关键词： Aluminum,Silicon Magic Numbers,Quantum Chemistry,Spectroscopy,Molecular Structure,Phosphorus Atoms
更新于2025-09-04 15:30:14
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AIP Conference Proceedings [Author(s) SolarPACES 2017: International Conference on Concentrating Solar Power and Chemical Energy Systems - Santiago, Chile (26–29 September 2017)] - Investigation of molecular and radical structures of ethylene glycol
摘要： In this study, the conformational variety of ethylene glycol molecule was theoretically searched via energy minimization calculations carried out by using the Merck Molecular Force Field (MMFF) and eight stable conformers were determined for the free molecule. For each of the found conformers, a geometry optimization calculation was performed by using the DFT/B3LYP method and 6-311++G(d,p) basis set. From these calculations some structural data of the conformers, such as optimized geometry, relative energy and dipol moment, were obtained and then they were used in evaluation of the stabilities of the investigated conformers. In the next step of the study, theoretically possible forteen radicals were derived from the most stable conformer of free ethylene glycol and geometry optimization was also performed for each of them. Afterwards, the “Electron Paramagnetic Resonance (EPR)” parameters of these radicals were calculated using DFT/B3LYP method and TZVP basis set and then the obtained values were compared to the corresponding experimental data reported in the literature. The results have demonstrated that the experimentally observed EPR parameters match the theoretical EPR values obtained for the model radical called here “Rad 5” (?OHCH2OH). The model “Rad 5” is an anion radical formed by abstraction of an hydrogen atom from the neutral free molecule.
关键词： DFT,EPR parameters,radical structures,ethylene glycol,molecular structure
更新于2025-09-04 15:30:14