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Thermally Induced Protonation of Conducting Polyaniline Film by Dibutyl Phosphite Conversion to Phosphate
摘要： The blue thin polyaniline base film changes its color to green after immersion of the film into dibutyl phosphonate. The green color of the film converts to a greenish-blue after heating up to 200 °C in air which is characteristic for protonated conducting form of polyaniline. This is in the contrast to the “standard” polyaniline hydrochloride which is transformed into a crosslinked polyaniline base under such conditions. To explain this unexpected observation, the interaction of polyaniline base with dibutyl phosphonate at ambient conditions and after heating to 200 oC was studied using UV–visible, FTIR and Raman spectroscopies. Based on these studies, we propose that dibutyl phosphite tautomeric form of dibutyl phosphonate which interacts with polyaniline base at 20 °C converts to the oxidized form, dibutyl phosphate, at 200 °C and subsequently protonates the film. Quantum-chemical modelling of the interaction of polyaniline base with dibutyl phosphite and dibutyl phosphate supports this explanation.
关键词： Thermally Induced Protonation,Quantum Chemistry,Conducting Polyaniline Film,Dibutyl Phosphite,Molecular Structure,Phosphate Conversion,Spectroscopy
更新于2025-09-10 09:29:36
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Low-Temperature Effect on the Electronic Structure and Spectral-Fluorescent Properties of Highly Dipolar Merocyanines
摘要： Absorption and fluorescence spectra of a vinylogous series of reversely solvatochromic merocyanines based on benzimidazole and malononitrile have been studied in frozen ethanol solutions at 77 K. It is found that they possess negative thermochromism – in contrast to both positively solvatochromic merocyanines and negatively solvatochromic symmetrical ionic polymethines – and even stronger negative thermofluorochromism. It has been deduced from the spectral data, that at low temperature their electronic structure becomes more dipolar, deviating substantially from the virtual ideal polymethine in both the ground and excited states. At that, owing probably to high polarity and ordering of frozen ethanol, the dipolarity of the studied merocyanines increases with the polymethine chain lengthening – the tendency not observed for them in common solvents. The conclusions, based on the spectra data analysis, have been verified by the (TD)DFT–PCM simulations of the dyes within the four-level scheme of electronic transitions.
关键词： Low-Temperature Effect,Spectral-Fluorescent Properties,Merocyanines,Quantum Chemistry,Electronic Structure,Molecular Structure,Spectroscopy
更新于2025-09-10 09:29:36
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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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Exciting vibrons in both frontier orbitals of a single hydrocarbon molecule on graphene
摘要： Vibronic excitations in molecules are key to the fundamental understanding of the interaction between vibrational and electronic degrees of freedom. In order to probe the genuine vibronic properties of a molecule even after its adsorption on a surface appropriate bu?er layers are of paramount importance. Here, vibrational progression in both molecular frontier orbitals is observed with submolecular resolution on a graphene-covered metal surface using scanning tunnelling spectroscopy. Accompanying calculations demonstrate that the vibrational modes that cause the orbital replica in the progression share the same symmetry as the electronic states they couple to. In addition, the vibrational progression is more pronounced for separated molecules than for molecules embedded in molecular assemblies. The entire vibronic spectra of these molecular species are moreover rigidly shifted with respect to each other. This work unravels intramolecular changes in the vibronic and electronic structure owing to the e?cient reduction of the molecule–metal hybridization by graphene.
关键词： single molecules,quantum chemistry,density functional theory,vibrational progression,graphene,scanning tunnelling microscopy,frontier orbitals
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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One-particle density matrix polarization susceptibility tensors
摘要： The electric field-induced change in the one-electron density has been expressed as a series of the one-particle density matrix susceptibilities interacting with the spatial distribution of the electric field. The analytic approximate expressions are derived at the Hartree-Fock theory, which serves as a basis for the construction of the generalized model that is designed for an arbitrary form of wavefunction and any type of one-particle density matrix. It is shown that it is possible to accurately predict the changes in the one-electron ground-state density of water molecule in a spatially uniform electric field, as well as in spatially non-uniform electric field distribution generated by point charges. When both linear and quadratic terms with respect to the electric field are accounted for, the electric field-induced polarization energies, dipole moments, and quadrupole moments are quantitatively described by the present theory in electric fields ranging from weak to very strong (0.001–0.07 a.u.). It is believed that the proposed model could open new routes in quantum chemistry for fast and efficient calculations of molecular properties in condensed phases.
关键词： polarization susceptibility tensors,one-particle density matrix,quantum chemistry,Hartree-Fock theory,electric field-induced polarization
更新于2025-09-09 09:28:46
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The Effect of Relativistic Interactions on the Spectral Characteristics of the Ground State of Carbon Monoxide
摘要： Using high-precision nonempirical methods of modern quantum chemistry, the effect of the weak relativistic interactions on the potential energy and the permanent dipole moment of the ground electronic state of the CO molecule is studied. The relativistic energy is calculated by the following three optional methods: within the first-order perturbation theory using the Cowan–Griffin operator containing the sum of the mass-velocity and Darwin corrections, within the framework of the approximate Douglas–Kroll–Hess scalar Hamiltonian, and the most rigid “four-component” relativistic Dirac–Coulomb–Gaunt Hamiltonian. The relativistic correction obtained by different methods agrees within a few percents and equals about 55–60 cm–1 in the region of an equilibrium internuclear distance of 1.128 ?. The addition of the relativistic correction decreases the equilibrium bond length by about 0.0002 ?. The magnitude of the Lamb shift estimated by the semiempirical scaling of the one-electron Darwin’s term does not exceed several inverse centimeters near eR. The relativistic correction to the dipole moment function is in the range from –0.001 to +0.003 D, which does not exceed 1% of the nonrelativistic component of the dipole moment.
关键词： dipole moment,relativistic interactions,carbon monoxide,spectral characteristics,quantum chemistry
更新于2025-09-09 09:28:46
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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