The Journal of Physical Chemistry A最新文献

The gas-phase reaction between ethylene glycol ((CH₂OH)₂) and the methylidyne radical (CH) has been investigated with the aim of understanding the competition between carbon addition and dehydrogenation processes under interstellar conditions. The former type of reaction increases the molecular complexity and leads to the formation of members of the C₃H₆O₂ family (with the hydrogen atom as coproduct), while the latter somewhat decreases the chemical complexity but opens the way toward the gas-phase production of C₂H₄O₂ isomers (together with the CH₃ radical as coproduct). An accurate investigation of the reactive potential energy surface indicates the formation of five isomers belonging to the C₂H₄O₂ family and six species belonging to the C₃H₆O₂ one. From a thermochemical point of view, the most stable product is acetic acid + CH₃, while 2-methoxyacetaldehyde + H is the least stable. However, because of the low temperatures of the interstellar medium, reactivity is ruled by kinetics. Kinetic simulations turn the tide, with the formation of 2-methoxyacetaldehyde becoming the fastest process. The title reaction also produces glycolaldehyde + CH₃, followed by the formation of methyl formate + CH₃ and methyl acetate + H to a lesser extent.

In this paper, an efficient implementation of the renormalized internally contracted multireference coupled cluster with singles and doubles (RIC-MRCCSD) into the ORCA quantum chemistry program suite is reported. To this end, Evangelista's Wick&d equation generator was combined with ORCA's native AGE code generator in order to implement the many-body residuals required for the RIC-MRCCSD method. Substantial efficiency gains are realized by deriving a spin-free formulation instead of the previously reported spin-orbital version developed by some of us. Since AGE produces parallelized code, the resulting implementation can directly be run in parallel with substantial speedups when executed on multiple cores. In terms of runtime, the cost of RIC-MRCCSD is shown to be between single-reference RHF-CCSD and UHF-CCSD, even when active space spaces as large as CAS(14,14) are considered. This achievement is largely due to the fact that no reduced density matrices or cumulants higher than three-body enter the formalism. The scalability of the method to large systems is furthermore demonstrated by computing the ground-state of a vitamin B₁₂ model comprised of an active space of CAS(12,12) and 809 orbitals. In terms of accuracy, RIC-MRCCSD is carefully compared to second- and approximate fourth-order n-electron valence state perturbation theories (NEVPT2, NEVPT4(SD)), to the multireference zeroth-order coupled-electron pair approximation (CEPA(0)), as well as to the IC-MRCCSD from Köhn. In contrast to RIC-MRCCSD, the IC-MRCCSD equations are entirely derived by AGE using the conventional projection-based approach, which, however, leads to much higher algorithmic complexity than the former as well as the necessity to calculate up to the five-body RDMs. Remaining challenges such as the variation of the results with the flow, a free parameter that enters the RIC-MRCCSD theory, are discussed.

Iron-sulfur clusters conduct a wide variety of biochemical reactions that are conserved across all domains of life. The detailed quantum spin structure of reactive ligands of these clusters can be studied experimentally and theoretically by means of magnetic hyperfine spectroscopy, which can reveal catalytic intermediates in these biochemical processes. Their theoretical prediction, however, requires either advanced methods that describe strongly correlated systems or Hamiltonian modeling based on symmetry-broken electronic structure methods. This work shows that the addition of electron-transfer interactions to the Heisenberg-Dirac-Van Vleck Hamiltonian model leads to a quantitative explanation of hyperfine coupling constants at active organic ligand sites. Comparison with experimentally available results confirms that our extended approach can be used in calculations aimed at describing cutting-edge systems.

We analyze the degradation of the C₆₀ fullerene under the action of hypochlorite (ClO^-). The ClO^- anion, produced by the human myeloperoxidase (hMPO) enzyme, is highly reactive and known for destroying bacteria and degrading nanostructured materials. In particular, previous studies show that hMPO can biodegrade nC₆₀ nanoparticles in a short time, with hypochlorite playing a key role, though the exact mechanism is still unknown. In this work, we use density functional theory (DFT) calculations to investigate ClO^- adsorption on water-covered fullerenes. We find that there is a strong tendency of hypochlorite to dissociate rather than remain molecularly adsorbed near the hydrated C₆₀ surface. As a consequence of this reaction, the fullerene cage can be oxidized through the adsorption of carbonyl, epoxy, molecular O₂, and ClO groups preferentially located in close proximity on the carbon network, while individual chloride ions remain hydrated and stabilized in the aqueous environment. The formation of domains of chemisorbed oxygen species, as reported here, reduces the number of C═C double bonds in the cage, thereby decreasing the structural stability of C₆₀. Chlorination of the carbon surface is not energetically favored following ClO^- bond cleavage. Most interestingly, our calculations reveal that the oxidation of the fullerene surface is frequently accompanied by the breaking of C-C bonds beneath the oxidized regions, resulting in hole formation in the carbon cage. We performed simulations of NMR, UV-vis, and ECD spectroscopies, which reveal well-defined spectral features that could be very helpful in identifying the structural transformations and chemical composition reported here for these nanosized carbon materials. According to our proposed atomistic mechanism, the dissociative adsorption of hypochlorite at various regions on the carbon network, along with the formation of molecular islands composed of oxidizing species, may lead to a generalized porous morphology of the cage consistent with experimental observations of significant structural transformations in hMPO exposed C₆₀ solutions.

Hydrogen shift reactions (H-shifts) are important unimolecular steps in the autoxidation of volatile organic compounds (VOCs). These potentially lead to the formation of highly oxygenated organic molecules (HOMs) that contribute to the formation of secondary organic aerosol (SOA). Although H-shift chemistry in hydrocarbon peroxy radicals (RO₂) is well-studied, the impact of incorporating a heteroatom, oxygen, nitrogen, sulfur, or phosphorus (O, N, S, or P), into the carbon backbone remains largely unexplored. We have used a multiconformer transition state theory approach to calculate H-shift rate coefficients in RO₂s that contain a heteroatom. We find that heteroatoms accelerate H-shift reactions, particularly when the heteroatom is positioned α with respect to the carbon from which the H atom is abstracted. For the 1,6 H-shift with the heteroatom β to the abstraction site, the rate coefficients are more similar to those in the hydrocarbon. For large H-shift spans, the variation in the H-shift rate coefficient with heteroatoms is large, and the reactions are faster than in hydrocarbons. For an ether, we find that the 1,8 H-shift is as fast as the 1,5 H-shift and both are competitive with bimolecular reactions under pristine atmospheric conditions. We explain the trends in selectivity and reactivity based on steric, inductive, and stereoelectronic effects. These findings highlight the importance of isomerization reactions in atmospheric chemistry, including those involving nonhydrocarbon peroxy radicals.

Organic radical emitters have recently emerged as promising alternatives to conventional singlet emitters, as they circumvent spin-statistical limits and can, in principle, achieve unity internal quantum efficiency in OLEDs. Here, we study the photophysics of a series of nitrogen-decorated triphenylmethyl radicals using the Pariser-Parr-Pople (PPP) model within the Restricted Active Space Configuration Interaction (RASCI) framework. By exploiting the PPP particle-hole difference operator introduced in J. Phys. Chem. C 2024, 128, 18158-18169, we quantify particle-hole symmetry breaking and relate it to the oscillator strength of the first absorption band. Systematic nitrogen substitution at the meta positions of the phenyl rings leads to increasingly bright doublet states. We further show that an effective difference operator value can be computed using ground-state DFT energies, enabling a fast and practical screening protocol for identifying potentially emissive radicals. Our results provide simple design rules and predictive indicators for engineering bright organic radicals through controlled particle-hole symmetry breaking.

Electrophilic dodecaborate fragment ions [B₁₂X₁₁]^- are among the most reactive anions known and are capable of converting inert reagents to charged compounds through covalent bonding. In this study, we investigate the reactivity of [B₁₂X₁₁]^- (X = Br, I) with a series of hydrocarbons in two environments: (1) in the gas phase, using ion-molecule reactions and collision-induced dissociation (CID), and (2) at vacuum/solid interfaces, via fragment ion deposition onto surface layers of reagents. In the gas phase, n-alkyl groups of hydrocarbons are bound to [B₁₂X₁₁]^- via the substitution of a proton by the electrophilic vacant boron atom. However, hydrocarbons with double and triple bonds are bound directly to [B₁₂X₁₁]^-, resulting in strongly bound adducts with characteristic fragmentation behavior. Aromatic compounds can bind to the vacant boron atom in [B₁₂X₁₁]^- by donating electron density from the aromatic unit, forming a quasi-tetrahedrally coordinated carbon atom, and the adduct fragments upon CID to regenerate [B₁₂X₁₁]^- ions. Contrary to the gas-phase results, [B₁₂X₁₁]^- reacts with all hydrocarbons at layer interfaces via proton substitution, regardless of the degree of saturation. Computational investigations rationalize that the thermochemically most favorable geometry of the [B₁₂X₁₁]^- ─hydrocarbon adduct can often explain their observed fragmentation behavior in the gas phase. However, in the condensed phase, low-lying transition states allow rearrangement into the proton substitution binding mode, which is favorable due to entropic effects (e.g., proton dissipation in the layer). These results emphasize the possible differences in the formation of products of reactive ions in the gas phase and at interfaces, contributing to the selectivity control in reactive ion deposition experiments and providing a foundation to apply the "universal binder" [B₁₂X₁₁]^- in analytical and preparative mass spectrometry for charge tagging of nonpolar organic molecules.

High-level G4 calculations show that the interaction of diborane(4) (B₂H₄) with nitrogen bases not only stabilizes the C_2v isomer with respect to the D_2d one, but more importantly retains and enhances the distinctive reactivity of the C_2v isomer. The formation of the complex results in a large enhancement of the donor ability of the diborane subunit. As a first consequence, the boron site is by far more basic than nitrogen in terms of enthalpy, leading to protonated complexes that can be viewed as the association of the different bases to the B₂H₅⁺ cation. Further analysis of the electron density redistribution upon complexation helps to rationalize the key factors behind the drastic basicity enhancement observed. The basicity of the B₂H₄-pyridine complex falls within the range of gas-phase superbases, with a calculated proton affinity (PA) exceeding 1000 kJ·mol^-1. Moreover, complexes with stronger bases, such as guanidine and methyl-substituted imidazoles, surpass the basicity of the prototypical proton sponge and superbase 1,8-bis(dimethylamino)naphthalene. Precisely, B₂H₄-1,2,5-trimethylimidazole is predicted to be a boron base 34 kJ·mol^-1 more basic than the proton sponge, corresponding to an increase in the protonation equilibrium constant of nearly 6 orders of magnitude.

The metal-complex formation of 1,4-dihydroxyanthraquinone (quinizarin, QNZ) and 6,11-dihydroxy-5,12-naphthacenedione (DHN) with Al(III) ions is investigated by stationary and time-resolved emission spectroscopy combined with quantum chemical calculations of optical properties. UV-vis and fluorescence spectra revealed small red-shifts of 200 and 60 meV for the QNZ and DHN metal complexes, respectively. The fluorescence quantum yield increases from 0.08 to 0.23 for QNZ, while for DHN it changes from 0.24 to 0.79 upon complexation, suggesting the presence of J-aggregate-like exciton coupling within the coordination structure. The average fluorescence lifetime of QNZ varies from 0.65 ns of the free ligand to 2.77 ns, and in the case of DHN it goes from 1.57 to 2.61 ns after Al(III) complexation. These results are consistent with formation of a more rigid molecular structure which effectively decreases the nonradiative rate constant. Confocal fluorescence microscopy images of Al(III) complexes adsorbed into the μmZeolite L structure gave similar red-shifted J type emission. Density functional theory, at the B3LYP/def2-TZVP level of theory, and the analysis of the electronic transition dipole moment, calculated with TDDFT at the CAM-B3LYP/def2-TZVP level, supports a near head-to-tail chromophore arrangement containing two metal centers coordinated with two chromophores. The Al(III)₂DHN₂ complex exhibits the stronger transition dipole coupling and a more pronounced J-type character when compared with Al(III)₂QNZ₂ complex. The radiative rate constant of Al(III)₂DHN₂ is twice that of the single DHN chromophore.

Laser vaporization of uranium in a pulsed supersonic expansion of carbon dioxide is used to produce complexes of the form U⁺(CO₂)_n, UO⁺(CO₂)_n, and UO²⁺(CO₂)_n. These ions are selected in a reflectron time-of-flight mass spectrometer and studied with visible laser photodissociation and tunable infrared laser photodissociation spectroscopy in the region of the CO₂ antisymmetric stretch. The dissociation patterns and spectroscopy of these ions indicate that CO₂ ligands are intact molecules. Although reaction products that form oxide-carbonyl or oxalate species are predicted to be stable, there is no direct evidence in the frequency range studied for the formation of these species. There is no clear indication for the coordination numbers for singly charged uranium and its oxide complexes with CO₂. However, there is strong support in the vibrational patterns for an eight-coordinate complex of the doubly charged UO²⁺ species, i.e., UO²⁺(CO₂)₈.