The Journal of Physical Chemistry A最新文献

The heterodinuclear AuNi(CO)₄^- complex is scrutinized in the gas phase by using mass-selected anionic photoelectron velocity-map imaging spectroscopy in conjunction with theoretical computations. The ground state of AuNi(CO)₄^- is characterized to have an Au-Ni bonded structure, consisting of an AuCO fragment attached to the Ni center of the Ni(CO)₃ fragment. Comprehensive quantum chemical studies reveal that the AuNi(CO)₄^- complex at equilibrium structure features a decentralized bonding scenario, where the exotic metal-metal σ bonding may be equally well described with dative bonding as with electron-sharing bonding between two fragments.

Few-femtosecond extreme-ultraviolet (EUV) pulses with tunable energy are employed to initiate the Jahn-Teller structural rearrangement in the ethylene cation. We report on a combined experimental and theoretical investigation of an unusual isotope effect on the low-energy competing H/D-loss and H₂/D₂-loss channels observed in the ultrafast dynamics induced by an EUV-pump pulse and probed by an infrared (IR) pulse. The relative production yields of C₂D₄⁺, C₂D₃⁺, and C₂D₂⁺ exhibit pronounced oscillations with a period of ∼50 fs as a function of the pump-probe delay, while the oscillatory patterns are less pronounced for C₂H₄⁺. By using surface hopping to model the nonadiabatic dynamics in the four lowest electronic states of the cation, we show that the enhanced oscillations in deuterated fragment yields arise from a synergy between the isotope effects on the wave packet relaxation through the network of conical intersections and on the vibrational frequencies of the cation.

Current developments in X-ray absorption spectroscopy (XAS) for liquid samples in the water window demand a rigorous understanding of the interactions between molecules or solute-solvent interactions observed in the spectra. Meanwhile, a theoretical description of such effects, in addition to inner-shell excitations, remains controversial. The controversy is mainly over whether the orbitals should be optimized in the final states or whether the orbital optimizations can be expressed by dynamic electron correlation. In the present work, we measured the XAS spectra of indole in aqueous solution at the carbon and nitrogen K-edges to compare them with those measured in the gas phase. Obvious solvatochromism was observed only in the XAS spectrum measured at the nitrogen K-edge. We then interpreted the observed solvatochromism by simulating spectra with both ΔSCF, where the orbitals were optimized in the final states, and the algebraic-diagrammatic construction through second order [ADC(2)], where the molecular orbitals optimized in the ground state were used throughout. The present results indicate that covalent interactions, such as hydrogen bonds, are the dominant causes of the solvation effects observed in XAS spectra. The present simulations with ΔSCF and ADC(2), in addition to some other reports, highlight the importance of optimizing the orbitals in the final inner-shell excited states for general inner-shell calculations with predictive accuracy.

The negative energy difference between singlet and triplet excited states (ΔE_ST) is currently attracting significant attention; however, molecular designs remain largely confined to azaphenalene structures, as reported by Leupin and Wirz in 1980. To show negative ΔE_ST, a maximally separated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) arrangement is crucial to minimizing the exchange interaction in the excited state. We revisited the electronic structure of cyclazine, consisting of cyclododecahexaene ([12]annulene) and a central nitrogen atom. The 12 π-electrons of the peripheral cyclic oligoene play an important role in achieving the less overlapping HOMO and LUMO arrangement, and the bridging by the nitrogen atom inside produces the energy difference between HOMO and LUMO while maintaining a stable planar structure. Based on these insights, we designed a set of 10 molecules in which the number of π-electrons (N) in the peripheral cyclic oligoene is 16, 20, and 24, satisfying N = 4·n (n = 4, 5, 6), and a further set of 11 molecules in which N in the peripheral cyclic oligoene is extended to 14, 18, 22, and 26, satisfying N = 4·n + 2 (n = 3, 4, 5, 6). HOMO, LUMO, exchange interaction (K), and ΔE_ST were calculated using configuration interaction singles, TD-DFT, and equation of motion coupled-cluster singles and doubles (EOM-CCSD), with the structure optimized without any symmetry constraint. Among the molecular structures with N = 4·n, only the molecules without bond alternation exhibit less overlapping HOMO and LUMO and a small K and ΔE_ST. In contrast, among the molecular structures with N = 4·n + 2, none of the molecules exhibit less overlapping HOMO and LUMO arrangement. The molecules with both N = 4·n and no bond alternation show negative ΔE_ST in the EOM-CCSD calculation. The findings of this study will pave the way for broader molecular designs of molecules exhibiting negative ΔE_ST, where a less overlap of HOMO and LUMO is essential.

We have developed a fully numerical method for calculating the response of the Hartree-Fock orbitals to an external electric field. The Hartree-Fock orbitals are optimized using Green's function methods by iterative numerical integration of the convolution with the Helmholtz kernel. The orbital response is obtained analogously by iterative numerical integration of the convolution with the Helmholtz kernel of the Sternheimer equation. The orbitals are expanded in atom-centered functions (bubbles), consisting of numerical radial functions multiplied by spherical harmonics. The remainder, i.e., the difference between the bubble expansion and the exact orbitals, is expanded in numerical tensorial local basis functions on a three-dimensional grid (cube). The methods have been tested by calculating polarizabilities for He, H₂, and NH₃, which are compared to the literature values.

We have developed a fully numerical method for calculating the response of the Hartree–Fock orbitals to an external electric field. The Hartree–Fock orbitals are optimized using Green’s function methods by iterative numerical integration of the convolution with the Helmholtz kernel. The orbital response is obtained analogously by iterative numerical integration of the convolution with the Helmholtz kernel of the Sternheimer equation. The orbitals are expanded in atom-centered functions (bubbles), consisting of numerical radial functions multiplied by spherical harmonics. The remainder, i.e., the difference between the bubble expansion and the exact orbitals, is expanded in numerical tensorial local basis functions on a three-dimensional grid (cube). The methods have been tested by calculating polarizabilities for He, H₂, and NH₃, which are compared to the literature values.

Information about the intermolecular potential energy surface for the interaction between solute and solvent molecules is required to understand the impact of solvation on reaction mechanisms and dynamics. In this study, we measured vibrational-specific infrared (IR) spectra of 4-aminobenzonitrile-(argon)_n cation clusters, 4ABN⁺-Ar_n (n = 1, 2), in the NH stretching range to elucidate the energetics of the photoionization-induced π → NH migration of Ar. The IR spectra of 4ABN⁺-Ar_n generated by resonant photoionization of neutral π-bonded clusters display the hydrogen-bonded NH₂ stretching vibration (ν_NH2) only when intermolecular vibrational levels are excited. This is the first observation of Ar migration from the aromatic ring toward the NH₂ group upon photoionization in the n = 1 cluster. From the vibrational-level dependence of the IR spectra, the activation barrier heights are determined to be 21-47 (34 ± 13) and <27 cm^-1 for 4ABN⁺-Ar₁ and 4ABN⁺-Ar₂, respectively. The potential energy surfaces and mechanism of the Ar migration are discussed with the help of complementary density functional theory calculations.

The ozonolysis reaction of trans-2-hexenal was studied theoretically on the basis of highly accurate CCSD(T)-F12b/AVTZ energy values obtained in M06-2X/AVTZ preoptimized nuclear configurations. The kinetics was modeled with the help of the master equation solver MESMER. Apart from the expected stable oxidation products 1-butanal (17%) and glyoxal (35%), a secondary ozonide is formed on the glyoxal channel, which is the principal first-generation product (49%). It is further shown that glyoxal is created on two competing pathways, one of which leads to simultaneous production of the ester propylformate (18%). The inclusion of all of these mechanisms explains the experimental findings and identifies for the first time the origin of the experimental carbon deficit.