The Journal of Physical Chemistry A最新文献

Noncovalent interactions and covalent bonds can be distinguished via quantum chemical topology analysis and molecular face theory, which are based on the potential acting on one electron in a molecule or molecular system (PAEM). A covalent bond forms when a PAEM bond critical point (BCP) occurs on the line connecting two atoms and when their molecular faces contact or fuse together, whereas a noncovalent interaction occurs between two adjacent atoms or chemical species when their molecular faces remain separate. The force acting on one electron within a molecule, which starts at infinity and ends at the BCP, forms nonoverlapping boundary surfaces that partition a molecule into distinct atomic regions. This is demonstrated with the following example reactions: H + H → H₂, H + X → HX (X = F, Cl, Br, I), O (¹D) + H₂ → H₂O, and S (¹D) + H₂ → H₂S. The exploration of the physical quantities at PAEM critical points, such as the eigenvalues of the Hessian matrix, ellipticity, and electron interflow frequency, reveals their changing trends during the transition from a noncovalent interaction to a covalent bond or vice versa. These changes can help predict chemical bond formation or breakage, providing insight into chemical bonding.

The mechanistic details of the DABCO-catalyzed diastereoselective cyclization between β-alkyl nitroolefins and alkylidene malononitriles were elucidated by using dispersion-corrected density functional theory (DFT-D3) calculations at the M06-2X/6-311G(d,p) level with SMD solvation (dichloromethane). The reaction proceeds through four consecutive steps: (i) deprotonation and Michael addition; (ii) 1,3-proton transfer; (iii) Pinner-type cyclization; and (iv) ring contraction. Our computational results demonstrate DABCO's dual catalytic role in both proton abstraction and proton shuttle mechanisms, rationalizing the observed diastereoselectivity. The rate-determining step (ring contraction) exhibits a moderate barrier of 24.9 kcal/mol, consistent with experimental conditions, and the exclusion of alternative pathways (ΔG^‡ > 70 kcal/mol) confirms the mechanistic preference. This study provides fundamental insights into stereochemical control and establishes a theoretical framework for designing related organocatalytic transformations.

Gas-phase photoelectron spectroscopy (PES) was performed on the mass-selected beams of [Au₂₅(PET)₁₈]^- and [Au₃₈(PET)₂₄]^- (PET = SC₂H₄Ph) desorbed by a matrix-assisted laser desorption/ionization (MALDI) process. The PE spectrum of [Au₂₅(PET)₁₈]^- was consistent with that generated by the electrospray ionization (ESI) method, indicating the compatibility of MALDI and ESI for the ionization method. The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of [Au₃₈(PET)₂₄]⁰ was estimated to be 0.78 ± 0.06 eV from the energy difference between the peaks at the lowest and second lowest electron binding energies. The HOMO-LUMO gap was smaller than those determined in solution by voltammetry (1.0 eV) and optical absorption spectroscopy (0.92 eV). Density functional theory calculations suggested that the difference in the HOMO-LUMO gap was due to the elongation of the bi-icosahedral Au₂₃ core of [Au₃₈(PET)₂₄]^- as compared to that in [Au₃₈(PET)₂₄]⁰ by the accommodation of an excess electron into an antibonding orbital.

Photoelectron spectroscopy and theoretical calculations are combined to elucidate the structures and chemical bonding of small boron clusters doped with a copper atom, CuB_x^- (x = 4-6). Relatively complex spectral features are observed and are interpreted by comparison with the theoretical results. Predicted global minimum structures of CuB_x^- (x = 4-6) evince that the Cu atom binds to an apex B atom in each cluster and does not significantly alter the planar boron framework of the corresponding B_x^- clusters. Multielectronic transitions (shakeup processes) are observed in all three systems, a manifestation of strong electron correlation effects. Chemical bonding analyses show that the copper atom preferentially binds to the apex sites with the highest electron localization to form a Cu-B covalent bond. The structures and bonding of CuB_x^- (x = 4-6) are compared with those of the bare B_x^- and the Cu₂B_x^- clusters, providing new insights into the structural and electronic evolution of Cu-doped boron clusters and the transition from Cu-B covalent bonding to ionic bonding.