Spectroscopic studies on excited-state proton transfer of a new chromophore 2-(2′-benzofuryl)-3-hydroxychromone (BFHC) have been reported recently. In the present work, based on the time-dependent density functional theory (TD-DFT), the excited-state intramolecular proton transfer (ESIPT) of BFHC is investigated theoretically. The calculated primary bond lengths and angles involved in hydrogen bond demonstrate that the intramolecular hydrogen bond is strengthened. In addition, the phenomenon of hydrogen bond reinforce has also been testified based on infrared (IR) vibrational spectra as well as the calculated hydrogen bonding energies. Further, hydrogen bonding strengthening manifests the tendency of excited state proton transfer. Our calculated results reproduced absorbance and fluorescence emission spectra of experiment, which verifies that the TD-DFT theory we used is reasonable and effective. The calculated Frontier Molecular Orbitals (MOs) further demonstrate that the excited state proton transfer is likely to occur. According to the calculated results of potential energy curves along O-H coordinate, the potential energy barrier of about 14.5 kcal/mol is discovered in the S 0 state. However, a lower potential energy barrier of 5.4 kcal/mol is found in the S 1 state, which demonstrates that the proton transfer process is more likely to happen in the S 1 state than the S 0 state. In other words, the proton transfer reaction can be facilitated based on the photo-excitation effectively. Moreover, the phenomenon of fluorescence quenching could be explained based on the ESIPT mechanism.
Density functional theory (DFT) calculations have been performed to investigate the detailed mechanism of Rh(III)-catalyzed redox-neutral C-H activation of pyrazolones with PhC≡CPh. It is found that (1) the methylene C-H activation is prior to the phenyl C-H activation, (2) the N-N bond cleavage is realized via Rh(III) → Rh(I) → Rh(III) rather than via Rh(III) → Rh(V) → Rh(III). The zwitterionic Rh(I) complex is identified to be a key intermediate in promoting the N-N bond cleavage. (3) Different from the Rh(III)-catalyzed hydrazine-directed C-H activation for indole synthesis, the rate-determining step of the reaction studied in this work is the Rh(III) → Rh(I) → Rh(III) process resulting in the N-N bond cleavage rather than the alkyne insertion step. The present theoretical study provides new insight into the mechanism of the conjugated N-N bond cleavage.
The mechanism of the cycloaddition reaction of forming a germanic hetero-polycyclic compound between singlet alkylidenegermylene and ethylene has been investigated with MP2/6-31G* method, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. The energies of the different conformations are calculated by CCSD(T)//MP2/6-31G* method. From the surface energy profile, it can be predicted that the dominant reaction pathway for this reaction consists of three steps: the two reactants first form a three-membered ring intermediate INT1 through a barrier-free exothermic reaction of 35.4 kJ/mol; this intermediate then isomerizes to an active four-membered ring product P2.1 via a transition-state TS2.1 with a barrier of 57.6 kJ/mol; finally, P2.1 further reacts with ethylene to form the germanic hetero-polycyclic compound P3, for which the barrier is only 0.8 kJ/mol. The rate of this reaction path considerably differs from other competitive reaction paths, indicating that the cycloaddition reaction has an excellent selectivity.
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