研究目的
To examine the photochemical pathways of N-(4,4′-dibromodiphenylamino)-2,4,6-trimethylpyridinium BF4? salt (DN) from photon absorption to the production of N,N-di(4-bromophenyl)nitrenium ion 2, and to investigate its nucleophilic adduct reaction with H2O in aqueous solutions.
研究成果
The formation of N,N-di(4-bromophenyl)nitrenium ion 2 occurs within about 4 ps in both MeCN and aqueous solutions, as directly observed by fs-TA. Ns-TR3 provides vibrational characterization of 2, showing enhanced Raman intensity due to halogen effects. In aqueous solutions, the nucleophilic adduct reaction with H2O produces intermediate 3 efficiently, confirming the proposed reaction pathway. These findings offer new insights for future studies on nitrenium ion reactions with nucleosides and other molecules.
研究不足
The strong Raman feature of MeCN at 1372 cm?1 obscures the C-N stretching feature of the nitrenium ion, limiting vibrational analysis in that region. Experiments were conducted in specific solvents (MeCN and aqueous mixtures), which may not represent all environmental conditions. The time resolution of ns-TR3 experiments is approximately 10 ns, potentially missing faster processes.
1：Experimental Design and Method Selection:
Femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy were employed to study the photochemical behavior of DN in acetonitrile (MeCN) and aqueous solutions. Density functional theory (DFT) calculations were used to interpret the spectroscopic data.
2：Sample Selection and Data Sources:
The sample DN was synthesized following a literature procedure. Spectroscopic grade MeCN and aqueous solutions (e.g., 1:1 MeCN:H2O) were used. Sample solutions were prepared with an absorbance of 1 at the excitation wavelength (266 nm or 267 nm).
3：List of Experimental Equipment and Materials:
Equipment includes a femtosecond regenerative amplified Ti:sapphire laser system for fs-TA, a commercial LP920 laser flash system for ns-TA, Nd:YAG lasers for ns-TR3, spectrometers, CCD detectors, photomultiplier tubes, and cuvettes (2 mm and 1 mm path-length). Materials include DN salt, MeCN, H2O, and CaF2 crystal for white-light continuum generation.
4：Experimental Procedures and Operational Workflow:
For fs-TA, a 267 nm pump beam and a white-light continuum probe (350-800 nm) were used with a flowing cuvette. For ns-TA, a 266 nm pump from a Nd:YAG laser and a Xenon arc lamp probe were used. For ns-TR3, pump wavelengths of 266 nm and probe wavelengths of 416 nm or 341.5 nm (from Raman shifting) were employed with synchronized lasers and time delays monitored. Data were collected at various time points post-excitation.
5：5 nm (from Raman shifting) were employed with synchronized lasers and time delays monitored. Data were collected at various time points post-excitation.
Data Analysis Methods:
5. Data Analysis Methods: Kinetics were modeled with exponential functions. Spectra were compared to DFT-calculated normal Raman spectra using Gaussian 09 software with B3LYP functional and 6-311G(d,p) basis set in PCM solvent mode. Vibrational frequencies were scaled by 0.975 for comparison.
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