<i>Ab Initio</i> Study of Decay Dynamics of 1‑Nitronaphthalene Initiated from the S₂(ππ* + n_NOπ*) State

Irradiation of nitro-PAHs in solution at ambient conditions leads to formation of its lowest excited triplet, dissociation intermediates nitrogen oxide (NO^•) and aryloxy radical (Ar–O^•). Experimental and theoretical studies demonstrated that Franck–Condon excited singlet state S_FC(ππ*) to a receiver, higher-energy triplet state T_n(nπ*) controlled the ultrafast population of the triplet state and, hence, the slight fluorescence yield of nitronaphthalenes. However, the detailed information about the curve-crossings of potential energy surfaces and the major channels for forming T₁ species and Ar–O^• radical were unclear. Here, by using the CASSCF//CASPT2 method, an efficient decay channel is revealed: S_2‑FC‑1NN → S_{2‑MIN‑1NN} or S₂T_{3‑MIN‑1NN} → T_{3‑MIN‑1NN} or T₃T_{2‑MIN‑1NN}→ T_{2‑MIN‑1NN} or T₂T_{1‑MIN‑1NN} → T_{1‑MIN‑1NN}. This explains the high yield of T_1–1NN species and minor yield of Ar–O^• and NO^• radicals. The calculation results suggest the bifurcation processes take place predominantly after the internal conversion to the T_1–1NN state via T₂T_{1‑MIN‑1NN}, one leads to T_{1‑MIN‑1NN}, while the other to T_{1‑MIN‑ISO} to produce Ar–O^• and NO^• radicals

Excited State Proton Transfer Dynamics of Thioacetamide in S₂(ππ*) State: Resonance Raman Spectroscopic and Quantum Mechanical Calculations Study

The photophysics and photochemistry of thioacetamide (CH₃CSNH₂) after excitation to the S₂ electronic state were investigated by using resonance Raman spectroscopy in conjunction with the complete active space self-consistent field (CASSCF) method and density functional theory (DFT) calculations. The A-band resonance Raman spectra in acetonitrile, methanol, and water were obtained at 299.1, 282.4, 266.0, 252.7, and 245.9 nm excitation wavelengths to probe the structural dynamics of thioacetamide in the S₂ state. CASSCF calculations were done to determine the transition energies and structures of the lower-lying excited states, the conical intersection points CI­(S₂/S₁) and CI­(S₁/S₀), and intersystem crossing points. The structural dynamics of thioacetamide in the S₂ state was revealed to be along eight Franck–Condon active vibrational modes ν₁₅, ν₁₁, ν₁₄, ν₁₀, ν₈, ν₁₂, ν₁₈, and ν₁₉, mostly in the CC/CS/CN stretches and the CNH₈,₉/CCH_5,6,7/CCN/CCS in-plane bends as indicated by the corresponding normal mode descriptions. The S₂ → S₁ decay process via the S₂/S₁ conical intersection point as the major channel were excluded. The thione–thiol photoisomerization reaction mechanism of thioacetamide via the S_2,FC → S′_1,min excited state proton transfer (ESPT) reaction channel was proposed

Intermolecular Hydrogen Abstraction from Hydroxy Group and Alkyl by T₁(ππ*) of 1‑Chloro-4-nitronaphthalene

Nanosecond transient absorption and theoretical calculations have been used to investigate the intermolecular hydrogen abstractions from alcohols and 1-naphthol by the lowest excited triplet (T₁) of 1-chloro-4-nitronaphthalene upon excitation of the compound in organic solvents. The hydrogen abstraction of T₁ from hydroxy group of 1-naphthol takes place through an electron transfer followed by a proton transfer through hydrogen bonding interaction with rate constants of ∼10⁹ M^–1 s^–1. Hydrogen-bonding is crucial in this process, indicated by the observation of a half reduction for T₁ yield when increasing the concentration of 1-naphthol. The hydrogen abstraction in this way can be decelerated by increasing solvent polarity and hydrogen-bonding donor ability. The T₁ of 1-chloro-4-nitronaphthalene can undergo one-step H atom abstraction from alkyl hydrogen in alcoholic solvents, with rate constants of ∼10⁴ M^–1 s^–1, and produce radical intermediates with the absorption maximum at 368 nm. DFT calculation results indicate both oxygens of the nitro group are active sites for hydrogen abstraction, and the difference of activation barriers for formation of two radical isomers is only 1.0 kcal/mol

UV and Resonance Raman Spectroscopic and Theoretical Studies on the Solvent-Dependent Ground and Excited-State Thione → Thiol Tautomerization of 4,6-Dimethyl-2-mercaptopyrimidine (DMMP)

The vibrational spectra of 4,6-dimethyl-2-mercaptopyrimidine (DMMP) in acetonitrile, methanol, and water were assigned by resonance Raman spectroscopy through a combination of Fourier-transform infrared spectroscopy (FT-IR), FT-Raman UV–vis spectroscopy, and density functional theoretical (DFT) calculations. The FT-Raman spectra show that the neat solid DMMP is formed as a dimer due to intermolecular hydrogen bonding. In methanol and water, however, the majority of the Raman spectra were assigned to the vibrational modes of DMMP­(solvent)_<i>n</i> (<i>n</i> = 1–4) clusters containing NH···O hydrogen bonds. The intermolecular NH···O hydrogen bond interactions, which are key constituents of the stable DMMP thione structure, revealed significant structural differences in acetonitrile, methanol, and water. In addition, UV-induced hydrogen transfer isomeric reactions between the thione and thiol forms of DMMP were detected in water and acetonitrile. DFT calculations indicate that the observed thione → thiol tautomerization should occur easily in lower excited states in acetonitrile and water

Direct Observation of 4‑Phenoxyphenylnitrenium Ion: A Transient Absorption and Transient Resonance Raman Study

Femtosecond (fs) and nanosecond (ns) transient absorption (TA) and single pulse transient resonance Raman spectroscopic investigation of the intermediates after laser photolysis of 4-phenoxyphenyl azide in acetonitrile and mixed aqueous solution is reported. fs-TA results show that the singlet 4-phenoxyphenylnitrene was produced immediately after photolysis of the azide. Then, the singlet nitrene underwent intersystem crossing (ISC) and ring expansion to generate triplet nitrene and ketenimine in acetonitrile with <i>t</i> = 346 ps or protonation in mixed aqueous solution with <i>t</i> = 37 ps, respectively, a little slower than the counterparts of the methoxy one (108 and 5.4 ps for ISC and protonation processes, respectively). The transient Raman spectrum combined density functional theory (DFT) calculation predicting the structure and vibrational frequencies suggested that phenoxyphenylnitrenium ion has a comparable quinoidal character to that of methoxy- and ethoxy-phenylnitrenium ions. All of these results indicated that the phenoxy substitution has some impact on the reactivity of phenylnitrene but a slight influence on the structure of phenylnitrenium ion

Excited State Structures and Decay Dynamics of 1,3-Dimethyluracils in Solutions: Resonance Raman and Quantum Mechanical Calculation Study

The resonance Raman spectroscopic study of the excited state structural dynamics of 1,3-dimethyluracil (DMU), 5-bromo-1,3-dimethyluracil (5BrDMU), uracil, and thymine in water and acetonitrile were reported. Density functional theory calculations were carried out to help elucidate the ultraviolet electronic transitions associated with the A-, and B-band absorptions and the vibrational assignments of the resonance Raman spectra. The effect of the methylation at N1, N3 and C5 sites of pyrimidine ring on the structural dynamics of uracils in different solvents were explored on the basis of the resonance Raman intensity patterns. The relative resonance Raman intensities of DMU and 5BrDMU are computed at the B3LYP-TD level. Huge discrepancies between the experimental resonance Raman intensities and the B3LYP-TD predicted ones were observed. The underlying mechanism was briefly discussed. The decay channel through the S₁(¹nπ*)/S₂(¹ππ*) conical intersection and the S₁(¹nπ*)/T₁(³ππ*) intersystem crossing were revealed by using the CASSCF­(8,7)/6-31G­(d) level of theory calculations