Laser-induced fluorescence (FE) and vibrationally resolved dispersed fluorescence (DF) spectra of a 1:1 water complex of p-fluorophenol (pFP) have been measured in a supersonic jet expansion. The hydrogen bond stretching fundamental (σ) of the complex appears in the FE spectrum as a doublet with band maxima at 155 and 161 cm. Emission spectra recorded upon excitations of the two components reveal that a Fermi resonance between σ and a combination involving a low-frequency intramolecular mode of pFP (mode 11) and a bending mode of water at the hydrogen bonded interface (mode ρ) is responsible for the observed splitting. The DF spectra of the Franck-Condon active 6a band (0 + 427 cm) of pFP reveals signatures of hydrogen bond induced vibrational energy relaxation (VER) predominantly from the bright (6a) to a dark (9b) level in S. The relative intensities of the emission bands from the locally excited and relaxed levels indicate that VER for excitation up to this level occurs at a time scale similar to the fluorescence decay time of the complex. However, complete VER at a much faster time scale occurs for excitation beyond 822 cm above S origin.

Conformational preferences of a binary hydrogen-bonded complex between p-fluorophenol (pFP) and 2,5-dihydrofuran (DHF) have been studied by means of laser induced fluorescence (LIF) spectroscopy in a supersonic jet expansion. Calculation predicts two major conformers for this complex, one having a nearly linear geometry in which the two molecular moieties are bound only by an O-H···O H-bond, but in the other an additional C-H···π type interaction between an ortho C-H group of pFP and ethylene group of DHF contributes to the binding stabilization and results in a folded geometry for the complex with respect to a global view, although the H-bond angle of the latter is relatively larger. This prediction is realized experimentally by identifying transitions corresponding to the two discrete conformers in a vibrationally resolved LIF excitation spectrum of the complex, and the red shifts of S-S origin band of pFP moiety of the two conformers are 542 and 659 cm, respectively. The assignments are corroborated by means of dispersed fluorescence (DF) spectroscopy. In comparison, the LIF spectral bands for the pFP-tetrahydrofuran complex can be corresponded to only one conformer, whose S-S origin transition shows a red shift (563 cm) somewhat similar to the linear conformer of pFP-DHF complex. Such similarities in spectral shifting behavior is consistent with the predictions of electronic structure calculations. The DF spectra also reveal that the energy threshold and pathways of vibrational dynamics in S of the two conformers show different behavior. Excitation to 6a level of pFP moiety of the folded conformer displays signatures of restricted intramolecular vibrational energy redistribution (IVR), whereas the linear form displays the emission feature for dissipative IVR.

The thermal reaction between nitrogen dioxide and phenol in the gas phase under anaerobic conditions has been investigated by diluting the reactants in dry nitrogen in a glass reaction vessel. Infrared spectroscopic analysis reveals that nitric acid, nitric oxide, and o-nitrophenol are the major products of the reaction. The kinetic analysis reveals the reaction stoichiometry as 3NO2 + PhOH → HNO3 + NO + o-nitrophenol, and the corresponding reaction enthalpy is Δr H 0 = −44.82 kcal/mol. Reaction monitoring by NO2 concentration variation shows that HNO3 formation is linearly correlated with the effective concentration of the nitrogen dioxide dimer (N2O4) formed, and the overall reaction follows a second-order kinetic behavior with respect to N2O4 and phenol, and the estimated rate constant value is (3.53 ± 0.56) × 10–18 cm3 molecule–1 s–1 at 298 K. In the presence of excess NO2, the reaction shows a pseudo-first-order kinetic behavior with a rate constant of (6.67 ± 0.12) × 10–3 s–1. The electronic structure calculation predicts that the N2O4–phenol complex can have multiple conformational minima, and in the lowest-energy conformer, the orientation of the two NO2 molecules about the phenolic −OH group is similar to that of the charge-separated asymmetric ONONO2 dimer of NO2. A radical mechanism has been ruled out, as HONO has not been identified as a product. To the best of our knowledge, the formation of o-nitrophenol in the gas-phase reaction between phenol and NO2 is reported here for the first time. The atmospheric implication of the reaction has been discussed.

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We have demonstrated here, for the first time to our knowledge, the formation of an emitting metastable species upon lowest electronic excitation (S1) of a hydrogen-bonded 1:2 complex of para-fluorophenol (pFP) with ammonia (NH3), which is known to be one of the smallest reactive complexes to undergo excited state H-atom transfer (HAT) reaction to produce •NH4(NH3) radical fragment. The emission spectrum of the species is characterized to be red-shifted, broad, and structureless. From the viewpoint of energy balance, an excited state proton transfer (ESPT) is unfavorable, but according to predicted electronic structure parameters, the metastable state species could be stabilized by charge transfer (CT) interaction at the hydrogen-bonded geometry of the complex. We propose that this species could act as an intermediate to the HAT process in the excited state. The observation of such a state could be valuable to understand the complex dynamics of similar events in biologically relevant systems.