Supersonic jets proton transfer

New IR techniques introduced for the study of prototropic tautomerism include IR dicroism for the photoinduced double proton transfer in por-phine 71 (Scheme 24) [89CPH(136)165], and IR spectroscopy in a supersonic jet (less than 50 K) for demonstrating the presence, in these conditions, of 2//-benzotriazole (57b) [96CPL(262)689]. 

Kijak M, Nosenko Y, Singh A et al (2007) Mode-selective excited-state proton transfer in 2-(2 -pyridyl)pyrazole isolated in a supersonic jet. J Am Chem Soc 129 2738-2739... 

Fuke and Kaya [1989] studied the tautomerization of dimer (6.15) formed in a supersonic jet to investigate the vibrational selectivity of concerted two-proton transfer in excited electronic state. Tunneling in the NH N fragments leads to broadening of certain vibrational bands in the fluorescence excitation spectrum (Figure 6.16). Tunneling is promoted by the symmetric intermolecular vibration with frequency 120 cm-1. The widths of bands with n = 0, 1, and 2 are 5, 10, and... 

In this chapter, we give a brief overview of several novel features of excited-state proton transfer in chromophore-solvent clusters which have been revealed by the interplay of computational chemistry and spectroscopy in supersonic jets. In the future, concerted efforts of theory and spectroscopy will be necessary to investigate the evolution of these phenomena with increasing cluster size towards liquid-phase photochemistry. 

Early theoretical work on FAD was concerned with the dimer equilibrium geometry and the electronic structure (see for example Refs. [32-36]). Ah initio molecular orbital studies on the structure of formic acid dimer in 1984 agreed very well with the experimental structures as determined by electron diffraction [37]. Due to the importance of the double proton transfer of FAD as a key prototype for multiple proton transfer reactions several theoretical studies have been reviewed in the literature [38]. Rotational constants for formic acid dimer were obtained by high resolution spectroscopy of (DCOOH)2 [39] and by femtosecond degenerate four wave mixing experiments in the gas cell at room temperature and under supersonic jet experiments by Matylisky et al. [40]. 

Mukherjee, Bandyopadhyay and Chakraborty " investigated the photophysics of 7-azaindole homodimer (Fig. 52) in Ar and N2 matrices and the souble-proton transfer reaction taken place upon UV excitation. The IR spectra revealed that a doubly hydrogen bonded dimer is predominantly produced upon annealing the seeded matrices at 28 K. The authors found that, unlike in hydrocarbon solutions or in a supersonic jet expansion, the matrix-isolated doubly hydrogen-bonded dimer emits exclusively in ultraviolet from the locally excited state, and the main photophysical process (excited state double proton transfer ESDTP) was slowed down dramatically. They interpreted these observations in terms of matrix effect on the double proton transfer potential, specifically the increase of the effective ESDPT barriers in the rigid inert gas matrices compared to those in the gas phase or hydrocarbon solution. 

EXCITED STATE INTRAMOLECULAR PROTON TRANSFER IN SUPERSONIC JETS... 

Figure 3.6a shows the structure and the scheme of excited-state intramolecular proton transfer (ESIPT) for a series of 2 -pyridylpyrroles la-c. For these systems, dual emission was observed in dry n-hexane and in a supersonic jet (Figure 3.6b)... 

Yoshihara s group has also measured the lifetimes of SA rotamers in a supersonic free jet expansion. The fluorescence decays of rotamer 11 were too fast to measure with their experimental time resolution (<1 ns) however, the excitation energy dependence of decay rates of rotamer I fluorescence could be observed. At low energy, the fluorescence decay was measured to be single exponential with a lifetime of 9.6 ns (12.0 ns for MSA). On the other hand, at higher energies above -1100 cm, the decay curves show bi-exponential behavior with lifetimes decreasing to -4.3 ns due to an efficient intramolecular nonradiative decay process, as observed in solution, which may be attributed to the proton transfer. The 9.6-ns component is nonlinearly dependent on SA concentration and is attributed to the dimer. 

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