Protonation, matrix isolation

Leszczynski et al. have shown interest in comparing experimental (matrix isolation) and calculated (B3LYP) IR data (frequency and intensity) to discuss the tautomerism of benzo-annelated pyridonone, pyrazinone, and pyrimidinone (144-148). These equilibria were well reproduced by theoretical calculations carried out at the QCISD and QCISD(T) levels. The combined experimental and theoretical results reveal links between aromaticity and tautomerism. Moreover, a UV-induced phototautomeric reaction transforming the oxo forms into the hydroxy tautomers was observed for all (except 3-hydroxyisoquinoline) studied compounds [144], The interest of Leszczynski in problems related to tautomerism, aromaticity, and proton transfer is also apparent in a study of (lH-aza-hetero-2-ylidene)-acetaldehyde and 2-azahetero-2-yl-ethanol tautomeric pairs [145],... 

The model put forward above for proton transfer by a tunneling mechanism based on naphthol/NH3 cluster data is both simple and sufficient to explain (nearly quantitatively) the observed cluster behavior. This same model can be used to explain cluster matrix isolation behavior as well (Brucker and Kelley 1987a,b, 1988, 1989a,b,c Brucker et al. 1991 Swinney and Kelley 1991). 

Amirand C, Maillard D (1988) Spectrumand structure ofwater-rich waterhydracid complexes from matrix isolation spectroscopy evidence for proton transfer. J Mol Struct 176 181-201... 

Del Bene, J. E., Person, W. B., and Szczepaniak, K., Ab initio theoretical and matrix isolation experimental studies of hydrogen bonding Vibrational consequences of proton position in 1 1 complexes ofHCl and 4-X-pyridines, Chem. Phys. Lett. 247, 89-94 (1995). 

Amirand, C., and Maillard, D., Spectrum and structure of water-rich water-hydracid complexes from matrix isolation spectroscopy Evidence for proton transfer, J. Mol. Struct. 176, 181-201 (1988). 

Infrared matrix isolation studies support the possibility that CH can act as a proton acceptor when complexed with HNOj. The presence of a H-bond is questionable, however, since the red shift of the bridging OH stretch is only 43 cm". ... 

UV-light-induced intramolecular proton transfer was observed in argon, xenon, and neon matrix-isolated 4-hydroxypyrimidine (88JST(175)91, 90SA(A)61,... 

In some cases, the identity of paramagnetic "satellite ions" was established by ODMR [42,44,48]. For example, 9,10-octalin + was identified in decalins and their solutions [42]. ODMR spectra of "satellite ions" in cyclohexane were related to EPR spectra of matrix-isolated cyclohexene + (Note that in the liquid cyclohexane, cyclohexene + undergoes a fast ring-puckering motion that averages hyperfine coupling constants for equatorial and axial protons, so the the EPR spectra of cyclohexene + in liquid and solid matrices are different) [42,44,48]. In both of these cases, the olefin radical cations were formed in spurs rather than in a reaction of the solvent hole with the olefin in the solvent bulk [42] (octalins gradually accumulate as radiolytic products). Olefin "satellite ions" were also observed in squalane [24]. 

In cyclohexane and decalins, reaction (1) is endothermic by 0.1-0.4 eV [60] and it seems reasonable that the excitation of the hole may facilitate the proton transfer. Fragmentation of matrix-isolated hydrocarbon radical cations upon excitation with 2-4 eV photons was observed by EPR (see review [61]). For cycloalkanes, the main photoreaction is reaction (3). For radical cations of methyl-branched alkanes, the loss of CH4 was also observed, while the radical... 

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. 

Fig. 52 Optimized geometries of the 7-azaindole homodimer in So, Si (normal species) and Si (tautomer) states The double proton exchange path leading to tautomeric conversion is denoted by arrows,Reprinted from M, Mukherjee, B. Bandyopadhyay and T. Chakraborty Ultraviolet and infrared spectroscopy of matrix-isolated 7-azaindole dimer Matrix effect on excited state tautomerization, Chem. Phys. Lett., 2012. 546. 74-79. Copyright (2012), with permission from Elsevier,...

The silicon complexes with oxygen donor molecules show strong kinetic isotope effects. Only the deuterated complexes are stable under matrix isolation conditions. With methanol the insertion reaction is so fast that the complex could not be observed. The reason might be that the insertion reaction in the protonated species is effected by quantum mechanical tunneling. In the case of HDO, only HSiOD is formed. 

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