Tautomer-selective spectroscopy

Tautomer-Selective Spectroscopy of Nucleobases, Isolated in the Gas Phase... 

I 7 Tautomer-Selective Spectroscopy ofNucleobases, Isolated In the Cas Phase... 

To access the ground state the IR-UV double resonant technique has proven to be very powerful. This approach provides ground state vibrations in the 500-4000 cm-1 range with isomer selection. This has made it possible to obtain tautomer selective and cluster structure selective spectroscopy. This ability to obtain... 

In solution the keto forms dominate. In matrix-isolation IR observation of C=0 stretch, frequencies points to keto forms but too many lines are present in the spectra to be able to exclude other tautomers as well [28]. The same problem manifests itself in resonance-enhanced multiphoton ionization (REMPI or R2PI) spectroscopy, where a dense spectrum is observed that potentially consists of contributions of multiple tautomers [29], These contributions can be separated by UV-UV hole-burning, revealing the presence of four different isomers in the gas-phase jet-cooled conditions of these experiments [30]. Three of these tautomer-selective REMPI spectra appear in Figure 7.3. Further identification is possible by IR-UV hole burning and by tautomeric blocking with methyl derivatives [30-32]. The spectra of species A (with the red-most origin at... 

In the first part of this article, we present a kinetic study on the isomerization of l-phenylazo-2-hydrozynaphthalene (1PA2N) measured by picosecond spectroscopy. The selection of this compound was based on its known photochemical properties of cis-trans isomerization and tautomerization by proton transfer. The phototautomerization of 1PA2N is evidenced by the fact that in solution this compound is a mixture of the azo tautomer A and the hydrazo tautomer as shown in Figure 2.1. It is thought that the inter-... 

However, it is clear that vibrational spectroscopy has considerable use beyond the identification of polymorphs and solvates. The infrared spectra obtained on the polymorphs of acetohexamide and selected derivatives has been used to study the tautomerism of the drug compound [127]. It was deduced in this work that Form A existed in the enol form, stabilized by the intramolecular bonding between the O—H and groups that produces a six-membered ring. Form B was characterized by the existence of the keto form, with the urea carbonyl group being intermolecular bonding to a sulfonamide N—H functionality. This behavior can be contrasted with that noted for spironolactone, where no evidence was found for the existence of enolic tautomers in any of the four polymorphs [128]. 

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