title: Direct observation of ground-state lactam-lactim tautomerization using temperature-jump transient 2D IR spectroscopy.
abstract: We provide a systematic characterization of the nanosecond ground-state lactam-lactim tautomerization of pyridone derivatives in aqueous solution under ambient conditions using temperature-jump transient 2D IR spectroscopy. Although electronic excited-state tautomerization has been widely studied, experimental work on the ground electronic state, most relevant to chemistry and biology, is lacking. Using 2D IR spectroscopy, lactam and lactim tautomers of 6-chloro-2-pyridone and 2-chloro-4-pyridone are unambiguously identified by their unique cross-peak patterns. Monitoring the correlated exponential relaxation of these signals in response to a laser temperature jump provides a direct measurement of the nanosecond tautomerization kinetics. By studying the temperature, concentration, solvent, and pH dependence, we extract a thermodynamic and kinetic characterization and conclude that the tautomerization proceeds through a two-state concerted mechanism. We find that the intramolecular proton transfer is mediated by bridging water molecules and the reaction barrier is dictated by the release of a proton from pyridone, as would be expected for an efficient Grothuss-type proton transfer mechanism.
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: Chunte Sam
authors.initials: CS
authors.lastname: Peng
authors.forename: Carlos R
authors.initials: CR
authors.lastname: Baiz
authors.forename: Andrei
authors.initials: A
authors.lastname: Tokmakoff
mesh_terms: D007536:Isomerism; D007700:Kinetics; D007769:Lactams / Q000737:chemistry*; D011728:Pyridones / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D007769:Lactams; D011728:Pyridones; D014867:Water
keywords: keto-enol tautomerism; multidimensional; time-resolved spectroscopy; ultrafast