Variable-Temperature Ultraviolet Photoelectron-Spectroscopy of the Keto-Enol Tautomers of Pentane-2,4-Dione

Abstract

The HeI photoelectron spectra of pentane-2,4-dione measured at various temperatures have been subjected to a spectrum-stripping technique to enable isolation of the complete HeI photoelectron spectra of the keto and enol tautomers. The resulting spectrum-stripping coefficients allow evaluation of the enolization equilibrium constant at each temperature and subsequently the enthalpy of enolization. Interpretation of the photoelectron spectra is achieved with the use of ab initio molecular orbital calculations. Photoionization of non-bonding oxygen, and π -type electrons, contribute to the low ionization energy region of the spectrum, with a πCC < no- <πCO ordering for the enol tautpmer , and no- < no+ < πCO+ for the keto tautomer . Because of different electronic relaxation effects, ∆SCF calculations are required to predict correctly the observed πCC/no- spacing for the enol tautomer.

The Tautomeric Persistence of Electronically and Sterically Biased 2‐Quinolinones

“…The structural analysis has also to take into account the physiological environment in which the compound is placed. Solvent [21][22][23] and phase effects can be critical. For example, an aqueous solution of 2-pyridone contains almost exclusively the amide form (K = 340, [24] ∆G°= 3.4 kcal/mol), whereas in the gas phase the 2-hydroxypyridine form is slightly favored (K = 2.2 [25][26] ∆G°= 0.47 kcal/mol).…”

The Thermochemistry of 2,4-Pentanedione Revisited: Observance of a Nonzero Enthalpy of Mixing between Tautomers and Its Effects on Enthalpies of Formation

“…13,[16][17][18][19][20][21][22][23][24][25][26][27][28] Both spectroscopic and chemical methods have been used to determine the enthalpy of tautomerization in the gas phase. Unlike the condensed phase studies, agreement between methods is less than satisfactory.…”

A New Macroporous Luminescent Hydrogel from Poly(allylamine)

“…[11] The enol content of pentan-2,4-dione was reported to be 95, 68, and 44% at 22, 180, and 275 8C, respectively. [12] This suggested that pentan-2,4-dione acted most likely as a monocarbonyl species in its reaction with PAL at room temperature (20 8C) and, hence, the reaction did not lead to crosslinking for the product to qualify as a hydrogel.…”