Keto-enol tautomerism and dissociation of ionized acetaldehyde and vinyl alcohol. A G2 Molecular Orbital Study

Abstract

Both studies 1,2 agree qualitatively in the estimates of the various critical energies separating each isomer and the dissociation products. However discrepancies of no less than 50 kJ/mol were apparent for some elementary steps, particularly hydrogen atom migrations. Furthermore, conflicting conclusions were drawn concerning the nature of the energy determining step for dissociation 3 of 1 Á . To clarify these questions, in view of the moderate size of this system, it was of interest to reinvestigate the energetic of reactions of Scheme 1 using a more sophisticated theoretical method such as the G2 theory. 4 COMPUTATIONAL SECTIONIt seems well established that accurate heats of formation of open-shell systems can be attained at the G2 level of theory 4a for species with low spin contamination. 5 Standard G2 theory employs a geometry optimized at the MP2(full)/ 6-31G(d) level and a scaled HF/6-31G(d) ZPE. A base energy calculated at the MP4/6-311G(d,p) level is corrected by several additivity approximations to QCISD(T) and to 6-311G(3df,2p) basis set. In an attempt to account for residual basis set deficiencies, G2 theory introduces higherlevel corrections (HLC) that depends on the number of paired and unpaired electrons. In the more economical G2(MP2) approach, the basis set-extension energy corrections are obtained at the 2nd order Molle-Plesset level.

General and Theoretical Aspects of Phenols

“…Keto-forms 22 and 24 are low-lying isomers which are situated 146 and 133 kJ mol −1 , respectively, above 21. This energy ordering within the pair 21 and 22 (or 24) is reminiscent of that encountered for simple keto-enol tautomers 459 is significantly more stable (about 60 kJ mol −1 ) than its keto ion counterpart 461,462 . The difference in energy observed here between ionized phenol and its keto tautomers is, however, more pronounced; this point will be examined below.…”

“…In fact, in the phenol series, the aromaticity renders the enol tautomer more stable; this situation is opposite to that observed in the aliphatic series. For example, neutral acetaldehyde is ca 40 kJ mol −1 below its enol form, namely the vinyl alcohol 459,461 . After removal of one electron, the enol structure becomes more stable than the keto form by 60 kJ mol −1 as recalled above 462 .…”

Exploring the potential energy surface of ion–molecule pairs by experiment and by theory: acetaldehyde and methanol

“…For many years, it has been known that the keto-enol isomerization of an ionized aldehyde or ketone in the gas phase involves a large energy barrier [1][2][3][4][5]. These barriers generally lie at energy above the lowest dissociation threshold for the carbonyl compound and so the isomerization is never directly observed for the isolated ion.…”

Isomeric recognition by ion/molecule reactions: The ionized phenol-cyclohexadienone case

“…In other words, enol ions are generally protected against dissociation or isomerization by significant energy barriers [3]. Consequently, ionized enols are predisposed model ions for fundamental studies in mass spectrometry such as associative ion/molecule investigations [4].…”

“…In that paper, we reported the reactivities of quite simple ionized enols and carbonyl compounds such as (for the carbonyl species) ionized acetaldehyde, acetone, acetic acid, acetamide, and acetophenone. These compounds share the property that, in the neutral state, the carbonyl isomer is more stable than the enol counterpart [1- 3,6].…”