# Theoretical Survey of the Intrinsic Reactivity of Functionalized (CH2=C(R)XH) Enols, Enethiols and Eneselenols: Potential Interstellar Species

## TL;DR

This paper explores the chemical behavior of certain unsaturated compounds in space, focusing on their conformations and reactivity.

## Contribution

The study provides a detailed theoretical analysis of the intrinsic reactivity and conformational properties of functionalized enols, enethiols, and eneselenols.

## Key findings

- Enols, enethiols, and eneselenols have nearly isoenergetic conformers governed by noncovalent interactions.
- Protonation typically occurs at the methylene carbon, except for specific derivatives.
- Deprotonation preferentially occurs at the X–H group, except in cyclopentadienyl-substituted enols.

## Abstract

The conformational properties and intrinsic reactivity of unsaturated CH2=C(R)XH systems (R = –H, –CH=CH2, –C≡CH, –C≡N, –Cl, –phenyl, –cyclopentadienyl, –pyrrole; X = O, S, Se)—namely enols, enethiols, and eneselenols—have been investigated using G4 and CCSD(T) calculations. All compounds exhibit antiperiplanar (ap) and anticlinal (ac)-conformers that are nearly isoenergetic, as their relative stabilities are governed by subtle noncovalent interactions, which are analyzed in detail. Both conformers are therefore expected to coexist in the gas phase, and because the rotational barriers are very low, their interconversion is effectively barrierless under typical conditions. In contrast, the corresponding protonated species display significantly higher barriers, approximately three to five times larger. The keto–enol tautomerization involves activation barriers exceeding 180 kJ·mol−1, confirming that, as in other keto–enol rearrangements, the process is not monomolecular. Protonation generally occurs at the methylene carbon, with the exceptions of the –C≡CH and –C≡N derivatives. Strong linear correlations are found among the proton affinities of the three families studied, which follow the trend: enols > enethiols > eneselenols. All systems behave as strong carbon bases; some are predicted to be 20–21 orders of magnitude more basic than ketene and 3–5 orders of magnitude more basic than vinylimine in terms of equilibrium constants. Deprotonation preferentially occurs at the X–H group in nearly all cases. The only exception is the cyclopentadienyl-substituted enol, for which deprotonation of the cyclopentadienyl moiety is favored due to enhanced aromatic stabilization of the resulting anion. Overall, acidity increases along the series O < S < Se.