CH₂CHOH₂⁺ + PN:  A Proton-Transfer Triple Play

Abstract

Quantum chemical calculations are used to explore the proton-transfer reactivity of O-protonated vinyl alcohol, CH2CHOH2+, with phosphorus nitride, PN. This reaction is relevant to the chemical evolution of interstellar clouds, since O-protonated vinyl alcohol has been postulated (and tentatively identified) as a product of the association reaction between interstellar H3O+ and C2H2, while PN is the most widespread and abundant phosphorus-containing molecule seen in astrophysical environments. Furthermore, the reaction exhibits an unusual mechanistic feature, namely, an extended "proton-transport catalysis" mechanism, which we characterize here as a "proton-transfer triple play". The reaction proceeds initially by proton transfer from CH2CHOH2+ to PN, then from PNH+ to CH2CHOH, and finally from CH3CHOH+ to PN, where the emphasized atom indicates the resultant site of protonation/deprotonation. Thus, the ultimate overall bimolecular proton-transfer reaction is expected to occur as CH2CHOH2+ + PN --> CH3CHO + PNH+; that is, the apparent favored product channel exhibits not only proton transfer but also keto/enol tautomerization. The triple-play mechanism can be rationalized in terms of the proton affinities of vinyl alcohol, acetaldehyde, and phosphorus nitride, which here are satisfactorily reproduced by high-level ab initio calculations. Other neutrals with a proton affinity appropriate for the possible triple-play mechanism converting CH2CHOH2+ to CH3CHO are also identified, with a view to encouraging experimental investigation of this mechanism.

Gas‐phase basicities of polyfunctional molecules. Part 1: Theory and methods

“…Molecular orbital calculations demonstrate that the favored protonation sites of these molecules are the β‐carbon atom for X = N, O, S and the heteroatom when X = P; intermediate situation is established for X = As (Mo et al, 1999). At the G2 level, structures eC and yC are favored over eX and yX (Scheme ) by 15 and 24 kJ mol −1 for X = N (Mo et al, 1999; see also Smith & Radom, 1992 for another theoretical level), by 96 and 173 kJ mol −1 for X = OH and by 54 and 100 kJ mol −1 for X = SH (this work; see also Fairley et al, 1996 and Petrie, 2005 for similar calculations on eC and eX ions with X = OH), while the situation is reversed for X = P with eX and yX being more stable than eC and yC by 11 and 7 kJ mol −1 (Mo et al, 1999).…”

Ions in space

“…It may well be that higherenergy isomeric ions are, in general, too rapidly lost through isomerization to permit their detection. One interesting variant on the general mechanism of protontransport catalysis has recently come to light (Petrie, 2005). Among feasible formation mechanisms for interstellar vinyl alcohol CH 2 CHOH, the association reaction between H 3 O þ and C 2 H 2 is thought to produce a mixture of isomeric adduct ions including an ion of the formula CH 2 CHOH 2 þ .…”

Prebiotic molecules formation through the gas-phase reaction between HNO and CH2CHOH2+

“…A value of 171 kcal mol −1 for vinyl alcohol is reported by Fairley et al (1996) at the G2 level; they also estimated an energy barrier of 41 kcal mol −1 for the isomerization process into the most stable isomer, protonated acetaldehyde. Later, Petrie (2005) reported PAs values for vinyl alcohol calculated at different theoretical levels which are included in the range 171-173 kcal mol −1 . In addition, we checked dissociation of protonated vinyl alcohol, CH 2 CHOH + 2 , into CH 2 CH + and H 2 O.…”