Thermodynamic Properties (Enthalpy, Bond Energy, Entropy, and Heat Capacity) and Internal Rotor Potentials of Vinyl Alcohol, Methyl Vinyl Ether, and Their Corresponding Radicals

Vinyl alcohols (enols) have been discovered as important intermediates and products in the oxidation and combustion of hydrocarbons, while methyl vinyl ethers are also thought to occur as important combustion intermediates. Vinyl alcohol has been detected in interstellar media, while poly(vinyl alcohol) and poly(methyl vinyl ether) are common polymers. The thermochemical property data on these vinyl alcohols and methyl vinyl ethers is important for understanding their stability, reaction paths, and kinetics in atmospheric and thermal hydrocarbon−oxygen systems. Enthalpies , entropies , and heat capacities (C_p(T)) are determined for CH₂CHOH, C^•HCHOH, CH₂C^•OH, CH₂CHOCH₃, C^•HCHOCH₃, CH₂C^•OCH₃, and CH₂CHOC^•H₂. Molecular structures, vibrational frequencies, , and C_p(T) are calculated at the B3LYP/6-31G(d,p) density functional calculation level. Enthalpies are also determined using the composite CBS-Q, CBS-APNO, and G3 methods using isodesmic work reactions to minimize calculation errors. Potential barriers for internal rotors are calculated at the B3LYP/6-31G(d,p) level and used to determine the hindered internal rotational contributions to entropy and heat capacity. The recommended ideal gas phase values calculated in this study are the following (in kcal mol^-1): −30.0, −28.9 (syn, anti) for CH₂CHOH; −25.6, −23.9 for CH₂CHOCH₃; 31.3, 33.5 for C^•HCHOH; 27.1 for anti-CH₂C^•OH; 35.6, 39.3 for C^•HCHOCH₃; 33.5, 32.2 for CH₂C^•OCH₃; 21.3, 22.0 for CH₂CHOC^•H₂. Bond dissociation energies (BDEs) and group additivity contributions are also determined. The BDEs reveal that the O−H, O−CH₃, C−OH, and C−OCH₃ bonds in vinyl alcohol and methyl vinyl ether are similar in energy to those in the aromatic molecules phenol and methyl phenyl ether, being on average around 3 kcal mol^-1 weaker in the vinyl systems. The keto−enol tautomerization enthalpy for the interconversion of vinyl alcohol to acetaldehyde is determined to be −9.7 kcal mol^-1, while the activation energy for this reaction is calculated as 55.9 kcal mol^-1; this is the simplest keto−enol tautomerization and is thought to be important in the reactions of vinyl alcohol. Formation of the formyl methyl radical (vinoxy radical/vinyloxy radical) from both vinyl alcohol and methyl vinyl ether is also shown to be important, and its reactions are discussed briefly.