(70e) Heat of Reaction Calculation for Substituted MEA Reaction with CO2
We have used high-level ab initio calculations at the MP2 level, coupled with geometries generated from B3LYP density functional theory to investigate heats of reaction of various substituted amines with CO2. Two reaction products were considered: carbamate, forming a 2:1 amine:CO2 reaction Stoichiometry, and the carbamic acid 1:1 stoichiometry. We have computed the heats of reaction for alpha- and beta- substituted monoethanolamine (MEA) for a variety of substitution groups, including: H, CH3, NH2, OH, OCH3 and F. We have found that it is imperative to account for solvent effects in order to generate physically meaningful results. We have used the conductor-like polarizable continuum model and have found good agreement with experimentally measured heats of reaction for test compounds. Our results indicate that the heat of reaction for alpha-substituted MEA for forming carbamate become less exothermic with increasing electronegativity of substitution group. In contrast, the heats of reactions for forming carbamic acid are relatively insensitive to the substituent group, except for F. Therefore, a crossing point was found at about an electronegativity of 3.7 and a reaction energy -7 kcal/mol. This means forming carbamic acid becomes thermodynamically more favorable than forming carbamate after the crossing point. Thus, highly polar substituent groups will favor 1:1 CO2 reactions. Computed heats of reaction are shown in the figure.
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