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The increasing CO2 concentration in the atmosphere, which has been attributed to the burning of fossil fuels, is considered a major reason for global warming and a great threat to the environment. Because plant exhaust emissions are one of the major sources of atmospheric CO2, the separation and collection of CO2 from the combustion of fossil fuel has attracted worldwide attention. Therefore, it is important to examine techniques for the capture and release of CO2. Lewis bases, such as water, amines and amides, have been used as CO2 absorbents because CO2 acts as a Lewis acid. A group of carboxylate anions is one type of popular Lewis base. Recently, the possibility of amino acids with a carboxylate motif as a potential absorbent of CO2 was identified in an experimental study. The carbonyl group in acetone can weakly interact with CO2 with a binding energy of 3-5.2 kcal/mol, which again suggests the possibility of the carboxylate anion as an electron donor. A stronger interaction between carboxyl groups and CO2 compared to carbonyl compounds is expected because the anion charge is delocalized over two O donors in the carboxylate anion. Nevertheless, the interaction between the carboxylate anion and CO2 has not been studied systematically. This study examined the interaction mode of CO2 to carboxylate functionality (CO2−) using ab initio calculations. The acetate anion (MeCO2−) was chosen as a model in this study. This study was initiated by performing HF/6-31+G(d) calculations for [MeCO2·CO2]− complex, where the CO2 molecule is located in the middle of the oxygen atoms of the carboxylate anion. The optimized geometry obtained by the HF/6-31+G* method remains relatively constant with the reoptimized geometry obtained using the MP2/6-31+G* method. The B3LYP/6-31+G* optimized structure and geometry, however, differs from the MP2/6-31+G* structure. m1 and b1, shown in Figure 1, are the optimized structure of the [MeCO2·CO2]− complex obtained by MP2/6-31+G* and B3LYP/6-31+G*, respectively. In the minimum energy structure, m1, the CO2 molecule is perpendicular to plane of O2-C1-O3 and the C1' atom of CO2 is positioned at the center between O2 and O3 of carboxylate. The angle θ1 (C1'-O2-C1) in m1 is 90.28. In b1, however, the position of the C1' atom of CO2 is predicted to be near one side. The angle θ1 (C1'-O2-C1) was 117.50°. To determine the other energy minima, the energy profile of the [MeCO2·CO2]− complex was examined by changing the relative position of CO2 to a carboxylate anion. The analysis was carried out using MP2/6-31+G* and B3LYP/631+G* methods by varying the angle θ1 (C1'-O2-C1) from 50 to 260 by 10°. This result is shown in Figure 2. The other energy minimum structure was found to be at an angle(θ) of 238.13 for b2 and 232.40 for m2 using B3LYP/ 6-31G* and MP2/6-31+G* methods, respectively. The geo-

Ab Initio Study on the Interaction of CO2to the Acetate

Ab Initio Study on the Interaction of CO₂to the Acetate