Hydration Structure of CO sub(2)-Absorbed 2-Aminoethanol Studied by Neutron Diffraction with the super(14)N/ super(15)N Isotopic Substitution Method

Neutron diffraction measurements were carried out for CO sub(2)-absorbed aqueous 11 mol % 2-aminoethanol (MEA) D sub(2)O solutions (corresponding to 30 wt % MEA solution) in order to obtain information on both the intramolecular structure and intermolecular hydration structure of the MEA carbamate molecule in the aqueous solution. Neutron scattering cross sections observed for (MEA) sub(0.11)(D sub(2)O) sub(0.89), (MEA) sub(0.11)(D sub(2)O) sub(0.89)(CO sub(2)) sub(0.06), and (MEA) sub(0.11)(D sub(2)O) sub(0.89)(DCl) sub(0.11) solutions with different super(14)N/ super(15)N ratios were used to derive the first-order difference function, Delta sub(N)(Q), which involves environmental structural information around the nitrogen atom of the MEA molecule. Intramolecular geometry and intermolecular hydration structure of MEA, protonated MEA (MEAD super(+)), and MEA carbamate (MEA-CO sub(2)) molecules were obtained through the least-squares fitting of the observed Delta sub(N)(Q) in the high-Q region and the intermolecular difference function, Delta sub(N) super(inter)(Q), respectively. In the aqueous solution, the MEA molecule takes the gauche conformation (dihedral angle, NCCO = 45 plus or minus 3 degree ), suggesting that an intramolecular hydrogen bond is formed. On the other hand, values of the dihedral angle NCCO determined for MEAD super(+) and MEA-CO sub(2) molecules were 193 plus or minus 4 degree and 214 plus or minus 8 degree , respectively. These results imply that the intermolecular hydrogen bonds are dominated for MEAD super(+) and MEA-CO sub(2) molecules. The intermolecular nearest neighbor N...O(D sub(2)O) distance for the MEA molecule was determined to be 3.13 plus or minus 0.01 Aa, which suggests weak intermolecular interaction between the amino-nitrogen atom of MEA and water molecules in the first hydration shell. The nearest-neighbor N...O(D sub(2)O) distances for MEAD super(+) and MEA-CO sub(2) molecules, 2.79 plus or minus 0.03 and 2.87 plus or minus 0.04 Aa, clearly indicate strong hydrogen bonds are formed among the amino group of these molecules and neighboring water molecules.