Scientific Publications 2001

L

Lea AS, JE Amonette, DR Baer, Y Liang, and NG Colton. 2001. "Microscopic Effects of Carbonate, Manganese, and Strontium ions on Calcite Dissolution." Geochimica et Cosmochimica Acta 65 (3):369-379. Abstract Aqueous dissolution of the (1014) surface of calcite was observed at pH near 9 using an atomic force microscope equipped with a fluid cell. The influence of carbonate, Sr, and Mn ion concentrations were observed. Carbonste ions were shown to have a step-specific effect on calcite dissolution. At ow levels (5 mu-M) of carbonate, the retreat rate of the more structually open [441]+steps was than the retreat rate of the structurally confined [441]-steps, leading to anisotropic dissolution. Increasing the carbonate level to 200 mu-M decreased the rate of retreat of both steps, but the [411]+step was slowed to a much greater extent making the dissolution nearly isotropic. At high levels (800 mu-M) of carbonate, the rate of retreat of the [441]+step was slower than that of the [441]-step making dissolution anisotropic in the opposite sense to that observed at low levels of carbonate. This decrease in step velocity at high carbonate levels was attributed to a corresponding increase in the reaction (i.e., precipitation) as the solution approached saturation with respect to calcite, and thus is related to the rate of incorporation of calcium cations into the structure. In addition to changing the rate, this back reaction also altered the shape of etch pits formed by dissolution. Strontium cations were also shown to have a step-specific effect on calcite dissolution similar to that of carbonate, suggesting that strontium is preferentially incorporated into the [441]-step to a greater extent than strontium. When the solution exceeded saturation with respect to rhodochrosite, calcite dissolution was nearly isotropic. These results suggest that the small manganese ion (r = 83 pm), is readily incorporated into both [441]+ and [441]-steps, in contrast to the larger Ca (r = 100 pm) and Sr (r = 131 pm) cations, which are preferentially incorporated into the [441]+step.

Li J, BE Bursten, and M Zhou. 2001. "A Combined Theoretical and Experimental Study of the Reaction Products of Laser-Ablated Thorium Atoms with CO: First Identification of the CThO, CThO-, OThCCO, OTh(eta(3)-CCO), and Th(CO)(n) (n=1-6) Molecules." Inorganic Chemistry 40(21):5448-5460. Abstract Laser-ablated thorium atoms have been reacted with CO molecules during condensation in excess neon. Absorptions at 617.7 and 812.2 cm-1 are assigned to Th-C and Th-O stretching vibrations of the CThO molecule. Absorptions at 2048.6, 1353.6, and 822.5 cm-1 are assigned to the OThCCO molecule, which is formed by CO addition to CThO and photochemical rearrangement of Th(CO)2. The OThCCO molecule undergoes further photoinduced rearrangement to OTh(Eta3-CCO), which is characterized by C-C, C-O, and Th-O stretching vibrations at 1810.8, 1139.2, and 831.6 cm-1. The Th(CO)n (n ) 1-6) complexes are formed on deposition or on annealing. Evidence is also presented for the CThO- and Th(CO)2- anions, which are formed by electron capture of neutral molecules. Relativistic density functional theory (DFT) calculations of the geometry structures, vibrational frequencies, and infrared intensities strongly support the experimental assignments.