Thermodynamics of Forming Water Clusters at Various Temperatures and Pressures by Gaussian-2, Gaussian-3, Complete Basis Set-QB3, and Complete Basis Set-APNO Model Chemistries; Implications for Atmosp
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- 作者：Meghan E. Dunn ; Emma K. Pokon ; and George C. Shields
- 刊名：Journal of the American Chemical Society
- 出版年：2004
- 出版时间：March 3, 2004
- 年：2004
- 卷：126
- 期：8
- 页码：2647 - 2653
- 全文大小：65K
- 年卷期：v.126,no.8(March 3, 2004)
- ISSN：1520-5126
文摘
The Gaussian-2, Gaussian-3, complete basis set- (CBS-) QB3, and CBS-APNO methods havebeen used to calculate 
hars/Delta.gif" BORDER=0 >H
and hars/Delta.gif" BORDER=0 >G values for neutral clusters of water, (H2O)n, where n = 2-6. Thestructures are similar to those determined from experiment and from previous high-level calculations. Thethermodynamic calculations by the G2, G3, and CBS-APNO methods compare well against the estimatedMP2(CBS) limit. The cyclic pentamer and hexamer structures release the most heat per hydrogen bondformed of any of the clusters. While the cage and prism forms of the hexamer are the lowest energystructures at very low temperatures, as temperature is increased the cyclic structure is favored. The freeenergies of cluster formation at different temperatures reveal interesting insights, the most striking beingthat the cyclic trimer, cyclic tetramer, and cyclic pentamer, like the dimer, should be detectable in the lowertroposphere. We predict water dimer concentrations of 9 × 1014 molecules/cm3, water trimer concentrationsof 2.6 × 1012 molecules/cm3, tetramer concentrations of approximately 5.8 × 1011 molecules/cm3, andpentamer concentrations of approximately 3.5 × 1010 molecules/cm3 in saturated air at 298 K. These resultshave important implications for understanding the gas-phase chemistry of the lower troposphere.
hars/Delta.gif" BORDER=0 >H and hars/Delta.gif" BORDER=0 >G values for neutral clusters of water, (H2O)n, where n = 2-6. Thestructures are similar to those determined from experiment and from previous high-level calculations. Thethermodynamic calculations by the G2, G3, and CBS-APNO methods compare well against the estimatedMP2(CBS) limit. The cyclic pentamer and hexamer structures release the most heat per hydrogen bondformed of any of the clusters. While the cage and prism forms of the hexamer are the lowest energystructures at very low temperatures, as temperature is increased the cyclic structure is favored. The freeenergies of cluster formation at different temperatures reveal interesting insights, the most striking beingthat the cyclic trimer, cyclic tetramer, and cyclic pentamer, like the dimer, should be detectable in the lowertroposphere. We predict water dimer concentrations of 9 × 1014 molecules/cm3, water trimer concentrationsof 2.6 × 1012 molecules/cm3, tetramer concentrations of approximately 5.8 × 1011 molecules/cm3, andpentamer concentrations of approximately 3.5 × 1010 molecules/cm3 in saturated air at 298 K. These resultshave important implications for understanding the gas-phase chemistry of the lower troposphere.