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On equilibrium structures of the water molecule
Attila G Császár1, Gábor Czakó, Tibor Furtenbacher
1Department of Theoretical Chemistry, Eötvös University, H-1518 Budapest 112, P.O. Box 32, Hungary. csaszar@chem.elte.hu
The Journal of Chemical Physics
|June 25, 2005
Summary
This study reveals that ab initio calculations of water's equilibrium geometry are more accurate than experimental methods. The Born-Oppenheimer approximation holds for water's structure to within 3 x 10(-5) Å and 0.02 degrees.
Area of Science:
- Molecular Sciences
- Quantum Chemistry
- Spectroscopy
Background:
- Equilibrium structures are crucial in molecular sciences, often determined via complex inverse procedures relying on assumptions like the Born-Oppenheimer approximation.
- Theoretical calculations offer a direct route to equilibrium geometries, potentially yielding higher accuracy than experimental data analysis.
Purpose of the Study:
- To analyze a high-quality ab initio semiglobal adiabatic potential-energy surface (PES) for water's electronic ground state.
- To determine the accuracy of theoretically derived equilibrium geometries compared to experimental methods.
- To investigate the impact of the breakdown of the Born-Oppenheimer approximation on water's structural parameters.
Main Methods:
- Utilized a recent high-quality ab initio semiglobal adiabatic potential-energy surface (PES) for water (CVRQD).
- Performed theoretical calculations to directly determine equilibrium geometries.
- Investigated the effects of isotopic substitution and the breakdown of the Born-Oppenheimer approximation.
Main Results:
- The mass-independent (Born-Oppenheimer) equilibrium bond length and angle for water are r(e)(BO) = 0.957 82 Å and θ(e)(BO) = 104.48(5)°.
- Adiabatic equilibrium parameters for H2¹⁶O are r(e)(ad) = 0.957 85 Å and θ(e)(ad) = 104.50(0)°, and for D2¹⁶O are r(e)(ad) = 0.957 83 Å and θ(e)(ad) = 104.49(0)°.
- Pure ab initio predictions of rotational levels are accurate to better than 0.002 cm⁻¹, and adjustments to the PES for experimental accuracy do not significantly alter adiabatic equilibrium structure parameters.
Conclusions:
- The concept of an isotope-independent equilibrium structure for water holds to approximately 3 x 10⁻⁵ Å and 0.02°.
- Theoretical calculations using high-quality PES provide highly accurate equilibrium geometries for water isotopologs.
- Small residual deviations in effective rotational constants are attributed to centrifugal distortion, electronic, and non-Born-Oppenheimer effects.