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water dimer
The water dimer consists of two water molecules loosely bound by a hydrogen bond. It is the smallest water cluster. Because it is the simplest model system for studying hydrogen bonding in water, it has been the target of so many theoretical 〔Buckingham, A. D. The hydrogen bond, and the structure and properties of water and the water dimer. ''Journal of Molecular Structure'' 1991, ''250'', 111-18.〕〔Goldman, N., Leforestier, C., and Saykally, R. J., Water Dimers in the Atmosphere II: Results from the VRT(ASP-W)III Potential Surface, ''Journal of Physical Chemistry A'', 2004, ''108'', p. 787-794.〕〔Schütz, M.; Brdarski, S.; Widmark, P.-O.; Lindh, R.; Karlström, G. The water dimer interaction energy: Convergence to the basis set limit at the correlated level, Journal Chemical Physics, 1997, ''107'', 4597-4605.〕 (and later experimental) studies that it has been called "a theoretical Guinea pig".〔Jeffrey, G. A.; ''An Introduction to Hydrogen Bonding (Topics in Physical Chemistry)''. Oxford University Press, USA (March 13, 1997). ISBN 0-19-509549-9〕
==Structure and properties==
The ab initio binding energy between the two water molecules is estimated to be 5-6 kcal/mol, although values between 3 and 8 have been obtained depending on the method. The experimentally measured dissociation energy (including nuclear quantum effects) of (H2O)2 and (D2O)2 are 3.16 ± 0.03 kcal/mol (13.22 ± 0.12 kJ/mol)〔Rocher-Casterline, B. E.; Ch'ng, L. C.; Mollner, A. K.; Reisler, H. ''Journal of Chemical Physics'' 2011, ''115'', 6903-6909 doi:10.1063/1.3598339〕 and 3.56 ± 0.03 kcal/mol (14.88 ± 0.12 kJ/mol),〔Ch'ng, L. C.; Samanta, A. K.; Czakó, G.; Bowman, J. M.; Reisler, H. ''Journal of American Chemical Society'' 2012, ''134'', 15430 doi:10.1021/ja305500x〕 respectively. The values are in excellent agreement with calculations.〔Shank, A.; Wang, Y.; Kaledin, A.; Braams, B. J.; Bowman. J. M. ''Journal of Chemical Physics'' 2009, ''130'', 144314 doi:10.1063/1.3112403〕〔Leforestier, C.; Szalewicz, K.; van der Avoird, A. ''Journal of Chemical Physics'' 2012, ''137'', 014305 doi:10.1063/1.4722338〕 The O-O distance of the vibrational ground-state is experimentally measured at ca. 2.98 Å;〔Scheiner, S. Ab initio studies of hydrogen bonds: the water dimer paradigm. ''Annual Review of Physical Chemistry'' 1994, ''45'', 23-56.〕 the hydrogen bond is almost linear, but the angle with the plane of the acceptor molecule is about 57°. The vibrational ground-state is known as the linear water dimer (shown in the figure to the right), which is a near prolate top (viz., in terms of rotational constants, A > B ≈ C). Other configurations of interest include the cyclic dimer and the bifurcated dimer.
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