Hydrogen-bonded complexes equilibrium geometries

The equilibrium angular geometry of a hydrogen-bonded complex B- HX... 

Fig. 2 The experimentally determined potential energy V(), expressed as a wavenumber for convenience, as a function of the angle in the hydrogen-bonded complex H20- HF. The definition of

For obtaining interaction energies and equilibrium geometries, local density approximation is even less adequate than it is in the case of hydrogen-bonded complexes. The intermolecular distances are too short and the interaction energies are overestimated.113,123,127-129,130,131 The overestimation of the interaction energy in the case of noble-gas dimers by factor three as it is the case for Ar2 or even ten for He2127 makes LDA rather useless for this type of systems. 

Concerning ab initio calculations on neutral hydrogen-bonded complexes we consider the simplest example, the hydrogen fluoride dimer (Table 3). Both the equilibrium geometry and energy of interaction strongly depend on the quality of the basis set... 

Table 8 contains the thermodynamic characteristics for the formation of various types of complexes, starting with a strong dipole-dipole complex, followed by different hydrogen-bonded complexes and ending with a true vdW molecule. For most complexes, the equilibrium geometry, harmonic frequencies, and AE were determined using ab initio SCF calculations in the 4-31G basis 43.54.65) Qjjjy fQj. H O... HF wcTC othcr basis sets used... 

The equilibrium angular geometry of a hydrogen-bonded complex B- HX can be predicted by assuming that the axis of the HX molecule lies ... 

Combining volumes, law of, 26, 236 Combustion, heat of hydrogen, 40 Complex ions, 392 amphoteric, 396 bonding in, 395 formation, 413 geometry of. 393 in nature, 396 isomers, 394 linear, 395 octahedral, 393 significance of, 395 square planar, 395 tetrahedral, 394 weak acids, 396 Compound, 28 bonding in, 306 Concentration and equilibrium, 148 and E zero s, 213 and Le Chatelier s Principle, 149 effect on reaction rate, 126, 128 molar, 72... 

Both species exhibit the expected linear geometry that maximizes the dominant n- - a interaction. However, these isomers are rather perplexing from a dipole-dipole viewpoint. The dipole moment of CO is known to be rather small (calculated Fco = 0.072 D), with relative polarity C- 0+. 40 While the linear equilibrium struc-ture(s) may appear to suggest a dipole-dipole complex, robust H-bonds are formed regardless of which end of the CO dipole moment points toward HF This isomeric indifference to dipole directionality shows clearly that classical dipole-dipole interactions have at most a secondary influence on the formation of a hydrogen bond. 

Figure 6 compiles the theoretical and experimental data on the equilibrium geometry of (HF)2. It reveals the commonly expected features of an ordinary hydrogen bond the complex is planar and exhibits Cs-symmetry. A small elongation of the HF bond in the proton donor molecule relative to the bond in free HF is found. The inter-... 

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