The ionic hydrogen bond and ion solvation. 5- OH.cntdot..cntdot..cntdot.O- bonds. Gas-phase solvation and clustering of alkoxide and carboxylate anions.
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Dissociation energies, ..delta..H/sup 0//sub D/, of RO/sup -/ x HOR, RCOO/sup -/ x HOR, and RCOO/sup -/ x HOOCR complexes range from 14 to 29 kcal/mol. Large values of ..delta..H/sup 0//sub D/ are observed for the symmetric dimers CH/sub 3/O/sup -/ x HOCH/sub 3/ (28.8 kcal/mol) and CH/sub 3/COO/sup -/ x HOOCCH/sub 3/ (29.3 kcal/mol). ..delta..H/sup 0//sub D/ decreases as the difference between the acidities of the components increase; e.g., for dimers with large ..delta delta..H/sup 0//sub acid/ such as CH/sub 3/COO/sup -/ x H/sub 2/O and HCOO/sup -/ x H/sub 2/O (..delta delta..H/sup 0//sub acid/ = 42.2 and 45.5 kcal/mol, respectively), ..delta..H/sup 0//sub D/ = 16.0 kcal/mol. For 13 dimers, a linear correlation of the form ..delta..H/sup 0//sub D/ = 28.4 - 0.29 ..delta delta..H/sup 0//sub acid/ is obtained. Some dimers deviate substantially from the correlation, suggesting special structural effects. For example, the large value of ..delta..H/sup 0//sub D/ = 36.8 kcal/mol for HCOO/sup -/ x HOOCH suggests double hydrogen bonding, involving a CH...O/sup -/ type bond. On the other hand, the data suggest that only one hydrogen bond is formed in RCOO/sup -/ x H/sub 2/O. Comparison between anionic and cationic hydrogen bonded systems shows that the bondingmore » energies are similar for RO/sup -/ x B and ROH/sub 2//sup +/ x B complexes involving the same ligand B, suggesting primarily electrostatic interactions in both, while RCOO/sup -/ x B is weaker by 4-6 kcal/mol than RCOOH/sub 2//sup +/ x B, probably due to charge delocalization in RCOO/sup -/.« less