Hydrogen-bond angle

NHb bond. As reported in the second row of Table 3, this bond becomes progressively longer as n is increased. The energy barriers to proton transfer from one N atom to the other show a marked dropoff as the strain of the n=l structure is relieved. The highly angularly distorted H-bond in H2N(CH2)iNH3+ leads to a barrier in excess of 30 kcal/mol, which drops below 10 kcal/mol as a second CH2 group is added. 

Comparison with the H-bond lengths in the last row of Table 3 illustrates how the effects of bond stretching and bending can conflict with one another. That is, the barrier in the H2N(CH2)iNH3+ system is by far the highest, even 

Malonaldehyde, with three C atoms intervening between the donor and acceptor oxygens, has a barrier of 10 kcal/mol, reduced to about 4 kcal/mol by correlation [69], although an earlier study placed the correlated barrier closer to 10 kcal/mol [70]. When the intervening carbon atoms are part of a larger aromatic system, as in tropolone, and when the system is overall neutral, the barrier is higher. At the SCF level, this barrier is computed to be 16-17 kcal/mol, lowered to 4 kcal/mol by correlation [71]. 

Hydrogen maleate ion, analogous to the larger H2N(CH2)4NH3+, and presumably with even less distortion imposed on the H-bond by virtue of its larger size, is computed to have a proton transfer barrier of less than 2 kcal/mol [72], a barrier that may disappear entirely after electron correlation is included [73]. 

Both the H3NH+—NH3 and H2OH+—OH2 systems obey very similar patterns so we will focus our discussion on the former. A deformation of 20° causes a rather minimal increase in proton transfer barrier, of only some 2-4 kcal/mol. However, the barrier jumps dramatically when the deformation is increased to 40°. Rotation of even one molecule, the proton acceptor, by 40° raises the barrier from 11 to over 17 kcal/mol, an increase of about 50%. 

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One after the other, examine methanol dimer and acetic acid dimer. Do the hydrogen-bond lengths in these systems differ significantly from the optimum distance in water dimer Are the hydrogen-bond angles in these compounds significantly different from those in water dimer Rationalize your results. 

Fig. 18.3 Ball and stick drawing of the centro-symmetric [H2NEt2]2Zr2Clio. Atom labels, bond distances and hydrogen bonds are labeled on the left side of the figure and Zr-CI-H-N hydrogen-bond angles are labeled on the right side of the figure.

Another example of a C—F bond participating in hydrogen bonding has been found in calcium 2-fluorobenzoate dihydrate (Karipides and Miller, 1984). In the crystal structure of this compound the Tip. . . q distance between the fluorine and a water molecule bound to the calcium as a ligand was 299.4 pm, and the hydrogen bond angle was 170°. 

These analyses also compared X-ray and neutron values of the hydrogen-bond angles and conclude that the agreement (N-fi- 0)x -(N- ft -0)N is generally within 10°. The plots of these error distributions are shown in Fig. 6.1. 

Fig. 19.14. Definition of out-of-plane (fi) and in-plane (y) hydrogen-bond angles as given in [596, 610]...

Amino acid Atom Hydrogen bonding Angle (°) Temperature factor B... 

It is interesting to note that the data in Figure 1 and Table I show that alcohols are better extractants for HDBP than are carboxylic acids. One might expect the reverse to be true because carboxylic acids probably form hydrogen-bonded complexes with HDBP which are similar in structure to the very stable HDBP dimer. Such structures have resonance stabilization and favorable hydrogen bond angles. However, one must consider the energy of association between the solvent molecules themselves. Association between solvent molecules must be broken in order for... 

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