Hydrogen-bond angle contour

Fig. 22.15. The Rn o and hydrogen-bond angle contour maps as functions of the dihedral angles (4>, ifi) for the Gly residue. The Rn o values and hydrogen-bond angles contour line are represented by the solid and dotted lines, respectively. The Rn -o and hydrogen-bond angle are expressed in A and degree (°), respectively.

Theoretical studies, based entirely on the local tetrahedral pentamer model of the liquid 47> but including isotropic expansion and contraction with the distribution of 00 separations matched to the observed 00 correlation function, cannot completely account for the observed OH stretching spectrum 90>. The addition of some weak hydrogen bonds improves the predicted spectrum 90>, but still leaves the widths of the band contours largely unaccounted for. It seems likely that inclusion of 000 angle distribution effects (i.e. bent hydrogen bonds as well as small dispersion about 109.5°) will improve the agreement between predicted observed spectra. 

Figure 3. Contour diagram of the base stacking potential energy Kg of sequential adenine bases and the hydrogen bonding potential energy VHB of the complementary A T base pairs as a function of the phosphodiester rotation angles

In both liquid water and ice, H2O molecules interact extensively via O— bonds. However, there are marked differences between the two phases. In the latter, H2O molecules are tetrahedrally hydrogen-bonded, and this local structure is repeated throughout the crystal. In liquid water, however, the O—H- O bond distance and angle vary locally, and the bond is sometimes broken. Thus, its vibrations cannot be described simply by using the three normal modes of the isolated H2O molecule. According to Walrafen et al. [444] an isosbestic point exists at 3403 cm in the Raman spectrum of liquid water obtained as a function of temperature, and the bands above and below this frequency are mainly due to non-hydrogen-bonded and hydrogen-bonded species, respectively. In addition, liquid water exhibits librational and restricted translational modes that correspond to rotational and translational motions of the isolated molecule, respectively. The librations yield a broad contour at 1000-330 cm while the restricted translations appear at 170 and 60 cm [445]. For more details, see the review by Walrafen [446]. 

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