Angstrom hydrogen bonding

The attraction for two neutral atoms separated by more than four Angstroms is approximately zero. The depth of the potential wells is minimal. For the AMBER force field, hydrogen bonds have well depths of about 0.5 kcal/mol the magnitude of individual van der Waals well depths is usually less. 

Monte Carlo and Molecular Dynamics simulations of water near hydrophobic surfaces have yielded a wealth of information about the structure, thermodynamics and transport properties of interfacial water. In particular, they have demonstrated the presence of molecular layering and density oscillations which extend many Angstroms away from the surfaces. These oscillations have recently been verified experimentally. Ordered dipolar orientations and reduced dipole relaxation times are observed in most of the simulations, indicating that interfacial water is not a uniform dielectric continuum. Reduced dipole relaxation times near the surfaces indicate that interfacial water experiences hindered rotation. The majority of simulation results indicate that water near hydrophobic surfaces exhibits fewer hydrogen bonds than water near the midplane. 

Fig. 4 Optimized structures (B3LYP/6-310(d)) of the most stable catalyst-substrate adducts. Bond distances characteristic for hydrogen bonds are given in Angstrom...

Figure 8.18. Optimized parameters for stationary points on water dimer and hydrogen fluoride dimer PES. Bond lengths are in angstroms and bond angles are in degrees. (From Smith et al. [1990].)...

Table 5. Hydrogen bonds for [Cu(imidazole)4Cl]Cl. [Angstroms andDegreesl ...

Whether or not two ions of the same kind have an attractive minimum, possibly of many kT, when they ate located at a few Angstroms apart, in water it is quite an interesting issue. But this issue has probably to be resolved by ab initio microscopic theories that include all the interactions (Coulomb, dispersion, hydrogen bonding, etc.) of a representative system. Today s computational capability (for up to about 20 water molecules) might be sufficient for such ab initio calculations. 

Figure 17 Distances between heavy atoms involved in hydrogen bonds formed by the phosphate group (central molecule) of every independent molecule existing in one asymmetric unit of NH4PThr given in angstroms. Dotted lines show hydrogen bonds. (A)-(D) correspond to A, B, C and D molecules, respectively, in one NH4PThr unit. For easier visualisation, the threonine chains are replaced by CH3 groups in this illustration. Taken from Ref. [75].

Figure 4-6. The interactions of (—)-HA with the active site of TcAChE. (a) The hydrogen bonding networks. The water molecules are represented by red balls, (b) The C—H... 7t hydrogen bonds. The distances are given in angstroms. Copy from Ref. 10.

Figure 1.4 (Home, 1969) shows the measured angle of 105° between the hydrogens and the direction of the dipole moment. The measured dipole moment of water is 1.844 debye (a debye unit is 3.336 x 10 ° C m). The dipole moment of water is responsible for its distinctive properties in the liquid state. The O—H bond length within the H2O molecule is 0.96 A (an angstrom unit, A, is 10 m). Dipole-dipole interaction between two water molecules forms a hydrogen bond, which is electrostatic in nature. The lower part of Figure 1.4 (not to the same scale) shows the measured H-bond distance of 2.76 A, or 0.276 nm.

Fig. 29 The elongated pentacoordinate phosphorus species observed in the X-ray structure of /1-PGM from Lactococcus lactis. Distances shown are in angstroms. In addition to the interactions shown, the P03 moiety also makes hydrogen-bonding contacts with two backbone amide N-H groups, and the hydroxyl of Ser-114.

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