Hydrogen bonding defined

If the command ACTA appears in the header of an. ins file, SHELXL generates such a. cif file. ACTA automatically sets the BOND, fmap 2, plan and LIST 4 instructions and ACTA cannot be combined with other fmap or LIST commands. Torsion angles defined by CONF and hydrogen bonds defined by HTAB are also written into the. cif file, while quantities defined by RTAB and MPLA are only tabulated in the. 1st file. 

Bonds to Oxygen.—Compounds of Lower Oxidation State. The chemistry of hypophosphorous acid and the metal hypophosphites has been reviewed. Improved dimensions for the HPOaOH" ion have been reported in a neutron-diffraction investigation of the structure of LiH2P03 in particular, the P— distance has been revised to 1.396 A and the presence of strong hydrogen bonds defined. 

To some extent, the structures discussed above resemble turn structures in peptides. A typical peptide turn structure is depicted in Scheme 5.5. Ring constraints by hydrogen bonds define y-turns (7-membered ring) or p-tums (10-membered ring), whereas 13-membered rings are present in a-helices. Before and after the turn, the peptide extends as anti-parallel strands, often in p-sheets... 

A fuzzier atom type participating in these descriptors has been defined that is pharmacologically relevant - the physicochemical type at near-neutral pH [24], which is one of the following seven binding property classes 1 = cation 2 = anion 3 = neutral hydrogen-bond donor 4 = neutral H-bond acceptor ... 

Equation (8.97) shows that the second virial coefficient is a measure of the excluded volume of the solute according to the model we have considered. From the assumption that solute molecules come into surface contact in defining the excluded volume, it is apparent that this concept is easier to apply to, say, compact protein molecules in which hydrogen bonding and disulfide bridges maintain the tertiary structure (see Sec. 1.4) than to random coils. We shall return to the latter presently, but for now let us consider the application of Eq. (8.97) to a globular protein. This is the objective of the following example. 

X-ray crystallographic studies (59) have defined the conformations and hydrogen bonding of the tetracyclines under nonpolar and polar conditions. These are shown ia Figure 3. It is beheved that the equiUbrium between the 2witterionic and nonioni2ed forms is of importance for the broad-spectmm antibacterial activity, membrane permeation, and pharmacokinetic properties. 

Zwitterionic L-alanine ( HjN—CfCHj)—CO2—) is a dipolar molecule that forms large well-ordered crystals in which the molecules form hydrogen-bonded columns. The strong interactions lead to the presence of well-defined intra- and intermolecular vibrations that can usefully be described using hannonic theory. 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

The coordination chemistry of the large, electropositive Ln ions is complicated, especially in solution, by ill-defined stereochemistries and uncertain coordination numbers. This is well illustrated by the aquo ions themselves.These are known for all the lanthanides, providing the solutions are moderately acidic to prevent hydrolysis, with hydration numbers probably about 8 or 9 but with reported values depending on the methods used to measure them. It is likely that the primary hydration number decreases as the cationic radius falls across the series. However, confusion arises because the polarization of the H2O molecules attached directly to the cation facilitates hydrogen bonding to other H2O molecules. As this tendency will be the greater, the smaller the cation, it is quite reasonable that the secondary hydration number increases across the series. 

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