Hydrogen-bond patterns Subject

An unusual photochemical reaction of 2-pyridones, 2-aminopyridinium salts and pyran-2-ones is photodimerization to give the so-called butterfly dimers. These transformations are outlined in equations (13) and (14). Photodimerization by [2+2] cyclization is also a common and important reaction with these compounds. It has been the subject of particular study in pyrimidines, especially thymine, as irradiation of nucleic acids at ca. 260 nm effects photodimerization (e.g. equation 15) this in turn changes the regular hydrogen bonding pattern between bases on two chains and hence part of the double helix structure is disrupted. The dimerization is reversed if the DNA binds to an enzyme and this enzyme-DNA complex is irradiated at 300-500 nm. Many other examples of [2+2] photodimerization are known and it has recently been shown that 1,4-dithiin behaves similarly (equation 16) (82TL2651). 

Hydrogen bonding vctsus proton transfer and the nature of the hydrogen bond, both in the liquid and in the gas phase, are subjects of extensive investigations and useful empirical rules to draw hydrogen bond patterns are well codified for organic compounds . 

Hydrogen bond patterns are often classified with graph set analysis, a notation describing the type of interaction and number of atoms involved, a subject we will briefly treat in Chapter 4 [28,29], It is useful in describing the synthon used in linking the net together. 

Oxidative attack on a carbon-hydrogen bond of an alkyl group a to a nitrogen atom is not restricted to saturated aliphatic amines. In fact X in an X-N-CH- structural subunit can be virtually any common atomic grouping that can be found in stable organic molecules. For example, w-carbon hydrogens of Aralkyl-substituted aromatic cyclic amines (119), aryl amines (120), amides (121), amidines (122), A-nitrosodialkylamines (123), etc. are all subject to oxidative attack, carbinolamine formation, and in most cases release of an aldehyde or ketone depending on the substitution pattern (1° or 2°)... 

In the epoxidation of acyclic allylic alcohols (Scheme 6), the diastereoselectivity depends significantly on the substitution pattern of the substrate. The control of the threo selectivity is subject to the hydroxyl-group directivity, in which conformational preference on account of the steric interactions and the hydrogen bonding between the dioxirane oxygen atoms and the hydroxy functionality of the allylic substrate steer the favored 7r-facial... 

The structural correlations are strongly enhanced in the under-cooled state as the temperature is reduced towaids the metastable limit of -40°C (to D2O) and various thermoph ical properties exhibit diverged behaviour [8]. The exact nature of this anomaly is still the subject of some controversy. However, the difiraction pattern indicates that the stmcture is evolving towards that of amorphous ice which is characterised as a continuous random networit of tetrahedral hydrogen-bonds [9]. Recent neutron measurements on amorphous ice [10] have re-infor the earlier conjectures tuid shown that the structure is similar to that of hyper-quenched glassy water produced by rapid cooling of micron-sized water droplets. It can now be realised that the CRN mo l for the disordered phase of ice is effectively the limiting stmcture of water at low temperatures. 

Especially the weak interaction patterns like van der Waals forces, weak hydrogen bonds (i.e. those with bond energies less than about 10 kJ mol ) and n-n stackings are often discussed in the light of hydrophobic effects. Such effects are strongly system-dependent, they can hardly be understood in an ad hoc fashion, and are thus subject of constant debate (see Refs. [199, 200] for examples and Ref. [201] for a review). 

The anions themselves in solution present a different picture since they are subject to much lower quadrupolar interactions. In aqueous solution the anions are solvated by hydrogen bonding. At any one instant the pattern of hydrogen bonded water molecules around the anion does not have spherical symmetry but is always close to it. Thus the quadrupolar couplings are much lower than for the covalently bound halogens but are not zero. Typically, in aqueous solution and in the absence of extraneous influences, both isotopes of chlorine show Cl" line widths of 10-15 Hz, both isotopes of bromine show Br" line widths of 400 Hz, I" has a line width of over 1000 Hz. 

---