Wrong-pair bonds

The so discrepant behaviour of the hydrogen atom has led to the introduction of a special term hydrogen bond for this interaction. Various explanations regarding the nature of this special form of bond, such as divalent hydrogen (thus with two electron pair bonds) or proton resonance similar to electron resonance as in H2+ (p. 124) have proved to be wrong. 

C-4 -C-5 bond is gauche (42.3°, -75.7°), Adjacent bases are linked together by pairs of N-6-H---N-7 and N-6-H-"N-l hydrogen-bonds. In both types of hydrogen-bonded pairs, the base planes make dihedral angles of 37°. Base stacking was not observed. The coordinates published correspond to the wrong enantiomorph. 

Sidgwick s discussion raises an important question What are the effective sizes and shapes of atoms in molecules From the viewpoint of the electride ion model of electronic structure, Sigdwick s circles for the fluoride ions in the first column of Fig. 15 are the wrong shape, if nearly the right overall size. In the electride-ion model a fluoride ion is composed of (approximately) spherical domains, but is not itself spherical, in the field of a cation, Fig. 16. Fig. 17 illustrates, correspondingly, the implied suggestion that, on the assumption that non-bonded interactions are not limiting, the covalency limits of an atom will be determined by the radius of the atom s core and by the effective radii, not of the overall van der Waals envelopes of the coordinated ions but, rather, by the radii of the individual, shared electron-pairs. 

The word resonance for this situation gives us an impression that the molecule resonates from one structure to the other and the electron pair jumps back and fourth from one bond to the other. This is totally wrong and the molecule has only one real electron structure which cannot be physically described. Thus the difficulty lies in the description and not in the molecule itself. 

Please note that the alternative conjugation1 shown in the structure below Is wrong. The structure with two adjacent doubte bonds in a six-membered ring is Impossible and, in any case, as you saw in Chapter 8, the lone pair electrons on nitrogen are in an sp2 orbital orthogonal to the p orbitals in the ring. There is no interaction between orthogonal orbitals. 

Covalent Solids. Interatomic potentials are the most difficult to derive for covalent solids. The potential must predict the directional nature to the bonding (i.e. the bond angles). Most covalent solids have rather open crystal stmctures, not close packed ones. Pair potentials used with diatomic molecules, such as the Lennard-Jones and Morse potentials, are simply not adequate for solids because atoms interacting via only radial forces prefer to have as many neighbors as possible. Hence, qualitatively wrong covalent crystal stmctures are predicted. 

Oxirene (75) is the other possible cyclic isomer of C2OH2. It involves the interaction of an oxygen 71-lone pair with a carbon-carbon double bond. However, if the molecule has C20 symmetry, this is an unfavorable interaction since delocalization from O to C=C cannot take place, the vacant n orbital of C=C having the wrong symmetry a ). This situation is similar to cyclopropene where we have already noted that hyperconjugation is inhibited for the same reason. In the STO-3G structure, we have maintained C21) symmetry and the lack of ji-electron delocalization is shown up in the longC—0 bonds (1.491 A compared with 1.433 A in methanol). The C—C bond is somewhat shorter than in cyclopropene. 

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