General and theoretical aspects

Abstract

Orbitals . 1. Slater-type atomic orbitals 2. Gaussian lobe functions representation G. Other than LCAO Methods . H. Conclusion . IV. REFERENCES . . . F. LCAO Self-consistent Field (Hartrec-Fock) Molecular . 44 45 45 47 48 48 50 50 52 52 c For discussion of the elFcctive radius of Ns7 in crystals see scction 1I.A.f For discussion of the chargc-transfer-to-solvent spectrum of h'c scc scction 1I.G. 9 The electroncgativity of N, is bctwecn that of Br a n d I (2.5). Thc h~lulliken scale is used herea. h En = E" + 2.60, where Eo is the clcctrodc potential; scc J. 0. Edwards, J. Am. Chcnr. SOL, 76, 1540 (1954). 4A. Treinin also capable of conjugation through their non-bonding electrons and thus can act as electron-donors, i.e. they exert a positive resonance efect ( -I-R). When this m-interaction is strong the pseudohalide group may become colinear with the atom to which it is bound6. This resonance effect is responsible for the effect of the azido group on a benzene ring, its ortho-para orienting character and its activating influence on electrophilic substitutbn. A significant difference between halides and pseudohalides lies in the 'unsaturation' of the latter; they contain low-lying, unfilled 7r orbitals which can accept electrons. This puts them higher than halides in the spectrochemical seriesfo.The reducing power of the halide and pseudohalide ions increases in the following orderff: F-, NCO-, C1-, Ng, Br-, SCN-, I-. In this and other respects the azido group shows close resemb!ance to bromine ( Table 1). The data recorded in Table 1 describe the behaviour of the azido group in its equilibrium nuclear configuration. However the main interest in this group lies in those properties which depend on changes in its geometry, its bending to form cyclic compounds7 and its dissociation to -N +N,. Obviously this has no parallel in the halogen series. B. Electronic Structure (Simplified Model)The general shape of covalent azides is shown in Figure 1. X I t L FIGURE I . The geometry of covalent azidcs (L is the atom to which N, is bound; all the nuclei lie in the xz plane).As late as 1944 azides were considered to have the classical structure RN=NdV, namely with pentavalent nitrogen ,l. This picture has been completely abandoned as it is now generally accepted that nitro-1. General and theoretical aspects 5 gen obeys the octet rule. keeping with the octet rule and the ' adjacent charge rule' 22 : One can write two canonical structures in Resonance of I and 11, with equal contributions, leads to bond order 1.5 and 2-5 for the bonds N,-N, and Nb-N, respectively, in agreement with the values derived from force constants (section 1I.E). The formal charges on N,, Nb and N, corresponding to the resonance hybrid 1-11 are -0.5, + 1 and -0-5 respectively. In both structures * For a recent discussion of hybridization and valence states of nitrogen see reference 27. General and theoretical aspects* Not to be confused with 'self-consistcnt-ficld'. t In this case the total energy of the T system is smaller than the sum of the orbital en...

Binding stoichiometry of a fluorescent cGMP analogue to membranes of retinal rod outer segments

“…The difference between 8-azido-and 8-bromoanalogues may be explained by the observation that the greater and more spheric bromo-favours the syn conformation of the nucleotide, whereas the smaller azido-group, with three nitrogen atoms in a linear array, is less likely to do so [19,201. For 8-thiobenzyl-and 8-(5-thioacetamidofluorescein)-analogues both size and hydrophobicity should be considered.…”