Valence Bond Studies of N2F+

The results of STO-6G valence bond (VB) calculations for the linear cation N2F+ show that (a) the dominant Lewis-type VB structures involve a single positive charge which is located on either of the nitrogen atoms, and (b) a small amount of N–F π-bonding arises primarily from the formation of one-electron N–F π-bonds. It is suggested that N–F π-bonding is one of several factors which are responsible for the shortening of the N–F bond of N2F+ relative to that of NF4+. Valence bond and bond length considerations imply that the extent of N–O π-bonding in N2O and NO43- exceeds the degree of N–F π-bonding in N2F+ and NF4+.

The chemistry of aliphatic unconjugated nitramines. II. Intramolecular properties and crystal packing

The VESCF(BJ)-MO electric dipole moments, molecular ionization potentials, electronic bond energies, charge distributions, and bond orders for nitramide, N-methylnitramine, and s- and as-N,N- dimethylnitramines are reported. The packing of nitramide, RDX, and HNX in their molecular crystals is rationalized in terms of electrostatic and hydrogen-bonding interactions. Simple VB structures do not readily predict their calculated MO charge distributions and bond orders.

The valence-bond theory of molecular structure II. Reformulation of the theory

The construction of spin eigenfunctions and the evaluation of matrix elements between ,them are discussed generally in preparation for a development of the valence bond (VB) theory along the lines indicated in I. The customary approximation of considering explicitly only the electrons outside a ‘closed shell’ is shown to be defensible. The reformulation of the VB theory is now straightforward, but its final description of bonding is quite new. Atomic orbitals (AO’s) are replaced, whenever they appear, by orthogonalized atomic orbitals (AO’s); but when the assumptions of the conventional theory are rigorously validated in this way the ‘covalent’ structures (now ‘VB’ structures) are found, quite generally, to indicate only strong repulsion between the ‘bonded’ atoms, and formal descriptions of bonding and of bond orders, in terms of ‘spin-pairing’, become nonsensical. Bonding can be described only by admitting into the wave functions polar VB structures; a bond between two atoms demands the appearance (with considerable weight) of pairs of structures differing by a ‘charge hop’ between the atoms concerned. The conventional VB structures are found to be equivalent to certain groupings of VB structures (non-polar and polar) and do, indeed, predict bonds between spin-paired atoms and repulsion between the atoms of different pairs. It is then possible to make full use of chemical intuition, using a plausible combination of conventional structures as a starting approximation in the more rigorous theory. A numerical illustration is provided by a discussion of the Kekulé structures of benzene. Some important characteristics of energy calculations in the VB theory are pointed out. Quantities of intra - and inter -atomic origin are well separated, and the method is apparently well suited to development along either ab initio or empirical lines.