The nature of bonding in the halogens: Hyperfine spectroscopy of iodine monochloride and bromine, and the separated atom theor

The hyperfine constants of a molecule depend on the details of the electronic motion and the electronic and nuclear charge distribution. Thus, by measuring the hyperfine constants, it is possible to directly measure the electronic wavefunction. In this work I present new measurements of the hyperfine constants of the A electronic state of iodine monochloride (ICl) and the B state of molecular bromine (Br$sb2$). The range of measurements for these two molecular states is herein extended to cover almost every vibrational level of the ICl A state between 7 and 34 (the highest $nu$ level of this state is 35). The range of Br$sb2$ B state measurements now extends from $nu$ = 6 to $nu$ = 41.;The vibrational dependence of the ICl hyperfine constants is used to determine the dependence on internuclear separation, R. A theoretical model based on the Separated Atom (SA, also known as Valence Bond) model is developed. This work extends previous versions of this model to include ion-pair states, which are necessary to describe the change in the A state dipole moment. General expressions are derived for the magnetic hyperfine constants for the $sp3Psb2, sp3Psb1,$ and $sp1Dsb2$ states of a positive halogen ion, The magnetic hyperfine constants are calculated specifically for the case of an I$sp+$ ion. The eight-basis-state SA model is shown to reproduce the observed hyperfine constants almost exactly over the range of internuclear separation from 6.0A to 2.8A.;The SA model can be used to understand the orientation of the bonding orbitals of the halogen atoms. This allows some insight into the mechanism of bond formation. Comparison of the SA and LCAO/MO models shows that the MO model appears to reproduce the equilibrium values of the quadrupole constants well for homonuclear halogen molecules, but not for heteronuclear ones. Some weaknesses of the MO model that lead to this failure are identified. Comparison of the radial variation of the quadrupole constants for different interhalogen molecules shows a strong trend for heteronuclear molecules, but only a slight variation in the homonuclear case.