Electronic structure of haloalkanes: a high resolution photoelectron spectroscopic study

High resolution He(I) photoelectron spectra (PES) of a number of bromo- and iodoalkanes have been measured. Linear relationships between ionization energies (E_i) and 1/n (where n is the number of carbon atoms in the chain) describe the trends reasonably well if n = 1 is excluded for dihaloalkanes. This property (i.e. line slopes) when joined the alternation of halogen lone pair bandwidths within a given spin-orbit doublet, can be used to provide a fully empirical assignment of the four non-bonding orbital ionizations of dihaloalkanes. The interactions between the halogen lone pair orbitals and the alkane σ orbitals dominate in dihaloalkanes for which n≥2, while through-space interaction is important only in dihalomethanes (n = 1). Vibrational fine structures were used to infer the existence of hyperconjugative effects.