Electronic excited molecular packing

The strong electronic absorption of alkane radical cations is readily understood in molecular orbital terms. Extending down from the highest occupied molecular orbital (HOMO) is a rather closely packed set of valence molecular orbitals, that are clearly displayed in the photoelectron spectra (PES) of neutral alkanes. The electronic absorption of alkane radical cations is due to transitions (induced by photon absorption) of electrons from such lower-lying molecular orbitals to the semi-occupied molecular orbital (SOMO), which is the highest-occupied molecular orbital in the ground-state ion. By illumination within the (broad and largely unstructured) absorption band of alkane radical cations, electronically excited states of alkane radical cations can thus be created in a quite convenient way. 

Returning to question (c), whether the difference in activity between Mo and W complexes is determined by molecular electronic "design" and/or by molecular packing in the crystal, it seems from the above data that the similarities between the crystal structures and 0m indicate that the explanation for the variations in SHG efficiency must lie in electronic factors. This is reasonable in view of the differences between Ef for the reduction of the respective complexes, the tungsten compounds having reduction potentials between 450 and 500 mV more cathodic than their Mo analogues. It would appear that on photo-excitation of these molecules, electron transfer from the ferrocenyl "donor" to the metal nitrosyl "acceptor" occurs, and the SHG efficiency mirrors the ease or otherwise of this process. 

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