Antibonding σ* orbitals in molecular spectroscopy

Abstract

This paper is dedicated to Professor Camille Sandorfy on the occasion of his 65th birthdayM. B. ROBIN. Can. J . Chem. 63, 2032Chem. 63, (1985. This paper discusses the role of a* antibonding valence MO's in the electronic spectra of large molecules. Using the term value and antishielding concepts, molecular absorption patterns terminating at a* MO's are systematized and rationalized; the frequencies of such excitations correlate with the ground-state bond strengths of the bonds giving rise to the a* MO's. In unsaturated systems, the 7~ + a* and np + a* excitations can be distinguished on the basis of their term values. Confusion in the past regarding the existence of excitations to a* relates to the fact that a* MO's often can be totally mixed into the surrounding sea of Rydberg levels of the same symmetry, and for all practical purposes can cease to exist except as perturbations on the Rydberg levels. For nonpenetrating Rydberg manifolds there is little or no mixing with a* MO's of the same symmetry, setting the stage for the appearance of valence-shell excitations to a* in high-symmetry molecules which are analogous to the d + f inner-well giant resonances of atomic spectroscopy.M. B. ROBIN. Can. J. Chem. 63, 2032Chem. 63, (1985. Dans ce travail, on discute du r61e des OM de valence antiliante a* dans les spectres Clectroniques des grosses molCcules. Utilisant les concepts de valeur de terme et d'antiblindage, on systimatise et on rationalise les types d'absorptions molCculaires qui se terminent au niveau des OM a*; il existe une correlation entre les frCquences de telles excitations et les longueurs de liaisons dans I'Ctat atomique fondamental qui donne lieu aux OM a*. Dans les systkmes non-saturCs, on peut distinguer les excitations 7~ + a* des excitations np + a* en se basant sur leurs valeurs de terme. La confusion qui a Ct C observte dans le passe relativement a I'existence d'excitations vers les niveaux a* provient du fait que les OM u* peuvent souvent Ctre complktement confondues avec les niveaux de Rydberg de m&me symCtrie qui les entourent et, a toutes fins pratiques, elles cessent d'exister, except6 comme perturbations des niveaux de Rydberg. Pour les Ctats non-pCnCtrants de Rydberg, il y peu ou pas du tout de mtlange avec les OM a* de mCme symCtrie et ceci permet, dans les molCcules de haute symCtrie, de noter ]'apparition d'excitations des couches de valence vers les Ctats a* qui sont semblables aux rksonances @antes d + f de la spectroscopie atomique.[Traduit par le journal]

The σ* molecular orbitals of perfluoroalkanes as studied by inner-shell electron energy loss and electron transmission spectroscopies

“…In C2H6, this band has been assigned as terminating at a mixed 3s/u* MO with predominantly R character (1,25). The increased term values of the transitions to 3s/u* upon fluorination of a wide variety of systems is well known (1,2). What is more interesting here is that in C2H6 the C 1 s + 3 s/u* transition is relatively weak and seems to be following the quasiatomic propensity rule forbidding 1s + 3s transitions, whereas what appears to be the corresponding transition in C2F6 has more than 10 times the oscillator strength and is the most prominent band in the spectrum!…”

“…valence character completely (1,2). Though valence transitions to u* in the ionization continuum d o appear as shape resonances (3,4) and cannot be confused with transitions to R orbitals, assignment still can b e ambiguous here as well, for other phenomena such as two-electron excitation, shake-up, and EXAFS interference also can contribute structure in this region (5).…”

“…(2) To what extent are u* orbitals stabilized by perfluorination and/or ring strain? ( 3 ) Are the inner-shell and negative-ion excitations terminating at u* in the perfluoroalkanes localized or delocalized?…”

“…In the case where inner-shell excitations to a * and R orbitals are nearly degenerate, term value arguments are less compelling and the question arises as to the separate existence of neardegenerate R and V levels (1,2). Although the answer to this is ambiguous in many cases, clear evidence for the coexistence of distinct V and R levels in the same spectrum has been presented for F,O (6).…”

“…Thus, if a molecule has a bond between atoms A and B which has a strength of 60 kcal/mol or less, then the u-u* splitting will be abnormally small and the a*(A-B) MO will be relatively low lying, as will inner-shell transitions such as A 1 s + u*(A-B) and outershell transitions such as B 2p + u*(A-B). This lowering of the inner-shell and outer-shell energies for excitations to u* has been called the weak-bond effect (2,6).…”