Vol.6, No 1, 2008 pp. 77 - 88
DOI 10.2298/FUPCT0801077I
UDC 539.196
DISSOCIATIVE ELECTRON ATTACHMENT IN ALCOHOLS
AND ETHERS: ITS RELATIOSHIP TO RYDBERG STATES
Bogdan C. Ibanescu, Michael Allan
Department of Chemistry, University of Fribourg, Chemin du Musée 9,
CH-1700 Fribourg, Switzerland
Abstract. A dramatic difference was observed in the reactivity of
alcohols and ethers toward free electrons. Whereas the lowest core-excited
state of the negative ion - a 2(n,3s2) Feshbach resonance - of the alcohols
readily dissociates by losing a hydrogen atom, ethers show no observable
signal from this resonance. This difference in reactivity has a parallel
in the anomalous shapes and energies of the parent states of the Feshbach
resonances, the 1(n,3s) Rydberg states of the neutral alcohols and ethers.
We explained this anomaly by means of potential surfaces of the alcohols
and ethers calculated using the TD-DFT method as a function of the dissociation
coordinate. The lowest excited state of alcohols was found to be repulsive,
as a consequence of an avoided crossing between the 3s and 3px Rydberg
configurations, whereas a barrier to dissociation was found in the ethers.
Rydberg-valence mixing and avoided crossings are decisive in determining
the shapes of the potential surfaces. It is concluded that the reactivities
of alcohols and ethers toward free electrons are rationalized by assuming
that the potential surfaces of the daughter Feshbach resonances closely
follow those of the parent Rydberg states, i.e., the lowest Feshbach resonance
is repulsive, but a barrier occurs in ethers. The potential surfaces of
both the Rydberg states and the Feshbach resonances thus differ dramatically
from the non-dissociative surface of the grandparent 2(n-1) positive ions,
despite the nominally non-bonding character of the Rydberg electrons.
DISOCIJATIVNI ZAHVAT ELEKTRONA KOD ALKOHOLA
I ETERA: POVEZANOST SA RIDBERGOVIM STANJIMA
Uočena je dramatična razlika kod rektivnosti alkohola i etera u odnosu
na slobodne elektrone. Kod alkohola, najniže stanje negativnog jona sa
pobuđenim atomskim ostatkom - a 2(n,3s2) Feshbach rezonanca – lako disosuje
gubljenjem vodonikovog atoma, dok kod etera nije opažen signal koji potiče
od ove rezonance. Ova razlika u reaktivnosti može se povezati sa anomalnim
oblikom i energijama roditeljskih stanja Feshbach rezonance, 1(n,3s) Ridbergovog
stanja kod neutralnih alkohola i etera. Ova anomalija je objašnjena u
ovom radu pomoću potencijalnih površina alkohola i etera, koje su računate
koristeći TD-DFT metod u funkciji koordinate disocijacije. Dobijeno je
da je najniže stanje alkohola odbojno, kao posledica izbegnutog presecanja
3s i 3px Ridbergove konfiguracije, dok kod etera postoji barijera za disocijaciju.
Mešanje Ridbergovih i valentnih stanja, kao i izbegnuto presecanje, su
odlučujući faktori koji definišu oblik potencijalnih površina. Zaključeno
je da se reaktivnost alkohola i etera u odnosu na slobodne elektrone može
racionalizovati pod pretpostavkom da potencijalne površine potomka Feshbach
resonance blisko prate ponašanje roditeljskog Ridbergovog stanja, t.j.
najniža Feshbach rezonanca je odbojna ali kod etera se javlja barijera.
Potencijalne površine Ridbergovog stanja i Feshbach resonance se tako
dramatično razlikuju od nedisocijativne potencijalne površine praroditeljskog
2(n-1) pozitivnog jona, uprkos nominalno nevezivnom karakteru Ridbergovih
elektrona.
