Multicon
figurational CASSCF and CASPT2 calculations were per
formed to investigate the enol
![](/images/entities/rarr.gi<font color=)
f"> ketotautomerization in the lowest singlet excited state o
f the 7-hydroxyquinoline·(NH
3)
3 cluster. Two di
fferentreaction mechanisms were explored. The
first one corresponds to that proposed previously by Tanner et al.(
Science 2003,
302, 1736) on the basis o
f experimental observations and CASSCF optimizations under
Cs-symmetry constraints. This mechanism comprises
four consecutive steps and involves nonadiabatic transitionsbetween the valence
1![](/images/gi<font color=)
fchars/pi.gi
f" BORDER=0 >
![](/images/gi<font color=)
fchars/pi.gi
f" BORDER=0 >* state and a
![](/images/gi<font color=)
fchars/pi.gi
f" BORDER=0 >
![](/images/gi<font color=)
fchars/sigma.gi
f" BORDER=0 >* Rydberg-type state, resulting in hydrogen-atom trans
fer. Single-point CASPT2 calculations corroborate that
for
Cs-symmetry pathways hydrogen-atom trans
fer is clearlypre
ferred over proton trans
fer. The second mechanism, predicted by CASSCF optimizations without constraints,implies proton trans
fer along a pathway on the
1![](/images/gi<font color=)
fchars/pi.gi
f" BORDER=0 >
![](/images/gi<font color=)
fchars/pi.gi
f" BORDER=0 >* sur
face in which one or more ammonia moleculesdepart signi
ficantly
from the molecular plane de
fined by the hydroxyquinoline ring. The results suggest thatboth mechanisms may be competitive with proton trans
fer being somewhat
favorable over hydrogen-atomtrans
fer.