Asymmetric Stretching CI Potentials
Other unusual features include the asymmetric stretch which shows
a minimum energy in the A state where the bond lengths are unequal. Using
a simple one dimensional vibrational model the previous experimental work estimated
the barrier to equal bond lengths to be 284 cm-1 with the
minimum at 0.110 Å, in good agreement with the ab initio values of
195 cm-1 and 0.104 Å. The source of this double minimum
potential is thought to be vibronic mixing with the higher lying 1Πg
state, as this will mix with the A1Πu state via the
Σu+ asymmetric stretch. Indeed the ab initio curves
show the 1Πg having significant anharmonicity, in
addition to the double minimum in the A1Πu state.