THE VIBRATIONAL-SPECTRUM OF H2O2-CATION - AN ILLUSTRATION OF SYMMETRY-BREAKING( RADICAL)

The H2O2+. radical cation has been investigated with ab initio MO meth ods using both the single reference and multiconfiguration based wave functions. For the electronic states of two low lying isomers of the t rans-(B-2(g)) and cis-((2)A(2)) forms, the effect of the choice of the reference wave function on the shape of the potential energy surfaces near the equilibrium geometry and also at the dissociation limit (OH. and OH+) was studied. In addition to the standard ab initio methods s uch as MP(n), CISD, CCSD(T), BD(T), and CASSCF, two different density functional methods were also employed. The force constants for the asy mmetrical vibrations evaluated with the MP2 method are very large and consequently the corresponding nu(5) (b(u), or b(2)) calculated freque ncies approach unphysical numbers, which are orders of magnitude large r than the expected. In addition the nu(2) (b(u)) frequency, which cor responds to the trans-conformer in the proper range around 1350 posses ses a huge IR intensity. For the cis-structure even the first asymmetr ical frequency nu(4) (b(2)), is too high (2216-2444 cm(-1)), and has a n unreasonable IR intensity. It is shown that the symmetry breaking of the wave function is fully responsible for this unphysical behavior w hich leads to unreliable predictions of the whole vibrational spectrum . Detailed analyses of the symmetry break in in the calculations of ha rmonic vibrational frequencies are presented. Finally, reliable, vibra tional spectra for both the H2O2+. isomers based on the full valence C ASSCF and BD(T) methods are presented. (C) 1997 American Institute of Physics.