THE (A)OVER-TILDE-(1)A(U) STATE AND THE T-2 POTENTIAL SURFACE OF ACETYLENE - IMPLICATIONS FOR TRIPLET PERTURBATIONS IN THE FLUORESCENCE-SPECTRA OF THE (A)OVER-TILDE STATE

The cis-trans isomerization reaction on the T-2 surface of acetylene a nd the lowest excited singlet state of acetylene, (A) over tilde (1)A( u), are investigated by ab initio electronic structure theory. We repo rt optimized geometries, dipole moments, and harmonic vibrational freq uencies of stationary points and adiabatic energy differences between them using basis sets as large as triple-zeta plus double polarization with higher angular momentum functions, TZ(2df,2pd), and theoretical methods up to coupled-cluster singles and doubles with a perturbative triples correction [CCSD(T)] and the equation-of-motion coupled-cluste r method (EOM-CCSD). Our theoretical predictions should aid the interp retation of observations from a series of recent spectroscopic studies involving excited electronic states of acetylene. In particular, the present theoretical results rule out several possible explanations for the anomalous sudden increase in detectable Zeeman anticrossings repo rted by Dupre et al. [Chem. Phys. 152, 293 (1991)]. (C) 1996 American Institute of Physics.