Excited singlet electronic states of acetylene:cisandtransstructures and energetics

摘要

Molecular electronic structure theory has been used to predict the equilibrium geometries and energies of acetylene in its excited singlet electronic states. A double zeta plus polarization basis set of contracted Gaussian functions was used in conjunction with selfhyphen;consistent field and large scale configuration interaction wave functions. The first excited singlet state of acetylene is thetrans1Austate, in agreement with the experimental studies of King, Ingold, and Innes. This result is particularly interesting because the lowest triplet state of C2H2is not the3Austate but rather thecis3B2state. The predicted geometry of the quest;1Austate isre(CC)=1.384 Aring;,re(CH)=1.096 Aring;, Vthgr;e(HCC)=121.7thinsp;deg;, in good agreement with available spectroscopic data. The predicted relative energies of the excited singlet states are 5.06 eV (1Au), 5.54 eV (cis1A2), 6.87 eV (1B2), and 7.29 eV (1Bu). Thus the energetic ordering of the singlet states isAuA2B2Bu, completely different from that predicted for the analogous tripletsB2BuAuA2. Electronic structures are discussed in terms of Mulliken populations and dipole moments predicted for the twocisexcited singlet states.