LOW-LYING TRIPLET ELECTRONIC STATES OF ACETYLENE - CIS (3)B(2) AND (3)A(2), TRANS (3)B(U) AND (3)A(U)

Ab initio molecular electronic structure theory has been used in conjunction with flexible basis sets to investigate the equilibrium properties of the four low-lying triplet electronic states of acetylene. Self-consistent-field (SCF) and configuration interaction with single and double excitations (CISD) levels of theory were employed with basis sets ranging from double zeta plus polarization (DZP) to quadruple zeta plus triple polarization with higher angular momentum polarization functions [QZ(3df, 3pd)]. Complete geometry optimizations of the equilibrium structures and vibrational analyses for the B-3(2), B-3(u), 3A(u), and 3A2 states as well as the ground 1SIGMA(g)+ state of acetylene were carried out at the SCF and CISD levels of theory. With the DZP basis set, configuration interaction with single, double, and triple excitations (CISDT) wavefunctions were also used to optimize geometries. At the CISD optimized geometries the total energies were determined using the correlated wavefunctions with higher excitations. Those wavefunctions include the triple zeta plus double polarization (TZ2P)CISDT, coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] methods. Although the energy ordering of B-3(2) < B-3(u) < 3A(u) < 3A2 remained unchanged, the excitation energies of these four triplet states relative to the 1SIGMA(g)+ ground state is increased by about 7.5 kcal/mol in comparison with previous theoretical work. At the highest level of theory, CCSD(T) with the QZ(3df, 3pd) basis set, the classical excitation energies of the four triplet states relative to the ground state were predicted to be 88.0(3.82; 30,790), 96.0(4.16; 33,590), 102.4(4.44; 35,830), and 109(4.76; 38,420) kcal/mol(eV; cm-1), respectively. For the first two triplet states, including the zero-point vibrational energies (ZPVE) the energy differences were 86.6(3.75; 30,270) and 94.8(4.11; 33,170) kcal/mol(eV; cm -1), respectively. The classical energy separation between the B-3(2) and 3A2 States was predicted to be 7630 cm-1. Including the estimated ZPVE correction of 50 cm-1 this energy difference became 7680 cm-1, which is in very good agreement with the experimental value of 7388 cm-1. The trans triplet states have never been observed in the laboratory, and it is hoped that these quantitative theoretical predictions will assist in their experimental identification.