Y. Yamaguchi et al., LOW-LYING TRIPLET ELECTRONIC STATES OF ACETYLENE - CIS (3)B(2) AND (3)A(2), TRANS (3)B(U) AND (3)A(U), Theoretica Chimica Acta, 86(1-2), 1993, pp. 97-113
Ab initio molecular electronic structure theory has been used in conju
nction with flexible basis sets to investigate the equilibrium propert
ies 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 se
ts ranging from double zeta plus polarization (DZP) to quadruple zeta
plus triple polarization with higher angular momentum polarization fun
ctions QZ(3df, 3pd)!. Complete geometry optimizations of the equilibr
ium 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 we
re carried out at the SCF and CISD levels of theory. With the DZP basi
s set, configuration interaction with single, double, and triple excit
ations (CISDT) wavefunctions were also used to optimize geometries. At
the CISD optimized geometries the total energies were determined usin
g the correlated wavefunctions with higher excitations. Those wavefunc
tions include the triple zeta plus double polarization (TZ2P)CISDT, co
upled cluster with single and double excitations (CCSD), and CCSD with
perturbative triple excitations CCSD(T)! methods. Although the energ
y ordering of B-3(2) < B-3(u) < 3A(u) < 3A2 remained unchanged, the ex
citation 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) wi
th the QZ(3df, 3pd) basis set, the classical excitation energies of th
e four triplet states relative to the ground state were predicted to b
e 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 tri
plet 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 th
e estimated ZPVE correction of 50 cm-1 this energy difference became 7
680 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 predi
ctions will assist in their experimental identification.