E. Orti et al., AB-INITIO DETERMINATION OF THE GEOMETRIC STRUCTURE AND INTERNAL-ROTATION POTENTIAL OF 2,2'-BITHIOPHENE, Journal of physical chemistry, 99(14), 1995, pp. 4955-4963
We report a detailed ab initio study of the molecular structure and co
nformational behavior of 2,2'-bithiophene. Fully optimized torsional p
otentials keeping planar the thiophene rings; are calculated at the HF
/3-21G, HF/6-31G*, and MP2/6-31G* computational levels. The optimized
geometries are analyzed in terms of conjugative effects and nonbondin
g interactions and are compared with gas-phase and solid-state experim
ental data. The reliability of a recent electron diffraction determina
tion of the molecular structure of 2,2'-bithiophene is discussed in li
ght of MP2 calculations, which provide a more delocalized structure th
an HF calculations. Very flat 4-fold potentials where minima correspon
d to s-cis- and s-trans-gauche structures are obtained at both the HF
and MP2 levels. The flatness of the potentials justifies the variety o
f conformations experimentally observed for thiophene oligomers. A tor
sional angle of about 147 degrees is predicted at the HF level for the
most stable s-trans-gauche conformer in agreement with electron diffr
action data. The inclusion of the electron correlation at the MP2 leve
l comparatively destabilizes the planar s-trans conformer and reduces
the torsional angle for the s-trans-gauche minimum to 142.2 degrees. A
dditional MP2 calculations using the 6-31G* basis set affect the rela
tive conformational energies by less than 0.2 kJ/mol. The torsional po
tentials are finally analyzed in terms of a Fourier decomposition trun
cated to the sixth term.