Structure and stability of isomeric C2GeH7+ ions. An ab initio post-SCF study

The structure and stability of isomeric C2GeH7+ ions have been investigated by ab initio calculations, using a split-valence quality basis set (BS). Ten different critical points have been located on the MF2(FULL)/BS potential energy surface, and their relative stability has been evaluated at the QCISD(T) level of theory. Four distinct minima have been identified, corresponding to the C-2 upsilon-symmetry CH3GeHCH3+ ion 7, the bisected and the eclipsed conformations of CH3CH2GeH2+, 1 and 2, almost degenerate and less stable than 7 by ca. 20 kcal mol(-1) and the high-energy ion 9, a hydrogen-bridged structure corresponding to protonated GeH3-CH=CH2, which is less stable than 7 by ca. 50 kcal mol(-1). The C-2 upsilon-symmetry GeH3CHCH3+ ion 8 was characterized as a second-order saddle point, less stable than 7 by ca. 52 kcal mol(-1), whereas the bisected conformations of GeH3CH2CH2+ and CH3GeH2CH2+ were identified as first-order saddle points, less stable than 7 by ca. 55 and 58 kcal mol(-1), respectively. No definitive conclusion could be drawn as to the character (minimum or transition structure) of the cyclic GeH3CH2CH2+ ion. The presently obtained results have been compared with the well-established ones concerning the C3H7+ ions.