Rhombic C4. Does it contain the shortest nonbonding C-C distance ?

• Published in: The Journal of chemical physics 1991-06, Vol.94 (12), p.8105-8111
• Main Authors: LAMMERTSMA, K, GÜNER, O. F, SUDHAKAR, P. V
• Format: Article
• Language: English
• Subjects: Atomic and molecular physics; Electron correlation calculations for atoms and molecules; Electronic structure of atoms, molecules and their ions: theory; Exact sciences and technology; Physics
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.460093

In this ab initio molecular orbital study the bonding properties between the inverted carbons in rhombic C4 and [1.1.1]propellane are discussed. Rhombic C4, its protonated forms C4H+ and C4H2+2, and its linear isomer have been studied with the 6-311G(2d,p) basis set and all electron correlation using Mo/ller–Plesset perturbation theory at second order. The rhombic form of C4 is estimated to be essentially isoenergetic with the linear form; the estimated harmonic vibrational frequencies for both are compared with experimental data. The molecular orbital and topological one-electron density analyses show that the cross-ring bond of rhombic C4 is topologically unstable (at various levels of theory) which suggests a frozen transition for C–C bond making/breaking. At MP4(SDTQ) this cross-ring distance is only 1.546 Å. The stabilizing properties of rhombic C4 (and C4H+, and C4H2+ ) are compared with those of the well known [1.1.1]propellane, which at MP2/6-31G* has a longer cross-ring bond.