Abstract: It is shown that the pseudo Jahn?Teller effect (PJTE) in combination with ab initio calculations explains the origin of instability of the planar configuration of tetrafluorocyclobutadiene, C4F4, with respect to a puckered structure and square-to-rectangle distortion of the carbon ring, and rationalizes its difference from the planar-rectangular geometry of C4H4 and nonplanar (puckered) structure of Si4H4. The two types of instability and distortion of the high-symmetry D4h configuration in these systems emerge from the PJT coupling of the ground B2g state with the excited A1g term producing instability along the b2g coordinate (elongation of the carbon or silicon square ring), and with the excited Eg term resulting in eg (puckering) distortion. A rhombic distortion b1g of the ring is also possible due to the coupling between excited A1g and B1g terms. For C4F4, ab initio calculations of the energy profiles allowed us to evaluate the PJTE constants and to show that the two instabilities, square-to-tetragonal b2g and puckering eg coexist, thus explaining the origin of the observed geometry of this system in the ground state. The preferred cis?trans (eg type) puckering in C4F4 versus trans?trans puckering (b2u distortion) in Si4H4 follows from the differences in the energy gaps to their excited electronic Eg and A1u terms causing different PJTE in these two cases.

Autoría

Descargar referencia