Model calculations on spiro-compounds with silicon and phosphorus as central atoms

Abstract

Spiro-compounds with silicon and phosphorus as central atoms have been studied using semiempirical molecular orbital calculations. The inclusion of 3d orbitals causes a considerable increase of the split between the antibonding 6, and a, orbitals.Darmstadt, West Germany THE special interaction (spiro-conjugation) of two mutually perpendicular x systems which are connected by a central carbon atom [e.g. in (1) of Scheme 11 is a topic of wide current interest .l-3 Using perturbation theory it can be shown that this spiro-conjugation is most important in those systems in which the x fragments (A) and (A') of (1) are identical (i.e. D 2 d symmetry). Within these conditions the total representation of x orbitals factorizes into three sets of A,, B,, and E symmetry. Those x orbitals with an even t number of nodal planes split into two symmetry adapted linear combinations of the irreducible representations A, and B,. The x orbitals with an uneven 7 number of nodes remain degenerate and belong to the irreducible representation E. The energy difference of the orbitals a, and b, is due to the spatial interaction between the centres p and v in (1) (Scheme 1). The b, linear combination is bonding (in phase), the a2 linear combination antibonding (out of phase).Recently we have pointed out that the interaction of

Ring strain in cyclopropane, cyclopropene, silacyclopropane, and silacyclopropene

“…A greater strain energy in silacyclopropane is also supported by the relative isomerization energies for the two rings, relative to their most stable acyclic analogues:13 x / \ --XHj-CH=CH2 (4) The 6-31G* 2s for eq 4 are -8.2 and -15.7 kcal/mol for X = C and Si, respectively. The latter value is similar to that obtained by Barthelat and co-workers.5…”

para‐Phenylene‐Bridged Spirobi(triarylamine) Dimer with Four Perpendicularly Linked Redox‐Active π Systems

“…Gleiter and Bçhm pointed out the possibility that the participation of 3d orbitals of the Si atom causes a considerable increase of spiro conjugation. [19] However, quantum chemical calculations predicted that the degeneracy of magnetic orbitals (fragment SOMOs originating from the two TAA subunits) could be maintained for the dication 1 2 + , and the two unpaired electrons could be ferromagnetically coupled. [17] In fact, 2 was a persistent diradical with a spin triplet multiplicity (J intra /k B = 16 K).…”

Helical electronic transitions of spiroconjugated molecules

“…Similar to allene, the parent spiropentadiene has D2d symmetry with four p-electrons. 31 A fairly unstable motif due to bond strain, 32 spiropentadiene has been synthesized with some variation including its sila-and germaanalogues. [33][34][35][36] Shown in the left column of Figure 3a, its HOMO and HOMO-1 are degenerate and each have clear p-character on one side of the molecule.…”