Density functional theory study of ultrashort and overlong CC single bondsand the lowest nonbonding C center dot center dot center dot C distance

Full conformational analysis on selected organic compounds containing the u ltrashort (bent and unbent) or overlong CC single bond or the shortest C... C nonbonding distance between saturated, tetravalent carbon atoms, reported in the literature so far, has been performed by means of the density funct ional theory (DFT) model B3LYP/6-31G(d,p). The patterns of the theoretical structural parameters are in excellent agreement with the X-ray diffraction data. The nature of these extraordinary bonds has also been rationalized w ith the composition of the atomic hybrid orbitals of the involved carbon at oms. Such unique structural features lead to unusual C-13 NMR spectroscopic properties, namely, chemical shifts and indirect nuclear spin-spin couplin g constants. The theoretical values of these observables, calculated by DFT methodologies, are fully consistent with available experimental evidence a nd expectation. In particular, theory predicts an unusually large value of (1)J(CC) for the ultrashort unbent bond in the bipolyhedron systems, a norm al cyclopropanic positive value of (1)J(CC) for the ultrashort bent bond in the tricyclo[2.1.0.0(2.5)]pentane derivative, a reduced alkanic value of ( 1)J(CC) for the overlong bond in the cyclobutaarene derivatives, and a larg e negative value of (2)J(CC) for the lowest nonbonding C...C interaction in the quaternary 1-bilyclo[1.1.1]pentane salt.