Computational chemical studies on thermochemistry and ring strains in cyclic [ n ]Metaphenyleneacetylenes, butadiyne-bridged [4_n] Metacyclophynes, and butadiyne-bridged [4_n]paracyclophynes

The thermochemical properties and ring strains in cyclic [n]metaphenyleneacetylenes ([n]CMPAs), butadiyne-bridged [4_n] metacyclophynes (B-B[4_n]MCs), and butadiyne-bridged [4 _n]paracyclophynes (B-B[4_n]PCs) were studied using a homodesmotic reaction scheme coupled with density functional theory (B3LYP/6-31G, mPW1PW91/6-31G, and M06-2X/6-31+G//B3LYP/6-31G). Strain energies of [n]CMPAs and B-B[4_n]MCs decrease first from very high values for small rings to become zero when n becomes 6, then increase with n, and finally decrease as n becomes larger than 8. In the case of B-B[4_n]PCs, strain energies decrease with increasing n. Heats of formation of [n]CMPAs, B-B[4_n]MCs, and B-B[4_n]PCs increase steadily with increasing numbers of phenylacetylene and 1-(buta-1,3-diynyl)benzene to reach a near-constant value per unit monomer as n increases. The geometries and (vibrational and nuclear magnetic resonance) spectroscopic properties of [n]CMPAs, B-B[4_n]MCs, and B-B[4_n]PCs were also studied. Geometrical parameters, Raman frequencies, and ¹H NMR chemical shifts of [3]CMPA and [4]CMPA are found to be in good agreement with compounds for which there are experimentally available values at the B3LYP/6-31G* level of theory. In addition, electronic structure calculations were carried out for [n]CMPAs, B-B[4_n]MCs, and B-B[4_n]PCs. Ring diameters were also calculated for B-B[4_n]PCs.