Density functional (M06-L and B3LYP) and multiconfigurational second-order perturbation (CASPT2) theories are applied to [n]-cyclacenes having n ¼ 6-12 in order to assess their strain energies and the degree of di-or polyradical character inherent in their electronic structures. In the case of density functional theory, a broken symmetry approach must be employed and the sensitivity of the results to choice of functional and broken-symmetry protocol is explored. Viewing the [n]-cyclacenes as monomeric building blocks which may be joined to grow finite length (n,0) single-walled carbon nanotubes, density functional calculations are further employed to explore changes in electronic structure associated with increasing nanotube length. Spin-state energy gaps are predicted to decrease both with increasing cyclacene size and with increasing nanotube length; [n]-cyclacenes with odd values of n are predicted to develop polyradical character for smaller n than is the case for even values n.

We have characterized the structural and energetic properties of CH3CN-BCl3via computations and matrix-IR spectroscopy. We find two equilibrium structures of the complex via computations. At the MP2/aug-cc-pVTZ level, the global minimum energy structure has a B-N distance of 1.601 Å, and a binding energy of 12.0 kcal mol(-1). The secondary structure lies 7.1 kcal mol(-1) higher in energy with a B-N distance of 2.687 Å and a binding energy of 4.9 kcal mol(-1). Computational scans of the B-N potential curve using both DFT and post-HF methods indicate that a significant barrier exists between these structures, and that it lies 1 to 2 kcal mol(-1) above the secondary minimum at a B-N distance of about 2.2 Å. We also observed several key, structurally-sensitive IR bands for six isotopic forms of the complex in neon matrices, including: the B-Cl asymmetric stretching band (ν) at 792 cm(-1) and the C-N stretching band (νCN) at 2380 cm(-1) (for the primary isotopomer, CH3C(14)N-(11)BCl3). These frequencies are consistent with computational predictions for the minimum-energy form of the complex. Energy decomposition analyses were conducted for CH3CN-BCl3 and also two related complexes, CH3CN-BF3 and CH3CN-BH3. These provide insight into the trend in Lewis acidity of the BX3 acceptors toward nitriles. Furthermore, these analyses indicate that the barrier along the B-N potential of CH3CN-BCl3 results from Pauli repulsion between the π electrons on the nitrile moiety and the chlorine atoms in BCl3, which is significant at relatively long distances where attractive bonding interactions fail to overcome it.