Weak metal-arene interactions have been investigated in Zn, Cd, Hg, and Ni complexes of meso-tetraaryl m- and p-benziporphyrin (1 and 2) and of the new compound, m-benziporphodimethene (3). Compounds 1-3 incorporate the phenylene moiety into a macrocyclic structure so as to facilitate the interaction between the arene and coordinated metal ion. X-ray studies performed on Cd(II) and Ni(II) complexes show that the arene fragment approaches the ion at a distance much shorter than the sum of van der Waals radii. In chloronickel(II) m-benziporphyrin, a weak agostic bond is actually formed. In the NMR spectra of the Cd(II) and Hg(II) species, unusual (1)H-M and (13)C-M scalar couplings have been observed that are transmitted directly between the metal and the arene. DFT calculations performed for two Cd(II) species and subsequent AIM analysis show that the accumulation of electron density between the metal and arene necessary to induce these couplings is fairly small and the interaction is steric in nature. In the paramagnetic Ni(II) complexes of 1 and 3, the agostic proton of the m-phenylene exhibits large downfield (1)H NMR shifts (386 and 208 ppm at 298 K, respectively). An agostic mechanism of spin density transfer is proposed to explain these shifts as resulting from electron donation from the CH bond to the metal. In chloronickel(II) p-benziporphyrin, the inner protons of the p-phenylene have a contrastingly small shift (0.0 ppm at 298 K), indicating that in this case the agostic interaction is inefficient, in agreement with the X-ray data.

Vibrational spectra of phenol are calculated with ab initio Hartree-Fock and MP2 methods as well as with density functional theory (DFT) using the 6-31G(d,p) basis set. A clear-cut assignment of the vibrational frequencies is reported on the basis of the potential energy distribution (PED) calculated at the three theory levels. These results are compared with the previously reported ab initio data and with the experiment. Several reassignments are suggested for the phenol modes: OH bend, 9b, 17a, 8a, and 8b. It is demonstrated that the MP2/6-31G(d,p) level fails in predicting the frequencies for two modes, labeled 14 and 4 in phenol. The calculated frequency of the former is about 140 cm -1 too high, and that of the latter is 220 cm -1 too low. Very similar results at the MP2 level have been reported earlier for the corresponding ω 14 and ω 4 in benzene. The HF/6-31G(d,p) method provides incorrect results for the modes related to the OH bend in phenol. It is remarkable that DFT with the BLYP functional gives excellent agreement between the calculated and observed frequencies for phenol. In particular, the modes 4 and 14 are predicted to within 11 and 6 cm -1 , respectively, which confirms the reliability of DFT (BLYP) in reproducing vibrational frequencies.

The complexes formed between cis- and trans-HONO isomers and ammonia have been observed and characterized in argon matrices. Five perturbed HONO vibrations and one perturbed NH3 deformation vibration were identified for the H3N···HONO-trans complex, and one perturbed HONO vibration and perturbed NH3 deformation vibration were identified for the H3N···HONO-cis complex. The OH stretching vibration in the H3N···HONO-trans complex is ca. 800 cm-1 red-shifted and NOH bending vibration is ca. 190 cm-1 blue-shifted with respect to the trans-HONO monomer, indicating formation of a very strong molecular hydrogen bond. Theoretical studies of the structure and spectral characteristics of the H3N···HONO-trans and H3N··· HONO-cis complexes were carried out on the electron correlation level and G-311+G(2df,2pd) basis set. The calculated binding energy at the MP2 level is −40.13 and −36.39 kJ mol-1 for the H3N···HONO-trans and H3N···HONO-cis complexes, respectively. The calculated spectra reproduce very well the frequencies and the intensities of the measured spectra.

Systematic studies of the ability of a broad family of density functional methods applied to hydrogen-bonded complexes have been carried out on the hydrogen fluoride dimer. Specifically, calculations have been performed using basis sets of triple-zeta quality with diffuse functions and multiple sets of polarization functions. Various local and nonlocal exchange-correlation functionals have been applied in order to study the structure, energetics, and vibrational properties of the hydrogen fluoride dimer. The comparison with the experimental data, and also with results coming from ab initio methods (Hartree–Fock, Mo/ller–Plesset second order, and quadratic configuration interaction with the single and double excitations) shows good performance of nonlocal density functional methods for the description of hydrogen-bonded systems. The calculated binding energy, with nonlocal Becke exchange and Lee–Yang–Parr correlation functionals and a 6-311++G(3df,3pd) basis set, is 4.48 kcal/mol and is in good agreement with experimental value and prior calculations.

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