CASPT2 molecular geometries of Fe(II) spin-crossover complexes.

Using fully internally contracted (FIC)-CASPT2 analytical gradients, geometry optimizations of spin-crossover complexes are reported. This approach is tested on a series of Fe(II) complexes with different sizes, ranging from 13 to 61 atoms. A combination of active space and basis set choices are employed to investigate their role in determining reliable molecular geometries. The reported strategy demonstrates that a wave function-based level of theory can be used to optimize the geometries of metal complexes in reasonable times and enables one to treat the molecular geometry and electronic structure of the complexes using the same level of theory. For a series of smaller Fe(II) SCO complexes, strong field ligands in the LS state result in geometries with the largest differences between DFT and CASPT2; however, good agreement overall is observed between DFT and CASPT2. For the larger complexes, moderate sized basis sets yield geometries that compare well with DFT and available experimental data. We recommend using the (10e,12o) active space since convergence to a minimum structure was more efficient than with truncated active spaces despite having similar Fe-ligand bond distances.

Orbital energy mismatch engenders high-spin ground states in heterobimetallic complexes.

The spin state in heterobimetallic complexes heavily influences both reactivity and magnetism. Exerting control over spin states in main group-based heterobimetallics requires a different approach as the orbital interactions can differ substantially from that of classic coordination complexes. By deliberately engendering an energetic mismatch within the two metals in a bimetallic complex we can mimic the electronic structure of lanthanides. Towards this end, we report a new family of complexes, [Ph,MeTpMSnPh3] where M = Mn (3), Fe (4), Co (5), Ni (6), Zn (7), featuring unsupported bonding between a transition metal and Sn which represent an unusual high spin electronic structure. Analysis of the frontier orbitals reveal the desired orbital mismatch with Sn 5s/5p primarily interacting with 4s/4p M orbitals yielding localized, non-bonding d orbitals. This approach offers a mechanism to design and control spin states in bimetallic complexes.

Beyond chemical accuracy in the heavy p-block: The first ionization potentials and electron affinities of Ga-Kr, In-Xe, and Tl-Rn.

A relativistic coupled-cluster version of the Feller-Peterson-Dixon composite method has been used to accurately calculate the first ionization potentials (IPs) and electron affinities (EAs) of the post-d, p-block elements Ga-Rn. Complete basis set extrapolations including outer-core correlation at the CCSD(T) level of theory were combined with contributions from higher order electron correlation up to CCSDTQ, quantum electrodynamic effects (Lamb shift), and spin-orbit (SO) coupling including the Gaunt contribution. Several methods for including SO were investigated, in which all involved the four-component (4c) Dirac-Coulomb (DC) Hamiltonian. The treatment of SO coupling was the contribution that limited the final accuracy of the present results. In the cases where 4c-DC-CCSD(T) could be reliably used for the SO contributions, the final composite IPs and EAs agreed with the available experimental values to within an unsigned average error of just 0.16 and 0.20 kcal/mol, respectively. In all cases, the final IPs and EAs were within 1 kcal/mol of the available experimental values, except for the EAs of the group 13 elements (Ga, In, and Tl), where the currently accepted experimental values appear to be too large by as much as 4 kcal/mol. The values predicted in this work, which have estimated uncertainties of ±0.5 kcal/mol, are 5.25 (Ga), 7.69 (In), and 7.39 (Tl) kcal/mol. For the EAs of Po and At, which do not have experimental values, the current calculations predict values of 34.2 and 55.8 kcal/mol with estimated uncertainties of ±0.6 and ±0.3 kcal/mol, respectively.

Intramolecular hydrogen bonding in malonaldehyde and its radical analogues.

High level Brueckner doubles with triples correction method-based ab initio calculations have been used to investigate the nature of intramolecular hydrogen bonding and intramolecular hydrogen atom transfer in cis-malonaldehyde (MA) and its radical analogues. The radicals considered here are the ones that correspond to the homolytic cleavage of C-H bonds in cis-MA. The results suggest that cis-MA and its radical analogues, cis-MARS, and cis-MARA, both exist in planar geometry. The calculated intramolecular O-H⋯O=C bond in cis-MA is shorter than that in the radical analogues. The intramolecular hydrogen bond in cis-MA is stronger than in its radicals by at least 3.0 kcal/mol. The stability of a cis-malonaldehyde radical correlates with the extent of electron spin delocalization; cis-MARA, in which the radical spin is more delocalized, is the most stable MA radical, whereas cis-MARS, in which the radical spin is strongly localized, is the least stable radical. The natural bond orbital analysis indicates that the intramolecular hydrogen bonding (O⋯H⋯O) in cis-malonaldehyde radicals is stabilized by the interaction between the lone pair orbitals of donor oxygen and the σ* orbital of acceptor O-H bond (n → σ*OH). The calculated barriers indicate that the intramolecular proton transfer in cis-MA involves 2.2 kcal/mol lower barrier than that in cis-MARS.

Spectroscopic characterization of the ethyl radical-water complex.

An ab initio investigation has been employed to determine the structural and spectroscopic parameters, such as rotational constants, vibrational frequencies, vertical excitation energies, and the stability of the ethyl-water complex. The ethyl-water complex has a binding energy of 1.15 kcal⋅mol-1. The interaction takes place between the hydrogen of water and the unpaired electron of the radical. This interaction is found to produce a red shift in the OH stretching bands of water of ca. 84 cm-1, and a shift of all UV absorption bands to higher energies.