Accurate structure, thermodynamics, and spectroscopy of medium-sized radicals by hybrid coupled cluster/density functional theory approaches: The case of phenyl radical

Abstract

The coupled-cluster singles doubles model with perturbative treatment of triples (CCSD(T)) coupled with extrapolation to the complete basis-set limit and additive approaches represent the "golden standard" for the structural and spectroscopic characterization of building blocks of biomolecules and nanosystems. However, when open-shell systems are considered, additional problems related to both specific computational difficulties and the need of obtaining spin-dependent properties appear. In this contribution, we present a comprehensive study of the molecular structure and spectroscopic (IR, Raman, EPR) properties of the phenyl radical with the aim of validating an accurate computational protocol able to deal with conjugated open-shell species. We succeeded in obtaining reliable and accurate results, thus confirming and, partly, extending the available experimental data. The main issue to be pointed out is the need of going beyond the CCSD(T) level by including a full treatment of triple excitations in order to fulfil the accuracy requirements. On the other hand, the reliability of density functional theory in properly treating open-shell systems has been further confirmed.

Dispersion corrected DFT approaches for anharmonic vibrational frequency calculations: nucleobases and their dimers

“…The B3LYP/N07D method has been employed to calculate anharmonic frequencies of several closed-and open-shell molecular systems 43,90,117,[125][126][127][128][129][130][131] , and a recent extension of N07D, the SNSD basis set, with the inclusion of diffuse s functions on all atoms and one set of diffuse polarized functions, d on heavy and p on hydrogen atoms, has improved its performance (Double and triple-ζ basis sets of SNS and N07 families, are available for download. 2012; visit http:// dreamslab.sns.it) 132,133 .…”

Radicalicity: A scale to compare reactivities of radicals

“…The significance of the large spin contamination for calculation of thermodynamic properties using the G3MP2B3 composite method is not known. However, we note that f H • (C 6 H 5 (g)) at T = 0 K has been obtained from a very thorough high level calculation [15] and from a G3MP2B3 calculation [10]. The difference between the two results is 0.7%.…”

“…The temperature and pressure are 298. 15 constructed using radii from the UFF force field. The dielectric constant for the water solvent was 78.3 and the electrostatic scaling factor for the radius of the atoms was 1.1.…”

An Accurate Molecular Structure of Phenyl, the Simplest Aryl Radical

“…[4] As the simplest aryl radical, it also serves as the benchmark for computational investigations of larger, open-shell ring molecules. [5] The reactivity of aryl radicals is due to the localization of the unpaired electron in a s-type orbital at the CÀH cleavage site, as indicated by the very high CÀH bond dissociation energy of benzene (465 AE 3 kJ mol À1 ). [6] Despite the fundamental importance of C 6 H 5 C, its structure has never been experimentally determined.…”

“…Owing to effects such as electron correlation and spin contamination, s-type aromatic radicals pose one of the more difficult challenges for quantum chemistry. [10] Because phenyl is an asymmetric rotor possessing a nonzero dipole moment (0.87 D) [5] along its intermediate principal (b) inertial axis, and its A and B rotational constants are similar in magnitude, its rotational spectrum is very close to that of an oblate symmetric top. Essentially the full gamut of theoretical approaches has been employed to predict the structure and properties of C 6 H 5 C, including Hartree-Fock, Møller-Plesset perturbation theory (MP2), CASSCF, coupled cluster, and density functional methods.…”