Stacked but not Stuck: Unveiling the Role of π→π* Interactions with the Help of the Benzofuran-Formaldehyde Complex

The 1:1 benzofuran-formaldehyde complex has been chosen as model system for analyzing π→π* interactions in supramolecular organizations involving heteroaromatic rings and carbonyl groups. A joint "rotational spectroscopy-quantum chemistry" strategy unveiled the dominant role of π→π* interactions in tuning the intermolecular interactions of such adduct. The exploration of the intermolecular potential energy surface led to the identification of 14 low-energy minima, with 4 stacked isomers being more stable than those linked by hydrogen bond or lone-pair→π interactions. All energy minima are separated by loose transition states, thus suggesting an effective relaxation to the global minimum under the experimental conditions. This expectation has been confirmed by the experimental detection of only one species, which was unambiguously assigned owing to the computation of accurate spectroscopic parameters and the characterization of 11 isotopologues. The large number of isotopic species opened the way to the determination of the first semi-experimental equilibrium structure for a molecular complex of such a dimension.

Determination of the "Privileged Structure" of 8-Hydroxyquinoline

An accurate semi-experimental equilibrium structure of 8-hydroxyquinoline (8-HQ) has been determined combining experiment and theory. The cm-wave rotational spectrum of 8-HQ was recorded in a pulsed supersonic jet using broadband dual-path reflection and narrowband Fabry-Perot-type resonator Fourier-transform microwave spectrometers. Accurate rotational and quartic centrifugal distortion constants and ¹⁴ N quadrupole coupling constants are determined. Rotational constants of all ¹³ C, ¹⁸ O and ¹⁵ N singly substituted isotopologues in natural abundance and those of a chemically synthesized OD isotopologue were used to obtain geometric parameters for all the heavy atoms and the hydroxyl hydrogen from a number of structure determination models. Theoretical approaches allowed for the determination of a semi-experimental equilibrium structure, r e SE in which computed rovibrational and electronic corrections were utilized to convert vibrational ground state constants into equilibrium constants. Despite the molecule having only a horizontal plane of symmetry and possessing 11 individual heavy atoms, microwave spectroscopy has allowed for a reliable and accurate structure determination. A mass dependent, r m 2 structure was determined and proved to be equally reliable by comparison with the B3LYP-D3(BJ)/aVTZ equilibrium structure.

Basis Set Extrapolations for Density Functional Theory

Basis set extrapolation is a common technique in wavefunction theory used to squeeze better performance out of the highest affordable level of theory by combining it with lower quality calculations. In this work, I present analogous techniques for basis set extrapolation in density functional theory, focusing on [2,3]-ζ calculations and including double hybrid and dispersion-corrected functionals. My recommendations are based on basis set limit data from finite element calculations and estimates of basis set limits for double hybrid corrections, and they are validated using the GMTKN55 and NCDT datasets. A short review of extrapolation methods for Hartree-Fock calculations based on modern finite element data is carried out to inform this work. Extrapolation of [2,3]-ζ calculations in def2-Xzvpd and cc-pvXz-pp basis sets with the proposed recipes routinely matches and sometimes outperforms 4-ζ calculations at a fraction of the cost. The methods are implemented in Psi4, allowing for an automated and intuitive application.