Herein we present the results of a blind challenge to quantum chemical methods in the calculation of dimerization preferences in the low temperature gas phase. The target of study was the first step of the microsolvation of furan, 2-methylfuran and 2,5-dimethylfuran with methanol. The dimers were investigated through IR spectroscopy of a supersonic jet expansion. From the measured bands, it was possible to identify a persistent hydrogen bonding OH–O motif in the predominant species. From the presence of another band, which can be attributed to an OH-π interaction, we were able to assert that the energy gap between the two types of dimers should be less than or close to 1 kJ/mol across the series. These values served as a first evaluation ruler for the 12 entries featured in the challenge. A tentative stricter evaluation of the challenge results is also carried out, combining theoretical and experimental results in order to define a smaller error bar. The process was carried out in a double-blind fashion, with both theory and experimental groups unaware of the results on the other side, with the exception of the 2,5-dimethylfuran system which was featured in an earlier publication.

This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED INProbabilistic performance estimators for computational chemistry methods: Systematic improvement probability and ranking probability matrix. I. Theory

We report on the vibronic structure of the ground state X̃(2)A″ of the thymine cation, which has been measured using a threshold photoelectron photoion coincidence technique and vacuum ultraviolet synchrotron radiation. The threshold photoelectron spectrum, recorded over ∼0.7 eV above the ionization potential (i.e., covering the whole ground state of the cation) shows rich vibrational structure that has been assigned with the help of calculated anharmonic modes of the ground electronic cation state at the PBE0/aug-cc-pVDZ level of theory. The adiabatic ionization energy has been experimentally determined as AIE = 8.913 ± 0.005 eV, in very good agreement with previous high resolution results. The corresponding theoretical value of AIE = 8.917 eV has been calculated in this work with the explicitly correlated method/basis set (R)CCSD(T)-F12/cc-pVTZ-F12, which validates the theoretical approach and benchmarks its accuracy for future studies of medium-sized biological molecules.

In the present work, we investigate the bonding, structures, stability and spectra of the Zn(q+)Im (where q = 0, 1, and 2) complexes, which are zeolitic imidazolate frameworks (ZIFs) and Zn-enzyme sub-units. Through a benchmark work, we used density functional theory (DFT) with dispersion correction and standard and explicitly correlated ab initio methods. For neutral Zn(0)Im, we found two stable weakly bound forms: (i) a stacked ferrocene-like complex and (ii) a planar σ-type complex. This is the first report of the Zn(0) organic compound with a stacked ferrocene-like structure. The most stable isomers of the ionic species consist of σ-type bonded complexes. The role of various types of covalent and noncovalent interactions within these complexes is discussed after performing vibrational, NBO, charge and orbital analyses. For neutral species, van der Waals (vdWs) and charge transfer through covalent as well as noncovalent interactions are in action; whereas the bonding is dominated by charge transfer from Zn to Im within the ionic species. These findings are important to understand, at the microscopic level, the structure and the bonding within the ZIFs and the Zn-enzymes. Moreover, we establish the ability and reliability of M05-2X and PBE0 functionals for the simultaneous correct description of covalent and noncovalent interactions since this DFT leads to a close agreement with post-Hartree-Fock methods. The newly launched M11 functional is also suited for the description of noncovalent interactions. Therefore, M05-2X and PBE0 functionals are recommended for studying the larger complexes formed by Zn and Im, such as the ZIFs and Zn-enzymes.

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