A new variant of the so-called "cheap" composite scheme has been purposely developed for the evaluation of the interaction energy of noncovalent molecular complexes, with its various contributions being tested for a set of 15 systems using the accurate interaction energies reported as reference values in the following: [R ̌ezać, J. et al. Phys. Chem. Chem. Phys. 2015, 17, 19268−19277]. The modified scheme, starting from the CCSD(T) method in conjunction with a triple-ζ-quality basis set augmented by diffuse functions, includes two additional terms computed at the MP2 level: (i) the extrapolation to the complete basis set (CBS) limit and (ii) the contribution of core−valence correlation effects (CV term). Various families of basis sets including diffuse functions have been tested for the CCSD(T) model as well as for the extrapolation to the CBS limit, with a mean absolute error of about 1% (below 0.2 kJ•mol −1 in absolute terms) obtained with the jun-cc-pVnZ and the jul-cc-pVnZ families. As far as the CV term is concerned, the cc-pCVTZ and cc-pwCVTZ basis sets provide comparable contributions, which are non-negligible in several cases. While the benchmark analysis has been carried out using accurate structures available in the literature, geometrical effects due to the use of reference B2PLYP(-D3BJ) geometries, optimized in conjunction with a triple-ζ-quality basis set, have been investigated, thus pointing out their suitability. Finally, the modified scheme has been applied to a number of test cases for which interaction energies were already available in the literature; among these, a number of molecular complexes bearing second-row atoms have been considered.

A recently developed model chemistry (denoted as junChS [Alessandrini, S.; et al. J. Chem. Theory Comput. 2020, 16, 988–1006]) has been extended to the employment of explicitly correlated (F12) methods. This led us to propose a family of effective, reliable, and parameter-free schemes for the computation of accurate interaction energies of molecular complexes ruled by noncovalent interactions. A thorough benchmark based on a wide range of interactions showed that the so-called junChS-F12 model, which employs cost-effective revDSD-PBEP86-D3­(BJ) reference geometries, has an improved performance with respect to its conventional counterpart and outperforms well-known model chemistries. Without employing any empirical parameter and at an affordable computational cost, junChS-F12 reaches subchemical accuracy. Accurate characterizations of molecular complexes are usually limited to energetics. To take a step forward, the conventional and F12 composite schemes developed for interaction energies have been extended to structural determinations. A benchmark study demonstrated that the most effective option is to add MP2-F12 core–valence correlation corrections to fc-CCSD­(T)-F12/jun-cc-pVTZ geometries without the need of recovering the basis set superposition error and the extrapolation to the complete basis set.

No abstract

By combining rotational spectroscopyinsupersonic expansion with the capability of state-of-the-art quantumchemical computations in accurately determining structural and energetic properties,the genuine nature of as ulfur-sulfur chalcogen bond between dimethyl sulfide and sulfur dioxide has been unveiled in agas-jet environment free from collision, solvent and matrix perturbations.ASAPT analysis pointed out that electrostatic S···S interactions play the dominant role in determining the stability of the complex, largely overcoming dispersion and CÀH···O hydrogen-bond contributions.Indeed, in agreement with the analysis of the quadrupole-coupling constants and of the methyl internal rotation barrier,the NBO and NOCV/CD approaches show am arked charge transfer between the sulfur atoms.B ased on the assignment of the rotational spectra for 7i sotopologues,a na ccurate semiexperimental equilibrium structure for the heavy-atom backbone of the molecular complex has been determined, whichis characterized by aS ···S distance (2.947(3) )w ell belowt he sum of van der Waals radii.

We present the first detection of (Z)-1,2-ethenediol, (CHOH)2, the enol form of glycolaldehyde, in the interstellar medium toward the G+0.693−0.027 molecular cloud located in the Galactic Center. We have derived a column density of (1.8 ± 0.1) × 1013 cm−2, which translates into a molecular abundance with respect to molecular hydrogen of 1.3 × 10−10. The abundance ratio between glycolaldehyde and (Z)-1,2-ethenediol is ∼5.2. We discuss several viable formation routes through chemical reactions from precursors such as HCO, H2CO, CHOH, or CH2CHOH. We also propose that this species might be an important precursor in the formation of glyceraldehyde (HOCH2CHOHCHO) in the interstellar medium through combination with the hydroxymethylene (CHOH) radical.

The accuracy of rotational parameters obtained from high-level quantum-chemical calculations is discussed for molecules containing second-row atoms. The main focus is on computed rotational constants for which two statistical analyses have been carried out. A first benchmark study concerns sulfur-bearing species and involves 15 molecules (for a total of 74 isotopologues). By comparing 15 different computational approaches, all of them based on the coupled-cluster singles and doubles approach (CCSD) augmented by a perturbative treatment of triple excitations, CCSD(T), we have analyzed the effects on computed rotational constants due to ( i) extrapolation to the complete basis-set limit, ( ii) correlation of core electrons, and ( iii) vibrational corrections to rotational constants. To extend the analysis to other molecules containing second-row elements, as well as to understand the effect of higher excitations, a second benchmark study involving 11 molecules (for a total of 54 isotopologues) has been performed. Finally, the rotational parameters of seven sulfur-containing molecules of astrochemical interest (CCS, CS, CS, CS, HCCS, HCS, and HOCS/HSCO) have been computed and compared to experimental data, when available, also addressing the direct comparison of simulated and experimental rotational spectra.

Polycyclic aromatic hydrocarbons (PAHs) and polycyclic aromatic nitrogen heterocycles (PANHs) are important and ubiquitous species in space. However, their accurate structural and spectroscopic characterization is often missing. To fill this...

State-of-the-art quantum-chemical computations have been employed to determine the accurate equilibrium structure of formamide and its symmetric dimer as well as the interaction energy of the latter, thus extending available reference data for the peptide (also denoted as amide) bond and the hydrogen-bond interaction that characterizes peptides and proteins. Equilibrium geometries and electronic energies have been evaluated by means of a composite scheme based on coupled-cluster calculations, including up to triple excitations, which also accounts for extrapolation to the complete basis set limit and core-correlation effects. This approach provides molecular structures with an accuracy of 0.001-0.002 Å and 0.05-0.1° for bond lengths and angles, respectively, and relative energies with an accuracy of about 1-2 kJ/mol.