p) calculations were employed in order to examine the molecular parameters of the C 2 H 3 XSÁÁÁNH 3 heterocyclic hydrogen-bonded complexes with X = H, F and CH 3 . Intermolecular criteria were taken into account when studying the formation of these hydrogen-bonded complexes, such as geometry analysis, charge density quantification and interpretation of the harmonic vibrational spectrum, in which case the appearance of red-shift and blue-shift effects was discussed. It was assumed from the outset that many hydrogen bond types may exist in these systems, and these were investigated using the results of topological integrations from the quantum theory of atoms in molecules (QTAIM) and intermolecular charge transfer calculations using the ChelpG scheme. The proton donor/acceptor behavior of C 2 H 3 XS was interpreted in terms of hydrogen bond energies, whose values were corrected using the basis sets superposition error (BSSE) and zero point energy (ZPE).
The geometries of three isomers of the C2H4O...2HF tri-molecular heterocyclic hydrogen-bonded complex were examined through B3LYP/aug-cc-pVDZ calculations. Analysis of structural parameters, determination of CHELPG (charge electrostatic potential grid) intermolecular charge transfer, interpretation of infrared stretching modes, and Bader's atoms in molecules (AIM) theory calculations was carried out in order to characterize the hydrogen bonds in each isomer of the C2H4O...2HF complex. The most stable structure was determined through the identification of hydrogen bonds between C2H4O and HF, (O...H), as well as in the hydrofluoric acid dimer, (HF(D-R)...HF(D)). However, the existence of a tertiary interaction (F(lambda)...H(alpha)) between the fluoride of the second hydrofluoric acid and the axial hydrogen atoms of C2H4O was decisive in the identification of the preferred configuration of the C2H4O...2HF system.
Theoretical investigations concerning the formation of hydrogen bonds in both homomeric, (HCN) n , and heteromeric clusters of the type (HCN) n OHF (n ϭ 1, 2, and 3) have been performed through ab initio molecular orbital calculations at the second-order Møller-Plesset (MP2) and density functional theory (DFT)/B3LYP levels, with the 6-311ϩϩG(d,p) basis set. The formation of hydrogen bonds is investigated in terms of changes in structural, electronic, and vibrational parameters of the free species. Important parameters include the increment in the distance of the HF proton donor species, the increment in the HC distance of HCN moiety, and the amount of intermolecular charge transfer between the HCN species in the (HCN) n group. It is interesting to point out the different behavior in the HC distance as HCN acts simultaneously as a proton acceptor, proton donor, and a proton donor and acceptor. Other important results concern the cooperative effect (CE) in terms of the stabilization energy and dipole moment. Both CEs increase with cluster size and are more pronounced for the heteroclusters. The HF stretching frequency is red-shifted on going in the direction (HCN)OHF 3 (HCN) 2 OHF 3 (HCN) 3 OHF. This trend is in
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