The hydrogen bond (HB) basicity of a series of ylides containing nitrogen, oxygen, or carbon as heavy atoms, as well as the influence of the formation of the HB complexes on their structure, has been studied. In addition, in this paper we propose the formation of some rather strong HBs (that could be considered low-barrier hydrogen bonds, LBHBs) between ylides and different neutral molecules. The ylides chosen for the study wereAs HB donors, classical donors such as HF, HCN, and HCCH were used. The analysis of the protonation energies of the ylides and the optimized geometries, interaction energies, and characteristics of the electron density of the complexes shows that these ylides are very good HB acceptors, forming stable complexes even with weak HB donors. With strong donors, when the proton transfer did not take place, very strong HBs were formed with quite large interaction energies and very short HB distances which could be considered as LBHBs. Moreover, we have found that the sign of the Laplacian of the electron density at the bond critical point (∇ 2 F BCP ) and that of the energy density (H BCP ) could characterize the strength of HBs. Thus, weak HBs (E I < 12.0 kcal/mol) show both ∇ 2 F BCP and H BCP > 0, and medium HBs (12.0 < E I < 24.0 kcal/mol) show ∇ 2 F BCP > 0 and H BCP < 0, while strong HBs (and therefore LBHBs; E I > 24.0 kcal/mol) show both ∇ 2 F BCP and H BCP < 0.

The complexes formed by a variety of anions with perfluoro derivatives of benzene, naphthalene, pyridine, thiophene, and furan have been calculated using DFT (B3LYP/6-31++G**) and MP2 (MP2/6-31++G** and MP2/6-311++G**) ab initio methods. The minimum structures show the anion interacting with the pi-cloud of the aromatic compounds. The interaction energies obtained range between -8 and -19 kcal mol(-1). The results obtained at the MP2/6-31++G** and MP2/6-311++G** levels are similar. However, the B3LYP/6-31++G** results provide longer interaction distances and smaller interaction energies than do the MP2 results. The interaction energies have been partitioned using an electrostatic, polarization, and van der Waals scheme. The AIM analysis of the electron density shows a variety of topologies depending on the aromatic system considered.

Hydrogen bonds (HBs) are the most important 'weak' interactions encountered in solid, liquid and gas phases. The HB can be defined as an attractive interaction between two molecular moieties in which at least one of them contains a hydrogen atom that plays a fundamental role. Classical HBs correspond to those formed by two heteroatoms, A and B, with a hydrogen atom bonded to one of them and located approximately in between (A-H•••B). Recently, knowledge of the number of functional groups which act as hydrogen bond donors or acceptors has increased considerably and most of these new groups are discussed.

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The nature of bifurcated or three-centered hydrogen bonds (HB) has been investigated. Different families of compounds were chosen:  monomers with intramolecular three-centered HB, dimers with a HB donor (HBD) and a molecule with two HB acceptor (HBA) groups, and trimers with one HBD and two HBAs. All the systems were optimized at the B3LYP/6-31G* level, and, in the case of the complexes, the interaction energies were evaluated and corrected with the basis set superposition error (BSSE). The electronic nature of these three-centered HBs was analyzed by means of the atoms in molecules (AIM) approach. The present study indicates the existence of bifurcated bond paths in the AIM analysis with electron densities that can be classified as follows:  (i) compounds with symmetric three-centered HBs presenting two symmetric bond critical points with equal values of electron density; (ii) compounds with asymmetric three-centered HBs presenting two bond critical points with different values of electron density; (iii) compounds with a regular HB and a van der Waals interaction showing two bond critical points with different electron density values one of which is very small; (iv) van der Waals complexes with two bond critical points having very small electron densities. Therefore, looking at the geometry, electron density, and energy results, the nature as HB of these three-centered interactions has been confirmed.

A theoretical study of the charge-transfer complexes formed by dihalogen compounds (F2, Cl2, Br2, FBr, FCl, and ClBr) and electron donors (FH, OH2, NH3, CO, NCH, and C2H2) has been carried out. The geometries, energies, and electronic and spectroscopic properties of these complexes have been compared with the corresponding properties of the hydrogen bonded complexes of FH with the same electron donors. The hybrid HF-DFT, B3LYP, and second-order Møllet−Plesset perturbation, MP2, methods have been used. The properties analyzed include geometry, energy, electron distribution using the atoms in molecules (AIM) methodology, and spectroscopic constants of the complexes and monomers. Similarities in the variations of the geometries, in the trends in the interaction energetic, and in the topological electron density characteristics between the properties of the HB complexes and the dihalogen charge-transfer systems are pointed out. The main differences correspond to the variation trend of the atomic properties and the NMR shielding when going from the monomers to the complexes.

We present theoretical proof that the nature of the interactions existent in the complexes formed between hydrogen fluoride and a series of π-systems (acetylene, ethylene, cyclopropene, cyclobutadiene, and benzene) and three-memebered-ring derivatives (cyclopropane and tetrahedrane) is that of a hydrogen bond between the hydrogen atom and the π-cloud. Geometric and energetic data and mainly the study of the topology of the electronic density within the frame of the theory of atoms in molecules established by Bader have been used for this analysis.

Theoretical calculations on a series of SiXY 3 ‚‚‚ZW complexes, where X and Y are H, F, and Cl, and Z corresponds to an electron donor atom (ZW ) NH 3 , NCH, CNH, OH 2 , FH), were performed. The calculations were carried out using B3LYP/6-311++G**, MP2/6-311++G** and MP2/6-311++G(2d,2p) computational methods. The electron density was characterized by means of the atoms in molecules (AIM) methodology, and the interaction nature was studied with the NBO method. Finally, the effect of the complexation on the nuclear chemical shieldings was evaluated with the GIAO method. The results display a wide range of interaction distances that vary from 2.1 to 4.1 Å. The complexes with shorter interaction distances (∼2.1 Å) show important distortion effects and large dipole moment enhancements. The NBO analysis indicates that in those complexes an ionic interaction is formed between the Si and Z atoms. Comparison of the chemical shieldings of the complexes and the monomers indicates that these interactions could be detected experimentally using 29 Si NMR. In addition, in the case of the complexes with NH 3 and OH 2 , the use of 15 N NMR and 17 O NMR could be adequate to check the potential formation of the corresponding complexes.