Identifying and Analyzing Intermolecular Bonding Interactions in van der Waals Molecules

Tetracyanometalates of Ni, Pd, and Pt with cyclic diquaternary cations of 2,2'-bipyridine and 1,10-phenanthroline. A vibrational, crystallographic, and theoretical study of intermolecular weak interactions

The compounds (bpy-dq)[Ni(CN)(4)].2H(2)O (1), (bpy-dq)[Pd(CN)(4)].2H(2)O (2), and ((bpy-dq)[Pt(CN)(4)].2H(2)O (3) (bpy-dq = (C(12)H(12)N(2))(2+), 6,7-dihydrodipyrido[1,2-a:2',1'-c]pyrazinediium) and (phen-dq)[Ni(CN)(4)] (4), (phen-dq)[Pd(CN)(4)].H(2)O (5), and (phen-dq)[Pt(CN)(4)].H(2)O (6) (phen-dq = (C(14)H(12)N(2))(2+), 5,6-dihydropyrazino[1,2,3,4-lmn]-1,10-phenanthrolinediium) have been synthesized and characterized by X-ray diffraction. The three bipyridinium diquaternary salt derivatives are isostructural. The crystal structures of these dihydrated compounds consist of columns formed by alternating anion complexes and diquaternary cations, pi-pi interacting through cyanide ligands and the aromatic rings, and stabilized by an extended hydrogen-bond network. On the other hand, the packing in the phenanthrolinium diquaternary salt derivatives is strongly dependent on the hydration degree. Thus, the anhydrous [Ni(CN)(4)](2-) compound presents a laminar arrangement and the hydrated salts show a columnar packing, similar but not the same as compounds 1-3. The anhydrous form of compound 5 is isostructural with compound 4. Vibrational (IR, Raman) and thermogravimetric studies of these compounds have been carried out. Finally, DFT calculations have been performed on the isolated tetracyanometalate anions and diquaternary cations to assign the fundamental modes in the vibrational spectra. The intermolecular weak interactions were studied through the analysis of the charge density by using the theory of atoms in molecules (AIM).

Ab initio and DFT studies on van der Waals trimers: the OCS.(CO2)2 complexes

Ab initio calculations [MP2, MP4SDTQ, and QCISD(T)] using different basis sets [6-31G(d,p), cc-pVXZ (X = D, T, Q), and aug-cc-pVDZ] and density functional theory [B3LYP/6-31G(d,p)] calculations were carried out to study the OCS.(CO2)2 van der Waals trimer. The DFT has proved inappropriate to the study of this type of systems where the dispersion forces are expected to play a relevant role. Three minima isomers (two noncyclic and one cyclic) were located and characterized. The most stable isomer exhibits a noncyclic barrel-like structure whose bond lengths, angles, rotational constants, and dipole moment agree quite well with the corresponding experimental values of the only structure observed in recent microwave spectroscopic studies. The energetic proximity of the three isomers, with stabilization energies of 1442, 1371, and 1307 cm-1, respectively, at the CBS-MP2/cc-pVXZ (X = D, T, Q) level, strongly suggests that the two unobserved structures should also be detected as in the case of the (CO2)3 trimer where both noncyclic and cyclic isomers have been reported to exist. The many-body symmetry-adapted perturbation theory is employed to analyze the nature of the interactions leading to the formation of the different structures. The three-body contributions are small and stabilizing for the two most stable structures and almost negligible for the cyclic isomer.

Self-discrimination of enantiomers in hydrogen-bonded dimers

The homochiral and heterochiral hydrogen-bonded (HB) dimers of a set of small model molecules (alpha-amino alcohols) have been studied by means of ab initio methods. The gas-phase calculations have been carried out with the hybrid HF/DFT B3LYP method and the 6-311++G** basis set. The electron density of the complexes has been analyzed using the atoms in molecules (AIM) methodology, which allows characterization of the HB interactions and additional intermolecular contacts. To take into account the water solvation effect, the polarized continuum model (PCM) method has been used to evaluate the Delta G(solv). The gas-phase results show that the heterochiral dimers are the most stable ones for each case studied, while in solution for several cases, the relative stability is reversed and the homochiral dimers become more stable. The AIM analysis shows the typical bond critical points characteristic of the HB and additional bond critical points denoting, in this case, destabilization of intermolecular interaction as CF(3)...F(3)C and CH(3)...H(3)C contacts.

The hydrogen bond in the solid state

The hydrogen bond is the most important of all directional intermolecular interactions. It is operative in determining molecular conformation, molecular aggregation, and the function of a vast number of chemical systems ranging from inorganic to biological. Research into hydrogen bonds experienced a stagnant period in the 1980s, but re-opened around 1990, and has been in rapid development since then. In terms of modern concepts, the hydrogen bond is understood as a very broad phenomenon, and it is accepted that there are open borders to other effects. There are dozens of different types of X-H.A hydrogen bonds that occur commonly in the condensed phases, and in addition there are innumerable less common ones. Dissociation energies span more than two orders of magnitude (about 0.2-40 kcal mol(-1)). Within this range, the nature of the interaction is not constant, but its electrostatic, covalent, and dispersion contributions vary in their relative weights. The hydrogen bond has broad transition regions that merge continuously with the covalent bond, the van der Waals interaction, the ionic interaction, and also the cation-pi interaction. All hydrogen bonds can be considered as incipient proton transfer reactions, and for strong hydrogen bonds, this reaction can be in a very advanced state. In this review, a coherent survey is given on all these matters.