AIM analysis of intramolecular hydrogen bonding in O-hydroxy aryl Schiff bases.

AIM analysis was applied to study the changes in such topological parameters as the electron density at critical points of all the bonds of the molecule during the so-called nonadiabatic proton transfer in intramolecular hydrogen bonding in o-hydroxy aryl Schiff bases. Proton transfer is presented by a stepwise elongation and fixing of the hydroxyl bond with complete optimization of the rest of the parameters of the molecule by the B3LYP/6-311++G(d,p) method. A more detailed study of electron density changes at the critical points of the chelate and phenol rings in the stepwise proton-transfer process is presented. It was shown that the dependency of the electron density at the critical point of the chelate ring on tautomeric equilibrium is of a complicated character, whereas it is linear for the phenol ring. A complex study of the changes in the total electron density at the hydrogen bond, the quasi-aromatic ring, and in the whole molecule has been accomlished. The calculations of the intramolecular hydrogen bond by means of conformational and topological methods are discussed.

Asymmetric hydrogen bonds in a centrosymmetric environment. III. Quantum mechanical calculations of the potential-energy surfaces for the very short hydrogen bonds in potassium hydrogen dichloromaleate.

In the crystal structure of potassium hydrogen dichloromaleate there are two short hydrogen bonds of 2.44 A. The 'heavy-atom' structure is centrosymmetric (space group P1) with centers of symmetry in the middle of the O-O bonds, suggesting centered hydrogen bonds. However, earlier unconventional types of refinements of the extensive neutron data taken at 30, 90, 135, 170 and 295 K demonstrated that the H atoms are actually non-centered in the hydrogen bonds, although the environment is centrosymmetric. Traditionally it has been assumed that the hydrogen distribution adopts the same symmetry as the environment. Reviewing these unusual results it was considered of great interest to verify that the non-centered locations of the H atoms are reasonable from an energy point of view. Quantum mechanical calculations have now been carried out for the potential-energy surfaces (PES) for both the centered and non-centered locations of the H atoms. In all cases the non-centered positions are closer to the energy minima in the PES than the centered positions, and this result confirms that the structure is best described with non-centered H atoms. There is virtually perfect agreement between the quantum-mechanically derived reaction coordinates (QMRC) and the bond-order reaction coordinates (BORC) derived using Pauling's bond-order concept together with the principle of conservation of bond order. [Part I: Olovsson et al. (2001). Acta Cryst. B57, 311-316; Part II: Olovsson et al. (2002). Acta Cryst. B58, 627-631.].

Structural manifestations of proton transfer in complexes of 2,6-dichlorophenols with pyridines.

DFT B3LYP/6-31G(d,p) calculations were performed to describe the proton transfer reaction pathway in the 2,6-dichlorophenolate of pyridine. The aim of these calculations was to establish the character of the dependence of the structure parameters on the proton transfer and comparing the results with known structures, e.g. the 2,6-dichloro-4-nitro- and pentachlorophenolates of pyridines. To make this comparison more reliable, the calculations were repeated with the use of a reaction-field correction with the Onsager radius and electric permittivity taken from the solid-state measurements. The calculations show that the second approach gives a better description of the structural modifications during the proton transfer.

Asymmetric hydrogen bonds in centrosymmetric environment. II. Neutron study of very short hydrogen bonds in potassium hydrogen dichloromaleate at 90 K and 170 K.

In our earlier neutron diffraction study of the title compound at 30 K and 295 K an unconventional strategy in the refinement of hydrogen was applied and the same procedure has now been followed in the present investigation at 170 K and 90 K. There are two short O...H...O hydrogen bonds [2.437 (2) A and 2.442 (2) A at 30 K] and the 'heavy-atom' structure is centrosymmetric (P1) with centres of symmetry in the middle of the O...O bonds. However, statistical significance tests clearly show that an asymmetric location of both H atoms gives the most satisfactory description of the structure at all temperatures. The shift of hydrogen from the centre of symmetry is 0.15, 0.14, 0.15 and 0.15 A for H2 at 30, 90, 170 and 295 K, respectively, and 0.15, 0.15, 0.15 and 0.12 A for H4 (sigma = 0.01 A). Furthermore, the behaviour of H2 is very interesting: at 295 K and 170 K it is located on one side of the symmetry centre but at 90 K and 30 K it is located on the other side. A detailed determination of the unit-cell parameters by X-ray diffraction in the whole temperature range from 30 K to 295 K has revealed that the data points of the cell parameters as a function of temperature fall on two different straight lines with a sudden change in the slope around 135 K. It appears likely that the change in the location of H2 as the temperature is lowered is related to this behaviour. At 170 K, R(F) = 0.029 for 1236 reflections; at 90 K, R(F) = 0.030 for 1457 reflections.

Asymmetric hydrogen bonds in centrosymmetric environment: neutron study of very short hydrogen bonds in potassium hydrogen dichloromaleate.

The structure of the title compound has been studied by neutron diffraction at 30 and 295 K, with the emphasis on the location of the protons. There are two crystallographically independent H atoms in two very short hydrogen bonds, 2.437 (2) and 2.442 (2) A at 30 K. The structure could be refined successfully in the centrosymmetric space group P1;, with the H atoms located at the centres of symmetry. However, the form of the thermal ellipsoids of hydrogen indicated either asymmetric hydrogen bonds or overlap of two closely spaced, partially occupied positions around the centres of symmetry. Several different types of refinements have then been applied, including unconventional models; with all atoms except hydrogen constrained in P1;, but with hydrogen allowed to refine without any constraints in P1, anisotropic refinement of all atoms resulted in clearly off-centred hydrogen positions. Significance tests clearly showed that the results from this constrained refinement give the most satisfactory description of the structure. This structure may be described as 'pseudo-centrosymmetric with non-centred protons'. The results demonstrate that it is very important to also include refinement models with non-centrosymmetric hydrogen in a centrosymmetric environment when studying very short hydrogen bonds. The shifts of the two H atoms from the centres of symmetry are 0.15 (1) and 0.12 (1) A, respectively, at 30 K, and 0.15 (1) A for both H atoms at room temperature. At 30 K: R(F) = 0.036 for 1485 reflections; at 295 K: R(F) = 0.035 for 1349 reflections.

Inelastic neutron scattering studies on low frequency vibrations of pentachlorophenol.

Inelastic neutron scattering (INS) and DFT theoretical studies on pentachlorophenol (PCP) and d-PCP were performed. IR and Raman spectra were also measured for comparison. A special attention was focused on low frequency modes in INS spectra, which provide information about modes into which the co-ordinates of the hydrogen and chlorine atoms are involved. The intensity of respective INS bands is discussed based on the cross-sections of nuclei and calculated relative amplitudes of vibrations. The appearance of overtones and summation frequencies in INS spectra was evidenced.

Vibrational spectra of the adduct of 1,8-bis(dimethylamino)naphthalene with dichloromaleic acid (DMAN x DCM).

The infra-red (IR), Raman (R) and inelastic incoherent neutron scattering (IINS) spectra, particularly in low frequency region, of the title ionic adduct were studied. It is shown that all low frequency vibrations (below 200 cm(-1)) of (CH3)2N groups of protonated 1,8-bis(dimethylamino)naphthalene (DMAN)--clearly observed in IINS spectra--are sensitive to the environment, i.e. to the type of counterion forming short contacts with C-H bonds of methyl groups. The internal frequencies were also calculated by ab initio method. The results are consistent with numerous observations of the counteranion effect on the geometry of the protonated DMAN. The conclusions are compared with structural and NMR studies reported recently for the 1,8-bis(dimethylamino)naphthalene with dichloromaleic acid (DMAN x DCM) adduct. The single crystal R polarized spectra taken over the frequency range 20-3200 cm(-1) were analyzed in detail. We have shown that a substantial difference in the IR spectrum of the dichloromaleic acid (DCM) anion in the DMAN adduct and in the potassium salt results from different geometries of OHO hydrogen bonds. In the case of potassium salt the chains of longer intermolecular hydrogen bonds are formed described by means of a double minimum potential.