E. G. Tarakanova

The IR spectra of solutions of hydrogen fluoride in (CH 3 ) 2 NCHO, (CH 3 ) 2 CO, and CH 3 CN with a molar ratio of the components of 0:1-10:1

Systematic measurements of the IR spectra of solutions of HF in N,N-dimethylformamide, acetone, and acetonitrile have been carried out for the first time in a wide range of variation of the molar ratio of the components (from 0:1 to 7:1, 8.9:1, and 10:1, respectively). This has not only scientific significance but also important applied significance. The absorption features of each of the binary liquid systems studied here and the spectral attributes of the heterocomplexes formed in it are analyzed. It is established that the molecular associates (HF)n⋅(CH3)2NCHO and (HF)n⋅(CH3)2CO contain the same large structural fragment the appearance of which is typical of solutions of HF in organic solvents.

Composition of heteroassociates formed in an HF-diethyl ketone liquid binary system

The IR spectra and solution densities in a binary liquid system (BLS) HF-diethyl ketone were measured for the first time upon variation of the component molar ratio from 1: 12 to 12: 1 and to 27: 1, respectively. Analysis of the concentration dependences of normalized (to the total number of moles of BLS components in 1 L) optical density and excess density of the solutions revealed the presence of 1: 1, 4: 1, and ∼12: 1 heteroassociates (HAs). For each associate, the concentration range in which it forms in the HF-(C2H5)2CO binary mixture was estimated. It was shown that, at component molar ratios from 2: 1 to 9: 1, all three types of HAs are present simultaneously.

Relationship between free molecules and molecules involved in various heteroassociates in HF-Me2CO solution

The experimental and computational methods to acquire the data on the relationship between free molecules and molecules involved in various heteroassociates (HA) in the HF—organic solvent binary liquid system (BLS) were developed and applied to the series of HF solutions in acetone. The first method is appropriate at the component molar ratios from 0 : 1 to 6 : 1 under the condition that the IR spectrum of the solvent molecule contains a band, whose frequency and intensity can be measured as the molecule passes from the free state to the composition of all HA formed in the BLS. The second method is based on the consideration of the balance between free molecules and molecules involved in HA of the solvent and HF. This method requires the knowledge of molar ratios of the components of the solution at which each HA appears in the solution. It is shown that in an HF-Me2CO solution the main part of the molecules is involved in the HA composition starting from the HF content ∼0.25 molar fraction, whereas 90–100% molecules are associated at an HF content of ∼0.50–0.93 molar fraction.

The effect of divalent cations on the vibrational properties of the water molecule in crystal hydrates

Based on the known experimental frequencies of three isotopic forms of the crystal hydrates M⋅Cl2⋅2H2O(M=Mn, Co, Fe, and Cu), the force constants of the M2+⋅OH2⋅2Cl− complexes are determined by solving the inverse spectral problems. A quantitative estimate is made of how the divalent cation interacting with the water molecule affects its dynamic properties. It is shown that the presence of the M2+ cation in the region of the undivided electron pairs of the H2O molecule increases the interaction force constant of its bonds by 0.065×106 cm−2. This value is an order of magnitude smaller than the change of the indicated force constant in hexagonal ice, caused by the influence of two adjacent proton-donor molecules on the water molecule.

Environment of the Al3+ ion in Water–Acetone Solutions of Aluminum Chloride

Water–acetone solutions in the AlCl3–(CH3)2CO–H2O and AlCl3–(CH3)2CO–H2O–HCl systems were obtained and studied. In both cases, the solution separated into layers to form liquid fractions of lower and higher density. The IR spectra of the fractions were studied. In the light fraction, no AlCl3 was present, but hydrogen-bonded acetone–water complexes were detected. The spectra of the heavy fraction exhibited a broad band at ~3020–3040 cm–1 corresponding to the OH groups of water from aluminum hexahydrate. As the solvent is evaporated, the band narrows down, with its position being retained. The IR spectrum of the obtained solid becomes identical to the IR spectrum of the crystal hydrate AlCl3.(H2O)6. The structures and spectral and energetic parameters of Al(H2O)63+, Al(H2O)6+123+, Al((CH3)2CO)63+, Al((CH3)2CO)43+, and (CH3)2CO.H2O were calculated by the density functional theory method (B3LYP/6-31++G(d,p)). Relying on the results, an explanation was proposed for the experimentlly observed absence of acetone molecules in the Al3+ first coordination shell.

Equilibrium Compositions of HF Solutions in N , N -Dimethylformamide

For HF solutions in DMF, concentration-dependent fractions of DMF molecules (α(DMF)) that remain unassociated and that enter heteroassociates (HAs) of 1 : 1, 4 : 1, and 12 : 1 molecular stoichiometries were obtained by two independent methods, namely, from an analysis of IR spectra and by calculating the material balance. The experimental way was shown to be enough exact in determining the ratio between the solvent molecules in four different states up to ~83 mol % HF. The equilibrium compositions of HF–DMF solutions were estimated over the entire range of concentrations. Starting with [HF] of ~25 mol %, more than one-half HF molecules are associated, and at [HF] of ~50–92 mol %, at least 90% of the HF molecules are associated. The equilibrium composition of HF–organic solvent (Solv) solutions in which HAs of 1 : 1, 1 : 4, and 1 : 12 molecular stoichiometries are formed, can be described by a single set of α(HF–Solv) versus concentration plots.

Using IR spectroscopy to determine the number of different heteroassociates in an HF-organic-solvent solution

Using the three binary liquid systems HF-(C2H5)2CO, HF-CH3CN, and HF-HCON(CH3)2 as examples, a simple method is proposed and tested for detecting stable heteroassociates (HAs) in solution and determining the number of various types of these HAs. When the variation of the vibrational frequencies of the multiple bonds of the solvent (Solv) molecules with the concentration of the solution was analyzed, it showed that the formation of various compositions of HAs is spectrally manifested by appreciable displacements (by about 15-170cm−1) of the CO(CN) vibration band. If the shape of this vibration remains approximately constant as the Solv molecule makes a transition from the pure solvent into the composition of the various types of associates, the corresponding frequency shifts have the same sign. A change in the shape of such a vibration when a solvent molecule is incorporated into any of the heterocomplexes can cause this sign to change.

Composition and spectral manifestations of molecular complexes formed in solutions of hydrogen fluoride in methanol

The IR spectra of solutions of HF in methanol have been measured as the molar ratio of the components is varied from 1:12 to 7:1. The features of the absorption of HF-CH3OH binary mixtures of various concentrations are analyzed, and the spectral attributes of the molecular heterocomplexes present in them are explained. It is established by analyzing the concentration dependence of the optical density of the test solutions, normalized to the total number of moles per liter, that associates of molecules are formed in them with stoichiometric HF:CH3OH ratios equal to 2:1 and about 6:1. These results are compared with analogous data obtained from the concentration dependence of the excess density of solutions of HF in methanol.

Semiempirical method of describing the dynamic and electrooptic properties of hydrogen bonds in molecular complexes

This paper explains the essence of a semiempirical method of describing the spectral properties of intermolecular bonds and their interaction in molecular complexes with various topologies. The method is based on the assumption, proven by numerous experimental and theoretical studies, that all the dynamic and electrooptic properties of interacting molecules are smooth functions of the strength of the intermolecular bond. Examples are given of studies that use this method, showing that it makes it possible for important and diverse information concerning the molecular structure and properties of a wide range of systems to be obtained from their vibrational spectra.