The interaction of fluorosilanes with hydrogen fluoride clusters: strongly blue-shifted hydrogen bonds

Electronic structure, molecular electrostatic potential and hydrogen bonding in DMSO-X complexes (X = ethanol, methanol and water)

In the present work, we have studied the electronic structure, molecular electrostatic potential (MEP) and hydrogen bonding in DMSO-ethanol, DMSO-methanol and DMSO-water complexes by employing the MP2 method. Different conformers were simulated on the basis of possible binding sites guided by molecular electrostatic potential topology. The stronger hydrogen bonded interaction lowers the energy of the conformer. Molecular electron density topology and natural bond orbital analysis were used to explain the strength of interactions. Experimental vibrations are also compared with the calculated normal vibrations. Blue shift is predicted for SC vibration in experimental and theoretical spectra as well. Molecular electrostatic potential and topology are used to understand the interaction strength of the conformer.

Ab initio studies of the vibrational spectra of some hydrogen-bonded complexes of fluoroacetylene

Ab initio molecular orbital theory has been used to compute the properties of a number of hydrogen-bonded complexes between fluoroacetylene as proton donor and ammonia, water, hydrogen fluoride, phosphine, hydrogen sulfide, and hydrogen chloride as proton acceptors. The properties considered were the vibrational spectra, the molecular structures, the hydrogen-bond energies, and the electron densities, and one of the aims of the study was to ascertain whether there was any evidence of blue-shifting hydrogen-bond character in the complexes formed. The adducts with NH3, H2O, PH3, and H2S were of the conventional CH···X kind (X = N, O, P, S), with hydrogen-bond energies decreasing in the order NH3 > H2O > PH3 ≈ H2S. Those formed with HF and HCl showed the presence of three alternative structures; in addition to the CH···F(Cl) complexes, adducts of the F(Cl)H···F and F(Cl)H···π type were also found to be stationary points on the potential energy surfaces, with stabilities in the order F(Cl)H···π > CH···F(Cl) > F(Cl)H···F. The hydrogen-bond energies of the CH···X series correlated with the gas-phase basicities of the proton acceptors; moreover, the CH bond-length changes, the wavenumber shifts, the complex–monomer infrared intensity ratios of the CH stretching modes, and the amounts of charge transferred on complex formation were all found to track with the hydrogen-bond energies. All those properties considered here are consistent with the formation of red-shifting hydrogen bonds, to the exclusion of the blue-shifting alternatives.

Blue-Shifted A−H Stretching Modes and Cooperative Hydrogen Bonding. 1. Complexes of Substituted Formaldehyde with Cyclic Hydrogen Fluoride and Water Clusters

The structures and vibrational spectra of the intermolecular complexes formed by insertion of substituted formaldehyde molecules HRCO (R = H, Li, F, Cl) into cyclic hydrogen fluoride and water clusters are studied at the MP2/aug-cc-pVTZ computational level. Depending on the nature of the substituent R, the cluster type, and its size, the C-H stretching modes of HRCO undergo large blue and partly red shifts, whereas all the F-H and O-H stretching modes of the conventional hydrogen bonds are strongly red-shifted. It is shown that (i) the mechanism of blue shifting can be explained within the concept of the negative intramolecular coupling between C-H and C=O bonds that is inherent to the HRCO monomers, (ii) the blue shifts also occur even if no hydrogen bond is formed, and (iii) variation of the acceptor X or the strength of the C-H...X hydrogen bond may either amplify the blue shift or cause a transition from blue shift to red shift. These findings are illustrated by means of intra- and intermolecular scans of the potential energy surfaces. The performance of the negative intramolecular coupling between C-H and C=O bonds of H(2)CO is interpreted in terms of the NBO analysis of the isolated H(2)CO molecule and H(2)CO interacting with (H2O)n and (HF)n clusters.

Red-, Blue-, or No-Shift in Hydrogen Bonds: A Unified Explanation

We provide a simple explanation for X-H bond contraction and the associated blue shift and decrease of intensity in IR spectrum of the so-called improper hydrogen bonds. This explanation organizes hydrogen bonds (HBs) with a seemingly random relationship between the X-H bond length (and IR frequency and its intensity) to its interaction energy. The factors which affect the X-H bond in all X-H...Y HBs can be divided into two parts: (a) The electron affinity of X causes a net gain of electron density at the X-H bond region in the presence of Y and encourages an X-H bond contraction. (b) The well understood attractive interaction between the positive H and electron rich Y forces an X-H bond elongation. For electron rich, highly polar X-H bonds (proper HB donors) the latter almost always dominates and results in X-H bond elongation, whereas for less polar, electron poor X-H bonds (pro-improper HB donors) the effect of the former is noticeable if Y is not a very strong HB acceptor. Although both the above factors increase with increasing HB acceptor ability of Y, the shortening effect dominates over a range of Ys for suitable pro-improper X-Hs resulting in a surprising trend of decreasing X-H bond length with increasing HB acceptor ability. The observed frequency and intensity variations follow naturally. The possibility of HBs which do not show any IR frequency change in the X-H stretching mode also directly follows from this explanation.

Theoretical Study of Hydrogen Bonding Interaction in Nitroxyl (HNO) Dimer: Interrelationship of the Two N−H···O Blue-Shifting Hydrogen Bonds

The hydrogen bonding interactions of the HNO dimer have been investigated using ab initio molecular orbital and density functional theory (DFT) with the 6-311++G(2d,2p) basis set. The natural bond orbital (NBO) analysis and atom in molecules (AIM) theory were applied to understand the nature of the interactions. The interrelationship between one N-H...O hydrogen bond and the other N-H...O hydrogen bond has been established by performing partial optimizations. The dimer is stabilized by the N-H...O hydrogen bonding interactions, which lead to the contractions of N-H bonds as well as the characteristic blue-shifts of the stretching vibrational frequencies nu(N-H). The NBO analysis shows that both rehybridization and electron density redistribution contribute to the large blue-shifts of the N-H stretching frequencies. A quantitative correlations of the intermolecular distance H...O (r(H...O)) with the parameters: rho at bond critical points (BCPs), s-characters of N atoms in N-H bonds, electron densities in the sigma*(N-H), the blue-shift degrees of nu(N-H) are presented. The relationship between the difference of rho (|Deltarho|) for the one hydrogen bond compared with the other one and the difference of interaction energy (DeltaE) are also illustrated. It indicates that for r(H...O) ranging from 2.05 to 2.3528 A, with increasing r(H...O), there is the descending tendency for one rho(H...O) and the ascending tendency for the other rho(H...O). r(H...O) ranging from 2.3528 to 2.85 A, there are descending tendencies for the two rho(H...O) with increasing r(H...O). On the potential energy surface of the dimer, the smaller the difference between one rho(H...O) and the other rho(H...O) is, the more stable the structure is. As r(H...O) increases, the blue-shift degrees of nu(N-H) decrease. The cooperative descending tendencies in s-characters of two N atoms with increasing r(H...O) contribute to the decreases in blue-shift degrees of nu(N-H). Ranging from 2.05 to 2.55 A, the increase of the electron density in one sigma*(N-H) with elongating r(H...O) weakens the blue-shift degrees of nu(N-H), simultaneously, the decrease of the electron density in the other sigma*(N-H) with elongating r(H...O) strengthens the blue-shift degrees of nu(N-H). Ranging from 2.55 to 2.85 A, the cooperative ascending tendencies of the electron densities in two sigma*(N-H) with increasing r(H...O) contribute to the decreases in blue-shift degrees of nu(N-H).

Hydrolysis of Fluorosilanes: A Theoretical Study

Hydrolysis and condensation of simple trifluorosilanes, HSiF3 and MeSiF3, was studied by quantum mechanical methods. Hydrolysis of fluorosilanes is highly endothermic. The Gibbs free energy of the first reaction step in the gas phase is 31.4 kJ/mol, which corresponds to an equilibrium constant of 10(-6). Hydrolysis of the subsequent fluorine atoms in trifluorosilanes is thermodynamically more unfavorable than the first step of substitution. No significant difference in thermodynamics of hydrolysis was found between HSiF3 and MeSiF3. The activation energy for hydrolysis by a water dimer is significantly lower than that for hydrolysis by a water monomer. The former reaction is also less unfavorable thermodynamically, due to a high binding energy of the HF-H2O complex formed as a product of hydrolysis. Self-consistent reaction field (SCRF) calculations show that hydrolysis of trifluorosilanes in aqueous medium has lower activation energy than in the gas phase. It is also thermodynamically less unfavorable, due to better solvation of the products. Homofunctional condensation of HSiF2OH is thermodynamically favored. The equilibrium mixture for hydrolysis/condensation of RSiF3 in water is predicted to contain ca. 2.3% disiloxane (HF2Si)2O, if 100-fold excess of water relative to silane is assumed. Further hydrolysis of (HF2Si)2O is negligible. The thermodynamics of fluorosilane hydrolysis contrasts with that of chlorosilanes, where both hydrolysis and condensation are strongly favorable. Moreover, in the case of trichlorosilanes each subsequent hydrolysis step is more facile, leading to the product of full hydrolysis, RSi(OH)3.

Strongly Blue-Shifted C−H Stretches: Interaction of Formaldehyde with Hydrogen Fluoride Clusters

The equilibrium structures, binding energies, and vibrational spectra of the cyclic, hydrogen-bonded complexes formed between formaldehyde, H(2)CO, and hydrogen fluoride clusters, (HF)(1< or =n < or =4), are investigated by means of large-scale second-order Møller-Plesset calculations with extended basis sets. All studied complexes exhibit marked blue shifts of the C-H stretching frequencies, exceeding 100 cm(-1) for n = 2-4. It is shown that these blue shifts are, however, only to a minor part caused by blue-shifting hydrogen bonding via C-H...F contacts. The major part arises due to the structural relaxation of the H(2)CO molecule under the formation of a strong C=O...H-F hydrogen bond which strengthens as n increases. The close correlation between the different structural parameters in the studied series of complexes is demonstrated, and the consequences for the frequency shifts in the complexes are pointed out, corroborating thus the suggestion of the primary role of the C=O...H-F hydrogen bonding for the C-H stretching frequency shifts. This particular behavior, that the appearance of an increasingly stronger blue shift of the C-H stretching frequencies is mainly induced by the formation of a progressively stronger C=O...H-F hydrogen bond in the series of H(2)CO...(HF)(1< or =n < or =4), complexes and only to a lesser degree by the formation of the so-called blue-shifting C-H...F hydrogen bond, is rationalized with the aid of selected sections of the intramolecular H(2)CO potential energy surface and by performing a variety of structural optimizations of the H(2)CO molecule embedded in external, differently oriented dipole electric fields, and also by invoking a simple analytical force-field model.