Thérèse Zeegers-Huyskens

Matrix isolation infrared spectra of the complexes between methylacetate and water or hydrochloric acid

Abstract The hydrogen-bonded complexes between methylacetate (MeAc) and water or hydrochloric acid have been studied by infrared spectrometry in a low temperature Argon matrix. The νCO, νCC and νCC ional modes of MeAc show a splitting to the low and to the high frequency side of the free molecule. This suggests that water and HCl interact with the keto and ether oxygens. This conclusions is supported by the appearance of two main absorptions in the νHCl region. These results are discussed as a function of the gas-phase basicity of the two oxygen atoms, derived from the O(1s) core electron binding energies and from the ionization potentials.

Theoretical study of the protonation and deprotonation of cytosine. Implications for the interaction of cytosine with water

The geometries, harmonic vibrational frequencies and energies of the two stable cyclic structures of the keto tautomer of cytosine complexed with water are computed using density functional theory (B3LYP) combined with the 6-3111G(d,p) basis set. The effect of complex formation with water on the pyramidalization of the amino group is discussed. The proton affinities of the oxygen and nitrogen atoms and the deprotonation enthalpies of the three NH bonds of cytosine are computed at the same level of theory. The deprotonation enthalpies of the two NH bonds of the amino group differ by 23 kJ mol 21 from each other and this reflects the asymmetric deformation of the amino group. The most stable hydrogen bond between cytosine and water is formed at the acceptor atom characterized by the lowest proton affinity and at the NH group having the highest acidity. The results are compared with data obtained at the same level of theory for the uracil‐ and thymine‐water complexes. For the three nucleobases, the intermolecular distances and the energies of the hydrogen bonds formed at the different sites depend on the proton affinity and the deprotonation enthalpy of these sites. The dominance of the proton donor capacity in determining the hydrogen-bond energies and the cooperativity in the cyclic structures are discussed. q 2000 Elsevier Science B.V. All rights reserved.

Theoretical investigation of the conformation, acidity, basicity and hydrogen bonding ability of halogenated ethers

MP2/6-311++G(d,p) calculations have been carried out to investigate the conformation, protonation and the hydrogen bonding interactions with water of several halogenated ethers (CH(3)OCH(2)Cl, CH(2)ClOCH(2)Cl, CH(3)OCHCl(2), CHFClOCHF(2)). The optimized geometries, ν(CH) harmonic vibrational frequencies and the SAPT decomposition of the interaction energies are studied. The interaction with one water molecule gives several stable structures characterized by O(w)H(w)...O and CH...O(w) hydrogen bonds or by O...Cl halogen bonding. The MP2/CBS calculated binding energies of different complexes between the halogenated ethers and water vary between 1.7 and 7.7 kcal mol(-1). The energies of these structures are discussed as a function of the proton affinity of the ethers and the deprotonation enthalpy of the CH bonds. The contraction of the CH bonds and blue shifts of the corresponding stretching vibrations in the O-protonated ethers and their O...H(w)O(w) complexes are compared. A natural bond orbital analysis has revealed that substitution of the H atoms by one or several halogen atoms has a great influence on the hyperconjugative effects from the two non-equivalent O lone pairs to relevant antibonding orbitals, and the subsequent geometry of the hydrogen bonded complexes.

Importance of the proton affinities in hydrogen bond studies. A new exponential expression

Abstract The gas-phase basicities and the proton affinities (PA) provide an excellent basicity scale for hydrogen bonds in solvents of low polarity. The usefulness of this scale is shown by several examples where the aqueous basicities are strongly affected by solvation effects. In a limited ΔPA range, the correlation between the energetics of hydrogen bond formation and ΔPA (the difference between the proton affinities of the two partners) can be considered as linear. However, in a broad ΔPA range, the association energy obeys the following expression − Δ H H B 0 = x e − x ′ | Δ P A | This function has been computed for several systems involving (NH⋯N) + , (OH⋯O) + , (NH⋯O) + , (NH⋯S) + and (OH⋯S) + hydrogen bonds. The results are compared with the Marcus equation. The derivative of the exponential expression seems to be related to the shape of the potential curve for the proton motion and to the barrier for the proton transfer.

Blue shifts of the CH stretching vibrations in hydrogen‐bonded and protonated trimethylamine. Effect of hyperconjugation on bond properties

The optimized geometry of isolated trimethylamine (TMA), its hydrogen bond complexes with phenol derivatives and protonated TMA is calculated at the B3LYP/6‐31++G(d,p) level. A natural bond orbital (NBO) analysis on these systems is carried out at the same level of theory. In isolated TMA, one of the CH bond in each of the three CH3 groups is more elongated than the two other ones. As revealed by the NBO data, this results from a hyperconjugative interaction from the N lone pair to the σ*(CH) orbitals of the CH bonds being in a transoid position with respect to the N lone pair. The formation of an intermolecular OH···N hydrogen bond with phenols results in a decrease of the lone pair effect. A linear correlation is found between the decrease in occupation of the σ*(CH) orbitals and the decrease in the hyperconjugative interaction energy in the complexes and isolated TMA. Complex formation with phenols results in a blue shift of 55–74 cm−1 of the CH stretching vibrations involved in the lone pair effect. Smaller blue shifts between 14 and 23 cm−1 are predicted for the other CH bonds. In these complexes, a linear correlation is found between the frequency shifts and the elongation of the CH bonds. Protonation of TMA results in a nearly equalization of all the CH distances and a blue shift of 180 cm−1 of the CH bonds involved in hyperconjugation with the N lone pair.

Influence of proton transfer on the formation of hydrogen-bonded complexes of higher stoichiometry and on their dissociation into free ions

Proton transfer in a given H-bond ⊂A—H—B—(H)→⊂A−— H+—B—(H) considerably enhances the strength of the electron donor sites of the first partner and that of proton donor sites possibly present in the second one. This leads to the formation of complexes of higher stoichiometry of the type B— H+—(A−—H—A--H—A--H--)or A−---(H—B+—H----B—H—B—H----) where the self-association bonds are much strengthened. This is due to the high stability of the homoconjugated anions or cations in the corresponding ion pairs. In polar solvents like acetonitrile, the ion pairs may dissociate into free ions. The variety of the entities that can be formed necessitates a diversification of the quantitative concepts connected with the proton transfer process. Besides the average value 〈x1〉 of the fractions of the various complexes in the ionized form, other quantities are defined that can also be used in the case of partial dissociation: (1) the percentage of ionized base molecules and (2) the fraction of donor molecules AH ionized after direct interaction with B. A further characteristic used in this generalized treatment is the average number 〈n〉 of proton donor molecules perturbed by one base molecule. Examples of determinations of these various parameters from calorimetric, infrared, or NMR data from the literature are presented and new quantitative correlations established.

An Infrared Study of the Interaction of Water with Nitrogen Bases

Abstract In the number of publications relating to the hydrogen bonded complexes between hydroxylic derivatives and bases, there is little quantitative work on the thermodynamic properties of these adducts when monomeric water is acting as a proton donor. The formation constants of 1:1 adduct of water and aliphatic amides1, dioxane2, tributylphosphine3, pyridazine4, some organic bases5 have been reported but there is sometimes a great discrepancy between the literature data; as for example, the reported values of the formation constant for the pyridine-water 1:1 adduct are respectively 100 (in CCl4)6, 6.8 (in CHCl3)7 and 2.63 (in CCl4)8. We report here the association constants for 1:1 complexes between a water molecule and several nitrogen bases. The frequency shift of the νOH stretching vibration is also determined and compared with the proton affinity of the base.

Theoretical investigation of the halogen bonded complexes between carbonyl bases and molecular chlorine

The halogen bonded complexes between six carbonyl bases and molecular chlorine are investigated theoretically. The interaction energies calculated at the CCSD(T)/aug‐cc‐pVTZ level range between −1.61 and −3.50 kcal mol−1. These energies are related to the ionization potential, proton affinity, and also to the most negative values (Vs,min) on the electrostatic potential surface of the carbonyl bases. A symmetry adapted perturbation theory decomposition of the energies has been performed. The interaction results in an elongation of the ClCl bond and a contraction of the CF and CH bonds accompanied by a blue shift of the ν(CH) vibrations. The properties of the Cl2 molecules are discussed as a function of the σ*(ClCl) occupation, the hybridization, and the occupation of the Rydberg orbitals of the two chlorine atoms. Our calculations predict a large enhancement of the infrared and Raman intensities of the ν(ClCl) vibration on going from isolated to complexed Cl2.

Hydrogen bonding equilibria in solution and gas-phase protonic acidities or basicities

The correlation between the free energy changes for hydrogen bonding formation in solution (−ΔG°HB) and the gas phase protonic acidity (GA) or basicity (GB) of the proton donor and proton acceptor is investigated. The equation −ΔG°HB = A − a(GA−GB) is established for OH…O and OH…N systems; this equation only holds for closely related complexes and is discussed as a function of geometrical, entropic and delocalization effects. The −ΔG°HB value is also compared with the free energies of ionization of the proton donor (ΔG°iA(H2O)) and of the proton acceptor (ΔG°iB(H2O)) in aqueous medium. In the correlation −ΔG°HB = A′ − a′ ΔG°iA(H2O) + b′ ΔG°iB(H2O) the a′ and b′ coefficients are not equal and this can be explained by the fact that the attenuation factor in solution is not the same for the proton donors and proton acceptors considered in this work.

Molecular complexes of HCl and H2O with aliphatic esters studied by matrix-isolation IR spectroscopy

Abstract The infrared spectra of matrix isolated mixtures of HCl or H 2 O with aliphatic formates and acetates show the existence of two types of H-bonded complexes : a CO…HA and a COO…HA species. In addition 1–2 interactions and “mixed” ester…H 2 O…HCl complexes are observed. The spectral band shifts Δv CO′ and Δv SHl are correlated with the proton affinity of the two oxygen atoms of the bases. The blue shift of v CH of the esters is discussed.