Proton Acceptor Molecules Research Articles

C–H⋯X interactions of fluoroform with ammonia, water, hydrogen cyanide, and hydrogen fluoride: conventional and improper hydrogen bonds

The structural changes, dipole moments, and interaction energies for a series of C–H⋯X hydrogen bonded complexes involving F3CH as proton donor and NH3, OH2, NCH, and FH as proton acceptors have been studied using ab initio calculations including electron correlation effect for various basis sets. The NH3 shows a conventional hydrogen bond, while the other molecules show improper hydrogen bonds as evidenced by the C–H bond length shortening and the blue-shift of C–H stretching vibrational frequency. In the complex of F3CH⋯NCH, both C–H and N–C bonds are contracted, and their corresponding stretching vibrational frequencies are blue-shifted compared with monomers. The blue-shifted stretching vibration in the proton acceptor molecule has not been much addressed until now. Both conventional and improper hydrogen bonds are noted in the nonlinear complex of F3CH⋯FH, where FH molecule plays a dual role of proton acceptor and donor at the same time. The interaction energies are almost equivalent for HF and MP2 calculations, which does not support a recent proposition that the dispersive interaction is the origin of improper H-bonding.

Interaction of triplet C60with p-tert-butyl-calixarenes and their complexes with pyridine derivatives

The interaction of triplet excited C60 with p-tert-butyl-calix[n]arenes (BCXn, n = 4, 6 or 8) and with their 2,4,6-trimethylpyridine (TMP) and pyridine complexes has been studied with laser flash photolysis experiments. It has been found that in polar solvents 3C60 is quenched by BCX6 via electron transfer, producing C60−˙ radical anion with a yield of 0.45 ± 0.07 in benzonitrile. In contrast, no reaction occurs in nonpolar media or with BCX4 and BCX8. Upon the addition of TMP, the rate of quenching is enhanced considerably due to the formation of TMP-BCXn complexes. The significant deuterium isotope effect indicates that the electron movement from the calixarene to the triplet excited C60 is coupled to the proton displacement from the calixarene to the base. In the presence of pyridine, which has weaker hydrogen-bonding power, the enrichment of the solvate shell in the proton acceptor molecules may play an important role in promoting triplet C60 quenching.

Acetylene as potential hydrogen-bond proton acceptor

Ab initio calculations are used to assess the ability of the CC triple bond in acetylene to act as proton acceptor to donor molecules HF, HCl, HCN and HCCH. The strength of the interaction energy varies from 3 kcal/mol for the strongest of these complexes FH⋯HCCH, to 1 kcal/mol for the weakest (HCCH) 2. The X–H bond of all donors undergoes the elongation characteristic of H-bonds, along with the red shift of its stretching frequency. In addition, electron density is transferred from the proton-acceptor molecule to the donor, and the bridging H becomes more positive. However, the magnitudes of these density shifts are larger than might be expected from a conventional H-bond. Opposite to a H-bond, the proton-accepting C atom loses density. Decomposition of the total interaction energy indicates the dominant role played by Coulombic attraction, as in a H-bond, but the exchange repulsion, charge transfer, and polarization terms are disproportionately larger than is typically seen in true H-bonds.

Anharmonicity and hydrogen bonding: II – A near infrared study of water trapped in nitrogen matrix

The infrared spectra of H 2 16O and H 2 18O trapped in solid nitrogen were measured in the range 8000–80 cm −1. Concentration and annealing effects enable monomer, dimer and polymer bands to be distinguished. Those due to the monomer, easily assigned on the basis of 16 O/ 18 O isotopic substitution, are subdivided in three classes: intramolecular fundamentals and multiquanta transitions, librations and combinations with librations, combinations with the matrix vibron. For the dimer the assignment of the bands of the proton acceptor (PA) molecule follows that of the monomer while the assignment of the proton donor (PD) bands has to take into account the decoupling between the two oscillators, one being hydrogen bonded (OH b), the other quasi-free (OH f). The same decoupling occurs for polymeric species (H 2O) n , n⩾3. The most remarkable result is the non-observation of the first overtone of the OH b oscillators while the corresponding fundamental transitions give rise to strong absorptions. The data analysis is focused on three points: determination of the vibrational spectral term after correction from resonances, determination of the dipole moment function (first and second derivatives of the dipole moment), and simultaneous transitions. For the monomer and PA the data are enough numerous for a comprehensive analysis. For PD the discussion is limited to the ν 1/2 ν 2, ν 1+ ν 2/3 ν 2 Fermi resonances and to the evolution of the dipole moment function accounting for the weakness of the 2 νOH b signal. For larger aggregates the same analysis as that carried out for PD can be developed, leading to comparable conclusions about the effect of hydrogen bonding on the dipole moment function.

Anharmonicity and hydrogen bonding: I. A near-infrared study of methanol trapped in nitrogen and argon matrices

The infrared spectra of six isotopic species of methanol trapped in solid nitrogen and argon were measured in the domain 7300–300 cm −1. Among the various combinations and overtones observed, the 2 ν 1 (OH or OD stretching), 2 ν 6 (OH or OD bending) and ν 1+ ν 6 transitions were examined as a function of the concentration and compared to the ν 1 and ν 6 fundamentals. Most of the bands of monomer and dimer were identified. For aggregates larger than the dimer no absorption assignable to 2 ν 1 was detected, which, taking into account the accuracy of the intensity measurements, leads to a ν 1/2 ν 1 intensity ratio greater than 1000 for these polymers, to be compared to ≈20 for the monomer and 400 for the proton donor molecule of the dimer. An opposite situation is encountered for the bending mode, the ν 6/2 ν 6 intensity ratio decreasing from monomer to polymers, possibly because of a Fermi resonance between 2 ν 6 and ν 1 whose frequencies get closer in case of aggregation. For the ν 1+ ν 6 combination the effects of hydrogen bonding are not as dramatic as in the case of the overtones. From the frequencies and relative intensities some anharmonicity parameters were deduced. Noticeable differences between the spectra of the proton donor and of the proton acceptor molecules of the dimer are explained by changes in the dipole moment function upon H-bond formation.

Spectroscopic studies of bifurcated hydrogen bonds in solution

The opportunity of the formation of molecular complexes with bifurcated hydrogen bonds (H-bonds) in liquid phase is discussed. It was shown that, practically in all previous studies, the idea of the bifurcation was used only for the explanation of the observed experimental facts. The IR and 1H NMR spectra of solutions containing 2,6-disubstituted phenol derivatives with strong intramolecular H-bond and various proton acceptors were obtained at 85–300 K. The distinct spectral manifestations of weak additional interaction between the bound OH group of proton donor and proton acceptor molecules were found confirming the real existence of the bifurcated H-bonds in the liquid state.

Matrix isolation infrared spectroscopy and DFT calculations of complexes between water and nitrogen

The mid-infrared spectra of water–nitrogen mixtures in argon matrices have been investigated. Evidence for the existence of several (H 2O) m (N 2) n , or m: n, complexes has been obtained from the appearance of many absorptions in the νOH, νNN and δOH spectral regions which are not present in the spectra of H 2O/Ar binary matrices. Concentration effects allow identification of the four (seven) intramolecular fundamentals of the 1:1 (2:1) species. The structural properties of these two complexes are discussed on the basis of ab initio calculations carried out within the framework of the density functional theory (DFT). The comparison of the calculated and observed vibrational properties of the 1:1 complex does not allow us to confidently deduce its structure because of the weakness of the perturbations. In the case of the 2:1 heterotrimer it is clearly established that the nitrogen molecule is fixed on the proton acceptor molecule of (H 2O) 2, in a hydrogen-bond type interaction. The frequency shifts with respect to the water dimer are accounted for in terms of cooperative effects.

A computational approach to intermolecular proton transfer in the solid state: assistance by proton acceptor molecules

Ab initio (B3LYP/6-311++G**) calculations have been carried out on the proton transfer of 2H-tetrazole and 5-phenyl-2H-tetrazole with and without the assistance of different nitrogen bases (hydrogen cyanide, ammonia and imidazole). In the absence of base, the proton transfer barrier amounts to 210 kJ mol–1 while in the presence of ammonia it is lowered to 119 kJ mol–1. Moreover, the inclusion of a solvent cavity of the Onsager type, which increases the first barrier, decreases the second one to 67 kJ mol–1 (for e = 5) which is consistent with experimental data for irbesartan (a 5-aryl-2H-tetrazole derivative).

Infrared photoisomerization of the methanol dimer trapped in argon matrix: monochromatic irradiation experiments and DFT calculations

In a first part we propose an accurate assignment of the vibrational spectra of (CH 3OH) 2 and (CH 3 18OH) 2 trapped in argon at 7 K, which differentiates the proton donor (PD) and proton acceptor (PA) molecules of four open chain conformers stable at this temperature. Then we describe the effects of monochromatic irradiations in the OH and CO stretching regions which give rise not only to interconversions between these conformers but also to the appearance of one highly unstable species with a lifetime of about 4 minutes at 7 K. Its structure is discussed in the framework of DFT calculations after having checked the reliability of the method on the stable dimeric form.

Vibrational spectroscopy of small water complexes embedded in large liquid helium clusters

Infrared molecular beam depletion spectroscopy has been employed to study the vibrational spectroscopy of water molecules and small water polymers [(H2O)n, n=2,3,4] embedded in large liquid helium clusters (HeN, 100≤〈N〉≤5000). The spectral region between 3500 and 3800 cm−1 was covered with an injection-seeded optical parametric oscillator. The following vibrational bands could be located and investigated: for the monomer the ν3 asymmetric stretch, for the dimer the asymmetric stretch of the proton acceptor molecule and the free and bonded O–H stretches of the donor molecule, for the trimer both free and bonded O–H stretches, and for the tetramer the free O–H stretch. By comparison with the data on free gas phase water complexes, it was found that the helium host clusters induce only minor perturbations in form of small frequency redshifts and that they constitute an ideal nano-matrix. Furthermore, it was found that both the water molecule and the dimer rotate almost freely in the host cluster and that the internal-rotationlike tunneling motion of the water dimer is not quenched. Due to the extremely low internal temperatures, a splitting of the trimer band associated with the O–H ring vibration could be resolved for the first time. This splitting indicates that the trimer structure is asymmetric, as has been predicted by theoretical calculations.

On the existence of bifurcated H-bond in liquids. Interaction of 2,6-disubstituted phenols with proton acceptors

The IR and 1H NMR spectra of solutions containing 2,6-disubstituted phenols with strong intramolecular H-bond and various proton acceptors have been obtained at 85 – 300 K. The distinct spectral manifestations of weak additional interaction between the bound OH-group of the phenols and proton acceptor molecules have been found. The Δ H values of the complex formation have been estimated. The data obtained confirmed the existence of a so-called bifurcated H-bonds in the liquid state.

High-resolution near-infrared spectroscopy of water dimer

High-resolution near-infrared spectra are reported for all of the O–H stretch vibrational bands of the water dimer. The four O–H vibrations are characterized as essentially independent proton-donor or proton-acceptor motions. In addition to the rotational and vibrational information contained in these spectra, details are obtained concerning the internal tunneling dynamics in both the ground and excited vibrational states. These results show that for tunneling motions which involve the interchange of the proton donor and acceptor molecules, the associated frequencies decrease substantially due to vibrational excitation. The predissociation lifetimes for the various states of the dimer are determined from linewidth measurements. These results clearly show that the predissociation dynamics is strongly dependent on the tunneling states, as well as the Ka quantum number, indicating that the internal tunneling dynamics plays an important role in determining the dissociation rate in this complex.

Charge distribution and proton transfer in hydrogen bonded complexes

Some problems related to the dipole moment enhancement (Δμ) caused by hydrogen bond formation between neutral proton donor and acceptor molecules are discussed. Special attention is paid to the relation between Δμ and the degree of proton transfer (PT). New results are presented with respect to the effects of temperature and solvent on the dipole moments of strongly hydrogen bonded systems. These effects can be entirely related to a shift of the proton transfer equilibrium. It was shown that, in the solid state, the dipole moment changes can be investigated using the nuclear quadrupole resonance (NQR) frequencies of quadrupole nuclei on the outside of the hydrogen bridge. The plot of NQR frequency versus the donor-acceptor properties of interacting components can be formally described by means of the PT equilibrium, although the shapes of the potential energy and free energy surfaces are different from those in solution. New crystallographic data concerning the complexes of phenols and car☐ylic acids with amines indicate a single potential energy minimum which moves from the donor to the acceptor molecule. The properties of hydrogen bonds at the inversion (critical) region, where the proton occupies (statistically) a position close to the centre of the bridge, are discussed. In solution a PT equilibrium appears, while in the solid state an asymmetric single flat minimum is postulated.

Theoretical investigation into the nature of the second vertical ionic state of the water dimer

Ab initio molecular orbital calculations have been carried out to resolve questions concerning the location of the second ionization in the water dimer. The results indicate that it is from the proton acceptor molecule of the water dimer.

Hydrogen bonding in methanol—organic solvent and methanol—water mixtures as studied by the vco and voh

At low concentrations of methanol in non-polar organic solvents, a splitting of the CO Raman band into monomeric and polymeric components has been observed. From a study of the interactions of CH 3OH monomers with proton-donor and proton-acceptor molecules, it is found that v CO decreases upon H bonding via the lone pairs while it increases upon bonding via the OH proton, and that v co is more sensitive to the latter type of interaction. The frequency shifts of the polymeric component upon dilution are associated with a shortening of methanol chains, the extent of which differs in CCl 4 and CHCl 3, solution. The cooperative properties of H bonding are clearly reflected both in the v oh and v CO band shapes. In methanol—water mixtures a decrease in the CO frequency is observed, accompanied by a gradual removal of the splitting in the I XXX — I ⊥ peak positions. This, together with the increase in the H 2O bending frequency, indicates, that methanol—methanol hydrogen bonds are progressively replaced by intercomponent bonds to water, mainly via the lone pairs of CH 3OH. The excess of lone pairs, introduced by the alcohol component, causes an increase in the overall strength of the H bonds in the mixtures and creates a significant fraction of free lone pairs at the water oxygen atoms.

Ab initio and localized molecular orbital studies of the integrated intensities of infrared absorption bands of polyatomic molecules. Part 4.—Complexes between chloroform and three aliphatic nitriles (CH3CN, CCl3CN and HCCCN); influence of hydrogen bonding on the CH and CN infrared characteristics in proton donor and acceptor molecules

STO-3G ab initio calculations are performed on the hydrogen-bond complexes between chloroform and three aliphatic nitriles (CH3CN, CCl3CN and HCCCN). The molecular electrostatic potential (m.e.p.) around the nitriles indicates the N lone-pair as the preferential site for H-bond formation. A partial inter- and intra-molecular geometry optimization and force constant evaluation is performed. Stabilization energies, inter-and intra-molecular geometry parameters and force constants are correlated with various properties of the isolated molecules such as the m.e.p. characteristics and the N lone-pair orbital energy. Vibration frequencies are obtained via the Wilson GF matrix procedure. The decrease in the CH stretching frequency in chloroform and the increase in the CN stretching frequency of the nitriles parallel the stabilization energies of the complexes and are in agreement with experimental infrared results. The form of the CN and CH stretching normal coordinates changes very little upon complexation. The equilibrium charge distribution is compared with experimental n.m.r. data and is related to the charge-transfer character of hydrogen bonding. The calculated CH intensity evolution upon complexation parallels the experimental results in solution, where a strong intensity increase is observed. The variation in intensity is seen to be linearly related to the frequency variation. The CN intensity results do not directly correspond to the experimental solution data. This is ascribed to a smaller contribution of the specific intermolecular interactions as compared with the CH bond.

Polarization model representation of hydrogen fluoride for use in gas‐ and condensed‐phase studies

AbstractThe polarization model, originally developed to describe deformable and ionizable water molecules, has been extended to hydrogen fluoride. Since electronic polarization is explicitly included, the interaction energy in aggregates of molecules (with or without ions) is nonadditive. The model properly describes the structure of (HF)2, including off‐axis bending of the proton acceptor molecule. Calculations are presented to illustrate elementary gas‐phase reactions involving proton transfer between HF and F−, and H2F+ and F−.

Molecular orbital theory of the hydrogen bond.: XVI. A comparative study of pyridine and the diazines as proton acceptors in ground and excited n → π* states

Ab initio LCAO MO SCF calculations with a minimal STO-3G basis set have been performed to determine the structures and energies of dimers having pyridazine, pyrimidine, and pyrazine as proton acceptor molecules, with HF and H 2O as proton donors. The structures of these dimers are consistent with structures anticipated from the General Hybridization Model. Differences in the relative stabilities of dimers in the two series which have HF and H 2O as proton donors and pyridine and the diazines as proton acceptors are attributed to different weightings of secondary effects which influence dimer stabilities. These azabenzeme molecules form stronger hydrogen bonds than HCN and weaker hydrogen bonds than NH 3 whether HF or H 2O is the proton donor. Configuration interaction calculations indicate that vertical excitation to n → π* states of these proton aceptor molecules results in various degrees of destabilization of hydrogen bonded dimers and trimers, depending upon the excited state electron densities at the nitrogen atoms and the excited state dipole moments. With respect to the proton acceptor molecule, computed blue shifts of the n → π* bands increase in the order pyrazine < pyradizine < pyrimidine < pyridine.

Molecular orbital theory of the hydrogen bond. XII. Amide hydrogen bonding in formamide–water and formamide-formaldehyde systems

A b initio SCF calculations with a minimal STO−3G basis set have been performed to determine the equilibrium structures and energies of dimers having formamide as the proton donor molecule and either water or formaldehyde as proton acceptor molecules. The structures of dimers in which the N−H proton ’’s−trans’’ to the carbonyl group is hydrogen bonded (t dimers) are consistent with structures anticipated from the general hybridization model. In these dimers, there is essentially free rotation of the proton acceptor molecule about the intermolecular line. When hydrogen bond formation involves the ’’s−cis’’ proton (c dimers), a single equilibrium formamide−water and formamide−formadehyde dimer exists, the structure of which is strongly influenced by the secondary factors of dipole alignment and long−range interaction. These factors are also responsible for the increased stability of c dimers relative to t dimers. A set of 1:2 formamide:water trimers has been constructed from the equilibrium formamide−water dimers, in which the formamide molecule forms two hydrogen bonds. Only in three open−chain trimers are the hydrogen bonds stronger than those of the corresponding dimers. CI calculations have also been performed to determine the effect of hydrogen bond formation on n→π* transition energies in the dimers and trimers. The n→π* transition energy of the proton acceptor formaldehyde molecule increases in the formamide−formaldehyde dimers. In these dimers, the magnitude of the blue shift is determined by the dimer hydrogen bond energy. The formamide n→π* band is also blue shifted to some extent in the formamide−water and formamide−formaldehyde dimers, even though the n→π* transition originates in the proton donor molecule. The blue shift of the formamide n→π* band in the open−chain trimers having formamide as the central molecule is equal to the sum of the blue shifts in the corresponding dimers.

Clays as Catalysts for Natural Processes

Clay minerals are important constituents of the earth's crust not only because of their abundance but merely because of their chemical activity. Clays found in soils and in sediments are distributed among three main mineralogical groups: the kaolin, hydrated micas, and smectite groups. Allophanes derived from volcanic glasses and amorphous Al-Si or Fe-Si mixed oxide gels are also abundant in large areas. These clays and clay-like materials share two common characteristics: a high specific surface area and the presence of exposed cations on their surface. In the natural environment these cations are either Na +, K +, Ca2+, or Mg2+, and/or polynucleic Al or Fe basic cations such as [AI(OH-)x(H20)yJ�+ or [Fe(OH-)x(H20)yJ�+. They balance the excess negative charge of the lattice, generated by isomorphic substitutions (for instance substitution of Si4+ by AI3+ in tetrahedral position, or AI3+ by Mg2+ in octahedral position). A clay microcrystal may thus be considered as the salt of a weak acid, whose anion (i.e. the lattice) has an infinite radius of curvature. In this situation the electrical charges of the cations generate a high electrostatic field because of the very incomplete screening of the positive charges by the anionic lattice charges. Since the lattice charges have fixed positions, the electrical neutrality is easily achieved by monovalent alkali cations, while for divalent cations the situation is much less favorable. Oxygen atoms or hydroxyl groups are the main surface constituents. They may play an important role by forming hydrogen bonds with proton donor or acceptor molecules. In addition, van der Waals (or dispersion) forces are expected to be quite active because of the magnitude of the surface area. In the natural environment a variable number of layers of water molecules but also polar organic molecules do form an adsorbed phase in which many kinds of chemical transformations may occur. The surface constituents and the adsorbed water may show a high catalytic activity in these transformations. The aim of this contribution is to review the recent progress of our knowledge in this field. After a paragraph in which the surface properties of clays and clay-like minerals