Vibrational relaxation in hydrogen bonded systems: I. Theory

A theory is proposed which expresses the influence of intermolecular interaction in H-bonded solutions on the solute IR band shape in terms of parameters characterising the modulation and strength of this interaction. It is found that appreciable vibrational relaxation can occur by interaction between oscillators with different frequencies, provided that the energy gap is closed by the hydrogen bond spectral density. Isotopic substitution of the solvent provides a means for experimental verification, which is the subject of a subsequent paper.     

Quadruple hydrogen bonded systems

In this feature article, the development of linear quadruple hydrogen bonded systems is discussed, emphasizing applications in supramolecular chemistry and self-assembly.     

Localized orbitals in hydrogen bonded systems

The localized molecular orbitals for a variety of hydrogen bonded systems are obtained from their INDO and CNDO/2 wavefunctions. The bonding of these systems is qualitatively discussed in light of the localized orbitals.     

NMR relaxation study of a hydrogen bonded complex

Spin-lattice relaxation times T₁ were measured over a large frequency region for H and D nuclei in the hydrogen bonded complex of trichloroacetic acid and dimethylsulphoxide. The measured relaxation times T₁ can be ascribed to the motion of the complex as a whole with a negligible contribution from the association—dissociation process.     

Vibrational spectroscopic properties of hydrogen bonded acetonitrile studied by DFT

Vibrational properties (band position, Infrared and Raman intensities) of the acetonitrile C[triple bond]N stretching mode were studied in 27 gas-phase medium intensity (length range: = 1.71-2.05 angstroms; -deltaE range = 13-48 kJ/mol) hydrogen-bonded 1:1 complexes of CH3CN with organic and inorganic acids using density functional theory (DFT) calculations [B3LYP-6-31++G(2d,2p)]. Furthermore, general characteristics of the hydrogen bonds and vibrational changes in the OH stretching band of the acids were also considered. Experimentally observed blue-shifts of the C[triple bond]N stretching band promoted by the hydrogen bonding, which shortens the triple bond length, are very well reproduced and quantitatively depend on the hydrogen bond length. Both predicted enhancement of the infrared and Raman nu(C[triple bond]N) band intensities are in good agreement with the experimental results. Infrared band intensity increase is a direct function of the hydrogen bond energy. However, the predicted increase in the Raman band intensity increase is a more complex function, depending simultaneously on the characteristics of both the hydrogen bond (C[triple bond]N bond length) and the H-donating acid polarizability. Accounting for these two parameters, the calculated nu(C[triple bond]N) Raman intensities of the complexes are explained with a mean error of +/- 2.4%.     

Proton transference in hydrogen bonded systems. I. Uracil tautomerism

We have made a detailed analysis of the adequacy of different theoretical approaches to the study of uracil tautomerism. We have considered the effect of molecular relaxation and the ability of several methods to reproduce it. We have also considered their performance in predicting the tautomerization energy barrier. We found that semiempirical methods are not adequate for predicting the energy value but that they can be used for obtaining geometry optimizations. We also found a strong discrepancy between the energy values predicted by a minimal basis set and a 2ζ basis set, the last one being 24.9 kcal/mol. The implications of these findings are discussed and the results compared with experimental evidence.     

Vibrational relaxation of ozone by para and normal hydrogen

The relaxation of O₃(001) by p- and n-H₂ was measured over the temperature range 167–424 K. n-H₂ quenched ozone more efficiently, with the difference between rate constants increasing with temperature. These observations are explained by a near-resonant V—R process, in agreement with the Sharma—Brau theory.     

Vibrational circular dichroism in hydrogen bond systems: Part II. Vibrational circular dichroism of the OH stretching vibration in 2,2-dimethyl-1,3-dioxolane-4-methanol

Vibrational circular dichroism (VCD) of the OH stretching band in 2,2-dimethyl-1,3- dioxolane-4-methanol has been studied. The OH stretching vibration for the intramolecularly hydrogen-bonded species in the (R)-enantiomer gives rise to a positive VCD band but no VCD band for the hydrogen-bond free species. It is shown that the G⁻G⁺ conformation in the intramolecular hydrogen bond system

gives a positive VCD band for the OH stretching vibration. The thermodynamic parameters in equilibrium between the free and intramolecularly hydrogen-bonded species have been determined as ΔH =−7.8 kJ mol⁻¹ and ΔS = −18 kJ K⁻¹ mol⁻¹.     

Correlated proton motion in hydrogen bonded systems: tuning proton affinities

The theorem of matching proton affinities (PA) has been widely used in the analysis of hydrogen bonds. However, most experimental and theoretical investigations have to cope with the problem that the variation of the PA of one partner in the hydrogen bond severely affects the properties of the interface between both molecules. The B3LYP/d95+(d,p) analysis of two hydrogen bonds coupled by a 5-methyl-1H-imidazole molecule showed that it is possible to change the PA of one partner of the hydrogen bond while maintaining the properties of the interface. This technique allowed us to correlate various properties of the hydrogen bond directly with the difference in the PAs between both partners: it is possible to tune the potential energy surface of the bonding hydrogen atom from that of an ordinary hydrogen bond (localized hydrogen atom) to that of a low barrier hydrogen bond (LBHB, delocalized hydrogen atom) just by varying the proton affinity of one partner. This correlation shows clearly that matching PAs are of lesser importance for the formation of a LBHB than the relative energy difference between the two tautomers of the hydrogen bond.     

Vibrational analysis of free and hydrogen bonded complexes of nicotinamide and picolinamide

Harmonic and anharmonic vibrations of free nicotinamide (NIA) and picolinamide (PIA) molecules together with their hydrogen bonded complexes H₂O–NIA and H₂O–PIA have been studied by means of density functional method. The calculation results of the vibrational spectra of free molecules have been investigated and are compared to the available experimental spectra. The vibrational wavenumbers of both molecules have also been calculated by polarizable continuum model (PCM) that represents the solvent as a polarizable continuum and places the solute in a cavity within the solvent (water is chosen as the solvent in this study). The results of PCM calculations and the H₂O–NIA, H₂O–PIA complexes, are used to investigate the H-bonding interactions of both molecules with the water molecule. The harmonic wavenumbers have been scaled by proper factors obtained by comparing the observed versus calculated wavenumbers and it is shown that anharmonic corrections on the vibrational spectra provided a better agreement between the observed and calculated wavenumbers compared to the results obtained by scaling factor method.     

The infrared-driven cis-trans isomerization of HONO. II: Vibrational relaxation and slow isomerization channel

In a recent paper [R. Schanz et al., J. Chem. Phys. 122, 044509 (2005)], we investigated the IR-driven cis-trans isomerization of HONO in a Kr matrix with the help of femtosecond IR spectroscopy. We found that isomerization occurs on a 20 ps time scale, however, with a cis-->trans quantum yield of only 10% that is significantly below the value reported in the literature (close to 100%). At the same time, we concluded that vibrational energy has not completely dissipated out of the molecule at the maximum delay time we reached in this study (500 ps). In order to verify whether additional, slower reaction channels exist, we extend the study here to delay times up to 100 ns. At a temperature of 32 K, we indeed find an additional isomerization channel on a 2 ns timescale, which increases the total cis-->trans quantum yield to approximately 30%. The trans-->cis quantum yield is approximately 7%. There is still a discrepancy between the quantum yields we observe and the literature values, however, we provide experimental evidence that this discrepancy is due to the different temperatures of our study. Vibrational cooling occurs on a 20 ns time scale, and cascades in a highly nonstatistical manner through one single normal mode (most likely the ONO bending mode nu(5)). Intermolecular energy dissipation into the rare gas matrix is more efficient than intramolecular vibrational energy redistribution and the matrix environment can certainly not be considered a weak perturbation.     

Methods of modulating hydrogen bonded interactions in synthetic host-guest systems

Hydrogen bonded interactions are among the most important non-covalent interactions in supramolecular chemistry. The strength, selectivity and directionality inherent in hydrogen bonding processes have allowed the creation of complex and efficient molecular hosts capable of selective binding to a wide variety of complementary guests. Major advances in controlling host-guest complexation have occurred in the last decade, principally through systematic modification of the electrostatic properties and/or geometry of the hosts, thereby fine-tuning the molecular recognition event. More recently, systems have been developed which allow the effectiveness and selectively of hydrogen bonding interactions to be reversibly modulated by an external stimulus, more accurately mimicking biological systems and providing building blocks for the construction of novel advanced materials, sensors and devices. In this review, we highlight some of the methods available for modulating the strength and selectivity of hydrogen bonded interactions in synthetic host-guest systems.     

Semiempirical quantum mechanical studies on some hydrogen bonded systems

The CNDO/2 method is used to study the hydrogen bond strength in several representative systems. Enthalpies of hydrogen bond formation are generally predicted fairly well although some exceptions are noted. The proton-proton acceptor distances are predicted to be about 10 percent too small in all cases where comparison with experiment can be made.

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Zusammenfassung Mit Hilfe der CNDO/2-Methode wird in verschiedenen repräsentativen Systemen die Stärke von Wasserstoffbrücken untersucht. Die Enthalpien werden bis auf wenige Ausnahmen recht gut vorausgesagt. In allen Fällen, in denen experimentelle Unterlagen vorhanden sind, werden die Abstände der Proton-Proton-Akzeptoren urn etwa 10% zu klein vorausgesagt.

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Résumé La méthode CNDO/2 est utilisée pour étudier le force de la liaison hydrogène dans quelques systèmes représentatifs. Les enthalpies de formation de la liaison hydrogène sont en général convenablement prédites à quelques exceptions près. La distance entre le proton et l'accepteur de proton est prédite avec une erreur par défaut d'environ 10% dans les cas où le résultat expérimental est connu.

     

On the electrical conductivity of hydrogen bonded biological systems

The LCAO MO method with Bloch eigenfunctions has been applied to the imidazole crystal to evaluate the anisotropy of current carriers mobilities. The results obtained for the exponent of 0.45 of the 2p hydrogen orbital of the hydrogen bond implied in the calculation are in qualitative agreement with experimental data for electrical conductivity. The possible role of the 2p hydrogen orbital of the hydrogen bond for the charge transfer of biological systems is pointed out.     

Vibrational relaxation in liquid nitrogen

The time constant for the collisional deactivation of the υ = 1 vibrational level of N₂ is found to be 1.5 ± 0.5 s in liquid nitrogen of 99.9995% purity of 78 K. This result is consistent with a simple binary collision theory of vibrational relaxation for liquids.     

Vibrational relaxation in liquid diethylamine

Two types of time resolved experiments have been performed on the intermediate sized polyatomic molecule diethylamine, in the liquid phase, in order to elucidate the pathway for vibrational relaxation of the ν = 3 level of the NH stretching mode, which has 9420 cm^?1 of energy. Both techniques had calibrated absolute sensitivities and were specific for vibrational modes of ca. 3000 cm^?1, yet with neither were transient populations in such modes observable. It is inferred that population relaxation in this highly excited room temperature system proceeds on the subpicosecond time scale to lower lying levels. The importance of intramolecular channels for this decay is suggested.     

Vibrational relaxation pathways of amide I and amide II modes in N-methylacetamide

We studied the vibrational energy relaxation mechanisms of the amide I and amide II modes of N-methylacetamide (NMA) monomers dissolved in bromoform using polarization-resolved femtosecond two-color vibrational spectroscopy. The results show that the excited amide I vibration transfers its excitation energy to the amide II vibration with a time constant of 8.3 ± 1 ps. In addition to this energy exchange process, we observe that the excited amide I and amide II vibrations both relax to a final thermal state. For the amide I mode this latter process dominates the vibrational relaxation of this mode. We find that the vibrational relaxation of the amide I mode depends on frequency which can be well explained from the presence of two subbands with different vibrational lifetimes (~1.1 ps on the low frequency side and ~2.7 ps on the high frequency side) in the amide I absorption spectrum.     

IR dispersion of hydrogen bonded systems III. Pentachlorphenol - dioxane-D8 complex

Transmission spectra of the H-bonded complex formed by pentachlorphenol with dioxane-D₈ were measured in the stretching vibration region of the proton-donor group in binary systems. From this data, the spectra of the complex refractive index and of the complex electric permittivity were obtained in the studied region. The molar atomic polarization and the vibrational polarizability related to the ν_s(OH) vibrational were determined and compared with earlier ones obtained for weaker complexes.     

Quantum field theoretical methods in chemically bonded systems II

Summary Many-body perturbation theory is derived for chemical bonds. Paired quasiparticles represent the bonds. Products of the paired quasiparticles define a model Bardeen-Cooper-Schrieffer function. The pairing force is added as a model interaction to the self-consistent problem. The starting model is based on valency and adiabatic symmetry correlation. Symmetries are enforced by the model Hamiltonian. Perturbative corrections are expressed as ordinary Feynman diagrams. The number of diagrams needed is the same as for particle-hole theory.This work was supported in part by the U.S. Department of the Navy, Space and Naval Warfare Systems Command under Contract N00039-89-C-0001, and in part by NATO Research Grant 1861. It was presented, in part, at the A.C. Wahl Memorial Session, Molecular Spectroscopy Symposium, Columbus, Ohio, 1984; and Midwest Theoretical Chemistry Conference, Milwaukee, Wisconsin, 1985.