Nuclear quantum effects and hydrogen bonding in liquids

We have employed ab initio path integral molecular dynamics simulations to investigate the role of nuclear quantum effects on the strength of hydrogen bonds in liquid hydrogen fluoride. Nuclear quantum effects are shown to be responsible for a stronger hydrogen bond and an enhanced dipole-dipole interaction, which lead, in turn, to a shortening of the H...F intrachain distance. The simulation results are analyzed in terms of the electronic density shifts with respect to a purely classical treatment of the nuclei. The observed enhanced hydrogen-bond interaction, which arises from a coupling of intra- and intermolecular effects, should be a general phenomenon occurring in all hydrogen-bonded systems.     

Analysis of nuclear quantum effects on hydrogen bonding

The impact of nuclear quantum effects on hydrogen bonding is investigated for a series of hydrogen fluoride (HF)n clusters and a partially solvated fluoride anion, F-(H2O). The nuclear quantum effects are included using the path integral formalism in conjunction with the Car-Parrinello molecular dynamics (PICPMD) method and using the second-order vibrational perturbation theory (VPT2) approach. For the HF clusters, a directional change in the impact of nuclear quantum effects on the hydrogen-bonding strength is observed as the clusters evolve toward the condensed phase. Specifically, the inclusion of nuclear quantum effects increases the F-F distances for the (HF)n=2-4 clusters and decreases the F-F distances for the (HF)n>4 clusters. This directional change occurs because the enhanced electrostatic interactions between the HF monomers become more dominant than the zero point energy effects of librational modes as the size of the HF clusters increases. For the F-(H2O) system, the inclusion of nuclear quantum effects decreases the F-O distance and strengthens the hydrogen bonding interaction between the fluoride anion and the water molecule because of enhanced electrostatic interactions. The vibrationally averaged 19F shielding constant for F-(H2O) is significantly lower than the value for the equilibrium geometry, indicating that the electronic density on the fluorine decreases as a result of the quantum delocalization of the shared hydrogen. Deuteration of this system leads to an increase in the vibrationally averaged F-O distance and nuclear magnetic shielding constant because of the smaller degree of quantum delocalization for deuterium.     

Some recent developments in the historiography of alchemy

The number of researchers and publications devoted to the history of alchemy has seen exponential growth and diversification in recent decades, to such an extent that some scholars speak of a "New Historiography of Alchemy". On the occasion of the seventy-fifth anniversary of the Society for the History of Alchemy and Chemistry, this paper outlines some highlights of the literature since 1990, with a view to identify current trends but also challenges for the future. Some of the most important changes identified are a marked awareness of the risks of presentism, a shift from ambitious histories to contextualised microhistories, a heightened recognition of the internal diversity of historical alchemy, and a greater emphasis on its practical dimensions and its role in the Scientific Revolution. Among the challenges, the paper underscores the potential risks of an excessive historiographical fragmentation, the need for further interdisciplinary training and cooperation, and the responsibilities of alchemy historians towards students and the general public alike.     

Some recent developments in pressure tuning spectroscopy

Pressure tuning spectroscopy has proved to be a powerful tool for studying a wide variety of electronic phenomena. In this review we present two recent examples of the application of high pressure luminescence: (1) a test of the validity of a kinetic model for the emission characteristics of molecules that emit from excited states with different degrees of charge transfer and different geometries (molecules with twisted intramolecular charge transfer (TICT)); (2) an elucidation of the mechanism for the large difference observed between the luminescence efficiencies of [Ru(bpy)₃]²⁺ and [Ru(bpy)₂(py)₂]²⁺ in solid media.In 1994, Prof. H. G. Drickamer was awarded theHonoris causa degree by the Russian Academy of Sciences.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1195–1202, July, 1995.     

Quantum theory of the structure and bonding in proteins: Part 7. The α helix and the hydrogen bonding in the tetrapeptide

Using the tetrapeptide as a model, formation of the C_xxx 1:4 or 3.6₁₃ hydrogen bond which stabilises the common α helix in proteins is demonstrated. The general methods, developed earlier in the smaller peptides, are based on conventional ab initio molecular orbital theory using the Gaussian 76 package. The extent to which the total energy of the tetrapeptide is additive with respect to the various possible fragments of the molecule (the 45° lines) is also demonstrated. The geometry and energetics of the hydrogen bond are established, the geometrical information probably being more reliable than the energetic data. An NO distance of about 2.9 Å does not always create a hydrogen bond. Investigation of the internal hydrogen bonds in polypeptides and proteins using purely quantum mechanical methods should prove practical.     

Some recent developments in SPM of crystalline polymers

Some recent advances in the application of atomic force microscopy to crystalline polymers are detailed. Ultra‐high resolution imaging of crystal surfaces, combined with the analysis of computer generated Connolly surfaces, enables the unambiguous identification of features on the cellulose crystal surface at near‐atomic resolution. The electronic enhancement of the quality factor of the cantilever when tapping in liquids enables a considerable improvement in force sensitivity to be obtained, allowing the fully saturated surface of an isotactic polystyrene gel to be imaged under the solvating molecule, at nanometre resolution. A series of experiments are detailed in which processes such as crystallization, crystal thickening and crystal deformation are followed in situ, in real time, providing significant new insights into long standing problems in polymer science.     

Some recent developments in the chemical synthesis of inorganic nanotubes

Inorganic nanotubes have been a subject of intensive research in the past decade. We recently developed a number of synthetic strategies for generating nanotubes from inorganic materials that do not have a layered structure. It is the intention of this contribution to provide a brief account of these research activities.     

Some recent developments in reference methodology within the United Kingdom

 The UK Valid Analytical Measurement Programme has pump-primed developments in chemical metrology, through a major project on reference methodology and reference materials. This paper provides an overview of developments during 1994–1997 and covers work on primary methods for trace inorganic and trace organic analysis, including the use of isotope dilution mass spectrometry; studies of sample pre-treatment, including digestion, extraction, and separation aimed at improving this weak link in the traceability chain; prioritisation, production and marketing of both pure substance and matrix reference materials; and international collaboration concerned with interlaboratory comparisons and the development of concepts, terminology and systems to underpin the international chemical measurement system. References are given to a number of papers covering specific parts of the programme. Received: 5 August 1998 · Accepted: 12 September 1998     

Quantum theory of the structure and bonding in proteins: Part 6. Factors governing the formation of hydrogen bonds in proteins and peptides

A detailed analysis of the factors which govern the energetics and geometry of the four C₁₀ hydrogen bonds in the tripeptide molecule and in the two monomeric amides suggests that the two main internal effects are the nitrogen—oxygen distance and the straightness at the hydrogen atom. At the oxygen atom, there seems to be a ball and socket joint effect in which the only restriction is that the HOC angle be greater than about 120°. At certain geometries, additional non-bonded repulsion effects arise.A mathematical expression is proposed for the energy of the hydrogen bond as a function of the main parameters and this may prove useful for semiempirical schemes in larger molecules.     

On the experimental validity of recent developments of solvent-shift theory

The validity of the equations reported by Amos and Burrows on solvent shifts has been tested in several solvents for seven different symmetrical compounds. The results confirm that their approximation introduced to simplify the interpretation of experimental data generally is not valid.     

Some recent developments in the surface-analytical application of X-ray fluorescence spectrometry

Summary Standard depth profiles of an analyte deposited into (diffusion or ion implantation) or on (thin-film deposition) a plane surface can be analyzed for profile type and centroid depth or film thickness by means of a standardless method in which the matrix-attenuated signals of the fiuorescing analyte measured at two different take-off angles are related to the mathematical distribution moments of the profile. For a binary thin film the element ratio can also be established. Results obtained on phosphorus profiles in silicon and on zinc sulphide optical coatings are referred to.The quantity or concentration level can be determined by use of a reference standard which may contain the analyte in an entirely different distribution. This simplifies the calibration of secondary reference standards.A good lateral resolution in the sub-millimeter range can be achieved with synchrotron radiation. A further improvement of lateral resolution is possible by direct excitation with electron microbeams, though at significantly inferior detection limits.

---

Neuere Entwicklungen bei der oberflächen-analytischen Anwendung der Röntgenfluorescenz-Spektrometrie

     

A generalized formalism of the quantum theory of valence and bonding

General definitions of valence, degree of bonding between pairs of atoms, and atomic anisotropy and reactivity are given. They can be applied to closed- or open-shell molecular wave functions (in the semiempirical, quasi or full ab initio SCF levels), as well as to GVB ones. The properties and usefulness of the definitions are discussed as well as their relation to former empirical notions. Examples of their application are also reported.     

The hydrogen molecule ion in the old quantum theory

The ground state of the H ₂ ⁺ molecule is studied using the equations of classical mechanics and the Einstein quantum condition, J _ζ+J _μ = h, where J _ζ and J _μ are the action integrals over a complete cycle of the elliptic coordinates. Strong bonding is found, but the quantitative results are good only for R<0.05 and R>4a ₀. The greatest error comes at R _eq and results from the coalesence of two classically allowed regions where the electron can exist.     

Density functional theory and atoms-in-molecules investigation of intramolecular hydrogen bonding in derivatives of malonaldehyde and implications for resonance-assisted hydrogen bonding

A density functional theory (DFT) and atoms-in-molecules (AIM) analysis has been applied to the intramolecular hydrogen bonding in the enol conformers of malonaldehyde and its fluoro-, chloro-, cyano-, and nitro-substituted derivatives. With the B3LYP/6-311++G(2d,p) method, good agreement between the DFT geometries and published experimental structures has been found. The donor-acceptor distance was also varied in a series of constrained optimizations in order to determine if energetic, structural, and topological trends associated with intermolecular hydrogen bonding remain valid in the intramolecular case. At very short donor-acceptor distances (<2.24 A), the hydrogen is symmetrically located between donor and acceptor; at distances longer than this, the hydrogen bonding is no longer symmetric. The AIM methodology has been applied to explore the topology of the electron density in the intramolecular hydrogen bonds of the chosen model systems. Most AIM properties for intramolecular hydrogen bond distances longer than 2.24 A show smooth trends, consistent with intermolecular hydrogen bonds. Integrated AIM properties have also been used to explore the phenomenon of resonance-assisted hydrogen bonding (RAHB). It is shown that as the donor-acceptor distance is varied, pi-electron density is redistributed among the carbon atoms in the intramolecular hydrogen bond ring; however, contrary to prior studies, the integrated atomic charges on the donor-acceptor atoms were found to be insensitive to variation of hydrogen-bonding distance.     

Cooperative hydrogen bonding effects are key determinants of backbone amide proton chemical shifts in proteins

A computational methodology for backbone amide proton chemical shift (delta(H)) predictions based on ab initio quantum mechanical treatment of part of the protein is presented. The method is used to predict and interpret 13 delta(H) values in protein G and ubiquitin. The predicted amide-amide delta(H) values are within 0.6 ppm of experiment, with a root-mean-square deviation (RMSD) of 0.3 ppm. We show that while the hydrogen bond geometry is the most important delta(H)-determinant, longer-range cooperative effects of extended hydrogen networks make significant contributions to delta(H). We present a simple model that accurately relates the protein structure to delta(H).