Hydrogen-bonded Crystals Research Articles

Force constants in hydrogen-bonded crystals

Hydrogen bond stretching force constants in crystals of imidazole, urea and cyanuric acid are calculated using both a modified CNDO/2 method and a constrained least squares fitting of interatomic pair potentials to the lattice vibrations. Results show that the modified CNDO/2 method gives closer agreement with the experimentally derived force constants than the normal CNDO/2 parameterization.

Calculation of intermolecular frequencies of some amide crystals

Empirical atom—atom intermolecular force fields derived recently for amide crystals are used without further elaboration to calculate the long wave lattice modes of urea, formamide and adipamide. The lattice frequencies of these hydrogen-bonded crystals. yielded by this model, are found to compare well with experimental results.

Elastic and thermoelastic properties of l-glutamic acid hydrochloride

Abstract The elastic constants, their temperature derivatives, and the thermal expansion of l-glutamic acid hydrochloride were measured. The crystals possess a remarkably high elastic and thermal anisotropy, which can be attributed to the preferential orientation of zigzag chains of hydrogen bonded molecules running parallel to the a3 axis. A similar behaviour was observed earlier in other hydrogen-bonded crystals like oxalic acid and acid salts of oxalic acid. Measurements of the piezoelectrical, electrooptical and nonlinear optical properties revealed only unexpected small effects, not high enough for a useful technical application.

Distribution of hydrogen bond angles in molecular crystals

THE structures of hydrogen-bonded molecular crystals have been studied extensively, and the resulting histograms of the distributions of the O–H ċ O bond angles θ show maxima at approximately 15° from the linear configuration1–3. At first sight this seems odd, because theoretically a linear hydrogen bond is stable4,5. Kroon and Kanters3 have indicated, however, that because the number of possible hydrogen bond configurations at any value of θ is proportional to sinθ, these histograms should not be interpreted to indicate that the configuration of θ = 15° is, energetically, the most stable. They applied the sinθ correction to the statistics of a series of 196 values of hydrogen bond angles in molecular crystals, and showed that the experimental distributions are not inconsistent with an assumed preference for linear hydrogen bonds.

Ferroelectric dispersion of some hydrogen-bonded crystals

The ferroelectric dispersion of the dielectric constant of TGS, TGSe and Colemanite, investigated up to 10 GHz and from 18 to 40 GHz, are of the monodispersive Debye-type. The characteristic relaxation frequencies, defined by fo = fe(es - e ∞), show a pronounced temperature dependence only in the ferroelectric phase. The experimental curves are compared with a recent phenomenological theory for the dispersion of “electrostrictive” ferroelectrics with a second order transition; qualitative, and in the case of Colemanite nearly quantitative, agreement is found.

Dynamics and structure of disordered hydrogen-bonded crystals

Since the problem of the structure and of the molecular and lattice dynamics of solid methanol seems yet unsolved, new contributions are presented coming from theoretical calculations and lattice dynamical calculations. The α and β structures of solid methanol are accounted for on the basis of CNDO and packing-forces calculations. It turns out that a structurally disordered model for the β phase is likely to occur. A mechanism for the λ-type phase transition at 157·4° K is proposed which qualitatively accounts for the data from specific heat, dielectric constants, N.M.R. line-width and X-ray studies. The vibrational spectrum is discussed and compared with the interpretations available in the previous literature. Attempts are made to find evidence in the infra-red of the disordered model proposed from theoretical calculations. k = 0 phonons are calculated and their geometry dependence studied. While the O-H stretching motion is independent from an out-of-plane deformation of the molecular chain, O-H … O de...

Molecular-Orbital Theory for Infinite Systems: Self-Consistent Field Perturbation Treatment of Hydrogen-Bonded Molecules

A self-consistent field perturbation theory of intermolecular interactions is introduced with a view to its application to the study of hydrogen-bonded molecular crystals. A numerical perturbation scheme is also introduced that permits a very detailed numerical check to be made on both the perturbation equations and the corresponding computer program. From a numerical study of the (HF)2 and (HCN)2 systems it is shown that, if taken to the third order, the perturbation method yields dimerization energies and changes in charge densities to within 7% of the values obtained by direct calculations. Structural data calculated from perturbation theory are still more accurate.

Ising Model with Second-Neighbor Interaction. I. Some Exact Results and an Approximate Solution

The square Ising lattice with unequal first- and second-neighbor interactions has been considered. Among the exact results discussed, we show that the lowest energy state can be ferromagnetic, antiferromagnetic, or superantiferromagnetic, and the transition temperature should vanish in some cases. It is also shown that this problem is a special case of a more general problem arising in the statistical consideration of the hydrogen-bonded crystals. A well-defined approximation procedure is then introduced to solve the latter problem and to derive the (approximate) critical condition and expressions for the thermodynamic functions. The critical temperature thus determined is exact for the regular Ising lattice and for the lattices with ${T}_{c}=0$ while for the equivalent neighbor model the error is less than 2%. The specific heat possesses the usual logarithmic singularity in all cases.

Neutron Spectroscopy: A Comparison with Raman and Infrared

The technique of neutron scattering has in recent years been applied with success to the study of molecular spectroscopy. The theoretical basis and experimental procedures of this technique are reviewed. Particular attention is paid to those aspects that clarify the role of neutron scattering as a complementary tool to the well-established methods of Raman and ir spectroscopy. Measurements on a variety of systems are presented as examples of its applicability. These various systems include (a) polymers, (b) torsional oscillators, (c) metal hydrides, (d) hydrogen-bonded crystals, and (e) dispersion curves of molecular crystals.

Hydrogen-bonded crystals and the anisotropic heisenberg chain

We show that the transfer matrix for Yang’s two-dimensional model of a hydrogen-bonded crystal in the presence of an electric field commutes with a generalized form of the Hamiltonian of the anisotropic Heisenberg chain. Thus the connection between the two problems initially found for the ground-state eigenvector is extended to all the eigenvectors.

Exact Solution of a Two-Dimensional Model for Hydrogen-Bonded Crystals

A model of two-dimensional hydrogen-bonded crystals satisfying the ice rule is presented and solved exactly in the presence of an external electric field. The transfer-matrix method is used. The model includes, for special values of the parameters, the ice problem, the Rys $F$ model of an antiferroelectric, and the Slater KDP model of a ferroelectric.

The infrared spectra of hydrogen-bonded crystals

A theoretical approach is proposed to explain the spectral changes in the X-H stretching region of hydrogen-bonded crystals. The model proposed here is restricted to a one dimensional crystal. The X-H stretching modes are supposed to be coupled to H-bond vibrations of lower frequency, and in general the separation of these modes is no longer possible. The fundamental hamiltonian describing this coupling is derived and formulae for quantities, such as intensity, bandwidth and center of gravity of the infrared line in the X-H stretching mode region are calculated.

Thermal Properties of Molecular Crystals. I. Heat Capacity and Thermal Expansion

The lattice heat capacity Cvs and especially the difference Cps-Cvs of molecular crystals composed of rigid molecules is shown to be well represented by a suitably modified form of the Leibfried and Ludwig theory of anharmonic crystals. Crystals of low packing density (ρ*<0.65) such as benzene require a (fairly large) heat capacity correction for the heat of lattice defect formation. An empirical correction of the theory is proposed to adapt it for the estimation of Cvs of crystals composed of flexible molecules. The expansion coefficient (αs) of molecular crystals composed of rigid molecules at 0.7Tm<T<0.9Tm is represented by a universal value of the dimensionless number αs*=αsΔHs/Cvs first proposed by Grueneisen and also derivable from Ludwig's theory of anharmonic oscillator crystals. Variants of αs* are discussed also for hydrogen-bonded crystals and for crystals composed of flexible molecules.

Cluster Approximations for Order-Disorder-Type Hydrogen-Bonded Ferroelectrics. I. Small Clusters

A high-temperature cluster series is derived for the partition function of an order-disorder-type hydrogen-bonded ferroelectric crystal, taking into account a simplified interaction between protons as well as the overlap of the protonic wave functions between the two sites in a hydrogen bond. The zero-order result is the Weiss molecular-field approximation, and the two-particle-cluster result reduces in the classical limit to the Bethe treatment of the Ising model. The specific properties of crystal lattices can be taken into account in higher order terms. The results of the above approximations as well as the results of an Oguchi-type treatment, valid at all temperatures, are compared with exact solutions in the case of a simple soluble model.

On the isotopic effects in the ferroelectric behaviour of crystals with short hydrogen bonds

An attempt to correlate the isotopic effects in the ferroelectric properties of hydrogen-bonded crystals with the observed anomalies in their i.r. and n.m.r. spectra has led to the conclusion that the protons in the hydrogen bonds are tunnelling in double-minimum potential fields. The ferroelectric transition is assumed to be the result of a deformation of the protonic distribution due to electrostatic interactions. It has been found that a quantum-mechanical extension of Mason's and Devonshire's long-range-forces model is able to explain, by the same mechanism, the results of i.r. and n.m.r. spectroscopy as well as the dependence of the Curie point on the mass of the hydrogen isotope, the sharper increase of the spontaneous polarization with falling temperature and the larger value of the spontaneous polarization at absolute zero for deuterated than for hydrogen compounds. The proposed model predicts that a ferroelectric transition occurs only if the dipole-dipole interactions, expressed as frequencies, are greater than the tunnelling frequency on the lowest vibrational level.

A study of thermal motion in hydrogen-bonded crystals

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Molecular Motion of Enclathrated Compounds as studied by Nuclear Magnetic Resonance

IT has been known for some time that hydrogenbonded crystals such as quinol and urea can enclathrate a variety of molecules. Many studies of the structures of clathrates by X-ray diffraction1–4 and infra-red spectroscopic techniques5 have yielded little information about the motions of the enclathrated molecules. In special cases, for example, oxygen and nitric oxide enclathrated in quinol, where the trapped molecule is paramagnetic, magnetic susceptibility measurements6–8 have indicated that the trapped oxygen and nitric oxide molecules behave rather like free gas molecules.

347. The vapour pressure and lattice energy of hydrogen-bonded crystals. Part II. α- and β-Anhydrous oxalic acid and tetragonal pentaerythritol

347. The vapour pressure and lattice energy of hydrogen-bonded crystals. Part II. α- and β-Anhydrous oxalic acid and tetragonal pentaerythritol

346. The vapour pressure and lattice energy of hydrogen-bonded crystals. Part I. Oxamide, oxamic acid, and rubeanic acid

346. The vapour pressure and lattice energy of hydrogen-bonded crystals. Part I. Oxamide, oxamic acid, and rubeanic acid

348. The vapour pressure and lattice energy of hydrogen-bonded crystals. Part III. 2-Thenoic, 2-furoic, and pyrrole-2-carboxylic acids

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