N. Neto

Lattice dynamics of molecular crystals

1 Lattice Dynamics.- 1.1 Introduction.- 1.2 The Dynamical Equations in Cartesian Coordinates.- 1.3 Dispersion Curves. Acoustic and Optical Modes.- 1.4 Invariance Conditions.- 1.5 Molecular Coordinates.- 1.6 The Dynamical Equations in Molecular Coordinates.- 2 Symmetry.- 2.1 Space Group Symmetry.- 2.2 Irreducible Representations of the Translational Group.- 2.3 Irreducible Representations of the Space Groups.- 2.4 Time Reversal.- 2.5 Symmetry of the Dynamical Matrix.- 2.6 Symmetry Properties of Vibrational States.- 2.7 Selection Rules.- 3 Intermolecular Potentials.- 3.1 The Crystal Potential.- 3.2 The Intramolecular Potential.- 3.3 The Intermolecular Potential.- 3.4 Intermolecular Force Constants.- 3.5 Lattice Sums and Ewalds Method.- 2.6 Calculation of Phonon Frequencies.- 4 Anharmonic Interactions.- 4.1 Introduction.- 4.2 The Crystal Hamiltonian.- 4.3 Quantum Field Treatment of Phonons.- 4.4 The Hamiltonian Renormalization Procedure.- 4.5 The Self-Consistent Phonon Method.- 4.6 The Method of the Greens Functions.- 4.7 Thermal Strain.- 4.8 Anharmonic Calculations.- 5 Two-Phonon Spectra of Molecular Crystals.- 5.1 Introduction.- 5.2 General Considerations.- 5.3 Two-phonon Density of States.- 5.4 Two-phonon Absorption Coefficients.- 6 Infrared and Raman Intensities in Molecular Crystals.- 6.1 Introduction.- 6.2 Historical Survey.- 6.3 Electrostatic Model of Infrared and Raman Intensities of Molecular Crystals.

Lattice dynamics of molecular crystals using atom—atom and multipole—multipole potentials

Abstract The theory of the lattice dynamics of a molecular crystal assuming an atom—atom and a multipole—multipole potential is developed. Expressions are d

Normal mode analysis of 2,2′-bipyridine—II. Crystal vibrations

Abstract A normal mode analysis of the crystal spectrum of 2,2′-bipyridine was carried out using a previously determined intramolecular force field and an intermolecular potential described in terms of interactions among non-bonded atoms. New i.r. and Raman data in the region of lattice vibrations are presented, which can be satisfactorily accounted for by frequency calculations in the rigid-body approximation. Frequency splittings and shifts of internal modes, mixing with external vibrations and relative intensity of out-of-plane modes are analyzed through calculations at k = 0 which include all vibrational degrees of freedom.

Normal mode analysis of 2,2′-bipyridine—I. Internal modes

Abstract The vibrational assignment of the internal modes of 2,2′-bipyridine is discussed using i.r. and Raman spectra in polarized light for a single crystal and for two different orientations of polycrystalline films. Predictions based on zero-order frequency calculations provide a useful guideline for the identification of the in plane fundamentals, while no satisfactory force field was available for the out-of-plane vibrations. A refined set of valence force constants was then obtained which, under the assumption of a planar stable conformation both in solution and in solid, can be used for further calculations of the whole crystal spectrum.

Lattice dynamics of crystalline ammonia and deutero-ammonia

Abstract The theory of lattice dynamics of molecular crystals using atom-atom and multipole-multipole potentials developed in a previous paper is applied to crystalline ammonia. It is found that the electrostatic terms play an important part in calculations of the crystal energy, structure and lattice vibrations.

Surface enhanced Raman scattering of pyrazine adsorbed on silver colloidal particles

Abstract The surface enhaced Raman scattering of pyrazine in a silver sol was investigated. Strong enhancement of the g modes and appearance of Raman forbidden vibrations were observed, in agreement with existing results obtained on electrode surface. Chemical reactions of pyrazine on Ag were monitored by Raman and mass spectroscopy.

Surface Enhanced Raman Scattering of Imidazole adsorbed onto Silver Colloidal Particles

Abstract SER spectra of imidazole adsorbed on silver sols have been investigated and compared with the Raman spectra of the neutral molecule and of the conjugated acid and base in aqueous solutions. Two different orientations of the adsorbate onto silver colloidal particles have been identified, depending on the adsorbate concentration and the presence of chloride anions in the colloidal dispersions. A face-on orientation of imidazole respect to the silver surface is favoured by the adsorbate dilution, whereas an edge-on orientation of the imidazolate anion is induced by addition of a small amount of NaCl.

Halide anion effect on surface enhanced Raman scattering of 2-amino,5-nitropyridine adsorbed on silver sols

Abstract 2-Amino,5-nitropiridine (ANP) can be adsorbed on silver sols as neutral molecule or ANP- anion, as detected by SERS. The adsorption of the ANP- is related to the presence of hydroxide ions on the metal surface. Strongly adsorbed halide anions are able to remove hydroxide ions from the silver surface, inducing the adsorption of ANP as neutral molecule.

Filters coated with colloidal silver for SER spectroscopy

Abstract 2,2′-bipyridine and 2-amino,5-nitropyridine were adsorbed on stable SERS-active substrates, prepared by filtration of silver colloidal dispersions through alluminum oxide membranes. SERS spectra were thus obtained and compared with those of corresponding experiments on colloidal dispersions.

Vibrational analysis and molecular structure of 2,2′-bipyrimidine

Abstract A complete vibrational analysis of 2,2′-bipyrimidine is presented, based on infrared and Raman data and on the predictions of a valence force field transferred from parent molecules. Infrared and Raman solution spectra gave no evidence of a non-planar molecular conformation which is, however, possible in non-polar solvents, as suggested by the ultraviolet spectra. A D 2 h point group symmetry was assumed to interpret the vibrational spectra of the isolated molecule. The vibrational assignment was used as a refinement of the zero-order force constants, to account for the presence of four nitrogen atoms characteristic of the molecule under consideration. Addition of an intermolecular potential of the Buckingham type allowed a complete description of the normal modes of the corresponding crystal for k =0. The results are discussed in connection with the observed splittings and shifts produced by the crystal field.