Showing papers on "Molecular solid published in 1996"

Intermolecular interactions in the molecular ferromagnetic NH4Ni(mnt)(2)center dot H2O

Abstract: Molecular solids that exhibit ferromagnetism are rare, and thus there is considerable interest in understanding the magnetic coupling mechanisms that operate in the few known examples(1). One such material is the charge-transfer salt NH4Ni(mnt)(2) . H2O, which consists of stacked planar metal ligands separated by ammonium cations. This salt is an insulator with localized spins that exhibit long-range ferromagnetic order at low temperatures (below 4.5 K)(2).3 Here we show that the Curie temperature demarcating the transition to the ferromagnetic state increases markedly with pressure until ferromagnetic order abruptly disappears at 6.8 kbar, indicating that the magnetic coupling is very sensitive to intermolecular separation. Using quantum-chemical calculations(3), we show that this pressure dependence arises from a competition between ferromagnetic coupling (resulting from nickel-sulphur intermolecular spin interactions), and antiferromagnetic coupling (from nickel-nickel interactions). We suggest that a similar interplay of spin-polarization effects might play a key role in determining the nature of the ground states (metallic, superconducting and so forth) observed in other molecular materials of this structural type(4,5).

Structure of Fullerenes

Abstract: This chapter illustrates the internal structure of the molecules. Many independent experiments show that the crystalline materials formed from fullerenes are molecular solids. Therefore the structure and properties of these solids are strongly dependent on the structure and properties of the constituent fullerene molecules. The 60 carbon atoms in C 60 are now known to be located at the vertices of a truncated icosahedron where all carbon sites are equivalent. This is consistent with the observation of a single sharp line in the nuclear magnetic resonance (NMR) spectrum. Since the bonding requirements of all the valence electrons in C 60 are satisfied, it is expected that C 60 has filled molecular levels. Because of the closed-shell properties of C 60 (and also other fullerenes), the nominal sp2 bonding between adjacent carbon atoms occurs on a curved surface, in contrast to the case of graphite where the sp 2 trigonal bonds are truly planar. This curvature of the trigonal bonds in C 60 leads to some admixture of sp 3 bonding, characteristic of tetrahedrally bonded diamond, but absent in graphite.

Prediction of a pure-carbon planar covalent metal.

Abstract: Carbon is well-known as an insulator, a semimetal, a molecular solid, and a one-dimensional semiconductor or low-density-of-states metal. We propose a pure-carbon planar structure composed of pentagons and heptagons that is metallic with a density of states at the Fermi level of \ensuremath{\sim}0.1 state per eV per atom. This structure, planar carbon pentaheptite, is metastable with a total energy per carbon atom comparable to that of ${\mathrm{C}}_{60}$. The structure can be rolled into tubes in a manner similar to graphite. Possible synthetic pathways are discussed.

Structure of the Ice Nanocrystal Surface from Simulated versus Experimental Spectra of Adsorbed CF4

Abstract: CF4 adsorbate uptake and spectra are simulated on various model icy surfaces and compared to experimental data measured for CF4 adsorbate on ice nanocrystals. Comparison suggests that surfaces of annealed ice nanocrystals are relatively smooth but laterally disordered, while surfaces of unannealed nanocrystals are disordered and rough. CF4 adsorbate structure and uptake display remarkable sensitivity to the structural features of the ice surface models and in particular to the extent of disorder. Asymmetric CF stretch spectroscopy of CF4 adsorbate is suggested as a useful probe of the surface structure of molecular solids and of large molecular clusters.

Neutron studies of vibrations in fullerenes

Abstract: Fullerenes are a remarkable new family of carbon phases in which the carbon atoms form closed cages weakly bounded to each other by van der Waals forces. Unique information about the motions of the molecules as a whole as well as about the atomic on-cage vibrations is obtained from inelastic neutron scattering experiments. The results are interesting both on their own for what they tell us about the binding properties of these compounds and, more generally, for the fact that fullerenes are prototypical examples of molecular solids. In particular, solid has extremely interesting properties as the molecules are more nearly spherical than in any other molecular solid. In the first section of this review, the structure of the most important members of the fullerene family is described. Then, an introduction is given into the field of vibrational properties and the experimental techniques used in this field are briefly presented. This is followed by a detailed discussion of the experimental results and their implications for our understanding of the intramolecular and intermolecular binding forces. This review was received in June 1995

Effects of Pressure on the Azafullerene (C59N)2 Molecular Solid to 22 GPa

Abstract: The recent synthesis and isolation in bulk quantities of the nitrogen-substituted fullerene solid, 1 (C59N)2, and its subsequent intercalation with alkali metals to afford azafulleride salts with stoichiometry A6C59N (A ) K, Rb)2,3 opened the way to new opportunities in the quest for the synthesis of fullerene-based materials with novel structural, electronic, and conducting properties. Solid (C59N)2 comprises dimeric molecular units linked by C-C bonds formed by C atoms neighboring the N atom on each monomer, 1 but, unlike solid C60, whose properties have been exhaustively studied in recent years, little is known at present about the physical properties of such condensed heterofullerene phases. We recently reported the first experimental information on the structure and morphology of azafullerene in the solid state by electron microscopy and X-ray diffraction.4 The as-prepared material showed a remarkable selforganization on the mesoscopic length scale, self-assembling into large (>1 μm), poorly crystalline, hollow spherical particles. In addition, a co-existing hexagonal crystalline phase was also present (space group P63/mcc, lattice constants:a ) 9.97 Å,c ) 16.18 Å at ambient temperature and pressure). In this communication, we report the results of high-pressure powder synchrotron X-ray diffraction experiments on a crystalline solid azafullerene sample up to a pressure of 22 GPa in an attempt to probe the diversity of bonding interactions present in this material with pressure tuning of the intermolecular distances. We derive the pressure-volume equation-of-state (EOS) and find that the hexagonal phase observed at ambient pressure remains stable under hydrostatic compression to 22 GPa with a bulk modulus ofK0 ) 21.5(8) GPa and a pressure derivative dK0/ dP) 4.2(1), signifying a somewhat less compressible solid than C60 in which5 K0 ) 18.1(1.8) GPa andK0/dP) 5.7(6). The intensities of selected X-ray reflections evolve with pressure in a way consistent with a very low compressibility of the nitrogensubstituted azafullerene molecules themselves. 6

TTF–TCNE a charge transfer π–molecular crystal with partial ionic ground state: Optical properties and electron‐molecular vibrations interaction

Abstract: The electronic ground state structure of the charge transfer molecular complex TTF–TCNE is investigated on the basis of its electronic and vibrational spectra. Highly oriented polycrystalline films and the spectra of the fully deuterated complex, TTF‐d4–TCNE, allow one to obtain a full exploitation of the spectra. Using the vibrational frequencies as local probes of the electronic structure one finds a value of 0.5±0.1 for the degree of charge transfer of this molecular solid. This partial degree of charge transfer and the alternation of self‐dimers of TTF and TCNE along the one‐dimensional electronic π‐structure reveal themselves in the vibrational spectra and particularly in the charge transfer vibronic resonances present in the infrared and Raman spectra. These resonances and the electronic spectrum related to the charge transfer excitations are understood on the basis of a Holstein–Hubbard model which allows the determination of the electron‐intramolecular vibration coupling constants of TTF and TCNE.

Chapter 11 – Vibrational Modes

Abstract: Publisher Summary This chapter reviews the lattice mode structure for the isolated fullerene molecules and for the corresponding molecular solid. The major experimental techniques for studying the vibrational spectra include Raman and infrared spectroscopy, inelastic neutron scattering, and electron energy loss spectroscopy. Of these techniques, the most precise values for vibrational frequencies are obtained from Raman and infrared spectroscopies. In most crystalline solids, Raman and infrared spectroscopy measurements focus primarily on first-order spectra which are confined to zone-center (q = 0) phonons. Because of the molecular nature of fullerene solids, higher-order Raman and infrared spectra also give sharp spectral features. Analysis of the second-order Raman and infrared spectra for C60 provides a good determination of the 32 silent mode frequencies and their symmetries. If large single crystals are available, then the most general technique for studying solid-state phonon dispersion relations is inelastic neutron scattering. Unfortunately, large single crystals are not yet available even for C60, so that almost all the reported inelastic neutron scattering measurements have been done on polycrystalline samples. Much of the emphasis of the inelastic neutron scattering studies thus far has been on the lowest-energy intermolecular dispersion relations, which have been studied on both single-crystal and polycrystalline samples.

Low-frequency Raman spectra of disordered molecular systems

Abstract: Low-frequency Raman spectra of liquids and amorphous solids of simple organic compounds were measured to study the molecular motion in disordered systems. Weak and broad bands were observed around 100 cm −1 in the spectra of amorphous solids made of aromatic molecules. These bands are not the so-called Boson peaks related to acoustic phonons in the crystal, but the bands arising from the optical phonons consisting of librational motions of molecules. For the liquids, similar broad bands due to the same origin were observed. In addition, a very-low-frequency component of scattering attributed to the relaxation motions of molecules was revealed for several liquids. The possibility of observing the related scattering for amorphous molecular solids is discussed.

The effect of surface molecular conformational change on morphology : a study of benzil (0001).

Abstract: Computational methods increasingly aid in the understanding of the processes which take place during crystal growth, especially in their use for crystal morphology predictions Such calculations, based on solid-state structure, rely on three assumptions which generally work well for molecular solids One of these assumptions is that the crystal surface is a perfect termination of the bulk, ie surface/bulk equivalence In this work, XAS data, which indicates a difference between the surface and the bulk structure of the benzil (0001) surface, has been rationalised through molecular surface minimisation techniques

Photochemistry of phosgene in the solid phase: Dissociation, ejection, and thermal desorption

Abstract: Understanding photochemistry and energy transfer mechanisms in molecular solid films is of interest to many scientific issues, ranging from matrix‐assisted laser desorption ionization mass spectrometry to photochemical processes on polar stratospheric cloud particles We present a study of a model system: the photochemistry (hν=12–64 eV) of a molecular Cl2CO solid film at low laser power density, 10 μJ–1 mJ/cm2 for ∼10 ns pulses At hν≥35 eV, photon absorption by Cl2CO leads to a major photodissociation channel resulting in CO (g) and Cl (g) and a minor molecular Cl2CO ejection channel Both photodissociation and molecular ejection are observed at the lowest laser power density and their yields depend linearly on pulse energy This result establishes a single photon photoexcitation mechanism The electronically excited Cl2CO in the surface region of the solid film can either dissociate or convert its electronic energy to translational motion in Cl2CO The translational energy distribution of CO (g) fro

Pressure-induced electron transfer in quasi-molecular solids

Abstract: A combination of Raman scattering and synchrotron X-ray powder diffraction on Group-V metal triiodides (XI3, X = As,Sb,Bi) has revealed a continuous isostructural transition from a molecular solid to a two-dimensional layered crystal under high pressure. The loss of molecular identity proceeds by gradual pressure-induced electron transfer from intramolecular to intermolecular bonds and suppression of sp3 hybridisation. Initial pressure increase has the effect of destroying molecular units while continued compression enhances intralayer cohesion. These two effects give rise to a non-monotonic pressure variation of the X-I bond-stretching frequency.

Optical probes of proton tunneling in molecular solids

Abstract: Low-temperature optical spectroscopy of guest-host molecular crystal systems provides a very sensitive tool to study proton tunneling processes. Examples include the light-induced creation and evolution of “proton defects”, translational tunneling along hydrogen bonds as well as rotational tunneling of methyl groups. Absorption spectra at 2 K of γ-picoline single crystals evolve over time scales of days due to the slow spin conversion of the methyl groups which leads to an ordering of the crystal.

Chapter 12 – Electronic Structure

Abstract: Publisher Summary The electronic structures of the crystalline phases are expected to be closely related to the electronic levels of the isolated fullerene molecules. The electronic structure for the solid is considered from both the standpoint of a highly correlated molecular solid and as a one-electron band solid. Theoretical work on the electronic structure of fullerenes had relied heavily on comparisons of these calculations with photoemission experiments, because, in principle, photoemission and inverse photoemission experiments are, respectively, sensitive to the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) electronic densities of states. Several models for the electronic structure of fullerene molecules have been developed, ranging from one-electron Hiickel calculations or tight-binding models to first principles models. While sophisticated models yield somewhat more quantitative agreement with optical, photoemission, and other experiments sensitive to the electronic structure, the simple Hiickel models lead to the same level ordering near the Fermi level and for this reason are often used for physical discussions of the electronic structure.

Magnets based upon the molecular solid state

Abstract: A few molecular solids comprised of radicals crystallize in such a manner that the spins ferromagnetically couple. In a rare cases ferromagnetic coupling can lead to ferromagnetic ordering and the formation of a bulk magnet. This occurs below a critical temperature, Tc. Several examples of molecular solid-based magnets have been prepared with Tc's exceeding room temperature. This short review summarizes some of these molecular solid-based magnets prepared in our laboratories.

Motional diminishing of optical activity: a novel method for studying molecular dynamics in liquids and plastic crystals

Abstract: Molecular dynamics calculations and optical spectroscopy measurements of weakly active infrared modes are reported. The results are qualitatively understood in terms of the "motional diminishing" of IR lines, a process analogous to the motional narrowing of a nuclear magnetic resonance (NMR) signal. In molecular solids or liquids where the appropriate intramolecular resonances are observable, motional diminishing can be used to study the fluctuations of the intermolecular interactions having time scales of 1psec to 100psec.