On the secondary structure of some vibrational bands of acetonitrile

The occurrence of shoulders on some Ramani bands of acetonitrile in relation to the formation of molecular clusters is discussed.     

Entangled polymers in condensed phases

We present Monte Carlo results on a model of polymers in a condensed phase, over a range of monomer densities. We imagine cutting a cube out of the system. This cube will typically have several polymer molecules running through its interior, and starting and ending on the boundary. These subchains will be mutually entangled and we present a way to assess the extent of entanglement complexity as a function of the monomer density and the number of subchains in the cube. The model is a set of k self-avoiding and mutually avoiding walks, properly embedded in the cube.     

Thermodynamic description of condensed phases

Phases may be smaller than visible to the human eye. In order to characterize a microphase, a phase smaller than 1 μm, one must consider surface area and free energy in addition to the standard thermodynamic functions. As one approaches nanometer sizes, one also needs to know the changing thermodynamic functions within the phases. The Gibbs–Thomson equation can be used to characterize microphases, but not nanophases. For the latter, the glass transition is needed to assess the properties in the interior. In order to classify condensed phases as liquid, solid, mesophase, or crystal, one needs to consider the molecular motion in addition to the molecular structure. Most important are large-amplitude displacements in form of translation, rotation, and conformational motion. An operational definition based on experiments and an updated classification of the phases is given. The surprising result is the observation that crystals, earlier assumed prime examples of solids, can have order–disorder transitions to more mobile mesophases, as well as a glass transition without change in crystal structure, i.e., under certain condition, they cannot be identified as a solid. To these observations, one has to add the fact that large-amplitude motion may start gradually to a more mobile phase without abrupt changes in structure. These observations limit the usefulness of the 80-year-old classification of transitions as being of first or second order. Quantitative thermal analysis is shown to be an important tool to identify the possible total of 57 different condensed states in terms of their macroscopic properties as well as molecular structure and motion.     

Inherent structures of condensed phases

The author’s subjective standpoint on the problem of inherent structure and on the history of this concept is expounded. The opinion is substantiated that basic regularities of the non-crystalline substances can be understood only on the level of inherent structures. In the case of structure of crystals this assertion is a triviality.     

Theoretical methods in thermodynamics of condensed phases

Theoretical possibilities of determining energetic and thermodynamic characteristics of chemical entities in gaseous and condensed (solid and liquid) phases are briefly reviewed. The considerations include quantum chemistry methods which enable evaluation of energetic quantities and statistical thermodynamics dependencies necessary for determining other thermodynamic characteristics. The possible applications of these methods are also discussed in brief.     

Kinetics of photochromic reactions in condensed phases

A review of kinetic data reported for a few organic photochromic systems is given. The kinetics of processes taking place in solid matrices and in crystals was briefly discussed. The effect of solid matrices manifests itself in the kinetics being controlled by distributions rather than by discrete rate constants. The photochromic reactions often require a substantial free volume to occur, hence they seldom take place in crystals without a destruction of the crystal lattice. The activation energies of thermally driven reactions are in this case related rather to crystal parameters than to the reactions themselves.     

Problems in molecular dynamics of condensed phases

A review of the recent theoretical and computational activity at the Chemistry Department of the University of Firenze in the field of molecular simulations of condensed phases is reported. The topics considered include quantitative methods for accurate free energy calculations, molecular dynamics of liquids and ionic solutions, chemical reactions in solutions, phase transformations and polymerization reactions at high pressures.     

On the structure of bilayer condensed phases confined between crystalline walls of triangular symmetry

Grand canonical ensemble Monte Carlo simulation method is used to study the structure of Lennard-Jones fluids confined between the parallel walls, formed by the (111) planes of the face centered cubic crystal. Thin slit pores with a width allowing for the formation of only two atomic layers are considered. It is shown that the structure of confined solidlike phases is very sensitive to the pore width, the misfit between the size of adsorbate atoms and the size of surface lattice, as well as the corrugation of the surface potential. In particular, when the misfit between the adsorbate atoms and the surface lattice is very small, even a weakly corrugated surface potential highly stabilizes solidlike phases of triangular symmetry. On the other hand, in the case of large misfit the packing effects become a dominating factor and lead to the appearance of solidlike phases of different structures and symmetries.     

Structure and polymerization kinetics of condensed phases

It is supposed that polymerization in the diffusion-controlled region occurs with chemically identical reaction centers in ordered and disordered parts kinetically nonequivalent, on account of differences in diffusion. Two types of medium are possible: in one the structure of the ordered regions favors the process, while defects act as kinetic traps in other defects more favorable to reaction than the ordered regions. The occurrence of several types of chemically identical but kinetically nonequivalent centers gives a qualitative explanation of various features of diffusion-controlled processes: stepwise loss of macroradicals, post-polymerization, anomalous temperature dependence of the molecular weight and polymerization rate near the phase transition point.     

Simulation of condensed phases using the Distributed Array Processor

Summary The use of a massively parallel computer for simulations of condensed matter systems at Bristol University is reviewed, with a discussion of the factors influencing the choice of algorithms. Emphasis is placed on the importance of adopting simple, easily-modifiable algorithms, based where possible on geometrical domain decomposition. Several examples of scientific applications are given.     

A re-assessment of the thermodynamic properties of iodine condensed phases

Thermodynamic properties of iodine have been calculated to 500 K. Specific heat anomalies accepted for the solid phase in previous reviews have been eliminated and a smooth specific heat curve derived. Corrections previously applied to high temperature solid and liquid enthalpy measurements were shown to be unnecessary.     

An investigation of vibrational exciton bands in solid nitrous oxide

Infrared spectra of N₂O crystals containing dilute to isotopic impurities are reported. Information on the second moments and other properties of the vibrational exciton bands has been determined from an analysis of the impurity modes, the LOTO splittings, and a comparison of the transition dipole moments of N₂O with those of CO₂. The effect of the random sense of the molecules on the spectra is discussed.     

Structure sensitive bands in the vibrational spectra of metal complexes of tetraphenylporphine

The i.r. and RR spectra of twenty Fe(TPP)LL′ type complexes have been measured to locate structure-sensitive bands. In i.r. spectra, band I (1350-1330 cm⁻¹) and band III (469-432 cm⁻¹) are spin-state sensitive whereas band II (806-790 cm⁻¹) is oxidation-state sensitive and slightly spin-state sensitive in the Fe(II) state. To examine the nature of these bands, the i.r. spectra of Co(TPP), (Fe(TPP))₂O and their d₈ and d₂₀ analogs have been measured, and empirical assignments proposed. In RR spectra, band C (1545-1498 cm⁻¹, ap) and band D (1565-1540 cm⁻¹, p) are spin-state sensitive whereas band E (391-376 cm⁻¹, p) is sensitive to both spin and oxidation states. These results on RR spectra are in good agreement with those of previous workers.     

Comment on the assignments of some vibrational bands of oxalyl fluoride

In view of existing contradictory assignments of the symmetrical stretching vibrations associated with the formal C-C and C-F bonds of trans/cis oxalyl fluoride, an additional theoretical analysis of the corresponding calculated wavenumbers was preformed on trans-C₂O₂F₂ and cis-C₂O₂F₂ based on previously calculated ab initio scaled force fields at the HF/6-31G computational level and new force fields calculated at the MP2/aug-cc-pVTZ level. This novel analysis included computational data from the isotopic shifts brought about by incorporating ¹³C and ¹⁴C atoms into the structure. A detailed examination of the calculated wavenumbers made it possible to validate the assignments of the ν₂ and ν₃ wavenumbers in the trans-C₂O₂F₂ and cis-C₂O₂F₂ molecules as the formal C-C bond stretching and the formal C-F bond symmetrical stretching vibrations, respectively.     

Coherence transfer processes in vibrational relaxation of polyatomic molecules in condensed media

The vibrational relaxation of a polyatomic molecule in a condensed host is studied by a consideration of two molecular vibrations. Relaxation processes, intermode coupling terms and vibrational frequency fluctuation contributions are retained. Population decay (T₁), dephasing (T₂), and coherence transfer rates are evaluated through second order in the limit where the host bath dynamics are rapid compared to these molecular timescales. The rates are expressed in terms of temperature and frequency dependent bath correlation functions. For the special case of a three level system (the ground state and ones where one of the two vibrational modes is excited) the important effects of anharmonicity are incorporated. It is shown that certain coherence transfer terms involve zero frequency bath correlation functions, so they should be larger than the high frequency ones which obey modified energy gap laws. A discussion is presented of the types of interactions which may contribute to these coherence transfer processes.     

Comment on bands due to simultaneous excitation in vibrational spectra

The existence of an in-phase and an out-of-phase OH stretching vibration in hydrogen bended alcohols is now a well established fact. It is shown that “simultaneous” or “double” excitation in two monomer molecules is a special case of this, not a different phenomenon. The reasoning is applicable to all cases of simultaneous excitation.     

Analysis of band broadening in vibrational high-resolution electron-energy-loss spectra of condensed methane

High-resolution vibrational electron-energy-loss spectra of multilayer condensed films of methane recorded at 20 K show a strong tailing of the vibrational bands that clearly exceeds the instrumental resolution. At low incident electron energy, this tailing is remarkably less important for the dipole-allowed bending vibration (nu(4)) than for other bands. Also, the tailing becomes less pronounced with increasing size of the molecule as demonstrated by spectra of ethane and heptane recorded under the same conditions. Dipole coupling, rotational broadening, and multiple inelastic scattering have been considered as origins of this band broadening. While the first two effects can be excluded, multiple scattering involving a low-frequency phonon band provides a reasonable explanation as demonstrated by simulations of the spectrum of methane using a classical two-stream model. A lower phonon frequency in the cases of the larger molecules is held responsible for the better resolved vibrational signals in the spectra of ethane and heptane.