Heteroatomic Molecules Research Articles

Evolution of surface morphology during epitaxial growth

We examine the type of information that can be obtained from Monte Carlo simulations of epitaxial growth. A basic model will be first introduced and some of the features that make it suitable for describing both atomic-scale processes and large-scale morphologies will be pointed out. The ability of this model to reproduce experimental data will then be addressed. The first example discussed will be growth on GaAs(OO1) vicinal surfaces, where the density of surface steps on the simulated surfaces reproduces quantitatively the evolution of the reflection high-energy electron diffraction (RHEED) intensity oscillations for appropriately chosen growth and diffraction conditions. This work will then be used as a basis for examining the predictions of the simulated surface morphologies on patterned substrates, based on comparisons with micro-RHEED measurements. Extensions of the basic model to more complex growth scenarios where the atomic constituents are delivered in the form of heteroatomic molecules will also be discussed.

Orientation and alignment of 2P [formula omitted] fragments following photodissociation of heteroatomic molecules

We theoretically treat the effect of orientation and alignment of the atomic photofragments produced in the photodissociation of diatomic molecules. The theory describes all possible nonadiabatic interactions in the decaying molecules as well as rotation of the molecular axis and coherent effects. The production of 2P 3 2 fragments, when the interaction between the atoms at large internuclear distance is of the quadrupole—quadrupole type, is discussed in detail. We discuss the results of two different types of experiments: (a) polarization of the fluorescence of excited Na (3 2P 3 2 ) atoms produced in the photodissociation of NaI and (b) orientation and alignment of metastable Tl(6 2P 3 2 ) atoms produced in the photodissociation of TlBr at 266 nm. The theory explains for (a) the results of the experiments that failed to detect a linear polarized Na fluorescence and for (b) the observed large orientation and alignment effects. We also discuss the orientation effect in case (a) if one uses circularly polarized dissociation light.

From Adatom Migration to Chemical Kinetics: Models for MBE, Mombe and MOCVD

AbstractThe application of Monte Carlo simulations to various epitaxial growth methods is examined from the standpoint of incorporating only those kinetics processes that are required to explain experimental data. A basic model for molecular-beam epitaxy (MBE) is first introduced and some of the features that make it suitable for describing atomic-scale processes are pointed out. Extensions of this model for cases where the atomic constituents of the growing surface are delivered in the form of heteroatomic molecules are then considered. The experimental scenarios that is discussed is the homoepitaxy of GaAs(001) using metalorganic molecular-beam epitaxy (MOMBE) with triethylgallium (TEG) and precursors and using MOCVD with trimethylgallium (TMG). For MOMBE, the comparisons between simulations and experiments are based on reflection high-energy electron diffraction intensities, by analogy with comparisons made for MBE, while for metalorganic chemical vapor deposition (MOCVD) the simulations are compared to in situ glancingincidence x-ray scattering measurements. In both of these cases, the inclusion of a second mobile species to represent the precursor together with various rules for the decomposition of this molecule (in terms of rates and local environments) with be shown to provide a useful starting point for explaining the general trends in the experimental data and for further refinements of the model.

Formation of exotic atoms by direct capture and via transfer from hydrogen

Exotic atoms like muonic atoms can be formed either by direct capture, often called Coulomb capture, or by muon transfer from a muonic hydrogen atom. The first estimates for the formation predicted, for both mechanisms, probabilities approximately proportional to the charge numberZ. The experiments did not confirm such a simpleZ-dependence. In direct capture, it turned out that the chemical bond played a decisive role, at least for lighter elements. In the formation via transfer, surprising data were obtained not only for heteroatomic molecules but also for noble gases. Whereas for direct capture, a model is able to reproduce quite satisfactorily a great number of measured capture ratios, including those in hydrogen compounds, the formation via transfer seems to put completely different and more fundamental questions.

Proton-tunnelling and dielectric relaxation spectroscopy

Dielectric relaxations of hetero-atom hydride molecules diluted in hydrocarbon media can often be observed down to liquid helium temperatures, where their interpretation is greatly simplified. In the absence of thermal activation, relaxation rates are determined by phonon-assisted tunnelling and become very sensitive to the relevant mass or moment of inertia. Significant skeletal movements are eliminated and only hydride rotations and nitrogen inversions occur. Relaxation data are intimately related to a tunnel splitting parameter which in favorable cases can also be found from the rotational spectrum of the free molecules. The barrier to internal movement is calculable from this parameter. The best applications have been to phenols and secondary amines and a particular example of each is described.

Heteroatomic molecules and heteroligand complexes within perturbation theory

Heteroatomic molecules and heteroligand complexes within perturbation theory

Software package for correlating structure and properties in heteroatomic molecules by means of weighted-graph invariants

Software package for correlating structure and properties in heteroatomic molecules by means of weighted-graph invariants

An extended version of boyd's force field method applicable to heteroatomic molecules: Part 2. l-Valine, dl-serine and l-threonine

The results of a theoretical study to determine the equilibrium structure, vibrational spectrum and thermodynamic properties of l-valine, dl-serine and l-threonine are presented. The force field method developed by Boyd and conveniently extended to include non-hydrocarbon molecules, i.e. those containing heteroatoms, is shown to be generally effective in predictive studies of large complex organic molecules.

An extended version of boyd's force field method applicable to heteroatomic molecules Part 3. Glycine, d-isoleucine, dl-norleucine

The equilibrium structures, vibrational spectra and thermodynamic properties of three aminoacids, glycine, d-isoleucine and dl-norleucine, have been determined by force field calculations. The minimal set of force constants between heteroatoms N and O is shown to yield calculated properties consistent with the results of previous work.

An extended version of Boyd's force field method applicable to heteroatomic molecules: Part 1. Adenine and uracil

The force field method developed by Boyd is extended to include molecules containing atoms other than C and H (e.g., N, O, P, S, Cl, Br, …). A new set of force field parameters is determined in order to redefine the potential energy functions that govern the dynamics of the internal (valence coordinates) degrees of freedom of a molecule. It is shown that the minimum of the partial potential energy surface is significantly affected by electrostatic intramolecular interactions. In this regard the non-bonded interactions appear to be less important than the dipole—dipole type interactions for a given interatomic distance when heteroatoms are present in the molecular framework. The reliability of the extended method as regards minimized structure, vibrational spectra and thermodynamic properties has been checked for more than 20 polyatomic molecules. From the correlation between calculated and experimental properties it is concluded that the method has good potential for further applications on polyatomic molecules with increasing size and topological compexities such as adenine and uracil.

An extended version of Boyd's force field method applicable to heteroatomic molecules: Part 1. Adenine and uracil

The force field method developed by Boyd is extended to include molecules containing atoms other than C and H (e.g., N, O, P, S, Cl, Br,…). A new set of force field parameters is determined in order to redefine the potential energy functions that govern the dynamics of the internal (valence coordinates) degrees of freedom of a molecule. It is shown that the minimum of the partial potential energy surface is significantly affected by electrostatic intramolecular interactions. In this regard the non-bonded interactions appears to be less important than the dipole-dipole type interactions for a given interatomic distance when heteroatoms are present in the molecular framework. The reliability of the extended method as regards minimized structure, vibrational spectra and thermodynamic properties has been checked for more than 20 polyatomic molecules. From the correlation between calculated and experimental properties it is concluded that the method has good potential for further applications on polyatomic molecules with increasing size and topological compexities such as adenine and uracil.

An extended version of Boyd's force field method applicable to heteroatomic molecules: Part 2. L-Valine, DL-serine and L-threonine

The results of a theoretical study to determine the equilibrium structure, vibrational spectrum and thermodynamic properties of L-valine, DL-serine and L-threonine are presented. The force field method developed by Boyd and conveniently extended to include non-hydrocarbon molecules, i.e. those containing heteroatoms, is shown to be generally effective in predictive studies of large complex organic molecules.

Calculations of ground- and excited-state potential surfaces for conjugated heteroatomic molecules

Calculations of ground- and excited-state potential surfaces for conjugated heteroatomic molecules

ChemInform Abstract: PHASE TRANSITION EFFECTS: A CRYSTALLOGRAPHIC CHARACTERIZATION OF THE TEMPERATURE DEPENDENCY OF THE CRYSTAL STRUCTURE OF THE 1:1 CHARGE TRANSFER COMPLEX BETWEEN ANTHRACENE AND TETRACYANOBENZENE IN THE TEMPERATURE RANGE 297 TO 119 K

Crystal structures of the charge transfer complex between anthracene and tetracyanobenzene, A:TCNB, C14H10: C10H2N4, at three temperatures (297, 234 and 226 K) above the order–disorder phase transition at 206 K and at four temperatures (202, 170, 138, and 119 K) below it have been determined from x‐ray diffraction data. The space group of the room temperature phase was assigned as Cm with a=9.528(2), b=12.779(3), c=7.441(2) A and β=92.39(2) degrees at 297 K and for the low temperature phase is P21/a with a=9.457(1), b=12.689(2), c=7.325(1) A and β=92.98(1)° at 119 K; Z=2 in both phases. The electron densities, plotted for both the donor and acceptor molecules at each temperature, indicate that there is dynamic librational disorder in the A molecules that gives rise to a single average orientation in the room temperature phase. Below the phase transition, two temperature dependent, symmetry related, orientations are observed; the libration diminishes slowly as a function of temperature in this phase. There are also modest reorientations, relative to the room temperature phase, in the TCNB molecules as the crystal is cooled. Crystal packing indicates that a network of intermolecular dipole–dipole interactions between the heteroatomic acceptor molecules accounts for their lack of libration comparable to that of the donors.

Evaluation of MINDO/3 calculations in simple amides, ureas, and heterocycles of biological interest

AbstractThe performance of the MINDO/3 method has been evaluated in simple heteroatomic molecules that are representative of several important biological systems. Molecular geometries are accurate to within 0.04 Å for bond lengths, and 3° for most bond angles. Dipole moments and charge distributions are estimated satisfactorily, and ionization potentials are predicted to an accuracy of 0.5 eV. The method is less accurate in estimating the ionization of some upper level σ orbitals. The overall evaluation shows that MINDO/3 provides a good description of the geometries and electronic properties of the molecules studied. The results of MINDO/3 calculations in several other chemical systems are also reviewed.

Phase transition effects: A crystallographic characterization of the temperature dependency of the crystal structure of the 1:1 charge transfer complex between anthracene and tetracyanobenzene in the temperature range 297 to 119 K

Crystal structures of the charge transfer complex between anthracene and tetracyanobenzene, A:TCNB, C14H10: C10H2N4, at three temperatures (297, 234 and 226 K) above the order–disorder phase transition at 206 K and at four temperatures (202, 170, 138, and 119 K) below it have been determined from x-ray diffraction data. The space group of the room temperature phase was assigned as Cm with a=9.528(2), b=12.779(3), c=7.441(2) Å and β=92.39(2) degrees at 297 K and for the low temperature phase is P21/a with a=9.457(1), b=12.689(2), c=7.325(1) Å and β=92.98(1)° at 119 K; Z=2 in both phases. The electron densities, plotted for both the donor and acceptor molecules at each temperature, indicate that there is dynamic librational disorder in the A molecules that gives rise to a single average orientation in the room temperature phase. Below the phase transition, two temperature dependent, symmetry related, orientations are observed; the libration diminishes slowly as a function of temperature in this phase. There are also modest reorientations, relative to the room temperature phase, in the TCNB molecules as the crystal is cooled. Crystal packing indicates that a network of intermolecular dipole–dipole interactions between the heteroatomic acceptor molecules accounts for their lack of libration comparable to that of the donors.

Application of the similarity measure. An estimation of the degree of fragmentation of a molecule in ground and excited states

The degree of fragmentation (based on the similarity measure [1] calculated using the SCF—SCI—PPP method) of 16 selected alternant, non-alternant and heteroatomic conjugated molecules is reported. The relative degree of fragmentation is scaled by means of the ground-state properties of the compounds considered, and tested using two well-investigated examples. Some properties of the low-lying excited states of these compounds are discussed in terms of their degree of fragmentation.

Gas chromatographic detection and identification of aromatic and aliphatic hydrocarbons in complex mixtures by coupling photoionization and flame-ionization detectors

A technique has been developed that is based on analyzing the effluent from a gas chromatograph with both photoionization (PID, with 10.2-eV lamp) and flame-ionization (FID) detectors. The relative molar response per mole of carbon (RMR) for the FID is similar for many types of carbon atoms, e. g., aromatic and aliphatic and therefore the FID is used to measure the relative levels of a particular hydrocarbon regardless of the degree of saturation. The RMR for the PID, however, increases with unsaturation. A ten-fold difference is observed between aromatics and alkanes, and the alkene response on the PID is midway between these two classes of compounds. When the PID and FID responses ( i. e., peak areas) are normalized to any alkene ( e. g, n-hexane) and are compared, those compounds with a normalized PID/FID ratio of 2–4 are alkenes, those with a ratio of 5–10 are aromatics and those with similar ratios (<2) are alkanes. Preliminary work on this technique was started with simple hydrocarbon mixtures, then extended to high-molecular-weight heteroatom molecules such as chlorinated and sulfur-based pesticides. Finally, a practical application of this approach was demonstrated with the successful analysis of aromatics in a light hydrocarbon feedstock for a synthetic natural gas plant. This technique should prove useful for analyses (nanogram levels) and for the identification of complex hydrocarbon mixtures.

The phenomenon of formation of prebiological compounds in volcanic processes.

Organic matter has been found in the juvenile ash of seven volcanoes in Kamchatka, the Kurile Islands and Indonesia. Its amount in one eruption is of the order of 1 00 000 tons. This matter constitutes a multicomponent mixture (more than 150 components) of, mainly, high boiling (b.p. over 250 degrees C) organic compounds of a complex structure. These are represented by hydrocarbons of saturated and aromatic nature, and, among them, polycyclic hydrocarbons, amino acids, amino sugars and other heteroatomic molecules, also containing N, O, S and C1. The formation of the above mentioned organic compounds is associated with volcanic processes--with abiogenous synthesis taking place in ash-gas clouds and, possibly, in the entrails of the Earth (hydrocarbons and their heteroatomic derivatives have also been found in volcanic bombs). On the strength of these facts, volcanic phenomena are regarded as the process which serves as the starting point of the chemical evolution from the inanimate to the animate matter.

Construction of a scheme for the degradation of conjugated hydrocarbons

1. The concept of degradation indices is introduced into the Huckel π-electron approximation treatment of conjugated systems, and the values of these indices used for comparative studies of various unsaturated molecule degradation schemes. 2. Theoretical thermodynamic stability sequences obtained for certain hydrocarbons and heteroatomic molecules prove to be identical with the corresponding experimentally developed sequences. Mechanisms are proposed for polyacene and polyphenylene decompositions.