Showing papers on "Molecular geometry published in 1981"

Vibrational Circular Dichroism

Abstract: Since the first discovery by Biot and Pasteur that some molecules rotate the polarization of light, this optical activity has been recognized to be a consequence of molecular structure [1] In fact, natural optical activity is one of the most structurally sensitive techniques available to the spectroscopist and, hence, has been extensively exploited Most spectroscopic techniques evidence sensitivity to detailed molecular geometry only via perturbation of energy levels and selection rules However, molecular optical activity comes about as the direct result of the geometrical arrangement and interaction of the atoms in a molecule [2] In a somewhat oversimplified manner, the observable optical activity can be viewed to have a first-order dependence on the molecular asymmetry

Microwave rotational spectrum, molecular geometry, and intermolecular interaction potential of the hydrogen‐bonded dimer OC–HCl

Abstract: The microwave rotational spectra of several isotopic species of a weakly‐bound dimer (CO,HCl) have been measured. For each isotopic species, values of the rotational, centrifugal distortion, and chlorine nuclear quadrupole coupling constants were obtained. These constants are consistent with a linear equilibrium geometry with the atom order OC–HCl, thus establishing the presence of a hydrogen bond to the carbon atom of the CO molecule.. The potential binding of the HCl and CO molecules is also discussed.

Development of a flexible intra‐ and intermolecular empirical potential function for large molecular systems

Abstract: The development of a flexible intra- and intermolecular empirical potential function is described, which is designed for investigating the geometric structure of large molecular systems. The intramolecular components in the potential consist of harmonic bond stretching and angle bending terms, out-of-plane deformation terms, and torsional terms; intermolecular components include nonbonding, hydrogen bonding, and electrostatic germs. Bond lengths, angles, and torsional angles are predicted to within 2% of experiment, with most cases being within 1%. The suitability of the intermolecular potential was tested by crystal packing calculations; in all cases the results obtained were in excellent agreement with experiment.

A closed three-center carbon-hydrogen-metal interaction. A neutron diffraction study of HFe4(.eta.2-CH)(CO)12

Abstract: The crystal and molecular structure of HFe/sub 4/(eta/sup 2/-CH)(CO)/sub 12/ has been determined at 173 K by x-ray diffraction and at 26 K by neutron diffraction techniques. The complex crystallized in the monoclinic space group with unit-cell dimensions of a = 8.694 (1) A, b = 32.920 (6) A, c = 13.757 (3) A, ..beta.. = 112.95 (1)/sup 0/, and V = 3625.7 A/sup 3/ at 26 K with Z = 8. Full-matrix least-squares refinement of the neutron data gave R(F/sub 0/) = 0.060 and R/sub w/(F/sub 0//sup 2/) = 0.079 for all 5663 data. The goodness-of-fit, with a data to parameter ratio of 10.1:1, was 1.876. The only significant structural differences in the two unique molecules of the asymmetric unit were the intermolecular contacts. The iron atoms were arranged in a butterfly conformation with a C-H group nestled between the wings. The most significant structural finding was a true C-H-Fe three-center interaction, containing both a very short Fe-H distance of 1.753 (4) A (1.747 (4) A, second molecule) and the longest reported C-H bond distance, 1.191 (4) A (1.176 (4) A). The results have been discussed in relation to the origin and nature of this C-H-Fe three-center interaction, the more » activation of C-H bonds in catalysis, and possible eta/sup 2/ bonding of a C-H fragment at a metal surface. « less

Structure of ice IV, a metastable high‐pressure phase

Abstract: Ice IV, made metastably at pressures of about 4 to 5.5 kb, has a structure based on a rhombohedral unit cell of dimensions a_R = 760±1 pm, α = 70.1±0.2°, space group R3c, as observed by x‐ray diffraction at 1 atm, 110 K. The cell contains 12 water molecules of type 1, in general position, plus 4 of type 2, with O(2) in a special position on the threefold axis. The calculated density at 1 atm, 110 K is 1.272±0.005 g cm^(−3). Every molecule is linked by asymmetric H bonds to four others, the bonds forming a new type of tetrahedrally‐connected network. Molecules of type 1 are linked by O(1)⋅⋅⋅O(1′) bonds into puckered six‐rings of 3 symmetry, through the center of each of which passes an O(2)⋅⋅⋅O(2′) bond between a pair of type‐2 molecules, along the threefold axis. The six‐rings are linked laterally by type‐2 molecules to form puckered sheets that are topologically similar to such sheets in ice I, but are connected to one another in a very different and novel way. One quarter of the intersheet bonds connect not directly between adjacent sheets but remotely, from one sheet to the second nearest sheet, through holes in the intervening sheet. These remote connections are the O(2)⋅⋅⋅O(2′) bonds, passing through the O(1)‐type six‐rings. The sheets are stacked in a sequence based on ice Ic, modified by reversal of the puckering to form the remote connections and by internal distortion of the sheets to complete the remaining intersheet bonds. Of the four nonequivalent H bonded O⋅⋅⋅O distance in the structure, two (279 and 281±1pm) are only moderately lengthened relative to the bonds in ice I (275 pm), whereas the O(1)⋅⋅⋅O(1′) bond (288±1pm) and O(2)⋅⋅⋅O(2′) bond (292±1pm) are lengthened extraordinarily. This is caused by repulsion between O(1) and O(2) at nonbonded distances of 314 and 329 pm in the molecular cluster consisting of the O(1)‐type six‐ring threaded by the O(2)⋅⋅⋅O(2′) bond. The mean O⋅⋅⋅O bond distance of 283.3 pm, which is high relative to other ice structures except ice VII/VIII, reflects similarly the accommodation of a relatively large number (3.75 on average) of nonbonded neighbors around each molecule at relatively short distances of 310–330 pm. Bond bending in ice IV, as measured by deviation of the O⋅⋅⋅O⋅⋅⋅O bond angles from 109.5°, is relatively low compared to most other dense ice structures. All H bonds in ice IV except O(1)⋅⋅⋅O(1′) are required to be proton‐disordered by constraints of space‐group symmetry. The x‐ray structure‐factor data indicate that O(1)⋅⋅⋅O(1′) is probably also proton‐disordered. Ice IV is the only ice phase other than ice I and Ic to remain proton‐disordered on quenching to 77 K. The increased internal energy of ice IV relative to ice V, amounting to about 0.23 kJ mole^(−1), which underlies the metastability of ice IV in relation to ice V, can be explained structurally as a result of extra overlap and bond‐stretching energy in ice IV, partially compensated by extra bond‐bending energy in ice V. The structural relation between ice IV and ice I offers a possible explanation for the reduced barrier to nucleation of ice IV, as compared to ice V, in crystallizing from liquid water.

Ab initio studies of structural features not easily amenable to experiment: Part III. The influence of lone pair orbital interactions on molecular structure

Abstract: The characteristic structural asymmetries and distortions of AXYB systems in which an electron lone pair is at Y are discussed on the basis of the completely relaxed ab initio equilibrium geometries of a number of representative systems including various conformations of methanediol, hydrazine, 1,2-dimethylhydrazine and of compounds with CH 3 groups adjacent to OH, OCH 3 , NH, NCH 3 and C(π). It is found that, regardless of quantitative overlap and energy gap factors, all calculated trends in the relative extensions of bond distances and bond angles can be correlated in every detail to qualitative predictions based only on the orientational aspects of orbital interaction concepts.

The g -Values of Defects in Amorphous C, Si and Ge

Abstract: The g-values of the ESR signal have been calculated for dangling bonds in amorphous C, Si and Ge and for weak bonds in amorphous Si by using the EHT method. Effects on g-values of changing the bond angle, bond length and dihedral angle have been calculated and found that the change of g-values due to fluctuations of the amorphous structure is dominated by the change of the bond angle. Although the calculated g-values depend on the cluster size, the results are useful for qualitative considerations on the dangling bonds and the weak bonds.

Vibrational Spectroscopies Applied to Chemisorption and Catalysis

Abstract: Vibrational (infrared) spectroscopy is often referred to as molecular spectroscopy to emphasize that it is the source of much of the known molecular-structure data, e.g., molecular symmetry, bond lengths, and bond angles [1]. Coupled with statistical mechanics, vibrational spectroscopy can also allow the calculation of thermodynamic properties [2]. Of course, catalysis is by definition a kinetic phenomena, and therefore it is noteworthy that vibrational spectroscopy has also been used to follow the transient response of kinetically significant (as opposed to stable, unreactive) intermediates on heterogeneous catalysts [3–5].

A structural model for the interface between amorphous and (100) crystalline silicon

Abstract: A structural model for the interface between amorphous and (100) crystalline Si has been constructed by building a model of a continuous random network (CRN) on a crystalline substrate. In relaxing the atomic coordinates to minimize the elastic energy, the Keating Potential was used for the interatomic interactions. The interface consists of 121 atoms on the amorphous and 230 atoms on the crystalline side. In the 121-atom CRN model, the r.m.s. deviations of the bond lengths and bond angles are 1·05% and 6·86° respectively, and the bond-angle distribution, dihedral-angle distribution, and reduced intensity function F(s) are similar to a bulk CRN model. The principal results characterizing the interface are (a) the r.m.s. bond-angle deviation in the crystal is 3·52° (not zero), and (b) the r.m.s. bond-angle deviation on the boundary surface between the amorphous and crystalline sections is 9·05° indicating a larger distortion than in the bulk CRN model.

Bond-lengths, bond angles and transition barrier in ice Ih by neutron scattering

Abstract: Although several studies1,2 of ordinary ice (Ih) have established its nearly perfect oxygen arrangement and the statistical distribution of hydrogen among two symmetrically equivalent sites (the ‘half-hydrogen’ model3), several fundamental problems remain. The O—H bond length found in ice Ih is considerably larger than in other polymorphs of ice determined precisely4,5, and many arguments have been put forward against the value observed5,6. Similarly, the tetrahedral H—O—H bond angle in ice Ih differs considerably from the value observed in the vapour phase, and this led Chidambaram7 to propose a bent hydrogen bond model which further splits the atom positions, and which has not yet been verified. Finally, the mean shape of the double potential governing the atom distribution and the barrier governing the mobility still remain to be evaluated. We show here how high-precision, short-wavelength neutron diffraction data from ice Ih can be used to evaluate the molecular structure and atomic density distribution at 60 K. The unusually long O—H distance is confirmed, but there is no evidence for bent hydrogen bonds. Indeed the hydrogen atom density distribution is well described by a librational motion of the hydrogen atom around the oxygen. The mean barrier height between the ‘half-hydrogen’ atom positions was obtained from the scattering density as 0.012 eV, which implies that the zero point motion is of high importance for the proton exchange at low temperatures.

The kinetics of conformational transitions: Effect of variation of bond angle bending and bond stretching force constants

Abstract: Conformational transitions in chain molecules have been shown to proceed via a reaction coordinate which is a localized mode involving rotations about bonds, and also bond angle bending and bond stretching. By investigating the kinetics as a function of the force constants (flexibility) for bond angle bending and bond stretching, the role of the localized mode is probed. The study reported here consists of computer simulations of the Brownian dynamics of chain motions, and of kinetic calculations of rates and reaction modes. The theory accurately predicts the relative effects of force constant variations on transition rates determined by simulation.

Structure determination of SO2 by electron diffraction

Abstract: A determination of the structural parameters of gaseous sulfur dioxide has been made with counting electron diffraction apparatus at an electron energy of 54.157 keV over the s range from 10 to 30 A−1. The structure parameters are found to be ra(S–O) = 1.4343(3) A, ra(O⋅⋅O) = 2.4718(28) A, l(S–O) = 0.0359(10) A, l(O⋅⋅O) = 0.0561(31) A, and ∢ OSO = 119.5 °(0.3 °). The quoted uncertainties are 2σ. Since the experimental results agree very well with the spectroscopic and theoretical structure parameters, it is proposed that SO2 be accepted as the standard reference molecule in place of CO2.

Structural investigation of hydrogenated amorphous silicon by X-ray diffraction

Abstract: X-ray diffraction measurements on hydrogenated amorphous Si (a-Si : H), containing 33 at.% H (the hydrogen content has been determined by Compton profile measurements), are interpreted in terms of an r-space transform function, G(r), that takes into account Si–Si as well as Si–H coordination. The one- and two-bond neighbour coordination of Si atoms are found to be almost the same as in vapour-deposited a-Si as far as bond length, r1, bond angle, α, and its standard deviation, Δα, are concerned (r1 = 2·363 ± 0·008) A; α = (109·5 ± 1·0)°; Δα = (7·9 ± 0·4)°). The reduction of the one-bond neighbour coordination number from ∼ 4 to 3·4 ± 0·1 and the reduction of the two-bond neighbour coordination number from ∼ 12 to 7·8 ± 0·2 can be understood mainly in the framework of a random-network model as a consequence of broken Si–Si bonds by assuming one hydrogen atom to be bound to each of the two Si atoms of the broken bond. But it cannot be excluded that there exists a small amount of structural units, di...

Low-temperature (20 K) neutron diffraction study of [K(crypt-222)]+[Cr2(CO)10(.mu.-H)]-. Influence of the lattice environment and the K+ ion on the anion's metal carbonyl framework and the ordered chromium-hydrogen-chromium bond

Abstract: A combination room-temperature x-ray and low-temperature (20 +- 1 K) neutron diffraction study of (K-(C/sub 18/H/sub 36/N/sub 2/O/sub 6/))/sup +/(Cr/sub 2/(CO)/sub 10/(..mu..-H))/sup -/ has been performed to determine the anion's configuration in the absence of crystallographic site symmetry constraints. Although the K/sup +/ ion is surrounded by the cryptate molecule, the ion influences the anion's solid-state structure. The relatively close separation of 2.966 (3) A between the K/sup +/ ion and 01 of the anion is nearly equal to the sum of the corresponding ionic radii of ca. 2.9 A and thereby suggests the presence of an appreciable cation-anion interaction. Although this attractive interaction does not noticeably perturb the local C/sub 4 sigma/ symmetry of the respective Cr(CO/sub 5/ fragment, it causes the anion's eclipsed metal carbonyl structure to undergo a twisting distortion toward a previously unobserved bent structure. The two independent Cr(CO)/sub 5/ groups of the anion are rotated ca. 19/sup 0/ with respect to each other about the Cr-Cr line. This configurational change in the metal carbonyl framework is accompanied by a reduction of the Cr-Cr separation to 3.300 (4) A and the Cr-H-Cr bond angle to 145.2 (3)/sup 0/, which collectively imply an increase in the metal-metalmore » bonding component of the closed three-center, two-electron Cr-H-Cr bond. The ordered bridging hydrogen atom resides in a symmetrical electronic environment as suggested by the two independent, but equivalent, Cr-H separations of 1.735 (5) and 1.723 (5) A. These Cr-H distances are comparable in magnitude to the corresponding distances in the (Et/sub 4/N)/sup +/ and ((Ph/sub 3/P)/sub 2/N)/sup +/ salts. The neutron diffraction data were measured with an automated four-circle diffractometer at the Brookhaven high-flux beam reactor.« less

A study of the arsenic, black phosphorus, and other structures derived from rock salt by bond-breaking processes. II. Band structure calculations and the importance of the gauche effect

Abstract: Band structure calculations based on the extended Huckel method order energetically the 36 arsenic‐like structures having the NaCl size unit cell in a way which is compatible with the observed crystal structures. Black phosphorus is the most stable of the 36, and the arsenic layer type is number seven. Energy differences among these structures can be explained by different energetic contributions from syn‐, gauche, and anti‐arrangements, of both bonded and nonbonded pairs of atoms, and by a destabilization accompanying small bond angles. The order of stability is gauche ≳ anti ≳syn. Lone pair repulsions are important in determining the total energy, but play only a minor role in the relative energies of these structures. The energies ascribed to all these local geometries are compatible with molecular orbital calculations on small molecules and with observed molecular conformations described by the ’’gauche effect.’’

Raman study of hydrogen bonding in α and β‐oxalic acid dihydrate

Abstract: The effects of different hydrogen bonding configurations upon the Raman spectrum of α-(COOH)2·2H2O, α-(COOD)2·2D2O, β-(COOD)2·2D2O and (COOK)2·H2O and the temperature stability of β- (COOD)2·2D2O are reported. The OH stretch frequencies correlate well to the changes in the hydrogen bond length and the bond angle. The band width of the stretch frequencies, however, is not related to the hydrogen bond strength. The α phase of oxalic acid dihydrate is the more stable, low temperature form and the β crystals were observed to transform to the α phase.

The (100) silicon—silicon dioxide interface. I. Theoretical energy structure

Abstract: The (100) Si---Si${\mathrm{O}}_{2}$ interface is characterized here in a systematic manner. A semiempirical tight-binding method is used to calculate the energy band structure and layer of density of states for (a) the free Si(100) surface, (b) the 2\ifmmode\times\else\texttimes\fi{}1 reconstructed surface, (c) the chemisorbed surface (both atomic and molecular), and (d) the interface between a thin oxide and Si. New states, typical of Si---O bonds, are identified. For the interface and for chemisorption, the Si---O bond lengths and the Si---O---Si bond angles are varied and the calculations repeated. The results show little variation in the density of states, which indicates that the variations in the results arising from differences in the actual bond lengths and bond angles are negligible.