Bond Angles Range Research Articles

Singlet–triplet surface crossings and low-temperature rate enhancement for O(3P)+H2→OH+H

Low-temperature curvature in Arrhenius plots of H-atom transfer reactions is usually attributed to tunneling. This paper investigates the feasibility of an alternative mechanism involving nonadiabatic transitions to enhance the low-T rate of O(3P)+H2→OH+H. 3A″ and 1A′ potential surfaces are calculated for the configuration OHH over a range of bond angles. The ab initio surface calculations use two basis sets: double-zeta + polarization (DZP), and DZP augmented by bond functions and Rydberg orbitals (BF), followed by extensive configuration interaction. The CI data points are fitted by a rotated Morse curve-cubic spline functional form. The surface crossing is analyzed in detail. The singlet surface crosses the triplet surface near 120° at an energy which is slightly above (2 kcal/mol) the energy of the lowest triplet barrier (13.2 kcal/mol for the linear geometry). The implications for low-temperature kinetics are discussed.

Structure of (1,10-phenanthroline)bis(4-toluenethiolato)zinc(II)

Zn(C12HsNz)(CTH7S)2), (Zn(phen)(4-tol- S)2), monoclinic, P21/c , a = 10.009 (3), b = 20.12 (1), c = 12.140(4)A, t= 107.73(1) ° , Pcalc = 1.403, Pobs = 1.396 Mg m -3, Z = 4; R = 0.058, R w = 0.061. The coordination geometry of the Zn 2+ central metal ion is distorted tetrahedral with a S-Zn-S bond angle of 126.2 ° and a N-Zn-N bond angle of 78.7 °. The N-Zn-S bond angles range from 107 to 113 °. Introduction. Complexes of Zn with the general formula (Zn(SL)2(N-N )) (N-N = substituted 1,10- phenanthroline or 2,2'-bipyridine, SL = substituted benzenethiol) are of spectroscopic interest (Truesdell, 1978; Koester, 1975). Truesdell has synthesized a large number of these complexes (including the title complex) and noted the presence of an unusual, broad, struc- tureless visible absorption band and its corresponding emission. The transitions were unique to the complexes and not present in the uncoordinated ligands. The transition was interpreted by Truesdell as interligand transmetallic charge transfer in nature. To account for this transition, Truesdell then proposed a molecular- orbital model based on C2v coordination geometry of the metal. The structural analysis reported here was undertaken to examine the coordination around the central metal atom. Bright, yellow chunks of (Zn(phen)(4-tol- S)2) were prepared by precipitation from an ethanol solution of stoichiometric mixtures of the metal and ligands. A small crystal (0.12 × 0.25 × 0.29 mm) with {010}, {010}, {011}, and {0ii} faces developed was mounted with the e axis collinear with the spindle axis of the goniometer. Systematic absences of the types hOl (l = 2n + 1) and 0k0 (k = 2n + 1) uniquely defined the space group as P2~/e. Intensity data were collected on an automated Picker full-circle diffractometer with Zr-filtered Mo Ka radiation. A 0-20 step scan was used with scans of 1.8 °, 20 steps deg -~ and 3.0 s step -1. Background measurements of 15 s were made before and after each scan. The standard deviation in intensity was calculated by oZ(Io) = TC + BC + (0.33)21o 2

The structure of rubredoxin at 1.2 Å resolution

Structural details of the model of Clostridium pasteurianum rubredoxin are presented, based on the refined model at 1.2 Å resolution. The molecule contains no extensive regions of pleated-sheet or helical structure. Regular secondary structure consists primarily of residues 3 to 7, 11 to 13 and 48 to 52 in a small region of pleated-sheet; and residues 14 to 18, 19 to 23, 29 to 33 and 45 to 49 in 3 10 helical corners. Interbond angles in the helical corners average as much as 10 ° greater than normally accepted values and a number of the peptide groups deviate significantly from planarity. Rubredoxin has a pronounced asymmetry in the distribution of charged groups on its surface. This would lead to highly favored molecular orientations when the protein interacts with other charged molecules. Bond lengths in the iron-sulfur complex range from 2.24 å to 2.33 Å, and bond angles range from 104 ° to 114 °.

A new phase of coesite SiO2

Abstract A coesite crystal synthesized from aqueous solution (Arndt and Rombach, 1976) under static high-pressure high-temperature conditions of 44 – 49 kb and 610°C was investigated by X-ray diffraction. From the lattice constants a = 7.148(2)Å, b = 12.334(3)Å, c = 7.112(2)Å, β = 120.30(2)°, and the space group P21/a we conclude the existence of another coesite phase, which is about 1 % denser than the known phase with C2/c (Gibbs et al., 1977). The overall arrangement of the atoms in the new phase is closely related to the C2/c phase, but the SiO4 tetrahedra are of considerably different sizes. The finding of a new, denser coesite phase implies that the silica phase diagram may consist of even more phases than presently known. The range of bond lengths and bond angles was used to check for linear correlations which would give supporting experimental evidence for the 7r-bond model in the silicate ion (Cruickshank, 1961).

The crystal and molecular structure of tetrakis(cyanomercuri)methane hydrate

The crystal structure of tetrakis(cyanomercuri)methane hydrate, C(HgCN) 4 · H 2O, has been determined from diffractometer X-ray intensity data by means of Patterson and Fourier methods and refined by the least-squares technique based on 911 independent reflections to the index R of 0.058 and R ω of 0.065. Crystals are monoclinic holohedral, space group P2 1/ n with Z 4 formula units in the unit cell of dimensions a 8.520(5), b 13.622(8), c 10.783(6) Å, β 92.48(5)°, D obs 4.99 g cm −3, D calc 4.97 g cm −3. The structure consists of discrete molecules of tetrakis(cyanomercuri)methane and molecules of water of crystallization. The bond angles at the methane carbon atom range from 105(2) to 114(3)° and the mean HgC(methane) bond length is 2.05(3) Å, while the distance of all the four mercury atoms from the geometrical centre of the tetrahedron is 2.053(3) Å. The CHgC bond angles range from 175(3) to 178(2)°. The mean value of the HgC(cyanide) bond length is 2.03 Å. The one OH⋯N hydrogen bond per water molecule is 2.77 Å long.

AB initio studies of the dependence on geometry of the deuteron quadrupole coupling constants and the nuclear diamagnetic shieldings in the bihalide ions FHF −, FHCl − and ClHCl −

Ab initio LCAO-MO-SCF calculations have been performed on each of the bihalide ions FHF -, FHC1 - and ClHCl - over a range of bond lengths and bond angles around their equilibrium geometries. The sensitivities of the computed total energies, deuteron quadrupole coupling constants and nuclear diamagnetic shieldings to variations in molecular geometry have been studied.

The crystal and molecular structure of N-(6-chlorophthalan-1-ylidene)-1-(methylazo)cyclohexylamine

The crystal and molecular structure of N-(6-chlorophthalan-1-ylidene)-1-(methylazo)cyclohexylamine, C15H18-CIN3O, has been determined by a three-dimensional single-crystal X-ray diffraction study. This compound crystallizes in space group P21/c with lattice parameters a= 7·990 ± 0·006, b= 10·812 ± 0·011, c= 16·987 ± 0·018 A, and β= 91·15 ± 0·03°(Z= 4). The intensity data were collected on a Picker automatic diffractometer (Cu-Kα radiation) and the structure was solved by iterative application of Sayre's equation. All hydrogen atoms were located by difference synthesis. Least-squares refinement of atomic positions, hydrogen isotropic thermal parameters, and anisotropic thermal parameters for all other atoms converged at a final R= 5·7% for 1340 reflections above background.The condensed ring system is found to be planar, while the cyclohexane ring takes a chair conformation with the azo-group equatorial and the condensed ring axial. The methyl and cyclohexyl substituents of the azo-group are trans. The molecule contains a localized CN double bond of length 1·243 A, and three C–N single bonds with distances 1·461, 1·472, and 1·494 A. The oxygen atom forms bonds of 1·445 A to the saturated carbon atom and 1·386 A to the unsaturated carbon. The length of the azo NN bond is 1·230 A. The carbon–chlorine bond distance, 1·720 A, is normal. Carbon–carbon bonds in the benzene ring vary from 1·355 to 1·424 A while the bond angles range from 118·8 to 122·4°; these variations may indicate some minor distortion.

Phosphorus–fluorine chemistry. Part XXVIII. Fluorophosphines with bulky substituents as ligands in transition metal carbonyl complexes

Bis(t-butyl)fluorophosphine, the first stable dialkylfluorophosphine, and the corresponding difluorophosphine, were evaluated with regard to their ligand properties. A series of transition-metal carbonyl derivatives of types (CO)3NiL, (CO)5ML, cis-(CO)4MoL2, cis-(CO)3MoL3(L = ButPF2 and But2PF; M = Cr, Mo, W) was prepared. Using i.r. and n.m.r. data (1H, 19F, 31P) the relative π-acceptor character of the phosphines in the series PF3–nButn(n= O–3) was assessed. It is proposed to use the quantity ΔJ/JL(ΔJ=Jc–JL; Jc= P–F or P–H coupling constant of the co-ordinated ligand; JL= P–F or P–H coupling constant of the free ligand) as a measure of the π-donor strength of the metal carbonyl fragments, M(CO)x(M = Ni, x= 3; M = Cr, W, x= 5; or M = Mo, x= 3–5). 31 P Chemical-shift values of the free phosphines, ButnPF3–n(n= 1–3) were calculated for a range of bond angles, on the assumption of a bond skeleton of C3v symmetry. Evidence for an increase in bond angle from ButPF2 to But3P was found.

Bond angle deformation and hybridization in H 2O

Results of minimum basis SCGF calculations on H 2O are reported for seven different HOH angles from 90 to 120°. Optimum hybridization is nearly constant over the range of bond angles considered, thus suggesting the existence of “bent bonds” for all but the equilibrium bond angle, where bond and hybrid angles coincide.

Structure of poly(N‐butyl isocyanate)

AbstractAn x‐ray study of poly(N‐butyl isocyanate) (PBIC) was undertaken with the aim of providing the structural information required for the calculation of its ultraviolet absorption spectrum. Well defined diffraction patterns are obtained from powder samples and oriented films of the polymer cast from cyclohexane solution. All the reflections can be satisfactorily indexed in terms of a pseudo‐hexagonal unit cell with a = 13.3 Å and c = 15.4 Å. This indexing and the intensity distribution lead to the conclusion that PBIC has a helical structure with a translation of 1.94 Å and a rotation of 135° per monomeric unit (i.e., the c axis corresponds to 8 units in 3 helical turns). In the absence of single‐crystal data on closely similar model compounds, a range of possible bond lengths and angles was assumed. The atomic coordinates of the chromophoric group of PBIC were then computed using the restrictions imposed by the helical parameters. From an investigation of all the assumed range of bond lengths and angles, a few closely similar structures having no unacceptably short van der Waals contacts were found. A comparison of calculated and measured densities indicates that there are two molecules per unit cell. Construction of space‐filling models of PBIC supports this conclusion.

LCAO SCF Computations for Ammonia

A series of LCAO SCF calculations with Slater-type atomic orbitals were performed for the NH3 molecule over a range of bond lengths and angles (keeping at least C3v symmetry). In addition to a minimal basis set (with two different values for the hydrogen 1s orbital exponent), two types of extended basis sets were examined for the planar and the experimental equilibrium configurations. One of these consisted of a ``double zeta'' set on N and one 1s function on each H (with exponent 1.20), while the other involved a minimal set augmented by four ``bond orbitals''−2s and 2p functions centered on N but having their maximum densities in the bond regions. The results for the minimal basis set are typical for this type of calculation, giving good ionization energies and an inversion barrier height which is twice the experimental value. The extended basis sets, though giving fairly low total energies, resulted in an excessive concentration of charge on the nitrogen atom, with a high dipole moment and a zero, or even small negative, inversion ``barrier''. Direct calculations were also performed for the NH3+ ion in the pyramidal and planar configurations.

Approximate Self-Consistent Molecular Orbital Theory. III. CNDO Results for AB2 and AB3 Systems

The approximate self-consistent molecular orbital theory with complete neglect of differential overlap (CNDO) presented in earlier papers has been modified in two ways. (a) Atomic matrix elements are chosen empirically using data on both atomic ionization potentials and electron affinities. (b) Certain penetration-type terms, which led to excess bonding between formally nonbonded atoms in the previous treatment, have been omitted. The new method (denoted by CNDO/2) has been applied to symmetrical triatomic (AB2) and tetratomic (AB3) molecules, for a range of bond angles. The theory leads to calculated equilibrium angles, dipole moments, and bending force constants which are in reasonable agreement with experimental values in most cases.