Relative Bond Strengths Research Articles

Experimental and theoretical determination of the magnetic susceptibility of C60 and C70

THE magnetic susceptibility of C60 and the possibility of magnetic-field-induced π-electron ring currents in this carbon spheroid have been of interest since the initial experiments on carbon clusters1. If the molecule is regarded as a sphere with a radius of 3.5 A, on which 60 electrons are free to move, the Pauling ring-current model predicts a ring-current diamagnetic susceptibility 41 times the π-electron ring-current magnetic susceptibility of benzene with the field normal to the plane of the six-membered ring2,3. London theory predicts, however, that the π-electron ring currents in C60 should be weakly paramagnetic or diamagnetic, depending on the relative bond strengths used in the calculation2,3. With the availability of macroscopic quantities of C60 (ref. 4), it is now possible to study experimentally the magnetic properties of the molecule. Here we report on such measurements. We find that the diamagnetism of C60 is small, a result that we attribute to excited-state paramagnetic contributions to the π-electron ring-current magnetic susceptibility. Thus C60 seems to be an aromatic molecule with a vanishingly small π-electron ring-current magnetic susceptibility. We have performed similar measurements on C70, which indicate an appreciable π-electron diamagnetism, consistent with theoretical calculations. We attribute the differences in magnetic properties of these two molecules to their different fractions of five-membered ring structures. The fullerenes may thus constitute a class of compounds of 'ambiguous' aromatic character, traditional measures of which will not provide an adequate classification.

Effects of the simultaneous presence of nitro and amine substituents in cubane and some azacubanes

We have carried out an ab initio self-consistent-field molecular orbital study of the structures and bond orders (as measures of relative bond strengths) of nitro and aminonitro derivatives of cubane and some azacubanes. The NO2 group is found to have, in general, a conformation-dependent effect on relative bond strengths. When the plane of the nitro group is perpendicular to that of an endocyclic bond, that bond is weakened. An aza nitrogen adjacent to C-NO2, however, eliminates the bond-weakening due to a “perpendicular” nitro group, except when a C-NH2 is also adjacent to the C-NO2. The NH2 group has a direction-specific bond-weakening effect on one [and only one] endocyclic bond; the particular bond undergoing this weakening is determined by the orientation of the amine nitrogen lone pair. The influences of the NO2 and NH2 are essentially independent; when these substituents are on adjacent carbons the effects are reinforcing. Thus, the combination of a “perpendicular” NO2 and an NH2 on adjacent carbons in cubane and azacubanes produces a marked bond weakening. Our findings suggest that strained cage systems that have alternating aza nitrogens and C-NO2 groups should be further investigated as potential high-energy systems.

Computational study of relative bond strengths and stabilities of a series of amine and nitro derivatives of triprismane and some azatriprismanes

An ab initio SCF-MO procedure has been used to compute the structures, bond orders, and isodesmic reaction energies of various NH2 and NO2 derivatives of triprismane and some azatriprismanes. Each of these substituents, individually, is found to have a weakening influence upon one or more bonds. The presence of aza nitrogens in the molecular framework enhances this tendency for the NH2 group but essentially eliminates it for NO2. No reinforcement (‘push-pull” effect) is observed when the two substituents are present simultaneously on adjacent carbons. The NH2 group is found to diminish the strain energies of these molecules; however the effect of the NO2 appears to depend upon the number of aza nitrogens.

Vibrational properties of CdGeP2.

We have investigated in great detail the infrared and Raman activity of ${\mathrm{CdGeP}}_{2}$. From experimental spectra collected at low temperature, we have first identified all ${\mathrm{\ensuremath{\Gamma}}}_{1}$, ${\mathrm{\ensuremath{\Gamma}}}_{3}$, ${\mathrm{\ensuremath{\Gamma}}}_{4}$, and ${\mathrm{\ensuremath{\Gamma}}}_{5}$ frequencies predicted by group-theory arguments. Second, comparing our experimental findings with theoretical predictions for the isoelectronic sequence ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$P to ${\mathrm{InGaP}}_{2}$ to ${\mathrm{CdGeP}}_{2}$, we have discussed the relative strengths of Cd-P and Ge-P bonds compared with In-P and Ga-P, respectively.

Heat of reaction of the Cr(CO)3(C5Me5) radical with hydrogen and related reactions. Relative and absolute bond strengths in the complexes H-Cr(CO)2(L)(C5R5)

Heat of reaction of the Cr(CO)3(C5Me5) radical with hydrogen and related reactions. Relative and absolute bond strengths in the complexes H-Cr(CO)2(L)(C5R5)

A computational analysis of the electrostatic potentials and relative bond strengths of hydrazine and some of its 1,1‐dimethyl derivatives

AbstractWe have carried out a computational study of hydrazine and five of its 1,1‐dimethyl derivatives, focusing on their electrostatic potentials and relative bond strengths. Our approach has involved the calculation of ab initio self‐consistent‐field molecular orbital wave functions and molecular properties using the GAUSSIAN 82 system of programs. The electrostatic potentials of the hydrazines possess negative regions of varying sizes and strengths associated with the nitrogens of the α‐diamino linkages. Through an analysis of the positions of the most negative potentials of these regions, we have obtained directly the dihedral angles between the nitrogen lone pairs in these systems. Our use of the electrostatic potential to obtain these angles is a direct and general approach, in contrast to indirect procedures used in the past. We find this dihedral angle to be close to 90° in hydrazine, with variations in the substituted hydrazines that depend on the nature of the substituents. A highly polar structure is found for 1‐chloromethyl‐1‐methylhydrazine, which involves a delocalization of electronic charge from the substituted nitrogen towards the CH2Cl group. We find that substituents able to withdraw significant amounts of electronic density from the central nitrogen lone pair regions, either through resonance or by induction, have a slight bond strengthening effect on the central N‐N bond. This is attributed to a decrease in the repulsion between the weakened nitrogen lone pair regions. The difficulties encountered in seeking the controlled oxidation of hydrazine to nitro derivatives may be due, in part, to the fact that two factors which would favor this, highly negative nitrogen potentials and strong N‐N bonds, are opposing in nature.

Chemistry of hydrotrioxides. A comparative ab initio study of the equilibrium structures of monomeric and dimeric hydrotrioxides (CH3OOOH, H3SiOOOH) and hydroperoxides (CH3OOH, H3SiOOH). Relative bond strengths in and the gas phase acidities of hydrotrioxides and hydroperoxides

Chemistry of hydrotrioxides. A comparative ab initio study of the equilibrium structures of monomeric and dimeric hydrotrioxides (CH3OOOH, H3SiOOOH) and hydroperoxides (CH3OOH, H3SiOOH). Relative bond strengths in and the gas phase acidities of hydrotrioxides and hydroperoxides

Regioselectivity of radical attacks on substituted olefins. Application of the state-correlation-diagram (SCD) model

The SCD model is used to derive regiochemical trends in radical addition to olefins. Regiochemistry is discussed by appeal to two fundamental properties of the radical and the olefin. The first factor is the relative spin density in the {sup 3}{pi}{pi}* state of the olefin. Thus, radical attack is directed toward the olefinic carbon which possesses the highest spin density. The second factor is the relative bond strengths of the radical to the olefinic carbons. This factor directs the regiochemistry toward the olefinic terminus which forms the strongest bond with the radical. When the two effects join up, regioselectivity will be large, e.g., for CH{sub 2}=CHX (X = NR{sub 2}, CR, Cl, CN, Ph). When the two effects oppose one another, regioselectivity will be smaller, and regioselectivity crossovers are expected, e.g., for CF{sub 2}=CHF. The normal regiochemistry is shown to coincide with the spin density rule which makes identical predictions to the HOMO rule.

Relative bond strengths in tetrahedrane, prismane, and some of their aza analogs

Ab initio molecular orbital calculations at the 3–21G level have been employed to explore structures, bond order, and bond deviation indices for a series of aza- and diazatetrahedranes and aza-, diaza-, triaza-, and tetraazaprismanes as well as their hydrocarbon analogs. The introduction of nitrogens has the effect of strengthening C-C and some of the N-C bonds, while the presence of N-N bonds introduces a decrease in relative thermodynamic stability in the aza-prismane series.

Plasma etching and modification of organic polymers

Abstract

A computational study of some nitrofluoromethanes

A group of eight nitrofluoromethanes has been studied by the ab initio SCF GAUSSIAN 82 computational procedure. All structures were optimized at the 3-21G level and then used to compute bond orders, as measures of relative bond strengths, and electrostatic potentials, as guides to the molecular charge distributions. The C-F bonds were found to be strengthened by the presence of other fluorines or nitro groups; this is attributed to inductive electron withdrawal by these other substituents, which leads to significant double-bond character in the C-F bond, through resonance back-donation by the fluorine. This simultaneously weakens or even eliminates the negative electrostatic potential normally associated with the fluorine. The interesting observation that there are marked differences between the strengths of the two N-O bonds in some nitro groups can be quantitatively related to the environment of each oxygen. The C-NO2 bond strengths are relatively insensitive to the presence of other-NO2or-F substituents. However, CHF(NO2)2 is found to be significantly more stable than CH(NO2)3, presumably because the crowding of three -NO2 groups together on the same carbon is partially relieved.

Adsorption of Para‐substituted Benzoates on Iron Oxides

AbstractIn order to understand the binding mechanism and adsorptivity of benzoic acids on Fe oxides, adsorption of parasubstituted amino, nitro, methyl, methoxy, and chlorobenzoic acids onto noncrystalline Fe hydroxide and goethite from aqueous solution and vapor state was studied. Adsorption on noncrystalline Fe hydroxide from solution phase was quantified by the determination of adsorption isotherms using UV spectrometry to measure the concentration of unadsorbed organic. Adsorption of the benzoic acids from the vapor phase was further studied, using Fourier‐transform infrared spectroscopy (FTIR) of self‐supported goethite films to identify the binding mechanism and relative bond strength of the organic anion‐oxide complex. Results showed that the higher the pKa of the benzoic acid (i.e., the smaller the Hammett Constant of the substituent) the more readily the organic was adsorbed from solution. From FTIR study, an inner‐sphere coordination of the carboxylate anion to surface Fe involving a ligand exchange process was identified. Quantity of adsorption from solution was correlated to surface Fe‐benzoate bond strength as deduced from FTIR of dry oxide films, with bond strengths of para‐substituted benzoate following the order: amino > methoxy > methyl > chloro > nitro. Bidentate coordination was deduced to be the binding mechanism for chemisorption on air‐dry surfaces; however, physical adsorption was also identified in the absence of competing anions. It is concluded that electron‐with‐drawing groups on the aromatic ring weaken the carboxylate bond to surface Fe atoms by withdrawing electron density from the carboxylate group. Conversely, electron‐donating groups increase adsorption by increasing the electron density of the carboxylate, strengthening its Lewis basicity.

A computational study of some 1,4-diaminobutanes and their protonated forms

A computational study of some 1,4-diaminobutanes and their protonated forms has been carried out with the ab initio self-consistent-field GAUSSIAN 82 procedure. An optimized structure was calculated for each system, and used to obtain its atomic charges, bond orders (as measures of relative bond strengths), and electrostatic potential (for the neutral molecules). Protonation of an amino nitrogen is found to considerably weaken its bond to the carbon chain, and to a much lesser extent, its bond to an N-methyl group. The introduction of a fluorine on C 2 strengthens the adjacent C-C and the C-N bonds. Strongly-negative electrostatic potentials are found to be associated with the nitrogens in the neutral diamines, which can be attributed to their lone pairs and indicate significant degrees of basicity.

Computational analysis of the structures, bond properties, and electrostatic potentials of some nitrotetrahedranes and nitroazatetrahedranes

The structures and properties of six nitro derivatives of tetrahedrane and its mono and diaza analogues have been determined by means of ab initio self-consistent field computations. Geometries optimized at the 3-21G level were used to calculate bond deviation indices (as measures of bond strain), bond orders (to determine relative bond strengths), and molecular electrostatic potentials, as guides to reactive behavior. The bond properties (length, degree of strain, and strength) are significantly influenced by the conformation of the nitro group, although there was found to be essentially no energy barrier to its rotation. For the polynitro derivatives, the general result is a slight strengthening of C-C bonds. The molecular electrostatic potentials are also considerably affected by the electron-withdrawing -NOz substituents; the negative potentials associated with the strained C-C bonds in unsubstituted systems are absent, and the basicities of the aza nitrogens are greatly reduced.

ChemInform Abstract: Comparison of Metal‐Hydrogen, ‐Oxygen, ‐Nitrogen and ‐Carbon Bond Strengths and Evaluation of Functional Group Additivity Principles for Organoruthenium and Organoplatinum Compounds.

AbstractThe relative bond strengths for the complexes (I) and (II) are determined from equilibrium measurements.

Conformations and charges distributions in 1,2-dinitrosoethylene and furoxan: 2.-ab initio electrostatic potentials and relative bond strengths

We present and discuss ab initio bond orders and electrostatic potentials for the cis (I) and the trans (II) 1,2-dinitrosoethylene, and furoxan (III).

Evaluation of some machining processes as adherend surface treatments in bonding aluminium

—This paper reports detailed studies on some machining processes used as adherend surface preparation methods. The two conventional treatments of sand blasting and chromic-sulphuric acid etching were also studied to compare their relative merits in making strong bonds. The nature of the adherend surface produced by these processes was examined by electron microscopy and surface profile records to determine its significance in the bonding process. Aluminium alloy 2014 (untempered) was used as the adherend material and a heat-curing, two-part structural epoxy as the adhesive. It was observed that different treatments render different bond strengths, and that variation of some process parameters in each treatment influences the joint strength with the same adhesive-adherend system. The surface profile records and electron micrographs of the treated adherends give an indication of the relative bond strengths which occur after the joints are formed. Some machining processes such as shaping, milling, and turnin...

The influence of ionic radii on the incorporation of trivalent dopants into BaTiO 3

The form of incorporation of different lanthanide ions into BaTiO 3 has been investigated as a function of the dopant concentration (0.15–3.0 at.%) and of the A-to-B site ratio (0.99–1.01); microstructural examination and electrical conductivity measurements have been used to assess the incorporation mode. The ionic radii of the dopants are significant in determining the form of incorporation. La 3+ to Ho 3+ substitute for Ba 2+ and hence act as donors, while Tm 3+ to Lu 3+ replace Ti 4+ and thus behave as acceptors. Er 3+ can be forced into either the barium or the titanium sublattice by adjustment of the A-to-B ratio (it is a donor for an A-to-B ratio less than unity, an acceptor for an A-to-B ratio greater than unity and self-compensated for an A-to-B ratio equal to unity). Similar features are found in a parallel study of group IIIb dopants (Sc 3+, Y 3+ and La 3+). Sc 3+ is a typical acceptor, while Y 3+ is analogous to Er 3+. The group IIIa trivalent cations are all acceptors (B site occupancy for size reasons). On similar grounds, Bi 3+ is always an A site donor. A phenomenon of “barium site preference” is found for ions of intermediate sizes. This is discussed in terms both of the “tolerance factor” t and of the relative strengths of the valence bonds.

Theoretical study on the difference in the relative strengths of metal-hydrogen and metal-methyl bonds in complexes of early transition metals and complexes of middle to late transition metals

Theoretical study on the difference in the relative strengths of metal-hydrogen and metal-methyl bonds in complexes of early transition metals and complexes of middle to late transition metals

Direct comparison of the strength of hydrogen bonds formed by H2O and D2O

Abstract The relative strength of hydrogen bonds formed by H2O and D2O was compared directly by measuring the tensile strength of strips of pure cellulose paper immersed in liquid water, heavy water, and a 1:1 mixture. The wet strength of paper was 8% lower in H2O than in D2O and intermediate in the mixture. The cohesion of paper is due to intrafiber and interfiber hydrogen bonds between cellulose chains. Water weakened the paper somewhat more than heavy water did because it forms somewhat stronger hydrogen bonds with the hydroxyl and ether groups of cellulose and, therefore, broke more interfiber hydrogen bonds.