The convergence of ab initio predictions to the one- and n-particle limits has been systematically explored for several conformational energy prototypes: the inversion barriers of ammonia, water, and isocyanic acid, the torsional barrier of ethane, the E/Z rotamer separation of formic acid, and the barrier to linearity of silicon dicarbide. Explicit ab initio results were obtained with atomic-orbital basis sets as large as [7s6p5d4f3g2h1i/6s5p4d3f2g1h] and electron correlation treatments as extensive as fifth-order Mo/ller–Plesset perturbation theory (MP5), the full coupled-cluster method through triple excitations (CCSDT), and Brueckner doubles theory including perturbational corrections for both triple and quadruple excitations [BD(TQ)]. Subsequently, basis set and electron correlation extrapolation schemes were invoked to gauge any further variations in arriving at the ab initio limit. Physical effects which are tacitly neglected in most theoretical work have also been quantified by computations of non...

In pursuit of the ab initio limit for conformational energy prototypes

A relation of the form k=aN(xAxB/d2)34+b has been found to hold accurately for a large number of diatomic and simple polyatomic molecules in their ground states. (The average deviation of k calculated from k observed for seventy‐one cases is 1.84 percent.) Here k is the bond‐stretching force constant, d the bond length, N the bond order, and xA and xB are the electronegativities of the bonded atoms. If k is measured in dynes/cm×10−5 and d in Angstrom units, a and b have the values 1.67 and 0.30, respectively, for stable molecules exhibiting their normal covalencies, except those in which both bonded atoms have only one electron in the valence shell; for diatomic molecules of the alkali metals, Na2, NaK, etc., a and b are 1.180 and −0.013, respectively; for hydrides of elements having a single electron in the valence shell, 1.180 and 0.040, respectively; and for diatomic hydrides of elements having two to four electrons in the valence shell, 1.42 and 0.08, respectively. Numerous applications of the relatio...

A Relation between Bond Force Constants, Bond Orders, Bond Lengths, and the Electronegativities of the Bonded Atoms

https://science.sciencemag.org/content/sci/153/3734/373.full.pdf

Framework Rearrangement in Boranes and Carboranes

Summary A detailed account is given of a quantum-mechanical treatment of the lower cycloparaffins, cyclobutadiene and ethylene. It is found that strained bonds (in the original sense of Baeyer) are to be described as bent. Strain energies comparable with those derived from thermochemical data are calculated; the properties of strained systems are discussed; and, in particular, a detailed description of the bonds in cyclopropane is given. The stability of cyclobutadiene has been considered with respect to that of the related molecules benzene, cyclooctatetraene and diphenylene; and hybridization in ethylene has been re-examined. The pairing approximation is used throughout, but an Appendix is added, in which the molecular orbital method is applied qualitatively to cyclopropane.

I. The properties of certain strained hydrocarbons

This year, 1976, the Nobel Prize in Chemistry has been awarded for research in pure inorganic chemistry, in particular the boranes. May I say that I am most pleased and profoundly grateful. My own orientation to this field has been, as it has in all of my studies, the relationships of the chemical behavior of molecules to their three-dimensional geometrical and electronic structures. The early work on the molecular structures of boranes by X-ray diffraction led to a reasonable basis for a theory of chemical bonding different from that which is typical in carbon chemistry, and yielded an understanding of the pleasing polyhedral-like nature of these compounds. Assimilated by the preparative chemists, the principles helped to establish a large body of a hitherto unknown chemistry, which made a reality of the expectation that boron, next to carbon in the periodic table, should indeed have a complex chemistry. In these nearly thirty years both the theoretical and experimental methods have been applied by us and others to areas of inorganic, physical, organic and biochemistry. For examples, these areas include low temperature X-ray diffraction techniques, and the theoretical studies of multicentered chemical bonds including both delocalized and localized molecular orbitals. An early example is extended Huckel theory, originally developed for studies of the boranes, and even now one of the most widely applicable approximate methods for theoretical studies of bonding in complex molecules. More soundly based theories are presently in use by my research students for studying how enzymes catalyze reactions, details of which are based on the three-dimensional structures by X-ray diffraction methods. Besides illuminating particular problems, these developments may contribute toward the redefinition of areas of chemisttry, and thereby broaden the chemist’s view. Our research in the boranes and their related molecular species crosses areas of inorganic, experimental physical, theoretical and organic chemistry, and includes applications in biochemistry. More simply stated, the area is the study of the relationships of molecular structure to function. BORANES, AND EARLY STRUCTURE STUDIES By now, large numbers of chemical compounds related to polyborane chemistry exist: boron hydrides, carboranes, metalloboranes, metallocarboranes, mixed compounds with organic moieties, and others. These discoveries of preparative chemists are relatively recent. Long ago, Alfred Stock established borane chemistry. He developed the experimental techniques which were required

http://science.sciencemag.org/content/sci/196/4294/1047.full.pdf

The Boranes and Their Relatives

The infra‐red spectrum of gaseous C2D6 has been investigated from 2 to 22μ, and the Raman spectrum of the liquid has been photographed. These data have been analyzed for both the symmetries D3d and D3h. It is concluded that the equilibrium configuration of ethane cannot be reliably determined from the analysis of the data now available on the vibrational spectra of C2H6 and C2D6. However, the ``uncertain frequency'' of C2H6 is determined by application of the product rule if the symmetry is D3d. A modified valence force type potential function containing interaction terms between the two methyl groups is found to fit the fundamental frequencies of the light and heavy ethanes quite well.

Infra‐Red and Raman Spectra of Polyatomic Molecules. VII. C2D6

The heat capacities of gaseous C2H6 and C2D6 have been measured down to 93°K with the low pressure thermal conductivity apparatus previously described. An improved cryostat and better designed conductivity cells were used. A new method of obtaining the heat capacities from the thermal conductivity data is described which does not require a knowledge of the accommodation coefficient α. It is based on the assumption that when the accommodating efficiency of the wire is changed, α changes in the same direction for the gases being compared and further that the percentage change in α is greater the more its absolute value deviates from unity. It is then possible to show that the unknown α for the gas being studied can be bracketed within narrow limits by the α's of comparison gases. The heat capacity results for light and heavy ethane indicate a potential barrier restricting internal rotation of essentially sinusoidal shape and with a depth of 2750 cal./mole.

The Low Temperature Gaseous Heat Capacities of C2H6

The infra‐red spectrum of diborane has been studied on a prism spectrometer from 400 to 4000 cm—1. Absorption occurs over much of the region investigated. A reliable and complete analysis of the infra‐red and Raman spectra of diborane must await further work, but two alternative tentative analyses of the main features are presented which are in accord with the heat capacity of gaseous diborane from 100 to 300°K. One of these analyses is based on the assumed existence of a singlet electronic state at 412 cm—1 above the ground state and requires the potential barrier to internal rotation within the molecule to be of the order of 15,000 calories per mole. The alternative analysis assumes that no electronic state lies less than 1600 cm—1 above the ground state and requires a potential barrier of about 5000 calories per mole. Both analyses assume an ethane‐like structure for the diborane molecule.

Infra‐Red and Raman Spectra of Polyatomic Molecules XV. Diborane

The Raman spectra of Si2H6(g) and of Si2Cl6(l) have been photographed. Polarization measurements of the stronger lines of Si2Cl6 have been made. From a consideration of the relations among the modes of vibration of similar molecules having different symmetry, the analysis of the observed lines is facilitated by a knowledge of the assignments of the fundamental frequencies of SiH4, HSiCl3 and BrSiCl3. A definite assignment is made for the three observed lines of Si2H6. A tentative assignment of the observed lines of Si2Cl6 is made and some evidence is presented for the existence of internal rotation in this molecule at room temperature.

/pdf/the-analysis-of-the-raman-spectra-of-si2cl6-l-and-of-si2h6-g-2msc71ktar.pdf

The Analysis of the Raman Spectra of Si2Cl6(l) and of Si2H6(g)

The infra‐red spectra of acetylene and its deuterium isotopes have been studied on a prism spectrometer from 450 to 4000 cm—1. All but two of thirty observed bands have been assigned as fundamentals, first and second overtones, and one‐one combination or difference bands. Most of the bands of these types which are infra‐red active according to the selection rules are either observed or occur in a region where they may be masked by the presence of super‐imposed bands.

Fred Stitt

Papers

Infra‐Red and Raman Spectra of Polyatomic Molecules. VII. C2D6

The Low Temperature Gaseous Heat Capacities of C2H6

Infra‐Red and Raman Spectra of Polyatomic Molecules XV. Diborane

The Analysis of the Raman Spectra of Si2Cl6(l) and of Si2H6(g)

Infra‐Red and Raman Spectra of Polyatomic Molecules X. C2D2, C2DH, and C2H2