Evaluation of strain in heterosiliranes: Systematics,surprises, and problems

A series of different isodesmic and homodesmic reactions defining strain energies in 3-membered rings are described and discussed. One isodesmic and one homodesmic reaction are applied numerically with the purpose of estimating strain in heterosiliranes including second-row and third-row heteroatoms. All molecules involved are optimized using SCF /6-31G **, and energies are calculated using MP 2/6-31G **//SCF /6-31G ** calculations. The results are discussed with reference to limitations of the chosen models. © 1996 John Wiley & Sons, Inc.     

Interplay of thermochemistry and Structural Chemistry, the journal (volume 17, 2006) and the discipline

Abstract  

In the current study we relate the contents of the journal of “Structural Chemistry” for the calendar year 2006 to thermochemistry. Each paper is briefly summarized and supplemented by a comment which explicitly interrelates the content of the study to chemical energetics.     

Paradoxes and paradigms: high oxidation states and neighboring rows in the periodic table—Lanthanides,Actinides, Exotica and Explosives

In this note, we briefly compare oxidation states associated with adjacent rows in the periodic table. Or more precisely, we make comparisons for a few compounds containing elements from the first two rows in which a given value of the quantum number “l” appears. We start with hydrogen and lithium, proceed through rows 2 and 3 of the main group elements, and then briefly discuss some transition metal chemistry. The discussion concludes with a contrast of the lanthanides and actinides.     

Where are the angles? Angular dependence (and independence) of orbitals and functions

Certain conditions under which a collection of atomic orbitals becomes angularly independent are considered. The results are applied to molecules of chemical interest. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Exact calculation of one-dimensional irreducible integrals

One-dimensional irreducible integrals (_k) are computed in the form of Mayerf-function polynomials for a general interparticle potential. Obeisance to the exact specification of the irreducible integral definition produces regularities in the interaction of star graphs with the integration process. Tables of _k fork 5 and test solutions are presented.     

The proton affinities of styrene,trsns-β-methylstyrene,and indene

We report experimental and theoretical AM1 proton affinities of styrene,-methylstyrenes, and indene. The computed AM1 proton affinities for the species of interest were in good agreement with the experimental values.trans--Methylstyrene was found to have a proton affinity slightly lower than that of styrene. This is an unusual result since methyl substitution in most classes of compounds increases the proton affinity by 2–4 kcal mol^–1. The lower basicity oftrans--methylstyrene compared to styrene is due to the greater stabilizing effect of the methyl group in the neutral species compared to the cation.     

Coumarins via the wittig reaction. Synthesis of methoxsalen-5-14C

A new and improved synthesis of methoxsalen ( 3 ) has been developed for the preparation of specifically carbon-14 labelled 3 . Introduction of the label by a Gattermann reaction on 6-hydroxy-7-methoxycoumaran ( 1 ) followed by dehydrogenation provides an intermediate 6 which, on reaction with carbethoxymethylenetriphenylphosphorane ( 8 ) and heat inversion, provides the title compound in overall yields of up to 50%. By these procedures, 3 has been prepared ¹⁴C labelled at C-5. Alternatively, 3 may be prepared from 6 and ethyl cyanoacetate in lower overall chemical yield but in potentially higher radiochemical yield if labelling at C-6 or C-7 is required since the Wittig process utilizes an excess of phosphorane.     

The gas phase enthalpies of formation of hydrazine, its methylated derivatives, and the corresponding values for ammonia and its methylated derivatives

Hydrazine and some of its methylated derivatives are highly important species. Despite their molecular simplicity and those of the other methylated derivatives, ammonia, and the corresponding amines, there are significant problems with the literature values of their gas phase enthalpies of formation. We present our high level quantum chemical calculations at the G4 level for all of these species.     

Short-Chain Branching in Polyethylene and Poly(vinyl Chloride) Using Pyrolysis Hydrogenation Gas Chromatography and 13C Nuclear Magnetic Resonance Analysis

This study compares the use of pyrolysis hydrogenation gas chromatography (PHGC) and ¹³C Fourier transform nuclear magnetic resonance (FTNMR) methods for the analysis of reference polyethylene (PE) samples, ethylene-α-olefin copolymers, and specially prepared poly(vinyl chloride) (PVC) samples which were reduced to their PE skeletal structures. The nature and relative quantities of the short branches along the polymer chains were determined using both techniques. Improved high-resolution PHGC data, obtained with a fused silica capillary separation column, gave results which were in satisfactory agreement with the ¹³C FTNMR data. This approach confirms that detailed microstructural information can be obtained with these methods by using carefully controlled experimental conditions and appropriate reference systems.