The addition of hydrogen atoms to diacetylene and the heats of formation of i-C4H3 and n-C4H3

In this article, we discuss in detail the addition of hydrogen atoms to diacetylene and the reverse dissociation reactions, H + C(4)H(2)<==>i-C(4)H(3) (R1) and H + C(4)H(2)<==>n-C(4)H(3) (R2). The theory utilizes high-level electronic structure methodology to characterize the potential energy surface, Rice-Ramsperger-Kassel-Marcus (RRKM) theory to calculate microcanonical/J-resolved rate coefficients, and a two-dimensional master-equation approach to extract phenomenological (thermal) rate coefficients. Comparison is made with experimental results where they are available. The rate coefficients k1(T, p) and k2(T, p) are cast in forms that can be used in chemical kinetic modeling. In addition, we predict values of the heats of formation of i-C(4)H(3) and n-C(4)H(3) and discuss their importance in flame chemistry. Our basis-set extrapolated, quadratic-configuration-interaction with single and double excitations (and triple excitations added perturbatively), QCISD(T), predictions of these heats of formation at 298 K are 130.8 kcal/mol for n-C(4)H(3) and 119.3 kcal/mol for the i-isomer; multireference CI calculations with a nine-electron, nine-orbital, complete-active-space (CAS) reference wavefunction give just slightly larger values for these parameters. Our results are in good agreement with the recent focal-point analysis of Wheeler et al. (J. Chem. Phys. 2004, 121, 8800-8813), but they differ substantially for DeltaH0(f 298)(n-C(4)H(3)) with the earlier diffusion Monte Carlo predictions of Krokidis et al. (Int. J. Chem. Kinet.2001, 33, 808-820).     

Interaction potentials for UF6 with itself and with rare-gas atoms

Potential curves have been calculated for the UF₆-UF₆ and UF₆-rare gas atom interactions using statistical models for the UF₆ charge density and the intermolecular interaction. Agreement with available experimental data is good.     

Kinetic-energy spectroscopy of high-rydberg fragments from keV collisions of H2+, D2+, N2+ and C2+ ions with rare-gas atoms

A method for measuring the kinetic-energy spectrum of high-Rydberg fragments from collisions of keV molecular ions with rare-gas atoms is described. The kinetic-energy spectra of high-Rydberg fragments from the collisions between D₂⁺, H₂⁺, N₂⁺ and C₂⁺ ions having 8 keV kinetic energy and thermal He and Xe are reported. Two single-collision processes for the generation of high-Rydberg fragments have been identified.     

Third virial coefficient and nonadditivity of the first-order repulsive interactions of rare-gas atoms

Corrections to the third virial coefficient derived from the first-order three-body exchange forces, ΔC₁(T), in helium, neon and argon have been calculated. Inclusion of ΔC₁ and the corrections due to the non-exchange three-body forces leads to a satisfactory agreement with experimental data.     

Fluorocyclohexanes-III 1H:4H/2H- and 1H/2H:4H-nonafluorocyclohexane and derived compounds

Two nonafluorocyclohexanes of b.p. 92° and 101°, made by fluorination of benzene,¹ have each been dehydrofluorinated to give the same six unsaturated products. These were identified by oxidation and other studies as 3H:4H- and 4H:5H-octafluorocyclohexene, 1H-1:4-, 1H-1:3-, and 2H-1:3-heptafluorocyclohexadiene, and hexafluorobenzene,² thus indicating a 1H:2H:4H-structure for the saturated precursors. The stereochemistry of the adjacent> CHF groups was established by resolution, via the brucine salt, of the 3H:4H-hexafluoroadipic acid obtained by oxidation of the 4H:5H-mono-olefin, showing that the hydrogens were trans. The complete stereochemistry of the nonafluorides was suggested by the dehydrofluorinations and confirmed by further fluorination³ to give known decafluorocyclohexanes. Treatment of 4H/5H-octafluorocyclohexene with lithium aluminium hydride in diethyl ether gave 1H,4H/5H-heptafluorocyclohexene, a precursor of pentafluorobenzene.⁴     

Metal cyanurate compounds formed directly from metal salts and urea

Compounds which have been identified as metal cyanurates have been prepared by heating certain metal oxides or salts directly with urea. Copper, nickel, cobalt, zinc, iron and manganese compounds have been formed in this manner. X-ray diffraction data and possible structures for some of these compounds are presented.     

Theory and simulation of organic solar cell model compounds: from atoms to excitons

We approach the electronic properties of a simple model of organic solar cells, a binary mixture of αα'-oligothiophenes and buckminsterfullerene, from a theoretical and numerical perspective. Close-packed model geometries are generated using a Monte Carlo method, the electronic structure is described by a reparametrized semiempirical Pariser-Parr-Pople Hamiltonian. All electronic properties, such as optical absorption spectra, tightly-bound charge transfer states and exciton bands, arise from the same atomistic Hamiltonian using a configuration interaction method involving single excitations. The absorption spectra are dominated by intramolecular contributions, whereas in the optical gap low-lying charge transfer states are predicted. The efficiency of the solar cell crucially depends on the structure of the charge-transfer exciton bands and on the relaxation mechanism. We discuss how these findings may help improve the design of organic solar cells from an excitonic view.     

Chemical Phosphorylation of Nucleosomes and Histone H4

Using N,N′-Dicyclohexylcarbodiimide (DCC) as a condensing agent, ³²P-cyanoethylphosphate and NH₃ were combined to yield ³²P-cyanoethylphosphoramidate. Treatment with NaOH resulted in a β-elimination step to yield ³²P-phosphoramidate and acrylonitrile. The phosphoramidate was purified via DEAE-cellulose chromatography. At pH 7.4 the histidine residues of histone H4 were chemically phosphorylated using ³²P-phosphoramidate as the phosphoryl donor. The nucleophilic nitrogen of the imidazole ring is attracted to the phosphorus of phosphoramidate resulting in the displacement of NH₃ and formation of the protein phosphoryl-nitrogen linkages. Chromatin monomers (nucleosomes) were prepared by a mild digestion of chromatin with micrococcal nuclease. This report also presents the chemical phosphorylation of these nucleosomes by ³²P-phosphoramidate.     

Kinetic isotope effect for the reaction of H atoms with GeH4 and GeD4

Rate constants for H atom attack on GeH₄ and GeD₄ have been measured in pulsed photolysis experiments in which H atoms were produced by the mercury-sensitized photolysis of H₂ and monitored by Lyman-α absorption. The values obtained for GeD₄ in the temperature range 293–550 K may be represented by the expression Combination of the rate constants for GeH₄ measured in this work with those previously determined in this laboratory gives, for the temperature range 293–455 K, The kinetic isotope effect indicated by these expressions is given by which yields . This is the first determination of the kinetic isotope effect for radical attack on a Ge(SINGLE BOND)H bond. The present results for the reaction of H atoms with GeH₄ have been combined with previously reported data, and the best value for the rate constant over the temperature range 210–473 K is recommended to be © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 237–243, 1997.     

Chemical synthesis with metal atoms: the reaction of chromium and nickel atoms with styrene

The preparation of C₈H₈Cr(CO)₃ (or C₈H₈Cr(PF₃)₃) and polystyrenechromium tricarbonyl (or polystyrenechromium tris(trifluorophosphine) from chromium atoms, styrene and carbon monoxide (or trifluorophosphine) is described; nickel atoms and styrene produce tristyrenenickel, which reacts with α,α′-bipyridyl to yield bipyridylstyrenenickel.     

Effective charges on atoms in some phosphorus-containing compounds from XPS data

We have calculated the charges on the atoms in 19 phosphorus compounds of the type PX₃, PX₃P, YPX₃, PX₅ by solution of the system of equations connecting the chemical shifts E_b of the inner electronic levels of the atoms with charges qA on the atoms. We have shown that for their compounds, we cannot choose a universal constant of proportionality between E_b(P) and qp. The technique used in this work, taking into account the molecular potential, allows us to describe well the effect of substitutents on the charge of the phosphorus atom. We observe a correlation between the charges qp obtained within the given approach and the MNDO method.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 4, pp. 485–490, July–August, 1990.     

Chemical ionization mass spectra of selected C3H6O compounds

The chemical ionization mass spectra of five isomers of C₃H₆O (acetone, propionaldehyde, oxetane, propylene oxide and allyl alcohol) have been determined using a variety of reagent gases (H₂, D₂, N₂/H₂, CO₂/H₂ and CO/H₂). The [C₃H₇O]⁺ ions produced by protonation of these isomers undergo very similar reactions to those reported for analogous [C₃H₇O]⁺ metastable ions; however, decomposing ions generated by chemical ionization appear to have somewhat higher internal energies. The results of ²H labelling studies (D₂ reagent gas or labelled analogues of C₃H₆O) indicate that protonation occurs mainly on oxygen and are consistent with previous investigations of metastable oxonium ions. The protonated acetone ion is particularly stable, in agreement with the higher activation energies for fragmentation of this isomer than for other [C₃H₇O]⁺ structures. As the calculated heat of protonation of C₃H₆O is reduced by changing the reagent gas, so the extent to which fragmentation occurs decreases. This is discussed in the context of competition between fragmentation and collisional stabilization of the excited [C₃H₇O]⁺* ion. It is concluded that on average a large fraction (approaching 1) of the exothermicity of the protonation reaction resides in the [C₃H₇O]⁺* ions produced initially.     

Infrared spectroscopic and density functional theory study on the reactions of lanthanum atoms with carbon dioxide in rare-gas matrices

Reactions of laser-ablated La atoms with CO2 molecules in solid argon and neon have been investigated using matrix-isolation infrared spectroscopy. On the basis of isotopic shifts, mixed isotopic splitting patterns, and CCl4-doping experiments, absorptions at 1839.9 and 753.6 cm-1 in argon and 1855.9 and 771.3 cm-1 in neon are assigned to the C-O and La-O stretching vibrations of the OLaCO molecule, respectively. Ultraviolet-visible photoinduced isomerization of OLaCO to La-(eta2-OC)O and OLa-(eta2-CO) have been observed under different wavelength photolyses in the solid matrix. The neon matrix experiments give the C-O and La-O stretching vibrations of the OLaCO- anion at 1769.5 and 779.3 cm-1, respectively. Density functional theory calculations have been performed on these products, which support the experimental assignments of the infrared spectra. The present study reveals that the C-O stretching vibrational frequencies of OMCO decrease from Sc to La, which indicates an increase in metal d orbital --> CO pi* back-donation in this series.