Limitations on mndo and mndo/ci computations of activation barriers

The ability of MNDO and MNDO/CI for predicting activation barriers of unimolecular reactions is tested for decompositions of methane, ethane, nitromethane, and methyl nitrite. MNDO is not useful for this purpose; MNDO/CI gives energy barriers in agreement with experiment to about ±10 $\text{kcal}\text{mole}$ for the types of reactions considered.     

Cidep in radicals undergoing electron transfer.A time-resolved flash photolysis ESR study

Unexpected effects in the time-resolved ESR signals from radicals exhibiting chemically induced dynamic electron polarization (CIDEP) are explained quantitatively by considering an electron-transfer reaction.     

Preferential energy transfer from N2(A) to specific energy levels of Cu atoms

Emission spectra resulting from reaction of “clean” N₂(A³ Σ_u⁺) with copper atoms were studied using a flowing afterglow apparatus. The population distribution of the Cu states was calculated from the spectrum; it indicates that Cu atoms are excited by nearly resonant energy transfer processes. N₂(A,v') + Cu(²S_{$\text{1}\text{2}$}) → N₂(X, v) + Cu^* , and that the transfer is most efficient for N₂(A,v') → N₂(X,v) transitions with large Franck-Condon factors. The preferential energy transfer results in population inversion between some of the Cu states.     

A quantitative study on the stereoselectivity of sigmatropic shift reactions in acyclic systems

The semiempirical MINDO/3 method has been used to study sigmatropic [1,3] shifts in propene, [1,5] shifts in pentadiene and [1,7] shifts in heptatriene, occurring in the suprafacial and antarafacial way. Hydrogen, fluorine and methyl (with retention or inversion of configuration) were taken as the shifting goups. For the [1,3] shifts some STO-3G and 4-31G calculations have also been performed. Good correspondence has been obtained with the stereoselectivity predictions of the Woodward-Hoffmann theory. The activation energy for the allowed reactions is 8–21 $\text{kcal}\text{mol}$ lower than the value for the forbidden modes. The shift of a F atom proceeds via an inversion-type mechanism.     

Intersystem crossing rates of pentacene in naphthalene at 1.9 K following single-mode laser excitation

The rates of nonradiativ decay and the spectra of pentacene in naphthalene are reported. The experiments were carried out at 1.9 K using a single-mode dye laser. The results in the naphthalene host are consistent with our earlier observation of transients in the p-terphenyl host, and suggest a slow rate for intersystem crossing in the molecule.     

Measurement and interpretation of the integrated infrared intensities of C2H2 and C2D2

The absolute IR intensities of acetylene and perdeuterio acetylene have been measured using the Wilson-Wells-Penner-Weber method. The intensity data have been analyzed in terms of dipole moment derivatives, bond charge parameters and atomic polar tensors. The experimentally determined intensity parameters have been compared with the corresponding values obtained from ab initio calculations.     

The formation of grignard compounds—V: The reaction of iodomethane with magnesium in allyl phenyl ether. A second-order cidnp spectrum

Analysis of product formation reveals that in the reaction of iodomethane and magnesium in anyl phenyl ether ethane and 1-butane are the major side products. The latter product is formed mainly by addition of methyl radicals to the terminal CH₂ group of allyl phenyl ehter, followed by β-scission under extrusion of a phenoxy radical. This mechanism_f is confirmed by an analysis of the CIDNP spectra during the reaction; CH₃ radicals escaping from $\text{CH}{}_3\text{CH}{}_3$ pairs and adding to allyl phenyl ether give rise to a very clear example of a second order CIDNP spectrum in the product 1-butane, while polarization in $\text{CH}{}_3\text{C}{}_6\text{H}{}_5\text{O-CH-CHCH}{}_2$ pairs, formed after α-H abstraction from allyl phenyl ether, leads to net effects in the products ethane (E) and 1-methylallyl phenyl ether (A).     

A new upper bound for a critical anomaly in the dielectric constant of SF6

The dielectric constant of SF₆ on the critical isochore was measured under conditions of high thermal stability and low gravitational disturbance. No direct evidence of a t^1-α-type anomaly was found, an upper bound for its amplitude is presented.     

Fourier transform method for collision dynamics: Model studies of one-dimensional double well and anharmonic potentials

The method described in the accompanying paper is applied to several one-dimensional model systems, separated into simpler subsystems by arbitrary reflection barriers. The examples include an asymmetrical potential well, arbitrarily subdivided, a symmetrical double well, and a gaussian potential barrier. A true second-order iterative process is used.     

A comparison of phototautomerism in different sites of free-base porphin (H2P) in n-alkane crystals

Selective laser-induced phototautomerism of free-base porphin in different n-alkanes (n-hexane, n-heptane, n-octane, nnonane, n-decane, n-undecane and n-dodecane) has been used to identify tautomer splittings in a number of sites in polycrystalline samples at 2 K. Pairs or lines arising from the two molecular orientations in each site can be reversibly transformed into each other. In the odd n-alkanes the occupation of different sites was not dependent on the rate at which the samples were frozen, but in the even n-alkanes metastable sites were only occupied following rapid quenching.     

Valence fluctuation in some Yb intermetallics by X-ray photoemission and X-ray absorption

The valence state of Yb in some of its intermetallics, YbNi₂Ge₂, YbCu₂Si₂ and YbPd₂Si₂ has been investigated by L_III(Yb) absorption edges and X-ray pnotoelectron spectra in the 4f and 4d regions. These studies establish the presence of mixed valence in all three systems and illustrate the utility of 4f and 4d spectra in the study of mixed valence in Yb compounds.     

Conformational studies of 1,2,3-trichloro- and 1,2,3-tribromopropane by nuclear magnetic resonance spectroscopy: attractive parallel (1:3) halogen-hydr

Analysis of the NMR spectra of 1,2,3-trichloropropane and 1,2,3-tribromopropane in various media shows the most stable conformer to be AG_-. The populations of several conformera have been estimated by using pure trans and gauche coupling constants obtained from closely similar molecules. The calculated populations found in non-polar solvents agree well with those obtained by electron diffraction studies in the gas phase. It is suggested that the AG_- form is stabilised relative to AG₊ by the former having two parallel (1:3) halogen-hydrogen attractions against one in the AG₊ form. Comparison is made to related molecules where the most stable conformers also have the greatest number of parallel (1:3) halogen-hydrogen interactions.     

On the reactivity of innercomplex-bonded palladium : A new specific test for cyanides in alkaline solutions

It has been found that the innercomplex salts of palladium with dimethylglyoxime, (benzildioxime and furildioxime), salicylaldoxime and 8-hydroxyquinolinc do not enter into redox reaction with carbon monoxide, cither in solid or in molecular dispersed form.Solutions of palladium-dimethylglyoxime and palladium-salicylaldoxime in alkali hydroxide or ammonia are masked with respect to many reactions of the palladium as well as of the organic components. In alkaline solutions, when adding alkali cyanide, demasking occurs, with the deliberation of the innercomplex forming organic components. For the processes of dissolution and demasking appropriated equations are suggested.The demasking of dimethylglyoximc in alkaline solutions of palladium dimethylglyoxime permits a new sensitive test for cyanide ion. This cyanide test seems to be appropriate for the detection of illuminating gas.Palladium hydroxyquinorate is insoluble in solutions of alkali cyanides, though it is not precipitable from cyanide containing palladium solutions. An explanation for this “false equilibrium” is suggested.     

Properties of strong hydrogen-bonded systems: The formation of hydrogen-bonded ion pair in amine·HCL systems

The hydrogen-bonded complexes between CH₃NH₂ and (CH₃)₂NH with HCl have been studied by the ab initio molecular orbital method using the 4–31G basis set. Calculations show that the proton potential curve has a single minimum near to the nitrogen atom in both complexes. This means that the proton has been transferred from HCl to the amine. ΔE and the dipole moment of the complexes studied are as follows: ?18.2 kcal mol^?1, 10.3 D for methylamine ·HCl, and ?21.7 kcal mol^?1 11.1 D for the corresponding dimethylamine complex. Other properties of the hydrogen-bonded ion pairs are discussed.     

Characterization of noise in inductively-coupled plasma emission spectrometry

The nebulizing process in inductively-coupled plasma emission spectrometry is a major source of the instrumental variability; an attempt has been made to isolate this source so that its influence on the final result could be evaluated. Direct measurements of the time-dependent variations of the sample supply to the plasma were achieved by using a device based on the light-scattering property of nebulized test solutions. The noise from the nebulizer and of the emission signal was characterized by autocovariance functions and power spectral densities. The types of noise observed showed f^{-$\text{1}\text{2}$} and f^{-$\text{1}\text{4}$} character, depending on the concentration of the test elements employed. Cross-covariance revealed a strong correlation between the noise sources considered. The applicability and limitations of possible correction and electronic filtering procedures are indicated.     

Preparation and aluminum trichloride-induced cationic rearrangements of bicyclo[2.2.0]hexene carboxylic esters

Reaction between the tetramethyl[4]annulene aluminum trichloride σ-complex and α, β-alkenic esters gave the corresponding 1,4,5,6-tetramethylbicyclo[2.2.0]hex-5-one-2-endo-carboxylic esters. AlCl₃-induced cationic rearrangements of these bicyclic esters yielded a number of bicyclo[3.1.0]hexene carboxylic esters and related lactones which were isolated and identified. The isomerization seems to proceed via stereospecific endo protonation and subsequent rearrangement to 2-methoxycarbonyl-1,4,5,6-exo-tetramethylbicyclo(3.1.0]hex-4-yl carbocations.     

Nuclear magnetic resonance studies of the phosphorus(v) pesticides : A rapid determination of the isomer ratio of systox

The integration circuit of the Varian A-60 nuclear magnetic resonance spectrometer is used in conjunction with vapor phase chromatography, to develop a rapid analysis technique for the determination of isomer ratios in technical Systox-Sulfotepp mixtures. The complete analysis requires less than 0.1 g of sample and only 15 min. The isomer ratio in the sample may be determined to an accuracy of ±1.5% and the undamaged sample may be recovered.