Element effect

Many of these sulphides occur naturally, for example iron(ll) sulphide, FeS (magnetic pyrites), and antimony(III) sulphide, Sb S, (stibnite). They can usually be prepared by the direct combination of the elements, effected by heating, but this rarely produces a pure stoichiometric compound and the product often contains a slight excess of the metal, or of sulphur. 

BeryUium aUoys ate usuaUy analyzed by optical emission or atomic absorption spectrophotometry. Low voltage spark emission spectrometry is used for the analysis of most copper-beryUium aUoys. Spectral interferences, other inter-element effects, metaUurgical effects, and sample inhomogeneity can degrade accuracy and precision and must be considered when constmcting a method (17). 

By the same token, in base-promoted E2 dehydrohalogenations, the rate of elimination of the halogen as a halide ion is expected to be I > Br > Cl F This element effect has indeed been documented in many instances, a few examples of which are listed in Table I... 

It should be pointed out that the existence of stable structures of the intermediate-complex type (also known as a-complexes or Wheland complexes) is not of itself evidence for their being obligate intermediates in aromatic nucleophilic substitution. The lack of an element effect is suggested, but not established as in benzene derivatives (see Sections I,D,2 and II, D). The activated order of halogen reactivity F > Cl Br I has been observed in quantita-tivei36a,i37 Tables II, VII-XIII) and in many qualitative studies (see Section II, D). The reverse sequence applies to some less-activated compounds such as 3-halopyridines, but not in general.Bimolecular kinetics has been established by Chapman and others (Sections III, A and IV, A) for various reactions. 

Pitting or local thinning of the measured element effectively puts a high resistance in series with the rest of the element and thus gives a highly inflated corrosion rate. 

This hypothesis of base catalysis of intermediate formation does no violence to and very conveniently accommodates all other experimental findings on these reactions. The element effect of Bunnett et al.56, 57 follows logically as the result of selecting all the reactions or carrying them out under conditions such that intermediate formation is the rate-determining step. 

The rate is first order with respect to [C5H10NH], showing that there is an intermediate. since the kinetic stoichiometry does not match the reaction stoichiometry. The second-order rate constant does not vary too much among six different substituents, five of which represent different elements. (Some would say that a kinetic element effect is absent.) One argues, therefore, that the C-X bond likely remains intact in the interme-... 

Kezdy-Swinboume method, 26-27 Kinetic element effect, 110-111 Kinetic isotope effects (kie), 214-219 inverse, 217... 

Mechanism Kinetic" Order P-Hydrogen Exchange Faster Than Elimination General or Specific Base Catalysis hAd Electron Withdrawal atCp Electron Release at C Leaving- Group Isotope Effect or Element Effect... 

For complicated samples where matrix or interelement effects are present, a linear calibration curve may not be valid, and one should consider using an empirical model for concentration correction. This usually requires a large set of standards of similar composition to the unknown, which generally makes analysis rather impractical. Inter-element effects can be calculated from a basic knowledge of physical parameters in combination with the appropriate use of samples of known composition, pure elemental standards or composite standards. 

XRF nowadays provides accurate concentration data at major and low trace levels for nearly all the elements in a wide variety of materials. Hardware and software advances enable on-line application of the fundamental approach in either classical or influence coefficient algorithms for the correction of absorption and enhancement effects. Vendors software packages, such as QuantAS (ARL), SSQ (Siemens), X40, IQ+ and SuperQ (Philips), are precalibrated analytical programs, allowing semiquantitative to quantitative analysis for elements in any type of (unknown) material measured on a specific X-ray spectrometer without standards or specific calibrations. The basis is the fundamental parameter method for calculation of correction coefficients for matrix elements (inter-element influences) from fundamental physical values such as absorption and secondary fluorescence. UniQuant (ODS) calibrates instrumental sensitivity factors (k values) for 79 elements with a set of standards of the pure element. In this approach to inter-element effects, it is not necessary to determine a calibration curve for each element in a matrix. Calibration of k values with pure standards may still lead to systematic errors for unknown polymer samples. UniQuant provides semiquantitative XRF analysis [242]. 

Cox AL, Johnston JN (2001) Use of the Vicinal Element Effect for Regiochemical Control of Quinone Substitutions and Its Implication for Convergent Mitomycin Construction. Org Lett 3 3695... 

Atomic absorption spectrometry is one of the most widely used techniques for the determination of metals at trace levels in solution. Its popularity as compared with that of flame emission is due to its relative freedom from interferences by inter-element effects and its relative insensitivity to variations in flame temperature. Only for the routine determination of alkali and alkaline earth metals, is flame photometry usually preferred. Over sixty elements can be determined in almost any matrix by atomic absorption. Examples include heavy metals in body fluids, polluted waters, foodstuffs, soft drinks and beer, the analysis of metallurgical and geochemical samples and the determination of many metals in soils, crude oils, petroleum products and plastics. Detection limits generally lie in the range 100-0.1 ppb (Table 8.4) but these can be improved by chemical pre-concentration procedures involving solvent extraction or ion exchange. 

The palagonite is thermodynamically unstable and, hence, reacts with seawater to form various clay minerals, including smectites (montmorillonite, nontronite, and saponite), micas (celadonite), and zeolites (phillipsite). This chemical weathering involves uptake of Si, Al, Mg, Ca, Na, and K and the release of water, reversing to some extent, the elemental effect of palagonitization. These mineral alterations tend to proceed progressively from the outer margin of the pillow basalts to their interior. 

Rappoport and co-workers work has continued in a study of the substitution of ( )-and (Z)-/3-bromo- or chloro-styrenes, (1) and (2), by MeS in DMSO-d 6 (sometimes in admixture with CD3OD) as solvent. Product studies indicated retention stereochemistry rate measurements found only a small Br/Cl element effect, slower reactions of the p-OMe bromo compounds, and retardation by CD3OD. These results are consistent with Tiecco s suggestion in 1983 that even this system, activated by only a single phenyl group, reacts through the nucleophilic addition-elimination multistep route. 

Isotope effects and element effects associated with hydron-transfer steps during methoxide promoted dehydrohalogenation reactions of jo-CF3C6H4C HClCH2X (X=Br, Cl, or F) have also been discussed, with regard to distinction between E2 and multi-step pathways. The Arrhenius behaviour of hydrogen isotope effects was used to calculate the amounts of internal hydrogen return associated with the two-step mechanism. 

ELECTROSTATIO BOND ELECTROSTATIO SUREAOE POTENTIAL ELECTROSTRIOTION ELECTROTAXIS ELECTROVALENT BOND ELEMENTARY OHARGE ELEMENTARY REACTION Elementary reaction stoichiometry, MOLECULARITY CHEMICAL KINETICS UNIMOLECULAR BIMOLECULAR TRANSITION-STATE THEORY ELEMENTARY REACTION Element effect,... 

As far as future research is concerned, one can anticipate a greater concern with the effects of the individual rare earth elements. Effects of cerium as opposed to lanthanum, or the other rare earths, may be more thoroughly researched. Perhaps the list of those elements specified by the foundry industry will eventually embrace those beyond neodymium. Nevertheless, much... 

Mechanism Kinetic order p-hydrogen exchange faster than elimination General or specific base catalysis ku ko Electron withdrawal at Cff Electron release at CJ Leaving- gronp isotope effect or element effect... 

The element effect is defined as kx/ky, where kx and ky are the rates of elimination of HX and HY (X and Y are different elements), respectively, from a single substrate. 

Ab initio calculations at the MP2/6-31+ G level have been performed for gas-phase El elimination reactions of CH3CH2X (X = NH3"1", Br, Cl, F, SH) promoted by NH . OH-, F-, PH2. SH-, and Cl- in order to determine how changes in transition-state geometry, from reactant-like to product-like, influence kinetic isotope effects.9 Secondary isotope effects (a-H) on leaving group departure are correlated with the hybridization at C7 in the transition state, whereas there is no such correlation between secondary (/5-H) isotope effects and the transition state hybridization at C/ . The primary deuterium isotope effect is influenced markedly by the nucleophilic atom concerned but approach to a broad maximum for a symmetric transition structure can be discerned when due allowance is made for the element effect. 

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