Other evidence

In a few cases, the structure of a protein has been obtained from more than one type of crystal. The resulting models were identical, suggesting that the molecular structure was not altered by crystallization. 

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It is clear, then, that the measurement of primary kinetic isotope effects will not give a wholly unambiguous clue to mechanism in the absence of other evidence. Nevertheless, the absence of a kinetic isotope effect is most easily understood in terms of the /S 2 mechanism... 

Selected for clinical trials as a compound to calm agitated patients, imipramine was relatively ineffective. However, it was observed to be effective in the treatment of certain depressed patients (38). Early studies on the mechanism of action showed that imipramine potentiates the effects of the catecholamines, primarily norepinephrine. This finding, along with other evidence, led to the hypothesis that the compound exerts its antidepressant effects by elevating norepinephrine levels at central adrenergic synapses. Subsequent studies have shown that the compound is a potent inhibitor of norepinephrine reuptake and, to a lesser extent, the uptake of serotonin, thus fitting the hypothesis that had been developed to explain the antidepressant actions ofMAOIs. 

Ceitification and records for components and materials. The examiner shall be assured, by examination of certification, records, or other evidence, that the materials and components are of the specified grades and that they have received reqiiired heat treatment, examination, and testing. The examiner shall provide the inspec tor with a certification that all quality-control requirements of the code and of the engineering design have been met. 

Review of reactive chemicals test data for evidence of flamma-bihty charac teristics, exotherms, shock sensitivity, and other evidence of instability... 

No other evidence of deterioration, such as corrosion, was apparent on either surface. The cracks are probably material defects. They may be laps or seams that were present on the external surface prior to the fin-rolling operation and were exaggerated during the rolling process. 

Bromination of 5j5-3-ketones yields the equatorial 4 -bromo compounds (22) as the thermodynamic or kinetic products,although the presence of a considerable amount of 2-bromo isomer has been reported in bromination with phenyltrimethylammonium bromide-perbromide. This is in keeping with other evidence that enolization of 5j5-3-ketones is not specifically directed to C-4. Cleaner results would probably be obtained via thermodynamic enol acelylation. ... 

As it passed checkpoints, the car was observed to be releasing plumes of carbon dioxide from the safety valve, after which the tank exploded 15 m (50 ft) in front of a group of other rail cars. Other evidence indicates that the explosion occurred after impact with these cars. 

It is important to point out one of the limitations of mechanism studies. Usually more than one mechanism is compatible with the same experimentally obtained rate expression. To make a choice between alternative mechanisms, other evidence must be considered. A classic example of this situation is the reaction between hydrogen and iodine... 

NMR and other evidence suggests [169] that these are classical octahedral hydrides (with, as usual, trans-phosphines). 

There are, however, short Rh—H contacts (2.77-2.84A) to ortho-hydrogens in phenyl groups. The Rh—P bond trans to Cl is some 0.1 A shorter than the others, evidence of the weak trans-influence of chloride [46]. 

Since the ratio of the two sulfones 2 and 3 increases with the polarity of the solvent (from 1 4 in benzene to 16 1 in formamide) a possible concerted [2,3]sigmatropic rearrangement for the formation of sulfone 3 was first considered. However, other evidence such as the effects of solvent and added salts seem to support an ionization mechanism, with the formation of the two sulfones by recombination from two different ion-pair species40. 

Both empirical formulae have hydrogen compositions assigned to agree with the composition chosen based on other evidence. 

Failure to observe polarization in a particular reaction is significant only to the extent that any negative evidence is significant. If other evidence points to a radical pathway for the reaction, it may well be worth checking that the nuclear relaxation times for nuclei in the product are not unexpectedly short and also that polarization is not observable in a different spectral region from that expected for the final product owing to the formation of a metastable intermediate. 

A singlet precursor has, however, been proposed for 4,4,4-triohloro-2-methyl-1-butene (17) produced in the photolysis of isomesityl oxide (18) in carbon tetrachloride solution on the basis of its all-emission spectrum (DoMinh, 1971). There remains some ambiguity, however, about the detailed route by which 17 is formed. Moreover there is other evidence suggesting that ketone photolysis in carbon tetrachloride is different CI3C. CHaCiMe) CHa CH3. CO. CHa. C(Me) CHa... 

In nickel and palladium dithiobenzoato complexes, four-membered chelate rings are formed (366), whereas, in the corresponding phenyl-dithio acetates [M2(S2CCH2Ph)4], the dithio ligands act as bridging groups between the two metal atoms, with the formation of binuclear units (367). The molecular structure of the latter compounds shows that each metal atom is coordinated to four sulfur atoms and to the other metal atoms in a square-pyramidal geometry. Other evidence for... 

We can therefore conclude that in 4n systems antiaromaticity will be at a maximum where a molecule is constrained to be planar (as in 59 or the dianion of 83) but, where possible, the molecule will distort itself from planarity and avoid equal bond distances in order to reduce antiaromaticity. In some cases, such as cyclooctatraene, the distortion and bond alternation are great enough for antiaromaticity to be completely avoided. In other cases (e.g., 96 or 103), it is apparently not possible for the molecules to avoid at least some p-orbital overlap. Such molecules show paramagnetic ring currents and other evidence of antiaromaticity, although the degree of is not as great as in molecules such as 59 or the dianion of 83. 

Since the central carbon of tricoordinated carbocations has only three bonds and no other valence electrons, the bonds are sp and should be planar. Raman, IR, and NMR spectroscopic data on simple alkyl cations show this to be so. In methylcycohexyl cations there are two chair conformations where the carbon bearing the positive charge is planar (9 and 10), and there is evidence that difference is hyperconjugation make 10 more stable. Other evidence is that carbocations are difficult to form at bridgehead atoms in [2.2.1] systems, where they cannot be planar (see p. 397). ° Bridgehead carbocations are known, however, as in [2.1.1]... 

There are two possible structures for simple alkyl radicals. They might have sp bonding, in which case the structure would be planar, with the odd electron in ap orbital, or the bonding might be sp, which would make the structure pyramidal and place the odd electron in an sp orbital. The ESR spectra of CHs and other simple alkyl radicals as well as other evidence indicate that these radicals have planar structures.This is in accord with the known loss of optical activity when a free radical is generated at a chiral carbon. In addition, electronic spectra of the CH3 and CD3 radicals (generated by flash photolysis) in the gas phase have definitely established that under these conditions the radicals are planar or near planar. The IR spectra of CH3 trapped in solid argon led to a similar conclusion. " °... 

The only groups in Table 9.5 with negative values of CT/ are the alkyl groups methyl and tert-butyl. There has been some controversy on this point. One opinion is that CT/ values decrease in the series methyl, ethyl, isopropyl, /ert-butyl (respectively, — 0.046, —0.057, —0.065, —0.074). Other evidence, however, has led to the belief that all alkyl groups have approximately the same field effect and that the a/ values are invalid as a measure of the intrinsic field effects of alkyl groups. Another attempt to divide ct values into resonance and field contributions is that of Swain and Lupton, who have shown that the large number of sets of ct values (cTm, <3p, a, a+, CT/, csp etc., as well as others we have not mentioned) are not entirely independent and that linear combinations of two sets of new values F (which expresses the field-effect contribution) and R (the resonance contribution) satisfactorily express 43 sets of values. Each set is expressed as... 

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