Strain ionization

Other than its kinetic and thermodynamic stability (above), several properties of tetrahedrane are of interest, and can be readily calculated by current computational chemical techniques. Here we will look at four strain, ionization energy, acidity, and basicity. 

A Comparative Survey of some Salient Characteristics of 1-7 Geometry, Strain, Ionization Energies, and IR and Electronic Spectra... 

Steric and Strain Effects on Substitution and Ionization Rates... 

SECTION 5.7 STERIC AND STRAIN EFFECTS ON SUBSTITUTION AND IONIZATION RATES... 

This interpretation is supported by results on the acetolysis of the bicyclic tosylates 9 and 10. With 9, after three months in acetic acid at 150°C, 90% of the starting material was recovered. This means that both ionization to a cyclopropyl cation and a concerted ring opening must be extremely slow. The preferred disrotatory ring-opening process would lead to an impossibly strained structure, the /ran -cyclohexenyl cation. In contrast, the stereoisomer 10 reacts at least 2x10 more rapidly because it can proceed to a stable cis-cyclohexenyl cation ... 

Intramolecular cycloadditions of substrates with a cleavable tether have also been realized. Thus esters (37a-37d) provided the structurally interesting tricyclic lactones (38-43). It is interesting to note that the cyclododecenyl system (w = 7) proceeded at room temperature whereas all others required refluxing dioxane. In each case, the stereoselectivity with respect to the tether was excellent. As expected, the cyclohexenyl (n=l) and cycloheptenyl (n = 2) gave the syn adducts (38) and (39) almost exclusively. On the other hand, the cyclooctenyl (n = 3) and cyclododecenyl (n = 7) systems favored the anti adducts (41) and (42) instead. The formation of the endocyclic isomer (39, n=l) in the cyclohexenyl case can be explained by the isomerization of the initial adduct (44), which can not cyclize due to ring-strain, to the other 7t-allyl-Pd intermediate (45) which then ring-closes to (39) (Scheme 2.13) [20]. While the yields may not be spectacular, it is still remarkable that these reactions proceeded as well as they did since the substrates do contain another allylic ester moiety which is known to undergo ionization in the presence of the same palladium catalyst. 

Mass Spectra. Obtained by Gillis et al (Ref 104). Field ionization and electron impact ionization mass spectra are given by Brunee et al (Ref 54) Mechanical Properties < Sound Velocity. Hoge (Ref 77) obtained the following ultimate stress as a function of strain rate for machined discs (1.77g/cc) of PETN (all failures were brittle fractures)... 

It was suggested that three alkyl groups attached to a central carbon atom would be strained because of steric crowding (20). Such strains would be relieved during ionization to form the less strained planar carbonium ion. 

B. anthracis and related species.41,44 6 Some of these peaks have been identified (e.g., as small acid soluble spore proteins and cyclic lipopeptides), but others remain uncharacterized. There is no agreement among different laboratories as to which markers are suitable for chemotaxonomic differentiata-tion of species (i.e., are consistently found in one species versus another) or for strain identification (i.e., are reproducibly found in one strain but not another). Further, although it might be anticipated that surface proteins can be preferentially ionized or extracted, the ultra-structural origin of some peptides within the cell is not always clear. 

Arnold, R. J. Reilly, J. P. Fingerprint matching of E. coli strains with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of whole cells using a modified correlation approach. Rapid Commun. Mass Spectrom. 1998,12,630-636. 

Other pattern recognition strategies have been used for bacterial identification and data interpretation from mass spectra. Bright et al. have recently developed a software product called MUSE, capable of rapidly speciating bacteria based on matrix-assisted laser desorption ionization time-of-flight mass spectra.13 MUSE constructs a spectral database of representative microbial samples by using single point vectors to consolidate spectra of similar (not identical) microbial strains. Sample unknowns are then compared to this database and MUSE determines the best matches for identification purposes. In a... 

Bernardo, K. Pakulat, N. Macht, M. Krut, O. Seifert, H. Fleer, S. Hunger, F. Kronke, M. Identification and discrimination of Staphylococcus aureus strains using matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Proteomics 2002, 2, 747-753. 

Nilsson, C. L. Fingerprinting of Helicobacter pylori strains by matrix-assisted laser desorption/ionization mass spectrometric analysis. Rapid Comm. Mass Spectrom. 1999,13,1067-1071. 

For these reasons we have developed a different approach that measures differential expression of intact proteins.21 In this approach the proteins are extracted from the cell, separated on an HPLC column, ionized via electrospray, and automatically deconvoluted into their respective uncharged nominal masses. By this methodology it is then possible to obtain accurate, intact protein profiles of the individual strains of bacteria. Because the masses of the detected proteins are accurate to +2 Da from run to run, it is possible to subtract protein profiles from known strains to quickly identify differences in protein expression among newly mutated strains. 

Subsequently 36 strains of aerobic endospore-forming bacteria, consisting of six Bacillus species and one Brevibacillus species could be discriminated using cluster analysis of ESMS spectra acquired in the positive ion mode (m/z 200-2000).57 The analysis was carried out on harvested, washed bacterial cells suspended in aqueous acidic acetonitrile. The cell suspensions were infused directly into the ionization chamber of the mass spectrometer (LCT, Micromass) using a syringe pump. Replicates of the experiment were performed over a period of six months to randomize variations in the measurements due to possible confounding factors such as instrumental drift. Principal components analysis (PCA) was used to reduce the dimensionality of the data, fol-... 

Summary Ab initio calculated bond dissociation energies of silicon compounds will be discussed by means of atomic ionization energies and atomic orbital overlap. Ring strain energies of C- as well as Si-rings are estimated by homodesmotic reactions. The hybridization concept is critically examined in the case of silicon compounds. From the most important results a set of basic rules will be presented. 

The use of ionizing irradiation on fruits and vegetables has not been intensively studied, but a few experiments have suggested that irradiation has potential for such an application (Allende et al., 2006 Zhang et al., 2006). Results from Buchanan et al. (1998) indicated that low-dose irradiation could easily eliminate E. coli 0157 H7 from fresh apple juice, but they also showed that acid-adapted strains needed higher doses of irradiation in order to obtain a 5 D inactivation in juice (73-value decimal reduction time, i.e. the amount of time it takes at a certain temperature to kill 90% of the organisms being studied (Wikipedia, 2006)). In experiments where sprouts were irradiated... 

A variety of other highly-strained electron-rich donors also form colored complexes (similar to homobenzvalene) with various electron acceptors, which readily undergo thermal cycloadditions (with concomitant bleaching of the color).209 For example, Tsuji et al.210 reported that dispiro[2.2.2.2]deca-4,9-diene (DDD), with an unusually low ionization potential of 7.5 eV,211 readily forms a colored charge-transfer complex with tetracyanoquinodimethane (TCNQ). The [DDD, TCNQ] charge-transfer complex undergoes a thermal cycloaddition to [3,3]paracyclophane in excellent yield, i.e.,... 

Some doubt is thrown on B strain as the sole explanation of the branching effect by the observation that the tri-te -butylboron-ammonia complex is actually less dissociated than the trimethylboron-ammonia complex.227 Since the products of the ionization of these highly branched compounds contain large amounts of rearranged material, another effect may be operating. As will be discussed in the next section, many ionization reactions produce directly an ion of structure different from that of the covalent parent compound. The transition state presumably resembles the new ion or a non-classical... 

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