Electron-impact studies

Early electron-impact experiments with diborane (67, 146) produced evidence for the whole gamut of singly charged ions, B2Hg, B2H5, B2Hij, B2H3,  

Negative ions have also been produced from dibor-ane. Although the earliest attempt (259) in this direction failed, very recently (91, 229) B2H5,  

and B have been detected, but not B2Hg. The B2H7 

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Electron impact mass spectrometry has been employed to study the fragmentation patterns of isoxazolylmethyl- and bis(isoxazolylmethyl)-isoxazoles and the results are in agreement with proposed pathways (79AC(R)8l). Electron impact studies of nitrostyryl isoxazole (6) show fragmentation in a variety of ways. The standard loss of NO2 from the molecular ion... 

Extensive mass spectral and electron impact studies have been reported for 3-hydroxy-1,2-benzisoxazole and its ethers. Similar work was also carried out with the isomeric A-alkyl-l,2-benzisoxazol-3-one (71DIS(B)4483). 1,2-Benzisoxazole A-oxide showed a mass spectral pattern than more closely resembled furoxans. The loss of NO predominated over the loss of O (Aft intense, [M— weak, [Af-30]" strong). 

The total reaction cross-sections of the individual primary and secondary species were derived and are compared in Table III with reactivities determined from previous electron impact studies (10, 31). 

When the counterion is complex, for example metal-halogen anions such as BF4-, the most electronegative portion of the counterion becomes attached to the silicon center. Because of this attachment, it is natural to consider the intermediacy of a silicenium cation (silylium or silylenium ion) intermediate in such reactions (Eq. 4). Bond energies derived from electron impact studies indicate that Eq. 4 is exothermic in the gas phase by about 8 kcal/mol.26,29 There seems little doubt that trivalent silicon-centered cationic species do exist in the gas phase30,31 or that processes similar to that shown in Eq. 4 do occur there.32,33... 

III, F. More phase diagrams of complex fluorides have been explored using Knudsen cell mass spectrometry (13, 15), and electron impact studies have yielded enthalpies and bond energies (1,3). The heat of formation of MoFmi) has been confirmed (12). Solid state cells have been used with lanthanide trifluorides (24) and NaNiF3 (21). 

Svec, H.J. Junk, G.A. Electron-Impact Studies of Substituted Alkanes. J. Am. 

An electron impact study of 1,3-dichlorotetrafluoroacetone has provided a value of 11.95 e.v. for the appearance potential of the CF2C1+ ion. There is at present no value for the ionization potential of the CF2C1 radical available but it would be safe to assume that it would lie between the limits of 8.78 e.v. for CC13 and 10.0 e.v. for -CF3, so that the C-C bond strength will not differ greatly from that of acetone itself. 

Three more estimates based on electron-impact studies have appeared recently. Hobrock and Kiser89 measured appearance potentials in CF2C1H and deduced AH298 CF2 = — 20 kcal/mole based on the assumption that the reaction was... 

The various results are listed in Table III and lead to a value of A/f298 CF2 between —43 and — 36 kcal/mole. The most reliable values are — 38 from electron impact studies,58,108,134 — 39.5 from Modica and LaGraff,121 —39.1 from Edwards and Small,55 and —39.3 kcal/mole from Zmbov et al.180 The value adopted here is —39.0 kcal/mole. With this value E> CF2—F and D CF2 CF2 become 95.6 and 77.0 kcal/mole, respectively. 

Malone and McGee114 performed electron impact studies on F202 and found D FO—OF = 103.5 4.6 kcal/mole. The enthalpy of formation of F202 has been measured calorimetrically" to be 4.73 + 0.30 kcal/mole A/f298 FO can be computed to be 54.1 kcal/mole. This value agrees with that obtained from A/f298 F20 = + 5.86 kcal/mole. 

Electron impact studies of ArHM(CO)396 and (AH)2M compounds81 are consistent with the pattern of enthalpies shown in Table 15, but as may be expected, the differences are overestimated. Thus, Z (Cr-C6Me6) is indicated to be greater than Z (Cr-C6H6) by ca. 150 kJ mol-1 by electron impact measurements80, three times the difference found by calorimetry. It is perhaps pertinent to note at this point that, even if the principle of transferability (Section 1.3.) is not accepted, so that E (ArH-M) = 0.25 A//D as a result of making E (ArH-M) = )(M-CO) for example, then the same pattern of relative values of E (ArH-M) shown in Table 15 is obtained but with EXCr-CeHs) = 125 kJ mol-1 andfs Cr-CsMes) = 138 kJ mol-1. There are a number of reasons, based on the chemical and spectroscopic properties of these compounds, why it would appear unlikely that E(ArH-M) should take the same value as D (M-CO). 

We will now review the studies of intramolecular Penning ionization and multiple scattering/ionization in vdW clusters, followed by our own electron impact studies on argon/methanol heteroclusters. 

R. B. King, Electron Impact Studies on Cyclopentadienyliron Carbonyl Tetramer A Novel Route to a Triple-decker Sandwich Ion, Chem. Comm. 1969, 436-437. 

The degenerate methyl group transfer in [5] has also been observed by Stahl et al. (1978) in electron-impact studies of C-labelled heptyl iodides. n-Heptyl and branched iodides all give ions that isomerize to [5] prior to fragmentation. 

The spectroscopic evidence on D(NO) is not conclusive. A value of 5 3 or 6-5 eV is obtained thermochemically depending on whether the lower or higher value of D(N2) is preferred. The lower value appears to the present author to be preferable. Electron impact studies on Z)(NO) lead unequivocally to 5 3 eV see Section 8.2.2.7) and this is possibly the strongest evidence for the low value of D(NO). 

The types of ion fragments produced by the decomposition of organic halogen compounds in the mass spectrometer have been summarised by McLafferty - . Brief reviews have also been given by Beynon and by Budzikiewicz et al . These authors have also summarised the results of other workers, such as the early studies on the monohalides by Stevenson and Hippie and by Dibeler and Reese (ref. 146) and on the polyhalides by Bernstein et by McDowell et al and by Dibeler et Electron impact studies on aromatic halogen compounds have been reported by Majer and Patrick . The interpretations of the mass spectra of halogen compounds have been provided in some detail by McLaffierty , and the main features only of these spectra are discussed very briefly here. 

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