Electron impact methods

Tlie molecular cation radical of 2-(3,5-di-f-butyl-4-oxopheiiyl)-l,3-dithiolaiie (59), which was generated by the electron impact method, split into ethylene and the cation radical of the corresponding dithiirane 60 (79DOK1030). 

The free t-butyl cation [7" ] in the gas phase is nothing more than a species detectable by the electron impact method (Yeo and Williams, 1970). However, it is not only an observable species by nmr studies in SbFs/FSOsH (Olah et al., 1964), but can be isolated from the solution in the form of its SbF or Sb2Ffi salt (Olah and Lukas, 1967a,b Olah et al., 1973 Yannoni et al., 1989). The crystal structure shows that this ion is planar and its carbon-carbon bonds are shortened to 144.2 pm (Hollenstein and Laube, 1993). Its particular electronic stabilization among aliphatic carbocations is attributed by physical organic chemists to the operation of both inductive and hyperconjugative effects in the cr bond system. 

The use of electron impact methods to determine bond enthapies, D(M-R), in the molecule MR from the relation... 

Table 11. Bond dissociation energies determined by electron impact methods. (kJ mol )...

Measurements by electron impact methods have led to experimental stabilization energies of a number of alkylcarbonium ions. Muller and Mulliken (1958) compared these with calculated values obtained by a procedure based on the LCAO-MO approximation, the large stabilization energies found for carbonium ions being attributed to the combined effects of hyperconjugation and charge redistribution (Table 1). 

The (CH3CO-CH3) bond dissociation energy has been measured by the kinetic, and more recently by the electron impact, methods, and concordant values40 41 of 71 kcal. mol.-1 were obtained so that when acetone suffers a photodecomposition of type A, the fragments still carry excess energy. 

Mass spectra of donepezil, carried out with electron impact method, were registered using Varian 320-GC/MS spectrometer. Figure 3.10 shows the detailed mass fragmentation pattern and Table 3.4 shows the mass fragmentation pattern of the drug substance. Clarke [2] reported the presence of the following principal peaks at m/z = 288,379,91,172,189,378,191 [2]. 

Routine MS data are available for 1,3,2( A2)-diazasiloles 83, 85, 89, and 90. Using electron impact methods the positive M+ ion was detected in 35-64% relative abundance <1994JA2691, 1995CC1931, 1996JOM211, 1998CEJ541>. In the case of the bis-silylene 86 the molecular ion was obtained as the base peak <2005ZFA1383>. Similarly, chemical ionization of compound 87 resulted in the detection of the molecular ion as the most abundant peak <1998ZFA295>. 

Measurements of ionization potentials by electron-impact methods have been studied for many years and the numerous experimental... 

Despite the difficulties, these electron-impact methods give ionization potentials in reasonable agreement with but somewhat higher than spectroscopic methods, and therefore may not measure the minimum energy required to remove an electron. 

Excess energy terms such as the value E in equation (8) must be either negligible or cancel for thermochemical arguments to lead to accurate bond dissociation energies. There is little evidence that excess energy terms and other errors of interpretation are not more important in more complex cases than the reaction (8). Further, the empirical assumptions used to determine ionization and appearance potentials by electron-impact methods work best for very simple molecules. 

Mass spectra of (+)-tadalafil (1), carried out with electron impact method, were registered at 70 eV using a ION TRAP GCQ FINNIGAN mass spectrometer (Fig. 8.9). El (MeOH) m/z 391.3 (M-l-2). Principal ions are presented in (Table 8.2). 

ELECTRON IMPACT METHODS and Warren and Stevenson 72 evidence relating to... 

Ions may also be produced with kinetic energy if there is a maximum in the potential energy curve for the stable excited state, as pointed out by Hippie and Stevenson 23 Xhe last possibility corresponds to an activation energy for the recombination reaction see Section 4.2.1). Measurement of kinetic energy, or proof that the ions are formed at rest, is essential for the reliable deduction of dissociation energies from the electron impact method. 

The electron impact method has been used in several different ways to determine D(GH3 -H). The results have been reviewed by Stevenson 3, Table 9.1.6.1 being taken from his paper. 

Franklin and Lumpkin deduce Z)(HS - SH) = 80 4 kcal using thermochemical data which may not be reliable and their value for the heat of formation of SH, determined by the electron impact method. We have given reasons for believing that their value for AHf SH) is about 4 kcal too large, which reduces their estimate of /)(HS-"SH) to 72 kcal. It is still noticeably larger than iJ)(HO-OH). 

Franklin and Lumpkin have measured the heat of formation of the RS radical by the electron impact method, and the heats of formation of some disulphides by combustion. They deduce the following values for D(MeS SMe), D(MeS-SEt), and Z)(EtS SEt) 73 2, 71 5, and 70 o kcal respectively. 

The above value supercedes all earlier values for the ionization potential of CPg which generally were obtained by electron Impact methods, which do not have the energy resolution available in the photoionization technique. For further details of the older values see references 3, 4, 5, 6 and 7. 

Using the photoionization technique, Matthews and Warneck (j ) measured the appearance potentials of HCO (g) from formaldehyde, formic acid and acetaldehyde as 11.95, 12.79 and 11.79 eV, respectively, whose average yields AjH (CHC0, g, 0 K) - 197.7 1.5 kcal mol", based on the following A H (0 K) data (in units of kcal mol" ) 51.63 for H(g), 9.35 for OH(g), 35.62 for CHg(g), -26.78 for HgCOCg) (2), -88.74 for HCOOH(g) (3) and -37.14 for CHgCHO (4). The appearance potentials obtained from photoionization are several tenths to 1 eV lower than the recent electron impact data (5, 6, 7, 8, 9). The appearance potentials determined by the electron impact method tend to be high because the fundamental nature of the process does not lead to a sharp o pet in contrast to the step-function behavior of photoionization onset. 

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