Mass spectrometric appearance potential

This experiment may be regarded as the forerunner of mass spectro-metric appearance-potential determination in that both are threshold techniques, that is they depend on slow variation in the energy supplied by the impacting electron until a change in the electron-molecule interaction is observed. Thus, just as the Hertz experiment did not distinguish between excitation and ionization potentials, mass spectrometric appearance potential measurements are subject to similar ambiguities in interpretation as between ionization and autoionization. 

The dissociation energies given in Table C were obtained by subtracting 0.6 kcal./mole (RT) from the heats of dissociation. For the charged NF+ species the mass spectrometric appearance potentials, A, listed in Table B were also used as follows—e.g.,... 

Another class of gas phase experiments begins with the molecular organometallic compounds of interest and measures the added energy required for ligand dissociation. The mass spectrometric appearance potential method relates the metal-ligand bond energy to the mass... 

The synthesis of triphenylcyclopropenium ion and related cyclopropenium ions by Breslow, coupled with its low reactivity toward nucleophiles, in contrast to other carbocations, demonstrates that the An + 2 rule applies even when n = 0. The heat of formation of cyclopropenium ion has been determined from the heat of combustion of cyclopropene and the mass spectrometric appearance potential of the C3H3 ion and indicates considerable stability. This led to an interest in the cyclopropenyl anion, which should not have any significant 7r-stabilization. As noted... 

The Heat of Formation of CF2. Several of experimental approaches have been used to determine AHf° (CF2).The most common technique involves mass spectrometric measurement of appearance potentials. The earlier appearance potential measurements indicated that A(CF2) = -30 10 kcal. mole 32), but it now appears this value is too high. Margrave and co-workers 33 reported a mass spectrometric study of the C2F4/CF2 equilibrium between 1127—1244 °K. Both second and third-law determinations of the enthalpy of reaction for C2F4 - 2 CF2 were made, yielding-39.3 3 kcal. mole 1 for A/fy-°298 (CF2, g). 

We have derived the dissociation energy of the dichloride gas from the mass-spectrometric data (20) for the appearance potential of Eu+/EuCl2 (15.0 + 0.5 eV) and the first ionization potential for Eu [5.64eV (40)] and obtain... 

A few estimates of metal-metal bond energies have been made on the basis of mass spectrometric measurements. These involve measurement of appearance potentials as, for instance (5), in eq 1-3. The energy required for eq 3 is the difference between the... 

Although, at first sight, the mass spectrometric methods do not establish the structure of the ionic species from the protonation of cycloalkanes, it must be recalled that a protonated cycloparaffin structure was first proposed by Bylander and Meyerson (1956) on the basis of appearance potential measurements. 

Such limitation does not detract from the mass-spectrometric approach, which has provided valuable information on the stability of the protonated arenes, and their dissociation patterns. Moreover, it seems appropriate to mention here that, as early as 1957, Field and Franklin established a correlation between the orienting influence of substituents and the appearance potential of the corresponding ionic species, and that their proposed structure of the arenium ions observed in chemical... 

It has been studied by IR spectroscopy (when trapped in a low-temperature matrix) and by mass spectrometric studies on the vapor. Isotopic studies ( Cl/ Cl and 0/ 0) allow the 0-P-Cl bond angle to be calculated from the IR spectra at ca. 105° (i.e. close to the bond angle of CH2PCI). From the observed appearance potential (20.9 eV) of P+ [AP(P+)j in the mass spectrum of OPCl, it is possible to estimate the enthalpy of atomization of OPCl(g) via the Bom Haber cycle (Scheme 6). Hence, since the standard enthalpies of formation of P(g), 0(g), and Cl(g) are all known, the standard enthalpy of formation of OPCl(g) [A // 9g(OPCl)] may be estimated as -250.7 kJmol . ... 

The flash vacuum pyrolysis and the mass spectrometric fragmentation of a series of five methyl substituted phenanthrenes were correlated with each other in a quaUtatively predictable way.i ) The major thermochemical products for these compounds generally corresponded to one of the five most intense fragment ions in the compound s electron impact mass spectrum. The importance of products which corresponded to ions with high appearance potentials increased with increasing temperature in the thermolysis system. 

Srlvastava and Farber ( 1), using a gas-inlet boron nitride effusion cell, performed a mass spectrometric study of the equilibrium vapor species over the system BFg(g) + B(cr). Ion intensities for BFg(g), BF(g) and BFgCg) were measured at ionizing electron energies slightly above appearance potentials ( 3 eV) in the temperature range 1477 to 1850 K and were used to obtain equilibrium data for the reaction BFg(g) + BF(g) = 2 BFgCg). We have analyzed their results and obtain ApH (298.15 K) = 17.1 ... 

The HBO ion has been detected mass spectrometrically by Sholette and Porter (2.) and Farber and Frisch (2). However, no appearance potential data have been reported for the ion. Kroto et al. ( ) recently attempted to measure the photelectron spectrum of HBO by passing water vapor over heated boron. The spectrum showed no bands which could be definitely assigned to HBO monomer. Unfortunately, the region of their spectrum ( 14-15 eV) where the first photoelectron band of HBO would be expected to lie shows a broad band which also appeared In the spectrum of HBS above 1150"C. Kroto et al. 4) have assigned this band to dlborane. He believe the HBO band may well be hidden under this broad band. 

The appearance potential of SrF from SrF(g) has been measured mass spectrometrically by Hildenbrand (1 ), Green et al. 

Porter and Schoonmaker (1 ) obtained an appearance potential curve near threshold for KOH which extrapolates to 7.8 eV at zero ion current. They presented evidence which Indicated that a large fraction of KOH was formed by dissociative ionization of the dimer, in a later study, these same workers (2) rejected their earlier results, since it was found that the presence of magnesia in their cell had reduced the activity of the hydroxide. Qusarov and Gorokhov (3) in a similar mass spectrometric study of the evaporation products of KOH have shown that KOH is formed primarily from direct ionization of the hydroxide. Very recently, Gorokhov et al. (4) reported the appearance potential of KOH from KOH as 7.5 0.2 eV. We adopt this result as A H for the process e + KOH(g) KOH (g) + 2e", since it is consistent with the expected mode of ionization. This value leads to AjH (K0H, g, 0 K) = 118 10 kcal mol" when used in conjunction with A H (K0H, g, 0 K) = -54.6 3.0 kcal mol" ( ). 

Gross and coworkers demonstrated that both Cf Hi2+ isomers reside in distinct potential wells and can be characterized by CID in both sector-field and FT-ICR mass spectrometers. The mass spectrometric experiments were in line with calculation in that the ionized bicyclic isomer appears to be more stable than 4-vinylcyclohexene ions, and with the radiolytic results in that a closely related bicyclic isomer, viz. ionized bicy-clo[2.2.2]octene, is not easily formed upon ionization of the other CgHn hydrocarbons. The retro-Diels-Alder reaction ionized bicyclo[2.2.2]octa-2,5-dienes leading to expulsion of the initially saturated bridge as an alkene gives rise to the base peak in the El spectra . ... 

Reduction of acetylene as well as ethylene, propylene, and cyclopropane, at positive potential was first reported by Langer and co-workers (25, 26, 33). Mass spectrometric and coulometric analyses showed quantitative hydrogenation of the unsaturates. Reductions were fastest in acidic electrolytes in agreement with the potentiometric results discussed earlier. The relative rates of the reactants on Pt black appeared to follow the strength of adsorption, with acetylene the most difficult to hydrogenate and with cyclopropane reducing readily. 

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