Ionization of acetylene

The acidity mcreases as carbon becomes more electronegative Ionization of acetylene gives an anion m which the unshared electron pair occupies an orbital with 50% s character... 

Ionization of Acetylene. Acetylene is known to possess acidic properties and when it is shaken with heavy water the density of the latter becomes less in alkaline solution indicating an exchange between the hydrogen of the acetylene and the deuterium ions of the water. The exchange does not take place with measurable velocity in acid solutions where the dissociation of the weak acetylene acid is repressed. 

Formation of long-lived Ar-atoms was studied with the same experimental setup between threshold and about 16 eV collision energy.90 The long-lived atoms were detected by Penning ionization of acetylene. The cross section for production of metastables is practically constant over the range studied from 11-5 to 16-3 eV. The absolute magnitude is 3-6 ( 2-4). 10-20 cm2. 

In the corresponding ionizations of ethylene and ethane the unshared pair occupies an orbital with 33% sp ) and 25% sp ) s character respectively Terminal alkynes (RC=CH) resemble acetylene m acidity... 

Alkali-metal graphites are extremely reactive in air and may explode with water. In general, reactivity decreases with ease of ionization of M in the sequence Li > Na > K > Rb > Cs. Under controlled conditions H2O or ROH produce only H2, MOH and graphite, unlike the alkali-metal carbides M2C2 (p. 297) which produce hydrocarbons such as acetylene. In an important new reaction CgK has been found to react smoothly with transition metal salts in tetrahydrofuran at room temperature to give the corresponding transition metal lamellar compounds ... 

Table II must be used with care in anomalous cases in which the transition probability for ionization of the molecule is very low in some energy ranges (e.g., acetylene, benzene, methylamine). In such cases higher RE s, not included in the table and normally of small importance, may be responsible for the charge exchange processes although with small cross-sections (cf. 9, 13).

Ervin et al. [27] have determined the electron affinity of the acetylide radical, HC = C-, to be equal to 2.969 + 0.010 eV and the enthalpy of the acid dissociation of acetylene in the gas phase to be equal to 377.8 + 0.6 Kcal mol Use these data, together with the ionization potential of the hydrogen atom, 13.595 eV, to calculate the enthalpy for the dissociation of the CH bond in acetylene. The ionization potentials are properly applied at 0 K, but a good approximation is to assume that they are equal to enthalpy changes at 298.15 K, the temperature at which the enthalpy of the acid dissociation was measured. 

Perhaps partly due to the difficulty of introducing solid salts into the ionization chamber, mass spectrometry has, to date, played an insignificant role in the study of quinolizinium compounds. It is of interest that a peak at m/e 130 which appears in the mass spectrum of 2-methylindolizine (11) (70OMS(3)1489) and in that of 6-(2-pyridyl)-3,6-dihydro-2i/-1,2-oxazine (12) (74BSB147) has been assigned to the quinolizinium ion (1). Evidence was presented that the ion next lost HCN to give a metastable ion of m/e 103 followed by loss of acetylene to afford a fragment of m/e 77. 

Vacuum ultraviolet photolysis of acetylene results in formation of triplet C2, as evidenced by its characteristic emission.139 Presumably, triplet acetylene is first formed and decomposes to C2 and H2. Saturated hydrocarbons undergo radiolytic reactions, but the relative importance of excited states versus ionized states has not yet been established with any certainty. 

Table 8. Comparison of experimental ionization potentials with calculated orbital energies of acetylenes and aromatic compounds...

Roithova and Schroder have demonstrated that a dicationic species can participate in carbon-carbon bond forming reactions in the gas-phase and that this is a potential route to polycyclic aromatic hydrocarbons.45 When C7H62+ is generated (from double ionization of toluene) and allowed to react with acetylene, new ions, CgRj1+ and CyHf,2+, are detected (eq 44). 

Duben et a/.616 have carried out extensive calculations of the correlation energy in the ground and ionized states of acetylene, C2H2. 

Soluble barium may be determined by S-198 (12) using a hot water leach, hydrochloric acid dissolution, and nitrous oxide/acetylene flame. In this method, 1000-2000 ppm sodium as sodium chloride is used to minimize ionization of barium in the flame. Although background correction is not mentioned in this method, it is strongly recommended when calcium is present. 

Calcium. Calcium like barium is best determined by AAS, since flame emission suffers from background effects where other alkali metal are present. P CAM 173 does not recommend the use of nitrous oxide/acetylene instead of air/acetylene, although the former offers greater sensitivity and detection limit when 1000 ppm potassium is added to the standards and samples. The nitrous oxide/acetylene flame needs the potassium to minimize the ionization of Ca. 

Although ionization of sodium is negligible and potassium small in an air—propane flame, some ionization is experienced in the recommended hotter air—acetylene flame. Ionization should be suppressed by the incorporation of excess potassium or cesium (for sodium determinations) or excess sodium or cesium (for potassium determinations), at concentrations of 1000/igml-1 or greater, in the form of chlorides or nitrates, in both sample and standard solutions. Cesium is the more effective but more expensive ionization suppressant. Extent of ionization is inversely related to analyte concentration with errors due to incomplete suppression thus being greater at low concentrations. As it is difficult to obtain alkali metal salts free from traces of other alkali metals, possible contamination must be considered, especially at low analyte levels. Use of a branched capillary for introduction of ionization buffer has been advocated for flame spectrometry to... 

Fig. 20.7. The upper panel shows the ion beam profiles of the C+ and H+ fragment ions in the z-direction, produced by electron impact ionization of methane and acetylene at an electron energy of 100 eV. The ion signal was measured as a function of the voltage on the z-deflector pair. In the lower diagram the initial kinetic energy distributions are displayed that were calculated from the z-profiles shown above. The initial kinetic energy distribution of both fragment ions consist of two parts designated as thin lines. The sum of the two contributions represents the total initial kinetic energy distribution and is displayed as a thick line in the same line style...

Most of the last-named difficulty has been removed in a simple mathematical treatment which effectively removes a major part of the high-energy side of the electron beam (Winters et al., 1966). An experimental factor b, accounting for the lack of knowledge of electron energy distribution, has been found to be quite close to that calculated from the Maxwell-Boltzmann distribution. Recently it has been claimed that the 0 0 ionization and vibrational fine structure of acetylene had been detected (J. H. Collins et al., 1968), and this energy distribution difference (E. D. D.) method appears to be both simple and accurate. 

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