Ionization potentials ethylene

The performance of many metal-ion catalysts can be enhanced by doping with cesium compounds. This is a result both of the low ionization potential of cesium and its abiUty to stabilize high oxidation states of transition-metal oxo anions (50). Catalyst doping is one of the principal commercial uses of cesium. Cesium is a more powerflil oxidant than potassium, which it can replace. The amount of replacement is often a matter of economic benefit. Cesium-doped catalysts are used for the production of styrene monomer from ethyl benzene at metal oxide contacts or from toluene and methanol as Cs-exchanged zeofltes ethylene oxide ammonoxidation, acrolein (methacrolein) acryflc acid (methacrylic acid) methyl methacrylate monomer methanol phthahc anhydride anthraquinone various olefins chlorinations in low pressure ammonia synthesis and in the conversion of SO2 to SO in sulfuric acid production. 

Enhancement of the total butene yield is observed when various additives whose ionization potential falls below about 9.4 e.v. are present during ethylene radiolysis (35). This is consistent with the above interpretation (Figure 2). In the vacuum ultraviolet photolysis of cyclobutane the yield of butenes varies with the ionization potential of the additives in the same way as observed here (12). The maximum enhancement corresponds closely to the yield of C4H8+, as expected from our mechanism. 

Figure 2. Enhancement of total butene yields from 100-torr ethylene with ionization potential of additive present in 10% concentration, 3 torr oxygen added when necessary to inhibit free radical reactions. The letter symbols indicate ionization potentials from Ref. 58 in parenthesis values in e.v.

Certain physical properties of substituted ethylenes may be correlated with the extended Hammett equation. Included in this category are dipole moments and ionization potentials. 

A number of correlations of ionization potentials for substituted benzenes (40-42), benzyl (43), phenoxy (44), and alkyl (45) radicals and substituted pyridines (46) with the simple Hammett equation have been reported. Charton (47) has studied the application of the extended Hammett equation to substituted ethylenes and carbonyl compounds. The sets studied here are reported in Table II (sets 2-10 and 2-11). Results of the correlations are set forth in Table 111. The results obtained are much improved by the exclusion of the values for X = C2 H3, Ac, F, H and OAc from set 2-10 (set 2-lOA) and the value for X = H from set 2-11 (set 2-11 A). The composition of the electrical effect corresponds to that found for the Op constants as is shown by the pR values reported in Table IV. 

The ionization potentials of substituted cyclopropanes also show a significant correlation with eq. (2). The value of pr obtained is comparable to that observed for substituted ethylenes and 1-substituted propenes (section II.A.2.) and is considerably above that found for substituted benzenes (for which a value of Pr = 59 is obtained). This result confirms the existence of a large resonance interaction between the cyclopropane ring and substituents. The magnitude of a is considerably greater for substituted cyclopropanes than it is for substituted ethylenes or benzenes. 

In our third example (52), dissociative chemisorption of Li2, B2, C2, 02, N2, F2, CO, NO and ethylene on (100)W and Ni surfaces was examined. The metal surfaces are represented by means of nine-atom clusters, arranged as in Fig. 35. Experimental geometry was used for the adsorbates. The standard EHT method was used, i.e. with charge-independent atomic ionization potentials. Charge transfer between adsorbate and surface was explored... 

SiM 3 Fig. 1. Correlation diagram giving electron attachment energies (AE) and ionization potentials (IP) of a-,—SiMe3 and jS-silyl substituted ethylenes... 

According to Koopmans theorem28, the first ionization potential of a disubstituted ethylene corresponds to the negative of the pi HOMO energy. From the previous discussion, the pi HOMO energy is predicted to vary in the following order ... 

It should be pointed out that the predicted higher ionization potential and lower electron affinity of a cis 1,2-disubstituted ethylene relative to the corresponding trans isomer is based upon the assumption that substantial pi nonbonded inter-... 

Ionization Potentials (IP) for various disubstituted ethylenes are shown in Table 24. As can be seen, the IP s for 1,2 cis and trans isomers are usually very close but the IP s of 1,1-disubstituted ethylenes relative to their 1,2-disubstituted counterparts are consistently higher, in agreement with our predictions. 

In this section we shall examine the effects of n—n and n—n interactions on the ionization potentials of substituted ethylenes and benzenes. A theoretical analysis has already been given in section 1.1. In the space below we survey some pertinent data. 

From the ionization potential listed in Table 40 it can be seen that substitution of ethylene or benzene by an electron releasing group of the first period lowers the ionization potential. Also, as the energy gap between the ethylene n MO and the substituent pz AO decreases the change in ionization potential increases. Thus, the energy change for F substitution is 0.22 eV while that for OMe substitution is 1.59 eV. 

From the ionization potential data for acrolein and acrylonitrile, it can be deduced that in both cases the major interaction of the ethylenic jt MO is with the n of the CH=0 and C N groups, respectively. 

Table 40. Ionization potentials of substituted ethylenes and benzenes (eV)...

For the G2-1 ionization potentials, the largest differences are 0.005 and 0.006 eV, respectively, for ethylene and acetylene. Differences in the G2-2 set are likewise small, although Si2H2 (0.009 eV) and CH3OF... 

Accepting the approximate validity of Koopmans theorem (129), UPS can be said to give information about the energies of the occupied orbitals in the ground state. Two UPS studies of push-pull ethylenes have been reported (130,131). In ref. 130, the ionization potentials (IPs) of 119 are discussed, where A denotes... 

Stabilization energies (S) of the ground and ionized states of alkylethylenes referred to ethylene. derived from observed changes in ionization potential (in e.v.)... 

Fia. 21. Improvement in agreement between calculated and observed ionization potentials for simple w systems as between values derived on the simple H.M.O. basis (left-hand end of arrows) and those derived using the oj technique (right-hand end of arrows). See text. 1, Methyl 2, Allyl 3, Pentadienyl 4, Benzyl 6, Ethylene 6, Butadiene 7, Benzene 8, Styrene 9, Naphthalene 10, Fhenanthrene. 

It is tempting to relate the thermodynamics of electron-transfer between metal atoms or ions and organic substrates directly to the relevant ionization potentials and electron affinities. These quantities certainly play a role in ET-thermo-dynamics but the dominant factor in inner sphere processes in which the product of electron transfer is an ion pair is the electrostatic interaction between the product ions. Model calculations on the reduction of ethylene by alkali metal atoms, for instance [69], showed that the energy difference between the M C2H4 ground state and the electron-transfer state can be... 

This result is similar to that obtained by Widing and Levitt for the normal alkanes and to that observed for alkyl-substituted ethylenes [171] in the latter case, linear correlations were obtained between the IPs and the sum of charges of the unsaturated carbon atoms, whereby any increase of their electron density due to substituent effects leads to a lowering of the molecular ionization potential. 

In the MS of oxetane the relative abundance of the ethylene radical ion is about eight times greater than that of the formaldehyde radical ion at the usual ionizing potential of 70 eV. However, 2,2-dimethyloxetane fragments much more to the radical ion of acetone... 

As discussed at the end of Chapter 3, one group orbital of a methyl or methylene group will always have the correct nodal characteristics to interact with an adjacent n orbital or with an adjacent spn orbital in fashion. The degree of interaction may be inferred from the energies of the orbitals, which may in turn be obtained by measurements of ionization potentials and application of Koopmans theorem. Thus, the methyl groups adjacent to the n bond in (Z)-2-butene (ionization potential IP = 9.12 eY [63]) raise the energy of the n orbital by 1.39 eV relative to that of ethylene (IP = 10.51 eV [87]). A similar effect is observed in cyclohexene [64]. 

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