Ionization Potential and Structure of Olefins

摘要

The method of photoionization has been used to measure the ionization potential of a number of simple, cyclic, exocyclic, bicyclic monohyphen; and diolefins. These data are compiled with others from the literature of related systems, and the results are compared. In monohyphen;olefins, the ionization potential of the olefinicpgr;electron is found to decrease as the size of the molecule increases, the change being largest with the first methyl substitution to ethylene. As the size of thesgr;system of a monofolefin increases, the energy of both the neutral molecule and the ion decreases by the extra bonds formed as well as by strengthening thepgr;system as a result of mixing it with the ionic form due to hyperconjugation. Both factors stabilize the ion more than they do the neutral molecule. The fact that cyclic and exocyclic olefins have ionization potential comparable to that observed for straight chain olefins of comparable size again strongly indicates that the stabilization of the ion by the electrons in thesgr;bonds (rather than by hyperconjugation) is an important factor in determining the ionization energy ofpgr;electrons in olefins. The effect of a second olefinic group greatly depends on whether or not strong interaction between the two groups occurs, but is independent of whether a chain or cyclic system is studied. Conjugation is found to decrease the I.P. values by sim;0.5 eV. Due to the increase in the effective nuclear charges of the atoms in the ion, resonance and Coulombichyphen;type interactions stabilize the ion upon conjugation more than they do the neutral species. Nonconjugated dienes have slightly lower I.P. than the corresponding monofolefins by an amount that depends on the separation and geometry of the diene. The effect of a second olefinic system in a bicyclic diene can be as great as that produced by the classic type of conjugation in favorable cases, such as in norbornadiene. In these systems, increasing the ring size increases the number of bonds, thus lowering the value of the I.P., but also increases the separation between the double bonds, which raises the value of the I.P. 1,4,7hyphen;cyclononatriene (crown) has an I.P. value which is 0.6 eV lower than that in benzene, but higher than trimethylhyphen;substituted benzene. This result is explained by the fact that strong conjugation is present in the crown but is not as effective as it is in the phenyl ring.