The chemical ionization mass spectra of benzyl acetate and t-amyl acetate have been determined as a function of the mass spectrometer ionization chamber temperature and the pressure in the ionization chamber of the esters. Most of the runs were made using i-C4H10 as reactant, but some measurements were made using CH4 as reactant. The results obtained in the study are the following. (A) As a check on the apparatus and method, measurements were made of the H(H2O)2+-H(H2O)3+, H(H2O)3+-H(H2O)4+, and H(H2O)4+-H(H2O)5+ equilibria in the gaseous water ionic system. Thermodynamic values in good agreement with the results of Kebarle, et al.,9 were obtained for 3,4 and 4,5 equilibria. (B) Ion intensities in i-C4H10 are presented for pressures of i-C4H10 between 0 and 1.0 Torr. At pressures above about 0.6 Torr, t-C4H9+ comprises about 95% of the total ionization of the compound. (C) Mass spectra of the two esters with i-C4H10 as reactant are given at low, intermediate, and high source temperatures. The spectra of benzyl acetate using CH4 as reactant is given at two temperatures. Extensive changes in the spectra occur as the temperature is changed when i-C4H10 is reactant. Several equilibria are observed at lower temperatures, and extensive amounts of dissociation occurs at high temperatures. With CH4 as reactant extensive dissociation is observed at both temperatures studied. (D) Residence times under chemical ionization conditions (reactant = i-C4H10, P.s = 0.70 Torr) are calculated and tabulated for the several ions investigated. An equation for calculating rate constants for unimolecular ionic decompositions occurring under chemical ionization conditions is given. (E) Rate constants for the formation of benzyl ion and t-amyl ion from protonated benzyl acetate and protonated t-amyl acetate determined at various temperatures. The activation energies and log A values for benzyl ion production are 12.3 kcal/mole and 11.2, and for production of t-amyl ion the values are 12.4 kcal/mole and 12.4. The relative magnitudes of the activation energies are tentatively identified with the relative energies of the charged centers in the carbonium ions, and it is suggested that the technique may comprise a useful new method for obtaining relative energies of gaseous ions. The lower A factor for the formation of benzyl ion is rationalized in terms of the postulate of the formation of a torsional vibration in the C6H54-CH2 bond in the transition state of the reaction. (F) Equilibrium reactions are observed in the formation of the following ions: the protonated dimers of the two esters, the association complexes between the protonated ester molecules and residual water in the mass spectrometer, and the association complex between benzyl acetate and C3H3+. The usual thermodynamic quantities are calculated from the equilibria. The entropies obtained for all these reactions are much higher than would be expected for association reactions, and the entropy change for the formation of the protonated dimer of benzyl acetate has the astonishingly high value of +14 eu. (G) The effect of repeller voltage on the chemical ionization spectrum of benzyl acetate was investigated. Dissociation processes increase with increasing repeller voltage, but one finds that a tenfold increase in repeller voltage has about the same effect in increasing the benzyl ion intensity as a 10% increase in temperature. (H) The pressure of benzyl acetate in the ionization chamber was varied, and a rate constant for the overall reaction of ions from i-C4H10 with benzyl acetate was determined. Investigations were made at two temperatures (99 and 178°). The values obtained are 5.1 × 109 (99°) and 3.2 × 10-9 cc/(mol sec) (178°). These values are in reasonable agreement with theoretical values which can be calculated for ion-permanent dipole reactions. (I) Rate constants for the total reaction of ions from i-C4H10 and CH4 with the two esters at various temperatures have been determined. A strong negative temperature coefficient for the rate constant obtained in the i-C4H10-benzyl acetate system is observed, and extraordinarily large values of the rate constants are observed for all the systems. © 1969, American Chemical Society. All rights reserved.
