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Kinetic Energy Release and Energy Partitioning During the Ionic Fragmentation in Metastable Ions F. H. Abd El-Kader and M. S. Abd El-Fattah Physics Department, Faculty of Science, Cairo University, Giza, Egypt

Z. Naturforsch. 38 a, 524-527 (1983); received November 19. 1982

The kinetic energy released in the unimolecular decomposit ion of some metastable transitions has been investigated. It was found to have a significant tempera ture dependence in cases of PF loss from molecular ions. This is attributed to the tunelling of FF atom through the centrifugal barrier.

The activation energies of the reverse reactions "e*" and the kinetic energy released "7b" have been determined for the loss of H" f rom acetonitrile and elimination of HCN from benzonitrile. The partitioning energy quotients q = TB/s* are found = 0.91 and 0.58 for acetonitrile and benzonitrile respectively.

Introduction

The investigations of metastable ions in mass spectrometer have been shown to reveal many mechanistic details of unimolecular reactions of organic ions [1]. The kinetic energy released in metastable fragmentations is an important thermo-chemical quantity. It has been widely used as a sensitive means of characterizing reaction mecha-nisms [2, 3]. Small changes in this kinetic energy can be measured accurately as a function of the tempe-rature of the sample molecules [4-7] ,

The partitioning of the excess potentional energy of the transition states (or the activation energy of the reverse reactions e*) between the kinetic energy T, released during the reaction, and the internal energy e* of the products depends strongly and systematically on the thermochemistry of the reac-tion [8] or the position of the potential energy barrier [9] on the reaction coordinate X*.

In this paper, further measurements of the released translational energy as a function of ion source temperature in the dissociation of metastable ions of some nitrile compounds are reported. In addition, the energy partitioning accompanying the loss of H' loss acetonitrile and elimination of H C N from benzonitrile are discussed.

Experimental

Metastable ion transitions were measured using the mass - analyzed ion kinetic energy spectrometer

Reprint requests to Dr. El-Kader, Physics Depar tment . Faculty of Science, Cairo University Giza, Ägypten.

(MIKES). The MIKE Varian MAT CH 5 reversed Nier-Johnson geometry was operated at 3 kV ion accelerating voltage, 70 eV electron energy, 1 mA total emission current and 8 x 10_ 6 torr sample pres-sure. The ion source temperature was varied over the range 150-300 °C. Fragmentations occurring in the second field-free region were detected by sector voltage scan procedure [10].

The metastable peak width at half-height, after correction for the energy spread in the parent ion beam [5], was used to determine the characteristic value of kinetic energy release. Multiple measure-ments were carried out. and the energy release values were reproducible to better than ± 5 mev. The maximum value of the kinetic energy, FB, was determined from the peak base-width. The re-producibility was = ± 8%. Thermochemical data were determined from the literature [11-13].

Results and Discussion

Temperature Dependence of the Translational Energy Release

The variation in kinetic energy release as a function of ion source temperature in metastable ion dissociation has been investigated for some molecular ions. The data are given in Table 1.

The values of the released kinetic energy in the elimination of HCN from the molecular ions of propionitrile, 2-chloroacrylonitrile and o-toluonitrile are not affected by the change in the ion source temperature. Also, the loss of chlorine atom from 2-chloracrylonitrile shows no change in the kinetic

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A. El-Kader and A. El-Fat tah • Kinetic Energy Release and Energy Par t i t ioning

Table 1. Dependence of kinetic energy release on the ion source t empera tu re .

525

C o m p o u n d Trans i t ion a 7~(mev)

150 ° C 200 ° C 250 ° C 300 ° C

Acetonitri le 41+ 40+ + 1 97 ± 0.5 100 ± 0 . 5 103 ± 0.5 106 ± 0.5

Propionitr i le 55+ -55+ -

5 4 + + 1 28+ + 27

91 ± 0.5 20 ± 2

94 ± 0.5 21 ± 1

98 ± 0.5 22 ± 0.5

101 ± 0.5 23 ± 1

Acrylonitri le 53+ 52+ + 1 144 ± 1 162 ± 2 202 ± 2 204 ± 3

2-chloroacrylo-nitri le

87+ -87+ ->

60+ + 27 52+ + 35

30 ± 1 14 ± 0 . 5

32 ± 2 13.5 ± 1

34 ± 2 14 ± 0.5

33 ± 2 14.5 ± 1

Benzonitri le 103+ 76+ + 27 28 ± 0.5 27.5 ± 1 28 ± 0.5 28.5 ± 1

o-Toluonitr i le 117+ -» 116++ 1 112 ± 1 130 ± 2 172 ± 2 226 ± 3

a T h e shape of metas table peak resulting in these reactions are of Gauss i an type.

energy released with the variation of ion source temperature. On the other hand, in the case of hydrogen atom loss from acetonitrile and propio-nitrile molecular ions, the kinetic energy released showed a slight variation with temperature. This is shown in Figure 1. In addition, the T released during the loss of a hydrogen atom from either acrylonitrile or o-toluonitrile is more sensitive to the change of ion source temperature. This is represent in Figure 2.

According to the quasi-equilibrium theory (QET), the rate of ionic dissociation is a function of the total energy transferred to the molecualr ion. This total energy originates from; (a) the thermal energy,

and (b) the excitation energy transferred by the ionization processes. Thus, the variation of the thermal energy by the source temperature has an effect on the angular momentum and the internal energy of the ion. Therefore, two competitive pro-cesses may be expected:

(i) A process involving a transition state depend-ing upon the angular momentum.

(ii) A process with different dependence upon rotational energy. As it has been reported [6] the second process exhibits a temperature-dependent energy release. The behaviour of the rotational energy distribution of molecular ion with tempera-ture, might form the basis for an inquiry into centri-

120

100

I 80-E

" 60|-

4 0 -

(a ) C 2 H 3 N + — C 2 H 2 N% H

( b) C 3H 3N

526 A. El-Kader and A. El-Fattah • Kinetic Energy Release and Energy Partitioning

fugal effect in mass spectrometric reactions. It is clear from the data presented in Table 1 that the F-value for the loss of H' atom from acetonitrile, propionitrile, acrylonitrile and o-toluonitrile is tem-perature dependent. This can be explained by as-suming tunnelling of the H' atom through the cen-trifugal barrier.

Energy Partitioning During the Fragmentation

Figure 3 represents the schematic reaction profile for the process:

M + e - > F + + N ' + 2e

and defines some energy data together with the symbols employed.

According to Miller's quantification [9] of the Hammond postulate [14] the position Xq of the transition state of an elementary reaction on the reaction coordinate can be expressed by the barrier height U* (here st) and potential energy of the reaction U{ (here AH^) according to equation:

y * _ 1 1

0 2-U{/U* 2-AH^/ef The experimental activation energy of the for-

ward reaction without correction of kinetic shift, is given as the difference between the appearance energy /1(F+) and the ionization energy 7(M). While the reaction enthalpy AH £ is given as the dif-ference between the heats of formation of the products and reactants.

Fig. 3. Potential-energy diagram of a unimolecular mass spectrometric fragmentation: / ( M ) = ionization energy; A(F+) = appearance energy of the fragment ion; AH£ = re-action enthalpy; AH{ = heats of formation; r B = maximum kinetic energy released; = Thermochemical activation energy of the forward reaction; = experimental activa-tion energy of the forward reaction; e* = non-fixed excess energy of the activated complex; = Thermochemical reverse activation energy; e* = experimental reverse acti-vation energy.

Table 2. Energy da ta a relating to the loss of H ' from acetonitrile and elimination of HCN from benzonitrile.

Compound Acetonitrile Benzonitrile

Transition 41+ -» 4 0 + + 1 103+ 76+ + 27 / ( M ) b 12.2 9.705 A (F+) 14.72c 13.38 d

4 2.52 3.575 ^ f ( M ) b 0.91 2.311 z l / / f ( N ' ) b 2.255 0.967 J / / f ( F + ) b 12.922 13.969

0.432 0.185 4 0.475 0.320 q 0.91 0.58 A Hi 2.085 3.36 n 0.85 0.94 a All values are in electron volt. b Value from [11], c Value from [12], d Value from [13],

The experimentally determined energy data for loss of H" from acetonitrile and elimination of HCN from benzonitrile, are summarized in Table 2. Also the partitioning energy quotient, reaction enthalpy and position of transition state derived from these data are shown in the same table.

Thus, in spite of the individual properties of both reactions discussed before, it has been established that the reactions are endothermic and associated with "symmetrical" or "late" transition states (A* > 0.4). Most of the energy of the transition state is accumulated in the stretched C - X bond and is released as kinetic energy of the product if the stretching of this bond proceeds toward dissociation [15], Therefore, it is difficult to correlate the tempe-rature dependence kinetic energy release with the transition state of the reaction and its thermochem-istry.

It should be mentioned that a difference in the values of et, AHR and q has been observed for the reactions of acetonitrile and benzonitrile molecular ion. These variations may give some insight into fundamental properties of elementary reactions of the organic ions. Thus, mass-spectrometric tech-niques seems to be well suited to reveal the charac-teristic of such elementary reactions of organic ions.

Conclusion

The results obtained for particular ions indicate that the kinetic energy release accompanying uni-

527 A. El-Kader and A. El-Fattah • Kinetic Energy Release and Energy Partitioning

m o l e c u l a r m e t a s t a b l e f r a g m e n t a t i o n of ions f o r m e d by e l ec t ron - impac t is a p p r o x i m a t e l y d e p e n d e n t on the ion source t e m p e r a t u r e . It is sugges t ed tha t t he ions a re d i s soc ia ted by a p rocess invo lv ing a t ransi-t ion s tate d e p e n d i n g u p o n r o t a t i o n a l energy .

T h e va r i a t i on o b s e r v e d in the energy p a r t i t i o n i n g q u o t i e n t q a n d t he r eac t i on e n t h a l p y AH £ for t he two reac t ions s t u d i e d is i n t e r p r e t e d as a n a l t e r a t ion in the pos i t ion of t he t r ans i t ion s tate . It m u s t b e e m p h a s i z e d tha t t he v a l u e of q d e p e n d s obv ious ly

on the deta i l s of t he energy h y p e r s u r f a c e of t he reac t ion nea r the t r ans i t ion s tate .

Acknowledgement

This work has b e e n p e r f o r m e d in t he M o l e c u l a r Research L a b o r a t o r i e s of P ro fe s so r M. E l - N a d i . T h e au tho r s are d e e p l y i n d e p t e d fo r his c o n t i n u o u s suppor t .

T h a n k s a re a lso to Dr . S. H. A l l a m for his in te res t and e n c o u r a g e m e n t .

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