International Journal of A4u.w Spectrometry and Ion Processes, 58 (1984) 259-271

Elsevier Science Publishers B-V., Amsterdam - Printed in The Netherlands 259

A STUDY OF THE ALLYL CATION THERMOCHEMISTRY BY PHOTOIONIZATION MASS SPECTROMETRY *

JOHN C. TRAEGER

Department of Physical Chemistry, La Trobe University, Bundoora, Vie. 3083 (Australia)

(Received 13 June 1983)

ABSTRACT

The ionization energies and C, Hl appearance energies for several C,H, and C,H, hydrocarbons have been measured by photoionization mass spectrometry. A 298 K heat of formation of 949.6 f 1.4 kJ mol-‘, based on the stationary electron convention, is derived for the ally1 cation in the gas phase; this results in a heat of formation for the ally1 radicai of 165.2+3.3 kJ mol-‘. From corresponding measurements for the ally1 halides, heats of formation at 298 K are obtained for 3-chloro-l-propene ( - 5.6 kJ mol-I), 3-bromo-1-propene (+47.7 kJ mol-‘) and 3-iodo-l-propene (+99.5 kJ mol-‘), together with values for the contributions of C-(C,)(H),(X) groups to heats of formation.

INTRODUCTION

The C,Hz cation is one of the most commonly observed fragments in organic mass spectra. Several theoretical studies [l] have shown that the most stable structure for this ion in the gas phase is the ally1 cation structure. This has been experimentally substantiated by Lossing [2,3] who showed that a variety of hydrocarbon precursors all fragmented under electron impact to produce the ally1 cation with little or no excess energy; this was found to be the case even when considerable skeletal rearrangements were required to form the ally1 structure. Although the heat of formation for the ally1 cation (945.6 & 8.4 kJ mol-‘) calculated by Lossing is sufficiently precise to dis- tinguish the structure from other possible isomers, the importance of the ally1 cation in gas-phase chemistry warrants a more accurate value. To this end, Buttrill et al. [43 carried out a photoionization study based on ap- pearance energy measurements for propene and cyclopropane. From their experiments, they concluded that a more reliable value for AH,(C,H,‘) was

* Dedicated to the memory of H.M. Rosenstock.

016%1176/84/$03.00 Q 1984 Elsevier Science Publishers B.V.

260

939.3 & 4.2 kJ mall I. However, this was based solely on the measurement for propene, their cyclopropane result of 942.7 kJ mol-’ being regarded as only an upper limit due to a possible kinetic shift effect, despite the unusually long ion residence time of 100 1~s for their quadrupole mass spectrometer. Although it is not clear how the C,Hc appearance energies were derived from the photoionization efficiency curves, no correction for thermal energy effects appears to have been made in the respective heat of formation calculations.

We have previously shown [5] that for the unimolecular fragmentation

ABi-hv+A++B+e-

the standard cationic heat of formation at temperature T can be obtained from

AHPT(A+) = AE, - AHPT(B) + AHt(AB) + AH_ (1) where AE, is the experimental appearance energy based on a threshold linear extrapolation of the photoionization efficiency (PIE) curve and the thermal energy correction term, AH,,,,, is given by

AH,,,, = AH,O(A+) i-AH;(B) - 2SRT (2)

where AH: represents H: - Hi and the stationary electron convention for cationic heats of formation [i.e. AH:(e-) = 0] has been adopted [5,6]. Any neglect of A Hcorr in calculating a 298 K heat of formation for a given cation should result in an underestimated value which will vary with B, i.e. with the fragmentation process being studied.

In this study, the technique of dissociative photoionization has been applied to a number of organic molecules which have a prominent m/z 41 peak in their mass spectrum, with the aim of determining‘a precise ionic heat of formation for the ally1 cation in the gas phase.

EXPERIMENTAL

Photoionization efficiency curves for the parent and fragment ions studied here were measured with a high-sensitivity photoionization mass spectrome- ter which has been described in detail previously [7]. Extensive modifications have recently been made to the data acquisition system which have greatly improved the flexibility and reliability of the instrument.

The PDP 11/40 and 6502 computer combination has been replaced with a Digital Equipment Corporation LSI f1/2 microprocessor with 64 Kb RAM and dual double density 20 cm floppy disk drives. An Analog Devices RTI 1251 combined 12-bit analog input/output subsystem is used to control the grating stepper motor drive and to monitor the photon intensity. A custom-

261

built l&bit programmable interval timer and asynchronous l&bit pulse counter is used to record the photoion count rate as well as to generate appropriate timing delays for the control of the photoionization mass spectrometer.

The processing of experimental data is carried out using a Visual Technol- ogy V500 graphics terminal and an Okidata Microline- dot matrix printer. With the exception of certain time-critical data acquisition subroutines, which have been written in assembly language, all programs used to collect and process data have been written in Fortran.

The hydrogen pseudocontinuum was used as the photon source for the present experiments with the band pass of the Seya-Namioka monochroma- tor set at 0.125 nm, which corresponds to a photon energy resolution of 10 meV at 10 eV. All compounds were of research grade purity and showed no significant impurities in their mass spectra. The experiments were performed at ambient temperature (296 K) with sample pressures of lob3 to 10U4 Pa in the ion source region.

RESULTS AND DISCUSSION

The threshold PIE curves for C,Hf produced from a series of C,H, and C,H, isomers are shown in Figs. 1-6. Because the photoion count rates were very low ( - 0.5 c.p.s.) in the threshold region it was necessary to collect the ra,w experimental data over extended periods of several days to obtain a sufficiently high signal-to-noise ratio in the curves; each such curve repre- sents the average of a number of separate experiments.

Propene {Fig. 1) shows a very sharp onset which gives a 298 K appearance energy of 11.86 eV. However, the pre-threshold tail, which should be just due to hot band structure (i.e. ionization and dissociation of thermally excited neutral precursor molecules), is more extensive than that observed in the corresponding PIE curve for the propene molecular ion and as a result, the “true” appearance energy may be lower than that indicated. Although the corresponding fragmentation process in cyclopropane was observed to have a lower cross-section, the onset is quite sharp (Fig. 2), contrary to the observations of Buttrill et al. [4]. The appearance energies for both C,H, isomers studied here are in excellent agreement with the results of Krassig et al. [S] who used a mass spectrometer in conjunction with synchrotron radiation for their photoionization experiments.

The C4Hx isomers are subject to a “competitive shift” because the onset of the dissociative process which leads to C,HT occurs in the same energy region [3]. The extrapolated sections of the PIE curves as indicated in Figs. 3-6 all give pre-threshold tails which can be correlated with the expected hot band structure. The 298 IS appearance energies are approximately 0.1 eV

262

,

PROPENE .

m/z 41

114 11-5 11.6 11.7 114 H-9 12,o 12.1 12.2 12 3

PHOTON ENERGY /eV

Fig. 1. Threshold photoionization efficiency curve for C,Hf fragment ions produced from propene.

E! n

CYCLOPROPANE m/z 41

. .

/

. l .

. .

i/

l *

. .

PHOTON ENERGY /eV

Fig. 2. Threshold photoionization efficiency curve for C,H: fragment ions produced from cyclopropane.

263

l-BUTENE . .

m/z 41 . .

.

. .

. .

. . .

10 9 11-O 11 1 11 2 11.3 11.4 115 11.6

PHOTON ENERGY /eV

Fig. 3. Threshold photoionization efficiency curve for C,Hz fragment ions produced from 1-butene.

trans-Z-BUTENE l

m/z 41 l *. . . .

. .

. .

. .

.

11 1 H-2 11-3 11,L 11fI 11.6 11.7 11-e 11.9

11.30

PHOTON ENERGY AA

+ .==

.

Fig. 4. Threshold photoionization efficiency curve for C,Hz fragment ions produced from trans-2-butene.

264

cis-2- BUTENE l . .

m/r 41 . .

l

.- . 0.

. l

l

l * .

. 11.25 .

I

. .

.

I . l a

e 11.2 11.3 II-6 11.7 11.8 i t9

PHOTON ENERGY /eV

Fig. 5. Threshold photoionization efficiency curve for C3Wc fragment ions produced from

cis-Zbutene.

2-METHYL PROPENE .

l

m/z 41 .

l

. .

I .

.

. .

. . 1133

. .

PH.OTON ENERGY /eV

Fig. 6. Threshold photoionization efficiency curve for C,Hf fragment ions produced from

2-methyl propene.

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lower than those measured by Lossing [3]. No corresponding photoionization measurements are available for comparison.

The ally1 halides are characterised by very sharp onsets in their PIE curves (Figs. 7-9) with threshold count rates of approximately 5 c.p.s. This is consistent with a simple bond cleavage process; a similar conclusion was reached by Bowen et al. for ally1 bromide [9] and by Buff et al. for ally1 chloride [lo]. No evidence was found for an ion pair process from the present experiments. The observed appearance energy of 11.04 eV for ally1 chloride is in good agreement with a previous photoionization value of 11.05 eV [lo]. However, no mass spectrometric studies of ally1 bromide or ally1 iodide by either photon impact or monoenergetic electron impact appear to have been made.

All adiabatic ionization energies listed in Table 1 were obtained by taking the first maximum of the first differential photoionization efficiency curves. Apart from the ally1 halides, these are in good agreement with previous photoionization measurements [l&21]. The present value of 10.05 eV for ally1 chloride is only 0.01 eV higher than a recent photoionization value of Buff et al. [lo]. However, their adiabatic ionization energy, which was taken as the first rise from background in the parent ion photoionization efficiency curve, will be an underestimate of the true adiabatic value because of contributions to the observed photoion current from the ionization of thermally excited ally1 chloride molecules (“hot bands”); the hot band

.

ALLY L CHLORIDE m/z 41

/

.

lo6 10.7 104 10.9 11.0 11.1 112 11.3 IId.

PHOTON ENERGY /eV

Fig. 7. Threshold photoionization efficiency curve for C,H,f fragment ions produced from ally1 chloride.

266

W

ii

ALLY L BROMIDE . . l

m/z 41 .

m-1 10.2 10.3 io4 105 10.6 10.7

PHOTON ENERGY /eV

Fig. 8. Threshold photoionization efficiency curve for C,Hf fragment ions produced from ally1 bromide.

ALLYL IODIDE m/z 41

9.6 9.7 94

PHOTON ENERGY /eV

Fig. 9. Threshold photoionization efficiency curve for C,Hz fragment ions produced from ally1 iodide.

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TABLE 1

Thermochemistry for the gas-phase reaction at 298 K

C,H,X+ hv -+ C,Hz +X+e-

C,H,X X IE AE AHarra A HE,, (kJ mol-‘)

(ev) (ev) (kJ mol-‘) C,H,Xb X’ C,H+ d 5

Cyclopropane H Propene H Ally1 chloride Cl Ally1 bromide Br Ally1 iodide I 1 -Butene CH3 Trans-2-butene CHs Cis-2-butene CH, a-Methyl propene CH,

9.90 9.73 11.86

10.05 11.04 9.96 10.39 9.27 9.80 9.58 11.20 9.09 11.30 9.13 11.25 9.19 11.33

11.43 11.3 53.3 218.0 949.4 11.3 20.2 218.0 957.8 11.3 -0.6 = 121.3 954.6 11.3 45.6 111.9 947.5 11.3 91.5 106.8 941.6 15.5 -0.4 143.9 r 951.9 15.5 - 12.2 143.9 f 949.7 15.5 - 7.8 143.9 f 949.3 15.5 - 16.9 143.9 f 947.9

a Calculated from Eq. (2); H&s - H,$i(C,Hz ) calculated using vibrational frequencies for C,H, from ref. 11; H& - H$(B> from ref. 12. b Ref. 13. ’ Ref. 14. d Calculated from Eq. (1). e Ref. 15. f Ref. 5.

structure observed for ally1 chloride in the present experiments extended down to 9.75 eV.

The photoelectron spectra of the ally1 halides have been measured by several workers 122,231, with only the adiabatic values for ally1 chloride and ally1 bromide being listed [22]; a value of 9.25 eV can be estimated for ally1 iodide from the experimental photoelectron spectrum [24]. With the excep- tion of ally1 bromide, these are in accord with the present photoionization measurements. Because the onset for the ally1 bromide photoionization efficiency curve was not clearly defined (unlike the other halides) the value listed in Table 1, although 0.10 eV lower than the photoelectron measure- ment [22], should be considered as an upper limit to the adiabatic ionization energy.

Thermochemistry

The C, H,X ionization energies and C, H f appearance energies measured in this work are summarized in Table 1 together with the C,Hl heats of formation calculated from eq. (1) and the supplementary thermochemical data used in these calculations. The heats of formation for all precursor molecules were obtained from the Sussex-N.P.L. compilation of Pedley and

268

Rylance [13], the only exception being ally1 bromide which is not listed; the

value used for ally1 bromide was obtained from the thermochemical tables of Stull et al. [15].

The calculation of absolute cationic heats of formation requires accurate appearance energies and auxiliary thermochemical data. The heats of forma- tion for the C,H, and C,H, precursors used in this study have been derived from a variety of experimental techniques [25] and, as a result, are well known, with uncertainties of less than 0.6 kJ mol -I [13]. Few thermochemi- cal studies have been made of the ally1 halides, however.

The values for 3-iodo-l-propene and 3-bromo-l-propene in Table 1 [13] have been based on the enthalpy of hydrolysis measurements of Gellner and Skinner [26] together with estimated heats of vaporization [25]. Although these heats of vaporization are in good agreement with experiment [27,28], Gellner and Skinner attributed an error of 8.4 kJ mol-’ to their experimen- tal technique, with a further uncertainty of 4.2 kJ mol-’ in the actual data. Similarly, the values tabulated by Stull et al. [15] for the ally1 halides are all based on the results of Gellner and Skinner. However, in selecting values for ally1 chloride and ally1 bromide, consideration was made of the electron impact appearance measurements of Lossing et al. [29], together with some very old combustion measurements [30]. A more recent equilibrium measure- ment by Benson and co-workers [31] yielded a heat of formation for ally1 iodide of 96.2 kJ mol-‘, which is 4.7 kJ mol-l higher than the value cited by Pedley and Rylance [ 131.

In order to obtain a reliable estimate of the heat of formation for C,Hc, several calculated values from Table 1 have been excluded. The uncertainty surrounding the heats of formation for the ally1 halides is sufficient to warrant their omission from any average, even though the appearance energies are well defined with no apparent excess energy involved at threshold.

The calculated C,Hc heats of formation for the C,H, and C,H, isomers listed in Table 1 are in excellent agreement, with the exception of propene which is some 8 kJ mol-l higher than the average. There is theoretical evidence [32] to suggest that the formation of C,Hl from propene requires additional energy to produce observable fragmentations in 1 ps, i.e. the appearance energy is subject to a kinetic shift. The low extended photoioni- zation cross-section in the immediate threshold region is consistent with this effect. For this reason, propene has also been excluded from the average.

The four C,H, isomers have been studied by threshold photoelectron- coincident photoion -mass spectrometry [33] and found to equilibrate com- pletely to a mixture of rapidly interconverting structures prior to fragmenta- tion; the threshold energy for isomerization was also shown to be less than that for fragmentation. The remarkable similarity of the photoionization

269

efficiency curves for these molecules (Figs. 3-6) and the derived C,H,f heats of formation are consistent with these observations.

Based on the appearance energy measurements for cyclopropane and the C,H, isomers, the average value for the 298 K heat of formation for C,Hl is found to be 949.6 + 1.4 kJ mol: I. This is slightly higher than the previous values of Lossing [3] and Buttrill et al. [4], but is to be expected as no thermal energy correction [Eq. (2)] was applied to these calculations,

The cationic heat of formation can be combined with the adiabatic ionization energy for the ally1 radical to provide an estimate of the radical heat of formation. Lossing [2] measured the adiabatic ionization energy using an energy-resolved electron beam and obtained a value of 8.07 t 0.03 eV. More recently, Houle and Beauchamp [34] made a photoelectron spec- troscopic study of the ally1 radical and obtained a value of 8.13 k 0.02 eV for the adiabatic ionization energy. They observed that the structural features in their spectrum at lower energies were temperature-dependent and. attributed them to hot bands. This ionization energy, when combined with the present C,H,f heat of formation, results in a value of 165.2 + 3.3 kJ mol-1 for the ally1 radical heat of formation, in excellent agreement with the recent measurements of Golden and co-workers [35,36] and the preferred value of Benson and O’Neal [37], but lower than the values suggested by other workers 138,391.

The calculated C,Hc heats of formation for the ally1 halides (Table 1) vary over a range of 14 kJ mol- ‘. Because the experimental appearance energies for these compounds are clearly defined (Figs. 7-9), with no evidence of excess energy being involved at threshold, we are forced to conclude that the heats of formation for the precursor molecules are in error. By using the above value of 949.6 kJ mol-’ for the C,H; heat of formation in Eq. (l), together with data from Table 1, 298 K heats of formation may be derived for 3-chloro-1-propene ( - 5.6 kJ mol-‘), 3-bromo-1-propene ( + 47.7 kJ mol-‘), and 3-iodo-l-propene (+99.5 kJ mol-‘), each having an esti- mated error of 2.4 kJ mol- ‘.

These values differ by up to 5 kJ mol-’ from the values of Stull et al. [15], which were used by Benson and co-workers [40] to estimate contributions of the C-(C,)(H),(X) groups to group additivity calculations of heats of formation [37]. Given the uncertainty surrounding the ally1 halide heats of formation listed by Stull et al., it is proposed that the following group contributions provide a more reliable set of values: C-(C,)(H,)(Cl) = - 67.7 kJ mol-‘, C-(C,)(H),(Br) = -14.4 kJ mol-’ and C-(C,)(H),(I) = +37.4 kJ mol-I. (These have been calculated using the present recommended heats of formation, together with C,-(C,)(H), = 26.2 kJ mol-’ and Cd-(C,)(C)(H) = 35.9 kJ mol-l [37].)

270

CONCLUSION

Photoionization studies have been made of a number of C3H, and C,H, hydrocarbons. From a discussion of the thermochemistry associated with formation of the C,Hc fragment ion, a 298 K heat of formation of 949.6 + 1.4 kJ mol-‘, based on the stationary electron convention, has been derived for the ally1 cation in the gas phase. This value can be used to estimate a heat of formation of 165.2 & 3.3 kJ mol-’ for the ally1 radical and, from the C,Hl appearance energies for the ally1 halides, to obtain heats of formation at 298 K for ally1 chloride (- 5.6 kJ mol-‘), ally1 bromide ( + 47.7 kJ mol-‘), and ally1 iodide ( + 99.5 kJ mol-‘). These values provide a reliable set of estimates for the C-(C,)(H),(X) group contributions to heats of formation.

ACKNOWLEDGEMENT

Financial support from the Australian Research Grants Scheme is grate- fully acknowledged.

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