International Journal of Mass Spectrometry and Ion Processes, 101 (1990) 11 l-120 Elsevier Science Publishers B.V., Amsterdam

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THRESHOLD C,H,+ FORMATION FROM THE BENZYL HALIDES BY PHOTOIONIZATION MASS SPECTROMETRY *

JOHN C. TRAEGER and BARBARA M. KOMPE

Department of Chemistry, La Trobe University, Bundoora, Victoria 3083 (Australia)

(First received 5 January 1990; in final form 30 April 1990)

ABSTRACT

Threshold photoionization mass spectrometry has been used to measure appearance energies for the C,H: fragment ions produced from benzyl chloride, benzyl bromide and benzyl iodide. Based on the stationary electron convention, the derived 298 K cationic heats of formation are 865 f 8 kJ mol-‘, 863 f 10 kJ mol-’ and 868 f 10 kJ mol-‘, respectively. These are consistent with a threshold tropylium structure, rather than the benzyl structure proposed from low ionizing electron energy collisional activation mass spectra. High level ab initio molecular orbital calculations for both the benzyl cation and tropylium cation support a value of 865 f 3 kJ mall’ for AH&, (tropylium+).

INTRODUCTION

The structures of the various gaseous C,H7f isomers have been studied extensively since the pioneering work of Rylander et al. [l]. It is generally accepted that the most stable isomer is the cyclic tropylium ion (Tr+) with the benzyl ion (Bz+) being slightly higher in energy [2].

Of the large number of precursor molecules that are capable of producing C,HT fragment ions, most can be regarded as consisting of either a sub- stituted toluene or a substituted cycloheptatriene. Their behaviour can be considered parallel to that of the toluene and cycloheptatriene radical cations. The toluene radical cation is the more stable structure, but rapid inter- conversion at the ionizing energies required for C,HT fragment ion formation results in preferential formation of the lower energy tropylium ion at threshold. At l-2eV above the threshold for Tr+ formation, production of Bzf becomes competitive, with isomerization occurring at slightly higher internal energies.

The proportion of each isomer is found to depend on both the identity of the precursor and the amount of internal energy. A case of interest is that of the benzyl halides. Although it is generally accepted that the proportion of

* Dedicated to Dr Fred P. Lossing on the occasion of his 75th birthday.

0168-1176/90/$03.50 0 1990 Elsevier Science Publishers B.V.

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Tr+ formed decreases with decreasing electron energy, the behaviour at threshold has been a matter of contention.

From the collisional activation (CA) mass spectra of benzyl iodide at 16 eV and 70eV, McLafferty and Winkler [3] found a higher proportion of Bz+ formed at the lower ionizing energy (80%, based on the relative abundance of the m/z 77 peak). They rationalized that loss of the halide to form Bz+ was favoured over the rearrangement reaction to form a substituted cycloheptatriene radical cation because of the lower activation energy required to break the weaker C-X bond compared with C-H bond rupture prior to rearrangement. At low electron energies the formation of nearly all Bz+ was ascribed to the insufficient internal energy requireS for Bz+ to Trf isomerization.

From an ion cyclotron resonance (ICR) study of the structures of C,HT ions Jackson et al. [4] found that, for benzyl chloride and benzyl bromide, the abundances of unreactive C,H7+ (Tr+ ) was low and also decreased with decreasing electron energy. However, their results of 93% Bz+ from benzyl chloride and 97% Bz’ from benzyl bromide at 14 eV were only in qualitative agreement with data from the CA spectra of McLafferty and Winkler [3].

In a subsequent study, McLafferty and Bockhoff [5] obtained results that were in better agreement with the ICR experiments. From the relative m/z 77: m/z 74 abundance in the CA spectra at 12 eV they found that benzyl chloride produced 92% Bz+, with essentially pure Bzf being obtained from benzyl fluoride. Although benzyl bromide gave 96% Bz+ when fragmented with 10.5 eV electrons, it was found that at the low internal energies associated with metastable decomposition the proportion of Bz+ decreased. They therefore concluded that benzyl bromide does not lead to pure Bz+ at any ionizing electron energy.

From a re-investigation of the CA mass spectra of C,H++’ ion mixtures, Buschek et al. [6] found that, at low ionizing electron energies, benzyl chloride produced 80% Bz+ whereas benzyl bromide gave rise to 100% Bz+. It was also proposed that the previous CA data for benzyl chloride and benzyl bromide [5] should be amended to 85% Bz+ and 100% Bz+ respectively, following the analytical correction procedure of Proctor and McLafferty [7].

More recently, Baer et al. [8] investigated the C,HT dissociation onset for benzyl bromide by photoelectron photoion coincidence (PEPICO). Based on the above observation that, near threshold, benzyl bromide ions dissociate exclusively to Bz +, they calculated a 298 K heat of formation of 897 f 5.kJ mol-’ for the benzyl cation. This is in excellent agreement with other previous experimental measurements [2,9-l I], but is lower than the theoretical values of 922 kJ mol-‘, obtained from semi-empirical MIND0/3 calculations by Cone et al. [12], and 908 kJ mol-‘, obtained by Abboud et al. [13] using ab initio molecular orbital calculations at the HF/4-31G level.

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Although the tropylium cation is more stable than the benzyl cation [2], its heat of formation is much less certain, with recent experimentally determined values ranging from 849 to 900 kJ mol-’ [ 11,14-l 71. The ionization energy of the tropyl radical is well defined at 6.24 + 0.01 eV [ 18,191 but the large uncertainty in its heat of formation precludes any direct accurate estimation of AHL9* (Tr+). The result of the MIND0/3 calculation [12] for the heat of formation of Tr+ is 818 kJ mol-‘, whereas the ab initio calculation [13] gives a corresponding estimate of 870 kJ mol-‘.

The aim of this study was to further investigate the ionization and dis- sociation of some benzyl halides using photoionization mass spectrometry. From the energetics associated with these processes it was hoped to resolve the problem of the threshold C,HT structure for benzyl bromide and to obtain reliable values for the heats of formation for both Bz+ and Tr+.

EXPERIMENTAL

Threshold photoionization efficiency (PIE) curves for the parent and frag- ment ions studied here were measured with a high-sensitivity photoionization mass spectrometer which has been described previously in detail [20,21]. The photon source used was the hydrogen pseudo-continuum with the band pass of the monochromator fixed at 0.125 nm; this corresponds to a resolution of 8-1OmeV for the range of photon energies used in this work. The absolute photon energy scale was calibrated with known atomic lines and found to be accurate to better than 0.003eV. Experiments were performed at ambient temperature (297 K) with sample pressures of 10e3 to lop4 Pa in the ion source region. There was no evidence for any collision-induced dissociative process occurring in the pre-threshold energy regions. The compounds were all of research grade purity. However, because of its low volatility, the sample of benzyl iodide was further purified by preparative layer chromatography (silica gel) using a chromatotron. A GC-MS analysis for each compound showed no contamination with fragment ions that would interfere with the present experiments.

RESULTS AND DISCUSSION

To establish the relative stabilities of Tr+ and Bz+ an ab initio molecular orbital calculation was performed using the GAUSSIAN 86 suite of programs [22]. This included a geometry optimization at the HF/6-31G level, with zero-point energies and vibrational frequencies being ob?ained with a 3-21G basis set. The 6-31G geometries for both Tr+ and Bz+ are presented in Fig. 1. The calculated normal coordinate vibrational frequencies are listed in Table 1 and, following the observations of Pople et al. [23], have been scaled by 0.89.

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im H

H H

128.6 115.7

I.390

H 3-7 H

H H

Tropylium cation Benzyl cation

Fig. 1. Calculated HF/6-3 1 G geometries for the tropylium cation and the benzyl cation. Bond lengths are given in 8, and bond angles in degrees.

The energy difference between the Bz+ and Tr+ structures (corrected for 3-21G zero-point vibrational energies) was found to be 28 kJ mol- ’ at the HF/3-21G level, which increased to 32 kJ mol-’ when the larger 6-3 1G basis set was used. Although it has been demonstrated that the inclusion of electron correlation in such calculations may significantly alter the relative stabilities of cyclic hydrocarbon cations [24], no such effect was observed here; a calculation at the MP2 level reduced the relative 3-21G stability by only 1 kJ mall’.

The threshold PIE curves for C,HT produced from benzyl chloride, benzyl bromide and benzyl iodide are shown in Figs. 2-4. If the stationary electron convention [25] for cationic heats of formation is adopted then, in the absence of any excess energy, AH” f298 (C,HT ) may be calculated from the expression

AH;,,(C,H:) = A&&H:) + AH&,(C,H7W - AK%g(X) + A&r (1)

TABLE 1

Calculated vibrational frequencies (cmm’)a

Benzyl cation

170 351 361 427 531 611 626 651 775 809 869 956 963 993 1027 1055 1087 1096 1134 1167 1187 1285 1335 1344

1422 1466 1513 1533 1559 2958 3003 3004 3007 3033 3037 3043

Tropylium cation

227 227 436 436 561 561 674 831 877 877 892 892 959 959 1071 1071 1111 1111 1218 1218 1281 1281 1431 1435

1435 1492 1492 1575 1575 2979 2980 2988 2988 2999 2999 3005

“Computed at HF/3-21G and scaled by 0.89.

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I’ 0 Benzyl chloride : F: _ ‘. 2 m/z 91 2 2. s

2 *... .

s .

i! . = .3 c =. . 2 : . . .

2. :-..

0

e .-.. .

9.6 9.7 9.8 9.9 10.0 10.1 10.2 10.3 10

Photon Energy /eV .4

Fig. 2. Threshold photoionization efficiency curve for C,HT fragment ions produced from benzyl chloride.

where AE298 is the experimental appearance energy obtained from a threshold linear extrapolation of the PIE curve. The thermal energy correction term, AH,,, , is derived from statistical mechanical calculations [25,26] and, where X represents an atomic species (as is the case for the benzyl halides), is simply given by the enthalpy change Ho,,, - W,, for the C,HT ion. This has been calculated using the vibrational frequencies for Tr+ from Table 1. The extrapolated sections of the PIE curves indicated in Figs. 2-4 each produce pre-threshold tailing which is consistent with the hot band structure observed for the corresponding molecular ion.

The thermochemistry for the unimolecular processes studied in this work is summarized in Table 2. The adiabatic ionization energies (IEs) were ob-

Photon Energy /eV

Fig. 3. Threshold photoionization efficiency curve for C,HT fragment ions produced from benzyl bromide.

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Photon Energy /eV

Fig. 4. Threshold photoionization efficiency curve for C,H: fragment ions produced from benzyl iodide.

TABLE 2

Thermochemistry for the gas-phase reaction at 298 K C,H,X + hv + C,H: + X + e-

G&X X IE” AE” AHmrb’C AH=&b

C,H,Xd X C,H:’

Benzyl chloride Cl 9.10 9.85 16.9 18.9 121.3’ 864.9 Benzyl bromide Br 9.02 9.21 16.9 63.3 111.9’ 862.7 Benzyl iodide I 8.13 8.84 16.9 104.6 106.8f 867.6 Isopropylbenzene CZ H, 8.72g 9.919 23.1’ 4.0 118.7’ 864.6 Propylbenzene GHs 8.71g 9.85g 23.1h 7.9 118.7’ 862.7

a * 0.02 eV. bkJ mol-’ ‘Calculated using vibrational frequencies for Tr+ from Table 1. dRef. 28. ‘Calculated from Eq. 1 and 1 eV = 96.487 kJ mall’. ‘Ref. 26. gRef. 16. hW298 - Ho0 (C,H,) = 12.4kJ mall’, from Ref. 25. ‘Ref. 29.

tained by taking the first maximum of the first differential PIE curves and are in good agreement with other photoionization studies [2].

The 298 K heat of formation for Bz+ has been critically evaluated as 899 kJ mall’ [2], with a more recent value of 897 f 5 kJ mol-’ being obtained by Baer et al. [S] from a PEPICO study of benzyl bromide (this latter value increases to 900 kJ mol-’ if the theoretically calculated vibrational frequencies

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for Bz+ from Table 1 are used to convert the 0 K experimental data to 298 K). Given that Tr+ is 32 kJ mol-’ more stable than Bz+ (see above), the corres- ponding heat of formation for Tr+ should thus be 867 kJ mol-‘. From the threshold C,HT AEs for benzyl chloride, benzyl bromide and benzyl iodide, we obtain 298 K heats of formation of 865 f 8 kJ mol-' , 863 + 10 kJ mol-’ and 868 f 10 kJ mall’ respectively (Table 2). The average AH&,(C,H,f) value of 865 f 3 kJ mol-’ clearly indicates that the benzyl halides produce Tr+, rather than Bz+, at threshold.

The C,HT heat of formation (897 kJ mall’) derived by Baer et al. [8] from the PEPICO breakdown diagram of benzyl bromide is in excellent agreement with the recent critically evaluated value of 899 kJ mol-’ [2] for the benzyl cation. How then can this be consistent with the present PIE data? One possible explanation is that, when it becomes energetically possible, the production of Bz+ rapidly overwhelms the threshold formation of Trf and that the observed breakdown curve is in fact dominated by Bz+. The low energy tail observed in the m/z 91 breakdown curve may be due to a small underlying contribution from Tr+ production; this would also be in accord with the low electron ionizing energy CA measurements [6] which indicated essentially “pure” Bzf at threshold.

An analysis of the m/z 91 PIE curves was made to see if any evidence could be found for an increase in C,H: production at the expected thermochemical threshold for Bz+. However, in each case, the region of interest was com- plicated by the onset of a higher electronic state for the molecular ion. From the photoelectron spectra [27] the first two excited electronic states (x2 and n,) occur in the regions of 9.6eV and 10.2 eV for benzyl chloride, 9.6eV and lO.OeV for benzyl bromide and 9.2eV and 9.9eV for benzyl iodide.

When a new excited ionic state becomes accessible the increased ionization transition probability leads to an increase in the total ion PIE which may appear as a break in the fragment ion PIE. The C,HT PIES relative to the precursor molecular ion for the benzyl halides studied here are shown in Figs. 5-7. For both benzyl bromide and benzyl iodide, the onsets of the x2 and n, electronic states appear solely as an increase in the production of C$H,+, with the-molecular ion PIE remaining constant. It is possible then that the thermochemical analysis of the PEPICO breakdown curve for benzyl bromide

y the appearance of this new fragmentation channel at tion from benzyl chloride, which occurs above the first

excited electronic state, also shows an increase at the onset of the second excited electronic state. However, unlike the other benzyl halides, this does not appear to be exclusively at the expense of production of the molecular ion.

In order to obtain the Bz+ heat of formation of 899 kJ mall’ the A,!?,,, value for benzyl bromide would have to increase from 9.27eV to 9.65 eV. Apart from the excessive amount of hot band structure that such an AE would

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Benzyl chloride

Photon Energy /eV

Fig. 5. Relative photoionization efficiency curves for C,H,Cl+’ and C,HT produced from benzyl chloride.

Photon Energy /eV

Fig. 6. Relative photoionization efficiency curves for C,H,Br+’ and C,H: produced from benzyl bromide.

require, there is no simple extrapolation that even closely correlates with the higher AE. The possible extrapolations of 9.52 eV and 9.86 eV that could be made from the PIE curve of Fig. 6 are a consequence of the onsets of the 7c2 and n, electronic states.

From an earlier study of the thermochemistry for C,Hj+ from a series of monosubstituted alkyl benzenes [16] we obtained a mean cationic heat of formation of 859 f 8 kJ mol-‘. However, in the absence of any excess energy, this was an underestimate because no thermal correction was made in the calculations. Although the C,H: PIE curves for the methyl, ethyl and butyl benzenes all had poorly defined thresholds, consistent with the presence of a kinetic shift effect, this was not the case for isopropyl benzene and n-propyl benzene, indicating that their measured 298K AEs should provide a

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b

o

O

o

Benzyl iodide ". ~A

m/z 91 •. •••"

.•• m/z 218

8.5 9.0 9.5 10.0 P h o t o n E n e r g y / e V

Fig. 7. Relative photoionization efficiency curves for C7H7 I+" and C7 H+ produced from benzyl iodide.

estimate of A/-Pf(tropylium + ). A recalculation in which the thermal correction term, AHco,, is included (Table 2) yields values of 864.6 and 862.7 kJ mol- i , in excellent agreement with the present results obtained for the benzyl halides.

CONCLUSIONS

Dissociative photoionization mass spectrometry has been used to measure the threshold C7H ~- AEs for benzyl chloride, benzyl bromide and benzyl iodide. These data have been used to derive 298 K heats of formation of 865 + 8kJ mol -~, 863 + 10kJ mo1-1 and 868 _ 10kJ mol -~ respectively, based on the stationary electron convention, which are 31-36 kJ mol-1 lower than the critically evaluated heat of formation for Bz + [2]. From ab initio molecular orbital calculations at the HF/6-31G level, the Tr + cation was found to be 32kJ mo1-1 more stable than the Bz ÷ cation, clearly indicating that the benzyl halides produce Tr +, rather than Bz +, at threshold. Although this is contrary to results obtained from low electron energy collisional activation mass spectra for benzyl bromide [6], it is proposed that the threshold formation of Tr ÷ is rapidly overtaken, and subsequently dominated, by the production of Bz ÷ at ionizing energies 30-40kJ mo1-1 in excess of threshold. AE measurements for n-propyl benzene and isopropyl benzene were found to support a value of 865 + 3kJ mo1-1 for AH~298 (tropylium ÷ ).

ACKNOWLEDGEMENTS

It is a pleasure to acknowledge the many great contributions of Fred Lossing to gas phase ion thermochemistry and, in particular, his most valued personal and scientific friendship during the past 18 years. We would like to

120

thank Dr. John Christie for his many helpful discussions. This work was financially supported by a grant from the Australian Research Council. One of us (B.M.K.) acknowledges the award of an Australian Postgraduate Research scholarship.

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