Heat of Formation for the SH Radical by - Photoionization Mass Spectrometry

John C. Traeger Department of Physical Chemistry, La Trobe University, Bundoora, Victoria 3083, Australia

Photoionization mass spectrometry has been used to measure the appearance energies for [C,H,]+ from ethanethiol, [C3H,]+ from 2-propanethiol and [C3H$ from 2-methylthiirane. From the known ther- mochemktry of these cations and their precursor molecules, a 298K heat of formation of 138.6*0.4 kJ mol-' for the SH radical has been derived.

INTRODUCTION

The energetics associated with the rupture, or forma- tion, of sulphur-containing bonds is of fundamental importance to the understanding of many important chemical processes. The 298 K heat of formation for the SH radical, which can be used to calculate R-SH bond dissociation energies, has been studied by a variety of experimental techniques over recent years, with the recommended value decreasing from 149k 12 kJ mol-' to 1 3 9 f 5 kJ mol".'~'

This latter value, cited in the JAJVAF Thermochem- ical Tables,' is based on a photoionization appearance energy ( A E ) measurement for [SH]+ produced from H2S,3 and spectroscopic ionization energy for the SH r a d i ~ a l . ~ Because of some uncertainty surrounding the actual 0 K AE, the error associated with the observed AE (14.27 eV) used in the thermochemical calcula- tions was arbitrarily increased from 0.02 to 0.04 eV.' A recent study of H2S by high resolution photoioniza- tion mass ~pectrometry,~ in which a molecular beam sample was used, has produced an A E for [SH]+ of 14.300f0.0024eV which should be a very good rep- resentation of the OK AE. This value, if used in the JANAF calculations, increases the 298 K SH heat of formation to 142 f 3 kJ mol-'.

There is some additional uncertainty surrounding the use of this particular reaction because the ioniza- tion and fragmentation of H2S producing [SH]' is not the lowest energy process; production of [S]+ from H2S occurs almost 1 eV 1 0 w e r . ~ ~ ~ It is, therefore, quite possible that there will be some competition between the two reaction channels of the parent cation, result- ing in a 'competitive shift' or increase in the observed A E for the higher energy process.6 To minimize this possible source of error, as well as any other excess energy effects, such as kinetic shift and/or reverse activation energy, it is preferable to use several differ- ent fragmentation processes for the thermochemical analysis. A further complication with the HzS frag- mentation is the interference to detection of C3'SH]+ ions by [33S]t ions in the mass spectrometer. The present study has investigated the thermochemistry of a number of organosulphur compounds by photoioni- zation mass specti-ometry, with the aim of determining an accurate heat of formation for the SH radical.

EXPERIMENTAL

Photoionization efficiency (PIE) curves were measured with a high-sensitivity photoionization mass spec- trometer which has been described in detail el~ewhere.~.' The photon source employed in the pres- ent experiments was the hydrogen pseudo-continuum, with the resolution of the monochromator being fixed at 0.125 nm; this corresponds to a photon energy resolution of 10 meV at 10 eV. A11 compounds were of research grade purity and contained no significant interfering impurities. The experiments were per- formed at ambient temperature (297 K), with sample pressures of 10-3-10-4Pa in the ion source region. Photoionization mass spectra were obtained by setting the monochromator in a total reflection mode, such that all usable light produced in the hydrogen dis- charge lamp could be transmitted to the ion source; this represents ionization by photons with an energy spread between approximately 8 eV and 14 eV.

RESULTS AND DISCUSSION

The [SH]+ cation is not a prominently observed frag- ment in the mass spectra of organosulphur com- pounds. Because the SH radical ionization energy is greater than most organic radicals, ionization and frag- mentation of R-SH molecules will preferentially pro- duce the [R]+ cation. Although it is preferable to use a dissociation process producing directly observable [SH]+ to calculate an accurate cationic (and, hence, radical) heat of formation, it should be possible to obtain reliable thermochemical information about the SH radical produced in a fragmentation where the daughter ion heat of formation is well known.'

The compounds chosen for this study were ethanethiol, 2-propanethiol and 2-methylthiirane, each of which undergoes a dissociative ionization pro- cess resulting in the formation of [R]' + SH. Accurate 298 K heats of formation for [C,H,]+, [2-C,H7]+ and [C3H,]+ have been established previously from photo- ionization studies in this laboratory.7310

In the absence of any excess energy, the 298 K heat of formation can be obtained from the A E for the

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514 ORGANIC MASS SPECTROMETRY, VOL. 19, NO. 10, 1984 OWiley Heyden Ltd, 1984

HEAT OF FORMATION FOR THE SH RADICAL

process

RSH + hv + R' + SH + e-

by means of the thermochemical expression lo

A%298(SH) = AE298 + AH&9!3(RSH) - A%Z~,([R]+) + AHCOX ( 1)

AE298 is the experimental appearance energy based on a threshold linear extrapolation of the PIE curve and the thermal energy correction term, AH,,, is given by

AHc,,= AH&8([R]+) + AH&8(SH)-745 R (2) where represents HZg8 - Hg and the stationary electron convention for cationic heats of formation (i.e. AH&(e-) = 0) has been adopted.'0,'' All appear- ance energies measured in this work have been ob- tained from linear extrapolations which yield hot-band structure consistent with that for the relevant molecu- lar ion.

Although the 298 K heat of formation for [CH,]' is well known," methanethiol was not a suitable precur- sor for the present study because the thermochemical A E (approximately 12.9 eV) is near the upper limit of photon energies available with the hydrogen pseudo- continuum. In addition, there would almost certainly be some competitive shift present, as the dissociative process leading to [CH,]+ + SH requires a minimum of 2.7 eV more energy than the corresponding produc- tion of [CH,S]+. + H2.12 A recent photoionization study of methanethiol', has in fact shown that there is an observed kinetic shift of approximately 0.4eV as- sociated with the [CH,]+AE.

The photoionization mass spectra for the RSH com- pounds studied here all showed a prominent [R]+ peak. For ethanethiol, the formation of [C,H,]+ was not the predominant fragmentation process; [CH,S]+ and [CH,S]+' each had more intense peaks in the spectrum. However, for both 2-propanethiol and 2- methylthiirane, the largest fragment peak corre- sponded to production of [R]+ + SH.

The PIE curve for [C2H5]+ produced from ethanethiol is shown in Fig. 1. The onset, although

ETHANETHIOL m/z 29

11.26

11.0 11.1 11.2 11.3 11.4 11.5 11.6

PHOTON ENERGY /eV

Figure I . Threshold photoionization efficiency curve for [C,H,]+ fragment ions produced from ethanethiol.

2-PROPANETHIOL m/z 43

10.35

I, -;-. 10.0 10.1 10.2 10.3 10.4 10.5 10.6 lo:,

PHOTON ENERGY /eV

Figure 2. Threshold photoionization efficiency curve for [C,H,]' fragment ions produced from 2-propanethiol.

reasonably well defined, was probably subject to a competitive shift, as the formation of [CH,]', [CH,S]+', [H2S]+. and [C,H,]+' were all observed at photon energies lower than 11.26eV. The lowest energy fragmentation was the production of [CH,S]+' + CH,, which had a sharp onset at 10.82 eV.

There are only two fragmentation processes ob- served at photon energies below the AE of 10.35 eV for [C,H,]' from propanethiol (Fig. 2). Of these, the formation of [C,H,]+' + H,S has a thermochemical threshold just 0.02 eV lower; the onset for production of [C,H,S]" was observed to occur in the region of 10.15 eV, which is 0.2 eV lower than the simple bond rupture producing the SH radical. As a consequence, any competitive shift associated with the [C,H,]+ A E should be minimal.

A similar conclusion can also be made for 2-methyl- thiirane. The two fragmentation processes competing with the production of [C,H,]+ + SH at 10.66 eV (Fig. 3) are [CH,S]+' + C2H4 and [C,H,S]'+ CH,, both of which have observed onsets at approximately 10.55 eV.

The RSH adiabatic ionization energies and [R]+

2-METHYLTHI IRANE m/z 41

10.66

.*........ ----.-----. 10.4 10.5 10.6 10.7 10.8 10.9 11

PHOTON ENERGY /eV 1

Figure 3. Threshold photoionization efficiency curve for [C,H,l+ fragment ions produced from 2-methylthiirane.

ORGANIC MASS SPECTROMETRY, VOL. 19, NO. 10. 1984 515

J. C. TRAEGER

~~ ~~ ~~~~ ~ ~

Table 1. Thermochemistry for the gas phase reaction at 298K RSH+ hv+ [R]'+SH+e-

AH;-(kJ mol-') RSH [Rl+ I € AE AH,,,: RSHb [RI' SHd

(eV) (eV) (LJmol-')

Ethanethiol [C,H,]+ 9.28 11.26 14.5 -46.3 903.7 150.9 2-Propanethiol [2-C3H7]+ 9.15 10.35 18.4 -76.2 802.5 138.3 2-Methylthiirane [C,H,]+ 8.85 10.66 14.3 45.8 949.6 139.1 Thietane" [C,H,]+ 8.61 10.50 14.3 60.7 949.6 138.5

aCalculated from Eqn (2); H&-H;([R]') from Ref. 7 and Ref. 10; H&-Hg(SH) from Ref. 2. bRef. 15.

Ref. 7 and Ref. 10. Calculated from Eqn (1). Results from Ref. 12.

appearance energies measured in this work are sum- marized in Table 1, together with the SH heats of formation obtained from Eqn (1) and the supplemen- tary thermochemical data used in these calculations. Although not studied here, the corresponding photo- ionization measurements for thietane of Butler and Baerl' have been included in Table 1. It should be noted that their AE for thietane was also determined by a linear extrapolation of the PIE curve in the threshold region.

Apart from ethanethiol, the calculated SH heats of formation are in excellent agreement. The high value for ethanethiol can be attributed to a competitive shift in the observed AE for [C,H,]+. If this value is excluded from the thermochemical discussion, an av- erage heat of formation for SH of 138.6k0.4 kJ mol-' is obtained. The small deviation associated with the three calculated heats of formation is good evidence that the fragmentation products are formed in their lowest energy states at threshold; it is highly improba- ble that such different systems would exhibit similar excess energies.

The exact agreement of the present result with the JANAF thermochemical tables' is probably fortuitous, particularly in view of the uncertain [SH]' AE used for the latter calculation. However, it is in accord with both the modified JANAF value of 142 f 3 kJ mol-l, discussed above, and a kinetic measurement of 141 k 5 kJ mol-' based on a study of the gas phase iodina- tion of H,S by I2.I4 The slightly higher value obtained by using the molecular beam photoionization meas-

urements for H,S in the JANAF calculation probably reflects an excess energy effect associated with the [SHI'AE. Given the excellent agreement between the present results and those of Butler and Baer," it is proposed that the 298 K heat of formation for the SH radical can be firmly established at 139 kJ mol-l.

CONCLUSION

Appearance energies have been measured from PIE curves for [C,H,]+ from ethanethiol, [C,H,]' from 2-propanethiol and [C,H,]+ from 2-methylthiirane. From the known heats of formation for these cations and their precursor molecules, calculations of the SH radical heat of formation have been made. Apart from ethanethiol, which has been shown to be affected by a competitive shift, the calculated values for the other two compounds are in excellent agreement with each other, as well as with a calculation based on an independent photoionization measurement of the [C,H,]'AE from thietane. A 298 K heat of formation of 138.6k0.4 kJ mo1-l for the SH radical has been derived.

Acknowledgement

Financial support of the Australian Research Grants Scheme is gratefully acknowledged.

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Received 16 December 1983; accepted 12 January 1983

516 ORGANIC MASS SPECTROMETRY, VOL. 19, NO. 10, 1984