10725
J. Phys. Chem. 1993,97, 10725-10731
Unimolecular Photochemistry of Alkanethiols Studied by Photodissociation-Photoionization Mass Spectrometry Philip L. Ross and Murray V. Johnston' Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716 Received: June 28. 1993"
Unimolecular photodissociation of isomeric alkanethiols in the C3 to c8 size range was studied by using photodissociation-photoionization mass spectrometry. Photodissociation was performed with 193- and 248-nm radiation. The primary products were photoionized with coherent vacuum ultraviolet radiation and detected in a time-of-flight mass spectrometer. Three basic reaction channels were observed: C S , C-C, and S-H bond dissociation. The branching ratios for fragmentation through thesechannels were found to be strongly dependent upon molecular structure and photodissociation wavelength. All compounds gave intense products of C S bond dissociation. Photoionization and secondary fragmentation characteristics of the primary products suggested that complete statistical partitioning of the excess internal energy did not occur. Small molecules gave C-C bond dissociation at the a position, while larger molecules showed increasing probability for cleavage a t the p and y positions. Cleavage of the a C-C bond was strongly suppressed relative to other reaction channels when the expected product was a methyl radical. Complex rearrangements were observed for molecules exhibiting suppression of C-C bond dissociation. Cleavage of the S-H bond was unambiguously observed only for small molecules ( I C s ) . The branching ratio for cleavage of the S-H bond was greater a t 248 nm than 193 nm. This behavior is consistent with the excited-state electronic configurations, which show a greater contribution from U*SH a t 248 nm than a t 193 nm. Both the S-H and C S bond dissociation characteristics suggest that the excited electronic state plays an important role in the photodissociation process.
Introduction The ultraviolet photodissociation chemistry of small molecules under collision-free conditions has been extensively studied over the past two decades. As these investigations are extended to molecules of increasing size and complexity, direct identification of the primary photodissociation products becomes difficult. In photodissociation-photoionization mass spectrometry (PDPIMS), molecular photodissociation is performed with an ultraviolet laser pulse. After a short time delay, the neutral photodissociation products are softly ionized with coherent vacuum ultraviolet radiation and detected in a time-of-flight mass spectrometer. Unlike high-pressure photochemical experiments under static (nonflow)conditions, PDPI permits direct observation of the entire product distribution on the microsecond time scale following absorption of a photon. Since secondary reactionsare significantly reduced, much larger molecules can be studied. Van Bramer et al.1-3 used PDPI to study the unimolecular photochemistry of linear and branched alkenes up to CIO. The predominant photodissociation pathway was found to be cleavage of the @ C-C bond. Other primary photodissociation reactions and secondary neutral and ionic reactions of the primary products were also observed, and the product distributions were directly related to molecular structure. These experiments illustrate the ability of PDPI to study the unimolecular photochemistry of relatively large molecules. Thiols are one of a wide variety of sulfur-containing species released into the atmosphere from natural and anthropogenic sources. Alkylsulfur radicals are known to be intermediates in atmospheric processes leading to acid rain and photochemical smog.4 Relatively little is known about the photochemical pathways of simple thiols. Liquid-phase and high-pressure gasphase photolysis of methanethiol, ethanethiol, and 2-methyl-2propanethiol have been r e ~ r t e d . 5 - ~In these experiments, bimolecular and biradical processes weredominant so the primary photodissociation reactions had to be inferred through stable Author to whom correspondence should be addressed. Abstract published in Advance ACS Absrracrs, September 15, 1993.
0022-365419312097-10725%04.00/0
product distributions. The primary processes identified are shown in reactions 1-3 below, using ethanethiol as an example: CH3CH,SH CH3CH,SH
--
CH,CH,S'
-
CH3CH,'
+ H' + 'SH
(1) (2)
+
CH,CH,SH C2H, H,S (3) More recently, unimolecular photodissociation studies of methanethiol and ethanethiol have been reported.IOJ1For ethanethiol, cleavage of the C-C bond was also observed: CH,CH,SH
-
CH,'
+ 'CH,SH
(4)
We have used PDPI to study the photochemistry of isomeric alkanethiols in the C3 to CSsize range. Reactions analogous to ( l ) , (2), and (4) above were directly identified. In some cases, molecular rearrangements were observed as well. The branching ratios for these primary reactions were found to be strongly dependent upon molecular structure and photodissociation wavelength.
Experimental Section The experimental setup for PDPI-MS has already been described in detail.l-' Samples were admitted into the mass spectrometer through a room temperature molecular leak. Photodissociation was performed at 193 or 248 nm withan excimer laser. Ionic products formed by multiphoton ionization during the photodissociation step were removed by applying a highvoltage pulse across the source region. After an adjustable time delay, the neutral photodissociation products and remaining undissociated parent molecules were then photoionized with coherent vacuum ultraviolet radiation and detected in a linear time-of-flight mass spectrometer. Ultraviolet radiation from a Nd:YAG pumped dye laser system was frequency tripled in variousraregasmixtures toproduce 118.2-nm(l0.49eV), 121.4nm (10.21 eV), and 128-nm (9.68 eV) radiation as described previously.12 Each spectrum was summed and averaged over at 0 1993 American Chemical Society
Ross and Johnston
10726 The Journal of Physical Chemistry, Vol. 97, No. 41 1993 I
nii
least 500 laser pulses. The data in Tables I-IV represent the average of at least three such spectra. Peak intensities are given relative to the most intense PDPI peak. Relative standard deviations of the peak intensities were typically f2% of the base PDPI peak intensity. High purity (99.9+%) 2-methyl-2-butanethio1, 3-methyl-2butanethiol, 2-methyl-2-pentanethio1, and 2,3-dimethyl-2-butanethiol wereobtained from A.P.I. Standard Reference Materials (Pittsburgh, PA) in vacuum-sealed ampules. Other chemicals were obtained from Aldrich (Madison, WI). All were used without further purification.
I
Results Photoionization Mass Spectra, Only a small fraction of the parent molecules (ca. 5%) was dissociated by the excimer laser pulse under normal operating conditions. Therefore, each PDPI spectrum was superimposed on the photoionization spectrum of the undissociated parent molecule. Since the ionization potentials of the alkanethiols studied were in the 9.0-9.2-eV range,I3 there was sufficient excess energy for ionic fragmentation of the parent ion to occur with the photoionization wavelengths used. With 118.2-nm photoionization, a significant amount of secondary ionic fragmentation leading to formation of numerous small mass ions was observed. This fragmentation was significantly reduced by using 121.4- and 128-nm photoionization. Some clear differences in wavelength-dependent ionic fragmentation patterns were observed over the range of compounds studied. Primary alkanethiols generally underwent loss of HIS with 10.21-eV photoionization, producing a metastably broadened (M - 34)*+peak. With9.68-eV photoionization, no fragmentation was observed. Therefore, the appearance energy for ionic loss of H2S can be bracketed between these two energies. Secondary alkanethiols also gave metastable loss of H2S with 10.21-eV photoionization that was eliminated with 9.68-eV photoionization. In addition, 2-butanethiol and 3-methyl-Zbutanethiol gave losses of ethane, (M - 30)*+, and propane, (M - 44)*+, respectively. Tertiary alkanethiols gave predominant loss of 'SH, (M - 33)+, with both 10.21-eV and 9.68-eV photoionization. The relative intensity of the ( M - 33)+ ion decreased with increasing photoionization wavelength. Primary Alkanethiols. The 193-nm PDPI mass spectra of 1-propanethiol, 1-butanethiol, and 1-hexanethiol using 10.21-eV photoionization are shown in Figure 1. The photodissociation products are easily identified by comparison to the corresponding photoionization mass spectra. The 193-nm PDPI spectra of these and other primary alkanethiols are summarized in Table I. The predominant neutral fragments of 1-propanethiol correspond directly to reactions 1,2, and 4 (S-H, C-H, and C-C bond cleavage, respectively) as indicated in Figure 1A. Cleavage of the S-H bond gives CH3CH2CH2S*, which is observed at m/z 75 after photoionization. Cleavage of the C-S bond yields 'SH and CHjCH2CH2*, which are observed at m/z 33 and 43, respectively, after photoionization. An intense m/z 41 peak most likely results from loss of H2 from the saturated alkyl radical. Cleavage of the a C-C bond yields 'CH2SH and CH3CH2' radicals, which are observed a t m / z 41 and 29, respectively. Large primary alkanethiols do not produce (M - H)+ ions, suggesting that S-H bond cleavage does not occur. However, one must consider the possibility that the thioradical produced by S-H bond cleavage undergoes complete secondary fragmentation. Several secondary pathways are possible. First, the thioradical could fragment by loss of CHIS. This pathway can be ruled out since no signal is observed at m l z 46, even though the ionization potential of CH2S is low (9.34 eV). Second, the thioradical could rearrange by an internal H-atom abstraction from the aliphatic chain followed by secondary fragmentation. The analogous H-atom abstraction reaction in large (LCd) alkoxy
m/z
Figure 1. PDPI mass spectra of (A) 1-propanethiol,(B) 1-butanethiol, and (C) 1-hexanethiol,using 193-nmPD and 10.21-eVPI. Peaksmarkd with an asterisk are observed in the 10.21-eV PI spectra of the corresponding parent molecules. Unmarked peaks in (C) arise from secondary fragmentation of the alkyl radicals produced by C-C bond
dissociation.
radicals is known to be facilitated when a six-member ring intermediate can be f0rmed.1~This mechanism can be ruled out for 2-methyl- 1-propanethiol, since no six-membered ring intermediate is possible. For the remaining alkanethiols, the alkyl radical produced by this rearrangement would be expected to give secondary products corresponding to C,H&lS+. Since these ions are not observed in the PDPI spectra, this pathway can be ruledout. A third possibility is ionicdecomposition of the primary photodissociation product after photoionization, perhaps by loss of H2S or CH2S. This pathway cannot be explicitly ruled out, so the absence of (M - H)+ from the spectra of large alkanethiols does not necessarily mean that cleavage of the S-H bond does not occur. The unsaturated alkenyl series a t m /z 55,69, and 83 appear as predominant fragments in the spectra of 1-butanethiol, 1-pentanethiol, and 1-hexanethiol, respectively. These products most likely result from C S bond cleavage followed by loss of HZ from the saturated alkyl radical, since the other C S cleavage product, 'SH, is observed at m/z 33 after photoionization. Cleavage of the C-S bond is also indicated in the spectra of 1-heptanethiol and 1-octanethiol by the presence of the photoionized 'SH radical. For 1-heptanethiol, the alkenyl series ion a t m/z97 isobserved, but thepeakisfairlyweakandismetastably broadened, indicating formation by a secondary ionic process. The neutral alkyl radical formed by C S cleavage apparently undergoes decomposition by competing secondary reactions that involve C-C cleavage rather than H2 loss. These competing secondary reactions yield ions corresponding to the formula C,H, where y = 2x - 1 to 2x + 1. For 1-octanethiol, the alkenyl series ion is no longer observed, but products of competing secondary reactions involving C - C bond cleavage are present in the PDPI spectra. For 2-methyl- 1-propanethiol, C S bond cleavage is indicated by the presence of the primary products (m/z 33 and 57) and secondary products of the alkyl radical (m/ z 55 and 56) in the PDPI spectra. Products of C-C bond cleavage are found in the PDPI spectra of all primary alkanethiols. For 2-methyl- 1-propanethiol, only
Unimolecular Photochemistry of Alkanethiols
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10727
TABLE I: Relative Intensities of PDPI/MS Fragments of Primary Alkanethiols' from 193-nm Pbotodissociation
1-propanethiol 1-butanethiol 2-methyl-1-propanethi01 1-pentanethiol 1-hexanethiol 1-heptanethiol 1-octanethiol 1-propanethi01 1-butanethiol
9.68-eV Photoionization 5 5
6
26 15
6 4 8
11
10 18 18
9 8
89 6
10 33 100 19 100 37
7 11 19
100 100
86
40
65
14 14
6
68 100 54
58 85
7
65
10 6
50
11
7
28
6 19
12
17 44
19 33
12
75
18
10.21-eV Photoionization 100 100 76 PIb 27 47 PIb 22 10 33
9
100 11 12 100
9
4
