Rearrangement Reactions Used To Probe Transient Intermediates
J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2963
dehydrogenate to 1,3-butadiene, or trap on the surface and
nonselectively decompose. Alternatively, heterolytic C-S bond
cleavage would yield the cyclopropylmethyl cation, which is
in equilibrium with both the 3-buten-1-yl cation and cyclobutyl
cation in the gas phase (Scheme 2).11-13 Hydrogenation
following heterolytic C-S bond cleavage would yield a mixture
of cyclobutane, methylcyclopropane, and 1-butene. Dehydro-
genation would yield a mixture of cyclobutene and 1,3-
butadiene. The reactions of 3-butene-1-thiol were studied as
an additional probe for a cationic mechanism. In this case, the
3-buten-1-yl cation would rearrange to a mixture of cyclopro-
pylmethyl, cyclobutyl, and 3-buten-1-yl cation (Scheme 2).14
Again, hydrogenation would yield a mixture of cyclobutane,
methylcyclopropane, and 1-butene, and dehydrogenation would
yield a mixture of cyclobutene and 1,3-butadiene. If C-H
bonds form more rapidly than rearrangement after sulfur
elimination, no rearrangement is expected, and only methylcy-
clopropane would be produced. Finally, C-S bond scission
forming a surface alkyl intermediate could follow one of three
pathways: (1) hydrogen addition, yielding methylcyclopropane;
(2) â-dehydrogenation to 1,3-butadiene; or (3) nonselective
Figure 1. Temperature-programmed reaction data following condensa-
tion of multilayers of (a) cyclopropylmethanethiol and (b) 3-butene-
1-thiol on initially clean Mo(110). The spectra for ions representative
of methylcyclopropane, m/e ) 56; 1,3-butadiene, m/e ) 54; 1-butene,
m/e ) 56; cyclopropylmethanethiol, m/e ) 88; 3-butene-1-thiol, m/e
) 88; and dihydrogen, m/e ) 2 are shown. The heating rate was nearly
constant at 4 K/s in the range shown.
decomposition to surface carbon, sulfur, and H2(gas)
. The
reactions of ethyl chloride and cyclopropylmethyl bromide,
which undergo carbon halogen bond cleavage at low temper-
atures to produce stable alkyl intermediates, are used to probe
for these pathways.
The crystal was positioned approximately 2 mm from the aperture
(3 mm) of the mass spectrometer shield during the collection of
temperature-programmed reaction data. The crystal was biased at -65
eV during temperature-programmed reaction to minimize reactions
induced by electrons generated by the quadrupole mass spectrometer.
The mass spectrometer was computer interfaced and the data were
collected with a program, which allowed collection of up to 10 separate
ion intensity profiles during a single experiment. The heating rate was
constant with dT/dt ) 4 K/s between 110 and 650 K.
Experimental Section
The experiments were performed in three ultrahigh vacuum chambers
described previously with base pressures of e2 × 10-10 Torr.15-17 All
three chambers were equipped with a UTI quadrupole mass spectrom-
eter, low energy electron diffraction (LEED) optics, and an Auger
spectrometer with cylindrical mirror analyzer. The high resolution
electron energy loss spectrometer (LK technologies, model LK2000)
was operated at a primary beam energy of 3 eV, with a spectral
resolution of 55-80 cm-1. The variation in the resolution was due to
the different reflectivities of the condensed and monolayer thiol phases
that were studied. The infrared spectra were collected using a single
beam, clean air purged Fourier transform infrared spectrometer (Nicolet,
Series 800) and averaged over 800 scans using an MCT detector at 4
cm-1 resolution; the scan time being approximately 5 min. Sample
spectra were ratioed against a background taken immediately after the
sample scan by flashing the crystal to 900 K. The background scan
was initiated after the crystal temperature had returned to ≈100 K.
The Mo(110) crystal (Metal Crystals Ltd.) could be cooled to 100
K, heated to 900 K radiatively, or heated to 2300 K by electron
bombardment. The Mo(110) surface was cleaned before each experi-
ment by oxidation at 1200 K in 1 × 10-9 Torr of O2 for 5 min. The
crystal temperature was allowed to return to ≈200 K and subsequently
flashed to 2300 K for 30 s to remove residual oxygen. No surface
carbon or oxygen were detected in the Auger electron spectra of the
surface recorded prior to cyclopropylmethanethiol adsorption. A sharp
(1 × 1) low energy diffraction pattern was also observed.
Results
Temperature Programmed Reaction Spectroscopy. Two
hydrocarbon products, methylcyclopropane and 1,3-butadiene,
are produced during temperature-programmed reaction of a
saturation coverage of cyclopropylmethanethiol (Figure 1a). The
only other gaseous product detected in a comprehensive search
for products in the range of 2-110 amu was gaseous dihydro-
gen. Condensed layers of cyclopropylmethanethiol sublime
from Mo(110) in a sharp peak at 150 K (Figure 1a, m/e ) 88).
This peak increases indefinitely with continued cyclopropyl-
methanethiol exposure, as expected. All of the products are
observed at saturation coverage and are identified by quantitative
analysis of mass spectrometer data.
Methylcyclopropane is the major gas phase hydrocarbon
product and evolves between 200 and 350 K (Figure 1a, m/e )
56). Identification of methylcyclopropane was made by com-
parison of the mass fragmentation patterns of the product and
an authentic sample of methylcyclopropane (Table 1). Forma-
tion of 1,3-butadiene is a minor pathway (Figure 1a, m/e )
54). 1,3-Butadiene evolves at 240 K and was identified by the
mass 54 signal, which is significantly greater than measured
for methylcyclopropane fragmentation. The methylcyclopro-
pane fragmentation pattern was subtracted from the pattern at
240 K. The resulting fragmentation pattern compares favorably
3-Butene-1-thiol18,19 and methylcyclopropane20 were synthesized
using established methodology. The cyclopropylmethanethiol was
purchased from Lancaster.21 The compounds were characterized by
mass spectrometry and nuclear magnetic resonance. Cyclopropylmethyl
bromide was purchased from Aldrich and was characterized by mass
spectroscopy.
(11) Caserio, M. C.; Graham, W. H.; Roberts, J. D. Tetrahedron 1960,
11, 171.
(12) Hehre, W. J.; Hiberty, P. C. J. Am. Chem. Soc. 1974, 96, 302.
(13) Mazur, R. H.; White, W. N.; Semenow, D. A.; Lee, C. C.; Silver,
M. S.; Roberts, J. D. J. Am. Chem. Soc. 1959, 81, 4390.
(14) Hehre, W. J.; Hiberty, P. C. J. Am. Chem. Soc. 1972, 94, 5917.
(15) Wiegand, B. C.; Friend, C. M.; Roberts, J. T. Langmuir 1989, 5,
1292.
(16) B. C. Wiegand, Ph.D. Thesis: Model Studies of Desulfurization
Reactions on Mo(110); Harvard University, Cambridge, 1991.
(17) Weldon, M. K.; Friend, C. M. Surf. Sci. 1994, 310, 95.
(18) Bain, C. D.; Troughton, E. B.; Tao, Y.; Evall, J.; Whitesides, G.
M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
(19) 1H NMR (CDCl3, 500 MHz) δ 5.81 (m, 1H), 5.06 (m, 2H), 2.74 (t,
J ) 7 Hz, 2H), 2.43 (m, 2H). The R hydrogen at δ 2.74 appeared as a
triplet, rather than a quartet, and the S-H proton (typically at δ 1.2-1.4)
was absent due to exchange of the latter with water.
(20) Demjanoff, N. Chem. Ber. 1895, 28, 21.
(21) 1H NMR (CDCl3, 500 MHz) δ 2.43 (t, J ) 7 Hz, 2H), 1.43 (t, J )
7 Hz, 1H), 0.97-1.10 (m, 1H), 0.49-0.60 (m, 2H), 0.13-0.28 (m, 2H).