Cyclopropylmethyl/cyclobutyl rearrangements on surfaces: Evidence for transient cation formation near O-covered Mo(110)

Article abstract of DOI:10.1021/ja0682428

Rearrangement reactions of C₄-alkoxides on O-covered Mo(110) have been studied using temperature-programmed reaction spectroscopy and reflection-absorption infrared spectroscopy. Cyclobutoxide on Mo(110), prepared from the corresponding alcohol or bromide, is described for the first time in detail. Several reaction mechanisms are considered for the ring-opening rearrangement of cyclopropylmethoxide during high-temperature annealing. In light of compelling new data, previous results are reinterpreted to support the formation of transient cations near O-covered Mo(110). For the first time, we present strong evidence for clean, heterolytic bond cleavage reactions over a metal surface. Our revised reactivity model is based on spectroscopic and reactivity data that show the rearrangement of cyclopropylmethyl groups to cyclobutyl groups and vice versa. Selectively deuterated 1,1-D ₂-cyclopropylmethanol was studied as a test of mechanism and as a probe for the lifetime of reactive intermediates. Isotopic scrambling observed for this substrate is consistent with the formation of a relatively long-lived carbocation during rearrangement. The intermediacy of transient cations is further invoked to explain the rearrangements that are now recognized to occur as alkyl bromides are transformed into alkoxides on Mo(110)-(1 x 6)-O. The observed ring expansion/contraction reactions are characteristic of a cationic process; carbon-centered radicals are not known to rearrange in this manner. However, in none of the cases discussed could contributions from radical pathways be completely ruled out. Our results are compared to analogous reactions in the vapor and solution phases. General trends governing rearrangement mechanisms on Mo(110) are presented with respect to metal-surface coverage, heteroatom incorporation, and temperature. Trends are discussed in the context of heterogeneous hydrocarbon oxidation.

Full text of DOI:10.1021/ja0682428

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Cyclopropylmethyl/Cyclobutyl Rearrangements on Surfaces:
Evidence for Transient Cation Formation near O-Covered
Mo(110)
Sean H. Wiedemann,†,§ Dae-Hyuk Kang,‡ Robert G. Bergman,§ and
Cynthia M. Friend*,‡
Contribution from the Department of Chemistry and DiVision of Engineering and Applied
Sciences, HarVard UniVersity, 12 Oxford Street, Cambridge, Massachusetts 02138, and
Department of Chemistry, UniVersity of California, and DiVision of Chemical Sciences,
Lawrence Berkeley National Laboratory, Berkeley, California 94720
Received November 17, 2006; E-mail: cfriend@deas.harvard.edu
Abstract: Rearrangement reactions of C4-alkoxides on O-covered Mo(110) have been studied using
temperature-programmed reaction spectroscopy and reflection-absorption infrared spectroscopy. Cyclo-
butoxide on Mo(110), prepared from the corresponding alcohol or bromide, is described for the first time
in detail. Several reaction mechanisms are considered for the ring-opening rearrangement of cyclopropyl-
methoxide during high-temperature annealing. In light of compelling new data, previous results are
reinterpreted to support the formation of transient cations near O-covered Mo(110). For the first time, we
present strong evidence for clean, heterolytic bond cleavage reactions over a metal surface. Our revised
reactivity model is based on spectroscopic and reactivity data that show the rearrangement of cyclopro-
pylmethyl groups to cyclobutyl groups and vice versa. Selectively deuterated 1,1-D2-cyclopropylmethanol
was studied as a test of mechanism and as a probe for the lifetime of reactive intermediates. Isotopic
scrambling observed for this substrate is consistent with the formation of a relatively long-lived carbocation
during rearrangement. The intermediacy of transient cations is further invoked to explain the rearrangements
that are now recognized to occur as alkyl bromides are transformed into alkoxides on Mo(110)-(1 × 6)-O.
The observed ring expansion/contraction reactions are characteristic of a cationic process; carbon-centered
radicals are not known to rearrange in this manner. However, in none of the cases discussed could
contributions from radical pathways be completely ruled out. Our results are compared to analogous reactions
in the vapor and solution phases. General trends governing rearrangement mechanisms on Mo(110) are
presented with respect to metal-surface coverage, heteroatom incorporation, and temperature. Trends are
discussed in the context of heterogeneous hydrocarbon oxidation.
Introduction
isotopic labeling to gain insight into the mechanisms of many
organic reactions.5
Compounds with cyclopropylmethyl groups have been used
to probe for both radical and ionic intermediates in a variety of
reaction systems.1 The cyclopropylmethyl radical undergoes ring
opening to yield 3-buten-1-yl radical on a nanosecond time
scale,2 whereas the cyclopropylmethyl-derived cation has been
observed to undergo partial rearrangement to a mixture of the
cyclobutyl, 3-butenyl, and unrearranged cyclopropylmethyl
products in the presence of a nucleophilic trap (Scheme 1).3,4
These rearrangements have been used in conjunction with
One of our groups has studied organic rearrangement reac-
tions that occur in the vicinity of metallic surfaces in an effort
to extend this approach to heterogeneous systems.6 Organic
radicals and cations are often proposed as intermediates in
important heterogeneous reactions, especially in catalysis, but
direct evidence for these short-lived intermediates is seldom
available.7 Recently, the rearrangement reactions of cyclopro-
(3) Treatment of cyclopropylcarbinylamine with sodium nitrite in aqueous
perchloric acid solution gives a 60% yield of an alcohol mixture containing
48% cyclopropylmethanol, 47% cyclobutanol, and 5% 3-buten-1-ol. 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.
† Present address: Department of Chemistry, Princeton University,
Princeton, NJ 08544.
(4) Reviews of C4H7+: (a) Lowry, T. H.; Richardson, K. S. Mechanism and
Theory in Organic Chemistry, 3rd ed.; Harper and Row: New York, 1987;
pp 454-463. (b) Wiberg, K. B.; Hess, B. A., Jr.; Ashe, A. J. In Carbonium
Ions; Olah, G. A., Schleyer, P. v. R., Eds.; Wiley-Interscience: New York,
1972; Vol. 3, pp 1295-1346. (c) Schleyer, P. v. R.; Maerker, C.; Buzek,
P.; Sieber, S. In Stable Carbocation Chemistry; Prakash, G. K. S., Schleyer,
P. v. R., Eds.; John Wiley and Sons: New York, 1997; pp 19-74.
(5) (a) Bowry, V. W.; Ingold, K. U. J. Am. Chem. Soc. 1991, 113, 5699. (b)
Newcomb, M.; Toy, P. H. Acc. Chem. Res. 2000, 33, 449.
‡ Harvard University.
§ University of California, Berkeley.
(1) (a) Newcomb, M.; Le Tadic-Biadatti, M.-H.; Chestney, D. L.; Roberts, E.
S.; Hollenberg, P. F. J. Am. Chem. Soc. 1995, 117, 12085. (b) Newcomb,
M.; Shen, R.; Choi, S.-Y.; Toy, P. H.; Hollenberg, P. F.; Vaz, A. D. N.;
Coon, M. J. J. Am. Chem. Soc. 2000, 122, 2677.
(2) (a) Kochi, J. K.; Krusic, P. J.; Eaton, D. R. J. Am. Chem. Soc. 1969, 91,
1877. (b) Maillard, B.; Forrest, D.; Ingold, K. U. J. Am. Chem. Soc. 1976,
98, 7024. (c) Newcomb, M.; Glen, A. G. J. Am. Chem. Soc. 1989, 111,
275.
(6) Wiegand, B. C.; Napier, M. E.; Friend, C. M.; Uvdal, P. J. Am. Chem.
Soc. 1996, 118, 2962.
9
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J. AM. CHEM. SOC. 2007, 129, 4666-4677
10.1021/ja0682428 CCC: $37.00 © 2007 American Chemical Society

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Cationic Rearrangements on O-Covered Mo(110)
A R T I C L E S
Scheme 1. Characteristic Cyclopropylmethyl Rearrangements
Scheme 2. Calculated Diradical Mechanisms of
Cyclopropylmethyl Rearrangement
pylmethoxide on Mo(110) (Mo-O-CH2-c-C3H7) were inves-
tigated.8 Alkoxides on Mo(110) are of special interest because
they undergo heteroatom removalsthe reverse of hydrocarbon
oxidationsto provide free hydrocarbons upon strong heating.9
Generally, alkoxides are generated on Mo(110)-(1 × 6)-O via
the corresponding alcohols or bromides. Evidence of cyclopro-
pylmethyl rearrangements can be detected immediately follow-
ing alkoxide formation (in the case of bromides)10 and upon
moderate heating (∼400 K).8 Eventually, at more elevated
temperatures, the C-O bonds of surface alkoxides are broken.
Alkenes are the predominant products of this reaction whenever
a â-hydride elimination pathway is accessible.11 Otherwise,
pathways including the reductive formation of alkanes become
significant.
Kretzschmar et al. previously reported that cyclopropyl-
methoxide (Mo-O-CH2-c-C3H7) on Mo(110) rearranges to
3-buten-1-oxide (Mo-O-CH2CH2CHdCH2) upon heating to
460 K.8a Originally, this isomerization was understood to occur
via homolytic C-O cleavage (Scheme 1). We proposed that
cyclopropylmethyl radical was formed in the vicinity of the
surface, rearranged to 3-butenyl radical, and then was captured
by radical recombination to re-form a C-O bond. Because the
ring-opening reaction of cyclopropylmethyl radical is known
to proceed with a well-defined rate constant under a variety of
reaction conditions, it is one of the best known “radical
clocks.”12 If a radical clock could be coupled to other well-
defined surface processes, one could begin to calibrate the rates
of surface reactions relevant to heterogeneous catalysis.
In an effort to provide support for our proposed mechanism
of cyclopropylmethoxide rearrangement on Mo(110), a theoreti-
cal study using density functional theory (DFT) was carried
out.13 Reactions were modeled for gas-phase species and for
species bound to a small MoO cluster meant to approximate an
extended surface. Two nearly isoenergetic, possible mechanisms
emerged for the observed reaction, (1) the so-called radical clock
mechanism shown in Scheme 1 and (2) a more unusual
bifurcated mechanism arising from homolytic C-C bond
cleavage (Scheme 2).14 Herein, we present data demonstrating
that 2-buten-1-oxide (Mo-O-CH2CHdCHCH3)san expected
product of one fork of the latter mechanism (Scheme 2, Path
A)scannot be involved in the surface rearrangements of
cyclopropylmethoxide. Therefore, diradical mechanisms are
ruled out in the present case.
In order to further test the radical clock model of radical
formation and rapid rearrangement in the vicinity of O-covered
Mo(110), we investigated the surface behavior of selectively
deuterated cyclopropylmethanol (c-C3H5-CD2-OH). For this
substrate and all others mentioned in this paper, temperature-
programmed reaction (TPR) spectroscopy and reflection-
absorption infrared (RAIR) spectroscopy were employed for
reaction monitoring and characterization. The results of the
isotope-labeling experiment cast serious doubt on the validity
of the radical clock mechanism. In the process of considering
necessary revisions to our mechanistic model, we were prompted
to study the surface behavior of cyclobutoxide (Mo-O-c-
C4H7). These, at first tangential, experiments now form the
centerpiece of our understanding of cyclopropylmethyl re-
arrangements near Mo(110). To our surprise, the spectroscopic
features characteristic of cyclobutoxide were found to be present
in numerous past rearrangement experiments, though at the time,
they were not assigned as such.8a,10 We now wish to reinterpret
those experiments in conjunction with the description of our
present findings. Together, data from the reactions of cyclo-
propylmethyl, cyclobutyl, and 3-buten-1-yl bromides and alco-
hols reveal that near O-covered Mo(110), cyclopropylmethyl
groups can rearrange to cyclobutyl groups and vice versa.
Because this behavior is uniquely attributable to cationic
intermediates, radical processes alone cannot account for our
results.
(7) (a) Bent, B. E. Chem. ReV. 1996, 96, 1361. (b) Lunsford, J. H. Angew.
Chem., Int. Ed. Engl. 1995, 34, 970.
(8) (a) Kretzschmar, I.; Levinson, J. A.; Friend, C. M. J. Am. Chem. Soc. 2000,
122, 12395. (b) Kretzschmar, I.; Friend, C. M. J. Phys. Chem. B 2002,
106, 8407.
(9) (a) Wiegand, B. C.; Uvdal, P.; Serafin, J. G.; Friend, C. M. J. Phys. Chem.
1992, 96, 5063. (b) Chen, D. A.; Friend, C. M. J. Am. Chem. Soc. 1998,
120, 5017.
(10) Levinson, J. A.; Kretzschmar, I.; Sheehy, M. A.; Deiner, L. J.; Friend, C.
M. Surf. Sci. 2001, 479, 273.
(11) In related work, gas-phase pyrolysis (820 K) of Ti(O-CH2-c-C3H7)4 led
to significant quantities of ether and alcohol products in addition to C4-
olefins. The presence of 3-butenyl products in this mixture indicates that
partial radical rearrangement occurred during pyrolysis. Nandi, M.;
Rhubright, D.; Sen, A. Inorg. Chem. 1990, 29, 3065.
(12) (a) Griller, D.; Ingold, K. U. Acc. Chem. Res. 1980, 13, 317. (b) Newcomb,
M. Tetrahedron 1993, 49, 1151.
Experimental Section
Apparatus. All surface experiments were performed in a stainless
steel ultrahigh vacuum (UHV) chamber with a base pressure of ∼1 ×
(14) Importantly, due to limitations of the modeling software, mechanisms
involving heterolytic bond cleavage could not be evaluated in comparison
to radical mechanisms.
(13) Flemmig, B.; Kretzschmar, I.; Friend, C. M.; Hoffman, R. J. Phys. Chem.
A 2004, 108, 2972.
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A R T I C L E S
Wiedemann et al.
10-10 mmHg, described in detail previously.15 The UHV chamber is
equipped with a UTI quadrupole mass spectrometer for temperature-
programmed reaction (TPR) experiments, low-energy electron diffrac-
tion (LEED) optics, and an Auger electron spectrometer with a
cylindrical mirror analyzer (Perkin-Elmer model 15-555). The mass
spectrometer is interfaced to a computer that allows the collection of
up to 16 separate ion intensity profiles during temperature-programmed
experiments. The heating rate was held nearly constant at ∆T/∆t )
(10 ( 2) K/s between 110 and 760 K. The crystal was biased at a
voltage of -100 eV during adsorption of all molecules and during
collection of temperature-programmed reaction data in order to retard
electrons emitted from the mass spectrometer and, thus, preclude
electron-induced processes. This practice is commonly used and does
not affect the outcome of our experiments because they are carried out
in vacuum. Infrared (RAIR) spectra were collected using a commercial
spectrometer (Nicolet, series 800) at a surface temperature of 110 K
using a MCT (HgCdTe) detector with a resolution of 4 cm-1. In cases
where the crystal was heated to a temperature above 110 K, the crystal
was then cooled back to 110 K before spectra were acquired. Specific
procedures for data collection are described elsewhere.15
Sample Preparation. Sample cleaning and preparation procedures
are described elsewhere.16 Briefly, the Mo(110) surface was cleaned
by oxidation at 1200 K under 1 × 10-9 mmHg of O2 for 1 min and
subsequently flashed to 2300 K to remove residual oxygen. Substrate
cleanliness was verified by Auger electron spectroscopy. A saturated
oxygen overlayer (θO ) 0.66 ML) was prepared by exposing the
Mo(110) surface to 1 × 10-9 mmHg of O2 for 1 min at 110 K, with
subsequent heating to 500 K.17
Reagents. Cyclopropylmethanol (c-C3H5-CH2-OH, 98%), cyclo-
butanol (c-C4H7-OH, 95%), and cyclobutyl bromide (c-C4H7-Br, 96%)
were obtained from commercial suppliers, stored in glass bottles, and
purified by several freeze-pump-thaw cycles before use. Mass
spectrometry was used to confirm reagent purity. Oxygen (O2, 99.998%,
Matheson) was used as received. For a detailed description of synthesis
and purification of 1,1-D2-cyclopropylmethanol, see Supporting Infor-
mation.
Figure 1. Temperature-programmed reaction (TPR) data comparing the
m/e ) 54 trace for O-covered Mo(110) treated with (a) 2-buten-1-ol, (b)
cyclopropylmethanol, (c) cyclobutanol, and (d) 3-buten-1-ol (reproduced
from ref 10). The m/e ) 54 signal corresponds to the parent ion and most
intense signal of 1,3-butadiene.
cm-1) is observed at a significantly higher frequency than that
assigned to ν(CdC) of 3-buten-1-oxide (1644 cm-1). No
experimental evidence we have collected suggests that 2-buten-
1-oxide is involved in the rearrangement reactions that are the
focus of this paper.
Cyclobutanol. The remaining data enable the radical clock
versus cationic mechanisms of rearrangement to be differenti-
ated. Experiments involving three well-defined surface species
are relevant in this regard: 3-buten-1-oxide, cyclopropyl-
methoxide, and cyclobutoxide. Cyclobutoxidesthe only one of
these that has not been previously reportedswas prepared
according to the usual protocol. First, multilayers of cyclobutanol
were condensed onto O-covered Mo(110) at 110 K. Subsequent
heating to 350 K led to transfer of H and alkoxide to Mo(110)-
(1 × 6)-O via O-H bond cleavage. This is a general reaction
for alcohols on this surface. The newly formed bonds are
indicated by the appearance of a sharp ν(O-H) peak at 3575
cm-1 in the RAIR spectrum. This resonance is characteristic of
OH adsorbed directly on the metal surface; therefore, it
evidences transfer of alcoholic protons to adsorbed oxygen
(Figure 2a). All other RAIR peaks can be assigned in terms of
the vibrational modes of cyclobutoxide. Of particular importance
to the following discussion are the strong resonances at 900,
917, 1048, 1243 (ring-CH2 scissor), and 2957 cm-1 (ν(C-H)).
The resonances at 900 and 1048 cm-1 are assigned to have a
significant component of ν(C-O). Note that, in this energy
range, intramolecular coupling commonly occurs between modes
of similar symmetry.18
Results
2-Buten-1-ol. The “diradical” rearrangement mechanism was
tested using 2-buten-1-ol. Following the usual reaction protocol
(vida infra), 2-buten-1-ol was introduced to O-covered Mo(110)
and annealed to yield the corresponding alkoxide (eq 1).
Upon further heating, hydrocarbon elimination took place to
form butadiene. This assignment is based on the single, sharp,
symmetric peak observed near 400 K in the TPR trace (m/e 54,
Figure 1a). The peak shape indicates a single mechanism for
C4-hydrocarbon formation. The peak temperature differs con-
siderably from the temperature at which C4-hydrocarbon evolu-
tion is observed from cyclopropylmethanol-treated O-covered
Mo(110) (550 K).8a
The RAIR spectrum of 2-buten-1-ol-treated O-covered Mo(110)
(not shown) is unambiguously attributable to 2-buten-1-oxide.
The RAIR peak assigned to ν(CdC) of 2-buten-1-oxide (1674
The infrared stretching frequencies of gas-phase cyclobutanol,
surface-adsorbed cyclobutanol, and surface-bound cyclobutoxide
are highly similar in frequency and magnitude (see Supporting
Information). Significantly, the RAIR spectrum of cyclobutoxide
remains constant following heating to 460 K (Figure 3b).
(15) Wiegand, B. C.; Friend, C. M.; Roberts, J. T. Langmuir 1989, 5, 1292.
(16) (a) Queeney, K. T.; Friend, C. M. J. Chem. Phys. 1998, 109, 6067. (b)
Queeney, K. T.; Friend, C. M. J. Phys. Chem. B 1998, 102, 5178. (c)
Queeney, K. T.; Friend, C. M. ChemPhysChem 2000, 1, 116.
(17) Colaianni, M. L.; Chen, J. G.; Weinberg, W. H.; Yates, J. T. Surf. Sci.
1992, 279, 211.
(18) Uvdal, P.; MacKerell, A. D., Jr.; Wiegand, B. C.; Friend, C. M. Phys. ReV.
B 1995, 51, 7844.
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Cationic Rearrangements on O-Covered Mo(110)
A R T I C L E S
provides evidence that cyclobutoxide undergoes heteroatom
removal by a single mechanism (Figure 1c).19 On the basis of
MS data alone, is not possible to know whether the primary
hydrocarbon product is butadiene or the direct product of
â-hydride elimination: cyclobutene.20 At the peak temperature
of hydrocarbon extrusion (560 K), cyclobutene readily under-
goes exergonic retro-electrocyclization to form butadiene.21
Reconsideration of Cyclopropylmethanol. To deconvolute
the behavior of surface-bound cyclopropylmethoxide, we will
rely on a basis set of characteristic data for cyclobutoxide and
3-buten-1-oxide. Previously published investigations of 3-buten-
1-oxide indicate simple surface behavior.8a To briefly sum-
marize, 3-buten-1-ol can be converted to 3-buten-1-oxide on
O-covered Mo(110) in the usual way. RAIR absorbances
observed after initial annealing (350 K) correlate well with gas-
phase FTIR data for 3-buten-1-ol (see Supporting Information).
Characteristic resonances of 3-buten-1-oxide relevant to the
present discussion are those assigned to ν(CdC) at 1645 cm-1
and ν(dCH2) at 3089 cm-1 (data not shown). The RAIR
spectrum of 3-buten-1-oxide remains unchanged upon further
heating to 460 K (Figure 3c). The only significant TPR event
for 3-buten-1-oxide observed at m/e 54 is a sharp, symmetric
peak corresponding to the extrusion of butadiene at a peak
temperature near 575 °C (Figure 1d). Like the case of cyclo-
butoxide, RAIR and TPR data together show that bond breakage
in 3-buten-1-oxide on O-covered Mo(110) is essentially limited
to a single process: high-temperature â-hydride elimination.
Figure 2. Infrared (RAIR) spectra obtained after condensing multilayers
of (a) cyclobutanol, (b) cyclopropylmethanol, (c) cyclobutyl bromide, and
(d) cyclopropylmethyl bromide (reproduced from ref 10) onto O-covered
Mo(110) and subsequently heating to 350 K. For all compounds, both
spectral regions were acquired in a single experiment.
With a satisfactory basis set in hand, the consideration of
cyclopropylmethoxide begins with its synthesis. Our most
reliable preparative method calls for adsorption and annealing
of cyclopropylmethanol onto O-covered Mo(110). Up to a
temperature of 350 K, the only C-containing surface-bound
species resulting from alkoxide deposition in this manner is
firmly believed to be cyclopropylmethoxide. The RAIR spec-
trum of cyclopropylmethoxide, which closely resembles the gas-
phase FTIR spectrum of cyclopropylmethanol (see Supporting
Information), does not significantly overlap with the RAIR
spectrum of any other C4-alkoxide in our basis set (Figure 2b).
Absorbances significant for the sake of this discussion are
observed at 1393 and 1434 cm-1 (assigned to ring-CH2 scissor)
and at 3010 cm-1 (assigned to ν(C-H)). Below 350 K, no
hydrocarbon evolution occurs on cyclopropylmethanol-treated
Mo(110) except for molecular layer desorption of c-C3H5-
CH2-OH around 200 K (partial data shown in Figure 1b).
When cyclopropylmethoxide is heated to 460 K, complete
skeletal rearrangement takes place. The characteristic RAIR
absorbances of cyclopropylmethoxide disappear, and a new set
of peaks appears in their place (Figure 3a). We previously
Figure 3. Infrared (RAIR) spectra obtained after condensing multilayers
of (a) cyclopropylmethanol, (b) cyclobutanol, and (c) 3-buten-1-ol (repro-
duced from ref 10) onto O-covered Mo(110) and subsequently heating to
460 K. For all compounds, both spectral regions were acquired in a single
experiment.
(19) Like other C4-alkoxides, cyclobutoxide can undergo heteroatom removal
via minor pathways to form trace amounts of cyclobutane or ethylene. The
C4H6/C4H8/C2H4 product ratio for cyclopropylmethoxide was estimated to
be 60:22:18.8a Compared to this ratio, the products derived from cyclo-
butoxide seem to be slightly enriched in C2H4 (see Supporting Information),
whereas the products derived from 3-buten-1-ol are known to be depleted
in C2H4.10 This relationship is consistent with our conclusion that
cyclopropylmethoxide is rearranged to a mixture of cyclobutoxide and
3-buten-1-oxide prior to heteroatom removal. Due to limitations of the mass
spectral data, a more detailed analysis of hydrocarbon product ratios is not
possible. The present discussion will be limited to the major â-hydride
elimination pathway of heteroatom removal.
(20) Butadiene, cyclobutene, and methylenecyclopropane, all of which can
produce a m/e 54 signal, have similar m/e 54:53 ratios of 1.3 ( 0.1, making
them indistinguishable in our mass spectrometer.
(21) (a) Cooper, W.; Walters, W. D. J. Am. Chem. Soc. 1958, 80, 4220. (b)
Carr, R. W., Jr.; Walters, W. D. J. Phys. Chem. 1965, 69, 1073.
Together, these data support the conclusion that, from the
temperature of alkoxide deposition to the temperature of
hydrocarbon elimination, cyclobutanol exists solely as cyclo-
butoxide on O-covered Mo(110).
TPR data further support a simple interpretation of the surface
behavior of cyclobutoxide. The single, sharp, symmetric peak
in the m/e 54 TPR trace, which corresponds to butadiene,
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J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007 4669