Sequential Hydrogen Tunneling in o-Tolylmethylene

Article abstract of DOI:10.1002/chem.202102010

o-Tolylmethylene 1 is a metastable triplet carbene that rearranges to o-xylylene 2 even at temperatures as low as 2.7 K via [1,4] H atom tunneling. Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopical techniq

Full text of DOI:10.1002/chem.202102010

Accepted Article
Accepted Article
Title: Sequential Hydrogen Tunneling in o-Tolylmethylene
Authors: Wolfram Sander, Thomas Lohmiller, Sujan K. Sarkar, Jörg
Tatchen, Stefan Henkel, Tim Schleif, Anton Savitsky, and
Elsa Sanchez-Garcia
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To be cited as: Chem. Eur. J. 10.1002/chem.202102010
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Sequential Hydrogen Tunneling in o-Tolylmethylene
Thomas Lohmiller,+[c, d] Sujan K. Sarkar,+[a, e] Jörg Tatchen,[b] Stefan Henkel,[a, f] Tim Schleif,[a]
Anton Savitsky,[c, g] Elsa Sanchez-Garcia,*[b] and Wolfram Sander*[a]
[a]
[b]
[c]
[d]
Dr. S. K. Sarkar,+ Dr. S. Henkel, Dr. T. Schleif, Prof. W. Sander
Lehrstuhl fꢀr Organische Chemie II, Ruhr-Universität Bochum, 44780 Bochum, Germany, E-Mail: wolfram.sander@ruhr-uni-bochum.de
Dr. J. Tatchen, Prof. E. Sanchez-Garcia
Fakultät für Biologie, Universitꢁt Duisburg-Essen, 45141 Essen, Germany, E-Mail: elsa.sanchez-garcia@uni-due.de
Dr. T. Lohmiller,+ Dr. Anton Savitsky
Max-Planck-Institut fꢀr Chemische Energiekonversion, 45470 Mꢀlheim an der Ruhr, Germany
Dr. T. Lohmiller
EPR4Energy Joint Lab, Abteilung Spins in der Energieumwandlung und Quanteninformatik, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH,
12489 Berlin, Germany
[e]
[f]
Dr. S. K. Sarkar
present address: The University of Hong Kong, Hong Kong SAR, China
Dr. S. Henkel
present address: Lehrstuhl für Physikalische Chemie II, Ruhr-Universität Bochum, 44780 Bochum, Germany
Dr. A. Savitsky
present address: Experimentelle Physik 3, Technische Universität Dortmund, 44221 Dortmund, Germany
These authors contributed equally to this work.
[g]
[+]
Supporting Information for this article is given via a link at the end of the document.
Dedicated to the memory of Professor Klaus Hafner
Abstract: o-Tolylmethylene 1 is a metastable triplet carbene that
rearranges to o-xylylene 2 even at temperatures as low as 2.7 K via
[1,4] H atom tunneling. Electron paramagnetic resonance (EPR) and
electron nuclear double resonance (ENDOR) spectroscopical
techniques were used to identify two conformers of 1 (anti and syn) in
noble gas matrices and in frozen organic solutions. Conformer-
specific kinetic measurements revealed that the rate constants for the
rearrangements of the anti and syn conformers of 1 are very similar.
However, the orbital alignment in the syn conformer is less favorable
for the hydrogen transfer reaction than the orbital configuration in the
anti conformer. Our spectroscopic and quantum chemical
investigations indicate that anti 1 and syn 1 rapidly interconvert via
efficient quantum tunneling forming a rotational pre-equilibrium. The
subsequent second tunneling reaction, the [1,4] H migration from anti
1 to 2, is rate-limiting for the formation of 2. We here present an
efficient strategy for the study of such tunneling equilibria.
the order of 10-6 s-1 were found. Temperature-independent
reaction rates are characteristic for QMT.[3b] For this reaction,
McMahon and Chapman stated that “the major rotomer decays
much more rapidly than the minor rotomer” since it has a “more
appropriate geometry for hydrogen migration”.[3] However, a
detailed analysis of the difference in kinetic behaviour of the two
conformers was not reported.
Introduction
Quantum mechanical tunneling (QMT) has long been recognized
as a general phenomenon in chemistry,[1] providing an alternative
mode of reaction to the classical thermal pathway. Due to its great
mass-dependency, QMT is most pronounced in reactions
involving the transfer of a hydrogen atom, with numerous
examples of hydrogen tunneling reported in the literature.[2] Matrix
isolation is a tool of choice to study such reactions, since
cryogenic temperatures suppress competing thermal reaction
pathways. An early example of a [1,4] hydrogen shift observed by
matrix isolation spectroscopy is the rearrangement of triplet
o-tolylmethylene 1 to o-xylylene 2 reported by McMahon and
Chapman (Scheme 1).[3] By using electron paramagnetic
resonance (EPR) and UV-vis spectroscopy, the reaction kinetics
was determined, and temperature-independent rate constants in
Scheme 1. Conformer-specific hydrogen tunneling in triplet o-tolylmethylene
1.[3b]
Recent studies renewed the interest in conformational control[4] of
tunneling reactions as exemplified by the investigations of the
[1,2]-hydrogen shift in hydroxycarbenes 3.[5] For the hydrogen
shift in hydroxycarbenes 3 or the ring expansion in fluorocarbene
5[6] to proceed (Scheme 2), the respective carbene center has to
be correctly aligned with respect to the adjacent proton (for 3) or
carbon atom (for 5). This is necessary to minimize the structural
rearrangement necessary to produce the corresponding products
1
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4 or 6, respectively. In both cases, no interconversion between
the respective conformers via thermal reaction or tunnelling was
detected. In addition, the tunnelling processes were observed to
be highly conformer-specific, with only the suitably aligned
conformer being able to undergo facile rearrangement.
Scheme 2. Examples for conformer-specific hydrogen or carbon tunneling in
carbenes: singlet hydroxycarbenes 3a/b[5] and singlet fluorocarbene 5,
respectively.[6]
Here we report the assignment of the syn and anti conformers of
1 based on EPR and electron nuclear double resonance
(ENDOR) spectroscopies in frozen solution and solid noble gas
matrices. Furthermore, we explore the conformer specificity of the
1 → 2 rearrangement at 5-10 K, aided by small curvature
tunnelling (SCT) calculations. Our findings reveal a consistent
reaction sequence including tunneling between syn and anti,
constituting a pre-equilibrium, and rate-limiting [1,4] H migration
from anti 1 to 2.
Figure 1. EPR spectra of 1 in different cryogenic environments. Top. a) X-band
CW EPR spectrum of 1 in argon at 4 K (ν = 9.578 GHz). b) Simulated triplet
spectra with D(major) = 0.5063 cm-1, |E(major)| = 0251 cm-1 and D(minor) =
0.505 cm-1, |E(minor)| = 0.0226 cm-1. Bottom. a) Pseudo-modulated Q-band
pulse EPR spectrum of 1 in MTHF at 5 K (ν = 34.03 GHz). b) Simulated triplet
spectra with D(major) = 0.5003 cm-1, |E(major)| = 0.0217 cm-1 and D(minor) =
0.5029 cm-1, |E(minor)| = 0.0249 cm-1. Insets show the most distinct
spectroscopic features of the two triplet components: x+1 at X-band (outside the
maximum B field at Q-band) and x-1 at Q-band (not resolved at X-band). As the
transitions relate to different sublevels, the relative positions of anti/syn 1 are
inverted. For the nomenclature of the transitions, see SI section S2.
Results and Discussion
EPR and ENDOR spectroscopy.
UV irradiation of o-tolyldiazomethane 7 in an argon matrix results
in an X-band continuous wave (CW) EPR spectrum that contains
a dominant set of characteristic triplet signals at magnetic field
positions of 196, 495, 598 and 878 mT (Figure 1, top). A smaller
triplet contribution with slightly different zero field splitting (zfs)
parameters is observed at 501 and 592 mT, though partially
overlapping with the signals of the major component. Based on
the x+1 signal, the ratio of the two triplet species is estimated to be
about 90:10. The zfs parameters from spectral simulations are in
agreement with those reported in literature (Table 1).[3b]
Calculations of the two rotamers of 1 place the syn conformer, in
which the carbene hydrogen atom is pointing towards the methyl
group (Scheme 1), 0.3 kcal mol-1 above the anti conformer
(BLYP-D3/6-311G** level of theory, in vacuo). Thus, the major
component observed in the matrix EPR spectrum is expected to
be anti 1. This assignment is supported by calculated zfs
parameters yielding different D and E values for the two
conformers of 1, with larger values for the syn conformer (Table
1, ΔD ≈ 1%, ΔE ≈ 10%).
Table 1. Zero-field splitting parameters of syn 1 and anti 1. Values shown in
bold are those of the major components.
anti 1
syn 1
D / cm-1
|E| / cm-1
D / cm-1
|E| / cm-1
Ar/4 K[a]
Ar/20 K[3b]
MTHF/5 K[a]
calc.[b]
0.5063
0.0251
0.5050
0.0226
0.503
0.5029
0.4861
0.0253
0.0249
0.0249
0.503
0.5003
0.4811
0.0244
0.0217
0.0223
[a] Based on linewidths and total spectral widths, relative errors of the fitted zfs
parameters are estimated to be significantly below 1 %. [b] TPSSh-D3/def2-
TZVP level of theory, E values are predicted to be negative.
A similar set of triplet signals is observed in a Q-band pulse EPR
spectrum obtained upon photolysis of precursor 7 in frozen 2-
methyltetrahydrofuran (MTHF). It contains 5 lines with higher
intensity at 524, 679, 876, 942 and 1421 mT and a second,
weaker set of signals, best identified by the distinct features at the
x-1 position, shifted to 868 mT (Figure 1, bottom). Comparison
with the spectrum obtained in solid argon reveals that the intensity
ratio of the two triplet species is inverted in MTHF with a ratio of
2
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approximately 15:85 in favor of the species with smaller simulated
zfs splitting parameters, which is assigned to the syn conformer
of 1 (Table 1, Figure S4). Accordingly, the behavior of 1 differs
significantly depending on whether the molecules are prepared in
a condensed noble gas matrix or a frozen organic glass with
respect to the ratio of the two conformers. This can be related to
either steric restrictions of the conformational isomerization of the
nascent carbene, as the precursor 7 is found exclusively in the
1-H syn conformation (vide infra), or to stabilization of syn 1 by
interactions with the frozen solvent. The ratio of the two
components in MTHF was found to be virtually unaffected by
increasing the temperature from 5 to 10 K (Figure S1) or by
changing the solvent to toluene-d8 (not shown), indicating that in
the frozen organic solvents, even more than in the noble gas
matrix, reaching the thermodynamic equilibrium is kinetically
hindered.
orientation. While the simulated spectral width of syn 1 agrees
well with the experiment, the calculated position of +13.4 MHz for
H4 in anti 1 lies clearly above the highest experimental resonance
frequency of +8.2 MHz. Thus, the major component in MTHF can
be unambiguously assigned to the syn conformer, while the minor
component represents the anti conformer, in agreement with the
assignment based on calculated zfs values.
IR spectroscopy
Next, we re-investigated the conformers syn 1 and anti 1 by IR
spectroscopy. The spectroscopic features of the diazo precursor
7 isolated in an argon matrix can be reproduced by assuming the
presence of only the less sterically congested conformer syn 7
(see SI). This assumption is consistent with the predicted relative
stability of syn 7 (1.9 kcal mol-1 at the BLYP-D3/6-311G** level of
theory). In agreement with the literature,[3b] irradiation of syn 7 with
λ = 254 nm results in the formation of carbene 1 as well as
secondary photolysis yielding o-xylylene 2. Despite some splitting
of the characteristic peaks of the carbene, it was not possible to
unambiguously assign IR signals to the individual rotamers of
carbene 1.
In the dark, 1 slowly rearranges to 2, and from the spectroscopic
changes in the IR spectrum, rates in the order of 2.1·10-5 s-1 were
determined in argon and xenon at 3 K (Table 2). These values are
in agreement with previously reported data, which was re-
evaluated using the same kinetic model.[3b] The lack of
deconvolution of the experimental IR spectrum with respect to the
contribution of the two conformers syn 1 and anti 1 limits any
kinetic information via IR (or UV) spectroscopy to an average rate
constant. Thus, if there is a difference in rates for the two
conformers of 1, the analysis will only yield the faster contribution.
Figure 2. Q-band Davies-type 1H-ENDOR. a) Experimental spectrum of 1 in
MTHF at the z+1 field position of the major spectral contribution of the
corresponding EPR spectrum (Figure 1). b), c) Simulations for syn 1 and anti 1,
respectively, employing the g, d and a matrices calculated at the TPSSh-
D3/def2-TZVP level of theory (Table S1). Spectra at the x-1 and y-1 positions are
shown in Figure S2.
Table 2. Rate constants for the rearrangement (d1-/d3-)syn/anti 1  2.
rate constants / 10-5 s-1
matrix/
solvent
syn
d1-1
anti
d1-1
syn
d3-1
anti
d3-1
syn 1
anti 1
[e]
[g]
[g]
[e]
Ar (5 K, UV)[a]
Ar (3 K, IR)[b]
1.4 ± 1.1
However, a definite assignment of the two conformers in the
different environments cannot be made based solely on the
comparison with theoretical zfs parameters from in vacuo DFT,
especially considering the small difference in D values and the
similar energies predicted for the two conformers. Therefore,
ENDOR spectroscopy was employed to investigate the spatial
and electronic structures of 1. The 1H electron-nuclear hyperfine
interactions provide insight into the spin density distribution
across the molecule and thus allow to characterize the
conformational distribution of a carbene molecule, as we recently
demonstrated.[7]
2.1 ± 0.1
2.1 ± 0.1
(3.1 ± 0.1)[f]
(2.9 ± 0.1)[f]
Xe (3 K, IR)[b]
Ar (4 K, EPR)[c]
2.3 ± 0.1
2.1 ± 0.2
[e]
[e]
[e]
[e]
[e]
[e]
[g]
[g]
[e]
[g]
[g]
[e]
2.1 ± 0.6
1.9 ± 0.3
1.2 ± 0.0
1.3 ± 0.2
MTHF (5 K, EPR)[d]
MTHF (10 K, EPR)[d]
0.9 ± 0.1
2.3 ± 0.7
[a] from re-evaluation of the UV intensity around 249 nm (1) reported by
McMahon and Chapman.[3b] [b] from fits of the time dependence of the IR
intensity around 741-743 cm-1 (1), 773-776 cm-1 (2) and 864-872 cm-1 (2),
slightly adjusted for deuterated species or experiments in xenon matrices. [c]
from fits of the time dependence of the CW EPR intensity around 592 mT (syn
1) and 597 mT (anti 1), deconvoluted by fitting two Voigt profiles. [d] from fits of
the time-dependence of the pseudomodulated pulse EPR intensity around
866 mT (syn 1) and 875-876 mT (anti 1). [e] not reported / not determined. [f]
from experiments using a broadband pass filter with a cutoff > 1500 cm-1. [g] no
reaction observed.
Figure 2 shows the Q-band Davies-type 1H-ENDOR spectrum of
1 in MTHF recorded at the z+1 magnetic field position of the major
component in the EPR spectrum, together with spin-Hamiltonian-
based simulations for the two conformers employing the
theoretical EPR parameters from the DFT computations (Table
S1). The spectra at the x-1 and y-1 positions are shown in Figure
S2. Considering that DFT calculations tend to slightly
EPR Kinetics
1
overestimate the magnitude of H hyperfine couplings in triplet
A kinetic analysis was carried out for the EPR time dependencies
at magnetic field positions specific for either syn or anti 1 (Table
2). McMahon and Chapman reported that "the major rotomer
decays much more rapidly than the minor rotomer".[3b] Indeed, a
comparison of EPR spectra in argon recorded at the beginning of
carbenes,[7] the major peaks in the experimental ENDOR
spectrum in MTHF are assigned to the syn conformer of 1. This
becomes particularly evident for the spectrum recorded at the z+1
position, which exclusively comprises
a single molecular
3
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the kinetic measurement and after several hours show a clear
decrease in the intensity ratio anti/syn (Figure 3, top). However,
rates of about 2.0·10-5 s-1, consistent with the result from the IR
measurements, are obtained for both isomers. The apparent
difference in their spectral decays is instead caused by different
ratios c [Eq. (1)] of non-reactive residuals of the two isomers (Ar,
4 K; syn 1: 0.73, anti 1: 0.11), which represent the lower limits at
which the decay curves saturate. This can be deduced from the
decrease in the relative intensities: For the dominant anti isomer,
the intensity decreases to a limiting value of roughly 30%, while
at the same time the syn isomer approaches 80%, despite nearly
identical reaction rates.
rates were found for d1-1, indicating that there is no pronounced
secondary kinetic isotope effect.
The presence of unreactive or trapped fractions within the
ensemble is not uncommon for molecules isolated at cryogenic
temperatures.[8] As already indicated by the differing anti/syn 1
ratio in noble gas and organic matrices, steric restrictions resulting
from interactions with the non-uniform molecular environment
kinetically impede processes towards the thermodynamic
minimum. Apparently, this is the case for both the isomerization
from syn 1 to anti 1 upon photolysis of 7, the syn conformation of
1-H being the energetic minimum of 7 (see SI section S8), as well
as for the [1,4] H shift, as indicated by the unreactive residuals of
anti 1 (c > 0). Steric hindrance removes part of the ensemble from
the anti/syn 1 equilibrium, resulting in differing residuals c(syn 1)
> c(anti 1). Restrained or frozen sites could be blocked both by
the matrix or solvent, as well as by N2 molecules partially still
occupying the position close to the divalent carbon after
photolysis. These effects are more pronounced in organic glasses,
which can be rationalized by stronger interactions with the solute
and higher matrix rigidity as compared to the noble gas matrices,
and lower mobility of residual N2.
Figure 3. Kinetic analysis of the tunneling rearrangement. Top: Decrease in
normalized intensities of the X-band EPR x+1 signals attributed to syn and anti
1 in argon at 4 K. Bottom: Decrease in normalized intensities of the Q-band EPR
x-1 signals attributed to syn and anti 1 in MTHF at 5 K (compare Figure 1). The
black lines show the best fit results obtained using the kinetic model [Eq. (1) in
the Experimental Section].
The reaction rate of the 1  2 reaction in frozen MTHF matrices
at 5 K is roughly half of those found in argon (Figure 3, bottom,
Table 2), with decay curves leveling off to 37% (anti 1) and 93%
(syn 1). At elevated temperatures of 10 K, the decay rates are
moderately increased for the syn isomer, but virtually unaltered
for the anti isomer. This increase can be attributed to temperature-
dependent side reactions with the organic matrix material, to
which the reactive carbene site is more exposed in syn 1.
A large primary kinetic isotope effect has been reported for CD3-
substituted 1,[3b] i.e., deuteration of the methyl group suppresses
the tunneling rearrangement to d3-2. This behavior was also found
in our experiments. Furthermore, the EPR spectra of d3-1 do not
show a change in the isomer ratio over time (Figure S3) and thus
no unidirectional syn→anti interconversion. Additionally, d1-1 with
the deuterium attached to the carbene carbon atom was
investigated to study the influence of the syn/anti interconversion
on the 1  2 tunneling rate. Based on IR measurements, identical
Figure 4. A) Calculated geometries for the tunneling rearrangement of 1. B)
Potential energy surface of the syn/anti isomerization of 1 and subsequent
rearrangement to 2. The values given are under consideration of the respective
zero-point vibrational energies. All calculations were performed at the BLYP-
D3/6-311G** level of theory.
4
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Computational studies
possible via vibrational excitation, and the SCT rate k(syn→anti
1) will outperform k(anti→syn 1) at very low temperatures.
Accordingly, k(anti→syn 1) exhibits a substantial rise by 3 - 4
orders of magnitude upon approaching 20 K, not found for
k(syn→anti 1) (see SI).
The rates of the [1,4] H migration were calculated only for anti 1
due to the lack of a reasonable transition structure for a direct syn
1 → T-2 reaction. The rate k(anti 1 → T-2) predicted at 20 K is
2.4 · 10-2 s-1, which deviates from the experimental value in Ar at
3 K by roughly three orders of magnitude. Graphical extrapolation
of the calculated rates to estimate the values below 20 K does not
bring them into agreement. The discrepancy between
experimental and computed rates is most likely a limitation of the
method, which does not describe the reaction barrier accurately
enough and which does not include solvent effects.
Geometrically, the transfer of a hydrogen atom from the methyl
group to the carbene center is not feasible for syn 1 and therefore
tunneling to 2 is expected to take place exclusively from anti 1.
The decrease of syn 1 is therefore attributed to a (tunneling)
conversion to the anti isomer, which in turn can react to 2. Since
no syn→anti interconversion is observed for the d3-isotopologue,
it is hypothesized that the two isomers are in equilibrium. A fast
(tunneling) anti/syn
disappearance rates for the two isomers reflecting the conversion
of anti 1 to 2.
For anti 1, rotation of the methyl group by 60° from its minimum
position will decrease the tunneling distance and thus increase
the tunneling probability (Figure 4). In our calculations, this
displacement is described by a valley-ridge inflection (VRI) point,
corresponding to the transitions state (TS) of the methyl group
rotation, connected to the TS of the hydrogen transfer reaction.
Experimentally, however, it is not possible to discriminate whether
tunneling takes place from the minimum structure of anti 1, from
the geometry with optimal CH3 rotation (i.e. the VRI point) or from
an intermediary state. The primary product obtained from the
rearrangement of 1 in its triplet ground state will be triplet T-2.
Here too, the geometry to which an in-plane hydrogen transfer is
leading corresponds to a VRI point, in which one of the methylene
groups of 2 is rotated by 90° from its minimum position.
Subsequent rotation and intersystem crossing will lead to the final
product, xylylene 2.
1
pre-equilibrium yields identical
Table 3. SCT rate constants at 20 K for the 1  T-2 reaction steps at the BLYP-
D3/6-311G** level of theory.
Figure 5. Reaction rates calculated via transition state theory (TST) only and
including tunneling (SCT) corrections at the BLYP-D3/6-311G** level of theory
for the (d1/d3-)anti 1  T-2 reaction.
calculated rate constants k / s-1
1
d1-1
d3-1
At temperatures below 100 K, the [1,4] H migration is predicted to
occur exclusively due to tunneling (Figure 5). However even at
300 K, tunneling still represents the main contribution to the
overall turnover. Consistent with the experiment, a small
secondary KIE is found for d1-1, while a large KIE of five orders of
magnitude is found upon deuteration of the methyl group,
effectively suppressing the rearrangement in d3-1.
syn→anti 1
anti→syn 1
anti 1 → T-2
1 → 2 (exp)
3.6 · 107
9.7 · 103
2.4 · 10-2
2.1· 10-5
1.0 · 106
3.1 · 102
9.3 · 10-3
2.1· 10-5
4.4 · 107
1.8 · 104
5.2 · 10-7
not observed
Tunneling rates were calculated with the SCT method for the
elementary reaction steps of the 1  T-2 tunneling reaction
(Table 3). The tunneling rate of the syn 1 → anti 1 isomerization
is predicted to a remarkably large value of 3.6 · 107 s-1. Such
highly efficient tunneling close to zero Kelvin is attributed to the
light weight of the tunneling atom, as the tunneling coordinate is
dominated by the movement of the methylene hydrogen atom and
a low barrier height of 3.2 kcal mol-1. Thus, the isomerization is
mostly occurring by tunneling at temperatures of 100 K and below,
while classical barrier passage is predicted to contribute
approximately half of the overall reaction turnover at room
temperature. Deuteration at the carbene position (d1-1) induces a
strong kinetic isotope effect (KIE) and slows down this rate by
approximately a factor of 36. However, the resulting rate constant
is still predicted to be too fast to be detected experimentally. A
slight inverse secondary KIE is found for d3-1.
Conclusion
The conformational isomerism and hydrogen tunneling of triplet
o-tolylcarbene 1, previously reported by McMahon and Chapman,
were re-investigated by using advanced spectroscopic and
quantum chemical techniques. ENDOR spectroscopy provides a
reliable method to identify the two conformers, which in turn
allowed us to confidently assign the conformers of 1 in the EPR
spectra as required for the conformer-specific kinetic experiments.
Tunneling rates are highly dependent on the tunneling distance.
To enable rapid [1,4] H tunneling in 1, it is therefore crucial that
the carbene adopts a proper conformation with a short distance
between the migrating H atom and the carbene center. To achieve
this, the carbene must adopt its anti conformation and the methyl
group has to rotate from its minimum conformation by 60° to
structure VRI(1TS) (Figure 4). Nevertheless, our EPR studies
performed reveal the same tunneling rates for both syn and anti
1. This observation is in notable contrast to the large differences
Tunneling rates were also calculated for the anti→syn 1
isomerization, which is endothermic. Hence, tunneling from the
anti conformer towards the more energetic syn conformer is only
5
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were employed. The Synchronous Transit-Guided Quasi-Newton (STQN)
method[20] with the QST3 algorithm was utilized in order to optimize the
geometries of first-order transition states (TS). Tunneling rates were
calculated using combination of transition state theory (TST) and
semiclassical Wentzel-Kramers-Brillouin (WKB) theory as implemented in
the Polyrate program.[21] The calculations were carried out using the
Gaussrate[22] interface of Polyrate to the Gaussian 09 package employing
the level of theory detailed above. The transmission coefficients for
tunneling were calculated with the small-curvature tunneling (SCT)
method.[23] The tabulated TST/SCT data refer to conventional (non-
variational TST) plus SCT corrections. Due to numerical issues arising at
very low temperatures, calculations were carried out at 20 K, where
classical barrier passage still is completely suppressed for all processes
under consideration.
in tunneling rates found for the conformers of singlet hydroxy- or
fluorocarbenes.[5a, 6]
We conclude that the (non-confined fractions of the) syn and anti
conformers are in rapid equilibrium even at temperatures between
3 and 5 K. The classical activation barrier is calculated to 2.9
kcal/mol, which should completely suppress this rearrangement
at temperatures below 5 K. This clearly indicates that the syn 
anti rearrangement is governed by tunneling, in agreement with
the predictions from our SCT calculations. Even after deuterium
substitution of the hydrogen atom at the carbene center, the
decrease of the tunneling rate is not sufficient to become rate
determining for the overall process. The hydrogen migration in the
syn conformer therefore requires a sequential tunneling reaction:
first synanti isomerization and second [1,4] H migration. If we
assume that the CH3 rotation is also governed by tunneling, we
must consider three consecutive tunneling movements required
Acknowledgements
for the 1  2 rearrangement. Thus, in contrast to the reactions in
6]
singlet hydroxy- or fluorocarbenes,[5a,
1 exhibits conformer-
This
Forschungsgemeinschaft (DFG, German Research Foundation)
under Germany’s Excellence Strategy EXC-2033
Projektnummer 390677874 RESOLV and the Research Training
Group “Confinement‐controlled Chemistry” (Grant
GRK2376/331085229).
work
was
supported
by
the
Deutsche
specific hydrogen tunneling to which the Curtin–Hammett
principle[9] applies. This describes
situation where the
-
-
a
(tunneling) barrier for conformer interconversion is considerably
lower than that of the actual reaction under consideration. For 1,
it allows for a rotational tunneling pre-equlibrium to exist, which in
turn disguises the conformer-specificity of the [1,4] H migration.
Tunneling reactions play a vital role in chemistry and biology,
particularly in C-H activation reactions.[2b, d, e] Conformational pre-
equilibria are important to achieve geometries with high tunneling
probabilities,[10] therefore our observation of sequential tunneling
will aid in the fundamental understanding of such complex
reaction sequences.
Keywords: carbenes • ENDOR spectroscopy • hydrogen
transfer • matrix isolation • tunneling
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Experimental Section
Experimental details
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dispersion coefficient β.
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I = I0 ∙ exp(-(k∙t)β) + c
with 0 < β < 1
(1)
Computational details
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6
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Entry for the Table of Contents
Who is first? Conformer specificity, although rarely evidenced, is expected to play a crucial role in tunneling reactions. [1,4]
hydrogen transfer in o-tolylmethylene is a puzzling example in this respect, as the apparent rate constants do not differ for the two
conformers. Our structural, kinetic, and energetic analyses demonstrate that conformer specificity is disguised by a rotational
tunneling pre-equilibrium.
8
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