Characterization of the Simplest Thiolimine: The Higher Energy Tautomer of Thioformamide

Article abstract of DOI:10.1002/chem.202005188

As sulfur-containing organic molecules thioamides and their isomers are conceivable intermediates in prebiotic chemistry, for example, in the formation of amino acids and thiazoles and resemble viable candidates for detection in interstellar media. Here, we report the characterization of parent thioformamide in the solid state via single-crystal X-ray diffraction and its photochemical interconversion to its hitherto unreported higher energy tautomer thiolimine in inert argon and dinitrogen matrices. Upon photogeneration, four conformers of thiolimine form, whose ratio depends on the employed wavelength. One of these conformers interconverts due to quantum mechanical tunneling with a half-life of 30–45 min in both matrix materials at 3 and 20 K. A spontaneous reverse reaction from thiolimine to thioformamide is not observed. To support our experimental findings, we explored the potential energy surface of the system at the AE-CCSD(T)/aug-cc-pCVTZ level of theory and computed tunneling half-lives with the CVT/SCT approach applying DFT methods.

Full text of DOI:10.1002/chem.202005188

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&
Prebiotic Chemistry
Characterization of the Simplest Thiolimine: The Higher Energy
Tautomer of Thioformamide
Bastian Bernhardt,[a] Friedemann Dressler,[a] AndrØ K. Eckhardt,[a] Jonathan Becker,[b] and
Peter R. Schreiner*[a]
Abstract: As sulfur-containing organic molecules thioamides
and their isomers are conceivable intermediates in prebiotic
chemistry, for example, in the formation of amino acids and
thiazoles and resemble viable candidates for detection in in-
terstellar media. Here, we report the characterization of
parent thioformamide in the solid state via single-crystal X-
ray diffraction and its photochemical interconversion to its
hitherto unreported higher energy tautomer thiolimine in
inert argon and dinitrogen matrices. Upon photogeneration,
four conformers of thiolimine form, whose ratio depends on
the employed wavelength. One of these conformers inter-
converts due to quantum mechanical tunneling with a half-
life of 30–45 min in both matrix materials at 3 and 20 K. A
spontaneous reverse reaction from thiolimine to thioforma-
mide is not observed. To support our experimental findings,
we explored the potential energy surface of the system at
the AE-CCSD(T)/aug-cc-pCVTZ level of theory and computed
tunneling half-lives with the CVT/SCT approach applying DFT
methods.
Introduction
that of HCN reacting with H2O, which has been investigated
computationally recently.[10] The general idea of constructing
organic molecules from simple building blocks has been
widely accepted since the famous Urey–Miller experiments.[11]
Mechanistically, addition of H2S to nitriles is expected to pro-
ceed via a thiolimine (2, methanimidothioic acid, R=H), which
is the thermodynamically disfavored tautomer of 1. To the best
of our knowledge, no data exist for 2, which prompted us to
study the tautomerization between 1 and 2 in cryogenic argon
(Ar) and dinitrogen (N2) matrices to provide first-hand evidence
for the existence and spectral identity of 2. We also report the
first single-crystal X-ray diffraction structure of 1.
Although thioformamide (1, methanethioamide, R=H) is the
simplest thioamide, it is not as well characterized as one
would assume. In nature as well as in synthetic chemistry[1]
thioamides resemble building blocks for thiazoles (4), a unit
that is present in various biologically important molecules
(e.g., thiamin) and drugs.[2] General syntheses towards thiazoles
build upon reacting thioamides with a-halogenated aldehydes
(3).[3] Parent thioformamide is an adduct of HCN and H2S,[4]
both of which were detected in interstellar media.[5–7] This
makes thioformamide a probable participant in the prebiotic
synthesis of sulfur containing compounds important for the
development of life (Scheme 1). Accordingly, Sutherland et al.
demonstrated that various thioamide derivatives might play a
role in the formation of amino acids through what has been
termed “cyanosulfidic protometabolism”.[8] In a prebiotic con-
text they can form from the reaction of nitriles with either H2S
or thiophosphate (5).[9] This chemistry displays similarities to
Various thioamide derivatives have been isolated under cryo-
genic conditions and characterized by infrared (IR) spectrosco-
[a] B. Bernhardt, F. Dressler, Dr. A. K. Eckhardt, Prof. Dr. P. R. Schreiner
Institute of Organic Chemistry, Justus Liebig University
Heinrich-Buff-Ring 17, 35390 Giessen (Germany)
E-mail: prs@uni-giessen.de
[b] Dr. J. Becker
Institute of Inorganic and Analytical Chemistry, Justus Liebig University
Heinrich-Buff-Ring 17, 35390 Giessen (Germany)
Supporting information and the ORCID identification numbers for the
authors of this article can be found under:
https://doi.org/10.1002/chem.202005188.
Scheme 1. Thioamides are considered key intermediates in the generation
of some highly relevant biologically active molecules like amino acids and
thiazoles (4) in prebiotic context. Their formation from nitriles and H2S or
thiophosphate (5) has been described.[9] Here we focus on parent thioform-
amide (1, R=H) and its tautomerization to 2 (R=H).
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py during the last decades. Several studies focused on the UV-
induced thioamide!thiolimine tautomerization, which has
been reported for 2(1H)-pyridinethione,[12] 4(3H)-pyrimidine-
thione and 3(2H)-pyridazinethione,[13] 2(1H)-quinolinethione,[14]
methimazole,[15] 2-thiobenzimidazole,[16] thiourea,[17,18] and thio-
acetamide.[19–21] In the case of thiourea the reverse thiolimine
! thioamide reaction has been observed even under cryogen-
ic conditions and the results were interpreted as to involve
quantum mechanical tunneling (QMT).[17,18] Dithiooxamide
shows UV-induced double proton transfer forming the dithiol-
imine and the reverse reaction due to QMT.[22] Thioacetamide,
on the other hand, does not display such a tunneling reaction
under experimental laboratory conditions.[19,20] Generally speak-
ing, QMT is an abundant, yet often underappreciated phenom-
enon in chemical reactions, and it is especially well observable
under cryogenic matrix isolation conditions when first order
tunneling half-lives range from a few seconds to several
days.[23–26] Due to the similarities regarding cryogenic tempera-
tures and very low concentrations, many of such reactions are
also highly relevant in interstellar media.
Scheme 2. Potential energy surface of thioformamide (1) and its thiolimine
tautomers (2) computed at the AE-CCSD(T)/aug-cc-pCVTZ level of theory. In
our nomenclature for 2 the first letter represents s-cis or s-trans orientation
of the SÀH bond while the second letter represents s-cis or s-trans orienta-
tion of the NÀH bond. Bond lengths are given in Š and angles in degrees of
geometries optimized at the level of theory mentioned above. Only those
internal coordinates that change throughout the displayed reactions are de-
picted, for exhaustive geometric data see the Supporting Information. Color
code: Carbon— black, hydrogen— white, nitrogen— blue, sulfur— yellow.
Owing to its quick decomposition at ambient conditions
(see Supporting Information) reports on experimental data of
1 are rare.[27,28] Only two infrared studies on 1 in the liquid and
solid phases exist, which provide IR spectra in good agreement
with each other, but are controversial in the assignment of the
observed bands.[29,30] However, no literature exists on the
(photo-)reactivity of 1. In stark contrast, the amide!enolimine
interconversion of the congener formamide induced by irradia-
tion with 248 nm in an Ar matrix at 10 K has been known since
2000[31] and its matrix IR spectra as early as 1970.[32,33] Irradia-
tion with wavelengths >160 nm leads to its photodecomposi-
tion into complexes of HCN, HNC, HNCO, and H2O.[34] The tau-
tomerizations of selenoformamide[35] and telluroformamide
have been investigated only theoretically.[36] For all homo-
logues this process has also been studied computationally by
modelling aqueous hydration shells, which yielded significantly
lower proton transfer barriers compared to the unimolecular
gas phase reaction.[37]
cluster methods (Scheme 2). All computed minima shown in
Scheme 2 display Cs symmetry at all levels of theory applied in
this study. According to the relative energies the ground state
structure at cryogenic temperatures is the thioamide tautomer
1, which is more than 8 kcalmolÀ1 lower in energy than its
thiolimine tautomers 2. There are four conceivable conformers
of 2 that are very close (within 1 kcalmolÀ1) in energy. The
order of stability is 2cc<2tt<2ct<2tc at every computation-
al level applied here. The conformers can interconvert through
CÀS bond rotations or inversion of the C=NÀH angle. The
latter requires overcoming high lying Cs symmetric transition
states.
The mechanism of the UV-induced tautomerization of thio-
amides has been discussed. Excited state intramolecular
proton transfer (ESIPT) has been ruled out by Lapinski et al.
due to the lack of intramolecular hydrogen bonds in these sys-
tems.[19] For methimazole[15] and 2-thiobenzimidazole[16] Brµs
and Fausto considered a photoinduced hydrogen-atom de-
tachment-association (PIDA) mechanism, which has been de-
scribed theoretically by Sobolewski et al.[38–41] Eventually, a
study combining Raman spectroscopy and theory reconsidered
the possibility of ESIPT to play a role in the tautomerization
mechanism in thioacetamide.[42]
We obtained single crystals of 1 in ethyl acetate and solved
their structure by X-ray diffraction and refined the data with
the independent atom model and Hirshfeld atom refinement
(Figure 1).[43] The excellent agreement of the computed geom-
etry of 1 (Scheme 2) with the X-ray structure lends credibility
to the theoretical approach. Within the crystal molecules of 1
are aligned in an array between ethyl acetate clusters. There
are hydrogen bonding contacts within these arrays, but also
between 1 and ethyl acetate. The structure of a single crystal
of pure formamide has already been reported in 1954.[44] Form-
amide forms C2h symmetric dimers bounded with two NÀH···O
contacts. Gas-phase B3LYP/6–311++G(3df,3pd) computations
predict that the analogous C2h dimer of 1 containing two NÀ
H···S contacts is 2.0 kcalmolÀ1 lower in energy than a Cs dimer
as depicted in Figure 1 containing only one such interaction.
Note that the CÀH···S contact in the crystal is 2.64 Š and,
hence, cannot be accounted for a relevant binding contact.[45]
The strong interaction between 1 and ethyl acetate (NÀH···O
contact of only 1.82 Š) is presumably the reason for the differ-
Results and Discussion
Computations and X-ray diffraction data
We computed the most significant part of the HC(S)NH2 poten-
tial energy surface with density functional theory (DFT; see
Supporting Information for details) as well as with coupled
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matrix. For the complete assignments see the Supporting In-
formation. Our results are in good agreement with the experi-
mental data by Davies and Jones[29] and Suzuki.[30] Additionally,
there is a good match with a computational study by Kowal.[48]
Our assignment is based on this latter study, which is in gener-
al accord with the one provided by Suzuki and the computa-
tions conducted herein.
The recorded matrix UV/Vis spectrum (Figure 3)
shows
a strong absorption at ca. 260 nm. TD-B3LYP/6–
311++G(3df,3pd) computations for thioformamide suggests
two strong absorptions at 231 nm (f=0.2027) and 225 nm (f=
0.1517 nm). After UV irradiation (vide infra) a small new feature
at ca. 225 nm appears. This is in accord with the computational
result that all thiolimine tautomers show absorptions between
210 nm and 215 nm (0.05<f<0.15). For an analogous spec-
trum obtained in N2 matrices as well as details of the nature of
the transitions see the Supporting Information.
Figure 1. Slice through a crystal containing 1 and ethyl acetate. Left: Geo-
metric details of a thioformamide molecule embedded in the crystal. Right:
Intramolecular interactions between molecules of 1 with each other and
ethyl acetate. Lengths are given in Š and angles in degrees. Color code:
carbon— black, hydrogen— white, nitrogen— blue, oxygen— red, sulfur—
yellow.
ence from the crystal structure in Figure 1 from the one of
formamide and the computations. In a different vein, analo-
gous S=CÀNÀH···O=C interactions resemble the central motif
in thiourea catalysis.[46,47]
Matrix isolation and characterization of thioformamide
After deposition of a sample of freshly synthesized thioform-
amide (see Supporting Information for details) together with a
large excess of Ar on the cold (3 K) matrix window, we mea-
sured IR and UV/Vis spectra of all matrix isolated species. The
IR spectrum displayed in Figure 2 shows excellent agreement
with the anharmonic spectrum computed at B3LYP/6–
311++G(3df,3pd). Using Ar as the host material, the strongest
bands are observed at 3519.4 (calc. 3507.2), 3402.2 (3391.1),
1597.6 (1596.7), 1432.0 (1424.2), 1287.6 (1283.3), and 393.1
(432.3) cmÀ1. We also recorded an IR spectrum of 1 in an N2
Figure 3. UV/Vis spectrum of the Ar matrix containing 1. Black: After deposi-
tion of 1 in an Ar matrix. Grey: After irradiation with 254 nm for 10 min.
Inset: Computed spectrum of 1 at TD-B3LYP/6–311++G(3df,3pd).
UV-induced thioamide to thiolimine interconversion
Irradiation of the matrix with UV light led to a decrease of the
thioformamide bands. New bands appear in the IR spectra that
can be assigned to 2. The four conceivable conformers of this
species can all be observed in these spectra. For ease of iden-
tification, we computed anharmonic IR spectra of 1 and 2 at
B3LYP/6–311+++G(3df,3pd). DFT in conjunction with a large
Pople basis set generally reproduces matrix IR spectra very rea-
sonably; recent precedence is provided in a study on thiotro-
polone.[49] Herein, the inclusion of anharmonicities leads to an
overall good agreement between computations and measure-
ments (see Supporting Information for details).
The computed intensities of the NÀH and the SÀH stretch-
ing vibrations of 2 are very low. The remainder of the simulat-
ed spectra is similar for all four conformers. The intensity pat-
tern, especially in the CÀH and NÀH bending regions, turns
Figure 2. Top: IR spectrum of thioformamide (1) after deposition in an Ar
matrix at 3 K. Bottom: Computed anharmonic IR spectrum of 1 at B3LYP/6–
311+ +G(3df,3pd).
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out to be very useful to distinguish between the two groups
2tc/2cc and 2ct/2tt. In the CN torsion region, the bands of
2ct and 2tt have nearly the same computed value. On the
other hand, 2tc and 2cc display large shifts, such that this
region is the most prominent for assignment (Figure 4). Ac-
cording to the anharmonic computations, only the CN torsion
region in the IR spectrum differs significantly for the four con-
formers: two bands at 940.0 and 912.7 cmÀ1 appear after UV ir-
radiation and only 2tc and 2cc are computed to show IR ab-
sorptions. We assign the first band to 2cc (calc. 944.2 cmÀ1)
and the second to 2tc (calc. 902.5 cmÀ1). The band at
1059 cmÀ1 only has a computed counterpart in conformers 2ct
and 2tt. As discussed below both conformers are present in
the matrix and their IR bands can only be resolved in differ-
ence spectra (1058.9 and 1059.9 cmÀ1, see Figure 6). The com-
puted values are 1048.8 and 1048.3 cmÀ1 for 2ct and 2tt, re-
spectively.
A study by Lapinski et al. demonstrated that the thioamide
to thiolimine interconversion of thioacetamide yields different
ratios of thiolimine conformers when the matrix is irradiated at
different wavelengths.[19] Following this strategy we performed
one experiment using 254 nm and another in which only
wavelengths >320 nm could pass through a cut-off filter in-
stalled in front of the matrix window. The difference spectra of
the spectra recorded before and after 10 min of irradiation are
presented in Figure 5. The intensity ratio of the new bands is
indeed sensitive to the applied wavelength. At lower energies
(>320 nm) 2tc and 2cc are the main products after UV excita-
tion while 2ct and 2tt are more easily accessible at lower
wavelengths. This can be rationalized by taking into account
the higher activation barriers for bending the N=CÀS angle
compared to rotating around the CÀS bond (Scheme 2). This
finding supports our assignment in the CN torsion region and
allows to assign other bands to the groups 2tc/2cc and 2ct/
2tt. Intensity patterns, comparing experimental with computa-
tional shifts, and interconversion of the conformers (see below)
serve to resolve these two groups further and enables a confi-
dent assignment of each observed band to one conformer.
In the CH stretching region at around 1350 cmÀ1 and the NH
bending region at around 1170 cmÀ1 four new bands appear
after UV irradiation. Taking the described strategy into account
we assign the strongest of these bands at 1346.2 cmÀ1 to 2ct
(calc. 1338.4 cmÀ1). With the same strategy the band at
1349.3 cmÀ1 is assigned to 2cc (calc. 1343.8 cmÀ1), the band at
1352.7 cmÀ1 to 2tc (calc. 1349.1 cmÀ1), and the band at
1358.0 cmÀ1 to 2tt (calc. 1352.2 cmÀ1). The different shifts and
intensities throughout the spectrum allow us to draw the con-
clusion that indeed 2ct and 2tt are both present. In the NÀH
bending region the band at 1162.2 cmÀ1 can be assigned to
2cc, the strong bands at 1171.8 cmÀ1 and 1173.2 cmÀ1 to 2ct,
and the band at 1176.9 cmÀ1 to 2tt. The computed values are
1149.7, 1161.3, and 1161.9 cmÀ1 for 2cc, 2ct, and 2tt, respec-
tively, which reproduce the experimental spectra in good
agreement.
Figure 4. Computed anharmonic IR spectra of the four conceivable conform-
ers of thiolimine 2 at B3LYP/6–311++G(3df,3pd).
Figure 5. IR difference spectra of spectra measured before and after UV irradiation of the matrix. The ratio of the groups 2tc/2cc and 2ct/2tt is sensitive to
the applied wavelength allowing for band assignment throughout the displayed spectral regions.
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Our assignment is further supported by the observation of a
tunneling process from 2tt to 2ct and the rotamerization of
2tc to 2cc (vide infra). In the 670–750 cmÀ1 range, all conform-
ers are computed to show several characteristic bands as well.
The assignment is possible by combining the above strategies.
Analogously, we observed all four thiolimine conformers in N2
matrices. Spectra and assignments are available in the Sup-
porting Information.
On the other hand, such phenomena have been assigned to
matrix effects before, namely that specific matrix environments
(or matrix sites) inhibit QMT.[50] Recent precedence is provided
in a study on thioacetamide.[20] Furthermore, for the N,N-dideu-
terated isotopologue of thioformamide dark reactions of the
thiolimine tautomers were not observed when installing the
cut-off filter. This provides another hint that the reaction 2tt!
2ct is due to QMT and resembles only the second example of
conformational QMT leading to a rotation of a CÀS bond.[20]
We obtained the experimental tunneling half-life of the reac-
tion 2tt!2ct by following the decay (increase) of the strong-
est bands of 2tt (2ct). A first-order kinetic model with the con-
centrations of 2ct and 2tt (that are proportional to the ob-
served band intensities I), the pre-exponential factor A (equal
to the band intensity at t=0) and the time t as in Figure 7 was
used. The offset B needs to be included because the bands of
2tt will not reach the asymptotical value of zero.
Especially when measuring the spectra with the 4.5 mm cut-
off filter the band intensities of 2 are rather small making an
exact analysis difficult. However, changing the temperature
from 3 to 20 K or placing the cut-off filter between the spec-
trometer and the matrix window does not change the results
significantly neither in Ar nor in N2 (see the Supporting Infor-
mation for details). The half-life remains between 25 and
45 min for both matrix materials.
After prolonged UV irradiation the new bands assigned to
the thiolimine tautomers vanished. The strongest bands after
irradiation for 1 h are at 3361.8, 3202.3, 2091.2, 2026.2, 1233.4,
647.2, and 638.9 cmÀ1. These bands cannot be confidently as-
signed to the decomposition products H2S, HCN, HNC, CS, NH3,
H2, HSCN, or HCNS because in the present mixture significant
shifts of the predicted bands of the pure compounds might
occur due to the formation of complexes. Bands at 1624.1,
1553.0, and 1408.1 cmÀ1 remain unchanged during UV irradia-
tion and can thus be assigned to unknown impurities in our
sample. The highest concentration of the thiolimine tautomers
is observed after 10 min of irradiation when 1 is still among
the main components.
Quantum mechanical tunneling (QMT)
After keeping the matrix for 2.5 h in the dark we observed a
decay of bands belonging to 2tt and a concomitant increase
of those assigned to 2ct. We measured the kinetics of this
process by collecting IR spectra in 5 min intervals. Figure 6
shows that IR measurements without the use of a 4.5 mm
(=^ 2222 cmÀ1) cut-off filter leads to the formation of 2cc from
2tc. This can be rationalized by the low activation barrier of
only 6.1 kcalmolÀ1. We notice that wavelengths >4.5 mm
(=^ 6.4 kcalmolÀ1) would in principle suffice to initiate this pro-
cess, however, we observe that when using such a cut-off filter
only the reaction 2tt!2ct occurs. It is important to stress that
even after longer times (up to 48 h) the intensities of bands of
2tt do not reach zero. This might on the one hand be due to
the IR Globar irradiation from the spectrometer (the activation
barrier for the reverse reaction 2ct!2tt is only 3.7 kcalmolÀ1).
We computed the tunneling half-lives for all conceivable re-
actions between the thiolimine conformers with canonical var-
iational theory (CVT) in conjunction with small-curvature tun-
neling (SCT). This method has often been demonstrated to
yield reliable results for matrix-isolated species.[24] Our results
are shown in Table 1. In agreement with the experiment, 2tt!
2ct is computed to be the fastest reaction. Even though ac-
cording to the computations 2cc!2tc should be observable
at laboratory time scales, we did not observe such a process.
This becomes apparent when applying a scaling factor (219.5)
to the computed half-lives accounting for the deviation of the
experimental (ca. 30 min) and the computed value of the reac-
tion 2tt!2ct. A somewhat more realistic value for 2cc!2tc
results (1361 d), far outside reasonable experimental time
Figure 6. IR difference spectra of spectra measured before and after recording the kinetics for 2.5 h. Usage of a 4.5 mm cut-off filter prevented the reaction
2tc!2cc induced by the Globar of the spectrometer while 2tt!2ct proceeds due to QMT.
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Substitution apparently has a strong effect on the tunneling
half-lives in thioamides. In this respect it is worth mentioning
the different reactivities of amides, thioamides, and seleno-
amides. Such studies were carried out by Nowak et al. on thio-
urea[17,18] and selenourea.[51] The tunneling half-lives in Ar at
10 K are 52 h and 16 h with activation barriers of 25.1 kcal
molÀ1 and 22.7 kcalmolÀ1, respectively, determined at the MP2/
6–311++G(2d,p) level of theory.[51] At this level the minima are
14.8 kcalmolÀ1 and 16.0 kcalmolÀ1 apart from each other with
the thioamide (selenoamide) form being more stable.[51] For
matrix isolated urea such a reaction has not been reported,
probably indicating a tunneling half-life too long to be observ-
able. A very recent computational study employing the least-
action tunneling (LAT) method and CVT/SCT supports the ex-
perimental trend in this series.[52]
As the tunneling rate increases for heavier homologues in
this series, we aimed to determine whether this trend is also
apparent in the parent structures. Table 2 shows the energies
associated with the enolimine!amide tautomerization. The
tunneling half-lives obtained with the Wentzel–Kramers–Bril-
louin (WKB) method are given in Table 2. Although the M06-
2X/6–311++G(2d,p) level of theory predicts a decrease in the
tunneling half-life, all values would be much too high to ob-
serve the corresponding reaction at laboratory time scales.
This result is in agreement with our experimental findings re-
ported herein as well as the absence of a tunneling reaction in
the formamide system, which has been investigated by Maier
and Endres.[31] We draw the conclusion that enolimine!amide
tunneling is enabled by the effects of the (remote) substituent,
whereas the parent systems themselves do not show such a
reactivity intrinsically. This also implies that 1 and 2 might co-
exist in interstellar media as quenching of 2 only seems possi-
ble through bimolecular reactivity, which is highly unlikely
under interstellar conditions.
Figure 7. Kinetic evaluation of bands of the conformers of 2 in an Ar matrix
at 3 K. A 4.5 mm cut-off filter was used to limit photochemistry induced by
the Globar of the spectrometer. The bands of 2tt vanish with a half-life of
ca. 30 min. Measurements were repeated every 5 min, but for clarity not all
spectra are displayed here.
Table 1. Unscaled computed tunneling half-lives at the CVT/SCT//B3LYP/
6–311++G(3df,3pd) level of theory. Experimentally, only 2tt!2ct is ob-
served.
Reaction
Symmetry number[a]
Tunneling half-life
Computation
Experiment
2tt!2ct
2cc!2tc
2tt!2tc
2cc!2ct
2
2
1
1
8.2 s
25–45 min
n.o.[b]
6.2 d
338 d
389 d
n.o.[b]
n.o.[b]
[a] Accounts for the degeneracy of the reaction coordinate
[b] Not observed.
Substituent effects have been shown to affect tunneling
rates before, most prominently in systematic studies on car-
boxylic acids[54–61] and highly reactive hydroxycarbenes.[62–69] In
hydroxycarbenes +I-donating substituents like methyl result in
reduced tunneling half-lives for [1,2]H-shifts towards the corre-
sponding aldehyde.[69] Contradicting results are obtained for
rotamerizations in carbonic acid[70] and its monomethyl ester.[71]
When comparing our result for the reaction 2tt!2ct (ca.
30 min) with the analogue reaction in thioacetamide (ca.
80 min)[20] a similar opposing trend is apparent. The absence of
thiolimine!thioamide tunneling in thioacetamide[19] and thio-
formamide in contrast to thiourea[18] is puzzling, since p-donat-
scales. Similar results were recently discussed for matrix isolat-
ed thioacetamide.[20] CVT/SCT underestimates tunneling half-
lives probably because the matrix environment has a stabiliz-
ing effect on trapped species. As expected, for the reactions
2tt!2tc and 2cc!2ct the computed tunneling half-lives are
much longer, due to the greater widths of the corresponding
reaction barriers.
In contrast to the findings for thiourea[17,18] but in agreement
with those for thioacetamide[20] no back-reaction to 1 was ob-
served after keeping the matrix for several days in the dark.
Table 2. Relative energies, barriers, and tunneling half-lives of the enol!ketone tautomerization at B3LYP/6-311++G(3df,3pd) (M06-2X/6-311+G(2d,p) in
brackets). The WKB method implemented in Tunnex[53] was used.
Formamide
Thioformamide
Selenoformamide
DDH0 between minima [kcalmolÀ1
]
12.8
10.0
10.4
(11.1)
26.4
(33.6)
2057
(44210)
(9.7)
27.1
(28.0)
2307
(7079)
(11.1)
25.4
(25.5)
4703
(1315)
°
DDH0 enolimine!amide [kcalmolÀ1
]
computed QMT half-life [years]
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Keywords: conformational interconversion · matrix isolation ·
prebiotic chemistry · tautomerism · tunneling
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Manuscript received: December 3, 2020
Revised manuscript received: January 21, 2021
Accepted manuscript online: January 26, 2021
Version of record online: March 12, 2021
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