The Journal of Physical Chemistry A
Article
3
5
Scheme 2. Reaction Products of Hydroxymethylene (3) and
Aminomethylene (4) and the Absence of These Products in
Their “Hybrid” Aminohydroxymethylene (1)
A gas phase study by Terlouw et al. hints toward the
possibility to isolate 1 under matrix isolation conditions but
also toward its facile isomerization to formamide (9) and/or
formimidic acid (10), because all three species were detected
after one-electron reduction of their cations in collision
induced dissociation mass spectra. The relationship between
a
1
, 9, and 10 has been the subject of several computational
3
6−38
studies.
The spectroscopic properties of 10 were
elucidated in an investigation on the photoreactivity of 9 in
39
an Ar matrix. According to this study, irradiation with a KrF
excimer laser (λ = 248 nm) induces a [1,3]H shift of 9 to yield
39
1
0. In contrast, irradiation of 9 at 193 nm yields the
photodecomposition products NH + CO as well as HNCO +
3
40
H . These findings are also in fair agreement with a study by
2
Duvernay et al., which shows that irradiation of formamide at
2
40 nm leads to 10 while broad-band UV irradiation
(
λ > 160 nm) results in dehydrogenation and dehydration of
41
1
0.
a
Carbene 1 instead decomposes to HNCO + H and NH + CO.
2
3
EXPERIMENTAL SECTION
■
system consisting of an RDK 408D2 closed-cycle refrigerator
cold head and an F-70 compressor unit was used for matrix
isolation experiments. A polished CsI window was mounted in
the cold head’s sample holder. The sample holder, connected
with silicon diodes for temperature measurements, was covered
by the vacuum shroud, which was equipped with KBr windows
Structurally, 1 combines the functionalities of hydroxy-
2
8
10
methylene (3) and aminomethylene (4), both of which
have been studied under matrix isolation conditions (Scheme
29−31
2
). As most hydroxycarbenes,
3 undergoes a [1,2]H shift
to yield formaldehyde (5) via quantum mechanical tunneling
28
(
QMT) or photochemical excitation. Carbene 4 reacts in a
similar fashion to methanimine 6 but only when the matrix is
10
irradiated with UV light. Herein we demonstrate that 1 does
not follow either of these reaction paths but instead
decomposes photochemically to NH + CO and HNCO + H .
to allow for IR measurements. In some experiments BaF2
windows were used due to their higher transparency when
3
2
measuring UV/vis spectra. The sample and the host gas (Ar,
purity of 99.999%) were co-deposited at 15 K. All spectral data
were collected at 3 K except for two kinetic experiments, which
were performed at 20 K. The pyrolysis zone was equipped with
a heatable 90 mm long quartz tube (inner diameter 7 mm),
controlled by a Ni/CrNi thermocouple. The travel distance of
the sample from the pyrolysis zone to the matrix was ∼45 mm.
Ar was stored in a 2 L gas balloon, which was evacuated and
filled three times before every experiment. The sample was
evaporated from a Schlenk tube at 55 °C (water) and reduced
Earlier studies demonstrated that the irradiation of thiazole-
3
2
2
(
-carboxylic acid (7a) and imidazole-2-carboxylic acid
33
7b) in an Ar matrix at 10 K yields the corresponding
carbenes 8a and 8b complexed with CO (Scheme 3). In
2
Scheme 3. Examples of Strategies To Generate and Isolate
Carbenes from Various Precursors under Matrix Conditions
−
6
pressure (∼3 × 10 mbar) and co-deposited with a high
excess of argon on both sides of the matrix window in the dark
(
preventing unwanted photochemistry) at a rate of ∼1 mbar
−1
min , based on the pressure inside the Ar balloon. Pyrolyses
were carried out at 700 °C. IR spectra were recorded between
−
1
−1
7
000 and 350 cm with a resolution of 0.7 cm with a Bruker
Vertex 70 FTIR spectrometer. A spectrum of the cold matrix
window before deposition was used as background spectrum
for the subsequent IR measurements. UV/vis spectra were
recorded between 200 and 800 nm with a resolution of 1 nm
with a Jasco V-760 spectrophotometer. A high-pressure-
mercury lamp equipped with a monochromator (LOT
Quantum Design) or a low-pressure-mercury lamp (Grantzel)
̈
fitted with a Vycor filter were used for irradiation of the matrix
contrast, 1 cannot be detected directly upon photolysis of
oxalic acid monoamide (2) but presumably is an intermediate
during photodecomposition of 2 into HNCO + H and NH +
2
3
3
4
CO. As we have demonstrated in multiple cases (Scheme
2
9−31
RESULTS AND DISCUSSION
3
),
decarboxylation via flash vacuum pyrolysis (FVP) of
■
α-keto carboxylic acids is the method of choice for the
generation, matrix isolation, and structural elucidation of
hydroxycarbenes. Accordingly, we generated 1 via pyrolysis of
oxalic acid monoamide (2).
Experimental Results. We pyrolyzed 2 at 700 °C in high
vacuum prior to trapping the resulting products in an Ar
matrix. Under the applied conditions the thermal fragmenta-
tion of 2 was not complete and complex IR spectra resulted
7
024
J. Phys. Chem. A 2021, 125, 7023−7028
Bernhardt, Bastian
