S. Lopes et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 241 (2020) 118670
5
spectra belonging to the individual conformers. Moreover, the good
agreement between the experimental and the calculated spectra
allowed a straightforward assignment of the fundamental bands. In
Table 2, the assignments are given using an approximate description,
which was established based on the performed normal coordinate
analysis. The full results obtained in these calculations are presented
in Tables S3‐S5 (Supporting Information).
absent in the spectrum shown in Fig. 3, showing that the spectra
correspond only to those of the monomeric species. The observed
band splitting in the ν(CH) stretching region is essentially due to
matrix-site splitting, though Fermi resonance may also contribute to
the splitting, since this effect has been observed for matrix-isolated
acetylene [47]. In the ν(CH) stretching spectral region, aggregation
can be expected to give rise to broader bands appearing below ca.
3280 cm−1, as reported previously for acetylene complexes isolated in
cryogenic matrices [47]. Accordingly, in the annealed N2 matrix of
ethyl propiolate, we observed a broad band with maxima at ca. 3250
and 3220 cm−1, which can then be ascribed to aggregates of the
compound.
As discussed above, since the energy difference between conformers
I and II is small (in the range 0.2 to 2.4 kJ mol−1, according to the
calculations; see Table 1) both conformers have significant populations
in the room temperature gas phase equilibrium (predicted relative II:I
population ratio in the range between 64.9%:35.1% and 43.2%:56.8%.).
However, as it was also pointed out before, the barrier for the
conversion of II into I is small (between ca. 1 and 4 kJ mol−1), and the
higher-energy conformer II can be expected to convert significantly
into the lower-energy form I during deposition of the matrices
[43–46]. According to the expectations, the experimental spectra of
the as-deposited matrices show that conformer I largely dominates in
both matrices, the II:I population ratios in Ar and N2 matrices being
ca. 1:4 and 1:2, respectively, as estimated from the relative intensity
ratios of pairs of bands assigned to individual conformers with similar
predicted IR intensities. Note that the predicted relative frequency of
the conformers for the most intense band in the spectra, ν(CO),
observed at ca. 1250 cm−1 is the opposite to that observed
experimentally (see Fig. 3). One can attribute this discrepancy to matrix
interactions, which shall reflect the most in large IR intensity vibrations
associated with more polarizable moieties and, consequently, very
much sensitive to medium.
The reason for the more efficient conformational cooling observed in
the Ar matrix compared with the N2 matrix can be tentatively explained
as resulting from the different thermal conductivities of solid Ar and N2.
Since the latter is approximately twice of the former, local heating of the
cold substrate at the place of landing of molecules being deposited is
reduced in the N2 matrix, because thermal dissipation is facilitated.
Under these circumstances, the locally available energy to surmount
the energy barrier for the conversion of conformer II into I is less, and
the conformational cooling is less efficient.
It is interesting also to note that the present results confirm the
assignment by Charles and coworkers [17] of the higher frequency
component of the pairs of bands observed in solution at 1112/1096
and 923/900 cm−1 to the anti conformer (I) and the lower component
to the gauche form (II) (see Table 2). The lower frequency component
disappears upon crystallization, indicating that in the crystal the
molecules of EP assume the structure of conformer I.
An additional observation that also agrees with the observations of
Charles et al. [17] concerns the high sensitivity of the frequencies of
the δ(C≡C) and γ(C≡C) modes to the medium. Accordingly, these two
modes are those exhibiting the largest frequency shifts in going from
the Ar to the N2 matrix, in particular, the γ(C≡C) vibration, which
changes by almost 50 cm−1 (see Table 2).
3.3. Photochemical experiments
After irradiation of EP isolated in N2 matrix with UV light
(λ N 235 nm) using a high-pressure Hg/Xe arc lamp for 120 min,
approximately half of the EP initially present in the matrix was
consumed, as shown by the changes of the IR intensities. New bands
of photoproducts appear in the spectra of the irradiated matrix.
Table 3 shows the experimentally observed bands of the photoproducts,
together with the corresponding calculated data (at the DFT(B3LYP)/6-
311++G(d,p) level) and previously reported literature data [48–52].
Full calculated data for the observed photoproducts are provided in
Tables S6-S9 (Supporting Information).
Characteristic intense bands of the photoproducts observed in the
2260–2100 cm−1 spectral region are presented in Fig. 4, which shows
the evolution of the spectrum in this region with the irradiation time.
Identification of characteristics bands due to monomeric carbon
monoxide (CO) at 2139.5 cm−1 and dimer (minor peaks at 2141.0 and
2136.5 cm−1) [48,49] allowed to conclude on the occurrence of
decarbonylation of EP (Fig. 5). Together with CO, ethoxyethyne
(HC≡COCH2CH3; EE) shall be formed and, accordingly, bands due to
this compound are observed in the spectra of the irradiated matrix,
specifically the predicted most intense bands: the strongest band is
observed at 2235.5 cm−1 (see Fig. 4) and is assigned to the C≡C
stretching vibration of EE (predicted at 2208.7 cm−1), while the bands
observed at 1379.5, 1114.0, 1103.0/1100.0, and 954.5 cm−1 correspond
to the
δas(CH3)´´, ν(CO), rock(CH2), and rock(CH3)´ vibrations
(predicted at 1393.0, 1129.3, 1097.4, and 993.1 cm−1), respectively.
In Fig. 4, two additional smaller bands are also shown. The first,
observed at 2120
cm−1, is due to aggregates of EP. It can be seen that aggregation of EP
was negligible and took place only in the initial states of irradiation. The
second band is observed at 2151.5 cm−1 and starts to grow visibly only
after ~20 min of irradiation, suggesting its origin in a secondary
photoproduct.
The dynamics of the photodissociation of EE at 193 nm has been
previously reported, namely describing EE as an efficient photolytic
precursor of ketenyl radical which acts as an intermediate in a wide
range of combustion reactions [53–55]. Though irradiation in the
present study has been carried on at longer wavelengths, it appears
still possible that the initially photoproduced EE subsequently
fragments into ketene (H2C=C=O; K) and ethene (H2C=CH2)
(Fig. 5). Under the conditions of our experiments, the process has very
low efficiency, as inferred from the low experimental infrared intensity
of the anti-symmetric C=C=O stretching vibration of the ketene (the
band at 2151.5 cm−1 observed in Fig. 4). This vibration is known to
give rise to a very intense band (see also Supporting Information
Table S9) and has been observed before at 2152.8 cm−1 in the gas
phase in several publications, including Duncan et al. and Moore and
Pimentel [50–52]. No bands ascribable to ethene could be detected in
the photolyzed matrix, but this could be anticipated because this
compound does not exhibit any strong IR band in a spectroscopic
clean region [56].
Moreover, all bands observed in Ar and N2 matrices have
counterparts in vapor or solution except for the rock(CH2) vibrational
mode. Some of the bands were not observed in the vapor phase, for
example, the band at 1236 cm−1 in solution assigned to the CO
stretching mode is the most intense vibration, however has no
counterpart in vapor.
Finally, the fact that the ν(CH) stretching bands (both in Ar and N2
matrices) show a multiplet profile shall also deserve here a brief
comment. We performed annealing experiments (up to 34 K) on the
matrices to check if the spectra shown in Fig. 3 contain bands due to
aggregated species. The annealing experiments allowed us to conclude
that aggregation is particularly visible in the ν(C=O) and ν(CO)
stretching regions of the spectra, where the aggregates give rise to
broad intense bands with main maxima at 1718/1715 and 1280/1269/
1261/1251 cm−1 respectively (values for N2 matrix; in argon, the
corresponding bands appear at similar frequencies). These bands are
Please cite this article as: S. Lopes, T. Nikitin and R. Fausto, Structural, spectroscopic, and photochemical study of ethyl propiolate isolated in
Fausto, Rui
