H. Rostkowska, et al.
ChemicalPhysicsLetters745(2020)137263
46dHP
converted also into the oxo-hydroxy Dewar isomer and into the open-
K
K
D
~
isolated in an H2 matrix did not induce any phototautomeric reaction
transforming the oxo-hydroxy form of the compound into the dihydroxy
tautomer (Figs. 3, 6 and S4). The major product of the UV-induced
Dewar oxo-hydroxy isomer. For the compound isolated in H2 matrix,
due to the lack of the competing phototautomeric transformation and
due to comparatively high population of the oxo-hydroxy substrate, the
amount of the photogenerated Dewar isomer of 2m46dHP was con-
siderable. It allowed a clear observation of a number of IR bands (at
3566, 3433, 1807, 1638, 869 cm−1) attributed to the photoproduced
Dewar isomer (Fig. 6). Good agreement between the spectral positions
of these bands and the bands theoretically predicted for the oxo-hy-
droxy Dewar isomer of 2m46dHP allowed a reliable identification of
this photogenerated species.
D
0.04
0.02
0.00
0.04
0.02
0.00
Ar
H2
2m46dHP
~
~
D
D
0.1
K
K
0.1
0.0
Ar
H2
0.0
The effects of UV (285 > λ > 270 nm) irradiation of 2m46dHP
monomers isolated in a Ne matrix (Fig. 6 and Fig. S4) were inter-
mediate between these observed for the compound isolated in Ar ma-
trices and those found for 2m46dHP monomers trapped in H2 matrices.
Though the phototautomeric oxo-hydroxy → dihydroxy conversion
occurred also for 2m46dHP isolated in solid Ne, it was significantly less
efficient than the analogous transformation observed for 2m46dHP in
solid Ar. For 2m46dHP isolated in Ne matrices and irradiated with UV
(285 > λ > 270 nm) light, total consumption of the oxo-hydroxy
substrate was accompanied only by a moderate increase of the popu-
lation of the dihydroxy form (to 128% of the initial abundance). This
effect of matrix environment on the phototautomeric transformation is
similar to that previously observed for 4(3H)-pyrimidinone [13]. For
monomers of this latter compound isolated in solid Ar, the oxo → hy-
droxy phototautomerism was the dominating photoreaction. Photo-
tautomerism occurred also for 4(3H)-pyrimidinone isolated in solid Ne,
although it was significantly less pronounced. But the molecules of
4(3H)-pyrimidinone trapped in solid H2 underwent no UV-induced
oxo → hydroxy phototransformation.
2200 2100 1800 1750
2200 2100 1800 1700
Wavenumber / cm-1
Fig. 5. Photochemical generation of the open-ring conjugated ketene and the
oxo-hydroxy Dewar isomers. Effects of UV (285 > λ > 270 nm) irradiation of
4,6-dihydroxypyrimidine (46dHP) and of 2-methyl-4,6-dihydroxypyrimidine
(2m46dHP) molecules isolated in Ar or H2 low-temperature matrices.
Fragments of the infrared spectra of matrix-isolated monomers of the studied
compounds recorded: (black) before any irradiation; (red) after 1 hour (for
46dHP) or 4 hours (for 2m46dHP) of UV irradiation. The characteristic bands
due to the oxo-hydroxy Dewar and the open-ring ketene photoproducts are
marked with “D” and “K”, respectively. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this
article.)
matrices at 2168/2158/2145/2144/2140/2139/2137 cm−1 (Ar),
2167/2161/2150/2146 cm−1 (Ne) and 2164/2157/2148/2146/2143/
2141 cm−1 (H2). Because the absolute intensity of this band is very
high (calculated intensities are ca. 900–1400 km mol−1, depending on
the isomer), appearance of the medium-intense experimental band at
2170-2130 cm−1 indicates that only a small portion of the oxo-hydroxy
substrate transformed into the open-ring ketene product.
Upon UV (285
> λ > 270 nm) irradiation, the oxo-hydroxy
monomers of 2m46dHP isolated in solid Ne transformed efficiently into
the oxo-hydroxy Dewar valence isomer. The IR spectral indications of
this Dewar photoproduct were found in the spectra of UV-irradiated Ne
matrices (Fig. 6) at similar positions as the analogous bands in the
spectrum of the Dewar isomer generated in solid H2 (Fig. 6). Similarly
as it was in the case of 46dHP, also for 2m46dHP isolated in Ar, Ne and
H2 matrices and irradiated with UV light, a small amount of the open-
ring conjugated ketene was produced, see Figs. 5 and S5.
4.4. Photochemical transformations of 2-methyl-4,6-dihydroxypyrimidine
isolated in Ar, Ne and n-H2 matrices
Monomers of 2m46dHP, isolated in Ar, Ne and H2 matrices, were
irradiated with UV (285 > λ > 270 nm) light. For the molecules
isolated in solid Ar, such irradiation resulted in an intensity decrease of
the IR bands assigned to the oxo-hydroxy tautomer accompanied by a
substantial intensity increase of the IR bands due to the dihydroxy form
(Fig. 3 and 4 and S4). After prolonged irradiation (lasting 4 hours),
tensities of the bands assigned to the dihydroxy tautomer reached 180%
of the values measured in the spectra recorded before any irradiation.
The observed change of the band intensities during UV irradiation al-
lows assessing the [oxo-hydroxy]:[dihydroxy] ratio in the freshly de-
posited matrices containing 2m46dHP as not lower than 0.8. The
higher value (> 0.8) of the [oxo-hydroxy]:[dihydroxy] ratio experi-
mentally assessed for 2m46dHP, in comparison to the lower value
(> 0.3) of the analogous ratio estimated for non-methylated 46dHP, is
in agreement with the theoretical predictions of relative energies of
tautomeric forms of the two compounds in question (Table 1). It is also
in accord with the experimental assessment that stems from the com-
parison of the intensities of the νOH and νNH bands assigned to the
dihydroxy and oxo-hydroxy forms of 46dHP and 2m46dHP (Fig. 1b, c).
of 2m46dHP isolated in Ar, Ne or H2 matrices and excited with UV light
In an additional series of experiments, matrices were irradiated with
UV light of longer (315 > λ > 295 nm) wavelengths. The qualitative
picture of the photoreactions induced by UV (315 > λ > 295 nm)
irradiation was the same as that observed after UV
(285 > λ > 270 nm) excitation (the same set of photoproducts was
generated). The only difference concerned the speed of the photo-
transformations, which was lower when the matrices were irradiated at
315 > λ > 295 nm. The probable reason for that can be related to
lower absorption coefficients at longer wavelengths.
5. Conclusions
Experimental investigation of 4,6-dihydroxypyrimidine (46dHP)
and 2-methyl-4,6-dihydroxypyrimidine (2m46dHP) monomers isolated
in low-temperature matrices revealed that the dihydroxy and oxo-hy-
droxy tautomeric forms of these compounds are populated in solid Ar,
Ne or n-H2. For both compounds in question, the energies of the di-
hydroxy and oxo-hydroxy forms are similar to each other. In the case of
2m46dHP, the energy difference is very small and, consequently, the
[oxo-hydroxy]:[dihydroxy] population ratio in freshly deposited ma-
trices is close to 1. For non-methylated 46dHP, the energy difference
6