Photochemistry of HNCO in Solid Xe
J. Phys. Chem. A, Vol. 103, No. 45, 1999 9161
After long photolysis, the yield is very small, indicating that
the new species does not correlate with NO, CO, or C2. It does
not correlate with NCO either because its maximum concentra-
tion occurs in annealing of the matrix before NCO reaches its
maximum.
the quartet structure of the ∼1800 cm-1 band, but the constant
ratio of the intensities of the four bands from experiment to
experiment suggests that the splitting is due to intrinsic
properties of the molecule rather than due to a site effect.
It is interesting to note the analogy to our previous experi-
ments with HCN in solid Xe and Kr.37 In those experiments,
we obtained H2CN from HCN by photolyzing the precursor
partially and mobilizing hydrogen atoms after photolysis in a
reaction H + HCN f H2CN. It appears that by controlled
photolysis and annealing of Xe matrices doped with small
hydrogen-containing molecules one can obtain interesting radical
species in a convenient way.
Formation of the new species correlates with the amount of
unphotolyzed HNCO, and we consider the products of H +
HNCO reaction. Actually, because the remaining concentration
of HNCO decreases typically by about 35% when hydrogen
atoms are mobilized, this reaction seems probable. Nguyen et
al. have studied by ab initio calculations the H + HNCO f
H2N + CO reaction and found two stable intermediates which
can be formed in an exothermic reaction corresponding to the
attachment of the hydrogen atom either to the oxygen or nitrogen
of HNCO.48 Formation of the oxygen bound form requires the
activation energy of about 100 kJ/mol, and even its thermody-
namical stability with respect to H + HNCO is questionable.
Therefore, we neglect this product in our consideration. Forma-
tion of the other configuration has an activation barrier of about
40 kJ/mol, and when the possibility of tunneling and uncertainty
in the computational results are taken into account, this reaction
seems possible in our matrix environment at about 50 K.
6. Conclusions
Photochemistry of HNCO is studied in Xe matrices by FTIR
and LIF methods. It is found that in the early stage main
products are H atoms, NCO, HXeNCO, HN-CO complexes,
and isolated CO and NH. Further photolysis leads to a
decomposition of NCO mainly into carbon atoms and NO and
to some extent into CN + O. Part of the carbon atoms move
-
and recombine to form C2, from which C2 is formed by
photoinduced charge transfer.
The radical H2NCO shown in Figure 8 is a global minimum
on the H + HNCO surface. Our calculated parameters are
presented in the figure, and they do not differ much from the
lower-level values of Nguyen et al.48 According to the calculated
spectrum (see Table 4), the four most intensive fundamentals
are exactly in the regions where the absorptions of the new
species are observed. For comparison, we have calculated the
spectrum of HNCO at the same level of theory. According to
those results, the asymmetric NH stretch of H2NCO is shifted
by +47 cm-1 from the corresponding HNCO value, and this is
In annealing of the photolyzed Xe matrix, new species are
formed upon a thermal mobilization of the isolated hydrogen
atoms. H2NCO radical is formed from the remaining precursor
HNCO by a reaction with a hydrogen atom. The IR spectrum
of H2NCO is presented experimentally for the first time. The
threshold for the photodecomposition of this radical lies between
365 and 405 nm. In addition, annealing leads to formation of
HCO, HXeNCO, HXeCN, HXeNC, and HXeH.
Acknowledgment. M. P. thanks the Academy of Finland
for the financial support. CSC-Center for Scientific Computing
is appreciated for providing computer mainframe time. Henrik
Kunttu is thanked for generously providing NO.
in a good agreement with the observed value of +39 cm-1
.
Additionally, our most intensive band is a quartet at the 1800
cm-1 region, and the same band is the strongest computationally.
The deuteration experiments support strongly the assignment
of the new species to H2NCO. For the strongest band, NCO
stretch, our calculations predict a shift of -0.9 cm-1 for
HDNCO and -3.2 cm-1 for D2NCO. Experimentally, we
observe several slightly shifted bands at 1793.9, 1796.3, 1797.8,
and 1800.2 cm-1. Although we cannot assign each band
specifically to HDNCO or D2NCO, the observed shifts are in a
good agreement with the computational predictions. A new weak
band at 3503.8 cm-1 is safely assigned to the N-H stretch of
HDNCO and the observed shift from the H2NCO band at 3518.5
cm-1 is -14.7 cm-1, in agreement with the calculated shift of
-25.7 cm-1. A band at 2634.4 cm-1 is assigned to the N-D
stretch of D2NCO. One more band at 1434.7 cm-1 is assigned
to the HDNCO analogue of the 1555.8 cm-1 band of H2NCO,
and the experimental shift of -121.1 cm-1 agrees well with
the computed shift of -129.5 cm-1. Other bands of the
deuterated forms are either too weak to be observed or
overlapped with other absorptions.
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Further support to the assignment is obtained by identifying
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On the basis of the facts presented above, we assign the new
bands to H2NCO. Currently, we do not have an explanation for