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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. March 2007, Volume 6, Number 3, 70-80 |
A DFT Study on the Low-Lying Excited States and Adiabatic Photodissociation Channels of Nitric Acid
Costantino Zazza and Luigi Bencivenni
Internet Electron. J. Mol. Des. 2007, 6, 70-80
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Abstract:
In this paper, the lowest valence electronic excited states of nitric acid (HNO3)
have been computed by means of density functional theory within its time
dependent formalism and compared with experimental data and previous first
principles calculations. Further, since nitric acid plays an important role in
atmosphere providing an important source of stratospheric OH radical, due to its
rapid photo-dissociation, the lowest adiabatic photo-dissociation channels have
been also studied. Time dependent density functional theory (TD-DFT) with
different functionals, in conjunction with different basis sets, has been applied to
calculate the vertical transition energies from the ground state to the low-lying
singlet and triplet electronic excitated states of nitric acid in vacuum. Moreover,
the adiabatic photo-dissociation channels of HNO3 molecule into OH· + NO2·
were investigated sampling the potential energy surfaces (PES) of the lowest
singlet excited states at TD-DFT(B3LYP)/aug-cc-pVQZ level of theory. This
work, one more time, confirms that the TD-DFT based calculations, together with
generalized gradient-corrected approximation (GGA), provide accurate and
physically consistent description of the lowest valence electronic excitation
transitions. Moreover, in line with our previous results on hexafluoropropene, the
comparison with more expensive calculations, such as MRDCI, CIS(D),
CASSCF, MCQDPT and CCSD, clearly shows the excellent performance of the
TD-DFT formalism with highly correlated systems with a limited extent of
multiple excitation character. Our TD-DFT results are in satisfactory agreement
with UV experimental absorption bands and in analogy with previous results
obtained by means of high level calculations. Interestingly, in this work we show
that at 248 nm, the potential energy barrier toward dissociation of the acid nitric
in OH + NO2 radical fragments strongly depends on the out-of-plane bending
vibration of the molecule in the 11A" excited state having a noticeable
contribution to the lowering of the barrier.
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