temperature range. As shown in the inset figure in Fig. 5b, the inversion of the
relative abundance of the H 2 O-embedded and attached conformers occurs at around
270 K. At around this inversion temperature, the numerical advantage of H 2 O-attached conformers contributes to the inversion dominantly, because the relative
abundances of H 2 O-embedded conformers are larger than that of H 2 O-attached
conformers. The large relative abundances of H 2 O-attached conformers in the high
temperature region are due to the entropic contributions from slow molecular
vibrations relevant to the attached H 2 O molecule ranging from 20 to 30 cm
−1 , e.g.
H 2 O rocking mode.
4 Conclusion
We theoretically analyzed the stable geometries of the atmospheric negative core
ion, NO
−
3 HNO 3
ð
Þ 2 , and its monohydrate, NO
−
3 HNO 3
ð
Þ 2 H 2 O, at MP2/6-31++G**
level of ab initio calculations, and discussed the relative energetic stability of
conformers for both ionic clusters. We found a total of 15 and 90 kinds of different
conformers for NO
−
3 HNO 3
ð
Þ 2 and its monohydrate, respectively.
Unlike the previous DFT study by Drenck and coworkers [1], in the most stable
geometry at 0 K, the core ion has the hydrogen-bonded structure in which one
oxygen atom on NO
−
3 has two hydrogen bonds with each HNO 3 . For the monohydrate, the most stable geometry is the hydrogen-bonded structure in which the
H 2 O molecule is located at the center of a cluster (H 2 O-embedded form) rather than
the structure in which H 2 O is located outside of the cluster (H 2 O-attached form).
This means that the hydrogen-bonding network of the core ion can be strongly
perturbed by a single water molecule. Analyzing the theoretical temperature
dependence of relative abundances of conformers, we also confirmed that the
conformers having the H 2 O-attached form become dominant at high temperature
region above 270 K.
Acknowledgements The present study was supported by Grant-in-Aid for Scientific Research
and for Priority Areas by Ministry of Education, Culture, Sports, Science and Technology, Japan
(KAKENHI). A part of the present computations were performed using Research Center for
Computational Science, Okazaki, Japan.
References
1. Drenck K, Hvelplund P, Nielsen SB, Panja S, Støchkel K (2008) Int J Mass Spectrom
273:126–131
2. Yu F, Turco RP (2000) Geophys Res Lett 27:883–886
3. Fend J, Möller D (2004) J Atmos Chem 48:217–233
4. Harrison RG, Carslaw KS (2003) Rev Geophys 41:1012–1027
5. Singh A, Agrawal M (2008) J Environ Biol 29:15–24
Ab Initio Investigations of Stable Geometries …
201
relative abundance of the H 2 O-embedded and attached conformers occurs at around
270 K. At around this inversion temperature, the numerical advantage of H 2 O-attached conformers contributes to the inversion dominantly, because the relative
abundances of H 2 O-embedded conformers are larger than that of H 2 O-attached
conformers. The large relative abundances of H 2 O-attached conformers in the high
temperature region are due to the entropic contributions from slow molecular
vibrations relevant to the attached H 2 O molecule ranging from 20 to 30 cm
−1 , e.g.
H 2 O rocking mode.
4 Conclusion
We theoretically analyzed the stable geometries of the atmospheric negative core
ion, NO
−
3 HNO 3
ð
Þ 2 , and its monohydrate, NO
−
3 HNO 3
ð
Þ 2 H 2 O, at MP2/6-31++G**
level of ab initio calculations, and discussed the relative energetic stability of
conformers for both ionic clusters. We found a total of 15 and 90 kinds of different
conformers for NO
−
3 HNO 3
ð
Þ 2 and its monohydrate, respectively.
Unlike the previous DFT study by Drenck and coworkers [1], in the most stable
geometry at 0 K, the core ion has the hydrogen-bonded structure in which one
oxygen atom on NO
−
3 has two hydrogen bonds with each HNO 3 . For the monohydrate, the most stable geometry is the hydrogen-bonded structure in which the
H 2 O molecule is located at the center of a cluster (H 2 O-embedded form) rather than
the structure in which H 2 O is located outside of the cluster (H 2 O-attached form).
This means that the hydrogen-bonding network of the core ion can be strongly
perturbed by a single water molecule. Analyzing the theoretical temperature
dependence of relative abundances of conformers, we also confirmed that the
conformers having the H 2 O-attached form become dominant at high temperature
region above 270 K.
Acknowledgements The present study was supported by Grant-in-Aid for Scientific Research
and for Priority Areas by Ministry of Education, Culture, Sports, Science and Technology, Japan
(KAKENHI). A part of the present computations were performed using Research Center for
Computational Science, Okazaki, Japan.
References
1. Drenck K, Hvelplund P, Nielsen SB, Panja S, Støchkel K (2008) Int J Mass Spectrom
273:126–131
2. Yu F, Turco RP (2000) Geophys Res Lett 27:883–886
3. Fend J, Möller D (2004) J Atmos Chem 48:217–233
4. Harrison RG, Carslaw KS (2003) Rev Geophys 41:1012–1027
5. Singh A, Agrawal M (2008) J Environ Biol 29:15–24
Ab Initio Investigations of Stable Geometries …
201