coworkers reported the structure classified into the group B at B3LYP/6-31++G**
level of DFT calculations [1]. We address here that their structure has a high
relative energy, ΔE ZPE = 2.1 kcal/mol, at MP2 level of ab initio calculations.
Our results clearly show that it is not necessary for the most stable geometry in
the monohydrate to maintain the most stable one of the core ion itself. To our
knowledge, the energetic stability of H 2 O-embedded form has not been reported so
far for NO
−
3 HNO 3
ð
Þ 2 H 2 O. Such energetic stability of the group C conformers can
be reasonably explained as resulting from the enhancement of polarization of the
embedded water molecule. In the structures (C) in Fig. 4, for instance, the NPA
charges on two hydrogen (δ H ) atoms of the embedded water molecule are +0.52
and +0.54, while those of the outer water molecule are +0.50 and +0.50 in the
structures (A) and (B) in Fig. 4. Such increment of the net charges on two hydrogen
atoms can strengthen the intermolecular HBs between H 2 O and other fragments.
(a)
(b)
(d)
(c)
Fig. 4 The most stable structures of NO
−
3 HNO 3
ð
Þ 2 H 2 O in the group A–D obtained with MP2/
6-31 ++G** calculations. The definitions of each group are given in the text. The ΔE ZPE (unit in
kcal/mol) means the relative energy with zero-point vibration correction from the most stable
structure. The value in the parenthesis is the maximum ΔE ZPE value in each group
Ab Initio Investigations of Stable Geometries …
199
level of DFT calculations [1]. We address here that their structure has a high
relative energy, ΔE ZPE = 2.1 kcal/mol, at MP2 level of ab initio calculations.
Our results clearly show that it is not necessary for the most stable geometry in
the monohydrate to maintain the most stable one of the core ion itself. To our
knowledge, the energetic stability of H 2 O-embedded form has not been reported so
far for NO
−
3 HNO 3
ð
Þ 2 H 2 O. Such energetic stability of the group C conformers can
be reasonably explained as resulting from the enhancement of polarization of the
embedded water molecule. In the structures (C) in Fig. 4, for instance, the NPA
charges on two hydrogen (δ H ) atoms of the embedded water molecule are +0.52
and +0.54, while those of the outer water molecule are +0.50 and +0.50 in the
structures (A) and (B) in Fig. 4. Such increment of the net charges on two hydrogen
atoms can strengthen the intermolecular HBs between H 2 O and other fragments.
(a)
(b)
(d)
(c)
Fig. 4 The most stable structures of NO
−
3 HNO 3
ð
Þ 2 H 2 O in the group A–D obtained with MP2/
6-31 ++G** calculations. The definitions of each group are given in the text. The ΔE ZPE (unit in
kcal/mol) means the relative energy with zero-point vibration correction from the most stable
structure. The value in the parenthesis is the maximum ΔE ZPE value in each group
Ab Initio Investigations of Stable Geometries …
199