242
H. Ohtsu et al.
12.1.10 Identifying Cyclo-S 3 Species in the Pore
Theoretical calculations predicted that cyclo-S 3 is less stable than bent-S 3 , yet energetically accessible. However, its experimental observation had never been reported
before. Hoffman used DFT calculations to show that the neutral form of cyclo-S 3
is 5.6 to 9.3 kJ/mol less stable than bent-S 3 , depending on the calculation method
[82]. Therefore, the cyclic form was predicted to be a metastable state. Indeed, the
interactive pores were able to temporarily trap cyclo-S 3 as an intermediate species.
This result represents the first structure of a cyclic S 3 allotrope determined using
single-crystal X-ray diffraction. The cyclo-S 3 was stabilized through a chemical
bond formation to create an I-(cyclo-S 3
2+ ) complex. Furthermore, the dication of the
cyclic trimer is the isoelectric state to a cyclo-SiS 2 molecule isolated by matrix isolation, supporting the possibility of cyclo-S 3
2+ existence [83]. The structural analysis
of the metastable sulfur allotrope trapped by the pore of the coordination network
opens up new avenues for producing and characterizing unusual sulfur species.
12.1.11 Theoretical Investigation of Sulfur Allotropes
in the Pores
The ability of interactive pore sites to trap and stabilize small sulfur allotropes was
investigated using theoretical calculations. Since both physisorbed and chemisorbed
S 2 species appeared in the same structure, the energetics of the chemisorption process
and electronic spin changes were evaluated. To obtain this information, the energy
of the I
−
· · · S–S complex was calculated.
In a triplet electronic state, the S 2 molecule is only physisorbed onto the iodide.
The interaction energy of this intermolecular interaction was calculated to be
−29.2 kJ/mol with the I–S bond order of 0.1294. As a result, this intermolecular
interaction cannot be considered as a covalent bond and is more similar in strength
to a halogen bond. The formation of this weakly interacting complex involves charge
transfer from the iodide primarily to the furthest sulfur atom. The equilibrium distance
for this interaction is 3.317 Å and the resultant species has a bent geometry with a
I· · · S–S angle of 127.5°.
On the other hand, in the singlet electronic state (Table 12.1 and Fig. 12.16), the
S 2 molecule is chemisorbed onto the iodide. In the process, a covalent I–S bond is
formed with a bond order of 0.9293 and the bond distance of 2.602 Å. The geometry
of chemisorbed complex is also bent, with an I–S–S angle of 112.5°. In addition,
there is a considerable charge transfer from the iodide to the sulfur atoms. The I–S–S
Mulliken charges are −0.372, −0.144, and −0.484, respectively. The distant sulfur
atom receives the highest negative charge. From these results, it can be concluded that
a donor-acceptor bond forms between the iodide and the sulfur atom. However, the
total energy gain in creating this intramolecular bond is only −19.4 kJ/mol, almost
10 kJ/mol less stabilizing than physisorption.
H. Ohtsu et al.
12.1.10 Identifying Cyclo-S 3 Species in the Pore
Theoretical calculations predicted that cyclo-S 3 is less stable than bent-S 3 , yet energetically accessible. However, its experimental observation had never been reported
before. Hoffman used DFT calculations to show that the neutral form of cyclo-S 3
is 5.6 to 9.3 kJ/mol less stable than bent-S 3 , depending on the calculation method
[82]. Therefore, the cyclic form was predicted to be a metastable state. Indeed, the
interactive pores were able to temporarily trap cyclo-S 3 as an intermediate species.
This result represents the first structure of a cyclic S 3 allotrope determined using
single-crystal X-ray diffraction. The cyclo-S 3 was stabilized through a chemical
bond formation to create an I-(cyclo-S 3
2+ ) complex. Furthermore, the dication of the
cyclic trimer is the isoelectric state to a cyclo-SiS 2 molecule isolated by matrix isolation, supporting the possibility of cyclo-S 3
2+ existence [83]. The structural analysis
of the metastable sulfur allotrope trapped by the pore of the coordination network
opens up new avenues for producing and characterizing unusual sulfur species.
12.1.11 Theoretical Investigation of Sulfur Allotropes
in the Pores
The ability of interactive pore sites to trap and stabilize small sulfur allotropes was
investigated using theoretical calculations. Since both physisorbed and chemisorbed
S 2 species appeared in the same structure, the energetics of the chemisorption process
and electronic spin changes were evaluated. To obtain this information, the energy
of the I
−
· · · S–S complex was calculated.
In a triplet electronic state, the S 2 molecule is only physisorbed onto the iodide.
The interaction energy of this intermolecular interaction was calculated to be
−29.2 kJ/mol with the I–S bond order of 0.1294. As a result, this intermolecular
interaction cannot be considered as a covalent bond and is more similar in strength
to a halogen bond. The formation of this weakly interacting complex involves charge
transfer from the iodide primarily to the furthest sulfur atom. The equilibrium distance
for this interaction is 3.317 Å and the resultant species has a bent geometry with a
I· · · S–S angle of 127.5°.
On the other hand, in the singlet electronic state (Table 12.1 and Fig. 12.16), the
S 2 molecule is chemisorbed onto the iodide. In the process, a covalent I–S bond is
formed with a bond order of 0.9293 and the bond distance of 2.602 Å. The geometry
of chemisorbed complex is also bent, with an I–S–S angle of 112.5°. In addition,
there is a considerable charge transfer from the iodide to the sulfur atoms. The I–S–S
Mulliken charges are −0.372, −0.144, and −0.484, respectively. The distant sulfur
atom receives the highest negative charge. From these results, it can be concluded that
a donor-acceptor bond forms between the iodide and the sulfur atom. However, the
total energy gain in creating this intramolecular bond is only −19.4 kJ/mol, almost
10 kJ/mol less stabilizing than physisorption.