Chapter 6
Theoretical Prediction of A New Type of Noble Gas Polymer
Containing the XeO3CC Units
Abstract
We have predicted a new type of xenon containing noble-gas polymer. The
general formula of these molecules is R(XeO3CC)nXeO3R , where R = H or F. The
MP2/aug-cc-pVDZ calculations showed the energies of F(XeO3CC)nXeO3F relative
to the most stable electronic state of F, XeO3 and CC fragments were 100.74,
131.79 and 162.93 kcal/mol for n = 1, 2 and 3 respectively. The stability was also
confirmed by CCSD(T)/aug-cc-pVTZ single point calculation for HXeO3CCXeO3H.
Regardless the positions in the molecules, the XeC and XeO bond distances are
fairly constant. By extrapolation it is reasonable to presume that the Xe-containing
polymers R(XeO3CC)nXeO3R would be stable and are good candidates for future
experimental synthesis.
182
Introduction
Xeon is the most chemically active noble-gas element in nature. Since the first
Xe compound XePtF6 had been synthesized in 1862 by Bartlett et al,1 a large variety
of noble gases containing compounds have been found in various laboratories.
Traditionally, stable Xe compounds are chemically bonded to electronegative atoms,
such as fluorine or oxygen atoms.2 Räsänen and coworkers3 have found many this
type xenon containing molecules of the type HXeY (where Y is usually an
electronegative group such as Cl, Br, I, OH, CN, NC, CCH, SH, NCO, etc.) using
matrix isolation/photolysis techniques. In particular,
the synthesis of the HXeCN
molecule in 1998 has suggested a novel way to build up the XeC bonding containing
noble-gas compound. The concept of Xe polymer was firstproposed by Gerber and
coworkers.4 They predicted that HXeCCXeCCXeH molecule was a metastable
species with the XeC bond lengths of ~2.3 Å. In 2003, Räsänen and coworkers
synthesized HXeCCH, HXeCC, and HXeCCXeH by photolysis and annealing of
C2H2/Xe solids. The HXeCCXeH molecule was the first synthesized noble-gas
compound containing two Xe atoms. It seems reasonable by extrapolation that the
Xe-containing polymers of similar types would be stable.
However, the XeC bond lengths are sensitive to the positions in the
HXeCCXeCCXeH from theoretical calculation. The terminal XeC bonds are longer
than that in the central part of the molecule. That could cause low stability for
large-sized H(XeCC)n XeH. In this study, we predicted a new type of noble gas
polymer containing XeO3CC units. The predicted the XeC bond lengths of
HXeO3CCH and HXeO3CCXeO3H were significantly shorter than that of HXeCCH
and HXeCCXeH. The XeC bond lengths were found to be insensitive to the
183
positions in the molecules. We also replaced the terminal H atoms by F atoms, the
results showed the F(XeO3CC)n XeO3F molecules were even more stable than the
H(XeO3CC)n XeO3H molecules.
184
Method
The molecular geometry was calculated using the MP25 theory and the hybrid
density functional theory B3LYP6 with the aug-cc-pVDZ basis sets for H, C, O and F
atoms. 7 For Xe atoms, the aug-cc-pVDZ-pp basis set was used. The “pp” means that
a pseudo-potential was used to replace the core electrons. Single-point energy
calculation was also performed at CCSD(T)/aug-cc-pVTZ(-pp) level using the MP2
geometry. For brevity, the basis sets will just be described as aug-cc-pVnZ (n = D, T)
for the rest of this article, and they are abbreviated as apnz in all the Tables. The
vibrational frequencies were calculated using the same level of theory for geometry
optimization. All calculations were performed using the Gaussian 03 program. 8 The
keyword ”Integral(Grid=UltraFine)“ was used in some DFT calculations to prevent
the numerical instability during geometry optimization.
185
Results and Discussion
(a) Geometry
For comparison, we started with the study of the HXeCCH, HXeCCXeH and
H(XeCC)2XeH molecules. The calculated structures of these molecules are shown in
the Figure 1 and 2. The geometry of HXeCCH, HXeCCXeH and H(XeCC)2XeH are
all linear. The calculated HXe and XeC bond length of HXeCCH are 1.751 and
2.349 Å at MP2/aug-cc-pVDZ level, which are similar to the corresponding values
1.767 and 2.351 Å by the CCSD(T)/LJ18/6-311++G(2d,2p) method in Gerber’s study.
At MP2/aug-cc-pVDZ level, the predicted XeH bond length of HXeCCXeH is ~0.03
Å longer and XeC bond length is ~0.01 Å shorter than that of the HXeCCH
molecule, respectively. The calculated structures of H(XeCC)2XeH showed the
terminal XeC bond is longer than the central XeC bond by ~0.1 Å. That is also
consistent with Gerber’s work.
For HXeCCH, FXeCCH and HXeCCXeH, we also performed the the geometry
optimization at the CCSD(T)/aug-cc-pVTZ level. The calculated HXe, XeC, CC
and CH bond length of HXeCCH are 1.755, 2.353, 1.226 and 1.066 Å, respectively.
For the HXeCCXeH, the XeC bond length predicted by MP2/aug-cc-pVDZ method
was in very good agreement with that by the CCSD(T)/aug-cc-pVTZ method with
difference only ~0.001 Å. However, the predicted XeC bond length of HXeCCH is
2.376 Å by B3LYP/aug-cc-pVDZ method, that is significantly shorter than that by the
CCSD(T)/aug-cc-pVTZ method of 0.023 Å. In the FXeCCH and HXeCCXeH,
MP2/aug-cc-pVDZ method also predicted HXe, FXe and XeC bond lengths better
agreement with the higher level CCSD(T)/aug-cc-pVTZ method. The
MP2/aug-cc-pVDZ method seems be a better method on predicting the bond lengths
186
that connect the Xe atom to other atoms. Thus, we will use the MP2/aug-cc-pVDZ
structures for other molecules in the following discussion.
In the current study, we replaced the terminal H in the HXeCCH, HXeCCXeH
and H(XeCC)2XeH by F atoms. The calculated structures of these molecules are also
shown in the Figures 1 and 2. The predicted XeC bond length of FXeCCH is ~0.25
(0.27) Å shorter than in the HXeCCH molecule at MP2/aug-cc-pVDZ
(CCSD(T)/aug-cc-pVTZ) level. The substitution of H atom by F atom significantly
shorten the terminal XeC bond. In the FXeCCXeH, the calculated XeC bond on the
the F side is also shorter than that on the H side by ~0.28 Å at MP2/aug-cc-pVDZ
level. For the FXeCCXeF, the XeC bond lengths are 2.106 Å which is also
significantly shorter (by 0.13 Å) than that in HXeCCXeH. It is noted that the central
XeC bond in H(XeCC)2XeH is ~0.10 Å shorter than the terminal XeC bond.
However, the central XeC bond in (XeCC)2XeF is 0.13 Å longer than the terminal
XeC bond. The FXeCCXeF and F(XeCC)2XeF also have slightly shorter CC bond
than the HXeCCXeH and H(XeCC)2XeH. Similar trends in XeC bond lengths were
also founded in the H(XeCC)3XeH, F(XeCC)3XeH and F(XeCC)3XeF.
The calculated structures of HXeO3CCH, FXeO3CCH and FXeO3CCF are
shown in the Figure 3. Compare to the HXeCCH, the HXe, XeC and CC bonds of
the HXeO3CCH are all shorter than that in the HXeOCCH. The XeC bond length of
HXeO3CCH is only 2.071 Å. For HXeO3CCH and FXeO3CCH, we found that the
XeC bond length is quite insensitive to the identity of the terminal atom. The XeC
bond lengths of HXeO3CCH and FXeO3CCH differs by only ~0.02 Å, which is
significantly less than the difference (~0.25 Å) between HXeCCH and FXeCCH.
The calculated structures of R(XeO3CC)nXeO3R are shown in Figure 4. The
187
predicted XeC bond length of HXeO3CCXeO3H is 2.089 Å at MP2/aug-cc-pVDZ
level, which is significantly shorter than the XeC bond of HXeCCXeH of 2.339 Å.
In H(XeO3CC)2XeO3H molecule, the central XeC bond is only 0.014 Å shorter than
the terminal XeC bond. However, the XeC bond lengths of different positions in
H(XeCC)2XeH differes by 0.094 Å. That also indicated the XeC bond is insensitive
to the change of positions in these XeO3 containing molecules. The XeC bond
lengths in the different position of H(XeO3CC)3XeO3H are also nearly the same, as in
the H(XeO3CC)2XeO3H. The XeH and CC bond lengths, which are ~1.66 and
~1.24 Å, respectively, are almost constants as the size of the molecule increases. The
XeO bond lengths are also fairly constant, 1.781.79 Å, in these molecules.
(b) Stability
The relative energies of RXeCCH (R = H or F) to the H (F) + Xe + CCH at
various theoretical levels were listed in Table 1. The dissociation energy of FXeCCH
to F + Xe + CCH at CCSD(T)/aug-cc-pVTZ level were found be 66.0 kcal/mol,
which is significantly higher than that of HXeCCH (~33 kcal/mol). The
B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ methods predicted similar results for
these two molecules. The dissociation energies of HXeCCH and FXeCCH calculated
at CCSD(T)/aug-cc-pVTZ level using the MP2 and B3LYP structures are both very
close to the energies using the CCSD(T)/aug-cc-pVTZ geometry. This indicated that
accurate dissociation energies can be obtained using structures calculated at more
economical levels.
Table 2 shows the relative energies of R(XeCC)nXeR (R = H or F) to their
dissociation products. The CCSD(T)/aug-cc-pVTZ single point energies of
188
HXeCCXeH calculated using MP2 and B3LYP structures are also very similar. Thus,
for the other middle sized molecules that the CCSD(T)/aug-cc-pVTZ single point
calculations can be performed, we used the structures calculated at the
MP2/aug-cc-pVDZ level.
Table 2 shows the calculated dissociation energies of R(XeCC)nXeR. The
dissociation energy of FXeCCXeH is higher than that of HXeCCXeH (~37 kcal/mol)
at CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVDZ level. The dissociation energy of
FXeCCXeF also increased ~29 kcal/mol from the FXeCCXeH at the same theoretical
level. Both for the H(XeCC)nXeH and F(XeCC)nXeF, the
CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVDZ calculations shows the dissociation
energies increased ~35 kcal/mol with with additional XeCC unit.
B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ methods predicted similar results for
these molecules.
The relative energies of RXeO3CCH (R = H or F) to their dissociation species
are shown in Table 3. The dissociation energies of HXeO3CCH and FXeO3CCH are
42.01 and 64.64 kcal/mol at CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVDZ level,
respectively. The dissociation energy of HXeO3CCH was higher than the HXeCCH
by 9.16 kcal/mol at the same theoretical level. Replacing the terminal H atom by an F
atom in the HXeO3CCH also increases the stability by 22.6 kcal/mol.
Table 4 shows the calculated dissociation energies of R(XeO3CC)nXeO3R (R =
H or F). We found that the dissociation energies of H(XeO3CC)nXeO3H were higher
than those of H(XeCC)nXeH molecules. For example, the dissociation energy of
HXeO3CCXeO3H is 61.1 kcal/mol at CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVDZ
level, which is higher than that of HXeCCXeH by 18.2 kcal/mol. The
189
MP2/aug-cc-pVDZ calculations also gave the same conclusions. However, the trend
was predictly differently for the F(XeO3CC)nXeO3F molecules. The dissociation
energy of the FXeO3CCXeO3F was slightly lower than the FXeCCXeF by ~10
kcal/mol. It is remarkable that the dissociation energy increases by 85.9 kcal/mol from
HXeCCXeH to FXeCCXeF at MP2/aug-cc-pVDZ level, but it only increases 49.2
kcal/mol from HXeO3CCXeO3H to FXeO3CCXeO3F. The dissociation energy is less
sensitive to terminal atoms for R(XeO3CC)nXeO3R molecules. This result was
consistent with the comparison of XeC bond lengths in R(XeO3CC)nXeO3R
molecules mentioned above. Base on the general concept of polymers, the chemical
environment of each structural unit in a polymer are almost equivalent. Thus,
R(XeO3CC)nXeO3R molecules with fairly constant XeC bond distances regardless
the positions or terminal atoms, are good candidates for synthesis of polymers.
For the HXeCCXeH, there is a barrier height of ~50 kcal/mol along the HXeC
bending coordinate for the process HXeCCXeH  Xe + HCCXeH at
MP2/aug-cc-pVDZ level. And for HXeO3CCH and FXeO3CCH, barrier heights of
~30 kcal/mol along the XeCC bending coordinate to the HXeO2…OCCH and
FXeO2…OCCH at MP2/aug-cc-pVDZ level were also found. With such barriers,
decay by the bending is totally negligible even at the room temperature.
190
Conclusion
We have predicted a new type of novel xenon-containing molecules
R(XeO3CC)nXeO3R. The predicted the XeC bond lengths of R(XeO3CC)nXeO3R
were significantly shorter than that of R(XeCC)nXeR. At the same n, the dissociation
energies of H(XeO3CC)nXeO3H were higher than those of H(XeCC)nXeH molecules
by ~27 kcal/mol, and the dissociation energy of the F(XeO3CC)nXeO3F was slightly
lower than the F(XeCC)nXeF by ~10 kcal/mol. However, the XeC bond length is
sensitive to the positions in the R(XeCC)nXeR, but it is fairly constant in
R(XeO3CC)nXeO3R molecules regardless the positions. The MP2/aug-cc-pVDZ
calculations shows the dissociation energies increased ~31 kcal/mol with additional
XeO3CC unit. It is reasonable to presume that the Xe-containing polymers
R(XeO3CC)nXeO3R would be stable and are good candidates for future experimental
synthesis.
191
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193
Table 1
The relative energies of RXeCCH (R = H or F) to R + Xe + CCH
B3LYP/apdz
MP2/apdz
CCSD(T)/aptz
HXeCCH
29.9
36.3
32.9 (32.9)a [32.8]b
FXeCCH
61.7
79.4
66.0 (65.8)a [65.6]b
related to H (F) + Xe + CCH
aSingle
point calculation using MP2/apdz structure.
bSingle
point calculation using B3LYP/apdz structure.
194
Table 2
The relative energies of R(XeCC)nXeR (R = H or F) to the dissociation fragments
B3LYP/apdz
MP2/apdz
CCSD(T)/aptz
HXeCCXeH
35.2
24.7
42.9 (42.9)a [42.9]b
H(XeCC)2XeH
61.2
56.4
(76.9)a
H(XeCC)3XeH
86.7
87.5
NAd
FXeCCXeH
71.6
71.4
80.2)a
F(XeCC)2XeH
96.1
101.3
NAd
F(XeCC)3XeH
120.7
131.6
NAd
FXeCCXeF
99.8
110.6
109.5)a
F(XeCC)2XeF
126.4
142.7
144.3)a
F(XeCC)3XeF
152.1
174.0
NAd
related to 2 H + n Xe + n CCc
related to H + F + n Xe + n CCc
related to 2 F + n Xe + n CCc
aSingle
point calculation using MP2/apdz structure.
bSingle
point calculation using B3LYP/apdz structure.
cRelated
dDue to
to the ground state CC, that is triplet state in B3LYP method and singlet state in MP2 method.
the large computational cost or less important.
195
Table 3
The relative energies of RXeO3CCH (R = H or F) to the dissociation fragments
B3LYP/apdz
MP2/apdz
CCSD(T)/aptza
HXeO3CCH
31.4
50.5
42.0
FXeO3CCH
50.4
76.6
64.6
related to
H (F) + XeO3 + CCH
aSingle point
calculation using MP2/apdz structure.
Table 4
The relative energies of R(XeO3CC)nXeO3R (R = H or F) to the dissociation
fragments
B3LYP/apdz
MP2/apdz
CCSD(T)/aptz
HXeO3CCXeO3H
38.3
51.6
61.1)a
H(XeO3CC)2XeO3H
53.1
82.8
NAd
H(XeO3CC)3XeO3H
67.9
114.0
NAd
FXeO3CCXeO3F
73.0
100.7
NAc
F(XeO3CC)2XeO3F
87.9
131.8
NAc
F(XeO3CC)3XeO3F
102.3
162.9
NAc
related to 2 H + n XeO3 + m CCb
related to 2 F + n XeO3 + m CCb
aSingle point
bRelated
calculation using MP2/apdz structure.
to the most stable electronic state CC, that is triplet state in B3LYP/apdz
method and singlet state in MP2/apdz method.
cDue to
the large computational cost.
196
Figure 1
The calculated structures of RXeCCH (R = H or F) at B3LYP/aug-cc-pVDZ (upper, in parentheses), MP2/aug-cc-pVDZ (upper) and
CCSD(T)/aug-cc-pVTZ level (lower)
(a) HXeCCH, C∞v symmetry
(b) FXeCCH, C∞v symmetry
197
Figure 2
The calculated structures of R(XeCC)nXeR (R = H or F) at MP2/aug-cc-pVDZ level
(a) HXeCCXeH, D∞h symmetry (at CCSD(T)/aug-cc-pVTZ level in parentheses)
(b) FXeCCXeH, C∞v symmetry
(c) FXeCCXeF, D∞h symmetry
198
(d) H(XeCC)2XeH, D∞h symmetry
(e) F(XeCC)2XeH, C∞v symmetry
(f) F(XeCC)2XeF, D∞h symmetry
199
(g) H(XeCC)3XeH, D∞h symmetry
(h) F(XeCC)3XeH, C∞v symmetry
(i) F(XeCC)3XeF, D∞h symmetry
200
Figure 3
The calculated structures of RXeO3CCH (R = H or F) at MP2/aug-cc-pVDZ level
(a) HXeO3CCH, C3v symmetry
(b) FXeO3CCH, C3v symmetry
201
Figure 4
The calculated structures of R(XeO3CC)nXeO3R (R = H or F) at MP2/aug-cc-pVDZ level
(a) HXeO3CCXeO3H, D3h symmetry
(b) H(XeO3)3(CC)2H, D3d symmetry
202
(c) H(XeO3)4(CC)3H, D3h symmetry
(d) FXeO3CC XeO3F, D3h symmetry
203
(e) F(XeO3)3(CC)2F, D3d symmetry
(f) F(XeO3)4(CC)3F, D3h symmetry
204