Chapter 3
Theoretical Prediction of Stable Noble-Gas Anions XeNO2 and
XeNO3 with very Short Xenon-Nitrogen Bond Lengths
Abstract
We have predicted a new type of noble-gas anions, XeNO2 and XeNO3 with very
short XeN bond lengths (~1.8 Å) using high-level electronic structure theory with extended
atomic basis sets. The chemical bonding between xenon and nitrogen atoms could formally
be assigned as triple bonds. The best estimates of the atomization energies of the two anions
were found to be 50 and 101 kcal/mol, respectively, and the lowest unimolecular dissociation
barriers were estimated to be approximately 42 kcal/mol. These anions were predicted to be
kinetically stable at low temperature. The possible neutral “salts” formed between the lithium
cation and these two anions were also discussed.
69
Introduction
Xeon is the most chemically active noble-gas element in nature. Bartlett synthesized the
first Xe compound XePtF6 in 1962.1 Since then, a large variety of Xe-containing compounds
have been made in various laboraories.2,3 More recently, Räsänen and coworkers4 have
observed many small Xe-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 the last decade, several molecules formed from lighter
noble gas atoms have also been experimentally identified such as HArF,5 HKrF,6 HKrCl,4a
HKrCN4g and HKrCCH.7 Xenon is also known to form ligands to transition-metal ions8 and
to form actinide complexes with the CUO molecule.9 Among the known Xe compounds, in
most cases the Xe atom is chemically bonded to fluorine or oxygen.2,3
The first compound
that contains the XeN bond, FXeN(SO2F)2, was synthesized in 1974 by LeBlond and
DesMarteau.10 The study on the XeN compounds was notably followed by Schrobilgen and
coworkers.11,12 In xenon oxides (e.g., XeO3 and XeO4),13-15 xenon form double bonds with
oxygen atoms with very short bond lengths of 1.71.8 Å. Recently, we have proposed that the
all noble-gas atoms can form very stable anions of the type FNgO (Ng = noble gas)16 where
the Ng=O bonds are stabilized by the polarizing effects of the fluoride ion, and the FXeO
might have been observed in experiment.17 One question naturally arises is whether xenon
can form multiple bonds with other elements besides oxygen? While it may seem improbable,
triple bonds between noble gas and nitrogen atoms have been proposed in the literature.3,18
Experimentally, the XeN bond lengths from X-ray diffraction in currently known
compounds are in the range of 2.02.7 Å, with the shortest bond of 2.02 Å in
[XeN(SO2F)2][Sb3F16],11c and all the XeN bonds seem to be single bond in nature.10-12 In
the current study, we seek computationally small molecules that might contain XeN
70
multiple bonds. Very simple molecules such as HXeN, FXeN, XeN+, NXeO were all found
to be unstable and showed no signs of strong XeN multiple bonds. However, when we tried
the anions XeNO2 and XeNO3 which are isoelectronic species of the stable XeO3 and
XeO4 molecules, we were surprised to find that the XeN bond lengths in the calculated
structures of these anions were very short (~1.8 Å). Thus we followed with detailed study
using high-level electronic structure calculation on these anions, which will be documented in
the following sections of this article.
Method
The molecular geometry was calculated using the MP219 and CCSD(T)20 methods and the
hybrid density functional theory B3LYP21 and MPW1PW9122 with aug-cc-pVDZ-pp and
aug-cc-pVTZ-pp basis sets.23 Single-point energy calculation was also performed at
CCSD(T)/aug-cc-pVTZ-pp and CCSD(T)/aug-cc-pVQZ-pp levels. The “pp” means that a
pseudo-potential was used to replace the core electrons of the Xe atom. For brevity, the basis
sets will just be described as aug-cc-pVnZ (n = D, T, Q) 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. To check if the multireference character
is important, full-valence CASPT224 energy and geometry optimization calculation was also
carried out for XeNO2. For transition state (TS) structure calculations that are not
computationally tractable using the CCSD(T) theory, the structures obtained at the
MP2/aug-cc-pVDZ level was used in the single-point calculation since previous studies16,25
suggested that for many noble-gas containing molecules geometry calculated at this level is
better than those obtained at the MP2/aug-cc-pVTZ level. The MP2, CCSD(T), and DFT
calculation was performed using the Gaussian 03 program,26 and the CASPT2 calculation
71
was performed using the Molpro 2009.1 program.27 We have also attempted an AIM28
analysis of the electron density. However, the AIM program in our Gaussian 03 program does
not support the use of effective core potentials, so the AIM analysis can not be done
automatically. Instead, we exported the calculated electron density, density gradients, and the
density laplacians from Gaussian calculations. The bond critical points29 were then located
manually by finding the positions with zero density gradients.
Results and Discussion
(1) XeNO2
The calculated structure of XeNO2, as depicted in Figure 1, is a triangular pyramid
with Cs symmetry. Table 1 shows the calculated bond lengths and angles at various
theoretical levels. At CCSD(T)/aug-cc-pVTZ level, the XeN bond length is 1.825 Å which
is significantly shorter than the XeO bond length of 1.860 Å. Calculation at CASPT2/
aug-cc-pVTZ level showed similar result with the XeN bond length of 1.849 Å. All
calculation in Table 1 predicted the XeN bond is shorter than XeO bond by approximately
0.04 Å. The predicted NXeO bond angle is also slightly larger than the OXeO angle at
all levels of theory. While the concept of the bond order is not very well-defined in quantum
chemistry, if one bases on the most plausible Lewis structure and on the predicted bond
lengths, the bonding between xenon and nitrogen atoms could still be tentatively assigned as
a triple bond. In comparison, in the HXeNC molecule, the predicted XeN single-bond length
is 2.3 Å by MP2 theory. 4g In a previous study by Schrobilgen et al.,12a the predicted XeN
bond length in the F5TeN(H)Xe+ ion at MP2/aug-cc-pVDZ level was 2.101 Å which was in
good agreement with experimental value of 2.044 Å. Thus, the theory applied in the current
study should also be able to give reasonably accurate estimation of the XeN bond lengths.
72
Using the aug-cc-pVTZ basis set, both MP2 and MPW1PW91 methods predicted
significantly shorter XeN bond lengths than that by the CCSD(T)/aug-cc-pVTZ theory
which should give the most accurate geometry. The calculated relative energies to the triplet
state (at the singlet structure), the most stable atomic species, and three sets of unimolecular
dissociation products on the singlet-state surface, and the transition states (TS) for two of the
unimolecular dissociation channels at various theoretical levels are listed in Table 2. At the
singlet structure of XeNO2, the triplet state is approximately 3 eV higher in energy, and thus
the calculated singlet state is the ground electronic state. Table 2 also shows that XeNO2 is
approximately 50 kcal/mol lower in energy than the most stable atomic species (ground-state
xenon, nitrogen, oxygen atom, and oxygen anion) at the CCSD(T)/aug-cc-pVQZ level. This
stabilization energy can also be described as the total atomization energy (TAE) of the anion.
This value is very similar to the calculated TAE of XeO3 (54 kcal/mol) at the same level of
theory. The unimolecular dissociation of XeNO2 to XeO + NO was found to have a
significant barrier of 42 kcal/mol at CCSD(T)/aug-cc-pVQZ level. As shown in Table 2, all
other theoretical methods predicted similar barrier heights. The calculated structure of the
transition state (TS) for this dissociation channel at MP2/aug-cc-pVDZ level is depicted in
Figure 2 where one of the NXeO bond angle decreases significantly from 113 degrees in
XeNO2 to 71 degrees in the TS. Another possible dissociation channel is to XeN + O2 (or
Xe + N + O2). The calculated barrier for this channel (67.6 kcal/mol) is approximately 25
kcal/mol higher than the XeO + NO channel. While the global energy minimum of the
system corresponds to Xe + NO2, it is unlikely that XeNO2would dissociate to the global
minimum through a lower energy path since all the XeN and XeO bonds would have to be
broken at the same time. Despite of extensive search, we could not locate a transition state
73
which corresponds to the direct dissociation to Xe + NO2. To our knowledge, there are no
other exoergic unimolecular dissociation channels on the singlet-state surface. Of course,
dissociation can occur following an intersystem crossing to the triplet state16 (see Table 2).
For example, the dissociation of XeNO2to XeO and triplet NO is exoergic by 57 kcal/mol.
However, by calculating the ST gap along the reaction paths (structures calculated at
MP2/aug-cc-pVDZ level and energies at CCSD(T)/aug-cc-pVTZ level) of the dissociation
reactions to XeO + NO and to XeN + O2, we estimated that for the intersystem crossing to
occur, the molecule would have to overcome a barrier at least as high as the dissociation
barriers on the singlet state. For example, we have located an ST crossing point along the
reaction path of the first dissociation channel (to XeO + NO). The geometry of the crossing
point is shown in Figure 3, and it is located on the far side of the exit channel. To reach this
point, the system has to first overcome the 42 kcal/mol barrier in the entrance channel on the
singlet state. Another crossing point was also located on the reaction path of the second
dissociation channel (to XeN + O2). It was found close to the TS on the singlet state and
require even higher energy (68 kcal/mol) to reach. (We noted that the crossing points
described above were located in an intuitive way and may not include the minimum-energy
ST crossing point which is in general very difficult to locate precisely on the entire potential
energy surface.) Direct dissociation into Xe + O(T) + NO(T) is highly exoergic but very
unlikely due to the large ST gap at the singlet minimum energy structure. Thus, the
gas-phase XeNO2 anion was predicted to be kinetically stable at low temperature.
The spin-orbital coupling could certainly have some effects on the ST gaps, relative
energies, and molecular geometry; and these in principle can be estimated using a
full-electron relativistic treatment.30 In practice, however, the sizes of the current systems
would make the calculation extremely difficult. In fact, the triplet character of the current
74
system comes mostly from the oxygen atom, and the NO or the O2 moiety. The typical
spin-orbital energies of these species are on the order of 1 kcal/mol or less.31 Thus, to a good
approximation, the spin-orbital coupling would not affect the calculated results significantly
in the current work.
(2) XeNO3
The calculated structure of XeNO3, as depicted in Figure 4, is a tetrahedron with the
C3v point group. Table 3 shows the calculated bond lengths and angles at various theoretical
levels. At CCSD(T)/aug-cc-pVTZ level, the XeN bond length is 1.800 Å and the XeO
bond length is 1.810 Å, both are shorter than those in XeNO2. The predicted NXeO bond
angle is significantly larger (~13 degrees) than the OXeO angle at all levels of theory. The
calculated relative energies to the triplet state, the most stable atomic species, four sets of
unimolecular dissociation products on the single-state surface, and the transition state of the
unimolecular dissociation channel to XeO2 + NO at various theoretical levels are listed in
Table 4. At the CCSD(T)/aug-cc-pVQZ level, the TAE of XeNO3 (101 kcal/mol) increases
by 100% as compared to that of XeNO2. This is reflected in the significantly shorter XeN
and XeO bond lengths. The triplet state is found to be 2.5 eV higher in energy at the
CCSD(T)/aug-cc-pVQZ level. The barrier for the dissociation to XeO2 + NO was calculated
to be 43 kcal/mol at CCSD(T)/aug-cc-pVQZ level, which is very similar to the barrier for
XeNO2  XeO + NO in Table 2. As shown in Table 4, all other theoretical methods
predicted similar barrier heights except that the MP2/aug-cc-pVTZ calculation predicted a
much higher barrier of 61 kcal/mol. The dissociation to XeNO + O2 is not considered here
because XeNO is not an energy minimum at the CCSD(T)/aug-cc-pVTZ level. The
75
dissociation channels to Xe + O2 + NO, XeO + NO2, and Xe + NO3 are unlikely to
proceed by a concerted step (these would require breaking at least three bonds at the same
time), and should have barriers at least as high as the XeO2 + NO channel. Despite of
extensive search, we could not locate other lower-energy transition states for the exoergic
dissociation channels other than XeO2 + NO. In fact, Table 4 shows that the XeO2 + NO
channel is predicted to be almost isoergic at the highest level of theory. Dissociation
following an intersystem crossing is possible (see Table 4). For example, the dissociation of
XeNO3to XeO2 and triplet NO is exoergic by 23 kcal/mol. We have located an ST
crossing point along the reaction path of the dissociation channel to XeO2 + NO. The
geometry of the crossing point is shown in Figure 5, and it is located very close to the TS on
the singlet state. To reach this point, the system has to first overcome the 43 kcal/mol barrier
on the singlet state. Direct dissociation into Xe + NO(T) + O2(T) is highly exoergic but very
unlikely due to the large ST gap at the singlet minimum energy structure. Thus, the
gas-phase XeNO3 anion was also predicted to be kinetically stable at low temperature.
76
(3) Vibrational Frequencies
The calculated harmonic vibrational frequencies of XeNO2 and XeNO3are listed in
Tables 5 and 6. The XeN stretching frequencies are calculated around 1000 cm1 in both
anions at the MP2 level and are 200 cm1 higher than the XeO stretching modes. This is
consistent with the calculated shorter XeN bond lengths. Thus for possible future
experimental identification, the “fingerprint” peak of the XeN stretching should be
well-separated from the XeO stretching modes. The frequencies of XeNO2 were also
calculated numerically using the CCSD(T)/aug-cc-pVTZ method, and the XeN and XeO
stretching frequencies were found to be 100200 cm1 lower than the MP2 values. However,
the predicted XeN stretching frequencies are also 150200 cm1 higher than the XeO
stretching modes, which is consistent with the MP2 results. (The frequencies of XeNO3
were not calculated at the CCSD(T)/aug-cc-pVTZ level due to resource limitation.) The
predicted IR intensities by the MP2 method are also shown in the tables. For XeNO2 the
XeN peak intensity is 1215% of the strong XeO peaks. Thus the XeN peak, although
relatively weak, should still be observable in a clean IR spectrum. For XeNO3 the XeN
peak intensity is approximately 20% of the strongest XeO peaks, and it should be relatively
easier to identify.
(4) XeNO2Li
The calculated structures of XeNO2Li, are depicted in Figure 6. Two minimum-energy
structures were located as shown in the Figure. The structure (a) in which the Li atom resides
among the two equivalent oxygen atoms (Cs symmetry) was found to be the lowest-energy
structure. The structure (b) in which the Li atom resides between the one oxygen atom and
77
the nitrogen atom was found to be approximately 19 kcal/mol higher in energy than (a). For
structure (a), at CCSD(T)/aug-cc-pVTZ level, the XeN bond length is 1.812 Å and the
XeO bond length is 1.906 Å. Upon forming the “salt”, the XeN bond length stays almost
the same as in XeNO2 while the XeO bond lengths increase by ~0.05 Å. The calculated
relative energies for the most stable form of XeNO2Li at various levels of theory are listed in
Table 7. The TAE increases significantly due to the ionic bonding between the oxygen and
lithium atoms. The unimolecular dissociation barrier toward Xe + NO + LiO is estimated to
be 31 kcal/mol at CCSD(T)/aug-cc-pVQZ level. As shown in Table 7, all other theoretical
methods predicted similar barrier heights. Thus this xenon salt is also predicted to be, to a
lesser extent, kinetically stable at low temperature.
(5) XeNO3Li
The calculated structures of XeNO3Li, are depicted in Figure 7. Three minimum-energy
structures were located as shown in the Figure. The structure (a) in which the Li atom resides
between two oxygen atoms (Cs symmetry) was found to be the lowest energy structure. The
structure (b) in which the Li atom resides among the three equivalent oxygen atoms (C3v
symmetry) was found to be approximately 6 kcal/mol higher in energy than (a). The structure
(c) in which the Li atom resides between the one oxygen atom and the nitrogen atom was
found to be approximately 14 kcal/mol higher in energy than (a). For structure (a), at
CCSD(T)/aug-cc-pVTZ level, the XeN bond length is 1.791 Å and the XeO bond lengths
are 1.851 and 1.794 Å. Upon forming the “salt”, the XeN bond length stays almost the same
as in XeNO3while two of the XeO bond lengths increase by ~0.04 Å. The calculated
relative energies for the most stable form of XeNO3Li at various levels of theory are listed in
Table 8. The energy barrier of the unimolecular dissociation to XeO + NO + LiO is estimated
78
to be 34 kcal/mol from structure (a) at CCSD(T)/aug-cc-pVQZ level. As shown in Table 8,
all other theoretical methods predicted similar barrier heights except that the
MP2/aug-cc-pVTZ calculation predicted a much higher barrier of 53 kcal/mol. Thus this
xenon salt might also be kinetically stable at low temperature. If the Li atom was replaced
with a Na atom in XeNO2Li and XeNO3Li, the calculated structures were similar and the
TAEs were predicted to be ~15 kcal/mol smaller.
(6) Other related molecules with strong XeN bonding
Several other related molecules were also investigated in the current study as shown in
Figure 8. The XeF2NO anion was found to have a see-saw structure which can easily be
predicted by the VSEPR theory if one assumes that the XeN bond is a triple bond. The
calculated TAE and the S-T gap at CCSD(T)/aug-cc-pVTZ level is 69.8 and 66.3 kcal/mol,
respectively. The neutral molecule XeFNO2 has a similar structure. However, the calculated
TAE is approximately 48 kcal/mol smaller than that of the XeF2NO anion. From XeNO2
and XeNO3, the oxygen atoms can be attached to a hydrogen or a methyl group as shown in
the Figure. However, this would decrease the XeO bond strength significantly and the
resulting molecules would thus be more susceptible to unimolecular dissociation. Another
interesting molecule is NXeO2BeO2XeN, which contains two XeNO2 units connected by a
divalent beryllium cation, as shown in Figure 8(e). Very similar structure was also obtained if
the beryllium was replaced by magnesium.
(7) Electron density and charge
Figure 9 shows the electron density (calculated at the MP2/aug-cc-pVDZ level) map of
XeNO2. The density map is consistent with a polar covalent bonding between Xe and N
79
atoms. The bonding between the Xe and O atoms looks more ionic in nature. Based on the
calculated electron density, we have located the bond critical points29 along the XeN and
XeO bonds at the MP2/aug-cc-pVDZ level. For XeNO2 the Laplacian (in atomic units) at
the critical points are 0.28 and 0.41 for the XeN and XeO bonds, respectively. For
XeNO3 the corresponding values are 0.22 and 0.39. These values also suggest that the XeN
bonds are highly polar but less ionic than the XeO bonds. Table 9 shows the calculated
atomic charges. The Xe atoms showed very positive charges and the nitrogen and the oxygen
atoms showed similar negative charges in all cases. The calculated Mulliken and NBO
charges agree very well with each other.
(8) Stability of NgNO2 and NgNO3 anions for other noble gases
Table 10 compares the calculated NgN and NgO bond lengths and atomization
energies of NgNO2 (Ng = Ar, Kr, and Xe). Using the MP2 theory, all the anions were found
to be local energy minima with very short NgN and NgO bonds. However, from the
calculated TAEs it is apparent that only the XeNO2 is stable. The Ar- and Kr-containing
anions have energies significantly higher than their most stable atomic constituents. In fact, at
CCSD(T)/aug-cc-pVTZ level, only the Xe-containing anion was true energy minimum, and
the other anions dissociated upon geometry optimization. The NgNO3 (Ng = Ar, Kr, and Xe)
were predicted to be energy minima by both MP2 and CCSD(T) theory. The ArNO3 and the
isoelectronic species ArO4 have been computationally studied by Pyykkö,18a and they were
found to have strong inner bonding. In the study, the calculated XeN and XeO bond
lengths for ArNO3 were 1.46 Å and 1.60 Å, respectively, at CCSD(T)/cc-pVTZ level. The
calculated bond lengths and atomization energies of NgNO3 at CCSD(T)/aug-cc-pVTZ level
80
from our current study are listed in Table 11. Although ArNO3 contain very short ArN and
ArO bonds, its energy is significant higher (by 66 kcal/mol) than its most stable atomic
constituents. (see Table 4) Thus, in a stricter sense, ArNO3 is not stable due to the negative
bond energy. However, the calculated TAE does not exclude the possibility that the
ArNO3could be kinetically stable at cryogenic conditions due to its strong inner bonding on
the singlet potential energy surface. The TAE of KrNO3 was calculated to be 31 kcal/mol.
Thus KrNO3 can be assigned as a bound anion but it is less stable than XeNO2 and
XeNO3.
(9) Is it really a XeN triple bond?
Although by the Lewis structures or from the calculated bond lengths, the XeN bond in
XeNO2 or XeNO3, can be formally assigned as a triple bond, we have to stress here again
that the bond order is not a well-defined quantity in quantum chemistry, although it is
nevertheless an extremely useful concept to understand and categorize the types of chemical
bonding. Thus, it is not our intention to defend or initiate a debate on the designation of the
XeN triple bond. Instead we just would like to further illustrate the plausibility for this
assignment by showing the calculated structures of two additional molecules, XeNHO2 and
XeNH2O2, which are isoelectronic to XeNO2. The two structures are shown in Figure 10.
At the MP2/aug-cc-pVDZ (B3LYP/aug-cc-pVTZ) level, the calculated XeN bond distances
(in Å) are 1.798 (1.825), 1.857 (1.895), and 2.019 (2.050), respectively, for XeNO2,
XeNHO2, and XeNH2O2.
In this series, it is clear that the XeN bond order could be
logically assigned as 3, 2, and 1, respectively, although the difference in the bond lengths
between the double and triple bonds, which is 0.059 (0.070) Å, is not very large. This is
81
actually expected since the XeN bond is weaker than those of the "normal" chemical bonds.
In comparison, the difference in the experimental CO bond lengths of CO and CO2 is only
0.034 Å. Recently, Pyykkö and Atsumi published a new set of self-consistent covalent radii
for all the atoms in the periodic table.32 According to the study, the triple-bond radii for
nitrogen and xenon are 54 and 122 pm, respectively. This leads to an approximate XeN
triple-bond length of 1.76 Å which is consistent with our predicted values at the
CCSD(T)/aug-cc-pVTZ level (see Tables 1 and 3).
Summary
In the current computational study we have discovered a new type of relatively strong
bonding between xenon and nitrogen atoms with very short bond distances in XeNO2 and
XeNO3. Based on the calculated structures and the Lewis structures, the XeN bonding can
be tentatively assigned as triple bonds. These anions and their lithium “salts” were predicted
to be kinetically stable at low temperature. It is thus anticipated that future experiments
would confirm the existence of this interesting new type of chemical species.
Acknowledgment.
The research described in this chapter has been published on The
Journal of Physical Chemistry A (Sun, Y.-L.; Hong, J.-T.; Hu, W.-P. J. Phys. Chem. A, 2010,
114, 9359.). This work is supported by the National Science Council of Taiwan, grant number
NSC-97-2113-M-194-004. We are grateful to the National Center for High-Performance
Computing (NCHC) for providing part of computing resources.
82
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86
Table 1. Calculated Bond Lengths (Å) and Bond Angles (º) of XeNO2
B3LYP/aptz
MPW1PW91/aptz
MP2/apdz
MP2/aptz
CCSD(T)/aptz
CASPT2/aptz
1.825
1.868
113.4
102.6
1.804
1.840
113.1
102.1
1.798
1.866
113.9
100.5
1.780
1.825
112.4
100.9
1.825
1.860
113.2
101.9
1.849
1.884
113.1
103.8
XeN
XeO
NXeO
OXeO
Table 2. Calculated Energies (kcal/mol) Relative to XeNO2
B3LYP/aptz
MPW1PW91/aptz
MP2/apdz
MP2/aptz
CCSD(T)/aptz
CCSD(T)/apqza
ST gap
57.4
64.1
90.4
98.5
69.3
69.3
Xe + N(Q) + O(T) + O(D)
32.5
30.9
19.2
63.5
40.9
50.1
XeO + NO (S)
TS
51.1
43.9
46.6
48.5
45.4
46.2
23.2
47.9
14.2
18.2
36.7
33.2
42.0b
26.0
61.5
38.9
12.6
81.5
36.8
41.9b
21.9
78.6
67.8d
67.6b
211.7
211.9
219.8
190.7
194.5
191.2
59.9
59.8
58.4
22.9
41.1
35.3
79.5
77.8
XeO + NO (T)
90.8
91.9
Xe + O(T) + NO(T)
aSingle point calculation using CCSD(T)/aptz structure.
71.0
1.2
46.8
54.7
61.4
72.7
57.3
66.6
XeN + O2 (S)
TS
Xe + NO2
XeN(T)
bSingle
+ O2 (T)
point calculation using MP2/apdz structure.
87
Table 3. Calculated Bond Lengths (Å) and Bond Angles (º) of XeNO3
Method
B3LYP/aptz
MPW1PW91/aptz
MP2/apdz
MP2/aptz
CCSD(T)/aptz
XeN
1.805
1.785
1.792
1.773
1.800
XeO
1.824
1.800
1.834
1.791
1.810
NXeO
115.7
115.6
116.1
115.9
115.9
OXeO
102.6
102.6
102.1
102.3
102.4
Table 4. Calculated Energies (kcal/mol) Relative to XeNO3
Method
B3LYP/aptz
MPW1PW91/aptz
MP2/apdz
MP2/aptz
CCSD(T)/aptz
CCSD(T)/apqza
ST gap
52.8
74.8
78.3
Xe + N(Q) + 2O(T) + O(D)
XeO2 + NO(S)
TS
46.1
74.8
58.8
75.5
119.0
59.7
87.0
58.5
100.7
19.5
39.2
11.9
44.6
7.6
56.0
13.4
61.3
Xe + NO3
260.7
261.4
271.3
232.5
3.4
40.8b
235.4
1.4
42.6b
230.4
XeO + NO2
158.1
153.8
160.0
127.3
136.9
131.3
104.9
97.4
114.9
73.5
87.2
80.4
47.9
43.1
33.3
10.2
27.9
22.7
Xe + NO(T) + O2(T)
171.7
169.2
aSingle point calculation using CCSD(T)/aptz structure.
bSingle point calculation using MP2/apdz structure.
171.1
126.1
141.5
134.0
Xe + NO + O2 (S)
XeO2
+ NO(T)
88
Table 5. Calculated vibrational frequencies (in cm1) of XeNO2.
symmetry
MP2/apdz
CCSD(T)/aptz
IR Intensitya
(km/mol)
260.2
278.0
319.8
240.0
237.6
272.5
14.3
12.0
37.8
mode
A'
A"
A'
Frequency
A"
XeO
stretching
778.3
681.7
457.3
A'
XeO
stretching
804.5
635.8
359.3
835.8
56.4
XeN
1065.9
stretching
aCalculated using MP2/apdz method.
A'
89
Table 6. Calculated vibrational frequencies (in cm1) of XeNO3.
symmetry
mode
E
E
A1
Frequencya
IR Intensitya
(km/mol)
255.4
307.4
342.9
0.0
30.5
48.9
E
XeO
stretching
820.5
261.1
A1
XeO
stretching
822.4
176. 8
XeN
1026.3
stretching
aCalculated using MP2/apdz method.
A1
54.1
90
Table 7. Calculated Energies (kcal/mol) Relative to XeNO2Li
Method
B3LYP/aptz
MPW1PW91/aptz
MP2/aptz
CCSD(T)/aptz
CCSD(T)/apqza
ST gap
Xe + N(Q) + 2O(T) + Li
Xe + NO + LiO
44.6
99.7
53.0
91.5
76.0
131.0
53.8
104.2
53.6
115.5
140.8
33.8
139.2
37.4
105.3
38.1
121.5
116.7
29.7b
64.9
217.9
36.9
191.7
27.5
199.3
30.7b
23.7
195.9
TS
XeO + NOLi
Xe + NO2Li
68.2
216.6
aSingle point calculation using CCSD(T)/aptz structure.
bSingle point calculation using MP2/apdz structure.
91
Table 8. Calculated Energies (kcal/mol) Relative to XeNO3Li
ST gap
Xe + N(Q) + 3O(T) + Li
XeO + NO + LiO
TS
Xe + NO2 + LiO
XeO2 + NOLi
XeO + NO2Li
Xe + NO3Li
B3LYP/aptz
MPW1PW91/aptz
MP2/aptz
CCSD(T)/aptz
CCSD(T)/apqza
42.0
124.7
47.9
118.2
73.4
167.7
55.2
133.3
54.5
149.3
104.5
27.9
98.4
32.5
60.7
52.6
81.0
73.7
190.3
47.4
177.1
278.3
152.7
19.2
147.1
246.9
31.0c
162.8
25.2
158.8
250.9
33.6b
155.7
19.8
152.9
245.5
193.0
53.8
180.2
276.7
aSingle point calculation using CCSD(T)/aptz structure.
bSingle point calculation using MP2/apdz structure.
92
Table 9. Calculated Atomic Chargesa
Xe
N
O
Li
XeNO2
2.54 (2.54)
1.36 (1.36)
1.09 (1.09)
XeNO3
3.51 (3.39)
1.28 (1.28)
1.08 (1.04)
XeNO2Li
2.53 (2.43)
1.12 (1.08)
1.00 (1.16)
0.59 (0.97)
1.01 (1.12)b
0.61 (0.97)
1.01 (0.95)c
aBy Mulliken and NBO (in parentheses) methods using electron density calculated at MP2/apdz level, in atomic unit, e.
The most stable conformations were used for XeNO2Li and XeNO3Li.
bOxygen atoms that are bonded to lithium.
cOxygen atoms that are not bonded to lithium.
XeNO3Li
3.52 (3.29)
1.10 (1.08)
Table 10. Comparison of bond lengthsa (Å) and TAEsb (kcal/mol) of NgNO2 (Ng Ar, Kr, and Xe)
ArNO2
KrNO2
XeNO2
NgN
NgO
TAEs
1.49
1.67
68.8
1.62
1.73
17.9
1.83
1.86
50.1
at MP2/apdz level for ArNO2 and KrNO2 and at CCSD(T)/aptz for XeNO2.
bCalculated at CCSD(T)/apqz level. See text and Table 2 for definition.
aCalculated
93
Table 11. Comparison of bond lengthsa (Å) and TAEsb (kcal/mol) of NgNO3 (Ng Ar, Kr, and Xe)
NgN
NgO
TAEs

1.47
1.63
66.2

1.62
1.69
31.4
XeNO3
1.80
1.81
100.7
ArNO3
KrNO3
aCalculated
bCalculated
at CCSD(T)/aptz level.
at CCSD(T)/apqz level. See text and Table 4 for definition.
94
Figure 1. The calculated structure of XeNO2at CCSD(T)/aug-cc-pVTZ level.
95
Figure 2. The calculated transition state structure of XeNO2 XeO + NOat
MP2/aug-cc-pVDZ level.
Figure 3. The calculated ST crossing point structure along the XeNO2 XeO + NO
reaction path.
96
Figure 4. The calculated structure of XeNO3 at CCSD(T)/aug-cc-pVTZ level.
Figure 5. The calculated TS and the ST crossing point structure along the XeNO3 XeO2
+ NO reaction path.
97
Figure 6. Two calculated structures of XeNO2Li at CCSD(T)/aug-cc-pVTZ and
MP2/aug-cc-pVDZ levels for forms (a) and (b), respectively.
(a)
(b)
98
Figure 7. Three calculated structures of XeNO3Li. Form (a) was calculated at
CCSD(T)/aug-cc-pVTZ level and forms (b) and (c) were calculated at MP2/aug-cc-pVDZ
level.
(a)
(b)
99
(c)
100
Figure 8. The calculated structure of other related molecules with strong XeN bonding at
MP2/aug-cc-pVDZ level.
(a) XeF2NO
(b) XeFNO2
101
(c) XeNO2H
(d) XeNO2CH3
102
(e) NXeO2BeO2XeN
103
Figure 9. The electron density map of XeNO2
104
Figure 10. The calculated structure of XeNHO2 and XeNH2O2. The labeled bond lengths
were obtained from the MP2/aug-cc-pVDZ calculation.
(a) XeNHO2
(b) XeNH2O2
105
TOC Graphics
Theoretical calculation predicted strong XeN bonding in XeNO2 and XeNO3 with very
short bond lengths.
106