Theoretical Prediction of Stable Noble-Gas Anions XeNO2â and

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Theoretical Prediction of Stable Noble-Gas Anions XeNO2- and XeNO3- with very Short Xenon-Nitrogen Bond Lengths Yi-Lun Sun, Jie-Ting Hong, and Wei-Ping Hu* Department of Chemistry and Biochemistry, National Chung Cheng UniVersity Chia-Yi, Taiwan 621 ReceiVed: June 3, 2010; ReVised Manuscript ReceiVed: July 27, 2010

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. 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, Ra¨sa¨nen and co-workers4 have observed many small Xecontaining 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 past decade, several molecules formed from lighter noble gas atoms have also been experimentally identified such as HArF,5 HKrF,6 HKrCl,4a HKrCN,4g 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 co-workers.11,12 In xenon oxides (e.g., XeO3 and XeO4),13-15 xenon formed 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 NgdO bonds are stabilized by the polarizing effects of the fluoride ion, and the F-XeO might have been observed in experiment.17 One question that naturally arises is whether xenon can form multiple bonds with other elements besides oxygen? Although 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 bonds in nature.10-12 In the current study, we seek computationally small molecules that might contain Xe-N multiple bonds. Very simple molecules such as HXeN, FXeN, XeN+, and 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 * To whom correspondence should be addressed. E-mail: chewph@ ccu.edu.tw.

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 augcc-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 pseudopotential 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 of the tables. The vibrational frequencies were calculated using the same level of theory for geometry optimization. To check if the multireference character is important, fullvalence 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/augcc-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 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 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.

10.1021/jp1050904  2010 American Chemical Society Published on Web 08/09/2010

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Figure 1. The calculated structure of XeNO2- at CCSD(T)/aug-ccpVTZ level. Figure 2. The calculated transition state structure of XeNO2- f XeO + NO- at MP2/aug-cc-pVDZ level.

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/augcc-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. Although 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, then 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. 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, 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. Dissociation energies for other less important channels and to other electronic states are listed in the Supporting Information. 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 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° in XeNO2- to 71° in the TS. The complete calculated geometrical parameters of the TS are listed in the Supporting Information. 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 + NOchannel. Although 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

TABLE 1: Calculated Bond Lengths (Å) and Bond Angles (°) of XeNO2Xe-N Xe-O N-Xe-O O-Xe-O

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

TABLE 2: Calculated Energies (kcal/mol) Relative to XeNO2S-T gap Xe + N(Q) + O(T) + O-(D) XeO + NO- (S) TS XeN- + O2 (S) TS Xe + NO2XeN-(T) + O2 (T) XeO + NO- (T) Xe + O(T) + NO-(T) a

B3LYP/aptz

MPW1PW91/aptz

MP2/apdz

MP2/aptz

CCSD(T)/aptz

CCSD(T)/apqza

57.4 32.5 -51.1 43.9 14.2 61.5 -211.7 -59.9 -79.5 -90.8

64.1 30.9 -46.6 48.5 18.2 38.9 -211.9 -59.8 -77.8 -91.9

90.4 19.2 -45.4 46.2 12.6 81.5 -219.8 -58.4 -71.0 -91.2

98.5 63.5 -23.2 47.9 36.7 78.6 -190.7 -22.9 -46.8 -54.7

69.3 40.9 -36.8 41.9b 21.9 67.8b -194.5 -41.1 -61.4 -72.7

69.3 50.1 -33.2 42.0b 26.0 67.6b -191.2 -35.3 -57.3 -66.6

Single-point calculation using CCSD(T)/aptz structure. b Single-point calculation using MP2/apdz structure.

Prediction of Noble-Gas Anions XeNO2- and XeNO3-

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Figure 3. The calculated S-T crossing point structure along the XeNO2- f XeO + NO- reaction path.

locate a transition state that 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 requires 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 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°) 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 singlet-state surface, and the transition state of the unimolecular dissociation channel to XeO2 + NO- at various theoretical levels are listed in Table 4. Dissociation energies for other less important channels and

Figure 4. The calculated structure of XeNO3- at CCSD(T)/aug-ccpVTZ level.

TABLE 3: Calculated Bond Lengths (Å) and Bond Angles (°) of XeNO3Method

B3LYP/ MPW1PW91/ aptz aptz

Xe-N 1.805 Xe-O 1.824 N-Xe-O 115.7 O-Xe-O 102.6

1.785 1.800 115.6 102.6

MP2/ apdz

MP2/ aptz

1.792 1.773 1.834 1.791 116.1 115.9 102.1 102.3

CCSD(T)/ aptz 1.800 1.810 115.9 102.4

to other electronic states are listed in the Supporting Information. 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- f 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 complete calculated geometrical parameters of the TS are listed in the Supporting Information. 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 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 NOis 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. 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

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TABLE 4: Calculated Energies (kcal/mol) Relative to XeNO3Method

B3LYP/aptz

MPW1PW91/aptz

MP2/apdz

MP2/aptz

CCSD(T)/aptz

CCSD(T)/apqza

S-T gap Xe + N(Q) + 2O(T) + O-(D) XeO2 + NO-(S) TS Xe + NO3XeO + NO2Xe + NO- + O2 (S) XeO2 + NO-(T) Xe + NO-(T) + O2(T)

46.1 74.8 -19.5 39.2 -260.7 -158.1 -104.9 -47.9 -171.7

52.8 74.8 -11.9 44.6 -261.4 -153.8 -97.4 -43.1 -169.2

78.3 58.8 -7.6 56.0 -271.3 -160.0 -114.9 -33.3 -171.1

75.5 119.0 13.4 61.3 -232.5 -127.3 -73.5 -10.2 -126.1

59.7 87.0 -3.4 40.8b -235.4 -136.9 -87.2 -27.9 -141.5

58.5 100.7 1.4 42.6b -230.4 -131.3 -80.4 -22.7 -134.0

a


Figure 5. The calculated TS and the S-T crossing point structure along the XeNO3- f XeO2 + NO- reaction path.

TABLE 5: Calculated Vibrational Frequencies (in cm-1) of XeNO2symmetry A′ A′′ A′ A′′ A′ A′ a

mode

frequency

IR intensitya

MP2/apdz CCSD(T)/aptz

(km/mol)

Xe-O stretching Xe-O stretching Xe-N stretching

260.2 278.0 319.8 778.3 804.5 1065.9

240.0 237.6 272.5 681.7 635.8 835.8

14.3 12.0 37.8 457.3 359.3 56.4

Calculated using MP2/apdz method.

TABLE 6: Calculated Vibrational Frequencies (in cm-1) of XeNO3symmetry E E A1 E A1 A1 a

mode

frequencya

IR intensitya (km/mol)

Xe-O stretching Xe-O stretching Xe-N stretching

255.4 307.4 342.9 820.5 822.4 1026.3

0.0 30.5 48.9 261.1 176. 8 54.1

Calculated using MP2/apdz method.

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

Figure 6. Two calculated structures of XeNO2Li at CCSD(T)/augcc-pVTZ and MP2/aug-cc-pVDZ levels for forms (a) and (b), respectively.

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 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-, whereas 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


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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 TS XeO + NOLi Xe + NO2Li

44.6 99.7 -140.8 33.8 -68.2 -216.6

53.0 91.5 -139.2 37.4 -64.9 -217.9

76.0 131.0 -105.3 38.1 -36.9 -191.7

53.8 104.2 -121.5 29.7b -27.5 -199.3

53.6 115.5 -116.7 30.7b -23.7 -195.9

a


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-, whereas 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 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 seesaw structure that 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 8e. Very similar structure was also obtained if the beryllium was replaced by magnesium. The calculated

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.

geometrical parameters of these molecules are listed in the Supporting Information.

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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 a

B3LYP/aptz

MPW1PW91/aptz

MP2/aptz

CCSD(T)/aptz

CCSD(T)/apqza

42.0 124.7 -104.5 27.9 -193.0 -53.8 -180.2 -276.7

47.9 118.2 -98.4 32.5 -190.3 -47.4 -177.1 -278.3

73.4 167.7 -60.7 52.6 -152.7 -19.2 -147.1 -246.9

55.2 133.3 -81.0 31.0c -162.8 -25.2 -158.8 -250.9

54.5 149.3 -73.7 33.6b -155.7 -19.8 -152.9 -245.5


Figure 8. The calculated structure of other related molecules with strong Xe-N bonding at MP2/aug-cc-pVDZ level.

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 atoms. The bonding between the Xe and O atoms looks more ionic in nature. On the basis of the calculated electron density, we have located the bond critical points29 along the Xe-N and Xe-O bonds at the MP2/aug-ccpVDZ 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.


J. Phys. Chem. A, Vol. 114, No. 34, 2010 9365 TABLE 9: Calculated Atomic Chargesa Xe -

XeNO2 XeNO3XeNO2Li XeNO3Li

2.54 3.51 2.53 3.52

(2.54) (3.39) (2.43) (3.29)

N -1.36 -1.28 -1.12 -1.10

(-1.36) (-1.28) (-1.08) (-1.08)

O -1.09 -1.08 -1.00 -1.01 -1.01

(-1.09) (-1.04) (-1.16) (-1.12)b (-0.95)c

Li

0.59 (0.97) 0.61 (0.97)

a By 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. b Oxygen atoms that are bonded to lithium. c Oxygen atoms that are not bonded to lithium.

TABLE 10: Comparison of Bond Lengthsa (Å) and TAEsb (kcal/mol) of NgNO2- (Ng ) Ar, Kr, and Xe) -

ArNO2 KrNO2XeNO2-

Ng-N

Ng-O

TAEs

1.49 1.62 1.83

1.67 1.73 1.86

-68.8 -17.9 50.1

a Calculated at MP2/apdz level for ArNO2- and KrNO2- and at CCSD(T)/aptz for XeNO2-. b Calculated at CCSD(T)/apqz level. See text and Table 2 for definition.

Figure 9. The electron density map of XeNO2-.

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 Pyykko¨,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 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 from our current study are listed in Table 11. Although ArNO3- contains 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 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, Pyykko¨ and Atsumi published a new set of selfconsistent covalent radii for all the atoms in the periodic table.32 According to the study, the triple-bond radii for nitrogen and

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Sun et al.

TABLE 11: Comparison of Bond Lengthsa (Å) and TAEsb (kcal/mol) of NgNO3- (Ng ) Ar, Kr, and Xe) -

ArNO3 KrNO3XeNO3-

Ng-N

Ng-O

TAEs

1.47 1.62 1.80

1.63 1.69 1.81

-66.2 31.4 100.7

a Calculated at CCSD(T)/aptz level. b Calculated at CCSD(T)/ apqz level. See text and Table 4 for definition.

194-004. We are grateful to the National Center for HighPerformance Computing (NCHC) for providing part of computing resources. Supporting Information Available: Tables of calculated geometry and relative energies. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 10. The calculated structure of XeNHO2 and XeNH2O2+. The labeled bond lengths were obtained from the MP2/aug-cc-pVDZ calculation.

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-ccpVTZ 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-. On the basis of 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. This work is supported by the National Science Council of Taiwan, grant number NSC-97-2113-M-

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