Noble Gas Monoxides Stabilized in a Dipolar Cavity: A Theoretical

Article pubs.acs.org/JPCA

Noble Gas Monoxides Stabilized in a Dipolar Cavity: A Theoretical Study Paweł Szarek* and Wojciech Grochala* Center for New Technologies, University of Warsaw, Ż wirki i Wigury 93, 02089 Warsaw, Poland S Supporting Information *

ABSTRACT: Encouraged by our previous theoretical results that indicated the stabilization of the HeO unit inside the ferroelectric cavity composed of two parallel LiF dipoles, we have now undertaken the theoretical study for the related noble gas systems, (NgO)(MF)2, Ng = Ar, Kr, Xe, M = Li, Na, K. The computational results indicate that all such molecules constitute local minima, which are protected by sizable energy barriers especially for M = Li, Na, and thus these systems might constitute interesting synthetic targets at low temperatures.

■

■

THEORETICAL METHODS

The minimum energy and transition state (TS) structures were calculated with the coupled cluster method using single and double substitutions from the Hartree−Fock determinant, CCSD, as well as a Hartree−Fock calculation followed by a Møller−Plesset correlation energy correction truncated at second-order, MP2. Dunning and co-worker’s1−3 valence double, triple, and quadruple-ζ quality basis sets with polarization and diffuse functions (aug-cc-pVDZ++, aug-ccpVTZ++, aug-cc-pVQZ++) for O, F, Li, Na, Ar, and Kr atoms were used. The Xe and K atoms were not represented in Dunning’s basis sets. The all-electron polarization consistent basis set of Jensen4 for the K atom (aug-pc-2, aug-pc-3, aug-pc4) has been chosen, due to similar composition of basis functions with respect to the closest noble gas atoms, such as in case of Dunning’s sets for Na and Li. The large Xe atom has been described with Peterson et al.’s5 basis set and effective core potential, ECP (aug-cc-pVDZ-PP++, aug-cc-pVTZ-PP++, aug-cc-pVQZ-PP++). We will simply refer to the respective combinations of these basis sets for each atom as double-, triple-, or quadruple-ζ. We found that the presence or absence of polarization functions does not have a noticeable effect on the ground state and TS geometry, the height of the energy barrier for decomposition reaction, nor Ng−O bond stretching IR frequencies or partial Mulliken charges on atoms. Moreover, after the analysis of systematic improvements of results for representative molecules, obtained at double-, triple-, and quadruple-ζ, we decided to include only the complete triple-ζ results as the best level of theory, because the differences with the quadruple-ζ set were very small, in both energy and geometry, in contrast to the computational effort. The nature of all transition states was confirmed by detection of a single imaginary harmonic frequency in each case studied. The calculations were performed with Gaussian’09.6 © XXXX American Chemical Society

INTRODUCTION

(HeO)(LiF)2 has been the first noble gas (Ng) monoxide molecule studied by theory.7 The He−O bond (otherwise unknown due to lack of stability of the free HeO molecule8,9) has been shown to be stabilized inside a molecular ferroelectric cavity composed of two LiF dipoles. (HeO)(LiF)2 turned out to exhibit all real vibrational frequencies. The corresponding local minimum of the C2v symmetry is characterized by a very short covalent He−O bond at 1.151 Å (CCSD(T)/6-311+ +G** results),7 the shortest helium−element bond in a neutral system studied so far.10 This is reflected in the high calculated He−O bond stretching frequency of 1112 cm−1. Obviously, singlet (HeO)(LiF)2 species should be metastable, with the ground state corresponding to He + 3O + (LiF)2.7 Still, (HeO)(LiF)2 and the previously described MFHeO (M = Cs, NMe4)11 constitute the first candidates for a small neutral chemical system that contains a helium atom bound chemically to another element via a covalent bond. (HeO)(LiF)2 is the only local minimum in the (HeO)(MF)2 (M = H to Cs) family. The (HeO)(MF)2 (M = Na to Cs) species supposedly exhibit too large a mismatch between the He−O and Na−F bond lengths or too small an ionicity of the MF bonding (for M = H) to constitute local minima. Moreover, the related Ne systems, (HeO)(MF)2 (M = Li, Na), do not constitute local minima either and they inevitably decompose to Ne + 3O + (LiF)2.7 This is one strong confirmation of a particularly pronounced “nobility” of neon as compared to that of helium. Similar results obtained in 2009 for MFNgO (M = Cs, NMe4) molecules11 gave rise to a postulate by Grochala11,12 that neon is the most inert noble gas whereas helium should be placed in group 2 of the periodic table, just above beryllium. This surmise was further supported by analysis Special Issue: Markku Räsänen Festschrift Received: August 30, 2014 Revised: November 6, 2014

A

dx.doi.org/10.1021/jp508786y | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

Article

of series of the first ionization potentials of group 1, 2, and 17 elements, as well as by the calculations for the OHeF− and OBeF− anions.11 Those turned out to share one important feature: the bond between the central atom and oxygen is considerably shorter in both species than that between the central atom and fluorine, despite a smaller size of F− as compared to O2−. In this succinct study we extend the concept of a pair of parallel electric dipoles hosting noble gas monoxide molecules to heavier Ng congeners of Ar, Kr, and Xe. The considered Ng species (NgO)(MF)2 contain alkali earth metal atoms M = Li, Na and K. No monoxides are known for any of the heavier Ng,13 and (as we will see) two parallel MF dipoles are indeed needed to stabilize the molecular system, as for (HeO)(LiF)2 described previously.7

Figure 2. Ng−O bond lengths depending on the metal atom, at the CCSD/triple-ζ level of theory.

or 1.59, 1.80, and 1.94 Å, from ref 15), thus pointing to substantially covalent character of Ng−O bonds in these systems; this is similar to the case of He species.7 The MP2 Ng−O bond lengths were ∼0.05 Å shorter than for the respective CCSD structures. The change in the quality of the basis set from double- to triple-ζ shortened the Ng−O distance by ∼0.05 Å using CCSD, and around 0.04 Å for the MP2 method. The difference between triple- and quadruple-ζ was found at most ∼0.01 Å, for MP2; hence, the CCSD calculations were not conducted with the CPU-demanding quadruple-ζ basis set. Figure 1 shows the structures of calculated complexes with respect to each other. It is clear that the Ng−O−M angle does not vary much with the change of the metal atom and it falls at 96.42 ± 0.48° for Ar, 99.43 ± 0.20° for Kr, and 103.02 ± 0.63° for Xe species, as calculated at the CCSD/triple-ζ level of theory. Similarly, the F−M−O angles seem to be only marginally affected by the change of a noble gas atom, being respectively 111.18 ± 0.41° in structures with the Li atom, 95.08 ± 0.79° with Na, and 82.66 ± 0.62° with K. Furthermore, the Ng−O distances change below 1%, i.e., around 0.01 Å with the substitution of metal atoms from Li via Na to K (Figure 2). This feature testifies to the robustness of the NgOM skeleton with the more ionically bound fluoride anion accommodating its position with respect to the remaining part of a molecule. A small charge transfer from NgO to MF is observed at diverse population analyses (Supporting Information). Usually, most of the electron density transfer takes place between NgO and MF units, leading to the (NgO)2δ+[(LiF)δ−]2 formulation. The calculated partial atomic charges on noble gas center are quite substantial with the following ranges: +0.25e < qAr < +0.75e, +0.25e < qKr < +0.90e, and +0.35e < qXe ≈ +1.25e, depending on the population analysis applied16 (cf. Supporting

■

RESULTS AND DISCUSSION As in the case of (HeO)(LiF)2, all the other noble gas and alkali earth metal molecules studied here yielded the planar structures of C2v symmetry for all levels of theory applied (Figure 1, Table

Figure 1. Superimposed geometries of (NgO)(MF)2 molecules at the CCSD/triple-ζ level of theory. The molecules with Ar, Kr, and Xe are in red, purple, and blue, respectively.

1). The NgO bond lengths are quite short, i.e., 1.73−1.75 Å for Ar, 1.81−1.83 Å for Kr, and 1.94 Å for Xe species, at the CCSD/triple-ζ level of theory (Figure 2, Table 1). These values are very similar to the sum of the covalent radii for oxygen (0.66 Å,14 or 0.63 Å from an alternative source15) and the corresponding Ng atoms (1.72, 1.82, and 2.06 Å, respectively,14

Table 1. Selected Properties of (NgO)(MF)2 Molecules at the CCSD/triple-ζ Level of Theory (ArO) RNg−O [Å] RM−F [Å] vNgO [cm−1] vMF [cm−1] vmin [cm−1] qNg [e] qO [e] qM [e] qF [e] μ [D] 3 E − 1E [eV]

(KrO)

(XeO)

(LiF)2

(NaF)2

(KF)2

(LiF)2

(NaF)2

(KF)2

(LiF)2

(NaF)2

(KF)2

1.730 1.613 738.8 813.5 818.9 24.2 +0.63 −0.34 +0.60 −0.75 7.17 1.57

1.748 2.025 782.3 493.6 493.8 23.3 +0.64 −0.51 +0.86 −0.92 12.83 1.39

1.746 2.311 726.4 368.3 370.9 23.3 +0.59 −0.52 +0.88 −0.92 15.21 1.34

1.814 1.619 713.8 803.9 809.3 20.60 +0.83 −0.51 +0.59 −0.75 5.86 1.98

1.824 2.031 712.7 485.3 488.1 17.3 +0.83 −0.71 +0.86 −0.92 11.63 1.95

1.825 2.318 713.5 363.7 366.2 16.9 +0.77 −0.71 +0.88 −0.91 14.40 1.74

1.936 1.626 695.6 792.5 798.5 13.0 +1.00 −0.65 +0.56 −0.74 4.52 2.21

1.942 2.039 697.5 481.4 484.3 3.1 +0.99 −0.89 +0.85 −0.90 10.30 2.16

1.944 2.321 714.8 365.8 368.6 1.2 +0.92 −0.89 +0.88 −0.90 13.49 1.91

B



Article

He to Xe series.17 The wavenumber of 739 cm−1 calculated for the Ar−O system may be compared with 750 cm−1 calculated for isoelectronic ArF+ cation.18 However, the ArF+ cation is probably too oxidizing to become a component of a neutral molecule, even with the AuF6− or SbF6− as counterions,19 whereas (ArO)(LiF)2 represents a neutral system. Interestingly, the M−F stretching modes of both MF subunits do not couple substantially with each other, and the in-phase stretching is split from the out-of-phase one by 6 cm−1 at most (Table 1). A similar feature was typical of the analogous Ng = He, M = Li system.7 The local stability of the C2v minimum was confirmed by single point calculations of the vertical excitations energies to triplet state. The energies of the vertically excited first triplet state are 1.3−2.2 eV higher than those of the singlet local minimum (at CCSD/triple-ζ level of theory, cf. Supporting Information). This feature testifies that, as for the related He system,7 the local minimum studied here is protected from decomposition, yielding the ground state 3O, Ng, and (MF)2 dimer. The ground state attains triplet character only at the larger Ng−O separations that correspond to the local minimum studied here. The relative stability of the (NgO)(MF)2 minimum with respect to diverse products of unimolecular decomposition reactions is certainly of interest while considering how realistic synthetic target one deals with. Similarly as for (HeO)(LiF)2,7 six diverse reasonable pathways for decomposition may be considered, as shown in eqs 1−6.

Information). Nevertheless, within each population analysis scheme, the Ng−O bond attains an increasing ionic character as one goes down group 18, which is in agreement with what is known about properties of Ng compounds. The Ar, Kr, and Xe equivalents of (HeO)(LiF)2 have much smaller dipole moments than the latter one (∼11 D): 7.2, 5.9, and 4.5 D, respectively. These properties do not show significant dependence on the level of theory used (including basis set or method). The reduction in the dipole moment can be traced back to the polarization of the NgO dipole (antiparallel to the LiF ones), which increases in the series from He to Xe, as discussed above. The electrostatic potential of the (NgO)(MF)2 molecules resembles a butterfly like that for the Ng = He, M = Li system,7 as exemplified here by the case of Ng = Ar, M = Li (Figure 3).

Δ1

(NgO)(MF)2 → 1O + 2MF + Ng Δ2

(NgO)(MF)2 → 3O + 2MF + Ng Δ3

(NgO)(MF)2 → MOF + MF + Ng Figure 3. Projection of the electrostatic potential at the CCSD/triple-ζ level of theory for the planar (ArO)(LiF)2 molecule.

Δ4

(NgO)(MF)2 → M 2O + F2 + Ng Δ5

(NgO)(MF)2 → 1O + (MF)2 + Ng

The local minimum character of the optimized C2v structures was confirmed by harmonic frequencies calculations, which showed no imaginary vibrational modes. The softest modes of vibrational frequencies corresponding to the 78 cm−1 mode of (HeO)(LiF)27 are red-shifted down to 24, 20, and 13 cm−1 for Ar, Kr, and Xe analogues, respectively. At the first sight, the progressive destabilization of this out-of-plane torsional mode (“butterfly wing motion”) as one goes down group 18 of the periodic table seems to reflect the increasing mass of the NgO units that participate in the normal vibration. However, closer inspection shows that this is the force constant that changes dramatically in the Ar, Kr, and Xe series rather than the reduced mass. It should be remembered that the “planar butterfly” geometry of the (NgO)(MF)2 molecules does not offer a sufficiently large coordination number neither for heavier Ng atoms nor for alkali earth metal ones; hence, the minimum in question is protected by relatively small energy barriers, as will be discussed below. Indeed, the (XeO)(KF)2 system is brought to the edge of dynamic stability, with the butterfly motion slightly exceeding a mere 1 cm−1. The Ng−O bond stretching frequencies compared to one for He−O, i.e., 1112 cm−1, were also found to downshifted to 739 cm−1 for Ar−O, 714 cm−1 for Kr−O, and 696 cm−1 for Xe−O. This can be understood by taking into account the square root of the reduced mass of the Ng−O oscillator decreasing in the

Δ6

(NgO)(MF)2 → 3O + (MF)2 + Ng

(1) (2) (3) (4) (5) (6)

The simplest scenario for decomposition is represented by three constituting dipoles falling apart from each other, with a simultaneous bond breaking of the (unstable, when free) Ng− O unit (eq 1). Equation 1 is relevant to reaction taking place on the singlet PES. A similar scenario (eq 2) involves a singlet− triplet crossing that yields same product except for 1O, which now becomes the ground state of the oxygen atom, i.e., 3O. These reactions could take place without breaking the C2v symmetry. Another important pathway is related to atomic motion, which breaks the C2v symmetry and yields the Ng atom, MF, and MOF, eq 3. This scenario has been shown to be stability-limiting for the (HeO)(LiF)2 system.7 Yet another possibility is that of formation of M2O and F2 along with the Ng atom, eq 4. Again, C2v symmetry is preserved during this process. This scenario is quite unrealistic, in agreement with chemical intuition, as reflected in very positive values of energy needed to reach these products for each system studied; hence, we will not discuss this any further. The final two scenarios (eqs 5 and 6) are analogues of eqs 1 and 2 where coupling of MF units to (MF)2 dimers (which is energetically favorable) is assumed. C



Article

Table 2. Energetics of Diverse Decomposition Reactions for (NgO)(MF)2 Systems at Both MP2 and CCSD/triple-ζ Levels of Theorya Δ1

Δ2

Ar Kr Xe Ar Kr Xe Ar Kr Xe

2.482 3.446 4.620 2.230 3.157 4.272 1.969 2.824 3.859

−0.384 0.580 1.754 −0.635 0.291 1.407 −0.896 −0.042 0.993

Ar Kr Xe Ar Kr Xe Ar Kr Xe

1.739 2.586 3.634 1.500 2.300 3.279 1.244 1.976 2.874

−0.664 0.184 1.232 −0.903 −0.103 0.877 −1.159 −0.427 0.472

Δ3

Δ4

Δ5

Δ6

2.365 3.329 4.503 2.464 3.391 4.506 3.262 4.117 5.152

−0.132 0.832 2.006 −0.165 0.761 1.877 −0.060 0.795 1.830

−2.998 −2.034 −0.860 −3.031 −2.104 −0.989 −2.925 −2.071 −1.036

2.561 3.408 4.456 2.874 3.674 4.653 3.615 4.347 5.245

−0.935 −0.087 0.961 −0.926 −0.126 0.853 −0.819 −0.086 0.812

−3.337 −2.490 −1.442 −3.329 −2.529 −1.549 −3.221 −2.489 −1.591

MP2 Li

Na

K

Li

Na

K

a

−0.925 0.039 1.213 −1.182 −0.255 0.860 −1.505 −0.650 0.385 CCSD −0.990 −0.142 0.906 −1.241 −0.441 0.538 −1.560 −0.828 0.070

Cf. Figure 4 and eqs 1−6. ΔN (N = 1−6) is given in eV.

seen in stabilization of the singlet NgO unit within the (MF)2 cavity even with respect to triplet oxygen and Ng atoms (eq 2). It follows from our calculations that LiF units are capable of energetically stabilizing both KrO and XeO, the ArO system being unstable by ca. 0.66 eV. Still, this is a substantial improvement with respect to uncomplexed ArO unit, which does not even have a bound minimum on the singlet surface.20,21 Moreover, the first excited triplet state falls at energy of 1.57 eV (Supporting Information) with respect to singlet local minimum of the (ArO)(LiF)2 system, which seems to warrant sufficient “local” dynamic stability of this molecule. Indeed, the CCSD calculations reveal that the crossing of the singlet and triplet PESs takes place at the Ar−O separation of ca. 1.94 Å, which falls about 0.27 eV above the local minimum on singlet PES (Figure 6). Thus, at least several vibrational levels of the Ar−O stretching fundamental (1/2 hν = 0.046 eV) should fall below the singlet−triplet crossing point, especially if anharmonicity is taken into account. The stability of connections of heavier (larger) noble gas atoms as measured using the singlet−triplet crossing barrier should be even larger than that for the argon species, given that higher oxidation states are usually more stabilized for heavier elements from the same group of the periodic table.22 The most interesting and likely stability-limiting decomposition pathway is related to the formation of the Ng atom and MF and MOF molecules, eq 3. This reaction channel has, indeed, proved to be important for the related helium system.7 Also for heavier Ng atoms, Ar, Kr, and Xe, this reaction has the most negative energy among all four “simple” reasonable reactions, eqs 1−4. For example, this reaction is exoenergetic for (ArO)(LiF)2 system by nearly 1 eV. This reaction turns out to be energetically uphill only for all Xe molecules studied (recall, oxidation of Xe to the divalent state is the most facile among all noble gases). Due to importance of this reaction channel we will discuss the energy barriers for this reaction in the last paragraph.

The computed energetics of all unimolecular processes considered here is collected in Table 2 and graphically illustrated in Figure 4 for an example of the (ArO)(LiF)2

Figure 4. Generalized reaction scheme for possible decomposition pathways of (NgO)(MF)2 molecules as exemplified by the Ng = Ar, M = Li molecule. For other systems cf. Table 2. Energy change, Δi, in eV.

system. The corresponding data for all molecules studied are collected in Figure 5. Only the CCSD/triple-ζ results are discussed in this section. It turns out that for all systems studied the reaction proceeding according to eq 1 is substantially uphill in energy (by ca. 1.2−3.6 eV, depending on the system). This reflects the success of the general approach taken here, as well as in the preceding study for the He system,7 namely the anticipated stabilization of the (otherwise unstable) NgO unit in the electric field of the two parallel dipoles. This is further D



Article

Figure 5. Electronic energy change, in eV, for decomposition reactions of (NgO)(MF)2 into products listed on the vertical axis, plus noble gas atom, Ng = Ar, Kr, Xe, with M = Li, Na, K, at (a) MP2/triple-ζ and (b) CCSD/triple-ζ levels of theory.

the corresponding bending mode sit below the energy barrier. For all systems studied here we, too, have detected energy barriers for decomposition processes of all C2v minima, as computed at the MP2/triple-ζ level of theory (Table 3).24 The Table 3. Energy Barriers for the (NgO)(MF)2 Decomposition Process (eV) at the MP2/triple-ζ Level of Theory (ArO) (KrO) (XeO)

Figure 6. Region of the PESs crossing for the singlet and triplet states of (ArO)(LiF)2 at the CCSD/triple-ζ level of theory. The single point energies on the triplet PES have been calculated for geometries corresponding to the singlet state, whereas the relaxed singlet PES has been obtained for incrementally changed Ar−O separations (by 0.05 Å) with all other internal coordinates optimized, while preserving the C2v symmetry.

(LiF)2

(NaF)2

(KF)2

0.20 0.20 0.20

0.19 0.18 0.15

0.06 0.06 0.04

transition states for molecules containing Li (Figure 7a) and Na atoms (Figure 7b) were planar (just like for the related He species7), in contrast to K-substituted species, which were nonplanar (Figure 7c). The computed barrier heights are quite small for all K-containing systems (0.04−0.06 eV), thus suggesting that the C2v minima are not dynamically stable because even v = 0 levels of certain high-energy oscillators do not sit inside the potential energy well. However, the calculated barriers are quite high for M = Na (0.15−0.19 eV) and M = Li (0.20 eV) and they are considerably larger than the one (0.04 eV) calculated previously for (HeO)(LiF)2.7 This suggests that (NgO)(MF)2 (Ng = Ar to Xe, M = Li, Na) should be robust enough to survive at low temperatures (once synthesized) and they may constitute a viable synthetic target.25 It is currently unclear how the (NgO)(MF)2 (Ng = Ar to Xe, M = Li, Na) and (HeO)(LiF)25 molecules and the related (HeO)(MF) (M = Cs, NMe4, PMe4)8 might be prepared. It is certain that, due to their lack of thermodynamic stability, they should be searched for in isolation at low temperature conditions, possibly in helium droplets or in noble gas (e.g.,

The last two reaction pathways are related to formation of (MF)2 dimers. These necessitate the out-of-plane motions of MF units as well as 180° rotation of one MF unit with respect to another, and therefore (and despite their seeming energetic feasibility), they are thought not to constitute the dynamic stability limiting scenarios but rather the subsequent stages of reactions described by eqs 1 and 2).23 We will now discuss the energy barriers for the decomposition reaction taking place according to eq 3. (HeO)(LiF)2 has been calculated to exhibit a rather small energy barrier for decomposition on the singlet surface via the antisymmetric bending channel (yielding He, LiOF, and LiF).7 In this case, only one (or maximum two) vibrational levels of E



Article

Figure 7. Transition state structures at the CCSD/triple-ζ level of theory. Planar: (a) (NgO)(LiF)2, (b) (NgO)(NaF)2. Distorted: (c) (NgO)(KF)2, top and side view. The molecules with Ar, Kr, and Xe are in red, purple, and blue, respectively.

■

neon) matrixes. One possible strategy relies on photochemical breaking of the fragile F−F bond of F2 in the presence of M2O molecules (note, the reaction described by eq 4 is endothermic for all Ng systems studied). Another viable strategy, discussed in earlier works, involves a similar photochemical rupture of the fragile O−F bond in the MOF hypofluorite species (the relevant reaction described by eq 3 is endothermic for heavier Ng atoms). Reactions using the in situ generated reactive singlet oxygen atoms (eq 1) constitute yet another option.

■

Corresponding Authors

*P. Szarek. E-mail: [email protected]. Tel: +48 22 55 40 828. *W. Grochala. E-mail: [email protected]. Tel: +48 22 55 40 828. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

CONCLUSIONS

Notes

In the above we have demonstrated using quantum mechanical calculations that a “ferroelectric cavity” composed of two parallel MF dipoles (M = Li, Na, K) holds a potential for stabilizing NgO species (Ng = Ar, Kr, Xe) whose dipole moment is aligned antiparallel to those of MF moieties, similarly as in the case of Ng = He.7 The so formed (NgO)(MF)2 molecules are planar and show C2v symmetry. All species studied constitute local minima on the potential energy surface and they are metastable with respect to He, 3O and (MF)2 products. Nevertheless, they are surrounded by considerable energy barriers (particularly for Li species) and as such they constitute viable synthetic targets. Preparation of NgO(MF)2 molecules could possibly be realized at low temperature conditions, thus enriching the chemistry of noble gases, which is now undergoing rapid development.26−30 This is of particularly importance for Ar chemistry, which is currently exemplified by only one neutral31 and thermally quite fragile species, the HArF molecule.32,33 Possibly, the use of giant electric fields generated by small ionic molecules (or laser radiation) will facilitate the formation of new Ng systems, not only for large and soft xenon or krypton but also for much less reactive argon, as well as the lightest one (helium).7,11,33,34 The question remains whether extension of this chemistry will be possible for the most stubbornly inert noble gas (neon).7,11

■

AUTHOR INFORMATION

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Authors acknowledge the Polish National Science Center (NCN) for financing (grant No. 64-01-00/501-66-RG 4521).

■

ABBREVIATIONS Ng, noble gas; TS, transition state; CCSD, coupled cluster method using single and double substitutions from the Hartree−Fock determinant; MP2, Møller−Plesset perturbation theory truncated at second-order; PES, potential energy surface

■

REFERENCES

(1) Dunning, T. H. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron Through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (2) Woon, D. E.; Dunning, T. H. Gaussian Basis Sets for Use in Correlated Molecular Calculations. III. The Second Row Atoms, Al− Ar. J. Chem. Phys. 1993, 98, 1358−1371. (3) Wilson, A. K.; Woon, D. E.; Peterson, K. A.; Dunning, T. H. Gaussian Basis Sets for Use in Correlated Molecular Calculations. IX. The Atoms Gallium Through Krypton. J. Chem. Phys. 1999, 110, 7667−7676. (4) Jensen, F. Polarization Consistent Basis Sets. VII. The Elements K, Ca, Ga, Ge, As, Se, Br and Kr. J. Chem. Phys. 2012, 136, 114107:1− 114107:7. (5) Peterson, K. A.; Figgen, D.; Goll, E.; Stoll, H.; Dolg, M. Systematically Convergent Basis Sets with Relativistic Pseudopotentials. II. Small-Core Pseudopotentials and Correlation Consistent Basis Sets for the Post-d Group 16−18 Elements. J. Chem. Phys. 2003, 119, 11113−11123. (6) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2009.

ASSOCIATED CONTENT

S Supporting Information *

The Z-matrices, optimized geometries, and calculated harmonic frequencies for all (NgO)(MF)2 minima, population analyses at the CCSD and MP2/triple-ζ level of theory, and singlet−triplet excitation eneergies. This material is available free of charge via the Internet at http://pubs.acs.org. F



Article

(7) Grochala, W. A Metastable He−O Bond Inside a Ferroelectric Molecular Cavity: (HeO)(LiF)2. Phys. Chem. Chem. Phys. 2012, 14, 14860−14868. (8) Krems, R. V.; Kłos, J.; Rode, M. F.; Szczęśniak, M. M.; Chałasiński, G.; Dalgarno, A. Suppression of Angular Forces in Collisions of Non-S-State Transition Metal Atoms. Phys. Rev. Lett. 2005, 94, 013202−1−013202−4. (9) Krems, R. V.; Buchachenko, A. A.; Szczęśniak, M. M.; Kłos, J.; Chałasiński, G. Dynamics of O(3Pj)+Rg Collisions on Ab Initio and Scattering Potentials. J. Chem. Phys. 2002, 116, 1457−1467. (10) The HeH+ cation is characterized by an even shorter bond length not exceeding 0.8 Å. (11) Grochala, W. On Chemical Bonding Between Helium and Oxygen. Pol. J. Chem. 2009, 83, 87−122. (12) A similar but not identical argument was used earlier by Bent, who considered changes in the first ionization potential as one goes down each group of the periodic table. Bent, H. New Ideas in Chemistry from Fresh Energy for the Periodic Law; AuthorHouse: Bloomington, IL, 2006. (13) We notice recent theoretical predictions of formation of Xe oxides at elevated pressures. XeO is predicted to form from Xe and O2 at pressures exceeding 83 GPa. Zhu, Q.; Jung, D. Y.; Oganov, A. R.; Glass, C. W.; Gatti, C.; Lyakhov, A. O. Stability of Xenon Oxides at High Pressures. Nat. Chem. 2013, 5, 61−65. (14) Cordero, B.; Gómez, V.; Platero-Prats, A. E.; Revés, M.; Echeverría, J.; Cremades, E.; Barragán, F.; Alvarez, S. Covalent Radii Revisited. Dalton Trans. 2008, 2832−2838. (15) Pyykkö, P.; Atsumi, M. Molecular Single-Bond Covalent Radii for Elements 1−118. Chem.Eur. J. 2009, 15, 186−197. (16) Cioslowski, J. A New Population Analysis Based on Atomic Polar Tensors. J. Am. Chem. Soc. 1989, 111, 8333−8336. (17) The reduced mass of this oscillator for molecules containing He is governed largely by atomic mass of He(4), whereas that for Xe systems is governed by the mass of oxygen(16); hence, the square root of the respective mO/mHe ratio is around 2. (18) Frenking, G.; Koch, W.; Deakyne, C. A.; Liebman, J. F.; Bartlett, N. The ArF+ Cation. Is It Stable Enough To Be Isolated in a Salt? J. Am. Chem. Soc. 1989, 111, 31−33. (19) Christe, K. O. A Renaissance in Noble Gas Chemistry. Angew. Chem., Int. Ed. 2001, 40, 1419−1421. (20) Li, T.-H.; Mou, C.-H.; Chen, H.-R.; Hu, W.-P. Theoretical Prediction of Noble Gas Containing Anions FNgO− (Ng = He, Ar, and Kr). J. Am. Chem. Soc. 2005, 127, 9241−9245. (21) Liu, Y.-L.; Chang, Y.-H.; Li, T.-H.; Chen, H.-R.; Hu, W.-P. Theoretical Study on the Noble-Gas Anions F−(NgO)n (Ng = He, Ar, and Kr). Chem. Phys. Lett. 2007, 439, 14−17. (22) Indeed, Xe−O is already weakly bound in its singlet 1Σ+ electronic state, with the dissociation energy of 0.36 eV (experimental value) to 0.70 eV (theoretical one), and the interatomic separation at minimum of 2.65 Å (experimental) to 2.08 Å (theoretical). Yamanishi, M.; Hirao, K.; Yamashita, K. Theoretical study of the low-lying electronic states of XeO and XeS. J. Chem. Phys. 1998, 108, 1514− 1521. (23) Because a risk exists that the C2v minima could possibly decompose via the low-frequency A2 mode while yielding low-energy (MF)2 dimer, Ng atom, and 1O (this reaction is exothermic for all cases studied, cf. eq 6), we have attempted to localize the transition state along this decomposition pathway. We have selected a relatively thermodynamically fragile (ArO)(LiF)2 molecule for this calculation. However, it turns out that even a very substantial deformation of the molecule along the normal coordinate of the A2 mode (which increases system’s energy by nearly 2 eV and thus dozens of this oscillator’s zero point energies) does not result in the tendency of a molecule to split into the products. I.e., each optimization on the singlet PES has returned to the C2v minimum. (24) Location of the transition states at the CCSD/triple-ζ level of theory proved to be a daunting task and it has not been pursued here. (25) As a referee correctly reminded us, in addition to six reaction equations considered here, some more might be added, which take

into account formation of the condensed phases. For example, reactions yielding MF(s) in the solid state and/or 3O2 in the liquid or solid state. Such reactions will obviously be severely exothermic, because the sublimation (or, jointly, melting and evaporation) enthalpies are large for alkali earth metal fluorides, and they vary between 2.5 eV for KF and 2.8 eV for LiF (data from www.nist.gov). However, such reactions are not relevant to the conditions of an isolated molecule with low mobility (at low temperature conditions). Although clustering of the MF molecules is clearly downhill in energy, these elementary reaction steps do not influence the kinetic stability of the starting (NgO)(MF)2 molecule because they constitute the late steps of (polymolecular) reactions. As we have explained in ref 23, even dimerization of LiF units separated by NgO moiety is not facile, and it is unrelated to dynamic stability of (NgO)(MF)2 species. (26) Grochala, W. Atypical Compounds of Gases, Which Have Been Called ‘Noble’. Chem. Soc. Rev. 2007, 36, 1632−1655. (27) Grochala, W.; Khriachtchev, L.; Räsänen, M. In Physics & Chemistry at Low Temperatures; Khriachtchev, L., Ed.; Pan Stanford Publishing: Singapore, 2011; p 421. (28) For older review, see: Frenking, G.; Cremer, D. Struct. Bonding (Berlin); Springer Verlag: Heidelberg, 1990; Vol. 73, pp 17−95. (29) Hope, E. Coordination Chemistry of the Noble Gases and Noble Gas Fluorides. Coord. Chem. Rev. 2013, 257, 902−909. (30) Lehmann, J. F.; Mercier, H. P. A.; Schrobilgen, G. J. The Chemistry of Krypton. Coord. Chem. Rev. 2002, 233−234, 1−39. (31) Formation of the cationic and anionic species of noble gases is much easier than the formation of neutral systems, as exemplified by HeH+; this has been extensively discussed in ref 11. The ArCF22+ dication is one example of charged species of argon, which has recently been prepared. Lockyear, J. F.; Douglas, K.; Price, S. D.; Karwowska, M.; Fijałkowski, K. J.; Grochala, W.; Remeš, M.; Roithová, J.; Schröder, D. Generation of the ArCF22+ Dication. J. Phys. Chem. Lett. 2010, 1, 358−362. (32) Khriachtchev, L.; Pettersson, M.; Runeberg, N.; Lundell, J.; Räsänen, M. A Stable Argon Compound. Nature 2000, 406, 874−876. (33) The HeBeO species, theorized over a quarter century ago, has not yet been observed in the laboratory: Frenking, G.; Koch, W.; Gauss, J.; Cremer, D. Stabilities and Nature of the Attractive Interactions in HeBeO, NeBeO, and ArBeO and a Comparison with Analogs NgLiF, NgBN, and NgLiH (Ng = He, Ar). A Theoretical Investigation. J. Am. Chem. Soc. 1988, 110, 8007−8016. CsFHeO, postulated in ref 7, also awaits synthesis. (34) Rzepa, H. S. The Rational Design of Helium Bonds. Nat. Chem. 2010, 2, 390−393.

G


Noble Gas Monoxides Stabilized in a Dipolar Cavity: A Theoretical

Recommend Documents