Helionitronium trication (NO₂He³⁺) and helionitrosonium trication (HeNO³⁺)

Abstract

The structures and stabilities of helionitronium trication NO₂He³⁺ and helionitrosonium trication HeNO³⁺ were calculated at the ab initio MP2/6–31G** level. The C_s symmetry structure was found to be a minimum for the NO₂He³⁺ trication, which is isoelectronic and isostructural with the previously studied NO₂H²⁺. Dissociation of the C_s symmetry structure into NO⁺ and OHe²⁺ is thermodynamically preferred by 183.1 kcal/mol (1 cal = 4.18 J), although a kinetic barrier of 12.4 kcal/mol has to be overcome. The C_∞v symmetry structure was also found to be a minimum for the HeNO³⁺ trication.

Nitronium ion (NO₂⁺) is the electrophile in the acid-catalyzed nitration of aromatics and activated aromatics with nitric acid (1, 2). Superelectrophilic (3) protonitronium dication (NO₂H²⁺), on the other hand, is responsible for nitration of highly deactivated aromatics with NO₂⁺BF₄⁻ in superacids, as shown by Olah et al. (4–7). Lower level calculations carried out by Simonetta (8) on the protonitronium dication indicated that the dication may not correspond to a minimum. However, our HF/6–31G*, MP2/6–31G**, and B3LYP/6–31G** levels calculations show that NO₂H²⁺ corresponds to a minimum (9, 10). Schwarz et al. (11) were indeed able to generate NO₂H²⁺ in the gas phase by dissociative electron impact ionization of HNO₃. The ¹⁷O NMR studies of nitronium ion in strong acid media have also been reported (10). The ¹⁷O NMR line broadening of nitronium ion peak in HSO₃F:SbF₅ has been attributed to proton exchange involving protonitronium dication NO₂H²⁺ (10).

Similar to NO₂⁺, NO⁺ can also be acitivated to give superelectrophilic protonitrosonium dication in superacids (12). Calculations show that only the N-protonated form HNO²⁺ corresponds to a stable minimum and the O-protonated form is unstable and dissociates into the nitrosonium ion and a proton (12).

The structures and stabilities of helium-containing polyatomic ions (13) were calculated by Wilson et al. (14, 15), Schleyer et al. (16), Koch and Frenking (17, 18), and Radom et al. (19). Unencumbered He²⁺ is an even stronger acid than H⁺.‡ Ab initio calculations show that helium is capable of forming strong bonds with carbon in cations such as in HeCCHe²⁺, as reported by Koch and Frenking (17, 18). Schleyer et al. (16) first calculated the quadruply charged tetraheliomethane tetracation CHe₄⁴⁺. Radom et al. have also presented theoretical evidence for the remarkable stability of CHe₄⁴⁺ (19). Olah et al. have also reported ab initio calculations that show that helium is capable of forming strong bond with carbon in heliomethonium dication CH₄He²⁺.

In continuation of our study of protonated onium dications (superelectrophiles) (3) we have now extended our theoretical investigations to helionitronium trication (NO₂He³⁺) and helionitrosonium trication (HeNO³⁺) and report our findings.

RESULTS AND DISCUSSION

Geometry optimizations and frequency calculations were carried out by ab initio method using the correlated MP2/6–31G** level (20). From calculated frequencies, the optimized structures were characterized as minima, saddle point, or transition structure. The Gaussian-2 (G2) method (21) was used for accurate energy calculations. The G2 theory is a composite method based on MP2(FU)/6–31G* geometry that is treated in single-point calculations with a variety of basis sets at the post-self-consistent field level. Atomic charges at the MP2/6–31G**//MP2/6–31G** were obtained by using the natural bond orbital analysis (22) (NBO) method.

NO₂He³⁺.

The C_s structure 1 and C_2v structure 2 (Fig. 1) were found to be minima on the potential energy surface of NO₂He³⁺ at the MP2/6–31G** level as indicated by frequency calculations (no. of imaginary frequencies = 0) at the same level. Calculated energies and frequencies are listed in Table 1 and Table 2, respectively. With the G2 theory, helium oxygen bonded 1, however, was found to be 99.3 kcal/mol (1 cal = 4.18 J) more stable than helium nitrogen bonded 2. Calculated structure of 1 and 2 together with NO₂⁺ 3 are given in Fig. 1.

Figure 1.

Calculated structures of 1–4 (calculated NBO charges).

Table 1.

Total energies (Hartree) and relative energies

Structure	Ion	MP2/6-31G** (ZPE)	G2	Relative energy, kcal/mol
1	NO2He3+	204.98651  (6.6)	205.26923	0.0
2	NO2He3+	204.88682  (11.7)	205.11102	99.3
3	NO2+	204.23933  (6.7)	204.48421	492.6
4	NO2He3+ (TS)	204.96464  (5.3)		12.4†
	OHe2+	76.00562  (1.9)	76.16218
5	HeNO3+	129.73938  (6.2)	129.85375	0.0
6	NO+	129.24259  (2.8)	129.39888	285.4

Zero point vibrational energies (ZPE) are at MP2/6-31G**//MP2/6-31G** level scaled by a factor of 0.93. Relative energies based on G2. 

^†Based on MP2/6-31G**//MP2/6-31G** + ZPE with G2, self-consistent field values did not converge. 

Table 2.

MP2/G-31G^** calculated frequencies and IR intensities

Structure	Frequencies, cm−1 (IR intensities in Km/mol)
1	330 (0); 339 (8); 415 (19); 807 (2); 1,347 (40); 1.706 (190)
2	562 (40); 643 (80); 916 (1); 1,269 (26); 2,421 (328); 3,015 (1,351)
4	i236† (268); 248 (2); 257 (1); 379 (7); 1,330 (112); 1,754 (968)
5	800 (1); 1,256 (160); 1,777 (16)

^†Imaginary frequency. 

The structure 1 is characterized by a long N—O₂ (1.586 Å) bond. This is 0.430 Å longer than N—O bond of NO₂⁺ 3. The O—He bond distance is 1.163 Å, which is slightly longer than that of calculated He₂O²⁺ (1.148 Å) (16). This observation indicates that helium is strongly bonded to the oxygen of 1. The O1—N—O2 bond angle is 171.0°. The trication 1 is isoelectronic with protonitronium dication NO₂H²⁺. The trication 1 is also isostructural with the previously calculated structure of NO₂H²⁺ (9, 10).

The NBO charge calculations (Fig. 2) show that the helium atom of 1 bears less positive charge than any of the oxygen or nitrogen atoms. The nitrogen atom of trication 1 bears almost the same positive charge as the nitrogen atom of the nitronium monocation NO₂⁺ 3. This observation shows that in the reaction NO₂⁺ 3 + He²⁺ → NO₂He³⁺ 1, most of the charges from helium transfer to the oxygen atoms of 1. These results also show that the unusually long N—O2 (1.586 Å) bond is probably due to charge–charge repulsion.

Figure 2.

Calculated structures of 5 and 6 (calculated NBO charges).

Two possible dissociation paths for the ion 1 were considered, and their energetics were calculated with the G2 method. The dissociation into NO⁺ and OHe²⁺ was calculated to be exothermic by 183.1 kcal/mol. We also have located a transition structure, 4 (Fig. 1), for the dissociation process. Structure 4 lies 12.4 kcal/mol higher in energy than 1. Thus, 1 has considerable kinetic barrier for such dissociation. In comparison, the barrier toward such a dissociation of protonitronium dication NO₂H²⁺ into NO⁺ and OH⁺ was calculated to be 38.5 kcal/mol. Another possible dissociation of 1 into NO₂⁺ 3 and He²⁺, however, was calculated to be highly unfavorable (endothermic by 492.6 kcal/mol).

HeNO³⁺.

Two possible structures, N-heliated and O-heliated forms, can be considered for the helionitrosonium trication. The N-heliated C_∞v structure 5 (Fig. 2) was found to be a minimum on the potential-energy surface of HeNO³⁺ at the MP2/6–31G** level, as indicated by their frequency calculations at the same level. The O-heliated structure is unstable, and dissociate to the NO⁺ 6 and He²⁺. Similar to O-heliated structure, the O-protonated nitrosonium dication NOH²⁺ is also unstable and dissociates into the NO⁺ 6 and H⁺ on optimization (12). The N—He bond distance of 5 is 1.269 Å, 0.106 Å longer than O—He bond in 1. The N—O (1.083 Å) bond is 0.043 Å longer than N—O bond in NO⁺ 6. Calculated NBO charges of 5 and 6 are given in Fig. 2. The dissociation of 5 into NO⁺ 3 and He²⁺ was calculated to be also highly unfavorable (endothermic by 285.4 kcal/mol).

Present ab initio molecular orbital study shows that the helionitronium trication NO₂He³⁺ 1 is a minimum on its potential energy surface. Its dissociation into NO⁺ 6 and OHe²⁺ is thermodynamically preferred by 183.1 kcal/mol, although a kinetic barrier of 12.4 kcal/mol has to be overcome. In structure 1, helium is strongly bonded to the oxygen. Similarly, helionitrosonium trication HeNO³⁺ 5 also was found to be a stable minimum on the potential energy surface of HeNO³⁺.

This is paper 52 in the series “Onium Ions”; paper 51 is ref. 23. Support of our work by the National Science Foundation and the Office of Naval Research is gratefully acknowledged.

ABBREVIATIONS

G2

Gaussian-2

NBO

natural bond orbital