Noble gas anions of general formula FNgBN- (Ng = He-Xe) have been investigated by MP2, coupled-cluster, and multireference-CI calculations with correlation-consistent basis sets. These species reside in deep wells on the singlet potential energy surface and are thermodynamically stable with respect to the loss of F, F-, BN, and BN-. They are unstable with respect to Ng + FBN-, but at least for Ng = Ar, Kr, and Xe, the involved energy barriers are high enough to suggest their conceivable existence as metastable species. The stability of FNgBN- arises from the strong F--stabilization of the elusive NgBN. The character of the boron-noble gas bond passes from purely ionic for FHeBN- and FNeBN- to covalent for FXeBN-.

The structure and stability of as yet unreported compounds with the general formula RNBeNg (Ng = He, Ne, Ar) have been theoretically investigated at various ab initio and B3LYP density functional (DFT) levels of theory. Exemplary species include the parent HNBeNg and saturated and unsatured compounds such as HONBeNg, FNBeNg, X−CH2−NBeNg, X−C(O)−NBeNg (X = H, OH, F), and C6H5−NBeNg. The thermochemical stability of these molecules, invariably characterized as true energy minima on the singlet potential energy surface, depends on two factors, namely, the energy of dissociation ΔE into singlet RNBe and Ng and the energy difference between the singlet and triplet states of RNBe. The values of ΔE are essentially independent of the nature of the substituent R and are around 6.5 kcal mol-1 for Ng = He, 8.5 kcal mol-1 for Ng = Ne, and 11.0 kcal mol-1 for Ng = Ar. In addition, for most of the investigated RNBeNg compounds, we have found that the singlet state of RNBe is more stable than the triplet state or for RNBe, with a triplet ground state, that the singlet−triplet gap is lower than the ΔE value given above. Therefore, our calculations support the prediction that this class of thermochemically stable RNBeNg compounds could actually be very large.

Ab initio calculations at the MP2 and CCSD(T) levels of theory disclose the conceivable existence of neutral complexes containing up to four helium atoms. These species are formally obtained by replacing the hydrogen atoms of parent molecules such as CH(4), SiH(4), NH(3), PH(3), H(2)O, H(2)S, C(2)H(2), C(2)H(4), and C(6)H(6) with -NBeHe moieties, which behave as monovalent functional groups containing helium. The geometries and vibrational frequencies of these M(NBeHe)(n) (n>1; M=central moiety) polyhelium complexes have been investigated at the MP2(full)/6-31G(d) level of theory, and their stability with respect to the loss of helium atom(s) has been evaluated by means of single-point calculations at the CCSD(T)/6-311G(d,p) level of theory. Molecules such as H(n)C(NBeHe)(4-n) and H(n)Si(NBeHe)(4-n) (n=0-3), C(2)(NBeHe)(2), and ortho-, meta-, and para-C(6)H(4)(NBeHe)(2) were invariably characterized as energy minima, and were found to be stable with respect to the loss of helium atom(s) by approximately 4-5 kcal mol(-1). On the other hand, species such as C(2)(NBeHe)(4) and C(6)(NBeHe)(6) were characterized as high-order saddle points on the potential-energy surface, and were unstable with respect to helium atom(s) loss owing to the bending motion of the -NBeHe groups. The molecules containing N, P, O, or S as the central atom also showed a variable topology and include second-order saddle points such as S(NBeHe)(2), third-order saddle points such as HN(NBeHe)(2), but also minimum-energy structures such as O(NBeHe)(2) and HP(NBeHe)(2), which are also stable by approximately 5 kcal mol(-1) with respect to the helium atom(s) loss. These results suggest the conceivable existence of an, in principle, very large class of M(NBeHe)(n) (n>1) polyhelium complexes, whose stability may be substantially affected by the nature and the size of the central moiety M. Atoms-in-Molecules (AIM) calculations on selected species invariably suggest that, in our investigated M(NBeHe)(n) (n>1) compounds, the beryllium-helium interaction is essentially electrostatic.

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