FULL PAPER DOI: 10.1002/chem.200600219 Neutral Helium Compounds: Theoretical Evidence for a Large Class of Polynuclear Complexes Stefano Borocci, Nicoletta Bronzolino, and Felice Grandinetti*[a] ACHTUNGRE ACHTUNGRE Abstract: Ab initio calculations at the (NBeHe)4Àn and HnSi(NBeHe)4Àn (n= the central atom also showed a variable ACHTUNGRE MP2 and CCSD(T) levels of theory 0–3), C2(NBeHe)2, and ortho-, meta-, topology and include second-order ACHTUNGRE ACHTUNGRE disclose the conceivable existence of and para-C6H4(NBeHe)2 were invari- saddle points such as S(NBeHe)2, neutral complexes containing up to ablyACHTUNGRE characterized as energy minima, third-order saddle points such as HN- ACHTUNGRE four helium atoms. These species are and were found to be stable with re- (NBeHe)2, but also minimum-energy ACHTUNGRE formally obtained by replacing the hy- spect to the loss of helium atom(s) by structures such as O(NBeHe)2 and HP- À1 ACHTUNGRE drogen atoms of parent molecules such approximately 4–5 kcalmol . On the (NBeHe)2, which are also stable by ap- À1 as CH4, SiH4,NH3,PH3,H2O, H 2S, other hand, species such as C2- proximately 5 kcalmol with respect ACHTUNGRE ACHTUNGRE C2H2,C2H4, and C6H6 with ÀNBeHe (NBeHe)4 and C6(NBeHe)6 were char- to the helium atom(s) loss. These re- moieties, which behave as monovalent acterized as high-order saddle points sults suggest the conceivable existence functional groups containing helium. on the potential-energy surface, and of an, in principle, very large class of ACHTUNGRE The geometries and vibrational fre- were unstable with respect to helium M(NBeHe)n (n> 1) polyhelium com- ACHTUNGRE quencies of these M(NBeHe)n (n>1; atom(s) loss owing to the bending plexes, whose stability may be substan- M=central moiety) polyhelium com- motion of the ÀNBeHe groups. The tially affected by the nature and the plexes have been investigated at the molecules containing N, P, O, or S as size of the central moiety M. Atoms-in- MP2(full)/6-31G(d)ACHTUNGRE level of theory, and Molecules (AIM) calculations on se- their stability with respect to the loss lected species invariably suggest that, Keywords: ab initio calculations · of helium atom(s) has been evaluated in our investigated M(NBeHe)ACHTUNGRE (n>1) atoms-in-molecules (AIM) theory · n by means of single-point calculations at compounds, the beryllium–helium in- helium · neutral complexes · the CCSD(T)/6-311G(d,p)ACHTUNGRE level of teraction is essentially electrostatic. stability theory. Molecules such as HnC- Introduction addition, over the years, several neutral complexes such as, [14–16] [17] [18] for example, ArW(CO)5, ArBeO, ArAgX, ArCuX + À [1,2] [19, 20] [21] [22] Since Bartletts discovery of “Xe PtF6 ” and the obser- (X=F, Cl, Br), ArAuCl, and CUO(Ar)n (n1) [3] vation of KrF2 one year later, the chemistry community re- have been experimentally observed in low-temperature ma- alized that krypton, xenon, and probably also radon, could trices. On the other hand, the isolation of neutral species have a promising chemistry. Numerous compounds of these containing helium and neon still remains a fascinating chal- elements (xenon in particular) have in fact been subsequent- lenge in the chemistry of the lightest noble gases.[23] Helium ly isolated and characterized,[4] and novel evidence of their in particular is the most inert among the inert gases. It has properties and reactivity is still emerging.[5–11] More recently, the highest ionization potential (24.587 eV) and the lowest another noble gas, argon, was observed in HArF,[12,13] a polarizability (0.205 3)[24] of all the chemical elements and matrix compound with a strong covalent (ArH) + bond. In therefore appears as a very hard sphere, strongly bound only by positively charged species.[25] As a matter of fact, apart from helium itself, with the formation of Hen (n2) [a] Dr. S. Borocci, Dr. N. Bronzolino, Prof. F. Grandinetti clusters or bulk He,[26] only rare neutral partners such as the Dipartimento di Scienze Ambientali [27,28] [29,30] Hg atom or the cage compound C60 have been ob- Università della Tuscia, L.go dell Università served to fix helium, and theory is invited to predict the ex- s.n.c., 01100 Viterbo (Italy) Fax : (+39)0761-357-179 istence of yet unknown compounds and to suggest viable E-mail: [email protected] routes to their preparation and structural characterization. Chem. Eur. J. 2006, 12, 5033 – 5042 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 5033 Over the years, these calculations have disclosed metastable [31–36] [37] [38] species such as HHeF, H3BOBeHe, and C6F5HeF, as well as thermodynamically stable species such as OBeHe[39–41] and SBeHe.[42] More recently, we have found a series of beryllium–helium complexes of general formula RNBeHe.[43] The residue R ranges from the monatomic H, F, and Cl to more complex aliphatic, carbonylic, and aromat- ic groups, and any species found has been invariably charac- terized as a minimum-energy structure on the singlet sur- face, and as stable or metastable with respect to dissociation into He and singlet RNBe. Generally speaking, the ÀNBeHe moiety behaves as a monovalent “functional group” containing helium, which combines with monovalentACHTUNGRE resi- dues RÀ to form a predictably very large class of RNBeHe molecules. This observation suggests the still unexplored possibility that neutral species that contain more than one helium atom do exist. These molecules should have the gen- ACHTUNGRE eral formula M(NBeHe)n (n>1), and we studied, in particu- lar, at the ab initio level of theory, the structure and stability ACHTUNGRE of exemplary structures such as HnC(NBeHe)4Àn (n=0–2), ACHTUNGRE ACHTUNGRE Figure 1. MP2(full)/6-31G(d)ACHTUNGRE optimized geometries (bond lengths in HnN(NBeHe)3Àn (n=0, 1), and O(NBeHe)2, as well as nu- ACHTUNGRE and bond angles in 8) of the HnC(NBeHe)4Àn (n=0–3) molecules. N is merous more complex organic molecules that contain up to the number of imaginary frequencies. six ÀNBeHe groups. Most of the investigated species were actually characterized as true minima on the singlet surface, thus providing the first evidence for stable or metastable emplary cases among the presently investigated polyhelium polynuclear helium complexes. The details of our calcula- complexes. tions will be discussed in the present article. They are formally obtained by replacing the H atoms of CH4 with ÀNBeHe moieties, which behave as monovalent functional groups containing helium. The BeÀN and BeÀHe Results and Discussion bond lengths are computed to be around 1.375 and 1.500 , respectively, and their corresponding harmonic frequencies The presently discussed helium complexes include the group all range around 1700 and 500 cmÀ1, respectively. In addi- ACHTUNGRE XIV molecules HnX(NBeHe)4Àn (n=0–3; X=C, Si), the tion, the frequency of the N-Be-He bending motions are in- ACHTUNGRE À1 group XV molecules HnX(NBeHe)3Àn (n=0–2; X= N, P), variably predicted to be around 150 cm . Concerning the the group XVI molecules HX- (NBeHe)ACHTUNGRE and X(NBeHe)ACHTUNGRE (X= 2 Table 1. MP2(full)/6-31G(d)ACHTUNGRE harmonic vibrational frequencies [cmÀ1] and CCSD(T)/6-311G(d,p)//MP2ACHTUNGRE (full)/6-ACHTUNGRE O, S), and other carbon-con- À1 ACHTUNGRE 31G(d) dissociation energies [kcal mol ] at 0 Kof the H nC(NBeHe)4Àn (n =0–3) molecules 1–4 (see Figure 1). taining molecules such as HC2- [a] ACHTUNGRE [a] ACHTUNGRE [a] ACHTUNGRE H3CNBeHe (1) H2C(NBeHe)2 (2) HC(NBeHe)3 (3) C(NBeHe)4 (4) (NBeHe),ACHTUNGRE C (NBeHe)ACHTUNGRE ,HC - 2 2 n 2 ACHTUNGRE À [b] [b] [b] [b] ACHTUNGRE n˜(Be He) 538.6 (A1,1.4) 468.8 (A1,1.1) 456.9 (A1,0.3) 454.0 (A1,0) (NBeHe)4Àn (n=0–3), [b] [b] [b] 537.7 (B2,7.1) 472.6 (E,0.003) 464.3 (T2,0.3) ACHTUNGRE [b] [b] [b] [b] C6H5NBeHe, and C6H4- n˜(BeÀN) 1722.8 (A1,29.5) 1689.9 (A1,16.1) 1660.4 (A1,5.6) 1633.5 (A1,0) ACHTUNGRE [b] [b] [b] (NBeHe)2. Their optimized ge- 1698.8 (B2,89.6) 1673.8 (E,104.4) 1649.7 (T2,102.1) ACHTUNGRE [b] [b] [b] [b] ometries, harmonic frequencies, d(N-Be-He) 161.0 (E,10.2) 143.9 (A2,0) 129.7 (A2,0) 112.0 (T1,0) [b] [b] [b] 148.7 (B2,10.6) 139.6 (E,7.8) 167.0 (E,0) and thermochemical data, as [b] [b] [b] 157.4 (B1,19.0) 184.7 (E,0.4) 173.5 (T2,0.3) well as the results of Atoms-in- [b] [b] 200.4 (A1,0.1) 186.8 (A1,0.01) ACHTUNGRE [c] [b] [b] [b] Molecules (AIM) calculations w(N-Be-He) 89.8 (A1,22.4) 83.7 (E,19.7) 73.1 (E,0) [b] [b] on selected species, are report- 95.3 (A1,54.1) 84.1 (T2,49.9) ACHTUNGRE [b] [b] [b] [b] n˜(CÀN) 1017.3 (A1,9.65) 978.3 (A1,1.6) 939.7 (A1,1.9) 759.5 (A1,0) ed in Figures 1-8 and in [b] [b] [b] 1026.2 (B2,112.9) 1024.6 (E,177.4) 1035.3 (T2,245.9) Tables 1–8 below. ACHTUNGRE [b] [b] [b] [b] d(C-N-Be) 299.9 (E,11.7) 274.4 (B2,1.4) 259.2 (E,0.5) 261.8 (T1,0) [b] [b] 274.7 (B1,14.0) 270.3 (A2,0) ACHTUNGRE [b] HnX(NBeHe)4Àn (n=0–3; X=C, 277.5 (A2,0) ACHTUNGRE [b] [b] [b] Si): The H C(NBeHe)ACHTUNGRE mole- d(N-C-N) 612.1 (A1,3.8) 585.1 (E,13.65) 487.8 (E,0) n 4Àn [b] [b] 678.2 (A1,0.2) 630.7 (T2,9.5) cules 1–4 shown in Figure 1 and [d] [e] [e] DE1 4.7 (5.5) 4.4 (5.1) Table 1, invariably character- [f] [e] DE2 4.4 (5.2) ized as true minima on the [a] The -CH motions (n=1, 2, or 3) are not included.