Beryllium–helium cations: computational evidence for a large class of thermodynamically stable species

Dedicated to Prof. Dr. Helmut Schwarz on the occasion of his 60th birthday.
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Abstract

Ab initio calculations, at the MP2, QCISD, CCSD, and CASSCF levels of theory, have been performed to investigate the structure, stability, and properties of a new class of thermodynamically stable cations containing helium. These species have general formula XBeHe+ (X: monovalent group) and arise from the ligation of a helium atom to singlet ground state BeX+. The presently investigated systems include prototype “inorganic” ions such as HBeHe+, FBeHe+, ClBeHe+, HOBeHe+, and H2NBeHe+, as well as “organic” species such as H3CBeHe+, F3CBeHe+, HC2BeHe+, H3C2BeHe+, and C6H5BeHe+. Irrespective of the substituent X, at any computational level, including the highly accurate Gaussian-3 (G3), the dissociation energies at 298.15 K of XBeHe+ into singlet ground-state BeX+ and He are predicted to be remarkably large and range from ca. 6 kcal mol−1 for C6H5BeHe+ to ca. 11 kcal mol−1 for FBeHe+. Thus, the electronic structure of the substituent X has an appreciable effect on the structure and stability of the XBeHe+ cations. We have also briefly examined the implications of our theoretical calculations for future gas-phase experiments aimed at the experimental observation and characterization of members of this new class of thermodynamically stable species of the lightest noble gas.

Introduction

The adducts between helium atoms and positively-charged ions Mn+ (n≥1) are of central interest in many areas of gas-phase ion chemistry and physics, ranging from the solvation and clustering of positive ions [1], [2], [3], [4] to the dynamics of the elastic and inelastic collisions between ionic projectiles and atomic targets [5], [6]. The interaction with helium atoms is also important for evaluating the transport properties of cations in a bath of the inert gas, which is of consequence in understanding the mobility of ions in plasma discharges [7], [8] and planetary atmospheres [9]. In the current years, the clusters of helium atoms with singly- and multiply-charged cations are investigated with renewed interest, stimulated by the possible use of ionic dopants as probes to investigate the superfluid properties of helium [10], [11], [12]. In addition, taking into account the reluctance of the lightest noble gas to form stable compounds in the condensed phase, the study of its gaseous ionic adducts is expected to provide telling information on the factors conceivably involved in the chemical fixation of this element [13]. It is therefore not surprising that, over the years, numerous research groups have employed various experimental techniques, including lasers, molecular ion beams, mass spectrometry, and molecular spectroscopy, to investigate the gaseous adducts and clusters of helium with a variety of monoatomic and polyatomic ions [2], [3], [4], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Pioneering observations include, for example, the reports by Tsong and coworkers on the ability of numerous transition metals to form mono- and dihelide cations [26], [27]. The structure, stability, and properties of the adducts of helium with Mn+ (n≥1) have been also the focus of intensive theoretical work, performed not only to aid the interpretation of the experiments but also to independently disclose novel fascinating features of this chemistry [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42].

Generally speaking, small-size multiply-charged ions Mn+ (n≥2) are able to fix helium with formation of Mn+–He complexes whose stability may be as large as some tens of kcal mol−1 [12], [31], [37], [43], [44], [45], [46]. On the other hand, apart from some remarkable exceptions such as the endohedral C60He+ cluster containing a helium atom “encapsulated” into the cage [17], [32], or simple diatomic and triatomic species such as HeNe+ and HeH2+, whose dissociation energies into Ne+ and He and H2+ and He amount to ca. 18 kcal mol−1 [15] and ca. 8 kcal mol−1 [41], respectively, the adducts of He with monoatomic and simple polyatomic ions M+ are usually characterized as extremely fragile complexes, which feature typical dissociation energies of less than 1 or 2 kcal mol−1 [1], [2], [3], [4]. Thus, the identification of novel M+–He adducts of appreciable thermodynamic stability is still posing stimulating questions of experimental and theoretical interest.

In the late 1980s, Frenking and coworkers [13], [47] reported that the diatomic BeO, in its singlet ground state 1Σ+, fixes the lightest noble gases with formation of OBeNg complexes (Ng=He, Ne, Ar, Kr, Xe), which are thermodynamically stable with respect to dissociation into BeO and Ng. The ObeNg bond energies were calculated to range from about 3 kcal mol−1 for Ng=He to about 13 kcal mol−1 for Ng=Xe, and the stability of the adducts was related with the small radius of the Be2+ cation in the neutral BeO, which leads to an electric field large enough to trap even a helium atom. This suggestion is consistent with the older idea, supported by early theoretical calculations [48], [49], [50], [51] and confirmed by more recent ones [31], [37], that it is possible to use the Be2+ dication to fix the unreactive helium. Stimulated by these findings, as part of our continuing interest in the chemistry of gaseous fluorinated cations [52], [53], [54], [55], [56], [57], [58], [59], [60], we have recently found that, in its singlet ground state 1Σ+, the diatomic BeF+, isoelectronic with BeO, forms thermochemically stable FBeNg+ adducts (Ng=He, Ne, Ar) whose dissociation enthalpies (at 298.15 K) into BeF+ and Ng are computed, at the Gaussian-3 (G3) level of theory, as large as 10.6 kcal mol−1 for Ng=He, 16.0 kcal mol−1 for Ng=Ne, and 34.5 kcal mol−1 for Ng=Ar [61]. The implications of these findings for the conceivable existence of stable salts of the lightest noble gases have been examined in our previous article [61]. In the present one, we wish to discuss the results of novel ab initio and density functional theory (DFT) calculations which indicate that BeF+ is just a member of a large class of BeX+ cations (X: monovalent group), which, in their singlet ground state, are able to fix helium with formation of thermodynamically stable XBeHe+ adducts. The detailed investigation of a series of exemplary “inorganic” and “organic” species such as HBeHe+, FBeHe+, ClBeHe+, HOBeHe+, H2NBeHe+, H3CBeHe+, F3CBeHe+, HC2BeHe+, H3C2BeHe+, and C6H5BeHe+ indicate that the substituent X has appreciable effects on the structure, stability, and properties of these ions. In addition, we will briefly examine the implications of our calculations for future conceivable gas-phase experiments aimed at the experimental observation and characterization of members of this new class of beryllium–helium cations.

Section snippets

Computational details

All the presently reported calculations have been performed using the Unix versions of the GAUSSIAN 98 [62] and the MOLPRO 2000.1 [63] sets of programs installed on a Alphaserver 1200 and a DS20E Compaq machine.

The geometries of the XBeHe+ ions and of their BeX+ fragments have been optimized at various ab initio levels of theory, including the second-order Møller–Plesset with inclusion of the inner electrons, MP2(full) [64], the quadratic configuration interaction with single and double

Structure, stability, and properties of XBeHe+ (X = H, F, Cl, OH, NH2, CH3)

The geometries of the XBeHe+ ions containing monoatomic and simple polyatomic substituents HBeHe+, FBeHe+, ClBeHe+, HOBeHe+, H2NBeHe+, and H3CBeHe+, in their singlet ground state, have been optimized, using various basis sets, at the B3LYP and at several ab initio levels of theory, including MP2, QCISD, and CCSD(T). The obtained parameters are collected in Table 1, Table 2, and the MP2(full)/6-31G(d) harmonic vibrational frequencies are listed in Table 3 together with those of the BeX+

Conclusions

The advances made recently in the mass spectrometric and spectroscopic investigation of fragile ionic complexes of the lightest noble gases, particularly helium, stimulate the theoretical investigation of still unexplored species which could be in principle experimentally investigated and characterized. In the present study, we have obtained evidence for a new large class of thermodynamically stable beryllium–helium cations. Although our calculations concern just a few number of “inorganic” and

Acknowledgements

The authors wish to thank the Italian Ministero dell’ Istruzione, dell’ Università e della Ricerca (MIUR) and Consiglio Nazionale delle Ricerche (CNR) for financial support.

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