Strong Closed-Shell Interactions - American Chemical Society

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Strong Closed-Shell Interactions: Observed Formation of BaRg2þ Molecules in the Gas Phase at Room Temperature Gregory K. Koyanagi and Diethard K. Bohme* Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada

ABSTRACT Closed-shell interactions between the rare gases He, Ne, Ar, Kr, and Xe and the Ba2þ dication (isoelectronic with Xe) have been explored both experimentally and computationally. Unprecedented observations are reported for the formation of the diatomic molecules BaHe2þ, BaAr2þ, BaKr2þ, and BaXe2þ in helium bath gas at room temperature in a selected ion flow tube (SIFT) tandem mass spectrometer. The Ba2þ dications were produced by electrospray ionization. Triatomic cations of the type Ba(Rg)22þ also were recorded with Rg = Ar, Kr, and Xe. Experiment and theory demonstrate that the rate of formation of BaRg2þ and the strength of the interaction between Rg and Ba2þ increase with increasing polarizability of the rare gas atom. SECTION Kinetics, Spectroscopy

he nature and strength of closed-shell interactions between two atoms in the absence and presence of charge remains an active topic of discussion in the scientific literature.1 Both s2-s2, as in He2, BeHe2þ, TiHe2þ, VHe3þ, and AlHe3þ,2-5 as well as d10-s2 or d10-d6 (s2 for He) interactions, as in AuHgþ and AuRgþ respectively,6,7 have been of interest, albeit largely from a theoretical viewpoint. The weak closed-shell interactions between two rare gas atoms, Rg-Rg, can be expected to be strengthened in the presence of charge as in their isoelectronic analogues XRgþ (where X is an alkali atom), YRg2þ (where Y is an alkaline earth atom), and so on, increasing in strength with increasing charge. The gas-phase production of such diatomic molecules by elementary termolecular association as those in reaction 1, slow in the absence of charge, will be enhanced in the presence of charge, which leads to increased bond strengths and therefore increased lifetimes of the intermediate adducts that are stabilized by collision.

very recent high-level ab initio calculations.9 The experiments were made possible through the production of Ba2þ by electrospray ionization (ESI). The electrosprayed Ba2þ ions are mass selected and injected into flowing helium bath gas at room temperature in a selected ion flow tube (SIFT) tandem mass spectrometer and then monitored with the addition of the rare gas of interest into the reaction region of the flow tube.10-13 Addition of large controlled amounts of the rare gases Ar, Kr, and Xe into the flow tube flushed with helium in which Ba2þ was established upstream as the dominant ion was seen to lead to the sequential formation of diatomic BaRg2þ and triatomic Ba(Rg)22þ dications according to reactions 3 and 4. The experimental results for the sequential addition of Xe to Ba2þ are illustrated in Figure 1. Ba2þ þ Rg þ He f BaRg2þ þ He

ð3Þ

RgðnþÞ þ Rg þ M f Rg2 ðnþÞ þ M

BaRg2þ þ Rg þ He f BaðRgÞ2 2þ þ He

ð4Þ

T

ð1Þ

2þ

ð2Þ Received Date: September 21, 2009 Accepted Date: October 21, 2009 Published on Web Date: November 05, 2009

Substantial bonding interactions that approach the magnitude of hydrogen-bond strengths are predicted for these species with our supporting ab initio calculations as well as

r 2009 American Chemical Society

Ba(Xe)22þ

The molecular ions BaKr , BaXe , and were identified unambiguously by their isotope patterns. No formation of BaNe2þ was observed with the addition of neon. Also, there was only the slightest indication of the formation of an ion with m/z equal to that for BaHe2þ in the helium buffer gas, but the signal was very small. Effective bimolecular rate coefficients were measured for reaction 3 at room temperature in the usual way and are summarized in Table 1. The rate coefficients are small,

Recent interest in the search for neutrino-less double beta decay in Xe gas, which produces the daughter Ba2þ dication,8 has prompted us to investigate the formation of diatomic BaRg2þ molecules (isoelectronic with XeRg) in gas-phase reactions of the type in reaction 2, and here, we are able to report the unprecedented experimental observation of such molecular dications with mass spectrometry as well as measurements of their rates of formation at room temperature. Ba2þ þ Rg þ M f BaRg2þ þ M

2þ

41

DOI: 10.1021/jz900009q |J. Phys. Chem. Lett. 2010, 1, 41–44

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Figure 2. Dependence of the measured effective bimolecular rate coefficient for the addition of rare gas atoms to Ba2þ measured at 294 K on the polarizability R of the rare gas atom.

Figure 1. Ion signals recorded for the reaction of selected Ba2þ cations derived by ESI with added Xenon. PHe = 0.36 Torr, and T = 294 K.

k < 2 10-12 cm3 molecule-1 s-1, but there is a clear trend of increasing rate as we move down the periodic table. The increase in rate also correlates with an increase in the computed bond energy as well as an increase in the polarizability of the rare gas atom (see Figure 2) that determines the magnitude of the charge/induced-dipole interaction. The secondary reaction 4 appeared to react faster than the primary reaction 3 in all cases, and the observed semilogarithmic decay of BaRg2þ always was rate-limited by its formation by reaction 3. The increase in the rate of secondary formation of the triatomic Ba(Rg)22þ can be attributed to an increase in the lifetime of the intermediate complex due to an increase in the number of vibrational degrees of freedom of this complex. The computed 1-D potential energy surfaces for the interaction of He, Ne, Ar, Kr, and Xe with Ba2þ are shown in Figure 3. Our observation of the formation of a doubly charged diatomic molecule of the type MRg2þ (where M is a metal atom) from a room-temperature reaction of its two constituent atoms (one doubly charged) is unprecedented. Others have performed measurements of other reactions of atomic dications with rare gas atoms but did not report the observation of such ions. For example, reactions of ground-state Hg2þ(d10) with Ar and Kr have been investigated using a flow drift tube technique at collision energies from 0.05 to 2 eV, but electron transfer with charge separation was observed instead of addition.16 Electron transfer is exothermic in this case because of the high electron recombination energy of Hg2þ; IE(Hgþ) = 18.756 eV is higher than IE(Ar) or IE(Kr) (see Table 1). Electron transfer and double electron transfer were the predominant product channels observed in a systematic


Figure 3. Computed 1-D potential energy surfaces for the bonding of rare gas atoms with Ba2þ, showing extrapolation with a 1/r4 function. Table 1. Measured Kinetics at 294 ( 2 K in Helium at 0.36 ( 0.01 Torr for Reactions of Ba2þ with Rare Gas (Rg) Atoms and Computed Binding Energies for the Observed Product Ions along with the Ionization Energies (IE), Polarizabilities (R) of the Rare Gas Atoms, and Computed Binding Energies (D298, De) of the BaRg2þ Adduct Ion Rg

IEa

Rb

kc

He 24.587 0.205 observedd -14

product

D298/Dee

Def

BaHe2þ

-/1.62

none

0/3.23

2.11 [3.32] 7.39 [10.1]

1.24 [1.82]

Ne

21.564 0.396 De for BaHe2þ (see text). f In kcal mol-1. Computed in ref 9 at two levels of theory, RHF and [RCCSD(T)].

study using the SIFT technique at 300 K of reactions of Rg2þ with Rg (Rg = Ne, Ar, Kr, and Xe).17 However, the production of dimer dications again was not reported. Electron transfer is thermodynamically forbidden in the special room-temperature reactions of Ba2þ with Rg that are reported here

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since IE(Baþ) = 10.004 eV, and this favors the formation of the novel adduct ions that were observed in our measurements. The unprecedented results reported here extend the observation of chemical compounds of rare gases in the gas phase at room temperature to diatomic and triatomic molecules containing Ba2þ dications. The p6-p6 orbital interactions in these novel BaRg2þ molecules are shown computationally to be sufficiently strong to account for their stability. Experiment and theory demonstrate that the rate of formation of BaRg2þ at room temperature, as well as the strength of the interaction between Rg and Ba2þ, increases with increasing polarizability of the rare gas atom. The experiments were performed with a multisector selected ion flow tube tandem mass spectrometer fitted with an electrospray ion source, symbolized as ESI/qQ/SIFT/QqQ, developed in the Ion Chemistry Laboratory at York University.10,11 Dications of Ba were generated by ESI from 30 μM solutions of barium chloride in water/methanol (80/20). A declustering potential of 150 V was applied to remove the water from the hydrated ions that emerge from the solution. The bare dications emerging from the ESI source are mass selected and injected through a Venturi type aspirator into the flow tube that is flushed with helium at 0.36 ( 0.01 Torr. Before reaching the reaction region, the ions undergo multiple collisions with helium (∼4 105) to ensure thermalization. The large number of collisions with the helium buffer gas atoms ensures that the atomic ions reach a translational temperature equal to the tube temperature of 294 ( 2 K prior to entering the reaction region. The ions are allowed to react with rare gases added into the reaction region, and then, they are sampled along with product ions and analyzed in a triple quadrupole mass spectrometer. The reactant and product ion signals are monitored as a function of the flow of the added rare gas. The Ne, Ar, Kr, and Xe are introduced into the reaction region of the SIFT as pure gases and are obtained commercially, Ne (99.999%, Air Products), Ar (99.999%, Air Liquide), Kr and Xe (99.995%, Spectra Gases, Brandburg, NJ 08876). Primary rate coefficients with an absolute accuracy estimated to be (30% are determined from the observed semilogarithmic decay of the primary reactant ion intensity in the usual manner using pseudo-first-order kinetics.12,13 Quantum chemical computations were performed using the Gaussian 03 package.18 As the interaction is between two closed-shell atomic species, the interaction was assumed to be mostly electrostatic in nature. Consequently, it was felt that the basis set used should accurately reproduce the polarizability of the noble gas atom. Initial trials with the SDD basis set19,20 indicated sufficient flexibility in the outer shell functions of both Kr and Xe to reproduce the majority of the experimentally observed polarizability; however, the computed polarizabilities of He, Ne, and Ar were unacceptably low. For these three atoms, the SDD basis set was supplemented by adding the diffuse functions of Clark et al.21 and by adding the polarization functions of Frisch


et al.22 (3pd) for helium and (3df) for neon and argon. The 1-D surfaces for BaHe2þ, BaNe2þ, BaAr2þ, BaKr2þ, and BaXe2þ were computed from 250 to 900 pm in 10 pm increments using the commonly employed density functional method and B3LYP.23 The region from 500 to 900 pm was fitted to a r-4 curve to determine the dissociation limit for use in determining the bond energy. Zero-point vibration energies (ZPE) were computed using the harmonic oscillator approximation. For BaHe2þ, the ZPE was found to be larger than De; ZPE = De for BaNe2þ.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: [email protected]. Tel: 1-416-736-2100 66188. Fax: 1-416-736-5936.

ACKNOWLEDGMENT Continued financial support from the Natural Sciences and Engineering Research Council of Canada is greatly appreciated. Also, we acknowledge support from the National Research Council, the Natural Science and Engineering Research Council, and MDS SCIEX in the form of a Research Partnership grant. As holder of a Canada Research Chair in Physical Chemistry, Diethard K. Bohme thanks the Canada Research Chair Program for its contributions to this research.

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