Theoretical Investigation of Selenium Interferences in Inductively

Article pubs.acs.org/JPCA

Theoretical Investigation of Selenium Interferences in Inductively Coupled Plasma Mass Spectrometry G. Bouchoux,*,† A. M. Rashad,‡,§ and A. I. Helal‡ †

Laboratoire des Mécanismes Réactionnels, Département de Chimie, Ecole Polytechnique and CNRS, 91128 Palaiseau, France Central Laboratory for Elemental and Isotopic Analysis, Nuclear Research Center, Atomic Energy Authority, Cairo 13759, Egypt

‡

S Supporting Information *

ABSTRACT: Structures, heats of formation, ionization energies, and proton affinities of selenium, argon dimer, argon-chlorine, and their hydrides (Se, SeH, SeH2, ArH, ArH2, Ar2, Ar2H, Ar2H2, ArCl, and ArHCl) are estimated by quantum chemistry calculations using G3, G4, and W1 composite methods and coupled cluster approach at the CCSD(T)/aug-cc-pVTZ levels. Thermochemistry of the reactions between ions A+ = Se•+, SeH+, SeH2•+, SeH3+, Ar2•+, Ar2H+, Ar2H2•+, Ar2H3+, ArCl+, ArClH•+, and ArClH2+ with various neutral gas G commonly used in dynamic reaction chamber-inductively coupled plasma-mass spectrometry (DRC-ICP-MS) (G = H2, CH4, NH3, O2, CO, CO2, NO, and N2O) has been investigated.

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leading to variable detection limit.8 For this reason, collision cell technologies have been developed and increasingly used in analytical plasma mass spectrometry in the last decade. These techniques in which a gas-filled multipole is located before the mass analyzer allow the elimination of interference by ion− molecule reactions or by collision induced dissociations. Strictly speaking, a collision cell is a device where part of the kinetic energy of the incoming ion is converted into internal energy after inelastic collision with the target gas. It is generally operating at low pressure and high kinetic energy of the incoming ions in order to induce its activation by transfer of a part of kinetic energy to internal degrees of freedom. Under these conditions, collision cells allow endothermic fragmentations to occur. By contrast, in a reaction cell, high pressure and low kinetic energies are operated in order to promote collision energy damping in order to attain near thermal conditions. Ideally, reaction cells allow exclusively the occurrence of exothermic chemical (ion−molecule) reactions. Ion−molecule reactions that may occur with the target gas in the gas cell of an ICP-MS device involve either the interfering ions or the element cation to be analyzed. Target gases such as NH3,8,10 CH4,11−13 H2,14,15 and mixtures of H2 in mixture with He7,8 have been used to remove the Ar2+ interfering ions. Shift of the Se+ signal to SeO+ by oxidizing agents such as O2 or N2O has been also reported.8,9,15 The mechanisms that may be involved inside the reaction/collision cell are charge exchange or proton, hydride ion, or atom (H, O, etc.) transfers, depending upon the element cation and the reaction gas. Understanding and predicting these reactions would be obviously facilitated by

INTRODUCTION Selenium is an element found in mineral form in sulfide ores (mainly pyrite) and in many organic species, particularly in foods (Brazil nuts, mushrooms, eggs, shellfishes, liver, kidney, etc.).1−3 Selenium is mainly used, at the industrial level, in glass manufacturing and as catalyst, pigment, or alloys component.2 Because of its fungicidal properties, selenium is also used in dermatology (antidandruff shampoo and body lotions). In human, selenium is present in selenoproteins (containing selenocysteine or selenomethionine), which act as antioxidant enzymes (glutathione peroxidase and thyroid hormone deiodinase). If at trace level, selenium is an essential element for animals and plants; it is, however, toxic at large doses (the present Tolerable Upper Intake Level is fixed to 400 μg per day).1,4 Determination of selenium by sensitive and accurate analytical methods in water, soils, and biological fluids is consequently of importance. The usual methods are atomic absorption5 and fluorescence detection6 and inductively coupled plasma/mass spectrometry (ICP-MS).7 Limitations in ICP-MS detection and quantization of selenium are due to interfering isobaric polyatomic cations.7−9 Accordingly, the six isotopes of selenium 74Se, 76Se, 77Se, 78Se, 80Se, and 82Se (relative natural abundances: 0.89, 9.37, 7.63, 23.77, 49.61, and 8.73%, respectively) are isobaric with argon containing ions such as Ar2•+, Ar2H+, Ar2H2•+, ArCl+, and ArClH•+ and, obviously, with selenium hydrogenated derivatives SeH+, SeH2•+, and SeH3+ (Table 1). Two types of experimental setups are generally used to separate spectral interferences: (i) sector field mass spectrometers operated in high-resolution mode and (ii) quadrupole or time-of-flight mass analyzers coupled with collision/reaction cells.7−9 However, under high resolution setting, the former instruments are generally less stable and less sensitive, thus © 2012 American Chemical Society

Received: May 14, 2012 Revised: August 21, 2012 Published: August 23, 2012 9058

dx.doi.org/10.1021/jp304644u | J. Phys. Chem. A 2012, 116, 9058−9070

The Journal of Physical Chemistry A

Article

Table 1. Isobaric Cations of the Six Isotopes of Se interfering ionsa

isotope (natural abundance) 74

Se Se 77 Se 78 Se 80 Se 82 Se

76

(0.89) (9.37) (7.63) (23.77) (49.61) (8.73)

36

38

•+

Ar Ar Ar38Ar•+/40Ar36Ar•+/38Ar36ArH2•+/40Ar35ClH•+ 38 38 Ar ArH+/40Ar36ArH+/40Ar37Cl+/40Ar35ClH2+/76SeH+/74SeH3+ 40 38 •+ 40 36 Ar Ar / Ar ArH2•+/38Ar38ArH2•+/ 40Ar37ClH•+/ 77SeH+/76SeH2•+ 40 40 •+ 40 38 Ar Ar / Ar ArH2•+/78SeH2•+/77SeH3+ 40 40 Ar ArH2•+/80SeH2•+ 38

a Assuming that only the 1H hydrogen isotope is a possible component of interfering ions. It is also of interest to recall the natural abundances of the element Ar and Cl: 36Ar(0.337), 38Ar(0.063), and 40Ar(99.600); 35Cl(75.77) and 37Cl(24.23).

the knowledge of the associated thermochemistry.9 However, these essential thermochemical parameters are not always experimentally available. By contrast, they can be obtained at comparable accuracy from high level quantum chemistry computation.16 Therefore, the goal of the present study was to provide theoretical estimates of the thermochemical properties (heats of formation, proton affinities, and ionization energies) relevant to selenium cation and its potentially interfering ions reported in Table 1. For this purpose, quantum chemistry composite methods (G3, G4, and W1) were used throughout this study.

mainly involve dispersive forces. This complementary investigation involves argon containing van der Waals complexes ArH, ArH2, Ar2, ArClH, ArHCl, ArH2Ar, and ArArH2. Basis set superposition error, as calculated using the counterpoise method, was found to be negligible (generally less than 1 kJ/ mol) for most of the investigated species. It may be finally noted that spin orbit corrections are expected to be important for Cl and Se containing species with degenerate electronic ground states. Since a first order spin−orbit correction is included for first, second, and third row main group elements in G3, G4, and W1 theories,16−18 it has been assumed that spin− orbit effect was satisfactorily accounted for by the calculation for chlorine and selenium containing species. All the calculations presented in the following lines were performed on the Gaussian0320 (W1 and G3 methods) and Gaussian0921 (G4 and CCSD(T) methods) series of programs. Theoretical heats of formation at 0 K were obtained from the computed atomization energies of the individual species combined with the experimental gas phase 0 K heats of formation of the constituent atoms.22 The ΔfH°0 of H, Ar, Cl, N, O, and Se gaseous atoms were taken as 216.035, 0.0, 119.6, 470.8, 246.8, and 242.2 kJ/mol, respectively. Temperature correction to enthalpy, H°298 − H°0, is obtained using the theoretical correction calculated for the species of interest and the experimental contribution for the constituent elements, i.e., for these latter: 8.468, 6.197, 9.181, 8.669, 8.680, and 5.52 kJ/ mol for H2(g), Ar(g), Cl2(g), N2(g), O2(g), and Se(solid), respectively.23 ΔfH°0 and ΔfH°298 calculated applying this procedure to W1, G4, G3, and CCSD(T)/aug-cc-pVTZ computational results are presented in Tables 2−4. Moreover, in order to account for residual discrepancies, heats of formation of van der Walls complexes were anchored to the experimental heats of formation of their separated partners. These estimates are included in Tables 3 and 4. Accuracy of the computations may be evaluated by comparison with available experimental ΔfH° values. The mean absolute deviation (MAD) obtained in the present study by comparing theory and experiment is 1.6, 3.1, 3.6, and 3.3 kJ/mol when using W1, G4, G3, and CCSD(T)/aug-cc-pVTZ methods, respectively. Comparison between heats of formation values provided by computational methods is also of interest. Interestingly enough, using averaged W1, G4, G3, and CCSD(T)/aug-cc-pVTZ results, an overall MAD value of 2.7 kJ/mol is obtained on ΔfH° values. Adiabatic ionization energies (IE) are calculated as the difference in total atomization energies at 0 K of the cation and the corresponding neutral at their respective optimized geometries. Proton affinities (PA(X), defined as the enthalpy of the reaction XH+ → X + H+) are obtained from the total atomization enthalpies at 298 K, H°298, of both the neutral and protonated species are calculated as follows: PA(X) = H°298(X)

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COMPUTATIONAL SECTION The choice of the theoretical level was obviously dictated by the need for a good accuracy on the calculation of thermochemical quantities such as heats of formation, ΔfH°, adiabatic ionization energies, IE, and proton affinities, PA. Computational chemistry methods that can achieve only few kJ/mol accuracy on these thermochemical quantities rely on composite approaches. These procedures are based on a series of quantum chemistry calculations combined assuming additivity of the energy terms. In a first step, geometry of the equilibrium structure is optimized and used to calculate harmonic frequencies. A series of single-point energies calculations is then performed at higher levels of theory including Hartree−Fock energy limit estimate, high-level electron correlation effects, use of diffuse and polarization functions, and sometime empirical corrections calibrated on accurate experimental data. The levels of theory used in the present work include the three composite methods G3,17a G4,17b and W1,18 which were demonstrated to provide accuracy better than 5 kJ/mol on ΔfH°, PA, and IE included in test sets of up to 454 energies.17 A comparison between the three methods is also of interest since geometry optimization is conducted at different levels. In the G316 method, geometry is optimized at the second order perturbation molecular orbital theory MP2(full)/6-31G(d), while G4 and W1 recipes use density functional theory with basis sets of higher quality (B3LYP/6-31G(2df,p) for G417 and B3LYP/cc-pVTZ for W118). Noncovalent complexes are a challenge for quantum chemistry calculation, particularly when only dispersive forces are involved such as in noble gases complexes.19 A number of neutral species presented in the following lines obviously fall into this category. For these peculiar species, it was of interest to control that composite methods such as those selected here bring an efficient way to estimate their thermochemical parameters. From this point of view, coupled cluster theory is a gold standard demonstrated to provide accurate results concerning geometries and energies of noncovalent complexes.19 We thus performed CCSD(T) calculations using the aug-cc-pVTZ basis set for neutral complexes suspected to 9059



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− H°298(XH+) + 6.2 kJ/mol, where the latter term is the enthalpy of the proton at 298 K. Hydrogen atom affinity (HAA(X) = ΔH°298 of reaction XH• → X + H•) and hydride ion affinity (HIA(X) = ΔH°298 of reaction XH → X+ + H−) are computed using corresponding heats of formations (experimental values of 218.0 and 145 kJ/mol were used for H• and H−, respectively).24 The following conversion factors were utilized when constructing Tables 2−4 and Tables S1 and S2 of the Supporting Information from experimental and computational data: 1 eV = 96.4853 kJ/mol; 1 hartree = 2625.5 kJ/mol; 1 kJ/ mol = 83.5935 cm−1.

The general thermochemical condition of occurrence of these reactions is (neglecting entropy effect) their exothermicity. Specifically, for each type of reaction, this condition may be expressed as an inequality between two thermochemical parameters (ionization energy, IE, proton affinity, PA, hydrogen atom affinity, HAA, and hydride ion affinity, HIA) as recalled in the right part of Scheme 1. Figures 1−4 report the calculated IE, PA, HAA, and HIA values of species relevant to ions A+ as bar graphs, and the corresponding values relevant to gas G as horizontal lines. These figures allow a simple comparison between values taken by these fundamental thermochemical parameters. They also offer a support for predicting occurrence of a given reaction between ions A+ and gas G. Selenium Systems. Selenium Thermochemistry. The ΔfH0°(Se,gas) value used in the calculations of the heats of formation reported in Table 2 may be briefly commented. The 0 K heat of formation of selenium atom in the gas phase reported in the NBS tabulation (ΔfH0°(Se,gas) = 226.4 kJ/ mol) in fact originates from a 1965 estimate.24 Since the 1980s, this value has been questioned by several authors.25−28 Accordingly, Berkowitz et al.25 obtained ΔfH0°(Se,gas) = 242.2 kJ/mol by combining the 0 K threshold energy for reaction: SeH2(gas) → Se•+ + H2 + e (11.916 ± 0.006 eV, their measurement), the adiabatic ionization energy of gaseous selenium, IE(Se,gas) = 9.75238 eV,29 and the selected ΔfH0°(SeH2,gas) value of 33.5 ± 0.8 kJ/mol kJ/mol.25 However, studies based on sublimation measurements yielded ΔfH0°(Se,gas) equal to 236.830 and 237.031 kJ/mol. Thus, rather than being close to 226.4 kJ/mol, experimental ΔfH0°(Se, gas) appears to be more probably situated between 237 and 242 kJ/mol. We decided to use the value of ΔfH0°(Se,gas) = 242.2(±1.1) kJ/mol as proposed in ref 25 since it seems to be nowadays the most widely used.25−28 Similarly, the experimental value ΔfH0°(Se•+,gas) = 1183.2 kJ/ mol (Table 2) is obtained by combining ΔfH0°(Se,gas) = 242.2 kJ/mol and IE(Se,gas) = 9.75238 eV.29 Another consequence is that all the G4 and G3 computed ΔfH0° of selenium containing species indicated in Table 2 are anchored to ΔfH0°(Se,gas) =

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RESULTS AND DISCUSSION This section will be separated into three parts dealing with selenium and selenium derivatives, argon dimer and related hydrides, and argon/chlorine containing species. Inside each of these parts, structures and thermochemistry of the analyte and interference ions is first presented and followed by examination of their possible reactions inside the cell with the target gas most commonly used during ICP-MS-DRC experiments, i.e., G = H2, CH4, NH3, O2, CO, CO2, NO, and N2O. Concerning this latter aspect, charge exchange, proton, hydrogen atom, and hydride ion transfers may be considered. A general overview of these reactions expected to occur between the incoming analyte or interference ion (denoted A+) and gas G is presented in Scheme 1. Scheme 1

Figure 1. Plot of adiabatic ionization energies IE(A) (vertical bars) and IE(G) (horizontal lines). Charge exchange reaction A+ + G → A + G+ (A+ = analyte ion; G = reactant gas) is occurring if IE(G) < IE(A). 9060



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Figure 2. Plot of proton affinities PA(A−H) (vertical bars) and PA(G) (horizontal lines). Proton transfer reaction A+ + G → (A−H) + GH+ (A+ = analyte ion; G = reactant gas) is occurring if PA(G) > PA(A−H).

Figure 3. Plot of hydrogen atom affinities HAA(A+) (vertical bars) and HAA(G−H) (horizontal lines). H atom transfer reaction A+ + G → AH+ + (G−H) (A+ = analyte ion; G = reactant gas) is occurring if HAA(G−H) < HAA(A+).

242.2 kJ/mol. Experimental heat of formation of SeH• has been estimated by Berkowitz et al.25 using the 0 K threshold energy for reaction: SeH2(gas) → SeH•+ + H• + e (13.266 ± 0.007 eV) and the ionization energy IE(SeH•,gas) = 9.845 ± 0.003 eV. The value of ΔfH0°(Se H•,gas) = 147.6 kJ/mol (Table 2) is based on the use of ΔfH0°(SeH2,gas) = 33.5 ± 0.8 kJ/mol kJ/ mol.20 Finally, experimental heats of formation of SeH+ and SeH2•+ were obtained by combining ΔfH0 and IE of the corresponding neutrals as determined by Berkowitz et al.25 Our calculated thermochemical data on Se hydrides compare reasonably well with previous computations performed at similar theoretical levels.26,32−35 For example, a calculated G2 ΔfH0°(SeH2,gas) of 38.1 kJ/mol has been reported in ref 26, while G4 and G3 methods provide 37.9 and 37.0 kJ/mol, respectively. However, larger deviations are observed for ΔfH0°(SeH•) and IE(SeH•). In the former case, our G4 and G3 estimate fall 8 and 6 kJ/mol above the literature value

ΔfH0°(SeH•) = 147.6 kJ/mol. As a consequence, calculated G4 and G3 IE(SeH•) are lower than the experimental value by ∼0.1 eV, in line with an overestimate of the heat of formation of the neutral SeH•. By contrast, it is noteworthy that G4 and G3 computed IE of Se and SeH2, as well as the proton affinity of the latter, PA(SeH2), and heats of formation of SeH+ and SeH2•+ are reproduced within less than 4 kJ/mol by comparison with experiment (Table 2). A summary of the computational results concerning heats of formation of neutral and ionized selenium and selenium hydrides is presented in Chart 1. Selenium Containing Ions: Reaction with Dihydrogen, Methane, and Ammonia. Use of H2 or mixtures of H2 and He in order to separate argon based interference ions from selenium isotopes in the collision cell of ICP-MS leads generally to a significant decrease of Ar2•+ signal accompanied by the marginal formation (i.e., from 5 to 9%) of SeH+ 9061



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Figure 4. Plot of hydride ion affinities HIA(A+) (vertical bars) and HIA(G−H)+ (horizontal lines). H− transfer reaction A+ + G → AH + (G−H)+ (A+ = analyte ion; G = reactant gas) is occurring if HIA(G−H)+ < HIA(A+).

Se•+ + CH4 → Se + CH4•+, are endothermic by ∼135 to 275 kJ/mol. Similarly, methyl cation transfer Se•+ + CH4 → SeCH3+ + H• needs ∼200 kJ/mol to occur. Formation of an adduct HSeCH3•+ by C−H insertion of the selenium cation has been also investigated. At the G4 level, this structure is situated 213 kJ/mol below the reactants but its dissociation to SeH+ and CH3• is endothermic by ca. 350 kJ/mol. This large dissociation energy excludes that the observation of small amounts of SeH+ ion may be due to collision induced dissociation of HSeCH3•+. This conclusion is in line with the suggestion that SeH+ may originate from reaction of Se+ with hydrogenated contaminants (sample matrix, water, or methanol).11 Ammonia has been reported to react only slowly with with Se•+ cations.8,10 An efficiency (kexp/kcoll) of 0.5% has been determined from ICP/selected ion flow tube (SIFT) tandem mass spectrometry experiments.10 Furthermore, the reaction leads to NH4+ product ions thus revealing a two-step process, most probably Se•+ + NH3 → SeNH3•+ followed by SeNH3•+ → SeNH2• + NH4+. Indeed, theoretical chemistry computations (at the B3LYP/aug-cc-pVTZ level) confirmed this expectation.10 Calculations conducted here are in qualitative agreement with these earlier results (Figure 6). Both formations of SeNH3•+ (or its insertion isomer HSeNH2•+) and NH4+ correspond to exothermic processes (of ca. −240 and −50 kJ/ mol, respectively). Finally, SeH+, SeH2•+, and SeH3+ are expected to react with ammonia by proton transfer due to the large PA value of the latter (Figure 2). Se•+ Reaction with Oxidizing Gases. O2,9 N2O,8,9,15 NO,14,15 and CO14,15 were tested with the expectation to shift the Se•+ signal by 16 mass units while forming SeO•+ ions. In fact, only N2O is able to form a sizable amount of SeO•+ in the collision/reaction cell, 8,9,15 in agreement with the exothermic character of the oxygen atom transfer reaction (ΔH°298 = −239 kJ/mol; Table S2 in the Supporting Information). Detection of SeO•+ at trace level was also reported using O2 as reagent gas9 (we note that it corresponds to the reaction of lower endothermicity in Table S2 in the Supporting Information, ΔH°298 = +92 kJ/mol). As expected, no reaction at all was observed with CO14,15 (ΔH°298 = +450 kJ/mol, Table S2 in the Supporting Information). Ionization

ions.7,8,36 These experimental observations are readily explained by efficient reactions between Ar2•+ ions and H2, as it will be described in part B, while Se•+ is practically nonreactive toward dihydrogen. Thermochemical data reported in Table 2 allows explaining this latter conclusion. In fact, the only exothermic process that may occur between Se•+ and H2 is the formation of the adduct SeH2•+ (ΔH°298 = −200 kJ/mol, Figure 5). The three other possible reactions, namely, electron, hydride ion and hydrogen atom exchanges, are endothermic by 131 to 547 kJ/mol as illustrated in Figure 5 on the ΔfH°298 scale (see also Figures 1,3, and 4). Further reactions involving the first generated SeH2•+ ions are presented in the right part of Figure 5. The H• atom transfer reaction, SeH2•+ + H2 → SeH3+ + H•, appears to be the most favorable process although it is endothermic by 86 kJ/ mol. It may be, however, noted that this quantity is smaller than the exothermicity of the SeH2•+ ions initially formed from Se•+ and H2, i.e., 200 kJ/mol; thus, SeH3+ ions may be formed spontaneously if thermalization of SeH2•+ in the collision cell is incomplete. Formations of SeH• + H3+ (proton transfer) and SeH2 + H2•+ (charge exchange) by reaction of SeH2•+ with H2 are by far more energy demanding processes (270 and 533 kJ/ mol, respectively). SeH+ ions are expected to be unreactive with H2 by charge exchange and H+, H•, or H− transfer (Figures 1−4). The only allowed process is the formation of the SeH3+ adduct (exothermicity ΔH°298 = −245 kJ/mol, exoergicity ΔG°298 = −206 kJ/mol at the G4 level). Considering the present thermochemical data, it turns out that the observation of a SeH+ signal after reaction between Se•+ or SeH2•+ with H2 cannot be simply explained. Occurrence of the collision induced dissociation process from SeH2•+ (dissociation into SeH+ + H• needs 325 kJ/mol) or SeH3+ ions (dissociation into SeH+ + H2 needs 239 kJ/mol) but also of reactions between Se•+ and contaminants in the cell may be suggested. Similarly to H2, methane is essentially unreactive with respect to Se•+ ions.9,11−13 Only a marginal formation of SeH+ (∼10% of the Se+ signal) has been reported.11 According to the present calculations (Table 2 and Table S1 in Supporting Information), hydrogen atom transfer, Se•+ + CH4 → SeH+ + CH3•, hydride ion transfer, Se•+ + CH4 → SeH• + CH3+, and charge exchange, 9062



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Table 2. Summary of the of Thermochemical Data Relevant to the Selenium Systemsa exptlc (kJ/mol) species

b

3

Se ( P2) Se•+ (4A1g) SeH• (2Π) SeH+ (3Σ−) SeH2 (1A1) SeH2•+ (2B1) SeH3+ (1) SeCH3• (2) SeCH3+ (1) HSeCH3 (1) HSeCH3+• (2) NH3 (1) NH4+ (1) SeNH (1) SeNH+ (2) SeNH2• (2) SeNH2+ (1) HSeNH• (2) HSeNH+ (1) SeNH3 (1) SeNH3•+ (2) HSeNH2 (1) HSeNH2•+ (2) SeO (3Σ−) SeO•+ (2) Se SeH• SeH2 SeCH3• HSeCH3 SeNH SeNH2 SeNH3 SeO Se SeH• SeH2 NH3 SeNH at Se SeNH at N SeNH2 at Se SeNH2 at N

ΔfH°0

G4 (kJ/mol)

evaluatedf (kJ/mol)

G3 (kJ/mol)

ΔfH°298

ΔfH°0

ΔfH°298

ΔfH°0

ΔfH°298

242.2 1183.2e 147.6e 1097.6e 33.5e 987.5e 843.6

242.9 1183.9 146.5 1096.6 29.4e 983.5 851.6

−38.9 644.8

−45.9 630.5

242.2 1180.1 155.6 1096.9 37.9 993.3 864.5 138.9 1104.5 34.9 908.2 −35.5 648.2 280.6 1136.4 164.5 992.1 258.5 1085.5 135.5 905.9 104.7 927.5 62.4 1028.5

242.9 1180.8 154.5 1095.9 34.0 989.3 856.5 130.8 1096.5 23.9 897.3 −42.5 636.9 281.0 1137.0 162.1 988.5 255.4 1081.9 128.4 895.4 98.2 921.4 61.4 1027.4

242.2 1179.1 153.7 1092.5 37.0 992.1 864.6 138.5 1108.7 33.5 908.7 −35.5 646.2 280.4 1141.0 165.6 991.9 256.9 1085.3 136.1 905.4 104.6 927.7 60.3 1028.5

242.9 1179.7 152.6 1091.4 33.0 988.1 856.6 130.5 1100.6 22.5 897.9 −42.5 634.9 276.4 1137.3 158.4 983.9 249.7 1077.4 124.8 893.5 93.8 917.2 59.3 1027.4

e

IE (eV)

IE (eV)

IE (eV)

9.75238d 9.845e 9.886e 9.892d

9.72 9.76 9.90 10.00 9.05 8.87 8.58 7.98 10.01

9.71 9.73 9.90 10.05 9.07 8.92 8.56 7.97 10.03

PA (kJ/mol)

PA (kJ/mol)

PA (kJ/mol)

675.3 693.0 707.8d 853.6

681.5 699.7 711.9 855.1 733.6 827.0 775.2 796.9

684.9 697.9 709.8 856.0

ΔfH°298 243 1184 147 1097 29 984 852 131 1099 23 898 −46 631 279 1137 160 986 253 1080 127 894 96 919 60 1027 IE (eV) 9.75 9.85 9.88 10.03 9.06 8.89 8.57 7.98 10.02 PA (kJ/mol) 675 693 708 854 734 827 775 797

a

Note that the W1 method is not available for selenium in Gaussian03 and Gaussian09 quantum chemistry program packages;20,21 thus, only the G4 and G3 results are presented in the Table. bMultiplicity of the fundamental electronic state is indicated in parentheses. cExperimental values are defined using the ion convention (or stationary electron convention), i.e., the value of zero is taken for the integrated heat capacity of the electron between 0 and 298 K. It is important to underline that ΔfH°298 obtained for positive ions using this convention are lower by 6.2 kJ/mol than the values quoted in certain thermodynamic compilations such as, for example, ref 52, where the electron convention (electron is considered as a standard chemical element) is preferred. dReference 29. eReference 25. fAverage of the computed ΔfH°298 values (MAD = 1.6 kJ/mol for this Table), except when an accurate experimental value is available.

energy of nitric oxide (9.2642 eV, Table S2 in the Supporting Information) is noticeably lower than that of selenium (9.75238 eV, Table 2), the charge exchange process Se•+ + NO• → Se + NO+ is consequently exothermic (ΔH°298 = −48 kJ/mol, Table S2 in the Supporting Information), thus allowing a very efficient reaction as indeed experimentally observed.14,15 It may be added that reaction Se•+ + NO• → SeO•+ + N• is

endothermic by 226 kJ/mol in keeping with the lack of observation of SeO•+ ions in ICP-SIFT experiments.14,15 Considering SeH+ and SeH2•+ ions, it may be observed that charge exchange is also predicted to be exothermic with G = NO (Figure 1). Argon Systems. Argon Monomer Hydrides. Proton transfer reaction rates of ArH+ to several reference bases 9063



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Chart 1. Computed Heats of Formation, ΔH°298 (kJ/mol), of Selenium Containing Species (Average of G3 and G4 Calculations, MAD = 1.0 kJ/mol); Experimental Values Are Indicated (in Red) in Parentheses

Figure 6. Enthalpy diagram associated with the Se•+ + NH3 system.

within ±2.3 kJ/mol. An averaged ΔfH°298(ArH2•+) value of 1328.4 kJ/mol is obtained. Of interest is the fact that ArH2•+ consists in an ionized dihydrogen molecule solvated by one argon atom in a linear structure. Stabilization energy of the ArH2•+ complex, ΔclusterH°298 (ArH2•+), deduced from W1, G3, G4, and CCSD(T) computations is equal to −166.1 ± 2.8 kJ/ mol revealing a strong stabilizing interaction between ionized dihydrogen and the polarizable argon atom. Concerning the neutral counterpart, theory gives averaged ΔfH°298(ArH2) = −1.0 ± 1.9 kJ/mol. As seen in Table 3, adiabatic ionization energy (IE) of dihydrogen is closely reproduced by the three composite methods since the MAD between experiment and W1, G4, and G3 calculations is equal here to 2.8 kJ/mol. It may be underlined that, when H2 is complexed by one argon atom, its IE is reduced by a considerable amount of ∼1.6 eV. This observation is obviously in keeping with a significantly more efficient stabilizing effect of the argon atom in the ionized complex ArH2•+ (ion−neutral interaction) than in its neutral counterpart ArH2 (neutral−neutral interaction). The increase of ∼35 kJ/mol in PA values observed between H2 and ArH2 results from comparable effect. Chart 2 summarizes the computed heats of formation of neutral and ionized argon and argon hydrides. Argon Dimer Hydrides. Vibronic spectra of argon dimer allows Herman et al.41 to determine its dissociation energy (De(Ar2) = 99.2 cm−1 = 1.19 kJ/mol) in its electronic ground state (1Σ+g). By using a fundamental frequency value of 31 cm−1, 41 this shallow van der Waals minimum corresponds to ΔH°0 ≈ 1.0 kJ/mol. From a theoretical point of view, a number of recent studies were devoted to argon dimer.42 A thorough investigation using couple-cluster method up to the quadruple excitation and a septuple-zeta level basis set allows the authors to nicely reproduce experiment.42f Satisfactory agreement is also found here at the CCSD(T)/aug-cc-pVTZ level (Table 3). As underlined in the computational section, composite methods are not expected to provide such precise results; however, averaged ΔH°298 correctly reproduces experiment (−0.9 against −1.0 kJ/mol) but with a standard deviation of 3.7 kJ/mol. It may be underlined that this deviation is within the expected precision of these composite methods (i.e., 5 kJ/mol).

Figure 5. Enthalpy diagram associated with the Se•+ + H2 system.

were measured using a flow drift tube after selection of the reacting ion by a quadrupole mass filter.37 Treatment of the data allows the authors to conclude that the proton affinity of argon is situated between 39 to 59 kJ/mol below that of H2 thus leading to PA(Ar) = 383−363 kJ/mol using PA(H2) = 422.3 kJ/mol. In the same study, a difference PA(H2) − PA(Ar) of 53 kJ/mol is derived from a linear fitting procedure from which the value PA(Ar) = 369.2 kJ/mol has been deduced and is presently retained in the NBS tabulation.29 One should note, however, that the accuracy on this estimate is not specified, but considering the data reported in the original paper,37 an error of at least ±10 kJ/mol may be suggested. Under these circumstances, the averaged theoretical value of 382.9 ± 11.8 kJ/mol matches the upper part of the experimental window. As a corollary, a 298 K heat of formation equal to 1147 kJ/mol may be proposed for ArH+ ion. The ionized adduct ArH2•+ has been the subject of a number of experimental and theoretical studies;38 by contrast, much less is known about its neutral counterpart.39,40 Both systems have no entry in the NBS thermochemical database.29 The four theoretical methods provide heats of formation of ArH2•+ 9064



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Table 3. Summary of Thermochemical Data Relevant to the Argon Dimer Systems b

exptl (kJ/mol) speciesa

ΔfH°0

H• (2S1/2) H+ (1) H2 (1) H2•+ (2) H3+ (1) Ar (1S0) Ar•+(2P3/2) ArH• (2) ArH+ (1) ArH2 (1) ArH2•+ (2) ArH3+ (1) ArAr (1) ArAr•+ (2) ArHAr• (2) ArArH• (2) ArHAr+ (1) ArArH+ (1) ArH2Ar (1) ArH2Ar•+(2) ArArH2•+ (2) ArH3Ar+(1) Ar3H3+(1)

216.0d 1528.1d 0 1488.3d 1107 0 1520.6d

218.0 1530.0 0 1488.5 1106.6 0 1520.6

ΔfH°298

1159

1157 ± 10

−1 1394

−2.4 1391.7

IE (eV) H• H2 Ar ArH• ArH2 ArAr ArHAr• ArArH• ArH2Ar

13.59844d 15.426d 15.759d

14.456e

PA (kJ/mol) H2 Ar ArH• ArH2 Ar2H2 ArAr ArHAr• ArArH•

422.5d 369 ± 10d

W1 (kJ/mol)

c

G4 (kJ/mol)

c

G3 (kJ/mol)

c

ΔfH°0

ΔfH°298

ΔfH°0

ΔfH°298

ΔfH°0

ΔfH°298

216.0 1528.8 −0.3 1488.6 1109.6 0 1526.8 216.1 1153.3 0.2 1328.3 1078.2 −0.6 1400.4 216.5 216.2 1090.0 1145.2 0.7 1284.7

218.0 1530.7 −0.1 1488.8 1106.8 0 1526.8 216.5 1151.5 (1149.8) 1.0 (1.1) 1323.2 (1322.9) 1070.2 (1070.0) −2.0 1398.1 217.7 217.3 1083.9 (1083.2) 1142.6 (1141.9) 0.8 (0.9) 1278.0 (1277.7)

216.0 1532.5 −1.6 1489.2 1113.4 0 1516.7 213.8 1153.3 1.0 1330.5 1083.9 2.7 1395.4 214.3 213.5 1091.8 1148.3 2.8 1287.8

218.0 1534.5 −1.4 1489.4 1110.6 0 1516.7 214.0 1151.5 (1147.0) 1.1(−0.3) 1325.3 (1324.4) 1075.8 (1071.8) 1.3 1393.1 215.0 214.1 1085.7 (1081.2) 1145.4 (1140.9) 2.6 (4.0) 1280.6 (1279.7)

216.0 1531.4 −2.1 1486.4 1113.8 0 1513.8 209.6 1150.9 −4.1 1325.5 1085.7 −4.7 1387.4 207.7

218.0 1533.4 −1.9 1486.6 1111.0 0 1513.8 210.1 1149.2 (1145.8) −3.2 (−1.3) 1321.2 (1323.1) 1079.7 (1075.3) −6.0 1385.0 209.2

1090.8 1140.3 −6.5 1280.8

1086.7 (1083.3) 1138.6 (1135.2) −5.9 (−4.0) 1274.3 (1275.2)

IE (eV) 13.606 15.432 15.825 9.71(9.70) 13.76(13.76) 14.521 9.05(9.04) 9.63(9.61) 13.31 (13.30) PA (kJ/mol)

1067.0 1056.7 (1052.7) 1050.8 1040.0 (1036.0) IE (eV) 13.644 15.451 15.719 9.74(9.71) 13.77(13.77) 14.434 9.09(9.07) 9.69(9.66) 13.32 (13.29) PA (kJ/mol)

423.9 379.2 (380.2) 423.9 (423.6) 461.5 (461.1) 444.8 (444.8) 470.4 (470.0)

422.5 383.0 423.2 459.8 480.4 450.1 468.9

(383.0) (419.6) (457.9) (481.3) (450.1) (465.3)

1063.4 1055.7 (1051.3) 1042.5 1034.5 (1030.1) IE (eV) 13.633 15.428 15.690 9.76(9.74) 13.78(13.73) 14.428 9.15(9.14) 9.66(9.65) 13.34 (13.34) PA (kJ/mol) 420.5 384.2 422.3 450.5 471.8 440.7 468.3

(384.2) (417.0) (453.4) (474.7) (440.7) (464.0)

CCSD(T)/augcc-pVTZ (kJ/mol)c,g

evaluatedf (kJ/mol)

ΔfH°298

ΔfH°298

218.0 1530.3 4.4 1489.4 1113.0 0 216.4 [216.5] 1146.3 (1146.0) 0.4 (−4.0) [−3.5] 1320.0 (1319.1) −2.4 [−2.1] 1379.3 216.1 215.2 1136.6 2.0 (−2.4) 1271.5 (1270.6) 1317.0 (1316.1)

IE (eV)

14.33 9.55(9.55) PA (kJ/mol) 383.9 (384.0) 426.6 (427.4)

428.2 (429.1)

218 1530 0 1488.5 1107 0 1521 216 1147 −3.5 1322 1072 −2.4 1392 216 215 1083 1139 −2.5 1276 1316 1052 1033 IE (eV)

13.60 15.43 15.76 9.71 13.76 14.46 9.05 9.60 13.30 PA (kJ/mol) 422 383 422 459 478 445 468 429

a

Multiplicity of the fundamental electronic state is indicated into parentheses. bExperimental values are defined using the ion convention (see footnote b in Table 2). cIn parentheses, ΔfH°298 anchored to the experimental heats of formation of H+, H2, and H2•+. dReference 29. eReference 43. f Average of the computed ΔfH°298 values (MAD = 3.1 kJ/mol for this Table), except when an accurate experimental or CCSD(T)/aug-cc-pVTZ value is available. gIn brackets, including basis set superposition errors as calculated using the counterpoise method.

give satisfactory values situated between −126.4 kJ/mol (W1 and G3 levels, Table 3) and −121.3 kJ/mol (G4 level, Table 3). Protonated argon dimer, Ar2H+, has been the subject of a number of studies, mainly theoretical.44,45 To the best of our knowledge, no information is available on the neutral system Ar2H. Two weakly stabilized (ΔclusterH°298 ≈ 0.5 and 1.5 kJ/ mol, respectively, with respect to Ar + ArH•) neutral complexes, ArHAr• and ArArH•, have been identified in the present study. As established earlier,44,45 we found that two isomeric forms of Ar2H+ are stable species. The most stable is the linear, centrosymetric, ArHAr+ cation; the second structure, ArArH+, is situated ∼59 kJ/mol above in the H°298 energy scale

Pulsed field ionization zero kinetic energy photoelectron spectra of argon dimer provide IE(Ar2) = 14.4558 ± 0.0009 eV.43 Correct agreement is obtained by computations at the G3, G4, and W1 levels since the average IE(Ar2) value is 14.46 ± 0.05 eV (Table 3). When comparing IE(Ar) and IE(Ar2), a lowering of ∼1.3 eV for the dimer occurs due to the more efficient complexation of an ionized species than a neutral by one argon atom as observed above with the ArH2•+ and ArH2 complexes. It may be noted that the stabilization energy of the Ar2•+ complex, ΔclusterH°0 (ArAr•+), deduced from experiment is equal to −125.0 ± 0.2 kJ/mol43 and that our computations 9065



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Chart 2. Computed Heats of Formation, ΔH°298 (kJ/mol), of Argon Containing Species (Average of W1, G3, G4, and CCSD(T)/aug-cc-pVTZ Calculations, MAD = 2.6 kJ/mol); Experimental Values Are Indicated (in Red) in Parentheses

Chart 3. Computed Heats of Formation, ΔH°298 (kJ/mol), of Argon Dimer Containing Species (Average of W1, G3, G4, and CCSD(T)/aug-cc-pVTZ Calculations, MAD = 3.3 kJ/ mol); Experimental Values Are Indicated (in Red) in Parentheses

(W1 and G4 calculations, Table 3). This energy difference may be accounted for by the more efficient charge-polarizable atom interaction in the inside protonated form of the argon dimer, ArHAr+, with respect to its outside homologue. The three composite methods remarkably agree in calculating ΔfH°298(ArHAr+) = 1085.4 ± 1.4 kJ/mol. Considering the evaluated ΔfH°298(ArH+) = 1147 kJ/mol proposed above, a complexation enthalpy at 298 K, ΔclusterH°298(ArHAr+), of 62 kJ/mol is calculated. Two possible Ar2•+ + H2 adducts were investigated, namely, the approach or outside structure ArArHH•+, and the inclusion structure ArHHAr•+. As shown in Table 3, the inside form is more stable than its outside homologue, probably because of a better charge-polarizable atom interaction. The complexation enthalpy of ArHHAr•+ with respect to the components ArHH•+ + Ar is equal to ΔclusterH°298(ArHHAr•+) ≈ 45 kJ/mol. At the G3 level, the outside structure ArArHH•+ does not converge toward a stable species, while it is predicted to be dissociative with respect to ArHH•+ + Ar products at the W1 and G4 levels. We consequently reexamined the two Ar2•+ + H2 adducts at the CCSD(T)/aug-cc-pVDZ level. It emerges from these calculations that indeed ArHHAr•+ is more stable than ArArHH•+ (by 36 kJ/mol in the H°298 scale) since this latter is more stable than ArHH•+ + Ar by ∼6 kJ/mol. Finally, the neutral counterpart ArHHAr has been identified here at the W1, G4, G3, and CCSD(T)/aug-cc-pVTZ levels but was found to be only marginally stabilized. Its adiabatic ionization energy is predicted to be close to 13.3 eV, thus ca. 0.5 eV lower than IE(ArH2) and ∼2.1 eV below IE(H2). Protonation of ArHHAr is associated with a PA value of ∼475 kJ/mol, i.e., 20 kJ/mol above PA(ArH2) and 55 kJ/mol above PA(H2). An overview of the computed heats of formation of neutral and ionized argon dimer and its hydrides is presented in Chart 3. Argon Interference Removal. Experimental removal of ArAr•+ interfering ion produced in the ICP ion source is based on its reactions with H27,8,14,15,46 or mixture of H2 and He.7,8 The products essentially observed when ArAr•+ and H2 are allowed to react in SIFT are ArH+ and ArH2•+ in the ratio ∼60/40.14,46 Reaction between ArH+ and ArH2•+ and a second molecule of H2 leads to H3+ + Ar and ArH3+ + H•, respectively.46 Summary of the thermochemistry associated with the ArAr•+ + H2 system is presented in Figure 7. Charge exchange reaction, ArAr•+ + H2 → ArAr + H2•+, may be clearly discarded since it is endothermic by 94 kJ/mol (Figure 1). Similarly, proton and hydride ion transfers may be excluded

(Figures 2 and 4). By contrast, several pathways leading to various argon hydride ions are thermodynamically allowed (Figure 7). Experimental observation of ArH2•+ product ions14,46 may be explained by Ar loss from either the transient collision complex ArArHH•+or the inclusion complex ArHHAr•+, the overall reaction being exothermic by 70 kJ/mol. Formation of ArH+ may simply originate from a direct dissociation of complex ArHHAr•+ since the complete process ArAr•+ + H2 → ArH+ + ArH• is exothermic by 29 kJ/mol (Figure 7). Beside formation of ArH+ and ArH2•+ ions, a third reaction allowed by thermochemistry between reactants ArAr•+ and H2 is the hydrogen atom abstraction leading to ArArH+ + H• (ΔH°298 = −35 kJ/mol) or ArHAr+ + H• (ΔH°298 = −91 kJ/ mol, Figure 3). Again, collision complexes ArArHH•+ and ArHHAr•+ may be involved as reaction intermediates during these processes. Surprisingly, formation of m/z 81 (ArArH+ or ArHAr+ ions) has not been reported in previous experiments. One explanation may be that, once produced by reaction between ArAr•+ and H2, these species spontaneously dissociate into ArH+ + Ar•. This may participate in the formation of ArH+ ions observed in ICP/SIFT experiments.14,46 In the same vein, some of the ArH2•+ product ions may expel one hydrogen atom to produce part of the detected ArH+ ions. Another explanation is the possibility of secondary reactions between ArArH+ or ArHAr+ ions and H2. Accordingly, formation of ArH3+ + Ar is thermochemically allowed even starting from the most stable ArHAr+ ion. It should be finally added that secondary reactions ArH+ + H2 → H3+ + Ar (proton transfer, Figure 2) and ArH2•+ + H2 → ArH3+ + H• (H atom transfer, Figure 3) are calculated to be exothermic by −40 and −32 kJ/mol, respectively, thus explaining observation of signals at m/z 3 and 43 in SIFT experiments.46 Considering now the other reagent gases usually utilized in argon interference removal (i.e., G = CH4, NH3, N2O, NO, O2, CO, and CO2), an efficient charge exchange process, ArAr•+ + G → ArAr + G•+, is expected. Accordingly, the ionization energies of these species, IE(G) (Tables 2 and S2, Supporting Information; Figure 1), are always lower than the ionization energy of argon dimer, IE(ArAr) = 14.456 eV (Table 3). This situation is in line with the considerable increase of the Se•+/ Ar2•+ signals ratio, which has been reported when using target gases G = CH4,11 N2O,8,9,15 O2,9 and CO.15 9066



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Figure 7. Enthalpy diagram associated to the ArAr•+ + H2 system.

Table 4. Summary of Thermochemical Data Relevant to the Argon−Chlorine Systems exptlb (kJ/mol)

W1 (kJ/mol)e

speciesa

ΔfH°0

ΔfH°298

ΔfH°0

Cl• (2P3/2) Cl+ (3P2) HCl (1) HCl•+ (2) H2Cl+ (1) ArCl• (2) ArCl+ (1) ArClH (1) ArClH•+ (2) ArHCl (1) ArHCl•+ (2) ArHClH+ (1) ClOH (1) ClOH•+ (2) ClNH2 (1) ClNH3+ (1)

119.6 1370.8 −92.1 1137.8 883.9

121.3d 1372.2 −92.3d 1137.6 880.8

119,6 1371,2 −96,3 1137,3 877.9 117.7 1295.0

121,2 1372,8 −96,4 1137,2 874.8 117.9 1293.6

1104.8

−75 998

ΔfH°298

ΔfH°0

ΔfH°298

1102.0 (1102.4)

119.6 1366.2 −90.3 1138.1 883.9 118.6 1292.4 −88.7 1107.3

121.2 1367.8 −90.5 1137.9 880.8 118.8 1291.0 −88.9 (−94.7) 1104.5 (1104.2)

119.6 1363.3 −91.5 1135.7 885.2 113.2 1291.7 −95.1 1104.0

121.2 1364.9 −91.7 1135.6 882.1 113.5 1290.6 −94.3 (−94.9) 1102.7 (1104.7)

−97.6 1109.5

−98.2 (−94.1) 1106.7 (1107.1)

−90.0 1110.1

−90.8 (−92.6) 1107.4 (1107.1)

−95.8 1110.6

−96.2 (−96.8) 1108.9 (1110.9)

858.7

852.9 (858.9)

866.3

860.6 (860.6)

865.3

861.3 (860.0)

−71.3 996.8 61.4 800.8

−74.3 993.8 54.4 789.5 IE (eV)

−69.9

−72.8

−78 995 53 IE (eV)

12.9676d 12.744d

12.971 12.785 12.202 12.51(12.47)

11.12 PA (kJ/mol)

Cl• HCl ArCl• ArHCl ClNH2

513.6d 556.9d

G3 (kJ/mol)e

ΔfH°0

IE (eV) Cl• HCl ArCl• ArClH ArHCl ClOH

ΔfH°298

G4 (kJ/mol)e

PA (kJ/mol)

12.919 12.731 12.165 12.39(12.41) 12.44(12.45) 11.07 PA (kJ/mol)

559.5 546.6 (545.5) 579.7 (577.0)

563.1 548.8 (544.6) 583.0 (576.8) 794.9

IE (eV) 12.890 12.719 12.213 12.43(12.45) 12.50(12.53) PA (kJ/mol) 559.6 544.2 (538.8) 575.9 (573.2)

CCSD(T)/aug-ccpVTZ (kJ/mol)e

evaluatedf

ΔfH°298

ΔfH°298 (kJ/mol)

121.2 −85.6 1132.9 118.0 1294.1 −95.1 (−94.6)

−95.1 (−94.0)

121.3 1372 −92 1137 881 118 1294 −95 1104 −94 1108 860 −74 994 54 789 IE (eV) 12.97 12.74 12.20 12.42 12.47 11.12 PA (kJ/mol) 514 557 545 577 795

a Multiplicity of the fundamental electronic state is indicated in parentheses. bExperimental values are defined using the ion convention (see footnote b in Table 2). cConverges on a transition structure. dReference 29. eIn parentheses, anchored to the experimental heat of formation of HCl or HCl•+. f Average of the computed ΔfH°298 values (MAD = 2.0 kJ/mol for this Table), except when an accurate experimental value is available.

Argon/Chlorine Systems. Complexes between noble gas and HY molecules (Y being an electronegative fragment such as

F, Cl, Br, OH, or CN) were known since the 1960s but were recently the object of regained attention.39 Among these 9067



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to a Ar···HClH + arrangement (Figure S3, Supporting Information). Rotation of the HClH moiety passes through two symmetric structures Ar···ClH2+ and Ar···H2Cl+ corresponding to saddle points both situated ∼15 and 8 kJ/mol above the global minimum in the H°298 scale. Calculated proton affinity of the Ar···HCl complex is calculated to be close to 580 kJ/mol (Table 4), i.e., ∼19 kJ/mol higher than PA(HCl) in keeping with a larger complexation energy of Ar···HClH+ (−21.0 ± 0.9 kJ/mol) with respect to Ar···HCl. ArCl+ and ArHCl•+ Interferences Removal. Computed ionization energies of ArCl• and ArHCl are situated between 12.2 and 12.5 eV, charge exchange reactions ArCl+ + G → ArCl• + G•+ and ArHCl•+ + G → ArHCl + G•+ are consequently expected with G = NH3, NO, and O2 (Figure 1). These reactions thus offer an efficient way to remove ArCl+ and ArHCl•+ interfering ions in selenium ICP/MS analysis. The second reaction expected to be strongly efficient, if thermochemically allowed, is proton transfer (Figure 2). Reaction ArHCl•+ + G → ArCl• + GH+ is allowed if PA(G) is larger than PA(ArCl•) (i.e., ∼545 kJ/mol, Table 4). This situation is clearly encountered for G = NH3, CO, and N2O, but we noted that the proton affinities of CH4 and CO2 are very close to that calculated for ArCl•, and considering the uncertainty associated with the computations, proton transfer reaction from ArHCl•+ to CH4 and CO2 cannot be discarded. Using reductive gases G = H2, NH3, or CH4, hydrogen atom transfer (Scheme 1) and hydride ion transfer (Scheme 1) should be considered. The former reaction is expected to occur only with ArHCl•+ ions (exothermicities ranging from −10 to −35 kJ/mol, Figure 3). By contrast, ArCl+ may react with NH3 or CH4 by hydride ion transfer (exothermicities −82 and −223 kJ/mol, respectively, Figure 4). ArCl+ ions may also react with reductive gases G = H2, NH3, or CH4 by formal Cl+ transfer and by H/Cl exchange leading to the formation of ArH+ ions. These reactions are predicted to be significantly exothermic (Figure S4, Supporting Information) and compete with charge exchange if G = NH3 and hydride ion transfer in the case of G = NH3 and CH4. Chlorine cation transfer is probably at the origin of the efficient removal of ArCl+ interference during analysis of selenium by ICP-MS reported by Bueno and co-workers.7b Similarly, ArHCl•+ may formally transfer a chlorine cation following reaction: ArHCl•+ + G → ArH• + GCl+, using gases G = H2 and CH4. The products of this reaction are situated 10 to 20 kJ/mol below the reactants, as illustrated in Figure S5 of the Supporting Information.

complexes appears ArHCl, which has been the subject of a number of experimental47,48 and theoretical40,49−51 studies. The present investigation, conducted at the W1, G4, G3, and CCSD(T)/aug-cc-pVTZ levels of theory, include ArCl• radical and its ionized form as well as ArClH2+, the protonated form of ArHCl. Results are reported in Table 4 and Chart 4. Chart 4. Computed Heats of Formation, ΔH°298 (kJ/mol), of Argon−Chlorine Containing Species (Average of W1, G3, G4, and CCSD(T)/aug-cc-pVTZ Calculations, MAD = 1.5 kJ/mol)

Thermochemistry and Structures. The potential energy surface associated with the ArHCl system presents two linear minima Ar···HCl (global minimum) and Ar···ClH (local minimum) calculated by Woon et al.40 to be situated 2.1 and 1.8 kJ/mol below the separated components Ar + HCl. A transition structure connecting these two minima, situated 0.8 kJ/mol above Ar···HCl, has been also identified.40 The very low complexation energies of Ar···HCl and Ar···ClH are indeed reproduced at the W1, G4, G3, and CCSD(T)/aug-cc-pVTZ levels since the average ΔH°298 is equal to −3.4 and −3.7 kJ/ mol, respectively. Photoionization of ArHCl cluster in a molecular beam allows the determination of its ionization energy, IE(ArHCl) = 12.55 ± 0.03 eV.48 The authors note that IE(ArHCl) is red-shifted by 0.22 eV relative to IE(HCl) and evaluate the complexation energy of ArHCl•+ to be 0.24 ± 0.04 eV.48 Quantum chemical calculation on ArHCl•+ system has been done using multireference configuration interaction calculations with extended basis sets containing 78 contracted Gaussian-type orbitals.51 Two minima in the potential energy surface (both situated ∼20 kJ/mol below Ar + HCl•+) were identified: a linear Ar···HCl•+ and an angular Ar···ClH•+ structure, a linear arrangement for the latter corresponds to a saddle point at this level of theory.51 Our computations confirm these observations; moreover, the three methods W1, G4, and G3 converge to predict Ar···ClH•+ to be slightly more stable than Ar···HCl•+ by ∼5 kJ/mol in the H°298 scale (Table 4). Computed complexation enthalpies at 298 K are equal to −33.8 ± 1.2 and −29.2 ± 2.2 kJ/mol for Ar···ClH•+ and Ar···HCl•+, respectively (average of the W1, G4, and G3 methods). Computed IE(ArHCl) fall in the range 12.4−12.5 eV slightly below, but in correct agreement with, the experimental value.48 The shift in IE from nude HCl to its complexed form ArHCl reported earlier48 is calculated here to be 0.26 ± 0.04 eV (average of the W1, G4, and G3 values). This situation, which has been encountered in the preceding section, is again explained by the better stabilization of ionized ArHCl•+ complexes (∼−30 kJ/mol) relative to their neutral forms (∼−5 kJ/mol). Protonation of ArHCl clusters was not previously documented. One stable structure was localized corresponding

■

CONCLUSIONS The present study provides several clues for the interpretation and development of experiments in DRC-ICP-MS analysis of selenium containing samples. Se•+ and its possible interferents, SeH+, SeH2•+, SeH3+, Ar2•+, Ar2H+, Ar2H2•+, Ar2H3+, ArCl+, ArClH•+, and ArClH2+, were theoretically examined. Composite quantum chemistry methods G3, G4, and W1 were used to compute 0 and 298 K heats of formation of about 30 species. The expected precision on these computations is better than ca. 4 kJ/mol, as revealed by comparison with experimental data, when possible, and with CCSD(T)/aug-cc-pVTZ calculations for crucial van der Waals complexes. For the major number of the investigated species, these ΔfH° values (see Charts 1−4) were not previously reported. This is in particular the case for Ar and Ar2 hydrides and ArCl containing neutral and ionized species. 9068



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Reaction between the above-mentioned ionized species (denoted A+) and a conveniently chosen target gas G can be used to resolve the interference problem during DRC-ICP-MS experiments. The general criterion of occurrence of ion− molecule reaction in the gas phase and under thermal equilibrium conditions is its exothermicity. Most of the time, reactions between A+ + G involve charge exchange, proton, H atom, or hydride ion transfer. In those cases, the thermal conditions rely on ionization energies, IE, proton affinities, PA, hydrogen atom affinities, HAA, and hydride ion affinities, HIA, of the relevant species. Computation of these quantities for the above-mentioned ions A+ and target gas G commonly used in DRC-ICP-MS experiments (G = H2, CH4, NH3, O2, CO, CO2, NO, and N2O) were performed at the G3, G4, and W1 levels in the present article. Results are presented as thermochemical ladders, in the form of bar graphs (Figures 1−4). Discussion concerning specific situations (e.g., Se•+ + H2, Se•+ + NH3 and ArAr•+ + H2 systems) are presented and illustrated by suitable enthalpy diagrams (Figures 5−7).

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ASSOCIATED CONTENT

S Supporting Information *

Auxiliary thermochemical parameters, structures of some relevant species, enthalpy diagram of ArCl+ + G and ArHCl•+ + G reactions (G = H2, CH4, NH3), and influence of argon complexation on the ionization energies and proton affinities of H, H2, and HCl. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 33 1 6933 4842. Fax: 33 1 6933 4803. E-mail: [email protected]. Present Address §

Department of Physics, Faculty of Science and Arts at Al-Rass, Qassim University, Kingdom of Saudi Arabia. Notes

The authors declare no competing financial interest.

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REFERENCES

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Theoretical Investigation of Selenium Interferences in Inductively

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