Ligation kinetics as a probe for relativistic effects: Ligation of atomic coinage metal cations with ammonia

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International Journal of Mass Spectrometry xxx (2016) xxx–xxx

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Ligation kinetics as a probe for relativistic effects: Ligation of atomic coinage metal cations with ammonia Voislav Blagojevic, Vitali V. Lavrov, Gregory K. Koyanagi, Diethard K. Bohme ∗ Department of Chemistry, Centre for Research in Mass Spectrometry and Centre for Research in Earth and Space Science, York University, Toronto, Ontario, M3J 1P3, Canada

a r t i c l e

i n f o

Article history: Received 14 February 2016 Received in revised form 18 May 2016 Accepted 17 June 2016 Available online xxx Dedicated in memory of Nico Nibbering, a superb ion chemist and a true friend. Keywords: Ammonia ligation of metal cations Relativistic effects Periodic trends in ion kinetics ICP/SIFT technique

a b s t r a c t The kinetics for ammonia ligation of the three (d10 ) transition metal coinage cations Cu+ , Ag+ and Au+ were measured in an attempt to assess the role of relativistic effects in reaction kinetics. Measurements of several main-group cations were included for comparison: the alkali (s0 ) cations K+ , Rb+ and Cs+ , the alkaline-earth (s1 ) cations Ca+ , Sr+ and Ba+ and the p0 atomic cations Ga+ , In+ and Tl+ . Measurements were performed at room temperature in helium bath gas at 0.35 Torr using an Inductively-Coupled Plasma/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer. The atomic cations are produced at ca. 5500 K in an ICP source and are allowed to decay radiatively and to thermalize by collisions with argon and helium atoms prior to reaction. Rate coefficients are reported for ammonia addition, the only reaction channel that was observed with all these cations. A strong enhancement in the rate of addition of NH3 to Au+ was observed for the coinage metal cation period in contrast to the continuous decline in rate down the periodic table that was seen for the main group s0 , s1 and p0 cations. We attribute this rate enhancement to the enhancement in the Au+ -NH3 binding energy expected from relativistic effects. Comparisons are made with the periodic trends that we have reported previously for the rates of ligation of the coinage metal cations with O2 , D2 O, N2 O, CO2 , CS2 , CH3 F and SF6 and that we measured with pyridine. For seven of the nine ligands that were investigated, rate enhancement with Au+ provided an indirect experimental measure of relativistic effects. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Relativistic effects in gas-phase ion chemistry are now well established and have been reviewed [1] in the context of relativistic effects in chemistry generally [2,3]. Structural properties, thermochemical data and reactivity patterns were discussed; much focus was directed to the ligation chemistry of Au+ (5d10 6s0 ). Of particular interest for the study reported here is an early, theoretical examination, albeit somewhat limited, of binding energies in the cationic gold(I) complexes with H2 O, CO, NH3 and C2 H4 with different ab initio and density functional methods in which relativistic effects are explicitly taken into account [4]. A large relativistic stabilization is found for Au+ (CO), Au+ (NH3 ) and Au+ (C2 H4 ), larger than that for Au+ (H2 O). Au+ (NH3 ) is computed to have a binding energy almost twice as high as in Cu+ (NH3 ) or Ag+ (NH3 ). We present here an experimental approach that provides an assessment of the relative magnitude of relativistic effects in ligated

∗ Corresponding author. E-mail address: [email protected] (D.K. Bohme).

cations with measurements of rate coefficients for their formation at room temperature. Why should such an experimental approach be informative in this way? The magnitude of the rate coefficient is sensitive to the binding energy of the adduct ion and therefore the presence of relativistic stabilization. In the gas phase (in a helium bath, for example), formation of ligated atomic cations M+ occurs via two steps: formation of an encounter complex and its subsequent collisional stabilization. K1

M + + L  (M + − L)∗ K−1

KS

(M + − L) ∗ +He→M + − L + He The third order rate constant k = k1 ks /k−1 can be derived when applying the steady-state assumption to the encounter complex (M+ –L)*. For the unimolecular dissociation of the encounter complex back to reactants, k−1 is a frequency or the inverse of the lifetime, ␶1 −1 . Classical statistical theories predict that the lifetime for unimolecular decomposition ␶ is related to the binding energy (well depth) D of the encounter complex according to ␶ = ␶0 ((D + rRT)/rRT)s−1 [5]. Here ␶0 is the collision lifetime (the

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Please cite this article in press as: V. Blagojevic, et al., Ligation kinetics as a probe for relativistic effects: Ligation of atomic coinage metal cations with ammonia, Int. J. Mass Spectrom. (2016), http://dx.doi.org/10.1016/j.ijms.2016.06.007

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inverse of the vibrational frequency along the reaction coordinate), r is the number of energy square terms contributing to the internal energy of the molecule (r = 3 for non-linear and 2.5 for linear molecules when contributions of vibrational degrees of freedom are ignored), and s is the number of coupled harmonic oscillators. So the rate coefficient for ligation k is related to the binding energy. An enhanced stability due to relativistic effects will therefore be manifested by an enhanced rate coefficient for ligation; when comparisons are made within a Group of atomic metal cations with the same ligand primarily the binding energy, and not the degrees of freedom or the number of energy square terms, will be decisive and trend setting. Both k1 and kc decrease with increasing mass of the cation, but only very slightly. We illustrate our approach first for ammonia ligation with new rate coefficient measurements for the Group 11 (coinage metal) cations Cu+ , Ag+ and Au+ . Also included, for comparison purposes, are new rate coefficient measurements for the ammonia ligation of the Group 1 (alkaline) cations K+ , Rb+ and Cs+ , the Group 2 (alkalineearth) cations Cs+ , Sr+ and Ba+ and the Group 13 (p0 ) cations In+ and Tl+ . We have previously reported the results of rate coefficient measurements for the ammonia ligation of Mg+ [6] and Ga+ [7]. Trends in ammonia ligation will then be compared with trends in the ligation of eight other ligands: O2 [8], D2 O [9], N2 O [10], CO2 [11], CS2 [12], CH3 F [13], SF6 [14] and pyridine. Seven of these have been reported previously. The rate coefficients for the ligation of the coinage metal cations with pyridine are reported here for the first time.

2. Experimental procedures The experimental results reported here were obtained with the ICP/SIFT tandem mass spectrometer that has been described in detail previously [15–18]. The atomic ions were generated within an atmospheric pressure argon plasma at 5500 K fed with a vapourized solution containing the metal salt. Solutions containing the metal salt of interest with concentration of ca. 5 ␮g L−1 were peristaltically pumped via a nebulizer into the plasma. The nebulizer flow was adjusted to maximize the ion signal detected downstream of the flow tube. The sample solutions were prepared using atomic spectroscopy standard solutions commercially available from SPEX, Teknolab, J.T. Baker Chemical Co., Fisher Scientific Company, Perkin-Elmer and Alfa Products. Aliquots of standard solutions were diluted with highly purified water produced in the Millipore Milli-Qplus ultra-pure water system. The final concentrations were varied between 5 and 20 ppm to achieve suitable intensity of the resultant ion beam. A stabilizing agent was usually added to each solution in order to prevent precipitation: KOH for base-stabilized salts, HNO3 or HCl for acid-stabilized salts. Atomic ions emerge from the ICP at a nominal plasma ion temperature of 5500 K with the corresponding Boltzmann distributions. After extraction from the ICP, the plasma ions may experience electronic-state relaxations via both radiative decay and collisional energy transfer. The latter may occur by collisions with argon, as the extracted plasma cools upon sampling, and with helium in the flow tube (ca. 4 × 105 collisions with helium) prior to the reaction region. However, the exact extent of electronic relaxation is uncertain. Clues to the presence of excited electronic states of the atomic ions in the reaction region can be found in the product ions observed and in the shape of the semi-logarithmic decay of the reacting atomic ion upon addition of neutral reactants. Curvature will appear in the measured atomic-ion decay when the ground state and excited state react at different rates even when they give the same product ions. An excited-state effect cannot be seen when the products and reaction rates are the same for both the ground and excited states, but in this case the measured atomic-ion decay defines the

103 Ag+ Cu+

Ion Signal /(s-1)

2

102

Cu+(NH 3)2

Cu+(NH3)3

Ag+ (NH3)2

Au+

Au+ (NH 3)2

Ag+(NH3)3

Au+(NH3)3

Au+(NH3) 101

Ag+(NH3) Cu+(NH3)

100 0.0 0.4 0.8 1.2 1.6 2.0 0.0 0.4 0.8 1.2 1.6 2.0 0.0 0.2 0.4 0.6 0.8 1.0

NH3 Flow /(1019 molecules s-1) Fig. 1. Ion profiles monitored for the ligation of the coinage metal cations with ammonia in He buffer gas at 0.35 ± 0.01 Torr. Symbols represent experimental data and lines represent kinetic fits to that data.

ground-state kinetics. There were no indications of excited state effects in the measurements reported here. The many collisions experienced by the atomic cations with the quite polarizable argon atoms as they emerge from the ICP and the ca. 4 × 105 collisions with helium atoms in the flow tube (the helium buffer gas pressure was 0.35 ± 0.01 Torr) appear to be sufficient to thermalize the excited states and to ensure that the atomic ions reach a translational temperature equal to the tube temperature of 295 ± 2 K prior to entering the reaction region. Reactions of Cu+ , Ag+ , Au+ , K+ , Rb+ , Cs+ , Ca+ , Sr+ , and Ba+ , In+ and Tl+ were investigated with NH3 at a helium buffer gas pressure of 0.35 ± 0.01 Torr and temperature of 295 ± 2 K. Reaction rate coefficients were determined in the usual manner using pseudo first-order kinetics. The highly pure NH3 gas was obtained commercially (Semiconductor Grade 99.999%, Matheson/Linde Canada), and introduced into the reaction region of the SIFT as a dilute (15%) mixture in helium. Reactions of Cu+ , Ag+ and Au+ also were measured with pyridine. The pyridine reagent for these reactions was obtained from Sigma–Aldrich (Oakville, ON, Canada) with 99.8% purity. Pyridine was mixed with helium by filling the reservoir system with pyridine vapour up to approximately 18 Torr and adding helium to a final mixture pressure of 760 Torr. Due to the larger than usual uncertainty in determining the concentration of the pyridine mixture, the rate coefficients reported here for pyridine have been assigned an uncertainty of ±50% rather than the usual ±30%. The low vapour pressure of pyridine restricted the pyridine flow range to a typical maximum flow rate of (0.5–1.0) × 1018 molecules s−1 [19]. 3. Results and discussion As an indication of the quality of our experimental measurements, we provide our experimental results in Fig. 1 for ammonia ligation of the coinage metal cations Cu+ , Ag+ and Au+ . Ammonia addition was the only reaction channel that was observed in each case. All three semi-logarithmic ion decays were observed to be linear, those for Cu+ and Au+ over more than a decade, so that there was no evidence for the presence of excited states. The change in the slope of the atomic cation decay is an indication of the change in the rate coefficient for the primary ammonia addition. Further higherorder additions of ammonia were observed for all three coinage metal ions. The measured rate coefficients for the ammonia reactions with the bare coinage metal ions are presented in Table 1. Also included

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Table 1 Effective bimolecular rate coefficients, k (in units of cm3 molecule−1 s−1 ), for the addition of one ammonia molecule to main-group and group 11 (coinage) atomic ions M+ measured at room temperature using the ICP/SIFT technique in our laboratory. The helium buffer gas pressure was 0.35 ± 0.01 Torr. Uncertainties in the rate coefficients are estimated to ±30%. M+ K+ Rb+ Cs+ a

M+

k

1.2 × 10−12 5.5 × 10−13 <10−13

Mg+, a Ca+ Sr+ Ba+

4.1 × 10−12 1.2 × 10−12 4.9 × 10−13 2.0 × 10−13

M+ Cu+ Ag+ Au+

M+

k 1.2 × 10−11 2.6 × 10−12 2.5 × 10−11

Ga+, b In+ Tl+

k 6.3 × 10−13 3.2 × 10−13 <10−13

Ref. [6]. Ref. [7].

Rate coefficient /(cm-3 molecule-1 s-1)

b

k

El e c t r o n i c En e r g y / ( k c a l m o l − 1 )

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10-10

60 50

Au + -NH3

40

Ag + -NH3

30 20 10 0 -10 100

20 0

Au+

Ag+ K+,Ca+ Rb+,Sr+ Ga+

Ba+

In+

3

4

5 00

60 0

Table 2 Effective bimolecular rate coefficients (in units of cm3 molecule−1 s−1 ) for the addition of one ligand molecule measured at room temperature using the ICP/SIFT technique in our laboratory. The helium buffer gas pressure was 0.35 ± 0.01 Torr. Uncertainties in the rate coefficients are estimated to ±30%, except for pyridine, for which it is estimated to be ±50%. Uncertainty. Ligand

Tl+,Cs+

10-13

40 0

Fig. 3. Computed relaxed 1-D potential energy surfaces for Au+ -NH3 and Ag+ -NH3 with De values (in kJ mol−1 ) of 235.6 and 196.6, respectively, and metal-nitrogen equilibrium bond distances of 2.106 Å and 2.197 Å, respectively.

Mg+

10-12

3 00

Met al-N Bond Dist ance / (pm )

Cu+ 10-11

3

5

6

Row in the periodic table Fig. 2. Periodic trends in the effective bimolecular rate coefficient for the ligation of atomic cations with ammonia at 295 ± 2 K in He at 0.35 ± 0.01 Torr. See Table 1 for numerical values.

N2 O NH3 D2 O SF6 CS2 CH3 F O2 Pyridine CO2 a

are new rate coefficient measurements for the ammonia ligation of the alkali cations K+ , Rb+ and Cs+ , the alkaline-earth cations Cs+ , Sr+ and Ba+ and the p0 cations In+ and Tl+ . Ammonia ligation was the only chemistry observed in each case. The trend down the periodic table is presented in Fig. 2. Clearly the rate coefficients for ammonia ligation of the s0 , s1 and p0 cations drop uniformly as the size of the cation increases. In sharp contrast, there is a dramatic rate enhancement for the d10 cations from Ag+ to Au+ . This enhancement can be attributed to the extra stabilization due to relativistic effects in the binding of NH3 to Au+ compared to Ag+ and Cu+ ; the number of degrees of freedom that also determine the magnitude of the lifetime of the encounter complex (and so the magnitude of the rate coefficient) remains unchanged in that the number of atoms and bonds stays the same. Calculations were performed with the Gaussian 03 suite of programs [20] using the B3LYP DFT method [21,22] and the SDD basis set [23] on the metals centers and the D95 basis set on hydrogen and nitrogen [24]. The SDD basis set employs a relativistically corrected effective core potential on the two metal centers studied. The Effective Core Potential (ECP) spans 60 electrons for Au and 28 for Ag. The geometry of the ion-molecule complexes was observed to have C3v symmetry, with metal-nitrogen equilibrium bond distances of 2.197 Å and 2.106 Å, for Ag and Au, respectively, a contraction of 0.091 Å (4.1%). 1-D relaxed potential energy surfaces were computed by varying the metal-nitrogen bond distances from 190 pm to 590 pm in 5 pm increments, while permitting other geometric parameters to be optimized to their lowest energy values. The potential energy surfaces are displayed in Fig. 3. The binding energy (De ) is seen to increase from 196.6 kJ mol−1 for Ag+ -NH3

b

Cu+

Ag+ −13

5.7 × 10 1.2 × 10−11 9.0 × 10−12 8.5 × 10−11 5.7 × 10−11 8.7 × 10−12 2.4 × 10−13 9.7 × 10−10 ≤4 × 10−13

Au+ −13

1.2 × 10 2.6 × 10−12 7.0 × 10−13 3.0 × 10−12 2.7 × 10−11 3.1 × 10−12 1.0 × 10−13 7.4 × 10−10 ≤5 × 10−13

−12

1.2 × 10 2.5 × 10−11 5.8 × 10−12 2.0 × 10−11 1.7 × 10−10 8.9 × 10−12, a 1.2 × 10−13 7.0 × 10−10, b ≤5 × 10−13

kAu+ /kAg+

Reference

10 9.6 8.3 6.7 6.3 2.9 1.2 0.95 –

[10] this work [9] [14] [12] [13] [8] this work [11]

Minor (12%) HF elimination channel is present. Minor (13%) electron transfer channel is present.

to 235.6 kJ mol−1 for Au+ -NH3 and this accounts for the observed increase in the measured rate of ligation. Given this realization of the influence of the relativistic effect on the kinetics of ligation of the coinage metal cations with ammonia, we expanded our view to other ligands for which we had previously measured and reported rate coefficients for addition to these coinage metal cations under similar operating conditions. These are presented in Table 2 along with previously unpublished results with pyridine. Fig. 4 shows the trends graphically. The overview offered by Table 2 and Fig. 4 provides some interesting insights. Seven of the nine ligands appear to exhibit a relativistic effect with Au+ in that an increase in reactivity is seen as we move down the periodic table from Ag+ to Au+ . If we adopt the ratio kAu+ /kAg+ as a measure of this effect we see from Table 2 that the effect is strong with NH3 , D2 O and N2 O, medium with CS2 and SF6 , weak with CH3 F and at best weak, within experimental uncertainty, with O2 and CO2 and not measurable with pyridine (ligation proceeds at almost unit efficiency, within the larger experimental uncertainty, with all three coinage cations). The hydration of the atomic coinage metal cations has been investigated computationally using relativistic spin-averaged Douglas-Kroll theory at the level of MP2 [25]. Results are shown in Table 3 together with those for parallel non-relativistic calculations. Increased relativistic stabilization of M+ (H2 O) accompanied by bond contraction is evident for all three coinage metal cations and is largest for Au+ (H2 O). Included in Table 3 are the results of our own relativistic calculations that converged to a planar C2v symme-

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4

SF6

Our state-of-the-art calculations with NH3 that include relativistic effects provide support for an enhanced binding energy and a contraction in binding length for Au+ over Ag+ . Also, available computational results for relativistic stabilization and contraction, as well as our own computations, are nicely consistent with the trend observed in the magnitude of the rate coefficient for the hydration of atomic coinage metal cations

CH3F

Acknowledgments

Rate coefficient /(cm-3 molecule-1 s-1)

10-9

Pyridine CS2

10-10

NH3

10-11

D2O N2O

10-12

CO2 O2

Continued financial support from the Natural Sciences and Engineering Research Council of Canada is greatly appreciated. As former holder of a Canada Research Chair in Physical Chemistry, D.K.B. (now retired) thanks the contributions of the Canada Research Chair Program to this research.

-13

10

Cu+

Ag+

References

Au+

Metal Cation Fig. 4. Periodic trends in the effective bimolecular rate coefficient for the ligation of atomic cations with various different ligands at 295 ± 2 K in He at 0.35 ± 0.01 Torr. See Table 2 for numerical values. Table 3 Computed binding energies (De in kJ mol−1 ) and bond distances (re (M-O) in pm) for M+ (H2 O) at the non-relativistic (NR) and relativistic (R) levels [25]. Values computed in our study are given in parentheses. De +

M (H2 O) Cu+ (H2 O) Ag+ (H2 O) Au+ (H2 O)

NR 174.5 120.5 121.3

re (M-O) R 189.1 (232.6) 131.0 (178.2) 189.1 (211.3)

NR 195 227 234

R 188 (191) 221 (219) 208 (214)

try for the geometry of M+ (H2 O). The two results agree reasonably well, certainly as far as trends in De and re are concerned, albeit our binding energies are larger by 12–36%. Our method is quite reliable and it should be noted that the Douglas-Kroll transformation has since been reformulated to the Douglas-Kroll-Hess method [26]. Both results track the experiment quite well and predict a minimum in the magnitude of the rate coefficient for the hydration of Ag+ . Our De results are more consistent with the highest rate coefficient in the series measured for the formation of Cu+ (H2 O). 4. Conclusions In a case study with the atomic metal coinage cations Cu+ , Ag+ and Au+ , a novel experimental approach has been demonstrated for the assessment of the role of relativistic effects in the binding of ligands to atomic metal cations from measurements of the kinetics of their formation at room temperature. New measurements with NH3 and earlier measurements with O2 , D2 O, N2 O, CO2 , CS2 and CH3 F show a positive response of the rate coefficient of ligation of Au+ in accord with an increased stabilization of the adduct ion due to relativistic effects. The magnitude of the response varies with the nature of the ligand. Using the relative magnitude of the measured rate coefficients of the ligation of Ag+ and Au+ as an indicator, relativistic effects appear to be strong with NH3 , D2 O and N2 O, medium with CS2 and SF6 , weak with CH3 F and at best weak, within experimental uncertainty, with O2 and CO2 .

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