Noble Gas Inserted Metal Acetylides (Metal = Cu, Ag, Au) - The

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Noble Gas Inserted Metal Acetylides (Metal = Cu, Ag, Au) Gourhari Jana, Sudip Pan, Gabriel Merino, and Pratim Kumar Chattaraj J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05404 • Publication Date (Web): 15 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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The Journal of Physical Chemistry

Noble Gas Inserted Metal Acetylides (Metal = Cu, Ag, Au) Gourhari Jana,1 Sudip Pan,2,* Gabriel Merino,3,* and Pratim K. Chattaraj.1,4,* 1

Department of Chemistry and Centre for Theoretical Studies Indian Institute of Technology Kharagpur, 721302, India

2

Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering,

Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing, China 3

Departamento de Física Aplicada, Centro de Investigación y de Estudios Avanzados, Unidad

Mérida. Km 6 Antigua Carretera a Progreso. Apdo. Postal 73, Cordemex, 97310, Mérida, Yuc., México 4

Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India

* Corresponding authors: [email protected] (SP); [email protected] (GM); [email protected] (PKC)

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Abstract Metal acetylides (MCCH, M = Cu, Ag, Au) were already experimentally detected in molecular form. Herein, we investigate the possibility of noble gas (Ng) insertion within C-H bond of MCCH and their stability is compared with those of the reported MNgCCH and HCCNgH molecules. Our coupled-cluster level computations show that MCCNgH (Ng = Kr, Xe, Rn) systems are local minima on the corresponding potential energy surfaces, whereas their lighter analogs do not remain in the chemically bound form. Further, their stability is analyzed with respect to all possible dissociation channels. The most favorable dissociation channel leads to the formation of free Ng and MCCH. However, there exists a high free energy barrier (29.3-46.9 kcal/mol) to hinder the dissociation. The other competitive processes against their stability include a two-body and a three-body neutral dissociation channels, MCCNgH → MCC + NgH and MCCNgH → MCC + Ng + H, respectively, which are slightly exergonic in nature at 298 K for Ng = Kr, Xe and M = Cu, Ag, and for AuCCKrH. But, the Xe analogs for Cu and Ag, and AuCCKrH would be viable at a lower temperature. AuCCNgH (Ng = Kr-Rn) molecules are the best candidates for experimental realization since they have higher dissociation energy values and higher kinetic protection in case of feasible dissociation channels compared to the Cu and Ag systems. A detailed bonding analysis indicates that the Ng-H bonds are genuine covalent bonds and there is also a substantial covalent character in Ng-C contacts of these molecules. Moreover, the possibility of insertion of two Xe atoms in AuCCH resulting in AuXeCCXeH, and the stability of XeAuXeCCXeH are also tested herein.

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Introduction The noble gas (Ng) inserted compounds have seen a great upsurge of interest in recent times, which contribute to the fascinating development in noble gas chemistry, starting with the production and characterization of a new class of molecules of type HNgY by Räsänen and coworkers1–12 and Feldman et al.,13,14 where Ng is a noble gas atom and Y usually represents an electronegative atom or a group of atoms. This type of Ng compounds can be prepared by photolysis of HY in a noble gas matrix. In general, HNgY can be represented as (HNg)+Y−, where the H-Ng bond is essentially covalent, whereas the interaction between (HNg)+ and Y− is predominantly ionic.15 Acetylene serves as an excellent motif for Ng insertion which resulted in the first observed Ng inserted hydrocarbon, HNgCCH (Ng = Kr, Xe), in cryogenic noble-gas matrices.1620

Thereafter, eventual experimental, as well as theoretical investigations, reported HNgCCX (X

= F, Cl, Br; Ng = Ar, Kr, Xe),9,12,21 where HNgCCX is claimed to have higher stability than the corresponding HNgCCH owing to the greater H-Ng stretching frequencies. Other few molecules with H-Ng-C or Ng-C bonding motifs, which are isolated, include HXeCC,18 HXeCCXeH,19,20 HNgCN (Ng = Kr, Xe),4 ClXeCN,22 BrXeCN,22 HNgC3N (Ng = Kr, Xe),23 and HNgC4H (Ng = Kr, Xe).24 Metal acetylides (MCCH; M = Cu, Ag, Au) in molecular form were characterized in experiments through rotational spectroscopy, which possess linear geometry with 1Ʃg ground electronic state.25-27 Moreover, they were reported to be effective in inserting Ng atom within MC bond, resulting in the first set of molecules having M-Ng-C fragment.28 Importantly, MCCH molecules have two possible sites M-C and C-H bonds for Ng insertion. Hence, the questions, 3



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that arise are whether MCCNgH (M = Cu, Ag, Au) is also viable, and which isomer between MNgCCH and MCCNgH is more stable. Further, it would also be worth understanding what type of variation in stability and bonding from that of HCCNgH is introduced due to the presence of M in MCCNgH. Finally, would it be also possible to insert two Ng atoms simultaneously resulting in an MNgCCNgH molecule? Even more, how would be the stability of the NgMNgCCNgH species? To check them, we have performed both ab initio and density functional theory (DFT) based computations and their thermochemical stability is elucidated by studying all the possible dissociation channels and the associated free energy barriers. The nature of bonding is investigated by employing natural bond orbital (NBO),29,30 electron density,31 adaptive natural density partitioning (AdNDP),32 and energy decomposition analyses (EDA).33-36

Computational Details Since the results of Ng insertion compounds are sensitive to the level of theory and only high-level computations provide reliable results, the geometry optimizations followed by the frequency computations are performed at the MPW1B95,37 MP2,38 and CCSD(T)39 levels in conjunction with the cc-pVTZ basis set,40-42 used for H, C, and Ar atoms, and cc-pVTZ-PP basis set along with the relativistic effective core potential (RECP) for Kr, Xe, Rn, Cu, Ag, and Au atoms. Henceforth, we have abbreviated this combination as VTZ. The RECPs used for computations are ECP10MDF for Cu and Kr, ECP28MDF for Ag and Xe, and ECP60MDF for Au and Rn.43,44 Note that in a benchmark study, MPW1B95 was reported to be one of the best functionals to reproduce the experimental and/or CCSD(T) results for Ng species.45 The T1 diagnostics46 from converged CCSD wave functions are evaluated to check whether a single4


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reference method is adequate to represent MCCNgH systems. In general, if T1 < 0.02 for main group elements and T1 < 0.05 for the systems involving transition metals, the system is considered to have negligible multi-reference character. In our cases, the corresponding T1 values range between 0.018 and 0.026 (see Table S1 in Supporting Information). So, the presently considered levels of theory should be appropriate in describing these systems. The NBO analysis is also performed at the same level to evaluate the natural charge and Wiberg bond index (WBI).47 All these computations are performed with the Gaussian 09 program.48 The electron density analysis31 is carried out at the MP2/cc-pVTZ/WTBS//CCSD(T)/VTZ level (all-electron WTBS for Cu, Ag, Au, Xe and Rn atoms)49 by using Multiwfn software.50 The nature of chemical bonding is analyzed using the AdNDP method developed by Zubarev and Boldyrev.31 This technique considers the electron pairs as the fundamental component of chemical bonding, describing the electronic structure in terms of n-center-twoelectron (nc-2e) bonds. So, AdNDP recovers both Lewis bonding concepts (lone pairs or 2c-2e bonds) and delocalized bonding elements. This analysis is also performed via Multiwfn software.50 The EDA analysis was carried out using the ADF (2017.101) program package51,52 at the PBE-D353/QZ4P54//CCSD(T)/VTZ level where scalar relativistic effects are considered for the heavy metals using the zeroth-order regular approximation (ZORA).55-59 The frozen core approximation was not employed in these computations. In the EDA method, the interaction energy (∆Εint) between two fragments is decomposed into four energy terms, viz., the electrostatic interaction energy (∆Eelstat), the Pauli repulsion

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(∆EPauli), the orbital interaction energy (∆Eorb), and the dispersion interaction energy (∆Edisp), Therefore, the interaction energy (∆Εint) between two fragments can be defined as: ∆Εint = ∆Eelstat + ∆EPauli + ∆Eorb + ∆Edisp.

(1)

∆Eelstat is computed classically by taking the two fragments at their optimized positions but considering the charge distribution is unperturbed on each fragment by other one. The next one is ∆EPauli, which appears as the repulsive energy between electrons of the same spin and it is computed by employing Kohn-Sham determinant on the superimposed fragments to obey the Pauli principle by antisymmetrization and renormalization. The ∆Eorb originates from the mixing of orbitals, charge transfer and polarization between two fragments. Lastly, the ∆Edisp represents the dispersion interaction between the two fragments. This method has been tuned out to be a very elegant tool to describe the qualitative nature of bonding in different systems including Ng compounds.60-68

Results and Discussion Structures and stability of MCCNgH Our CCSD(T) computations reveal that MCCNgH (Ng = Kr, Xe and Rn) systems, having linear geometries and 1Ʃg electronic state, are found to be minima on the corresponding potential energy surfaces (see Figure 1). However, although Ar inserted analogs are minimum energy structures at the MP2 and MPW1B95 levels, at CCSD(T) level they get dissociated into MCC + Ar + H upon optimization. As reflected from the geometrical parameters provided in Figure 1, upon the changes in M, the Ng-H and Ng-C bonds show very little variation. The Ng-H bond 6


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distance is somewhat shorter in Au case than those in Cu and Ag systems. In comparison to the corresponding Ng-H covalent bond distances (rcov),69 the Kr-H bond in these systems is slightly longer (by 0.154-0.208 Å), whereas the Xe-H and Rn-H bond lengths are almost similar to the corresponding rcov values. On the other hand, C-Ng bonds are noticeably longer than the rcov value for Kr, and the deviation gradually diminishes along Kr (~0.33 Å) to Rn (~0.13 Å).

Figure 1. CCSD(T)/VTZ structures of MCCNgH (M = Cu, Ag, Au). The bond lengths are in Å unit and the values within the first bracket, second and square brackets are computed for Cu, Ag, and Au, respectively. The corresponding transition states for the MNgCCH → Ng + MCCH process at the MPW1B95/VTZ level.

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To check the thermochemical stability of MCCNgH compounds, we have considered the possible dissociation channels leading to all probable cationic, anionic, and neutral fragments (see Table 1 for CCSD(T) results, and Tables S2 and S3 for MP2 and MPW1B95 ones, respectively). There are three possible three-body (3-B) and five two-body (2-B) dissociation channels where the resulting fragments are noted to have minimum energy structures. Similar to all Ng inserted molecules, the most exergonic dissociation process is the release of free Ng producing the parent MCCH molecule (free energy change at 298 K, ∆G: from -103.2 to -135.1 kcal/mol). For a given M, the degree of exergonicity for such dissociation gradually diminishes along Kr to Rn, presumably because of the increasingly larger stabilizing interaction along the same. Nevertheless, MCCNgH → MCCH + Ng cannot proceed easily at room temperature as the associated free energy barrier (∆G‡) is reasonably high, being 30.9-51.9 kcal/mol at the MP2 level and 29.3-46.9 kcal/mol at the MPW1B95 level. For a given M, the kinetic stability gradually enhances in moving from Kr to Rn. Therefore, there is no doubt that these systems cannot undergo such dissociation at ambient temperature. The related TSs are also depicted in Figure 1 where the NgH is almost located horizontally with MCC unit and Ng center is loosely interacting with both the C centers. The C-Ng bonds in TSs are significantly longer than those in minimum energy structures. The imaginary single frequency is involved in the C-Ng-H bending mode of vibration which brings the C and H centers closer. Among other 2-B channels, the most probable process, which is likely to occur, is MCCNgH → MCC + NgH. This is exergonic with a ∆G value of -9.9 (Au) − -21.7(Cu) kcal/mol for Kr cases and with very small values of -2.6 kcal/mol for CuCCXeH and -1.2 kcal/mol for AgCCXeH. The remaining systems are stable with respect to this dissociation. Another related 8


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dissociation is a 3-B channel, MCCNgH → MCC + Ng + H, which is exergonic by -9.4 (Au) − 23.8 (Cu) kcal/mol for Kr cases, and CuCCXeH and AgCCXeH have this value as -4.6 and -3.3 kcal/mol, respectively. Note that the latter 3-B dissociation is more probable than the former 2-B channel, and, therefore, this 3-B dissociation makes MCCKrH (M = Cu, Ag) molecules vulnerable to spontaneous dissociation. Low exergonicity for those dissociations of MCCXeH (M = Cu, Ag) and AuCCKrH indicate that these molecules would be viable at a lower temperature. Since such molecules are usually isolated at low temperature (less than 40-45 K in many cases),18 we have studied the feasibility of such 3-B dissociation at 77 K (see Table S4). The corresponding ∆G values indicate that Xe species are stable with respect to this 3-B dissociation. In fact, they would be stable at much higher temperature as well. AuCCKrH would also likely to be stable at a slightly lower temperature than 77 K. Only CuCCKrH and AgCCKrH would be kin to undergo such 3-B dissociation at cryogenic temperature. Therefore, the possible metastability of Kr species would depend on the kinetic stability along this 3-B dissociation. Previous studies70 indicate that multireference methods such as multireference configuration interaction and complete active space perturbation theory are required to correctly describe such 3-B dissociation pathways. Moreover, the results of different multireference methods differ significantly from each other. Since these methods are computationally expensive, particularly for the present systems involving heavier elements, this aspect is not explored herein. Among the studied species, AuCCNgH molecules are the best candidates for the experimental realization as they are least prone to get dissociated and have the highest kinetic protection along the feasible dissociation channels. Rest of the dissociation processes are not at all possible as reflected in their very high endergonicity.

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Table 1. ZPE corrected dissociation energy (D0, kcal/mol) and free energy change at 298 K (∆G, kcal/mol) for different dissociation channels of MCCNgH (M = Cu, Ag, Au and Ng = Kr-Rn) at the CCSD(T)/VTZ level. Processes

MCCNgH → CCNg+ + H-

MCCNgH → MCCH + Ng

∆E‡ (∆G‡)a

∆E‡ (∆G‡)b

MCCNgH → MCC + NgH

MCCNgH → MCC+ + NgH-

MCCNgH → MCC- + NgH+

D0

∆G

Kr

Xe

Rn

Kr

Xe

Rn

Cu

188.5

187.5

190.8

184.0

181.4

184.7

Ag

185.4

184.6

188.0

180.7

178.6

182.0

Au

200.0

198.4

201.4

195.7

192.4

195.5

Cu

-128.7

-109.4

-99.1

-135.1

-116.0

-105.7

Ag

-128.6

-109.4

-99.1

-135.0

-116.0

-105.7

Au

-127.0

-107.1

-96.7

-133.3

-113.7

-103.2

Cu

42.7

50.3

53.7

(41.0)

(48.2)

(51.4)

Ag

43.2

50.9

54.2

(41.5)

(48.8)

(51.9)

Au

31.5

44.4

47.5

(30.9)

(42.1)

(45.7)

Cu

35.2

46.7

47.7

(29.3)

(42.8)

(44.1)

Ag

36.1

47.4

51.1

(31.0)

(43.4)

(46.9)

Au

35.4

44.3

47.8

(31.6)

(40.8)

(44.3)

Cu

-9.3

10.0

20.3

-21.7

-2.6

7.8

Ag

-7.8

11.4

21.7

-20.3

-1.2

9.1

Au

2.1

23.6

34.1

-9.9

12.3

22.8

Cu

235.6

253.5

262.7

226.2

244.1

253.3

Ag

222.2

240.0

249.2

213.1

230.9

240.1

Au

246.5

265.0

274.4

237.2

255.5

264.9

Cu

152.8

156.2

159.8

144.5

147.5

151.0 10


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MCCNgH → MCC + Ng + H

MCCNgH → MCC+ + Ng + H-

MCCNgH → MCC- + Ng + H+

a

Ag

153.1

156.5

160.0

144.7

147.7

151.1

Au

140.8

144.8

148.5

132.6

136.2

139.8

Cu

-9.3

10.0

20.3

-23.8

-4.6

5.6

Ag

-7.8

11.4

21.7

-22.4

-3.3

6.9

Au

3.8

23.6

34.1

-9.4

10.2

20.7

Cu

237.1

256.4

266.7

224.8

243.9

254.1

Ag

223.7

242.9

253.2

211.6

230.7

240.9

Au

248.1

267.9

278.4

235.7

255.3

265.8

Cu

256.9

276.2

286.5

244.1

263.3

273.5

Ag

257.2

276.4

286.7

244.3

163.4

273.6

Au

244.9

264.7

275.2

232.3

251.9

262.3

The activation barriers, ∆E‡ and ∆G‡, for the process, MCCNgH → Ng + MCCH are at MP2

level; bat MPW1B95 level.

Nature of bonding in MCCNgH In MCCNgH compounds, NgH unit attains a positive natural charge of 0.55-0.65 |e|, whereas MCC unit possesses a negative charge with the same magnitude (see Table 2). Therefore, the electron distribution suggests that there would be a significant ionic interaction between MCC and NgH fragments. On the other hand, the Ng center in these molecules acts almost as Ng+● and, hence it can form an electron-shared covalent bond with H. This is also reflected in the corresponding WBI values where the bond orders of Ng-C bonds (0.36-0.44) are lower than those in Ng-H bonds (0.60-0.70). Note that the Ng-C bonds are not exclusively ionic in nature but significant covalent interaction is also present there.

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Table 2. The natural charges on M, Ng, C and H centers (q, au), WBI for Ng-C and Ng-H bonds, for MCCNgH compounds at the CCSD(T)/VTZ level. q

WBI

Systems

M

C

C

Ng

H

MCC

NgH

C-Ng

Ng-H

CuCCKrH

0.76

-0.81

-0.50

0.60

-0.06

-0.55

0.55

0.36

0.61

CuCCXeH

0.77

-0.77

-0.58

0.77

-0.19

-0.58

0.58

0.44

0.68

CuCCRnH

0.76

-0.77

-0.61

0.86

-0.24

-0.62

0.62

0.42

0.67

AgCCKrH

0.76

-0.78

-0.52

0.60

-0.06

-0.54

0.54

0.36

0.60

AgCCXeH

0.77

-0.74

-0.60

0.77

-0.19

-0.58

0.58

0.44

0.67

AgCCRnH

0.77

-0.75

-0.63

0.86

-0.25

-0.61

0.61

0.42

0.66

AuCCKrH

0.52

-0.63

-0.50

0.62

-0.01

-0.60

0.60

0.33

0.64

AuCCXeH

0.54

-0.60

-0.56

0.77

-0.15

-0.62

0.62

0.40

0.70

AuCCRnH

0.53

-0.60

-0.59

0.86

-0.21

-0.65

0.65

0.39

0.69

We have computed different descriptors of electron density to corroborate with the above inference (see Table 3). In general, while a large electron density (ρ(rc))) and negative Laplacian of electron density (∇2ρ(rc)) at the bond critical point (BCP) imply a covalent bond, a reverse situation indicates non-covalent bond. However, ∇2ρ(rc) does not fulfill the criterion particularly for the bonds involving heavier elements.71,72 This is because ∇2ρ(rc) derives from the three curvature values (λ1, λ2 and λ3) where the first two terms are negative but λ3 is positive. For heavier elements, the term λ3 often dominates over the other two terms making the overall ∇2ρ(rc) value positive. For these cases, the use of total electronic energy density (H(rc)),73 a sum 12


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of local kinetic energy density (G(rc)) and local potential energy density (V(rc)), and the ratio of G(rc) and ρ(rc) are recommended, where a negative H(rc) value and G(rc)/ρ(rc) < 174 indicate covalent bond and a reverse situation represents non-covalent bond. The contour plots of the ∇2ρ(r) at the molecular plane are depicted in Figure 2, where the spherical electron density around H is clearly perturbed in the molecule and it gets depleted towards the Ng center. Thus, in case of Kr-H bond the ∇2ρ(rc) values becomes negative. However, in all the remaining cases although the electron distribution is quite similar, the BCP is located just outside the electron accumulated region. On the other hand, in Ng-C bonds no clear region of electron accumulation is noted. Now, if we look on the values of H(rc) and G(rc)/ρ(rc), for all cases while the former is considerably negative, the latter is very much less than 1. Nevertheless, for a given M and Ng, the related values show that Ng-H is more covalent in nature than the Ng-C bonds. Further, the plot of electron localization function (ELF) shows that the ELF value is higher in the former bond than in the latter one, but it is also considerable in latter, clearly showing covalent nature of interaction in both the bonds where the degree of covalency is larger in Ng-H bond than in Ng-C bond (see Figure S1).

Table 3. Electron density descriptors (in au) like electron density (ρ(rc)), Laplacian of electron density (∇2ρ(rc)), local kinetic energy density (G(rc)), local potential energy density (V(rc)), electron

energy

density

(H(rc)),

the

ratio

of

G(rc)

and

ρ(rc) at the MP2/cc-

pVTZ/WTBS//CCSD(T)/VTZ level. Systems

BCP

ρ(rc)

∇2ρ(rc)

G(rc)

V(rc)

H(rc)

G(rc)/ρ(rc)

CuCCKrH

Kr-C

0.080

0.104

0.050

-0.073

-0.024

0.621

CuCCXeH

Xe-C

0.082

0.084

0.051

-0.081

-0.030

0.626 13



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CuCCRnH

Rn-C

0.078

0.097

0.053

-0.081

-0.028

0.676

AgCCKrH

Kr-C

0.080

0.103

0.050

-0.073

-0.024

0.619

AgCCXeH

Xe-C

0.082

0.083

0.051

-0.081

-0.030

0.622

AgCCRnH

Rn-C

0.082

0.083

0.051

-0.081

-0.030

0.622

AuCCKrH

Kr-C

0.079

0.106

0.050

-0.073

-0.023

0.628

AuCCXeH

Xe-C

0.079

0.085

0.050

-0.078

-0.028

0.626

AuCCRnH

Rn-C

0.076

0.096

0.051

-0.078

-0.027

0.673

CuCCKrH

Kr-H

0.118

-0.040

0.060

-0.130

-0.070

0.118

CuCCXeH

Xe-H

0.110

0.021

0.067

-0.128

-0.062

0.110

CuCCRnH

Rn-H

0.100

0.079

0.070

-0.120

-0.050

0.100

AgCCKrH

Kr-H

0.116

-0.034

0.060

-0.128

-0.068

0.116

AgCCXeH

Xe-H

0.109

0.022

0.066

-0.127

-0.060

0.109

AgCCRnH

Rn-H

0.099

0.079

0.069

-0.119

-0.050

0.099

AuCCKrH

Kr-H

0.130

-0.075

0.067

-0.153

-0.086

0.130

AuCCXeH

Xe-H

0.115

0.020

0.072

-0.140

-0.067

0.115

AuCCRnH

Rn-H

0.104

0.085

0.075

-0.130

-0.054

0.104

14


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Figure 2. Contour diagrams of the Laplacian of the electron density of MCCNgH (M = Cu-Au; Ng = Kr-Rn) at the MP2/cc-pVTZ/WTBS//CCSD(T)/VTZ level.(Green solid lines stand for ∇2ρ(r) > 0, whereas blue-dashed lines stand for ∇2ρ(r) < 0.)

To get more insight into the bonding situation the results of the AdNDP analysis are provided in Figures 3 and S2. It identifies five d lone-pairs (LPs) on M and three LPs on Ng center. There are one C-C σ-bond and two degenerate C-C π-bonds, representing the C≡C unit. The M is connected through one M-C σ-bond. Most importantly, it recovers Ng-H σ-bond with high ON of 1.99 e-. On the other hand, the consideration of Ng-C 2c-2e σ-bond results in a low 15



Page 16 of 31

ON of 1.75-1.81 e-, corroborating with lower covalency than Ng-H bond. Ideal ON of 1.97-1.98 e- can be attained if one considers a delocalized C-Ng-H 3c-2e σ-bond (Figure S2). Therefore, all these computations indicate that there is also a considerable covalent character in Ng-C contacts.

Figure 3. The bonding elements recovered by the AdNDP analysis for MCCNgH molecules (M= Cu, Ag and Au; Ng = Kr- Rn).

16


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Finally, the results of EDA are provided in Table 4 to corroborate with the above the results. For Ng-H electron-shared bonds, the ∆Eorb term contributes around 57-59% of the total attraction, whereas ∆Velstat contributes the rest (41-43%). Dispersion contribution is not at all important in the present cases. Therefore, despite electron-shared description in Ng-H bond, the electrostatic nature is also substantial. On the other hand, in the ionic description of C-Ng bond, while 61-67% of the total attraction results from the electrostatic contribution, the orbital contact is responsible for 33-39% of stabilizing energy. In fact, the magnitude of ∆Eorb value in Ng-C bond is even larger than that in Ng-H bond. Therefore, it is clear both in covalent and ionic descriptions that the other factor also plays important role in stabilizing the interaction. This is the reason to have a covalent interpretation for Ng-C bond in electron density analysis.

Table 4. EDA results of MCCNgH at the PBE-D3/QZ4P//CCSD(T)/VTZ level. All energy values are in kcal/mol. aThe values in parentheses are percentage contribution toward the total attraction, ∆Velstat + ∆Eorb + ∆Edisp. a

a

a

Systems CuCCKrH

Fragments CuCCKr + H

∆Eint -27.1

∆EPauli 94.6

∆Velstat -50.3 (41.3%)

∆Eorb -71.3 (58.6%)

∆Edisp -0.1 (0.1%)

CuCCXeH

CuCCXe + H

-40.3

96.0

-56.3 (41.3%)

-80.0 (58.7%)

-0.1 (0.1%)

CuCCRnH

CuCCRn + H

-44.3

86.3

-55.3 (42.3%)

-75.2 (57.6%)

-0.1 (0.1%)

AgCCKrH

AgCCKr + H

-27.5

91.2

-49.0 (41.2%)

-69.7 (58.7%)

-0.1 (0.1%)

AgCCXeH

AgCCXe + H

-39.9

95.4

-55.8 (41.2%)

-79.4 (58.7%)

-0.1 (0.1%)

AgCCRnH

AgCCRn + H

-43.9

86.1

-55.0 (42.3%)

-74.9 (57.6%)

-0.1 (0.1%)

AuCCKrH

AuCCKr + H

-29.5

104.5

-54.8 (40.9%)

-79.0 (59.0%)

-0.1 (0.1%)

AuCCXeH

AuCCXe + H

-40.6

101.6

-58.6 (41.2%)

-83.5 (58.7%)

-0.1 (0.1%)

AuCCRnH

AuCCRn + H

-44.6

90.4

-57.3 (42.4%)

-77.7 (57.5%)

-0.1 (0.1%) 17



Page 18 of 31

CuCCKrH

CuCC− + KrH+

-164.2

120.0

-175.7 (61.8%) -108.1 (38.0%)

-0.5 (0.2%)

CuCCXeH

CuCC− + XeH+

-162.4

144.2

-195.6 (63.8%) -110.4 (36.0%)

-0.5 (0.2%)

CuCCRnH

CuCC− + RnH+

-163.8

139.5

-201.1 (66.3%) -101.6 (33.5%)

-0.6 (0.2%)

AgCCKrH

AgCC− + KrH+

-165.4

114.0

-170.9 (61.2%) -108.0 (38.7%)

-0.4 (0.1%)

AgCCXeH

AgCC− + XeH+

-163.2

142.5

-192.7 (63.0%) -112.5 (36.8%)

-0.5 (0.2%)

AgCCRnH

AgCC− + RnH+

-164.6

137.6

-197.9 (65.5%) -103.7 (34.3%)

-0.6 (0.2%)

AuCCKrH

AuCC− + KrH+

-154.0

109.9

-166.2 (63.0%)

-97.3 (36.9%)

-0.5 (0.2%)

AuCCXeH

AuCC− + XeH+

-153.8

132.8

-183.8 (64.2%) -102.1 (35.6%)

-0.6 (0.2%)

AuCCRnH

AuCC− + RnH+

-155.5

129.0

-189.2 (66.5%)

-94.7 (33.3%)

-0.7 (0.2%)

Comparison with MNgCCH (Ng = Xe, Rn) In case of the possibility of Ng insertion into M-C bond of MCCH, only Xe and Rn inserted analogs were reported to be viable, whereas Kr species are not even minima. For a given M and Ng, MNgCCH is energetically more stable (by 36.0-48.2 kcal/mol) than MCCNgH isomer (see Table S5). This is because in MCCNgH the molecule gives up the C-H bond which is stronger than the M-C bond. However, MNgCCH possesses considerably less kinetic protection (∆G‡ = 14.0-34.8 kcal/mol) along the exergonic dissociation channel, MNgCCH → Ng + MCCH, than that in MCCNgH. On the other hand, MNgCCH shows greater resistance towards 3-B dissociation, MNgCCH → M + Ng + CCH than the similar neural dissociation in MCCNgH isomer. In other words, each isomer has its own pros and cons, but in general, MNgCCH shows less exergonicity (or higher endergonicity) towards the most competitive dissociation channels than that in MCCNgH.

18


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Comparison with HCCNgH (Ng = Kr-Rn) Now, to understand the differences inducted because of replacement of H by M, we have optimized HCCNgH (Ng = Kr-Rn) at the CCSD(T) level. Table S6 summarizes the change in Ng-C and Ng-H bond lengths present in HCCNgH and its M analog. In MCCNgH, the C-Ng bonds get shortened whereas the Ng-H bonds become longer in comparison to those in HCCNgH. Both the MCCNgH → MCCH + Ng and HCCNgH → HCCH + Ng are almost equally exergonic for a given M and Ng. However, because of the weakened Ng-H interaction, the corresponding free energy barrier gets lower to some degree in the former cases than those in the later ones (see Table S7). Note that this decrease is not so much substantial that it would destabilize the molecules along this dissociation. Other interesting aspect would be to compare the stability along the neutral 2-B and 3-B dissociation channels, HCCNgH → ΗCCNg + H and HCCNgH → ΗCC + Ng + H. In these cases, unlike the MCCNgH, only Kr analog is noted to have such dissociations spontaneous in nature. Importantly, for a given Ng, AuCCNgH has slightly enhanced stability along these dissociation channels in comparison to that involving HCCNgH (see Table S8). The partial natural charges and WBI values for HCCNgH are provided in Table S9. A comparison with MCCNgH indicates that the charge separation between HCC and NgH fragment is slightly larger in HCCNgH than that in MCCNgH. Consequently, it results in a slightly higher WBI value in C-Ng bond of later species than the former. On the other hand, the Ng-H bond in HCCNgH is more covalent in nature than that in its M analogs. Similar to that in MCCNgH, both for C-Ng and Ng-H bonds in HCCNgH the corresponding H(rc) values are negative, and the value is more negative in the latter bond than that in the former (see Table S10). Further, for a given Ng, the H(rc) value in Ng-H bond is more negative in HCCNgH than 19



Page 20 of 31

its M analog. On the other hand, the H(rc) value in C-Ng bond in HCCNgH is quite comparable with that in MCCNgH.

Stability of AuXeCCXeH and XeAuXeCCXeH The experimental report on detecting multiple Ng atoms inserted systems is limited. HXeCCXeH is an excellent example, related to our present systems, which was prepared in a low-temperature Xe matrix using UV photolysis of acetylene in 2003 by Khriachtchev et al.18 Therefore, in our present cases since both MNgCCH and MCCNgH isomers are viable, it would be interesting to check whether MNgCCNgH are also stable enough for experimental realization. The CCSD(T) optimization on MXeCCXeH shows that only Cu and Au analogs remain in the bound state, whereas Ag analog gets dissociated into AgXe and CCXeH fragments. Since Au is the best-recommended species in the present cases, the detailed stability analysis with respect to important dissociation channels of AuXeCCXeH at the MP2 level is provided in Table 5 (see the results of some other dissociation channels at the MP2 and MPW1B95 levels in Tables S11 and S12 respectively). Figure 4 depicts the structures and geometrical parameters of these two Ng bonded species.

20


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Figure 4. The minimum energy structures of AuXeCCXeH, XeAuCCXeH, and XeAuXeCCXeH molecules and the corresponding transition state for the process, AuXeCCXeH → XeAuCCXeH. The bond lengths (in Å) at CCSD(T), MP2 and MPW1B95 levels are given in parentheses, braces and brackets, respectively. WBI and partial natural charges (q) are also given at the CCSD(T) level.

Two 2-B dissociation channels, AuXeCCXeH → AuXeCCH + Xe and AuXeCCXeH → AuCCXeH + Xe, are noted to be spontaneous at 298 K where the former process is more exergonic than the latter one. However, the release of Xe from C-Xe-H bonding unit involves larger ∆G‡ value than that from Au-Xe-C unit. Therefore, AuXeCCXeH → AuCCXeH + Xe process is likely to occur first. In fact, such dissociation would first lead to the conversion, AuXeCCXeH → XeAuCCXeH, via a barrier of ∆G‡ = 29.0 kcal/mol. The Xe-Au bond dissociation energy in XeAuCCXeH is 15.1 kcal/mol (∆G value of 7.7 kcal/mol). So, more energy would be required to break the Xe-Au bond which would lead to free Xe and AuCCXeH.

21



Page 22 of 31

Therefore, the present results indicate that two Ng inserted analogs, AuXeCCXeH and even XeAuCCXeH are viable to be detected, particularly via low-temperature matrix isolation.

Table 5. ZPE corrected dissociation energy (D0, kcal/mol) and free energy change at 298 K (∆G, kcal/mol) for some important dissociation channels of AuXeCCXeH and XeAuCCXeH at the MP2/VTZ level.

a

Processes

D0

∆G

AuXeCCXeH → Au + XeCCXeH

81.1

68.7

AuXeCCXeH → AuXe + CCXeH

81.0

68.6

AuXeCCXeH → AuXeCC + XeH

52.2

41.3

AuXeCCXeH → AuXeCCXe + H

41.0

30.3

AuXeCCXeH → AuXeCCH + Xe

-103.4

-109.8

AuXeCCXeH → AuCCXeH + Xe

-71.4

-78.5

AuXeCCXeH → Au + XeCCXe + H

118.8

120.2

AuXeCCXeH → AuXe + CCXe + H

72.5

54.0

AuXeCCXeH → AuXeCC + Xe + H

52.2

39.2

AuXeCCXeH → XeAuCCXeH

-86.5

-86.2

∆E‡ (∆G‡)a

32.2

(29.0)

XeAuCCXeH → Xe + AuCCXeH

15.1

7.7

The activation barrier is for AuXeCCXeH → XeAuCCXeH process at the MP2 level.

The Xe center bound to Au in AuXeCCXeH carries a positive charge of 0.55 e-, whereas the other Xe is even more electropositive (0.77 e-) in nature (see Figure 4). Both the Au-Xe bond and Xe-H bond have high WBI values (0.63 and 0.69, respectively), showing the covalent nature 22


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of the bonds, while the Xe-C and C-Xe bonds have relative lower WBI values than the former bonds. But still, the covalent contribution in the later bonds is considerable. On the other hand, in XeAuCCXeH, the bonding and charge distribution in C-Xe-H fragment are nearly same to that in AuXeCCXeH, but as expected the non-inserted Xe-Au bond is relatively weaker and has a WBI value of 0.30. The Xe also has a positive charge of 0.18 e- which gets transferred to Au. The details about the topological descriptors in these systems are provided in Table 6. For AuXeCCXeH, for all the bonds involving Xe have negative H(rc) values and G(rc)/ρ(rc) values less than1. For C-Xe-H moiety, the Xe-H has much lower H(rc) value than the C-Xe bond, reflecting the higher degree of covalency in former bond. However, in Au-Xe-C fragment, despite significant ionic interaction in Xe-C bond the H(rc) values and G(rc)/ρ(rc) values signify higher covalent character than the Au-Xe bond. Such anomaly was also noted previously.28,75 Nevertheless, this is clear that substantial covalent character is involved in all bonds involving Xe atom. In contrary, the non-inserted Xe-Au bond in XeAuCCXeH has very low negative H(rc) value and the corresponding G(rc)/ρ(rc) value is greater than 1, implying the poor covalent contribution in this bond. Lastly, we have checked the stability of XeAuXeCCXeH and because the Xe-Au bond dissociation energy is very low (1.4 kcal/mol), such three Ng bound analogs are not viable.

Table 6. Electron density descriptors (in au) like electron density (ρ(rc)), Laplacian of electron density (∇2ρ(rc)), local kinetic energy density (G(rc)), local potential energy density (V(rc)), electron

energy

density

(H(rc)),

the

ratio

of

G(rc)

and

ρ(rc) at the MP2/cc-

pVTZ/WTBS//CCSD(T)/VTZ level.

23



Page 24 of 31

Systems

BCP

ρ(rc)

∇2ρ(rc)

G(rc)

V(rc)

H(rc)

G(rc)/ρ(rc)

AuXeCCXeH

Au-Xe

0.058

0.184

0.055

-0.064

-0.009

0.959

AuXeCCXeH

Xe-C

0.103

0.130

0.079

-0.126

-0.047

0.767

AuXeCCXeH

C-Xe

0.075

0.089

0.048

-0.073

-0.025

0.633

AuXeCCXeH

Xe-H

0.124

0.012

0.081

-0.159

-0.078

0.655

XeAuCCXeH

Xe-Au

0.048

0.212

0.057

-0.060

-0.003

1.188

XeAuCCXeH

C-Xe

0.084

0.085

0.053

-0.085

-0.032

0.633

XeAuCCXeH

Xe-H

0.116

0.029

0.076

-0.144

-0.068

0.651

Conclusions Ng insertion within the C-H bond of metal acetylides (MCCH; M = Cu, Ag, Au) would provide viable Ng inserted molecules. The Xe and Rn for Cu and Ag cases and Kr-Rn for the Au system should be the preferable ones. Among all, AuCCH is the best to facilitate insertion of Ng. The most exergonic dissociation channel, MCCNgH → MCCH + Ng, is clearly prohibited by a high kinetic barrier at ambient temperature; however, there are two other radical dissociation processes, MCCNgH → MCC + NgH and MCCNgH → MCC + Ng + H, which would decide their overall stability. These latter two processes are slightly exergonic in nature at 298 K for Ng = Kr, Xe and M = Cu, Ag, and for AuCCKrH. A lower temperature would be able to stabilize these molecules, except for Kr inserted Cu and Ag systems for which the exergonicity is larger. This is not the case that the inclusion of M causes such destabilization; even in HCCNgH (Ng = Kr-Rn) species, they have similar exergonic channels but only for Ng = Kr. In fact, AuCCNgH 24


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possesses slightly enhanced stability than its acetylene analog. The bonding situation in these molecules shows that the Ng takes part in substantial covalent interaction with the atoms located at either side of it. Further, insertion of two Xe atoms into the Au-C bond and C-H bond of AuCCH would also be possible. The resulting AuXeCCXeH molecule is noted as kinetically stable enough to exist. The other isomer, XeAuCCXeH is also stable against the dissociation, Xe + AuCCXeH. On the other hand, XeAuXeCCXeH is not much stable against dissociation. Therefore, the present results would certainly enrich the number of viable Ng inserted species and expand our knowledge about the interesting bonding that Ng inserted species possess.

Supporting information Supporting Information Available: T1 diagnostic values for MCCNgH, results at the MP2 and MPW1B95 levels, ∆G values at 77 K for neutral 3-B dissociation, relative energies between MCCNgH and MNgCCH, results for HCCNgH, bonding analysis data for HCCNgH, MP2 and MPW1B95 results for AuXeCCXeH, ELF and AdNDP plots, and Cartesian coordinates of the studied systems. This material is available free of charge via the Internet at http://pubs.acs.org.

Notes The authors declare no competing financial interest.

Acknowledgements

25



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PKC would like to thank DST, New Delhi for the J. C. Bose National Fellowship. GJ thanks IIT, Kharagpur for his fellowship. SP thanks Nanjing Tech University for the postdoctoral fellowship and the High-Performance Computing Center of Nanjing Tech University for supporting the computational resources. This work is supported by Conacyt (Grant CB-2015252356). ABACUS at Cinvestav (Conacyt grant EDOMEX-2011-COI-165873) is acknowledged for allocation of computational resources.

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Noble Gas Inserted Metal Acetylides (Metal = Cu, Ag, Au) - The

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