alexa A Density-Functional Study on the Interaction of C 2 H Radical with Silver Clusters Ag n 0/- (n =1–4) | OMICS International
ISSN: 2157-7048
Journal of Chemical Engineering & Process Technology

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A Density-Functional Study on the Interaction of C 2 H Radical with Silver Clusters Ag n 0/- (n =1–4)

Xu Qiu-Hong*, Li Da-Zhi, Song Ming-Zhi and Zhang Shi-Guo
Binzhou Key Laboratory of Material Chemistry, Department of Chemistry and Chemical Engineering, Binzhou University, Binzhou 256603, P.R. China
Corresponding Author : Xu Qiu-Hong
Binzhou Key Laboratory of Material Chemistry
Department of Chemistry and Chemical Engineering
Binzhou University, Binzhou 256603, P.R. China
E-mail: [email protected]
Received July 11, 2013; Accepted September 20, 2013; Published September 23, 2013
Citation: Qiu-Hong Xu, Da-Zhi Li, Ming-Zhi S, Shi-Guo Z (2013) A Density- Functional Study on the Interaction of C2H Radical with Silver Clusters Agn0/-(n =1–4). J Chem Eng Process Technol 4:173. doi:10.4172/2157-7048.1000173
Copyright: © 2013 Qiu-Hong Xu, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Abstract

The interaction between C2H radical and silver clusters Agn0/-(n=1-4) has been studied based on a systematic density functional theory (DFT) investigation. The DFT calculated results show that C2H radical inclines to interact with silver clusters Agn0/-(n =1-4) as an integrity in the most stable structures of C2HAgn0/-(n=1-4) rather than being divided by Agn clusters. The Agn0/-(n =1-4) clusters remain their structural integrities as units in the ground states of C2HAgn0/-(n=1-4). Detailed natural resonance theory (NRT) and natural bond order analyses show that the interaction between the C2H radical and the Agn0/-(n =1–4) in the most stable structures of C2HAgn 0/-(n = 1–4) are mainly ionic. Compared with that of C2H radical and the Aun0/-(n = 1–4), the electronic component in C2HAgn0/-(n = 1-4) increased due to the strong relativistic effects of Au. The stretching vibrational frequencies of C≡C and C–H in the ground states of C2HAgn0/-(n =1–4) occur red shifts compared with those of the C2H radical due to the interaction between the Agn0/- clusters and C2H radical. The photoelectron spectra (PES) of the most stable structures of C2HAgn0/-(n=1–4) have been simulated to facilitate their future experimental characterizations.

Keywords
DFT; C2H radical; Silver clusters; Interaction
Introduction
The past few years have witnessed increasing research activities in coin metal clusters [1-6] due primarily to their important implications in electronics, telecommunications, aerospace, chemicals, and complicated physical and chemical properties. Many reactions using supported coin metals represented by Au nanoparticles as catalysts have been reported, including the combustion of hydrocarbons [7-10]. C2H radical is one of the important reactive intermediates in hydrocarbon combustion processes [11-14]. So investigating the interaction of silver clusters with C2H radical is valuable for understanding the remarkable catalytic effects discovered for silver nanoparticles [15-17].
In recent years, experimental and theoretical works have been conducted to study the interaction between metals and organic molecules because of its significant role for both heterogeneous and homogeneous catalytic systems. Mark B [18] reported the infrared molecular beam photo dissociation spectra of complexes formed from the reaction of silver clusters with methanol by pulsed laser vaporization from a target of the corresponding pure metal within a continuousflow cluster source. Interactions of small neutral coinage metal clusters Ag3 and Ag4 with chalcogen dihydrides (H2O, H2S, H2Se) have been investigated using the DFT-B3LYP method [19], while the low-energy silver-cation-water clusters Ag+(H2O)n=1-6 have been investigated using the B3LYP and MP2 levels [20]. Jiang [21] investigated the reactions of laser-ablated silver atoms with carbon monoxide molecules in solid argon and neon using matrix-isolation IR spectroscopy, while CO adsorption on small neutral, anionic, and cationic silver clusters Agn has been studied with use of the PW91PW91 density functional theory (DFT) method [22]. The reactions of Ag3 clusters with CH3Br as well as the photo dissociation of the resulting complexes at 266 nm were studied in a radio frequency ion trap under multiple collision conditions [23]. The interaction between Fe and C2H radical has been studied based on the PES analyses [24]. High-level ab initio calculations, using the CPd-G2 thaw and CP-G2 composite computational procedures (combined with spin projection techniques when appropriate), are used to explore the bonding between the metal mono cations Na+, Mg+, Al+, K+, and Ca+ and the radicals C2H [25]. Brugh [26] found that both the ground and the excited state of CrC2H had liner structure and their vibronic spectrum were complicated in the 11100-13300 cm-1 region. The first spectroscopic identification and characterization of YbC2H has been reported by combining resonance- enhanced two photon ionization, laser-induced fluorescence and photoionization efficiency spectroscopy with density functional calculations [27]. Yuan [28] investigated the adsorption of C2H radical on small cobalt clusters by mass spectrometry and PES of ConC2H- (n =1-5) cluster anions.
To the best of our knowledge, there have been no studies reported to date on the interaction between C2H radical and Agn0/- clusters. In a very recent DFT study, our group investigated the interaction of C2H radical with gold clusters Aun0/-(n = 1-4) [29]. Considering the same electron configuration between Au and Ag, which all have a closed d shell and a single s valence electron, it is natural to anticipate that silver clusters may have similar properties as gold clusters. In this work, we present an investigation on the structures and bonding characteristics of C2HAgn0/-(n = 1-4) complexes in hopes of understanding the interaction between Agn0/-(n = 1-4) clusters with C2H radical. Detailed natural resonance theory (NRT) and IR spectra analyses are also used to show the interaction between Agn0/-(n = 1-4) clusters and C2H radical.
Theoretical Method
Structural optimizations and vibrational analyses were performed employing the B3LYP method [20,30]. For comparison, calculations were also carried out by the PBE1PBE with symmetry constraints [31,32]. The Stuttgart quasi-relativistic pseudo potential and the basis set augmented with two f-type polarization functions and one g-type polarization function (Stuttgart_rsc_1997_ecp+2f1g (α(f)=0.498, α(f)=1.464, and α(g)=1.218) were employed for Ag and the 6-311+ G(d, p) basis set implemented in Gaussian 03 was used for C and H [33,34]. Both the B3LYP and PBE1PBE produced similar ground-state structures and relative energies with slightly different bond parameters. Therefore, the following discussions were mainly focused on the B3LYP results. Further, relative energies for the isomers were refined using the coupled-cluster method with triple excitations (CCSD(T)) at the B3LYP structures [35,36]. Natural resonance theory (NRT) was used to calculate the bond orders and bond polarities. Adiabatic detachment energy (ADE) values were calculated as the energy difference between the anion and its neutral molecule at their groundstate structures, while vertical electron detachment energies (VDEs) of the anions were calculated from the ΔSCF energy difference between the neutral and anion ground states and the excitation energies of the neutral (triplet excited states only) calculated by time-dependent DFT (TDDFT) [37,38]. The simulated spectra were constructed by fitting the distribution of calculated VDE values with unit-area Gaussian functions of 0.04 eV at full width. The NBO 5.0 [39,40] program was used to calculate the bond orders and atomic charges.
The low-lying isomers of C2HAgn- (n=1-4) anions and the groundstate structures of neutral C2HAgn (n=1-4) are depicted in Figure 1-5, respectively. Figure 6 shows the simulated photoelectron spectra. Natural atomic charges (q/|e|), wiberg bond indexes calculated at the B3LYP level are listed in Table 1. Calculated NRT bond orders, covalent (CNRT) and ionic (INRT) of C(c)-Ag(c) in the ground states of C2HAgn0/-(n=1-4) clusters at B3LYP level as well as ADE and VDE are tabulated in Table 2. C-C and C-H stretching vibration and C-H rocking vibration of the C2H radical and the ground states of C2HAgn- (n=1-4) clusters at B3LYP level are summarized in Table 3.
Results and Discussion
Geometries
We start from C2HAg- and C2HAg, the simplest clusters discussed in this work. As shown in Figure 1, the Ag-terminated C2HAg- (2Σg, 1A) is the ground state which lies 1.91, 2.00 and 1.98 eV lower than the Aginserted C1 C2HAg- (2A, 1B) at B3LYP, PBE1PBE and CCSD(T)//B3LYP levels, respectively. The C2HAg- (2Σg, 1A) with C∞v symmetry proves to be similar with linear CoC2H- [28] and C2HAu- [29] in geometry in which C2H as a unit combines with Co or Au by its terminal C atom. Interestingly, the ground states of neutral C2HAg (1Σg, 1N) possesses the similar linear structure which can be obtained by replacing one of the H atom in C2H2 with an Ag atom and appears to be analogous in geometry to the linear CoC2H and C2HAu at the same level [28,29]. The ground state of C2HAg- (1A) has a bond length of rC-Ag=2.13 Å, whereas the rC-Ag is 2.02 Å in C2HAg (1N). The shorter rC-Ag in C2HAg (1N) compared with that of C2HAg- (1A) indicates the extra electron of the anion is primarily localized among the C≡C-Ag conjugate system. The calculated natural atomic charges (qAg(c)= -0.23 and +0.66 |e|, and qC(c)= -0.46 and -0.55|e| in C∞v C2HAg- (1A) and C2HAg (1N), respectively) confirm the conference. The WBIC-Ag=0.40 in C2HAg- (1A) and WBIC-Ag=0.64 in C2HAg (1N) (see Table 1) can also support the prediction. The next two isomers C2HAg- (1C) and C2HAg- (1D) is 6.40 and 8.54 eV higher than the ground state of C2HAg- (1A) at CCSD (T)//B3LYP levels, respectively.
When C2H radical interacts with Ag2- cluster, the C∞v (1Σg, 2A) (depicted in Figure 2) is obtained which lies 2.21, 2.24 and 1.81 eV lower than C2v (1A1, 2B) at B3LYP, PBE1PBE and CCSD (T)//B3LYP levels, respectively. The perfectly linear structure contains a C≡C triple bond with rC≡C = 1.22 Å, C-Ag σ-bond with rC-Ag = 2.09 Å, and an Ag-Ag bond with rAg-Ag = 2.67 Å. Our calculated Ag-Ag bond lengths in 2A and 2N are longer than the experimental value 2.53 Å and 2.61 Å in single Ag2 cluster [41,42] due to the interaction between C2H radical with Ag20/- cluster. The Ag atoms connected to C of C2H radical in 2A and 2N carry the net atomic charges of +0.22 and +0.38 |e|, respectively. It is interesting to notice that our calculation produces nearly the same second low-lying isomers with Y-shaped structure for Ag2C2H- 2B, Co2C2H- and Au2C2H- [28,29].
For C2HAg3- clusters, as shown in Figure 3, C2HAg3- anion possesses a ground state of C2v (2B2, 3A), which lies 0.28, 2.51 and 5.97 eV lower than C∞v (2Σg, 3B), Cs (2A”, 3C) and Cs (2A”, 3D) at CCSD(T)//B3LYP level, respectively. It is interesting that the ground state of C2v C2HAg3-(2B2, 3A), as well as C2v C2HAg3 (1A”, 3N) possesses a perfect triangular Ag3 unit similar to that of the ground state Ag3 clusters [43]. Distinctive similarities were also observed in the ground states of C2v C2HAu3 and C2v C2HAu3- [29]. The Ag-Ag bond length in C2HAg3- (1A) is 2.77 or 2.78 Å, which is shorter than that in D3h Ag32+ (2.88 Å) [44] but longer than that in C2v Ag3 (2.67 Å) and D3h Ag3- (2.69 Å) [42]. Similar to Au3 triangular [29], Ag3 triangular unit can serve as substituent to replace one H atom in C2H2 to form the head-on coordinated C2HAg3- 3A and C2HAg3 3N. Apparently, the C2H unit exists as an integrity with the C≡C triple bond length of 1.22 Å and rC-H=1.06 Å in the ground state of C2HAg3- 3A, C2HAg3 3N, and the second low-lying isomer C∞v (2Σg, 3B). The third isomer Cs (2A’, 3C) appears to be similar to acetylene in geometry, while the C-C bond length is 1.28 Å, longer than the C≡C triple bond of acetylene (1.20 Å) and shorter than the C=C bond (1.33 Å) of ethene. The Ag atoms connected to C of C2H radical in 3A and 3N carry the net atomic charges of +0.17, and +0.21 |e|, respectively.
Given the proved fact that C2H radical inclines to interact with silver clusters Agn0/-(n =1-3) as an integrity and Agn0/-(n =1-3) clusters remain their structural integrities as units in the most stable structures of C2HAgn0/-(n = 1-3), we design a series of isomers. The four low-lying isomers are listed in Figure 4. Isomer 4A with C2H attaching directly to one Ag atom of a distorted tetrahedral Ag4 cluster is the ground state, which lies 0.01 and 0.12 eV lower in energy than isomer 4B and 4C at CCSD(T)//B3LYP level, respectively. 4A and 4B can be considered to be essentially degenerate in energy and may coexist in experiments. It is interesting that the Ag2 units attaching to C atom exists in the third lowest-lying structure 4C. For C2HAg4, isomer 4N possesses a planar diamond Ag4 unit similar to Ag4 neutral cluster [43]. The C-C bond length of isomer 4N is 1.23 Å, longer than that of the C≡C bond (1.20 Å) and shorter than the C=C bond (1.33 Å) of ethene. The calculated WBIC-C=2.83 well supports the bonding pattern. Both C2H radical and Ag4 cluster still incline to maintain their integrities as structural units when C2H radical interacts with Ag4 cluster.
Clearly, combined with the reported results on Agn interacting with CO [21,22], CD3OH [18], OH2, SH2, SeH2 [19], Agn clusters incline to maintain as integrities in the ground states. Simultaneously, C2H radical prefer to maintain its integrity when C2H radical interacts with Fe, Cr, Yb, Co and Au [25-29].
Bonding characteristics
Based upon analyses above, we can easily conclude that C2H radical inclines to interact with silver clusters Agn0/-(n =1-4) as integrities in the most stable structures of C2HAgn0/-(n=1-4). The calculated wiberg,s bond index, WBIC-C larger than 2.83 (listed in Table 1) can also support the bonding pattern. Further, NRT analyses give some deep understanding of the bonding characteristics in these systems. As shown in Table 2, The C(c)-Ag(c) (Ag(c) and C(c) represents atoms connected with C or Ag) bonds in the ground states of C2HAgn0/- (n=1-4) have the covalent component under 45 % (0.33 and 0.36 in C2HAg- anion (1A) and C2HAg (1N) , 0.30 and 0.44 in C2HAg2- 2A and C2HAg2 2N, 0.32 and 0.38 in C2HAg3- 3A and C2HAg3 3N, 0.30 and 0.13 in C2HAg4- 4A and C2HAg4 4N). So the interactions between Agn 0/-(n=1-4) clusters and C2HA radical in the ground states isomers are mainly ionic. Compared with that of C2HAun0/-(n=1-4) [29], the electronic component in C2HAgn,0/-(n=1-4) increased due to the strong relativistic effects of Au.
The interaction of Agn0/-(n=1-4) clusters with C2H radial brings about IR spectral changes of the C2H moiety. Table 3 lists C-H rocking vibration, C-C and C-H stretching vibration of the ground states of C2HAgn- (n=1-4) clusters at B3LYP level. Distinctly, both C≡C and C-H bond stretching vibrations of C2HAgn-(n=1-4) occur red shifts compared to that of C2H radical, similar to the situation in C2HAun-(n=1-4) series. Furthermore, as a result of the increasing d→π* interaction of the occupied Ag 4d orbital to the empty antibonding π* of C2H radical, smaller red shift in the C≡C and C-H bond stretching bands of C2HAgn- (n=1-4) occur with an increase in n. However, with the increasing in the number of Ag atoms, the blueshifts of C-H rocking vibration become increasingly obvious due to the increasing interaction between the Agn- clusters and C2H radical. These calculated values may help identify C2HAgn-(n=1-4) systems in future IR measurements.
Electron detachment energies
As listed in Table 2, for C2HAgn-(n=1-4), the calculated ADEs and VDEs at B3LYP level are between 1.21 and 3.28 eV. The electronic binding energies of C2HAgn- (n=1-4) well fall within the energy range of conventional excitation lasers used in PES measurements (266 nm, 4.661 eV) [45-48]. We notice that the difference between ADE and VDE of C2HAg4- is the largest (0.73 eV). This reflects the different covalent components between anionic and neutral species. It can be confirmed from the calculated NRT results that the covalent percentage of C2HAg4- 4N and C2HAg4 4A is 0.13, and 0.30, respectively. To facilitate future experiments, the PES spectra of the ground structure of C2HAgn-(n=1-4) were simulated based on the time-dependent density functional theory (TDDFT) calculations. Several important features are observed from the comparison of the simulated PES of the C2HAgn- (n=1-4) ground states demonstrated in Figure 6. Among these anionic clusters, C∞v C2HAg2- and Cs C2HAg4- have the highest first VDE values of 3.17 and 3.28 eV, with small X-A energy gaps of 0.46 and 0.69 eV, respectively. However, C∞v C2HAg- and C2v C2HAg3- possess an exceptionally wide X-A energy gaps which are 2.46 and 1.91 eV, respectively. These calculated values may help identify C2HAgn- (n=1-4) systems in future PES measurements.
Conclusion
The interaction of C2H radical with small Agn-/0(n=1-4) clusters using DFT calculations was investigated based on the study of the geometrical and electronic properties of C2HAgn-/0(n=1-4) isomers. The C2H radical prefer to interact with the Agn-/0(n=1-4) clusters through its terminal C atom. The terminal C atom inclines to connect with one or two silver atoms of the silver clusters. The NBO and NRT analyses indicate that the interaction between C2H radical and Agn-/0(n=1-4) clusters is mainly ionic. Just the interaction, the C≡C and C-H stretching bands of the global minima C2HAgn-(n=1-4) occur smaller red shifts compared to C2H radical. To facilitate the future experiment, the PES of stable structures were simulated. The current study provides further insight into the interaction between C2H radicals and silver clusters, which may lead to understanding the remarkable catalytic effects discovered for silver nanoparticles.
Acknowledgements
This research was supported by the Research Fund of Binzhou University, China. (2012Y02)
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