Probing the Electronic Structure and Property of Neutral and Charged

ARTICLE pubs.acs.org/JPCA

Probing the Electronic Structure and Property of Neutral and Charged Arsenic Clusters (Asn(+1,0,
1), n e 8) Using Gaussian-3 Theory Gang Liang,† Qiang Wu,† and Jucai Yang*,‡ †

College of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing, 10083, People's Republic of China ‡ School of Energy and Power Engineering, Inner Mongolia University of Technology, Hohhot, 010051, People's Republic of China ABSTRACT: The structures and energies of Asn (n = 2
8) neutrals, anions, and cations have been systematically investigated by means of the G3 schemes. The electron affinities, ionization potentials, binding energies, and several dissociation energies have been calculated and compared with limited experimental values. The results revealed that the potential surfaces of neutral Asn clusters are very shallow, and two types of structural patterns compete with each other for the ground-state structure of Asn with n g 6. One type is derived from the benzvalene form of As6, and another is derived from the trigonal prism of As6. The previous photoelectron spectrum (taken from J. Chem. Phys. 1998, 109, 10727) for As3 has been reassigned in light of the G3 results. The experimental electron affinities of As3 were measured to be 1.81 eV, not 1.45 eV. We inferred from the conclusion of G3 and density functional theory that the experimental electron affinities of 1.7 and 3.51 eV for As5 are unreliable. The reliable electron affinities were predicted to be 0.83 eV for As2, 1.80 eV for As3, 0.54 eV for As4, 3.01 eV for As5, 2.08 eV for As6, 2.93 eV for As7, and 2.02 eV for As8. The G3 ionization potentials were calculated to be 9.87 eV for As2, 7.33 eV for As3, 8.65 eV for As4, 6.68 eV for As5, 7.97 eV for As6, 6.58 eV for As7, and 7.65 eV for As8. The binding energies per atom were evaluated to be 1.99 eV for As2, 2.01 eV for As3, 2.61 eV for As4, 2.39 eV for As5, 2.51 eV for As6, 2.55 eV for As7, and 2.67 eV for As8. These theoretical values of As2, As3, and As4 are in excellent agreement with those of experimental results. Several dissociation energies were carried out to examine relative stabilities. This characterized the even-numbered clusters as more stable than the odd-numbered species.

’ INTRODUCTION Over the past two decades, the compounds of arsenic have been studied experimentally and theoretically because they have been used as pigments since ancient times and used as components to modify the mechanical properties of lead and copper alloys and to eliminate unwanted coloration of glasses in modern industries.1,2 The arsenic clusters have played prominent roles not only in the early development of both forensics and chemotherapy but also in many different fields of modern science, such as solid-state physics, biochemistry, physical chemistry, surface phenomena, and catalysis.1
17 The knowledge of the fundamental properties, such as the ground and low-lying electronic states of neutral and charged arsenic clusters, electron affinities (EAs), and ionization potentials (IPs), should lead to a more complete understanding of their role in many different fields. With this motivation, we have carried out a detailed study of the structures and properties of neutral and charged arsenic clusters by means of the higher level of the Gaussian-3 (G3) theory.18,19 There have been previous experimental studies on arsenic clusters. For example, Bennett et al.12 have measured the appearance potentials and ion translational energies for the negative ions As
, As2
, and As3
formed by dissociative resonance capture of As4. Lippa et al.1 have probed the EAs of Asn
(n = 1
5) using photoelectron spectra in 1998. In 2002, Zhai et al.4 r 2011 American Chemical Society

have studied the electronic structures and EAs of Pn5
(Pn = P, As, Sb, and Bi) with photoelectron spectroscopy and ab initio calculations. Recently, Walter et al.5 have measured the binding energy and fine-structure splitting of the As
using infrared photodetachment threshold spectroscopy. For IPs, Bhatia and Jones20 have measured the IPs of As by optical spectroscopy. Using gas-phase charge-transfer reactions, Zimmerman et al.3 have determined the IPs of arsenic clusters (Asn, n = 1
5). Using high-resolution photoelectron spectra and photoionization mass spectrometry, the IPs of As2 and As4 have been measured by Wang et al.21
23 and Yoo et al.,10 respectively. Brumbach and Rosenblatt24 have investigated the vibrational modes of As4 with Raman spectroscopy. There are many different methods to study the structures and properties of Asn clusters, especially for small clusters with n e 6. For instance, Scuseria25 studied the bond length, harmonic vibrational frequency, and IP for As2 with the CCSD method. Balasubramanian et al.11 investigated the electronic structure and potential energy surface of As3 and its positive ion using CASMCSCF, followed by MRCI. Meier et al.26 explored the structures, Received: April 18, 2011 Revised: May 30, 2011 Published: June 23, 2011 8302

dx.doi.org/10.1021/jp203585p | J. Phys. Chem. A 2011, 115, 8302–8309

The Journal of Physical Chemistry A

ARTICLE

Table 1. Zero-Point Vibrational Energy (ZPVE) Corrected Adiabatic Electron Affinities (EAs) and Adiabatic Ionization Potentials (IPs) for Asn (n = 2
8)a species

Figure 1. Neutral and charged As2(0,
1,+1) geometries optimized with the MP2(full) scheme and 6-31G(d) basis set. The bond lengths are in angstroms.

stabilities, and ionization potentials of As2 and As4 at the MRDCI level of theory. Warren et al.27 studied the geometry of As6 with ab initio SCF-MO calculations. BelBruno28 explored the bonding properties of Asn (n = 2
5) clusters using the B3PW91 scheme. Igel-Mann et al.29 probed the structures and vertical IPs of Asn (n = 1
6) using SCF and CI methods with energy-adjusted pseudopotentials. Zhao et al.8 investigated the structures and EAs of Asn (n = 1
5) using various density functional theories (DFTs). For medium-sized Asn clusters, Ballone et al.30 explored the structures of Asn clusters up to n = 11 using local and nonlocal density functionals. Zhao et al.7 reported the study of structural and electronic properties of Asn clusters in the size range of n e 28 using DFT with the PBE exchange-correlation functional and all-electron basis set of the double-numerical-plus-d-polarization (DND) type. Guo6 investigated the geometrical and electronic properties of neutral and charged Asn clusters with n = 2
15 using the B3LYP/6-311+G(d) method. Besides these, Schaefer and co-workers9 investigated the stability of Asn clusters with n = 2, 4, 12, and 20 using HF, MP2, CISD, and CCSD methods. Baruah et al.31 explored the geometry, polarizabilities, and infrared and Raman spectra of fullerene-like arsenic cages with n = 4, 8, 20, 28, 32, 36, and 60 by means of GGA. The objective of the present study is to apply a higher level of G3 theory to the determination of the EAs and other properties of Asn (n = 2
8) species. Of specific interest is (a) the comparison of the electron affinities with the limited available experimental results and (b) the predictions of other properties, including IPs, binding energies (BEs), and several dissociation energies (DEs). We would like to establish reliable theoretical predictions for those arsenic clusters in the absence of experimental results and, in some cases, to challenge existing experiments.

’ COMPUTATIONAL METHODS All of the calculations at the extension of G3 theory18,19 have been performed using the Gaussian 03 package.32 The combined G3 methods are ab initio calculations of molecular energies of compounds containing first-, second-, and third-row atoms. The average absolute deviation from experiment for the 423 reactions, including enthalpies of formation, ionization potential, electron affinities, and proton affinities, is 1.06 kcal/mol.19 Recently, we calculated the EAs of Sin clusters up to n = 10 using this G3 theory. The average absolute deviations from experiment are only 0.97 kcal/mol (0.042 eV).33 ’ RESULTS AND DISCUSSION As2 and Its Charged Molecules. The optimized geometries of the ground states of As2 and its charged species are given in Figure 1. The As2 in its 1∑g+ ground state has an experimental As
As bond length of 2.103 Å.34 The As
As bond length in As2 is 2.150 Å predicted by MP2(full)/6-31G(d), which overestimates the As
As bond distance by 0.047 Å. The best theoretical estimates for this bond length are CCSD results of 2.107 Å

As2

method G3 exptl.

As3

EAs

9.87, 10.18b

0.83 0.739

IPs

c

9.89 ( 0.10, 10.230d

G3

1.80

7.33

exptl.

1.81e

7.46 ( 0.10 f,