A theoretical and experimental study of Sb4O6: vibrational analysis, infrared, and Raman spectra

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Abstract

The first ab initio theoretical study of tetraantimony hexoxide (Sb4O6) is reported. The normal mode frequencies, intensities, and the corresponding vibrational assignments of Sb4O6 in Td symmetry were calculated using the gaussian 98 set of quantum chemistry codes at the Hartree-Fock (HF)/CEP-121G, Møller-Plesset (MP2)/CEP-121G, and density functional theory (DFT)/B3LYP/CEP-121G levels of theory. By comparison to experimental data deduced by our laboratory and others, correction factors for the calculated vibrational frequencies were determined and compared. Normal modes were decomposed into three non-redundant motions (SbOSb stretch, SbOSb bend, and SbOSb wag). Percent relative errors found for the HF, DFT, and MP2 corrected frequencies when compared to experiment are 5.8, 6.1, and 5.7 cm−1, respectively. Electron distributions for selected molecular orbitals are also considered.

Introduction

The group V oxides comprise a varied and diverse set of inorganic chemistry. Of particular note are those compounds that form adamantanoid cages (X4Y6–10, X=Sb, P, As; Y=O, S) [1], [2], [3]. Interest in these species has been significant due, in part, to their suitability as general models for solid-state systems [4], [5], and for a wide variety of industrial applications. For instance, P4O10 is used in the development of high-energy laser glasses [6], whereas As4O6 has shown promise in treating solid tumors associated with acute promyelocytic leukemia [7].

Another group V oxide, Sb2O3, has generated considerable interest for several reasons. These include: use as a flame retardant in polymers, coatings, and textiles when added along with halogenated compounds [8]; as a catalyst in the production of polyethylene terephthalate, and polyester resins and fibers [9]; to increase stability and decrease wear of fluid lubricants [10]; and its application in the manufacture of semiconductor devices [11].

As such, the electronic structure and vibrational characteristics of Sb2O3 in both its solid and vapor forms are of interest. The solid phases of Sb2O3 include vitreous, valentinite, and senarmontite. Valentinite has an orthorhombic unit cell. It is the predominant form from 570 °C to the melting point at 656 °C [12], [13]. At temperatures below 570 °C, the most stable phase is senarmontite, a cubic “molecular” crystal. This material is composed of spherical-top Sb4O6 “dimers” that form an adamantanoid cage with Td symmetry [14], [15]. In the vapor phase, antimony trioxide has been shown by Akishin and Spiridonov [16] to also exist in this Td symmetry dimer.

Presented here is the first of our modeling and experimental studies to better understand the nature of Sb2O3 in its various phases. This first study was concerned solely with the Sb4O6 molecule, and the goal was to determine its structure, molecular orbital electron distributions, and normal vibrational modes.

Previous semiclassical calculations, using valence force constants to determine the structure and vibrational modes, have been performed by Mueller et al. [17], and Sourisseau and Mercier [18]. To the best of our knowledge, this is the first reported ab initio calculation on Sb4O6.

The calculated structure results at three levels of theory are compared with X-ray diffraction data [14], [15] and are shown to be in excellent agreement.

Lastly, a comparison of the calculated and experimentally determined vibrational frequencies from our laboratory are compared with those reported from other infrared [19], [20] and Raman [21], [22], [23] studies. After correcting with the appropriate scaling factors, the calculated results are shown to agree well with the experimental data. Similar to our earlier work on an analogous molecule, P4O6 [24], the best results were obtained when scaling factors were determined for specific groups of vibrational motions.

Section snippets

Computational details

The molecular ground state structure, orbitals, and the vibrational frequencies of Sb4O6 were determined at the Hartree-Fock (HF) [25], second-order Møller-Plesset (MP2) [26], and density functional theory (DFT) [27] using the exchange correlation function B3LYP [28], [29]. For each level of theory, the standard Stevens/Basch/Krauss effective core potential triple-split basis set, CEP-121G, was used [30], [31], [32]. These calculations were then repeated using the SDD basis set [33]. All were

Experimental method

Sb4O6 was purchased from the Aldrich Chemical Company (purity 99.997% metals basis). To determine the proportions of cubic and orthorhombic phases in this sample, an X-ray powder diffraction pattern was recorded using a Scintag (now Thermo Arl) XDS-2000 diffractometer. The details of this method have been previously reported [35], [36].

High-resolution Raman spectra were then recorded from samples sealed in glass capillaries under a dry nitrogen atmosphere. The Raman spectrometer consisted of a

Molecular structure

Sb4O6 is isostructural with As4O6 [38] and P4O6 [39] and is of Td symmetry. A molecular modeling representation of the minimized energy structure is presented in Fig. 1. The details of the geometry, for each of the three levels of theory, along with the experimental results determined from X-ray diffraction data by Svensson [14] and Wood et al. [15] are given in Table 1. The calculations using the SDD basis set gave essentially identical results.

As may be seen from the table, each of the three

Conclusion

This study has considered the prediction of the normal mode vibrational frequencies and assignments based on calculations done at the HF/CEP-121G, DFT/B3LYP/CEP-121G, and MP2/CEP-121G levels of theory for Sb4O6. Based on use of experimentally determined normal mode frequencies for this compound and others, correction constants have been developed for the predominant vibrational motion in each normal mode. For Sb4O6, it is observed that the global correction factors for the three non-redundant

Acknowledgements

The authors would like to express their gratitude for the funding provided by the US Army Edgewood Chemical and Biological Center. The authors also wish to thank the Materials Research Center at the University of Missouri at Rolla for the use of their X-ray powder diffractometer.

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