TDDFT Studies on the Electronic Structures and Chiroptical Properties

Article pubs.acs.org/JPCA

TDDFT Studies on the Electronic Structures and Chiroptical Properties of Mono-Tin-Substituted Wells−Dawson Polyoxotungstates Yuan-Mei Sang, Li-Kai Yan,* Jian-Ping Wang, and Zhong-Min Su* Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun, 130024, P. R. China S Supporting Information *

ABSTRACT: The UV/CD spectra of tin-bearing acetonyl-substituted Wells−Dawson polyoxotungstates α1- and α2-[P2W17O61{SnCH2CH2C(O)}]6− were systematically investigated using the time-dependent density functional theory (TDDFT) method. The electronic circular dichroism (ECD) spectra were produced over the range of 3.3−5.8 eV. The calculated ECD spectra of the α1-R isomer were generally in agreement with the experimental spectra. The CAM-B3LYP hybrid functional was found to predict the excitation energies of tin-containing polyoxotungstates well. The fact that the UV/ECD spectra of α1-isomers are different from those of α2-isomers demonstrates the effect of the tin substitution site on the chiroptical properties of the studied isomers. The origins of the ECD bands are mainly ascribed to charge-transfer (CT) transitions from oxygen atoms to W atoms, from organic fragments to W atoms, or from the combination of two CT transitions. The results suggest that the organic fragment and polyoxometalate (POM) cage are chiroptical chromophores.

1. INTRODUCTION Polyoxometalates (POMs) constitute an immense class of inorganic oxo-metal clusters with defined structures based on octahedral early-transition-metal oxides. POMs exhibit remarkable chemical and physical properties and have been applied in a variety of fields, including catalysis, medicine, biology, analytical chemistry, and materials science.1,2 In recent years, chirality has become a crucial issue in many fields ranging from chemistry to materials science and biology. Chiral POMs, or chiral structures including POMs, have captured the attention of chemists, as chiral POMs could provide ultimate control of the synthesis of nanosized objects. The chirality of POMs can derive from (1) stereogenic arrangements in the solid state or at the supramolecular level, (2) chiral metal frameworks, or (3) functionalization of the ligands or the introduction of stereogenic side chains in hybrid structures.3 In several research groups,4 different strategies have been developed to obtain chiral POM architectures, as summarized in a recent review.3 To obtain a chiral POM skeleton, the following methods are usually used: changing the bond length, distorting the structure, forming a vacancy, substituting other metals, and eliminating the molecular symmetry center or plane by organic decoration.5−7 Many chiral POM derivatives have been synthesized and characterized by ECD spectra.8,9 Although experimental works on chiral POMs have been continuously developed, theoretical studies are few.5 Chiral molecules can be detected through a variety of optical and spectroscopic effects, such as optical rotation (OR), optical rotatory dispersion (ORD), and circular dichroism (CD), which © 2012 American Chemical Society

disclose the relationship between chirality and electronic structure. Electronic circular dichroism (ECD) measures the difference between the absorptions of left and right circularly polarized light and provides information such as the origin of optical activity and the electronic and geometric structures of chiral molecules. To explain the CD spectra of chiral isomers, theoretical approaches are very helpful.10,11 Many different theoretical methods, such as Hartree−Fock,12 coupled cluster theory,13 configuration interaction methods,14 and time-dependent density functional theory (TDDFT)15 have been applied to calculate the CD spectra of chiral isomers.16,17 Compared with ab initio methods, TDDFT methods greatly increase the accuracy and reduce the computational cost, in particular, when only lowlying excitations are of concern.18 Our group has focused on understanding the electronic, redox, bonding, and optical properties of POMs using quantum chemistry calculations.5,19 Recently, Lacôte and co-workers synthesized tin-substituted Dawson polyoxotungstates α1-and α2-[P2W17O61{SnCH2CH2COOH}]7− using a new strategy.20 Later, molecular recognition on the chiral metal oxide surface of a tin-substituted Dawson polyoxotungstate was carried out, which enabled its kinetic resolution.21 This type of chiral hybrid has potential applications in catalysis (chiral anions), chemistry, and materials science. The following questions motivated us to investigate the chiroptical properties of tin-bearing Received: November 22, 2011 Revised: March 22, 2012 Published: March 23, 2012 4152

dx.doi.org/10.1021/jp211262b | J. Phys. Chem. A 2012, 116, 4152−4158

The Journal of Physical Chemistry A

Article

acetonyl-substituted Dawson polyoxotungstates α1- and α2[P2W17O61{SnCH2CH2C(O)}]6−. (1) What are the origins of the ECD spectra for these polyanions? (2) Does the organic fragment or POM cage contribute to the ECD spectra? (3) Does the spatial arrangement of the Sn−O−W unit affect the chiroptical properties of studied polyanions? In this work, the electronic properties of these polyanions were analyzed, then the ECD spectra were calculated, and the origins of the ECD spectra of six tin-substituted POMs were assigned. This article is organized as follows: The computational details are outlined in section 2. Section 3 presents the computational results and discussion, and section 4 presents some concluding remarks.

Figure 1. Calculation models for the six studied isomers.

in α2-[P2W17O61{SnCH2CH2C(O)}]6−, the tin atom substitutes the W atom in the polar position, so two isomers (α2-R and α2-U) are formed. The frontier molecular orbital (FMO) energy diagrams of the six isomers are shown in Figure 2. The energies of the

2. COMPUTATIONAL DETAILS The studied isomers20 were optimized using the ADF2010.01 program.22 The local density approximation (LDA) characterized by the Vosko−Willk−Nusair (VWN) parametrization23 for correlation was used. The generalized-gradient approximation (GGA) was employed using the Becke24 and Perdew25 exchange correlation (XC) functional. The zeroth-order regular approximation (ZORA)26 was adopted in all calculations to account for scalar relativistic effects. The basis functions were Slater-type sets. Triple-ζ plus polarization (TZP) basis sets were used to describe the valence electrons of all atoms, whereas for transition-metal W and Sn atoms, a frozen core composed of 1s−4spd shells was described by means of single Slater functions. Moreover, the value of the numerical integration parameter used to determine the precision of numerical integrals was 6.0. All of the structures were optimized in the presence of a model solvent, the conductor-like screening model (COSMO)27 implemented as part of the ADF code. To define the cavity surrounding the molecules, we used the solvent-excluding-surface method and a fine mesh. The van der Waals radii of the POM atoms, which actually define the size of the solvent cavity where the target molecule remains, were chosen to be 2.10 Å for W; 2.33 for Sn atom; and 1.40, 1.49, 1.92, and 1.08 Å for O, C, P, and H, respectively. The dielectric constant (ε) utilized in the computations was set to 37.5 for modeling the solvent of acetonitrile. TDDFT is one of the most popular methods for studying excitation properties in quantum chemistry because of its efficiency and accuracy. The Coulomb-attenuated CAM-B3LYP hybrid functional is effective for describing charge-transfer excitations and able to give a uniform description of excited states bearing different characters. The ECD spectra of the six isomers were calculated using the CAM-B3LYP functional combined with the effective core potential (ECP) basis set LANL2DZ28 as implemented in the Gaussian 09W program package.29 The ECD spectrum of each isomer was simulated by calculating 200 excited states. In comparisons of the calculated CD spectra with the experimental values, Gaussian bandshapes with a bandwidth of 0.11 eV were used to simulate the UV/CD spectra.

Figure 2. Molecular orbital energy diagram for the six studied isomers.

highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) and the energy gaps between the HOMOs and LUMOs for α1- and α2[P2W17O61{SnCH2CH2C(O)}]6− are listed in Table 1. The Table 1. Energies of HOMOs and LUMOs and Energy Gaps (HOMO − LUMO) of α1- and α2-[P2W17O61{SnCH2CH2C(O)}]6− α1-R α1-U α1-T α1-L α2-R α2-U

HOMO (eV)

LUMO (eV)

energy gap (eV)

−6.388 −6.127 −6.275 −6.238 −6.342 −6.248

−4.075 −4.098 −4.120 −4.101 −4.121 −4.153

2.313 2.029 2.155 2.137 2.221 2.095

HOMO energies of α1-R, α1-U, α1-T, and α1-L are −6.388, −6.127, −6.275, and −6.238 eV, respectively, and the LUMO energies are −4.075, −4.098, −4.120, and −4.101 eV, respectively. Compared with that of α1-R, the HOMO energies of the other three α1-isomers increase, whereas the LUMO energies decrease. This demonstrates that the Sn−O−W units bearing different shared oxygen atoms influence the FMO energies, as well as the HOMO−LUMO energy gaps. The HOMO energies of α2-R and α2-U are −6.342 and −6.248 eV, respectively, and the LUMO energies are −4.121 and −4.153 eV, respectively. This supposes that the different shared oxygen atoms between the Sn and W atoms influence the HOMO and LUMO energies

3. RESULTS AND DISCUSSION 3.1. Geometrical and Electronic Structure. The geometrical structures for Sn-substituted Wells−Dawson-type polyoxotungstates α 1 - and α 2 -[P 2 W 17 O 61 {SnCH 2 CH 2 C(O)}]6− are shown in Figure 1. There are four and two isomers for α1- and α2-[P2W17O61{SnCH2CH2C(O)}]6−, respectively.20 In α1-[P2W17O61{SnCH2CH2C(O)}]6−, the tin atom substitutes the W atom in the belt, and four isomers (α1-R, α1U, α1-T, and α1-L) are generated, in which each isomer bears a Sn−O−W unit with a different shared oxygen atom. Similarly, 4153



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Figure 3. Calculated UV−vis spectra of the six isomers. The band half-width is σ = 0.11 eV.

(ΔE ≈ 4.24 eV). For isomer α1-R, the UV−vis band at λ ≈ 292 nm is mainly from excited state 21; the band at λ ≈ 274 nm is mainly from excited states 38, 46, and 74; and the high-lying band at λ ≈ 232 nm is from state 200. Each of these states is composed of several electronic transitions; thus, analysis of the individual transition molecular orbitals is complicated. To clarify the origins of the UV−vis spectral peaks, electron density difference maps (EDDMs) of these states were calculated using the Gauss-Sum2.2.3 software package.31 An EDDM is a representation of the changes in electron density that occur for a given electronic transition. The calculated EDDMs together with their corresponding transition symmetries, molecular orbitals, and coefficients are listed in Tables S1−S6 (see the Supporting Information). For isomer α1-R, the transitions predicted at 274 and 292 nm involve CT not only from oxygen atoms to W atoms in the polyanion but also from π-conjugated organic fragments to W atoms. The transition nature of the band at 232 nm is CT from oxygen atoms to W atoms in the polyanion. The other three α1-isomers (α1-U, α1-T, and α1-L) show absorption bands similar to those of α1-R. The corresponding transition natures for α1-U, α1-T, and α1-L are assigned to CT from oxygen atoms to W atoms or from π-conjugated organic fragments to W atoms. For α2-isomers, the shapes and intensities of absorption bands are different from those of α1-isomers. The α2-U isomer

of α1- and α2-[P2W17O61{SnCH2CH2C(O)}]6−. The HOMOs of the studied isomers formally delocalize over the organic group, whereas the LUMOs are contributed by the d orbitals of the W atoms. 3.2. UV−Vis and ECD Spectra of Six Isomers. POMbased chiral isomers are large in size and usually low in symmetry; therefore, quantum chemical calculations of their excitation energies and optical rotatory strengths are limited. The TDDFT/CAM-B3LYP method has been demonstrated to be a reliable tool to describe the ECD spectra of POM derivatives.30 Herein, we present the ECD calculations of six chiral tinsubstituted polyanions. The experimental ECD spectra of isomer α1-R were reproduced by CAM-B3LYP calculations. To investigate the factors that potentially affect the chiroptical properties of the studied isomers, the ECD spectra of the five other isomers were further predicted. To obtain reliable results, the lowest 200 excited states were calculated to simulate the ECD spectra of each isomer. The UV−vis spectra of studied conformers were first examined. The calculated excitation energies, oscillator strengths, and simulated UV−vis spectra for six isomers are shown in Figure 3. The α1-R isomer exhibits a shoulder at λ ≈ 232 nm (ΔE ≈ 5.34 eV), an intense absorption band at λ ≈ 274 nm (ΔE ≈ 4.53 eV), and a weak absorption band at λ ≈ 292 nm 4154



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Figure 4. ECD spectra of the six isomers. The calculated ECD spectra are uniformly shifted by +0.1 eV. The band half-width is σ = 0.11 eV.

experimental data, the simulated ECD spectra of isomer α1-R are blue-shifted by 0.1 eV. For α1-R, the calculated rotatory strengths are in agreement with the experimental results.21 Because of a lack of experimental data for the other five isomers, we compare the simulated ECD spectra only with the experimental results of α1-R. This comparison confirms that the TDDFT/CAM-B3LYP method is sufficient to describe the excitation energies of tin-substituted chiral POM derivatives. Compared with the experimental data, the simulated ECD spectrum of isomer α1-R shows overall bathochromically shifted bands: a small broad negative ECD band at ΔE ≈ 3.82 eV (band A); a weak positive ECD band at ΔE ≈ 4.10 eV (band B); a small negative ECD band at ΔE ≈ 4.24 eV (band C); a ternary-Cotton-effect band composed of a broad positive Cotton effect at ΔE ≈ 4.50 eV, a small negative Cotton effect at ΔE ≈ 4.70 eV, and a strong sharp positive Cotton effect at ΔE ≈ 4.95 eV,

exhibits an extra band at λ ≈ 271 nm (ΔE ≈ 4.58 eV) comparing with α2-R. Therefore, the electronic transition properties of studied polyanions are sensitive to the position of SnCH2CH2C(O) fragment. The main UV−vis bands of α2-U are at around 286 nm (ΔE ≈ 4.33 eV), 271 nm (ΔE ≈ 4.58 eV), and 238 nm (ΔE ≈ 5.21 eV). The transitions at 271 and 286 nm are ascribed to CT from oxygen atoms to W atoms, whereas the transition at 238 nm (ΔE ≈ 5.21 eV) is a combination from oxygen atoms to W atoms and organic fragments to W atoms. The calculated excitation energies and corresponding optical rotatory strengths, the simulated ECD spectra for six isomers, and the experimental spectrum of α1-R are shown in Figure 4. The ECD bands were simulated by using a Gaussian band shape with a bandwidth of σ = 0.11 eV and are indicated by capital letters. To make a convenient comparison with the 4155



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Table 2. Excitation Energies (E, eV), Transition States, Rotatory Strengths, and Charge Transfers for the ECD Bands of the Studied Compounds E (eV)

transition state(s)

A

3.82

3

−13.63

B C

4.10 4.24

15 21

87.62 −217.14

D

4.50 4.70

38, 46 54, 59

E A B

4.95 5.22 3.76 4.16

91 182 3 14, 19

84.23, 61.91 −63.18, −26.04 14.80 −27.32 −6.08 71.56, 85.30

C

4.40

35, 37

D

4.56

49

E

4.76

62, 76

F G H

5.01 5.19 5.41

99 137 185

A

3.86 4.03 4.09 4.28

8 14 21 32, 36

B C

4.55 4.72

37, 51 58

−14.164 −48.54 −131.91 −85.09, −76.06 72.25, 74.33 −60.75

D

4.89; 5.11 5.35

110 151 186

26.27 28.63 −11.23

compound band α1-R

α1-U

α1-T

E

rotatory strength(s)

−69.28, −68.67 85.42 −76.90, −40.35 16.64 −15.71 18.63

compound band

charge transfer(s)

α1-L

O to W and organic fragments to W O to W O to W and organic fragments to W O to W and organic fragments to W

O to W O to W O to W and organic fragments to W O to W and organic fragments to W O to W and organic fragments to W O to W and organic fragments to W O to W O to W O to W and organic fragments to W O to W and organic fragments to W

α2-R

α2-U

O to W O to W and organic fragments to W O to W and organic fragments to W O to W and organic fragments to W

which is marked as band D; and a weak negative ECD band at ΔE ≈ 5.22 eV (band E). The first three small Cotton effects in the experimental ECD spectrum of α1-R correspond to simulated bands A−C and are well reproduced by our calculations. The ternary-Cotton-effect band D is assigned to the large sharp experimental ECD band. This is reasonable because of the potentially low resolution in the experiments, as the three Cotton effects overlapped in the experimental ECD spectra. Band E is assigned to the experimental ECD band at ΔE ≈ 5.30 eV. It confirms that the TDDFT/CAM-B3LYP method reproduces the crucial experimental spectrometry features of studied isomers. ECD bands A and B correspond to the rotatory absorptions of the excitations at ΔE ≈ 4.24 eV in the UV−vis spectra. Two pairs of Cotton effects around ΔE = 4.24−5.10 eV originate from the rotatory absorptions of the excitation at ΔE ≈ 4.53 eV in the UV−vis spectra. The calculated excitation energies, transition states, rotatory strengths, and charge-transfer characters for the ECD bands of the studied isomers are listed in Table 2. ECD bands A−C of the α1-R isomer are mainly simulated from excited states 3, 15, and 21, respectively, whereas band D of α1-R is simulated from excited states 38, 46, 54, 59, and 91, and band E is mainly from the contributions of excited state 182. According to the EDDMs of these states, the origins of ECD bands B and E are assigned to CT from oxygen atoms to W atoms. Bands A, C, and D are

E transition (eV) state(s)

rotatory strength(s)

A B

3.77 4.09

3 12

−7.18 18.60

C

4.27

23, 28

D

4.53

47, 52

−122.68, −53.83 45.43, 59.78

E

4.72

58, 68

F A B

4.98 5.20 4.06 4.24

86 153 15 22, 31

C D

4.49 4.79

E F G

5.00 5.15 5.31

A B

3.77 4.17

C

4.39

D E

4.56 4.76

F

5.01

G

5.23

H

5.43

−40.04, −30.50 32.01 40.70 86.95 −23.89, −82.39 65.50, 67.65 −62.74

charge transfer(s) O to W O to W and organic fragments to W O to W O to W, O and organic fragment to W O to W, O and organic fragment to W O to W, O and organic fragment to W

O to W O to W and organic fragments to W 44, 46 O to W 69 O to W and organic fragments to W 92 16.55 O to W 140 −17.04 O to W 168 13.72 organic fragments to W 3 −6.71 O to W 17, 20 32.21, 142.17 O to W, O and organic fragment to W 31, 43 −169.19, O to W −45.72 48, 52 43.22, 53.98 O to W 61, 71 −30.28, O to W −28.68 93 16.97 organic fragments to W 136, 175 −21.99, O to W and organic −11.95 fragments to W 198 13.88 O and organic fragment to W

identified as transitions from oxygen atoms to W atoms in the POM, as well as organic fragments to W atoms. The α1-U isomer presents eight strengthened ECD bands: a weak negative ECD band at ΔE ≈ 3.76 eV (band A), a broad positive ECD band at ΔE ≈ 4.16 eV (band B), a negative ECD band at ΔE ≈ 4.40 eV (band C), a small positive band at ΔE ≈ 4.56 eV (band D), a broad negative ECD band at ΔE ≈ 4.76 eV (band E), a positive ECD band at ΔE ≈ 5.01 eV (band F), a small negative ECD band at ΔE ≈ 5.19 eV (band G), and a small positive ECD band at ΔE ≈ 5.41 eV (band H). Band A is simulated from excited state 3, band B from excited states 14 and 19, and band C from excited states 35 and 37. Band D is simulated from excited state 49, band E from states 62 and 76, band F from state 99, band G from state 137, and band H from state 185. According to the analysis of EDDMs, the origins of bands A, F, and G are mainly assigned as CT transitions from oxygen atoms to W atoms, and those of bands B−E and H are from the combination of oxygen atoms to W atoms and organic fragments to W atoms. The ECD spectrum of α1-L echoes that of α1-U both in shape and magnitudes: a weak negative ECD band at ΔE ≈ 3.77 eV (band A), a positive ECD band at ΔE ≈ 4.09 eV (band B), a negative ECD band at ΔE ≈ 4.27 eV (band C), a small positive band at ΔE ≈ 4.53 eV (band D), a large sharp negative ECD band at ΔE ≈ 4.72 eV (band E), a large broad positive ECD band at ΔE ≈ 4.98 eV, and a sharp positive ECD band at 4156



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ΔE ≈ 5.20 eV (band F). The detailed EDDMs of the main excited states that contribute to the CT transition for α1-L are shown in Table S3 (Supporting Information). The origins of bands A and C are mainly ascribed to CT transitions from oxygen atoms to W atoms, band B is the combination of CT from oxygen atoms to W atoms and from organic fragments to W atoms. For bands D−F, there is an extra CT transition from oxygen atoms and organic fragments to W atoms. This illustrates that the organic fragment plays a role as an optically active chromophore and contributor to the absorptions of ECD spectra. In comparison with the above three α1-isomers, the α1-T isomer shows unique chiroptical properties. The ECD spectrum of the α1-T isomer features the following bands: a broad ECD band ranging from ΔE ≈ 3.54 eV to ΔE ≈ 4.42 eV (band A), which is considered as four overlapped small peaks. There are a steep band at ΔE ≈ 3.86 eV, a small negative ECD band at ΔE ≈ 4.03 eV, a small upward ECD band at ΔE ≈ 4.09 eV, and a broad negative ECD band at ΔE ≈ 4.28 eV, which are contributed by the excited states 8, 14, 21, 32, and 36; a large positive ECD band at ΔE ≈ 4.55 eV (band B) originating from excited states 37, 43, and 51; a small negative ECD band at ΔE ≈ 4.72 eV (band C) from the contribution of excited state 58; a steep positive ECD band at ΔE ≈ 4.89 eV; a large positive ECD band at ΔE ≈ 5.11 eV (band D); and a negative ECD band at ΔE ≈ 5.35 eV (band E). From Table S4 (Supporting Information), the analysis of EDDMs shows that the origins of band B are mainly ascribed to CT transitions from oxygen atoms to W atoms, bands C−E are from the combination of CT from oxygen atoms to W atoms and from organic fragments to W atoms. For the α1-T isomer, the oxygen atom in the Sn−O−W unit is in the pole. However, the oxygen atoms in the Sn−O−W unit in the α1-R, α1-L, and α1-U isomers are in the belt. Therefore, the site of the shared oxygen atom in the Sn−O−W unit is an important factor that affects the chiroptical properties of the studied isomers. The deviations of the ECD spectra between α1-R and α2-R confirm that the chiroptical properties of the studied isomers are potentially switched by the substitution site of the Sn atom on the polyanions. The ECD spectra characters of α2-R are as follows: a broad positive ECD band at ΔE ≈ 4.06 eV (band A), a small negative ECD band at ΔE ≈ 4.24 eV (band B), a positive ECD band at ΔE ≈ 4.49 eV (band C), a large broad negative band at ΔE ≈ 4.79 eV (band D), a positive ECD band at ΔE ≈ 5.00 eV (band E), a small negative ECD band at ΔE ≈ 5.15 eV (band F), and a large positive ECD band at ΔE ≈ 5.31 eV (band G). According to the EDDMs in Table S5 (Supporting Information), the origins of ECD bands A, C, E, and F are mainly ascribed to CT transitions from oxygen atoms to W atoms; the CT transition of band G comes from organic fragments to W atoms; and bands B and D are mainly ascribed to the combination of oxygen atoms to W atoms and organic fragments to W atoms. The ECD spectra of isomers α1-U and α2-U show similar shapes and absorption band sites, but different rotatory strengths. For isomers α2-R and α2-U, the two isomers are different from each other in shape, rotatory strengths, and absorption sites. This demonstrates that the site of the shared oxygen atom in the Sn− O−W unit is an important factor affecting the chiroptical properties of the studied isomers.

The TDDFT method was used to calculate the ECD spectra of these chiral POM derivatives. The CAM-B3LYP functional was found to be applicable to predictions of the excitation energies of tin-containing POMs. The simulated UV−vis spectra of the α1-isomers have observable differences compared with those of the α2-isomers. The calculated ECD spectra are generally in agreement with the experimental spectra. Among α1-isomers, the ECD spectra of α1-R, α1-U, and α1-L are similar in shapes and magnitudes, but vary slightly in absorption sites. The α1-T isomer exhibits a deviated ECD spectral shape, absorption sites, and magnitudes of rotatory strengths in comparison with the other three α1-isomers. Isomers α2-R and α2-U show different shapes, rotatory strengths, and absorption sites in their ECD spectra. This suggests that both the shared oxygen atom in the Sn−O−W unit and the substitution site of the Sn atom affect the ECD spectra of the studied isomers. The ECD spectral deviations between α1-R and α2-R confirm that the chiroptical properties of the studied isomers are potentially switched by changing the substitution site of the Sn atoms in the polyanions. It is worth noting that the ECD spectra of isomers α1-U and α2-U show similar shapes, rotation strengths, and absorption sites. The origins of the ECD bands are mainly ascribed to CT transitions from oxygen atoms to W atoms and organic fragments to W atoms or the combination of two CT transitions. The organic fragment plays a role as an optically active chromophore and contributor to the absorptions of ECD spectra.

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ASSOCIATED CONTENT

S Supporting Information *

Calculated EDDMs together with their corresponding transition symmetries, molecular orbitals, and coefficients for α1-R, α1-U, α1-L, α1-T, α2-R, and α2-U isomers (Tables S1−S6) and optimized Cartesian coordinates of systems a1-R. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-431-85684009. Fax: +86-431-85684009. E-mail: zmsu@ nenu.edu.cn (Z.-M.S.), [email protected] (L.-K.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by NSFC (20971020, 21073030, and 21131001), Program for New Century Excellent Talents in University (NCET-10-318), Doctoral Fund of Ministry of Education of China (20100043120007), and Science and Technology Development Planning of Jilin Province (20100104 and 20100320).

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REFERENCES

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4. CONCLUSIONS In this work, we investigated the structures, electronic properties, and ECD spectra of chiral Sn-substituted Wells−Dawson-type polyoxotungstates α1- and α2-[P2W17O61{SnCH2CH2C(O)}]6−. 4157



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