TDDFT Studies on the Determination of the Absolute Configurations

Article pubs.acs.org/JPCA

TDDFT Studies on the Determination of the Absolute Configurations and Chiroptical Properties of Strandberg-Type Polyoxometalates Yuan-Mei Sang, Li-Kai Yan,* Na-Na Ma, 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 electronic circular dichroism (ECD) and UV−visible absorption (UV−vis) spectra of Strandberg-type polyoxometalates (POMs) (R, R)[(R*PO3)2M5O15]2‑ (R* = CH3CH(NH3), (M = Mo, W)) have been explored using the time-dependent density functional theory (TDDFT) method. It demonstrates that the absolute configurations of chiral systems can be determined by chiroptical spectroscopic methods combined with DFT calculations. The calculated ECD spectra of the Strandberg-type molybdate were produced over the range of 3.3− 6.5 eV, which are generally in agreement with the experimental spectra. In addition, the ECD spectra of (R, R)-[(R*PO3)2W5O15]2‑ (R* = CH3CH(NH3)) were produced over the range of 4.5−8.5 eV. The Becke’s half-and-half hybrid exchange-correlation functional (BHandHLYP) with the HF exchange fraction to 55% hybrid functional was found to well predict the excitation energies of studied systems. The origins of the ECD bands of two systems are mainly ascribed to charge-transfer (CT) transitions from oxygen atoms to metal atoms in polyanion. The results suggest that the polyanion are chiroptical chromophores. The polyanion plays a role as an optically active chromophore and contribute to the absorptions of ECD spectra. The difference of the UV−vis/ ECD spectra between two systems shows that the transition metal atom significantly influences on the chiroptical properties of the studied Strandberg-type POMs.

1. INTRODUCTION Polyoxometalates (POMs) are a vast class of polynuclear molecular oxide anions usually formed by W, Mo, or V.1 In recent years, there has been increasing interest in the chemistry of main-group element, organic, and organically derivative POMs.2 During the past few years, chirality has become a crucial issue in chemistry, material science, and biology. Chiral POMs and chiral structures containing POM, which integrate the functionalization of chiral material with POM, have attracted considerable attention. It is not only because of their intriguing variety of architectures and topologies3 but also because of their numerous potential applications in stereoselective catalysis, chiral recognition, and medicine.4 The chirality of POM can derive from chiral arrangements of POM, chiral POM framework, and chiral organic side chain on POM. In several research groups, many successful synthetic strategic methods for chiral POMs have been continually reported, which were summarized in a recent review.3d The following strategies are commonly used: distorting the structure, forming a vacancy, replacing with other metals, and eliminating the molecular symmetry center or plane by organic decoration. 5 The Strandberg-type heteropolyanion [S2Mo5O23]4‑ is dissymmetric and hence expected to be chiral. Both enantiomers of [S2Mo5O23]4‑ were succeeded to observe through Pfeiffer effects.6 Kwak et al.7 reported the synthesis of several anions [(RP)2Mo5O21]n‑, where R = H, CH3, C2H5, C6H5, etc. Those are the first polyanions in which the organic groups are covalently bonded to a phosphorus heteroatom. © 2013 American Chemical Society

This combination with the chemical versatility of organic functional groups is expected to produce new species with interesting chemical and physical properties.8 Chiral Strandberg-type molybdates [(RPO3)2Mo5O15]2‑ (R = CH3CH(NH3)) have been reported and it sheds light on phosphonate exchange dynamics that might be exploited for recognition of biological targets in water,9 as well as for the preparation of new chiral materials via soft-chemistry routes.10 Chiral POM is often characterized by the circular dichroism (CD, the difference in absorptivity of left and right circularly polarized light) spectrum over ultraviolet−visible (UV−vis) region, which is related to the electronic excitations and named as electronic CD (ECD) spectrum. Since the discovery of circular dichroism and optical rotation in transition metal complex solutions by Cotton in the 1890s,11 chiroptical properties have become a useful tool for the characterization of these compounds. Up to now, many chiral POM derivatives have been synthesized and characterized by ECD spectra.12 In recent years, there are many chiral complex frameworks that have been extensively studied in experiments, whereas the theoretical studies on the ECD spectra of chiral POMs by quantum chemical calculations are few.5 As we know, it is a costly job to calculate the ECD spectrum of chiral POM due to its complexity in geometric and electronic structures. However, Received: January 16, 2013 Revised: February 26, 2013 Published: March 1, 2013 2492

dx.doi.org/10.1021/jp400506z | J. Phys. Chem. A 2013, 117, 2492−2498

The Journal of Physical Chemistry A

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theoretical studies on chiral POMs are necessary and invaluable to understand the electron transition origins. Theoretically reproducing and predicting the ECD spectrum are helpful for specifying the absolute molecular conformation in solution.13,14 Time-dependent density functional theory (TDDFT)15 has been used successfully to calculate ECD spectra.16 It proves that TDDFT methods could provide reliable estimates of transition energies, in particular only low-lying excitations are of concern,17 and enable the ECD calculations for large size molecules and metal complexes at reasonable computational cost. TDDFT is the only viable approach to deal with POMs due to their large sizes, many transition metal atoms involved, relativistic effects spanning all energy levels, and high negative charge, which requires including solvation in the calculation.18 In this paper, geometry structures of the Strandberg-type molybdates (R, R)-[(R*PO3)2M5O15]2‑ (R* = CH3CH(NH3), (M = Mo, W), abbreviated as (R, R)-1) and (R, R)-2)) were studied by DFT calculations. On the basis of the optimized geometries, exchange-correlation functionals and basis sets were first assessed. By the aid of ECD calculations, we specified the absolute conformations of flexible chiral POM (R, R)-1 in solution and discussed the effect of transition metal on the UV−vis and ECD spectra.

octahedra, with four edges and one corner junctions yields enantiomorphic Mo5O15 rings with formal “Δ” or “Λ” helical handedness. Each face of the MoO5 ring is capped by the tetrahedral R*PO3 (R*CH3CH(NH3)) group. Thus, there are two interchangeable conformations (R, Δ, R)-1 and (R, Λ, R)-1 for each enantiomer in solution. Structural and atomlabeling schemes for studied polyanions are presented in Figure 1. The optimized partial geometrical parameters are listed in

Figure 1. Calculation models for two conformations (R, Δ, R)-1 and (R, Λ, R)-1.

Table 1. Experimental and Theoretical Optimized Geometrical Parameters for (R, Δ, R)-1 and (R, Λ, R)-1

2. COMPUTATIONAL DETAILS All of the calculations in this work were carried out using the Gaussian 09W program package.19 The ground-state geometries of all studied compounds were optimized using the Becke20 and Perdew21 exchange correlation functional. The effective core potential (ECP) basis set LANL2DZ22 associated with the pseudopotential was used to describe the Mo and W atoms, whereas the basis sets of 6-31+G(d) was used for F, P, C, N, O, and H atoms. The solvent effects of water have been taken into account in the optimization calculations via the polarizable continuum model (PCM).23 The Becke’s half-andhalf hybrid exchange-correlation functional (BHandHLYP)24 is effective for describing charge-transfer (CT) dominating excitations and able to give a uniform description for excited state properties of large metal-containing systems.25 When the portion of Hartree−Fock exchange in the BHandHLYP functional is enhanced to 55%, the calculated spectra are in excellent agreement with the experimental spectra both in the relative ordering and magnitude of the transitions as well as the signs of the Cotton effect. Thus, to describe the absorption properties and electronic transition of POM derivatives for giving close excitation energies to the experimental data, the electronic excitation energies, oscillator strengths, and rotational strengths were calculated using BHandHLYP (55% HF exchange function) method in combination with def2-SVP26 basis set and LANL2DZ for Mo and W atoms. The solvent effects of acetonitrile for ECD calculations were considered with PCM using the integral equation formalism variant. The lowest 200 excited states were calculated to simulate the ECD spectra of studied polyanions. TDDFT calculations were performed by using Gaussian 09W. Gaussian band shape with a bandwidth of 0.25 eV was used to simulate the UV−vis/ECD spectra.

(R, Δ, R)-1 parameter dihedral angle (deg) bond length (Å)

Φ1 Φ2 N1−C1 P1−C1 P1−O1 N2−C2 P2−C2 P2−O2 Mo4−O1 Mo2−O2

(R, Λ, R)-1

exp

cal

exp

cal

−84.2 −84.4 1.517 1.856 1.521 1.517 1.856 1.521 2.330 2.329

−89.2 −88.3 1.523 1.877 1.551 1.524 1.876 1.550 2.283 2.288

92.8 92.1 1.514 1.869 1.527 1.519 1.867 1.525 2.292 2.296

91.7 91.7 1.522 1.881 1.553 1.522 1.881 1.553 2.269 2.269

Table 1, the experimental data10 were also included. The calculated dihedral angles Φ1 and Φ2 of (R, Δ, R)-1 are −89.2° and −88.3°, which are in reasonable agreement with the experimental data, −84.2° and −84.4°. Compared with experimental ones, the Mo−O bonds are slightly shortened, whereas the other calculated bonds are slightly lengthened. Clearly, the important structural parameters are wellreproduced in the theoretical calculations. It confirms that the BP86 exchange correlation functional in combination with LANL2DZ for metal atoms and 6-31+G(d) for F, P, C, N, O, and H atoms we chose was sufficient to describe the structures of studied chiral POM derivatives. 3.2. Simulated ECD Spectra and the Determination of Absolute Configurations of (R, R)-1. Enantiopure transition metal complexes have been used in a wide range of areas, including enantioselective catalysis27 and materials applications,28 for example, as materials for chiral induction.29 It is important to determine the absolute configuration of the complex if the optically active Δ and Λ isomers are resolved. While it is difficult to obtain these complexes in experiments, moreover, the conformation and configuration of these complexes are changeable in solution. The theoretical calculations can determine the absolute configuration, which would greatly assist with the structural assignments. Usually the

3. RESULTS AND DISCUSSION 3.1. Geometrical Structure. The (R, R)-1 polyanion composes of MoO6 octahedra jointed by sharing edges or corners. The nonplanar arrangement of the five MoO6 2493



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Figure 2. Experimental and calculated ECD spectra of (R, R)-1 in CH3CN CAM-B3LYP functional, LANL2DZ basis set (a) and CAM-B3LYP functional, LANL2DZ, def2-SVP basis set (b).

Figure 3. Experimental and simulated ECD spectra of (R, R)-1 in CH3CN. The simulated ECD spectra from BHandHLYP functional//LANL2DZ, def2-SVP (a) and BHandHLYP (55% HF) functional//LANL2DZ, def2-SVP (b).

combined with the effective core potential (ECP) basis set LANL2DZ as well as the normalized Bf of two conformations (R, Δ, R)-1 and (R, Λ, R)-1 (0.826 and 0.174) were used to simulate the ECD spectra of studied polyanions. The simulated Boltzmann-factor-weighted ECD spectra of two interchangeable conformations are shown in Figure 2a. For convenient comparison with the experiments, the simulated spectra are blue-shifted by 3.5 eV. Although the shape and magnitudes of spectra are well agreement with the experimental spectra except the small magnitudes at ΔE ≈ 4.5 − 5.0 eV, while the shift (3.5 eV) was larger than those reported works.32 Thus, the CAMB3LYP/LANL2DZ result is not acceptable in this work. The impact of the basis sets was considered. LANL2DZ performing on Mo atom, and def2-SVP basis set for C, H, O, N, P atoms, combined with CAM-B3LYP method were employed to calculate the ECD spectra. The simulated spectra without shifting are shown in Figure 2b. Although the spectra shape was not very well agreement with experimental data, especially in high-lying band, and the rotation strength at ΔE ≈ 5.5 eV, 6.2 eV are not matched well, while the magnitudes of bands at ΔE ≈ 4.0 and 5.0 eV are well reproduced. Comparing with the result from CAM-B3LYP/LANL2DZ, the ECD spectra obtained by CAM-B3LYP/LANL2DZ, def2-SVP are much improved. It means that the def2-SVP basis set is favorable for the ECD calculations of studied polyanions, thus the def2-SVP basis set for nonmetal atoms and LANL2DZ for Mo atoms were used in the following study. Autschbach33 indicated that the choice of functional can influence the bands in CD spectra. Recent studies presented

absolute configurations in these molecules are determined by their CD spectra. Comparing the experimental spectra and calculated ECD spectra, the structural assignments can be accurate. Conformational energies were used to create weighting factors for the interchangeable structures. It supposes that there are two likely interchangeable conformations for each studied polyanion in solution, therefore Boltzmann factor (Bf) can be expressed as eq 1 Bf (i) = 1/[1 + exp(ΔEj /(KBT ) + exp(ΔE k /KBT ))] (1)

Bf is used to weight the ECD spectra of two conformations for obtaining the final ECD spectra of studied polyanion.30 KB is a constant, and the value is 8.617343(15) eV/K. Herein, the relative total bonding energies (ΔE) of two conformations, (R, Δ, R)-1 and (R, Λ, R)-1, are 0.00 and 0.04 eV. Based on the relative total energies of two conformations of studied polyanions, the normalized Boltzmann factors at 298 K are calculated to be 0.826 and 0.174, respectively. In present work, the ECD spectra of (R, R)-1 is discussed as the enantiomers present the same physicochemical properties and ECD absorptions with opposite signs. In order to obtain the accurate ECD spectra, different XC functionals and basis sets were first assessed. Recent work confirmed that 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.31 Therefore, the TDDFT/CAM-B3LYP functional 2494



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Figure 4. Calculated UV−vis spectra of (R, R)-1 (a). ECD spectra of (R, R)-1 (b). UV−vis spectra of W atom substitutes (R, R)-2 (c). ECD spectra of W atom substitutes (R, R)-2 (d).

accuracy and computational requirements, the BHandHLYP with 55% HF exchange combined with LANL2DZ/def2-SVP basis set was selected for the following study. 3.3. Analysis of the UV−vis and ECD Spectra. The UV− vis spectra of the (R, R)-1 polyanion was simulated in CH3CN, and the calculated excitation energies and oscillator strengths of simulated UV−vis spectra for the (R, R)-1 polyanion are also presented in Figure 4a. The (R, R)-1 polyanion exhibits absorption band A at λ ≈ 245 nm (ΔE ≈ 5.05 eV), absorption band B at λ ≈ 214 nm (ΔE ≈ 5.80 eV), and band C at λ ≈ 184 nm (ΔE ≈ 6.75 eV), the oscillator strengths of the three bands are almost equal. For (R, R)-1 polyanion, the UV−vis band at λ ≈ 245 nm is mainly from excited states 37 and 42, the band B at λ ≈ 214 nm is mainly from excited states 81 and 106, and the high-lying band C at λ ≈ 184 nm is from excited states 189 and 193. Each state is composed of several electronic transitions, thus the analysis of the individual transition molecular orbitals is complicated. To clarify the origins of the absorption peaks, electron density difference maps (EDDMs) of these states were plotted using the Gauss-Sum2.2.3 software package.37 EDDM is a representation of the differences in electron density for a given electronic transition. The calculated EDDMs together with their corresponding transition symmetries, molecular orbitals, and coefficients are listed in Tables S1 (Supporting Information). According to these results, the corresponding transition natures for (R, R)-1 were assigned to charge-transfer (CT) from oxygen atoms to Mo atoms in the polyanion. The calculated excitation energies and corresponding optical rotatory strengths, the simulated ECD spectra, and the experimental spectra of (R, R)-1 are shown in Figure 4b. The ECD bands were simulated by using a Gaussian band shape with a bandwidth of σ = 0.25 eV and are indicated by capital

that DFT calculations give the reliable results for choosing the right DFT XC-functional.34 In particular, hybrid functionals including a linear combination of Hartree−Fock (HF) exchange with DFT exchange correlation are successful. In hybrid functionals, BHandHLYP was recently reported to provide the excellent performance.35 In TDDFT calculations, the functionals with low HF exchange fraction such as CAMB3LYP (20%) often favor with strong CT character. However, functionals with high HF exchange fraction such as BHandHLYP (50%) is more suitable to treat the electronic excitation.36 Thus, the BHandHLYP functional was tested for simulating the ECD spectra of studied polyanions. Figure 3a presents the ECD spectra for (R, R)-1 by BHandHLYP/ LANL2DZ, def2-SVP. After red-shifted by 0.15 eV, the spectra shape and the band positions were greatly improved, especially the positive bands at the range ΔE ≈ 5.5−6.5 eV, and the band shape at ΔE ≈ 4.7 eV, which are better than the ECD spectra by CAM-B3LYP functional (Figure 2b). It should be stressed that BHandHLYP contains 50% HF exchange, whereas CAMB3LYP is characterized by 20% amount of HF exchange. So the XC functional and the proportion of HF in the exchange functional have impact on the ECD simulation. From Figure 3a, the magnitudes in high-lying bands do not well reproduce with the experiment. In the following, the amount of HF exchange in BHandHLYP was enhanced to 55% for simulating the ECD spectra of (R, R)-1, and the results are shown in Figure 3b. It is clearly seen that the calculated results are in well agreement with the experimental spectra not only the band positions, relative ordering and magnitude of the transitions but also the signs of the Cotton effect. In summary, both XC functional and basis set have impact on the ECD spectra of studied systems. Considering the 2495



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at λ ≈ 176 nm is mainly from excited states 79 and 106, and the high-lying band C at λ ≈ 159 nm is mainly from excited state 144. The calculated EDDMs together with their corresponding transition symmetries, molecular orbitals, and coefficients are listed in Tables S2 (see the Supporting Information). According to these results, the corresponding transition nature for (R, R)-2 was mainly assigned to CT from oxygen atoms to W atoms in the polyanion. The calculated excitation energies and corresponding optical rotatory strengths and the simulated ECD spectra of (R, R)-2 are shown in Figure 4d. The simulated ECD spectrum of (R, R)-2 shows a weak positive ECD band at ΔE ≈ 5.40 eV (band A), a ternary-negtive band composed of a broad negtive ECD band at ΔE ≈ 6.10 eV (band B), a positive ECD band at ΔE ≈ 6.93 eV (band C), and a strong positive ECD band at ΔE ≈ 7.60 eV (band D). The calculated excitation energies, transition states, rotatory strengths, and charge-transfer characters for the ECD bands of the studied (R, R)-2 are listed in Table 2. For (R, R)-2 polyanion, ECD bands A is mainly from excited states 14, band B is mainly from excited states 28, 32, 37, 39, 40 and 50, band C mainly from the excited states 69 and 79, whereas band D is mainly from the contributions of excited states 106, 143, and 193. According to the EDDMs of these states, the origins of ECD bands are assigned to CT from oxygen atoms to W atoms in the polyanion. The CT character of (R, R)-2 is same with that of (R, R)-1. In order to clearly present the effect of transition metal on the optical properties, the UV−vis spectra and ECD spectra of (R, R)-1 and (R, R)-2 were illustrated together (Supporting Information, Figures S1 and S2). The absorption bands in UV− vis spectra of (R, R)-1 are similar to those of (R, R)-2, which the oscillator strengths of bands B and C are almost equal. For (R, R)-2, the sites and intensities of absorption bands are different from those of (R, R)-1. Comparing with (R, R)-1, the UV−vis spectra of (R, R)-2 are clearly hypsochromic. The oscillator strengths of bands A, B, and C significantly increase. The ECD spectra of (R, R)-2 are different from those of (R, R)-1. The rotatory strengths of (R, R)-2 obviously increase, and the whole ECD spectra are evidently hypsochromic. The difference of the UV−vis/ECD spectra between (R, R)-1 and (R, R)-2 confirm that the chiroptical properties of the studied polyanions are potentially switched by transition metal atoms.

letters. To make a convenient comparison with the experimental data, the simulated ECD spectra of (R, R)-1 was red-shifted by 0.3 eV. The calculated rotatory strengths, band shapes, and absorption positions are in excellent agreement with the experimental results except ECD band C. The calculated band C was lower than experimental one, which results from the superposition of positive and negative rotatory strengths near ΔE ≈ 4.61 eV. The simulated ECD spectrum of (R, R)-1 shows a weak positive ECD band at ΔE ≈ 4.00 eV (band A), a weak negative ECD band at ΔE ≈ 4.35 eV (band B), a small negative but upward trend ECD band at ΔE ≈ 4.61 eV (band C), a strong negative ECD band at ΔE ≈ 4.96 eV (band D), a positive ECD band at ΔE ≈ 5. 44 eV (band E), and a strong positive ECD band at ΔE ≈ 6.05 eV (band F). The calculated excitation energies, transition states, rotatory strengths, and charge-transfer characters for the ECD bands of the studied (R, R)-1 are listed in Table 2. For (R, R)-1 Table 2. Excitation Energies (ΔE, eV), Transition States, Rotatory Strengths, and Charge Transfers for the ECD Bands of the (R, R)-1 and (R, R)-2 compound

band

ΔE (eV)

transition state(s)

(R, R)-1

A B C D

4.00 4.35 4.61 4.96

E

5.44

8 22 29, 32, 33 36, 37, 43, 54 70, 72, 82

F A B

6.05 5.40 6.10

C D

6.93 7.60

(R, R)-2

115, 129 14 28, 32, 37, 39, 40, 50 69, 79 106, 143, 193

rotatory strength(s) 3.57 −10.00 −20.45, 16.81, 22.64 86.38, −137.58, −68.86, −53.74 −39.41, 73.71, −55.41 35.37, 57.11 11.10 −18.72, −28.00, 49.00, −35.10, 85.91, −38.30 82.01, 45.76 57.59, 50.61, 33.48

charge transfer O O O O

to to to to

Mo Mo Mo Mo

O to Mo O to Mo O to W O to W O to W O to W

polyanion, ECD bands A−C are mainly from excited states 8, 22, 29, 32, and 33, respectively; band D is from the opposite rotatory absorptions excited states 36 and 37 as well as excited states 43 and 54; band E is mainly from the contributions of excited state 70, 72 and 82; and band F is mainly from the contributions of excited states 115 and 129. According to the EDDMs of these states, the origins of ECD bands are assigned to CT from oxygen atoms to Mo atoms in the polyanion. The polyanion plays a role as an optically active chromophore and contributes to the absorptions of ECD spectra. The EDDMs show that the metal Mo atoms in POM have obvious contribution charge transfer. So how is the transition metal effect on ECD spectra, UV−vis spectra or charge transfer? Thus, we use W atom substitutes the Mo atom and named this system as (R, R)-2. The TDDFT calculations were performed based on the optimized geometry structure. The calculated UV−vis spectra, ECD spectra, excitation energies, and corresponding optical rotatory strengths are shown in Figure 4c,d. The UV−vis spectra of (R, R)-2 polyanion exhibits absorption band A at λ ≈ 203 nm (ΔE ≈ 6.11 eV), absorption band B at λ ≈ 176 nm (ΔE ≈ 7.10 eV), and band C at λ ≈ 159 nm (ΔE ≈ 7.80 eV), the oscillator strength of the band A is minimum, while the oscillator strengths for bands B and C are almost equal. For (R, R)-2 polyanion, the UV−vis band at λ ≈ 203 nm is mainly from excited states 31, 33 and 37, the band B

4. CONCLUSIONS In this work, we theoretically investigated the structures, chiroptical properties of chiral Strandberg-type molybdates (R, R)-[(R*PO3)2M5O15]2‑ (R* = CH3CH(NH3), (M = Mo, W)). The TDDFT method was used to calculate the ECD spectra, and the results showed that the choice of functional and basic sets is necessary for simulating the ECD spectra. It is found that the hybrid functional with suitable Hartree−Fock portion can improve the ECD calculations of the studied system. The simulated ECD spectra, by using BHandHLYP (55% HF exchange function) functional with the def2-SVP/LANL2DZ basis set, are in good agreement with the experimental spectra not only the band positions, relative ordering and magnitude of the transitions but also the signs of the Cotton effect. The absolute configurations of (R, R)-1 were determined, which are composed of 0.826 (R, Δ, R)-1 and 0.174 (R, Λ, R)-1. The origins of the ECD bands of (R, R)-1 and (R, R)-2 are mainly ascribed to CT transitions from oxygen atoms to metal atoms in the polyanion. The polyanion plays a role as an optically active chromophore and contributes to the absorptions of ECD 2496



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spectra. The difference of the UV−vis/ECD spectra between (R, R)-1 and (R, R)-2 demonstrates that the substitution metal atom affects the chiroptical properties of the studied systems.

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ASSOCIATED CONTENT

S Supporting Information *

Calculated EDDMs together with their corresponding transition symmetry, molecular orbitals, and coefficients for (R, R)1 (Table S1) and (R, R)-2 (Table S2). The UV−vis spectra and ECD spectra of (R, R)-1 (Figure S1) and (R, R)-2 (Figure S1) are illustrated. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-431-85684009. Fax: +86-431-85684009. E-mail address: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by NSFC (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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TDDFT Studies on the Determination of the Absolute Configurations

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