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J. Phys. Chem. A 2010, 114, 3551–3555

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TDDFT Study of the Optical Properties of Cy5 and Its Derivatives Anu Bamgbelu, Jing Wang, and Jerzy Leszczynski* Interdisciplinary Nanotoxicity Center, Department of Chemistry, Jackson State UniVersity, Jackson, Mississippi 39217 ReceiVed: September 2, 2009; ReVised Manuscript ReceiVed: January 21, 2010

Cyanine dyes which show a very strong π-π* type electronic absorption have attracted attention as active components for optoelectronic applications. In the present study, the optical properties of Cy5 and Cy5-X (X ) F, Cl, Br, and I) have been theoretically explored by applying the time-dependent density functional theory (TDDFT) method. The geometries of the ground state of the studied models have been fully optimized at the B3LYP/LanL2DZ and B3LYP/SDD levels of theory. Based upon the optimized structures of the ground state, the TDDFT calculations were performed to compute the vertical singlet excitation energies and the absorption spectra of the studied species. Compared with the optical properties of Cy5, halogen ions lower the vertical excitation energies and cause the maximum absorption wavelength to be red-shifted for the Cy5-X derivatives. By employing the polarizable continuum model (PCM) approach for all the gas-phase-optimized structures, the influence of solvent effects on the spectra of Cy5 and Cy5-X was considered in three solvents (ethanol, DMSO, and water). The obtained results suggest that the solvents may lower the excitation energy of Cy5. However, for the Cy5-X species the excitation energies are higher in solvents than in the gas phase. 1. Introduction Cyanine dyes represent chemical compounds utilized by industry for over 200 years. They have attracted considerable interest since 1856 due to their applications in the field of photography, in addition to other dye applications.1 It has been established that cyanine dyes have notable fluorescent properties, and they have been applied in various devices, such as photographic sensitizers, lasers, nonlinear optical materials, and fluorescent probes.2 Cyanine dyes are characterized by a conjugated (polymethine) chain terminated at each end with an aromatic or heterocyclic moiety. They are classified as symmetrical and nonsymmetrical. Symmetrical cyanine dyes draw more attention than nonsymmetrical species. The well-known examples of symmetrical cyanine dyes are represented by Cy3 and Cy5, which are distinguished by a five-carbon conjugated chain separating the heterocyclic moiety in the latter and a threecarbon conjugated chain separating the heterocyclic moiety in the former (Figure 1). Cy3 and Cy5 are reactive water-soluble fluorescent dyes that have been applied in biochemistry to label nucleic acids and proteins.3 The lifetimes of their excited state are shorter (∼picoseconds) than those of ordinary dyes. Cyanine dyes absorb and emit light mostly in the visible region of the optical spectrum, with a wavelength being a function of the chain length and the terminating moieties. In order to be able to apply cyanine dyes in photonics, it is essential to understand their quantum efficiencies. Increasing the polymethine chain in cyanine dyes can yield higher quantum efficiency. Cyanine dyes were shown to be utilized in dyesensitized solar cells (DSSCs) as first reported by O’Regan et al.4 They are especially handy when they replace some metal complexes, such as rubidium bipyridyl complexes, used in solar cells. Their advantages over rubidium bipyridyl complexes include superior molar extinction coefficients, low cost, and a diversity of molecular structures. It was revealed that their impressive photovoltaic performance in solar cells has been * Corresponding author. E-mail: [email protected].

Figure 1. Backbone schemes of Cy3 and Cy5.

obtained when cyanine dyes are utilized with efficiencies in the range of 5-9%.5 For bulk heterojunction photovoltaic devices, cyanine dyes can play the role of electron acceptors while the combined MEH-PPV (methoxy-substituted polyphenylene vinylene)6 acts as the electron donor. Cyanine dyes have bright colors, which makes them one of the most interesting classes of dyes.7 Recently, a new group of cyanine dyes known as the bichromophoric cyanine dyes has been shown to have important applications in medicine and in industry.8 These species have molar absorption coefficients ε(λ) close to 105 M-1 cm-1 in the 500-700 nm spectral range and yield relatively high quantum yields of the triplet state. Cyanine dyes have been applied in cellular fluorophores and imaging “Forster resonance energy transfer” (FRET) studies.9 Due to their specific electronic properties, Cy3 and Cy5 were used as donor-acceptor pair of fluorophores for FRET measurements.10 The molecular structures of cyanine dyes in their ground and excited states largely influence their roles in all their applications. Their properties in electronically excited states depend on the structures of the molecules and their surroundings. To shed light on how cyanine dyes yield colors by absorbing in the visible range and exhibit certain optical properties, the vital features of these dyes are of importance to be investigated. According to simple resonance theory, color arises from an electron delocalization over a conjugated system. In the case of the investigated species, the two identical nitrogens on the cyanine fragment are separated by a conjugated bridge. The wavelength of absorption has a direct relation to the length of this conjugated bridge. A longer conjugated bridge between the two nitrogens facilitates a greater electron delocalization and thus a longer absorption wavelength.11

10.1021/jp908485z  2010 American Chemical Society Published on Web 02/15/2010

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Figure 2. Scheme of Cy5 and Cy5-X with atoms labeled (X ) F, Cl, Br, and I).

The optical properties of cyanine dyes vary for different species. For further understanding of the role cyanine dyes play in various optoelectronic applications, we performed theoretical investigations of the optical properties of cyanine dye Cy5 and anionic Cy5 derivatives (with halogen group substituent) (Figure 2) through applying the time-dependent density functional theory (TDDFT) approaches. The influences of three solvents (water, dimethyl sulfoxide (DMSO), and ethanol) on the spectra of Cy5 and anionic Cy5 were investigated by performing calculations using the polarizable continuum model (PCM) self-consistent reaction field approach of Tomasi and co-workers.12 2. Methods The density functional theory (DFT) with Becke’s three parameter (B3)13 exchange functional along with the Lee-YangParr (LYP)14,15 nonlocal correlation functional (B3LYP) and two effective core potential (ECP) basis setssLos Alamos ECP plus double zeta (LanL2DZ)16,17 and Stuttgart-Dresden ECP plus DZ (SDD)shave been employed in the present study.18 The ground state geometries of Cy5 and Cy5-X (X ) F, Cl, Br, and I) have been fully optimized using the above-mentioned theoretical level without any symmetry constraints. Time-dependent density functional theory has been developed for theoretical studies of excitation energies, absorption wavelengths, and oscillator strengths.19 The advantages of density functional theory and time-dependent formalism are combined in TDDFT to make it well-suited for efficient and reasonably accurate determination of excited state properties. TDDFT is

Bamgbelu et al. hence widely applied in both large and medium size molecule optical investigations. The TDDFT approach has been employed herein to calculate the vertical excitation energies based upon the optimized geometry of the studied models in the ground state. The PCM model20 was applied for all gas-phase-optimized structures to evaluate the solvation effects on the absorption spectra of the studied species with three different solvents. All calculations were carried out by the Gaussian 03 package of programs.21 3. Results and Discussion 3.1. Ground State Geometry Structure of Cy5 and Cy5-X (X ) F, Cl, Br, and I). The Cy5 model in this study contains a five-carbon polymethine chain (Figure 2) with two heterocyclic rings of indoles which are attached by a sulfonate group (SO3-), respectively. The two nitrogens on the indole ring are each linked with a methyl group. Four Cy5-X models represent quaternary ammonium halides in which the halogen ions (F-, Cl-, Br-, and I-) bind to the quaternary ammonium from the indole ring of the Cy5 model. All five models were fully optimized at the B3LYP/LanL2DZ and B3LYP/SDD levels in their ground states. The optimized geometries are local minimum energy structures which were ascertained by the fact that all the harmonic frequencies are real. The geometric parameters obtained at the B3LYP/LanL2DZ level are shown for Cy5 in Figure 3 and Cy5-X in Figure 4. The molecular structure of Cy5 possesses a C1 symmetry geometry. The molecule adopts mostly a planar structure, with DC15C1C2C3 and DC6C5C4C3 dihedral angles predicted as -179.6° and -178.7°, respectively. The π-electrons are evenly delocalized on the polymethine bridge separating the two indole rings. The fluorine ion (F-) is located in the middle of the two nitrogens of the indole rings of Cy5-F. Both atomic distances, from N19 to F and from N10 to F, amount to 4.04 Å at the B3LYP/LanL2DZ level. Cy5-F also possesses the C1 symmetry structure. The dihedral angles DC15C1C2C3 and DC6C5C4C3 (-179.0° and -176.4°) establish an almost planar molecular structure. Similar characteristics are predicted for Cy5-Cl, Cy5-Br, and

Figure 3. Optimized structures of Cy5 obtained at B3LYP/LanL2DZ (normal font) and B3LYP/SDD (underline) levels (bond lengths in Å).

Figure 4. Optimized structures of Cy5-X obtained at B3LYP/LanL2DZ level (bond lengths in Å; bold for Cy5-F, italic for Cy5-Cl, normal font for Cy5-I, and underline for Cy5-Br).

Optical Properties of Cy5 and Its Derivatives

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TABLE 1: NPA Results for Cy5 and Cy5-X (X ) F, Cl, Br, and I) Obtained at the B3LYP/LanL2DZ Level gas phase

water

ethanol

DMSO

N19 N10 SO3SO3X N19 N10 SO3SO3X N19 N10 SO3SO3X N19 N10 SO3SO3X

Cy5

Cy5-F

Cy5-Cl

Cy5-Br

Cy5-I

-0.4 -0.4 -0.74 -0.74

-0.42 -0.42 -0.79 -0.79 -0.84 -0.41 -0.41 -0.91 -0.91 -0.86 -0.41 -0.41 -0.9 -0.9 -0.86 -0.41 -0.41 -0.9 -0.9 -0.86

-0.41 -0.41 -0.79 -0.79 -0.88 -0.41 -0.41 -0.9 -0.9 -0.91 -0.41 -0.41 -0.9 -0.9 -0.91 -0.41 -0.41 -0.9 -0.9 -0.91

-0.41 -0.41 -0.79 -0.79 -0.91 -0.41 -0.41 -0.9 -0.9 -0.94 -0.41 -0.41 -0.9 -0.9 -0.94 -0.41 -0.41 -0.9 -0.9 -0.94

-0.41 -0.41 -0.78 -0.78 -0.93 -0.41 -0.4 -0.9 -0.9 -0.96 -0.4 -0.41 -0.89 -0.89 -0.96 -0.4 -0.4 -0.9 -0.9 -0.96

-0.4 -0.4 -0.92 -0.92 -0.4 -0.4 -0.89 -0.89 -0.4 -0.4 -0.89 -0.89

Cy5-I. The halogen ions are positioned in the middles of models with planar geometries. However, the atomic distance between the N19 and the halogen ions varies for the different considered ions. These distances amount to 4.65 Å for Cy5-Cl, 4.88 Å for Cy5-Br, and 5.15 Å for Cy5-I, respectively. The Cy5 fragments of Cy5-X structures reveals insignificant differences compared to that of the Cy5 model (the relative bond length differences are less than 0.004 Å). As a comparison, the structures of Cy5 and Cy5-X were also fully optimized at the B3LYP/SDD level (shown in Figure SI1

in the Supporting Information). One can see that the geometry characteristics of the studied models are consistent with those obtained at the B3LYP/LanL2DZ level. The shortest atomic distance between the N19 and X ion is 4.04 Å for Cy5-F, while the longest one is revealed in Cy5-I (5.15 Å). The natural population atomic (NPA) charges were determined at the same theoretical levels for all studied species. Table 1 lists the NPA charges of Cy5 and Cy5-X obtained at the B3LYP/LanL2DZ level (NPA results obtained at B3LYP/SDD level are depicted by Table SI in the Supporting Information as a comparison). The NPA charges resulting from the two basis sets are similar. For Cy5-F, the NPA data for the gas phase show that the negative charge distributed around the fluorine ion amounts to -0.84 while each of the two nitrogens carries the charge of -0.42. For the Cy5-Cl model the NPA charge on the chlorine ion is -0.88 and the nitrogens have the NPA charge of -0.41. The NPA charges for Br and I ions in the Cy5-Br and Cy5-I models are predicted to be -0.91 and -0.93, respectively. 3.2. Excitation Spectra of Cy5 and Cy5-X (X ) F, Cl, Br, and I) in the Gas Phase. The optimized structures of the studied models were used to calculate the electronic vertical singlet excitation energies at the TDDFT level applying the B3LYP functional with LanL2DZ and SDD basis sets. At the TD-B3LYP/LanL2DZ level, the dominant absorption band of Cy5 is found to be associated with the 13th excited state (with oscillator strength of 1.71). The electronic excitation to the lowest singlet ππ* excited state is dominated by the transition from molecular orbital (MO) MO121 to the lowest unoccupied molecular orbital LUMO (MO134), together with MO117 f LUMO transition. As shown in Figure 5, MO121 and MO117 illustrate an evenly delocalization of electrons around the Cy5.

Figure 5. Molecular orbitals related to the lowest ππ* excitation of all the systems in the gas phase obtained from the TDDFT calculations employing the LanL2DZ basis set.

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TABLE 2: Optical Properties for the Lowest ππ* Excitation of Cy5 and Cy5-X (X ) F, Cl, Br, and I) at TDDFT/B3LYP Level with LanL2DZ and SDD Basis Sets, Respectively (with the Related Maximum Absorption Wavelength λ, Excitation Energies E, and Oscillator Strengths f) gas phase (ε ) 1.0) molecule Cy5 Cy5-F Cy5-Cl Cy5-Br Cy5-I

λ (nm) E (eV) f λ (nm) E (eV) f λ (nm) E (eV) f λ (nm) E (eV) f λ (nm) E (eV) f

ethanol (ε ) 24.55)

DMSO (ε ) 46.7)

water (ε ) 78.39)

LanL2DZ

SDD

LanL2DZ

SDD

LanL2DZ

SDD

LanL2DZ

SDD

515 2.4 1.71 583 2.12 0.35 562 2.2 0.93 580 2.13 0.82 601 2.05 0.71

515 2.4 1.71 582 2.13 0.36 562 2.2 0.94 593 2.08 0.75 613 2.02 0.67

530 2.3 2.4 522 2.37 1.87 523 2.36 2.23 532 2.32 0.86 562 2.2 0.26

531 2.33 2.38 522 2.37 1.87 523 2.36 2.23 529 2.34 2.32 534 2.32 1.84

537 2.3 2.4 527 2.34 1.92 529 2.34 2.29 535 2.31 2.35 541 2.29 2.13

537 2.3 2.4 527 2.34 1.92 529 2.34 2.29 537 2.3 1.55 556 2.22 0.6

527 2.34 2.39 520 2.38 1.93 521 2.37 2.29 526 2.35 2.33 532 2.32 2.18

527 2.35 2.39 520 2.38 1.93 521 2.37 2.29 528 2.34 2.14 541 2.29 0.99

The LUMO shows a more localized electron distribution than the highest occupied molecular orbital (HOMO), in which the polymethine chain plays a role as the electron acceptor. This transition is characterized by the excitation energy of 2.40 eV and the absorption wavelength of 515 nm (Table 2). The second excited state was assigned to the lowest singlet ππ* excitation of Cy5-F (with oscillator strength of 0.35). The excitation transition is mainly caused by the MO136 f LUMO transition, combined with four other transitions (MO129 f LUMO, MO131 f LUMO, MO132 f LUMO, and MO133 f LUMO). Similar to that of Cy5, the LUMO of Cy5-F shows that the polymethine bridge also acts as the electron acceptor during the excitation process. The maximum absorption wavelength of Cy5-F is calculated to be 583 nm (with an excitation energy of 2.1 eV), which is red-shifted by 68 nm compared with that of Cy5. This suggests an easier excitation for Cy5-F than that of Cy5. For the Cy5-Cl model the sixth excitation state contributes to the lowest singlet ππ* excitation with an oscillator strength of 0.93. This excitation includes the main contributions of the MO130 f LUMO and MO132 f LUMO transitions. The vertical excitation energy for Cy5-Cl amounts to 2.2 eV (562 nm), which is higher than that of Cy5-F and lower than that of Cy5. Cy5-Br has a maximum absorption wavelength of 580 nm (the vertical singlet excitation energy is 2.13 eV). The transition that caused this ππ* lowest singlet excitation is associated with the sixth excitation state, which is due to the contributions of the MO132 f LUMO and MO130 f LUMO transitions. The absorption wavelength of Cy5-Br is by about 65 nm red-shifted compared to that of the Cy5 model. The lowest singlet ππ* excitation of Cy5-I near 601 nm occurs at the sixth excited state. This excitation is dominated by the transitions of MO132 f LUMO, MO130 f LUMO, and MO128 f LUMO (MO138). The orbitals with lower energy are evenly delocalized around the Cy5-I model system, and the LUMO demonstrates delocalization around the conjugated polymethine. The lowest vertical singlet ππ* excitation is predicted to be 2.05 eV (oscillator strength of 0.71). Compared to the Cy5, Cy5-I also reveals a red-shifted absorption spectrum. The above data suggest that the polymethine chain acts as the electron acceptor during the electronic excitation. With the halogen ions, Cy5-X facilitates red-shifted absorption spectra and lowers the singlet excitation energies compared to that of Cy5. It is noted that the lowest singlet ππ* excitation differs

significantly for Cy5 and Cy5-X. The smallest excitation energy required was predicted for the Cy5-I model, and the related maximum absorption wavelength amounts to 601 nm. Meanwhile, Cy5 has the shortest absorption wavelength (515 nm). Compared with the experimental spectrum of Cy5 (the maximum absorption wavelength is around 649 nm), our computational results reveal a shorter absorption wavelength. However, from our results one can deduce that the surroundings play an important role in the optical properties of Cy5. This finding may be useful for designing different Cy5 derivatives in order to obtain the desired absorption spectral features. Table 2 also lists the electronic vertical singlet excitation energies for all the models obtained at the B3LYP/SDD level. The assignment of the lowest singlet ππ* excitation appears to be the same as those obtained at the B3LYP/LanL2DZ level. The lowest singlet ππ* excitation energies for Cy5, Cy5-F, Cy5Cl, Cy5-Br, and Cy5-I amount to 2.40 eV (515 nm), 2.13 eV (582 nm), 2.20 eV (562 nm), 2.08 eV (593 nm), and 2.02 eV (613 nm), respectively. 3.3. Solvent Effects on the Optical Properties of Cy5 and Cy5-X (X ) F, Cl, Br, and I). To evaluate the influences of solvents on the optical properties of the studied species, the PCM models were applied for all the gas-phase optimized structures at both B3LYP/LanL2DZ and B3LYP/SDD levels of theory. Three solvents characterized by different dielectric constants were considered for the evaluation: ethanol (ε ) 24.55), DMSO (ε ) 46.7), and water (ε ) 78.39). The results are also listed in Table 2. At the B3LYP/LanL2DZ level, the vertical excitation energy for the lowest singlet ππ* excitation of Cy5 is calculated to be 2.33 eV (531 nm) in ethanol, 2.30 eV (537 nm) in DMSO, and 2.34 eV (527 nm) in water, respectively. Compared with the gas-phase excitation (2.40 eV), it is expected that Cy5 is easier to be excited with the lower excitation energy in solvents. The lowest singlet ππ* excitation energy for Cy5-F at the B3LYP/LanL2DZ level appears to be higher than that in the gas phase (2.37 eV in ethanol, 2.34 eV in DMSO, and 2.38 eV in water, respectively). Similar variations are observed for Cy5Cl, Cy5-Br, and Cy5-I, whose solvent involved lowest singlet ππ* excitation energies are larger than those in the gas phase. For Cy5-Cl, the lowest singlet ππ* excitation energy is 2.36 eV in ethanol, 2.34 eV in DMSO, and 2.37 eV in water. Cy5Br has lowest singlet ππ* excitation energies of 2.32 eV (532 nm) in ethanol, 2.30 eV (537 nm) in DMSO, and 2.34 eV

Optical Properties of Cy5 and Its Derivatives (528 nm) in water, while those of Cy5-I amount to 2.20 eV (562 nm) in ethanol, 2.22 eV (556 nm) in DMSO, and 2.29 eV (541 nm) in water, respectively. The above results suggest that the considered solvents may raise the related excitation energies for Cy5-X. However, the solvents lower the excitation energy for Cy5 to make it easier to be excited in solvents. The solvent involved optical properties of Cy5 and Cy5-X obtained at the B3LYP/SDD level are consistent with those predicted at the B3LYP/LanL2DZ level (as shown in Table 2). NPA analysis was also performed for all the studied models in three considered solvents using the two applied basis sets (Tables 1 and 2). The obtained results illustrate that the influences of the investigated solvents on the NPA charge are identical. The two nitrogens on the indole rings of the Cy5 species carry a negative charge of -0.41. For Cy5-F, at the LanL2DZ level calculations, the negative charge distributed around the fluorine ion amounts to -0.86 for all three solvents. The two nitrogens are characterized by an NPA charge of -0.41 each. For Cy5-Cl, the chlorine ion has a negative charge of -0.91, while the two nitrogens on the indole rings have a charge of -0.41. The bromine ion on Cy5-Br carries a negative charge of -0.94, and the two nitrogens on the indole rings possess a charge of -0.41. For Cy5-I, the iodine ion has a charge of -0.40. From the above analysis, it was concluded that Cy5-I possesses the least negative charge on the halogen ion (I-), and Cy5-F carries the highest charge on the halogen ion (F-). 4. Conclusions The present study suggests that in the gas phase Cy5-X (X ) F, Cl, Br, and I) yields red-shifted absorption spectra and a lower singlet excitation energy compared to that of Cy5. This indicates that the excitation of the Cy5-X species is easier than in the case of the parent Cy5 compound. The polymethine chain acts as the electron acceptor during the excitation process. The considered chemical modifications influence vertical excitation energies and the current data provide guides for knowledgebased design of cyanine dyes with desired optical properties. Solvent effects on the absorption spectra of the studied models were also investigated by performing PCM model calculations. The obtained results reveal that Cy5 has a lower vertical excitation energy in solvents than in the gas phase. Meanwhile, the studied solvents have an opposite effect on Cy5-X, which suggest that a higher energy is needed to excite Cy5-X in solvents than in the gas phase. Therefore, it can be concluded that desirable optical properties of the investigated types of Cy5 dye molecules can be obtained by modifying their structures and selecting an appropriate solvent environment. Acknowledgment. This work was financially supported by the NSF-PREM program (Grant 0611539). We would like to

J. Phys. Chem. A, Vol. 114, No. 10, 2010 3555 thank the Mississippi Center for Supercomputing Research for a generous allotment of computer time. Supporting Information Available: Optimized structures of Cy5-X, the molecular orbitals related to the lowest ππ* excitations of the models in the gas phase at B3LYP/SDD level, and the NPA results for Cy5 and Cy5-X (X ) F, Cl, Br, and I) obtained at the B3LYP/SDD level. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Mishra, A.; Behera, R.; Behera, P.; Mishra, B.; Behera, G. Chem. ReV. 2000, 100, 1973–2011. (2) Casalboni, M.; Matteis, F.; Prosposito, P.; Quatela, A.; Sarcinelli, F. Chem. Phys. Lett. 2003, 373, 372–378. (3) Fegan, A.; Shirude, P. S.; Balasubramanian, S. Chem. Commun. 2008, 2004–2006. (4) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737–739. (5) Ma, X.; Wu, W.; Zhang, Q.; Guo, F.; Meng, F.; Hua, J. Dyes Pigm. 2009, 82, 353–359. (6) Castro, F.; Faes, A.; Geiger, T.; Graeff, C. F. O.; Nagel, M.; Nuesch, F.; Hany, R. Synth. Met. 2006, 156, 973–978. (7) Jedrzejewska, B.; Rudnicki, A. Dyes Pigm. 2009, 80, 297–306. (8) Schaberle, F.; Sergio, E.; Borissevitch, I. E. Spectrochim. Acta, Part A: Mol. Biomol. Spectrosc. 2009, 72, 863–867. (9) Li, E.; Hristova, K. Langmuir 2004, 20, 9053–9057. (10) Fu, Y.; Lakowicz, J. R. Anal. Chem. 2006, 78, 6238–6245. (11) Levine, I. N. Quantum Principles, 5th ed.; Prentice Hall: Upper Saddle River, NJ, 2000; pp 627-629. (12) Caricato, M.; Menucci, B.; Tomasi, J. J. Chem. Phys. 2006, 124, 124520(1-3). (13) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (14) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789. (15) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200–206. (16) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (17) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (18) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. (19) Scalmani, G.; Frisch, M. J.; Mennucci, B.; Tomasi, J.; Cammi, R.; Barone, V. J. Chem. Phys. 2006, 109 (7), 2798–2807. (20) Cossi, M.; Barone, V.; Cammi, R.; Tomasi, J. Chem. Phys. Lett. 1996, 255, 327–335. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.

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