Conformational Analysis and Vibrational Circular Dichroism of Tris

Dec 11, 2009 - Department of Chemistry, University of Kashmir, Srinagar, Kashmir, India 190006, and Theoretical Sciences Unit and New Chemistry Unit, ...
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J. Phys. Chem. A 2010, 114, 87–92

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Conformational Analysis and Vibrational Circular Dichroism of Tris(ethylenediamine)ruthenium(II) Complex: A Theoretical Study Altaf Hussain Pandith† and Swapan K Pati*,‡ Department of Chemistry, UniVersity of Kashmir, Srinagar, Kashmir, India 190006, and Theoretical Sciences Unit and New Chemistry Unit, Jawaharlal Nehru Centre For AdVanced Scientific Research, Jakkur, Bangalore, India 560064 ReceiVed: May 10, 2009; ReVised Manuscript ReceiVed: NoVember 11, 2009

The conformational preferences and vibrational circular dichroism of tris(ethylenediamine)ruthenium complex in two main configurations (Λ) and (∆), have been performed using density functional theory. We find that for the free [Ru(en)3]2+ ion in the ∆-configuration, the conformational stability order is ∆(δδδ) > ∆(λδδ) > ∆(λλδ) > ∆(λλλ) and that for the Λ-configuration it is Λ(δδδ) < Λ(λδδ) < Λ(λλδ) < Λ(λλλ). The energy differences between the four conformers for both the configurations ∆ and Λ are relatively small, but the activation barriers for ring inversion from one conformation to another are significant, as compared to other such systems. We trace the origin of these results to the lower oxidation state of Ru and relatively larger Ru-N bond length. We have also studied the effect of counterions on the conformational stability for Ru(en)3Cl2. Our results indicate a reverse stability order for the associated complex, Ru(en)3 Cl2 and higher activation barriers for ring inversion as compared to the free complex ion Ru(en)32+. It is because of larger hydrogen bonding interactions between the three N-H bonds and the chloride ion in these two conformers as compared to other conformations, which is also evident from the VCD spectra of N-H stretching modes. We also investigate IR spectra for all conformations in ∆- and Λ-configurations and together with energetics and VCD spectra elucidate the spectroscopic characteristics of Ru(en)32+ complexes with and without the associated counterions. 1. Introduction Many molecules and complexes exist in more than one conformational form by virtue of rotation about one or more single bonds at ambient temperatures. The structural preference for a relatively more stable conformation over the other is determined by a variety of interactions that exist in the molecules, which include the steric class of repulsions, conjugation, and hyperconjugation among σ and π electrons.1 Recently, a renewed academic interest has been generated about the origin of the conformational preferences in different molecules. Ethane, being a textbook problem, is known to possess extra stability in the staggered form over the eclipsed form due to the repulsive exchange interactions between the electrons in the two methyl groups.2,3 However, some workers have reported that the Pauli exchange energy actually stabilizes the eclipsed conformer of ethane relative to the gauche conformer, and it is only because of the stabilization through hyperconjugative interaction that yields the staggered conformation of ethane as a preferred one.4–8 Some other works9–11 have demonstrated that the steric hindrance dominates the ethane rotation barrier, although the hyperconjugation interaction does favor the staggered conformation. This has generated a renewed interest in the conformational analysis of many other homonuclear and heteronuclear molecular systems during the past few years.12–15 Investigations on conformational preferences of chiral chelate rings in transition metal complexes have been investigated by many workers recently, as it provides an insight into the structural and functional aspects of these complexes.16,17 * Corresponding author. E-mail: [email protected]. † University of Kashmir. ‡ Jawaharlal Nehru Centre For Advanced Scientific Research.

Tris(ethylenediamine)metal complexes exist in two enantiomeric mirror image configurations, designated as ∆ and Λ, depending on the orientation of three ethylenediamine rings about a central metal ion. Each chiral ethylenediamine ligand can assume one of the two gauche conformations, giving rise to eight conformations, such as Λ(λλλ), Λ(λλδ), Λ(λδδ), Λ(δδδ), ∆(λλλ), ∆(λλδ), ∆(λδδ), and ∆(δδδ). On the basis of their analysis, Corey et al.18 predicted that the most stable conformer of a Λ-tris(ethylenediamine)metal complex would have each ethylenediamine ligand in the δ conformation, with the C-C bond axis of each ligand parallel to the C3 axis of the complex, in preference to the λ conformation in which the C-C bond forms an oblique angle with the C3 axis. These results agreed well with the observed crystal structure of Co(en)33+ in several salts19 and with that of [Ni(en)3] · (NO3)220 wherein the metal-ethylenediamine rings were found in the δδδ conformation for the Λ-configuration about the central metal ion. However, on a statistical basis,21 the intermediate ones, (λλδ) and (λδδ), are suggested to be more favored over the other two in a ratio of 3:1 by NMR studies.22 The relative ring conformer populations are also found to vary from system to system. X-ray, NMR, IR, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD) techniques have been used as probes to identify the absolute configuration, assignment of solution conformations, and the effects of temperature and counterion concentrations on the structure. In fact, the VCD technique contains unique and important information about the molecular structures and has been shown to be particularly useful for the study of the conformational characteristics of transition metal complexes,23 and in conjunction with ab initio calculations, it can determine the absolute configurations of metal complexes. Recently, many calculations have been carried out successfully

10.1021/jp904339p  2010 American Chemical Society Published on Web 12/11/2009

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on organic molecules24–28 and inorganic complexes16,17,29 at DFT level with the B3LYP or BPW91 functional and 6-31G or LanL2DZ basis set. Interestingly, the NMR spectra of diamagnetic tris(ethylenediamine) transition metal complexes show widely varying CH resonances depending on the central metal ion. The Co(III) complex exhibits a broad unresolved band, Ir(III) and Rh(III) show a narrow band with definite fine structure, and the platinum(IV) shows a very sharp peak. In contrast to this, the N-deuterated NMR spectrum of the tris(ethylenediamine)ruthenium(II) ion30 consists of a complex spectrum of about twenty lines, which is expected only if the different gauche conformations of the ethylenediamine ligand with different axial and equatorial protons are considered. The absence of such expected multiplets in the case of Co(III), Pt(IV), or Rh(III) has been attributed to rapid conformational equilibration between the λ and δ forms during which the axial forms become equatorial, and vice versa. The unusual spectral features of Ru(en)32+ were thought to be either due to higher activation energy barriers between different conformers so as to prohibit conformational interchange even at higher temperatures or due to one conformation being strongly thermodynamically favored. This line of reasoning was subsequently challenged by other workers31 who concluded that all of the M(en)3 complexes undergo rapid ring inversion at room temperature as it would need an activation energy of 12-15 kcal mol-1 to prevent rapid ring inversion on the NMR time scale as compared to an estimated maximum value of 5-7 kcal mol-1. However, there exist no extensive studies to support either point of view. We attempt here to understand the possible reason for the unusual NMR spectral features and the conformational preferences of Ru(en)32+, with and without the associated counterions. We analyze the optimized geometries, vibrational frequencies, and VCD intensities to identify the most stable conformers in different configurations from the VCD spectra, estimate the activation barrier for ring inversion from one conformation to another and assess the counterion effect on the stability of different conformers. 2. Computational Method The optimized geometries and IR and VCD spectra for the four ∆-Ru(en)32+ conformers and the four Λ-Ru(en)32+ conformers, were calculated by using the GAUSSIAN 03 set of codes,32 at the density functional theory level with exchange functional B3LYP33,34 and LANL2DZ basis set.35,36 To assess the effect of the associated counterions, the optimized geometries and IR and VCD spectra of all eight conformers of the complex [Ru(en)3]Cl2 in ∆- and Λ-configurations, with two chloride ions along the C3 axis of D3 symmetry conformers, were also calculated with the same level of theory. The VCD intensities were calculated with the magnetic field perturbation method37 incorporated into the Gaussian set of codes, using gauge invariant atomic orbitals. To obtain the real time spectral frequencies, all the calculated frequencies were uniformly scaled by 0.97 for all the conformers, both in the free state and in complex form. 3. Results and Discussion The lowest energy optimized structures among all the four conformations of ∆-Ru(en)32+ and Λ-Ru(en)32+ ion each, with and without the two chloride ions put along the C3 axis of the D3 symmetry conformers, are shown in Figure 1. The frequency analysis performed for all the optimized geometries reveal no imaginary frequencies (see Figure S1-S4 and Table S1 in the

Pandith and Pati Supporting Information). The relative energies (kcal/mol) calculated for the ∆-Ru(en)32+ are 0.00, +0.03, +0.10, and +0.20 and for the Λ-Ru(en)32+ are +0.20, +0.09, +0.04, and 0.00 for the (λλλ), (λλδ), (λδδ), and (δδδ) conformations, respectively. On the other hand, the relative energies (kcal/mol) calculated for the ∆-Ru(en)3Cl2 complex are 0.00, +3.95, +8.00, and +12.29 and for the Λ-Ru(en)3Cl2 complex are +12.14, +7.96, +4.00, and 0.00 for the (λλλ), (λλδ), (λδδ), and (δδδ) conformations, respectively. We have calculated the relative energy changes along the conformational pathway and the activation energy barrier for ring inversion from one conformation to another by varying the dihedral angle N-C-C-N (Dω), in one of the three ethylenediamine rings, in small increments. Since a rigid scan results in significant deviations of the structure from the optimized geometries, the Dω was varied step by step (38 separate runs for single point energy), and for each of the variations in dihedral angle, the rest of the bond angles in that particular en ring were restored to their initial optimized values, without disturbing the rest of the complex. The results are shown in Figure 2 (see also Table S2 and Table S3 in the Supporting Information). As can be seen in Figure 2, the activation barrier for the inversion of one ring conformation to another is on the order of 9-10 kcal/mol, which is significant, compared to other such metal complexes studied so far.23 The small relative energy difference between the four conformers for the ∆-configuration and a comparatively large activation barrier for ring inversion from one conformation to another, as compared to other such systems, is due to the larger Ru-N bond length. The conformational energy of each of the conformers arises from the individual energies of each ring (the ring strain energy) and from the interaction between three pairs of rings that are cis to each other. As the M-N bond length increases, the magnitude of interaction energy between rings diminishes, and as a result, the difference between the energy of the four conformations decreases. However, the ring strain energy of the transition state structure with Dω ) 0 markedly increases with an increase in M-N bond length as compared to the decrease in the ring interactions, giving rise to a higher energy activation barrier. With the increase in M-N bond length, the ring strain also increases slightly in the ground state structures, but the decrease in ring interaction energies is higher in magnitude than the increase in ring strain, thus decreasing the overall energy of the ground state conformers. Therefore, the activation energy for ring inversion leading to a transition from one conformation to another is higher in this complex than in many other M(en)3 systems with smaller M-N bond lengths. To assess the effect of the counterions on the stability of different conformations, we consider two chloride ions along the C3 axis of the complex ion, for all the conformations, and calculated the optimized geometries. The energy order of the four conformations in both the configurations is inverse to that of the conformations of the free metal complexes, and the energy difference between any of the two conformers is on the order of 4-12 kcal/mol. Also, in the case of the associated complex with two chloride ions, the activation barrier increases quite significantly from about 11 to 24 kcal/mol (Figure 2). The greater stability of the (λλλ) conformer in the ∆-configuration and that of (δδδ) conformer in the Λ-configuration can be attributed to the stabilizing hydrogen bonding interactions between the amine hydrogens and the chloride ions. Since, in the ∆-configuration, the (λλλ) conformer (and in the case of the Λ-configuration, the (δδδ) conformer) has more N-H bonds directed toward the two chloride ions along the C3 axis, it is

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Figure 1. Geometries for the lowest energy conformers of (a) ∆-[Ru(en)3]2+, (b) Λ-[Ru(en)3]2+, (c) ∆-[Ru(en)3]Cl2, and (d) Λ-[Ru(en)3]Cl2.

Figure 2. Relative energy change along the conformational pathway for (λλλ), (λλδ), (λδδ), and (δδδ) conformers of ∆-[Ru(en)3]2+ and ∆-[Ru(en)3]Cl2.

expected that in this configuration the hydrogen bonding stabilizes the (λλλ) conformation much more compared to the other conformations and increases the activation barrier for ring

inversion. In fact, the relatively larger ionic size of ruthenium, coupled with lower metal oxidation state, enhances the hydrogenbonding interactions between the amine hydrogens and the chloride ions, in comparison to the other systems, which is also reflected by the changes on the stretching vibrational modes of the NH bonds (discussed below). Thus, the greater stability of one particular isomer over the other and larger activation barrier for ring inversion, due to combination of hydrogen bonding interactions with lower metal oxidation state may explain the special features of the NMR spectra of Ru(en)32+ complex. We have also calculated the normal modes of vibration for both Ru(en)32+ and Ru(en)3Cl2. The high frequency normal modes that arise due to the excitation of the vibrational motion of functional groups are comparatively easy to assign, but the low frequency and skeleton modes have contributions from a large number of vibrations spread throughout the complex. The assignments of 105 (3 × 37 - 6) modes for Ru(en)32+ [and 111 modes for Ru(en)3Cl2] represent only the major contributions, particularly, in the wavenumber region 66-1200 cm-1 (see also Table S4 and Table S5 in the Supporting Information). In fact, the conformational transitions change the overall symmetry [D3 for (λλλ) and (δδδ), C2 for (λλδ) and (λδδ)] and modify the electronic structure and bonding due to structural changes in the complex. This change is better reflected by

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Figure 4. Mid-IR absorbance spectra for (a) the four conformers of ∆-[Ru(en)3]2+ and (b) the four conformers of Λ-[Ru(en)3]2+. A scale factor of 0.97 is used. Figure 3. VCD spectral intensity for CH and NH stretching modes for (a) the four conformers of ∆-[Ru(en)3]2+ and (b) the four conformers of Λ-[Ru(en)3]2+. A scale factor of 0.97 is used.

electronic CD and VCD measurements for chiral molecules such as M(en)3 complexes. In the case of the Ru(en)32+ complex ion, we find no significant changes in the IR absorptions along the conformational change, but the VCD spectra of the complex are a characteristic feature of the conformation as well as the configuration of this ion. The calculated VCD spectra for the four conformers of each configuration, ∆-Ru(en)32+ and Λ-Ru(en)32+, are presented in Figure 3 for the NH and CH stretches. As can be seen, the NH2 antisymmetric stretches at 3410 cm-1 (scaled frequency) generate a positive VCD signal for all the four conformers in the Λ-configuration, with small increments of intensity from the Λ(λλλ) to Λ(δδδ) ring conformation. However, in the case of the ∆-configuration, the NH2 antisymmetric stretches generate a net negative VCD signal for all the four conformers. Interestingly, about one-quarter of the negative polarization is lost along the conformational change from (λλλ) to (δδδ) for this configuration. The symmetric NH2 stretches at 3329 cm-1 generate a net negative VCD signal for the Λ-configuration, with intensity increasing smoothly from (λλλ) to (δδδ) ring conformations. In the case of the cobalt complex in the Λ-configuration, qualitatively similar results were found.16 For the ∆-configuration, the symmetric NH2 stretches generate a net positive VCD for all the four conformers, with an intensity order inverse to that of the Λ-configuration (see Figure 3). Thus, the VCD of the symmetric and antisymmetric NH2 stretching modes are good markers of the configuration and comparison of these with the experimental spectra can provide an identification of the absolute configuration of the complex. The CH stretching VCD spectra shows a strong sensitivity to ring conformation and is quite independent of the configuration of the metal complex, as is expected for such systems.16,23 Symmetric CH2 stretching at 2988 cm-1 for the (λλλ) conformation is left (positive) polarized where as antisymmetric CH2 stretching at 3050 cm-1 for this conformer is right (negative) polarized (Figure 3). In fact, the (λλδ) conformer loses threefourths of the intensity from that of the (λλλ) conformer at both frequencies. The VCD signal actually changes sign for both symmetric and antisymmetric stretching for the (λδδ) conforma-

Figure 5. VCD spectral intensity in the mid-IR region for (a) the four conformers of ∆-[Ru(en)3]2+ and (b) the four conformers of Λ-[Ru(en)3]2+. The scale factor is 0.97.

tion, and for the (δδδ) conformation, the VCD spectra is the perfect opposite of the (λλλ) conformation, for both the frequency regions. Thus, the VCD spectra of CH2 stretching modes are good conformation markers and can aid in identification of the dominant ring conformation in the complex. We also find that the IR absorption peaks in NH2 stretching regions (3300-3420 cm-1) and CH2 stretching regions (2986-3060 cm-1) are almost similar in the two configurations except for the intensity of absorption (see also Figure S5 in the Supporting Information). The IR and VCD spectra in the 1100-1700 cm-1 region exhibits characteristic features and provides important information regarding the conformational analysis and configuration identifications. They are shown in Figures 4 and 5, respectively. The IR absorption bands in the region 1641-1661 cm-1 (see Figure 4) are due to excitation of six normal modes (1641.5, 1651.7, 1653.5, 1654.1, 1660.7, and 1661.8 cm-1). For each conformational transition from (λλλ) to (δδδ) for ∆-configuration and (δδδ) to (λλλ) for Λ-configuration, the intensity of the higher frequency lines decreases and the lower and middle frequency lines gain intensity, so that only one strong band

Tris(ethylenediamine)ruthenium(II) Complex

Figure 6. NH/CH stretching VCD spectral intensity for (a) ∆(λλλ), ∆(δδδ) and (b) Λ(λλλ), Λ(δδδ) conformers of the complex [Ru(en)3]Cl2. The scale factor is 0.97.

appears for (δδδ) conformation in ∆-configuration and for (λλλ) conformation in Λ-configuration. The VCD spectral lines due to NH2 scissoring motion and NH2 wagging motion (see Figure 5) are good markers for configuration identification. The positive VCD band due to NH2 scissoring motion at 1641 cm-1 in the ∆-configuration shows an irregular intensity pattern along the (λλλ), (λλδ), (λδδ), and (δδδ) conformational change, with the least intensity for the latter conformation. The negative VCD band at 1661 cm-1 loses one-half of the intensity during the (λλλ) to (δδδ) conformational transition, whereas in the Λ-conformation, the NH2 scissoring modes generate a negative band at 1641 cm-1 and a positive band at 1661 cm-1 with the intensity pattern completely inverse to that of the ∆-configuration. Additionally, the NH2 wagging motions generate a positive VCD band at 1124 cm-1 (see Figure 5a), with the intensity increasing in larger increments from the (λλλ) conformer to the (δδδ) conformer. On the other hand, at 1166 cm-1, the NH2 wagging modes generate a negative band with intensity almost equal for ∆(λλλ), ∆(λλδ), and ∆(λδδ) conformers and one-third lower for the ∆(δδδ) conformation. In the case of the Λ-conformation (see Figure 5b), the wagging modes at 1124 cm-1 give a negative VCD band and at 1166 cm-1, give a positive VCD band with intensity distribution perfectly inverse to that for the ∆-configuration. Furthermore, we find that the VCD bands originating predominantly from CH2 scissoring modes at 1483 cm-1, CH2 Wa + NH2 Tw modes at 1395 cm-1, and CH2 + NH2 twisting modes at 1312 and 1284 cm-1 are ring conformation markers. The band due to CH2 + NH2 twisting modes at 1284 cm-1 for the (δδδ) conformer loses about 50% of its polarization upon transition to (λδδ) conformation and changes sign, with increasing intensity from the (δλλ) to (λλλ) conformation, in both the configurations (see Figure 5). The effect of the hydrogen bonding interaction between NH2 groups of the complex and the chloride ions is clearer in the VCD spectra of different conformations of the associated complex in NH stretching regions, which are presented in Figure 6. As can be seen, two absorption bands due to NH2 antisymmetric and symmetric stretching modes at 3410 and 3329 cm-1

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Figure 7. NH/CH stretching absorption spectra for (a) ∆(λλλ), ∆(δδδ) and (b) Λ(λλλ), Λ(δδδ) conformers of the complex [Ru(en)3]Cl2. The scale factor is 0.97.

in the free complex (see Figure 3) are shifted to lower frequencies with increased intensity in the associated complex. The NH2 symmetric stretching modes give rise to two very intense VCD peaks of opposite signs. The VCD calculations show that the symmetric stretches of the NH bonds associated with the chloride ion lying on the C3 axis generate an intense (() VCD couplet for the ∆-configuration, and an intense (-) VCD couplet for the Λ-configuration. Interestingly, the VCD couplet for the ∆(δδδ) conformer at 3158 (+) and 3165 (-) cm-1 are displaced toward lower frequency by more than 100 cm-1 to 3056 (+) and 3064 (-) cm-1 in the case of the ∆(λλλ) conformer (see Figure 6a). In fact, from the animation of the vibrational modes, we find that the N-H symmetric stretching modes are associated with the two chloride ions for the ∆(λλλ) conformer, whereas in the case of the ∆(δδδ) conformer, all the stretching modes are in a direction away from the two chloride ions. Similarly, in the Λ-configuration, the VCD couplet at 3167 (-) and 3191 (+) cm-1 for the (λλλ) conformer are displaced to 3055 (-) and 3068 (+) cm-1 in the case of the (δδδ) conformer. This observation is consistent with the earlier studies performed on similar systems with different metal ions16,17 In our case, such shifts are precisely because of the association of chloride ions with the N-H bonds in the Λ(δδδ) conformer (see Figure 6b). The intensity is, however, more for the (λλλ) conformation than for the (δδδ) conformation in both the configurations. The sign of the VCD spectra generated by C-H symmetric and antisymmetric modes remains the same as in free complex ions in both the configuration, although there are slight deviations in absorption frequencies along the conformational change. The absorption spectra due to NH/CH stretching vibrational modes of the two conformers, (λλλ) and (δδδ), of the associated complex in the ∆- and Λ-configurations are compared in Figure 7. It is clear from the figure that the frequency shift is quite prominent in IR absorption spectra along the conformational transition. The absorption intensity is higher for (λλλ) in the ∆-configuration and for (δδδ) in the Λ-configuration. The VCD spectra due to NH2 antisymmetric stretches for the different conformers in the ∆-configuration show an intense right

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polarization at 3356 cm-1 and a weak left polarization at 3368 cm-1 (see Figure S6 in the Supporting Information). The right polarization decreases rapidly for ∆(λλδ) and ∆(λδδ), and dies out for the ∆(δδδ) conformer, with gains in left polarization almost in the same proportion. The main point is that the relative stabilization of four conformers in two different configurations in free ion and associated chloride complex and the associated activation barrier heights for conformational transitions provide complex spectral features that are captured quite accurately by VCD spectra. 4. Conclusion Energy calculations indicate the (δδδ) conformation as the most stable one out of the four possible conformations for [Ru(en)3]3+ ion in the ∆-configuration, whereas in the Λ-configuration, the (λλλ) conformer is the lowest in energy. However, the relative conformational energy difference between different conformers is relatively small in this complex, as compared to other M(en)3 complexes, with a high activation barrier for the conformational transition between different conformers. The calculations for [Ru(en)3]Cl2 indicate that the (λλλ) conformer is the most stable one for the ∆-configuration and the (δδδ) conformer is the most stable one for the Λ-configuration, with an activation barrier ranging from 12 to 24 kcal/mol for the conformational transition between the different conformations. The relative stabilization of conformers by the differing level of hydrogen bonding between the N-H and chloride ions in different conformers and higher values for the activation barrier for conformational transitions provide an insight into the possible reasons for the unusual NMR spectral features of this complex. Calculations of the VCD spectra indicate that, in [Ru(en)3]2+ ion, the VCD peaks due to NH stretching modes, NH2 scissoring modes, and NH2 wagging modes are good markers for identification of the absolute configuration. The VCD spectral peaks generated by C-H stretching modes, CH2 scissoring modes, CH2 wagging modes, and CH2 + NH2 twisting modes are good markers for conformational identification. The association of chloride ions with the NH bonds through hydrogen bonding shifts the IR absorbance and VCD peaks toward lower frequencies with higher intensities. The relative shift is in agreement with the energy order for the four conformers in each configuration, which is affected by the extent of hydrogen bonding between the amine hydrogens and the chloride ions in different conformers. Acknowledgment. A.H.P. thanks JNCASR for providing him visiting scientist fellowship. S.K.P. thanks CSIR, DST, and the Government of India for research grants. Supporting Information Available: Optimized geometries for all the conformers, IR absorbance spectra for CH and NH stretching modes for the conformers of [Ru(en)3]2+ in ∆- and Λ-configurations, absorbance and VCD spectra generated by NH antisymmetric stretches for all the four conformers of the ∆-[Ru(en)3]Cl2 complex, frequency assignments of the normal vibrational modes for the conformers of [Ru(en)3]2+ and [Ru(en)3]Cl2, a few specific and important bond lengths and bond angles of optimized structures of the ∆-[Ru(en)3]Cl2 complex, and the relative conformational energy changes along the conformational pathways between different conformational isomers of ∆-[Ru(en)3]2+ and ∆-[Ru(en)3]Cl2. This material is available free of charge via the Internet at http://pubs.acs.org.

Pandith and Pati References and Notes (1) Mo, Y.; Song, L.; Wu, W.; Cao, Z.; Zhang, Q. J. Theor. Comput. Chem. 2002, 1, 137. (2) Sovers, O. J.; Kern, C. W.; Pitzer, R. M.; Karplus, M. J. Chem. Phys. 1968, 49, 2592. (3) Hoyland, J. R. J. Am. Chem. Soc. 1968, 90, 2227. (4) Porphristic, V.; Goodman, L. Nature 2001, 411, 565S. (5) Goodman, L.; Pophristic, V.; Weinhold, F. Acc. Chem. Res. 1999, 32, 983. (6) Goodman, L.; Gu, H.; Pophristic, V. J. Chem. Phys. 1999, 110, 4268. (7) Pophristic, V.; Goodman, L.; Wu, C. T. J. Phys. Chem. A 2001, 105, 7454. (8) Goodman, L.; Pophristic, V.; Wang, W. Int. J. Quantum Chem. 2002, 90, 657. (9) Bickelhaupt, F. M.; Baerends, E. J. Angew. Chem., Int. Ed. 2003, 42, 4183. (10) Mo, Y.; Wu, W.; Song, L.; Lin, M.; Zang, Q. J. Angew. Chem., Int. Ed. 2004, 43, 1986. (11) Song, L.; Lin, Y.; Wu, W.; Zang, Q. J. Phys. Chem. A 2005, 109, 2310. (12) Tanwar, A.; Pal, S. J. Phys. Chem. A 2004, 108, 11838. (13) Leoboeuf, M.; Russo, N.; Salahub, D. R.; Toscano, M. J. Chem. Phys. 1995, 103, 7408. (14) Feller, D.; Craig, N. J. Phys. Chem. A, in press. (15) Sairam, S. M.; Datta, A.; Pati, S. K. J. Phys. Chem. A. 2006, 110, 5156. (16) Freedman, T. B.; Cao, X.; Young, D. A.; Elmer, D. L.; Nafie, L. A. J. Phys. Chem. A 2002, 106, 3560. (17) Norani, N.; Rahemi, H.; Tayyari, S. F.; Riley, M. J. J. Mol. Model. 2009, 15, 25. (18) Corey, E. J.; Bailar, J. C. J. Am. Chem. Soc. 1959, 81, 2620. (19) Nakatsu, K.; Shiro, Y.; Saito, Y.; Kuroya, H. Bull. Chem. Soc. Jpn. 1957, 30, 158. (20) Nakatsu, K. Bull. Chem. Soc. Jpn. 1962, 35, 832. (21) Piper, T. S.; Karipides, A. G. J. Am. Chem. Soc. 1964, 86, 5039s. (22) Hawkins, C. J.; Palmer, J. A. Coord. Chem. ReV. 1982, 44, 1. (23) Young, D. A.; Freedman, T. B.; Elmer, D. L.; Nafie, L. A. J. Am. Chem. Soc. 1986, 108, 7255. (24) Nafie, L. A.; Freedman, T. B. In Infrared and Raman Spectroscopy of Biological Materials; Yan, B., Gremlish, H., Eds.; Marcel Dekker: New York, 2001; pp 15-54. (25) Nafie, L. A.; Freedman, T. B. In Circular Dichromism, Theory and Practice; Nakanish, K., Berova, N., Woody, W., Eds.; Wiley: New York 2000; pp 97-131. (26) Stephens, P. J.; Devlin, F. J. Chirality 2000, 12, 172. (27) Gigante, D. M. P.; Long, F.; Bodack, L. A.; Evans, J. M.; Nafie, L. A.; Freedman, T. B. J. Phys. Chem. A 1999, 103, 1523. (28) Delvin, F. J.; Stephens, P. J.; Cheeseman, J. R.; Frisch, M. J. J. Phys. Chem. A 1997, 101, 9912. (29) He, Y.; Cao, X.; Nafie, L. A.; Freedman, T. B. J. Am. Chem. Soc. 2001, 123, 11320. (30) Elsbernd, H.; Beattie, J. K. J. Am. Chem. Soc. 1970, 92, 1940. (31) Gollogly, J. R.; Hawkins, C. J.; Beattie, J. K. Inorg. Chem. 1971, 10, 317. (32) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montogomery, 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.; Adamo, C.; Jaramillo, J.; Homperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochtreski, 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. M.; Gill, P. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision B.01; Gaussian: Pittsburgh, PA, 2003. (33) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (34) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1998, 37, 785. (35) Hay, P. J.; Wadt, R. W. J. Chem. Phys. 1985, 82, 299. (36) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (37) Stephens, P. J. Phys. Chem. 1985, 89, 748.

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