Article pubs.acs.org/JPCA
Simultaneous Resonance Raman Optical Activity Involving Two Electronic States Christian Merten,*,†,‡ Honggang Li,§ and Laurence A. Nafie§,∥ †
Department of Chemistry, University of Alberta, Edmonton, Alberta, T6G2G2, Canada Fraunhofer Institute for Manufacturing Technology and Advance Materials (IFAM), 28359 Bremen, Germany § BioTools, Inc., Jupiter, Florida 33458, United States ∥ Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100, United States ‡
S Supporting Information *
ABSTRACT: In the present work, the first observation of strong resonance Raman optical activity (RROA) involving more that one resonant electronic state is reported. The chiral transition metal complex bis-(trifluoroacetylcamphorato) copper(II), abbreviated Cu(tfc)2, exhibits both resonance Raman (RR) and RROA spectra with laser excitation at 532 nm. Vibrational assignments for this complex were carried out by comparing the non-RR spectra of Cu(tfc)2 excited at 1024 nm to density functional theory (DFT) calculations. The theory of the single-electronic-state (SES) RROA is extended to the next simplest level of theory involving two resonant electronic states (TES) without interstate vibronic coupling as an aide to the interpretation of the observed TES-RROA spectra. Based on measured UV−vis electronic absorbance spectra and corresponding TD-DFT calculations, the most likely two states associated with the RROA spectra are identified.
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INTRODUCTION Raman optical activity (ROA) is the chiroptical version of Raman spectroscopy. It can be measured as a small difference in the Raman scattering of right- and left-circularly polarized incident (incident circular polarization, ICP), Raman scattered (scattered circular polarization, SCP), or both incident and scattered (in-phase, DCPI, or out-of-phase, DCPII) radiation.1,2 Due to its high sensitivity to conformations and conformational changes, one of its major fields of application is the study of the structures of biomolecules such as proteins, peptides, and carbohydrates in solution.3,4 Smaller molecules such as αpinene and phenylethyl amine have been studied as well.5,6 Very recently, the first experimental spectra of a helical chiral polymer and a chiral metal complex under far-from-resonance conditions as well as gas phase ROA have been reported. Those studies highlight the applicability of ROA for the investigations of a broad variety of types of molecules.7−9 In resonance Raman (RR) spectroscopy of biomolecules, the incident laser light can excite electronic transitions of appropriate chromophores in the sample and thereby enhance the Raman intensities of vibrations related to the excited chromophore.10 The theory of ROA has been extended for the case of resonance with a single electronic state (SES)11 and shortly after was confirmed experimentally.12 The theory predicts that the resonance ROA (RROA) spectrum will be monosignate, either positive or negative, and to feature the same relative band intensities as the parent Raman spectrum. The sign of the RROA bands and the ratio of RROA/RR are © 2012 American Chemical Society
determined by the electronic circular dichroism (ECD) band of the corresponding resonant electronic state. The RROA sign is opposite to the ECD sign and the ratio of RROA/RR equals the negative ratio of ECD/2UV for backscattering ICP and SCP ROA (and equal to ECD/UV for DCPI ROA). The change in sign is related to the opposite sign conventions in CD spectroscopy and ROA. The corresponding extension of the SES theory of RROA covering the simultaneous resonance with two electronic states (TES) has been reported recently2 and will be briefly summarized in the following theory section. Somewhat earlier, the SES theory of RROA was extended by Luber et al. to include interference effects arising from more than one nearby excited electronic state.13 The theoretical development of this paper is at the same level of formalism of ROA as the original SES theory11 and includes a new formulation of SES theory using the time-dependent wave packet evolution theory of RR scattering of Lee and Heller.14 When the time-dependent SES formalism is used, the theory is extended to include the influence of multiple excited electronic states. The aim of their paper is to address deviations of the RROA of (S)(+)-naproxen-OCD3 and (S)-(+)-naproxen from the simple predictions of the SES theory where the RROA spectra of these Received: April 14, 2012 Revised: May 26, 2012 Published: June 4, 2012 7329
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contributions from the electric quadrupole interactions, strictly absent from the SES theory, are not included in the TES theory, again for simplicity. A more general presentation of the TES theory, including B-term interstate interactions, has recently been published.2 The Raman polarizability tensor in the limit of strong resonance with a single electronic state, e, is given by
molecules was originally used to confirm the basic theoretical predictions of the SES RROA theory.12 The present work reports the first clear experimental observation of RROA originating from two different excited electronic states. The sample used was the chiral transition metal complex bis(3-trifluoroacetyl-camphorato) copper(II), Cu(tfc)2, the structure of which is shown in Scheme 1. In Scheme 1. Chemical Structure of the trans-Conformer of (R)-Cu(tfc)2
(α̃ αβ)ag1, g 0
1 = ℏ
∑ υ
⟨ϕga1|⟨g |μ̂α|e⟩|ϕeυ⟩⟨ϕeυ|⟨e|μ̂β|g ⟩|ϕga0⟩ ωeυ , g 0 − ω0 − i Γeυ
(1)
Here, the nonresonance term is not needed. The tensor is automatically a complex quantity due to the presence of the imaginary damping term that prevents the denominator from becoming unrealistically small as resonance with any particular excited vibronic state (eυ) of a given normal mode a (or the e0 level) is approached. In the case of one a single excited electronic state, one may choose the z-axis of the molecular reference frame to coincide with the direction of the dipole transition moment of the excited electronic state. With this change the RR tensor is given by
Results and Discussion, the observed RR and RROA spectra obtained using a laser excitation wavelength of 532 nm are presented. This is followed by a comprehensive vibrational analysis of the far-from-resonance (FFR) Raman spectrum using a laser excitation at 1064 nm. Next, the experimental and calculated UV/vis and electronic CD data are discussed in detail. Finally, based on the UV/ECD data, it is shown that resonance of the incident laser light with two particular electronic states of Cu(tfc)2 is the likely explanation for the observed RR and RROA spectra. For completeness, it is noted that in 2007 Johannessen et al. published the RR and RROA spectra of cyotchrome c as background for their report of surface-enhanced RROA (SERROA) in this molecule.15 The reported RROA spectra exhibited many positive and negative ROA intensities relatively evenly across the spectrum. Clearly more than one excited electronic state was involved in the generation of these RROA spectra, but these results were not interpreted using the SES or any other theory.
(α̃ zz)ag1, g 0
1 = ℏ
∑ υ
⟨ϕga1|⟨g |μ̂z|e⟩|ϕeυ⟩⟨ϕeυ|⟨e|μ̂z|g ⟩|ϕga0⟩ ωeυ , g 0 − ω0 − i Γeυ
(2)
Notice that only a single element of the RR tensor is nonzero, a simplification that is possible only for a single excited electronic state (or if more than one such state is present then all such states must be exactly parallel to one another). If only a single electronic excited state is present, then only RR A-terms with no vibronic coupling between states can occur, and the RR polarizability can be factored into a pure electronic contribution and a summation over all vibronic sublevels of the excited electronic state (α̃ zz)ag1, g 0
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⟨ϕga1|ϕeυ⟩⟨ϕeυ|ϕga0⟩ 1 = ⟨g |μ̂z|e⟩⟨e|μ̂z|g ⟩ ∑ ℏ ωeυ , g 0 − ω0 − i Γeυ υ
(3)
The RR invariant associated with a tensor with a single element is simply its absolute square
THEORY OF RESONANCE WITH TWO ELECTRONIC STATES As outlined above, the SES theory of RROA, proposed in 1996,11 confirmed experimentally in 1998,12 was recently further confirmed computationally in 2007.16 The SES RROA theory is remarkably simple in that the RROA spectrum is exactly proportional to the parent RR spectrum but each band in the RROA is smaller than its parent RR band by the ratio of the electronic circular dichroism (ECD) to the parent UV−vis absorbance and having the opposite sign of the ECD spectrum as mentioned above. Departure from the SES theory quickly leads to complexity of the RROA theory that precludes simple analysis, as previously demonstrated,2,13 although Jensen et al.16 did demonstrate computationally that the SES limit can be realized for more than one relatively nearby resonant electronic state without noticeable interferences between the two states. Below, the simplest extension of the SES theory to two excited electronic states is presented. The basic expressions of the SES theory are first presented as background for a theory of RROA with two adjacent noninteracting resonant electronic states (TES theory). The TES theory is expressed as Albrecht A-term (Franck−Condon) vibronic integrals without the complications of interstate vibronic coupling between the two states via the Albrecht B-term (Herzberg−Teller) vibronic coupling integrals between these two states.17 In addition, the normally small
|(α̃ zz)ag1, g 0 |2 = (1/ℏ)2 (Deg )2 U (ω0 , ωa)
(4)
where the electronic part is recognized to be the dipole strength of the resonant electronic transition that is proportional to the intensity of its absorbance spectrum Deg = |⟨g |μ̂|e⟩|2
(5)
and U(ω0,ωa) is a resonance line shape function. The same steps can be carried out for the RROA magnetic dipole tensor defined by direct analogy to eq 3 by substitution of one of the electric-dipole moment operators for a magnetic-dipole moment operator. ̃ )ag1, g 0 = 1 (Gzz ℏ
∑ υ
⟨ϕga1|⟨g |μ̂z|e⟩|ϕeυ⟩⟨ϕeυ|⟨e|m̂ z |g ⟩|ϕga0⟩ ωeυ , g 0 − ω0 − i Γeυ
(6)
The corresponding invariant associated with this RROA tensor is ̃ )∗ ]ag1, g 0 = − (1/ℏ)2 (Deg Reg )U (ω0 , ωa) Im[(αzz ̃ )(Gzz
(7)
and the rotational strength proportional to the ECD of the resonant excited state is 7330
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Reg = Im(⟨g |μ̂|e⟩·⟨e|m̂ |g ⟩)
The RR and RROA tensors are each comprised of two SES terms. If either of the two terms is neglible, then the theory reverts back to the orginal SES theory. The most obvious change is that the RR and the RROA tensors are no longer comprised of single zz terms, but rather have generally nonzero contributions for all nine Cartesian components. This number for the RR tensor elements could be reduced from 9 to 4 components if the two electric-dipole transition moments lie in a plane defined by two molecular Cartesian component axes, say the z and x axes. In any case, if significant resonance enhanced contributions occur for more than a single excited electronic state, the RR and RROA tensors no longer have a single nonzero element (zz), hence, their simple connection to the UV−vis absorbance and ECD spectra is necessarily lost. In particular, there are interference terms when the RR tensor is squared to create a Raman invariant. Nevertheless, one can achieve almost SES behavior for individual bands in the spectrum if for those vibrations only one or the other of the two states considered is important. In this case, there may be some minor interferences from the second state that causes a noticeable deviation of the RROA/RR intensity ratio from that of minus one-half the anisotropy ratio (−ΔεECD/2εUV‑vis) of the dominant resonant electronic state. This appears to be the case for the RROA spectra presented below, where all but one of the bands exhibit SES-like behavior with a single dominant resonant electronic state, but for that one band, the RROA intensity must originate mostly from a second resonant electronic state with opposite ECD sign to that of the dominant electronic state of the other bands.
(8)
For a single excited electronic state there is only one RR invariant and one RROA invariant. The electric quadrupole operator cannot contribute to ROA without at least one additional excited state. Under these limiting conditions, backscattering SCP-Raman and SCP ROA intensities are given by IRU (180o) + ILU (180o) = 48K |(α̃ zz)ag1, g 0 |2
IRU (180°) − ILU (180°) =
(9)
96K ̃ )∗ ]ag1, g 0 Im[(α̃ zz)(Gzz c
(10)
When eqs 4 and 7 are used, the ratio of RROA to RR intensities is given by IRU (180°) − ILU (180°) IRU (180°) + ILU (180°) ̃ )∗ ]ag1, g 0 geg ⎛ 2 ⎞ Im[(α̃ zz)(Gzz 2 RegDeg ⎜ ⎟ = − = − = ⎝c⎠ c (Deg )2 2 |(α̃ zz)ag1, g 0 |2 (11)
Here geg is the anisotropy ratio, the ratio of ECD to UV-vis absorbance intensity of the resonant excited electronic state given by geg =
4Im(⟨g |μ̂|e⟩·⟨e|m̂ |g ⟩) 2
c |⟨g |μ̂|e⟩|
=
4Reg cDeg
=
ΔεECD εUV‐vis
(12)
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Thus, for all bands in the RROA spectrum, the ratio of RROA to RR is equal to −1/2 times the anisotropy ratio of the single resonant state. If considering two resonant electronic states, e1 and e2, with vibronic sublevels ν1 and ν2 located at ωe1ν1,g0 and ωe1ν2,g0, respectively, the RR and magnetic RROA tensors in the absence of vibronic coupling between the two states can be written as (α̃ αβ)ag1, g 0 =
1 [ℏ⟨g |μ̂z|e1⟩0 ⟨e1|μ̂z|g ⟩0 ℏ2 ⟨ϕga1|ϕe υ ⟩⟨ϕe υ |ϕga0⟩
∑ υ1
∑ υ2
̃ )ag1, g 0 = (Gαβ
1 1
1 1
ωe1υ1, g 0 − ω0 − i Γe1υ1 2 2
ωe2υ2 , g 0 − ω0 − i Γe2υ2
1 [ℏ⟨g |μ̂z|e1⟩0 ⟨e1|m̂ z |g ⟩0 ℏ2 ⟨ϕga1|ϕe υ ⟩⟨ϕe υ |ϕga0⟩
∑ υ1
+ℏ⟨g |μ̂α|e 2⟩0 ⟨e 2|μ̂β|g ⟩0
⟨ϕga1|ϕe υ ⟩⟨ϕe υ |ϕga0⟩ 2 2
1 1
1 1
ωe1υ1, g 0 − ω0 − i Γe1υ1
⟨e 2|m̂β |g ⟩0 ∑ υ
] (13)
+ℏ⟨g |μ̂α|e 2⟩0
⟨ϕga1|ϕe υ ⟩⟨ϕe υ |ϕga0⟩ 2 2
EXPERIMENTAL SECTION All chemicals were obtained from Sigma Aldrich and used without further purification. Bis(3-trifluoroacetyl-camphorato)copper(II), Cu(tfc)2, was synthesized according to published procedures.18 Briefly, a solution of 0.588 g (2.4 mmol) of 3trifluoroacetyl camphor in 10 mL of ethanol was added to a solution of 0.236 g (1.2 mmol) of Cu(OAc)2·H2O in 10 mL of water. After stirring for 30 min at room temperature, the green precipitate was filtered off and sublimated several times. Raman spectra with an excitation wavelength of 1064 nm were recorded with a Bruker IFS 66/S spectrometer equipped with an FRA 106/S Raman module and a collection time of 30 min. All ROA data were acquired using 532 nm excitation with a ChiralRAMAN-2X SCP-ROA spectrometer from BioTools Inc. The incident laser line was time-averaged to zero net circular and linear polarization with a set of half-wave plate optics, and the difference of left and right circularly polarized scattered light from the sample was measured simultaneously. Samples with a concentration of 0.18 M in chloroform were measured with 100 mW excitation and an exposure time of 0.7 s. A long-pass filter was used to cut off the spectrum below 800 cm−1 in order to suppress the solvent signals. No background correction has been carried out and the spectra are presented as obtained. The average resolution across the ROA/Raman spectrum is in the range of 6−8 cm−1. The UV/vis spectra were recorded using a Specord 200 photometer from Analytik Jena AG, Germany, using a quartz cuvette with a path length of 1 cm at various concentrations as given in the text. The ECD spectra were measured with 1 mm path length using a Jasco J-815 spectrometer. The concentrations were 4 mM for the lower UV region and 44 mM for the region from ∼380−700 nm.
2 2
ωe2υ2 , g 0 − ω0 − i Γe2υ2
] (14)
Here, in addition to neglecting interstate coupling B-terms, for the same reasons of simplicity, any contributions from the electric quadrupole ROA tensors have been omitted. This is further rationalized by the typically small contributions of the electric-quadrupole mechanism, relative to the magnetic-dipole mechanism, in calculated RROA intensities. 7331
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association of chloroform to Cu(tfc)2. A similar solvent association has been reported, with supporting DFT calculations, for VCD from the C−D stretching mode of the solvent CDCl3 hydrogen-bonded to the carbonyl group of the chiral molecules camphor and pulegone.20,21 A similar possibility for Cu(tfc)2 is currently under theoretical investigation. Further band assignments are given below. In the 532 nm Raman spectrum, some indications for resonance enhancement can be found for instance when comparing the intensities of the Raman band at 1519 cm−1 with the bands in the range from 1500 to 1425 cm−1. Other bands show intensity ratios comparable to the 1064 nm measurement. Slight intensity differences might also be attributed to the different spectral resolutions. The recorded ROA spectra are almost monosignate in the range from 830 to 1600 cm−1. However, it is noticeable that the ROA band corresponding to the Raman band at 1640 cm−1 shows the respective opposite sign. One might suspect that this opposite sign behavior originates in the change of sign of the electronic CD background at 610 nm, but this is far removed from this vibrational frequency and carries no influence on the ROA sign change. Furthermore, the backgrounds of both ROA spectra feature a bias with the same sign as the dominating band sign. The ROA to Raman ratios for the intense bands are in the range of ∼1−5 × 10−4. For weaker Raman bands it is more complicated to determine exact values because the trend of the background has to be estimated. For a comparison of the relative ROA and Raman intensity, Figure 2 shows the
The geometry optimizations, vibrational frequency calculations, and TD-DFT calculations were carried out using the Gaussian 03 E01 software package19 at the density functional theory level using the B3PW91 functional and the 6-311+ +G(2d,p) basis set. Line broadening of the predicted vibrational spectra was introduced using a Lorentzian broadening function with a half-width at half height of 4 cm−1.
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RESULTS AND DISCUSSION Experimental Raman and Raman Optical Activity Spectra. The Raman and ROA spectra of Cu(tfc)2 in chloroform solution obtained for a laser excitation wavelength of 532 nm are presented in Figure 1. The 532 nm spectra are
Figure 1. Experimental Raman (lower panel) and ROA spectra (upper panel) obtained with an excitation wavelength of 532 nm compared to the 1064 nm Raman spectrum.
presented without any baseline corrections to show that the Raman spectra of both enantiomers feature a fluorescence background. The absence of mirror symmetry of the intensities of the ROA of the enantiomers for the 1640 cm−1 band indicates the possible presence of some degree of polarization artifact that may originate from a highly polarized nature of this band, but the difference in the ROA spectra for the enatiomers, upon which our considerations are based, eliminates any such artifact. For comparison, the experimental Raman spectrum obtained using a laser excitation wavelength of 1064 nm is shown as well. It has been recorded for a chloroform solution and a solid sample of Cu(tfc)2 but the spectra were basically identical. Therefore, only the solid state spectrum is shown since it is not overlapped with solvent bands. The strong band at 1219 cm−1 in the 532 nm Raman spectra can hence be assigned to the C−H bending mode of chloroform. The presence of a large ROA band for this mode is noted but is not understood at this time. It may be due to a specific binding
Figure 2. Normalized Raman (gray) and [(S)−(R)]/2-ROA (black) spectra.
normalized Raman and ROA spectra. The scaling factor used is 2.3 × 103; hence, all ROA bands are lower in intensity by a factor of 4.3 × 10−4 relative to those shown in the figure. From this comparison, the RROA/RR intensity ratios for the bands at 968, 1333, 1522, and 1640 cm−1 are approximately 1.1 × 10−3, 4 × 10−4, 4.3 × 10−4, and 2.4 × 10−4, respectively. Band Assignments and Vibrational Analysis. Being a square-planar complex of the type ML2 with two identical nonsymmetric ligands, Cu(tfc)2 features two C2-symmetric geometrical isomers denoted as trans- and cis-Cu(tfc)2. The structure of Cu(tfc)2, shown in Scheme 1, is that of the transisomer, while for the cis-isomer the CF3 groups point in the 7332
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same direction. The computed energy difference between these two isomers is only 0.43 kcal/mol with the trans-isomer being the more stable isomer. From this energy difference, a population of 67.5% for the trans- and 32.5% for the cis-isomer can be deduced. For both isomers, the Raman spectrum has been calculated and the obtained population-weighted spectrum is compared to the experimental spectrum in Figure 3. The experimental 1064 nm Raman spectrum is a typical far-
Table 1. Band Assignments for the Infrared and Raman Spectra of Cu(tfc)2 νtheo
νRaman
968 994 1001 1058 1082 1103 1132, 1148 1161, 1176 1186 1202, 1223
963 996 1004 1061 1081 1103 1132, 1149 1166, 1181 1191 1200, 1230
1233 1269 1286, 1298 1330 1377 1442, 1454, 1472 1484 1521 1652
1230 1268 1284, 1297 1324 1371 1409, 1451, 1469 1475 1519 1640
assignment C−C stretch CH3 deformation vibration C−C stretch CCO bend complex CHx deformations C−F stretch C−C ring deformations (CCH bend) C−F stretch CH3 rocking complex CHx and aliphatic C−C deform. CH2 twisting CH2 wagging C−H deformation C−C stretching sym CH3 bending asym CH3 bending CH2 scissoring in-phase CC stretch in-phase CO stretch
Scheme 2. Atomic Displacements Originating from the Raman Active In-Phase CO Stretching Modes Figure 3. Experimental (excitation wavelength 1064 nm) and calculated Raman spectra of Cu(tfc)2. The gray spectra are expanded by a factor of 3 in the region
