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Resolution of overlapping charge-transfer transitions by a combined absorption-MCD–MLD approach
Chemical Physics Letters 365 (2002) 164–169 www.elsevier.com/locate/cplett
Resolution of overlapping charge-transfer transitions by a combined absorption-MCD–MLD approach Jim Peterson *, Terrence J. Collins, Eckard M€ unck, Emile L. Bominaar
1
Department of Chemistry, Mellon Institute, Carnegie Mellon University, Box 170, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA Received 19 July 2002; in final form 2 September 2002
Abstract The general lack of resolution in molecular electronic absorption spectra has been a major obstacle to development in this field for decades, resulting in gradual loss of interest and effort compared to virtually all other areas of spectroscopy. Ironically, magnetic linear dichroism (MLD) has been known to atomic spectroscopists for more than a century, but ignored as a technique suitable for molecular studies. Here we demonstrate that the concerted application of MLD, in conjunction with magnetic circular dichroism and absorption spectroscopy, represents a significant advance in deconvoluting overlapping electronic transitions of randomly oriented chromophores. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The electronic spectra of transition metal ions can provide important information concerning non-crystalline structures and likely reactivities of the complexes, metalloproteins and other materials in which they are either constituents, contaminants, or doped probe species. To unambiguously extract this kind of knowledge, at least some of the multiple overlapping transitions under the spectral envelope need to be resolved. There are numerous examples in the literature concerning the resolution of electronic transitions arising from planar organic molecules by a combination of absorption, *
Corresponding author. Fax: +412-268-1061. E-mail addresses: [email protected] (J. Peterson), [email protected] (E.L. Bominaar). 1 Also Corresponding author.
orientational linear dichroism (LD) and emission anisotropy measurements [1]. In the case of inorganic chromophores, particularly paramagnetic ones, deconvolution of molecular spectra by simultaneously fitting electronic absorption, natural circular dichroism (CD) and magnetic circular dichroism (MCD) data with multiple Gaussian bands has been by far the most commonly applied resolution method [2,3]. Here we present evidence that the latter approach can yield incomplete solutions and that many of the elusive transitions can be detected by applying magnetic linear dichroism (MLD) spectroscopy. The intrinsic MLD, which is a consequence of the Zeeman effect, was first detected in a condensed phase with lanthanide samples [4,5] and later with matrix-isolated atoms [6,7]. These are chromophores in which the MLD is strong due to the presence of unquenched orbital angular momenta
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 4 2 7 - 6
J. Peterson et al. / Chemical Physics Letters 365 (2002) 164–169
associated with the circulating electrons. More recently, we have started to consider the MLD effect in transition-metal complexes and metalloproteins [8,9], systems exhibiting much weaker signals due to the quenching of orbital momentum by the prevailing ligand field. (There is a phenomenologically similar effect in which particles may be oriented by a magnetic field and, consequently, exhibit a natural LD. This is usually referred to as the ÔCotton–Mouton effectÕ [10,11], even though it was actually first observed by Kerr [12] and reported as the Ôbrush-grating experimentÕ.) Several criteria were originally considered in selecting the sample for this case study. The presence of high-valent iron species is proposed in relation to numerous biochemical and non-biological catalytic oxidation cycles. However, the majority of the intermediates involved are not well characterized, due partly to a paucity of reports concerning studies with suitable model compounds. To date, the best characterized model compound containing iron in its tetravalent oxidation state is [Et4 N][FeIV Clðg4 -MAC*)] ([H4 (MAC*)] ¼ 1,4,8, 11-tetraaza-13,13-diethyl-2,2,5,5,7,7,10,10-octamethyl-3,6,9,12,14-pentaoxocyclo-tetradecane) [13,14]. The MLD of this species (spin S ¼ 2) was anticipated to be readily measureable as the technique has been predicted to be particularly well suited to studying integer-spin systems [8,9]. Additionally, the number of visible region charge transfers was expected to be small due to the presence of only a single chloride ligand.
2. Methods Cryogenic absorption, MCD and MLD spectra were recorded using an Aviv Associates (Lakewood, NJ) 41DS circular dichroism spectrometer in conjunction with a Cryomagnetics (Oak Ridge, TN) cryomagnet as previously described [8]. Our current equipment does not allow us to record data in the ultraviolet, hence we show herein spectra from 25,000 cm1 (400 nm) to lower energy. The spectral simulations were performed with a Mathematica algorithm developed for this purpose. The expansion coefficients for a set of bands with given energies and bandwidths were obtained
165
by a least-squares fit to the data for each type of spectrum; that is, three sets of linear equations were solved. The three coefficient sets were used to evaluate simulations and the sums of the squares of deviations from the experimental data. The three sums were multiplied by appropriate weight factors to ensure fits of comparable quality for the three data sets. By adding the weighted sums, a quantity which is a function of the bandwidths and transition energies was obtained. The determination of the optimal values for these parameters, at which the bandwidth-energy function is a minimum, involves the solution of a set of non-linear equations. This step of the protocol was performed by a numerical routine provided by the Mathematica package. For reasons of computational efficiency, the simultaneous fits were first performed on the low-energy halves of the spectra, using bands with transition energies in this spectral range only. The resulting simulations were extrapolated to the upper-energy halves of the spectra and subtracted from the data, whereupon the residual spectra were then fit using only bands centred in the upper energy range. A provision was made to avoid the occurrence of physically unreasonable negative amplitudes in the deconvolution of the absorption spectrum by assigning large values to the minimizable function where these arose. [Et4 N][FeIV Clðg4 -MAC*)] was synthesized as previously described [15] and dissolved in 50% (v/v) acetonitrile/propionitrile to obtain glassing solutions upon freezing. To ensure samples were free of the FeIII -containing analogue, solutions of the complex were prepared in the presence of a slight excess of the oxidant CeðSO4 Þ2 H2 O. Control experiments verified that this reagent did not lead to any significant interference in the visible-region spectra of samples. All reagents and solvents were purchased from Aldrich or Fluka and used without further purification. Concentrations of the complex were determined spectrophotometrically, using the extinction coefficient e440 ¼ 11 mM1 cm1 [14].
3. Results and discussion The 4.2 K absorption spectrum of [Et4 N] [FeIV Clðg4 -MAC*)] exhibits transitions through-
166
J. Peterson et al. / Chemical Physics Letters 365 (2002) 164–169
out the visible region (Fig. 1). The complex has roughly square-pyramidal geometry with a chloride in the apical position [15]. If the chloride is stripped by addition of monovalent silver, or replaced with fluoride by metathesis, the visible bands are lost (not shown) indicating that chloride-to-metal charge-transfer transitions are responsible for the spectrum observed in this region. The absorption spectrum (symbols) is very well simulated (solid trace) by summation of four Gaussian components (dashed traces) representing transitions at about 450 nm (22,300 cm1 ), 530 nm (18,800 cm1 ), 650 nm (15,400 cm1 ) and 740 nm (13,500 cm1 ). The presence of four halide-tometal charge-transfers can reasonably be expected in the case of chromophores with fourfold symmetry [16] if we apply the conventional wisdom of expecting any transitions from the 3pz orbital (along the chloride–iron axis) to be in the ultraviolet. Thus, in the absence of any additional information, the temptation is great to take the data analysis no further. Indeed, this kind of simple
Fig. 1. Electronic absorption spectrum of [Et4 N][FeIV Cl ðg4 -MAC*)] at 4.2 K in acetonitrile/propionitrile (50% v/v). Experimental data are given by symbols; the dashed curves represent Gaussian bands; the solid curve is a simulation (best fit) constructed by summing 4 Gaussian bands. This is an example of an unacceptable deconvolution that does not take the MCD and MLD data into account – see text.
symmetry-based argument has been common in the ÔassignmentÕ of the electronic spectra of transition metal complexes. However, in the present case, it has been inferred from M€ ossbauer spectroscopy that the structures of [Et4 N][FeIV Clðg4 MAC*)] must be nearly identical in frozen solution and the crystalline state [14] and so, the assumption of fourfold symmetry in solution is clearly invalid. MCD spectra of all paramagnetic chromophores are expected to be dominated by strongly temperature-dependent C terms at 4.2 K [17] and so, the envelope should approximate to a series of overlapping positive and negative Gaussian bands, each centred close in energy to the maxima of the absorption spectra. Consequently, by straightforward comparison of the 4.2 K absorption and MCD spectral characteristics of the complex (Fig. 2A,B) one is again led to suspect that there are four transitions to be resolved. In fact, to simultaneously fit the absorption and MCD spectra of Fig. 2A,B, we find it necessary to include a minimum of five Gaussian components (not shown). However, if it is acknowledged that the complex has no real symmetry, then five transitions also seems too small a number; since, in a simple orbital model, an electron moving from one of three Cl 3p orbitals to one of five FeIV 3d orbitals equates to fifteen possible transitions. In addition, there is actually a non-obvious procedural problem to be considered. The signal-to-noise ratios of the present data set are very much better than those normally obtained in the case of less well characterized samples, where the chromophore is likely to be dilute. Were the current spectra to exhibit our more typically encountered experimental signal-to-noise, we would be unable to distinguish between the quality of theoretical fits to the absorption and MCD data employing four or five Gaussian components. Given it is a necessary theoretical assumption that the electronic transitions under study will have Gaussian (or some other idealized) band shape(s), it is clearly undesirable that any fitting procedure be guided by vague inflexions or slightly distorted bands; comparisons with strong spectral features, such as peaks and troughs, are much better indicators of the validity of deconvolutions.
J. Peterson et al. / Chemical Physics Letters 365 (2002) 164–169
Fig. 2. MLD in 5.0 T field applied perpendicular to the light path (C), MCD in 1.0 T field applied parallel to the light path (B), and absorption in 0 T field (A) spectra of [Et4 N][FeIV Cl ðg4 -MAC*)]. The three data sets were recorded at 4.2 K using the same sample as described in the legend to Fig. 1. Experimental data are given by symbols; the dashed curves represent the Gaussian bands; solid lines are simulations (best fits) constructed by summing 8 Gaussian bands. The simulations are based on a simultaneous least-squares fit of the three spectra in which the energies, widths, and amplitudes of the individual Gaussian bands are treated as adjustable parameters.
167
Interestingly then, the predominant feature in the MLD spectrum of [Et4 N] [FeIV Clðg4 -MAC*)] (Fig. 2C) is a peak centered around 680 nm (14,700 cm1 ). This strong positive band is located between the two lowest energy maxima of the absorption spectrum and corresponds to a position in the MCD spectrum where overlapping transitions almost completely cancel. It follows, therefore, that the MLD spectrum will yield information not reliably obtainable from the combined absorption and MCD data, even before any detailed analysis is attempted. In the rigid-shift approximation [18], each electronic transition gives rise to a signal that is the weighted sum of the absorption band and its derivatives with respect to energy. That is, absorption contains one term (f, here a Gaussian), MCD two (f þ f 0 ) and MLD three (f þ f 0 þ f 00 ), but at cryogenic temperature the zeroeth-derivative terms (f) are predominant in all three measurements [8]. The three spectra can thus be simulated as linear combinations of a single set of bands, differing only in their associated coefficients, which are technique specific. To simultaneously fit the three spectral envelopes using this method, we find it necessary to take into consideration a minimum of eight transitions; a considerable improvement in spectral resolution. The individual transitions (dashed traces) and resulting simulations of the spectral envelopes (solid traces) are given in the three panels of Fig. 2. The calculated spectral parameters associated with the resolved transitions are given in Table 1. The obvious next task is to assign the resolved transitions, but a description of the methodology to be employed is beyond the scope of the present article. Nevertheless, it should be noted that preliminary analysis suggests a unique assignment for the transitions presented in Table 1 can be deduced from the signs and magnitudes of the associated MCD and MLD bands (work in progress). 4. Concluding remarks Other groups have reported convincing electronic spectral deconvolutions by various combinations of absorption, orientational LD, emission
168
J. Peterson et al. / Chemical Physics Letters 365 (2002) 164–169
Table 1 Resolved electronic transitions of [Et4 N][FeIV Clðg4 -MAC*)] Energy ðcm1 Þ
Bandwidtha ðcm1 Þ
Absorptionb
MCDb
MLDb
12,894 13,583 14,667 15,370 17,111 18,727 21,844 22,260
661 1175 2615 1825 1393 2206 2194 5757
3:74 103 5:08 102 1:61 102 9:42 102 3:67 102 8:28 102 4:31 103 1.00
)0.061 +0.510 +0.879 )1.000 )0.075 )0.007 )0.799 +0.298
)0.053 )0.285 +1.000 )0.547 )0.044 )0.135 )0.033 +0.135
a b
Width at half maximum height. Relative intensities.
anisotropy, natural CD and MCD measurements. However, these studies concerned special cases where either rather few transitions were present [2], the spectra involved were unusually well resolved [3], or the chromophores were diamagnetic and of at least planar symmetry [19]. In the more general case of unsymmetric (and possibly paramagnetic) chromophores exhibiting broad and extensively overlapping transitions in the visible region, the present results clearly illustrate that the inclusion of MLD data in the protocol leads to a dramatic improvement in resolution. It is to be stressed that all three data sets (absorption, MCD and MLD) must be included in the analysis. Omitting any one data set and simultaneously fitting the other two results (in this particular case) in about half as many transitions being found. Together, the MCD and MLD identify principally those transitions polarised xy with respect to the molecular g tensor, while the remaining z-polarised transitions tend to dominate the absorption spectrum. Of course, if samples can be usefully prepared as single crystals, or otherwise oriented, then the improved resolution of the absorption spectrum may be obtainable with non-magnetic polarised light measurements [1,20]. However, the protocol described here is applicable to the more generally useful case of randomly-dispersed chromophores prepared as frozen solutions (glasses) and is experimentally much less troublesome than dealing with oriented samples. In addition to the spectrum of Fig. 2C, we have now recorded MLD signals associated with n–p , p–p , d–d transitions and other charge transfers exhibited by several types of molecular
chromophore. Consequently, we anticipate that MLD spectroscopy will prove to be an extremely useful tool for deconvoluting (and subsequently assigning) the electronic spectra of a great many transition-metal-ion centres, exhibiting various oxidation levels and spin states, in a wide range of samples.
Acknowledgements Supported by the National Institutes of Health awards HL61411 (to J.P.) and GM22701 (to E.M.).
References [1] J. Michl, E.W. Thulstrup, in: Spectroscopy with Polarized Light: Solute Alignment by Photoselection, in Liquid Crystals, Polymers, and Membranes, VCH Publishers Inc., New York, 1986, p. 354, and references cited therein. [2] E.I. Solomon, E.G. Pavel, K.E. Loeb, C. Campochiaro, Coord. Chem. Rev. 144 (1995) 369. [3] J. Mack, M.J. Stillman, Inorg. Chem. 40 (2001) 812. [4] B.E. Argyle, J.C. Suits, M.J. Freiser, Phys. Rev. Lett. 15 (1965) 822. [5] A.C. Boccara, J. Ferre, B. Briat, M. Billardon, J.P. Badoz, J. Chem. Phys. 50 (1969) 2716. [6] M. Vala, J.-C. Rivoal, C. Grisolia, J. Pyka, J. Chem. Phys. 82 (1985) 4376. [7] M. Vala, M. Eyring, J. Pyka, J.-C. Rivoal, C. Grisolia, J. Chem. Phys. 83 (1985) 969. [8] J. Peterson, L.L. Pearce, E.L. Bominaar, J. Am. Chem. Soc. 121 (1999) 5972. [9] E.L. Bominaar, J. Peterson, J. Chem. Phys. 111 (1999) 7512.
J. Peterson et al. / Chemical Physics Letters 365 (2002) 164–169 [10] A. Cotton, H. Mouton, Annales de Chimie et de Physique 11 (8) (1907) 145. [11] A. Cotton, H. Mouton, Annales de Chimie et de Physique 11 (8) (1907) 289. [12] J. Kerr, Reports of the British Association (1901) 568. [13] T.J. Collins, K.L. Kostka, E. M€ unck, E.S. Uffelman, J. Am. Chem. Soc. 112 (1990) 5637. [14] K.L. Kostka, B.G. Fox, M.P. Hendrich, T.J. Collins, C.E.F. Rickard, L.J. Wright, E. M€ unck, J. Am. Chem. Soc. 115 (1993) 6746.
169
[15] T.J. Collins, B.G. Fox, Z.G. Hu, K.L. Kostka, E. M€ unck, C.E.F. Rickard, L.J. Wright, J. Am. Chem. Soc. 114 (1992) 8724. [16] A.B.P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amsterdam, 1968. [17] P.J. Stephens, Adv. Chem. Phys. 35 (1976) 197. [18] M. Kreglewski, M. Vala, J. Chem. Phys. 74 (1981) 5411. [19] P. Lincoln, B. Norden, J. Phys. Chem. B 102 (1998) 9583. [20] W.A. Eaton, J. Hofrichter, Methods Enzymol. 76 (1981) 175, and references cited therein.
Resolution of overlapping charge-transfer transitions by a combined absorption-MCD–MLD approach Jim Peterson *, Terrence J. Collins, Eckard M€ unck, Emile L. Bominaar
1
Department of Chemistry, Mellon Institute, Carnegie Mellon University, Box 170, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA Received 19 July 2002; in final form 2 September 2002
Abstract The general lack of resolution in molecular electronic absorption spectra has been a major obstacle to development in this field for decades, resulting in gradual loss of interest and effort compared to virtually all other areas of spectroscopy. Ironically, magnetic linear dichroism (MLD) has been known to atomic spectroscopists for more than a century, but ignored as a technique suitable for molecular studies. Here we demonstrate that the concerted application of MLD, in conjunction with magnetic circular dichroism and absorption spectroscopy, represents a significant advance in deconvoluting overlapping electronic transitions of randomly oriented chromophores. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The electronic spectra of transition metal ions can provide important information concerning non-crystalline structures and likely reactivities of the complexes, metalloproteins and other materials in which they are either constituents, contaminants, or doped probe species. To unambiguously extract this kind of knowledge, at least some of the multiple overlapping transitions under the spectral envelope need to be resolved. There are numerous examples in the literature concerning the resolution of electronic transitions arising from planar organic molecules by a combination of absorption, *
Corresponding author. Fax: +412-268-1061. E-mail addresses: [email protected] (J. Peterson), [email protected] (E.L. Bominaar). 1 Also Corresponding author.
orientational linear dichroism (LD) and emission anisotropy measurements [1]. In the case of inorganic chromophores, particularly paramagnetic ones, deconvolution of molecular spectra by simultaneously fitting electronic absorption, natural circular dichroism (CD) and magnetic circular dichroism (MCD) data with multiple Gaussian bands has been by far the most commonly applied resolution method [2,3]. Here we present evidence that the latter approach can yield incomplete solutions and that many of the elusive transitions can be detected by applying magnetic linear dichroism (MLD) spectroscopy. The intrinsic MLD, which is a consequence of the Zeeman effect, was first detected in a condensed phase with lanthanide samples [4,5] and later with matrix-isolated atoms [6,7]. These are chromophores in which the MLD is strong due to the presence of unquenched orbital angular momenta
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 4 2 7 - 6
J. Peterson et al. / Chemical Physics Letters 365 (2002) 164–169
associated with the circulating electrons. More recently, we have started to consider the MLD effect in transition-metal complexes and metalloproteins [8,9], systems exhibiting much weaker signals due to the quenching of orbital momentum by the prevailing ligand field. (There is a phenomenologically similar effect in which particles may be oriented by a magnetic field and, consequently, exhibit a natural LD. This is usually referred to as the ÔCotton–Mouton effectÕ [10,11], even though it was actually first observed by Kerr [12] and reported as the Ôbrush-grating experimentÕ.) Several criteria were originally considered in selecting the sample for this case study. The presence of high-valent iron species is proposed in relation to numerous biochemical and non-biological catalytic oxidation cycles. However, the majority of the intermediates involved are not well characterized, due partly to a paucity of reports concerning studies with suitable model compounds. To date, the best characterized model compound containing iron in its tetravalent oxidation state is [Et4 N][FeIV Clðg4 -MAC*)] ([H4 (MAC*)] ¼ 1,4,8, 11-tetraaza-13,13-diethyl-2,2,5,5,7,7,10,10-octamethyl-3,6,9,12,14-pentaoxocyclo-tetradecane) [13,14]. The MLD of this species (spin S ¼ 2) was anticipated to be readily measureable as the technique has been predicted to be particularly well suited to studying integer-spin systems [8,9]. Additionally, the number of visible region charge transfers was expected to be small due to the presence of only a single chloride ligand.
2. Methods Cryogenic absorption, MCD and MLD spectra were recorded using an Aviv Associates (Lakewood, NJ) 41DS circular dichroism spectrometer in conjunction with a Cryomagnetics (Oak Ridge, TN) cryomagnet as previously described [8]. Our current equipment does not allow us to record data in the ultraviolet, hence we show herein spectra from 25,000 cm1 (400 nm) to lower energy. The spectral simulations were performed with a Mathematica algorithm developed for this purpose. The expansion coefficients for a set of bands with given energies and bandwidths were obtained
165
by a least-squares fit to the data for each type of spectrum; that is, three sets of linear equations were solved. The three coefficient sets were used to evaluate simulations and the sums of the squares of deviations from the experimental data. The three sums were multiplied by appropriate weight factors to ensure fits of comparable quality for the three data sets. By adding the weighted sums, a quantity which is a function of the bandwidths and transition energies was obtained. The determination of the optimal values for these parameters, at which the bandwidth-energy function is a minimum, involves the solution of a set of non-linear equations. This step of the protocol was performed by a numerical routine provided by the Mathematica package. For reasons of computational efficiency, the simultaneous fits were first performed on the low-energy halves of the spectra, using bands with transition energies in this spectral range only. The resulting simulations were extrapolated to the upper-energy halves of the spectra and subtracted from the data, whereupon the residual spectra were then fit using only bands centred in the upper energy range. A provision was made to avoid the occurrence of physically unreasonable negative amplitudes in the deconvolution of the absorption spectrum by assigning large values to the minimizable function where these arose. [Et4 N][FeIV Clðg4 -MAC*)] was synthesized as previously described [15] and dissolved in 50% (v/v) acetonitrile/propionitrile to obtain glassing solutions upon freezing. To ensure samples were free of the FeIII -containing analogue, solutions of the complex were prepared in the presence of a slight excess of the oxidant CeðSO4 Þ2 H2 O. Control experiments verified that this reagent did not lead to any significant interference in the visible-region spectra of samples. All reagents and solvents were purchased from Aldrich or Fluka and used without further purification. Concentrations of the complex were determined spectrophotometrically, using the extinction coefficient e440 ¼ 11 mM1 cm1 [14].
3. Results and discussion The 4.2 K absorption spectrum of [Et4 N] [FeIV Clðg4 -MAC*)] exhibits transitions through-
166
J. Peterson et al. / Chemical Physics Letters 365 (2002) 164–169
out the visible region (Fig. 1). The complex has roughly square-pyramidal geometry with a chloride in the apical position [15]. If the chloride is stripped by addition of monovalent silver, or replaced with fluoride by metathesis, the visible bands are lost (not shown) indicating that chloride-to-metal charge-transfer transitions are responsible for the spectrum observed in this region. The absorption spectrum (symbols) is very well simulated (solid trace) by summation of four Gaussian components (dashed traces) representing transitions at about 450 nm (22,300 cm1 ), 530 nm (18,800 cm1 ), 650 nm (15,400 cm1 ) and 740 nm (13,500 cm1 ). The presence of four halide-tometal charge-transfers can reasonably be expected in the case of chromophores with fourfold symmetry [16] if we apply the conventional wisdom of expecting any transitions from the 3pz orbital (along the chloride–iron axis) to be in the ultraviolet. Thus, in the absence of any additional information, the temptation is great to take the data analysis no further. Indeed, this kind of simple
Fig. 1. Electronic absorption spectrum of [Et4 N][FeIV Cl ðg4 -MAC*)] at 4.2 K in acetonitrile/propionitrile (50% v/v). Experimental data are given by symbols; the dashed curves represent Gaussian bands; the solid curve is a simulation (best fit) constructed by summing 4 Gaussian bands. This is an example of an unacceptable deconvolution that does not take the MCD and MLD data into account – see text.
symmetry-based argument has been common in the ÔassignmentÕ of the electronic spectra of transition metal complexes. However, in the present case, it has been inferred from M€ ossbauer spectroscopy that the structures of [Et4 N][FeIV Clðg4 MAC*)] must be nearly identical in frozen solution and the crystalline state [14] and so, the assumption of fourfold symmetry in solution is clearly invalid. MCD spectra of all paramagnetic chromophores are expected to be dominated by strongly temperature-dependent C terms at 4.2 K [17] and so, the envelope should approximate to a series of overlapping positive and negative Gaussian bands, each centred close in energy to the maxima of the absorption spectra. Consequently, by straightforward comparison of the 4.2 K absorption and MCD spectral characteristics of the complex (Fig. 2A,B) one is again led to suspect that there are four transitions to be resolved. In fact, to simultaneously fit the absorption and MCD spectra of Fig. 2A,B, we find it necessary to include a minimum of five Gaussian components (not shown). However, if it is acknowledged that the complex has no real symmetry, then five transitions also seems too small a number; since, in a simple orbital model, an electron moving from one of three Cl 3p orbitals to one of five FeIV 3d orbitals equates to fifteen possible transitions. In addition, there is actually a non-obvious procedural problem to be considered. The signal-to-noise ratios of the present data set are very much better than those normally obtained in the case of less well characterized samples, where the chromophore is likely to be dilute. Were the current spectra to exhibit our more typically encountered experimental signal-to-noise, we would be unable to distinguish between the quality of theoretical fits to the absorption and MCD data employing four or five Gaussian components. Given it is a necessary theoretical assumption that the electronic transitions under study will have Gaussian (or some other idealized) band shape(s), it is clearly undesirable that any fitting procedure be guided by vague inflexions or slightly distorted bands; comparisons with strong spectral features, such as peaks and troughs, are much better indicators of the validity of deconvolutions.
J. Peterson et al. / Chemical Physics Letters 365 (2002) 164–169
Fig. 2. MLD in 5.0 T field applied perpendicular to the light path (C), MCD in 1.0 T field applied parallel to the light path (B), and absorption in 0 T field (A) spectra of [Et4 N][FeIV Cl ðg4 -MAC*)]. The three data sets were recorded at 4.2 K using the same sample as described in the legend to Fig. 1. Experimental data are given by symbols; the dashed curves represent the Gaussian bands; solid lines are simulations (best fits) constructed by summing 8 Gaussian bands. The simulations are based on a simultaneous least-squares fit of the three spectra in which the energies, widths, and amplitudes of the individual Gaussian bands are treated as adjustable parameters.
167
Interestingly then, the predominant feature in the MLD spectrum of [Et4 N] [FeIV Clðg4 -MAC*)] (Fig. 2C) is a peak centered around 680 nm (14,700 cm1 ). This strong positive band is located between the two lowest energy maxima of the absorption spectrum and corresponds to a position in the MCD spectrum where overlapping transitions almost completely cancel. It follows, therefore, that the MLD spectrum will yield information not reliably obtainable from the combined absorption and MCD data, even before any detailed analysis is attempted. In the rigid-shift approximation [18], each electronic transition gives rise to a signal that is the weighted sum of the absorption band and its derivatives with respect to energy. That is, absorption contains one term (f, here a Gaussian), MCD two (f þ f 0 ) and MLD three (f þ f 0 þ f 00 ), but at cryogenic temperature the zeroeth-derivative terms (f) are predominant in all three measurements [8]. The three spectra can thus be simulated as linear combinations of a single set of bands, differing only in their associated coefficients, which are technique specific. To simultaneously fit the three spectral envelopes using this method, we find it necessary to take into consideration a minimum of eight transitions; a considerable improvement in spectral resolution. The individual transitions (dashed traces) and resulting simulations of the spectral envelopes (solid traces) are given in the three panels of Fig. 2. The calculated spectral parameters associated with the resolved transitions are given in Table 1. The obvious next task is to assign the resolved transitions, but a description of the methodology to be employed is beyond the scope of the present article. Nevertheless, it should be noted that preliminary analysis suggests a unique assignment for the transitions presented in Table 1 can be deduced from the signs and magnitudes of the associated MCD and MLD bands (work in progress). 4. Concluding remarks Other groups have reported convincing electronic spectral deconvolutions by various combinations of absorption, orientational LD, emission
168
J. Peterson et al. / Chemical Physics Letters 365 (2002) 164–169
Table 1 Resolved electronic transitions of [Et4 N][FeIV Clðg4 -MAC*)] Energy ðcm1 Þ
Bandwidtha ðcm1 Þ
Absorptionb
MCDb
MLDb
12,894 13,583 14,667 15,370 17,111 18,727 21,844 22,260
661 1175 2615 1825 1393 2206 2194 5757
3:74 103 5:08 102 1:61 102 9:42 102 3:67 102 8:28 102 4:31 103 1.00
)0.061 +0.510 +0.879 )1.000 )0.075 )0.007 )0.799 +0.298
)0.053 )0.285 +1.000 )0.547 )0.044 )0.135 )0.033 +0.135
a b
Width at half maximum height. Relative intensities.
anisotropy, natural CD and MCD measurements. However, these studies concerned special cases where either rather few transitions were present [2], the spectra involved were unusually well resolved [3], or the chromophores were diamagnetic and of at least planar symmetry [19]. In the more general case of unsymmetric (and possibly paramagnetic) chromophores exhibiting broad and extensively overlapping transitions in the visible region, the present results clearly illustrate that the inclusion of MLD data in the protocol leads to a dramatic improvement in resolution. It is to be stressed that all three data sets (absorption, MCD and MLD) must be included in the analysis. Omitting any one data set and simultaneously fitting the other two results (in this particular case) in about half as many transitions being found. Together, the MCD and MLD identify principally those transitions polarised xy with respect to the molecular g tensor, while the remaining z-polarised transitions tend to dominate the absorption spectrum. Of course, if samples can be usefully prepared as single crystals, or otherwise oriented, then the improved resolution of the absorption spectrum may be obtainable with non-magnetic polarised light measurements [1,20]. However, the protocol described here is applicable to the more generally useful case of randomly-dispersed chromophores prepared as frozen solutions (glasses) and is experimentally much less troublesome than dealing with oriented samples. In addition to the spectrum of Fig. 2C, we have now recorded MLD signals associated with n–p , p–p , d–d transitions and other charge transfers exhibited by several types of molecular
chromophore. Consequently, we anticipate that MLD spectroscopy will prove to be an extremely useful tool for deconvoluting (and subsequently assigning) the electronic spectra of a great many transition-metal-ion centres, exhibiting various oxidation levels and spin states, in a wide range of samples.
Acknowledgements Supported by the National Institutes of Health awards HL61411 (to J.P.) and GM22701 (to E.M.).
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