Time-Resolved Resonance Raman Spectroscopic Studies on the

Oct 10, 1996 - Quinones and their radical ion intermediates have been much studied by vibrational spectroscopy to understand their structure−functio...
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J. Phys. Chem. 1996, 100, 16472-16478

Time-Resolved Resonance Raman Spectroscopic Studies on the Radical Anions of Menaquinone and Naphthoquinone Gurusamy Balakrishnan, Pothukattil Mohandas, and Siva Umapathy* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India ReceiVed: February 23, 1996; In Final Form: June 18, 1996X

Quinones and their radical ion intermediates have been much studied by vibrational spectroscopy to understand their structure-function relationships in various biological processes. In this paper, we present a comprehensive analysis of vibrational spectra in the structure-sensitive region of both the naphthoquinone (NQ) and 2-methyl1,4-naphthoquinone (MQ, menaquinone) radical anions using time-resolved resonance Raman and ab initio studies. Specific vibrational mode assignments have been made to all the vibrational frequencies recorded in the experiment. It is observed that the carbonyl and CdC stretching frequencies show considerable coupling in NQ and MQ radical anions. Further, the asymmetric substitution present in MQ with respect to NQ shows important signatures in the radical anion spectrum. It is concluded that assignments of vibrational frequencies of asymmetrically substituted quinones must take into consideration the influence of asymmetry on structure and reactivity.

Introduction Quinones and their reactive intermediates have been studied widely because of their importance to various biological redox processes and also because of their potential for applications in various industrial processes.1-3 Although benzoquinone (BQ)4-9 and anthraquinone (AQ)10,11 intermediates have been thoroughly investigated using both the transient absorption and Raman methods, naphthoquinone (NQ) intermediates have received little attention. The reactive intermediates of NQ and its derivatives participate in a number of biological electron transfer processes. For example, the NQ biological analogue, 2-methyl-1,4-naphthoquinone (MQ, menaquinone) radical anion, is known to be an active intermediate in various biological processes, such as photosynthesis and transport of electrons to cytochrome.1-3 NQ and MQ have been studied by transient absorption techniques,12-21 but apart from spectroelectrochemical FTIR studies (Vide infra) of bacterial reaction centers involving various naphthoquinone analogues22-26 no structural details of the intermediates have been reported in detail. The photosynthetic reaction centers (RC’s) are complex membrane-bound pigment systems, consisting of various chromophoric units such as bacteriopheophytins, bacteriochlorophylls, carotenoids, and quinones. There have been a number of studies on RC’s using various techniques including resonance Raman spectroscopy.3,22-26 The interpretation of the spectroscopic experimental data on these systems is facilitated when the structural details of the individual chromophoric units become available. Our interest is to provide some insight into the structural details of one of the chromophores, namely menaquinone. In this paper we present the resonance Raman spectra of naphthoquinone and menaquinone radical anions in aqueous solution state using TR3S, which can further be used in interpreting the data on RC and therefore help in elucidating structure-function relationships in the biological processes. In addition, the effect of the presence of one substituent at the position ortho to carbonyl in the radical anion of MQ, in comparison to NQ, is of interest. It has been observed27 previously that such asymmetric substitution has led to the * Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, September 15, 1996.

S0022-3654(96)00568-0 CCC: $12.00

observation of two different stretching frequencies for the carbonyl group in the neutral state of MQ. The influence of asymmetric substitution on the redox process is particularly relevant here, since the addition of an electron in the reduced state of quinone can be expected to preferentially locate itself in one of the carbonyls, albeit with some amount of delocalization. Thus, the assignment of vibrational frequencies for the radical anion spectra would have to take into consideration such effects. Such asymmetric substitution is present in all the biological quinone analogues, such as ubiquinone, duroquinone, and the vitamin K series, e.g., menaquinone (vitamin K3). Therefore, the influence and presence of such asymmetric substitution on the structure and reactivity of the model system have to be understood. This paper is the first in a series toward addressing such issues. Time-resolved resonance Raman (TR3) spectroscopy has been well demonstrated to be an ideal technique to study reactive intermediates, particularly quinone radical anions.28,29 Although the traditional time-resolved absorption method has been used to study such intermediates,9,10 it provides information only on the kinetics and electronic spectra of the intermediates with little or no information on the structural aspects, whereas TR3 spectroscopy can be used to understand the vibrational structure of unstable intermediates.28,29 Further, IR spectroelectrochemical studies have also been used to observe the vibrational structure of the radical anions generated electrochemically.22-26 Compared to the resonance Raman technique, IR has some disadvantages: (a) it precludes the use of an aqueous medium (most biological processes take place in aqueous media), (b) IR spectra are recorded using a differential absorption method, which allows observation of only intense bands, and (c) it is difficult to discriminate bands from a mixture of species, such as radical anion, protonated radical, etc. On the other hand, the TR3 approach provides specific advantages. For example, by selecting a probe wavelength within the absorption spectral profile of the radical anion, it is possible to study the vibrational spectra with no interference from the neutral molecules or other intermediates; and the vibrational spectra of the species can be recorded at very short time scales, typically from picoseconds upwards, thus providing an opportunity to follow the formation and decay of intermediates. © 1996 American Chemical Society

Radical Anions of Menaquinone and Naphthoquinone

J. Phys. Chem., Vol. 100, No. 41, 1996 16473

Figure 1. Experimental setup used for time-resolved resonance Raman spectroscopy: DOUBLE MONO ) double monochromator, OMAC ) optical multichannel analyzer controller, COM ) computer, PD ) photodiode, OSC ) oscilloscope, DG ) delay generator, THG ) third harmonic generator, FHG ) fourth harmonic generator, PB ) Pellin Broca prism, L ) lens, RS ) Raman shifter, DM ) dichroic mirror, PLOT ) plotter, GCR250, DCR11 ) Nd:YAG lasers.

In this paper, we present the radical anion spectra of both NQ and MQ using TR3 spectroscopy. The observed Raman spectra are then analyzed with the help of ab initio calculations. Ab initio studies have also been extended to investigate the effect of isotopic substitution of 16O to 18O and to provide appropriate potential energy distributions (PEDs) in the normal modes involved. Finally, our results have been critically compared with the reported FTIR data on the NQ derivatives, and the vibrational assignments for all the experimentally observed bands are presented. Methods Experimental Procedure. The experimental setup used for the TR3S is shown in Figure 1. Briefly, the fourth harmonic output of 266 nm from a Nd:YAG laser (DCR-11, Spectra Physics) was used as the photoexcitation (pump) source. The 416 nm output (probe) from a home-made H2 Raman shifter was used to probe the resonance Raman scattering of the photogenerated intermediates. The Raman shifter was pumped by the third harmonic output of 355 nm from another Nd:YAG laser (GCR250, Spectra Physics). The laser pulses were of about 8-10 ns in temporal width and the energies of about 1.0 and 0.5 mJ, respectively, for pump and probe. The delay between the laser pulses was provided by a Stanford DG535 delay generator. A SPEX 1404 double monochromator was used with two 600 groove gratings to disperse the scattered light. A liquid N2 cooled CCD (Princeton Instruments) with 576 × 378 pixels was used as the multichannel detector. A photodiode (Electrooptics Technology, Model ET 2010) in combination with a 500 MHz oscilloscope (HP 54520A) was used to follow and then control the time delay between the laser pulses. The recorded Raman spectra were calibrated using known solvent bands as reference and the spectral resolution is estimated as 5 cm-1. The experimental sequence involves recording, first, the probe only spectrum (A), second, the pump-probe spectrum (B), and then, finally, the pump only spectrum (C). The result, namely the TR3 spectrum, was obtained by subtracting the pump and the probe spectra from the pump-probe spectrum, in that order (TR3S ) B-C-A). After the completion of each cycle of experiment, a probe only spectrum was recorded to confirm that the sample did not contain any photoproducts. The sample solutions were circulated through a capillary at a rate of about

Figure 2. UV-visible absorption spectra of (a) 1,4-naphthoquinone (inset: its radical anion15 spectra) and (b) 2-methyl-1,4-naphthoquinone (inset: its radical anion15 spectra) in water at pH of 6.

10 mL/min. In order to avoid possible accumulations of photoproducts, samples were replaced regularly. All the sample solutions were deoxygenated to avoid quenching of the intermediates by oxygen. A typical sample solution contained 1.0 mM of the quinone and 0.1 M of sodium nitrite in water. NQ and MQ were obtained from Aldrich and were sublimed before use. The water used was of Millipore grade and sodium nitrite was of analytical grade and used as received. Ab Initio Studies. Ab initio calculations were performed with the Gaussian-92 program30 on an IBM RS6000 computer system employing the standard 6-31G split-valence basis set. Calculations for the neutral molecules and radical anions were carried out using restricted Hartree-Fock (RHF) and unrestricted Hartree-Fock (UHF) formalisms, respectively. Moderate spin contamination was seen in the UHF wave function for the radical anions, but previous studies on similar systems5 have shown that this contamination does not lead to serious effects on the results. The equilibrium geometries of the systems considered were located by search procedures contained within the program. In all the cases a complete geometry optimization was performed. The stationary points thus obtained were characterized by the evaluation of the eigenvalues of the force constant matrix. All the systems reported in the present study represent minima in the potential energy surface. The normal modes and vibrational frequencies are then evaluated at the computed equilibrium geometries by analytic evaluation of energy second derivatives. Potential energy distributions (PED) of the normal modes are calculated by transforming the normal mode displacements from Cartesian to internal coordinates. As suggested by Pople et al.31 for a better comparison with experiment, the calculated frequencies are multiplied by an empirical factor of 0.89. This scaling factor accounts for systematic errors caused by basis set incompleteness, neglect of electron correlation, and vibrational anharmonicity.

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Figure 4. Time-resolved resonance Raman spectra of 2-methyl-1,4naphthoquinone radical anion, time delays (λpump 266 nm, λprobe 416 nm): (a) 60 ns, (b) 120 ns, (c) 270 ns, (d) 1 µs, and (e) 3 µs.

Figure 3. Time-resolved resonance Raman spectra of 1,4-naphthoquinone radical anion, time delays (λpump 266 nm, λprobe 416 nm): (a) 100 ns, (b) 300 ns, (c) 700 ns, (d) 1.2 µs, and (e) 3 µs.

Results and Discussion The radical anions of both NQ and MQ were generated photochemically, in water. Although ubiquinone,32 a biological analogue of BQ, is known to form radical anions through oxidation of water from its triplet state, both NQ and MQ required a reducing agent to produce radical anions. This is not surprising since at pH ) 6 (the present experimental condition) the triplet state redox potentials calculated based on Rehm-Weller equation of both the quinones are not sufficient to oxidize water.33 Thus, we have used sodium nitrite as a reducing agent to generate the radical anions, as has normally been used for other quinones including NQ.10,16,32 In Figure 2, UV-vis absorption spectra of the neutral and radical anion15 of both the NQ and MQ are shown. The arrows marked indicate the positions of the photoexcitation (λpu) and resonance Raman excitation (λpr) laser wavelengths used in the experiment. In the TR3 experiment the radical anions were generated by photoexciting the solution containing the quinone and sodium nitrite in water. In Figures 3 and 4 are shown the timedependent resonance Raman spectra of the radical anions of NQ and MQ, respectively. The lifetime of the radical anion spectra observed here are consistent with the literature data on the transient absorption spectra.16 In the case of NQ, the radical anion bands are observed at 1603, 1537, 1441, 1327, and 1158 cm-1, whereas MQ radical anion bands are present at 1605, 1539, 1468, 1442, 1339 and 1139 cm-1. The assignments of the neutral and radical anion vibrational frequencies have been carried out with the use of both the results obtained from ab initio calculations and by qualitative comparison with literature data on other quinones as discussed below. We note that, although the presence of protonated radical is also expected, it is not unreasonable to assume that the protonated radical species is not observed in our experiment

Figure 5. Numbering of the atoms of (a) 1,4-naphthoquinone and (b) 2-methyl-1,4-naphthoquinone.

for the following reasons. Firstly, the pKa15 for protonation is 4.1 for NQ and for MQ is 4.4 and therefore under our experimental conditions of pH 6.0 protonated radical concentration is unlikely to be high. Further, the protonated radical has an absorption maximum at 370 nm with a bandwidth of 35 nm,14 and thus the excitation wavelength of 416 nm is expected to be out of resonance to yield significant Raman intensity. Geometrical Structure. The definitions of the geometrical parameters are given in Figure 5. The optimized equilibrium geometries of NQ and NQ•- correspond to C2V symmetry and those of MQ and MQ•- to Cs symmetry. The structural parameters of interest for NQ, NQ•-, MQ, and MQ•- are given in Table 1 along with the available crystallographic data34 for NQ. A close examination of the bond distances obtained for NQ shows that the carbonyl part of the molecule (O5-C3-

Radical Anions of Menaquinone and Naphthoquinone

J. Phys. Chem., Vol. 100, No. 41, 1996 16475

TABLE 1: Optimized Geometrical Parametersa of NQ, MQ, and Their Radical Anions Obtained at RHF/6-31G (Neutral) and UHF/6-31G (Radical Anion) Levels bond length (Å) structural parameter

NQ

NQ•-

MQ

MQ•-

C1-C2 C1-C3 C2-C4 C3-O5 C4-O6 C3-C7 C4-C8 C7-C8 C1-C9 C2-C10 C9-C11 C10-C12 C11-C12

1.397 (1.39)b 1.483 (1.43) 1.483 (1.46) 1.222 (1.21) 1.222 (1.22) 1.477 (1.48) 1.477 (1.45) 1.327 (1.31) 1.388 (1.39) 1.388 (1.36) 1.386 (1.41) 1.386 (1.43) 1.388 (1.37)

1.406 1.463 1.463 1.276 1.276 1.418 1.418 1.371 1.407 1.407 1.372 1.372 1.405

1.395 1.483 1.482 1.223 1.224 1.490 1.473 1.332 1.388 1.388 1.386 1.386 1.389

1.405 1.459 1.460 1.278 1.275 1.426 1.420 1.373 1.409 1.408 1.371 1.371 1.406

a For the definition of parameters see Figure 5. b Crystallographic data from ref 34.

Figure 7. Raman spectra of (a) 2-methyl-1,4-naphthoquinone radical anion (λexc 416 nm) and (b) 2-methyl-1,4-naphthoquinone (λexc 488 nm).

Figure 6. Raman spectra of (a) 1,4-naphthoquinone radical anion (λexc 416 nm) and (b) 1,4-naphthoquinone (λexc 488 nm).

C7-C8-C4-O6) corresponds to enedione structure and the other part to benzenoid structure. The bond length values of 1.222 and 1.327 Å, respectively, for C-O and C7-C8 clearly show their double bond character. Other C-C bond lengths (1.477 Å) in the enedione part of the molecule falls in the single bond region. The benzenoid part can be visualized from the almost identical C-C bond lengths (1.38-1.39 Å). This enedione and benzenoid description of the NQ structure is consistent with the X-ray and theoretical studies on NQ derivatives.1 For NQ•-, the ab initio electron density distribution data confirms that the added electron is found to be mostly in the enedione part of the radical anion. This is reflected in the relative changes of the bond distances in NQ•-. For example, the CdO and CdC bonds are elongated by 0.054 and 0.044 Å, respectively, compared to those of the neutral. The C-C bonds of the enedione part (C3-C7 and C4-C8) are shortened by 0.059 Å. The geometrical changes associated with benzenoid part of the radical anion shows its less aromatic character compared to that of the neutral molecule. However, the changes, due to the addition of an electron, in the bond lengths for benzenoid part (0.014-0.019 Å) are minor compared to those seen in the enedione part of the neutral molecule.

In the case of MQ and its radical anion, the influence of asymmetry (due to the introduction of a methyl group in NQ) is not so obvious in the calculated geometrical parameters, whereas the observed vibrational frequencies of neutral MQ reflect the presence of asymmetry.27 For MQ, the two CdO bond lengths are almost identical, with values of 1.223 and 1.224 Å. The other bond distances in the quinonoid part of the molecule, such as C3-C7, C4-C8, and C7-C8, also show a similar trend. However, the benzenoid part is hardly affected by the methyl substitution. The geometrical changes associated with the reduction of MQ to its radical anion follows a pattern similar to that of NQ and NQ•-. These changes, however small they may be, are reflected in significant shifts in the corresponding vibrational frequencies on going from NQ to MQ. For both the quinones and their radical anions, the calculations indicate only minor changes in C-H bond lengths and in the bond angles when passing from neutral to anion. However, the significant changes of CdO as well as enedione CdC and C-C bond lengths for the anions suggest that the corresponding frequencies should show large shifts compared to those of the neutral molecule. Vibrational Frequencies and Assignments In Figures 6 and 7, the neutral and radical anion spectra of NQ and MQ respectively are shown for comparison. The neutral species were recorded for solid samples utilizing normal scattering process, unlike the radical anion spectra which were observed under resonance excitation (Vide supra). Neutral Molecule. The Raman spectrum of the neutral NQ is shown in Figure 6b. The spectrum contains bands at 1667, 1599, 1445, 1407, 1357, 1323, 1187, and 1177 cm-1. The calculated vibrational frequencies, PEDs, and isotopic shifts of NQ and MQ are listed in Table 2, together with the available experimental values. Only the structure-sensitive vibrational modes of interest (1300-1700 cm-1 region) are given in the table for the sake of brevity. In the case of NQ, the CdO stretching modes are strongly coupled with CdC stretching modes and the extent of coupling can be visualized from the corresponding PEDs and isotopic shifts (δ1) given in the table. Based on a pseudodiatomic mode description for the CdO, the expected isotopic shift for 16O to 18O is about 40 cm-1. The

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TABLE 2: Ab Initio Vibrational Frequenciesa (cm-1) and Isotopic Shifts (δ1)a (cm-1) of 1,4-Naphthoquinone (NQ) and Menaquinone (MQ) As Obtained by RHF/6-31G Calculations (Experimental Values in Parentheses) NQ sym

12C16O

A1 B2 A1 B2 A1 A1 B2 B2 A1

1712 (1667) 1681 (1660) 1624 1604 (1599) 1579 1481 (1445) 1456 (1407) 1367 (1357) 1294 (1323)

a

MQ PED

(%)b

CdO (21), CdC (27) CdO (52) CdO (18), CdC (73) CdC (56) CdC (47) CdC (30), C-C (20) CdC (45), C-C (17) C-C (20) C-C (48)

δ1

sym

12C16O

13 27 15 2 4 1 1 1 0

A′ A′ A′ A′ A′ A′′ A′ A′′ A′

1707 (1669) 1673 (1672) 1639 (1624) 1604 (1599) 1581 1456 (1447) 1354 (1362) 1297 (1305) 1267 (1267)

PED (%)b

δ1

CdO (26), CdC (29) CdO (50) CdO (32), CdC (48) CdC (43) CdC (59) CdC (20) C-C (70) C-C (34) C-C (64)

10 27 17 2 5 1 0 1 0

Values are scaled by 0.89. δ1 ) ν12C16O - ν12C18O. b Stretching modes with PED values greater than 10 are listed.

isotopic shift of 27 cm-1 and the PED of 52 for the 1681 cm-1 band clearly indicate that it is predominantly CdO stretching. The bands at 1624 and 1712 cm-1 given in the table exhibit isotopic shifts of 15 and 13 cm-1, respectively. The corresponding PED values of these modes are 18 (CdO) + 73 (CdC) and 21 (CdO) + 27 (CdC), respectively, suggesting that the CdO and CdC stretching modes are coupled to a significant extent. Therefore, we assign these two frequencies to CdO + CdC modes. The aromatic CC stretching modes are found to be at 1604 and 1579 cm-1. The frequencies at 1481 and 1456 cm-1 were assigned to C-C/CdC stretching modes of both aromatic and enedione character. The frequencies at 1367 and 1294 cm-1 correspond to the C-C stretching modes of enedione part of the molecule. As can be seen from Table 2, the calculated frequency values for NQ are in good agreement with the experimental values. The Raman spectrum of neutral MQ is shown in Figure 7b and the Raman bands are observed at 1669, 1599, 1447, 1362, 1385, 1305, 1267, 1171, and 1109 cm-1. In general, the relative values of the calculated frequencies and isotopic shifts of MQ are found to be similar to those of NQ. However, the electronic effect of the methyl group present in MQ is reflected in the changes of CdO and CdC stretching frequencies of the enedione moiety. The difference between the asym and sym CdO stretching frequencies of MQ (1673 and 1639 cm-1) and NQ (1681 and 1624 cm-1) are 34 and 57 cm-1, respectively. This shows that these two CdO stretching frequencies of MQ are shifted toward each other in comparison with those of NQ. Meyerson27 has assigned the IR bands at 1673 and 1665.5 cm-1 to the two CdO stretching modes of MQ, whereas Bauscher and Mantele22 assigned bands at 1664 and 1626 cm-1 to these modes. The significant difference between these two sets of experimental values suggests that the band observed at 1626 cm-1 by Bauscher and Mantele may be due to CdC stretching mode coupled with CdO mode. The band at 1707 cm-1 is assigned to the asymmetric νCdO analog of NQ and the PED of CdO (26) + CdC (29) shows the strong coupling between CdO and CdC stretching modes. The bands at 1581 and 1604 cm-1 correspond to the aromatic CC stretching frequencies. The bands between 1500 and 1300 cm-1 region are assigned to the various C-C and CdC coupled stretching frequencies of both the aromatic and enedione parts of the molecule. In general, the calculated frequency values of MQ are in agreement with the experimental results. Radical Anion. The calculated vibrational frequencies, PEDs, and isotopic shifts along with the available experimental frequencies of NQ and MQ radical anions are summarized in Tables 3 and 4, respectively. The calculated NQ anion spectrum has a symmetric CdO frequency at 1406 cm-1, which is in good agreement with the experimentally observed band at 1441 cm-1. Clark et al.26 observed the asymmetric νCdO IR band at

TABLE 3: Ab Initio Vibrational Frequenciesa and Isotopic Shifts (cm-1) of 1,4-Naphthoquinone Radical Anion (NQ•-) As Obtained by UHF/6-31G Calculations frequencies (cm-1) PED (%)b

sym

12C16O

B2 A1 A1 A1 A1 B2 A1

1615 1600 (1603) 1522 (1537) 1418 1406 (1441) 1376 (1515) 1283 (1327)

d

CdC (44) CdC (46) CdC (55) C-C (28) CdO (48), CdC (26) CdO (36), C-C (34) C-C (89)

isotopic shifts δ1c

∆νQ•--Qd 12C16O

1 2 1 2 20 12 2

11 -112 -57 115 -218 -305 -11

a Values are scaled by 0.89. b As in Table 2. c As in Table 2. Frequency shift ) radical anion - neutral molecule.

TABLE 4: Ab Inition Vibrational Frequenciesa and Isotopic Shifts (cm-1) of Menaquinone Radical Anion (MQ•-) As Obtained by UHF/6-31G Calculations frequencies (cm-1) PED (%)b

sym

12C16O

A′ A′ A′ A′ A′ A′ A′ A′

1615 1600 (1605) 1527 (1537) 1465 (1339) 1424 (1442) 1414 (1468) 1395 1379 (1505)

CdC (43) CdC (40) CdC (44), C-C (20) CdC (31), C-C (10) CdO (48) CdO (35), C-C (19) C-C (36) CdO (29), C-C (30)

isotopic shifts δ1c

∆Q•--Qd 12C16O

1 2 1 1 17 6 1 8

11 -107 -54 -212 -259 98

a Values are scaled by 0.89. b As in Table 2. c As in Table 2. d As in Table 3.

1515 cm-1, but the ab initio studies show asymmetric CdO band at 1376 cm-1. The assignment of 1515 cm-1 band to CdO/CdC coupled mode is more reasonable than pure CdO mode. The experimental spectrum of NQ radical anion has symmetric and asymmetric CdO stretching frequencies downshifted to 226 and 155 cm-1, respectively, compared to the neutral molecule, which is similar to the case of BQ6 with the shifts of about 228 and 158 cm-1. As expected, the extent of coupling between CdC and CdO stretching modes is larger for the radical anion (because of the delocalization of the added electron throughout the enedione part) than the neutral molecule. This is clearly manifested in the PEDs as well as in the isotopic shifts of these modes. The significant decrease in these frequencies is consistent with the weakening of the CdO and CdC bonds seen in the geometrical structure of the radical anion. The calculated CdC(quin) stretching frequency of 1600 cm-1 is in very good agreement with the observed value of 1603 cm-1. The enedione C-C stretching frequency is increased by 115 cm-1 (1303 to 1418 cm-1), which reflects the increase in the corresponding bond order. The aromatic C-C stretching frequencies show a small variation compared to those observed in the neutral molecule. These shifts in the frequency values

Radical Anions of Menaquinone and Naphthoquinone

J. Phys. Chem., Vol. 100, No. 41, 1996 16477 TABLE 5: Experimental Vibrational Frequencies (cm-1) in Assignments of NQ, MQ, and Their Radical Anions NQ anion

neutral

anion

vibrational assignment

1667

1515

1669

1660

1441 1603 1537

1672 1624 1599 1447 1385 1362 1305

1505 1468 1442 1605 1539

asym νCdOa νCdO+C-C/δCH3 sym νCdO quin νCdC arom νCdC

1339

arom/quin νC-C/νCdC

1599 1445 1407 1357 1323 a

Figure 8. Calculated atomic displacements of normal modes involving CdO motions and their frequencies (cm-1) of 2-methyl-1,4-naphthoquinone radical anion.

are in accordance with the changes associated with the corresponding bond parameters given in Table 1. The description of the vibrational modes of MQ radical anion (MQ•-) follow a pattern similar to that of NQ. In the case of MQ•-, the observed/calculated asymmetric and symmetric CdO stretching frequencies are shifted to lower wave-numbers by 158/259 and 225/212 cm-1, respectively, compared to those of MQ. Interestingly, for MQ•-, the extent of coupling between the two CdO stretching modes is found to be different from that of other quinones such as BQ, AQ, and NQ. From the ab initio data, the stretching mode of CdO adjacent to the methyl group is coupled with the enedione C-C stretching modes giving rise to two bands at 1424 and 1379 cm-1 with significant contribution of CdO stretching. Another band at 1414 cm-1 is found to be νCdO, which is strongly coupled with CdC/C-C stretching motions. The calculated atomic displacements of these three modes are shown in Figure 8. From Figure 8, it can be seen that the two CdO bonds are contracting (symmetric νCdO) for the 1424 cm-1, whereas for 1379 cm-1 one is contracting and the other is elongating (asymmetric νCdO). Therefore, it is reasonable to expect the symmetric νCdO to manifest itself in the considerable intensity in the resonance Raman spectrum and the asymmetric νCdO in the IR spectrum. With this rationale, we assign the observed (calculated) Raman band at 1442 (1424) cm-1 to symmetric νCdO and the observed24 (calculated) IR band at 1505 (1379) to asymmetric νCdO. The large difference between the observed and calculated frequency for the asymmetric νCdO is perhaps not so surprising, considering similar studies on BQ5 and NQ (Vide supra) have also exhibited such differences. Since the theoretical frequencies of asymmetric νCdO are considerably underestimated in all these cases, the corresponding PEDs and coupling involved must be used with caution. Further, a direct comparison of the ab initio data with the experimental ones, in the absence of the atomic displacement description, would suggest a more reasonable agreement for the

MQ

neutral

1327

For radical anion, the IR bands are taken from ref 26.

calculated frequency at 1379 cm-1 being correlated to the experimental value of 1339 cm-1. However, we believe that the measurement of the depolarization ratio of the band at 1339 cm-1 would help us in categorically identifying the normal modes to be either the asymmetric νCdO or otherwise. Such a study is not possible at present, but it will be carried out in future. Assuming that the calculated 1379 cm-1 is due to asymmetric νCdO as observed in IR at 1515 cm-1, the calculated frequency of 1414 cm-1, which has contributions from both C-C and CdO, is correlated to the experimentally observed Raman band at 1468 cm-1. Interestingly, this band at 1468 cm-1 is only observed for MQ•- and has no equivalent Raman band in NQ•-. This suggests that this band might involve the δ(CH3) coupling along with the CdO and C-C stretch. Thus, we assign this band to the CdO and C-C coupled with δ(CH3). The observation of three carbonyl-coupled vibrational spectral features (including Raman and IR) of menaquinone is significant in that the qualitative approaches generally used to assign vibrational frequencies for substituted (biological relevant) quinones need due care. In all these cases the extent of coupling of CdO with other modes is also evident from the relative values of calculated isotopic shifts (δ1) and PEDs. The shift in the CdC stretching frequency (also shifted to lower values) is 107 cm-1 (1707 to 1600 cm-1) and the enedione C-C stretching frequency is shifted to a higher value by 98 cm-1 (1297 to 1395 cm-1) as expected for a completely delocalized enedione part. The aromatic CdC stretching frequencies increased by 11 cm-1. Bauscher and Mantele,21 from their FTIR spectroelectrochemical studies, have assigned the bands at 1502 and 1444 cm-1, respectively, to the CdO and CdC stretching fundamentals of MQ•-. Our calculations show that CdO stretching modes are present in the range of 1350-1450 cm-1 and the CdC stretching in the range of 1525-1600 cm-1. The calculated frequencies, isotopic shifts and PEDs are consistent with the assignment of Mantele et al. Thus, the bands at 1444 and 1502 cm-1 are assigned to the CdO and CdC coupled stretching modes. The Raman band observed at 1339 cm-1 can be assigned to the CdC/C-C stretching mode. In Table 5, the final assignments of the experimental vibrational frequencies observed in our experiment of NQ and MQ are summarized. These assignments are also consistent with the qualitative trend observed in other quinones. In summary, we have analyzed the vibrational spectra of NQ and MQ in their neutral and radical anion forms. Ab initio data and literature data of similar systems have been used to assign all the experimentally observed Raman frequencies. These assignments will be of use in identifying the vibrational frequencies observed in Raman studies of photosynthetic systems involving a number of chromophores, including MQ and their analogues. Further work is in progress in identifying

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