J. Phys. Chem. B 2000, 104, 10885-10899
10885
Calculation of the Vibrational Spectra of Linear Tetrapyrroles. 2. Resonance Raman Spectra of Hexamethylpyrromethene Monomers† Maria-Andrea Mroginski,‡,§ Ka´ roly Ne´ meth,‡ Ildiko´ Magdo´ ,‡,| Martin Mu1 ller,‡ Uwe Robben,‡ Carlos Della Ve´ dova,§ Peter Hildebrandt,*,‡ and Franz Mark*,‡ Max-Planck-Institut fu¨ r Strahlenchemie, Postfach 101365, D-45413 Mu¨ lheim a.d. Ruhr, Germany, and Departamento de Quı´mica, Facultad de Ciencias Exactas, UniVersidad Nacional de La Plata, 47 esq. 115, 1900 La Plata, Argentina ReceiVed: February 3, 2000; In Final Form: June 14, 2000
The resonance Raman (RR) spectra of monomeric 3,3′,4,4′,5,5′-hexamethylpyrromethene (HMPM) were measured upon excitation in resonance with the strong 436 nm absorption band. The experimental spectra were analyzed by comparison with calculated RR spectra that were obtained on the basis of scaled quantum chemical force fields in combination with the transform theory. The ground-state structure and force field of HMPM were calculated by density functional theory (DFT) using the B3LYP exchange functional and the 6-31G* basis set. The monomeric HMPM adopts a planar structure in contrast to HMPM dimers in which the intermolecular hydrogen-bonding interactions induce a slight torsion of the methine bridges as revealed by both the experimental and the calculated structures. The force fields were scaled by using a global set of scaling factors determined previously (Magdo´, I.; Ne´meth, K.; Mark, F.; Hildebrandt, P.; Schaffner, K. J. Phys. Chem. A 1999, 103, 289). To account for the effect of the intramolecular hydrogen bond between the pyrrolic N-H group and the pyrroleninic nitrogen in the monomeric HMPM, only the scaling factor for the N-H in-plane bending force constant required a slight adjustment. Electronic transitions were calculated by means of CNDO/S, Hartree-Fock single configuration interaction (HF-CIS), and time-dependent DFT, which all predict one strong and one or two adjacent weak transitions for the lowest electronic excitations. This pattern is in line with band fitting analyses of the 436 nm absorption band. The best agreement in excitation energies was obtained by time-dependent DFT calculations. Excited-state displacements as required for evaluating RR intensities were determined for the lowest excited singlet state S1 using the equilibrium geometries optimized for the ground and excited states by means of the HF and HF-CIS methods, respectively. For the second lowest excited state (S2), only an approximate equilibrium geometry could be used for determining the excited-state displacements as the S2 state became quasi-isoenergetic with the S1 state during the geometry optimization. Employing the transform theory, RR spectra were calculated for resonance enhancement via the S1 and S2 states. The experimental RR spectrum of HMPM excited at 413 nm agrees well with the calculated S1-RR spectrum, allowing a plausible and consistent vibrational assignment for most of the observed bands of HMPM and its isotopomer deuterated at the pyrrolic nitrogen. The root-meansquare deviations between the experimental and calculated frequencies are 7 and 5 cm-1 for nondeuterated and deuterated HMPM, respectively. Experimental RR intensities and their dependence on the excitation wavelength are reproduced in a semiquantitative manner. The only significant exceptions refer to the CdC stretching and CsH rocking modes of the methine bridge, ν21 and ν49. On one hand, these discrepancies may reflect intrinsic deficiencies of the HF/HF-CIS method in calculating excited-state displacements. On the other hand, the unique deviation of the experimental excitation profile of ν21 from the expected behavior suggests a destructive interference of the S1 and S2 states in the resonance enhancement specifically of this mode.
I. Introduction Methine-bridged open chain tetrapyrroles constitute the chromophoric sites of various biological photoreceptors.1 Elucidating the molecular basis of the biological function of these chromoproteins requires a detailed knowledge of the structure †
Part of the special issue “Thomas Spiro Festschrift”. * To whom correspondence should be addressed. Max-Planck-Institut fu¨r Strahlenchemie. § Universidad Nacional de La Plata. | Present address: Computer Assisted Drug Discovery, Gedeon Richter Ltd., P.O. Box 27, H-1475 Budapest, Hungary. ‡
of the chromophore and its structural changes during the photoinduced processes. In this respect, resonance Raman (RR) spectroscopy is a particularly powerful method as it selectively probes the vibrational spectra of the chromophore.2-5 However, the structural information that is contained in these spectra can only be extracted on the basis of a reliable vibrational analysis for this class of compounds, which is the goal of our combined experimental and theoretical studies.6-8 The theoretical approach is based on the quantum chemical calculation of force fields which are corrected by a set of global scaling factors.7,9 In our previous work,7 these scaling factors
10.1021/jp000444h CCC: $19.00 © 2000 American Chemical Society Published on Web 10/04/2000
10886 J. Phys. Chem. B, Vol. 104, No. 46, 2000 have been optimized for a variety of small training molecules which include internal coordinates similar to those in tetrapyrroles. The best performance was achieved with density functional theory (DFT) using the B3LYP exchange functional and the 6-31G* basis set. This method (B3LYP) required only 10 scaling factors and afforded a root-mean-square (rms) deviation for the calculated frequencies of 11 cm-1. Furthermore, B3LYP calculations also permit satisfactory semiquantitative predictions of IR and off-resonance Raman intensities. Such an approach has also successfully been employed for calculating the vibrational spectra of larger albeit highly symmetric cyclic tetrapyrroles, i.e., porphines.10,11 The good agreement with the experimental spectra suggests that the reliability of the scaled quantum mechanical force field calculations is not necessarily lowered with increasing size of the molecules. Moreover, preliminary but promising results have been recently obtained also for linear tetrapyrroles, which do not possess molecular symmetry.8 Although these studies provide increasing confidence in the reliability of ab initio methods for vibrational analyses in general, further systematic studies are required prior to extending the approach to the protein-bound tetrapyrrole. In this work, two problems associated with the vibrational analysis of prosthetic groups in chromoproteins will be examined. First, for molecules with extended conjugated π-electron systems such as tetrapyrroles, true nonresonant Raman spectra cannot be measured as even excitation lines in the near-infrared (e.g., 1064 nm) provide a preresonance enhancement for vibrational modes.3,6 Specifically, for chromoproteins RR spectroscopy represents the only technique for measuring vibrational spectra of the chromophoric group.2-5 Thus, it is necessary to develop an approach in which scaled quantum mechanical force field calculations are combined with a reliable method for predicting RR intensities. The transform theory may represent such a method,12-14 inasmuch as previous studies have demonstrated that RR intensities can be obtained with a satisfactory accuracy.15-17 Second, in the protein-bound state, the chromophore is involved in specific interactions with the immediate molecular environment. Among them, hydrogen-bonding interactions are known to have a strong impact on the structural and electronic properties of the chromophore, thereby affecting the RR spectrum.18 Therefore, it is of particular importance to assess the effect of well-defined hydrogen-bonding interactions on the vibrational spectra. The methine-bridged dipyrrole 3,3′,4,4′,5,5′-hexamethylpyrromethene (HMPM; Figure 1) represents an appropriate test molecule to develop an approach for calculating RR intensities as well as to study the response of the vibrational spectra on hydrogen-bonding interactions. In this respect, HMPM offers several advantages. At first, the crystal structure of HMPM has been determined,19 and vibrational spectra can be obtained by different techniques, providing a large set of experimental data for a comparison with the calculations. In addition, the N-H group of the pyrrolic ring and the nitrogen of the pyrroleninic ring of HMPM may serve as hydrogen bond donor and acceptor, respectively. In this way, HMPM is capable of forming both intra- and intermolecular hydrogen bonds. Furthermore, HMPM exhibits a strong electronic transition at ca. 440 nm which is in a spectral range appropriate for probing the RR spectra under rigorous resonance conditions. On the other hand, the relatively small size of HMPM permits systematic quantum chemical calculations to optimize the theoretical approach. Finally,
Mroginski et al.
Figure 1. Structural formula of monomeric HMPM (top) and the experimentally determined structure of dimeric HMPM as derived from the crystal structure (bottom).
HMPM can be regarded as a model for the inner rings of linear tetrapyrroles7 so that the vibrational analysis may also constitute a sensitive test prior to extending the studies to the target molecules. This paper is dedicated to the vibrational analysis of monomeric HMPM, whereas the results obtained for HMPM dimers will be reported in a subsequent paper.20 Since HMPM forms dimers in the solid phase and such dimers even prevail in Ar matrixes,20 monomeric HMPM could exclusively be obtained in diluted solutions so that RR spectroscopy is the only applicable technique. In this work, special emphasis is laid on the calculation of the electronic transitions and the excited-state geometries, which are required for the evaluation of RR intensities. Since there is only limited experience in this field so far, we have employed various quantum chemical methods for a comparison. As a compromise between the desired accuracy and the available computational time, we have chosen ab initio Hartree-Fock based methods for the RR intensity evaluation, whereas the ground-state structure and the force fields were calculated by B3LYP. II. Syntheses and Methods A. Syntheses. HMPM was obtained in a condensation reaction from 2-(ethoxycarbonyl)-3,4,5-trimethylpyrrole (ETP). For the synthesis of ETP, which was carried out similar to published procedures,21,22 a cold saturated NaNO2 solution (16.8 g in 30 mL of H2O) was slowly added within 2 h to an icecooled solution of ethyl acetoacetate in acetic acid (31.2 g in 180 mL) under stirring so that the temperature did not exceed 8 °C. After the solution was stirred at ambient temperature for a further 2 h and addition of 25 g of 3-methylpentane-2,4-dione, a mixture of zinc powder (31.2 g) and sodium acetate (40 g) was added portionwise under vigorous stirring such that the temperature was rapidly raised to ca. 95 °C and kept at this temperature during the reaction. The solution was allowed to cool within 1 h under continuous stirring and poured into 2 L of a water/ice mixture to let the product precipitate for 12 h. The precipitate was washed with water and dried in a vacuum. Recrystallization from ethanol yielded white crystals (21.7 g, yield 49%) with a melting point of 127 °C (lit.23 mp 128 °C). For the synthesis of HMPM from ETP, the procedure published by Johnson et al.24 was modified. A 4.2 g sample of ETP was suspended in 10 mL of formic acid (98% w/w), combined with 10 mL of aqueous HBr solution (50% w/w),
Vibrational Spectra of Linear Tetrapyrroles and kept at 100 °C under stirring in an Ar atmosphere for 8 h. The reaction mixture became a homogeneous solution after 10 min of heating, accompanied by a color change from pale yellow to red, and a red solid started to precipitate after 4 h. The suspension was cooled and stirred overnight. The precipitate was filtered off, washed with formic acid and subsequently with water, and finally redissolved in a 1:1 mixture of CHCl3 and aqueous ammonia solution (15% w/w). After the organic phase was washed three times with aqueous ammonia, the solvent was evaporated. Recrystallization of the solid in CH2Cl2/CH3OH afforded red crystals (yield 53%, 1.4 g) that melt under decomposition at 168 °C (lit.23 mp 172 °C; lit.24 mp 168 °C). For the present RR experiments, solutions of monomeric HMPM are required. According to Guy and Jones,25 the IR spectrum of a 1 mM HMPM solution in CCl4 displays the N-H stretching at 3281 cm-1, which is close to the calculated value (3300 cm-1), whereas the corresponding modes of the dimer are found and calculated at ca. 3250 and 3230 cm-1.20 In the present experiments, the HMPM concentration was even much lower (60 µM in n-hexane) so that dimerization is expected to be negligibly small. In fact, the RR spectra reveal no indication for HMPM dimers (vide infra). Deuterium exchange at the pyrrolic nitrogen (i.e., HMPM-d1) was achieved by addition of aliquots of D2O to the solution. In this way, a deuteration grade of ca. 70% was obtained in various sample preparations as judged from the relative RR intensities of the characteristic marker bands (vide infra). Note that the RR spectra of nondeuterated HMPM (HMPM-d0) were not affected by addition of H2O to the solution. B. RR Experiments. RR spectra were obtained with 413 and 457 nm excitation (Kr+ and Ar+ lasers) using a spectrograph (U1000, ISA) equipped with a liquid-nitrogen-cooled CCD detector. The spectral slit width was 2.8 cm-1, and the wavenumber increment was 0.5 cm-1. For the RR spectra excited with the 442 nm line of a He/Cd laser, a double monochromator (Spex 1403) equipped with a photon-counting detection system was used. In these experiments, the spectral slit width and the wavenumber increment were 2.8 and 0.2 cm-1, respectively. Details of the experimental setups are given elsewhere.26 In all experiments that were carried out at ambient temperature, the samples were contained in a rotating cell to avoid photoinduced degradations. The scattered light was collected in a 90° scattering geometry. The integrity of the samples during the experiments was checked by comparing the RR spectra of the individual measurements, which were eventually combined if no spectral differences were noted. The structureless background of the spectra was removed by polynomial subtraction. Unless noted otherwise, in the spectra shown in this work, the contributions of the Raman bands of n-hexane were subtracted. In the HMPM samples the wavelength-dependent attenuation of the Raman scattered light causes small changes in the relative intensities (
