J. Phys. Chem. 1990, 94, 5195-5196 Finally we wish to emphasize that second harmonic generation from the electrode-electrolyte interface depends on several parameters. One needs to check and explain the effects of all of them to ensure the effectiveness of this technique as a probe of the electrochemical interface properties.
5195
TABLE I: Experimental and Theoretical Shifts of Vibrational Frequencies of the CuTPP Molecule on Isotope Substitution of the Nitrogen Atoms I4N by 15N and the Carbon Atoms “C in the Meso Positions by Au(I2C-”C)
Registry No. Ag, 7440-22-4; KC104, 7778-74-7.
Laboratoire d’Electrochimie Interfaciale du CNRS I , Place A . Briand F- 921 95 Meudon CPdex. France LURE, Centre Unioersitaire Paris S u d F-91405 Orsay, France
our
A. Tadjeddine*
P. Guyot-Sionnest
Receiued: September 20, 1989; In Final Form: January 18, 1990
On the Appllcabllity of the Valence Force Field of the Copper Porphine Molecule Obtained by Solving the Inverse Spectral Problem for the Description of Isotopic Effects in the Resonance Raman Spectra of Metalloporphyrins Sir: In our worksI4 the valence force fields of the molecules of porphine and copper porphine have been determined by solving the inverse spectral problem. The vibrational frequencies of these molecules and those of a series of their deuterated derivatives were used in the process of their calculations. The choice of the compounds mentioned was determined by the possibility of using the obtained force fields in the calculations of molecular vibrations of porphyrins and metalloporphyrins substituted at the @-positions of the pyrrole rings and/or at the methine bridges. In fact, the calculated force fields have been used for the theoretical interpretation of vibrational and vibronic spectra of a number of @substituted porphyrins, including tetramethylporphyrins and their N H tautomer^,^ octamethylporphine and octaethylporphine2 and their metal complexes,16isomers of copper etioporphyrin7 and zinc tetramethylporphine,8 and hemoprotein^;^ they served as a basis for the molecular force fields of chlorins (dihydr~porphyrins)~J*~~ and tetrahydr~porphine.’~On the basis of these force fields the changes in vibrational frequencies on substituting the meso positions of the porphyrin ring (methine bridges) have been correctly described, and the IR and resonance Raman spectra of mesotetraphenylporphine (TPP) metal complexes’s-’7 and vibronic
( I ) Gladkov, L. L.; Solovyov, K. N. Preprint No. 303. Institute of Physics, BSSR Academy of Sciences, Minsk, 1983 (in Russian). (2) Solovyov, K . N.; Gladkov, L. L.; Starukhin, A . S.; Shkirman, S. F. Spektroskopiya porfrinou: kolebatelnyye sostoyaniya (Spectroscopy of porphyrins: vibrational stores); Nauka i Tekhnika, Minsk, 1985 (in Russian). (3) Gladkov, L. L.; Solovyov, K . N. Spectrochim. Acta 1985,41A, 1437. (4) Gladkov, L. L.; Solovyov, K. N. Specfrochim.Acta 1985,4/A, 1443. ( 5 ) Shulga, A. M.; Gladkov, L. L.; Stanishevsky, 1. V.; Starukhin, A. S. Teor. Eksp. Khim. 1985, 21, 554. (6) Gladkov, L. L.; Solovyov, K. N. Specfrochim. Acta 1986, 42A. I . (7) Gladkov, L. L.; Solovyov, K. N. Zh. Prikl. Spektrosk. 1986, 44, 64. (8) Shulga, A. M.; Gladkov, L. L.; Stanishevsky, 1. V.; Starukhin, A. S. Teor. Eksp. Khim. 1985, 21, 431. (9) Gladkov, L. L.; Solovyov, K . N . Mol. Biol. 1983, 17, 13 12. (IO) Gladkov, L. L.; Solovyov, K . N.; Starukhin, A. S.; Shulga, A. M.; Gradyushko, A. T. Z h . Prikl. Spektrosk. 1983, 38, 598. ( I I ) Gladkov, L. L.; Starukhin, A. S.;Shulga, A. M. Zh. Prikl. Spektrosk. 1987, 46, 23 1. (12) Gladkov. L. L.; Baranov, V. I. Opt. Spektrosk. 1987.63, 669. (13) Gladkov. L. L.; Starukhin, A. S.; Shulga, A. M. Spectrochim. Acta 1987, 43A, 1125. (14) Gladkov, L. L. Zh. Prikl. Spektrosk. 1988, 49, 806. ( I 5 ) Gladkov, L. L.; Solovyov, K. N . Dokl. Akad. Nauk BSSR 1986,30, 983.
0022-3654/90/2094-5 195$02.50/0
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spectra of meso-tetraethylporphine and mesetetrapropylporphine18 as well as those of meso-tetraphenyltetrahydr~porphine’~ have been reasonably interpreted. Thus, the valence force fields of the molecules of porphine and copper porphine obtained by solving the inverse spectral problem may serve as the basis for the detailed interpretation of vibronic and vibrational (IR and Raman) spectra of the compounds belonging to the class of porphyrins. Therefore, we were greatly surprised after having read in ref 20 the following statement: “We found this force field to be unsatisfactory in that it completely failed to reproduce the observed ISNand meso-13Cisotope shifts”. In the paper” dealing with the resonance Raman spectra of the Cu TPP molecule and the Zn TPP anion as well as with the normal-coordinate analysis of these molecular systems, the inverse spectral problem is solved for the Cu TPP molecule, the frequencies of the vibrations, which are active in the resonance Raman spectra of Cu TPP and its isotope-substituted analogues Cu TPP-IsN4, Cu T P P - ( ~ ~ S O - ’ ~ C ) ~ , and Cu TPP-d8,being used for optimization of the force constants. Before giving direct evidence against the statement of ref 20, we would like to mention two circumstances. First, the direct comparison of the potential energy distributions of the corresponding modes of the Cu TPP molecule obtained in our workt7 and in ref 20 shows that in most cases they are almost identical. Consequently, our force field should not be worse than that obtained in ref 20. Second, it seems rather strange that in ref 20 there is no reference to our paper,” published in an international journal, in which the resonance Raman spectra of the same subject (Cu TPP) are interpreted on the basis of normal-coordinate analysis with the use of the copper porphine force field. To solve the question of the applicability of our force field to the description of changes of vibrational frequencies on isotope substitution, we have carried out the direct calculations of the normal-mode frequencies of Cu TPP-IsN4and Cu TPP-(mesoI3C4). The input data for the calculations-the masses and coordinates of the atoms, the vibrational coordinates, and the force field-were taken the same as in refs 15-1 7 . For those interested in reproducing the calculations, the listing of the input data is submitted as Supplementary Material (see the paragraph at the (16) Gladkov, L. L.; Solovyov, K. N. Preprint No. 413, Institute of Physics BSSR Academy of Sciences, Minsk, 1986 (in Russian). (17) Gladkov, L. L.; Solovyov, K. N. Spectrosc. Lett. 1986, 19, 905. (18) Gladkov, L. L.; Stanishevsky, I . V.; Starukhin, A . S.;Shulga, A. M., in press. (19) Gladkov, L. L.; Egorova, G . D.; Stanishevsky, I. V . Z h . Prikl. Spectrosk. 1989, 5 1 , No. IO. (20) Atamian, M.; Donohoe, R . J.; Lindsey, J. S.; Bocian, D. F. J . Phys. Chem. 1989, 93, 2236.
0 1990 American Chemical Society
5196
Comments
The Journal of Physical Chemistry, Vol. 94, No. 12, I990
end of the paper). The obtained frequency shifts on isotope substitution are presented in Table I along with the calculation results from ref 20 and the experimental data from the same work (only those modes are included in Table I, for which experimental data are available). It should be emphasized that, contrary to ref 20, we did not vary the force constants of the Cu TPP molecule with the aim of achieving a better description of isotopic shifts but directly used the force field of copper porphine found earlier. The analysis of the data given in the table shows that the frequency shifts obtained by us correspond to the experimental ones at least not worse than the results of ref 20. They are slightly inferior to the data of ref 20 on 15N but are more exact in the case of 13C. If we sum up the absolute values of the deviations of the calculated frequency shifts from the experimental ones, X . , ~ A V ~-"AviQPI, '~ we obtain, for the isotope substitution '2C-13C, 41 cm-' (our calculation) and 57 cm-' (data of ref 20). For the isotope substitution I4N-l5N this sum is 28 and 25 cm-l, respectively. Thus, the valence force field of the copper porphine molecule obtained as a result of the solution of the inverse spectral p r ~ b l e m ' *gives ~ - ~a sufficiently correct description of the dynamics of the metalloporphyrin macrocycle. Hence, the criticism of our force field in ref 20 is ungrounded and misleading.
At our request, in Professor L. A. Gribov's laboratory expert calculations of normal modes have been carried out for C u TPP ' ~ Cour ) , force field (the program for the and C U T P P - ( ~ ~ S ~ -with normal-coordinate analysis used in Gribov's laboratory and that used in our Institute are different). The calculated values of vibrational frequencies differ from ours by fractions of a wavenumber, and the changes of frequencies on isotope substitution completely coincide in the two calculations. Registry No. CuTPP, 141 72-91-9; I5N, 14390-96-6; ISC,14762-74-4. Supplementary Material Available: Input data from the calculations (2 pages). Ordering information is given on any current masthead page. Institute of Physics Academy of Sciences of the Byelorussian S S R 70 Leninsky Prospekt 220072 Minsk, USSR
L. L. Cladkov K. N. Solovyov*
Received: October 24, 1989; In Final Form: February 27, 1990
