J. Phys. Chem. 1990, 94, 47-61 to a phenyl mode because of its larger dzo shift. Other phenyl modes are also seen and their intensity is likewise attributable to coupling with nearby porphyrin modes. There is no qualitative evidence for significant electronic coupling between the porphyrin and phenyl ?r systems, although a quantitative intensity analysis is needed to evaluate the extent of coupling. The surprisingly large intensity of the CoH bending mode of NIP implies a significant change in the C,H bond angle in the excited state, while the intensity of the B,, C,H bending mode implies a significant change in the C,H bond angle via an excited-state Jahn-Teller effect. Substitution of methyl or ethyl groups at the meso-carbon atoms produces somewhat different kinematic effects than phenyl substitution; modes of the alkyl substituents can be seen. The alkyl groups lower the effective molecular symmetry, as reflected in the C,C, stretching band polarization, probably due to orientation effects. All the Ni porphyrins show strong overtone and combination band en-
47
hancements in resonance with the QI transition, reflecting the small QOtransition moment and the resulting importance of C term RR scattering.
Acknowledgment. This work was supported by N I H grants G M 33576 (T.G.S.) and DK 35153 (J.R.K.). Registry No. Nip, 15200-33-6; NiTPP, 14172-92-0; D, 7782-39-0; N, 14390-96-6; I T , 14762-74-4. Supplementary Material Available: Atom numbering schemes for NiP (35 atoms) and NiTPP (77 atoms) with D4hsymmetries and characteristics of the porphyrin in-plane valence force field (Figures 1-3), Cartesian coordinates for NiP and NiTPP, definition of internal coordinates for NiP and NiTPP, and unnormalized U matrices for A,,, A2,, B,,, B2,, and E, symmetry blocks of NiP and NiTPP (Tables 1-6) (21 pages). Ordering information is given on any current masthead page.
Consistent Porphyrin Force Field. 2. Nickel Octaethylporphyrin Skeletal and Substituent Mode Assignments from 15N, Meso-d4, and Methylene-d,, Raman and Infrared Isotope Shifts Xiao-Yuan Li,t Roman S. Czernuszewicz,t James R. Kincaid,t Paul Stein,$ and Thomas G. Spire**+ Department of Chemistry, Princeton University, Princeton, New Jersey 08544, Department of Chemistry, Marquette University, Milwaukee. Wisconsin 53233, and Department of Chemistry, Duquesne University, Pittsburgh, Pennsylvania I5282 (Received: December 20, 1988)
Resonance Raman spectra with variable-wavelength excitation are reported for nickel octaethylporphyrin and its isotopomers containing 15N,and 2Hat the methine (meso-d,) and methylene (methylene-d,,) carbon atoms. The lsN, meso-d, double isotopomer is also examined. The infrared spectrum of the methylene-d,, isotopomer is reported, and the frequencies are combined with recently published infrared results for the other isotopomers. Essentially all of the porphyrin skeletal modes have been assigned and have been allocated to local coordinates which recognize the pyrrole rings as cooperative vibrational units. The assignments are supported by a normal-coordinate analysis with a valence force field involving standard ethyl force constants and porphyrin in-plane force constants which are transferred nearly intact from Ni porphine and Ni tetraphenylporphyrin. Many vibrational modes of the NiOEP ethyl substituents have also been located in the spectra and assigned. Bands assignable to ethyl C-C stretching and C-H bending modes are surprisingly strong in the resonance Raman spectra and suggest appreciable involvement of the ethyl groups in the porphyrin T-T* excited states. The conformations of the ethyl substituents have a marked influence on the low-frequency vibrational spectra.
Introduction Nickel octaethylporphyrin (NiOEP) has played a key role in the characterization of heme proteins by resonance Raman (RR) spectroscopy.'v2 Like all physiological porphyrins, OEP has carbon substituents at the eight pyrrole positions, and it retains the 4-fold symmetry of the porphyrin skeleton (neglecting questions of ethyl orientation), so that symmetry considerations can be brought fully to bear on the vibrational analysis. Kitagawa and co-workers3 carried out an important base-line study of NiOEP using I5N and meso-d, isotope shift data to assign most of the in-plane skeletal modes of the porphyrin ring in a consistent fashion, and they carried out a normal-coordinate calculation with a modified Urey-Bradley force field. They assumed that the ethyl substituents were isolated from the porphyrin electronic system and did not contribute directly to the R R spectra; the substituents were in fact treated as point masses. While this was a reasonable starting approximation, there have since been indications that it *Author to whom correspondence should be addressed. +Princeton University. Marquette University. 8 Duquesne University. f
0022-3654/90/2094-0047$02.50/0
breaks down, especially in the 900-1300-~m-~and in the lowfrequency region of the spectrum, where porphyrins with different pyrrole substituents show appreciable spectral difference^.^ We have therefore undertaken to examine the question of ethyl-substituent involvement in the NiOEP vibrational spectra via replacement of the methylene hydrogen atoms with deuterium. The results are striking; some of the strongest bands in the 1000-cm-' region are attributable to modes which are predominantly ethyl in character, as revealed by their methylene-d,, shifts. These enhancements imply involvement of the ethyl groups in the porphyrin P-P* excited states. The low-frequency region of the
-
(1) Spiro, T. G.; Li, X.-Y. In Biological Applications of Raman Spectroscopy; Spiro, T. G . , Ed.; Wiley-Interscience: New York, 1988; Vol. 111, Chapter 1. (2) Kitagawa, T.; Ozaki, Y. Struct. Bonding (Berlin) 1987, 64, 71-1 14. (3) (a) Kitagawa, T.; Abe, M.; Ogoshi, H. J. Chem. Phys. 1978, 69, 4516-4525. (b) Abe, M.; Kitagawa, T.; Kyogoku, Y. J. Chem. Phys. 1978, 69,4526. (c) The calculations in ref 3a and 3b have been revised slightly in a recent review by: Abe, M. In Spectroscopy of Biological Systems; Clark, R. J. H., Hester, R. E., Eds.; Wiley: New York, 1986; Vol. 13, Chapter 7. (4) (a) Choi, S.; Spiro, T. G.; Langry, K. C.; Smith, K. M. J. A m . Chem. Soc. 1982, 104, 4337. (b) Choi, S.; Spiro, T. G.; Langry, K. C.; Smith, K. M.; Budd, L. D.;La Mar, G. N. J. A m . Chem. Soc. 1982, 104,4345.
0 1990 American Chemical Society
48
The Journal of Physical Chemistry, Vol. 94, No. I , 1990
spectra is also influenced strongly by the ethyl groups and their orientations. We have also reexamined the resonance Raman spectra of 15N and meso-d4 isotopomers of NiOEP using variable-wavelength laser excitation. The study of Kitagawa and co-workers was limited to excitation at 488.0 and 514.5 nm, neither of which is fully resonant with any of the F A * electronic transitions. We have employed 406.7-nm excitation, near-resonance with the B (Soret) band, where polarized (Alg)totally symmetric modes are dominant, 568.2-nm excitation, near-resonance with the Qoband, where depolarized (Blgand BZg)modes are dominant, and 530.9 nm, in between the Qo and QItransitions, where anomalously polarized bands (Azg)are dominant. In addition we have recorded spectra for the I5N, meso-d, double isotopomer in order to secure the assignments of modes with large meso deuteration shifts from their I5N sensitivities. The new data permit a more complete assignment of the NiOEP vibrations than has previously been available. Normal-coordinate calculations have been carried out with explicit inclusion of the methylene hydrogen atoms, in order to support the assignments. The porphyrin skeletal force constants are nearly the same as those used in the preceding study5 of Ni porphine and Ni tetraphenylporphyrin. The success of this analysis means that we now have an empirical force field which is truly representative of the porphyrin macrocycle, independent of the peripheral substituents. This force field should be applicable to protoheme, the prosthetic group of most heme proteins, when due allowance is made for the altered substituents, and for the replacement of Ni by Fe. It should also serve as the starting point for more complex force fields, e.g., in heme a, the prosthetic group of cytochrome oxidase, where the delocalizing effect of the formyl substituent will have to be taken into account,6a or in the hydroporphyrins (chlorophyll, bacteriochlorophyll, siroheme) in which one or more of the pyrrole double bonds is reduced.6b
Experimental Section Octaethylporphine (OEP) was purchased from Strem Chemical Co. The meso-deuterated analogues, OEP-d4 and [I5N]OEP-d4, were obtained by exchange in 2H2S04 (99% 2H) as described by Smith.7a The 15N-labeled compound, [ 15N]OEP, was prepared from NaI5NO2(99% 15N,ICON Services, Inc., Summit, NJ) and ethylpropionyl acetate (Pfaltz and Bauer, Stamford, CT) by the reliable procedure described by Paine and co-~orkers.'~Methylene deuterated sample,8 OEP-d16,was kindly provided by Professor H. M. Goff, University of Iowa. Nickel was incorporated into the free-base porphyrins in hot (120 "C) dimethylformamide (DMF).9a The dried solids were chromatographed on alumina (Grade IV) with methylene chloride as eluent. The nonfluorescent fractions were combined and evaporated to dryness. The residue was recrystallized from refluxing toluene solution upon cooling to room temperature. Raman spectra were obtained in backscattering geometry from CS2, CH2CI2,or CH2Br2solutions at room temperature by using the spinning N M R tube techniquegband the Raman setup described in the preceding paper.5 UV-vis absorption spectra were obtained in 1 -mm quartz cells by using a Hewlett-Packard 8450A diode array spectrophotometer. The infrared spectra in KBr pellets were recorded at room temperature on a Digilab FTS-2OC Fourier-transform infrared spectrophotometer. Normal-mode calculations were performed with the CF matrix methodiaaand a valence force field as described in the preceding ( 5 ) Li,
X.-Y.; Czernuszewicz, R. S.; Kincaid, J. R.; Su, Y . 0.;Spiro, T.
G . J. Phys. Chem., preceding paper in this issue.
(6) (a) Choi, S.; Lee, J . J.; Wei, Y . H.; Spiro, T. G. J. Am. Chem. Soc. 1983, 105, 3692-3707. (b) Boldt, N . J.; Donohoe, R. J. S.; Birge, R . R.; Bocian, D. I . J. Am. Chem. SOC.1987, 109, 2284-2298. (7) (a) Smith, K. Porphyrins and Metalloporphyrins; Elsevier: New York, 1975. (b) Paine, J . B.; Kirshner, W. B.; Maskowitz, D. W.; Dolphin, D. J. Org. Chem. 1976, 41, 3857. (8) (a) Hickman. D.L.; Goff. H. M. J. Am. Chem. SOC.1984, 106, 5013. (b) Godziela, G. M.; Kramer, S. K.; Goff, H. M. Inorg. Chem. 1986, 25, 4286. (9) (a) Adler, A.; Lango, R. F.; Kampus, F.; Kim, J. J. Inorg. Nucl. Chem. 1970. 32. 2443 (b) Walters. M . A. Appl. Specrrosc. 1983, 37, 299.
Li et al. paper.5 Molecular parameters were obtained from the crystal structure datal3-I5 from which Cartesian coordinates were determined via simple trigonometric relationships developed by holding the porphyrin ring planar and the ethyl groups in selected orientations (see Discussion). The 16-methylene hydrogen atoms were included explicitly in the calculation; the methyl groups were approximated by 15 amu point masses. The internal coordinates (R,)consisted of Wilson-type bond stretching, angle bending, and out-of-plane wagging (ethyl groups) coordinates.'0 The symmetry were taken as symmetry adapted linear combicoordinates (Si) nations of internal coordinates. A C2, local group coordinate, symmetrically adapted to the overall DZd(or D4) symmetry of NiOEP was used to derive Sifor ethyl substituents. The corresponding U matrices (S = UR) containing unnormalized linear transformation coefficients and the definition of force constants and of internal coordinates are given in the supplementary material. Schachtschneider's programslobwere used to construct the C matrices and to solve the secular equations, ICF - EX1 = 0, for each symmetry species on a VAX-I1/780 computer. The observed RR frequencies were found to be slightly (1-3 cm-I) solvent dependent (see figures and tables) and those from CS2 solution were used to fit the calculated ones. The final force field (see preceding paperS for refinement procedures) contained 43 freeadjustable force constants, 3 1 for the porphyrin core and 12 for the ethyl groups. It reproduced the 307 observed frequencies (including isotopic frequencies) with an average error of 14 cm-'. Large deviations were mainly from ethyl-related modes.
Results and Discussion A. Normal-Coordinate Analysis: Force Field and Structure. A normal-coordinate analysis was carried out to support the spectral assignments. Table I lists the valence force constants employed, while Table I1 gives structure parameters and Table 111 compares observed with calculated frequencies for all the assigned modes of NiOEP and its isotopomers. For the ethyl substituents, the methylene groups were included explicitly, in order to account for the m e t h ~ 1 e n e - disotope ~~ shifts, but the methyl groups were approximated as 15 amu point masses for simplicity. Force constants for ethyl C H and C C stretching and bending coordinates were taken from the work of Snyder and Schachtschneider." The porphyrin in-plane skeletal force constants are very similar to those used in the preceding study for Ni porphine (NIP) and Ni tetraphenylporphyrin (NiTPP).5 Indeed the force fields for these three porphyrins were developed simultaneously in order to achieve maximum uniformity, recognizing that there may be slight variations due to the electronic effects of the peripheral substituents. These variations were taken up by small adjustments of the interaction force constants and of two principal constants, K(C,CB), which varies from 5.02 to 5.27 to 5.43 mdyn/A for NiTPP, Nip, and NiOEP, and H(C,C,C,), which varies from 1.25 to 1.45 to 1.10 mdyn A/radz in the same order. The remaining principal constants are identical for the three porphyrins. The present valence force field is not comparable to the UreyBradley force field employed by Kitagawa and c o - ~ o r k e r sand ,~~ in any event the assignments are altered and extended significantly in the present work. Gladkov and S o l o ~ y o vcarried ~ ~ ~ out a calculation for CuOEP which grafted an ethyl force field onto the valence force field they developed for Cu porphine;12bthe porphine force field differs from ours in significant respects, as (10) (a) Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations, McGraw-Hill: New York, 1955. (b) Schachtschneider, J. H . Shell Development Co., Technical Report No. 57-65 and 231-264, 1962. ( I 1) (a) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1965, 21, 169. (b) Schachtschneider, J. H.; Snyder, R. G . Spectrochim. Acra 1%3, 19, 117. (Slight modifications are made to account for the -CH, .. point mass approximation.) (12) (a) Gladkov, L. L.; Solovyov, K. N. Specrrochim. Acta 1986, 42A, 1 . (b) Gladkov, L. L.; Solovyov, K. N. Spectrochim. Acra 1985,41A, 1443. (13) Cullen, D. L.; Meyer. E. F., Jr. J. Am. Chem. SOC.1974, 96, 2095. (14) Brennan, T. D.; Scheidt, W. R . ; Shelnutt, J. A. J. Am. Chem. Soc. 1988, 110, 3919-3924. ( 1 5 ) Meyer. E F., Jr. Acra Crysrallogr., Sect. B 1972, 828, 2162
*
Consistent Porphyrin
Force Field
The Journal of Physical Chemistry, Vol. 94, No. 1, 1990 49 SA
2
N-NI-N
Triclinic-A (CZh)
Triclinic-B (CZh)
Tetragonal-C (D2J
Model-D (D4) Figure I . Structural diagram of NiOEP showing the ethyl orientations for the known crystal structures, triclinic, AI3 and B,I4 and tetragonal, CI5 (the porphyrin ring is ruffled in structure C, but was kept flat for the normal-mode calculation; see text). D is a model structure chosen because of the alternating up and down ethyl orientations.
TABLE I: Valence Force Constants for NiOEP K(Ri)/(mdyn/A) H(aj)/(mdyn A/rad2) K(C&)I = 7.12 K(C,C,)2 = 6.98 K(C,N), = 5.64 K(CaCp)4 = 5.43 K(C,H)5 = 5.02 K(C?Cl), = 4.20 K ( N I N ) ~= 1.68
interaction force constantsb
H(C,NC,), = 1.62 H(C&,C,), = 1.37 H(CBC,N), = 1.37 H(C,C,C,)p = 1.10 H(C&,C,)s = 0.83 H(NC,C,)6 = 0.83 H(C&,Cl), = 1.20 H(C,C&1)8 = 1.20 H(C,C,H)g = 0.50 H(C,NNi)lo = 0.30 H ( N N i N ) I I = 0.25
‘K(R,) and H(aj)are principal stretching and bending force constants for the indicated bonds and angles. The subscripts ( i = 1-9, j = 1, ..., 16) are used to label the interaction force constants. Stretch-stretch interactions in mdyn/A; stretch-bend interactions in mdyn/rad; bend-bend interactions in mdyn A/rad2. cfi,2(Ri,Ri)= 1,2 stretch-stretch interaction (common aj); e.g., f(1,4) = fi,,(C,C,, C,C& dfi,,(R,,R,) = 1,3 stretchstretch interaction between pyrrole ring and methine bridge; e.g.,f(l,2) = fi,3(CgCB,C,C,). efi,S,(Ri,Ri) = 1,3 stretch-stretch interaction within the pyrrole ring; e.g., f(1,3) = fI,,(CgCB, C,N). f f ( R i , ~ j ) = stretch-bend interaction between R, and aj sharing one common atom; e.g., f(2,3) = fi(C,C,, C,C,N). gf2(Rr,aj)= stretch-bend interaction between R, and aj sharing two common atoms; e.g.,f(l,2) = f2(CBCB,C,C,C,). hfi(aj,aj) = bend-bend interaction between ais sharing one common atom; e.g.,f(13,14) = fl(C,CIH, C2ClH). ‘f2(aj,aj)= bend-bend interaction between ais sharing two common atoms; e.g., f(4,6) = f2(C,C,C,, NC,C,). jK(C,C,) is larger than K(C,C,) due to the point mass approximation for the methyl groups. r(C,C,) is out-of-plane wagging of C& bond relative to pyrrole ring. ‘ h ( a j , y )= bend/out-of-plane wag interaction between aj and y. discussed in t h e preceding a r t i ~ l e .Although ~ Gladkov a n d Solovyov did not have ethyl isotope d a t a , they did identify several ethyl modes correctly. Figure I is a structural d i a g r a m of t h e model used in our calculation. T h e r e a r e t h r e e crystal structures of N i O E P , A,I3 B,I4 a n d C,I5 which show isomerism of two kinds. S t r u c t u r e s A and B, which occur in triclinic crystals, have planar porphyrin cores, while structure C , found in tetragonal crystals, is ruffled. T h e ruffling permits the NiN-(pyrrole) bonds to shorten, a t the expense of some loss in T conjugation of the ring. T h e accessibility of these alternative core structures reflects the fact that the natural cavity size of a flat porphyrin ring is slightly larger t h a n the optimum low-spin Ni2+ radius.I6 T h e consequences of porphyrin ruffling for the R R spectra a r e discussed in subsequent articles,17 (16) Hoard, J. L. In Porphyrins and Metalloporphyrins; Smith, K. N., Ed.; Elsevier: New York, 1975; pp 317-376.
in which crystal a n d solution spectra a r e analyzed. In solution N i O E P adopts an intermediate structure, with diminished ruffling.’7b T h e isotope shifts, a n d therefore t h e normal-mode compositions, d o not differ significantly for planar a n d ruffled Ni0EP.I’ T o maintain a tractable analysis (separation of in- a n d out-of-plane modes) t h e normal-mode calculations were carried o u t for a flat porphyrin. T h e second kind of isomerism involves t h e ethyl orientations. In all three structures the ethyl substituents a r e oriented with the C&C2 planes perpendicular to the porphyrin plane (see structural d i a g r a m in Figure 1 for a t o m labeling). Presumably this is t h e minimum-energy orientation, steric interactions with t h e C, hydrogen a t o m s being minimized. However, t h e relative orien(17) (a) Li, X.-Y.; Czernuszewicz, R. S.; Kincaid, J. R.; Spiro, T. G. J . A m . Chem. Soc., in press. (b) Czernuszewicz, R. S.; Li, X.-Y.;Spiro, T. G.
J . A m . Chem. Soc., in press.
50
The Journal of Physical Chemistry, Vol. 94, No. 1, 1990
Li et al.
TABLE 11: Structural Parameters Used in Normal-Coordinate Analysis of NiOEP"
distance. A 1.346
bond
1.371 1.376 1.443
1.090
bond
C,-N-C,
c,-c,-c, N-C,-C, c,-c,-c, c,-c,-c,
1.958
N-C,-C, C,-c+Q-cl
1.506 1.100
C,-C,-H
1.495
c,-c0-c1 C,-N-Ni N-Ni-N
c,-C I-c2 Co-C 1-H C2-CI-H H-Cl-H
anale. dea 104.0 106.5 111.5
125.0 124.1 124.4 125.5
128.0 117.5 128.0
90.0 109.5b
109.5 109.5
109.5
Bond distances and angles were averaged from the crystal structure of NiOEP in triclinic A" and BI4 forms with slight modifications to maintain a D4,,geometry of the porphyrin core. bTetrahedral angles (l09.5") were used around C1 carbon atoms for the ethyl substituents.
tations of the eight ethyl groups differ in the three crystal structures, as illustrated in Figure 1. For A and C the two ethyl groups on a given pyrrole ring point in the same direction, but for A the ethyls point up on one pair of adjacent pyrroles and down on the other pair, giving overall c2h symmetry, while for C the ethyls point up on one pair of opposite pyrroles and down on the other pair, giving overall DU symmetry (S, if the porphyrin core ruffling is considered). The B structure departs from this pattern in that two of the pyrroles, which are opposite to one another, have one ethyl pointing up and the other down, while the remaining pyrroles retain the same mutual orientation of the pairs, up on one pyrrole and down on the opposite one; the overall symmetry is again C2h (although the 2-fold axis is rotated 45' relative to A).
For convenience in the calculation we chose the D , arrangement of the ethyl groups (structure C), but with a planar porphyrin. The Dzd subgroup of the D4h point group maintains the symmetry relationship of the in-plane vibrational modes, i.e., A,,, A2g,B,,, B2,, E, (D4,,) A,, A2, B2, B,, E ( D Z d ) .This sorting is lost in the Clh subgroup. Because of evidence from the crystal spectra that the low-frequency modes are sensitive to whether the ethyl groups are pointed in the same or opposite directions on a given pyrrole (vide infra) we also calculated the modes for a D4 arrangement of the ethyl groups, alternatively pointing up and down at successive positions around the ring; this model is shown as D in Figure I . B. Resonance Raman Enhancement Pattern. Figure 2 presents survey RR spectra of NiOEP in CHzClzsolution with excitation at 406.7, 530.9, and 568.2 nm, as well as the absorption spectrum (inset A). The RR bands are labeled according to the mode assignments which are discussed below. In the low-frequency region some of the bands arise from out-of-plane modes (labeled as oop); these are discussed in a separate There are also some weak bands which remain unassigned; these are listed in Table IV. The very different enhancement patterns can be understood in relation to the nature of the resonant electronic transitions (see Discussion in the preceding article5). The first and last wavelengths are near-resonance with the strong B and weak Qo transitions. Polarized (p) bands, arising from totally symmetric (A,,) modes are seen in both spectra, the overall enhancement ( A term) being much higher with 406.7-nm excitation. As noted for the relative intensities of these polarized bands change dramatically with wavelength. The high-frequency u2, v3, and Y., bands dominate the 406.7-nm spectrum but are much weaker than the 4, u6, and u7 bands in the 568.2-nm spectrum. The reversed relative intensities imply marked differences in the shape of the potential surfaces of the B and Q states. Remarkably, one of the strongest ethyl polarized bands in the Qo-resonant spectrum is the 1025-~m-~ mode assigned to ClC2stretching. This point is discussed below.
-
Figure 2. RR spectra in parallel (11) and perpendicular ( I ) scattering, of NiOEP (-I mM) in CH2CI2,at the three indicated excitations, which bring out selectively the A, (p) (4067 A), AO (ap) (5309 A), and B,,, B2g(dp) (5682 A) modes, wbose assignments are given by the labels (oop = out-of-plane, see ref 17a). Solvent bands are indicated with asterisks. Conditions: backscattering from spinning NMR tube; 150 mW laser power; 3 cm-' slit widths; one scan, 1 s integration time at 0.5-cm-I increments. Insets: (A) UV-visible spectrum of NiOEP in CHZCl2 (arrows indicate excitation wavelengths used for RR measurements); (B) exploded view of the 1250-1350-~m-~ region of RR spectra of NiOEP in CH2CI2solution and tetragonal crystals showing enhancement of the v 2 , (Azg)skeletal mode with B-band excitation (4067 A).
The strongest bands in the 568.2-nm spectrum are depolarized (dp), and arise from B!, and B2, vibrations. They are enhanced via Qo-B vibronic mixing ( B term), which also gives rise to the Q, absorption sideband. Also enhanced via vibronic mixing are A2, modes, which produce anomalously polarized (ap) bands. These are, however, much stronger when excited at 530.9 nm, between the Qo and Q I resonances. This A2dB1,(B2J selectivity is due to interference between Qo and Q I contributions to the Raman scattering tensor. The interference is constructive for A2g but destructive for B,, (B2,) modes at wavelengths between Qo and Q,, while the signs reverse for wavelengths above Q, or below QO.l8 The appearance of several depolarized bands in the 406.7-nm spectrum is likely due to Jahn-Teller activity in the B excited state.19 We note that one AZgmode, u2, at 1309 cm-I, is detectable with 406.7-nm excitation, as seen in the Figure 2, inset B. (Other A2, frequencies fall under enhanced p or dp bands.) The Q-B vibronic mixing should produce some enhancement in resonance with the B, as well as the Q transition, but this is overwhelmed by the stronger Franck-Condon and Jahn-Teller mechanisms. By comparison with the 704-cm-I band of the CH2C12solvent we estimate that the u2, intensity ratio between 406.7- and 530.9-nm excitation is about 1/50. The 406.7-nm wavelength is somewhat off resonance (Figure 2, inset A), and the intensity is expected to increase on resonance (395 nm). Thus Q-B mixing can explain the u 2 , enhancement. (18) Spiro, T. G.; Stein, P. Reu. Phys. Chem. 1977, 28, 501. (19) Shelnutt, J . A,; Cheung, L. D.; Cheng, C. C.; Yu,N.-T.; Felton, R. H.J . Chem. Phys. 1977, 66. 3381.
The Journal of Physical Chemistry, Vol. 94, No. 1. 1990 51
Consistent Porphyrin Force Field
>* OEP-n.a.
900
1100
1300 A? i cm”
1500
17(
Figure 3. 4067-&excited RR spectra of natural abundance (na) NiOEP (top) and its meso-d, (middle) and methylene-d16(bottom) isotopomers, in CS2. I5N shifts are given in parentheses. The dotted lines correlate polarized bands (A,*). Conditions as in Figure 2.
These wavelength-dependent selectivities are very helpful in unravelling the complex and overlapping mode structure of the porphyrin vibrational spectrum. Essentially all of the Ramanactive in-plane fundamental modes can be identified from the spectra obtained with these three excitation wavelengths. C. Skeletal Mode Assignments. Figures 3,4, and 5 compare RR spectra obtained with 406.7-, 530.9-, and 568.2-nm excitation, respectively, for NiOEP and its meso-d, and methylene-dl, isotopomers in CS, solution. The dashed lines correlate modes having similar compositions, as judged by the relative intensities, and by the 15N isotope shifts, shown in parentheses, which are available for the natural abundance and meso-d, species. The I5Nspectra, which are of the same quality as those shown, are omitted for clarity from Figures 3-5. They are included, however, in Figure 6, in order to show how they help to resolve the complex 9001 2 0 0 - ~ m -region ~ (vide infra). Figure 7 shows 600-900-~m-~RR spectra with 568.2-nm excitation (CH2Br2solution), which serves to expose all the modes assignable in this region, while Figure 8 shows RR spectra below 600 cm-I, obtained with 530.9-nm excitation (CS2solution). We note that out-of-plane modes are also seen below 900 cm-l; these are discussed ~ e p a r a t e l y . ~Finally, ’~~~ Figure 9 shows IR spectra for NiOEP and the methylene-dl, isotopomers; IR spectra for the other isotopomers have been reported recently.20 Table V lists our in-plane porphyrin skeletal mode assignments using the local-coordinate scheme introduced in the preceding article, in which the phasings of adjacent bond stretches and the collective nature of the pyrrole ring vibrations are taken into account. The mode numbering scheme is the one introduced by Kitagawa and c o - ~ o r k e r s .Our ~ assignments modify and extend theirs in several significant respects, as discussed in the following paragraphs. Ddhsymmetry labels are used for the skeletal modes, (20) Kincaid, J. R.; Urban, M. W.; Watanabe, T.; Nakamoto, K. J . Phys. Chem. 1983, 87, 3096.
A? i cm-’
Figure 4. As Figure 3, but with 5309-A excitation. Correlations are shown for anomalously polarized bands’ (A2,).
AV i cm”
Figure 5. As Figure 3, but with 5682-A excitation. Correlations are shown for depolarized bands (B,,, B2,).
to conform with previous usage, recognizing that the symmetry is lowered by the ethyl groups in their energetically favored conformations.
52
Li et al.
The Journal of Physical Chemistry, Vol. 94, No. 1 , 1990
200
400
300
600
500
A? I cm-1 Figure 8. 5309-&excited RR spectra of NiOEP in CS2 in the 200600-em-’ region. 15Nshifts given in parentheses. The pairs of bands marked v8 and u9 are believed to arise from molecules with different ethyl orientations (see text). Conditions as in Figure 2, but three scans.
NiOEP
Y
NiOEP-d,8
I 00
l 1500
l
l 1300
l
l 1100
l
l 900
l
l 700
l E
cm.1 I
600
7 50 A? I cm.1
900
Figure 7. 5682-A-excited RR spectra of NiOEP in CH2Br2(solvent band indicated by asterisk) in the 550-950-em-’ region. Conditions as in Figure 2.
Aside from the unobserved C,H stretches at -3000 cm-l, all skeletal modes have been identified with the exception of one A,,
Figure 9. IR spectra of NiOEP and its methylene-& isotopomer obtained in KBr pellets with a Digilab m 2 0 C Fourier transform infrared spectrophotometer.
mode (v,~) and three E, modes (u3,, u41, us,,). 1 . A , , Modes. a. Conformation-Dependent us and ug. In solution NiOEP displays a prominent doublet of polarized bands at 360 and 343 cm-I. Because only one A,, skeletal mode, us, is
The Journal of Physical Chemistry, Vol. 94, No. 1. 1990 53
Consistent Porphyrin Force Field
"9
250
Solid plaw-NIOEP, 12K
300 A; / cm-1
350
Figure 10. 5682-A-excited low-frequency 12 K RR spectra of two samples of solid NiOEP (planar form) obtained by evaporation from CH2CI,. The 357- and 350-cm-' us bands are assigned to the A and B structures, on the basis of the single-crystal spectra of Brennan et a1.;I4assignments of u9 and v17 to A and B forms are based on the intensity correlations with us. Conditions: backscattering from KCI pellet attached to a cold finger of Model CSA-202E closed-cycle liquid He refrigerator (Air Products, Allentown, PA),26100-mW laser power, 2 cm-' slit widths, 1 scan, I-s integration time at 0.5-cm-' increments.
expected in this region, Abe et al. assigned these features as a Fermi doublet, arising from the near-resonant interaction of v g with an overtone mode, suggested to be 2v35.3bHowever, Brennan et aI.l4 have recently found that RR spectra from single triclinic crystals containing the A or B forms of NiOEP show only a single band, at 342 cm-l for A and 354 cm-I for B. Consistent with this result, we find, as shown in Figure 10, that the low-temperature ( 1 2 K) RR spectra of different solid NiOEP samples, obtained by rapid evaporation of CHzClzsolvent, have very different relative intensities of the 350- and 357-cm-I bands. The variability among samples is in line with the observation of Brennan et al.I4 that it is difficult to isolate pure crystalline A or B, and that mixtures are usually obtained. Figure 10 reveals additional closely spaced doublets at 283/289 cm-' and 316/309 an-', the relative intensities of which also differ for the different preparations. The 289- and 309-cm-I components are strong when the 350-cm-' band is strong, and likewise for the 283-, 316-, and 357-cm-I bands. Thus we associate the 283-, 316-, and 357-cm-l bands with B and the 289-, 309-, and 350-cm-' bands with A. The 283/289-~m-~ bands correspond to the 263/274-cm-I solution bands which are assigned48to vQ while the 309/316-cm-I bands are assigned to ~ 1 7 . (In solution only a weak shoulder can be seen, at -305 cm-I.) The v8 and ug modes are strongly coupled Ni-pyrrole breathing and Cgethyl bending motions, as can be seen from their eigenvectors, shown in Figures 13-15. They are both strongly perturbed by methylene deuteration (Figure 8); in solution the v8 bands shift down from 360 and 343 cm-l to 350 and 324 cm-l while the v9 bands disappear into a complex envelope of bands around 238 cm-I. Thus it is reasonable that these bands might be sensitive to the ethyl orientations, which differ in A and B, as discussed above. In particular a kinematic difference might be produced by having one ethyl point up and the other down on a given pyrrole, as in B, instead of having both point in the same direction as in A. To test this hypothesis we calculated the vg and v9 frequencies and methylene-dl, isotope shifts for a D4 model in which the ethyl
pairs point in opposite directions on all four pyrrole rings (pattern D, Figure 1). As shown in Table VI, the calculated frequencies and d16 shifts did differ significantly for the D2dand D4 models (Figure 1); the force constants were left unaltered for this comparison. These calculations do not accurately model the A and B structures, which have lower symmetries and different arrangements of the ethyl groups, but they demonstrate that the kinematic effects of ethyl orientation influence v8 and v9. The B1, mode /)I7 is calculated (Table 111) to be mainly CB-ethyl bending in character and is therefore also subject to orientation effects. On the basis of this evidence for sensitivity to ethyl orientation, we attribute the v8 and v9 doublets observed in solution to ethyl conformational isomerism. It is not obvious, however, why there are two bands of nearly equal intensity. One might have expected a statistical distribution of rotamers in solution with a continuous range of v8 and v9 frequencies, producing a single broad envelope. Either the distribution is not statistical, or, more likely, the frequencies are not continuously distributed but cluster around the two observed values. The kinematics of the various possible conformers remain to be investigated. b. Other A , Modes. Abe et al.'s assignment of v5 to the 1024-cm-' R R bands3b has been challenged by Choi et aL4 and by Gladkov and Solovyov,lz who pointed out that the 1138-cm-' band is a better candidate since it is more strongly enhanced with B-band excitation (see Figure 3). The isotope data clearly show that the 1024-cm-' band is a composite of overlapping ethyl modes (see below), and confirm the 1138-cm-' assignment of v5. The remaining AI, assignments are in agreement with those of Abe et al.,3balthough the normal-mode descriptions differ somewhat. Thus the prominent v4 band, which is the AI, pyrrole half-ring stretch, is calculated by Abe et al. to have a significant (21%) methine bridge C,H bending contribution. Their predicted 5-cm-I meso-d, shift is unsupported by the data, however, which show no more than a 1-cm-' shift. Likewise their calculated 1 1-cm-' meso-d, shift for the 804-cm-l pyrrole breathing mode v6 is too high, the actual shift being 5 cm-I. 2. B , , Modes. In the B,, block Abe et al.3bassigned only four at 1655, 1577, 1220, and 751 cm-I. modes, vlo, vI1, v13, and We have now identified all nine modes in the RR spectra. One of them, vlz, is found at 1331 cm-' in the meso-d, but not in the natural abundance spectrum (see Figure 5). Its assignment to the B,, half-ring stretch is secured via the I5N shift (6 cm-I). It is calculated to mix appreciably (18%) with C,H bending, and a 12-cm-I meso-d, downshift is expected; adding this to the observed meso-d4 frequency gives 1343 cm-' as the assigned value for the natural abundance species. Another newly assigned mode, vi4 at 1 131 cm-I, is quite weak in the natural abundance spectrum but is seen clearly in the perpendicularly polarized spectra, especially for the I5N isotopomer (see Figure 6,406.7-11111 excitation). In the meso-d4 spectrum it shifts up to 1186 cm-l due to relief of its interaction with the C,H bending mode, vI3, at 1220 cm-I, which shifts down to 948 cm-' upon meso deuteration. The altered coupling also produces a switch in the I5N shifts of vi3 and ~ 1 4 , from 2 and 12 cm-I in the natural abundance molecule to 10 and 2 cm-l in the meso-d, isotopomer. In effect the 6(C,H) and v(C,C,) (mixed with pyrrole breathing) modes switch character in concert with the large deuteration frequency shifts. Coupling among all three modes, v12, ~ 1 3 ,and v I 4 is manifested in the intensity pattern. In the natural abundance molecule ~ 1 is3 strong (568.2-nm excitation, Figure 4 and 6) while ~ 1 is4 very weak and v l z is undetectable, but in the meso-d, isotopomer all three bands are of medium strength; the total intensity is conserved. Evidently the eigenvectors of the three modes are phased in a way that concentrates the vibronic coupling strength in v13 but redistributes it upon meso deuteration. at -751 and -742 cm-I, respectively. We assign v15 and The two modes are unresolved in solution but can both be seen in low-temperature crystal They are readily distinguished in the meso-d, spectrum, shifting up to 762 cm-' and v I 5 shifting down to 683 cm-' (Figure 7). As discussed above, v 1 7is seen as a weak -305-cm-I shoulder but is resolved into a
Li et al.
54 The Journal of Physical Chemistry, Vol. 94, No. 1, 1990
TABLE III: Observed and Calculated Vibrational Band Frequencies and I5N Isotope Shifts ( c d ) for Skeletal and Etbyl Substituent Modes of NiOEP and Its Meso-dr and Metbylene-dlr Isotomerso Ob." nhrd cakd - obsd calcd "I AI5N met-dI6 met-d,, A,, Skeletal Modes
1599
3044 1597
I 508
1
1376
7
1519 1380
1512 1378
1118
9 9
1068 780
1048 770
0
1601
1
2263 1598
1602
1
1520 1383
I 7
1517 1380
1 7
1512 1382
1
7
I138 a04
7
1120 827
9 9
1138 799
7 4
5
0
0
3041 1604
824
0
674 360/343*
3
690 346
0 1
668 353/343b
2 010
672 345
0
211
1
674 350/324b
686 316
263/274b
211
256
0
262/274b
21 1
256
0
238
223
0 1 2 0
1456 1345 1276 763 1019
0 1
1149 842
1
1456 1345 1276 764
0 1
643 1189
1167 878 901 660 1220
0 1
1655 1571
2
1642 1578 1318 976 1 I90
6 7 3
1243 1075
1657 1567 1329 1283 1053
1464 1316 1258 776 1024
0 0 1 0
0 1
6 2 10
1645 1576 1331 948 1 I86
759
5
683
2
658
1
754
742
0
733
1
762
3
753
2
7376
71 1
305 168
0
315 168
0
0
o
305 168
0 0
315 168
0 0
164
274 166
CHI scis CH2wag
1463 1312
0 0
1459 1359
0
1463 1312
0
1459 1357
0 0
1171
0
1174 903
CH2 twist CH2rock
1276 781
0 1
1284 782
3
78 1
1
1276 782
I 3
a32d 643
871 663
r(CIC2)
1024
3
1019
0
1031
0
1019
0
1207
1208
"19
1603 I393
1 0
1601 1402
0 3
1581 1393
1 0
1579 1395
0
"20
2
1601 1389
1601 1394
"21
I307
4
1327
4
887
4
a55
1
1302
1284
JJ22 "23 y24
1121
15
1202
1
1058
597
1
3
582
12 0 1
1217 1075 623
a
0
1127 1055 626
9
1058
1109 991 576
1104 999 546
"25
55 I
0
2
545
0
559 243
1 0
504
526 227
1461 I309
0 0
1459 1350
0 0
1149
1 I55
1260
0
1306
2
903 809
CHI scis CH2wag CH, twist CH2rock v(CIC2)
1464 1316 1258 770 1024
0 0 1
3
1018
1
"IO
1655 1577
0 1
0
y14
1220 1131
2 12
1658 1578 1330 1238 1150
"IS
75 I
3
-740'
"1 I
"12 "I3
p16
y17 VI8
1
1
3
6
IO
1
2
1 2
566 243
0
1459 1351
0 0
1236
2
CHI rock
842
6
a4 I
7
U(CIC2)
1014
0
1016
1
3041 1486 1403
0
0
"26
CHI scis CH, wag
1462
CHI twist
1252
0 0
A2 Ethyl Substituent Modes 6(HCH) (76) + B(C2C,H) (11) ~ ( C & I H )(41) + B(C,C,H) (35) + u(C&,) (16)
856
1177
1225
I
1479 1405
3041 1485 1383
B,. Skeletal Modes "27 "28
1478 1405
2 1
226 1 1475 1403
11
1159 1003
9 IO
1160 980
6 12
1096 988
1062 999
945
0
934
0
945
0
973
968
515
I
490
1
514
1
472
466
3
"29
1483 1407
"30 y3 I
I I59
IO
1015
9
1160 1004
"32
938
0
u33
493
1
1
6 1
6
3
Consistent Porphyrin Force Field
The Journal of Physical Chemistry, Vol. 94, No. 1, 1990 55
TABLE 111 (Continued)
y34
obsd na AI5N 197 0
u35
144
VI
0
calcd na AI5N assignment (PED, %) 204 0 u(C,C~)(11) + u(C,C,) (10) + 6(CnCiC,) _ . - (12) + a'cc,c,, (9) + S(C,C,C,) (9) 151 0 b(C,NNi) (28) + 6(C,Cm) (21) + 6(C8C,) (13) + B(NNiN) (12)
obsd calcd meso-d4 AI5N meso-d4 AI5N 197 0 203 0 141
0
150
0
obsd met-d16 194 144
calcd met-d16 198 137
CHI scis CH2 wag
1459 1350
0
1458 1349
0
0
0
1178 845
CH2 twist CH2 rock 4CIC2)
I269 657 1023
0 0 2
1269 657 1021
0 0 1
878 569 1231
b6 y37 Y38 y39
0 0 0 3 2
1604 1494 1392
0 0 0
2262 1620 1586 1484 1400
0 0 0 4 1
1380
304 1 1637 1579 1493 1391
948
8
I342 888
7 2
1233
1349 1289
1071 1018/ 1026 974
1080 1043 990
1604 1501 1396
0 0
3041 1637 1588 1496 1404
1231
3
1346 1240
5 1
8 12 5
1144 1130 1004
IO
y45
1153 1133 996
6 3
1185 1151 996
8 11 5
1212 1129 1048
7 6 4
v46
927
3
922
7
919
3
915
10
791
5
766
5
79 1
5
698
699
667
554
355/341' 31P 26V
484 342 313 260 151
y4fl
"4 I
y42
y43 y44
0
y47
Y48
605
0
615
0
600
0
603
0
u49
544
0
534 358 317c 293' 167
2
543
0
2
y50
y51 y53
CH2 scis CH2 wag
0
322' 26V 212e
0 1 0
531 357 315 292 167
0 0 0
1454 14401 1380
0 0 0
1459 1456 1355
0
0
1459 1457 1356
0
1354
3
1322
0
1350
0
854
1307
2
1287
3
90 1
1273 76 1 69 1
2
1015 1019
1
328' 263' 212e
0
1456 14401 1378
0 0
1323
2 0
CHI twist CHI rock V(ClC2)
935
1
1 0
1275 754 726
0 0 4
1273 761 728
3
1021
0
1029 1019
2 1
+ u(C,N) + 6(C,H)
(25)
+
(16)
+ 1267
1
1
6(NC,C&(19)
+
726
0 4
1
1 0 0
0 0
1 1 1
1137f
677 646
1172 1163 866
881 664 72 1 1229 1215
#Observed values from CS2 solution RR spectra (A and B modes), and from matrix-isolated (na and meso-d4molecules and their "N isotopomers)20and KBr pellet (met-d16isotopomer, room temperature) IR spectra (E modes). na = natural abundance; meso-d, and met-d16 = 2H substituted analogues at meso carbon and methylene group positions, respectively. AI5N = shifts upon I5N-pyrrole substitution in na and meso-d, species; not available for methylene-d16isotopomer. PED = potential energy distribution (%). *Pairs of frequencies attributed to ethyl orientational isomers. 'Hidden by uIs in na solutions, "These bands might also arise from the out-of-plane modes.17* 'Observed in 12 K RR spectra of tetragonal crystals.17P/Seen in both solution and crystal RR spectra.
A/B doublet at 309/316 cm-l in triclinic crystals. The remaining B,, mode, u I 8 , is found at 168 cm-I (not shown). 3. A2, Modes. Abe et al.3b assigned five out of the eight expected A2, fundamentals, u19-u2z and ~ 2 4 . We agree on the u ~ ~ assignments. - u ~ ~ Their ~ 2 candidate, 4 739 cm-I, corresponds in our spectra to a depolarized band, not an anomalously polarized band; we assign this band to an out-of-plane mode.17a The u24 mode is assigned to the ap band at 597 cm-I (Figure 8); the calculated frequency is 626 cm-I. This mode, whose natural frequency is -750 cm-', interacts strongly with a methylene rocking mode (calculated at 842 cm-', Table 111) and is shifted down for this reason. We also identify u25 with the ap band at 551 cm-I (Figure 8). (It may be that the ~ 2 and 4 u25 assignments should be reversed, since the 551-cm-I band has the larger d16 sensitivity, as expected for u24. In this event one would have to 4 the A2 methylene conclude that the interaction between ~ 2 and rocking mode is substantially underestimated in the calculation.)
The ~ 2 mode, 3 which involves CBClstretching, is identified with a weak ap band at 1058 cm-' (Figure 4) on the basis of its 67-cm-' methylene-d16 shift. Only the lowest frequency A2, band, v26, predicted at 243 cm-', remains unidentified. The intensities of A2, RR bands are expected to diminish with decreasing frequency due to interference between 0and 0-1 resonances.l* The lowest frequency A2 mode so far detected is the 433-cm-I u25 band of Ni porphine. ! 4. BZgModes. Abe et al.3bfound candidates for three of the 9 1407 cm-I, ~ 3 at 0 1159 cm-I, and ~ 3 at 2 785 nine B2 modes, ~ 2 at cm-I. b e agree with the ~ 2 and 9 ~ 3 assignments 0 but not with the ~ 3 assignment 2 since the 785-cm-l band (781 cm-' in our spectra; see Figure 7) disappears upon methylene deuteration and is assigned to a methylene rocking mode. Instead we assign a weak dp band at 938 cm-I (Figure 6 ) to ~ 3 2 . Another weak dp band in this region, at 1015 cm-', which shifts 9 cm-I upon 15N substitution is assigned to ~ 3 1 . A fairly prominent 1483-cm-l dp band
56
Li et ai.
The Journal of Physical Chemistry, Vol. 94, No. I , 1990 C
TABLE IV: Currently Unassigned RR Frequencies ( c d ) for NiOEP and Its Meso-d, and Metbylene-d16 Isotopomers' P na A% meSO-d4
p ap aP ap
1502 1368b
7 0
p
1126 (sh)< 1091
aP aP p p dp ap
1060 960' 960' 922'
0 IO 0 3 3 0
met-dl6
1368b 1230 1125 1090
0 0
1060 960
0 3
938
0
0 8
1506 1366b 1280 1 177d 1085 1080 1036 1053
"See footnote a in Table H I . bThese bands migth be due to the methyl in-phase deformation modes, which are expected near 1370 cm-1.20Coverlapswith v22 band (1 121 cm-I), but is clearly resolved in the 15N spectrum. dThis band might be due to the A, C,-C2 stretching mode (see Table 111). (These bands, which disappear in the met-d16 spectra are tentatively assigned to C H 3 rocking modes; see discussion of ethyl mode assignments.
in the 568.2-nm spectrum (Figure 5) is v28 as previously assigned by Choi et aL4* Candidate bands for v j 3 , v34r and v j 5 are found at 493, 197, and 144 cm-I (Table 111). 5. E, Modes. The E, assignments are taken from the recent study by Kincaid et aL20 Three low-frequency modes, vS1, vsz, and u s ) , are assigned from low-temperature RR spectra of tetragonal crystals, where they are activated by the loss of the molecular center of symmetry.17aSeveral E, modes have not been assigned, and the calculated values are listed in Tables I11 and V. There is an assignment problem concerning u37, which is calculated to be close to the B1,asymmetric C,C, stretch, vlo, at 1655 cm-l. No TR band is seen in this region, but a meso-d4sensitive (1 1 cm-') band is seen at a much lower frequency, 1575 cm-l. A RR band in the 1550-cm-I region was assigned to v37 for Ni and Fe protoporphyrin complexes on the basis of its core-size sensitivity, which indicates a large C,C, stretching contribution. The asymmetric disposition of the protoporphyrin vinyl substituents destroys the molecular inversion center and permits R R activation of E, modes. I n the calculation, however, it is very difficult to produce the 100-cm-I separation between vi0 and v37 required to bring the frequency of the latter into agreement with the experimentally indicated value. The asymmetric C,C, stretches mix very little with other modes, so that the only distinction between vIo and vj7 is the phasing of the bridge motions on opposite sides of the porphyrin ring (same phase for B,,, opposite phase for E,,). Thus
CH,
Scissors
CH,
Wagging
CH,
Twisting
CH,
Rocking
c/
Ir" '
Out-of.Phase C-C Stretch In-Phase C-C Stretch Figure 11. Diagram of ethyl local modes. In O E P the three C atoms are C,, C , (methylene), and C2 (methyl). In the normal-coordinate calculation the methyl groups were treated as 15 amu point masses. The 6(CBC1C2)coordinates (not shown) were included, but their local modes are considered separately as part of the out-of-plane mode analysis.17a
a significant cross-porphyrin interaction force constant would be required to separate the modes. We are reluctant to invoke such an interaction, since all the other frequencies can be satisfactorily accounted for with interaction constants involving no more than a one-bond separation. This problem is left for future investigation. We note that Abe et al.3bfit the 1604-cm-' IR band to the C,C, asymmetric stretch and the 1575-cm-I band to the C,C, stretch. However, the 1604-cm-I band shows no sensitivity to meso deuteration,20 while the meso-d, shift of the 1575-cm-l band is 11 cm-I. The calculated shifts of Abe et al. give the reverse pattern, showing that these assignments are unsatisfactory. A point of some interest is the mode crossing induced by meso deuteration in those blocks which contain C,H bending modes, B,, (discussed above), AZg,and E,. These bending modes are found at 1220 ( ~ 1 3 ) , 1307 (vZI), and 1231 (v4J cm-l, respectively, and shift down in the meso-d4 isotopomer to 948, 887, and 948
TABLE V: Allocation of NiOEP Skeletal-Mode Frequencies (cm-I) to Local Coordinates" local modeb AI, B', A2s 4Cm-H) 4Cu-Cm)asym 4C,-C,) V(Cu-Cm)sym u(Pyr quater-ring) u( Pyr half-ring),,, 6(Cm-H)
V I [3041]'
v(c,-Cl)s
~5
m
u(Pyr hal[-ring)a8ym ~ ( C O - C)asym I ~ ( P Ydef)asym F u(Pyr breathing) 6 ( P ~ def),,, r 6(Pyr rot.) u(NiN) 6 ( C,-C I 1arym J(C,-C, ),ym 6(Pyr transl)
uII
vi9
v4 1383
1138
1603 ~ 2 81483
w l 2 1343d V I 3 1220 v14 1131
1121 v2, 1058 P24 597 804 674
VIS
~7
uI6
751 746'
vg
3601343'
PI8
168
ug 2631274'
~ 2 91407
~ 2 11307
v22
u6
[30411
1577 ~ 2 01393
E"
B2* *27
u I o 1655
1602 u3 1520 ~2
1
c,
C
1159 v3' 1015 ~ 3 938 2
uj0
P~~
551
Y33
493
"26
[2431
Y~~
197
v I 7 305 ~ 3 5144
[30401 v37 [I6371 ~ 3 81604 ~ 3 91501 u40 1396 "41 113461 ~ 4 21231 vM 1153 w4, 1133 ~ 4 5996 v46 927 v47 766d u48 605 v49 544 Y ~ O[3581 uSI 328' ~ 5 263' 2 Y 5 , 212c Y36
"Observed values from CS2 solution R R (A, and B, modes) and matrix-isolated IRZ0(E, modes) spectra. bSee Figure 2 of the preceeding paper for illustration of the local coordinates. [ ] calculated frequencies; not observed. dObserved only in the meso-d4 isotopomer and its I5N double isotopomer; not observed in the natural abundance species. Adding the calculated d4 shift, 12 cm-I, to the 1331-cm-l meso-d, frequency gives 1343 cm-' as the assigned value for u 1 2 . 'These frequencies from 12 K RR spectra of tetragonal ~ r y s t a 1 s . l ~/Pairs ~ of frequencies attributed to ethyl orientational isomers.
Consistent Porphyrin Force Field
The Journal of Physical Chemistry, Vol. 94, No. I, 1990 57
TABLE VI: Ethyl Orientational Effects on u8 and u9 Frequencies (cm-I) of NiOEP
TABLE VII: In-Plane Skeletal-Mode Frequencies (cm-') and Local-Mode Assignments for Ni(I1) Complexes of OEP, Porphine, and T P P
obsd"
triclinic A
A(dI6)'
triclinic B
y8
350 289
29 8
357 283
y9
calcdc y8
y9
D2d
346 256
A(d16)
30 33
A(dl6) 15
sym
ui
description*
11
0 4
385 284
20 35
"Observed RR frequencies for the A and B triclinic forms at 12 K. bA(d16)= shifts upon 2H substitution at the methylene positions. 'Calculated with ethyl orientations corresponding to models C (D2& and D (D4) in Figure I . cm-'. The modes immediately below them, v14 (1 131 cm-I), uzz (1 121 cm-I), and v43 (1 153 cm-I) show meso-d, upshifts, to 1186, 1202, and 1 185 cm-I, reflecting the relief of 6(C,H) coupling. D. Ethyl Modes. The RR and IR spectra contain several extra bands which are attributable to the ethyl substituents on the basis of their sensitivity to methylene deuteration. In the calculation the ethyl groups were modeled with full inclusion of the CH, atoms but with the methyl groups approximated as 15 amu p i n t masses. Figure I I shows local modes of the ethyl groups in this model (not including the C-H stretching modes, expected in the 2900-cm-I region). The 6(C&Cz) coordinate is omitted in the figure because it is allocated to the out-of-plane modes, which are treated in the next paper;17athe coordinate was included in the present calculation, however. Internal modes of the methyl group?' aside from C,Cz stretching, are not represented in the model, but they do influence the spectra. The IR spectra contain bands that may be due to methyl in-phase and out-of-phase deformation modes, which are expected at 1378 and 1463 cm-', respectively; they do not shift upon methylene deuteration. A weak anomalously polarized band at 1368 cm-l may be due to in-phase CH3 deformation (Table IV). More significantly, the methyl rocking mode manifests itself in the RR spectra, as discussed below. Not only are candidate bands found for the remaining local modes (Table 111) as evidenced by large methylene deuteration effects, but in each case there are multiple bands with polarized, depolarized, and anomalously polarized representatives. The multiple bands are associated with the phasing of the local modes on the eight ethyl substituents, the resulting couplings with each other and with the skeletal modes affecting the normal-mode frequencies somewhat. The polarizations indicate that the overall symmetry of the NiOEP skeleton is preserved at least approximately. They are consistent with the DZdmodel used in our calculation, in which the mode symmetries are AI (p), Az (ap), and BI and B, (dp). Lower symmetry is not excluded, however, since the polarization properties could be retained approximately even if the symmetry labels are formally lost due to ethyl group rotations. The methylene deformation (CH, scissors) is expected2' at 1460 cm-I. Weak bands are seen near this frequency which are polarized, anomalously polarized, and depolarized with 406.7-, 530.9-, and 562.8-nm excitation, respectively. All of these bands are expected to shift dramatically upon methylene deuteration, down to 1 170 cm-I, and p, dp, and ap bands are found in this frequency region in the methylene-d16spectra. They are therefore assigned to overlapping phasings of the CH, deformation modes corresponding to A,, Az, and B1or B2 symmetry, respectively. An additional weak ap band is seen at 1440 cm-' in the 530.9-nmexcited spectrum (Figure 4), which shifts down to 1137 cm-I upon methylene deuteration. We tentatively assign this band to the expected E CH, deformation; the E representation in the DU point group can contain antisymmetric scattering tensor components. The band was not detected in the IR spectrum, in which this region is dominated by the methyl deformations.*' Methylene wagging and twisting modes cover a range of frequencies in different compounds2' but are generally not far from
-
-
(21)Colthup, N. B.; Daley, L. H.; Wiberley, S. E. Introduction to Infrared Raman Spectroscopy; Academic Press: New York, 1975;pp 223-233.
NiOEP
NIP
NiTPP
[3041]' 1602 1520 1383 I138 804 674 3601343 2631274 1655 1577 1331f 1220 1131 75 1 7468 305 168 1603 1394 1307 1121 1058 597 55 1 [2431 [3041] 1483 1407 1160 1015 938 493 197 144 [3040] [1637] 1604 1501 1396 [ 13461 1231 1153 1133 996 927 766e/ 605 544 W81 3288 263s 2128
130421 -157Sd 1459' 1376 [3097] 995 73 1 369 1066 1650 1505
1235 1572'
1319,
1185 [3097] 1003 730 1060 237 1611 1354 1 139h 1005 [3087] 806 429 1317h [3041] I504 1368 1003 [3088] 819 435 1193 197 [3042] 1624 1547 1462 1385 1319 1150 [3097] 1033 [3087] 806 995 745 366 420 1064 1250 282
1470d 1374 [3097] 1004 889t 402 1079 1594 1504 1302 238 [3097] 1009 846 1084 277 1550 1341 P571 1016 [3087] 828 560 1230 1269 [1481] 1377 1004 [3087] 869 450 1191 109 [1254] [ 15861 [1552] [1473] [1403] [1331] [2331 [3097] [loo31 [3100] [a641 [I0231 [8951 [5121 [4361 [lo931 [1213] [3061
A and B mode frequencies from CS2solution (NiOEP, Nip) and crystal (NiTPP) RR spectra at room temperature; E mode frequencies from matrix-isolated (NiOEP)20and room temperature KBr pellet (NIP) IR spectra. bSee preceding paper for local coordinates definitions. X, Y = H, H (Nip); H, C2HS (NiOEP) and C6HS,H (NiTPP). [ ] calculated frequencies; not observed. dSee preceding paper for the detailed discussion of the C,-C,/ C,-C, mixing in NiP and NiTPP. preceding paper for the discussion of the mixing of phenyl and pyrrole deformations. /Observed only in the meso-d4 isotopomers. 8These frequencies from 12 K RR spectra of tetragonal ~ r y s t a 1 s . l h~S~e e preceding paper for the discussion of the 6(CB-H)/
b(C,-H) mixing in Nip.
-
1300 cm-l in alkyl groups. Polarized, depolarized, and anomalously polarized bands are found at 1315 cm-I, assignable to CH2 wagging, and at 1250-1280 cm-I, assignable to CH2 twisting. These modes are expected to fall into the 850-cm-' region upon D / H replacement. Candidate polarized and depolarized bands are found at 842 and 832 cm-I in the methylene-d16 spectrum. For the CH, rocking modes, expected in the 760-cm-l region, candidate polarized and depolarized bands are found at 770 and 781 cm-l (Figure 7). The CDz counterparts are expected near 670 cm-I, a position obscured in the methylene-d,, spectrum by another band due to a skeletal mode (q). In alkyl chains there is pronounced mixing between adjacent CC stretches to produce in-phase and out-of-phase combinations.
Li et al.
58 The Journal of Physical Chemistry, Vol. 94, No. 1, 1990
(a)
(b)
JI (Porphyrin) + On(Ethyl)
'-GI., Figure 12. Diagram of hyperconjugative interaction in O E P which is proposed to account for ethyl mode RR enhancements. The out-of-plane ethyl orientation optimizes the orbital alignment (a). The ethyl arporphyrin a,, interaction (b) modulates the electronic transition energy and induces excited-state origin shifts and vibronic couplings along ethyl coordinates.
In addition the rocking mode of the terminal methyl group mixes strongly with the in-phase CC stretching combination. The result is a set of three modes,2' at 1140 cm-l (in-phase stretching primarily), 1050 cm-l (out-of-phase stretching primarily), and
-
-
-900 cm-l (methyl rock primarily). This pattern is manifested in the Raman spectra of NiOEP, as can be seen most clearly in the data with 568.2-nm excitation (Figures 5 and 6 ) . There are three prominent polarized bands, at 1138, 1024, and 960 cm-I, all of which are strongly perturbed in the methylene-& spectrum. These are assigned respectively to the A, combination of C,C, stretching (skeletal mode v5), ClC2stretching and methyl rocking, recognizing that these modes are strongly mixed (see Figure 16). The B, combinations of these coordinates can also be seen. The C,C, stretch is skeletal mode v i 4 at 1131 cm-I, while the ClC2 stretching and methyl rocking modes are at the same frequency as the A I modes (reflecting very weak couplings among these spatially distant coordinates, as expected). The mode overlaps can be seen from the mixed polarization (-0.5 depolarization ratio) of the 1024- and 960-cm-' bands and is clearly demonstrated by detectable mode separation in the meso-d4 spectra (Figure 6). The depolarized components shift up and down, to 1031 and 934 cm-I, respectively, upon meso deuteration (see 568.2-nm excitation), leaving only the polarized components at the original position (the 1024-cm-I band splitting is especially clear in the ISN,"Id4 isotopomer). These shifts are induced by the interaction with the C,H(D) bending coordinate (also responsible for the vi4 upshift, to 1186 cm-I, on meso deuteration, as discussed above), which is limited to the B,, block, there being no C,H bending mode of
Alg Modes NiOEP
NIP
NIOEP
NIP
1604
1564
1517
1459
1380
1384
1120
3097
a27
1020
690
735
346
378
256
1079
v4
Figure 13. Selected porphyrin skeletal mode eigenvectors compared for NiOEP and NiP (left and right members of each pair). The arrows are scaled displacements of the atoms. For NiOEP the terminal methyl carbon atoms (methyl H's not included in the calculation) are directly above or below the methylene carbon atoms.
Consistent Porphyrin Force Field
The Journal of Physical Chemistry, Vol. 94, No. 1, 1990 59
Modes
1658
@#
#e @e I
"'
1649
1578
1197
759
..
1529
.a
1238
"13
168
733
1015
%R
245
710
Modes
%\12@ 1403
1486
-6 1160
1381
1492
"30
1036
204
.'.,
202
Figure 14. As in Figure 13.
A,, symmetry. Likewise the 922-cm-' a p band (Figure 4) may be the A2 methyl rocking mode, interacting with v23 (v(C,C,)), which shifts down and gains the intensity of both modes upon methylene deuteration, when the 922-cm-, band disappears. The A, C,C2 stretch, which is calculated at 1014 cm-I, may be hidden under the 1024-cm-I B, and A, bands in the na and meso-d, spectra, but a candidate band can be located at 1177 cm-' in the methylene-d,, spectrum (Figure 4, Table 111). The 466-cm-l p band (Figure 8) is assigned to C C C bending and is considered more fully in a subsequent pa;er;I7: E . Ethyl Mode Intensities. The strength of the ethyl modes in the 568.2-nm-excited RR spectrum (Figure 5) is remarkable. Among the depolarized bands only uIo, Y,,, and (in the natural abundance spectrum) v 1 3are stronger than the depolarized component of the 1024-cm-' band (v(C,C2)); the remaining skeletal modes have comparable or weaker intensity. The polarized 1024-cm-, component is also quite strong in the 568.2-nm spectrum. Only v6 is comparable. Since the 1024-cm-' modes are mainly C,C2 stretching in character (67% contribution to the
potential energy distribution, in our calculation), the intensities cannot be explained merely by mixing in of porphyrin ring mode character through kinematic coupling, as might have been expected. The ethyl coordinates must themselves be important contributors to both vibronic and Franck-Condon scattering mechanisms.18 Saturated alkyl substituents are not ordinarily thought to couple electronically to aromatic chromophores, although the influence of inductive and hyperconjugative effects is readily seen in shifts of the electronic transition energies.22 The present results suggest that the ethyl groups of NiOEP do significantly delocalize the Q excited state, presumably by mixing of ethyl u character into the porphyrin T * orbitals.23 We remark that the out-of-plane (22) For example, the positions of the Soret and Q bands are different for Ni porphine and Ni octaethylporphyrins. The influence of the substituents on the electronic transition energies was studied by: Spellane, P. J.; Gouterman, M.; Antipas, A,; Kim, s.;Liu, Y. C. Inorg. Chem. 1980, 19, 386, and by: Shelnutt, J . A,; Ortiz, V. J . Phys. Chem. 1985, 89, 4733.
60
The Journal of Physical Chemistry, Vol. 94, No. 1, 1990
A2g
Li et al.
Modes
VII0,
1601
1602
1351
1402
@
21
1327
,e 1120
1127
992
796
566
41 9
24
626 Figure 15. As in Figure 13.
orientation of the ethyl substituents places the G orbital in the proper orientation to interact with the .R orbital, as illustrated in Figure 12. Specifically the G, ethyl orbitalsz3 interact with and raise the energy of the a,, porphyrin orbital, which is the HOMO of NiOEP.24 Consequently, the ethyl coordinates, especially C,C, stretching, can induce significant excited-state origin shifts and Q / B vibronic coupling. While the ethyl mode activation is not as dramatic with 406.7-nm excitation, the 1024-cm-' band (now mostly polarized) is clearly seen (Figure 3), and the u5 skeletal mode (C& stretching) has about half the intensity of the strongest bands ( u2, u3, v4), suggesting significant electronic involvement of the ethyl groups in the B state, as well as the Q state. Interestingly the CH, twisting A, mode, at 1258 cm-', is also prominent in the 406.7-nm spectrum, whereas it is barely detectable in the 568.2-nm spectrum. These relative intensity differences of the symmetric modes reemphasize the different shapes of the B and Qo excited-state potentials, as discussed above. F. Mode Comparison for Nip,NiTPP, and NiOEP. Table VI1 compares the frequencies of corresponding modes for NiOEP, Nip, and NiTPP, while Figures 13-1 5 show selected eigenvectors for porphyrin skeletal modes appearing in the RR spectra. Except for the phenyl/H substitution the skeletal mode eigenvectors of NIP and NiTPP are nearly indistinguishable, and the latter have been omitted. Figures 13-1 5 also show the motions of the ring atoms to be very similar for NiP and NiOEP (except, of course, when the C,H atoms are the primary contributors to NiP modes, e.g., u5 and vg). For example, u2 and u3 are similar mixtures of C,C, and C,C, stretching, despite 40-60-cm-' frequency differences. The mixing is quantitatively greater in NiP (difficult to see in the qualitative representations in Figures 13-1 5) leading to a near-cancellation of the u3 RR inten~ity.~ The mixing of these coordinates is much less in the B,, block, vlo and v i , being mainly C,C, and C,C, stretching, respectively. (23) Libit, L.; Hoffmann, R. J . Am. Chem. SOC.1974, 96, 1370. (24) Czernuszewicz, R. S.; Macor, K. A.; Li, X.-Y.; Kincaid, J. R.; Spiro, T.G.J . Am. Chem. SOC.1989, I l l , 3860.
v5(Alg), 1120 cm-1
v(C&J(A,), 1019 CW'
v(c,-c,)(~,), 1019 cm-1
Figure 16. Selected ethyl substituent-mode eigenvectors for NiOEP.
Figure 16 shows eigenvectors of important ethyl modes; u5 and vi., involving C,-C, stretching mixed with ring coordinates, and
the A, and B, modes involving CI-CZ stretching, primarily. The half-ring stretching character of the u4 modes is clearly evident in the out-of-phase motions of the C,C, and C,N bonds. This mode generally gives rise to the strongest band in Soretexcited metalloporphyrin RR spectra, implying an especially large origin shift along this coordinate. We can see why this should be the case by comparing the eigenvector with the orbital patternZ5 of the eg* upper orbital of the Soret transition, in Figure 17. For two of the pyrroles, the orbital is bonding with respect to the C,N bonds but antibonding with respect to the C,C, bonds. Thus the (25) Longuet-Higgins, 1950, 18, 11 74.
H. C.; Rector, C . W.; Platt, J. R. J . Chem. Phys.
( 2 6 ) Czernuszewicz, R. S. Appl. Speclrosc. 1986, 40, 571.
Consistent Porphyrin Force Field
The Journal of Physical Chemistry, Vol. 94, No. 1, 1990 61 Conclusions
eg’ LUMO
v4(Als) vibrational
mode
Figure 17. Comparison of the porphyrin u4 eigenvector (C, substituents omitted) with the e,* orbital (the sizes of the circles are proportioned to the orbital coefficient^^^) showing the matching of the phases. u4 eigenvector is especially well matched to the bonding change associated with the electronic transition. The eigenvectors of ~ 1 9 uzO, , and u2,, the strongest A,, modes, show large amplitudes for all the atoms of the 16-membered inner porphyrin ring as well as the methine H atoms. When the latter are replaced with D, u21 is shifted out of the region, and the 9 u22 (see Figure 4). intensity accumulates in ~ 1 and With Q-band excitation, the strongest totally symmetric modes, aside from the C,-C2 and u5 ethyl modes, are and u, (Figure 2), which are breathing and deformation modes of the pyrrole rings. These also have nearly the same forms for NiP and NiOEP, despite the large frequency differences brought about by interaction with the C,C, stretching coordinates in NiOEP. This interaction also accounts for the accidental near-degeneracy of the corresponding B,, modes, u I s and u I 6 ; the NIP modes are separated by over 300 cm-I. ~ 1 is5 among the strongest B,, modes (Figure 2). Ni-pyrrole stretching character is clearly evident in the eigenvectors of us and u18 (all pyrrole atoms move in or out in concert). As noted earlier,5 the uls frequency for Nip, 245 cm-l, is what one would expect for the stretching of metal-pyrrole bonds, while the elevated frequency of us, 378 cm-I, reflects the contribution of C,C,C, angle bending to the breathing mode force constant. For NiOEP there is additional complexity due to the involvement of ethyl bending coordinates, leading to extensive mixing between us and u9 in the A,, block, as discussed above (the us and u9 eigenvectors of NiOEP are nearly the same, except for 7 shown) in the B,, block. phasing), and between u l s and ~ 1 (not We call attention to the interesting motions displayed by q 5 (BZg),in which the pyrrole rings move tangentially to the porphyrin 5 in which the pyrrole rings rotate circumference, and by ~ 2 (A2,), in place, producing a pinwheel effect.
Almost all of the in-plane skeletal modes of NiOEP have now been reliably assigned, with the aid of variable-excitation and polarization RR measurements, and IR spectra of lSN,meso-& 1SN/meso-d4,and methylene-d16isotopomers. They have been classified with a local-coordinate scheme that recognizes the pyrrole rings as cooperative vibrating units, exposing similar frequencies for similar kinds of vibrations. The frequencies and isotope shifts are reproduced with a skeletal force field which is very similar to one developed for Ni porphine and NiTPP, and the skeletal mode eigenvectors are similar for all three porphyrins. A notable feature of these eigenvectors is the recognition that uq, the pyrrole half-ring stretch, matches the phasing of the e,* acceptor orbital of the resonant electronic transition, thereby accounting for its dominance of Soret-enhanced R R spectra. The NiOEP R R spectra contain several bands due to modes which are assignable to the ethyl substituents on the basis of large methylene-deuteration effects. Their appearance in polarized (A,) depolarized (B, or B2) and anomalously polarized (A2) scattering indicates that the overall porphyrin symmetry is at least approximately preserved. The large enhancements seen for bands assignable to C,C2 as well as CbCl stretching cannot be explained simply on the basis of coupling to other skeletal modes and suggest significant delocalization of ethyl electrons into the porphyrin A* orbitals in the excited states. The relative orientations of the ethyl groups markedly influence the low-frequency modes, and the appearance of doublets for us and u9 is attributed to rotational isomerism.
Acknowledgment. We are deeply grateful to Dr. Harold Goff, at the University of Iowa, for providing us with a sample of NiOEP(methylene-d16), to Dr. Richard Mathies (University of California, Berkeley) for providing us with a graphics computer program for plotting eigenvectors, and to Dr. Robert Scheidt (Notre Dame University) and Dr. John Shelnutt (Sandia Laboratories) for communicating results of their work on NiOEP crystals prior to publication. This work was supported by N I H grant GM 33576 (to T.G.S.) and N I H grant DK 35153 (to J.R.K.). Registry No. NiOEP, 24803-99-4; IsN, 14390-96-6; D, 7782-39-0. Supplementary Material Available: Atom numbering scheme (60 atoms) (Figure l), characteristics of the porphyrin in-plane valence force field (Figure 2), Cartesian coordinates, definition of internal coordinates, and unnormalized U-matrices (A,,, A2,, B,,, B2,, and E, blocks) for NiOEP, tetragonal C ( D Z d )model (Tables 1-3) (15 pages). Ordering information is given on any current masthead page.
