Ruffling Effects on Porphyrin Vibrations: Normal ... - ACS Publications

7441

J. Phys. Chem. 1993,97, 7441-1450

Ruffling Effects on Porphyrin Vibrations: Normal-Mode Analysis for Nickel Octaethyltetraphenylporphinefrom Resonance Raman and IR Spectra of Isotopomers Christine Piffat, Dan Melamed, and Thomas G. Spiro' Department of Chemistry, Princeton University, Princeton, New Jersey 08544 Received: February 18, 1993; In Final Form: April 22, 1993

Resonance Raman [RR] and infrared spectra are reported for nickel octaethyltetraphenylporphine(NiOETPP) and its lSN, meso-13C, and phenyl-d2o isotopomers. Assignments were made of most of the in-plane skeletal modes and the substituent modes using a normal-coordinate calculation for a reference planar porphyrin. For the calculation, the force field was transferred from nickel tetraphenylporphine (NiTPP)' and nickel octaethylporphine (NiOEP).2 The effects of steric crowding and the associated saddle distortion in NiOETPP are manifested in large, ca. 70 cm-*, downshifts in a number of porphyrin skeletal modes, relative to the frequencies calculated for a planar skeleton. Three prominent low-frequency R R bands are assigned to the out-of-plane modes 715, 716, and 84, which have Bzu symmetry in the idealized Dqh point groupa3 This is the same symmetry as that of the saddling distortion. The 7 1 6 mode, which involves tilting of the pyrrole rings, gives rise to one of the strongest bands in the Soret-excited R R spectrum.

Introduction There is much current interest in the possibility that out-ofplane distortions of the macrocycle modulate the biological roles of tetrapyrrole prosthetic groups. Such distortions are evident in structures available from protein crystallography. For example, the protein environment deforms the tetrapyrrole rings of the bacteriochlorophylls a that comprise the light-harvestingantenna of Prosthecochloris a e ~ t u a r i i .The ~ tetrapyrrole rings of the bacteriochlorophylls b of the special pair are also determined to be nonplanar in the reaction center of Rhodopseudomonas viridis.596 Possible implications for the mechanism of photoinduced electron transfer have been disc~ssed.~ To elucidate the effects of nonplanarity, these systems are being modeled by highly nonplanar porphyrins. The porphyrin skeleton is relatively flexible, and distortions of the macrocycle can be imposed by variations in the metal-nitrogen bond lengths or by substituent steric interaction^.'-^ Examples of both these types of distortions have been observed in the X-ray structures of NiOEP,l0J1 ZnOETPP,12 and NiOETPP.13 NiOEP X-ray structures have shown that both ruffledlo and planar" forms of the molecule exist due to the tradeoff between delocalization in the planar structure and shortened Ni-N bonds in the ruffled structures. Because the low-spin NiZ+ ion is smaller than the planar porphyrin cavity size, the pyrrole rings twist to produce a smaller core via S4-ruffling of the skeleton.* Out-of-plane distortions can also occur when peripheral substituents crowd the porphyrin rings, as in octaethyltetraphenyl porphyrin (OETPP).lZJ4Js For ZnOETPP,12X-ray data show the molecule to be saddle-shaped due to steric interactions between the mesoand &carbon substituents. There are also consequences for bond distances and angles. Relative to Hoard's "average" planar porphyrin values,9 the C,-Cp, CrCp, and C,-C, bonds in ZnOETPP increase in length by 0.16-0.26 A; the bond angles C,NC, and CpC,C, also increase by 2.2 and 4.0°, respectively, while the MNC, and NC,C, angles decrease by 4.3 and 3 . 3 O , respectively. Because Zn lies above the porphyrin plane, this saddling is not due to the metal; rather, the distortion helps minimize the steric interactions.~*~14~15 Since the distortions are due to a crowded periphery, ZnOETPP models biological tetrapyrroles, whose ruffling is due to the protein matrix. A recent NiOETPP X-ray structure shows that this molecule is also saddled.13 Effects of saddling on NiOETPP can be

* To whom correspondence should be addressed.

TABLE I: Comparison of Selected Bond Distances and Angles NiOEP NiOETPP planarb ruffled' ZnOETPPd avg porphyrin' Bond Distances, A Ni-N N-C, CU-C,

c,-ClU Cm-Cph

1.906 1.381 1.451 1.365 1.395 1.491

1.958 1.376 1.443 1.346 1.371

1.929 1.386 1.449 1.362 1.372

2.063 1.371 1.461 1.370 1.416 1.494

1.379 1.443 1.354 1.390 1.499

Bond Angles, deg NNiN C,NC, C,C&p NiNC, NC,C, C,C,C, NC,C, C&,C,

90.6 105.9 106.8 125.3 109.8 121.4 121.3 128.3

90.15 103.9 106.5 128.0 111.6 125.1 124.4 124.1

90.0 105.1 106.8 127.4 110.6 124.1 124.0 125.0

89.4 107.7 106.8 122.9 109.2 124.4 122.4 128.2

89.7 105.5 107.0 127.2 110.2 123.8 125.7 124.2

Taken from ref 13. b Taken from ref 11. Taken from ref 10. d Taken from ref 12. e Taken from ref 9. @

determined by comparing the crystal structure with thoseof planar and ruffled NiOEPll (Table I). In NiOETPP, the C&p and C,-C, bonds expand by 0.02 A; the C,NC, and CpC,C, angles increase by 2.0 and 4.2O, while the NiNC, and NC,C, angles decrease by 2.7 and 3.1 O . Thus, NiOETPP shows similar bond distance and angle distortions as does ZnOETPP. These distortions indicate that nonplanarity occurs through substituent steric interactions. Distortion of the skeleton through saddling changes both the porphyrin electronic structure and its vibrational frequencies. INDO/s calculations indicate that porphyrin puckering destabilizes the ?r system of the macrocycle, raising the energy of the HOMO relative to the LUM0.12 This appears as a red-shift in the absorption spectrum of NiOETPP relative to both NiTPP or NiOEP. Thevibrational frequencies are expected to alter because of kinematic interactions produced by skeletal distortions and because of changes in bond lengths. In addition, distortion from planarity lowers the symmetry of the porphyrin from D4h and changes the Raman and IR selection rules as shown in Table 11. Some of these effects have been discussed by Shelnutt and coworkers, who have reported resonance Raman spectra and molecular mechanics calculations on a series of nickel complexes of sterically crowded porphyrins.14

0022-3654/93/2091-7441%04.00/00 1993 American Chemical Society

Piffat et al.

7442 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993

TABLE II: Correlation Table for the Species of the D4, Grow and Its SubmouW C2’ C2’ C2’ C2’ D4h DZd s4 D4h Dw s4

-

AI, -4% B1, B2s

AI A2 BI Bz

-

~

A A B B

AI, A2u B I ~ Bzu Ey

BI B2

AI Az

B B A A

E E E E Table is taken from Wilson et a1.16 The C2‘ axes are defined as the axes through the N atoms, while the dihedral C2” axes are defined as the axes through the meso-carbons. E,

0

In this study, we examine the effect of saddlingon the in-plane modes of NiOETPP through detailed assignment of the RR and IR spectra, as well as through normal-modecalculations. Nickel was chosen as the central metal because it forms stable 4-coordinatecomplexes that provide high-quality, fluorescencefree resonance Raman spectra.’ Also, force fields for NiTPP and NiOEP have been determined by Li et al.l.z and can be used to construct a NiOETPP force field. In this work, we compare the frequencies calculated for a hypothetical planar NiOETPP molecule with those actually observed for NiOETPP in solution. Distinctive signatures of out-of-planedistortion are seen in large frequencydecreasesof certain skeletal modes, as well as substantial RR intensity for bands assignable to out-of-planeporphyrin modes.

Experimental Section Octaethyltetraphenylporphine and Its Derivatives. Octaethyltetraphenylporphine (OETPP) was synthesized according to the method of Barkigia et al.,Iz starting with benzaldehyde and 3,4-diethylpyrrole. The preparation of OETPPJT4 and OETPPdzowere accomplished by reacting 3,4-diethylpyrrolewith I3Clabeled benzaldehyde (carbonyl-T, 99% Cambridge Isotope Laboratories) and benzaldehyde-ds (98%ZH,Cambridge Isotope Laboratories), respectively. OETPP-lSN4 was synthesized from 3,4-diethylpyrroleJSN and benzaldehyde. The porphyrin was purified by column chromatography on alumina (Fisher, 80-200 mesh). In each case, the Ni was incorporated by using the DMF procedure16and purified on a silica plate (Analtech, Silica Gel

G). Spectroscopy. Resonance Raman spectra were obtained in backscattering geometry from CDCl, or CHzClz solutions in a spinning NMR tube. The scattered light was collected and analyzed with a triple monochromator (SPEX 1451N) equipped with a cooled diode array detector (Princeton Instruments IRY1024). The backslit was set to 30 mm, and data were collected for 10 min at each window. Laser excitation at 441.6 nm was obtained from a Liconix 4240NB HeCd laser, while 568.2 nm was produced by a Coherent Innova I100 Kr+ ion laser. 587.8nm excitation was provided by a Coherent 590 tunable dye laser [rhodamine-6-G]. Power at the source for the 568.2- and 587.8nm lines was 100mW. Polarizationmeasurements wereobtained for the samples. UV/vis absorption spectra were obtained with a 1-cm quartz cell using a Hewlett-Packard 8452A diode array spectrophotometer. The infrared spectra in KBr pellets were recorded on a Nicolet 730 Fourier-transforminfrared spectrophotometer. Spectra were taken from 4000 to 400 cm-l, although no modes were seen below 700 cm-1 in the natural-abundance NiOETPP. Normal-CoordinateAnalysis. Normal-modecalculationswere performed with the GF matrix method and a valence force field.” The molecule was constructed with a planar porphyrin ring, the phenyl rings perpendicular to the porphyrin plane, and the ethyl substituents pointing up and down to maintain D u symmetry, as illustrated in Figure 1. As in the previous analysis of NiOEP,2 the D~arrangementof ethyl groups was chosen in order that the

H- H

k

H‘

H

H

Figure 1. Structural Diagram of NiOETPP as a planar molecule.

TABLE L E Structural Parameters Used in Normal-Coordinate Analysis of NiOETPP’ bond distance, A bond angle, deg 1.347 Cu-N-Cu 103.1 CBCP 106.8 Ca-Cm 1.385 C4&, 111.7 Cu-N 1.389 N-CCI-C, CU-C, 1.437 Cu-Cm-Ca 124.1 1.500 CBCu-Cm 123.8 C&I 1.509 N-Cu-Cm 124.5 Cmxph Ni-N 1.955 CBC&I 125.3 128.0 CI-CZ 1.503 CU-CBCI CI-H 1.098 Cu-Cm-Cph 118.0 Cphxph 1.380 C,-N-Ni 128.4 Cph-H 1.089 N-Ni-N 90.0 CBCrC2 109.1 CBC1-H 109.0 CrC1-H 109.7 H-CI-H 109.7 Cmxphxph 120.0 Cphxph-Cph 120.0 Cphxph-H 120.0 Bond distances and angles were taken from Li’s calculations for NiTPP.’ sorting of the ethyl modes into the four D4h-derived in-plane vibrational mode symmetries would be maintained. Bond distances and angles, taken from those used for NiTPPl and NiOEP,Z are listed in Table 111. As in the analysis of NiOEP,Z the 16-methylenehydrogen atomswere included in the calculation, while the methyl groups were approximated by 15 amu point masses. The internal coordinates (RJ consisted by Wilsontype1’J8 bond stretching, angle bending, torsion of the phenyl groups, and out-of-planewagging of the ethyl and phenyl groups as described by Li et al.ls2 The symmetry coordinates (Si)and corresponding U matrices (S= UR)were taken from those used for NiTPP and NiOEP.1.2 The potential energy was expressed by a general valence force field that included bond stretching ( K ) , angle bending (H), and valence interaction cr) force constants. These interactions included all possible adjacent coordinate pairs sharing at least one atoms or pairs which are separated by no more than one bond. In addition, out-of-planewagging (y), torsion (4,waggingwagging (f(r,y)),wagging-torsion and torsion-torsion ( f ( 7 , 7 ) ) interaction force constants were included for the phenyl substituents, and out-of-planewagging (y) and bending-wagging Cf(a,y))interaction force constants were included for the ethyl substituents. The force constants were transferred from NiTPP and NiOEP1sZand are listed in Table IV. The values were averaged for the four skeletal force constants that differ slightly between NiTPPandNiOEP, K(CuC~)4,H(CuNCU)1,H(C~aN)3, and H(CuC,Cu)4. Schactschneider’s programs18 were used on a VAX- 1 1/780 computer to construct the G matrices and to solve the secular equations, IGF - EX1= 0, for each symmetry species. The modes were classified as skeletal, phenyl, and ethyl modes, depending on which region of the molecule was primarily involved in the

wr,.)),

Ruffling Effects on Porphyrin Vibrations

The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 7443

TABLE Iv: Valence Force Constants for NIOETPP. K(R0 / (mdyn/A) H ( 4 / (mdyn A M 2 )

interaction force constantsb Poruhvrin Core fi,z(Ri,R,): f(1,4) = 0.435;j(i,6) =f(4,6) = 0.43;f(2,2) =f(2,3) =f(2,4) = 0.56;f(2,5) = 0.30;f(3,4) = 0.44;f(3,7) = 0.41;f(3,3) = 0.44;f(7,7) = 0.15 f1,3(R,,R,): f(1,2) = -0.26;f(3,6) =f(4,6) = -0,.25;f(2,3) =A2,4)

-0).20;f(2,7) -0.18

fi,3’(R&): f(1,3) =f(3,4) =f(4,4) = 0.22 fi(R,,a,): f(2,3) =f(4,6) = -0.23;f(2,9) -0.10;f(3,2) = -0.025;f(3,3) -0.12;f(3,4) f(3,5) = -0).232;f(3,6) =f(4,1) = -0.12;f(3,9) = O.O7;f(4,7) -0.22;A6,3) = -0.30

-0.09;

f2(Rip,): f(1,2) =f(4,2) 0.30;f(1,7) =f(6,7) = 0.32;f(2,4) =f(2,5) =f(2,6) = 0.21; f(2,9) =f(3,3) =f(3,6) =f(3,10) = O.lQf(3,l) =f(4,3) =f(4,5) =f(4,8) =f(6,8) = 0.24

K(CC)u = 6.312 K(CH)g = 5.116

H(CCC)12 = 0.959 H(CCCm)13 0.755 H(CCH)14 0.535

fI(a,,a,):A2,4) =f(5,5) = 0.15;f(6,6) = 0.1 1 f2(a,,a,): f(2,3) = -0.lO;f(3,6) =f(5,6) = 0.025;&4,6) = -0.015;f(4,9) = 0.20 Phenyl Substituents In-Planec fi,z(R,,R,): f(5,8) 0.15;f(8.8) = 0.74 fi,o(Ri,Ri): f(898) = -0.346 fi,&,Ri): f(8,8) = 0.30 fi(Rl,a,): f(5,2) = -0.30;f(5,13) = 0.25 h(Ri,a,): f(8,12) = 0.112;f(8,13) = 0.108;f(8,14) 0.25 Out-of-Planed f(~i.7,):f0(15,15) = 0.025;fm(15,15) -0.005;fp(15,15) 0.035 f ( y j ~ / ) fo(15,16) : = -0.02;fm(15,16) -0.005 f(7,,7,): fo(16,16) -0.005

Ethyl Substituentd fis(R1,Ri): f(6,lO) 0.43 f2(R,,c~j):f(6,20) =/110,21) = 0.11 fi(c~j,c~j): f(20,21) = 0.07 f2(9,a,): f(20,21) = -0.07 f3(~~,,7):f(7,23) =f(8,23) = -0.088 0 K(RJ 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. b Stretch-stretch interactions in mdyn/A; stretch-bend interactions in mdyn/rad; bend-bend interactions in mdyn A/radz. fl,z(R,,R,) = 1,2 stretch-stretch interaction (common a,);e.g.,f(l,4) =fi,2(c&&&#). fi,3(R,,RJ = 1,3 stretch-stretch interaction between pyrrole ring and methine bridge; e.g.,f(1,2) =fi,3(C&.&Cm). fi,3’(RiR,) = 1,3 stretch-stretch interaction within the pyrrole ring; e.g.,f(1,3) = f i ~ ( c & & ~ N ) . fi(Ri,a,) = stretch-bend interaction between R, and a, sharing one common atom; e.g., f(2,3) = fi(C,C,,C&,N). fz(R,,u,) = stretch-bend interaction between R, and a] sharing two common atoms; e.g.,f(l,2) = f2(C&a~,,C&&,). fi(a,,a,) = bend-bend interaction between ajs sharing one common atom; e.g.,f(20,21) =fl(C&1H,C2ClH). fz(a,,a,)= bend-bend interaction between ajs sharing two common atoms; e.g., f(4,6) =f2(CaCmCa,NCaCm). C Simplified in-plane valence force field for phenyl vibrations was transferred from the work on biphenyl by Zcrbi and Sandroni as described by Li et a1.l The out-of-plane phenyl force field was transferred from the work on biphenyl by Eaton and Steele as described by Li et al.1 y(CH) = out of the phenyl plane C-H bond wagging. s(CC) = C-C-C-C torsions within the phenyl rings; s(C1C) = Cm-C1-C-C torsions which are equivalent to y(CICm) out-of-plane wagging of CI-C, bond relative to the porphyrin ring; T(CmCI) = C&,-Cl-C and C&,-Cl-C torsions. fly,,?,) = out-of-plane wag-wag interactions between y j s within the phenyl rings; fa, fm, and fp refer to interactions of the two indicated coordinates at respectively ortho, meta, and para positions on a given phenyl ring. f(y,,s,) = out-of-plane wag-torsion interactions between 7, and s, within the phenyl rings. f2(~,,a,)= torsion-bend interactions between 71 and a, sharing two common atoms. f(sj,s,) = torsion-torsion interactions bond. Its value, 0.05 mdyn A/rad2, is estimated within the phenyl rings. e This force constant describes the phenyl ring rotation around the from a one phenyl rotational barrier of about 15 kcal/mol as described by Li, et a1.I fK(ClC2) is larger than K(C&1) due to the point mass approximation for the methyl groups. y(C&1) is out-of-plane wagging of C& bond relative to pyrrole ring. gf2(a,,y) = bcnd/out-of-plane wag interaction between and y.

vibration. These modes were also classified into the four Ramanallowed in-plane symmetries for D4h (Alg,AZg, Big, and BZg) and one IR-allowed in-plane symmetry (E,,).For the ethyl substituents, which have DU symmetry,these modes become AI,Az,B I , Bz, and E, respectively.

Results and Discussion and Force Field. Normal-Coordinate A M ~ Y S ~Structure : Previous normal-coordinateanalyses were performed for the inplane modes of NiOEPl and NiTPPe2 These analyses provide an opportunity to calculate what the vibrational spectrum of NiOETPP would be in the absence of steric crowding and of out-of-plane distortion. Thus, the vibrational spectrum of a hypothetical planar NiOETPP can be calculated. Assuming this planar structure, the vibrational modes were calculated for NiOETPP using the inputs from NiOEP and NiTPP. At this stage, no attempt was made to adjust the

calculation to fit the observed frequencies. Nevertheless, the frequency match and the isotope shift patterns were close enough to permit reliable assignment of essentially all the RR and IR bands. These assignments are shown in Tables V-VIII. In previous work, Shelnutt, et al. assigned v2, u3, v4, and 4 4 for crystalline NiOETPP.14 Our independent assignments match Shelnutt’s. Sparks et al. found the RR frequencies to be unaltered between solution and crystalline samples.15 Consequently,they inferred that the NiOETPP macrocycle maintains its crystal conformation in solution. Therefore,NiOETPP should have been saddled for our RR experiments,while we performed calculations on a planar molecule. The frequencydiscrepanciesobserved give us insight into the effects of the distortions experienced by NiOETPP due to saddling. As in the previous analyses, the vibrations are conveniently divided into two groups: the porphyrin skeletal modes, which extend to thebonds connectingtheporphyrin with thesubstituents,


TABLE V Comparison of NiOETPP Calculated and Observed Phenyl Mode Frequencies (cm-l) with Observed Phenyl Frequencies of NiTPP (cm-9’ Phenyl in-Plane Modes AI1

v1

[A1JN,AW,Adl

BD [A1JN,AI3C,Adl

E. [AIJN,A13C,Adl

Y(CH)(Y~) 613071 [0, 0,7781 61’3071 [0, 0,7791 61’’ 3071 [0, 0,7781 3071 [-, -,7781 3071 [-, -,7791 3071 [-, -,7781 v(CH)(vis) 62 3070 [O, 0,7791 6i 3069 [O, 0,7801 42’’ 3070 [0, 0,7791 3069 [-,-,7801 3070 [-, -,7791 3070 [-,-,7791 v(CH)(v=) 63 3069 [O, 0,7841 63’ [O, 0,7861 63” 3069 [O, 0,7841 3068 [-,-,7 8 4 3069 [-, -,7841 3069 [-, -,7841 v(CC)(VC.) 64 1600 [O, 0,261 6; 1599 [0, 1,261 64” 1603 [ l , 5,311 1599 [l,0, -1 1598 [1, 1,331 1605 [-,-,271 1607 [-, -,491 1599 [-,-,27l is00 [i, 3,901 V(CC)(V~,) bJ1503 [o; 1, iosj 6Jfis01 [112,i06j 1491 [l, 0, 1041 1% [I, 8 9 -1 1513 I-. 1131 1525 I-. 7.261 1513 I-. 1251 66 1200 [ko;337j 66‘ 1200 [o, o,337j 46’’ 1201 [iio; 298j v(cc)(v,) 1199 I-, 1199 [-, -,3371 1200 I-, -,3381 -,3371 v(CC)(V~~,) 61 1033 [0, 1,2151 97’ 1052 [2,4,234] 67’’ 1029 [O,O, 2131 1002 [O, 1, -1 999 [I, 0,421 1037 [-, -,2 2 4 1063 [-, -,2481 1057 [-, -,2421 ~ ( C C C ) ( V I ~ ) 68 945 [ l , 2,301 ${ 921 [ l , 1,251 6 ~ ” 9 2 7[2,3,64] 921 [z -5 -1 905[O, -3,8171 941 [-,-,291 925 [-,-,251 939 [-, -,271 6(CCC)(vd & 618 [O, 2, 141 &’ 526 [2,2,3] &” 658 [2,0, 151 647 io, 0,171 619 [-, -,1-51 650 [-, -,181 650 [-,-,181 4CmPh)(Cm+) 610186 [O, 0941 610’390 [2,1,31 610”232 [O, 0,281 190 IO,& 61 [% 1,31 215 [-, -,31 215 [-, -,-31 195 [-, -,4

-.

-.

Phenyl Out-of-Plane Modes AD [ALJN,AL3C,W [A1’N,AI3C,Adl BII

vi

7(CH)(v3

TI

965 [ l , 1,241 964 [-, -,2 4 Y ( C H ) ( V ~ ~ T~Z ) 829 [ l , 3,981 823 [-, -,931 y(CH)(vll) ~3 718 [O,O, 1701 T(CC)(VII)

~4

T(CC)(Vl6b)

TJ 389

& & c)(6

718 [-, -,1711 477 [O, 1,641 474 [-,-,651 [o, 0,911 411 .--r-1.0. -1 388 [-,-I,k j 7 6 45 [o, 0,21 45 [-, -,31

E l

[AlJNN,A13C,Adl

962 [0, 0,281 ~ l ” 9 6 4[ l , 1,261 962 [-, -,2 4 961 [-,-,271 ~ 2 846 ’ [0, 0, 851 irfl859 [2,2, 1241 837 [-, -,901 850 [-, -,1051 7 ) )719 [0, 0, 1721 i g ” 718 [0, 0,1671 700 [0, 1,1551 718 [-, -,1681 718 [-, -,I551 ~ 4 ~ ~ [0, 4 91,421 3 T; 497 [l, 0,381 503 r.2 0. -1 441 (-,L, ji j 430 [-, -,451 T I ) 413 [o, 0,105) TII) 412 [o, 0,1191 TI’

384 [-, -,821 [o, 0,21 83 [-, -,31

76)65

381 [-,-,941

Td‘ 48 [o, 0,21

49 [-, -,31 For each mode frequency, plain text is the calculated frequency for NiOETPP, bold face text indicates the observed frequencies for these NiOETPP modes, and italics indicatecalculated frequenciesfor N1TPP.l bFrequency shifts for ISN, mesoJ3C, and phenyl-d20 are given in parentheses. 4

and the substituent modes, which are more or less localized on the phenyl rings or ethyl groups. The phenyl modes, listed in Table V, are readily identified by their large d20shifts. They are divided into two types of modes: modes which are in the planes of the phenyl rings, &, and modes which are out of the phenyl planes, ?rl. [Only half of all the phenyl modes of each type are directed parallel to the porphyrin plane; the other half are parallel to the porphyrin normal and classify as porphyrin out-of-plane modes.] The 4i modes have AI, B2, and E symmetry, while the ?rt modes have B1, A2, and E symmetry. For each type of mode, in-the-phenyl-planeor out-of-the-phenyl plane, the three different symmetry classes differ only in the phasings of the motions on the successive phenyl rings. This relationship is indicated by using the same index, i, with a prime for the B2 and A2 classes, and a double prime for the E classes. Because the phenyl rings are separated by several intervening bonds, the modes differing only in phase are calculated to have nearly the same frequencies with only a few exceptions. The frequencies calculated for NiOETPP are essentially the same as

Piffat et al. those calculated for NiTPP; these are shown in italics in Table V. Exceptions to this rule involve modes which interact significantly with porphyrin skeletal modes if these interactions depend on the symmetry of the mode. These interactions become increasingly noticeable at lower frequencies-below 1200 cm-l in the case of 4 modes and below 500 cm-1 in the case of r modes. The ethyl modes, listed in Table VI, are divided according to the following local mode descriptions: CH2 scissors, CH2 wag, CH2 twist, CH2 rock, and stretching of the C 1 4 2 bond. Since the methyl groups have been approximated as point masses, methyl modesareomitted [althoughsomemethylmodescanbeidentified in the spectra, as in the case of NiOEP2]. Because there are eight ethyl groups, these local modes each contribute to two E modes and to one each of the AI, BI, A2, and B2 modes. Since the different symmetry classes differ only in phase, the frequencies are again essentially the same for modes arising from a given local mode and nearly the same as those calculated for NiOEP, which are listed for comparison. Interactions of ethyl modes with porphyrin skeletal modes are expressed in frequency differencesamong the symmetry classes. These interactions are less marked for ethyl modes than for the phenyl modes, being noticeable mainly for the CH2 rocking modes. Table VI1 lists the calculated porphyrin skeletal modes and potential energydistributionsof these modes, divided by symmetry class. Also listed are the frequencies calculated for the corresponding modes of NiOEP and NiTPP. The mode labeling scheme, vi, was introduced for NiOEP by Kitagawa and c o - w ~ r k e r sand ~ ~ adapated to NiTPP by Li et al.’ Modes with the same label differ in frequency between NiOEP and NiTPP because of different interactionswith the peripheral substituents, as well as differences in the force field that reflect electronic influences of the phenyl and ethyl groups. Large differences are, of course, seen for those models that involve bonds connecting H atoms to the porphyrin ring in OEP which are replaced by C atoms in TPP and vice versa. For these modes, the corresponding NiOETPP frequency is at the lower value, since all the substituent bonds are to C atoms. Otherwise, the calculated NiOETPP frequencies are expected to be intermediate between those of NiOEP and NiTPP, since the force constants were averaged in the calculation. This is generally found to be the case, except for the three highest frequency A1 modes v r v 4 , the B1 mode v l l r the A2 mode ~ 2 2 ,the Bz mode v28, and the E modes v38 and ~ 3 9 .For these relatively high-frequency modes, the calculated frequency is higher for NiOETPP than either NiOEP or NiTPP, evidently reflecting an altered interaction pattern due to the absence of H atom substituents. TableVIIIallocatestheskeletal modes to thevarious symmetry classes on the basis of the predominant local coordinate contributor, as described previously for NiTPPl and NiOEP.2 For a given local coordinate, the frequencies aresimilar in the different symmetry classes, although there are some notable differences. These differencesreflect symmetry-specific interactions with other skeletal modes or with modes of the ethyl or phenyl groups. Resonance Raman Enhancement Pattern. The resonance enhancement pattern of the NiOETPP Raman spectra is similar to that observed for NiOEP2and N1TPP.l In all three molecules, A-term scattering is observed when the sample is excited in the B (Soret) band, while B-term scattering is observed when the sample is excited in the Q band region. Figure 2 shows the absorption spectrum of NiOETPP in methylene chloride with B (Soret), Q1 (p), and QO(a)bands at 434, 552, and 588 nm, respectively. The solution was excited at 441.6,568.2, and 587.8 nm to produce the resonance Raman spectra shown in Figures 3-8. The bluest and reddest excitation lines are near-resonance with the B and QObands, respectively; the 568.2-nm line falls between the two Q bands. The three excitation lines were chosen to enhance modes of



TABLE VI: Comparison of NiOETPP Calculated and Observed Ethyl Made Frequencies (cm-l) with Observed Made Frequencies of NiOEP (cm-1)’ .

I

vi

A1 [A’5N4, A’3C4, A&]

CH2 scissors

1464 [0, 0,-21 1469 [O, -,-1 1456 [0,-,-1

1465 [0, 1,0] 1458 [l,-,-1 1459 [0,-,-1

1338 [ l , 0, 11

1346 [O,O, 01

1345 [ l , -,-1

1281 [3,0,0] 1260 [3,0,0] 1276 [2,-,-1

CH2 wag

CH2 twist

CHI rock

BI [A15N4, A13C4, ad,]

A2 [AL5N4,AI3C4, A&]

B2 [A1’N4, A”C4, Ada]

1513 [O,O, 01

1465 [0, l,O]

1459 [0,-,-1

1459 [0,-,-1

1343 [0, O,O]

1359 [ l ,-,-1

1347 [0, l,O] 1303[-, 0, 01 1351 [0,-,-1

1350 [0,-,-1

1282 [5,0,0]

1294 [3, 3,0]

1256 [3,5,33]

1284 [ I , -,-1

1236 [2,-,-1

1269 [0,-,-1

762 [0, 0, 01 779 [1, 0, 01 764 [0,-,-1

780 [3, 1, 01 747 [2, 2, -121 782 [3,-,-1

816 [3, 1, -11

661 [0, 0, 01

1014 [O, 0,-11 1026 [3,0,0] 1018 [I,-,-]

1015 [O,O, 01 1027 [2,1,0] 1019 [0, -,-1

842

[a, -,-1

657 [0,-,-1

E [AL5N4,A”C4, A&] 1462 [O,O, 01 1447 10, -,-1 1459 [0,-,-1 1348 [0, l,O] 1442 [l, 0, -7 1457 [0,-,-1 1335 [O,O, 01 1351[0, -,41 1356 [0,-,-1 1331 [ l , O,O] 1354 [3, -,-1 1288 [2,2,2] 1307 [2, -,-1 1276 [2,1, 11 1261 [l, 0, 01 1273 [3, -,-1 766 [0, 0, 01 760 129%21 761 [ I , -,-1 754 [l, 1, -41 740 ri. 0. -1 728 11; -j 1042 [ l , 1, 51 1053[0,1, -11 1029 [2, -,-1 1016 [ l , 1,0] 1021 [2, 1,1] 1019 [ l ,-,-1

-I

1017 [0, 1,0]

1016 [0, 0, -11

1014 [0,-,-1

1021 [2, -,-1

I, For each mode frequency, plain text is the calculated frequency for NiOETPP, bold face text indicates the observed frequencies for these NiOETPP modes, and italics indicate calculated frequency for Ni0EP.Z NiOETPP

400

450

500

550

e00

X/nm

F i p e 2 . Absorptionspectra of NiOETPP in methylenechloride. Arrows indicate wavelengths used for RR measurements.

different symmetry classes.20 The totally symmetric A,, modes appear as polarized bands which gain intensity via A-term scattering.21 This scattering mechanism depends on the square of the electronic transition moment. As a result, the B-bandresonant spectra (Figures 3 and 4) are dominated by the AI, modes. All nine skeletal AI, modes, v1-v9, are seen, as are several phenyl (t#I,,t#I5,417,t#Ig) and ethyl modes.I4 The totally symmetric modes are less intense for QOexcitation and even less intense for excitation between the Q bands, where the absorption strength is small. As in the case of NiOEP2 and NiTPP,’ the relative intensities of theA1, modes differ for B- and Q-resonant excitation, reflecting different geometries of the successive r-r* states. The higher-frequencyAI, modes are stronger for B band excitation, while some of the lower frequency AI, modes are stronger for Q band excitation.

1600

Figure 3. The 441.6-nm excited RR spectra of natural abundance (na) NiOETPP and its m e ~ o - ~ ~and C .dm isotopomers. Dotted linea correlate bands.

The strongest bands in the 587.8-nm spectrum are depolarized (dp) bands from B1, and B2, vibrations,lgwhich are enhanced by Q-B vibronic mixing. This mixing produces B-term scattering, which depends on the product of the oscillator strengths of the two electronic transitions involved.21 In Figures 7 and 8, the B1, modes (VIG-VIZ, V14-VI6, VI^, and T S ) and B2, modes (V29, V309 and 410’) are particularly enhanced. Anomalously polarized (ap)

Piffat et al.


TABLE W: ComprrisOa of NiOETPP Calculated and Observed Skeletal Mode Frequencies with Calculated Mode Frequencies of NiOEP and NiTPP (cm-l) NiOETPP Obsd

(A”N4,

AW4,

Calcd

Ad20)

(AI5N4,AW4, Adm)

assignment (PED, %)

NiOEPb NiTPF calcd ~ a l c d(A’5N4) (AI5N4, AI3C4, A&)

1563 (1,3, -1) 1506 (1,7,15)

1634 (0,8, 1) 1536 (1,8, 10)

1604 (0) 1517 (1)

1573 (0,10,-16) 1471 (0,7,-9)

1360 (5,4,13)

1442 (6,2,-3)

1380 (7)

1378 (9,2,6)

1234 (3,7,46)

1233 (4,7,52)

3041 (0)

1227 (3,6,53)

1131 (5,0,5) 905 (0,12,6) 731 (6,0,9) 379 (O,O, O ) / 363 (0,0,4) 267 (2,0,0)

1121 (8,1, 13) 900 (1,9,19) 804 (7,1,7) 352 (1,0,1)

1120 (9) 827 (9) 690 (0) 346 (1)

3101 (0,0,O) 1006 (13, 1,2) 875 (1, 11, 16) 403 (3,0,1)

268 (0,0, 1)

256 (0)

1097 (l,O,9)

1563 (0,10,-1) 1507 (1, 16,-1) 1355 (4,2,1) 1170 (0,0,-1) 855 (0,9,-15)

1632 (1,29,0) 1579 (l,O, 0) 1328 (9,0,0) 1178 (3,4,0) 916 (6,9,11)

1658 (0) 1578 (1) 1330 (6) 1150 (10) 759 (5)

1607 (1,31,0) 1505 (0,0,O) 1308 (15, 1,O) 3100 (O,O, 0) 1021 (5,5,0)

761 (4,8,7)

750 (1, 0,-1)

733 (1)

900 (7,7,12)

250 (-, 0,27) 153 (0,1,3)

317 (1,0,0) 238 (O,O, 31) 171 (O,O, 0)

315 (0) 1238 (2) 168 (0)

1088 (O,O, 0) 235 (0,0,29) 256 (1,0,1)

1491 (0,22,2) 1376(1,0,0)

1557 (0,30,0) 1389 (1,2,0)

1601 (0) 1402 (3)

1540 (0,30,0) 1348 (0,1,0)

1185 (13,0,0) 1 1 16 (3,0,0)

1207 (9,2,0) 1083 (1,4,0)

1127 (9) 1055 (1)

1018 (9,3,0) 3096 (0,0,O)

613 (7,0,7)

566 (2)

565 (4,0,39)

599 (0,1, 1)

626 (3)

818 (2,5,-7)

291 (0,0,27) 210 ( O , O , 15)

1327 (4) 243 (0)

257 (0,0,33) 1239 (10,4,0)

1539 (1,12,11)

1486 (6)

1481 (1,10,-18)

1401 (O,O, -9)

1403 (1)

1368 (0,3,7)

1283 (4,9,15)

3041 (0)

1267 (9,10,38)

1145 (9,-1,5)

1147 (6,0,3) 994 (2,5,20) 949 (0,0,O)

1160 (6) 1004 (11) 945 (0)

1010 (5,2,8) 3096 (0,0,O) 858 (3,4,6)

627 (2,4,19)

644 (2,0,16)

515 (1)

427 (0,0,O)

186 (0,1, 1)

204 (0)

1169 (0,0,2)

105 (0,1, 2)

151 (0)

112 (0,1,3)

1618 (0,44,1) 1594 (0,-16,-3) 1535 (1,8,12)

1637 (0) 1588 (0) 1496 (3)

1586 (1,28,-2) 1552 (0,12,9) 1473 (2,10,-13)

1371 (1, 1,O)

1402 (0,2,0)

1404 (2)

1403 (3, 1, -6)

1169 (8,3,3) 1134 1072(1,0,-) 960 (2,3,-2)

1381 (6,2,8) 1240 (4,7,34) 1198 (6,3,11) 1132 (5,0, 5) 1060 (3,4,10) 955 (1,4,24)

1346 (5) 3041 (0) 1144 (10) 1130 (6) 1004 (3) 922 (7)

1331 (7, 1,O) 1254 (8,8,22) 31OO (0,0,O) 1003 (11, 1, 5) 3097 (0,0,O) 864 (3,3,7)

564 (3, 10,5)

1398 (1, 1,4)



TABLE VII. (Continued) NiOETPP obsd (A1’N4. A”C4, Adm)

calcd (A”N4, A13C4, Adm)

797 (5, 1,O)

800 (-1, 0,2)

823 (3,4,-18) 551 (2,2,7) 368 (1,0,1) 342 (0,0,O) 299 (0,0,-1) 217 (0,0,O) 154 (0,0,1) a Observed frequencies from CHzClz solution RR (Ag and B modes) and KBr pellets (E,,modes) spectra at room temperature. Observed and = l5N substitution on the pyrrole nitrogens; 13C4= I3C substitution at the meso calculated isotope shifts (cm-1) downfield are placed in bracket~;~SNd carbons; d20 = deuterium substitution on the phenyl carbon positions. Taken from ref 2. Taken from ref 1.

TABLE VIIk Allocation of NiOETPP in-Plane Skeletal Mode Frequencies (cm-l) to Local Coordinates’ local coordinates

vi

A18 calc (obs)

1634 (1 563)

vi

BI8 calc (obs) 1579 (1 507) 1632 (1563)

A28 calc (obs)

vi

1389 (1376) 1328 (1355)

1121 (1131)

238 (250) 1178 (1170)

VI

1539 1401 (1398) 1283

900 (905) 804 (731)

916 (855) 750 (761)

352 (3791363)

171 (153)

268 (267)

317

29 1 1207 (1185) 1083 (1116) 599 (564)

1147 (1145) 994 949

613

644 (627)

210

186 105

a

E,, calc (ob)

1594 1618 1535 1402 (1371) 1381 1240 342 1198 (1169) 1132(1134) 1060 (1072) 955 (960) 797 (800) 823 551 368 299 154 217

1557 (1491)

1536 (1506) 1442 (1360) 1233 (1234)

B2g calc (obs)

vi

Observed values from CH2C12 solution RR (A, and B, modes) and matrix-isolated IR (E,,modes) spectra.

bands are enhanced by vibronic mixing involving A2, modes,lg which are strongest at 568.2-nm excitation. In Figures 5 and 6, the A2, modes ( ~ 1 9 , ~20,~22-24,and T { ) are enhanced. The B1, and B2, versus A?, selectivity in the Q band region is due to interference between QOand Q I contributions to the Raman scattering tensor.20 At wavelengths between QO and Q1, the interference is constructive for A2, but destructive for Bl, and Bz8. The signs are reversed for wavelengths above Q I or below

1 NiOETPP

Qo.

Normal-Mode Assignments. Figures 3-8 compare resonance Raman spectra for 441.6-, 568.2-, and 587.8-nm excitation for NiOETPP and its meso-W and phenyl-d20 isotopomers. The dashed lines correlate modes of similar composition as judged by intensity and polarization. Although the molecular symmetry is reduced from the idealized DU point group by the porphyrin saddling and by rotations about the porphyrin+thyl bonds, the polarizations are still recognizably related to the expected symmetryclasses. The perpendicular scattering components are very low for AI modes, while they are higher than the parallel components for A2 modes; for the BI and B2 modes, the perpendicularcomponent is approximately three-quarters of the intensity of the parallel component. The l5N spectra (not shown) were similar in quality to the other spectra; the l5N isotope shifts are given in Tables V-VII. Figure 9 shows the infrared spectra of NiOETPP and its isotopomen. Assignments of specific modes are based on comparisons with the calculated frequencies and isotope shift patterns, shown in Tables V-VII. Many skeletal modes have been assigned. All nine calculated Al, skeletal modes have been identified in the RR spectra, as well as seven of the nine BI, modes, five of the eight A2, modes, and

1

I

900

500

700 cm

Figure 4. Conditions the same as in Figure 3.

300

Piffat et al.


NiOETPP 568.2 nm

ax=

1500

NiOETPP

h

ON

N

1350

1200

1050

900

cm-' Figure 7. As for Figure 3, but with 587.8-nm excitation.

cm-' Figure 5. As for Figure 3, but with 568.2-nm excitation.

/I

INiOETPP hex=568.2~14

1

2-

NiOETPP

1

I

800

650

500

350

200

950

750

550

-1

350

150

cmFigure 6. As for Figure 3, but with 568.2-nm excitation.

cm Figure 8. As for Figure 3, but with 587.8-nm excitation.

four of the nine B2&modes. Six of the eighteen E.skeletal modes have been assigned in the IR spectra. Most of the unassigned Raman and IR modes are calculated to fall at frequencies where they might overlap with other bands in the spectra. As in the case of NiOEP,Z the Y E band is doubled, probably because of multiple conformers involving alternative orientations of the ethyl groups. The YE mode involves a mixture of Ni-N [pyrrole] stretching and porphyrin-ethyl bending; this mode has been shown to be sensitive to the ethyl orientations in the spectra of NiOEP polymorphs.2 In summary, most of the skeletal modes have been

assigned for NiOETPP, and they tend to follow patterns similar to those seen for NiOEP and NiTPP. Several phenyl modes are also seen in the RR and IR spectra. The phenyl modes are identified via their large 40shifts. The RR intensities of the phenyl modes are mostly quite low, and as in NiTPP, these intensities probably derive from vibrational mixing with nearby skeletal modes.' However, the $10 band [ 190 cm-l] is strong in the Q band excited region, indicating that the Q state must be significantly displaced along the $10 coordinate. This mode involves an essentially rigid body breathing motion of



TABLE M: Suggested out-of-Plane Assignments. obsd freq obsd freq cm-1 [AISN,, assignp AW4, Adm] ment NiOEP NiTPP description P 726 I-, 0,-41 s 715, B2,, 704 14, -, -1 605 [3, 4 9 1 (pyr fold), p 474 [2,0,0] s 64, Bin 477 [2, -, -1 6(C@9C2),m p 314 [0, 0, 01 s 716, B2u 270 [2, -, -1 414 [3,0,4] (pyr tllt) Observed frequencies and isotope shifts from solution RR at room temperature as describedin Table V. Frequenciesfor out-of-planemodes of NiOEP and NiTPP are taken from assignments by Li et al.1.3 Symmetries for out-of-planemodes are AI, (dp), Azu (dp), B1, (p), and B2u

cm-'

Figure 9. IR spectra of NiOETPP and its meso-l3C4and d20 isotopomers in KBr pellets.

the phenyl rings against the porphyrin ring.1 Thus, phenyl modes are observed through interactions of these modes with nearby skeletal modes. The ethyl modes were identified via their closefrequency match with the correspondingmodes of NiOEP, in which the ethyl groups had been deuterated at the methylene positions.2 As noted for NiOEP, the intensitiesof some of the ethyl groups are surprisingly strong and cannot be explained by vibrational mixing with skeletal modes. In particular, the C I - C stretching ~ band [ 1026 cm-l] is among the strongest in the 587.8- and 568.2-nm excited spectra; it is quite noticeable with 441.6-nm excitation as well. The intensity has been suggested to arise from an electronic effect involving hyperconjugation.2.22 The orientation of the ethyl substituents allows the u orbital to interact with a porphyrin ?r orbital, particularly the filled alu porphyrin orbital.2 Consequently, the ethyl coordinates, especially the C1-C2 stretch, can induce significant excited-state origin shifts.2 Eight of the ten calculated E-symmetry ethyl modes were identified in the IR spectrum. As in NiOEP, the methyl rocking mode was observed in the Q-band-excited RR spectra at 965 cm-l.2 There are also 6(CH3) symmetric and asymmetric deformation modes at 1358 and 1476 cm-1. (These modes were not calculated, since the methyl groups were approximated as point masses.) Mode Frequencies and Skeletal Distortion. Although the availability of polarization and isotope shifts makes the mode assignments quite secure, examination of the tables reveals substantial discrepancies between calculated and observed frequencies for some of the modes. We ascribe these differences to the effects of substituent crowding and porphyrin saddling in NiOETPP, inasmuch as these effects were deliberately ignored in the calculation. Both kinematic and electronic effects are expected. For NiOETPP, the four highest frequencies, v2, v10, vll,and v19,are all -70 cm-1 below the calculated values. These modes involve primarily stretching of the C& and C,C, bond. As seen in Table 1, these bonds lengthen -0.02 A in NiOETPP relative to planar NiOEP; consequently, the modes are expected to downshift. The CBCBbonds also lengthen by -0.02 A in the ruffled form of NiOEP, although the C,C, bonds stay the same.

(PI.

Likewise, the four high-frequency modes also decrease in ruffled relative to planar NiOEP, although to a smaller extent, 9-21 cm-l ,8 The larger discrepancies between observed and calculated frequencies in NiOETPP reflect the additional C,C, expansion, attributable to the steric crowding; larger out-of-plane displacements in NiOETPP vis-a-vis ruffled NiOEP may also be a contributing factor. With the exception of v4, v7, and V15, most of the NiOETPP modes observed at middle and low frequencies are in reasonable agreement with the calculated values. However, 60-80-cm-1 discrepancies are seen for v4, v7, and ~ 1 5 . These modes involve displacements primarily within the pyrrole rings, and the discrepanciesmay reflect kinematic changes due to the porphyrin saddling. Normal mode calculations on the nonplanar structure will be required to sort out these effects. As might be expected, the ethyl and phenyl modes are all quite close to the calculated values. To the extent that these modes are localized on the substituents, they should not be affected by distortions of the porphyrin skeleton. Out-of-Plane Modes. Three prominent low-frequency bands do not correspond to any of the calculated in-plane modes and are tentatively assigned to out-of-plane modes, as indicated in Table IX. On the basis of comparisonswith NiOEP and NiTPP, they are suggested to arise from 7 1 5 [726 cm-'1, 7 1 6 [314 cm-'1, and 64 [474cm-I], which have principal contributionsfrom pyrrole folding,pyrrole tilting and ethyl bending coordinates, respectively.' None of the three bands shift upon 13Cor dzo isotope substitution, which is consistent with the proposed assignments of all three to the Bzu symmetry block in the parent D4h point group. The Bzu representation is invariant to rotations about the 2-fold axes passing through the methine bridges, so that out-of-plane motions of either the methine C atoms or of the phenyl rings is precluded. Although lowering the point group to S 4 via porphyrin saddling converts the Bzublock to A symmetry,the eigenvectorsare unlikely to change enough to alter the isotope pattern significantly. The A symmetry does explain, however,why the bands are prominent, since the excited state can experience displacement along totally symmetric coordinates. The saddling of the porphyrin itself derives from a BzUdistortion of the planar D4h porphyrin. In particular, the 716 mode, which primarily involves tilting of the pyrrole rings,3 carries the saddled porphyrin back toward a planar structure; therefore, it should modulate the U-T* transition moments significantly. The 314-cm-l band, assigned to 716, is one of the strongest bands in the 441.6-nm excited spectrum. It is substantially stronger than the skeletal modes vg and v9, which usually give rise to the strongest bands in the Soret-excited RR spectra of metalloporphyrins. We note that this Franck-Condon [A-term] scattering mechanism differs significantly from the activation mechanism for out-of-plane modes in NiOEP and NiTPP.3 For those molecules, the out-of-plane modes were most prominent with Q-band excitation, presumably reflecting a vibronic mechanism, in which the out-of-planemodes were enabled to mix the Q and B transitions vibronically as a result of the out-of-plane distortion. In contrast, the three identified NiOETPP out-of-plane modes give strong bands with Soret excitation but significantly weaker bands with


Piffat et al.

Q band excitation. This difference in mechanism must reflect the different forces responsible for the out-of-planedistortion. In all three porphyrins, the Ni-N [pyrrole] bonds contract, but in the case of NiOETPP, additional steric clashes between the peripheral substituents occur.

In addition, they assign the phenyl modes 4's and 4''s to bands at 1506 and 1475 cm-1, but these phenyl modes are well isolated and cannot differ by 30 cm-I (we calculate them at 1501 and 1500 cm-l); the 1506-cm-' band is observed in the IR in both studies and is assigned to 4"s.

Conclusions Most of the in-plane skeletal modes have been assigned for NiOETPP on the basis of isotope shifts and Raman polarizations. Many of these modes, as well as the ethyl and phenyl substituent modes, are satisfactorily calculated with a planar porphyrin skeleton and a force field transferred from NiOEP and NiTPP. Effects of the steric crowding of the substituents can be seen in large downshifts of ca. 7 0 cm-' relative to the calculated frequencies both in the out-of-phase C.C, stretching modes, u10 and u19, and in the C& , stretching modes, u2 and u11. For the C,C, stretching modes, these downshifts are attributed to kinematic effects of the porphyrin saddling, while for the C&S stretching modes, they are attributed to the lengthening of the C&p bonds seen in the crystal structure. Certain other modes involving the pyrrole rings, u4, u7, and Ul6, also experience large downshifts. Three out-of-plane modes, 715, 716, and 6 4 are tentatively assigned to bands which are strongly enhanced in the Soret-excited RR spectrum. They are of Bz,,symmetry in the D4h point group, the same symmetry as the saddling distortion itself. The 7 1 6 band is one of the strongest in the RR spectrum; it involves tilting of the pyrrole rings, a motion that leads directly to saddling.

References and Notes

Acknowledgment. We thank Dr. Kristine Prendergast for help with the normal-coordinatecalculations and Dr. Jack Fajer for helpful discussions. This work was supportedby NIH Grant GM 12526.

Noted Added in hoof. Since this article was accepted for publication, Stichternath et al.23 published a vibrational study of NiOETPP, and its phenyl-dzo isotopomer, in which mode assignmentswere suggested,althoughwithout a normal coordinate calculation. These assignmentsare in good agreement with ours except for um,u12, U6, and V I S , which they assign at 1352, 1292, 844, and 760 cm-l while we assign them at 1376, 1355,905, and 855 cm-' and calculate them at 1389, 1328,900, and 916 cm-1.

(1) Li, X-Y.; Czernuezewicz, R. S.;Kincaid, J. R.; Su,Y.0.; Spiro, T. G. J. Phys. Chem. 1990,94,31.

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