369

J. Phys. Chem. 1984,88, 369-374

Resonance Raman Spectra of S,TPP, SSeTPP, Se,TPP, and H,TPP: Extended Tetraphenylporphine Vibrational Assignments and Bonding Effects Paul Stein,+$Avi Ulman,s and Thomas G. Spire*+ Department of Chemistry, Princeton University, Princeton, New Jersey 08544, and Isotope Department, Weizmann Institute of Science, Rehovot. Israel (Received: May 23, 1983)

Resonance Raman (RR) spectra of dichalcogen (S or Se) substituted and free-base tetraphenylporphine (TPP), obtained with B-band excitation, show features corresponding to most of the expected in-plane porphyrin skeletal vibrations, and a pair of bands due to phenyl vibrations (1598, 640 cm-I). H2TPP (free base) also shows three depolarized bands (1383,441, 167 cm-') and one anomalously polarized band (1 135 cm-'), and two extra bands, at 1327 and 674 cm-', which disappear in D2TPP and are assigned to in- and out-of-plane NH bending modes. The porphyrin skeletal modes can all be correlated with previously recorded modes of (FeTPP),O and its d8 and d,, isotopic forms, and the symmetry lowering (&, DZh) in the X,TPP molecules allows assignment of depolarized D4kparent modes to B,, or B,, symmetry classes. The previous assignments of the TPP skeletal modes are extended with the aid of the present data. The RR relative intensities of SSeTPP are very different for excitation at 4579 or 5145 A, consistent with x polarization (the x axis passes through S and Se) of the higher energy Q transition and y polarization of the lower energy B transition. A general lowering of all skeletal frequencies between SzTPP and Se2TPP, greater than expected from the mass change alone, supports the inference from previous electrochemical studies and from MO calculations that Se is more effective than S in draining ?r electrons from the porphyrin ring. Two specifically X-sensitive bands are assigned to C,X stretching and symmetric X-ring deformation, occurring at frequencies close to analogous modes in thiophene and selenophene. Both pairs of bands are observed for SSeTPP, at slightly shifted frequencies, indicative of some inter-ring interaction. The interaction may be linked to direct Xes .X bonding or may be transmitted via the porphyrin conjugation. One of the low-frequency porphyrin deformations is attributed to a radial motion of the X-rings, showing a systematic decrease from S2TPP(330 cm-') to SSeTPP (295 cm-') to Se,TPP (280 cm-I). The decrease is less than expected from the increased mass of the selenophene ring and is consistent with stronger Sese than SS bonding. Differential mixing with other porphyrin deformation coordinates could also explain the trend, however. SSeTPP shows two weak bands, at 1106 and 1019 cm-I, which are coincident with IR bands and are attributable to activation of E,-derived modes. Their weakness, and the absence of the many other E,-derived modes, implies that there is no large displacement of the porphyrin pseudosymmetry center in the excited state of SSeTPP.

-

Introduction The recent availability of tetraphenylporphine analogues in which the two oppositely positioned pyrrole NH groups are replaced by chalcogen atoms, S or Se,' has stimulated research on the alterations in the porphyrin electronic structure induced by this substitution.1-5 A particularly arresting conclusion from electrochemical results2and from the molecular orbital calculations of Gouterman and co-workers3 is that the chalcogen atoms drain a electrons from the porphyrin ring and take part in a direct bonding interaction across the ring, the effects being greater for Se than for S. The a-electron shift is supported by the measured proton N M R chemical shift^,^ while an appreciable Sese bond is suggested by the short Se-OSe contact, 2.91 A, found in Se2TPP, compared with 3.02 A for the S . 8 separation in S2TPP.5 In this study we report resonance Raman (RR) spectra for S2TPP, SSeTPP, and Se,TPP, and H2TPP, the tetraphenylporphine free base, obtained with B (Soret) band laser excitation, and also (for SSeTPP) with excitation in the highest energy component (520 nm) of the Q band. Porphyrin fluorescence, coming mostly from the bottom of the Q band, was sufficiently red shifted that high-quality R R spectra could be obtained. Correlation of the observed frequencies lends additional support to the a-draining tendency of Se2TPP,relative to S2TPP. Specific modes of the thiophene and selenophene rings have been identified, and an assignment is suggested for a mode involving the relative displacement of these rings, whose frequency lends plausible, although not unambiguous, support to the existence of a Sese bond. Additionally, the X2TPP spectra correlate with previously studied metallo-TPP spectra6-8 in a manner, consistent with well-established porphyrin scattering mechanisms,'-10 that extends the TPP vibrational assignments.

Experimental Section The chalcogen TPP's were prepared as described previously' and purified on basic alumina. HzTPP was purchased from Mann-Win; it was converted to D2TPP by dissolution in a 5:l dimethylformamide-D20 mixture." Raman spectra were recorded on solutions contained in a spinning cell, by using a Spex 1401 monochromator with a cooled photomultiplier and photon counting electronics, with excitation by argon or krypton (Spectra Physics 170,171) lasers. IR spectra were recorded on Nujol mulls, by using a Perkin-Elmer Model 283B spectrophotometer. Results Figures 1 and 2 compare RR spectra, obtained with Soret (4579 A) excitation, for S2TPP, Se2TPP, and SSeTPP, while Figure 3 compares their IR spectra, in the 1000-1 150-cm-' region. Figure 4 shows the effect on the R R spectrum of SSeTPP of shifting the excitation wavelength to 5145 A, close to the highest energy component (520 nm) of the Q band. Figure 5 shows the R R spectrum of H2TPP. The quality of the RR spectra is high, and (1) Ulman, A,; et al. J. Am. Chem. SOC.1975,97,6540; Tetrahedron Lett. 1968, 167; 1978, 1885; J. Chem. SOC.,Perkin Trans. I 1979, 1066. (2) Ulman, A.; Manassen, J.; Frolow, F.; Rabinovich, D. Inorg. Chem. 1981, 20, 1987.

(3) Hill, R.; Gouterman, M.; Ulman, A. Inorg. Chem. 1982, 21, 1450. (4) Ulman, A.; Manassen, J.; Frolow, F.; Rabinovich, D. J. A m . Chem. SOC.1979, 101, 7055.

( 5 ) Frolow, F.; Rabinovich, D.; Ulman, A,; Manassen, J., in preparation. (6) Gaughan, R. R.; Shriver, D. F.; Boucher, L. J. Proc. Nutl. Acad. Sci. U.S.A. 1975. 72. 433. - - .. -(7) Shelnutt, J. A,; Cheung, L. D.; Chang, R. C. C.; Yu, N.-T.; Felton, R. H. J. Chem. Phys. 1977, 66, 3387. (8) Burke, J. M.; Kincaid, J. R.; Spiro, T. G. J. Am. Chem. SOC.1978,100,

-. -.

hn77. .

+Princeton University. *Current address: Department of Chemistry, Duquesne University. Weizman Institute; for previous papers in a continuing series on tetraphenylporphyrins with heteroatoms other than nitrogen, see ref 1-5.

0022-3654/84/2088-0369$01.50/0

(9) Spiro, T. G.; Stein, P. Annu. Reo. Phys. Chem. 1977, 28, 501. (10) Spiro, T. G. In 'Iron Porphyrins", Part 11, Lever, A. B. P., Gray, H. B., Eds.; Addison-Wesley: Reading, MA, 1983; pp 82-160. (1 1) Mason, S. F. J . Chem. SOC.1958, 976.

0 1984 American Chemical Society

370 The Journal of Physical Chemistry, Vol. 88, No. 3, 1984

Stein et al.

I

J

Figure 4. High-frequency Raman spectra of SSeTPP in benzene (0.20 mM) compared for 4579- and 5145-A (near the highest energy Q band, 520 nm) excitation. Conditions: as in Figure 1.

I

I

I

1

I

I

I200

I000

I

1

I

1400

IS00

C M-I

Figure 1. Soret-excited (A, = 4579 A) Raman spectra of Se2TPP, SSeTPP, and S2TPPdissolved in benzene (0.21, 0.20, and 0.14 mM, respectively), in the high-frequency region. Solvent bands are marked S. Conditions: 200 mW, 8-cm-' slit width, 0.5 cm-'/s scan rate. *OE

40,

IO0

600

coo

100

100

600

I

CM-

I

I

I

1

230

I

Figure 5. Soret-excited (4131 A) Raman spectrum for H2TPPin benzene (0.24 mM). Conditions: 75 mW, 8-cm-'slit width, 0.5 cm-l/s scan rate.

1

400

600

I

I

800

I

C M-' Figure 2. Spectra as in Figure 1, in the low-frequency region.

Discussion Vibrational Mode Assignments. Since the visible absorption spectra of porphyrins are dominated by in-plane T-T* transitions, RR enhancements are expected, and observed, primarily for inplane vibrations of the porphyrin skeleton. In the analysis of the XzTPP RR spectra, it is useful to correlate the bands with those of metallo-TPP's, which have been extensively If one assumes D4hsymmetry and counts the phenyl rings as point masses, a metallo-TPP has 71 in-plane vibrational degrees of freedom, classified as = 9Al, + SAzg + 9B1, + 9B,, + 18E, The E, modes are infrared active only. Depolarization ratios can be used to identify the symmetry species of the Raman modes ( p < ,/4, AI,; p = 3 / 4 , BI, or BZg;p > 3/4, A,,), although there is an ambiguity between B,, and Bag modes, both of which give depolarized bands. The chalcogen porphyrins have idealized Dlh symmetry as does free base H,TPP (protons on opposite pyrrole N atoms). The Ddh DZhcorrelations are as follows: A,,, B,, Alg ( p 5 3/4); A,,, B2, B,, ( p L 3/4); E, B,, + B,, (Raman inactive). Since the symmetry center is preserved, infrared modes remain Raman inactive, but polarized Raman bands (A,) arise from both AI,and B,,-type modes, while depolarized or anomalously polar bands (B1J arise from both Azg-and B,,-type modes. Thus, correlating MTPP with X2TPP RR spectra provides a means for distinguishing B,, from BZgmodes, as has been demonstrated for physiological-typeporphyrin~.'~J~ In SSeTPP, the inversion center is lost, allowing Raman activation of infrared modes. The idealized symmetry is now C,,, and the D2h C,, correlations are A,, BZu A I and B1,, B,, B,. Since the central metal ion is missing in the chalcogen porphyrins, there are two fewer in-plane degrees of freedom, corresponding to the E, mode of metalloporphyrins involving off-center displacement of the metal ions. This mode is likewise missing for H,TPP, but there are additional modes involving the protons on the pyrrole N atoms. With the aid of these correlations we can proceed to analyze the RR bands in terms of the porphyrin internal coordinates, using

--

IIJ

I

-+

1100

1000

c m-' Figure 3. IR spectra in the 1000-1 150-cm-' region for S2TPP,SSeTPP, and Se,TPP in Nujol mulls. Bands coincident with SSeTPP Raman bands are marked.

almost all the expected polarized bands are observed (vide infra). In addition the HzTPP spectrum shows depolarized ( p = 0.75) bands, at 1383, 441, and 167 cm-I, and an anomalously polarized band at 1135 cm-I, even though nontotally symmetric modes are not usually observed with Soret excitation.

-

-

-

-

(12) Spiro, T. G.; Strekas, T.C. Proc. Naatl. Acad. Sci. U.S.A. 1972, 69, 2622. (1 3) Kitagawa, T.; Ogoshi, H.; Watanabe, E.; Yoshida, 2. J . Phys. Chem. 1975, 79, 2629.

The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 371

Dichalcogen-Substituted and Free-Base TPP

TABLE I: RR Frequencies (cm-')a and Assignments for X2TPP and (FeTPP),O -

X2TPP assignmen tb

Se2TPP

SSeTPP

S2TPP

1598 1397 1521

1598 1407 1534

1598 1406 1539

1597 1438 1550

- 1432 1246L.i

1442, 1253' 1314

1456 -1262iJ 1316

1499 1292 1357

1308 527

1167 1240

I s: I 1170 1247

630

1178 1253

H2TPPC

(FeTPP),Od

Adae

AdZue

1599 1561 (dp) 1553 151 1 (ap) 1495 (dp) 1450 1359

0 0 21 0 51 22 11

1004h

1271 (dp)

8

0 0 1 0 0 3 0

1383 (dp)

70

+5

1234 1135 (dp)

1368 (dp) 1333 (ap) 1189 (dp) 1237 1234 (ap)

3

50 9

1076 1030

1087 (dp) 1083 1030

92 92 137 0 4 0 +3

32

67

(1094)' 1068' 1064

1106 1071 1066

(1118)' 1074' 1068

(1015)'

1019

(1024)'

975

972

975

962

862

867

868

886

1014 (dp) 1005 995 886

821

iE1

837

833

848 (dp)

0

23

5 00

638 500 441 (dp)

640

490

4 05 280

4 06 295

405 330

334

8 0

0 0

175

174

176

390 257 (dp) 195 (PI" 195 (dp)

620 483

197 167 (dp)

(FeTPP),O assignmentf Dhenvl

1 0 166

0

0 10 183

a All bands are polarized except those marked dp (depolarized) or ap (anomalously polarized. Suggested major contributing coordinates; (N) and (X) label bonds in the pyrrolenine and pyrrole (or thio- or selenophene) rings, respectively. Additional bands, at 1327 and 674 cm-', disappear for D,TPP and are assigned to in- and out-of-plane NH bends. Data from ref 8, except 1368- and 1189-cm-' (dp) bands, reported in ref 14, and the 195-cm-' (p) band reported for (FeTPP),N in ref 17. e Frequency downshifts (or upshift, +) f o r d , (pyrrolenine C-D's) and d,,, (phenyl C-D's) substitution in (FeTPP),O; from ref 8. Suggested major contributing coordinate and symmetry label; s and as are symmetric and antisymmetric combinations (see Table 11). g The mode numbering system has been altered from ref 8 to conform with that adopted by Abe et a1.I' for octaethylporphyrin. The in-plane skeletal modes are numbered consecutively, in order of decreasing frequency, v I , i ~ , , ..., within each symmetry block, in the order A l g , A2g,Big, BZg. The first number in each symmetry block is reserved for a C-H stretching mode, although these have not been observed. Phenyl modes are labeled with capital letters (Table 111). Shifts down 10 cm" for D,TPP. ' Observed with 5145 A excitation. Frequency uncertain due to overlapping band. Observed in the IR spectrum only. Out-of-plane folding of the pyrrole rings. Suggested to be a radial mode of the X-rings, with a contribution from X..X stretching. Band observed for (FeTPP),N (ref 17).

(FeTPP)*O as a reference molecule. Its RR bands have been tentatively assigned with the aid of d8 (pyrrole) and d20(phenyl) isotope shifts6 These assignments are extended and modified in the ensuing discussion and are summarized in Table I. (The mode numbering has been altered from that of ref 8, to accord with the system used by Abe et al.I4 in analyzing octaethylporphyrin vibrations, as described in the Table 1 footnotes.) Although the porphyrin internal coordinates are expected to mix appreciably in the normal modes, the deuterium shifts for (FeTPP)20 are sufficiently selective (Table I) to allow a meaningful distinction between modes involving the pyrrolenine rings primarily (d6 sensitive) and those concentrated at the methine bridges or the phenyl rings (d20sensitive). Furthermore, it is useful to discuss the modes in terms of contributions from the various bond stretching and bending coordinates. Their contributions to the symmetry blocks of a D4h metalloporphyrin are given in Table 11, while the atom labels are defined in Figure 6. The coordinates fall into sets of four or eight equivalent members. Those with eight members are conveniently divided into two subsets, involving combinations that are symmetric or antisymmetric with respect to the pyrrole 2-fold axes (v(cbH), 6(CbH), v(C,N)) or the methine 2-fold axes (v(c,cb)). The low-frequency in-plane deformation modes of the entire porphyrin skeleton involve many redundant angle bending coordinates and (14) Abe, M.; Kitagawa, T.; Kyogoku, Y.J . Chew. P h p . 1978,69,4526.

TABLE 11: In-Plane Internal Coordinatea Contributions t o the Symmetry Blocks of a D 4 h Metallo-TPP CbH

Vsb

A*, 1

Big

"as CbH s s

1

1

6 as Cacb *s

1

1

C,N v s

1

1

J'as v(cbcb) 11(cm-P h) 6 (Cm-Ph)

s v(MN) 6 (porld rin-plane

1 1

1 (lie 1 9

*2,

B*g

1 1

1

1

1

1

1

1

1 1

1 1

1

1 1

1

1 9

2 8

1

3 9

E, 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3 18

See Figure 6 for atom labeling. s and as refer to symmetric and asymmetric combinations with respect to the pyrrole or, for v(CaCm), methine 2-fold axes. Symmetric (with respect to the pyrrole 2-fold axis) deformation of the pyrrole rings. Collective designation for in-plane deformations of the porphyrin ring as a whole. e These contributions are redundant with porphyrin deformations.

372 The Journal of Physical Chemistry, Vol. 88, No. 3, 1984

Stein et al. below). Rather, a d20-sensitive (FeTPP)20 mode at 640 cm-I, seen at 638 cm-’ in H,TPP and 620 cm-’ in Se2TPP,is assigned to the phenyl mode, G, in this region. Two phenyl modes, B and D, are expected at 1500 and 1200 cm-’ but have not been observed. The phenyl mode C, 1237 cm-l in (FeTPP)20 and 1234-1253 cm-I for X2TPP, is a strong RR band and was earlier assigned as the C,-phenyl stretch.’ The eigenvector of the corresponding biphenyl model6 does show a large displacement of the inter-ring bond, and a large contribution from the C,phenyl coordinate is likewise implicated. It is the lowest frequency biphenyl mode (H), at 315 cm-’, however, that has all the atoms moving outward from the center in phase16 and therfore best qualifies as the inter-ring stretch. The mode involving motion of the phenyl groups as rigid bodies vibrating against the porphyrin ring, which seems best suited to the v(C,-Ph) assignment, is expected at even lower frequency (the porphyrin ring being more massive). While no polarized band below 300 cm-’ has been detected in the (FeTPP)’O R R spe~trum,~J’ fairly prominent polarized bands have been reported at 205, 204, and 195 cm-l for (CrTPP)C1,18CuTPP,” and (FeTPP)2N,17which we identify as mode H, or v(C,-Ph), along with the strong polarized bands at 197 cm-’ for HzTPP and 175-176 cm-I for the chalcogen porphyrins. As expected, porphines, which have only H atoms as peripheral substituents, have no counterpart mode; the lowest frequency Ai, modes of copper(I1) and nickel(I1) porphinelg are at 368 and 371 cm-l. Methine Stretching. The asymmetric v(CaCm)modes (A2, + B,, + E,, Table 11) are expected at relatively high frequency, as in physiological-type porphyrins,’ because of the substantial abonding through the methine bridges. For (FeTPP)20they have been assigned8at 1561 (B,,, v,,) and 1511 (A2,, vzo)cm-I. These should correlate with A, and B,, modes of X’TPP. The B,, counterpart of vz0 has not been located, but the A, mode correlating with v,, is assigned to a band at substantially lower frequency, 1397-1436 cm-’, in X’TPP. The frequency lowering is attributed to the alteration in the conjugation pathway. The symmetric v(C,C,) modes are expected at much lower frequencies, because of strong interaction with the methine-phenyl stretches (which by symmetry cannot interact with the asymmetric modes). The A, mode was assigned’ to a strong d,,-sensitive band at 1004 cm-’ of (FeTPP)’O, and the strong band observed at 962-995 cm-l for XzTPP (obscured by the strong solvent band in Figure 1, but seen in deuteriobenzene) is similarly assigned. The corresponding B2, mode (BIBfor X,TPP) has not been observed. Pyrrole and Pyrrolenine Modes. CbCb Stretching. The c b - c b stretching modes (A!g + B1, E.,,) are expected at high frequencies and were assigned in (FeTPP)20 to ds-sensitive bands at 1553 (A,,, v2) and 1495 (Big, vlZ) cm-I. They correlate with A, modes of XzTPP at 1521-1550 and 1432-1495 cm-’. However, it seems likely that the higher of the two v(Cb-Cb) modes predominantly involves the pyrrolenine rings (cbcb(N)) while the X-ring (c&b(x)) predominates in the lower mode. Consistent with this interpretation, the lower of the two frequencies shows a larger decrease, 24 vs. 18 cm-’, between S2TPPand S,TPP; Se,TPP is calculated3to have lower a-bond orders than S’TPP, and the effect is greatest in the X-rings. CON and C,J Stretching. The strong 1359-cm-I band of (FeTPP),O is analogous to the lSN-sensitiveband at 1370 cm-I in physiological-type metalloporphyrinsI4 and has been assigned’ to the breathing mode of the C,N bonds (Alg). Strong bands at 1292-1308 cm-’ in the X,TPP spectra are likewise assigned to pyrrolenine Ca-N stretching; the higher frequency for (FeTPP),O is attributed to interaction with Fe-N stretching. The second symmetric v(C,N) mode (B,,, ~ 1 3 )of (FeTPP),O is assignable to the 1271-cm-’ depolarized band; the two symmetric v(CaN) modes show similar d, shifts, 11 and 8 cm-’. vi3 correlates with

-

Figure 6. Structural formulas for a metallo-TPP, and for XYTPP (X, Y = NH, S,Se), with atom labeling, and showing the dominant 16- and 18-membered conjugation pathways. TABLE 111: Correlation of (FeTPP),O RR Bands (cm-‘) Assigned as Phenyl Modes with the A , Modes of Biphenyl’‘ label biphenyl Ada (FeTPP),O Ada

__

-

A B C D E F G H

1612 1507 1285 1190 1030 1003 742 315

41 95 96 230 16 1 168 52 15

1599

32

1237

50

1030 995 640 194b

166 183 23

a rrequency dounshift upon phenyl perdeuteration. observed for (FeTPP),N” (see text).

Band

are lumped together in a single line of Table 11. The CH stretching modes are in the region near 3000 cm-’ and are neither expected nor observed to be resonance enhanced. The CH bending modes are expected near 1000 cm-’ and can mix with the stretching modes of the ring bonds, which are expected from 1000 to 1600 cm-’. For metalloporphyrins, the dominant conjugation pathway is the 16-membered inner ring, as illustrated in Figure 6. X-ray structure datals confirm that the CbC, and CaCb bonds are close to being double and single bonds, while the C,N and C,C, bonds are in between. The expected order of stretching force constants, and natural frequencies, is CbCb > C,c,, C,N > C,cb, but the various interactions in the porphyrin ring produce considerable interleaving of the normal modes having major contributions from these coordinates. For H2TPP the conjugation shifts mainly to the 18-membered pathway including the outer parts of the pyrrole rings. The bonds in the pyrrole (NH) rings are not equivalent to those in the pyrrolenine (N) rings, bond orders now falling in the order cbcb(N)> cbcb(”), CaCb(”), C,N(N), C,C, > C,Cb(N), C,N(NH). Thus, in addition to symmetry lowering per se, some alteration in compositions is expected for modes which formally correlate between metalloTPP’s and H2TPP. Similar alterations are expected for the chalcogen porphyrins, and additional changes should be induced by the substitution of S and Se, due to altered masses and bonding effects. Phenyl Modes. Modes internal to the phenyl rings are expected to occur at essentially the same frequencies as in other monosubstituted benzenes. Biphenyl is a particularly apt model compound, since the inter-ring bond is similar to the C,-phenyl bonds of TPP. Table I11 lists the eight totally symmetric modes (excluding C-H stretching) of biphenyl,I6which we label A-H. These are analogous to the phenyl modes which can mix with the C,-phenyl bond stretching coordinate, and which may thereby gain enhancement in the RR spectrum. The (FeTPP),O R R spectrum contains bands which correlate satisfactorily with modes A, C, E, F, and G, both in frequency and in the shift observed upon phenyl perdeuteration, as shown in Table 111. Three of these, 1598, 1030, and 995 crn-’, were earlier identified as phenyl modes! and called A, B, and C, but are now relabeled. A fourth (FeTPP),O band, at 866 cm-’, was tentatively assigned as a phenyl mode* but is now reassigned to a porphyrin skeletal mode (see (15) Hoard, J. L. In “Porphyrins and Metalloporphyrins”; Smith, K. N., Ed.; American Elsevier: New York, 1975; pp 317-66. (16) Zerbi, G.; Sandioni, S. Spectrochim. Acta, Parr A 1968, 24, 511.

N

+

-

(17) Schick, G. A,; Bocian, D. F. J . Am. Chem. Soc. 1983, 105, 1830. (18) Cheung, L. D.; Yu,Nai-Teng; Felton, R. H. Chem. Phys. Lett. 1978, 55, 527. (19) Susi, H.; Ard, J. S. Spectrochim. Acta, Part A 1977, 33, 561.

Dichalcogen-Substituted and Free-Base TPP the symmetric C,-X stretch of the X-ring in the X2TPP molecules, which is expected at a much lower frequency because of the lowered C,X bond order, associated with the 18-membered conjugation pathway, and, for the chalcogen porphyrins, the greater mass of the X atom. For H,TPP, v(C,X) is located at 1004 cm-I, via the 10-cm-' downshift of this strong band in D2TPP. For S,TPP and Se2TPP, v(C,X) is assignable to strong bands at 630 and 527 cm-I, since SSeTPP shows two strong bands close to these frequencies, at 622 and 517 cm-'. In thiophene17and selenophene18 this mode occurs at 606 and 457 cm-'. The asymmetric C,N stretches of (FeTPP),O are expected at lower frequencies than the symmetric modes because they do not interact with FeN stretching. The A,, mode, u2,, was assigneds to the anomalously polarized band at 1234 cm-'. The 1135-cm-I band of H,TPP is also anomalously polarized ( p = 1.O at 4067 A) and is suggested to arise from the corresponding asymmetric pyrrolenine C,N stretch. Neither the B2, C,N mode of (FeTPP),O nor the asymmetric pyrrole C,N mode of H2TPP has been located. C,CbStretching. The A, and A,, CaCb modes were assigned8 to the remaining > 1100-cm-f polarized and anomalously polarized bands of (FeTPP),O, at 1450 and 1333 cm-', both of which show large (22 and 70 cm-') ds shifts. HzTPP has a depolarized band at 1383 cm-' which is suggested to correlate with the Bzg c,cb mode, assigned to the 1368-cm-' depolarized band observed for (FeTPP),O by Chottard and Mansuy.,O They also found a 1189-cm-' depolarized band, which we assign to the B,, C,Cb mode. There are nearby (1 167-1 178 cm-') bands of XzTPP which appear to correlate with this mode, while the 1246-1262-cm-' bands of XzTPP are suggested to correlate with the A,, CaCb mode. Since the dominant 18-membered conjugation pathway increases the CaCb double-bond character of the pyrrole relative to the pyrrolenine rings, it is likely that the higher frequency mode is mainly C,Cb(x) and the lower frequency mode is mainly C,Cb(N) in character. The ds-sensitive anomalously polarized (FeTPP),O band at 1333 cm-' is assigned to the AZgasymmetric C,Cb mode. C& and NH Deformation. The 6(CbH) modes should show large downshifts in the d8form of (FeTPP),O. Three such bands were located,8 at 1087 (dp), 1083 (p), and 1014 (dp) cm-'. The 1083-cm-' band is the A,, mode; it correlates with XzTPP bands at 1026-1046 cm-I. The 1087-cm-' band correlates with polarized (A,) bands of X,TPP, at 1068-1074 cm-I, and is therefore, the B,, mode, leaving the 1014-cm-' band as the B,, mode. The A,, mode has not been located (although the 67-cm-' d8 shift of the 1234-cm-' C,N mode requires substantial 6(CbH) character). Two HzTPP bands, 1327 and 674 cm-I, disappear in D2TPP and are assigned to in-plane and out-of-plane N H bending, respectively. Ring Deformation. Five-membered aromatic heterocycles have a symmetric in-plane ring deformation mode at -800 cm-'. For thiophene,' and selenopheneZ2these are found at 832 and 760 cm-', and the polarized S,TPP and SezTPPbands at 837 and 821 cm-' are assigned to corresponding (in-phase) breathing modes of the X-rings. They correlate with the polarized 833-cm-' band of H2TPP and the depolarized 848-cm-' band of (FeTPP),O, which is therefore assigned to the B,, combination of the pyrrolenine symmetric ring deformations. The Al, combination is assigned to the polarized 886-cm-' band of (FeTPP),O, which correlates with the X,TPP bands at 862-886 cm-'. Consistent with these correlations, SSeTPP shows two X-ring deformations, 8 18 and 845 cm-', close to the S,TPP and Se2TPP frequencies, but only one pyrrolenine deformation, 867 cm-', as expected. The 886-cm-' (FeTPP)20 band had been assigneds to a phenyl mode, since it was not located in the dz0spectrum, although it was recognizedS that other monosubstituted benzenes do not show polarized Raman bands in this region. In view of the present correlations, the (20) Chottard, G.; Battioni, P.; Battioni, J.-P.; Lange, M.; Mansuy, D. Inorg. Chem. 1981, 20, 1718.

(21) Rico, M.; Orza, J. M.; Morcillo, J. Spectrochim. Acta 1965, 21, 689. (22) Magdesiera, N. N. Adu. Heterocycl. Chem. 1970, 12, 1.

The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 373 pyrrolenine deformation assignment is more plausible; the absence of this band in the dzospectrum is not understood, however. The weak X2TPP bands at 483-500 and 405 cm-l do not have (FeTPP),O counterparts. At this frequency in-plane modes are not expected, but out-of-plane folding modes (ypyrrole) do occurz3 and have been identifiedz4in R R spectra of protohemes via their vinyl deuteration shifts. This assignment is suggested for X,TPP, their ruffled structures5plausibly providing a mechanism for the out-of-plane enhancement. It is possible that the 405-cm-l modes, not seen for H,TPP, are localized on the X-rings. Porphyrin Deformations. The two low-frequency depolarized bands of H2TPP, at 441 and 167 cm-I, must correlate with A,, or B,, porphyrin deformations. The latter band correlates with the 195-cm-I depolarized band of (FeTPP)20, which is therefore identified as a B f g mode. The strong 390-cm-' polarized band of (FeTPP),O is identified as the sole Alg deformation mode, us. It is suggested to correlate with the prominent (especially with Q-band excitation-not shown) bands at 334, 330, 295, and 280 cm-' for H,TPP, S,TPP, SSeTPP, and SeTPP. The strong shift in frequency along this series is evidence for major involvement of the X-rings, as discussed below. E , Modes in SSeTPP. The SSeTPP R R spectrum (Figure 4) shows two weak bands, at 1019 and 1106 cm-', which are not seen for SzTPPor Se,TPP, and are therefore candidates for u-derived modes. This assignment is confirmed by the IR spectra in this region (Figure 3) which show 1019- and 1106-cm-' bands for SSeTPP. Corresponding S,TPP and Se2TPP IR bands are seen at 1024, 1015 and 1118, 1094 cm-I and are missing in the RR spectra. These bands are therefore assignable to BZumodes (A, for SSeTPP), correlating with (unobserved) E, modes of (FeTPP),O. The weakness of these SSeTPP RR bands and the lack of evidence for any other of the numerous u-derived modes indicate that the origin shifts for these modes are very small and that the pseudosymmetry center of SSeTPP is not appreciably displaced in the excited state. Strictly speaking, the pairs of SSeTPP bands at 845, 818 and 622, 517 cm-', assigned to the ring deformation and C-X stretching modes of the thiophene and selenophene rings, are inand out-of-phase combinations, derived from the corresponding g and u (unobserved) modes of SzTPP and Se2TPP. The comparable intensities of the bands in each pair suggest that these thiophene and selenophene coordinates couple nearly independently to the excited state. Raman Intensities. Figure 4 shows a large change in the R R relative intensities of SSeTPP when the excitation wavelength is changed from 4579 A, near the B absorption band, to 5145 A, close to the highest energy component (520 nm) of the Q band. The pattern of changes suggests that the higher energy Q transition is x polarized, the x axis being defined by the line joining the X atoms in X,TPP, whereas the lower energy B transition is y polarized. The evidence is that bands arising from modes which are localized mainly on the pyrrolenine rings lose most of their intensity at 5145 A whereas X-ring localized modes maintain their intensity at 5145 A. A particularly clear example is the pair of bands at 1534 and 1442 cm-' assigned to V(CbCb(N)) and u(Cbcb(x)), respectively. At 4579 A, the u(CbCb(N))is stronger, but it disappears at 5145 A, while the u(cbCb(x)) intensity is unaltered. (Since the Q-band oscillator strength is much weaker than that of the B band, the V(CbCb(N)) origin shift must actually be larger for the former, in order to account for the undiminished intensity.) Moreover, both of the G,(ring(X)) modes, at 845 (S) and 818 (Se) cm-', are seen at 5145 A (spectrum not shown) as well as 4579 A whereas the 867-cm-' G,(ring(N)) mode disappears at 5145 A. X2TPPBonding Effects. A striking trend revealed by Table I is that nearly all the porphyrin frequencies decrease in a continuous manner from S,TPP to SSeTPP to Se2TPP. Part, but (23) Colthup, N. B.; Daily, L. H.; Wiberley, F. E. "Infrared and Raman Spectroscopy", 2d ed.; Academic Press: New York, 1975; p 275. (24) Choi, S. H.; Spiro, T. G. J . Am. Chem. Soc. 1983, 105, 3692. (25) Gouterman, M. In "The Porphyrins"; D. Dolphin, Ed : Academic Press: New York, 1979; Vol. 111, Part A, pp 1-156.

J . Phys. Chem. 1984, 88, 374-380

374

not all, of the trend is attributable to the heavier mass of Se and the mixing of S and Se motions into the various porphyrin modes. This can be seen from the product which can be written in the following form: nFi,Se2TPP/nFi,S,TPP

= mSenV,,Se2TPp2/(mSllu,,S2TPP2)

where ms and mse are the S and Se masses, u, and F, are the frequency and force constant for the ith normal mode of the indicated molecule, and the products are taken over all the modes in a given symmetry block. When evaluated for the 15 assigned A, modes of S2TPP and Se2TPP, the right-hand side of the equation has a value of 0.93, implying a 7% reduction in the product of the A, force constants between S2TPP and Se2TPP. The calculation leaves out of account the unobserved A, mode, vl, which is CbH stretching. Since vI does not involve appreciable S(Se) motion, its omission does not affect the conclusion that the porphyrin restoring forces are significantly weakened in Se2TPP, relative to S2TPP. This conclusion directly supports the results of M O calculation^,^ indicating that net a bonding is lower in Se2TPP than S2TPP because Se is more effective than S in draining a electrons from the ring. The modes most directly involving the X atoms are those as) to the symmetric deformations signed to C,X stretching ( q 3 and of the X-rings (u16). As noted above, these are found at frequencies close to the analogous modes of thiophene and selenophene themselves. SSeTPP shows both sets of bands, implying essentially localized motions of the X-rings. Some interaction between the rings is indicated however by the lack of coincidence between the SSeTPP frequencies and the corresponding frequencies for S2TPP and Se2TPP. In particular the C,S and Case frequencies ( q 3 ) are both lower, by 8 and 10 cm-’, than those of S2TPP and Se2TPP. It is possible to explain these lowerings by taking into account the cross-porphyrin X. -X bonding interactions which the M O calculations3 suggest to be significant, particularly for

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(26) Wilson, E. B.; Decius, J. C.;Cross, P. C. “Molecular Vibrations, The Theory of Infrared and Raman Vibrational Spectra”; McGraw-Hill: New York, 1955.

Se2TPP, which has a short Se.. .Se separation, 2.80 A, compared to 3.04 A for the .S separation in S2TPP.5 An attractive force is implied by the direction of the shifts. However, the required perturbation could arise via the porphyrin conjugation as well as a direct X. -X interaction. Stronger evidence for X. * .X bonding might be the identification of a mode attributable to XX stretching. Disulfides and diselenides show XX stretching bands at -520 and ~ 2 9 cm-’, 0 the frequency lowering being due to the greater Se mass and lower Sese force constant. In porphyrins, however, the number of vibrational modes is completely determined by the structure of the ring and is independent of any cross-ring interactions. Stretching of the XX bond, if present, is redundant with the deformation coordinates of the ring. Nevertheless, the XX interaction, if present, should contribute significantly to the low-frequency deformation modes. In particular, it is reasonable to expect that one of these modes involves a radial motion of the X-rings themselves and would be principally affected by the X- .X interaction. We tentatively identify Y8 (Table I) with this mode, noting the strong decrease from S2TPP(330 cm-I) to SSeTPP (295 cm-I) to Se2TPP (280 cm-’). The decrease, however, is less than might be expected from mass effects alone. If this mode is regarded as being due to a hypothetical diatomic oscillator with dynamical masses equal to those of thiophene (94) and selenophene (141), then the required force constants would be 3.02, 2.90, and 3.25 mdyn/A. If the dynamical masses are just those of S and Se, the force constants would be 1.03, 1.17, and 1.82 mdyn/A. In either case the trend is consistent with stronger Se...Se than S...S bonding. In the absence of a reliable normal-mode calculation, however, one cannot rule out that the trend reflects differential mixing of the ring displacements with other deformation coordinates. S a

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Acknowledgment. This work was supported by NSF Grant CHE-8106084 and N I H Grant H L 12526 (to T.G.S) and by funds provided to P.S. through a N I H Biomedical Research Support Grant, 11H-2940-8751, to Washington State University. Registry No. Se2TPP, 66951-06-2; SSETPP, 66951-07-3; S2TPP, 5751 1-57-6; H2TPP,917-23-7; (FeTPP),O, 12582-61-5.

Conformational Study and Vibrational Analysis of Ethyl Thiocyanate J. R. Durig,* J. F. Sullivan, and H. L. Heuself Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received: May 25, 1983)

The infrared spectra (3500-40 cm-I) of gaseous and solid CH3CH,SCN and the Raman spectra (3200-10 cm-’) of the liquid and solid have been recorded. The vibrational spectrum has been assigned on the basis of a gauche form of C, symmetry, but several lines observed in the Raman spectrum of the liquid have been attributed to the less stable trans conformer. From a variable-temperature Raman study of the liquid, an enthalpy difference of 586 f 23 cm-I (1.68 f 0.07 kcal/mol) between the gauche and trans conformers was determined by measuring, as a function of temperature, the intensities of the lines attributed to the S-CN stretches of the two conformers. The methyl torsion was observed as a broad, nondescript band at approximately 230 cm-I in the spectrum of the gas which gives a periodic barrier of 3.5 kcal/mol(l220 cm-I). A reassignment of several of the low-frequencymodes has been presented and normal-coordinatecalculations have been carried out for both conformers. The results of this study are discussed in relation to the previous studies on ethyl thiocyanate and compared to corresponding studies of similar molecules.

Introduction Many of the early studies on rotational isomerism have centered on the rotation about carbon-carbon single bonds; however, more attention is now being directed to rotational isomerism about carbon-heteroatom single bonds. We have recently extended our ‘Taken in part from the thesis of H. L. Heusel which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree.

0022-3654/84/2088-0374$01.50/0

structural and vibrational studies of group 4A compounds with pseudohalogen linkages’ to include those alkyl isocyanates2 and i s o t h i ~ c y a n a t ewhich s ~ ~ ~ can exhibit rotational isomerism about (1) J. R. Durig, M. R. Jalilian, J. F. Sullivan, and J. B. Turner, J. Ramon Spectrosc., 11, 459 (1981), and references therein. (2) J. R. Durig, K. J. Kanes, and J. F. Sullivan, J . Mol. Struct., 99, 61 (1983). (3) J. R. Durig, J. F. Sullivan, and T. S. Little, J . Mol. Struct., in press.

0 1984 American Chemical Society