Resonance Raman, infrared, and normal coordinate analysis of free-base tetraphenylbacteriochlorin: A model for bacteriopheophytins

Article abstract of DOI:10.1021/jp962479g

Resonance Raman (RR) and FT-IR spectra are reported for free-base tetraphenylbacteriochlorin and its pyrrole-¹⁵N₄, β-D₈, meso-¹³C₄, and phenyl-D₂₀ isotopomers. A normal mode analysis is car

Full text of DOI:10.1021/jp962479g

472
J. Phys. Chem. B 1997, 101, 472-482
Resonance Raman, Infrared, and Normal Coordinate Analysis of Free-Base
Tetraphenylbacteriochlorin: A Model for Bacteriopheophytins
Ching-Yao Lin and Thomas G. Spiro*
Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544
ReceiVed: August 14, 1996; In Final Form: October 25, 1996X
Resonance Raman (RR) and FT-IR spectra are reported for free-base tetraphenylbacteriochlorin and its pyrrole-
15N4, â-D8, meso-13C4, and phenyl-D20 isotopomers. A normal mode analysis is carried out in order to assign
observed RR and IR modes, using a NiTPP-based force field, with bond-distance scaling of force constants
and the introduction of long-range interaction constants. The modes correlate well with those of NiTPP,
once due allowance is made for direct effects of pyrrole ring reduction: inequivalent Câ-Câ and CR-Câ
bonds and extra Câ-H bending modes. Although the CR-Cm and CR-N bonds of the 16-membered inner
ring are not strictly equivalent, there is little tendency for mode localization with respect to these bonds. The
absence of a central metal lowers the frequencies of CR-Cm stretching and pyrrole translational modes. The
RR enhancement pattern is quite different than for porphyrins. Totally symmetric modes dominate spectra
in resonance with the Qx band, due to a larger transition moment than in porphyrins, while non-totally symmetric
modes are strongly enhanced in the region of the close-lying Bx and By transitions, reflecting vibronic coupling
between them.
Introduction
Bacteriochlorophylls (BChl) and bacteriopheophytins (BPh)
play important roles in light harvesting, energy transduction,
and charge separation in the purple bacteria.1,2 Crystal structures
of photosynthetic reaction center complexes have been reported,3-5
and spectroscopic techniques are providing a detailed under-
standing of the electronic and molecular structure of these
tetrahydroporphyrin derivatives and their interactions with the
protein hosts. EPR, ENDOR, and TRIPLE have been used to
study the radical species within the chromatophore,6,7 and FT-
IR spectra have also been reported for the anion radicals of
bacteriochlorophylls.8 Resonance Raman (RR)9 and FT-Ra-
man10 spectroscopies have been employed because of their
sensitivity and selective enhancement for vibrational modes of
the chromophores.11-13
Figure 1. Molecular structures of (a) bacteriopheophytin a (BPh) and
(b) free-base tetraphenylbacteriochlorin (H2TPBC). Atom labels are
shown for H2TPBC.
Interpretation of the Raman spectra is hampered by their
complexity and by incomplete analysis of the vibrational modes.
There is good understanding of the vibrational modes of
al.24 However, we found that using excess reducing agent (3 g
of toluenesulfonylhydrazine to 200 mg of H2TPP) dramatically
shortened the reaction time from the reported 6.5 h to less than
half an hour.
porphyrins, thanks to extensive experimental and computational
studies of many derivatives and isotopomers, and this analytical
framework is being extended to hydroporphyrins, including
chlorins,14-19 isobacteriochlorins,20-22 and bacteriochlorins.18b,23
The bacteriochlorin normal mode studies have focused on nickel
1,5-dihydroxy-1,5-dimethyloctaethylbacteriochlorin [Ni(HOE-
BC)]23 and copper tetraphenylbacteriochlorin (CuTPBC).18b We
now report a comprehensive study of free-base tetraphenylbac-
teriochlorin (H2TPBC, Figure 1) including IR and variable
wavelength RR spectra of four isotopomers: [pyrrole-15N4]-,
[â-D8]-, [meso-13C4]-, and [phenyl-D20]H2TPBC. Assignments
are made with the aid of a normal coordinate analysis, and the
extensive data permit the development of a high-quality force
field. Because the analysis is for a bacteriochlorin free-base,
the results are expected to be useful in interpreting RR studies
of the bacteriopheophytin primary acceptor in reaction centers.1-5
Free-base [pyrrole-15N4]-, [â-D8]-, [meso-13C4]-, and [phenyl-
D20]TPBC were obtained from the respective porphyrin isoto-
pomers, which were prepared from isotope-labeled pyrroles (D5
and 15N) and benzaldehydes (D5 and 13C) according to the
method of Lindsey et al.25 Deuterated H2TPBCs (D8 and D20)
were checked by NMR spectroscopy to ensure the quality of
isotope labeling.
Spectroscopy. Resonance Raman spectra were obtained in
backscattering geometry either from THF solution in a spinning
NMR tube or from a rotating KBr pellet under vacuum.
Excitation lines were provided with a Coherent Innova 100K3
krypton ion laser (520.8 nm) and a Spectra-Physics 2025 argon
ion laser (363.8 nm). The scattered light was collected and
dispersed with a Spex 1404 double monochromator equipped
with a cooled photomultiplier (RCA 31034A-02). To minimize
laser-induced degradation, solutions were prepared with freshly
distilled THF, which was vigorously degassed with at least three
freeze-pump-thaw cycles.
Experimental Section
Synthesis. H2TPBC was prepared from H2TPP (Mid-
Century, Posen, IL) according to the methods of Whitlock et
* To whom correspondence should be addressed.
X Abstract published in AdVance ACS Abstracts, December 15, 1996.
S1089-5647(96)02479-0 CCC: $14.00 © 1997 American Chemical Society

Spectroscopy of Tetraphenylbacteriochlorin
J. Phys. Chem. B, Vol. 101, No. 3, 1997 473
Figure 3. Frontier orbitals of bacteriochlorins.
TABLE 1: Structure Parameters Useda in
Normal-Coordinate Analysis of H2TPBC
Bacteriochlorin Core
pyrrole rings
pyrroline rings
Figure 2. UV-vis spectrum of H2TPBC in benzene. The arrows
Bond Length, Å
indicate the positions of excitation laser lines in the RR experiments.
Câ-Câ
CR-Cm
CR-Câ
CR-N
Câ-H
N-H
1.366
1.404
1.447
1.378
1.087
0.997
1.488
1.384
1.493
1.371
1.088
The electronic absorption spectrum was obtained with a 1-cm
quartz cell on a Hewlett-Packard 8452A diode array spectro-
photometer. The FT-IR spectra of H2TPBC were recorded on
a Nicolet 730 Fourier transform infrared (FT-IR) spectropho-
tometer. All the spectral data were imported into and processed
with Labcalc software (Galactic Industries Co., Salem, NH).
Normal Coordinate Analysis (NCA). Normal mode cal-
culations were performed with the empirical GF matrix method
of Wilson.26 The molecular geometry was taken from the crystal
structure of five-coordinated zinc tetraphenylbacteriochlorin27
and slightly modified to retain D2h symmetry. The internal
coordinates are Wilson-type bond-stretching and angle-bending
coordinates. The symmetry coordinates (Si) and the corre-
sponding U matrix were obtained from symmetry-adapted linear
combination of internal coordinates. A menu-driven, graphically
interfaced version of Schachtschneider’s program, developed
in our laboratory and implemented on a R3000 Indigo worksta-
tion (Silicon Graphics Inc.),28 was used to set up the G matrix
and to solve the secular equation |GF - Eλ| ) 0.
C1-Cm
1.500
Bond Angle, deg
CR-Cm-CR
CR-Câ-Câ
CR-N-CR
Câ-CR-Cm
Câ-CR-N
Cm-CR-N
CR-Cm-C1
CR-Câ-H
Câ-Câ-H
CR-N-H
H-Câ-H
119.2
106.3
104.6
121.0
111.4
127.7
120.0
128.7
125.0
127.7
103.5
105.7
118.1
113.6
128.3
120.9
112.5
109.2
109.7
Phenyl Substituents
Bond Length, Å
Bond Angle, deg
C1-C2
C2-C3
C3-C4
Cp-Hp
1.372
1.390
1.361
1.100
C1-C2-C3
119.4
121.1
118.6
122.8
119.4
120.6
120.2
119.8
C2-C1-C2
C2-C3-C4
C3-C4-C3
Cm-C1-C2
C1-C2-Hp
C3-C4-Hp
C2-C3-Hp
Results and Discussion
A. Electronic Structure. The UV-vis absorption spectrum
of H2TPBC (Figure 2) resembles those of bacteriochlorins and
bacteriopheophytins1,6a,b and differs from those of porphyrins29
in having a split Soret band (356 and 378 nm, By and Bx) and
two strong Q bands (521 and 740 nm, Qx and Qy). The Qy
band is as strong as the Soret bands, while the Qx band has
nearly half of the Soret band intensity. These features can be
understood on the basis of Gouterman’s four-orbital model for
porphyrins and their derivatives.29 For D4h metalloporphyrins,
the two degenerate lowest unoccupied molecular orbitals
(LUMO, eg*) and two nearly degenerate highest occupied
molecular orbitals (HOMO, a1u and a2u) give rise to a strong
Soret band in the near-UV region and weak Q bands in the
visible region, as a result of configuration interactions. This
interaction is lifted in H2TPBC (Figure 3) because of the
reduction of two opposite pyrrole rings. The degenerate eg*
(D4h) LUMOs split into x and y components (b3g and b2g, under
D2h). The b3g and au orbitals are more destabilized relative to
the b2g and b1u orbitals because of their orbital nodal pattern.29
As a result, the two y polarized electronic transitions (au × b2g
a Taken from ref 28.
) b2u and b1u × b3g ) b2u, under D2h symmetry) are
energetically well separated, and the lowest (Qy) and highest
(By) energy absorption bands have nearly equal intensity. The
two x polarized transitions (au × b3g ) b3u and b1u × b2g
)
b3u) are sufficiently close in energy to experience some
configuration interactions, leaving the Qx band with half the
intensity of the Bx band.
B. Normal Coordinate Analysis. A planar model was
employed to carry out the normal mode calculation in order to
avoid the mixing of in-plane and out-of-plane vibrations and to
keep the number of calculated frequencies tractable. The
structure parameters used in the normal coordinate analysis were
taken from the crystal structure of (Py)ZnTPBC27 (Table 1).
We note that the phenyl C-C bonds have different lengths

474 J. Phys. Chem. B, Vol. 101, No. 3, 1997
Lin and Spiro
TABLE 2: Valence Force Constants for H2TPBC
Bacteriochlorin Core
pyrroline
Stretch, mdyn/Å
pyrrole
pyrrole
pyrroline
4.600
K(CâCâ)
K(CRCm)
K(CRN)
K(CRCâ)
7.350
6.500
5.700
5.020
3.900
K(CâH)
K(NH)
5.200
6.300
6.900
5.800
4.300
K(C1Cm)
4.900
Bend, mdyn/rad2
H(C1CmCR)
H(CRCâCâ)
H(CRNCR)
H(CmCRN)
H(CâCRCm)
H(CRCmCR)
0.900
1.370
1.600
1.000
0.730
1.100
1.200
1.750
1.100
0.630
H(CâCRN)
H(CâCâH)
H(CRCâH)
H(HCâH)
H(CRNH)
1.400
0.440
0.400
1.200
0.694
0.716
0.475
0.275
1.250
1,2 Stretch Stretch Interactions, mdyn/Å
(CRCm)(C1Cm)
(CRCm)(CRCâ)
(CRN)(CRCâ)
(CRN)(CRCm)
0.308
0.628
0.480
0.653
0.420
0.300
0.314
0.680
(CRN)(CRN)
(CâCâ)(CRCâ)
(CRCm)(CRCm)
0.432
0.600
0.595
0.316
0.422
Stretch Bend Interactions (Two Atoms Common), mdyn/rad
(C1Cm)(C1CmCR)
(CRCm)(C1CmCR)
(CRCâ)(CRCâCâ)
(CRCâ)(CâCRCm)
(CRCâ)(CâCRN)
(CRCm)(CRCmCR)
(CRCm)(CâCRCm)
(CRCm)(CmCRN)
0.150
0.090
0.220
0.220
0.230
0.190
0.180
0.180
0.150
0.100
0.070
0.150
0.150
0.200
0.150
0.220
(CRN)(CRNCR)
(CRN)(CâCRN)
(CRN)(CmCRN)
(CâCâ)(CRCâCâ)
(CRCâ)(CRCâH)
(CâCâ)(CâCâH)
(CRN)(CRNH)
0.250
0.300
0.290
0.200
0.072
0.090
0.050
0.320
0.300
0.320
0.100
0.160
0.224
Stretch Bend Interactions (One Atom Common:Central), mdyn/rad
(CRCâ)(CmCRN)
(CRCm)(CâCRN)
(CRN)(CâCRCm)
(CRCm)(C1CmCR)
-0.230
-0.220
-0.230
-0.090
-0.200
-0.230
-0.240
-0.100
(CâCâ)(CRCâH)
(CRCâ)(CâCâH)
(C1Cm)(CRCmCR)
-0.080
-0.020
-0.300
Stretch Bend Interactions (One Atom Common:Side), mdyn/rad
(CRCâ)(CRNCR)
(CRN)(CRCâCâ)
(CRN)(CRCmCR)
-0.115
-0.115
-0.120
-0.100
-0.110
-0.130
(CRN)(CâCRN)
(CRN)(CmCRN)
(CRCâ)(CâCâH)
-0.115
-0.115
-0.079
-0.115
-0.130
Bend Bend Interactions (Two Atoms Common), mdyn/rad2
(CRCmCR)(C1CmCR)
(CâCRN)(CâCRCm)
(CmCRN)(CâCRCm)
(CmCRN)(CâCRN)
0.090
0.040
0.030
0.030
0.090
0.020
0.020
0.020
(CRCâH)(CRCâCâ)
(CâCâH)(CRCâH)
(CâCâH)(CâCâH)
0.020
0.025
-0.028
1,3 Stretch Stretch Interactions, mdyn/Å
(CRCâ)(CRCâ)
(CRCâ)(CRN)
(CRN)(CâCâ)
(CRCm)(CRN)
0.456
0.290
0.172
0.180
0.270
0.083
(CRCm)(CâCâ)
(CRN)(CRCm)
(CRCm)(CRCâ)
-0.300
-0.148
-0.034
-0.111
-0.168
-0.173
-0.150
-0.100
Other Stretch Stretch Interactions, mdyn/Å
1,4 (CRCm)(CRCm)
1,4 (CâCâ) (CRCm)
1,5 (CRCm)(CRCm)
-0.041
0.100
0.057
-0.059
1,6 (CRCm)(CRCm)
1,9 (CRCm)(CRCm)
-0.068
-0.052
-0.117
-0.021
0.132
Phenyl Substituents
1,2 Stretch Stretch Interactions, mdyn/Å
Stretch, mdyn/Å
Stretch Bend Interactions (Two Atoms Common)
K(C1C2)
6.220
6.200
6.300
5.116
(C1C2)(C1Cm)
(C1C2)(C1C2)
(C2C3)(C1C2)
(C2C3)(C3C4)
(C3C4)(C3C4)
0.150
0.770
0.770
0.770
0.800
(C1Cm)(CmC1C2)
(C1C2)(CmC1C2)
(C1C2)(C1C2Hp)
(C2C3)(CpCpHp)
(C3C4)(CpCpHp)
0.250
0.108
0.275
0.210
0.335
K(C2C3)
K(C3C4)
K(CpHp)
Bend, mdyn/rad2
H(CmC1C2)
H(CpCpCp)
H(C1C2Hp)a
H(C2C3Hp)a
H(C3C4Hp)a
0.755
0.970
0.525
0.565
0.450
Other Stretch Stretch Interactions, mdyn/Å
Stretch Bend Interactions (One Atom Common)
(C1Cm)(C2C1C2) -0.300
1,3 (CpCp)(CpCp)
1,4 (C1C2)(C3C4)
1,4 (C2C3)(C2C3)
-0.300
0.250
0.320
a H(CxCyHp) ) H(CyCxHp).
(1.361, 1.372, and 1.390 Å, a feature also seen in biphenyl30,31).
We found it necessary to use different force constants as well
as bond lengths for these bonds in order to reproduce the phenyl
vibrational frequencies.
constants taken from free-base porphine.33 The C-C and C-N
stretching force constants were scaled to the bond distances
using the equation of Burgi-Dunitz.34 The bending force
constants and the stretch-stretch interaction terms were adjusted
in parallel with the changes made to the stretching force
constants. Slight alterations were made to improve the fit to
The starting point of the calculation was the force field
developed for NiTPP32, with the addition of N-H bending force

Spectroscopy of Tetraphenylbacteriochlorin
TABLE 3: Observed and Calculated Skeletal Mode Frequencies (cm-1) for H2TPBCa
obsd calcd
mode N.A. ∆15N ∆13C ∆D8 ∆D20 N.A. ∆15N ∆13C ∆D12 ∆D20
Ag Skeletal Modes
J. Phys. Chem. B, Vol. 101, No. 3, 1997 475
assignment (PED, %)
νN-H
3383
3099
2870
1568
1521
1444
1368
9
0
0
0
1
0
7
0
0
0
20
13
7
3
0
4
1
3
2
8
6
9
0
0
0
0
778
762
8
8
0
0
0
0
0
3
99% ν(N-H)
98% ν(Câ-H)
99% ν(Câ-H)*
ν5
ν14
ν10
ν2
ν3
ν4
ν12
ν1
ν9
ν11
ν6
ν15
ν7
1565
1521
2
1
18
11
14
10
(1402)
20
4
-7
307
-155
13
0
2
50% ν(CRCm)*, 18% ν(CRCm), 14% ν(CâCâ), 6% ν(CRCâ)*
42% ν(CRCm), 13% ν(CâCâ), 8% ν(CRCâ), 8% ν(C1Cm), 7% δ(CâCRN)
-5 35% δ(HCâH)*, 17% ν(CRCm)*, 10% ν(CâCâ), 7% δ(CRCâH)*
5
0
1367 14
3
1
10
0
0
2
3
6
5
2
9
1
7
9
18% ν(CRCâ), 17% ν(CRCâ)*, 16% ν(CRN)*, 10% δ(CmCRN)*, 10% δ(CRNCR)*
31% ν(CRN), 29% ν(CRN)*, 12% ν(CRCâ), 6% δ(CmCRN)
1297
1226
1076
1026
995
968
856
822
335
7
9
0
0
8
9
4
7
4
1300 10
50 1225
2
0
2
5
6
4
8
2
0
1
-5
308
-124
-3
30
50 32% ν(C1Cm), 15% δ(C2C3Hp), 9% ν(CRN), 7% ν(CRN)*, 6% δ(C1C2Hp)
1
2
0
1
1
1074
1028
983
948
862
3
0
0
9
39% δ(CâCâH), 30% δ(CRCâH), 8% ν(CâCâ)
23% ν(CâCâ)*, 10% δ(CâCâH)*, 9% ν(CRCâ), 8% ν(CRN)
23% ν(CâCâ)*, 21% ν(CRCâ), 7% ν(CRN), 6% ν(CRCâ)*
22% ν(CRCâ)*, 15% ν(CRN)*, 11% ν(C1C2)
31
-31
9
23 -2 26% ν(CRCâ)*, 8% δ(CâCRN), 8% ν(CRCâ), 8% ν(C1C2), 6% δ(CRNCR)
-16 -2 15% δ(CRNCR)*, 15% ν(CâCâ)*, 12% δ(C1CmCR)*, 10% δ(C1CmCR)
ν16
ν8
ν13
ν18
12 814
3
330
298
157
7
0
3
2
0
0
23% δ(CRCmCR), 10% δ(C1CmCR)*
31% δ(C1CmCR), 26% δ(C1CmCR)*, 7% ν(CRCm)*
25% δ(CmCRN)*, 24% δ(CmCRN), 16% δ(CâCRCm), 15% δ(CâCRCm)*
166
0
0
3
2
B1g Skeletal Modes
ν23
ν31
ν19
ν28
ν29
ν27
ν20
ν34
ν22
ν30
ν32
ν24
ν25
ν35
ν33
ν21
3095
2869
1561
0
0
1
2
0
0
0
32
12
1
13
4
1
3
3
2
4
788
761
2
5
75
-2
-19
346
-172
-84
44
0
0
1
98% ν(Câ-H)
99% ν(Câ-H)*
1560
1421
1381
1320 10
1267 10
4
2
1
20
19
5
12
3
0
7
127
0
0
48% ν(CRCm)*, 43% ν(CRCm)
-5 1421
-14 27% ν(CRCm)*, 20% ν(CRCm), 16% ν(CRN), 11% ν(C1Cm), 10% ν(CRN)*
7
1379
2 44% ν(CRCâ), 29% δ(CâCâH), 11% δ(CRCâH), 11% δ(CRCâCâ), 7% ν(CRN)
14 1314 13
18 1272 12
17 35% ν(CRN), 23% ν(C1Cm), 10% δ(CRNH), 6% δ(C2C3Hp)
16 64% ν(CRN)*, 8% ν(CRCâ)*, 7% ν(C1Cm)
-9
1146
1023
893
1
5
2
4
1
6
0
2
0
0 15% δ(CâCâH), 15% δ(CRCâH), 15% ν(CRCâ), 13% ν(CRCâ)*, 8% ν(CRCm)
11 11% δ(C1C2C3), 10% δ(C2C3C4), 9% ν(CRN)*, 8% δ(CRNH), 6% ν(CRCâ)*
1
1
1
0
0
6
0
34% δ(CRNH), 13% ν(CRCâ), 9% ν(CRN), 8% ν(CRCâ)*
865
-6 735
30% δ(CRCâCâ), 22% ν(CRN), 7% δ(CRCâH), 6% δ(CRCâCâ)*
27% δ(CRCâCâ)*, 13% δ(CâCRN)*, 9% ν(CRCâ), 7% δ(CâCRN), 6% ν(CRN)*
18% δ(C1CmCR)*, 16% δ(C1CmCR), 13% δ(CmCRN)*, 13% δ(CmCRN)
11% ν(CRCm), 9% ν(CRCm)*, 7% δ(CâCRN)*, 6% δ(CâCRCm), 6% ν(CRCâ)*
17% ν(C1Cm), 10% δ(CmCRN), 10% δ(CâCRCm), 9% δ(C2C1C2)
20% δ(C1CmCR)*, 20% δ(C1CmCR), 18% δ(CâCRCm)*, 17% δ(CmCRN)*
738
317
0
1
3
0
63
11
30
16
33
2
538
377
376
308
0
0
1
0
5
13
B2u Skeletal Modes
ν43y
ν45y
3095
2870
-2 1581
0
0
0
1
1
0
0
24
8
1
3
9
3
0
8
2
2
7
1
0
0
0
788
762
3
0
0
0
98% ν(Câ-H)
99% ν(Câ-H)*
ν37y 1581
ν39y 1460
ν40y 1381
ν41y 1365
ν36y
2
18
9
6
1
64% ν(CRCm)*, 17% ν(CRCm), 8% ν(CRCâ)*
1446
1382
1357 14
1270 10
-10 -6 34% δ(HCâH)*, 14% ν(CRCm), 9% ν(CRCm)*, 7% δ(CRCâH)*, 7% ν(CRN)
0
7
58
5
73
-2
7
383
-185
114
-52
53
5
7
41% ν(CRCâ), 27% δ(CâCâH), 12% δ(CRCâH), 6% δ(CRCâCâ)
32% ν(CRN), 19% ν(CRN)*, 11% ν(CRCâ)*, 8% δ(CmCRN)*, 7% δ(CRNCR)*
2
3
28 26% ν(C1Cm), 21% ν(CRN)*, 7% δ(C2C3Hp), 7% δ(CâCâH)*, 7% ν(CRN)
8
0
ν52y
(1149) 1166
2
1
4
2
5
4
4
1
0
0
10% δ(C2C3Hp), 8% δ(CâCâH), 8% δ(CRCâH), 8% ν(CRN), 7% ν(CRCâ)
50% ν(CâCâ)*, 11% δ(CâCâH)*
ν38y 1020
ν44y
3
3
-190
0
1007
957
889
864
804
453
377
248
226
17 13% ν(CRN)*, 13% ν(C1C2), 11% ν(CRCâ)*, 8% δ(CRNH), 7% ν(C2C3)
ν47y
0
0
5
2
1
0
4
33% δ(CRNH), 18% ν(CRCâ)*, 14% ν(CRCâ), 12% ν(CRN)
24% ν(CRCâ)*, 22% δ(CRCâCâ), 16% ν(CRN)
13% δ(CRNCR)*, 12% ν(CâCâ)*, 10% δ(CRCâCâ), 9% δ(CâCRN)
19% δ(C1CmCR)*, 11% ν(CRCm), 10% δ(CmCRN), 10% δ(C1CmCR)
22% δ(CâCRCm), 14% δ(CmCRN), 10% δ(C1CmCR)*, 6% δ(CRCmCR)
22% δ(C1CmCR), 12% δ(CâCRCm)*, 9% δ(C1CmCR)*, 8% ν(CRCm)*
13% ν(C1Cm), 8% δ(CmCRN)*, 7% δ(CmCRN), 7% δ(CâCRCm), 7% δ(CRCmCR)
ν46y
ν48y
44
4
24
2
ν49y
ν50y
ν42y
ν53y
5
B3u Skeletal Modes
νN-Hx
3383
3099
2869
9
0
0
0
0
1
7
9
3
0
6
8
4
3
4
1
1
0
0
0
0
29
4
16
1
5
4
1
4
3
6
7
2
0
0
0
0
778
761
4
33
6
0
0
0
99% ν(N-H)
98% ν(Câ-H)
99% ν(Câ-H)*
ν43x
ν45x
ν37x
(1572) (1574) 1576
-2 47% ν(CRCm), 32% ν(CRCm)*, 6% ν(CâCâ)
51% ν(CâCâ), 13% ν(CRCm), 9% δ(CRCâH), 9% ν(CRCâ), 8% δ(CâCRN)
-14 36% ν(CRCm)*, 12% ν(CâCâ), 12% ν(C1Cm), 10% ν(CRN)*, 8% ν(CRN)
ν38x 1511
ν39x 1420
ν41x 1344
ν36x 1266
ν40x 1244
ν51x
4
0
5
3
5
10
4
1
8
1
27
3
1515
1424
14 1320
1
9
-13
-8
4
2 25% ν(CRCâ), 21% ν(CRN)*, 17% ν(CRN), 9% δ(CmCRN), 9% δ(CRNCR)
3
8
1264
1227
1074
1025
986
864
743
460
350
-31
-15
308
6
11 43% ν(CRN)*, 14% ν(C1Cm), 8% ν(CRCâ)*
30 20% ν(CRN), 17% ν(C1Cm), 10% ν(CRCâ)*, 7% δ(C2C3Hp), 6% ν(CRN)*
5 37% δ(CâCâH), 28% δ(CRCâH), 6% ν(CâCâ)
ν47x
10 12% δ(C1C2C3), 11% δ(C2C3C4), 10% ν(CRN)*, 8% ν(CRCâ)*, 6% ν(C3C4)
22% ν(CRCâ), 12% ν(CRN), 8% ν(C2C3), 8% ν(C1C2), 7% ν(C3C4)
ν44x
ν48x
ν46x
ν49x
ν50x
ν42x
ν53x
981
9
7
12
5
45
0
42 -2 11% δ(CâCRN), 10% ν(CRCâ), 9% δ(CRCâCâ)*, 9% ν(CRCâ)*,8% δ (CRNCR)
24
6
22
1
6
0
3
1
0
4
31% δ(CRCâCâ)*, 13% δ(CâCRN)*, 9% δ(C1CmCR), 7% ν(CRN)*
13% δ(C1CmCR), 12% δ(CmCRN), 10% δ(C1CmCR)*, 7% ν(CRCm)*
20% δ(CâCRCm)*, 12% δ(C1CmCR), 9% δ(CmCRN)*, 8% δ(CâCRN)*
25% δ(C1CmCR)*, 14% δ(CâCRCm), 10% δ(CmCRN)*, 7% δ(CmCRN)
12% ν(C1Cm), 11% ν(CRCm), 7% ν(CRN), 6% δ(CmCRN)*, 6% δ(CmCRN)
256
219
a The asterisk indicates atoms located at or adjacent to pyrroline rings. b Observed only in the 520.8-nm excitation RR spectra (not shown).

476 J. Phys. Chem. B, Vol. 101, No. 3, 1997
Lin and Spiro
TABLE 4: Observed and Calculated Pyrroline δ(CâH) and Pyrrole δ(NH) Mode Frequencies (cm-1) for H2TPBCa
obsd
calcd
mode
N.A. ∆15N ∆13C ∆D8 ∆D20 N.A. ∆15N ∆13C ∆D12 ∆D20
assignment (PED, %)
Pyrroline Câ-H Bending Modes
Ag
sym scissors
sym wag (ν17) 1278
(1148)
1479
(1284) 1267
1
1
2
2
367 -5 25% δ(HCâH)*, 14% δ(CRCâH)*, 13% ν(CâCâ)
505
2
43% δ(CâCâH)*, 29% δ(CRCâH)*, 22% ν(CâCâ)*
B1g
asym scissors
asym wag (ν26) 1358
B2g
asym twist
asym rock
B3g
1469
1357
0
1
1
2
415
356
0
2
53% δ(HCâH)*, 36% δ(CRCâH)*, 6% ν(CRCâ)*
54% δ(CâCâH)*, 16% ν(CRCâ)*, 12% δ(HCâH)*
0
4
0
1212
1069
0
0
0
0
0
0
1213
1069
0
0
0
0
296
227
0
0
72% δ(CâCâH)*, 27% δ(CRCâH)*
72% δ(CRCâH)*, 23% δ(CâCâH)*
sym twist
sym rock
B1u
sym twist
sym rock
1216
0
0
338
190
1215
878
0
0
0
0
326
218
0
0
65% δ(CRCâH)*, 36% δ(CâCâH)*
59% δ(CâCâH)*, 34% δ(CRCâH)*
1213
3
4
1215
878
0
0
0
0
326
218
0
0
65% δ(CRCâH)*, 36% δ(CâCâH)*
59% δ(CâCâH)*, 34% δ(CRCâH)*
B2u
sym scisssors
1474
1278
1
1
5
3
351 -7 28% δ(HCâH)*, 17% ν(CRCm), 15% δ(CRCâH)*,
9% δ(C2C3Hp)
185
sym wag
1277
3
2
5
0
0
39% δ(CâCâH)*, 26% δ(CRCâH)*, 23% ν(CâCâ)*
B3u
asym scisssors
asym wag
1470
1362
0
0
1
2
428
218
0
3
52% δ(HCâH)*, 36% δ(CRCâH)*, 7% ν(CRCâ)*
49% δ(CâCâH)*, 16% ν(CRCâ)*, 12%δ(HCâH)*
Pyrrole N-H Bending Modes
B1g
sym δ(N-H)
B2u
asym δ(N-H) 1103
1102
5
2
11
92
23 23% δ(CRNH), 10% δ(C2C3Hp), 9% ν(C1C2), 7% ν(CRCm)
17 33% δ(CRNH), 10% δ(CRCâH), 9% ν(CRCâ)
93
3
1107
5
1
a The asterisk indicates atoms located at or adjacent to pyrroline rings.
the observed frequencies. In order to retain D2h symmetry, the
â-D12 isotopomer was used in the calculation, instead of the
â-D8 molecule which was actually measured. The â-D12
calculation provides a reasonable comparison because the Câ-H
bending modes at the reduced pyrrole rings are not extensively
coupled with other skeletal modes of the macrocycle.
Counting the hydrogen and phenyl C1 atoms (Figure 1) which
are bonded to the macrocycle, H2TPBC has 42 skeletal atoms
and 2n - 3 ) 81 in-plane vibrations:
calculated to be 60 cm-1 higher than ν19, while ν37y and ν37x
were calculated to be lower than ν10 and ν19. Long-range
interactions have also been introduced for free-base porphine,33
etio-porphyrins,35 and Ni(HOEBC).23
Although a single phenyl C-C force constant was used in
the NiTPP calculation,32 we scaled the three phenyl C-C
stretching force constants to their bond lengths in order to fully
reproduce the phenyl vibrations, especially those with symmetry
of B1u, B2g, and B3g (Table 5). For example, the calculated
values for Ψ3 and Ψ4, 1581 and 1446 cm-1, are in much better
Γin-plane ) 19Ag + 18B1g + 2B2g + 2B3g + 2Au + 2B1u
+
agreement with the observed frequencies (1577 and 1442 cm-1
)
18B2u + 18B3u
than was possible with a single C-C force constant (1600 and
1470 cm-1).
Seventy-nine of these modes are detectable through vibrational
spectroscopy: Ag, B1g, B2g, and B3g vibrations are Raman-active,
while B1u, B2u, and B3u vibrations are IR-active (Au are Raman-
and IR-inactive). The final force field, listed in Table 2,
reproduced 79 skeletal Raman and infrared frequencies for
natural abundance (NA-), [pyrrole-15N4]-, [â-D12]-, [meso-13C4]-
, and [phenyl-D20]H2TPBC. The average error for NA-H2TPBC
vibrational frequencies was 1.9 cm-1, and the isotope shifts were
satisfactorily calculated (Tables 3-5). Most of the skeletal
modes (Table 3) are labeled according to the mode-numbering
scheme developed for NiTPP.32 Although the pyrroline Câ-H
and the pyrrole N-H bending modes are also skeletal vibrations,
they are tabulated separately (Table 4) for clarity. The phenyl
vibrations are listed separately in Table 5. Skeletal modes are
allocated to local coordinates32 in Table 6.
Long-range interactions between CRCm bonds, i.e. 1,4-, 1,5-,
1,6-, and 1,9-(CRCm)(CRCm) interactions (Table 2) were required
by the data. These interactions mainly affect the separation
among CRCm stretching modes, e.g. ν10, ν19, ν37y, and ν37x. The
introduction of 1,4-, 1,5-, and 1,6-(CRCm)(CRCm) interactions
narrowed the separation between ν10 and ν19, while the 1,9-
(CRCm)(CRCm) interaction mainly affected the separation be-
C. Spectral Assignments. Figure 4 compares Soret-,
Q-band-excited RR and FTIR spectra of H2TPBC in a KBr
pellet, while Soret-excited RR spectra in a KBr pellet and in
THF solution are compared in Figure 5 (depolarization ratios
in parentheses). Figure 6 and 7 display high-frequency (1700-
900 cm-1) RR spectra of H2TPBC and its four isotopomers
obtained with 363.8- and 520.8-nm excitation, and Figure 8
shows low-frequency (900-100 cm-1) RR spectra obtained with
520.8-nm excitation. High-frequency FTIR spectra of the
isotopomers are collected in Figure 9. Isotope shifts, selective
enhancement patterns, depolarization ratios, and correspondence
with NiTPP frequencies were used to assign the observed modes
to the calculated vibrations. Eigenvectors of selected modes
are plotted in Figure 10-12. In the following paragraphs, the
modes are considered with respect to the dominant local
coordinates:
CR-Cm Stretching Modes. The eight CR-Cm stretches divide
into four in-phase (ν10, ν19, ν37y, and ν37x) and four out-of-phase
modes (ν3, ν28, ν39y, and ν39x) which are found near 1550 and
1440 cm-1, respectively. All of them have been identified Via
isotope-labeling and depolarization-ratio measurement, although
ν3 is very weak and has been seen only in the D8 isotopomer
(1402 cm-1, Figure 6).
tween Raman-active (ν10 and ν19) and IR-active (ν37y and ν37x
)
CRCm stretching modes. Without these interactions, ν10 was

Spectroscopy of Tetraphenylbacteriochlorin
J. Phys. Chem. B, Vol. 101, No. 3, 1997 477
TABLE 5: Observed and Calculated Phenyl Mode Frequencies (cm-1) for H2TPBCa
Phenyl Substituent Modes
obsd
calcd
mode N.A. ∆15N ∆13C ∆D8 ∆D20 N.A. ∆15N ∆13C ∆D12 ∆D20
assignment (PED, %)
Ag
φ1
φ2
φ3
3071
3070
3069
0
0
0
0
0
0
0
0
0
0
0
0
0
1
2
1
1
1
2
0
0
0
0
0
7
0
0
-3
1
777 98% ν(C-Hp)
777 98% ν(C-Hp)
779 98% ν(C-Hp)
φ4 1599
φ5 1492
φ6
φ7
φ8 1003
φ9
0
1
0
1
0
6
32 1599
99 1491
(838) 1176
1046
43 1000
642
30 41% ν(C2C3), 22% ν(C1C2), 13% δ(C1C2Hp), 8% δ(C2C3Hp),8% ν(C1Cm)
103 28% ν(CâCâ), 16% δ(C2C3Hp), 13% ν(C3C4), 8% δ(C3C4Hp)
329 41% δ(C2C3Hp), 17% δ(C3C4Hp), 12% δ(C1C2Hp), 9% ν(C2C3)
241 29% ν(C3C4), 10% δ(C3C4Hp), 10% δ(C1C2Hp), 7% δ(C2C3Hp)
43 15% δ(C2C3C4), 15% δ(C1C2C3), 9% δ(C3C4C3), 9% ν(C3C4),8% δ(C2C1C2)
14 21% δ(C3C4C3), 12% δ(C2C1C2), 11% δ(C2C3C4), 7% ν(C2C3),6% ν(C1C2)
0
0
-1
-1
φ10 197
0
1
6
185
0
5 16% ν(C1Cm), 11% ν(CRCm), 10% ν(CRCm)*
B3u
φ1′
φ2′
φ3′
φ4′ 1598
φ5′ 1487
φ6′ 1174
φ7′ 1032
φ8′ 1001
φ9′
3071
3070
3069
0
0
0
0
0
0
1
2
1
0
0
0
1
1
1
1
1
1
0
0
0
0
777 98% ν(C-Hp)
777 98% ν(C-Hp)
779 98% ν(C-Hp)
33 40% ν(C2C3), 21% ν(C1C2), 13% δ(C1C2Hp), 8% δ(C2C3Hp),8% ν(C1Cm)
0
0
0
0
-3
0
3
0
0
-12
0
35 1599
91 1490
1178
1051
44 997
672
-7 110 27% δ(C2C3Hp), 21% ν(C3C4), 13% δ(C3C4Hp), 9% ν(C1C2),7% ν(CâCâ)
0
-1 236 20% ν(C3C4), 9% δ(C2C3Hp), 9% ν(C1C2), 7% δ(C3C4Hp),6% δ(CâCâH)
5
8
330 42% δ(C2C3Hp), 17% δ(C3C4Hp), 14% δ(C1C2Hp), 8% ν(C2C3)
6
0
0
48 10% ν(C3C4), 9% ν(C2C3), 8% δ(C2C3C4), 8% δ(C1C2C3),8% ν(CRCâ)
18 20% δ(C3C4C3), 11% δ(C2C3C4), 10% ν(C1Cm), 9% ν(C1C2), 8% δ(C2C1C2)
B2g, B3g, B1u (Ψ, Ψ′, and Ψ”, respectively)
Ψ1
Ψ1′
Ψ1′
Ψ2
Ψ2′
Ψ2′
Ψ3 1577
Ψ3′
Ψ3′
3069
3069
3069
3068
3068
3068
1581
1581
1581
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
783 98% ν(C-Hp)
783 98% ν(C-Hp)
783 98% ν(C-Hp)
787 98% ν(C-Hp)
787 98% ν(C-Hp)
787 98% ν(C-Hp)
0
0
0
0
0
0
36 47% ν(C1C2), 34% ν(C3C4), 10% δ(C2C3Hp)
36 47% ν(C1C2), 34% ν(C3C4), 10% δ(C2C3Hp)
36 47% ν(C1C2), 34% ν(C3C4), 10% δ(C2C3Hp)
101 28% ν(C2C3), 20% δ(C3C4Hp), 14% δ(C1C2Hp), 13% δ(C2C3Hp), 9% ν(C3C4)
101 28% ν(C2C3), 20% δ(C3C4Hp), 14% δ(C1C2Hp), 13% δ(C2C3Hp), 9% ν(C3C4)
101 28% ν(C2C3), 20% δ(C3C4Hp), 14% δ(C1C2Hp), 13% δ(C2C3Hp), 9% ν(C3C4)
99 41% δ(C2C3Hp), 27% δ(C3C4Hp), 24% δ(C1C2Hp)
99 41% δ(C2C3Hp), 27% δ(C3C4Hp), 24% δ(C1C2Hp)
99 41% δ(C2C3Hp), 27% δ(C3C4Hp), 24% δ(C1C2Hp)
204 70% ν(C3C4), 49% ν(C1C2), 45% ν(C2C3)
204 70% ν(C3C4), 49% ν(C1C2), 45% ν(C2C3)
204 70% ν(C3C4), 49% ν(C1C2), 45% ν(C2C3)
320 53% δ(C3C4Hp), 36% δ(C2C3Hp)
Ψ4 1442
Ψ4′
124 1446
1446
Ψ4′
1446
1331
1331
1331
Ψ5
(1334)
Ψ5′
Ψ5′
Ψ6
(1033) 1239
1239
Ψ6′
Ψ6′
1239
1157
1157
1157
1073
1073
1073
Ψ7 1160
Ψ7′
0
0
0
3
0
5
0
0
320 53% δ(C3C4Hp), 36% δ(C2C3Hp)
320 53% δ(C3C4Hp), 36% δ(C2C3Hp)
Ψ7′
Ψ8 1070
Ψ8′
277 44% δ(C3C4Hp), 33% ν(C2C3), 8% δ(C2C3Hp)
277 44% δ(C3C4Hp), 33% ν(C2C3), 8% δ(C2C3Hp)
277 44% δ(C3C4Hp), 33% ν(C2C3), 8% δ(C2C3Hp)
19 34% δ(C2C3C4), 32% δ(C1C2C3), 10% ν(C3C4), 9% ν(C1C2), 7% δ(C3C4Hp)
19 34% δ(C2C3C4), 32% δ(C1C2C3), 10% ν(C3C4), 9% ν(C1C2), 7% δ(C3C4Hp)
19 34% δ(C2C3C4), 32% δ(C1C2C3), 10% ν(C3C4), 9% ν(C1C2), 7% δ(C3C4Hp)
Ψ8′
Ψ9 598
Ψ9′
13 582
582
Ψ9′
582
a For the definition of C1, C2, C3, and C4, see Figure 1.
Câ-Câ Stretching Modes. The two CâdCâ stretches give
rise to a 1521-cm-1 RR band and a 1511-cm-1 IR band. The
latter is assigned to ν38x (B3u, x polarized) because the CâdCâ
bonds are aligned with the x axis, but the former can be either
ν2 or ν11; ν2 is chosen arbitrarily. The two Câ-Câ single-bond
stretches, ν11 and ν38y, are observed at 1026 and 1020 cm-1 in
the RR and FTIR spectra, respectively, and are calculated to
be ∼1020 cm-1. Large upshifts of ν11 and ν38y from ∼1020 to
∼1200 cm-1 upon deuteration (Figure 6 and 9) are similar to
those of the C1-C2 stretching modes of the NiOEP ethyl
groups.36 These upshifts result from mode crossing of the Câ-
Câ single-bond stretching modes (Figure 10) and the symmetric
pyrroline CH2 wagging modes (Figure 11) upon deuteration of
the Câ positions.
Pyrrole Câ-H, Pyrroline Câ-H, and Pyrrole N-H Bending
Modes. In NiTPP, the eight Câ-H bends give rise to four
Raman modes, ν9, ν17, ν26, and ν34, and two (degenerate) IR
modes, ν51 and ν52.32 In the case of H2TPBC, four of the Câ-H
bonds are replaced by CH2 groups, leaving two Raman and two
IR Câ-H bends: ν9 (assigned to the 1076-cm-1 RR band), ν34
(not observed, calculated at 1146 cm-1), ν51x (calculated at 1074
cm-1), and ν52y (calculated at 1166 cm-1).
The four pyrroline CH2 groups give rise to 14 Raman and
IR bending vibrations (Table 4). Like the ethyl C-H bends of
NiOEP,36 the CH2 bends on the pyrroline rings are classified
as (symmetric and asymmetric) scissoring, wagging, twisting,
and rocking modes. The scissoring and the wagging modes
have Ag, B1g, B2u, and B3u symmetry, while the twisting and
the rocking modes have B2g, B3g, Au, and B1u symmetry (Au
modes are Raman and infrared inactive). The twisting and the
rocking modes do not mix with other skeletal vibrations because
they are in separate symmetry blocks. However, the wagging
coordinates are somewhat mixed with the pyrroline Câ-Câ
stretching coordinates in the ν11 (Figure 10 and 11) and ν38y
modes. This mixing is reflected in the mode-crossing phenom-
ena upon deuteration of the Câ positions addressed above. The
frequencies of these modes are very similar to the NiOEP ethyl
groups, except for the asymmetric rocking mode (observed at
1069 cm-1). This mode is ∼300 cm-1 higher than that of

478 J. Phys. Chem. B, Vol. 101, No. 3, 1997
Lin and Spiro
B3u (Eu)
TABLE 6: Allocation of H2TPBC Skeletal Mode Frequencies (cm-1) to Local Coordinatea
local coordinate
symmetry
Ag (A1g)
Ag (B1g)
B1g (A2g)
B1g (B2g)
27 1320
1269
B2u (Eu)
ν(C1Cm)
D2h
D4h
D2h
D4h
D2h
D4h
D2h
D4h
D2h
D4h
D2h
D4h
D2h
D4h
D2h
D4h
D2h
D4h
D2h
D4h
D2h
D4h
D2h
D4h
D2h
D4h
D2h
D4h
D2h
D4h
D2h
D4h
ν1 1226
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
36y [1270]
ν
36x 1266
1235
[1254]
ν(CRCm)asym
ν(CâCâ)
ν
10 1565
1594
11 1026
1504
ν
19 1560
37y 1581
38y 1020
39y 1460
40y 1381
41y 1365
42y [248]
44y [957]
46y [864]
47y [889]
48y [804]
49y [453]
50y [377]
ν
37x [1576]
1550
[1586]
ν2 1521
1572
ν3 [1444]
1470
ν
ν38x 1511
1538
ν(CRCm)sym
ν
28 1421
1485
ν
39x 1420
[1473]
ν(Pyr quarter-ring)
ν(Pyr half-ring)sym
δ(C1Cm)
ν
1341
20 1267
ν
29 1381
1377
ν
40x 1244
[1403]
ν4 1367
1374
ν
12 1297
1302
13 [298]
238
ν41x 1344
1308
ν
ν
21 307
ν
[233]
ν
42x [256]
[255]
ν(Pyr half-ring)asym
δ(Pyr def)asym
ν(Pyr breathing)
δ(Pyr def)sym
δ(Pyr rot)
ν
1016
ν
828
22 [1020]
ν
30 [893]
998
44x 981
1004
ν
24 738
ν
32 [865]
869
46x [743]
[864]
ν
ν6 995
1004
ν7 856
889
ν
15 968
1005
47x [1025]
[1023]
ν
16 822
846
ν48x [864]
879
ν
523
ν
445
ν
560
25 [538]
ν33[376]
450
49x [461]
50x [350]
51x [1074]
ν(Pyr transl)
δ(CâH)asym
ν8 335
402
ν
18 166
277
(asym wag 1358; ν26) ν34[1146] (sym wag, 1277)
1230 1190
ν
1071
(asym wag, [1362])
1204
δ(CâH)sym
ν9 1076
1079
(sym wag 1278; ν17)
1084
ν
52y [1166]
53y [226]
δ(Pyr transl)
ν
35 [377]
109
ν
ν53x [219]
[306]
a The calculated frequencies are given in square brackets. For comparison, the frequencies of metallo-TPP are also listed in italic font under D4h
symmetry (the Raman and calculated IR frequencies are from NiTPP, and the observed IR frequencies are from CuTPP).
Figure 4. Soret- and Qx-band-excited RR spectra and the FT-IR
spectrum of H2TPBC in a KBr pellet. Experimental conditions: 4-cm-1
spectral slit, 1-cm-1/s scan rate, 3 scans for 363.8 nm at 30 mW, and
Figure 5. 363.8-nm-excited RR spectra of H2TPBC in a KBr pellet
(top) and in THF solution (bottom). Both parallel and perpendicular
traces are shown for the solution RR spectrum, and the depolarization
ratios are marked in parentheses. Experimental conditions: same as in
Figure 4 for the KBr spectrum; for the solution spectrum, 4-cm-1
spectral slit, 0.5 cm-1/s, 2 scans at 20 mW for 363.8 nm with the use
of a cylindrical lens.
2 scans for 520.8 nm at 50 mW. FT-IR spectrum resolution: 4 cm-1
.
NiOEP ethyl groups36 probably because the vibration involves
conformational changes of the pyrroline rings.37

Spectroscopy of Tetraphenylbacteriochlorin
J. Phys. Chem. B, Vol. 101, No. 3, 1997 479
Figure 6. High-frequency Soret-excited RR spectra of natural abun-
dance (NA-) H2TPBC and its pyrrole-15N4, â-D8, meso-13C4, and phenyl-
D20 isotopomers. Experimental conditions are as in Figure 4.
Figure 8. As in Figure 7, but in the low-frequency region.
Figure 9. FT-IR spectra of NA-, [15N]-, [13C]-, [D8]-, and [D20]H2-
TPBC in KBr pellet at 4-cm-1 resolution.
Figure 7. As in Figure 6, but with 520.8-nm excitation.
H2TPBC, and the band at 1103 cm-1 does shift for the D8
isotopomer.
There are two pyrrole (symmetric and asymmetric) N-H
bending modes (Table 4). Only one mode was observed at 1103
cm-1 in the FTIR spectrum (Figure 9). This mode has been
observed at 1209 cm-1 for free-base porphine.33 Interestingly,
these modes are calculated to be Câ-deuteration sensitive for
Low-Frequency Modes. Several RR bands were observed
in the low-frequency region (Figure 8). These modes are mainly
rotations and translations of the pyrrole and pyrroline rings. The
pyrrole and pyrroline rotations (ν25, ν33, ν49y, and ν49x) were

480 J. Phys. Chem. B, Vol. 101, No. 3, 1997
Lin and Spiro
Figure 10. Eigenvector plots of selected skeletal vibrations.
Figure 12. Eigenvector plots of four low-frequency skeletal vibrations.
The phenyl vibrations of B2g, B3g, and B1u symmetry (Ψ,
Ψ′, and Ψ′′ modes) have been reported for monosubstituted
benzenes38 and biphenyl31 but were not observed in the NiTPP
RR spectra.32 The respective modes in each symmetry block
have exactly the same frequency, isotope shift, and potential
energy distribution, but differ in phasing.
Figure 11. Eigenvector plots of three pyrroline CH2 bending modes.
observed at ∼500 cm-1 and the translations at ∼350 cm-1 (ν8,
ν18, ν50y, ν50x, ν35, ν53y, and ν53x). The eigenvector plots of four
selected modes are depicted in Figure 12.
Phenyl Modes. The vibrations of the phenyl groups are not
affected by the reduction of the pyrrole rings, and their
frequencies and isotope shifts are very similar to those of
NiTPP32 (Table 5). The B1g and B2u phenyl vibrations are not
listed because they are nearly identical to phenyl modes with
Ag and B3u symmetry, respectively.
Out-of-Plane Vibrations. There are candidates for out-of-
plane vibrational modes in the region from 750 to 200 cm-1
.
The frequencies and isotope shifts of these modes are similar
to the phenyl out-of-plane vibrations (π3, π4, and π5) reported
for NiTPP.32
D. Comparison with NiTPP and CuTPBC. The mode
correlation with NiTPP works well, once allowance is made

Spectroscopy of Tetraphenylbacteriochlorin
J. Phys. Chem. B, Vol. 101, No. 3, 1997 481
TABLE 7: Symmetry Correlation between D4h and D2h
Groups
for the effects of pyrrole reduction to pyrroline. Thus, four of
the Câ-H bends are replaced by a set of pyrroline CH2 modes:
scissors, wags, twists, and rocks. The CâCâ stretches are now
divided into double-bond (ν2 and ν38x at 1521 and 1511 cm-1
)
and single-bond (ν11 and ν38y at 1026 and 1020 cm-1) pairs.
Likewise, modes with predominant CRCâ stretching contribu-
tions are divided into pyrrole and pyrroline pairs, although the
frequencies are less widely separated, e.g. ν40y (1381 cm-1) and
ν40x (1244 cm-1).
However, the eigenvectors of the modes primarily involving
the CRCm and CRN bonds of the 16-membered inner ring are
essentially unaltered from NiTPP (Figure 10). For example,
the (CRCm)asym Raman modes, ν10 (Ag) and ν19 (B1g), look very
much like their B1g and A2g parents in NiTPP. So too do their
IR counterparts, ν37x and ν37y, which resemble degenerate
components of the Eu parent; indeed the degeneracy is split by
only 5 cm-1. The bond distances of adjacent CRCm bonds differ
by 0.02 Å, and the force constants differ by 0.4 mdyn/Å (Tables
1 and 2), but these differences do not produce mode localization.
The eigenvectors show little tendency to segregate on the basis
of long Vs short CRCm bonds. This is true also for the modes
involving large CRN contributions. These modes also resemble
the NiTPP counterparts. We note that Donohoe et al.18b did
report localization of CRCm and CRN modes in CuTPBC, using
a different empirical force field. The reason for this variance
in mode characters is unclear.
The absence of a metal ion at the center of the ring also has
little influence on the eigenvectors, but some frequencies are
lowered significantly. Thus, the ν2 and ν10 frequencies are 20-
30 cm-1 lower in H2TPBC (1521 and 1562 cm-1) than in
CuTPBC18b (1543 and 1598 cm-1). Parallel increases have been
observed between H2TPP (1555 and 1556 cm-1) and NiTPP
(1572 and 1594 cm-1) and between free-base (1546 and 1610
cm-1) and nickel porphine (1574 and 1650 cm-1).
As might be expected, the pyrrole translational modes are
also affected. ν8 and ν18 are 335 and 116 cm-1 for H2TPBC,
but 385 and 202 cm-1 for CuTPBC. The binding of Cu at the
center of the ring adds Cu-N stretching character to these
modes.
E. RR Enhancement Pattern. Although the crystal struc-
ture of (Py)ZnTPBC shows a slightly buckled conformation,27
the mutual exclusion rule holds well for H2TPBC (Figure 4):
vibrational modes found in RR spectra were not observed in
the FTIR spectrum, and Vice Versa. This evidence for an
inversion center justifies the assumption made in the normal
mode calculation that H2TPBC has effective D2h symmetry.
The symmetry correlation between the D4h and D2h point
group is given in Table 7, along with the Raman depolarization
ratios and the directions of IR polarizations. As a result of
symmetry lowering, A1g and B1g (under D4h) merge to Ag (under
D2h), A2g and B2g merge to B1g, and Eg (Eu) splits into B2g and
B3g (B2u and B3u). The H2TPBC spectra are consistent with
these symmetry properties. As seen in Figure 5, the depolar-
ization ratio is <0.75 for Ag modes (ν10, ν2, ν9, ν11, ν6, and
ν15), and >0.75 for the B1g modes (ν28, ν29, ν27, and ν20).
There is an interesting reversal in the RR enhancement pattern
for H2TPBC, relative to porphyrins. RR spectra of porphyrins
are dominated by totally symmetric modes when excited in the
B band, but non-totally symmetric modes when excited in the
Q bands.32,36 The weakness of the Q transition moment resulting
from the configuration interaction allows vibronic (B-term)
scattering to dominate, whereas Frank-Condon (A-term) scat-
tering dominates in resonance with the allowed Soret transition.
The lifting of configuration in H2TPBC induces large Q
transition moments and allows the dominance of totally sym-
metric modes (ν10, ν2, ν4, and ν9ssee Figure 4) Via Frank-
a The depolarization ratios are given for Raman active modes, while
the directions of polarization are given for IR active modes.
Condon scattering with 520.8-nm excitation; the non-totally
symmetric modes are weak. In contrast, the non-totally
symmetric modes (ν19, ν28, and ν20) are quite strong with 363.8-
nm excitation (Figure 5), being exceeded only by ν10. These
bands arise from vibronic scattering, which must result from
vibronic mixing of the close-lying Bx (378 nm) and By (356
nm) transitions (Figure 2). Similar evidence for Bx/By vibronic
mixing was found for the Câ-ethyl-substituted bacteriochlorin,
Ni(HOEBC).23 Thus vibronic scattering is seen with Q excita-
tion in porphyrins, as a result of Q forbiddenness and Q/B
mixing, but with B excitation in bacteriochlorin, as a result of
the close spacing and mixing of the Bx and By transitions.
There is also evidence for RR mode selectivity Via the
transition-moment direction in H2TPBC. For example, ν2 is
the strongest band in the 520.8-nm-excited spectrum but ν6 is
unobservable, while the two modes have comparable intensity
with 363.8-nm excitation (Figure 2). The eigenvectors (Figure
10) show bond stretching mostly in the x direction for ν2 and
the y direction for ν6. It is reasonable to suppose that the excited
state displacement in the Qx state is large for ν2 (large Rxx′ tensor
element) but small for ν6, accounting for the intensity disparity
with 520.8-nm excitation. A reversal would be expected for
the Qy state, but Qy-excited RR spectra are unobtainable because
of intrinsic fluorescence. The comparable ν2 and ν6 intensities
observed with 363.8-nm excitation are attributable to this
wavelength lying between the Bx and By transitions; Bx is
expected to enhance ν2, while By is expected to enhance ν6.
Similar directional selectivity was reported for CuTPBC by
Donohoe et al.,18b who were able to demonstrate the effect with
Qy excitation since fluorescence is quenched by the Cu2+
.
Acknowledgment. We thank Dr. Songhzou Hu for helpful
discussions on the synthesis and the normal mode calculation.
This work was supported by DOE Grant DE-F G02-93ER-
14403.
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(36) Li, X.-Y.; Czernuszewicz, R. S.; Kincaid, J. R.; Stein, P.; Spiro,
T. G. J. Phys. Chem. 1990, 94, 47-61.
(37) The asymmetric rocking mode involves pseudorotation of the
pyrroline C
â-Câ single bond and changes the pyrroline rings from one
enVelop conformation to another Via an eclipsed conformation in which
the ring atoms are all in-plane. This eclipsed conformation results in different
bond angles of the (sp3) CH2 groups from 109.5° (tetrahedral bond angle)
and creates instability of the pyrroline rings. The elevated frequency of the
asymmetric rocking mode is attributed to the high-energy state of the
eclipsed conformation. To fit the frequency, the H(CRCâH) and H(CâCâH)
force constants were elevated to ∼0.7 mdyn/rad2 (Table 2) from the ethyl
value, 0.625 mdyn/rad2.
(38) Green, J. H. S. Spectrochim Acta. 1961, 17, 607-613.