14254
J. Phys. Chem. 1995, 99, 14254-14260
Polarized Infrared Spectra of Photooriented Matrix-Isolated Free-Base Porphyrin Isotopomers Juliusz G. Radziszewski,laMilos NepraStb V. Balaji,'c Jacek Waluk,ld Emanuel Vogel,'e and Josef Michl* Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215 Received: February 14, 1995; In Final Form: July 17, 1995@ Polarized single-site IR spectra of free-base porphyrin-& -&, 412, and -I5N4, isolated in xenon matrix and aligned by action of linearly polarized visible light, have been measured and compared with the previously obtained spectra of unlabeled porphyrin and porphyrin-&. There is much agreement but also a fair amount of disagreement with the symmetry assignments previously proposed in the literature. The results are compared with those of the empirical force fields proposed in the literature and with a b initio calculations at the 3-21G level of Hartxee-Fock theory. It is concluded that experimental information on additional isotopomers and particularly, on gerade symmetry vibrations, will be needed before a reliable harmonic force field can be developed. The calculations require the introduction of electron correlation before the equilibrium ground state geometry and force constants can be calculated reliably enough to be useful.
Introduction Although the measurement and assignment of vibrational transitions in free-base porphyrin (1) have been reported in numerous experimental and theoretical papers?-35 the force field is still far from being known reliably and accurately. This is due to many reasons, experimental and theoretical. Free-base porphyrin is poorly soluble in most solvents, attempts to orient it by conventional methods have not been reported, and very few isotopically labeled analogues have been studied. The molecule has 108 normal modes. A reliable a priori calculation of the vibrational pattem for the molecule of that size still remains a formidable task, and the difficulties inherent to the development of a purely empirical force field with a large number of adjustable parameters are well recognized. Until recently, the information on vibrational symmetries was derived from rather poorly resolved polarized resonance Raman and visible absorption and fluorescence spectra. No polarized IR spectra were available, and conclusionsregarding vibrational symmetry assignments were drawn on the basis of comparison of computed and measured isotopic shift^.^,^ It seemed to us that ultimately a reliable force field will be derived from an ab initio calculation scaled to fit the experimental data on a large number of isotopomers and that the values of the experimental frequencies must be accompanied by experimental information on the symmetries of the corresponding vibrations. We have, therefore, initiated a program of study32-34that combines the advantages of the rare gas matrix isolation technique (high spectral resolution and very inert solvents transparent in the IR and W / v i s regions) with the possibility of photoorienting free-base porphyrin in a rigid matrix.36 The latter is achieved as a result of photoinduced double proton transfer r e a ~ t i o n , ~ ' -a~ ~process discovered originally in 7-azaindole dimers!3 The shift of two inner protons in porphyrin is equivalent to a rotation of the molecule by 90" about an axis perpendicular to the molecular plane. Irradiating a low-temperature rigid sample of porphyrin with linearly polarized light in the visible region induces linear dichroism (LD) in absorption spectra. Its degree uniquely reflects the polarization of each transition moment along one of the three molecular symmetry axes ( x along NHHN, z out of @
Abstract published in Advance ACS Abstracts, September 1, 1995.
plane) provided that the polarization of the visible transition used for the photoorientation is known.& By applying this procedure, we have been able to determine symmetries of many vibrational and vibronic bands, the latter in both absorption33 and emission.34 The results were incorporated in the development of the latest empirical force field for the in-plane vibrations of free-base porphyrin.35 It was our original intention to develop a much larger base of experimental data with assigned symmetries before attempting to develop a harmonic force field, but due to funding uncertainties it is not entirely clear how far we shall be able to carry the project. In this paper we compare the reported32 single-site symmetry-assigned IR spectra of unlabeled porphyrin (1) and porphyrin-d2 (1-dz) with those of three additional deuterated porphyrins-porphyrin-d4 (l-d4, D in meso), porphyrin-dg (1ds, H in meso), and porphyrin-dl2 (l-d,z)-and of porphyrin15N4 (l-I5N4). As before, we find much agreement but also some errors in the previously p r o p o ~ e d ~assignments , ~ , ~ ~ of vibrational symmetries. We also present the results of our ab initio calculations of the equilibrium geometries and vibrational frequencies of free-base porphyrin at the Hartree-Fock level. The results are useful for the qualitative understanding of the measured spectra, but the introduction of some degree of electron correlation will be absolutely necessary for further progress.
Experimental Section Parent porphyrin was purchased from Porphyrin Products. 1-dz was obtained by shaking a solution of 1-do in CSz with
C2H50D. Other deuterated derivatives were prepared according to ref 2. The synthesis of the ISN-labeledporphyrin is described else~here.4~ Matrices were prepared using a closed-cycle helium refrigerator (APD-Cryogenics DE-202). Typically, porphyrin was sublimed at -180 "C into a stream of 0.5-2.0 mmollmin of Xe (99.999%) and condensed on a CsI substrate, held at about 56 K in order to produce samples of optimal optical quality. Deposition times varied between 1.5 and 4.5 h, depending on the required optical density. All spectra were measured using the Nicolet 60-SXR interferometer. MCT or InSb detectors were used with CsI or CaF2 beam splitters to cover the range between 7000 and 210 cm-l. In this work we employed wire-
0022-365419512099-14254$09.00/0 0 1995 American Chemical Society
IR Spectra of Free-Base Porphyrin Isotopomers
I .
goo
1300
1100
"
J. Phys. Chem., Vol. 99, No. 39, 1995 14255
"
' .
"
1-d2
500
I"
'
'
"I
1-d8
I;:
A -
I
.I
.I
I
L
1700
Figure 1. Portions of experimental IR spectra of 1-do, 1-15N4,and 1-d2 isolated in Xe matrix at 10 K. Symmetries determined from LD measurements are indicated by triangles (blU),dots (bz"), and circles (b3u). 1500
1700
1300
900
1100
100
500
Figure 4. Calculated IR spectra of 1-do, l-d4,1 4 , and 1-dlz grouped according to symmetry species. See caption to Figure 3. TABLE 1: 0-0 Energies of the Q Transition of Porphyrin Isotopomers in a Xe Matrix
500 ij
1-42 isolated in Xe matrix at 10 K. Symmetries determined from LD measurements are indicated by triangles (b~"),dots (bz,,), and circles (b3J. 1700
1300
500
900
I* ' 4 'I ' x0.4
0 0 0
I klii
II
7iii
'
l-dZ
x0.4
: 1
.ii i 0
I.?,% i
1700
$ 0
,o 1300
1I;
I 4,. iI
0
0
I
900
500
v ("1)
Figure 3. Calculated IR spectra of 1-do, and 1-dz grouped according to symmetry species. To improve legibility, the intensities of weak transitions were enhanced. For the correct values, see Table VI (supporting information). grid polarizers (Cambridge Physical Sciences) on either CaF2 (IGP-227) or KRS-5 (IGP-225) substrates. For simultaneous observation of the visible and IR spectra, we used a quartz beam splitter and a Si diode as a detector. All IR measurements were done with 0.5 cm-I resolution. Attempts to record the far-IR absorption of porphyrin failed, in part due to extremely low extinction coefficients, despite the fact that we have used the highest sensitivity liquid helium cooled Si bolometer.
(cm-I)
site
1-do
I-&
I-&
I-&
l-dlz
1-15N4
A X
16271 16319 16364
16272 16320 16365
16291 16338 16385
16285 16333 16379
16305 16353 16400
16274 16322 16367
B
Figure 2. Portions of experimental IR spectra of 1-do, l d 4 , 1-d~,and
1
Partial site population conserving phot~orientation~~ of the samples was achieved using a high-pressure Xe lamp equipped with a band-pass filter transmitting light covering uniformly the entire group of sites corresponding to the 0-0 transition for each investigated compound. The use of an unsymmetrical filter permits a partial site resolution. For the full clear-cut resolution of single sites and site population altering phot~orientation,~~ we have used the narrow-band output of a ring dye laser (Coherent 699-21, R6G) pumped with a Coherent Innova-100 Ar ion laser. The laser was tuned to the desired frequency using the Nicolet interferometer.
Results and Discussion All the compounds reveal the same site pattem in absorption to the first excited singlet state Two main sites, which we label A and B, are observed throughout the spectrum. A weaker site is labeled X. Table 1 gives detailed information about positions of the electronic origins and magnitudes of the site splittings. The similar spectral behavior of all compounds enabled us to use the same experimental procedures as previously applied to the parent free-base p o q ~ h y r i n ~for ~ - the ~~ separation of sites and determination of transition symmetries. As reported earlier for 1and l-d2,32IR frequencies exhibit small site effects as well. The results for Site A are collected in Tables 2-5. The experimental results for 1-do, 1-I5N4 and 1 4 ,grouped into symmetry blocks, are shown in Table 2. Tables 3-5 present the data for the derivatives substituted on the perimeter ( 1 4 , 1-dg, and ld12). For all compounds but 1 4 ,intensities were scaled to that of the N-H stretching\mode; for l-d2, they are normalized to the intensity of the strongest band observed at 853 cm-'. Equilibrium geometries were computed at the 3-21G HF level, both fully optimized (C2,) and under a D a symmetry constraint (which gave an energy higher by 2.6 kcdmol). The frequencies and intensities computed in the 3-21G HF approximation, with D2h symmetry assumed, are presented in Tables VI and VI1 (supporting information). For convenience, the isotopic shifts with respect to the parent compound are given in parentheses. The geometry optimization under a Dah symmetry constraint
(a).
Radziszewski et al.
14256 J. Phys. Chem., Vol. 99, No. 39, 1995 TABLE 2: Vibrations of Xenon Matrix Isolated 1 4 0 , l-1sN4,and 1 4 2 do SYm
biu
b2u
6
219 357 540 639 691 731 773 774 785 852 938 335 745 944 951 977 986 990 1054 1156 1216 1220 1222 1228 1255 1357 1365 1406 1490 1535 1540 1549 1576 1609 1686 1693 1747 1786 1792 1904 1915 1939 2040 2097 2110 2168 2192 2233 2242 2265 2288 2324 2331 2382 2398 2427 2450 2493 2522 2562
2571 2644 2671 2758 2778 2865 2888 2905 2978 2996 3045 3112 3124 3421 3646 4343
0.1 18.2 11.0 0.2 30.3 10.5 36.1 30.0 14.8 90.6 0.4 8.2 28.0 2.7 79.0 3.4 4.7 0.5 52.5 0.6 0.2 0.7 4.2 41.6 0.2 1.2 0.3 9.9 1.7 0.8 10.4 0.9 0.2 0.6 0.7 0.1 0.3 0.8 0.4 0.1 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 vw vw vw vw 0.2 vw 0.8 vw vw vw vw vw 0.6 2.0 1.7 vw vw vw
6
I
6
I
686 731 771 774 783 852
21.0 9.1 26.0 21.0 13.0 81.9
693 713 762 766
28.3 5.8 36.0 20.0
853 866
100.0 3.5
744 930 938 974 982
27.3 3.1 85.1 2.2 1.7
744
15.4
950
90.5
1053 1152
53.2 2.8
1215 1218 1223 1253 1353 1365 1400
0.2 8.3 7.1 0.1 2.7 0.1 7.2
1053
1537 1547
7.6 0.4
1607 1686 1697 1746 1785 1791 1902
0.4 0.7 0.1 0.3 0.8 0.3 0.1
2109
2323
2483
27.0
1098
6.5
1255 1352 1360
2.0 2.0 2.5
/
1479
2.5
0.1
sYm
b3"
6
I
4417 4454 4508 4527 4576 310 723 735 780 956 964 971 994 1028 1032 1043 1134 1138 1143 1177 1282 1287 1369 1396 1412 1420 1425 1430 1456 1489 1500 1507 1522 1561 1585 1662 1683 1697 1787
vw vw vw vw vw 3.7 46.3 0.7 5.9 1.0 0.6 66.8 vw 0.1 0.1 45.3 28.0 1.1 0.2 7.6 0.2 0.4 2.3 5.9 13.3 3.5 3.3 4.1 0.4 0.2 0.8 0.7 0.6 2.6 0.4 0.1 0.7 0.6 1.1
1899 1957 1959 2049 2130 2220 2227 2237 2353 2394 2468
0.1 0.2 vw vw vw vw vw vw 0.2 vw vw
2584 2609 2658 2673 2691 2790 2823
vw vw vw 0.1 vw 0.4 vw vw vw vw vw vw vw vw 0.2 0.5 0.5 vw vw vw 100.0 vw vw
t
d2 i
0.1
2642 2669
vw vw
2771
0.5
0.6 2.0 1.5
3045 3112 3124
2.0 vw vw
2853 2925 2947 2950 2953 2965 3042 3088 3118 3128 3191 3210 3324 3344 3354
6
i
722
53.0
723
22.4
778 954 963 959
4.1 0.8 0.6 70.0
957 962
5.0 2.0
1027 1028 1043 1132 1125
0.1 0.1 53.1 21.2 0.3
1025
vw
1043 1134
25.4 11.2
1172 1281 1286
8.4 0.1 0.3
1172
5.0
1392 1410 1415 1425 1429 1454 1487
7.4 11.7 3.2 2.7 0.5 0.3 0.2
1394 1406
23.0 6.8
1506 1519 1560 1583 1661 1683 1697 1786
1.7 0.7 2.5 0.1 0.1 0.8 0.5 1.1
1685 1696 1786 1794
5.0 4.0 2.0 2.0
1893
0.1
2343
0.2 2475
20.3
2.0
0.1
2828
3045 3114 3123
i5n4
do
d2
I5N4
I
2786
0.3
2960 3042
vw 0.2
3042
3118 3128
0.5 vw
3118 3128
vw vw
3316
100.0
2475
m
IR Spectra of Free-Base Porphyrin Isotopomers
J. Phys. Chem., Vol. 99, No. 39, 1995 14257
TABLE 3: IR Vibrations and Symmetries of Porphyrin44 sym 9 (cm-I) I sym 6 (cm-I) I b3u(~)
b2uCY)
680 687 738 770 914 981 990 1044 1158 1161 1236 1262 1264 1313 1330 1343 1352 1376 1393 1396 1409 1418 1463 1498 1505 1592 1673 1783 2004 2266 2371 2590 2759 2923 3117 3179 3324 3345 3355 6643 6648 357 694
35.2 vw vw vw vw 60.3 12.2 33.7 vw vw vw vw 22.0 vw vw vw 10.0 vw vw vw 11.8 vw vw 6.5 5.5 vw vw 8.0 vw vw vw vw vw vw vw vw 100.0 vw vw vw vw vw 15.0
bldz)
923 955 96 1 996 1058 1062 1203 1205 1235 1241 1253 1339 1347 1479 1486 1518 1537 1542 1597 1685 1784 201 1 2258 2273 2354 2542 2576 2631 2744 2748 2850 3082 3110 3122 3426 627 647 699 745 796 799 93 1
48.9 vw 40.0 12.2 15.0 15.0 8.0 6.0 vw 21.1 vw 17.0 vw vw vw vw vw 24.6 vw vw 6.0 vw vw vw vw vw vw vw vw vw vw vw vw vw vw 1.7 5.7 5.9 22.0 vw 77.8 vw
was repeated at the 6-31G* level of HF theory (total energy -983.250772 hartrees). At the 3-21G HF optimized C2v geometry, a single-point 6-31G* HF calculation gave an energy lower by 2.06 kcdmol (total energy -983.254 049 hartrees). The geometrical parameters characterizing the optimized geometries are collected in Table VI11 (supporting information). The experimental IR spectra in the fingerprint region are shown in Figures 1 and 2, and the corresponding results of computations are shown in Figures 3 and 4. Assignments. Although we prefer to wait with a detailed analysis until a larger base of symmetry-assigned experimental data becomes available, in particular, for gerade vibrations, some assignments appear obvious already at this stage. The labels given here are oversimplifications, due to the likely extensive coupling of the various local motions. N-H(D) Stretching Vibrations. b3,: 3324 cm-' ( 1 - 4 , l-d4, l-d8, l-dl2); 2475 cm-' (1-d2);3316 cm-l (l-I5N4). P-C-H(D) Stretching Vibrations. (i) b2": 3112, 3124 cm-' (l-do, 1 - 4 ) ; 3114, 3123 cm-' (1-I5N4);3110, 3122 cm-I (14 , 2 3 3 8 cm-I (l-ds);2324,2328 cm-' ( 1 4 2 ) . (ii) b3,: 3118, 3128 cm-' (l-do, 1 4 , 1-I5N4);3117, (l-d4); 2335, 2336 (1d8); 2334, 2338 (l-dl2). p-C-H(D) Stretching Vibrations. (i) b2,: 3045 cm-' (l-do, l-d2, 1-IsN4); 3046 cm-' (148); 2258 cm-' 1 4 1 2 ) . (ii) b3,: 3042 cm-' (l-do, 1 - 4 1-I5N4);3040 cm-' ( l - d ~ )2266 , cm-' (l-d4); 2269 cm-' (1412).
TABLE 4: IR Vibrations and Symmetries of Porphyrin48 sym
9 (cm-I)
I
b3u(~)
700 704 773 884 959 1075 1078 1149 1159 1162 1210 1359 1363 1379 1381 1382 1406 1427 1433 1515 1690 1698 2335 2336 257 1 2686 2690 2777 3040 3128 3315 3324 3333 3335 3340 3346
16.9 vw 63.6 vw 81.4 vw 29.3 vw vw 7.3 vw vw vw vw vw 6.7 25.0 4.0 8.0 2.0 vw vw 2.0 2.0 vw 2.0 1.o 1.o 6.0 2.0 2.0 100.0 3.0 1.o vw 4.0 vw 6.0 53.4
784
sym
bldz)
9 (cm-l)
I
881 938 1204 121 1 1225 1324 1329 1331 1400 1402 1447 1491 1494 1496 1505 1507 1521 1590 1595 1600 1696 1700 2338 2371 2375 2392 2519 2521 2675 2747 2755 2761 3046 566 722 753 847 933
66.3 175.0 vw vw 116.3 vw vw vw 2.0 25 .O 12.7 vw vw vw vw vw 45 .O vw vw vw vw 40.5 2.0 vw vw vw vw vw vw vw 1.o 4.0 vw 40.2 14.2 75.0 56.5 15.1
p-C-H(D) Out-ofplane Bending. bl,: 691 cm-' (1-4);693 cm-' ( 1 4 2 ) ; 686 cm-' (1-IsN4);699 cm-' (l-d4); 566 cm-' (l-d8);563 cm-' (l-dl2). p-C-H(D) Out-of-Plane Bending. 852 cm-' ( 1 - 4 , 1-I5N4); 853 cm-' ( 1 4 ) ;847 cm-' (l-dg);799 cm-' ( 1 4 ) ;764 cm-' (l-di2). N-H(D) Combinations of Symmetric and Asymmetric Stretches. 6639, 6643, 6648 cm-' ( 1 - 4 ) ; 6638, 6643, 6648 cm-' ( 1 - 4 ) ;4949, 4953 cm-' (l-d2). A band of b2, symmetry which shifts strongly upon passing from 1 4 0 to 1 4 2 , and thus may be associated with NH(D) bending, lies at 1222 cm-' ( 1 - 4 ) ; 1098 cm-' ( 1 4 ; 1218 cm-' (1-I5N4). Still, the assignments of NH(D) in-plane and out-ofplane bending vibrations cannot be made unequivocally. Both experiment and calculation show that many bands are strongly influenced upon deuteration of the inner nitrogen atoms. Actually, the calculations predict large shifts of as many as six bands of b2" and six bands of bl, symmetry (Table VI, supporting information). In contrast, the b3" NH(D) stretch is localized in a single mode. Symmetry Reassignments. For a large number of weaker bands symmetries are assigned for the first time. Previously proposed symmetry assignments for a number of stronger bands have now been found to be incorrect. The disagreements with the assignments proposed in refs 3 and 4 are fairly extensive. For l-do, we find bl, symmetry for the transition at 357 cm-I, which was previously assigned to either b2" or b3,. The 990 and 1351 cm-I vibrations are assigned to b2,. not b3". The band at 1177 (1 1833)cm-' has b3,, not bzu, symmetry. In I-&, the
Radziszewski et al.
14258 J. Phys. Chem., Vol. 99, No. 39, 1995 TABLE 5: IR Vibrations and Svmmetries of Pomhvrin42 ~~
sym
e(cm-')
I
b3u(x)
667 74 1 775 920 969 1114 1166 1168 1173 1225 1291 1315 1350 1369 1399 1414 1471 1500 1531 1549 2260 2269 2334 2338 2347 677 752 870 918 934 952 1003 1197
43.2 7.0 27.0 vw 86.8 vw 7.8 vw 12.5 2.0 1.o vw 4.5 6.0 1.o 1.o 2.0 1.o 1.o 2.0 5.0 5.0 2.0 2.0 vw 20.7 5.0 2.0 191.6 10.0 37.5 vw vw
MY)
sym
b I u(z)
i j (cm-I)
I
1205 1211 1223 1261 1284 1310 1324 1352 1357 1378 1408 1441 1447 1454 1490 1513 1522 1533 1552 1591 2258 2263 2324 2328 2693 2761 563 635 709 745 764 93 1
2.0 77.9 2.0 vw 1.o vw 10.0 3.0 1.o 2.0 vw 3.0 4.5 vw 1.o 9.0 27.0 1.o 3.0 5.0 5.0
5.0 2.0 2.0 6.0 3.0 21.6 5.8 16.0 6.0 50.0 15.0
vibrations at 923 and 1339 cm-I are definitely y-polarized (bz,,), and the bands at 1352 and 1409 cm-I are x-polarized (b3,). The 1162 cm-' vibration in 1-dg is of b3,, not b2,, symmetry. In l - d ~ z the , transition at 745 cm-' is z-polarized (bl,,). It was previously assigned3 to b2,, symmetry; we find a weaker band of the latter symmetry species at 752 cm-I. The most recently published empirical force field, limited to in-plane vibration^?^ benefited from the experimental symmetry assignments for porphyrin and p0rphyrin-d2~~9~~ that were not available to the earlier workers. Comparison for the two symmetry classes available, bz, and b3u, is hampered by discrepancies between the experimental frequencies assumed to correspond to the fundamental modes of vibration in the development of the empirical force field and collected from a variety of literature sources,35and our values, all of which are in Xe matrix. Most of our numbers agree within a few cm-' with those listed as experimental in ref 35. This is also true for the A, and B I , frequencies derived from phosphorescence spectra in Xe and fluorescence spectra in Ne matrices.34 These were not available to the authors of ref 35, who based these symmetry assignments primarily on reported Raman depolarization ratios. The good agreement between most of the frequencies we observe and those collected35 from various sources in the literature is encouraging and suggests that no strong solvent shifts intervene. In contrast, some of the IRactive b2,, and b3,, peaks used in ref 35 differ by as much as 20-30 cm-' from the nearest peaks of the appropriate symmetry that we can find in our IR spectra. In many cases, the discrepancy can be most easily accounted for as a symmetry misassignment in ref 35. Thus, in 1-4,the reported 1506 cm-' peak assigned35as b2, probably corresponds to a 1507 cm-l vibration in our spectrum, which is of b3,, symmetry, and not to the bz,, peaks at 1490 or 1535 cm-' that we also observe. The reported 793 cm-' peak also assigned as bz,, and the reported
802 cm-I peak assigned as b3, are absent in our spectra. Instead, we observe a b3u peak at 780 cm-I and out-of-plane polarized bl, peaks at 773,774, and 785 cm-I. Since the assignments of out-of-plane polarized peaks were not attempted in ref 35, we are at a loss as to how to relate our results to those described there. The 357 cm-' vibration in 1-do is a particularly difficult case. We found it to be of bl, symmetry.32 This result was obtained by site population altering photoselection, exciting either a b2,, or a b3, polarized visible band to various degrees of conversion. The induced dichroism is always that of other bl,, vibrations and unmistakably different from those of b2,, and b3,, vibrations and from a superposition of overlapping b2, and b3, vibrations (this could mimic a bl, vibration at a particular degree of conversion, but not at many different degrees of conversion.) This result was disregarded in the proposed35assignment of this band, reported as observed at 353 cm-I, to overlapping b2,, and b3,, vibrations. Admittedly, the empirical force field35as well as our own HF calculation (Table VI, supporting information) predicts such a degeneracy of a b3,, and a b2,, vibration in this general region, but we find it difficult to see how the experimental result for the polarization of the 357 cm-' peak can be ignored. Related problems occur in the labeled porphyrins. The 1390 cm-' vibration assigned35 as bz,, in 1 4 2 is absent in our spectrum, the closest possibility being a b2,, peak at 1360 cm-I. Perhaps it corresponds to our b3,, polarized peak at 1394 cm-I, in which case the 1402 cm-I (presumably b3J vibration of ref 35 corresponds to our 1406 cm-' b3,, vibration and the 1418 cm-I (presumably b3,,) vibration of ref 35 remains unobserved in our spectrum. In 1 - 4 , some of the larger discrepancies between the experimental frequencies used in ref 35 and those we find in xenon matrix are 1337 versus our 1352 cm-I, 1418 versus our 1409 cm-I, and 3305 versus our 3324 cm-' for b3,, vibrations; 681 versus 694 cm-I, 1005 versus 996 cm-I, 1352 versus 1339 cm-I, and 3095 versus 3122 cm-' for bzu vibrations. The 768 cm-I vibration assigned as bz,, in ref 35 has no counterpart among our bZu peaks at all and in all probability represents a misassigned 770 cm-' b3,, vibration observed in our spectra. In 1-ds, similar problems occur. The experimental 785,921, and 1593 cm-' frequencies assigned as b3, in ref 35 cannot be identified in our list of observed b3, peaks. (We observe an intense bzu peak at 784 cm-I, a bl, peak at 933 cm-I, and bzu peaks at 1590, 1595, and 1600 cm-', one of which presumably corresponds to the observed ''b3u)' peak at 1593 listed in ref 35.) Among our b3,, peaks, the closest correspondence to the 1320 cm-' b3, peak of ref 35 lies as far as 1359 cm-', but we observe three weak bZu peaks at 1324, 1329, and 1331 cm-I. One of these may, however, correspond to the 1340 cm-' peak assigned as bzu in ref 35. The observed peak at 1165 cm-' attributed to a bz, vibration in ref 35 might correspond to a weak 1204 cm-' bz,, peak in our spectra, but it is much more likely that it corresponds to the fairly prominent 1162 cm-I peak that we observe to be of b3, symmetry. We do not observe the 760 and 775 cm-l peaks assigned as bZu in ref 35. In l - d ~ the ~ , 743 cm-' peak assigned as b2, in ref 35 most likely corresponds to the 745 cm-' peak that we observe to be bl, polarized, and the 893 cm-' b2, peak of ref 35 may correspond to our 870 cm-' b2, peak. Finally, for 1-I5N4, no experimental data are referred to in ref 35, making this kind of comparison impossible. Force Field Development. We find the situation outlined in the previous section quite discouraging. It is evident that a fair fraction of the symmetries are misassigned even in the most
IR Spectra of Free-Base Porphyrin Isotopomers recent treatment;35 actually, many of the misassignments date from the earlier ~ o r k . It~ is, ~most likely true that none of the frequencies of the assumed fundamentals in the published empirical force field^^,^,^^ are off by more than 50-100 cm-’ and that in that sense the starting point for the vibrational force field analysis is almost correct. There are many combination bands of b2u and b3u symmetries in the congested region from about 700 to 1700 cm-I, and the forest of lines predicted by any halfway reasonable force field can be said to agree with experimental frequencies if intensities are not calculated. However, while the published force fields may be considered satisfactory for a global reproduction of the IR spectrum, we suspect that for many peaks in the spectra there is little assurance that they actually correspond to the normal modes to which they have been assigned. This may have disastrous effects for any attempt to understand the response of a particular peak to substitution or environmental effects. Even if all the frequencies were to be taken from a single source, such as our xenon matrix data, and all polarization data were to be properly heeded, the large number of combination bands present makes the selection of peaks to be assigned to fundamental vibrations rather arbitrary. Ultimately, the choice should be such as to account for all the combination and overtone bands as well, but this has not been attempted. We believe that the situation is likely to be at least equally difficult for vibrations of gerade symmetry, for which far fewer polarization data are available at present. Overall, it seems to us that it would be premature to attempt to develop an empirical force field better than those already a ~ a i l a b l until e ~ ~a ~much ~~~ larger amount of experimental information is available, such as reliable symmetry assignments of a much larger number of gerade vibrations, and information on additional isotopomers. After all, the problem is woefully underdetermined for a purely empirical fitting procedure, and guidance available from comparison with force fields of related compounds, bond lengthstretching force constant relations, and the like, even though very valuable, cannot be expected to remove all ambiguities. Ab Znitio SCF Calculations. While we considered it premature to attempt yet another empirical force field fit at this time, we felt that in spite of all its shortcomings a nonempirical calculation of the IR spectrum would provide some overall qualitative guidance to the understanding of trends in isotope shifts and of the nature of the vibrational motions corresponding to the various observed peaks. The ab initio programs available to us presently permitted only calculations at the Hartree-Fock level. Since porphyrin is closely related to [4N 21annulenes in its x-electron structure, it appeared likely that the HF solution will exhibit singlet instability at the maximum D2h symmetxf6 and distort to one of the two symmetry-related C2v structures corresponding to two KekulB-type structures, as had been observed earlier at the semiempirical level of c a l ~ u l a t i o nThis .~~ indeed occurred. Both with the 3-21G and the 6-31G* basis set, the optimized D2h geometry corresponds to a saddle point and not to a minimum on the potential energy surface. The geometrical parameters are listed in Table VI11 (supporting information). We have searched for the C2u minimum at the 3-21G level and found it to lie 2.6 kcal/mol below the D2h structure. In view of the symmetry breaking in the HF wave functions of the related [4N 2lann~lenes:~we believe that the distortion from D2h to CzVgeometry is an artifact of the approximation and that there is little point in trying to specify the CzV geometry more accurately by the use of the better 6-31G* basis set. With this basis set, the HF energy difference between the D2h constrained geometry optimized at the 6-31G* HF level, and the unconstrained geometry optimized at the
+
+
J. Phys. Chem., Vol. 99, No. 39, 1995 14259 3-21G level (C2,) is 2.06 kcal/mol. What is clearly needed is a wave function of the GVB or CAS SCF Since we were not able to compute those, we must view the calculated frequencies with even more suspicion than would normally be the case for the HF approximation. The frequency that is affected the most is the one that corresponds to the Kekult vibration, which is imaginary at the D2h geometry and about whose actual value nothing can be said from the calculation. Other modes of the same b3u symmetry are also likely to be affected strongly, and the differences between the harmonic 3-21G frequencies obtained at the optimized C2, and D2h geometries indeed range up to 75 cm-I. In contrast, it is quite likely that normal modes of other symmetries should not be affected noticeably. In fact, the differences are less than 10 cm-’ for vibrations of symmetries other than b3,,. In spite of these complications, the uniformly scaled calculated frequencies and intensities (3-21G) actually reproduce most of the observed patterns in a surprisingly reasonable way (Figures 1-4).
Conclusions We see the main contribution of this paper in providing a large amount of accurate data for the frequencies and symmetries of porphyrin isotopomers under the fairly good resolution singlesite conditions of xenon matrix isolation. We believe that an extension of this type of measurement to additional isotopomers, to the region of lower frequencies, and to Raman and luminescence work is necessary before it will make sense to attempt the construction of a definitive harmonic force field for prophyrin free base, presumably in a way that would rely on empirical scaling of a higher-quality ab initio force field than we have been able to produce so far. We find a sufficiently large number of discrepancies with the assignments used in the development of existing force that we doubt their ability to make detailed statements about many of the observed individual IR peaks. However, there is little doubt that they assign most bands correctly and that they provide valuable overall guidance to the spectrum as a whole.
Acknowledgment. This work was supported by NSF grants CHE-9318469 and CHE-9412767 and by a NATO Research Grant. 1-I5N4 was synthesized by M. Kijcher. Supporting Information Available: Tables VI (calculated IR-active vibrations of 1-do, 1-I5N4,and 1 4 , VI1 (calculated IR-active vibrations of 1-do, 1 4 , 1-d~,and 1-42), and VIII (calculated geometry of free-base porphyrin, optimized at the FWF level) (8 pages). Ordering information is given on any current masthead page. References and Notes (1) (a) Present address: Department of Chemistry, Vanderbilt University, Nashville, TN 37235. (b) Permqent address: Department of Chemical Technology, University of Pardubice, Cs. Legii 565,53210 Pardubice, Czech Republic. (c) Present address: NCSA, 4151 B e c k ” Institute, 405 N. Mathews Ave., Urbana, IL 61801. (d) Permanent address: Institute of Physical Chemistry, Polish Academy of Sciences, 01-224, Warsaw, Kasprzaka 44,Poland. (e) Department of Organic Chemistry, University of Cologne, 5000-Cologne, Greinstrasse 4, Germany. (2) Solov’ev, K. N.; Gladkov, L. L.; Gradyushko, A. T.; Ksenofontowa, N. M.; Shulga, Z. M.; Starukhin, A. S. J . Mol. Strucr. 1978, 45, 267. (3) Gladkov, L. L.; Solovyov, K. N. Spectrochim. Acta 1985, 41A, 1437; Zh. Prikl. Spektrosk. 1984, 40, 275. (4) Gladkov, L. L.; Gradyushko, A. T.; Shulga, A. M.; Solovyov, K. N.; Starukhin, A. S. J. Mol. Struct. 1978, 47, 463. (5) Gladkov, L. L.; Gradyushko, A. T.; Sivchik, V. V.; Solovyov, K. N. Zh. Prikl. Spektrosk. 1976, 25, 94.
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