J . Phys. Chem. 1991, 95, 1555-1563
1555
Tracking Chlorin and Porphyrin Modes with a General Valence Force Field: Normal-Mode Analysis of Nickel Octaethylchlorin Kristine Prendergast and Thomas G. Spiro* Department of Chemistry, Princeton University, Princeton, New Jersey 08544 (Received: April 10, 1990; In Final Form: July 17, 1990)
An empirical general valence force field, previously developed to describe the Raman and infrared vibrational frequencies of nickel octaethylporphyrin (NiOEP), has been applied to nickel octaethylchlorin (NiOEC) by scaling the stretching force constants to the known changes in bond distance between porphyrin and chlorin. Raman band frequencies and the shift pattern upon ISNsubstitution and upon deuteration at the y,6 meso positions are satisfactorily calculated. The eigenvectors show a clear correlation between corresponding porphyrin and chlorin modes, in contrast to the conclusion from a recent QCFF/PI calculation that mode localization in chlorin obscures such correlations. The present analysis leads to straighforward assignments of the observed NiOEC Raman bands from their isotope shifts, depolarization ratios, and enhancement patterns. These patterns reveal a dominant vibronic scattering mechanism in resonance with the weak Q, electronic transition, and dominant Franck-Condon scattering in resonance with the strong Qy and B,, transition. The enhancement pattern is nevertheless strikingly different for Qy and B, resonances, the former enhancing mid-frequency (1 100-1 300 cm-I) and the latter enhancing high-frequency (1350-1650 cm-( modes. This indication of different geometries in the B and Q states is similar to the pattern reported previously for NiOEP. The chlorin-porphyrin correlations support a substantial body of chlorin vibrational spectroscopy and provide a rationale for analogous core-size sensitivities of chlorin and porphyrin skeletal mode frequencies.
Introduction Resonance Raman (RR) spectroscopy has played an important role in the elucidation of the structure and dynamics in metalIoporphyrins and in heme proteins.' There has recently been considerable interest in extending the technique to metallochlorins especially in connection with the structural analysis of chlorophyll and chlorin-containing protein^.^-^ Metallooctaethylchlorins (OEC) have been extensively surveyed because of their resemblance to naturally occurring @-substitutedchlorins. These studies have suggested substantial homology in the RR spectra of chlorins and porphyrins, when due allowance is made for the symmetrylowering effects of the reduced pyrrole (pyrroline) ring in chlorins. Moreover, an inverse dependence on core size for the macrocycle vibrational modes between 1400 and 1650 cm-I, previously detected for protoporphyrin (PP) and octaethylporphyrin (OEP) complexes, has also been noted for c h l ~ r i n s . ~ * ' ~ A recent calculation of the NiOEC normal modes using the semiempirical electronic structure program QCFF/PI has suggested, however, that these chlorin-porphyrin analogies are illusory, since the calculated chlorin modes were found to be localized on separate fragments of the macrocycle, rendering arbitrary any correlations with the delocalized porphyrin macrocycle.2 This localization was suggested to arise from the bond alternation around the Idmemberd inner .rr system, a known feature of chlorin structure.I1-I3 On the other hand, another calculation by the same authors on copper tetraphenylchlorin (CuTPC), this time using an empirical force field, yielded fewer modes which were markedly l ~ c a l i z e d . ' ~It was suggested that the altered mode localization pattern might result from structural differences between 8-alkyland meso-aryl substituted chlorins. An empirical general valence force field has recently become available for NiOEP which satisfactorily accounts for almost all of the in-plane fundamental modes, as well as pyrrole-I5N, meso-*H4,and methylene-*H,, isotopic frequency shifts.I5 We now report the application of this force field to NiOEC, adjusting the bond stretching force constants in accord with the alternating bond distances around the chlorin ring. This calculation leads to a satisfactory understanding of the NiOEC R R spectra, including isotopic shifts upon I5N and y,6 meso-2H2substitution,
* Author to whom correspondence should be addressed. 0022-3654/91/2095-1555$02.50/0
and the resonance enhancement pattern. We find that the normal-mode eigenvectors resemble those of NiOEP quite closely; only small differences are found, which result from the mode mixing that is permitted by the symmetry lowering. The similarity of the eigenvectors provides a rationale for the observation of core-size sensitivities for skeletal modes in the 1450-1650-cm-' region in chlotins as well as porphyrins. The present analysis supports the bulk of empirical chlorin vibrational studies, which have generally been based on the chlorin-porphyrin analogy. Gladkov et al.9 also report successful calculation of the OEC vibrational modes using a porphyrin-based force field, although details of the force constants or eigenvectors were not given.
(1) Spiro, T. G., Ed. Biological Applications of Raman Spectroscopy; Wiley: New York, 1988;Vol. 111. (2)Boldt, N. J.; Donohoe, R. J.; Birne, R. R.; Bocian, D. F. J. Am. Chem. SOC.1987,109,2284. (3)Tasumi, M.; Fujiwara, M. In Spectroscopy of Inorganic Based Materials; Clark, R. J. H., Hester, R. E., Eds.; Wiley: New York, 1987;Vol. 14,p 407. (4) (a) Andersson, L. A.; Sotiriou, C.; Chang, C. K.; Loehr, T. M. J . Am. Chem. SOC.1987,109,258.(b) Andersson, L. A.; Loehr, T. M.; Lim, A. R.; Mauk, A. G . J . Biol. Chem. 1984,259,15340. (c) Andersson, L. A.; Loehr, T. M.; Chang, C. K.; Mauk, A. G . J . Am. Chem. SOC.1985,107,182. (d) Andersson, L. A.; Loehr, T. M.; Sotiriou, C.; Wu, W.; Chang, C. K. J. Am. Chem. SOC.1986,108, 2908. (e) Andersson, L. A.; Sotiriou, C.; Chang, C. K.; Loehr, T. M. J . Am. Chem. SOC.1987,109,258. (f) Andersson, L. A.; Loehr, T. M.; Cotton, T. M.; Simpson, D. J.; Smith, K. M. Biochim. Biophys. Acta 1989,974, 163. (9) Andersson, L. A.; Loehr, T. M.; Stershic, M. T.; Stolzenberg, A. M. Inorg. Chem. 1990,29,2278.(h) Andersson, L. A.; Loehr, T. M.; Thompson, R. G . ; Sttauss, S. H. Inorg. Chem. 1990,29, 2142. (5) Babcock, G.T.; Ingle, R. T.; Bolscher, B. G.; Wever, R. Biochim. Biophys. Acta 1985,828, 58. (6) Lutz, M. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E.,Eds.; Wiley-Heyden: London, 1984; Vol. 1 I , p 21 1. (7) Cotton, T. M.; Van Duyne, R. P. J . Am. Chem. Soc. 1981,103,6020. (8) Ozaki, Y.;Kitagawa, T.; Ogoshi, H.Inorg. Chem. 1979, 18, 1772. ( 9 ) Gladkov, L. L.; Starukhin, A. S.; Shulga, A. M. Spectrochim. Acfa 1987,43A. 1 125. (IO) Ozaki, Y.; Iriyama, K.; Ogashi, H.; Ochiai, T.; Kitagawa, T. J. Phys. Chem. 1986, 90,6105. ( I I ) Straws, S. H.; Silver, M. E.; Long, K. M.; Thompson, R. G.; Hudgens, R. A.; Spartalian, K.; Ibers, J. A. J . Am. Chem. SOC.1985,107,4207. (12)Spaulding, L. D.; Andrews, L. C.; Williams, G . J. B. J . Am. Chem. SOC.1977,99,6918. ( I 3) Gallucci, J. C.; Swepston, P. N.; Ibers, J. A. Acta Crystallogr. 1982, 838, 21 34. (14)Donohoe, R.J.; Atamian, M.; Bocian, D. F. J . Phys. Chem. 1989,93, 2244.
0 1991 American Chemical Society
1556 The Journal of Physical Chemistry, Vol. 95, No. 4, 1991
Prendergast and Spiro
?
Experimental Methods
NiOEC and the I5N isotopomer were kindly provided by Prof. S. H. Strauss, at Colorado State University. Resonance Raman spectra were obtained as solid samples in KBr pellets, using a Spex 1401 double monochromator equipped with a RCA C31034A photomultiplier tube and photon counting detection. Spectral slit widths were set to 6 cm-l for each wavelength and the monochromator was stepped at 0.5 cm-' s-l, integrating for 5 s at each point, and averaging 2 scans. Laser excitation at 406.7 nm was obtained from a Coherent lnnova 100K3 Kr+ ion laser, while 488.0-nm excitation was produced by a Spectra Physics Ar+ ion laser, and 607-nm excitation was provided by a Coherent 590 tunable dye laser pumping Rhodamine 6-G. Power at the source for each line was 50 mW. Polarization measurements were obtained for samples in CH2C12solution. Results
Normal-Mode Analysis. To analyze the chlorin normal modes we utilized the general valence force field recently developed for NiOEPI5 and adapted it to the chlorin molecular structure, taking into account the expected dependence of bond stretching force constants on the bond distance. For this purpose geometric parameters were obtained from the crystal structure of Fe"OEC,' those of NiOEC being unavailable; the bond lengths and angles of Fe"0EP are quite similar to those of NiOEP. The Fe"0EC structure is ruffled but a planar macrocycle was employed in the calculation, to avoid the difficulties of combining in- and outof-plane force fields. Because ruffling is expected to influence the vibrational frequencies, an exact fit to the observed spectra is not expected. The approximation should not affect the forms of the normal modes, however; it is known that isotope shifts are essentially the same for ruffled and planar forms of NiOEP, despite appreciable frequency differences, implying that the eigenvectors are largely unaltered.I6 As in the NiOEP calculation, the ethyl substituents were included as methylene groups with point mass methyl groups. The methyl groups were oriented out of the plane, pointing alternately up and down to maintain C, molecular symmetry. Likewise, the trans isomer (H atoms on the pyrroline ring on opposite sides) was chosen, although the spectra were obtained on a cis/trans mixture. This is another possible source of discrepancy between observed and calculated frequencies. Anderson et al.48 have recently found some frequency and intensity differences between cis and trans isomers of NiOEC and for CuOEC in the solid state. This idealized molecular structure for NiOEC is shown in Figure I . Thc X-ray derived FeiiOEC structural parameters that are related by the 2-fold molecular axis were averaged; these averaged values, which fall within 0.005 A and 0.5' of the individual bond lengths and angles, are listed in Table I. The most obvious effect of the pyrrole ring reduction, aside from the lengthening of the C,C, and C,C, bonds in the reduced ring, is the division of each pair of C,C, bonds into a short and a long bond, the difference being fully 0.040 A. The remaining bond distances are similar to those found in Ni0EP.I' The stretching force constants were initially scaled to the bond distances by using the equation developed by Johnston1*and reformulated by Burgi and Dunitz.Ig In addition, the stretchstretch interaction constants were scaled according to the percent change in the geometric mean of the stretching force constants. Angle bending force constants, stretch-bend, and bend-bend interaction constants were carried over directly from NiOEP,IS except that force constants for the (15) Li, X.Y.; Czernuszewicz, R. S.; Kincaid, J. R.; Stein, P.; Spiro, T. G.J . Phys. Chem. 1990, 94, 47. (16) Czernuszewicz, R. S.; Li, X.Y.; Spiro, T. G.J . Am. Chem. Soc. 1989, 111, 7024.
(17) Cullen, D. L.; Meyer, E. F., Jr. J . Am. Chem. SOC.1974, 96, 2095. ( 1 8 ) Johnston, H. S. Adu. Chem. Phys. 1960, 3, 131. (19) Burgi, H.; Dunitz, J. D. J . Am. Chem. SOC.1987, 109, 2924.
Figure 1. Structural diagram for NiOEC. The reduced ring (R) on the right is the trans isomer: one ethyl group is above and one is below the
plane. The remaining ethyl substituents have their CI-C2bonds pointing alternately up and down around the ring, preserving the molecular C2
axis. The bonds and angles are labeled in the tables according to whether they belong to the reduced ring (R) or the trans ring (T), or are adjacent (cis) to R (CR) or to T (CT). The methine bridge bonds (m)are labeled in a bond-alternant pattern, starting at the reduced ring: short (s) and long (I). The a,& and m positions are also indicated. Hydrogen atoms are omitted for clarity, except on the reduced ring. TABLE I: Structural Parameters for the NiOEC Calculation' internal reduced cis to R cis to T trans coord ring (R) (CR) (CT) ring (T) I .380 1.370 1.389 aN 1.368 a8
88 ma mH
81
NiN 12 aNa
Nab aBP
NiNa Nam ma@ ama a81
PP 1 812
NNiN
1.516 I SO8 I .40 1 1.098 1.531 2.002 1,502 108.2 112.2 103.2 125.9 124.4 122.9 125.4 109.8 117.2 117.5 90.0
I .437 1.363 1.363 1.098 1 .SO4 1.969 1.529 105.0 110.5 106.8 127.4 124.4 125.0 125.4 124.9 128.2 128.3 90.0
1.433 1.363 1.401 1.098 1.SO4 1.969 1S29
105.0 110.5 106.8 127.4 124.4 125.0 125.4 124.9 128.2 128.3 90.0
1.446 1.363 1.363 1.098 1 SO4 1.987 1.529 105.0 110.5 106.8 127.4 124.4 125.0 125.4 124.9 128.2 128.3 90.0
a Bond lengths (angstroms) and angles (degrees). Experimentally determined FdIOEC bonds and angles related by the molecular C2axis were averaged (ref 9). Labels: a, 0,and m are the C,, C,, and C, atoms; 1 and 2 refer to the first and second C atoms of the ethyl
groups.
reduced ring were estimated from force fields of saturated hydrocarbons.20*21Normal-mode calculations were performed with the GF matrix method.22 An updated version of Schachtschneider's programs were used to construct the G matrix and in the solution of the vibrational secular equation.23 All calculations were performed on a VAX-I 1/780. After the NiOEC force field was constructed in this way, small adjustments were made in some of the force constants in order to improve agreement of calculated isotope shifts with those observed by Boldt et ale2 (see below). The final force constants are listed in Table I1 and Table 111. Twenty-six of them (indicated by asterisks) were adjusted, giving an average error in the calculated frequencies (20) Snyder. R. G.;Schachtschneider, J. H . Spectrochim. Acta 1965.21, 169. (21) Kartha, V. B.; Mantsch, H. H.; Jones, R.N. Can. J. Chem. 1973,51, 1749. (22) Wilson, E. B.; Decius, J . C.; Cross, P. C. Molecular Vibrations; McGraw-Hill: New York. 1955. -~~ (23) Fuhrer, HT; Kartha, V.B.; Kidd, K. G.;Krueger, P. J.; Mantsch, H. H. NRCC Bull. 1975, No. 15. ~
Normal-Mode Analysis of Nickel Octaethylchlorin
The Journal of Physical Chemistry, Vol. 95, No. 4, 1991
TABLE 11: Diagonal Force Constantsa for the NiOEC Calculation internal reduced cis to R cis to T trans coord ring (R) (CR) (CT) ring (T) 5.685*b 4.184* 3.829. 6.652* 4.560 3.688. 1.503. 4.600 4.560 4.560 1.620 1.370 1.170. 0.300 0.830 0.730* 1.100 0.900. 0.900' 1.200 0.250 0.500 0.625 0.625 0.520 0.400 0.400 0.400
5.635. 5.5555 6.980. 7.100* 4.560 4.064' 1.636. 4.600 4.560
5.339. 5.655. 6.980 6.652 4.560 4.064 1.636 4.600 4.560
5.5455 5.755. 6.980 7.100 4.560 4.064 1.561* 4.600 4.560
1.620 1.370 1.370 0.300 0.830 0.830 1.100 1.200 1.200 1.200 0.250 0.500 0.625 0.625 0.520
1.620 1.370 1.370 0.300 0.830 0.830 1.100 1.200 1.200 1.200 0.250 0.500 0.625 0.625 0.520
1.620 1.370 1.370 0.300 0.830 0.830 1.100 1.200 1.200 1.200 0.250 0.500 0.625 0.625 0.520
TABLE 111: Off-Diaeonal Force Constants for NiOEC ~
~~~~
reduced ring (R)
nonreduced rings
1-1, 1-4, 2-3
0.430
0.430
4-6, 2-6, 4-4 1-7, 6-8 2-4 7-7
0.040. 0.150
0.430 0.150
0.250 -0.250
0.250 -0.250
0.245
0.245
0.160. 0.020* 0.320 0.100 0.080. 0.1 I O
0.320 0.320 0.320 0.100 0.150. 0.110
internal coord' (K-K)
I .2
(K-K)1.3
1-2, 2-2 1-4, 4-7, 2-4 2-6, 1-6
(K-H)~ 4-17, 2-16, 2-1 3 3-1 3 3-19, 1-12, 4-22 6-23,
1-11, 2-12 4-16, 2-18
6-18, 6-19 1-15, 1-14 8-24
(K-H)i
1-15, 1-12, 2-1 1 6-12 1-12, 2-15, 4-12 2-19, 6-13 1-22
(H-H)2
15-17 23-24
0.400
0.400
0.400
'In m d y n / A for stretches ( K ) and mdyn A/rad2 for bends (H.7). bThese force constants were adjusted in the refinement (*), while the remainder wcrc fixed at the values taken from the NiOEP force field of ref 15.
of 10.7 cm-' for 32 observed frequencies of the natural abundance species, and an average error of 3.3 cm-l for 38 observed isotope shifts. We note that this is a substantial improvement over the QCFF/PI calculation,2 which gave an average error of 25 cm-' for 32 frequencies; isotope shifts were not calculated. A plot of the final stretching force constants against bond distance (Figure 2) shows a small negative divergence from the scaling relationship employed; a similar divergence was also found for NiOEP. The results of the calculation are given in Table IV, where the calculated frequencies and isotope shifts for the chlorin skeletal modes are compared with the available experimental data; the assignments are based on NiOEP (see below) for which the corresponding frequencies are also listed. C-H stretching modes, which are not observed in the RR spectra, have been omitted. As in the case of NiOEP,IS modes mainly involving internal coordinates extending beyond the methylene carbon atoms are classified as ethyl modes and are listed separately in Table V, along with approximate mode descriptions. In most cases the calculated I5N and meso-deuterium isotope shifts are very small, as expected for ethyl modes, but they become large in a few cases, due to substantial mixing with skeletal modes (see especially the 687-cm-I "CH2 rock"). While the general pattern of 15N and d, shifts is satisfactorily reproduced by the force field, there are some discrepancies, the most notable of which are the d2 shifts of v~~~ and q7,,.These are calculated to be IO and 14 cm-I, respectively, but the candidate RR bands, at 1614 and 1608 cm-I, are reported not to shift at all. The absent shifts pose a problem for any chlorin calculation, since the eight C,C, bond stretches must produce a greater aggregate shift of the RR bands above 1500 cm-', no matter how localized the modes, than has so far been observed. It is desirable that the experimental d, shifts should be reinvestigated with this in mind. The two H atoms of the OEC pyrroline ring introduce six extra vibrational modes, relative to OEP. Two of these are C-H stretches, and the remaining four are mainly C-H bends. These bending coordinates are assigned to modes at 970, 841,693, and
1557
-0.120
-0.120
-0.245
-0.245
0.080
0.080
0.060 -0.070
0.060 -0.070
i-j, interactions between internal coordinates i and j , labeled according to the numbering scheme in Table 11. (H-H)2 bend-bend interaction between angles sharing two common atoms. ( K - H ) stretch~ bend interaction between bond and angle sharing one common atom. stretch-bend interaction between bond and angle sharing two (K-H)~ common atoms. ( K - K ) ~ , ~stretch-stretch interaction between two bonds separated by one atom. ( K - K ) ~ , ~stretch-stretch interaction between two bonds sharing one atom. bThese force constants were adjusted in the refinement (*), while the remainder were taken from the NiOEP force field (ref 15).
1.300
1.400
1.500
1.600
C-C Bond Length (A)
Figure 2. C-C stretching force constants. The points are the final values used in the normalmode calculation, and the curve is the empirical relationship derived by JohnstonI8and reformulated by Burgi and Dunitz:19 k = ~ O - l ( ~ 1 . 8 5 ) / 0 . 5 5 1 ,
672 cm-l on the basis of their eigenvectors (Figure 6), and are labeled "pyrroline" in Table IV. Except for the 970-cm-l band, there are appreciable I5N or meso-deuteration shifts, however, indicating substantial mixing with other skeletal modes.
1558 The Journal of Physical Chemistry, Vol. 95, No. 4, 1991 The remaining modes are highly isomorphous with the skeletal modes of NiOEP. This is demonstrated in Table VI, in which calculated frequencies are compared for NiOEC (italicized) and NiOEP (roman) for NiOEP modes classified in D4h symmetry on the basis of the predominant local coordinate. These coordinates were definedISto take into account appropriate symmetry combinations of bond stretching or bending coordinates, and collective motions of the pyrrole rings; entries corresponding to the same local coordinate in different symmetry classes have ,, similar frequencies. The single exception concerns the CC stretching modes. Since the pyrroline C,C, bond of chlorin IS a single bond, it contributes mainly to an A symmetry mode at 11 14 cm-l. The remaining three C,C, stretches combine into a B symmetry mode, which correlates with E, in D4h symmetry, and two A symmetry modes one of which is in-phase and correlates with AI,. The out-of-phase A-symmetry mode correlates with either B,, or &; the B,,(A) correlation has been arbitrarily chosen for Table V, leaving the 1 114-cm-' pyrroline C,C, stretch classified as E,(A) (the correlations between D4*and C2 representations are given in Table VII). The close correspondence between O E P and OEC vibrations is emphasized by comparing the eigenvectors (Figure 3). All of the high-frequency modes are very similar for O E P and OEC; the eigenvectors are often nearly superimposable if the pyrroline outer bonds are ignored. This result is in contrast to that of Boldt et who reported eigenvectors showing marked localization of the atomic motions to various fragments of the macrocycle. The discrepancy may indicate a limitation of the QCFF/PI approach in exaggerating the symmetry lowering, although the calculated geometry was not reported. The localization may also have been overemphasized partially by restricting the eigenvector plots to internal coordinate contributions exceeding Ozaki et aL8 found inverse linear correlations between core size and frequency for metallo OEC R R bands above 1450 cm-I, similar to those established previously for p r o t ~ p o r p h y r i n and s ~ ~OEP.2S This parallelism is readily understood as resulting from the OEP/OEC eigenvector correspondences established in this work. There are, of course, some NiOEC/NiOEP eigenvector differences. For example, the C,H bending coordinate contributes noticeably to the NiOEC modes correlating with A,,, whereas such contributions are prevented by symmetry in NiOEP. In the 900-1 300-cm-' region pattern recognition is somewhat problematic because of large contributions from pyrroline stretching and deformation coordinates. Nevertheless the frequency correspondence in Table IV remain impressive even in this frequency region. Assignments and the RR Enhancement Pattern. The assignments on which our analysis rest are based on the R R band frequencies and the isotopic shifts reported for NiOEC substituted with ISN,and with 2H at the meso positions distal to the pyrroline ring (Figure 1). Other key data were the enhancement patterns of the bands and their depolarization ratios. The enhancement patterns are illustrated in Figure 4, showing R R spectra at three different wavelengths, 406.7,488.0, and 607.1 nm; Figure 5 shows where these wavelengths fall with respect to the NiOEC absorption spectrum. The RR band frequencies and relative intensities are in good accord with those reported by Bocian and co-workers at these wavelengthse2 The relationship of chlorin and porphyrin electronic transitions:6 , ~ ~ been discussed and the consequences for RR ~ c a t t e r i n ghave elsewhere and are summarized briefly here in order to show how the assignments support these expectations. The first two porphyrin transitions, Q and B, show a large intensity disparity due to configuration interaction between the first two orbital excitations, alu-eg and a2,-eg, the a,, and azuorbitals having nearly the
(24) Parthasarathi, N.; Hansen, C.; Yamaguchi, S.; Spiro, T. G . J . Am. Chem. SOC.1987, 109, 3865. (25) Oertling, W. A.; Salehi, A.; Cheng, Y. C.; Leroi, G. E.; Chang, C. K.; Babcock, G. T. J . Phys. Chem. 1987, 91, 5887. (26) Gouterman, M. J . Mol. Spectrosc. 1961, 6, 138. (27) Kitagawa, T.; Ozaki, Y. Sfrucf.Bonding (Berlin) 1987, 64, I .
Prendergast and Spiro same energy. The transition moments add for the strong B transition but nearly cancel for the much weaker Q transition, which however steals 10% of the intensity back into a vibronic side band, Q,. In resonance with the B absorption band, the RR spectra are dominated by totally symmetric modes (A,,), enhanced via Franck-Condon ( A term) scattering, while resonance with the Q bands enhances non totally symmetric modes (A2g,B,,, and B2,) which are effective in vibronic mixing. The replacement of a pyrrole ring in porphyrin by a pyrroline ring in chlorin lifts the in-plane (x,y ) degeneracy. The B electronic transition nevertheless remains nearly degenerate (see the strong B band in Figure 5), but the Q transition is strongly split into x and y components.26 The higher energy Q, transition ( x is the short axis of the a system, along the pyrroline N-M bond (Figure 1)) remains weak, and a vibronic side band, Qxl, is evident. Consequently, RR scattering in resonance with Qa or QxIis expected to be vibronic in character. The lower energy component, Q,,,is much stronger, however, and is therefore expected to support Franck-Condon scattering of totally symmetric modes, like the B transition. The Raman mode symmetries are also altered by the pyrroline ring, however. The ideal point group is lowered from D4,,to C2; consequently B,, as well as A,, modes become totally symmetric (A), while A2, and Bagremain non totally symmetric (B) (Table VII). Meanwhile the E, infrared modes become Raman-active, due to the lost symmetry center, and split into two components, one of which (E,) is totally symmetric, while the other (Eb) is non totally symmetric. The NiOEC R R enhancement pattern can be readily interpreted on the basis of these considerations. The 488.0-nm-excited spectrum, being resonant with the weak Qxl, is expected to involve vibronically active modes, and the prominent bands do indeed correlate with the NiOEP B,,, B2,, and AZgmodes (Figure 4). The bands correlating with Aag( ~ 1 9 ,u 2 , , and u22) and B2, (u2*, ~ 2 9 ,and u30) are anomalously polarized and depolarized, respectively, thus retaining their D4,-derived distinction, even though all are of B symmetry in the C2 point group. The B, modes ( u I 0 , uII, and q 4 ) are polarized, reflecting the lowerec! symmetry, but are nevertheless strong in the 488.0-nm-excited spectrum, cvidently maintaining their effectiveness in vibronic mixing (of the Q, and B, transitions). Al,-derived modes, however, are weak in the 488.0-nm-excitedspectrum. This parallels NiOEP, where the A,, modes are ineffective in vibronic mixing of the Q and B transit~ons.l~ Strong enhancement of A,,-derived modes is seen with 40.7-nm excitation, in resonance with the B transition. The u2, u3, and u4 bands are the strongest ones in the spectrum, as they are in the B-band-resonant NiOEP s p e c t r ~ m and , ~ ~ the us band is also prominent. Interestingly, the B,,-derived mode u I I is also very strong, and evidently has a large excited-state origin shift. It is notable that the eigenvector of uI1 (Figure 3) bears a marked resemblance to that of the u2 mode, save for the phasing of the displacements of the pyrrole ring trans to the pyrroline ring. Both u2 and uI1 involve a large symmetric displacement of the C,C, stretches on the cis pyrrole rings, no doubt accounting for the large origin shifts. The remaining B,,-derived modes are weaker and have about the same intensity as the B2,- and A2,:derived modes. These intensities are appreciable, however, and indicate significant activity in the B transition. Possibly this activity reflects mixing of the nearly degenerate B, and B, components, which are unresolved in the broad B absorption band. In NiOEP these components are strictly degenerate, and their coupling is due to the Jahn-Teller effect, which does in fact produce significant B,, and B2, activity;2s intrastate A2, activity is suppressed, however, in the limit of x,y degeneracy. Candidate bands are also seen for the E,-derived modes u37a, Y37b, V38b, uNa, ~ 4 0 b ,and u&b, as detailed in Table IV. These bands have comparable relative intensities in the 406.7- and 488-nm-excited spectra, and seem to be vibronically active.
(28) Shelnutt, J. A.; Cheung, L. D.;Cheng, C. C.; Yu, N. T.; Felton, R.
H.J . Chem. Phy.7. 1977, 66, 3387.
Normal-Mode Analysis of Nickel Octaethylchlorin
The Journal of Physical Chemistry, Vol. 95, NO. 4, 1991
1559
TABLE I V Calculated vs Observed In-Plane Frequencies and Isotope Shifts" for NiOEC Compared to NiOEP
NiOEC (calc) assigncm-' AI5N AyJd-2 Ad-4 ment 1650 0 9 18 V I 0 0 IO 1632 l7 h l a 1631 14 0 2o b 1 b 1590 0 5 29 v i 9 1586 2 0 7 v2 1572 0 5 0 v38b I569 1 0 1 VI1 1507 1 3 12 v3 1491 1 3 l 4 v39b 2 1483 8 14 ~ 3 9 s I456 22 2 22 Y28 1403 2 0 "40b 2 4 1401 7 v4on 1392 2 1 3 v20 I390 0 0 Y29 1382 4 1 5 v4 1355 5 1 v41b I344 4 1 5 Y41a 6 1336 452 474 Y21 1300 3 22 23 Y12 2 1257 32 1 276 v 4 2 ~ 1252 1 237 245 Y42b 1 1235 299 336 v i 3 1208 +I8 8 +22 4 4 235 I I55 8 234 V43b 7 1 I40 5 4 Y5 8 1138 0 v30 1 I27 11 +24 +62 v43a 1117 6 0 "44b 6 1116 +IO0 +I02 u22 1114 IO 9 9 v3sa I IO3 5 +I3 + I 4 vMP +I 1 2 1041 +22 ~ 2 3 9 24 I004 29 v31 2 +48 I003 +50 ~ 4 5 s +29 9 984 +31 v45b 0 0 0 pyrroline 970 IO 12 918 14 V G a
v,
89 1 885 841 816 808 795 794 74 1 730 708 693 669 630 623 610 579 565 434 386 38 1 352 343 308 297 273 269 264 253 183 172 160 157
4 4 3 6 2 4 4
I50
1
+5
0 1 1 5
IO0 1
2
+54 +3 3 +2 8
1
0
I
2 3 2
1
3 1
0
1 1
3 0 1 1 0 1 1
1 0 0 0 1 1 0
0 0
1 0 0 0 0 0
2 1
0 0 0 0 1 0 0 0 1
NiOEC (obs) cm-l A15N A ~ , 6 d - 2 ~ percent PED* 1648 p 0 8 37% am,,45% aml,16% amH 1614 p 0 0 21% am,,51% aml,14% amH 1608 0 0 32% am,,SI% aml,l7% amH 1590 ap 0 5 32% am3,49%aml,21% amH 1588 p 0 45% @@,20% am,,lO% @@Cl 1572 0 57% @@,12% am,,ll% @@Cl 1546 p 0 0 65% @@,ll%@@CI 1512 p 1 4 25% @@,20% am,,lO% ami,13% @CI 17% aN,,I 8% am,,23% am, 1481 1 25% am3,18%aml 1478 dp 2 11% aN,,14% am,,26% HCIH 13% aN,,44% a@,,13%a@@,ll% @ClC2,16%@ClH 1382 p 4 4 16% aNc,,14% (~@~,,18% @ClH,I2%C2ClH 1392 4 11% aNC,,33%(~@,,,11% c ~ @ @ , I l %@@C1,16% @ClH 1402 dp 0 4 21% a@,,26% @CIH,17%C2ClH 1367 p 4 5 12% aN,,15% a@,,,20%a&10% @C, 12% aNc,,20% aN,,IO% a&,18% a@,,IO% CzCIH 1349 p 2 13% aNcr,18% aN,, 18% abC, 1308 ap 4 14% amH,14% @CIH,Il%C2ClH 11% aNr,16%amH,2l% C2CIH,21%@CIH 21% amH,I 1% a@,,13%@CIH 23% amH,ll% @CIH,12%a@H 1232 p 2 286 23% a@,,24% amH,16% @CIH 1204 p +I8 10% aN,,10% aNa,17% amH,16% Cl@H 17% aN,,28%amH 1146 p 13 6 12% @Cl,13%amH 1152dp 7 0 12% aNC,,14%aN,,26% @Ci,11%a@@ 16% aNc1,12%",,IS% @Cl,ll%amH 22% aN,,24% @Cl 1 I24 ap 5 +62 19% aN,,12% @C1,18%amH 23% @@,,IO% @CI, 24% @@,IO% C2CIH 1026 ap 37% ClC2 17% C,C,,Il% Nam,l2% ama 29% CiC,,lO% @CIH,I5%@Cl 10% aN,,IO% a@,,12%CIC2 22% &3H,,28% C;C2,41%.@Cl, 926 p 8 6 21% a@,,14%a@CIr,12% @@Ci,,ll% @CIH,18% C2CIH 10% a@,,29%@C11,12% Nam,,lI% ama v32 18% a@,,12%C2ClH l 3 Y46b 3 pyrroline 13% pCIH,l8% C2CIH,IO% @OH 798 p 14% @CIC2,26%@Cl Y6 19% C2ClH,22%@C1,,12%@@H "47b 149 V I S 778 23% @CI,20%@CIC,,I7%y@Cl 18% @CI,lI%y@Ci,14%@CIC2 1 V47a +53 V I 6 1 17% C$lH,l5% @ClH 746 p 1 pyrroline 20% @@H,12% j3CIC2 23 V, 681 p 3 18% ama,lO% y@Cl +7 pyrroline 15% @ClC2,16%@@H 26% @CiC2,11%CY@@ v24 20% @c1,41%')'@c~ 0 v48b 22% @C,,43%y@C1 +4 Y4Be 14% Nam,lO% ama,ll% a@CI v25 11% @CIC2,25%@CIH,20%C2ClH,l3% ~r@Cl v49b 16% ~@C1,21% C2CIH,16% @ClH 3 ~49s 14% @Cl,18%@CiC2,20%y@Cl 1 Y33 37% NiN,,13% @C,C2,15%y@Ci "Sob 16% NiNc,,17%@CIC2,18%y@C, 0 Ysoa 362f 342 p 46% @@Cl,40% @ClC2 0 V8 25% @CiC2,17%y@CI v17 11% NiN,,lO% Nam,l2% a@CI V51b 25% NiN,,l8% NiN,,ll% NiNa 1 v5ia 13% a@Cl,ll%@@Cl 1 bza 34% @@c1,34% a@cI v5Zb 27% @@c1,29% a@Cl,16%@CIC~ 0 v9 20% y@CI,26%j3ClC2 v26 15% @Cic2,24%~ @ C I 1 v34 18% Nam,ll% NiN,,I2% NiN, 0 Vi8 10% Nam,20% ma@,IO%a@C, 0 v53a 46% @@CI v53b 19% NiNa,l3% ma@,lO%NNiN 1 u35 Y,
NiOEP Y , cm-' Ad-4 1655d IO (1637)d (17) 1603 1602 1604 1577 1520
22
1501
7 7 15 16 16 0 2
1501
1483 1396 1396 1393 1407 1383 (1346) (1346) 1307 1330 1231 1231 1220 1131 1153 1138 1159 1153 1133 1121 1133 1058
1
0 1
8
1
(4) (4) 420 12 289 283 283 +55
+32 0 1
+32 +I8 +81 +I8 0
1015 996 996
12 0
927
8
938 927
4 8
804 79 1 751 79 1 740
0 68 0 +22
674
6
597 615 615 55 1
534 534 493 358 358 343 305 328 328 263 263 274 243 197 168 167 167 144
0
5
15
12 12 6 3 3 13 1
1
0 0 6 6 3 3 0 0 0 0 0 0
3
"AI5N shifts upon substitution at all four N atoms. Ay,6d-2 shifts after deuterating two (yJ) or all four (Ad-4) of the methine bridges. bPotential energy distributions, coordinate definitions (see Figure 1 and Table I for atom labeling): X-Y stretching of XY bond. XYZ bending of XYZ angle. eNiOEC-d2 data from Boldt et aL2 dNiOEP from Li et al.ls Values in parentheses are calculated frequencies for NiOEP.
Prendergast and Spiro
1560 The Journal ofPhysica1 Chemistry, Vol. 95, No. 4, 1991
NiOEP
NiOEC
NiOEP
NiOEC
Alg
@
v2
*2g v19
v3
v20
v4
v10
v29
VI 1
NiOEC
%*
NiOEP %b
v37a
v37b
v38a
v38b
E,
V17
v38
Figure 3. Comparison of eigenvectors calculated for NiOEC (present work) and NiOEP15for modes in the high-frequency region. Only one E, component is shown for NiOEP. while both components are shown for NiOEC. The uj8, mode (1 114 cm-I) is chosen somewhat arbitrarily from the modes having largc C,C, reduced ring stretching character.
The Journal of Physical Chemistry, Vol. 95, No. 4, 1991 1561
Normal-Mode Analysis of Nickel Octaethylchlorin
TABLE V Etbvl Freauencies and Isotope Shifts for NiOEC
NiOEC (calc)
NiOEC in KBr
NiOEC (obs)
u,
V,
cm-I AI5N AY,bd-2 Ad-4
607.1 nm loo0
12w
14CQ
1600
Figure 4. Resonance Raman spectra of NiOEC in KBr pellets: 50 mW, 0.5 cm-’ s-l, 5-s scan, 6-cm-l slits: (a) 406.7-nm excitation, (b) 488.0-nm excitation, (c) 607.1 -nm excitation. Bands are marked with frequency
(cm-I), polarization (p = polarized, dp = depolarized, ap = anomalously polarized), and NiOEP-derived mode assignments.
NiOEC in CHzClz
/ B
Wavelength (nm)
Figure 5. Absorption spectrum of NiOEC in CH2CI2. The electronic assignments (B, Qy,Qm, QXI) are shown, and the arrows indicate the RR laser wavelcngth.
The enhancement pattern differs strikingly for the 607.1- and 406.7-nm-excited spectra, even though Franck-Condon activity should be dominant in resonance with the strong Q, as well as the B transition. The strong bands in the 607.1-nm-excited spectrum are indeed polarized, but the strongest B-resonant bands, v2, ujr and v4, are very weak, while the strongest Q-resonant skeletal modes, u5 and vI3, are of only moderate strength at 406.7-nm. The situation is entirely analogous to that found in NiOEP, for which the Q-resonant polarized bands (whose absolute intensity is much lower than in NiOEC because of the lower Q transition moment) are also stronger in the mid-frequency than in the high-frequency region. In both cases the geometric distortion must be quite different for the B and Q excited states. Prominent bands in all three spectra are also assigned to ethyl modes: C,-C2 stretching, at 1020 cm-I, and CH2 twisting at 1261
1453 1452 1451 1450 1450 1446 1441 1440 1375 1374 1368 1366 1362 1357 1357 1337 1325 1315 1306 1296 1292 1292 1285 1229 1049 1049 1028 1024 1024 1024 1021 1013 918 752 751 740 738 672 730 123 707 687 631 484 482 451 445 415 299 263 258 229 224 210 198 196
0 0 0 0 0 2 0 0 1 1 0 0 0 0 0 1 3 2 2 4 2 5 1 1 0 1 2 0 2 0 2 1 1
0 0 0 0 1 0 1 0 2 2 4 2 1 1 0 1 0 1 0 0 0 0 0
0 0
0 0 0 +3 0 0 1 0 0 1 0 0 0 0 +2 +4 3 0 +3 1 1 1 5 9 4 0 0 2 8 0 +2 1 0 1 0 +15 0 +1 2 7 1 1 1 0 0 0
0 0 0 0 0 +3 0 0 1 0 0 1 1 1 1 0 9 9 0 0 1 2 2 1 1 2 4 0 0 3 7 +1 +2 1 0 1 1 +13 0 +4 2 27 1 3 3 3 1 0
assignment CH, scissors CH, scissors CH, scissors CH2 scissors CH2 scissors CH, scissors CH2 scissors CHI scissors CH2wag CH2wag CH,wag CH,wag CH,wag CH2wag CH2wag CHzwag CHI twist CH, twist CHI twist CHI twist CHI twist CH, twist CH, twist CH2 twist C1-C2 C&
c,-c, C& q-c, C,-C, q-c,
C,-C, CH, rock CH, rock CH, rock CH2 rock BCIC2 pCIC2 W,C2 CH,rock CH,rock CH2 rock CH2rock BCIC, BCIC2 BCIC2 PCIC2 PClC2
cm-l AI5N A7,bd-2‘
1427
0
1328
0
1274 1261
0 0
1062
0
1020 961
0 0
4
0
1
0 1 0 0 0 0 0
“AI5Nshifts upon substitution at all four N atoms. A7,bd-2 shifts after deuterating two (7J) or all four (Ad-4) of the methine bridges. NiOEC-d2 data from Boldt et aL2 and 1274 cm-I. These assignments are made on the basis of the ISN and meso-2H insensitivity, and close frequency matches with the corresponding ethyl modes of NiOEP, which were definitively assigned via methylene de~teration.’~ These bands are comparably strong for NiOEP and NiOEC. In both molecules the CI-C2 stretch is one of the strongest bands in the Q-resonant spectrum, implying a large origin shift along this coordinate in the Q state. A hyperconjugative mechanism has been proposed to account for this effect, involving interaction of the methyl or orbital with the porphyrin (or chlorin) al, orbital, an interaction which is max-
1562 The Journal of Physical Chemistry, Vol. 95, No. 4, 1991
Prendergast and Spiro
TABLE VI: Comwrison of NiOEP and NiOEC (in Italics) Calculated Skeletal Mode Freouencies (cm-')
v(CruCm) asym 1604 1586 1517 1507
"(CBCP) "(CaCm)sym
1658 1650 1578 1569
6(CmH)
1330 I300 1238
1120 1140
v2 1
u(C@Cl)asym
v23
d(pyr def) asym
"24
d(pyr def) sym
346 352 256 264
1 I27 1116 1055 1041 626 669
"30 "31
v32
1 I60 1138 1004 I004 945 891
566 610
"33
493 434
243 253
"34
"35
204 I83 151
I50
C,
84 1cm-1
730cm-1
693cm-l
Figure 6. Eigenvectors of the four vibrations allocated to C,H bending in the reduced ring.
imized when the ethyl groups are in their sterically favored out-of-plane ~rientation.'~ Gladkov et al. have noted strong ethyl mode enhancement for Z n O E C 9
"46
v49
"5 I "52
@(pyr trans)
DA'
"45
"50
315 343
TABLE VII: Correlation of Du and C2In-Plane Modes
"4 3
"48
"26
6(CflCI)sym
"42
168 I72
d(C@C I ) asym
"40
"41
"25
u(NiN)
1321 I336
759 795 733 741
6(pyr rot)
"39
"44
u22
827 816 690 708
"29
1150 I208
"(pyr 112 ring) asym
"(pyr breath)
1402 I392
1486 1456 1403 I390
"41
I235
u(CBC1)sym
"37 "38
"20
1381 I382
1601 1590
"28
u(pyr 114 ring) u(pyr 112 ring) sym
"19
"53
1631 I631 11632 1588 157211 I I4 1496 I491 / I 483 1404 140311 401 1346 135511 344 1240 I252/1257 1 I44 I I 1711 I03 1 I30 I IS511 I27 1004 9841 I003 922 8851918 791 8081794 615 6301623 534 5791565 358 3861381 317 3081297 293 2691273 161 1571160
Conclusions When due allowance is made for the bond distance changes between O E P and OEC, the empirical valence force field developed for NiOEP does a good job in calculating the NiOEC vibrational frequencies and isotope shifts. The resulting assignments provide a satisfying account of the RR enhancement pattern, which is similar to that of NiOEP when symmetry-loweringeffects are taken into account. In particular the excited-state distortion differs between the B and Q states in the same way for NiOEC and NiOEP, selective enhancement being seen for high-frequency and mid-frequency symmetric skeletal modes, respectively, in resonance with the B and Q states. Strong enhancement is also seen for ethyl mode in both molecules, especially the C1-Czstretch, and is suggested to arise from a hyperconjugative mechanism. With the exception of modes allocated to the pyroline ring, the NiOEC vibrations are very similar to those of NiOEP, as reflected in frequency correspondence and direct comparison of the calculated eigenvectors. This similarity provides strong support for the analysis of metallochlorin vibrational spectra by analogy with the spectra of the corresponding metalloporphyrin spectra. Note Added in Proof. Babcock and co-workers (Fonda et al. J . Am. Chem. SOC.1990,112,9497) have reported new data on zinc, copper, and nickel chlorins, with isotope shifts for selective deuteration of the meso positions in CuOEC, which will be very useful in further refinements of the chlorin force field. They note that the shifts produced by deuterating two methine positions deviate significantly from half the value of the meso-d, shifts and suggest that these deviations support the idea of mode localization. Such deviations, however, are evident in our calculation (Table IV) and are attributable to symmetry reduction per se and not to localization. The hydrogen atom motions are very sensitive to details of the mode composition. Some of the reported CuOEC deuteration shifts deviate significantlyfrom the values we calculate, but they are also different from the reported NiOEC deuteration
J . Phys. Chem. 1991, 95, 1563-1572
1563
shifts, indicating that the mode compositions are somewhat metal-sensitive.
providing NiOEC samples, and to Laura Anderson for helpful comments. This work was supported by N I H grant GM 33576.
Acknowledgment. We are indebted to Prof. S.H. Strauss for
Registry No. NiOEC, 39001-94-0; ISN, 14390-96-6;D2,7782-39-0.
Infrared and Raman Spectra, Conformational Stability, Barriers to Internal Rotation, Normal-Coordinate Calculations, and Vibrational Assignment for Vinylsilyl Chloride J. R. Durig,* J. F. Sullivan, G. A. Guirgis,t and M. A. Qtaitat' Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received: April 26, 1990; In Final Form: September 1 I , 1990)
The infrared (3250-40 c d ) and Raman (3250-10 cm-I) spectra of the gaseous and solid states of vinylsilyl chloride (CH,CHSiH,CI) have been recorded. The Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. From the far-infrared spectrum of the gas, the fundamental asymmetric torsions for both the cis and gauche conformers have been observed at 101.9 and 74.8 cm-I, respectively, with each having several upper state transitions. From these transitions, the potential function to internal rotation has been determined with the following values: VI = IO 7, Vz = 160 8, V, = 606 4, and V6 = -33 2 cm-', with the cis conformer thermodynamically preferred by 124 19 cm-' (354 f 54 cal/mol). The cis to gauche, gauche to gauche, and gauche to cis barriers are 731 (2.09 kcal/mol), 492 ( I .4 I kcal/mol), and 607 cm-' (1.74 kcal/mol), respectively. A variable-temperature study of the liquid was carried out and the enthalpy difference between the two conformers was found to be 98 f 11 cm-l (280 f 31 cal/mol) with the cis form being more stable. The data are also consistent with the gauche conformer being the only form present in the solid phase. A complete vibrational assignment is proposed based on infrared band contours, depolarization ratios, and group frequencies. The assignment is supported by an ab initio calculation utilizing the 3-21G* basis set to obtain the force constants and potential energy distribution. These results are compared with the results of some related molecules.
*
*
*
Introduction
The vibrational and rotational spectra of the 3-halopropenes have been the subject of several studies for the past few years,'-* from which it was concluded that these molecules exist as a mixture of cis and gauche conformers. The silicon analogue, vinylsilyl chloride (CH2CHSiH2CI), has been previously studied.+l6 It has been concluded from vibrational'OJ1 and microwave12spectroscopic studies that this molecule exists as a mixture of cis and gauche (skew) conformers in the fluid phases, while the gauche conformer is the only form present in the solid phase. In one study, Goreva et a1.I0 attributed a Raman line at 188 cm-' in the spectrum of the liquid to the overtone of the torsional fundamental of the cis conformer. However, in another study, they" assigned the bands at 167 and 124 cm-I to the torsional fundamentals of the cis and gauche conformers, respectively, whereas the band at 188 cm-' was assigned to the CSiCl bending mode for the cis conformer. The potential function governing the barrier to internal rotation of the SiHzCl moiety in vinylsilyl chloride has been previously reported.I3-l5 These calculations were based on theoretical treatments and experimental data for the liquid phase. From these calculations, the cis to gauche and gauche to gauche barriers were determined to be 1470 and 945 cm-I, respectively. However, these investigators did not calculate the gauche to cis barrier, as no attempt was made to obtain the torsional transitions of vinylsilyl chloride from the far-infrared spectrum in the gaseous phase. Moreover, the Raman spectrum in the vapor phase has not been
' Permanent address: Mobay Corp., Dyes and Pigments Division, Bushy Park Plant, Charleston, SC 2941 1 'Taken in part from the thesis of M. A. Qtaitat which was submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree, May 1990.
0022-3654/91/2095-1563$02.50/0
*
reported, and the previous vibrational were based on spectral data from the liquid and solid phases. In two independent controversial variable-temperature studies, Khristenko et al.l0*l6reported the enthalpy difference between the two conformers, cis and gauche, to be 87 f 21 and 175 f 94 cm-I. However, in the first study,1° they reported that the gauche conformer is the most stable form, and in the second study,16the cis is the most stable conformer when the isotopic species,
(1) Durig, J. R.; Zhen, Mengzhang; Heusel, H. L.; Joseph, P. J.; Groner, P.; Little, T. S. J. Phys. Chem. 1985, 89, 2877. (2) Durig, J. R.; Jalilian, M. R. J . Phys. Chem. 1980, 84, 3543. (3) Durig, J. R.; Zhen, Mengzhang; Little, T. S. J. Chem. Phys. 1984,81, 4259. (4) Stolevik, R.; Thingstad, 0.J . Mol. Struct. (THEOCHEM) 1984, 106, 333. (5) Schei, S. H.; Shen, Q. J. Mol. Struct. 1985, 128, 161. (6) Barnes, A. J.; Holroyd, S.; George, W. 0.;Goodfield, J. E.;Maddams, W. F.Spectrochim. Acta 1982, 38A, 1245. (7) Durig. J. R.; Geyer, T. J.; Little, T. S.; Durig, D. T. J . Mol. Struct. 1988, 172, 165. (8) Durig, J. R.; Durig, D. T.; Jalilian, M. R.; Zhen, Mengzhang; Little, T. S. J . Mol. Struct. 1989, 194, 259. (9) Sullivan, J. F.; Qtaitat, M. A.; Durig, J. R. J . Mol. Struct. ( T H E 0 CHEW 1989, 202, 159. (10) Goreva, V. I.; Khristenko, L. V.; Pentin, Yu. A. Vopr. Stereokhim. 1972, 2, 57. (1 I ) Khristenko, L. V.;Pentin, Yu. A. Vestn. Mosk. Unio.,Ser. 2 Khim. 1976, 31, 304. (12) Imachi, M. J. Sci. Hiroshima Unto. Ser. A 1978, 42, 43. (13) Khristenko, L. V.; Zenkin, A. A,; Pentin, Yu.A.; Tyulin, V. I. Zh. Strukt. Khim. 1979, 20, 809. (14) Pentin, Yu. A. J. Mol. Struct. 1978, 46, 149. (15) Pentin, Yu. A.; Tyulin, V. I. Vestn. Mosk. Unio., Ser. 2 Khim 1977, 32, 580. (16) Khristenko, L. V. Ph.D. Dissertation, Moscow State University, 1974.
0 1991 American Chemical Society
