Langmuir 1992,8, 942-946
942
Spectroscopic Studies of Langmuir-Blodgett Monolayers of Praseodymium Bisphthalocyanines J. Souto, L.Tomilova,+and R. Areca* Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario, Canada N9B 3P4
J. A. DeSaja Fisica de la Materia Condemada, Universidad de Valladolid, Valladolid 47100, Spain Received August 13, 1991.I n Final Form: January 2, 1992 Lan uir-Blodgett (LB) monolayers of praseodymium bisphthalocyanine (PrPcz) and a tetra-tertbutyl G v a t i v e [4-(t-Bu)4Pcl~Pr(orPrPctz)have been prepared and characterized using Raman scattering (RS), surface enhanced resonant Raman scattering (SERRS),and infrared and visible absorption spectroscopies. The green and blue forms of PrPctzstarting material were separated,and the oxidationreduction and acid-base reactivity of the green form of PrPcz and PrPctzin solution is reported. Well-characterized LB multilayer assemblies of the green form of PrPcz and PrPctz were exposed to NO, (N02/N20r)gas, and the effect of gas adsorptionwas monitoredusingvisible and Raman spectroscopies. With the observation of a reversible chemicaladsorption of NO2 on a mononomolecular LB layer of PrPcz and PrPctzcomplexes, the reversibility of the adsorption reaction for early and late rare earth bisphthalocyanine is confirmed. Introduction The present study is part of our research program to study the fabrication and chemical and physical properties of Langmuir-Blodgett (LB) monolayers of bisphthalocyanine derivatives. In particular, an attempt is made to understand the effect of adsorption of small electron acceptor molecules on the molecular properties of complexes of rare earth bisphthalocyanines in LB films. The effect of NO, on crystals and films of monometalated phthalocyanine (Pc) derivatives has been recently reviewed by Wright.l The thermal stability of most Pc materials allows the fabrication of evaporated thin solid films, and a number of film studies have been reported, in particular, the electronic absorption spectra.24 A much more challenging proposition has been the fabrication of a stable LB layer of unsubstituted and substituted bisPc complexes. Very recently, the formation of LB layers of LuPc2 and YbPc2 has been rep~rted."~Mixed monolayers of YbPc2 and stearic acid were studied by Petty et al.? and LB layers of HoPc2 and DyPc2 and arachidic acid mixtures were fabricated in our group.8 The vibrational spectra, surface enhanced resonant Raman scattering, LB layer fabrication, and interaction with the NO, gas are reported here for the first time for praseodymium bisphthalocycompounds. anine (PrPc2) and [4-(t-Bu)4Pc12Pr(PrPct2) Chemical oxidation-reduction and acid-base behavior of the molecules in solution were investigated, and the results
* To whom correspondence should be addressed.
+ Permanent address: Organic, Intermediates and Dyes Institute, B. Sadovaya, 1/4,103787 Moscow, USSR. (1) Wright, J. D. Prog. Surf. Sci. 1989, 31, 1. (2) Moskalev,P. N.; Alimova, N. 1.Russ.J.Inorg.Chem. (Engl. Trawl.) 1975,20, 1474. (3) Markovitsi,D.;Tran-Thi, T.;Even, R.; Simon,J. Chem. Phys.Lett. 1987,137, 107. (4) Sidorov, A. N.; Moskalev, P. N. Russ. J. Phys. Chem. 1988, 62, 1569. (5) Liu, Y.; Shigehara, K.; Yamada, A. Thin Solid Films 1989, 179, Rng.
(6) Petty, M.; Lovett, D. R.; OConnor, J. M.; Silver, J. Thin Solid Films 1989,179, 387. (7) Clavijo, R. E.; Battisti, D.; Aroca, R.; Kovacs, G. J.; Jennings, C. A. Langmuir 1992,8, 113. (8)Souto, J.; Aroca, R.; DeSaja, J. A. J.Raman Spectrosc. 1991,22, 349.
were used to help in the interpretation of gas adsorption experiments. Previously assigned fundamental vibrational frequencies in evaporated films of LuPc2 and YbPc2,9 were used here as a reference for the identification of characteristic vibrations. Experimental Section The sample of PrPc2 was prepared according to Kirin et al.lo The synthesisof PrPct2 has been previously reported,ll and uses a mixture of 4-tert-butylphthalonitrileand a Pr salt in a 8 1 molar ratio. Purification was carried out chromatographically using an aluminum oxide column and benzene as the solvent. Blue and green forms of the bisphthalocyanine were obtained. The separationwas attainedon thin-layer chromatography (TLC) aluminum sheets, silica gel 60. The green form12of PrPc2 and PrPct2 with a characteristic electronic absorption spectrum in solution as shown in Figures 1 and 2 was the starting material in our experiments. LB monolayers of PrPcP and PrPc$ were prepared at 10 O C and transferred to Corning 7059 glass slides, and glass slides with a Au island film, in a Lauda Langmuir film balance equippedwith an electronicallycontrolled dipping device, Lauda Filmlift FL-1. Monolayers were transferred at a constant speed of 3 mm/min, and at surface pressures of 20 mN/m. Au island films were prepared by evaporation of 4 nm of Au onto glass heated at 200 O C . The thickness was monitored by a XTC Inficon quartz crystal oscillator. Spectra Physics Model 2020 Kr+and Ar+ ion laserswere used to obtain the inelasticscattering of light. Typical spectral band-passand laser power were 5 cm-1 and 50 mW, respectively. Raman shifts were measured with a Spex-1403double spectrometer. Infraredspectrawere measured on a BOMEM DA3 FT-IR spectrometer. Electronic absorption spectra were recorded on a Response UV-vis spectrophotometer. Results and Discussion Langmuir-Blodgett Monomolecular Layers. LB monolayers of PrPc2 were formed reproducibly by spread(9) Clavijo, R. E.; Aroca, R.; Kovacs, G. J.; Loutfy, R. 0. Spectrochim. Acta 1989, &A, 957. (10) Kirin, L. S.;Moskalev, P. N.; Makashev, Yu. A. Zh.Neorg. Khim. 1965,10, 1951. (11) Tomilova, L. G.; Chernykh, E. V.; Ioffe, N. T.; Luk'yanets, E. A. Zh.Obshch. Khim. 1983,53, 2594. (12) MacKay, A. G.; Boas, J. F.; Troup, G. J. Aust. J.Chem. 1974,27, 955.
0 1992 American Chemical Society
Langmuir, Vol. 8, No. 3, 1992 943
LB Monolayers of Praseodymium Bisphthalocyanines
0
0
1
2
5
4
5
6
7
Arca/Molecula (nm2)
Figure 4. Surface pressure-area isotherm for PrPc$ on a water subphase at 10 "C. 600
400
Wovelength ( n m )
Figure 1. Electronic absorption spectra of PrPcz: toluene solution (1)and LB multilayer (2). ,320
1
2
-
Solution in benzene 15 LE film
600
400
800
Wavelength ( n m )
Figure 2. Electronic absorption spectrum of PrPctz: toluene solution (1)and LB multilayer (2).
-- I
I
El
f
10
v)
0 0.00
0.25
0.50
0.75
Area /MOIOCUIO
1.00
1.25
1.50
1.75
(nm2)
Figure 3. Surface pressure-area isotherm for PrPcz on a water subphase at 10 "C.
ing between 2 X 10l6 and 4 X 10l6 molecules in toluene solution onto a triple-distilled and deionized water subphase. Consistent and reproducible results for the surface pressure versus area isotherm for the neat layer were obtained at 10 OC, and are shown in Figure 3. The collapse pressure was about 40 mN/m, and a relatively high compressibility value was observed for the full range of pressure. Monomolecular layers were transferred to glass slides by Z-deposition with a nearly unity transfer ratio. The transfer ratio for the second, third, and subsequent
layers was less than unity, and high-quality multilayers of the neat material were not obtained. Mixed monolayers of the PrPcz molecules and arachidic acid were also prepared and transferred to glass substrates with a unity transfer ratio, and good multilayer assemblies of mixed monolayers were formed on a glass substrate. Structural data have shown that the lanthanide ion is at the center of two Pc ligands facing each other in a staggered arrangement.13 However, the metal coordination may not be totally symmetric with respect to both macrocycles as was reported for N d P ~ 2 . l The ~ latter geometry would prevent a D4d symmetry, but an approximate CgU point group symmetry would be possible, as has been postulated for the interpretation of the observed vibrational spectra? Assuming PrPcz molecules to be standing (edge-on) with the Pc ring perpendicular to the water surface, and assuming a molecular thickness of 0.5 nm, an area per molecule of about 0.65 nm2 was estimated. The area per molecule obtained by extrapolation of the high-pressure section of the isotherm to 0 mN/m was 0.75 nm2. Similar values were found by Petty et a1.6 for YbPcz (0.74 nm2) and by Clavijo et al. for LuPcz' (0.75 nm2), indicating an edge-on (or tilted) organization of molecular units in the floating LB layer. The LB monolayer of PrPctz was prepared from a toluene solution spread on a water surface maintained at 10 "C, and gave a reproducible isotherm as shown in Figure 4. The floating layer should be assigned to the type of film known as liquid-expanded15 due to a difficulty in packing. The limiting area estimated for the low compressibility region between 25 and 40 mN/m was 1.55 nm2, which also indicates a tilted edge-on configuration for the ordered monolayer. Electronic Spectra. The electronic absorption spectra of PrPcz and PrPctz in solution agreed, in the number of frequencies and their relative intensities, with previously reported spectra of the green form for lanthanide bisphthalocyanine molecules.2-4J1J2The electronic spectra of a LB multilayer assembly has been included in Figures 1 and 2, together with the solution spectra. Notably, the absorptions of 456 and 478 nm, which are attributed to the Pc radical in the green form (asin L ~ P c ) ~ ~ ' ~ are not seen in the spectrum of the blue form of the material.12 The electronic absorption spectrum of the LB multilayer of PrPc2 (21 mixed layers on glass) closely resembles that of the solution in toluene as shown in Figure 1,and it can be said that the LB film contains the same (13)Song, S. A.; O'Connor, J. M.; Barber, D. J.; Silver, J. J . Cryst. Growth 1988,88, 477. (14)Kasuga, K.; Tsutsui, M.; Pettersen, R. C.; Tataumi, K.; Van Opdenbosh, N.; Meyer, E. F. J. Am. Chem. SOC.1980, 102, 4835. G.,Ed.;Plenum (15)Hann,R. A. InLangmuir-BEodgettFiIms;Roberta, Press: New York, 1990;p 17. (16)Turek, P.; Andre, J. J.; Giraudeau, A.; Simon, J. Chem. Phys. Lett. 1987, 134, 471.
Souto et al.
944 Langmuir, Vol. 8, No. 3, 1992 green form of the starting material. However, the electronic spectrum of the PrPct2 LB multilayer was different from that of the starting material. The green and the blue forms of PrPct2 were separated and identified by their distinct electronic spectra. The green form presented a Pc radical band a t 484 nm, a band at 602 nm, and the Q-band at 680 nm as can be seen in Figure 2. The blue form has a strong Q-band at 650 nm, and there are no absorptions a t 484 or 602 nm. In the spectrum of the LB multilayer of PrPct2, the characteristic Q-band was broadened and blue-shifted by ca. 18 nm, and the relative intensity of the Pc radical band (484 nm) was weakened. The observed spectra seem to indicate that part of the starting material (green form) has been interconverted to the blue form,11J2which has a Q-band centered a t 650 nm. As part of our thin solid studies, the PrPc2 green material was evaporated onto KBr substrates to form a 200-nm film. The spectral analysis of the evaporated film showed that both the green and the blue forms were present, and at the same time the formation of HzPc was detected by the characteristic N-H stretching vibrational frequency. The latter results illustrate the effect of heating on the interconversion of green-blue forms and the relative stability of the PrPcz complex. The green form of PrPc2 and PrPct2 can also be converted into the blue form, for instance, by reacting its toluene solution with N2H4. Oxidation of the green form of PrPct2 with quinones leads to dissociation of the complex with the formation of HzPct as determined from the electronic spectrum. Sustained oxidation produced a red product that regenerated the H2Pct by treatment with N2H4. Similar results were obtained by reacting PrPct2 with acids. Strong acids such as HN03 also dissociate the green form, producing H2Pct. It could be argued that the latter reaction could be due to an oxidation of the PrPct2 that dissociates the complex. However, weak organic acids such as CH3COOH were also able to carry out the dissociation given a longer reaction time (48 h). The stability and interconversion phenomenon between the blue, green, and red forms in the series lanthanide bisphthalocyanines will be the subject of a separate communication. Surface-Enhanced Resonant Raman Scattering. From the electronic absorption it can be seen that excitation with 647.1 nm is in resonance with the Q-band and would give rise to resonant Raman scattering (RRS). The RRS spectrum of PrPc2 and PrPct2 showed a fluorescence signal a t 698 nm that was observed in both the pellet and the unenhanced RRS of the LB multilayer. In order to measure the RRS frequencies without fluorescence interference, 3 nm of the material was evaporated onto a Au island film or one monomolecular layer was transferred onto a Au island film on glass. For one LB monolayer and the 3-nm evaporated film, the energy transfer to the metal was more effective than the fluorescence enhancement and SERRS was obtained without fluorescence background as can be seen in Figures 5 and 6. The SERRS spectrum may be regarded as the molecular spectrum produced by the average scattered emission from molecular units physisorbed onto Au particles. Therefore, the SERRS spectrum may serve as a reference to be compared with the Raman scattering of solid samples as can be seen in Figure 6, where the RRS from PrPct2 in a KBr pellet has been added. In Tables I and 11, characteristic vibrational frequencies and their interpretation in terms of internal coordinates are listed for PrPc2 and PrPct2, respectively. The RRS spectra of PrPc2 and PrPct2 are very similar as can be seen in Figures 5 and 6 (top traces), since in both cases the vibrational
1
C
500
1000
1500
Wovenumbers (cm-1)
Figure 5. SERRS of a single LB monolayer of PrPcz on a Au island film (A). SERRS after NO, gas adsorption (B). SERRS after gas desorption (C). SERRS of an evaporated film on Au
SERRS of an LB monolayer on Au
500
1000
1500
Wavenumbers (“1)
Figure 6. SERRS of 3 nm of PrPc‘z evaporated onto 4 nm of Au (upper trace),RRS from a KBr pellet of PrPcB and SERRS of a single monolayer on Au (lower trace). fundamentals of the chromophore (Pc macrocycle) are selectively enhanced. Therefore, frequencies of the alkyl substituent would not be seen, and their identification would be possible using the IR spectral data. Characteristic vibrations of the macrocycle were identified,17for instance, the macrocycle breathing at 677 and 686 cm-l and pyrrole stretching at 1329 and 1326 cm-’ and 1512 and 1514 cm-I for PrPcz and PIP&, respectively. The off-resonanceRaman spectrum of PrPctz was also obtained with a 514.5-nm laser line, where the most intense Raman band corresponded to the benzene stretching vibration a t 1608cm-l, which is a common observation in a number of Pc molecules. A close examination of the infrared and Raman data listed in Tables I and I1 showed that the mutual exclusion rule was not observed, supporting the assumption of a staggered conformation of the Pc rings. Infrared Spectra. Infrared spectra of the KBr pellet of PrPc2 follow closely the previously published data for (17)Aroca, R.; Zeng, Z. Q.;Mink, J. J. Phys. Chem. Solids 1990,51, 135.
LB Monolayers of Praseodymium Bisphthalocyanines
Langmuir, Vol. 8, No. 3, 1992 945
Table I. Characteristic Vibrational Frequencies of PrPcz (Numbers in Parentheses Are Relative Intensities)
SERRS (647.1nm)
IR (pellet)
478 (23) 574 (27) 624 (4) 677 (100)
interpretation benzene radial
625 (14) 674 (12) 729 (100) 739 (74) 759 (13) 776 (27) 809 (11) 879 (28)
740 (68) 772 (12) 813 (51)
Pc breathing C-H wag C-H wag Pc ring Pc ring
SERRS (647.1nm)
benzene ring C-H bending C-H bending C-H bending C-H bending pyrrole breathing C-H bending
1059 (25) 1103 (19) 1110 (67) 1139 (28) 1155 (13)
interpretation
1315 (76)
C-H bending C-H bending pyrrole stretch pyrrole stretch
1352 (24) 1357 (25) 1418 (6) 1442 (40) 1479 (21) 1495 (24)
isoindole stretch isoindole stretch
1329 (32) 1337 (24) 1417 (15) 1445 (11)
933 (16) 1044 (9)
IR (pellet)
1197 (14) 1216 (24) 1302 (14)
1499 (32) 1512 (24)
1523 (3) 1537 (8) 1593 (8) 1607 (12)
isoindole stretch pyrrole stretch pyrrole stretch C=N aza stretch benzene stretch benzene stretch
Table 11. Characteristic Vibrational Frequenciee of PrP& SERRS (647.1nm) 523 (25) 542 (14) 584 (13) 596 (22) 618 (20)
IR (pellet)
interpretation tert-butyl def tert-butyl def tert-butyl def benzene radial
675 (35) 685 (23)
686 (100) 717 (12) 745 (65) 756 (33) 782 (15) 822 (28)
Pc breathing tert-butyl wag Pc ring Pc ring Pc ring
752 (58) 785 (15)
950 (16) 1090 (15) 1100 (15)
(I 1000
C-H bend (t-Bu) C-H bend (t-Bu) benzene ring C-C stretch (t-Bu) C-H bend (Pc) C-H bend (t-Bu) C-H bend (Pc) C-H bend (t-Bu)
Fb.pc** 1500
Wavenumbers ("1)
Figure 7. Middle infrared transmission spectra of PrPcz and PrPctz in a KBr pellet.
LuPcp or Y ~ P CA~comparison . ~ between the PrPcz and PrPct2 spectra as shown in Figure 7 allows a clear identification of the vibrational fundamentals due to the tert-butyl moieties. However, an important difference should be pointed out. The most intense band in the IR spectrum of the unsubstituted bisphthalocyanine observed at 729 cm-l, which has been assigned to an out-of-plane vibration of the C-H bonds, was not seen in the spectrum of the substituted PrPct2. Partial substitution of C-H by a C-C bond should decrease the relative intensity of the
IR (pellet)
1135 (22)
interpretation pyrrole breathing
1200 (25) 1214 (23) 1219 (27) 1227 (20) 1284 (16)
1254 (30) 1281 (29) 1314 (100)
1326 (27)
tert-butyl def
832 (33) 853 (16) 895 (12) 915 (25) 948 (6) 1022 (11) 1046 (14) 1090 (20) 1100 (8) 1125 (15)
853 (17)
SERRS (647.1nm)
1408 (17) 1438 (13) 1491 (16)
1351 (26) 1364 (26) 1384 (14) 1393 (19) 1409 (20) 1463 (26) 1483 (30) 1489 (31) 1501 (23)
1514 (38) 1524 (5) 1611 (18)
C-H bend (t-Bu) C-H bend (Pc) C-H bend (t-Bu) C-H bend (t-Bu) C-H bend (t-Bu) C-H bend (Pc) pyrrole stretch C-H bend (t-Bu) isoindole stretch isoindole stretch isoindole stretch isoindole stretch pyrrole stretch pyrrole stretch C=N aza stretch benzene stretch
out-of-plane C-H vibration. The latter observation supports the assignment. Multilayer assemblies of PrPc2 and PrPct2 were transferred to AgCl and ZnS substrates for IR transmission measurements. For an ordered LB film, the molecular alignment favors a vibrational transition where the component of the dipole moment derivative is parallel to the incident electric field component of radiation, i.e., parallel to the surface of the substrate. The results are illustrated in Figure 8, where a region of the 1R spectra obtained for each multilayer film is presented. The molecular organization within the LB film affected the relative intensities in the IR spectrum. In the case of PrPcz the relative intensity of isoindole ring stretching vibrations (1442, 1479, and 1495 cm-') was strongly dependent on the orientation in the monolayer. Further, the out-ofplane C-H vibration at 729 cm-' was observed with a strong relative intensity in PrPcz, giving support to an edge-on rather than face-on organization. Stretching C-H vibrations of the tert-butyl groups were also sensitive to orientation in the LB film. In the spectrum of the pellet of PrPct2 (random orientation), a strong C-H stretching at 2956 cm-l and two weak bands at 2926 and 2860 cm-' were observed. However, the symmetric stretching at 2926 cm-I was the most intense C-H stretch in the LB film, giving indirect support to the two-dimensional alignment of the transferred film. Similar intensity changes were seen in the spectral region of the bending vibrations of the tert-butyl groups (1000-1400 cm-l). For instance, in the
Souto et al.
946 Langmuir, Vol. 8,No. 3, 1992
V
1000
I
PrPci 1500
Wavenumbers (“1)
Figure 8. Infrared transmission spectra of LB multilayers of PrPcz and PrPc‘z on ZnS substrate.
PrPctz spectrum of the pellet, the 1255- and 1281-cm-1 C-H bending frequencies of the tert-butyl group were observed with equal intensity, but in the LB film the relative intensity of the 1254-cm-’ band was considerably weaker than the 1281-cm-1band. Finally, the 948-cm-l band of the benzene ring was observed with an enhanced relative intensity in the LB film, supporting the assumption that the edge-on organization was maintained during the transfer of the floating monolayer to the solid substrate. Adsorption of NO, Gas in LB Monolayers. It is well known that the Pc complexes interact strongly with electron acceptors such as NOx.lJ8 The adsorption process and the nature of the interaction macrocycle complexelectron acceptor molecule are important issues in determining the potential applications of Pc complexes for selective binding of small molecules. Recently,lgwe have shown, using SERRS, that the adsorption of NO, on a single LB monolayer of mono- and bisphthalocyanine is reversible. In order to extend our understanding of the adsorption process, well-characterized LB films of PrPcz and PrPctz were exposed to NO, gas and the interaction was monitored by the absorption in the visible region and SERRS on Au island films. The electronic absorption spectrum of the exposed films of PrPcp and PrPctz clearly showed a red-shifted Q-band for both molecules, a shift that is commonly associated with the oxidation of the PCZ complex (formation of the “red form”), and in correspondence with electrochemical observation.” LB multilay-
ers of PrPcz and PrPctz exposed to NO, gas (atotal pressure of 400 Pa) showed a red-shifted electronic absorption at 720 nm and an intense charge-transfer band at 510 nm. Evacuation of the adsorbed gas at 10 Pa (or self-desorption over a period of hours) restored the electronic adsorption spectrum of the original multilayer. The high sensitivity of SERRS allows one to monitor the adsorption of NO, gas by a single LB monomolecular layer on Au island films. The results obtained with the 647.1-nm line for PrPc2 are shown in Figure 5B. A dramatic change in the intensity of the vibrational frequencies with a large contribution of bridged C=N bonds (aza groups) was seen in agreement with the reported observations for YbP~2.l~ The interpretation of the latter results is given in terms of a resonant Raman effect with an increasingcontribution from the n-a* resonance. Since the n-a* transition is to be found in the high-energy side of the Q-band, and the laser line used (647.1 nm) is in that spectral region, it may be concluded that the NO2 interaction affects mainly the a-a* electronic transition.20 The chemisorption of NO2 could then be explained in terms of a direct interaction of the NO2 with the metal with a transfer of electron density from the inner macrocycle to the electron acceptor ligand. LB multilayers with an IR spectra shown in Figure 8 were also treated with NO, gas and their infrared transmission spectra recorded after exposure to gas and after gas desorption. Minor changes in relative intensities were observed in the IR spectrum due to gas chemisorption. However, the information encoded in the IR data is not as straightforwardly related to the nature of the adduct as the SERRS or electronic absorption. In both cases the relative intensity of the 1314-cm-’ band increases, which could be due to an overlapping with the NO2 stretching vibration that may be seen in the same region.21 Conclusions Langmuir-Blodgett monolayers of PrPcz and PrPctz materials were prepared and transferred to glass and AgCl and ZnS substrates. Transmission IR spectra of LB multilayer assemblies were obtained. The relative intensity pattern in the IR spectra indicated that the transferred monolayer preserved a large degree of molecular organization with respect to the surface of the substrate. A reversible adsorption of NO, gas on a single monolayer and multilayer assemblies of PrPcz and PrPct2 was established by SERRS and visible spectra. From the electronic spectra it was inferred that the NO2, acting as electron acceptor, forms a complex with both PrPcz and PrPctz that causes the oxidation of the Pc rings.
Acknowledgment. Financial assistance from NSERC of Canada and NATO scientific affair division (Grant CGR 900582) is gratefully acknowledged. (20)Henriksson, A.;Roos, B.; Sundbom, M. Theor. Chim. Acta 1972, 27, 303.
(18)Honeybourne, C.L.;Even, R.J. J. Chem. Phys. Solids 1983,44, 833.
(19)Battisti, D.;A r m , R.J. Am. Chem. SOC.1992, 114, 1201.
(21)Herzberg, G.,I11 Electronic Spectra and Electronic Structure of
Polyatomic Molecules; D. Van Nostrand Co., Inc.: Princeton, 1986; p
602.
