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Resonance Raman and quantum chemical studies of short polyene radical cations
Journal of
MOLECUIAR STRUCTURE ELSEVIER
Journal of Molecular
Structure 410-411
(1997) 339-342
Resonance Raman and quantum chemical studies of short polyene radical cations Tam& Keszthelyi”, Robert Wilbrandt”‘*, Thomas Ballyb aDepartment of Environmental Science and Technology, Ris# National Laboratory MIL - 313, DK - 4000 Roskilde, Denmark blnstitut de Chimie Physique, UniversitP de Fribourg, PCrolles, CH - 1700 Fribourg, Switzerland Received 26 August 1996; accepted 6 September
1996
Abstract The results of our investigations of the geometric and vibrational structures of some short conjugated polyene radical cations are reported. The radical cations of 1,3 - butadiene and three of its deuterated isotopomers, rruns - and cis - I,3 - pentadiene, 2 - methyl - I,3 - butadiene, and E - and Z - 1,3,5 - hexatriene have been studied. The radical cations were generated radiolytically in a glassy Freon matrix and investigated by optical absorption and resonance Raman spectroscopy. Ab initio and density functional molecular-orbital calculations have been carried out to predict equilibrium structures and to assist assignment of the resonance Raman spectra. A new and improved scaled quantum mechanical force field for the butadiene radical cation was also determined. The presence of more than one rotamer was observed in all the polyene radical cations we investigated. 0 1997 Elsevier Science B.V. Keywords:
Molecular orbital calculations;
Polyenes;
Radical cations; Resonance
1. Introduction The neutral precursors of the radical cations investigated in this work belong to the class of molecules called linear conjugated polyenes. Conjugated polyenes are hydrocarbons containing alternating single and double carbon - carbon bonds. The limiting case of a polyene with an infinite number of conjugated double bonds is provided by polyacetylene, which is the prototype conducting polymer. The interest in the properties of polyene radical cations is in part due to their proposed role in the conductivity of polyacetylene [ 11. Molecular systems with conjugated double bonds * Corresponding
author.
0022-2860/97/$17.00
0 1997 Elsevier
PII SOO22-2860(96)09632-9
Raman spectroscopy
also play an important role in biology and biochemistry, and some investigations point towards the possible involvement of their radical cations in biological redox reactions. It has been shown that light-excitation of chloroplasts can lead to the formation of carotenoid radical cations at the photosynthetic reaction centre PSI1 [2,3]. The active role of carotenoid radical cations in artificial photosynthetic systems that would convert solar energy into chemical energy has also been demonstrated [4,5]. Here we report our investigations of the equilibrium geometries and vibrational structures of the radical cations of some short polyenes. The radical cations under study were those of 1,3 - butadiene, tmns - 1,3 - pentadiene (truns - piperylene), cis I,3 - pentadiene (cis - piperylene), 2 - methyl - 1,3 -
Science B.V. All rights reserved.
340
T. Keszthelyi et al./Journal of Molecular Structure 410-411
butadiene (isoprene), E - 1,3,5 - hexatriene, and Z 1,3,5 - hexatriene. The experimental method used in these studies was resonance Raman spectroscopy. Theoretical methods have also been employed; ab initio and density functional theory molecular-orbital calculations have been performed in order to determine the equilibrium geometries and harmonic vibrational frequencies of the ground and some excited states of these radical cations. The radical cations investigated in our work are the smallest members of the family of linear conjugated polyene radical cations and can therefore serve as useful models of biologically or technically important longer polyene radical cations. Nevertheless, these radical cations are important not only as prototypes of potentially functional compounds, but are very fundamental compounds which deserve to be studied in their own right.
300
a)
(1997) 339-342 400
500
600
nm
J\
b)
2. Results The butadiene radical cation, being the simplest and most fundamental of those we investigated, was subjected to the most detailed treatment both experimentally and theoretically [6,7]. Four isotopomers with CZh symmetry were studied, namely butadiene - do, butadiene - 2,3 - dz, butadiene - 1,1,4,4 - dq, and butadiene - de. We measured their Raman spectra in resonance with the first excited electronic state (Fig. la and b), and assigned them with the help of our own density functional theory calculations and an IR-based scaled quantum mechanical force field which was available from the literature [8]. The spectra could not be assigned by taking into account only one rotamer of the butadiene radical cation. We propose therefore that apart from the most stable s - tram rotamer, a higher energy rotamer was also present in our sample. Our experimental and theoretical results suggested that this second rotamer has a planar s - cis structure. Using our combined data together with those from the previous IR study we determined a new and improved scaled quantum mechanical force field for the butadiene radical cation in its s - tram rotameric form (Fig. lc). The set of optimized scaling factors was applied to the force field of the s - cis rotamer calculated by density functional theory (Fig. Id). The frequencies obtained from the
1600
14bo
ii00
’ 1000
cm-l
Fig. 1. Electronic absorption (a) and resonance Raman (b) spectra of the butadiene - do radical cation in Freon glass at 77 K, together with the predicted positions of the vq-v8 fundamentals of the two planar rotamers from the scaled UB3-LYP/6-3 lG* force field (c and d). The assignments of the observed resonance Raman bands are indicated.
new SQM force field were in excellent agreement with experimental data, and confirmed our earlier assignment of several observed resonance Raman bands to the planar s - cis rotamer of the butadiene radical cation. For the s - tram rotamer we also carried out a theoretical estimation of the relative intensities of the resonance Raman bands. While the theoretical treatment yielded good agreement with the observed absorption spectrum, the calculated resonance Raman intensities were less satisfactory. In particular, all calculations strongly underestimated the intensity of the strongest observed Raman band in the 1600 cm-’ region. The reason for this discrepancy is not yet clear. Electronic absorption and resonance Raman spectra
T. Keszthelyi et al/Journal of Molecular Structure 410-411
of the cis - and tram - 1,3 - pentadiene and isoprene radical cations were also recorded, and the resonance Raman spectra assigned on the basis of harmonic frequencies from density functional theory calculations [9,10]. The absorption and the resonance Raman spectra are different for the radical cations derived from the two neutral 1,3 - pentadiene isomers. However, a one - way photoinduced cis - tram isomerization was observed during the resonance Raman experiments. Furthermore, in accordance with our results for the butadiene radical cation, the presence of more than one rotamer was observed in all three monomethyl-substituted butadiene radical cations. The radical cations of E - and 2 - 1,3,5 - hexatriene were subjected to a similar treatment with the exception that the theoretical vibrational frequencies were obtained from ab initio molecular orbital calculations [ Ill. Identical optical absorption and resonance Raman spectra were observed for the radical cations derived from the two neutral isomers. Of the possible six planar rotamers of the hexatriene radical cation two were identified in the resonance Raman spectrum. More recently we have calculated the force fields of the hexatriene rotamers by density functional theory, and applied the scaling factors we had obtained for the butadiene radical cation to these force constants. Comparison of the scaled density functional theory frequencies with the experimental ones suggests new assignments for some bands and the presence of additional rotamers in minor concentrations.
3. Discussion The analysis and interpretation of our data have led to the important conclusion that under the cryogenic conditions of our experiments, all the radical cations we investigated exist in more than one rotameric form. There are, however, several differences between the individual cases covered by this general statement. The neutral precursor of the butadiene radical cation possesses only one stable geometrical isomer, since by rotation about the double bonds identical atoms are interchanged. Ionization leads to an increase in the height of the barrier to rotation about the nominally single central C-C bond. As a result of
(1997) 339-342
341
the increased barrier height, there can most probably be no thermal rotamerization of the butadiene radical cation under cryogenic conditions. We suggest that the s - cis rotamer is possibly formed in the process of y - radiolysis, since upon positive charge transfer from the Freon solvent enough excess energy may be imparted onto the butadiene radical cation to enable it to surmount the barrier. The ratio in which the s tram and s - cis rotamers are formed is preserved, and is apparently not influenced by laser irradiation during the Raman measurements. This seems to imply that there is also a barrier to rotamerization in the first excited state of the radical cation. We envision a similar situation in the isoprene radical cation, which also has only one stable neutral geometrical isomer. On the other hand, a different situation arises in the 1,3 - pentadiene radical cations. The two stable and separable geometrical isomers, cis and trans, of the neutral species yield markedly different electronic absorption and resonance Raman spectra upon ionization. Hence, under cryogenic conditions the radical cations of cis - and trans - pentadiene do not equilibrate, and their interconversion is not induced during y - radiolysis either. However, upon photoexcitation of the radical cation a cis apparently takes trans one - way isomerization place. This indicates that the potential energy barrier with respect to torsion of the C3C4 bond in the ground state of the radical cation is relatively high, while it is lowered upon excitation to the resonant electronic state. Furthermore, in this excited state the tram form apparently has the lowest energy. As far as the stable rotamers, differing in their arrangements about the nominally single C2C3 bond, are concerned, the situation in the 1,3 - pentadiene radical cations is probably not different from that in the butadiene and isoprene radical cations. Also in this case we observe a mixture of two rotamers, their relative amounts unchanged during the resonance Raman measurements. Just as for 1,3 - pentadiene, there exist two stable isomers of neutral 1,3,5 - hexatriene. In contrast with the 1,3 - pentadiene isomers, E - and Z - 1,3,5 - hexatriene yield identical electronic absorption and resonance Raman spectra upon ionization. This resonance Raman spectrum shows the presence of two rotamers of the hexatriene radical cation, ttt and tct, corresponding to the two isomers of neutral
342
T. Keszthelyi et al/Journal of Molecular Struciure 410-411
hexatriene. From the observed relative band intensities these are assumed to be present in our sample in very similar amounts. We think it is likely that equilibration of the rotamers occurs in the process of radiolytic generation of the radical cation, the equilibrium mixture being dominated by the trt and ret rotamers. This difference in behaviour upon ionization in the Freon matrix does not necessarily imply that the barrier to rotamerization is lower in the hexatriene radical cation than in the pentadiene radical cation. The ionization potential of hexatriene is -0.3 eV lower than of 1,3 - pentadiene, thus positive charge transfer from the Freon leaves the hexatriene radical cation with approximately 0.3 eV more excess energy than I,3 pentadiene. The resonance Raman spectrum of the hexatriene radical cation was recorded with excitation in resonance with the second excited state. No change in the ratio of the ftt and feet rotamers was observed during the resonance Raman measurements, but since the system starts from a nearly 1:l ratio of the two rotamers, we cannot draw any real conclusions concerning the height of the barrier to interconversion of the rotamers in this state.
4. Conclusions According to our knowledge, resonance Raman spectroscopy has been used in combination with the radiolytic generation of radical cations in polyatomic matrices on few occasions [12-141. Owing to the sensitivity and selectivity provided by the resonance effect, resonance Raman spectroscopy appears to be very well suited to the investigation of radical cations, which usually absorb at longer wavelengths than the corresponding neutral species, With our investigations we have further demonstrated the power and general applicability of this technique. For the s - tran~ rotamer of the butadiene radical cation we obtained a scaled quantum mechanical force field which is more complete and balanced than its previous version [8]. The final set of optimized scaling factors is expected to be transferable to radical cations of longer polyenes, thus assisting the studies of their vibrational structures. We also demonstrated that the optimized scaling factors of
(1997) 339-342
the s - trans rotamer were transferable to the s - cis rotamer, a result with possible applications in future conformational studies. With regard to the performance of the density functional theory calculations in predicting the vibrational frequencies of the radical cations, we note that agreement with the experimental values is completely satisfactory even without any scaling of the force field. Moreover, the UB3 - LYP/6 - 31G* calculated force field of the butadiene radical cation provided a more suitable basis for the scaling procedure than the computationally more demanding MP2 force field. Our studies thus contribute new examples to the observation that density functional theory calculations offer a cost - effective yet remarkably accurate means of obtaining first estimates of harmonic frequencies of organic molecules and radical ions.
References [l] T. Bally, K. Roth, W. Tang, R.R. Schrock, K. Knoll and L.Y. Park, J. Am. Chem. Sot., 114 (1992) 2440. [2] CC. Schenck, B. Diner, P. Mathis and K. Satoh, Biochim. Biophys. Acta, 680 (1982) 216. [3] P. Mathis and A.W. Rutherford, Biochim. Biophys. Acta, 767 (1984) 217. [4] T.A. Moore, D. Gust, P. Mathis, J. - C. Mialocq, C. Chachaty, R.V. Bensasson, E.J. Land, D. Doizi, P.A. Liddell, W.R. Lehman, G.A. Nemeth and A.L. Moore, Nature, 307 (1984) 630. [5] D. Gust, T.A. Moore, P.A. Liddell, G.A. Nemeth, L.R. Makings, A.L. Moore, D. Barrett, P.J. Pessiki, R.V. Bensasson, M. Rougte, C. Chachaty, F.C. de Schryver, M. van der Auweraer, A.R. Holzwarth and J.S. Connolly, J. Am. Chem. Sot., 109 (1987) 846. [6] T. Keszthelyi, R. Wilbrandt and T. Bally, J. Phys. Chem., submitted for publication. [7] T. Keszthelyi, R. Wilbrandt, T. Baby and J.-L. Roulin, J. Phys. Chem., submitted for publication. ]8] W. Tang, X. - L. Zhang and T. Bally, J. Phys. Chem., 97 ( 1993) 4373. 191 T. Keszthelyi and R. Wilbrandt, J. Mol. Struct., 379 (1996) 211. [lo] T. Keszthelyi and R. Wilbrandt, J. Phys. Chem., in press. [I 11 T. Keszthelyi, R. Wilbrandt, R.J. Cave and J.L. Johnson, J. Phys. Chem., 98 (1994) 5632. [ 121 S.J. Sheng and G. Hug, Chem. Phys. Lett., 57 (1978) 168. [ 131 T. Kato, N. Muraki and T. Shida, Chem. Phys. Lett., 164 (1989) 388. 1141 H. Kawashima, T. Kato and T. Shida, Chem. Phys. Lett., 165 ( 1990) 59.
MOLECUIAR STRUCTURE ELSEVIER
Journal of Molecular
Structure 410-411
(1997) 339-342
Resonance Raman and quantum chemical studies of short polyene radical cations Tam& Keszthelyi”, Robert Wilbrandt”‘*, Thomas Ballyb aDepartment of Environmental Science and Technology, Ris# National Laboratory MIL - 313, DK - 4000 Roskilde, Denmark blnstitut de Chimie Physique, UniversitP de Fribourg, PCrolles, CH - 1700 Fribourg, Switzerland Received 26 August 1996; accepted 6 September
1996
Abstract The results of our investigations of the geometric and vibrational structures of some short conjugated polyene radical cations are reported. The radical cations of 1,3 - butadiene and three of its deuterated isotopomers, rruns - and cis - I,3 - pentadiene, 2 - methyl - I,3 - butadiene, and E - and Z - 1,3,5 - hexatriene have been studied. The radical cations were generated radiolytically in a glassy Freon matrix and investigated by optical absorption and resonance Raman spectroscopy. Ab initio and density functional molecular-orbital calculations have been carried out to predict equilibrium structures and to assist assignment of the resonance Raman spectra. A new and improved scaled quantum mechanical force field for the butadiene radical cation was also determined. The presence of more than one rotamer was observed in all the polyene radical cations we investigated. 0 1997 Elsevier Science B.V. Keywords:
Molecular orbital calculations;
Polyenes;
Radical cations; Resonance
1. Introduction The neutral precursors of the radical cations investigated in this work belong to the class of molecules called linear conjugated polyenes. Conjugated polyenes are hydrocarbons containing alternating single and double carbon - carbon bonds. The limiting case of a polyene with an infinite number of conjugated double bonds is provided by polyacetylene, which is the prototype conducting polymer. The interest in the properties of polyene radical cations is in part due to their proposed role in the conductivity of polyacetylene [ 11. Molecular systems with conjugated double bonds * Corresponding
author.
0022-2860/97/$17.00
0 1997 Elsevier
PII SOO22-2860(96)09632-9
Raman spectroscopy
also play an important role in biology and biochemistry, and some investigations point towards the possible involvement of their radical cations in biological redox reactions. It has been shown that light-excitation of chloroplasts can lead to the formation of carotenoid radical cations at the photosynthetic reaction centre PSI1 [2,3]. The active role of carotenoid radical cations in artificial photosynthetic systems that would convert solar energy into chemical energy has also been demonstrated [4,5]. Here we report our investigations of the equilibrium geometries and vibrational structures of the radical cations of some short polyenes. The radical cations under study were those of 1,3 - butadiene, tmns - 1,3 - pentadiene (truns - piperylene), cis I,3 - pentadiene (cis - piperylene), 2 - methyl - 1,3 -
Science B.V. All rights reserved.
340
T. Keszthelyi et al./Journal of Molecular Structure 410-411
butadiene (isoprene), E - 1,3,5 - hexatriene, and Z 1,3,5 - hexatriene. The experimental method used in these studies was resonance Raman spectroscopy. Theoretical methods have also been employed; ab initio and density functional theory molecular-orbital calculations have been performed in order to determine the equilibrium geometries and harmonic vibrational frequencies of the ground and some excited states of these radical cations. The radical cations investigated in our work are the smallest members of the family of linear conjugated polyene radical cations and can therefore serve as useful models of biologically or technically important longer polyene radical cations. Nevertheless, these radical cations are important not only as prototypes of potentially functional compounds, but are very fundamental compounds which deserve to be studied in their own right.
300
a)
(1997) 339-342 400
500
600
nm
J\
b)
2. Results The butadiene radical cation, being the simplest and most fundamental of those we investigated, was subjected to the most detailed treatment both experimentally and theoretically [6,7]. Four isotopomers with CZh symmetry were studied, namely butadiene - do, butadiene - 2,3 - dz, butadiene - 1,1,4,4 - dq, and butadiene - de. We measured their Raman spectra in resonance with the first excited electronic state (Fig. la and b), and assigned them with the help of our own density functional theory calculations and an IR-based scaled quantum mechanical force field which was available from the literature [8]. The spectra could not be assigned by taking into account only one rotamer of the butadiene radical cation. We propose therefore that apart from the most stable s - tram rotamer, a higher energy rotamer was also present in our sample. Our experimental and theoretical results suggested that this second rotamer has a planar s - cis structure. Using our combined data together with those from the previous IR study we determined a new and improved scaled quantum mechanical force field for the butadiene radical cation in its s - tram rotameric form (Fig. lc). The set of optimized scaling factors was applied to the force field of the s - cis rotamer calculated by density functional theory (Fig. Id). The frequencies obtained from the
1600
14bo
ii00
’ 1000
cm-l
Fig. 1. Electronic absorption (a) and resonance Raman (b) spectra of the butadiene - do radical cation in Freon glass at 77 K, together with the predicted positions of the vq-v8 fundamentals of the two planar rotamers from the scaled UB3-LYP/6-3 lG* force field (c and d). The assignments of the observed resonance Raman bands are indicated.
new SQM force field were in excellent agreement with experimental data, and confirmed our earlier assignment of several observed resonance Raman bands to the planar s - cis rotamer of the butadiene radical cation. For the s - tram rotamer we also carried out a theoretical estimation of the relative intensities of the resonance Raman bands. While the theoretical treatment yielded good agreement with the observed absorption spectrum, the calculated resonance Raman intensities were less satisfactory. In particular, all calculations strongly underestimated the intensity of the strongest observed Raman band in the 1600 cm-’ region. The reason for this discrepancy is not yet clear. Electronic absorption and resonance Raman spectra
T. Keszthelyi et al/Journal of Molecular Structure 410-411
of the cis - and tram - 1,3 - pentadiene and isoprene radical cations were also recorded, and the resonance Raman spectra assigned on the basis of harmonic frequencies from density functional theory calculations [9,10]. The absorption and the resonance Raman spectra are different for the radical cations derived from the two neutral 1,3 - pentadiene isomers. However, a one - way photoinduced cis - tram isomerization was observed during the resonance Raman experiments. Furthermore, in accordance with our results for the butadiene radical cation, the presence of more than one rotamer was observed in all three monomethyl-substituted butadiene radical cations. The radical cations of E - and 2 - 1,3,5 - hexatriene were subjected to a similar treatment with the exception that the theoretical vibrational frequencies were obtained from ab initio molecular orbital calculations [ Ill. Identical optical absorption and resonance Raman spectra were observed for the radical cations derived from the two neutral isomers. Of the possible six planar rotamers of the hexatriene radical cation two were identified in the resonance Raman spectrum. More recently we have calculated the force fields of the hexatriene rotamers by density functional theory, and applied the scaling factors we had obtained for the butadiene radical cation to these force constants. Comparison of the scaled density functional theory frequencies with the experimental ones suggests new assignments for some bands and the presence of additional rotamers in minor concentrations.
3. Discussion The analysis and interpretation of our data have led to the important conclusion that under the cryogenic conditions of our experiments, all the radical cations we investigated exist in more than one rotameric form. There are, however, several differences between the individual cases covered by this general statement. The neutral precursor of the butadiene radical cation possesses only one stable geometrical isomer, since by rotation about the double bonds identical atoms are interchanged. Ionization leads to an increase in the height of the barrier to rotation about the nominally single central C-C bond. As a result of
(1997) 339-342
341
the increased barrier height, there can most probably be no thermal rotamerization of the butadiene radical cation under cryogenic conditions. We suggest that the s - cis rotamer is possibly formed in the process of y - radiolysis, since upon positive charge transfer from the Freon solvent enough excess energy may be imparted onto the butadiene radical cation to enable it to surmount the barrier. The ratio in which the s tram and s - cis rotamers are formed is preserved, and is apparently not influenced by laser irradiation during the Raman measurements. This seems to imply that there is also a barrier to rotamerization in the first excited state of the radical cation. We envision a similar situation in the isoprene radical cation, which also has only one stable neutral geometrical isomer. On the other hand, a different situation arises in the 1,3 - pentadiene radical cations. The two stable and separable geometrical isomers, cis and trans, of the neutral species yield markedly different electronic absorption and resonance Raman spectra upon ionization. Hence, under cryogenic conditions the radical cations of cis - and trans - pentadiene do not equilibrate, and their interconversion is not induced during y - radiolysis either. However, upon photoexcitation of the radical cation a cis apparently takes trans one - way isomerization place. This indicates that the potential energy barrier with respect to torsion of the C3C4 bond in the ground state of the radical cation is relatively high, while it is lowered upon excitation to the resonant electronic state. Furthermore, in this excited state the tram form apparently has the lowest energy. As far as the stable rotamers, differing in their arrangements about the nominally single C2C3 bond, are concerned, the situation in the 1,3 - pentadiene radical cations is probably not different from that in the butadiene and isoprene radical cations. Also in this case we observe a mixture of two rotamers, their relative amounts unchanged during the resonance Raman measurements. Just as for 1,3 - pentadiene, there exist two stable isomers of neutral 1,3,5 - hexatriene. In contrast with the 1,3 - pentadiene isomers, E - and Z - 1,3,5 - hexatriene yield identical electronic absorption and resonance Raman spectra upon ionization. This resonance Raman spectrum shows the presence of two rotamers of the hexatriene radical cation, ttt and tct, corresponding to the two isomers of neutral
342
T. Keszthelyi et al/Journal of Molecular Struciure 410-411
hexatriene. From the observed relative band intensities these are assumed to be present in our sample in very similar amounts. We think it is likely that equilibration of the rotamers occurs in the process of radiolytic generation of the radical cation, the equilibrium mixture being dominated by the trt and ret rotamers. This difference in behaviour upon ionization in the Freon matrix does not necessarily imply that the barrier to rotamerization is lower in the hexatriene radical cation than in the pentadiene radical cation. The ionization potential of hexatriene is -0.3 eV lower than of 1,3 - pentadiene, thus positive charge transfer from the Freon leaves the hexatriene radical cation with approximately 0.3 eV more excess energy than I,3 pentadiene. The resonance Raman spectrum of the hexatriene radical cation was recorded with excitation in resonance with the second excited state. No change in the ratio of the ftt and feet rotamers was observed during the resonance Raman measurements, but since the system starts from a nearly 1:l ratio of the two rotamers, we cannot draw any real conclusions concerning the height of the barrier to interconversion of the rotamers in this state.
4. Conclusions According to our knowledge, resonance Raman spectroscopy has been used in combination with the radiolytic generation of radical cations in polyatomic matrices on few occasions [12-141. Owing to the sensitivity and selectivity provided by the resonance effect, resonance Raman spectroscopy appears to be very well suited to the investigation of radical cations, which usually absorb at longer wavelengths than the corresponding neutral species, With our investigations we have further demonstrated the power and general applicability of this technique. For the s - tran~ rotamer of the butadiene radical cation we obtained a scaled quantum mechanical force field which is more complete and balanced than its previous version [8]. The final set of optimized scaling factors is expected to be transferable to radical cations of longer polyenes, thus assisting the studies of their vibrational structures. We also demonstrated that the optimized scaling factors of
(1997) 339-342
the s - trans rotamer were transferable to the s - cis rotamer, a result with possible applications in future conformational studies. With regard to the performance of the density functional theory calculations in predicting the vibrational frequencies of the radical cations, we note that agreement with the experimental values is completely satisfactory even without any scaling of the force field. Moreover, the UB3 - LYP/6 - 31G* calculated force field of the butadiene radical cation provided a more suitable basis for the scaling procedure than the computationally more demanding MP2 force field. Our studies thus contribute new examples to the observation that density functional theory calculations offer a cost - effective yet remarkably accurate means of obtaining first estimates of harmonic frequencies of organic molecules and radical ions.
References [l] T. Bally, K. Roth, W. Tang, R.R. Schrock, K. Knoll and L.Y. Park, J. Am. Chem. Sot., 114 (1992) 2440. [2] CC. Schenck, B. Diner, P. Mathis and K. Satoh, Biochim. Biophys. Acta, 680 (1982) 216. [3] P. Mathis and A.W. Rutherford, Biochim. Biophys. Acta, 767 (1984) 217. [4] T.A. Moore, D. Gust, P. Mathis, J. - C. Mialocq, C. Chachaty, R.V. Bensasson, E.J. Land, D. Doizi, P.A. Liddell, W.R. Lehman, G.A. Nemeth and A.L. Moore, Nature, 307 (1984) 630. [5] D. Gust, T.A. Moore, P.A. Liddell, G.A. Nemeth, L.R. Makings, A.L. Moore, D. Barrett, P.J. Pessiki, R.V. Bensasson, M. Rougte, C. Chachaty, F.C. de Schryver, M. van der Auweraer, A.R. Holzwarth and J.S. Connolly, J. Am. Chem. Sot., 109 (1987) 846. [6] T. Keszthelyi, R. Wilbrandt and T. Bally, J. Phys. Chem., submitted for publication. [7] T. Keszthelyi, R. Wilbrandt, T. Baby and J.-L. Roulin, J. Phys. Chem., submitted for publication. ]8] W. Tang, X. - L. Zhang and T. Bally, J. Phys. Chem., 97 ( 1993) 4373. 191 T. Keszthelyi and R. Wilbrandt, J. Mol. Struct., 379 (1996) 211. [lo] T. Keszthelyi and R. Wilbrandt, J. Phys. Chem., in press. [I 11 T. Keszthelyi, R. Wilbrandt, R.J. Cave and J.L. Johnson, J. Phys. Chem., 98 (1994) 5632. [ 121 S.J. Sheng and G. Hug, Chem. Phys. Lett., 57 (1978) 168. [ 131 T. Kato, N. Muraki and T. Shida, Chem. Phys. Lett., 164 (1989) 388. 1141 H. Kawashima, T. Kato and T. Shida, Chem. Phys. Lett., 165 ( 1990) 59.