- Email: [email protected]
Formation of 3,4-dihydrofuran radical cation through intramolecular H-shift: quantum chemical calculations and low-temperature EPR study
Radiation Physics and Chemistry 67 (2003) 243–246
Formation of 3,4-dihydrofuran radical cation through intramolecular H-shift: quantum chemical calculations and low-temperature EPR study S. Naumov*, I. Janovsky! , W. Knolle, R. Mehnert Institut fur (IOM), Permoserstrasse 15, D-04318, Leipzig, Germany . Oberflachenmodifizierung .
Abstract Intramolecular transformations of dihydrofuran (DHF) radical cations radiolytically generated in Freon matrix were investigated using low temperature EPR spectroscopy. Radical cation 2,4-DHF+d has been identified earlier as a product of intramolecular rearrangement of unstable primary oxygen-centred radical cation of 2,5-DHF. 2,4-DHF+d is stable at 77 K but its further intramolecular rearrangements through 2-3 and 3-4 H-shifts can be induced by illumination with visible light. Both transformations proceed simultaneously, with more-or-less same yields of the two isomers of the DHF radical cation, namely 2,3-DHF+d and 3,4-DHF+d. The latter is a newly identified species, stable at 77–145 K, characterised by hfs splitting constants að2 HÞ ¼ 1:59 mT and að4 HÞ ¼ 2:82 mT and is the only of the four isomers of DHF radical cation showing a strong optical absorption in visible. The interpretation of the experimental results is strongly supported by DFT quantum chemical calculations. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Dihydrofuran; Radical cations; Photobleaching; Intramolecular H-shift; EPR-spectra; DFT calculations
1. Introduction Recent theoretical and experimental results indicate that conventional radical cations produced by ionisation of their neutral precursors are often less stable than their non-conventional isomers, which do not have a neutral parent molecule. Such isomers may be spontaneously formed by a single intramolecular hydrogen transfer, resulting in a distonic form, characterised by spatially separated charge and radical site. The technique of low-temperature ESR spectroscopy in Freon matrix is well established and has been used successfully to investigate a large variety of radical cations, generated radiolytically through positive charge transfer from matrix to solute. In our previous study (Knolle et al., 1999), dealing with radical cations of 2,3- and 2,5dihydrofuran (DHF), was observed that the primary radical cation 2,5-DHF+d is not stable even at 77 K and
undergoes irreversible transformation via intramolecular 5-4 H-shift within the molecular ring to the double bond; the product of this rearrangement, the 2,4-DHF+d radical cation, is quite stable up to temperatures around 145 K. Though according to the calculations the radical cation 2,3-DHF+d is by about 15 kcal/mol more stable than 2,4-DHF+d, there was no indication of the formation of 2,3-DHF+d. The aim of the present study was to investigate the rearrangements of DHF radical cations and stability of 2,4-DHF+d further, namely by attempting to surpass energy barriers of H-shift by photobleaching irradiated samples with visible light. Quantum chemical calculations were used to support the interpretation of the experimental results and to assist in identification of transient species, by comparison of the experimental and computed hfs splitting constants.
2. Experimental *Corresponding author. Tel.: +49-341-235-2046; fax: +49341-235-2584. E-mail address: [email protected] (S. Naumov).
The EPR experiments were performed using a Bruker 300E spectrometer (9.5 GHz, 100 kHz modulation)
0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00045-8
244
S. Naumov et al. / Radiation Physics and Chemistry 67 (2003) 243–246
equipped with either a finger dewar (77 K) or a variable temperature control unit ER 4121 VT (above 95 K). Spectra were recorded at a microwave power of 0.1 mW and modulation amplitude of 0.05 or 0.1 mT. 2,5-DHF (Aldrich, 97%) and CF3CCl3 (Acros, 99%) were purified by passing them through a column filled with neutral alumina. Further details on sample preparation and irradiation with the 10-MeV electron beam of a Linac are described elsewhere (Knolle et al., 1999). A tungsten lamp (250 W) equipped with a water heat-filter and colour glass filters was used for photobleaching of irradiated samples. Spectra simulations were performed using the WinSim software (Duling, 1994).
accepted as appropriate to describe the transformations of DHF radical cations in Freon matrix.
4. Results and discussion In agreement with our previous results (Knolle et al., 1999), the EPR spectrum recorded with irradiated solution of 2,5-DHF in CF3CCl3 at 77 K shortly after irradiation (Fig. 1(a)) corresponds to two species. The primary oxygen-centred radical cation 2,5-DHF+d, characterised by four strongly coupling protons (að4 HÞ ¼ 8:5 mT) is clearly indicated by the most outer spectrum lines. This primary species transforms via intramolecular 5-4 H-shift (first-order process with a
3. Computation method (a)
To provide an accurate calculation of isotropic hfs splitting constants, the accurate description of the geometrical structure of molecule, good electron correlation and well-defined basis set are needed. Quantum chemical calculations were performed using Gaussian 98, Revision 11 (Frisch et al., 1998) program. Molecular and electronic structures of radical cations and neutral radicals were investigated by the Density Functional Theory (DFT) Hybrid B3LYP (Becke, 1993, 1996), because the hfs splitting constants calculated using the B3LYP method had shown very good agreement with experiment for different transients derived from DHF both in Freon matrix (Knolle et al., 1999) and in aqueous solution (Geimer et al., 2002). Standard 631G(d) and 6-311+G(2d,p) basis sets were used for geometry optimisation. The frequency calculations were also used to locate transition state geometries, to determine the nature of stationary points found by geometry optimisation, and to obtain thermochemical parameters such as Zero-Point Energy, activation energy Ea (height of H-shift barrier) and reaction enthalpy. The frequency job on all DHF radical cations studied does not produce any negative frequencies, indicating that these structures are minima on their potential surface, whereas all optimised transition structures (intramolecular H shift) produce only one negative (imaginary) frequency, indicating that these conformations are first-order saddle points (transition states). The electronic transition spectra of transients were calculated with the Unrestricted Time Dependent (UTD DFT) (Bauernschitt and Ahlrichs, 1996) B3LYP/ 6-311+G(2d,p) method. To test the possible influence of the polarized continuum (Freon, e ¼ 2:4) on molecular structure, the geometry optimisations were also made using COSMO model (Barone and Cossi, 1998). Since no essential solvent effect both on geometrical and electronic parameters were found, the calculations in vacuum were
(b)
(c)
(d)
(e)
(f) (g) 10 mT Fig. 1. EPR spectra of irradiated 103 mol % 2,5-DHF in CF3CCl3: (a) 77 K, 2.5 min after irradiation, (b) 77 K, 22 h after irradiation, (c) 77 K, after photobleaching with l > 600 nm; (d) 145 K, after photobleaching with l > 600 nm, (e) spectrum simulation with parameters given in Table 1. Stick diagrams (f) and (g) correspond to radical cations 3,4-DHF+d and 2,3DHF+d, respectively.
S. Naumov et al. / Radiation Physics and Chemistry 67 (2003) 243–246
half-life of 2.3 min at 77 K) into the 2,4-DHF+d radical cation, which dominates in the spectrum. The spectrum measured later (Fig. 1(b)) corresponds to the 2,4DHF+d only. In CF3CCl3 matrix is the 2,4-DHF+d radical cation stable at 77 K for days, and also persists at temperatures up to 145 K. Only at temperatures above the matrix softening point (145 K) the spectrum of the 2,4-DHF+d disappears and the spectrum of the dihydrofuryl radical (DHFd), resulting from the deprotonation process, is observed (Knolle et al., 1999). The schematic potential energy profile showing the transformations of 2,5-DHF+d is given in Fig. 2. As DFT calculations show, there are two possibilities of further rearrangements of 2,4-DHF+d, namely H-shifts leading either to 2,3-DHF+d or to 3,4-DHF+d, the latter having a somewhat higher activation energy. To induce possible transformations, the following experiments with samples of 103 mol% 2,5-DHF in CF3CCl3 were performed. The samples irradiated at 77 K were kept at the same temperature, until the transformation of 2,5-DHF+d into 2,4-DHF+d was completed, and only then photobleaching (at 77 K) was performed. The illumination was repeated until no further change in the EPR spectrum was observed. Two kinds of experiment were performed: (a) photobleaching with the full spectrum of the lamp, (b) photobleaching with the light with l > 600 nm. Since the results of both experiments were essentially the same, only EPR spectra recorded during experiment (b) are shown. Fig. 1(c) shows the spectrum recorded at 77 K after complete photobleaching. In order to obtain a better resolved spectrum, sample was then warmed up to 145 K (just below the matrix glass-transition point) and
245
the resulting spectrum is shown in Fig. 1(d). The latter spectrum is perfectly simulated (Fig. 1(e)) assuming a mixture of 3,4-DHF+d (41%) and 2,3-DHF+d (59%). Stick diagrams (Fig. 1(g,f)) correspond to radical cations 3,4-DHF+d and 2,3-DHF+d, respectively. Comparison of the experimental and calculated hfs splitting constants is shown in Table 1. The spectrum of the photobleached sample recorded at 77 K is somewhat less resolved, nevertheless it can be also well simulated, using the same sets of hfs splitting constants, somewhat larger line-width (0.15 and 0.23 mT were taken at 145 and 77 K, respectively) and again approximately the same amounts of each of the two species. The spin density in 3,4-DHF+d is distributed between the carbon atoms 2 and 5 (Table 1) and since the structure is planar, coupling of two equivalent a-protons (a ¼ 1:59 mT) and four equivalent b-protons (a ¼ 2:82 mT) occurs. The transformation of the EPR spectrum at 77 K during photobleaching proceeded without significant loss of the total spin concentration (not more than about 8%, integration of the whole spectrum). This is also evidence of occurrence of a quantitative 1:1 transformation of the radical cation 2,4-DHF+d. As calculated (see Table 1) the first exited state of 2,4-DHF+d lies at 725 nm (165 kJ/mol), which is higher than energy barriers for H-shifts leading to formation of both 3,4-DHF+d and 2,3-DHF+d (Fig. 2). The energy of the light of wavelength >600 nm (200 kJ/mol) is therefore sufficient to induce both 2-3 and 2-4 H-shifts. Transformation then proceeds apparently on statistical basis and moreor-less the same yields of both isomers (3,4-DHF+d and 2,3-DHF+d) can be expected, as was actually observed. For this reason also the relative yields of the two radical
Fig. 2. Transformation scheme of 2,5-DHF+d radical cation and spin density distributions of different structures of dihydrofuran radical cation as calculated at B3LYP/6-311+G(2d,p) level: Tr. St.-transition state, Ea-activation energy, DH-reaction enthalpy.
S. Naumov et al. / Radiation Physics and Chemistry 67 (2003) 243–246
246
Table 1 Calculated values (B3LYP/6-31G(d)) of the spin densities r and hfs splitting constants a(mT) for individual H atoms for different structures of DHF radical cation. l—first three exited states calculated with UTD/B3LYP/6-311+G(2d,p) (in parenthesis is oscillator strength) 2,5-DHF+d Calc.
2,4-DHF+d
2,3-DHF+d
Exp.
Calc.
a(H,C3)
0.297 0.021 0.197 0.197 0.021 7.934 7.934 0.521
8.49 8.49 0.57
0.003 0.076 1.021 0.076 0.001 3.966 3.969 2.580
3.85 3.85 2.27
a(H,C4)
0.521
0.57
a(H,C5)
7.934 7.934 671(0.032) 397(0.001) 308(0.000)
8.49 8.49 Greenish
3.805 3.805 0.37
3.82 3.82 0.30
725(0.016) 232(0.002) 223(0.000)
Yellowish
rO1 rC2 rC3 rC4 rC5 a(H,C2)
l; nm
Exp.
cations were not influenced by the wavelength of the light used (in the case of photobleaching with full light, simulation of the resulting EPR spectrum gave relative yields of 3,4-DHF+d and 2,3-DHF+d radical cations of 51% and 49%, respectively). The photobleaching was accompanied by the pronounced colour change of the sample: the almost colourless sample turned during illumination intense blue. This complies also with the results of the UTD/ B3LYP/6-311+G(2d,p) quantum-chemical calculations of the optical absorption spectra of individual DHF cations, according to which only 3,4-DHF+d has a very strong absorption band in visible at 516 nm (B 2.4 eV, which is very close to the energy of about 2.1 eV, corresponding to blue light).
Acknowledgements Authors greatly appreciate interest of Professor Ffrancon Williams (Department of Chemistry, University of Tennessee) in their earlier investigations on dihydrofuran radical cations, as well as his initiation of the present study.
Calc.
3,4-DHF+d Exp
Calc.
Exp.
0.276 0.006 0.037 0.584 0.094 0.385
0.52
0.133 0.578 0.042 0.042 0.578 1.797
1.59
1.501
1.35
4.183 4.183 1.556 1.556 308(0.000) 287(0.050) 260(0.000)
4.28 4.28 1.46 1.46 Colourless
2.853 2.853 2.853 2.853 1.897
2.82 2.82 2.82 2.82 1.59
516(0.056) 210(0.000) 203(0.001)
Blue
References Barone, V., Cossi, M., 1998. Quantum calculations of molecular energies and energy gradients in solution by a conductor solvent model. J. Phys. Chem. A 102, 1995–2001. Bauernschitt, R., Ahlrichs, R., 1996. Treatment of electronic excitations within the adiabatic approximation of dependent density functional theory. Chem. Phys. Lett. 256, 454–464. Becke, A.D., 1993. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 98, 5648–5652. Becke, A.D., 1996. Density-functional thermochemistry, IV. A new dynamical correlation functional and implications for exact-exchange mixing. J. Chem. Phys. 104, 1040–1046. Duling, D.R., 1994. Simulation of multiple spin-trap EPR spectra. J. Magn. Reson. Ser. B 104, 105–110. Frisch, M.J., et al., 1998. Gaussian 98, Revision A.11 J.A. (1998). Gaussian, Inc., Pittsburgh PA. Geimer, J., Bernhard, K., Naumov, S., Beckert, D., 2002. Fourier transform EPR study and quantum chemical calculations of dihydrofuran radicals formed by triplet sensitized electron transfer in aqueous solution. Phys. Chem. Chem. Phys. 4, 1738–1745. Knolle, W., Janovsk"y, I., Naumov, S., Mehnert, R., 1999. Lowtemperature EPR study of radical cations of 2,5- and 2,3dihydrofuran and their transformations in freon matrices. J. Chem. Soc. Perkin Trans. 2 (11), 2447–2453.
Formation of 3,4-dihydrofuran radical cation through intramolecular H-shift: quantum chemical calculations and low-temperature EPR study S. Naumov*, I. Janovsky! , W. Knolle, R. Mehnert Institut fur (IOM), Permoserstrasse 15, D-04318, Leipzig, Germany . Oberflachenmodifizierung .
Abstract Intramolecular transformations of dihydrofuran (DHF) radical cations radiolytically generated in Freon matrix were investigated using low temperature EPR spectroscopy. Radical cation 2,4-DHF+d has been identified earlier as a product of intramolecular rearrangement of unstable primary oxygen-centred radical cation of 2,5-DHF. 2,4-DHF+d is stable at 77 K but its further intramolecular rearrangements through 2-3 and 3-4 H-shifts can be induced by illumination with visible light. Both transformations proceed simultaneously, with more-or-less same yields of the two isomers of the DHF radical cation, namely 2,3-DHF+d and 3,4-DHF+d. The latter is a newly identified species, stable at 77–145 K, characterised by hfs splitting constants að2 HÞ ¼ 1:59 mT and að4 HÞ ¼ 2:82 mT and is the only of the four isomers of DHF radical cation showing a strong optical absorption in visible. The interpretation of the experimental results is strongly supported by DFT quantum chemical calculations. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Dihydrofuran; Radical cations; Photobleaching; Intramolecular H-shift; EPR-spectra; DFT calculations
1. Introduction Recent theoretical and experimental results indicate that conventional radical cations produced by ionisation of their neutral precursors are often less stable than their non-conventional isomers, which do not have a neutral parent molecule. Such isomers may be spontaneously formed by a single intramolecular hydrogen transfer, resulting in a distonic form, characterised by spatially separated charge and radical site. The technique of low-temperature ESR spectroscopy in Freon matrix is well established and has been used successfully to investigate a large variety of radical cations, generated radiolytically through positive charge transfer from matrix to solute. In our previous study (Knolle et al., 1999), dealing with radical cations of 2,3- and 2,5dihydrofuran (DHF), was observed that the primary radical cation 2,5-DHF+d is not stable even at 77 K and
undergoes irreversible transformation via intramolecular 5-4 H-shift within the molecular ring to the double bond; the product of this rearrangement, the 2,4-DHF+d radical cation, is quite stable up to temperatures around 145 K. Though according to the calculations the radical cation 2,3-DHF+d is by about 15 kcal/mol more stable than 2,4-DHF+d, there was no indication of the formation of 2,3-DHF+d. The aim of the present study was to investigate the rearrangements of DHF radical cations and stability of 2,4-DHF+d further, namely by attempting to surpass energy barriers of H-shift by photobleaching irradiated samples with visible light. Quantum chemical calculations were used to support the interpretation of the experimental results and to assist in identification of transient species, by comparison of the experimental and computed hfs splitting constants.
2. Experimental *Corresponding author. Tel.: +49-341-235-2046; fax: +49341-235-2584. E-mail address: [email protected] (S. Naumov).
The EPR experiments were performed using a Bruker 300E spectrometer (9.5 GHz, 100 kHz modulation)
0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00045-8
244
S. Naumov et al. / Radiation Physics and Chemistry 67 (2003) 243–246
equipped with either a finger dewar (77 K) or a variable temperature control unit ER 4121 VT (above 95 K). Spectra were recorded at a microwave power of 0.1 mW and modulation amplitude of 0.05 or 0.1 mT. 2,5-DHF (Aldrich, 97%) and CF3CCl3 (Acros, 99%) were purified by passing them through a column filled with neutral alumina. Further details on sample preparation and irradiation with the 10-MeV electron beam of a Linac are described elsewhere (Knolle et al., 1999). A tungsten lamp (250 W) equipped with a water heat-filter and colour glass filters was used for photobleaching of irradiated samples. Spectra simulations were performed using the WinSim software (Duling, 1994).
accepted as appropriate to describe the transformations of DHF radical cations in Freon matrix.
4. Results and discussion In agreement with our previous results (Knolle et al., 1999), the EPR spectrum recorded with irradiated solution of 2,5-DHF in CF3CCl3 at 77 K shortly after irradiation (Fig. 1(a)) corresponds to two species. The primary oxygen-centred radical cation 2,5-DHF+d, characterised by four strongly coupling protons (að4 HÞ ¼ 8:5 mT) is clearly indicated by the most outer spectrum lines. This primary species transforms via intramolecular 5-4 H-shift (first-order process with a
3. Computation method (a)
To provide an accurate calculation of isotropic hfs splitting constants, the accurate description of the geometrical structure of molecule, good electron correlation and well-defined basis set are needed. Quantum chemical calculations were performed using Gaussian 98, Revision 11 (Frisch et al., 1998) program. Molecular and electronic structures of radical cations and neutral radicals were investigated by the Density Functional Theory (DFT) Hybrid B3LYP (Becke, 1993, 1996), because the hfs splitting constants calculated using the B3LYP method had shown very good agreement with experiment for different transients derived from DHF both in Freon matrix (Knolle et al., 1999) and in aqueous solution (Geimer et al., 2002). Standard 631G(d) and 6-311+G(2d,p) basis sets were used for geometry optimisation. The frequency calculations were also used to locate transition state geometries, to determine the nature of stationary points found by geometry optimisation, and to obtain thermochemical parameters such as Zero-Point Energy, activation energy Ea (height of H-shift barrier) and reaction enthalpy. The frequency job on all DHF radical cations studied does not produce any negative frequencies, indicating that these structures are minima on their potential surface, whereas all optimised transition structures (intramolecular H shift) produce only one negative (imaginary) frequency, indicating that these conformations are first-order saddle points (transition states). The electronic transition spectra of transients were calculated with the Unrestricted Time Dependent (UTD DFT) (Bauernschitt and Ahlrichs, 1996) B3LYP/ 6-311+G(2d,p) method. To test the possible influence of the polarized continuum (Freon, e ¼ 2:4) on molecular structure, the geometry optimisations were also made using COSMO model (Barone and Cossi, 1998). Since no essential solvent effect both on geometrical and electronic parameters were found, the calculations in vacuum were
(b)
(c)
(d)
(e)
(f) (g) 10 mT Fig. 1. EPR spectra of irradiated 103 mol % 2,5-DHF in CF3CCl3: (a) 77 K, 2.5 min after irradiation, (b) 77 K, 22 h after irradiation, (c) 77 K, after photobleaching with l > 600 nm; (d) 145 K, after photobleaching with l > 600 nm, (e) spectrum simulation with parameters given in Table 1. Stick diagrams (f) and (g) correspond to radical cations 3,4-DHF+d and 2,3DHF+d, respectively.
S. Naumov et al. / Radiation Physics and Chemistry 67 (2003) 243–246
half-life of 2.3 min at 77 K) into the 2,4-DHF+d radical cation, which dominates in the spectrum. The spectrum measured later (Fig. 1(b)) corresponds to the 2,4DHF+d only. In CF3CCl3 matrix is the 2,4-DHF+d radical cation stable at 77 K for days, and also persists at temperatures up to 145 K. Only at temperatures above the matrix softening point (145 K) the spectrum of the 2,4-DHF+d disappears and the spectrum of the dihydrofuryl radical (DHFd), resulting from the deprotonation process, is observed (Knolle et al., 1999). The schematic potential energy profile showing the transformations of 2,5-DHF+d is given in Fig. 2. As DFT calculations show, there are two possibilities of further rearrangements of 2,4-DHF+d, namely H-shifts leading either to 2,3-DHF+d or to 3,4-DHF+d, the latter having a somewhat higher activation energy. To induce possible transformations, the following experiments with samples of 103 mol% 2,5-DHF in CF3CCl3 were performed. The samples irradiated at 77 K were kept at the same temperature, until the transformation of 2,5-DHF+d into 2,4-DHF+d was completed, and only then photobleaching (at 77 K) was performed. The illumination was repeated until no further change in the EPR spectrum was observed. Two kinds of experiment were performed: (a) photobleaching with the full spectrum of the lamp, (b) photobleaching with the light with l > 600 nm. Since the results of both experiments were essentially the same, only EPR spectra recorded during experiment (b) are shown. Fig. 1(c) shows the spectrum recorded at 77 K after complete photobleaching. In order to obtain a better resolved spectrum, sample was then warmed up to 145 K (just below the matrix glass-transition point) and
245
the resulting spectrum is shown in Fig. 1(d). The latter spectrum is perfectly simulated (Fig. 1(e)) assuming a mixture of 3,4-DHF+d (41%) and 2,3-DHF+d (59%). Stick diagrams (Fig. 1(g,f)) correspond to radical cations 3,4-DHF+d and 2,3-DHF+d, respectively. Comparison of the experimental and calculated hfs splitting constants is shown in Table 1. The spectrum of the photobleached sample recorded at 77 K is somewhat less resolved, nevertheless it can be also well simulated, using the same sets of hfs splitting constants, somewhat larger line-width (0.15 and 0.23 mT were taken at 145 and 77 K, respectively) and again approximately the same amounts of each of the two species. The spin density in 3,4-DHF+d is distributed between the carbon atoms 2 and 5 (Table 1) and since the structure is planar, coupling of two equivalent a-protons (a ¼ 1:59 mT) and four equivalent b-protons (a ¼ 2:82 mT) occurs. The transformation of the EPR spectrum at 77 K during photobleaching proceeded without significant loss of the total spin concentration (not more than about 8%, integration of the whole spectrum). This is also evidence of occurrence of a quantitative 1:1 transformation of the radical cation 2,4-DHF+d. As calculated (see Table 1) the first exited state of 2,4-DHF+d lies at 725 nm (165 kJ/mol), which is higher than energy barriers for H-shifts leading to formation of both 3,4-DHF+d and 2,3-DHF+d (Fig. 2). The energy of the light of wavelength >600 nm (200 kJ/mol) is therefore sufficient to induce both 2-3 and 2-4 H-shifts. Transformation then proceeds apparently on statistical basis and moreor-less the same yields of both isomers (3,4-DHF+d and 2,3-DHF+d) can be expected, as was actually observed. For this reason also the relative yields of the two radical
Fig. 2. Transformation scheme of 2,5-DHF+d radical cation and spin density distributions of different structures of dihydrofuran radical cation as calculated at B3LYP/6-311+G(2d,p) level: Tr. St.-transition state, Ea-activation energy, DH-reaction enthalpy.
S. Naumov et al. / Radiation Physics and Chemistry 67 (2003) 243–246
246
Table 1 Calculated values (B3LYP/6-31G(d)) of the spin densities r and hfs splitting constants a(mT) for individual H atoms for different structures of DHF radical cation. l—first three exited states calculated with UTD/B3LYP/6-311+G(2d,p) (in parenthesis is oscillator strength) 2,5-DHF+d Calc.
2,4-DHF+d
2,3-DHF+d
Exp.
Calc.
a(H,C3)
0.297 0.021 0.197 0.197 0.021 7.934 7.934 0.521
8.49 8.49 0.57
0.003 0.076 1.021 0.076 0.001 3.966 3.969 2.580
3.85 3.85 2.27
a(H,C4)
0.521
0.57
a(H,C5)
7.934 7.934 671(0.032) 397(0.001) 308(0.000)
8.49 8.49 Greenish
3.805 3.805 0.37
3.82 3.82 0.30
725(0.016) 232(0.002) 223(0.000)
Yellowish
rO1 rC2 rC3 rC4 rC5 a(H,C2)
l; nm
Exp.
cations were not influenced by the wavelength of the light used (in the case of photobleaching with full light, simulation of the resulting EPR spectrum gave relative yields of 3,4-DHF+d and 2,3-DHF+d radical cations of 51% and 49%, respectively). The photobleaching was accompanied by the pronounced colour change of the sample: the almost colourless sample turned during illumination intense blue. This complies also with the results of the UTD/ B3LYP/6-311+G(2d,p) quantum-chemical calculations of the optical absorption spectra of individual DHF cations, according to which only 3,4-DHF+d has a very strong absorption band in visible at 516 nm (B 2.4 eV, which is very close to the energy of about 2.1 eV, corresponding to blue light).
Acknowledgements Authors greatly appreciate interest of Professor Ffrancon Williams (Department of Chemistry, University of Tennessee) in their earlier investigations on dihydrofuran radical cations, as well as his initiation of the present study.
Calc.
3,4-DHF+d Exp
Calc.
Exp.
0.276 0.006 0.037 0.584 0.094 0.385
0.52
0.133 0.578 0.042 0.042 0.578 1.797
1.59
1.501
1.35
4.183 4.183 1.556 1.556 308(0.000) 287(0.050) 260(0.000)
4.28 4.28 1.46 1.46 Colourless
2.853 2.853 2.853 2.853 1.897
2.82 2.82 2.82 2.82 1.59
516(0.056) 210(0.000) 203(0.001)
Blue
References Barone, V., Cossi, M., 1998. Quantum calculations of molecular energies and energy gradients in solution by a conductor solvent model. J. Phys. Chem. A 102, 1995–2001. Bauernschitt, R., Ahlrichs, R., 1996. Treatment of electronic excitations within the adiabatic approximation of dependent density functional theory. Chem. Phys. Lett. 256, 454–464. Becke, A.D., 1993. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 98, 5648–5652. Becke, A.D., 1996. Density-functional thermochemistry, IV. A new dynamical correlation functional and implications for exact-exchange mixing. J. Chem. Phys. 104, 1040–1046. Duling, D.R., 1994. Simulation of multiple spin-trap EPR spectra. J. Magn. Reson. Ser. B 104, 105–110. Frisch, M.J., et al., 1998. Gaussian 98, Revision A.11 J.A. (1998). Gaussian, Inc., Pittsburgh PA. Geimer, J., Bernhard, K., Naumov, S., Beckert, D., 2002. Fourier transform EPR study and quantum chemical calculations of dihydrofuran radicals formed by triplet sensitized electron transfer in aqueous solution. Phys. Chem. Chem. Phys. 4, 1738–1745. Knolle, W., Janovsk"y, I., Naumov, S., Mehnert, R., 1999. Lowtemperature EPR study of radical cations of 2,5- and 2,3dihydrofuran and their transformations in freon matrices. J. Chem. Soc. Perkin Trans. 2 (11), 2447–2453.