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Polarized neutron diffraction study of the spin density distribution in an indolinonic nitroxide radical
Physica B 180 & 181 (1992) North-Holland
PHYSICA M
76-78
Polarized neutron diffraction study of the spin density distribution in an indolinonic nitroxide radical R. Caciuffo”,
0.
Francescangeli’,
L. Greci”,
S. Melone”,
“Dipartimento di Scienze dei Materiali e della Terra. Universitci di Ancona. hLaboratoire LPon Brillouin, CENS, Saclay, France ‘Dipartimento di Fisica, Universitic di Parma, Parma, Italy
B. Gillonh
and G. Amoretti”
Via Brecce Bianche.
l-601.31 Ancona.
Italy
The spin density distribution in the indolinonic nitroxide radical 1,2-dihydro-2-methyl-2-phenyl-3H-indole-3-oxo-l-oxyl has been determined by polarized neutron diffraction measurements. An analytical description of the spin density obtained from the experimental data using the multipole expansion method. The results show that the unpaired electron not confined to the N-O group but is delocalised over the indolinic moiety.
1. Introduction Polarized neutron diffraction (PND) experiments have been successfully used in the past few years to study the spin density of 2p unpaired electrons in organic molecules [l-.5]. In contrast to nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) methods, which provide local information on the electron spin density at the atom position, the PND method gives the distribution of the spin density over the whole molecule and, therefore, provides a very stringent test for semi-empirical or ab initio theoretical calculations [ 1,6]. In this paper, we present the results of a PND study of the indolinonic nitroxide radical 1,2-dihydro-2methyl-2-phenyl-3H-indole-3-oxo-l-oxyl. This is a stable compound with the nitroxide function involved in conjugative interactions through the adjacent fused benzene ring [7,8]. The N-O bond length of 1.262 A corresponds to an electronic configuration with the unpaired electron on the m antibonding orbital formed from p, atomic wave functions. 2. Experimental
with the crystallographic c-axis parallel to a 5 T magnetic field oriented along the w axis of the diffractometer. All measurements were performed with a neutron wavelength A = 0.858 A and a polarization P = 0.98. A typical measuring time was of 1 h per flipping ratio. A Er filter was used to eliminate A/2 contamination in the incident beam. The experiment consists of measuring, for different Bragg reflections, the flipping ratio R, i.e. the ratio between the peak intensities with incident neutrons polarized parallel or antiparallel to the sample magnetization. The measured values of R lead directly to the ratio y between the magnetic and the nuclear structure factors F,(Q) and F,(Q), from which FM can be calculated if F, is known. Neutron measurements were performed at T= 9.8 K. A total of 63 independent R values were obtained from the average of at least three equivalent reflections with Miller indices of the type (h k 0), (h k 1) and (h k 2). No corrections for extinction have been applied. The nuclear structure factors have been
details and results
The studied indolinonic nitroxide radical (fig. 1) is obtained by the method described in the literature [7]. Large single crystals of good quality are easily obtained from ethanol solution by slow evaporation. The system crystallizes in the monoclinic space group P2, / c with room temperature lattice parameters a = 10.159(2) A, b = 12.751(2) A, c = 10.394(2) w and p = 116.8” [9]. Full crystallographic details may be found elsewhere [9]. Polarized neutron diffraction measurements have been performed on a single crystal of about 30 mm’ in volume using the 5-Cl diffractometer of the Laboratoire L&on Brillouin, Centre d’Etudes NuclCaires de Saclay, France. The sample was mounted 0921-4526/92/$05.00
is is
0
1992 - Elsevier
Science
Publishers
Fig. 1. The molecular structure of the studied indolinonic nitroxide radical showing the crystallographic (arbitrary) numbering. The reference frame used in the multipolar expansion of the spin density is also shown. The x-axis is nearly parallel to the C( 1)-C(8) direction.
B.V. All rights
reserved
R. Caciuffo
et al.
I Spin density
distribution
calculated using the room temperature X-ray diffraction crystallographic data [9]. The thermal parameters have been scaled to 9.8 K using the Debye model. 3. Spin density model The expansion of the spin density in a molecule in terms of a series of nucleus centred multipoles [lo] has been successfully used in the past few years to study the chemical bonding in free radicals and other complex molecular systems [l]. The model consists of a superposition of aspherical atomic spin density distributions, each atomic density being represented as a series expansion in real spherical harmonics Y,,,,(B, 4) (?zj”+‘)
p,(r)
=
2 ,L::: ,=,I
,
+2)!
?7=,
yn:” e_GJl’ x P;;'Y;;'(e, $J) m=m/ (1)
P,,,, is the weight of the Y,,,,(& 4) component and the radial functions R,(r) are of Slater type. In practice, only a few terms in the sum over 1 need to be included, depending on the type of orbital occupied by the electrons. The magnetic structure factors may be obtained as a Fourier transform of eq. (1) and the set of parameters (i, P,,), for each atom, may then be refined to obtain the best fit of the experimental data. In particular, the population coefficient of the spherical term gives the value of the magnetic moment carried by the atom. In the present case, due to the limited set of experimental data, it is not possible to model the shape of the spin density on all the atoms of the molecule. We then analyzed the data assuming a spherical spin distribution for the C and O(2) atoms and forcing the spin density on the N and O(1) atoms to have the shape of a 2p orbital. Moreover, we assumed for the exponents of the Slater radial functions the values given in ref. 6. The least-squares refinement was then performed on the population of the monopole terms and on the orientation of the 2p orbitals with respect to the reference frame shown in fig. 1 (the x-axis lies on the principal plane of the radical and is nearly parallel to the C( 1)-C(8) bond, the y-axis is along the N-O direction and the z-axis lies on the n-plane of the radical). The goodness of the fit is x2 = 1.4. The magnetic moments obtained for the different sites are reported in table 1. Figure 2 shows the projection of the model spin density on the principal plane of the radical (z = 0 plane) while the projection on the r-plane (x = 0 plane) in proximity of the nitroxide function is shown in the inset. The results show that only 58% of the total spin density is located around the N-O bond, 24% is associated with the C-O group and the remaining part is distributed over the indolinic moiety. In contrast to the results obtained on the tanol and tanol suberate
in an indolinonic
nitroxide
radical
77
Table 1 Magnetic moments on the different molecular sites as determined by a multipolar expansion analysis of polarized neutron diffraction data. The corresponding normalized spin populations (column a) arc compared with (b) ENDOR spectroscopy results [14] and (c) INDO calculations [8]. The numbers in parentheses are the errors on the last significant digit.
0.047(6) 0.068(6) 0.034(l) -0.004(6) 0.015(S) 0.007(S) -0.010(2) 0.026(5) -0.010(2) 0.019(8) 0.002(S) 0.005(7)
0.24(3) 0.34(2) 0.17(4) -0.02(3) 0.075(40) 0.035(40) -0.05( 1) 0.13(3) -0.05( 1) O.lO(4) O.Ol(4) 0.025(35)
0.111
-0.045 0.14 -0.045 0.13
-0.039 0.047 -0.036 0.055
radicals [2,3], the spin localised on the nitroxide function is not equally shared between the N and 0 radical centres, 59% of it being on the oxygen. The 2p orbitals on the N and O(1) atoms are almost perpendicular to the N-O bond, as expected. The results obtained by PND are compared in table 1 with those previously determined by magnetic resonance methods or intermediate neglect of differential overlap (INDO) calculations [8]. The second column of table 1 reports the PND monopole population after normalization to 1~~ per molecule. This quantity is directly comparable to the spin population determined from the hyperfine coupling constant by using the
4.56
’ -4.40
3.16
Fig. 2. Projection of the principal plane of the molecule of the spin density map obtained by a multipolar refinement of the polarized neutron diffraction observations. Positive contour levels go from 0.0025 to 0.16&?1’. Negative contours are at -0.0025 and -O.OOS&~‘. No contour is drawn at zero. The inset shows the projection in the m-plane of the radical.
R. Caciuffo et al. I Spin density distribution in an indolinonic r&oxide
78
semi-empirical McConnell relation [l l] (the values used for the McConnell constant were QcH = ~23 G for the hydrogens bound to carbons and Q, = 48 G for the nitrogen atom [12]). The third column of table 1 reports the values deduced in such a way from the trace of the proton hyperfine tensor determined by ENDOR spectroscopy with the radical in a glassy matrix [13]. The INDO calculations given in ref. 8 are reported in the last column. The agreement between PND and magnetic resonance results is quite good. Additional information is however provided by PND concerning the partition of the spin between the N and 0 atoms of the nitroxide function and the spin population on atoms which are not visible in magnetic resonance experiments. For the spin population of the N atom, the value calculated in ref. 8 is lower than the experimental one by almost a factor 2. The calculated hyperfine coupling constants for the hydrogen nuclei are also lower than the experimental values showing that the INDO method for predicting the hyperfine coupling constants in nitroxide radicals should be used with care. References [l]
B. Gillon and J. Schweizer, in: Molecules in Physics, Chemistry and Biology, Vol. 111. ed. J. Maruani (Kluwer Academic. Dordrecht, 1989) p. 111.
PI
radical
P.J. Brown, A. Capiomont, B. Gillon and J. Schweizer, J. Magn. Magn. Mater. 14 (1979) 289. I31 J.X. Boucherle. B. Gillon and J. Schweizer, Proc. Internat. Symp. Neutron Scattering (AIP, New York, 1982). 141 J.X. Boucherle. B. Gillon. J. Maruani and J. Schweizer. Acta Crystallogr. C 43 (1987) 1769. ISI J.X. Boucherle, B. Gillon, J. Maruani and J. Schweizcr, Mol. Phys. 60 (1987) 1121. Paris VI (1983) [61 B. Gillon. These d’Etat, Universite unpublished. L. Greci and L. Marchetti. [71 C. Berti, M. Colonna, Tetrahedron 31 (1975) 174.5. G.D. PI R. Benassi, F. Taddei. L. Greci, L. Marchetti, Andreetti, G. Bocelli and P Sgarabotto, J. Chem. Sot.. Perkin Trans. 2 (1980) 786. M.F. Ottaviani, C. Rizzoli. [91 A. Maniero, M. Brustolon. P. Sgarabotto, F. Ugozzoli. P. Carloni and L. Greci. Mol. Phys. 73 (1991) I. A 34 [lOI N.K. Hansen and P. Coppens. Acta Crystallogr. (1978) 909. [Ill H.M. McConnell and J. Stradhdee, Mol. Phys. 2 (1959) 129. and P.T. Narashiman, 1121 D.N. Nanda. J. Subramanian Theor. Chim. Acta 22 (1971) 369. [I31 M. Brustolon, A.L. Maniero, U. Segre and L. Greci, J. Chem. Sot., Faraday Trans. 1 83 (1987) 69.
PHYSICA M
76-78
Polarized neutron diffraction study of the spin density distribution in an indolinonic nitroxide radical R. Caciuffo”,
0.
Francescangeli’,
L. Greci”,
S. Melone”,
“Dipartimento di Scienze dei Materiali e della Terra. Universitci di Ancona. hLaboratoire LPon Brillouin, CENS, Saclay, France ‘Dipartimento di Fisica, Universitic di Parma, Parma, Italy
B. Gillonh
and G. Amoretti”
Via Brecce Bianche.
l-601.31 Ancona.
Italy
The spin density distribution in the indolinonic nitroxide radical 1,2-dihydro-2-methyl-2-phenyl-3H-indole-3-oxo-l-oxyl has been determined by polarized neutron diffraction measurements. An analytical description of the spin density obtained from the experimental data using the multipole expansion method. The results show that the unpaired electron not confined to the N-O group but is delocalised over the indolinic moiety.
1. Introduction Polarized neutron diffraction (PND) experiments have been successfully used in the past few years to study the spin density of 2p unpaired electrons in organic molecules [l-.5]. In contrast to nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) methods, which provide local information on the electron spin density at the atom position, the PND method gives the distribution of the spin density over the whole molecule and, therefore, provides a very stringent test for semi-empirical or ab initio theoretical calculations [ 1,6]. In this paper, we present the results of a PND study of the indolinonic nitroxide radical 1,2-dihydro-2methyl-2-phenyl-3H-indole-3-oxo-l-oxyl. This is a stable compound with the nitroxide function involved in conjugative interactions through the adjacent fused benzene ring [7,8]. The N-O bond length of 1.262 A corresponds to an electronic configuration with the unpaired electron on the m antibonding orbital formed from p, atomic wave functions. 2. Experimental
with the crystallographic c-axis parallel to a 5 T magnetic field oriented along the w axis of the diffractometer. All measurements were performed with a neutron wavelength A = 0.858 A and a polarization P = 0.98. A typical measuring time was of 1 h per flipping ratio. A Er filter was used to eliminate A/2 contamination in the incident beam. The experiment consists of measuring, for different Bragg reflections, the flipping ratio R, i.e. the ratio between the peak intensities with incident neutrons polarized parallel or antiparallel to the sample magnetization. The measured values of R lead directly to the ratio y between the magnetic and the nuclear structure factors F,(Q) and F,(Q), from which FM can be calculated if F, is known. Neutron measurements were performed at T= 9.8 K. A total of 63 independent R values were obtained from the average of at least three equivalent reflections with Miller indices of the type (h k 0), (h k 1) and (h k 2). No corrections for extinction have been applied. The nuclear structure factors have been
details and results
The studied indolinonic nitroxide radical (fig. 1) is obtained by the method described in the literature [7]. Large single crystals of good quality are easily obtained from ethanol solution by slow evaporation. The system crystallizes in the monoclinic space group P2, / c with room temperature lattice parameters a = 10.159(2) A, b = 12.751(2) A, c = 10.394(2) w and p = 116.8” [9]. Full crystallographic details may be found elsewhere [9]. Polarized neutron diffraction measurements have been performed on a single crystal of about 30 mm’ in volume using the 5-Cl diffractometer of the Laboratoire L&on Brillouin, Centre d’Etudes NuclCaires de Saclay, France. The sample was mounted 0921-4526/92/$05.00
is is
0
1992 - Elsevier
Science
Publishers
Fig. 1. The molecular structure of the studied indolinonic nitroxide radical showing the crystallographic (arbitrary) numbering. The reference frame used in the multipolar expansion of the spin density is also shown. The x-axis is nearly parallel to the C( 1)-C(8) direction.
B.V. All rights
reserved
R. Caciuffo
et al.
I Spin density
distribution
calculated using the room temperature X-ray diffraction crystallographic data [9]. The thermal parameters have been scaled to 9.8 K using the Debye model. 3. Spin density model The expansion of the spin density in a molecule in terms of a series of nucleus centred multipoles [lo] has been successfully used in the past few years to study the chemical bonding in free radicals and other complex molecular systems [l]. The model consists of a superposition of aspherical atomic spin density distributions, each atomic density being represented as a series expansion in real spherical harmonics Y,,,,(B, 4) (?zj”+‘)
p,(r)
=
2 ,L::: ,=,I
,
+2)!
?7=,
yn:” e_GJl’ x P;;'Y;;'(e, $J) m=m/ (1)
P,,,, is the weight of the Y,,,,(& 4) component and the radial functions R,(r) are of Slater type. In practice, only a few terms in the sum over 1 need to be included, depending on the type of orbital occupied by the electrons. The magnetic structure factors may be obtained as a Fourier transform of eq. (1) and the set of parameters (i, P,,), for each atom, may then be refined to obtain the best fit of the experimental data. In particular, the population coefficient of the spherical term gives the value of the magnetic moment carried by the atom. In the present case, due to the limited set of experimental data, it is not possible to model the shape of the spin density on all the atoms of the molecule. We then analyzed the data assuming a spherical spin distribution for the C and O(2) atoms and forcing the spin density on the N and O(1) atoms to have the shape of a 2p orbital. Moreover, we assumed for the exponents of the Slater radial functions the values given in ref. 6. The least-squares refinement was then performed on the population of the monopole terms and on the orientation of the 2p orbitals with respect to the reference frame shown in fig. 1 (the x-axis lies on the principal plane of the radical and is nearly parallel to the C( 1)-C(8) bond, the y-axis is along the N-O direction and the z-axis lies on the n-plane of the radical). The goodness of the fit is x2 = 1.4. The magnetic moments obtained for the different sites are reported in table 1. Figure 2 shows the projection of the model spin density on the principal plane of the radical (z = 0 plane) while the projection on the r-plane (x = 0 plane) in proximity of the nitroxide function is shown in the inset. The results show that only 58% of the total spin density is located around the N-O bond, 24% is associated with the C-O group and the remaining part is distributed over the indolinic moiety. In contrast to the results obtained on the tanol and tanol suberate
in an indolinonic
nitroxide
radical
77
Table 1 Magnetic moments on the different molecular sites as determined by a multipolar expansion analysis of polarized neutron diffraction data. The corresponding normalized spin populations (column a) arc compared with (b) ENDOR spectroscopy results [14] and (c) INDO calculations [8]. The numbers in parentheses are the errors on the last significant digit.
0.047(6) 0.068(6) 0.034(l) -0.004(6) 0.015(S) 0.007(S) -0.010(2) 0.026(5) -0.010(2) 0.019(8) 0.002(S) 0.005(7)
0.24(3) 0.34(2) 0.17(4) -0.02(3) 0.075(40) 0.035(40) -0.05( 1) 0.13(3) -0.05( 1) O.lO(4) O.Ol(4) 0.025(35)
0.111
-0.045 0.14 -0.045 0.13
-0.039 0.047 -0.036 0.055
radicals [2,3], the spin localised on the nitroxide function is not equally shared between the N and 0 radical centres, 59% of it being on the oxygen. The 2p orbitals on the N and O(1) atoms are almost perpendicular to the N-O bond, as expected. The results obtained by PND are compared in table 1 with those previously determined by magnetic resonance methods or intermediate neglect of differential overlap (INDO) calculations [8]. The second column of table 1 reports the PND monopole population after normalization to 1~~ per molecule. This quantity is directly comparable to the spin population determined from the hyperfine coupling constant by using the
4.56
’ -4.40
3.16
Fig. 2. Projection of the principal plane of the molecule of the spin density map obtained by a multipolar refinement of the polarized neutron diffraction observations. Positive contour levels go from 0.0025 to 0.16&?1’. Negative contours are at -0.0025 and -O.OOS&~‘. No contour is drawn at zero. The inset shows the projection in the m-plane of the radical.
R. Caciuffo et al. I Spin density distribution in an indolinonic r&oxide
78
semi-empirical McConnell relation [l l] (the values used for the McConnell constant were QcH = ~23 G for the hydrogens bound to carbons and Q, = 48 G for the nitrogen atom [12]). The third column of table 1 reports the values deduced in such a way from the trace of the proton hyperfine tensor determined by ENDOR spectroscopy with the radical in a glassy matrix [13]. The INDO calculations given in ref. 8 are reported in the last column. The agreement between PND and magnetic resonance results is quite good. Additional information is however provided by PND concerning the partition of the spin between the N and 0 atoms of the nitroxide function and the spin population on atoms which are not visible in magnetic resonance experiments. For the spin population of the N atom, the value calculated in ref. 8 is lower than the experimental one by almost a factor 2. The calculated hyperfine coupling constants for the hydrogen nuclei are also lower than the experimental values showing that the INDO method for predicting the hyperfine coupling constants in nitroxide radicals should be used with care. References [l]
B. Gillon and J. Schweizer, in: Molecules in Physics, Chemistry and Biology, Vol. 111. ed. J. Maruani (Kluwer Academic. Dordrecht, 1989) p. 111.
PI
radical
P.J. Brown, A. Capiomont, B. Gillon and J. Schweizer, J. Magn. Magn. Mater. 14 (1979) 289. I31 J.X. Boucherle. B. Gillon and J. Schweizer, Proc. Internat. Symp. Neutron Scattering (AIP, New York, 1982). 141 J.X. Boucherle. B. Gillon. J. Maruani and J. Schweizer. Acta Crystallogr. C 43 (1987) 1769. ISI J.X. Boucherle, B. Gillon, J. Maruani and J. Schweizcr, Mol. Phys. 60 (1987) 1121. Paris VI (1983) [61 B. Gillon. These d’Etat, Universite unpublished. L. Greci and L. Marchetti. [71 C. Berti, M. Colonna, Tetrahedron 31 (1975) 174.5. G.D. PI R. Benassi, F. Taddei. L. Greci, L. Marchetti, Andreetti, G. Bocelli and P Sgarabotto, J. Chem. Sot.. Perkin Trans. 2 (1980) 786. M.F. Ottaviani, C. Rizzoli. [91 A. Maniero, M. Brustolon. P. Sgarabotto, F. Ugozzoli. P. Carloni and L. Greci. Mol. Phys. 73 (1991) I. A 34 [lOI N.K. Hansen and P. Coppens. Acta Crystallogr. (1978) 909. [Ill H.M. McConnell and J. Stradhdee, Mol. Phys. 2 (1959) 129. and P.T. Narashiman, 1121 D.N. Nanda. J. Subramanian Theor. Chim. Acta 22 (1971) 369. [I31 M. Brustolon, A.L. Maniero, U. Segre and L. Greci, J. Chem. Sot., Faraday Trans. 1 83 (1987) 69.