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Environmental effects on phenoxyl free radical spin densities and hyperfine couplings
Journal of Molecular Structure: THEOCHEM 730 (2005) 251–254 www.elsevier.com/locate/theochem
Environmental effects on phenoxyl free radical spin densities and hyperfine couplings Pete Wu, Patrick J. O’Malley* School of Chemistry, University of Manchester, North Campus, Sackville St., Manchester, M60 1QD, UK Received 6 June 2005; revised 10 June 2005; accepted 13 June 2005 Available online 8 August 2005
Abstract The effect of the environment, as modelled by hydrogen bonding, ion-pairing and/or general continuum model effects, is investigated for the phenoxyl free radical. All are shown to lead to a redistribution of spin density from the phenoxyl O atom to the Cipso position. Isotropic and anisotropic hyperfine couplings are calculated at the B3LYP level of theory confirming this trend. Introduction of the continuum model has a significant effect the Na-O bond length of the ion-pair model significantly altering its calculated EPR properties in comparison with gasphase values. The trends identified are of immediate significance for biological environmental effects on tyrosyl free radicals. q 2005 Elsevier B.V. All rights reserved.
1. Introduction The phenol/phenoxyl couple, PhOH/PhO†, plays crucial roles in a variety of chemical and biologically related electron transfer reactions [1–4]. This ranges from electron transfer in photosynthesis and respiration to antioxidant activity. Great effort has been expended to characterise both the phenol and phenoxyl moiety via spectroscopic methods. The most important of these is Electron Paramagnetic Resonance (EPR) which has been widely used to characterize environmental influences such as hydrogen bonding and to monitor phenolic electron and hydrogen transfer reactions essential to the catalytic activity of many enzymes [2,3]. It is now becoming apparent that the environment plays a crucial role in moderating the properties of biological free radicals. This is often manifested in altered redox potentials and EPR spectroscopic properties. Assessing environmental influences on radical properties using experimental probes is hampered by the inability to vary the external environment systematically, making individual contributions difficult to quantify. The phenoxyl free radical, in particular, can be generated under only certain specific conditions in vitro and, in its biological environment, changes to its surrounding are often * Corresponding author. E-mail address: [email protected] (P.J. O’Malley).
0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2005.06.034
difficult if not impossible to perform. Density functional based electronic structure methods have enabled accurate prediction of hyperfine couplings for larger sized free radicals, principally in the gas phase [5–10]. Using environment modelling it is also possible to account for specific environmental influences in altering the free radical properties of interest [11,12]. Here we investigate the role of the environment on the phenoxyl free radical properties. Explicit ion-pairing and hydrogen bonded models, continuum models and combinations thereof are examined. The fundamental electronic reasons for spin density and resultant hyperfine coupling changes in the phenoxyl free radical as a function of its environment are elucidated.
2. Methods All calculations were performed using the Gaussian 03 and Spartan programs [13,14]. Geometry optimisations were performed at the B3LYP/6-31CG(d,p) level of theory and hyperfine couplings were calculated using B3LYP/EPR-II// B3LYP/6-31CG(d,p). The models used are given in Fig. 2. Continuum dielectric effects were treated within the polarisable continuum model (PCM) of Tomasi et al. [15] specifically using the conductor like PCM model [16], CPCM, as implemented in Gaussian 03. Calculations in water were performed using standard UAHF radii for the construction of the cavity.
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H
O
O
H
H
[Na] H
O
O
H
H
H
H
H
H
H H
H
H
(a)
(b)
Fig. 1. Models used for (a) P–2H2O and (b) P–Na. Table 1 Principal bond lengths, angstroms.
CO CC CC CC O–-H O–Na
P
P_W
P–2H2O
P2H2O_W
P–Na
P–Na_W
1.260 1.454 1.380 1.412 – –
1.266 1.454 1.379 1.414 – –
1.270 1.452 1.378 1.414 1.916 –
1.271 1.451 1.378 1.412 1.883 –
1.272 1.450 1.376 1.415 – 2.105
1.267 1.452 1.378 1.416 – 2.296
The models used are shown in Fig. 1. Inclusion of the water continuum model is denoted by _W in the tables and text. 3. Results and discussion 3.1. Geometries
Fig. 3. Spin density difference plot for {P–Na}–{P}. Contours values (a) C0.002 e/au and (b) K0.002 e/au.
bond length, which is lengthened on ion-pairing, hydrogen bonding and/or inclusion of the water solvation. This corresponds to the expected polarisation of the CO bond by the dielectric medium or by direct hydrogen bonding/ion pairing. All lead to an increase in negative charge at the O reflecting increased electronegativity. The most dramatic effect on geometry is exhibited by the P–Na complex where
The principal bond lengths are presented in Table 1. In the internal phenoxyl the major change occurs in the CO
Fig. 2. Electron spin density contour at 0.002 e/au for the P model. Blue indicates positive spin density and green represents negative values.
Fig. 4. Spin density difference plot for {P–Na}–{P}. Contours values (a) C0.0002 e/au and (b) K0.0002 e/au.
P. Wu, P.J. O’Malley / Journal of Molecular Structure: THEOCHEM 730 (2005) 251–254
253
Table 2 Mulliken spin populations. RO denotes Restricted Open Shell Kohn-Sham. U denotes Unrestricted Kohn-Sham P
O Cpara Cmeta Cortho Cipso
P–2H2O
P_W
P2H2O_W
P–Na
P–Na_W
RO
U
RO
U
RO
U
RO
U
RO
U
RO
U
0.33 0.24 0.01 0.18 0.06
0.42 0.45 K0.20 0.35 K0.11
0.29 0.25 0.01 0.17 0.09
0.37 0.46 K0.19 0.32 K0.05
0.30 0.25 0.01 0.17 0.09
0.37 0.46 K0.21 0.34 K0.04
0.30 0.25 0.01 0.17 0.09
0.34 0.47 K0.18 0.30 K0.01
0.24 0.28 0.02 0.15 0.13
0.29 0.48 K0.18 0.27 0.06
0.28 0.26 0.01 0.16 0.10
0.34 0.47 K0.18 0.30 K0.01
solvation in H2O leads to an increase in the O–Na bond length of 0.191 angstrom due to dielectric shielding. 3.2. Spin density and spin population The spin density distribution for the phenoxyl free radical, P, is demonstrated by the contour plot in Fig. 2. In the phenoxyl free radical positive spin density is located primarily at the O, Cortho and Cpara positions around the phenoxyl ring. Negative spin density is located at the Cipso and Cmeta positions, see Fig. 2. The effect of ion complex formation on the spin density distribution is demonstrated by the spin density difference plot {P}–{P–Na} in Fig. 3. Similar plots can be obtained for the explicit hydrogen bonded complex and continuum models. In Fig. 4, the more concentrated spin density difference plot reveals electron transfer from O to Cipso on ion complex formation while the more diffuse plot illustrates additional spin density increase at Cpara and Cmeta and decrease at Cortho,on ion formation. The spin density distribution arises from direct unpaired spin delocalisation effects of the odd electron plus secondary spin polarisation effects. It is possible to separate these different contributions by comparing unrestricted Kohn-Sham and restricted open- shell KohnSham calculations. In the restricted open-shell calculation, only the primary delocalisation effect is operative whereas this and secondary spin polarisation effects on the spin density distribution are accounted for in the unrestricted calculations. For restricted and unrestricted calculations the spin populations for each atom are given in Table 2. The restricted open shell calculations provide a map of the SOMO electron density allowing the influence of the environment, without the effect of spin polarisation, to be assessed. From Table 2, the primary effect of environmental interactions with the O atom is a redistribution of spin from O to Cipso. More moderate increases in spin population at Cpara and decrease at Cortho are also found particularly for the P–Na complex. Ion complex formation, hydrogen bonding and/or solvation therefore leads to a primary redistribution of spin density from O to Cipso. This in turn leads to a secondary redistribution at the ortho, meta and para positions caused by spin polarisation, see Fig. 2. The phenoxyl SOMO orbital can be qualitatively interpreted as overlap between the DOMO of the phenyl fragment and the out of plane pz orbital of the O atom [9]. The relevant energies of these fragments determines
their participation in the SOMO. Previous estimates for the free phenoxyl free radical shows that the SOMO is predominantly due to the phenyl DOMO with a 12% contribution from the O orbital [9]. Hydrogen bonding and ion pairing will be expected to lower the O orbital energy decreasing its contribution to the SOMO. This lower contribution to the SOMO is the underlying reason for the decreased spin density observed on hydrogen bonding/ ion-pairing. This spin density is primarily transferred to the Cipso position on the ring. 3.3. Anisotropic and isotropic hyperfine couplings The anisotropic and isotropic hyperfine couplings, hfcs, are shown in Tables 3 and 4, respectively. The anisotropic hfcs give a direct measure of the spin density distribution as probed by the magnetic nuclei and can be taken as a quantitative measure of the spin density distribution shown in Fig. 2. Their absolute magnitude determines the size of the contour plot around a particular nucleus and for the p electron systems used here their axial nature reflects the cylindrical distribution of spin around each nucleus. Table 3 Anisotropic hyperfine couplings, principal values, in MHz
O
Cpara
Cmeta
Cortho
Cipso
Hmeta
Hortho
P
P_W
P– 2H2O
P2H2O_W
P–Na
P–Na_W
K125 62 63 56 K29 K27 K18 8 10 43 K22 K21 K12 3 9 4 K3 K1 12 K10 K2
K113 57 57 58 K30 K28 K17 7 9 41 K21 K20 5 K1 K4 4 K3 K1 12 K9 K3
K113 56 57 57 K30 K28 K17 8 9 41 K21 K20 3 K2 K1 4 K3 K1 12 K9 K3
K106 53 53 59 K30 K28 K15 7 8 39 K20 K19 K4 1 3 4 K3 K1 12 K9 K3
K92 46 46 61 K32 K30 K14 6 8 36 K19 K17 11 K7 K4 4 K3 K1 12 K9 K3
K106 53 53 59 K30 K28 K16 7 9 39 K20 K19 K3 1 2 4 K3 K1 12 K9 K3
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Table 4 Isotropic hyperfine couplings for the phenoxyl models in MHz. Experimental values from Ref. [17]
O Cpara Cmeta Cortho Cipso Hmeta Hortho
P
P_W
PK 2H2O
P2H2O_W
P–Na
P– Na_W
EXPT
K26 35 K27 23 K38 8 K20
K26 35 K27 23 K38 8 K20
K27 34 K26 21 K31 7 K20
K26 34 K25 18 K27 7 K19
K24 35 K24 16 K21 6 K18
K25 34 K25 19 K28 7 K19
– – – – – 7 K20
As expected for all systems the concentration of spin density at O, Cortho and Cpara leads to the largest magnitude values at these positions. For Cipso and Cmeta the values reflect the negative spin density at these positions for P. The changes that occur for the interaction complex models reflect the changes in spin density distribution described above. Ionpairing and/or hydrogen bonding lead to a reduction in the O values, an increase in the Cipso value, a decrease in the Cortho values and an increase in the Cmeta values reflecting the decreased spin density at the oxygen position and the increased spin density at Cipso. The isotropic values, Table 4 do not show the more dramatic changes of the anisotropic data. Here the spin density being monitored is that directly at the nuclear position (Fermi Contact) and arises via an indirect spin polarisation mechanism. The O values are practically unchanged for each model. Here the loss in p spin at the O is compensated by the increased spin at Cipso. The most notable changes occurs for the Cipso position which increases from K38 for P to K21 for P–Na. Absence of significant p spin at Cipso means that the spin density at the nuclear position arises from spin density at the O and two Cortho atoms. Hence a large negative isotropic hyperfine coupling is calculated. On ion complex formation or hydrogen bonding or solvation we have shown that spin density is transferred to the Cipso position. This positive spin density transfer lowers the net amount of negative spin at the Cipso nucleus leading to the reduced magnitude of the isotropic hfc for this position for the solvated or interacting P. The 1H values are practically unchanged for each model showing the insensitivity of these couplings to environmental influences. Unfortunately these are the most commonly reported in experimental studies of these systems.
4. Conclusions Environmental influences on the spin density and hyperfine coupling of the phenoxyl free radical are investigated using explicit, continuum models and combinations thereof. Environmental influences are mainly characterised by spin density transfer from the O atom to the Cipso position. Such changes are attributed mainly to an increased electronegativity for the O atom leading to decreased overlap of the O pz orbital with the phenyl ring DOMO.
References [1] G.W. Burton, K.U. Ingold, Acc. Chem. Res. 19 (1986) 194. [2] G.W. Burton, Y.L. Page, E.J. Gabe, K.U. Ingold, J. Am. Chem. Soc. 102 (1980) 7791. [3] B.P. Roberts, J. Chem. Soc. Perkin Trans. 2 (1996) 2719. [4] A.A. Zavitsas, C. Chatgilialoglu, J. Am. Chem. Soc. 117 (1995) 10645. [5] P.J. O’Malley, S.J. Collins, Chem. Phys. Lett. 259 (1996) 296. [6] P.J. O’Malley, Chem. Phys. Lett. 262 (1996) 797. [7] P.J. O’Malley, J. Phys. Chem. A. 101 (1997) 6334. [8] Y. Qin, R.A. Wheeler, J. Chem. Phys. 102 (1995) 1689. [9] C. Adamo, R. Subra, A. Di Matteo, V. Barone, J. Chem. Phys. 109 (1998) 10244. [10] V. Barone, M. Cossi, J. Phys. Chem. A. 102 (1998) 1995. [11] I. Ciofini, R. Reviakine, A. Arbuznikov, M. Kaupp, Theor. Chem. Acc. 111 (2004) 132. [12] R. Improta, V. Barone, Chem. Rev. (2003). [13] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G. E. Scuseria, M. A. Robb, J.R. Cheesman, V.G. Zakrzewski, J.A.J. Montgomery, R.E. Stratmann, J.C.Burant,S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin,M.C. Strain, O. Farkas, V. Barone, J. Tommass, M. Cossi, R. Cammi, B. Mennuci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachani, J.B. Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanaykara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B.G. Johson, W. Chen, M.W. Wong, J.L. Andres, M. Head-Gordon, E.S. Replogle, J.A. Pople, In Gaussian98, revision A.7; Gaussian Inc.: Pittsburgh, PA. 1998. [14] Spartan Ver. 4.0 Wavefunction Inc. [15] J. Tomasi, M. Persico, Chem. Rev. 94 (1995) 2027. [16] A. Klamt, G. Schuurman, J. Chem. Perkin Trans. 2 (1993) 799. [17] P. Neta, R.W. Fessenden, J. Phys. Chem. 78 (1974) 523.
Environmental effects on phenoxyl free radical spin densities and hyperfine couplings Pete Wu, Patrick J. O’Malley* School of Chemistry, University of Manchester, North Campus, Sackville St., Manchester, M60 1QD, UK Received 6 June 2005; revised 10 June 2005; accepted 13 June 2005 Available online 8 August 2005
Abstract The effect of the environment, as modelled by hydrogen bonding, ion-pairing and/or general continuum model effects, is investigated for the phenoxyl free radical. All are shown to lead to a redistribution of spin density from the phenoxyl O atom to the Cipso position. Isotropic and anisotropic hyperfine couplings are calculated at the B3LYP level of theory confirming this trend. Introduction of the continuum model has a significant effect the Na-O bond length of the ion-pair model significantly altering its calculated EPR properties in comparison with gasphase values. The trends identified are of immediate significance for biological environmental effects on tyrosyl free radicals. q 2005 Elsevier B.V. All rights reserved.
1. Introduction The phenol/phenoxyl couple, PhOH/PhO†, plays crucial roles in a variety of chemical and biologically related electron transfer reactions [1–4]. This ranges from electron transfer in photosynthesis and respiration to antioxidant activity. Great effort has been expended to characterise both the phenol and phenoxyl moiety via spectroscopic methods. The most important of these is Electron Paramagnetic Resonance (EPR) which has been widely used to characterize environmental influences such as hydrogen bonding and to monitor phenolic electron and hydrogen transfer reactions essential to the catalytic activity of many enzymes [2,3]. It is now becoming apparent that the environment plays a crucial role in moderating the properties of biological free radicals. This is often manifested in altered redox potentials and EPR spectroscopic properties. Assessing environmental influences on radical properties using experimental probes is hampered by the inability to vary the external environment systematically, making individual contributions difficult to quantify. The phenoxyl free radical, in particular, can be generated under only certain specific conditions in vitro and, in its biological environment, changes to its surrounding are often * Corresponding author. E-mail address: [email protected] (P.J. O’Malley).
0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2005.06.034
difficult if not impossible to perform. Density functional based electronic structure methods have enabled accurate prediction of hyperfine couplings for larger sized free radicals, principally in the gas phase [5–10]. Using environment modelling it is also possible to account for specific environmental influences in altering the free radical properties of interest [11,12]. Here we investigate the role of the environment on the phenoxyl free radical properties. Explicit ion-pairing and hydrogen bonded models, continuum models and combinations thereof are examined. The fundamental electronic reasons for spin density and resultant hyperfine coupling changes in the phenoxyl free radical as a function of its environment are elucidated.
2. Methods All calculations were performed using the Gaussian 03 and Spartan programs [13,14]. Geometry optimisations were performed at the B3LYP/6-31CG(d,p) level of theory and hyperfine couplings were calculated using B3LYP/EPR-II// B3LYP/6-31CG(d,p). The models used are given in Fig. 2. Continuum dielectric effects were treated within the polarisable continuum model (PCM) of Tomasi et al. [15] specifically using the conductor like PCM model [16], CPCM, as implemented in Gaussian 03. Calculations in water were performed using standard UAHF radii for the construction of the cavity.
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P. Wu, P.J. O’Malley / Journal of Molecular Structure: THEOCHEM 730 (2005) 251–254
H
O
O
H
H
[Na] H
O
O
H
H
H
H
H
H
H H
H
H
(a)
(b)
Fig. 1. Models used for (a) P–2H2O and (b) P–Na. Table 1 Principal bond lengths, angstroms.
CO CC CC CC O–-H O–Na
P
P_W
P–2H2O
P2H2O_W
P–Na
P–Na_W
1.260 1.454 1.380 1.412 – –
1.266 1.454 1.379 1.414 – –
1.270 1.452 1.378 1.414 1.916 –
1.271 1.451 1.378 1.412 1.883 –
1.272 1.450 1.376 1.415 – 2.105
1.267 1.452 1.378 1.416 – 2.296
The models used are shown in Fig. 1. Inclusion of the water continuum model is denoted by _W in the tables and text. 3. Results and discussion 3.1. Geometries
Fig. 3. Spin density difference plot for {P–Na}–{P}. Contours values (a) C0.002 e/au and (b) K0.002 e/au.
bond length, which is lengthened on ion-pairing, hydrogen bonding and/or inclusion of the water solvation. This corresponds to the expected polarisation of the CO bond by the dielectric medium or by direct hydrogen bonding/ion pairing. All lead to an increase in negative charge at the O reflecting increased electronegativity. The most dramatic effect on geometry is exhibited by the P–Na complex where
The principal bond lengths are presented in Table 1. In the internal phenoxyl the major change occurs in the CO
Fig. 2. Electron spin density contour at 0.002 e/au for the P model. Blue indicates positive spin density and green represents negative values.
Fig. 4. Spin density difference plot for {P–Na}–{P}. Contours values (a) C0.0002 e/au and (b) K0.0002 e/au.
P. Wu, P.J. O’Malley / Journal of Molecular Structure: THEOCHEM 730 (2005) 251–254
253
Table 2 Mulliken spin populations. RO denotes Restricted Open Shell Kohn-Sham. U denotes Unrestricted Kohn-Sham P
O Cpara Cmeta Cortho Cipso
P–2H2O
P_W
P2H2O_W
P–Na
P–Na_W
RO
U
RO
U
RO
U
RO
U
RO
U
RO
U
0.33 0.24 0.01 0.18 0.06
0.42 0.45 K0.20 0.35 K0.11
0.29 0.25 0.01 0.17 0.09
0.37 0.46 K0.19 0.32 K0.05
0.30 0.25 0.01 0.17 0.09
0.37 0.46 K0.21 0.34 K0.04
0.30 0.25 0.01 0.17 0.09
0.34 0.47 K0.18 0.30 K0.01
0.24 0.28 0.02 0.15 0.13
0.29 0.48 K0.18 0.27 0.06
0.28 0.26 0.01 0.16 0.10
0.34 0.47 K0.18 0.30 K0.01
solvation in H2O leads to an increase in the O–Na bond length of 0.191 angstrom due to dielectric shielding. 3.2. Spin density and spin population The spin density distribution for the phenoxyl free radical, P, is demonstrated by the contour plot in Fig. 2. In the phenoxyl free radical positive spin density is located primarily at the O, Cortho and Cpara positions around the phenoxyl ring. Negative spin density is located at the Cipso and Cmeta positions, see Fig. 2. The effect of ion complex formation on the spin density distribution is demonstrated by the spin density difference plot {P}–{P–Na} in Fig. 3. Similar plots can be obtained for the explicit hydrogen bonded complex and continuum models. In Fig. 4, the more concentrated spin density difference plot reveals electron transfer from O to Cipso on ion complex formation while the more diffuse plot illustrates additional spin density increase at Cpara and Cmeta and decrease at Cortho,on ion formation. The spin density distribution arises from direct unpaired spin delocalisation effects of the odd electron plus secondary spin polarisation effects. It is possible to separate these different contributions by comparing unrestricted Kohn-Sham and restricted open- shell KohnSham calculations. In the restricted open-shell calculation, only the primary delocalisation effect is operative whereas this and secondary spin polarisation effects on the spin density distribution are accounted for in the unrestricted calculations. For restricted and unrestricted calculations the spin populations for each atom are given in Table 2. The restricted open shell calculations provide a map of the SOMO electron density allowing the influence of the environment, without the effect of spin polarisation, to be assessed. From Table 2, the primary effect of environmental interactions with the O atom is a redistribution of spin from O to Cipso. More moderate increases in spin population at Cpara and decrease at Cortho are also found particularly for the P–Na complex. Ion complex formation, hydrogen bonding and/or solvation therefore leads to a primary redistribution of spin density from O to Cipso. This in turn leads to a secondary redistribution at the ortho, meta and para positions caused by spin polarisation, see Fig. 2. The phenoxyl SOMO orbital can be qualitatively interpreted as overlap between the DOMO of the phenyl fragment and the out of plane pz orbital of the O atom [9]. The relevant energies of these fragments determines
their participation in the SOMO. Previous estimates for the free phenoxyl free radical shows that the SOMO is predominantly due to the phenyl DOMO with a 12% contribution from the O orbital [9]. Hydrogen bonding and ion pairing will be expected to lower the O orbital energy decreasing its contribution to the SOMO. This lower contribution to the SOMO is the underlying reason for the decreased spin density observed on hydrogen bonding/ ion-pairing. This spin density is primarily transferred to the Cipso position on the ring. 3.3. Anisotropic and isotropic hyperfine couplings The anisotropic and isotropic hyperfine couplings, hfcs, are shown in Tables 3 and 4, respectively. The anisotropic hfcs give a direct measure of the spin density distribution as probed by the magnetic nuclei and can be taken as a quantitative measure of the spin density distribution shown in Fig. 2. Their absolute magnitude determines the size of the contour plot around a particular nucleus and for the p electron systems used here their axial nature reflects the cylindrical distribution of spin around each nucleus. Table 3 Anisotropic hyperfine couplings, principal values, in MHz
O
Cpara
Cmeta
Cortho
Cipso
Hmeta
Hortho
P
P_W
P– 2H2O
P2H2O_W
P–Na
P–Na_W
K125 62 63 56 K29 K27 K18 8 10 43 K22 K21 K12 3 9 4 K3 K1 12 K10 K2
K113 57 57 58 K30 K28 K17 7 9 41 K21 K20 5 K1 K4 4 K3 K1 12 K9 K3
K113 56 57 57 K30 K28 K17 8 9 41 K21 K20 3 K2 K1 4 K3 K1 12 K9 K3
K106 53 53 59 K30 K28 K15 7 8 39 K20 K19 K4 1 3 4 K3 K1 12 K9 K3
K92 46 46 61 K32 K30 K14 6 8 36 K19 K17 11 K7 K4 4 K3 K1 12 K9 K3
K106 53 53 59 K30 K28 K16 7 9 39 K20 K19 K3 1 2 4 K3 K1 12 K9 K3
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Table 4 Isotropic hyperfine couplings for the phenoxyl models in MHz. Experimental values from Ref. [17]
O Cpara Cmeta Cortho Cipso Hmeta Hortho
P
P_W
PK 2H2O
P2H2O_W
P–Na
P– Na_W
EXPT
K26 35 K27 23 K38 8 K20
K26 35 K27 23 K38 8 K20
K27 34 K26 21 K31 7 K20
K26 34 K25 18 K27 7 K19
K24 35 K24 16 K21 6 K18
K25 34 K25 19 K28 7 K19
– – – – – 7 K20
As expected for all systems the concentration of spin density at O, Cortho and Cpara leads to the largest magnitude values at these positions. For Cipso and Cmeta the values reflect the negative spin density at these positions for P. The changes that occur for the interaction complex models reflect the changes in spin density distribution described above. Ionpairing and/or hydrogen bonding lead to a reduction in the O values, an increase in the Cipso value, a decrease in the Cortho values and an increase in the Cmeta values reflecting the decreased spin density at the oxygen position and the increased spin density at Cipso. The isotropic values, Table 4 do not show the more dramatic changes of the anisotropic data. Here the spin density being monitored is that directly at the nuclear position (Fermi Contact) and arises via an indirect spin polarisation mechanism. The O values are practically unchanged for each model. Here the loss in p spin at the O is compensated by the increased spin at Cipso. The most notable changes occurs for the Cipso position which increases from K38 for P to K21 for P–Na. Absence of significant p spin at Cipso means that the spin density at the nuclear position arises from spin density at the O and two Cortho atoms. Hence a large negative isotropic hyperfine coupling is calculated. On ion complex formation or hydrogen bonding or solvation we have shown that spin density is transferred to the Cipso position. This positive spin density transfer lowers the net amount of negative spin at the Cipso nucleus leading to the reduced magnitude of the isotropic hfc for this position for the solvated or interacting P. The 1H values are practically unchanged for each model showing the insensitivity of these couplings to environmental influences. Unfortunately these are the most commonly reported in experimental studies of these systems.
4. Conclusions Environmental influences on the spin density and hyperfine coupling of the phenoxyl free radical are investigated using explicit, continuum models and combinations thereof. Environmental influences are mainly characterised by spin density transfer from the O atom to the Cipso position. Such changes are attributed mainly to an increased electronegativity for the O atom leading to decreased overlap of the O pz orbital with the phenyl ring DOMO.
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