Toward an Understanding of the Ambiguous Electron Paramagnetic

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Towards an Understanding of the Ambiguous EPR Spectra of the Iminoxy Radical from o-Fluorobenzaldehyde Oxime: DFT and AB Initio Studies Maciej Witwicki, and Julia Jezierska J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b06143 • Publication Date (Web): 10 Aug 2015 Downloaded from http://pubs.acs.org on August 12, 2015

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The Journal of Physical Chemistry

Towards an Understanding of the Ambiguous EPR Spectra of the Iminoxy Radical from o-Fluorobenzaldehyde Oxime: DFT and Ab Initio Studies

Maciej Witwickia,*, Julia Jezierskaa a

Faculty of Chemistry, Wroclaw University, Joliot-Curie 14, 50-383 Wroclaw, Poland * [email protected]

Abstract: Iminoxy radicals (R1R2C=N-O•) possess an inherent ability to exist as E and Z isomers. Although isotropic hyperfine couplings for the species with R1 = H allow one to distinguish between E and Z, unequivocal assignment of the parameters observed in the EPR spectra of the radicals without the hydrogen atom at the azomethine carbon to the right isomer is not a simple task. The iminoxyl derived from o-fluoroacetophenone oxime (R1 = CH3 and R2 = o-FC6H5) appears to be a case in point. Moreover, for its two isomers the rotation of the o-FC6H5 group brings into existence the syn and anti conformers, depending on the mutual orientation of the F atom and C=N-O• group, making ascription of hyperfine couplings to structure even more challenging. To accomplish this, a vast array of theoretical methods (DFT, OO-SCS-MP2, QCISD) was used to calculate the isotropic hyperfine couplings. The comparison between experimental and theoretical values revealed that the E isomer is the dominant radical form, for which a fast interconversion between anti and syn conformers is expected. In addition, the origin of the significant AF increase with solvent polarity was analysed.

Keywords: isomers, hyperfine coupling, solvent, OO-SCS-MP2, QCISD

– 1 –Environment ACS Paragon Plus


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1. Introduction 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Iminoxyls, whose general structure is R1R2C=N-O•, can be considered σ-type radicals as their singly occupied molecular orbital (SOMO), lying in the nodal plane of the C=N π–bond, is derived from a nitrogen sp2 orbital and an in-plane p orbital of oxygen. The nitrogen hybridization and characteristic nonlinearity of the C=N-O• structural unit in the iminoxyls with the C-N-O angle varying from 137 to 143o were first determined on the basis of EPR spectra.1

Figure 1. General structure of iminoxyls as well as Z and E isomers for the iminoxyl derived from o-fluoroacetophenone oxime

Iminoxy radicals are relatively stable and were detected by means of electron paramagnetic resonance (EPR) spectroscopy in different biological systems inter alia in photosystem II2 and prostaglandin H synthase-2 systems.3 These radicals were demonstrated to originate from tyrosine as iminoxyls were derived from the peroxidase oxidation of 3-nitroso-N-acetyl-L-tyrosine and peroxidase oxidation of free L-tyrosine in the presence of nitric oxide4; the mechanism of the reaction was later proposed on the basis of density functional theory (DFT).5 Iminoxyls were found to be the intermediate product of the reaction between nitrogen oxide and the anticancer drug etoposide (VP-16) diminishing its cytotoxic activity toward cancer cells.6 These radicals were also identified in lichens thalli collected from atmospherically polluted environments and also generated in laboratory conditions as a result of the nitrogen dioxide action on these composite organisms.7 Moreover, iminoxyl radicals derived from unsaturated ketoximes are the starting reagents in organic synthesis. Owing to the unpaired electron density


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on both the O and N atoms, these iminoxyls were utilized in double-heteroatom-containing cyclizations 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

leading to differently functionalized isoxazolines and cyclic nitrones regarded as important reagents for pharmacology.8,9 The angular structure of the C=N-O• moiety, theoretically confirmed for various R1R2C=N-O• radicals by our DFT calculations10–15, provides an exceptional ability of the iminoxyls with two different substituents at azomethine carbon (R1 ≠ R2) to exist as two isomers, E and Z, with the possibility of interconversion. It is characteristic that when one of the substituents is a phenyl ring its rotation is manifested in the EPR spectrum. At high temperature, when the rotation is fast, a 1 : 2 : 1 triplet of hyperfine splitting from the two ortho protons is observed, while at low temperature, when the rotation is restricted, the spectrum consists of a 1 : 1 doublet from one ortho proton. Both the isomerization and rotation processes depend on the energy barriers which were estimated experimentally for the iminoxyl from p-chlorobenzaldehyde oxime (R1 = H and R2 = para-ClC6H4) in toluene as ∆Eisom = 12.5 kcal/mol and ∆Erot = 6.2 kcal/mol.16 The barriers of isomerisation and rotation for the iminoxyls from 2-, 3- and 4pyridinealdoximes were the subject of our DFT studies; the calculated energy barriers closely resembled those determined experimentally.10,11 It should be mentioned that the electronic structure of iminoxyls was analysed by means of electron localisation function (ELF)17 and the hybrid DFT method B3LYP was employed to study the iminoxyl/oxime self-exchange reaction, revealing that the mechanism consists of proton transfer between electron pairs on the oxygens and the electron migration occurs between in-plane orbitals on the two nitrogens18. The rotation of the phenyl ring differently substituted at the two ortho positions can also lead to syn and anti conformers (rotamers) of Z and E isomers, making the radical characteristics more complicated.19 This is exactly the case of the iminoxy radical derived from o-fluorobenzaldehyde oxime (R1 = H, R2 = oFC6H4) for which the assignment of the EPR spectra to the particular isomers was unequivocal.20 Two isomers, E and Z, observed in the EPR spectrum exhibited a very different isotropic hyperfine coupling (IHFC) due to the hydrogen at the azomethine carbon, Aiso(1H) =28.0 G and 6.75 G. Furthermore, the interaction due to the ortho-F substituent of the phenyl was detected only for the E isomer and assigned to the Esyn conformer, whereas Zanti conformation was more stable than Zsyn, showing the IHFC due to one hydrogen atom at the ortho position.20 In contrast, unambiguous assignment of the 1H and

19

F IHFC to the particular radical form was

impossible for the iminoxyls derived from o-fluoroacetophenone oxime (R1 = CH3 and R2 = o-FC6H5) as the IHFC from the methyl protons were the same for the two isomers (ACH3 = 1.5 G).21 The hyperfine splitting due to ortho-F, seen in the dominant spectrum for the radical in solution, was assigned to Zsyn isomer. Although its geometry was suspected of being deviated from planar owning to the steric repulsion between the o-FC6H5 and C=N-O• group, this conformation allowed, according to the authors, for hyperfine interaction with 19F resulting in AF = 6.6 G splitting (in benzene solution), much higher than that possibly expected for an ortho-proton. Moreover, a characteristic surge in AF was observed on rising – 3 –Environment ACS Paragon Plus


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solvent polarity (from 6.6 G in benzene to 10.6 G in DMSO)21,22 and was explained by increasing 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

contribution of the planar dipolar canonical structures of Zsyn isomer with the solvent polarity enhancement21. This ascribing of IHFC to Zsyn was questioned twelve years later on the basis of further experiments showing that the halogen splitting is much larger in isomers with the C=N-O• group inversely oriented to ortho-halogen (for Esyn) than when both groups were facing each other (for Zsyn )19.

Theoretical methods were frequently and successfully employed to investigate various radical systems10–17,23–47 The controversies over the iminoxy derivative of o-fluoroacetophenone oxime prompted us to undertake theoretical research on its geometry, stability and IHFCs of its various forms shown in Figure 1. It seemed to be intriguing how mutual orientations of C=N-O• and the ortho-FC6H5 substituent affect the structures of the isomers and consequently are reflected in the calculated IHFCs. We hoped that accurate prediction of IHFCs, including with respect to the changes in AF induced by the solvent polarity, will allow for conclusive identification of the EPR detected isomers. Another aim of this work was to scrutinise the origin of the solvent effect on the hyperfine couplings in the iminoxyls and compare it with that intensively studied for the EPR parameters of other radicals, that is for semiquinones23-31 and nitroxides32-36. In addition, the performance of various DFT and ab initio methods in prediction of IHFCs was tested.

2. Computational details The geometry optimizations were carried out at the DFT level using the hybrid functional B3LYP48-50 in the framework of RIJCOSX approximation51 and in concert with the TZVP basis set52 and TZVP/J auxiliary basis set53. Each of the stationary points was fully characterized as a minimum or transition state through a vibrational analysis. The isotropic hyperfine coupling (IHFC), also referred to as the Fermi contact term, arises from the spin density on the nucleus under investigation. For a nucleus X it can be calculated as54: A X=

where

βe

value,

⟨S Z ⟨

is the Bohr magneton,

βX

4π β β β g g ⟨S ⟨− 1 ρα− X 3 e X e X Z

the nuclear magneton,

ge

(1)

the electron g-value,

gX

the nuclear g-

α− β

the expectation value of the z-component of total electronic spin, and ρ X

the spin density at

the position of nucleus X. The calculations of IHFCs are widely known for having rather strict basis set requirements, especially in the core region.54-56 Therefore, in this work the EPR-II and EPR-III basis sets tailored for this purpose were used55 in combination with a vast array of theoretical methods. The density functional theory was represented by the GGA (BP8657,58), meta-GGA (TPSS59), hybrid (B3LYP48-50) and double hybrid (B2PLYP60) approximations. For the perturbation theory based functional B2PLYP the relaxed densities were utilised61,62 and the resolution of the identity (RI)63 was applied to the MP2 part. – 4 –Environment ACS Paragon Plus

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Two wave function based methods were also employed, namely the orbital-optimized spin-component1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

scaled MP2 method (OO-SCS-MP2)64,65 combined with the RI approximation63 and the quadratic configuration interaction including single and double substitutions (QCISD)66. In the case of B2PLYP and OO-SCS-MP2 the RI approximation was used, so the appropriate correlation fitting basis sets were utilised67. The choice of methods is discussed in Section 3.4. In order to establish accurate energy differences between the isomers, their conformers and localised transition states, the single-point energy calculations on the structures from the RIJCOSXB3LYP/TZVP optimisations were carried out at the B3LYP/TZVP, B2PLYP/TZVP and QCISD/TZVP levels. In this attempt the RIJCOSX-B3LYP/TZVP zero point energy corrections (ZPE) were used. The Gaussian 09 program package68 was employed to perform the QCISD calculations, while the other ones were carried out with the ORCA suite of programs69. Solvent effects were introduced into the calculations via the continuum solvation models: COSMO70,71 in the case of ORCA and IEF-PCM72,73 in the case of Gaussian. All the calculations were conducted in the unrestricted protocol. The direct contributions to the IHFCs, that is contributions from the SOMO, were not estimated from restricted openshell calculations but calculated according to Equation 1 using the spin density evaluated exclusively from the SOMO. 3. Results and discussion 3.1 Molecular structure The selected geometric parameters predicted by the DFT calculations for the studied radical are collected in Table 1. The lengths of N-O, N-C1, C1-C2 and C1-C3 bonds in vacuum are substantially independent of the isomer, and their values vary between 1.223 and 1.230 Å for N-O, and 1.286 and 1.289 for N-C1. The angles formed by the atoms of the C=N-O• group, on which a substantial proportion of spin density is located, are between 133.4° for Eanti and 136.1° for Zanti. These values deviate from those expected for the sp2 hybridization of nitrogen orbitals but are in agreement with the angle of 134° reported for nitrogen dioxide74, the molecule considered a precursor of iminoxyl radicals. On the other hand, angles NC1C2 and NC1C appear to be distinctly different, not only for the two isomeric forms but also for their two rotamers, for instance 120.9o for Zanti, 121.8o for Zsyn, 116.9o for Eanti and 119.7° for Esyn. Larger values of NC1C2 for syn rotamers were also observed in the case of the radical with R1 = P(O)(OCH3)2 and R2 = C6H512. The alterations to the bond lengths and angles brought about by the increase in the solvent polarity are limited, except perhaps for the N-O bonds, which become longer going from toluene to DMSO from 1.233 Å to 1.237 Å for Zanti, from 1.229 Å to 1.235 Å for Zsyn , from 1.232 Å to 1.236 Å for Eanti and from 1.228 Å to 1.234 Å for Esyn.



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Table 1. B3LYP/TZVP calculated angles (in degrees) and energy differences corrected for ZPE (∆E0 in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

kcal/mol). Numbering of atoms is shown in Figure 1. Zanti

Zsyn

∠

∠

∠

Φ

∆E0

∠

∠

∠

Φ

∆E0

C1NO

NC1C3

NC1C2

NC1C2C3

[kcal/mol]

C1NO

NC1C

NC1C2

NC1C2C3

[kcal/mol]

vacuum

136.1

115.5

120.9

-163.5

0.80

135.5

117.6

121.8

47.8

2.70

toluene

136.0

116.0

120.8

-156.6

0.84

135.6

117.8

121.6

48.1

2.57

CH2Cl2

135.9

116.5

120.5

-151.2

0.96

135.7

118.0

121.5

48.2

2.26

CH3CN

135.9

116.7

120.5

-149.1

1.10

135.8

118.0

121.5

48.1

2.34

DMF

135.9

116.7

120.5

-149.0

1.11

135.8

118.0

121.5

48.1

2.33

DMSO

135.9

116.7

120.5

-148.9

1.11

135.8

118.0

121.5

48.1

2.32

Esyn

Eanti ∠

∠

∠

Φ

∆E0

∠

∠

∠

Φ

∆E0

C1NO

NC1C

NC1C2

NC1C2C3

[kcal/mol]

C1NO

NC1C

NC1C2

NC1C2C3

[kcal/mol]

vacuum

133.4

119.5

116.9

-151.1

0

133.9

119.1

119.8

9.6

0.73

toluene

133.4

119.5

116.8

-149.1

0

134.0

119.2

119.7

9.6

0.34

CH2Cl2

133.5

119.6

116.8

-147.2

0.10

134.2

119.3

119.6

9.6

0

CH3CN

133.5

119.6

116.8

-146.7

0.24

134.3

119.4

119.5

9.6

0

DMF

133.4

119.6

116.8

-146.5

0.27

134.3

119.4

119.5

9.6

0

DMSO

133.5

119.6

116.8

-146.4

0.35

134.3

119.4

119.5

9.7

0

The structural feature most markedly differentiating the forms of the studied radical is their degree of deviation from planarity (Table 1 and Figure 2), which can be monitored by the dihedral angle NC1C2C3. According to our calculations the greatest divergence from the value corresponding to a planar structure is observed for Zsyn. This effect results from the strong steric repulsion between the N–O• radical centre and the voluminous o-FC6H4 group and additionally from the electrostatic repulsion between the F and O atoms, as is distinctly shown by the Löwdin population analysis (B3LYP/TZVP/COSMO(DMSO)) giving the atomic charges on F and O as – 0.123 and – 0.177, respectively. Similarly, the Zanti and Eanti isomers are not planar, this time because of the steric repulsion between the o-FC6H4 and CH3 groups. Only for Esyn does the NC1C2C3 value indicate nearly planar structure. In comparison with Zsyn, for this radical form the steric effect is reduced as the N–O• moiety is oriented in the direction opposite to o-FC6H4. Moreover, the steric repulsion in Esyn is alleviated by the N-F electrostatic interaction as the Löwdin atomic charges on N and F are +0.082 and –0.122, respectively. Unexpectedly, inclusion of the solvent had no effect on NC1C2C3 in the case of Zsyn and Esyn, while for Zanti and Eanti the divergence from planar structure increases with the solvent polarity. These findings contradict the previous suggestion that the increased solvent polarity leads to a greater contribution of planar dipolar canonical structures of Zsyn isomer21. – 6 –Environment ACS Paragon Plus

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Deviation from planar geometry for isomers of iminoxy radical derived from ofluoroacetophenone oxime and distances between O and F as well as between N and F atoms calculated at B3LYP/TZVP level for DMSO.

3.2 Hyperfine couplings The isotropic hyperfine couplings calculated using various methods and the EPR-II basis set are listed in Table 2; for EPR-III the results are given as Supporting Information. In general all the used methods give qualitatively similar results, and quantitative differences will be discussed in section 3.4. The EPR experiment on the radicals generated from o-fluoroacetophenone oxime revealed the existence of two iminoxyl forms with different IHFC patterns: AN = 32.6 G, AF =7.5 G, ACH3 = 1.5 G for the dominant form; AN = 32.0 G, ACH3 = 1.5 G for the secondary form.21 These two forms were assumed to arise from E and Z isomers. As mentioned in the Introduction, the assignment of the parameters to the particular isomers was problematic and questionable.19 The identification of radicals’ structures by means of comparison between experimental and theoretical EPR parameters was proven effective, e.g. in refs. 1113, 45, 46 and 75-78, but application of this approach here must be preceded by verification of the usefulness of AN, ACH3 and AF for such a purpose. The hyperfine coupling is the EPR parameter that probably carries the largest amount of structural information. Therefore, it is not surprising that the predicted distortion from planar geometry has interesting implications for AF. On the basis of simple structures shown in Figure 1, the largest AF value should be expected for Zsyn since this is the case in which the F atom seems to be the closest to the N–O• radical centre20-22. However, in the nearly planar Esyn the distance between the N and F atoms is about 0.16 Å shorter than the distance between O and F in Zsyn (Figure 2). In other words, the N–O• radical centre in Esyn is closer to the F atom and hence the calculated AF is far larger for this radical structure (the AF origin



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will be discussed in detail in Section 3.5). On the other hand, for Eanti and Zanti the F atoms are distant from 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

N–O• and the observed values of AF are therefore small. Table 2. Isotropic hyperfine couplings calculated using EPR-II basis set; experimental values are the ones determined from the dominant EPR spectrum of unknown isomer/conformer; results obtained with EPR-III are given as Supporting Information. E

method

anti

E

syn

Z

anti

Z

syn

AN

ACH3

AF

AN

ACH3

AF

AN

ACH3

AF

AN

ACH3

AF

0.42 -0.35 0.14 5.77 -0.01 0.50

28.69 30.54 27.98 29.84 32.84 29.86

-1.52 -2.06 -1.52 0.23 -0.92 -2.27

8.16 6.58 7.14 9.82 7.08 9.61

vacuum ε = 1.00

BP86 B3LYP TPSS B2PLYP OO-SCS-MP2 QCISD

28.90 30.78 28.11 28.95 32.97 30.37

-1.52 -2.18 -1.43 0.56 -1.21 -3.08

-0.37 -0.69 -0.51 4.67 -0.27 1.05

31.96 34.04 31.24 31.93 36.68 34.38

-1.52 -2.17 -1.42 0.87 -1.26 -3.34

22.33 21.62 20.23 30.06 21.93 20.60

31.78 32.01 29.18 30.11 34.11 31.94

-1.52 -2.08 -1.52 0.93 -0.99 -2.91

toluene ε = 2.40

BP86 B3LYP

29.09

-1.48

-0.34

31.78

-1.49

22.67

29.93

-1.55

0.54

29.02

-1.55

8.22

31.13

-2.15

-0.65

33.99

-2.14

22.02

32.08

-2.10

-0.17

31.03

-2.08

6.62

TPSS

28.31

-1.39

-0.48

31.07

-1.39

20.57

29.20

-1.56

0.28

28.31

-1.54

7.21

B2PLYP

29.46

0.40

4.15

31.89

0.77

29.91

30.50

0.63

4.87

30.39

0.12

9.60

OO-SCS-MP2

32.91

-1.21

-0.27

36.65

-1.25

21.88

33.85

-0.99

0.14

32.69

-0.92

7.01

QCISD exptl.a,b

30.49 31.95

-2.95 1.50

1.17 6.60

34.10 31.95

-3.27 1.50

20.66 6.60

31.63 31.95

-2.73 1.50

1.06 6.60

30.07 31.95

-2.20 1.50

9.42 6.60

CH2Cl2

BP86

29.24

-1.44

-0.31

31.60

-1.46

23.07

30.01

-1.58

0.66

29.37

-1.56

8.39

ε = 9.08

B3LYP

31.46

-2.12

-0.60

33.97

-2.11

22.47

32.24

-2.12

-0.01

31.55

-2.09

6.74

TPSS

28.49 29.96

-1.36 0.25

-0.45 3.69

30.90 31.94

-1.36 0.64

20.95 29.80

29.28 30.98

-1.59 0.34

0.41 4.11

28.66 30.98

-1.56 -0.01

7.38 9.50

B2PLYP OO-SCS-MP2

32.85

-1.20

-0.23

36.63

-1.25

21.89

33.63

-0.99

0.28

32.57

-0.92

7.03

QCISD exptl.a

30.52 32.25

-2.82 1.50

1.31 7.50

33.79 32.25

-3.19 1.50

20.85 7.50

31.40 32.25

-2.55 1.50

1.64 7.50

30.30 32.25

-2.13 1.50

9.37 7.50

CH3CN

BP86

29.29

-2.12

-0.30

31.54

-1.44

23.24

30.04

-1.59

0.71

29.37

-1.56

8.39

ε = 36.6

B3LYP

31.58

-2.10

-0.58

33.96

-2.10

22.67

32.32

-2.12

0.06

31.76

-2.09

6.81

TPSS

28.56

-1.35

-0.44

30.85

-1.34

21.12

29.32

-1.59

0.47

28.80

-1.57

7.46

B2PLYP

30.14

0.20

3.55

31.97

0.58

29.77

31.18

0.23

3.84

31.22

-0.06

9.50

OO-SCS-MP2

32.91

-1.22

-0.16

36.63

-1.24

21.91

33.54

-0.99

0.34

32.53

-0.91

7.06

QCISD exptl.a

30.51 32.35

-2.78 1.50

1.35 9.25

33.67 32.35

-3.15 1.50

20.95 9.25

31.32 32.35

-1.01 1.50

1.88 9.25

30.40 32.35

-2.11 1.50

9.38 9.25

BP86 B3LYP

29.29

-1.42

-0.30

31.54

-1.44

23.25

30.04

-1.59

0.71

29.50

-1.56

8.49

31.59

-2.10

-0.58

33.96

-2.10

22.67

32.32

-2.12

0.06

31.76

-2.08

6.81

TPSS

28.56

-1.34

-0.44

30.85

-1.34

21.13

29.32

-1.59

0.47

28.80

-1.57

7.46

B2PLYP OO-SCS-MP2

30.15 32.91

0.20 -1.22

3.53 -0.16

31.97 36.63

0.58 -1.24

29.77 21.92

31.19 33.54

0.23 -0.99

3.83 0.34

31.22 32.52

-0.06 -0.91

9.50 7.06

QCISD

30.50

-2.78

1.37

33.67

-3.15

20.95

31.32

-2.47

1.89

30.40

-2.11

9.38

exptl.a

32.35

1.50

10.00

32.35

1.50

10.00

32.35

1.50

10.00

32.35

DMSO

BP86

29.29

-1.42

-0.30

31.53

-1.44

23.26

30.04

-1.59

0.71

29.51

-1.56

8.50

ε = 42.7

B3LYP TPSS

31.59 28.56

-2.10 -1.34

-0.58 -0.44

33.97 30.85

-2.10 -1.34

22.69 21.13

32.32 29.32

-2.12 -1.59

0.07 0.47

31.78 28.80

-2.09 -1.57

6.82 7.46

B2PLYP

30.16

0.19

3.52

31.98

0.58

29.77

31.20

0.22

3.81

31.24

-0.06

9.50

OO-SCS-MP2 QCISD

32.83 30.50

-1.19 -2.77

-0.23 1.38

36.63 33.66

-1.24 -3.15

21.92 20.96

33.53 31.32

-0.99 -2.47

0.34 1.91

32.52 30.40

-0.91 -2.11

7.07 9.38

exptl.a

32.40

1.50

10.60

32.40

1.50

10.60

32.40

1.50

10.60

32.40

DMF ε = 38.3


1.50 10.00

1.50 10.60

Page 9 of 24 a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

b


taken from ref. 22 experimental values determined in benzene Regardless of the used computational method, the IHFC due to the protons of CH3 (ACH3) were

found not to be affected by the E-Z isomerism and anti-syn conformation, e.g. for Eanti, Esyn, Zanti and Zsyn these values calculated at B3LYP/EPR-II for DMSO are -2.10, -2.10, -2.12 and -2.09 G, respectively. Therefore, ACH3 cannot be an objective criterion for identification of the radical forms detected in the experiment. The isomers and rotamers of iminoxyl are differentiated by the AN values. In the case of Eanti the AN value is moderately lower than for Esyn, and this relation is reversed for anti and syn rotamers of the Z isomers, in agreement with our previous observation.10,14 However, the changes in AN are of clearly smaller scale in comparison with AF, and for the two isomers and their conformers the AN values stay relatively close to the experimental counterpart, making its potential use in radical structure identification inconclusive. All in all, only finding the best fit between AF determined experimentally and predicted theoretically can allow for the identification of radical structures observed in the experiment. From the data in Table 2, it is apparent that only AF predicted for Zsyn is in reasonably good agreement with the experiment. Therefore, let us take the concept that Zsyn is the dominant radical observed in the experiment as a working hypothesis. From the energetic point of view it is the less profitable structure (see Table 1), but the situations in which the calculated spectroscopic properties (especially at the DFT level) react much more sensitively to structural fluctuations than the total and relative energies themselves are not rare.76,77,79 A matter of deeper concern is the solvent effect on AF. In a series of EPR experiments it was demonstrated that the AF values sharply increases with the solvent polarity from 6.6 G in benzene to 10.6 in DMSO.20,22 Regardless of the method used, the performed calculations fail to reproduce this trend for Zsyn. Furthermore, no significant changes in AF with the solvent polarity are predicted for the other forms of radical. This might be attributed to the fact that the continuum solvation models do not include any specific solvent-solute interactions, i.e. hydrogen bonding, as in the case of computational reproduction of the EPR properties of semiquinones23-31 and nitroxides32-36. However, such doubts here can be dispelled because all the solvents used in the experiments were aprotic and in consequence the continuum solvation models should be able to emulate the AF increase.

3.3 Hyperfine couplings averaged over rotamers According to Table 1 the solvent polarity is clearly reflected in the relative stability of isomers and their rotamers, suggesting the association between the distribution of possible radical forms and the observed IHFCs. Let us assume that the paramagnetic molecule (S = ½) giving rise to hyperfine interaction with a nucleus with spin I exists in two forms, µ and ν. If the interconversion between µ and ν is slow (in



Page 10 of 24

EPR timescale) then two, likely superimposed, spectra should be observed, each of them consisting of 2I+1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

lines.80,81 Obviously, the intensity of these spectra will depend on molar fractions of µ and ν. This is the case of, inter alia, the E and Z isomers of iminoxyls19-22, the radical anions of terephthaldehyde82 or bis-Nmethyl-3,4-fulleropyrrolidine83. In the opposite case, if the interconversion between µ and ν is fast, then one spectrum consisting of 2I+1 lines is observed and the hyperfine splitting becomes the weighted average of its extreme values for the µ and ν isomers80,81: Ā = xµ Aµ + xν Aν

(2)

where Aµ and Aν are hyperfine couplings and xµ and xν the molar fractions of µ and ν, respectively. Such averaging was observed e.g. for the axial and equatorial β protons in nitroxides obtained from piperidine-1oxyl and morpholine-4-oxyl84, the β protons in the radical cation of n-propylamine85 or for the two ortho protons of the phenyl ring in the iminoxyl from benzaldehyde oxime (R1 = H and R2 = C6H5)19 The interconversion between E and Z isomeric forms of the iminoxy radical derived from ofluoroacetophenone oxime was proved to be slow by the EPR experiments since two different radical forms were observed. The situation appears to be different for rotamers. Table 3 shows relative energies between anti and syn conformers for Z and E isomers (∆Eanti/syn), their molar fractions derived from the Boltzmann distributions (x) and values of hyperfine couplings averaged for E and Z isomers over their rotamers (Ā) according to Equation 2 (fast interconversion assumed). Regardless of the theory level, in the absence of solvent effects for the E and Z isomers their anti conformations are found to be energetically beneficial. The relative stability of anti rotamers decreases with solvent polarity and as a consequence the molar fractions of these conformers decline. In the case of B3LYP for the E isomer the syn conformation even becomes the more abundant one.

10 –Environment ACS Paragon –Plus

Page 11 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Relative energies between anti and syn rotamers for E and Z isomers (∆Eanti/syn, in kcal/mol), molar fractions of the rotamers determined from Boltzmann distribution (x), and hyperfine coupling constants for E and Z isomers averaged according to molar fractions of rotamers (Ā, in Gauss). Ā values were calculated using the EPR-II values (the EPR-III counterparts are given as Supporting Information), and ∆Eanti/syn values at the B2PLYP/TZVP and QCISD/TZVP theory levels were calculated using geometries and the ZPE corrections from B3LYP/TZVP calculations. B3LYP/TZVP

vacuum ε = 1.00 ∆E x

anti/syn


B2PLYP/TZVP

Eanti Esyn 0.00 0.73 0.774 0.226

Eanti 0.00 0.886

Esyn 1.21 0.114

ĀN ĀF 29.59 4.76 31.51 4.35 28.82 4.17 29.62 10.41 33.81 4.75 31.27 5.47

ĀN 29.25 31.15 28.47 29.29 33.40 30.82

ĀF 2.22 1.86 1.86 7.57 2.27 3.29

B3LYP/TZVP

toluene ε = 2.40 ∆Eanti/syn x BP86 B3LYP TPSS B2PLYP OO-SCS-MP2 QCISD exptl.a,b

B2PLYP/TZVP

Eanti Esyn 0.00 0.34 0.638 0.362 ĀN ĀF 30.06 7.99 32.17 7.56 29.31 7.14 30.34 13.47 34.26 7.75 31.79 8.23 31.95 6.6

Eanti 0.00 0.812 ĀN 29.59 31.67 28.83 29.92 33.61 31.17 31.95

B3LYP/TZVP

CH2Cl2 ε = 9.08


Eanti 0.10 0.456 ĀN 30.52 32.82 29.80 31.03 34.91 32.30

exptl.a

32.25

∆E x

anti/syn

B2PLYP/TZVP

Esyn 0.00 0.544 ĀF 12.40 11.94 11.19 17.89 11.80 11.93

B3LYP/TZVP Zsyn 1.90 0.039

Zanti Zsyn 0.00 1.62 0.939 0.061

Zanti 0.00 0.965

Zsyn 1.96 0.035

ĀN 29.31 31.21 28.53 29.35 33.47 30.90

ĀN 31.66 31.95 29.13 30.10 34.06 31.85

ĀF 0.72 -0.07 0.42 5.93 0.27 0.74

ĀN 31.59 31.92 29.10 30.09 34.03 31.81

ĀN 31.67 31.95 29.13 30.10 34.07 31.86

ĀF 0.69 -0.10 0.39 5.92 0.24 0.72

ĀF 2.66 2.29 2.26 8.06 2.70 3.66

QCISD/TZVP Eanti Esyn 0.00 0.83 0.803 0.197 ĀN ĀF 29.61 4.18 31.69 3.81 28.85 3.66 29.94 9.21 33.64 4.09 31.20 5.00 31.95 6.60 QCISD/TZVP

B3LYP/TZVP Zanti 0.00 0.948 ĀN 29.89 32.03 29.15 30.49 33.79 31.54 31.95

Zsyn 1.72 0.052 ĀF 0.94 0.18 0.64 5.11 0.50 0.82 6.60

B3LYP/TZVP

QCISD/TZVP

Zsyn 1.13 0.129 ĀF 1.66 0.86 1.31 4.81 1.15 1.31

Zanti 0.00 0.926 ĀN 29.96 32.19 29.23 30.98 33.55 31.31

Zsyn 1.49 0.074 ĀF 1.24 0.50 0.93 4.51 0.79 0.99

7.50 32.25

7.50

32.25

7.50

32.25

7.50

32.25

7.50

32.25

7.50

B2PLYP/TZVP

Esyn Eanti 0.00 0.00 0.598 0.646

ĀF

ĀN

ĀF

ĀN

ĀF

8.04 7.65 7.19 12.84 7.65 8.29 9.25

29.98 32.31 29.26 30.70 34.04 31.47 32.35

6.89 6.52 6.15 11.56 6.58 7.34 9.25

29.97 32.26 29.26 31.19 33.43 31.22 32.35

1.56 0.81 1.24 4.46 1.08 1.20 9.25

B2PLYP/TZVP

Esyn Eanti 0.00 0.00 0.611 0.635

ĀF

Esyn 0.33 0.365

Eanti 0.00 0.695

Esyn 0.49 0.305

B3LYP/TZVP

ĀN

B3LYP/TZVP

Esyn 0.36 0.354

QCISD/TZVP

30.09 32.43 29.37 30.79 34.22 31.63 32.35

ĀN

Zsyn 1.49 0.074 ĀF 1.12 0.34 0.79 5.22 0.65 0.96 6.60

Zanti 0.00 0.871 ĀN 29.92 32.15 29.20 30.98 33.49 31.25

ĀF

30.66 33.04 29.96

Zanti 0.00 0.926 ĀN 29.86 32.00 29.13 30.49 33.77 31.51 31.95

Zsyn 1.30 0.100 ĀF 1.44 0.67 1.11 4.65 0.96 1.14

13.78 13.32 12.45 19.23 13.04 13.07 9.25

BP86 B3LYP TPSS

B2PLYP/TZVP

QCISD/TZVP

Zanti 0.00 0.900 ĀN 29.94 32.17 29.22 30.98 33.52 31.29

ĀN

/

Zanti Zsyn 0.00 1.38 0.911 0.089 ĀN ĀF 29.85 1.23 31.99 0.44 29.12 0.89 30.49 5.29 33.75 0.75 31.49 1.04 31.95 6.60

Esyn 0.55 0.283 ĀF 6.31 5.93 5.61 11.08 6.03 6.84

30.63 33.01 29.93 31.24 35.13 32.40 32.35

∆E x

B2PLYP/TZVP

Eanti 0.00 0.717 ĀN 29.91 32.17 29.17 30.52 33.92 31.44

BP86 B3LYP TPSS B2PLYP OO-SCS-MP2 QCISD exptl.a

anti syn

ĀF 0.89 0.08 0.57 6.02 0.43 0.88

Esyn 0.48 0.308 ĀF 6.88 6.50 6.14 11.73 6.57 7.32

∆Eanti/syn x

Eanti 0.27 0.389

QCISD/TZVP

Zanti 0.00 0.961

Eanti 0.24 0.402

DMF ε = 38.3

B2PLYP/TZVP

Eanti Esyn 0.00 1.11 0.866 0.134

Eanti 0.00 0.692 ĀN 29.96 32.23 29.23 30.57 34.01 31.52

B3LYP/TZVP

CH3CN ε = 36.6

Esyn 0.48 0.188 ĀF 3.99 3.62 3.48 8.99 3.90 4.84 6.60

QCISD/TZVP

QCISD/TZVP Eanti 0.00 0.687

Esyn 0.46 0.313

Zanti 0.00 0.890

Zsyn 1.24 0.110

B3LYP/TZVP Zanti 0.00 0.887

Zsyn 1.22 0.113

ĀN

ĀF

ĀN

ĀF

ĀN

ĀF

14.10 30.11 13.64 32.45 12.75 29.39

8.31 7.92 7.44

30.00 32.33 29.27

7.08 6.71 6.32

29.98 32.26 29.26

1.59 0.82 1.26

– 11 –

ACS Paragon Plus Environment

B2PLYP/TZVP Zanti 0.00 0.849 ĀN 29.94 32.23 29.24 31.19 33.39 31.18 32.35

Zsyn 1.02 0.151 ĀF 1.87 1.08 1.52 4.69 1.35 1.44 9.25

B2PLYP/TZVP Zanti 0.00 0.840 ĀN 29.96 32.23 29.24

Zsyn 0.98 0.160 ĀF 1.95 1.14 1.59

QCISD/TZVP Zanti 0.00 0.922 ĀN 29.99 32.27 29.28 31.18 33.46 31.25 32.35

Zsyn 1.46 0.078 ĀF 1.31 0.59 1.01 4.28 0.86 1.01 9.25

QCISD/TZVP Zanti 0.00 0.916 ĀN 30.00 32.27 29.28

Zsyn 1.41 0.084 ĀF 1.37 0.63 1.06


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

B2PLYP OO-SCS-MP2 QCISD exptl.a

anti

∆E x

anti/syn

BP86 B3LYP TPSS B2PLYP OO-SCS-MP2 QCISD exptl.a

b

19.58 13.34 13.35 10.00

B3LYP/TZVP

DMSO ε = 42.7

a

31.26 35.18 32.44 32.35

E 0.35 0.356

30.81 34.27 31.66 32.35

13.12 7.91 8.53 10.00

B2PLYP/TZVP

syn

anti

syn

E E 0.00 0.00 0.644 0.602

E 0.24 0.398

30.72 34.07 31.50 32.35

11.76 6.76 7.51 10.00

QCISD/TZVP anti

E 0.00 0.657

syn

E 0.39 0.343

31.19 33.43 31.22 32.35

Page 12 of 24 4.47 1.10 1.23 10.00

B3LYP/TZVP anti

Z 0.00 0.886

syn

Z 1.21 0.114

ĀN

ĀF

ĀN

ĀF

ĀN

ĀF

ĀN

ĀF

30.74 33.12 30.03 31.33 35.28 32.54 32.40

14.87 14.40 13.45 20.43 14.04 13.99 10.60

30.19 32.54 29.47 30.88 34.34 31.76 32.40

9.08 8.68 8.15 13.97 8.59 9.17 10.60

30.06 32.41 29.34 30.78 34.13 31.58 32.40

7.77 7.39 6.95 12.52 7.36 8.09 10.60

29.98 32.26 29.26 31.20 33.42 31.21 32.40

1.60 0.84 1.27 4.46 1.11 1.23 10.60

31.19 33.38 31.17 32.35

4.74 1.41 1.50 10.00

B2PLYP/TZVP anti

Z 0.00 0.826 ĀN 29.95 32.23 29.23 31.21 33.36 31.16 32.40

syn

Z 0.92 0.174 ĀF 2.06 1.24 1.68 4.80 1.51 1.58 10.60

31.19 33.45 31.24 32.35

4.31 0.91 1.06 10.00

QCISD/TZVP anti

Z 0.00 0.913 ĀN 30.00 32.28 29.28 31.20 33.45 31.24 32.40

syn

Z 1.40 0.087 ĀF 1.38 0.65 1.07 4.31 0.92 1.07 10.60

taken from ref. 22 experimental values determined in benzene

For the Z isomer the ĀF value predicted using all methods slightly rises with the solvent polarity, but the scale of this increase is small in comparison with the experiment. More important, the magnitudes of the predicted ĀF are clearly lower than those revealed by the experiment (except for that calculated with the B2PLYP functional; this will be commented on in section 3.4). In contrast, in the case of the E isomer both the magnitude of ĀF and its increase with the solvent polarity are in line with the experiment. However, the ĀF values averaged using molar fractions determined at the B3LYP/TZVP level are noticeably overestimated for polar solvents. The situation is far more favourable if the conformer quantities from the B2PLYP/TZVP or QCISD/TZVP calculations are used to obtain ĀF. This is not surprising, as the double hybrid functionals are considered to be an enhancement over standard hybrid functionals in terms of energy calculation whereas highly correlated ab initio methods are obviously also superior to standard hybrid functionals in this matter.86,87 Hence, the more accurate estimation of molar fractions at these two levels leads to ĀF values which better mimic the experiment. All in all, this good agreement between theory and experiment allows one to draw a logical conclusion that the dominant radical observed in the EPR experiments20-22 is the E isomer. However, this hypothesis has to be additionally verified against two factors. First, in the experiments the hyperfine coupling due to the hydrogen atom bonded to C4 (see Figure 1) was not observed, and this should also be reproduced by the computations. The values of this coupling obtained at the QCISD/EPR-II theory level in toluene are 0.73 G and 0.61 G for Eanti and Esyn, respectively; in DMSO they are 0.82 G and 0.66 G for Eanti and Esyn, respectively; after averaging using molar fractions derived from QCISD/TZVP calculations they are 0.71 and 0.77 G in toluene and DMSO, respectively. The hyperfine couplings of such magnitude could have been difficult to detect. The second aspect that needs verification is the ad hoc assumption that the energy of the rotational barrier is so low that the

– 12 –


Page 13 of 24


anti and syn rotamers are not perceptible as different chemical species on the EPR time scale, while the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

isomerisation barrier between E and Z is high enough. Therefore, these barriers should be inspected. The diagram in Figure 3 compares the relative energies (computed at QCISD/TZVP theory level using geometries and ZPE corrections from the B3LYP/TZVP calculations) of the two isomers, their rotamers and transitions states between all these iminoxyl species. It is apparent that the potential energy barrier between the isomers is always at least fivefold higher than between the pairs of rotamers and hence transition between the rotamers should occur at a much higher rate. The predicted isomerisation barriers closely resemble those predicted for the radicals from pyridinealdoximes10,11 and estimated experimentally for iminoxyl from p-chlorobenzaldehyde oxime16. On the other hand, the barriers of rotations for the studied iminoxyl are sharply decreased in comparison to these reference radicals. This decrease can be explained by the significant distortions from planar geometries not present in the case of iminoxyls from the pyridinealdoximes and p-chlorobenzaldehyde oxime. Reaching the non-planar geometry of the transition states related to the rotation along the C1–C2 bond is energetically easier if the initial structure is already twisted in the direction of this rotation. The lower barrier in the case of the investigated radical shows that the syn-anti rotation is less likely to be hindered at lower temperatures. This is in line with the results of Gilbert and Norman21, who did not observe symptoms of hindered rotation in the EPR spectra recorded even at –50° C in CH2Cl2.

Figure 3. Rotation/isomerisation energy diagram computed at the QCISD/TZVP theory levels (using geometries and ZPE corrections from B3LYP/TZVP calculations); results for other solvents were omitted for the sake of clarity.

3.4 Comparison between methods

– 13 –



In spite of all the progress in quantum chemistry, isotropic hyperfine coupling remains a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

challenging property62, especially if a significant proportion of spin density on the nuclei of interest results from spin polarization. When DFT methods are considered, it has not been identified which functional tends to give the most reliable results62, but for various nitrogen radicals, including iminoxyls, the highly satisfactory performance of hybrid functionals was demonstrated.10-13,42,43 The OO-SCS-MP2 method is a relatively new tool of quantum chemistry and has not been tested as intensively as the GGA and hybrid functionals. The predictions made using highly correlated QCISD or CCSD methods are generally excellent32,88, but this excellence frequently comes with prohibitive computational price. Nevertheless, the effectiveness of QCISD for iminoxyls was demonstrated.89 The differences between the hyperfine couplings obtained using the EPR-II and EPR-III basis sets are insignificant (see Supporting Information), which is in accordance with previous observations for iminoxyls.10,11,13,14 Therefore, the further discussion will be focused on various methods. A direct comparison with the experimental data is hindered since errors in Ā were introduced not only by calculation of hyperfine coupling but also by the estimation of the molar fractions. These two kinds of errors can cumulate or cancel and cannot be separated. Therefore to evaluate the sole effectiveness in the hyperfine couplings prediction (not affected by estimation of molar fractions) it is sensible to compare the results obtained for the two isomers and their conformers using DFT and OO-SCS-MP2 with those calculated via the QCISD method (Table 2). In general, the employed DFT methods perform reliably for AN of iminoxyls, and the predicted values are not far off their counterparts from QCISD. This is in contrast to the case of nitroxides32-36, having, like iminoxyls, the unpaired electron delocalised between nitrogen and oxygen atoms. However, the SOMO of nitroxides is essentially the π* orbital, and as isotropic hyperfine coupling is a singular property that depends on the spin density on the nucleus14 then it stems for them from the spin polarizations of doubly occupied orbitals. This effect is frequently called the indirect contribution, and its accurate prediction might be a challenge at the DFT level. On the other hand, iminoxyls are σ type radicals and the direct contribution, from the SOMO, is expected to predominate AN. This is true for Esyn and Zsyn, but in the case of the two anti conformers the direct and indirect effects are comparable (Figure 4). The calculations carried out with double hybrid B2PLYP and meta-GGA TPSS functionals provided satisfactory AN values, but these functionals do not appear as an improvement in comparison with the standard hybrid and GGA. Among the DFT methods tested here, B3LYP seems to be the best choice for the AN calculations, although its performance in the estimation of the molar fractions was not entirely satisfactory.

– 14 –


Page 14 of 24

Page 15 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Direct and indirect contributions to AN (top chart) and AF (bottom chart) calculated at B3LYP/EPR-II level for toluene. Table 2 shows that for Esyn and Zsyn, which are the two structures giving rise to noticeable AF, good agreement between the AF values calculated at the QCISD and DFT levels is observed. The B2PLYP functional is, however, an exception to this trend. The values obtained using the double hybrid functional tend to be higher than those predicted with B3LYP or QCISD. This overestimation, consequently leading to a significant inaccuracy in ĀF, seems to originate primarily from unbalanced spin polarization on the 19F nucleus, as indicated by indirect contribution. For instance, in the case of Esyn in toluene the indirect contribution to AF amounts to 15.15 G and is nearly fivefold higher than the counterpart from the B3LYP calculation (3.45 G). Although the hyperfine couplings obtained with B2PLYP are usually excellent (provided that the calculations are based on the relaxed density approach), this functional was found to suffer occasionally from the unstable MP2 component62. The observed problems with AF are probably another case in point. (AF predicted with B2PLYP is close to the QCISD counterpart only in the case of Zsyn, so this result should be treated as accidental.) Interestingly, the ACH3 values, which are practically independent of E-Z isomerism and antisyn conformations, are in better accordance with the experiment when calculated at the DFT level. This fact must be interpreted with caution. The ACH3 values in this work were evaluated as the arithmetic average of the hyperfine coupling due to the three protons of the CH3 moiety only to demonstrate that ACH3 is not a good criterion for identification of the isomers observed in the EPR experiments. A more accurate approach to ACH3, which, however, was not the aim of this work, would probably require inclusion of CH3 rotation90.

– 15 –



The performance of OO-SCS-MP2, which can be labelled as an affordable wave function based 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

approach, needs a separate comment. The accuracy of this method here was comparable to DFT, but slightly inferior to B3LYP. Particularly interesting is the fact that OO-SCS-MP2 did not overestimate AF in the same manner as B2PLYP did. In the OO-SCS-MP2 method the orbitals are optimized alongside with the double excitation amplitudes, which is a source of greatly improved accuracy and stability for open-shell molecules64,65,91. Since the poor performance of B2PLYP towards AF is suggested here to originate from the MP2 instability, it indicates that the inclusion of the orbital optimisation in the B2PLYP calculations of hyperfine couplings could lead to a noticeable improvement. Another point of concern is that, in spite of higher computational cost of the B2PLYP and OOSCS-MP2 methods, they did not provide results closer to QCISD than the standard GGA and hybrid functionals did. However, the general usefulness of these two methods should not be underestimated. B2PLYP was shown to clearly outperform the standard DFT methods in calculations of hyperfine couplings due to transition metal ions62, and OO-SCS-MP2 is an attractive alternative as it is not troubled by self-interaction problems that occasionally can interfere DFT calculations64,65, for instance in the case of radicals with extended π-systems.

3.4 Nature of solvent effect and AF origin Even the first EPR studies of organic radicals compellingly proved that the magnetic properties of these molecules are modified by the chemical environment, particularly by the solvent. For example, nitrogen hyperfine coupling in nitroxides can increase by 2.5 G when going from apolar solvents to water.92 Therefore, it is not surprising that numerous computational studies have investigated the solvent effect on the EPR properties of organic radicals, among which semiquinones2331

and nitroxides32-36 principally drew the attention of researchers. For these two types of radicals the

mechanism of solvent-induced changes in the EPR parameters included alteration of molecular structure followed by the flow of spin density from the oxygen atom towards the nitrogen for nitroxides and from the oxygen atoms towards the carbon atoms for semiquinones. For the investigated iminoxyl the solvent effect was experimentally exposed only in hyperfine coupling due to nucleus 19F (increase from 6.60 G in toluene to 10.60 G in DMSO; for AN the effect was minimal)22. The lack of noticeable change in AN clearly indicates that the solvent does not significantly affect the spin distribution in the C=N-O• radical centre, which differentiates iminoxyls from both nitroxides and semiquinones. The calculations conducted in this work accurately reproduced these experimental observations and revealed that the mechanism of AF rise with solvent polarity is entirely different than that reported for the EPR parameters of semiquinones and nitroxides. The solvent induced change of AF does not stem from the structural fluctuations or spin density alterations

– 16 –


Page 16 of 24

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but from variation of molar fractions of the two fast interconverting rotamers Eanti and Esyn, i.e. by 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

increasing the contribution of Esyn with much greater AF than that for Eanti. The last problem that can be discussed in detail on the basis of the presented calculations is the origin of AF. As mentioned above, AF depends on the spin density on the 19F nucleus, i.e. s-type spin density. As exemplified for Esyn in Figure 5, the spin density on the F atom is present and its noticeable proportion is of s-type. The source of this density should be explored. For the iminoxyl radical derived from acetophenone oxime it was suggested that the isotropic hyperfine coupling due to the ortho protons is introduced by spin polarisation (indirect effect).16 By analogy, one might suspect that the situation is similar in the case of AF for the studied iminoxy radical, but as indicated in Figure 4 the AF value is mainly brought about by the direct effect; therefore the spin density on the F atom must result from contribution to the SOMO from the s-type atomic orbitals of the fluorine. A modest contribution of 0.3% from the 2s atomic orbital was found, but taking into account the large A0 value for the

19

F

80

nucleus (18865.3 G; hyperfine interaction for unit s-type spin density) it is understandable than even such a small 2s contribution can give rise to significant AF. These 2s contributions to the SOMO for the other radical structures were even smaller leading to the lower AF values.

Figure 5. Spin density and SOMO isosurfaces, contributions from fluorine to SOMO (according to Löwdin reduced orbitals) and Löwdin spin populations of s- and p-type for Esyn; all calculated at the B3LYP/EPR-II level.

4. Conclusions Interpretation of EPR spectra of the iminoxy radical from o-fluoroacetophenone oxime has been a subject of debate. Gilbert and Norman proposed that the source of the dominant EPR spectrum is the Z isomer in syn conformation20-22, while Alberti et al. suggested the E isomer, also in syn conformation19. This theoretical investigation was meant to confirm one of the two theories. It was – 17 – ACS Paragon Plus Environment


demonstrated that only the AF values differentiate the isomers and their conformers. Unexpectedly, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

neither Zsyn nor Esyn or any other single molecular structure was sufficient to computationally reproduce the experimental rise of AF with the solvent polarity. An attempt was then made to calculate the IHFCs assuming fast interconversion between the anti and syn conformers. It revealed that both the magnitude of AF and its solvent-induced increase stem from such a process for the E isomer. The possibility of the interconversion was confirmed by the computational approach to the anti-syn potential energy barrier, which was found to be low, substantialy lower than in the case of other iminoxyls10,11,16. This decrease was explained by the significant distortions from planar geometries not present in the case of the other iminoxyls for which appropriate barriers are known. The mechanism of solvent effect on AF was compared to the reasons for EPR parameters changes reported for semiquinones23-31 and nitroxides32-36. For these two types of radicals the solvent was shown to significantly alter their molecular and electronic structures, which brought about changes in the EPR parameters. The situation was entirely different for the solvent-induced rise in AF originating not from such structural modifications but from variation of molar fractions of the two fast interconverting conformers, that is by increasing abundance of Esyn with AF much greater than that for Eanti. In addition, the efficiency of various theoretical methods in the IHFC calculations of iminoxyls was elaborated via comparison to the QCISD results. The DFT methods appeared to be a good choice for such a task. Among them the B3LYP functional showed particularly excellent performance although it was somewhat mediocre in the estimation of the molar fractions. The B2PLYP functional tended to calculate AN accurately but overestimated the AF values owing to unbalanced spin polarization on the 19F nucleus. The accuracy of OO-SCS-MP2 was comparable to DFT, but somewhat inferior to B3LYP.

Supporting Information. Isotropic hyperfine couplings calculated using the EPR-III basis set. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work was financed from the National Science Centre (NCN) funds allocated on the basis of decision DEC-2011/03/B/ST5/01742 and partially by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wrocław University. The computations were partially performed using computers of the Wrocław Centre for Networking and Supercomputing (Grant No. 47).

References – 18 –


Page 18 of 24

Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(1) Fox, W. M.; Symons, M. C. R. Unstable Intermediates. Part XXXIX. The Structure of Iminoxy-Radicals Deduced from Their Electron Spin Resonance Spectra in Rigid Media. J. Chem. Soc A. 1966, 1503-1507. (2) Sanakis, Y.; Goussias, C.; Mason, R. P.; Petrouleas, V. NO Interacts with the Tyrosine Radical YD• of Photosystem II to Form an Iminoxyl Radical. Biochemistry 1997, 36, 1411-1417. (3) Gunther, M. R.; Hsi, L. C.; Curtis, J. F.; Gierse, J. K.; Marnett, L. J.; Eling, T. E.; Mason, R. P. Nitric Oxide Trapping of the Tyrosyl Radical of Prostaglandin H Synthase-2 Leads to Tyrosine Iminoxyl Radical and Nitrotyrosine Formation. J. Biol. Chem. 1997, 272, 17086–17090. (4) Sturgeon, B. E.; Glover, R. E.; Chen, Y. R.; Burka, L. T.; Mason, R. P. Tyrosine Iminoxyl Radical Formation from Tyrosyl Radical/Nitric Oxide and Nitrosotyrosine. J. Biol. Chem. 2001, 276,. 45516–45521. (5) Papavasileiou, K. D.; Tzima, T. D.; Sanakis, Y. Melissas, V. S. A DFT Study of the Nitric Oxide and Tyrosyl Radical Interaction: A Proposed Radical Mechanism. ChemPhysChem 2007, 8, 2595–2602. (6) Sinha, B. K.; Bhattacharjee, S.; Chatterjee, S.; Jiang, J. J.; Motten, A. G.; Kumar, A.; Espey, M. G.; Mason, R. P. Effect of Nitric Oxide on the Anticancer Activity of the Topoisomerase-Active Drugs Etoposide and Adriamycin in Human Melanoma Cells. Chem. Res. Toxicol. 2013, 26, 379−387. (7) Jezierski, A.; Bylinska, E.; Seaward, M. R. D. Electron Paramagnetic Resonance (EPR) Investigations of Lichens – 1: Effects Of Air Pollution. Atmos. Environ. 1999, 33, 4629−4635. (8) Peng, X.; Deng, Y. J.; Yang, X. L.; Zhang, L.; Yu, W.; Han, B. Iminoxyl Radical-Promoted Dichotomous Cyclizations: Efficient Oxyoximation and Aminooximation of Alkenes. Org. Lett. 2014, 16, 4650−4653. (9) Yang, X. L.; Chen, F.; Zhou, N.-N.; Yu, W.; Han, B. Synthesis of Isoxazoline-Functionalized Phenanthridines via Iminoxyl Radical-Participated Cascade Sequence. Org. Lett. 2014, 16, 6476−6479. (10) Jaszewski, A. R.; Jezierska, J.; Jezierski, A. Hyperfine Parameters of Pyridyl-Containing Iminoxy Radicals Generated from Oximes by γ-Irradiation and in Zeolite Lattice: EPR Spectroscopy and Density Functional Study. Spectrochim. Acta A 1999, 55, 1699-1709. (11) Jaszewski, A. R.; Jezierska, J.; Krowicka, M.; Kalecinska, E. EPR and Density Functional Studies on 3Pyridylmethaniminoxy Radical: 1H Hyperfine Couplings as a Structural Criterion for Iminoxyls Derived from Pyridinealdoximes. Appl. Magn. Reson. 2000, 18, 85-100. (12) Jaszewski, A. R., Siatecki, Z.; Jezierska, J. EPR Spectroscopy and Hybrid Density Functional Studies of Dialkoxyphosphinyl-phenyl Iminoxy Radicals. Chem. Phys. Lett. 2000, 331, 403-412. (13) Jaszewski, A. R.; Jezierska, J.; Jezierski, A. Hybrid Density Functional Approach to the Structures and EPR Parameters of σ-Type Iminoxy Radicals: The Case of 3-Oxobutan-2-iminoxyl. Chem. Phys. Lett. 2000, 319, 611-617. (14) Tabaka, K.; Jezierska, J. Molecular Geometry and Hyperfine Interactions in Iminoxy Radicals with C‚ O or CH2 Group – DFT and EPR Studies in Liquid and Rigid Media. Chem. Phys. Lett. 2004, 394, 298-306. (15) Tabaka, K.; Jezierska, J. Positional and Angular Dependence of Hyperfine Interactions in Cyclic and Bicyclic Iminoxy Radicals with C=O or CH2 Group – DFT and EPR Studies. Chem. Phys. Lett. 2005, 410, 391-399. (16) Lucarini, M.; Pedulli, G. F.; Alberti, A. Geometrical Isomerization and Restricted Rotation in Iminoxyl Radicals from Benzaldoximes. J. Org. Chem. 1994, 59, 1980–1983. (17) Berski, S.; Jaszewski, A. R.; Jezierska, J. The View on the C=N-O Group in the RHC=NO (R = H, BH2; CH3; NH2; OH; F) Iminoxy Radicals by Means of Electron Localisation Function (ELF). Chem. Phys. Chem. 2001, 341, 168-178. (18) DiLabio, G. A.; Ingold, K. U. A Theoretical Study of the Iminoxyl/Oxime Self-Exchange Reaction. A Five-Center, Cyclic Proton-Coupled Electron Transfer. J. Am. Chem. Soc. 2005, 127, 6693-6699. (19) Alberti, A.; Barbaro, G.; Battaglia, A.; Dondoni, A.; Pedulli, G. F. Configuration of Arylchloromethaniminoxy Radicals. A Reassignment. J. Org. Chem. 1980, 45, 4223-4225. (20) Gilbert, B.C.; Norman, R.O.C. Electron Spin Resonance Studies of Oxidation. Part VII. Iminoxy Radicals. Part II. Radicals from Benzaldoximes and Related Oximes. J. Chem. Soc. (B) 1966, 722-726. (21) Gilbert, B.C.; Norman, R.O.C. Electron Spin Resonance Studies. Part XV. Iminoxy-Radicals from Acetophenone Oxime and Related Oximes. J. Chem. Soc. (B), 1968, 123-130. (22) Norman, R.O.C.; Gilbert, B. C. Electron Spin Resonance Studies of Oxidation. IX. Some Interesting Properties of Iminoxy Radicals J. Phys. Chem. 1967, 71, 14-19. (23) Asher, J. R.; Kaupp, M. Hyperfine Coupling Tensors of the Benzosemiquinone Radical Anion from Car–Parrinello Molecular Dynamics ChemPhysChem 2007, 8, 69-79. (24) Ciofini, I.; Reviakine, R.; Arbuznikov, A.; Kaupp, M. Solvent Effects on g-Tensors of Semiquinone Radical Anions: Polarizable Continuum Versus Cluster Models. Theor. Chem. Acc. 2004, 111, 132–140. (25) Mattar, S. M. Role of the Solvent in Computing the 1,4-Benzosemiquinone g-Tensor by the Coupled-Perturbed Kohn−Sham Hybrid Density Functional Method. J. Phys. Chem. B 2004, 108, 9449–9455. (26) O’Malley, P. J. Density Functional Calculated 13C and 17O Hyperfine Couplings for the Ubisemiquinone Anion Radical in an Alcohol Solvent Model and in a Model of the Qa Binding Site of Rb Sphaeroides. Chem. Phys. Lett. 1998, 285, 99–104. (27) Witwicki, M.; Jezierska, J.; Ozarowski, A. Solvent Effect on EPR, Molecular and Electronic Properties of Semiquinone Radical Derived from 3,4-Dihydroxybenzoic Acid as Model for Humic Acid Transient Radicals: High-Field EPR and DFT Studies. Chem. Phys. Lett. 2009, 473, 160–166.; (28) Witwicki, M.; Jezierska, J. Protic and Aprotic Solvent Effect on Molecular Properties and g-Tensors of oSemiquinones with Various Aromacity and Heteroatoms: A DFT study. Chem. Phys. Lett. 2010, 493, 364–370. (29) Witwicki, M.; Jezierska, J. Effects of Solvents, Ligand Aromaticity, and Coordination Sphere on the g Tensor of Anionic o-Semiquinone Radicals Complexed by Mg2+ Ions: DFT Studies. J. Phys. Chem. B 2011, 115, 3172–3184.

– 19 –



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(30) O’Malley, P. J. Effect of Hydrogen Bonding on the Spin Density Distribution and Hyperfine Couplings of the pBenzosemiquinone Anion Radical in Alcohol Solvents: A Hybrid Density Functional Study. J. Phys. Chem. A 1997, 101, 9813–9817. (31) Witwicki, M.; Jezierska, J. DFT Insight into o-Semiquinone Radicals and Ca2+ Ion Interaction: Structure, g Tensor, and Stability Theor. Chem. Acc. 2013, 132, 1383-1396. (32) Improta, R.; Barone, V. Interplay of Electronic, Environmental, and Vibrational Effects in Determining the Hyperfine Coupling Constants of Organic Free Radicals. Chem. Rev. 2004, 104, 1231–1254. (33) Owenius, R.; Engstrom, M.; Lindgren, M.; Huber, M. Influence of Solvent Polarity and Hydrogen Bonding on the EPR Parameters of a Nitroxide Spin Label Studied by 9-GHz and 95-GHz EPR Spectroscopy and DFT Calculations J. Phys. Chem. A 2001, 105, 10967-10977. (34) Pavone, M.; Cimino, P.; De Angelis, F.; Barone, V. Interplay of Stereoelectronic and Enviromental Effects in Tuning the Structural and Magnetic Properties of a Prototypical Spin Probe: Further Insights from a First Principle Dynamical Approach. J. Am. Chem. Soc. 2006, 128, 4338–4347. (35) Pavone, M.; Sillanpää, A.; Cimino, P.; Crescenzi, O.; Barone, V. Evidence of Variable H-Bond Network for Nitroxide Radicals in Protic Solvents J. Phys. Chem. B 2006, 110, 6189–16192. (36) Pavone, M.; Biczysko, M.; Rega, N.; Barone, V. Magnetic Properties of Nitroxide Spin Probes: Reliable Account of Molecular Motions and Nonspecific Solvent Effects by Time-Dependent and Time-Independent Approaches. J. Phys. Chem. B 2010, 114, 11509–11514. (37) Zamora, P. L.; Rockenbauer, A.; Villamena, F. A. Radical Model of Arsenic(III) Toxicity: Theoretical and EPR Spin Trapping Studies. Chem. Res. Toxicol. 2014, 27, 765−774. (38) Kim, S.-U; Villamena, F. A. Reactivities of Superoxide and Hydroperoxyl Radicals with Disubstituted Cyclic Nitrones: A DFT Study. J. Phys. Chem. A 2012, 116, 886–898. (39) Lin, T.-J.; O’Malley, P. J. An ONIOM Study of the Spin Density Distribution of the QA Site Plastosemiquinone in the Photosystem II Reaction Center. J. Phys. Chem. B 2011, 115, 4227–4233. (40) Lin, T.-J.; O’Malley, P. J. Binding Site Influence on the Electronic Structure and Electron Paramagnetic Resonance Properties of the Phyllosemiquinone Free Radical of Photosystem I. J. Phys. Chem. B, 2011, 115, 9311–9319. (41) Witwicki, M. Theoretical Characterisation of Phosphinyl Radicals and Their Magnetic Properties: g Matrix. ChemPhysChem 2015, 16, 1912-1925. (42) Hermosilla, L.; de la Vega, J. L. M.; Sieiro, C.; Calle, P. DFT Calculations of Isotropic Hyperfine Coupling Constants of Nitrogen Aromatic Radicals: The Challenge of Nitroxide Radicals. J. Chem. Theory Comput. 2011, 7, 169–179. (43) Hermosilla, L.; Calle, P.; de la Vega, J. M. G.; Sieiro, C. Density Functional Theory Study of 14N Isotropic Hyperfine Coupling Constants of Organic Radicals. J. Phys. Chem. A 2006, 110, 13600-13608. (44) Mattar, S. M. Accurate Calculation of the Phenyl Radical's Magnetic Inequivalency, Relative Orientations of Its Spin Hamiltonian Tensors, and Its Electronic Spectrum. J. Phys. Chem. A 2007, 111, 251-260. (45) Solc, R.; Gerzabek, M. H.; Lischka, H.; Tunega, D. Radical Sites in Humic Acids: A Theoretical Study on Protocatechuic and Gallic Acids. Comput. Theor. Chem. 2014, 1032, 42-49. (46) Severino, J. F.; Goodman, B. A.; Kay, C. W. M.; Stolze, K.; Tunega, D.; Reichenauer, T. G.; Pirker, K. F. Free Radicals Generated During Oxidation of Green Tea Polyphenols: Electron Paramagnetic Resonance Spectroscopy Combined with Density Functional Theory Calculations. Free Radical Biol. Med. 2009, 46, 1076-1088. (47) Li, X.; Rinkevicius, Z.; Kongsted, J.; Murugan, N. A.; Ågren, H. Binding Mechanism and Magnetic Properties of a Multifunctional Spin Label for Targeted EPR Imaging of Amyloid Proteins: Insight from Atomistic Simulations and FirstPrinciples Calculations J. Chem. Theory Comput. 2012, 8, 4766–4774. (48) Becke, D. Density-functional thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648-5652. (49) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785-789. (50) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98, 11623-11627. (51) Neese, F.; Wennmohs, F.; Hansen, A.; Becker, U. Efficient, Approximate and Parallel Hartree–Fock and Hybrid DFT Calculations. A ‘Chain-of-Spheres’ Algorithm for the Hartree–Fock Exchange. Chem. Phys. 2009, 356, 98-109. (52) Schaefer, A.; Horn, H.; Ahlrichs, R. Fully Optimized Contracted Gaussian Basis Sets for Atoms Li to Kr. J. Chem. Phys. 1992, 97, 2571-2577. (53) Weigend, F. Accurate Coulomb-Fitting Basis Sets for H to Rn. Phys. Chem. Chem. Phys. 2006, 8, 1057–1065. (54) Munzarová, M. L. in Kaupp, M.; Bühl, M.; Malkin, V. G. Calculation of NMR and EPR Parameters. Theory and Applications. Wiley-VCH, Weinheim, 2004. (55) Barone, V. in Chong, D. P. Recent Advances in Density Functional Theory, World Scientific, Singapore, 1995. (56) Neese, F. Theoretical Study of Ligand Superhyperfine Structure. Application to Cu(II) Complexes. J. Phys. Chem. A 2001, 105, 4290-4299. (57) Becke, A. D. Density-Functional Exchange Energy Approximation with Correct Asymptotic-Behavior. Phys. Rev. A 1988, 38, 3098–3100. (58) Perdew, J. P. Density Functional Approximation for the Correlation Energy of the Inhomogeneous Electron Gas. Phys. Rev. B 1986, 33, 8822-8824. (59) Tao, J. M.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Climbing the Density Functional Ladder: Nonempirical Meta–Generalized Gradient Approximation Designed for Molecules and Solids. Phys. Rev. Lett. 2003, 91, 146401-146404. (60) Grimme, S. Semiempirical Hybrid Density Functional with Perturbative Second-Order Correlation. J. Chem. Phys. 2006, 124, 34108-34124.

– 20 –


Page 20 of 24

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(61) Neese, F.; Schwabe, T.; Grimme, S. Analytic Derivatives for Perturbatively Corrected “Double Hybrid” Density Functionals: Theory, Implementation, and Applications. J. Chem. Phys. 2007, 126, 124115-124130. (62) Kossmann, S.; Kirchner, B.; Neese, F. Performance of Modern Density Functional Theory for the Prediction of Hyperfine Structure: meta-GGA and Double Hybrid Functionals. Mol. Phys. 2007, 105, 2049-2071. (63) Vahtras, O.; Almlof, J.; Feyereisen, M. W. Integral Approximations for LCAO-SCF Calculations. Chem. Phys. Lett. 1993, 213, 514-518. (64) Neese, F., Schwabe T., Kossmann S., Schirmer B., Grimme S. Assessment of Orbital-Optimized, Spin-Component Scaled Second-Order Many-Body Perturbation Theory for Thermochemistry and Kinetics. J. Chem. Theory Comput. 2009, 5, 3060-3073. (65) Kossmann, S.; Neese, F. Correlated ab Initio Spin Densities for Larger Molecules: Orbital-Optimized SpinComponent-Scaled MP2 Method. J. Phys. Chem. A 2010, 114, 11768-11781. (66) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. Quadratic Configuration Interaction. A General Technique for Determining Electron Correlation Energies. J. Chem. Phys. 1987, 87, 5968-5975. (67) Weigend, F.; Häser, M.; Patzelt, H.; Ahlrichs, R. RI-MP2: Optimized Auxiliary Basis Sets and Demonstration of Efficiency. Chem. Phys. Lett. 1998, 294, 143-152. (68) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, Revision B.1; Gaussian, Inc.: Wallingford. CT, 2009 (www.gaussian.com, access date: 20 May 2015). (69) Neese, F. The ORCA Program System. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2012, 2, 73-78. (70) Sinnecker, S.; Rajendran, A.; Klamt, A.; Diedenhofen, M.; Neese, F. Calculation of Solvent Shifts on Electronic gTensors with the Conductor-Like Screening Model (COSMO) and Its Self-Consistent Generalization to Real Solvents (Direct COSMO-RS) J. Phys. Chem. A 2006, 110, 2235-2245. (71) Klamt, A.; Schüürmann, G. COSMO: A New Approach to Dielectric Screening in Solvents with Explicit Expressions for the Screening Energy and Its Gradient. J. Chem. Soc. Perkin Trans. 2 1993, 799-805. (72) Cancès, E.; Mennucci, B.; Tomasi, J. A New Integral Equation Formalism for the Polarizable Continuum Model: Theoretical Background and Applications to Isotropic and Anisotropic Dielectrics. J. Chem. Phys. 1997, 107, 3032-3041. (73) Tomasi, J; Mennucci, B.; Cancès, E. The IEF Version of the PCM Solvation Method: An Overview of a New Method Addressed to Study Molecular Solutes at the QM ab Initio Level. J. Mol. Struct. (Theochem) 1999, 464, 211−226. (74) Atkins, P. W.; Symons, M. C. R. 936. Oxides and Oxy-Ions of the non-Metals. Part IV. Nitrogen Derivatives. J. Chem. Soc. 1962, 4794-4797. (75) Witwicki, M.; Jezierska, J. Protonated o-Semiquinone Radical as a Mimetic of the Humic Acids Native Radicals: A DFT Approach to the Molecular Structure and EPR Properties. Geochim. Cosmochim. Acta 2012, 86, 384-391. (76) Witwicki, M.; Jerzykiewicz, M.; Ozarowski, A. Understanding Natural Semiquinone Radicals – Multifrequency EPR and Relativistic DFT Studies of the Structure of Hg(II) Complexes. Chemosphere 2015, 119, 479-484. (77) Benisvy, L.; Bittl, R.; Bothe, E.; Garner, C.D.; McMaster, J.; Ross, S.; Teutloff, C.; Neese, F. Phenoxyl Radicals Hydrogen-Bonded to Imidazolium: Analogues of Tyrosyl D• of Photosystem II: High-Field EPR and DFT Studies. Angew. Chem. Int. Ed. 2005, 117, 5448–5451. (78) Jerzykiewicz, M.; Witwicki, M.; Jezierska, J. pH-Dependent Formation of Hg(II)-Semiquinone Complexes from Natural Phenols. Chemosphere 2015, 138, 233–238. (79) Neese, F. A Critical Evaluation of DFT, Including Time-Dependent DFT, Applied to Bioinorganic Chemistry. J. Biol. Inorg. Chem. 2006, 11, 702–711. (80) Weil, J. A.; Bolton, J. R. Electron Paramagnetic Resonance. Elementary Theory and Practical Applications. (2nd Edition), Wiley-Interscience, Hoboken, 2007. (81) Bolton, J. R.; Carrington, A.; Todd, P. F. An Electron Resonance Study of Rotational Isomerism in the Naphthazarin Semiquinone Cation. Mol. Phys. 1963, 6, 169-177. (82) Maki, A.H. Rotational Isomerism in Terephthalaldehyde Anion. J. Chem. Phys. 1961, 35, 761-762. (83) Brustolon, M.; Zoleo, A. Agostini, G.; Maggini, M. Radical Anions of Mono-and bis-Fulleropyrrolidines: An EPR Study. J. Phys. Chem A 1998, 102, 6331-6339. (84) Windle, J. J.; Kuhnle, J. A.; Beck, B. H. ESR Study of Interconversion in Substituted Piperidine Iminoxyls. J. Chem. Phys. 1969, 50, 2630. (85) Janovsky, I.; Knolle, W.; Naumov, S.; Williams, F. EPR Studies of Amine Radical Cations, Part 1: Thermal and Photoinduced Rearrangements of n-Alkylamine Radical Cations to their Distonic Forms in Low-Temperature Freon Matrices. Chem. Eur. J. 2004, 10, 5524–5534. (86) Neese, F. Prediction of Molecular Properties and Molecular Spectroscopy with Density Functional Theory: From Fundamental Theory to Exchange-Coupling. Coord. Chem. Rev. 2009, 253, 526-563. (87) Grimme, S. Seemingly Simple Stereoelectronic Effects in Alkane Isomers and the Implications for Kohn–Sham Density Functional Theory. Angew. Chem. Int. Ed. 2006, 45, 4460-4464. (88) Chipman, D. M. The Spin Polarization Model for Hyperfine Coupling Constants. Theor. Chim. Acta. 1992, 82, 93115. (89) Jaszewski, A.; Jezierska, J. An ab Initio Approach to the Structure and EPR Parameters of Formaldiminoxy Radical. Chem. Phys. Lett. 2001, 342, 239-248. (90) Mattar, S. M.; Emwas, A. H.; Stephens, A. D. Accurate Computations of the Methyl Isotropic Hyperfine Coupling Constants in 2-Methyl-1, 4-Benzosemiquinone Radical Intermediate. Chem. Phys. Lett. 2002, 363, 152-160. (91) Kurlancheek, W.; Head-Gordon, M. Violations of N-Representability from Spin-Unrestricted Orbitals in Møller– Plesset Perturbation Theory and Related Double-Hybrid Density Functional Theory. Mol. Phys. 2009, 107, 1223-1232.

– 21 –



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(92) Knauer B. R.; Napier, J. J. The Nitrogen Hyperfine Splitting Constant of the Nitroxide Functional Group as a Solvent Polarity Parameter. The Relative Importance for a Solvent Polarity Parameter of Its Being a Cybotactic Probe vs. Its Being a Model Process. J. Am. Chem. Soc. 1976, 98, 4395-4400.

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Toward an Understanding of the Ambiguous Electron Paramagnetic

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