Effect of substituents on spin density in benzimidazole nitronyl nitroxide radicals studied by electron spin resonance

Article abstract of DOI:10.1002/mrc.2443

Several novel benzimidazole-3-oxide-1-oxyl radicals with substituents at 5 and/or 6 position were synthesized. The ESR analysis of nitrogen hyperfine coupling constants (hfccs) revealed that substituents at 5 and 6-position affect the spin density to greater extent than substituents on the phenyl ring at 2-position. Density functional theory calculations of nitrogen hfccs were performed using several different Pople type basis sets, as well as double and triple zeta quality individual gauge for localized orbital (IGLO-II, IGLO-III) and electron paramagnetic resonance (EPR-II, EPR-II) basis sets. Experimental and theoretical hfccs are compared.

Full text of DOI:10.1002/mrc.2443

Research Article
Received: 22 September 2008
Revised: 27 March 2009
Accepted: 2 April 2009
Published online in Wiley Interscience: 11 May 2009
(www.interscience.com) DOI 10.1002/mrc.2443
Effect of substituents on spin density in
benzimidazole nitronyl nitroxide radicals
studied by electron spin resonance
Burak Esat,a∗ Ismail Fidan,b Sumeyye Bahceci,a Yusuf Yerlic and Levent Sarid
Several novel benzimidazole-3-oxide-1-oxyl radicals with substituents at 5 and/or 6 position were synthesized. The ESR analysis
of nitrogen hyperfine coupling constants (hfccs) revealed that substituents at 5 and 6-position affect the spin density to
greater extent than substituents on the phenyl ring at 2-position. Density functional theory calculations of nitrogen hfccs were
performed using several different Pople type basis sets, as well as double and triple zeta quality individual gauge for localized
orbital (IGLO-II, IGLO-III) and electron paramagnetic resonance (EPR-II, EPR-II) basis sets. Experimental and theoretical hfccs are
c
compared. Copyright ꢀ 2009 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: ESR; 14N; Benzimidazole-3-oxide-1-oxyl; benzimidazole nitronyl nitroxide; spin density; DFT
Introduction
(BNN) type radicals (Fig. 1). BNN radicals are analogues of
nitronyl nitroxide radicals (NNs) which are known for their high
stability and for easy modification of their properties through
different substituents at the 2-position.[4] However, bulkiness
of the tetramethyl-substituted imidazoline ring in NNs leads to
decreased close-packing of molecules in the solid phase. This
causes an undesirable low spin density and weak intermolecular
magnetic interactions in the bulk of the material. BNNs offer to
overcome, at least partly, these problems by providing a more
planar structure for better close-packing and by delocalization of
the spin density over a greater area that may lead to stronger and
more effective intermolecular magnetic interactions as compared
with NNs having spin density squeezed into the imidazoline
ring only.[5] Furthermore, their substitution at benzimidazole
benzenoid ring offers additional chance for modification of
structural and magnetic properties compared with NNs.
BNNs, although were first synthesized about 40 years ago,[6]
havenotbeeninvestigatedfortheirmagneto-structuralproperties
until recently.[7–10] Detailed structural studies on the representa-
tives of these molecules, indeed, showed better close-packing of
theradicalmoleculesandshorterintermolecularcontactdistances
between the ONCNO moieties, where the majority of the spin den-
sity resides, as compared with nitronyl nitroxides in general.[8,9]
The use of organic molecular building blocks for magnetic ma-
terials is one of the several strategies to achieve purely organic
magnetic materials that has been adopted by many research
groups worldwide. Many different types of spin-bearing organic
stable free radicals, such as phenoxyl, galvinoxyl, verdazyls, nitrox-
ide, dithiazoyl and nitronyl nitroxide radicals, have been proposed
as building blocks for organic ferromagnetic materials. However,
a purely organic ferromagnetic material of practical and com-
mercial value has not yet been achieved by using this strategy.
The main reason for this is the failure in establishment of a
viable methodology for designing ferromagnetic intermolecu-
lar exchange coupling, although the conditions for such coupling
were first described by McConnell in 1963.[1] Studies have revealed
the importance of intramolecular spin density distribution, close
contacts and relative orientations of molecules in the solid state.[2]
Ferromagnetic interactions have to be favored among radical
molecules in their crystal packing to attain bulk ferromagnetic be-
havior. Relativeorientationandclosecontactscanbecontrolledby
utilizing crystal engineering strategies, through hydrogen bond-
ing and van der Waals contacts. This strategy has been proved to
be successful in a few cases where ferromagnetic interactions and
materials could have been achieved.[3] However, the critical tem-
perature (Curie temperature, Tc) below which the materials show
spontaneous magnetization was very low (<50 K). This showed
that intermolecular magnetic interactions mediated via hydrogen
bonds and van der Waals interactions are quite weak. The strength
of intermolecular interactions also depends on the spin densities
on the interacting sites on the neighboring radical molecules.
Therefore, factors that affect the spin density distribution in the
molecule should also affect the strength of magnetic interac-
tions. These factors include the substituent effects and degree of
delocalization of spin density over the molecule via conjugation.
In this study, we aim to investigate the effects of substituents
on spin density distribution in benzimidazole nitronyl nitroxide
∗
Correspondence to: Burak Esat, Department of Chemistry, Fatih University,
Buyukcekmece, 34500 Istanbul, Turkey. E-mail: besat@fatih.edu.tr
a
b
c
Department of Chemistry, Fatih University, Buyukcekmece, 34500 Istanbul,
Turkey
Department of Chemistry, GebzeInstituteof Technology, Gebze, 41400 Kocaeli,
Turkey
Department of Physics, Gebze Institute of Technology, Gebze, 41400 Kocaeli,
Turkey
d
DepartmentofPhysics, FatihUniversity, Buyukcekmece, 34500Istanbul, Turkey
c
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B. Esat et al.
Theoretical calculations were done using Gaussian 03 program
running parallel with Linda on our Linux cluster. All calculations
were done by density functional theory (DFT) method at the
UB3LYP level of theory, employing both Pople type basis sets
and a couple of special basis sets designed for magnetic property
calculations. The Pople type basis sets that we used are, in the
order of increasing basis set size, 6-31G, 6-31+G, 6-31+G(d,p)
and 6-311+G(d,p). The special basis sets used are double and
triple zeta quality individual gauge for localized orbital (IGLO-II,
IGLO-III) basis sets and double and triple zeta quality electron
paramagnetic resonance (EPR-II, EPR-III) basis sets. These basis
sets describe the nuclear region much better than the Pople type
basis sets, as this is very important to get a reliable spin density
at the nucleus. The IGLO-II and IGLO-III basis sets are developed
by Kutzelnigg and co-workers,[11] whereas the EPR-II and EPR-III
basis sets are developed by V. Barone.[12] The EPR-II and EPR-III
basis sets are specially optimized for the calculation of hfccs at
the DFT level of theory, having very tight s functions. EPR-II is a
double zeta basis set, whereas EPR-III is at the triple-zeta quality
including double d-polarizations, a set of f-polarization functions
and diffuse functions.
Figure 1. Structures of benzimidazole nitronyl nitroxides and nitronyl
nitroxides.
Molecular dimer formation through π –π stacking appears as
more general structural pattern together with occasional nearly
orthogonal arrangement of molecules between adjacent dimers
in the solid phase of BNN radicals. Also, more extensive spin den-
sity delocalization was evidenced by the decrease in the electron
spin resonance (ESR) nitrogen hyperfine coupling constant (hfcc;
which is proportional to the spin density on the atom responsible
for hyperfine splitting) from around 7.00 G in NNs to about 4.50 G
in BNNs.[7–10] In spite of the fact that intradimeric intermolecular
contacts lead to the stronger magnetic interactions within the
bulk of material in BNN, interdimer magnetic interactions have
been observed to be weak and overall magnetic interactions are
generally weakly antiferromagnetic.[7–10]
First of all, geometry optimizations were done at each basis
set. Once the optimized geometries were obtained, single point
calculations were carried out with the property calculations to get
the isotropic Fermi contact couplings. The results are tabulated in
Table 2.
In this study, we synthesized BNN radicals substituted at 5-
and/or 6-position of benzimidazole ring (on benzimidazole ben-
zenoid ring) and investigated the effect of this kind of substitution
on spin density distribution in comparison to such effects of
substituents on a phenyl ring at 2-position. Increase or decrease in
spin density on the benzenoid ring and the associated decrease or
increase in the spin density on the ONCNO moiety will most likely
have a large impact on the intermolecular magnetic interactions.
Syntheses
We accomplished the synthesis of nitrones(2),[13] substituted
benzofuroxanes (BFO)(6),[14,15] 2-aryl-1-hydroxy-benzimidazole-
3-oxides
(HBNN)(8)
and
benzimidazole-3-oxide-1-oxyls
(BNN)(9)[16,17] according to literature procedures as summarized
in Figs. 2, 3 and 4. General procedures are provided below. More
detailed information is provided in supplementary information.
Synthesized phenyl azides showed characteristic azide infrared
(IR) bands in 2200–2100 cm−1 region. Synthesized BFOs showed
characteristicIRbandsin1625–1600 cm−1 region(forC N+ –O−)
and in 1480–1410 cm−1 region (for N+(O)–O−).[18] The number
Experimental
Materials and Methods
1
of H NMR peaks in the aromatic region of BFOs were generally
All reagent chemicals and solvents were purchased and used as
supplied without further purification unless otherwise is stated.
All reactions are performed under dry nitrogen atmosphere.
Nuclear magnetic resonance (NMR) spectra were recorded on a
Varian UNITY – INOVA 500MHz NMR spectrophotometer. Fourier
transform infrared spectra were obtained on a PERKIN ELMER BX-
II spectrophotometer. Mass spectrometric measurements were
recorded on a MICROMASS ULTIMA API spectrometer. ESR
studies were done on a BRUKER EMX series spectrometer
designed for measurements in the X-band (9.5 GHz). ESR nitrogen
hyperfine values were obtained via simulations performed using
WinSim2002 program by Bruker.
broad. This was attributed to the presence of the two resonance
forms of BFOs. Synthesized nitrones showed characteristic singlet
H–C N–proton peaks in 8.00–8.50 ppm region of their 1H NMR
spectra. They also showed characteristic C N+ –O− peaks in
1600–1575 cm−1 region in their IR spectra.
N-Phenyl-aldehyde oximes (N,α-Diarylnitrones) (2)
The nitrone was synthesized from N-phenylhydroxylamine and
aldehyde by refluxing in EtOH under nitrogen as described in the
literature.[13,19,20]
Figure 2. Synthesis of nitrones (2).
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Effect of substituents on spin density in BNNs
Figure 3. Synthesis of benzofuroxanes (6).
Figure 4. Synthesis of Benzimidazole nitronyl nitroxides (9).
2-Nitro-aryl azides (5)
to violet, and then to orange-yellow. A yellow-colored solid
was collected by filtration and washed with ice-cold water.
Drying gave a yellow solid.[23,24]
The corresponding 2-nitroaniline derivative was dissolved in
aqueous(aq.)HClandtreatedslowlywithaq. NaNO2 whilekeeping
the temperature of the mixture in 0–5 ◦C range. After stirring for
1 h at this temperature and filtration of the precipitate, aq. NaN3
was added slowly to this solution in an ice-bath. The solid formed
was collected by filtration. The solid was recrystallized to yield the
solid-substituted aryl azide.[21]
HBNN (8)[16,17]
Only 4-Py-HBNN was prepared by the condensation of
o-benzoquinone dioxime with an aldehyde in the presence
of HBr as catalyst in EtOH solution under reflux conditions
and inert atmosphere as stated in the literature (Fig. 4). The
others were synthesized using the nitrone method shown
(Fig. 4).
BFOs (6)
(a) Substituted-2-nitrophenyl azide was refluxed in toluene
under nitrogen until N2(g) evolution ceased to yield a yellow
solid after cooling.[21,22]
BNN (9)
(b) 5-Substituted-2-nitroaniline was dissolved in 7.5% ethanolic
KOH solution. The solution was cooled to 0 ◦C while
mechanically stirred. At this temperature, 6–14% aq. NaOCl
was added slowly. The solution changed from red color, first
All BNN radicals were obtained by PbO2 oxidation of CHCl3
suspension of HBNNs (8) for 30 min. The resultant mixture was
filtered to get rid of excess PbO2 and unreacted HBNNs to yield
generally yellow, orange, red or green CHCl3 solutions.
c
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B. Esat et al.
Figure 5. The experimental(top) and the simulated electron spin resonance spectra of 9c.
Results and Discussion
ically equivalent nitrogen atoms (Fig. 5). Although substitution at
5-position seems to generate a chemical nonequivalence in the
two benzimidazole nitrogens, this nonequivalence is very small,
possibly due to the presence of two resonance structures render-
ing unpaired electron densities on these two nuclei almost equal
(see Fig. 6), as suggested by the experimental observations and
theoretical calculations of hfccs and spin densities (see Tables 1
and 2). A similar near equivalence case of these two nitrogens
is suggested by the observation of a pentet pattern, instead of
a triplet-of-triplet pattern, in the ESR spectra of the BNNs substi-
tuted with unsymmetrical aryl groups at 2-position.[9,10] The ESR
spectrum of 5-OCH3-substituted 2-4ꢁ-pyridyl-BNN shows some
unresolved splitting (Fig. 5) possibly due to hydrogen hfccs and
2-4ꢁ-pyridyl group nitrogen atom. A better resolution of these hy-
In general, HBNNs (8), the precursors to BNN radicals (9), are
synthesized from the reaction of nitrones (2) and BFOs (6).[16]
Nitrones were obtained via condensation of aldehydes with
N-phenylhydroxylamine (1).[13] BFOs were prepared from ortho-
nitroanilines (3) either through the azide route (Fig. 3) or directly
via reaction of ortho-nitroanilines with basic aq. or ethanolic
hypochlorite solution (Fig. 3).[14,15] The diazonium salts (4) were
never isolated, but immediately converted to azides (5) in situ.
The BNNs were generally quite stable in CHCl3 solutions. ESR
spectra of these solutions showed characteristic five-line hyper-
fine pattern with the relative intensities of 1 : 2 : 3 : 2 : 1 and with
nitrogen hfcc (AN) of about 4.00–4.50 G due to two nearly chem-
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Effect of substituents on spin density in BNNs
Figure 6. Resonance forms of benzimidazole nitronyl nitroxides.
Table 1. Summary of experimental ESR analysis results of 2-Aryl benzimidazole nitronyl nitroxide CHCl3 solutions (9)
O
9a: R=H, Ar=Ph
9b:R=H, Ar=4-Pyridyl
9c; R= OCH3, Ar=4-Pyridyl
9d: R=F, Ar=4-Pyridyl
9e: R= NO2, Ar=4-Pyridyl
4
7
9k: R= NO2, Ar=Ph
9l : R= 5,6-DiCH3, Ar=Ph
9f : R=Cl, Ar=4-Pyridyl
9g: R= CH3, Ar=4-Pyridyl
9h: R= 5,6-diCH3, Ar=4-Pyridyl
9i: R= OCH3, Ar=Ph
3
R
5
N
Ar
2
6
9j: R= F, Ar=Ph
N
1
O
2-Aryl-BNN (9)
Mulliken spin
A
N (mT)
Exp.
Spin
density (ρN)
Percentage of spin
density changea
density (ρN)b
N(1) – N(3)/Average
Entry
R =
Ar =
1
H
H
4-Pyridyl
Ph
0.436
0.2068
0.2133
0.2099
0.2156
0.2056
0.2087
0.2103
0.1997
0.1863
0.1924
0.2118
0.2175
0.2085
0.2137
0.2166
−3.05
–
0.2109–0.2109/0.2109
2
0.450
–
3
5-OCH3
5-OCH3
5-F
4-Pyridyl
Ph
0.443
−1.59
+1.08
−3.61
−2.16
−1.41
−6.38
−12.66
−9.80
−0.70
+1.97
2.25
0.2156–0.2168/0.2162
4
0.455
–
5
4-Pyridyl
Ph
0.434
0.2098–0.2094/0.2096
6
5-F
0.440
–
7
5-CH3
5-Cl
4-Pyridyl
4-Pyridyl
4-Pyridyl
Ph
0.444
0.2141–0.2146/0.2144
0.2089–0.2085/0.2087
0.1977–0.1991/0.1984
–
8
0.421
9
5-NO2
5-NO2
5,6-CH3
5,6-CH3
H-
0.393
10
11
12
13
14
15
0.406
4-Pyridyl
Ph
0.447
0.2141–0.2146/0.2144
–
0.462
4-NO2-Ph-
4-OCH3-Ph-
4-N(CH3)2-Ph-
0.440[32]
0.451[32]
0.457[32]
H-
+0.19
+1.55
–
–
H-
[ρN (R = X, Ar = Y) − ρN (R = H, Ar = Ph)]
ρN (R = H, Ar = Ph)
a % SDC =
× 100
b Calculated Mulliken atomic spin densities at the B3LYP/IGLO-III level of theory. The values separated by a dash mark are spin densities at
benzimidazole nitronyl nitroxide nitrogens (i.e. N(1) and N(3) atoms). The values after the slash mark are the averages of these two values.
perfinecouplingscouldnotbeobtaineddueeithertofactthatthey
arequitelargeandoverlappingortothelowinstrumentsensitivity.
The results of ESR studies are summarized in Table 1. The AN
values are expected to show a margin of error in the range of
0.05G ( 0.005 mT) within the reported values as suggested
by ESR studies performed on radical samples with different
concentrations and different signal broadening. The nitrogen
hfcc (AN) is known to vary linearly with the spin density according
to Eqn (1), where QNN (a constant specific for BNN nitrogen) is 2.11
and ρN is the spin density.[7]
atoms for different BNN radicals were also calculated using Eqn (2)
in reference to the spin density on the nitrogen atom of 2-Ph-BNN
radical (9a) with R = H, Ar = Ph (Entry 2 in Table 1). These results
are tabulated in Table 1 as well. The calculated spin densities and
% SDC values reflect the effect of substituents on the spin density
on the ONCNO group. The negative and positive values for % SDC
denote either a decrease or increase, respectively in spin density
on nitrogen atoms of BNN molecules compared with spin density
on the nitrogen atoms of 2-Ph-BNN (9a).
[ρN(R = X, Ar = Y) − ρN(R = H, Ar = Ph)]
% SDC =
× 100 (2)
AN(mT) = QNNρN
(1)
ρN(R = H, Ar = Ph)
The following conclusions can be drawn by analyzing the data in
Table 1. The negative % SDC values and lower AN values observed
with respect to those of 9a indicate the spin density reducing
effects of 2-4ꢁ-pyridyl-, 5-fluoro, 5-chloro and 5-nitro groups as
Spin densities on the nitrogen atoms of the spin-bearing
ONCNO moiety were calculated for each BNN radical using the
experimentally obtained AN values and literature QNN values.
Percentage of spin density change (% SDC) on these nitrogen
c
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B. Esat et al.
c
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Effect of substituents on spin density in BNNs
expected. Aryl groups at 2-position carrying electron-withdrawing
substituents have smaller effect on the spin density on the
ONCNO nitrogen atom compared with electron-withdrawing sub-
stituents on the benzimidazole benzenoid ring. Strongly, electron-
withdrawing –NO2 group on the benzenoid ring of benzimidazole
ring (entry 10 in Table1) decreases the spin density on nitrogen by
about 10% compared with only a 2% decrease in spin density ob-
served for 4ꢁ-NO2-phenyl group at 2-position (entry 13 in Table 1).
This is an expected result since, as seen in the literature, varying
the 2-aryl group was shown to have little effect on the spin density
and nitrogen hyperfine values.[9,10] Only about 2.5% change in
nitrogen hyperfine was reported when 2-phenyl ring was replaced
by a 2-4ꢁ-pyrdiyl or 2-2ꢁ-pyridyl group,[10] or about 1–2% change
in nitrogen hyperfine when the substituents changed from 2ꢁ,6ꢁ-
difluorophenyl to pentafluorophenyl or to 3ꢁ-cyanophenyl, and so
on.[9] We have also shown that 4ꢁ-pyridyl substitution is about as
effective as 4ꢁ-NO2-phenyl substitution at 2-position in influencing
the spin density on nitrogen atoms. The fluoro group at 5-position
was found to be less influential in decreasing the spin density on
the nitrogen atoms than the chloro group at 5-position (entries
5 and 8 in comparison to entry 1 in Table1) as evidenced by
the experimental AN values obtained for 5-fluoro-2-4ꢁ-pyridyl BNN
(9d)(0.434 mT) and for 5-chloro-2-4ꢁ-pyridyl BNN (9f) (0.421 mT).
About 3% smaller AN value for 5-chloro derivative (9f) compared
with the AN value for 5-fluoro derivative (9d) indicates a smaller
spin density on the nitrogen atoms of 9f. These findings are in
accord with the calculated average AN values (isotropic Fermi
contact couplings) obtained from DFT calculations on molecules
9d, 9f using Gaussian 03 program (Table 2).[25] Similar differences
in hyperfine values were observed in the literature for fluorine
and chlorine substitution in BNN radicals and substituted benzyl
radicals.[9,26] However, this is contrary to what is expected qualita-
tively from chloro and fluoro substitution since fluorine, as being
a more electronegative atom compared with chlorine, should
decrease the AN and spin density more than chlorine does. This
unexpected result may be attributed to the stronger interaction
of larger chlorine atom orbitals with the π-orbital system of the
benzimidazole ring compared with smaller fluorine atom orbitals.
Furthermore, chlorine atom has extra third-quantum shell orbitals
to accommodate more of the delocalized spin that are lacked by
fluorine atom. The chloro group was experimentally found to be
about one third effective in lowering spin density as compared
with the nitro group when substituted on the 5-position of the
benzenoid ring (compare entries 8 and 9 in Table 1).
The experimental data show that OCH3 group increases the spin
density only weakly. The nitrogen hyperfine value of 5-OCH3 BNNs
(9c and 9i) was determined to be slightly larger than the related
structures 9a and 9b. The comparison of % SDC values in entries 4
and 14 in Table 1 shows that the effect of OCH3 group is also more
pronounced if it is substituted at 5-position (although to a very
small extent) rather than on the phenyl ring at 2-position. The 5-
OCH3 groupiselectron-donatingthroughresonanceandexpected
to lead to a larger nitrogen hyperfine value than 5-H group. How-
ever, its effect on spin density on BNN nitrogen and hfcc was mea-
suredtobenotaslargeasexpected(slightlylessthanthatofweakly
electron-donating CH3 group). This may be due to high elec-
tronegativity of the oxygen and the related electron-withdrawing
inductiveeffectoftheOCH3 groupcounteractingandoutweighing
the resonance effects when it is situated at 5-position of BNNs.
CH3 substituent at 5-position increases the spin density on
nitrogen atoms by 1.64% (compare entries 1 and 5 in Table 1).
Electron-donating 5,6-dimethyl substitution (entry 12 in Table 1)
Figure 7. N-hyperfine versus σp for 5-substituted 2-4ꢁ-pyridyl-BNNs.
Figure 8. N-hyperfine versus σN (dot) for 5-substituted 2-4ꢁ-pyridyl-BNNs.
was observed to be about 27% more effective in increasing the
spin density compared with dimethylamino group (a powerful
electron-donating group) substituted at para position of a phenyl
group at 2-position (entry 15 in Table 1).
We have also investigated the correlation between nitrogen
hfccs and the usual Hammett substituent parameters (σp, σm, σ−
and σ+). Among these, σp parameters gave the best correlation
(Fig. 7). However, the substituent effects on nitrogen hfccs were
observed to be too small. A better correlation (in fact, a perfect
fit) has been found if a new parameter (similar to the ones used
to express substituent effects on benzyl radicals[25]) has been
utilized. These new parameters (σN·) for different substituents at
5-position of 2-4ꢁ-pyridyl BNN and 2-phenyl BNN radicals were
calculated according to Eqn (3) below. These are tabulated against
regular Hammett substituent parameters and those calculated for
substituted benzyl radicals[26] in Table 3.
hfc . . . of . . . 5 − X − substituted
σN· = 1 −
(3)
hfc . . . of . . . 5 − H − substituted
Considering a correlation between nitrogen hyperfines and
Hammet parameters (or alike) would be in order because hfcc
values as well can be attributed to relative stability as the kinetic
rate constants or equilibrium constants used in ordinary Hammett
c
Magn. Reson. Chem. 2009, 47, 641–650
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B. Esat et al.
Table 3. Substituent parameters for benzimidazole nitronyl nitroxide (BNNs) and other Hammett parameters
a
b
[26]
[26]
c
c
Substituent
σN·
σN·
σp
σm
σp
σm
σp+
σp−
σm+
5,6-di CH3
5-CH3
5-OCH3
5-H
−0.025
−0.017
−0.015
0.000
−0.025
–
0.015
0.034
−0.001
−0,171
−0,268
0,000
−0,066
0,115
0,000
0,337
0,373
0,710
−0,311
−0,778
0,000
−0,150
−0,130
0,000
−0,066
0,047
0,000
0,352
0,399
0,674
−0.011
0.000
0.022
–
0.000
0.000
−0.018
−0.001
5-F
0.006
−0.011
0.017
0,062
−0,073
0,114
0,020
5-Cl
0.034
0,227
0,190
5-NO2
0.099
0.098
0,778
1,240
1,240
a For 2-4ꢁ-pyridyl-BNN.
b For 2-Phenyl-BNN.
c Subscripts p and m denote substituent positions as para and meta, respectively. Values are taken from Refs [26,27].
studies of substituent effects on 13C NMR shifts, where measured
chargedelocalizationandcarbocationstabilitywereassociated.[27]
According to Eqn (3), σN·value for 5-H is zero. Substituents that
decrease spin density have positive values and substituents
that increase spin density have negative values. The correlation
betweenthesenewσN·valueswithnitrogenhfccsisshowninFigs 8
and 9. A new correlation equation similar to regular Hammett
correlations can be set up as given in Eqn (5).
AN = ρ σN·
(5)
where ρ is the slope of these AN versus σ graphs and reflects the
sensitivityofradicalnitrogenhfccstopolarsubstituenteffects.Very
closeρ valuesfoundfor2-4ꢁ-pyridylBNNand2-phenylBNNradicals
(−4.36 and −4.50, respectively) show that the sensitivity of BNN
radicals’ nitrogen hfcc values to polar substituent effects is very
similar and quite consistent among BNNs differently substituted
at 2-position. The ρ value obtained for 2-4ꢁ-pyridyl BNN (Fig. 10)
when σp is tabulated against AN is −0.51, which is much less than
those obtained when σN· values are used indicating the greater
sensitivity of AN to these new substituent parameters (σN·).
Figure 9. N-hyperfine versus σN (dot) for 5-substituted 2-phenyl-BNNs.
The electronic structure calculations predict quite scattered
values for the isotropic hyperfine values, as seen in Table 2.
Although the values obtained with the 6-31G and 6-31+G basis
sets seem to be the closest values to the experimental ones,
they should not be considered as the reliable estimates. As seen
in Table 2, the values of around 4.50-5.00 Gauss calculated with
the 6-31G and 6-31+G basis sets. Addition of the polarization
to functions, to get 6-31+G(d,p), give much smaller values of
around 3.00 Gauss. The largest Pople type basis set that we used,
6-311+G(d,p), produced values of around 2.00 Gauss, which are
lessthanhalfoftheexperimentalvalues. Therefore, weseethatthe
Pople type basis sets cannot reliably estimate the Fermi contact
couplings, as expected. This is fundamentally because of the fact
that these basis sets cannot describe the nuclear region very well,
as they focus on the valence shells, causing unreliable estimates
of the spin densities on nucleus. However, the IGLO basis sets of
Kutzelinnig[11] andEPRbasissetsofBarone[12] producedconsistent
values, as we see clear approach to the experimental values when
we go from IGLO-II to IGLO-III and from EPR-II to EPR-III. Thus, we
believe that the most reliable estimates we have are seen to be the
predictions of the EPR-III basis set. If we compare the B3LYP/EPR-III
values with the experimental nitrogen hfccs, we see that there is
still a difference of around 1.50 Gauss. The experimental values
are around 4.50 Gauss while the B3LYP/EPR-III predictions are
around 3.00 Gauss. The difference got smaller when we included
Figure 10. N-hyperfine versus σp (minus) for 5-substituted 2-4ꢁ-pyridyl-
BNNs.
plots as suggested by its connection with the spin delocalization
energy. Hfcc values are related to an energy value via Eqn (4).[26]
E = hν = gβ(H δH),
(4)
where E = energy of electromagnetic transition causing transition
between magnetic energy levels, H = field strength without
hyperfine splitting and 2δH = hfcc. This reasoning is similar to
c
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Magn. Reson. Chem. 2009, 47, 641–650

Effect of substituents on spin density in BNNs
the solvent effects. Although we could not do B3LYP/EPR-III
calculations on all molecules with solvent effects included, due
to the computational cost, we carried out such calculations on
the 5-F-substituted 2-4ꢁ-pyridyl BNN inside chloroform (dielectric
constant taken to be 4.90) as a representative case. We also looked
for the effects of different solvation methods, as implemented in
Gaussion03 program package. We get the value of 3.29 Gauss
with a polarizable continuum solvation model as suggested by
Barone and Cossi,[28] 3.24 Gauss with the polarizable continuum
solvation model using the integral equation formalism[29] and 3.12
Gauss with a model known as self-consistent isodensity polarized
continuum model.[30] These results show that the inclusion of
the solvent increases the results by about 0.4 Gauss (from 2.92
Acknowledgements
WewouldliketoexpressourspecialthankstoBekirAktasandSinan
Kazan of Gebze Institute of Technology. This work is financially
supported by the Scientific and Technological Research Council
of Turkey (Project No: 106T496) and by Fatih University (Scientific
Investigation Project No: P50020601).
Supporting information
Supporting information may be found in the online version of this
article.
to 3.29). To see the effect of other DFT functionals, we carried References
out a test calculation with the PBE0[31]/EPR-III level of theory on
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3.11 Gauss for both nitrogen atoms (a 0.16 Gauss improvement
over B3LYP/EPR-III level calculations). Considering all of these,
we can say that the electronic structure calculations can predict
the nitrogen hyperfine couplings in these radicals with about
25% deviation from the experimental values. We think that this
is an appreciable amount of deviation, and we will analyze its
causes in detail in a future theoretical study. When we look at
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smallest coupling is seen to be in 5-NO2 substitutions, which is
about 0.20–0.30 Gauss less than the others. This is similar to the
experiment where the reduction is about 0.50 Gauss. As far as the
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5-OCH3 substitution gave a value of 4.43 Gauss, which is only 0.04
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to that of 5-CH3 (4.44 Gauss). As far as the spin densities are
concerned, the spin densities calculated at the B3LYP/IGLO-III
level of theory match those calculated from experimental data,
as seen in Table 1. However, the calculations of the Fermi contact
couplings with EPR-III basis sets rather than those with IGLO-III
basis sets are the closest ones to the experimental hfcc values.
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In conclusion, we proved that substituents on the benzenoid
benzene ring are generally more effective in changing the
spin density on the ONCNO moiety of BNN radicals where
most of the spin density resides. The spin density was shown
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