Spectroscopic and electrochemical investigation with coordination stabilities: Mononuclear manganese(II) complexes derived from different constituents macrocyclic ligands

Article abstract of DOI:10.1016/j.saa.2007.01.032

Since the manganese(II) complexes are known as having a high degree of stability, some of them may be able to play a very important role in biosystems. We prepared manganese(II) complexes with different chromospheres containing macrocyclic ligands bearing N, S and O like functional donor atoms in order to obtain different models of compounds. So these new manganese(II) complexes were derived from macrocyclic ligands by chelating them with metal ions. Thus, two macrocyclic ligands, L¹: 2,4-diphenyl-1,5-diaza-8,12-dioxo-6,7:13,14-dibenzocyclo tetradeca-1,4-diene[N₂O₂]ane; L²: 2,4,9,11-tetraphenyl-6,13-dimethyl-1,5,8,12-traazacyclotertr-adeca-1,4,8,11-tetraene[N₄]ane; and two more different form first one viz.-L³: 1,7-diaza-4-monothia-10,14-dioxo-8,9:15,16-cyclohexadecane[N₂O₂S]ane and L⁴: 4,13-diaoxa-1,7,10,16-hexazacyclooctadecane[N₄O₂]ane were prepared and their capacity to retain the manganese(II) ion in solid as well as aqueous solution was determined from various physiochemical techniques viz: characterized by elemental analyses, molar conductance measurements, magnetic susceptibility measurements, mass, IR, electronic, ESR spectral studies and cyclic voltammetric measurements.

Full text of DOI:10.1016/j.saa.2007.01.032

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Spectrochimica Acta Part A 68 (2007) 1250–1255
Spectroscopic and electrochemical investigation with coordination
stabilities: Mononuclear manganese(II) complexes derived
from different constituents macrocyclic ligands
Rajiv Kumara,b,∗, S. Chnadrab, Parashuram Mishraa
a Department of Chemistry, University of Delhi, New Delhi 110007, India
b Department of Chemistry, Zakir Hussain College, University of Delhi, New Delhi 110002, India
Received 15 July 2006; accepted 25 January 2007
Abstract
Since the manganese(II) complexes are known as having a high degree of stability, some of them may be able to play a very important role in
biosystems. We prepared manganese(II) complexes with different chromospheres containing macrocyclic ligands bearing N, S and O like functional
donor atoms in order to obtain different models of compounds. So these new manganese(II) complexes were derived from macrocyclic ligands by
chelating them with metal ions.
Thus, two macrocyclic ligands, L1: 2,4-diphenyl-1,5-diaza-8,12-dioxo-6,7:13,14-dibenzocyclo tetradeca-1,4-diene[N2O2]ane; L2: 2,4,9,11-
tetraphenyl-6,13-dimethyl-1,5,8,12-traazacyclotertr-adeca-1,4,8,11-tetraene[N4]ane; and two more different form first one viz.—L3:
1,7-diaza-4-monothia-10,14-dioxo-8,9:15,16-cyclohexadecane[N2O2S]ane and L4: 4,13-diaoxa-1,7,10,16-hexazacyclooctadecane[N4O2]ane were
prepared and their capacity to retain the manganese(II) ion in solid as well as aqueous solution was determined from various physiochemical tech-
niques viz: characterized by elemental analyses, molar conductance measurements, magnetic susceptibility measurements, mass, IR, electronic,
ESR spectral studies and cyclic voltammetric measurements.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Macrocyclic ligands; 1H NMR; IR; EPR; Manganese(III) complexes
1. Introduction
during the past decades due to their wide applications in biolog-
ical and sensor fields [1,2]. Different neutral donors have been
incorporated into the ligand backbone [3–6]. Macrocycles find
wide applications in medicine, cancer diagnosis and in treatment
of tumors, in metal ion techniques and treatment of kidney stone.
Macrocyclic ligands have been used successfully for diverse
processes such as separation of ions by transport through arti-
ficial and natural membrane, liquid–liquid or solid–solid phase
transfer reaction, preparation of ion-selective electrodes, isotope
separation and in the understanding of some natural processes
through mimicry of metalloenzymes.
Very often the incorporation of a functional group into the lig-
and backbone is accompanied by changes in reactivity of the cor-
responding metal complex. Therefore, investigations of model
compounds, which are closely related, electronically and struc-
turally, to the supported catalysts need to be performed [7–13].
Our findings are quite interesting about the geometrical or
geometrical classification with stereochemical of anions and
centre on their coordination abilities which are further used
for the investigation of model complexes. Needless to write,
We adopt a native point of view in our discussion in the spec-
troscopic characterization of our synthesized complexes with all
the findings which are inventively clear. By spectroscopic stud-
ies we mean a whole characterization sets of such techniques
which are sufficiently powerful to analysis our needs through
this article and our intentions are clear to understand the capac-
ity of our synthesized complexes for their vast applications in
different fields and intricate importance for the development in
this field. The synthesis of macrocyclic ligands with their man-
ganese(II) metal complexes is a growing area of research in
inorganic and bioinorganic chemistry, it is due to their impor-
tance in many biologically and chemical process. Coordination
compounds containing macrocyclic ligands have been studied
∗
Corresponding author at: Department of Chemistry, University of Delhi,
New Delhi 110007, India. Tel.: +91 1234276530; fax: +91 1234276530.
E-mail address: chemistry rajiv@hotmail.com (R. Kumar).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.01.032

R. Kumar et al. / Spectrochimica Acta Part A 68 (2007) 1250–1255
1251
Fig. 2. UV spectrum of MnL1(NO3)2 complexes.
Fig. 3. UV spectrum of MnL2(NCS)2 complexes.
mental analyses (CHN) of all these complexes were carried
out on a Carlo-Erba 1106 Elemental Analyzer. Molar conduc-
tance was measured on an ELICO conductivity bridge (Type
CM82T). Magnetic susceptibility measurements were made on
Gouy Balance at room temperature using CuSO4·5H2O as cel-
ebrant. Infrared spectra were recorded on a Perkin Elmer 137
instrument as KBr pellets. Electronic spectra were recorded in
DMSO solution on a Shimadzu UV mini-1240 spectrophotome-
ter. EPR spectra of the chromium(III) complexes were recorded
as powder samples at room temperature on an E-4 EPR spec-
trometer using DPPH as the g-marker.
Fig. 1. Structures of macrocyclic ligands.
we follow the ligands reported earlier we report the synthesis
and characterization as a combined way of manganese(II)
complexes and our purpose in this article to develop some new
manganese(III) complexes with macrocyclic ligands (Fig. 1)
containing different donor atoms viz.—ligand L1: 2,4-diphenyl-
1,5-diaza-8,12-dioxo-6,7:13,14-dibenzocyclotetradeca-1,4-
diene[N2O2]ane; ligand L2: 2,4,9,11-tetraphenyl-6,13-dimet-
hyl-1,5,8,12-traazacyclotertr-adeca-1,4,8,11-tetraene[N4]ane;
ligand L3: 1,7-diaza-4-monothia-10,14-dioxo-8,9:15,16-cyclo-
hexadecane[N2O2S]ane and ligand L4: 4,13-diaoxa-1,7,10,16-
hexazacyclooctadecane[N4O2]ane. Reported complexes are
sufficiently powerful to develop required modeling for biosys-
tems in the rest of this article.
All solvents used were purified before use according to stan-
dard procedures.
All the ligands are prepared as reported earlier [12].
2. Experimental
2.1. Materials and instrumentation
All the chemicals used in the present investigation were of
AR grade, purchased from Sigma Chemical Co., USA. Ele-
Fig. 4. UV spectrum of [Mn(L3)(NCS)](NCS) complexes.

1252
R. Kumar et al. / Spectrochimica Acta Part A 68 (2007) 1250–1255
Table 1
Analytical data of the manganese(II) complexes
Complexes/empirical formula
Yield (%)
Molar conductance
Color
Elemental analysis calcd. (found%)
Mn
7.92
(ꢀ−1 cm2 mol−1
)
C
H
N
[Mn(L1)(NCS)2]
MnC36H36N4
55
52
51
46
68
58
74
71
12.0
Light yellow
Light yellow
Yellow
67.82
4.51
7.45
(7.79)
(7.90)
(97.23)
(4.59)
[Mn(L1)(NO3)2]
Mn C36H36N4
06.0
05.0
7.84
63.92
4.20
11.00
(11.11)
(7.80)
(63.25)
(4.26)
[Mn(L2)(NCS)2]
Mn C30H26N2O2
8.39
64.97
4.28
11.10
(11.2)
(8.12)
(64.71)
(4.31)
[Mn(L2)(NO3)2]
Mn C30H26N2O2
11.0
Yellow
7.88
64.25
3.90
7.85
(7.92)
(7.80)
(64.20)
(3.99)
[Mn(L3)NCS]NCS
MnC40H28N6O2S2
85.0
Light yellow
Light yellow
Yellow
7.39
64.59
3.50
10.65
(10.7)
(8.01)
(64.20)
(3.60)
[Mn(L3)NO3]NO3
MnC38H28N6O8
82.0
7.31
60.72
3.75
11.00
(11.17)
(7.25)
(60.42)
(3.65)
[Mn(L4)](NCS)2
MnC40H28N6O2S2
165.0
168.0
7.39
64.59
3.69
7.46
(7.58)
(7.26)
(64.50)
(3.82)
[Mn(L4)](NO3)2
MnC38H28N6O8
Light yellow
7.31
60.72
3.50
10.56
(10.61)
(7.28)
(59.00)
(3.56)
2.2. Preparation of manganese(II) complexes with
macrocyclic ligands
3. Results and discussion
3.1. Manganese(II) complexes
A hot (∼65 ◦C) ethanolic solution (20 ml) of the ligands
(0.02 mol) and a hot (∼55 ◦C) ethanolic solution (20 ml) of
the manganese(II) metal salt (2 mmol) were mixed together
with constant stirring. The mixture was refluxed for 5–6 h at
65–90 ◦C. On cooling the content to ∼5 ◦C, the colored com-
plexes were precipitated out. They were filtered, washed with
cold EtOH and dried under vacuum over P4O10. The reactions
(1), (2), (3) and (4) are quite facile and can be completed in 5–6 h
of refluxing.
On the basis of the elemental analyses, the complexes were
assigned the compositions shown in Table 1. The molar conduc-
tance measurements of all the complexes in DMSO of L1 and
L2 correspond to the non-electrolytic nature but complexes of
ligands L3 and L4 are 1:1 and 1:2 electrolyte in nature. Thus,
these complexes may be formulated as [Mn(L1 or L2)X2], and
[Mn(L4)]X2 [X = NCS− and NO3−].
The manganese(II) complexes show a magnetic moment at
room temperature in the range of (μeff = 5.83–6.02 B.M.) corre-
sponding to five unpaired electrons.
L1 + MnX2 · nH2O → Mn(L1)X2 + nH2O,
L2 + MnX2 · nH2O → Mn(L2)X2 + nH2O,
L3 + MnX2 · nH2O → Mn(L3)X2 + nH2O,
L3 + MnX2 · nH2O → Mn(L3)X2 + nH2O,
where X = NO3− and NCS−.
(1)
(2)
(3)
(4)
3.2. Bands due to thiocyanate anions
We adopt a native point of view in our discussion on complex-
ation behavior of the complexes as a result, all the important IR
bands corresponding to various fragments of the ligands which
are shifted to lower frequency as compared to the macrocyclic
ligands. We assume that the coordination of ligand has occurred
Table 2
Important I.R. absorption bands (cm−1) of nitrato and thiocyanato manganese(II) complexes s of L1, L2, L3 and L4
Complex
υC
υNi–N
υNi–O
υNi–S
υNO
υNCS
N
3
[Mn(L1)(NO3)2]
[Mn(L1)(NCS)2]
[Mn(L2)(NO3)2]
[Mn(L2)(NCS)2]
[Mn(L3)(NO3)](NO3)
[Mn(L3)(NCS)](NCS)
[Mn(L4)](NO3)2
[Mn(L4)](NCS)2
1588
1587
1582
1596
1581
1584
620
620
620
620
620
620
–
–
–
–
–
–
310
–
–
–
315
–
–
–
–
–
–
–
2080, 860, 410
–
–
2050
–
509, 521
518, 522
1384

R. Kumar et al. / Spectrochimica Acta Part A 68 (2007) 1250–1255
1253
Table 3
Magnetic moment and electronic spectra of the manganese(II) complexes
Complex
Meff. BM
υ1 (cm−1
)
υ2 (cm−1
)
Y3 (cm−1
)
υ4 (cm−1
)
[Mn(L1)(NO3)2]
[Mn(L1)(NCS)2]
[Mn(L2)(NO3)2]
[Mn(L2)(NCS)2]
[Mn(L3)(NO3)](NO3)
[Mn(L3)(NCS)](NCS)
[Mn(L4)](NO3)2
[Mn(L4)](NCS)2
5.83
5.98
5.93
5.89
5.89
6.02
6.01
6.00
17,550
17,755
17,865
17,621
18,023
17,421
18,911
17,868
24,390
24,650
22,222
23,145
24,910
24,449
23,555
23,690
28,615
28,977
29,001
28,554
28,633
28,944
27,895
28,012
34,462
31,600
32,258
–
32,894
31,550
–
31,007
through the donor atoms of the groups. It intensively cleared the
coordination abilities of studied complexes.
These values are calculated from the second and third transitions
because these transitions are free from the crystal field splitting
and depend on B and C parameters.
Orgel [19] calculates the values of Dq with the help of
curve, transition energies versus Dq, as given by the using
3.3. IR spectra of the manganese(II) complexes
The IR spectra of nitrato complexes of L1 and L2 dis-
play three (N–O) stretching bands, at ∼1418–1427 cm−1 (ν5),
1303–1311 cm−1 (ν1) and 1003–1012 cm−1 (ν2).
the energy due to the transition A1g → T1g(4G). Parame-
ters B and C are linear combinations of certain coulombs
and exchange integral, which are generally, treated empirical
parameters obtained from the spectra of the free ions. Slater
Condon-shortly repulsion parameters F2 and F4 are related to
Racah parameters B and C as: B = F2–5F4 and C = 35F4, The
electron–electron repulsion in the complexes is more than in the
free ion, resulting in an increased distance between electrons,
and thus, affects the size of the orbital. On increasing delo-
calization, the value of β decreases up to less than one in the
complexes.
6
4
The position of bands in the IR spectra of the complexes indi-
cates that both nitrate groups are coordinated to the central metal
ion in a unidentate manner [14]. The reported nitrato complex of
L3 shows three IR spectral bands at 1420, 1310 and 1028 cm−1
.
.
The separation of the two highest frequency bands is 110 cm−1
The position of these bands indicates a monodentate coordi-
nation of this group as well as displays a sharp and shows a
strong band at 1388 cm−1 which suggests that the nitrate groups
are uncoordinated. In the infrared spectra of Mn(L4)(NO3)2
complexes show sharp bands at 1383 cm−1 corresponding to
an uncoordinated nitrate group and this extends our previous
results which are reported earlier [15] (Table 2).
3.5. Ligand field parameters
The value of β can be calculated from the Nephelauxetic
parameter for the ligand (hx) and the Nephelauxetic parameter
for the metal ion (km) as (1 − β) = hx × km. The value of the
parameter hx for manganese(II) complexes have been calculated
by using the co-valency contribution of manganese(II), while
for the calculation of β, we used the numerical value of B for
manganese(II) free ion which is 786 cm−1. The observed values
forparameterβ andhxsuggestthatthecomplexes, reportedhere,
have appreciable ionic character [19]. We have observed in this
spectroscopic characterization that one of the basic concept of
transitions and nephelouxetric parameters analysis is to study the
bounded metal to donor atoms which are defined the geometries
of our complexes to understand their stereochemistry. Related
data are given in Tables 2 and 4 respectively.
3.4. Electronic spectra of manganese(II) complexes
The electronic spectra of the manganese(II) com-
plexes exhibit four weak-intensity absorption bands in
the ranges 17,421–18,911, 22,222–24,910, 27,895–29,001 and
31,007–34,462 cm−1 which may be assigned to the transi-
6
4
6
4
4
tions: A1g → T1g(4G), A1g → Eg, A1g(4G) (10B + 5C),
6A1g → Eg(4D) (17B + 5C) and A1g → T1g(4P) (7B + 7C),
4
6
4
respectively [16,17] (Figs. 2–4).
The parameters B and C were calculated and reported which
have a good agreement with data reported earlier [18]. The cal-
culated values of the ligand field parameters are given in Table 3.
Table 4
Ligand field parameters of the manganese(II) complex
Complex
Dq (cm−1
)
B (cm−1
)
C (cm−1
)
β
F4
F2
979
Hx
[Mn(L1)(NO3)2]
[Mn(L1)(NCS)2]
[Mn(L2)(NO3)2]
[Mn(L2)(NCS)2]
[Mn(L3)(NO3)](NO3)
[Mn(L3)(NCS)](NCS)
[Mn(L4)](NO3)2
[Mn(L4)](NCS)2
1775
1780
1730
1795
1795
1886
1830
1760
539
614
760
629
680
636
620
644
3800
2702
2924
3371
3622
3617
3471
3450
0.68
0.78
0.96
0.80
0.86
0.80
0.78
0.81
108
77
83
4.5
3.1
–
2.8
2.0
2.8
3.1
2.7
999
1175
1109
1195
1151
1115
1134
96
103
103
99
98

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R. Kumar et al. / Spectrochimica Acta Part A 68 (2007) 1250–1255
3.6. EPR spectra of manganese(II)
Virtually, all the theory of this spectroscopy had its origin in
characterization patterns. Our present interest, however, lies in
the study of stability and oxidation state, not in exploring the
many applications of this resonance technique to specific use of
such complexes. This technique is therefore strongly oriented
towards the stability and geometrical aspects of the synthesized
complexes. ESR spectra were recorded at room temperature for
a polycrystalline sample and in a solution of DMSO. And give
ESR spectra containing the six lines arising due to the hyper-
fine interaction between the unpaired electron with the 55Mn
nucleus (1 = 5/2). The nuclear magnetic quantum number M1,
corresponding to these lines, are −5/2, −3/2, −1/2, +1/2, +3/2
and +5/2 from low to high field. ESR [19,20] parameters of
the manganese(II) complex (g , g , A0 and giso) are calculated
Fig. 6. Structures of manganese(II) complexes with L2.
ꢀ
⊥
and their values are found in the range 3.95–3.64, 1.99–1.87,
111–105 and 2.64–2.45.
3.7. Cyclic voltammetery
On comparing the cyclic voltammograms we observe that the
variation in the oxidation and reduction potentials may be due to
distortioninthegeometrythatarisesduetodifferentanionscoor-
dinated. Cyclic voltammetric data for complexes in (CH2Cl2)
Ep,a1 (V), E1/2 (mV), ꢁEp1 (mV), Ep,a2 (V) and E1/2 (mV) are
calculated and found in the range 0.580–0.570, 495.0–490.0,
110.0–105.0, 0.8950–0.8940 and 800–798. The MnL complex
shows two oxidation processes at Ep,a = 0.58 and 0.895 V. The
first wave is nearly reversible with ꢁEp = 110 mV. This process
is consistent with a one-electron oxidation to form the mixed
valence Mn(II, III) species. The second wave is irreversible at
0.895 V. The observed reaction voltages of the complexes of
CuL and CoL are lower than that of the ligand, while the MnL
one is higher [19].
We have prepared and fully characterized a family of four
macrocyclic ligands which have several potential donor atoms,
but due to different fragments of the constraints all the ligands
can provide a approach to their coordination behavior through
which they can effect the stability of the manganese(II) com-
plexes and their importance in coordination chemistry as well
as their biophysical systems. The maximum priority of taking
such donor atoms which are characterized by such different
spectroscopic techniques having different stereochemistry can
be arranged to manage different structures with metal. On the
Fig. 7. Structures of manganese(II) complexes with L3.
basis of donor atoms of the ligands are coordinated to the metal,
formed different models of complexes, it follows that the com-
plexes would be four, five and six coordinate with respect to
the all ligands. As these ligands are potentially teradentate, pen-
tadentate and hexadentate. It is quite feasible that the metal ions
are six coordinate with different anions, as shown in Figs. 5–8
Fig. 8. Structures of manganese(II) complexes with L4.
Fig. 5. Structures of manganese(II) complexes with L1.

R. Kumar et al. / Spectrochimica Acta Part A 68 (2007) 1250–1255
1255
around the metal were obtained, suggesting that an octahedral or
distorted octahedral configuration around the metal is possible.
The possibility of a square pyramidal configuration around the
metal has been discarded from the conductivity measurements,
which show corresponding different types of electrolytes. IR
spectral studies are very helpful to explore the probability related
to the geometrical findings.
possible with out the help of Mohit Krishan, Computer Pro-
grammer, CSL Delhi University, Delhi. Thanks to the University
Grants Commission, New Delhi for financial assistance. Thanks
to I.I.T. Bombay for recording ESR spectra. Thanks are also
due to the Solid State Physics Laboratory, India for recording
magnetic moments.
References
4. Conclusion
[1] S. Chandra, R. Kumar, Trans. Met. Chem. 29 (2004) 269.
[2] R. Kumar, R. Singh, Russ. J. Coord. Chem. 32 (2006) 192.
[3] J. Lewis, M.J. Schro¨der, Chem. Soc., Dalton Trans. (1982) 1085.
[4] P.R. Seeber, A.G. Cinquantini, L. Mazzocchin Fabbrizzi, J. Chem. Soc.,
Dalton Trans. (1982) 893.
[5] A. Fabbrizzi, V. Poggi, P. Zanello, J. Chem. Soc., Dalton Trans. (1983)
2191.
[6] F.F. Lovecchio, E.S. Gore, D.H. Busch, J. Am. Chem. Soc. 96 (1974) 3109.
[7] C.W.G. Ansell, J. Lewis, M.C. Liptrot, P.R. Raithby, M. Schro¨der, J. Chem.
Soc., Dalton Trans. (1982) 1593.
[8] J. Lewis, T.D. O’Donoghue, P.R. Raithby, J. Chem. Soc., Dalton Trans.
(1980) 1383.
[9] P.S. Bryan, J.C. Dabrowiak, Inorg. Chem. 14 (1975) 299.
[10] S.-G. Kang, M.-S. Kim, J.-S. Choi, D. Whang, K. Kim, J. Chem. Soc.,
Dalton Trans. (1995) 363.
[11] P.S. Bryan, J.C. Dabrowiak, Inorg. Chem. 4 (1975) 296.
[12] S. Chandra, R. Kumar, Spectrochim. Acta Part A 66 (2007) 74.
[13] L.F. Lindoy, The Chemistry of Macrocyclic Ligand Complexes, Cambridge
University Press, Cambridge, 1989, p. 14.
[14] C.N.R. Rao, Chemical Applications of Infrared Spectroscopy, Academic
Press, New York, 1963.
A series of manganese(II) complexes with macrocyclic lig-
ands have been prepared and fully characterized. It is some times
said that coordination chemistry of manganese(II) is the study
has its importance in biological functions and catalytic activities
which are effected in the environment of macrocyclic cavities.
By using spectroscopic techniques our observation provides a
deeper look on the stability and characterization ability of man-
ganese(II) with donor atoms in a cyclic environments. These
observations will be used in the designing with approach towards
their application in chemical, biological and industrial aspects.
Naturally our simplifies matters; but it does come as close to the
truth as an aphorism come. The studies of spectroscopy and their
importance lead us to understand the molecular arrangements of
the studied macrocyclic complexes. The coordinating behavior
in the complexes of manganese(II) effected by the coordination
stereochemistry of the macrocyclic ligands NCS and NO3. Due
to these coordinating sites of the ligands are affecting coordi-
nation behaviors of the NO3 and NCS ions. On the basis of
above studies suggested structures of the complexes are given
in Figs. 5–8.
[15] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, Willey, New York, 1970.
[16] A.B.P. Lever, Inorganic Electronic Spectroscopy, Elesevier, Amsterdom,
1984.
[17] A.B.P. Lever, Crystal Field Spectra, Inorganic Electronic Spectroscopy,
first ed., Elsevier, Amsterdam, 1968, p. 249.
[18] S. Chandra, R. Kumar, Synth. React. Inorg. Met.-Org. Chem. 35 (2) (2005)
161.
Acknowledgments
Co-authors (Rajiv Kumar) gratefully acknowledge his
younger brother Bitto and special thanks to my friends for moti-
vation and his interest in my research topic. This work is not
[19] I.F. Orgel, J. Chem. Phys. 23 (1855) 1004.
[20] A. Schweiger, G. Jeschke, Principles of Pulse Electron Paramagnetic Res-
onance, Oxford University Press, Oxford, 2001.