supplementary materials


HTML versionpdf versioncif file3d viewstructure factorssupplementary materialsCIF check reportsimilar papers Open access

Acta Cryst. (2009). E65, o1045-o1046    [ doi:10.1107/S1600536809013476 ]

2,2-Diphenylbenzo[c]quinoline-1-oxyl

C. Rizzoli, E. Marku, L. Greci and P. Astolfi

Abstract top

In the title compound, C25H18NO, a stable phenanthridinic nitroxide, the ring containing the nitroxide function assumes a twist-boat conformation and the dihedral angle formed by adjacent benzene rings is 21.78 (5)°. The phenyl substituents at position 2 are approximately orthogonal to each other, forming a dihedral angle of 81.04 (4)°. The crystal structure is stabilized by an intramolecular C-H...O hydrogen bond and by C-H...[pi] interactions.

Comment top

Most nitroxides (aminoxyls) are stable radicals that have received a great attention since the second half of the last century for the variety of their applications. In fact, in biology they have been used as relatively stable spin-adducts for studying short-lived radicals such as superoxide (Carloni et al., 1996), hydroxy and alkylperoxy radicals (Greci, 1982) that are typical for peroxidation processes. In this field, nitroxides have also been used as spin probes/spin labels for studying membranes and proteins (Likhtenshtein et al., 2008). In medicine, they have been studied as mimics of superoxide dismutase (Damiani et al., 2008), catalase (Krishna et al., 1996) and as contrast agents of NMR-imaging. In pharmacology, they have been used to study the metabolism of drugs (Setjurc et al., 1995). As antioxidants they have been used in polymers, in stabilising monomers for polyaddition during controlled radical polymerization, for the synthesis of living polymers and in large hydrocarbon distilleries for preventing polymerization and incrustation of pipes (Guillaneuf et al., 2007). As antioxidants, they have also been studied in the medical (ischemia-reperfusion) and cosmetic field for protecting against free radical damage (Greci et al., 2007). In chemistry, they have been used as inhibitors of radical processes, in radical synthesis and as mediators of controlled oxidations of primary alcohols and aldehydes (Arends et al., 2006). Recently, nitroxides have found applications in supramolecular chemistry (Franchi et al., 2008), in nanomaterials (Bailly et al., 2006) and in other technologies such as the construction of free radical batteries (Bugnon et al., 2007). In view of its potential application in cosmetics and as a precursor of alkoxyamines used in the controlled radical polymerization, the title compound has been synthesized and its crystal structure is reported here.

In the molecule of the title compound (Fig. 1), geometric parameters are usual. The value of the N1-O1 bond length (1.2811 (13) Å) corresponds well to the mean value of 1.286 (1) Å found from 891 observations yielded by the Cambridge Crystallographic Database (version 5.30; Allen, 2002) for the N—O single bond, and is in agreement with the radical character of the oxygen atom evidenced by an ESR study (Fig. 3). The benzoquinoline ring system is not planar, the dihedral angle between the C2–C7 and C8–C13 benzene rings being 21.78 (5)° as a result of the sp3 character of the C1 carbon atom. The ring containing the nitroxide function assumes a twist-boat conformation, with puckering parameters Q = 0.443 (2) Å, θ = 110.23 (18) ° and φ = -142.62 (18) ° (Cremer & Pople, 1975). The dihedral angle formed by the phenyl substituents at C1 is 81.04 (4)°. The molecular conformation is stabilized by an intramolecular C—H···O hydrogen bond (Tab. 1) generating an S(5) ring motif (Etter et al., 1990). In the crystal packing (Fig. 2), weak C—H···π interactions are observed ranging from 2.97 to 3.27 Å (Table 1).

Related literature top

For applications of nitroxides in biology, see: Carloni et al. (1996); Greci (1982); Likhtenshtein et al. (2008). For their applications in medicine, see: Damiani et al. (2008); Krishna et al. (1996). For their use in pharmacology and cosmetics, see: Krishna et al. (1996); Setjurc et al. (1995); Greci et al. (2007). For their applications in chemical processes and materials science, see: Guillaneuf et al. (2007); Arends et al. (2006); Franchi et al. (2008); Bailly et al. (2006); Bugnon et al. (2007). For a description of the Cambridge structural Database, see: Allen (2002); For puckering parameters, see: Cremer & Pople (1975). For graph-set motifs, see: Etter et al. (1990). For the synthesis, see: Colonna et al. (1980). Cg1, Cg2 and Cg3 are the centroids of the C2–C7, C14–C19 and C20–C25 aromatic rings, respectively.

Experimental top

A dried tetrahydrofuran solution (30 ml) of 2-phenyl-3,4-benzoquinoline-N-oxide (2.72 g, 10 mmol) prepared according to the literature method (Colonna et al.,1980), was reacted at room temperature with phenylmagnesium bromide (3.62 g, 20 mmol; a commercial compound produced by Aldrich). The reaction mixture was then poured into a 10% ammonium chloride water solution (100 ml) and extracted with diethyl ether. The dried organic layer was oxidised with lead dioxide (4.8 g, 20 mmol) and, after filtration, evaporated to dryness. The residue was chromatographed on silica gel column eluting with cyclohexane/ethyl acetate (8:2 v/v; 300 mL). The expected nitroxide was isolated from the red fraction in 83% yield (2.9 g): m.p 176-7°C (175°C in Colonna et al., 1980). Single crystals suitable for X-ray analysis were obtained by slow evaporation of an ethanol solution. IR in KBr, ν, cm-1: 1926, 1771, 1732, 1667, 1589, 1732. Mass, calcd. for C25H18NO, 348.44; found: m/z = 349(M++1, 22.8), 348(60.6), 318(87.1), 272(47.4), 254(66.8), 240 (100). EPR, hfccs in Gauss: aN = 10.75; aH = 2.76; aH = 2.67; aH = 1.03; aH = 0.88; aH = 0.38; aH = 0.27; g-value = 2.00577. The melting point was measured on a Mitamura Riken Kogyo Mp. D electrochemical apparatus and was not corrected. IR spectrum were recorded in KBr with a Perkin–Elmer MGX1 spectrophotometer equipped with Spectra Tech. Mass spectrum was recorded on a Carlo Erba QMD 1000 mass spectrometer in positive electron impact (EI) mode. The electron-spin resonance (ESR) spectrum (Fig. 3) was simulated by using WinSim program in the NIEHS Public ESR Software Tools package (Duling, 1994).

Refinement top

Though all the H atoms were discernible in the difference electron density maps, the H atoms were positioned into idealized positions with C—H = 0.93 Å, and refined using a riding model approximation with Uiso(H) = 1.2 Ueq(C).

Computing details top

Data collection: SMART (Bruker, 1998); cell refinement: SAINT-Plus (Bruker, 1998); data reduction: SAINT-Plus (Bruker, 1998); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997) and SCHAKAL97 (Keller, 1997); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008), PARST95 (Nardelli, 1995) and WinSim (Duling, 1994).

Figures top
[Figure 1] Fig. 1. The molecular structure of the title compound. The displacement ellipsoids are drawn at the 50% probability level. The intramolecular C—H···O hydrogen bond is shown as a dashed line.
[Figure 2] Fig. 2. Crystal packing of the title compound viewed approximately along the b axis. Intramolecular C—H···O hydrogen bonds are shown as dashed lines.
[Figure 3] Fig. 3. Experimental and simulated ESR spectrum of the title compound.
2,2-Diphenylbenzo[c]quinoline-1-oxyl top
Crystal data top
C25H18NOF(000) = 732
Mr = 348.40Dx = 1.278 Mg m−3
Monoclinic, P21/cMelting point = 449–450 K
Hall symbol: -P 2ybcMo Kα radiation, λ = 0.71073 Å
a = 12.6188 (12) ÅCell parameters from 1226 reflections
b = 8.8704 (8) Åθ = 3.2–24.8°
c = 16.6083 (15) ŵ = 0.08 mm−1
β = 102.998 (2)°T = 295 K
V = 1811.4 (3) Å3Block, red
Z = 40.16 × 0.14 × 0.08 mm
Data collection top
Bruker SMART 1000 CCD
diffractometer		3548 independent reflections
Radiation source: fine-focus sealed tube2115 reflections with I > 2σ(I)
graphiteRint = 0.047
ω scansθmax = 26.0°, θmin = 1.7°
Absorption correction: multi-scan
(SADABS; Bruker, 1998)		h = −15→15
Tmin = 0.972, Tmax = 0.990k = −10→10
18472 measured reflectionsl = −20→20
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.037Hydrogen site location: difference Fourier map
wR(F2) = 0.080H-atom parameters constrained
S = 1.01 w = 1/[σ2(Fo2) + (0.0321P)2]
where P = (Fo2 + 2Fc2)/3
3548 reflections(Δ/σ)max < 0.001
244 parametersΔρmax = 0.11 e Å−3
0 restraintsΔρmin = −0.14 e Å−3
72 constraints
Crystal data top
C25H18NOV = 1811.4 (3) Å3
Mr = 348.40Z = 4
Monoclinic, P21/cMo Kα radiation
a = 12.6188 (12) ŵ = 0.08 mm−1
b = 8.8704 (8) ÅT = 295 K
c = 16.6083 (15) Å0.16 × 0.14 × 0.08 mm
β = 102.998 (2)°
Data collection top
Bruker SMART 1000 CCD
diffractometer		3548 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 1998)		2115 reflections with I > 2σ(I)
Tmin = 0.972, Tmax = 0.990Rint = 0.047
18472 measured reflectionsθmax = 26.0°
Refinement top
R[F2 > 2σ(F2)] = 0.037H-atom parameters constrained
wR(F2) = 0.080Δρmax = 0.11 e Å−3
S = 1.01Δρmin = −0.14 e Å−3
3548 reflectionsAbsolute structure: ?
244 parametersFlack parameter: ?
0 restraintsRogers parameter: ?
Special details top

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
N10.26974 (9)0.84150 (12)0.17884 (7)0.0436 (3)
O10.29038 (8)0.90766 (11)0.24920 (6)0.0590 (3)
C20.29264 (10)0.61901 (15)0.09755 (8)0.0406 (3)
C200.13820 (10)0.64080 (15)0.17300 (7)0.0386 (3)
C130.22201 (11)0.92305 (16)0.10729 (9)0.0443 (3)
C80.21679 (11)0.85600 (16)0.03064 (8)0.0458 (4)
C140.33392 (11)0.60524 (16)0.25222 (8)0.0445 (4)
C10.25883 (11)0.67236 (14)0.17575 (8)0.0391 (3)
C70.26972 (11)0.70981 (15)0.02684 (8)0.0424 (3)
C60.30162 (12)0.66030 (18)−0.04392 (9)0.0526 (4)
H60.28960.7217−0.09050.063*
C250.07658 (13)0.54387 (17)0.11658 (9)0.0550 (4)
H250.10750.49640.07750.066*
C30.34171 (12)0.48029 (16)0.09411 (9)0.0537 (4)
H30.35600.41860.14060.064*
C210.08830 (12)0.71122 (17)0.22877 (9)0.0512 (4)
H210.12800.77850.26680.061*
C22−0.01884 (13)0.68398 (18)0.22926 (10)0.0595 (4)
H22−0.05060.73280.26750.071*
C50.35039 (12)0.52270 (19)−0.04600 (9)0.0596 (4)
H50.37030.4908−0.09380.072*
C23−0.07899 (13)0.58527 (19)0.17372 (10)0.0610 (4)
H23−0.15120.56560.17440.073*
C120.18124 (13)1.06672 (18)0.11352 (10)0.0621 (4)
H120.18841.11220.16490.074*
C150.30015 (13)0.48928 (18)0.29547 (9)0.0603 (4)
H150.22850.45600.28070.072*
C24−0.03136 (13)0.51648 (19)0.11758 (10)0.0647 (5)
H24−0.07180.45010.07930.078*
C90.16276 (13)0.9348 (2)−0.03924 (10)0.0646 (5)
H90.15620.8914−0.09110.078*
C190.44123 (12)0.65176 (19)0.27533 (9)0.0614 (4)
H190.46580.72910.24640.074*
C40.36960 (13)0.43243 (19)0.02258 (10)0.0622 (4)
H40.40160.33840.02100.075*
C100.11911 (15)1.0748 (2)−0.03309 (13)0.0808 (6)
H100.08211.1245−0.08040.097*
C110.13017 (15)1.1411 (2)0.04279 (13)0.0782 (5)
H110.10281.23750.04650.094*
C180.51180 (15)0.5847 (2)0.34067 (11)0.0777 (6)
H180.58350.61780.35580.093*
C160.37243 (17)0.4214 (2)0.36115 (10)0.0838 (6)
H160.34910.34260.38990.101*
C170.47781 (18)0.4703 (3)0.38350 (11)0.0866 (6)
H170.52600.42570.42770.104*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
N10.0451 (7)0.0437 (7)0.0430 (7)−0.0050 (5)0.0120 (5)−0.0072 (6)
O10.0672 (7)0.0597 (7)0.0499 (6)−0.0065 (5)0.0130 (5)−0.0191 (5)
C20.0382 (8)0.0459 (9)0.0375 (8)−0.0026 (7)0.0083 (6)−0.0042 (7)
C200.0412 (8)0.0409 (8)0.0328 (7)−0.0012 (6)0.0067 (6)0.0045 (6)
C130.0407 (8)0.0414 (9)0.0518 (9)−0.0035 (7)0.0126 (7)0.0040 (7)
C80.0435 (9)0.0484 (9)0.0465 (9)−0.0048 (7)0.0121 (7)0.0064 (7)
C140.0442 (9)0.0538 (9)0.0347 (7)0.0060 (7)0.0073 (6)−0.0024 (7)
C10.0428 (8)0.0391 (8)0.0350 (7)−0.0008 (6)0.0081 (6)−0.0008 (6)
C70.0383 (8)0.0501 (9)0.0386 (8)−0.0081 (7)0.0085 (6)−0.0028 (7)
C60.0482 (9)0.0708 (11)0.0396 (9)−0.0065 (8)0.0115 (7)−0.0004 (8)
C250.0578 (10)0.0598 (10)0.0492 (9)−0.0158 (8)0.0159 (8)−0.0097 (8)
C30.0627 (10)0.0526 (10)0.0460 (9)0.0061 (8)0.0128 (8)−0.0018 (7)
C210.0474 (9)0.0599 (10)0.0467 (9)−0.0056 (7)0.0113 (7)−0.0060 (7)
C220.0504 (10)0.0726 (11)0.0592 (10)−0.0016 (8)0.0205 (8)−0.0002 (9)
C50.0520 (10)0.0823 (13)0.0468 (9)−0.0012 (9)0.0159 (8)−0.0138 (9)
C230.0452 (9)0.0765 (12)0.0610 (10)−0.0096 (9)0.0112 (8)0.0115 (9)
C120.0625 (11)0.0494 (10)0.0782 (12)−0.0006 (8)0.0241 (9)−0.0012 (9)
C150.0582 (10)0.0713 (11)0.0513 (9)0.0106 (8)0.0119 (8)0.0165 (8)
C240.0612 (11)0.0725 (12)0.0577 (10)−0.0262 (9)0.0076 (9)−0.0069 (9)
C90.0687 (11)0.0689 (12)0.0558 (10)0.0025 (9)0.0129 (9)0.0158 (9)
C190.0491 (10)0.0752 (12)0.0560 (10)0.0062 (8)0.0035 (8)−0.0030 (9)
C40.0652 (11)0.0634 (11)0.0591 (10)0.0113 (8)0.0164 (8)−0.0149 (9)
C100.0855 (14)0.0726 (13)0.0831 (14)0.0150 (11)0.0166 (11)0.0324 (11)
C110.0866 (14)0.0491 (11)0.0998 (15)0.0135 (9)0.0229 (12)0.0181 (11)
C180.0564 (11)0.0990 (15)0.0687 (12)0.0196 (11)−0.0048 (10)−0.0123 (12)
C160.0889 (16)0.0997 (15)0.0638 (12)0.0323 (12)0.0192 (11)0.0338 (11)
C170.0782 (15)0.1184 (18)0.0552 (11)0.0486 (13)−0.0020 (11)0.0070 (12)
Geometric parameters (Å, °) top
N1—O11.2811 (13)C22—C231.371 (2)
N1—C131.4057 (17)C22—H220.9300
N1—C11.5064 (16)C5—C41.369 (2)
C2—C31.3844 (18)C5—H50.9300
C2—C71.3994 (18)C23—C241.362 (2)
C2—C11.5305 (17)C23—H230.9300
C20—C251.3763 (18)C12—C111.375 (2)
C20—C211.3810 (18)C12—H120.9300
C20—C11.5383 (18)C15—C161.392 (2)
C13—C121.387 (2)C15—H150.9300
C13—C81.3933 (18)C24—H240.9300
C8—C91.3942 (19)C9—C101.372 (2)
C8—C71.4665 (19)C9—H90.9300
C14—C151.3760 (19)C19—C181.375 (2)
C14—C191.385 (2)C19—H190.9300
C14—C11.5244 (17)C4—H40.9300
C7—C61.3961 (19)C10—C111.369 (2)
C6—C51.371 (2)C10—H100.9300
C6—H60.9300C11—H110.9300
C25—C241.387 (2)C18—C171.363 (3)
C25—H250.9300C18—H180.9300
C3—C41.3796 (19)C16—C171.368 (3)
C3—H30.9300C16—H160.9300
C21—C221.375 (2)C17—H170.9300
C21—H210.9300
O1—N1—C13119.72 (11)C21—C22—H22119.8
O1—N1—C1119.05 (10)C4—C5—C6119.76 (14)
C13—N1—C1117.73 (11)C4—C5—H5120.1
C3—C2—C7119.14 (13)C6—C5—H5120.1
C3—C2—C1121.50 (12)C24—C23—C22118.95 (15)
C7—C2—C1119.32 (12)C24—C23—H23120.5
C25—C20—C21117.79 (13)C22—C23—H23120.5
C25—C20—C1122.53 (12)C11—C12—C13119.18 (16)
C21—C20—C1119.68 (12)C11—C12—H12120.4
C12—C13—C8121.22 (14)C13—C12—H12120.4
C12—C13—N1120.37 (13)C14—C15—C16120.49 (16)
C8—C13—N1118.42 (13)C14—C15—H15119.8
C13—C8—C9117.42 (14)C16—C15—H15119.8
C13—C8—C7119.23 (12)C23—C24—C25121.02 (15)
C9—C8—C7123.33 (14)C23—C24—H24119.5
C15—C14—C19118.45 (14)C25—C24—H24119.5
C15—C14—C1121.29 (13)C10—C9—C8121.41 (16)
C19—C14—C1120.02 (13)C10—C9—H9119.3
N1—C1—C14109.01 (10)C8—C9—H9119.3
N1—C1—C2107.22 (10)C18—C19—C14120.63 (17)
C14—C1—C2110.28 (11)C18—C19—H19119.7
N1—C1—C20105.34 (10)C14—C19—H19119.7
C14—C1—C20112.28 (10)C5—C4—C3120.32 (15)
C2—C1—C20112.42 (10)C5—C4—H4119.8
C6—C7—C2118.75 (13)C3—C4—H4119.8
C6—C7—C8122.32 (13)C11—C10—C9119.86 (17)
C2—C7—C8118.88 (12)C11—C10—H10120.1
C5—C6—C7121.16 (14)C9—C10—H10120.1
C5—C6—H6119.4C10—C11—C12120.78 (17)
C7—C6—H6119.4C10—C11—H11119.6
C20—C25—C24120.47 (14)C12—C11—H11119.6
C20—C25—H25119.8C17—C18—C19120.67 (18)
C24—C25—H25119.8C17—C18—H18119.7
C4—C3—C2120.80 (14)C19—C18—H18119.7
C4—C3—H3119.6C17—C16—C15120.05 (18)
C2—C3—H3119.6C17—C16—H16120.0
C22—C21—C20121.45 (14)C15—C16—H16120.0
C22—C21—H21119.3C18—C17—C16119.70 (17)
C20—C21—H21119.3C18—C17—H17120.2
C23—C22—C21120.30 (15)C16—C17—H17120.2
C23—C22—H22119.8
Hydrogen-bond geometry (Å, °) top
D—H···AD—HH···AD···AD—H···A
C21—H21···O10.932.423.044 (2)124
C24—H24···Cg1i0.933.273.864 (3)136
C6—H6···Cg2ii0.933.163.932 (4)142
C10—H10···Cg3iii0.932.973.839 (4)154
Symmetry codes: (i) −x, −y+1, −z; (ii) x, −y+3/2, z−1/2; (iii) −x, −y+2, −z.
Table 1
Hydrogen-bond geometry (Å, °) top
D—H···AD—HH···AD···AD—H···A
C21—H21···O10.932.423.044 (2)124
C24—H24···Cg1i0.933.273.864 (3)136
C6—H6···Cg2ii0.933.163.932 (4)142
C10—H10···Cg3iii0.932.973.839 (4)154
Symmetry codes: (i) −x, −y+1, −z; (ii) x, −y+3/2, z−1/2; (iii) −x, −y+2, −z.
Acknowledgements top

Financial support from the Universitá Politecnica delle Marche and the Universitá degli Studi di Parma is gratefully acknowledged.

references
References top

Allen, F. H. (2002). Acta Cryst. B58, 380–388.

Altomare, A., Burla, M. C., Camalli, M., Cascarano, G. L., Giacovazzo, C., Guagliardi, A., Moliterni, A. G. G., Polidori, G. & Spagna, R. (1999). J. Appl. Cryst. 32, 115–119.

Arends, I., Li, Y.-X. & Sheldon, A. R. (2006). Biocatal. Biotransform. 24, 443–448.

Bailly, B., Donnenwirth, A.-C., Bartholome, C., Beyou, E. & Bourgeat-Lami, E. (2006). J. Nanomater. pp. 1–10.

Bruker (1998). SMART, SAINT-Plus and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.

Bugnon, L., Morton, C. J. H., Novak, P., Vetter, J. & Nesvadba, P. (2007). Chem. Mater. 19, 2910–2914.

Carloni, P., Damiani, E., Greci, L., Stipa, P., Marrosu, G., Petrucci, R. & Trazza, A. (1996). Tetrahedron, 52, 11257–11264.

Colonna, M., Greci, L. & Poloni, M. (1980). J. Heterocycl. Chem. 17, 1473–1477.

Cremer, D. & Pople, J. A. (1975). J. Am. Chem. Soc. 97, 1354–1358.

Damiani, E., Astolfi, P., Carloni, P., Stipa, P. & Greci, L. (2008). Oxidants in Biology: a Question of Balance, edited by G. Valacchi & P. Davis, pp. 251–266. Vienna: Springer Verlag.

Duling, D. R. (1994). J. Magn. Reson. B, 104, 105–110.

Etter, M. C., MacDonald, J. C. & Bernstein, J. (1990). Acta Cryst. B46, 256–262.

Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565.

Franchi, P., Faní, M., Mezzina, E. & Lucarini, M. (2008). Org. Lett. 10, 1901–1904.

Greci, L. (1982). Tetrahedron, 38, 2435–2439.

Greci, L., Damiani, E. & Astolfi, P. (2007). WO Patent 2007 068 759.

Guillaneuf, Y., Gigmes, D., Marque, S. R. A., Astolfi, P., Greci, L., Tordo, P. & Bertin, D. (2007). Macromolecules, 40, 3108–3114.

Keller, E. (1997). SCHAKAL97. University of Freiburg, Germany.

Krishna, M. C., Samuni, A., Taira, J., Goldstein, S., Mitchell, J. B. & Russo, A. (1996). J. Biol. Chem. 271, 26018–26025.

Likhtenshtein, G. (2008). Nitroxides: applications in Chemistry, Biomedicine and Materials Science, edited by G. Likhtenshtein, J. Yamauchi, S. Nakatsuji, A. I. Smirnov & R. Tamura, pp. 331–363. Weinheim: Wiley-VCH Verlag.

Nardelli, M. (1995). J. Appl. Cryst. 28, 659.

Setjurc, M., Swartz, H. M. & Kocherginsky, N. (1995). Nitroxide Spin Labels: Reactions in Biology and Chemistry, edited by H. M. Swartz & N. Kocherginsky, pp. 199–206. Boca Raton: CRC Press.

Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122.