An experimental and theoretical study of 1J(13C–14N) coupling constants in nitro-aromatic and nitro-heteroaromatic compounds

An experimental and theoretical study of 1J(13C–14N) coupling constants in nitro-aromatic and nitro-heteroaromatic compounds

Journal of Molecular Structure 979 (2010) 180–185 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.els...

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Journal of Molecular Structure 979 (2010) 180–185

Contents lists available at ScienceDirect

Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc

An experimental and theoretical study of 1J(13C–14N) coupling constants in nitro-aromatic and nitro-heteroaromatic compounds Guy Jacob a, Grégoire Hervé a, Ibon Alkorta b,*, José Elguero b a b

SNPE Matériaux Energétiques, 9 rue Lavoisier, 91710 Vert Le Petit, France Instituto de Química Médica (CSIC), Juan de la Cierva, 3, E-28006 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 5 May 2010 Received in revised form 15 June 2010 Accepted 16 June 2010 Available online 22 June 2010 Keywords: Nitropyrazoles Nitrofurazans Nitrobenzenes 13 C–14N couplings 6-311++G(d,p) calculations

a b s t r a c t Data for 12 nitro derivatives (benzenes, pyrroles, furazans and pyrazoles) are reported, of which some furazans and the three pyrazoles are new. These couplings, in the 9–18 Hz range, were compared with B3LYP/6-311++G(d,p) calculations. Although the agreement is not very good, several interesting consequences can be drawn: the value of the coupling constant is not related to the position in the ring nor to the torsion angle but the dimensionless parameter g. For large g (slow quadrupole relaxation) the triplets are observed while for small g (rapid quadrupole relaxation) a broad triplet or even a broad singlet is observed. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction The number of molecules with C–N is very large (in the order of millions, probably) and their 13C NMR spectra very common. However, the number of reported 1J(13C–14N) for unlabeled compounds spin–spin coupling constants (SSCC) is very small and concerns nitro derivatives and highly symmetric ammonium salts, like tetraethylammonium [1]. Two reference books reported several 1 13 J( C–15N) SSCC, one of them, relevant for the present work, is that of nitrobenzene (1), 1J(13C–15N) = 14.6 Hz [2] and 14.57 Hz [3]. Using the equation J(15N, X) = 1.4027, J(14N, X), the value 1 13 J( C–14N) = 10.4 Hz can be calculated. Having observed these couplings for a number of unlabeled nitro heterocycles (nitrofurazans and nitropyrazoles) we have searched the literature for other examples and carried out a theoretical study of their absolute 13C shieldings and 13C–14N coupling constants. The 12 studied compounds are gathered in Fig. 1. 2. Methods 2.1.

13

C NMR

We have recorded the 13C NMR spectra in solution for compounds 5, 6, 8–12. The literature data concerns compounds 1

* Corresponding author. E-mail address: [email protected] (I. Alkorta). 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.06.022

(15N transformed into 14N) [2,3], 2 [4], 3 [4], 4 [4], 5 [5], 6 [4] and 7 [5]. The spectra of compounds 5–12 were recorded on a Bruker Avance 400 instrument with a 10 mm BBO ATM probe using as solvents nitromethane and acetone. Two relevant examples are given in Fig. 2. The carbon atoms bearing nitro groups appear as triplets with intensities close to the 2:3:2 theoretical ratio [6].

2.2. Computational The geometry of the systems has been fully optimized with the hybrid HF/DFT, B3LYP, computational method [7] and the 6-31G(d) basis set [8]. Frequency calculations have been carried out at the same computational level to confirm that the structures obtained correspond to energetic minima (all real frequencies) or to transition states (only one imaginary frequency) [9]. A further optimization has been performed at the B3LYP/6-311++G(d,p) computational level [10]. These geometries have been used for the calculation of the absolute chemical shieldings and indirect coupling constant at the B3LYP/6-311++G(d,p) level with the GIAO method [11]. For those calculations where the evolution of the magnetic properties with the dihedral angle have been studied, the geometry of the molecules has been optimized at the B3LYP/6-311++G(d,p) level while keeping fixed the desired dihedral angle. All the calculations have been carried out with the Gaussian-03 package [12].

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G. Jacob et al. / Journal of Molecular Structure 979 (2010) 180–185

NO2

NO2 1

O2N 2

2 3

O2N 1

4

O

NO2

2N 1

N5

O

F 3

N5

2N 1

O

N 5 NO2 H

NC

NO2

3

4

2N 1

O

4

2N 1

5

N 5 NO2 NH2

10

N5

8 NO2

3

N NO2

9

N5

O2N

4

2N 1

2N 1

4

7 NO2

3

O2N 5 N 2 NO2 CH3

4 NO2

4

O2N

4

3 1

NO2

6 NO2

3

O2N

NO2

4

3

3

5 O2N

6

H2N

4

2N 1

NO2

4 5

O2N NO2

1

2 NO2

3

O2N

NO2

1 O2N

3

NO2 NO2

11

O2N

NO2

3

4

2N 1

N 5 NO2 CH3

12

Fig. 1. The 12 C-nitro derivatives.

Fig. 2.

13

C NMR spectra at 100.61 MHz of compounds 9 (left) and 10 (right).

13

C–14N coupling constants

3. Results and discussion

3.2.

3.1. Absolute shieldings and chemical shifts

We have collected in Table 2 the 1J(13C–14N) SSCC of the compounds of Fig. 1. The signal of the C3 carbon atom of 10 at 145.1 ppm (Fig. 2 right) is represented enlarged in Fig. 4 [15]. A 2J(13C–15N) has been measured in 15N labeled nitrobenzene ( 1.672 Hz) [16] that we have transformed into a 2J(13C–14N) = 1.2 Hz. The value is similar to the value we have measured (1.35 Hz, Fig. 4) and calculated (1.06 Hz) for 1,3,4-trinitropyrazole (10). The agreement of the values (Fig. 5) is not very good, exp. = (1.4 ± 2.1) + (1.0 ± 0.14) calc., n = 16, R2 = 0.78; since the intercept is not significant, we can assume that it is 0, then

The 13C NMR experimental chemical shifts (only sp2 carbon atoms) and the corresponding GIAO/ B3LYP/6-311++G(d,p) calculated absolute shieldings are reported in Table 1. The correlation is acceptably good taking into account that the range of chemical shifts covers only 36.4 ppm. When there are several values for a given compound, we have used those determined in this work. The trendline (Fig. 3) is similar to that reported for a large number of carbon atoms (n = 461): d13C = 175.7–0.963 r13C [14].

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G. Jacob et al. / Journal of Molecular Structure 979 (2010) 180–185

Table 1 13 C experimental chemical shifts (d, ppm) and calculated absolute shieldings (r, ppm). Compd. 1

2

3 4

5 5 6 6 7

9

10

11

12

a

d, ppm

Refs.

r, ppm

C1 (CNO2) C2 (CH) C3 (CH) C4 (CH) C1, C5 (CNO2) C2, C4 (CNO2) C3 (CNO2) C6 (CH) C1 (CNO2) C2 (CNO2) C3 (CNO2) C4 (CNO2) C5 (CNO2) Aver C2, C5 (CNO2) Aver C3, C4 (CNO2) C3 (CNO2) C3 (CNO2) C3 (CNH2) C4 (CNO2) C3 (CNH2) C4 (CNO2) C3 (CF)

148.4 123.6 129.4 134.6 141.5 139.8 138.2 125.8 138.7 – – – – 128.8 123.6 153.5/152.9 153.8 152.3 153.9 151.6 153.2 159.0 1 JCF = 291.2 Hza 150.5 129.2b 160.1 – – 143.2 122.6 145.1 128.4 (br) 128.6 140.8 (br) 123.1 136.5 142.8 (br) 123.5 137.6

[13] [13] [13] [13] [4] [4] [4] [4] [4]

27.40 53.59 49.63 43.38 32.73 33.60 35.57 51.69 35.35 51.27 47.41 55.35 42.23 46.75 51.38 21.76 21.76 26.82 24.80 26.82 24.80 14.73

C4 (CNO2) C3 (CCN) C4 (CNO2) C3 (CNO2) C5 (CNO2) Aver C3, C5 (CNO2) C4 (CNO2) C3 (CNO2) C4 (CNO2) C5 (CH) C3 (CNO2) C4 (CNO2) C5 (CNO2) C3 (CNO2) C4 (CNO2) C5 (CNO2)

8

b

Atom

[4] [4] [5] This work [4,5] [5] This work This work [5]

Table 2 Experimental and calculated Compd. 1 2

3 4

5 5 6 6 7 8 9

10 11

[5] This work This work

This This This This This This This This This This This

work work work work work work work work work work work

26.65 48.88 15.76 29.51 38.60 34.05 53.12 31.33 48.59 54.94 38.04 51.93 44.44 33.47 50.38 38.36

12

13 14 15

13

Atom C1 C1, C5 C2, C4 C3 C1 C2 C3 C4 C5 Aver C2, C5 Aver C3, C4 C3 C3 C4 C4 C4 C4 C3 C5 Aver C3, C5 C4 C3 C4 C3 C4 C5 C3 C4 C5 C3 C4 C5

C–14N SSCC (Hz) of CNO2 carbon atoms. Exp. a

10.4 9 14 14 14 – – – – 17 17 Not reported 20.9 17 17.0 Not reported 19.45 – – br 17.5 16.9b br br 17 17 br 18.7 18.0 – – –

Refs.

Calc.

[2,3] [4] [4] [4] [4]

9.1 11.1 11.7 11.4 11.5 16.7 15.6 16.0 16.2 16.4 15.8 17.7 17.7 16.8 16.8 17.4 16.6 17.8 16.0 16.9 16.2 17.9 15.5 18.3 16.2 16.4 18.0 16.0 15.4 16.4 15.2 15.5

[4] [4] [5] This work [4,5] This work [5] This work

This This This This This This This This This This – – –

work work work work work work work work work work

a

Calculated from a 15N labeled compound, not observed in the unlabeled sample. A 2J(13C–14N) with the nitro group at position 4 is observed (1.35 Hz); calc. 1.06 Hz. b

Calculated: 1JCF = 351.5 Hz. CN 104.5 ppm (calc: r = 74.6951 ppm).

170

160

δ (ppm)

150

140

130 Fig. 4. The signal of the C3 carbon atom of compound 10.

120

110 CN (8)

100 10

20

30

40

50

60

70

80

σ (ppm) Fig. 3. Scatter of experimental chemical shifts vs. calculated absolute shieldings, the trendline equation is: d (ppm) = (174.8 ± 1.1) (0.958 ± 0.026) r (ppm), n = 29, R2 = 0.98.

exp. = (1.09 ± 0.02) calc., n = 16, R2 = 0.992. The main conclusions are: (i) the difference between experimental and calculated 1 13 J( C–14N) SSCC is comprised between 2.1 and +3.2 Hz, which is acceptable; (ii) the six- and five-membered rings are clearly differentiated. Besides, the triplets observed in 13C NMR only represent the true value of 1J(13C–14N) when they are narrow. If they are large, the broadening is accompanied by the two lateral bands of the triplet moving towards the central one with an ‘‘apparent” decrease of 1J(13C–14N).

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G. Jacob et al. / Journal of Molecular Structure 979 (2010) 180–185

19

22

18

5

17

20

12 4

16

10

6

15

Calc J (Hz)

Exp. (Hz)

18

16

8

12

14 13 12

Benzenes

11 heterocycles

10

14

2,3

benzenes

9 8 -10

12

1

0

10

20

30

40

50

60

70

80

90

100

Dihedral angle θº

10

2 (C1,C5)

1

Fig. 6. Plot of calculated values of J(13C–14N) vs. the calculated dihedral O–N–C–C angle h.

8 12

14

16

18

20

22

Calc. (Hz) Fig. 5. Plot of experimental vs. calculated values of 1J(13C–14N) in Hz.

Table 3 Calculated dihedral O–N–C–C angle h, rotational barriers of the NO2 groups about planar and perpendicular transition states (TS in kJ mol 1). ‘‘Largest TS” is the largest of the two values (0° and 90°). Compd.

Atom



TS(0°)

TS(90°)

Largest TS

1 2

C1 C1, C5 C2, C4 C3 C1 C2 C3 C4 C5 Aver C2/C5 Aver C3/C4 C3 C4 C4 C4 C3 C5 Aver C3, C5 C4 C3 C4 C3 C4 C5 C3 C4 C5 C3 C4 C5

0.0 33.3 63.2 52.8 55.7 8.4 78.8 16.5 65.6 37.0 47.6 37.7 3.4 0.0 0.0 0.0 0.0 0.0 90.1 62.6 10.4 7.7 78.0 12.7 0 90 0 0.0 0.0 0.0

0.0 11.5 22.9 11.5 27.5

31.5 21.5 14.7 14.7 26.6

31.5 21.5 22.9 14.7 27.5

2.8 2.4 2.5 0.1 0.0 0.0 0.0 0.0 0.0 17.5 11.6 2.2 0.1 14.2 0.1 0.0 15.5 0.0 0.0 0.0 0.0

2.4 2.8 12.0 34.4 14.6 13.9 18.4 28.1 23.2 0.0 2.2 12.2 14.2 0.2 14.2 15.5 0.0 15.5 26.6 38.5 40.0

2.8 2.8 2.5 34.4 14.6 13.9 18.4 28.1 23.2 17.5 11.6 12.2 14.2 14.2 14.2 15.5 15.5 15.5 26.6 38.5 40.0

3 4

5 6 7 8 9a

10 11

12

13 14 15

Of the calculated TSs of Table 3, only the largest TS barrier (either through 0° or through 90°) shows a tendency with the calculated 1J(13C–14N) of heterocycles: when the TS energy increases, 1 13 J( C–14N) decreases. We then calculated the variation values of 1J(13C–14N) vs. the dihedral O–N–C–C angle h for four model compounds: nitrobenzene (1), 3-nitro-1H-pyrazole (13), 4-nitro-1H-pyrazole (14) and 5-nitro-1H-pyrazole (15) (Fig. 7). The effect of the dihedral angle on 1J(13C–14N) is weak, about ±1 Hz (Fig. 8). From the conformation of minimum energy, h = 0° to the TS, 90° or close to it, 1J(CN) increases for 1 and 13 and decreases for 14 and 15. The effect in nitrobenzene is the same as that observed in Fig. 5. Note that the proximity of a pyridine-like nitrogen 13 or a pyrrole-like nitrogen 15 is not determinant for the slope of the curves in Fig. 8.

O

a The X-ray molecular structure of 9 has been recently reported [17]: there are two independent molecules A and B in the unit cell with torsion angles of 0.3° (C3), 0.3° (C5), 80.7° (C4) for A and 9.4° (C3), 8.1° (C5), 67.2° (C4).

N

O O N

O

O N O

3

N

1

4

N

N H

N

N H

14

13

O N 5 N O H

15

Fig. 7. Model compounds used for rotational studies.

17.0 16.5

C6H5NO2 3NO2Pz 4NO2Pz 5NO2Pz

16.0 15.5

J (CN)

10

1

8

15.0 14.5 14.0 10.0 9.5

We have examined whether the calculated values are related to the dihedral O–N–C–C angle h (Table 3). To find some trends, it is necessary to separate the heterocycles from the benzenes, but only the last ones are linearly correlated to h (Fig. 6, when h increases, J increases). For the heterocycles, there seems to be a slight decrease of J for high values of h.

9.0 0

20

40

60

80

100

120

140

160

180

Dihedral Angle Fig. 8. Variation of 1J(13C–14N) with the dihedral O–N–C–C angle h°. The ordinate line is broken to separate nitrobenzene (about 10 Hz) from nitropyrazoles (about 16 Hz).

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G. Jacob et al. / Journal of Molecular Structure 979 (2010) 180–185

The tetranitro derivative 4, also a geared compound [17], is very interesting. The rotation of the four nitro groups transforms 4 into its quasi-mirror image 4, which differs only in the conformation of the methyl group; this explains why the corresponding barrier is very low since the rotation of the four nitro groups (from 4 to 4) and the rotation of the methyl group (from 4 to 4) are identical processes.

A final comment about the transition states. It is expected when the dihedral angle h is close to 0° that the TS(0°) should be close to 0 kJ mol 1 and when the dihedral angle h is close to 90° the TS(0°) should be large; the reciprocal should be true for the other TS: when the dihedral angle h is close to 0° the TS(90°) should be large and when the dihedral angle h is close to 90° the TS(90°) should be close to 0 kJ mol 1.

O

O N

O

O N

O

O

N

N

O

O N

N

O

C

O

O

N

H

H

15

TS (0)

14

TS (90)

6 1 9 13

25 pyrazoles

20

9 (C4) 9 11 (C4)

11 11 7 8

N

N

H

O

C H

4

The case of compound 5 is also interesting: the 0° and 90° barriers are identical because the TS is the same for both: O

O

O O

O

5 min

O

O

O

O

5 TS

O

O

5 min

O

4. Conclusions The observation of the characteristic triplets in the 13C NMR spectra of nitro-aromatic (or heterocyclic) compounds is neither related to the position in the ring nor to the torsion angle since the calculated values are always close to 11 Hz in benzenes and 17 Hz in heterocycles. Thus, we conclude that the non-observation of 13C–14N couplings is due to broadening [4,6]. In the case of 3,4,5-trinitro1H-pyrazole (9), the annular tautomerism of NH-pyrazoles makes C3 and C5 equivalent but broad at room temperature (Fig. 2 left) preventing the possible observation of the triplet, at least for one of these carbons. In 1,3,4-trinitropyrazole (10) while C3 is well resolved, C4 is broad (Fig. 2 right) and in 1-amino-3,4,5-trinitropyrazole (11), C3 is broad while C4 and C5 are well resolved triplets. These observations are related to the dimensionless parameter g (defined by Pople as 2psJ) [6a]. For large g (slow quadrupole relaxation) the triplets are observed while for small g (rapid quadrupole relaxation) a broad triplet or even a broad singlet is observed. It has been suggested that the quadrupole relaxation of the 14N atom is related to the steric hindrance to rotation (largest TS), this explanation is consistent with the observation reported in the present work.

45

TS (kJ/mol)

N

4*

If we exclude compounds 2, 3, 4 and 5, we obtain Fig. 9, which satisfies the previous expectations, i.e. the complementary red and blue zones. There is a considerable dispersion of TS(90) values in the 0° dihedral angle zone to the point that the blue trendline [TS(90°) = (23.4 ± 2.4) (0.29 ± 0.06) dihedral (°), n = 18, R2 = 0.57] is not very significant. On the other hand, the red trendline correspond to a good fit [TS(0°) = (0.24 ± 0.20) + (0.185 ± 0.005) dihedral (°), n = 18, R2 = 0.988]. For the polynitrobenzenes 2 and 3 that have the nitro groups geared, both TS are high and similar [18].

15

O

O

H

H

4

O

N

N C

H

H

H

30

N

O N

35

N

TS

TS

40

O

O

12 (C4)

Acknowledgements

10

10 (C3)

10

This work was carried out with financial support from the Ministerio de Ciencia e Innovación (Project No. CTQ2009-13129-C0202) and Comunidad Autónoma de Madrid (Project MADRISOLAR2, reference S-2009/PPQ/1533). Thanks are due to the CTI (CSIC) for allocation of computer time. Pyrazoles were synthesized under contract from French DGA which is greatly acknowledged.

5 9&12 (C4) 10 (C3)

0

11 (C4)

References

-5 -10

0

10

20

30

40

50

60

70

80

90

100

Dihedral (º) Fig. 9. Variation of TS(0°) and TS (90°) with the dihedral O–N–C–C angle h°.

[1] M.-L. Jimeno, I. Alkorta, J. Elguero, J.E. Del Bene, Magn. Reson. Chem. 44 (2006) 698–707. [2] H.-O. Kalinowski, S. Berger, S. Braun, Carbon-13 NMR Spectroscopy, John Wiley and Sons, Chichester, 1988, pp. 569.

G. Jacob et al. / Journal of Molecular Structure 979 (2010) 180–185 [3] S. Berger, S. Braun, H.-O. Kalinowski, NMR Spectroscopy of the Non-Metallic Elements, John Wiley and Sons, Chichester, 1997, pp. 265. [4] M.D. Coburn, C.B. Storm, D.W. Moore, T.G. Archibald, Magn. Reson. Chem. 28 (1990) 16–20. [5] L. Larina, V. Lopyrev, Nitroazoles: Synthesis, Structure and Applications, Topics in Applied Chemistry, Springer, Dordrecht, 2009, pp. 210. [6] (a) J.A. Pople, Mol. Phys. 1 (1958) 168–174; (b) M. Franck-Neumann, J.M. Lehn, Mol. Phys. 7 (1964) 197–199; (c) M. Suzuki, R. Kubo, Mol. Phys. 7 (1964) 201–209; (d) M. Witanowski, G.A. Webb, Nitrogen NMR spectroscopy, in: E.F. Mooney (Ed.), Ann. Rep. NMR Spectrosc., vol. 5, 1972, pp. 414. [7] (a) A.D. Becke, Phys. Rev. A 38 (1988) 3098–3100; (b) A.D. Becke, J. Chem. Phys. 98 (1993) 5648–5652; (c) C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785–789. [8] P.A. Hariharan, J.A. Pople, Theor. Chim. Acta 28 (1973) 213–222. [9] J.W. McIver, A.K. Komornicki, J. Am. Chem. Soc. 94 (1972) 2625–2633. [10] (a) R. Ditchfield, W.J. Hehre, J.A. Pople, J. Chem. Phys. 54 (1971) 724–728; (b) M.J. Frisch, J.A. Pople, R. Krishnam, J.S. Binkley, J. Chem. Phys. 80 (1984) 3265–3269. [11] (a) R. Ditchfield, Mol. Phys. 27 (1974) 789–807; (b) F. London, J. Phys. Radium 8 (1937) 397–409. [12] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.

[13] [14] [15]

[16] [17] [18]

185

Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian-03, Revision E01, Gaussian, Inc., Wallingford, CT, 2004. H.-O. Kalinowski, S. Berger, S. Braun, Carbon-13 NMR Spectroscopy, John Wiley and Sons, Chichester, 1988, pp. 315. F. Blanco, I. Alkorta, J. Elguero, Magn. Reson. Chem. 45 (2007) 797–800. Three 1J(13C–15N) coupling constants (sign unknown) have been reported for 15 N labeled nitroimidazoles: 2,4-dinitroimidazole (30.6 Hz, 2-nitro); [S. Bulusu, R. Damavarapu, J.R. Autera, B. Behrens, L.M. Minier, J. Villanueva, K. Jayasuriya, T. Axenrod, J. Phys. Chem. 99 (1995) 5009–5015] 5-nitroimidazole4-carboxylic acid (26.4 Hz) and 5-nitroimidazole-4-carboxamide (25.9 Hz). [M.K. Jain, Y.-P. Tsao, N.-L. Ho, J.-W. Cheng, J. Org. Chem. 66 (2001) 6472–6475] The calculated 1J(13C–15N) values (at the same level than the others compounds of this paper) are: 28.1, 22.9 and 22.8 Hz, that is, about 90% of the experimental ones. L. Ernst, E. Lustig, V. Wray, J. Magn. Reson. 22 (1976) 459–464. G. Hervé, C. Roussel, H. Graindorge, Angew. Chem. Int. Ed. 49 (2010) 3177– 3181. (a) C. Roussel, A.T. Balaban, U. Berg, M. Chanon, R. Gallo, G. Klatte, J.A. Memiaghe, J. Metzger, D. Oniciu, J. Pierrot-Sanders, Tetrahedron 39 (1983) 4209–4219; (b) C. Uncuta, I. Paun, C. Deleanu, M. Plavetti, A.T. Balaban, C. Roussel, New J. Chem. 21 (1997) 1055–1065.