The solid-state structure of primary fatty amines: True amines or ammonium amides?

Journal of Molecular Structure 969 (2010) 106–110

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The solid-state structure of primary fatty amines: True amines or ammonium amides? Dionisia Sanz a,*, Rosa M. Claramunt a, Ibon Alkorta b,*, José Elguero b a b

Departamento de Química y Bio-orgánica, Facultad de Ciencias, UNED, Senda del Rey 9, E-28040 Madrid, Spain 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

a b s t r a c t

Article history: Received 15 December 2009 Received in revised form 20 January 2010 Accepted 26 January 2010 Available online 1 February 2010

A computational and experimental NMR study of octadecylamine and hexadecylamine proves that these compounds behave in the solid-state like all other primary amines and not as ammonium amide inner salts. Ó 2010 Elsevier B.V. All rights reserved.

Keywords: Amines Ammonium amide inner salts IR NMR CPMAS DFT calculations

1. Introduction Probably many erroneous results are forever forgotten in the chemical literature if they are reasonable (se non è vero, è ben trovato). On the contrary, a surprising result would probably lead to its verification or dismissal [1]. Recently Pohle and Gauger [2] reported that octadecylamine and related primary amines belong to ‘‘new” type-I amines that, contrary to most amines (‘‘classical” type-II amines) exist in the solid-state as ammonium amide inner salts. H R N H Type-II amines 2

H H N R R N H H Type-I amines: ammonium amide

This result is astonishing. It is known that ammonia under pressures from above 1 mbar forms the ammonium amide salt (NH2NH4+) but under normal conditions no proton transfer occurs. According to Pohle and Gauger [2] classical amines exhibit two stretching-vibration bands due to the antisymmetric (masNH) and symmetric modes (msNH) situated in the 3500–3200 cm1 region * Corresponding authors. E-mail addresses: [email protected] (D. Sanz), [email protected] (I. Alkorta). 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.01.049

and one band due to the scissoring vibration (dNH) around 1600 cm–1. For instance, octadecylamine [stearylamine, CH3(CH2)17NH2] when melted behaves normally: 3380 cm1 (masNH), 3300 cm1 (msNH) and 1615 cm1 (dNH) but in the solidstate the spectrum is very different: 3332 cm1 (mNH of NH) and 2880 cm1 (mNH of NH3+). The masNH and msNH stretching frequencies of the NH2 obey the Bellamy–Williams relationship

ms NH ¼ 0:876 mas NH þ 345:5 ðcm1 Þ;

ð1Þ

calculated msNH = 3306, experimental msNH = 3300 cm1 [3], proving that it is a type-II amine. 2. Experimental Samples of octadecylamine (ODA, m.p. 50–52 °C) and hexadecylamine (HDA, m.p. 43–45 °C) were purchased from Aldrich and used without further purification. The NMR results are given in Table 1. The assignment of the 13C signals of the central methylene signals is based on the calculated values (see Section 4.2). Columns 2, 3 and 4 reports the 15N and 13C chemical shifts and some 1J(13C–1H) coupling constants for ODA in CDCl3 (neutral molecule, NH2), for the trifluoroacetate of ODA in CDCl3 (partly protonated molecule) and for ODA in trifluoroacetic acid (protonated molecule, NH3+). Columns 5 and 6 are similar to columns 2 and 4 but for HDA. Column 7 reports the solid-state results.

107

D. Sanz et al. / Journal of Molecular Structure 969 (2010) 106–110 Table 1 Chemical shifts (d, ppm) and some coupling constants (J, Hz) of ODA and HDA. TFAA, trifluoroacetic acid (+DMSO-d6 capillary). Concentration: 55 mg/0.5 mL of solvent. Atom

15 N NH2 13 C C1 (C–NH2)

ODA CDCl3 (NH2)

ODAH+ CDCl3 TFAc

ODA TFAA (NH3+)

HDAa CDCl3 (NH2)

HDAa TFAA (NH3+)

ODA/HDAb CPMAS (NH2)

330.0

347.5

349.2

330.0

349.2

347.8

42.29

40.44 J = 142.4 31.93 1 J = 124.1 26.17 1 J = 125.5 29.30 27.42 1 J = 125.6 29.37 29.45 29.60 29.67 29.67 29.71 29.71 29.71 29.71 29.71 29.71

40.98 J = 144.2 26.28

42.29 J = 133.3 33.92 1 J = 126.8 26.88

41.00 J = 145.5 26.27

43.35

1

C2

33.93

C3

26.88

C4 C5

29.66 29.33

C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16

29.50 29.60 29.63 29.62 29.64 29.65 29.66 29.66 29.66 31.89 33.93

C17

22.65

C18 (C–CH3)

14.06

1

24.87 1 J = 126.4 28.33 27.73

22.68 1 J = 125.0 14.08 1 J = 124.5

28.33 28.48 28.57 28.62 28.62 28.62 28.64 28.64 28.64 28.64 30.92 1 J = 127.5 21.47 1 J = 125.3 11.90 1 J = 124.5

7.40 (br) 2.98 (m)

6.5 (br) 3.09 (m)

1.65 J = 7.5 1.25 0.88 J = 6.7

1.65 (q) J = 7.5 1.17 (br) 0.74 (m) J = 6.8

1

1

24.85 J = 127.3 28.09 27.69

38.97 29.04

1

29.66 29.33 29.49 29.60 29.62 29.62 29.64 – – 29.66 29.66 31.89 33.92 22.65 1 J = 125.2 14.06 1 J = 124.3

28.27 28.28 28.42 28.55 28.55 – – 28.55 28.55 28.55 30.87 1 J = 123.9 21.42

32.19 32.19 32.19 32.19 32.19 32.19 32.19 – – 32.19 32.19 32.19 33.66 24.04

11.82 J = 124.3

14.35

6.5 (br) 3.10 (m)

– –

1.65 (q) J = 7.4 1.17 (br) 0.74 (m) J = 6.8

–

1

1

H NH2 CH2-1 CH2-2

(CH2)n3 CH3 a b

1.06 (s) 2.65 (t) J = 6.9 1.41 (m) J = 6.8 1.24 (br) 0.86 (t) J = 6.9

1.05 (s) 2.66 (t) J = 7.0 1.40 (m) J = 6.9 1.24 (br) 0.86 (t) J = 6.8

– –

HDA has only 16 carbon atoms: we have removed two central ones. Both amines yield identical CPMAS NMR spectra. C CF3 115.5, 1J = 288.4 Hz, CO2 161.7, 2J = 38.7 Hz (see discussion for this species).

c 13

2.1. NMR spectroscopy Solution NMR spectra were recorded on a Bruker DRX 400 (9.4 Tesla, 400.13 MHz for 1H, 100.62 MHz for 13C and 40.56 MHz for 15N) spectrometer with a 5-mm inverse-detection H-X probe equipped with a z-gradient coil, at 300 K. Chemical shifts (d in ppm) are given from internal solvent, CDCl3 7.26 for 1H and 77.0 for 13C, and for 15N NMR nitromethane (0.00) was used as external standard. The spectra done in TFA solution was recorded with a lock capillary with DMSO-d6 2.49 for 1H and 39.5 for 13C. Typical parameters for 1H NMR spectra were spectral width 3500 Hz and pulse width 7.5 ls at an attenuation level of 0 dB. Typical parameters for 13C NMR spectra were spectral width 7 kHz, pulse width 10.6 ls at an attenuation level of 6 dB and relaxation delay 2 s; WALTZ-16 was used for broadband proton decoupling; the FIDS were multiplied by an exponential weighting (lb = 2 Hz) before Fourier transformation. 2D inverse proton detected heteronuclear shift correlation spectra, (1H–13C) gs-HMQC, (1H–13C) gs-HMBC, were acquired and processed using standard Bruker NMR software and in non-phasesensitive mode. Gradient selection was achieved through a 5% sine truncated shaped pulse gradient of 1 ms. Selected parameters for (1H–13C) gs-HMQC and gs-HMBC spectra were spectral width 3500 Hz for 1H and 7000 Hz for 13C, 1024  256 data set, number of scans 2 (gs-HMQC) or 4 (gs-HMBC) and relaxation delay 1 s. The FIDs were processed using zero filling in the F1

domain and a sine-bell window function in both dimensions was applied prior to Fourier transformation. In the gs-HMQC experiments GARP modulation of 13C was used for decoupling. Solid-state 13C (100.73 MHz) and 15N (40.60 MHz) CPMAS NMR spectra have been obtained on a Bruker WB 400 spectrometer at 300 K using a 4 mm DVT probehead. Samples were carefully packed in a 4-mm diameter cylindrical zirconia rotor with Kel-F end-caps. Operating conditions involved 3.2 ls 90° 1H pulses and decoupling field strength of 78.1 kHz by TPPM sequence. 13C spectra were originally referenced to a glycine sample and then the chemical shifts were recalculated to the Me4Si [for the carbonyl atom d(glycine) = 176.1 ppm] and 15N spectra to 15NH4Cl and then converted to nitromethane scale using the relationship: d 15N(MeNO2) = d 15N(NH4Cl)  338.1 ppm. Typical acquisition parameters for 13C CPMAS were: spectral width, 40 kHz; recycle delay, 15 s; acquisition time, 30 ms; contact time, 2 ms; and spin rate, 12 kHz. Typical acquisition parameters for 15N CPMAS were: spectral width, 40 kHz; recycle delay, 15 s; acquisition time, 30 ms; contact time, 5 ms; and spin rate, 6 kHz. 3. Computational details The optimization of the compounds was carried out at the B3LYP/6-31G(d) computational level [4,5]. In all the cases, harmonic frequency calculations have been carried out to confirm that the geometries obtained correspond to energetic minima

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D. Sanz et al. / Journal of Molecular Structure 969 (2010) 106–110

[6]. On these minimum energies we have calculated, at the same level, absolute shielding within the GIAO approximation [GIAO/ B3LYP/6-31G(d)] [7,8]. All these calculations have been carried out with the Gaussian-03 package [9]. The IR/Raman frequencies were scaled using the scaling factor (0.9614) proposed for the computational level used [9].

NH2

4. Results Since, as expected, both amines behave similarly we will discuss only the case of ODA. 3000

4.1. IR and Raman

3100

3200

We have reported in Table 2 the experimental and calculated values of the frequencies related to the NH2, NH and NH3+ groups. The Raman data are from a paper of Amorin da Costa, Geraldes and Teixeira-Dias and concern dodecylamine (DCA) [10]. Fig. 1 represents the 3000–3500 cm1 part of the three calculated spectra. The experimental data for the amino form (3380, 3300 and 1615 cm1 of the melted ODA) are well reproduced by the calculations

3300

3400

3500

3300

3400

3500

3300

3400

3500

NH3

Exp: ¼ ð25:6  1:0Þ þ ð1:003  0:004Þ Calcd: n ¼ 3; R2 ¼ 1:000:

3000

3100

3200

ð2Þ On the other side, the bands assigned to the ammonium amide inner salts are very different from those calculated for the sum of the anion and the cation. They could be closer to a structure containing strong hydrogen bonds or even to amine hydrates [11].

NH

4.2. NMR Concerning the NMR study we have represented in Fig. 2 the results obtained by 13C CPMAS NMR spectroscopy. The protonation effects of ODA and HDA in solution are comparable to those reported for other primary amines [12–15]. The trendlines of Fig. 2 including the TMS (r = 189.6911) [16,17] are:

3000

3100

3200

Fig. 1. Scaled IR spectra of the ODA: neutral (NH2), protonated (NH3+) and deprotonated (NH).

Table 2 Wavenumbers (in cm1) of IR and Raman bands of ODA and DCA related to NH2, NH and NH3+ groups. Abbreviations: s, strong; m, medium; w, weak. Calculations: in parenthesis, IR intensities and in brackets, Raman intensities (only for the NH2 group). Exp. IR ODA [2] Melted masNH (NH2) Melted msNH (NH2) Melted dNH (NH2) Solid mNH (NH) Solid mNH (NH3+)

Solid dNH (NH) Solid dasNH (NH3+) Solid dsNH (NH3+)

3380 w–m 3300 w–m 1615 m 3332 m 2880 m–s

1646 w 1568 m 1486 m–s

Exp. Raman DCA [10]

Calcd. IR ODA (scaled, this work)

3380 (liquid) m 3323 (liquid); 3250 (solid) s

NH2 NH2 NH2 NH NH3+

NH NH3+ NH3+

3396.8 (1.4) [56.2] 3316.3 (2.6) [93.9] 1636.2 (20.2) [9.6] 3093.9 (325.2) 3259.1 (mas) (50.0) 3347.0 (mas) (87.3) 3348.8 (ms) (90.5) Nothing – 1635.8 (41.0) 1629.9 (59.1)

D. Sanz et al. / Journal of Molecular Structure 969 (2010) 106–110

(+10.6 ppm) [15]. At least, one of the values in Table 1 (330.6 and 349.2, +18.6 ppm) is anomalous, probably due to a TFAA solvent effect. Such value in the solid-state (347.8 ppm) could correspond to either the neutral (NH2) or to the protonated ODA (NH3+). Using the equation [7],

H H 3C

N

H

Octadecylamine, ODA (stearylamine) H H 3C

N

H H

Protonated octadecylamine, ODAH+

d15 N ¼ 131:6  1:01

H3C

N

H

45 C1-NH2 (red), C-NH3 (blue), C-NH– (black)

Experimental 13C CPMAS (ppm)

r15 N;

ð7Þ

we have transformed the absolute shieldings into chemical shifts: propylamine 355.5, propylammonium 349.0, butylamine 355.8, butylammonium 349.8, ODA 355.6, ODAH+ 349.7, ODA (NH) 331.9 ppm. The value for the same 15N signal in CDCl3 (330.6 ppm) is also anomalous but must correspond to the neutral molecule; in the solid-state it is close to that of ODA cation but the corresponding anion (expected at 331.9 ppm) is clearly absent.

Deprotonated octadecylamine, ODA–

40

109

C2

35 C16

5. Conclusions

30

C3

Both the vibrational and the nuclear magnetic resonance spectroscopies demonstrated that ODA and other long-chain saturated amines exist in the solid-state as well as in solution in the classical amine form disproving the existence of the so-called type-I amines (ammonium amide inner salts).

25 C17

20

Acknowledgements 15

s13C NH2 s13C NH3+ s13C NH–

CH3

10 120

130

140

150

160

170

180

Calculated sigma 13C for NH2, NH3+ and NH– (ppm) Fig. 2. Scatter of the 13C CPMAS chemical shifts vs. the calculated absolute shieldings (all values in ppm) for the neutral (red), protonated (blue) and deprotonated (black) ODA. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)

d13 C CPMAS ¼ ð184:7  1:6Þ  ð0:977  0:010Þ r13 CðNH2 Þ; n ¼ 19; R2 ¼ 0:998

ð3Þ

d13 C CPMAS ¼ ð177:4  9:9Þ  ð0:928  0:062Þ r13 CðNHþ3 Þ;n ¼ 19;R2 ¼ 0:929

ð4Þ

d13 C CPMAS ¼ ð139:6  10:6Þ  ð0:683  0:068Þ r13 CðNH Þ;n ¼ 19;R2 ¼ 0:856

ð5Þ

It is evident that not only the solid-state 13C CPMAS spectrum corresponds to the neutral form (Eq. (3)) but that an equimolar mixture of cation and anion (the inner salt) should have some signals splitted: C1 (48.5 and 66.7 ppm), C2 (29.4 and 43.7 ppm) and C3 (27.1 and 30.2 ppm), respectively. However, in the spectrum of ODA in TFAA (that includes solvent effects) the signals of C1, C2 and C3 appear at 40.98, 26.28 and 24.87 ppm, respectively. The differences are due to the fact that we are comparing CPMAS and TFAA chemical shifts. For this solvent, and including TMS, the regression equation is: d13 C TFAA ¼ ð163:1  2:8Þ  ð0:862  0:018Þ r13 CðNHþ3 Þ;n ¼ 19;R2 ¼ 0:993 ð6Þ

The worse signal belongs to the methyl group (11.90 ppm) that should appear at 13.6–13.9 ppm, like other amines [12,13]. This anomaly may be related to a folding or a head-to-tail dimerization of ODA in TFAA. We have also recorded the spectrum of ODA in CDCl3 after adding two drops of TFAA (Table 1, third column) but the chemical shifts correspond to a 40% of protonated ODA (ODAH+ in TFAA, fourth column) and 60% of neutral ODA (in CDCl3, second column). The 15N chemical shifts are rather insensitive to amine protonation [15]. For instance, propylamine, 359.6, hydrochloride 349.0 (+10.6 ppm), butylamine, 359.4, hydrochloride, 348.8

This work was supported by the Ministerio de Educación y Ciencia (Project No. CTQ2009-13129-C02-02), the Spanish MEC (CTQ2007-62113), and the Comunidad Autónoma de Madrid (Project MADRISOLAR, Ref. S-0505/PPQ/0225). Thanks are given to the CTI (CSIC) for allocation of computer time. Appendix A. Supplementary data Supplementary data associated with this article (IR frequencies and intensities as well as absolute shieldings) can be found, in the online version, at doi:10.1016/j.molstruc.2010.01.049. References [1] I. Alkorta, F. Blanco, J. Elguero, J. Mol. Struct. (THEOCHEM) 896 (2009) 92–95. [2] W. Pohle, D.R. Gauger, J. Mol. Struct. 924–926 (2009) 144–147. [3] (a) L.J. Bellamy, R.L. Williams, Spectrochim. Acta 9 (1957) 341–345; (b) L.J. Bellamy, Advances in Infrared Group Frequencies, Methuen, London, 1969; (c) L.K. Dyall, G. L’abbé, W. Dehaen, Spectrochim. Acta A 53 (1997) 377–378; (d) I. Alkorta, J. Elguero, H.H. Limbach, I.G. Shenderovich, T. Winkler, Magn. Reson. Chem. 47 (2009) 585–592. [4] (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. [5] P.A. Hariharan, J.A. Pople, Theor. Chim. Acta 28 (1973) 213–222. [6] J.W. McIver, A.K. Komornicki, J. Am. Chem. Soc. 94 (1972) 2625–2633. [7] (a) R. Ditchfield, Mol. Phys. 27 (1974) 789–807; (b) F. London, J. Phys. Radium 8 (1937) 397–409. [8] (a) P.v.R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N.J.R.v. Hommes, J. Am. Chem. Soc. 118 (1996) 6317–6318; (b) P.v.R. Schleyer, M. Manoharan, Z.X. Wang, B. Kiran, H. Jiao, R. Puchta, N.J.R.v. Eikema Hommes, Org. Lett. 3 (2001) 2465–2468. [9] 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. Adao, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. 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. AlLaham, 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, Gaussian, Pittsburgh, 2003.

110

D. Sanz et al. / Journal of Molecular Structure 969 (2010) 106–110

[10] A.M. Amorim da Costa, C.F.G.C. Geraldes, J.J.C. Teixeira-Dias, J. Raman Spectrosc. 13 (1982) 56–62. [11] S. Janeda, D. Mootz, Z. Naturforsch. 53b (1998) 1197–1202. [12] J. Llinares, J. Elguero, R. Faure, E.J. Vincent, Org. Magn. Reson. 14 (1980) 20–24. [13] R. Faure, J. Llinares, J. Elguero, An. Quím. 81C (1985) 167–172. [14] H.O. Kalinowski, S. Berger, S. Braun, Carbon-13 NMR Spectroscopy, Wiley, Chichester, 1997. p. 221.

[15] S. Berger, S. Braun, H.O. Kalinowski, NMR Spectroscopy of the Non-Metallic Elements, Wiley, Chichester, 1997. p. 117. [16] A. Frideling, R. Faure, J.-P. Galy, A. Kenz, I. Alkorta, J. Elguero, Eur. J. Med. Chem. 39 (2004) 37–48. [17] O. Prakash, A. Kumar, A. Sadana, R. Prakash, S.P. Sing, R.M. Claramunt, D. Sanz, I. Alkorta, J. Elguero, Tetrahedron 61 (2005) 6642–6651.