Intramolecular interactions in nitroamines studied by 1H, 13C, 15N and 17O NMR spectral and quantum chemical methods
1H, 13C, 15N and 17O NMR chemical shifts are used for the characterization of the intramolecular interactions in several nitramines of the Me2N-G-NO2 type. The charge of lone electron pair of the amino group in N,N-dimethylnitramine, N,N-dimethyl-2-nitroethenamine, N,N-dimethyl-p-nitroaniline, 4-nitro-β-dimethylaminostyrene, 4-N,N-dimethylamino-β-nitrostyrene, 4-(N,N-dimethylamino)-4′-nitrobiphenyl, and 4-(N,N-dimethylamino)-4′-nitrostilbene is transferred not only to the nitro oxygens, but also to the vinylene and benzene carbons of the G spacer and to N-methyl carbons as well. Decreased nuclear shielding is found to be qualitatively related to the decreased atomic charge around a nucleus. This finding was further verified and quantified by comparison of the NMR data with those obtained by ab initio quantum chemical calculations. 17O NMR chemical shift changes seem to be more significant when the interacting NMe2 and NO2 groups are separated by a short spacer. On the other hand, 15N NMR chemical shifts suggest that a decrease of the charge at the amino nitrogen is not related to the length of the spacer alone. A lack of the linear dependence between the 17Onitro and 15Namino chemical shifts suggests that the charge lost by the amino nitrogen was only partially gained by the oxygens in the nitro group. The increased shieldings of the aryl carbons in 4-(N,N-dimethylamino)-4′-nitrobiphenyl indicate that atoms of the p,p-biphenylene spacer also gain some charge originating from the amino nitrogen. 3JH,H spin–spin coupling constant shows that among different vinylene compounds, the charge transfer to the nitro group is practically effective only in N,N-dimethyl-2-nitroethenamine where the bond between the vinylene carbons is significantly of low order by character. The calculated Natural Population Analysis (NPA) data confirms that except the nitro oxygens, other atoms that receive the negative charge lost by NMe2 in the compounds studied are the aryl and N-methyl carbons.
KeywordsNitroamines Resonance interaction Charge transfer Molecular structure Ab initio calculations
The separation of various electron-donors and electron-acceptors by the conjugated π-electron spacer stabilized by resonance results in the nonlinear optical (NLO) response . Large application possibilities of such push–pull systems in various photonic technologies deserve the structures of this type of compounds to be carefully studied . Therefore, intramolecular interactions in such systems have been studied for a long time [3, 4, 5, 6, 7, 8, 9, 10, 11].
Among various more common electron-donors and electron-acceptors, Me2N and NO2 groups are the extreme cases and thus a molecule bearing both of them, Me2N-G-NO2, is a classical push–pull system. Generally, an elongation of the spacer G was found to weaken the conjugation between the NMe2 and NO2 groups . However, it is still effective in 4-N,N-dimethylamino-β-nitrostyrene and it is argued to be responsible for stacking of such molecules in dimers and tetramers with antiparallel dipoles . Similar aggregation was observed for the Brooker’s merocyanine structurally related to (E)-N,N-dimethyl-4-(4-nitrostyryl)benzeneamine , which is a resonance hybrid weighed toward a zwitterionic form even when dissolved in solvents of low dielectric constant .
According to the valence bond theory, the negative charge from the amino group in p-nitroaniline is transferred merely to the benzene ring than to the nitro group . So, in spite of the criticism towards the basic concept of intramolecular charge transfer [15, 16], this phenomenon seems still to be important factor that affects the molecular structure. Recent findings [17, 18, 19, 20] suggest, however, that this hypothesis should be more or less modified.
X-ray and electron diffraction structural data show that a significant amount of the charge from the amino nitrogen in N,N-dimethylnitramine, N,N-dimethyl-2-nitroethenamine, N,N-dimethyl-p-nitroaniline, 4-nitro-β-N,N-dimethylaminostyrene and 4-N,N-dimethylamino-β-nitrostyrene was transferred to the nitro group . Effectiveness of the ground-state charge transfer in N,N-dimethylnitramine was found to be very high. This effect is much weaker in 4-N,N-dimethylamino-β-nitrostyrene. An electron donating power of the amino nitrogen atom in 4-N,N-dimethylamino-β-nitrostyrene is weaker than in 4-nitro-β-N,N-dimethylaminostyrene although these two compounds are isomers. NPA shows that charge transfer from the amino to nitro oxygen atoms is again very effective in N,N-dimethylnitramine, Me2N-NO2. The oxygens of NO2 are not the only acceptors of the negative charge lost by the amino nitrogen atom. In addition to the nitro oxygens, the negative charge acceptors are the vinylene, benzene as well as N-methyl carbons.
Results and discussion
1H, 13C, 15N and 17O NMR chemical shifts (δ/ppm) for 0.1–0.2 M solutions of 1–7 in DMSO-d6 and in CDCl3 (in italic) at 303 K [3J(1H,1H)/(Hz) values are given in parentheses]
The charge transfer in N,N-dimethylaniline is expected to be insignificant [δ(15N15 = −337.7 ppm in CDCl3] . However, an insertion of the strong electron acceptor p-nitro group into its molecule resulting in formation of compound 3 deshields N15 considerably (Table 1). As can be seen in Table 1, δ(15N15) varies significantly throughout the series. On the other hand, δ(15Nnitro) is much less sensitive to the spacer (Table 1). It is known that δ(15Nnitro) possesses low inherent sensitivity to the substituent  owing to the mutual canceling of the changes of principal components of the chemical shift tensor of the nitro group . Some other explanations have also to be considered. Thus, charge transfer from the amino nitrogen to nitro oxygen atoms (see, e.g., the resonance structure in Scheme 1) may not affect the nitro nitrogen itself. However, decreased C1–N18 bond lengths in 3 and 4 , as compared to the averaged CAr–NO2 distance , prove that the nitro group in these nitroarenes is really conjugated with the aromatic moiety.
The sensitivity of 17O NMR chemical shifts in meta- and para-substituted nitrobenzenes is rather low . 17O NMR resonance of nitromethane (605 ppm from external H2O ) is deshielded when compared with the shifts of 1–7 (Table 1). Thus, electron density around the nitro oxygens in 1–7 is higher than that in nitromethane (no significant transfer of the charge to the oxygen atoms is expected to take place in its molecule). The difference of δ(17O) for 5 (561.0 ppm, Table 1) and β-nitrostyrene, Ph–CH=CH–NO2, (578.8 ppm ) show the same trend as the difference of δ(17O) of NH2 and NO2 substituted benzenes being close to 40 ppm . Although decreased nuclear shieldings (larger chemical shifts) are usually associated with lower atomic charges , the correlations of these two parameters are usually fortuitous. Linear dependence between them may be treated as rational only in a narrow range of similar compounds . Keeping in mind this warning, we would like to see at least how the 17O NMR chemical shifts change in 1–7 and such an analysis is worthy to be done, even if the found relationship has a qualitative character only. Moreover, this approach can be verified by comparison of conclusions that neglect the above warning with these based on, e.g., molecular geometries .
δ(17O) (Table 1) suggests that the charge at NO2 in 1–7 to be more significant when the interacting groups, i.e., NMe2 and NO2, are separated by the shorter spacer. On the other hand, δ(N15) (Table 1) reveal that decrease of the charge at NMe2 is not simply related to the length of the spacer. As it is shown by its correlation coefficient (0.927, CDCl3), linear dependence between the chemical shifts of O18 and N15 is rather low. Thus, one can see that the charge lost by N15 was only partially gained by O18.
A comparison of δ(17O) for 4-(N,N-dimethylamino)-4′-nitrobiphenyl, 6, (Table 1) and 4-nitrobiphenyl  shows that the dimethylamino group shields the C11/13, C3/5 and C4 atoms by 16.43, 1.62 and 1.66 ppm, respectively. These chemical shift differences show that the charge from the NMe2 group in 6 was transferred mainly to C11/13, C3/5 and C4.
3JH,H coupling constant for trans-RCH=CHR′ (12–18 Hz ) depends on the order of the vinylene carbon–carbon bond. Low values of 3JH7,H8 = 10.6 Hz (Table 1, 10 Hz ) for (E)-N,N-dimethyl-2-nitroethenamine, 2, supports intramolecular charge transfer of the O2N–CH=CH–NMe2 ↔ ⊖O2N=CH–CH=N⊕Me2 type to take place  (JHa,Hb = 9–13 Hz for the >C=CHa–CHb=C< system ). As it is indicated by the coupling constants for 4 and 5 (Table 1), the respective bond in their molecules is significantly more double by character. Thus, the charge transfer in these two compounds is less effective than in 2. Comparison of the 3JH7,H8 values for 4 and 5 (Table 1) with these for β-nitro- and β-(dimethylamino)styrenes (both are equal to 13.9 Hz [37, 38]) suggests that effect of the p-NO2 and p-NMe2 groups on the magnitude of the intramolecular charge transfer in 4 and 5 is insignificant. Interestingly, effects of the β-NO2 and β-NMe2 substitution on 3JH7,H8 in styrene are comparable. Influences of the p-NO2 and p-NMe2 substituents on this coupling constant in styrene (ca. 18 Hz [39, 40]) and stilbene (ca. 16 Hz [41, 42]) are also similar. Large 3JH7,H8 coupling constant show that there is an almost fully double bond between C7 and C8 in compound 7 and in 4-nitro- and 4-dimethylaminostilbenes. Thus, effectiveness of the charge transfer in these three compounds seems to be very low.
1H and 13C NMR spectral data (Table 1) show that it is the case only in 2 where the charge is much more effective than in 4, 5 and 7. The X-ray data taken from literature show that Me2N–C bond in 2  is really much shorter than in 3 [43, 44], 4  and 5 , being in all of them similar. Nonequivalent N-methyl groups were also found in some related compounds. Thus, due to increased contribution of the p-⊖O2N=C6H4=N–CH=N⊕Me2 quinoid form, two different 1H N-methyl signals are observed in the NMR spectrum of N,N-dimethyl-N-(4-nitrophenyl)formamidine below the coalescence temperature .
Correlations between 15N15 NMR chemical shifts and geometrical parameters [bond lengths (pm) and valence angles (deg)] for compounds 1–7
∠C3C4C5 or C11C12C13
d(C8N15) or d(C12N15)
An ab initio level of theory using the Gauge-Including Atomic Orbitals (GIAO)  approach seems to be very convenient method to calculate the nuclear shielding  for 1–7. Our experiences [48, 49, 50] show that these calculations predict the experimental chemical shifts well although very sophisticated basic functions are not used. The reliability of this procedure is usually high for 13C NMR shifts. On the other hand, 15N NMR chemical shifts of non-quaternary nitrogen atoms are known to be sensitive to the solvent, concentration and temperature . For similar reasons, one would expect the 17O NMR chemical shifts to depend strongly on the solvent. Since the protons are always located on the periphery of the molecule, they are also subjected to more efficient intermolecular (solvent–solute) effects than the carbon atoms . The calculated 15N chemical shifts for 1–7 performed with different B3LYP methods are in agreement with experimental data; the correlation coefficients varying between 0.96 and 1.00 (see Supporting Information). It is noteworthy that the order of the calculated δ(15N15)s are 1 > 2 > 3 > 4 > 5 > 6 > 7. Thus, electron density at the amino nitrogen is expected to be lowest and highest in 1 and 7, respectively.
Among different methods used, B3LYP/6-31G(2d,p)//M05/6-31G(d,p) was found to give the best results in comparison with the experimental 17O chemical shifts (see Supporting Information). Coefficients of the linear correlations between the calculated and experimental data (solutions in CD3CN and CDCl3) for five correlated points [δ(17O)s were available only for the limited number of compounds, see Table 1] slightly exceed 0.8. It is noteworthy that δ(17O) change in the following order: 1 < 2 < 3 < 4 < 5 < 6 < 7. Thus, both the calculated and experimental values of δ(17O) show that electron density at the nitro oxygen atoms in 1 is significantly higher than in 2–7 further suggesting that the gain of the charge in 1 is more efficient than in 2–7.
Natural Population Analysis (NPA) charges at the heavy atoms in compounds 6 and 7
As this can be seen in our recent paper  and in Table 3, except O18 other atoms that accept the negative charge lost by N15 in the molecules of compounds 2–7 are C3, C5, C7, C9, C10, C11, C13 and C14. Relatively high NPA values for the methyl carbon atoms, C16 and C17, are also noteworthy.
Mulliken population analysis of the total and π electron densities at the oxygen and nitrogen atoms in NO2 of nitrobenzenes were earlier found independent on the substitution . The calculated NPA charges at N18 and O18 (Table 3) show this to be the rule also for compounds 3, 4, 6 and 7 which all are substituted nitrobenzenes. Interestingly the C1–N18 bonds in the molecules of compounds 3 and 4  are relatively shorter than the averaged CAr–NO2 distances . This suggests that the nitro group in nitramines possessing an aromatic spacer is really conjugated with the aryl moiety.
Although the classical concept of intramolecular charge transfer needs some modification, it is still an important factor that affects molecular structure of the Me2N-G-NO2 type nitroamines. Both the 1H, 13C, 15N and 17O NMR chemical shifts, and 3JH,H spin–spin coupling constants are indicative of the charge donors and acceptors in the molecule (results of the ab initio calculations show that correlation between the nuclear shieldings and atomic charges are not fortuitous). Thus, lone electron pairs of the amino nitrogen in N,N-dimethylnitramine, N,N-dimethyl-2-nitroethenamine, N,N-dimethyl-p-nitroaniline, 4-nitro-β-dimethylaminostyrene, 4-dimethylamino-β-nitrostyrene, 4-(N,N-dimethylamino)-4′-nitrobiphenyl and 4-(N,N-dimethylamino)-4′-nitrostilbene are transferred not only to the oxygens in NO2 but also to the vinylene, benzene and N-methyl carbons. The calculated Natural Population Analysis (NPA) values show that the amount of the charge at the amino nitrogen in N,N-dimethylnitramine is much lower than in other compounds (where it is almost constant). Since the above findings are consistent with those drawn from molecular geometries, the nuclear shieldings in this series of nitroamines can at least qualitatively be related to the atomic charges.
Melting points were measured on a Boetius table and are uncorrected. (E)-N,N-dimethyl-2-nitroethenamine, 2; N,N-dimethyl-4-nitroaniline, 3; 4-(N,N-dimethylamino)-4′-nitrobiphenyl, 6 and (E)-4-(N,N-dimethylamino)-4′-nitrostilbene, 7 are commercial products. N,N-Dimethylnitramine (dimethylnitramide), 1, was obtained by nitrodephosphorylation of hexamethylphosphoramide . Other compounds studied were prepared according to the modified literature methods.
(E)-4-Nitro-β-(N,N-dimethylamino)styrene, 4. Mixture of p-nitrotoluene (13.72 g, 0.1 mol), N,N-dimethylformamide dimethyl acetal (15.49 g, 17.2 mL, 0.13 mol) and N,N-dimethylformamide (10 mL) was refluxed for 2 h under N2. In order to remove the unreacted p-nitrotoluene, dry residue obtained after evaporation of the solvent under diminished pressure was extracted with two 20-mL portions of 18 % hydrochloric acid. The combined extracts were made alkaline with conc. aqueous sodium hydroxide and the precipitated solid was recrystallized from methanol. The product (13.50 g, 62 %) melts at 137.5–139.5 °C (lit. mp 137–139 °C , 132–133 °C ).
(E)-4-(N,N-Dimethylamino)-β-nitrostyrene, 5. Mixture of p-(N,N-dimethylamino)benzaldehyde (7.46 g, 0.05 mol), nitromethane (4.88 g, 3.84 mL, 0.08 mol), ammonium acetate (2.31 g, 0.03 mol) and glacial acetic acid (20 mL) was refluxed for 2 h. The cold reaction mixture was poured on the crushed ice (100 mL), the precipitated solid filtered off and recrystallized from methanol twice. The product (6.63 g, 69 %) melts at 176–180 °C (lit. mp 182–183 °C , 179–180.5 °C ).
1H, 13C and 15N NMR experiments were run with a Bruker Avance DRX 500 spectrometer equipped with an inverse 5-mm-diameter probe head with a z-gradient for 0.1–0.2 M DMSO-d6 and CDCl3 solutions at 303 K. Acquisition and processing parameters of the NMR experiments are those reported earlier . 2D pulsed field z-gradient (PFG) selected 1H,13C HMQC and 1H,13C HMBC experiments were run to assign reliably the 13C NMR spectra . 1H and 13C chemical shifts were referenced to an internal TMS, δ = 0.0 ppm. 15N NMR chemical shifts (referenced to an external neat 15N-natural abundance nitromethane, δ = 0.0 ppm) were obtained with the PFG 1H,15N HMBC experiments . 17O NMR spectra were recorded with a Varian Inova 500 MHz NMR spectrometer for 1 M solutions in CD3CN at 348 K and in CDCl3 at 323 K. 17O NMR signals are referenced to external neat 17O-natural abundance water, δ = 0.0 ppm.
Natural population analysis [52, 53] was performed using MP2/6-31G(2d,p) method for the isolated molecules (vacuum) with Gaussian 03 . Molecular geometries were optimized at the B3LYP/6-31G(2d,p) level (no intermolecular interactions were considered). The frequencies were calculated to make sure that geometry is in the energy minimum (no imaginary frequencies). HF/3-21G, M05/6-31G(d,p), MP2/6-31G(2d,p) and B3LYP/6-31G(d,p) geometries were used for the GIAO computations  using the B3LYP functional and various basis sets (see Supporting Information).
We are very much indebted to the CI TASK Gdańsk for supply of computer time and providing programs. The authors thank Spec. Lab. Technician Reijo Kauppinen for his help in the NMR experiments. This work was supported by Polish Ministry of Science and Higher Education (Grant No. IP2011 010171).
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