Structure-based evaluation of the resonance interactions and effectiveness of the charge transfer in nitroamines
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Structural data for five nitroamines of general formula Me2N–G–NO2 show effectiveness of the ground-state charge transfer to be most and least efficient in N,N-dimethylnitramine and in 4-N,N-dimethylamino-β-nitrostyrene, respectively. Electron-donor power of the amino nitrogen atom in the latter compound is less than that in 4-nitro-β-N,N-dimethylaminostyrene (these two compounds are isomers). Natural population analysis shows that the charge transfer from the amino to the nitro oxygen atoms is most effective in N,N-dimethylnitramine, Me2N–NO2. The nitro oxygen atoms are not the only acceptors of the negative charge lost by the amino nitrogen atom. The nitro group in two substituted nitrobenzenes studied was found to be independent on substituent (nitro group attached to the benzene ring withdraws a constant electron density regardless the substitution).
KeywordsNitroamines Resonance interaction Charge transfer Molecular structure Quantum-chemical calculations
Numerous properties of the conjugated push–pull systems, e.g. basicities, dipole moments, and spectral parameters, are significantly affected by the intramolecular charge transfer . A series of such compounds has been recently studied by us . Since nitro and amino groups are among the most common acceptors and donors known, nitramines R2N–NO2 are subjected to extremely effective electron transfer. For this reason, intramolecular interactions in these compounds have often been studied [3, 4, 5, 6, 7]. p-Nitroanilines also contain these two strongly interacting groups where they are separated by the p-phenylene moiety. There is a question about the effect of conjugated spacers on the charge transfer in compounds of general formula R2N–G–NO2. This problem has been studied only occasionally. Thus, comparison of the dipole moments and spectral (IR and UV–vis) parameters for Me2N–NO2 and p-Me2N–C6H4–NO2 was found helpful to prove that the said interaction is really weaker when the NMe2 and NO2 groups are separated with the system of conjugated π bonds [8, 9]. Effective intramolecular charge transfer in the molecules of 4-N,N-dimethylamino-β-nitrostyrene is responsible for their stacking in dimers and tetramers with antiparallel dipoles .
Results and discussion
The literature data show that N,N-dimethylnitramine, 1, is planar both in the crystal and gas states [17, 18]. Length of the N–N bond (126 pm) proves that it has significantly double-bond character [8, 17, 9]. In the crystal state, the C2–C3 and C5–C6 bonds in the molecule of N,N-dimethyl-p-nitroaniline, 3, are significantly shorter than other bonds in the ring [19, 20]. The C1–N18 and C4–N15 bonds in the compounds studied are shorter as compared with the average values for CAr–NO2 and CAr–NMe2 distances . Thus, there is a significant quinoid contribution to the electronic structure of N,N-dimethyl-p-nitroaniline.
X-ray bond lengths [pm] in 2–5
Delocalization of the lone electron pair of N15 shortens the N15–C8 bond in 4 and 5 (crystalline state) to 135.9 pm (it is much shorter than the standard single N–C bond) [10, 21, 23]. The molecule of 4-N,N-dimethylamino-β-nitrostyrene, 5, is almost planar, indicating significant conjugation between the amino and nitro groups . Analysis of the bond lengths supports significant quinoid character of the six-membered ring in this compound. The C13–C14 (139.0 pm) and C10–C11 (137.3 pm) bonds in 5 are significantly shorter than C12–C13 (141.8 pm), C12–C11 (142.3 pm), C14–C9 (140.45 pm), and C9–C10 (140.57 pm). For the above mentioned reason, C9–C8 bond in this compound (144.8 pm) is shorter than the standard single C–C bond . One should pay attention to asymmetry of the benzene rings in compounds 4 and 5 (the lines passing by C1 and C4 and by C9 and C12 are not their symmetry axes) which is a result of the asymmetry of the CH=CH–NMe2 and CH=CH–NO2 moieties.
Analysis of the bond alternation [24, 26] in compounds 4  and 5  (crystalline state) shows that although the sum of differences between the longest bond and residual bonds in the benzene ring are comparable (12.6 pm for 4 and 12.3 pm for 5), differences between the CAr–Cexo and C7=C8 bond lengths are much more differentiated (0.3 pm for 4 and 11.2 pm for 5). This clearly shows that charge transfer in 4 is much more effective than in 5.
Sums of the bond angles around the amine nitrogens in 2 (359.8° ), 3 (359.9° ), 4 (359.7° ), and 5 (359.2° ) reveal that these atoms are not fully sp2 hybridized. The nitro groups in the molecules of compounds 1 [24, 25], 2 , and 4  (crystal state) are always planar (sum of the bond angles around the nitro nitrogen is equal to 360.0°). Analysis of the bond lengths confirms that the charge transfer in 4 is much more effective than in 5. It is also proved by sum of the angles around the amino nitrogen being equal to 359.7° in the molecule of 4  and 359.2° in the molecule of 5  (sums of the angles around the nitro nitrogen atoms in these molecules are equal to 360.0° [10, 23]).
NPA charges at the heavy atoms in compounds 1–5 (in vacuum)
Electron-withdrawing ability of the nitro group in a series of nitrobenzenes seems to be independent on substituent. Mulliken population analysis of the total and π electron densities at the nitro oxygen and nitrogen atoms in such compounds was really found independent on substitution [35, 37]. The calculated NPA charges at N18 and O18 (Table 3) show this to be the rule also for compounds 3 and 4 (these compounds are substituted nitrobenzenes). The nitro group attached to the benzene ring seems to withdraw a constant electron density regardless the ring is electron rich or electron deficient . Shortening of the C1–N18 bonds in the molecules of compounds 3 and 4 (Table 1), as compared with the averaged CAr–NO2 distances , supports the nitro group in nitroarenes to be really conjugated with the aromatic moiety.
X-ray and electron diffraction structural data show that significant amount of the charge from the amino nitrogen atom in N,N-dimethylnitramine (1), N,N-dimethyl-2-nitroethenamine (2), N,N-dimethyl-p-nitroaniline (3), 4-nitro-β-N,N-dimethylaminostyrene (4), and 4-N,N-dimethylamino-β-nitrostyrene (5) was transferred to the nitro group. In general, effectiveness of the charge transfer changes in the following order: 1 > 2 > 3 > 4 > 5. Evaluation of this effect is based mainly on the bond lengths (bond angles are less useful). More severe steric interactions between the ortho and α hydrogen atoms in 5, with respect to these in 4, are responsible for more effective charge transfer in the later compound. The calculated NPA charges (Natural Population Analysis) at the amino and nitro nitrogen atoms show that amount of the charge at the former in compound 1 is much lower than in other compounds studied (it is almost constant in compounds 2–5). Except the nitro oxygens, other atoms that accept the negative charge lost by the amino nitrogen in the molecules of compounds 2–5 are the vinylene, benzene as well as methyl carbons. The calculated NPA charges at the nitro nitrogen and oxygen atoms in compounds 3 and 4 show electron-withdrawing ability of the nitro group in substituted nitrobenzenes to be independent on substituent (nitro group attached to the benzene ring seems to withdraw a constant electron density regardless the substitution). Shortening of the CAr–Nnitro bonds in compounds 3 and 4 (as compared with the averaged CAr–NO2 distances) supports the nitro group in these nitroarenes to be really conjugated with the aromatic moiety.
Natural population analysis [35, 36] was performed with Gaussian 03  with use of MP2/6-31G(2d,p) method. 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).
We are very much indebted to the ACK CYFRONET AGH, Kraków (MNiSW/SGI3700/UTPBydg/042/2007) and CI TASK Gdańsk, for supply of computer time and providing programs.
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