Article pubs.acs.org/JPCA
Aromaticity of Overcrowded Nitroanilines Irena Majerz*,† and Teresa Dziembowska‡ †
Faculty of Chemistry, University of Wrocław, 50-383 Wrocław, Poland Institute of Chemistry and Environmental Protection, West Pomeranian University of Technology, 70-061 Szczecin, Poland
‡
S Supporting Information *
ABSTRACT: For a series of 2,4,6-trinitroanilines substituted with bulky groups, the influence of intramolecular hydrogen bonds, electronic substituents effect, and steric hindrance on aromaticity of the molecules in crystals and their analogues optimized at the B3LYP/6-311++G** level were studied. The HOMA index was used as a measure of the aromaticity, while the parameter ΔP was a description of the distortion of the benzene ring from planarity. Conformation of the nonplanar ring in crystal and optimized structures was also described using the puckering parameters. A comparison of the data for crystal and optimized structures showed an important effect of the intermolecular interactions on aromaticity of the overcrowded nitroanilines. The packing effects were analyzed using the simplified PCM model of solvents. NBO analysis illustrated the changes of orbitals upon dearomatisation.
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INTRODUCTION Aromaticity, as one of the fundamental concepts in organic chemistry characterizing physical properties and chemical reactivity of compounds, still attracts a considerable attention.1,2 Traditionally, planarity and bond equalization have been used as two structural characteristics of benzene, basing on expectation that substantial π-electron delocalization can take place only in a planar system. However, numerous experimental and theoretical studies undertaken in the last decades have brought a strong evidence that aromaticity of benzene or heterocyclic compounds can be preserved in spite of considerable in-plane and out-of-plane distortions of the aromatic ring.1−4 Conformational flexibility of the benzene and heterocyclic aromatic rings was shown by experimental and theoretical methods.3,5−7 Recently, dynamical nonplanarity of benzene has been evidenced by molecular dynamic studies; conformational analysis in terms of puckering parameters has shown that the benzene molecule in the gas phase exists mainly in a flattened boat and twist-boat conformations.7 Numerous studies on overcrowded substituted benzene derivatives,3,4,8,9 as well as aromatic nonplanar polycyclic hydrocarbons like twisted acenes,10 pyracylene,11 [n]pyrenophanes,12,13 or highly distorted π-electron systems like fullerenes and nanotubes14 were published. Considerable effort has been undertaken to understand how in-plane and out-of-plane deformations of the benzene ring affect the π-electron delocalization and to find the limits of the aromatic ring deformability. Benzene molecules containing annulated alicyclic rings were studied as a model of the 6-membered aromatic rings under geometric constraints.4 Annelation with saturated alicylic clamps caused a bond alternation from small (ΔR ≤ 0.025 Å) up to significant (ΔR = 0.089 Å) values for highly strained compounds. Analysis of the ring current maps evidenced that aromaticity persisted in all these compounds. However, tris(benzocyclobutadieno)© 2012 American Chemical Society
benzene (ΔR = 0.159 Å) has lost its aromatic character. Authors stated that electronic properties of the cyclobutadieno clamp were responsible for the loss of the aromaticity. The theoretical study of the effect of the ring deformation on aromaticity of the benzene molecule in a boat conformation showed that the molecule persists the aromatic nature up to a deformation of 55°, but at 60°, the aromaticity was completely lost.15 In order to evaluate the aromaticity, numerous experimental and theoretical methods have been used, including analysis of structural, energetic, magnetic, and electronic properties and the reactivity of the compounds.1−3,5,16−22 Several aromaticity indices were also introduced: structurally based HOMA index,16−18 magnetic index of aromaticity (NICS),19 and others, like PDI,20 FLU,21 or Edef.5 As aromaticity is not a directly measurable feature, each of these indices has some limitations, so using several aromaticity indices was recommended.3,11,22 Recently, Feixas et al.3 investigated the effects of the bond length alteration, bond length elongation, and clamping, as well as the out-ofplane deformation of benzene molecules employing seven aromaticity indices. For [n](2,7)pyrenophanes with nonplanar pyrene moieties, it was shown that an increase of the bending angle up to 39.7° led to HOMA index decrease from 0.742 to 0.672, which meant a retention of the 90% aromaticity.12 A significant decrease of aromaticity described by the HOMA index was found for 2,4,6-trinitroanilines with bulky substituents.23 In this work, we have selected a series of overcrowded nitroanilines with strongly nonplanar rings (the torsion angle higher than 30°) from the Crystal Data Base. We have Received: December 24, 2011 Revised: May 22, 2012 Published: May 23, 2012 5629
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compared those structures with the respective single molecule structures optimized at the B3LYP/6-311++G** level as well as with analogous compounds without intramolecular hydrogen bonds. The main goal of this work has been to show the principal factors responsible for an increase of the nonplanarity and a destruction of aromaticity of overcrowded nitroanilines in the crystals.
Scheme 1. Chemical Structures of the Investigated Compounds
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RESULTS AND DISCUSSION Geometry of the Investigated Molecules: Intra- and Intermolecular Hydrogen Bonds. X-ray structures of the investigated compounds and their chemical structures are shown in Figure 1 and Scheme 1, respectively. The list of
proton, engaged in the intramolecular hydrogen bond, additionally interacts with the nitro group of the neighboring molecule; the N···O interatomic distance amounts to 2.665 Å. In order to verify if this interaction may be classified as an intermolecular hydrogen bond, we have applied the AIM method, delivering criteria of the hydrogen bond existence.30 It was found that, for this N···O interaction, there was a bond path passing through the bond critical point with the electron density of 0.0043 au and Laplacian of 0.0047 au. The distance between this ring’s critical point and the hydrogen bond critical point is 0.076 Å. All these values fulfill the criteria of the hydrogen bond existence, and the presence of a weak intermolecular NH···O hydrogen bond can be considered as confirmed. VUHJOX (IV) molecule crystallizes with two molecules of methylene chloride. The amine 6-NH proton is engaged in the bifurcated hydrogen bond to an oxygen atom of the nitro group in position 5 and also to that of the neighboring molecule; this N···O distance amounts to 2.174 Å. These two intermolecular hydrogen bonds present in neighboring molecules lead to the formation of a dimer structure. Another intermolecular hydrogen bond is formed between the proton of
Figure 1. Crystal structures of investigated molecules.
compounds is collected in Table 1; numbering of substituents is shown in Scheme 1. All the investigated compounds are substituted by nitro groups in 2,4,6-positions (Scheme 2). Other substituents are aliphatic tertiary or secondary amine groups. JARLOD (I) is an example of a compound without a hydrogen bond. In the others, because of the presence of the NH groups, various intra- and intermolecular hydrogen bonds are present. The hydrogen bonds are known to be one of the most important factors determining the structure of the compounds. The intramolecular hydrogen bonds in the investigated compounds are listed in Table 2 and depicted in the Figure 1. In the crystal structures of TEWHOS (II), two unidentical molecules are present, differing in the intramolecular H···O distances. In the molecule of TEWHUY (III), the 3-NH amine 5630
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Table 1. List of the Investigated Compounds refcode (no)
compound name
ref
substituents
JARLOD (I) TEWHOS (II)
1,3,5-tris(diethylamino)-2,4,6-trinitrobenzene 1,3-bis(dimethylamino)-5-(isopropylamino)-2,4,6-trinitrobenzene
24 25
TEWHUY (III)
1-(dimethylamino)-3,5-bis(isopropylamino)-2,4,6-trinitrobenzene
25
VUHJOX (IV)
(2,4,6-trinitro-5-piperidinyl)benza(1,3)-1,11-diazacyclodecane dichloromethane solvate 1,3,5-tris(t-butylamino)-2,4,6-trinitrobenzene 1,3,5-tris(neopentylamino)-2,4,6-trinitrobenzene 1,3,5-tris(isopropylylamino)-2,4,6-trinitrobenzene toluene solvate 1,3,5-tris(isopropylylamino)-2,4,6-trinitrobenzene 1,3,5-tris(isopropylylamino)-2,4,6-trinitrobenzene isopropylamine
26
R1R2R3R4R5R6C2H5 R1R2R3R4CH3; R5H; R6 CH(CH3)2 R1R4H; R2R3CH(CH3)2; R5R6 CH3 R1∼R6(CH2)10; R2R5H; R3∼R4C5H10
27 27 28 29 27
R1R4R5C(CH3)3; R2R3R6H R1R4R6CH2CH(CH3)3; R2R3R6H R1R4R5H; R2R3R6CH(CH3)2 R1R4R5H; R2R3R6CH(CH3)2 R1R4R5H; R2R3R6CH(CH3)2
KIYBUP (V) KIYBOJ (VI) KOGCOY (VII) PEVYEU (VIII) KIYBID (IX)
to an unfavorable conformation (the distance of the proton from the NCCNO plane is 0.7046 Å). In crystals of PEVUEU (VIII), there are two nonequivalent molecules in which three intramolecular hydrogen bonds are present (Table 2). In the second molecule, one of the NH groups forms a long intramolecular hydrogen bond (2.090 Å). The proton of this group additionally forms an intermolecular hydrogen bond to the nitro group in the neighboring molecule; the H···O distance equals 2.206 Å. KIYBID (IX) crystallizes with a molecule of isopropyl amine, which forms the intermolecular hydrogen bonds with the oxygen atoms of the nitro group in positions 2 and 6, of 2.03 Å and 2.09 Å H···O distances, respectively. Additionally, three intramolecular hydrogen bonds link the NH amine groups with the nitro ones. Nonplanar Conformation and Aromaticity of Investigated Molecules. The aim of our work has been to investigate how steric and electronic effects of the substituents, intramolecular hydrogen bonds, and intermolecular interactions in crystal state can modify the geometry and aromaticity of the overcrowded nitroanilines. The planarity of benzene ring was characterized by the ΔP parameter, previously defined23 as the sum of the squares of the deviations of the carbon atoms of the benzene ring from the its averaged plane
Scheme 2. General Structure of the Investigated Compounds
Table 2. Distances of the Proton to Acceptor in Intramolecular Hydrogen Bonds (Å) refcode (no) TEWHOS (II) TEWHOS (II) TEWHUY (III) VUHJOX ((IV) KIYBUP (V) KIYBOJ (VI) KOGCOY (VII) PEVYEU (VIII) PEVYEU (VIII) KIYBID (IX)
crystal structure 1.984 2.053 1.982, 1.959, 1.769, 1.619, 1.618, 1.831, 2.090, 1.906,
optimized structure 1.781
2.028 2.057 1.784 1.842, 1.756 1.814, 1.879, 2.179,
1.988
1.791, 1.821, 1.695, 1.742, 1.736,
1.791 1.821 1.663 1.737, 1.802 1.795, 1.795
ΔP = Σ(dC)2 To compare the conformation of the nonplanar benzene ring in optimized and crystal structure molecules, a quantitative description in terms of puckering parameters is necessary.7,31,32 In our work, the approach proposed by Cremer and Pople has been applied.31 The puckering parameters (Q, ϕ, θ) for crystal and optimized structure have been calculated and are listed in Table 4. The total puckering amplitude Q expresses the maximal out-of-plane deviation. Polar angle θ represents the particular type of chair or boat conformation, while the phase angle ϕ expresses how the molecule is skewed. For the θ = 0° or 180°, the ring has a typical chair conformation. For θ = 90°, the ring conformation is boat-like. The puckering parameters for crystal and isolated structures are given in Table 4. Aromaticity is commonly expressed by the HOMA index,1,15−17 based on an assumption that the geometry is strongly related to the electron distribution, and their EN and GEO terms are in the following relationship to the HOMA index: HOMA = 1 − EN − GEO. The HOMA index is defined as shown below:
1.869 1.864 1.966
methylene chloride and the oxygen atom of the nitro group in position 2. In KIYBUP (V) molecule, three secondary amine groups are present. Two of them are engaged in the intramolecular hydrogen bonds. For the third NH group, the distance of 2.093 Å between the proton and the oxygen atom of the NO2 might suggest an existence of a hydrogen bond, but the proton is located 0.627 Å above the NCCNO plane. The AIM analysis has shown unambiguously that this interaction did not have a hydrogen bond character. The existence of a free NH group in a crystal is rather exceptional. In the KIYBOJ (VI) structure, only intramolecular hydrogen bonds are present. Three structures: KOGCOY (VII), PEVUEU (VIII), and KIYBUP (V) possess the same substituents but different geometries. In KOGCOY (VII), two amine groups in positions 3 and 5 form intramolecular hydrogen bonds to the nitro group in position 4, but the third NH group is not engaged in a hydrogen bond due
n
HOMA = 1 − α /n ∑ (R opt − R ij)2 i=1
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Table 3. Parameters Describing Nonplanarity and Aromaticity of the Investigated Compounds crystal structure
optimized structure
optimized methyl control
refcode (no)
EN
GEO
HOMA
ΔP
EN
GEO
HOMA
ΔP
EN
GEO
HOMA
ΔP
JARLOD (I) TEWHOS (II) TEWHOS (II) TEWHUY (III) VUHJOX (IV) KIYBUP (V) KIYBOJ (VI) KOGCOY (VII) PEVYEU (VIII) PEVYEU (VIII) KIYBID (IX)
0.7938(226) 0.8719(94) 0.7986(90) 0.6020(87) 0.8719(150) 0.8275(244) 0.6486(113) 0.6615(153)
0.1684(37) 0.1116(1) 0.1554(3) 0.0180(1) 0.0623(2) 0.0409(2) 0.0200(1) 0.0767(6)
0.0378(263) 0.016(89)5 0.0460(99) 0.3800(88) 0.0658(149) 0.1316(242) 0.3310(113)5 0.2618(159)
0.2566(4) 0.2953(3) 0.2540(3) 0.2585(2) 0.2810(3) 0.1835(3) 0.1464(3) 0.1559(4)
0.9537 0.7843
0.1150 0.0630
−0.0687 0.1527
0.2382 0.2157
0.3721
0.0725
0.5554
0.0801
0.9072 0.9277 1.0944 0.8130
0.0244 0.0562 0.0229 0.0102
0.0683 0.0161 −0.1173 0.1769
0.2000 0.2346 0.1767 0.1307
0.1526 0.2093 0.0517 0.0172
0.0923 0.1036 0.0001 0.0000
0.7551 0.6870 0.9482 0.9828
0.0361 0.0704 0.0010 0.0001
0.6703(84) 0.7654(94) 0.9800(80)
0.0330(1) 0.0468(1) 0.2052(3)
0.2967(84) 0.1878(94) −0.1852(83)
0.1523(2) 0.1644(2) 0.2584(2)
0.8769
0.0145
0.1086
0.1448
0.0266
0.0006
0.9727
0.0001
where n is the number of bonds taken into summation, and α is a normalization constant (α = 257.7), fixed to give HOMA = 0 for a nonaromatic Kekule structure of benzene, and HOMA = 1 for the system with all CC bonds equal to the Ropt = 1.388 Å. Rij stands for the bonds lengths. The EN term measures the increase of the bond lengths of the ring: EN = α(Ropt − Rav)2, while the GEO term is related to the bond lengths alternation: GEO = α/n Σ(Rav − Rij). Aromatic benzene ring is characterized by HOMA equal to 0.9911; negative values of HOMA were found for antiaromatic annulenes1. The EN, GEO, and HOMA indices and ΔP parameter for the crystal structures of compounds under study are collected in Table 3. The relationship between HOMA and ΔP values for crystal and optimized structures has been presented in Figure 2. The data obtained previously23 have also been included. Although a decrease of aromaticity was connected with an increase of the aromatic ring nonplanarity, the spread of the points in Figure 2 did not let us identify a simple correlation. The survey of the data for crystal structures indicates a significant loss of aromaticity up to the antiaromaticty for
KIYBID (IX). The strong distortion of the central ring is reflected in the exceptionally large ΔP parameter, being in most cases much higher than that found for nitroanilines in the previous study.23 A comparison of the HOMA values as well as ΔP for two nonequivalent crystal structures of TEWHOS (II) and PEVYEU (VIII) and for four crystal structures of 1,3,5-tris(isopropyloamine)-2,4,6-nitroaniline (VII−IX) clearly show an influence of the crystal packing and the hydrogen bonds on the planarity of the benzene ring and aromaticity of the compounds. However, undoubtedly, the substituent effect plays a crucial role in the destruction of the aromaticity of compounds and the deviation of the benzene ring from planarity. In order to investigate these effects, the HOMA and ΔP parameters have been calculated for isolated optimized molecules. In all these structures but JARLOD (I), the intramolecular hydrogen bonds are present (Table 2). To eliminate the influence of the hydrogen bond, so-called methylene structures, where the amine NH groups were substituted by the CH2 groups of a similar volume, have been studied. The respective data are presented in Tables 3 and 4. The analysis of HOMA indices and ΔP parameters for methylene structures KIYBUP (V), KIYBOJ (VI), and KIYBID Table 4. Puckering Parameters of Investigated Molecules refcode (no) Optimized Structure JARLOD (I) TEWHOS (II) TEWHUY (III) VUHJOX (IV) KIYBUP (V) KIYBOJ (VI) KIYBID (IX) Crystal Structure JARLOD (I) TEWHOS (II) TEWHOS (II) TEWHUY (III) VUHJOX (IV) KIYBUP (V) KIYBOJ (VI) KOGCOY (VII) PEVYEU (VIII) PEVYEU (VIII) KIYBID (IX)
Figure 2. Correlation of ΔP and HOMA. Dots, X-ray structure; triangles, optimized structure; squares, compounds without an intramolecular hydrogen bond. Filled shapes, the results of the present work; empty shapes, ref 23. 5632
Q (Å)
ϕ (deg)
θ (deg)
0.4880 0.4645 0.4472 0.4844 0.4203 0.3615 0.3805
181.93 191.86 179.99 60.01 158.49 195.67 342.81
100.05 84.40 86.86 87.21 89.35 85.49 86.35
0.5065 0.4973 0.5040 0.5085 0.5306 0.4285 0.3827 0.3949 0.3904 0.4055 0.5083
298.65 233.49 187.19 300.48 56.88 273.86 325.51 94.70 311.69 99.92 57.06
80.04 85.37 84.82 91.72 87.05 89.85 97.02 95.35 92.62 99.75 92.69
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(IX) reveals that the presence of three nitro and three bulky alkyl groups only slightly decrease the aromaticity (HOMA > 0.95) and do not disturb the planarity of the benzene ring. A similar small effect on the decrease of aromaticity expressed by HOMA1,33 and Edef5 parameters can be found in the literature. However, in other compounds, where two electron-donating tertiary amine groups remained (TEWHOS (II), TEWHUY (III), and VUHJOX (IV)), a significant decrease of HOMA and an increase of ΔP have been observed. It is worth noting that, for the optimized JARLOD (I) structure with three nitro and three diethylamine groups substituted to the benzene ring, a negative HOMA value has been found indicating an antiaromaticity and an important deviation from planarity. These results show an essential role played by the presence of bulky amine substituents in decreasing the HOMA index and the distortion of the planarity of the 2,4,6-trinitrobenzene benzene ring. Introduction of the electron-donating amine groups in positions ortho and para to the nitro groups in the benzene ring induces a perturbation of the π and σ electron distribution and, in consequence, a change of the geometry of the benzene ring and a decrease of its aromaticity. For the optimized structures, the very small HOMA values indicate a substantial loss of aromaticity in comparison to the analogous methylene structure. A negative HOMA value for KIYBID (IX) indicates its antiaromatic character. The EN term for these structures is the main contribution to the HOMA and is significantly greater than that in the structures without intramolecular hydrogen bonds. Presence of the intramolecular hydrogen bonds is responsible for a decrease of the aromaticity of the optimized structures of nitroanilines. The influence of the intramolecular hydrogen bond on the decrease of the HOMA values was well evidenced.34,35 However, considering compounds under study, one must not forget about the electronic effect of the amine group, discussed above. In our studies, it was not possible to separate the effects of the hydrogen bond and the mesomeric effect of an amine group. Nonplanarity of the benzene ring in optimized structures is higher than in methylene structures. The smallest values of ΔP have been observed for KIYBUP (V), KIYBOJ (VI), and KIYBID (IX) compounds, where two intramolecular hydrogen bonds, linking the protons of the two amine groups with the oxygen atoms of the same nitro group, flattened the molecule.23 In the crystal structures, the influence of the packing effect and the intermolecular hydrogen bonds come additionally into a play. Comparison of the results for crystal and optimized structures given in Table 2 and 3 reveals some unexpected facts. The intramolecular hydrogen bonds in all crystal structures but one are longer than the respective bonds in optimized structures. This stands in a contradiction with the popular view that, in an optimized single molecule, the hydrogen bond is elongated compared to the hydrogen bond in a crystal. For all molecules, with the exception of TEWHOS (II) and KIYBID (IX), the transition from the isolated molecules to the crystal leads to a small increase of the HOMA values. The EN, being here a dominant term in HOMA, decreases in comparison to the optimized structures, indicating that the bond lengths in the benzene ring in crystal structures are a bit shorter than in the isolated molecules. Comparison of the ΔP values (Table 3) for crystal and optimized structures leads to an unexpected conclusion that crystal packing forces favor a deformation of the aromatic ring. According to a close packing rule, the molecules in a crystal should be more planar than the separate molecules in vacuum.36
Analysis of the conformation of optimized and crystal structures (Table 4) confirms a greater nonplanarity of the crystal structures; the values of total puckering amplitude Q correlates with the ΔP parameter (ΔP = 0.874Q − 0.1884; R2 = 0.9977). The values of ϕ and θ parameters suggest the molecules to be close to a boat conformation both in the optimized and crystal structures for JARLOD (I), TEWHOS (II), and TEWHUY (III), but the significant difference in the θ for JARLOD (I) and in the ϕ values for other compounds indicates some changes of the torsional angle induced by interactions in the crystals. For other compounds with exception of the crystal structure of KIYBID (IX), the puckering parameters indicate a greater deformation of the ring and conformation close to twist-boat. It is worth noting that, for KIYBID (IX), the biggest increase in distortion parameter Q has been observed. In order to elucidate the origin of differences in planarity and aromaticity observed when isolated molecules have been placed in the crystal lattice, the packing effect and other intermolecular interactions have been discussed. There are two possible mechanisms of the packing effect: the first connected with increasing of the density in a crystal and the second one connected with a polarization of the investigated molecule by its environment. To check the importance of both mechanisms, the simplified model calculations were performed. The influence of the density of the crystal was simulated by modification of the cavity radius going from optimized structures to the crystal ones. For simulating the effect of polarization, the polarizable continuum model (PCM) was used. In the first step of the calculations, the molecular radii of all single molecules under study were calculated with the Gaussian09 program. These molecular radii were compared with those derived from the crystal cell volume per one molecule. Comparison of both molecular radii in Table 5 shows Table 5. Comparison of Molecular Radii refcode (no) JARLOD (I) TEWHOS (II) TEWHUY (III) VUHJOX (IV) KIYBUP (V) KIYBOJ (VI) KOGCOY (VII) PEVYEU (VIII) KIYBID (IX)
experimental molecular radius (Å)
calculated molecular radius (Å)
5.0777 4.6612 4.7545 5.5056 5.0266 5.2634 5.3028
6.04 5.64 5.72 5.98 6.09 6.25 5.75
4.8893
5.82
5.1766
5.68
that packing of the molecule in the crystal decreases its molecular radius by about 10%. In the second step of the calculations, the investigated molecules were optimized using both molecular radii and three solvents with different electric permittivity: heptane (ε = 1.9113), acetone (ε = 20.493), and acetonitrile (ε = 35.688). Results of the optimization show that an increase of packing expressed by decreasing of the molecular radius has hardly any influence on the aromaticity (Table 6). For all molecules, independently of the molecular radius, the HOMA values are the same or very close. An increase of the electric properties of the solvent surrounding the aromatic molecule induces only small and irregular changes of the HOMA value and ΔP parameters. Comparison of the 5633
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Table 6. Parameters Describing the Aromaticity and Nonplanarity of the Investigated Compounds (For Experimental Molecular Radii) Optimized with PCM refcode (no) n-Heptane JARLOD (I) TEWHOS (II) TEWHUY (III) VUHJOX (IV) KIYBUP (V) KIYBOJ (VI) KIYBID (IX) Acetone JARLOD (I) TEWHOS (II) TEWHUY (III) VUHJOX (IV) KIYBUP (V) KIYBOJ (VI) KIYBID (IX) Acetonitrile JARLOD (I) TEWHOS (II) TEWHUY (III) VUHJOX (IV) KIYBUP (V) KIYBOJ (VI) KIYBID (IX)
Table 7. Puckering Parameters of Investigated Molecules of the Investigated Compounds (For Experimental Molecular Radii) Optimized with PCM
EN
GEO
HOMA
ΔP
0.9511 0.9077 0.9021 0.9226 1.0435 0.8188 0.8759
0.1145 0.0663 0.0244 0.0545 0.0199 0.0100 0.0143
−0.0656 0.0259 0.0734 0.0229 −0.0634 0.1712 0.1098
0.2383 0.2165 0.2005 0.2353 0.1770 0.1306 0.1448
0.9537 0.9133 0.9057 0.9246 1.0473 0.8096 0.8774
0.1143 0.0642 0.0238 0.0551 0.0192 0.0095 0.0133
−0.0679 0.0224 0.0706 0.0203 −0.0665 0.1809 0.1093
0.2384 0.2181 0.2019 0.2365 0.1779 0.1306 0.1448
0.9537 0.8981 0.9062 0.9246 1.0462 0.8096 0.8774
0.1143 0.0626 0.0236 0.0551 0.0193 0.0096 0.0131
−0.0679 0.0394 0.0702 0.0203 −0.0655 0.1809 0.1095
0.2384 0.2182 0.2019 0.2365 0.1778 0.1305 0.1448
refcode (no) n-Heptane JARLOD (I) TEWHOS (II) TEWHUY (III) VUHJOX (IV) KIYBUP (V) KIYBOJ (VI) KIYBID (IX) Acetone JARLOD (I) TEWHOS (II) TEWHUY (III) VUHJOX (IV) KIYBUP (V) KIYBOJ (VI) KIYBID (IX) Acetonitrile JARLOD (I) TEWHOS (II) TEWHUY (III) VUHJOX (IV) KIYBUP (V) KIYBOJ (VI) KIYBID (IX)
Q (Å)
ϕ (deg)
θ (deg)
0.4881 0.4653 0.4478 0.4851 0.4207 0.3614 0.3805
61.86 71.88 60.00 300.00 38.25 75.85 317.41
79.77 95.73 93.21 92.85 90.74 94.65 93.80
0.4883 0.4670 0.4493 0.4863 0.4217 0.3613 0.3805
301.77 71.93 300.00 240.00 277.94 316.17 317.82
100.58 95.97 86.68 92.96 89.15 85.09 94.09
0.4883 0.4672 0.4494 0.4864 0.4217 0.3613 0.3805
301.76 71.93 300.00 240.00 277.89 76.21 317.85
100.60 95.98 86.67 92.97 89.14 94.93 94.11
molecule is not sufficient to predict their HOMA value and a distortion from planarity in the crystal. NBO Analysis. Changes of the investigated molecules from aromatic to nonaromatic ones must be connected with the rearrangement of electrons in an aromatic ring. NBO analysis, which refers to a traditional Loewdin structure for the investigated compounds has found typical benzene structure only if the amino groups were replaced by methylene groups. For these compounds, NBO analysis reproduced one of Kekule structures of benzene. Electronic structure of other molecules is very far from aromaticity. Although delocalization of orbitals is common for all investigated compounds, there is no similarity between typical benzene structure and the investigated compounds. Typical NLMOs are depicted in Figure 3. Double bonds are found for the bonds linking amino groups with the benzene ring. Lone electron pairs are located on the carbon atoms of the aromatic ring. All these π electrons are strongly delocalized. Ratio of parent NBOs in the NLMOs is rather low and for the π electrons of the double bonds is about 96%. Lone pairs in the aromatic ring are delocalized to the large extent with ratios of parent NBOs going down to 66%. KIYBUP (V), TEWHOS (one molecule), and PEVYEU (one molecule) belong to another group in which the orbitals are more delocalized. Double bonds are found not only for the CN bond linking the amino group with the central ring, but also on the CN bond to a nitro group. A ratio of the parent NBOs in the NLMOs in the above cases is about 85%. At the carbon atoms in the central ring not engaged in a double bond, lone electron pairs can be found, but the ratio of the parent NBOs in these NLMOs is very low, dropping even to 62%. For this group of compounds, the result of NBO analysis is dependent on the calculation method.
puckering parameters for molecules optimized in the solvent cavity evidence that the Q value slightly increases with the raising electric permittivity for all compounds but KIYBOJ (VI) and KIYBID (IX) (Table 7). The above results do not allow us to draw any firm conclusion concerning the impact of the packing effect on the destruction of the aromaticity of the nitroanilines under study. Hence, in order to explain the obtained results, the specific intra- and intermolecular interactions have to be considered. The molecular structure of the isolated molecules of overcrowded nitroanilines is the result of a balance between the steric hindrance of the bulky substituents causing a tilting of some substituents from the benzene ring’s plane and an electronic effect of the amine groups and intramolecular hydrogen bonds tending to form a planar chelate ring. One may suppose that, in the crystal structure, the substituents are more tilted from the averaged molecular plane than in the isolated molecules. Additionally, the NH groups are engaged in the bifurcated intermolecular hydrogen bonds. As a consequence, the electronic effect of the amine group is diminished, and intramolecular hydrogen bonds are elongated (Table 2). Both of these effects may be responsible for an increase of the HOMA values in the crystal structure. The intermolecular hydrogen bonds are known to decrease aromaticity of benzene ring.37 The influence of the bifurcated intermolecular hydrogen bonds seems to be negligible, but a stronger intermolecular hydrogen bond between the nitro group and NH proton of isopropylamine in KIYBID (IX) is undoubtedly responsible for the largest nonplanarity and an exceptional antiaromatic character of this compound. All of these observations show that the aromatic ring in the studied compounds can be easily modified by intermolecular interactions. Knowledge of the geometry of an isolated 5634
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amino group was replaced by a methylene group to obtain analogous compounds without intramolecular hydrogen bonds. HOMA14 and ΔP23 parameters were calculated to characterize the aromaticity and planarity of the investigated compounds. The AIM analysis with the AIM2000 program40 was used to check the existence of hydrogen bonds in solidstate and optimized molecules. NBO analysis of all investigated molecules was performed using the ADF program.41
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ASSOCIATED CONTENT
S Supporting Information *
Benzene ring bond lengths (Å) of investigated compounds. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
Figure 3. Selected NLMO molecular orbitals of the optimized structure of KIYBOJ (VI). Upper part, double bonds; lower part, lone pairs.
*Fax: (+) 48713282348. E-mail:
[email protected]. Notes
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The authors declare no competing financial interest.
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CONCLUSIONS (1) The very small HOMA indices, down to negative ones, indicate a substantial loss of aromaticity of crystal structures of 2,4,6-trinitrobenzenes substituted by three bulky amine groups as well as isolated optimized molecules. (2) ΔP parameter indicates an important distortion of the benzene ring from planarity, greater for crystal structures than for the optimized structures. The ΔP decreases with the HOMA increasing in the crystal structures and isolated molecules. (3) Presence of the bulky amine groups and intramolecular hydrogen bonds are essential for the loss of aromaticity of the 2,4,6-trinitroanilines. (4) The planarity of the benzene ring in the studied compounds as well as its aromaticity are shown to be easily modifiable by intermolecular interactions, particularly intermolecular hydrogen bonds, in a crystal structure. (5) The nonspecific interactions analyzed in the frame of the PCM method are not sufficient to explain the differences in the HOMA values between the isolated molecules and the molecules in a crystal. Changes in electronic interactions, the intramolecular hydrogen bonds, and the formation of the intermolecular hydrogen bonds in a crystal lattice play an important role. (6) In two crystal structures, the exceptional presence of the free NH groups was observed. (7) NBO analysis confirmed the importance of the mesomeric effect of the amine groups for the changes of the electron density in the studied nitroanilines.
ACKNOWLEDGMENTS We thank the Wroclaw Centre for Networking and Supercomputing for being generous by letting us use their computer resources.
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REFERENCES
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EXPERIMENTAL SECTION The CSD crystal database38 was used to find the compounds with very big deformation of the aromatic ring. These solidstate structures were used as the starting point in the optimization at the DFT B3LYP/6-311++G** calculation level with the Gaussian03 program.39 Vibrational frequency calculation was used to verify that the absolute minimum on the potential energy surface was reached. Identical optimization with verification of vibrational frequencies was performed for the compounds in which the 5635
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