Substituents and Environment Influences on Aromaticity of peri- and

Article pubs.acs.org/JPCA

Substituents and Environment Influences on Aromaticity of peri- and para-Substituted Naphthalenes Irena Majerz*,† and Teresa Dziembowska‡ †

Faculty of Pharmacy, Wroclaw Medical University, Borowska 211a, 50-556 Wroclaw, Poland Institute of Chemistry and Environmental Protection, West Pomeranian University of Technology, 70-061, Szczecin, Poland

‡

ABSTRACT: The influence of substituents and environment on the aromaticity of the naphthalene ring is shown for a series of peri- and para-substituted naphthalenes. Crystal structure geometries are compared with the single molecule structures in vacuum (optimized at the B3LYP/6-311++G** level) and with structures determined in media of different polarity. The harmonic oscillator model of aromaticity (HOMA) index of the naphthalene rings has been used to characterize the aromaticity of the investigated molecules. It has been shown that the ellipticity of the C2−C3 (C6−C7) bonds can be applied as a measure of participation of the quinoid resonance structure and through-resonance effect between the para-substituents.

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INTRODUCTION Peri-interaction in 1,8-disubstituted naphthalenes has attracted attention for a long time, due to their influence on physical and chemical properties of the compounds.1−6 Two substituents in peri-positions of naphthalene are forced to be in close contact, generally closer than the sum of their van der Waals radii, leading to strong through-space interactionsattractive or repulsive depending on their properties. To release the steric strain, the substituents generally adopt nonplanar conformations. This through-space interaction leads to a change of exocyclic bonds and torsion angles and a disturbance of the naphthalene skeleton. When into the 4 and/or 5 position of the naphthalene ring having a pair of the peri-substituents in the 1,8 positions we introduce substituents with opposite electronic properties, the through-resonance effect may appear.7,8 The through-resonance effectassociated with a charge transfer from electron-donor to electron-acceptor group in the para positionresults in an increase of the weight of the canonical quinoid structure. The electronic structure and geometry of the naphthalene derivatives with two pairs of peri substituents of opposite electronic properties in the para positions is the effect of compromise between the effects of through-space (peri) and through-bonds (through-resonance) interactions.4,7,8 One of the best known groups of naphthalene derivatives, with substituents in peri and para positions, are 1,8bis(dialkylamino)naphthalenes, called the “proton sponges”.4−6 Exceptional basicity of 1,8-bis(dimethylamino)-napthalene (pKa = 12.19) results from the repulsive steric and electrostatic interactions between the dimethylamino groups. The 1,8bis(dialkylamino)naphthalenes additionally substituted with electron-accepting groups in 4,5 positions, called “the pushpull proton sponges”, exhibit some interesting properties caused by a through-resonance (push−pull) interaction.7,8 It © 2014 American Chemical Society

was shown that these compounds are very sensitive to the solvent effect.7,8,10 The through-space and through-bond interaction between the pairs of substituents in peri and para positions in the naphthalene ring affects both the π-electron delocalization and geometry of the aromatic ring. A consequence of these interactions is a change of aromaticity of the naphthalene ring. A π-electron delocalization is known to be responsible for many physicochemical, chemical and biochemical properties of the aromatic compounds. In the past decades, the relation between the substituent effect and aromaticity has been the subject of numerous studies, by using a variety of structural, magnetic, energetic or electronic indices of the aromaticity.11−21 The influence of the medium polarity on the harmonic oscillator model of aromaticity (HOMA) index of some parasubstituted benzene derivatives was recently shown.22 The purpose of this paper is to analyze the influence of peri and para interactions in naphthalene rings on aromaticity, described by the geometry-based aromaticity index HOMA.11 To analyze this problem, we have chosen a series of naphthalenes with amino and nitro substituents in peri and para positions (Scheme 1). The influence of different environment of the aromatic molecule has also been investigated. Crystal structure geometries were compared with the optimized structures of a single molecule in vacuum and with the optimized structures determined in media of different polarity. Received: May 14, 2014 Revised: August 2, 2014 Published: August 5, 2014 7118

dx.doi.org/10.1021/jp5047672 | J. Phys. Chem. A 2014, 118, 7118−7129

The Journal of Physical Chemistry A

Article

Scheme 1. Investigated Naphthalene Derivatives with peri and para Interaction

measures the increase of the bond lengths in the ring, while the GEO term is related to the bond lengths alternation. The comparison of these values may permit one to differentiate between the two mechanisms of the aromaticity decrease. One must have in mind that the HOMA values based on measured bond length determined by X-ray or neutron diffraction may be biased by some, generally small, systematic errors caused by the precision of the diffraction experiment, thermal motions, and the static or dynamic disorder. The HOMA values for naphthalene calculated for geometries measured at 239, 143, and 92 K amount to 0.8220, 0.8132, and 0.8070, respectively.11 The HOMA index calculated for the optimized structure of the naphthalene equals 0.7840. The ΔP parameter, describing the deviation of the atoms from the aromatic ring plane, has been also calculated.21 It is defined as a sum of the squares of the deviation of the carbon atoms positions from the averaged plane of a given aromatic ring (dC): ΔP =

∑ (dC 2)

In order to investigate the effect of the medium polarity on the aromaticity of studied peri- and para-substituted naphthalenes, the polarizable continuum model (PCM) has been used. To model the effect of medium polarity, three solvents acetonitrile (ε = 35.688), acetone (ε = 20.493), and heptane (ε = 1.9113)have been applied taking into account two molecular radii: in isolated molecule (ro) and in the crystal (rc). As a starting point for discussing the aromaticity of bis- and multisubstituted naphthalenes, we have calculated the HOMA indices for 1-(dimethylamino)naphthalene and 1-nitronaphthalene. The aromaticity of the substituted derivatives of benzene was recently widely studied and comprehensively discussed.13−17 These investigations have shown that, in monosubstituted benzenes, substituents very weakly influence the HOMA values,13,15−17 and this effect does not depend on the substituent conformation.16 As we know, the substituent effect on the naphthalene aromaticity has never been studied in a systematic way, and only few papers concerning this effect have been published.18−20 For a naphthalene ring with a charged CH2+ group in position 1, the HOMA index was shown to depend strongly on the torsion angle; for 90° it was equal to 0.794, while for 0° it decreased to 0.514.19 The calculated dependencies of the HOMA value on the angle between the aromatic ring plane of naphthalene and the CNC plane for dimethylamino or ONO plane for the nitro group, respectively, are presented in Figure 1a. Similar dependency for EN and GEO terms is shown in Figure 1b,c. The results show that the effect of the substituents on the HOMA value depends on the conformation of the substituent and that electron-donating amine group exerts a stronger effect on decreasing of the HOMA in comparison to the nitro group. The numbering of the atoms in a naphthalene molecule is presented in Scheme 2. The naphthalene derivatives used as model structures with peri and/or para interactions between the amino and nitro substituents are shown in Scheme 1. The crystal structures of these compounds have been taken from the CSD database.24 We have chosen typical naphthalene derivatives with the simplest electron-donating groups (dimethylamino-, diethylamino-, pyrrolidino-) and with the nitro group as electron acceptor. The structures taken into account were measured at room temperature and for the compounds with more than one

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RESULTS AND DISCUSSION The influence of peri- and para- interactions between the electron-donating: dimethylamino, diethylamino, and pyrrolidino group, of increasing basicity,23 and the electron-accepting nitro groups on aromaticity of naphthalenes (Scheme 1) has been investigated. For the molecules in crystals, in vacuum (optimized at the B3LYP/6-311++G** level) and in media of different polarity the HOMA index has been calculated for a particular ring:11 n

HOMA = 1 − α /n ∑ (R opt − R i)2 i=1

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 Ropt = 1.388. Ri stands for the ith bond length. The HOMA index consists of two contributions: HOMA = 1 − EN − GEO. The EN term 7119



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Scheme 2. Numbering of the Atoms in a Naphthalene Ring

peri interactions: 1,8-dinitronaphthalene (DNTNAP),25 1,8bis(dimethylamino)-naphthalene (DMANAP),26,27 and 1,8bis(pyrrolidino)naphthalene (ROKRAL);28 peri interactions and resonance interactions in the one aromatic ring: 1,8-bis(dimethyloamino)-4-nitronaphthalene (ZOSKAT),29 1,8-bis(pyrrolidino)-4-nitro-naphthalene (GIBWEV);8 peri and resonance interactions in both rings: 1,8-bis(dimethylamino)-4,5-dinitronaphthalene (SORYEE),7 1,8-bis(diethylamino)-naphthalene (SORYII), 7 and 1,8-bis(pyrrolidino)-4,5-dinitro-naphthalene (GIBVEU);8 resonance interactions: 1- pyrrolidino-2,4-dinitronapfthalene (WAPGEZ).30 The HOMA and the ΔP planarity index for a particular aromatic ring in naphthalene derivatives in crystal, in optimized structures in gas phase as well as in media of different polarity have been calculated and presented in Tables 1−10. The selected geometric parameters for all these structures: ϕ - the angle between the two naphthalene aromatic ring planes, α the angle between particular aromatic ring plane and the CNC plane for amino substituents and ONO plane for the nitro groups, C−N [Å] - the bond length between the C atom in an aromatic ring and the N atom of the substituent, and N···N [Å] - the distance between the N atoms of amine groups in the 1 and 8 positions are collected in Tables 1−10. The calculations show that only for naphthalene derivatives with the resonance interactions between the amino and nitro groups in para positions is the HOMA index significantly sensitive to the medium polarity. Only for these compounds are the data for the structures optimized in different medium polarity and at different molecular radius given in tables (Table 4−10). In the optimized and crystal25 structure of DNTNAP (Table 1), the repulsive, electrostatic and steric interaction between the NO2 groups leads to a distortion of some external and internal bonds and angles. The angle between the NO2 group plane and the aromatic ring suggests a possibility of a mesomeric interaction of the substituent with the π-electron system. The comparison of the data for optimized molecule and the crystal structure25 shows an influence of the intermolecular interaction on differentiation of both aromatic rings. The HOMA values for 1,8-dinitronaphthalene optimized and crystal structure are similar and indicate a small decrease of the aromaticity. The value of HOMA for the optimized structure is close to that for 1-nitronaphthalene.20 No influence of the solvent polarity on HOMA values has been observed. The structure of DMANAP (Table 2) was the object of numerous experimental26,27 and theoretical studies.4,5 The

Figure 1. Correlation of theoretical values of HOMA of the substituted ring (a), EN (b), and GEO (c) with the angle between the naphthalene aromatic ring plane and the CNC plane for dimethylamino group in 1-(dimethylamino)naphthalene (empty triangles) and ONO for nitro group in 1-nitronaphthalene (full points). HOMA value of the unsubstituted ring changes from 0.7880 to 0.7761 for 1-(dimethylamino)naphthalene and from 0.7855 to 0.7518 for 1-nitronaphthalene.

structure in the CSD base, this with lower R factor has been chosen. The choice of the compounds gives four groups of the substituted naphthalenes varying the type of interactions: 7120



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Table 1. HOMA, ΔP, and Geometric Parameters for DNTNAP: Optimized and Crystal Structures25 optimized structure DNTNAP01 DNTNAP 02 a

HOMA

EN

GEO

ΔP

ϕ [deg]

α [deg]

CN [Å]

N···N [Å]

0.7578 0.7369a 0.7659a 0.7432a 0.7464a

0.0798 0.0458 0.0361 0.0542 0.0716

0.1624 0.2172 0.1980 0.2026 0.1820

0.0037 0.0027 0.0036 0.0045 0.0032

7.757 3.796

39.975 41.654 44.133 41.411 44.428

1.4811 1.4715 1.4725 1.4737 1.4625

2.9860 2.9458

4.236

2.9615

Data for two nonequivalent aromatic rings of naphthalene.

Table 2. HOMA, ΔP, and Geometric Parameters for DMANAP01, DMANAP02, and DMANAP10: Optimized Structures and Crystal Structures26,27 optimized structure DMANAP01 DMANAP10 a

HOMA

EN

GEO

ΔP

ϕ [deg]

α [deg]

CN [Å]

N···N [Å]

0.6474 0.6380a 0.6572a 0.7150a 0.6039a

0.1513 0.1833 0.1698 0.0194 0.0351

0.2013 0.1788 0.1730 0.2657 0.3610

0.0104 0.0104 0.0098 0.0088 0.0104

9.128 7.494

56.807 54.311 54.323 54.538 54.816

1.4205 1.4015 1.4051 1.3954 1.3994

2.9195 2.8035

7.674

2.7919

Data for two nonequivalent aromatic rings of naphthalene.

Table 3. HOMA, ΔP, and Geometric Parameters for ROKRAL: Optimized Structures, Crystal Structures28 optimized structure ROKRAL

HOMA

EN

GEO

ΔP

ϕ [deg]

α [deg]

CN [Å]

N···N [Å]

0.6468 0.6832

0.1543 0.0940

0.1990 0.2228

0.0142 0.0249

9.270 12.415

55.695 52.069

1.4087 1.3938

2.8991 2.8771

Table 4. HOMA, ΔP, and Geometric Parameters for ZOSKAT: Optimized Structures, Crystal29 and Media of Different Polarity Calculated for Molecular Radii rO = 5.22 Å and rc = 4.26 Å optimized structure ZOSKAT optimized structure ro ε = 35.688 optimized structure, ro ε = 20.493 optimized structure, ro ε = 1.9113 optimized structure, rc ε = 35.688 optimized structure, rc ε = 20.493 optimized structure, rc ε = 1.9113 a

HOMA

EN

GEO

ΔP

ϕ [deg]

0.5762 0.6481 0.4931 0.6909 0.4929 0.6337 0.4999 0.6517 0.5619 0.6521 −0.6272 0.4369 −0.1837 0.2979 0.5383 0.6558

0.2069 0.1499 0.1853 0.0362 0.2918 0.1094 0.2855 0.1665 0.2250 0.1524 0.9396 0.2250 0.7505 0.3490 0.2501 0.1559

0.2169 0.2020 0.3216 0.2729 0.2153 0.2570 0.2146 0.1818 0.2131 0.1956 0.6876 0.3381 0.4332 0.3531 0.2116 0.1883

0.0249 0.0140 0.0312 0.0161 0.0351 0.0214 0.0346 0.0209 0.0315 0.0177 0.0242 0.0246 0.0374 0.0378 0.0315 0.0177

11.259 12.113 11.258 12.872 12.448 11.258 11.500 12.448

α [deg] 46.980a 56.196 38.552a 48.856 37.925a 45.203 38.443a 46.171 41.722a 52.160 27.051a 46.321 30.584a 46.171 44.410a 54.449

32.119b 27.462b 17.664b 18.634b 24.622b 32.375b 5.06b 28.867b

CN [Å] 1.3868a 1.4111 1.3713a 1.3834 1.3647a 1.3896 1.3657a 1.3913 1.3727a 1.4017 1.3176a 1.3379 1.3425a 1.3447 1.3727a 1.4017

N···N [Å]

1.4631b

2.8441

1.4281b

2.8592

1.4327b

2.8399

1.4404b

2.9117

1.4499b

2.8794

1.3541b

2.8399

1.3701b

3.0922

1.4499b

2.8794

Data for dimethylamino group. bData for NO2 group.

group and the aromatic ring. Shortening of the CN bond in the optimized structure comparing with that in the compound discussed previously (Table 2) is in agreement with the greatest electron-donor properties of the pyrrolidine group in comparison to the dimethylamine group (Table 2). The deviation from planarity of the aromatic ring and ϕ angle are more significant in comparison to the 1,8-bis(dimethylamino)naphthalene molecule. The HOMA index shows a similar medium aromaticity, greater for the crystal. No influence of the solvent polarity on the HOMA value is observed. ZOSKAT (Table 4) is asymmetrically substituted naphthalene molecule with peri-repulsive interaction between dimethylamino groups and resonance interaction between the 1- N(CH3)2 and 4-NO2 group. Structural and spectroscopic

structure of DMANAP10 determined at room temperature shows a distinct difference in HOMA values for two aromatic rings. The HOMA values indicate the medium aromaticity. A loss of aromaticity, in this compound, is higher when compare to 1,8-dinitronaphthalene, which may be attributed to the mesomeric effect of the dimethylamine group. The ΔP values indicate some deviation from planarity for aromatic rings. No correlation between the C−N bond length and HOMA values is observed, which indicates that the HOMA value is influenced by the intermolecular interaction in a crystal. In ROKRAL, peri interaction between two 1-pyrrolidinyl groups are present. The value of the α angle and C−N distance (Table 3) in the optimized and crystal structure28 indicate the mesomeric intraction between the N atom of the pyrrolidinyl 7121



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Table 5. HOMA, ΔP, and Geometric Parameters for GIBWEV: Optimized Structures, Crystal8 and Media of Different Polarity Calculated for Molecular Radii ro = 5.35 Å and rc = 4.51 Å optimized structure GIBWEV optimized structure ro ε = 35.688 optimized structure, ro ε = 20.493 optimized structure, ro ε = 1.9113 optimized structure, rc ε = 35.688 optimized structure, rc ε = 20.493 optimized structure, rc ε = 1.9113 a

HOMA

EN

GEO

ΔP

ϕ [deg]

0.5409 0.6524 0.6645 0.7461 0.4413 0.6318 0.4397 0.6334 0.5336 0.6497 −0.2169 0.2964 −0.1630 0.4044 0.5202 0.6450

0.1921 0.1702 0.1636 0.0978 0.3383 0.1930 0.3380 0.1916 0.2658 0.1755 0.7786 0.3547 0.7501 0.3023 0.2809 0.1812

0.2671 0.1774 0.1718 0.1561 0.2204 0.1752 0.2223 0.1750 0.2006 0.1748 0.4383 0.3489 0.4129 0.2933 0.1988 0.1738

0.0430 0.0282 0.0398 0.0341 0.0466 0.0319 0.0465 0.0317 0.0444 0.0293 0.0390 0.0387 0.0437 0.0374 0.0453 0.0304

14.339 12.864 14.132 14.17 14.409 11.444 11.986 14.381

α [deg] 38.824a 41.446 33.858a 37.490 34.926a 35.817 35.075a 36.060 37.493a 39.757 30.093a 29.289 30.476a 29.624 36.484a 38.273

CN [Å]

25.742b 5.000b 11.223b 11.708b 21.383b 5.124b 5.568b 17.287b

1.3722a 1.3905 1.3476a 1.3802 1.3542a 1.3732 1.3550a 1.3740 1.3667a 1.3857 1.3392a 1.3427 1.3376a 1.3457a 1.3620a 1.3812

N···N [Å]

1.4546b

3.0264

1.4202b

3.0012

1.4266b

3.0679

1.4282b

3.0662

1.4475b

3.0397

1.3691b

3.1183

1.3747b

3.1133

1.4407b

3.0504

Data for pyrrolidino group. bData for NO2 group.

Table 6. HOMA, ΔP, and Geometric Parameters for SORYEE: Optimized Structures, Crystal,7 and Media of Different Polarity Calculated for Molecular Radii ro = 5.59 Å and rc = 4.38 Åa EN

GEO

ΔP

ϕ [deg]

α [deg]

CN [Å]

d [Å]

N···N [Å]

optimized structure

0.6195

0.1720

0.2086

0.0294

16.699

SORYEE

0.6379

0.1524

0.2097

0.0277

16.278

SORYEE 01

0.6582

0.1363

0.2054

0.0281

16.450

optimized structure, ro ε = 35.688

0.5661

0.2395

0.1944

0.0382

18.703


0.5709

0.2353

0.1938

0.0379

18.657

optimized structure ro ε = 1.9113

0.6140

0.1862

0.1998

0.0321

17.363

optimized structure, rc ε = 35.688

−0.9195

1.1730

0.7465

0.0242

14.535


0.5970

0.2098

0.1932

0.0356

18.188

29.642b 44.149c 27.117b 35.574c 27.076b 35.623c 24.617b 32.542c 24.817b 32.907c 28.236b 40.768c 24.838b 32.711c 26.285b 36.042c

1.4710 1.3891 1.4534 1.3669 1.4440 1.3669 1.4489 1.3638 1.4501 1.3648 1.4656 1.3814 1.4147 1.3192 1.4576 1.3719

−0.2590 0.2379 −0.2704 0.2572 0.2723 −0.2686 0.2433 −0.3543 0.2437 −0.3494 0.2527 −0.2688 0.2169 −0.2163 0.2472 −0.3132

3.0019 2.8799 2.9757 2.9171 2.9823 2.9239 3.0770 3.0038 2.0739 2.9988 3.0228 2.9102 3.3287 3.0011 3.0516 2.9592

HOMA

a

It is not possible to optimize the structure for rc, ε = 20.493. bData for NO2 groups. cData for dimethylamino groups.

crystal molecular radius (rc) in polar medium of ε equal to 35.688 or 20.493 indicate antiaromatic character of the investigated ring. It is noteworthy that the through-resonance interaction in 1,8-bis(dimethyloamino)-4-nitronaphthalene in solutions was evidenced by an experimental observation of the long-wave absorption bands in the UV/vis spectrum (476 nm) in the methanol solution.7 GIBWEV30 (Table 5) is another example of asymmetrically substituted naphthalene derivatives. A decrease of the HOMA value is greater in the para-disubstituted aromatic ring (Table 5) than in the second ring without the para effect. The deviation of planarity and the ϕ angle between the aromatic ring planes is greater than in 1,8-bis(dimethyloamino)-4nitronaphthalene. The geometric parameters and the HOMA values are sensitive to the molecular radius and polarity of the medium. Similarly to 1,8-bis(dimethyloamino)-4-nitronaphthalene, the negative values of HOMA indicate antiaromatic character of compound in polar media and with rc molecular radius (Table 5). Spectroscopic investigation performed by

properties of this compound have been recently studied by Gordon at al.4 and Ozeryanskii at al.7,8 The most striking effect is a decrease of the HOMA value to 0.5762 in optimized and to 0.4929 in the solid state molecule, indicating a considerable decrease of aromaticity in the para-disubstituted aromatic ring. The respective HOMA values for monosubstituted ring in this molecule are 0.6481 and 0.6909. A comparison of the geometric parameters for those rings shows a shortening of the C−N bond, decreasing the α angles associated with the through-resonance between the substituents in para positions. Nonplanarity of the para-disubstituted ring is more distinct than that for the second ring in the molecule. Both effects, the through-resonance between the amino and nitro group and the deviation from planarity, seem to be responsible for significant decrease of the aromaticity. The HOMA value of the parasubstituted aromatic ring changes with an increase of relative permittivity (ε) and a decrease of the molecular radius in contrast to the ring without the peri interaction. The negative values of the HOMA obtained for the molecule optimized at 7122



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Table 7. HOMA, ΔP, and Geometric Parameters for SORYII: Optimized Structures, Crystal,7 and in Media of Different Polarity Calculated for Molecular Radii ro = 5.59 Å and rc = 4.38 Åa

a

HOMA

EN

GEO

ΔP

ϕ [deg]

α [deg]

CN [Å]

N···N [Å]

optimized structure

0.5947

0.1796

0.2257

0.0271

6.196

SORYII

0.1223

0.2654

0.2343

0.2049

0.0355 0.0198 0.0410

6.486


0.6123 0.6243 0.5608

6.751


0.5647

0.2299

0.2054

0.0402

6.748


0.5916

0.1907

0.2178

0.0304

6.464


0.0144

0.5925

0.3930

0.0827

4.345


0.1680

0.4989

0.3331

0.0764

4.886


0.5861

0.2020

0.2118

0.0335

6.616

45.009b 55.469c 39.898b 51.370c 40.774b 47.895c 41.036b 48.288c 44.043b 53.623c 31.167b 34.817c 32.040b 36.058c 43.101b 51.802c

1.4695 1.3962 1.4467 1.3794 2.4501 1.3723 1.4513 1.3736 1.4650 1.3898 1.3926 1.3334 1.4016 1.3373 1.4607 1.3843

2.9491 2.8597 2.8984 2.8520 3.0180 2.9349 3.0140 2.9301 2.9663 2.8751 3.1409 3.1451 3.1311 3.1201 2.9816 2.8916

In optimization as a starting point, the crystal structure has been used. bData for NO2 groups. cData for diethylamino groups.

Table 8. HOMA, ΔP, and Geometric Parameters for SORYII: Optimized Structures, Crystal,7 and in Media of Different Polarity Calculated for Molecular Radii ro = 5.59 Å and rc = 4.38 Åa

a

HOMA

EN

GEO

ΔP

ϕ [deg]

α [deg]

CN [Å]

N···N [Å]

optimized structure

0.6011

0.1856

0.2134

0.0294

16.699

0.6123

0.1223

0.2654

0.0355

6.486


0.5403

0.2469

0.2128

0.0460

20.280


0.5446

0.2431

0.2122

0.0458

20.248


0.5909

0.1989

0.2102

0.0418

19.553


0.2511

0.4467

0.3022

0.0429

19.224


0.2758

0.4290

0.2952

0.0445

19.608


0.5803

0.2113

0.2084

0.0430

19.780

1.4684 1.3867 1.4467 1.3794 1.4497 1.3683 1.4507 1.3692 1.4638 1.3816 1.4139 1.3451 1.4166 1.3476 1.4600 1.3774

2.8799

SORYII

28.051 44.899 39.898b 51.370c 25.552 33.834 23.720 34.142 26.282 39.027 20.481 26.546 34.401 27.014 20.481 26.546

2.8984 2.8520 3.0899 3.0858 3.0243 3.1559 3.1534 3.0410

In optimization as a starting point, the twisted structure has been used. bData for NO2 groups. cData for diethylamino groups.

“push-pull” proton sponges. The experimental UV/vis studies evidenced existence of the through-resonance in solutions. It is to note that the long-wave absorption band for this compound in the methanol solution is observed at 12 nm lower wavenumber in comparison to the asymmetrically substituted 1,8-bis(dimethylamino)-4-nitronaphthalene.8 The HOMA value for 1,8-bis(dimethylamino)-4,5-dinitronaphthalene falls dramatically with increasing polarity of the medium and decreasing molecular radius, up to 0.5661 and −0.9195 for ro and rc, respectively, in medium of ε equal to 35.688. In the crystal structure of 1,8-bis(diethylamino)-4,5dinitronaphthalene7 (Tables 7 and 8), the deviation from coplanarity of both aromatic rings is very small, contrary to its analogue with dimethylamino groups discussed above (Table 6). Ozeryanskii et al.7 assigned it to differences in the crystal packing forces. Surprisingly, also in the optimized structure the ϕ angle is very small (Table 7). The optimized structure with a more twisted naphthalene planes can be expected to be more stable in the situation where there are no intermolecular

Ozeryanskii at al. has evidenced the intramolecular charge transfer between the para substituents in this compound.8 The third group of the investigated compounds contains naphthalene derivatives called “push-pull proton sponges”: 1,8bis(dialkylamino)-4,5-dinitronaphthalenes (SORYEE, SORYII) and GIBVEU. Recently, Ozeryanskii at al.7,8 stated the existence of the through-resonance effect in these naphthalene derivatives basing on comprehensive analysis of their crystal structure and UV and NMR spectra in solutions. The crystal7 and optimized structure of SORYEE (Table 6) shows nonplanarity of the both aromatic rings and large ϕ angle between their planes. The comparison of the geometric parameters with those for 1,8-bis(dimethylamino)naphthalene (Table 2) shows a shortening of the C−N bond length and decreasing of the α angle, which is related to the resonance interaction between the amine and nitro group in both aromatic rings. The HOMA value indicates a moderate decrease of aromaticityidentical for both aromatic rings. However, the decrease of aromaticity is not as large as could be expected for 7123



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Table 9. HOMA, ΔP, and Geometric Parameters for GIBVEU: Optimized Structures, Crystal,8 and in Media of Different Polarity Calculated for Molecular Radii ro = 5.59 Å and rc = 4.38 Åa

a

HOMA

EN

GEO

ΔP

ϕ [deg]

α [deg]

CN [Å]

N···N [Å]

optimized structure

0.6255

0.1907

0.1838

0.0416

19.312

0.6295

0.2069

0.1637

0.0484

19.848


0.5532

0.2576

0.1892

0.0444

19.673


0.5587

0.2533

0.1880

0.0443

19.671


0.6125

0.2061

0.1813

0.0436

19.606


0.5916

0.2263

0.1821

0.0436

19.606

1.4652b 1.3749c 1.4399b 1.3518c 1.4444b 1.3556c 1.4455b 1.3565c 1.4596b 1.3690c 1.4532b 1.3628c

3.0527

GIBVEU

26.732b 36.099c 27.330b 34.072c 23.371b 31.004c 23.525b 31.217c 25.743b 34.434c 24.672b 32.828c

3.0786 3.0944 3.0928 3.0802 3.0802

It is not possible to optimize the structure for rc, ε = 20.493, and ε = 35.688. bData for NO2 groups. cData for pyrrolidino groups.

Table 10. HOMA and Geometric Parameters for WAPGEZ: Optimized Structures, Crystal,30 and in Media of Different Polarity Calculated for Molecular Radii ro = 5.59 Å and rc = 4.38 Å EN

GEO

ΔP

ϕ [deg]

α [deg]

CN [Å]

optimized structure

0.4291

0.3127

0.2582

0.0882

14.253

24.224a 27.102b 36.836c

1.4623 1.4520 1.3486

WAPGEZ

0.8188 0.4360

0.0674 0.2739

0.1139 0.2901

0.0007 0.0600

14.171

16.504a 24.335 41.814

1.4466 1.4270 1.3294


0.8575 0.3308

0.0056 0.3843

0.1369 0.2849

0.0025 0.0611

13.740

17.332a 27.378b 35.206c

1.4504 1.4482 1.3382


0.8288 0.3363

0.0650 0.3803

0.1062 0.2834

0.0029 0.0611

13.774

17.688a 27.344b 35.295c

1.4511 1.4483 1.3386


0.8286 0.4023

0.0650 0.3324

0.1064 0.2652

0.0029 0.0606

14.133

22.392a 27.097b 36.393c

1.4590 1.4505 1.3452


0.8226 −0.0552

0.0662 0.6387

0.1111 0.4165

0.0026 0.0421

10.148

0.710a 33.703b 27.057c

1.4087 1.4616 1.3222


0.8219 0.0211

0.0704 0.5896

0.1076 0.3892

0.0041 0.0473

10.851

1.148a 32.036b 1.148c

1.4159 1,4571 1.3236


0.8267 0.3724

0.0686 0.3544

0.1047 0.2732

0.0042 0.0610

13.986

20.250a 27.164b 35.897c

1.4554 1.4492 1.3419

0.8260

0.0653

0.1087

0.0027

HOMA

a

NO2 group in position 4. bNO2 group in position 2. cPyrrolidino.

interactions. In our calculation in the optimization procedure, the crystal structure has been used as a starting point (Table 7). Optimization starting from a twisted structure similar to that for 1,8-bis(dimethylamino)-4,5-dinitronaphthalene, permits one to find a nonplanar conformation (ϕ = 16.699) (Table 8). The nonplanar conformation (Table 8) corresponds to a global minimum when the previously obtained structure is a

saddle point. The differences in the HOMA values for both planar and nonplanar conformer are not significant and indicate a moderately aromatic character for conformers. No difference between the two aromatic rings of naphthalene has been found. The HOMA values decrease with an increasing ε value of the medium and a decrease of the molecular radius (Tables 7 and 8). For the most polar medium (ε = 35.688), the HOMA value 7124



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calculated for ro radius is 0.5608, while for the rc radius it is definitely smaller and equals 0.0144. It is noteworthy that contrary to 1,8-bis(dimethylamino)-4,5-dinitronaphthalene, a complete destruction of the aromaticity is not observed. In GIBVEU (Table 9), in the crystal as well as in the single molecule, the peri interaction between the pyrrolidine groups caused a significant nonplanarity of both rings (ΔP) and twisting the of the naphthalene molecule (ϕ). The distance between the N atoms of the pyrrolidine groups is also the greatest in the series of the compounds under study. The HOMA values, similar in crystal and in the optimized structures, indicate moderate aromatic character. WAPGEZ (Table 10) is an example of the naphthalene derivatives where the peri effect is absent and the aromaticity is influenced by the through-resonance interaction between the amine and nitro group in para positions as well as the substituent effect of the nitro group in the ortho position. The angle between the 4-nitro group and the aromatic ring (α) is the smallest in the series of the compounds under study. The C−N bonds of the pyrrolidine group in the optimized and crystal structures are the shortest in the series of studied compounds. The C−N bond length indicates the presence of the through-resonance interaction of the amino with the nitro group. The HOMA values for the disubstituted ring, equal to 0.4291 for the optimized molecule and 0.4360 for the molecule in crystal, indicate a substantial decrease of the aromaticity. For unsubstituted ring the HOMA values are 0.8188 in the optimized and 0.8575 in the crystal molecules. The distortion from planarity (ΔP) of the aromatic ring is quite big. For the most polar medium the HOMA value is 0.3308 for the molecular radius ro and −0.0552 for the crystal structure rc radius. The values of HOMA indices for investigated compounds reveals that the decrease of aromaticity for 1,8-bis(amine)-4,5dinitronaphthalenes is unexpectedly small in vacuum, solid state as well as in nonpolar medium and is significant in polar medium for crystal molecular radius. Analysis of the HOMA components shows that the main contribution to the observed destruction of aromaticity in the polar medium is connected with the EN term, that is, the elongation of the mean bond length. For optimized and crystal structure, the dominating term is the GEO term describing a decrease in the π-electron delocalization due to an increase in the bond alternation. In order to analyze the effect of the through-resonance on πelectron delocalization in the aromatic rings of naphthalenes under study, we have applied the Atom in Molecules method.31 Aromaticity is directly related to the π electron delocalization, so we have chosen the bond ellipticity ε(r) as the most useful parameter among the parameters proposed by this method. The ellipticity at the bond critical point (BCP) can be interpreted as a measure of the anisotropy of the curvature of the electron density in the direction normal to bond and therefore may serve as a sensitive index monitoring π character of the double bond. The ellipticity is defined as

where c is the normalization constant (c = 10.0588 for the BLYP method of the molecule optimization), n is the number of bonds, εi is the ellipticity of particular bond, and εref is the ellipticity of the bond in benzene (εref = 0.199). The relationship of the HOMA and EL parameters for the compounds investigated in this paper is shown in Figure 2.

Figure 2. Correlation of EL parameter versus HOMA: open triangles − SORYII, ZOSKAT, GIBWEV (ε = 35.688, 20.493, rc); open circles − GIBWEU, SOREE, SORYII ZOSKAT (all except ε = 35.688, 20.493, rc); open squares − unsubstituted ring of WAPGEZ; filled triangles − substituted ring of WAPGEZ; filled dots − DNTNAP, DMANAP, ROKRAL.

It is not possible to find a common correlation between EL and HOMA values, which would include all investigated compounds. There is no group of the compounds where the EL parameter would decrease with decreasing HOMA value, as it had been suggested for unsubstituted aromatic rings.32 This result shows that the EL parameter cannot be used as an aromaticity measure for naphthalenes with electron-donating and electron-accepting substituents. Instead of the EL parameter, we have applied the ellipticity of the bonds in a naphthalene ring to investigate the π electron delocalization in the compounds under study. A comparison of the ellipticity values for selected bonds with those calculated for naphthalene proves to be quite informative concerning the changes in the π electron delocalization. In Figure 3a−c, the relationships between the ellipticity of selected bonds of the substituted naphthalenes investigated in this paper are shown. In the naphthalene molecule, the highest ellipticity values ε(r) are found for C1−C2 and C3−C4 bonds, indicating the greatest double bond character of these bonds. Decreasing of aromaticity due to the through-resonance interaction between the NR2 and NO2 group in the para position is a result of an increasing weight of the quinoid canonical structure with of the charge separation. In this situation, the double bond character of the bonds C2−C3 (C6−C7) and C9−C10, and, as a consequence, the ellipticity of these bonds, is expected to increase. We have adopted ellipticity of the C2−C3 (C6−C7) bonds as a measure of contribution of the quinoid canonical structure and through-resonance interaction between the amine and nitro groups.

ε(r ) = λ1/λ 2 − 1 where λ1 and λ2 are the negative eigenvalues of the Hessian of electron density at BCP (ρ(r)). Recently, Palusiak at al.32 proposed the sum of ellipticities of the bonds in the aromatic ring as a sensitive measure of the ring aromaticity and introduced a new aromaticity index EL: EL = 1 − c /n ∑ |εi − εref | 7125



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1,8-bis(dimethylamino-4-nitronaphthalene) and 1,8-bis(dialkiloamino)-4,5-dinitro-naphthalenes permits one to observe changes in the π-electron delocalization induced by through-resonance. For 1,8-bis(dimethylamino)-naphthalene, the increasing ellipticity of the C2−C3 (C6−C7) bonds to 0.1740 is a result of conjugation between the electron-donor amine groups and the π electron system. An increase of ellipticity of the C1−C9 (C8−C9) bond is noted. Similar ellipticity values are observed for the monosubstituted aromatic ring in 1,8-bis(dimethylamino)-4-nitronaphthalene, where the para interactions are absent. In the monosubstitution of 1,8bis(dimethylamino)-4-nitronaphthalene, ε(r) for the C6−C7 bond equals 0.1737. For the para-disubstituted ring in this compound, an increase of the elipticity of the C2−C3 bond (0.1919) indicates greater weight of quinoid resonance structure. For 1,8-bis(dimethylamino)- and 1,8-bis(diethylamino)-4,5-dinitronaphthalene, the ellipticity of C2− C3 and C6−C7 bonds (0.1862) is smaller than that in 1,8bis(dimethylamino-4-nitronaphthalene), which corresponds with the higher value of the HOMA in the last compounds. It is also in agreement with experimentally stated differences in frequency of the long-wave absorption bands in methanol.7 This observation confirms a rather unexpected result that the presence of the two pairs of para substitutents of opposite electronic properties does not lead to a significant decrease of aromaticity. This result could suggest that the dispersion of the charge in both identical aromatic rings may leads to decrease the weight of the quinoide structures with charge transfer between the substituents in para positions. Comparison of the ellipticity for optimized structure of series of 1,8-bis(pyrrolidino)naphthalenes leads to a similar conclusion. The respective values of ε(r) are greater in comparison to the respective 1,8-bis(dialkyloamino)naphthalenes, which is in agreement with the statement of Ozeryanskii at al.8 that the pyrrolidin-1-yl is a more effective electrondonor in comparison to the dialkylmethylamine group. The highest ε(r) value of the C2−C3 (0.2118) was found for the para-disubstituted ring in 1-pyrrolidino-2,4-dinitronaphthalene, where the HOMA value (0.4291) indicates a significant loss of the aromaticity. However, even for this compound the comparison the ellipticity values in the aromatic ring does not show a dominance of the quinoid structure. For unsubstituted ring in this compound the ellipticity of C6−C7 bonds equals only 0.1700 and the HOMA value of 0.8188 indicates its high aromaticity. One may suggest that delocalization of the π electrons in the substituted ring compensates the loss of the aromaticity in another ring caused by the through-resonance interactions. Analysis of the ellipticities of the C2−C3 (C6−C7) bonds in the solid state structure shows that, in most cases, these values are slightly greater than for optimized structures. These ellipticities correlate with the CN bond lengths. It is noteworthy that the HOMA values are generally greater in the crystal. The exception is the lower HOMA value (0.4931) for the para-substituted ring in 1,8-bis(dimethylamino)-4nitronaphthalene). This indicates that not only the resonance interactions but also interactions in crystal and changes in conformation influence the aromaticity of aromatic rings of in the naphthalene molecules under study. The most significant effect on the ellipticity values for the investigated naphthalenes with para amino and nitro groups is produced by an increase of the medium polarity. For 1,8bis(dimethylamino)-4,5-dinitronaphthalene, in the most polar

Figure 3. Mutal correlations of bond ellipticities related to respective naphthalene bonds: (a) C1−C9 versus C4−C10 and C8−C9 versus C5−C10, (b) C1−C2 versus C3−C4 and C7−C8 versus C5−C6, (c) C 9-C10 versus C6−C7 and C9−C10 versus C2−C3. Atom numbering according to Scheme 2. Theoretical and experimental structures are included.

For 1,8-dinitronaphthalene, the ε(r) values are close to those found for unsubstituted naphthalene; this suggests that the NO 2 group does not strongly affect the π electron delocalization. Comparison of the ellipticity values in the optimized structures of 1,8-bis(dimethylamino)-naphthalene, 7126



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medium and with crystal molecular radius, the ε(r) value of C2−C3 and C6−C7 bonds of 0.2334 exceeds those of C3−C4 and C1−C2 bonds. At the same time, the ε value of the C9− C10 bond increases to 0.2287, which indicates a dominate contribution of the quinoid canonical structure. A similar situation has been found for para-disubstituted rings of 1,8bis(dimethylamino)-4-dinitronaphthalene, 1,8-bis(pyrrolidino)4-nitronaphthalene and 1-pyrrolidino-2,4-dinitronaphthalene. In all these compounds the negative HOMA values indicate an antiaromatic character of the ring. The obtained results show that the increasing medium polarity causes significant change in the π-electron delocalization in the aromatic ring. This favors the through-resonance interactions and the charge transfer from amine to nitro group in the naphthalenes under study. This conclusion is in agreement with the experimentally observed solvent effect in UV/vis spectra of “push−pull proton sponges”7,8 and finding that 1,8-bis(dimethylamino)-4,5diformylonaphthalene has a surprisingly high dipole moment (μ = 9.21 D) in a water solution, suggesting almost ionic structure.10 Summing up, the ellipticity of the C2−C7 (C6−C7) bonds has been shown to be an efficient index of an increase in the weight of the quinoid canonical structure and thereby a measure of the through-resonance interaction between the substituents in para positions in nitroaminonaphthalenes under study. An increase of ε(r) value of the C2−C3 (C6−C7) is accompanied by the shortening of the C−N bond length to amine and nitro group, respectively (Figure 5). A correlation

Figure 5. Correlation of the C−N bond length with ellipticities of the C2−C3 and C6−C7 bonds. Empty points: C−N to amino group (y = −1.135x + 1.5989, R2 = 0.8127), full points: C−N to nitro group (y = −17.167x2 + 5.8295x + 0.9768R2 = 0.849).

DFT B3LYP/6-311++G** level of calculation using the Gaussian 09 program.33 To verify that the absolute minimum of the potential energy surface was reached, all vibrational frequencies were checked for being positive. The HOMA11 parameter was calculated to characterize the aromaticity of the investigated compounds. To investigate the effect of the polarity of the medium, the PCM33 was used. The calculation was performed for two molecular radii: ro of single molecules calculated with the Gaussian 09 program, and rc resulting from the crystal cell volume per one molecule. The investigated molecules were optimized using the calculated radii and three different polarities expressed by the following electric permittivities: ε = 1.9113, ε = 20.493 and ε = 35.688. To calculate the bond ellipticity, the wave function evaluated for each X-ray and the optimized structures were used as an input to the AIMALL program.31 In the next step, the ellipticity of the bonds of the naphthalene ring was used to calculate the EL parameter according to the literature.33

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CONCLUSIONS The molecular structure and the aromaticity of the naphthalenes derivatives under study, expressed with HOMA index, are a result of the compromise between the effects of through-space repulsive interaction between the two amine and/or two nitro groups in peri positions (1,8 and 4,5, respectively) and through-resonance interactions between these groups as well as the intermolecular interaction and the molecular packing in the solid state. The ellipticity of the C2− C3 (C6−C7) bonds, applied as a measure of throughresonance effect between the para substituents, correlates with the C−N bond length. The peri effect does not reduce the mesomeric interaction between the substituents and the π electron system. Comparison of the HOMA and ellipticity of C2−C3 (C6−C7) bond for 1,8-diaminonaphthalenes and 1,8diamino- 4,5-dinitronaphthalene reveals that the throughresonance interaction between the amines and nitro groups modestly reduces the aromaticity of the naphthalene ring. A considerable effect has been observed for 1,8-diamino-4,5-

Figure 4. Correlation of ellipticities of the C2−C3 and C6−C7 bonds related to respective naphthalene bond elipticities with the HOMA values. Atom numbering according to Scheme 2. Theoretical and experimental structures are included.

between the ellipticity of the C2−C3 (C6−C7) bond related to naphthalene values and the HOMA value is presented in Figure 4. Dispersion of the points in this correlation results from the fact that HOMA values depend not only on through-resonance effect but also on the geometry of the molecules and intermolecular interactions.

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EXPERIMENTAL SECTION The structure of the investigated compounds was chosen from the CSD crystal database,24 and the solid-state structures were used as the starting point in the geometry optimization at the 7127



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(12) Krygowski, T. M.; Stępień, B. T. Sigma- and Pi-Electron Delocalization: Focus on Substituent Effects. Chem. Rev. 2005, 105, 3482−3512. (13) Shishkin, O. V.; Omelchenko, I. V.; Krasovska, M. V.; Zubatyuk, R. I.; Gorb, L.; Leszczyński, J. Aromaticity of Monosubstituted Derivatives of Benzene. The Application of Out-of-Plane Ring Deformation Energy for a Quantitative Description of Aromaticity. J. Mol. Struct. 2006, 791, 158−164. (14) Omelchenko, I. V.; Shishkin, O. V.; Gorb, L.; Hill, F. C.; Leszczyński, J. Properties, Aromaticity, and Substituents Effects in Poly Nitro- and Amino-Substituted Benzenes. Struct. Chem. 2012, 23, 1585−1597. (15) Krygowski, T. M.; Ejsmont, K.; Stepień, B. T.; Cyrański, M. K.; Poater, J.; Solà, M. Relation Between the Substituent Effect and Aromaticity. J. Org. Chem. 2004, 69, 6634−6640. (16) Krygowski, T. M.; Stępień, B. T. Changes in Aromaticity in the Ring of Monosubstituted Benzene Derivatives. Polym. J. Chem. 2004, 78, 2213−2217. (17) Krygowski, T. M.; Dobrowolski, M. A.; Zborowski, K.; Cyranśki, M. K. Relation Between the Substituent Effect and Aromaticity. Part II. The Case of meta- and para-Homodisubstituted Benzene Derivatives. J. Phys. Org. Chem. 2006, 19, 889−895. (18) Krygowski, T. M. Crystallographic Studies of Inter- and Intramolecular Interactions Reflected in Aromatic Character of πElectron Systems. J. Chem. Inf. Comput. Sci. 1993, 33, 70−78. (19) Krygowski, T. M.; Cyrański, M. K.; Nakata, K.; Fujio, M.; Tsuno, Y. Separation of the Energetic and Geometric Contributions to Aromaticity. Part VII. Changes of the Aromatic Character of the Rings in Naphthalene Induced by the Charged Substituent CH2+. The Dependence on the Position of the Substitution, Torsion Angle and the Exocyclic Bond Length Variation. Tetrahedron 1998, 54, 3303− 3310. (20) Krygowski, T. M.; Palusiak, M.; Płonka, A.; Zachara-Horeglad, J. E. Relationship Between Substituent Effect and Aromaticity - Part III: Naphthalene as a Transmitting Moiety for Substituent Effect. J. Phys. Org. Chem. 2007, 20, 297−306. (21) Majerz, I.; Dziembowska, T. Aromaticity of Overcrowded Nitroanilines. J. Phys. Chem. A 2012, 116, 5629−5636. (22) Cysewski, P.; Jeliński, T.; Krygowski, T. M. Factors Influencing Aromaticity: PCA Studies of Monosubstituted Derivatives of Pentafulvene, Benzene and Heptafulvene. Curr. Org. Chem. 2012, 16, 1920−1933. (23) Hall, H. K., Jr. Correlation of the Base Strengths of Amines. J. Am. Chem. Soc. 1957, 5441−5444. (24) Allen, F. H. The Cambridge Structural Database: A Quarter of a Million Crystal Structures and Rising. Acta Crystallogr. 2002, B58, 380−388. (25) Ciechanowicz-Rutkowska, M. An Independent Investigation of the Crystal Structure of 1,8-Dinitronaphthalene (Orthorhombic Form) at 22 and 97°C. J. Solid State Chem. 1977, 22, 185−192. (26) Mallinson, P. R.; Woźniak, K.; Wilson, C. C.; McCormack, K. L.; Yufit, D. S. Charge Density Distribution in the “Proton Sponge” Compound 1,8-Bis(dimethylamino)naphthalene. J. Am. Chem. Soc. 1999, 121, 4640−4646. (27) Einspahr, H.; Robert, J.-B.; Marsh, R. E.; Roberts, J. D. Peri Interactions: An X-ray Crystallographic Study of the Structure of 1,8bis(dimethylaminonaphthalene). Acta Crystallogr. 1973, B29, 1611− 1617. (28) Ozeryanskii, V. A.; Shevchuk, D. A.; Pozharskii, A. F.; Kazheva, O. N.; Chekhlov, A. N.; Dyachenko, O. A. Protonation of Naphthalene Proton Sponges Containing higher N-alkyl Groups. Structural Consequences on Proton Accepting Properties and Intramolecular Hydrogen Bonding. J. Mol. Struct. 2008, 892, 63−67. (29) Pozharskii, A. F.; Kuzmenko, V. V.; Alexandrov, G. G.; Dmitrienko, D. V 1,8-bis(dimethylamino)naphthalene 0.13. Solvatochromism and Molecular Structure of 4-nitro-1,8-bis(dimethylamino)naphthalene and Its Salts with Chloric Acids. Zh. Org. Khim. 1995, 31, 570−578.

nitronaphthalenes where the through-resonance interaction is present only in the para-substituted ring. The effect of the medium polarity on the aromaticity has been observed only for aromatic ring where conjugation between the para-amino and nitro group is possible. Analysis of the ellipticities of C2−C3 (C6−C7) bond shows that the polar medium facilitates the through-resonance interaction between the amine and nitro group in para positions. With increasing polarity and decreasing molecular radius aromaticity significantly decreases up to its complete destruction.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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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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