Aromaticity of peri- and para-Substituted Naphthalene-1-carbaldehyde

Article pubs.acs.org/JPCA

Aromaticity of peri- and para-Substituted Naphthalene-1carbaldehyde. Comparison with 1‑Nitronaphthalene Irena Majerz*,† and Teresa Dziembowska‡ †

Faculty of Pharmacy, Wroclaw Medical University, Borowska 211a, 50-556 Wroclaw, Poland Department of Organic and Physical Chemistry, West Pomeranian University of Technology, 70-061, Szczecin, Poland

‡

S Supporting Information *

ABSTRACT: Aromaticity of the naphthalene ring substituted by formyl or nitro and secondary amine groups in peri and para positions has been investigated. HOMA (harmonic oscillator model of aromaticity) index of the naphthalene rings has been used to characterize the aromaticity of the investigated molecules. Ellipticity of C2− C3 (C6−C7) bonds, obtained by means of topological analysis of the electron density, has been used as a measure of the through-resonance effect between the para substituents. Dependence of the HOMA values on the rotation angle of formyl and nitro groups to the aromatic plane for naphthalene-1-carbaldehyde and their derivatives with dimethylamino group in para position has been analyzed. It has been shown that the nitro group has a stronger resonance effect and causes greater decrease of aromaticity than the formyl group only for planar or close to planar conformation. The attractive and repulsive peri interactions have been analyzed by using the QTAIM and NCI methods. Interrelation between the peri and para interactions has been discussed.

1. INTRODUCTION

aromatic ring tends to keep the substituent in the naphthalene plane. When two substituents of opposite electron properties are included in the para position of the naphthalene ring, the through-resonance effect between the para substituents appears.17,20−22 The through-resonance effect associated with a charge transfer from electron-donor to electron-acceptor group in para position results in an increase of the weight of canonical quinoid structure and a decrease of their aromaticity.2,20−22 Naphthalene derivatives with π-donor and π-acceptor groups in the para-position belong to the family of push−pull compounds, widely studied due their application in optoelectronics and other functional materials.20 The push− pull interaction in the naphthalene derivatives also leads to the specific optical properties and their possible practical applications as a fluorescence switch,20,23 as fluorofores in biological studies and in fluorescence microscopy, and recently in medical studies in positron-emission tomography.24,25 The through-resonance interactions in peri-substituted naphthalene derivatives were recently discussed.17,21,22 In the previous paper,17 we investigated the influence of throughspace and through-resonance interactions in the series of naphthalenes with amino and nitro substituents in peri and para positions on aromaticity of the aromatic rings expressed by HOMA index. As a measure of the quinoind-resonance structure we applied the ellipticity of the electron density at

1,2

Aromaticity of the mono- and disubstituted naphthalene and its heterocyclic analogues3 depends on the electronic properties of the substituent and location of the heteroatom in the naphthalene ring. As was shown,1 also the conformation of the substituents influences the aromaticity of the naphthalene ring. Two substituents in positions 1 and 8 in the naphthalene ring are forced to be in a close contact, generally closer than the sum of their van der Waals radii, which leads to attractive and/or repulsive peri interaction. It is to be noted that in 1-substitued naphthalenes, because of the proximity of the 8-H atom, interaction of the substituent and the 8-H atom is effectively also observed. The peri interaction brings changes in exocyclic bonds lengths and torsion angles and a disturbance of the naphthalene skeleton.5−16 Deviation from planarity of the groups in positions 1 and 8 affects the resonance interaction of the substituent with naphthalene π-electron system and the aromaticity of the naphthalene ring.4,5,6,16,17 The physicochemical properties of the peri- substituted naphthalenes are strongly influenced by the peri interactions.4,18 The exceptional reactivity of the substituents of opposite electronic properties in peri positions was stated.4,7,8,11,14,19 Application of the perinaphthalene system as a model for chemical reactions between the groups in close proximity was used.11,13,14 The electronic structure and geometry of the peri-substituted naphthalenes is a balance between two competing interactions. A steric repulsion due to the proximity of substituents tends to cause deviation of the substituent from the aromatic ring plane, while the resonance interaction of the substituents with the © 2017 American Chemical Society

Received: November 28, 2016 Revised: March 9, 2017 Published: March 10, 2017 2627

DOI: 10.1021/acs.jpca.6b11926 J. Phys. Chem. A 2017, 121, 2627−2635

Article

The Journal of Physical Chemistry A

for the system with all CC bonds equal to the Ropt = 1.388. Ri stands for the ith bond length. The HOMA index calculated for the optimized structure of naphthalene equals 0.7840. The ellipticity of the C2−C3 (C6−C7) bonds at the bond critical point (BCP) has been applied as an indicator of the decreasing of the aromaticity of naphthalene rings and a measure of through-resonance effect between the parasubstituents.17 The ellipticity, calculated in the frame of the quantum theory of atom in molecules method (QTAIM)30 is define as

the bond critical point of C2−C3 (C6−C7) bonds in naphthalene aromatic ring obtained by topological analysis of the electron density in frame of QTAIM method. It was shown that the through-resonance interaction between peri-diamine groups and peri-dinitro groups reduces the aromaticity of the naphthalene ring only modestly.26 The object of our present paper is investigation of the influence of the peri and para interactions between the formyl or nitro and amino groups in naphthalene molecules on the aromaticity expressed by the HOMA index, taking into consideration the result for the respective peri-dinitro derivatives.17 There are some controversies concerning the substituent effect of the nitro and formyl group on π-electron delocalization in the aromatic ring20,23 that prompted us to closely investigate the effect of these groups on decrease of aromaticity of the naphthalene ring. The nitro group exhibits much stronger electronegative properties than the formyl group; the σp constant equals 0.42 for NO2 and 0.22 for CHO group. On the other hand, the resonance constant σR that describes the interaction of a substituent with the π electrons of the aromatic ring are greater for the formyl (σR = 0.24) than for the nitro group (σR = 0.17).27 In the presence of the strong electron-donor substituent in para position, the π-electron acceptor effect of the CHO and NO2 group increases and the respective substituent constant σp− equals 1.03 for the formyl group and 1.27 for the nitro group.27 Recently Cyrański et al.28 have shown that for para-nitrophenolate the substituent constant σp− depends on the conformation of the NO2 group with respect to the aromatic plane and decreases to 0.72 for the twist angle of 90°. Dependence of HOMA values for parasubstituted nitrobenzene derivatives on rotation angle of the NO2 group was investigated.28,29 In naphthalene derivatives with 1-NO2 and 1-CHO groups, the peri interaction with the 8H atom may influence the conformation of these groups with respect to the aromatic plane and hence the resonance interaction with the π-electron system. In the first part of our investigation we compare the peri interaction in the models naphthalene-1-carbaldehyde and 1nitronaphthalene and peri and para interactions in their derivatives with the dimethylamino group substituted in position 4. The peri interaction is analyzed using the QTAIM30 method and the recently developed NCI31 method. The influence of the rotational angles of these groups in respect the aromatic plane on the peri interactions and HOMA values are presented. In the second part, the influence of the peri and para interactions on HOMA values for crystal and optimized structure of few naphthalene-1-carbaldehydes and naphthalene1,8-dicarbaldehydes substituted with dimethylamino groups are analyzed.

ε(r ) = (λ1/λ 2) − 1

where λ1 and λ2 are the negative eigenvalues of the Hessian of electron density at BCP (ρ(r)). To analyze and visualize the nonbonded attractive and repulsive interactions, the noncovalent interaction (NCI) method has been applied.31 The objects of our investigation are the optimized model naphthalene derivatives: naphthalene-1-carbaldehyde; naphthalene-1,8-dicarbaldehyde, 4,5-bis(dimethylamino)-naphthalene1-carbaldehyde and their 1-nitronaphthalene analogous as well as the crystal structures taken from the CSD database:32,33 8-dimethylaminonaphthalene-1-carbaldehyde (BAZWUU),7 4,5-bis(dimethyloamino)naphthalene-1,6-dicarbaldehyde (KUPHOS),34 4,5-bis(dimethylamino)naphthalene-1,8-dicarbaldehyde (KUPHIM),18,20 and 4,5-bis(pyrolidino)naphthalene1,8-dicarbaldehyde (GIBTUI).7 The numbering of the naphthalene ring is given in Scheme 1 Scheme 1. Numbering of the Atoms in the Naphthalene Ring

The analyzed geometric parameters are ϕ, the angle between the two naphthalene aromatic ring planes; αONO, αCHO, and αCNC, the angles between the aromatic ring plane and the plane of the nitro, formyl, and dialkyamino substituents, respectively; C−N and C−C, the exocyclic bond lengths between the C atom in the aromatic ring and the N and C atoms of the substituent groups; and C···C, N···N, C···N, and H···O(N), the distances between the atoms in the peri positions, as given in Tables S1−S10. 2.1. peri and para Interactions in the Models Naphthalene-1-carbaldehyde, 1-Nitronaphthalene, and Their Derivatives with Two Dimethylamino Groups in Positions 4 and 5 and Dependence of the Ring Aromaticity on the Conformation of the Substituent. Ozeryanskii et al.20 compared the crystal structure of 4,5bis(dimethylamino)naphthalene-1,8-dicarbaldehyde with 4,5bis(dimethylamino)-1,8-dinitronaphthalene and stated that, unexpectedly, the formyl group exerted a stronger π-acceptor effect than the nitro group. The authors assigned this result to the greater steric demand of the nitro group with respect to the formyl group. Hence, we have undertaken a more comprehensive investigation of peri and para interactions in the model optimized structures of naphthalene-1-carbaldehyde and 1nitronaphthalene and their derivatives with the N-dimethyla-

2. RESULTS AND DISCUSSION In order to estimate the aromaticity of a particular naphthalene ring of peri and para naphthalenecarbaldehydes and compare it with the results for nitro analogues, the HOMA index (harmonic oscillator model of aromaticity) has been calculated for the naphthalene rings:1,32 n

HOMA = 1 − (α /n) ∑ (Ropt − 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 2628


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which is denoted by the values of the σp− substituent constant: 1.27 for NO2 and 1.03 for the CHO group.27 The observed effect has to be assigned to the greater deviation of nitro group from the naphthalene plane than the deviation of the formyl group. In order to compare the influence of the conformation of 1CHO and 1-NO2 group against the naphthalene ring plane on the aromatic character of the naphthalene rings, we have calculated the respective HOMA values as a function of α, the rotation angle of these groups with respect to the aromatic plane. Figure 1a presents the dependence of HOMA value on

mino groups in positions 4 and 5, taking into account the conformation of the nitro and formyl group and their influence on the HOMA value of particular rings. The molecule of naphthalene-1-carbaldehyde (Table S1) is almost planar, with the oxygen of the carbonyl group directed toward the 8-H atom, suggesting the existence of the peri interaction between these groups. This conformation is more stable, by 1.9504 kcal/mol, comparing to the conformation with the formyl group rotated by180°. The H···O distance between the carbonyl O atom and the 8-H atom suggests the presence of a weak H···O hydrogen bond. Because the QTAIM method delivers criteria for the existence of a hydrogen bond,27 to confirm the presence of a weak H···O hydrogen bond in naphthalene-1-carbaldehyde, the QTAIM approach has been applied.30 The presence of the bond path linking the 8-H atom and the O atom of the 1-CHO group with the bond critical point (BCP) of charge density (ρBCP) equal to 0.0173 au confirms the presence of a weak H···O hydrogen. The conformation of the formyl group, coplanar with the aromatic ring, is favorable for resonance interaction with the π-electron system which leads to a small decrease of the aromaticity of the naphthalene ring connected with the CHO group (Table S1). Because of the steric repulsive interaction with peri-H atom, the NO 2 group in 1-nitronaphthalene (Table S2) is considerably twisted from the aromatic plane (α equals 35.197°). The H···O distance between the 8-H and O atom of the nitro group suggests the existence of an attractive H···O interaction that has been evidenced by QTAIM analysis. HOMA value of 0.7614 indicates that the resonance interaction of the NO2 group with the π-electron system is weaker comparing to the CHO group. In the most stable conformation of 4,5-bis(dimethylamino)naphhtalene-1-carbaldehyde (Table S3), the H···O distance between the peri H atom and the O atom of the formyl group is shortened in comparison to the naphthalene-1-carbaldehyde. The QTAIM analysis shows the existence of H···O bond paths with the ρBCP of 0.0188 au, confirming the existence of the weak hydrogen bond. The geometric parameters of the molecule: almost planar conformation of the formyl group, decreased angle between the CNC planes of 4-dimethylamine group and the aromatic ring plane, and relatively short exocyclic C−C bond of the formyl and C−N of 4-dimethylamino group indicate the presence of the through-resonance interaction between the 1-formyl and 4-dimethylamino group (Table S3). HOMA values of 0.5751 for the ring substituted with CHO group show grater decrease of aromaticity in comparison to the naphthalene-1-carbaldehyde. In 4,5-bis(dimethylamino)-1-nitronaphthalene,17 the H···O distance between peri H atom and the oxygen atom of the NO2 group equal to 2.1517 Å (Table S4) and electron density at the BCP (ρBCP) of 0.0199 au indicate a strengthening the H···O interaction in comparison to 1-nitronaphthalene. The presence of an attractive interaction between the N atoms of the dimethylamine groups is confirmed by the QTAIM analysis. The HOMA value of 0.5932 indicates a significant decrease of aromaticity caused by through-resonance interactions between the nitro and dimethylamino group. It is to note that for analogous naphthalene-1,8-dicarbaldehyde the HOMA value indicates greater decrease of aromaticity. The above results show that the formyl group displays a stronger resonance effect and more effectively reduces the aromaticity of the naphthalene ring in comparison to the nitro group. On the other hand, the NO2 group is a better π-electron acceptor than the CHO one,

Figure 1. Dependence of HOMA value on the angle between the naphthalene aromatic ring and the CHO (NOO) plane for (a) naphthalene-1-carbaldehyde (Δ, broken line) and 1-nitronaphthalene (●, solid line) and for (b) 4,5-bis(dimethylamino)naphhtalene-1carbaldehyde (▲, broken, dotted line) and 4,5-bis(dimethylamino-1nitronaphthalene (○, dotted line).

the angle between the substituent plane and the naphthalene ring plane for 1-naphthalenecarbaldehyde and 1-nitronaphthalene. The HOMA value gradually decreases with rotation of both groups from the aromatic plane, but for the NO2 the steepest dependence is observed. For small deviation from planarity, the nitro group affects the HOMA values more strongly, while for the angles greater than 40° the effect of the conformation of NO2 group is smaller than that for the formyl group. The dependence of HOMA values on the rotational 2629


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attractive H···O component to the peri interactions has been demonstrated. The existence of a weak H···O hydrogen bond between the 8-H and O atoms of the CHO and NO2 group has been evidenced by the QTAIM method. In order to examine the interrelation between the attractive H···O and the repulsive interaction in 1-substituted naphthalene derivatives we have examined the dependence of the H···O distance and the electron charge density at the BCP (ρBCP) on the rotation angle α of the nitro and formyl group with respect to the aromatic plane (Figures 3 and 4). The dependencies presented in

angles for 4,5-bis(dimethylamino)-1-naphthalene-1-carbaldehyde and its 1-NO2 analogue is presented in Figure 1b. As can be expected, through-resonance interaction between the CHO and NO2 groups with the dimethylamino groups in para positions leads to a greater decrease of the HOMA values and stronger dependence of these values on the deviation of the CHO and NO2 group from the aromatic ring plane. The resonance interaction of the NO2 group with the π-electron system responsible for the decrease of aromaticity is stronger than that for the CHO group only for almost coplanar conformation. When rotation angle α increases to about 20°, the π-electron accepting properties of the CHO group become more important. As it was shown previously,17 information concerning the decrease of aromaticity and the weight of the resonance quinoide structure may be obtained from the ellipticity of the electron density at the bond critical point (BCP) for 2−3 (ε2−3) and 6−7 (ε6−7) bonds.17 The ellipticity value is known to be dependent on the double-bond character of the bond and hence may be a measure of the weight of the quinoide resonance structure. In Figure 2, the dependence of the

Figure 3. Dependence of H···O distance on the angle between the naphthalene aromatic ring and the CHO (NOO) plane for naphthalene-1-carbaldehyde (Δ, broken line), 1-nitronaphthalene (●, solid line), 4,5-bis(dimethylamino)naphhtalene-1-nitronaphthalene (▲), and 4,5-dimethylamino-1-carbaldehyde (○, dotted line).

Figures 3 and 4 show that the increasing deviation of CHO and NO2 from the naphthalene ring plane causes a decrease of the Figure 2. Dependence of the ε2−3 values on the angles between the aromatic ring planes and the substituent CHO and ONO planes for naphthalene-1-carbaldehyde (Δ, broken line), 1-nitronaphthalene (●, solid line), 4,5-bis(dimethylamino)naphhtalene-1-carbaldehyde (▲, broken, dotted line), and 4,5-bis(dimethylamino-1-nitronaphthalene (○, dotted line).

ellipticity of 2−3 bonds in naphthalene-1-carbaldehyde,1nitronaphthalene and in their derivatives with the dimethylamino groups on the rotation angle α of the CHO and NO2 groups is presented. As can be seen from Figure 2, the ε2−3 ellipticity values for naphthalene-1-carbaldehyde and its nitro analogous are closer to the values in nonsubstituted naphthalene (0.1621)17 and are not sensitive to the rotation the functional group. For 4,5bis(dimethylamino)naphhtalene-1-carbaldehyde and 4,5-bis(dimethylamino)-1-nitronaphthalene, where the through-resonance between the substituents in para position appears, the ε2−3 value decreases with increasing α angle. The effect of rotation of the CHO and NO2 group on HOMA and ε2−3 values shown above indicates the importance of repulsive peri interactions, responsible for the increasing α angle. For 1-naphthalene derivatives, the presence of the

Figure 4. Dependence of electron density at H···O BCP on the angle between the naphthalene aromatic ring and the CHO (NOO) plane for naphthalene-1-carbaldehyde (Δ, broken line), 1-nitronaphthalene (●, solid line), 4,5-bis(dimethylamino)naphhtalene-1-nitronaphthalene (▲ , broken, dotted line). and 4,5-bis(dimethylamino)naphhtalene-1-carbaldehyde (○, dotted line). 2630


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Figure 5. QTAIM diagram, NCI plot and NCI surface for optimized naphthalene-1-carbaldehyde (a), 1-nitronaphthalene (b), 4,5bis(dimethylamino)-naphthalene-1-carbaldehyde (c), and 4,5-bis(dimethylamino)-1-nitronaphthalene (d). Carbon, hydrogen, nitrogen, and oxygen atoms are marked as gray, white, blue, and red dots, respectively. Red, yello,w and green dots represent BCPs (3, −1), RCPs (3, +1) and CCP (3, 3) respectively.

hydrogen bond H···O strength. For the angle greater than 60°, the hydrogen bond disappears (Figure 4). Comparison of the H···O distances and the ρBCP values for 1-nitronaphthalene and naphthalene-1-carbaldehyde shows that hydrogen bond is stronger in first of these compounds, which is in accordance with stronger electron-accepting properties of the NO2 than those of the CHO group. Introduction of the electron-donating dimethylamino group in para position with respect to the CHO and NO2 causes a decrease of the H···O distance and an increase of the charge density at the BCP, more significant for 1-nitronaphthalene than for naphthalene-1-carbaldehyde. This may be assigned to an increase of negative charge density on O

atoms caused by the through-resonance interaction between the CHO or NO2 and N(CH3)2 groups. In order to obtain information on the repulsive and attractive nonbonded peri interaction in the investigated naphthalene derivatives, we have applied the recently developed NCI method.28 In Figure 5a−d, the QTAIM diagrams, NCI plots and NCI surfaces for the investigated model compounds have been presented. The comparison of the NCI plots and NCI surfaces for the optimized naphthalene-1-carbaldehyde and 1nitronaphthalene shows the existence of the attractive H···O hydrogen bond and several repulsive interactions. Introduction of two dimethylamino groups in positions 4 and 5 results with an increase of both: attractive and repulsive interaction. It is to 2631


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N atom of dimethylamino group (ρBCP equal 0.02082 au). Exceptionally large values of the angle between the aromatic ring plane and the plane of the formyl (61.811°) and dimethylamino group (78.58°) exclude a possibility of resonance interaction of the substituents with the π-electron system. The HOMA values of 0.8050 and 0.8118 for both rings confirm the high aromaticity of the naphthalene ring. In the optimized structure of 8-dimethylaminonaphthalene1-carbaldehyde, the short C···N distance is also stated (Table S7). Also the presence of C···N bond path with the electron density of 0.0208 au indicates the existence of the C···N interaction. The attractive and repulsive peri interactions are visualized using the NCI surface and the NCI plot (Figure S2). The ellipticity value for electron density at 2−3 and 6−7 naphthalene bonds are close to that for the unsubstituted naphatalene molecule (ε2−3 = 0.1681 and ε6−7 = 0.1610), which indicates decreasing resonance interactions of the formyl and dimethylamino group with the π-electron systems. The values of the α angles and the exocyclic bond lengths, greater than those in the crystal state, indicate the presence of a resonance interaction of the substituents with the π-electron system. The HOMA values of 0.7655 and 0.7771 indicate a small loss of aromaticity. It is to note that in the crystal state the peri attractive interactions are stronger and prohibit the resonance interactions of the substituents with the π-electron system and a decrease of aromaticity. In 4,5-bis(dimethyloamino)naphthalene-1,6-dicarbaldehyde (KUPHOS),31 two formyl groups are present: 1-CHO in position para with respect to 4-dimethylamino group and the second 6-CHO group in ortho position to the 5-dimethylamino group (Table S8, Figure S3). A decrease of the α angles between the 1-CHO group and aromatic ring plane and shortening of the exocyclic C−C bond in comparison to the respective values for 6-CHO group are a result of the throughresonance interaction between the 1-formyl and 4-dimethylamino group. The ellipticity values of the 2−3 (0.1902) and 6−7 (0.1524) bond confirm the through-resonance interaction between the substituents in para positions. HOMA index of 0.6141 indicates a decrease of aromaticity. The conformer of 4,5-bis(dimethylamino)naphthalene-1,6dicarbaldehyde, with the short distance between the 8-H atom and the O atom of the formyl group, is more stable by 1.4930 kcal/mol than that obtained by rotation of the CHO group by 180°. Topological analysis of the electron density locates the bond path between the 8-H and O atom of the formyl group, indicating the presence of a very weak H···O hydrogen bond (ρBCP equal to 0.0186 au). Additionally, attractive N···N component to repulsive interaction between the dimethylamino groups has been evidenced by the presence of the N···N bond path with ρBCP equal to 0.0143 au The attractive and repulsive interactions between the 8-H atom and formyl group and between the dimethylamino groups in peri position are visualized in the QTAIM diagram, NCI plot and NCI surface (Figure S4). The difference in the exocyclic C−C bond length for the formyl group in positions 1 and 6, as well as twisting of these groups relatively to the aromatic ring planes clearly shows the presence of the through-resonance interaction between the substituents in para position. The ellipticity value for 2−3 bond of 0.1877, much larger than that for 6−7 bond (0.1522), confirms the presence of this interaction. HOMA values of 0.5886 and 0.6379 for two rings indicate a decrease of aromaticity caused by through resonance interaction between the formyl and dimethylamino group.

note that in the QTAIM diagram, an additional bond path has been found, indicating the attractive N···N interaction between the nitrogen atoms. The values of electron density ρBCP at N··· N critical point are equal to 0.0149 au for 4,5-bis(dimethylamino)naphthalene-1,8-dicarbaldehyde and 0.0151 au for 4,5-bis(dimethylamino)-1,8-dinitronaphthalene. 2.2. peri Interactions in the Model Naphthalene-1,8dicarbaldehyde and Comparison with the Optimized Structure of 1,8-Dinitronaphthaldehyde. peri interaction between the substituents in positions 1 and 8 of the naphthalene rings was an object of numerous studies for a long time.4,6−8,10,12,15 In this work we have investigated the peri interaction between two formyl groups in the model structure of naphthalene-1,8-dicarbaldehyde and have compared them with the peri interaction in the previously investigated 1,8dinitronaphthalene.17 Three conformations of the formyl with the C···O, H···H and O···O short distances between the respective atoms in the formyl groups in positions 1 and 8 have been considered (Table S5, Figure S1a−c). The QTAIM method shows a possibility of the existence of attractive interactions only between the C and O atom in the formyl groups. The ρBCP electron density at the BCP of the C···O path is 0.0172 au, but the bond path is curved, which signifies a structural instability. The repulsive peri interaction between two formyl groups causes important deviation of these groups from the naphthalene ring plane (Table S5). For the conformation with the α angle of 20.427°, the smallest HOMA value, 0.7142 au, has been found. Grater deviation of the formyl group from the naphthalene ring plane, to about 30°, reduces the resonance interaction with the π-electron system, which results in a higher HOMA value. For 1,8-dinitronaphthalene (Table S6) steric repulsion between two NO2 groups in peri position leads to deviation of these groups from the aromatic plane. The HOMA value equal to 0.7578 indicates only a very small decrease of aromaticity, comparable to that found for conformer of naphthalene-1,8-dicarbaldehyde, in which the α angle is close to 30°. 2.3. peri and para Interactions in Crystal and Optimized Structures of Substituted Naphthalene-1carbaldehydes: 8-Dimethylaminonaphthalene-1-carbaldeh yde (BAZWUU), 4,5-Bis(dimeth yloamino)naphthalene-1,6-dicarbaldehyde (KUPHOS), 4,5-Bis(dimethylamino)naphthalene-1,8-dicarbaldehyde (KUPHIM), and 4,5-Bis(pyrrolidino)naphthalene-1,8dicarbaldehyde (GIBTUI). The presence of an attractive peri interaction in 8-dimethylaminonaphthalene-1-carbaldehyde (BAZWUU) between the N atom of dimethylamine and O atom of the formyl group (Figure S2) was suggested basing on the analysis of the crystal structure.7 Similarly, the CO···N attractive interaction was found for several naphthalene derivatives with dimethylamine and carbonyl group in peri postion.6−8,10,12 In the 8-dimethylaminonaphthalene-1-carbaldehyde, the C···N distance of 2.4886 Å is the shortest in the series of the investigated compounds.7 The exocyclic C−C bond to the formyl group is splayed outward, while the dimethylamine group is splayed inward. Such orientation of the groups, similar to other compounds of this series, permits a lone electron pair of the nitrogen atom to be directed toward the adjacent carbonyl group. This arrangement was interpreted as an attractive interaction involving overlap of the lone pair of the nitrogen with the LUMO orbital of the carbonyl group.12 QTAIM analysis confirms the existence of a significant attractive interaction between the C atom of the formyl and 2632


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investigated naphthalene derivatives, suggesting the greatest participation of the quinoid resonance structure. The HOMA value shows the medium aromaticity of the naphthalene rings; however, it is to note that the decrease of the aromaticity is greater than for 4,5-bis(dimethylamino)-1,8-dicarbaldehyde. This observation is in accord with the statement of Ozeryanskiiet al.21 that the pyrrolidino group is a more effective π-electron acceptor than the dimethylamine group. Aromaticity of 4,5-bis(pyrrolidino)naphthalene-1,8-dicarbaldehyde is smaller than for 4,5-bis(dimethylamino)-1,8-dinitronaphthalene (HOMA equals 0.629517). It is to note that the angle between the plane of the nitro group and the aromatic ring equal to 27.330° is greater than the respective angle for the formyl group (17.502°). For the optimized 4,5-bis(pyrrolidino)naphthalene-1,8-dicarbaldehyde, three conformations, that differ in the mutual positions of the formyl groups, have been considered (Table S10, Figure S5). The short distances between the C···O, H···H, and O···O atoms of the peri formyl groups in respective conformers have been stated; however, the QTAIM analysis has not confirmed the presence of stable attractive interactions between these atoms. What has been confirmed is the weak N···H interaction between the H atom of the methylene group and N atom. The attractive and exceptionally strong repulsive peri interactions are visualized in the NCI plots and NCI surface (Figure S5). For every analyzed conformation, two naphthalene rings are not equivalent and are strongly twisted. The repulsive interaction between the peri formyl groups causes deviation of these groups from the naphthalene ring plane. The ellipticity values for 2−3 and 6−7 bonds (19.8−20.7) indicate considerable participation of the quinoid type resonance structure that is the largest in series of investigated compounds. The greatest decrease of HOMA value is observed for the C···O conformer, where the strongest through-resonance interaction is stated basing on the lowest value of the αCHO angle and the exocyclic C−C bond length.

4,5-Bis(dimethylamino)naphthalene-1,8-dicarbaldehyde (KUPHIM)20 is an example of the so-called “push−pull” proton sponges. The structure of this compound and conjugation between the substituents was discussed by Ozeryanskii et al.20 Experimental UV/vis studies evidenced the existence of the through-resonance between peri-diformyl groups and peri-dialkylamino groups in solutions.20 Short distance between the C···O atoms between two formyl groups suggests the presence of the peri attractive interactions. In the solid state structure of this compound, two nonequivalent rings are present with large angle between the naphthalene planes. Decrease of the exocyclic C−C bonds and deviation of the formyl groups from the aromatic ring plane in comparison to the naphthalene-1,8-dicarbaldehyde show an increase of the conjugative interaction between the CHO group and the aromatic ring. The ellipticity of 2−3 and 6−7 bonds (from 0.1974 to 0.19986), similar to those for 4,5−bis(dimethylamino)-1,8-dinitronaphthalene 17 indicates the through-resonance interaction between substituents in para positions. HOMA values show that through-resonance interaction between the two pairs of para substituents causes a moderate decrease of aromaticity. A similar modest decrease of aromaticity was found for analogous 1,8-bis(dimethylamino)-4,5-dinitronaphthalene.17 HOMA values for these two structures were 0.6397 and 0.6582 and the angles between the nitro ONO and aromatic plane groups were 27.117° and 27.076°.17 Three possible conformations with different mutual orientations of the formyl groups have been considered as the optimized structure of 4,5-bis(dimethylamino)naphthalene1,8-dicarbaldehyde (Table S9; Figure S4). Possible attractive interaction between the H···H atoms of two formyl groups in most stable conformer has not been confirmed by the QTAIM analysis. Similarly, the existence of attractive interactions between the O atoms of the peri formyl groups in the least stable conformer has not been evidenced. For the conformer with the short distance between the C and O atom of two formyl groups, QTAIM analysis shows the presence of the curved C···O bond path with ρBCP equal 0.0163 au, which indicates a weak nonstable C···O interaction. Indeed, the existence of the N···N interactions between the nitrogen atoms of the dimethylamino groups in all the analyzed conformers has been stated. The attractive and repulsive interactions between the two peri formyl and two peri dimethylamino groups are presented in the AIM diagram and NCI plot (Figure S4). The molecule is not planar. Shortening of the exocyclic C−C and C−N bonds indicates the through-resonance interactions between the formyl and dimethylamino group. The ellipticity values for 2−3 and 6−7 bonds, of 0.1901 and 0.1901, indicate participation of a quinoid type resonance structure. HOMA values of 0.6315 and 0.63621 indicate a moderate aromatic character. 4,5-Bis(pyrrolidino)naphthalene-1,8-dicarbaldehyde (GIBTUI) also belongs to the group of ”push−pull” sponges.20 The compound crystallizes in two crystal forms and in both of them two nonequivalent rings are present. Twisting of the naphthalene molecule (17.502° and 18.414°) is the greatest in the series of the compounds under study. The small values of αCHO angles and the lengths of the exocyclic C−C bonds in comparison to those for naphthalene-1−8-dicarbaldehyde indicate the presence of conjugative interactions between the formyl and pyrrolidino group. Ellipticity values for 2−3 and 6− 7 bonds from 0.1976 to 0.2064 are the greatest for all

3. CONCLUSIONS 1. Influence of the peri and para interaction on aromaticity, expressed by HOMA and ellipticity of the C2−C3 bond of naphthalene-1-carbaldehyde and 1-nitronaphthalene derivatives has been investigated. 2. Existence of the attractive and repulsive interaction between the 8-H atom and the formyl, nitro and amino group in peri position has been shown using the QTAIM and NCI method. The presence of a weak hydrogen bond between the 8-H and the oxygen atom of the formyl and nitro groups has been evidenced. 3. Comparison of the HOMA values of the naphthalene rings for the crystal and the optimized naphthalenecarbaldehydes with analogous compounds with the NO2 group shows weaker π-acceptor properties of the NO2 group in comparison to the CHO groups. 4. Weakening of the mesomeric effect of the NO2 with respect to the CHO group has been explained by the greater deviation of the NO2 group from the aromatic ring plane and a stronger dependence of the HOMA value on the α dihedral angle in comparison to the CHO group. 5. Analysis of the dependence of HOMA values on the α rotation angle with respect to aromatic plane has shown that the NO2 group exhibits stronger resonance effect 2633


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molecule. Traditionally a blue color of isosurface for attractive, red for repulsive, and green for intermediate interaction are used.36−38

than the CHO group only for planar or nearly planar conformation. For the α angles above 20°, the formyl group appears as a stronger π-electron acceptor and causes a more effective decrease of the HOMA value of the aromatic ring. 6. The value of the α angles between the CHO/NO2 group and the aromatic ring plane is a result of a competition between the repulsive and attractive peri interactions from the one side, and the through-resonance interactions with the para-amino groups, from the other side.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b11926. HOMA, geometric parameters, QTAIM diagrams, NCI plots, and NCI surfaces for the investigated structures (PDF)

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4. EXPERIMENTAL SECTION The crystal structures were taken from the CSD crystal database.30,33 Optimization of the investigated naphthalenes was performed at DFT B3LYP/6-311++G**-DG3 level of calculation using Gaussia0935 with including Grimme dispersion.33,36 For the compounds for which the crystal structure was known, it was used as a starting point in the optimization. The wave functions evaluated for optimized and crystal structure molecules were used as an input to the AIMALL program.34,37 to perform QTAIM analysis. The quantum theory atom in molecules method (QTAIM)27,30 has been applied as a tool to investigate the nature of peri interaction and π-electron delocalization in the aromatic rings. This theory describes the molecule as electron density, ρ(r), characterized by a system of critical points (CP), where the gradient of the electron density vanishes. Two atoms are bonded if they are located at the end of a common bond path with a BCP. A bond path in the frame of QTAIM theory can be understood as an interaction of two atoms including the chemical bond or weak interaction. The topological parameters of the critical points on a bond path are a quantitative description of the interatomic interactions existing in the molecular system. The most significant QTAIM parameter is the electron density at the BCP (ρBCP) that informs on the strength of the interactions. The linearity of bond path is also an important parameter; the curved bond path signified the structural nonstability. The ellipticity (ε) of the electron clouds at the BCPs is connected with stability of the bond or its double character.27 The high ellipticity value at the BCP of path joining the H···A atoms in the hydrogen bond signify very weak or lack of interactions. The ellipticity of the C2−C3 (C6−C7) bonds has been applied as a measure of through-resonance effect between the para-substituents.17 Because in the frame of AIM method very weak interactions are not evident, the NCI analysis was performed with the NCI program.35,38 van der Waals, hydrogen bonds and steric repulsion can be analyzed and visualized with the NCI method. The main parameter used in this method is the first derivative of the electron density that describes the deviation from a homologous electron distribution s = 1/ (2(3π2)1/3)|∇ρ|/ρ4/3). In the case of very weak interactions, the main problem is to differentiate between repulsive and attractive interactions. In the frame of NCI method the electron density is multiplicated by the sign of the second Hessian eigenvalue. For the repulsive interaction (sign λ2)ρ > 0 and for the attractive (sign λ2) ρ < 0. Additionally, the spikes of the electron density in the plot of s versus (sign λ2)ρ may be an indicator of the interactions strengths. Noncovalent interactions in NCI method are visualized by gradients isosurfaces in real space for the

AUTHOR INFORMATION

Corresponding Author

*(I.M.) E-mail: [email protected]. ORCID

Irena Majerz: 0000-0001-6049-2017 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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Aromaticity of peri- and para-Substituted Naphthalene-1-carbaldehyde

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