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Density Functional Investigations on the Selective Binding of an endo-Functionalized Bis-urea Macrocycle Maneesha N. Shewale, Dipali N. Lande, and Shridhar P. Gejji* Department of Chemistry, Savitribai Phule Pune University, Pune 411 007, India S Supporting Information *

ABSTRACT: The preferential binding of syn and anti configurational isomers of endo-functionalized bis-urea molecular receptor to 1,2-dinitrobenzene (G1) and 1,4-dioxane (G2) guests has been explained using dispersion-corrected M06-2X-based density functional theory. The host−guest binding is facilitated via hydrogen bonding, C−H···π, dipole− dipole, C···C and O···O (chalcogen−chalcogen) interactions. The formation of an inclusion complex is spontaneous and thermodynamically favorable. The molecular electrostatic potential and quantum theory of atoms in molecules in conjunction with the noncovalent interactions reduced density gradient have been employed to characterize the noncovalent interactions. The encapsulation of G1 or G2 within the πelectron-rich cavity of the bis-urea macrocycle reflects the frequency shift of the characteristic N−H and C−H vibrations of their vibrational spectra. It has also been shown that binding of the bis-urea isomers to G1 and G2 emerges with a signature in the upfield signals of the guest protons confined to the host cavity in 1H NMR spectra.

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INTRODUCTION The design and synthesis of newer-generation synthetic macrocycles in continuation of the family of cyclodextrin1 or cucurbituril2 hosts have spurred significant interest in the area of host−guest chemistry.3−11 A variety of artificial macrocyclic arene hosts, namely, calixarenes,6 resorcinarenes,7 cyclotriveratrylenes,8 pillararenes,9,10 or modified hosts thereof incorporating pyrrole,11 pyridine,12 and imidazolium heterocycles13 instead of phenyl moieties in their architecture have been synthesized and characterized with X-ray diffraction and spectroscopy experiments. Recently, naphthalene-based molecular containers endowed with unique structural attributes such as water-soluble cyclic tetramer cyclotetrachromotropylene (CTCT) wherein the four naphthyl units are linked together by methylene bridges have been synthesized.14−16 Along parallel lines, Georghiou and co-workers17 derived the calix[n]naphthalenes comprising methylene linkages by the direct cyclocondensation reaction of 1-naphthol with formaldehyde under basic conditions. These macrocyclic hosts possess short methylene linkages, and their use has been rather limited owing to steric hindrance from two naphthalene rings that engenders an ill-defined cavity and hence poor complexation ability. The binding efficiencies of such molecular containers can be improved by incorporating functionalities having longer chains with substitution of the heteroatom(s) in their CH2−X−CH2 (X = O, S) framework.18,19 Pursuant to this, Glass et al. demonstrated that molecular receptors with the naphthalene backbone comprising amide and allyl linkages can be used as fluorescent sensors.20,21 Jiang et al.22 successfully © 2016 American Chemical Society

synthesized naphthotube hosts in which a pair of dynamic imine-based configurational isomers are connected via covalent bonds. In this direction, the recently developed oxatub[4]arene receptor has conformational isomers that reveal a distinct preference for guest binding.23,24 In supramolecular chemistry, the artificial receptors, in particular, the converging functional group are constructed by taking into account their complementarity of size, shape, and charge distribution with the substrate, mimicking biological functionalities in enzyme−substrate binding.25 In light of this, Jiang et al. successfully synthesized endo-functionalized bis-urea macrocyclic hosts with convergent urea/thiourea functionalities combined with bis-naphthalene cleft.26,27 These authors concluded that these hosts show efficient and selective binding toward neutral molecules and can be explored for potential applications in materials science and biomedical chemistry. Interestingly, the naphthotubes are highly reminiscent of enzyme binding pockets where the rigid bis-naphthalene cleft rendered with the deep π-electron-rich cavity with its curvature facilitating guest binding. The low symmetry of the cleft engenders two configurational isomers: one with both bisnaphthalene clefts in parallel orientation (syn) labeled as SBU and the other with antiparallel orientation (anti) referred to as ABU. The molecular containers of this type demonstrated remarkable configuration selectivity and structural reconfiguraReceived: October 12, 2016 Revised: December 15, 2016 Published: December 15, 2016 288

DOI: 10.1021/acs.jpca.6b10310 J. Phys. Chem. A 2017, 121, 288−297

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The Journal of Physical Chemistry A

Figure 1. Optimized structures of (a) the bis-naphthalene cleft, (b) SBU, (c) ABU, (d) G1, and (e) G2.

further reveals the strong affinity toward neutral guests. In view of this, 1,2 dinitrobenzene (G1) and 1,4 dioxane (G2) with multiple intermolecular interaction sites and appropriate size should serve as good examples for understanding host−guest complexation.

tion, with the methylene bridges between urea groups and clefts facilitating NH protons to direct within the host cavity, which brings about guest encapsulation via noncovalent and hydrogen bonding interactions. The hollow π-electron-rich cavity of the SBU and ABU configurational isomers of the bis-urea host 289


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Figure 2. (a, b) Top and side views of optimized G1@SBU and (c, d) G2@ABU complexes.

The present work precisely focuses on unraveling supramolecular interactions accompanying such complexation, in particular, remarkable configuration selectivity of the bis-urea macrocycles toward G1 and G2 as prototype organic guests, employing the topography of the molecular electrostatic potential in conjunction with the quantum theory of atoms in molecules (QTAIM) approach within the framework of the dispersion-corrected density functional theory (DFT). Structural changes of the bis-urea macrocyclic host accompanying the guest encapsulation and their ramifications for characteristic normal vibrations in the infrared and 1H NMR spectra obtained from theory have been analyzed. The present investigations should prove useful in modeling new supramolecular assemblies in a myriad of their chemical applications.

The molecular electron density topography and accordingly the bond critical points are identified by employing the QTAIM approach with the help of AIMAll software.32,33 Furthermore, the chemical shift (δH) parameters in 1H NMR spectra were calculated by subtracting the nuclear magnetic shielding tensors of protons in the individual hosts, guests, and their complexes from those for tetramethylsilane (reference) within the framework of self-consistent reaction field (SCRF) theory incorporating the polarization continuum model (PCM)34 using chloroform as the solvent. Noncovalent interaction reduced density gradients (NCI-RDG), s, defined through |∇ ρ| 1 s= 2 1/3 4/3 within the QTAIM approach, further elicit the 2(3π )

ρ

underlying interactions accompanying the complexation.35

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COMPUTATIONAL METHOD All optimized structures of SBU and ABU isomers and their complexes were derived using M06-2X/6-31+G(d,p) employing the Gaussian 09 suite of programs.28 The hydrogen bonding and dispersive interactions typifying noncovalent host−guest binding are simulated well by this level of theory.29,30 To obtain further molecular insights for the complexation of bis-urea cavitands, the charge distribution within the cavity was characterized in terms of the molecular electrostatic potential (MESP) topography. Stationary-point structures on the potential energy surfaces were confirmed to be the local minima from the frequencies of normal vibrations, all of which turn out to be real. The normal vibrations were assigned by visualizing the displacements of all of the atoms around their equilibrium (mean) positions using the Gaussview-5 program.31

RESULTS AND DISCUSSION

Optimized structures of SBU and ABU configurational isomers of the bis-urea macrocycle and guests G1 and G2 are displayed in Figure 1a−e along with an atomic labeling scheme. These hosts are stabilized through intramolecular hydrogen bonding. The chalogen−chalcogen (O···O) and the C−H···O interactions at the lower rim provide additional stability to the SBU isomer, bringing two naphthalene clefts closer and engendering a bowl-shaped cavity (cf. Figure 1b). However, the absence of such interactions renders the ABU isomer with a tubularshaped cavity (cf. Figure 1c). To gain molecular insights for the host−guest binding, the MESP was used as a tool. The dimensions of the host cavity can be estimated from these considerations.36 The charge distribution therein has been further characterized. The rich topographical features of MESP 290


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Table 1. Selected Geometrical Parameters (Bond Distances in angstroms and Angles in Degrees) in the Guests, Host, and Their Inclusion Complexes from M06-2X/6-31+G(d,p) Theory G1

G1@SBU

G2 1.102

C−He C−C C1−C1 C1−C2 C2−C3 C3−C3 C−Ha C−Hb C1−N N−H

1.093 1.520

C−O C−O−C N−C−N H−N−C O−N−O O2−N−C1 O1−N−C1 H−N−C−O

O1−N−C1−C1 O2−N−C1−C1 C−Ca a

SBU

C−Ha

1.391 1.385 1.394 1.392 1.0844 1.084 1.473

1.400 1.384 1.397 1.388 1.0839 1.087 1.468 1.012 1.012 1.234

1.009 1.011 1.229

ABU

G2@ABU 1.101 1.102 1.094 1.519

1.010 1.012 1.226 114.2 119.2

1.015 1.017 1.228 110.6 114.3 116.6

168.3 −168.3 160.3 −160.3

153.3 171.1 −173.6 −163.4

10.396

9.884

110.6 115.1 120.7

114.7 119.8 124.9 117.8 117.3 164.9

126.4 116.8 116.7 −172.7 166.8 −172.7 166.8

176.4

−143.8 39.2

−148.7 34.6 10.868

10.476

Refers to the distance between two readily opposite carbonyls of urea.

structural parameters in the lowest-energy complexes, along with those in the isolated SBU and ABU hosts and guests, are compared in Table 1. As may readily be noticed, the N−H bond distances in the isolated SBU fall in the range of 1.011− 1.009 Å, which on encapsulation with G1 are elongated by ∼0.003 Å consequent to the N−H···O interactions. The G2@ABU complex reveals longer N−H bond distances. Likewise, the G1@SBU complex reveals an increase in the carbonyl bond distance from 1.229 to 1.234 Å. However, the interaction of G1 with SBU brings about a large distortion of the nitro group with the N−O bond length increasing from 1.214 to 1.222 Å compared to that of the free G1 guest. The C−O bond distance in the G2 guest was elongated by 0.010 Å upon complexation. Generally, the bond angles are nearly insensitive to complexation and show a deviation of up to 2°. The cavity size (dimensions) can be estimated from a separation of radially opposite atoms in the upper and lower rims of the macrocycle. The encapsulation of the G1 guest within SBU leads to increasing separation of radially opposite atoms of the bridging urea linkages. Contrary to this, the confinement of G2 within the ABU engenders the contraction of its cavity consequent to stronger N−H···O hydrogen bonding. Noncovalent interactions turn out to be the driving force leading to the formation and stabilization of G1@SBU and G2@ABU complexes with significant alterations in their structure and properties. Detailed analyses of such interactions at the molecular level have been carried out using the QTAIM approach, as evidenced by the electron density at the bond critical point (ρbcp) parameters and its Laplacian (∇2ρ). Table 2

succinctly bring out the critical points (CPs) shown in Figure S1 of the Supporting Information. A closer observation of the MESP topography further revealed that the (3, +3) local minima are attributed to lone pair domains of the heteroatoms (N, O) and the delocalized π-cloud corresponding to the bisnaphthalene cleft. It is noteworthy that the CPs from the πdelocalization reveal shallow minima compared to those signifying the lone pair(s). The lower rim of the SBU isomer further reveals a more electron-rich nature for parallel orientation of bis-naphthalene clefts with the bridging oxygens being in one plane. The regions of maximum positive electrostatic potential are evident through the NH hydrogens of urea linkages in the host isomer which are evident from the (3, −3) CPs, with the corresponding Vmax being ∼170 kJ mol−1. It may therefore be conjectured that the binding of the neutral guests can be attributed to electrostatic complementarity. Taking a cue from this the complexes of G1 and G2, guests with SBU and ABU isomers of bis-urea macrocycles were generated by rotation and translation of the guest in the neighborhood of the MESP maxima/minima parameters within the isolated host. Interestingly, the complexation of the centrosymmetric G2 guest with the centrosymmetric ABU host is energetically favored over the SBU isomer whereas G1@ SBU turns out to be the lowest-energy structure. The energy lowering was attributed to intermolecular hydrogen bonding (C−H···O, N−H···O, C−H···N) and C−H···π interactions. These inferences agree well with X-ray single-crystal structure data.24 Energy-minimal optimized geometrical configurations of the G1@SBU and G2@ABU complexes from the M06-2X/631+G(d,p) theory are illustrated in Figure 2a−d. Selected 291


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The Journal of Physical Chemistry A Table 2. QTAIM Parameters for the Nonbonding Interactions in the G1@SBU and G2@ABU Complexes r C−H···π

N−H···O

C−H···O C−H···N

C−H···π

N−H···O

ρ

∇2ρ

2.582 2.874 2.582 2.874 2.982 2.982 2.438 2.294 2.294 2.438 2.381 2.381 3.016 3.016

0.0096 0.0063 0.0096 0.0063 0.0049 0.0048 0.008 0.0126 0.0126 0.008 0.0108 0.0108 0.0045 0.0045

0.0327 0.0190 0.0327 0.0190 0.0146 0.0146 0.0333 0.0455 0.0455 0.0333 0.0410 0.0410 0.0141 0.0141

2.709 2.769 2.915 3.004 2.702 3.274 1.938 2.096 2.429 2.513

0.0075 0.0076 0.0049 0.005 0.0072 0.0031 0.0267 0.0192 0.0092 0.0082

0.0237 0.0247 0.0157 0.0155 0.0221 0.0088 0.0833 0.0563 0.0347 0.0310

G

V

G1@SBU 0.0066 0.0038 0.0066 0.0038 0.0029 0.0029 0.0072 0.0106 0.0106 0.0072 0.0091 0.0091 0.0030 0.0030 G2@ABU 0.0049 0.0051 0.0031 0.0031 0.0045 0.0018 0.0215 0.0149 0.0080 0.0069

shows topological parameters, viz., ρbcp, ∇2ρ, G, V, H, |V|/G, and |λ1/λ3| for intermolecular interactions along with the corresponding distances in the host, guest, and their complexes. The presence of C−H···O, C−H···N, and bifurcated N−H···O···H−N intermolecular interactions in G1@SBU as well as in G2@ABU complexes is evident from the ρbcp parameters that fall in the range of 0.027 to 0.004 au. The Laplacian ∇ 2 ρ parameters are > 0. Here, bifurcated N−H···O···H−N hydrogen bonding results in the formation of a six-membered ring and contributes to the stability of the complex. Interestingly, the aromatic ring carbon atoms of the SBU isomer of the host and G1 guest participate in the C···C interaction with a bond distance of 3.241 Å, with the corresponding ρbcp value being 0.007 au, well within those observed for the weak noncovalent interactions.37 The exponential dependence of ρbcp as a function of the corresponding bond separations is depicted in Figure S2 of the Supporting Information, with the correlation coefficient being ∼0.92. It has also been recognized that the energy density distribution has been well described through the total energy density (H) having contributions from kinetic (G > 0) and potential (V < 0) electron energy densities. Deeper insights for the bond interaction and the corresponding bond strength can be obtained. Figure 3 and Figure S3 in the Supporting Information display the correlation between the hydrogen bond dissociation energy (De) as a function of intermolecular distance together with G and V.38 As may readily be noticed from the figures, G and V components at the bcp that counterbalance each other show an exponential dependence on the hydrogen bond distances. The |V|/G ratio for both the complexes studied here falls in the range of 0.728−0.924 au, implying high ionic character for the corresponding intermolecular interactions.37 Furthermore, an examination of

−0.0050 −0.0029 −0.0050 −0.0029 −0.0021 −0.0021 −0.0061 −0.0098 −0.0098 −0.0061 −0.0080 −0.0080 −0.0024 −0.0024 −0.004 −0.004 −0.002 −0.002 −0.003 −0.001 −0.022 −0.016 −0.007 −0.006

|V|/G

|λ1/λ3|

0.0016 0.0009 0.0016 0.0009 0.0008 0.0008 0.0011 0.0008 0.0008 0.0011 0.0011 0.0011 0.0005 0.0005

0.7607 0.7565 0.7607 0.7565 0.7282 0.7282 0.8456 0.9238 0.9238 0.8456 0.8765 0.8765 0.8190 0.8190

0.1588 0.1429 0.1588 0.1429 0.1343 0.1343 0.1487 0.1924 0.1924 0.1487 0.1688 0.1688 0.1292 0.1292

0.0011 0.0011 0.0008 0.0008 0.001 0.0004 −0.0007 −0.0008 0.0007 0.0008

0.7820 0.7928 0.7355 0.7575 0.7744 0.7476 1.0319 1.0533 0.9095 0.8802

0.1669 0.1721 0.1244 0.1727 0.1613 0.1553 0.1511 0.1010 0.1671 0.1746

H

Figure 3. G, V, and De parameters as a function of intermolecular distance for the G1@SBU complex. See the text for details.

|λ1/λ3| values reveals this ratio to be ∼0.145 for SBU and ∼0.167 in the ABU complex, which is indicative of the fact that the charges are not accumulated between the atoms in the bonding region and are usually more on the atoms. It may therefore be inferred that C−H···π interactions act as an attractive molecular force for polarized CH fragments and aromatic rings. The dominant electrostatic contributions can thus be inferred. To delve further into intra- and intermolecular noncovalent interactions underlying the host−guest binding, we employ the NCI-RDG method based on bonding topology described through the QTAIM theory. A pictorial representation of NCI isosurfaces and 2D plots for the SBU and ABU complexes are shown in Figure 4a,b. The color-coding scheme is as follows: 292


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Figure 4. Color-filled RDG isosurfaces depicting noncovalent interaction (NCI) regions in (a) G1@SBU and (b) G2@ABU complexes. Green regions refer to van der Waals interactions. The steric interactions are shown in red. See the text for details.

isosurfaces. MESP textured on electron density isosurfaces (0.001 au) for the free host, guest, and their complexes are depicted in Figure 5a−f. The most negative potential (red) is located near the oxygen atoms of the carbonyl and ether functionality, and the positive MESP (dark-blue region) appears around the NH groups from urea linkages (cf. Figure 5a,b). Table S1 of the Supporting Information summarizes the Vmin and Vmax parameters in the MESP topography for the host, guest, and complexes. The presence of bifurcated hydrogen bonding (N−H···O···H−N) is evident from the MESP minima (Vmin = −130.4 kJ mol−1) located around the oxygen atoms of G2, which engenders a shallow MESP minima (−59.0 and −42.9 kJ mol−1) in its complex. Moreover, the C−H···π interactions render stability to the complexes and are reflected in the Vmin parameters corresponding to the π-delocalized electron cloud of the naphthalene moiety, which range from

the strong repulsive nonbonded overlap is shown in red, attractive interactions shown in blue, and the green regions suggest electrostatic interactions. The red cigar-shaped surfaces noticed to be elongated along the directions of decreasing density (cf. the central region aromatic rings in Figure 4a,b) represent strong destabilizing steric interactions for ρ > 0.01 au and λ2 > 0, consequent to the strain from the ring structure with the covalent bonds being responsible for ring cohesion. It is thus evident that the density and gradient parameters for the G1 and G2 complexes are relatively small (ρ < 0.01 au), and second, λ2 turns out to be nearly zero. The existence of weaker delocalized C−H···π van der Waals interactions thus are deciphered from the greenish sheetlike extended region. A quick glance at the electron density redistribution associated with noncovalent interactions has been further examined through the lens of MESP mapped density 293


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Figure 5. Electron density isosurfaces (0.001 au) overlaid with the MESP (−0.05 au) in (a) SBU, (b) ABU, (c) G1, (d) G2, (e) G1@SBU, and (f) G2@ABU.

Table 3. Selected Vibrational Frequencies (cm−1) (ν = Stretching) of G1, G2 Guests, SBU, and ABU Hosts and Their Complexes frequency ν(N−H) ν(N−O) ν(C−O) ν(C−He) ν(C−Ha) ν(C−Hb)

G1

SBU

G1@ SBU

3532

3501 1495

1499

3113 3138

3151 3088

−80 to −68 kJ mol−1, and a concomitant disappearance of Vmax can also be noticed. The charge redistribution can be envisaged from Figure 5f. Similar corollaries are drawn for the G1@SBU complex (cf. Figure 5e).

G2

1209 3019 2920

ABU

G2@ABU

3500

3431 1189 3018−3040 2917−2930

To probe accompanying molecular interactions, vibrational frequencies within the framework of the M06-2X/6-31G(d, p) theory were computed for the individual host, guests, and their complexes. The G1 and G2 guests encaged within host cavity induce structural changes that are reflected in characteristic 294


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Figure 6. Frontier orbital isosurfaces (±0.04 au).

normal vibrations. The calculated infrared spectra portraying the molar absorption coefficient (or, molar absorptivity in units of 0.1 m2 mol−1) versus the frequency (in cm−1) of the G1@SBU and G2@ABU complexes in different regions, namely, (i) 3900−3000, (ii) 1900−1000, and (iii) 1000− 500 cm−1, are portrayed in Figures S4 and S5 of the Supporting Information, respectively. The prominent stretches in free SBU, ABU, G1, G2, and their complexes are reported in Table 3. A significant decrease of 90 cm−1 for the free N−H stretch (3496 cm−1) was observed concurrent with the intermolecular bifurcated N−H···O···H−N hydrogen bond formation in the G2@ABU complex. In the G1@SBU complex, the corresponding vibration was observed at 3501 cm−1. It may as well be inferred that the carbonyl functionality protruding outside the cavity is nearly insensitive to complexation and shows only marginal change in frequency. The C−H···π interactions emerge with their signature in the IR spectra, and a blue shift of 40 cm−1 for the C−Ha stretching in G1 can be noticed. The G1@SBU complex reveals characteristic NO stretching near 1495 cm−1 which corresponds to the 1499 cm−1 band in G1. The direction of the frequency shift accompanying the complexation can further be rationalized from the NBO parameters given in Table S2 of the Supporting Information. As may be noticed, N−H···O−N hydrogen bonds emerge with a large increase in the electron density for N−H σ* and N−O π* antibonding orbitals of the G1@SBU complex

engendering the bond weakening which leads to the frequency downshift. These arguments are further extended to the G2@ABU complex. In an effort to understand the electronic properties of the G1@SBU and G2@ABU complexes at the molecular level, time-dependent DFT (TDDFT) was adopted to investigate the ground- to excited-state transitions.39 The frontier orbitals in the bis-urea receptor and its complexes are illustrated in Figure 6. HOMOs of both isomers are found to be largely localized around the naphthalene moiety, with the smallerintensity part also observed near nitrogen atoms of urea linkages. The molecular electron density reorganization accompanying G1 guest encapsulation within SBU renders electron-rich regions on the naphthalene clefts. On the other hand, the LUMO resides over the guest exclusively. It may therefore be inferred that the HOMO to LUMO absorption transition corresponding 403 nm, stems from the intermolecular charge transfer. Moreover, the HOMO is localized on one of the bis-naphtahlene clefts whereas the LUMO is distributed over the other cleft, which points to the intramolecular charge transfer in the G2@ABU and further shows a band at 246 nm. The wavelength absorption maxima, oscillator strengths, and orbital descriptors of different electronic transitions in both complexes are summarized in Table S3 of the Supporting Information. The HOMO−LUMO energies, global indices of reactivity, namely, the chemical 295


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potential (μ), hardness (η), and electrophillicity index (ω), and the thermodynamic parameters are given in Table S4 of the Supporting Information. The chemical potential parameters turn out to be negative, corroborating the inferences on the spontaneity of the encapsulation. The chemical structure, conformational behavior, and underlying molecular interactions in bis-urea macrocycles embedded with the guest can be monitored through chemical shifts (δH) in the 1H NMR spectra. Accordingly, δH values of the isolated SBU and ABU hosts, G1 and G2 guests, and their inclusion complexes in chloroform (solvent) from the M062X/6-31+G(d, p) theory are reported in Table S5 of the Supporting Information. The δH parameters of the SBU and ABU hosts show only a little dependence on the orientation of bis-napthalene clefts. The protons of the bis-urea macrocycle are broadly classified as naphthalene cleft protons (H1 to H7), butoxy groups (H8−H11), methylene linkages (H12, H13), and N−H from the urea linkages (H14) illustrated in Figure 1a. As may be observed the aromatic H1−H4 protons of the naphthalene moiety reveal deshielding, whereas the butoxy protons reveal an upfield signal. The N−H protons of hosts facilitate bifurcated intermolecular hydrogen bonding interactions and thus engender a large deshielding of ∼1.0 ppm in the complex. The methylene protons of the isolated G2 guest split into a near doublet as 3.62 and 3.56 ppm, typifying axial and equatorial protons. Moreover, the G1 and G2 guest protons emerge with the upfield signals, which suggests that the guests are encapsulated within the host cavity.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.P.G. acknowledges support from a research project (37(2)/ 14/11/2015-BRNS) from the Board of Research in Nuclear Sciences (BRNS), India. M.N.S. is thankful to the University Grants Commission, India for the faculty development programme. D.N.L. is thankful to Savitribai Phule Pune University for the award of a research fellowship through the University of Potential excellence scheme from the University Grants Commission, New Delhi, India. The authors thank the Center for Development of Advanced Computing (CDAC), Pune, India for providing access to the National Param Supercomputing Facility.

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CONCLUSIONS The electronic distributions in the SBU and ABU configurational isomers of the endo-functionalized bis-urea macrocycle have been characterized from density functional theory. The SBU is endowed with a bowl-shaped cavity whereas ABU possesses a tubular cavity. It has been shown that G1 binds strongly to SBU whereas G2 favors complexation with the ABU isomer, which concur with the experiment. These inclusion complexes are stabilized through hydrogen bonding and CH···π and O···O interactions. Accompanying noncovalent interactions are analyzed by employing the NCI approach, an extension of QTAIM theory reflected in the 1H NMR and infrared spectra. The present theoretical investigations underline the role of noncovalent interactions for the host−guest binding of hazardous G1 and G2 with the bis-urea host. The molecular recognition of these hosts can be explored for trapping toxic organic molecules.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b10310. MESP maxima and minima, ρbcp parameter, vibrational spectra, 1H NMR, and TDDFT (PDF)

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REFERENCES

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Shridhar P. Gejji: 0000-0003-3305-2475 296


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