Experimental and theoretical investigation of p–n alkoxy benzoic acid based liquid crystals – A DFT approach

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 511–523

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Short Communication

Experimental and theoretical investigation of p–n alkoxy benzoic acid based liquid crystals – A DFT approach P. Subhapriya a, K. Sadasivam a, M.L.N. Madhu Mohan b, P.S. Vijayanand a,⇑ a b

Department of Physical sciences, Bannari Amman Institute of Technology (Autonomous), Sathyamangalam, Erode 638 401, Tamil Nadu, India Liquid Crystal Research Laboratory (LCRL), Bannari Amman Institute of Technology, Sathyamangalam, Erode 638 401, Tamil Nadu, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 NBO analysis provides information

about delocalization charge of mesogen.  The observed and theoretical FT-IR spectra corroborated the H- bond in the mesogen.  Analysis of energy gap revealed the reactivity of all the reactants.  Solvent effect investigations provide dipole moment and stability of mesogen.  Nematic phase stability of mesogen is evidenced by local charge distribution.

a r t i c l e

i n f o

Article history: Received 19 November 2013 Received in revised form 5 January 2014 Accepted 10 January 2014 Available online 23 January 2014 Keywords: Liquid crystal DFT Hydrogen bonding NBO MEP

a b s t r a c t In the present study structural effects of alkoxy chain lengths and mesogen properties of hydrogen bonded (nOBASA) complexes (n = 5, 6, 7) have been studied by density functional theory (DFT) calculations and Fourier Transform Infrared (FT-IR) spectrum. The B3LYP/6-311G(d,p) level of theory has been adopted for all the computations. The experimental FT-IR (400–4000 cm1) spectrum was recorded on the solid phase of the molecule. The intermolecular hydrogen bond formation has been conformed from the optimized geometry. The vibrational assignments, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies were calculated. The stability of molecule arising from hyper-conjugative interactions and charge delocalization were analyzed using natural bond orbital (NBO) analysis. The electron density (ED) in the r and p anti-bonding orbital and second order delocalization energies confirmed the occurrence of intermolecular charge transfer. The energetic behavior of the title compounds in solvent phase is examined using the B3LYP/6-311G(d,p) method by applying the Onsager and polarizable continuum model. The molecular electrostatic potential (MEP) surface was generated over the optimized geometry of the molecule to obtain the chemical reactivity of the molecule. The charge distribution of the mesogen molecules has been calculated. The reliability of the methods used has been assessed by comparing the theoretical results obtained from the experimental findings. Moreover, the mesomorphic behavior and the nematic phase stabilities for each molecule have been predicted using calculated local charge distribution. The simulated FT-IR spectrum of 5OBASA was agreed with experimentally observed spectrum. Ó 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +91 4295 226304, mobile: +91 99426 37573; fax: +91 4295 226666. E-mail address: [email protected] (P.S. Vijayanand). http://dx.doi.org/10.1016/j.saa.2014.01.074 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

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Introduction Liquid crystal (LC) materials have great potential for functional molecular systems because of their self-organized dynamic structure [1–3]. Interest on LC materials have attracted to a greater extent in recent years due to their prospective optical characteristics. They exhibit a wide spectrum of applications and some of them are used as displays in digital watches, calculators, cell phones and laptops etc. [4–9]. Hydrogen bonded LC complexes have attracted a great deal of interest, due to their structural and dynamic properties as they play significant roles in many chemical and biological systems [10,11]. By choosing an appropriate proton acceptor and proton donor, stable intermolecular hydrogen bonds can be formed and thereby they provide a novel system with new properties [12,13]. In such LC compounds, the effect of hydrogen bonding is very important for the material properties. LC materials are generated by inter-molecular hydrogen bonds which have been studied and investigated extensively to design novel LC materials [14–21]. The syntheses of hydrogen-bonded LCs are relatively easier than that of conventional LCs and have the potential for developing optical properties. Manipulation of supramolecular LCs in self-organizing material is of an importance in order to achieve desired properties in liquid crystalline state. Diverse molecular architectures such as different spacer length, functional and the rigid groups are being explored which have dramatic effects on the physical properties of LCs. It has been reported [22,23] that the lower melting point, greater mesophase and thermal stability were observed for the supramolecular hydrogen-bonded complexes with 4-alkoxybenzoic acids by increasing the terminal alkyl group. Further, alkyl chain length is a very important parameter which influences the properties of mesogen molecules. It has been shown that LC properties depend upon the number of methylene units in the alkyl chain and the authors have reported such hydrogen bonded complexes earlier. There has been an increasing development of computational chemistry in the past decade. Many imperative chemical and physical properties of biological systems can be predicted by various computational techniques [24,25]. In recent years, density functional theory (DFT) with a better exchange-correlation functional has made it possible to calculate many molecular properties with comparable accuracies to traditional correlated ab initio methods [26]. Literature survey also reveals that the DFT has a great accuracy in reproducing the experimental results [27–31]. Even a slight modification in the bonding can have a higher influence in the vibrational frequencies and geometrical arrangement of atoms. Vibrational spectroscopy is one of the most powerful tools for the study of molecular environment and to study the type of flexible spacers and the mesogenic moieties in these LCs. With the aid of FT-IR absorption techniques, the minute modifications can be detected and it provides very distinct information about thermal stability and molecular structural changes of LCs [32]. In particular, the frequency shift of carbonyl stretching bands is found to be sensitive to bonding parameters, molecular conformation and intermolecular interactions [33,21]. No previous theoretical literature is available for an in-depth analysis of hydrogen bonded LCs utilizing DFT as a tool. In this present work, molecular structure conformations, electronic properties, and solvent effect of inter molecular hydrogen bonding of n-alkoxy benzoic acid (nOBASA, n = 5,6,7) and suberic acid complexes that exhibit LC characteristics have been computed and analyzed. The harmonic vibrational frequencies, natural bond orbital (NBO) analysis, molecular electrostatic potential (MEP) surfaces are also studied to elucidate the intermolecular hydrogen bond interactions of supramolecular hydrogen bonded benzoic acid LCs. Finally, electronegativity, chemical hardness and electrophilicity index of these LC molecules are calculated and interpreted

with the use of HOMO and LUMO energies. The local charge distribution analysis and frontier molecular orbitals have been simulated and interpreted. Experimental details All complexes presented in this paper have been synthesized with the aid of procedure available in the previous literature [23]. Intermolecular hydrogen bonded LCs were synthesized using two moles of p–n-alkoxybenzoic acids (nOBA) with one mole of suberic acid in N,N-dimethyl formamide (DMF) respectively. Further, they have been subjected to constant stirring for 12 h at ambient temperature (30 °C) till a white precipitate in a dense solution is formed. The white crystalline crude complexes have been obtained by removing excess DMF and recrystallized using dimethyl sulfoxide (DMSO) and the yield was around 80%. The FT-IR spectrum was recorded in the frequency region 400– 4000 cm1 using KBr pellets (ABB FTIR MB3000) and analyzed by the MB 3000 software. Computational details Density functional theory computations have been performed using Gaussian 03W program package [34] with Becke-3–Lee– Yang–Parr hybrid exchange-correlation three-parameter functional (B3LYP) [35,36] level with standard 6-311G(d,p) basis set. The geometry optimization has been performed [37] using same level of theory and the frequencies were calculated to check the presence of any imaginary frequency and to evaluate the zero point corrections. In all the computations unrestricted open-shell approach was adopted. The interaction energy during mesogen formation through the hydrogen bonded interface was calculated by super molecule method [38,39]. The interaction energy was calculated using Eq. (1).

DEinteraction ¼ EðA1 ; A2 ; A3 ; . . . ; AN Þ 

N X EðAi Þ

ð1Þ

i¼1

where E (A1, A2, A3, . . ., AN) are the energies of the isolated monomers. The natural bonding orbital (NBO) calculations [40] have been performed using NBO 3.1 program (Gaussian 03 package). The intermolecular delocalization or hyperconjugation leads to various second order interactions between the filled orbitals of one subsystem and the vacant orbitals of another subsystem. The hyperconjugative interaction energy was inferred from the second-order perturbation approach E(2) as follows [41]:

Eð2Þ ¼ gr

F 2ij hrjFjri2 ¼ gr er  er DE

ð2Þ

where hrjFjrii2 or F 2ij is the Fock matrix element between i and j is NBO orbitals, er and er are the energies of r and r NBO’s, and nr is the population of the donor r orbital. To predict the reactive sites for electrophilic and nucleophilic attacks for the investigated compounds, the MEP surface have been generated from the optimized geometry. The HOMO–LUMO analysis has been carried out to explain the charge transfer within the molecule. The chemical hardness (g), the chemical potential (l), the softness (s) and the electrophilicity index (x) of all the three title compounds have also been evaluated as follows:

Electronegativity ðvÞ :

l  v ¼ ðionization potential þ Electron affinityÞ=2 Hardness g  ðIP  EAÞ=2

ð3Þ ð4Þ

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Softness s ¼ 1=ð2gÞ

ð5Þ

Electrophilicity index x ¼ l2 =2g

ð6Þ

Solvent effects have been computed in the frame work of self-consistent reaction field polarizable continuum model [42–45] (SCRF– PCM) implemented in the Gaussian 03 code. Single point estimation of the solvation energy is sufficient to pronounce the behavior of the studied systems in benzene, acetonitrile and ethanol since they are characterized by conformational rigidity. Charge distributions of the molecules have been calculated by performing Mulliken analysis. The mesomorphic behavior and the nematic phase stability have been predicted through the calculated local charge distributions. The harmonic vibrational wavenumbers were calculated using the similar level of theory. Further, the calculated vibrational wavenumbers are scaled down using the scaling factor 0.9608 to offset the systematic error caused by neglecting anharmonicity and electron density [46,47]. Results and discussion Hydrogen bonding The energies, energy gap, dipole moment of all the reactants, their intermolecular hydrogen bonded mesogens (Table 1) and their optimized geometrical structures are presented in Figs. 1 and 2. The formation of intermolecular hydrogen bonds in 5OBASA mesogen, 6OBASA mesogen and 7OBASA mesogen are shown in Fig. 2. The geometrical parameters of all the studied compounds are given in Table 2. The hydrogen bond interactions among O16  H22AO21, O17AH18  O20 and O43AH44  O58, O42  H57AO59

groups have been observed. The intermolecular hydrogen bonds are found to be linear and the bond lengths are 1.652 Å, 1.673 Å, 1.651 Å and 1.674 Å for O16  H22, H18  O20, O58  H44 and H57  O42 respectively. This clearly revealed the double hydrogen bond formation of the 5OBASA mesogen. Similarly in 6OBASA mesogen, the distances between O16  H22 is 1.651 Å, H18  O20 is 1.674 Å and O58  H44 is 1.651 Å, H57  O42 is 1.674 Å and 7OBASA mesogen, distance between O16  H22 is 1.651 Å, H18  O20 is 1.674 Å and O58  H44 is 1.651 Å and H57  O42 is 1.675 Å. From the computed parameters, it is found that the hydrogen bond length is less than 3.0 Å which shows the hydrogen bond interaction between the molecules [48]. The bond angle between various hydrogen bonds and the non-planar structures of intermolecular hydrogen bonded complexes are depicted in Table 2. It has been observed that there is no significant change in hydrogen bond length due to the addition of alkyl group. The molecules in 5OBASA, 6OBASA and 7OBASA mesogens are bounded together via doubly hydrogen-bonded interactions and the total hydrogen-bonded interaction energies are 0.06, 0.062 and 0.054 Hartree, respectively, as estimated using Eq. (1). This interaction arises largely through the four equivalent stable hydrogen bonds such as O16  H22AO21, O17AH18  O20, O43AH44  O58 and O42  H57AO59 resulting in increased stabilization. This was well- reflected as distortion in the molecular geometry with respect to the isolated molecule. The OAH bonds in which the hydrogen bonds are formed, have a significant increase in the bond lengths of all OBAs (0.03 Å) and in SA (0.032 Å) due to the formation of dimerization complexes. The shortening of the CAO bond (single) upon dimerization may be responsible for the redistribution of partial charges on the O atoms, as the unpaired electron is significantly delocalized and thereby the CAO bond shows significant double bond character which is

Table 1 Calculated total energies, energy gap and dipole moment values of all the reactants and mesogens. Parameter

5OBA

6OBA

7OBA

SA

5OBASA complex

6OBASA complex

7OBASA complex

Energy (in Hartree) Fronterier orbital energy gap (in Hartree) Dipole moment (D)

692.798 5.279 3.949

732.122 5.281 4.007

771.446 5.282 4.002

614.404 7.648 0.016

2000.059 5.189 1.260

2078.706 5.187 1.339

2157.354 5.184 1.166

(a) 5OBA

(c) 7OBA

(b) 6OBA

(d) SA

Fig. 1. The theoretical geometry structure and atomic numbering scheme of all the reactants.

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(a) 5OBASA mesogen

(b) 6OBASA mesogen

(c) 7OBASA mesogen Fig. 2. The theoretical geometry structure and atomic numbering scheme of all the hydrogen bonded mesogens.

a characteristic of a carbonyl group. Similar effect was also noticed in the bond angle of CAOAH with an increase of 5° approximately. NBO analysis NBO theory allows the assignment of the hybridization of atomic lone pairs and the atoms involved in bond orbitals. Interaction between atomic orbitals can be analyzed using NBO theory. In order to explain the hyperconjugative interaction energy, inter, intra molecular hydrogen bonding, intermolecular charge transfer (ICT), electron density transfer (EDT) and cooperative effect due to delocalization of electron density from the filled lone pairs n(Y) of ‘‘Lewis base’’ Y into the unfilled anti-bond r(XH) of ‘‘Lewis acid’’ XAH in XAH  Y hydrogen bonding systems, NBO analysis has been an effective tool [49]. In the present work, NBO analysis has been performed on 5-OBA, SA reactant and 5OBASA mesogen to study the intermolecular hydrogen bonding, ICT, delocalization of electron density and cooperative effect due to n(O) ? r(OAH) using NBO 3.1 program as implemented in Gaussian 03 W package. The intermolecular OAH  O hydrogen bonding is formed by the orbital

overlap between the n(O) and the r(OAH) which results ICT causing stabilization of the H-bonded systems. Hence, hydrogen bonding interaction leads to an increase in electron density (ED) of OAH anti-bonding orbital. The increase of population in OAH anti-bonding orbital reduced the strength of OAH bond. Thus the nature and strength of the intermolecular hydrogen bonding can be explored by studying the changes in electron densities in the vicinity of O  H hydrogen bonds. The NBO analysis of 5-OBA, SA (Fig. 1) and dimer complex mesogen (Fig. 2) vividly gives the evidences for the formation of four strong H-bonded interactions between oxygen lone pair electrons and r(OAH) anti-bonding orbitals. The occupancies, energies of oxygen lone pairs and anti-bonding orbitals are responsible for the stabilization of H-bonded mesogens. The reactants and mesogen entities are given in Table 3. The magnitude of charge transfer from the lone pairs of Lewis base sites from n1(O16), n1(O20), n1(O42) and n1(O58) into anti-bonding Lewis acid sites r(O17AH18), r(O21AH22), r(O43AH44) and r(O59AH57), have significantly increased upon dimerization. It provides unambiguous evidence about the weakening of bonds, elongation and concomitant red shifts of their stretching frequen-

P. Subhapriya et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 511–523 Table 2 Optimized geometrical parameters of stable reactants and intermolecular hydrogen bonded mesogens. Parameters

B3LYP/6-311G(d,p) 5OBA

Bond lengths (A) O11AC12 O11AC6 C6AC1 C1AC2 C2AC3 C3AC4 C4AC5 C5AC6 C3AC15 C15AOI6 C15AO17 O17AH18 H22AO21a O21AC19a C19AO20a C19AC23a C38AC41a C41AO43a C41AO42a O43AH44a H57AO59 O59AC60 C60AO58 C60AC61 C61AC63 C63AC65 C65AC66 C66AC64 C64AC62 C62AC61 C66AO71 O71AC72

1.432 1.355 1.4 1.391 1.396 1.403 1.381 1.404 1.479 1.21 1.36 0.968 0.969 1.357 1.204 1.511 1.511 1.357 1.204 0.969 0.968 1.36 1.21 1.479 1.404 1.381 1.403 1.396 1.391 1.4 1.355 1.432

5OBASA 1.432 1.355 1.401 1.389 1.396 1.404 1.382 1.404 1.478 1.231 1.324 0.998 1.001 1.319 1.225 1.511 1.511 1.319 1.225 1.001 0.998 1.324 1.231 1.478 1.404 1.381 1.404 1.401 1.391 1.397 1.355 1.434

6OBA 1.432 1.355 1.4 1.391 1.396 1.403 1.381 1.404 1.479 1.21 1.36 0.968 0.969 1.357 1.204 1.511 1.511 1.357 1.204 0.969 0.968 1.36 1.21 1.479 1.404 1.381 1.403 1.396 1.391 1.4 1.355 1.432

Inter-molecular H bond lengths O16  H22AO21 1.652 O17AH18  O20 1.673 O43AH44  O58 1.651 O42  H57AO59 1.674 Bond angles (°) C2AC3AC4 C2AC3AC15 C4AC3AC15 C3AC15AO16 C3AC15AO17 O16AC15AO17 C15AO17AH18 H22AO21AC19a O21AC19AO20a O21AC19AC23a O20AC19AC23a C38AC41AO43a C38AC41AO42a C41AO43AH44a O43AC41AO42a H57AO59AC61 O59AC60AO58 O59AC60AC61 O58AC60AC61 C60AC61AC62 C60AC61AC63 C63AC61AC62

119.03 118.6 122.6 125.26 113.08 121.66 105.79 106.45 122.4 111.3 126.3 111.3 122.4 106.45 126.3 105.79 121.66 113.08 125.16 122.6 118.38 119.03

119.04 119.08 121.88 122.31 114.46 123.25 110.3 110.3 123.9 112.93 123.15 112.92 123.13 110.32 123.96 110.09 123.25 114.56 122.2 121.86 119.1 119.04

Inter-molecular H bond angles O16  H22AO21 179.58 O17AH18  O20 179.64 O43AH44  O58 179.65 O42  H57AO59 179.69 a

6OBASA 1.432 1.355 1.401 1.389 1.396 1.404 1.382 1.404 1.478 1.231 1.324 0.998 1.001 1.319 1.225 1.511 1.511 1.319 1.225 1.001 0.998 1.324 1.231 1.478 1.404 1.381 1.404 1.401 1.391 1.397 1.355 1.434

7OBA 1.432 1.355 1.4 1.391 1.396 1.404 1.381 1.404 1.479 1.21 1.36 0.968 0.969 1.357 1.204 1.511 1.511 1.357 1.204 0.969 0.968 1.36 1.21 1.479 1.404 1.381 1.404 1.396 1.391 1.4 1.355 1.432

1.651 1.674 1.651 1.674 119.03 118.38 122.6 125.16 113.08 121.66 105.79 106.45 122.4 111.3 126.3 111.3 122.4 106.45 126.3 105.79 121.66 113.08 125.16 122.6 118.38 119.03

119.04 119.05 121.91 122.29 114.4 123.25 110.12 110.3 123.92 112.91 123.17 112.9 123.14 110.32 123.96 110.09 123.25 114.56 122.19 121.87 119.09 119.04 179.46 179.51 179.66 179.61

7OBASA 1.434 1.355 1.401 1.389 1.396 1.404 1.382 1.404 1.478 1.231 1.324 0.998 1.001 1.32 1.225 1.511 1.511 1.319 1.225 1.675 0.998 1.324 1.231 1.478 1.404 1.381 1.404 1.401 1.391 1.397 1.355 1.434 1.651 1.674 1.651 1.675

119.03 118.37 122.6 125.26 113.09 121.66 105.79 106.45 122.4 111.3 126.3 111.3 122.4 106.45 126.3 105.79 121.66 113.09 125.26 122.6 118.37 119.03

119.04 119.05 121.91 122.29 114.46 123.25 110.12 110.3 123.92 112.91 123.92 112.9 123.14 110.32 123.96 110.09 123.25 114.56 122.19 121.87 119.09 119.04 179.32 179.37 179.68 179.64

The values taken from SA unit.

cies. Similar conclusion can be obtained while considering the energy of each orbital.

515

The stabilization energy E(2) associated with hyperconjugative interactions n1(O16) ? r(O21AH22), n1(O16) ? r(O21AH22), n1(O20) ? r(O17AH18), n2(O20) ? r(O17AH18), n1(O42) ? r(O59AH57), n2(O42) ? r(O59AH57), n1(O58) ? r(O43AH44) and n2(O58) ? r(O43AH44) are obtained as 8.41, 22.67, 7.73, 20.83, 7.67, 20.76, 8.43 and 22.74 kcal/mol respectively (Table 4) which quantify the extent of intermolecular hydrogen bonding. The difference in E(2) energies was judicious, due to that, the accumulation of electron density in the OAH bond is not only haggard from the n(O) of hydrogen acceptor but also from the whole molecule. Generally as observed in alkoxy benzoic acid, the carboxyl CAO bond transfer lesser energy to the anti-bonding CAO. Since the oxygen atom of the hydroxyl group has lost ‘full-control’ of this lone pair, it acquires a small positive charge. The same instant, the oxygen atom of the CAO group has gained slight share of this lone pair, leading to acquiring a low magnitude of negative charge. The greater electron attracting power of oxygen in hydroxyl group over that of an oxygen atom alone is due to the sharing of one pair of electrons in the bond formation process by the hydrogen atom. This, in turn decreases the electron density on that oxygen atom to some extent [50]. It is evidenced from Table S1, both the reactants and hydrogen bonded mesogen carbonyl oxygen atoms possess less negative charge with respect to OAH bond of oxygen atom. In 5OBA compound it has been observed that the higher electronegative charges are accumulated (0.69816 e and 0.60873 e) on O2 and O3 atoms and at the same moment the C13 atom possess more positive charge (0.81222 e). The O1 (0.69494 e), O2 (0.69494 e), O3 (0.60063 e) and O4 (0.60063 e) atoms of SA contains more negative charges and C11 and C12 carbon atoms possess greater positive charge (0.8404 e). The same characteristics have been observed for hydrogen bonded mesogens also. The strength of O17AH18 and its contraction is due to the s-character nature of O17AH18 bond and increases to 4.72% (sp3.78 to sp2.90) as evidenced from Table 5. This shows the existence of a mesomeric structure characterized by delocalization of electron density into the r(O17AH18) anti-bonding orbital from the remaining part of the molecule. This is quite possible because of the energy of r(O17AH18) anti-bonding orbital was (0.3944 a.u.) lower than the energy of r(C15AO17) anti-bonding orbital (0.3972 a.u.) which supports the probability of delocalization of ED from the CAO to the OAH region. The contraction of bond length C15AO17 to 0.036 Å supports the above finding with respect to the reactants in optimized geometry. The natural atomic hybrids corresponding to the H-bonded NBO also showed that the redistribution of natural charges in OAH bond which destabilizes the H-bond. Since hyper conjugation and rehybridization act in the opposite directions, the compression and elongation of the bond OAH bond was due to the balance of the two effects. However, the hyper conjugative interaction is dominant and overshadows the rehybridization effect resulting in a significant elongation in OAH bond (0.03 Å) and a concomitant red shift in the stretching frequency. The same effect has also been observed in O21AH22 and C19AO21. In addition, the s character of spn hybrid orbital for the C15AO16, C19AO20 bond decrease from sp1.41 to sp2.05 and sp1.26 to sp2.03 respectively. The intermolecular hydrogen bonding authenticates the weakening of C15AO16, C19AO20 bond and its elongations. Molecular Electrostatic Potential (MEP) surface MEP is used as a tool for interpreting and forecasting the reactive behavior of a wide variety of chemical systems in both electrophilic and nucleophilic reactions, the study of biological recognition processes and hydrogen bonding interactions [51]. In the MEP surface, red and blue colors refer to the electron-rich and electron-poor regions respectively whereas the green color

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Table 3 Occupancies and energies of stable 5OBA reactants and 5OBASA hydrogen bonded mesogens. Parameters

rC3AC15 rC15AO16 pC15AO16 rC15AO17 rO17AH18 rC61AC60 rC60AO58 pC60AO58 rC60AO59 rO59AH57 n1(O16) n2(O16) n1(O17) n2(O17) n1(O58) n2(O58) n1(O59) n2(O59) rC23AC19 rC19AO20 pC19AO20 rC19AO21 rO21AH22 rC38AC41 rC41AO42 pC41AO42 rC41AO43 rO43AH44 n1(O20) n2(O20) n1(O21) n2(O21) n1(O42) n2(O42) n1(O43) n2 (O43) a

Occupancy (e)

Energy (a.u.)

Reactants

5OBASA mesogens

Docc. (e)

Reactants

5OBASA mesogens

Docc (a.u.)

0.06602 0.01807 0.25764 0.0983 0.00994 0.06602 0.01807 0.25764 0.0983 0.00994 1.9794 1.83233 1.97834 1.84696 1.9794 1.83233 1.97834 1.84696 0.01482a 0.02093a 0.20845a 0.10081a 0.01078a 0.01482a 0.02093a 0.20845a 0.10081a 0.01078a 1.97768a 1.84599a 1.97892a 1.82636a 1.97768a 1.84599a 1.97892a 1.82636a

0.06015 0.02374 0.31427 0.07031 0.06727 0.06015 0.02374 0.31427 0.07031 0.06727 1.95685 1.85086 1.97081 1.77989 1.95682 1.85086 1.97081 1.77989 0.05544 0.0268 0.26398 0.0721 0.0719 0.05544 0.0268 0.26398 0.0721 0.0719 1.95782 1.85146 1.96999 1.77128 1.95796 1.85159 1.96993 1.77078

0.00587 0.00567 0.05663 0.02799 0.05733 0.00587 0.00567 0.05663 0.02799 0.05733 0.02255 0.01853 0.00753 0.06707 0.02258 0.01853 0.00753 0.06707 0.04062 0.00587 0.05553 0.02871 0.06112 0.04062 0.00587 0.05553 0.02871 0.06112 0.01986 0.00547 0.00893 0.05508 0.01972 0.0056 0.00899 0.05558

0.435 0.6141 0.01471 0.34457 0.38793 0.435 0.6141 0.01471 0.34457 0.38793 0.62276 0.33239 0.68614 0.26162 0.62276 0.33239 0.68614 0.26162 0.39383a 0.61958a 0.00842a 0.34416a 0.37998a 0.39383a 0.61958a 0.00842a 0.34416a 0.37998a 0.69243a 0.27065a 0.62635a 0.33874a 0.69243a 0.27065a 0.62635a 0.33874a

0.43367 0.57078 0.00406 0.39719 0.39435 0.43367 0.57078 0.00406 0.39719 0.39435 0.66964 0.31504 0.57602 0.3219 0.66992 0.31504 0.57602 0.3219 0.38149 0.58251 0.00399 0.40647 0.39107 0.38149 0.58251 0.00399 0.40647 0.39107 0.67504 0.31374 0.5738 0.32172 0.67561 0.31397 0.57393 0.32192

0.00133 0.04332 0.01877 0.05262 0.00642 0.00133 0.04332 0.01877 0.05262 0.00642 0.04688 0.01735 0.11012 0.06028 0.04716 0.01735 0.11012 0.06028 0.01234 0.03707 0.01241 0.06231 0.01109 0.01234 0.03707 0.01241 0.06231 0.01109 0.01739 0.04309 0.05255 0.01702 0.01682 0.04332 0.05242 0.01682

The values taken from SA unit.

signifies the zero electrostatic potential. The increment of electrostatic potential magnitude increases in the following sequence:

Red < orange < yellow < green < blue It is found that in 5OBA compound the atoms O1, O2 and O3 exhibits high electronegativity (0.48179, 0.60776, 0.54575 a.u.) which in turn related to high electrophilic reactivity. This is shown in yellow (O1 and O2) and red (O3) regions of the MEP surface presented in Fig. 3. The p bonded electron between C13 and O3 is entirely fascinated by O3 and hence O3 acts as electrophile. However, a high spread of positive region is localized on the H24 atom and the magnitude being +0.4250 a.u. indicating a possible site for nucleophilic attack. The same phenomena have also been observed in 6OBA (O1 = 0.4963 a.u., O2 = 0.6060 a.u., O3 = 0.5452 a.u.) and 7OBA (O1 = 0.4885 a.u., O2 = 0.6082 a.u., O3 = 0.5416 a.u.) compounds with a difference of charge concentrations in each of the atoms as depicted in Fig. 3. At the same instant, H24 atom of 6OBA (+0.4240 a.u.) and 7OBA (+0.4270 a.u.) reveal high magnitude of positive charges which is highly susceptible for nucleophilic attack. In SA, the oxygen atoms O1, O2, O3 and O4 are highly electronegative (0.6192, 0.5538 a.u.) as indicated by yellow and red colors in the MEP contour whereas the two hydrogen atoms (H25 and H26) are highly electropositive (0.4213 a.u.) witnessed from the MEP surface. It is well proved from computational analysis, that the hydrogen bonds that have been formed at H24 (OBA-blue region) and O3 or O4 (SA-red region) and H25 or H26 of SA and O3 of OBA. Negative poten-

tial sites are concentrated on electronegative atoms whereas the positive potential sites are spread over the hydrogen atoms and these sites provide information about the region from where the compound can have intermolecular interactions. It is interesting to know that the absence of blue region in Fig. 4 indicated the existence of intermolecular hydrogen bonding and was highly supportive for the presence of intermolecular hydrogen bond formation. Frontier molecular orbitals With the help of frontier orbital gap, it is easy to understand the characteristics of a molecule regarding its interaction with other species. The frontier orbital gap helps to characterize the chemical reactivity and kinetic stability of the molecule. A molecule with small frontier orbital was more polarizable and is generally associated with high chemical reactivity and low kinetic stability due to which the molecule can be termed as soft molecule [52]. The contours of HOMO and LUMO for the 5OBA, 6OBA and 7OBA and their hydrogen bonded complexes are displayed in Fig. 5. The HOMO which is an electron donor represents its capability to contribute an electron as an electron acceptor and the LUMO represents the attitude to accept the electron. The smaller HOMO and LUMO gaps, the electrons are straight away excited from the HOMO and it is uncomplicated for LUMO to accept these electrons. The frontier energy gap for 5OBA, 6OBA and 7OBA were 5.279 eV, 5.281 eV and 5.282 eV respectively and these energies are quite sufficient for electron transitions. Hence, the lowest energy absorption is assigned as the p–pcharge transfer [53].

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E(2) (kcal/mol)

E(j)–E(i) (a.u.)

F(i,j) (a.u.)

Table 5 Composition of hydrogen bonded NBO’s in terms of natural atomic hybrids. Hydrogen bonded NBOs n

Within unit 1 LP (1)C3 ? BD(2)C1AC2 LP (1)C3 ? BD(2)C4AC5 LP (1)C3 ? BD(2)C15AO16 LP (1)O11 ? BD(1)C1AC6 LP (1)O11 ? BD(1)C5AC6 LP (1)O11 ? BD(1)C12AH13 LP (1)O11 ? BD(1)C12AH14 LP (1)O11 ? BD(1)C12AC45 LP (1)O11 ? BD(1)C45AC48 LP (2)O11 ? BD(1)C12AH13 LP (2)O11 ? BD(1)C12AH14 LP (1)O16 ? BD(1)C15AO17 LP (2)O16 ? BD(1)C3AC15 LP (2)O16 ? BD(1)C15AO17 LP (1)O17 ? BD(1)C15AO16 LP (2)O17 ? BD(2)C15AO16

78.85 68.42 86.76 7.24 0.65 0.71 0.71 1.11 0.72 5.35 5.34 4.9 16.96 19.11 8.06 56.99

0.14 0.15 0.11 1.1 1.1 0.93 0.93 0.93 0.94 0.7 0.7 1.07 0.75 0.71 1.15 0.32

0.112 0.108 0.104 0.08 0.024 0.023 0.023 0.029 0.023 0.057 0.057 0.065 0.103 0.106 0.086 0.123

From unit 1 to unit 2 LP (1)O16 ? BD(1)O21AH22 LP (2)O16 ? BD(1)O21AH22 LP (1)O16 ? BD(1)C19AO21

8.41 22.67 0.09

1.06 0.71 0.72

0.085 0.115 0.007

From unit 1 to unit 3 (None above threshold) From unit 2 to unit 1 LP (1)O20 ? BD(1)O17AH18 7.73 LP (1)O20 ? BD(1)C15AO17 0.09  LP (2)O20 ? BD (1)O17AH18 20.83

1.07 0.71 0.71

0.082 0.007 0.111

Within unit 2 LP (1)O20 ? BD(1)C19AO21 LP (2)O20 ? BD(1)C19AO21

4.8 19.65

1.08 0.72

0.065 0.108

Within unit 1 LP (2)O20 ? BD(1)C19AC23 LP (1)O21 ? BD(1)C19AO20 LP (2)O21 ? BD(1)C19AO20 LP (1)O42 ? BD(1)C41AO43 LP (2)O42 ? BD(1)C38AC41 LP (2)O42 ? BD(1)C41AO43 LP (2)O43 ? BD(1)C38AC41 LP (1)O43 ? BD(1)C41AO42 LP (2)O43 ? BD(1)C41AO42

18.14 8.38 59.84 4.8 18.12 19.63 0.5 8.4 60.02

0.7 1.16 0.32 1.08 0.7 0.72 0.95 1.16 0.32

0.103 0.088 0.124 0.065 0.103 0.108 0.02 0.088 0.124

From unit 2 to unit 3 LP (1)O42 ? BD(1)H57AO59 LP (2)O42 ? BD(1)H57AO59 LP (2)O42 ? BD(1)O59AC60

7.67 20.76 0.09

1.07 0.71 0.71

0.081 0.11 0.007

From unit 3 to unit 1 None above threshold From unit 3 to unit 2 LP (1)O58 ? BD(1)O43AH44 8.43 LP (2)O58 ? BD(1)C41AO43 0.09 LP (2)O58 ? BD(1)O43AH44 22.74

1.06 0.72 0.71

0.085 0.007 0.115

Within unit 3 LP (1)O58 ? BD(1)O59AC60 LP (2)O58 ? BD(1)O59AC60 LP (2)O58 ? BD(1)C60AC61 LP (1)O59 ? BD(1)O58AC60 LP (1)O59 ? BD(1)C60AC61 LP (1)O59 ? BD(1)O58AC60 LP (1)O71 ? BD(1)C64AC66 LP (1)O71 ? BD(1)C65AC66 LP (1)O71 ? BD(1)C72AH73 LP (1)O71 ? BD(1)C72AH74 LP (1)O71 ? BD(1)C72AC75 LP (1)O71 ? BD(1)C75AH77 LP (2)O71 ? BD(1)C64AC66 LP (2)O71 ? BD(1)C72AH73 LP (2)O71 ? BD(1)C72AH74

1.07 0.71 0.75 1.15 1.01 0.32 1.1 1.1 0.94 0.93 0.94 0.095 0.34 0.71 0.7

0.065 0.106 0.103 0.086 0.024 0.123 0.08 0.024 0.022 0.023 0.029 0.022 0.1 0.057 0.056

4.92 19.12 16.94 8.05 0.53 56.81 7.21 0.66 0.62 0.74 1.08 0.65 32.7 5.29 5.24

The HOMO–LUMO gap between 5OBA and 6OBA is found to be 0.002 eV and 6OBA and 7OBA is 0.001 eV. Energy gap between the 5OBA compound is less than 7OBA compound and it is concluded that the reactivity decreases from 5OBA to 7OBA and conversely

5OBA

5OBASA 2.90

DNBO

sp (O17AH18) % of s character % of p character of O17 % of p character H18 q(O17)/e q(H18)/e

3.78

sp 20.88 74.41 25.59 0.69816 0.47781

sp 25.6 78.1 21.9 0.6897 0.5058

+s +4.72 +3.69 3.69 +0.0085 0.02780

spn (C15AO16) % of s character % of p character of O16 % of p character C15 q(O16)/e q(C15)/e

sp1.41 41.52 65.4 34.6 0.6087 0.8122

sp2.05 32.76 74.04 25.96 0.6843 0.8395

s 8.76 8.64 8.64 0.0756 +0.0273

spn (O21AH22) % of s character % of p character of O21 % of p character H22 q(O21)/e q(H22)/e

sp3.83 20.69 74.31 25.69 0.69494 0.47521

sp2.92 25.5 78.21 21.79 0.6890 0.5045

+s +4.81 +3.9 3.9 0.0059 0.0293

spn (C19AO20) % of s character % of p character of C19 % of p character O20 q(C19)/e q(O20)/e

sp1.36 42.26 69.77 30.23 0.84047 0.60063

sp2.03 32.97 73.36 26.64 0.8688 0.67513

s 9.29 +3.59 3.59 0.0283 0.0745

and the stability increases from 5OBA to 7OBA because there is an increase in the methylene group and it enhances the stabilization of the title compounds. As shown in the Fig. 5, the charge density concentration of the HOMO contours of all the three compounds are found to be more on the benzene ring than any other part of the compounds. In other words, the HOMO is p nature (i.e. aromatic ring) and delocalized over the whole CAC bond of all the reactant of the title compounds. In contrast, the charge density is localized over the aromatic ring as observed in the LUMO contours of all the three compounds respectively. This is the consequence of the charge transfer which takes place from oxygen containing CH2 group and carboxylic groups from the aromatic ring. As observed in Fig. 5, charge delocalization was found to be maximum in the HOMO contour of suberic acid on either side. It clearly revealed that the charge transfer can easily be relocated to alkoxy benzoic acids, which, in turn, shows the ready formation of hydrogen bonded complex. The LUMO contour also supports the above fact. Another thing to be noticed in HOMO is minimum charge concentration existing in the middle part of the chain where it is not available in the LUMO contour. A major deviation has been observed in the LUMO contours of all the hydrogen bonded complexes that charge delocalization was found to be maximum on the right side of the contour. Molecular orbital studies The molecular orbital (MO) theory is most widely used by theoretical chemists. It is important that ionization potential (IP), electron affinity (EA), hardness (g), softness (s) electronegativity (ve) and electrophilic index (x) be put into an MO framework. These are readily being done within the limitations of Koopman’s theorem. The potential energy required to remove an electron completely from its atom is known as IP. EA refers to the ability of a ligand to accept precisely one electron from a donor. The chemical hardness and softness of a molecule is a good indicator of the chemical stability of a molecule. In quantum theory, changes in the electron density of a chemical system result from the mixing of suitable excited-state wave function with the ground-state wave function.

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-0.5458 a.u

0

+0.4250 a.u

(a) 5OBASA mesogen MEP (a) 5OBA MEP -0.5458 a.u

0

+0.4240 a.u

(b) 6OBASA mesogen MEP

(b) 6OBA MEP

-0.5416 a.u

0

0.4270 a.u

(c) 7OBASA mesogen MEP Fig. 4. Calculated 3D molecular electrostatic potential contour map of all the hydrogen bonded mesogens.

(c) 7OBA MEP

-0.5538 a.u

0

+0.4243 a.u

(d) SA MEP Fig. 3. Calculated 3D molecular electrostatic potential contour map of all the reactants.

The mixing coefficient is inversely proportional to the excitation energy between the ground and excited states. Obviously a small HOMO–LUMO gap demands less excitation energies to the manifold of excited states. In terms of chemical reactivity, soft molecule will be more reactive than hard molecule. Parr et al. [54] have defined a new descriptor to quantify the global electrophilic power of the compound as electrophilic index (x), which defines a quantitative classification of the global electrophilic nature of a compound. In other words, electrophiles are electron deficient which tends to accept electrons and forms bonds with nucleophiles. Thus electrophilic is a useful structural descriptor of reactivity and is frequently used in the analysis of the chemical reactivity of molecule [55]. The electronegativity and the hardness are of course used extensively to make predictions about chemical behavior and these are used to explain aromaticity in organic compounds [56]. All the chemical

descriptors which are described above have been computed and displayed in Table 6. It was found that all the reactants are soft in nature and increased of methylene group minimize the hardness of the molecule. Similarly it is also observed that in hydrogen bonded mesogens higher the softness lower is the hardness. The IP values of all the reactants and hydrogen bonded mesogens are found to be decreasing from 5OBA to 7OBA the exceptional being the suberic acid. Among all the reactants, 5OBA and its hydrogen bonded mesogens records highest IP than the other two reactants and their corresponding hydrogen bonded mesogens. As displayed in Table 6, all the hydrogen bonded mesogens are possess higher magnitudes of IPs than their reactants. It is because of the IP of SA which is found to be higher and it is one of the reactants which bridges the two reactants (nOBA) which in turn enhances the IP of all the hydrogen bonded mesogens. The same behavior has been observed for all the reactants and their hydrogen bonded mesogens with respect to the remaining molecular characteristics. The exception may be due to SA which showed a low degree of EA.

Role of solvent effects on the intermolecular hydrogen bonding Electrostatic nature of hydrogen bonds through which mesogens are associated and the hydrogen bonded mesogens stability is influenced by the dielectric properties or polarity of the surrounding environment in which they are found [57]. Qualitatively, the interaction between solutes and a dielectric constant value of the solvent can be correlated with the magnitude of solute dipole moment [58]. Inspection of Table 7 showed that all the hydrogen bonded mesogens are stable in polar solvents comparable to gas phase and in the presence of non-polar solvents. As the magnitude of dielectric constant of the solvent increase, there was a corresponding hike in the values of dipole moment that has also been observed in all hydrogen bonded mesogens.

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(a) 5OBA HOMO

(b) 5OBA LUMO

(c) 6OBA HOMO

(d) 6OBA LUMO

(e) 7OBA HOMO

(f) 7OBA LUMO

(g) SA HOMO

(h) SA LUMO

(i) 5OBASAmesogen HOMO

(j) 5OBASA mesogen LUMO

(k) 6OBASAmesogen HOMO

(l) 6OBASA mesogen LUMO

(m) 7OBASA mesogen HOMO

(n) 7OBASA mesogen LUMO

Fig. 5. Density plot of the HOMO and LUMO compositions of all the reactants and all hydrogen bonded mesogens.

Table 6 Properties related with the chemical potential in the optimized structures of reactants and all mesogens.

5OBA 6OBA 7OBA SA 5OBASA mesogen 6OBASA mesogen 7OBASA mesogen

Ionization potential (eV)

Electron affinity (eV)

Hardness

Softness

Electronegativity

Electrophilic index

6.4628 6.4604 6.4590 7.7251 6.4761 6.4745 6.4732

1.1932 1.1916 1.1902 0.0765 1.2865 1.2876 1.2887

2.6348 2.6344 2.6342 3.8243 2.5943 2.5935 2.5922

0.1898 0.1899 0.1901 0.1307 0.1927 0.1928 0.1929

3.8280 3.8260 3.8246 3.9008 3.8819 3.8811 3.8809

2.7808 2.7782 2.7763 1.9894 2.9043 2.9040 2.9051

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Table 7 Total energy (a.u.) and dipole moment of the all the mesogens with different solvent environment. Medium

5OBASA

Gas phase Benzene Ethanol Acetonitrile

6OBASA

7OBASA

Energy

Dipole moment

Energy

Dipole moment

Energy

Dipole moment

2000.058 2000.069 2000.083 2000.083

1.2603 1.4087 1.7143 1.7325

2078.71 2078.72 2078.73 2078.73

1.3387 1.4995 1.836 1.8561

2157.35 2157.36 2157.38 2157.38

1.1661 1.2865 1.5788 1.5971

The energy difference between the gas phase and the non-polar solvent is 0.02(a.u.) for 5OBASA compound and the difference in energy is 0.03(a.u.) in the presence of polar solvent. Simultaneously, the energy variance concerning the gas phase and nonpolar solvent is 0.01(a.u.) and in polar environment is 0.02(a.u.) for 6OBASA. The energy inconsistency between the nonpolar atmosphere and gas phase in 7OBASA is found to be 0.01(a.u.) and modification of energy between the gas phase and the polar solvent is 0.03(a.u.). From the above discussions, it is found that 5OBASA and 7OBASA exhibit higher stability in polar atmosphere than 6OBASA in the same environment. It has been observed that the differences in dipole moment values of 5OBASA and 6OBASA are slightly higher than that of the 7OBASA mesogens. In general all the three hydrogen bonded mesogens demonstrate the following sequence:

the gaseous phase of the complex while experimental FT-IR has been carried out at solid phase. It has been observed that a few disagreements exist between the calculated and observed vibrational wave numbers. To overcome these discrepancies, a scale factor of 0.9608 is introduced at the time of computation and this reduces the deviation to a greater extent with respect to the experimental findings. The complex 5OBASA consists of 88 atoms which have 258 normal vibrational modes and the molecule belongs to C1 symmetry. The experimental and calculated IR data are depicted in Fig. 6 and the calculated IR intensities are plotted against harmonic vibrational wave numbers. The experimental wave numbers are displayed in Table 9 along with the calculated wave numbers of 5OBASA complex.

Polar > Non  polar > gas phase

Ring vibrations The aromatic ring vibrational modes of 5OBASA complex have been analyzed based on the vibrational spectra of previously published vibrations of the benzene molecule which is helpful in the identifications of the phenyl ring vibrational modes [60,61]. The ring stretching vibrations are very prominent, as the double bond is in conjugation with the ring in the vibrational spectra of benzene and its derivatives [62]. The ring carbon–carbon stretching vibration occurs in the region of 1650–1200 cm1. In general, from the previous literature it has been observed that these bands are of variable intensities and observed at 1625–1590, 1590–1575, 1540–1470, 1465–1430 and 1380–1280 cm1 [63]. In the present

It is expected that the specific charge distributions and electrostatic interactions in LC molecules play an interfacial role in the formation of various mesophase. Subsequently atomic charge is not a quantum mechanical observable; all methods for computing it are necessarily arbitrary. However, there is much greater agreement among the methods when it comes to the group charges than the charges on each individual atom [59]. Since group charges are needed in order to explain the behavior of the mesogen molecules, Mulliken population analysis which partitions the total charge among the atoms in the molecule is performed for each of the molecules and investigated in this study. As for the 5OBASA, 6OBASA and 7OBASA molecules, the charge distribution occurs between the acid group and the core structure. From Table 8, positive charge in the center of the core structure attracts the negatively charged acid group of the neighboring molecules causing the formation of almost perpendicular pairs. These shorter units attribute to the medium stability of the nematic phase and thus lower the TN?I transition temperature. The charge distribution between the molecules 6OBASA and 7OBASA mesogens are found to be almost same and it is less for 5OBASA mesogen. The results show that a methylene unit in alkoxy chain plays a minor role in the formation of the charge distribution. Vibrational analysis

FT-IR Solid phase

Transmittance

Molecular charge distributions

B3LYP/6-311G(d,p) (Theoretical)

With the intention of acquiring the spectroscopic monogram of 5OBASA complex, the frequency calculation has been performed in

Table 8 Group charges for the mesogen molecules and transition temperatures TN?I. Molecules

Acid group

Core

Alkoxy group

TN?I (°C)

5OBASA 6OBASA 7OBASA

0.1951 0.1953 0.1953

0.2723 0.2731 0.2732

0.0773 0.0779 0.0779

119 125 124

4000

3500

3000

2500

2000

1500

1000

500

Wave number (cm-1) Fig. 6. Comparison of observed and computed FT-IR spectrum of 5OBASA mesogen.

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521

Table 9 Comparison of the experimental wavenumbers and theoretical harmonic wavenumbers (cm1) of 5OBASA hydrogen bonded mesogens. Experimental frequencies

3124 3116 2924 2916

2910

2890 1706 1658 1620, 1566

1458 1450 1411, 1419 1373

1326

1280

1095 1103

990 933 930 902

825

617

Theoretical frequencies Unscaled

Scaled

3237, 3235, 3231 3221, 3215, 3221, 3214 3214, 3214 3114, 3113 3101, 3099 3096 3091, 3073, 3065, 3060 3056, 3054 3043 1778 1724 1668, 1667, 1624, 1623 1549, 1542 1538, 1530 1529 1510 1480, 1477, 1468, 1466 1442, 1441 1435, 1431 1429 1373 1351, 1350 1349 1346, 1343 1338 1326, 1325 1322, 1317 1304, 1302 1209, 1162, 1161 1153, 1152 1145, 1143 1137, 1113 1091, 1089 1075, 1074 1071, 1067 1054 1050 1046 1030, 1020 1000, 990, 977 972 942, 930, 909 918, 917 887, 880 864, 861 856 832, 830 805, 787 783 775 771, 745, 741 701 679, 675 655, 646 644 571, 570 556, 553, 533 517, 507 482, 462 427, 426 365 264 173 115 85, 86 59, 58, 51, 48 27, 24, 16, 11 6, 4

3112, 3109, 3106 3098, 3096, 3090, 3089 3089, 3087 2993, 2994 3979, 3979 3976 2971, 2954 2947, 2942 2937, 2935 2925 1708 1657 1603, 1602, 1561, 1560 1489, 1482 1478, 1470 1469 1451 1422, 1420, 1411, 1409 1386, 1385 1380, 1376 1374 1320 1299, 1298 1297 1294 1291 1286 1275, 1274 1254, 1253 1162, 1117, 1116 1109, 1108 1101, 1098 1093, 1070 1048, 1047 1033, 1032 1029, 1026 1014 1009 1006 990, 998 961, 951, 939 934 906, 894, 874 883, 881 853, 846 831, 828 823 800, 798 774, 756 753 745 741, 716, 712 673 653, 649 630, 621 619 549, 548 535, 532, 512 497, 487 464, 444 411, 410 349 254 166 110 83, 82 57, 55, 49, 44 23, 21, 15, 10 5.8, 3.6

Symmetry species

Assignment

A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0

ms CH(ring) mas CH(ring) mOH masCH3 mas CH2(SA unit) mOH + asmCH2(OBA unit) mas CH2(OBA unit) + mas CH2(SA unit) ms CH2(OBA unit) ms CH2 (SA unit) ms CH3 mC=O + bCAOH bOH + mCAO mCAC(ring) aCH2 (OBA unit) aCH2(SA unit) cCH3 bOH mCAC(ring) bCH3 x CH2 (OBA unit) x CH2(SA unit) mCAC(ring) tCH2 (OBA unit) tCH2(SA unit) bCAH(ring) tCH2 (OBA unit) + tCH2(SA unit) mCAC + sOH(ring) rCH2(OBA unit) mCAO(alkyloxy link) bCAH(ring) brCH3 bCAH(ring) mCH2(SA unit) mCAC(OBA unit) + sCAC(SA unit) mCAC(OBA unit) bmCAC mCAO mCAC(OBA unit) mCAC(SA unit) bCAH cCAH cOAH b rCH3 cOAH tCH2 cCAH RBM cCAH rCH2 bOAH cCAC rCH2 + rCH cCAH bCACAO bCAO bring cCAOAH bring bring bCAOAH cCAH cCACAO rIMHB c x IMHB m IMHB cring + t IMHB tCH3 t ring butt

ms, symmetry stretching; mas, asymmetry stretching; b, in-plane bending; c, out-plane bending; x, wagging; r, rocking; t, twisting or torsion; buttfly mode, butt; RBM, ring breathing mode, IMHB, intermolecular hydrogen bonding.

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study, the wavenumbers which have been obtained from FTIR spectrum at 1620, 1566, 1419, 1411, 1326 and 1280 cm1 are in well agreement with the previously observed results. The computed wave numbers for CAC stretching vibrations are found in the range of 1600–1270 cm1 and observed values are in good correlation with the literature data of 1622, 1427, 1341, 1325, 1278, 1258 and 1122 cm1 [64]. The in-plane bending mode of 5OBASA complex is occurring at 617 cm1 and the predicted wave numbers of ring out-plane bending modes have been found at 83 and 82 cm1 respectively. The ring torsion has been calculated at 23, 21, 15, 10 cm1 in B3LYP method. CAH vibrations The carbon hydrogen stretching vibrations typically give upswing to bands in the region of 3100–3000 cm1 in all aromatic compounds [65,66]. In this region, the bands are not affected considerably by the nature of the substituents. All the aromatic CAH stretching bands are found to be weak and this is due to decrease of dipole moment caused by reduction of the negative charge on the carbon atom. This reduction occurs because of the electron is withdrawal on the carbon atom by substituent due to the decrease of inductive effect which in turn by the increase in chain length of the substituent [67]. In this 5OBASA complex, CAH stretching vibrations are diverse due to sixteen methylene groups. The observed CAH stretching vibrations in benzene ring are at 3124, 3116 cm1 and are in good agreement with the computed values 3112, 3098 cm1. Methylene group vibrations are obtained theoretically at 2971, 2954, 2937 cm1 and experimentally found at 2910 cm1. The in-plane CAH bending vibrations appear in the range of 1300–1000 cm1 in the substituted benzenes and the out of plane bending vibrations occur in the frequency range of 1000–750 cm1 [68,69]. Theoretically observed in-plane CAH bending vibrations are 1162, 1117 and 1098 cm1 and out-plane CAH bending vibrations are 961, 798 cm1 and these values are matches with the experimental values. Experimental in-plane CAH bending vibrations at 1103 cm1 and out-of-plane bending vibrations appear at 933 and 825 cm1. Methyl group vibrations Methyl groups are generally referred to as electron donating substituents in the aromatic ring system [62]. Normally, the CAH stretching modes of the methyl group produce bands in the region 2840–2975 cm1 [60,70]. For the 5OBASA, asymmetric stretching is observed at 2916 cm1 and symmetric stretching is observed at 2890 cm1 experimentally. The theoretically computed value for asymmetric stretching is 2993, 2994 cm1 and symmetric stretching is 2925 cm1 coinciding with experimental value. The observed band at 1373 cm1 in FT-IR is assigned to in-plane bending mode of the methyl group and 1458 cm1 is assigned to out-plane bending mode of the methyl group and these peaks are well matched with calculated values. Normally, aromatic compound displays the methyl in-plane and out-of plane rocking vibration bands in the neighborhood of 1045 cm1 and 970 ± 70 cm1 respectively [71]. In the present investigation, the band seems at 1095 cm1 and the weak band at 902 cm1 is assigned to in-plane and out-of-plane rocking vibrations respectively. The calculated values 59, 48 cm1 are assigned to the twisting mode of the methyl group. Carboxylic group vibrations The carboxylic acids form strong intermolecular hydrogen bonding in the solid state. Vibrational analysis of carboxylic acid group is made on the basis of carbonyl group and hydroxyl group. The C@O stretch of carboxylic acid is identical to the C@O stretch in ketones, which is expected in the region 1740–1660 cm1 [72]. The

C@O bond formed by P p–P p between C and O, intermolecular hydrogen bonding reduces the frequencies of the C@O stretching absorption to a greater degree than does by intermolecular H bonding because of the different electro negativity of C and O, the bonding is not equally distributed between the two atoms. The lone pair of electrons on oxygen also determines the nature of the carbonyl group. The strong band at 1706 cm1 in FT-IR is assigned to C@O stretching and calculated value of this mode at B3LYP/6-311G(d,p) method is found to be consistent with the experimental data. A band related to CAO stretching mode of carboxylic acid group is highly coupled with the vibrations of adjacent groups i.e. OAH in plane bending. Hence the wavenumber region for the existence of CAO is based on the nature of the nearby substitution. The band observed at 1658 cm1 is in excellent agreement with the computed value. The in-plane bending modes of C@O are computed at 653, 649 cm1 and out-plane bending mode is at 349 cm1 and these vibrational modes are agreement with the literature values [73,74]. The OAH group vibrations are likely to be most sensitive to the environment and hence they show pronounced shifts in the IR spectra of the hydrogen bonded species. The OAH stretching band is characterized by a very broad-shoulder band appearing approximately at 3400 cm1. The observed experimental value is 2924 cm1 and calculated value is 3089 cm1 which is a positive deviation from the literature due to the presence of strong intermolecular hydrogen bonding. In general the OAH in-plane bending vibrations and OAH out-of-plane deformation vibrations occurs approximately at 1440 and 750 cm1, respectively. The observed and computed values are found to be in the same line at 1450 cm1 and are not much affected due to hydrogen bonding unlike the stretching and out-of-plane deformation frequencies. In both inter-molecular and intra-molecular associations, the frequency is at a higher value than in free OH. The frequency increases with respect to hydrogen bond strength because of larger amount of energy required to twist the OAH bond out-of-plane [75]. In the present case, OAH out-of-plane deformation vibration is experimentally observed at 930 cm1 and the calculated value is 934 cm1.

Conclusions The optimized geometry shows that shorter bond length in C15AO17 bond shows double bond character due to delocalization of unpaired electron of oxygen atom. This is an indicator for the formation of the intermolecular hydrogen bonding. It is also observed that the strength of the hydrogen bond remains constant in all the hydrogen bonded mesogens. The NBO analysis clearly demonstrates the formation of four strong H-bonded interactions. Hyper conjugation and re-hybridization act in opposite directions within the OAH bond, but the hyper conjugative interaction is dominant and overshadows the re-hybridization effect resulting in a significant elongation in OAH bond and a concomitant red shift in stretching frequency. The MEP surface shows that the negative potential sites are on electronegative atoms and the positive potential sites are around the hydrogen atoms. These sites give information about the region from where the reactants can have intermolecular interactions. From the HOMO–LUMO analysis it is concluded that the reactivity decreased from 5OBA to 7OBA and conversely the stability increased from 5OBA to 7OBA due to the fact that increased in the methylene group enhances the stabilization of the title compounds. It has been found that all the reactants are soft in nature and hardness has decreased with the increase in methylene group. At the same instant, the hardness is found to be decreasing with the increased of softness in the formed hydrogen bonded mesogens.

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