Structure and NLO properties of halogen (F, Cl) substituted formic acid dimers

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 821–832

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Structure and NLO properties of halogen (F, Cl) substituted formic acid dimers P. Umadevi, L. Senthilkumar ⇑, M. Gayathri, P. Kolandaivel Department of Physics, Bharathiar University, Coimbatore 641 046, 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

 Trans–trans form of halogenated

formic acid dimer is more stable.  Non-symmetric structures enhance

polarizability and hyperpolarizability.  Fluorine and chlorine atoms in formic

acid dimer increase NLO properties.  Chlorinated formic acid dimers are

good candidate for nonlinear optical materials.  Hydrogen bonding increases the Rayleigh intensity components of the light.

a r t i c l e

i n f o

Article history: Received 18 December 2013 Received in revised form 13 May 2014 Accepted 18 May 2014 Available online 11 June 2014 Keywords: Formic acid SAPT NLO CBS TD-DFT Hyperpolarizability

a b s t r a c t In this work, using ab initio and density functional theory (DFT) methods halogen substituted formic acid (FA) dimer is studied. The dimer stability is due to the hydrogen bonds, either conventional (OAH  O, OAH  F, OAH  Cl) or non-conventional (CAH  O, CAH  F, CAH  Cl). Among all the dimers, trans– trans form is more stable than the trans–cis, and cis–cis form. Basis set extrapolated counterpoise corrected interaction energy results for the FA dimer are in excellent agreement with BSSE corrected MP2 interaction energy. Symmetry Adopted Perturbation Theory (SAPT) analysis reveals that the electrostatic effect plays a dominant role in stabilization among the dimers with maximum interaction energy. Chlorine substituted FA dimer has high hyperpolarizability, which makes them excellent candidate for nonlinear optical materials (NLO). The halogen substituted formic acid dimers have higher stability and polarizability value than the unsubstituted formic acid dimer. The hyperpolarizability values depend on the geometrical structures of halogenated formic acid dimers than the type of hydrogen bonds. The small excitation energy and HOMO–LUMO gap in the halogenated formic acid dimer has led to the strong nonlinear optical response. The depolarization ratio and Rayleigh scattering increases in formic acid dimer after the halogen atom substitution. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Nonlinear optical (NLO) materials play a key role in the domains of photoelectronic modulators for optical telecommunications [1], ⇑ Corresponding author. Tel.: +91 9443702753. E-mail address: [email protected] (L. Senthilkumar). http://dx.doi.org/10.1016/j.saa.2014.05.080 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

and photonic applications such as optical data manipulations, storage, and electro optical devices [2–4]. Hence, searching for the excellent NLO molecular materials has become the spotlight for researchers. A key objective to develop the materials for nonlinear optical applications is to find highly active materials with large second-order polarizabilities b. The hyperpolarizability and the second-order NLO response relates to electronic intramolecular

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charge transfer (ICT) of the molecule. Both theoretical and experimental studies have shown that large hyperpolarizabilities arise from an amalgamation of a strong electron donor and acceptor positioned at opposite ends of a suitable conjugation path. Few studies have shown that aggregation of electron donor and acceptor in the form of dimers, trimers, and clusters enhances b property of their individual molecules. This enhancement of NLO property depends on the geometry, which follows from the nature of interaction that exists between binary (dimers), ternary (trimer) and large clusters [5–11]. The inter-molecular interactions that mainly exist in NLO organic molecules are dipole–dipole interactions, hydrogen bonding, p–p stacking, and hydrophobic effects. Studies have shown that hydrogen-bonding interactions produce pronounced changes in the geometry and thereby affect the NLO property of the complex [12–14]. Considering the above fact, organic molecules appear as promising NLO candidates, since a variety of molecules containing different donor–acceptor units has been reported [15–29]. In addition, organic molecules provide a few advantages over the inorganic molecule: (i) their dielectric constant and refractive index is much smaller, (ii) their polarizability are purely electronic, and (iii) they are compatible with the polymer matrix for flexible devices, and so forth [30–34]. Formic acid (FA), a major organic constituent in cloud, fog water as well as in interstellar medium plays a vital role in astrochemistry and atmospheric science [35]. FA is of particular interest in its electronic and molecular structures along with reactivity from both experimental and theoretical points of view [36,37], since it is one of the simplest model molecules for studying biological systems exhibiting organic acidic type bonding. Apart from this, FA also exhibit distinct electrostatic properties that play an important role in the processes of molecular cluster aggregation [38]. FA exists as two conformers shown in Fig. 1 [39], trans and cis, which differ by the orientation of the hydroxyl group (AOH). FA dimer forms a variety of hydrogen-bonded structures, which is of primary importance for many processes, including the science of life [40]. Earlier studies on the NLO property of formic acid dimers [41], have shown that the value of dipole polarizability are twice when compared with their monomer form. To, further increase the NLO property of formic acid dimers, halogen substitution to the carbon atom might be a solution, as we know halogen substituent draws the electrons in the CAX bond toward itself, giving the carbon a partial positive charge (d+) and the halogen a partial negative charge (d). The presence of the resulting polar covalent bond makes the molecule a polar compound. This polar nature could increase dipole moment and thereby manifold NLO property. However, all this depends on the symmetry of the formic acid dimer. Therefore, molecular level modelling has come to be a critical tool for obtaining and understanding of NLO property in formic acid dimer with halogen substitution. In this perspective, we study the halogen substituted (F, Cl) formic acid dimers in cis–cis, trans–trans, and trans–cis orientation. The structures formed are due to both conventional (OAH  O, OAH  F, OAH  Cl) and non-conventional (CAH  O, CAH  F, CAH  Cl) hydrogen bonds. The study of structure, nature of

Fig. 1. Cis and Trans Conformers of the Formic acid.

hydrogen bond is important because we could infer the possible effect of both conventional and non-conventional hydrogen bond on NLO properties. Furthermore, a study of Rayleigh light scattering on the halogenated formic acid dimers would be of interest to study the influence of halogens and hydrogen bond on elastic light scattering parameters. Computational methods The structures of all the halogen substituted formic acid dimers with trans and cis form ZCOOH (Z = F, Cl), as well as the individual molecules were fully optimized using the aug-cc-pVTZ basis sets with both Møller–Plesset perturbation (MP2) [42] and Density Functional theory (DFT) methods with B3LYP [43,44] functional. To ensure true minima of the monomers and dimers on the potential energy surface, vibrational frequency calculations were performed. The choice of DFT method is justified as it offers an attractive alternate to demanding high level ab initio methods, and offers useful molecular properties. In addition, previous literature show that the predictive capability of the B3LYP functional for electronic polarizability is fair [45,46] and even better than MP2 method in some case [47]. Having the optimized structure, the frequency independent (static) dipole polarizabilities ð~ aÞ, and first order hyperpolarizabilities (b) are calculated with B3LYP functional using the finite field method [48–51]. The calculations of the hyperpolarizabilities require flexible and adequately polarized and diffuse added basis set. Davidson and Feller [52] states that the choice of suitable basis sets is a key element in molecular property calculations and it depends on the system [53–55]. The computations in this work is performed by using the flexible basis set such as aug-cc-pVTZ basis set, which is of reasonable size and is well tested for the electric property calculations by Maroulis [56]. The Dunning basis set is more suitable for the hyper polarizability calculations than the Pople basis set [57]. Using the TD-DFT (TD-B3LYP/aug-cc-pVTZ) framework the ground state absorption energies for B3LYP optimized geometries were obtained. All the calculations were carried out using Gaussian 03 package [58]. The basis set superposition error (BSSE) corrected interaction energies of the optimized geometries were calculated using the methods of Boys and Bernardi [59]. Further, to overcome the slow convergence of the correlation energy, the interaction energies were computed using complete basis set extrapolation (CBS) at MP2 level. The HF energy of the system were calculated using the scheme of Karton and Martin [60]

pffiffiffiffi EHF X ¼ A þ BðX þ 1Þ expð9 X Þ The CBS extrapolation computations are performed with MOLPRO [61] software. In addition to the above, the energy decomposition analysis is carried using the symmetry adapted perturbation theory (SAPT) calculations with the SAPT2002 program implemented in MOLPRO software [61]. Light scattering cross-section and depolarization are related to the anisotropy of the electronic polarization of a molecular system. In the elastic Rayleigh scattering, the degree of depolarization (r) of the incident light scattered at right angles to the direction of incidence for natural, plane-polarized, circularly polarized light and Rayleigh activity (R) can be expressed in terms of the mean polarizability and polarizability anisotropy [41,62– 65] as follows:

rn ¼

6ðDaÞ2  Þ2 þ 7ðDaÞ2 45ða

 Þ2 þ 13ðDaÞ2 Rn ¼ 45ða

rp ¼

3ðDaÞ2  Þ2 þ 4ðDaÞ2 45ða

 Þ2 þ 7ðDaÞ2 Rp? ¼ 45ða

Rpll ¼ 6ðDaÞ2

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where Rn is the Rayleigh activity for the natural light, while Rp\ and Rpll are the polarized light with polarization being perpendicular and parallel to the scattering plane, respectively. The elastic Rayleigh scattering cross section and depolarization, related to the anisotropy of the dipole polarization (Da) were calculated using DFT method. Results and discussion Structure and hydrogen bond Various possible orientations of formic acid dimers with cis and trans forms were considered, and only the minimum energy orientations are reported. In all, we found four trans–trans, five trans– cis and one cis–cis formic acid dimer forms, each substituted with a halogen atom (X = F, Y = Cl) at carbonyl and hydroxyl position. Fig. 2 shows the optimized structures of FA dimer obtained using

TC3

TC4

TC5

CC1 Fig. 2 (continued)

TT1

TT3

TC1

TT2

TT4

TC2

Fig. 2. Halogen substituted formic acid dimers (ZCOOH)2 with cis and trans forms (Z = F, Cl). The value in the parentheses indicates the bond length for chlorine substituted formic acid.

the MP2 method with aug-cc-pVTZ basis set. The structure has been labelled with general notation XTT, XTC, XCC, YTT, YTC, and YCC, where X, Y indicates F, Cl respectively, and TT, TC, CC represent trans–trans, trans–cis and cis–cis orientation respectively. Fig. 2 shows the two centre and three centre hydrogen bonds using the dotted lines. These geometric interatomic descriptions are based on the criteria, (i) distance between acceptor (A) and donor (D) is less than the sum of their van der Waal radii (ii) H-bond is a distance of less than 4.0 Å between D and A with a minimum DAHAA angle of 90° [66,67]. Table 1 presents the computed structural parameters viz., hydrogen bond length and bond angles. From the structural parameters given in Table 1, we see that either one or two hydrogen bonds bind the dimers. In the structures with two hydrogen bonds either both the hydrogen bonds are conventional (OAH  O, OAH  F, and OAH  Cl) or one of them is non-conventional (CAH  O, CAH  F, and CAH  Cl) bond. The trans–trans form TT1, TT3 and trans–cis form TC2 are cyclic structures with two OAH  O hydrogen bonds. The structure TT2, TC1 and TC3 are also cyclic structure, but with one conventional OAH  O and one non-conventional [either CAH  F or CAH  Cl] hydrogen bond. The dimer TT4, TC5 and CC1 are with one and two conventional bond respectively. The dimer TC4 has two weak non-conventional (CAH  O, CAH  F or CAH  Cl) hydrogen bonds. In all the trans–trans dimers carbonyl oxygen atom acts as electron donor as well as proton acceptor in all OAH  O bonds. For trans–cis dimers TC1 and TC3, in OAH  O hydrogen bond carbonyl oxygen acts as an electron donor and hydrogen attached to the carbonyl oxygen as electron acceptor. In the dimer TC2 both carbonyl oxygen and carbonyl hydrogen acts as an electron donor and acceptor which results in two conventional OAH  O hydrogen bonds. Further, in the CAH  O hydrogen bond present in all trans– trans dimers, carbonyl oxygen atom acts as a donor but carbonyl hydrogen acts as electron acceptor

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Table 1 Optimized structural parameters, BSSE corrected interaction energy and CBS extrapolate interaction energy with and without counterpoise correction for halogen substituted formic acid dimers, where X = F, Y = Cl calculated at MP2/aug-cc-pVTZ level. Structure

Bond

Bond length (Å)

Bond angle (°)

Interaction energy (kcal/mol)

DEMP2 CBS (kcal/mol)

DEMP2 CBS-CP (kcal/mol)

XTT1

OAH  O OAH  O OAH  O CAH  O OAH  O OAH  O OAH  F

1.641 1.640 1.751 2.296 1.864 1.953 2.046

176.946 176.869 173.701 121.860 147.479 136.354 161.103

19.39 (17.79)a 10.38

21.31

19.70

11.60

10.33

9.41

8.35

5.61

4.65

OAH  O CAH  O OAH  O OAH  O OAH  O CAH  F CAH  F

1.758 2.320 1.831 1.949 1.765 2.408 2.756

173.553 120.756 146.632 138.037 179.398 112.520 126.272

9.85

11.05

9.82

8.83

10.19

9.09

8.30

9.45

8.25

0.94

3.28

2.23

OAH  F OAH  O OAH  F OAH  F OAH  O OAH  O OAH  O CAH  O OAH  O OAH  O OAH  Cl OAH  O OAH  O CAH  O OAH  O OAH  O OAH  O CAH  Cl CAH  Cl

1.990 2.874 1.890 1.889 1.656 1.656 1.739 2.328 1.937 1.889 2.447 2.724 1.740 2.320 1.952 1.833 1.741 2.822 3.160

169.277 122.207 176.026 176.028 178.437 178.424 176.114 122.511 149.670 134.753 173.665 122.297 176.004 122.437 148.075 138.266 168.715 115.016 127.658

1.01

4.36

3.32

1.26

6.43

5.12

19.51

17.69

11.51

10.20

7.85

9.20

8.02

1.23

7.41

4.62

10.06

12.46

10.02

8.43

10.01

8.65

8.88

10.16

8.82

1.02

4.28

3.00

OAH  Cl OAH  O OAH  Cl OAH  Cl

2.368 2.724 2.258 2.259

172.665 122.297 170.016 170.032

1.24

5.40

3.86

1.60

6.96

5.03

XTT2 XTT3 XTT4 XTC1 XTC2 XTC3 XTC4 XTC5 XCC1 YTT1 YTT2 YTT3 YTT4 YTC1 YTC2 YTC3 YTC4 YTC5 YCC1 a

8.13 (8.51) 0.89

17.44 (16.71) 10.21

Taken from the reference data [71].

The conventional hydrogen bond OAH  O is the strongest among all the hydrogen bonds in all dimers, with the bond length value in the range 1.640–2.874 Å. The complex XTT1 has the strong OAH  O hydrogen bond (1.640 Å, 1.641 Å). The YTC2 dimer possesses weak OAH  O hydrogen bond (values 1.952 and 1.833 Å). The OAH  F and OAH  Cl are the second most strong bond with bond length value in the range 1.889–2.447 Å. These bonds are formed in dimers XCC1, XTT4, YTT4, YCC1. The non-conventional CAH  O hydrogen bond lengths from all dimers are in the range 2.296–2.328 Å and are comparatively weaker than OAH  F, OAH  Cl in bond strength but are stronger than the CAH  F and CAH  Cl hydrogen bonds. The hydrogen bonds CAH  F and CAH  Cl have the bond length value of 2.408, 2.756 Å (XTC3, XTC4) and 2.822, 3.160 Å (YTC3, YTC4) for F and Cl substitution respectively. Comparison between CAH  F and CAH  Cl hydrogen bonds show that Cl substitution results in the weak hydrogen bond. The above results for the bond lengths ascertain that conventional hydrogen bonds are stronger than non-conventional hydrogen bonds, which earlier studies report as well [68–71]. Further, on comparing the OAH bond length in OAH  O hydrogen bonds in XTT1 and YTT1 dimer with that of the monomer, the elongation of OAH bond is by an amount 0.029 and 0.027 Å respectively. In YTC2 dimer, which possess a weak OAH  O hydrogen bond (values 1.952 and 1.833 Å), the OAH bond length elongation from its corresponding monomer is marginal, in the order 0.001– 0.004 Å. In all complexes the CAH bond length in CAH  O

hydrogen bond contracts when compared with corresponding monomer bond from 0.0019 to 0.0027 Å. Similarly, the CAH bond length in CAH  F and CAH  Cl bond has also contracted by about 0.0005–0.0058 Å. The bond angle for conventional hydrogen bond in all complexes is almost linear and ranges between 136.354° and 179.398°. The strong hydrogen bond OAH  O (1.640 Å, 1.641 Å) found in the complex XTT1 has the linear bond angle 176.946° and 176.869°. Similarly, in the complex XTC3, the bond angle is 179.398°, which is almost linear with the bond length of 1.765 Å. This shows that linearity in bond augments strong overlap of orbitals, thereby increase bond strength. The non-conventional hydrogen bond angles are in the range of 120°, which indicates that hydrogen bonds are distorted and nonlinear. Table 1 presents the interaction energies calculated using the procedure developed by Boys and Bernardi [59] and by the complete basis set limit by Karton and Martin [60]. From the interaction energy values given in Table 1, in general, it is apparent that trans–trans complexes are more stable than the trans–cis, cis–cis form. The counter poise corrected interaction energy indicates that fluorine substituted FA dimer XTT1 is the most stable structure with the interaction energy value 19.39 kcal/mol. The high stability of dimer XTT1 is due to the strong hydrogen bond [two conventional OAH  O] and the linear bond angle. The second most stable structure is XTT2, with interaction energy value 9.01 kcal/mol lesser than XTT1 structure. This XTT2 structure consists of one conventional and non-conventional hydrogen bond. The XTT4

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structure is the least stable form with the interaction energy value 0.89 kcal/mol. The XTT4 structure has only one conventional (OAH  F) hydrogen bond present in the structure. In chlorine substituted FA dimer, YTT1 structure is the most stable form having the interaction energy value 17.44 kcal/mol. This cyclic structure consists of two conventional hydrogen bonds. The YTC4 dimer is the least stable form with low interaction energy value 1.02 kcal/mol. This is because of the presence of two weak nonconventional hydrogen bonds (CAH  Cl and CAH  O). The cis– cis form (XCC1 and YCC1) in both fluorine and chlorine form are not very stable either. Thus, trans–trans form is more stable among all the forms in both fluorine and chlorine substituted structure followed by trans–cis forms. On comparing fluorine and chlorine, fluorine forms are slightly more stable than chlorine forms. The overall stability order is as follows

XTT1 > YTT1 > XTT2 > YTT2 > YTC1 > XTC1 > YTC3 > XTC2 > YTC2 > XTC3 > XTT3 > YTT3 > YCC1 > XCC1 > YTC5 > YTT4 > YTC4 > XTC5 > XTC4 > XTT4 Further, for all the above systems Table 1 shows the interaction energy with and without counterpoise correction (CP) calculated using CBS method with MP2 geometry with aug-cc-pVTZ and aug-cc-pVDZ basis sets. From the results we see that the CP corrected CBS interaction energy values are similar to BSSE corrected interaction energy. However, we see deviations in interaction energy values for some structures, but well below 4 kcal/mol. On the other hand, the CBS interaction energy without CP correction shows deviation in the interaction energy value to a maximum of 6 kcal/mol. From the above observations, we conclude that BSSE corrected interaction energies from MP2 method are on par with CP corrected CBS interaction energy values. In addition, the overall stability order obtained from CBS method is same for first eleven structures as that of BSSE interaction energy order, except for few changes later on, the order obtained is

XTT1 > YTT1 > XTT2 > YTT2 > YTC1 > XTC1 > XTC2 > YTC3 > YTC2 > XTT3 > XTC3 > YTT3 > XCC1 > YCC1 > XTT4 > YTT4 > YTC5 > XTC5 > YTC4 > XTC4 Subsequently, from the results described above, it is evident that the trans - trans orientation form stronger dimers than the trans– cis or cis–cis. Earlier studies by Marushkevich et al. [72] and Senthilkumar et al. [73] also confirm that the trans–trans dimers interact strongly. SAPT analysis SAPT method is a practical tool for investigating the nature of the intermolecular interactions. Based on DFT-SAPT energy decomposition scheme [74], the two-body binding energy is decomposed as:

ESAPT ¼ E1pol þ E1ex þ E2ex-ind þ E2ind þ E2ex-disp þ E2disp int Some of these terms combine in order to define values that correspond to physical quantities.

Eelec ¼ E1pol Eexch ¼ E1ex Eind ¼ E2ind þ E2ex-ind Edisp ¼ E2disp þ E2ex-disp The terms commonly combined are shown above, where Eelec is the first-order electrostatic term describing the classical columbic

interaction of the occupied orbitals of one monomer with those of another monomer, Eexch is the repulsive first-order exchange component resulting from the anti symmetrization (symmetry adaption) of wave function, Eind and Edisp correspond to induction and dispersion effects respectively. The induction component is the energy of interaction of the permanent multipole moments of one monomer and the induced multipole moments on the other, whereas the dispersion part comes from the correlation of electron motions on one monomer with those on the other monomer. For the DFT-SAPT calculations were performed with B3LYP functional, using MP2/aug-cc-pVTZ optimized geometries. Table 2 shows the DFT-SAPT decomposition analysis of the interaction energy (IE) in halogen substituted formic acid dimer. The trans– trans (XTT1, YTT1) formic acid dimer forms have high interaction energy values due to strong conventional (OAH  O) hydrogen bonds. The second most stable form is trans–trans form (XTT2, YTT2) having one conventional and one non-conventional hydrogen bonds (OAH  O and CAH  O), the difference in the interaction energy from most stable form is 2.1, 1.0 kcal/mol for F and Cl terminal substitution respectively. The TT dimers are followed by the trans–cis (TC1 and TC3) formic acid dimer form in the stability order with an energy difference of 2.6, 3.7 kcal/mol for F and 1.3, 2.3 kcal/mol for Cl substitutions respectively. The cis–cis dimeric form of formic acid with two hydrogen bonds involving halogens each (OAH  F and OAH  Cl), is found to have least interaction energy for Cl substitution (YCC1), while for F substitution (XCC1) it is second least stable form. From the decomposed interaction energy components, we notice a clear dominance of electrostatic effect in most complexes, which accounts for the attraction in all dimers. The most stable dimers (XTT1, YTT1) have a large contribution from electrostatic energy and induction energy. In contrast, the least stable dimer YCC1 has the largest contribution from the induction energy. However, in all other dimers the contribution of dispersion energy along with electrostatic energy is slightly larger than induction energy. The exchange energy that accounts for the repulsion due to Pauli’s exclusion principle and antisymmetry of wave function increases with a decrease in the monomers distance. In both F and Cl, the exchange energy magnitudes are similar; this is because in most dimers carbonyl group has been the proton acceptor. Similarly, the proton donor, OAH, CAF, CACl moiety also cause small variation in exchange energies values. The stability order

Table 2 SAPT (B3LYP/aug-cc-pVTZ) interaction energy (DE), and their decompositions such as electrostatic (Eelec), exchange (Eexch), Induction (Eind), dispersion (Edisp) energies in kcal/mol, for the halogen substituted formic acid dimers. Structure

Eelec

Eexch

Eind

Edisp

DE

XTT1 XTT2 XTT3 XTT4 XTC1 XTC2 XTC3 XTC4 XTC5 XCC1 YTT1 YTT2 YTT3 YTT4 YTC1 YTC2 YTC3 YTC4 YTC5 YCC1

32.55 16.60 14.74 5.60 15.29 16.02 12.82 3.16 4.37 7.32 30.37 16.63 14.48 4.61 15.92 15.35 13.55 5.16 4.61 7.22

43.15 19.58 19.76 6.26 18.65 21.24 15.96 5.25 6.86 11.98 42.14 20.31 19.93 9.53 19.95 21.27 17.79 9.73 9.53 16.73

10.80 4.32 4.06 1.21 4.18 4.42 3.55 0.32 3.43 2.82 10.20 4.30 3.91 6.22 4.35 4.39 3.82 0.60 6.20 11.74

7.46 4.67 4.52 2.86 4.60 4.69 3.99 3.01 3.59 3.64 7.65 4.88 4.76 5.49 4.92 5.01 4.64 4.07 5.49 6.75

8.44 6.39 3.94 3.56 5.80 4.29 4.70 1.38 1.84 2.01 6.96 5.94 3.65 1.36 5.69 3.94 4.62 0.38 1.36 0.06

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for the fluorine and chlorine remains same except for the last two terms which are interchanged. The order is as follows for fluorine followed by chlorine,

XTT1 > XTT2 > XTC1 > XTC3 > XTC2 > XTT3 > XTT4 > XTC5 > XCC1 > XTC4 YTT1 > YTT2 > YTC1 > YTC3 > YTC2 > YTT3 > YTT4 > YTC5 > YTC4 > YCC1 Figs. 3a and 3b presents the variation of various components of interaction energy for F and Cl substituted formic acid dimers. Polarizability (a) The dipole polarizability tensor gives information about the response of the molecular dipole moment to an external electric field. It is a symmetric tensor with six independent components, diagonalized and represented in terms of three components aii (i = x, y, z) called the principal values. In theories of opto-electronics and intermolecular forces, certain quantities of experimental interest turn out to be associated with various combinations of these principal components. In particular, the mean polarizability

Fig. 3b. DFT-SAPT graph for chlorine substituted formic acid dimer, in X-axis legend 1–10 represents the structure names such as YTT1, YTT2, YTT3, YTT4, YTC1, YTC2, YTC3, YTC4, YTC5, YCC1.

Table 3 Calculate dipole moment (l) in Debye, mean (a) and anisotropy (Da) of the static dipole polarizabilities, hyperpolarizability (b) in a.u. and Band gap (Egap) in eV at B3LYP level of theory using aug-cc-pVTZ basis set.

1 3

a ¼ ðaxx þ ayy þ azz Þ where axx, ayy and azz are three components of dipole polarizabilities respectively. The polarizability anisotropy Da is given by

ðDaÞ2 ¼

i 1h ðaxx  ayy Þ2 þ ðayy  azz Þ2 þ ðazz  axx Þ2 2 h i þ 3 ðaxy Þ2 þ ðaxz Þ2 þ ðayz Þ2

which are invariant with respect to rotation. Table 3 shows the computed values of polarizability (a). In addition, Table S1 in the Supporting information provides the components of polarizability along x, y and z direction. The halogen substituted FA dimer exhibit C1 symmetry and hence the axx component has the largest value in comparison with ayy and azz components. This indicates a major contribution to electron redistribution is along the length of the molecule. Hence, the polarizability of the studied complexes is predominately along the x-direction. The polarizability (a) value is directly proportional to the transition dipole moment and is inversely proportional to the excitation energy. Among the trans–trans FA dimer structure with fluorine substitution, XTT2 structure has

a b c

Fig. 3a. DFT-SAPT graph for fluorine substituted formic acid dimer, in X-axis legend 1–10 represents the structure names such as XTT1, XTT2, XTT3, XTT4, XTC1, XTC2, XTC3, XTC4, XTC5, XCC1.

Structure

l

a

Da

b

Egap (eV)

XTT1

0.005

24.49 (23.31)b

0.12

8.90 (7.49)b

XTT2 XTT3 XTT4 XTC1

1.74 2.15 2.35 1.67

6.00 8.57 8.36 5.90

1.75 3.98

29.38 24.43 9.28 31.01 (27.54)b 25.15 31.60

63.87 9.19 22.59 33.46

XTC2 XTC3 XTC4 XTC5 XCC1 YTT1 YTT2 YTT3 YTT4 YTC1 YTC2 YTC3 YTC4 YTC5 YCC1

45.79 (45.11)b (45.40)c 49.00 46.31 45.23 48.97 (34.36)b 46.44 49.33

34.90 34.03

8.62 5.82

a

a

a

a

a

4.01 0.04 0.004 2.64 2.06 1.94 2.75 1.75 4.85

46.01 46.86 75.51 77.03 75.03 73.67 77.09 75.05 77.17

18.57 28.69 55.47 55.80 43.63 21.59 53.71 41.91 46.18

1.21 0.01 0.017 176.50 64.64 41.84 104.36 1.88 170.98

8.03 8.80 7.64 5.09 7.54 7.57 4.94 7.58 5.03

a

a

a

a

a

2.90 0.72

73.96 76.22

20.76 41.10

2.13 36.31

7.44 7.92

Data not available. The formic acid dimer without substitution of halogen B3LYP/aug-cc-pVTZ. Data for the formic acid dimer using MP2/aug-cc-pVDZ from Ref. [41].

the highest polarizability value 49.00 a.u. and its corresponding excitation energy is low 5.32 eV (given in Table 4 obtained from TD-DFT study). The reason is the non-symmetric arrangement of fluorine atoms in XTT2 structure. The XTT4 structure has low polarizability value (45.23 a.u.) even though the dipole moment is higher than other trans–trans forms. In trans–cis form, XTC3 structure has the larger polarizability (49.33 a.u.) with high dipole moment and low excitation energy value (5.26 eV). The cis–cis XCC1 form has higher polarizability than XTT1 with high excitation energy (9.08 eV) and low dipole moment. Comparing trans–trans, trans– cis and cis–cis form, the trans–cis form has the maximum polarizability due to the structural arrangements of the molecules. The polarizability values of halogen substituted FA dimers are slightly more than the unsubstituted formic acid dimer. In the case of chlorine substitution, trans–trans dimer YTT2 has high polarizability value 77.03 a.u. From the TD-DFT study (values given in Table 4), YTT2 structure has the least excitation energy of

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Table 4 Calculated absorption energies (Ecal, kcal), dominant orbital excitations along with weight percentage, oscillator strength (f) and transition dipole moment (le) of the halogen substituted formic acid dimer from TD-B3LYP calculation using aug-cc-pVTZ basis set.

a

Structure

State

Excitation

Ecal (eV)

kcal (nm)

Character

f (a.u.)

(le) (Debye)

XTT1 XTT2 XTT3 XTT4 XTC1 XTC2 XTC3 XTC4 XTC5 XCC1 YTT1 YTT2 YTT3 YTT4 YTC1 YTC2 YTC3 YTC4 YTC5 YCC1

S6 S2 S5 S4 S3 S5 S2

HOMO1 ? LUMO+1 (+93%) HOMO ? LUMO (+70%) HOMO1 ? LUMO (+96%) HOMO1 ? LUMO (+92%) HOMO2 ? LUMO (+78%) HOMO1 ? LUMO+1 (+93%) HOMO ? LUMO (+82%)

8.52 5.32 8.21 7.72 5.50 8.33 5.26

145.4 233.3 151.0 160.6 225.3 148.9 235.5

p ? r r ? r p ? r p ? r r ? r p ? r p ? p

0.0793 0.0228 0.0416 0.0231 0.0140 0.0274 0.0065

0.3796 0.1753 0.2066 0.1219 0.1039 0.1344 0.0507

7.73 9.08 7.62 4.60 7.45 6.94 6.99 7.59 5.36

160.5 136.6 162.8 269.6 166.3 178.5 177.4 163.5 231.3

n ? r n ? p r ? r p ? p r ? n r ? r p ? p r ? r r ? r

0.0245 0.1147 0.2274 0.0204 0.0209 0.0281 0.0215 0.1069 0.0206

0.1294 0.5157 1.2184 0.1812 0.1147 0.1650 0.1253 0.5751 0.1572

7.03 6.99

176.3 177.5

p ? r p ? r

0.0255 0.0486

0.1479 0.2839

a

S4 S9 S10 S2 S8 S4 S5 S10 S6

HOMO2 ? LUMO (+91%) HOMO2 ? LUMO+1 (+49%) HOMO1 ? LUMO+2 (+36%) HOMO ? LUMO (+95%) HOMO2 ? LUMO+1 (+32%) HOMO1 ? LUMO (+49%) HOMO ? LUMO+1 (+89%) HOMO4 ? LUMO (+49%) HOMO2 ? LUMO (+52%) a

S5 S4

HOMO2 ? LUMO+1 (+43%) HOMO1 ? LUMO (+48%)

Data not available.

4.60 eV and corresponding transition dipole moments is also low, hence has large polarizability. Trans–cis form, YTC3 structure has the highest polarizability value 77.17 a.u. due to low the excitation energy 5.36 eV. The dipole moment of the YTC3 is 4.85 Debye and it is higher than other trans–cis form structure. The cis–cis form has polarizability comparable with trans–trans and trans–cis form with the low dipole moment. Comparing the three forms, trans–cis form has the highest polarizability value due to the structural arrangements. As we know polarizability is directly proportional to transition moment, but in the present study from values of le in Table 4 no particular trend was observed both between le and a or its components given in Tables 3 and S1. Overall, the chlorine substituted FA dimer has the highest polarizability value when compared to the fluorine substituted FA dimer. The dipole moment of the chlorine substituted FA dimer is greater than the fluorine substituted. In both halogen substituted FA dimer, trans–cis form has the highest polarizability value. More specifically, the TC3 form has the highest polarizability value for both F, Cl substitution. In both trans–trans and cis–cis form, the presence of halogen atom in hydrogen bond increases the polarizability value. To conclude, both fluorine and chlorine atom substitution have improved the polarizability of formic acid dimer than the unsubstituted formic acid dimer. The anisotropy of the static dipole polarizability (Da) shown in Table 3 indicates that values of chlorine substituted dimers have large anisotropy than fluorine forms. Hyperpolarizability (b) The significance of the first order hyperpolarizability of molecular systems is dependent on the efficiency of electronic communication between the donor and the acceptor groups as that will be the key to intra molecular charge transfer [75]. The mean b value can be calculated by following the procedure adopted by Maroulis [76] wherein for the lower symmetries a mean hyperpolarizability can be defined along the direction of the permanent dipole moment. The unit vector in the direction of the dipole moment is lx/l, ly/l, and lz/l. The first hyperpolarizability along the direction of the dipole moment for the formic acid dimer is

b¼

  3 bx lx þ by ly þ bz lz 5 l

where

bx ¼ bxxx þ bxyy þ bxzz by ¼ byyy þ bxxy þ byzz bz ¼ bxxz þ byyz þ bzzz Tables 3 and S2 (in Supporting information) present the calculated hyperpolarizability values and its components along different direction respectively. The general observation of the b components shows major charge transfer to occur along x-direction, but in a few cases, y-direction is also noted. Among the fluorine substituted FA dimer, trans–trans XTT2 structure has the highest hyperpolarizability value 63.87 a.u. This is because the structure XTT2 is not symmetric and the fluorine atoms present at the ends are oriented normal to each other. This kind of structural arrangement might have led to the larger dipole moment and high hyperpolarizability value. Also from TD-DFT calculations, the higher hyperpolarizability of XTT2 is due to the minimum excitation energy of 5.32 eV, this also come from the fact that the excitation energy (Ecal) is inversely proportional to the hyperpolarizability [8]. Further, the TD-DFT result shows that the electron transition in XTT2 complex mainly arises from HOMO to LUMO with the oscillator strength 0.02 a.u. In addition the above, the low-lying HOMO–LUMO energy gap (6.00 eV) is also responsible for the higher hyperpolarizability value of XTT2. On the contrary, the XTT1 structure has the lowest b value (0.12 a.u.) with maximum excitation energy of 8.52 eV. The XTT1 structure is also symmetric and the HOMO–LUMO gap is high of about 8.90 eV. This may be the reason for such a low b value, because b and Egap are inversely proportional. In trans–cis complex, XTC2 structure possesses the highest b value. The oscillator strength and the corresponding transition dipole moment are higher than other structure in the trans–cis form and the corresponding values are 0.0274 a.u. and 0.1344 Debye respectively. The lowest b (1.21 a.u.) value among trans–cis is observed for dimer XTC5, the corresponding excitation energy and dipole moment is 7.73 eV and 4.01 Debye respectively. The cis–cis (XCC1) form possesses the low b value similar to XTT1 structure, with high excitation energy of 9.08 eV and the band gap value of 8.80 eV respectively. On comparing the trans–trans, trans– cis and cis–cis form, trans–trans form exhibits good NLO property.

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In chlorine substitution, the trans–trans, FA dimer YTT2 structure has the highest b value 176.50 a.u. The excitation energy (4.60 eV) and band gap (5.09 eV) is minimum for the YTT2 complex. In contrast, the YTT1 structure has the least b value 0.017 a.u.; the corresponding excitation energy is higher than the other trans–trans complex. The structure YTT1 is symmetric and cyclic; hence, its hyperpolarizability value is almost zero. Among trans–cis forms, the structure YTC3 has the highest b value (170.98 a.u.) due to the low excitation and HOMO–LUMO gap energy of 5.36 eV and 5.03 eV respectively. The stacked structure YTC5 has low b value 96.63 a.u., due to the high excitation and HOMO–LUMO gap energy of 7.03 eV and 7.44 eV respectively.

Unlike fluorine cis–cis form here in the case of chlorine substitution, YCC1 structure has higher hyperpolarizability value 60.55 a.u. On comparing trans–trans, trans–cis and cis–cis form, trans–cis with chlorine substitution has the highest NLO property. In summary, the chlorine substituted FA dimer has the high NLO property than fluorine substituted. Our results follow the previous studies by Maroulis et al. [77,78] on the halogen substituted organic molecules, which shows the hyperpolarizabilities increases from fluorine to chlorine. Both fluorine and chlorine substitution in FA dimer have enhanced NLO property than the normal formic acid dimer, which is shown in Table 3. The cyclic complexes, which consist of two (OAH  O) and (OAH  F) hydrogen bonds, have low

HOMO

HOMO-1

HOMO

LUMO

XTT1

LUMO+1

XTT2

HOMO

HOMO-1

LUMO

LUMO

XTC2

LUMO+1

Fig. 4a. HOMO, LUMO and dominant transition orbitals of the highest hyperpolarizability for the fluorine substituted formic acid dimers. Violet colour represents the positive lobe and blue colour represents the negative lobe in HOMO. In LUMO positive and negative lobes are indicated as orange and green colour. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

P. Umadevi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 821–832

hyperpolarizability values since the net dipole moment is almost zero. Finally, we observe that large anisotropy does not correspond to maximum hyperpolarizability in the present study. HOMO LUMO The Frontier molecular orbitals play an important role in the chemical reactions, UV–Vis spectra, electrical and optical properties [79]. Figs. 4a and 4b show the molecular orbitals such as HOMO, LUMO along with the dominant transitions for the XTT1, XTT2, XTC2, YTT1, YTT2, and YTC3 structures, which have high and low polarizability value, obtained from B3LYP level of theory.

Fig. S1 in the Supporting information provides the HOMO–LUMO plots for the remaining structures. The violet and blue colours represent HOMO with contour value of 0.04. The violet and blue colours indicate positive lobe and negative lobe respectively. The LUMO is in red and green colour, where red and green are positive lobe and negative lobe respectively. The structure XTT1 and YTT1 has low polarizability value in trans–trans form because the HOMO, HOMO1 and LUMO, LUMO+2 distributions on the donor and acceptors is even and symmetric. Therefore, the application of electric field will cause the charge distribution to be equal and opposite, resulting in the cancellation of the dipole moment, leading to low a and b values. On

HOMO

HOMO-1

829

LUMO

YTT1

HOMO

LUMO+2

YTT2

HOMO

LUMO

LUMO

HOMO-1

YTC3

Fig. 4b. HOMO, LUMO and dominant transition orbitals of the highest hyperpolarizability for the chlorine substituted formic acid dimers. Violet colour represents the positive lobe and blue colour represents the negative lobe in HOMO. In LUMO positive and negative lobes are indicated as orange and green colour. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 5 Depolarization ratios (rn, rp, rc) and Rayleigh intensities (Rn, Rp\, Rpll) of the halogen substituted formic acid dimers for the incidence of natural light (n) as well as plane-polarized (p) and circularly polarized (c) light in a.u., calculated at B3LYP level of theory by using aug-cc-pVTZ basis set. Structure XTT1 XTT2 XTT3 XTT4 XTC1 XTC2 XTC3 XTC4 XTC5 XCC1 YTT1 YTT2 YTT3 YTT4 YTC1 YTC2 YTC3 YTC4 YTC5 YCC1 a b

rn 0.0365 (0.036) 0.0454 0.0355 0.0062 0.0504 0.0374 0.0514

b

rp

rc

Rn (105)

Rp\ (105)

Rpll (105)

0.0186 (0.018) 0.0232 0.0181 0.0031 0.0258 0.0190 0.0264

0.0379 (0.037) 0.0475 0.0368 0.0062 0.0531 0.0388 0.0542

1.0217 (1.002) 1.1929 1.0430 0.9334 1.2047 1.0531 1.2252

0.9857 (0.9677) 1.1411 1.0072 0.9276 1.1468 1.0151 1.1652

0.0359 (0.0345) 0.0518 0.0358 0.0057 0.0578 0.0379 0.0599

a

a

a

a

a

a

0.0211 0.0472 0.0662 0.0646 0.0428 0.0113 0.0602 0.0396 0.0452

0.0107 0.0242 0.0343 0.0334 0.0218 0.0056 0.0310 0.0202 0.0231

0.0210 0.0490 0.0710 0.0692 0.0447 0.0114 0.0640 0.0413 0.0473

0.9977 1.0952 2.9700 3.0747 2.7811 2.5032 3.0493 2.7635 2.9573

0.9770 1.0458 2.7853 2.8879 2.6668 2.4752 2.8762 2.6581 2.8293

0.0206 0.0493 0.1846 0.1868 0.1142 0.0279 0.1731 0.1054 0.1280

a

a

a

a

a

a

0.0103 0.0371

0.0052 0.0188

0.0105 0.0385

2.5177 2.8340

2.4918 2.7326

0.0258 0.1013

Data not available. Data for the formic acid dimer using MP2/aug-cc-pVDZ from Ref. [41].

the contrary, HOMO and LUMO is localized in XTT2 and YTT2 structures, on either of the formic acid monomers. Hence, the charge distribution is uneven, which has enhanced the both a and b value. In XTC2 structure which has the high a and b with large band gap, the HOMO is majorly localized in the cis form while the LUMO is localized on the trans part of the structure. In contrast, the HOMO1 is majorly localized on the trans part of the molecule while the LUMO+1 is meagrely distributed. The structure YTC3 predicted to have the highest a and b value among all substituted dimers has HOMO, HOMO1, and LUMO localized on the cis and the trans part of FA respectively. Further, in YTC3 the distribution of HOMO on the cis is more dominant than the LUMO distribution on the trans part. Hence the band gap between HOMO to LUMO is minimum (5.03 eV) for this structure, which has augmented the highest a and b value of 77.17 a.u. and 349.1 a.u. respectively. Table 4 TD-DFT calculations present the HOMO–LUMO transitions characters for all the dimers. For the dimers with high a and b values (XTT2, YTT2 and XTC3), the transitions are found to occur between p ? p (for YTT2, localized on C@O bonds on either monomers) and r ? r (for XTT2, YTC3, localized on CAF, CACl bonds) orbitals respectively. In the case of structures with low a and b values (XCC1 and YTT1), n ? p (for XCC1, localized on C@O bonds on either of the monomer) and r ? r (for YTT1, localized on CACl bond) orbital transitions are observed. In general, uneven distribution of HOMO–LUMO along with minimum energy gap augments high a and b values. Rayleigh scattering The molecular scattering of light provides a great spread of information about the details of phenomena observed in the interaction of radiation with matter [80–83]. The scattering of light off molecules occurs in two ways, elastic and inelastic. The elastic scattering of light or otherwise called Rayleigh scattering is a classic effect caused by optical inhomogeneity in the medium. The Rayleigh scattering technique is a useful tool to study the changes in the internal hydrogen bond strength for cis–trans photo isomerisation process [83]. Further, the technique is useful for obtaining the average size of clusters formed and molecular species concentration in molecular beams under different expansion conditions

[83–85]. Thus, for molecules the state of polarization of the scattered light relates to the molecular anisotropy [81] and the Rayleigh scattering properties along with both the dipole polarizability and the polarizability anisotropy [86]. In Table 5, the Rayleigh scattering parameters of the halogen substituted FA dimer for natural, plane and circular polarized light calculated using B3LYP method are given. The depolarization ratio value (given in Table 5) for circularly polarized light is higher in magnitude followed by natural and plane polarized light. The depolarization ratio values increase in most of the halogen substituted FA dimers when compared with unsubstituted formic acid dimer. The reason for such increase in depolarization values among substituted dimers attributes to hydrogen bond as well as halogen atoms. Earlier studies [39] have shown that hydrogen bonds increase the depolarization ratios. The increase in depolarization ratio values for chlorine substitution is larger than fluorine. In dimer YTT1 structure the depolarization, values are almost twice the formic acid dimer values. The YTC3 dimer, which has strong NLO due its b values, has higher depolarization ratio values for fluorine and chlorine substitutions, suggesting strong Rayleigh activity. The Rayleigh intensity values in Table 5 correlate with depolarization values of the dimers, that is the dimers with are large depolarization ratio values have large intensities. Here, once again, the dimers with chlorine substitution have large intensities when compared with fluorine dimers. Further from Table 5, it is evident that the hydrogen bond formed within the dimers has significant influence on the Rayleigh intensities for all components of light. In the complexes (both for X,Y) TT1, TT2, TT3, TC2, having an OAH  O bond, the intensities of natural light increases about 4%, while for the structure TT2, increases by 13%. The non-conventional hydrogen bond involving complexes such as TC1, TC4 and TC3 the scattering of natural light has increased by 18%. Similarly, for the CC1 dimer with halogen substitution, the natural light scattering has increased by 8% than normal FA. To summarize, the scattering intensities are high for natural, plane and circularly polarized light. Among the three forms of light scattering intensities, natural intensities are more dominant than the plane and circular polarized light. The halogen substitutions have increased the Rayleigh scattering intensities for all forms of lights when compared with normal formic acid dimer.

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Conclusion All the structures considered in this study were optimized using MP2 and B3LYP level of theory with aug-cc-pVTZ basis set. Both the BSSE corrected MP2 as well as CBS extrapolated interaction energies are similar and confirm that trans–trans form is more stable than the trans–cis, cis–cis for both fluorine and chlorine substitutions. The higher interaction energy in the trans–trans dimer form is due to the large contribution of electrostatic energy. The mean polarizability is large for chlorine substituted formic acid dimer. The introduction of halogens (F, Cl) in the formic acid has led to larger b values, thereby invoking strong second-order NLO response in formic acid dimers. In particular, the chlorine substitution in FA dimers (YTC3) has the higher NLO response than fluorine dimers. In addition, it is worth mentioning that the non-symmetrical geometrical arrangement plays a vital role in enhancing NLO response than the type of hydrogen bond. The HOMO and LUMO localized on either of the monomer with lower excitation energy has led to the higher hyperpolarizability. The complex, which involves the non-conventional hydrogen bond, has the higher degree of depolarization. The hydrogen bonding among the molecules increases the Rayleigh intensity for all the components of light significantly. Acknowledgement The authors are grateful for the use of computational resources HPCF (High Performance Computing Facility) at CMSD, University of Hyderabad, Hyderabad, India. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.05.080. References [1] N.P. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects in Molecules and Polymers, Wiley, New York, 1991. [2] L.R. Dalton, A.W. Harper, R. Ghosn, W.H. Steier, M. Ziari, H. Fetterman, Y. Shi, R.V. Mustacich, A.K.Y. Jen, K.J. Shea, Chem. Mater. 7 (1995) 1060–1081. [3] J. Zyss, Molecular Nonlinear Optics: Materials, Physics and Devices, Academic Press, Boston, 1994. [4] S. Di Bella, Chem. Soc. Rev. 30 (2001) 355–366. [5] D. Vitalini, P. Mineo, S.D. Bella, I. Fragala, P. Maravigna, E. Scamporrino, Macromolecules 29 (1996) 4478–4485. [6] G. Tang, Y. Cao, J. Zhang, Y. Zhang, Y. Song, F. Bei, L. Lu, C. Zang, Synth. Met. 158 (2008) 264–272. [7] Y. Bai, Z.J. Zau, J.J. Wang, Y. Li, D. Wu, W. Chen, Z. Ru li, C.C. Sun, J. Phys. Chem. A 117 (2013) 2835–2843. [8] M.R.S.A. Janjua, M.U. Khan, B. Bashir, M.A. Iqbal, Y. Song, S.A.R. Naqvi, Z.A. Khan, Comput. Theor. Chem. 994 (2012) 34–40. [9] G.J. Perpetuo, J. Janczak, J. Mol. Struct. 1031 (2013) 14–21. [10] G. Maroulis, Atoms, Molecules and Clusters in Electric Fields Theoretical Approaches to the Calculation of Electric Polarizability, vol. 1, Imperial College Press, London, 2006. [11] G. Maroulis, D. Begue, C. Pouchan, J. Chem. Phys. 119 (2003) 794–797. [12] K. Wu, J.G. Snijders, C. Lin, J. Phys. Chem. B 106 (2002) 8954–8958. [13] T.R. Cundari, H.A. Kurtz, T. Zhou, J. Chem. Inf. Comput. Sci. 41 (2001) 38–42. [14] A. Haskopoulos, G. Maroulis, Chem. Phys. Lett. 397 (2004) 253–257. [15] V. Keshari, S.P. Korna, N.P. Prasad, J. Phys. Chem. A 97 (1993) 3525–3529. [16] J.O. Morley, M.G. Hutchings, J. Zyss, I. Ledoux, J. Chem. Soc., Perkin. Trans. 2 (1997) 1139–1142. [17] T.J.J. Muller, J.P. Robert, E. Schmalzlin, K. Meerholz, Org. Lett. 2 (2000) 2419– 2422. [18] S.-S.P. Chou, G.T. Hsu, H.C. Lin, Tetrahedron Lett. 40 (1990) 2157–2160. [19] S.-S.P. Chou, D.J. Sun, H.C. Lin, P.K. Yang, Chem. Commun. 9 (1996) 1045–1046. [20] H.E. Katz, K.D. Singer, J.E. Sohn, C.W. Dirk, L.A. King, H.M. Gordon, J. Am. Chem. Soc. 10 (1987) 6561–6563. [21] B.R. Cho, N.K. Son, S.J. Lee, T.I. Kang, M.S. Han, S.J. Jeon, N.W. Song, D. Kim, Tetrahedron Lett. 39 (1998) 3167–3170. [22] K. Belfield, C. Chinna, K. Schafer, Tetrahedron Lett. 38 (1997) 6131–6134. [23] S. Sun, C. Zhang, L. Dalton, S. Garner, A. Chen, W. Steier, Chem. Mater. 8 (1996) 2539–2541.

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