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Spectrochimica Acta Part A 81 (2011) 72– 78

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

jou rn al hom epa ge: www.elsev ier .com/ locate /saa

robing the compoundE)-5-(diethylamino)-2-[(4-methylphenylimino)methyl]phenol mainly from theoint of tautomerism in solvent media and the solid state by experimental andomputational methods

¸ igdem Albayraka,∗, Gökhan Kas tas b, Mustafa Odabas ogluc, Rene Frankd

Faculty of Education, Department of Science Education, Sinop University, 57000 Sinop, TurkeyDepartment of Physics, Faculty of Arts and Sciences, Ondokuz Mayıs University, 55139 Kurupelit-Samsun, TurkeyChemical Technology Program, Pamukkale University, 20070 Kınıklı- Denizli, TurkeyFaculty of Chemistry and Mineralogy, Leipzig University, D-04103 Leipzig, Germany

r t i c l e i n f o

rticle history:eceived 22 February 2011eceived in revised form 2 May 2011ccepted 17 May 2011

eywords:chiff baseomputational method

a b s t r a c t

In this study, the molecular structure and spectroscopic properties of (E)-5-(diethylamino)-2-[(4-methylphenylimino)methyl]phenol were characterized experimentally by X-ray diffraction, FT-IR andUV–vis spectroscopic techniques and computationally by DFT method. It is concluded on the basis ofX-ray diffraction and FT-IR analyses that the title compound exists in enol form in the solid state. UV–visspectra of the title compound were recorded in different organic solvents to investigate the dependenceof tautomerism on solvent types. The tautomerism-solvent relation was also studied by computationalmethods to have more insight on structural properties. The geometry optimization of the title compound

automerismntramolecular proton transferpectroscopic investigation

in gas phase was performed by using DFT (B3LYP) method with 6-311G(d,p) basis set. The geometryoptimizations in solvent media were carried out at the same theory level by the polarizable continuummodel (PCM). In the calculation of excitation energies, TD-DFT calculations were carried out in both gasand solution phases. The computational investigation of non-linear optical properties indicates that thetitle compound has a good second order nonlinear optical property. The thermodynamic properties wereobtained in the range of 100–500 K.

© 2011 Elsevier B.V. All rights reserved.

. Introduction

o-Hydroxy Schiff bases derived from the condensation ofrimary amines with carbonyl compounds have a strong

ntramolecular hydrogen bond. These compounds are of inter-st because of their thermochromism and photochromism in theolid state, which can involve reversible intramolecular protonransfer from an oxygen atom to the neighboring nitrogen atom.ntramolecular proton transfer mechanism of both excited-statend ground-state is still the subject of intensive research. It wasroposed on the basis of thermochromic and photochromic Schiffases that the molecules exhibiting thermochromism are planar

hile the molecules exhibiting photochromism are non-planar

1,2]. Photochromic compounds are important because of their uses optical switches and optical memories, variable electrical cur-

∗ Corresponding author. Tel.: +90 368 2715532; fax: +90 368 2715530.E-mail address: calbayrak@sinop.edu.tr (C . Albayrak).

386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2011.05.046

rent, ion transport through membranes [3]. In addition, they havewidespread usage as ligands in the field of coordination chemistry[4] as well as in diverse fields of chemistry and biochemistry owingto their biological activities [5]. o-Hydroxy Schiff bases can existin two tautomeric structures as enol and keto forms in the solidstate [6,7]. o-Hydroxy Schiff bases were studied in solvent mediaand found to be existed in both enol and keto forms. The absorp-tion band peaking at wavelength greater than 400 nm is observedin polar solvents while this band is not observed in apolar solvents.The absorption band peaking above 400 nm belongs to the ketoform of o-hydroxy Schiff bases [8,9].

In this study, the crystal structure of (E)-5-(diethylamino)-2-[(4-methylphenylimino)methyl]phenol was determined by singlecrystal X-ray diffraction technique. The structural properties werealso characterized by FT-IR and UV–vis spectroscopic methods. The

computational studies at Density Functional Theory (DFT) levelwere carried out in order to get more insight on tautomerism and itsdependence on solvent type. The excitation energies were obtainedat TD-DFT level at the optimized geometry.

C . Albayrak et al. / Spectrochimica Acta Part A 81 (2011) 72– 78 73

Table 1Crystal data and structure refinement for title compound.

Empirical formula C18H22N2OFormula weight 282.38Temperature 130(2 KWavelength 71.073 pmCrystal system MonoclinicSpace group P 21/cUnit cell dimensions a = 890.41(2) pm, = 90◦

b = 1119.89(3) pm, = 100.662(2)◦

c = 1572.94(4) pm, � = 90◦

Volume 1.54140(7) nm3

Z 4Density (calculated) 1.217 mg/m3Absorption coefficient 0.076 mm−1F(0 0 0) 608Crystal size 0.4 × mm × 0.4 × mm × 0.4 mmTheta range for data collection 2.95–26.37◦ .Index ranges −11 ≤ h ≤ 11, −13 ≤ k ≤ 13, −19 ≤ l ≤ 18Reflections collected 10364Independent reflections 3136 [R(int) = 0.0200]Completeness to theta = 26.37◦ 99.9%Absorption correction Semi-empirical from equivalentsMax. and min. transmission 1.00000 and 0.91267Refinement method Full-matrix least-squares on F2

Data/restraints/parameters 3136/0/278Goodness-of-fit on F2 1.050Final R indices [I > 2sigma(I)] R1 = 0.0356, wR2 = 0.0940

2

2

a((sad

2

aawe

2

gtdwmphwFfp

2

p

The tautomerism appears in o-hydroxy Schiff bases as a result ofintramolecular proton transfer from oxygen atom to nitrogen atom.This proton transfer resulted in two tautomeric structures as enoland keto forms in the solid state. These tautomeric forms are related

Table 2The selected bond lengths, angles and torsion angles ( ´A,◦).

X-ray DFT/B3LYP

Enol form Keto form

C2–O1 1.358 (1) 1.340 1.262N2–C7 1.289 (1) 1.293 1.336C1–C7 1.438 (2) 1.438 1.387C1–C2 1.412 (2) 1.424 1.474C2–C3 1.383 (2) 1.391 1.427C3–C4 1.408 (2) 1.409 1.391C4–C5 1.420 (2) 1.425 1.449C5–C6 1.368 (2) 1.377 1.360C1–C6 1.404 (2) 1.406 1.425C8–C9 1.393 (2) 1.401 1.399C9–C10 1.382 (2) 1.392 1.391C10–C11 1.392 (2) 1.397 1.397C11–C12 1.390 (2) 1.401 1.400C12–C13 1.383 (2) 1.387 1.387C8–C13 1.389 (2) 1.403 1.402N1–C4 1.371 (2) 1.380 1.380N2–C8 1.418 (2) 1.405 1.401C7–N2–C8 120.3(1) 121.3 128.4O1–C2–C1 120.3(1) 121.3 121.2O1–C2–C3 118.2(1) 118.2 122.1N2–C7–C1 122.1(1) 122.7 122.5

R indices (all data) R1 = 0.0502, wR2 = 0.0986Largest diff. peak and hole 0.288 and −0.170 e.Å−3

. Experimental and computational methods

.1. Synthesis

The title compound was prepared by refluxing a mixture of solution containing 5-(diethylamino)-2-hydroxybenzaldehyde0.5 g, 2.59 mmol,) and a solution containing 4-methylaniline0.28 g, 2.59 mmol) in 20 mL ethanol. The reaction mixture wastirred for 1 h under reflux. The crystals of the title compound suit-ble for X-ray analysis were obtained by slow evaporation fromiethylether (yield 72%; m.p. 369–371 K).

.2. Instrumentation

The melting point was determined by StuartMP30 melting pointpparatus. FT-IR spectrum of the title compound was recorded on

Bruker 2000 spectrometer in KBr disk. UV-vis absorption spectraere recorded on a Thermo scientific BioGenesis UV-vis spectrom-

ter using a 1 cm path length of the cell.

.3. X-ray crystallography

All diffraction measurements were performed at 130 K usingraphite monochromated MoK� radiation and an Xcalibur diffrac-ometer. Semiempirical absorption correction was applied usingata previously available for similar compounds. The structureas solved by direct methods using SHELXS-97 [10]. The refine-ent was carried out by full-matrix least-squares method on the

ositional and anisotropic temperature parameters of the non-ydrogen atoms. All non-hydrogen atom parameters were refinedith anisotropically. All H atoms were located in a difference

ourier map, and their coordinates and Uiso (H) values were refinedreely. The data collection conditions and parameters of refinementrocess are listed in Table 1.

.4. Computational procedure

All computations were performed using Gaussian 03W programackage running under Windows XP [11]. Ground state geom-

Fig. 1. A view of the title compound, with the atom numbering scheme.

etry optimization of the title molecule was initiated from thecrystal structure and performed in both and solution phases byusing DFT method with Becke’s three-parameters hybrid exchange-correlation functional (B3LYP) [12] employing 6-311G(d,p) basisset [13–15] as implemented in Gaussian 03W. Total molecularenergy and dipole moment were obtained from the optimiza-tion output. The ground state geometry optimization of the titlecompound for gas phase was calculated by using DFT methodwith B3LYP adding 6-311G(d,p). The polarizable continuum model(PCM) was used for solution phase calculations [16,17]. Verti-cal excitation energies were calculated in both gas and solutionphases at TD- B3LYP/6-311G(d,p) level at the corresponding opti-mized geometries. In addition, gas-phase vibrational frequencies,non-linear optical properties and thermodynamic properties werecalculated at the corresponding optimized geometry using thesame theory level.

3. Results and discussion

3.1. Structure determination

The molecular structure of the title compound is shown inFig. 1. The selected bond lengths and angles are given in Table 2.

N2–C8–C13 117.7(1) 118.2 117.7C1–C7–N2–C8 −179.3(1) −177.1 179.6C7–N2–C8–C13 −149.1(1) −146.2 177.4C7–N2–C8–C9 33.2(2) 36.1 −2.5

74 C . Albayrak et al. / Spectrochimica Acta Part A 81 (2011) 72– 78

Fig. 2. The possible resonance structures of the title compound.

F hydro−

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ei[

H

TH

S

ig. 3. A partial packing diagram for the title compound, with C–H· · ·� and C–H· · ·Oz].

o two types of intramolecular hydrogen bonds as O–H· · ·N in enolorm (Fig. 2(a)) and N–HO in keto form (Fig. 2(b)). Our investigationshow that there is not an elongation of C7–N2 compared with C Nouble bond and of C1–C2, C2–C3, C4–C5 and C1–C6 compared withhe normal values for the salicylidene ring and the observed O1–C2ond length is consistent with O–C single bond. Furthermore, the19 atom is located on atom O1, thus the title compound exists

n the enol form in the solid state. The C N double bond and C–Oingle bond lengths compare well with previously reported val-es for similar compounds [18–20]. It is found that the moleculedopts E configuration about the C N double bond (Fig. 1). Thetrong intramolecular interaction between the phenolic atom O1nd nitrogen atom N2 constitutes a six-membered ring defined as(6) in Etter’s notation [21]. This strong hydrogen bond is charac-erized by O1· · ·N2 distance of 2.607(1) A (Table 3), being shorterhan the sum of the van der Waals’ radii for N and O [22].

One another way to confirm that the title compound exists innol form is to calculate the harmonic oscillator model of aromatic-ty (HOMA) index by using the following equation for the rings23,24]:

OMA = 1 −[

˛

n

n∑i=1

(Ri − Ropt)2

](1)

able 3ydrogen bonding geometry ( ´A,◦).

D–H· · ·A D–H H· · ·A D· · ·A ∠D–H· · ·AO1–H19 N2 0.96 (2) 1.744 (19) 2.607 (1) 148.0 (17)C16–H16 A O1i 0.996 (16) 2.513 (16) 3.450 (2) 156.5 (12)

X–H Cg H· · ·Cg (Å) X–H· · ·Cg (◦) X· · ·Cg (Å)

C15–H15A Cg1ii 2.956 (15) 116.0 (10) 3.508 (14)

ymmetry code: (i) x,−y + 1/2, z − 1/2, (ii) −x,−y, −z.

gen bonds shown as dashed lines [symmetry code: (i) x,−y + 1/2, z − 1/2, (ii) −x,−y,

where n is the number of bonds in the ring, is a constant equal to257.7 and Ropt is equal to 1.388 A for CC bonds. For purely aro-matic compounds HOMA index is equal to1 while it is equal to0 for non-aromatic compounds. The HOMA indexes in the rangeof 0.900–0.990 or 0.500–0.800 correspond to aromatic or the nonaromatic rings, respectively [25,26]. In this study, the calculatedHOMA indexes corresponding to C1–C6 and C8–C13 rings are 0.884and 0.995, respectively. These results show that C8–C13 ring in thetitle compound has purely aromatic character while C1–C6 ring isslightly deviated from aromaticity.

The crystal packing of the title compound is mainly stabilized byintra- and inter-molecular hydrogen bonds. The inter-molecularC16–H16A· · ·O1i (i): x,−y + 1/2, z − 1/2 interaction take placesbetween the methyl group and phenolic oxygen. It is seen fromFig. 3 that these C–H· · ·O interactions form C(8) chain runningthrough [0 0 1] direction. Further analysis of noncovalent interac-tions show that the crystal packing is also supported by C–H· · ·�interactions which are geometrically characterized by the distanceof 2.956 A between H15A and Cg1 ring centroid (Fig. 3). Theseinteractions form R2

2(10) and R66(30) ring motifs and interconnect

the C(8) chains formed by C–H· · ·O hydrogen bonds (Fig. 3). The

details of noncovalent interactions are given in Table 3.

The optimized geometry of enol form (blue in Fig. 4) has onlyslight deviation from the experimental geometry (red) with r.m.s.

Fig. 4. Superimposition of gas phase optimized (blue) and experimental geometry(red) of the title compound. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of the article.)

C . Albayrak et al. / Spectrochimica Acta Part A 81 (2011) 72– 78 75

oitcmTmiowl11liabftofp

3

shsbriiaort7tftC1ttao

Table 4The experimental and the scaled computational vibrational frequencies (cm−1).

Assignments Experimental DFT/B3LYP

� (O–H) 3000–1700 3076�aromatic(C–H) 3023 3055�methyl(C–H) 2971 3017

2873 2923�methylene(C–H) 2931 2939� (C=N) str. 1625 1609�aromatic(C=C) 1584 1582�aromatic(C=C) 1519 1502�aromatic(C–H) 1129 1102�aromatic(C–H) 821 811�methylene(C–H), �methyl(C–H) 1425 1413�methylene(C–H), �methyl(C–H) 1392 1345

1336

Fig. 5. FT-IR spectrum of the title compound.

f 0.135 A. This can be attributed to the presence of crystal pack-ng effects in the crystal structure. The optimizations pertainingo enol and keto forms of the title compound were performed toompare each other. Selected bond lengths and angles for the opti-ized structures and X-ray geometry of the molecule are listed in

able 2. The effects of the intramolecular proton transfer on theolecular geometry can be seen better via examining the changes

n bond lengths for both forms. Table 2 shows that the changesccurred in the lengths of C2–O1 single and C2 N7 double bondsith the transfer process from enol form to keto form. While bond

engths of C2–O1 and N1–C7 for enol tautomer were found as.340 A and 1.293 A, these lengths for the keto tautomer were 1.262,.336 A, respectively. Table 2 clearly indicates that the intramolecu-

ar proton transfer affects the single and double characters of thesendicative bonds. In addition, if the C1/C6 ring bond lengths of enolnd the keto forms in Table 2 are compared with each other, it cane said that the results obtained from the optimization for enolorm are more compatible to the experimental results. However,he experimental bond lengths are slightly different from thosef optimized geometry since the theoretical calculations are per-ormed for gas phase while experimental results belong to solidhase.

.2. FT-IR spectrum

FT-IR spectrum of the title compound was given in Fig. 5. The OHtretching vibration is very sensitive to inter- and intra-molecularydrogen bonds and lies in the region 3000–3500 cm−1 in FT-IRpectrum. It gives rise to the three vibrations as stretching, in-planeending and out-of-plane bending vibrations. In 1700–3000 cm−1

egion, the absorption band is attributed to the �(O–H) stretch-ng vibration which broaden owing to the formation of strongntramolecular hydrogen bonding O–H· · ·N in the structure. Gener-lly, the OH in-plane bending and out-of-plane bending vibrationsf phenols lie in the region 1300–1400 cm−1 and 517–710 cm−1,espectively [27]. The band observed at 1346 cm−1 was assignedo OH in-plane bending vibration and the band observed at87 cm−1 was assigned to OH out-of-plane bending vibration forhe title compound. The absorption band located at 1625 cm−1

or the title compound is attributed to �(C N) stretching vibra-ion. The aromatic C–H stretching, C–H in-plane bending and–H out-of-plane bending vibrations appear in 3000–3100 cm−1,100–1500 cm−1 and 800–1000 cm−1 frequency ranges, respec-

−1

ively [28]. The absorption band at 3023 cm corresponds tohe aromatic CH stretching vibrations of the title compound. Inddition, in-plane bending and out-of-plane CH vibrations werebserved at 1129 cm−1 and 821 cm−1, respectively. The asymmet-

�(C–O) 1194 1222�methylene(C–H) 708 768

ric and symmetric stretching vibrations of the aliphatic CH2 andCH3 group of the title compound were observed at 2971, 2931and 2873 cm−1. The scissoring mode of these groups was observedat 1425 cm−1 while twisting modes were observed at 1392 cm−1,rocking modes at 708 cm−1. The C–O stretching vibrations in phe-nols are expected to appear as a strongest band in 1300–1100 cm−1

frequency ranges [28]. The absorption band observed at 1194 cm−1

for the title compound was assigned to O–C stretching mode ofvibration. The absorption bands at 1600–1400 cm−1 are due toCC stretching vibrations of the aromatic compounds [28]. The CCstretching modes of aromatic rings of the title compound wereobserved at 1584, 1519, and 1451 cm−1, which agree well with theliterature data.

The vibrational frequencies of the title compound were cal-culated by using B3LYP/6-311G(d,p) method. The vibrationalfrequencies were scaled by 0.9682 [29]. The vibrational bandsassignments have been made by using Gaussview molecular visual-ization program. The experimental and the calculated frequenciesare given in Table 4. The calculated results show slight deviationsfrom experimental values due to the intramolecular hydrogen bondbetween N and O atoms.

3.3. UV–vis absorption spectra

It is seen in Table 5 that the total energies decrease with theincrease in the polarity of the solvent. In other words, the stabilityof molecule increases. The energy gap (�E) between the highestoccupied molecular orbital (HOMO) and the lowest-lying unoccu-pied molecular orbital (LUMO) decreases in enol form when anincrease occurs in the polarity of the solvent. On the other hand,the change of dipole moment is directly proportional to the polar-ity of solvent. HOMO and LUMO called Frontier molecular orbitals(FMO) are the most important ones in terms of their roles in reac-tions between molecules and in electronic spectra of a molecule.The frontier molecular orbitals for both enol and keto forms of thetitle molecule are shown in Fig. 6 in EtOH.

In addition, the first 10 spin-allowed singlet–singlet excitationsfor both enol and keto forms were calculated by TD-DFT approach.TD-DFT calculations were started from optimized geometry usingthe same level of theory and performed for gas phase to calcu-late excitation energies. The percentage contributions of molecularorbitals to formation of the bands were obtained by using SWizardProgram [30]. For both enol and keto forms of the title compound,wavelength (�), oscillator strength (f), major contributions of cal-

culated transitions are given in Table 6. The absorption peakingat 360–370 nm arises excitation from HOMO to LUMO transitionin solution for enol form. The absorption peaking above 400 nmarises excitations from HOMO and HOMO−1 to LUMO for the keto

76 C . Albayrak et al. / Spectrochimica Acta Part A 81 (2011) 72– 78

Table 5Calculated total energies, frontier orbital energies and dipol moments of title compound for enol and keto form.

Gas phase Benzene EtOH DMSO

Enol Form Etotal (a.u.) −884.129970 −884.135275 −884.143307 −884.143808ELUMO (eV) −1.41 −1.47 −1.57 −1.58EHOMO (eV) −5.25 −5.28 −5.33 −5.34�E (eV) 3.84 3.81 3.76 3.76� (D) 3.36 3.98 4.92 4.98�E (kcal/mol)a 8.7 5.4 0.3 0.0

Keto form Etotal (a.u.) −884.123648 −884.131252 −884.142831 −844.143456ELUMO (eV) −1.75 −1.80 −1.90 −1.91EHOMO (eV) −5.21 −5.31 −5.44 −5.45�E (eV) 3.46 3.51 3.53 3.53� (D) 3.02 3.74 4.99 5.08�E (kcal/mol)a 12.7 7.9 0.6 0.2

a Relative energies in comparison with enol form in DMSO.

Table 6For enol and keto forms wavelength, oscillator strength, major contributions of calculated transitions.

Experimental Calculated

Enol Form Keto Form

� (nm) � (nm) (f) Major Contribution � (nm) (f) Major Contribution

Gas Phase – 349 (1.1) H → L (81%) 369 (0.5) H → L(64%), H-1 → L(23%)367 (0.7) H-2 → L (56%) H-1 → L (26%)

H → L (8%)414 (0.03) H-2 → L(40%), H-1 → L(36%) H → L(10%)

Benzene 374 363 (1.3) H → L (84%) 383 (1.3) H-1 → L(55%) H → L(28%)412 (0.1) H → L(55%) H-1 → L(33%)

EtOH 388 363 (1.3) H → L (83%) 380 (1.3) H → L(66%) H-1 → L(17%)432 401 (0.1) H-1 → L(72%) H → L(16%)

DMSO 382 365 (1.3) H → L (84%)

Fig. 6. The molecular orbital surfaces of the title c

383 (1.3) H → L(64%) H-1 → L(20%)401 (0.1) H-1 → L(70%) H → L(18%)

ompound for enol and keto forms in EtOH.

C . Albayrak et al. / Spectrochimica

F(

fF4

s2bt3ttceabek

bitbNidetftirffosiaf

gfpwd

ig. 7. The solvent effect on UV–vis spectra of the title compound in (- - - -) DMSO,—) EtOH, (– . . –) benzene.

orm. These transitions are attributed � → �* transition shown inig. 6. Hence, the absorption band peaking in the range greater than00 nm belongs to the keto form.

The UV–vis spectra of the title compound in various organicolvents (EtOH, DMSO and benzene) were recorded within00–600 nm range (Fig. 7). The characteristic UV–vis absorptionands of the molecule in EtOH, DMSO and benzene are compara-ively given in Table 6. The absorption bands at 382 (in DMSO) and74 nm (in benzene) are attributed � → �. However, new absorp-ion band peaking at 432 nm was observed in the spectrum of theitle compound in EtOH (Fig. 7), which was not present in thease of DMSO and benzene. Referring to the computational andxperimental studies new absorption bands above 400 nm can bettributed to the keto form of o-hydroxy Schiff bases [8,31]. It cane said that the absorption band peaking at 370–390 nm arises fromnol form and the absorption band peaking at 432 nm arises frometo form in the solution.

The title compound exists in enol form in the solid state whileoth enol and keto forms are shown in EtOH. The keto form is

mportant in the solution and stabilized by the polar solventshrough solute and solvent interactions. Fig. 2 shows the possi-le resonance structures of the title compound in the solution. The,N-diethylamino group is an electron-donating group. The zwitter

onic form arises before the formation of keto form. The electron-onating N,N-diethylamino group stabilizes zwitter ionic form andases the formation of keto form with resonance. However, not onlyhe electron-donating group but also the solvent is effective in theormation of keto form. If the formation of keto form depended onhe electron-donating group, the keto form would have observedn all solvents. In addition, the formation of keto form can not beelated to the polarity of solvent. Such a relation would require theormation of keto form in DMSO unlike observed results. There-ore, the proton transfer is affected by H-donor–acceptor propertyf solvent instead of solvent polarity. EtOH is a polar and proticolvent. This property causes both enol and keto forms to be stablen solvent. The hydrogen bond formation between the compoundnd solvent plays an important role in the stabilization of the ketoorm.

The computational studies also show that the strong hydro-en bond interaction decreases the activation energy between enol

orm and transition state (TS) [32,33]. Benzene is a solvent of lowolarity. On the other hand, DMSO is a polar and aprotic solventhile EtOH is a polar and protic solvent acting as both hydrogenonor and acceptor. Because the intramolecular proton transfer

Acta Part A 81 (2011) 72– 78 77

from enol form to keto form occurs in TS, protic solvents as EtOH canfacilitate the proton transfer and molecules are strongly solvatedby EtOH. TS structure of the title compound will be stabilized byhydrogen bond involving EtOH, and the activation energy neededto overcome the barrier of intramolecular proton transfer betweenenol form and TS structure will decrease. This decrease in the acti-vation energy enables the formation of keto form in EtOH. That ketoform is not present in UV–vis spectra in DMSO can be attributed tothe transition states energy that is high for the formation of ketoform.

3.4. Non-linear optical (NLO) properties

Having the knowledge of non-linear optical properties is ofcrucial importance in the design of materials in communicationtechnology, signal processing, optical switches and optical mem-ory devices [34]. The non-linear optical properties are associatedwith delocalized � electrons of a molecule. That is, the increase ofthe conjugation on molecule is resulted in an increase in the non-linear optical properties. The addition of donor and acceptor groupsto conjugated systems also affects nonlinear optical properties inincreasing way. The delocalization of � electron cloud on organicmolecules increases in the case of powerful donor and acceptorgroups. This results in an increase in the polarizability and firsthyperpolarizability of organic molecules [35].

The energy gap between HOMO and LUMO is related to thepolarizability of a molecule [36]. The increment in the strengthof donor and acceptor groups causes an increase in the non-linear optical properties of organic molecules due to the decreasethe energy gap between HOMO and LUMO. The molecule witha small energy gap is more polarizable than that having a largeenergy gap. Small HOMO–LUMO gap requires small excitationenergy. Absorption bands of a molecule with small energy gapshift towards the visible region. Quantum chemical calculationscan be used to describe the relationship between the electronicstructure of molecules and their non-linear optical properties.The title compound includes delocalized � electron system withelectron-donating substituent –N(Et)2 group. In this study, the titlecompound was computationally studied at DFT (B3LYP) level inorder to investigate the effect of � electron system and donor groupon its non-linear optical properties.

The total static dipole moment �, the average linear polarizabil-ity ˛, and the first hyperpolarizability can be calculated by usingthe following equations [34]:

� = (�2x + �2

y + �2z )

1/2(2)

= 13

(˛xx + ˛yy + ˛zz) (3)

= [ˇxxx + ˇxyy + ˇxzz)2 + (ˇyyy + ˇxxy + ˇyzz)2

+ (ˇzzz + ˇxxz + ˇyyz)2]1/2 (4)

The dipole moment, polarizability and the first hyperpolariz-ability were calculated using polar = ENONLY at B3LYP/6-311G(d,p)level. The results are given in Supplementary data.

The calculated polarizability and first hyperpolarizability forthe title compound are 40.214 A3 and 34.98 × 10−30 cm5/esu thatare greater than those of urea ( and of urea of 3.8312 A3 and0.37289 × 10−30 cm5/esu), respectively [37]. In addition, it is foundthat the first hyperpolarizability and polarizability of the title com-

pound are greater than those of related Schiff base [37,38]. Theenergy gap between HOMO and LUMO of the title compound is3.838 eV for gas phase. The decreased value of energy gap (3.838 eV)shows that the title compound has a smaller energy gap than those

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[

8 C . Albayrak et al. / Spectrochi

f previously reported compounds [37,38]. Therefore, the absorp-ion bands in the electronic spectra of the title compound arehifted towards the visible region. The movement of � electronloud and the increase in the � electron system with the effectf electron donating –N(Et)2 group (through C N bridge of theitle compound) lead to an increase in the conjugation and, con-equently, to an increase in its non-linear optical properties. Theseesults show that the title compound can be used as a good non-inear optical material.

.5. Thermodynamic properties

The standard thermodynamic functions, namely, the heat capac-ty (C0

p,m), entropy (S0m) and enthalpy (H0

m), were calculated at3LYP/6-311G(d,p) level for varying temperatures from 100 to00 K. The results obtained on the basis of vibrational analysis arehown in Supplementary data. The results reveal that an increasen temperature causes an increase in heat capacities, entropies andnthalpies due to increasing intensities of molecular vibrations.

The correlation equations between heat capacities, entropies,nthalpies and temperature are shown below and can be usedor analyzing heat capacities, entropies and enthalpies in differentemperature.

0p,m = 7.7455 + 0.24363T − 2.20645 × 10−5T2, R2 = 0.99931

0m = 66.05964 + 0.32003T − 7.55698 × 10−5T2, R2 = 0.99987

0m = 0.04201 + 0.01011T + 1.1752 × 10−4T2, R2 = 0.99999

. Conclusion

In this study, the title compound was experimentally character-zed by X-Ray diffraction, IR and UV-vis spectroscopic techniquesnd computationally by DFT method. It is concluded on the basisf X-ray and FT-IR investigations that the title compound exists innol form in the solid state. UV-vis spectra of the title compoundere recorded in different organic solvents in order to investi-

ate the dependence of tautomerism on solvent types. The resultshow that the title compound exists in both keto and enol formsn EtOH while it has enol form in benzene and DMSO. In addition,he geometry optimizations for both keto and enol forms were per-ormed using DFT method with B3LYP applying 6-311G(d,p) basiset based on X-ray geometry. The results obtained from the opti-ization for enol form are more compatible with the experimental

esults. TD-DFT calculations starting from optimized geometryere carried out in both gas and solution phases to calculate

xcitation energies of title compound. Considering TD-DFT calcula-ions it can be said that the absorption bands at 370–390 nm ariserom enol form and the absorption band peaking above 400 nmrises from keto form in the solution. The computational studiesn the non-linear optical properties of the title compound showhat the title compound can be used as a good non-linear optical

aterial.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.saa.2011.05.046.

[[

[

cta Part A 81 (2011) 72– 78

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