Vibrational spectral analysis, electronic absorption and non-linear optical behavior of (E)-1-(2,4,6-trimethoxyphenyl)pent-1-en-3-one

Vibrational Spectroscopy 79 (2015) 1–10

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Vibrational spectral analysis, electronic absorption and non-linear optical behavior of (E)-1-(2,4,6-trimethoxyphenyl)pent-1-en-3-one S. Alen a,b , D. Sajan a, * , K. Job Sabu a , Tom Sundius c, K. Chaitanya d , Frank Blockhuys e , V. Bena Jothy b a

Department of Physics, Bishop Moore College, Mavelikara, Alappuzha 690110, Kerala, India Department of Physics, Women’s Christian College, Nagercoil 629001, India Department of Physics, University of Helsinki, P.O. Box 64, FIN-00014 Helsinki, Finland d Department of Chemistry, Nanjing University of Science and Technology, Xialingwei 200, Nanjing, PR China e Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Wilrijk, Belgium b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 8 December 2014 Received in revised form 14 April 2015 Accepted 14 April 2015 Available online 17 April 2015

Vibrational spectral investigations of (E)-1-(2,4,6-trimethoxyphenyl)pent-1-en-3-one (TMPPO) have been carried out using the techniques of FT-IR and FT-Raman spectra with the aid of density functional theory (DFT) calculations. Normal coordinate analysis (NCA) has been performed to obtain the vibrational modes. NBO analysis has been carried out and it reveals the presence of hydrogen bonding. The calculated first hyperpolarizability and the HOMO–LUMO energies confirm the nonlinear optical activity of TMPPO. UV–vis spectral analysis has been carried out. ã 2015 Elsevier B.V. All rights reserved.

Keywords: TMPPO SQMFF NLO DFT

1. Introduction Organic NLO (nonlinear optical) materials are expected to be active materials for optical communication and optical electronics because of their applications in high-speed and high-density data processing. They have a great impact on information technology and industrial applications. Organic nonlinear optical materials have been investigated due to their potentially high nonlinearities and rapid response in the electro-optic effect compared to inorganic NLO materials. Recent efforts have been focused on developing organic molecules with large molecular NLO response, improved optical transparency and good thermal stability. p-Bridged donor– acceptor–donor (D–A–D) systems are excellent candidates for organic non-linear optics (NLO) media with a high second-order hyperpolarizability because of their high degree of conjugation [1]. They are expected to have electronic properties and hence they are promising candidates for electronic applications such as organic light-emitting diodes [2–3]. The (E)-1-(2,4,6-trimethoxyphenyl) pent-1-en-3-one (TMPPO) crystal is such a novel NLO material whose NLO property is reported for the first time. In order to expound the relationship between the molecular structural features and NLO

properties, vibrational spectral analysis has been carried out on TMPPO with the aid of density functional theory (DFT) computations. Vibrational spectra with the help of DFT methods have proved to be an essential tool for interpreting and predicting the various molecular properties of NLO materials [4–6]. Natural bond orbital (NBO) analysis has been performed to investigate charge transfer interactions and the hydrogen bonding within the molecule. UV–vis spectral analysis has also been carried out to understand the various electronic transitions. 2. Experimental A solution of sodium (1.0 g, 0.04 mol) in ethanol (50 ml) was added dropwise to a solution of 2,4,6-trimethoxybenzaldehyde (5.6 g, 0.04 mol) and 2,5-dimethylpyrazine (2.2 g, 0.02 mol) in ethanol (150 ml) at room temperature and the reaction mixture was heated under reflux for 4 h. The resulting fluorescent yellow solution was poured into 500 ml of ice water and the precipitate was filtered off. Crystals were grown by slow evaporation of a THF solution. 2.1. Second harmonic generation (SHG) measurement

* Corresponding author. Tel.: +91 9495043765; fax: +91 4792303230. E-mail addresses: [email protected], [email protected] (D. Sajan). http://dx.doi.org/10.1016/j.vibspec.2015.04.002 0924-2031/ ã 2015 Elsevier B.V. All rights reserved.

The second harmonic generation (SHG) measurement has been carried out on TMPPO by the Kurtz and Perry technique [7]. The

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A. S. et al. / Vibrational Spectroscopy 79 (2015) 1–10

2.2. FT-IR, FT-Raman, UV–vis spectra and thermal studies

Fig. 1. Optimized geometry of TMPPO calculated at B3LYP/cc-pVTZ level of theory.

fundamental beam of 1064 nm from a Q-switched Nd:YAG laser is used to test the second harmonic generation (SHG) property of the TMPPO crystals. The input laser beam was passed through the sample after reflection from an IR detector. The output from the sample was filtered by an IR filter to eliminate the fundamental, and the second harmonic was detected using a monochromator and PMT. The second harmonic efficiency of TMPPO was calculated to be 2.1 times that of urea.

FT-IR spectrum of the synthesized material was recorded in the wavenumber range 400–4000 cm
1 by the KBr pellet technique using a Thermo-Nicolet 6700 FT-IR spectrometer. The NIR–FTRaman spectra were recorded using a Bruker RFS 27 spectrometer. The measurements were carried out in the range of 50–4000 cm
1 (Happ-Genzel apodization, 2 cm
1 resolution, 1064 nm Nd:YAG laser excitation, 450 mW power at the sample). The UV–vis absorption spectrum of the sample was recorded in a chloroform solution using a Shimadzu UV-1800 UV–vis spectrophotometer in the spectral region of 200–700 nm. Thermal analysis of TMPPO was carried out using a PerkinElmer simultaneous thermo gravimetric/ differential thermal (TGA/DTA) analyzer. The sample was scanned in the temperature range 50–900  C at a rate of 10  C/min in an inert nitrogen atmosphere. 3. Computational details All the quantum chemical computations were carried out using the Gaussian 09 program [8] package using the B3LYP method with cc-pVTZ [9] as basis set. The Raman activities (Si) calculated with the Gaussian 09 program have been suitably adjusted in the scaling procedure with MOLVIB and then converted to relative Raman intensities using the following relationship:

Table 1 Optimized geometrical parameters of TMPPO. Bond length

Values (Å) B3LYP/cc-pVTZ

XRD

C1–C2 C2–C3 C3–C4 C4–C5 C5–C6 C2–O7 C3–H8 C4–O9 C5–H10 C6–O11 C1–C12 O7–C13 C13–H14 C13–H15 C13–H16 O9–C17 C17–H18 C17–H19 C17–H20 O11–C21 C21–H22 C21–H23 C21–H24 C12–H25 C12–C26 C26–H27 C26–C28 C28–O29 C28–C30 C30–H31 C30–H32 C30–C33 C33–H34 C33–H35 C33–H36 O11–H25 O7–H27

1.419 1.387 1.392 1.391 1.393 1.352 1.077 1.357 1.075 1.360 1.451 1.420 1.092 1.092 1.086 1.419 1.092 1.092 1.086 1.417 1.092 1.092 1.086 1.079 1.348 1.077 1.470 1.222 1.524 1.090 1.089 1.535 1.089 1.090 1.090 2.204 2.218

1.412 1.382 1.385 1.381 1.390 1.358 0.930 1.362 0.929 1.363 1.452 1.425 0.960 0.959 0.96 1.422 0.96 0.96 0.96 1.425 0.96 0.96 0.96 0.93 1.332 0.93 1.463 1.221 1.507 0.971 0.970 1.514 0.96 0.96 0.96 2.272 2.178

Angle

C1–C2–C3 C2–C3–C4 C3–C4–C5 C4–C5–C6 C1–C2–O7 C2–C3–H8 C3–C4–O9 C4–C5–H10 C1–C6–O11 C2–C1–C12 C2–O7–C13 O7–C13–H14 O7–C13–H15 O7–C13–H16 C4–O9–C17 O9–C17–H18 O9–C17–H19 O9–C17–H20 C6–O11–C21 O11–C21–H22 O11–C21–H23 O11–C21–H24 C1–C12–H25 C1–C12–H26 C12–C26–H27 C12–C26–H28 C26–C28–O29 C26–C28–C30 C28–C30–H31 C28–C30–H32 C28–C30–C33 C30–C33–H34 C30–C33–H35 C30–H33–H36

Values ( )

Torsion angle

B3LYP/cc-pVTZ

XRD

121.8 119.8 120.7 118.8 116.1 122.0 115.3 120.7 115.8 125.5 119.6 111.3 111.3 105.6 119.0 111.4 111.4 105.9 119.7 111.5 111.5 105.7 112.4 130.7 122.1 124.5 119.3 121.5 106.2 111.9 111.1 110.3 111.4 110.7

121.46 119.64 121.70 117.92 115.86 120.17 114.17 121.01 115.05 124.89 118.43 109.49 109.48 109.47 118.08 109.48 109.58 109.48 118.99 109.47 109.47 109.51 114.62 130.79 117.76 124.58 118.99 120.63 108.95 108.95 113.05 109.44 109.49 109.46

C1–C2–C3–C4 C2–C3–C4–C5 C3–C4–C5–C6 C6–C1–C2–O7 C1–C2–C3–H8 C2–C3–C4–O9 C3–C4–C5–H10 C2–C1–C6–O11 C3–C2–C1–C12 C1–C2–O7–C13 C2–O7–C13–H14 C2–O7–C13–H15 C2–O7–C13–H16 C3–C4–O9–C17 C4–O9–C17–H18 C4–O9–C17–H19 C4–O9–C17–H20 C1–C6–O11–C21 C6–O11–C21–H22 C6–O11–C21–H23 C6–O11–C21–H24 C2–C1–C12–H25 C6–C1–C12–H26 C1–C12–H26–H27 C1–C12–H26–H28 C12–C26–H28–O29 C12–C26–H28–C30 C26–C28–C30–H31 C26–C28–C30–H32 C26–C28–C30–C33 C28–C30–C33–H34 C28–C30–C33–H35 C28–C30–C33–H36

Values ( ) B3LYP/cc-pVTZ

XRD


0.0 0.0 0.0 179.9 179.9
179.9
179.9
179.8 179.8
179.9 61.2
61.3 179.9 179.9 61.2
61.3 179.9
179.8 61.4
61.3
179.9
179.0
179.2 0.2
179.3 179.5 1.5
146.2
28.8 94.7 56.5
63.5 176.3


0.77 1.28
0.90 179.95 179.26
178.56 179.13 179.49 179.45 177.93 58.93
61.08 178.94 176.20 62.57
57.53 177.42 173.01 63.54
56.44
176.49
178.08
178.63 0.20
179.79
179.71 0.09
62.00
65.34 176.69 60.04 60.04
179.91

A. S. et al. / Vibrational Spectroscopy 79 (2015) 1–10

f ðn0
ni Þ Si h
i ni ni 1
exp
hc kT

3

(a)

Absorbance(arbt. units)

(b)

(c)

309

where y0 is the exciting wavenumber (in cm ), yi is the vibrational wavenumber of the ith normal mode, h,c and k are universal constants, and f is the suitably chosen common scaling factor for all the peak intensities [10]. The TD-DFT method was used to calculate energies, oscillator strengths of electronic singlet–singlet transitions and the absorption wavelengths. Solvent effects were considered using the polarizable continuum model (PCM) developed by Tomasi and co-workers [11–13]. Natural bond orbital (NBO) analysis has also been carried out.
1

329

(1)

(d)

247

330

Ii ¼

306

4

200

4. Results and discussion

300

400

500

600

700

Wavelength(nm) 4.1. Optimized geometry The optimized geometrical parameters of (E)-1-(2,4,6-trimethoxyphenyl)pent-1-en-3-one calculated at the B3LYP/DFT level with the cc-pVTZ basis set are compared with the experimentally available XRD data [14] and are given in Table 1. The optimized geometry of TMPPO is given in Fig. 1. From Table 1, it can be seen that there is a deviation between the computed geometrical parameters and the corresponding experimental data. This may be due to intermolecular interactions in the crystalline state. The existence of intramolecular C

H  O hydrogen bonding is evident from the strengthening and contraction of C26–H27 (0.147 Å) and C12–H25 (0.149 Å) bond lengths. This can be inferred from the short H  O distances (H27–O7 = 2.218 Å and H25–O11 = 2.204 Å). The DFT computation gives a decrease of the angle C6–C1–C12 by 1.68 and increase of the angle C2–C1–C12 by 5.53 from 120 at C1 position. This asymmetry of the exocyclic angles reveals the repulsion between CH2CH3 and the phenyl ring. There is a decrease in the endocyclic angle C6–C1–C2 at the junction of the phenyl ring and the propenoate group and an increase in the two neighboring angles (C1–C6–C5 and C1–C2–C3) around the ring when compared to XRD data. This is due to the charge transfer interaction of the phenyl ring and ester group through C12 = C26 double bond. The increase in the angle C26–C28–O29 from the experimental value is due to the repulsion between O29 and C26–H27.

Fig. 2. (a) Simulated UV–vis spectrum of TMPPO in chloroform at CAM-B3LYP/ccpVTZ level. (b) Simulated UV–vis spectrum of TMPPO in chloroform at B3LYP/ccpVTZ level. (c) Simulated UV–vis spectrum of TMPPO in gas phase. (d) Experimental UV–vis spectrum of TMPPO in chloroform.

4.2. Natural bond orbital analysis The NBO analysis has been performed with the NBO 3.1 program [15]. The most important interactions between ‘filled’ (donors) Lewis-type NBO’s and ‘empty’ (acceptors) non-Lewis NBO’s are reported in Table 2. For each donor NBO (i) and acceptor NBO (j), the stabilization energy E(2) associated with delocalization or hyperconjugation is estimated. [16]. The hyperconjugative interaction energy was deduced from the second-order perturbation approach: Eð2Þ ¼
ns

F ij 2 hs jFjs i2 ; ¼
ns es 
es DE

(2)

where hs jFjs i2 or Fij2 is the Fock matrix element between i and j NBO orbitals, es and es * are the energies of s and s * NBO’s and ns is the population of the donor s orbital. The most important interaction (n–s *) energies, related to the resonance in the molecules, are electron donation from the LP2O atoms of the electron donating groups to the anti-bonding acceptor s *(C

C) of the phenyl ring (LP2O29 ! s *(C28–C30)) = 20.60 kcal

Table 2 Second order perturbation theory analysis of Fock matrix in NBO basis. Donor(i)

ED(i) (e)

Acceptor(j)

ED(j) (e)

E(2)a (kcal mol
1)

E(j)–E(i)b (a.u.)

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

p(C1–C6) p(C1–C6) p(C2–C3) p(C2–C3) p(C4–C5) p(C4–C5) p(C1–C6)

1.62811 1.62811 1.70534 1.70534 1.69085 1.69085 1.62811 1.95827 1.95793 1.95793 1.83558 1.96733 1.96733 1.96733 1.95827 1.95954 1.95954 1.89366 1.89366

p*(C2–C3) p*(C4–C5) p*(C1–C6) p*(C4–C5) p*(C1–C6) p*(C2–C3) p*(C12–C26) s *(C26–H27) s *(C21–H24) s *(C21–H22) s *(C21–H23) s *(C1–C6) s *(C1–C12) s *(C2–C3) s *(C1–C2) s *(C4–C5) s *(C5–C6) s *(C26–C28) s *(C28–C30)

0.38713 0.43422 0.44967 0.43422 0.44967 0.38713 0.13978 0.13978 0.00996 0.02006 0.02007 0.02568 0.02186 0.02411 0.03013 0.02928 0.02543 0.05991 0.06854

27.05 13.63 11.58 27.78 27.51 11.40 18.46 0.51 2.96 5.79 5.79 2.17 2.72 2.81 0.55 8.38 7.60 18.58 20.60

0.27 0.26 0.28 0.27 0.28 0.29 0.31 1.02 0.93 0.67 0.67 1.19 1.16 1.23 1.07 1.10 1.09 0.70 0.62

0.077 0.054 0.053 0.081 0.082 0.052 0.071 0.020 0.047 0.058 0.058 0.045 0.050 0.053 0.022 0.086 0.082 0.103 0.102

LP1 O7 LP1 O11 LP1 O11 LP2 O11 s (C1–C2) s (C1–C2) s (C1–C2) LP1 O7 LP1 O9 LP1 O9 LP2 O29 LP2 O29 a b c

E(2) means energy of hyperconjugative interactions; cf. Eq. (2). Energy difference between donor and acceptor i and j NBO orbitals. F(i,j) is the Fock matrix element between i and j NBO orbitals.

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A. S. et al. / Vibrational Spectroscopy 79 (2015) 1–10

Table 3 Experimental and calculated absorption wavelengths, energies and oscillator strengths of TMPPO using the TD-DFT method at the B3LYP/cc-pVTZ and CAM-B3LYP level. Excitation

CI expansion coefficient

Excited state1 66–>68 66–>70

Singlet A

Excited state2 65–>68 67–>68 Excited state3 65–>68 67–>68 67–>69

0.68801 0.14638

Wavelength (nm) calc. gas phase

Oscillator strength (f)

Excitation CI expansion coefficient Singlet A

0.0001

Excited state1 65–>68 65–>70

Singlet A

0.7038

Excited state2 66–>68 67–>68 Excited state3 66–>68 67–>68 67–>69

Singlet A

367

Singlet A
0.13595 0.68503

309

Singlet A 0.65987 0.14729
0.19972

298

0.0784

0.69007 0.14112

0.11743 0.69219

0.68112
0.12275 0.1374

Wavelength Oscillator (nm) calc. strength (f) choloroform B3LYP/cc-pVTZ

348 0.0003

330

Excitation CI expansion coefficient Excited state1 65–>68 65–>70

Singlet A

Excited state2 67–>68

Singlet A

0.64208 0.25763

314

mol
1. This larger energy compared to other lone pair interaction energies shows the hyperconjugation between the electron donating groups and the phenyl ring. Another intramolecular hyperconjugative interaction is formed by the orbital overlap between p(C

C) bond orbital to p*(C

C) antibonding orbital, which results in an intramolecular charge transfer (ICT) causing stabilization of the system. This strong ICT is one of the causes for the NLO activity. These interactions are observed as increase in electron density (ED) in the C

C antibonding orbital that weakens the respective bonds. In the phenyl ring the ED at the two conjugated p bond (1.67e) and p* bond (0.38e) clearly demonstrate the strong delocalization, which leads to a stabilization of 19.63 kcal mol
1. The NBO analysis clearly gives evidence for the formation of strong H-bonded interaction between oxygen lone electron pairs and s *(C

H) antibonding orbitals. The

0.0968

Oscillator strength (f)

Expt. (nm)

316 0.0006

0.69639

0.8325 Excited state3 66–>68 66–>70 67–>69

Wavelength (nm) calc. chloroform CAM-B3LYP

306

0.9331

330

273

0.0486

247

Singlet A 0.6417
0.16022 0.23281

importance of hyperconjugation and electron density transfer from lone electron pairs of the Y atom to the X

H antibonding orbital in the X

H  Y system has been reported [4–6]. In general such interaction leads to an increase in population of X

H antibonding orbital. The energy contribution of LP1O11 ! s *(C21– H24), LP1O11 ! s *(C21–H22), LP2O11 ! s *(C21–H23) values are 2.96, 5.79, and 5.79 kcal mol
1, respectively. These energy E(2) values are chemically significant and can be used as a measure of the intramolecular C

H

O hydrogen bonding interaction between the oxygen lone pair and the antibonding orbitals. 4.3. UV–vis analysis To understand the electronic transitions on TMPPO, the electronic spectra were computed in the gas phase and in the

Fig. 3. HOMO and LUMO orbital of TMPPO by B3LYP/cc-pVTZ level of theory.

A. S. et al. / Vibrational Spectroscopy 79 (2015) 1–10

5

Fig. 4. TGA/DTA curve of TMPPO.

chloroform medium. The solvent effect was calculated using PCM– TD-DFT method by employing the B3LYP/cc-pVTZ and CAM-B3LYP/ cc-pVTZ functional. The observed and simulated (in gas phase and chloroform) UV–vis spectra are shown in Fig. 2. The results are given in Table 3. The molecular orbital geometry calculations show that the first excited state originates from the HOMO (highest occupied molecular orbital) to LUMO (lowest unoccupied molecular orbital) transition that corresponds to the lmax absorption band in the UV–vis spectrum. The absorption band at 330 nm corresponds to an n–p* transition with a strong oscillator strength of 0.9331. The corresponding calculated values using the B3LYP/ccpVTZ and CAM-B3LYP/cc-pVTZ functional are found to be 330 nm and 306 nm, respectively. The next strong transition is observed at 247 nm corresponds to p ! p* transition. 4.4. HOMO–LUMO analysis Both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is the main orbital take part in chemical stability [17]. The HOMO represents the ability to donate an electron, LUMO as an electron acceptor represents the ability to obtain an electron. The HOMO and LUMO energy calculated by B3LYP/6-31G(d,p) method as shown below. HOMO energy =
0.21119 a.u.

LUMO energy =
0.05825 a.u.

Table 4 First hyperpolarizability (b) components of TMPPO and urea (a.u) calculated using Cartesian coordinates in the Gaussian standard orientation.

b components

Static (l = 1) TMPPO

bxxx bxxy bxyy byyy bxxz bxyz byyz bxzz byzz bzzz b (total)


2403
149 235
60
84
0.17 12 60
54 23 2124

Dynamic (l = 1064 nm) Urea

TMPPO

Urea

HOMO–LUMO energy gap =
0.15294 a.u. The low value of the HOMO and LUMO energy gap explains the charge transfer interactions taking place within the molecule and reflects its NLO activity. The HOMO and LUMO orbital are shown in Fig. 3. 4.5. Thermal analysis The TGA/DTA method was used for the study of the thermal behavior of TMPPO, which gives information about the thermal stability of the material. The thermo analytical curves of TMPPO are presented in Fig. 4. The DTA curve implies that the material undergoes an irreversible endothermic transition, where the melting begins. The endothermic peak of the DTA at 239.35  C corresponds to the first phase of the weight loss in the TG curve, indicating decomposition of the sample. This endothermic peak is followed by an exothermic peak at 279.05  C indicating evaporation of the decomposed compound. 4.6. Molecular hyperpolarizability The first hyperpolarizability (bo) of TMPPO and its components is also calculated by choosing the origin of the Cartesian coordinate system (x, y, z) = (0, 0, 0) at the center of the compound. In the presence of an applied electric field, the energy of a system is a function of electric field. First hyperpolarizability is a third rank tensor that can be described by a 3  3  3 matrix. The 27 components of the 3D matrix can be reduced to 10 components by the Kleinman symmetry [18]. The components of b are defined as the coefficients in the Taylor series expansion of the energy in the external electric field. When the external electric field is weak and homogeneous, 1 1 1 E ¼ E0
mi F i
aij F i F j
bijk F i F j F k
g ijkl F i F j F k F l þ . . . 2 6 24

(3)

0

6
1 16 1 0 0 0 7 0 0 29

4191 145 138
247 2
71 67
12 61
26 3897

6
1 18 1 0 0 0 9 0 0 32

where E is the energy of the unperturbed molecules, Fi is the field at the origin, mi, aij, bijk and g ijkl are the components of dipole moment, polarizability, the first hyperpolarizabilities, and second hyperpolarizabilities, respectively. The mean polarizability a0, the anisotropy of the polarizability Da and the mean first hyperpolarizability bijk, using the x, y, z components are defined as

a0 ¼

 1 axx þ ayy þ azz 3

(4)

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A. S. et al. / Vibrational Spectroscopy 79 (2015) 1–10

Table 5 Vibrational assignment of TMPPO by normal mode analysis based on SQM force field calculations. Experimental wavenumbers (cm
1) IR

Raman

3106 w

3110 w 3052 w

3048 w 2990 m

2948 w

2944 vw

2909 w 2881 w

2915 w

2879 vw 2853 vw 2837 vw 1658 m 1593 s

1653 m 1596 s 1568 w 1493 m

1461 m 1412 m

1417 w

1327 s

1329 s 1264 m 1233 m

1202 s 1194 s

1149 m

1111 s 1066 w

1067 m

1028 m 983 w 948 w

951 w 926 vw

867 w 828 m 802 vw 786 vw

685 vw 628 w 556 m

Scaled wavenumbers (cm
1)

IRa intensity

Ramanbintensity

Characterization of normal modes with PEDc (%)

3103 3095 3086 3054 2993 2990 2988 2961 2950 2935 2928 2927 2893 2878 2874 2872 2871 2840 1655 1598 1580 1561 1529 1487 1483 1482 1480 1473 1470 1469 1467 1467 1465 1464 1434 1414 1388 1362 1339 1324 1306 1273 1231 1216 1208 1197 1190 1185 1155 1153 1153 1152 1145 1119 1078 1055 1048 1036 1016 963 948 924 898 831 804 793 787 777 700 684 624 597 564 546

0.066 0.004 0.003 0.013 0.069 0.086 0.080 0.055 0.089 0.100 0.138 0.136 0.082 0.212 0.287 0.284 0.261 0.039 0.492 0.262 1.000 0.630 0.049 0.130 0.209 0.216 0.201 0.159 0.179 0.184 0.176 0.176 0.149 0.133 0.153 0.443 0.041 0.040 0.070 0.185 0.374 0.034 0.506 0.662 0.376 0.218 0.333 0.188 0.353 0.331 0.331 0.304 0.255 0.155 0.178 0.107 0.114 0.049 0.059 0.013 0.026 0.009 0.007 0.008 0.05 0.035 0.067 0.022 0.010 0.003 0.010 0.002 0.025 0.049

1.74 1.56 2.08 0.88 5.71 7.11 6.38 2.01 2.61 2.85 3.74 3.68 5.54 9.77 12.70 12.30 11.20 4.34 24.60 97.70 100 67.40 4.25 7.90 8.98 9.25 9.92 17.20 16.10 15.60 15.40 15.40 15.20 14.60 6.13 6.13 1.51 1.44 10.90 17.70 14.10 1.35 11.70 23.40 14.20 50.00 19.70 9.33 3.53 3.37 3.37 3.11 1.49 1.00 16.30 5.77 5.69 3.01 1.27 4.38 6.22 2.18 2.31 3.48 0.57 1.62 0.87 1.03 0.45 0.24 0.45 1.54 0.97 2.38

nCH (99) nCH (99) nCH (99) nCH (99) nCH3 (100) nCH3 (100) nCH3 (100) nCH3 (99) nCH3 (99) nCH3 (100) nCH3 (100) nCH3 (100) nCH3 (99) nCH3 (100) nCH2 (97) nCH3 (99) nCH3 (100) nCH2 (99) nC¼O (79) nCC (75), bCH (15) nCC (66) nCC (71), bCO (11) bCH2 (93) CH3IPB (66) CH3IPB (81) CH3IPB (87) CH3IPB (87) CH3SB (45), nCC (19), CH3IPB (13) CH3OPB (88) CH3SB (54), nCC (12), CH3IPB (11) CH3OPB (92) CH3OPB (89) CH3OPB (83) CH3SB (73), CH3OPB (10) CH3SB (51), nCC (23), bCH (13) nCC (33), CH3SB (25), bCH (12), nCO (11) CH3SB (89) bCH2 (58), nCC (22) nCC (62), nCO (12) nCC (43), nCO (19) bCH2 (40), nCO (20), nCC (14) bCH (57), nCC (20), bCCC (10) nCC (38), bCH2 (17), bCO (13) nCO (25), CH3OPR (25), nCC (16) CH3OPR (33), bCH (18), nCO (13), CH3IPR (12) nCC (32), bCH (29), CH3OPR (15) bCH (37), nCC (11), nOC (11) CH3OPR (60), CH3IPR (23) bCH (34), CH3IPR (18), nCO (16), nOC(13) CH3IPR (68), CH3OPR (26) CH3IPR (60), CH3OPR (31) CH3IPR (66), CH3OPR (28) bCH (31), nOC (23), nCO (3), CH3OPR (11) bCH (30), nOC (28), nCC (15) bCH2 (34), CH3IPR (23), nCC (19) nOC (57), nCC (17), bCH (16) CH3OPR (47), nCC (18) nOC (55), bR2tri (15), nCC (14), bCH (11) gCH (53), TCC (40) nCC (86) nOC (31), nCO (30), nCC (10) nOC (37), nCO (26), nCC (10) gCH (68), tR2asy (13) bCH2 (18), CH3IPR (18), bCCC (18), nCC (17) tR2asy (56), gCH (21), tR2tri (14) nCC (25), bCH2 (23), CH3IPR (19) tR2tri (44), tR2sym (31), gCH (21) nCC (56), bCCC (11) tR2asy (32), tR2tri (30), bCO (16) tR2asy (49), tR2tri (48) tR2tri (50), tR2asy (26) bCOC (33), nCC (14), nCO (12) tR2sym (61), gCO (15) bCCC (23), bCOC (18), bR2sym (17)

A. S. et al. / Vibrational Spectroscopy 79 (2015) 1–10

7

Table 5 (Continued) Experimental wavenumbers (cm
1) IR

Raman

512 vw

521 vw

420 vw 373 vw

253 vw

123 vw

66 m

Scaled wavenumbers (cm
1)

IRa intensity

0.038 0.004 0.016 0.004 0.005 0.003 0.006 0.007 0.004 0.016 0.011 0.005 0.009 0.004 0.004 0.003 0.003 0.003 0.003 0.006 0.006 0.003 0.005 0.005 0.007 0.001 0.001 0.004

541 517 502 471 430 370 365 338 274 265 259 253 241 228 207 193 184 181 177 168 142 118 93 88 72 38 33 19

Ramanb intensity

1.91 0.71 3.36 1.02 5.13 7.34 14.60 0.82 1.78 6.14 6.57 8.14 3.3 2.61 1.98 6.71 7.2 8.19 8.01 6.32 3.56 3.11 7.37 12.30 4.03 88.20 8.00 95.60

Characterization of normal modes with PEDc(%)

gCO (27), bCO (24), tR2sym (14) tR2asy (58), tR2sym (39) bR2asy (32), bCOC (14) bCOC (32), bR2asy (17), bCCC (15), bCO (10) bCCC (38), bR2sym (13), bCO (11) tR2asy (21), tR2tri (20), gCC (17), gCO (16) bCOC (34), yCC (24), bCO (15) bCOC (53), bCC (11) tR2asy (56), tR2sym (38) bCCC (21), tR2sym (16), TCH3 (13), bCC (12) TCH3 (36), tR2asy (24), tR2tri (24) tR2asy (26), tR2sym (14), TCH3 (12) TCH3 (32), tR2sym (25), tR2asy (16) TCH3 (49), gCO (13), bCCC (12) TCH3 (52), bCO (14), bCCC (10) tR2asy (67), tR2sym (24) tR2asy (49), tR2sym (30) tR2asy (52), tR2sym (37) bCO (44), bCOC (17), tR2asy (13) tR2sym (57), tR2tri (25), tR2asy (10) tR2asy (46), tR2sym (31) TCCOC (52), TCH3 (15), TCC (12) tR2asy (54), tR2sym (21), TCCOC (12) bCCC (43), bCC (16), tR2asy (10) tR2asy (31), TCCOC (23), tR2sym (19) tR2asy (34), tR2sym (24), TCCH2 (23) tR2asy (46), tR2sym (32) TCC (65)

RMS value for the scale factor calculation is 8.86 cm
1. n: stretching; b: bending; SB: symmetric bending; IPB: in-plane bending; OPB: out of plane bending; OPR: out plane rocking; IPR: in-plane rocking; g: out of plane bending; T: Torsion; tRsym: symmetric ring torsion; tRasym: asymmetric ring torsion; tRtri: trigonal ring torsion. a Calculated IR intensities. b Relative Raman intensities calculated by Eq. (1) and normalized to 100. c Only PED contributions >10% are listed. For definition of local symmetry coordinates see Table S2 provided as Supplementary material (Appendix A).

h

Da ¼ 2
1=2 axx
ayy

2

i1=2  2 þ ayy
azz þ ðazz
axx Þ2 þ 6a2xx (5)




bijk ¼ b2x þ b2y þ b2z

1=2

bx ¼ bxxx þ bxyy þ bxzz

(7)

by ¼ byyy þ bxxy þ byzz

(8)

bz ¼ bzzz þ bxxz þ byyz

(9)

(6)

where bx, by and bz are the components of first hyperpolarizability given by,

The components of the static and dynamic hyperpolarizability tensor components of TMPPO and urea molecule are shown in Table 4. From Table 4 it is found that the calculated first hyperpolarizability value of urea is 32 a.u. (at l = 1064 nm). The experimental data obtained from the EFISH (electric-field-induced second harmonic generation) measurement in water solution is (0.45  0.12)  10
30 esu, which ranges from 0.33  10
30 to 0.57  10
30 esu (38–66 a.u.) [19]. Therefore the first hyperpolarizability value calculated with B3LYP/cc-pVTZ level is in agreement with the experimental value. Hence it can be shown that the static and dynamic first hyperpolarizability values of TMPPO are nearly 73 and 122 times larger than urea (Table 4). 4.7. Vibrational spectral analysis

Fig. 5. (a) Experimental FT-IR spectrum of TMPPO, and (b) theoretical IR spectrum of TMPPO.

The vibrational spectral analysis of (E)-1-(2,4,6-trimethoxyphenyl)pent-1-en-3-one (TMPPO) was performed on the basis of the characteristic vibrations of the methoxy group, carbonyl group, methyl group, methylene group, phenyl ring and ethylenic bridge. The vibrational spectral assignments have been carried out with the help of normal coordinate analysis using the MOLVIB program version 7.0 written by Sundius [20,21]. The internal coordinates have been described (Table S1, see in Supplementary material) according to Pulay et al. commendations [22]. The computed

8

A. S. et al. / Vibrational Spectroscopy 79 (2015) 1–10

and back-donation, which cause the decrease of stretching wavenumbers and IR intensities, as reported in literature [29] for similar molecular systems. The asymmetric and symmetric stretching modes of the CH3 group are expected to occur about 2962 and 2872 cm
1 [25]. The methyl symmetric stretching vibration is active in IR as a weak band at 2881 cm
1. The weak bands at 2948 cm
1 in IR and at 2944 cm
1 in Raman are assigned to methyl asymmetric stretching vibrations. The simultaneous IR and Raman activation of the CH3 asymmetric stretching vibrations is due to the presence of intramolecular charge transfer in the molecule. The methyl rocking, wagging and twisting vibrations are expected to appear in the region 1422–719 cm
1. The CH3 rocking mode of methyl is observed as a medium intensity band at 1202 cm
1 in the IR spectrum, which is coupled with C

H bending and carbonyl stretching vibrations. The CH3 out of plane bending vibrations are also present and are listed in Table 5.

Fig. 6. (a) Experimental FT-Raman spectrum of TMPPO, and (b) theoretical Raman spectrum of TMPPO.

wavenumbers are selectively scaled according to the SQMFF (scaled quantum mechanical force field) procedure comprising a set of 11 transferable scale factors (Table S2) suggested by Rauhut and Pulay [23]. The detailed vibrational assignments of fundamental modes along with the calculated IR and Raman intensities and normal mode descriptions (characterized by PED) are reported in Table 5. The observed and simulated FT-IR and FT-Raman spectra are given in Figs. 5 and 6 for visual comparison. 4.7.1. Methoxy group vibrations When a CH3 group is directly attached to oxygen atom, the C

H stretching and bending bands can shift their position due to electronic effects [24]. Vibrational studies on the aryl methoxy group have shown that the asymmetric and symmetric methyl stretching bands can be observed around 2960 and 2846 cm
1, respectively [25–28]. Since the calculations were done on a free molecule and the experimental spectra were measured on solid samples, at least some differences between calculated and experimental wavenumbers may exist due to intermolecular interactions and crystal effects in the samples. In TMPPO, the asymmetric stretching mode of the methoxy group is observed in IR as a medium intensity band at 2990 cm
1 and the symmetric stretching mode is observed as a very weak band in Raman at 2853 cm
1. The calculated values of asymmetric and symmetric stretching modes are found at 2990 cm
1 and 2871 cm
1, respectively. This lowering of stretching wavenumber from the computed values and the decrease in intensity may be due to the electronic effects caused by back-donation [26] and induction due to the presence of an oxygen atom [29–31]. The asymmetric bending vibrations of methoxy groups are expected to appear around 1460 cm
1 [26]. The medium intensity band at 1461 cm
1 in the IR spectrum is assigned to CH3 asymmetric bending mode. There is a lowering of symmetric bending mode by about 3 cm
1 in comparison with the calculated results (Table 5) which is due to the electronic effects. The CH3 rocking vibrations are mixed with C

O stretch and the C

H bending mode, which results in a weak band at 1149 cm
1 in IR. The band observed at 253 cm
1 in IR corresponds to the torsional modes of CH3, which is in agreement with the computed values. 4.7.2. Methyl group vibrations Methyl groups are generally referred to as electron donating substituents in the aromatic ring system. The methyl hydrogen atoms in TMPPO are subjected simultaneously to hyperconjugation

4.7.3. Carbonyl group vibrations The C¼O absorption is almost always one of the most characteristic bands in the entire spectrum, and it is also likely to be the most intense spectral feature. Hence such bands have been the extensively studied [32–34]. Due to conjugation the intensity of these bands increase. In saturated esters the carbonyl stretching vibrations are expected to appear in the region 1750– 1735 cm
1. The medium intensity bands at 1658 cm
1 in the IR spectrum and at 1653 cm
1 in the Raman spectrum are assigned to the C¼O stretching mode. This lowering of carbonyl stretching wavenumbers is due to the conjugation of C¼O bond with the aromatic ring [35]. 4.7.4. Methylene group vibrations The asymmetric and symmetric stretching bands of the methylene groups occur near 2926 and 2853 cm
1, respectively [25,26,34]. The symmetric stretching mode of methylene group of TMPPO is observed as a very weak band in IR at 2837 cm
1 (computed at 2840 cm
1). The downshifting of the stretching wavenumber and the weakening of the methylene stretching intensities is due to the existence of electronic effects of hyperconjugation and back-donation. The twisting, wagging, and rocking vibrations of the methylene groups normally appear in the region 1362–793 cm
1. The in-plane and out-of plane deformation modes have also been observed and are presented in Table 5. 4.7.5. Vibrations of phenyl ring Various normal modes of vibration of the substituted phenyl ring have been broadly studied according to Wilson’s numbering convention [36]. For benzene, there are two doubly degenerate C

C ring stretching modes e2g (8a, 8b) and e1u (19a, 19b). The ring mode 8a is observed as strong bands in both IR and Raman spectra at 1593 cm
1 and 1596 cm
1, respectively. The phenyl ring mode 8b is active in Raman at 1568 cm
1 as a weak band. The intensity difference between modes 8a and 8b is large because the intensities do not depend on the algebraic differences of the electronic effects of the substituent. The ring mode 19a is expected in the region 1460–1530 cm
1 and 19b, between 1370 and 1470 cm
1 [27]. Mode 19a is calculated to be at 1473 cm
1. The band observed at 1412 cm
1 in the IR spectrum and at 1417 cm
1 in the Raman spectrum corresponds to the 19b mode. The mode 14 is observed in the IR spectrum at 1327 cm
1 and in the Raman spectrum at 1329 cm
1 as strong bands. The simultaneous IR and Raman activation of the phenyl ring modes of 8, 19 and 14 provide evidence for the charge transfer interaction between the donor and the acceptor group through the p-system. In tetra-substituted benzene, the C

H

in-plane bending modes 3, 9a, 18a and 18b can be expected in the region 1300–

A. S. et al. / Vibrational Spectroscopy 79 (2015) 1–10

1000 cm
1 [27]. The strong band in IR at 1111 cm
1 corresponds to ring mode 18b. Mode 9a is observed as a strong band in the Raman spectrum at 1194 cm
1. The intensity enhancement of these modes is due to the presence of the strong electron donor substituent (OCH3 groups) [25–26]. The C

H out of plane bending vibrations is expected to occur in the region 1000–675 cm
1 [26–27]. The 17a C

H out of plane bending vibration is observed as a weak band at 867 cm
1 in IR spectrum and 17b is observed at 802 cm
1 in the Raman spectrum. The modes corresponding to 10a are observed as a weak band at 786 cm
1 in Raman spectrum, which is also in agreement with the DFT results. In all the aromatic compounds, the carbon–hydrogen stretching vibrations occur in the region 3100–3000 cm
1 [26–27]. The C

H stretching vibrations in the benzene derivatives arise from two nondegenerate modes a1g (3072 cm
1), b1u (3060 cm
1) and two degenerate modes e2g (3047 cm
1), e1u (3099 cm
1), i.e., the vibrations 2, 13, 7 and 20, respectively. A weak band at 3052 cm
1 in the Raman spectrum corresponds to the C

H stretching mode 20a. The weak band observed in the IR spectrum at 3048 cm
1 corresponds to mode 7b. In benzene, the fundamentals a1g (997 cm
1) and b1u (1010 cm
1) represent the ring breathing and trigonal bending modes, respectively. The trigonal ring bending mode can be observed as a medium intensity band in the IR spectrum at 1028 cm
1. The ring breathing modes for the substituted phenyl ring with entirely different substituents [27–28] have been reported to be strongly IR active with typical bands in the interval 780–960 cm
1 and are given in Table 5. 4.7.6. Ethylenic bridge vibrations The vibrations of the ethylenic bridge are highly sensitive to the degree of charge transfer between the donor and the acceptor groups and hence such stretching modes are of special interest [37]. In TMPPO the C12¼C26 stretching is observed as a strong band at 1593 cm
1 in the IR spectrum and as a strong band at 1596 cm
1 in the Raman spectrum, and the corresponding calculated mode is at 1598 cm
1. In most cases, even in the absence of inversion symmetry, the strong bands in the Raman spectrum are weak in the infrared spectrum and vice versa. But the intramolecular charge transfer from the donor to acceptor group through the single-double bond conjugated path can induce large variations of both the molecular dipole moment as well as the molecular polarizability, making the IR and Raman activity considerable at the same time [29]. Thus in TMPPO, simultaneous infrared and Raman activation of the C12¼C26 stretching modes indicates the charge transfer interaction between the

COOCH3 group and phenyl ring through the ethylenic bridge. From Table 5 it can be seen that the calculated modes at 1580 and 1561 cm
1 also have large intensities both in Raman and IR. 5. Vibrational contribution to NLO activity With the aid of vibrational spectroscopy the p-conjugated systems that are having large values of molecular second order polarizabilities (b) were analyzed. It is found that some of the vibrational modes are simultaneously active in both IR and Raman [38–39]. The vibrational mode 8 splits into 8a and 8b and mode 8a possess higher wavenumber. The phenyl ring modes 8 and 14 are simultaneously active in both the IR and Raman spectrum which is important for the NLO activity of the molecule. The bands observed in IR at 1593, 1412, and 1327 cm
1 are active in the Raman spectrum at 1596, 1417, and 1329 cm
1, respectively. This shows that the relative IR and Raman intensities are comparable due to the electron cloud movement through p-conjugated frame work from electron donor to electron acceptor group.

9

6. Conclusions The detailed interpretation of the vibrational spectra has been carried out with the aid of the scaled quantum mechanical method. The calculated static first molecular hyperpolarizability is found to be 1.83  10
29 esu which is 73 times larger than urea. The second harmonic generation efficiency was found to be 2.1 times that of urea. These results show that TMPPO can be used for nonlinear optical applications. The natural bond orbital analysis provides evidence of the formation of H-bonded interaction. The lowering of carbonyl stretching wavenumber indicates the presence of conjugation. The electronic effects of hyperconjugation and back-donation have also been established. The thermal analysis shows that the sample is stable up to 239.35  C. The low value of HOMO–LUMO energy gap explains the charge transfer interactions taking place within the molecule. The simultaneous IR and Raman activation of the phenyl ring modes of 8, 19 and 14 also provides evidence for the charge transfer interaction. Acknowledgement D.S. (D. Sajan) thanks the Council of Scientific and Industrial Research (CSIR), New Delhi-110012, India for the financial support (No. 03(1247)/12/EMR-II). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. vibspec.2015.04.002. References [1] D.S. Chemla, Nonlinear Optical Properties of Organic Molecules and Crystals, Academic Press, Boston, 1987. [2] A.C. Grimsdale, R. Cervini, R.H. Friend, A.B. Holmes, S.T. Kim, S.C. Moratti, Synth. Met. 85 (1997) 1257–1258. [3] M.W. Liu, X.H. Zhang, W.Y. Lai, X.Q. Lin, F.L. Wong, Z.Q. Gao, C.S. Lee, L.S. Hung, S.T. Lee, H.L. Kwong, Phys. Status Solidi A: Appl. Res. 185 (2001) 203–211. [4] C. Ravikumar, I. Hubert Joe, V.S. Jayakumar, Chem. Phys. Lett. 460 (2008) 552– 558. [5] C. Ravikumar, I. Hubert Joe, D. Sajan, Chem. Phys. 369 (2010) 1–7. [6] C. Ravikumar, I. Hubert Joe, Phys. Chem. Chem. Phys. 12 (2010) 9452–9460. [7] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798–3813. [8] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman Jr., J.A. Montgomery, T. Vreven, K.N. Kudin, J.C. Burant, J.M. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 09, Revision B.01, Gaussian, Inc., Wallingford, CT, 2010. [9] T.H. Dunning Jr., J. Chem. Phys. 90 (2) (1989) 1007–1023. [10] G. Keresztury, S. Holly, J. Varga, G. Besenyei, A.Y. Wang, J.R. Durig, Spectrochim. Acta A 49 (1993) 2007–2017. [11] S. Miertuš, E. Scrocco, J. Tomasi, Chem. Phys. 55 (1981) 117–129. [12] S. Miertuš, J. Tomasi, Chem. Phys. 65 (1982) 239–245. [13] M. Cossi, V. Barone, R. Cammi, J. Tomasi, Chem. Phys. Lett. 255 (1996) 327–335. [14] A. Collas, F. Blockhuys, Acta Crystallogr. E 66 (2010) o2525–o2526. [15] E.D. Glendening, A.E. Reed, J.E. Carpenter, F. Weinhold, NBO Version 3.1, TCI, University of Wisconsin, Madison, 1998. [16] A.E. Reed, L.A. Curtiss, F. Weinhold, Chem. Rev. 88 (1988) 899–926. [17] S. Gunasekaran, R.A. Balaji, S. Kumaresan, G. Anand, S. Srinivasan, Can. J. Anal. Sci. Spectrosc. 53 (2008) 149–161. [18] D.A. Kleinman, Phys. Rev. 126 (1962) 1977–1979. [19] I. Ledoux, J. Zyss, Chem. Phys. 73 (1982) 203–213. [20] T. Sundius, J. Mol. Struct. 218 (1990) 321–326. [21] T. Sundius, Vibr. Spectrosc. 29 (2002) 89–95. [22] P. Pulay, G. Fogarasi, F. Pang, J.E. Boggs, J. Am. Chem. Soc. 101 (1979) 2550–2560. [23] G. Rauhut, P. Pulay, J. Phys. Chem. 99 (1995) 3093–3100.

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