Influence of the bromo group on the vibrational spectra and macroscopic properties of benzophenone derivatives

Journal of Molecular Structure 887 (2008) 87–91

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Influence of the bromo group on the vibrational spectra and macroscopic properties of benzophenone derivatives L.M. Babkov a, J. Baran b, N.A. Davydova c,*, D. Drozd b, O.S. Pyshkin d, K.E. Uspenskiy a a

Saratov State University, Saratov, Russia Institute of Low Temperature and Structure Research, PAS, Wroclaw, Poland c Institute of Physics, 46 Pr. Nauki, NANU, 03028 Kiev, Ukraine d Institute for Low Temperature Physics and Engineering, NANU, Kharkov, Ukraine b

a r t i c l e

i n f o

Article history: Received 14 September 2007 Received in revised form 1 February 2008 Accepted 1 February 2008 Available online 22 March 2008 Keywords: 2-Bromobenzophenone 4-Bromobenzophenone Computer simulation Differential scanning calorimetry IR spectra Structure Raman spectra

a b s t r a c t The effects of a minor chemical modification such as a change in the position of a Br atom within the same phenyl ring on the optical and macroscopic properties of benzophenone derivatives are investigated by spectroscopic and calorimetry methods. More specifically, we have studied IR and Raman spectra of the two isomers of monosubstituted benzophenones: 2-bromobenzophenone (2BrBP) and 4-bromobenzophenone (4BrBP) in the wide spectral and temperature regions. It has been found that the substitution of a Br in an ortho position leads to some changes of the anharmonicity of the m(C@O) vibrations. Full geometry optimization and vibrational spectra modeling for 2BrBP and 4BrBP isolated molecules have been calculated by the density functional method (B3LYP/6–31+G(d)) using GAUSSIAN’03 software. Quantum-mechanical calculations for the isolated molecules have shown that the shape of 2BrBP molecule is strongly asymmetric in comparison with the shape of 4BrBP molecule. A change in the molecular shape translates into rather different macroscopic properties such as the crystal melting points. Namely, the melting point of 2BrBP (318 K) was found to be lower than that of 4BrBP isomorphs (358 K). Moreover, 2BrBP exhibits a large reluctance to crystallize, while 4BrBP crystallizes immediately below the melting point as a liquid is cooled. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Over the last 20 years much effort has been directed to organic nonlinear optical materials due to their promising applications in optoelectronics technology [1,2]. Recently, during systematic searching for nonlinear optical materials it has been found [3,4] that the highly conjugated organic compounds can easily crystallize in the noncentrosymmetric structures when the H atom in the phenyl is substituted with the Br atom. That is to say, the Br atom is an effective atom for the macroscopic second-order nonlinearities [4]. Most of the studies of Br-substituted compounds were directed towards registration of a second harmonic generation. However, the questions remain as to the role of the Br atom in changing of the vibrational and macroscopic properties of bromo-substituted compounds. We have performed IR, Raman and differential scanning calorimetry (DSC) measurements for the two isomers of Br-substituted benzophenones: 2-bromobenzophenone and 4-bromobenzophenone (referred to as 2BrBP and 4BrBP). More specifically, we have studied the influence of the position of a Br atom within the same phenyl ring (ortho or para) on the IR and

* Corresponding author. Tel.: +380 44 525 3515; fax: +380 44 525 1589. E-mail address: [email protected] (N.A. Davydova). 0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2008.02.045

Raman spectra and on the thermal properties, such as melting and glass transition temperatures. Nothing changes in these experiments but only the structure (the position of Br atom) so all changes in the vibrational and macroscopic properties must be due to differences in structure. It should be noted that the IR and Raman spectra and thermal properties of benzophenone (BP) has been studied previously [5–7]. As related to 2BrBP and 4BrBP compounds they have been studied with different techniques including luminescence spectroscopy, single-crystal and powder X-ray diffractometry [8–10]. However, to our knowledge, no temperature-dependent IR and Raman vibrational spectra have been measured previously for 2BrBP and 4BrBP, and no information concerning their thermal stability and the glass transition temperatures have been obtained. 2. Experimental procedure FTIR absorption spectra were recorded at different temperatures with Bruker IFS-88 Fourier transform infrared spectrometer, with resolution 2 cm 1 and 32 scans were typically co-added for an individual spectrum. Data processing was performed with OPUS software. For the IR measurements, the powder samples were inserted in a cell with two KBr windows at room temperature, then

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melted and crystallized into thin film. Spectra were collected at temperatures between ca. 12–300 K at roughly 30 K intervals. The Raman spectra were measured using Bruker IFS-88 infrared spectrometer equipped with FRA-106 Raman attachment, and 32 scans were taken for each spectrum. The 1064 nm line of an Nd:YAG laser (ca. 200 mW) was used as exciting light. The back scattering geometry was applied for the FTRaman spectra measurement. The resolution for the Raman experiments was set up to 2 cm 1. The thermal properties were measured with a Perkin-Elmer DSC-7 differential scanning calorimeter equipped with the CCA-7 low temperature accessory, using sealed Al pans. Liquid nitrogen was used as a coolant and the measurements were carried out in the temperature range 104 up to 343 K. The quantum mechanical calculations were performed with GAUSSIAN’03 program package. Full geometry optimization and vibrational spectra modeling for 2BrBP and 4BrBP isolated molecules were performed at B3LYP levels with the 6–31+G(d) basis set. 3. Results and discussion

Table 1 Some torsion angles calculated for molecule in this work in comparison with the respective angles measured by X-ray in [8,9] and calculated dipole moments Torsion angles

2BrBP (this 2BrBP Values work) from X-ray [8]

O1C1C1aC6a O1C1C1bC6b C2aC1aC1C1b C2bC1bC1C1a Dipole moment

58.7 15.7 65.0 16.1 3.242

68.3 17.6 70.0 22.4

4BrBP (this 4BrBP Values work) from X-ray [9]

BP (from [5])

29.5 25.8 33.7 29.3 3.027

28.3 28.3 32.1 32.1 3.239

21.6 28.1

the unsubstituted aromatic ring ( 15.7°). Such very large the torsion angle difference results in a strong asymmetry of the shape of 2BrBP molecule. In contrast to 2BrBP the conformation of benzophenone is not appreciably changed by the introduction of Br into the 4-position. Comparison of a few calculated torsion angles with the X-ray crystallographic measurements [8,9] listed in Table 1 shows their good agreement. The calculated values of the dipole moments of the molecules are also given in Table 1.

3.1. Optimized structures 3.2. Thermal behavior of 2BrBP and 4BrBP Figs. 1 and 2 show the optimized structures of 2BrBP and 4BrBP molecules, correspondingly. Any conformation of these molecules can be specified by two dihedral torsion angles (h, /) which define the orientations of the phenyl rings relative to the plane of the Car–CO–Car group. Quantum-mechanical calculations for the isolated molecules of 2BrBP and 4BrBP give the next values for the torsion angles. The torsion angles of the unsubstituted /OACACB1ACB6 and substituted hOACACA1ACA6 aromatic rings for 2BrBP were found to be 15.7° and 119.0°, respectively. The same torsion angles for 4BrBP are equal: 25.8o and 29.5°. The angles h and / in BP were found to be 28.3° [5]. It is seen that the torsion angle involving atoms of the substituted aromatic ring in 2BrBP is unusually large (119.0°), compared with the torsion angle of

The thermal behavior of two bromo-substituted benzophenones is shown in Fig. 3 by the corresponding DSC measurements from 293 to 380 K at a heating rate of 20 K min 1. Curve 1 corresponds to the heating of crystalline 2BrBP with a mass of 7.68 mg and curve 2 to the heating of crystalline 4BrBP with a mass of 8.4 mg as crystalline phase converted to liquid. Only sharp endothermic peaks are seen on these curves, which correspond to the melting of the crystalline samples. The crystalline 2BrBP starts to melt at the onset temperature 316.97 K (peak temperature 317.35 K), and the melting enthalpy is 68.23 J/g. The crystalline 4BrBP starts to melt at the onset temperature 355.6 K (peak temperature 357.9 K) and the melting enthalpy is 92.10 J/g. From Fig. 3 it is seen that the melting point of 2BrBP is appreciably lower than that of 4BrBP analogue. For comparison, the melting point of the stable phase of BP is 328 K [6,7]. From experimental results we can deduced that the contribution of Br to the melting point depends on the position of the Br atom: ortho or para. Fig. 4 shows two cooling DSC curves 1 and 3 of 4BrBP and 2BrBP, respectively, and one heating DSC curve (2) of 2BrBP scanned with rate of 20 K/min. The cooling curve 1 shows a rel-

Tmelt = 318 K Fig. 1. Geometry of 2BrBP molecule optimized at B3LYP/6–31+G(d) level.

Tmelt = 358 K

Endo. heat flow (a.u.)

10000

8000

4BrBP

2BrBP

6000

4000

1

2

2000

0

300

310

320

330

340

350

360

370

380

Temperature (K) Fig. 2. Geometry of 4BrBP molecule optimized at B3LYP/6–31+G(d) level.

Fig. 3. Two heating DSC curves for crystalline 2BrBP with a mass of 7.68 mg (curve 1) and 4BrBP with a mass of 8.4 mg (curve 2). The heating rate is 20 K/min.

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4BrBP

Tmelt

Endo. heat flow (a.u.)

8000

6000

Tg

3.3. IR and Raman spectra

1

2BrBP

2 3

4000

2000

0

180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340

Temperature, K Fig. 4. DSC curves for 4BrBP (1) and 2BrBP (2 and 3). Cooling of the liquid phase from 250 to 104 K (curves 1 and 3); heating of the glassy phase from 104 to 343 K (curve 2). In the cooling–heating cycle the scanning rate is 20 K/min. The sample mass is 8.4 mg for 4BrBP and 19.5 mg for 2BrBP.

atively sharp single exothermal peak corresponding to crystallization of the liquid sample of 4BrBP (with mass 8.4 mg). The crystallization starts from the supercooled liquid region at 311.27 K (peak temperature 310.06 K). The enthalpy of crystallization is 75.54 J/g. It is seen from Fig. 4 (curve 3) that the liquid 2BrBP (m = 19.5 mg) does not crystallize on cooling and only a distinct glass transition at 225.1 K (onset) is clearly seen, indicating the freezing of the supercooled liquid. The entropy of the glass transition is 0.29 J/g K. Curve 2 shows the heating of the glassy sample from 104 to 370 K. Three thermal anomalies are seen. The first one, small endothermic rise corresponds to a glass transition at 224.8 K (onset) with the entropy of 0.70 J/g K, the second is a small endothermic peak at 300.48 K, and third endothermic peak at 316.07 K corresponds to the melting of the crystalline phase. The enthalpy of melting at 316.07 K (3.78 J/g) (Fig. 4, curve 2) is about 17 times smaller than the melting enthalpy of the crystalline sample (Fig. 3, curve 1). That means that we did not achieve the complete crystallization using the scanning rate of 20 K/min. In other words, 2BrBP exhibits a large reluctance to crystallization. The values of the melting, crystallization and glass transition temperatures for 2BrBP and 4BrBP and those for BP [6,7] are listed in Table 2. Thus, we can conclude that not only the melting point of monosubstituted isomers BP depends on the position of Br atom within the same phenyl ring (ortho or para), but also their reluctance to crystallization depends on the position of Br atom. The Br in para position gives the high preference for 4BrBP molecules to crystallize (Fig. 3), while Br in ortho position determines a high thermal stability of 2BrBP molecules against crystallization. It should be noted that the small endothermic peak at 300.48 K always accompanies peak at 316.07 K. Its peak enthalpy (0.53 J/K) is very small in comparison with the enthalpy of melting of the endothermic peak at 316.07 K. In our opinion it comes from the surface effects or strain on small crystallites in liquid.

Before discussing the results we wish to note that there are strong dipoles at C@O bonds in the studied compounds, which are responsible for the dipole–dipole interaction between the neighboring molecules. As was pointed out in [9], usually, in many substituted BP derivatives any two C@O groups are too far apart. In this respect, the 2BrBP proved to be a rare exception. At room temperature the distance between two C@O groups is comparatively short (about 3.4 Å´). In the IR and Raman spectra of a crystal the intermolecular interaction should decrease the m(C@O) stretching vibration frequency compared to that in an isolated molecule. Thus, we should expect the difference between the experimental and calculated values of the m(C@O) frequency. Another point that should be mentioned is that the C@O and the Br atoms are closely placed groups (Fig. 1). Thus we should expect the anharmonic coupling of the C@O group vibrations with the low-frequency stretching and out of plane deformation vibrations of the Br atom. We will focus on an analysis of the room temperature IR and Raman spectra in the C@O and CAC ring stretching bands region and their temperature dependences. Fig. 5 shows fragments of the IR spectra in the 1500–1750 cm 1 region, where the main characteristic stretching vibrations of the C@O (1645–1670 cm 1) and CAC (1561–1600 cm 1) manifest themselves, for BP (curve 1), 2BrBP (curve 2) and for 4BrBP (curve 3). Fig. 6 shows fragments of the Raman spectra in the 1500–1750 cm 1 region. The results of the calculations of the IR spectra for BP, 2BrBP and 4BrBP in the same region are presented in Fig. 7. The computed harmonic vibrational frequencies, relative intensities and their assignments in the 1561– 1670 cm 1 frequency region are presented in Table 3. An analysis of Table 3 shows, that vibrations are not ‘pure’ stretching vibrations (Q) of particular bonds, but mixed with the in-plane deformation (b,c) vibrations (the variations of the angles formed by two bonds: CAC and CAH). In consequence of mixed character of vibrations the forms of the calculated and experimental bands are complicated. The comparison of the experimental and calculated IR and

2BrBP (2) 1.0

Transmittance (a.u.)

Tcryst

4BrBP (3) BP (1) 0.5

1500

1550

1600

1650

1700

1750

-1

Wavenumber (cm ) Fig. 5. Fragments of the IR spectra for crystalline BP, 2BrBP and 4BrBP recorded at room temperature in the CAC and C@O stretching band regions.

Table 2 The values of the melting (Tmelt), and crystallization (Tcryst) temperatures in the maximum of the peaks, of the onset of the glass transition Tg (onset), of the enthalpy of melting (Emelt, J/g) and crystallization (Ecryst, J/g) and entropy of the glass transition (Eglass transition, J/gK) Compounds

Tmelt (peak), K

Emelt, J/g

Tcryst (peak), K

Ecryst, J/g

2BrBP 4BrBP BP

317.35 357.9 328

68.23 92.1 74.4

310.06

75.54

Tg (onset), K on cooling

Eglass transition, J/gK, on cooling

Tg (onset), K on heating

Eglass transition, J/gK, on heating

225.1

0.29

224.8

0.70

216.8

0.36

211.7

1.00

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4BrBP (3)

Raman spectra at room temperature reveals that the frequencies in the CAC phenyl ring stretch region are almost identical for BP, 2BrBP and 4BrBP compounds. In contrast, significant differences are seen between measured and calculated frequencies for C@O stretching vibration in the studied compounds. The experimental m(C@O) frequencies are much lower than that of the calculated frequencies (for isolated molecules) and this shift lies in the range of 15–20 cm 1. We consider this shift as a manifestation of the intermolecular C@O dipole–dipole interaction in the crystals. In order to elucidate the effect of temperature on the m(C@O) and m(CAC) vibrations we have performed variable temperature measurements of the IR spectra for the 2BrBP which are shown in Figs. 8 and 9, respectively. As can be seen the C@O band shifts from 1664 to 1659.5 cm 1 as temperature is decreased from 293 to 12 K (Fig. 8). In contrast, the complicated CAC band structure (Fig. 9) does not vary very strongly with temperature. Only the intensity of the band slightly increases with decreasing temperatures (Fig. 9). For comparison temperature dependence of the (C@O) band in BP is shown in Fig. 10. As can be seen from Fig. 10, the frequency and form of this band practically does not change with temperature. It is possible to explain the shift of the C@O band in 2BrBP by the effect of the mechanical anharmonic coupling between the high-frequency Q(C@O) stretching vibration and low-frequency Q(CABr) stretching and the v (CCACABr), v (CACA CABr), v (HACACABr) out-of-plane deformation vibrations. According to paper [11] the shift of the high-frequency vibrational band to the low-frequency side with decreasing temperature can be attributable to the third-order anharmonic constant in a power series of the potential energy of the crystal and the high-frequency shift to the fourth-order anharmonic constant. Thus, we can conclude that the Br atom in the ortho position effectively increases the third-order anharmonic interaction of the stretching C@O mode with the low-frequency vibrations.

2BrBP (2)

4. Conclusion

1649 1595.36

Relative Raman Units

0.6

1666.7 0.5

1649

0.4

1576.92

0.3

BP (1) 0.2

2BrBP (2) 0.1 0.0 1500

4BrBP (3) 1550

1600

1650

1700

1750

-1

Raman shift (cm ) Fig. 6. Fragments of the Raman spectra for crystalline BP (curve 1), 2BrBP (curve 2) and 4BrBP (curve 3) recorded at room temperature in the CAC and C@O stretching band regions.

Calculated IR Spectra

1671

300

IIR, Km/Mole

1683 200

BP (1)

100

0

1500

1550

1600

1650

ν, cm

1700

1750

1

Fig. 7. Fragments of the calculated IR spectra for crystalline BP (curve 1), 2BrBP (curve 2) and 4BrBP (curve 3) in the CAC and C@O stretching band regions.

In order to clearly understand the ‘structure–properties’ relations, we compare two single-bromated benzophenones, 2BrBP and 4BrBP with BP. We have studied their thermal properties such as melting and glass transition temperatures and their IR and Raman vibrational spectra.

Table 3 Calculated harmonic (B3LYP/6-31G(d)) IR and Raman vibrational frequencies (m), relative intensities (IIR and IRaman) and assignments for No.

Symm

m, cm

BP 52 53 54 55 56

A B A B A

1583 1585 1604 1606 1671 1

1

IIR

IRaman

Assignment

11.9 2.4 20.8 25.0 188.9

13.3 2.4 203.3 84.7 142.6

Q (C C), b (C C H), Q (OC@O CC@O), c (C C C) b (C C H), Q (C C), c (C C C) Q (C C), b (C C H), c (C C C), Q (CC@OC), c (CC@O C C) Q (C C), b (C C H), c (C C C), Q (CC@OC) Q (OC@O CC@O), c (C CC@OC)

IIR

IRaman

Assignment

1570 1586 1595 1604 1683

8.4 9,2 19.4 33.1 201.6

14.2 8,2 55.1 136.7 99.7

Q (CA CA), b (CA CA HA), c (CA CA CA) Q (CB CB), b (CB CB HB), c (CB CB CB) b (CA CA HA), c (CA CA CA), b (CB CB HB), c (CB CB CB), Q (O C), c (CB C CA)

1567 1584 1592 1605 1671

9.9 11.1 124.1 30.3 195.7

11.2 18.9 327.3 160.0 230.7

Q (CA CA), b (CA CA HA), c (CA CA CA) Q (CB CB), b (CB CB HB), c (CB CB CB) b (CA CA HA), c (CA CA CA), Q (C CA) b (CB CB HB), c (CB CB CB), Q (C CB) Q (O C), c (CB C CA)

No.

m, cm

2BrBP 53 54 55 56 57 4BrBP 53 54 55 56 57

For 2BrBP and 4BrBP index A corresponds to the substituted ring, index B corresponds to the unsubstituted ring. Coordinates of the in-plane deformation vibrations are denoted by b and c (the variations of the angle formed by two bonds, CCC and CCH). Coordinates of the stretching vibrations of OAC and CAC bonds are denoted by Q.

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2BrBP

0.9

Transmittance (a.u.)

012 100 150 200 250 293

Temperature dependence

0.8 0.7 0.6

0.9

Transmittance (a.u.)

1.0

Temperature dependence benzophenone

0.8

0.7

10 70 130 190 293

0.6

0.5

1656.23

0.5

0.4

1656.8 0.4 1630

1640

1650

1660

1670

1680

1690

1700

1640

Fig. 8. Fragments of the IR spectra for crystalline 2BrBP measured at T = 012, 100, 150, 200, 250 and 293 K (from left to right) in the C@O stretching band region.

Temperature dependence

Transmittance (a.u.)

2BrBP 0.9

012 100 150 200 250 293

0.8

0.7

0.6 1570

1580

1590

1600

1610

1670

Wavenumbers (cm )

Wavenumber (cm )

1560

1660 -1

-1

1.0

1650

1620

Fig. 10. Fragments of the IR spectra for crystalline BP measured at T = 012, 70, 130, 190 and 293 K in the C@O stretching band region.

set of quantum-chemical methods. It has been found, that the shape of 2BrBP molecule is strongly asymmetric in comparison to the shape of 4BrBP molecule. The temperature effect on the wavenumbers of the C@O band in 2BrBP we consider as a result of the anharmonicity responsible for the coupling between the high-frequency vibrations of the C@O group (C@O) with the low-frequency stretching and out-of-plane deformational vibrations of the Br atom. Acknowledgement The authors are grateful to Professor A. Yaremko for helpful discussions.

1630

-1

Wavenumber (cm ) Fig. 9. Fragments of the IR spectra for crystalline 2BrBP measured at T = 012, 100, 150, 200, 250 and 293 K in the CAC stretching band region.

It has been found that the melting point of monosubstituted isomers of the BP depends on the position of the Br atom within the same phenyl ring: ortho or para. Namely, Br atom in a para position is propitious to the higher melting temperature, while in ortho position to the lower melting temperature in comparison with BP. Furthermore, the Br atom in an ortho position determines a high thermal stability of 2BrBP molecules against crystallization. We have also performed full geometry optimization and vibrational spectra modeling at B3LYP levels with the 6–31+G(d) basis

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