Dielectric relaxation of the diaryl ketones

CHEMICAL

Volume 153, number 4

DIELECTRIC

RELAXATION

E. JAKUSEK,

H. KOLODZIEJ

PHYSICS LETTERS

16 December

1988

OF THE DIARYL KETONES

Znst,vtut Chemli, Umwersytet Wrockwski, ul. Joliot-Curie 14, 50-383 Wrociaw, Poland

and S. SORRISO Dipartimento di Chimica. University di Pemgia. via E/cc di Sotto, 8. 06100 Per&a, Italy Received 23 March 1988; in final form 5 October

1988

The dielectric relaxation of the three symmetrically substituted diary1 ketones: 2,2’-dipyridyl ketone (2,2’-DPyCO), dimes&y1 ketone (DMCO) and 3,3’-dinitromesityl ketone (3,3’-DNDMCO) in pxylene was examined over the frequency range 2-30 GHz, at 293-323 K. A distribution of relaxation times was observed for 2,2’-DPyCO and the average value for TVwas found to is monodisperse. A comparison of the be lower than for fluorenone. The absorption in the case of DMCO and 3,3’-DNDMCO relaxation times of these molecules suggests the possibility of a contribution from the intramolecular rotation of the pyridyl rings in 2,2’-DPyCO. The relaxation mechanism of 2,2’-DPyCO is discussed in terms of the Fong and Goulon-Rivail theories.

1. Introduction

Studies of the dielectric relaxation of Ar-X-Ar type diary1 systems (where X=0, S, SOz, CH2 or similar atoms or groups) reveal that in some cases the relaxation time (e.g. for diphenyl ether and its derivatives) is relatively short [ 11. One could suggest that, in addition to the overall molecular rotation, other mechanisms could be responsible for the dielectric relaxation of the systems. Among others, the inversion of a molecule [ 21 and the n conjugation [ 3,4] between the phenyl rings and the lone electron pairs of the bridge atoms have been suggested. Also the rotation of the aryl rings may modulate the dipole moment. Recent studies of the dispersion of the positive nonlinear dielectric effect (NDE) for dimesityl sulphones [ 5 ] seem to confirm the rotation of mesityl rings around the C-S bond. This rotation, however, appeared to be much slower in comparison with the over-all rotation and as a consequence the relaxation in the microwave region is of the Debye-like pattern and the estimated relaxation times are tens of pi0 009.2614/88/$ ( North-Holland

03.50 0 Elsevier Science Publishers Physics Publishing Division )

sulcoseconds (7= 89 ps for 3,3’-dinitromesityl phone in p-xylene, at 293 K). It seemed justified to obtain more information on the dielectric relaxation of molecules, in which, besides the dipole moment along the direction determined by the bisectrix CXC, there exists an additional dipole moment, perpendicular to the C-X-C plane. For such molecules the rotation of the aryl rings should reduce the effective relaxation time, according to the theoretical considerations of Fong [ 1 ] and Goulon and Rivail [ 61. For investigations we have chosen molecules large enough to study the contributions to dielectric relaxation deriving from the over-all and internal rotations, provided steric effects are not predominant.

2. Experimental The dielectric relaxation of 2,2’-dipyridyl ketone (2,2’-DPyCO), dimesitylene ketone (3,3’-DMCO) and 3,3’-dinitromesitylene ketone (3,3 ‘-DNDMCO) in p-xylene was measured in the frequency range 2-30 GHz at 293-323 K. The real t’ and imaginary B.V.

341

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CHEMICAL

6’ parts of the complex dielectric permittivity were measured by means of the modified Roberts-Hippel method [ 7 1. In the range 2-4 GHz the coaxial slotted line of Orion EZM-1 type was used, while in the range 4-30 GHz the rectangular wave-guides laboratory systems of Unipan X-100 were applied. The “static” dielectric measurements were performed at the frequency 1.59 kHz by means of the autobalance bridge Tesla DM-494. The error of E’ and c” was less than + 2% and + 0.5O& respectively.

PHYSICS LETTERS

Table I The experimental values of the real (E’) and imaginary (e”) parts of the dielectric permittivity as a function of temperature and frequency for 2,2’-DPyCO in pxylene at a concentration W-O.05 (weight fraction) T (K)

w (GHz)

e’

C”

293

static 2.0 6.1 1.5 31.3

2.547 2.522 2.470 2.440 2.336

0.046 0.087 0.096 0.061

303

313

323

342

static

2.523

_

2.0 6.1 7.5 31.3

2.516 2.462 2.439 2.343

0.039 0.085 0.090 0.068

static 2.0 6.1 1.5 31.3

2.503 2.495 2.456 2.438 2.325

0.038 0.076 0.087 0.060

static 2.0 6.1 1.5 31.3

2.483 2.447 2.422 2.422 2.307

0.025 0.066 0.081 0.064

1988

Table 2 The experimental real (e’ ) and imaginary (c”) pans of the dielectric permittivity as a function of temperature and frequency for DMCO inp-xylene at a concentration W= 0.053 T(K)

w (GHz)

E’

e”

293

static 2.09 3.00 7.48 9.91

2.481 2.402 2.312 2.315 2.305

_

static 2.09 3.00 7.48 9.97

2.456 2.398 2.366 2.306 2.291

0.084 0.088 0.060 0.048

static 2.09 3.00 7.48 9.91

2.431 2.386 2.356 2.292 2.219

0.076 0.082 0.064 0.054

static 2.09 3.00 7.48 9.97

2.406 2.372 2.348 2.281 2.271

0.064 0.080 0.066 0.055

303

3. Results and discussion The measured e’ and e’ values are presented in tables 1-3 and plotted as Cole-Cole diagrams in fig. 1. The analysis of c’ and C” values for 2,2’-DPyCO and in particular the distribution of the relaxation times (fig. 1) reveals the temperature dependence of the distribution parameter cy (table 4). For two other

16 December

313

323

0.090 0.090 0.054 0.044

ketones the dispersion was of the Debye-like type, with (Y= 0. It should be noted that both the relaxation time and the activation energy change markedly going from 2,2’-DPyCO to 3,3’-DNDMCO (table 4). The activation energy has been calculated from the Eyring-Kauzmann relationship [ 8 ] In(rT)=ln(h/k)-ASE+/R+AHg/RT,

where AHE’ is the molar activation enthalpy barrier, h is Planck’s constant and k is Boltzmann’s constant. For dimesityl ketones the dielectric increment changes linearly with the reciprocal temperature, while for 2,2’-DPyCO this relation is non-linear (fig. 2). Such a behaviour confirms that in the case of mesityl ketones we are dealing with a constant dipole moment reorientation, while in the case of 2,2’DPyCO the reorientation of molecules either proceeds between the non-equivalent positions or the dipole moment changes during rotation. Vector analysis shows two components of the dipole moment in 2,2’-DPyCO when the nitrogen atoms are located out of the plane of the >CO of the

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CHEMICAL

Table 3 The experimental real (t’) and imaginary (e”) parts of the dielectric permittivity as a function oftemperature and frequency for 3,3’-DNDMCO at a concentration W=O.O36 w (GHz)

e’

.z”

293

static 2.06 2.89 9.97

2.504 2.382 2.352 2.306 2.300

0.104 0.092 0.047 0.037

static 2.06 2.89 7.48 9.96

2.478 2.377 2.354 2.294 2.287

static 2.06 2.89 7.48 9.96

2.452 2.371 2.343 2.219 2.272

0.096 0.092 0.056 0.044

static 2.06 2.89 7.48 9.96

2.426 2.359 2.339 2.265 2.258

0.092 0.086 0.060 0.048

303

313

323

=/6/p:

1

where 70,, is the correlation

if 020

3’23K

0.096 0.05 I 0.040 0.10-

303K

0.05 -

WOO

2AO

2.30

2.50

Fig. I. Cole-Cole diagram for 2,2’-DPyCO W~0.05. The density of the solvent is d,.

d

in pxylene

at

(1)

where C, is the relative contribution deriving from the dipole moment component ,u,,parallel to the C=O bond. C, is the relative contribution originating from the dipole component pL perpendicular to the C=O bond. Provided that the dipole moment of the C=O group equals 2.94 D and the angle between the pyridyl rings in the C-CO-C plane is equal to 120”, CJC, ~0.23. It should be noted that the parallel component changes only with the over-all rotation, while the perpendicular component could also change with the mutual rotation of the pyridyl rings. According to the Fong [ 1 ] and Goulon and Rivail [6] theories, the correlation time describing the correlation of the perpendicular component will be given by 1/7=1/705,,+l/7k,

1988

_

molecule. The contribution assuming the independence of rotations of the molecule about the main axes of the moment of inertia will be given by the ratio of squares of the appropriate dipole moments: WC,

16 December

0.05.

T(K)

1.48

PHYSICS LETTERS

(2) time of the rotation

Table 4 Relaxation times r,,, distribution parameters a at various temperatures, and the activation energy for diary1 ketones in pxylene System

2,2’-DPyCO

DMCO

3,3’-DNDMCO

T (K)

70 (ps)

(y

293 303 313

16.2 15.1 14.2

0.08 0.10 0.02

Affz

323

12.4

0.0

293 303 313

65.5 54.2 45.1

0.0 0.0 0.0

323

39.7

0.0

293 303 313

90.3 75.1 65.4

0.0 0.0 0.0

323

54.9

0.0

W/mol)

5.02

10.62

10.45

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CHEMICAL

Volume 153. number 4

PHYSICS LETTERS

16 December

1988

25csg?-.,o3 ( E, + 2)d,

u

Y

-cl-

3.3’-DNDMCO

5[

’ B 3d0

”

’

.. a 3.20

ti

”

”

DMCO

”

I-

3.30

8

’

8

I

a 3.40 ;.Io’

Fig. 2. Dependence

of the dielectric increment

around the axis changing the direction ,uLL,i.e. the rotation around the GO bond; rk is the correlation time combined with rotation of the pyridyl rings. In practice, the second mechanism of absorption, connected with the rotation of the pyddyl rings, could be observed Only at fk< r,,, because of the calculated C,/C, ratio. At t orx tk both absorption areas should overlap and a small deviation from the monodispersive character of the relaxation should be observed. However, the average relaxation time r, should be shorter than that of a rigid molecule of a similar geometry. The fluorenone molecule is the best to compare with 2,2’DPyCO. The relaxation time of fluorenone in benzene at 243 K is equal to 19.9 ps [9]; it exceeds that of 2,2’-DPyCO, which could be indicative for pyridyl ring rotation in a 2,2’-DPyCO molecule. According to eq. (2), this rotation is slower than the over-all rotation because

“=

16.2x 19.9 = 87.5 ps . 19.9- 16.2

Thus, the dielectric relaxation of 2,2’-DPyCO will be dominated by the over-all rotation. Vector analysis of 3,3’-DNDMCO with pNo2 = 3.98 D shows that the contribution of the perpendicular component should be predominant be344

[K-“)

upon the reciprocal temperature for the diary1 ketones in pxylene. cause CJC, z 13. It means that in 3,3’-DNDMCO the rotation of the mesitylene rings should lead to a short relaxation time. In fact a high value is observed, ~,=90.3 ps at 293 K. Such a long relaxation time and the value of the calculated dipole moment suggests that the dielectric relaxation here is connected with the reorientation of rigid molecules in which the mesitylene rings are perpendicular to the C-CO-C plane, or that the ring rotation is much slower than the over-all rotation. This conclusion is supported by a comparison of reIaxation times of DMCO and 3,3’-DNDMCO. The reason for the difference in relaxation times is that in DMCO the dipole moment is directed along the C=O bond only, i.e. along the short molecular axis, which must decrease the relaxation time of DMCO, compared with 3,3’-DNDMCO, where the moment directed along the molecular >C=O axis prevails. Finally, it can be concluded that in the systems under study the dielectric relaxation is dominated by the over-all reorientation. If in the mesitylene ketones the steric hindrance is most probably decisive, in the case of 2,2’-DPyCO the most essential is the electron coupling of the pyridyl rings with the x: electrons of the carbonyl groups. This interaction drastically inhibits internal rotation.

Volume 153, number 4

CHEMICAL

Acknowledgement The authors are indebted to the Polish Academy of Sciences and the Italian Research Council for financial support.

References [ 11 F.K. Fang, J. Chem. Phys. 40 (1964) 132. 121 E. Fischer, Z. Naturforsch. 4a ( 1949) 707.

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16 December

1988

[3] K. HigasiandCP. Smyth, J. Am. Chem. Sot. 82 (1960) 4759. [ 41 K. Higasi, Bull. Chem. SOC.Japan 35 ( 1962) 692. [ 5] A. Koll and L. Hellemans, private communications. [ 6 ] J. Goulon and J.L. Rivail, Proton and ions involved in fast dynamic phenomena, ed. P. Laszlo (Elsevier, Amsterdam, 1978). [ 7 ] M.G. Corfield, J. Hone& and A.H. Price, Brit. J. Appl. Phys. 12 (1961) 680. [8] N.E. Hill, W.E. Vaughan, A.H. Price and M. Davies, Dielectric properties and molecular behaviour (Van Nostrand, Pknceton, 1969). [9]A.D.PittandC.P.Smyth,J.Am.Chem.Soc.80(1958)1061.

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