Vibrational relaxation of proton acceptor in H-bonded complexes

277

Journal of Molecular Liquids, 36 (1987) 277-292 Elsevier Science publishers B.V., Amsterdam - Printed in The Netherlands

Vibrational

Relaxation

of Proton

Acceptor

in,H-Bonded

Complexes

H. Abramczyk*. D. Samios, Th. Dorfmiiller Universitat Bielefeld, Fakultat fiir Chemie Postfacb 8640, D-4800 Bielefeld 1

(Received

7 April

1987)

Abstract The isotropic benzonitrile a function

Raman band-widths

of

the ucN stretching

mode and u,(A,)

mode of

complex with various phenols have been studied as band of free and temperature. The u CN and uiz benzonitrile has been analyzed in terms of the isolated binary collision model. has been analyzed in terms of the The u12 band of the complexed benzonitrile coupling of the oscillators and the dipolar coupling theory. The changes in the spectral profiles produced by the sub- stituent in phenols can be understood in It was shown that both the strength of terms of the dipolar coupling theory. the H-bond and the viscosity of the solution are important factors for the band broadening in H-bonded complexes.

*

in the H-bonded of concentration

Permanent address: Technical University, Institute Chemistry, Pt-93590 Lodz, Wroblewskiego 15. Poland

of

Applied

Radiation

218 I Introduction Experimental evidence for the formation of hydrogen-bonded complexes of phenols with nitriles has been presented in the literature [l-4]. The frequency shift u(0l-I) of the IR band of phenols has usually been regarded as a measure of the strength of the H-bonding with ni triles [4-61. More information about the nature of H-bonding can be obtained by band shape analysis of spectra of proton donors as well as of proton acceptors. The mechanism of broadening of IR or isotropic Ramsn bands in liquids has received considerable attention. The various theoretical models for vibrational dephasing have been reviewed by Oxtoby [7], Bratos [S] and Robertson [9]. The aim of this paper is to study the effect of substituents in phenols on vibrational relaxation in proton acceptors. It is interesting to see, whether the presence of a substituent in the benzene ring of phenols will sufficiently affect their acidity to render the influence on the band of proton acceptors observable and what is the mechanism of vibrational relexation of the proton acceptor modes engaged directly in intermolecular H-bond. We have studied the stretching mode ua and the uiZ mode of benzonitrile according to the assignments [lo] in the presence of different substituted phenols in benzene solutions. The acidity of the phenols can be extensively varied with different meta- and par-a-ring substituents. The enthalpy of intramolecular H-bonding ten thus be changed from weak to strong, making these systems an appealing target for theoretical considerations. After presenting the necessary experimental details and the results (section II and IIa) we shall first discuss in section IIIa the isotropic band width of the IJ~ and uls modes of the free (uncomplexed) benzonitrile applying the isolated binary collision model. Subsequently in section IIIb we discuss the width of the u,s mode of the complexed benzonitrile. In this section we apply the oscillators coupling and the dipolar coupling models aiming to explain the observed dependence of the width from the concentration of benzonitril and the phenol substituents.

II

Experimental The Ramsn band profile

to the totally

synnnetric nm stretching

mode and

uiZ (A,) mode of benzonitrile have been recorded as a function of the benzonitrile concentration and temperature in the following ternary mixtures: benzobenzonitrile/o-chlorophenol/benzene. benzonitrile/phenyl/benzene, nitrile/p-methoxyphenol/benzene and benzonitrile/onitrophenol/benzene. The concentration of the phenols was kept constant (3 Mel/l) in all solutions. ‘lhe relative concentrations of them have been varied by varying the concentration of the other two components (benzene end benzonitrile). Data for the binary mixture benzonitrile-phenol have been taken from Ref. (29). The binary mixtures benzonitrile-o-chlorophenol. benzonitrile-m-chlorophenol and benzonitrile-o-bromophenol have been studied only for the mole Benzonitrile and phenols (Merck, W.Germany) were fraction 0.5 at 15V. destilled under reduced pressure. Spectrograde benzene was used without was determined by further purification. The composition of the mixtures weight. Raman spectra were recorded with a double-grating monochromator (Spex. model 1403). The 488 nm line of an argon ion laser (Spectra Physics. model Right-angle scattering geometry was 171) was chosen as the exiting line. the scattered light collecting used. A description of the sample holder. system and the detection system was published elsewhere [ll]. The temperature was controlled to within 0.5 K with a Haake thermocirculator-controller.

279 Corrections formula:

for

the

finite

At = Aa [

slit

have

width

been

estimated

1-2 (s/A~)]~”

using

the

(I)

where the subscripts t and a indicate the “true” and “apparent’half-width at half-height. s is the slit width. Polarised (IW) and depolarized (IllV) spectra have been measured under identical

conditions.

The isotropic

IisoW The leakage

of

= Iw(4

the polarized

spectra

-

were obtained

from the equation

3 +q (0)

light

from the

IW

into

the

IllV-spectrum

was smaller than l’/oo. The signal-to-noise ratio achieved under these conditions was about 40 for W spectra and about 20 for HV. A contour analysis was carried out to separate the bands of free and of complexed benzonitrile. The digitalized data were fitted with two Lorentzians using a Marquard least square fit procedure. The data analysis was made on a Hewlett-Packard 9000/320 UNIX computer System. Errors of the isotropic band width were estiwrated from spectra measured 3 times under identical conditions. Viscosities of the solutions were measured with a Micro-XPC- Ubbelohde viscosimeter (Schott Company, West Germany) and densities with a Hereaus/Paar DMA 6OlHP-DMA 026 densitymeter as a function of the temperature.

IIa

Experimental

results

The ucN and ui2 bands added. This additional of neat benzonitrile. the H-bond formation.

1500-

of benzonitrile

splitting

Fig.

5

f 1000 -

L

is

1

Raman W spectra of benzonitrile-phenol in benzene solution at 15%. a) ucN mode, b) ui2 benzo-

soo-

2198

when phenol

to the frequency and arises from 1.

aI

;

,Y E

exhibit

band, shifted by 6 cm-’ with respect is not observed in neat benzonitrile Typical spectra are presented in Fig.

2206

2216

2222

2230

2238

2266

2254

2262

0

430 440 450

660

470

480 cm-'

490

nitrile

mode

280 In Fig. benzonitrile benzonitrile

2 we present the isotropic band-widths of in various phenols for the ui2 mode as mole fraction XhZ.

free and complexed a function of the

For o-nitrophenol no splitting of the benzonitrile band was observed. An explanation of this finding is that o-ni trophenol , in contrary to orto-halophenols. form strong intramolecular H-bonds and at room temperature only the syn-isomer can exist excluding intermolecular H-bonding with benzonitrile. For comparison, the enthalpy of formation -AH of an intermolecular H-bond in halophenols is about 8 KJ/mol [12-141, whereas in o-nitrophenol, this quentitiy was estimated by different methods [14-153 to be approximately equal to 30 KJ/mol.

Ais'/& Fia.

9.0 8.0 7.0 6.0

x\ 0

isotropic bend-width Aiso of the vi2 mode of benzonitrile at 15OC in ternary mixtures with substituted phenols and benzene versus the benzonitrile mole fraction. (0) phenol.

A-A-

A-

a

\

0

0

0

(0) o-chlorophenol. (A) p-methoxyphenol. (0) o-nitrophenol, (1) free bands of uncomplexed benzonitrile. complexed bands. (2)

5.0 1.0 3.0

0.2

OL

0.6

2

08

1.0

X8*

In Fig. 2 we can see a significant difference in the behaviour of the free and complexed bsnds. The presence of a substituent in the benzene ring of phenols does not influence the free band of the proton acceptor end, as a the isotropic band-width of benzonitrile is nearly insensitive consequence, the band-width of complexed to the substituent in phenols. Contrary to this, benzonitrile markedly depends on the substituent in phenol. This provides evidence that a substituent in the benzene ring of phenols affects the electronic structure of the complex to a sufficient degree to produce an observable effect on the band of the proton acceptor.

281

Fig.

3

Isotropic band-width Aiso of the vcpI mode of the free benzonitrile at 15% in ternary mixtures with substituted phenols and benzene solutions versus the benzonitrile mole fraction. (0) phenol,

IJJC model for benzonitrile in phenol, o-chlorophenol and o-nitrophenol.

In

Fig.

3 the

isotropic

hand widths

mode are shown. The results here, because the bands are the fitted spectrum.

2

*

50 -

free

benzonitrile

0

.=-a

9-

0

for

the V~

are not presented poor statistics for

Fig. 4 Temperature dependence of the isotropic band-width of the vi2 nitrile at 15% in ternary mixtures with substi tuted phenols and benzene free bands (1) (2) complexed bands (0) pheno 1

ii] &

LO-

of

for complexed benzonitrile not well separated giving

o-chlorophenol p-methoxyphenol

1

A

-26 /

3.0LIJ--I;oI 20

LO

60 T/ OC

In Fig. 4 we present the temperature dependence of the isotropic bandwidths for the free and complexed v *a bands of benzonitrile in the presence of phenol, o-chlorophenol and p-methomhenol. The band width slightly increases with temperature for free as well as for complexed bends in ochlorophenol and phenol. It shows the opposite trend for the complexed band in p-methoxyphenol.

282 III.

a)

Discussion

Free band of benzonitrile The

vibrational

in phenol-benzene

relaxation

of

the

solutions

ucrr and

ulc

modes of

uncomplexed

benzonitrile ten be snalysed in terms of dephasing theories for simple liquids [16-201. The general features of the density and temperature dependence of the isotropic Raman bands in a number of molecules have been successful 1 interpreted [21- 241 in terms of the Kubo stochastic line shape 17 and the isolated binary collision (IDC) model as proposed by l%%r and Laubereau [lS]. In the framework of the IBC model the following expression is given for the dephasing band-width 6 in a multicomponent system ph

ITI

(‘,h)

=

iz16i

j

where the subscript i denotes the component under study. j is taken over all due to the collisions between molecules of components. The band width 6 ij the components i and j csn be expressed as

6

ij

=

M is

z;:;L

[*Y

o2 Pi gij [ 1 - *

]

(4)

ij the reduced

mass of

the oscillator

with

frequency

w. Mij

is

the

reduced mass of the colliding molecules i.j. 7 is the amplitude factor. L. ij measures the range of interaction of colliding molecules, vi is the u. . =( ui + c j )/2, where ai and aj are the hard sphere diameters, iJ number density. The anharmonicity effect is included through the anharmonic force-constant f. In our calculation the contact values of the radial disequations [27]. tribution function g ij were obtained from the Percus-Yervick The hard sphere diameters have been calculated from the viscosity data according to the procedure given by Chandler [28]. The dephasing theory of Fischer-Laubereau [lS] can only describe relative changes of the vibrational dephasing rate, but not the absolute values, because of the uncertainties of For the molecular parameters M, 7, f. which are used in the calculations. we have normalized the dephasing band width with respect to these reasons, neat benzonitrile. The parameters used for calculations from the IEC model are given in Table 1.

Table

1

Parameters used binary collision

for calculating model oi

benzonitrile phenol o-chlorophenol o-ni trophenol benzene

0)

5.55 5.30 6.04 6.41 5.13

the

vibrational

lo== ‘K&P)

0.226

band

width

from

lo==+lu~s(g)

1.46

the

283 The result of the calculations are plotted in Figs. 3.5. We can see from Figs. 3.4 that for both bends of benzonitrile in o-chlorophenol the agreement between the IBC model and the experiment is good.

Fig.

6.0

L.0

I_&

2

A-

ff-TG

1

3.0

2.01

a2

0.L

Q6

08

5

Isotropic band-width Aiso of the ui2 mode of free benzoni tri le in ternary with substitutet mixtures phenols and benzene versus benzonitrile mole the fraction pheno 1 (0) o-chlorophenol p-methoxyphenol o-nitrophenol from IBC 1 *I’;I - calculated model for phenol, o-chlorophenol and o-nitrophenol. ii;

x0* For o-nitrophenol the calculated dephasing bend widths are larger than phenol shows an additional broadening those obtained from the experiment. which is due to the inhomogeneous mechanism from the interaction between benzonitrile and solvent (benzene). In order to estimate this effect we present (Fig. 6) the result for the two component mixture benzonitrile-phenol.

A'SO,&

Fig. 6 Isotropic band-width of the free benzoVi2 mode of nitrile at 15’C versus the benzonitrile mole fraction benzonitrile-phenol(*1 benzene (a) benzonitrile - phenol

TO 6.0 L-7 5.0 . LQ

. Q

. 0

q

-calculated IEC model

from

the

3.0

We can see that the band width decreases with increasing mole fraction whereas for the three-component mixture benzonitrileof benzonitrile. phenol-benzene we observe the opposite trend. This opposite trend is alsc predicted by the IEC model. illustrating that this model is able to describe detils of molecular interactions.

284

Fin. 7 Temperature dependence of the isotropic band-width of the uiZ band of benzonitrile at 15% in o- chlorophenol and benzene mixtures (0) free band (0) complexed band 1.2 calculated from the IRC model free and for complexed benzonitrile.

6.0 5rl

0

LO 3.0

20

LO

60

80 Tl'C

In Fig. 7 we present the temperature dependence of the isotropic bandwidths calculated by the IRC model for the free and complexed ols band and together with the experimental data. This illustrates clearly that this model fails for complexed benzonitrile in the temperature range under study. Despite the uncertainties of the IRC model, which do not allow more precise conclusions, there is evidence from the results presented in Fig. 3-7 that the band broadening of free benzonitrile is dominated by a mechanism which can, to some extend, be described by the binary collision model. On the contrary, this model cannot explain the behaviour of the complexed band of benzonitrile. This suggests, that vibrational relaxation of the proton acceptor modes which are directly engaged in H-bonding is affected by a particular process like the relaxation of the us (XH) mode of the proton donor. Two questions arise: first, to what extent can the vibrational lines of proton acceptor molecules be understood in terms of presently accepted mechanisms for H-bonding systems? Second. is there any correlation between the strength of H-bonding and the shape and width of the proton acceptors bands. In the following section we attempt to give an answer to these questions.

b)

Complexed band of benzonitrile

There are two groups of theories of vibrational complexes. The first is based on the idea of strong

relaxation in H-bonded coupling of the uS(XH)

the bridge oscillator and the low frequency stretching oscillator of (X-H...Y) [30-321. In the second it is assumed that the dipolar coupling between the dipole moment of a molecule engaged in a H-bond and the increment dipole moment Au reduced by H-bonding, is the dominant mechanism of vibrational relaxation P33-343. In the theory developed by Robertson et al. [31] the dipole auto-coreq. 2.26) depends on three parameters: A. ;lzonr function (in ref. [31], C, where A is the angular amplitude of modulation of the uS(X-H)

mode, w2 and rC are

the angular frequency of

the

uo(X-H..

. .Y) mode and a

characteristic time of modulation. respectively. Sakun [32] proposed an extension of the Robertson and Yarwood theory. relations between the parameters A. rCand w2 and the band

has recently He has found shape of the

uo(X-H...Y)

IR absorption

profile.

The knowledge

of

these

parameters

from

285 data would reduce the number of unknown variables in the fitting procedure. thus significantly increasing the reliability of a test of the theory. Unfortunately. the strong absorption in the mid and far infrared region of the involved polar molecules overlap the possible band of the bridge resulting in a seriously decrease of the accuracy of pertinent spectral data. Only in favorable cases we can obtain a reliable spectrum of the u mode. We o were unable so far, to record in a satisfactory way far infrared spectra for the studied systems, because of the strong absorption of benzonitrile between 100 - 200 cm-‘, the absorption of the bridge OH...0 of phenols lying of the bridge O-H. _.Cl of o-chloroat 163 cm-’ [36 ] and the absorption phenol at 84 cm-’ respectively [3~]. For these reasons, we restrict ourselves to a qualitative interpretation of the observed concentrationand substituentdependences. there are strong indications that diAccording to the second theory, mechanisms of polar coupling plays an essential role in the broadening for proton donors. In a previous paper [34] we H-bonded systems, at least have found that the vibrational correlation function due to electrostatic coupling can be expressed as @(t)

= exp

This

eq.

of a proton molecules, rotation

[

b2rs2 ( ew (iDt-‘))]

-

describes

two-particle

donor and a proton where

around

(5)

correlations

acceptor.

between

D = 2(Daa A

are the rotational $, and %b the axis a (D, = Dlb /IIcc ) for

+%b) diffusion

a proton

the

reorientation

for

symmetric top

coefficients

for

donor end a proton

acceptor, respectively. The angle is the average angle in the equilibrium configuration between the dipole moment of the proton donor or the proton acceptor and the increment dipole moment Ap proced by H-bonding. In the static limit (ATE >> 1) we obtain A% Y 2(21n2)’ while in the rapid limit ( for the diffusion coefficients A

‘v

H

B A n Vc << 1) using we obtain

2BAp2 D

(6) a hydrodynamical

approximation

(7)

B is a constant given in [34]. We have found [34] between the band width of the ug(0 H) band of phenols

a linear correlation and A pz n. which is

predicted from the theory of dipolar coupling in the rapid limit region (ref. [s], Fig. 2). Because it may be an important step in understanding the mechanisms of vibrational dephasing in H-bonded systems, we would check if there exists such a correlation for the bends of the proton acceptor.

b,)

Concentration

dependence

The theory of Robertson and Yarwood predicts that increasing the solvent the amplitude of the modulation parameter A can increase, if the polarity, structure of the complex is affected by the solvent. Specific interactions between solvent molecules and the complex can lead to a strengthening of the A strengthening of the hydrogen bond hydrogen bond in a more polar solvent. may be expected to increase the anharmonic coupling constant Ki2 and to

286 reduce the force constant Kii, both effects tending to increase A. The damping factor 7 (ref. [31] (eq. 2.5)) also increases with solvent polarity. This would result in a broadening of the bend even if A remained constant. The change of solution viscosity is of minor importance in this model, because -r does not appear to be directly related to the viscosity of the solvent. We can see (Fig. 2) that for the benonitrile-o-chlorophenol and the benzonitrile-p-methoxyphenol solutions in benzene the isotropic band width of the complexed vi2 mode is nearly independent on concentration, whereas for benzonitrile-phenol it decreases with increasing benzonitrile concentration, in other words with increasing polarity.

Fig. 8 Isotropic band-width of the vi2 mode of co?lexed benzonitrile at 15 C in phenol versus the benzonitrile mole fraction XSZ. (1) benzonitrile-phenol, (2) benzonitrile-phenolbenzene

10.090 8.0 7.060 5.0L.0 I

0.0

02

0.1

06

0.8

1.0

xBz

The dipolar coupling model predicts that the band width is independent on concentration (F&. 6) in the static limit and increases with increasing viscosity (Fq. 7) in the rapid limit. In Fig. 8 we present data for the binary mixture benzonitrile phenol and data from the ternary mixture benzonitrile phenol benzene. Going from low concentration benzonitrile to neat benzonitrile in the binary mixture with phenol the viscosity decreases and the band width should decrease, as it is observed. However. using benzene tration we invert the trend of solutions, with constant phenon concenviscosity, but the trend of the bend width remained the same as previously, All this suggests the presence of an addialthough significantly smaller. tional relaxation mechanism in this system, which is not accounted for either by Robertson’s model or by the dipolar coupling mechanism.

bs Substituent

effects

that the It is possible, vo(X-H.... Y) oscillator [30 -

coupling 321 will

theories be able

with the to predict

low frequency accurately a

variation of the band-width of proton donnors and proton acceptors with errors as well variable substituents. However, until now both, experimental as unsatisfactory fitting procedures do not allow an accurate test of the theoretical model. To apply the dipolar coupling theory we must determine the increment dipole moment Au for the systems under study. The increment dipole moment can directly be calculated if we know the dipole moments na, nb of isolated

287 molecules, according

uab of [37]

An = (nib

the complex and

- n>inea

- nacosea

geometry

- %“ixBb

- 2+lalbSineaSinCjb

of

the complex


- llbCO.&Sb

The geometric parameters listed in table 2.

Table

the equilibrium

to the expression.

end dipole

(9) moments needed

for

calculation

of

A are

2

Parameters values for

used for calculation of the increment dipole moment Au and the linear and perpendicular H-bond in phenol-benzonitrile. 1inear

nb I”LI nab *b k An = 1.48 D

3.66 D [50] 1.56 D [59-j 3.79 D [57] O0 155O [49] O0

perpendicular

1.56 D

*a nab

3.79 D 90°

9b ea &OS o> AJJ= 2.14 D

It seems, that the value 2.14 D obtained for is too high as compared with the correlation OH.. .O or OH... N systems [39, 391. As a matter than 20 KJ/mol. remains relatively small and is 8~ D. We have chosen for the calculations Au = metry of H-bond. In order to calculate An for the substituted stituted) we can assume in a good approximation with the energy of the bond obeys the equation C

3.66 D

%a

= A (-All+)

155O O0

the perpendicular structure plots Al-l/An for different of fact, when -AH is lower expected to be lower then 2 1.48 D for the linear geophenols (except ortho-subthat the variation of An

(9)

with

An = 0 for AM = 0. The validity of this assumption is supported by the results of Huyskens et al. [39.39] in the range AH = 0 - 30 KJ/mol. For larger enthalpies the curve becomes sigmoidal. indicating a nonnegligible proportion of proton transfer complexes. The enthalpy of H-bond formation-AH for the benzonitrilephenol system obtained by different experimental techniques is equal to 19.4 KJ/mol [37] or 13.8 KJ/mol [40]. respectively. The former value is determined by IR. the latter by W spectroscopy. In our calculation we have taken -AH = 19.4 KJ/mol taking into account the results obtained by Eppley and Drago [41] indicating considerable errors in the UV spectrometric evaluation of the -AH values for acid-base adducts. For the substituted phenols we have used a linear relationship between AH end AUK with a variety of donors [42] for energies in the range of 16-40 KJ/mol. -AH = 0.0431 AuOH + 12.9

(10)

288 Despi te earlier objections [43] it was found that all phenolic compounds (without intramolecular H-bond) fall within experimental error on the same correlation line with phenol [42]. In table 3 we show the AH values calculated from relation (12) using the experimental peak frequency shifts AuOR obtained by Jawed [4].

Table

3

The peak values

frequency

for

shift

benzonitrile

pheno 1

taken from Ref.

with

acids

intermolecular

-AH (KJ/mol)

142 148 163 194 207

19.0 19.4 20.0 21.3 21.8

that for a given ortho-substituents

in water.

-AH and

the pKa

PK, a) 10.20 9.95 9.39 7.95 7.15

of

AHexp = &nter

base, the H values of the complexes are linearly related to the pKa of

For ortho-substituted

H-bond AHlnter

py AHexp. because

formation phenols

[57]

It is known [38]. phenols without

these

of

AuOH(cm-‘)

p-methoxyphenol pheno 1 p-chlorophenol p-cyanophenol p-ni trophenol a)

AuOH, the enthalpy in complex with various

the presence _ &ntra

phenols

is no longer of

equal

intramolecular

the proper

enthalpy

to the experimental hydrogen

x

of

enthal-

bond AHintra (11)

intra

For this reason, the values of AH(AH = AHinter in this case) for orthophenols have been estimated from the comparison of AH for the complexes without ortho-substituents but with the same ps value as for ortho-substituents. The results are given in Table 4.

Table The

4 enthalpy

benzonitrile

of

intermolecular

H-bond

formation

in complex with ortho-substituent

-AH

and

pKa values

for

phenols

-AH(Kj/mol) phenol o-chlorophenol o-bromophenol o-fluorophenol a)

taken from ref.

In

Table

5 are

19.4 21.1 21.1 21.0

9.95 8.48 8.44 8.8

[571 shown AH,

An.

the

viscosity

of

the

soluAion

and

the

isotropic

band width

of

Aiso

the complexed ui2 mode. In Figs.

9a end 9b the

in complex with plots of the AlSo vs. Au2n and Au are shown for benzonitrile We can see that the static limit fails complevarious substituted phenols. tely, whereas the rapid modulation limit gives the correct trend of the band width on the substituent.

Table

5

The enthalpy

of

intermolecular

moment, the solution

H-bond

viscosity

formation

and the isotropic

-Ah.

the

increment

Ramen band width

dipoole

Aiso

8ystem

Bz-phenol-B= Bz-pheno 1 Bz-o-chlorophenol-B Bz-o-chlorophenol Bz-p-methoxyphenol-B Bz-m-chlorophenol BZ-o-bromophenol

0.460 0.501 0.496 0.500 0.496 0.502 0.500

19.4 19.4 21.1 21.1 19.0 25.5 21.0

1.48 1.48 1.61 1.61 1.46 1.56 1.61

inter * The enthalpy of intermolecular H-bond AH calculated from pKa values in aqueous media )(3( Bz - benzonitrile. B - benzene

1.946 2.784 1.774 2.224 3.796 2.349 2.006 for

4.26 6.10 4.59 5.76 8.09 5.72 5.20

orth-substituted

6.92 8.58 6.00 6.48 8.35 7.03 6.60 phenols

A’sojCm-’

90 -

‘00) Q’

80 -

/ Fig. 9 Dependence of the isotropic band-width of the ui2 mode of benzonitrile in substipheno 1s tuted a) Ap2n (rapid limit ATE << 1)

5.0LO 40

I 80

69 A$ll/

101 3

D2- cP

A’so/CKi’ WO-

b)

80 70 60 -

1:. 1D

0$ ‘0 4 0Y 03

1.5

Ap.lD

20

b)

Au (static

limit

ATE << 1)

290 As we can see from table 5 the isotropic band-width of the uIZ band of the complexed benzonitrile depends on the substituent on the phenols in the following order p-OCHs > H > m-Cl > o-Br > o-Cl. The theory of dipolar coupling allows us to understand, at least qualitatively, this finding. If the strength of intermolecular H-bond OH.. .N would be the only determinant of the vibrational relaxation of proton acceptors, then according to the -AH values we should observe 0-Br > O-Cl > m-Cl > H > p- 0CHV3 (Table 5). We have found [34], however, that the strength of the H-bond is the main determinant of the vibrational relsxation only in the static limit. In the rapid limit the band-width is proportional to the product Anz. Thus. for real systems. where dynamic processes very often follow mechanisms lying between the extreme regimes both factors may affect the band width. The overall effect of broadening depends on the magnitude both of the increment dipole moment and the viscosity. Our results illustrate this competition between the strength of the H-bond and the viscosity of the solvent. The steric hindrance for reorientation, due to the large OCHs group gives the strongest contribution to the band broadening (the highest viscosity) despite the fact that the increment dipole moment is the lowest (the highest pKa value). ‘lhs, the product An2q is the largest entailing the largest broadening in p-O&. On the other hand, ortho-chlorophenol has the highest increment dipole moment (the lowest pRa value) and the reorientation motions are very fast (the viscosity of the solution is smaller than in all other substituented phenols at the same concentration). That gives the smallest An2~ entailing the smallest broadening. It is interesting to notice that the IR band-width of the uS(OH) mode of proton donors in benzonitrile-substituted phenol systems exhibits the oppoin ref [4]. This trend can be site trend p-Cl > H > p-OC& as reported understood in the framework of the dipolar coupling theory. The IR band-width of proton donors were measured in very diluted solutions, so that the solutions with different substituents have the same viscosity, equal to the viscosity of the solvent. According to eq. (7). the band-width depends on A$ (rapid limit) only, because 7) does not change. In this case the strength of the H-bond is the only determinant of the broadening. Para-chlorophenol is the strongest acid with the largest increment dipole moment and consequently the largest broadening of the band, while paramethoxy-phenol is the weakest one.

IV Conclusions In this paper we have studied vibrational relaxation of a proton accepphenols using tor (benzonitrile) in H-bonded complexes with substituted of uncomplexed Raman spectroscopy. We have found that the uls band-width in the phenolring benzonitrile is nearly insensitive to the substituent while the complexed band exhibits significant differences for various phenols. We have found that the dominant mechanism of dephasing of the free band can be described by an isolated binary collision model. However, this mechanism fails completely for the complexed band. We have considered two mechanisms of broadening in H-bonded complexes: First the coupling between the u(Y-B) mode (in our case uC.$uid uiz) with the interactions low frequency mode of the bridge (X-H.. .Y). Second the dipolar between the molecules of the complex. The model based on the coupling of oscillators predicts, that the viscosity is of minor importance to the vibrational dephasing in H-bonded is not directly related to the viscosicomplexes, since the damping factor ty. Our results on the contrary show that the band-width is significantly The opposite trend observed in affected by the viscosity of the solution.

291 the Raman band-width of benzonitrile in substituted phenols in concentrated solutions to that obtained with IR spectra in diluted solution provides a strong indication on the influence of viscosity on the band broadening. It seems, that the conclusions about the independence of the band-width on viscosity in H-bonded systems which have been previously drawn should be modif ied. According eqs. 5 - 7. the broadening depends on both factors: polarity and viscosity. In the rapid limit the band-width depends on the product of the increment dipole moment Au2 end the solution viscosity g. Thus, A’ increases with polarity and viscosity. To conclude, strong indication has been found that the vibrational relaxation in weakly H-bonded systems is determined by electrostatic interactions between the molecules of the complex. The dynamics of the bridge is governed by the fluctuations of the angle between the dipole moment of the molecule ~_rand the increment dipole moment Ap of the complex. The time dependence of these fluctuations described by the two-particle correlation function @(t) (eq. 5). is the dominant mechanism of vibrational dephaing of the proton acceptor modes engaged in the H-bonded complex.

This work was carried out within the project “Complex Liquids” of the Centre for Interdisciplinary Research of the University of Bielefeld. We also gratefully acknowledge financial support of the “Fonds der Chemischen Industrie”.

References 1. 2. z: 5. :: 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

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