Isotropic and anisotropic Raman scattering study in liquid p-dioxane

Structure, 143 (1986) 291-296 E1sevierSciencePublishersB.V.. Amsterdam-Printed inTheNetherlands

291

Journal of Molecular

ISOTROPIC AND ANISOTROPIC RAMAN SCATTERING STUDY INLIQUID

p-DIOXANE

M.FhTIMA R.M.FERREIRA MARQUES' and A.M.AMORIM da COSTA' 'Se&o

de Qutica, InstitutoSuperiorde mgenharia, 3000 Coinbra (Portugal)

to de Quimica,Universidade de Coimbra,3000 Coimbra (Portugal) 2Departamen

ABSTRACT The isotropicand anisotropicprofilesof the 835 and 2965 cm-' m1ines of p-dioxanein the neat liquidand in solutionhave been studiedas a function of temperatureand concentration. From the correlationfunctionsobtained by Fourierinversionof band.contours,the possibleinteraction responsiblefor the vibrationalaephasingof the oscillatorsand their reorientational relaxationare considered.It is shmn that the p+!lioxane moleculettiling about the C2 axis in then-olecularplaneperpendicularto theoxygen-oxygendirectionproceedsby small-stepBrowniandiffusionassociatedwith an Arrheniusactivationenergy of 9.0 kJ ml-l. The vibrationalrelaxationmechanismof the tm modes is interpreted in terms of pure aephasingdue to weak collisions. INTRODUCTION Physicalinteraction betweenmoleculesin liquidshas been a persistentchallengeto chemists.Particularly, it is known that the spectraof som armatic materialsare perturbedto varyinga-s

by differentsolutes (refs.l-3).The

extentto which the perturbations are due to specificinterrmlecular forces has remaineda highlycontroversial subject.Vibrationalspectroscopic studies of this kind of systemscan yield importantinformation regardingthe natureof such interactions. Themainpurpose of thepresentwxkhas

been to study thevibrational and

orientational relaxationof p+dioxaneon the basis of band contoursin m spectra.For this purposewe have selectedthe totallysymetric v (A ) and u1 8 g (Ag)&es, at 835 and 2965 m-', respectively, corresponding to the ring ana to the C-H symmetricstretchings. The mechanismswhich controlvibrationaland orientationalband broadeningfor these tvm &man mxles is investigated by studying the relaxationfunctionsand correlationtimes as a functionof temperature, solventand concentration. EXPERIMENTAL PART The experimental set-upused for recordingthe Ramn spectrahas bsendescribed elsewhere(ref.4). The Raman spectrophotamzter Warian
of this slitwidth on the observed vibrational linewidths was corrected for according to (ref.51 :

IV

obs is the corwhere r: is the true Raman vibrational halfwidth at halfheight, rv responding observed Paman linewidth and s is the halfwidth at halfheight of the spectral slitwidth. Fmaan lineshapes due to the reorientationalrelaxation were obtained by deconvolution of the isotropic component from the depolarized spectrum. Numerical Fourier transforms of the spectral lines were performed in order to obtain the corresponding correlation functions. The reproducibilityof the measured halfwidths was within 3 to 5 %. Viscosity measurements were carried out using an Ostwald viscosimater. The densities were measured using an Anton-Paar DMA 60 digital densiraeter fitted with a DMA 601 cell.

RESULTS

AND DISCUSSION

i) Reorientationalrelaxation The temperature dependence of the reorientationalrelaxation times obtained from the pdioxane 835 and 2965 cm-' Ran-anmodes is presented in Fig.1.

The reorientationalrelaxation times decrease as the temperature increases. The activation energies associated with the reorientationalprocess, deduced -1 for the 835 cm-' and 7.9 frcanthe slope of log ~~~ vs. l/T are 9.0 kJ mol kJ m-l

for the 2965 cm-'. These two activation energy values are quite similar

and agree very well with previously reported results obtained either by Paman spectroscopyor calorimetric methods (ref.6); there is, however, a significant difference relatively to the results of depolarized Rayleigh light scattering (ref.71 and of NMR (ref.8).Probably the observed motion in the NMR and.the Ramn

Fig. 1. Dioxane reorientational relaxation tims as a fun$ion of teqerazye: 000 835 cm ,u8; l++2965 cm ,v,.

2.8

3.0 (103/T)/K-’

3.2

3.4

293

o---+$

I

’

I

1

.;.:

-

Fig. 2. Reorientational correlation functionsof 835 cm-i p-dioxanebaud at 298 X :experinlentalvalues (m-1, theXutc stochastictheory simulation (_____) and the free rotor (-.-*-).

i;‘.~.. C

i i i i

W0' (1

'. l. . '5l. . '.‘.

i

E

l.

.

1

O-8

i i

t

I

1.2 0

I

i

1 I 1.0 t/ps

1

I

2.0

--

study is not exactlythe same; and the cooparativeeffectsare quite significant in the case of Rayleigh~imants

(ref.9).

The time evolutionof the experimntal reorientational correlationfunction at 298 X is shmn in Fig.2. In this figureare also presentedthe free rotor correlationfunction(ref.101and the correlationfunctiono&ained by using the X&c stochasticlineshalze theory. It is obviousthat the recrientational motion in pure p-dioxaneat the considered temperaturedoes not followa free rotor behaviourfor times longer than of the eqerimental results about 0.1 ps. This seems to favourthe interpretation in tems of a "collisional" reorientation process.Consideringthat the Xub theoryproducesresultssimilartc tbse of J-diffusimrmdeland consideringthe discrepancybetweenthe experimental correlationfunctionand the Xub results, it seems that the J-diffusionMel

does not appearas the most adeguateto des-

cribe the reorientationalprocess inpurep-dioxane. The so-calledx test (ref.111leads to a value of 6.3 for x(t.he ratio between the orientational and the free rotor correlationtimes)at 298 X, using a value of 0.46 ps as the free rotor correlationtime. Such a value of X wauld indicate that n~~lecular reorientation prcceedsthroughmall angle rotationaldiffusion. Under this assumptionwe detemkned that about 86 collisionsare requiredfor a net reorientation of one rad, with a mean angle turnedper collisionofQ4degrees. The exparimantaror as a functionof rl/Tis s.hom in Fig.3.The extrapolated zero viscosityvalue of 'caris 0.75 ps. If this value ware used in the abve mentionedx test as the free rotor correlationtine, a value of 3.8 would be produced,suggestingrathera rmlecularreorientation under inertialeffects (ref.11). The slope of ~~~ vs. n/T in Fig.3, (5.3L0.6).102 ps X cp-', is ten times smallerthan the value calculatedusing the Stokes-Einstein theoryunder s~xk lmundaryconditions.The use of slip bmndary conditions(refs.12,13), by consi-

Fig. 3. Dioxane reorientationalrelaxation times as a function of n/T for the 835 cm-i Raman node.

1

3

2 qT-‘/

(cp

4 K-‘.103)

dering separately the rotations around each of the three principal axes of inertia, leads to the values presented in Table I. Frm

the values one can conclude

that a notion consisting essentially of tumbling about the C2 axis would account quitewell for theobsemedbehaviourof

the piiioxane exparimentalreorienta-

tional times.

TABLE

I

Theoretical Stokes-Einsteinslopes of 'carvs. n/T for p-dioxane.

Stokes-Einsteinslope for a sphere using stick boundary conditions . . . . . Stokes-Einsteinslops for an ellipsoid using slip boundary conditions . . . . .

a) Kotationover 0xygeGoxygen b) Rotation over c) Rotation over

6.1*103 ps K cp-' 6.1.10* 2.4*10* I 0.9*10*

;; ,,

(a) (b) (c)

anaxis in themolecular plane and perpendicular to the axis: the oxygen-oxygen axis: an axis perpendicular to those considered in a) and b).

ii) Vibrational relaxation The observed linewidth for the isotropic cmponents of 2965 and 835 cm-' Ramanbands of p-dioxanemsasured in the pureliquidand

inwater, carbon tetra-

chloride and chloroform solutions, are presented in Figs. 4 and 5. -1 In the pure liquid, the isotropic linewidth of the 2965 cm C-H stretching band decreases as the temperature increases, whereas the linewidth of the 835 cm-l band increases with temperature. In case of line broadening by weak collisions the vibrational halfwidth at halfheight TV can be calculated by perturbation theory (ref.14) from the freguency dispersion and frcmCthe correlation time 'cc,i. e. :

-I

280

270 ': E 260 U \ NA

I

1

0

T/K

Dioxane

Fig. 4. Changesof (***)and. rv (++) forvlandvsof p-dioxane as a functionof temperature.

r”

=

.2

I

.4 mole

I

.6

.8

1

fraction

Fig. 5. Vibrationallinewidthfor theandvebands of p-dioxaneas a function of concentration in H20 (++), CC14( -1 and CHC13 (001. l

TC

The correlationtime 'ccdecreaseswith increasingtemperature, while the frequencydispersion, usuallyincreases.The prevailingtrend de+

on how

fast the close distanceof approachof particlesin a head-oncollisionchanges with temperature. When the broadeningis controlledby short-range attractive interactions the distanceof approachof particlesis almostconstant;then 'cc prevails.When the broadeningis controlledby short-range repulsiveinteractions that distancereducesappreciably with temperatureand prevails
relaxationmhanism seems tobe controlledby

short-rangeattractiveinteractions and there is a notionalnarrowingas the temperatureincreases,whereasthe relaxationof the 835 Cm' mdeappearstobe controlledby short-rangerepulsiveinteractions. In solution,both the increaseof the vibrationallinewidthof the 2965 cm-' band upon dilutionwith water and its decreaseupon dilutionwith carbontetrachloridecanbe interpretedonthebasisofpuredephasingmxhanisnas

follows:

296 the p-dioxanemoleculeshave two oxygenelectronegative atcnns adjacentto the C atoms of the C-H bonds responsiblefor the observed. stretchingRaman band. Hydrogenbondingbetwaenthe oxygenatoms of the pdioxane and thewatermolecules leads to preferentially intermolecular orientations which decreasethe efficiency of the phase relaxationdue to molecularcollisions.Carbontetrachloridedoes not form hydrogenbondinywith p-dioxanemolecules.Thus, the phase relaxation becomesmore effectiveupon dilutionwith this solvent. The changeof the C-H vibrationalfrequencyfrcm the pure liguid (g2965cn?') -1 to aqueoussolutions(-2975cm for 0.24 @ioxane; 0.76 water) just confirmed the existenceof hydrogenbondingin p-dioxaneagueoussolutions. Vibrationalrelaxationof the 835 cm-' Ramanscde inwaterandchloroformsolutionsdoes not differ frcanthe vibrationalrelaxationof the 2965 cm-' modein solutions ofwater and carbontetrachloride. An explanation on the basis of existence and non-existence of hydrcgenbondingseemsonce more satisfactory.

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