PMR study of some quinol clathrates

: Volume 30, number 2

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CHEMICAL ~HYSIC~LETTERS

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15 Januxy ..

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1975

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,Jea‘n GALtIER,

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iXportemerrr de Physique Cristalline et Chihie Stnrcturole. E.R.A. au CNX 35031.Rennet-Cedea, France Received 20 May 1974 Revned m&uscript received 18 September

no. 015, Unisersitb de Rennes.

1974

Thi motion of trapped molecules in some deutcrated quinol &&rates is studied by proton magnetic resonance. Theoretical wlcularions of second moments and line widths are made,for s:veral reorientation assumpiions of the

trapped mo!eculcs.

1. Introduction A previous study on quinol ciathrates allowed us to deteimine the.origin of certain very narrow Lines in wide line proton resonance spectra and deduce the motion of molecules trapped in the lattice by cc_mparing expkiimental and theoretical second moments. This study has been performed again using deuterated quinol*I The proton resonance spectrum is then mainly produced by encaged mo!ecules because of the deuterated lattice. Both second moment and l&e width &pend greatly qn the dynamics of the trapped molecules; Sq there is another experimental parameter that can be compared to its theoretical valu.e,for severd. reorientation assumptions of the : @lusion.

terated quinol molecules. During crystal growth, an exchange phznomenon conceinti~ the hydroxyl group i.nvolves formic acid deuteration and deuterium loss for the methanol. Before preparation of the crystals, the quinol deuteration rate was 86%. Deuteration rates in clathrates are: - lattice ceuteration rate: CH,CN 78%, CH,OD 92.4%, HCOOH 66.5%; - trapped molecules deuteration rate: HCOOH < lo%;, CH,OH 13.5%. The cage filling rate is measured by thermal analysis. We obtain wilh a recently prepared sample: CH3 CN 85%, CH, OQ 85%, HCOOH 670/o, 56%.

3. PMR line shapes Structure can be observed tiith.!he wide line spectra tid theii width depends both on the kind’of

2: F’repae&n

and analysis of single crystal samples

: ‘Single crystals are produced by evaporation of the detitemfed quinol solutions. The solvents are’ HCOOH, CH3 CN cd Cl33OD. The experiment-has been perforrngi in an insulated dry b6x. IMassq&ogkaphy makes it possible to.investigte the-glob;al deuterzition rate and ‘the percentages df differently deu.. ;_: ,j ,. 1 Kindly.&vided by Mrs Kahane’s laboratory (Gregol+). /i’O6 : ‘..:,, ‘,‘- : ‘.‘. ,, ‘,. .‘.‘I ‘,‘.:, _., ‘.’ ._ ..“i.’ -,: ..-. ,.‘. .:

clathrate considered and on the crystal.orientation in the magnetic field:Tke absorption lines obtained by the integraLion

of experimental

spectra produce

a central line and two lateral symmetric lines. Each line may’be ati:ributed,to‘the lattice or to the inciusion. in a different ijay for the thrk clathrates investigated. 3.1. ‘S&dy of cIa&tes con~aitlitg aceidnittiie and mqhal1,ol : .; ., : .. : ‘., .. I?le distmci: A betw’een the &IO lateral lines of’ ,, :.’ ., ,. .: ._ ,‘. ” ., ..,.. :. .: .. .. .I:’ ,, .. :.. ‘_._. ., ,: : “. ,/‘_ ., ,. ‘. -.

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-VoIume 30. number 2

15 Sanuaq

CHEMICAL PHYSICS LETTERS

I975

important lattice influence is to limit the volume of the cages and therefore.the possibilities of motion of the included .molecules. The preceding results confirm ,those previously obtained [ 11, but they are established with much more sensitive ambiguously.

experimental

3.2. Sru~‘y of clathate

P

cl Fig. 1. Variation of A and Ath with crystal orientation in the magnetic tield. -Separation A between the experimental spectrum hteral lines, (1) clathrate HQ(D) + W&N, (2) clathrate HQ(D) f CH3OD. I 7 T Separation A & between the theor&iul spectrum lateral lines of an isohted methyl group. - -- assumption A: the methyl group reorients around its ternary alris coinciding with the !-attice c asis, a.. assumption B: the methyl goup reorients around its lernary atis which besides moves around any axis fat least tkry) perpendicularly to the lattice c axis.

parameters

colztatninillgformic

snd

so uni

acid

A is always greater than the central line width. It never vanishes. Its variation with crystal orientation iri the magnetic field does not agrke with any df the. curves representing the variation of the theoretical separation Aa between the two peaks of the formic acid moIecuIar spectrum in severaI molecular conformations. Furthermore, the integrated are2 of the laterablines gives a protonic rate which cannot be assigned to the protons of the inclusion. CLX.E~,the cenird.l Iine ii narrower than that allowed for a lattice line. Therefore, we may not assign the lateral lines to the trapped molecules. They are due to the lzttice and the A parameter represents the lattice widthTherefore, Iine shape studies cannot provide any information concerning inclusion dynamics in this clathrate.

4. P!#R second moment the spectra vanishes for some crystal orientations. This parameter may not be identified with the lattice composant width. On the contrary, the attribution of the lateral lines td the trapped molecules accounts for the relative inter&ties of these lines. Thus, we assign the lateral lines to the trapped moleclrles and the centf$.Iine both to fattice and incIusion:Therefore, the-A parameter is available for the’study of the dynamics of the trapped molecules. We compare it with the theoretical separation Aa between the lateral lines of an,isolated methyl group, calculated for several reorientation a~ump~ions. Fig. 1 represents the variations of A.and Ati for the two &&rates;, depen~ng on crystal o~entation in the magnetic field., The& is good agreement between theoretical ,,and experimental results with assumptioti A conearning CH3CM inclusion tid with assumption B con... &rning%H3~D inclusion..This’co~fums the f&that the encaged molectiies are &rJst is&ted. ‘P-heonly ..

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Second moment detestation is carried out accordin; to.the method explained + the previous work [I] _In order to make ‘the calculations easier, we had to postulate,the equality of molecules’ and cages’ de~teration rates - that is we assume that each cage is constituted with identically deuterated molecules. Second moment studies produce the same con&tsions as those provided by the line shape study and perfect it in the case of thi formic acid &&rate by precising conformation and dynamics of the tiapped molecules.

5.

Conclusion... .,

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,Our PMR line-shape and second’moment studies of ihe.(’ thrPe d+pied quii~l~ck&ates &ow.us to

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J. Gall& and J. hleinnel; 3. Chim. Phys:’ .’‘

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