Análisis de Las Variables

Z3 ro DO X $ lU Z

120

obtained under the above aeeumptlone for the effect of ring Inver­ sion on the IntraHHoleoular contribution was c reduction from 6.19

P y. 2 2

gauss to 2,o1 gausa . This value was only reduced to about 1 gauss " If the plane throu^i the four carbons 5, 6^, 8 and 9 (see figure 7*5) was peiimltted to tilt between the Inversion positions.

It Is difficult to estimate the effect of this type of motion P on the Inter contribution, but a lower limit of about 0.5 gauss" would appear probable, since there Is no translational motion of

p the molecules. On this basis an Intra value much less than 1 gauss" is required for agreement with the experimental value of 0.81 - 0,05

2 gau&s ,

P

The major part of the remaining intra contribution of 1 gauss" comes from tlie almost unchanged contributions to the second mcanent of the proton pairs 15-1^^ 15-16, 17-12* These contributions would be gi^eatly reduced by any rotation about the y-axj.8. The molecular envelope of a rapidly inverting CHT ring is approximately '^doughnut" shaped, and this would make rotation about the y-axls more facile, so that a combination of these two motions could explain the

observed second moment,

Further information about the molecular motion has been obtained from an activation energy analysis of the line width reduction

between 8 0 and I^C^K, In figure 7*6 the observed values of the line width In this temperature region are shown. The smooth curve through the expeidmental points was drawn by the computer digital

plotter from a computer least sc^uares analysis of the experimental data. This analysis was based on the Kubo and Tomlta (1954)

relationship 2,10, and the Arhennlus equation, Tlie details of the computer programmes are given In Appendices A8 and A9» The computed activation energy from 14 data points was 6,8 - ,5 koal,/ mole. It Is Interesting to note that this value is very close to the activation energy found for ring Invorslon In GIfT 6,5 - 0,5 kool,/hole. This gives further support to the proposal that ring Inversion Is the dominating molecular motion between 80**K and 154 *K,

(c) Spectrum above 154**K, Phase I,

The unit cell structure of phase I of GHT Is Imovjn, and so It Is possible to obtain a second moment value for quasl-lsotroplo reorientation of the molecules on their lattice sites. This cal­ culation is based on equation 2,15^

. g « 558.1 N« 2.15

The calculation is very similar to -(iliat made for oyclo-octane since their unit cell structures are identical (Budman and Post 1968), Using the value of the lattice summation computed in Appendix A12^

o

and the GHT crystal unit cell edge a 10.6 A obtained by Reed end Lipscomb (I955)j# c. value of 0,89 gauss^ is obtained for the second moment. This agrees well with the expérimenta], value of 0,8 Ï 0.1

2

gauss observed just after the II-I transition.

The smooth reduction in both the line ivldth and second moment

P

In phase 1, to values less than 0.2 gauss and 0,05 gauss";, Is characteristic of the onset of self-diffusion of the molecules. This motion probably begins limedlately after the H-I transition

k

at a rate of the order of the line widths 10 jumps per second. Self-diffusion In phase II Is most likely prevented by a tighter packing of the molecules in the different crystal structure. The tighter packing In phase II Is suggested by the large entropy

'I 1

change at the II-I transition^ 5*64-5 cal, dog, ' mole^ (o,f, «,1 «.'I

entropy of fusion1.402 cal, deg, mole ),

It now remains to explain the fine structure observed In the spectrum above 145%. The most obvious answer Is the presence of some hydrogen bearing Impurity. The presence of such an Impurity^ particularly toluene an Isomer of CHT^ was carefully checked before sealing the samples In the glass specimen tubes. However there are certain conditions when small quantities of an Impurity con escape detection by hl^-resolutlon n*m.r, A particular example occurs when the Impurity spectral peeks are coincident with the main con­ stituent peaks. The methyl peak of toluene for example occurs almost In the middle of the methylene triplet peaks of OIIT^ and althou^ spectra were obtained from two samples Td.th different 6o Mc/s spectrometersj, Varlan A6o and a Perkln-Elmer R-10., it Is pos- slble that small traces of toluene (/v 1^) escaped detection.

æ

However the relative areae of the two components of the wide- line spectre seemed to Indicate a greater proportion than 10 for the narrow component, The narrow line width;, < 0,4 geiues;, suggests the preeenoe of a very mobile hydrogen group ^ and an alternative explanation Is that the rapid ring Inversion oaueee the GHT methy­ lene proton spins to become decoupled from their surrounding Inter­ actions, Certainly the contribution of the two methylene protons to the second moment Is one of those most drastically reduced by ring inversion, The narrow oomponent could then be Identified ivith the contribution from these two protons. It Is interesting to note

that the Mgh-resolutlon n,m,r, studies of dilute solutions of GHT show that the methylene triplet is unchanged from room temperature to 153%j* and between 15^ % and 155% a single peak is observed. Only below 152% Is the ring inversion rate slow enougli to show non-equivalence of the methylene protons by the separation of the peaks,

7.4,2 Spin Lattice Relaxation time T.

The general variation of T. in the two solid phases of CRT closely mirrors the behaviour of the absorption spectrum;, with significant changes associated ivlth the heat capacity transitions at 82-92% and 154%, see figure 7*4, In phase IIa T. becomes

longer as the temperature Is decreased. It was orlglnalJy Intended to follcMf the variation beloif 50%., using the helium cryostat, to ensure that T. continued to Increase for lower temperatures In a

CYCLÜ-HEPTflTRIENE