J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2769-2774 Molecular Conformation of n-Alkyloligo(oxyethy1ene)s in the Solid State studied by Raman Spectroscopy Effect of the End Group Sei Masatoki and Hiroatsu Matsuura* Department of Chemistry, Faculty of Science, Hiroshima University, Kagamiyama , Higashi-Hiroshima 724,Japan The molecular conformation of a-hexadecyl-o-methoxyoligo(oxyethy1ene)s CH3(CH,),,(OCH,CH,),0CH3 (C,,E,C,) (rn = 1-4) in the solid state has been studied by Raman spectroscopy. The conformations of these methoxy group terminated compounds were compared with those of the corresponding hydroxy group terminat- ed compounds (C16E,,,). The conformational change of the inner oxyethylene segments from the extended to the helical structure takes place at rn = 4 for C, 6E,C,, while it takes place at rn = 3 for C,,E,. These observations, together with those for other homologous compounds, indicate that the end group of the oxyethylene chain affects the conformation of the molecule.The difference in the conformational behaviour between the com- pounds ending in a methoxy group and those ending in a hydroxy group results from the difference of the layer structures of crystals. The conformational variety of n-alkyloligo(oxyethy1ene)s in the solid state is associated primarily with the high flexibility of the oxyethylene chain which assumes the most appropriate conformation by adapting itself to its environment. The structure of block oligomers consisting of the conforma- tionally contrasting n-alkyl and oxyethylene chains has attracted much attention in recent years; the former chain prefers the extended conformation, while the latter prefers the helical conformation.The molecular conformation of a series of 7 1 a-n-alkyl-o-hydroxyoligo(oxyethy1ene)s CH,(CH,),-,(OCH,CH,),OH (C,E,) with n = 1-16 and rn = 1-8 in the solid state has been investigated systemati- cally in our Raman spectroscopic studies, and a number of interesting conformational features have been revealed. '-' These studies have shown that the molecular conformation of the C,E, compounds with n < 4 is basically helical, while the conformation of those with n > 5 depends significantly upon the oxyethylene-chain length. For the latter compou'nds, the conformational transition takes place at rn = 3-4 from the highly extended form to the helical/extended diblock form as the number of oxyethylene units increases. The polymorphic conformational behaviour of C1& in the solid state has been reported in detail in a separate paper.5 The effect of the end group of the molecular chain on the conformation of the whole molecule is another important structural feature of the block oligomeric compounds.The study of the compounds ending in a methoxy group, a-n-alkyl-o-methoxyoligo(oxyethy1ene)s CHJCH,), -1(OCH2CH2), OCH, (C,E,C,), is thus relevant to this problem for comparison with the C,E,, ending in a hydroxy group. In our recent work,, we studied the conformation of C,E,C, (rn = 1-4) in the solid state by Raman spectroscopy and discussed the difference in the conformational behaviour between the compounds ending in methoxy and hydroxy groups.Booth and c~-workers,~-~ on the other hand, have studied the morphology and crystallinity of a variety of C,E,C, compounds by means of Raman spectroscopy, X-ray diffraction and differential scanning calorimetry and dis-cussed the molecular structure of these substances. In the present work, we have extended the conformational studies of diblock n-alkyloligo(oxyethy1ene) compounds in the solid state to another series of compounds terminated by a methoxy group, C, ,E,C, [a-hexadecyl-o-methoxy-oligo(oxyethylene)s] with rn = 1-4. These compounds contain a longer alkyl chain than the C,E,C, compounds studied previously.6 The results of the two series of compounds, C,E,,,C, and C16EmC1, provide further information on the effect of the terminal methoxy group on the conformation of the molecule, in comparison with the effect of the hydroxy group in C,E, and C,,E, .3*4 Experimental Materials C1,E,Cl with rn = 1-4 was synthesized in the present work.C,,E3Cl and C,,E4Cl were prepared by the conventional method of Williamson ether synthesis, while C16ElCl and C,,E,C, were prepared by an improved method using a phase-transfer catalyst.".' 'The materials thus prepared were purified by repeated distillation in vucuo. Raman Spectroscopy The samples of CI6EmC1 were contained in sealed glass ampoules, and their Raman spectra were measured in the solid state at liquid-nitrogen temperature.The spectra were recorded on a JEOL JRS-400D Raman spectrophotometer equipped with a Hamamatsu R649 photomultiplier. The 514.5 nm line of an NEC GLG3200 argon ion laser was used for Raman excitation. A bandpass filter was used to eliminate the laser plasma emission. For spectral calibration, the neon emission lines were utilized. The solid phases of Cl6EmC1 (rn = 1-4) for the Raman measurements were obtained by three different methods of solidification : (1) The liquid sample was cooled rapidly, reaching near liquid-nitrogen temperature in <3 min. (2) The liquid sample was cooled more slowly, reaching the same temperature in >30 min. In this case, the solidified substance was then annealed by warming it to a temperature slightly below the melting point and maintaining it at around this temperature for > 1 h.It was cooled to liquid-nitrogen tem- perature before recording the spectra. (3) When the liquid sample of C16E,Cl was cooled to just below the melting point, a waxy solid was obtained. This waxy solid was cooled rapidly to near liquid-nitrogen temperature. 2770 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Results and Discussion obtained by methods (l), (2) and (3) will be called solids 1, 2 Spectral Analysis and Conformation Determination of and 3, respectively. The observed spectra were analysed in relation to the Cl ,E,Cl molecular conformation by utilizing the conformational key The Raman spectra of C,,E,C, (rn = 1-4) in the solid state bands that had been established previously for C,E,C,.are shown in Fig. 1. For C,,E,C, and C,,E,C,, the spectra Most of these key bands were derived from the con-obtained by methods (1) and (2) are the same, but differ from formation-spectrum correlations for oxyethylene chains. '' was also utilized for the determi- the spectrum obtained by method (3). The compound The LAM-1 ~ibration'~''~ C,,E,C, gave three different spectra depending upon the nation of the molecular structure. Normal-coordinate calcu- solidification method, whereas C,,E,C, gave essentially the lations were performed on a number of possible same spectra irrespective of the method. The solid phases conformations of C,,E,C, molecules in order to confirm the spectral interpretation. In the calculations, the force con- stants established previously for the alkyl and oxyethylene chains' 5.l6 and a programme, MVIB,' were used. The conformational key bands observed for various solid phases of C,,E,C, are listed in Table 1, where the conforma- tional and vibrational assignments of the respective bands are indicated.The distinctive conformational fragments of the C,E,C molecules are those of the oxyethylene-adjoining CH,CH,CH,O' 'Isegment, the inner OCH,CH,O segment and the terminal OCH,CH,OCH, segment. On the basis of the observed key bands, the molecular conformations of C,,E,C, in the solid phases have been determined as sum- marized in Table 2, where the conformations of the corre- sponding C,&, corn pound^^-^ are also given.The skeletal molecular models of C,,E,C, representing the conforma- tions thus determined are shown in Fig. 2. The conformations of C,E,C, and C,E,, as determined in the previous w~rk,~.~ are given in Table 3 for comparison of the molecular confor- mations of the relevant compounds. A brief discussion is given below on the conformation determination of the C,,E,C, compounds studied. C16E1C1 This compound gives the same type of spectra when the solid phase is obtained by the three different methods. The observed key bands indicate that both of the alkyl and oxy- ethylene chains adopt the extended conformation [Fig. 2(a)]. The wavenumber of the LAM-1 band, 116 cm-', is consis- tent with the fully extended molecular structure consisting of 21 coplanar backbone atoms, in conformity with the LAM-1 wavenumber (1 15 cm-') for solid CH,(CH,),,CH, .18 The Raman band at 1415 cm-' corresponds to the band at Y i 1 iIf 0 1200 800 400 0 wavenumber/cm-' (a1 (b) (c) (a (elFig.1 Raman spectra of C,,E,C, in the solid state: (a) CI6E,C, (solids 1-3); (b) C,,E,C, (solids 1 and 2); (c) C,,E,C, (solid 3); (d) Fig. 2 Skeletal molecular models of C,,E,C, : (a) C,,E,C, (solids C,,E,C, (solids 1 and 2); (e) C,,E3C, (solid 3); (f)C,,E,C, (solid 1-3); (b) C,,E,C, (solids 1-3); (c) C,,E3C, (solids 1 and 2); (d) 1); (9)C,,E,C, (solid 2); (h) C,,E,C, (solid 3) C,,E,C, (solid 1);(e) C,,E,C, (solid 2) Table I Conformational key bands" and assignments for C,,E,C, (m = 1-4) in the solid state Raman wavenumber/cm -c v,v,"A 16E2C 1 C16E3C1 1 6E4C 1 C16E1C1 solids 1-3 solids 1, 2 solid 3 solids 1 and 2 solid 3 solid 1 solid 2 solid 3 conformational assignmentb vibrational assignmentc 6 r \oinner OCH,-CH,O in t CH, scissors 0 1497 m 1498 m 1499 m 1415 m 1415 m 1416 m 1414 mw 1411 m extended alkyl chain in orthorhombic CH, scissors or monoclinic cell 1252 w 1253 w 1254 vw terminal (C)O-CH,-CH,-OCH, in tgt CH, twist 1238 w, br 1232 mw 1238 w, br inner (C)O-CH,-CH,-O(C) in tgt CH, twist 1170 w 1166 w 1170mw 1168 w 1170 mw 1170 w 1166 w oxyethylene-adjoining (C)CH,-CH,-CH,-O(C) CH, rock in ttt 954 w 955 w 957 w terminal OCH,-CH,0CH3 in t C-O(CH,) stretch 939 vw OCH,-CH,-O-CH,-CH,O in gt-tg CH, rock 918 vw oxyethylene-adjoining (C)CH ,-CH,-CH,-O(C) C-0 stretch, C-C stretch in tgt 893 mw 891 mw 890mw 890mw 891 mw 890 mw 888 m 889 mw terminal CH,CH,-CH,CH, in t CH, rock, (CH,)C-C stretch 852 mw 854 mw, br 852 mw 850 mw 851 mw, br terminal OCH,-CH,OCH, in gd C-O(CH,) stretch, CH, rock 294 mw oxyethylene chain in tgt helical conformation helix breath 116 vs 105 s 105 s 90 s 83 s molecular chain, fully or partly, LAM-1 (accordion) in the extended conformation a Approximate relative intensities: vs, very strong; s, strong; m, medium; mw, medium-weak; w, weak; vw, very weak; br, broad.* t and g denote trans and gauche conformations, respectively. Conformational assignment is based on the conformation-spectrum correlations for oxyethylene chains" and the normal coordinate analysis. Vibrational assignment is based on the normal coordinate analysis.Weak bands at 852 cn-' forC,,E,C, (solids 1 and 2) and at 857 cm-' for C16E,C, (solid 3) are assigned to the alkyl chain (CH, rock). N44CL J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 compound' Cl6ElCl rapid cooling (solid 1) slow cooling (solid 2) C16E2C1 rapid cooling (solid 1) slow cooling (solid 2) C16E3C1 rapid cooling (solid 1) slow cooling (solid 2) C16E4C1 rapid cooling (solid 1) slow cooling (solid 2) rapid cooling slow cooling C&2d rapid cooling slow cooling C16E3d rapid cooling slow cooling 1 6E4d rapid cooling slow cooling Table 2 Conformations" of C,,E,C, alkyl chain: oxyethylene-adjoining (C)CH,-CH,-CH,-O(C) ttt ttt ttt ttt ttt ttt ttt tgt alkyl chain,b oxyeth ylene-adjoining (C)CH2-CH,-CH,-O(C) ttt ttt ttt ttt ttt tgt and C,,E, (rn= 1-4) in the solid state oxyethylene chain inner terminal (C)O- CH -CH,-O(C) (C)O-CH -C H,-OCH t t t t t t ttt t t t ttt t t t ttt t 9 t ttt t 9 t ttt t 9 t tgt t 9 t oxyethylene chain inner terminal (C)O-CH,-CH,-O(C) (C)O-CH,-CH,OH t g+t t g+t " t and g denote trans and gauche conformations, respectively.Conformation of the alkyl chain is successive trans (t),with the conformation of the oxyethylene-adjoining segment as indicated. ' For the molecular structure of solid 3, see text.Ref. 3-5. similar wavenumbers for orthorhombic and monoclinic n-alkanes.lg This band is interpreted as a crystal-field split component of the CH, scissoring mode of the extended alkyl chain; the other component is observed for C16ElC, at 1444 cm-l. Cl6E2Cl The essential difference between the two spectra, one observed for solids 1 and 2 and the other for solid 3, is present only in the 1400-1510 cm-l region. The former spec- trum shows prominent bands at 1497 and 1503 cm-', while the latter shows a band at 1490 cm-'. A distinct band at 1415 cm-' is observed only for the latter. An examination of the conformational key bands leads to a fully extended molecular structure for the three solids [Fig. 2(b)]. This struc- ture, consisting of 24 coplanar backbone atoms, is further evi- denced by the LAM-1 wavenumber of 105 cm-', which coincides substantially with 104 cm-' for solid CH3(CH,),,CH,.'8 C16E3C1 The two spectra of this compound, one observed for solids 1 and 2 and the other for solid 3, are noticeably different in two ways: thz appearance of a distinct band at 1416 cm-' for salid 3 iiad the broad spectral feature in the region below JWCI crn for solid 3.The key bands of solids 1 and 2 give the molecular structure in these solids phases, in which the dkyl chain is fully extended and the oxyethylene chain is i" :tended except for the terminal oxyethylene segment in the gauche conformation [Fig. 2(c)]. In the spectrum of solid 3, the bands characteristic of the oxyethylene chain are broad or ill-defined.The fact that there are no distinct spectral fea- tures in the low-wavenumber region suggests that the whole molecule is not fully ordered. These considerations indicate for solid 3 that the alkyl chain is ordered by taking the extended conformation while the oxyethylene chain is mostly disordered. cl 6E4C1 Of the three different spectra of this compound, the spectrum of solid 1 resembles the spectra of solids 1 and 2 of C16E3C,, except for a crystal-field split band at 1414 cm-' for solid 1 of C16E,Cl. It is shown for this solid that the molecular chain is extended except for the terminal oxyethylene segment that takes the gauche conformation [Fig. 2(d)]. The spectrum of solid 2 exhibits several distinctive features which are not observed in other spectra of C16E,C,.The bands at 294 and 1232 cm-are obvious indications of the presence of the trans-gauche-trans helical conformation in the oxyethy- lene chain. These key bands, together with others, establish the molecular conformation for solid 2; the oxyethylene chain adopts the helical conformation, while the alkyl chain is extended except for the oxyethylene-adjoining segment in the gauche conformation [Fig. 2(e)]. The overall spectral feature of solid 3 of C16E4C, coincides essentially with that of solid 3 of C,,E,C,. This shows that the alkyl chain takes the ordered extended conformation but the oxyethylene chain is mostly disordered. J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 Table 3 Conformations" of C8E,Cl and C8E, (rn = 1-4) in the solid state alkyl chain: oxyethylene-adjoining compound (C)CH,-CH ,-CH ,-O(C) C8E1ClC rapid cooling (solid 1) slow cooling (solid 2) C8E2C1C rapid cooling (solid 1) slow cooling (solid 2) C8E3C1c rapid cooling (solid 1) slow cooling (solid 2) C8E4ClC rapid cooling (solid 1) slow cooling (solid 2) ttt ttt ttt ttt ttt ttt tgt ttt alkyl chain: oxyethylene chain inner terminal (C)O -CH ,-CH, -O(C) (C)O-CH,-CH,-OCH, t t ttt tg t ttt tt t ttt t ttt t t t oxyethylene chain (C)CH, -CH ,-CH,-0( C) oxyethylene-adjoining inner (C)O-CH,-CH,-O(C) terminal (C)O-CH,-CH,OH C8E Id rapid cooling slow cooling t t t t t t t g+t t g+t C8E2d rapid cooling t t t t t t slow cooling t t t t t t rapid cooling C8E,d t t t t t t slow cooling t g t t g t rapid cooling C8E,d slow cooling " t and g denote trans and gauche conformations, respectively.Conformation of the alkyl chain is successive trans (t), with the conformation of the oxyethylene-adjoining segment as indicated. Ref. 6. Ref. 3. Conformational Behaviour of C,E,C, and C,E,: Effect of the End Group The important confdrmational features of C,,E,C, (m= 1-4) in the solid state are given in Table 2, in comparison with those of C16Em.3--5The conformations of C,,E,C, are summarized as follows: The molecules of C,,E,C, and C,,E,C, are in the all-trans extended conformation, while the molecules of C,,E3C, in the rapidly cooled and slowly cooled solids and of C,,E,C, in the rapidly cooled solid adopt the extended conformation except for the methoxy group terminated oxyethylene group.The C,,E,C, mol-ecules in the slowly cooled solid, on the other hand, take the helical conformation for the oxyethylene chain and the extended conformation for the alkyl chain, the oxyethylene- adjoining alkyl part being transformed into the helix. When the waxy solid of C,,E,C, and C16E4C1, obtained by cooling the liquid just below the melting point, is refrigerated, only the alkyl chain is crystallized by taking the extended conformation, but the oxyethylene chain is disordered. These conformational features of C16E,C, are compared with those of the hydroxy group terminated compounds C16Em.3--5The effect of the end group and the chain length on the molecular conformation is noted mostly in the oxyethylene-chain part.The conformational change of the inner oxyethylene segments from the extended to the helical structure takes place at rn = 4 for C16E,C1, while it takes place at m = 3 for C,&, (Table 2). The boundary com-pounds C,,E,C1 and C,,E, in fact adopt two conformations depending upon the solidification conditions. This indicates, under the notion of the conformational competition,20 that four or more oxyethylene units are necessary for C,,E,C, to establish the stable helical structure of the oxyethylene chain in competition with the stability of the extended structure of the hexadecyl chain.For C,&,, on the other hand, three or more oxyethylene units suffice to stabilize the helical struc- ture of the oxyethylene chain. According to the previous work on the shorter homologues C,E,C, and C,E,,, a similar conformational transformation of the oxyethylene chain from the extended to the helical structure is observed between m = 3 and 4for the methoxy group terminated com- pounds and at m= 3 for those terminated by a hydroxy group (Table 3). On the basis of these conformational charac- teristics associated with the end group, it is apparently shown that the oxyethylene chain with a terminal methoxy group, -(OCH,CH,),OCH,, is less effective than that with a ter- minal hydroxy group, -(OCH,CH,),OH, in retaining the intrinsic helical structure against the conformational effect of the alkyl chain to propagate into the oxyethylene chain.Booth and co-worker~~.~ have investigated the crystallinity of n-alkyloligo(oxyethy1ene)s and have shown that the hydroxy group terminated C,E, molecules crystallize into bilayers while the methoxy group terminated C,E,C, mol-ecules crystallize into monolayers. Two C,E, molecules are linked to each other through strong hydrogen bonds across the end-group plane to form the bilayer ~tructure.~ The more effective retention of the helical structure of the oxyethylene chain in C,E, than in C,E,C, molecules can be explained by the linkage structure of the two oxyethylene chains of a hydrogen-bonded dimer in the bilayer; the stabilities of the helical structures of the two oxyethylene chains are mutually reinforced.The different conformational behaviour of the methoxy group terminated and hydroxy group terminated n-alkyloligo(oxyethy1ene)s is thus explained by the different layer structures of the crystals. The previous spectral observa- tion that the LAM-1 wavenumbers of the helical oxyethylene chain for C,E, molecules are about half those for the corre- sponding C,E,Cl molecules has also been explained by the doubled chain length in the bilayer ~tructure.~ Disordered Conformation of the Oxyethylene Chain In solid 3 of C16E,Cl and C16E4Cl,the alkyl chain takes the ordered extended conformation, while the oxyethylene chain is mostly disordered. For C16ElCl and C16E,Cl, however, no such structures containing the disordered portion are found.Our previous study' on the polymorphic C16E3com-pound showed the existence of the same structure in the solid phase obtained by the same method (solid B). These experi- mental findings suggest that the C,E,Cl and C,E, com-pounds that contain a long alkyl chain and a relatively short oxyethylene chain tend to yield the solid phase in which the oxyethylene moiety is non-crystalline. The crystallization of the oxyethylene block in n-alkyloligo(oxyethy1ene)s is thus greatly influenced by the length of each of the two blocks constituting the molecule and the process yielding the solid phase. Booth and co-workers21*22have in fact classified the structures of the tri- block compounds C,E,C, on the basis of the crystallinity of the alkyl and oxyethylene chains ; the crystallinity depends significantly upon the relative lengths of the three blocks.Conformational Variety of n-Alkyloligo(oxyethylene)s The molecular conformation of a number of n-alkyloligo(oxyethy1ene)s in the solid state has been studied extensively by Raman spectroscopy in the present and pre- vious ~ork.'-~,~~ These studies have revealed that C,E, and C,E,C, compounds exhibit a rich variety of conformations in the solid state. The chain segments that characterize the con- formational state of the molecule are the oxyethylene-adjoining alkyl segment (C)CH,-CH,-CH,-O(C), the oxy- ethylene segment in the inner part (C)O-CH,-CH,-O(C), the hydroxy group terminated oxyethylene segment (C)O-CH,-CH,OH and the methoxy group terminated oxyethylene segment (C)O-CH, -CH, -OCH, .The alkyl chain generally takes the extended structure, but in some cases the oxyethylene-adjoining segment adopts the trans-gauche-trans conformation (e.g. fi form of C,Em4).A much greater variety is noted for the conformation of the oxyethylene chain. While the intrinsically stable conformation of repeated trans-gauche-trans is observed for the successive inner oxyethylene segments in a relatively long oxyethylene chain (a and fi forms of C,Em4; C8E4Cl and C16E4Cl),the extended trans-trans-trans conformation is usually attained in a short oxyethylene chain except for the terminal part, when a long alkyl chain is linked to the oxyethylene (y and yt forms of C,Em4).The conformation of the hydroxy group ter- minated oxyethylene segment in C,E, is either trans-gauche-trans (y form) or trans-trans-trans (yr form).For some of the C,E,Cl compounds with a short oxyethylene chain, the methoxy group terminated oxyethylene segment adopts the trans-gauche-trans conformation, whereas the inner oxyethy- lene segments are in the trans-trans-trans conformation (C,E2Cl, C8E,C, and CI6E3C,).Other C,E,Cl compounds with a short oxyethylene chain prefer the trans-trans-trans J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 conformation for the methoxy group terminated oxyethylene segment together with the inner oxyethylene segments (C16E1C1 and C16E2C1).The conformational variety as revealed for a large number of C,E, and C,E,C, compounds is associated primarily with the high flexibility of the oxyethylene chain which assumes the most appropriate conformation by adapting itself to its environment.The molecular conformation of the block com- pounds consisting of the oxyethylene and alkyl chains is practically determined by the relative lengths of the constitu- ent blocks and by the end group of the oxyethylene chain. Conclusions Raman spectroscopic studies of n-alkyloligo(oxyethy1ene)sin the solid state have shown that the end group of the oxyethy- lene chain affects the conformation of the molecule. The dif- ference in the conformational behaviour between the compounds terminated by methoxy and hydroxy groups results from the difference of the layer structures of the crys- tals.The strong hydrogen bonds between the hydroxy group terminated oxyethylene chains in the bilayer are responsible for the more effective retention of the helical structure of the oxyethylene chain in the C,E, molecules in competition with the stability of the extended structure of the alkyl chain. Conformational variety is one of the most prominent struc- tural features of n-alkyloligo(oxyethy1ene)s. This property stems from the high flexibility of the oxyethylene chain. The peculiar conformational behaviour plays an important role in various functional substances that contain the oxyethylene chain.We thank Dr. Koichi Fukuhara of Hiroshima University for valuable discussions. References 1 H. Matsuura and K. Fukuhara, Chem. Lett., 1984,933. 2 H. Matsuura and K. Fukuhara, J. Phys. Chem., 1986,90,3057. 3 H. Matsuura and K. Fukuhara, J. Phys. Chem., 1987,91,6139. 4 H. Matsuura, K. Fukuhara, S. Masatoki and M. Sakakibara, J. Am. Chem. SOC., 1991,113,1193. 5 S. Masatoki, K. Fukuhara and H. Matsuura, J. Chem. SOC., Faraday Trans., 1993,89,4079. 6 S. Masatoki, H.Matsuura and K. Fukuhara, J. Raman Spectro-sc., 1994,25, in the press. 7 K. Viras, F. Viras, C. Campbell, T. A. King and C. Booth, J. Chem. SOC., Faraday Trans. 2,1987,83,917. 8 J. R. Craven, 2.Hao and C. Booth, J. Chem. SOC., Furaday Trans., 1991,87, 1183. 9 J. R. Craven, K. Viras, A. J. Masters and C. Booth, J. Chem. SOC., Faraday Trans., 1991,87,3677. 10 H. H. Freedman and R. A. Dubois, Tetrahedron Lett., 1975, 3251. 11 T. Gibson, J. Org. Chem., 1980,45, 1095. 12 H. Matsuura and K. Fukuhara, J. Polym. Sci., Part B: Polym. Phys., 1986,24,1383. 13 S. Mizushima and T. Shimanouchi, J. Am. Chem. SOC., 1949,71, 1320. 14 R. F. Schaufele and T. Shimanouchi, J. Chem. Phys., 1967, 47, 3605. 15 T. Shimanouchi, H. Matsuura, Y. Ogawa and I. Harada, J. Phys. Chem. Ref: Data, 1978,7,1323. 16 H. Matsuura, K. Fukuhara and H. Tamaoki, J. Mol. Struct., 1987,156,293. 17 H. Matsuura, Comput. Chem., 1990,14,59. 18 H. G.Olf and B. Fanconi, J. Chem. Phys., 1973,59,534. 19 F. J. Boerio and J. L. Koenig, J. Chem. Phys., 1970,52, 3425. 20 K. Fukuhara and H. Matsuura, Chem. Lett., 1987, 1549. 21 R.C. Domszy and C. Booth, Makromol. Chem., 1982,183,1051. 22 H. H. Teo, T. G. E. Swales, R.C. Domszy, F. Heatley and C. Booth, Makromol. Chem., 1983,184,861. 23 H. Matsuura, K. Fukuhara and 0. Hiraoka, J. Mol. Struct., 1988,189,249. Paper 4/02904F; Received 16th May, 1994