J. Phys. Chem. 1985,89,4799-4803 directly comes from the small upper shifts of the unperturbed fundamentals in distorted molecules. Indeed the fundamentals get closer to the manifold of overtones and combinations crowded near 2930 cm-' and the latter become thus more intense. As a consequence, the ratio Z2930/Z2850is still shown to be physically meaningful. It can be taken as approximately proportional to the average concentration of G units as was first pointed out in our previous s t ~ d i e s . ~ . ~
Discussion The physical validity of the numerical results presented in the previous section of this paper must be judged on the basis of the acceptability of the model chosen. We have good reasons to think that interaction terms up to the second-nearest-neighbor may m u r in polymethylene systems and may be revealed by their Raman spectra in the CH stretching region. Indeed the experimental data on solid heptadeuteriopropane-1-Hby McKean and the Raman data on the CH2 group in the middle of an otherwise perdeuterated molecule of propane and pentane do not seem to be easily accounted for in terms of first-neighbor interactions, as discussed previously in this paper. On the other hand, one may think that the observed spectra could be justified by simply changing the diagonal force constant KCH in going from one C H 2 to the neighboring ones. We think that whichever way one projects physics into a model, the physical phenomenon we have to cope with is the change in the properties of the bonds which feel the length of the polymethylene chain at least as far as the second neighbor. This fact has never been brought up and used explicitly by other authors. Quantum-mechanical calculations have so far focused on the first-nearest-neighbor interaction and found relevant interaction terms. The existence of second-neighbor interactions, which we have assumed in this work is, at present, a challenge to "ab initio" calculations. We do not deny the possible changes of diagonal force constants, but we cannot so far refuse the existence of longer range interactions. In this paper we try to bring up the effects on the
4799
vibrational spectra only of first- and second-neighbor interactions, keeping the diagonal force constant unchanged. Once this model is accepted as a working hypothesis, one can try to judge whether calculations account for the experimental observations. We think that the results are very encouraging. In agreement with experiments, we find that, in going from the all-trans to a distorted conformation, the "a"mode near 2880 cm-' loses its intensity, becomes broader35and somewhat ill-defined, and shifts upward36 by about 10 cm-'. This is due to a geometry-dependent interplay of d- and d+ in the Fermi resonances, which generates several overlapping bands, whose intensities and frequencies depend on the population and topology of the G defects in an otherwise T lattice. d+, instead, keeps its overall dominant intensity at almost constant frequency. It is interesting to point out that a few short trans segments are enough for giving rise to the characteristic "d'" scattering. However, its peak frequency does shift with the length of the longest trans chain in the sample under consideration. Finally the feature at 2930 cm-' can still be related to the concentration of G structures. In conclusion, this work brings up two relevant problems, which require further consideration. The first one is specifically spectroscopic, in the fact that the Raman spectrum in the C H stretching region may be a manifestation of Fermi resonances and intramolecular harmonic couplings, thus becoming a probe for structural diagnosis in simple alkane molecules as well as in complex biological systems. The second problem more relevant to various fields of chemistry is the existence of intramolecular interactions in bonds extending at least up to the second-nearest neighbor in molecules for which inductive effects due to substituents are not predicted. This fact is not an obvious matter either theoretically or experimentally. Further experiments and calculations are required to solve the dichotomy of whether the effects used in this paper must be described in terms of a physical phenomenon localized on each single C H or extended over a few C H groups. Registry No. CD3(CH2),CD3, 97860-57-6.
Order-Disorder Transitlons of Urea Inclusion Adducts: A Study by Vibrational Spectroscopy+ H. L. Casal Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K1A OR6 (Received: April 10, 1985)
The solid-phase behavior of the urea inclusion adducts of 1-eicosanol and eicosanoic acid has been studied by vibrational (infrared and Raman) spectroscopy. The urea adduct of 1-eicosanolundergoes a phase transition centered at 210 K while the transition is centered at 250 K in the case of eicosanoic acid. From the vibrational spectra it is possible to differentiate between the effects of the transition on the urea channel and on the guest molecules. In this manner, it is found that the distortion of the urea channel structure occurs in all cases abruptly in narrow (3-4 K) temperature ranges while the onset of orientational disorder of the included chains occurs typically over wider (20-30 K) temperature ranges. The nature of these transitions is the same as previously found for other urea adducts.
Introduction Urea inclusion adducts of long-chain compounds continue to be a subject of interest.',* These adducts form readily with compounds such as fatty acids and n-paraffins. The inclusion adducts are known to undergo solidsolid phase tran~itions,~ the temperature of which increases with the length of the included chain^.^ These transitions are generally viewed as order-disorder transitions regarding the orientations of the included guest molecules. 'Issued as NRCC No. 24872.
0022-3654/85/2089-4799$01.50/0
However, the crystal structure of the urea host also changes. The well-known hexagonal structure found in urea adducts at room temperature converts to an orthorhombic s y ~ t e m .With ~ respect to the urea molecules the transitions involve anisotropic contraction (1) Fetterly, L. C.; In "on-Stoichiometric Compounds"; Mandelcorn, L., Ed.; Academic: New York, 1964. (2) Takemoto, K.; Sonoda, N. In "Inclusion Compounds"; Atwood, J. L., Davies, J. E. D., McNicol, D. D., Eds.; Academic: New York, 1984; Vol. 2. (3) Chatani, Y.; Taki, Y.; Tadokoro, H. Acta Crystaflogr.,Sect. B 1977, 833, 309. (4) Chatani, Y.; Anraku, H.; Taki, Y . Mol. Cryst. Liq. Crysr. 1978, 48, 219.
Published 1985 American Chemical Society
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The Journal of Physical Chemistry, Vol. 89, No. 22, 1985
of the lattice in the basal plane. While the adducts of n-alkanes have been studied in detail by X-ray d i f f r a ~ t i o n ,no ~ detailed crystallographic studies of the temperature dependence of urea adducts of fatty acids or n-alkanols have been reported. In this work, infrared and Raman spectra have been used to study the thermal behavior of the urea adducts of 1-eicosanol and eicosanoic acid. Vibrational spectroscopy is well suited to these studies since information relating to the structure of the urea host lattice and the dynamics of the guests can be obtained simultaneously. In a previous studyS of the stearic acidurea adduct, it was found that the orientational order-disorder transition of the included stearic acid occurs continuously from 223 to 253 K. This transition is manifested in the Raman spectrum as a continuous increase in the rate of chain rotation. However, the distortion of the urea channel occurs abruptly at 234 K covering only a 4 K temperature interval. Recent Raman spectroscopic studied of the urea inclusion adducts of n-alkanols suggest that the channel distortion also covers a wide temperature range paralleling the temperature interval in which the orientational order-disorder transition of the guest alkanol takes place. This suggests a difference in behavior between adducts of alkanols and fatty acids. In the present comparison of adducts of alkanol and fatty acid of the same chain length ( n = 20) the same general thermal behavior is found for both types of compounds.
Experimental Section 1-Eicosanol, eicosanoic acid, (from Aldrich), and urea (from Fisher) were used without further purification. The inclusion adducts were crystallized from hot methanol following standard procedures.'%*A few drops of CHC1, were added to facilitate the dissolution of the long-chain compounds. Elemental analysis of the adducts gave urea/guest molar ratios of 15.2 and 15.6 for 1-eicosanol and eicosanoic acid, respectively. These ratios are consistent with a small amount of excess guest present relative to full inc1usion.l The Raman spectra of these adducts indicate that inclusion has occurred and that the amount of urea not forming adducts (Le., tetragonal) is very sthall.' Infrared spectra at 2-cm-I resolution were collected with a Digilab FTS-15 Fourier transform infrared spectrometer equipped with a HgCdTe detector. The samples consisted of the finely divided powdered adducts held between KBr windows in an infrared cell which was placed in a thermostated holder* inside an evacuable chamber. The temperature was varied by flowing nitrogen through the holder and measured with a copperconstantan thermocouple in contact with the cell windows. The Raman spectra were excited with 150 mW of 5145-A radiation from a CR12 Argon ion laser and collected with a Spex 1401 double monochromator equipped with a cooled RCA C31034 photomultiplier. The spectral slit width was -2 cm-I and frequency and intensity calibrations were applied to each spectrum. The samples were held in sealed capillaries and the temperature controlled by using a laminar flow of cold n i t r ~ g e n . ~
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Results and Discussion Infrared and Raman spectra of urea inclusion adducts have been analyzed in previous p u b l i c a t i ~ n s , ~and ~ ~assignments ~ ~ ~ J ~ of the bands are available. Since the crystal structure in neat pure urea is different from that in the inclusion adducts, the vibrational spectra can be used for analyzing the extent of inclusion.I0 The spectra of the included chain compounds are also well-known. Therefore, the present spectra will not be discussed regarding ( 5 ) Casal, H. L.; Cameron, D. G.: Kelusky, E. C.; Tulloch, A. P. J . Chem. Phys. 1984, 81, 4322. (6) Le Brumant, J.: Jaffrain, M.; Lacrampe, G. J . Phys. Chem. 1984,88, 1548. (7) Kutzelnigg, W.; Mecke, R.; Schrader, B.; Nerdel, F.; Kresse, G. Z . Elektrochem. 1961,65, 109. Fawcett, V.;Long, D. A. J . Raman Spectrosc. 1975, 3, 263. (8) Cameron, D. G.; Jones, R.N. Appl. Spectrosc. 1981, 35, 448. (9) Scherer, J. R.; Snyder, R. G. J . Chem. Phys. 1980, 7 2 , 5798. (10) Casal, H. L. Appl. Speczrosc. 1984, 38, 306. (1 1) Barlow, G. B.; Corish, P. J. J . Chem. SOC.1959, 1706. Fischer, P. H. H.; McDowell, C. A. Can. J . Chem. 1960, 38, 187.
Casal
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t C
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700 1000
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Wavenumber shift, cm-' Figure 1. Raman spectra (A: 500-700-cm-l and B: 1000-1200-~m-~ regions) of the 1-eicosanol-urea inclusion adduct at 163, 190, 240, and 298 K. The band a t 1084 cm-' marked with an asterisk is an artifact introduced by the spectrometer. The intensity scales in both spectral regions have been scaled for display and no direct comparison of intensities can be made.
assignments or general characteristics. Of interest to the present work are those spectral regions yielding information on the structure of the urea channel and on the dynamics of the included chains. It has been shown that in the Raman spectrum the N C O skeletal deformation bands of urea (at 530 and 610 cm-I) are good markers for determining the integrity of the adductsk0and for monitoring distortions of the ~ h a n n e l . Figure ~ 1A shows the 500-700-cm-' region of the Raman spectrum of the 1-eicosanol adduct at several temperatures. The two bands at 530 and 610 cm-' are the N C O skeletal bands of urea; the weak shoulder at -550 cm-I is probably due to urea not forming the inclusion adduct. The Raman spectra of the eicosanoic acid adduct in this region are almost the same as those of the 1-eicosanol adduct. As the temperature is varied the main effect observed in the spectra of Figure 1A is a change in the relative peak heights of the two N C O skeletal deformation bands. Figure 2 shows the temperature dependence of the peak height ratio of the two skeletal deformation bands (h530/h610) measured from the spectra of the adducts of 1-eicosanoland eicosanoic acid. In both sets of data there are abrupt discontinuous changes in the values of h530/h610; for 1-eicosanol-urea the change occurs at -210 K while for eicosanoic acid-urea it is at -250 K. The discontinuous changes observed in the values of hs30/h6,0 indicate that a phase change occurs for the two adducts at 2 10 and 250 K, respectively. The magnitude of the changes are similar for both cases and are also comparable to the change observed for the inclusion adduct of stearic acid5 at the orthorhombichexagonal transition at 234 K. Thus, the inclusion adducts of 1-eicosanol and eicosanoic acid undergo transitions involving changes in the channel structure of the orthorhombic-hexagonal type. From the Raman spectra of the inclusion adduct of cetyl alcohol (n-CI6H3@H)Le Brumant et aL6 have also measured the tem-
Transitions of Urea Inclusion Adducts
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985 4801 transition temperature. It appears that for the adducts of hexadecane and cetyl alcohol AR is larger. This could be a consequence of the special stability found when the length of the guest is compatible with the length of the c axis of the urea lattice such as n-hexadecane.'V2 Regarding the structure of the urea channel, other regions of the Raman spectra of 1-eicosanol-urea and eicosanoic acid-urea are also affected by the transitions, in particular, the regions of lattice vibration below 200 cm-', the C - 0 stretching and N H 2 bending bands between 1500 and 1700 cm-l, and the N-H stretching regions between 3100 and 3500 cm-I. The strong band due to C-N stretching and appearing at 1024 cm-I (see Figure 1B) does not show any discontinuous behavior at the transition temperatures. A linear dependence of the frequency of the C-N stretching mode is found as the temperature is lowered. The same general behavior for the frequency of this mode was found in the spectra of the stearic acid-urea5 and cetyl alcohol-urea6 adducts. The Raman spectral features of the included 1-eicosanol and eicosanoic acid yield information relating to their conformation and structure. The C-C stretching region (1000-1 150 cm-I) of the Raman spectrum of 1-eicosanol-urea is shown in Figure 1B. The corresponding spectral region for the eicosanoic acid-urea adduct is practically the same. In this region, bands are observed at 1063, 1110, and 1130 cm-' which are assigned to C-C stretching vibration^.'^ The position of these C-C stretching bands are characteristic of chains in the all-trans conformation and no evidence of C-C stretching bands due to gauche conformers (- 1080-1090 cm-') is found.15 This is expected since it is known that inside urea channels the included guests are in the fully extended all-trans conformation. The C-C stretching bands are not much affected by temperature (see Figure 1B). As the temperature is raised there is slight broadening of the three C-C stretching bands due to increased chain mobility. The strong band at 1130 cm-' shows a very slight frequency shift with temperature such that it is at 1131.3 cm-' at 160 K and at 1129.8 cm-l at 300 K. In contrast to the behavior of the C-C stretching bands is the temperature dependence of the band at 1174 cm-'. This band can be assigned to the CH2 rocking vibration of the all-trans chain.16 The band is only observed in the spectra recorded below 200 K in the case of 1-eicosanol-urea and below 235K for eicosanoic acid-urea. As the temperature is raised the CH2 rocking bands broaden very rapidly and to such an extent that they cannot be distinguished from the background. This behavior has also been observed in the Raman spectra of n-hexadecane-ureaI6 and stearic a c i d - ~ r e a . ~This marked temperature dependence of the CH2 rocking mode is paralleled by the strong CH2 antisymmetric stretching band at -2885 cm-l. These two vibrational modes are particularly sensitive to temperature effects which affect the rate of chain rota ti or^.^.^^ Their temperature dependence serves as confirmation that the major change introduced at the phase change of 1-eicosanol-urea and eicosanoic acid-urea is an increase in chain rotational freedom as the temperature is raised. The increase in chain rotational freedom can be followed by measuring the bandwidth of the CH2 scissoring mode' (at 1430 cm-l) as a function of temperature. These data are shown in Figure 3 for the 1-eicosanol-urea adduct. There are three distinct regions in the plot; between 160 and 200 K the bandwidth remains almost constant with increasing temperature, and similar behavior is observed between 230 and 310 K. The bandwidth increases by -0.9 cm-l continuously in the 200-230 K temperature range. A similar behavior is also found in the temperature dependence of the peak height ratio h2850/h2885 of the two CH2 stretching bands. The behavior of the C H 2 scissoring bandwidth demonstrates that the rotational freedom of the included chains is increasing
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230
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TEMPERATURE, K Figure 2. Temperature dependence of the peak height ratio h530/h610 of the two NCO skeletal deformation bands obtained from the spectra of 1-eicosanol-urea (0)and eicosanoic acid-urea ( 0 )inclusion adducts. In both cases the peak heights were measured after subtraction of a linear base line extending from 500 to 665 cm-'.
perature dependence of the relative peak heights of the N C O skeletal bands. The band at -530 cm-I increases in height with respect to the band at 610 cm-', but no clear discontinuous change was found between 80 and 300 K. However, spectra were measured at 20 K intervals.6 From the data of Figure 2 the transition is found to occur at lower temperature for the adduct of the alcohol (210 K) than for that of the acid (250 K) of the same chain length. This is in accord with the earlier findings of Chatani et al.334 Since fatty acids are forming dimers12inside the urea channel and the transition involves a change in chain rotational freedom, the fatty acids actually behave as the corresponding chain with double length. The values of the h530/h611 peak height ratios are of comparable magnitude for both inclusion adducts above the transition temperatures. Below the transition temperature hSso/hsl is slightly higher for the fatty acid. Higher h530/h611 values could be correlated with a more pronounced distortion of the lattice in the case of the fatty acid adduct than in the alkanol adduct. The values of h530/h611 can be correlated with the kinetic diameter of the guests.I3 The eight-membered dimer moiety of the fatty acid guest behaves as a larger diameter guest able to introduce further distortion of the urea channel. This could be especially effective at low temperature where the channel has a smaller diameter and the fatty acid dimer moiety is forming stronger H bonds than in the room temperature hexagonal form of the adduct (see below and ref 5). The changes induced in the channel dimension and structure are reflected in the length of the N-H-0 bonds formed by the urea molecules. A variation of 0.045 A was found in the length of the hydrogen bonds between 98 and 300 K in adducts of nhe~adecane.~ From the temperature dependence of the frequency of the N H stretching band a t -3380 cm-' the variation ( A R ) in hydrogen bond length between 80 and 300 K of the adduct of cetyl alcohol was calculated to be -0.04 A.6 In the present case AR is found to be 0.02 A for both adducts in the 160-280 K range for the alkanol and in the 208-300 K range for the acid. Thus, AR is the same at comparable temperature differences from the (12) Laves, F.; Nicolaides, N.; Peng, K. C. 2.Krist. 1965, ZZZ, 258. (1 3) The Raman spectra of urea inclusion adducts of aromatic compounds yield hs30/hsl,values at room temperature of the order of 1.2-1.4 (unpublished results from this laboratory).
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(14) Levin, I. W. "Advances in Infrared and Raman Spectroscopy"; Clark, R. J. H., Hester, R. E., Eds.; Wiley: New York, 1984; Vol. 11, pp 1-48. (15) The peak at 1084 cm" is an artifact introduced by the spectrom-
eter.
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(16) Snyder, R. G.; Scherer, J. R.; Gaber, B. P. Biochim. Biophys. Acto 1980, 601, 41.
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4802 The Journal of Physical Chemistry, Vol. 89, No. 22, 1985
A
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TEMPERATURE, K Figure 3. Temperature dependence (in cm-I) of the full width at 0.90
peak height of the CH2 scissoring band (-1430 cm-I) in the Raman spectra of 1-eicosanol-urea inclusion adduct. 1000 1100
1100
I
B 150 K
8S
8
c
G
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0
0 v)
\
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w -
1 10
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Wavenumber, cm-'
I
I
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1000
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Wavenumber, cm-' Figure 4. The 1400-800-~m-~ region of the infrared spectra of eicosanoic acid-urea at 170, 193, 254, and 275 K.
Figure 5. The 1100-1000-cm-' region of the infrared spectra of l-eicosanol-urea adduct and neat 1-eicosanol at 300 K (part A) and 150 K (part B). The right-hand side shows the spectra after deconvolution with a 4-cm-' wide Lorentzian band.
continuously in the range 200-230 K for 1-eicosanol-urea. The corresponding plot for the eicosanoic acid-urea adduct shows that in this case the bandwidth remains constant from -200 to 235 K and from 206 to 300 K with the largest rate of change observed between 235 and 265 K. The temperature behavior of the included guests, monitored by the bandwidth of the CH2 scissoring bands, is in marked contrast with that shown by the urea channel structure monitored by the peak height ratio h530/h610 (Figure 2). In the latter case the distortion introduced by the transitions covers less than 4 K and it is fairly abrupt. A similar contrast in temperature behavior was found for the stearic acid-urea a d d ~ c t These .~ results can be interpreted as indicating that the increased rate of chain rotation
induces strain in the low-temperature urea lattice until this reaches a critical point (210 K and 1-eicosanol and 250 K for eicosanoic acid) in which the lattice changes. The present results are comparable to those obtained from the stearic acid-urea a d d ~ c t .This ~ "dual" temperature behavior of urea inclusion adducts in which the channel distortion occurs abruptly while the included guests show a "smooth" dependence seems to emerge as a fairly general property of urea inclusion adducts of long-chain compounds. The infrared spectra of the urea adducts of 1-eicosanol and eicosanoic acid can be used to provide information regarding the chain conformations, nature of the fatty acid dimer, and the possibility of hydrogen bonding between the included alcohol and the urea lattice.
Transitions of Urea Inclusion Adducts Figure 4 shows the 1400-800-cm-' region of the infrared spectrum of the eicosanoic acid-urea adduct at 170, 193, 254, and 275 K. In this spectral region there are two strong bands due to the urea host at 1017 and 1161 cm-l. The position of these bands show a very slight temperature variation. The CH2wagging band progression between 1180 and 1360 cm-I is clearly evident in all the spectra. These bands broaden with increasing temperature and shift slightly (- 1 cm-') to higher frequencies as the temperature is raised. The more noticeable change in the spectra of Figure 4 as the temperature increases is in the band at -956 cm-I which shows a very pronounced broadening as the temperature is raised such that it is seen as a broad base-line feature in the spectrum recorded at 254 K and not distinguishable in the 275 K spectrum. The band at -956 cm-' is the OH out-of-plane bending mode of the dimeric fatty acid moiety.17 In the spectra of neat fatty acids it appears at -940 cm-I when the fatty acids pack in their C polymorph. It is found at lower frequencies in other crystalline modification^.'^*^* The appearance of the OH out-of-plane bending mode at higher frequencies in the spectra of the lowtemperature form of the urea inclusion adducts shows that the dimeric moiety is involved in shorter, stronger H bonds than in neat fatty acids. This is most likely a consequence of the steric crowding induced by the contraction of the urea channel. The difference of the bands due to the dimeric moiety between the urea inclusion adducts and neat fatty acids is correlated with a different geometry of the eight-membered dimer in urea adducts and in neat crystals. The OH out-of-plane band, however, is not observed in the spectra of the urea adduct at temperatures above -250 K; this corresponds to the high-temperature crystal form of the adduct in which the rate of chain rotation is higher.5J9 The nonobservation of the O H bending band (or at least its very low intensity) is an indication that the hydrogen bonds between the two fatty acid molecules forming the dimer are very weak. This conclusion contrasts with the results of X-ray diffraction studies which showed short distances for the dimer moietyI2 (i.e., -H,COOH-.HOOCCH2-) in fatty acids included in urea. The other possible explanation for not observing the OH out-of-plane band in the spectra of room temperature adducts is that the geometry is such that the vibration is not active. Regardless of the reasons for the characteristics of the OH out-of-plane bending mode the infrared spectra show (Figure 4) that the transition at -250 K of the eicosanoic acid-urea adduct involves a major geometrical change of the acid dimer moiety. Although unlikely, the possibility of hydrogen bonding to the urea lattice cannot be discarded. The conformation of the included 1-eicosanol chains in the urea channel is also all-trans as demonstrated by the Raman spectrum (Figure 1). The infrared spectra also demonstrate this point by the observation of the full CHI wagging band progression between 1180 and 1360 cm-'. Furthermore, the conformation of the C-OH bond can also be found to be trans with respect to the C-C-C chain. This is seen in the infrared spectra by examining the C-OH stretching band. Figure 5 compares the 1100-lOOO-cm-l region of the infrared spectra of 1-eicosanol-urea and neat 1-eicosanoL Figure 5A compares these spectra at 300 K and Figure 5B compares them at 150 K. The right-hand side of Figure 5A,B shows the spectra after Fourier self-deconvolution20with a Lorentzian line of 4-cm-' bandwidth at half-height. At room temperature the spectra of the neat alkanol and of the 1-eicosanol-urea adduct are practically identical (Figure 5A) with the exception of the band at 1014 cm-I which is the C-N stretching band of urea. The C-O stretching mode give two bands at 1067 and 1062 cm-I (see deconvolved spectra); the other bands in this region are due to C-C stretching.21 The sample of neat
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(17) Holland, R. F.; Nielsen, J. R. J . Mol.Spectrosc. 1962, 9, 436. (18) Conti, G.; Minoni, G.; Zerbi, G. J . Mol. Strucr. 1984, 118, 237. (19) Casal, H. L.; Cameron, D. G.; Kelusky, E. C. J . Chem. Phys. 1984, 80, 1407. (20) Kauppinen, J. K.; Moffatt, D. J.; Mantsch, H. H.; Cameron, D. G. Appl. Spectrosc. 1981, 35, 271.
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985 4803 1-eicosanol from which the spectra of Figure 5 were collected was recrystallized from the melt. This yields the y crystal modification of 1-eicosanol whose infrared spectrum has been described in detail by Tasumi et al.I1 This y crystal form of 1-eicosanol is composed of all-trans chains in which the C-OH bond is trans with respect to the alkyl chain. The all-trans arrangement is manifested in the position of the C-OH stretching band and the relative intensity of the C-C stretching bands and also by the observation of a band at 1427 cm-I which is also observed in the infrared spectra of the 1-eicosanol-urea adduct. The spectrum of the adduct thus confirms once more the conformation of the included guest chains in urea inclusion adducts as being all-trans. However, from the spectra of the adduct it can also be concluded that the O H group of 1-eicosanol is involved in the same type of hydrogen bonding as in the y crystal form of neat 1-eicosanol. The spectra at low temperatures (Figure 5B) are different, however. The spectrum of neat 1-eicosanol at 150 K shows two bands at 1069 and 1062 cm-I for the C-OH stretching. The spectrum of the 1-eicosanol-urea adduct shows three bands at 1069, 1066, and 1062 cm-I for the same mode. This difference can be ascribed to a different hydrogen-bonding situation between the neat 1-eicosanol and the adduct a t low temperature. As in the case of eicosanoic acid this is probably due to the contraction of the urea channel inducing changes in the geometry of hydrogen bonds between two 1-eicosanol molecules. The conformation of the included 1-eicosanol remains all-trans in the low-temperature form of the urea adduct.
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Conclusions From the data presented herein several conclusions about the solid-phase behavior of urea inclusion adducts can be derived. The adducts of 1-eicosanol and eicosanoic acid undergo phase transitions a t 210 and 250 K. These phase transitions involve predominantly an increase in rotational freedom of the included chains as the temperature is increased. In accord with previous measurements the onset of the disorder with respect to the chain orientation in the urea channel is weakly cooperative and occurs gradually over a -30 K temperature interval. The induced channel distortion, however, occurs over a very narrow temperature interval (3-4 K) and seems to be very similar for the alkanol and fatty acid of the same length. The difference between the fatty acid or alkanol adducts is the transition temperature. The temperature of transition seems to be primarily a function of the effective chain length which is double for the case of the fatty acid due to dimer formation. Chain length alone is not the only factor determining the phase transition temperature. In the case of n-eicosane the transition temperature was measured22at 189 K for n-nonadecane a value between 160 and 170 K was found.19 For 1-eicosanol-urea the present study shows that the transition temperature is 210 K, Le., some 20 K higher than the corresponding n-alkane. In the case of the alkanol compared to the n-alkane the restrictions to molecular chain rotation which eventually induce the transition to the low-temperature orthorhombic structure are possibly assisted by hydrogen bonding between the O H group and the urea molecules or between two l-eicosanol molecules. This argument is supported by the infrared spectra of the 1-eicosanol-urea adduct at low temperature. The geometry of the fatty acid dimer moiety in the low-temperature form of the urea adduct is different from that at room temperature and also different from those found in neat crystals of fatty acids. This points to a geometrical arrangement for the dimer moiety which is not attainable in the neat crystals; a nonplanar dimer moiety possibly involving hydrogen bonding to the urea lattice can be proposed. Registry No. 1-Eicosanol urea inclusion adduct, 97975-87-6; eicosanoic acid inclusion adduct, 97975-88-7. (21) Tasumi, M.; Shimanouchi, T.; Watanabe, A,; Goto, R. Spectrochim. Acta 1964, 20, 629. (22) Pemberton, R. C.; Parsonage, N. G. Trans. Faraday SOC.1965.61, 1,,1
LIIL.
