J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1313-1322 Conformational and Vibrational Properties of a,w-Dihalogenoalkane/Urea Inclusion Compounds :A Raman Scattering Investigation Sharon P. Smart Department of Chemistry, University of St. Andrews, St. Andrews, Fife, Scotland, UK KY16 9ST Abdelkrim El Baghdadi and FranGois Guillaume* Laboratoire de Spectroscopie Moleculaire et Cristalline, Universite de Bordeaux I, CNRS URA 124, 33405 Talence Cedex, France Kenneth D. M. Harris* Department of Chemistry, University College London, 20 Gordon Street, London, UK WCIH 0AJ Raman spectroscopic investigations of urea inclusion compounds containing a,o-dihalogenoalkane [X(CH,),X ; n = 8 for X = CI; n = 7-11 for X = Br; n = 8 for X = I] guest molecules are reported. In these inclusion com- pounds, the urea molecules form a tunnel structure within which the guest molecules are located.Vibrational modes due to the urea confirm the structural identity of the inclusion compounds, and lattice modes of the urea host structure are assigned tentatively. Investigations of the vibrational properties of the X(CH,),X guest mol- ecules included within this host structure have focused on the longitudinal acoustic mode (LAM-l) and the C-X stretching vibrations. Bands in the Raman spectrum due to the v(C-X) mode have been studied as a function of: (i)the length (n) of the guest molecule; (ii) the identity of the terminal substituent X; (iii) temperature and (iv) pressure. From these results, trends in the relative amounts of gauche and trans end-groups for the X(CH,),X guest molecules in their urea inclusion compounds have been assessed.Urea inclusion compounds have been widely investigated from the viewpoint of their fundamental physico-chemical properties.’ The ‘host’ structure in these crystalline solids is constructed from an extensively hydrogen-bonded arrange- ment of urea molecules, and this structure contains a regular arrangement of one-dimensional, parallel tunnel^.^.^ Guest molecules of appropriate dimensions can be accommodated within these tunnels. Because the cross-section of the tunnels (defined by the van der Waals surface of the tunnel wall) is ca. 5.3-5.7 A, only guest molecules based on a sufficiently long n-alkane chain, with an appropriately limited degree of sub- stitution, can fit within these tunnels.Of relevance to the present paper is the fact that a,o-dihalogenoalkanes [X(CH,),X] are known2-’ to form inclusion compounds with urea. In such a ‘constrained’ environment, these guest mol- ecules must adopt a linear, extended conformation and, as a consequence, urea inclusion compound formation has often been used as a means of isolating molecules in such confor- mations, which may differ substantially from the preferred conformations of the same molecules in other solid-state environments or in dispersed phases. The urea inclusion com- pounds have thus been exploited as prototypical materials for understanding the structural, dynamic and spectroscopic properties of molecules in linear, extended conformations.The physico-chemical properties of urea inclusion com-pounds containing alkane guests have been studied extensively’ via a wide range of experimental techniques, although much less is known, at present, about the corre- sponding properties of urea inclusion compounds containing functionalized alkane guests. We are currently addressing this question by carrying out extensive studies of the structural and dynamic properties of urea inclusion compounds con- taining guests such as a,o-dihalogenoalkanes, diacyl per- oxides, and carboxylic acid anhydrides. In this paper, we consider the vibrational properties of the host and guest com- ponents in a,o-dihalogenoalkane/urea inclusion compounds. The alkane/urea inclusion compounds undergo a phase transition from a high-temperature (HT) phase in which the host structure is hexagonal to a low-temperature (LT) phase in which the host structure is orthorh~mbic.~*~ Differential scanning calorimetry has shown that the a,o-dibromoalkanel urea inclusion compounds undergo a similar phase tran-sition, and powder X-ray diffraction* has shown that this transition is associated with the same distortion of the host tunnel structure as that established previously6~’ for the alkane/urea inclusion compounds. For all of the guest mol- ecules considered here, the phase-transition temperature is in the range CQ.145-170 K. At sufficiently high temperature, urea inclusion compounds decompose to produce the ‘pure’ crystalline phase of urea (which differs in structure from the host structure in urea inclusion compounds, and is not a tunnel-containing structure).The decomposition temperature is a function of the length of the guest molec~le;~ urea inclu- sion compounds with short guests such as octane are not stable if left in the atmosphere at room temperature. The ‘pure’ crystalline phase of urea is often referred to as ‘tetragonal urea’ and the conventional urea inclusion com- pounds are often referred to as ‘hexagonal urea’ (as a conse- quence of the different crystal systems of these structures). We have discovered recently,4*10 by X-ray diffraction and other techniques, that urea inclusion compounds containing functionalized alkane guests exhibit new structural proper- ties, particularly concerning the three-dimensional packing arrangement of the guest molecules.In particular, single- crystal X-ray diffraction4 has shown that, at room tem-perature, the guest molecules in the urea inclusion compounds containing Br(CH,),Br guest molecules with n = 7-10 exhibit a characteristic three-dimensionally ordered packing arrangement with A, = ~$3, where c, denotes the periodic repeat distance of the guest molecules along the tunnel and A, denotes the offset, along the tunnel axis, between the positions of guest molecules in adjacent tunnels (Fig. 1). This guest structure is rhombohedral, and a given single crystal of the inclusion compound usually contains two domains of this guest structure, differing in orientation with respect to the host structure.Furthermore, the M Br(CH,),Br/urea inclusion compounds also contain regions in which the guest molecules are ordered only along the tunnel axis; the periodic repeat distance is the same (within experimental error) for these one-dimensionally ordered regions and the three-dimensionally ordered regions dis- cussed above. It is relevant to note that, in contrast to this situation for the Br(CH,),Br guest molecules, the molecular packing in the three-dimensionally ordered regions of the guest structure in alkane/urea inclusion compounds corre- sponds to A, = 0. It is important to consider whether the presence of terminal bromine atoms uis-ci-uis methyl groups on the guest molecule gives rise to a similarly marked differ- ence in the local structural properties of the guest molecule, with particular interest, within the context of this paper, in the conformational properties of the guest molecule.It is important to stress that the X-ray diffraction studies dis- cussed above have elucidated only the relative positioning of guest molecules within the urea tunnel structure, and have not led to structure determination of the guest structure (i.e. determination of atomic coordinates, from which the confor- mational properties could clearly be deduced). Finally, we note that the periodic repeat distance of the guest molecules along the tunnel (c,) is usually incommensurate with the periodic repeat distance of the host structure along the tunnel ;a detailed discussion of incommensurate uersus com-mensurate behaviour in one-dimensional inclusion com-pounds (typified by the urea inclusion compounds) has been given elsewhere.11-' The periodic repeat distance c, of the guest molecules in the alkane/urea inclusion compounds is always ca. 0.5 %i shorter than the 'van der Waals length' of the alkane mol- ecule in its all-trans conformation, and similar behaviour has also been observed for Br(CH,),Br/urea inclusion com-pound~.~This arises" from the fact that, in the energetically most stable state of the inclusion compound, there is a repul-sive interaction between adjacent guest molecules in the same tunnel. It is clearly relevant to consider whether the propor- tion of conformational defects (i.e.differing from a trans end- group) at the ends of the guest molecules is influenced by this repulsive intejaction between adjacent end-groups.Our recent incoherent quasielastic neutron-scattering studie~'~.'~of the dynamic properties of the guest molecules in Br(CH,),Br/urea inclusion compounds have shown that both reorientational motions of the guest molecules about the tunnel axis and translational motions of the guest mol- ecules along the tunnel axis are effective on the picosecond timescale. The translation length depends critically upon temperature and is ca. 2.3 %i for 1,9-dibromononane/urea at 280 K. The motivation underlying the work described in this paper was to investigate, uia Raman spectroscopy, the confor- mational and vibrational properties of the a,u-dihalo-genoalkane/urea inclusion compounds. One specific aim was J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 to derive an understanding of the effect of the dense packing of guest molecules, and the translational motions of the guest molecules, on their conformational properties. The aspects of the Raman spectrum that can be used to probe the structural properties of the host are discussed, and the Raman-active vibrational modes of the guest molecules, which depend criti- cally upon the molecular conformation, are analysed. In this regard, two specific vibrational modes have been considered in detail: (a)the longitudinal acoustic mode LAM-1 and (b) the v(C-X) stretching mode, the frequency of which is partic- ularly sensitive to the conformation of the -CH,-CH,-X end-group.These vibrational properties have been studied as a function of: (i) the identity of the halogen atom (X); (ii) the length (n)of the guest molecule [X(CH,),X] ;(iii) temperature and (iv) hydrostatic pressure. Because our previous X-ray diffraction4 and neutron ~cattering'~? l5 investigations were focused on Br(CH,),Br/urea inclusion compounds, particu- lar emphasis [concerning points (ii), (iii) and (iv)] has been given here to the study of Br(CH,),Br guest molecules. Experimental Urea inclusion compounds containing X(CH,),X guest mol- ecules (n= 8 for X=C1; n=7-11 for X=Br; n= 8 for X = I) were prepared by slowly cooling warm solutions of X(CH,),X and urea in methanol.Powder X-ray diffraction confirmed that all samples had the characteristic tunnel host structure of the conventional urea inclusion compounds. The phase-transition temperatures for the Br(CH,),Br/urea inclu- sion compounds, established by differential scanning calorim- etry, are in the range ca. 145-170 K. Polarized Raman spectra were recorded for these X(CH,),X/urea inclusion compounds on a triple-mono-chromator Dilor 224 spectrometer and on a double-mono- chromator Jobin-Yvon Ramanor spectrometer. The specific polarizations used were (in the Porto notation) X(ZZ)Y, X(ZX)Y, X(YZ)Y and X(YX)Y in direct goemetry. Unless otherwise stated, the samples used in these experiments were single crystals of the urea inclusion compounds, with typical crystal dimensions of ca.0.5 x 0.5 x 3 mm3. Each crystal was sealed in a thin glass tube with its long axis [which corre- sponds to the urea tunnel axis (crystallographic c axis)] parallel to the Z axis of the laboratory reference frame (see Fig. 2). The incident radiation was the 514.5 nm line of an Ar' ion Spectra Physics laser, with power ca. 150-300 mW at the sample. Spectra were generally recorded between 20 and 2000 cm-'. The spectral resolution (full width at half maximum height) was 1.5 cm-' for the spectral range 20 < V/cm-' < 300 and 2.8 cm-' for the spectral range V > 300 cm-'. A =r I incoming laser beam I INKII y X \IT-X Fig.2 Configuration of a single crystal of a urea inclusion com- pound in the polarized Raman experiments. The axes (X,Y, Z) are fixed in space (laboratory reference frame), whereas the axes (x, y, z) are associated with the guest molecule within the inclusion com- pound. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 liquid-nitrogen cryostat was used to perform experiments in the temperature range 80-300 K (temperature stability ca. +3 K). Raman spectra were also recorded at non-ambient pressure (at room temperature) using a home-built device described in detail elsewhere. l6 These experiments were performed on polycrystalline samples, and only the Br(CH,), ,Br/urea inclusion compound was studied. The pressure range was 1 bar to 6 x lo3 bar and helium gas was used as the compres- sing fluid.Background Vibrational Modes due to the Urea Host The symmetry of an isolated urea molecule is described by the point group C,,. In the high-temperature phase of the urea inclusion compounds, the space group of the urea host structure is P6,22 and there are six urea molecules per unit ell.^.^ The site symmetry of the urea molecule is C, and the factor group is D, . There are 45 Raman-active internal vibra- tions and 13 Raman-active external vibrations: I'fiman9Al= + 18E1 + 18E2; = 2A1 + 5E1 + 6E2. Below the phase-transition temperature for the urea inclu- (using superscript g to represent gauche and superscript t to represent trans): These intensity ratios have been used in our assessment (uide infia) of the relative amounts of the gauche and trans end- groups in the guest molecules. A potential problem in the study of Raman bands associ- ated with the LAM-1 modes of the guest molecules is that these can be hidden beneath bands due to urea lattice vibra- tions.In order to derive a precise estimate of the LAM-1 fre- quencies, the LAM-1 bands in the spectra were fitted using several Lorentzians convoluted with a triangular instrumen- tal resolution function. The vibrational modes of the X(CH,),X guest molecules will necessarily reflect the geometrical constraints imposed upon them by the confined character of their one-dimensional host environment. In the all-trans conformation, the symmetry of the molecules with odd n is described by point group C,, and the symmetry of the molecules with even The space group is P2,2,2,, with 12 urea molecules per unit cell (and three independent urea molecules in the asymmetric unit).In this structure, the site symmetry of the urea molecule is C, and the factor group is D,.There are 72 Raman-active internal vibrations and 2 1 Raman-active external vibrations : = 18A + 18B1+ 18B2 + 18B3; I'&Lan= 6A + 5B1+ 5B2 + 5B3. Vibrational Modes due to the a,a-Dihalogenoalkane Guest For the scattering geometries used in this work, the following components of the derivative of the polarizability tensor for the X(CH,),X guest molecules can be obtained:17 From X(ZZ)Y experiments: (alzz)* = (1) From X(2X)Yand X(Y2)Yexperiments: = 3c<a:z)2 + (2) From X(YX)Yexperiments: In these expressions, a:j = (2)Qo where Q is the normal coordinate of the vibration.As shown in Fig. 2, Oz represents the main axis of the guest molecule (assumed, on average, to be parallel to the urea tunnel axis). The Raman intensity (IIJ) is proportional to the mean (averaging over rotation about the z axis) of the square of the relevant component of the derivative of the polarizability tensor: '1, cc (4) The relative intensities of the bands due to the v(C-X) stretching vibrations for the gauche and trans end-groups (-CH,CH,CH,X) were determined by numerical integra- tion. The intensity ratios Yftand 9$were evaluated from the spectra recorded in the different polarizations as follows sion compounds, the urea host structure is orth~rhombic.~,~ n is described by point group C,, .LAMS correspond to stationary vibrational waves with one or more nodes in the longitudinal displacements of the skeletal carbon atoms." Only LAM vibrations with an odd number of nodes are Raman active, and the Raman intensity decreases rapidly as the number of nodes is increased. The LAM-1 vibration gives rise to the most intense LAM band in the Raman spectrum and is of A, symmetry when n is odd and A, symmetry when n is even. The LAM-1 vibration is an intramolecular mode, and the term 'longitudinal accordion motion' is probably a more accurate description. LAM-1 vibrational modes have been modelled (primarily for polymers) using various different approaches (see later).In this paper, we consider whether the LAM-1 frequency is affected by the structural constraints imposed upon the guest molecules by the urea tunnel structure. Considering now the end-group conformations of the X(CH,),X guest molecules, we refer to situations in which the end-group is in the gauche conformation as 'end-gauche defects'. For a molecule in the all-trans conformation, the symmetries of the v(C-X) stretching modes are A, and B, when n is even, and A, and B, when n is odd. The point symmetry of a molecule containing one gauche end-group is C, and all vibrations are thus Raman active. The v(C-X) stretching modes for the conformations with trans and gauche end-groups have different characteristic frequencies in the 300-1000 cm-' region of the Raman spectrum," and these modes can therefore be used to probe the existence of end- groups in the gauche conformation.Results and Discussion Assessment of Crystal Integrity As shown previously,20 Raman spectroscopy can be used to determine the integrity of urea inclusion compound single crystals (hexagonal urea). In our work, this has been assessed from the following features of the spectrum recorded at 300 K: (i) At low wavenumbers, the pure crystalline phase of urea (tetragonal urea) is characterized by a sharp, strong band at 60 cm-'. As shown in Fig. 3A(b), this band was observed in the spectrum of the 1,7-dibromoheptane/urea inclusion com- pound, indicating partial decomposition of this inclusion A I 40 60 80 100 120 140 160 180 200 980 990 1000 1010 1020 1030 1040 Fig.3 Raman spectra [recordedin X(Z2)Ypolarization] for (a)the Br(CH,),,Br/urea inclusion compound and (b) a partially decom- posed Br(CH,),Br/urea inclusion compound. The spectral regions shown are for: A, the lattice modes and By the v(C-N) stretching mode. The asterisks indicate the Ar' emission lines (77 an-' and 117 cm-' for A, = 514.5 nm). The arrows indicate bands due to tetragonal urea. compound. This band was not observed for the other urea inclusion compounds studied [see e.g. the spectrum of 1,lO- dibromodecane/urea shown in Fig. 3A(a)]. (ii) In the spectral region corresponding to the bending modes 6(NCO) of urea, a strong band at 556 cm-' and a weak band at 570 cm-' are characteristic of tetragonal urea, whereas the corresponding bands for the urea inclusion compounds are at 530 cm-' and 608 cm-'.(iii) The v,(C-N) stretching vibration is very intense and occurs at 1010 cm-' for :etragonal urea and at 1024 cm-' for hexagonal urea.,' As shown in Fig. 3, the integrity of the inclusion compounds can be checked readily by considering this spectral region. As expected, the polarization properties of a urea inclusion compound single crystal are lost if it decomposes to tetrago- nal urea. Modes due to the Urea Host In general, the total number of bands observed for urea vibrational modes is consistently lower than the number pre- dicted theoretically, both in the HT and LT phases.The number of observed Raman bands is the same below and above the phase-transition temperature, and is consistent with the fact that the changes in the host structure associated with the phase transition are small. Polarization of the Raman bands is also unaffected by the phase transition. These observations are probably related to the fact that a J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 single crystal in the HT phase (hexagonal structure) becomes triply twinned in the LT phase (orthorhombic structure). The frequencies of the internal vibrational modes of the urea are essentially independent of the length of the guest molecule. As discussed previously,21 only the highest-frequency internal vibrational modes [v(N-H) stretching region above 3200 cm-'1 have a significant temperature dependence. Some of the lattice modes reflect changes in the tunnel structure which occur at the phase transition (see Fig.4). The band at 185 cm-' in the LT phase (at 93 K), which is pol- arized X(YX)Y (i.e.E, symmetry), moves considerably to 164 cm-' in the HT phase (at 298 K). E, modes arise from coupled translations and rotations about different axes, and although this band cannot be assigned definitively to a par- ticular lattice vibration, it may be compatible with an oscil- lation of the urea tunnel with a large component in the plane perpendicular to the tunnel axis. At the phase transition there is a major change in the frequency of this mode, as a conse- quence of the structural changes in the urea tunnel at the phase transition.The band at ca. 135 cm-'in the LT phase (at 93 K) and at ca. 130 cm-' in the HT phase (at 298 K) has A, symmetry and is strongly X(ZZ)Y polarized; A, modes result from coupled translations along and rotations about the axis of the C-0 bond of each urea molecule. In contrast to the E, mode discussed above, the frequency of this A, mode is not sensitive to change of temperature, and may represent a vibration with a substantial component along the tunnel direction. The broad band at ca. 101 cm-' [symmetry El; polarized X(ZY)Y] does not shift significantly with change of tem- perature and is also assigned as a urea lattice vibration. A f/cm -' BiI ~~ ~ 10 68 126 184 242 300 f/cm-' Fig.4 Raman spectra showing the spectral region for lattice modes of the Br(CH,),Br/urea inclusion compound at different polariza- tions: (a)X(Zz)Y,(b)X(2X)Yand (c)X(YX)Yfor: A, the LT phase (T = 93 K) and B, the HT phase (T = 298 K) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Other bands below 100 cm-’ arise from lattice vibrations, although definitive assignments of these modes are difficult. Modes due to the a,o-Dihalogenoalkane Guest LAM-1 Mode LAM-1 vibrations have been widely studied for polyethylene and alkanes in the all-trans conformation, and have been identified for alkane guest molecules in their urea inclusion compounds.22 A variety of different approaches have been used to model these vibrational modes.For example, Mizushima and Schiman~uchi~~approximated the all-trans polyethylene molecule by an elastic rod, and several worker^'^.^^ have retained this approximation to study the effect of end-groups on LAM-1 frequencies. The model of Minoni and Zerbi2’ is more realistic and more appropriate in view of the aims of the work discussed here. In this model, the system is treated as an infinite, one- dimensional crystal, the repeat unit of which is a single molecule (Fig. 5). In employing this model for the Br(CH,),Br/urea inclusion compounds, we have represented each Br(CH,),Br molecule by a strictly one-dimensional arrangement of point masses, with each terminal substituent (in this case CH,Br) having mass M and each CH, unit having mass rn.Interactions are considered only between adjacent point masses, and only longitudinal displacements are considered. The intramolecular CH,-CH, force con- stant is denoted F and the intermolecular force constant is denoted f: For simple harmonic oscillator behaviour, the fol- lowing dispersion relation can be derived :” (7) where V is the LAM frequency (in wavenumbers) and 8 is the molecular phase. A value of F = 5.2 mdyn A-’ is obtained -by equating eqn. (7) to the empirical function[?(a)= A8 + ... where A = 2495 cm- ’and 8 = 8/n]given by Schaufele and Schimanouchi26 for polyethylene. The molecu- lar phase relation for Raman scattering with wavevector k z0 can be shownz5 to be: sin[(N -1)8](cos 8 -1) x [(2K -1 -2K2)cos 8 + 2K2 -2K + R -2RK] + cos[(N -1)Blsin 8[(1 -~K)(cos8 -1)-R] + R sin 8 where R =flF, K = M/m, and N is the number of point masses per molecule.This equation has N solutions in 8. Since F,f,rn and M are known, the molecular phase relation can be solved to give values for 8. LAM-1 wavenumbers can then be calculated from eqn. (7). Using the value f = 0.05 mdyn A-determined24 for alkanes at low temperature, a value F = 4.2 mdyn A-1 was determined27 by fitting the calculated wave- numbers to experimental results28 at 168 K for the range of alkanes CH,(CH,),CH, with n = 6-26. In other work,,’ MmmmmM M M Fig. 5 Definition of the parameters used in the infinite one-dimensional model of a linear chain developed by Minoni and Zerbi’’ force constants have been calculated for various alkanes CH,(CH,),CH, and a,o-disubstituted alkanes X(CH,),X with even n, with the aim of investigating the effect of the end-group X on the LAM-1 frequency for these molecules in urea inclusion compounds.Experimental data on LAM-1 vibrations for a,o-dibromoalkanes in their pure crystalline phases have been reported.27 In this work, theoretical LAM- 1 wavenumbers (Table 1) for Br(CH,),Br molecules with n = 7-10 were determined via the Minoni-Zerbi model, using an intramolecular force constant F = 4.2 mdyn A-’ and an intermolecular force constantf = 0.05 mdyn A-’. Raman spectra recorded in X(ZZ)Y polarization in the present work for Br(CH,),Br/urea inclusion compounds illus- trating the LAM-1 bands in the LT phase (at 93 K) and the HT phase (at 298 K) are shown in Fig.6 and 7, respectively. The experimental wavenumbers (f)for LAM- 1, deduced from fitting these bands, are reported in Table 1. The uncertainties in V are greater at 298 K owing to the larger bandwidths, and the error in the measurement of V at 93 K is estimated to be ca. &0.5 cm-’. Furthermore, for 1,lO-dibromodecane/urea at both 93 and 298 K and for 1,7-dibromoheptane/urea at 298 K, there is a larger uncertainty in V since the band due to the LAM-1 vibration overlaps bands due to urea lattice modes. Comparison between our results and those for the pure crystalline a,o-dibromoalkanest7 suggests that, in the Br(CH,),Br/urea inclusion compounds, the Br(CH,),Br guest molecules are predominantly in the all-trans conformation, and the proportion of guest molecules containing kinks in the middle portion of the molecule is negligible.For the Br(CH,),Br molecules with even n, the theoretical and experi- mental wavenumbers are in acceptable agreement (Table 1). However, for the Br(CH,),Br molecules with odd n, the theo- retical values [calculated via eqn. (7) and (S)] are significantly higher; even reducing the value off to zero does not lower the theoretical wavenumbers sufficiently to give agreement with the experimental wavenumbers. The failure of the Minoni-Zerbi model to predict correctly the observed wave- numbers for the compounds with odd n arises from the fact that the model considers only the longitudinal mode (i.e.lon-gitudinal displacement of atoms).Normal mode calculations on short alkanes CH,(CH,),CH, with odd n (n = 7, 9, ll), however, have shown3’ that the LAM-1 vibration cannot be Table 1 Measured wavenumbers (7)of LAM-1 for Br(CH,),Br mol-ecules in their urea inclusion compounds (UIC) and their pure crys- talline (PC) phases, and calculated values (as discussed in the text) Br(CH2)7 Br UIC (LT phase) UIC (HT phase) PC 132 133 140 calculated 158 Br(CH,),Br UIC (LT phase) UIC (HT phase) PC 151 151 150 calculated 145 Br(CH,),Br UIC (LT phase) UIC (HT phase) PC 115 116 121 calculated 135 Br(CH,),,Br UIC (LT phase) UIC (HT phase) PC 122 126 126 calculated 127 Experimental results for the urea inclusion compounds were deter- mined (in this work) at 93 K (LT phase) and at 298 K (HT phase).Experimental results for the pure crystalline phases are taken from ref. 25 and 29. t 4-.-v) Q,+ C 80 90 100 110 120 130.140 150 160 F/cm-Fig. 6 Raman spectra [recorded in X(Z2)Ypolarization] showing the spectral regions for LAM-1 vibrations (indicated by arrows) of Br(CH,),Br/urea inclusion compounds at 93 K (LT phases). n = (a) 7, (b)8, (c) 9 and (6)10. described by such longitudinal displacements alone, and the origin of this effect is attributed to the difference in mass between the terminal CH, groups and the CH, repeat units in alkanes with odd n. Similarly, the presence of heavy CH,Br end-groups on the Br(CH,),Br molecules will have an important effect on the LAM-1 vibration.In order to give a physical interpretation to the calculated LAM-1 frequencies for alkanes with odd n, it has been suggested3’ that coupling between an ‘unperturbed’ LAM-1 mode (as described by the linear chain model) and an ‘unperturbed’ transverse acoustic mode (TAM-4) is effective. Evidence for such coupling has also been obtained from normal mode calculations on 1-br~moalkanes.~~Unperturbed LAM and TAM modes, as defined by a linear chain model, can interact provided they belong to the same symmetry species and provided their unperturbed frequencies are sufficiently close to each other. This coupling therefore cannot occur for the molecules with even n since LAM-1 has symmetry A, and TAM-4 has sym- metry B, (although LAM-1 and TAM-3 do have the same symmetry and, in principle, they could interact if their fre- quencies were sufficiently close).For the molecules with odd n, on the other hand, LAM-1 and TAM-4 both belong to symmetry species A,. Assuming that coupling occurs through a Fermi-type resonance3’ (anharmonicity due to difference in masses), the maximum coupling for the alkanes occurs when n = 11. In our work, low intensity bands were observed at wavenumbers between 150 and 300 cm-’ only for Br(CH,),Br/urea inclusion compounds with odd n [as shown in Fig. 8 for Br(CH,),Br/urea] and can be assigned tentati- J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 80 90 100 110 120 130 140 150 160 f/cm -Fig. 7 Raman spectra [recorded in X(2z)Y polarization] showing the spectral regions for LAM-1 vibrations (indicated by arrows) of Br(CH,),Br/urea inclusion compounds at 298 K (HT phases). (a)-(d) as Fig. 6. vely as TAM modes (on the basis of comparing the Raman spectra of the Br(CH,),Br/urea inclusion compounds with those of the pure solid phases of a,o-dibr~moalkanes).’~-~~ From Table 1, it is clear that there is no significant tem- perature dependence of the LAM-1 frequency for the Br(CH,),Br/urea inclusion compounds. Furthermore, our experimental LAM-1 frequencies are similar to those re-for theported previou~ly~~*~~ corresponding a,o-dibro-moalkanes in their pure crystalline phases.Thus, the 100 140 180 220 260 300 ;/cm -’ Fig. 8 Raman spectrum [recorded in X(22)Ypolarization] for the Br(CH,),Br/urea inclusion compound at 93 K (LT phase) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ' ?/cm- I B 500 540 580 620 660 700 ?/cm-' Fig. 9 Raman spectra [recorded in (a)X(ZZ)Y,(b) X(ZX)Yand (c) X(YX)Ypolarizations] showing the spectral regions for the v(C-Br) stretching vibrations of the Br(CH,),Br/urea inclusion compound :A, at 108 K (LT phase); B, at 298 K (HT phase) 'constrained ' environment of the urea tunnel structure appar- ently has little effect on the frequency of the LAM-1 mode of the Br(CH,),Br molecule, in comparison with the same mol- ecule in its pure crystalline phase. C-X Stretching Mode Study US a Function of the Length of the Guest Molecule and Temperature.Raman spectra were recorded in the X(ZZ)Y, X(2X)Y and X( YX) Y polarizations for Br(CH,),Br/urea inclusion compounds with n = 7-10 in both the LT phase and the HT phase. Representative spectra illustrating the v(C-Br) stretching modes in the X(ZZ)Y, X(2X)Y and X( YX)Y polarizations for Br(CH,),Br/urea at 108 K (LT phase) and at 298 K (HT phase) are shown in Fig. 9. The v(C-Br) band for the trans end-group is polarized X(Z2)Y (asexpected for symmetry A,) and is at ca. 656 cm-at 298 K. The v(C-Br) band for the gauche end-group is at ca. 570 cm-' at 298 K; this band is depolarized and is much less intense than the corresponding band for the trans end-group. The ratios 97 for each sample at 90 K (LT phase) and at 298 K (HT phase) are reported in Table 2.Despite our limited knowledge regarding the relationship between 9ftand the Table 2 Values of 97 [defined in eqn. (5)] measured for Br(CH,),Br/urea inclusion compounds at 90 K (LT phase) and at 298 K (HT phase) sample LT HT Br(CH ,),Br/urea 0.17 0.15 Br(CH,),Br/urea 0.05 0.08 Br(CH ,),Br/urea 0.09 0.13 Br(CH,),,Br/urea 0.10 0.13 proportion of end-groups in the gauche conformation, we assess that the proportion of end-groups in the gauche con-formation is in the range 5-14% for the LT phase and in the range 7-13% for the HT phase. From Table 2, no well defined relationship between 9;'and molecular length (n) is apparent. Raman spectra were recorded as a detailed function of temperature for Br(CH,),Br/urea in polarization [X(ZZ)Y + X(2X)Y)(in this configuration, both horizontally and vertically polarized components of the scattered radi- ation are analysed together).From the results (Fig. lo), the intensity ratio 9f[defined in eqn. (6)]increases slightly with temperature but exhibits no detectable discontinuity at the phase-transition temperature, and we conclude that the pro- portion of end-groups in the gauche conformation has only a weak temperature dependence. Study as a Function of the Terminal Functional Group on the Guest Molecule. The Raman spectra recorded in the region of the v(C-X) stretching modes for 1,8-dichlorooctane/urea, 1,8-dibromooctane/urea and 1,8-diiodooctane/urea in the X(Zz)Y, X(ZX)Y and X(YX)Y polarizations at 298 K are shown in Fig.11. From the mea- sured intensity ratios 9r,the proportion of end-groups in the gauche conformation is calculated to be ca. 51% for Cl(CH,),Cl/urea, 7% for Br(CH,),Br/urea and 1% for I(CH,),I/urea. Thus, the proportion of end-groups in the gauche conformation decreases as the size of the terminal substituent increases. Presumably a major factor here is that the gauche conformation becomes relatively more difficult to accommodate within the urea tunnel structure on moving from Cl to Br to I as the end-group. In deriving this conclu- sion from the Raman results, we have assumed that the mea- sured intensity ratios 9f*[defined in eqn. (5)] for C1(CH2),C1/urea, Br(CH,),Br/urea and I(CH,),I/urea can be compared directly. This is considered justified in view of the fact that the depolarization ratio (measured for the pure liquid) for the v(C-X) stretching vibration of the trans con-formation is similar in magnitude for the X(CH,),X mol-ecules containing the different end-groups X, and the depolarization ratio for the v(C-X) stretching vibration of the gauche conformation is similar in magnitude for the X(CH,),X molecules containing the different end-groups X.Study as a Function of Pressure. Experiments to probe the pressure dependence of the Raman spectrum were carried out on a polycrystalline sample of Br(CH,),,Br/urea (as a conse- quence of using a polycrystalline sample, the concept of polarization in these experiments is not relevant).With increase of pressure, the urea vibrational modes shift to 0.05 o.04/ 0.03t ;,., I I I I I 50 100 150 200 250 300 TIK Fig. 10 9;'us. temperature for the Br(CH,),Br/urea inclusion com- pound 1320 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 F A h(c) 1 r\1 trans 1 500 540 580 620 660 700 trans Jc) I 450 490 530 570 610 650 i/cm-' Fig. 11 Raman spectra [recorded in (a)X(ZZ)Y,(b)X(2X)Yand (c) X(YX)Y polarizations] showing the spectral regions for the v(C-X) stretching vibrations of the X(CH,),X/urea inclusion com-pounds at 298 K (HT phases): A, X = C1; B, X = Br; C, X = I higher frequencies (Fig. 12), and the intensity of the band for the v(C-Br) stretching vibration of the gauche end-groups increases markedly (Fig.13). The results of these experiments were reproducible, and the variation of the spectrum with change of pressure was identical for the low-to-high and high-to-low pressure cycles. From consideration of the v(C-N) stretching vibration and the bands at ca. 530 and 610 cm-' (characteristic of urea in the urea inclusion compounds), it is concluded that there was no pressure-induced decomposition of the Br(CH,), 'Br/urea inclusion compound under the pressures exerted in these experiments. We now consider in more detail the v(C-Br) stretching mode [at V = 570 cm-'for the gauche end-group, and at V x 660 cm-' for the trans end-group (Fig. 13)]. As seen from 129 cm-' 165 cm-' I I *II(1:141 cm-' 183 cm-l- L...#...,...,,, ,...y 80 104 128 152 176 200 i/cm-Fig.12 Raman spectra showing the spectral region for the lattice modes of the Br(CH,),,Br/urea inclusion compound at 298 K (HT phase). p = (a)1 and (b)6 x lo3bar. Fig. 13, the intensity ratio: p-Z(570 cm -') 3-I(660 cm -I) (9) increases markedly as the applied pressure is increased. Con- sidering 99 as a measure of the equilibrium constant for trans s gauche interconversion, and using AVE to denote the difference in molar volume between the gauche end-group and the trans end-group (AVE = V:gauche -V?"'), it can be shown (assuming that AVE is independent of pressure over the restricted range of pressures considered here) that Sg varies with the applied pressure (p) according to : where C is a constant independent of p.Fig. 14 shows a graph of ln(ff) us. p at T = 298 K; the value of AVE deter-mined from the best-fit line is AVE = -7.4 cm3 mol-'. It has been suggested32 that the cross-sectional area of the urea tunnel decreases with increase in pressure, and it is also prob- able that the periodic repeat distance (ch) of the urea tunnel structure along the tunnel axis also decreases with increase in pressure. In view of this reduction in the space available within the urea tunnel structure on increasing the pressure, it may be expected that reduction of the volume of the guest molecule (for example, by conversion from a trans end-group to a gauche end-group) will be favoured on increasing the pressure.While this explanation is consistent with the observed pressure dependence of Yf,we make no attempt in this paper to pursue this argument further in view of the fact that the different shapes (as well as the different volumes) of the gauche and trans end-groups must be taken into account in assessing the feasibility of fitting these different conforma- tions of the guest molecule within the urea tunnel structure at increased pressure. This issue clearly requires a more detailed future investigation. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I I trans I 500 540 580 620 660 700 i/crn-' Fig. 13 Raman spectra showing the change in spectral region for the v(C-Br) stretching vibration, as a function of pressure, for the Br(CH,),,Br/urea inclusion compound at 298 K (HT phase): p = (a) 1, (b)4 x lo3and (c)6 x lo3bar.Finally, we stress that the results reported here refer to urea inclusion compounds under hydrostatic pressure gener- ated by helium gas as compressing fluid. Further experiments using other compressing fluids (e.9. N,, CO,, Ar, Xe) are 0.0 .a , h 'lo -0.5-B' '"s v L' -C -1 .o -I' , I -----I--L I 140i---l ' 1 0 1000 2000 3000 4000 5000 6000 7000 p/bar Fig. 14 ln(Yf) us. pressure for the Br(CH,), Br/urea inclusion com- pound at 298 K (HT phase) required before a full understanding of the effect of the com- pressing fluid on the results from such experiments can be obtained; at this stage, we cannot rule out the possibility that atoms of the compressing fluid enter the tunnels of the inclu- sion compound during these experiments.Concluding Remarks From the Raman investigations reported here, some general conclusions can be drawn regarding the structural and con- formational properties of the a,o-dihalogenoalkane/urea inclusion compounds. The fact that changes in spectral characteristics on passing through the phase transition are small suggests that the LT phase is structurally very similar to the HT phase (as sug- gested previ~usly~~~ for alkane/urea inclusion compounds). Two strongly polarized urea lattice modes were identified (in agreement with other authors);21,33.34 an X( YX)Y polarized mode at ca.180 cm-' (LT phase) and ca. 163 an-' (HT phase), and an X(Z2)Y polarized mode at ca. 130 cm-' (LT phase) and ca. 129 cm-' (HT phase). It is important to con- trast the strong temperature dependence of the X(YX)Y pol-arized mode with the weak temperature dependence of the X(Z2)Y polarized mode. In view of the known changes in the urea tunnel structure upon crossing the phase transition temperature, the differing temperature dependences of these modes may imply that the mode which is polarized X(YX)Y may involve translations of the urea molecules in a plane almost perpendicular to the tunnel axis whereas the mode which is polarized X(Z2)Y may represent a vibration with a large component along the tunnel direction. The latter band has been attributed34 to the 'totally symmetric breathing mode of the lattice', with the proposal that this mode relates to the periodic change in the cross-section of the urea tunnel; such an assignment would imply that the frequency of this mode should have a larger temperature dependence than that observed in our experiments.The LAM-1 modes due to the a,o-dihalogenoalkane guest molecules [X(CH,),X] were identified in the LT phase and less readily in the HT phase. As expected, the LAM-1 fre- quency decreases as the length of the guest molecule is increased within the series with odd n, and decreases as the length of the guest molecule is increased within the series with even n. On applying the Minoni-Zerbi model2' to these systems, the calculated frequencies were in reasonable agree- ment with the experimental values for the Br(CH,),Br guest molecules with even n, suggesting that a large proportion of the included Br(CH,),Br molecules are in the fully extended all-trans conformation.However, the experimentally observed LAM- 1 frequencies for the Br(CH,),Br molecules with odd n cannot be described by the Minoni-Zerbi model. Substantial further work is required to interpret fully the observed bands in the low-frequency region of the Raman spectra of the Br(CH,),Br/urea inclusion compounds and also the pure crystalline phases of the a,o-dibromoalkanes. The C-X stretching modes due to the trans and gauche end-groups were identified for all of the X(CH,),X/urea inclu- sion compounds studied.The intensity ratios suggest that the proportion of end-groups in the gauche conformation is ca. 5-14%. Studies of the evolution of the measured intensity ratios as a function of terminal substituent, temperature and pressure show the following: (a)9":decreases with increase in the size of the terminal substituent; (b)Yf increases slightly with increasing temperature [for Br(CH,),Br/urea inclusion compounds]; (c) 9!t increases markedly with increase in applied pressure [for Br(CH,), ,Br/urea]. In the current study, changes in the measured intensity ratio 9y were used 1322 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 to assess changes in the proportion of end-groups in the gauche conformation. In view of the differences in polariza- tion of the v(C-X) modes for the trans and gauche end-groups, this approach for assessing trends in the proportion 6 7 8 Y.Chatani, H. Anraku and Y. Taki, Mol. Cryst. Liq. Cryst., 1978,48,219. K. D. M. Harris, I. Gameson and J. M. Thomas, J. Chern. SOC., Faraday Trans., 1990,86,3135. A. E. Aliev, I. J. Shannon, S. P. Smart and K. D. M. Harris, in of gauche end-groups has substantially more justification than the use of this approach to assess the absolute value of the proportion of gauche end-groups. It is clear from the results reported here that intercon- version between the gauche and trans conformations of the end-group is slow with respect to the timescale of the Raman measurement at all temperatures investigated here, and other techniques are required to elucidate quantitative dynamic information on this conformational exchange process.In this regard, it is relevant to note that our high-resolution solid- state ' 3C NMR studies of a,o-dihalogenoalkane/urea inclu-sion compounds35 are consistent with the guuche-trans interconversion being rapid with respect to the timescale of 9 10 11 12 13 14 15 16 preparation. H. G. McAdie, Can. J. Chem., 1962,40,2195. K. D. M. Harris and M. D. Hollingsworth, Proc. R. SOC.London A, 1990,431,245. A. J. 0. Rennie and K. D. M. Harris, Proc. R. SOC. London A, 1990,430,615. A. J. 0. Rennie and K. D. M. Harris, J. Chem. Phys., 1992, %, 7117. I. J. Shannon, K. D. M. Harris, A. J. 0. Rennie and M. B. Webster, J. Chem. SOC.,Faraday Trans., 1993,89,2023. S. P. Smart, F.Guillaume, K. D. M. Hams, C. Sourisseau and A. J. Dianoux, Physica B, 1992,180,181,687. F. Guillaume, S. P. Smart, K. D. M. Harris and A. J. Dianoux, J. Phys. : Condens. Matter, in the press. J. J. Martin, R. Cavagnat, J. C. Cornut, M. Couzi, G. Daleau, J. this technique at all temperatures investigated. The existence of end-groups in the gauche conformation in alkane/urea inclusion compounds has recently been the source of considerable controversy. IR spectroscopy2' and molecular dynamics simulation36 have both suggested that the proportion of end-groups in the gauche conformation is small (less than 3% and 5%, respectively). Raman s~attering~~?~~(on the basis of bond polarizability model calculation^^^) and 2H NMR3' experiments have shown that the amount of gauche end-groups in a series of alkane/urea inclusion compounds is ca.5%. In contrast to these results, it has been propo~ed,~'.~' on the basis of 2H NMR, 13C NMR, 17 18 20 21 22 19 Devaure, M. Maissara and R. Mokhlisse, Appl. Spectrosc., 1986, 40,217. R. G. Snyder, J. Mol. Spectrosc., 1971,37, 353. The Theory of Vibrational Spectroscopy and its Application to Polymeric Materials, ed. P. C. Painter, M. M. Coleman and J. L. Koenig, Wiley, New York, 1982, p. 336. Characteristic Raman Frequencies of Organic Compounds, ed. F. R. Dollish, W. G. Fateley and F. F. Bentley, Wiley, New York, 1974. H. L. Casal, J. Phys. Chem., 1990,94,2232. J. Le Brumant, M. Jaffrain and G. Lacrampe, J. Phys. Chem., 1984,95, 1548. V. Fawcett and D. A. Long, in Ado. Raman Spectrosc., 1973, 1, 570.molecular mechanics (MM2) and molecular dynamics simula- tions, that the amount of gauche end-groups may be as large as 40%. Despite the diversity of results obtained from these experi- ments, it is nevertheless clear that alkane and a,o-dihaloge- noalkane guest molecules within the tunnels of urea inclusion 23 24 25 26 S. Mizushima and T. Schimanouchi, J. Am. Chem. SOC.,1949,71, 1320. S. L. Hsu and S. Krimm, J. Polm. Sci., Polym. Phys. Ed., 1977, 15, 1769. G. Minoni and G. Zerbi, J. Phys. Chem., 1982,86,4791. R. F. Schaufele and T. Schimanouchi, J. Chem. Phys., 1967, 47, 3605. compounds contain some amount of end-groups in the gauche conformation, and it is proposed that the existence of these conformational defects depends strongly on the longitu- dinal packing of the guest molecules within the tunnels.There is clearly considerable scope for future work to extend further our understanding of the conformational properties of the guest molecules in urea inclusion compounds. 27 28 29 30 31 32 K. Viras, F. Viras, C. Campbell, T. A. King and C. Booth, J. Phys. Chem., 1989,93,3479. H. G. Olf and B. Fanconi, J. Chem. Phys., 1973,59,534. H. G. M. Edwards, V. Fawcett and M. T. Lung, J. Inclusion Phenom. Mol. Recognit. Chem., 1991,11,267. J. Mazur and B. Fanconi, J. Chem. Phys., 1979,71,5069. F. Viras, K. Viras, C. Campbell, T. A. King and C. Booth, J. Polym. Sci., Part B, Po1ym.-Phys., 1991,29, 1467. K. Fukao, T. Horiuchi, S. Taki and K. Matsushige, Mol. Cryst. The authors wish to thank Dr R. Cavagnat (CNRS, LSMC) for technical assistance and Dr C. Sourisseau (CNRS, LSMC) for fruitful discussions. The SERC is thanked for financial support (studentship to S.P.S. and general support to K.D.M.H.),and the University of St. Andrews is thanked for 33 34 35 Liq. Cryst., 1990, lWB, 405. V. Fawcett and D. A. Long, J. Raman Spectrosc., 1975,3,263. M. Kobayashi, H. Koizumi and Y. Cho, J. Chem. Phys., 1990, 93,4659. A. E. Aliev, S. P. Smart and K. D. M. Harris, unpublished results. the award of an Ettie Steele Travel Scholarship to S.P.S. 36 K-J. Lee, W. L. Mattice and R. G. Snyder, J. Chem. Phys., 1992, 96,9138. References 1 K. D. M. Harris, J. Solid State Chem., 1993,106,83. 2 A. E. Smith, Acta Crystallogr., 1952,5,224. 3 K. D. M. Harris and J. M. Thomas, J. Chem. SOC., Faraday Trans., 1990,86,2985. 4 K. D. M. Harris, S. P. Smart and M. D. Hollingsworth, J. Chem. 37 38 39 40 A. El Baghdadi, Ph.D. Thesis, University of Bordeaux I, 1993. Y. Kim, H. L. Strauss and R. G. Snyder, J. Phys. Chem., 1989, 93,485. G. M: Cannarozzi, G. H. Meresi, R. L. Vold and R. R. Vold, J. Phys. Chem., 1991,95,1525. F. Imashiro, D. Kuwahara, T. Nakai and T. Terao, J. Chem. Phys., 1989,90,3356. SOC.,Faraday Trans., 1991,87,3423. 5 S. P. Smart, Ph.D. Thesis, University of St. Andrews, 1993. Paper 3/07016F; Receioed 25th November, 1993