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In this study, the useful information for the design of side-chain liquid crystal polymers was described. Side-chain liquid crystal PEIs with different spacer lengths (n) were newly synthesized. In the case of short spacers, only a smectic A phase with a bilayer structure was formed. The long spacers led to the formation of a smectic A, C and F phases with a monolayer structure.

The characterization of an active complex in a soluble Ti(O-n-Bu)4-AlEt3 system is reported in connection with its polymerization activity for styrene. The catalytic activity strongly depends on the Al/Ti ratio in the catalytic system. The optimal Al/Ti ratio for polymerization reaction is estimated as 1,5. Active titanium species are present in trivalent state. Further reduction of titanium species to a valency of less than 3 results in no activity for styrene polymerization. Two types of active species are detected by the IR and ESR studies of the catalytic system. One is Ti(O-n-Bu)2Et, and the other is a complex formed by 1 mole of Ti(O-n-Bu)2Et and 2 moles of (n-BuO)-AlEt2. The differences in activity between these are now under investigation. IR and ESR studies of the Ti(O-n-Bu)3-AlEt3 system are also reported.

Ionic and non-ionic liquid crystalline polymethacrylates were synthesized, and their thermal and liquid crystalline properties were compared. Both the ionic and non-ionic liquid crystalline polymethacrylates exhibited a thermotropic smectic A phase. An isotropization temperature of the ionic liquid crystalline polymethacrylate was higher than for the non-ionic liquid crystalline polymethacrylate. Moreover, the ionic liquid crystalline polymethacrylate showed a higher glass transition temperature when compared to the non-ionic liquid crystalline polymethacrylate. The ionic and non-ionic liquid crystalline polymethacrylates formed a partial bilayer structure in the smectic A phase. Copyright © 2000 John Wiley & Sons, Ltd.

ポリアミン系分岐高分子を構成単位とするイオンコンプレックス型の液晶系について, 液晶性, 熱的性質および配向構造を検討した. 分岐高分子とアルカン酸あるいはカルボキシル基をもつアゾベンゼン誘導体とのイオンコンプレックスは, 流動性液晶相を形成する. イオン化した分岐高分子は, イオン性液晶に特有なイオン性凝集層をアニオンとともに形成し, 液晶性および熱安定性に直接影響を与える. 分岐鎖の存在は, イオン性凝集層の熱安定性を減少させる. このため, 液晶-等方相転移温度および液晶-等方相転移エンタルピーが低下する. これらの結果は, 適度な分岐鎖の導入が, 液晶性および相転移挙動の制御に利用できることを示している.

電子アクセプター性の二量体液晶 (TN-n) をベースとする二成分液晶システムの液晶形成能および配向特性を二量体液晶成分のコンポメーションと関連づけて検討した, 二成分液晶システムとして次の二つのタイプを構築した, 一つはニトロ末端基をもつTN-nにブチル末端基をもっ二量体液晶を混合した液晶系 (T-n-x) であり, もう一方はTN-nにエチルァゾベンゼン基を側鎖にもつ高分子液晶を混合した液晶系 (P-n-x) である. これらの二成分液晶システムは, 電子ドナー・アクセプター効果によって誘起スメクチック相を形成し, 誘起されるスメクチック相の構造は, 二量体液晶分子の中央メチレン鎖長の偶奇性に依存する. T-n-xとP-n-xの両方とも, 中央メチレン鎖長が偶数で, 二つのメソゲン基が比較的平行に配置した形状をもつ偶数系二量体液晶を混合成分としてもつ場合には, SmA相を誘起する. それに対し, 一つのメソゲン基がもう一方のメソゲン基に対して傾いた形状をもつ奇数系二量体液晶を混合成分とするT-n-xの場合には, 誘起SmCA相が形成される. 側鎖型高分子液晶と奇数系二量体液晶との混合系のP-n-xでは, SmCA相に加えSmA相を誘起する. 以上のように, 電荷移動力に加え, 分子の立体的な効果を組み合わせることによって新しい液晶集合体の構築が可能であることが明らかになった.

The thermal and liquid crystalline properties of the new binary systems, consisting of a polycarbonate type of a main-chain liquid crystal polymer (PC) and a twin liquid crystal (TN-n: n = number of central methylene units) with a nitro terminal unit, were examined. The binary systems having PC and TN-n with even-numbered central methylene units (n = 6, 8) showed an induced smectic A phase with batonnets and a fan texture. However, the binary systems consisting of PC and TN-n with odd-numbered central methylene units (n = 5, 7) formed an induced smectic C phase in addition to an induced smectic A phase. In the induced smectic C phase, the schlieren with two and four brushes was observed.

Thermotropic liquid-crystalline polyurethanes were synthesized by the polyaddition reaction of para-substituted 2,5-tolylene diisocyanate (2,5-TDI) with 1,4-bis(ω-hydroxyalkoxy)benzenes (BHBm : HO(CH 2 ) m OC 6 H 4 O(CH 2 ) m OH ; m is the carbon number of the hydroxyalkoxy group). Intrinsic viscosities of the polymers were in the range of 0.22-0.41 dL/g. Polyurethanes 2,5-TDI/BHBm's (m = 3, 5, 6, 8) prepared from BHBm and 2,5-TDI, exhibited monotropic liquid crystallinity, although these polyurethanes contained no mesogenic core unit. For example, polyurethane 2,5-TDI/BHB5 with [η] = 0.33 prepared from 1,4-bis(5-hydroxypentoxy)benzene (BHB5) and 2,5-TDI exhibited an anisotropic phase from 119 to 51 °C. In contrast, for the series of 1,4-PDI/BHBm (m = 2, 3, 5, 6, 8, 11) having no substituent in a phenylene unit, no explicit mesomorphic behavior was observed by DSC measurement and polarized microscopic observation.

The dielectric properties of the liquid crystallline polymethacylates with side chain containing a terminal alkyloxy chain have been over wide ranges of frequency and temperature. The temperatures corresponding to the peak of the dielectric loss tangent for PM6BP and PM6BP5 in its temperature dependence agreed well with those phase transition. The activation energies at low temperature agreed with Δ-relaxation which is e-relaxation by rotation of mesogenic gruop around the backbone chain. The dipole moment have been calculated by using the Frohlich modification of Onsager's equation. The alignment of both materials in the present of an a.c. electric field are also discussed.

A chirally ionic liquid crystal (ICLC) was prepared by the complexation of L(+)-tartaric acid and an achiral mesogenic group (NLC) with an amine unit, and its liquid crystalline behavior was examined by DSC, polarizing microscopy, and X-ray techniques. Ionic ICLC formed smectic A and chiral smectic C* phases. Due to an ionic aggregation, ionic ICLC exhibited a higher mesomorphic-isotropic phase transition temperature when compared to nonionic NLC.

Two types of liquid crystalline polymethacrylates, having 4-(4-substituted phenylazo)phenyl benzoate in a mesogenic side chain, were prepared. The relationship between the shape of the mesogenic side chain and the liquid crystallinity was studied.

The liquid crystalline polyacrylamides (PAAD-R) having secondary or tertiary amide groups were synthesized and the thermal properties of PAAD-R were studied. PAAD-H with the secondary amide group exhibited enantiotropically smectic phases with enhanced thermal stability through the formation of hydrogen bonding between amide groups when compared to PAAD-M with the tertiary amide group. Also, a liquid crystalline polyacrylate, which has the same mesogenic side chain as PAAD-H, was prepared and the thermal properties of PAAD-H and PA were compared.

A chiral smectic C* side chain polymer (LCPTT) with a chiral polytartrate skeletal main chain exhibited cholesteric (Ch) and chiral smectic C* (SmC*) hases. The maximum value of spontaneous polarization was 5 mC/m2 in the SmC* phase. In the Ch phase, LCPTT exhibited electric-field-induced ferroelectric behavior. The reciprocal response time in the Ch phase was directly proportional to the strength of the applied electric field, in the same manner as the SmC* phase. In the SmC* phase, the ferroelectric response time was 103 times shorter than the paraelectric response time.

Hydrogen-bonded ferroelectric liquid-crystalline complexes have been obtained by self-assembly of S-(+)-4-(2-methylbutoxy)benzoic acid and trans-4-alkoxy-4'-stilbazoles. Chiral smectic C phases are induced by the formation of the hydrogen bonds between the chiral nonmesogenic benzoic acid and the achiral stilbazoles. The chiral smectic C phase of the complex has been characterized by X-ray measurement. The hydrogen-bonded complex shows stable ferroelectric behavior in electric fields.

An ionic liquid-crystalline polymethacrylate [P-Pi/MeI having a piperidinium skeleton in a mesogenic side-chain was synthesized. Its nonionic family [P-Pi was also prepared, and phase transitions of P-Pi/ MeI and P-Pi were compared. Both P-Pi/MeI and P-Pi exhibit enantiotropically smectic phases. Ionic P-Pi/MeI shows enhanced thermal stability in comparison with nonionic P-Pi. A temperature of P-Pi/MeI, at which an isotropic phase forms, is higher than that of P-Pi. P-Pi/MeI also shows a higher glass transition temperature when compared to P-Pi. Also, nonionic and ionic low molecular weight model compounds were prepared, and their thermal properties were compared.

Formation of Smectic Orientational Order in an Ionic Thermotropic Liquid-Crystalline Side-Chain Polymer

メソゲン側鎖末端にアルキル鎖を有し, エチレン鎖をスベーサとする側鎖型高分子液晶を合成し, 末端アルキル鎖長の液晶形成への影響にっいて調べた. 側鎖型高分子液晶の液晶相から等方相への転移温度は, 末端アルキル鎖長に対してほとんど依存しなかった. アゾベンゼン基をもつ側鎖型高分子液晶は, 末端アルキル鎖- (CH2) -mHの長さmが2以下の場合にはネマティック相を示し, mが3以上の末端アルキル鎖の場合にはスメクティック相を示した. ベンジリデンアニリン基をもつ側鎖型高分子液晶の場合には, mが3以下および4以上において, それぞれネマティック相およびスメクティック相を形成した. また, 末端アルキル鎖長がヘキシル鎖以上ではチルトスメクティック相が形成された.

The iodide (I/CA) and poly(vinylsulfonate) (PA/CA) of bis(2-hydroxyethyl)ethyl(6-(4-(4-nitrophenyl)azo)phenyl)oxy)hexyl)ammonium were prepared, and their liquid-crystalline behavior was compared. I/CA and PA/CA exhibited enantiotropically smectic A phases. The ionic groups play an important role in forming the smectic layer structure and enhance the thermal stability of the smectic state. I/CA and PA/CA exhibit an oriented overgrowth on a glass plate by an orientational adsorption of ammonium ion groups. The homeotropic structures form spontaneously in the smectic phase of I/CA and PA/CA and remain unchanged on cooling in the solid phase

The liquid-crystalline comb-like polyether (PECB) was synthesized by reactions of 4-cyanobiphenyl mesogenic moieties with chloromethyl pendant groups in the repeating unit of polyepichlorohydrin. PECB exhibited an enantiotropically nematic phase. Isotropization temperatures and isotropization entropies in PECBs increased with increasing molecular weight. PECB was spinable from its nematic or isotropic melt. In the stretched sample, mesogenic pendant groups were aligned perpendicular to the stretching direction. The orientational behavior was characterized by X-ray technique, IR dichroic measurement and orthoscopic observation.

The thermal and liquid crystalline properties of comb-shaped polymers, having side-groups with an azo dye unit, were estimated by DSC and polarizing microscopy. The liquid crystalline comb-shaped polymers (LCP) showed a smectic phase with a focal conic fan texture on heating and cooling. LCP with an amide terminal group in the side-chain exhibited a 62°C higher glass transition point than LCP with nitro terminal groups and the thermal decomposition at 300.0°C, which is lower than an isotropization temperature.

Ferroelectric liquid crystalline polytartrate [CO-CHR-CHR-COO(CH 2 ) 4 O] n (R = O(Ch 2 ) 6 OPhN=NPhNO 2 ) was prepared by melt polycondensation and studied. Cholesteric and chiral smectic (SmC * ) phases were observed. LCPTT exhibited the large spontaneous polarization value of 5 mC/m 2 in the SmC * phase because of a stereopolar coupling of the chiral tartrate moiety.

Liquid-crystalline ionic polyethylenimine derivatives have been synthesized, and their thermal properties and dielectric behavior have been studied. They exhibited thermotropic smectic phases. The ferroelectric like behavior was confirmed by dielectric measurement and observation of the hysteresis loop with the Sawyer-Tower circuit.

In order to explore the motion of low molecular compound (LMW) of which chemical structure is similar to the mesogenic groups of liquid crystalline main-chain (LCP) in LCP/LMW composite system was studied by means of dielectric measurements and director order parameter (Sd) determinations by dielectric relaxation spectroscopy. The unaligned PLC exhibits a dielectric absorption of the α-relaxation process occurs in the glassy state. On the other hand, the unaligned material including 40 mol% of LMW in composite system exhibits a new dielectric absorption, which is based on the λ-relaxation process as judged by the activation energy. The director order parameter (Sd) varied from 0.16 to -0.30. It found that the dielectric behavior of the PLC/LMW composite system is very similar to that of liquid crystalline side-chain polymers.

Thermotropic liquid-crystalline copolymers containing an ionic group have been synthesized, and the effects of the ionic component for their liquid-crystalline properties have been studied.

Thermotorpic liquid-crystalline ionic polymers formed between the anionic polymer and the cationic low-mass compound have been prepared, and their thermal properties and orientational structures have been studied. They spontaneously organized homeotropic structures in the smectic phases.

アンモニウムイオン基を構成基とするポリエチレンイミン系及びイオンコンプレックス系のイオン性高分子液晶を合成し, それらの熱的性質及び配向特性について調べた. またアンモニウムイオン基をもつモデル低分子液晶も合成して比較検討した. 合成したイオン性高分子液晶及びモデル低分子液晶はすべてエナンシオトロピックなスメクティック相を示した. アンモニウムイオン基の導入はスメクティック相の熱安定性を向上させ, 垂直配向構造の自発的形成を促進させることが明らかとなった. 液晶相において自発的に形成された垂直配向構造は固体状態で保持された. 垂直配向試料のX線回折測定から, スメクティック層内ではイオン性基部分及びメソゲン基部分はそれぞれ分離して凝集状態を形成することが見いだされた. イオンコンプレックス系においても安定なスメクティック相が形成されることから, イオン結合を用いた機能複合系液晶の構築が可能であることがわかった.

Chiral liquid-crystalline polytartrate (LCPTT) with 2R,3R-(+)-tartrate moiety in the skeletal main chain and two mesogenic pendant groups in the repeating unit was prepared. Thermal and ferroelectric properties of LCPTT were dependent on those of the chiral structure. LCPTT exhibited cholesteric and smectic C* phases. The mode value of the spontaneous polarization was 5 mC/m2.

Thermotropic liquid-crystalline polyethylenimines with an ammonium halide moiety and those low-mass model compounds exhibited enantiotropically smectic phases. They were homeotropically aligned with the formation of an ammonium bilayer. The homeotropic structure of the liquid-crystalline polyethylenimine remained unaltered below Tg on cooling.

Etude des mouvements moleculaires sur le domaine de temperature des transitions de phase cristal/cristal liquide. Le volume segmentaire des chai←nes laterales est estime a partir de la theorie du volume libre

The thermal polymerization of aziridine monomers without a mesophase was performed in rubbing and non-treated sandwich cells. Cross-linked polymers, having homogeneous and homeotropic structures, were prepared. The thermal polymerization process was examined by X-ray diffraction measurements and polarizing microscopy. The cross-linked polymers exhibited a smectic A layered structure. In the rubbing sandwich cell, an aziridine monomer with an ethanol terminal group gave a cross-linked polymer having a homogeneous alignment. The homeotropic structure was obtained by thermal polymerization of an aziridine monomer with a nitro terminal group in a non-treated sandwich cell. The homeotropic structure was also formed on the surface of a glass plate and an aluminum thin film. A uniaxial-optical property of the homeotropic structure was characterized by conoscopic observation. The X-ray diffraction measurements showed the formation of the smectic A domain within an isotropic fluid by thermal polymerization at 100°C as well as optical texture observations. The X-ray diffraction pattern of the homeotropic structure was composed of first, second, and third inner reflections and a wide-angle halo, corresponding to the smectic A fluid phase.

The thermal and mesomorphic properties of the new liquid crystalline binary systems, consisting of a nematic main chain liquid crystalline polymer and a nematic twin liquid crystal, have been studi...

The binary systems consisting of di[4-(4-substituted phenylazo)phenyl]alkanedionates, which are twin liquid crystals, were prepared. The twin liquid crystals having a nitro or a butyl terminal substituent exhibited only a nematic phase with a schlieren texture. The liquid-crystalline binary system (BS-8-n) containing di{4-[4-(nitro)phenylazo]phenyl}sebacate and di{4-[4-(butyl)phenylazo]phenyl}sebacate, which involve an even number of central methylene units, exhibited an induced smectic A phase with a fan texture. However, the liquid-crystalline binary system (BS-7-m) containing di{4-[4(nitro)phenylazo]phenyl}azelate and di{4-[4-(butyl)phenylazo]phenyl}azelate, which have an uneven number of central methylene units, showed an induced smectic C phase with broken fan and schlieren textures, as well as a nematic phase. This different liquid crystal formation can be related to the molecular shapes of the twin liquid crystals, due to the conformation of the central methylene chain.

The thermal behavior and order parameter of two kinds of model compounds for liquid crystalline polyesters were studied by DSC and ESR spin probe methods, respectively. The transition temperatures for the twin model compounds which composed of structural sequences with a mesogen-flexible chain-mesogen demonstrated a dramatic odd-even alternation as increasing the number of methylene unit in the central flexible part. Both melting (Tm) and isotropization (Ti) temperatures for the twin model compounds with an even number of methylene lengths were higher than those with an odd number of methylene lengths. The strong alternation in the isotropization entropy change(ΔSi) and in the order parameter (S) were also observed in these twin compounds with variable spacer lengths. However, an inverse alternation regarding with the number of methylene length was found in Si and S for the azobenzene type of twin models which are composed of the inverse order of atoms in the ester linkage comparing with that of ...

The reaction of MAN with AlMe3 was studied by the 13C- and 1H-NMR spectral measurement. In the presence of an excess of AlMe3, it was found that a 1:1 complex of MAN and AlMe3 is initially formed and subsequently dimethyl(α-methyl isopropylidene amino)aluminum is produced by the reaction of the complex with AlMe3. The isolated product was pale yellow crystal (mp 39–40°C). The kinetics of the reaction were also examined.

The azoxydibenzoate copolyesters were obtained by the combination of azoxybenzene group as a mesogenic core with two different flexible chain lengths of oxyethylene unit. Their liquid crystalline behavior was studied by measuring of polarized microscopy, X-ray diffraction and differential scanning calorimetry. Smectic phase was observed for all the copolyesters. These copolyesters have a wider mesomorphic temperature range than that of corresponding homopolyesters.

Polyurethanes were synthesized by reaction of a rigid molecule, 3,3′-dimethyl-4,4′-biphenyldiyl diisocyanate (DBD), with flexible molecules, oligoethylene glycols (di-, tri-, and tetraethylene glycol) or α,ω-alkanediols, and some of their thermal properties were studied. Poly(DBD-oligoethylene glycol)s are amorphous polymers which exhibit no mesomorphic phase. Poly(DBD-alkanediol)s are found to show a mesomorphic phase depending on the number of methylene units in the alkanediol. The polymers having methylene units from five to twelve are crystalline and show a mesomorphic phase. On the contrary, polymers having less than four methylene units do not show a mesomorphic phase since thermal decomposition of the polymers occurs before melting. The transition temperature from the mesomorphic phase to the isotropic liquid decreases with increasing length of the flexible chain of alkanediol. Ternary polymers are obtained by reaction of DBD with a mixture of hexanediol (HD) and dodecanediol (DD). The ternary polymers containing a small amount of HD and DD give a clear mesomorphic phase.

The 13C relaxation times T1 and T2 for all resolved carbon lines are reported as a function of temperature in the nematic and isotropic phases of MBBA. Also the NOE measurements were performed in the both nematic and isotropic phases. The discontinuity of 13C T2's was observed at the nematic-isotropic transition point. From T1 and T2 data, it is shown that a short range order remains in the small temperature range just above the isotropic-nematic transition point. The region of small temperature dependence was found in the plots of 13C T1 vs reciprocal temperature. The rotational motion around the molecular axis was estimated to be three orders faster than the motion around the axis perpendicular to the molecular axis.

Preparation du polycarbonate par reaction du dihydroxy-4,4' azoxybenzene avec l'α-chloroformyl-ω-chloroformyloxyoligo(oxyethylene)

前報で示唆されたスメクチック液晶状態におけるchain foldingの可能性を追求するために, アゾキシジベンゾアートと長さが異なるポリオキシエチレン (重合度n=2, 6, 13) との組合せからなる3種のコポリエステルを試料として, 液晶の押出紡糸を行い, 繊維内における分子鎖及びメソーゲン層の配向挙動を調べた. 小角及び広角X線回折より, 繊維内で (1) メソーゲン基は押出方向 (繊維軸) に垂直に, (2) メソーゲン層の広い板面は押出方向に平行に選択配向することが分かった. せん断流動下でこのような配向が生ずることは, 高分子スメクチック液晶状態においても, 低分子スメクチック液晶の場合と同様に, 流動の基本的構造単位が板状のメソーゲン層であることを意味している. この事実, ならびに固相 (結晶状態) 押出しにおいても液晶押出物と同様な配向を示すこと及び結晶化挙動は, コポリエステルのスメクチック状態においてchain foldタイプのメソーゲン層が生成していることを支持している.

This paper is concerned with the synthesis and thermal properties for five families of side-chain type liquid crystal- line polymers. All these side-chain type polymers showed an enantiotropic mesomorphic nature. A sharp reflection was observed in the small angle region, whereas a broad halo or a relatively sharp peak was also detected in the wide angle region for all the polymers. The experimental values of the thickness of the smectic layer which can be easily assigned by the polarizing microscopic measurements were smaller than that of the calculated ones. This difference would be caused from the tilted arrangement in the smectic layer or smectic A like structure in which the side chains would be overlapped.

Azoxy and azo type polyesters were obtained by combination of an azoxybenzene or an azobenzene group as a mesogenic core with different flexible chain lengths of ethylene glycol groups. The thermal properties of these mesomorphic polymers are discussed related to their chemical structures. The mesomorphic temperature region becomes narrower with increasing flexible chain length. Further, the transition temperature from the mesomorphic phase to the isotropic liquid is higher for the azoxy type polyester than for the azo type polyester.

Polymerization activities of the soluble Ziegler-type of catalyst systems, Ti(OR)4-AlEt3, Ti(NEt2)4-AlMe3, and V(NEt2)4-AlEt3, were investigated. In the catalyst system of Ti(OR)4-AlEt3, formation of two types of Ti(III) compounds, i.e., Ti(OR)2Et and its bridged complex with aluminum alkyl, was confirmed by IR and ESR measurements. With the addition of donor molecule to the system, it was found that the polymer yield decreased remarkably and that the bridged complex dissociated into a single or uncomplex Ti(III) paramagnetic species. It has been concluded that the bridged structure of Ti(III) species was responsible for the polymerization activity of styrene. Two reaction products of Ti(NEt2)3Me and Al(NEt2)Me2 were found by NMR spectroscopic observation with the Ti(NEt2)4-AlMe3 catalyst system. From the kinetic study of polymerization of styrene, it was found that Ti(NEt2)3Me is an active species. An anionic mechanism was proposed for the styrene polymerization by Ti(NEt2)3Me. In the polymeriza...

The complex of acrylonitrile (AN) with trimethylaluminum (AlMe3) was successfully isolated as a white needlelike crystal with a melting point of 42.5°C. In this article the preparation of the complex is described in detail. Cryoscopic and spectral investigations indicated that the complex is formed by an equimolar ratio of AN and AlMe3. From the measurement of the x-ray diffraction photograph of the crystal the repeat period was determined to be 9.47 ± 0.05 A.

Electro-optical properties of the comb-like and the main chain type liquid crystalline polymers were investigated in terms of the molecular weight and the polymer structures. The comb-like polymer has acrylic or methacrylic skeletal chain with 4-cyanobiphenylbenzoate as a mesogenic pendant group. The response time on application of DC field was longer for the methacrylic polymer owing to its less mobility of the skeletal chain. The main chain type polymer with α-cyanostilbene as a mesogenic moiety exhibited a relatively short response time as low as 200ms. This is likely originated from a large dielectric anisotropy of the mesogenic moiety.

The infrared spectra of meso- and racemic-2,3-dibromobutanes in solution and in gaseous states were obtained in the study of the rotational isomers. The occurrence of three rotational isomers was confirmed for both meso- and racemic-isomers. The quantitative estimation of the equilibrium concentrations of the rotational isomers was also made in various solvents. The energy difference between the rotational isomers was determined by measuring the temperature-dependence of the absorption intensities of v(C–Br) vibrations.