Trifluoroacetic Acid Solution Research Articles

Electron transfer from anthracenes. Comparison of photoionization, charge-transfer excitation and electrochemical oxidation

Quantitative measures of electron detachment from anthracene and various meso-substituted derivatives (D) are examined in the gas phase and in solution. The photoelectron spectra show well-resolved bands for E1 corresponding to the first ionization [Formula: see text] from a π-molecular orbital (HOMO) with maximum electron density at the 9,10-positions. The same series of anthracenes form 1:1 electron donor–acceptor complexes with tetracyanoethylene [D,TCNE], which exhibit distinctive colors associated with charge transfer transitions hvCT to the ion pair[Formula: see text]. These measures of the gas phase energetics of electron transfer from the anthracenes are compared with their standard oxidation potentials [Formula: see text] determined from the reversible cyclic voltammograms in methylene chloride and trifluoroacetic acid solutions. Although there are direct, linear free energy relationships between E1and hvCT as well as E1and [Formula: see text], the best correlation is observed between [Formula: see text] and hvCT. The validity of the latter is discussed in terms of offsetting effects arising from differences in solvation energy and steric effects attendant upon 9,10-substitution in anthracenes.

Structure of 2-amino-4-thiazolinone

It was shown by the methods of IR and NMR spectroscopy that 2-amino-4-thiazolinone (“pseudothiohydantoin”) exists in an amino form in the crystalline state and in solutions in dimethyl sulfoxide, water, and trifluoroacetic acid, and in this amino form all the nitrogen-carbon bonds are partially double. In dimethyl sulfoxide and trifluoroacetic acid there is an autoassociation with the formation of dimers. Inhibited rotation of the amino group around the exocyclic nitrogen-carbon bond was detected. The results of a calculation of the IR spectrum of 2-amino-4-thiazolinone according to the force-field method agrees with the experimental data.

Study of the electrical double-layer between mercury and aqueous solutions of trifluoroacetic acid

The adsorption of CF 3COO − ions at the Hg-aqueous CF 3COOH interphase was studied, using both differential and electrocapillary measurements. The hump of the C d- E curve diminishes as the concentration increases from the 0.2 M solution, which is attributed to adsorption of undissociated CF 3COOH molecules. The predictions of the Esin-Markov coefficients are not valid in this case. The representations of φ M−2 vs q are not linear and present negative slopes for concentrations above 0.2 M. This last result has no physical meaning, indicating, if anything, an ion radius greater than the diameter of a molecule of water.

Blends of poly(ethylene terephthalate) and cellulose

AbstractPoly(ethylene terephthalate) (PET) (intrinsic viscosity 0.59) and cellulose (Whatman) are compatible in up to 7.5% (w/v) solutions in trifluoroacetic acid and in mixtures of trifluoroacetic acid and methylene chloride. Evaporation of the solutions yielded films that did not contain cellulose per se, but rather partial esters of cellulose and trifluoroacetic acid. Clear films were cast from these solutions with compositions of 100/0, 75/25, 50/50, 25/75, and 0/100 PET. cellulose (w/w). Infrared spectra and DSC measurements indicate specific polymer‐polymer interaction although two Tg were observed. Hydrolysis of the trifluoroacetate films to blends of PET and regenerated cellulose was accomplished by suspending the films in water at the boil. Infrared spectra indicate no interaction between the two polymers, although the films of the 50/50 and 25/75 PET. cellulose compositions were clear. The 25/75 composition, from its Tg and melting‐point behavior appears to be a dispersion of very small‐particle PET in a cellulose matrix. The 75/25 composition became opalescent during the hydrolysis and may be a dispersion of large‐particle cellulose in a PET matrix. The regenerated cellulose appears to be a mixture of cellulose II and IV polymorphs.

New catalyst for the synthesis of dialkyl ketones from olefins, carbon monoxide, and hydrogen

Dialkyl ketones are formed in mild conditions (30–70°C, 1 atm) from H2+CO + C2H4 or C3H6 in aqueous trifluoroacetic acid solutions of a PdII triphenylphosphine complex.

An improved procedure for the quantitative determination and characterization of sulfatides in rat kidney and brain by high-performance liquid chromatography

A significant improvement has been made in the desulfation step of our previously published HPLC determination of cerebrosides, sulfatides, and monogalactosyi diacylglycerols (Nonaka G. and Kishimoto Y., Biochim. Biophys. Acta 572 (1978) 423–431). Instead of the original two-phase reaction, a solution of trifluoroacetic acid in ethyl acetate is used for the solvolysis in the new method. The revised method was used to determine the levels of cerebrosides and sulfatides in rat kidney. Among four individual glycosphingolipids studied, hydroxysulfatide was present at the highest level (0.7–1.3 nmol/mg of dry tissue), followed by nonhydroxysulfatide (0.3–0.8 nmol/mg of dry tissue). Hydroxycerebroside (0.09–0.16 nmol/mg of dry tissue) and nonhydroxycerebroside (0.03–0.09 nmol/mg of dry tissue) were present in smaller quantities. There appear to be no significant differences between male and female animals of different ages (30–120 days), although the amounts decreased slightly in older animals and there was a higher concentration in female than in male kidney. Tissue size was significantly smaller in females. The homolog composition of rat kidney sulfatide was studied by reverse-phase HPLC, and was found to be significantly different from that reported in human kidney. Rat sulfatides contained fatty acids with a higher degree of saturation and longer chain length. Preliminary studies indicated that rat kidney contained unusually large quantities of C25:1 and C27:1 fatty acids and also that there was more C26:1 than C24:1 acid. In brain of the same animals the ratio of nonhydroxy to hydroxysulfatide decreased with age (1.5:1 in 30-day-old brain; 1:1 at 90 days).

Gel formation and liquid crystalline order in cellulose triacetate solutions

The formation of a liquid crystalline state of cellulose triacetate solution in trifluoroacetic acid was studied using optical microscopy, polarimetry and circular dichroism measurements. Below a critical concentration of 34 g 100 ml a clear isotropic solution was formed. Solutions more concentrated than this were in the cholesteric liquid crystalline state. If water was added to the solution, a gel phase was formed if the polymer concentration was above a critical value. We interpret our observations in terms of the Pincus-de Gennes theory and show that the formation of liquid crystalline order involves both inter-and intramolecular forces.

Secondary structure of peptides, 10. FT‐IR and 13C NMR CP/MAS study on tacticity and secondary structure of poly(D,L‐leucine)s

AbstractA series of poly (L‐leucine)s with average degrees of polymerization in the range between 10 and 100 and various poly (D,L‐leucine)s were prepared by polymerization of L‐leucine‐N‐carb‐oxyanhydride (NCA) or D,L‐Leu‐NCA. The FT‐IR spectra measured in KBr show that the amide I and II bands are exclusively sensitive to the secondary structure of the poly (leucine)s and not to their tacticity. The tacticities of the poly(D,L‐leucines) were determined in solution of trifluoroacetic acid by means of 13C NMR spectra, while 13C NMR CP/MAS spectra of the solid polymers revealed that the poly(D,L‐leucine)s contain 40 – 60 wt.‐ % α‐helix structure. For this reason the broad amide I and II bands do not enable one to distinguish isotactic and more or less atactic poly(leucine)s. However, the IR‐bands between 500 and 900 cm−1 allow a differentiation between poly(L‐leucine) and poly(D,L‐leucine)s and also between poly(D,L‐leucine)s of different tacticity. The IR‐spectra confirm that primary amine‐initiated polymerizations of D,L‐Leu‐NCA do not lead to long stereo blocks.

Synthesis and solution properties of poly(trans‐5‐ethylproline). A slow mutarotating substituted polyproline

AbstractAn optically active polypeptide, poly(trans‐5‐ethylproline) (PT5EP) was synthesized and its solution properties were observed to investigate the conformational changes with various conditions. The trans‐5‐ethyl substitution on polyproline showed noticeable perturbed effects on the conformations of the polypeptide as well as mutarotation of the polypeptide in solution. Circular dichroism (CD) spectra suggested that the polypeptide existed in a poly(L‐proline) form‐I‐type helix and mutarotated slowly to an intermediate conformation in which some of the amide bonds had rotated to a trans conformation. In trifluoroethanol (TFE) solution the polymer took more than 20 days to change from a form‐I‐type helix conformation, in which CD bands for D‐PT5EP are at 199.5 ± 1.0 (positive), 115.5 ± 0.5 (negative), and around 238 nm (positive), to an intermediate conformation. Upon addition of trifluoroacetic acid (TFA) to a TFE solution, the polymer was transformed to form‐II‐type polymers. Even a greater change in conformation was observed in a solution of TFA or in LiClO4‐TFE. The overall change of the intensity ratio RCD of positive to negative CD bands of D‐PT5EP was from 0.6–0.7 to 30. Reverse mutarotation toward the original form I was observed when n‐butyl alcohol, water, or THF was added to a solution containing the form II polymer. A blue shift of the UV spectra and a change in the NMR spectrum also supported the concept of this conformational change.

Rheological studies of cellulose derivatives solutions

AbstractThe rheological behavior of solutions of cellulose derivatives, acetate, and hydroxypropyl was studied as a function of different parameters; molecular weight, concentration,…, at T = 25°C in the vicinity of the liquid crystal phase transition. A variation of the strain rate sensitivity parameter with concentration is found. A degradation of the solution in trifluoroacetic acid is demonstrated.

Kinetics and mechanism of the reaction of 9-arylfuoren-9-yl cations with polymethylbenzenes in trifluoroacetic acid solution

9-Arylfluoren-9-yl cations, ArFl+, generated from the alcohol in CF3CO2H solution, are reduced to ArFIH by the polymethylated aromatic hydrocarbons (Ar1CH3) hexamethylbenzene, pentamethylbenzene, 1,2,3,4-, 1,2,3,5-, and 1,2,4,5-tetramethylbenzene, 1,3,5-trimethylbenzene, and 9,10-dimethylanthracene. Simple hydride-ion transfer, ArFI++ Ar1CH3→ ArFIH + Ar1CH2+, is thermodynamically unfavourable, but analysis indicates that Ar1CH2+ reacts further, to give stable products. Rates of disappearance of ArFI+(=k2[ArFI+][Ar1CH3]) measured spectrophotometrically show wide variation depending on the nature of Ar and Ar1, and kinetic isotope effects in the range 2.8–4.2 at 30°C resulting from experiments using perdeuteriohexamethylbenzene. The rate variation with changing Ar1CH3 correlates best with the ability of Ar1CH3to form charge-transfer complexes with fluoranil or to undergo one-electron oxidation. It is concluded that in the transition state for hydride transfer, electron transfer may have progressed further than nuclear motion of the migrating hydrogen. The possibility of intermediate formation of the pair ArFI˙–Ar1CH3+˙ is also considered. 4-Substituents in the phenyl group of PhFI+ have a remarkably constant effect on reaction rates with a range of Ar1CH3 of varying reactivity.

The effect of casting solvent and water on gas permeability of poly(γ‐methyl L‐glutamate) membrane

AbstractGas permeability of poly(γ‐methyl L‐glutamate) (PMLG) membranes prepared by using dichlorethane, trifluoroacetic acid, and formic acid as solvents was studied. The membranes prepared by casting dichloroethane and trifluoroacetic acid solutions of the polymer, designated as PMLG—DCE and PMLG—TFA, respectively, had α‐helical structures according to infrared absorption spectra, while the membranes prepared by allowing the PMLG—TFA membranes to swell in formic acid, designated as PMLG—FA, had mainly a β‐sheet structure. The diffusion coefficients of each gas studied for PMLG—DCE, PMLG—TFA, and PMLG—FA decreased in that order, and the apparent activation energies of diffusion increased in that order. The apparent heats of solution for Ne,O2, N2, and CO2 were negative in PMLG—FA. These results were discussed in connection with the molecular conformations of PMLG in the membranes. It is suggested that the diffusion of gases in PMLG—DCE takes place in the side‐chain regions between helices, while in PMLG—FA the diffusion occurs across the polymer main chain whose mobility is depressed by the intermolecular hydrogen bonds. The effect of water on oxygen permeability of PMLG—DCE was small; on the contrary that of PMLG—FA was very large. Furthermore, it is assumed that the random‐coil conformation partly exists in PMLG—TFA.

The free-radical phenylation of isoquinolinium cation

Free-radical phenylation of isoquinolinium cation has been effected by photolysis of phenylthalliurn(III)di(trifluoroacetate) in a trifluoroacetic acid solution of isoquinoline. Analysis (g.l.c. and h.p.l.c.) of the isomeric phenylisoquinoline mixture showed that the dominant product (81.1 %) was 1-phenylisoquinoline, in accordance with two published theoretical predictions.

Mesomorphic solutions of cellulose triacetate in halogenated organic acids and mixtures of trifluoroacetic acid and dichloromethane

AbstractCellulose triacetate (CTA) forms liquid‐crystalline solutions in trifluoroacetic acid, dichloroacetic acid, and mixtures of trifluoroacetic acid and dichloromethane. Brilliant iridescent coloration, high optical rotations, and birefringent regions with swirl‐like fingerprint patterns suggested the formation of cholesteric liquid crystals. Trifluoroacetic acid–dichloromethane is a particularly excellent solvent mixture to form liquid‐crystal solutions of cellulose acetate. Crude bulk viscosity measurements confirm the formation of an anisotropic phase, and the temperature dependence of the critical concentration for formation of the anisotropic phase, obtained by bulk viscosity measurements at various temperatures, provides confirming evidence. Viscosity decreases with aging, suggesting that CTA is slowly degraded in the solvents studied. This is also confirmed by optical rotatory measurements and by polarizing microscope observations. The miscibility gap is observed as a function of molecular weight, and the differences between the experimental data and Flory's prediction are discussed.

The preparation of some 2-nitroacridines and related compounds

The reaction of 2-fluoro-5-trobenzaldehyde with a wide variety of arylamines gives, in general, mixtures of the corresponding 2-arylamino-5-nitrobenzaldehydes and their related anils. Both aldehydes and anils readily underwent acid-catalysed cyclization to give the corresponding 2-nitro-acridines. The effect of substituent on the rate of cyclization of these aldehydes and of some related anilino-benzaldehydes and -acetophenones has been studied in trifluoroacetic acid solution by means of 1H n.m.r. spectroscopy. A solution of 2-(N-methylanilino)-5-nitrobenzaldehyde in trifluoroacetic acid appears to contain a substituted acridinium ion; treatment of this solution with alkali gave 10-methyl-2-nitroacridone. 1H n.m.r. data for a wide variety of substituted 2-nitro-acridines are discussed.

Kinetic hydrogen isotope effects in intermolecular hydride transfer from arylalkanes to 9-arylfluoren-9-yl cations

Rates of hydride-ion transfer to a series of 9-phenylfluoren-9-yl cations, substituted in the phenyl group, from triphenylmethane and 4,4′-dimethoxydiphenylmethane have been measured at 30°C in trifluoroacetic acid solution containing 6% v/v acetic acid. Deuterium kinetic isotope effects have been measured directly and show evidence of a broad maximum, the peak value occurring when the difference in pKR+ values between the product and reactant carbenium ions is around zero. Application of Marcus theory suggests that the intrinsic barrier to hydride transfer in these systems is substantially larger than for hydride transfer from formate ion to a series of triphenylmethyl cations. A limited series of measurements has also been made of tritium kinetic isotope effects on hydride transfer from 4,4′-dimethoxydiphenylmethane to 9-arylfluoren-9-yl cations and these have been used to derive deuterium–tritium isotope effects. The relevance of the results to the question of the origin of isotope-effect maxima is briefly discussed.

The effect of medium acidity on the rate of protonation of 6-dimethylaminofulventricarbonyl-chromium, -molybdenum, -tungsten and comparison of their basicity

Protonation of 6-dimethylaminofulvenic complexes of chromium (I), molybdenum (II) and tunsten (III) carbonyls in trifluoroacetic and acetic acid solutions of differing acidity in methylene chloride has been studied using PMR and IR spectroscopy. It has been established that the complexes are protonated at the metal atom and that the protonation rate is linearly dependent on the acidity of the medium. Comparison of the basicities of complexes I–III has shown that their basicity ratio is I/II/III ⋍ 1/150/1800.

Helical poly(L‐alanine) in trifluoroacetic acid

AbstractIn contrast to previous reports of other authors different 1H NMR chemical shifts were found for poly(L‐alanine) and poly(D,L‐alanine) in pure trifluoroacetic acid. The 13C and 15N NMR signals of poly(L‐alanine) are likewise different from those of poly(D,L‐alanine), and they are also different from the signals of the L‐alanine blocks in random copolypeptides such as (Ala/Gly)n, (Ala/Leu)n, (Ala/Val)n, and (Ala/Phe)n. It is concluded that poly(L‐alanine) is helical in trifluoroacetic acid (TFA); however, the optical purity, and thus, the helix stability is dependent on the reaction conditions used for the polymerization of L‐alanine‐NCA. The base initiated polymerization of L‐alanine‐NCA can cause substantial racemization in contrast to the primary amine‐initiated NCA polymerization. Sulfonic acids were found to be better solvents for polypeptides than trifluoroacetic acid, and their addition to TFA solutions of L‐alanine containing polypeptides exerts a strong influence on the chemical shifts and on the optical rotation. However, no clear experimental evidence was found for the helix‐coil transition of poly(L‐alanine) in TFA/sulfonic acid.

The effect of casting solvent on the sorption and diffusion of water vapor in poly(γ‐methyl L‐glutamate)

AbstractWater vapor permeability of poly(γ‐methyl L‐glutamate) (PMLG) membranes prepared by using dichloroethane, trifluoroacetic acid, and formic acid as solvents was studied. The membranes prepared by casting dichloroethane and trifluoroacetic acid solutions of the polymer, designated as PMLG–DCE and PMLG–TFA, respectivley, had α‐helical structures according to infrared absorption spectra, while the membranes prepared by allowing the PMLG–TFA membranes to swell in formic acid, designated as PMLG–FA, had mainly a β‐sheet structure. The amounts of water sorbed by PMLG–DCE, PMLG–TFA, and PMLG–FA increased in that order. The isotherms of PMLG–TFA and PMLG–FA were sigmoidal‐shaped isotherms, and the heat of sorption for PMLG–TFA and PMLG–FA was larger than that for PMLG–DCE, which suggested the presence of the sorption sites. The diffusion coefficients of water for PMLG–DCE increased and then decreased with increasing concentration. On the other hand, the diffusion coefficients for PMLG–TFA and PMLG–FA increased with concentration. The activation energies of diffusion for PMLG–DCE, PMLG–TFA, and PMLG–FA increased in that order. These results were discussed in connection with the molecular conformations of poly(γ‐methyl L‐glutamate) in the membranes. From these results, it is assumed that the molecular chains in the PMLG–TFA membranes are mainly in α‐helical and partly random‐coil conformations.

Activation of hydrocarbon molecules via Co(III)RH interaction in trifluoroacetic acid solution

Abstract The formation of the charge-transfer complex between cobalt(III) and 14 aromatic hydrocarbons in trifluoroacetic acid solution has been proved on the basis of spectrophotometric investigations. In the model system containing Co3O (O2CCH3)6(HO2CCH3)3 and various aromatic hydrocarbons (RH) the characteristic absorption bands of the charge-transfer complex are observed. Their positions (ΔECT) indicate the linear dependence on hydrocarbon ionisation potential (IPD). The cationic RH(+) radical and the reduced cobalt complex are the dissociation products of the charge-transfer complex [Co(III)RH]. The charge-transfer complex formation between the aromatic hydrocarbon and the cobalt(III) complex may be the explanation of the activation mode of CH bondings at low temperatures. This result sheds new ligh on the initiation mechanism of oxidation reactions of hydrocarbons.