Lowell M. Schwartz

Acid dissociation of cyclohexaamylose and cycloheptaamylose

Abstract Acid dissociation constants of aqueous cyclohexaamylose (6-Cy) and cycloheptaamylose (7-Cy) have been determined at 10–47 and 25–55°C, respectively, by pH potentiometry. Standard enthalpies and entropies of dissociation derived from the temperature dependences of these pKas are ΔH0 = 8.4 ± 0.3 kcal mol−1, Δ S 0 = −28. ± 1 cal mol −1 0 K −1 for 6-Cy and ΔH0 = 10.0 ± 0.1 kcal mol−1, Δ S 0 = −22.4 ±0.3 cal mol −1 0 K −1 for 7-Cy. Intrinsic 13C nmr resonance displacements of anionic 6- and 7-Cy were measured at 30°C in 5% D2O ( v v ). These results indicate that the dissociation of 6- and 7-Cy involves both C2 and C3 20-hydroxyl groups. The thermodynamic and nmr parameters are discussed in terms of interglucosyl hydrogen bonding.

Acid dissociation of cyclooctaamylose

Abstract Acid dissociation constants of aqueous cyclooctaamylose (8-Cy) have been determined at 15–45°C by pH potentiometry. Standard enthalpies and entropies of dissociation are derived from the temperature dependences of these p K a s. These results are compared to corresponding measurements of aqueous cyclohexaamylose and cycloheptaamylose, and the observed trends are interpreted in terms of complexation of cycloamylose with hydroxide ion. 13 C-nmr spectral measurements are reported for 8-Cy in 99.8% D 2 O solution, and assignments of the observed lines are made with the help of deuterium-induced differential isotopic shift experiments.

Complexation of carboxylic acids and anions by alpha and beta cyclodextrins

A pH potentiometric method is used to measure complex formation constants of aqueous alpha- and/or beta-cyclodextrin with several carboxylic acids and carboxylate anions: butyric acid/butyrate; valeric acid/valerate; hexanoic acid/hexanoate; octanoic acid/octanoate; decanoic acid/decanoate; cyclohexanecarboxylic acid/cyclohexanecarboxylate and benzoic acid/benzoate. Standard enthalpies and entropies of complex formation are calculated from the temperature dependencies of the equilibrium constants. These thermodynamic parameters of the alpha-cyclodextrin complexes largely conform to a correlation based on complexes with other substrate species previously reported. Both standard enthalpies and entropies of formation of beta-cyclodextrin complexes are found to be more positive than the corresponding complexes of alpha-cyclodextrin with the same substrates. These observations lead to insights into the bonding mechanism of cyclodextrin complexation.

Complexation of adamantane-ammonium substrates by beta-cyclodextrin and itsO-methylated derivatives

A spectrophotometric method is used to determine formation constants of complexes ofβ-cyclodextrin, the 2,6-di-O-methylated and 2,3,6-tri-O-methylated derivatives ofβ-cyclodextrin as hosts with adamantan-1-ylammonium, adamantan-2-ylammonium and adamantan-1-ylmethylammonium as substrate species. The spectrophotometric method uses methyl orange anion and acid forms as indicator species. Complexes of the cyclodextrins with these species are determined as well as with the adamantane derivatives. Standard enthalpies and entropies of formation of all complexes are calculated from the temperature variation of the equilibrium constants.β-Cyclodextrin and its 2,6-O-methyl derivative have comparable complex strengths with adamantaneammonium substrates and these strengths are about two orders of magnitude stronger than the corresponding complexes of the permethylated derivative. Thermodynamic parameters are interpreted in terms of differing intramolecular properties of the cyclodextrin complexes.

The complexation chemistry of cycloamyloses: equilibrium constants by novel spectrophotometric methods.

Abstract Formation constants for binary complexes of cyclohexaamylose (Cy) and p-nitrophenol/p-nitrophenolate ( PNP PNP − ) are determined by means of a novel acid-base spectrophotometric analysis. Thermodynamic parameters ΔH0 and ΔS0 are derived from the temperature dependences of these constants in the range 20–40°C. The Cy/PNP/PNP− system is then employed as a spectrophotometric indicator to estimate Cy complex formation constants with acid-base inactive substrates potassium perchlorate and sodium benzoate at 25°C and pH 6.8. The advantages and limitations of these methods in comparison with spectrophotometric analyses relying on the Hildebrand-Benesi equation are discussed.

Multiparameter models and statistical uncertainties

Abstract When more than one parameter is found by a least-squares calculation, the statistical uncertainties of the parameters are generally interdependent. If the uncertainty of one such parameter is quoted as a confidence interval based on the standard error estimate of that parameter and the Student t-statistic, this interval tends to be an underestimate. A suggestion is made to quote more conservative parameter uncertainties as the extreme points on the 95% joint confidence ellipsoid and it is shown that these joint parametric uncertainties are easily calculated from the standard error estimates. Both linear and nonlinear multiple regression are discussed. Nonlinear parameter uncertainties are found after an iterative search for the minimum sum-of-squares of residuals; searches by the Gauss and simplex methods are considered. A joint parametric uncertainty calculation is illustrated by a four-parameter nonlinear regression involving a pH potentiometric titration.

Photoconductivity in p-diiodobenzene

Abstract Castro and Hochstrasser1 have measured the singlet-triplet absorption spectrum of crystalline p-diiodobenzene and interpreted their findings as evidence of strong vibronic coupling between molecular states. Within the band-theory model of excess charge transport this implies a wide conduction band in comparison to other molecular crystals and hence a relatively long mean-free path and high drift mobility of the excess charge. In light of these implications we have undertaken a preliminary study of p-diiodobenzene photoconductivity primarily to measure the drift mobilities. Such data are particularly important at this time because of the uncertain state of charge transport theory in molecular crystals as discussed recently by Glaeser and Berry.2 Our study has indeed revealed relatively high drift mobilities. In addition we have found (1) that the action spectrum for charge generation bears no apparent relation to the known portion of the absorption spectrum of crystalline p-diiodobenzene, and (2) ...

Nonlinear calibration curves

LITERATURE CITED (10) H. G. Streim, E. A. Boyce, and J. R. Smith, Anal. Chem., 33, 85 (1961). (1 1) H. S. Knight and F. T. Weiss, Anal. Chem., 34, 749 (1962). (12) J. W. Forbes, Anal. Chem., 34, 1125(1962). (13) L. F. Fieser, and M. Fieser, “Reagents for Organic Synthesis”, J. Wiley, (1) A. S. Brown, J. Chem. Phys., 19, 1226 (1951). (2) J. H. Bower, J. Res. Nat. Bur. Stand., 12, 241 (1934). (3) F. Trusell and H. Diehl, Anal. Chem., 35, 674 (1963). (4) R. L. Meeker, F. Critchfield, and E. T. Bishop, Anal. Chem., 34, 1510 New York, 1967.

Complexation of aliphatic dicarboxylic acids and anions by alpha-cyclodextrin

Inclusion complexes are formed in aqueous solution between α-cyclodextrin and several straight-chain alkanedioic acids (ethanedioic, propanedioic, butanedioic, pentanedioic, hexanedioic, heptanedioic and octanedioic acids), several corresponding anions, several straight-chain alkenedioic acids (cis-butanedioic,trans-butanedioic andt,t-2,4-hexadienedioic acids) and several corresponding anions. Formation constants for these complexes were determined by measuring the effect of complexation on the pH of cyclohexaamylose/acid/base buffer equilibria. Enthalpies and entropies of complexation were calculated from the temperature dependences of the formation constants. The observed trends in the thermodynamic parameters lead to hypotheses about the structures of the complexes.

13 C nuclear magnetic resonance study of acid–base tautomeric equilibria

Acid–base tautomerisation equilibria are reported for o-, m-, and p-aminobenzoic acids, p-aminophenylacetic acid, pyridine-3-carboxylic (nicotinic) acid, and pyridine-4-carboxylic (isonicotinic) acid. These equilibria are determined by modelling the displacements of 13C n.m.r. chemical shifts of each carbon due to protonation of basic sites. The models are based on the corresponding displacement due to protonation of as many as twelve related compounds. Each such model compound yields several independent estimates of the tautomeric partitioning ratio corresponding to the several carbon resonances. It is observed that the mean tautomeric partitioning ratio and its estimated uncertainty are independent of the nature or the location of ring substituents and are unaffected by heteroatom substitution in the aromatic ring. This observation has led to the development of generic models for 13C n.m.r. chemical shift protonation displacements based on collections of similar model compounds. The use of generic models summarises and simplifies the determination of tautomeric equilibria.