Protonation Entropy Research Articles

Protonation Thermochemistry of α,ω-Alkyldiamines in the Gas Phase: A Theoretical Study

The proton affinities (PA) of the three first members of the series of α,ω-alkyldiamines, 1,2-ethanediamine (1), 1,3-propanediamine (2), and 1,4-butanediamine (3), were calculated at the G2(MP2) level. [PA(M) = 947.7, 977.7, and 999.8 kJ/mol for M = 1, 2, 3, respectively.] Protonation entropies, ΔS°p = S°(MH+) − S°(M), were estimated by explicitly considering the rotational barriers of the torsional modes in both the neutral molecules M = 1, 2, 3 and their protonated forms MH+. Calculated protonation entropy values are −17, −29, and −46 J·mol-1·K-1 for 1, 2, 3, respectively. Combining the calculated PA and ΔS°p lead to calculated gas-phase basicities [GBcalc(M) = 910.3, 936.9, and 953.6 kJ/mol for 1, 2, 3, respectively] in excellent agreement with experiment. [GBexp(M) = 912.4, 940.0, and 954.4 kJ/mol for 1, 2, 3, respectively.]

Thermodynamics and hydration of the europium complexes of a nitrogen heterocycle methane-1,1-diphosphonic acid

The enthalpies for formation of five Eu3+ complexes with N-piperidinomethane-1,1-diphosphonic acid (H4pmdp) have been determined by titration calorimetry in 2 M (Na,H)ClO4 media at 25 °C. Both protonation enthalpies and entropies, and the structure of H4pmdp in the solid state indicate protonation of the piperidine nitrogen to give a zwitterion in the free ligand. Luminescent lifetime measurements show that the formation of the 1∶1 and 1∶2 Eu∶L complexes of a variety of methane-1,1-diphosphonic acids liberates an average of 3.3 ± 0.1 water molecules per ligand from the Eu inner hydration sphere. The number of water molecules in the inner coordination sphere and the thermodynamic parameters of the complexes suggest that the structure of the Eu3+–pmdp complexes and the strength of the Eu3+–pmdp bonds are similar to those of other methane-1,1-diphosphonic acids despite the presence of the piperidine ring. However, the complexation entropies argue for weaker solvation of the Eu–pmdp complexes probably due to the hydrophobic character of the piperidine ring.

Proton affinities of the N- and C-terminal segments arising upon the dissociation of the amide bond in protonated peptides

Dissociation of the amide bonds in a protonated peptide leads to N-terminal sequence fragments with cyclic structures and C-terminal sequence fragments with linear structures. The ionic fragments containing the N-terminus (b n ) have been shown to be protonated oxazolones, whereas those containing the C-terminus (y n ) are protonated linear peptides. The coproduced neutral fragments are cyclic peptides from the N-terminus and linear peptides from the C-terminus. A likely determinant of these structural choices is the proton affinity (PA) of the described peptide segments. This study determines the PA values of such segments (Pep), i.e., cyclic and linear dipeptides and a relevant oxazolone, based on the dissociations of proton-bound dimers [Pep + B i ]H + in which B i is a reference base of known PA value (Cooks kinetic method). The dissociations are assessed at different internal energies to thereby obtain both proton affinities as well as entropies of protonation. For species with comparable amino acid composition, the proton affinity (and gas phase basicity) follows the order cyclic peptide ≪ oxazolone ≈ linear peptide. This ranking is consistent with dissociation of the protonated peptide via interconverting proton-bound complexes involving N-terminal oxazolone (O) or cyclopeptide (C) segments and C-terminal linear peptide segments (L), viz. O ⋯ H + ⋯ L ⇄ C ⋯ H + ⋯ L. N-terminal sequence ions (b n ) are formed with oxazolone structures which can efficiently compete for the proton with the linear segments. On the other hand, N-terminal neutral fragments detach as cyclic peptides, with H + now being retained by the more basic linear segment from the C-terminus to yield y n .

Protonic and Native Conduction in Sr‐Substituted LaPO4 Studied by Thermoelectric Power Measurements

Measurements of the thermoelectric power were applied to investigate the electrical conductivity in 5 mol % Sr‐substituted as a function of temperature (600 to 900°C), water vapor partial pressure (0.2 to 6 kPa), and oxygen partial pressure (4 to 100 kPa). Expressions of the thermoelectric power for materials conducting protons, oxide ions and electronic defects were derived, based on a thermodynamic treatment of entropy production by heat and charge transfer. The experimental data for 5 mol % Sr‐substituted were interpreted in terms of protonic conduction, and some additional conduction, possibly by electron holes. In air, the proton transport number is unity at 600°C when , and at 700°C when . In wet atmospheres, the transported entropy of protons in this material was found to be nearly constant, at 600 to 800°C in wet atmospheres.

Proton Affinity and Gas-Phase Basicity of Urea

Urea undergoes O-protonation in the gas phase to yield a product that is thermodynamically more stable than the N-protonated isomer, as also is the case in aqueous solution. The proton affinity and gas-phase basicity of urea, determined by using the kinetic method, are 873.5 ± 5.0 and 841.6 ± 5.0 kJ/mol, respectively. These values are in excellent agreement with G2(MP2) calculations, which give PA = 872.4 kJ/mol. The entropy requirements for the competitive dissociation channels of the proton-bound heterodimers of urea and the chosen reference compounds are measured and lead to the conclusion that urea and these references have almost equal protonation entropies (Δ(ΔS°) = 0.80 J/kmol). In comparison with proton affinities of acetone and acetamide, the proton affinity of urea is understandably enhanced by resonance stabilization in both neutral and protonated urea, and an upper limit to the PA value is established by a resonance saturation effect. These considerations provide a basis for the explanation of some aspects of the reactivity of urea.

Calorimetric determination of the enthalpies of coordination of competitive ternary mixed-ligand complex compounds. Copper(II)-12-(2′-hydroxy-5′-bromo-benzyl)-1,4,7,10-tetraazacyclo-tridecane-11,13-dione-α-aminoacids and 5-substituted 1,10-phenanthrolines systems

The enthalpies of protonation of 12-(2′-hydroxy-5′-bormo-benzyl)-1,4,7,10-tetraazacyclotridecane-11,13-dione(HBTADO) have been determined in aqueous solution at 25.0±0.1°C and ionic strength 0.1 mol dm −3 KNO 3 by means of model RD-1 automatic conduction calorimeter. The enthalpies of coordination of the binary complex compounds of this ligand with copper(II) and the ternary complex compounds of this ligand copper(II)-5-substituted 1,10-phenathrolines have also been determined under identical conditions of the ternary complex compounds of the type Cu(II)-α-aminoacids-HBTADO have been evaluated through “overall calculation”. Linear enthalpiesrelationships, linear enthalphy-entropy relationships have been found to exist between the enthalpies of formation of ternary complex compounds Cu(II)-R′Phen-HRTADO and the enthalpies, entropies of protonation of ligands HRTADO. In addition, linear enthalpy relationships have been found to exist between the enthalpies of formation of ternary complex compounds Cu(II)-α-aminoacids-HRTADO and the enthalpies of formation of binary complex compounds Cu(II)H −1A and linear entropy relationships have been found to exist between the entropies of formation of ternary complex compounds Cu(II)-α-aminoacids-HRTADO and the entropies of formation of binary complex compounds Cu(II)H −1A.

Gas-Phase Proton Affinity and Protonation Entropy Scale between Acetaldehyde and Methyl Acetate. An Experimental Evaluation and Comparative Analysis

The thermochemistry associated with the gas-phase proton affinity scale between acetaldehyde and methyl acetate has been reinvestigated by variable-temperature equilibrium measurements using pulsed electron beam high-pressure mass spectrometry. This experimentally difficult and sometimes kinetically inaccessible subset is particularly important since it provides the only bridge between the expansive lower and upper scales. Data were taken from 497 to 665 K, with an average temperature for all base pair combinations of 584 K. Special care was taken to ensure that the measurements accurately reflected both the ΔH° and ΔS° values within this temperature range. The present data indicate that earlier values from this laboratory within the same subset were expanded by ∼13% in both ΔG° and ΔH°, although no evidence was found for a similar expansion in our original ladder for bases with proton affinities above isobutene, which was used as the reference base in that study. A detailed analysis and comparison betwee...

Entropies and Free Energies of Protonation and Proton-Transfer Reactions

Entropies, enthalpies, and free energies for gas-phase protonation and proton-transfer reactions have been calculated and compared with state-of-the-art experimental values. Statistical entropies have been determined by using ab initio molecular parameters and several previously defined models (E1, E2, and E3). The accuracy of the ab initio proton-transfer entropies (ΔS) obtained with the E3 procedure (1 J mol-1 K-1) is significantly better than that normally attainable for ΔS values derived from van't Hoff plots of experimental equilibrium data for proton-transfer reactions (5−10 J mol-1 K-1). In fact, even the simplest E1 procedure produces proton-transfer entropies that are accurate to about 1−2 J mol-1 K-1. The commonly used isoelectronic approach to estimating entropies of gas-phase ions has been tested. Errors associated with this approximation are generally about 1−3 J mol-1 K-1, but can increase to 5−10 J mol-1 K-1 for systems that have low energy torsional or other floppy modes. G2 enthalpies and E3 entropies have been used to obtain free energies (ΔG) for 25 experimentally observed proton-transfer reactions. The ab initio free energies are in very good agreement with experimental values, the mean absolute deviation being 2.2 kJ mol-1 and maximum deviation 4.9 kJ mol-1. There is also very good agreement between theory and experiment for the enthalpies of these proton-transfer reactions with a mean absolute deviation and maximum deviation of 2.7 and 8.5 kJ mol-1, respectively. Theoretical proton affinity (ΔH) and gas-phase basicity (ΔG) scales have been constructed on the basis of G2 energies for 39 molecules and corresponding protonated species. There is generally good agreement between theory and experiment. The small deviations that do exist between theoretical and experimental values appear to correlate with the proton affinity magnitude.

Combined Experimental and Theoretical Study of the Protonation of Polyfluorobenzenes [C6H6-nFn] (n=0-6)

In a recent high-pressure mass spectrometric revision to the gas-phase basicity scale (J. E. Szulejko and T. B. McMahon, J. Am. Chem. Soc. 115, 7839 (1993)), it was observed that the proton affinity for hexafluorobenzene was 24 kcal mol-1 (1 kcal=4.184 kJ) lower than the accepted National Institute of Science and Technology (NIST) database value of 177.7 kcal mol-1 (S. G. Lias et al., J. Phys. Chem. Ref. Data 17, Suppl. 1 (1988)). Furthermore, the proton affinities for most other polyfluorobenzenes were also found to differ substantially from the NIST values. For many of the polyfluorobenzenes large protonation entropy changes were noted, which were substantially greater than then those expected from rotational symmetry number changes alone. In view of these observations, MP2/6–31G**//HF/6–31G** ab initio calculations were undertaken to investigate further the proton affinity and entropy changes with respect to the degree of fluorine substitution. The present proton affinity variations found for the polyfluorobenzenes (hexaflorobenzene excepted) can be interpreted with the aid of the ab initio results in terms of a simple additivity scheme. Each fluorine substituent para, meta, ortho or ipso to the ring protonation site will induce an incremental proton affinity change with respect to benzene of 1.5, -7.0, -1.5 and -19.0 kcal mol-1, respectively. This additivity scheme can also be used to rationalize the re-evaluated proton affinities for the polymethylbenzenes and m- and o-fluorotoluenes. The corresponding methyl increments are 7.5, 5.5, 2.5 and 1.0 kcal mol-1 for para, meta, ortho and ipso protonation respectively. From the present ab initio statistical thermodynamic analysis of the various protonation entropy components, it was concluded that the low-frequency vibrations are almost exclusively responsible for the large excess entropy changes observed experimentally. Ab initio calculated barriers for 1,2 proton shifts in arenium species available in the literature are concluded to be too large to allow the existence of a so-called dynamic proton. Large excess protonation entropies are noted from the literature for polymethylbenzenes, naphthalene and 1-methylnaphthalene. © 1997 by John Wiley & Sons, Ltd.

Linear thermodynamic function relationships in coordination chemistry: Calorimetric study on nickel(II)—, cobalt(II)—, zinc(II)— N-( p-substituted phenyl)iminodiacetic acid binary systems and nickel(II)—, cobalt(II)—, zinc(II)—nitrilotriacetic acid— N-( p-substituted phenyl)iminodiacetic acid ternary systems

The heats of protonation of N-( p-substituted phenyl)iminodiacetic acids (B,R— ▪—N(CH 2COOH) 2, RPhIDA: R is CH 3O, CH 3, H, Cl) and nitrilotriacetic acid (A, N(CH 2COOH) 3, NTA) and the heats of formation of the M(II)—B and M(II)—A binary systems (M is Ni, Co, Zn), and of the M(II)—A—B ternary systems have been determined at 25.0 ± 0.2°C, I=0.1 mol dm −3 (NaClO 4) in aqueous solution using an improved model RD-1 automatic conduction calorimeter. The entropies of protonation and of formation for these binary and ternary systems have been calculated from their free energies. The results show that both linear enthalpy relationships (LERs) and linear entropy relationships (LSRs) exist between the heat (and entropy) of protonation of B and the heat (and entropy) of formation of both binary and ternary complex compounds. It is noted that the heats of protonation of RPhIDA and the heat of formation for their metal complexes are all positive, i.e. endothermic, and a hypothetical structure of the polyring for the hydrated ligands B has been suggested to interpret the experimental results.

Proton affinity ladders from variable-temperature equilibrium measurements. 1. A reevaluation of the upper proton affinity range

An interlocking ladder of relative proton affinities of 47 compounds, over a range of 54 kcal/mol, was obtained from the enthalpies of proton-transfer equilibria, by using variable-temperature pulsed high-pressure mass spectrometry. From C 3 H 6 (PA=179.5 kcal/mol) to i-C 4 H 8 (PA=195.9 kcal/mol) the results agree well with tabulated values, but for the upper PA range the present values are increased significantly. For example, the present vs literature PA values are NH 3 , 208.3 vs 204.0; CH 3 NHI2, 219.6 vs 214.1; and t-C 4 H 9 NH 2 , 229.2 vs 220.8 kcal/mol. The new value for t-C 4 H 9 NH 2 is also confirmed by the association thermochemistry of t-C 4 H 9 + with NH 3 . The entropies of protonation are in reasonable agreement with expected rotational symmetry changes. Aromatics and olefins show a positive entropy of protonation of 3-6 cal/mol K due to structural effects, but no anomalous effects are observed that would indicate a dynamic proton

Shock heating of the solar wind plasma

We present a study of all shocks observed from Pioneers and Voyagers in 1973–1982. The average shock strength increases with the heliocentric distance outside 1 AU, reaches a maximum near 5 AU, and then decreases with the distance. The increase in the entropy of the solar wind protons across shocks also reaches a maximum near 5 AU. When an average shock propagates through the solar wind, the shock heating increases the entropy of the solar wind protons by approximately 0.8×0−23 J/K/proton. We also use plasma data from Voyagers and Pioneers between 1 and 30 AU and data from IMP at 1 AU to calculate the increase in the average entropy of solar wind protons with the heliocentric distance. When the distance increases by a factor of 10, the entropy increases by about 4×10−23 J/K/proton. In order to evaluate the role played by shocks for the heating of the solar wind, we use a MHD simulation model to calculate the entropy changes for the November, 1977 event. Shock heating is the only heating mechanism included in the model. The calculated entropy increase agrees reasonably well with that calculated from observational data. The simulation result suggests that shocks are chiefly responsible for the heating of the solar wind plasma between 1 and 15 AU.

Linear enthalpy relationships between heats of formation of complex compounds and heats of protonation of ligands. Copper(II)- N-( para-substituted phenyl)iminodiacetic acid-α-amino acids ternary system and copper(II)- N-( para-substituted phenyl)iminoacetic acids binary system

The heats of protonation of N-( para-substituted phenyl)iminodiacetic acids (A, p-RPhIDA: RCH 3O, CH 3, H, Cl) and the heats of formation of Cu(II)- p-RPhIDA binary complex compounds and of Cu(II)- p-RPhIDA-α-amino acids (B, AA: L-proline, L-isoleucine, L-valine, L-serine, glycine and α- aminoisobutyric acid) competitive ternary complex compounds have been determined at 25.0 ± 0.2 °C, and I = 0.1 mol dm −3 (KNO 3) by means of a model RD-1 automatic conduction calorimeter. In the meantime the entropy of protonation of the ligand A and the entropy of formation of the binary complex compounds Cu(II)- p-RPhIDA-AA have been calculated. The results showed the existence of linear enthalpy relationships between the heats of formation of these ternary complex compounds and the binary complex compounds and the heats of protonation of the ligands p-RPhIDA and the existence of linear entropy relationships between the entropy of formation of binary complex compounds, Cu(II)- p-RPhIDA and the entropy of protonation of ligands, p-RPhIDA.

Gas-phase proton transfer in halogenotoluene mixtures. Evidence for a ‘dynamic’ ion structure

Gas-phase proton transfer equilibria between fluoro- and chloro-toluene mixtures have been studied as a function of temperature to determine relative proton affinities and entropy changes. Some of the entropy changes are much greater than any values previously reported for proton-transfer reactions. It is suggested that this can only be explained if the ortho- and para-halogenotoluene isomers have a ‘dynamic’ protonated ion structure in which a mobile hydrogen atom is shared by two or more unsubstituted ring carbon atoms. There is also an apparent linear correlation between the proton affinities measured and the corresponding entropy of proton bonding of these species. This ‘isokinetic’ relationship has a slope of 589 ± 50 K and is consistent with the suggestion in these compounds that the internal translational movement of the proton within the plane of the aromatic ring decreases the more tightly bound it becomes. This appears to be the first time such a relationship has been observed for gas-phase proton transfer. The following proton affinities relative to Ap(p-chlorotoluene) were measured: p-chlorotoluene, 0; p-fluorotoluene, 1.2 ± 0.2; o-chlorotoluene, 16 ± 0.8; o-fluorotoluene, 28 ± 1.5; m-chlorotoluene, 33.9 ± 1.4; m-fluorotoluene, 36.9 ± 1.4 kJ mol–1.

A potentiometric and calorimetric study of the protonation and complexation with Cu 2+ and Ni 2+ of some sulphur-containing α,ω-amino acids

The behaviour in aqueous solution of some aminocarboxylates of the type NH 2(CH 2) n S(CH 2) m-1 COO − (abbreviated as n,m-NSO; n and m = 2 or 3) in equilibria with H +, Cu 2+ and Ni 2+ ions has been investigated potentiometrically and calorimetrically at 25° C in a 0.5 M KNO 3 medium. The protonation of the amino function and especially the carboxylate function is attended by strong desolvation effects, which are characterized by low exothermic enthalpies and strongly positive entropies of protonation. In [Cu( n,m-NSO)] + and [Ni( n,m-NSO)] + the aminocarboxylates act as tridentate ligands, forming complexes with a strong hard—hard character. The biligand species [Ni( n,m-NSO) 2] behave as six-coordinated complexes whereas in [Cu( n,m-NSO) 2] the second ligand is bound only through the N and S donor, forming a five-coordinated species. Finally, the n,m-NSO ligands also form protonated species with the Cu 2+ ion.

Metal chelates of phosphonate-containing ligands—II Stability constants of some β-styrylphosphonic acid metal chelates

The synthesis and properties of β-styrylphosphonic acid (SPA) are reported. The values of the protonation constants of the ligand were determined potentiometrically at different ionic strengths and in the temperature range 0–65°. The enthalpy and entropy of protonation have been calculated by using the van't Hoff isochore. The protonation process is endothermic and is stabilized by a relatively large positive entropy change. The stability constants of the complexes formed between SPA and the bivalent Mg, Ca, Ba, Co, Ni and Pd ions at 25° and ionic strength of 0.12 M KNO 3 were also determined. All measurements were carried out in 18% dimethylformamide—water mixture.

Double-double effect and the coordination number

The thermodynamics of protonation of the lanthanide(III) chelates of trans-l,2-diaminocyclohexanetetraacetic acid (LnDCTA') was studied by potentiometric titration at the ionic strength of 1 (KC1). The protonation constants determined change monotonically and the double-double effect in the log K against Z variation is manifested in all four segments of the lanthanide series. The variations of the protonation enthalpy and entropy with Z are estimated to be also monotonical. The direction of the double-double effect observed indicates that the coordination number in the protonated chelates is higher by 1 in the entire series than in non-protonated chelates i.e. the protonation is accompanied by the coordination of the additional water molecule. The coordination number 10 is expected for the HLnDCTA chelates at least at the beginning of the series. Presumably the present conclusions are valid also for the LnEDTA- system.

The basicities of substituted N-trimethylammoniophenylacetamidates and N-trimethylammoniocinnamamidates. The hammett correlations and the thermodynamics of protonation

The pKa values of a number of substituted N-trimethylammoniophenylacetamidates and N-trimethylammoniocinnamamidates have been determined over a range of temperature and the thermodynamic functions of protonation (ΔH° and ΔS°) derived. Both reaction series obey the Hammett equation very well, the ρ values at 25 °C being 0.639 ± 0.026 and 0.709 ± 0.049, respectively. It is found that the ratio of these ρ values to the ρ values for the ionization of the corresponding carboxylic acids is constant at 1.50 ± 0.04, regardless of the structure of the rest of the molecule, and is thus a characteristic of the amidate group. An analysis of the pKa data for these acid–base series and comparison with the pKa data for the corresponding carboxylic acid series leads to the conclusion that mesomeric electron donation by the benzene ring to the basic groups (a) and (b) is more important in the carboxylates than in the amidates, consistent with the greater overall electronegativity of the carboxylate group. The thermodynamic functions of protonation, ΔH° and ΔS°, broadly show a similar behaviour to that of the N-trimethylammoniobenzamidate series, although the entropies of protonation are more positive.

Solution chemistry of macrocycles. Part 2. Enthalpic and entropic contributions to the proton basicity of cyclic tetra-amine ligands: 1,4,8,11-tetra-azacyclotetradecane, 1,4,8,12-tetra-azacyclopentadecane, and 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane

The enthalpy changes of stepwise protonation of the cyclic tetra-amines: 1,4,8,11-tetra-azacyclotetradecane, [14]aneN4, 1,4,8,12-tetra-azacyclopentadecane, [15]aneN4, and 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane, Me4[14]aneN4, have been determined at 25 °C in 0.5M-KNO3 by calorimetry using the continuous titration procedure. Through a combination of the values of the enthalpy changes with the corresponding free-energy changes, reported elsewhere, the entropies of protonation have been obtained. In order to explain these thermodynamic functions two factors, which are not involved in the case of open-chain polyamines, are to be taken into account: (i) the presence of internal hydrogen bonds of the type N–H ⋯ N and (ii) conformational rearrangements of the macrocyclic ring due to the nitrogen inversion, the lone pairs being directed either towards the inside or the outside of the ring.

The basicities of N-trimethylammonioacetamidate and of substituted N-trimethylammoniobenzamidates. The Hammett correlation and the thermodynamics of protonation

The pKa values of N-trimethylammonioacetamidate and a number of substituted N-trimethylammoniobenzamidates have been determined over the temperature range 15–55 °C. The pKa, values show a correlation with Hammett σ constants with ρ 1.51 ± 0.03 at 25 °C. These facts are consistent with an N-protonated cation structure, i.e. the cations are N-trimethylammoniobenzamides, as deduced earlier from spectroscopic studies. The thermodynamic functions of protonation have been compared with those for tertiary amines and a close similarity of the entropies of protonation has been observed. The variation of the entropies of ionisation of N-trimethylammoniobenzamides with change of substituent is analogous to that in the benzoic acid series, except that solvation effects are smaller. The substantial difference in the enthalpies of ionisation and their variation with substitution in the two reaction series is largely attributable to that difference in solvation.