Experimental ΔH Research Articles

A corrected benzene nitration three-step mechanism derived by DFT calculation and MO theory

Density-functional theory (DFT) calculations at the LC-wHPBE/6-311++G(d,p) level found that the textbook three-step nitration mechanism of benzene in mixed acids was seriously wrong. Step 1 of generating nitronium ion (NO2+) is not spontaneous, the NO2+ is generated by Lewis collision, and needs to overcome a barrier Ea = 18 or 22 kcal/mol in mixed acid or in nitric acid. Obtaining the Ea of the Lewis collision by quantum chemical calculations is a highlight of the study. The reaction system (NO2+ + H2O) + HSO4⎺ or + NO3⎺ or + nH2O (n ≥ 1) can make NO2+ spontaneously change to HNO3 through a poly(≥3)-molecular acidification. Sulfuric acid can greatly reduce [H2O] and increase [NO2+]. Therefore, the nitration rate in mixed acid is much faster than that in nitric acid. Step 2, C6H6 + NO2+, is an electrophilic addition, follows the transition state theory, and needs to overcome a low barrier, ΔE* = 7 kcal/mol. The product of Step 2 is the σ-complex C6H6-NO2+. The essence of the electrophilic addition is the transfer of HOMO-1 electrons of C6H6 to LUMO of NO2+. Step 3 is a spontaneous Lewis acid-base neutralization without any barrier, and generates the target product nitrobenzene C6H5NO2. NO2+ and σ-complex are the two active intermediates in nitration. The benzene nitration rate control step is not Step 2 of generating σ-complex, but is Step 1 to generate NO2+. The DFT calculation obtains the barriers Ea and ΔE*, the reaction heats ΔHσ and ΔHp of each step of the nitration, resulting in the total nitration reaction heat ΔH = -35 kcal/mol. It is consistent with the experimental ΔH = -34 kcal/mol. Based on the results, a corrected benzene nitration three-step mechanism proposed.

A hill-type muscle model expansion accounting for effects of varying transverse muscle load

Recent studies demonstrated that uniaxial transverse loading (FG) of a rat gastrocnemius medialis muscle resulted in a considerable reduction of maximum isometric muscle force (ΔFim). A hill-type muscle model assuming an identical gearing G between both ΔFim and FG as well as lifting height of the load (Δh) and longitudinal muscle shortening (ΔlCC) reproduced experimental data for a single load.Here we tested if this model is able to reproduce experimental changes in ΔFim and Δh for increasing transverse loads (0.64 N, 1.13 N, 1.62 N, 2.11 N, 2.60 N). Three different gearing ratios were tested: (I) constant Gc representing the idea of a muscle specific gearing parameter (e.g. predefined by the muscle geometry), (II) Gexp determined in experiments with varying transverse load, and (III) Gf that reproduced experimental ΔFim for each transverse load.Simulations using Gc overestimated ΔFim (up to 59%) and Δh (up to 136%) for increasing load. Although the model assumption (equal G for forces and length changes) held for the three lower loads using Gexp and Gf, simulations resulted in underestimation of ΔFim by 38% and overestimation of Δh by 58% for the largest load, respectively. To simultaneously reproduce experimental ΔFim and Δh for the two larger loads, it was necessary to reduce Fim by 1.9% and 4.6%, respectively. The model seems applicable to account for effects of muscle deformation within a range of transverse loading when using a linear load-dependent function for G.

Thermodynamics of substrate binding to the metal site in homoprotocatechuate 2,3-dioxygenase: Using ITC under anaerobic conditions to study enzyme–substrate interactions

BackgroundExtradiol dioxygenases are a family of nonheme iron (and sometimes manganese) enzymes that catalyze an O2-dependent ring-opening reaction in a biodegradation pathway of aromatic compounds. Here we characterize the thermodynamics of two substrates binding in homoprotocatechuate 2,3-dioxygenase (HPCD) prior to the O2 activation step. MethodsThis study uses microcalorimetry under an inert atmosphere to measure thermodynamic parameters associated with catechol binding to nonheme metal centers in HPCD. Several stopped-flow rapid mixing experiments were used to support the calorimetry experiments. ResultsThe equilibria constant for 4-nitrocatechol and homoprotocatechuate binding to the iron(II) and manganese(II) forms of HPCD range from 2×104 to 1×106, suggesting there are distinctive differences in how the enzyme–substrate complexes are stabilized. Further experiments in multiple buffers allowed us to correct the experimental ΔH for substrate ionization and to fully derive the pH and buffer independent thermodynamic parameters for substrate binding to HPCD. Fewer protons are released from the iron(II) dependent processes than their manganese(II) counterparts. ConclusionsCondition independent thermodynamic parameters for 4-nitrocatechol and homoprotocatechuate binding to HPCD are highly consistent with each other, suggesting these enzyme–substrate complexes are more similar than once thought, and the ionization state of metal coordinated waters may be playing a role in tuning redox potential and in governing reactivity. General significanceSubstrate binding to HPCD is a complex set of equilibria that includes ionization of substrate and water release, yet it is also the key step in O2 activation. This article is part of a Special Issue entitled Microcalorimetry in the BioSciences — Principles and Applications, edited by Fadi Bou-Abdallah.

Gas-phase acidities of nitrated azoles as determined by the extended kinetic method and computations.

Making use of the extended kinetic method and the alternative method for data analysis, we have experimentally determined ΔH°acid (kcal/mol) for six mononitrated azole species (2-nitropyrrole = 337.0, 3-nitropyrrole = 335.8, 3-nitropyrazole = 330.5, 4-nitropyrazole = 329.5, 2-nitroimidazole = 327.4, and 4-nitroimidazole = 325.0). We report an absolute uncertainty of ±2.2 kcal/mol that arises from the uncertainties of the reference acids; the relative values are known within 0.4 kcal/mol. Combining these experimental ΔH°acid values with ΔS°acid values calculated at the B3LYP/aug-cc-pVTZ level of theory, we report ΔG°acid (kcal/mol) for the nitroazoles (2-nitropyrrole = 329.4, 3-nitropyrrole = 328.4, 3-nitropyrazole = 323.1, 4-nitropyrazole = 322.0, 2-nitroimidazole = 319.7, and 4-nitroimidazole = 317.6); the absolute uncertainties are ±2.4 kcal/mol. In addition to the experimental studies, we have computationally investigated the gas-phase acidities and electron affinities of the azoles in this work, as well as higher-order aza- and dinitro-substituted azoles. We discuss trends in the stabilities of the deprotonated azoles based on aza substitution and nitro group placement. 4-Nitroimidazole has already found use as the anionic component in ionic liquids, and we propose that the additional nitrated azolate ions are potential candidates for the anionic component of ionic liquids.

High-level ab initio predictions for the ionization energy, bond dissociation energies, and heats of formation of cobalt carbide (CoC) and its cation (CoC+)

The ionization energy (IE) of CoC and the 0 K bond dissociation energies (D0) and the heats of formation at 0 K (ΔH°f0) and 298 K (ΔH°f298) for CoC and CoC(+) are predicted by the wavefunction based coupled-cluster theory with single, double, triple and quadruple excitations (CCSDTQ) and complete basis set (CBS) approach. The CCSDTQ∕CBS calculations presented here involve the approximation to the CBS limit at the coupled cluster level up to full quadruple excitations along with the zero-point vibrational energy, high-order correlation, core-valence (CV) electronic, spin-orbit coupling, and scalar relativistic effect corrections. The present calculations provide the correct symmetry, (1)Σ(+), for the ground state of CoC(+). The CCSDTQ∕CBS IE(CoC) = 7.740 eV is found in good agreement with the experimental IE value of 7.73467 ± 0.00007 eV, determined in a two-color laser photoion and pulsed field ionization-photoelectron study. This work together with the previous experimental and theoretical investigations support the conclusion that the CCSDTQ∕CBS method is capable of providing reliable IE predictions for 3d-transition metal carbides, such as FeC, CoC, and NiC. Among the single-reference based coupled-cluster methods and multi-reference configuration interaction (MRCI) approach, the CCSDTQ and MRCI methods give the best predictions to the harmonic frequencies ωe (ωe (+)) = 956 (992) and 976 (1004) cm(-1) and the bond lengths re (re (+)) = 1.560 (1.528) and 1.550 (1.522) Å, respectively, for CoC (CoC(+)) in comparison with the experimental values. The CCSDTQ∕CBS calculations give the prediction of D0(Co(+)-C) - D0(Co-C) = 0.175 eV, which is also consistent with the experimental determination of 0.14630 ± 0.00014 eV. The theoretical results show that the CV and valence-valence electronic correlations beyond CCSD(T) wavefunction and the relativistic effect make significant contributions to the calculated thermochemical properties of CoC∕CoC(+). For the experimental D0 and ΔH(o) f0 values of CoC∕CoC(+), which are not known experimentally, we recommend the following CCSDTQ∕CBS predictions: ΔH(o) f0(CoC) = 775.7 kJ∕mol and ΔH(o) f0(CoC(+)) = 1522.5 kJ∕mol, ΔH(o) f298(CoC) = 779.2 kJ∕mol and ΔH(o) 298(CoC(+)) = 1526.0 kJ∕mol.

Conformational isomerism of 1-butene secondary ozonide as studied by means of matrix isolation infrared absorption spectroscopy

Abstract Theoretical calculations of structures, stability and vibrational spectra of 1-butene secondary ozonide (SOZ) conformers were performed using DFT method B3LYP with a 6-311++G(3df, 3pd) basis set. The calculations predict six staggered structures of 1-butene SOZ. The FTIR spectra of 1-butene SOZ isolated in Ar, N2 and Xe matrices were recorded. It was found that nitrogen is the best suited for the matrix isolation of 1-butene SOZ. The bandwidth of the spectral bands of the ozonide isolated in nitrogen was as narrow as 2 cm−1. For the first time the existence of five conformers of 1-butene SOZ were confirmed experimentally by means of matrix isolation infrared absorption spectroscopy. The equatorial gauche (∠OCCC=−66.1°) conformer was proved theoretically and experimentally to be the most stable. It was found that due to high potential barriers of the conformational transitions annealing of the matrix is useless for the assignment of spectral bands to various conformers of 1-butene SOZ. Using the hot nozzle technique the van’t Hoff experimental plots were made for three additional conformers of 1-butene SOZ and experimental ΔH values for these additional conformers were established. The crystallization problems of 1-butene SOZ are discussed which accounts for the rich conformational diversity of the ozonide as well as high conformational barriers for axial-equatorial transitions.

Time Resolved Infrared Spectroscopy: Kinetic Studies of Weakly Binding Ligands in an Iron–Iron Hydrogenase Model Compound

Solution photochemistry of (μ-pdt)[Fe(CO)(3)](2) (pdt = μ(2)-S(CH(2))(3)S), a precursor model of the 2-Fe subsite of the H-cluster of the hydrogenase enzyme, has been studied using time-resolved infrared spectroscopy. Following the loss of CO, solvation of the Fe center by the weakly binding ligands cyclohexene, 3-hexyne, THF, and 2,3-dihydrofuran (DHF) occurred. Subsequent ligand substitution of these weakly bound ligands by pyridine or cyclooctene to afford a more stable complex was found to take place via a dissociative mechanism on a seconds time scale with activation parameters consistent with such a pathway. That is, the ΔS(‡) values were positive and the ΔH(‡) parameters closely agreed with bond dissociation enthalpies (BDEs) obtained from DFT calculations. For example, for cyclohexene replacement by pyridine, experimental ΔH(‡) and ΔS(‡) values were determined to be 19.7 ± 0.6 kcal/mol (versus a theoretical prediction of 19.8 kcal/mol) and 15 ± 2 eu, respectively. The ambidentate ligand 2,3-DHF was shown to initially bind to the iron center via its oxygen atom followed by an intramolecular rearrangement to the more stable η(2)-olefin bound species. DFT calculations revealed a transition state structure with the iron atom almost equidistant from the oxygen and one edge of the olefinic bond. The computed ΔH(‡) of 10.7 kcal/mol for this isomerization process was found to be in excellent agreement with the experimental value of 11.2 ± 0.3 kcal/mol.

Rates of Water Exchange on the [Fe4(OH)2(hpdta)2(H2O)4]0 Molecule and Its Implications for Geochemistry

The ammonium salt of [Fe(4)O(OH)(hpdta)(2)(H(2)O)(4)](-) is soluble and makes a monospecific solution of [Fe(4)(OH)(2)(hpdta)(2)(H(2)O)(4)](0)(aq) in acidic solutions (hpdta = 2-hydroxypropane-1,3-diamino-N,N,N',N'-tetraacetate). This tetramer is a diprotic acid with pK(a)(1) estimated at 5.7 ± 0.2 and pK(a)(2) = 8.8(5) ± 0.2. In the pH region below pK(a)(1), the molecule is stable in solution and (17)O NMR line widths can be interpreted using the Swift-Connick equations to acquire rates of ligand substitution at the four isolated bound water sites. Averaging five measurements at pH < 5, where contribution from the less-reactive conjugate base are minimal, we estimate: k(ex)(298) = 8.1 (±2.6) × 10(5) s(-1), ΔH(++) = 46 (±4.6) kJ mol(-1), ΔS(++) = 22 (±18) J mol(-1) K(-1), and ΔV(++) = +1.85 (±0.2) cm(3) mol(-1) for waters bound to the fully protonated, neutral molecule. Regressing the experimental rate coefficients versus 1/[H(+)] to account for the small pH variation in rate yields a similar value of k(ex)(298) = 8.3 (±0.8) × 10(5) s(-1). These rates are ∼10(4) times faster than those of the [Fe(OH(2))(6)](3+) ion (k(ex)(298) = 1.6 × 10(2) s(-1)) but are about an order of magnitude slower than other studied aminocarboxylate complexes, although these complexes have seven-coordinated Fe(III), not six as in the [Fe(4)(OH)(2)(hpdta)(2)(H(2)O)(4)](0)(aq) molecule. As pH approaches pK(a1), the rates decrease and a compensatory relation is evident between the experimental ΔH(++) and ΔS(++) values. Such variation cannot be caused by enthalpy from the deprotonation reaction and is not well understood. A correlation between <Fe(III)-OH(2)> bond lengths and the logarithm of k(ex)(298) is geochemically important because it could be used to estimate rate coefficients for geochemical materials for which only DFT calculations are possible. This molecule is the only neutral, oxo-bridged Fe(III) multimer for which rate data are available.

Site selectivity and reversibility in the reactions of titanium hydrazides with Si–H, Si–X, C–X and H+ reagents: TiNα 1,2-silane addition, Nβ alkylation, Nα protonation and σ-bond metathesis

We report a combined experimental and computational comparative study of the reactions of the homologous titanium dialkyl- and diphenylhydrazido and imido compounds Cp*Ti{MeC(N(i)Pr)(2)}(NNR(2)) (R = Me (1) or Ph (2)) and Cp*Ti{MeC(N(i)Pr)(2)}(NTol) (3) with silanes, halosilanes, alkyl halides and [Et(3)NH][BPh(4)]. Compound 1 underwent reversible Si-H 1,2-addition to Ti=N(α) with RSiH(3) (experimental ΔH ca. -17 kcal mol(-1)), and irreversible addition with PhSiH(2)X (X = Cl, Br). DFT found that the reaction products and certain intermediates were stabilised by β-NMe(2) coordination to titanium. The Ti-D bond in Cp*Ti{MeC(N(i)Pr)(2)}(D){N(NMe(2))SiD(2)Ph} underwent σ-bond metathesis with BuSiH(3) and H(2). Compound 1 reacted with RR'SiCl(2) at N(α) to transfer both Cl atoms to Ti; 2 underwent a similar reaction. Compound 3 did not react with RSiH(3) or alkyl halides but formed unstable Ti=N(α) 1,2-addition or N(α) protonation products with PhSiH(2)X or [Et(3)NH][BPh(4)]. Compound 1 underwent exclusive alkylation at N(β) with RCH(2)X (R = H, Me or Ph; X = Br or I) whereas protonation using [Et(3)NH][BPh(4)] occurred at N(α). DFT studies found that in all cases electrophile addition to N(α) (with or without NMe(2) chelation) was thermodynamically favoured compared to addition to N(β).

Temperature and Length Scale Dependence of Tetraalkylammonium Ion Solvation in Water, Formamide, and Ethylene Glycol

This paper focuses on the temperature and length scale dependence of solvation of familiar hydrophobic solutes: tetraalkylammonium ions in highly associated solvents with H-bond networks. Standard enthalpies and heat capacities of solution of symmetrical tetraalkylammonium bromides in formamide (FA) and ethylene glycol (C(2)H(4)(OH)(2)) have been computed using experimental ΔH(m)(sol) values and compared with previously reported enthalpies and heat capacities in pure water. Heat capacities of hydration are found to be large, positive, and increase almost linearly up to tetrapentylammonium bromide. Hydration of tetrahexyl- and tetraheptylammonium bromides is accompanied by much smaller heat capacity changes than would be expected from the length scale dependence observed. The same behavior reveals enthalpies and heat capacities of solute transfer from FA and C(2)H(4)(OH)(2) to water. The results obtained rather indicate that solvation of large tetraalkylammonium ions containing more than twenty carbon atoms is accompanied by much less water orientational ordering than for solutes of a moderate size. It results in smaller fluctuations in the water-water pair energy in a hydration shell and consequently reduces the heat capacity of hydration. The comparison of tetraalkylammonium ion behavior in water and the nonaqueous solvents studied shows that the length scale dependence of solvophobic solvation is strongly pronounced in water but not in nonaqueous media.

Local protein backbone folds determined by calculated NMR chemical shifts

NMR chemical shifts (CSs: δN(NH), δC(α), δC(β), δC', δH(NH), and δH(α)) were computed for the amino acid backbone conformers (α(L), β(L), γ(L), δ(L), ε(L), α(D), γ(D), δ(D), and ε(D) [Perczel et al., J Am Chem Soc 1991, 113, 6256]) modeled by oligoalanine structures. Topological differences of the extended fold were investigated on single β-strands, hairpins with type I and II β-turns, as well as double- and triple-stranded β-sheet models. The so-called "capping effect" was analyzed: residues at the termini of a homoconformer sequence unit usually have different CSs than the central residues of an adequately long homoconformer model. In heteroconformer sequences capping effect ruins the direct applicability of several chemical shift types (δH(NH), δC', and δN(NH)) for backbone structure determination of the parent residue. Experimental δH(α), δC(α), and δC(β) values retrieved from protein database are in good agreement with the relevant computed data in the case of the common backbone conformers (α(L), β(L), γ(L), and ε(L)), even though neighboring residue effects were not accounted for. Experimental and computed ΔδH(α)-ΔδC(α), ΔδH(α)-ΔδC(β), and ΔδC(α)-ΔδC(β) maps give qualitatively the same picture, that is, the positions of the backbone conformers relative to each other are very similar. This indicates that the H(α), C(α), and C(β) chemical shifts of alanine depend considerably on the backbone fold of the parent residue also in proteins. We provide tabulated CSs of the chiral amino acids that may predict the various structures of the residues.

Hydration of Mononucleotides

The sequential addition of water molecules to protonated and deprotonated forms of the four mononucleotides dAMP, dCMP, dGMP, and dTMP was studied experimentally by equilibrium measurements using an electrospray mass spectrometer equipped with a drift cell and theoretically by computational methods including molecular modeling and density functional theory calculations. Experiments were carried out in positive and negative ion mode, and calculations included the protonated and deprotonated forms of the four nucleotides. For deprotonated anionic nucleotides the experimental enthalpies of hydration (DeltaH degrees n) were found to be similar for all four systems and varied between -10.1 and -11.5 kcal mol-1 for the first water molecule (n = 1) and -8.3 and -9.6 kcal mol-1 for additional water molecules (n = 2-4). Theory indicated that the first water molecule binds to the charge-carrying phosphate group. Simulations of deprotonated mononucleotides with four water molecules yielded a large number of structures with similar energies. In some of the structures all four water molecules cluster around the phosphate group, and in other structures the four water molecules each hydrate a different functional group of the nucleotide. These include the phosphate group, the deoxyribose hydroxyl group, and various functional groups on the nucleobases. Experimental DeltaH degrees 1 values for the protonated cationic mononucleotides ranged from -10.5 to -13.5 kcal mol-1 with more negative values (< or =-12 kcal mol-1) for dCMP, dGMP, and dTMP and the least negative value for dAMP. For n = 2-4 DeltaH degrees n values varied from -6.9 to -9.7 kcal/mol and were similar in value to the deprotonated nucleotides except for dAMP. Theory on the protonated nucleotides indicated that the first water molecule binds to the charge-carrying group for dCMP, dGMP, and dTMP. For protonated dAMP, on the other hand, the charge-carrying N3 group is well self-solvated by the phosphate group and not readily available for a hydrogen bond with the water molecule. The insight gained on nucleotide stabilization by individual water molecules is used to discuss the competition between hydration of individual nucleotides and Watson-Crick base pairing.

C≡H ⋅s O Hydrogen Bonds in Small Ring Carbonyl Compounds: Vibrational Spectroscopy and Ab initio Calculations

The effect of the molecular structure on the properties of C-H⋅s O bonds was investigated in a set of small cyclic carbonyl compounds, using vibrational spectroscopy and ab initio calculations. Two main effects were studied: the size of the ring and the inclusion of oxygen atoms in the ring. The analysis of the band profile of the carbonyl stretching mode reveals the presence of single dimerization equilibria in the series cyclobutanone-cyclopentanone-cyclohexanone. The smallest cyclobutanone was found to have the lowest dimerization energy, with an experimental ΔH value of 3.7± 0.6 kJ mol−1. The systems with oxygen atoms in the ring, γ-butyrolactone and ethylene carbonate were found to present more complex equilibria. Ab initio calculations suggest that this complexity is due to the presence of additional dimer structures and higher oligomers.

Solid‐state amidization and imidization reactions in vapor‐deposited poly(amic acid)

AbstractThe condensation polymerization of 4,4′‐oxydianiline with pyromellitic dianhydride for the formation of poly(amic acid) and the subsequent imidization for the formation of polyimides were investigated for films prepared with vapor‐deposition polymerization techniques. Fourier transform infrared spectroscopy, thermal analysis, and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry of films at different temperatures indicated that additional solid‐state polymerization occurred before imidization. The experiments revealed that, upon vapor deposition, poly(amic acid) oligomers formed that had a number‐average molecular weight of about 1500 Da. Between 100–130 °C, these chains underwent an additional condensation reaction and formed slightly higher molecular weight oligomers. Calorimetry measurements showed that this reaction was exothermic [enthalpy of reaction (ΔH) ∼ −30 J/g] and had an activation energy of about 120 kJ/mol. The experimental ΔH values were compared with results from ab initio molecular modeling calculations to estimate the number of amide groups formed. At higher temperatures (150–300 °C), the imidization of amide linkages occurred as an endothermic reaction (ΔH ∼ +120 J/g) with an activation energy of about 130 kJ/mol. The solid‐state kinetics depended on the reaction conversion as well as the processing conditions used to deposit the films. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5999–6010, 2004

Raman spectroscopy, surface‐enhanced Raman spectroscopy and density functional theory studies of 2‐formylfuran

AbstractThe Raman spectrum of 2‐formylfuran in the liquid phase shows pairs of bands whose temperature and solvent dependence behavior can be interpreted in terms of cis–trans conformational equilibrium. Density functional theory (DFT) calculations performed at the BPW91/6–311+G* and B3LYP/6–311+G* theoretical levels show that the trans‐isomer is more stable than the cis‐isomer by 2.81 and 3.12 kJ mol−1, respectively. The theoretical ΔH values obtained at these theoretical levels [2.99 kJ mol−1 (BPW91) and 3.2 kJ mol−1 (B3LYP)] are very close to the experimental value ΔH = 2.52 kJ mol−1 determined from the plots of logarithmic relative intensities of the band pairs 1673/1692, 1477/1465 and 1396/1371 cm−1 of these isomers against reciprocal temperature. The surface‐enhanced Raman spectrum of 2‐formylfuran in silver colloid suggests the chemisorption of this molecular species on the silver surface both through the ring oxygen and the oxygen atom of the substituent group, the cis‐form being preferred in the adsorption state. The adsorbed molecules are oriented perpendicular or least tilted with respect to the silver surface. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Structural transformations and phase transitions in aqueous solutions of gadolinium nitrate in the course of freezing

The microstructure of Gd(NO3)3 aqueous solutions in the course of cooling at different rates is investigated using electron paramagnetic resonance. The local concentrations of Gd3+ ions in microregions are estimated, the concentration boundary at which the solution is completely vitrified throughout its volume is revealed, and the temperature of vitrification of liquid microregions (eutectic temperature) and the eutectic composition are determined. It is found that the adsorption of Gd3+ aqua ions on the capillary surface affects the crystallization of water and the process of concentration of the solute. It is demonstrated that the anomalous experimental dependence δH(C0), which is observed for vitrified water-glycerol solutions with a uniform distribution of paramagnetic centers over the sample, agrees well with the theoretical curve in the case when allowances are made not only for the dipole-dipole interactions but also for the inhomogeneous broadening of the EPR lines in the spectra with an incompletely resolved fine structure.

Density Functional Investigation of Hydrated V(II) and V(III) Ions: Influence of the Second Coordination Sphere; Water Exchange Mechanism

Two aspects of the aqueous chemistry of [V(H2O)6]2+/3+ have been studied at the B3LYP level of theory. The first one concerns the role of an explicit second coordination sphere of 12 water molecules on the structures of hexahydrated V(II) and V(III) ions. For the two cations, the most stable conformers correspond to a wheel-like structure in which the second hydration shell can be viewed as being constituted of three open quadrimers, each one being connected by one hydrogen bond to the other two and to three ligands of the first sphere through four additional hydrogen bonds. The [V(H2O)6]3+ subunit in this conformer is found to mimic the arrangement observed in [V(H2O)6][H5O2](CF3SO3)4 (Cotton, F. A; Fair, C. K.; Lewis, G. E.; Mott, G. N.; Ross, F. K.; Schultz, A. J.; Williams, J. M. J. Am. Chem. Soc. 1984, 106, 5319). The others conformers investigated, for which the second hydration shell consist of four noninteracting cyclic trimers, were calculated to be 15.6 kcal/mol (V3+) and 13.6 kcal/mol (V2+) higher in energy at the in vacuo level. These differences were increased to 22.8 kcal/mol (V3+) and 18.5 kcal/mol (V2+) when taking into account the bulk solvent using a polarizable continuum model. For the water-exchange mechanisms, we find that, for both cations, the D mechanism is the preferred one at the gas-phase level using density functional theory calculations. The effect of the solvent on the activation barriers (ΔE⧧) is emphasized because it induces inversions in the A−/D− and I−/D− reaction-path orderings. On the basis of the calculated ΔE⧧ values in solution, the water-exchange mechanism is expected to proceed via a limiting A pathway and an I-activation process for V3+ and V2+, respectively. The differences between the experimental ΔH⧧ values and the computed activation barriers were found to be 0.6 kcal/mol and −2.6 kcal/mol, respectively. The effect of the hydration energy on the stability and, hence, the lifetime of the various intermediates is also discussed.

Structural energetics of MgADP binding to the isolated β subunit of F 1-ATPase from thermophilic Bacillus PS3

The energetics of binding of MgADP to the isolated β subunit of F 1-ATPase from thermophilic Bacillus (Tβ) was characterized by high-precision isothermal titration calorimetry. The reaction was enthalpically driven, with a ΔCp of −36 cal ( mol K ) −1 . To gain insight into the molecular basis of this small ΔCp, we analyzed the changes in accessible surface areas ( ΔASA) between the structures of empty and MgADP-filled β subunits, extracted from the crystal structure of bovine heart F 1. Consistent with the experimental ΔCp, the ΔASA was small ( −775 A ̊ 2 ). We used a reported surface area model developed for protein reactions to calculate ΔCp and ΔH from ΔASA, obtaining good agreement with the experimental values. Conversely, using the same model, a ΔASA of −770 A ̊ 2 was estimated from experimental ΔCp and ΔH for the Tβ–MgADP complex. Our structural–energetic study indicates that on MgADP binding the isolated Tβ subunit exhibits intrinsic structural changes similar to those observed in F 1.

Tautomerism and Dimerization of Acetamidothiazole Derivatives − UV/Vis and NMR Spectroscopic Investigation

Amido (A)/imido (B) tautomerism has been investigated by UV/Vis and NMR spectroscopic methods for a number of 2-acetamidothiazoles and 2-acetamidobenzothiazoles, without the use of the physical properties of “fixed parents”. The addition of polar substances to solutions of selected compounds in apolar solvents (carbon tetrachloride, dichloromethane) strongly affects the [B]/[A] ratios. Data show that the shift of the tautomeric equilibrium A/B towards the B form has two main causes: (i) increase of the polarity of the medium, and (ii) a base effect on the stabilization of the B form. The experimental ΔH and ΔS values indicate (in agreement with 1H NMR spectroscopic data) that the self-assembly of 2-acetamido derivatives is a very important factor in determining the position of the tautomeric equilibrium.

Estimating the Activation Energy for Hydrogen Abstraction Reactions by a Combination of Semiempirical Calculation and Family Correlation

A semiempirical method, the PM3-family-correlation (PM3-FC) method, has been developed to estimate the activation energies for hydrogen abstraction reactions between hydrocarbon radicals and hydrogen donors. The method combines semiempirical PM3 calculations of transition state enthalpies [ΔH°f(TS)] with family correlations between the PM3-calculated and experimental ΔH°f(TS) values on the basis of regression analysis. For a test set of 40 hydrogen abstraction reactions, including alkyl + alkyl−H, alkyl + allylalky−Hα/arylalkyl−Hα, and benzyl + allylalkyl−Hα/arylalkyl−Hα, it is found that the deviations between the PM3-calculated and experimental ΔH°f(TS) values are systematic and dependent on the transition state structures. The structurally homologous transition states show excellent linear correlation. Using the obtained linear regression parameters to scale the PM3-calculated ΔH°f(TS) values leads to a very significant increase in estimation accuracy. The activation energies for the whole test set are...