Hydration energies of sodiated amino acids from gas-phase equilibria determinations

The sequential hydration of a number of sodiated amino acids is investigated using a high-pressure mass spectrometer. Ions produced continuously by electrospray are injected into the reaction chamber in the pulsed mode where the hydration equilibria, AANa+(H2O)n-1+H2O=AANa+(H2O)n (AA=Val, Pro, Met, Phe, and Gln), and the temperature dependence of the equilibrium constants are measured in the gas phase at 10 mbar (N2 bath gas and known pressure of H2O). The thermochemical properties, DeltaH degrees n, DeltaS degrees n, and DeltaG degrees n, for the hydrated systems are determined and discussed in conjunction with the structural forms. The results show that the binding energies of water to the AANa+ complexes decrease with the increasing number of water molecules. The present results from equilibrium measurements are compared to those from earlier studies obtained by other techniques. A correlation between the free energy changes for the addition of the first and second water molecules to AANa+, and the corresponding sodium ion affinities, is observed. Generally, the hydration free energy becomes weaker as the AA-Na+ bond strength increases.     

Thermodynamic ionization energies and charge transfer reactions in benzene and substituted benzenes

Ionization energies of 11 substituted benzenes of CS₂ related to the ionization energy of benzene were obtained by measurements of the charge exchange equilibrium constants for C₆H₅X⁺ + C₆H₅Y ? C₆H₅Y⁺ + C₆H₅X at 450 and K. Thermodynamic ionization energies of substituted benzenes, related to that of benzene, are found to be higher by 0.5–2.0 kcal/mole than the corresponding photoionization (0—0) values. Exothermic charge transfer reactions between substituted benzenes are found to proceed with rate constants of (1.3–1.6) × 10^?9 cm³/mol s, which agree well with calculated collision rates.     

Analysis of variance applied to determinations of equilibrium constants

The statistical analysis of variance has been applied to the values of the equilibrium constants of the glycinate-proton and glycinate-nickel systems, determined in different laboratories by pH-titration in aqueous solution. The analysis shows how the main part of the error derives from the variability from one titration to another even in the same laboratory. Therefore the data for a single titration (k) must be processed separately, thus yielding a mean value for the equilibrium constant logbeta (pqr)(k) of the species M(p)H(q)L(r); from these mean values for different titrations in each laboratory l, a within-laboratory grand average, logbeta (pqr)(l), can be calculated; the variance of this grand average measures the experimental error. A further analysis of the data from the different participating laboratories shows that there were no significant differences between laboratories for the constants reported. From these results it can be inferred that all the values of the mean constants logbeta (pqr)(k) for one species, as determined separately for each titration in four laboratories, belong to the same population. A chi(2) analysis of these populations demonstrates that the stability constants of the species HL, H(2)L(+), NiL(+), NiL(2) (with L(-) = glycinate) are normally distributed, but not that for NiL(-)(3). Therefore, general mean values of the first four constants can be calculated and proposed as reliable standard values at 25 degrees and I = 1.0M Na(Cl): protonation of glycinate, log beta(011) = 9.651(12), log beta(021) = 12.071(26); nickel-glycinate complexes, log beta(101) = 5.615(35), log beta(102) = 10.363(62). These values indicate that the standard deviations are rather higher than those often reported in the literature.     

Determination of equilibrium constants for atom transfer radical polymerization

Atom transfer radical polymerization (ATRP) equilibrium constants (K(ATRP)) were determined using modified Fischer's equations for the persistent radical effect. The original Fischer's equations could be used only for low conversion of Cu(I) to X-Cu(II) and consequently for relatively low values of K(ATRP). At higher conversion to X-Cu(II) (>10%) and for larger values of K(ATRP) (>10(-)(7)), modified equations that take into account the changes in catalyst and initiator concentrations should be used. The validity of new equations was confirmed by detailed kinetic simulations. UV-vis spectrometric and GC measurements were used to follow the evolution of X-Cu(II) species and the initiator concentration, respectively, and to successfully determine values of K(ATRP) for several catalysts and alkyl halides. The effect of structure on reactivities of ATRP components is presented.     

Relative cross-sections of charge transfer and ionization processes between multiply-charged ions and He at low and intermediate energies

The relative cross-sections of double (DI) to single ionization (SI) σ_DI/σ_SI, transfer ionization (TI) to single capture (SC) σ_TI/σ_SC and double capture (DC) to single capture σ_DC/σ_SC of helium by multiply-charged ions A^q+ (q = 2–5) at low and intermediate energies are calculated by considering the distribution of target electron and the interaction time between the projectile and the target electron. The calculated results are compared with published experimental data by our group and others.     

Hartree-Fock calculation of cohesive energies and equilibrium lattice constants for solid Li and Be

A method is described for calculating cohesive energies of solids in the single-determinant approximation including the full Hartree-Fock exchange. The method involves (1) the construction of a rapidly convergent series in vectors of the direct and reciprocal lattice for the Fock matrix, (2) a decoupling procedure for the k -dependence of the Fock matrix, which works even in the case of strong interatomic overlap. An application to Li and Be is given. Agreement with experiment to 10% is achieved for the cohesive energies and to 5% for the equilibrium lattice constants.     

Comparison of ribonucleic acid homopolymer ionization energies and charge injection barriers

Thin films of guanosine and uridine ribonucleic acid (RNA) homopolymers (poly rG, poly rU) were grown in high vacuum in several steps on highly oriented pyrolytic graphite (HOPG) using electrospray deposition. Between deposition steps, the sample surface was characterized with X-ray and ultraviolet photoemission spectroscopy (XPS, UPS). The resulting spectra series allowed the determination of the orbital alignment at the HOPG interface, as well as the ionization energies of the homopolymer thin films. Comparison with earlier results on cytidine and adenosine RNA homopolymers (poly rC, poly rA) indicates significant ionization energy and charge injection barrier differences between purines and pyrimidines.     

溶剂对电荷转移复合物平衡常数的影响

研究了溶剂与电荷转移复合物平衡常数K_(CT)之间的关系,并建立了简略模型,选择N-乙基咔唑作为给体,分别与2,4,7-三硝基芴酮和2,4,5,7-四硝基芴酮作为受体进行复合,得到了两对电荷转移复合物。用VPO法测量了在不同溶剂中复合物的K’_(CT)。发现,K’_(CT)随溶剂偶极矩μs的减小而增大,InK'_(CT)和μ_s~2间存在着良好的线性关系。其关系可表达为InK'_(CT)=In K_(CT)—Bμ_s~2-C或InK'_(CT)=A—Bμ_s~2。     

On Slater's transition state for ionization energies

For theXα- and the Xαβ-exchange appropriation the transition state for concept for estimating ionzation energies is analyzed mainly in atomic systems. Comparisons with HF results and experimental data are presented to put the different theoretical approach in proper perspectives.     

Electron correlation as the driving force for charge transfer: charge migration following ionization in N-methyl acetamide

A hole charge created in a molecule, for instance, by ionization, can migrate through the system solely driven by electron correlation. The migration of a hole charge following ionization in N-methyl acetamide (a molecular system containing a peptide bond) is investigated. The initial hole charge is localized at one specific site of the molecule. Ab initio calculations show that nearly 90% of the hole migrates to a remote site of the molecule in 4.2 fs. This migration of charge is highly efficient and ultrafast. The underlying mechanism for this migration of a hole charge is identified and compared with a simple model.     

A versatile method for molecular structure determinations from ground state rotational constants

A new procedure is described to determine the geometrical structure of a molecule. It starts from measured ground state rotational constants of isotopically substituted species. Internal structural parameters such as bond lengths, bond angles and dihedral angles are directly fitted with a suitable least-squares algorithm. The new method for structure determination is compared to the usual Costain—Kraitchman substitution method. It contains less stringent conditions than the latter, permits a broad range of applications and provides a reliable molecular structure.     

Determining equilibrium constants for dimerization reactions from molecular dynamics simulations

With today's available computer power, free energy calculations from equilibrium molecular dynamics simulations "via counting" become feasible for an increasing number of reactions. An example is the dimerization reaction of transmembrane alpha-helices. If an extended simulation of the two helices covers sufficiently many dimerization and dissociation events, their binding free energy is readily derived from the fraction of time during which the two helices are observed in dimeric form. Exactly how the correct value for the free energy is to be calculated, however, is unclear, and indeed several different and contradictory approaches have been used. In particular, results obtained via Boltzmann statistics differ from those determined via the law of mass action. Here, we develop a theory that resolves this discrepancy. We show that for simulation systems containing two molecules, the dimerization free energy is given by a formula of the form ΔG ∝ ln(P(1) /P(0) ). Our theory is also applicable to high concentrations that typically have to be used in molecular dynamics simulations to keep the simulation system small, where the textbook dilute approximations fail. It also covers simulations with an arbitrary number of monomers and dimers and provides rigorous error estimates. Comparison with test simulations of a simple Lennard Jones system with various particle numbers as well as with reference free energy values obtained from radial distribution functions show full agreement for both binding free energies and dimerization statistics.     

Coulomb crystals of few particles: Closed-form expressions for equilibrium energies and geometries, and vibrational force constants

Analytical expressions for equilibrium energies and geometries, and the corresponding vibrational force constants are derived for clusters of between three and eight particles interacting through a pairwise potential of the form (r)=(1/3)(r²+2r^-1). In the case of five-, seven-, and eight-particle clusters, these expressions are functions of single parameters that are the larger real roots of respective twelfth-degree polynomials.     

Investigation of equilibria in solution. Determination of equilibrium constants with the HYPERQUAD suite of programs

A new suite of 10 programs concerned with equilibrium constants and solution equilibria is described. The suite includes data preparation programs, pretreatment programs, equilibrium constant refinement and post-run analysis. Data preparation is facilitated by a customized data editor. The pretreatment programs include manual trial and error data fitting, speciation diagrams, end-point determination, absorbance error determination, spectral baseline corrections, factor analysis and determination of molar absorbance spectra. Equilibrium constants can be determined from potentiometric data and/or spectrophotometric data. A new data structure is also described in which information on the model and on experimental measurements are kept in separate files.     

Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes

Reported herein are the hydrogen atom transfer (HAT) reactions of two closely related dicationic iron tris(alpha-diimine) complexes. FeII(H2bip) (iron(II) tris[2,2'-bi-1,4,5,6-tetrahydropyrimidine]diperchlorate) and FeII(H2bim) (iron(II) tris[2,2'-bi-2-imidazoline]diperchlorate) both transfer H* to TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) to yield the hydroxylamine, TEMPO-H, and the respective deprotonated iron(III) species, FeIII(Hbip) or FeIII(Hbim). The ground-state thermodynamic parameters in MeCN were determined for both systems using both static and kinetic measurements. For FeII(H2bip) + TEMPO, DeltaG degrees = -0.3 +/- 0.2 kcal mol-1, DeltaH degrees = -9.4 +/- 0.6 kcal mol-1, and DeltaS degrees = -30 +/- 2 cal mol-1 K-1. For FeII(H2bim) + TEMPO, DeltaG degrees = 5.0 +/- 0.2 kcal mol-1, DeltaH degrees = -4.1 +/- 0.9 kcal mol-1, and DeltaS degrees = -30 +/- 3 cal mol-1 K-1. The large entropy changes for these reactions, |TDeltaS degrees | = 9 kcal mol-1 at 298 K, are exceptions to the traditional assumption that DeltaS degrees approximately 0 for simple HAT reactions. Various studies indicate that hydrogen bonding, solvent effects, ion pairing, and iron spin equilibria do not make major contributions to the observed DeltaS degrees HAT. Instead, this effect arises primarily from changes in vibrational entropy upon oxidation of the iron center. Measurement of the electron-transfer half-reaction entropy, |DeltaS degrees Fe(H2bim)/ET| = 29 +/- 3 cal mol-1 K-1, is consistent with a vibrational origin. This conclusion is supported by UHF/6-31G* calculations on the simplified reaction [FeII(H2N=CHCH=NH2)2(H2bim)]2+...ONH2 left arrow over right arrow [FeII(H2N=CHCH=NH2)2(Hbim)]2+...HONH2. The discovery that DeltaS degrees HAT can deviate significantly from zero has important implications on the study of HAT and proton-coupled electron-transfer (PCET) reactions. For instance, these results indicate that free energies, rather than enthalpies, should be used to estimate the driving force for HAT when transition-metal centers are involved.     

Adiabatic and non-adiabatic corrections to rovibrational energies of diatomic molecules: variational calculations with experimental accuracy

Using the variational technique, eigensolutions of the radial Herman-Asgharian equation accounting for non-adiabatic terms are determined within the experimental accuracy of the high-resolution spectroscopy. This method, which is independent of the algebraic and numerical approaches currently used in the literature for a "direct-potential-fit" of diatomic rovibrational spectra, is shown to be useful for validation of available calculations and for resolving some controversial issues. Comparative discussions are reported in this paper for a dozen diatomic molecules.     

Polarization energies,transport gap and charge transfer states of organic molecular crystals

A self-consistent calculation of electronic polarization in organic molecular crystals and thin films is presented in terms of charge redistribution in nonoverlapping molecules in a lattice. The polarization energies P₊ and P₋ of a molecular cation and anion are found for anthracene and perelynetetracarboxylic dianhydride (PTCDA), together with binding energies of ion pairs and transport gaps of PTCDA films on metallic substrates. The 500 meV variation of P₊+P₋ with film thickness agrees with experiment, as do calculated dielectric tensors. Comparisons are made to submolecular calculations in crystals.     

A computational method for approximate evaluation of ionization constants from potentiometric data

A general equation is given for the evaluation of proton dissociation constants from potentiometric data. The method is applicable to monobasic acids, and also polybasic acids, where the successive dissociation steps may or may not overlap. A computer program was employed for the calculations. The method was tested by evaluation of the constants for 22 acids, from monobasic to octobasic, with satisfactory agreement with literature data.     

Determination of equilibrium constants from chromatographic and electrophoretic measurements

Chemical interactions, such as acid-base, complex-forming, ion association and other equilibria, are widely exploited to improve the separation efficiency in liquid chromatography as well as in electrophoresis. On the other hand, these techniques can be advantageously used to study the chemical equilibria affecting the separations. If the equilibium is sufficiently fast in comparison with the separation process, then the retention characteristics in chromatography (retention factors) or the migration characteristics in electrophoresis (effective mobilities) may be expressed as functions of the composition of mobile phase or background electrolyte (BGE), respectively. Using a proper experimental arrangement, the dependencies of retention (migration) characteristics on the mobile phase (background electrolyte) composition can be measured and utilized to calculate the equilibrium constants for equlibria taking place in the mobile phase (background electrolyte). Although principles of these measurements have been known for a long time, only more recent studies utilizing HPLC and capillary electrophoretic techniques are reviewed in this paper.