Reliability of association constants of 1:1 molecular complexes from spectrophotometric data

Some basic factors affecting the reliability of K (association constant) and ? (molecular extinction coefficient) estimates for 1:1 molecular complexes are studied. To accomplish this, the error matrix is examined and a new variable G=K^?1((a+b+K^?1)2?4ab)^{?$\text{1}\text{2}$} (where a and b are the concentrations of the reagents) is introduced. Whenever G has the same value for all the experimental points, K and ? are undetermined. A great dispersion of G values, on the contrary, contributes to more reliable estimates. The variable G has in fact the same role as the independent variable in a linear relationship; therefore the problem of the optimal choice of experimental points is reduced to one of a simple linear regression.     

The calculation of stability constants of weak complexes from spectrophotometric data

Formation constants, K_ML, of weak 1:1 complexes can be calculated from spectrophotometric data by systematically varying the value assumed for ϵ_ML, the experimentally inaccessible value for the molecular extinction coefficient of the complex, until the standard deviation in the calculated average value of K_ML for all the data points is minimised. The conditions and procedure leading to a most convenient and rapid computer-based solution are discussed and illustrated by data for complexes of bis(acetylacetonato) copper(II) with quinoline and isoquinoline.The results are compared with those of a second procedure in which assumed values of K_ML are varied systematically until the sum of the squares of the deviations between calculated and experimental values of absorbancies has been minimised.     

SPECA: a program for the calculation of thermodynamic equilibrium constants from spectrophotometric data

A computer program called SPECA and written in compiled BASIC (Microsoft QuickBasic Ver. 4.5) has been written for the calculation of thermodynamic constants in solution equilibria studies from spectrophotometric measurements. In order to calculate the thermodynamic constants it is necessary to introduce different theories to estimate the activity coefficients of the system involved under investigation. In this version of the program the equations proposed in the models of Debye-Hückel. Specific Interaction Theory (SIT) or the Modified's Bromley Theory can be chosen. To achieve its purpose, the program uses absorbance and activity or free concentration data of one of the components, normally H(+), and it is not limited with respect to the number of components that can be studied. Minimization of the error square sum in the absorbance, which can be absolute or relative, is carried out using the Levenberg-Marquardt-Nash algorithm at three different levels: equilibrium constants in the first level, molar absorptivities in the second and interaction coefficients in the third for those systems using the SIT or the Modified Bromley's Theories.     

A program for evaluating equilibrium constants from spectrophotometric data by non-linear regression analysis

A new program, CFTSP, is described for computing stability constants and molar absorptivities from data on the absorbances of mixtures of a metal ion with a ligand. It is written in FORTRAN 77, can run on personal computers, and has facilities for interactive data analysis and presentation that ease the operator's task in searching for the equilibrium model that best fits the experimental data. A critical evaluation of the performance of the program is presented, and some general criteria for selecting equilibrium parameters, making use of both synthetic and experimental data, are described.     

Determination of molecular constants of scandium triiodide monomer and dimer from electron diffraction data

The molecular intensity function of a ScI₃+SC₂I₆ mixture is solved using gas-phase electron diffractometry and taking into account the theoretical values of l(Sc-I). The symmetry of geometrical configuration, the internuclear distances, and vibration frequencies of the scandium triiodide monomer and dimer are determined. Translated fromZhurnal Struktumoi Khimii, Vol. 38, No. 3, pp. 489–494, May–June, 1997.     

Thermodynamic ionization constants of p-halogenophenols from spectrophotometric measurements

Standard free energies, enthalpies, and entropies of ionization of phenol, p-chlorophenol, p-bromophenol and p-iodophenol in aqueous solution have been determined from spectrophotometric measurements within the temperature range 5–60°C. Difficulties that arise in data-processing of the results have been discussed and a re-assessment of previous values of ΔC_p has also been made.     

Large shift and small broadening of Br2 valence band upon dimer formation with H2O: an ab initio study

Valence electronic excitation spectra are calculated for the H(2)O···Br(2) complex using highly correlated ab initio potentials for both the ground and the valence electronic excited states and a 2-D approximation for vibrational motion. Due to the strong interaction between the O-Br and the Br-Br stretching motions, inclusion of these vibrations is the minimum necessary for the spectrum calculation. A basis set calculation is performed to determine the vibrational wave functions for the ground electronic state and a wave packet simulation is conducted for the nuclear dynamics on the excited state surfaces. The effects of both the spin-orbit interaction and temperature on the spectra are explored. The interaction of Br(2) with a single water molecule induces nearly as large a shift in the spectrum as is observed for an aqueous solution. In contrast, complex formation has a remarkably small effect on the T = 0 K width of the valence bands due to the fast dissociation of the dihalogen bond upon excitation. We therefore conclude that the widths of the spectra in aqueous solution are mostly due to inhomogeneous broadening.     

Model hysteresis dimer molecule I: Equilibrium properties

A Hamiltonian system describing hysteresis behavior in a dimeric chemical reaction is modeled in a MD simulation utilizing novel two-body potentials with switches that is particularly suitable for numerical thermodynamical investigations. It is surmized that such reaction mechanisms could exist in nature on the basis of recent experiments, which indicate that electromagnetic hysteresis behavior is exhibited at the molecular level, although experimental interpretations tend to construct models that avoid such mechanisms. Numerical results of various common equilibrium thermodynamical and kinetic properties are presented together with new algorithms that were implemented to compute these quantities, where no unusual thermodynamics was observed for the chemical reaction which might be interpreted as not being “time reversible invariant” and therefore susceptible to manifesting unusual thermodynamical phenomena, which might contradict any of the known laws of thermodynamics. A revision of the concept of “time reversibility” to accommodate the above results is suggested. The general design of the reaction mechanism also allows for the use of conventional potentials and by the utilization of switches, overcomes the bottleneck of computations which involves multi-body interactions. The initial theoretical, program and algorithm development was done at Norwegian University of Science and Technology-NTNU, Institute of Physical Chemistry, (Trondheim, Norway) during a sabbatical visit 2000–2001 financed by University of Malaya. The method of MD used here conforms to the periodic boundary conditions and thermostatting algorithms developed or refined by Ikeshoji and Hafskjold for standard, non-reacting particles. Their approach is non-synthetic using traditional Verlet integration of Newton’s equations of motion.     

Equilibrium constants for the formation of Cu(I) halide complexes

The stoichiometric equilibrium constants, K ₃ ^* , for the formation of CuX ₃ ²⁻ from CuX ₂ ⁻ +X⁻ where X=Cl and Br, have been determined from spectral measurements. The measurements were made in NaCl and NaBr solutions from I=0.5 to 6.0m at 5, 25 and 45°C. The measured constants were extrapolated to infinite dilution using the Pitzer equations. The Pitzer parameters, β⁰, β¹ and C^φ, for the interaction of Na⁺ with CuX ₂ ⁻ and CuX ₃ ²⁻ are briefly discussed.     

Equilibrium constants and center of formation of H-complexes of hydrazones with p-nitrophenol

Conclusions The center of formation of H-complexes of hydrazones with p-nitrophenol is the imine nitrogen atom. This direction of interaction is determined by the potential of the electrostatic field. Experimental values of the complex formation constants vary in proportion to the calculated values of the charge on the imine nitrogen atom.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2493–2497, November, 1981.The authors wish to thank T. N. Pylaeva and L. V. Ermolaeva for assistance in synthesis of the hydrazones and for their discussion of the results.     

Miniquad-A general computer programme for the computation of formation constants from potentiometric data

A new computer programme for the computation of formation constants of complex species in solution is presented. The programme can be applied to all kinds of potentiometric titration data, including multi-reactant and multi-electrode systems. A statistical analysis is made of the results in order to assess their validity, and to assist in hypothesis testing.     

Complementary face-to-face dimer formation from meso-aryl subporphyrins bearing a 2-carboxyphenyl group

meso-Aryl-substituted subporphyrins bearing a 2-carboxyphenyl group were prepared and shown to form complementary face-to-face dimers upon azeotropic removal of methanol.     

A new algorithm for the determination of protolytic constants from spectrophotometric data in multiwavelength mode: Calculations of acidity constants of 4-(2-pyridylazo)resorcinol (PAR) in mixed nonaqueous-water solvents

A new efficient, simple and versatile algorithm is presented for determination of the protolytic constants from spectrophotmetric data in multiwavelength mode based on the combining of hard and soft modeling. The algorithm was checked by determining the acidity constants of a triprotic acid from theoretical and real absorption-pH data. The real spectral data are obtained from photometric titration of different solutions of 4-(2-pyridylazo)resorcinol (PAR) by a standard base solution under an inert atmosphere. The algorithm starts the minimization process using an user supplied number of components and initial guesses of the unknown parameters and refined in a least squares manner. New algorithm is implemented in the new version of DATAN package (version 3.1). The validity of the obtained results was checked by some well known programs such as DATAN 2.1, SPECFIT/32, SQUAD, a modified version of difference spectra and a A-pH curve method. The comparison of the outputs of the DATAN 3.1 with the other programs reveals that there is a very good agreement between the obtained results and mentioned programs.     

An improved computer program for calculating formation constants of ligand complexes from pH data

Several improvements are made to available computer programs for calculating formation constants from pH data. The most important features are wide portability of program among computers, ability to optimize any parameter in the model equations and to fix any parameters (including stepwise formation constants) to a constant, and options to allow for variations in thermodynamic activity of ions and to fit pH after fitting the volume of added reagent. The latter feature was used with synthetic data to explore the effects of random errors in either reagent volume or pH or both. Whereas fits to reagent volume will usually be more accurate for systems with pK_a values near 7, this is not the case for systems with pK_a values which differ by > 2.0 from 7.     

Thermodynamic constants of ionization processes by spectrophotometric methods: Meta-halogenophenols

Thermodynamic ionization constants over the temperature range 10–60° together with values for the standard free energy, enthalpy and entropy of ionization of m-chlorophenol, m-bromophenol and m-iodophenol are reported. These results in conjunction with an extensive collection of similar results from the literature show that substituent effects in the ionization of phenols are due primarily to entropy changes possibly involving a change of solvation state. In contrast to the free energies of ionization the entropies are well correlated by σⁿ values.     

Computation of acidity constants of a polyprotic acid from nuclear magnetic resonance or u.v.-visible spectrophotometric data

A least-squares program in BASIC is presented for the rigorous calculation of the equilibrium parameters in the dissociation of a polyprotic acid H_qA and of the spectra of all the individual species at equilibrium. The behaviour of the program is discussed for a simulated case involving n.m.r, and for a spectrophotometric study of eriochrome black T.     

Calculation of stability constants from photometric data

A method originally designed for the determination of stability constants by using a wedge calorimeter has been adapted to normal photometric measuring techniques. The calculations are simple and the method is especially suitable for use when the absorptivities of the absorbing species are unavailable because these species cannot be obtained alone in solution. Results of some determinations are given.     

Equilibrium constants in reactions of 2-aminoethanol and ammonia with isophthalaldehyde and terephthalaldehyde

Equilibria established in reactions between isophthalaldehyde (IPA) and terephthalaldehyde (TPA) on one side and 2-aminoethanol and NH3 on the other side were followed in solutions, where these amines acted both as reagents and as buffers. The equilibrium between TPA and 2-aminoethanol was followed spectrophotometrically; all others, of reactions of TPA with NH3 and of IPA with both NH3 and H2N-CH2CH2-OH, were followed by using polarography. Separate limiting reduction currents of the dialdehyde and its imine enable simultaneous determination of the starting material and the product. The equilibria are shifted more in favor of the imine in reactions of TPA than those of IPA. Equilibrium constants for reactions of both TPA and IPA with 2-aminoethanol are 3 orders of magnitude higher than those observed for the reactions of dialdehydes with NH3, even when the pKa values of these two reagents differ only slightly.     

Multiparametric curve fitting-XI: POLET computer program for estimation of formation constants and stoichiometric indices from normalized potentiometric data

The FORTRAN computer program POLET(84) analyses a set of normalized potentiometric titration curves to find a chemical model, i.e., the number of species present and their stoichiometry, and to determine the corresponding stability constants log beta(pqrs) and unknown stoichiometric indices p, q, r, and s of up to quaternary M(p)L(q)Y(r)H(s) complexes. The program belongs to the ABLET family, based on the LETAG subroutine, and can use an algorithmic and/or heuristic minimization strategy, or a beneficial combination of both. The data, a set of potentiometric titration curves plotted as volume and potential, are converted into normalized variables (formation function, pH) and then a computer-assisted search for a chemical model by POLET(84) is applied. The procedure for efficient application of POLET(84) in an equilibrium analysis is described and the program is validated by use of literature and simulated data. The reliability of the chemical model and its parameters is established by the degree-of-fit achieved, and the closeness of the stoichiometric indices to integral values.