UV/vis, 1H, and 13C NMR spectroscopic studies to determine mangiferin pKa values

The acid constants of mangiferin (a natural xanthonoid) in aqueous solution were determined through an UV/vis spectroscopic study employing the SQUAD program as a computational tool. A NMR study complements the pK(a) values assignment and evidences a H-bridge presence on 1-C. The chemical model used was consistent with the experimental data obtained. The pK(a) values determined with this procedure were as follows: H(4)(MGF)=H(3)(MGF)(-)+H(+), pKa1 (6-H)=6.52+/-0.06; H(3)(MGF)(-)=H(2)(MGF)(2-)+H(+), pKa2 (3-H)=7.97+/-0.06; H(2)(MGF)(2-)=H(MGF)(3-)+H(+), pKa3 (7-H)=9.44+/-0.04; H(MGF)(3-)=(MGF)(4-)+H(+), pKa4 (1-H)=12.10+/-0.01; where it has been considered mangiferin C(19)H(18)O(11) as H(4)(MGF). Mangiferin UV/vis spectral behavior, stability study in aqueous solution as well as NMR spectroscopy studies: one-dimensional (1)H,(13)C, 2D correlated (1)H/(13)C performed by (g)-HSQC and (g)-HMBC methods; are also presented. pK(a) values determination of H(4)(MGF) in aqueous solution is a necessary contribution to subsequent pharmacokinetic study, and a step towards the understanding of its biological effects.     

Determination by fluorimetric titration of the ionization constants of ciprofloxacin in solution and in the presence of liposomes

A fluorescence titration method was applied for the determination of pKa of ciprofloxacin (CPX) in solution. Values of 6.18 +/- 0.05 and 8.76 +/- 0.03 were obtained for pKa1 and pKa2, respectively. The method was used to determine the ionization constants in the presence of liposomes of dipalmitoylphosphatidylcholine (DPPC) and DPPC with 10 mol% of dipalmitoylphosphatidylglycerol. A dependence on the surface charge of liposomes was found which supported the existence of a basic electrostatic interaction between CPX and the phospholipid bilayer. Both pK values for the N-4 butyl-piperazinyl derivative (BCPX) of the parent compound were also determined in solution and in the presence of liposomes. The competition of both drugs for the same binding site as 1-anilino-8-naphtalene sulfonate demonstrate that the interaction is governed by electrostatic forces.     

Kinetic and thermodynamic properties of the aminoxyl (NH2O*) radical

The product of one-electron oxidation of (or H-atom abstraction from) hydroxylamine is the H2NO* radical. H2NO* is a weak acid and deprotonates to form HNO-*; the pKa(H2NO*) value is 12.6+/-0.3. Irrespective of the protonation state, the second-order recombination of the aminoxyl radical yields N2 as the sole nitrogen-containing product. The following rate constants were determined: kr(2H2NO*)=1.4x10(8) M-1 s-1, kr(H2NO*+HNO-*)=2.5x10(9) M-1 s-1, and kr(2HNO-*)=4.5x10(8) M-1 s-1. The HNO-* radical reacts with O2 in an electron-transfer reaction to yield nitroxyl (HNO) and superoxide (O2-*), with a rate constant of ke(HNO-*+O2-->HNO+O2-*)=2.2x10(8) M-1 s-1. Both O2 and O2-* seem to react with deprotonated hydroxylamine (H2NO-) to set up an autoxidative chain reaction. However, closer analysis indicates that these reactions might not occur directly but are probably mediated by transition-metal ions, even in the presence of chelators, such as ethylenediamine tetraacetic acid (EDTA) or diethylenetriamine pentaacetic acid (DTPA). The following standard aqueous reduction potentials were derived: E degrees (H2NO*,2H+/H3NOH+)=1.25+/-0.01 V; E degrees (H2NO*,H+/H2NOH)=0.90+/-0.01 V; and E degrees (H2NO*/H2NO-)=0.09+/-0.01 V. In addition, we estimate the following: E degrees (H2NOH+*/H2NOH)=1.3+/-0.1 V, E degrees (HNO, H+/H2NO*)=0.52+/-0.05 V, and E degrees (HNO/HNO-*)=-0.22+/-0.05 V. From the data, we also estimate the gaseous O-H and N-H bond dissociation enthalpy (BDE) values in H2NOH, with BDE(H2NO-H)=75-77 kcal/mol and BDE(H-NHOH)=81-82 kcal/mol. These values are in good agreement with quantum chemical computations.     

Infrared and Raman spectra,conformational stability,vibrational assignment and ab initio calculations of methylvinyl silyl chloride

The infrared spectra (3200-50 cm(-1)) of gaseous and solid and Raman spectra (3200-10 cm(-1)) of the liquid and solid methylvinyl silyl chloride, CH(2)=CHSiH(CH(3))Cl, and the Si-d isotopomer have been recorded. The three expected stable conformers (the three different groups eclipsing the double bond) have been identified in the fluid phase, but it was not possible to obtain an annealed solid with a single conformer. Variable temperature (-105 to -150 degrees C) studies of the infrared spectra of the sample dissolved in liquid krypton has been carried out. From these data the enthalpy differences between the most stable conformer with the hydrogen atom (HE) eclipsing the double bond to that with the chlorine atom (ClE) and the methyl group (ME) eclipsing the double bond have been determined to be 17+/-4 cm(-1) (203+/-48 Jmol(-1)) and 80+/-12 cm(-1) (957+/-144 Jmol(-1)), respectively. However in the liquid state the ME conformer is the most stable form with enthalpy differences of 13+/-4 and 27+/-7 cm(-1) to the HE and ClE rotamers, respectively. It is estimated that there is 39% of the HE conformer, 35% of the ClE conformer, and 26% of the ME conformer present at ambient temperature. A complete vibration assignment is proposed for the HE conformer which is based on infrared band contours and group frequencies, which is supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G(d) calculations. Additionally, several of the fundamentals for the other two conformers have been assigned. The optimal geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies are reported for all three conformers from MP2/6-31G(d,p) ab initio calculations with full electron correlation. Optimized geometrical parameters and conformational stabilities have been obtained from MP2/6-311+G(d,p) calculations. At this highest level of calculations, the HE conformer is predicted to be more stable by 62 and 84 cm(-1) than the ME and ClE conformers, respectively. The coefficients from the potential function governing the conformational interchange have been obtained from the MP2/6-31G(d) ab initio calculations. By utilizing the frequency of the SiH stretching mode, the r(0)-H distance has been determined to be 1.481 A for the HE conformer. The ab initio calculated quantities are compared to the experimentally determined values where applicable, as well as to some corresponding results for some similar molecules.     

Deprotonation of calixarenes in acetonitrile

[Structure: see text]. The pKa values for calixarenes in MeCN have been determined by selective titration with bases using a spectroscopic method. These values are as follows: calix[4]arene pKa(1) = 19.06 +/- 0.22, pKa(2) > 33; calix[6]arene pKa(1) = 15.59 +/- 0.06, pKa(2) = 23.85 +/- 0.35, pKa(3) > 33; calix[8]arene pKa(1) = 17.20 +/- 0.20, pKa(2) = 20.32 +/- 0.31, pKa(3) > 33. The trends in acidity are rationalized using structures generated by a DFT model. For mono-deprotonation, the degree and nature of hydrogen bonding in the anion is the dominant factor; for di-deprotonation, spatial separation of the anionic charges becomes important.     

Tautomeric and conformational properties of malonamic acid methyl ester, NH2C(O)-CH2-C(O)OCH3: electron diffraction and quantum chemical study

The tautomeric and conformational properties of malonamic acid methyl ester, NH2C(O)-CH2-C(O)OCH3, have been investigated by means of gas-phase electron diffraction (GED) and quantum chemical calculations (HF, B3LYP, and MP2 approximations with different basis sets up to 6-311++G(3df,pd)). Both quantum chemistry and GED at 360(8) K result in the existence of a single diketo conformer in the gas phase. According to GED refinement, this conformer possesses an (ac, sc) conformation with dihedral angles C-C-C(NH2)=O of 140.3(3.0) degrees and C-C-C(OCH3)=O of 31.1(7.2) degrees. The experimental geometric parameters are reproduced very closely by MP2 and B3LYP methods with large basis sets.     

UV and fluorescence spectral changes induced by neodymium binding of N,N'-ethylenebis[2-(o-hydroxyphenolic)glycine] and N,N'-di(2-hydroxybenzyl)ethylenediamine-N,N' diacetic acid

In 0.01 M 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (Hepes), pH 7.4 and room temperature, the binding of neodymium to N,N'-ethylenebis[2-(o-hydroxyphenolic)glycine] (EHPG), or N,N'-di(2-hydroxybenzyl)ethylenediamine-N,N' diacetic acid (HBED) had been studied from 210 to 330 nm by means of difference UV spectra. Two peaks at 240 and 292 nm appear in difference UV spectra after neodymium binding to EHPG or HBED. The 1:1 stable complex can be confirmed from spectral titration curves. The molar extinction coefficient of Nd-EHPG and Nd-HBED complexes are Deltaepsilon(Nd-EHPG)=(12.93+/-0.21) x 10(3)cm(-1)M(-1), Deltaepsilon(Nd-HBED)=(14.45+/-0.51) x 10(5)cm(-1)M(-1) at 240 nm, respectively. Using EDTA as a competitor, the conditional equilibrium constants of the complexes are logK(Nd-EHPG)=11.89+/-0.09 and logK(Nd-HBED)=12.19+/-0.15, respectively. At the same conditions, fluorescence measurements show that neodymium binding to EHPG leads to a quenching of the fluorescence of EHPG at near 310 nm. However, there is no obvious fluorescence change of HBED at 318 nm with the binding of neodymium to HBED.     

Conformational stability from variable temperature FT-IR spectra of krypton solutions, r0 structural parameters, vibrational assignment, and ab initio calculations of 4-fluoro-1-butene

Variable temperature (-115 to -155 degrees C) studies of the infrared spectra (3200-400 cm-1) of 4-fluoro-1-butene, CH2=CHCH2CH2F, dissolved in liquid krypton have been carried out. The infrared spectra of the gas and solid as well as the Raman spectra of the gas, liquid, and solid have also been recorded from 3200 to 100 cm-1. From these data, an enthalpy difference of 72 +/- 5 cm-1 (0.86 +/- 0.06 kJ x mol-1) has been determined between the most stable skew-gauche II conformer (the first designation refers to the position of the CH2F group relative to the double bond, and the second designation refers to the relative positions of the fluorine atom to the C-C(=C) bond) and the second most stable skew-trans form. The third most stable conformer is the skew-gauche I with an enthalpy difference of 100 +/- 7 cm-1 (1.20 +/- 0.08 kJ x mol-1) to the most stable form. Larger enthalpy values of 251 +/- 12 cm-1 (3.00 +/- 0.14 kJ x mol-1) and 268 +/- 17 cm-1 (3.21 +/- 0.20 kJ x mol-1) were obtained for the cis-trans and cis-gauche conformers, respectively. From these data and the relative statistical weights of one for the cis-trans conformer and two for all other forms, the following conformer percentages are calculated at 298 K: 36.4 +/- 0.9% skew-gauche II, 25.7 +/- 0.1% skew-trans, 22.5 +/- 0.2% skew-gauche I, 10.0 +/- 0.6% cis-gauche, and 5.4 +/- 0.2% cis-trans. The potential surface describing the conformational interchange has been analyzed and the corresponding two-dimensional Fourier coefficients were obtained. Nearly complete vibrational assignments for the three most stable conformers are proposed and some fundamentals for the cis-trans and the cis-gauche conformers have been identified. The structural parameters, dipole moments, conformational stability, vibrational frequencies, infrared, and Raman intensities have been predicted from ab initio calculations and compared to the experimental values when applicable. The adjusted r0 structural parameters have been determined by combining the ab initio predicted parameters with previously reported rotational constants from the microwave data. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.     

Spectrophotometric determination of acidity constants of salicylaldoxime in aqueous solution at 25 degrees C and ionic strength of 0.5 M controlled with NaCl

The equilibrium constants of salicylaldoxime in water at 25 degrees C, 0.5 M of ionic strength with NaCl and concentration of 1x10(-4) M were determined spectrophotometrically. The spectral data were processed using SQUAD program. The salicylaldoxime in acid medium has the value of pKa1=1.224+/-0.027. In alkaline medium the salicylaldoxime has the values of pKa2=8.551+/-0.024 and pKa3=11.728+/-0.016.     

Conformational stability from variable temperature infrared spectra of rare gas solutions of 2-chloro-3-fluoropropene

Variable temperature (-55 to -155 degrees C) infrared spectra of rare gas solutions of 2-chloro-3-fluoropropene, H2C=C(Cl)CH2F, have been recorded from 3500 to 400 cm(-1). The relative intensities of 16 conformer pairs at ten different temperatures of a krypton solution have been measured and from these data an enthalpy difference of 271+/-27 cm(-1) (3.17+/-0.32 kJ x mol(-1)) has been obtained with the cis conformer the more stable form. Similar studies were also carried out in xenon and an enthalpy difference of 334+/-33 cm(-1) (4.00+/-0.39 kJ x mol(-1)) was obtained again with the cis conformer the more stable form. It is estimated that there is 35+/-2% of the gauche conformer present at ambient temperature. However, in the crystalline solid the gauche conformer is the stable rotamer. Extensive ab initio calculations with full electron correlation by the perturbation method at the MP2 level with a variety of basis sets as well as density functional theory calculations (DFT) by the B3LYP method have been carried out. Several of these calculations predict an energy difference in the range of 400 cm(-1) with the cis form the more stable conformer but most of the predicted energy differences are significantly larger than the experimentally determined value. The spectroscopic and theoretical results are discussed and compared with the corresponding quantities for some similar molecules.     

Infrared and Raman spectra, conformational stability, ab initio calculations of structure, and vibrational assignment of ethynylmethyl cyclobutane.

The infrared spectra (3200-300 cm(-1)) of the gas and solid and the Raman spectra (3200-30 cm(-1) of the liquid with quantitative depolarization values and the solid have been recorded of ethynylmethyl cyclobutane (cyclobutylmethyl acetylene), c-C4H7CH2C[ triple bond]CH. Both the equatorial and the axial conformers have been identified in the fluid phases and both the gauche and trans conformations of the methyl acetylenic group have been identified for each ring conformer. Variable temperature (-105 to -150 degrees C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data the enthalpy differences have been determined to the 112 +/- 11 cm(-1) (1.34 +/- 0.13 kJ mol) between the most stable equatorial-trans (Et) conformer and the equatorial-gauche (Eg) conformer which is the second most stable conformer and 327 +/- 35 cm(-1) (3.91 + 0.42 kJ/mol) with the axial-gauche (Ag) conformer which is the least stable conformer. The enthalpy difference between the axial-trans (At) and the equatorial-gauche(Eg) is 56 +/- 6 cm(-1). At ambient temperature there is approximately 33% of the Et conformer, 38% of the Eg form, 15% of the At and 14% of the Ag conformer. For the polycrystalline solid the Eg conformer is the only form present which probably results form the packing in the crystal. A complete vibrational assignment is proposed for both equatorial conformers and additionally a few of the fundamentals of the At and Ag conformers have been assigned. The vibrational assignments are supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G(d) calculations. Complete equilibrium geometries have been determined for all four possible rotamers by ab initio calculations employing the 6-31G(d) and 6-311 + + G(d,p) basis sets at levels of restricted Hartree-Fock (RHF) and /or Moller-Plesset (MP2) with full electron correlation by the perturbation method to second order. The results are discussed and compared to those obtained for some similar molecules.     

Vibronic spectroscopy of unsaturated transition metal complexes: CrC2H, CrCH3, and NiCH3

Vibronically resolved resonant two-photon ionization and dispersed fluorescence spectra of the organometallic radicals CrC(2)H, CrCH(3), and NiCH(3) are reported in the visible and near-infrared wavelength regions. For CrC(2)H, a complicated vibronic spectrum is found in the 11 100-13 300 cm(-1) region, with a prominent vibrational progression having omega(e) (')=426.52+/-0.84 cm(-1), omega(e) (')x(e) (')=0.74+/-0.13 cm(-1). Dispersed fluorescence reveals a v(")=1 level of the ground state with DeltaG(1/2) (")=470+/-20 cm(-1). These vibrational frequencies undoubtedly pertain to the Cr-C(2)H stretching mode. It is suggested that the spectrum corresponds to the A (6)Sigma(+)<--X (6)Sigma(+) band system, with the CrC(2)H molecule being linear in both the ground and the excited state. The related CrCH(3) molecule displays a vibronic spectrum in the 11 500-14 000 cm(-1) region. The upper state of this system displays six sub-bands that are too closely spaced to be vibrational structure, but too widely separated to be K structure. It is suggested that the observed spectrum is a (6)E<--X (6)A(1) band system, analogous to the well-known B (6)Pi<--X (6)Sigma(+) band systems of CrF and CrCl. The ground state Cr-CH(3) vibration is characterized by omega(e) (")=525+/-17 cm(-1) and omega(e) (")x(e) (")=7.9+/-6 cm(-1). The spectrum of NiCH(3) lies in the 16 100-17 400 cm(-1) range and has omega(e) (')=455.3+/-0.1 cm(-1) and omega(e) (')x(e) (')=6.60+/-0.03 cm(-1). Dispersed fluorescence studies provide ground state vibrational constants of omega(e) (")=565.8+/-1.6 cm(-1) and omega(e) (")x(e) (")=1.7+/-3.0 cm(-1). Again, these values correspond to the Ni-CH(3) stretching motion. (c) 2004 American Institute of Physics.     

Spectra and structure of silicon containing compounds. XXXIX. Raman and infrared spectra,conformational stability,vibrational assignment and ab initio calculations of n-propyltrifluorosilane

The infrared (3100-40 cm(-1)) spectra of gaseous and solid and Raman (3200-20 cm(-1)) spectra of liquid with qualitative depolarization values and solid n-propyltrifluorosilane, CH(3)CH(2)CH(2)SiF(3), have been recorded. Additionally the infrared spectra of the sample in nitrogen and argon matrices have been recorded. Both the anti and gauche conformers have been identified in the fluid phases but only the anti conformer remains in the solid. Variable temperature (-105 to -150 degrees C) studies of the infrared spectra of the sample dissolved in liquid krypton have been recorded and the enthalpy difference has been determined to be 135+/-14 cm(-1) (1.62+/-0.17 kJ mol(-1)) with the anti conformer the more stable form. At ambient temperature it is estimated that there is 51+/-2% of the gauche conformer present. Also the enthalpy difference in the liquid was obtained from variable temperature studies of the Raman spectra and from three conformer pairs an average value of 179+/-18 cm(-1) (2.14+/-0.22 kJ mol(-1)) was obtained again with the anti form the more stable conformer. Relatively complete vibrational assignments are proposed for both conformers based on the relative infrared and Raman spectral intensities, infrared band contours, depolarization ratios which are supported by normal coordinate calculations. The geometrical parameters, harmonic force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios, and energy differences have been obtained for the anti and gauche conformers from ab initio MP2/6-31G(d) calculations. Structural parameters and energy differences have also been obtained utilizing the larger 6-311+G(d, p) and 6-311+G(2d, 2p) basis sets. By utilizing the previously reported microwave rotational constants for five isotopomers of CH(3)SiF(3) along with ab initio predicted structural values, r(0) parameters have been obtained for methyltrifluorosilane. Similarly, from the ab initio predicted parameters "adjusted r(0)" parameters have been estimated for both conformers of n-propyltrifluorosilane. The results are discussed and compared with those obtained for some similar molecules.     

In silico calculation of acidity constants of carbonic acid conformers

In order to explore the aqueous acid chemistry of carbonic acid, we employ a constrained ab initio molecular dynamics (AIMD) technique to study acid dissociations of its three conformers including CC (cis-cis), CT (cis-trans), and TT (trans-trans). The simulations of reagent states reveal similar hydration characteristics for them: the hydroxyls donate H-bonds to solvating waters but no obvious H-bonding exists between hydroxyl oxygen atoms and waters. It is found that the CC conformer dissociates spontaneously to bicarbonate within picoseconds whereas the other two can stay for relatively long simulation times. This suggests that CC has the strongest acidity among the three conformers and it is not stable in water. The simulations indicate that the symmetrical hydroxyls of TT conformer have a pKa value of 3.11 and the two asymmetrical hydroxyls of CT show different pKa values: 2.60 and 3.75, respectively. Overall, these results confirm the recent experimental measurement: about 4.0 for deuterated carbonic acid. By analyzing the dissociation processes, it is revealed that the differences of the acid constants stem from the initial steps of hydroxyls stretches. This simulation study provides a quantitative and microscopic basis for better understanding the reactivity of aqueous carbonate species.     

Acidity constants in different media (I=0 and I=0.1 M KCl) from the uncertainty perspective

Procedures for estimating the measurement uncertainty for the acidity constant Ka (or the pKa value) in different media (I=0 and I=0.1 mol L(-1) KCl), as determined by potentiometric titration, are presented. The uncertainty budgets (the relative contributions of the different input quantities to the uncertainty in the result) of the pKa (I=0) and pKa (I=0.1 mol L(-1) KCl) values are compared. Unlike the values themselves, the uncertainties and uncertainty budgets of the values are comparable. The uncertainty estimation procedures are based on mathematical models of pKa measurement and involve the identification and quantification of individual uncertainty sources according to the ISO GUM approach. The mathematical model involves 52 and 48 input parameters for pKa (I=0) and pKa (I=0.1 mol L(-1) KCl), respectively. The relative importance of each source of uncertainty is discussed. In both cases, the main contributors to the uncertainty budget are the uncertainty components due to the hydrogen ion concentration/activity measurement, which provide 63.7% (for pKa (I=0)) and 89.3% (for pKa (I=0.1 mol L(-1) KCl)) of the uncertainty. The remaining uncertainty contributions arise mostly from the limited purity of the acid. From this work, it is clear that the uncertainties of the pKa (I=0.1 mol L(-1) KCl) values tend to be lower than those of the pKa (I=0) values. The main reasons for this are that: (1) the uncertainty due to the residual liquid junction potential is nominally absent in the case of pKa (I=0.1 mol L(-1) KCl) due to the similarly high concentrations of background electrolyte in the calibration solutions and measured solution; (2) the electrode system is more stable in solutions containing the 0.1 mol L(-1) KCl background electrolyte and so the readings obtained in these solutions are more stable.     

pKa determinations of xanthene derivates in aqueous solutions by multivariate analysis applied to UV-Vis spectrophotometric data

Xanthenes form to an important class of dyes which are widely used. Most of them present three acid-base groups: two phenolic sites and one carboxylic site. Therefore, the pKa determination and the attribution of each group to the corresponding pKa value is a very important feature. Attempts to obtain reliable pKa through the potentiometry titration and the electronic absorption spectrophotometry using the first and second orders derivative failed. Due to the close pKa values allied to strong UV-Vis spectral overlap, multivariate analysis, a powerful chemometric method, is applied in this work. The determination was performed for eosin Y, erythrosin B, and bengal rose B, and also for other synthesized derivatives such as 2-(3,6-dihydroxy-9-acridinyl) benzoic acid, 2,4,5,7-tetranitrofluorescein, eosin methyl ester, and erythrosin methyl ester in water. These last two compounds (esters) permitted to attribute the pKa of the phenolic group, which is not easily recognizable for some investigated dyes. Besides the pKa determination, the chemometry allowed for estimating the electronic spectrum of some prevalent protolytic species and the substituents effects evaluation.     

Molecular structure and conformational composition of 1,3-dihydroxyacetone studied by combined analysis of gas-phase electron diffraction data, rotational constants, and results of theoretical calculations. Ideal gas thermodynamic properties of 1,3-dihydroxyacetone

The molecular structure of 1,3-dihydroxyacetone (DHA) has been studied by gas-phase electron diffraction (GED), combined analysis of GED and microwave (MW) data, ab initio, and density functional theory calculations. The equilibrium re structure of DHA was determined by a joint analysis of the GED data and rotational constants taken from the literature. The anharmonic vibrational corrections to the internuclear distances (re-ra) and to the rotational constants (B(i)e-B(i)0) needed for the estimation of the re structure were calculated from the B3LYP/cc-pVTZ cubic force field. It was found that the experimental data are well reproduced by assuming that DHA consists of a mixture of three conformers. The most stable conformer of C2v symmetry has two hydrogen bonds, whereas the next two lowest energy conformers (Cs and C1 symmetry) have one hydrogen bond and their abundance is about 30% in total. A combined analysis of GED and MW data led to the following equilibrium structural parameters (re) of the most abundant conformer of DHA (the uncertainties in parentheses are 3 times the standard deviations): r(C=O)=1.215(2) A, r(C-C)=1.516(2) A, r(C-O)=1.393(2) A, r(C-H)=1.096(4) A, r(O-H)=0.967(4) A, angleC-C=O=119.9(2) degrees, angleC-C-O=111.0(2) degrees, angleC-C-H=108.2(7) degrees, angleC-O-H=106.5(7) degrees. These structural parameters reproduce the experimental B(i)0 values within 0.05 MHz. The experimental structural parameters are in good agreement with those obtained from theoretical calculations. Ideal gas thermodynamic functions (S degrees (T), C degrees p(T), and H degrees (T)-H degrees (0)) of DHA were calculated on the basis of experimental and theoretical molecular parameters obtained in this work. The enthalpy of formation of DHA, -523+/-4 kJ/mol, was calculated by the atomization procedure using the G3X method.     

Determination of formation constants of hydroxo and carbonate complexes of Pr(3+) in 2 M NaCl at 303 K

The hydrolysis of praseodymium III in 2 M sodium chloride at 303 K was studied. Two methods were used: pH titration followed by a computational refinement and solvent extraction in the presence of a competitive ligand. The hydrolysis constants obtained by pH titration were: logbeta(1,H)=-7.68+/-0.07, logbeta(1,2H)=-15.10+/-0.03, and beta(1,3H)=-23.80+/-0.04. The stability constants of praseodymium carbonate complexes were determined by pH titration as well and were: logbeta(1,CO(2-)(3))=5.94+/-0.08 and logbeta(1,2CO(2-)(3))=11.15+/-0.15. Praseodymium carbonate species were taken into consideration for calculating the first hydrolysis constants by the solvent extraction method and the value obtained was: logbeta(1,H)=-7.69+/-0.27. The values for logbeta(1,H) attained by both methods are the same. The species-distribution diagram was obtained from the stability constants of praseodymium carbonate complexes and hydrolysis products in the conditions of the present work.     

A potentiometric and 51V NMR study of the aqueous H+/H2VO4-/H2O2/L-alpha-alanyl-L-histidine system

The speciation in the quaternary aqueous H+/H2VO4-/H2O2/L-alpha-alanyl-L-histidine (Ah) system has been determined from quantitative 51V NMR measurements and potentiometric data (glass electrode). The study was performed in 0.150 M Na(Cl) medium at 25 degrees C. Data were evaluated with the computer program LAKE, which is able to treat combined EMF and NMR data. The pKa values for Ah were determined as 8.06, 6.72 and 2.64. In the ternary H+/H2VO4-/Ah system, two complexes, (H+)p(H2VO4-)q(Ah)r, for which (p, q, r) values of (0, 1, 1) and (1, 1, 1) with log beta(0,1,1) = 2.52 +/- 0.03 and log beta(1,1,1) = 9.40 +/- 0.05 (pKa = 6.88), respectively, explain all data. The errors given are 3sigma. In the quaternary H+/H2VO4-/H2O2/Ah system, eight complexes were determined in addition to all binary and ternary complexes, four with a V/X/Ah ratio 1:1:1 and four with a ratio 1:2:1 (X = peroxo ligand). VX2Ah2- and VX2Ah- (pKa = 8.19) are the main complexes and predominate in the pH range 5 to 9. Three additional minor species have also been found but their compositions could not be determined owing to their small amounts. Equilibria are slow, significant decomposition of peroxide occurs only in acidic solutions. Data in the pH range 5 to 10 have been used for the LAKE calculations. Chemical shifts, compositions, and formation constants for the eight quaternary complexes are given, and equilibrium conditions are illustrated in distribution diagrams. Structural proposals for VX2Ah2- and VX2Ah- are made from 1H and 13C NMR measurements.     

Conformational stability from variable temperature infrared spectra of krypton solutions, ab initio calculations, vibrational assignment, and r0 structural parameters of 1,3-difluoropropane

The infrared spectra (3200-400 cm(-1)) of krypton solutions of 1,3-difluoropropane, FCH2CH2CH2F, at variable temperatures (-105 to -150 degrees C) have been recorded. Additionally, the infrared spectra (3200-50 cm(-1)) of the gas and solid have been recorded as well as the Raman spectrum of the liquid. From a comparison of the spectra of the fluid phases with that in the solid, all of the fundamental vibrations of the C2 conformer (gauche-gauche) where the first gauche indicates the form for one of the CH2F groups and the second gauche the other CH2F, and many of those for the C1 form (trans-gauche) have been identified. Tentative assignments have been made for a few of the fundamentals of the other two conformers, i.e. C2v (trans-trans) and Cs (gauche-gauche'). By utilizing six pairs of fundamentals for these two conformers in the krypton solutions, an enthalpy difference of 277 +/- 28 cm(-1) (3.31 +/- 0.33 kJ mol(-1)) has been obtained for the C2 versus C1 conformer with the C2 conformer the more stable form. For the C2v conformer, the enthalpy difference has been determined to be 716 +/- 72 cm(-1) (8.57 +/- 0.86 kJ mol(-1)) and for the Cs form 971 +/- 115 cm(-1) (11.6 +/- 1.4 kJ mol(-1)). It is estimated that there is 64 +/- 3% of the C2 form, 34 +/-3% of the C1 form, 1% of the C2v form and 0.6% of the Cs conformer present at ambient temperature. Equilibrium geometries and total energies of the four stable conformers have been determined from ab initio calculations with full electron correlation by the perturbation method to second order as well as by hybrid density functional theory calculations with the B3LYP method using a number of basis sets. The MP2 calculations predict the C1 conformer stability to be slightly higher than the experimentally determined value whereas for the C2v and Cs conformers the predicted energy difference is much larger than the experimental value. The B3LYP calculations predict a better energy difference for both the C1 and C2v as well as for the Cs conformers than the MP2 values. A complete vibrational assignment is proposed for the C2 conformer and many of the fundamentals have been identified for the C1 form based on the force constants, relative intensities and rotational-vibrational band contours obtained from the predicted equilibrium geometry parameters. By combining previously reported rotational constants for the C2 and C1 conformers with ab initio MP2/6-311 + G(d, p) predicted parameters, adjusted r0 parameters have been obtained for both conformers. Comparisons are made with the parameters obtained for some other molecules containing the FCH2 group. The spectroscopic and theoretical results are compared to the corresponding properties for some similar molecules.