Isotope exchange study of the dissociation of metal —humic substance complexes

Isotope exchange was employed to study dissociation of metal cations from their complexes with humic substances (HS). Dissociation of cation from HS controls the rate of isotope exchange between two identical metal-HS solutions (but for the presence of a radiotracer) divided by a dialysis membrane. The rate of isotope exchange of Eu/¹⁵²Eu and Co/⁶⁰Co in the systems with various HS was monitored as a function of pH, ionic strength, and the degree of HS loading with metal. The apparent rate of Eu-HS dissociation was found to be enhanced by decreasing pH, increasing ionic strength, and increasing metal loading. Co-HS dissociation was too fast to be followed by the method. For interpretation of the experimental kinetic data, the multiple first order law has been applied. Based on the results, a concept of HS as a mixture of two types of binding sites is discussed.     

Probing the proton-transfer coordinate of complexes with F--H...P hydrogen bonds using one- and two-bond spin-spin coupling constants

Scalar coupling constants have been computed using the EOM-CCSD method for equilibrium structures of complexes stabilized by F--H...P hydrogen bonds, as well as structures along the proton-transfer coordinates of these complexes. Variations in the signs and absolute values of (1)J(F--H), (1h)J(H--P) and (2h)J(F--P) have been analyzed and interpreted in terms of changing hydrogen bond type. Of the three phosphorus bases (phosphine, trimethylphosphine and phosphinine) investigated in this study, trimethylphosphine forms the strongest complex with FH, and has the largest two-bond F--P coupling constant. Among the relatively simple phosphorus bases, it would appear to be a leading candidate for experimental NMR study. Similarities and differences are noted between the corresponding coupling constants (J) and the reduced coupling constants (K) across F--H...P and F--H...N hydrogen bonds.     

HC[triple bond]P and H3C-C[triple bond]P as proton acceptors in protonated complexes containing two phosphorus bases: structures, binding energies, and spin-spin coupling constants

Ab initio calculations at the MP2/aug'-cc-pVTZ level have been carried out to investigate the structures and binding energies of cationic complexes involving protonated sp, sp2, and sp3 phosphorus bases as proton donor ions and the sp-hybridized phosphorus bases H-C[triple bond]P and H3C-C[triple bond]P as proton acceptors. These proton-bound complexes exhibit a variety of structural motifs, but all are stabilized by interactions that occur through the pi cloud of the acceptor base. The binding energies of these complexes range from 6 to 15 kcal/mol. Corresponding complexes with H3C-C[triple bond]P as the proton acceptor are more stable than those with H-C[triple bond]P as the acceptor, a reflection of the greater basicity of H3C-C[triple bond]P. In most complexes with sp2- or sp3-hybridized P-H donor ions, the P-H bond lengthens and the P-H stretching frequency is red-shifted relative to the corresponding monomers. Complex formation also leads to a lengthening of the C[triple bond]P bond and a red shift of the C[triple bond]P stretching vibration. The two-bond coupling constants 2pihJ(P-P) and 2pihJ(P-C) are significantly smaller than 2hJ(P-P) and 2hJ(P-C) for complexes in which hydrogen bonding occurs through lone pairs of electrons on P or C. This reflects the absence of significant s electron density in the hydrogen-bonding regions of these pi complexes.     

Anab initio molecular orbital study of substituted carbonyl compounds

Ab initio SCF calculations with a minimal STO-3G basis set have been performed to determine the equilibrium geometries of two series of carbonyl compounds, RCHO and R₂ CO. For the mono-substituted compounds, R may be CH₃, NH₂, OH, F, CHO, and C₂ H₃. In the disubstituted compounds, R has been restricted to the isoelectronic saturated groups. The computed equilibrium geometries of these compounds are in satisfactory agreement with the experimentally-determined geometries, with an average difference of 0.021 Å between computed and experimental bond lengths, and 1.9 ° in corresponding bond angles. An analysis of the effect of the substituent on the electronic structure of the carbonyl group has also been made. The saturated groups are found to be electron-withdrawing groups relative to H, with the electron withdrawing ability increasing in the order CH₃ 2 2H₂ are also electron withdrawing relative to H, and comparable to CH₃ in acetaldehyde. Vertical ionization potentials andn* transition energies have also been calculated for these molecules at their optimized geometries, experimental geometries, and geometries given by a standard model. The effect of changes in molecular geometry on these computed properties has been analyzed.     

Proton-bound homodimers involving second-row atoms

High-level ab initio quantum chemical calculations (G4(MP2)//MP2/6-311+G(2df,p)) have been used to examine homodimers of second-row bases, and to compare the results with those obtained previously for the first-row analogs. The relationship between the binding energies of the dimers and the proton affinities (PAs) of the bases follows the same pattern as that for the first-row systems, with the binding energies initially increasing with increasing proton affinity but subsequently decreasing. This may be attributed to the opposing effects of increased PA on the hydrogen-bond donor and hydrogen-bond acceptor. The binding energies are generally smaller for the second-row dimers than for the corresponding first-row dimers. There is an increased tendency for asymmetrical hydrogen bonds in homodimers of the second-row compared with first-row dimers. This may be attributed to the lower electronegativities of second-row atoms relative to their first-row counterparts, and to the longer internuclear separation between the hydrogen-bonded second-row atoms.     

Ab initio study of 4-monosubstituted pyrimidines in ground and excited n → π* states

Ab initio SCF and SCF -CI calculations have been performed to investigate substituent effects on ground- and excited-state properties of 4-R-pyrimidines, and to compare these with substituent effects in 2- and 4-R-pyridines, with R including the π donating and σ withdrawing groups CH₃, NH₂, OH, F, and C₂H₃ and the σ and π electron-withdrawing groups CHO and CN. Substitution leads to significant changes in the internal angles of the pyrimidine ring, which are independent of the nature of the substituent. The geometry of the pyrimidine ring is more sensitive to substitution in the 4 position than the pyridine ring geometry is to substitution in either the 2 or the 4 position. The isodesmic reaction energies for substituent transfer from the 4 position of pyrimidine to the 2 or 4 position of pyridine indicate that all R groups except CN have a relative stabilizing effect in pyrimidine. The presence of a π donating group leads to an increase in the n→π* transition energy of 4-R-pyrimidines, while the π withdrawing group CN leads to a decrease in the transition energy relative to pyrimidine. Orbital energy differences and virtual excitation energies tend to correlate with n→π* transition energies of 4-R-pyrimidines with saturated R groups, but such correlations are masked by π conjugation, n orbital interaction, and configurational mixing when the unsaturated groups C₂H₃, CHO, and CN are present. The electronic effects of a π donating group are stronger when the group is bonded to pyrimidine than to pyridine, but those of a π withdrawing group are weaker when the group is bonded to pyrimidine.     

An ab initio study of cooperative effects in ternary complexes X:CNH:Z with X, Z = CNH, FH, ClH, FCl, and HLi: structures, binding energies, and spin-spin coupling constants across intermolecular bonds

A systematic ab initio investigation has been carried out to determine the structures, binding energies, and spin-spin coupling constants of ternary complexes X:CNH:Z and corresponding binary complexes X:CNH and CNH:Z, for X, Z = CNH, FH, ClH, FCl, and HLi. The enhanced binding energies of ternary complexes X:CNH:Z for fixed X as a function of Z decrease in the same order as the binding energies of the binary complexes CNH:Z. In contrast, the enhanced binding energies of the ternary complexes for fixed Z as a function of X do not decrease in the same order as the binding energies of the binary complexes X:CNH, a consequence of the increased stabilities of ternary complexes FCl:CNH:Z due to very strong chlorine-shared halogen bonds. For complexes in which the X···CNH interaction is a D-H···C hydrogen bond for D-H the proton-donor group (N-H, F-H, or Cl-H), spin-spin coupling constants (1)J(D-H) and (2h)J(D-C) in ternary complexes X:CNH:Z decrease in absolute value as the binding energies of binary complexes CNH:Z and the enhanced binding energies of the ternary complexes for fixed X as a function of Z also decrease. However, (2X)J(F-C) increases as the enhanced binding energies of the ternary complexes FCl:CNH:Z decrease, a consequence of the nature of the chlorine-shared halogen bond. The one-bond coupling constants (1)J(N-H) for the CNH···Z interaction in ternary complexes vary significantly, depending on the nature of the X···CNH interaction. The largest values of (1)J(N-H) are found for ternary complexes with FCl as X. Two-bond coupling constants (2h)J(N-A) for A the proton-acceptor atom of Z, and (2d)J(N-H) decrease in absolute value in the order of decreasing enhancement energies of ternary complexes X:CNH:Z for fixed Z as a function of X.