Steric effects in mono- and polynitroalkane molecules

Conclusions The equilibrium configurations, strain energies, and atomization enthalpies were calculated for a number of mono- and polynitroalkane molecules. A steric hindrance criterion was derived, which could be one of the parameters that characterize the stability. The steric effects, caused by nitro groups, increase in a nonlinear manner with increase in the number of nitro groups in the molecule and elongation of the carbon skeleton.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 182–184, January, 1977.     

Steric effects on diffusion of associated molecules in acetone

The effects due to steric hindrance on solute-solvent interactions and on diffusion of associated molecules were found by comparing the diffusion coefficients of different aromatic isomers in acetone at 298.2 K; there exists a correlation between the isomeric effects of intermolecular association on diffusion and the molecular scales of overall hydrogen-bond acidity of the isomers studied.     

Conductometric studies on the stability of copper complexes with different oligosaccharides

Bioactive copper complexes with pullulan or dextran oligosaccharides are the subject of intensive research mainly because of their possible application in veterinary and human medicine. The thermal stability and stability under oxidative conditions of the Cu(II) complexes with reduced low-molar pullulan or dextran were investigated in this paper, using a conductometric method. The influence of ligand constitutions on the stability of the complexes was examined on the basis of ligand property. Forced degradation studies were performed on bulk sample of complexes by using heat (25, 40 and 60°C) and an oxidation agent (0.1, 0.5, 1.0 and 10.0% v/v hydrogen peroxide). It can be concluded, according to the results obtained (by examining conductivity during the forced degradation studies), that Cu(II) complexes show a large pharmaceutical stability for both tests.      

The effects of hydrogen-bonding environment on the polarization and electronic properties of water molecules

Adequate representation of the interactions that take place between water molecules has long been a goal of force field design. A full understanding of how the molecular charge distribution of water is altered by adjacent water molecules and by the hydrogen-bonding environment is a vital step toward achieving this task. For this purpose we generated ab initio electron densities of pure water clusters and hydrated serine and tyrosine. Quantum chemical topology enabled the study of a well-defined water molecule inside these clusters, by means of its volume, energy, and multipole moments. Intra- and intermolecular charge transfer was monitored and related to the polarization of water in hydrogen-bonded networks. Our analysis affords a way to define different types of water molecules in clusters.     

The effects of transition metal complexes on the permeation of small gas molecules through cellulose acetate membranes

Cellulose acetate (CA) membranes containing RuCl₃·3H₂O and RhCl [P(C₆H₅)₃]₃ were prepared reproducibly. Such membranes, on treatment with CO, formed metal-carbonyl species at relatively low temperature. The Ru-carbonyls formed in CA were quite stable at 40°C in comparison with the Rh-carbonyl species and, interestingly, there was no permeation of CO gas through the ruthenium-containing CA membrane at 40°C. However, the permeation of other gas molecules, such as H₂, N₂ and O₂, through the same membrane was reduced only slightly, probably due to the cross-linking effect of the transition metal complexes in CA. It was found that essentially pure H₂ gas could be recovered from a 1: 1 mixture of H₂ and CO gases using ruthenium-containing cellulose acetate membranes.     

Hydrolysis of Fischer carbene complexes. Steric effects of the pi-donor group

Rate constants for the hydrolysis of Fischer carbenes (CO)5Cr=C(OR)Ph (R = n-propyl, neopentyl, isopropyl, and menthyl) in 50% MeCN-50% water (v/v) at 25 degrees C are reported. The rate constants for the addition of -OH to the carbene carbon are 5.3, 3.7, 0.84, and 0.01 M(-1) s(-1), respectively. These rate constants give linear correlations with the corresponding rate constants for the hydrolysis of esters such as acetate, benzoate, and formiate. The slopes of the plots of the observed rate constants for the carbenes vs the rate constants for the esters are 1.4 and 1.2 for acetate and benzoate, respectively, indicating that the factors that decrease the reactivity of the two types of compounds are similar, but the carbenes show higher sensitivity. The rate constants are well correlated with several steric parameters giving a value of -3.84 for the Charton's psi parameter. The results show that the steric bulkiness of the R group is the main factor determining the reactivity differences for these carbene complexes.     

Steric effects in the aerobic oxidation of pi-allylnickel(II) complexes with N-heterocyclic carbenes

Pi-allylchloro(NHC)nickel(II) complexes were synthesized and their reactions with O2 were studied. Ligand steric effects were found to determine the difference between rapid oxidation of the allyl group to produce bis-mu-hydroxonickel complexes and no observable reaction. The ability of the metal-NHC bond to rotate correlates with the ability of the complex to react with O2. In the limiting cases, conformationally restricted complexes are stable to O2 and complexes with rapid Ni-NHC bond rotation react rapidly with O2. Complexes with intermediate conformational flexibility were found to exhibit lesser reactivity with O2. On the basis of the observed inertness of complexes with saddle-shaped ligands to O2, we propose the adoption of a nonplanar geometry upon reaction with O2 to be required. The issue of conformational flexibility versus rigidity is expected to directly impact the catalytic behavior of metal-NHC complexes.     

Metal Complexes with Macrocyclic Ligands. Part XXXVIII. Steric effects in the copper(II) and nickel(II) complexes with tetra-N-alkylated 1,4,8,11-tetraazacyclotetradecanes

A series of tetra-N-alkylated 1,4,8,11-tetraazacyclotetradecanes have been synthesized and their complexation potential towards Ni²⁺ and Cu²⁺ studied. In the case of sterically demanding alkyl substituents, such as i-Pr, PhCH₂, or 2-MeC₆H₄CH₂, no metal complexes are formed, whereas for substituents such as Me, Et, and Pr, the metal ion is incorporated into the macrocycle. The spectroscopic properties of the Ni²⁺ and Cu²⁺ complexes in aqueous solution indicate that, depending on the sterical hindrance of the N-substituents, the complexes are either square planar or pentacoordinated. All these Ni²⁺ and Cu²⁺ complexes react with N to give ternary species, the stability of which have been determined by spectrophotometric titrations. The tendency to bind N decreases with increasing steric hindrance of the alkyl substituents. The X-ray studies of the Ni²⁺ complex with the macrocycle 11 , substituted by two Me and two Pr groups, and that of the Cu²⁺ complex with the tetraethyl derivative 8 show that in the solid state, the metal ions exhibit square planar coordination with a small distortion towards tetrahedral geometry.     

Steric effects of the alkyl groups: evaluation on isolated molecules by means of isodesmic reactions

Several possible scales of steric effects of the alkyl groups were suggested on the basis of isodesmic model reactions, in which a sterically crowded compound is formally synthesized from simpler derivatives. The reaction energies were calculated within the framework of the density functional theory at the level B3LYP/6-311+G(d.p)//B3LYP/6-311+G(d.p) for 6 model systems and 7 various alkyl groups. The most important systems were cis-1,2-dialkylcyclopropanes 1 synthesized from two mono derivatives and sterically crowded derivatives of bicyclo[2.2.2]octane 2 with C(3) symmetry. The scales of steric effects evaluated from the two models were rather different: the first scale depended in effect only on the C atoms in the alpha and beta positions and the effects were almost equal for all primary alkyls. The second scale depended also on the gamma position and the effect of the CH(2)-t-Bu group was much greater than that of the ethyl group. Any relationship between various systems was found rarely, only in the case of very similar reaction series; even in such cases the relationship was sometimes linear, sometimes distinctly curvilinear. It is concluded that any universal scale of steric effects is in principle not possible since these effects depend specifically on the surroundings of the substituent in a particular reaction. Nevertheless, there is a similarity between various scales; a bulky group appears as bulky in any scale. Therefore, very rough correlations of steric effects are possible.     

Steric influence on the excited-state lifetimes of ruthenium complexes with bipyridyl-alkanylene-pyridyl ligands

The structural effect on the metal-to-ligand charge transfer (MLCT) excited-state lifetime has been investigated in bis-tridentate Ru(II)-polypyridyl complexes based on the terpyridine-like ligands [6-(2,2'-bipyridyl)](2-pyridyl)methane ( 1) and 2-[6-(2,2'-bipyridyl)]-2-(2-pyridyl)propane ( 2). A homoleptic ([Ru( 2) 2] (2+)) and a heteroleptic complex ([Ru(ttpy)( 2)] (2+)) based on the new ligand 2 have been prepared and their photophysical and structural properties studied experimentally and theoretically and compared to the results for the previously reported [Ru( 1) 2] (2+). The excited-state lifetime of the homoleptic Ru (II) complex with the isopropylene-bridged ligand 2 was found to be 50 times shorter than that of the corresponding homoleptic Ru (II) complex of ligand 1, containing a methylene bridge. A comparison of the ground-state geometries of the two homoleptic complexes shows that steric interactions involving the isopropylene bridges make the coordination to the central Ru (II) ion less octahedral in [Ru( 2) 2] (2+) than in [Ru( 1) 2] (2+). Calculations indicate that the structural differences in these complexes influence their ligand field splittings as well as the relative stabilities of the triplet metal-to-ligand charge transfer ( (3)MLCT) and metal-centered ( (3)MC) excited states. The large difference in measured excited-state lifetimes for the two homoleptic Ru (II) complexes is attributed to a strong influence of steric interactions on the ligand field strength, which in turn affects the activation barriers for thermal conversion from (3)MLCT states to short-lived (3)MC states.     

Steric and hydrophobic effects of substituents in extraction of metal complexes with O,O-dialkyldithiophosphoric acids

The two-phase stability constants (equilibrium constants) of metal complexes MA(n) [M(n)(+) = Ni(II), Zn(II), Cd(II), Pb(II), Tl(I), In(III)] with a series of O,O-dialkyldithiophosphoric acids in the system water-organic solvent) have been determined. By use of correlation analysis the role of the steric and hydrophobic effects of the substituents at the phosphorus atom on the stability constant beta(n) and distribution constant P of the complexes has been elucidated. The data obtained are of use for determining the relationships in the influence of structure of a reagent on its extraction properties and on the conditions for practical application of O,O-dialkyldithiophosphoric acids for metal extraction.     

Effect of a water-ethanol solvent on the stability of copper(II) complexes with L-tyrosine

Stability constants of copper(II) mono- and bis-complexes with L-tyrosine were determined by the potentiometric titration method. Gibbs energies of the transfer (Δ_tr G ⁰) of a ligand and a complex ion from water into water-ethanol solvents were calculated. Stability of the complexes [CuHTyr]⁺ and [Cu(HTyr)₂] increases as the ethanol concentration in solutions increases. Increasing stability of the complexes is promoted by weakening solvation of ligand donor groups entering into coordination.