Complexes Of Donors Research Articles

Quenching of triplet states by Schiff base nickel(II) complexes

Rate constants for the quenching of triplet states of several molecules by certain Schiff bases and their Ni(II) chelate complexes have been measured using the technique of laser photolysis. In the case of the parent ligands the dependence of the quenching efficiency on the energy of the triplet states being quenched suggests triplet–triplet energy transfer as the quenching mechanism and allows estimates of the energies of the triplet states of the ligands which are often too short-lived to be calculated. The observed variation in rate constants for quenching by diamagnetic trans-planar Schiff base Ni(II) complexes is also explained as being due to triplet–triplet energy transfer with high energy triplet donors exciting triplet ligand states and low energy donors exciting triplet ligand field states in the quencher. It is suggested that the much lower efficiency of quenching by paramagnetic, tetrahedral and octahedral Ni(II) complexes of low energy triplet donors is due to the stereochemical changes and demonstrates the need for good overlap with d orbitals before spin-allowed exchange transfer of high efficiency can occur to lower lying ligand field states in coordination complexes.

Étude, par mesures de moment dipolaire, de la structure des complexes nitriles-donnhurs de proton

The method of Few and Smith was applied to the study of complexes of various proton donors with nitriles. Interpretation of the experimental results leads to the conclusion that there is a preferred form for these complexes in which the polar moments of the associated molecules are perpendicular. This indicates that the association takes place by means of the π electrons of the triple CN bond, which is in accordance with the values of the ionization potentials of the electronic orbitais of the CN group.

Substituenteneffekte und störung von π-systemen—IXX: Der einfluss von R 3Si- und R 3C-gruppen auf die elektronenstruktur donator-substituierter benzole

The charge transfer maxima of the tetracyanoethylene complexes of donor substituted trimethylsilyl and t-butyl benzenes XC 3H 4D(X  Si(CH 3) 3, C(CH 3) 3; D  OH, OCH 3, OSi(CH 3) 3, N(CH 3) 2) are discussed on the basis of qualitative MO schemes. The increased excitation energies of the silyl derivatives can be attributed to an electron delocalization into unoccupied silicon atomic orbitals of π symmetry. The intramolecular charge transfer transitions in the UV spectra of the para substituted derivatives R 3SiC 6H 4D are shifted to longer wavelengths and thus also demonstrate that the electron transfer to silicon increases with the strength of the donor D (OH < OCH 3 < NH 2 < SH < O ⊝ < N(CH 3) 2).

Crystal field aspects of vibrational spectra: II. First-row transition metal(II) complexes with nitrogen donors

The M-N stretching frequencies and certain metal-sensitive ligand vibrations of complexes of nitrogen donors with first-row divalent transition metal ions vary with d orbital population in a way which is consistent with crystal field theory. The effects on the frequencies of tetrahedrally co-ordinated Co(II) and Zn(II) and of tetragonally distorted Cu(II) are discussed.

Solvent facilitated proton transfer in hydrogen bonded complexes of proton donor and acceptor groups in model protein systems

The tautomeric equilibrium between a hydrogen bonded complex and a proton transfer complex (hydrogen bonded ion pair) in a number of phenol-amine systems, is dependent, among other factors, on the polarity of the solvent. The formation of a proton transfer complex is accompanied by a decrease in free energy of 2–6 kcal/mole. Based on spectrophotometric studies of the base dependent ionization of several phenol derivatives in aqueous dioxane solutions, it is likely that proton donor sidechain groups possessing a p K a in the range 3–10, can participate in proton transfer complexes with basic groups whose conjugate acids possess a greater p K a, in nonpolar and partially polar regions of a protein. It is suggested, that the decrease in free energy accompanying the formation of a proton transfer complex can affect the direction of a protein conformational transition.

Nuclear magnetic resonance studies of ketone•BF3 complexes. II. The boron trifluoride catalyzed condensation of acetone

A detailed study of the reactions of the acetone•BF3 complex in excess acetone has been carried out by "F nuclear magnetic resonance (n.m.r.) spectroscopy. It is shown that rapid reversible fluorine exchange processes occur in solution, in addition to the slower aldol condensation–dehydration reaction. Two of the new 19F absorptions which appear during the reaction are shown to arise from BF3 complexes of donors which are formed by condensation and dehydration of acetone. The rate of the aldol condensation reaction is shown to fall off markedly as an increasing proportion of the BF3 becomes complexed with water; thus BF3 catalysis of the reaction appears to depend on the existence of a ketone–BF3 donor–acceptor bond. 1H and 19F n.m.r. studies of the acetone•BF3 complex in methylene chloride throw some light on the apparent greatly decreased reactivity of the complex in this solvent. Evidence is presented for the reversibility of the first step of the condensation reaction.

Dipole moments and charge-transfer parameters of the iodine complexes of some oxygen and sulfur donors

Dipole moments of the charge-transfer complexes of some sulfur (sulfide and thiocarbonyl) and oxygen (ether and carbonyl) donors with iodine have been determined and the various charge-transfer parameters have been calculated by making use of the charge-transfer transition energies and heats of formation of the complexes. The contribution of the charge-transfer structure in the ground state appears to be greater in the complexes of sulfur donors than in the complexes of oxygen donors.

Charge transfer interaction between tetrahalo p-benzoquinones and polynuclear aromatic hydrocarbons

The molecular charge-transfer absorption bands have been detected in mixture of tetrahalo p-bezoquinones and polynuclear aromatic hydrocarbons in chloroform, carbon tetrachloride and methyl acetate solutions. The plot of charge-transfer interaction energies against ionization energies of hydrocarbons was found to be linear. For the isolated complexes, the charge transfer interaction was found to be in the order hγ (iodanil) > (bromanil) > hγ (chloranil) for all hydrocarbons, although in solution the order was anomalous. For the complexes of the same donor with different acceptors the oscillator strength of the charge-transfer absorption band was found to increase with an increase in equilibrium constant. No such correlation was noticed with complexes of different donors with the same acceptor. The equilibrium constant increased with the lowering of the ionization energy of the hydrocarbon. There was nearly a hundredfold increase in dipole moment of the complexes on electronic excitation. The evaluation of Mulliken's a and b co-efficients showed that the charge transfer-complexes of tetrahalo p-benzoquinones with polynuclear aromatic hydrocarbons are weak complexes.

Nuclear Magnetic Resonance Studies of Boron Trifluoride Complex Formation

HIGH-resolution nuclear magnetic resonance spectroscopy affords a uniquely penetrating method of studying co-ordinate complex formation between boron trifluoride and electron donor substances. The examples investigated so far comprise oxygen compounds (water, alcohols, ethers and ketones), but the method promises wide application. Details of a typical experiment follow. Fluorine magnetic resonance (at a frequency of 34.2 Mc./s.) is observed in a quantitatively prepared liquid mixture of two donor substances (as, say, methanol and ethanol) with boron trifluoride, the latter in quantity insufficient to saturate both donors. The temperature is controlled during a given measurement, the usable range extending from somewhat above room temperature to about − 100° C. At sufficiently low temperatures two relatively narrow resonance peaks are seen, separated in the case cited by some 70 c./s. These correspond to the co-ordination complexes of the respective donors. Measurement of the peak areas (assumed to be proportional to the volume concentrations) in mixtures of various composition indicates that reversible equilibrium is established between the two donors and their respective complexes. The mass law constant may thus be measured. The values at a series of different temperatures yield the corresponding enthalpy and standard entropy changes for the interconversion reaction.