Showing papers on "Substituent published in 1983"

Metal Isocyanide Complexes

Abstract: Publisher Summary This chapter discusses metal–isocyanide chemistry. In the interim, reviews have appeared on specific aspects of isocyanide chemistry. The generally accepted valence bond and molecular orbital (MO) approach to the bonding of metal isocyanides has been well described in Treichel's review, and has been used to rationalize variations in infrared (IR) stretching frequencies between bonded and nonbonded isocyanides and the better π-acceptor qualities of aryl versus alkyl isocyanide groups. A number of new synthetic routes to isocyanide complexes of chromium, molybdenum, and tungsten have been investigated. A number of attempts have been made over the years to develop reproducible synthetic routes to six- and seven-coordinate isocyanide complexes of molybdenum and tungsten. A number of routes have been employed for the synthesis of metal–isocyanide complexes by generating the isocyanide ligand on the metal atom. A number of monomeric complexes have been prepared, which may be considered as substitution products of the [Co(CNR) 5 ] + and [M(CNR) 4 ] + cations. Electrochemical investigations have been reported on a range of homoleptic and mixed carbonyl–isocyanide complexes, in attempts to rationalize substituent effects on the isocyanide with the electronic structure of the metal. Insertion reactions of isonitriles into metal–alkyl or metal–aryl bonds are now well established, occurring with metal–alkyl or –aryl groups from group IVA to IB and, recently, with uranium and thorium carbon bonds.

Photoelectron spectra of acenes. Electronic structure and substituent effects

Abstract: The valence-electronic structure of the first three members of the acene series, benzene, naphthalene and anthracene, seems to be confirmed now by their photoelectron (PE) spectra. This especially holds for the r-ionizations which, according to their specific behaviour, can be assigned to different ionization modes. The vibrational fine structure of the observed systems is discussed. The effects of a series of substituents on the electronic structure of the parent molecules were traced in their PE spectra and intercompared. The results for methyl, chloro, methoxy and cyano substitution are described in detail. The effect is most pronounced for the two lowest energy -ionizations (i.e. HOMO and SHOMO in the Koopmans' picture) and can be described by the shift of the mean energy ("center of gravity") of HOMO and SHOMO, and the additional splitting compared to the unsubstituted molecule. Both parameters show good linear dependence for the three series of compounds, thus allowing to formulate general substituent parameters. A linear dependence of the lowest ionization energy with Hammett constants is found to exist as well. INTRODUCTION Acenes are a series of molecules whose properties often can be described as a monotonic function of the number of benzene rings, e.g. the similarities in position and intensity of bands in electronic spectra (1,2), first ionization energy (3), electron affinity (4-7), as well as reduction and oxidation potentials (8-10). It is known that these properties depend mainly on the characteristics of the outer, occupied and unoccupied, molecular orbitals, rather than on the total electronic configuration. Ionization energies of the valence electrons measured by molecular photoelectron (PE) spectroscopy provides, within the limitations of Koopmans' theorem, a valuable method for the determination of energies of the outer occupied molecular orbitals. In combination with quantum chemical calculations other characteristics as symmetry, space distribution and "origin" are obtained, all of them yielding a picture which is usually described as the electronic structure of a molecule. In this study the electronic structures of a number of monosubstituted benzenes, naphthalenes and meso-substituted anthracenes are compared with that of the non-substituted parent molecules in order to determine common effects of substituents which would enable to predict these effects in a quantitative way. Such an approach has been used several times as yet, but these studies have concerned mainly the effect of substituents on one of the molecules mentioned above. Pioneer work in the comparison of these effects can be found in the approach of Heilbronner (11) who indicated the parallelism of substituent effects on certain energy levels of benzene, naphthalene and anthracene. However, it is important to notice that until recently even the location of all n-ionizations in the PE spectra of naphthalene and anthracene was based mainly on considerably diverging calculations and thus was rather uncertain. Now consistent experimental evidence exists for the assigment of ir -ionizations which certainly is necessary to rely on such correlations and makes the intended comparison more than worthwhile. EXPERIMENTAL Compounds: All compounds were of high purity, redistilled or recrystallized before use and checked by mass spectrometry. The 9-Iand 9-CF3-anthracene were synthesized by Dr. M. Mintas. PE spectra: All spectra were recorded on a Vacuum Generators UV-G3 spectrometer (12) using He! (FWHM of 35 meV and 15 meV for fine structure determinations) and He!! (FWHM of 50 meV) excitation. The inlet system was heated if necessary and the spectra calibrated by addition of argon and xenon to the sample gas flow. Calculations: Standard PPP calculations of the n -electron levels were performed for most of the compounds. The correlation with assigned lower n -ionizations up to 15 eV is reasonably good. 289 RESULTS AND DISCUSSION ELECTRONIC STRUCTURE OF THE PARENT MOLECULES: BENZENE, NAPHTHALENE AND ANTHRACENE For the first three members of the acene series, benzene, naphthalene and anthracene, the valencr electronic structure can now be considered as well established; different techniques of obtaining their PE spectra were of great help in the assignment. Thus, a comparison of Hel and Hell spectra (fig. 1)) shows that cross sections of T-ionizations are relatively enhanced under Hell radiation (13,14). On the other hand, in the perfluorinated derivatives TI-ionizations change their energy much less than G -ionizations which are shifted to higher energy by 2-3 eV (15). Especially under Penning ionization the -ionizations observed are pronounced (16). All this in combination with quantum chemical calculations yielded the following iT-electronic structures (fig. 2); the listed energies correspond to vertical ionizations and the wave numbers to observed vibrational modes. E/eV 7 (44o 1) 1812, !32b9cII !)::i b29 b19 -•22_bl(1O4O 1ol82SJj I11) b9 b3.QOJ b3f 290 L. KLASINC et

Self-termination and electronic spectra of substituted benzyl radicals in solution

Abstract: Several substituted benzyl radicals have been generated in liquid cyclohexane by the photolysis of symmetrically substituted dibenzyl ketones. The rate constants for self-termination to substituted bibenzyls, and absorption spectra of the transient radicals, have been measured by optical modulation spectroscopy. The termination rates are generally well described by the von Smoluchowski equation with a spin factor of 1/4. Deviations are discussed in terms of steric retardation and the unreliability of the estimated reaction diameters of radicals with bulky substituent groups. Absorption spectra of the three monomethyl-, the three monochloro-, two dichloro- and the three monomethoxybenzyl radicals demonstrate the influence of the substituents on the energy levels of the benzyl system.

Kinetics of acid-catalyzed cleavage of procyanidins

Abstract: Comparisons of the rates of cleavage of isomeric procyanidin dimers in the presence of excess phenylmethane thiol and acetic acid showed that compounds with a C(4)–C(8) interflavanoid bond were cleaved more rapidly than their C(4)–C(6) linked isomers, that 2,3-cis isomers with an axial flavan substituent were cleaved more rapidly than a 2,3-trans isomer with an equatorial substituent, and that cleavage rate was independent of the stereochemistry in the terminal unit. Cleavage rate constants followed Arrhenius temperature dependence. The C(4)–C(8) interflavanoid bond in 2,3-cls procyanidins should be labile to cleavage at 20°C over a pH range of 3.6 to 5.4. Reaction of loblolly pine bark tannins in the presence of excess (+)-catechin with acetic acid rapidly produced oligomeric procyanidins. The ratio of the procyanidin Bl to B7 decreased from 2.9:1 after 4 hours to 1.3:1 after 50–100 hours at 90°C. The structure and reactions of condensed tannins are interpreted in light of these results.

Process for sizing paper with anionic hydrophobic sizing agents and cationic retention aids

Abstract: Sizing agents, some of which are known compounds and some of which are novel aromatic compounds which contain in the aromatic nucleus a single hydrophobic substituent containing at least 5 carbon atoms and at least one anionic group, said hydrophobic substituent being linked to the aromatic nucleus through an ester, amide, urethane or urea bridge, with the CO group of ester and amide bridges being attached to the hydrophobic radical and a methylene group being optionally present between said bridges and the aromatic nucleus, are particularly suitable, together with commercially available retention aids, for use in a process for the mass sizing of paper or cardboard or for the surface sizing of paper.

An all-nitrogen aromatic ring system: structural study of 4-dimethyl-aminophenylpentazole

Abstract: The crystal structure of the title compound at 128 K confirms the presence of the pentazole ring which, as a substituent, appears to have an inductive effect similar to that of the nitro group.

Phloretin and phloretin analogs: Mode of action in planar lipid bilayers and monolayers

Abstract: Phloretin and other neutral phloretin-like molecules are able to decrease the electrostatic potential within neutral lipid bilayers and monolayers. The relationship between the change in the dipole potential and the aqueous concentration of the molecule is well described by a Langmuir isotherm. From the Langmuir isotherm, the apparent dissociation constants (K D A ) and the maximum dipole potential change (ΔΔψ max) are obtained for the different phloretin-like molecules tested. Considering the phloretin analogs as derivatives of acetophenone containing two kinds of substituents, one on the benzene ring and another on the carbon chain, it is found that (a)K D A is related to the hydrophobicity of the compound and is also a function of the position of the hydroxyl substituent in the ring; (b) from the dependence ofK D A on the length of the acyl chain, it is estimated that the free-energy change is ∼650 cal/mole CH2; (c)ΔΔψ max is not a simple function of the dipole moment of the molecule but depends on the substituent on the carbon chain and on the position and number of hydroxyl groups on the benzene ring; (d) phloretin adsorption parameters are a function of membrane lipid composition. The results are discussed in terms of the effect of these compounds on chloride transport in red blood cells.

Investigations of the nature of dehydrogenation of the α-carbon atom in the oxidation of amines coordinated to ruthenium

Abstract: The oxidations of the amine complexes Ru(bpy)₂(A-B)²⁺ (where A-B = ampy [2-(aminomethyl)pyridine], d₂-ampy [2-(1,l-dideuterioaminomethyl)pyridine], and Meampy [2-( 1-aminoethy1)pyridinel and bpy = 2,2'-bipyridine) in aqueous solution to the corresponding imine species have been studied by using flash photolysis techniques. The rate-determining step of the reaction involves removal of a hydrogen from the a-carbon atom and the concomitant intramolecular transfer of two electrons from ligand to metal. Deuterium exchange and deuterium isotope effect studies, and stereochemical arguments, are consistent with a mechanism involving either (i) a hydride transfer from C to Ru or (ii) a base (solvent water)-assisted proton abstraction. Chirality studies indicate no loss of configurational integrity about the metal center during oxidation, which favors the second proposal. The X-ray crystal structure of the more rapidly oxidized diastereoisomer of [Ru(bpy)₂(Meampy)]S₂O₆.5H₂O it to be the λS(δR) form, in which the methyl substituent on the a-carbon atom is equatorial and the hydrogen substituent axial.

Stereochemical investigation on asymmetrically modified Raney nickel catalyst. Mode of interaction between modifying reagent and substrate in the enantioface-differentiating process.

Abstract: The mode of enantioface-differentiating hydrogenation with asymmetrically modified Raney nickel (MRNi) was investigated by using various sorts of modifying reagents and substrates. The optical yields (O.Y.) of the hydrogenation of methyl acetoacetate over MRNi for different modifying reagents occured in the following order: (Remark: Graphics omitted.), (Remark: Graphics omitted.). The hydrogenation of various substrates of the structure, (Remark: Graphics omitted.), over tartaric acid-modified Raney nickel (TA–MRNi), gave high optical yields only when the substituent –Y functioned as a good hydrogen bond acceptor like a carbonyl group, and two pairs of hydrogen bondings between substrate and two hydroxyl groups of tartaric acid were available. As for the stereochemistry of the reaction in such a case, (R,R)-TA–MRNi always gave (R)-product in excess. Based on the stereochemistry of the reaction and the role of hydrogen bonds between modifying reagent and substrate, a stereochemical model is proposed to elu...

Helical Configuration of Poly(Iminomethylenes) - Synthesis and Cd Spectra of Polymers Derived from Optically-Active Isocyanides

Abstract: Each of nine chiral isocyanides, R*-N=C, in enantiomerically pure or almost pure form, is polymerized to the corresponding poly(iminomethylene), (R*-Ne<),. The group R* either contains at least one substituent (carboxylic ester, phenyl, phosphinyl) or is unsaturated (C=C, CeC). The polymers are of the rigid-rod type with a helical main chain of carbon atoms. Because of the chirality of R*-N=C either the P or the M helix is preferentially formed. CD spectra and optical rotation at 578 nm are determined and compared with those of the model compounds R*-N=CH(t-C,H9) and the monomers. This comparison and earlier calculations on (t-C,H,-N=C<), allowed the assignment to each polymer of the excess screw sense. The screw sense is also theoretically predicted by two approaches, a thermodynamic and a kinetic one. The latter approach gives an appreciably better fit with the experimental results.

Ab initio MO calculations and 17O NMR at natural abundance of para‐substituted acetophenones

Abstract: 17O NMR chemical shifts and calculated (ab initio MO theory) electron densities are reported for a series of para-substituted acetophenones, X[BOND]C4H6[BOND]COCH3, where X = NH2, OCH3, F, Cl, CH3, H, COCH3, CN, NO2. The 17O shifts are very sensitive to the para substituent and cover a range of some 51 ppm. Donors induce upfield shifts and acceptors downfield shifts. The substituent chemical shifts (SCS) correlate precisely with sI and sR+ using the Dual Substituent Parameter (DSP) method. The derived transmission coefficients rI and rR indicate that polar and resonance mechanisms contribute approximately equally to the observed substituent effects. The shifts also correlate well with calculated p-electron densities (slope = 1500 ppm per electron) confirming their electronic origin. lmath image values are also reported, and the role of the average excitation energy, DE, in determining 17O SCS values is discussed. It is concluded that variations in DE are minor and that the local D-electron density is the dominant feature controlling 17O SCS values.

Radiolabeled angiotensin converting enzyme inhibitors for radiolabeling mammalian organ sites

Abstract: Radiolabeled angiotensin converting enzyme inhibitors having the general formula: ##STR1## wherein X is a radiolabeling substituent, preferably a radiolabeling halogen substituent such as 123 I, 125 I, 131 I, 75 Br or 77 Br, Y is hydroxy, alkoxy containing from 1 to 6 carbon atoms, phenoxy, benzyloxy, or amino, R1 is hydrogen or methyl, R2 is hydrogen, an ester moiety hydrolyzable under mammalian in vivo conditions, such as alkyl of 1 or 2 carbon atoms, phenyl or benzyl, or an ionically bonded anion of a physiologically acceptable non-toxic salt, and m is an integer of from 1 to 6, are useful as in vivo radioimaging compounds for mammalian sites.

Substituent-induced chemical shifts in 3- and 4-substituted styrenes: definition of substituent constants and determination of mechanisms of transmission of substituent effects by iterative multiple linear regression

Abstract: Iterative multiple linear regression analysis of substituent-induced chemical shifts in 3- and 4-substituted styrene derivatives is used to redefine σF and substituent constants (with respective standard errors of ca. 0.01 and 0.005). Correlations with the data for various model systems indicate that these substituent constants are applicable to data sets involving both chemical shift and acidity measurements in aprotic media. The approach we have used involving a homogeneous data set with a large number of variables is particularly suited for providing well-defined dual substituent parameter constants for the more common neutral substituents.Correlations for individual nuclei are entirely consistent with a previously proposed model of substituent effects involving field, resonance, and polarization effects. From these results and the results for other model systems, it is concluded that σF and are respectively pure field and resonance substituent constants. It is pointed out that a combination of detaile...

Carbon‐13 NMR spectra of hydroxylated bile acid stereoisomers

Abstract: Carbon-13 NMR signals were assigned for the complete set of the 26 theoretically possible isomers of methyl 5β-cholanates having one to three hydroxy groups at positions C-3, C-7 and/or C-12 in the nucleus. Substituent effects on the 13C NMR shielding data serving to characterize the position and configuration of the hydroxy groups are discussed.