Showing papers in "Israel Journal of Chemistry in 1972"
TL;DR: The lattice controlled solid-state polymerisation of three different modifications of 2,4-hexadiin-1,6-diol-bis (phenylurethane) was investigated by X-ray and optical methods.
Abstract: The lattice-controlled solid-state polymerisation of three different modifications of 2,4-hexadiin-1,6-diol-bis(phenylurethane) was investigated by X-ray and optical methods. The polymerisation is a homogeneous reaction. The polymer grows in the form of single chains within the crystal of the monomer. The chains extend along a definite crystallographic direction. Monomer and polymer are isomorphous and monomer-polymer single crystals of various compositions are obtained up to a quantitative conversion in the case of modification I or III. Phase separation into a mesomorphic polymer and oriented monomer phase was observed on annealing partially polymerised single-crystals of modification II below the transition point to modification III.
93 citations
TL;DR: The first recorded direct Stevens rearrangement of a phosphorusylid was reported in this article, where the ylid is bonded to the nickel through the C-atom with resulting hybridization from sp2 towards sp3.
Abstract: The Stevens rearrangement of the triarylphosphorusylids R3 P=CHR′ (III) (R′ = H, CH3, C2 H5) to diarylbenzylphosphines (V) is catalyzed by nickel complexes (e.g., (COD)2 Ni (I)). Attempts to prepare trimesitylphosphine-methylene result in direct formation of dimesityl (mesitylmethyl) phosphine (XII) — the first recorded direct Stevens rearrangement of a phosphorusylid. Phosphorusylids react with nickeltetracarbonyl without rearrangement to give complexes of the type R3P=CHa · Ni(CO)3 (XIV). The structure of XIV has been established by the IR and NMR spectra and confirmed by an X-ray structural determination.1 The ylid is bonded to the nickel through the C-atom with resulting hybridization from sp2 towards sp3.
73 citations
TL;DR: In this paper, it was shown that the PE spectrum of a molecule M of Cs or higher symmetry can lead either to the natural order of the corresponding delocalized orbitals or to the inverted order.
Abstract: The competitive through-space and through-bond interaction of two symmetry-equivalent, semi-localized orbitals φa, φb, (= π or lone-pair orbitals) in a molecule M of Cs or higher symmetry can lead either to the natural order of the corresponding delocalized orbitals, i.e., ψ− ≈ (φa - φb)/√2 above ψ+ ≈ (φa - φb)/√2 or to the inverted order, i.e., ψ+ above ψ−. It is shown that one can discriminate between these two situations through an analysis of the PE spectrum of a molecule M′ which differs from M only by an additional semi-localized orbital φc of known symmetry behaviour. The method is exemplified by an investigation of the dependence of the n-orbital sequence in 3,6-bridged 1,4-cyclohexadienes on the dihedral angle between the two CCHCHC moieties.
63 citations
TL;DR: In this paper, it was shown that hypoxanthine and 6th-thiopurine are present in aqueous solution predominantly as 1H, 9H-tautomers.
Abstract: Ultraviolet spectra show that hypoxanthine and 6-thiopurine are present in aqueous solution predominantly as 1H, 9H-tautomers. However, a 3-methyl group causes the imidazole ring to assume the 7-NH form. The sequence of anion formation in hypoxanthine is 9(7) 1 and in 6-thiopurine 1 9(7). The NMR spectra suggest that the mono-anion of hypoxanthine is a mixture of tautomers. Protonation always takes place in the imidazole ring, with the exception of 3-methyl derivatives. In the latter, formation of mono-cations involves the pyrimidine moiety.
60 citations
TL;DR: In this paper, Bacillus megaterium, in suspension, is used to study the biological effects of the three principal products of radiolysis of water, i.e., e−aq, OH and H2 O2.
Abstract: Spores of Bacillus megaterium, in suspension, are used to study the biological effects of the three principal products of radiolysis of water, i.e., e−aq, OH and H2 O2. A model is proposed accounting for the action of OH and H2 O2. This model involves two reactions:
in which RH2 is a “target” molecule, and is a lethally damaged molecule. Experimental evidence is given supporting this model. These studies of the effects of radiolysis products of water are extended by studies using sensitizers such as biacetyl, NO−3, and Ag+ in varying concentrations. Evidence for a possible relationship between peak sensitivities in anoxic conditions and chemical properties of the sensitizers is discussed. These results are also discussed with regard to the oxygen effect, and the general chemical characteristics of an efficient tumor sensitizer are outlined.
52 citations
TL;DR: The molecular structure of kinamycin C has been determined by means of an X-ray study of its p-bromobenzoate derivative, C31H23N2O11Br as discussed by the authors.
Abstract: The molecular structure of kinamycin C has been determined by means of an X-ray study of its p-bromobenzoate derivative, C31H23N2O11Br. The crystals are monoclinic, P21, with four molecules of the derivative and four molecules of benzene in a unit cell of dimensions: a = 18.404 ± 0.005, b = 21.299 ± 0.004, c = 9.049 ± 0.001 A, and β = 90.07 ± 0.03°. Reflection intensities were measured visually from equi-inclination Weissenberg photographs taken with Cu Ka radiation. The structure was solved by the heavy-atom method, and was refined by the block-diagonal-matrix least-squares method using anisotropic temperature factors for all the atoms. The final R factor is 8.9%. The stereostructures of the two crystallographically independent molecules are almost identical. The skeleton of the molecule is made up of four fused rings: two of these form a 1,4-naphthoquinone system and the rest are a pyrrole and a cyclohexene ring. A cyano group is bonded to the nitrogen atom of the pyrrole ring, which is rather unusual in an antibiotic.
50 citations
TL;DR: In this article, it was shown that increasing concentrations of ketonic agents, such as diacetyl, acetone, acetophenone and p-nitroacetophenone, cause an increase in the radiosensitivity of aqueous anoxic spores to an extent that is some 40% of that achieved by O2 alone; no effect on radiosensitivity was observed with agents in the presence of O2.
Abstract: Increasing concentrations of the ketonic agents, diacetyl, acetone, acetophenone and p-nitroacetophenone, cause regular increases in the radiosensitivity of aqueous anoxic spores to an extent that is some 40% of that achieved by O2 alone; no effect on radiosensitivity is observed with agents in the presence of O2. The effectiveness of these agents and O2 as radiosensitizers ranks on a molar basis in the same order as that of their electron affinities, indicating that their modes of action are in some way associated with radiation-induced electrons. A lack of radiosensitizing action of diacetyl and p-nitroacetophenone in dried spores and of p-nitroacetophenone in suspensions of spores in non-aqueous liquids supports the notion that the origin of their actions is within the water inside the cell.
48 citations
TL;DR: In this paper, the spatial arrangement of subunits in protein complexes is determined by an application of a new method based on X-ray experiments, where the subunits with heavy atoms are labeled and the distances between subunits can be found from Xray experiments.
Abstract: The determination of the spatial arrangement of subunits in protein complexes is possible by an application of a new method. After appropriate labelling of the subunits with heavy atoms the distances between the subunits can be found from X-ray experiments. Theoretical and experimental considerations allow us to deduce these distances and to find the relative positions of the subunits within the protein complex.
46 citations
TL;DR: Combined pulse radiolysis and inactivation studies on the reactions of inorganic radical-anions with several enzymes lead to identification of amino acid residues essential to enzymic activity.
Abstract: Some inorganic radical-anions, which have very strong absorption spectra, have some oxidative reaction specificity with amino acids. Combined pulse radiolysis and inactivation studies on the reactions of these radicals with several enzymes lead to identification of amino acid residues essential to enzymic activity. Data are reviewed for the enzymes, lysozyme, ribonuclease, trypsin, α-chymotrypsin, papain and carboxypeptidase. The reducing radical, CO2−, is more selective than the hydrated electron in its reactions with enzymes and proteins. Results from pulse radiolysis studies in electron-transfer and other reactions involving CO2− are compared with results from inactivation studies with lysozyme and ribonuclease.
37 citations
TL;DR: In this article, a heptane-3-one was synthesized by the reaction of cyclopentene and chlorosulphonyl isocyanate, and the structure of the product was established by its hydrolysis to cis-2-amino-cyclopentanecarboxylic acid.
Abstract: 2-Azabicyclo [3.2.0] heptane-3-one was synthesized by the reaction of cyclopentene and chlorosulphonyl isocyanate. The structure of the product was established by its hydrolysis to cis-2-amino-cyclopentanecarboxylic acid.
33 citations
TL;DR: In this paper, two-component solid solutions of β-arylacrylamides, prepared by melting and crystallising equimolar quantities of the amides tr-ArCH:CHCONE where Ar=C6H5, p-MeOC6H4, pClC6h4,p-MeC6 h4, and 2-thienyl, which appeared in the α crystal form, gave three photoproducts for each system studied: two α-truxillic homodimers, derived from the respective
Abstract: Irradiation (λ >300 nm) of eight two-component solid solutions of β-arylacrylamides, prepared by melting and crystallising equimolar quantities of the amides tr-ArCH:CHCONE where Ar=C6H5, p-MeOC6H4, p-ClC6H4, p-MeC6H4, and 2-thienyl, which appeared in the α crystal form, gave three photoproducts for each system studied: two α-truxillic homodimers, derived from the respective monomers, and an α-truxillic heterodimer. No other isomers were found. Similarly, two-component solid solutions were prepared by melting and crystallising ring-substituted cinnamic acids (tr-ArCH:CHCO2H, where Ar=C6H5 (α-form), p—ClC6H4(β), p-MeC6H4(α), p-MeOC6H4(γ), o-ClC6H4(β) and o-CH3C6H4(γ)) and irradiated. A solid solution of two α-type acids (each photodimerising in the solid to α-truxillic acid) behaved like the amides, affording two α-truxillic homodimers and an α-truxillic heterodimer. A solid solution of an α-type acid with a β-type acid (photodimerising to β-truxinic acids) gave six photoproducts: three α-truxillic acid dimers and three β-truxinic acid dimers (i.e., four homodimers and two heterodimers). Irradiation of a solution of a γ-type acid (non-photodimerising in the solid state) with an α-type acid afforded α-truxillic dimers (two homodimers and one heterodimer). The above results are consistent with mutual miscibility of the two components which gives rise to mixed crystal formation and subsequent photo-behaviour within each crystal which is characteristic of the particular lattice type. However, irradiation of the mixed crystals from p-methoxycinnamic acid (γ-type) and p-chlorocinnamic acid (β-type), which afford two α-truxillic dimers (methoxy-homodimer and methoxy, chloro-heterodimer) and two β-truxinic dimers (chloro-homodimer and methoxy, chloro-heterodimer) indicates that new phases, unobserved for the pure compounds, can also be generated. The new dimers were all characterised by the NMR and mass spectra of their dimethyl ester derivatives and, when possible, by conversion to known compounds.
TL;DR: In this paper, the CT band of Eu3+ in aqueous solution, in the presence of scavengers for H atoms, gives rise to formation Eu2+ and evolution of molecular hydrogen.
Abstract: Irradiation in the CT band of Eu3+ in aqueous solution, in the presence of scavengers for H atoms, gives rise to formation of Eu2+ and evolution of molecular hydrogen. The kinetics of these processes were studied as functions of the concentrations of Eu3+ and scavenger, of pH, and of light intensity. The results are interpreted by means of a complex scheme in which an excited Eu2+ donates an electron to a proton, the resulting H* initiating a series of dark reactions which lead to evolution of H2 and oxidation of the scavenger.
TL;DR: The mechanisms by which γ-radiation produces strand breaks in DNA were investigated using model systems: breakage of the sugar phosphate bond of deoxynucleotides irradiated in aqueous solution was equated to a reaction which would produce a strand break in DNA.
Abstract: The mechanisms by which γ-radiation produces strand breaks in DNA were investigated using model systems: breakage of the sugar phosphate bond of deoxynucleotides irradiated in aqueous solution was equated to a reaction which would produce a strand break in DNA. Release of inorganic phosphate from 5′ deoxynucleotides is caused by a radiation-produced hydroxyl radical abstracting a hydrogen atom from the deoxyribose moiety. Decay of the sugar radical so formed leads to phosphate release. After release of inorganic phosphate from deoxycytidylic acid two products are formed which consist of the cytosine base moiety attached to two or three carbon atoms of the sugar. Radioprotective sulphydryl compounds can repair the sugar radical by donation of a hydrogen atom to the damaged site. In thymidine 3′5′ diphosphate, reaction of OH* radicals with the base moiety as well as with the sugar can lead to inorganic phosphate release, both reactions leading to breakage of both 3′ and 5′ sugar phosphate bonds in the same molecule. When thymidine 3′5′ diphosphate is irradiated in the presence of oxygen, labile phosphate esters are produced in yields which are large compared to the inorganic phosphate yield. Reaction of 5-bromouracil moieties in DNA with hydrated electrons can also lead to strand break production. Transfer of the electron to 5-bromouracil from the thymine anion radical was found to be an efficient reaction. Other species such as HCO2−, O2−, and the cytosine anion radical can also donate electrons to 5-bromouracil. The results and conclusions of these experiments are discussed with reference to reactions which could occur in DNA.
TL;DR: The reversible photocyclisation of cis-stilbene analogues to the corresponding dihydrophenanthrenes is not slowed down at high viscosities of the medium.
Abstract: The reversible photocyclisation of cis-stilbene analogues to the corresponding dihydrophenanthrenes is not slowed down at high viscosities of the medium. Its attenuation at low temperatures is a true temperature effect. Methylation affects the photocylisation significantly only if it results in strong steric interactions in the cyclisation product. Photocyclodehydrogenation of stilbenes with oxygen to the corresponding phenanthrene analogues passes through the 4a, 4b-dihydrophenanthrenes, and can be sensitized by various compounds at low concentrations. Photo-oxidation with iodine probably involves dehydrogenation of ground-state dihydrophenanthrene with atomic iodine. Electronically excited dihydrophenanthrenes may also be involved in the photo-oxidation.
TL;DR: In this article, the applicability of the pulse radiolysis technique for understanding the election transfer mechanism in redox proteins is presented, and it is also suggested that the observed redox reactivity of the protein is correlated to its structural features.
Abstract: The applicability of the pulse radiolysis technique for understanding the election transfer mechanism in redox proteins is presented. It is also suggested that the observed redox reactivity of the protein is correlated to its structural features.
TL;DR: In this article, the γ-radiolysis of ethylene glycol, meso-erythritol, 2-deoxy-ribose, and alkyl phosphates has been studied in diluted (10−2 M), N2 O-saturated, deoxygenated aqueous solutions.
Abstract: The γ-radiolysis of ethylene glycol, meso-erythritol, 2-deoxy-ribose, and alkyl phosphates has been studied in diluted (10−2 M), N2 O-saturated, deoxygenated aqueous solutions. Products and their G values have been determined. Reaction schemes were derived from complete material balances. In the polyols the primary attack of the radiolytically formed OH radicals and H atoms leads to α, β-dihydroxyalkyl radicals (A) which eliminate water to give substituted α-carbonyl-methyl radicals: (B)
Disproportionation reactions of the radicals A and B yield products having the structural units COCHOH and COCH2 —. In ethylene glycol a chain reaction is induced, radical B abstracts a hydrogen from ethylene glycol to give acetaldehyde and radical A. In 2-deoxy-ribose the major attack is at C-1. Products from this radical are 2-deoxy-ribonic acid and 2,5-dideoxy-ribonic acid. In the formation of the latter a rearrangement is involved. With trimethyl phosphate, the reaction of the solvated electron is only small (2 × 10s 1/mole sec), as shown by pulse radiolysis. Dimethyl and methyl phosphates do not react with the solvated electron to a measurable extent. The attack of the OH radical apparently leads to a cleavage of the alkyl phosphate linkage.
TL;DR: In this paper, the decay of superoxide radicals in the presence of the enzyme was found to be first order in both enzyme and superoxide concentrations, with an apparent second order reaction rate constant of 2 × 109 M−1 sec−1, decreasing slightly as pH is increased from 5 to 9.5.
Abstract: The pulse radiolysis technique has been employed in the investigation of the dismutation of superoxide radicals, O−2 and HO2, in the presence of superoxide dismutase in aqueous solutions. The decay of superoxide radicals in the presence of the enzyme was found to be first order in both enzyme and superoxide concentrations. An apparent second order reaction rate constant was found to be about 2 × 109 M−1 sec−1, decreasing slightly as the pH is increased from 5 to 9.5. A mechanism which accounts for all our observations is proposed. It includes two steps: (1) formation of a product (EO−2 or E−) from one enzyme (E) molecule and one O−2 radical ion; (2) regeneration of E by a reaction of this product with an additional O−2 ion radical. The reaction rate constants k = (1.4 ± 0.2) × 109 and k = (1.9 ± 0.6) × 109 M−1 sec−1 were measured at pH = 7 in an oxygenated 0.16 M sodium formate solution.
TL;DR: In this paper, the softness parameters σM and σX were calculated as dimensionless quantities for approx. 90 cations and 18 anions for binary common-ion molten salt mixtures.
Abstract: Softness parameters σM for cations and σX for anions, have been calculated as dimensionless quantities for approx. 90 cations and 18 anions. They are given by σM = [σA (Mm+) - σA(H+)]/σA(H+) and σX = [σB(Xa−) - σB(OH−)]/σA(H+) where σA = [σIi(M) + ΔH0h(Mm+)]/m and σB = [-Ea(X) + ΔH0h(Xa−)]/a are Ahrland's parameters. The new normalized and comparative (to the test ions H+ and OH−) softness parameters are positive for soft ions and negative for hard ones. These parameters, obtained independently, are used with a four-coefficient equation to calculate coordinate bond energies for metal halides with acceptable accuracy. Considerations of the average coordination in reciprocal molten salt mixture lead to an expression for the metathesis energy change as proportional to the product of the differences in softness parameters of the two cations and the two anions. An empirical one-coefficient equation involving the softness parameters is proposed to deal with next-nearest-neighbor interactions in binary common-ion molten salt mixtures. These relationships are then used with Blander and Topol's equation to predict the occurrence of irascibility gaps in uni-univalent reciprocal salt mixtures. The gaps found in other systems are also discussed in terms of the softness of the constituent ions.
TL;DR: It is suggested that the earliest coding polynucleotides contained a high proportion of alternating sequences of purines and pyrimidines, and that these sequences coded for polypeptides in which hydrophobic and hydrophylic amino acids alternated.
Abstract: It is suggested that the earliest coding polynucleotides contained a high proportion of alternating sequences of purines and pyrimidines, and that these sequences coded for polypeptides in which hydrophobic and hydrophylic amino acids alternated. Structural properties of such alternating polypeptides are discussed.
TL;DR: In this article, the dipole moments of xanthone, thioxanthone and their halogeno-derivatives were analyzed and it was shown that these molecules are not planar.
Abstract: A study of the dipole moments of xanthone, thioxanthone and their halogeno-derivatives leads to the conclusion that these molecules are not planar. In the xanthene and thioxanthene series, the dipole moments as well as the NMR spectra can be explained by a planar or a non-planar but rapidly inverting model. In the acridan series, the NMR spectra of N-acylacridans show that the molecule is non-planar and inverts. The barrier is 11.7, 13.1 and 13.1 Kcal/mole for the N-acetyl-, N-chloroacetyl- and N-iodoacetyl derivatives, respectively. Equally, it has been observed that at low temperature the rotation around the XCH2 CO bond in the halogenoacetylacridans is hindered.
TL;DR: The crystal structure of cyclo-di-β-alanyl (monoclinic; a = 11.85, b = 5.20, c = 10.66 A, β = 94.83°; space group C2/c; Z = 4) was determined by X-ray analysis, using direct methods, and refined by full-matrix least squares calculations as discussed by the authors.
Abstract: The crystal structure of cyclo-di-β-alanyl (monoclinic; a = 11.85, b = 5.20, c = 10.66 A, β = 94.83°; space group C2/c; Z = 4) has been determined by X-ray analysis, using direct methods, and refined by full-matrix least-squares calculations. The two cis-peptide groups, related by a twofold axis, are almost exactly planar, and the molecule occurs in a “flexible” chair conformation with N-CH2CH2-C torsion angle of 27°.
TL;DR: In this paper, the average F ··· F distance, after correction, is found to be 2.292 ± 0.004 A. Best agreement was obtained for a physically impossible model which must be rejected on other grounds.
Abstract: Further refinement has been carried out on the neutron diffraction data for potassium bifluoride. Best agreement was obtained for a physically impossible model which must be rejected on other grounds. Following Ibers, the hydrogen atom is assumed to be centrally located. Combination of spectroscopic and diffraction data from sodium, potassium, and ammonium bifluorides allows a detailed analysis of the thermal motions to be made. The average F ···F distance, after correction, is found to be 2.292 ± 0.004 A.
TL;DR: A sapogenin isolated from the acid hydrolysates of roots or tops of alfalfa (Medicago sativa) has properties and constants identical to those of hederagenin this article.
Abstract: A sapogenin isolated from the acid hydrolysates of roots or tops of alfalfa (Medicago sativa) has properties and constants identical to those of hederagenin. The corresponding saponin was isolated from a mixture of alfalfa saponins by successive chromatography on Al2 O3 and DEAE-Sephadex columns. Upon acid hydrolysis it yields glucose, arabinose and hederagenin, similarly to the saponin isolated from Hedera helix.
TL;DR: The results from structure studies on 6a-thiathiophthenes show that different substituent groups perturb the bonding in the three-sulphur sequence to different degrees as discussed by the authors.
Abstract: The results from structure studies on 6a-thiathiophthenes show that different substituent groups perturb the bonding in the three-sulphur sequence to different degrees. Both equal and unequal SS bonds occur in symmetrically as well as in unsymmetrically substituted derivatives.
The specific effect of methyl and phenyl substituents on the sulphur-sulphur bonding may be described by the results from CNDO/2 calculations on mono-methyl and mono-phenyl substituted 6a-thiathiophthenes. Molecular packing and intermolecular close contacts also seem to affect the SS bonding.
The structures of equally substituted 6a-thiathiophthene analogues with
and
show, when compared with the equivalent 6a-thiathiophthene, that
and
cause a pronounced change in the SS bonding, while the perturbation caused by
is negligible.
In symmetrical 6a-thiathiophthene analogues with
and
, the average bond length in the linear three-atom sequence is 9 – 12% longer than the respective single bond.
TL;DR: In this paper, the photochemistry of the reaction of thiobenzophenone with acrylonitrile, following excitation into both the n, π* and π-π* absorption bands, has been examined.
Abstract: The photochemistry of the reaction of thiobenzophenone with acrylonitrile, following excitation into both the n, π* and π, π* absorption bands, has been examined. The products obtained are different, indicating dual mechanisms. It has been shown that the thietane is not a primary product, but is the result of thermal decomposition of a 1,3-dithiane which can be isolated at low temperatures. This is the main product at the shorter wavelengths, but at longer wavelengths a 1,4-dithiane and a thiatetrahydronaphthalene are obtained. A disulphide is also produced as a result of a secondary photochemical attack of thiobenzophenone on the thietane. The mechanistic implications of these results are discussed.
TL;DR: The spectrum of the RSSR radical was obtained by direct attack of e−aq on glutathione and the rate constants of these processes were also measured in this article, where the first observed product is RS, the spectrum of which is obtained.
Abstract: Reaction of H atoms with glutathione leads rapidly to H + RSSR RS · + RSH. The first observed product is RS, the spectrum of which is obtained. The spectrum of the RSSR radical was obtained by direct attack of e−aq on glutathione. The rate constants of these processes were also measured.
ke−aq + RSSR = (2.7 ± 0.3) × 109 M−1 sec−1
kH + RSSR = (1.0 ± 0.2) × 1010 M−1 sec−1
When the OD of RSSR is plotted vs pH a titration curve is obtained. This is due to the protonation of RSSR with a rate constant of 2.6 × 1010 M−1 sec−1 which is probably followed by a cleavage to RS and RSH. In both cases the RSSHR radical cannot be detected. The spectrum attributed to the RSSHR radical is more likely to be that of RS.
TL;DR: The free radicals in irradiated solid amides, fatty acids, and similar compounds react rapidly with ambient gases such as O2, NO, SO2, etc..
Abstract: The free radicals in irradiated solid amides, fatty acids and similar compounds react rapidly with ambient gases such as O2, NO, SO2, etc. The gas seems able to penetrate the lattice and reach the reaction site. This apparently depends on a particular kind of crystal structure which we term the “bilayer structure.” Some characteristics of these reactions are discussed.
TL;DR: In this article, the role of the crystal field in stabilizing dynamically distorted electronic states of aromatics is discussed and conditions under which these distortions are bound to occur are discussed with an emphasis on qualitative understanding.
Abstract: In this paper we present some ideas concerning the role of the crystal field in stabilizing dynamically distorted electronic states of aromatics. It is shown that a crystal field term of appropriate symmetry reveals in a dramatic way the result of strong vibronic coupling in emission spectra. Conditions under which these distortions are bound to occur are discussed with an emphasis on qualitative understanding. In this context the role of predissociating modes in spectra of aromatics is touched upon and the ideas are demonstrated through discussions of the emission spectra of pyrazine, p-dibromobenzene and m-dichloropyrazine.