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Repair of two major forms of DNA damage, single strand breaks and base modifications, are dependent on XRCC1. XRCC1 orchestrates these repair processes by temporally and spatially coordinating interactions between several other repair proteins. Here we show that XRCC1 contains a central DNA binding domain (CDB, residues 219–415) encompassing its first BRCT domain. In contrast to the N-terminal domain of XRCC1, which has been reported to mediate damage sensing in vitro, we demonstrate that the DNA binding module identified here lacks binding specificity towards DNA containing nicks or gaps. Alanine substitution of residues within the CDB of XRCC1 disrupt DNA binding in vitro and lead to a significant reduction in XRCC1 retention at DNA damage sites without affecting initial recruitment. Interestingly, reduced retention at sites of DNA damage is associated with an increased rate of repair. These findings suggest that DNA binding activity of XRCC1 plays a significant role in retention at sites of damage and the rate at which damage is repaired.

Pseudomonas aeruginosa uses long, thin fibres called type IV pili (T4P) for adherence to surfaces, biofilm formation, and twitching motility. A conserved subcomplex of PilMNOP is required for extension and retraction of T4P. To better understand its function, we attempted to co-crystallize the soluble periplasmic portions of PilNOP, using reductive surface methylation to promote crystal formation. Only PilOΔ109 crystallized; its structure was determined to 1.7 Å resolution using molecular replacement. This new structure revealed two novel features: a shorter N-terminal α1-helix followed by a longer unstructured loop, and a discontinuous β-strand in the second αββ motif, mirroring that in the first motif. PISA analysis identified a potential dimer interface with striking similarity to that of the PilO homolog EpsM from the Vibrio cholerae type II secretion system. We identified highly conserved residues within predicted unstructured regions in PilO proteins from various Pseudomonads and performed site-directed mutagenesis to assess their role in T4P function. R169D and I170A substitutions decreased surface piliation and twitching motility without disrupting PilO homodimer formation. These residues could form important protein-protein interactions with PilN or PilP. This work furthers our understanding of residues critical for T4aP function.

Expansion into new host niches requires bacterial pathogens to adapt to changes in nutrient availability and to evade an arsenal of host defenses. Horizontal acquisition of Salmonella Pathogenicity Island (SPI)-2 permitted the expansion of Salmonella enterica serovar Typhimurium into the intracellular environment of host cells by allowing it to deliver bacterial effector proteins across the phagosome membrane. This is facilitated by the SsrA-SsrB two-component regulatory system and a type III secretion system encoded within SPI-2. SPI-2 acquisition was followed by evolution of existing regulatory DNA, creating an expanded SsrB regulon involved in intracellular fitness and host infection. Here, we identified an SsrB-regulated operon comprising an ABC transporter in Salmonella. Biochemical and structural studies determined that the periplasmic solute-binding component, STM1633/DalS, transports D-alanine and that DalS is required for intracellular survival of the bacteria and for fitness in an animal host. This work exemplifies the role of nutrient exchange at the host-pathogen interface as a critical determinant of disease outcome.

We have found that, while 2-oxetanone undergoes thermal decarboxylation at 350°C to give CO 2 and ethene, 2-azetidinone proceeds in two different modes at 550°C, giving isocyanic acid, ethene, ketene, hydrogen cyanide and carbon monoxide among the pyrolysis products. B3LYP/6-31G ** calculations were performed to investigate the two alternative cycloreversion pathways for the two compounds. Theoretical thermodynamic data predicted that decarboxylation of 2-oxetanone has among the lowest activation barrier (159 kJ mol −1 ) and free energy (Δ G =−183 kJ mol −1 ). While the cycloreversion to ethene can be classified as a concerted asynchronous process, natural bond orbital (NBO) analysis suggested that the cycloreversion to ketene is more asynchronous for 2-azetidinone than for 2-oxetanone.

The interconversion and decomposition of furan-2(3H)- and -2(5H)-one and their methylated derivatives were studied by following the changes in their photoelectron spectra during pyrolysis. Interconversion occurred at 300–400 °C and decomposition at around 600 °C giving CO and acrolein as the only products for the unsubstituted furanones. The experimental results suggest that decarbonylation takes place through the 2(3H) form as the common precursor.

He II photoelectron spectra of tetrahydrothiophen-2-one, tetrahydrothiophen-3-one and 2(5H)-thiophenone have been recorded along with the corresponding He I spectra. The relative He I/He II band intensities provide clear evidence for the assignment of the first few bands, which are also assigned on the basis of bandshape and SCF MO calculations. Correlations with IPs of tetrahydrothiophene, 2,5-dihydrothiophene and formaldehyde show that the interaction of the carbonyl group and the ring heteroatom is primarily inductive in nature in these thiophenones, though there may be a small degree of conjugative interaction between the carbonyl group and the ethylenic moiety in 2(5H)-thiophenone.

The valence electronic structures of the compounds γ-thionobutyrolactone (X=S,Y=O,R=R″=H), γ-butyrolactone (X=O,Y=O,R=R″=H) and their respective 3- and 5-methyl-substituted derivatives have been investigated by photoelectron spectroscopy. Two distinct bands are observed in the ionization potential (IP) range 8–11 eV in the spectra of all these compounds, with the first ionization band exhibiting clear vibrational progressions. Variation in the He I/He II relative intensities in the spectra of the γ-thionobutyrolactones suggests the assignment of the first band to the lone-pair ns orbital, the second band to theπ ocs * orbital and the third band to theπ ocs orbital. The first two bands in the spectra of the γ-butyrolactones, on the other hand, are ascribed to the nO(C O) and the ethereal no(π) orbitals, respectively. The assignments are aided by the results from semi-empirical PM3 and ab initio calculations using the 6–31G** basis sets. Correlation with the IPs of thioformaldehyde, cyclopentanone and tetrahydrofuran indicates that the interaction between the carbonyl group and the ring oxygen atom is mainly inductive in nature. In the γ-thionobutyrolactones, the conjugative interaction is more significant. Methyl substitution causes mainly inductive effects on the parent heterocyclics.

The reaction profiles of the two decomposition modes of γ-thiobutyrolactone, named as concerted and consecutive pathways of decarbonylation or decarboxylation, have been explored with the UMP4/6–31G ∗∗ //UHF/6–31G ∗∗ method. Overall adiabatic energy and enthalpy barriers of the four pathways have been calculated. Decarbonylation leading to CO, C 2 H 4 and CH 2 S is the main process and decarboxylation leading to COS and C 3 H 6 is the minor process in both adiabatic energy and enthalpy.

A two-level orthogonal array design is used to study the effects of water content in the samples on the precision of gas chromatographic analysis. Six relevant parameters and possible interactions are examined simultaneously in the statistically designed experiments. Based on the results of the experiments, the following conclusions are drawn: water contained in a sample can reduce the precision of gas chromatographic analysis with electron capture detection; the adverse effect is more serious with less stable analytes; the adverse effect can be minimized by adopting a lower injector temperature and a higher initial oven temperature, reducing the injection volume, adding a cosolvent with a higher boiling point than the water-containing sample, and using a switching valve to discharge the solvent before it reaches the detector. Under the proposed conditions, water has little effect on the analysis of lindane, a very stable pesticide, even when water content reaches 50%. However, the effect is still noticeable with methyl parathion, a less stable pesticide

Through a number of strategies nonribosomal peptide assembly lines give rise to a metabolic diversity not possible by ribosomal synthesis. One distinction within nonribosomal assembly is that products are elaborated on an enzyme-tethered substrate, and their release is enzyme catalysed. Reductive release by NAD(P)H-dependent catalysts is one observed nonribosomal termination and release strategy. Here we probed the selectivity of a terminal reductase domain by using a full-length heterologously expressed nonribosomal peptide synthetase for the dipeptide aureusimine and were able to generate 17 new analogues. Further, we generated an X-ray structure of aureusimine terminal reductase to gain insight into the structural details associated with this enzymatic domain.

The quantum yields for direct photolysis of 16 organophosphorus pesticides and their phenolic products in aqueous solutions were determined at 254 and 313 nm. Highly significant correlation between the quantum yields at 254 nm and those at 313 nm was obtained (r=0.89,n=14). There also exists a highly significant correlation between the quantum yields of the organophosphorus pesticides and those of their phenolic products (r=0.92, n=15). The pesticides tend to have quantum yields similar to those of their phenolic products. The relationship between the quantum yields and the molecular structure was also investigated

Most solid-phase microextraction methods are based on poly (dimethylsiloxane)-coated silica fibers. Is the present study, pencil lead was used as an alternative sorbent for solid-phase imcroextraction. Its application to three organic pollutants with different polarity in water was investigated. The detection limit for determination by gas chromatography with electron capture detector was 0.005 ng ml−1 for lindane, 0.05 ng ml−1 for methyl parathion and 1 ng ml−1 for 2-chlorophenol. The presence of dissolved humic substances (10 mg l−1) in water did not affect the extraction of the three analytes.

The promotion of hydrolysis of some organophosphorus pesticides by mercury(II) ion was studied. The presence of mercuric chloride at 20 mg litre−1 increased the initial hydrolysis rates of malathion, fenitrothion, fenthion and parathion-methyl in pH 5.5 buffer by two to three orders of magnitude. The hydrolysis was found to be first order with respect to both mercury(II) ion and the pesticide. The main hydrolysis product was 3-methyl-4-nitrophenol for fenitrothion, and 4-nitrophenol for parathion-methyl. It is postulated that the reaction involves the formation of a 1:1 complex and a very slow regeneration of the mercury(II) ion from the reacted complex. Mercury(II) ion had little effect on the organophosphorus pesticide dichlorvos.

Orthogonal array desings were applied to the optimization of liquid chromatographic analysis of pesticides used in golf courses. Three variables related to the composition of the mobile phase were studied: the percentage of acetonitrile, percentage of methanol, and the buffer pH. In the first experiment, each variable was tested at three levels using an OA9 (34) matrix, in which the interactions between the parameters were temporarily neglected. Based on the results of the first experiment, the second experiment was carried out following a two-level orthogonal array design with an OA8 (27) matrix, in which more exact levels for each variable were chosen, and the interaction effects were considered separately. Finally, the optimum conditions for liquid chromatographic analysis of the pesticides were proposed. The advantages and the disadvantages of three-level and two-level orthogonal array designs were discussed.

Ethenethiol and 1-propene-1-thiol were produced through the thermolysis of thiirane and 2-methylthiirane, respectively, under reduced pressure in a flow system. The photoelectron spectra of the olefin thiols are characterised by the presence of a sharp first band which is well separated from the other bands by more than 2 eV. The spectra are discussed with the aid of AM1 and ab initio SCF MO calculations, and a diagram which correlates the binding energies of the first three orbitals, viz. π CCS * , π CCS and σ CS , with those of the corresponding orbitals of related molecules.

A simple method for preventing unsuitable solvents and other substances from entering gas chromatographic detectors is described. A split vent was constructed by connecting the outlet of the column to the unused injector and the detector with a press-tight Y-splitter and two short capillary columns. Unsuitable solvents and other substances can be discharged through the unused injector by keeping the injector open for a few minutes. When the nut on the injector is tightened, all the effluent from the analytical column enters the detector. Use of this split vent can avoid or reduce the adverse effects of unsuitable solvents on the detector performance.

A simple and inexpensive multi-residue method is described for the determination of organochlorine and pyrethroid pesticides in vegetables. Pesticides in vegetables were extracted with ethanol and partitioned into toluene. A mini-column packed with 0.5 g of Florisil was used for further clean-up prior to gas chromatographic determination. The detection limits were 0.02-0.05 microgram/g without concentrating the extract, which are below the maximum residue limits set by the Singapore government. The recoveries of the pesticides from fortified samples were 65-97% at the 0.1 microgram/g level and 87-114% at the 0.5 microgram/g level. The amounts of the reagents required for analysing one sample are only 100 ml of ethanol, 6 ml of toluene and 0.5 g of Florisil. Among fifteen vegetable samples collected from the Singapore local market and were analysed by this method, five were found to contain detectable amount of organochlorine pesticides. One sample contained 22 micrograms/g of endosulfan but the residue levels in other four samples were below 1 microgram/g.

A method for determining the phenolic compounds from the hydrolysis of some organophosphorus pesticides by liquid chromatography/ electrochemical' detection has been developed. A carbon fibre micro-electrode was used for the detection. Phenol, 4-nitrophenol, 3-methyl-4-nitro-phenol, and 2,4,5-trichloropyridinol in water were analyzed by the method. The responses of the four phenolic compounds increased steadily with the increased working electrode potential within the potential range of 1.2 to 1.4 V. The detection limits were 0.1 ng for phenol, 0.5 ng for 2,4,5-trichloropyridinol, 1 ng for 4-nitrophenol, and 2 ng for 3-methyl 4-nitrophenol.

He I photoelectron spectra of title compounds were recorded and assigned by MNDO-PM3 calculations and comparison with spectra of related derivatives. Substituents were found to noticeably affect the electronic structure, depending on their type and position of attachment to the ring. MNDO-PM3 calculation results become unreliable in cases of thiophenes with widely different substituent types.

The photo-induced cis-trans isomerization of Pt(CO)(PR 3 )X 2 , where X=Cl, Br and I; PR 3 =PEt 3 , PMePh 2 and PPh 3 , has been studied in chloro-hydrocarbon solvents, with emphasis on the chloro-triethyl complexes. Photo-induced dimerization was a concurrent reaction to varying degrees for the complexes. In chloroform and carbon tetrachloride, an induction period was observed for isomerization. Addition of carbon monoxide enhanced isomerization but suppressed dimerization, while continuous purging with nitrogen produced the opposite effects. For cis -Pt(CO)(PEt 3 )Cl 2 , the quantum yield of cis-trans conversion was about 0.6 at different wavelengths of incident light; it was invariable towards the presence of oxygen or 1,3-pentadiene, but increased to about 0.8 in the presence of carbon monoxide introduced at different pressures. These results indicate that isomerization takes place from a singlet state of relatively long lifetime, with metal-carbonyl bond breaking as the first step, followed by catalysis of the reaction by the free ligand.

In the gas phase the title lactone pyrolyses via decarboxylation, while the thiolactone pyrolyses via decarbonylation, as the major modes of decomposition.

He I and He II photoelectron spectra of all isomeric mono- and diethynylthiophenes were recorded. The electronic structure was analyzed and spectra assigned on the basis of empirical considerations and semiempirical MO calculations. The spectra indicate various interactions taking place between thiophene π and ethynyl π orbitals. The relationship between deduced electronic structure and formation of novel polymer materials is discussed

published in Advance ACS Abstracts, April 15, 1994. 0022-3654/94/2098-5240$04.50/0 l2.P He 7 9 11 13 15 17 EileV Figure 1. He I and I1 photoelectron spectra of 22BSe 22FSe. selenophe0eyl)thiophene (22TSe), 3-( 2’-selenopheney1)thiophene (32TSe), 3-(2’-selenopheneyl)furan (32FSe), 5-chloro-2-( 5’- chloro-2’-selenopheneyl)thiophene (55C1), and 5-bromo-2-(5’- bromo-2’-selenopheneyl)thiophene (55Br) are shown in Figures 1-3. The ionization energies (Ei) quoted in Figures 1-3 and Table 1 refer to vertical energies. UV spectra were obtained in hexane solvent. Theoretical Calculations. MO eigenvalues and eigenvectors were calculated at the semiempirical level (PM3 Hamiltonian) with the MOPAC 6.0 packages running on a Silicon Graphics Iris platform. The selenium atom parametrization was only available with the PM3 method, and thus all calculations were performed at this level of theory. The experimental ring geometries for 0 1994 American Chemical Society

He II photoelectron spectra of isomeric bifurans and thienylfurans have been recorded and are assigned by comparing He I/He II intensity variations. The assignment agrees with the previous study of these molecules, where spectral interpretation was based on MO calculations and MO correlations within the title series. He I and He II spectra are reported for the new 3,3′-bifuran compound. Trends in the electronic structure within the bichalcophene series are discussed.

He * (2 3 S) Penning ionization electron spectra (PIES) of some nitroalkanes, nitropropenes and nitrobenzene have been recorded and compared with their respective He(I) photoelectron spectra (UPS). The spatial electron density distributions of the out-of-plane π orbitals of the nitro group are more extended than the in-plane n 0 orbitals, resulting in relatively strong π bands in the PIES. Similarly, the π CC bands of nitropropenes and the π ring bands of nitrobenzene are enhanced in the PIES owing to the more extended nature of these orbitals. This has enabled the unequivocal assignment of the first three ionic states of nitromethane. The assignment is also supported by multi-deference determinant configuration interaction (MRD-CI) calculations. Variations of the intensity enhancement among these compounds are discussed in terms of local steric shielding effects.

He(II) photoelectron spectra of nitromethane, nitroethane, 1- and 2-nitropropane have been recorded and compared with the corresponding He(I) spectra. The relative He(I)/He(II) spectral band intensities provide clear evidence for ascribing the first two maxima in the range 10–12 eV to ionization processes involving the removal of electrons from the orbitals n 0 + , n 0 − and π 2 . Similarly the band at ≈ 17 eV is attributed to ejection of electrons from the π 1 and σ NO − orbitals. Detailed assignment of spectral bands is made on the basis of bandshape and intensity, as well as ab initio SCF MO calculations using the 6-31G basis set. In the He(II) spectra, two bands in the region 18–22 eV are assigned to σ NO + and an orbital with 2s N and 2s O character.

Vertical ionization energies of the 1 : 1 molecular complexes of boron trifluoride with the three picolines and 3,5-lutidine are reported along with assignments based on their HeI photoelectron spectra and semi-empirical molecular orbital calculations. The shift in the lone pair (n N ) orbital of the donor molecules towards a higher ionization energy (IE) upon complexation is found to be essentially constant in all the complexes (nearly 2.8 eV) except in 3,5-lutidine·BF 3 where the shift is 3.3 eV. Such shifts in the IEs of the π orbital are about 1.1 eV for the 2- and 3-picoline complexes and 1.3 eV for the complexes of 4-picoline and 3,5-lutidine. These shifts reflect the relative stabilities of the complexes. The complex 2,6-lutidine·BF 3 undergoes dissociation into the free donor and acceptor molecules under the conditions of our experiment. A comparison of the shifts is made with the experimental heats of reaction of the pyridine bases with boron trifluoride. Relative stabilities of the complexes as deduced from ab initio and AM1 calculations are compared with predictions of gas-phase basicities of the bases.

The photo-induced cis-trans isomerization of Pt(CO)(PR 3 )X 2 , where X=Cl, Br and I; PR 3 =PEt 3 , PMePh 2 and PPh 3 , has been studied in chloro-hydrocarbon solvents, with emphasis on the chloro-triethyl complexes. Photo-induced dimerization was a concurrent reaction to varying degrees for the complexes. In chloroform and carbon tetrachloride, an induction period was observed for isomerization. Addition of carbon monoxide enhanced isomerization but suppressed dimerization, while continuous purging with nitrogen produced the opposite effects. For cis -Pt(CO)(PEt 3 )Cl 2 , the quantum yield of cis-trans conversion was about 0.6 at different wavelengths of incident light; it was invariable towards the presence of oxygen or 1,3-pentadiene, but increased to about 0.8 in the presence of carbon monoxide introduced at different pressures. These results indicate that isomerization takes place from a singlet state of relatively long lifetime, with metal-carbonyl bond breaking as the first step, followed by catalysis of the reaction by the free ligand.

The thermal decomposition of the three isomers of nitropropene has been investigated in a flow system at low pressures, by using the characteristic bands of the photoelectron spectra of the products for their identification. Of the three isomers, 1- and 2-nitropropene decomposed to give products analogous to those found in the thermolysis of nitrostyrenes, implying some similarities in mechanism. On the other hand, 3-nitropropene gave similar products as 3-propenyl nitrite, indicating the isomerization of the nitro compound to the nitrite form as a probable first step for decomposition.

The unstable species N-methylvinylamine was prepared by the thermal decomposition of gaseous 2-methylaziridine in a flow system and its UV photoelectron spectrum was recorded. The first two bands were observed at 8.00 and 11.00 eV. The methyl substituent shows the effect of reducing the degree of conjugation between the nN and πCC orbitals, as revealed by a comparison with the spectrum of the parent compound vinylamine.

The thermal decomposition of the three isomers of nitropropene has been investigated in a flow system at low pressures, by using the characteristic bands of the photoelectron spectra of the products for their identification. Of the three isomers, 1- and 2-nitropropene decomposed to give products analogous to those found in the thermolysis of nitrostyrenes, implying some similarities in mechanism. On the other hand, 3-nitropropene gave similar products as 3-propenyl nitrite, indicating the isomerization of the nitro compound to the nitrite form as a probable first step for decomposition.

In the gas phase the title lactone pyrolyses via decarboxylation, while the thiolactone pyrolyses via decarbonylation, as the major modes of decomposition.

The thermal decomposition of thiirane and 2-methylthiirane was investigated in a flow system by following the changes in their photoelectron spectra at various temperatures. Similar decomposition patterns were observed in both cases. Thus at 600 °C, formation of sulfur and an alkene was detected. At higher temperatures, conversion of thiirane into ethenethiol and thioacetaldehyde, and of 2-methylthiirane into cis- and trans-prop-1-ene-1-thiol was observed. Hydrogen sulfide, alkynes, carbon disulfide and thiophene were detected as products of secondary reactions. The experimental data pertaining to the rearrangement processes are compared with potential energy profiles, calculated by using the MNDO/CHAIN procedure. A mechanism involving initial C–S bond breaking, followed by 1,2-hydrogen shift, is suggested by the results.

He I and He II UV photoelectron spectra (UPS) of isomeric thienylpyridines and thienylethynylpyridines have been recorded. Their electronic structures were analysed and spectra assigned on the basis of He II/He I intensity variations, comparison with UPS of pyridine, thiophene and ethyne and semiempirical MO calculations. The spectra indicate that there is considerable interaction between π orbitals of thiophene, pyridine and ethynyl moieties. The relationship between electronic structure and formation of novel polymer materials is discussed.

The new molecule chlorophosphaethyne, Cl—CP, has been produced by flow thermolysis of trichloromethylphosphonous dichloride, Cl3CPCl2, over granulated zinc at 530 °C and its photoelectron spectrum has been recorded. All four bands expected below 21 eV have been detected and the first vertical ionization energy at 10.10 eV is assigned to the degenerate 4π orbital. The assignments are supported by ab initio SCF calculations.

The He II photoelectron spectra of 1-, 2- and 3-nitropropene have been recorded and compared with the corresponding He I spectra. The variation in relative He I/He II band intensities allows unequivocal classification of bands into those associated with orbitals of the nitro group and those with orbitals of the hydrocarbon moiety. Band assignment is also aided by observed vibrational structure, band shape and SCF MO calculations. The variation of the relative energies of the orbitals πCC, n+0, n–0 and π2 among the isomers is discussed in terms of inductive, hyperconjugation and resonance effects.

The photolysis of CO(Me4[14]tetraeneN4)(OH2)3+2 in an aqueous acidic medium has been carried out with incident wavelengths at 254, 265, 313 and 365 nm. Spectrophotometric measurements in the uv, visible, ir and nmr region show that reduction of the metal centre takes place, but the macrocyclic ligand remains intact. The quantum yield of CoII (N4) formation is small and decreases with decreasing photon energy. Addition of alcohol leads to an appreciable increase in the quantum yield, and the effect of glycerol is comparable to that shown by 2-propanol or t-butanol. The results are interpreted within the context of the radical-pair model with formation of hydroxyl radicals.

Photoelectron spectroscopy has been employed to study the thermal decomposition of phenylnitromethane in the gas phase. The results indicate at least two processes for the decomposition. Thus at low temperatures ( ca. 400°C) water, nitric oxide and benzaldehyde are formed, together with a small amount of carbon monoxide. Possible pathways of reactions are discussed. The thermal decomposition of nitroethane has also been investigated.

The He I and He II photoelectron spectra of 2(3H)-furanone and 5-methyl-2(3H)-furanone have been recorded. They show a third band not reported earlier. The first three bands in the spectrum of 2(3H)-furanone at 9.76, 10.65 and 12.83 eV are assigned to ionizations from the π CC–O– *, n O(CO) , and π –O–CO * orbitals, respectively. Correlation with orbitals in ethene and acetic acid reveals significant π-conjugation in the furanones.

published in Advance ACS Abstracts, April 15, 1994. 0022-3654/94/2098-5240$04.50/0 l2.P He 7 9 11 13 15 17 EileV Figure 1. He I and I1 photoelectron spectra of 22BSe 22FSe. selenophe0eyl)thiophene (22TSe), 3-( 2’-selenopheney1)thiophene (32TSe), 3-(2’-selenopheneyl)furan (32FSe), 5-chloro-2-( 5’- chloro-2’-selenopheneyl)thiophene (55C1), and 5-bromo-2-(5’- bromo-2’-selenopheneyl)thiophene (55Br) are shown in Figures 1-3. The ionization energies (Ei) quoted in Figures 1-3 and Table 1 refer to vertical energies. UV spectra were obtained in hexane solvent. Theoretical Calculations. MO eigenvalues and eigenvectors were calculated at the semiempirical level (PM3 Hamiltonian) with the MOPAC 6.0 packages running on a Silicon Graphics Iris platform. The selenium atom parametrization was only available with the PM3 method, and thus all calculations were performed at this level of theory. The experimental ring geometries for 0 1994 American Chemical Society

He I and He II photoelectron spectra of all isomeric mono- and diethynylthiophenes were recorded. The electronic structure was analyzed and spectra assigned on the basis of empirical considerations and semiempirical MO calculations. The spectra indicate various interactions taking place between thiophene π and ethynyl π orbitals. The relationship between deduced electronic structure and formation of novel polymer materials is discussed