Type Of Halogen Research Articles

Roles of polycyclic compounds in increasing breakdown strength of LDPE film

Polycyclic compounds of acene series without radicals were added to the material of LDPE (low density polyethylene) film to investigate the roles of aromatic compounds in an increase in the breakdown strength (BDS) of the film. The additives used were naphthalene, anthracene, tetracene and pentacene (group A). Dc and impulse BDSs of the film with anthracene were the highest among the films with the group A additives. It was estimated from our experiments that dc breakdown of the film in this study is mainly due to the electron avalanche mechanism. Furthermore, the dependence of BDS upon the type of radical connected to anthracene (An) was measured using three kinds of additive (group B), which are 9,10-dibromoanthracene (Br-An), 9-nitroanthracene (NO-An) and 1-aminoanthracene (NH-An). DcBDS of the films with radicals are arranged in the following order: (PE/NO-An)ges(PE/Br-An)>(PE/An)>(PE/NH-An). DcBDS of (PE/NO-An) was approximately 1.65 times higher than that of the film without an additive. The dependence of dcBDS upon the type of radical connected to An was discussed using the experimental results with UV absorbance, DSC and TSC. The dependence of dcBDS on the kind of group B additive corresponds to the dependence of trap depth for electron carriers on the type of connected radical: the electron-accepting or electron-donating, or halogen type. The excitation of the additive at the collision with electron carriers is considered to increase dcBDS for all the films with the additives of group B and An. The modification of the morphology is not observed significantly in the films studied

SU-FF-T-236: Experimental Validation of the Inner Shell Ionisation Model to Predict the Radiosensitisation Induced by the IDU and BrDU Halogenated Pyrimidine

Purpose — Halogenated pyrimidines (IDU and BrDU) are heavy atoms compounds used as radiosensitizers. For IUdR, it has been questionned if inner shell ionisations (ISI) and Auger cascades that are rare events using megavoltage beams could be at the origin of the radiosensitization. The purpose of this work is to validate experimentally the ISI model previously published by the authors. This model involves the photon spectrum degradation in the tissues and the ISI induced by secondary electron knock‐on. Material and Methods — The ISI model predicts that the primary photon energy, irradiation depth, and type of halogen influence the Sensitization Enhancement Ratio (SER). Radiobiological assay were used to evaluate cell survival of the radioresistant CHO cell line using 192 Ir or 60 Co irradiation at various depths in water phantoms (2, 5 and 10 cm). Cells were grown in exponential or in plateau phase to account for different ratios, and exposed 3 days before irradiation to 4 mol of IDU or BrDU. The experimental validation of the ISI model was realized by testing for each experimental condition the correlation between i/− the theoretical SER calculated using Monte Carlo simulation of the overdosage due to Auger cascades; and ii/− the SER calculated from the radiobiological assays. Results: — The experimental SER values are in the same range than the theoretical one, and they are significantly correlated (p=0.017). As predicted by the ISI model, there is an influence of depth and energy that is stronger for the IDU than the BrDU. Also, as predicted by the model, the largest SER is found using IDU and 192 Ir . Conclusions — The ISI model is robust in predicting the radiosensitisation for a given experimental situation. Based on a pure particle‐matter interaction modelisation, the ISI model could be extended to other particles type, including electrons, protons of heavy ions.

Effects of π bond type, backbone size, and halogen on structural and spectroscopic properties of hydrogen‐bonded complexes of the XH … π type between alkenes or alkynes and haloacids (HF and HCl)

AbstractThe effects of CC bond type (double or triple), substituent (H or methyl), and halogen (F and Cl) on three properties of hydrogen‐bonded complexes formed between unsaturated hydrocarbons and HX (XF, Cl) are studied. The properties comprise hydrogen bond distances (RH), stabilization energies (SE), and frequency shifts (Δν). A 23 factorial design technique, along with ab initio (HF and MP2) and DFT (B3LYP and PBE1PBE) calculations, has been employed. All three responses are mainly affected by the halogen, and when it is changed from F to Cl, RH tends to increase, while SE tends to decrease. Surprisingly, the type of substituent is more important than the type of CC bond, for all three responses. Both effects tend to decrease RH. Significant interaction effects are obtained for the type of CC bond along with the type of substituent, and for the type of substituent along with the type of halogen. Both interaction effects are smaller than the main effects and also tend to decrease RH. The greatest SE values are obtained with PBE1 functional (BSSE + ZPE corrected values). Again, the next more important effect is due to the type of substituent, and the replacement of H by CH3 group tends to increase SE. The effect due to the CC bond type is not significant, at all computational levels. The only interaction effect that is significant for SE (corrected) and Δν is between factors 1 (CC bond type) and 2 (substituent), but only at HF and B3LYP levels, and it tends to increase both properties. As the halogen changes from F to Cl, Δν tends to decrease. In contrast, changing the substituent from H to CH3 leads to greater values of Δν. The effect of CC bond type is not significant at HF level, and when it is changed from double to triple Δν is decreased, at B3LYP and PBE1 levels. A suggestion as to how the results may point toward a better experimental detection of similar (π‐type) complexes is also given. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

The tailoring of crystal structures via the self-assembly of organic coordination compounds by N⋯I non-covalent halogen bonds: co-crystals of sterically hindered N-heterocycles and 1,4-diiodo-tetrafluorobenzene

The N-containing heteroaromatics 2,3,5,6-tetramethylpyrazine (tmpyr), 2,2′-bipyridine (2,2′-bpy), 2,4′-bipyridine (2,4′-bpy) and o-phenanthroline (phen) were each cocrystallized with 1,4-diiodo-tetrafluoro-benzene (TFDIB). The effect of the sterically constrained aza cycles on supramolecular assembly and the crystal structure determining interactions is discussed. The N⋯I non-covalent halogen bonds are the main directing interactions responsible for the observed structures. Nevertheless, steric hindrance of the donor sites increases the impact of weaker intermolecular interactions such as CH⋯F bonding and π–π interactions provoking changes in the coordination behaviour of the donor modules. In spite of steric constraints, the co-crystals tmpyr·TFDIB (1) and 2,2′-bpy·TFDIB (2) exhibit polymeric structures consisting of infinite one-dimensional chains of alternating electron donors (D) and acceptors (A). In contrast, the structures of the cocrystals 2,4′-bpy·TFDIB (3) and phen·TFDIB (4) reveal a packing of termolecular [A1D2] complexes. It follows that the structure type of such halogen bonded supramolecular assemblies can be tuned by choosing the appropriate precursors.

Novel knowledge-based potentials for ligand docking: variational approach to the old problem

The variational approach of evaluation for knowledge-based potentials is considered for the first time. In this approach, the problem to derive knowledge-based potentials is solved as the optimization task in the multiparametric model of atom types, reference states and interaction cutoff radii. Using analogy to liquid state theory we offered four new reference states and derived corresponding knowledge-based potentials. The cutoff radii and atom types are optimized to minimize averaged root-mean square deviations (RMSD) of the ligand docked positions regarding to the experimentally determined poses. The number of atom types is varied on the developed atom type tree with 6 root (C, N, O, S, P and the halogen type) and 49 apical atom types. We showed a pronounced effect of atom type choice on docking accuracy and proved that splitting of elements C, N and O of the periodic system up to the 18 optimal atom types essentially improves docking accuracy.

Chamigrenelactone, a Polyoxygenated Sesquiterpene with a Novel Structural Type and Devoid of Halogen from Laurencia obtusa.

A biogenetically interesting halogen-devoid metabolite chamigrenelactone 1, with a high oxygen-content, has been isolated from Laurencia obtusa from Isla Grande (Caribbean Panama). Its structure and stereochemistry were determined on the basis of spectral studies.

Chamigrenelactone, a polyoxygenated sesquiterpene with a novel structural type and devoid of halogen from Laurencia obtusa

A biogenetically interesting halogen-devoid metabolite chamigrenelactone 1 , with a high oxygen-content, has been isolated from Laurencia obtusa from Isla Grande (Caribbean Panama). Its structure and stereochemistry were determined on the basis of spectral studies.

Accurate torsional potentials in conjugated systems: ab initio and density functional calculations on 1,3-butadiene and monohalogenated butadienes

Accurate calculations of the torsional potentials for rotation around the carbon–carbon single bond of all conceivable monohalogenated 1,3-butadienes C4 H 5X, (X ∈ [F, Cl, Br]), are presented. The parent compound, 1,3-butadiene, is also included as a benchmark and reference case. Large-scale ab initio calculations were performed at the second-order Møller–Plesset perturbation theory (MP2) and the coupled cluster, CCSD(T), levels. Additionally, density functional methods were applied. In all compounds considered, the anti- or s-trans-conformation is the most stable. For all three halogens, the 2-halo-1,3-butadiene is the most stable isomer, followed by the cis-1-halo-1,3-butadiene. Depending on the position and the type of halogen, the original 1,3-butadiene torsional potential is modified in a different manner. The modifications are particularly visible in the region of the syn- or s-cis and the gauche structures and in the barrier heights.

Tris(tetraethylammonium) tris(μ3-sulfido)bis[bromocopper(I)][dibromocopper(I)]oxomolybdate(VI) acetone solvate

The title compound, (NEt4)3[(MoOS3)(CuBr)2(CuBr2)]·(CH3)2CO, contains a central pseudo-tetrahedral MoOS32− unit with Cu atoms bridging all three S⋯S edges while the Mo=O bond remains terminal. The Mo, 3 Cu and three triply bridging S atoms occupy seven of the vertices of a distorted cube, the vertex opposite Mo being vacant. Two of the Cu atoms carry one terminal halogen atom each and have distorted trigonal planar coordination, while the third carries two and has distorted tetrahedral coordination. One of these two halogen atoms on the third Cu atom has a secondary interaction with one of the other Cu atoms, so it may be considered as semi-bridging. Three of the four halogen sites show disorder as a result of minor substitution of Cl for Br, the semi-bridging halogen atom being purely or essentially Br. This semi-bridging type of halogen is unprecedented in MOS3Cu3Xn cluster anions.

Self-assembly via (N⋯I) non-covalent bonds between 1,4-diiodo-tetrafluoro-benzene and a tetra-imino ferrocenophane

The tetra-imino ferrocenophane ( 1) was cocrystallized with 1,4-diiodo-tetrafluoro-benzene (TFDIB). In the resulting compound 2, two of the four ferrocenophane nitrogens show interactions with the iodine atoms of TFDIB leading to a polymeric structure with extended linear alternating electron donor ( 1) and acceptor (TFDIB) molecule chains. For the first time, imino nitrogens are involved in this type of halogen bonding. The N⋯I non-covalent bonds (N⋯I distances 2.879(5) and 2.896(5) Å; N⋯I–C angles 171.1(2) and 178.1(2)°) are the directing interactions responsible for the observed self-assembly. The ferrocene fragments of the macrocycle are in an almost perpendicular conformation. Mössbauer spectroscopy indicates the sole presence of low spin iron (II). The temperature dependence of the magnetic susceptibility is corresponding to a quasi-diamagnetic compound.

Effects of Alkaline Metal Salts on Viscosity of Gellan Aqueous Solutions.

Effects of alkaline metal salts on zero-shear viscosity (η0) of gellan aqueous solutions were investigated. The temperature (T) dependence curves of η0 showed a steep change due to the coil-helix transition of the gellan molecules. The transition temperature (TTr) depended on the salt concentration while it was independent of the cationic species. At temperatures higher than TTr, η0 decreased with increasing salt concentration. At temperatures lower than TTr, T-η0 curves at various salt concentrations produced a kind of envelope corresponding to the T-η0 curve for gellan with double-helical conformation. The addition of alkaline salts enhanced the aggregate formation at temperatures lower than TTr, and the efficiency for the aggregate formation depended on the cationic species used. At a fixed salt concentration, the degree of aggregation increased in the order of Cs +>Rb+>K+>Na + >Li. Defining the temperature at which the double helices start to aggregate as Ta, both TTr and Ta were not affected by the type of halogens, Cl −, Br− and I−.

Dielectric Spectroscopy and Low Temperature Molecular Dynamics of Thermotropic Side-Chain Polymethacrylates with Side Halogen Terminal Groups

Local forms of molecular mobility of LC side-chain halogen-containing polymers (P-Hal) where Hal is F, Cl, Br, or J, with caproyloxyphenyl benzoate mesogenic groups were studied by dielectric relaxation spectroscopy (DRS) and by thermally stimulated depolarization currents (TSDC) techniques. For all halogen-containing polymers investigated herein three relaxation ranges of dipole polarization were detected at temperatures below Tg, β, γ1 and γ2 processes in the order of decreasing temperature. The molecular mechanisms of these processes correspond to the mesogen motion about the long axis, to the spacer motion, and to the mobility of the mesogen end part, respectively. For all P-Hal, the kinetic characteristics for each of the β, γ1 and γ2 processes can be described by a single curve in the Arrhenius plot, log fmax = φ (1/T). In other words, the local molecular mobility in the series P-F, P-Cl, P-Br, and P-J does not change with halogen type.

Heteronuclear and homonuclear surface abstraction reactions of Cl, Br, and F

Surface reactions of atomic halogen atoms play important roles in various plasma etching processes, commonly used in microlectronics manufacturing. However, relatively little is known about the surface chemistry of these key reactive intermediates. Previous measurements of the recombination coefficients of Cl, Br, and F on various surfaces in a molecular beam apparatus indicated that the recombination reaction is pseudofirst order [G. P. Kota, J. W. Coburn, and D. B. Graves, J. Vac. Sci. Technol. A 16, 270 (1998); 16, 2215 (1998)]. One mechanism that would result in pseudofirst order kinetics is a two-step process in which the first halogen atom adsorbs into a relatively strongly bound chemisorbed state, and the second atom reacts with it either through a direct reaction, or after being physisorbed onto the halogenated surface. In this article, we report experiments in which surfaces are first exposed to a molecular beam of one type of halogen atom, then the surface is exposed to a second type of halogen. During the second exposure, the heteronuclear reaction product is monitored with a mass spectrometer. Finally, the surface is sputtered and the mass spectrometer is used to detect any remaining presence of the original halogen atom. Analogous experiments were also performed with isotopically enriched mixtures of chlorine. These experiments unambiguously demonstrate that halogen atom surface recombination involves a two step adsorption-abstraction mechanism. Under all conditions studied, the surface recombination reactions proceeded at rates on the order of surface collision frequencies. The relative magnitudes of the heteronuclear rates (as a function of surface composition and halogen atom type) scaled in the same way as the homonuclear recombination probabilities measured previously. In every case examined, after the second halogen exposure, the surface retained a significant coverage of the halogen that had been originally exposed to the surface. This leads to the conclusion that only a fraction of the strongly bound surface sites are available for abstraction by free radical attack. Absolute calibration of the incident and evolved species fluxes allowed an estimate to be made of the reactive site densities for several surfaces. These ranged from 1012 to 1015 cm−2 depending on the surface.

Reactions of Saturated and Unsaturated Tertiary Alkyl Halides and Saturated Secondary Alkyl Iodides with Lithium Aluminum Deuteride. Convincing Evidence for a Single-Electron-Transfer Pathway

Reactions of saturated secondary and tertiary alkyl halides with LiAlH4 (LAH) and LiAlD4 (LAD) have been carried out, and convincing evidence for a single-electron-transfer (SET) pathway has been obtained. Reactions involving saturated alkyl halides with LAD provide a model system in which a halogen-atom radical chain process is not possible, and therefore, the observation of large quantities (in some cases >90%) of protium in the reduction product provides strong evidence for a radical intermediate and a SET pathway. Specifically, the reaction of 2-iodooctane (10) with LAD produced octane with a deuterium content as low as 21%. Also, the reaction of the tertiary halide 2-iodo-2-methylheptane (15) with LAD produced 2-methylheptane (16) with a deuterium content as low as 8%. The effect of stoichiometry, halogen type, and reaction vessel surface on these reactions was studied. Reaction of the unsaturated tertiary halide 6-iodo-6-methyl-1-heptene (25) with LAD was also studied and was found to proceed predom...

P2G217 - The pharmacological study of benfuracarb

A series of 31 N,N-disubstituted 2-amino-5-halomethyl-2-thiazolines was designed, synthesized, and evaluated for inhibitory potential against acetylcholinesterase (AChE), butyrylcholinesterase (BChE) and carboxylesterase (CaE). The compounds did not inhibit AChE; the most active compounds inhibited BChE and CaE with IC50 values of 0.22–2.3 μM. Pyridine-containing compounds were more selective toward BChE; compounds with the para-OMe substituent in one of the two dibenzyl fragments were more selective toward CaE. Iodinated derivatives were more effective BChE inhibitors than brominated ones, while there was no influence of halogen type on CaE inhibition. Inhibition kinetics for the 9 most active compounds indicated non-competitive inhibition of CaE and varied mechanisms (competitive, non-competitive, or mixed-type) for inhibition of BChE. Docking simulations predicted key binding interactions of compounds with BChE and CaE and revealed that the best docked positions in BChE were at the bottom of the gorge in close proximity to the catalytic residues in the active site. In contrast, the best binding positions for CaE were clustered rather far from the active site at the top of the gorge. Thus, the docking results provided insight into differences in kinetic mechanisms and inhibitor activities of the tested compounds. A cytotoxicity test using the MTT assay showed that within solubility limits (<30 μM), none of the tested compounds significantly affected viability of human fetal mesenchymal stem cells. The results indicate that a new series of N,N-disubstituted 2-aminothiazolines could serve as BChE and CaE inhibitors for potential medicinal applications.

Bromine-, chlorine-, and mixed halogen-substituted 4-methyl-2(5H)-furanones: synthesis and mutagenic effects of halogen and hydroxyl group replacements.

The versatility of 4-(hydroxymethyl)-2(5H)-furanone as a starting point for the synthesis of several bromine and mixed halogen analogues of the potent water mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) has been demonstrated. However, in some preparations the yields of desired products were lower for bromine- than chlorine-substituted counterparts. A total of 12 bromine-, chlorine-, and mixed halogen-substituted 4-methyl-2(5H)-furanones were tested repeatedly in 10 independent experiments for levels of Salmonella typhimurium (TA100) mutagenicities. The purpose of these experiments was to determine the mutagenic response to changing halogen content, type, and position as well as to learn the measure of these responses in the presence and absence of the C-5 OH group. Mutagenicities reached levels of 10(3) and 10(2) rev/nmol for all trihalo- and dihalo-4-methyl-5-hydroxy-2(5H)-furanones, respectively, notwithstanding substitutions by bromine or chlorine. Trihalides lacking the C-5 hydroxyl group possessed mutagenicities of the order of 10(2) rev/nmol, while hydroxyl group absence in the dihalides resulted in potency levels of slightly less than 10 rev/ nmol. Pairwise comparisons of compound mutagenicities showed that overall the C-5 H-by-OH replacement and, next in importance, increasing the number of C-6 halogens from one to two resulted in the greatest enhancements of mutagenicities. However, in comparing compound pairs within two different sets of four di- and trihalides, it was observed that replacement of a C-5 H by OH enhanced mutagenicity more for the dihalides than the trihalides indicating that increasing the C-6 halogen number simultaneously with replacing C-5 H by OH results in a nonlinear, additive enhancement. For fewer than half of the compound pairs compared, changing the C-6 halogen from chlorine to bromine resulted in small increases in mutagenicity, and for the remaining compound pairs, no increase could be discerned. This result points to the relative unimportance of only C-6 halogen type as a determinant of mutagenicity. Similarly, no impact on mutagenicity was observed for changing only the halogen type attached to C-3.

Activity profiles of carcinogenicity data: Application in hazard identification and risk assessment

Animal cancer data play a primary role in human risk assessment due to the limited epidemiological data. The current database of test results from the NCI/NTP rodent bioassays provide valuable information concerning the carcinogenic potential of hundreds of environmental agents. An approach is presented to reduce and graphically display these data as activity profiles to allow visualization and assessment of tumor response trends across multiple parameters, e.g. species, sex, target site, and route of exposure. Spreadsheet graphics are used to construct the profiles organized on the multiple parameters of carcinogenicity in a format that enables comparative analysis among chemicals. Several example applications are described to illustrate the value of activity profiles in hazard identification and risk assessment. The NCI/NTP data used in developing this concept are from the Carcinogen Potency Database (CPDB) compiled by Gold et al. (Environ. Health Perspect. 103 (Suppl. 8) (1995) 3–122). Computer links to the underlying details in the CPDB are maintained such that specific histopathologies at individual tumor sites, duration of the study, dose–response data, and notes related to diet, survival, treatments, and the authors evaluation are available to aid in the assessment process. The profiles display carcinogen potency based on the tumorigenic dose rate 50 (TD 50), i.e. the chronic dose rate that would induce tumors in half of the test animals at the end of their standard lifespan adjusting for spontaneous tumors. The TD 50 values provide an index for establishing a relative potency ranking of the chemicals for any specific parameter, such as species or target site. An example ranking of hepatocarcinogens is presented to illustrate relative potencies for chemical analogs. The rank order indicates that the degree and type of halogenation of alkanes has a direct bearing on the carcinogenic potency of these compounds.

Cross-linking of hydropolysilanes for use as silicon carbide precursors

Cross-linking reactions of hydropolysilanes with alkyl halides as cross-linking agents were evaluated in order to obtain high yields of oxygen-free silicon carbide ceramics. On the basis of data from infrared spectroscopy, thermogravimetry, solubility, gel permeation chromatography, and ultraviolet spectroscopy, it has been found that the cross-linking reactions of the polymer occurred to varied extents depending on the type of alkyl halides used. The reactivities of alkyl halides to hydropolysilanes were found to be dependent on the type of halogen, the number of halogen atoms attached to the same carbon atom, the size of the alkyl group, and the reaction temperature.

The First Mixed-Halide Zirconium Cluster Compounds: Zr6Cl1.6I10.4Be, Zr6Cl1.3I10.7B, and Zr6Cl11.5I1.5B. Matrix Effects and Halogen Substitution in Compact Network Structures

Investigations of the effect of halogen size on structure stability have been conducted in well-reduced and heavily interbridged zirconium chloride-iodide cluster systems. The title compounds are obtained in good yields from reactions of Zr, ZrCl(4), ZrI(4), and B or Be in sealed Ta tubes for approximately 4 weeks at 850 degrees C. Single-crystal diffraction at room temperature established these as Zr(6)Cl(1.65(4))I(10.35(4))Be and Zr(6)Cl(1.27(3))I(10.73(3))B [R&thremacr;, Z = 3, a = 14.3508(8), 14.389(1) Å, c = 9.8777(9), 9.915(2) Å, respectively] and Zr(6)Cl(11.47(2))I(1.53(2))B [P4(2)/mnm, Z = 2, a = 12.030(1) Å, c = 7.4991(8) Å]. These are derivatives of the Zr(6)I(12)C and orthorhombic Zr(6)Cl(13)B structures, respectively, the latter containing unusual linear chains of clusters interbridged by Cl(i-i) that are in turn interconnected by three-bonded Cl(a-a-a) atoms. The random substitution of fractional Cl at specific I sites in the first two, and I for certain Cl in the third, was positionally resolved in all cases. The replacement always occurs at two-bonded X(i), so that single types of halogen are left in sites that interconnect clusters and generate the three-dimensional array. Structural changes seen in both structures are specifically related to relief of X.X crowding in the parent structure (matrix effects). Substitution of Cl for I(i) in the Zr(6)I(12)C type greatly reduces intercluster I.I repulsions and allows, among other things, a 0.20 Å (5.8%) reduction in Zr-I(a-i) intercluster bond lengths. Increased Cl.I repulsions caused by I substitution in orthorhombic Zr(6)Cl(13)B (Pnnm) convert the twisted chains and angular Cl(a-a-a) interchain bridges to planarity in tetragonal Zr(6)Cl(11.5)I(1.5)B. Phase widths found are 0 </= x </= 1.4 for Zr(6)Cl(x)()I(12)(-)(x)()Z (Z = B, Be) and 0 </= x </= 1.5 for Zr(6)Cl(13)(-)(x)()I(x)()B. The limit for iodine substitution in the chlorine-rich rhombohedral Zr(6)Cl(12)(-)(x)()I(x)()Be is about x = 2.5.

Effect of halogen substitution of indole-3-acetic acid on biological activity in pea fruit

Auxins (a class of plant growth hormones naturally present in all plants) have been implicated in fruit growth of pea. Pea ( Pisum sativum L.) fruit contain the auxins indole-3-acetic acid (IAA) and 4-chloroindole-3-acetic acid (4-C1-IAA). Fruits grow poorly and subsequently abscise when seeds are removed two days after anthesis, but 4-C1-IAA can substitute for the seeds in maintaining growth of deseeded fruit (pericarp) in planta. Applications of 4-C1-IAA promoted pericarp growth, the effect increasing with concentration from 1 to 100 μM, but IAA was ineffective in stimulating growth when tested from 0.1 to 100 μM. The effect of the position of the halogen on pericarp growth was examined by assaying the activities of 4-, 5-, 6- and 7-chloro- and fluoro-substituted IAA. The position and type of halogen dramatically affected auxin activity, with the natural product 4-C1-IAA being most effective. Of the other compounds tested, only 5-C1-IAA stimulated pea pericarp elongation, and then only moderately. Fluoro-substituted IAAs did not stimulate pericarp growth, and 4-F-IAA was inhibitory. This study is unique in that it reports the biological activity of 4-C1-IAA and halogen-IAA analogues in tissues of intact plants known to contain 4-C1-IAA. The relative activity of the compounds is discussed in reference to previous reports of auxin activity in other systems, and 4-C1-IAA's possible importance in pea fruit growth.