bluuebunny/crossref_metadata_2025_split huggingface dataset:
Magnetic Property and Electronic Structure of Tris(2,2′-bipyridine) Complexes of Manganese in Low Oxidation States
Authors: Inoue Motomichi , Hara Ken-ichi , Horiba Tatsuo , Kubo Masaji 
Published: July 2006

Abstract
The magnetic susceptibilities of tris(2,2′-bipyridine)manganese complexes, [Mn(bpy)3]·2THF and Li[Mn(bpy)3]·4THF (THF: tetrahydrofuran), have been determined over a temperature range of 1.6–320 K. The susceptibility of Li[Mn(bpy)3]·4THF can be explained with a radical ion model, in which three 2,2′-bipyridine radical ions are coordinated to a manganese(II) ion. A similar model is applicable to [Mn(bpy)3]·2THF as well. The presence of antiferromagnetic interaction indicates the formation of a weak covalent bond between manganese and bipyridine. This can be elucidated consistently by the molecular orbital theory.
Total Synthesis of Ferrichrome
Authors: Isowa Yoshikazu , Ohmori Muneki , Kurita Hideaki 
Published: July 2006

Abstract
Two different sequence hexapeptides for the synthesis of ferrichrome were prepared using Nδ-tosyl-Nδ-benzyloxy-l-ornithine by means of N-hydroxysuccinimide plus N,N-dicyclohexylcarbodiimide or Woodward reagent K method. The linear hexapeptides (XII·HCl and XIX·HCl) were cyclized by excess N,N-dicyclohexylcarbodiimide to give cyclo-triglycyltri-Nδ-tosyl-Nδ-benzyloxy-l-ornithyl (XX). Detosylation, acetylation and successive reductive debenzylation of XX afforded deferri-ferrichrome (XXIV). Addition of ferric iron to XXIV gave ferrichrome, which had the same crystalline form, superimposable infrared and well-agreeing visible spectra and the same value in optical rotation compared with the data in the literature of natural product.
A Shock-tube Investigation of Ignition in Ethylene–Oxygen–Argon Mixtures
Authors: Hidaka Yoshiaki , Kataoka Tsuyoshi , Suga Masao 
Published: July 2006

Abstract
The ignition of ethylene-oxygen mixtures highly diluted with argon was studied in a shock tube by monitoring the emission from CH* and C2* or OH*. The measurements covered the temperature range of 1400–2100 K at the C2H4/O2 ratios of 0.33–1.00. From the observation of the CH*, C2*, and OH* emissions, a mechanism for the production of chemiluminescent species upon ethylene oxidation was briefly discussed, and from the observation of the induction period of the CH* emission, the following equation was found:
logτ_CH[O_2]=-11.45+27.5×10^3/4.58T
in units of s mol l−1, the rate-determining step being C2H3+O2→C2H3O+O.
Linear Free Energy Relationships in Heterogeneous Catalysis. XII. Interpretation of the Catalytic Activity of Decomposition of Hydrogen Peroxide over Nickel Oxide on the Basis of Oxidation Power Distribution
Authors: Uchijima Toshio , Yoneda Yukio 
Published: July 2006

Abstract
Kinetic studies have been carried out on the heterogeneous decomposition of hydrogen peroxide over several nickel oxide catalysts calcined at different temperatures. Kinetic parameters such as reaction order and apparent activation energy depend somewhat on the reaction conditions, but show practically no change with catalyst. The catalytic activity changes with calcination temperature, becoming minimum at 550 °C and maximum at 600 °C. The trend does not change with pretreatment of the catalyst, resembling the behavior of surface excess oxygen having a relatively higher oxidation power. Thus the distribution of oxidation power is closely related to catalytic activity for the decomposition of hydrogen peroxide. We therefore applied the regional analysis assuming that the surface excess oxygen represents active sites for catalysis, and a specific regional activity corresponds to each regional oxygen. It was found that the fifth region (the range of free energy change of adsorption of oxygen being from −11.1 to −12.6 kcal/mol) markedly contributes to the reaction rate, and the calculated rates agree with the observed ones. The nature of the active site was discussed as regards pretreatment of the catalyst.
Kinetics of Multidentate Ligand Substitution Reactions. XVII. Substitution Reactions of Diethylenetriaminepentaacetate (DTPA) with Aminopolycarboxylate Complexes of Nickel(II) and Cobalt(II)
Authors: Kodama Mutsuo 
Published: July 2006

Abstract
The kinetics of the substitution reactions of DTPA with N-(2-hydroxyethyl)iminodiacetate (HIDA) and N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetate (EDTA-OH) complexes of nickel(II) and EDTA-OH and 2,2′-ethylenedioxydi(ethylamine)-N,N,N′,N′-tetraacetate (GEDTA) complexes of cobalt(II) were studied systematically by employing the polarographic technique. The reaction of ethylenediaminetetraacetate (EDTA) with the nitrilotriacetate (NTA) complex of nickel(II) in an alkaline medium was also studied. All the substitution reactions were found to have a common reaction mechanism. They were first-order with respect to DTPA or EDTA and also first-order with respect to the 1:1 ratio metal (II) complex of HIDA, NTA, EDTA-OH, or GEDTA. From a comparison of the observed rate constants with those estimated on the basis of the proposed reaction intermediate, the detailed reaction mechanism and the steric effect of the DTPA anion on the reaction rates were studied. The DTPA anion had practically no effect on the rates of the substitution reactions of the aminopolycarboxylate complexes of metal (II) ions.
The Chemistry of Protactinium. XI. A Study of the Solvent Extraction of Protactinium(IV) Reduced with Europium(II) from a Perchloric Acid Solution, Using Thenoyltrifluoroacetone as the Extractant
Authors: Kawasuji Isamu , Sato Akiko , Suzuki Shin 
Published: July 2006

Abstract
Europium (II) was found to be an excellent reductant for reducing protactinium in microquantities to the tetravalent state. The behavior of Pa(IV) in the perchloric acid solution was investigated by the TTA-benzene extraction method. It was found that the PaOH3+ ion existed predominantly in a 0.03–3 M perchloric acid solution at the ionic strength of 3. The reaction of TTA extraction may proceed as:
&PaOH^3++4HA=PaA_4+H_2O+3H^+
\intertextor
&PaOH^3++4HA=Pa(OH)A_3(HA)+3H^+
The distribution ratio of Pa(IV) in the system using Eu(II) as the reductant is about ten times as large as that in the system using Cr(II) as the reductant. It was considered that a non-extractable complex may be formed between Pa(IV) and Cr(III), while such a complex may not be formed when Eu(II) is used as the reductant.
The Ionic Reaction in the Chlorophosphonation of Alkylbenzenes
Authors: Okada Toshihiro , Okamoto Yoshiki , Sakurai Hiroshi 
Published: July 2006

Abstract
The chlorophosphonation of toluene, p-xylene, ethylbenzene, and diphenylmethane gave methyldiphenylmethanes; 2,4′,5-trimethyldiphenylmethane and ethyl-1,1-diphenylethanes; and p-benzyltriphenylmethane respectively as the main products. Isopropylbenzene gave α-methylstyrene, 4-methyl-2,4-diphenylpentene-1, 4-methyl-2,4-diphenyIpentene-2, and 1,1,3-trimethyl-3-phenylindane, in addition to the ordinary chlorophosphonated products. Those diphenylmethane derivatives may be formed by the electrophilic attack of the benzylic cations, presumably resulting from electron transfer from the benzylic radicals to phosphorus trichloride with the participation of the oxygen or oxy radical, upon toluene, p-xylene, ethylbenzene, or diphenylmethane. α-Methylstyrene may be formed by the elimination of a proton from the intermediate 1,1-dimethylbenzyl cation, resulting from isopropylbenzene in a manner similar to that above. Diphenylpentene and phenylindane derivatives may be formed by the acid-catalyzed dimerization of α-methylstyrene.
Aromatic Nucleophilic Substitution. III. Meisenheimer Complexes in the Reactions of 1-(N,N-Dialkylamino)-2,4-dinitronaphthalenes with Sodium Methoxide
Authors: Sekiguchi Shizen , Shinozaki Katsuo , Hirose Tetsuo , Matsui Kohji , Sekine Kiyoshi 
Published: July 2006

Abstract
The intermediates in the reaction of 1-(N,N-dimethylamino)-2,4-dinitronaphthalene (9) with sodium methoxide have been studied in the mixtures of DMSO and CH3OH. The NMR spectra of the equimolar mixture of 9 and sodium methoxide indicate that 9 and a 1,1-disubstituted Meisenheimer complex coexist immediately after mixing, and that nearly all 9 changed into the Meisenheimer complex 15 min after mixing. The addition of 2 mol of sodium methoxide to 1 mol of 9 gave no 1,1,3,3-tetrasubstituted 2,4-dinitronaphthalene Meisenheimer complex. 1-Piperidino- or 1-(N-methyl-N-n-butylamino)-2,4-dinitronaphthalene reacted with sodium methoxide to give a similar Meisenheimer complex.
Base-induced Reactions of 2-Substituted 2-Bromodimedones
Authors: Wakui Tadahiro , Otsuji Yoshio , Imoto Eiji 
Published: July 2006

Abstract
The reactions of 2-substituted 2-bromodimedones with various bases were studied. The behavior of the dimedones changed with the base and substituent attached to the dimedones. Treatment of 2-alkyl-2-bromodimedones (1a and 1b) with a weak nucleophilic base such as sodium acetate afforded 2-alkyl-4,4-dimethyl-2-cyclopenten-1-ones (2a and 2b), 2-alkyl-5,5-dimethyl-3-oxo-1-cyclopentene-1-carboxylic acids (3a and 3b), 2-alkyl-5,5-dimethylcyclohexane-1,3-diones (4a and 4b) and carbon monoxide. A similar treatment of 2,2-dibromodimedone (7) with the weak nucleophilic base gave 2,3-dibromo-4,4-dimethyl-2-cyclopenten-1-one (8), 2-bromo-5,5-dimethylcyclohexane-1,3,4-trione (9) and 2-bromodimedone (10). However, the reactions of 1a and 7 with a strong nucleophilic base such as sodium ethoxide gave mainly the open-chain products, ethyl 3,3-dimethyl-5-oxoheptanoate (5a) and diethyl 3,3-dimethylglutarate (11). On treatment of 1a with silver acetate, a debrominative dimerization occurred to produce the dimer 6. The mechanisms of these reactions were discussed.
A Photochemical Reaction of Lupan-3-one
Authors: Hirota Hiroshi , Tsuyuki Takahiko , Tanahashi Yoshiaki , Takahashi Takeyoshi 
Published: July 2006

Abstract
Lupan-3-one (5) was irradiated in n-hexane under an argon atmosphere using a high pressure mercury lamp. Besides three seco compounds, namely 10α-(2-formylethyl)-5β-isopropenyl-des-A-lupane (6), 10α-(2-carboxyethyl)-5β-isopropyl-des-A-lupane (8), and 10α-(2-carboxyethyl)-5β-isopropenyl-des-A-lupane (9), two seco-nor compounds, 10α-(carboxymethyl)-5β-isopropyl-des-A-lupane (7) and lactone of 10α-(carboxymethyl)-5α-hydroxy-5β-isopropyl-des-A-lupane (10), have been isolated and their structures have been determined. NMR signals of 10 were examined using Eu(fod)3-d27 as a shift reagent.
Dielectric Studies of ortho- and paro-Bromoanisoles and β-Bromophenetole in the Liquid State
Authors: Ghatak Alpana , Das Arati , Roy Subir Kr 
Published: July 2006

Abstract
Complex dielectric permittivities in ortho- and para-bromoanisoles and β-bromophenetole in the liquid state have been measured in 1.6, 3.17, and 3.49 cm microwave regions at different temperatures. The dielectric data were analysed in terms of molecular and intramolecular relaxation times. The values of methoxy group relaxation times and its contribution to total polarization obtained in the liquid bromoanisoles have been compared to the corresponding values in dilute solutions reported in literatures. Activation energy for dielectric relaxation and viscous flow in all the cases have also been obtained in all the liquids.
The Mechanism of the Reduction of Tosylhydrazones with B2H6 and with NaBH4 in Aprotic Solvents
Authors: Cacchi S , Caglioti L , Paolucci G 
Published: July 2006

Abstract
Cyclohexanone tosylhydrazone was reduced with NaBH4 and B2H6 in aprotic solvents; results indicate that the reduction affords cyclohexyl-tosylhydrazine or an equivalent organometallic adduct confirming the mechanism previously proposed by us in the case of NaBH4 in protic solvents.
Simple Mechanical Model Calculations of the Elastic Constants and Normal Frequencies of the NaNO2 Crystal
Authors: Yamamoto Akio , Shiro Yuji , Murata Hiromu 
Published: July 2006

Abstract
The optically-active vibration was analyzed for the NaNO2 crystal on the basis of a simple mechanical model. Force constants which interpret fairly well both the observed elastic constants and the vibrational frequencies of the optically-active modes were determined.
The Diazo-coupling Reaction of Phenol in Organic Solvents. The Isomer Ratio
Authors: Yamada Hideo , Hashida Y\={o}ji , Sekiguchi Shizen , Matsui Kohji 
Published: July 2006

Abstract
The ortho : para-ratios in the reaction of phenol with benzenediazonium fluoroborate have been investigated in organic solvents. It has been found that the o/p ratios in protic solvents were smaller than those in aprotic solvents. The results were interpreted in terms of the difference in solvation toward the phenolate ion.
Layered Compounds. XVIII. Synthesis of a Paracyclophane Containing the 1,2,3,4,5-Hexapentaene Group and the Stereochemistry of Its Formation
Authors: Negi Taichi , Kaneda Takahiro , Mizuno Hiroshi , Toyoda Toshihiro , Sakata Yoshiteru , Misumi Soichi 
Published: July 2006

Abstract
Novel cyclic 1,2,3,4,5-hexapentaene compounds were synthesized from cyclic diyne-diol IX via diallene-dibromide X in order to study transannular electronic interaction. A diallene-dibromide Xc-I obtained from a diyne-diol isomer IXc-I was debrominated with active zinc to give cyclic hexapentaene IIc, whereas the other diallene-dibromide Xc-II derived from the second diyne-diol isomer IXc-II was not. The X-ray crystallographic analysis of the dibromide Xc-I was then carried out, and the s-trans structure was assigned to the dibromide. On the basis of this result, the stereochemistry of the transformation from diyne-diol to the hexapentaene group was discussed. The electronic spectrum of a cyclophane IIa containing the hexapentaene group showed a transannular π-electronic interaction between the benzene ring and the hexapentaene group, unlike those of the other hexapentaene derivatives, IIc and XV.
Effects of Substituents on the Formation of Acetals of 4-Substituted 1,3-Indandiones
Authors: Sakane Kazuo , Otsuji Yoshio , Imoto Eiji 
Published: July 2006

Abstract
Treatment of 4-nitrol,3-indandione (1) with an equivalent of ethylene glycol in benzene in the presence of p-toluenesulfonic acid afforded 4-nitro-1,3-indandione-1-ethylene acetal (2), 3-ethylene acetal (3) and 1,3-bisethylene acetal (4). Kinetic studies revealed that 2 is a kinetically preferred product and 3 is a thermodynamically preferred product. However, the reaction of 4-acetamido-1,3-indandione (5) with ethylene glycol under similar conditions gave 4-acetamido-1,3-indandione-1-ethylene acetal (6) as a product both kinetically and thermodynamically controlled. Thioacetalization of 1 with an equivalent of 1,2-ethanedithiol afforded selectively 4-nitro-1,3-indandione-1-ethylene thioacetal (9).
Phosphorimetric Investigation of Triplet-Triplet Energy Transfer in Fluid Solution
Authors: Takemura Takeshi , Aikawa Masayuki , Baba Hiroaki 
Published: July 2006

Abstract
The mechanism of triplet-triplet (T-T) energy transfer in fluid solution has been studied by means of phosphorimetry in the temperature range 0–60 °C with acetophenone as energy donor, 1,4-dibromonaphthalene as acceptor and isooctane, isopentane or carbon tetrachloride as solvent. From the temperature dependence of the energy transfer, α and kdif could be evaluated separately, kdif denoting the diffusion-controlled rate constant for the encounter of donor and acceptor molecules and αkdif corresponds to the rate constant for the overall energy transfer reaction. The α values are 0.28 at 25 °C in isopentane and 0.43 at 21 °C in isooctane, indicating that the T-T energy transfer process is not strictly diffusion-controlled. The kdif values are nearly equal to those predicted by a modified Debye equation.
The Electrodepositions of Copper and Nickel from Their Trifluoroacetate–Formamide Baths
Authors: Takei Tatsuko 
Published: July 2006

Abstract
The present author has been studying the electrodeposition of metals from non-aqueous solutions of metal trifluoroacetates. In the present study, the properties of the Cu(CF3COO)2–HCONH2 bath and the Ni(CF3COO)2–HCONH2 bath, and the mechanism of the electrodeposition of copper and nickel from these baths, were studied. Both Cu(CF3COO)2 and Ni(CF3COO)2 were easily dissolved in formamide. The specific conductance of these solutions was high. Satisfactory electrodeposits are obtained from these solutions, with a high current efficiency. The cross-section of the electrodeposits showed a granular structure. The rate of the electrodeposition of copper from the Cu(CF3COO)2–HCONH2 bath and that of nickel from the Ni(CF3COO)2–HCONH2 bath were controlled by the charge-transfer reaction at low current densities. The activation overvoltage of the hydrogen-evolution reaction for the electrolysis of the CF3COOH–HCONH2 bath was higher than that for the electrolysis of the CF3COOH–H2O bath.
Properties and Configuration of Manganese(II), Cobalt(II), Nickel(II), Copper(II) and Zinc(II) Chelates of Acetoacetylferrocene
Authors: Imai Hiromu , Ota Tadamasa 
Published: July 2006

Abstract
Physical and chemical properties of manganese(II), cobalt(II), nickel(II), copper(II) and zinc(II) chelates of acetoacetylferrocene were investigated. The solubility decreased in the order CHCl3&gt;CH2Cl2&gt;C6H5Cl&gt;C6H4(CH3)2&gt;C6H6, the chelates being more soluble in pyridine. It was found from magnetic data that the configuration is octahedral for the aquo or pyridine adduct of manganese(II), cobalt(II) and nickel(II) chelates, square planar for anhydride of copper(II) chelate, distorted octahedral for its pyridine adduct, and quasi-tetrahedral for the anhydride of manganese(II) chelate. The d-d transition of electronic spectra was analysed by Ohsymmetry for cobalt(II) and nickel(II) chelates, and by C2h symmetry for copper(II) chelate. Assignment of the d-d transition for manganese(II) chelate was difficult.
The Polymorphism of Inosine. II. The Measurement of Solubilities in Water
Authors: Suzuki Yoshihisa 
Published: July 2006

Abstract
The solubilities of anhydrous inosine crystals in the α-form and in the β-form in water were determined in the temperature range between 20 °C and 60 °C. The results may be formulated as follows:
& α-form & logS=0.0190 t-0.065
& β-form & logS=0.0198 t-0.161
These data indicate that the β-crystals are more stable than the α-crystals over the whole temperature range examined. However, the difference in solubility is too small to induce the transition without seed crystals in the β-form, so that the α-crystals themselves are practically stable in water. The rate of the transition from the α-form to the β-form varies in proportion to the amount of the seed β-crystals. At low temperatures the dihydrate form of inosine is more stable than both of these two anhydrous forms. The solubility may be formulated as follows:
dihydrate-form logS=0.0314 t-0.276
Polymorphism of Inosine. III. The Equilibrium for the Inosine–Dimethyl Sulfoxide–Water System
Authors: Suzuki Yoshihisa , Hara Kentaro 
Published: July 2006

Abstract
The phase diagram for the inosine–dimethyl sulfoxide–water system was determined at 30 °C. Inosine in the α-form is more soluble than that in the β-form in aqueous DMSO. The difference in the solubilities between the two forms is small when the concentration of DMSO is below 50%. However, it becomes several times larger in 70% DMSO than in water. This is the reason why inosine in the β-form is often obtainable with ease from 70% DMSO in the absence of seed β-crystals, while it is with difficulty obtained only from water. When the concentration of DMSO becomes higher than 82%, a new solid phase is obtained. It consists of inosine with two mol of DMSO of crystallization. Inosine in the β-form is also obtainable with ease through the decomposition of these new crystals.
The Thermodynamic Properties of Tetraalkylammonium Halides in Acetone
Authors: Imura Masahiro , Nomura Hiroyasu , Kawaizumi Fumio , Miyahara Yutaka 
Published: July 2006

Abstract
The vapor-pressure depressions of solutions of four tetraalkylammonium halides in acetone were determined by means of a vapor-pressure osmometer. The experimental results were discussed by analyzing the relationship between the h(=1000lna1⁄M1m+1) function and the concentration. The following conclusions are obtained: tetraalkylammonium salts are dissociated in acetone, and the thermodynamic behavior of the dissociated species is described by the Debye-Hückel-Onsager limiting formula with sufficient accuracy, where the undissociated species are assumed to behave ideally.
Except for (n-Bu)4NCl, the dissociation constants determined by the present work are in good agreement with those found in the literature, which were determined conductometrically.
The Preparation of 3,5-DicMoro-2,4,6-trifluorophenyl(cyclopentadienyl)dicarbonyliron and Its Derivatives
Authors: Ishikawa Nobuo , Ibuki Shuji 
Published: July 2006

Abstract
3,5-Dichloro-2,4,6-trifluorophenyl(cyclopentadienyl)dicarbonyliron (4) was prepared by the nucleophilic substitution of sym-trichlorotrifluorobenzene with the (cyclopentadienyl)dicarbonyliron anion. One of the chlorine atoms of 4 was lithiated and the resulting lithio compound was hydrolyzed or carboxylated to give 3-chloro-2,4,6-trifluorophenyl(cyclopentadienyl)iron (6) or its 5-carboxylic acid (7) respectively.
The Solubility Study of Ion-pairs in Organic Solvents
Authors: Takamatsu Takejiro 
Published: July 2006

Abstract
The solubilities of alkali picrates were measured in alcohols (13 species) and esters (19 species) at 25±0.2°C. It was found that the solubility data are well correlated with the solubility parameters of the solvents. It is also pointed out that the distribution ratios of the cesium picrate between the aqueous phase and the organic phase are apparently correlated with the solubility parameters of the extraction solvents.
The Equilibria of Protonation Reaction of N,N′-Ethylenebis(salicylideneiminato) cobalt(II) and of Its Oxygenated Binuclear Complex by Perchloric Acid in Dimethyl Sulfoxide
Authors: Abe Yuriko , Wada Goro 
Published: July 2006

Abstract
Acid dependence of oxygen uptake by Co(II)salen in DMSO was studied. As the concentration of perchloric acid increases in the absence of O2, Co(II)salen forms 1: 1 and 1: 2 protonated complexes, with the formation constants K1=8.3×103 M−1 and K2=1.6×103 M−1 measured spectrophotometrically by the linearity method. The oxygen uptake ratio n of Co(II)salen in the absence of acid is 0.5, indicating formation of a 2: 1 O2-bridged binuclear complex, with an immeasurably large formation constant K3. The oxygenated complex is protonated by addition of acid with a formation constant K4=3.3×105 M−1 as observed again spectrophotometrically. When Co(II)salen is oxygenated in the presence of acid, n decreases considerably from 0.5 with increasing acid concentration. This is because the protonated Co(II)salen complexes are inert to oxygenation and both the rate of oxygenation of Co(II)salen and that of deoxygenation of oxygenated binuclear species are slow, as compared to the rapid processes of protonation.
Ringspaltungsreaktion von Thiadiazolo- und Triazolopyrimidin-Derivaten
Authors: Okabe Takayuki , Taniguchi Eiji , Maekawa Kazuyuki 
Published: July 2006

Abstract
The ring-opening and rearrangement reaction of 7-methyl-5H-1,3,4-thiadiazolo[3,2-a]pyrimidin-5-ones (I) and the alternative isomeric 7-ones (XI) has been studied. I (R=H) and XI (R=H) gave quantitatively through N–N bond cleavage methylthiouracil (III) by treating with 5% NaOH, while they yielded 4-methyl-2-thiocyanato-6(1H)pyrimidone (II) by treating with liquid ammonia. By treating I and XI (R=alkyl) with 5% NaOH, the S–C bond cleaved to produce the corresponding acylaminopyrimidine derivatives (IV and IV′) and 3-aminopyrimidinethione (V and V′). The treatment of 2-hydrazino derivative (I; R=NHNH2) with 5% NaOH gave 1-amino-2-mercapto-7-methyl-5H-s-triazolo[1,5-a]pyrimidin-5-one (VIII) through ring-opening and recyclization. On the other hand, the pyrimidine moiety of I (R=Et and i-Pr) cleaved by the action of chlorine to afford 2-amino-1,3,4-thiadiazole (X; R=Et and t-Pr).