bluuebunny/crossref_metadata_2025_split huggingface dataset:
Optical Resolution and Ternary System Solubility Isotherms of Cobalt(III) Complex Salts
Authors: Shimura Yoichi , Tsutsui Katsuyuki 
Published: July 2006

Abstract
The characteristics of three-component phase diagrams consisting of water and a pair of enantiomeric or diastereomeric cobalt(III) complex salts have been clarified experimentally by constructing the solubility isotherms of the following 6 ternary systems at 25 °C: H2O–Δ-[Co(ox)(en)2]X–Λ-[Co(ox)(en)2]X, where X stands for Cl−, I−, (R,R)-C4H5O6–, Δ-[Co(edta)]−, 1/2[Sb2{(R,R)-C4H2O6}2]2− or d-C10H14OBrSO3−. It has been found that the latter two anions form an active racemate, rac-[Co(ox)(en)2]2[Sb2{(R,R)-C4H2O6}2]·5H2O and rao[Co(ox)(en)2](d-C10H14OBrSO3)·H2O, respectively. Solubility of the [Co(ox)(en)2]+ salts containing optically active counterions was determined at 5–60 °C in water.
Kinetic Study of Oxidative Addition and Replacement Reactions of Chlorotris(triphenylphosphine)rhodium(I) in Benzene
Authors: Ohtani Yoshimi , Fujimoto Masatoshi , Yamagishi Akihiko 
Published: July 2006

Abstract
Chlorotris(triphenylphosphine)rhodium (I) exists in benzene as RhCl(PPh3)3, a dimeric species [RhCl(PPh3)2]2 and a reaction intermediate RhCl(PPh3)2. The rates of oxidative addition and replacement reactions were examined in benzene. For the oxidative addition reaction, RhCl(PPh3)3, RhCl(PPh3)2, or [RhCl(PPh3)2]2+X1\oversetkadd\ightleftarrowsProducts (X1=H2, O2, C2H4, CHCl=CCl2 CH3I), it was found that the reaction intermediate RhCl(PPh3)2 is most reactive, the degree of its reactivity depending largely on the reactants (X1). A significant enhancement of the rate (ksub&gt;kadd) was observed for the ligand substitution reaction, RhClX1(PPh3)m+X2\oversetksub\ightleftarrowsRhClX2(PPh3)n+X1 (m, n=3 or 2), where X1=H2 and X2=olefin and vice versa. The results are discussed in relation to the mechanism of hydrogenation.
The Fluorometric Determination of Indium with 8-Quinolinethiol
Authors: Watanabe Kunihiro , Fujiwara Akio , Kawagaki Ky\={o}z\={o} 
Published: July 2006

Abstract
8-Quinolinethiol reacts with indium in the presence of acetic acid to give a yellow ternary complex. The complex, which has a greenish-yellow fluorescence with an emission maximum at 515 nm, can be extracted into chloroform at pH 2–4. In the ternary complex, the fluorescence intensity was about two times as large as that of the indium 8-quinolinethiolato complex containing no acetate ion. The fluorescence was stable for at least 1 h. By the use of a 0.2 μg/ml uranine solution as the setting reagent, 0–25 μg of indium in 10 ml of chloroform was determined. The coefficient of the variation was 3% for 12 μg of indium. The extractability of the ternary complex from 50 ml of an aqueous solution into 10 ml of chloroform was 99%. Iron, mercury, and silver, which interfere with the determination of indium, were masked with ascorbic acid, potassium iodide, and thiourea. Other interfering elements, including zinc, cadmium, gallium, nickel, palladium, cobalt, antimony, gold, copper, bismuth, and vanadium, must be removed before any analysis.
The Effect of Steric Condition on the Structure of Cobalt(II), Nickel(II), and Copper(II) Complexes with Schiff Bases Obtained from Salicylaldehyde Derivatives and 9-Aminofluorene
Authors: Yamada Shoichiro , Tanaka Hideaki , Yamanouchi Kuniko 
Published: July 2006

Abstract
Cobalt(II), nickel(II), and copper(II) complexes with N-(9-fluorenyl)salicylideneaminates (abbreviated as X-sal-fl) have been examined in relation to the effect of steric condition on the structure of the metal complexes. Compounds of the type M(X-sal-fl)2 and their pyridine adducts were isolated as crystals. Complexes of the type Cu(X-sal-fl)Cl were also obtained. Both in the solid state and in non-donor solvents, the complexes Co(X-sal-fl)2 have a tetrahedral configuration, and Ni(X-sal-fl)2 and Cu(X-sal-fl)2 have a square-planar configuration. Possible structures of the pyridine adducts are discussed. The results indicate that the steric hindrance caused by fluorenyl is greater than that by diphenylmethyl.
Secondary Ion Mass Spectrometry for Diffusion Studies in Glass
Authors: Mizuike Atsushi , Iino Akira 
Published: July 2006

Abstract
A procedure has been established for obtaining quantitative diffusion profiles by means of an ion microanalyzer in the narrow region within 1 μm of the glass surface. The interdiffusion of silver and sodium ions in Pyrex glass immersed in molten silver nitrate was studied as an example.
Metal Complexes of Sulfur–Nitrogen Chelating Agents. IV. Complexes of Ni(II), Pd(II), Pt(II), and Rh(III) with Methyl Ester of 2-Amino-1-cyclopentene-1-carbodithioic Acid
Authors: Joardar D S , Mondal S K , Nag K 
Published: July 2006

Abstract
The methyl ester of 2-amino-1-cyclopentene-1-carbodithioic acid (HE) acts both as an unidentate and a bidentate ligand. With Ni(II) and Pd(II) the deprotonated chelates of the ME2 type are obtained readily, the corresponding Pt(II) complex being obtained on the addition of an equivalent amount of alkali. In these compounds, bond formation takes place from the nitrogen and the sulfur atoms and there is strong delocalization in the chelate ring. Unidentate bonding behavior of the ligand is observed in the complexes Pt(HE)2Cl2 and Rh(HE)3Cl3, where the bonding takes place through the nitrogen atom only.
Kinetic Studies of the Inversion of η2-trans-2-Butene in Platinum(II) Complexes Containing Various Amino Carboxylates
Authors: Terai Yoshiro , Kido Hiroaki , Kashiwabara Kazuo , Fujita Junnosuke , Saito Kazuo 
Published: July 2006

Abstract
Platinum(II) complexes containing S,S- or R,R-trans-2-butene and various l-amino carboxylate, i.e. cis(N,olefin)[PtCl(l-prolinate)(S,S-trans-2-butene)] and trans(N,olefin)[PtCl(l-am)(S,S- or R,R-trans-2-butene)], were synthesized (l-am=l-prolinate, N-methyl-l-prolinate, N-benzyl-l-prolinate, l-alaninate and l-valinate). The kinetics of the inversion reaction of the coordinated S,S- or R,R-trans-2-butene was investigated in acetone in the presence of an excess of trans-2-butene at 8.0 °C. Second order rate law was obeyed with respect to the concentrations of the complex and free olefin, and no solvent path was observed. In the trans (N,olefin) complexes, interactions steric around the coordinated nitrogen atoms seems to be responsible in determining the ease of inversion via an associative mechanism. On the other hand, smaller rate for the cis(N,olefin) complex than the corresponding trans isomer seems to be due to the trans effect.
A One-step Synthesis of 2,7-Dimethyl-5-silaspiro[4.4]nona-2,7-diene and the Synthesis of Its Derivatives
Authors: Terunuma Daiyo , Hatta Satoshi , Araki Tsunao , Ueki Tadashi , Okazaki Tsutomu , Suzuki Yasutaka 
Published: July 2006

Abstract
The in situ reaction of dialkoxydichlorosilanes, such as dimethoxy-, diethoxy-, dipropoxy-, dibutoxy-, and diisobutoxydichlorosilane, with isoprene and magnesium in tetrahydrofuran (THF) was investigated. It was found that the double annelation product, i.e., 2,7-dimethyl-5-silaspiro[4.4]nona-2,7-diene (1), was obtained from diethoxy-, dipropoxy-, and dibutoxydichlorosilane in 87.0, 65.2, and 66.5% yields respectively. On the other hand, the reaction of dimethoxy- and diethoxydichlorosilane with butadiene and magnesium in THF gave 5-silaspiro[4.4]nona-2,7-diene (7) in 20.7 and 35.4% yields respectively. Several silaspirononane derivatives were synthesized from 1.
A Synthesis of 3′,4′-Dideoxykanamycin B
Authors: Nishimura Tomo , Tsuchiya Tsutomu , Umezawa Sumio , Umezawa Hamao 
Published: July 2006

Abstract
3′,4′-Dideoxykanamycin B was prepared from kanamycin B via 4″,6″-O-cyclohexylidenation, N-benzyloxycarbonylation, 3′,4′,2″-tri-O-benzylsulfonylation, double bond formation at C-3′,4′, removal of the cyclohexylidene group, simultaneous removal of the N,O-protecting groups with sodium metal in liquid ammonia and hydrogenation of the resulting 3′,4′-dideoxy-3′-eno-kanamycin B.
Synthesis of Mesoionic Triazolopyridine. I. N-Alkylation of 1,2,4-Triazolo[4,3-a]pyridin-3(2H)-one
Authors: Saito Akio , Shimizu Bunji 
Published: July 2006

Abstract
Mesoionic anhydro-1-alkyl-3-hydroxy-1,2,4-triazolo[4,3-a]pyridinium hydroxide (6) was synthesized by selective alkylation at the N1-position of 1,2,4-triazolo[4,3-a]pyridin-3(2H)-one (8). The selectively depends on the use of a mercuric salt catalyst in combination with the trimethylsilyl derivative of 8, and its mechanistic features were also studied with reference to the benzylation of 8.
Synthesis of Mono-O-tritylraffinoses and Deca-O-acetylraffinoses
Authors: Suami Tetsuo , Otake Toshiki , Ikeda Toshihide , Adachi Ryoichi 
Published: July 2006

Abstract
Starting from raffinose (1), its 6″-O-trityl (2) and 6′-O-trityl derivative (6) have been prepared. Acetylation of 2 and 6 gave deca-O-acetyl-6″-O-tritylraffinose (3) and deca-O-acetyl-6′-O-tritylraffinose (7) respectively. Detritylation of 3 gave two products: 1′,2,2″,3,3′,3″,4,4′,4″,6′-deca-O-acetylraffinose (4) as a main product and 1′,2,2″,3,3′,3″,4,4′,6′,6″-deca-O-acetylraffinose (5) as a minor product, which were successfully separated by a column chromatography. On the other hand, detritylation of 7 gave 1′,2,2″,3,3′,3″,4,4′,4″,6″-deca-O-acetylraffinose (8) as a sole product. Their structures were established by NMR spectra, together with a chemical evidence.
Kinetics and Mechanism of the Oxidation of Substituted Benzyl Alcohols by Chloramine-T in Acid Solution
Authors: Banerji Kalyan K 
Published: July 2006

Abstract
The oxidation of benzyl alcohol and eight monosubstituted benzyl alcohols by chloramine-T in acid solution has been studied. The reaction is of first order with respect to the concentration of alcohol and oxidant. The rate constant is proportional to the square of hydrogen ion concentration. The kinetic isotope effect (kH⁄kD) is 4.13 at 303 K. The solvent isotope effect (k(D2O)/k(H2O)) is 5.93 at 303 K. The activation enthalpies and entropies are linearly related. The reaction exhibited a reaction constant ρ+=−2.08±0.08 at 303 K. The active oxidizing species is considered to be (H2OCl)+. A mechanism involving transfer of a hydride ion to the oxidant is suggested.
Branched-Chain Deoxy Nitro and Amino Sugar Derivatives from Ketose
Authors: Abdel-Rahman M M A 
Published: July 2006

Abstract
The nitromethane addition to 1,2-O-isopropylidene-5-O-trityl-α-d-erythro-pentofuranos-3-ulose (1) proceeded smoothly to give the corresponding branched-chain sugar (2), which on acetylation afforded (3) followed by elimination of acetic acid in presence of a base catalyst, resulted in the formation of the corresponding nitro olefin (4). The nitro olefin when subjected to borohydride reduction gave 5-O-acetyl-3-deoxy-1,2-O-isopropylidene-3-(nitromethyl)-α-d-ribofuranose (5), which on catalytic hydrogenation followed by acetylation yielded the 3-(acetamidomethyl)-5-O-acetyl-3-deoxy-1,2-O-isopropylidene-α-d-ribofuranose (6). By the study of the NMR spectra of 3-deoxy derivatives (5) and (6), their d-ribo configurations were established.
The Microwave Spectra, Molecular Structures, and Quadrupole Coupling Constants of Methyltrichlorosilane and Trichlorosilane
Authors: Takeo Harutoshi , Matsumura Chi 
Published: July 2006

Abstract
The microwave spectrum of methyltrichlorosilane has been re-examined in order to determine the precise molecular structure by observing the spectra due to the isotopic species. The previously reported value of the nuclear quadrupole coupling constant of trichlorosilane has been corrected by the reinvestigation of the hyperfine structure.
Additions of Dibenzyl Selenoxides and Selenonium Ylides to an Electron-Deficient Ethylene and Acetylene
Authors: Tamagaki Seizo , Akatsuka Ryuji , Kozuka Seizi 
Published: July 2006

Abstract
Dibenzyl selenoxides reacted with dimethyl acetylenedicarboxylate and tetracyanoethylene to give new ylides, while the reaction of selenonium ylides produced new ylides or cyclopropane derivatives.
Cation Distribution in Mixed Formates. I. Change in Lattice Constants and in Infrared Spectra with the Chemical Constitution of (Cu,MII)(HCOO)2·2H2O
Authors: Ogata Toshiko , Taga Tooru , Osaki Kenji 
Published: July 2006

Abstract
Mixed crystals of formates, with the general formula of (Cu,MII)(HCOO)2·2H2O, were found to show non-uniform changes in lattice constants with the change in the chemical constitution. The reason for this unusual change was sought in relation to a possible non-uniform distribution of the cations over the two non-equivalent sites for metal ions. A simple model based merely on the difference in the radii of metallic ions did not agree with the results of IR measurements nor with those of an ESR study by Wagner et al. Another model, incorporating the change in the direction of the longest axis of the octahedron coordinating to the Cu2+ ion at the Ml-site, as suggested from the results of a related work (Part II), was found not only to explain the changes in lattice constants, but also to conform with other evidences; it seems to be an essentially correct interpretation.
Cation Distribution in Mixed Formates. II. The Structure of the Mixed Crystal Cu0.5Zn0.5(HCOO)2·2H2O
Authors: Ogata Toshiko , Taga Tooru , Osaki Kenji 
Published: July 2006

Abstract
The structure of the 1 : 1 mixed crystal between Cu(HCOO)2·2H2O and Zn(HCOO)2·2H2O has been determined on the basis of 878 visual intensity data. The space group is P21/c, with a=8.77, b=7.25, c=9.17 Å, β=98.0°, and Z=4. The structure is similar to that of Cu(HCOO)2·2H2O, except that the two metal ions are distributed over two metal ion sites, the Cu2+ ions being mostly localized in the [Cu(HCOO)2] sheet, and except that the longest axis of the coordination octahedron around the Cu2+ ion in this sheet is nearly perpendicular to it. It is suggested that the type of sheet found in the mixed crystal as well as in Cu(HCOO)2·4H2O is more favorable than that in Cu(HCOO)2·2H2O for an independent [Cu(HCOO)2] sheet.
Ultrasonic Study of Aqueous Solutions of Amines
Authors: Nishikawa Sadakatsu , Otani Unpei , Mashima Mitsuo 
Published: July 2006

Abstract
Ultrasonic absorptions in aqueous solutions of allylamine have been measured in the frequency range from 8.5 to 220 MHz at 20 °C. Only a single relaxational absorption has been observed in the concentration range from 0.0562 to 4.00 M; this absorption has been attributed to the proton-transfer reaction. The rate constants and the standard volume change due to the reaction have been determined, and the results are compared with those for aqueous solutions of other amines previously published. It has been elucidated from the ultrasonic absorption data that the presence of the intermediate in the proton-transfer reaction is required in order to analyze the ultrasonic absorption mechanisms quantitatively. The hydrophobic effect in the aqueous solutions of amines has also been discussed.
Electrical Conductivity of Single Crystals of N-Methylphenazinium–TCNQ and Rb–TCNQ
Authors: Fujii Genshiro , Shirotani Ichimin , Nagano Hiroshi 
Published: July 2006

Abstract
Temperature and pressure dependence of the electrical conductivity of NMP(I)–, NMP(II)–, Rb(I)– and Rb(II)–TCNQ single crystals was studied. NMP(I)–TCNQ showing diffuse streaks was found to have a semiconductive behavior at atmospheric pressure and no conductive maximum appeared at low temperatures. Moreover the metallic behavior was not shown even at 100 kbar. The crystal structure of NMP(II)–TCNQ is not similar to that of NMP(I)–TCNQ. The difference of physical properties of two phases is discussed. The anomalous electrical behavior in the pressure range from 1 bar to 40 kbar was observed for Rb(I)–TCNQ. The resistivity of Rb(II)–TCNQ decreased monotonously with increasing pressure. The magnitude of their activation energies was 0.15 eV for Rb(I) salt and 0.11 eV for Rb(II) salt at 100 kbar. The insulator to metal transition could not be observed at least up to 100 kbar.
Bimolecular Quenching of Triplet Exciton in Crystalline p-Dibromobenzene
Authors: Kobayashi Takayoshi , Hirota Noboru 
Published: July 2006

Abstract
The rate constants of the bimolecular quenching (γ), the hopping rate (λ), and the diffusion coefficient (D) of triplet exciton in crystalline p-dibromobenzene at temperatures between 160 and 77 K were obtained by analyzing the phosphorescence decay curves at high and low density excitations with the use of an N2 laser as an exciting pulsed light source. Estimated values of γ, λ, and D at room temperature (300 K) are (2±1)×10−12 cm3· s−1, 9±4×109s−1, and 2±1×10−4 cm2 s−1, respectively. The activation energy of the exciton hopping was obtained to be 290±50 cm−1. This implies that the triplet exciton in p-dibromobenzene crystal is described in terms of a localized exciton model.
Color and Precipitation Reactions of Lanthanoids with Chlorophosphonazo III
Authors: Taketatsu Tomitsugu 
Published: July 2006

Abstract
Aqueous solutions of pH 1–3 containing Chlorophosphonazo III chelates of lanthanoids give an absorption spectrum of the α-type, the spectrum showing two maximum peaks at approximately 620 and 670 nm, in the presence of an excessive amount of the reagent. However, the chelates of heavier lanthanoids give an absorption band of the β-type, which extends from 700 to 800 nm with a maximum at 745 nm, with an increase in the mole ratio of the metal against the reagent. A dark-violet precipitate forms from the solution, giving the β-type spectrum under certain experimental conditions. The combining ratio of the metal against the reagent in the precipitate is estimated to be 1:1. Since a reaction rate from the α-type to the β-type corresponds to the second order, the combining form between the metal and the reagent in the β-type chelate is assumed to be 2:2, i.e., a dimer. The chelates of lanthanum to samarium do not give the β-type spectrum under the same conditions.
Kinetics of the Oxidation of Formate by Iodine in the Water–Methanol and the Water–Ethanol Mixed Solvents
Authors: Hayakawa Katumitu , Sato Makoto 
Published: July 2006

Abstract
The kinetics of the iodine–formate reaction gives an identical rate equation in several water–alcohol mixed solvents: rate=k[HCOO−] [I3−]/(a[I−]+1). The values of k/M−1 s−1 and 10−3 a/M−1 are 5.5 and 16 in ethanol at 35 °C, 3.6 and 12 in methanol at 30 °C, and 0.06 and 0.7 in water at 30 °C, respectively. The values of a are comparable to the formation constants of I3− in the water–alcohol mixtures. The rate-determining step is the reaction between an iodine molecule and a formate ion. The activation parameters, ΔH\eweq/kJ mol−1 and ΔS\eweq/J K−1 mol−1, are 71 and −1 in the 0.95 mole fraction of methanol, 60 and −32 in the 0.92 mole fraction of ethanol, and 75 and −20 in water, respectively. The entropy term contributes to the acceleration by alcohol in the water–methanol mixtures, whereas both the entropy term and the enthalpy term contribute to the acceleration in the water–ethanol mixtures. These effects of the mixed solvents suggest that the specific interactions of the solvent molecule with the reactants and the activated complex, such as a specific solvation or a hydrogen bond, play an important role in the rate-determining step.
Crystal and Molecular Structure of a Chelating Ylide Complex of Palladium: [(Benzoylmethylene)diphenyl-2-(diphenylphosphino)ethylphosphorane]dichloropalladium(II) Toluene Solvate
Authors: Takahashi Hirofumi , Oosawa Yoshinao , Kobayashi Akiko , Saito Taro , Sasaki Yukiyoshi 
Published: July 2006

Abstract
The crystal structure of the title compound was solved by single-crystal X-ray diffraction techniques. Crystals are monoclinic, space group P21/c, with cell dimensions a=13.39(5), b=16.88(1), c=19.85(6) Å, β=123.54(1)°, Z=4. The final R was 0.065. The ylide ligand coordinates to palladium through the ylide carbon Pd–C, 2.115(15) Å and phosphine phosphorus Pd–P, 2.230(8) Å forming a six-membered chelate ring. The chelating ligand together with two chlorine atoms forms a square-planar geometry around the palladium. The Pd–Cl distances are discussed in terms of trans-influence of the ylide carbon.
Stereoselectivity in Mixed Ligand Copper(II) Complexes with Electrostatic Ligand-Ligand Interactions. Application to Optical Resolution of α-Amino Acids with a Charged Side Chain
Authors: Yamauchi Osamu , Sakurai Takeshi , Nakahara Akitsugu 
Published: July 2006

Abstract
Optical resolution of Dl-aspartic acid and Dl-glutamic acid (abbreviated as Dl-A) has been achieved via formation of ternary copper(II) complexes composed of a Dl-A and l-arginine, l-lysine, or l-ornithine (l-B). For every pair of ligands Dl-A and l-B, a neutralized solution containing Cu(ClO4)2, Dl-A, and l-B in the molar ratio of 1 : 2 : 1 gave the mixed ligand complex abounding in [Cu(d-A)(l-B)] as crystals, from which incorporated A was isolated through a Dowex CCR-2 column after treating with H2S. In a very similar manner d-enantiomerrich B was obtained from Dl-B by using an l-A. The optical purities of the resolved amino acids were as high as 90 and 70% for aspartic acid and glutamic acid, respectively, and 40–50% for the basic amino acids. The finding shows that the electrostatic ligand-ligand interactions in the mixed ligand complexes give rise to geometric isomerism around copper(II) and hence preferential formation and crystallization of the meso complexes, [Cu(l-A)(d-B)] and [Cu(d-A)(l-B)], probably with a cis configuration.
13C NMR Spectroscopy. Substituent Effects in the Ethyl trans-2-Substituted Cyclopropanecarboxylates
Authors: Kusuyama Yoshiaki , Ikeda Yoshitsugu 
Published: July 2006

Abstract
13CFT NMR spectra have been obtained for ethyl trans-2-substituted cyclopropanecarboxylates. The chemical shifts have been compared with those of saturated and unsaturated compounds and with substituent constants. The chemical shifts of the methylene carbon of the ethyl group have been correlated linearly with σm.
The Intramolecular OH···π Bonding in [n]Paracyclophanols
Authors: Mori Nobuo , Takemura Tetsuo , Ohkuma Takayoshi 
Published: July 2006

Abstract
The intramolecular OH···π bonding, the probable conformations, and the internal rotation in two series of [n]paracyclophanols, I (1–4) and II (5–8), are investigated by means of IR, UV, and PMR spectroscopies. The IR results indicate that (1) there is the OH···π bonding in 1, 2, 3, 5, and 7 in CCl4, and (2) the integrated intensity ratio of the π-bonded OH species to the free OH, (Ab/Af), hardly changes in going from 1 to 3, while it decreases in going from 5 to 7. The ratio is, then, related to both ΔH and ΔS. In the PMR spectra, aliphatic-proton signals are assigned by means of the spin-decoupling and/or the shift-reagent technique; the results agree with those predicted, with a few exceptions. In the aromatic region, 3, 4, and 8 show singlets, while the others show AB-like quartets. These results are discussed in relation to the internal rotation of the benzene ring and/or the alkylene chain.
Photochemical Reactions between Dibromomaleic Anhydride and Benzo[b]thiophene
Authors: Matsuo Taku , Mihara Satoru 
Published: July 2006

Abstract
On the irradiation of dibromomaleic anhydride in the presence of benzo[b]thiophene, both photosubstitution and photocycloaddition are simultaneously induced. The ratio of the substituted product to the cycloadduct increased with solvent polarity, and the same Stern-Volmer plots were obtained with both of the reactions. Kinetic parameters, obtained under various concentrations of benzo[b]thiophene, indicate that the reactions proceed via a common triplet exciplex. The activation energy for the photosubstitution is higher than that for the cycloaddition by 1–2 kcal/mol, where the former reaction is considered to proceed via a more polar transition state than the latter.