Polarized infrared spectra of isotactic polypropylene (IPP) , (-CH2-CH-)n, I IPP-l, 1, 2-d., (-CD2-CD-) .. , I and IPP-ds, (-CD2-CD-)n I CHI CHI CD.were measured in the region from 4000 to 80 em-I. The normal vibrations of these three polymers were calculated for the infrared-and Raman-active A and E species, by the use of the GF-matrix method with the Urey-Bradley force field. Tlte results show a fairly good agreement between the observed and calculated frequencies. The assignments of the absorption bands were given, and discussions were made on the vibrational modes of the bands characteristic of the isotactic helical structures. A computation program for calculating the characteristic values and vectors of a Hermitian matrix was presented. From the normal coordinate treatment applied to the threo-and erythro-model of di-isotactic polypropylene-l-d,it was found that the polymers prepared from trans-and cis-propylene-l-d, CHD, CH-CHa, have threo-and erythro-di-isotactic structures, respectively, suggesting cis-opening mechanism in the coordinated anionic polymerization [a-TiCla-Ai (C.H6h catalyst].The absorption bands characteristic of crystalline IPP are also observed in the 1: 1 mole mixture of IPP and IPP-l, 1, 2-da, while these bands disappear in the copolymer with the same chemical composition. This fact shows that these characteristic bands arise from the intramolecular interactions in a helical molecule, and the sequence of a certain number of the same monomeric units in the helix is necessary for appearance of these bands. The 1152 and 975 cm-I bands, which appear also in molten IPP and an ether-soluble fraction of a commercial polypropylene, are ascribed to the chemical structure of head-to-tail sequence of -CRLCH(CHa)-units.
Two primary photoprocesses in the flash photolysis of Mn2(CO)10 are established with the use of a 10-ns N2 laser.One is the cleavage of the Mn-Mn bond to form • (00)5 radicals, and the other is the cleavage of the Mn-CO bond to form Mn2(CO)9. The reactivity of Mn2(CO)9 toward ligands is found to decrease in the following order: P(n-Bu)3 >> r-BuNC = EtCN >> CO. The substitution of CO in the .Mn(CO)5 radical with P(n-Bu)3 is shown to be associative. The reactivity of Mn2(CO)9 toward P(n-Bu)3 is higher than that of • (00)5.Photoexcitation of transition-metal carbonyls containing a metal-metal bond has been consistently interpreted to result in preferential homolysis of the metal-metal bond generating radical species in the primary process.2 Other photoprocesses, however, are not explicitly exempted from the dinuclear systems.3 Hughey et al.4 observed nonradical species besides • (00)5 in the conventional flash photolysis of Mn2(CO)m. The participation of another photointermediate in the reaction of Mn2(CO)10 with CC14 was verified kinetically by Fox and Poe.5 During the progress of our research,63 the flash photolysis study of M2(CO)10 (M = Mn and Re) using a flash duration of about 35 µ$ was reported by Wegman et al.7 in which under 1 atm of CO pressure the only process is the formation of • (0 )5 radicals, whereas in the thoroughly degassed solution a second intermediate is observed. They assigned the second intermediate absorptions to M2(CO)" ( = 8 and 9) formation by facile loss of CO from the sole primary photoproduct • (0 )5, followed by thermal recombination of the resulting • (00)4 radicals with themselves or with • (00)5. Their assignment gives rise to controversy in the understanding of the primary photoprocesses of Mn2(CO)10.We report here the laser flash photolysis study66 of Mn2(CO)10in cyclohexane using a 10-ns laser pulse to elucidate the primary photoprocesses in degassed conditions. A closely related study using picosecond flash photolysis by Rothberg et al.8 has recently appeared. Experimental SectionMn2(CO)m was synthesized by the method described in the literature9 and purified by sublimation. Mn2(CO)9(MeCN) and Mn2(CO)9(EtCN)
Re( b ~y ) ( C O ) ~B r l (bpy = 2,2'-bipyridine) and [Re(terpy)(CO),Br] (terpy = 2,2' : 6',2"-terpyridine) complexes can work as efficient catalysts for C02 electroreduction t o produce formic acid and CO in an aqueous medium when incorporated into a coated Nafion membrane.
Direct photolysis of 1,2,3-trimethyl-5,6-dicyanonorbornadiene (4) at 366 nm induces valence isomerization to the corresponding quadricyclene compound, 5, with a quantum yield of 0.68 ±0.01. The same transformation occurs in the presence of a number of triplet photosensitizers. Thus Ru(bpy)32+ (bpy is 2,2'-bipyridine), whose emissive metal-to-ligand charge-transfer excited state is quenched by 4 with a rate constant of 2.0 ± 0.2 X 108 M"1 s'1, sensitizes the production of 5 at 546 nm with a limiting quantum yield of 0.06 ± 0.01. Limiting yields at 436 nm for the organic sensitizers biacetyl and 9-fluorenone are 0.11 ± 0.01 and 0.15 ± 0.01, respectively. The finding that direct photoisomerization occurs with much higher efficiency than the triplet-sensitized process suggests that the lowest excited singlet state of 4 is significantly more reactive than the triplet state. Interestingly, the opposite reactivity pattern obtains in the case of the unsubstituted norbornadiene molecule. Possible reasons for this substituent effect are discussed in terms of the excited-state potential energy surfaces that interconnect the norbornadiene and quadricyclene structures.
The transient spectroscopy technique was applied for the determination of the quantum yields for the following two processes, and their dependences on the excitation wavelength were found. The quantum yield for process I, increases with the excitation wavelength while that of process II, Y2, decreases. The ratio R = Yi/Y2 obtained for different excitation wavelengths (Xex) is as follows: R = 0.19 ± 0.05 (Xex = 266 nm), R = 0.43 ± 0.02 (Xex = 337 nm), and R = 1.1 ± 0.2 (Xex = 355 nm). The sum of Y1 and Y2 were Mn2(CO)i0 Mn2(C0)9 fiv --2-Mn(CO)5 (I) --Mn2(C0)9 + CO (II) --•Mn(C0)s + • (00)4 (III) -Mn2(C0)8 + CO (IV)found to be unity for 355-, 337-, and 266-nm excitations. These results are discussed in terms of the properties of their excited states. Using two Q-switched Nd:YAG lasers, the following successive photolysis of Mn2(CO)9 by the second pulse was found to result in further CO elimination (process IV).
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