Electronic spectra solution

The electronic spectrum of a compound arises from its 7r-electron system which, to a first approximation, is unaffected by substitution of an alkyl group for a hydrogen atom. Thus, comparison of the ultraviolet spectrum of a potentially tautomeric compound with the spectra of both alkylated forms often indicates which tautomer predominates. For example, Fig. 1 shows that 4-mercaptopyridine exists predominantly as pyrid-4-thione. In favorable cases, i.e., when the spectra of the two alkylated forms are very different and/or there are appreciable amounts of both forms present at equilibrium, the tautomeric constant can be evaluated. By using this method, it was shown, for example, that 6-hydroxyquinoline exists essentially as such in ethanol but that it is in equilibrium with about 1% of the zwitterion form in aqueous solution (Fig. 2). 

The single-electron spectrum for the antisoliton solution, A,U)=-Av(x ), is exactly the same as for the soliton, except that now the wave function of the midgap state is given by... 

Scaiano and Kim-Thuan (1983) searched without success for the electronic spectrum of the phenyl cation using laser techniques. Ambroz et al. (1980) photolysed solutions of three arenediazonium salts in a glass matrix of 3 M LiCl in 1 1 (v/v) water/acetone at 77 K. With 2,4,5-trimethoxybenzenediazonium hexafluorophos-phate Ambroz et al. observed two relatively weak absorption bands at 415 and 442 nm (no e-values given) and a reduction in the intensity of the 370 nm band of the diazonium ion. The absence of any ESR signals indicates that these new bands are not due to aryl radicals, but to the aryl cation in its triplet ground state. 

Occasionally, some bands which might otherwise be expected to be weak are observed to be quite strong. Two examples are shown in Fig. 4-4. The first shows the electronic spectrum of a solution containing [CoC ] ions in nitromethane. For this cT system, we expect three spin-allowed transitions and these are observed at roughly 3500, 7000 and 14,000 cm h They correspond (see Chapter 3) to the excitations M2 —> Ti F) and T P) respectively. Note, however, that the... 

Methyl-5-amino-l-formylisoquinoline thiosemicarbazone, 22, also yields cobalt(II) complexes from unheated methanol solution [202]. However, due to this ligand s added steric requirements, a complex, [Co(22)Cl2], with one ligand per metal ion center is formed. This brown solid has a magnetic moment of 4.42 B.M., is a non-electrolyte, has coordination of a neutral NNS ligand, and the electronic spectrum indicates approximate trigonal bipyramidal stereochemistry. 

In a solution where a nonzero volume change between the electronic isomers, HS and LS, is encountered, the position of the spin equilibrium will depend on pressure. The volume change, usually denoted here AF°, may be obtained from the study of the pressure dependence of equilibrium properties such as the magnetic susceptibility or the electronic spectrum. In favorable cases, A F° values may be derived from the amplitude of sound absorption observed in ultrasonic relaxation measurements of a spin equilibrium as will be shown in the... 

Solutions of the alkylammonium salts of Cl , Br , r in acetonitrile show no visible absorptions beyond 300 nm. The aromatic it-acceptor, tetracyanopy-razine (TCP) is characterized by strong absorptions in the 220-300 nm range and a shoulder at 350 nm. However, the electronic spectrum of a mixture of the bromide salt and TCP reveals a new absorption band at Xct = 400 nm... 

In this contribution, we describe work from our group in the development and application of alternatives that allow the explicit inclusion of environment effects while treating the most relevant part of the system with full quantum mechanics. The first methodology, dubbed MD/QM, was used for the study of the electronic spectrum of prephenate dianion in solution [18] and later coupled to the Effective Fragment Potential (EFP) [19] to the study of the Claisen rearrangement reaction from chorismate to prephenate catalyzed by the chorismate mutase (CM) enzyme [20]. 

Although low energy structures for prephenate have been reported before [40], these have been optimized using gas-phase quantum mechanics, and are not compatible with the structure determined for the prephenate inside the active site of CM [41], The first calculation of the electronic spectrum of prephenate inside the active site of the enzyme was done by our group [18]. Using the MD/QM method described, we were also able to obtain an electronic spectrum for prephenate in solution. 

Solutions of CrCl2-2CH3CN and the wet solid are extremely sensitive to oxygen. When dried, however, the complex remains unchanged in air for several minutes before oxidation becomes apparent. It is virtually insoluble in acetonitrile but quite soluble in ethanol. The room-temperature magnetic moment (4.8 B.M.) and electronic spectrum are consistent with a high-spin distorted octahedral Cr2+ ion.15... 

Potassium hexafluororhenate (IV), K2ReF6, is, obtained from potassium perrhenate via K2Re(I)6 and subsequent fusion with KHF2 (45), and shows a magnetic moment of 3.3—3.4 B.M. at 298 °K (31, 46). The electronic spectrum in aqueous solution has been studies in some detail by Jergensen and Schwochau (29), and the principle features of their results are listed in Table 11. (See also Fig. 6). 

The electronic spectrum of the complex consists of a combination of the spectra of the parent compounds plus one or more higher wavelength transitions, responsible for the colour. Charge transfer is promoted by a low ionization energy of the donor and high electron affinity of the acceptor. A potential barrier to charge transfer of Va = Id — Ea is predicted. The width of the barrier is related to the intermolecular distance. Since the same colour develops in the crystal and in solution a single donor-acceptor pair should be adequate to model the interaction. A simple potential box with the shape... 

Figure 2. The electronic spectrum of a typical phenoxyl in MeCN solution l,l-bis[2-(l-methylimidazolyl)]-l-(3,5-di-/c/t-butyl-4-oxyphenyl)ethane (BIDPhE). [Adapted from (136, 137)].

The electronic spectrum of the fractions containing the pure tridehydro [18]annulene exhibits the strongest absorption maximum (in benzene) at 342 nm. (e 155,000) and the spectroscopic yield, based on the molar extinction coefficient, is 1.17 g. (2.40% from 1,5-hexadiyne). The yield of tridehydro[18]annulene in the mixed fractions, based on the 342 nm. maximum,is 0.27g.(0.55%). The tridehydro [18]annulene is best stored in solution in the refrigerator. 

The stability of C60 and C70 solutions in vegetable oils has been examined also towards the action of UV light. A C60 solution in linseed oil has been irradiated in a quartz reactor with UV light from a 12 W low-pressure Hg lamp having its main emission at 254 nm under N2. In less than 1 hour irradiation, all the visible part of the electronic spectrum of C60 with bands at about 530 and 600 nm have been bleached. Simultaneously, a growth in absorption intensity as function of the irradiation time has been observed at about 410nm. 

Dibenzothiophene acts as a 7r-electron donor and readily forms complexes with known electron acceptors. In such cases the electronic spectrum of a solution of the two compounds shows a new absorption band, usually in the visible region. The order of donor strengths of several o,o -bridged biphenyls has been estimated from their respective charge-transfer spectra and found to be carbazole > fluorene > dibenzothiophene >dibenzofuran. Dibenzothiophene forms complexes with tetracy-anoethylene, various polynitro derivatives of fluorenone, > naphthalene-1,4,5,8-tetracarboxylic acid dianhydride, and tetra-methylmic acid. ... 

The simplest method is probably colorimetric, based on its electronic spectrum [215], It can also be determined gravimetrically by addition of diphenylsulfide or ethanol to a solution of RuO this gives RuO which is then reduced to the metal [236], Alternatively addition of 2-propanol to a solution of RuO solution generates RuO. nHp [237],... 

Electronic spectra (Table 1.1, Fig. 1.2) have been measnred for the orange soln-tions of (RuO ] in aqueous base from 250-600 nm. [212-215, 222], and reproduced [215, 222]. There are two at 460 and 385 nm. [212, 213, 222] or three bands in the visible-UV region, at 460, 385 and 317 nm [214, 215]. These appear to be at the same positions as those for [RuO ] but the intensities and hence the general outline of the two spectra are very different. Woodhead and Fletcher reviewed the published molar extinction coefficients and their optimum values / dm (mol" cm" ) are 1710 for the 460 nm. band, 831 for the 385 nm. band and 301 for the 317 nm. band - the latter band was not observed by some workers [214]. The distinctive electronic spectrum of ruthenate in solution is useful for distinguishing between it, [RuO ]" and RuO [212, 222]. Measurements of the electronic spectra of potassium ruthenate doped in K CrO and K SeO and of barium ruthenate doped into BaSO, BaCrO, and BaSeO (in all cases the anions of these host materials are tetrahedral) indicate that in that these environments at least the Ru is tetrahedrally coordinated. Based on this evidence it has been suggested that [RuO ] in aqueous solution is tetrahedral [RuO ] rather than franx-[Ru(0H)3(0)3] [533, 535]. Potential modulated reflectance spectroscopy (PMRS) was used to identify [RuO ] and [RuO ] " in alkaline aqueous solutions during anodic oxidation of Ru electrodeposited on platinum from [Ru3(N)Clg(H30)3] [228]. 

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