Optics early studies

In many aqueous solutions nickel has the ability to become passive over a wide range of pH values. The mechanism of passivation of nickel and the properties of passive nickel have been studied extensively—perhaps more widely than for any other element, except possibly iron. In recent years the use of optical and surface analytical techniques has done much to clarify the situation . Early studies on the passivation of nickel were stimulated by the use of nickel anodes in alkaline batteries and in consequence were conducted in the main in alkaline media. More recently, however, attention has been directed to the passivation of nickel in acidic and neutral as well as alkaline solutions. 

Early experimental spectroscopic investigations on Rg- XY complexes resulted in contradictory information regarding the interactions within them and their preferred geometries. Rovibronic absorption and LIF spectra revealed T-shaped excited- and ground-state configurations, wherein the Rg atom is confined to a plane perpendicular to the X—Y bond [10, 19, 28-30]. While these results were supported by the prediction of T-shaped structures based on pairwise additive Lennard-Jones or Morse atom-atom potentials, they seemed to be at odds with results from microwave spectroscopy experiments that were consistent with linear ground-state geometries [31, 32]. Some attempts were made to justify the contradictory results of the microwave and optical spectroscopic studies, and... 

Tanford (1968) reviewed early studies of protein denaturation and concluded that high concentrations of Gdm-HCl and, in some cases, urea are capable of unfolding proteins that lack disulfide cross-links to random coils. This conclusion was largely based on intrinsic viscosity data, but optical rotation and optical rotatory dispersion (ORD) [reviewed by Urnes and Doty (1961) ] were also cited as providing supporting evidence. By these same lines of evidence, heat- and acid-unfolded proteins were held to be less completely unfolded, with some residual secondary and tertiary structure. As noted in Section II, a polypeptide chain can behave hydrodynamically as random coil and yet possess local order. Similarly, the optical rotation and ORD criteria used for a random coil by Tanford and others are not capable of excluding local order in largely unfolded polypeptides and proteins. The ability to measure the ORD, and especially the CD spectra, of unfolded polypeptides and proteins in the far UV provides much more incisive information about the conformation of proteins, folded and unfolded. The CD spectra of many unfolded proteins have been reported, but there have been few systematic studies. 

Solid-state NMR methods have been used to characterize both elemental and compound bulk semiconductors for half a century, but a significant upturn in the level of activity and the sophistication of methods and analyses is taking place, not to mention the availability of much higher magnetic fields than were available for many of the early studies. In many cases, the availability of well-defined and optically and electrically well-characterized bulk samples will prove crucial. 

Initially, progress in this area was hampered by the lack of suitable analytical methods for chiral hydrocarbons. Early studies relied on optical rotation to determine enantiomeric excess (ee) values, but with the development of chiral gas chromatography (GC) and high-performance liquid chromatography (HPLC) columns, chromatographic methods have become more common. 

Early studies of the asymmetric reduction of prochiral ketones by chiral aluminum alkoxides have been reviewed by Morrison and Mosher (1). Doering and Young (123) reported the reduction of methyl cyclohexyl ketone with chiral 3-methyl-2-butanol in the presence of a catalytic amount of aluminum alkoxide to give the (S)-( + )-carbinol in a 22% optical yield. Jackman and co-workers (124) similarly reduced methyl n-hexyl ketone with chiral 3,3-dimethyl-2-butanol to the (S)-( - )-carbinol in a 6% optical yield. Other attempts resulted in similar low optical yields or gave only racemic products. Since the reductions were carried out under equilibrium conditions, racemization could have accounted for the low optical yields. 

In related work, Sasai developed several bifunctional BINOL-derived catalysts for the aza-Morita-Baylis-Hillman (aza-MBH) reaction [111]. In early studies, careful optimization of the catalyst structure regarding the location of the Lewis base unit revealed 41 as an optimal catalyst for the aza-MBH reaction between acyclic a,P-unsaturated ketones and N-tosyl imines. Systematic protection or modification of each basic and acidic moiety of 41 revealed that all four heterofunctionalities were necessary to maintain both chemical and optical yields. As seen in Scheme 5.58, MO calculations suggest that one hydroxyl groups forms a... 

The lifetime of the RSSR radical anions is usually very short on the microsecond timescale in water. However, they have been detected and characterized by time-resolved optical methods. In one early study, y irradiation of matrices containing alkyl and aryl disulfides provided spectroscopic evidence for the formation of the corresponding radical anions. Subsequently, the formation of RSSR radical anions has been well documented, particularly by EPR, flash photolysis, and pulse radiolysis. In fact, 2a/ a three-electron bonded radical anion species, particularly from sulfur compounds, constitute significant and interesting intermediates. The RSSR radical anions may be obtained from different approaches. One is by one-electron reduction of disulfides (equation 75), such as by pulse radiolysis. However, the most common approach is by association of RS and RS (equation 79). ... 

There are many studies on photoconductivity in these films, many of them early ones and focused on annealed films (since air annealing is necessary for optimal photoconductivity). The use of a chemical oxidant (which never seems to be specified) gives much higher photosensitivity for as-deposited films than for films deposited without oxidant, although even here annealing is used to obtain maximum performance. Some studies on photoconductivity in as-deposited PbSe films have shown shifts in photoconductivity spectral response, with onsets shifted to 2.2 (xm instead of the ca. 4.5 xm more typical of annealed films. As with optical absorption studies, these shifts can be attributed to size quantization. 

Steroids represent rigid chiral systems which are convenient substrates for mechanistic studies of geometric details. Early studies on the difacial selectivity of ketene to steroidal alkene cycloadditions led to the preparation of optically pure cyclobutanones. The addition of dichloroketene to 2- or 3-methyl-5a-cholcst-2-ene (1) generates the cyclobutanones 2 and 3 with regio- and stereoselectivity. The cycloadditions proceed to give the adducts resulting from ketene approach to the a-face.4... 

In early studies of these reactions, the turnover efficiency was not always high, and stoichiometric amounts of the promoters were often necessary to obtain reasonable chemical yields (Scheme 105) (256). This problem was first solved by using chiral alkoxy Ti(IV) complexes and molecular sieves 4A for reaction between the structurally elaborated a,/3-unsaturated acid derivatives and 1,3-dienes (257). Use of alkylated benzenes as solvents might be helpiul. The A1 complex formed from tri-methylaluminum and a C2 chiral 1,2-bis-sulfonamide has proven to be an extremely efficient catalyst for this type of reaction (258). This cycloaddition is useful for preparing optically active prostaglandin intermediates. Cationic bis(oxazoline)-Fe(III) catalysts that form octahedral chelate complexes with dienophiles promote enantioselective reaction with cyclopentadiene (259). The Mg complexes are equally effective. 

Essentially all of the early studies were directed towards enantioselective cyclopropanation and Maas has reviewed the literature up to 198 54. The most successful of these early studies were those of Aratani and coworkers"2 174 who developed chiral copper(II) chelates of type 153 from salicylaldehyde and optically active amino alcohols with which to catalyse intermolecular cyclopropanation with diazoesters. Enantioselectivities exceeding 90% ee could be achieved in selected cases (equations 133 and 134) including the synthesis of permethrinic acid 154 and /ram-chrysanthemic acid 155. 

Carbon-Nitrogen Bonds. Several groups have studied the synthesis of optically active a-amino acids from inexpensive and readily available a-haloesters by displacement with phthalimide in the presence of chiral cinchona catalysts [1 le,24h,24i,47e,60d,77]. Early studies, with chiral, non-racemic starting material, showed that this reaction occurs with partial... 

The acyclic aldehydes and ketones are expected to give substantially weaker CD than that of their cyclic counterparts because of the conformational mobility of the former, as was borne out by Djerassi and Geller75 in an early study of a series of optically active methyl-substituted aldehydes and ketones. Since then only a few studies on acyclic aldehydes and ketones have appeared76,77. 

There has been no controversy about the structure of fluorene (31) but its true conformation was in doubt for a number of years. From an early X-ray analysis, Iball (1936a) concluded that the fluorene molecule had a folded conformation and, in a review, Cook and Iball (1936) discussed further evidence for a non-planar conformation, provided by optical activity studies of unsymmetrically substituted fluorene derivatives. Later stereochemical studies (Weisburger et al., 1950) suggested that fluorene had, in fact, a planar conformation. A reinvestigation of the crystal structure by Burns and Iball (1954, 1955) and, independently, by Brown and Bortner (1954) showed that the early X-ray work was in error and confirmed the planar conformation. The refinement of the crystal structure (Burns and Iball, 1954, 1955), by two-dimensional Fourier and least-squares methods, reveals that the maximum deviation of the carbon atoms from the mean molecular plane is 0-030 A, the r.m.s. deviation being 0-017 A. This deviation, 0-017 A, is taken by Burns and Iball to be a measure of the accuracy of their analysis, assuming now that the molecule is strictly planar. 

The late 30 s brought a further important step in the investigation of the interstellar medium — the discovery of the first molecular species. In the optical region, the electronic spectra of the diatomic radicals CH, CH+, and CN, seen in absorption against the continuum spectra of bright background stars, furnished the first evidence that the interstellar medium was not devoid of molecules but contained at least some simple ones. However, the intensities of the molecular spectral peaks seen via optical absorption studies were quite weak compared with the spectra of atoms, indicating that the sources observed in these early studies were not rich in molecules. These sources, now labeled diffuse interstellar clouds, possess very low gas densities (n 102 cm-3) and are of limited interest chemically. 

Heat and alkaline treatments have been known since the early part of the century to raoemize amino acid residues in proteins (1,2,). Dakin and Dudley (3) also studied digestibility of casein in vitro and in vivo after hydroxide treatment. Heating casein with 0.5 N NaOH at 37° for about 30 days completely prevented enzymatic hydrolysis and intestinal absorption when the treated casein was fed to a dog. The kinetics of base-catalyzed racemization of proteins was investigated by Levene and Bass (4-6). In these early studies, the extent of racemization was measured by changes in optical rotation. 

Stereospecific Effects. Early studies of Co (III) complexes of optically active propylenediamine 62, 65, 66), 1,2-cyclopentanediamine 28, 29, 30, 31), and 1,2-cyclohexanediamine 32) indicated that one did not get all combinations of +) and (—) ligand configurations with the two configurations of chelate rings in the tris complexes. The composition of the cyclopentanediamine complexes reported by Jaeger have been found... 

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