Plain curve

In Cotton effect curves, we use the terms peaks and troughs. The use of maxima and minima is not recommended because of the confusion that may arise. Since these terms are employed to describe ultra violet spectra, the sign of rotation is unimportant. Sometimes we also use the terminology amplitudes and widths of Cotton effects. The meanings of these terms are clear from Fig. (10.6). The plain curves are much less useful than Cotton effect curves. 

Between 1926 and 1962, numerous ORD curves of optically active sulfoxides were described in the literature for the visible and ultraviolet (UV) regions. However, down to about 250 nm all compounds investigated showed plain curves with [0] increasing toward shorter wavelengths, and this limited their diagnostic value for the determination of the absolute configuration of these compounds. 

On the other hand, many of those plates generally considered modern fakes were made by a repousee technique and involved an outer, plain-curved back that was folded over at the rim to form a double-walled hollow plate. Occasionally a separate (cast) piece, usually a raised front leg of the horse, was attached, probably by soldering. These fakes and copies of existing museum plates all have less than 0.6% Au in Ag (in agreement with the fake Sasanian coins, Figure 5) and 14 of the 18 fakes in Figure 6 have less than 94% Ag + Au. 

Here aromatic bonds are designated with dotted curved lines. Plain curved lines are n- bonds, three dots signify weaker %- bonds (as in 2,3-butadiene), the dotted arrow (->) represents shifting of n- electrons into the cation field, and the curved arrow ( r )) the displacement of electrons. These denote the most important forms of the conjugations. 

ABA absorbs ultraviolet with maxima at 240 nm (e 2.1 x 104, a shoulder peak), 260 nm (e 2.6 x 104), and 320 nm (e 50) in an acidic methanol solution.590 Irradiation with UV with a wavelength shorter than 305 nm isomerizes the 2-(Z)-double bond to (E) to give an equilibrium mixture of ABA and its 2-( )-isomer with a ratio of 1 1, and also causes decomposition of ABA to unidentified compounds by the excitation of the 7t-7t transition of the side chain and the enone groups. ABA has a strong optical activity, and its specific optical rotation is +430° in an acidic methanol solution.591 In the optical rotatory dispersion (ORD)591 and the circular dichroism (CD) spectra,558 ABA shows a positive Cotton effect from 300 to 200 nm. Phaseic acid and dihydrophaseic acid and its epimer, which did not have the enone group, show a small specific optical rotation with a minus value and also a negative plain curve in the ORD. 

The negative optical rotation of 5-acetamido-5-deoxy-a-D-xylopyra-nose is in contradiction to its existence as the a-D anomer. As Fig. 2 shows, the methyl D-xylopyranoside that corresponds to the a-D form, according to the Hudson isorotation rule, shows a positive plain curve and the p-D form shows a negative plain curve in optical rotatory dispersion. On the other hand, 5-acylamido-5-deoxy-a-D-xylop)franoses and their glycosides exhibit negative plain curves. Obviously, the... 

Benzyloxycarbonyl)amino]-5-deoxy-D-ribopyranose (208) was separable into its two anomers. In accordance with the above concept, the a-D anomer shows an upward mutarotation and a negative plain curve, whereas the )8-d anomer exhibits a downward mutarotation and a positive plain curve. A further deviation from the Hudson isorotation rule is observable with 4-acetamido-l-0-acetyl-4-deoxy-2,3,5-tri-0-p-nitrobenzoyl-/8-D-ribofuranose. ... 

If the amide chromophore is removed from methyl 5-[(benzyloxy-carbonyl)amino]-5-deoxy-a-D-xylopyranoside (40), or from its non-glycosidic form (31),a positive plain curve of optical rotatory dispersion is found for 5-amino-5-deoxy-D-xylopyranose (17) and its methyl glycoside (41). Because, at least with the glycoside 41, no reversal of configuration at C-1 is to be expected under these conditions, 5-amino-5-deoxy-D-xylopyranose and its methyl glycoside likewise show a tendency to assume the o-D anomeric form, and show plain curves in agreement with the Hudson isorotation rule. ... 

The anomalous positive ORD curve in Fig. 2b is rounded at finite values for the maximum (peak) and minimum (trough) extremities. The crossover wavelength, where a = 0, generally coincides with the wavelength of the maximum absorbance. An anomalous curve is always superimposed on a fundamental plain curve that is alluded to as the background rotation. Media confirmed to have just a single anomalous dispersion can be solved for 2,. Historically, this procedure was used to predict the wavelength maximum for an incomplete absorbance band that could not be observed in its entirety because of instrumental limitations. This particular application of ORD is now obsolete. 

Figure 12. Time evolution of the hamming distance between the hinary codes corresponding to stimulus 1 and 2 obtained at each peak of the LFP. The x and y axis is the peak of the LFP and the number of different hits between the 2 codewords. Plain curve is for an intact network and dashed curve is for a network without any...

Figure Al. Instantaneous firing frequency vs. applied current for a PN (left) and a LN (right). Plain curves are for the simulations of the conductance based models from [6] and dotted curves are for the simulations of the corresponding fitted theta models.

As a consequence of a CD the normal plain curve of the wavelength dependence of a, the "optical rotatory dispersion" (ORD) becomes anomalous and the plain curve is superposed by an S-shaped curve. Both, CD and anomalous ORD, are called "Cotton effect". Quantitatively CD and ORD are linked through the Kronig-Kramers relationship. No stringent coupling of the extinction coefficient (Emax) with the CD effect exists, because contains... 

Earlier ORD investigations of all possible diastereomeric di-, tri-, and tetrapeptides of alanine, serine (Beacham et al., 1966 Scopes et a/., 1966), and leucine (Dunstan and Scopes, 1968) showed that the mostly plain curves of these peptides, which were obtained down to 227 nm, are increment curves i.e., they arise from the arithmetic addition of contributions from all chiral chromophores present in the molecule. For instance, for an lddl tetrapeptide in water, the sum of contributions would be equal to -h nLD — mLL — mLD -h CL, where nLD is the NH2-terminal amide between an l center and a d center, mLL is the middle amide unit between two l centers, mLD is the middle amide between an l and a d center, and cl is the terminal car-boxylate chromophore adjacent to an l center. This approach is similar to the one used for the calculation of rotation at various mercury (Schechter and Berger, 1966) and sodium lines of oligopeptides (Sachs and Brand, 1954 Goodman et a/., 1963). This additivity was also found when amino, carboxyl, and/or hydroxyl groups in these small peptides were protected (Scopes et aU 1967). 

Because of the predictability of CD spectra, in earlier times, CD was frequently used as a means of establishing the absolute configuration of chiral molecules, and extensive correlations of CD spectra with molecular structure were developed based upon empirical rules. The shapes of the curves, called either plain curves or curves possessing positive and/or negative Cotton effects, can be correlated with structure. In more recent times, x-ray crystallography has become the most common way to establish absolute configuration (see below). One area in which CD has remained quite a powerful and commonly used tool is in studies of protein secondary structure. We will discuss this application of CD later in this chapter. 

Figure 8.12 Optimized (DFT/B3LYP) geometries of the [CpMo( -H2)H2(dpe)] [ROHOR] and [CpMo(H)4(dpe)]" [ROHOR] ion pairs (RO = (CF3)2CHO) and potential energy curve for their interconversion. Plain curves and squares in the gas phase dashed lines and triangles in dichloromethane solution. The H-H length of the coordinated dihydrogen has been taken as the reaction coordinate.

The most common form of optical activity was first measured at a constant wavelength by the angle of rotation of linearly polarized light. More recently the measurements have been extended to the entire range of visible and attainable ultraviolet regions where electronic transitions are observed, giving rise to the ORD technique (Optical Rotatory Dispersion). The Cotton effects appear in the region of optically active absorption bands outside of these bands the plain curve spectrum is also dependent on all the electronic transitions of the chromophores. 

The isolated 2-hydroxy-5-methylhexanoic acid belongs to the L-series with S-configuration, as indicated by the positive ORD plain curve (41). It follows that (-) humulone would be the L-form. However, because in the Cahn-Ingold-Prelog nomenclature (37) the sequence of the substituents at the chiral centre in humulone differs from that in the hydroxycarboxylic acid, (-) humulone possesses the R-configuration. This result confirms the conclusion reached from the chiroptical studies. 

Fig. 3. Differential enthalpy of adsorption of N2 at 77 or 87 K on a microporous ica gel outgassed at 423 K, as determined by the isosteric nts) or microcalorimetric (plain curve) methods.

It IS well known, since the work of Neumann J, Jagendorf AT (1964), that the illumination of a suspension of chloroplasts in an unbuffered.medium induces an increase of the pH of the suspension. When the light is switched off the pH returns to its initial value (plain curve, fig. 2). This light induced pH variation is completely reversible in darkness. 

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