Bisignate bands

This knowledge and understanding may be helpful to characterize local conformations of other optically active polysilanes in solution. For example, poly(methyl-(-)-(3-pinanylsilane) [(+)-7 Mw = 10,200] prepared by Shinohara and co-workers.281 showed a bisignate CD band at 280 and 303 nm, associated with a broad UV absorption at 300 nm in chloroform at 15°C. Since the spectroscopic features are quite similar to those of i,28d-28e it is possible that the main chain in 7 may contain diastereomeric helical motifs with opposite screw senses and different screw pitches. 

Chromatograms of the optical resolution of (I) and (II) Eire shown in Figure. 1. Compound (II) is completely resolved into its enantiomers chromatographically. The g-factor remains at its constant optimum value throughout the elution band of each enantiomer. Compound (I) in contrast is incompletely resolved, giving a single absorbance elution band, but a double bisignate difference absorbance band. The fractions of (I) eluted in the volume between the vertical dashed lines are incompletely resolved. Values for the g-max factors have been determined from the CD and absorption spectra of (I) and (II). 

A stereospecific synthesis of [6]paracyclophane-8-carboxylic acid (2) was reported, and its CD spectrum was compared with that of [8]paracyclophane-10-carboxylic acid (3, R = CO2H) [17]. The former afforded a pair of bisignate CD patterns in the 200-400 nm region, while only a pair of Cotton effect peaks of the same sign were observed for the latter. The CD intensity (IAsI) of 2 was in the order of 10 M 1 cm4 for the main band at 220 nm. [6]Paracyclophane (2) is one of the most strained cyclophanes, and its boat-type deformation of the benzene ring led to the red-shift of ca. 30 nm in its UV and CD spectra. The theoretical spectrum obtained at the TD-DFT-B3-LYP/TZV2P level well reproduced the experimental spectrum except for the sign of the weak 1Lh band around 320 nm [8]. The CD spectrum of structurally similar methyl 3,6-hexanooxepin-4-carboxylate was also reported [16]. 

In Figure 19 the VCD and infrared absorption spectra of the dimethyl ester 11 in the C=0 region is displayed. The strong 1730 cm absorption band gives rise to a bisignate... 

The di-LCPAs in solution and in cast film showed monosignate and bisignate Cotton effects in their CD spectra, respectively. These results suggest the formation of intrachain and interchain helical structures, respectively. In spite of this, the di-LCPAs in solution and in cast film exhibited no CPL [21]. However, (/f)-/(S)-Poly-4 in an N -LC film showed bisignate CPL bands centered around the PL emission band (Fig. 11.12a). (/f)-Poly-4 exhibited positive and negative CPL bands at wavelengths shorter and longer than 530 nm, respectively. (S)-Poly-4 exhibited CPL bands with handedness opposite to those (/ )-Poly-4. The CPL spectra of (/ )-/(S)-Poly-4 in an N -LC film showed gem values of 1.8 x 10 and... 

Since the respective progressions of lines differ for different orientations within the molecule, the bisignate behaviour (ref. 19) of many CD-band systems within one single benzene transition becomes understandable. In fig. 6 two extremely pronounced finestructure series within the a-band are shown for 2 related compounds 6 (ref. 20) and 7 (ref. 21) (see later). 

Another way of looking into this problem of bisignated CD-curves is the following the a-band of benzene with D -symmetry is electrically as well as magnetically forbidden, the two branches (8,8 ) of the degenerate E -transi-tion are magnetically forbidden, but electrically allowed (and polarized in x-and y-direction). Addition to the two rings as in fig. 7 reduces symmetry to... 

C2 thus the 02 - transition becomes an A-type (p and m both being allowed), the degenerate - transition is split into a B- (p 0, 0) and an A-transition (p 0, 0). Since excited one-electron configurations of same symmetry mix with each other, this will also be so with the two A-transitions, but by this the finestructure is barely influenced. In order for the B-state to mix in, one needs one quantum of the non-totally symmetric vibration of symmetry b, thus the 0-0 - lines of these two partial band systems will not coincide, and their CD-signs must not be the same. In this latter case the "bisignate" picture of the CD within the a-band may be observed. 

Fig. 14 Bisignate CD-curves within the a-band of 3 related oxygensubstituted decahydrophenanthrenes, and CD of a related glycol allowing internal hydrogen bridging.

Polyribonucleic acids can be measured in aqueous solution using the windows at 1750-1550 cm "i. In contrast to the monomers, which do not show VCD in this region, the dimers and higher polymers show bisignate VCD bands. The spectra have been calculated for the dimers ApA and CpC using the coupled oscillator model. 

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