N->7t* absorbances

Only the n — 7r, 7t —> n, and more rarely the n -> near ultraviolet and visible regions, which are the available regions for ordinary spectrophotometers. Species which absorb in the visible region are colored, and black is observed when all visible light is absorbed. 

Even in the absence of 7t-delocalization, azoalkanes absorb in the near-UV region, although the corresponding molar absorption coefficients (n,Jt ) are small. In the absence of competing photoreactions, some rare sterically hindered Z-isomers can be prepared. For example, (Z)-l,l -azodinorbornane (352) was prepared in toluene at 0°C from its / -precursor (Scheme 158).1083 

The fine structure of the absorption band associated with the n-7T electronic transitions in A -3-ketosteroids is better resolved using chiroptical detection methods compared to absorbance spectrophotometry. Selectivity in the determination of these steroids, therefore, is increased using the solvent-dependent CD spectra of the compounds being investigated, and enhanced even more if difference CD procedures are developed [24], 

The long wavelength band near 220 nm is readily assignable to the n-7t transition because of its energy but also because of the low absorbance yet high rotational 

Weakly absorbing chomophores (i.e., simple ketones, sulphides etc., or n a or n -> 7t transitions) are indicated by weak bands in the absence of strong chromophores. 

The triene absorbs at a longer (275 nm). Since the cyclic n system of benzene has a lower enei than the linear 7t system of the triene, benzene absorbs radiation of shorter wavelength. 

The Hamiltonian H(p,x,n, J ) is a model of diatomic molecules. h(p,x) represents the translational degrees of freedom and / ,(7t, Sj represents the internal vibrations of the molecules. If all the molecules are identical, we can assume that all frequencies are set to be equal. The internal part h n, E,) takes the form of uncoupled harmonic oscillators, so it looks specific. But this is not the case because all the nonlinear terms can be absorbed into the coupling term f P,X, 7I, ). 

Wavelengths between A, = 280 nm and 290 nm or longer are absorbed by carbonyl compounds. Here the free n-electrons of the oxygen enter the antibonding 7t molecular orbital. 

Acetophenone, benzophenone and benzaldehyde absorb at higher wavelengths (325, 348 and 328 m/x respectively) compared to acetone (277 m/x). This is due to conjugation in these molecules. The n->7T bands in these systems arfe seen immediately next to the very intense 7r-> tt bands, and often the former bands completely merge into the latter in polar solvents. 

Nitrates absorb very strongly below 2500 A and an ill-defined weaker band centered at about 2600 A (for nitromethane) extends up to 3200 A as in the case of nitrocompounds, this transition is probably an (n, 7t ) . Very little work has 

FIG. 6.21 Observed ti-tc l nd maximum and absorbance at the maximum for the %-t band of a regated (323 nm) and nonaggregated (348 nm) trans-chromo ores, as well as the ab rbance at the n-7t tend (characteristic for the cis-isomer) as a funt on of the irradiation time (at X 360 nm) of a LBK film of polymer 38 (reproduced with permission from reference 93). 

Here we are interested in escape out of the domain L specified by a single cycle of the potential that is out of a domain of length n that is the domain of the well. Because the bistable potential of Eq. (5.42) has a maximum at x = n/2 and minima at x = 0, x = 7t, it will be convenient to take our domain as the interval —7t/2 < x < n/2. Thus we will impose absorbing boundaries at x = —n/2, x = n/2. Next we shall impose a second condition that all particles are initially located at the bottom of the potential well so that x0 = 0. The first boundary condition (absorbing barriers at —n/2, n/2) implies that only odd terms in p in the Fourier series will contribute to Y (x). While the second ensures that only the cosine terms in the series will contribute because there is a null set of initial values for the sine terms. Hence 

The C-17 side-chain of corticosteroids does not contain a chromophoric group suitable for spectrophotometric measurement, nor does oxidation of the chain lead directly to spectrophotometrically active derivatives. However, the 20-keto group of the 17 a-ketol side-chain, as a chirally perturbed chromophoric group, has an optically active absorbance band in the interval 270-300 nm that is characteristic of the n-7T electronic transition for saturated ketones. An intense positive Cotton effect is observed in the CD spectra see the CD spectra for hydrocortisone and cortisone in Figures 8 and 9. 

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