Fading behaviour

The function iKt-t ) may be interpreted as a memory function having a form as shown in Figure 3.14. For an elastic solid, iff has the value unity at all times, while for a purely viscous liquid iff has the value unity at thfe current time but zero at all other times. Thus, a solid behaves as if it remembers the whole of its deformation history, while a purely viscous liquid responds only to its instantaneous deformation rate and is uninfluenced by its history. The viscoelastic fluid is intermediate, behaving as if it had a memory that fades exponentially with time. The purely elastic solid and the purely viscous fluid are just extreme cases of viscoelastic behaviour. 

Behaviour towards Alkalis.—Add a few drops of sodium hydroxide solution to the aqueous solution of a basic triphenylmethane dye. For a short time the colour remains unchanged, but soon fades and at the same time a faintly coloured flocculent precipitate separates. In all cases this is the carbinol to which repeated reference has already been made when pure it is colourless. In the case of crystal violet the carbinol is [(CHg N.CgHJjCOH. 

The photochromic behaviour of the reduced xanthone (587) when irradiated has been studied. The yellow or orange colour thus produced is due to ring opening to the chromanone (588). This colour fades gradually as the reverse reaction proceeds (69JOC2407). 

The electronic configuration of free atoms is an important factor in the interpretation of atomic spectra, but less so for the understanding of chemical behaviour. Chemistry happens in crowded environments, which means that atomic electron densities fades to zero far from infinity. SCF wave functions are therefore not appropriate for atoms in a chemical environment. More suitable wave functions are obtained by terminating the SCF calculations at some fixed distance p from the nucleus, rather than infinity. The effect of such a new boundary condition is like applying hydrostatic pressure to the atom. 

Colourless crystals of 2.5-dimethylisophthalaldehyde [29] change to red when placed in sunlight. The red colour fades when the sample is kept in the dark for more than five minutes. The back reaction is accelerated by warming the sample in the dark. This whole process occurs repeatedly at least for several cycles. Similar photochromic behaviour is observed in 2-methyl-5-isopropylisophthalaldehydes [30] (Raju and Krishna Rao, 1987). It is claimed... 

Incidentally the photochromic behaviour of anils cannot be observed in fluid media. This is because, even though isomerization around the C-N bond occurs photochemically, the reverse isomerization occurs readily in a thermal process through the inversion of the nitrogen atom. Thus, fading of colours takes place rapidly in fluid media. 

In addition to their irregular fadings thought to be due to directed mass-ejections, R CrB stars show low-amplitude variability with periods of 30 - 60 d and associated with pulsations. All show a substantial infrared excess attributed to a warm dust shell, while a few show evidence of a more extended nebula. It will be seen that these stars are extraordinarily rich in exotic behaviour [132]. 

Cr(CO)a in methyl methacrylate polymers showed photochromic behaviour a yellow colour appeared on irradiation which faded slowly at room temperature but fast at 100 °C. The corresponding species has been regarded as Cr(CO)s 322). 

An analogous product is obtained with SH . Alkaline mercaptan solutions give red-purple colourations (equation 19) and similar behaviour is observed for alkaline solutions of ketones or other species containing acidic hydrogen bound to carbon. Here, however, colourations are often transient and fade as the NO-containing ligand is cleaved from the metal by hydrolysis (equation 20). Oximes and related organic products are isolable from such solutions. ... 

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