SULPHUR (sulfur)-Why is it coloured

[Alan "Uncle Al" Schwartz]

One imagines that sulfur is colored because the predominant S(sub)8 rings
possess a low-lying n-pi* transition.

Heating sulfur much above its melting point leads to progressive ring
scission via homolytic bond breaking. The so-created free-radical tipped
chains are deeper in color, polymerize and entangle, and the viscosity
increases with temperature. Near the boiling point the chains shorten
and viscosity decreases.

One can test this hypothesis by adding a rather small amount of elemental
iodine to an elemental sulfur melt. Capping chain ends with iodine
should measureably lighten the color and lessen the viscosity.

--

Alan "Uncle Al" Schwartz
uncl...@ix.netcom.com ("zero" after "uncleal")
http://vvv.com/adsint/freehand/uncleal/
"Quis custodiet ipsos custodes?" The Net!

[James Tearle]

Help please-why is sulphur coloured ?,and why does its colour
change when it is heated ?
Hubble bubble toil and trouble..................

[Pavel Afanasiev]

In article <46me0h$29r$1...@mhadg.production.compuserve.com>,
10071...@CompuServe.COM says...

>
>Help please-why is sulphur coloured ?,and why does its colour
>change when it is heated ?
>Hubble bubble toil and trouble..................

Sulfur has empty d-orbitals. Since they are not too high in E,
the e- transition from uncompletely filled P do empty d-orbital
should take place in near-UV and visible, somewere around
150-250 nm. When heated, S8 polymerizes to chains. Formation
of chain leads to decrease of gap between LUMO and HOMO. It
means a shift of light absorption from UV to visible, therefore
deepening of S color. Moreover, rupture of chains lead to
Sx radicals having even higher LUMO than chains. Formation
of radicals is strongly promoted by the impurities of ionic or
oxydative nature. By this reason "dirty" sulfur melt is black.
When passing to gas phase, Sx again depolymerizes and S vapor is
light - yellowish.

PA

[Pavel Afanasiev]

In article <46nst7$8...@cismsun.univ-lyon1.fr>,
afa...@catalyse.univ-lyon1.fr says...

>
..............
>
>Sulfur has empty d-orbitals. Since they are not too high in E,
>the e- transition from uncompletely filled P do empty d-orbital
>should take place in near-UV and visible, somewere around
>150-250 nm.

Ooops..Sorry, Correction. I doubt if d-levels do form
LUMO of S; rather just pi-orbitals composed from empty p-ones.

The rest is O'K