Subject: Re: Are P2(1), P4(2), P6(3) chiral space groups?
From: glazer
Date: 15 Dec 1996 11:45:45 GMT
As the person who started this problem I think I should explain. In my
first book I erroneously wrote that in soace group P4(2) optical
activity is not possible. This was removed from the second edition
because I later learnt that it is not true. To determine whether a
crystal is potentially optically active it is the POINT GROUP that
matters, not the space group. Thus P4(2) belongs to point group 4 and
optical activity is allowed in this group. (see the book, for examplke
by Nye on Physical Properties of Crystals). Now, another point in which
there is misunderstanding is that even in groups with mirror planes. for
example point group m, optical activity is still possible. What happens
in such a crystal is that the optical rotation changes sign depending on
the direction of view. However this is in principle because if you try
to do the experiment using a polarising microscope generally you will
not see any rotation of the light at all. This is because such crystals
are also birefringent and the birefringence is usually several orders of
magnitude greater than the effect of optical rotation. It can however be
measured by specialised polarimeters in which careful intensity or
polarisation measurements are made with very sophisticated techniques.
One of them, known as the HAUP technique (devised originally by
Kobayashi in Japan) is used in my own laboratory for the express purpose
of separating the linear birefringence from optical rotation.
Mike Glazer
Oxford
Subject: Rietveld Mailing List Homepage and Other Rietveld Info.
From: lachlan@melbpc.org.au (Lachlan Cranswick)
Date: Mon, 16 Dec 1996 03:19:44 LOCAL
Hi All,
A quick message that the new non colour co-ordinated Rietveld Mailing
List homepage with the updated mailing list address at Rietveld_l@ill.fr
is available for viewing at :-
http://www.unige.ch/crystal/stxnews/riet/welcome.htm
-------------------
The Web Archive of Past Messages, thanks to John Quintana of Argonne National
Labis at:-
http://tomato.dnd.aps.anl.gov/~jpq/maillists/rietveld/rietveld.html
-------------------
Repeating Alan's Message, To respectively subscribe or
signoff from the list send a message without subject or signature
to listserv@ill.fr.
SUBscribe Rietveld_l
SIGnoff Rietveld_l
To send a message to the whole list of subscribers, address it to:
Rietveld_l@ill.fr (_l for list).
---------------
This includes pointers to the list of Rietveld FAQs :-
(a suitable candidate for expansion)
http://www.unige.ch/crystal/stxnews/riet/faq/rietfaqs.htm
---------------
Which includes the pointer to Rietveld programs available for PCs
and Macs including related software.
Cheers,
Lachlan.
PS : If any of the above pages seem to terminate prematurely, could you
please contact me as the Australia - the Europe link is rather unreliable.
Lachlan Cranswick _--_|\
Phone/Fax : (613) 9455-1345 / \
E-mail : lachlan@melbpc.org.au \_.--._/
Mobile Phone/Voice Mail : 0412-1141-31 v
Crystallographic WWW : http://www.unige.ch/crystal/stxnews/stx/volnteer.htm
