Jacob, ITC vol A (5th ed), sect. 10.2 (pp 804-7) contains the full low-down on physical methods of detecting anisotropy & chirality of crystals (see sect 10.2.4 for optical properties).
-- Ian > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Ian Tickle > Sent: 16 November 2007 23:19 > To: Jacob Keller > Cc: CCP4BB@jiscmail.ac.uk > Subject: RE: Malonate Crystals? > > Hi Jacob, birefringence, the variation in refractive index > depending on > the direction of the plane of polarisation of the incident light has > nothing to do with chirality, it's a consequence of anisotropy of the > refractive index tensor, thus a crystal in any crystal system > other than > cubic may be birefringent, whereas a crystal in any system > may be chiral > but only if the space group is enantiomorphic. Chirality produces the > effect observed in a polarimeter, i.e. rotation of the plane of > polarised light, and also circular dichroism, the differential > absorption of left- & right-handed circularly polarised light. In all > trigonal, rhombohedral, tetragonal & hexagonal point groups one > eigenvalue of the refractive index tensor in the unique axis direction > is different from the other two and you get uniaxial birefringence; in > that case if you happen to be looking down the unique axis the crystal > appears isotropic in projection and you don't see > birefringence. In all > triclinic, monoclinic & orthorhombic point groups all 3 > eigenvlaues are > different and you get biaxial birefringence, and you may see > birefringence in any direction. Of course whether birefringence is > actually observable will depend on the degree of anisotropy. > > Cheers > > -- Ian > > > -----Original Message----- > > From: [EMAIL PROTECTED] > > [mailto:[EMAIL PROTECTED] On Behalf Of Jacob Keller > > Sent: 16 November 2007 19:49 > > To: CCP4BB@JISCMAIL.AC.UK > > Subject: Malonate Crystals? > > > > Dear Crystallographers, > > > > I have just found some crystals in a 1 year old screen, and, > > having been repeatedly trained to be very skeptical, think > > that probably they are salt. So the question: has anybody > > seen malonate crystals before? I was wondering whether they > > could possibly be birefringent, given that the molecular > > structure is not chiral. The crystals I see are very > > birefringent. None of the other ingredients is chiral either. > > In general, my understanding had been that the birefringence > > was due to the chirality of the molecules/crystals...is this > > misguided? > > > > Jacob > > > > ******************************************* > > Jacob Pearson Keller > > Northwestern University > > Medical Scientist Training Program > > Dallos Laboratory > > F. Searle 1-240 > > 2240 Campus Drive > > Evanston IL 60208 > > lab: 847.467.4049 > > cel: 773.608.9185 > > email: [EMAIL PROTECTED] > > ******************************************* > > > > > Disclaimer > This communication is confidential and may contain privileged > information intended solely for the named addressee(s). It > may not be used or disclosed except for the purpose for which > it has been sent. If you are not the intended recipient you > must not review, use, disclose, copy, distribute or take any > action in reliance upon it. 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