Revision history [back]
click to hide/show revision 1
initial version

posted Apr 01 '10 at 16:47

Soumya's gravatar image

Soumya
21

As far as I understand, a partially oriented sample can be identified from the quadrupolar splitting of deuterium. If you do not have deuterium channel (example in many Bruker instruments with cryoprobe) you can observe the lock channel which should show two different wings. I have used this technique a lot and for an unstable sample in liquid crystal media like PEG, which is losing its partial alignment, the two wings observed in the lock channel will slowly changes into a single one. The amount of quadrupolar splitting directly depends on the degree of alignment. You can find more about these relation in this paper: Rueckert, M.; Otting, G. J. Am. Chem. Soc. 2000, 122, 7793-7797. One other way of determining the orientation of the molecule (although this will be more tedious) is to do some C-H or N-H coupling experiments in an isotropic and anisotropic sample and measure some of the splittings. A measurable difference in between two different splittings will give you an idea of whether partial orientation has occured. Although one should be careful in terms of stumbling across those peaks where there might not be any significantly measurable RDC (that is at the magic angle orientation). There are no hard and fast rule about the acceptable difference in coupling constant between isotropic and anisotropic samples. It depends more on the degree of order, the nuclei for which couplings are measured and also their relative distance and finally the type of nuclei you are looking at. Long range couplings will have significantly smaller magnitudes than single bond interactions in a given ordering media at a given concentration.
click to hide/show revision 2
No.1 Revision

posted Apr 02 '10 at 04:34

Soumya's gravatar image

Soumya
21

As far as I understand, a partially oriented sample can be identified from the quadrupolar splitting of deuterium. If you do not have deuterium channel (example in many Bruker instruments with cryoprobe) you can observe the lock channel which should show two different wings. I have used this technique a lot and for an unstable sample in liquid crystal media like PEG, which is losing its partial alignment, the two wings observed in the lock channel will slowly changes into a single one. The amount of quadrupolar splitting directly depends on the degree of alignment. You can find more about these relation in this paper: paper Rueckert, M.; Otting, G. J. Am. Chem. Soc. 2000, 122, 7793-7797. 7793-7797. One other way of determining the orientation of the molecule (although this will be more tedious) is to do some C-H or N-H coupling experiments in an isotropic and anisotropic sample and measure some of the splittings. A measurable difference in between two different splittings will give you an idea of whether partial orientation has occured. Although one should be careful in terms of stumbling across those peaks where there might not be any significantly measurable RDC (that is at the magic angle orientation).

There are no hard and fast rule about the acceptable difference in coupling constant between isotropic and anisotropic samples. It depends more on the degree of order, the nuclei for which couplings are measured and also their relative distance and finally the type of nuclei you are looking at. Long range couplings will have significantly smaller magnitudes than single bond interactions in a given ordering media at a given concentration.

click to hide/show revision 3
fixed the url

posted Apr 02 '10 at 05:57

Evgeny%20Fadeev's gravatar image

Evgeny Fadeev
5471

As far as I understand, a partially oriented sample can be identified from the quadrupolar splitting of deuterium. If you do not have deuterium channel (example in many Bruker instruments with cryoprobe) you can observe the lock channel which should show two different wings. I have used this technique a lot and for an unstable sample in liquid crystal media like PEG, which is losing its partial alignment, the two wings observed in the lock channel will slowly changes into a single one. The amount of quadrupolar splitting directly depends on the degree of alignment. You can find more about these relation in this paper Rueckert, M.; Otting, G. J. Am. Chem. Soc. 2000, 122, 7793-7797. One other way of determining the orientation of the molecule (although this will be more tedious) is to do some C-H or N-H coupling experiments in an isotropic and anisotropic sample and measure some of the splittings. A measurable difference in between two different splittings will give you an idea of whether partial orientation has occured. Although one should be careful in terms of stumbling across those peaks where there might not be any significantly measurable RDC (that is at the magic angle orientation).

There are no hard and fast rule about the acceptable difference in coupling constant between isotropic and anisotropic samples. It depends more on the degree of order, the nuclei for which couplings are measured and also their relative distance and finally the type of nuclei you are looking at. Long range couplings will have significantly smaller magnitudes than single bond interactions in a given ordering media at a given concentration.

powered by CNPROG