title: Assignment of the (13)C NMR spectrum by correlation to dipolar coupled proton-pairs and estimation of order parameters of a thiophene based liquid crystal.
abstract: Materials with widely varying molecular topologies and exhibiting liquid crystalline properties have attracted considerable attention in recent years. (13)C NMR spectroscopy is a convenient method for studying such novel systems. In this approach the assignment of the spectrum is the first step which is a non-trivial problem. Towards this end, we propose here a method that enables the carbon skeleton of the different sub-units of the molecule to be traced unambiguously. The proposed method uses a heteronuclear correlation experiment to detect pairs of nearby carbons with attached protons in the liquid crystalline core through correlation of the carbon chemical shifts to the double-quantum coherences of protons generated through the dipolar coupling between them. Supplemented by experiments that identify non-protonated carbons, the method leads to a complete assignment of the spectrum. We initially apply this method for assigning the (13)C spectrum of the liquid crystal 4-n-pentyl-4'-cyanobiphenyl oriented in the magnetic field. We then utilize the method to assign the aromatic carbon signals of a thiophene based liquid crystal thereby enabling the local order-parameters of the molecule to be estimated and the mutual orientation of the different sub-units to be obtained.
authors.affiliation: NMR Research Centre, Indian Institute of Science, Bangalore 560012, India. kvr@nrc.iisc.ernet.in.
authors.forename: Y Jayasubba
authors.initials: YJ
authors.lastname: Reddy
authors.forename: K V
authors.initials: KV
authors.lastname: Ramanathan