Investigation of Ortho Effect, Relaxation and Selective Excitation Techniques in Nmr

Abstract

NUCLEAR spin relaxation is central to the nuclear magnetic resonance phenomenon, providing a wealth of information on global and local motions. There are several mechanisms by which nuclear spins relax. Although known for quite some time, rapid development of NMR methodology and its use for structure determination of biomolecules has in recent years rejuvenated interest in the cross terms between various relaxation mechanisms. This motivated us to exploit such experiments for the determination of physicochemical properties of complex organic systems. In this thesis we exploit cross correlation for the study of the C-H--N hydrogen bonding and the ortho effect in fluorobenzenes. We also examine some novel sequences for cross relaxation spectroscopy and selective excitation. An application of the cross-correlation measurements for providing an evidence for the existence of weak hydrogen bond is described here. The ability of carbon atoms to act as proton donors in a hydrogen bond has been the subject of debate for many years, An increasing number of spectroscopic studies indicate that C-H—X (especially C-H—0) hydrogen bonds occur in many systems. Although intermolecular C-H—X hydrogen bonds are well authenticated and have been the subject of several systematic studies, intramolecular interactions of this type have received scant attention. We report here a cross-correlated relaxation study aimed at probing the existence of such interaction in liquids, specifically in 4,4Bisphenylsulphonyl-N,N-dimethylbutylamine in which an intramolecular hydrogen Abstract bond has already been reported. These experiments provide a means of measuring a parameter related to the orientation of the chemical shift anisotropy (CSA). This single parameter provides an overall idea about the magnitude and orientation of the CSA tensor. These CSA orientation parameters are measured via determination of the cross correlation rate and the correlation time. We show that the strength of the hydrogen bond reduces in the presence of trifluoroacetic acid, which is reflected in a significant decrease in the CSA orientation parameter of the methine proton, as compared to a minor change in the isotropic chemical shift value. An increased CSA orientation parameter is observed in pyridine due to a competition between the basic solvent and the N atom of the dialkyl-amino group for the methine proton. To our knowledge, this is the first kind of investigation where the variation in the CSA tensor is observed due to the presence of weak hydrogen bond. We have also performed a systematic study for the assessment of orthosubstitutioja effect on CSA tensors in fluorobenzenes. The variation in the magnitude and orientation of the CSA tensors, on chemical substitution was at first recognized in solid state NMR. Recently it has also been perceived in liquid state through the cross correlation between the tensor (CSA) and dipolar coupling. No systematic study of the ortho effect on CSA tensors has been carried out in liquid state NMR before. Experiments we report here for the determination of the cross correlation between the CSA of F and its dipolar interaction with the nearby protons are based on the longitudinal relaxation of F. The correlation time is measured independently from the C relaxation studies. A detailed computational study indicating the variations in the value of CSA orientation parameters, as a function of different o-substitution, is also presented in this thesis.bond has already been reported. These experiments provide a means of measuring a parameter related to the orientation of the chemical shift anisotropy (CSA). This single parameter provides an overall idea about the magnitude and orientation of the CSA tensor. These CSA orientation parameters are measured via determination of the cross correlation rate and the correlation time. We show that the strength of the hydrogen bond reduces in the presence of trifluoroacetic acid, which is reflected in a significant decrease in the CSA orientation parameter of the methine proton, as compared to a minor change in the isotropic chemical shift value. An increased CSA orientation parameter is observed in pyridine due to a competition between the basic solvent and the N atom of the dialkyl-amino group for the methine proton. To our knowledge, this is the first kind of investigation where the variation in the CSA tensor is observed due to the presence of weak hydrogen bond. We have also performed a systematic study for the assessment of orthosubstitutioja effect on CSA tensors in fluorobenzenes. The variation in the magnitude and orientation of the CSA tensors, on chemical substitution was at first recognized in solid state NMR. Recently it has also been perceived in liquid state through the cross correlation between the tensor (CSA) and dipolar coupling. No systematic study of the ortho effect on CSA tensors has been carried out in liquid state NMR before. Experiments we report here for the determination of the cross correlation between the CSA of F and its dipolar interaction with the nearby protons are based on the longitudinal relaxation of F. The correlation time is measured independently from the C relaxation studies. A detailed computational study indicating the variations in the value of CSA orientation parameters, as a function of different o-substitution, is also presented in this thesis.

Cite this paper

@inproceedings{Kurur2005InvestigationOO, title={Investigation of Ortho Effect, Relaxation and Selective Excitation Techniques in Nmr}, author={Narayanan . D . Kurur}, year={2005} }