Two-Dimensional (2D) NMR Methods

6 Nuclear Overhauser Effect Spectroscopy

P.K. Madhu

Tata Institute of Fundamental Research Hyderabad, Hyderabad 500 046, India

6.1 Introduction

Nuclear magnetic resonance is an excellent spectroscopic tool to obtain the structure of molecules and elucidation of dynamics within them. Determination of internuclear distances is vital in both of these. Dipole-dipole (DD) couplings between nuclear spins are essential in this regard as they carry an dependence in the expression for the DD Hamiltonian [1], where is the internuclear distance. Whilst normally in solution-state NMR all anisotropic (orientation-dependent) interactions that carry structural information are averaged to zero on account of rapid molecular tumbling, they lead to relaxation through higher-order effects. Nuclear Overhauser effect (NOE) is an elegant consequence of this that can yield distance information in molecules [2, 3]. Two-dimensional (2D) analogue of NOE, termed as nuclear Overhauser effect spectroscopy (NOESY), is often used for large molecules [3] in place of the one-dimensional (1D) NOE experiments. NOESY can also detect chemical and conformational exchange, the technique in this case named as exchange spectroscopy (EXSY) [4, 5]. The NOESY experiment can lead to distance information between nuclear spins ...