Two-Dimensional (2D) NMR Methods

15 Chemical Exchange

Ashok Sekhar¹ and Pramodh Vallurupalli²

¹ Molecular Biophysics Unit, Indian Institute of Science, Bengaluru - 560012, Karnataka, India² Tata Institute of Fundamental Research Hyderabad, 36/P, Gopanpally Village, Serilingampally Mandal, Ranga Reddy District, Hyderabad 500046

15.1 Introduction

The chemical shift of a nucleus in a molecule is extremely sensitive to its electronic environment and consequently the chemical shift of the nucleus changes if the chemical environment around it changes. The change in chemical environment can be due to chemical reactions which involve the making and breaking of bonds or due to conformational changes that involve dihedral rotations. Hence, this phenomenon is called “Chemical Exchange” in the NMR literature as the nucleus of interest is exchanging between different chemical environments. Solution NMR spectra often report on the chemical shifts of various sites in the molecule and hence chemical exchange can have a large effect on the NMR spectrum of the molecule. This is illustrated in Figure 15.1 for an azapropazone derivative where the two N-methyl groups exchange with each other due to rotation about the C–N bond. At very low temperatures (223 K), when this interconversion is essentially nonexistent, the N-methyl ¹H spectrum consists of two sharp peaks with one arising from each of the N-methyl groups. As the temperature is increased and the rate of interconversion increases, the peaks first broaden, then coalesce ...