Nuclear Magnetic Resonance (NMR) NMR arises from the fact that certain atomic nuclei have a property called “spin” In analogy with other forms of spectroscopy, like UV-VIS, for example, where the electron can occupy either a ground state or excited state, certain types of NMR spins can assume two possible possible orientations, aligned or opposed to the static magnetic field, B o. Aligned state is designated , opposed state is designated .

Excitation of NMR Spin Energy

NMR Chemical Shifts  = physical constant for a given type of nucleus (ratio of magnetic moment and angular momentum) h = Planck’s constant B o = static magnetic field strength

Predictions Do Not Match Reality  = chemical shielding tensor Frequency Ile in D 2 O 11 22 11   HDO   11 22 11 (Acquisition time = 4 hr)

Chemical Shielding Shielding arises from the various ways by which electrons “shield” the nuclear spin from the external magnetic field (B o ) Physical mechanism relates to induced circulation of electrons that oppose static magnetic field (Lentz’ Law) Shielding (tensors) can in principle be determined through ab initio calculations. This, however, is computationally expensive, and realistically not applicable to large molecules

Classic Approaches to Shielding Local electronic structure; electronegativity of attached groups, bond lengths, bond angles, and conformation (dihedral angles) Anisotropy of local groups (circulating electrons from aromatic rings for example) Hydrogen bonds Electric field effects that polarize bonds

Chemical Shielding Trends for Protons Functional Groups Proteins Frequency

Chemical Shifts Can Change Dramatically with Changes in Conformation 8 M Urea

Chemical Shielding & Chemical Shifts Recall B o field dependence of frequency makes comparison of spectra difficult from one instrument to another Hence, report relative ’s, not absolute ’s Chemical Shift (ppm) =  = peak = frequency of signal of interest ref = frequency of reference signal IUPAC-IUB Shift Standard for Proteins Sodium-2,2-dimethyl-2- silapentane-5-sufonate (DSS)

J-coupling

J-couplings in Ile Ile in D 2 O 11 22 11     11 22 11 11

NMR Active Nuclei

Sensitivity of NMR  &  spin states will assume a Boltzman distribution Implications: Highest sensitivity w/ higher  & higher B o

1D 13 C Natural Abundance Spectrum of Ile Ile in D 2 O ( 1 H Decoupled) (Acquisition time = 4 hr)   11 22 11 13 C ppm   11 11 22 COCO