MOLECULAR STRUCTURE ANALYSIS NMR Spectroscopy VCE Chemistry Unit 3: Chemical Pathways Area of Study 2 – Organic Chemistry

 NMR stands for Nuclear Magnetic Resonance  NMR is one of most powerful techniques used by chemists for the determination of molecular structure.  NMR is also used in medicine where its called MRI. NMR Spectroscopy - Introduction

 NMR uses energy in the radio frequency range of the electromagnetic spectrum.  The energy of the radiation is too low to cause electronic, vibrational or rotational transitions.  The radio waves used in NMR cause a change in the ‘spin’ of nucleons.  Protons, neutrons and electrons can be thought of as spinning on their axes in either and up (↑) or down direction (↓).  In many nuclei, the orientation of the spins of all the nucleons are paired and so cancel out.  However, in atoms with an odd number of nucleons, the nucleus has an overall spin NMR Spectroscopy - Theory

NMR Spectroscopy - Introduction

 This spin creates a magnetic field around certain nuclei (e.g. 1 H & 13 C ), and causes them to act like tiny bar magnets.  When these nuclei are placed in a strong external magnetic field, most of these nuclei will align with the field.  We say they have a parallel alignment.  Those that are in a higher energy state, will be aligned in the opposite direction (anti-parallel alignment). NMR Spectroscopy - Theory

 Nuclei in the lower energy state can be ‘spin-flipped’ into their higher energy state by supplying radiofrequency energy.  A precise amount of energy needs to be supplied, or this excitation to a higher energy state will not occur.  The amount of energy required can be supplied by the absorption of radio waves.  The frequency of the radio waves required to produce the ‘spin-flip’ is termed the resonance frequency. NMR Spectroscopy - Theory  h aligned with the field   aligned against the field ENERGY

NMR Spectroscopy - Theory  h aligned with the field   aligned against the field ENERGY

Main features of a basic NMR spectrometer include:  A radio transmitter coil that produces a short powerful pulse of radio waves  A powerful magnet that produces strong magnetic fields  Samples are placed in a glass tube that spins in a uniform magnetic field.  A radio receiver coil that detects the radio frequencies emitted as nuclei relax to a lower energy level.  A computer that analyses and records the data. NMR Spectroscopy - Instrument

RADIOFREQUENCY OSCILLATOR RADIOFREQUENCY OSCILLATOR

NMR spectra provide information about the structure of organic molecules from:  Number of different signals in the spectrum  Position of the signals (chemical shift)  Splitting pattern of the signals  Intensity of the signals 1 H & 13 C Spectra Interpretation - Introduction

 Liquid samples are placed in a NMR tube which spins in a magnetic field  Solid samples are first dissolved in a solvent that will not give a signal, for example D 2 O, or CDCl 3.  A small amount of TMS, tetramethylsilane, is added to provide a reference signal  When the spectrum has been run, it can be integrated to find the relative peak heights. 1 H & 13 CSpectra Interpretation – Sample Preparation 1 H & 13 C Spectra Interpretation – Sample Preparation

 Each nuclei type is said to be chemically shifted relative to a standard (usually TMS).  Chemical shift is the difference between the field strength at which it absorbs and the field strength at which TMS protons absorb.  TMS is assigned a value of ZERO (  = 0.00)  All peaks of a sample under study are related to it and reported in ppm. 1 H & 13 C Spectra Interpretation – Chemical Shift

1 H & 13 CSpectra Interpretation – Chemical Shift 1 H & 13 C Spectra Interpretation – Chemical Shift DATA BOOK page 5 – 6 (HNMR) page 7 (CNMR)

 The spin of one nucleus affects the nuclei on adjacent atoms.  Low resolution NMR gives 1 peak for each different proton environment  Being in the same chemical environment, H atoms must be attached to the same kind of atom in the same way.  The areas under the peaks give us the proportion of H atoms in each different chemical environment. 1 H & 13 C Spectra Interpretation – Low Resolution NMR

Example:

 Any 1 H nuclei present in an environment adjacent to the environment of another 1 H nucleus, will affect the strength of the external magnetic field to which it is subjected.  This effect is known as spin-spin coupling.  What appears to be a single peak on a low resolution spectrum now splits into several smaller peaks.  The signal produced indicates the number of protons on adjacent carbon atoms.  If a 1 H atom has n protons as its nearest neighbours, its absorption peak will be split into (n +1) peaks.  This is known as the “n + 1 rule”. 1 H & 13 C Spectra Interpretation – High Resolution NMR

Number of peaks = Number of chemically different H’s on adjacent atoms neighbouring H2 peaks“doublet”1:1 2 neighbouring H’s 3 peaks“triplet”1:2:1 3 neighbouring H’s 4 peaks“quartet”1:3:3:1 4 neighbouring H’s 5 peaks“quintet”1:4:6:4:1 Signals for the H in an O-H bond are unaffected by protons on adjacent atoms You get a singlet peak

Interpretation of high res NMR

Example:

Example: Given that the molecular formula is C 4 H 8 O 2

The carbon-13 NMR shows peaks representing different carbon atom environments.