NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY Sulistyo Saputro

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References: Day, R.A. dan Underwood, A.L., Quantitative Analysis, Fifth edition, terjemahan oleh: A. Hadyana Pudjaatmaka, Jakarta: Erlangga. Pecsok, R.L., Shields, L.D., Cairns, T., dan MacWilliam, I.G., Modern Methods of Chemical Analysis, New York: John Wiley & Sons. Skoog, D.A., Principles of Instrumental Analysis, Third edition, New York: Saunders College Publishing. Vogel, A.I., Jeffery, G.H., Basset, J., Mendam, J., dan Denney, R.C Textbook of Quantitative Chemical Analysis, New York: Longman Scientific & technical. Other books and open-sources.

INTRODUCTION  Basic Concept of NMR Spectroscopy  Instrumentation  Application NMR is based upon the measurement of absorption of electromagnetic radiation in the radio frequency region (4 to 600 MHz), which correspond to λ 75 to 0.5 m. In contrast to UV, Visible, and IR spectroscopy, nuclei of atoms rather than outer electrons are involved in the absorption processes. In principle, NMR is applicable to any nucleus possessing spin.spin Futhermore, in order to cause nuclei to develop the energy states required for absorption to occur it is necessary to expose the analyte to an intense magnetic field of several thousand Gauss. NMR Spectroscopy --- one of the most powerful tools for elucidating the structure of both organic (and inorganic species). The most important applications for the organic chemist are proton NMR and carbon-13 NMR spectroscopy.proton NMRcarbon-13 NMR

History On 1945 : Purcell, Torry and Pond (Harvard Univ.) & Bloch, Hansen and Packard (Stanford Univ.) :  Independently announced a far-reaching discovery about the behavior of the atomic nucleous. Most nuclei, including p and e-, possess inherent magnetic fields, though the effects of the nuclear fields are too small to be observed in the ambient magnetic field of the earth.  Bloch & Purcell (physicians, get Noble Price 1952) invented techniques to detect the minute amount of energy absorbed or emitted as the nuclei jump from one energy level to another.

Magnetic Properties of the Nucleus It is necessary to assume: nuclei rotate about an axis and thus have the property of spin. Futhermore, it is necessary to postulate that the angular momentum associated with the spin is a half-integral multiple of h/2π (h=Planck’s constant). The maximum spin component for a particular nucleus is its spin quantum number, I. I = half-integral values 0, ½, 1, 3/2, …, 9/2 depending on the particular nucleus. It is found that a nucleus will have (2I+1) discrete states. The component of angular momentum for these states in any chosen direction will have values -I, (-I+1), …, (I-1), I

Examples: Proton has spin number of ½, thus the spin states is (2(1/2)+1)=2 and corresponding to I=+1/2 and I=- 1/2. For Chlor: I=3/2, the spin states are (2(3/2)+1)=4 and corresponding to -3/2, -1/2, +1/2, and +3/2. In the absence of an external field, the various states have identical energies.

Spinning For I=1/2, there are 2 possible levels or orientation With the applied field (lower energy) --- E= - µ Ho (parallel) or against the applied field (higher energy) --- E=+ µ Ho (anti-parallel)

Nuclear Resonance This separation into 2 levels with an energy of +µHo or -µHo (Ho is the intensity of applied magnetic field). ΔE=2 µHo (I=1/2) ……………………. (1) The general relationship: ΔE=µHo/I (Ho commonly used Gauss) For 1 H in this field: ΔE is 5.7 x cal/mole (very small) At room temperature: a thermal energy is considerably larger and sufficient to maintain nearly equal populations in the two levels. In fact, at 25 C Boltzmann distribution tells us that for every million 1 H, there is excess only 3 1 H in the lower level. So, ΔE=hv=2 µHo …………………………………(2) (v=60 x 10 6 Hz or 60 MHz --- a radio frequency range with λ = 5 m

Nuclear Resonance We are able to supply energy of just this frequency. A radio- frequency transmitter is an appropriate energy source. A 1 H in the lower level may absorb this energy to jump to the upper level. This absorption processes is called “magnetic resonance” or the nucleous “resonates” at the proper “resonance frequency”. From eq. (1) and (2) we get: 2Π.ν = 2Π.µ.Ho/hI = γHo (2Π is to convert linear frequency to angular unit of frequency) Obviously, γ = 2Π.µ/hI is characteristic property of the nucleous, called the magnetogyric ratio. Another definition: magnetogyric ratio = γ = 2Π.µ/Ho --- fundamental eq for NMR it defines the “resonance condition”– “a function of the ratio of frequency to field”

Some data: Note: If atomic number odd, mass number odd or both are odd --- has spin number. If both are even, has no spin number

Questions: How about the spin quantum number, I for: 1. Nuclei which have either odd number of protons or an odd number of neutrons? Give some examples! 2. Nuclei which the number of neutrons and proton are both odd? Give some examples! 3. Nuclei which the number of neutrons and proton are both even? Give some examples! Note: To be successful in using nmr as an analytical tool, it is necessary to understand the physical principles on which the methods are based.

The Chemical Shift Sample contains charged species (nuclei & electrons). Each charged particle is subject to the influence of the surrounding magnetic field. Electrons in covalent bond normally have paired spins and have no net magnetic field. But an applied magnetic field induces additional modes of circulation for these paired electron which generate a small local magnetic field proportional to but opposing the applied field This phenomenon is called “diamagnetic shielding” (it shields the nucleous to some degree from the effects of the applied field.

The Chemical Shift The nucleous thus finds itself in an effective field, Heff. H eff = Ho - .Ho or H eff = Ho (1 -  )  = shielding parameter  Depends on the electron density around the proton. Example: methanol, CH 3 OH 2 kinds of protons: CH 3 and OH (O atom is more electronegative than the C atom, therefore the e- density around methyl proton is higher than that around the hydroxyl proton). The shielding parameter,  CH 3 >  OH, so H eff proton CH 3  H eff proton OH The variation of the resonance line with chemical structure is called the chemical shift.

Instrumentation

The NMR sample is prepared in a thin- walled glass tube

Magnet The sensitivity and resolution of an NMR spectrophotometer are critically dependent upon the strength & quality of its magnet. Sweep Generator A pair of coils located parallel to the magnet faces permits alteration of the applied field in a small range. The field strength is changed automatically & linearly with time.

The Radio-Frequency source The signal from a radio-frequency oscilator (transmitter) is fed into a pair of coils mounted at 90° to path of the field. A fixed oscillator of exactly 60, 90, or 100 MHz is ordinarily employed. For high- resolution work, the frequency must be constant to about 1 part in 10 9.

Detector 2 types of detector: a. cross-coil (based on nuclear induction), b. single-coil (based on nuclear absorption) ---the attenuation of the source signal by the sample is the quantity measured). Signal Recorder Peak areas are important because they permit estimation of the relative number of absorbing nuclei in each chemical structure.

Fourier-Trasform Instrument: Conventional NMR is not so sensitive, so FT- NMR has resulted dramatic increases in sensitivity and widespread application of C- 13, H, F, P, Si. FT-NMR is a time-domain spectrum (not a frequency-domain). The resolution elements of the spectrum are observed in a very brief period by measurements.