Nuclear Magnetic Resonance (NMR) Wade Textbook

Transcription

1 Nuclear Magnetic Resonance (NMR) Wade Textbook

2 Background Is a nondestructive structural analysis technique Has the same theoretical basis as magnetic resonance imaging (MRI) Referring to MRI as nuclear magnetic resonance would scare patients, hence the name change to MRI Is the most important structure determination tool we have in organic chemistry We will learn the theory and how to interpret NMR spectra (there is a lot going on in these spectra)

3 The Theory? The theory behind NMR is a little complicated, please bear with me as we work through this The theory behind modern Fourier Transform NMR is even more difficult to understand and is beyond the scope of this class, I will not get into the details of FT-NMR

4 The Theory Not all nuclei are NMR active, NMR will only work for nuclei that have an odd atomic number or an odd mass number The most important active nucleus in NMR is 1 H (standard, vanilla hydrogen) The second most important (arguably, tied with hydrogen ) is 13 C, which is NOT the dominant isotope in a carbon sample (hence, carbon NMR is a little bit more challenging than hydrogen NMR (HNMR)) Other nuclei: 19 F, 31 P, and 33 S are (fairly) common nuclei in NMR

5 The Theory Higher Energy Place the sample in a large, external magnetic field. E B 0 Lower Energy No external magnetic field, random nucleus spins Applied magnetic field with strength B 0 1.) E B 0 (the greater the strength of the external magnetic field, the greater the gap in energy ( E) is... 2.) There are more nuclei in the lower energy spin state at any given time...

6 E = hν = γ h 2π B 0 E between the alpha (lower energy) and beta (higher energy) is directly related to the strength of the magnetic field, B 0 The gyromagnetic ratio (γ) is a constant for each nucleus; for hydrogen the value is 26,753 s -1 Gauss-1, which means that a 60 Mhz (radio frequency) is required to bring a hydrogen atom into resonance when B 0 is 14,092 Gauss Now if every hydrogen nucleus in a molecule absorbed an exactly 60 Mhz photon, NMR wouldn t be so useful but

7 Radio Frequency Generator The Theory Radio Photons h E = hν = γ B0 2π Detector External Magnetic Field Higher Energy Resonance E B0 Lower Energy Applied magnetic field with strength B0

8 Local, Induced Magnetic Fields Electrons are in motion around hydrogen nuclei and induce local magnetic fields, which oppose the external magnetic field; this in turn affects E and the frequency of absorption NMR is useful since each hydrogen group in a molecule is potentially in a different, local magnetic environment! Terms frequently used in NMR are shielded or deshielded, which refer to how much a particular nucleus is shielded by electrons from the external magnetic field

9 Shielding Less shielded More shielded Higher Energy Higher Energy B (induced) 0 (By circulating electons) E hydroxyl E methyl B (induced) 0 (By circulating electons) Lower Energy B (external) 0 Lower Energy

10 Tetramethylsilane (TMS) Serves as a reference compound ; this molecule is added to nearly all NMR samples TMS is added to NMR samples as all 12 methyl hydrogens have the same chemical shift as well as being very shielded The TMS signal is always set to zero and every other signal is downfield from TMS

11 Typical NMR Solvents Deuterochloroform - Cheapest and most widely used Heavy water - Used for dissolving polar compounds (and other uses too) DMSO-d6 - Used to dissolve very stubborn compounds; tends to have water contamination though Deuterated methanol - Used for dissolving polar organic molecules; very expensive

12 What NMR Tells Us 1.The number of signals in the NMR spectrum tells us how many different kinds of hydrogen 2.The location (or shift ) of the hydrogen signal tells us how much shielding the hydrogen nucleus is experiencing 3.The intensity of the signal tells us how many hydrogens are represented by that signal 4.The splitting (or multiplet ) pattern of the signal tells us information about neighboring hydrogen nuclei

13 Bromination of 2-Methylbutane (Review) Let s review: How many unique monobromo products result from radical bromination of 2-methylbutane?

14 NMR Signals How many unique hydrogen and carbon signals do we expect for 2- methylbutane?

15 NMR Signals How many hydrogen and carbon signals do we expect for the following molecules?

16 Stereochemical Nonequivalence

17 Stereochemical Nonequivalence

18 Homotopic, Enantiotopic, and Diastereotopic Hydrogens Homotopic hydrogens in a group have the same chemical shift and therefore are counted as one signal Enantiotopic hydrogens in a group have the same chemical shift and are also counted as one signal Diastereotopic hydrogens attached to the same carbon have different chemical shifts and are counted as two different signals! With that being said, how many signals would (R)-2- bromobutane have?

19 NMR Signals How many hydrogen signals are expected for the following molecules? Label (relevant) hydrogens as homotopic, enantiotopic, or diastereotopic.

20 Cyclohexanol

21 Pinene