Organic Structures from 2D NMR Spectra

L. D. Field, H. L. Li, A. M. Magill

• Publisher: Wiley
• Year: 2015
• ISBN:
  9781118868935
• Edition: 1
• Topic: Scienze fisiche
• Subtopic: Chimica analitica

The derivation of structural information from spectroscopic data is now an integral part of organic chemistry courses at all Universities. Over recent years, a number of powerful two-dimensional NMR techniques ( e.g. HSQC, HMBC, TOCSY, COSY and NOESY) have been developed and these have vastly expanded the amount of structural information that can be obtained by NMR spectroscopy. Improvements in NMR instrumentation now mean that 2D NMR spectra are routinely (and sometimes automatically) acquired during the identification and characterisation of organic compounds.

Organic Structures from 2D NMR Spectra is a carefully chosen set of more than 60 structural problems employing 2D-NMR spectroscopy. The problems are graded to develop and consolidate a student's understanding of 2D NMR spectroscopy. There are many easy problems at the beginning of the collection, to build confidence and demonstrate the basic principles from which structural information can be extracted using 2D NMR. The accompanying text is very descriptive and focussed on explaining the underlying theory at the most appropriate level to sufficiently tackle the problems.

Organic Structures from 2D NMR Spectra

• Is a graded series of about 60 problems in 2D NMR spectroscopy that assumes a basic knowledge of organic chemistry and a basic knowledge of one-dimensional NMR spectroscopy
• Incorporates the basic theory behind 2D NMR and those common 2D NMR experiments that have proved most useful in solving structural problems in organic chemistry
• Focuses on the most common 2D NMR techniques – including COSY, NOESY, HMBC, TOCSY, CH-Correlation and multiplicity-edited C-H Correlation.
• Incorporates several examples containing the heteronuclei 31 P, 15 N and 19 F

Organic Structures from 2D NMR Spectra is a logical follow-on from the highly successful " Organic Structures from Spectra " which is now in its fifth edition. The book will be invaluable for students of Chemistry, Pharmacy, Biochemistry and those taking courses in Organic Chemistry. Also available: Instructors Guide and Solutions Manual to Organic Structures from 2D NMR Spectra

Chapter 1

NMR Spectroscopy Basics

1.1 THE PHYSICS OF NUCLEAR SPINS

Any nucleus that has an odd number of protons and/or neutrons has a property called “nuclear spin”. Such nuclei are termed “NMR-active nuclei” and, in principle, these nuclei can be observed by Nuclear Magnetic Resonance (NMR) spectroscopy.

Any nucleus that has an even number of protons and an even number of neutrons has no nuclear spin and cannot be observed by NMR. Nuclei with no nuclear spin are “NMR-silent nuclei”. Common nuclei that fall into the NMR-silent category include carbon-12 and oxygen-16. Fortunately, with a few exceptions, most elements do have at least one isotope that has a nuclear spin, and so while ¹²C and ¹⁶O are NMR-silent, we can observe NMR spectra for the less abundant isotopes of carbon and oxygen, ¹³C and ¹⁷O. So even the elements where the most abundant isotope is NMR-silent can usually be observed via one or more of the less abundant isotopes.

Each nucleus has a unique nuclear spin, which is described by the spin quantum number, I. Nuclear spin is quantised, and I has values of 0, 1/2, 1, 3/2 etc. NMR-silent nuclei have I = 0. Each nuclear spin also has a magnetic moment, μ. The nuclear spin and the magnetic moment are related by Equation 1-1:

(1-1)

The constant of proportionality, γ, is known as the magnetogyric ratio, and γ is unique for each NMR-active isotope. Table 1-1 provides a summary of the nuclear spins of some of the common NMR-active nuclei.

Table 1-1 Nuclear spins and magnetogyric ratios for some common NMR-active nuclei.

The combination of spin and charge means that NMR-active nuclei behave like small magnets and when a nucleus with a nuclear spin I is placed in an external magnetic field, that nucleus may assume one of 2I + 1 orientations relative to the direction of the applied field.

So, for a nucleus with I = 1/2 like ¹H or ¹³C, there are two possible orientations, which can be pictured as having the nuclear magnet aligned either parallel or antiparallel to the applied field. For nuclei with I = 1 there are three possible orientations; for nuclei with I = 3/2 there are four possible orientations and so on.

The various orientations of a nuclear magnet in a magnetic field are of unequal energy, and the energy gap (ΔE) is proportional to the strength of the applied magnetic field (B₀) according to Equation (1-2:

(1-2)

where h is the Planck constant.

Nuclei in a lower energy orientation can be excited to the higher energy orientation by a radiofrequency (Rf) pulse of the correct frequency (v) according to Equation (1-3:

(1-3)

It follows from Equations (1-2 and (1-3 that the fundamental equation that relates frequency (v) to magnetic field strength (B₀) is Equation (1-4 which is known as the Larmor Equation:

(1-4)

The Larmor equation specifies that the frequency required to excite an NMR-active nucleus is proportional to the strength of the magnetic field and to the magnetogyric ratio of the nucleus being observed. For magnetic fields that are currently accessible routinely for NMR spectroscopy (up to about 21 T), the frequencies required to observe most common NMR-active nuclei fall in the Rf range of the electromagnetic spectrum (up to about 900 MHz).

Table 1-2 summarises the NMR frequencies of common NMR-active nuclei.

Table 1-2 Resonance frequencies for some common NMR-active nuclei in different magnetic fields.

1.2 BASIC NMR INSTRUMENTATION AND THE NMR EXPERIMENT

Samples for NMR spectroscopy are typically liquids (solutions) or solids. In order to observe Nuclear Magnetic Resonance, the sample must be placed in a strong magnetic field.

Magnets for NMR spectroscopy may be either permanent magnets or electromagnets. Most modern magnets are electromagnets based on superconducting solenoids, cooled to liquid helium temperature.

NMR spectrometers require an Rf transmitter which can be tuned to the appropriate frequency for the nucleus one wishes to detect (Equation (1-4) and an Rf detector or receiver to observe the Rf radiation absorbed and emitted by the sample. In most modern instruments, the Rf transmitter and the Rf receiver are controlled by a computer and the detected signal is captured in a computer which then allows processing and presentation of the data for analysis.

Chapter 2

One-Dimensional Pulsed Fourier Transform NMR Spectroscopy

A short pulse of radiofrequency radiation will simultaneously excite all of the nuclei whose resonance frequencies are close to the frequency of the pulse. If a sample placed in a magnetic field of 9.395 T contains ³¹P nuclei, then a pulse whose frequency is close to 161.9 MHz will excite all of the ³¹P nuclei in the sample. Typically, the excitation pulse is very short in duration (microseconds). Once the pulse is switched off, the magnetisation which builds up in the sample begins to decay exponentially with time. A pulsed NMR spectrometer measures the decrease in sample magnetisation as a function of time, and records the free-induction decay (FID) (Figure 2-1).

Figure 2-1 ¹H NMR spectra: (a) time domain spectrum (FID); (b) frequency domain spectrum obtained after Fourier transformation of (a).

The FID is a time domain signal (i.e. a signal whose amplitude is a function of time), and contains information for each resonance in the sample, superimposed on the information for all the other resonances. The FID signal may be transformed into the more easily interpreted frequency domain spectrum (i.e. a signal whose amplitude is a function of frequency), by a mathematical procedure known as Fourier transformation (FT). The frequency domain spectrum is the typical NM...