13.3 Introduction to ' H NMR Spectroscopy As shown in Figure 13. 4, the energy difference between the two states is directly proportional to the strength of the applied field. Net absorption of electromagnetic radi- The SI unit for magnetic field ation requires that the lower state be more highly populated than the higher one, and strength is the tesla (m) detectable signal. A magnetic field of 4.7T. which is about 100,000 times stronger than contemporary of Thomas o quite strong magnetic fields are required to achieve the separation necessary to give a named after Nikola Tesla, earth's magnetic field, for example, separates the two spin states of ' h by only 8X 10-5 Edison and who, like Edison as an inventor of electrical kJ/mol(1.9 10 kcal/mol). From Planck's equation AE= hv, this energy gap cor- devices responds to radiation having a frequency of 2 X 10 Hz(200 MHz) which lies in the radio frequency(rf) region of the electromagnetic spectrum(see Figure 13.1) Frequency of Energy difference electromagnetic is proportional to. between nuclear is proportional to Magnetic field adiation spin states (s or Hz) (kJ/mol or kcal/mol) PROBLEM 13.1 Most of the Nmr spectra in this text were recorded on a spec rometer having a field strength of 4.7 T(200 MHz for H). The first generation of widely used NMR spectrometers were 60-MHz instruments What was the mag netic field strength of these earlier spectrometers? The response of an atom to the strength of the external magnetic field is different for different elements, and for different isotopes of the same element. The resonance fre quencies of most nuclei are sufficiently different that an NMR experiment is sensitive only to a particular isotope of a single element. The frequency for H is 200 MHz at 4.7 T, but that of C is 50.4 MHz. Thus, when recording the NMr spectrum of an organic compound, we see signals only for H orC, but not both; H andC NMR petra are recorded in separate experiments with different instrument settings PROBLEM 13.2 What will be the C frequency setting of an NMR spectrome ter that operates at 100 MHz for protons? The essential features of an NMR spectrometer, shown in Figure 13.5, are not hard to understand. They consist of a magnet to align the nuclear spins, a radiofrequency (rf) transmitter as a source of energy to excite a nucleus from its lowest energy state to the next higher one, a receiver to detect the absorption of rf radiation, and a recorder to print ut the spectrum. Nuclear magnetic moment antiparallel △E moment par FIGURE 13. 4 An external magnetic field causes the two nuclear spin states to in absence of extemal have different ies. th magnetic field e In energy△Eis Increasing strength of proportional to the streng Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteAs shown in Figure 13.4, the energy difference between the two states is directly proportional to the strength of the applied field. Net absorption of electromagnetic radi￾ation requires that the lower state be more highly populated than the higher one, and quite strong magnetic fields are required to achieve the separation necessary to give a detectable signal. A magnetic field of 4.7 T, which is about 100,000 times stronger than earth’s magnetic field, for example, separates the two spin states of 1 H by only 8  105 kJ/mol (1.9  105 kcal/mol). From Planck’s equation E h, this energy gap cor￾responds to radiation having a frequency of 2  108 Hz (200 MHz) which lies in the radio frequency (rf) region of the electromagnetic spectrum (see Figure 13.1). PROBLEM 13.1 Most of the NMR spectra in this text were recorded on a spec￾trometer having a field strength of 4.7 T (200 MHz for 1 H). The first generation of widely used NMR spectrometers were 60-MHz instruments. What was the mag￾netic field strength of these earlier spectrometers? The response of an atom to the strength of the external magnetic field is different for different elements, and for different isotopes of the same element. The resonance fre￾quencies of most nuclei are sufficiently different that an NMR experiment is sensitive only to a particular isotope of a single element. The frequency for 1 H is 200 MHz at 4.7 T, but that of 13C is 50.4 MHz. Thus, when recording the NMR spectrum of an organic compound, we see signals only for 1 H or 13C, but not both; 1 H and 13C NMR spectra are recorded in separate experiments with different instrument settings. PROBLEM 13.2 What will be the 13C frequency setting of an NMR spectrome￾ter that operates at 100 MHz for protons? The essential features of an NMR spectrometer, shown in Figure 13.5, are not hard to understand. They consist of a magnet to align the nuclear spins, a radiofrequency (rf) transmitter as a source of energy to excite a nucleus from its lowest energy state to the next higher one, a receiver to detect the absorption of rf radiation, and a recorder to print out the spectrum. Frequency of electromagnetic radiation (s1 or Hz) Magnetic field (T) Energy difference between nuclear spin states (kJ/mol or kcal/mol) is proportional to is proportional to 13.3 Introduction to 1 H NMR Spectroscopy 491 0 0 ' E1 E1 ' E2 E2 ' No energy difference in nuclear spin states in absence of external magnetic field Nuclear magnetic moment antiparallel to 0 Nuclear magnetic moment parallel to 0 Increasing strength of external magnetic field ∆E ∆E' FIGURE 13.4 An external magnetic field causes the two nuclear spin states to have different energies. The difference in energy E is proportional to the strength of the applied field. The Sl unit for magnetic field strength is the tesla (T), named after Nikola Tesla, a contemporary of Thomas Edison and who, like Edison, was an inventor of electrical devices. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website