Spectrometer, Bloch

Fig. 1 SSNMR spectra of ibuprofen, 75.6 MHz. (A) Bloch decay experiment (single pulse), no decoupling, static, 240-sec pulse delay, 100 scans, 400-min experiment time. (B) Same as (A) but with decoupling ( 60 kHz). (C) Same as (B) but with 5-kHz MAS. (D) Cross-polarization experiment, with H decoupling ( 60 kHz), 5-kHz MAS, 1.5-msec contact time, 3-sec pulse delay, 100 scans, 5-min experiment time. (E) Same as (D) with the TOSS pulse sequence applied to suppress spinning sidebands. Note Asterisk ( ) denotes spinning sidebands sharp ( ) denotes spectrometer background artifact.

Fig. 16. Block diagram of the crossed coil nuclear magnetic resonance spectrometer due to Bloch,...

In NMR-SIM the simulation of an NMR experiment is based on the density matrix approach with relaxation phenomena implemented using a simple model based on the Bloch equations. Spectrometer related difficulties such as magnetic field inhomogenity, acoustic ringing, radiation damping or statistical noise cannot be calculated using the present approach. Similarly neither can some spin system effects such as cross-relaxation and spin diffusion can be simulated. 

Today the most useful chemical instrument is probably the nuclear magnetic resonance (NMR) spectrometer. Magnetic resonance imaging (MRI), vital in modern medicine, is derived from NMR. In late 1945, a physics group at Stanford, led by Felix Bloch (1905-83) (with William W. Hanson [1909-49] and Martin W. Packard), and one at Harvard, led by Edward M. Purcell (1912-97) (with Henry C. Tbrrey [1911-99] and Robert V. Pound [1919- ]), independently discovered the phenomenon of nuclear magnetic resonance. In order to manifest NMR an atomic nucleus must have nonzero nuclear spin. Of the roughly 100 stable isotopes that have nonzero nuclear spin, H, present in the vast majority of... 

The NMR methods was introduced in 1946 by Bloch and Purcell (Nobel Prize 1952). Ten years later NMR spectrometers were available, but only with weak magnetic field, yielding spectra of poor resolution. 

Sason Already Bloch realized the potential of his NMR technique for chemistry. But it took a chemist like Shooley, who was the first to appreciate the huge potential of NMR for probing molecular structure. He joined the company Varian which built the first NMR spectrometer. And soon after, the technique was taken over by the community of chemists, who found it to be immensely useful for identification of molecular structure. The chemist who developed the technique immensely and received the Nobel Prize in 1991 was Richard R. Ernst (Figure 9.4g). 

Faced with a broad range of prospective spin-lattice relaxation times, the investigator needs two types of spectrometers, a situation that is further complicated if multi-frequency measmements are required. Furthermore, the phenomenological descriptions of measurements made by cw and transient spectrometers differ, as they correspond to separate solutions to Bloch s equations. This chapter describes refinements of both instrumental and theoretical/computational techniques that facilitate the measurement of spin-lattice relaxation times. 

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