Bar magnet analogy

Figure 8.4. The bar-magnet analogy for a spin- /2 nucleus in which the magnetic dipole is viewed as possessing a magnetic North and South pole.

A fundamental quantum law is that In a uniform magnetic field, a nucleus of spin I may assume 21+1 orientations. Thus, for a nucleus of 7 = 1/2 (for instance, the proton), there are 2(1/2) +1=2 permissible orientations. This makes a nucleus of 7 = 1/2 analogous to a bar magnet in a magnetic field (Fig. 12.3). Since we shall deal exclusively with nuclei of spin 7 = 1/2 in this chapter, and almost exclusively with protons, the bar-magnet analogy will be useful. 

Figure 12.10. The bar-magnet analogy for spin-spin coupling.

Since heteroatoms are generally more electronegative than carbon, a carbon-heteroatom bond will have an electron density that is localized towards the heteroatom. The result is a permanent dipole that is analogous in its electronic properties to a bar magnet. This permanent dipole can induce a dipole in an otherwise neutral portion of another molecule in the same manner that a magnet induces a dipole in a piece of steel. The resulting attraction is referred to as a dipole-induced dipole, or Debye interaction. 

As an analogy, consider pairs of small bar magnets constrained into a specific spatial relation to each other (as nuclei are in real molecules) and which can assume just two North-South directions (as nuclei of spin 7=1/2 do). It is immediately obvious that, for the relative arrangements depicted in Figure 12.10, the antiparallel arrangement A has a lower energy than the parallel arrangement B. 

The nucleus is analogous to a tiny bar magnet and so has an associated magnetic moment p. When placed in an external magnetic field H, the nuclear spins tend to line up either parallel to the field direction or in the opposite direction (antiparallel). These two spin states have different associated energies separated by... 

Magnetic dipoles are found to exist in magnetic materials, which, in some respects, are analogous to electric dipoles (Section 18.19). Magnetic dipoles may be thought of as small bar magnets composed of north and south poles instead of positive and negative... 

Trituration of the crude cw-azide with hexane at ambient temperature caused detonation to occur. Isolation of low molecular weight carbonyl azides should be avoided, or extreme precautions taken. A similar incident with the analogous cyclobutane diazide was reported [1], During use as an intermediate in preparation of the 1,2-diamine, the explosive diazide was never isolated or handled free of solvent [2]. The trans isomer (0.25 mole) was prepared by the latter technique, but as a 45 w/v% solution in toluene, rather than as the recommended 10% solution. When a magnetic spin-bar was introduced, the solution detonated. Initiation may have involved friction from adventitious presence of iron filings adhering to the magnetic bar [3],... 

A 100-W immersion heater (such as VWR Scientific Inc. 33897-140 a 200-W to 300-W tea/coffee immersion heater can also be used), controlled by a Variac magnetic stirrer and stir bar 1000-mL beaker thermometer with a resolution of 0.02 K (platinum resistance thermometer or calibrated thermistor such as Vernier model TMP-BTA) 1- to 5-mW He-Ne laser (633 nm) or a red (545 mn) or green (532 nm) laser — a battery-operated laser pointer is suitable, but a module with a separate 3-volt power supply, available from Z-Bolt and other sources, will give better power stability for long rans two Polaroid sheets in rotation holders photodetector such as photomultiplier tube or Thorlabs model 201/579-7227 silicon photodiode analog-to-digital data collection system such as Vernier LabPro. 

---