Spin isomerization

As in the case of hydrogen and tritium, deuterium exhibits nuclear spin isomerism (see Magnetic spin resonance) (14). However, the spin of the deuteron [12597-73-8] is 1 instead of S as in the case of hydrogen and tritium. As a consequence, and in contrast to hydrogen, the ortho form of deuterium is more stable than the para form at low temperatures, and at normal temperatures the ratio of ortho- to para-deuterium is 2 1 in contrast to the 3 1 ratio for hydrogen. 

Ortho-Para Tritium. As in the case of molecular hydrogen, molecular tritium exhibits nuclear spin isomerism. The spin of the tritium nucleus is S, the same as that for the hydrogen nucleus, and therefore H2 and T2 obey the same nuclear isomeric statistics (16). Below 5 K, molecular tritium is... 

Most physical properties are but little affected by nuclear-spin isomerism though the thermal conductivity of P-H2 is more than 50% greater than that of 0-H2, and this forms a ready means of analysing mixtures. The mp of P-H2 (containing only 0.21% (3-H2) is 0.15 K below that of normal hydrogen (containing 75% 0-H2), and by extrapolation the mp of (unobtainable) pure... 

Long-Lived (Persistent) Spin isomerism. The singlet of A-tosyl-3,4-dimethylenepyrrole (46f, Scheme 5.10) is a blue, ESR-silent species that is stable over many days in cold matrices. The triplet is also stable for days at low temperatures. This type of long-lived spin isomerism has been attributed... 

Chromium(II) complexes of bipyridyls, terpyridyl and the phenanthrolines have been discussed in Section 35.2.2.1. Complexes of the ligands 2-aminomethylpyridine (pic, 2-picolyl-amine) and 8-aminoquinoline (amq), which have one heterocyclic and one amino nitrogen donor atom, have been prepared by methods similar to those in Scheme 10. The bis(amine) complexes are typical high-spin, distorted octahedral complexes, and the mono(amine) complexes, from their antiferromagnetic behaviour and reflectance spectra, are six-coordinate, halide-bridged polymers (Table 15).103 No tris(amine) complexes could be prepared so the attempt to find spin isomeric systems in octahedral chromium(II) systems was unsuccessful ([Cr(en)3]X2 are high-spin and [Cr(bipy)3]X3 and [CrX2(bipy)2] low-spin). 

By the same technique of resonance and pulsed ultrasonic perturbation Beattie et al.176) have also studied the spin crossover dynamics in aqueous solutions of [Fe(paptH)2]Cl2- Relaxation times of ca. 40 ns were measured at 25 °C, again independent of concentration and thus presumably arising from a unimolecular spin isomerization process. Some of the thermodynamic parameters of particular interest are here AH0 = 3.91 kcal mol-1, AS° = 14.8 eu, AH = 7.63 kcal mol-1, AH = 3.72 kcal mol-1,, , = 10-2S. 

At one time Taylor considered7 that if adsorption on a solid surface would catalyse the interconversion of ortho and para hydrogen, it was an indication that chemisorption with dissociation into atoms occurred the temperature at which such interconversion took place would (if this were true) have been some indication of the activation energy for chemisorption. No doubt chemisorption does promote this spin isomerization but it is now known that adsorption of the van der Waals type can result in speedy attainment of the equilibrium mixture of ortho and para hydrogen, provided that the surfaces are of some paramagnetic substance the close proximity to a paramagnetic surface can catalyse the interconversion,8 without dissociation of the adsorbed molecules. 

Ah diatomic molecnles for which the constituent nnclei have spin exhibit the phenomenon of spin isomerism. The nnclear spins can be parallel (the ortho isomer) or opposed (the para isomer). For most diatomic molecules, which might be expected to exhibit, spin isomerism the energy separation of the rotational states is small compared to kT, even at low temperatures. However, in the case of hydrogen molecules, which have the smallest moment of inertia of any diatomic molecnles, the energy difference between the rotational energy levels is relatively large and only the lowest states are popnlated at room temperature. 

Electrostatic and polarization interactions occurring within the protein matrix and the solvent environment were investigated in a second study for each of the 10 simple BS states and the additional two states arising from spin isomerization within the BS6 state. " " Moreover, redox-potential calculations were also performed. When the protein and solvent environment were taken into account, BS6-3 was found to be lowest in energy, followed by BS2 (+0.39 kcal mol" ), BS6-1 (+1.70 kcal molX and BS6-2 (+2.34 kcal mol ). For the most favorable BS state, the electrostatic influence of the protein field... 

Here, one has to be careful because spin isomerism behaviour in solution may well be different to that in the solid state. In the solid state the individual magnetic ions couple weakly together but often sufficiently strongly for the phenomenon to have a cooperative aspect and to show hysteresis the stronger the cooperativity the more abrupt the transition. The spin-crossover may be induced not only thermally but also by application of pressure, for small structural changes accompany the spin change. 

For both of the last two classes of isomerism we have detailed—fluxional and spin isomerism—the lifetime of individual isomers may be rather short. Spin isomers, for instance, typically live for about 10 s (but see Section 3.4.1 and the Further Reading at the end of this chapter). Some would argue that classical isomerism refers only to species capable of physical separation, and so of long lifetime. However, with the increasing use of methods which explore short lifetimes—NMR and EPR, particularly, in the present context—it seems sensible to ignore this limitation. 

The fact that spin equilibria (spin isomerism) has been the subject of much recent work is indicated by the almost simultaneous appearance of three reviews on the subject ... 

Structural isomerism observed due to different electron spins in complexes of the same geometry is known as spin isomerism. In some of the complexes of Fe(II), Fe(III) and Co(II), the metal ions are known to exist in both high-spin and low-spin states. Fe(III) ion in an octahedral ligand field can have either a high-spin (t2g) (eg) or low-spin (t2g) (Og) configuration, as shown in Figure 4. 

Spin isomerism is known to be present in a complex [Fe (S2CNMe2)3], as shown in Figure 5. 

Spin isomers coexist in the sample, but their lifetimes are very short s) and hence are very difficult to distinguish. The spin isomerism behaviour in solution and solid states are found to be different. 

---