Spin isomers

Despite its very simple electronic configuration (Is ) hydrogen can, paradoxically, exist in over 50 different forms most of which have been well characterized. This multiplicity of forms arises firstly from the existence of atomic, molecular and ionized species in the gas phase H, H2, H+, H , H2" ", H3+. .., H11 + secondly, from the existence of three isotopes, jH, jH(D) and jH(T), and correspondingly of D, D2, HD, DT, etc. and, finally, from the existence of nuclear spin isomers for the homonuclear diatomic species. 

Fig. 9.14 Effective thickness derived from NFS measurements on [Fe(tpa)(NCS)2]. The open squares solid circles) denote measurements with a fraction of high-spin (low-spin) higher than 95%. The open circle at 40 K denotes the trapped high-spin isomer obtained after rapid cooling. The inset shows the step in the transition region the upward directed triangles downward directed triangles) denote measurements recorded with decreasing (increasing) temperature. The lines are guides to the eyes. (Taken from [41])...

Figure 2.26. Energy landscape (BP/DNP) for the reaction of the copper-oxo species Cu—OJZSM-5 with N20, N02, and NO molecules, including associated spin isomers calculated for the M5 site (the values are given in kcal x mol-1).

Since interconversions between different states of symmetry (i.e., between ortho- and parahydrogen) are forbidden, the adjustment of the relative ratios of the two spin isomers to the values corresponding to the thermal equilibrium at an arbitrary temperature is normally very slow and, therefore, must be catalyzed. In the absence of a catalyst, dihydrogen samples retain their once achieved ratio and, accordingly, they can be stored in their enriched or separated forms for rather long periods (a few weeks or even a few years in favorable cases). 

Fig. 12.5 Fraction of spin isomers at thermal equilibrium between o-H2 and p-H2.

N tion state geometry intermediate between spin isomers ... 

As already mentioned, molecular hydrogen consists of two spin isomers, one of which has a total spin of 1=0 (singlet), and the other a total spin of 1=1 (triplet). The first is named parahydrogen, the latter orthohydrogen (Figure 9.8). 

It is obvious that this constitution/ configuration dichotomy does not readily accommodate isomerism of the special types (e.g. spin isomers)13 mentioned above but it does form a very convenient basis for the consideration of the most frequently encountered and most important... 

There have been very few reports of the Raman spectra of spin-equilibrium complexes. In one experiment the presence of both high-spin and low-spin isomers of an iron(II) Schiff base complex was observed by the resonance Raman spectra of the imine region (11). The temperature dependence of the spectra was recorded for both solid and solution samples. Recently differences were described in the resonance Raman spectra of four- and six-coordinate nickel(II) porphyrin complexes which undergo coordination-spin equilibria. These studies are extensions of a considerable literature on spin state effects on the Raman spectra of iron porphyrins and hemes. There are apparently no reports of attempts to use time-resolved Raman spectra for dynamics experiments. 

All intramolecular spin equilibria have a nonzero enthalpy of reaction. This occurs because the high-spin isomer possesses greater entropy, both from its higher spin degeneracy and from its larger vibrational partition function, than does the low-spin isomer. Because AG is approximately zero, AH is therefore positive. Consequently, the equilibria are temperature dependent and can be perturbed by a rapid change in temperature. 

The cobalt(II) complexes which undergo spin equilibrium are of several different types. Octahedral high-spin complexes with a T ground state are subject to Jahn-Teller distortion in the low-spin d1 2E state. This effect is best documented in structures of the Co(terpy)22+ spin-equilibrium complex. The high-spin isomer is nearly octahedral, with a difference in Co N bond lengths between the central and distal nitrogens of only 6 pm. In the Jahn-Teller distorted low-spin state this difference has increased to 21 pm (58). 

The EPR spectrum of a spin-equilibrium complex can be used to establish a lower limit to the spin state lifetimes of the order of 10 10 second. In an important paper in 1976, Hall and Hendrickson reported observation of EPR signals for both the high-spin and the low-spin isomers of iron(III) dithiocarbamate complexes at 4 12 K as powders, glasses, and doped solids (71). This resolved the question whether these complexes possess distinct high-spin and low-spin states. It also sets a lower limit on their interconversion lifetimes. Similarly, the observation of signals for both the high-spin and low-spin states of [Co(terpy)22+] (97) leads to the same conclusions about this complex. In both cases the interconversion rates in solution have proved too fast to measure, with lifetimes of less than 10-9 second indicated. The solution measurements were undertaken, of course, at room temperature and the EPR measurements at close to 4 K. Significant differences in the rates of solid and solutions at room temperature are still possible. 

The results obtained from thermal spin equilibria indicate that AS = 1 transitions are adiabatic. The rates, therefore, depend on the coordination sphere reorganization energy, or the Franck-Condon factors. Radiationless deactivation processes are exothermic. Consequently, they can proceed more rapidly than thermally activated spin-equilibria reactions, that is, in less than nanoseconds in solution at room temperature. Evidence for this includes the observation that few transition metal complexes luminesce under these conditions. Other evidence is the very success of the photoperturbation method for studying thermal spin equilibria intersystem crossing to the ground state of the other spin isomer must be more rapid than the spin equilibrium relaxation in order for the spin equilibrium to be perturbed. 

It cannot be excluded that for first-row transition metal complexes, in addition to the above factors, the central atom spin state also plays a significant role in the stability of individual electronic and/or spin isomers, and in their ability to undergo photochemical interconversions. For some iron complexes such spin-state photoisomerizations are well-known [1, 121]. 

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