Spin susceptibility singlet-triplet

The Mossbauer spectra (between 4.2 K and 298 K) show two overlapping doublets with temperature dependent relative intensities. In conjunction with the data from magnetic susceptibility measurements (between 83 and 313 K) the Mossbauer spectra were interpreted in terms of a singlet triplet spin transition of the central iron(II) ion. No internal magnetic field was observed in a Mossbauer experiment under applied field at 4.2 K. The authors consider this complex the first authentic example of a singlet < triplet spin transition in the 3 d6 configuration of iron(II). Support has been received from UV-vis spectra and IR vibrational spectra201). 

Magnetic susceptibilities of 10a and 10b were measured on a SQUID suscep-tometer in microcrystalline form. %T-T plots are shown in Fig. 9.5. The data were analyzed in terms of a modified singlet-triplet two-spin model (the Blea-ney-Bowers-type), in which two spins (S = V2) couple antiferromagnetically within a biradical molecule by exchange interaction J. The best-fit parameters obtained by means of a least-squares method were 2J/kB = -2.2 + 0.04 K for 10a and -11.6 + 0.4 K for 10b. Although the interaction (2J/kB = -2.2 K) between the two spins in the open-ring isomer 10a was weak, the spins of 10b showed a remarkable antiferromagnetic interaction (2J/kB = -11.6 K). 

Calculations on the singlet, triplet and quintuplet states allow us to evaluate the spin susceptibility [3] of a cluster of N = 8 sites. Since the size of the system is finite, there is a gap in the spin excitation spectrum. The resulting temperature-dependent susceptibility curve cannot reproduce the macroscopic curve near T = 0 K, but at least we may compare the absolute value of the susceptibility at 300 K, and the temperature of the maximum of the susceptibility, to the experimental values, as this has already been done for Heisenberg phenomenological Hamiltonians [11]. 

Doping of conducting polymers leads to the increase of the paramagnetic susceptibility, %. This parameter consists of the temperature-independent Pauli susceptibility of the Fermi gas %p, a temperature-dependent contribution of localized Curie PC %o and the term %sr becoming due to a possible singlet-triplet spin equilibrium in the system, ... 

The Ni(mnt)2 anions in this compound are arranged as dimers, [Ni(mnt)2]2, and the formal oxidation of Ni(III) is in good agreement with the temperature-dependent paramagnetic susceptibility that can be well described by a singlet-triplet model of pairs of antiferromagnetically coupled 5=1/2 spins, with Ms = -226 K (Figure 2.63) [118]. 

The susceptibility of a carbene to reaction with oxygen can be used as a probe of its spin multiplicity. If a carbene is insensitive to oxygen, this is taken to be an indication that it is a ground-state singlet and that the singlet does not readily form the triplet carbene. 

Another study (12) concerns the nominally tetrahedral complex, bis(/V-isopropylsalicylidene)nickel(II) shown in Fig. 14. The principal crystal susceptibilities over the temperature range 20-300°K have been fitted, as shown in Fig. 14, within the complete spin-triplet basis 3F + 3P (further calculations demonstrated the unimportance of including spin-singlet terms in the model) using the parameter set B f k, e (0), eCT(N), e n(N), e i(N), e,m(0), e i(0). 

In the presence of Umklapp scattering, the following susceptibilities will be considered for the Peierls channel at 2k% one has the on-site p, = SDW for spin-density-wave and p = BOW for the bond-order-wave in the Cooper channel, one has singlet p = SS and triplet p=TS superconducting correlations. At two-loop level, the auxiliary responses are governed by the flow equations... 

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