Experimental evidence of the vibrational coupling of nearest neighbours in 1D spin crossover polymers of rigid bridging ligands

The nuclear inelastic scattering signatures of the low spin centres of the methanosulphonate, tosylate and perchlorate salts of the spin crossover polymer ([Fe(II)(4-amino-1,2,4-triazole)₃]⁺²) _n have been compared for the pure low-spin phase, for the mixed high-spin and low-spin phases, as well as for Zn(II) diluted samples. Within this series a change in the spectral pattern in the 320–500 cm^?1 region is observed involving the decrease of the intensities of a band at ～320 cm^?1 and those over 400 cm^?1 as the molar fraction of the low-spin centres decreases. On the basis of the DFT calculations (B3LYP/CEP-31G) this effect is interpreted in terms of vibrational coupling of the iron centres of the same spin.