ATR or transmission—A variable temperature study comparing both techniques using [Fe(3ditz)3](BF4)2 as model system☆
Graphical abstract
Introduction
Especially for inorganic and materials chemists infrared spectroscopy has become a powerful, today’s inevitable, routine characterization technique [1]. Many of the samples investigated feature structural changes and/or phase transitions at non-ambient temperatures. Although infrared spectroscopy could provide valuable information on such transformations, mainly due to the necessary non-ambient equipment only for a minor percentage thereof temperature dependent infrared studies are reported [2], [3], [4], [5], [6], [7], [8], [9], [10].
For spectroscopic investigations at elevated temperatures a small selection of various commercial solutions from different suppliers are available. But, especially, for low-temperature spectroscopy only a very limited number is offered, mainly in form of custom-tailored prototype equipment.
A very prominent class of inorganic coordination compounds featuring a first order phase transitions often also reported in temperature dependent spectroscopic investigations is the one of spin crossover compounds [11]. For 3d4-3d7 transition metal complexes with a spin-pairing energy in the same order of magnitude as the ligand-field splitting energy a spin transition, leading from a high-spin to a low-spin compound and vice versa can be introduced by various external stimuli (temperature, light, pressure, …). This transition is accompanied by notable changes in the materials properties, as e.g. colour, magnetic moment, dielectric constant and bond-length [11], [12], [13], [14]. Especially, the changes in the bond-strengths inducing changes in the bond-lengths between the coordinating ligand-atoms to the metal centre have a notable impact on the geometry of the molecule. This behaviour is perfectly observable in the temperature-dependent MIR and FIR spectra [15], [16], [17].
Using the example of [Fe(3ditz)3](BF4)2 [3ditz = 1,3-bis(1H-tetrazol-1-yl)propane] [18]. We compare the performance of a non-ambient temperature prototype device using ATR technology with a thermostatable transmission equipment. The comparison of their performance includes the MIR and FIR spectra, obtained in the range within liquid-nitrogen temperature up to 477 K. Advantages and drawbacks of both instrumentation setups are discussed.
Section snippets
Instrumentation
All shown spectra were recorded on a Perkin-Elmer 400 FIR/MIR FTIR spectrometer (see Fig. S1), using a DTGS MIR and a DTGS FIR detector (MIR 15,000 cm−1–370 cm−1, FIR 720 cm−1–30 cm−1) with a MIR Opt-KBr beamsplitter (7800 cm−1–400 cm−1) and FIR GRID beamsplitter (700 cm−1–30 cm−1). The spectrometer is continuously purged with a flow of dry air, obtained from a Parker BALSTON purge gas generator (see Fig. S2).
All MIR spectra were recorded with a spectral resolution of 2 cm−1 as accumulation of 8 single
Results and discussion
The iron(II) spin crossover complex [Fe(3ditz)3](BF4)2 is characterized by a sharp spin transition with a T1/2 of 159 K. Going along with the first order phase transition from a paramagnetic high-spin to a diamagnetic low-spin state the molecular geometry around the iron atom is distorted by a contraction of the coordinating Fe-N bonds about 8.51%. This change can be followed by positional shifts of the ligands characteristic MIR-bands. Especially, the ν(CHTz), usually located around 3100 cm−1
Conclusion
The spin crossover complex [Fe(3ditz)3](BF4)2 was used for a comparison of two non-ambient temperature units, suitable for MIR and FIR spectroscopy. A prototype, using ATR-technology, for the range between 77 K and 473 K was lined up with a transmission cell, suitable for temperatures between 135 K and 477 K.
In the MIR spectra the transmission unit is the clear favourite, as it combines a satisfying temperature range with good spectral quality and signal intensities over the full temperature and
Acknowledgements
This work was supported by the Austrian research fund FWF, project number P24955-N28, and the COST action CM1305 “Explicit Control Over Spin-states in Technology and Biochemistry (ECOSTBio)”.
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Selected paper from 8th International Conference on Advanced Vibrational Spectroscopy, 12–17 July 2015, Vienna, Austria.