The UV photofragmentation spectroscopy of the metal dication complex [Mn(pyridine)₄]²⁺

Abstract

A theoretical and experimental study has been undertaken of the gas phase UV photoexcitation spectrum of the metal dication complex [Mn(pyridine)₄]²⁺. The ions have been prepared using a combination of the pick-up technique and high energy electron impact, and then held in a cold ion trap where they were excited with tuneable UV radiation. Spectra have been recorded by monitoring fragment ion yields as a function of laser wavelength following the excitation of [Mn(pyridine)₄]²⁺ at wavelengths between 225 and 348 nm. For the range 225–263 nm the summation of photofragment ion intensities resulted in a spectrum that contained considerable structure. Although it has been possible to give regions of the spectrum a qualitative assignment to either metal-based or ligand-based transitions, a detailed comparison with results from the application of adiabatic time-dependent density functional theory (TDDFT) to provide an accurate assignment of the electronic transitions involved is considered unsound. This failure to model the excited state spectrum of the open-shell complex [Mn(pyridine)₄]²⁺ is attributed to the significant double- and higher-excitation character of the excited states, which cannot be modelled by the strict single-electron excitations available from adiabatic TDDFT.

Keywords:

• manganese
• pyridine
• ion trap
• UV spectrum
• dication

Acknowledgements

The authors would like to thank EPSRC for financial support and for the award of a studentship to HS. HC and FDL would like to thank the Nuffield Foundation for the award of a Nuffield Science Bursary for FDL and the EPSRC National Service for Computational Chemistry Software for computer time: URL http://www.nsccs.ac.uk. Financial assistance from Nottingham University for the program of experimental work is also gratefully acknowledged.