Optical spectroscopy of (C₂H₅NH₃)₂CdCl₄:Cu²⁺ under pressure: Study of Cu²⁺ local structure from theoretical calculations

Resumen

Abstract The variations experienced by the energy E u (π) of the e u (π)→ b 1 g (∼ x 2 – y 2 ) charge‐transfer transition of (C 2 H 5 NH 3 ) 2 CdCl 4 :Cu 2+ upon pressure in the 0‐ to 40‐kbar range have been measured at room temperature by means of a sapphire anvil cell. These data reveal that E u (π) undergoes a red shift of 1400 cm −1 on passing from ambient pressure to 40 kbars. To understand this puzzling result theoretical calculations of ∂ E u (π)/∂ R eq and ∂ E u (π)/∂ R ax have been performed where R eq and R ax mean the equatorial and axial Cu 2+ –Cl − distances of the elongated CuCl 6 4− complex, respectively. All results indicate that ∂ E u (π)/∂ R eq and ∂ E u (π)/∂ R ax for R eq =228 pm and R ax =297 pm are indeed negative. Moreover ab initio complete active space self‐consistent field (CASSCF/CASPT2) and density functional calculations lead to ∂ E u (π)/∂ R ax values, which are about 10 times smaller than those of ∂ E u (π)/∂ R eq . From the ensemble of experimental and theoretical results, it is concluded that a pressure of 40 kbars gives rise to a decrement of ≈25 pm of the axial distance and at the same time to an increase of ≈7 pm of the equatorial one. It is stressed that the present study on a diluted Jahn–Teller impurity lies far beyond the current possibilities of X‐ray absorption structure techniques. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001