Optical spectroscopy of (C<INF>2</INF>H<INF>5</INF>NH<INF>3</INF>)<INF>2</INF>CdCl<INF>4</INF>:Cu<SUP>2+</SUP> under pressure: Study of Cu<SUP>2+</SUP> local structure from theoretical calculations

The variations experienced by the energy E<INF>u</INF>(π) of the e<INF>u</INF>(π)→b<INF>1g</INF> (~x²–y²) charge-transfer transition of (C<INF>2</INF>H<INF>5</INF>NH<INF>3</INF>)<INF>2</INF>CdCl<INF>4</INF>:Cu²⁺ 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<INF>u</INF>(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<INF>u</INF>(π)/∂R<INF>eq</INF> and ∂E<INF>u</INF>(π)/∂R<INF>ax</INF> have been performed where R<INF>eq</INF> and R<INF>ax</INF> mean the equatorial and axial Cu²⁺–Cl⁻ distances of the elongated CuCl<INF>6</INF>⁴⁻ complex, respectively. All results indicate that ∂E<INF>u</INF>(π)/∂R<INF>eq</INF> and ∂E<INF>u</INF>(π)/∂R<INF>ax</INF> for R<INF>eq</INF>=228 pm and R<INF>ax</INF>=297 pm are indeed negative. Moreover ab initio complete active space self-consistent field (CASSCF/CASPT2) and density functional calculations lead to ∂E<INF>u</INF>(π)/∂R<INF>ax</INF> values, which are about 10 times smaller than those of ∂E<INF>u</INF>(π)/∂R<INF>eq</INF>. 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