Detailed absorption, reflectance, and UV photoelectron spectroscopic and theoretical studies of the charge-transfer transitions of tetrachlorocuprate(2-) ion: correlation of the square-planar and the tetrahedral limits

Abstract

Low-temperature (7 K) polarized single-crystal absorption and room-temperature polarized specular reflectance spectra have been obtained of the chloride-to-copper charge-transfer region of the three known square-planar salts of CuCl42", bis(methadonium) tetrachlorocuprate(II), bis(TV-methylphenethylammonium) tetrachlorocuprate(II), and bis(creatinium) tetrachlorocuprate(II) and of the tetragonal monomer, bis(ethylammonium) tetrachlorocuprate(II). These spectra show two intense, x-y polarized, x-y split transitions in the regions 26000-28 000 and 37 000-39 000 cm"1. These bands are assigned to the allowed 2EU *-2Blg (4eu(ir) -3blg) and 2EU <-2Blg (3 "( ) -3blg) transitions, respectively. In addition, a weaker band is observed between 22000 and 25000 cm'1 and has been assigned to the 2A2g 2Blg (la2g(nb) -3blg) transition. These assignments are compared to the results of SCF-Xa-SW ground-and transition-state calculations; although the calculated transition energies are too low and there is overlap of the calculated d-d and charge-transfer manifolds of states (in contrast to the clean separations observed experimentally), the differences between the calculated energies of the three charge-transfer transitions agree well with those observed experimentally. The assignment is also supported by He(II) ultraviolet photoelectron spectroscopic data on single crystals of (C2H5NH3)2CuCl4 and (CH3NH3)2CuC14. In light of these assignments for the square planar (Dih) CuCl42" ion, the charge-transfer spectrum of the flattened tetrahedral (Om) CuCl42" ion has been reassigned (the tetrahedral parentage of the excited state is indicated in parentheses): 22 700 cm"1 (sh), 2A2(2Thnb) <-2B2(2T2); 24730 cm"1, 2E(2T,,nb) -2B2(2T2); 28 880 cm"1 (sh), 2E(2T2,t) -2B2(2T2); 33480 cm"1, 2E(2T2,<r) -2B2(2T2); 43 000 cm"1,2 ,(2 " ) *-2B2(2T2). These assignments require all of the chloride ir-and -derived charge-transfer transitions to contribute to the visible-ultraviolet absorption spectrum within a 20 000-cm"1 region. This spread in energy is supported by both SCF-Xa-SW ground-and transition-state calculations and UPS studies on single crystals of Cs2CuCl4. In addition, the lack of intensity of the group theoretically allowed 2A, •*-2B2 (Sa^ir) -* 5b2) transition in D^a nd the allowed 2B2u <-2Blg (lb2u(Tr) -3blg) transition in DAh is explained by a simple model for charge-transfer transition intensities. The contributions of many-electron reorganization effects to optical and UPS transitions are examined in a comparison of both experimental and theoretical data.The results indicate that, because of differences in electron-electron repulsion and orbital relaxation, one must be cautious in attempting to correlate UPS and optical transition energy differences. Finally, the factors that contribute to the differences between energies of related charge-transfer transitions in the )4 and Dlc¡ geometries are evaluated by using the experimental and theoretical results. It is concluded that while crystal-field effects are quite significant, shifting th...

Metal-mediated diradical tuning for DNA replication arrest via template strand scission

“…2, Inset), confirming the presence of chloride in the equatorial plane (45,(49)(50)(51)(52)(53) (Fig. 3A).…”

Growth and characterization of hybrid (CnH2n+1NH3)2CuCl4 self‐assembled films

“…In this framework by fitting the experimental data we estimated the gap energy of the (C 2 H 5 NH 3 ) 2 CuCl 4 film, E g , to be 3.32±0.02 eV Optical absorption measurements The optical absorption was measured at room temperature for quartz substrate deposited films, in the 9000-52000 cm -1 range. The absorbance spectra were consistent with previous results [1,7,14,18,19], thus confirming that the films have sufficient structural and optical quality. The absorption spectra are shown in fig.…”

“…The two UV bands were assigned to charge transfer (CT) transitions occurring in the axially elongated CuCl 6 4-octahedron. In particular, the 35500 cm -1 band was claimed to correspond to the σ→d x 2 -y 2 transition and the 26000 cm -1 one to the π→d x 2 -y 2 (e u →b 1g ) transition [18,19]. In the case of (C 2 H 5 NH 3 ) 2 CuCl 4 film we found that the first UV band is centred at 26200 cm -1 .…”

Investigations on the EPR Parameters for the Square Planar Cu2+ Centers in K2PdX4 (X = Cl, Br)

“…For the ligand Br, the above contributions rapidly increase to almost 4 and 8 times those of the respective CF terms. This can be ascribed to the much larger ζ p 0 (≈ 2460 cm −1 [34]) and lower CT energy levels for [CuBr 4 ] 2− cluster than that for [CuCl 4 ] 2− cluster [18,36]. The importance of the CT contributions for the hyperfine structure constants is less significant than that for the g factors due to the large isotropic part characterized by the core polarization constant.…”

“…The core polarization constants in the formulas of the hyperfine structure constants are taken as κ ≈ 0.3 [32,35], the expectation value for Cu 2+ in tutton salts. The CT energy levels E CT1 and E CT2 are about 38000 cm −1 and 35000 cm −1 [36] for K 2 PdCl 4 , and 30500 cm −1 and 27500 cm −1 [18] for K 2 PdBr 4 , respectively. Substituting these values into (8), the theoretical EPR parameters (Cal.…”