Spin–orbit absorption spectroscopy of transition metal hydrides: A TD‐DFT and MS‐CASPT2 study of HM(CO)₅(M = Mn, Re)

Abstract

The spin-orbit (SO) effects on the absorption spectra of HMn(CO) 5 and HRe(CO) 5 have been studied by means of ab initio multistate complete active space perturbation second order (MS-CASPT2) and time-dependent density functional theory (DFT) calculations. For both molecules and in contrast to the experimental spectra, the spin-free theoretical absorption spectra differ significantly, especially in the region of the intense band observed at 47,000 cm À1 . The main features of the lowest and highest parts of the absorption spectra are well reproduced making a reasonable assignment of the bands observed at 34,500 and 51,300 cm À1 for HMn(CO) 5 and 37,000 and 50,630 cm À1 for HRe(CO) 5 . Whereas, the lowest band of HMn(CO) 5 has a significant metalcentered (MC) character, the band centered around 37,000 cm À1 in HRe(CO) 5 corresponds to nearly pure metal-to-ligand-charge-transfer (MLCT) states. The intense bands observed above 48,000 cm À1 in both complexes have mixed character with MLCT, sigma bond to ligand charge transfer, sigma bond to sigma bond charge transfer, and metal to sigma bond charge transfer contributions, which may vary as a function of the level of calculation (DFT, MS-CASPT2, SO). This is attributed to the ability of the method at describing electronic correlation effects in various types of excited states in a balanced way. The SO absorption spectra are more realistic for both molecules with a red shift of the start of the absorption and a broadening of the bands due to a large mixing between the singlet-triplet states and an increase of the density of electronic states contributing to the absorption. This red shift is more important in HMn(CO) 5 than in HRe(CO) 5 due to the MC contribution to the lowest states in first-row transition metal complexes. Our calculations point to the presence of an absorption of intermediate intensity, not observed experimentally, around 40,000-42,000 cm À1 , mostly of MLCT character.The geometries of HMn(CO) 5 and HRe(CO) 5 were optimized at the DFT(B3LYP: Becke, three-parameter, Lee-Yang-Parr) level with the LANL2DZ: Los Alamos National Laboratory 2-double-zeta pseudopotential [20] on the rhenium atom and the BRAHIM, DANIEL, AND RAHMOUNI INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 2091 experimental spectrum of HMn(CO) 5 . The upper states of mixed MSBCT/MLCT/SBSBCT character calculated at 48,150 cm À1 (f ¼ 0.02), 49,680 cm À1 (f ¼ 0.028) and 51,090 cm À1 (0.09) compose the most intense band observed at 51,300 cm À1 .The band observed at 34,500 cm À1 could be attributed to the 1 E (MC/MSBCT) state calculated at 37,130 cm À1 . The region in between 41,000 and 46,820 cm À1 is more difficult to assign. FIGURE 3. Spin-free (dashed lines) and spin-orbit (solid lines) MS-CASPT2 absorption spectra of HMn(CO) 5 (a) and HRe(CO) 5 (b).

Description of excited states in [Re(Imidazole)(CO)3(Phen)]+ including solvent and spin‐orbit coupling effects: Density functional theory versus multiconfigurational wavefunction approach

“…Moreover, several E i states of main triplet character acquire modest oscillator strengths (< 0.01) by mixing with the singlet states. As a consequence, the absorption spectrum is shifted to the red by spin‐orbit effects as observed in a number of transition metal complexes recently investigated . Moreover, the first singlet excited state S 1 (a 1 A″) largely mixes with T 2 (A 3 A′) resulting in almost half‐and‐half E 4 and E 7 “spin‐orbit” states, and the main absorbing state S 2 (a 1 A′) becomes significantly mixed with T 4 (B 3 A′) (15%) in the spin‐orbit state E 11 calculated at 370 nm versus 371 nm without SOC (Table ).…”

“…Because of the fast development of these methods it is now possible to assign and calculate with reasonable accuracy electronic absorption spectra, to optimize the structures of the low‐lying excited states and to interpret luminescent properties . Whereas solvent effects can be simulated practically by polarized continuum model (PCM) approaches, SOC should be taken into account, especially in third row transition metal complexes . Potential of wave function methods such as complete‐active‐space self‐consistent field (CASSCF) supplemented by perturbation theory has not been used extensively until now since the construction of a correct active space for describing excited states of various character is still a challenge.…”

Validation of a computational protocol to simulate near IR phosphorescence spectra for Ru(II) and Ir(III) metal complexes

“…Indeed, it would allow the computation of absolute (instead of relative) intensities together with Herzberg-Teller effects. Recent developments in this specific area open more refined feasible computations [54][55][56]. However, the computational cost increases dramatically when one wants to fully treat the targeted compounds with this feature.…”

Spin–orbit effects in square-planar Pt(ii) complexes with bidentate and terdentate ligands: theoretical absorption/emission spectroscopy

“…[40][41][42][43][44][45][46][47][48] Herein the electronic and geometrical structures of the ground state and lowest S n and T n excited states of bidentate complexes 1 and 2 and of terdentate complexes 3, 4 and 5 depicted in Scheme 1 and their absorption spectra are studied at the TD-DFT level, including solvent and SOC effects. Our goal is to compare and interpret the remarkable disparity in the emissive properties of these complexes, cyclometalated or not and representative of a wide class of square planar Pt(II) complexes investigated for their luminescence properties.…”