The ultraviolet−visible light (UV-vis) absorption spectrum of ferrocene is modeled with time-dependent density functional theory employing LSDA, BLYP, B3LYP, and CAM-B3LYP functionals in combination with 6-31G*, 6-31+G*, CC-PVTZ, and aug-CC-PVTZ basis sets. With the exception of LSDA, all functionals predict a reasonable Fe-CP distance of ∼1.67 Å. Diffuse functions are essential for the strongly allowed states at high energy but of lesser consequence for the visible range of the spectrum. Dipole forbidden states are examined with vibrationally excited structures, obtained from the normal modes of the infrared (IR) spectrum. Despite earlier claims, TDB3LYP predicts the UV−vis spectrum of ferrocene quantitatively correct. TDBLYP predicts a large number of spurious charge-transfer states, TDCAM-B3LYP and TDwB97XD are correct in the low-energy region but overestimate the energy of strongest peak of the spectrum by 0.8 eV. The amount of charge transfer involved in "d−d transitions" is equal to that in "charge-transfer states".
■ INTRODUCTIONFerrocene was discovered in 1951 1 and has revolutionized the views of chemists about how metals bind to organic π-systems. Its electronic structure and ultraviolet-visible light (UV-vis) absorption spectrum have become textbook 2 examples, and one would assume that not much new can be said about them. However, comparison between results obtained with timedependent density functional theory (TDDFT) by the author and theoretical data from the literature 3−15 revealed several discrepancies, which prompted the present investigation.Pure ferrocene is a very stable light orange powder, and the UV−vis absorption spectrum of ferrocene has been investigated in detail. 16−19 Upon cooling, the color changes from orange to lemon yellow. 16,17 The orange color stems from a dipoleforbidden absorption at 440 nm (2.81 eV) and a shoulder at 528 nm (2.34 eV), 16−18 both of which gain oscillator strength only through vibronic coupling. 8 Outside the visible range, a weak peak occurs at 324 nm (3.83 eV) and several relatively weak but allowed absorptions are present between 265 nm and 230 nm. The strongest absorption of ferrocene is observed at 202 nm (6.12 eV) in solution and 6.31 eV in the vapor phase. 16−19 The oscillator strength of this allowed absorption is 350 times stronger than that of the peak at 440 nm.B3LYP produces the structural parameters of ferrocene with errors of 2 pm or less, 20 and there is consensus that, in agreement with experiment, theory predicts the eclipsed form to be more stable than the staggered one. Several authors have cautioned, however, against using B3LYP for excited-state calculations of ferrocene. 6,10,12,15 Boulet et al., 6 Li et al., 10 and Scuppa et al. 12 promoted gradient-corrected pure DFT functionals, and Fromager et al. 15 promoted the rangeseparated CAM-B3LYP functional. In this investigation, TDDFT results with several functionals and different allelectron basis sets are analyzed. It is demonstrated that B3LYP works perfectly fine and that t...