Vibrational Spectroscopy and Theory of Fe⁺(CH₄)_n (n = 1−4)

Abstract

Vibrational spectra are measured for Fe(+)(CH(4))(n) (n = 1-4) in the C-H stretching region (2500-3200 cm(-1)) using photofragment spectroscopy. Spectra are obtained by monitoring CH(4) fragment loss following absorption of one photon (for n = 3, 4) or sequential absorption of multiple photons (for n = 1, 2). The spectra have a band near the position of the antisymmetric C-H stretch in isolated methane (3019 cm(-1)), along with bands extending >250 cm(-1) to the red of the symmetric C-H stretch in methane (2917 cm(-1)). The spectra are sensitive to the ligand configuration (η(2) vs η(3)) and to the Fe-C distance. Hybrid density functional theory calculations are used to identify possible structures and predict their vibrational spectra. The IR photodissociation spectrum shows that the Fe(+)(CH(4)) complex is a quartet, with an η(3) configuration. There is also a small contribution to the spectrum from the metastable sextet η(3) complex. The Fe(+)(CH(4))(2) complex is also a quartet with both CH(4) in an η(3) configuration. For the larger clusters, the configuration switches from η(3) to η(2). In Fe(+)(CH(4))(3), the methane ligands are not equivalent. Rather, there is one short and two long Fe-C bonds, and each methane is bound to the metal in an η(2) configuration. For Fe(+)(CH(4))(4), the calculations predict three low-lying structures, all with η(2) binding of methane and very similar Fe-C bond lengths. No single structure reproduces the observed spectrum. The approximately tetrahedral C(1) ((4)A) structure contributes to the spectrum; the nearly square-planar D(2d) ((4)B(2)) and the approximately tetrahedral C(2) ((4)A) structure may contribute as well.

Near ultraviolet photodissociation spectroscopy of Mn+(H2O) and Mn+(D2O)

“…However, the UV beam only makes one pass through the ion cloud due to absorption by the mirrors. 35 The computations use the Gaussian 09 suite of programs. 36 The geometries, energies, and vibrational frequencies of the ground and excited states of Mn + (H 2 O) and Mn + (D 2 O) are calculated with the B3LYP hybrid density functional with the 6-311++G(3df,3pd) basis set.…”

Gas phase chemistry of neutral metal clusters: Distribution, reactivity and catalysis

“…In this review we focus on the latter approach, employing VUV lasers [78][79][80][81][82][83][84][85] (118 nm, 10.5 eV) and an EUV laser [35,86] (46.9 nm, 26.5 eV). Some studies of emission [67,[87][88][89][90], photoelectron [91][92][93][94][95], and vibrational [96,97] spectroscopies have been employed for the detailed characterization of small cluster structure and electronic states.…”

Vibrational Spectroscopy of Co+(CH4)n and Ni+(CH4)n (n = 1–4)

“…29,30 Those experiments show that Mn + ( 7 S, 3d 5 4s 1 ) and sextet Fe + ( 6 D, 3d 6 4s 1 ) bind methane in η 3 coordination while Co + ( 3 F, 3d 8 4s 0 ) and Ni + ( 2 D, 3d 9 4s 0 ) bind in η 2 coordination. One would expect that the binding energies would be correlated with the red shifts in the lowest C−H stretching frequency of M + (CH 4 ) (compared to bare CH 4 ).…”

“…The first method, infrared multiple-photon dissociation spectroscopy (IRMPD), was previously used to measure spectra of Fe + (CH 4 ) n (n = 1, 2). 30 Unfortunately, IRMPD of Co + (CH 4 ) n and Ni + (CH 4 ) n leads to no dissociation for n = 1 and to very low dissociation and broad peaks for n = 2. Therefore, a second method, argon-tagging, 40−44 was used to measure the spectra for n = 1, 2.…”

“…30 Also noteworthy is the recent measurement of the vibrational spectra of V 5 O 12 + (CH 4 ) and V 5 O 13 + (CH 4 ), in which the methane binds to one vanadium, with η 2 coordination. 31 Computations carried out in conjunction with the thermodynamic measurements of Zhang et al 19,21 predict that many of the clusters have several potential isomers with similar energies.…”