Article pubs.acs.org/JPCA
Vibrational Spectroscopy of Co+(CH4)n and Ni+(CH4)n (n = 1−4) Abdulkadir Kocak, Zachary Sallese, Michael D. Johnston, and Ricardo B. Metz* Department of Chemistry, University of Massachusetts Amherst, Amherst, Massachusetts 01003, United States S Supporting Information *
ABSTRACT: Vibrational spectra of M + (CH 4 ) m (Ar)3−m and M+(CH4)n (M = Co, Ni; m = 1, 2; n = 3, 4) in the C−H stretching region (2500−3100 cm−1) are measured using photofragment spectroscopy, monitoring the loss of argon or methane. Interaction with the metal leads to large red shifts in the C−H stretches for proximate hydrogens. The extent of this shift is sensitive to the coordination (η2 vs η3) and to the metal−methane distance. The structures of the complexes are determined by comparing measured spectra with those calculated for candidate structures at the B3LYP/ 6-311++G(3df,3pd) level. Binding energies are also computed using the CAM-B3LYP functional. In all cases, CH4 shows η2 coordination to the metal. The m = 1 complexes show very large red shifts of 370 cm−1 (for M = Co) and 320 cm−1 (for M = Ni) in the lowest C−H stretch, relative to the symmetric stretch of free CH4. They adopt a C2v structure with the heavy atoms and proximate hydrogen atoms coplanar. The m = 2 complexes have slightly reduced red shifts, and Tee-shaped structures. Both Tee-shaped and equilateral (or quasi-equilateral) structures are observed for the n = 3 complexes. The measured photodissociation onset and significantly reduced intensity for low-frequency C−H stretches imply a value of 2650 ± 50 cm−1 for the binding energy of Ni+(CH4)2−CH4. The Co+(CH4)4 complexes have two low-lying structures, quasi-tetrahedral and distorted square-planar, which contribute to the rich spectrum. In contrast, the symmetrical, square-planar Ni+(CH4)4 complex is characterized by a very simple vibrational spectrum.
I. INTRODUCTION Methane is a permanent gas, which makes it awkward to transport and store and precludes its use as a fuel for vehicles. Thus, direct, efficient conversion of methane to a liquid such as larger hydrocarbons or methanol has been a long-standing goal of catalysis.1 The indirect process first reacts methane with steam to produce synthesis gas (steam re-forming, which is endothermic and inefficient), followed by the Fischer−Tropsch process, catalyzed by iron and cobalt, to produce liquid hydrocarbons. Nickel is widely used as a catalyst for steam reforming, which is also the primary process for industrial hydrogen production. The catalytic importance of these reactions has also prompted extensive fundamental studies of methane activation on nickel surfaces2,3 and by gas-phase metal atoms and ions.4−11 Activation of methane by gas-phase metal ions
endothermic by 147 and 155 kJ/mol, respectively, and that there is an additional barrier to reactivity, associated with the formation of the (H2)MCH2+ exit channel complex. Their calculations, as well as those by Hendrickx et al.,17 predict that the [H−M+−CH3] intermediate is not a local minimum for these metals. However, the M+(CH4) entrance channel complexes are quite strongly bound and their binding energies have been measured in both collision-induced dissociation15,16,18 and equilibrium19−21 experiments, as shown in Table 1. The binding of methane to M+ includes electrostatic and covalent contributions. The strength of the interaction depends on the electronic configuration of the metal. In particular, metals with a 3dn4s0 ground state (such as Co+ and Ni+) interact more strongly with ligands than those with 3dn−14s1 (such as Mn+). This is because the 3d orbitals are smaller than the 4s, and they are directional, so they can orient to minimize repulsion with ligands. Previous spectroscopic studies of M+(CH4)n complexes include the electronic spectra of Mg+(CH4),22 Ca+(CH4),23 V+(CH4),24 and Zn+(CH4)25 and vibrational spectroscopy of Li + (CH 4 ) 1−6 , 26,27 Al + (CH 4 ) 1−6 , 28 Mn + (CH 4 ) 1−6 , 29 and Fe+(CH4)1−4.30 Also noteworthy is the recent measurement of the vibrational spectra of V5O12+(CH4) and V5O13+(CH4), in
M+ + CH4 → M+(CH4) → [H−M+−CH3] → (H 2)MCH 2+ → MCH 2+ + H 2
(1)
is exothermic and occurs under thermal conditions for most third-row transition metals, while it is endothermic for the firstand second-row metals.10,12−14 Gas-phase metal cations are thus an excellent model system as they have the desired reactivity and can be studied in detail by experiment and theory. Armentrout and co-workers investigated the forward and reverse of reaction 1 as a function of collision energy for cobalt15 and nickel16 cations. They find that the reactions are © 2014 American Chemical Society
Received: January 18, 2014 Revised: April 12, 2014 Published: April 16, 2014 3253
dx.doi.org/10.1021/jp500617n | J. Phys. Chem. A 2014, 118, 3253−3265
The Journal of Physical Chemistry A
Article
Table 1. Experimental and Calculated 0 K Binding Energies (cm−1) of M+(CH4)n−1−(CH4) and M+(CH4)n(Ar)m−1−Ar (M = Co, Ni; n = 1−4; m = 1−2)a M = Co
M = Ni calcdf
exptl M −CH4 M+(CH4)−Ar M+(CH4)(Ar)−Ar M+(CH4)−CH4 M+(CH4)2−Ar M+(CH4)2−CH4 M+(CH4)3−CH4 +
B3LYP
7500 ± 500 , 8080 , 8010 ± 250 b
c
d
8100 ± 400b, 8850c, 8670 ± 280d 3300 ± 400b, 2550c, 3850d,e,
