Is Spin Conserved in Heavy Metal Systems? Experimental and Theoretical Studies of the Reaction of Re⁺with Methane

Abstract

A guided ion beam tandem mass spectrometer is used to study the reactions of atomic 187 Re + with CH 4 and CD 4 and collision-induced dissociation (CID) of ReCH 4 + with Xe. These studies examine the activation of methane by Re + in a low-pressure environment free of ligand supports or other reactive species. In the bimolecular reaction, ReCH 2 + is efficiently produced in a slightly endothermic process and is the only ionic product observed at low energies, whereas at higher energies, ReH + dominates the product spectrum. Other products observed include ReC + , ReCH + , and ReCH 3 + . Modeling of these endothermic reactions yields 0 K bond dissociation energies in eV of D 0 (Re + -C) ) 5.12 ( 0.04, D 0 (Re + -CH) ) 5.84 ( 0.06, D 0 (Re + -CH 2 ) ) 4.14 ( 0.06, D 0 (Re + -CH 3 ) ) 2.22 ( 0.13. Analysis of the behavior of the cross sections suggests that formation of ReH + , ReCH 2 + , and ReCH 3 + occurs via an H-Re + -CH 3 intermediate. CID of ReCH 4 + reveals a bond energy of 0.53 ( 0.15 eV for Re + -CH 4 . The experimental bond energies compare favorably with theoretical calculations at the B3LYP/HW+/6-311++G(3df,3p) level with the exception of the singly bonded species (ReH + , ReCH 3 + ), where the Becke-half-and-half-LYP functional performs much better. Theoretical calculations also elucidate the reaction pathways for each product and provide their electronic structures. Overall we find that the dehydrogenation reaction, which occurs with an efficiency of 86 ( 10%, must involve three facile spin changes (2s + 1 ) 7 f 5 f 3 f 5) indicating that little hint of spin conservation remains in this heavy-metal system. † Part of the special issue "Tomas Baer Festschrift".

The most reactive third-row transition metal: Guided ion beam and theoretical studies of the activation of methane by Ir+

“…1). This is typical behavior for the reaction of bare metal ions with hydrogen-containing polyatomic molecules [4,9,16,[18][19][20][21]. The observation that the IrH + + CH 3 (IrD + + CD 3 ) channel dominates the nearly isoenergetic IrCH 3 + + H (IrCD 3 + + D) channel (Table 1) is largely a result of angular momentum constraints [9][10][11]18].…”

“…Formation of IrD 2 + has the highest threshold of all processes observed, which is consistent with formation of the high-energy CD 2 neutral. The formation of MD 2 + product ions is unusual and has not been detected for reaction of methane with first-row [8][9][10][12][13][14][15]18], second-row [7,11,[16][17][18], and several thirdrow (La + , W + , Re + , Lu + ) transition metal ions [18,20,21]. However, this product has been observed in reactions of methane with Pt + [19].…”

“…The B3LYP functional was used for all the calculations done here because it provides reasonable results for the analogous Pt + , W + , and Re + with CH 4 systems [19][20][21]. In all cases, the thermochemistry reported here is corrected for zero-point energy effects using unscaled frequencies.…”

“…This type of mechanism is also available for both W + ( 6 D) and Re + ( 7 S) reacting with methane, where multiple spin changes are also required [20,21].…”

“…Such ion beam studies also provide insight into the electronic requirements for the activation of methane by transition metal ions and periodic trends in this reactivity. We have applied this methodology to all the first-row and second-row transition metals except Tc [3,[7][8][9][10][11][12][13][14][15][16][17][18] and more recently to several of the third-row metals as well [18][19][20][21]. A significant advantage of these comprehensive studies is the comparison of bond energies and reactivities of all transition-metal complexes, thereby establishing periodic trends.…”