Unimolecular Reactivity of the [Urea-Sr]²⁺Complex, a Metastable Dication in the Gas Phase: An Experimental and Theoretical Perspective

Abstract

The interactions between urea and Sr(2+) in the gas phase have been investigated by combining electrospray ionization/mass spectrometry techniques and density functional and high-level ab initio molecular orbital calculations. Our theoretical survey indicates that [Sr(urea)](2+) adducts are thermodynamically stable with respect to direct Coulomb explosions. However, after isomerization, some of the local minima of the PES are thermodynamically unstable with respect to the formation of NH(4)(+), but kinetically metastable. The loss of neutral fragments with the concomitant generation of lighter doubly charged fragment ions, namely, [(H(3)N)Sr](2+) and [(HNCO)]Sr(2+), compete with the aforementioned Coulomb explosion processes yielding NH(4)(+) + [(NCO)Sr](+) and [(H(2)N)Sr](+) + [H(2)NCO](+), although the former processes dominate. Hence, both singly and doubly charged species are detected as dissociation products. Quite importantly, the observed eliminations of NH(3) or HNCO lead to the formation of new doubly charged species, which turn out to be thermodynamically stable.

Electron and ion spectroscopy of the cyclo-alanine–alanine dipeptide

“…The release of neutral carbon monoxide (CO) and isocyanic acid (HNCO), 79 have been also observed in the fragmentation of urea, 80,81 uracil, 82-84 thymine 85 and larger samples like flavins. 86 Together with the theoretically predicted rearrangement of the molecular ion into a 4-imidazolidinone structure, 35 these represent particularly relevant channels: in addition to the already discussed path leading to the release of charged fragments in the so-called 'amino acid-like' structure, [A-H-OH] + , with a peculiar predicted reactivity, 19 these neutral and charged fragments are crucial in astrochemistry, confirming how cyclo dipeptides may have played a role as prebiotic molecules.…”

A RRKM study and a DFT assessment on gas-phase fragmentation of formamide–M2+ (M = Ca, Sr)

“…However, [Sr(NH 2 )] + has been detected in similar experiments with the urea-Sr 2+ system. 27 Further investigations are needed in order to clarify this point. It should also be taken into account that RRKM is a statistical theory that can only describe reactive mechanisms in which the energy flows through the ion's vibrational modes because it assumes that dissociation takes place after complete intramolecular vibrational energy redistribution (IVR).…”

“…[23][24][25][26] However, this is not the case with alkaline-earth dications such as Ca 2+ and Sr 2+ , since [CaB] 2+ and [SrB] 2+ are stable and detectable in the gas phase. 27 This opened the possibility of studying for the first time the unimolecular reactivity of complexes formed by these metal dications with a Departamento de Quı ´mica, Facultad de Ciencias, Mo ´dulo 13. Universidad different neutral bases such as urea, glycine, thiourea, selenourea, uracil and its thio derivatives.…”

Can Supported Reduced Vanadium Oxides form H2 from CH3OH? A Computational Gas-Phase Mechanistic Study

“…However, CH 3 OH has been widely studied and utilized as a smart chemical probe molecule to establish the fundamental structure–activity relationships for metal oxide catalysts. − Despite its vast use, there is still a lack of understanding on the nature of the reaction mechanisms involved in the catalytic transformations, due mainly to the difficulty for obtaining representative model systems on which to perform detailed chemical and kinetic analyses. In this context, extensive research efforts have been made both experimentally and theoretically to search for suitable catalysts to improve chemical processes concerning alcohol transformations; , likewise, gas-phase experiments combined with theoretical works − have provided a wealth of information of elementary steps and intermediates for various catalytic reactions, − and they provide ideal model systems for local reactive sites of condensed-phase catalysts. Therefore, small transition-metal oxides clusters may well be effective models of real surface species or active sites over the bulk transition-metal oxide materials, which are capable of providing a conceptual framework for modifying or controlling the chemical properties of mixed-metal oxides and for engineering industrial catalysts.…”