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

Abstract

A kinetic study of the unimolecular reactivity of formamide-M(2+) (M = Ca, Sr) systems was carried out by means of RRKM statistical theory using high-level DFT. The results predict M(2+), [M(NH2)](+) and [HCO](+) as the main products, together with an intermediate that could eventually evolve to produce [M(NH3)](2+) and CO, for high values of internal energy. In this framework, we also evaluated the influence of the external rotational energy on the reaction rate constants. In order to find a method to perform reliable electronic structure calculations for formamide-M(2+) (M = Ca, Sr) at a relatively low computational cost, an assessment of different methods was performed. In the first assessment twenty-one functionals, belonging to different DFT categories, and an MP2 wave function method using a small basis set were evaluated. CCSD(T)/cc-pWCVTZ single point calculations were used as reference. A second assessment has been performed on geometries and energies. We found BLYP/6-31G(d) and G96LYP/6-31+G(d,p) as the best performing methods, for formamide-Ca(2+) and formamide-Sr(2+), respectively. Furthermore, a detailed assessment was done on RRKM reactivity and G96LYP/6-31G(d) provided results in agreement with higher level calculations. The combination of geometrical, energetics and kinetics (RRKM) criteria to evaluate DFT functionals is rather unusual and provides an original assessment procedure. Overall, we suggest using G96LYP as the best performing functional with a small basis set for both systems.

Solvation effects drive the selectivity in Diels–Alder reaction under hyperbaric conditions

“…A detailed discussion for this choice is reported in the Supporting Informations (SI, Section S1). Note that it is not uncommon that a small basis set can provide results better than a larger one: 31 convergence in basis set sometimes occur only for a very large one, which would be then inadequate to perform QM/MM MDs.…”

Gas-phase reactivity of [Ca(formamide)] 2+ complex: an example of different dynamical behaviours

“…A detailed analysis of the PES at B3LYP/cc-pWCVTZ level of theory allowed one to establish the mechanisms behind the aforementioned fragmentation process [15,16]. [Ca(formamide)] 2+ complexes undergo a charge separation reaction yielding [Ca(NH 2 )] + + HCO + .…”

“…For [Ca(formamide)] 2+ electronic structure theory, we used G96LYP and BLYP functionals with the 6-31G(d) basis set. We previously assessed the level of theory in order to better reproduce CCSD(T) barriers [16].…”

“…This simple fact does not allow one to explain the most obvious experimental outcome clearly showing that the dominant reactive channel is that associated precisely with loss of formamide, yielding Ca 2+ . This apparent contradiction between the topology of the PES and experimental evidence moved us to re-explore the unimolecular reactivity of [Ca(formamide)] 2+ ion through a deep investigation of its dynamics on the reactive PES in terms of kinetics [16] and dynamics approaches [17,18]. In this paper, we will review these studies.…”

Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces

“…They give an overview of their recent work [50][51][52] in which they show that, to correctly understand collision-induced dissociation obtained in mass spectrometry experiments, it is necessary to go beyond the description of dissociation pathways only by minima and TSs. In particular, two non-statistical effects are reported: (i) the so-called shattering mechanism dealing with high-energy fragments, observed experimentally and which cannot be deduced only from the inspection of the PES, and (ii) the dynamics after the TS which do not follow the minimum energy path, such that a single TS leads to more than one mechanism.…”