Dynamical and stereodynamical studies of alkaline-earth atom–molecule reactions

Abstract

Alkaline-earth atom molecule reactions have been extensively investigated since the early days of reaction dynamics. Though these reactions have many similarities to those of corresponding alkali atoms, as one would expect from their low ionization potential, there are important differences due to the presence of the two valence electrons in the alkaline-earth atom. This feature confers a distinct dynamical or stereodynamical behaviour whose basic description and analysis is the main objective of this article. The adopted approach is phenomenological based on the huge body of data revealed by modern crossed-beam and laser pump and probe techniques with no intention to be comprehensive but rather to put into context both authors' work. The paper starts with a resume´of the experimental approaches followed by a visit to full-collision studies under gas cell, beam-gas and crossed-beam arrangements. Detailed analysis and discussion is given of the M þ H 2 (M ¼ Ca, Mg) ! MH þ H reaction family whose dynamics is discussed using their Potential Energy Surfaces. In these studies care was taken to discuss the most relevant features of the molecular reaction dynamics with emphasis on the energy selectivity and on the stereodynamical (vectorial) nature of the reaction. To this end the available data are discussed in light of topological features of the underlying potential energy surface favouring either insertion or abstraction mechanisms. Particular attention is paid to the discussion of alkaline-earth atom reactions with halogen containing molecules. In these reactions the presence of two valence electrons leads to reaction channels, as chemiluminescent or chemi-ionization reactions, which are specifically associated with the transfer of the more energetic (inner harpooning) electron, whose dynamics or stereodynamics pattern differs from those of reaction channels associated to the transfer of the first, less energetic electron as, for example, in ion-pair reactions. A full section of the paper is dedicated to photo-induced intracluster reactions and, in particular, to the transition state dynamics of the prototype Ba . . . FCH 3 þ h ! BaF þ CH 3 reaction investigated in great detail by both nanosecond and femtosecond photo-ion and photo-electron pump and probe techniques. For this system the combination of these ultrahigh resolution techniques with isotopic labelling and ab initio calculations allowed the deciphering of its ultrafast reaction mechanism. Finally, the paper ends with some concluding remarks based on the observed propensity rules or general trends found in these alkaline-earth atom reactions.

Can a gas phase contact ion pair containing a hydrocarbon carbocation be formed in the ground state?

“…The importance of the orientational effect for the harpoon mechanism has been recognized in several systems. [86][87][88][89][90][91][92] Clearly, the occurrence of H bonds is very important for the stability of HBCIP. These bonds can be interpreted as a "lock" that holds the opposite charged ions relatively close to each other (as can be seen in Fig.…”

Rotational and vibrational state distributions of NaH in the reactions of Na(4S2,3D2,and6S2) with H2: Insertion versus harpoon-type mechanisms

“…In contrast, the low rotational component is subject to a weaker anisotropic interaction; the leaving-H atom departs along the Mg-H stretching coordinate at a rate faster than that for the change of the HMgH angle. 42,43 C. Reaction pathway for the Na"6 2 S… +H 2 reaction Different from the bimodal feature obtained above, the Na͑6 2 S͒ plus H 2 reaction yields a rotational state distribution of NaH ͑vЉ =0͒ peaking at J = ϳ 10, slightly hotter than the oven temperature at 670 K ͓Fig. 3͑b͔͒.…”

“…In the latter reaction, MgH is produced similarly via a nonadiabatic transition. [40][41][42][43] The collision complex MgH 2 is exerted by two forces after going through the surface crossing. When the bent intermediate is affected by strong anisotropy of the ground state potential, it may decompose via a linear HMgH geometry.…”

Chemiluminescence from the Ba(P3)+N2O→BaO(A Σ1+)+N2 reaction: Collision energy effects on the product rotational alignment and energy release

“…For each laser ablation fluence reported, the experimental values probing the 3 P 1 substate alone are given in the left column, while values ͑in italics͒ in the right column are rough estimates ͑see Ref. 8 for details͒ of the overall compositions including the relative contribution of the 3 P 0,2 states, as computed by assuming 2J + 1 degeneracies at the ablation volume and properly accounting for Ba͑ 3 …”

“…[1][2][3] An important dynamical aspect of these electron-transfer reactions concerns the role that internal motion of the molecular partner plays in determining the reaction outcome, especially when oxidant molecules having a small positive or a negative electron affinity are involved. In such circumstances, the electron transfer may be prevented from occurring instantly but it would first require either the stretch of a bond or the bend of an angle connecting to a halogen or an oxygen atom of the molecule to increase its electron affinity to the extent that the charge transfer can proceed.…”