Reactions of ground-state alkaline earth atoms

3 Reactions of ground-state alkaline earth atoms 

The reactions producing alkaline earth halides are comprehensively reviewed by Herm [216]. Table 3 summarises measurements of energy disposal in these reactions. 

Reactions of ground-state alkaline earth metals (see Appendix 1)  

Reaction Reagent preparation System Product detection 

Studies [327] of the effect of reagent translational energy in the range 8—50 kJ mole 1 on the reactions 

Varying the initial translational energy of the reagents for Ba + HF in the range 13—54 kJmole shows [51] that the reaction cross section has a low threshold (4 kJ mole ) and increases to a maximum at 30 kJ mole . The variation of the fractional energy disposal is shown in Fig. 

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Sr -t-HCl, extremely narrow vibrational distribution. It appears that a much higher fraction of the reaction energy appears as product vibration than is the case for the reactions of ground state alkaline earth atoms with HX. In the case of the excited atom, reaction takes place on the triplet rather than the singlet surface correlating with ground state products. The transition from covalent reagents... 

The gas-phase chemistry of ground-state transition metals is much less well understood and has attracted far fewer studies than the reaction dynamics of alkali and alkaline earth metal atoms. Three reasons explain this situation. The first is expert-... 

One of the surprising aspects of the chemistry of alkaline earth atoms is that ground-state atoms can react readily in the gas phase with molecules such as H20 and CH3OH. In solution, the overall reaction... 

There have been several studies of the reaction dynamics of the ground and excited states of the alkaline earth atoms with various oxygen-containing molecules under single collision conditions. Although these studies are not directly applicable to the multiple collision regime in the Broida oven, they clarify the dynamics of a single encounter between a metal atom and an oxidant molecule. Oberlander and Parson [43] looked at the reactions of Ca and Sr with water, alcohols, and peroxides. Similar studies... 

In our laboratory, a series of studies of spin-orbit effects in reactions of alkaline earth atoms has been carried out [36-42]. Optical pumping state selection has been employed in order to determine the relative reactivity of the individual spin-orbit levels of metastable Ca( P ), Sr( pO), and Ba( D) atoms. For most of the reactions studied, the products can be formed in the ground electronic state,... 

Many of these vapours will break down spontaneously to atoms in the flame. Others, particularly diatomic species such as metal monoxides (e g. alkaline earth and rare earth oxides), are more refractory. Monohydroxides which can form in the flame can also give problems. The high temperature and enthalpy of the flame aid dissociation thermodynamically, as does a reducing environment. The role of flame chemistry is also important. Atoms, both ground state and excited, may be produced by radical reactions in the primary reaction zone. If we take the simplest flame (a hydrogen-oxygen flame), some possible reactions are the following ... 

Dramatic effects of electronic excitation on the reaction mechanisms have been demonstrated in several cases. One of the first reported examples must be recalled here also as it falls outside the scope of this chapter. Electronically excited 0( D) is much more reactive than ground-state 0( P) and inserts into the C-H bonds of methane [162]. Similar state specificity in the reactivity has also been encountered in electron-transfer reactions and seems to be the rule in light systems. Its origin has been explored systematically in alkali and alkaline earth metal atom reactions. Before discussing some of the studies, it is appropriate to survey a much simpler situation where electronic excitation affects the dynamics of the reaction just by changing the location of the electron-transfer region. 

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