Reactive alkaline earth atoms

Both alkali metal and alkaline earth atoms are reactive with oxygen-bearing terminal groups but the latter appear to be far more destructive. K deposition on -COOCH3 and -COOH terminated SAMs results in a 1 1 reaction to form -COO species, which in the ester case requires cleavage of the -CH3 group [45, 46, 58, 59, 61]. Walker and co-workers have reported that Ca deposition on OCH3... 

Angular distribution measurements of reactive scattering of alkaline earth atoms A by halogen molecules X2 have been reported80 for Ba + Cl2 and more extensively81-82 for Ba, Sr, Ca, Mg with Cl2, Br2, IC1, BrCN. The reactions form monohalide products AX, thus following primarily the reaction path... 

Although we cannot directly detect HCaOH because it probably has a dissociative UV spectrum [14], we can detect another predicted reaction intermediate in some of our experiments. Mechanism A predicts that the CaH molecule will be present in the Broida oven, and with some oxidants we have detected it by laser-induced fluorescence. The CaH molecule is seen when carboxylic acids such as formic acid are used to make the monocar-boxylates such as Sr02CH [42]. Curiously, CaH is not detected [41] when water or alcohols such as CH3OH are used to make alkoxides such as CaOCH3. More experimental and theoretical work is necessary to establish the chemical mechanisms involved in the reactivity of the alkaline earth atoms. 

Restructured programs have been used for a systematic investigation of the reactive properties of some atom-diatom systems for which accurate ab initio PESs are available. Some illustrative results obtained for the alkali/alkaline earth atoms -f hydrogen halide family of reactions are reported in this paper. In addition to reaction Rl the following systems are considered... 

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,... 

Alexander [126] has taken a somewhat different approach to model in a fully quantum mechanical fashion spin-orbit effects in reactions of alkaline earth atoms with halogen molecules. Here the reactive encounter is simulated by a one-dimensional atom-atom collision in which the diabatic covalent curves correlating... 

Our theory regarding atomic size and reactivity holds true for the alkaline earth metals. As we move down a group on the periodic table, as the atomic size increases, the chemical reactivity increases. Calcium is more reactive than beryllium and magnesium. Neither the alkali metals nor the alkaline earth metals would be good candidates for jewelry making. We would not want to wear metal jewelry that might react violently to oxygen or water vapor in the air. 

The alkali metals are the most chemically active metals. Alkaline earth metals are second in reactivity. Elements in the same family have the same outermost electron configuration. Alkali metals have one outermost electron that is easily transferred to needy atoms. Alkaline earth metals have two outermost electrons to share or transfer. The transition metals are the least active. 

The inclusion of impurity atoms in MgO is much more interesting from a chemical point of view when alkali metals are used to replace Mg ions. In fact, this results in trapped-hole centers. The MVO pairs have been extensively studied in the bulk of alkaline-earth oxides by optical studies, EPR and ENDOR measurements [185,186] as well as by embedded cluster calculations [187]. The LiVO ions create an effective dipole which polarizes the surrounding lattice, with the two ions moving toward each other. The presence of an O radical, however, is most interesting when one is dealing with surface properties. This center in fact is very reactive and is the subject of the next paragraph. 

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. 

Measuring the polarization of the reaction product is also an important issue in stereodynamics. A lot of the activity in this field concerns the reactivity of alkaline earth metal atoms since the corresponding reaction products are easy to probe by optical techniques. A full account of methods to measure product alignment and orientation in bimolecular collisions has been given by Orr-Ewing and Zare [18]. Such measurements, with the help of simple models such as the DIPR-DIP model considered Section 2.3.2, give insight into the shape of the reactive system at the moment where forces are released [86, 87, 184, 195, 233, 234]. 

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