Electron parity

6 Mass and Electron Parities, Closed-Shell Ions and Open-Shell Ions 6.6.1 Electron Parity 

Common molecules have an even number of electrons. Stable radicals are rare exceptions, such as NO. In classical chemistry, we most often meet active species that are ions with an even number of electrons, or radicals, an uncharged species with an odd number of electrons. In mass spectrometry, we observe ions with an even number of electrons, but we also often meet radical ions, a species uncommon in solution chemistry and having specific characteristics. 

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Thus, the chemical interconversion for equal electronic parity channels has four separated aspects i) activation via molding of reactants ii) population of TS rovibrational quantum states iii) population of reactants molded into configurations covered by the TS, and iv) relaxation towards products in their ground states. All such changes are submitted to energy and angular momentum conservation rules. 

The electronic quantum state ofthe pair H,ls> H+>= in> remains invariant at all distances. The electron transfer will not take place in a direct manner because the electronic parity is equal for both channels. The interconversion process requires aTS with parity -1. Among the states available to a system decomposable in one electron and two protons (or proton deuterium, etc) there are the hydrogen molecule ion species. The first electronic excited state (leu) ofthe molecular ion H2+ provides an "intermediate" (Q-state) for the interconversion once angular momentum conservation rules are fulfilled. The state (lau) is found above the in> and out> states leading to resonance in the cross section. This state may either relax to the (lrg) state yielding the hydrogen molecule ion and emitting a photon as this state is 2.8eV below dissociation, or it may take the product channels. This is a FC-like process. The reaction (27) is a prototype of electron transfer (ET). Thus, for any ET reaction whose in> and out> asymptotic electronic states share the same parity, the actual interconversion would require the mediation of a TS. 

Fig. 5.5. Magnetic field-induced e/f mixing between 1 and 1+, 0+ state levels with A J = 1. For an even isotope J possesses even integer values only, thus the levels shown with a dashed line are not realized , denote total parity, +, — (or e, /) denote electronic parity.

Abstract Understanding the origin of chirality in nature has been an active area of research since the time of Pasteur. In this chapter we examine one possible route by which this asymmetry could have arisen, namely chiral-specific chemistry induced by spin-polarized electrons. The various sources of spin-polarized electrons (parity violation, photoemission, and secondary processes) are discussed. Experiments aimed at exploring these interactions are reviewed starting with those based on the Vester-Ulbricht hypothesis through recent studies of spin polarized secondary electrons from a magnetic substrate. We will conclude with a discussion of possible new avenues of research that could impact this area. 

The nitrogen rule requires that the molecular mass is always even when the number of nitrogen atoms is even or zero. This results from the fact that nitrogen has a different mass parity and valence electrons parity mass 14 u, five peripheral electrons. Both of these parities are identical in the case of any other atom. It should be noted that this holds only if we consider the mass of the predominant isotope. Thus, the chemical mass of bromine is 80 u, an even number, but its predominant isotope is that of mass 79 u, an odd mass. In the same way, isotopically labelled compounds do not always obey this rule. 

An odd number of nitrogen atoms brings about an odd molecular mass in daltons such as is defined in mass spectrometry NH3 17, CH3NH2 31, and so on. Thus, in the case of an odd number of nitrogens, the earlier rule must be inverted for the ion, the mass parity is the same as the electron parity. 

McLafferty rearrangement through a six-atom ring intermediate. Figure 6.11 shows another example and details the rules linking the mass and electron parities. 

The QTS is a part of a reaction mechanism. These species may be found related to stationary arrangements of the external Coulomb sources. Solutions to eq.(8) coming as saddle points of at least index one (one imaginary frequency) are natural candidates to play the role of QTS. If the saddle point wave function has closed electronic shell structure, its electronic parity is positive. In this case one would expect a situation similar to the symmetry-forbidden electronic absorption bands. The intensity is borrowed from the excited states having the correct parity via couplings at second-order perturbation theory [21]. 

Let us examine a simple model for chemical reactions now. The first two states define asymptotic reactants (R1+R2, represented as llk>) and products (P1+P2, or 12k >) as collision pairs. In real low energy chemistry the reactant and product channels have the same electronic parity. In particular, this is the case when partners in both systems have closed shell electronic structures. The... 

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