Krypton, electron density

The compound KrF2 is of great importance in the chemistry of krypton, and the nature of the binding is a challenge to the theoretical chemist. Early calculations at the minimal-basis-set level were reported by Collins et al,389>39<> using STO-3G expansions. Extra valence orbitals were added to the basis set. Electron-density plots showed the three-centre nature of the bond. 

Shown in Fig. 7.9 is the 2-3 spectrum of neon-like krypton (Kr26+), with satellites [36] previous observations can be found in [8,28,37,38]. This is a composite spectrum obtained by scanning in wavelength during a sequence of reproducible Alcator C discharges [36], with a peak electron density of... 

RbCl2, and RbBr2 gives a krypton atom in all cases, but small chemical isomer shifts can be detected which are the result of a change in the j-electron density due to orbital overlap [15]. 

Fig. 1 Electron density of Krypton given by Eqs. (14), (24), and (28) (ralid line), with the parameters obtained from the minimization of Eq. (56), compared to Hartree-Fock values [9]...

This would mean that there are 10 electrons in the valence shell of the Ng atom in xenon difluoride or krypton difluoride and 12 or 14 electrons for xenon tetrafluoride or hexafluoride, and even more for the octafluoroxenate ion, [XeFs] . Since one s and three p orbitals can accommodate only eight electrons, this would require the participation of d orbitals. In fact, the currently favored model uses only s and p atomic orbitals [16]. For example, XeF2 can be constmcted with a three-center-two-electron (3c-2e) bond, like NF5 (Chapter 4, Fig. 4.5) without using d orbitals (Fig. 5.1). Perhaps one should not worry much about which orbitals are involved, because as has been pointed out, bonding is not an observable quantity only bonding distancies and electron density are amenable to observation although... 

The density dependence of Vg in Kr was determined by field ionization of CH3I [62] and (0113)28 [63]. Whereas previous studies found a minimum in Vg at a density of 12 X 10 cm [66], the new study indicates that the minimum is at 14.4 x 10 cm (see Fig. 3). This is very close to the density of 14.1 x 10 cm at which the electron mobility reaches a maximum in krypton [67], a result that is consistent with the deformation potential model [68] which predicts the mobility maximum to occur at a density where Vg is a minimum. The use of (0113)28 permitted similar measurements of Vg in Xe because of its lower ionization potential. The results for Xe are also shown in Fig. 3 by the lower line. 

Table I. Experimental conditions for the Auger analyses Sputtering gas krypton, gas pressure 5.10 3 Torr (recycled gas) Ion beam energy 3 keV Ion current density 3 iA/cm2 Electron energy 9 keV Electron current density 90 mA/cm Analyzer (semi-dispersive) energy resolution 1.5 eV...

Sowada U, Warman JM, de Haas MP. (1982) The density dependence of hot-electron thermalization in liquid argon, krypton and xenon. Chem Phys Lett 90 239-241. 

In supercritical krypton the formation of excimers has also been time resolved but the results contrast with those for xenon.As in xenon, electron-ion recombination should occur rapidly. Again, electrons remain hot for many nanoseconds in krypton and the mobility of hot electrons is in the range of 150 to 400 cm /Vs. This leads to a theoretical range for of 1.4 to 3.6 x 10 m" s" at a density of 0.48 g/cm. In pulse radiolysis studies using optical detection, the concentration of intermediates is around 0.5 to 1 pM, thus the first half-life for recombination of electrons with ions is less than 10 ps in krypton. What has been observed is that an excimer species (A), the... 

Most surface area determinations are based on measurements of the low temperature adsorption of nitrogen or krypton on the solid and use of the BET theory. This procedure may not give reliable results because the products are chilled well below reaction temperature, possibly resulting in the sealing of internal pores. Volumes of gases adsorbed are sometimes small, as observed for dehydrated alums [37] and decomposed ammonium perchlorate [48], where the areas are consistent with product crystallites of linear dimensions between 1 and 3 pm. The results indicate, however, that little, if any, zeolitic material is formed [36]. The surface area of a solid may also be estimated from electron micrographs. Density measurements may be used to complement area measurements. 

Figure 15.5 shows the dependence of calculated cohesive energy on volume of the unit cell. Local density approximation of the exchange-correlation energy of electrons fails for these molecular systems. Minima at curves based on improved theory (Figure 15.5, small squares) correspond well to experimental quantities (diamonds) for argon and krypton (error of the order of 9%). For neon the error of the cohesive energy calculation equals to 39%. 

Reininger, R., Asaf, U., and Steinberger, I. T., The density dependence of the quasifree electron state in fluid xenon and krypton, Chem. Phys. Lett., 90, 287,1982. 

Jacobsen, F.M., Gee, N., and Freeman, G.R., 1986, Electron mobility in liquid krypton as functions of density, temperature, and electric field strength, Phys. Rev. A, 34 2329. 

In previous chapters, we saw that the volume of an atom is taken up primarily by its electrons (Chapter 2) occupying quantum-mechanical orbitals (Chapter 7). We also saw that these orbitals do not have a definite boundary but represent only a statistical probability distribution for where the electron is found. So how do we define the size of an atom One way to define atomic radii is to consider the distance between nonbonding atoms that are in direct contact. For example, krypton can be frozen into a solid in which the krypton atoms are touching each other but are not bonded together. The distance between the centers of adjacent krypton atoms—which can be determined from the solid s density—is then twice the radius of a krypton atom. An atomic radius determined in this way is called the nonbonding atomic radius or the van der Waals radius. The van der Waals radius represents the radius of an atom when it is not bonded to another atom. 

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