Theory of InertGas FluoridesR. K. Nesbet Citation: The Journal of Chemical Physics 38, 1783 (1963); doi: 10.1063/1.1776953 View online: http://dx.doi.org/10.1063/1.1776953 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/38/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Growth Kinetics of InertGas Bubbles in Polycrystalline Solids J. Appl. Phys. 43, 2047 (1972); 10.1063/1.1661452 Field Adsorption of InertGas Atoms J. Chem. Phys. 55, 2884 (1971); 10.1063/1.1676510 Growth of InertGas Bubbles in Solids with Constant Rate of Gas Generation J. Appl. Phys. 40, 1986 (1969); 10.1063/1.1657888 Visible Spectra of Lithium in InertGas Matrices J. Chem. Phys. 47, 2905 (1967); 10.1063/1.1712314 InertGas Compounds Phys. Today 16, 94 (1963); 10.1063/1.3050838

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THE JOURNAL OF CHEMICAL PHYSICS

Letters to the Editor

T HE Leiters to the Editor section is subdivided into three parts entitled Communications, Comments and Errata, and Notes.

These three sections are all subject to the same limitations on length. The textual material of each Leiter is limited to a number of words equal to 950 minus the following: (a) 200 words for each average-sized figure,' (b) 50 words for each displayed equation; (c) 7 words for each line of table including headings and horizontal rulings . No proof will be sent to the authors of Communications. Proof will be sent to authors of Comments, Errata, and Notes. The publication charge for Letters is $40 per page, with a minimum of $40 per Letter. The publication charge, if honored, entitles the author's Institution to 100 reprints without covers at no extra charge. See the issue of December 1961 for a flttler description of Letters to the Editor.

Communications

Theory of Inert-Gas Fluorides

R. K. NESBET

IBM Research Laboratory, San Jose, California

(Received 16 January 1963)

RECENTLY, XeF2 and XeF4 have been found to be stable,t with XeF distances 2.00 and 1.93 A,

respectively.2 This note suggests that the principal binding mechanism is identical with the so-called "superexchange" mechanism in antiferromagnetic ox­ides such as MnO. The analysis of the superexchange interaction given in an earlier paper3 can be applied directly. There are two kinds of configurations that interact selectively with the singlet state of two other­wise uncoupled electronic spins, not interacting with the triplet state. The interaction is described by a Heisenberg exchange integral J such that, if S=!, 2J is the singlet-triplet splitting, equal to the binding energy of the singlet state if the triplet is assumed to be unbound (neglecting van der Waals interactions).

The theory is expressed in terms of localized ortho­normal transforms of Hartree-Fock molecular orbitals, asymptotically Hartree-Fock orbitals of the separated atoms. For a linear difluoride, consider orbitals a, a' to be singly occupied 2p orbitals on F and F', respec­tively, and b to be a doubly occupied p orbital on Xe.

VOLUME 38, NUMBER 7 1 APRIL 1963

The dominant contributions to the binding energy of XeF2, with respect to Xe+F2 are either, for AD>O,

or, for ~D<O,

BE=2(ba I ba')2[ -1/~DJ-~D-BE(F2)' (2)

where (3)

and ( 4)

Here ~D is the energy of configuration a2a'2 minus that of ab2a' in a point-charge electrostatic model. AE is the energy of a2b2 or b2a'2 minus that of ab2a'. II and 12 are, respectively, the first and second ionization potentials of Xe. A = 3.50 eV,4 the electron affinity of F. I F =17,42 eV,5 the ionization potential of F. BE(F2) is 1.68 eV.6 The XeF distance is d angstroms. (ba I ba') is the electrostatic energy of charge distribu­tion ba interacting with charge distribution ba'. Inte­grals containing a ( 1 ) a' ( 1) are neglected as in the Pariser-Parr theory.7 If the criterion given in an earlier paper is evaluated, it is found that the localized or­bital configuration ab2a' lies well below the traditional molecular orbital ground-state configuration (a+a') 2b2,

so that the weak interaction limitS is appropriate for parameters considered here.

Table I gives a survey of inert-gas difluorides, all assumed linear (FXF) since if b is a p orbital (ba I ba') is largest for the linear configuration. The XF distances d are assumed to be equal to the atomic radius9 of the halogen next to X, plus that of F, plus 0.10 A, to give the observed value for XeF2. Two values of (ba I ba') are used. They give just zero stability for XeF2 and KrF2, respectively. Although this integral is unknown, these numbers are of reasonable magnitude.

From the numbers derived in Table I, it can be con­cluded that, first, the present formalism gives a reason­able explanation of the stability of XeF2; second, sta­bility increases with nuclear charge, so RnF2 should be more stable than XeF2 ; third, it is unlikely that any of the lighter inert-gas fluorides are stable; fourth, the role of the unusually small binding energy of F2 is im­portant, so other halides may not be stable. A more de­tailed conclusion that follows from the theory is that when ~D is positive (neutral configuration lowest),

TABLE r. Parameters and computed binding energies relative to X + F2 of linear inert-gas difluorides FXF. Ionization potentials are from reference 5. Energies are in electron volts.

X d II 12 AD AK BE:(ba I ba' ) =0.883 1.22

Ne 1.10 A 21.56 41.07 9.81 7.37 -1.10 -0.57 Ar 1.60 15.76 27.62 4.88 9.42 -1.03 -0.44 Kr 1. 75 14.00 24.56 2.76 9.81 -0.80 0.00 Xe 2.00 12.13 21.2 1.13 10.32 0.00 1.54 Rn 2.20 10.75 [17.5?] -1.66 10.65 0.92 1.77

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1784 LETTERS TO THE EDITOR

XF2, XF4, XFo in linear, square planar, and octahedral configurations, respectively, will have stabilities with respect to X + F2, XF2+ F2, etc., that decrease gradu­ally. This decrease is due only to the perturbing ionic component of the wavefunction which brings in the rapidly increasing higher ionization potentials, since to second-order-of-perturbation theory (starting with the neutral configuration), the successive increments of binding energy are equal. If .1n is negative, binding energies for successive F2 additions decrease sharply, since the ionic configuration lies lowest. Thus RnF2 is expected to be stable, but RnF4 may not be. A stable radon fluoride has been observed. 1

The integral (ba I ba') is unknown, but since it is essentially a dipole-dipole interaction, it must vary roughly as d-3• Thus it is certainly different for the different inert gases. A more precise estimate can be made when dissociation energies are established ex­perimen tally.

I C. L. Chernick et at., Science 138, 136 (1962) ; H. H. Claassen, H. Selig, and J. G. Maim, J. Am. Chern. Soc. 84, 3593 (1962).

2 H. A. Levy and P. A. Agron, J. Am. Chern. Soc. (to be pub­lished); D. H. Templeton, A. Zalkin, J. D. Forrester, S. M. Williamson, J. Am. Chern. Soc. (to be published); J. A. Ibers and W. C. Hamilton (private communication).

3 R. K. Nesbet, Phys. Rev. 119, 658 (1960). 4 L. M. Branscomb, in A tomic and Molecular Processes, edited

by D.R. Bates (Academic Press Inc., New York, 1962); B. Edlen, J. Chern. Phys. 33, 98 (1960).

5 C. E. Moore, "Atomic Energy LevelS," National Bureau of Standards Circ. No. 457 (U. S. Department of Commerce, Washington, D. C., 1958).

6 R. P. Iczkowski and J. L. Margrave, J. Chern. Phys. 30, 403 (1959).

7 R. Pariser and R. G. Parr, J. Chern. Phys. 21, 466,767 (1953). 8 R. K. Nesbet, Phys. Rev. 122, 1497 (1961). 9 J. C. Slater, Quarterly Progress Rept., Solid State and Molec­

ular Theory Group, MIT, October 15, 1962, p. 6 (unpublished).

Chemi-Ionization in the Room-Temperature Reaction of Oxygen Atoms with Acetylene*

A. FONTIJN AND G. L. BAUGHMAN

AeroChem Research Laboratories, Inc., P. O. Box 12, Princeton, New Jersey

(Received 18 January 1963)

THE presence of ions in excess of their equilibrium concentration in hydrocarbon combustion flames

is well established. This phenomenon is attributed to chemi-ionization and has also been observed behind shock waves in acetylene-oxygen mixtures. 1 There is considerable agreement that an important ion-forming re3.ction in these high-temperature systems isl- 4

(1)

in which CH may be in an electronically excited state. Other reactions have also been suggested.2.5

To obtain a further understanding of this phenom­enon we have reacted 0 atoms with C2H2 at room tem­perature and also observe chemi-ionization, which has been found to be due to a reaction involving hydro­carbon free radicals.

One apparatus used in this study was a conventional flow system containing a 22-mm-i.d. Pyrex reaction tube in which oxygen atoms fed from one sidearm were mixed with acetylene molecules supplied from another. The 0 atoms were produced upstream in an "02-free" system by passing N2 through a 24S0-Mc 12S-W, micro­wave discharge, followed by titration of the resulting N atoms with NO.6.7 Total positive-ion concentrations along the tube were measured with either a single Lang­muir probe consisting of a S X 1O-2-cm-radius gold disk surrounded by a screen or a double probe consisting of two, 1.3-cm long, 6 X 1O-4-cm radius, parallel Pt-lO% Rh wires. At 0.7 mm Hg, average gas velocity 7 m seeI, and an initial concentration of both reactants of about 1014 molecule ceI, the maximum ion con­centration was estimated to be 109 to 1010 ions cel. In all experiments the acetylene flow was adjusted to give maximum ionization; larger flows decreased the ion current. The positive-ion current before acetylene addition8•9 was less than 1 % of that after addition.

In further experiments free-radical scavengers,1O-l2 NO or O2, were added through a movable nozzle just downstream from the mixing zone and the amount of scavenger required to produce a SO% and a 90% de­crease in ion concentration was measured. It was found that as a result of NO addition, after reaction times of approximately 10-2 sec, the ion concentration had de­creased at least 10 times as much as corresponded to the 0 atom decrease because of recombination by the 0+NO+M[k=6.3(±1.3) X 10-32 cc2 molecule-2 seeljI3 mechanism; upon O2 addition the decrease was at least 103 times larger than expected on the basis of 0 atom recombination via 0+02+ M[k::; 2.2 X 10-34 cc2 mole­cule-2 seelJ.13.14 These results demonstrate that free radicals produced initially in the reaction between 0 atoms and C2H2 11.15 are involved in the chemi-ionization reaction. The O2 concentration required to produce the same decrease in ion concentration was roughly 10 times the NO concentration required. With CH3 radi­cals under somenhat different conditions, it was ob­served that the reaction with O2 is slower than with NO.12 Further extension of the technique presented would allow measurement of the rate constants of the scavenging reactions. The fact that O2, calculated6 •l4 to be present at 1010 molecules cel due to recombina­tion, is not involved in the chemi-ionization reaction is indicated by the continuous decrease in ion concentra­tion observed upon O2 addition. The same decreases in ion concentration upon scavenger-gas addition were obtained when oxygen atoms produced by discharging 97%He-3%02 mixtures were used.

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