The relative energies of SF− 6 and SF6 as a function of geometryP. Jeffrey Hay Citation: The Journal of Chemical Physics 76, 502 (1982); doi: 10.1063/1.442696 View online: http://dx.doi.org/10.1063/1.442696 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/76/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Energy dependences of reactions between some atomic ions and SF6, and reevaluation of appearancepotentials (SF+ 5/SF6) and D(SF5–F) J. Chem. Phys. 87, 4615 (1987); 10.1063/1.452873 Optoacoustic effect in SF6 J. Acoust. Soc. Am. 77, S104 (1985); 10.1121/1.2022141 SF6 in a xenon matrix: Vibrational energy relaxation J. Chem. Phys. 75, 495 (1981); 10.1063/1.441849 Multiplephoton isotope separation in SF6: Effect of laser pulse shape and energy, pressure, and irradiationgeometry J. Chem. Phys. 67, 4545 (1977); 10.1063/1.434595 A vibrationalbath model for the dynamics of SF6 absorption near 10.4 μm as a function of wavelength andabsorbed energy J. Appl. Phys. 48, 4435 (1977); 10.1063/1.323472

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The relative energies of SFs and SF 6 as a function of geometry

P. Jeffrey Hay

Theoretical Division, Los Alamos National Laboratory, University of California, Los Alamos, New Mexico 87545 (Received 20 August 1981; accepted 24 September 1981)

The potential curves as a function of symmetric stretch have been computed for SF, and SF, at the SCF level. The calculated electron affinity of 0.90-1.00 eV and the photodetachment threshold of 3.0 eV appear to be in good agreement with existing experimental data.

Electron attachment to sulfur hexafluoride has been extensively studied1

-S since SFs is known to have a large

cross section for absorption of low energy electrons

e- +SFs - [sF~l*MSF~ .

At energies above 0.2 eV production of SF;; can also ensue2

e-+SFs-SF;+F.

Studiess~9 of the threshold for charge -exchange reactions such as

Cs +SFs - Cs+ +SF~ ,

have led to values for the adiabatic electron affinity of 0.5 to 0.7 eV for SFs, although higher10 and lowerS val­ues have also been reported, There appears to be little information on the geometry of SF~ or on the potential energy curves of SFs and SF;; in these processes.

This work was inspired by the intriguing results of Freiser and Beauchamp, 11 who studied the wavelength dependence of electron photodetachment from SF;;:

SF;; +hv- SFs +e- •

Despite the fact that the thermodynamic threshold for this process is only 0.6 eV, they found no detaChment below 370-380 nm (3.25-3.35 eV), but observed a monotonically increasing photodetachment cross sec­tion at shorter wavelengths (350-220 nm). In this work we report the results of SCF calculations on SFs and SF;; as a function of symmetric stretch coordinate. These curves should be reasonably accurate in the vicinity of the equilibrium geometries for each speCies. The re­gions of the potential surface corresponding to dissocia­tion to SF;; + F were not investigated here, since an ac­curate description would require both the addition of 3d polarization functions on the fluorines as well as the in­clusion of correlation effects, both of which are absent in the present treatment.

SC F calculations were carried out at the restricted Hartree-Fock level for the ground states of SF6 and SF;; having the following configurations in octahedral sym­metry

SF SeAl,.) : ••• (1t2,,)S (5t 1U )6 (1t2,.)6 ,

S F;;(2A 1,.): .•• (1t2.)6 (5t1.)6 (1t2,)6 (6ah,)!

(9s5p) and (l1s7p1d) primitive Gaussian basis sets12,13

were contracted to [3s2p] and [4s3p1d] on fluorine and sulfur, respectively, 14 using the general contraction scheme. 1S This basis set is identical to the set used in our previous study16 of SF6 and SFa. The addition of

diffuse functions 14 appropriate for S- or r was found to have only a small effect on S Fs and hence these functions were not included in the calculations. For example, the addition of a set of diffuse F 2p functions lowered the total energies of SF6 and SF~ by only 0.00601 and 0.01079 a.u., respectively, at 3.25 bohr, resulting in a lowering of 0.13 eV of SF~ relative to SF6.

The calculated potential energy curves of S F6 and SF;; along the symmetric stretch mode are shown in Fig. 1, from which the following features are apparent (see Table I). SF;; has a significantly longer bond length (cal­culated at 1. 710 A) compared to S F6 (calculated at 1. 567 A, experimental at 1. 564 A).17 The adiabatic electron affinity of SF6 is calculated to be 0.90 eV without zero­point corrections. The addition of diffuse p functions on F resulted in a slightly higher electron affinity (EA) of 1. 03 e V . The SC F values are thus in rather good agree­ment with the majority of the measurements in the range 0.5-0.7 eV and actually in slighly better agreement with a higher value recently reported of 1.1 ± O. 2 eV (see Table I).

The magnitude of the contribution to molecular elec­tron affinities from electron correlation, which has not been included here, is troublesome to estimate. In

> Q)

5

4

r 3 CI 0:: W

~ 2

o

1.4 1.5 1.6 1.7 1.8 1.9

R(S-FI,A

FIG. 1. Calculated potential energy curves of SF 6 and SFs as a function of symmetric stretch coordinate from SCF calcula­tions.

502 J. Chem. Phys. 76(1),1 Jan. 1982 0021·9606/82/010502-03$02.10 © 1982 American Institute of Physics

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P. Jeffrey Hay: Relative energies of SFs and SFs 503

:ases where the extra electron occupies a nonbonding Irbital (as in alkali halides), correlation corrections :an be negligible. 18,19 While one normally associates a . arger correlation energy with the negative ion having an ldditional electron, for UFa we observed a decrease in the calculated EA in going from SCF to modest CI calcu­lation. 2o This decrease was attributed to an "excluded volume" effect by the f orbitals in UF;;.

At the equilibrium geometry of SFs, however, the SF;; curve lies 1. 57 eV higher in energy but crosses below the SFa curve near 1. 61 A at a crossing point about 0.23 eV above the bottom of the neutral well. This curve crossing is also reflected in the orbital energy of the un­paired electron (see Table II), which is negative to the right of the crossing but is positive (unbound) for shorter distances. The only previous calculation on SF;; of which we are aware, an Xa scattered-wave approach2l yielded a vertical EA of + 0.7 e V as compared to our value of -1. 57 eV.

The fact that SFa attaches zero-energy electrons would indicate that the ground state vibrational energy distribu­tion of SFa must sample the crossing region. An exam­ination of processes leading to SF 5 + F - or SF 5- + F production, however, would require further calculations involving regions of potential energy surface along the asymmetriC stretch (V3) leading ultimately to dissociation.

The longer bond length in SF;; is consistent with the qualitative picture in which the electron in the negative ion occupies an antibonding 6all' orbital comprised of sul­fur 38 and fluorine 2p character. This reduced bonding

TABLE I. Calculated and experimental spectroscopic con­stants for SF s and SF;;.

Calculated (this work

SFs

Re (A) 1. 567

we (em-I) 844

SF~

Re (A) 1.710

We (em-I) 652

Relative energies (eV)b

Adiabatic electron 0.90 affinity 1. 03c

Vertical electron affinity - 1. 57

Vertical photode-tachment threshold 2.99

SF s - SFii crOSSing point 0.23 above SF s

aReference 17. ~uoted experimental quantities have reported uncertainties of 0.1 to 0.2 eV.

cCalculated value with added diffuse functions (see the text).

Experimental

1. 565a

769a

0.54, d O. 75,· O. 32f 0.46,' 1. 10h

dReference 6. ·Reference 7. fReference 8. ~eference 9. ~eference 10. iReference 11.

TABLE II. Total energies (in a.u.) of SFs and SF; as a function of S-F bond distance in the symmetric stretch mode. Also shown is the orbital energy of the singly-occupied orbital of SFii •

R (bohr) E (SFs) E (SF;) € (6al,)

2.75 - 993. 949431 - 993.693980 + O. 2077 2.85 - 994. 012 270 - 993.858297 + 0.1036 2.9555 - 994.031604 - 993. 970 357 +0.0112 3.05 - 994.019835 - 994.029151 - O. 0579 3.125 - 993. 996 574 - 994.054177 - 0.1047 3.25 - 993. 939363 - 994.064062 -0.1690 3.375 - 993. 868 040 - 994.045603 - O. 2188 3.50 -993.789712 - 994.008597 - O. 2571

character in the negative ion is also manifested by the lower symmetric stretch frequency (calculated at 652 em-I) for SF; vs SFa (calculated at 844 cm-I, experi­mental at 769 cm- l

). The 6all orbital in SF;; is also qual­itatively similar to the 6all orbital occupied in the low­est excited electronic states of SFs which produce the broad continua in the UV absorption spectrum of SFa• 22

Although there exists the possibility that SF;; may have a structure with lower than octahedral symmetry, the to­tally symmetric nature of the 2AlI state of SF;; does not require a Jahn-Teller distortion. Lower symmetries were not studied here.

The marked difference in geometries of SFa and SF;; leads directly to the explanation for the large threshold for photodetachment of SF;;, since neutral SFs lies 2.99 eV above the negative ion at the latter's equilibrium geometry. This is very close to the threshold (3.3 eV) observed by Freiser and Beauchamp.ll The photodetach­ment cross section as a function of energy should ex­hibit outgoing p-wave behavior, which is the only dipole­allowed channel for an electron initially occupying an 8-

like (all) orbital.

ACKNOWLEDGMENTS

We wish to thank Professor J. L. Beauchamp for sug­gesting this study and for providing unpublished experi­mentaldata. We also thank Dr. G. E. Streitforfurnishing results prior to publication and for helpful discussions.

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