A study of the orbital electronic structure of the P4 molecule by photoelectron spectroscopy

Iniern~t~ohd Journd of Mars Spectromet&and

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Elsevier ptiblisbing Company, Amst&dam;-Prjnted

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A STUDY 6F THE ORBITAL ELECTRONIC STRUCTIJPA MOLECULE BY PHOTOELECTRON SPECTROSCOPY

OF THE% ..

S. EVANS; I’. J. JOACHIM AND A. F. ORCHARD Inorganic Chemistry Laboratory, South Parks Road, Oxford (Englcnci) D. W. TURNER Physical Chemistry L&oratory,

South Parks Road, Oxford (England)

(Received July 2nd, 1971)

ABSTRACT

The one-electron structure of the valence shell of the P4 molecule has been investigated by means of helium(I) photoelectron spectroscopy. The inferred orbital structure is in accord with the simplest expectations. Very substantiai Jahn-Teller distortions are evkknt in the orbitally degenerate states of the molecular ion. Analogy with the symmetrical trimer, cyclopropane, casts doubt on the previous assignment of the cyclopropane photoelectron spectrum.

INTRODUCTION

There has recently been a revival of interest in the properties of the phocphorus tetramer, P4’ - 4. This prompts us to report here the vapour phase hehi? (I) photoelectron spectrum of the P4 molecule, which exhibits a number of interesting features.

Comketial samples of white phosphorus were pur5e.d by vacuum distilla: tion (lo- 3 ton) to prodWe colourless kystals which slowly -turneti.yellow .on exposure to light.. The helium(I) pho?Gelect.rGn.(PE) speck vksmeasured on -. two difIemnt instruments, one ik which incorporatks a high _reSc&ttiOn 127” eleckrp&ati~ analysers; the other- a similar but knaller analyser’ c&b&d &th a .-variable retarding 6eld6.- -~.; ..- .. : I _ ___._ -1.:; _./.. : ... .. -.

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1.

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-41 _. .: _::_~_:-$~,:~_.. .:./ :.:--: : ._ -_ ~-~:-.~ -. __-__;_..-. .~ _,~ ;: ..._. _- _. -. -. -__. .-_c. _. I:_

Fig. 1. The helium(I) photoelectron spectrum of the phosphorus P,

C

8 12

speck*.

IA

I

I

1

17

70

9

W

Fig. 2. Part of the helium(I) photoeIectron spectrum of P4 measured on the spectrometer described in ref. 5.

RESULTS

No major differences were observed between the two sets of PE spectra. The relative intensities of the Srst four bands were similar (Figs. 1 and 2) in spite of the different geometrical characteristics of the electron analysers. However, a very weak and broad band centred at about 18.5 eV that was apparent in the spectra7 obtained on the first spectrometer5 has no counterpart in the spectrum reproduced in Fig. 1* which was measured more recently on the medium-resolution instrument. The background signal in this spectrum continues to increase quite steadily between 17.5 and20.5 eV without any significant superimposed structure. The 18.5 eV band observed in the earher work is almost certainly spurious. * The vertical axis in Fig. 1 (and also in Figs. 2 and 5) refers tdelectron flux (counts per second), ionisation energy being plotted on the horizontal axis. The maximum of the most intense fE band (0 in Fig. 1 corrkponds’to ca. 100 c/s, whi!e the resolution was ca. 40 meV throughout the spectrum. 42

Int. 3. Mass

Specrrom.

Ion Phys.,

9 (1972)

The P4 photoeiectron spectrum exhibits six well resolved band envelopes, designated A-F in Fig:. 1. A vibrational progression in a normal mode-of frequency 508 & 10 cm-’ is clearly discernible in band D (see inset to Fig. 1): while band C shows an incipient splitting of ca. 0.14 eV together with a barely perceptible inflection to higher ionisation energy. Since the weak bands, E and F, are situated on a rising background (possibly due in part to electron scatter in the spectrometer) we include in Fig. 1 the normalised contour (N) for these bands. The ionisation energies and relative band intensity data are collected in Table 1. The onset of the first ionisation of P4 occurs at 9.10+ 0.05 eV, in excellent agreement with the photoionisation threshold of 9.08 2 0.05 eV measured by Watanabe**‘. TABLE

1

PHOTOIONISATION

PE band

DATA

FOR THE Ph MOLECIXE

Vrrticui IE (eV)*

A B

9.54 9.90

Relarive band areas**

Assignment

7

6fti

6

2et

1

Sal

0.8 0.8

-2

10.40 C

10.54 (10.74) 11.79 1 l-85**+

D

11.91 (11.97) (12.03)

E

15.39

F

16.m

5tz

* iO.05 eV. Figures in parentheses relate to sho$ders. ** ADproximate estimates only since the bands & B and C overlap and bands E a&F relative to the background. *** Most intense element in a vlbrational progression with a spacing of 63 & 1 meV. t The assignment of 6t, and 2e is uncertain (see text).

are weak

-c_. _-

DiSCUSSlON

The assignment of the diifferent bands in the P1 photoelc+tron spectrum to particular states of the molecular ion - or, vi2 Koopmans’ approximation1 O, to distinct molecular orbitals (MO's) of P4 - is not entirely straightforward. We choose to describe the LCAO-MO structure of the tetrahedral” P4 molecule in terms of the 3p and 3s valence atomic orbitals (Ao's),distinguishingp, orbitais directed towards the centroid of the tetrahidron from the perpendicularly oriented p, orbitals. We have argued elsewhere’ 2, essentially from overlap considerations, that the relative energies of the tetrahedral symmetq.orbitals (2”’ point group) should be 5 P.3: t2 > a,

p= :t, Int. J- Mks

>ea

Spectrom.

f2_ Ion Phys.,

9 (1972)

43

Fig. 3. A qua!itative molecular orbital energy diagram for tetrahedral Pd. (Occupied levels are ringed.)

This type of argument proved very helpful in the interpretation of the PE spectra of symmetrical tetrahedral species12*13_ One concludes immediately that the orbital electronic structure of P4 must be basically of the form given in Fig. 3 that is - - - (dc$)* (5t,)6(5a,)2(2e)4(6r2)6. where the relative energies of the 6t2,2e and- 5~~ molecular orbit& are uncertain a priori. The 6f,- 2e separation will be determined by two effects: p,-pn mixing (in the t2 representation) will tend to impose the energy sequence 2e > 6f2, while s-p, interactions (also in the t2 representation) will destabilise the 6t2 orbital relative to 2. The position of 5a, will depend on both the difference between the individualp, andp, Fock integrals and also on the strength of the s-p= interactions (in the a1 representation). Ho Fever. as regards the MO’S of predominantly 3s character, we can be reasonably sure l2 that the 9, orbital will have a less negative energy than the 4a, orbital. There are more ban.ds in the photoelectron spectrum in Fig. 1 than can be accounted for by applyitig Koopmans’ approximation” to the energy level diagram in Fig. 3. Multiplet fine structure is unlikely to be resolved* given the magnitude of the spin-orbit coupling constant fez the phosphorus 3p shell, JSp < 45 meV14. The additional detail in the P4 photoelectron spectrum must therefore be effect) in the orbita!Iy degenerate ‘E due to vibronic interactions l5 ( J&n-Teller and ‘T2 states of the molecular ion. Group-theoretical considerations show that the 2E state is susceptitile to vibronic distortion through the e normal modes, while the ‘T2 states can distort through vibrations of both e and f2 symmetry. If a single normal vibration (e or t2) happens to be vibronically dominant then the ‘T2 states are expected to distort asymmetric&) to doubly degenerate (E) and non-degener: ate (+I> Jahn-TAl.er cotigurations. * ti arty case only -the 2Tr states of the:P4 molecular icn are ksceptiblh to multiplet splitting.. gko that no mul’riplet structure is-evident in the PE sp&ctra-of.CCL and SiCL “.

Note

-41

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..

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./:_ -I

Int. 3. kfqS;r~SpeCtrOn. Ion P&s., 9_(1972) ..-. ._

A further point should be borne in mind at this-stage. Considering molecular orbitals of similar localisation properties (in particular, similar Ati composition). one anticipates .&at the relative ionisation cross-sections will approximately reflect tbe orbital degeneracies16-18. Now the 6t,, 2e and 5a, MO% ze expected- to be composed mainly of phosphoms 3p orbitals so that baud D, withi& rather small integrated intensity,. may be assigned with some confidence to~ionisation of the 5a, (non-degenerate) orbital. This interpretation is supported by the observed vibrational progression of frequency 508 cm- ‘, which must be the totally symmetric stretching mode, at (ca. 600 cm- ’ in the ground state of neutral Pa ‘* ’ ‘)*_ It is of course the a, vibration that one expects to be most strongly excited on ionisation of a non-degenerate orbital since the 2A1 state of the molecular ion is vibronically stable. The decrease in the a, frequency observed in the molecular ion is indicative of a somewhat bonding molecular orbitall6 despite the rather narrow width (ca. 2OOmeV at half-height) of the photoelectron band. The band D is thus reminiscent of the 2a, band in the helium(II) photoelectron spectrum of methane reported by Potts et al.“: a vibrational series was detected with a spacing of ca. 2000 cm- ’ as compared with an a, frequency of 29 14 cm- ’ in the neutral molecule. n

Fig. 4. The LCAO form of the a1 &‘inner 0” molecular orbitals. (The 5aX linear combination of ul (p,) and a1 cs).)

MO

of P., is essentially a

The relatively low integrated intensity of band D may be correlated with the totally symmetric character of the 5a, molecular orbital, which is of the “inner 0” type (Fig. 4). Particularly iow cross-sections, together with narrow band widths, have also been encountered for the photoiotiisation of totally symmetric bonding * The mostrecentRamanstudy’ gave the following fundamentaifrequenciesfor Ph i al 600 cm-‘, tz 450 cm-‘, e 360 cm-‘. Jt seems most improbable that the vr%ration.kxcited in band D is eithe? of the t2 or e n&ma1 modes, since the frequency change-in the ion wo-dG imply si&ficant antibonding character for the orbital ionised. :

orbitals in a number of symmetrical inorganic species, including TiC1412n1’, SiC&‘*, BC1120-22 and SF6**. But the most powerful analogies to band D are perhaps to be found in the organic field. In benzene, for example, a low intensity PE band at ca. 16.8 eV, displaying pronounced vibrational structure, has been reliably assigned to ionisation of the 3a,, (radial inner G) molecular orbital”. The tie structure arises from strong excitation of the v2 (a,,) normal mode together with weak excitation of vl (ai& the frequency of the former vibration (928 cm -‘) being reduced relative to its value in the ground state of the neutral berzene molecule (992 cm- ‘). A similar phenomenon has been observed in the photoelectron spectrum of neopentane, C(CH,), “_ A weak band at 17.6 eV exhibits a marked vibrational progression with spacing 1363+30 cm-’ and also !ess prominent structure consistent with coexcitation of a vibration with frequency 658130 cm-‘. The normal modes concerned are probably the symmet&ai C-H deformation, y2 (al), and the symmetrical breathing mode, vl (al),the neutral molecule frequencies being ca. 1430 cm- ’ and 733 cm- ‘, respectively. The ionisation energy of the PE band, and the reduction in the vibrational frequencies in the molecular ion state produced, suggest assignment to an a, bonding XG (which may be approximately viewed as a totally symmetric combination of localised C-C : G bonding orbita;s). Returning to the photoelectron spectrum of P4, the broad bands, E and F (Fig. l), observed at higher ionisation energy, must relate to the ionisation of strongly bonding orbitals. These bands are separated by ca. 1.2 eV, band E being perhaps the more intense. Most probably they arise from ionisation of the 5tt MO'S, yielding the distinct E and A Jahn-Teller configurations of the *T2(5t2) state of P&+. A very large vibronic distortion, even greater th&n that observed for the tz ionisations in methane24n20, must then be accepted. The alternative possibility that one of these two bands concerns the 4a, ionisations can be eliminated, partly on intensity considerations; but mainly on semi-empirical grounds since approximate calculations of various kinds*’ (including the chro method26 and the approach of Fenske and Radtke2’) suggest a very large ener,oy separation between the 5t, and 4a, orbitals. We assume tlrerefore that the ionisation of the 4a, electrons requires exciting photons with energy in excess of 21.21 eV. The very low ionisation cross-sections observed for the 5f2 orbitals may be correlated with their presumed majority 3~ character: the 3s AO'S are thus supposed to have lower cross-sections for photoionisation than the antisymmetric and spatially more extensive 3p Ao's. Similar effects have been observed elsewher&20. There are now two likely assignments for the low ionisation energy bands, A, B and C. First there is the interpretation ventured in Table 1, where bands A and B are associated with the Jahn-‘Feller components of *T2 (6t2), while band C, with only partially resolved vibronic components, is assigned to the 2E(2e) state of pa+. The main point in favour of this assignment is the distinctive shape of bands A and B, their overall structure resembling that of bands E and F. The

46

Id.

J. Mass Specrrom. Ion Phys., 9 (1972)

obvious alternative interpretation is to associate bands A and B with the two JahnTeller configurations of ‘E (2e), band C being assigned to ‘T2 (6f2): assuming Koopmans’ approximation, the relative energies of the 6r, and 2e orbitals would then be the reverse of that suggested in Fig_ 3. The latter assignment has the merit of providing a possible explanation for the slight shoulder discernible on the high ionisation energy side of band C. If this band arises from the 6t, ionisations then the inflection may be due to further splitting of the E Jahn-Teller component through the second vibronically active normal mode. In this band C resembles the highly asymmetric t, PE band of methane”. The observed integrated band intensities, which are ca. (A+ B):C:D = 7:6: 1, only marginally favour the f&t assignment. The poor agreement with simple theory may arise in part from substantial (and differing) admixtures of the phosphorus 3s AO’S through the s-p, (a,) and s-p, (f2) interactions_ In view of the very low 52, ionisation cross-sections one suspects that s-p mixing will reduce the 6t, and 5a; cross-sections relative to that of 2e. However, we cannot at present rule out the possibility of other effects, in particular autoionisation”, influencing the intensities of the photoelectron bands in question. A complete, unambiguous assignment of the P4 heliuml.1) photoelectron spectrum is unfortunately not possible with the available experimental data. The main problem, which concerns the 6r, and 2e ionisations, will not be easily resolved by either experimental or theoretical means. However, the principal features of the P4 spectrum are not difficult to interpret. The orbital electronic structure of the P4 molecule (as inferred by use of Koopmans’ approximation) is understandable in very simple terms. The relative PE band widths suggest that the princi- . pal bonding MO’S are 5t, and (presumably) 4a,. These orbitals are prcbably predominantly of phosphorus 3s character.

APPENDIX

Our discussion of the 5a, ionisation (band D) in P4 suggests that the assignment proposed by Basch et al.” for the He(I) photoelectron spectrum of cyclopropane may require some revision. We present in Fig. 5 a PE spectrum of cyclopropane measured on the spectrometer described in ref. 6. This spectrum shows clearly a high ionisation energy .band (F) at 19.3tO.l eV that has not been previously discussed. Basch et al.2p, arguing on the basis of ab initio molecular orbital calculations, assign the low intensity PE band at ca. 16.5 eV (band E) to ionisation of the lat” bonding molecular orbital, while the more intense 15.7 eV band (D) is attributed to ionisation of the 3a,’ ,inner 5 orbital. However, the relative intensities of bands D and E, and the vibrational structure resolved in band E, argue for a reversal of this assignment. The low intensity of band E is more readily understood if it is assigned to the 3a,'ionisations since, in the tist place, an imzer 0 orbital may now Znt. J. Mass Spectrum.

Zon Phys., 9 (1972)

47

20

18

li

Fig. 5. The helium(I)

14 photoelectron

12

IO

spectrum

of cyclopropane,

C3H6.

be expected to have a relatively low ionisation cross-section and, secondly, because the 3aI’ orbital (unlike l+“j may have appreciable carbon 2s character, s-p, mixing also leading to a reduction in the photoionisation probability_ A comparison with the photoelectron spectrum of C3D, I7 shows that the vibrational progression in band E arises from the al’ (v2) symmetrical CH, scissor mode. The vibrational frequency in the molecular ion (1130 cm-‘) is reduced relative to the neutral molecule value (1504 cm-l), an effect which is consistent only with the iomsation of a bonding a,’ molecular orbital with its symmetrical combination of hydrogen 1s orbit&. Photoionisation of the la,” orbital should not stimulate the CH, bending mode (v2) to any great extent. The revised interpretation of the cyclopropane PE spectrum is indicated in Fig. 5. We agree with Basch et aL2’ and with Haselbach30 in their assignment of bands A and B to Jahn-Teller components of the 2E’ (3e’) ground state of the molecularion, and of band C to the e” ionisations. Band F, not previously assigned, probably arises from the 2e’ ionisations.

ACKNOWLEDGEMENTS

We thank the Science Research Council for financial support and for a Research Studentship (to SE.)

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Int. 3. Mass Specrrom. Ion Phys., 9 (I 972) ..

6 7 8 9 10 11 12

S. EVANS, A. F. ORCHARD AND D. W. T~I~%R, Znt. J. Mass Spectrom. Ion Phys., 7 (1971) 261. P. J. JOACHIM,Part II Thesis, Oxford, 1969. K. WATANME, quoted iu ref. 9. R. R. HART, M. J!&ROBIN AND N. k KUEBLER, J. &em. P/lys..,42 (1965) 3631. T. KOOPMANS, Physica, 1 (1933) 104. L. R. M&LL, J. B:HE&DR& AND V. M. MOSELY, J. Chem. Phys., 3 (1935) 699. 3. C. GREEN, M. L. H. GREEN, P. 1. JOACHIM,A. F. ORCHARD AND D. W. TURNER, Phil. Trans. Roy. SOL (London), A268 (1970) 111. 13 P. A. Cox, S. EVA=, A. HAMNE-~~ AND A. F. ORCW, Chenz. Z%y.r. Lett., 7 (i970) 414. 14 C. E. MOORE, Atomic Energy Lwefs, Nat. Bur_ Std. Circ. No. 467, Washington, DC.. 194915 M. D. SIURGE, Solid State Phys., 20 (1967) 92. 16 D. W. TWINER, in H. A 0. HILL AND P. DAY (Editors), PhysicaZ Methods in Advanced Znorganic Chemistry, Interscience, London, 1968. 17 D. W. TURNER, A. D. BAKER, C. BAKER AND C. R. BRUX~LE, Molecular Photoelectron Specfrocopy, Wiley, London, 1970. 18 P. A. Cox AND A. F. ORCHARD, Chem. phys. Lett., 7 (1970) 273. 19 H. S. GUTOWSKY AND C. J. HOFFMAN, J. Amer. Chem. SOL, 72 (1950) 5751. 20 A. W. Pans, H. J. LEMP~~A,D. G. STREEX AND W. C. PRICE, Phil. Trans- Roy. Sot. (Lu,xakn), A268 (1970) 59. 21 D. R. LLOYD, private communication. 22 3. D. H. ELAND, A. F. ORCHARD AND N. F. POWELL, unpublished work. AND D_ W. TUR~R, to be published. 23 S. EVANS, P. J. JOACHIM, A. F. ORCHARD 24 A. D. BAKER, C. BAKER, C. R. BRUNDLE AND D. W. XIJR~ZR, Znt. J. Mass Spectrom. Zon Phys., 1 (1968) 285. 25 A. F. ORCHARD AND P. J. ROBERIS, unpublished work. 26 J. A. POPLE AND G. A. &GAL, J. Chem. Phys., 44 (1966) 3289. 27 R. F. FENSKEAND D. D. RADTI(E, Znorg. Gem., 7 (1968) 479. 28 A. L. SMITIT,Ph!L Trans. Roy. Sot. (London), A268 (1970) 169 and refs. therein. TURNER, J. Chem. Phys., 51 29 H. BAXH, M. B. ROBIN, N. A. KLJEBLER, C. BAKER AND D. -5%‘. (1969) 52. 30 E. HASELBACH, Chem. Phys. Lett., 7 (1970) 428.

Znf. J. Mass Spectrom. Zon Phys., 9 (1972)

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