Photoelectron spectra and electronic structure of furan homologues computed by the ms scf Xα method

Volume

77, number 2

CHEMICAL

PHOTOELECTRON

COMPUTED

SPECTRA

AND ELECTRONIC

and

Plero

Trreste 341177. ltah

Recencd

8 December

The

16 October

~oruzat~on

analyst

STRUCTURE

OF FURAN

1980

energtcs

obtalncd

of the nature

in tirul

form

of turan,

thlophcnc,

and 3 wmparlson

x\lth

of the molcculrr

1950

selcnophcnc

and tcllurophcnc

prel IOU) czkulrtlons

orb!tals

arc ev~u.ttcd

an nwpnmcnt

0i correlation

wtli

the first

of the correspondmg

The little molecules form an interesting axles of heterocychc compounds well studled from the expertmental pomt of view The photoelectron speLtrJ furnosh dtrect tnformatton on the structure ot the energ) levels [l-5] The assignment of the spectra is greatly helped by the avatlabthty of accurate calculattons The first two molecules have been the ObJect of several theoretiwl

with different

by tilt

of cxpcrmwntal

11s Xa

spccrra

I‘roni

method

IS proposed

\n

5s prczLntcd

introduction

mvestlgatlons

HOMOLOCUES

DECLEVA

Istrtuto dl Clmz~ca. Unwersmf dl Trrrstc

the results

LETTERS

BY THE MS SCF Xa METHOD

C~ancarlo DE ALTI

1.

PHYSICS

degree of soplustlcatlon

compnrrson

w~tll

hydrldcs

the spcctr.!

ionlzat10n

potential

On the other

(IP)

hand

ot the corresponding

trahydro derlvatlvcs [9] su ggcsted fourth and fifth band respectively,

asslgnnient

te-

to the

lIta1 IS, below ow or two orbttals of u type Follov.w~g an earlier Investlgatlon on tursn. we llavc thercfore undertaken the study 01 tllesc molecules wtth tk hlS Xa method which permits the treatment of the whole series wth the wme degree ot nccurac) The MS Xa method has been shown to be rather reltable for the calculntlon of tonl/Jtlon energies The exlstence of accurate computations for furan and tlllophene [6] pcrmlts an assessment of tllc quality of the

In partxular a very accurate calculatton, performed wth the Green’s function (GF) technlqbe [6], IS avndable m which a detatled revtcw of the previous results and asstgnments IS reported No general agreement IS apparent for the level ordering of these molecules In fact new assignments have recently been suggested, both expertmentally [7] and theorerlcally [S]. Much less mformatton IS avatlable for tile other two members. Special attentton has been devoted only to

h% Xa results which can be used JS a gutde tn the asslgnnient of the spectra of the last two molecules The pattern ol‘thc energy levels oitordcd b) the present results displays separate groupings hnvmg drfferent behavlour along the series An r\planJtlon of these trends can be gtveu m terms ot the composltlon

the behavlour

of the relative

mverston strongly

of the two highest

of their supported

levels which

order in tellurophene Thts by experlmental evidence

show an

orbltals

fact IS

[9] On the contrary, no ektennve studies have been performed on the lower part of the valence region which pre-

2 Details of the computations

sents nonetheless interesting features For exampie. the relative energy positlon of the Innermost n (1 b t ) MO, which IS relevant for the aromatic properties of

The expertmental geometrtes [IO] are adopted tar ail the molecules The theory of the hlS SCF Xa method IS well re-

these molecules, is stti uncertam attnbuted m ref. [S ] to the thrd below

the two

0 009-26

uppermost

14/31/0000-0000/S

Thti orbital has been band, mimedlately

n orbltals,

03.50

on the grounds

0 Nortll-Holland

ported

m the literature

[ 1 1 ,I 21 Overlappmg

spllcres

are used In order to correct pnrtmlly the deflclencles of the muffm-tin npproxuiiation

Pubhshmg

Company

Volume 77, number

CHEMICAL

I?

Table 1 Sphere radu (au) and exchange factors a)

&drus

16263 3 19 2 34 262 167 1 29

0 s Se Te C H

‘1 The Inter-sphere

Outer sphere

0

0 0 0 0 0 0

74367 72436 70606 70007 75847

rzdius

o!

5 s 5 5

0 0 0 0

05 50 60 JO

75794 75346 74926 74788

77725

LYfactor IS set equal to the outer-sphere

ralue

As concerns the chorce of atomrc sphere radu, the values for the butadrene skeleton are kept fixed along the whole series and equal to those employed rn the previous work on furan [13] The ratto between the radius of the heteroatom then determuted adoptmg

and that of the Co sphere IS the Norman procedure [14]

The exchange parameters qT are taken from Schwarz [ 15 1, for hydrogen the value proposed by Siater [ 161 IS adopted A common mean value, wetghted accordrng to the number of valence electrons, is employed for the utter- and outer-sphere regrons Sphere radu and cr values are reported m table I Partial waves up to I = 2 for the outer sphere and heteroatom, I= 1 for the carbons and I = 0 fo: the hydrogens are employed The ronrzatron potentrais are evaluated wrth the transitton state formaltsm 3. Results

and discussion

The tomzatton energies, obtamed by the MS Xa method and the experunental data are collected m fig. 1 For the last two molecules the values were deduced from the reported spectra [5] except for the first three IPs The expertmental value for the 3at orbttal of selenophene IS assumed on the basis of the energy stits along the serves as shown by the dashed ime. The core regron is not taken mto account m the numbermg of the orbttais so that there are thirteen valence orbitals tn all the molecules The assignment of the experrmental romzatron ener@es of furan and thtophene 1s that obtained by the GF calculation [6] which IS the most accurate at the pres414

15 January

PHYSICS LETTERS

1981

ent time. For the other two congeners the assignment IS that proposed on the basis of the results of the Xcr calculations and of addttronal expenmental ewdence. The spectrum above 20 eV for all the compounds can be roughly drvtded mto three regions. Starting from the lowest IPs, the first region IS formed by two ionizations, the second by five and the last one by three. The Xo results reproduce satrsfactorlly the groupmg of the levels in these regtons and their energy separatron Expenmental data are avallable for furan and tluophene also tn the regton below 20 eV [3J _ The orderrng of the energy levels rn furan has been drscussed [13]. It 1s suffictent here to summartze the mam results The Xa ordermg IS m accord wrth the GF calculatron except for the posrtron of the 1 bl level m the second region Wlule the latter places tt below the 3b2 orbrtal, the present results locate It between the 5at and 4bl orbttals The ordermg obtamed both wtth the ab u-utlo method usmg Koopmans’ theorem and with the transition operator method [17], which IS equtvalent to a ASCF calcuiatton IS Identical to that gtven by the GF method It IS therefore apparent that both reiavatlon and correlation are not relevant for the ordering in this molecule Looking at the results obtatned for thtophene, the orderrng of the energy levels IS agam tdenttcal to that proposed by the GF calculatton, except for the lb1 level. As rn furan, rt IS shdted upwards so that It rs located at the top of the second region. The exchange ofthe5a1,4b,.3al,2b,and lb2,2al predtctedby the GF calcula~lons IS correctly reproduced by the present results Also m this case the ab rmtro method gives the correct ordermg An ordermg ldentlcal to that offered by the Xol results was proposed by Derrtck et at. [2] on the basis of the experimental evtdence and correlation wrth

other

molecules

In partrcular

the

1 b L level was

to he at the top of the second regron as proposed also by Schafer et al [5] _On the other hand, angular dlstrlbutlon analysrs [7] locates correctly the

beheved

lb,

level In thlophene

too low, m the thud

whte

It places the same level

regton. m furan The Xo results for selenophene and tellurophene show the la, level practtcally constant wrth respect to throphene, as elperlmentally evrdent. On the other hand the upward shift of the 2bl level m selenophene

Volume 77, number 2

CHEMICAL PHYSICS LETTERS

\

1.5 January 1981

I&

E~P

S

Se

Fe

0

SS

fe

2r Id

/ Zb, ?

,

I

/

2b,/

t-g

1. tT\Penmental

and cc?lcut;ltcd tof112;ltton energies

In the region below 2@eV the SC& IS halved 415

Volume

CHEhlICAL

77, number 2

PHYSICS

IS not reproduced so that the relative ordering is mamtamed as suggested expenmentally, while the energy separation IS too large Due to this fact the shift m tellurophene, which IS correctly evaluated, IS not able to reverse the ordermg of the two levels wluch are caic&ted to be very near. On the other hand an ab mltlo computation [ 181 performed on selenophene gwes the reversal of thes ordenng m tha molecule As regards the rest of the valence part of the spectrum, the tiscusslon has been confined to the positIon of the lb, level In the second region as aiready mentioned The evaluated sequence IS lb, 6a, 4b, Sal 3b, in selenophene and 6ai lb1 4b2 Sal 3b, III tellurophene The t\ko lowest levels Sa, and 3b2 remam roughly constant while the other three are shfted upwards The stift IS less pronounced for the 1b I level, which comes out

below

the 6al

WI tellurophene

In addltlon

thus be-

havlour causes a progressive separation of the levels U-Ito two groups, as 1s recogruzable also m the experunental spectrum As already stated, the Xa and GF orderings differ only in the powon of the lb, level* Lookmg at Its behavlour along the series, the Xa calculations indicate that it undergoes a large skft m passmg from furan to tluophene while this 1s less pronounced for the other members The GF calculations also display a large shft, so that it IS reasonable to tlunk that the Xa results reproduce, at least quahratively, the correct behawour of this level for aLl the congeners. Both m furan and tiuophene, the Xcu computations locate it too high and the correct order should be obtamed lowermg It by almost the same amount If a slmliar correctlon IS also made for the other congeners, the 1b, , 6al levels m seienophene and 1 b I, 4b2 m tellurophene should be inverted As a consequence, the ordermg

should be like that of thophenc

first case whde in the second

in the

the I b, level should

be

still lower This assignment Indicates that the lb, level 1s responsible for the second and third band tn selenophene and tellurophene respectively, confiimg m thrs way the attnbutlon previously suggested [9] The assignment proposed here is reported in fig 1 with dashed connection lines. * It is worth mentionmg that the same behavlour IS sho\bn by the results of an XP calcularion performed on the pyrrole molecule

416

LETTERS

15 January

As concerns

the

remammg

levels, the 3al,

1981

3_b7 and

1 b,,

2a t are inverted m passing from furan to tGophene. The same ordermg IS afforded by the GF and ab n-utlo results The same order IS then mamtamed for the other two molecules and we beheve it to be the correct one The quantitative accord ts quite satlsfactory

also

therefore

m the lower region of the spectrum_ It IS apparent that the electron reorgamzatlon

which should be particularly important m this region IS adequately taken mto account by the transition state formahsm The proposed assignment permits an e\ammatlon of the trends along the series The

mam feature

IS a general upward shft

of the

energy levels LLSexpected on the grounds of the decreasing electronegatlwty of the heteroatom, so that dewatlons from this behavlour become very mteresttng. As concerns

the

first

la, level, which IS almost has been attributed [4,5] to whch

the heteroatom

region,

the behavlour

of the

constant along the senes, to the nature of thus orbital cannot

partxlpate

except

by

d orbltals

An analysis of Its nature, as determmed by the present method, displays an almost constant composItIon In all compounds wth a negllglble d contrlbutlon, conflrmtng m this way the earlier explanation The calculated IP follows closely the experimental data except for furan m which tt IS surprlsmgly high This could be attributed to a less adequate choice of the radu for this molecule The expermiental blndmg energies of the other n levels, 2b L and 1b I , display a regular decrease along the series, the latter showing a more marked shift in passmg from furan to thiophene Their nature IS tndicated by the atomic charges wIuch are reported m table 2 The partlclpatlon of the heteroatom 1s progreswely siufted from the 1 b t to the 2b I orbltal, the larger varlatlon being from furan to thlophene At the same time the p carbon partlclpatlon changes m the opposite dlrectlon In tlus way the lb1 level IS largely

heteroatom in character in furan wtule it becomes mostly rmg In the other molecules The reverse behaviour IS apparent for the 2b, level so that the two orbltals ewhange their nature, the rmg b, orbital IS essentially 2b, in furan and lb1 m the other congeners 211s 1s reasonably attributable to the difference In the relative energy posltlons of the b 1 rmg and p heteroatom levels along the series. This situation turns out

Volume

77, number

Table 2 Partml-wave

~nalvs~~

CHEMICAL

2

for the bl

molecular

lb1

a) d orbltals

15 January

LETTERS

orbltals L_I____--

Outer

7b,

PHYSICS

_--____ Xl

sphere

P

d

0 008

funn thlophene selenophcne tellurophenc

0 0 0 006

0 0 0 0

furaIl thlophenc selenophene tellurophene

0 004 0011 0 013 0.015

0 0 0 0

_ Inter

ca P

5

P

cphcrc

0 -IO?

d J)

P 008 022 025 044

1981

0 192

_

0 345 0 383 0431

0 007 0001 0

0 036 0011 0 010 0001

0 771 0 234 0 165

0 0 0 0

0 0 0 0

0 00.5 0 004 0 00-I

0 0 0 0

0051 0 163 0 188 0 255

0 279 0391 0 409 0 426

502 233 195 10’

163 197 197 193

353 341 347 357

are not used In furAn

to be ChdraCterlstIc of other orbital& even IF less well defined due to their more complex composltlon. The large charge fraction m the inter-sphere region IS characterMic of the TTorbltals This could account for the less satisfactory descrlptlon of these orbltals 111 the present method. The other four orbitals m the second region can be separated Into two couples The first one, 6a, -lb2, shops a regular and marked upward shift, while the second, Sal 3b,, displays an irregular behawour remammg practlcallq unshifted The behawour of the couples 15 shown m a more marhed way by the present calculations The 5at and 6at levels in furan show a slmllar partlclpatlon of oxygen wtule they become only rmg and heteroatom m character respectwely, so that the first remains practically constant and the second goes up m energy In the case of the 3b2, 4b, levels the heteroatom partlclpatlon IS m 3b2 for furan and 4b, for the others, so that an analogous coxlderatlon IS vahd for tlus pair The ring nature of the 4b2 in furan and 3b, III the other congeners is reflected m the sumlanty of their energy The 4at level 1s always essentmlly rmg In nature and accordmgly Its cakulated energy varies little. The largest tls orbital partmpatlon of the heteroatom IS UI la, for furan, 111?a, for thiophene and selenophene and m 3a, for tellurophene In fact the la, energy shows a large increase from furan to Theophene and then mcreases slowly, almost reaching the ener,T posItion of the 2al level m furan. Indeed their composltlons become quite smidar

An analogous behavlour, although less regular due to the different particlpatlon of the heterontom, c.m be observed for the ?a, and sat levels The remammg 1b2 and 2b, levels are mamly rmg m character, except in furan I; which they display a shght oxygen partiCIp3tioii

Acknowledgement

Thanks

are due to C N R ot Italy

ior the flnanclal

support

References 111 P J Derrick, VI I31 [41

151

[61 [71 ISI

L &brmk, 0 Edqwst B 0 Jonsson Jnd E Lmdholm, lntcrn J Xlnss Specrrom lon 1’11~s 6 (1971) i61 P J Derrick, L Asbrmh, 0 Edqvlst, B 0 Jonsbon and E Lmdhobn. Intern J Xlnss Spectrom Ion Phys 6 (1971) 177. U Gchus, C J Allan, G Johansson, H SlefbJhn, D 4 Allison Jnd K Slegbahn. Physu Scripta 3 (1971) 237 G Dlstefano, S Plgnataro, G Innorta, F ~nngucll~, G hlarmo and A TJtlcchI Clwm Phys Letter5 12 (1973) 132 W Schater, A SLhwclg, S Gronowtz, A Tztlcclu and I‘ Fr~ngucll~, J Chem Sot Chcm Commun (1973) 541 W. van Nwssen, W P Kmemer and L S Ccdcrbwm, J Electron Spectry 8 (1976) 179 J A Sell and A Kuppermann, Chem Phys Letters 61 (1979) 355 L Asbnnk, C rndh and E Lmdholm, J Electron Spectry 16 (1979) 65

417

Volume

77, number

3-

CHEhlICAL

[9] F FrmgueLh, C hlarmo, A_ Tat~cti, G Dlstcfano, F-P. Colonna and S Psnataro, J Chem Sot Perkm 11 (1976) 276. [ 101 B Bak, D Christensen, W B DLxon. L. HansertNyga,trd. J Rastrup-Andersen and hl Shotthnder, I hloL Spectry 9 (1962) 124. B Bak, D. Christensen, 1. Hansen-Nygaard and J Rastrup-Anderserr. J hfol Spectry. 7 (1961) 58, N.hl Dozdeev, 0-B. Akuhnm, A _A.Shapkln and N N hfagdeseva, Doll Akad Nauk SSSR 185 (1969) 384, R D. Brown and J.C Crofts, Chem Phys 1 (1973) 217

418

PIIYSICS

LETTERS

15 January

1981

[ 111 K 11 Johnson, Advan Quantum Chem 7 (1973) 143. [ 121 J C. Slater, The self-conststent field for molecules and solids, Vol. [ 131 [I41 [ 15

]

[ 161 [ 171 [18]

4 (hlcGraw-Ha,

New York,

1974).

G de Alto. P. Decleva and A SgameUottb Gazz Chem ItaL 110 (1980) 49. J G. Norman Jr, J. Chem Phyn 61(1974) 4630 K. Schwarz, Phys Rev B5 (1972) 2466,Theoret Chem. Act-a 34 (1974) 225. J C Slater, Intern J Quantum Chem 7 (1973) S533 hl. Hehenberger, Chem Phys Letters 46 (1977) 117 R H. Fmdlay, J. Chem. Sot Faraday II (1975) 1397