Angular distributions in the photoelectron spectroscopy of furan, thiophene, and pyrrole

Volume 61, number 2

ANGULAR OF FURAN,

_

CHEMICAL

DISTRIBUTIONS THIOPHENE,

91 IX.

IN THE PHOTOELECTRON

1.5 February

1979

SPECTROSCOPY

Glifornia

IItstitute

off Technology.

USA

Received 10 May 1978 Revised manuscript received 9 November

The photoelectron

LETTERS

AND PYRROLE*

Jeffrey A_ SELL** and Aron KUPPERMANN ArrJzurAmos No_~esLaboratory of Ozenzical Ph,vScsT. Pasadena_ Gdifornia

PHYSICS

1978

angular distributions of furan, thiophene, and pyrrole are reported. The trends in the value of the

asymmetry parameter p and in the shapes and positions of bands in tile spectra are used to assign the peaks thar correspond to ionization from the ?r orbitals in these molecules.

I_ Introduction

shown that the value of p for ionization of R orbitals is usually 0.2 to 1 .O units higher than for ionization of

Investigations of the angular distributions in the photoelectron spectroscopy of small molecules have been recently performed in this laboratory [I-6] _ These studies have shown that much information on the eiectronic structure of molecules which is not obtainable from fixed angle spectra alone can be obtained from rhe angular distribution data_ Briefly [7, 81) the differential cross section do/dQ for photoionization from a given initial state i of an atom or molecule to a given lima1 state f of the corresponding ion using unpolarized light has the form

u orbitals. We have also found this to be true for several molecules [ 1,4,1 I] _This difference in 13values for ‘in and u orbitals has previously [4,12--141 been used to assign the peaks in the photoelectron spectra of x electron molecules. In this paper the results of the measurements of photoelectron angular distributions in furan, thiophene, and pyrrole are reported for the first time. The photo-

doti/dfi = (Q4ir)

[I - +flif P,(COS

e)] .

0)

where QIf is the integral photoionization cross section, &(cos 6) is the second Legendre polynomial, 0 is the angle between the momentum vector of the ejected electron and the propagation direction of the incident light, and p is the asymmetry parameter which ranges from -1 to +2. Carlson and co-workers [9,10] have * This work was supported by a contract (EY-76-S-03-767) from the Department of Energy. Report Code: CALT767P4-176. ** Work performed in partial fulftient of the requirements for the Ph.D. in Chemistry at the California Institute of Technology. ? Contriiution No. 5784.

!WOfl

tniophena

p)rrole

electron spectra of these molecules have been published several times [Is-241 but there is still uncertainty regarding the energy ordering of the o and zr orbitals. Since the understanding of their electronic structure is important to the whole field of heterocyclic chemis?ry, we felt that a study of their photoelectron angular distributions would provide useful information_ Complementary information about the electronic energy levels of these molecules and of their aromatic character hzs previously been obtained by low energy variable angle electron impact spectroscopy [25] _ 3.55

VoIume

61, number 2

15 February

CHEMICAL PHYSICS LETTERS

1979

by fractional

2. Experimental

distillation under 1 atm of argon using AlI three sampIes were subjected to three liquid nitrogen freezepump-thaw cycles before being ioaded into the spectrometer-

a spinning band distillation coIumn [26].

The apparatus used in the present study is essentially the same as the one previousIy described in detail [I ] and will therefore be reviewed here only briefly. A helium discharge Iamp produces mostly 584 A radiation which interacts with the sample vapor in the scattering chamber_ The photoelectrons produced are energy-seIected by a hemispherical electrostatic analyzer and detected by a Spiraltron electron multiplier. The analyzer and detector are rotated about the center of the sample chamber which permits scanning of scattering angles over the range of 45” to 120”_ The main vacuum chamber is well shielded by LI metal and three pairs of HeImhoItz coils which reduce the residual magnetic field to about 03 mG_ The energy resolution of the apparatus was set at about 55 meV as measured from the full width at half maximum of the argon ‘P3p peak_ Samples of the three heterocyclic molecules were obtained from the Aldrich Company. The stated purity of pyrrole was 98% while that of thiophene and furan was > 99%. The pyrrole sample was further purified

2

12 L

‘

IO

1

ELECTRON 8

i

L

ION IZATION

3. Results

The photoelectron spectrum and a plot of II as a function of ionization potential (P) and electron energy (hereafter called II-spectra) for fumn are shown in fig. 1_ The W’s and values of B are also listed in tabIe I_ The spectrum is similar to the ones reported previously by Eland [ 191, Derrick et al_ [20], and Raker et al. [ 161, except that some of the relative band intensities are different. One cause for this difference is that our spectra were obtained at a detector a&e of 54.7” for which Pz(cos 0) = 0. At this angle, ratios of relative differential cross sections are equal to ratios of integral cross sections. Most other spectra

ENERGY

6

POTENTIAL

(eV)

L

4

6

L

2

*

fev)

F@ t _ ~h~r~~lectron spectrum (10~~ panel) and @spectrum (upper panel) for funn using He1 (21.22 ev) ndiation. The spectrum NQSobtained at a detector ar@e of 54.P using adjacent 18 meV wide channeLs and 60 scans. Total dwell time per channel ~2s 60 s.

356

Volume 6 1, number 2 Table 1 B values and

CHEMICAL PHYSICS LETTERS

ionization potentials for furan at

Band

I II III IV V VI VII

VIII

584 A

Initial

Vertical

orbital

W)

--_--.-_--~ laa 2br

lb1

15 February 1979

IP a)

__ - ________ 8.9 IO.4 12.9 13.8 14.4 15.1 175

~18.8 c)

P vertical

rant across band b, 0.63 t 0.09 to l-23 + O-18 0.43 r G-06 to 1.16 i 0.06 -0.18 i 0.06 to 0.20 0.04 r 0.05 to 0.36 0.11 t 0.07 to 0.30 0.09 r 0.07 to 0.44 0.19 5 0.09 to 0.61

+ 5 2 f +

1.07 i 0.05 0.56 c 0.11

0.10 0.11 0.07 0.08 0.05

-0.08 = 0.08 0.25 5 0.07 0.26 r 0.03 0.09 + 0.07 0.49 i 0.13

-0.70 + 0.09 to 0.28 r 0.09

0.13 f 0.19

a) This corresponds to the band maximum and has an absolute accuracy of to.05 eV_ b) The fast and second values quoted are respecthely the lolxest and highest ralues. The uncertainties giLen are twice the standard deviation (see ref. [ 1 I)_ C) The value of this IP is uncertain by kO.7 eV due to the low intensity and brUad shape of the band. reported for the molecules considered in this paper have been obtained at a detector angle of 90” for which a nonzero value of/l causes the observed relative intensities to deviate from those obtained at 54.7”. Bands I and Ii (see fig. 1) have vertical II? of 83 eV and 10.4 eV respectively, and have resolved vibrational structure. Bands III through VI are fairly broad and have vertical Ip’s of 12_9,13.8,14_4, and 15.1 eV, respectively. Band VII is located at 17.5 eV and looks remarkably similar to bands I and II. Finally, band VIII is very broad and featureless and has maximum somewhere near 18.8 eV. There are several interesting features in the plot of p as a function of IP. Band I has relative high values of f? which range from 0.63 to 122. Similarly, band II has values which range from 0.43 to 1.16. If straight lines are drawn through the 0 points for each of these bands one finds that the slope of these lines is -1 .O eV_’ for both bands. These results will be discussed in section 4. Bands III through VI have lower values of fl which are in the range -0.18 to 0.44. fi is again relatively high for band VII decreasing from 0.61 to 0.19 , as the IP increases at an approximate rate of -0.45 eV_’ , and is in the range -020 to 0.28 for band VIII. 3.2. Thiophene The photoelectron and p-spectra for thiophene are shown in fig. 2. The IP’s and values of fl are also listed

in table 2. Again the spectrum is similar to those reported previously by Eland [ 191, Derrick et al_ [21] _ Baker et ai. [16] and Baloalais et al. [24], except for minor disagreements on relative band intensities, as discussed for furan. The I? of band 1 is 8.9 eV, the same as in furan. but band II has moved downwards in energy from 10.4 eV to 9.5 eV. Bands III through VII are all fairly broad and have II% of 11.9, 12.4, 13.1, 13.5, and 14.2 eV, respectively. Finally, band VIII is located at 16.4 eV. The value of fl for bands I and II ranges between 0.78 and 1 _I 1 with no discernible systematic trends in its energy dependence. p is relatively low, from 0.23 to 0.42, for band 111 and then rises to a maximum of 0.74 at the maximum intensity of band IV, decreasing thereafter_ For bands V, VI, and VII the value of fl is in the range 0.25 to 0.46 and then varies between 0.21 and 0.46 for band VIII. 3.3. &-role The photoelectron and P-spectra for pyrrole are shown in fig. 3. The IP’s and values of p are also listed in table 3_ The spectrum is similar to those reported earlier [16,19,22] _ The first two bands (see fig. 3) have resolved vibrational structure and have IP’s of 8.2 eV and 9.2 eV. Bands III through Vi overlap each other and are very broad, having Ills of 12-7.13-7, 14.3, and 14.8 eV, respectively. Band VII is weak and 3.57

Volume 61. number 2

CHEMICAL

PHYSICS LETTERS

ELECTRON

ENERGY

15 February 1979

(eV)

L

2;j4 30

f

p_/;“’ f

Ii ION tZATiON

9

I

13 POTENTIAL

15 (eV1

17

Fig_ 2_ Photoelectron spectrum (Io\\er panel) and p-spectrum (upper panel) for thiophene using He1 radiation. The spectrum xx obtained at a detector a&e of 54-T using adjacen: 18 meV wide channels and 50 scans. Total d\%eIItime per channel was 50 s.

extremeIy broad and relatively flat between 172 eV and 182 eV_ P decreases from 1 .I 5 to 0.79 across band I at an approximate rate of -1.5 eV-’ _SimilarIy for band II, fl decreases from 1.09 to 0.76 at an approximate rate of -I -5 eV_’ _The values of fl for bands III through VI are al1 in the range-O.07 to i-033 and the vafues for band VII range from -0.17 to 034. Table2 p valuesxrd ionktfon potenthds for thiophrnu rrt584 A Band

Inirirrl orbital

I II III IV V VI VII VIII

Ins 2bt Ibr

-

89 9.5 11.9 12.4 13.1 135 X4.2 16.4

The moiecules furan, thiophene and pyrrole belong to the Czv point group for which the standard coordinate system 1271 designates the z and y axes to lie in the molecular plane, the first one being the C, axis_ In all three cases the heteroatom has a lone pair of electrons in an orbital which is directed out of the molecu-

-__----.

Vertical IP a) feV) .-

4_ Discussion

_-----

range across band b, ___~__. ___ 0.78 r 0.89 2 0.23 + 0.46 + 0.28 2 0.35 2 0.25 f 0.21 f

0.09 0 07 0.04 0.05 0.04 0.04 0.04 0.12

to l-1 1 f to 1.11 f to 0.12 i to 0.74 f to 0.40 + to 0.44 + to 0.46 2 to O-46 *

a) This corresponds to the band maximum and has an absolute nccuncy of +0.05 eV_ b) The range and uncertainties have the same meaning as in table 1.

358

--

P vertical 0.09 0.06 0.10 0.05 0.04 0.03 0.03 O-11 -___

0.99 + 0.07 1.11 150.06 0.28 f 0.02 O-74 f 0.05 0.28 f 0.04 0.35 + 0.03 0.25 2 0.04 O-46 k 0.11

Volume 61. number 2

15 February 1979

CHEMICAL PHYSICS LETTERS ELECTRON

2

I2

,

,

10

1

8

ENERGY 8

1

1

!eV) 6

L

4 i

t

1

,

I-I

n

G ~60-1 ?

II

m

n

-

LpPx!t

-

I

e405 s

H

-

200) 8

IO

14

I2 IONIZATION

16

POTENTIAL

18

(eV)

3. PhotoeIectron spectrum (Holier panel) and p-spectrum (upper panel) for pyrrole using He1 radiation_ The spectrum was obtained at a detector angle of 54.7 using adjacent 18 meV wide channels and 40 scans TotaI daell time per channel uas 40 s. fig.

lar plane. There are six out-of-plane electrons, and previous work [25 ] has shown that they form a ring with aromatic character_ Three of the corresponding molecular orbitals are of the n type and have a symmetry described by either the a2 or b, irreducible representations. The shapes and nodal characteristics of these orbitals are shown below [ 151 I

Table 3 p vahtes and ionization Band

potentials for pyrroie at 584 A Initial orbital

Vertical IP a) (eV)

--__

-

range across bandF_ i

Ia2

II III IV V VI VII

2bt

8.2

9.2 12.7 13.7 14.3 14.8 17.4 c) ~_

-____--

B

0.79 0.76 -0.07 0.04 0.18 0.09 -0.17

I r f + i + +

0.17 0.09 0.07 0.14 0 08 0.16 0.11

to 1.15 i 0.07 to 1.09 r 0.06 to 0.18 i- 0.09 to 0.24 f 0.06 to 0.31 f 0.10 to 0.33 I 0.10 to 0.34 + 0.16 _____

\erticaI 1.15 1.09 0.11 0.04 0.26 0.33 0.00

i 0.07 L 0 06 f 0.07 f 0.14 f 0.08 -c 0.10 i 0.12

al This corresponds to the band maSmum and has an absolute accuracy of kO.05 eV_ b) The range and uncertainties have the same meaning as in table 1. c) This band is very broad and flat making an assignment of the vertical IP quite uncertain. 359

Volume 61, number 2

CHEMICAL PHYSICS LETTERS

The rest of the discussion wilI focus on our use of the angular distribution data to assign the peaks in the spectra that correspond to ionization from these orbitals and will include a comparison of our assignments to those made previously [ 15-241 using different techniques. C I_ Furan

IS February 1979

in average sufficiently high and the vibrational structure sufficiently analogous for their assignment as x bands to be reliable. These assignments agree with those given by the experimentai work and calculations of Eland [I91 and also the calculations of Siegbahn 1301. However, our assignment of the 175 eV band (VII) to the 1 b, orbital does not agree with Derrick et al. [20], who associate that orbita to the band at 14.4 eV N)_

Due to the relatively high vaIues of fl at the verticai IP of bands !, II, and VII we assign these peaks as the ones that correspond to the three rr orbitak The assignment for band VII is sIightIy more uncertain than those for bands I and II since the highest value of p for this band is O-61 which is onIy slightly higher than the highest value in band VI (O-44)_ However, the variation of p across band VII is much more simiiar to that for bands I and II than for any o?her band. In addition, the vibrational spacing and intensity distribution of band VII is remarkabIy similar to those of bands I and II_ These arguments suggest that the assignment of band VII to a x orbital is a reasonable one_ The 1 bl orbital is nodeless and therefore the most stable (lowest energy, highest IP) of the three z orbitals displayed above_ The reIative ordering of the Iat and 2bI orbitaIs is more difficult to ascertain using simple arguments_ However, the calculations of Derrick et al_ [20-221 predict that the 2bt is more stable than the Ia, for al: three moIecuIes.This order is reflected in the orbitzl assignments given in table I_ One interesting aspect of the acgular distrrbution data is that the change in fl across each of the bands I, II and VII is essentiaIIy monotonic (deviations from monotonicity are due to statistica fluctuations and are not reproducible) and the slopes of the lines drawn through the corresponding parts in fig_ 1 are, as stated above -equal to --I_0 eV_‘, - 1-O eV_’ . and -0.45 eV_’ respectiveIy_ Previously, values of such slopes for a electron ionizations were typically -0.1 to -0.2 eV_’ for benzene [ 14,281, linear dienes [9,1 I] and methq I substituted ethylenes [29] _ However, higher values for this sIope have been recently observed for TTionizations in the halobenzenes [3] as we11 as in cyclopropane [ 13 ] _ Therefore, the expectation that /3 varies onIy gradually across a t photoionization band may hare to be modified and this behavior may have to be taken into account when 0 is used to make orbital assignments_

itio SCF CI calculation for the ground and excited states of pyrroIe and its positive ion. They found that aIthough the third 71state of the positive ion is formed mostly by ionization from the Ibt orbital (lb4 + a), two shakeup transitions (lag * 3bt m and 2bT + 3b, m) contribute significantly to this state. This configuration mixing wouid affect 0 1333, but since a11 three orbitals involved in the ionization (Ibt, Ia2 and 2bt) have x

Nevertheless, the p vaIues for bands I, II and VII are

symmetry, the resuiting value of this parameter would

360

4.2. ThiopJzerre T’he reIativeIy high values of fl at the vertical IP of bands I, II, and IV lead to the association of these bands with n- orbitals- Consideration of the nodal characteristics (which are simiIar to those of furan) leads to the assignments giver, in table 2_ With the exception of the Ib, orbital, these assignments agree with those of Derrick et al. [21], Gelius et al. [3 I], EIand [ 193, and Baker et al. [ 16]_ There is essentiaIIy no agreement among these authors on the assignment of the 1 b, orbital, but our angular distribution data indicate that this orbital is associated with the peak at 12-4 eV_ 4.3. mffole The vertical 0 values for pyrrole are relatively high only for bands I and II (see fig. 3) which are associated with the Ia? and 2b, orbitals, respectiveiy, in anaiogy with furan and thiophene. The higb negative slopes of the lines drawn through the fl values across these features (-1.5 eV_‘) are significant for the same reason mentioned in the discussion of the corresponding bands in furan. The fact that another transition which has a relativeIy high value of 0 is not observed is very interesting since a third z orbital is expected to be accessibIe to 21.2 eV photons.

Tanaka et al. 1321, have recently reported an ab in-

Volume 61, number 3

CHEMICAL

PHYSICS

still be characteristic of a s ionization. Their results yidd an IP of 13.3 eV for the lb1 orbital. This type of calculation can give an IP which is in error by up to 1 eV, but the importani consideration is that the band which corresponds to this orbital probably lies in the 12.2 to 15.5 eV region where several bands can be seen in fig. 3. The fact that there are several u bands in this region which could overlap this n band may explain why only relatively low values of fl are observed. When two transitions overlap one another without interfering, having characteristic asymmetry parameters /3r and &, the observed fl value will be a lseighted average of these:

P

ohs

=

(Q,P,

+

Q2P2Y(Ql

f

Q,>.

(2)

where Qr and Q2 are the integral photoionization cross sections for the two bands [2,3] _Thus, depending on the ratio of cross sections, it is possible for the observed p value of an overlapped c transition to be lower than one would normally expect.

5. Summary and conchrsions The photoelectron spectra of furan, throphene, and pyrrole have been obtained using 584 A light at scattering angles in the range of 45O to 1 20°, including the magic angle of 54.7”. The anisotropy parameters p have been determined as a function of IP for each molecule, and in some cases show a rapid variation across individual photoelectron bands. This angular distrrbution data is used to make assignments of the two lowest IP ITbands in all of these molecules in agreement with previous identifications. In addition, it is used to label the third IP ‘ITband in furan and thiophene, for which previous assignments are in disagreement with each other For all these eight rr bands the value of fl at the vertical IP is relatively high compared to those of the u bands. For pyrroie. the third IP rr band seems to occur in a re- ’ gion of the spectrum heaviIy overlapped by o bands, making the observed values of 0 ineffective for determining its precise Iocation.

Acknowledgement We wish to thank Professor David A_ Evans for assistance in the purification of the pyrrole sample.

15 February

LEITERS

1979

References (11 DC. Mason, D.M. Mintz and A. Kuppermann. Rev. Sci. Instr. 48 (1977) 926. [2] D.&I. Mintz and A. Kuppermann, Enersy Dependence of the Differential Photoelectron Cross Sections of Jioleculsr Nitrogen, J. Chem. Phls , to be published. [3] J.A. Sell. A. Kuppermann and D.M. Mintz, Angular Distrtbutions in the Photoelectron Spectroscopy of Carbon XlonoGde. J. Electron Spectry., to be pub!tshed_ [4] J.A. SelI and A. Kuppermann, Chem. Ph>s. 33 (1978) 367. (51 J.A. Sell and A_ Kuppermann, Chem. Phys. 33 (1978) 379. (61 J.A. Sell, A_ Kuppermann and D 31. Xfinrz, Chem. Phls. Letters 58 (1978) 601. [7] J. Cooper and R.N. Zxe, J. Chem. Phys. 48 (1968) 942. [S] J. HaII and &I. Siegel, J. Chem. Phys. 48 (1968) 943. [9] R.M. Wtite. T-.X_ Carlson and D.P. Spears, J_ Electron Spectry. 3 (1974) 59_ [IO] T.A. CarIson and C.P. Anderson, Chem. Phys. Letters 10 (1971) 561. [ II] D.&l. Mmtz and A. Guppermann. Variable-An$e Photoelectron Spectroscopy of AUene, 1,3-Butadiene, 1,4Pentadiene. and 1.5-He~diene, in prepnation. [ 121 T. Kobaqashi and S. Nagakura. J. Electron Spectry. I1 (1977) 293. [ 131 FJ_ Leng 2nd G-L_ Nyber_e, J_ Electron Spectrv_ I1 (1977) 293. [ 141 V.B. Sldyaev snd F I. Vdeso\. High Energy Chem. 7 (1973) 414. [IS] A.D. Baker and D. Bettcridge. Photoelectron spectroscopy: chemical and nnalytical aspects (Pergamon, Oxford. 1972) pp_ 80-82. [ 161 A.D. Baker, D. Betteridge, N-R. Kemp and R-E_ IGrb>, Anal. Chem. 42 (1970) 1064. [ 171 V ii. Porapor and B A. Bazhenos, Khim. V) s. Energ. 4 (1970) 553. [IS] 3-W. Turner, C. Baker, A D. Baker and CR. Brundle, Molecular photoelectron spectroscopy (Wlcy-Interscience, New York, 1970) pp 329-331. [ 191 J.H_D_ Eland. Intern. J. Mass Spectrom. Ion Phys. 2 (1969) 471. [20] P-J. Derrick, L. Xsb:ink, 0. Edqwst. B.& Jonsson and E. Lindholm, Intern_ J. Mass Spectrom. Ion Phys. (1971) 161. 1211 P.J Derrick, L. &brink, 0. Edqvist, B 6. Jonsson and

E. Lindholm. [22]

Intern. J. Mass Spectrom

Ion Phys. 5 (1971)

177. PJ. Derrick. L. Asbrink. 0. Edqvist. B-6. Jonsson and E. Lindholm. Intern. J. Mass Spectrom. Ion Phys. 6 (1971)

191_

[ 23 ] P.J. Derrick, L. Asbnnk, 0. Edq\ ist and E. Lindhotm, Spectrochim. Acta 27A (1971) 2525. [24 ] J.W_ Rabaiab, L-0. Werme. T. Bergmark, L. Karlsson and I;. Siegbshn. Intern. J. Mass Spectrom. Ion Phys. 9 (1972) 185. [25] \\‘.\I. nicker. 0-A. hlosher and A. Kuppermann, J. Chem. Phys. 64 (1976) 13i5.

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CHEMICAL

f26] T-P. Carney. Laboratory fractional disti8ation Wv%i&n. New York, 1949). p_ 81. 1271 KS. MuIliken, 3. Chem_ Phys- 23 (1955) 1997. [ZS] J.A_ Kinsinger and J-W_ Taylor, Intern. J_ Mass Spectrom. ion Phys- 10 (1972/73) 44% [ZS] D.&I_ Mintz and A_ Kuppermann. Variable-Angie Photoelectron Spectroscopy of EthyIene, IsobutyIene. 2Methyl. 7,Butene and 2.3-Dimethyl, ZButene. in preparation.

362

PHYSICS LE’ITERS

1.5 February

1979

1301 3_ Siegbahn, C&m_ Phys- Letters 8 (1971) 245 [31] V_ Gelius. B. Roos and P. Siegbahn, Theoret. Chim. Acta 27 (1972) 171_ [32] K. Tanaka. T. Nomura, T. Noro, H. Tatewaki, T_ Takada, H. Kashiwagi. F. Sasaki and K. Ohno. J_ Chem_ Phys. 67 (1978) 5738. [33] J-Cooper and RN Zare. in: Lectures in theoretical physics, eds. S. G&man. K-T_ Mahanthappa and W-E. Britten (Gordon and Breach, New York, 1969) pp_ 317-337.