XV. Photoelectron, uv, mass and electron impact spectra of pyrazine

International 3oournal of Mass Spectrometry und Zon Physics Eketier Publishing Company, Amsterdam. Printed in theNetherlands

RYDBERG

SERIES

IN SMALL

XV. PHOTOELECTRON, OF PYRAZINE

C. FRIDH, L. _&BRINK, Department

of Physics,

MOLECULES

UV, MASS AND

B. 6

101

ELECTRON

IMPACT

SPECTRA

JOISSON AND E. LlNDHOLM

The Ro>ol

Institute of Technology,

S-100 44 Stockhohn

70 (&w.ib~)

(ReceivedMay IOth,1971)

ABSTRAST

The electronic structure of pyrazine has been studied in a photoelectron spectrometer, an electron spectrometer and a tandem mass spectrometer. The Rydberg transitions from electron spectrometry and ultraviolet spectrometry make possible an interpretation of the photoelectron spectrum. The first and third ionization potentials correspond to “lone-pair” electrons and the second and fourth to R electrons. The “lone-pair” electrons are bonding and the YCelectrons nearly nonbonding. The mass-spectrometric breakdown is discussed.

INTRODUCllON

In a previous paper’ the electronic structure of s-triazine was studied by use of photoelectron, ultraviolet, mass and electron impact spectroscopy. It was found that the highest molecular orbital was of n-type. This result motivates similar studies of the other azines. In the present paper pyrazine will he studied in the same way as striazine. For de*ails of our method we will therefore refer to the earlier paper.

-n-iEoBETlCAL

!XRUCTURE

OF PYFWZlNE

The molecular orbitals of pyrazine are given in Table 1 in the same manner as in the triazine paper r_ The heavy atoms are numbered consecutively around the ring and the hydrogen atoms have the same number as the nearest carbon atoms. To describe the orbitafs reference is made to the symmetry of pyridine (C,,) in the same way as in some theoretical calculations 2. The numbering of the orbitals from our study of pyridine’ has been used and will be retained in this paper indepenZnt. J. Mass

Specrrom.

Zon Ph_vs., 8 (1972)

101-l 18

102

c_ FRIDH,

TABLE -ED

FOR-

HikKEL AKD

THE

AND

ORBITAL

CALCULATfON

ALLOWED

ZP

ZP

/ictic (this paper)

Vertical (this paper)

meby

(ev)

tev)

ii.659 10.17 13.1 10.93 9.216 -16.5 -13.5 -14.7 -16.5 16.1 -20.5 20.0 23

ii.7 102 13.3 ii.4 9.4 -17 14.0 15.0 -17 1

pyridine c2,

ENTRCIES (EHM)

RYD!3ERG

orbit&J refmed to symof

B. 6. JONSSON.

E LINDHOLM

1

APPROXIhfAIE (IP)

I_ ASBRINK,

OF THE _MOLECULAR

-

WTXi

RiE

ORBlT’ALS

MEASKED

OF PYRAZlh’E IONIZ4TION

FROM AN

POTENTI=

TR.4XSITlO?JS

&proximate

form

(=A#)

Type

Species

Orbital ener

W~ect)

ENM

D 2h

(r/h

.7

162,

z

Ibis

-7

1b3m

t

363,

t t

2

r

462,

r

5&u

r

5+

2u

S

-21 20.6 24

S

g

S

3s

S

262,

s

3&u

S

3%

16.0 13.3 16.7 14.6 12.9 16.7 14.7 15.7 17.2 16.2 20.4 20.0 23.3 27.9 29.7

a Cannot be determined.

dentiy of the values of the orbital energiesdeduced. The correct species in I&,, has also been indicated_ In our calculation of the orbital energy and the approximate form of the orbitals an extended Hiickel method was used._Some other calculations of orbital energies are also included in Table 1. CIementi’ and Petke, Whitten and Ryan4 used ab initio SCF calculations_ Del Bene and JafF&‘*6 used a CNDO method, Yonetiwa, Kato and Kate’ and Sundbom’ used semi-empirical methods. The allowed Rydberg transitions have further been included in Table 1 together with the expected quantum defects (CL ref. 9). We expect larger quantim defects for npbz (e.g. 0.61inTable L) than for xpu, (0.50in Table 1) as the hydrogens infiuence the quantum defects more in the former case.

THE PHOTOELECTR ON SPECTRUM OF PYRAZINE

The photoelectron spectra obtained with the 304 dL and 584 A heiium lines are shown in Figs. 1 and 2_ Details of the photoelectron spectrum are presented in Table 2. The O-@bands 9.216 eV and Il.659 eV are measured accurately (calibration -with argon). The vibrationai energies of the &t progression may be wrong ZriLJ.

i%zs

SpccirOM.

Ion

?%ys.,

8

(19X!) 101-118

paper)

RYDBERG

SERIES

IN

SMALL

MOLECULE:.

XV.

Allow& RJdberg trunritionr (RJaBerg orbitals tabulated (C,,))

gies (negarice) Clementi’

103

Petke*

Del Beness6

Yonezawa7

Sundboma 6 M 0.8

13.4 12.6

13.1 12.0

17.7 16.6

17.2 16.4

12.0 20.5

12-2 20.3

18.6 14.5 21.2 19.7 26.0 25.3 30.8 34.6 37.4

18.7 14.7 21.5 20.1 26.0 25.4 30.5 34.8 37.3

IO.9 11.1

12.2 10.7

10.1

-

val*

n&l

-

nsal -

npal

npbl -

nd -

-

m&*

npal’

npbl

-

nsa;

-

-

-

nd

nsaI

-

-

-

nd

nw71 -

wb2

nwl

-

nd -

-

13.1

10.1

11.2

12.5

13.8

6M 0

npb2* npb2

10.3 9.3

12.6 11.6

6 M 0.61 d w 0.50 6 M 0.2

rrpbl

* Obsened.

a, s

18

16

lOal r

6b2

r

lb1

12

14

H

Fig. 1. Photoelectron spectnm

2bl ?b2 -n t

-IT

of pyrazine

”

F

“c\

la2

Yt

llal

t

10

eV

N\

N’

? ,C”

(a) With 304 A helium line. (b) With 584 A helium line. fnt. J. Mass

Spectrom.

Ion Phys.,

8 (1972) 101-l I8

104 TABLE D-

9.216 *I meV

IO.17

IO.93

il.659

C_FRIDH,

&~ONSSON,E

2 BANDS

IN -SHE PHO-ON

ilal

Ia2

762

zbb,

ilmeV

13.1

16.1

L_ ASERINK,B_

9al

SPECTRUM

0 75 132 150 170 207 225 245 282 300 320 340 375 39s 41; 450 470 49O 525 54s 565 0 70 128 175 0 70 138 206 270 338 0 70 110 140 180 220 250 288 358 0 72 I35 195 262 340 380 0 B-u)0

OF PYRAZINE

60 95

9 9s SO 10 60 100 10 40 90 20 25 40 35 9 2S 2S S 10 IO 100 2O? 40 30

o-o I

QO 10’ Q02

&O’

IO’ 6ao’ 3

&O

8a01 6ao’ lo'6ao* -a &O

8ao' 4au2 ho2 6aOS &lo’

6u0=

&02

6aOl 6

600

8ao' 6a04 8a02 6a02 &O’ 800’

6aO5

8a02

6a03

04 QO’ lo’ 8ao’

0-o QO’ 640 6aO &O

2 1 4

&OS

100 4s 7s 20 so IS 15 30 S 100 40 25 60 25 3s 25

Int.1. Ma.ss.Spec~o~.ZonPhys.,8(1972)1OI-118

0 k0’ 212 lo’ IO

6ao’ 2

IO’

6002

102

6aO’

102

ciao*

04 QO1 lo’:

&IO2

8ao' lo*:8u01 6a01 102 6ao' 3 IO

0-o 2or

L~NDHOLM

RYDBERG

3

SERIES

t

,

IN

i

_/-A

XV.

105

,

Ia4

142

118

120

122

MOLECULES.

I

la6

lo.8

SMALL

la0

116

ti4

112

110

10.6

r‘

=_-

-_

-~v_” lb, 136

,

13.4

li2

-L -< 190 eV

Fig. 2. Photoelectron spectrum of pyrazine with 584 A helium line showing the vibrational structure of the first five bands. A study of the first band at 9.216 eV with the 1215 A hydrogen line has been published earlier’=. The vibrational energies are given in meV.

by +3 “/dowing to drift during the measurement. This error is the same for ail bands in the progression. For the other progressions the same is valid but here the breadth of the bands influences the uncertainty more. Our spectra agree well with earlier studies at lower resohttion10-12. The vibrations are numbered according to the scheme used by Lord, Marston and Miller’ 3 and in other studies of vibrational spectra 14-16. The Crst band at 9.216 eV (cf. ref. 17 and Yencha and El-!Sayedl*, who obtained 9.29 eV) exhibits a long vibrational progression which was anaIyzed earlier”. The progression is very regular without anharmonicity, The vibrations 75 meV and 132 meV were interpreted as v6, and vl, respectively, but for the vibration 170 meV two interpretations are possible: v,, and qro_ To solve the problem directly, deuterated pyrazine should be studied, but we have not been able to do

106

L’. FRIDH, L. ASBRINK,

B. 6. JONSSON, E. LINDHOLM

this. However, Parkin and Inneslg studied Rydberg transitions with the same vibrational structure as the 9.216 eV band. As, in their study, the vibration mentioned had about the same energy in ordinary {Table 4) and deuterated (Table 6) pyrazine, the vibration must bc of ring type: ash_ In a study of the changes in vibrational frequencies Turner et aLzoD1’ interpreted the vibration 73 meV in this band as the ring-breathing mode v1 _ To discuss this we will make use of the combined information from our photoelectron band (Table 2} and Parkin and Innes’ Rydberg transitions (Tables 3-6), which shows that four fundamentals (Ij6=, y1 , v,, , and us,) are present in this band. As the fifth fmdamental, the C-H stretching vibration v2, has much higher energy, all possible fundamentals (A,) have thus been observed With the assumption that v,, has the lowest vibrational energy amongst them, Turner’s interpretation of the vibration 75 meV as vi is evidently impossible. On the other hand, the parahel bond vibration, yga, has a much lower enera (170 meV) than in the Raman spectra (I96 meV). This shows that the ionized electron is strongly bonding with regard to the parallel bonds, but in disagreement with Turner’s rule no long progression is obtained in this vibration. Obviously, the rules obtained by Turner for tLe changes in vibrational frequencies are not important fur pyrazine although he has estabiished their importance for smaller molecules. No explanation cao be given for this deviation. We will, however, assume that our result is valid also for similar six-membered rings (ttazine’, pyrimidine”, pyridazine *‘, and pyridine23). The band at 10.17 eV has non-bonding character with a strong O-O band. The diffuse vibrational band indicates the presence of several unresolved fundamentals, probably also v&. The third band at 10.93 eV is probably similar to the first band with a long vibrational progression, but as it :s diffuse the analysis is incomplete. The fourth band at II .659 eV is of non-bonding type (cf. ref. 17). It is somewhat similar to the benzene band at 9.241 eV with the same fundamentals excited (especiaily v& and zrl). The fifth band with adiabatic IP 13.1 eV exhibits vibrational structure (Fig. 4) with a strong parallel bond vibration vg, (195 meV). As it has appeared that extended Hiickel calculations in the case of five- and six-membered rings usually give good ionization potentials for s-type orbitals, the orbitals with high 1~ can be interpreted immediately. The very broad peak with maximum around 24 eV will be interpreted as due to 3bz, and the band with larger area and maximum at about 20.8 eV as the two nearly degenerate orbitals 7al and 4b,. The peak with maximum at 16.1 eV has too small an area to be interpreted as a molecular orbital of pyrazine. It has also been observed in other studies’2*‘oThe explanation is probably that this peak is the first in a progression and that the other bands of this progression are overlapped by the broad band around 17 eV. The vibrational energy must then amount to about 300 meV. In pyridazine this Int. 3. .MzssSpecrrom.

Ion Phys.,

8 (1972) 101-l

18

RYDBERG

SERfES

IN

SMALL

MOLECULES.

107

XV.

progression is not overlapped, and the vibrational energy is 320 meVz2. In benzeneI’ the vibrational energy is 290 meV_ Comparison with benzeneQ*24 and the extended HGckeI calculation indicates that this band is due to the s-type orbital 9a,. According to Table I this orbital is strongly CH-bonding. Also in the case of r-type orbitals the extended Hiickel method has been found to give reliable ionizatioa potentials ‘_ We will therefore interpret part of the peak with maximum at 17.0 eV as due to 8a,. The z orbital lb1 has in benzene the energy 12.2 eV ‘s2’, in pyridine 12.5 eV3 and in s-triazine 13.2 eV’. As our EHM calculations indicate a slow change with the number of nitrogenatoms, we expect the IP in pyrazine at about 12.8 eV and identify this orbital as the band with adiabatic IP 13.1 eV_

ELECTRON

1h~Ac-T

ENERGY

LOSS SPEXX-RUM

AND

ULTRAVIOLET

SPECTRUM

OF PYRAZINE

In the electron spectrometer, which will be described separatelyt6 (see also ref. 1), pyrazine was studied at an electron energy of 150 eV and scattering angie 0”. The electron spectrum is shown in Fig. 3. It gives a good overall view of the transitions of pyrazine. Part of the energy range was studied at higher resolution in the vacuum ultraviolet between 2800 A and 1500 A by Parkin and I~es’~. They also studied the deuterated compound_ For interpretation of the electron spectrum (Fig. 3) reference is made to earlier studies. McWeeny and Peacock2’, Favini, Vandoni and Simonetta”, Fischer-Hjalmars and Sundbom”, Yonezawa, Karo and Kato7 and others (see Innes, Byrne and Ross’~) have studied IC+ 3~*and n + IL*transitions theoretically, and Innes, Byrne and ROSS’~ have reviewed and interpreted the spectroscopic studies. The strongest transition in the spectrum at 7.68 eV corresponds probably to the strongly allowed x --, z* transition ‘El” in benzene and to the ‘E’ transition in s-triazine’, but in pyrazine splitting is expected. The theoretical studies indicate a splitting of 0.05 eV27, 0.16 eV2’, 0.8 eV7 or 0.9 eVZQ. We will interpret the strongest bandin Fig. 3 at 7.68 eV as ‘B2, and I&,_ That the major part of the intensity of the strongest band is due to a n --, IE* transition and not to a Rydberg transition could be proved in the case of s-triazinel. However, some of the theoretical calculations7~‘Q*30 indicate a large splitting in the case of pyrazine but smaller splittings for pyridine, pyrimidine, pyridazine and s-triazine, and there is a large hump at 8.3 eV in pyrazine which has no correspondence in the last mentioned molecules. Further, Fischer-Hjalmars and Sundbom2’ give a smaller f-value for the high-energy transition. This suggests that part of the intensity at 8.3 eV might be due to the it --, IL* ‘Blu transition. On the other hand, the lla, + np Rydberg transitions are quite strong in pyrazine. Our measurements are not conclusive on Int. J. Mass Spectrom. Ion Phys., 8 (1972) 101-l 18

C. FRIDH,

108

,

7

f,

-,

8

,

I,,

;

1

10

11

L. &BRINK,

*

,

12

,

B. 0. JONSSON,

I,

*

13

14

E. LINDHOLM

eV

Fig- 3. The electron impact energy loss spectrum of pyrazine. (a) Low resolution. The impact energy is 150 eV and the scattering angle is 09 (b) H&her resolution. The impact energy is 200 eV and the scattering angle is 0”. The high-intensity peak has been drawn to the same scafe as the rest of the curve.

this poIz~t_The hump cannot be seen in Parkin and 1n11es”~ curve owing to drop in intensity_ Also, the other valence transitions have high intensities in Fig. 3. According to tk review by Tnnes,Byrne and Ross l6 the following transitions can be identified: Maximum 3.9 eV: n 3 3t* ‘BJ, Maximum 4.8 eV: it - a* ‘BZ, Maximum 6.5 eV: x-x+ ‘B1, Maximum 7.68 eV: x - K* ‘B2,, and IBIb. The x - z* transition at 6-5 eV is partly overlapped by a strong Rydberg transition (see below), but the maximum is free from overlap and not due to the Rydberg transition. Znr_X Mau Spec&om. Zon Piz_xs.,8 (1972) 101-l 18

RYDBERG

SERIES

IN

SMALL

MOLECULES.

XV.

109

The Rydberg transitions will be discussed in detail. ila,

-+ np (+

9.216 eV with 6 = 0.61 and 6 = 0.50)

These transitions have been found both by Parkin and Innes and in the eIectron spectrogram. TABLE

3

RYDBERGTRAY!IIToNsINUV'~IN

Py~~zxm

(cm-')

I

Energy

(ev)

Vibrational energy (meV)

Interpretation

IZal --;F3pb2 (-+ 9.2i6 eV with 6 = 0.61) 55154 6.838 50 55786 917 100 56145 961 5 56336 985 2 56418 995 50 56597 7.017 20 56965 063 3 57055 074 20 57228 095 30 57393 Ii6 5 57591 140 2 57691 153 3 57857b I73_ 20 58014 193 4 58481 251 15 llal + 4pb, (-+ 9.216 eV with 6 = 0.55) 8.073’ 65746 8.152 ZZal + 5pb, (+- 9.216 eV with 6 = 0.52)

69504

8.538. 8.617

0 79

123 147 157 179 225 236 257 278 302 315 335 355 413

o-o 6%’

10’ 9ao' 66-I

2

So,,’ 90,’ 6a01 3 6QO

8ao’ 6ao’ lo’ 6ao2 9ao’ 6aQ2 k04

8a01 6a02 8a02 8ao’ 6a03

0 79’

WII

0 79’

o-0

6Q01

600’

Ertimation in this paper. b Overlap. a

Parkin and Innes found a very long progression of sharp intense bands at 55 i54 cm-'. We have found it necessary to divide the long progression into two progressions (Tables 3 and 4) and obtain in this way very good correspondance with the vibrational structure of the photoeiectron band for both progressions. The two progressions will be interpreted as two Rydberg transitions for R = 3 with quantum defects 0.61 and 0.50, respectively_ Also in pjrazine-d, two similar progressions are obtained with the same quantum defects (Tables 5 and 6). In our interpretations the hydrogen-breathing vibration vt is not excited. This is satisInc.3. MQSSSpectrom. Ion Phys., 8 (t97f) lOl-1_18

C. FRIDH,

110 TABLE

L. &BRINK,

B. ij. JONSSON,

4

RYDBERG

-IRA??Il-ION

13~~ -+ 3pal (+

IN WI9

IN I’-

9.216 eV with 6 = 0.50)

(cm-‘)

I

Vfbrationol energy
Interpretation

56855 57491 57857* 58114

7.053 128 173 20s

10 20 20 so

0 75 120 152

O-o 6mx’ 10’

58319 53431 58716

231 24s 280

10 5 80

178 192 227

8~~~ lo’ 6&l 6QO 3

55902 59059 59325 59484 59925 6m89 60542

303 322 355

20 8 80 2

250 269 392

SO,’ 6a0’ 101 6ao2 soo4

322

cto,' t&J' &OS SaoE 6aoJ

375 -

60693

TABLE

+-

(cm-

‘)

430 450 506

20 3

377 397 453

525

10

472

8a01 6a04

Vibrarional energy Wev)

Interpretation

0 75 104 124 151 174 180 227 249 256 277 325 352 399 423 500

O-O 6ao’ 9a01 lo’

h0

6

5

RYDBERG 1 la,

SpO 2

TRAXSITION

IN UV1’

IS PtrRuIXE-d,

3pb2 (+ 9.216 eV with 8 = 0.61)

5528-S 55896 56126 56286 56507

6.855 930 959 979 7.006

50

IO0 20 5 50

56691

ow

m

56741 57118 57298 57350 57527 57909 58130 58509 58697 59324

035

20

fnt_ J_ M’

082

5

104 Ill 132 180 207 2S4 278 35s

20 1G f5 10 20 5 IO s

-*

Spectronr, Ion Phys., 8 (1972) 101-118

Sa0=

8ao’ 9a01 600’ sao5 800’ 60,’ 900’ 6a02 9ao’ 800’ 8ao’ 6a02 9a01 8ao’ 6a01 8aoL 6a03 9ao’ 8ao’ &to2 9ao’ Sao’ 6ao’

E. LINDHOLW

RYDBERG TABLE RYDBERG

SERIES

IN

SMALL

MOLECULES.

111

XV.

6 TRANSITION

f In, --t 3pa,

IX W”

(+- 9.216

P-4 P?%AZINE-&

eV with 6 = 0.50)

Energy

(cm-‘)

Interpretation

Vibrational energy

I

(eV)

(me v) 7.065

56979 57605 58210 58390 58825 58985 59442 59576

o-o

0

5 50

142 217 239 293 313 370 387

77 IS2 174 228 248 30s 322

100 10 so 20 so 50

6ao’ e0

2

8a01 3 &O

8a01 6ao’ 4 6aO

8a01 6a02

factory for, in Parkin and Irmes’ interpretation, v2 of pyrazine-d, had to be given the improbable value of 2317 cm-‘. From the study of the deuterated compound it follows that the vibrations vg, and vga can be distinguished. From a comparison with the photoelectron spectrum the vibration with energy 236 meV will be interpreted as 6a03 and not as 102. The isotope shifts of the Rydberg transitions are given in Table 7. TABLE

7

ISOTOPE SH1Fl-S OF RYDBERG

1 la1 + II& +

3pb. 3put

TRANSITIONS

IN PYRAZINE

P> razine

Pyrazine_di

Shift

(ev)

(e V!

(eV)

6.838 7.@53

6.855 7.065

0.017 0.012

*-

r,

In the eJectron spectrum all major bands in these two progressions can be found (Table 8). For II = 4 and n = 5 the information is less complete. Greer, Parkin and Innes (see ref. 16) have observed also the 4pb2 and the 5pb2 transitions (Table 3). They remark that the 0-O bands are more pr&ninent than in the 3pb2 progression. We iri%pret these bands instead as GO1 and obtain in this way acceptable quantum defects. The extrapolation by Greer et al. resulted in a somewhat too large ionization potential. In the electron spectrum the two n = 4 progressions overlap partly (Table 8) but ail major bands that can be expected are found ** From the quantum defects it follows that the Rydberg orbitals are probably Int. f- Mass Spectrom.

Ion Phys.,

8 (1972)

101-l

18

112

C.FRIDH,L.

ASBRINK,B_&JONSSON,E.LINDHOLM

TABLES RYDBERG

TFU?SrA-IONS

Energy

IN THE El&-ON

SPECTRUM

OF PYRAZIhZ

(Fig. 3)

- Interpretation

I


llal +3pb2 artd3pq (+9_216 eV) 3pb2 (c5= 0.61) 6.M SO O-o 6.92 100 Qo' 629 50 6uo':&lo' 50 7.09 6a03: &,,I 6aO’ ?_I3 20 7_20 50 7_29 80 7.37 80 7.44 40 IIal

+

4pbz and 4pal

8.06 8.15 8.23 8.32 8.41 8.49 8.57 5.64

1 6 S 5 3 2 1 0

4pb, (CS= 0.57) O-0 6QlJ' 6ao' 6%=

4pul ta = 0.43)

o-o 6a01 6a02 ha

3

2:

6a06

0.61) and npn, (S = 0.50). This makes possible a determination of the symmetry of the molecular orbital at 9.216 eV. From Table 1 it fohows that the only molecular orbitals giving transitions to these two Rydberg states are 7b,, 1la, or 8a, _ As, according to the theoretical calculations, 7b, and 8a, have higher m’s, the orbital with IP 9.216 eV must be 1la, _

vhf (6 =

It seems to be impossible to take la2 as the highest orbital, as according to Table 1

only one Rydberg transition with this quantum defectt would be obtained. The state of the molecule after the Rydberg transition at 55 154 cm-’ is, with our interpretation, ‘Ball (in Dzh) in agreement with the conclusion by Parkin and lnnes from a band contour analysis_ Tt follows from the extensive vibrational structure of the band-at 9.216 eV that the “lone-pair” orbital has not much “lone-pair” character (cf. refs. 2, 31-33). Pyraxine is simiIar to s-triazine’ with a “Ione-pair” orbital as the highest orbital_ w-5 f + 11.659 eV with 6 = 0.45) The fourth band at I I.659 eV in the photoelectron spectrum is of “lone-pair” type. 1t is connected with one Rydberg series in the electron spectrogram (Fig. 3

2b1 +

ht.

L

M&s

S)ectrom-

Zon Phys_, 8

(1972)101-l18

RYDBERG

SERIES IN SMALL

MOLECULES.

I13

XV.

TABLE 9 RYDBERG

TRANSITIOEis

IN THE ELECTRON

SPEClRUM

OF PYRAZIhz

(Fig. 3)

2b, -+ npaz (3 11.659 eV) Energy 957

Infensify

Transition

Very strong

3pa, (a =

Weak Weak overlapped

4pal (6 = 0.42) 5pu, (6 = 0.46)

Vibrations 0.45)

9.72

O-O lo1

10.60 11.00

and lox 6uoz

and Table 9). The band at 9.57 eV seems to have the expected vibrational structure and gives 6 = 0.45 for n = 3. Also the n = 4 band and probabIy also the n = 5 band are present. IO.17 eV with 6 = 0.56) The second band at 10.17 eV in the photoelectron spectrum is also of “ionepair” type_ No certain Rydberg transition can be found either in the curves published by Parkin and 1nnes or in Fig. 3. Important conclusions can, however, be drawn from the observation that no s-type Rydberg transition is present. As such a transition is expected to have a quantum defect of about 0.8, it should be observed at about 7.4 eV as an almost single strong peak amongst the bands of the previously discussed Rydberg transitions (Tables 3 and 4). As no unexplained bands are present in this spectral region, we can be sure that the s-transition is

la, + npb, (+

absent. From Table 1 it then follows that p-transitions must be present. The Rydberg transition la, + 3p is overlapped by the very strong IZ--, xc* transition. Possibly the hump at 7-81 eV in the electron spectrogram (Fig_ 3) indicates this transition with 6 = 0.56. In such a case, the weak uncertain peak at 9.03 eV might indicate the 4p transition with 6 = 0.54 (Table 10). ln the uhraviolet, Parkin and Innes also found some structure around 7.85 eV. TABLE 10 RYDBERG

Ia2 +

TRANSITIONS

zpb,

Enersy

(+

IO-17

IN THE ELECTRON

SPECTRUM

OF PYRAZIhz

(Fig. 3)

eV)

Intensity

Transition

Strong Uncertain

4pb2 (d =

(eV) 7.81

9.03

3pbz (6 = 0.56) 0.54)

Discussion of the Rydberg transitions of 2bl and la, As the photoelectron bands at 10.17 eV and Il.659 eV are not connected with s-type Rydberg transitions it follows from Table 1 that they must be 26,) la,, In?. J. Mass

Specn-om.

Ion Phys.,

8 (1972) 101-118

c. FRIDH, L. ASBRINK, B. 0.

114

Jomsoh’, E. L~NDHOLM

7b2 or 1la,. It is important to find that lOa,, which is the “lone-pair” orbital that has been discussed in several theoretical studies, stems to be out of question, as it gives rise to an s-type and not a p-type Rydberg series. To choose amongst the four possibilities we first remark that lla, has a lower IP (see above) and is bonding. This finding, together with information from benzene and triazine’, shows that the “non-bonding” photoelectron bands probably correspond to x electrons. As the theoreticai calculations (Table 1) show that la, has a lower LP than 2bI we interpret the 10.17 eY band as la, and the 11.659 eV band as %,_ It is finally satisfactory (see Table 1) that the quantum defect 0.45 of the 2b, --+np transition is smaller than the defect 0.56 of the ia? ---,np transirion. It is remarkable that no other Rydberg transitions can be identified. Especially, no Rydberg transition connected wirh the z-type orbital lb1 can be seen. A similar observation was made in the case of pyridine3.

ELECTRONICSTRUCTUREOF

PYRXZINJS

The third orbital with an adiabatic IP 10.93 eV has not yet been discussed. The Iong vibrational progression (Table 2) is similar to that of the 9.216 eV orbital and thus indicates t-type, i.e. 7bz. As EHM calculations seem to give good approximations of IP’S of r-type orbitals for ring cornFounds’, we expect 6bz at about 14.7 eV and lOa, at about 15.7 eV and obtain the assignments given in Table 1 and Fig. 1. That the energy difference between the orbitals 1la, and IOa, shouId be large has been pointed out by Hoffmann32. 33. Finally, the “one band per orbital” principle” may be used to identify the t-type orbital Sb, at about 16.5 eV_

COLMPAFWSONWITH

OTHER MOLECULES

A comparison of pyrazine and s-triazine’ with regard to the orbitals and orbital energies is of interest. Firstly, the t-type orbitals can be compared. The two pyrazine orbitals 7b2 and lla, merge into the triazine orbital 6e’. It is then satisfactory that the triazine IP ,O.Ol eV is the mean of the pyrazine P ‘c, 3.216 eV and 10.93 eV, and that all three photoelectron bands have vibratic=lal structures of a similar type. Secondly, the vertical IP of the r-type orbital of triazine, 14.7, is the approximate mean of the IP’S of the corresponding orbitals of pyrazine, 6b, and l&r,. Thirdly, the 7~band 2b, at 11.659 eV in pyrazine has approximately the same IP as the z band le” at 11.69 eV in triazine. The explanation is that both orbitals have a large percentage nitrogen character (Table 1 and ref. 1). 11%J. Mass Specwom.

Ion Phys., 8 (1972) 101-l 18

RYDBERG

SERIES

IN

ShiALL

MOLECULES.

115

XV.

A consequence (see e.g. Dewar and Worley34) of this observation is that in pyridine the ionization potential of 2b, should be the mean of the value in pyrazine and in benzene, i.e. about 10.5 eV. It is satisfactory that there is a narrow peak in the photoelectron spectrum of pyridine at this energy (10.4 eV). This peak ti earlier been interpreted as due to a lone-pair orbital of pyridine3*10*35.

CHARGE

EXCHANGE

MASS SPECTRUM

OF PYRAZINE

Charge exchange mass spectra have been measured with a tan&m mass spectrometer9 in the same way as in earlier papers in this series (e.g. ref. 1). The mass spectra corrected with regard to ’ 3C and “N are presented in Table 11. The mass spectrum as a function of the energy absorbed during the ionization is displayed in Fig. 4.

TABLE

11

MASS SPECIRA

OF PYRAZRJJ

KINETXC ENERGY

Znciifenf

ion

COS’

OBTAINED

IN CH4XGE

EXCHAKGE

WI-IX

INCIDENT

POSiTIVE

IONS OF LOW

(KE)

Recombinafioz energy (eev?

KE (ev)

11.2

30 10 3 I 30 10 3 1 30 10 3 1 30

5 4 2 1 7

10 3 1 30 10 3 1 30

4 5 3 I2 12 9 8 76

1 PI 89 I 92 2 86 2 86 2 89 290 12 12

10 3 I

78 76 58

11 9 14

XC+

12.13 13.43

CO2’

13.8

CO’

14.0

Kr’

14.00 14.67

Arf

15.76 15.94

Electron impactJ6

mle 26

Q 52

53

79

80

51

2 1 1 1 2

96 98 100 100 55 53 52 50 13 11 il 10 4

(2) (2) (2) (2) (3) (3) (3) (2) (3) (3) (3) (3) (4)

25 30 30 50 64 67 64 68 22 28 28 35 20

2 3 2

(3) (3)

(3)

9 40 20 20 22 36 7

(3) (3)

1: 23

4 2

2

45 47 48 48 80 84 86 88 85

11 15 52

2 2 2

1 100

Inr. 1. Mass Spectrom. Ion Whys., 8 (1972) 101-118

116

C_ FRIDH, L. ASBRINK, B. ij. JONSSON, E. LZNDH0L.M

Fig- 4. Mass spectrumof pyrazineas a function of the energyabwrbcd during charge exchange with kckient positive ions of low kinetic energy.

The characteristic, feature of the mass spectrum of pyrazine is that HCN is Iost giving the fragment ion 53+ (C3NH3+)_ This is especially obvious with the incident ions COzi and CO’. The photoelectron spectrum (Fig. 1) shows that at these energies (13.8 eV and 14.0 eV) the ionized electron comes from the lb: or 6bz orbitals. If the ionized electron comes from 6bz the C-H bonds are weakened and we expect possibly loss of hydrogen and formation of (p- I)*_ This process seems to take place to a very limited extent (Table 1 I and ref. 36) but we cannot imagine ring breakage to take pIace after this ionization_ The ring breakage must therefore be due to the ionization of lb, _ As pointed out in our study of pyridazine22, ring breakage does not seem to be caused by weakening of the ring bonds. We wilf therefore explain the ring breakage in the same way as in s-triazine’, pyrimidine2’, and i>yridazine22_ We observe that, according to Table 1, lb, is localized mainly on the nitrogens. Ionization therefore means ionization of one of the nitrogen atoms: CH=CH _

After the ring breakage HCN is lost giving the fmgment ion 53+_ With incident Ar_’ ions (RE 15.3 eV) ring breakage gives 26) (C,H, ‘) probably after loss of HCN twice. The process is similar although the ionized electron is now 1&r, which, according to Table 1, means a lone-pair eIectron Iocalized on the nitrogens. TabIe 3 shows that below about 12.5 eV no ring breakage takes place-with COSt only parent ions are formed, and with Xef the recombination energy 12.13 eV gives only parent ions and RE 13.44 eV mainly ring cleavage. The process leading to breaking of the ring therefore does not occur with the R electron 2b, _ This r-mbles the situation in s-triazine and pyrimidine but is different from the situation in pyridazine. hzr_;T.Muss S@ectrom_ ion Pity-s., 8 (1972) 101418

RYDBERG

SERIES

IN

SMALL

MOLECULES.

XV.

117

IMPLICATIONS FOR THE JAHN-TELLER EFFECT IN BENZENE In an earlier paperl’ the vibrational structure of the photoelectron band in benzene at 9.241 eV (Ie,, of ‘ir type) was compared with the photoelectron bands in pyrazine which, according to the generally accepted interpretation, were supposed to be n-type. According to the present paper, however, these bands are instead of “lone-pair” type. The comparison should, therefore, be performed instead with the pyrazine bands a: 10.17 eV and 11.659 eV. The similarity between these two bands is large although one is more diffuse than the other, and they are, in turn, rather similar to the benzene band with approximately the same vibrational energies and approximately the same Franck-Condon factors, although in the benzene band the ring deformation vibration v6, (70-85 meV) is somewhat stronger_ It seems therefore to be impossible to study the .Ialm-TelIer effect in benzene by use of this comparison

ACKNOWLEDGEMENTS

A grant from Malmfonden- Swedish Foundation for Scientific Research and Industrial Development - has made possible the construction of the tandem mass spectrome?er and a grant from Knut och Alice Wallenbergs Stiftelse made possible the construction of the electron impact spectrometer. The work has further been supported by The Swedish Natural Science Research Council. The authors wish to thank Dr. Rolf Manne for help with the extended Hiickel calculations.

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I#. J. McYs Spertrom. Ion P&s.,

3 (1972) 101-118