Absolute vacuum ultraviolet absorption spectra of picolines

JOURNAL OF MOLECULAR SPECTROSCOPY lu),

11-22

(1986)

Absolute Vacuum Ultraviolet Absorption Spectra of Picolines C. KOSMIDIS, A. BOLOVINOS, AND P. TSEKERIS Physics Departmenl, University of Ioannina. Ioannina. Greece

Absolute extinction coefficientand oscillatorstrengthvalues of 2-, 3-, and Qpicolines were found from their medium-resolutionspectrain the region of 4.0-9.5 eV. Valence * + T* and n + ** transitionsas well as a 3s Rydbergband are presentedand discussed. o 1986 Academic Press Inc.

INTRODUCTION

Picolines are major parts of biochemical compounds. It is expected that their structure has many similarities with that of benzene and even more with toluene and pyridine. The main K + ?r* transitions seen in benzene should also appear, modified more or less in their energy, intensity, and vibrational structure. In addition, as in pyridine, n -+ ?r* transitions already have been observed in the UV, to the red of their lowest ‘K--* ?r* transition. In this work we present the gas phase absorption spectra of 2-, 3-, and Cpicolines (or CX-,@-, and y-picolines, respectively) in the UV-VUV region, up to their first ionization potential (IP). Values of transition energies, vibrational assignments, absolute extinction coefficients c, and oscillator strengthsfare found. The observed bands are due to singlet u + ?r*, n + **, and Rydberg transitions. Comparison is also made with the absorption spectra of toluene (I) and pyridine (2). There is not much work done on the excited electronic states of picolines. There is mainly an early exhaustive analysis of the near-UV n + 7r* and a + ?r* bands, seen with the photographic method (3), and a theoretical CNDO energy calculation of the lowest two n + ?r* and three a + 7r* transitions (4). EXPERIMENTALDETAILS

The l-m Rank-Hilger 776 monochromator (linear dispersion 16.6 &mm), the McPherson hydrogen light source, dual-beam sample chamber and detection system, the gas handling system, and the measuring procedure used previously (5) were employed to record the spectra. The samples studied were bought from Janssen Chimica and their stated purities were 98, 99.5+, and 99+% for 2-, 3-, and Cpicolines, respectively. They were used without any further purification, but they were degassed by several freeze-pump-thaw cycles. RESULTS AND DISCUSSION Fii 1a-f show the absolute absorption spectra of benzene (5), toluene (I), pyridine (2), and 2-, 3-, and 4-picolines. The first three spectra are included for comparison. 11

0022-2852186$3.00 Copy&t 0 1986 by Academic Press, Inc. AU rightsof reproductionin my form ramed.

KOSMIDIS, BOLOVINOS, AND TSEKERIS

I

x0.1

x

El”

5

6

7 ENERGY---*

a

9

EV

RYDBERG

5

6

7

a

9

ENERGY-EV

FIG. 1. The gas absorption spectra of (a) benzene, (b) toluene, (c) pyridine, (d) Z-picoline, (e) 3-picoline, (f) Cpicoline. Instrumental bandpass IB = 2.5 A.

WV

13

ABSORPTION SPECTRA OF PICOLINES

‘B, x0 .

5

6

7

ENERGY+

EV

6

7

8

9

A',

5

ENERGY---rEV FIG. I-Continued.

a

9

KOSMIDIS, BOLOVINOS, AND TSEKERIS

o-o

-I

5

6

7

8

9

ENERGY-eEV

1

L

o-

q

o-o

dr 1

x1/8

‘4”

x1/10

n-n’

IA 1180

'8" x1/20

J

3

‘6

.

f

.

a

_

4

4

ENERGY&V FIG.

I-Continued.

The instrumental bandpass of all spectra is 2.5 A. Spectra with smaller bandpasses were also taken in order to investigate possible additional structure. The error in the e values is 65% at the peaks and larger at the valleys of the spectra.

VUV ABSORPTION SPECTRA OF PICOLINES

15

u + I? Transitions The a +

?r* bands, which correspond

to the benzene valence shell transitions

‘AIg+ ‘Bw ‘Al, --* ‘Blu9and ‘Al, + ‘El,,, are clearly seen. They are ascribed to the benzene Dbh symmetry species for presentation reasons and in order to emphasize their close relationship to the parent molecule. In the C, (2- and 3-picolines) and CZV (Cpicoline) point groups, the symmetry of the Alg, Bzu, BIU, El, states of benzene becomes A’, A’, A’, A’ and A,, B2, A,, A, + B2, respectively, and the corresponding transitions from the ground state are all allowed. Table 1 contains the O-O transition energies and oscillator strength values found in this work, as well as values from other theoretical and experimental studies. Especially, experimentalfvalues in the literature are from liquid phase measurements. The errors in the energy position of the different bands are due to the wavelength calibration uncertainty and in some cases to the broadness of the respective transitions. The calculation offfor overlapping bands was carried out by approximating each side of the stronger band with a tail extending under the weaker one, as described in detail in Ref. (5). In this way we separated the spectrum into different bands, which we could integrate. The errors in the respectivefvalues are d 10% for the El, and B, u and =S15% for the Bzubands, and they are due to the errors of the 6 values as well as the approximate shape of the artificially separated bands. In the following we examine each ?r * rr* transition separately.

‘Alg + ‘El,, Transitions These transitions are presented for the first time. The O-O energies have values in the interval between the O-O values of toluene and pyridine, which is reasonable. TABLE I O-O Energy Values and Oscillator Strengthsfof

4-Pico’ine

R + ?r* Transitions

/ tie;r. / 6.3

a: This work, the number in parentheses gives the error in the last digit. b: From literature.Theor.valuesfrom Ref. 4, exp. values: m

from Ref. 3, B from Ref. 7 and I from Ref. 8.

16

KOSMIDIS, BOLOVINOS, AND TSEKERIS

There is also very good agreement with the theoretical predictions (4) for 2- and 3-picolines. On the other hand, we do not see any splitting due to the lifting of the degeneracy of the ‘Elu state, although the same calculations predict one, for 4-picoline. The other spectral features can be assigned to different vibrational transitions which, tentatively at least, are the ones shown in Fig. 1 and Table II. The uncertainty in the values of the excited state vibrational frequencies is not less than 10 cm-‘, because of the instrumental resolution and/or the diffuseness of some of the transitions. The vibrational structure of 4-picoline is quite similar to that of pyridine, 3-picoline is less similar, while that of 2-picoline is much closer to toluene.

‘A,, + ‘Blu Transitions These bands are also presented for the first time in vapor phase. Their O-O energies are also in the interval between the O-O ones of toluene and pyridine. In 2-picoline there is a scarcely discernible shoulder at the theoretically predicted 5.90-eV position which might be the O-O transition instead of our assigned one at 6.01 eV. As far as the structure appearing on top of B1, is concerned, we can propose a more or less detailed, yet tentative, analysis. For 4-picoline there are some Rydberg transitions which are much sharper than the valence ones and are discussed later. In 3-picoline we see a broad structured background on which lots of small and narrower transitions (seen mainly in a spectrum taken with higher resolution) are superimposed. In 2-picoline we can distinguish only one vibrational valence transition at -6.12 eV (which may be due to two excited vibrational quanta, if the O-O transition is the weak shoulder at -5.90 eV). The rest of the structure is attributed to Rydberg transitions as in the case of Cpicoline. The energy and proposed symmetry of the excited vibrations of the B1, state are given in Table II.

‘A,* + ‘B2,,Transitions The gaseous absorption spectra of these bands have been recorded photographically and were analyzed many years ago (3), but they were not given any symmetry assignments. Their absorption spectrum in solutions was also recorded and theirfvalues were evaluated (7). In 2-picoline there is a more recent absolute absorption spectrum andfvalue (8). The fvalues we found are in agreement with Platt’s perturbation theory of “spectroscopic momentum” (9). The O-O transitions of 2- and 4-picoline (Figs. 2a and c) are found to be between the toluene and pyridine ones. The case for 3-picoline (Fig. 2b) is not so clear because of the overlapping n + a* transitions. In Ref. (3) the transition at 2675 A (4.635 eV) is suggested as the most probable candidate for the O-O assignment. But this peak is further to the red of the toluene O-O transition, while we would expect it to the blue, because of the nitrogen contribution. Furthermore, the shape of the 2675-A transition is narrow enough and similar to the other peaks on its red side. We therefore think that the peak at 2665 A (4.652 eV) is more probable for assignment to the O-Otransition. It is broader and is also seen in solutions (7), where the n + ti transitions are absent. The different excited vibrations and their possible symmetries are also given in Table II.

WV

17

ABSORPTION SPECTRA OF PICOLINES TABLE II

Frequency Values and Symmetry of Vibrations in A --, r* Transitions Molecule

Vibration

Vibrational frequency in excited state (cm-l)

Vibrationala frequency in ground state (cm-l)

'A 'g +

Symnetry

Corresponding Benzene Vibration

'El" 1003

Toluene

A

960

Pyridine

A B

810 '190

L-Picoline

A

760

797

a'

12

3-Picoline

A

1010

1049

a'

1

4-Picoline

A B

910 '160

996 1276

a1 a1

4

A

900

1003

Pyridine

A B

370 850

405 990

2-Picoline

A

900

1050

3-Picoline

A

900

4-Picoline

A 6

820 1100 lAlg +

A :

1 9a

'Blu

Toluene

Pyridine

12

a,

'A + 14

Toluene

a1

a,

12 16b 1

a'

1

1049

a'

1

9965 1276

a1 a1

1 9a

'B2"

456 528

522 623

al

6a 6b

932 751 964 1189

1% 1030 1208

a1 b2 z; a,

1: 18a 13

327 950 1215

374 992 1218 884 1050 633

a1 a1 a1

16a ia

2-Picoline

A :

810 960 630

3-Picoline

X :

490 820 1000

534 922 1049

a' a" a'

6a 10a 1

250

388 678 996.5 1276

a_

1.53 .__ 6b 1 9a

A 4-Picoline

a. Ref. 6.

: 0

600 920 1180

a" a' a'

b; :I

10a 1 I%

KOSMIDIS, BOLOVINOS, AND TSEKERIS

ENERGY-EV

ENERGY-EV Fs. 2. n + T* and ‘A,, --DlBzugas absorption bands of (a) 2-picoline, (b) 3-picoline, and (c) 4-picotine. Instrumental bandpass IB = 2.5 A.

WV

ABSORPTION SPECTRA OF PICOLINES

19

5

4

ENERGY

-

EV

FIG. 2-Continued.

n + ?r* Transitions Figures 2a-c show in greater detail the n + ?r* bands of 2-, 3-, and 4-picoline. In 3-picoline we can assign the different peaks in two ‘A’ + ‘A” transitions. The O-O energy of the first is at 2885 A (4.297 eV) (3) and that of the second at 2740 A (4.525 eV) [which is also a possibility according to Ref. (3)]. One can see in Fig. 2b that these two bands have different shapes; specifically the lower energy peaks are broader. In 4-picoline we see only the ‘Ai + ‘Bi n --* ?r* band system. The ‘A,-I.42 one is symmetry forbidden. In 2-picoline, finally, the n * ‘lr* system is very weak. Table III gives the O-O energies andfvalues from our work and from the literature. It is clearly seen that these transitions are weaker compared to the case of pyridine. The main features of the observed vibrations and their possible symmetry assignments are given in Table IV. In all cases there appear the same vibrations 16a, 6a, lOa, and 12. The last three appear in pyridine, too. In pyridine, 16b is the weaker vibration assigned (I I), but in picolines its ground state values (6) are smaller than the excited state ones we see. Vibrations 15 and 16a are the weaker vibrations of picolines (6) which could be compatible. We propose the 16a since it is seen now and then in azabenzenes (2) while 15 is never seen. Unfortunately, our spectra do not have high enough resolution to base our assignments in other factors, like bandshapes, etc. As far as the other excited vibrations are concerned, 6a and 12 are close to their ground state values, while 10a suffers a large reduction. Actually, we would have great difficulty in assigning this vibration, if we did not have the case of pyridine, where 10a is also strongly reduced, and is considered responsible for the vibronic coupling

20

KOSMIDIS, BOLOVINOS, AND TSEKERIS TABLE III O-O Energy Values and Oscillator Strengthsfof Molecule

O-O?e")

O-O;e")

Pyridine

n --+ ** Transitions fa x1o-4

fb.c

4.31

P-Picoline

-4

30

exp. 4.30ga

4.308(2)

x10

exp. 9

9.2

L&z. 30

theor. 4.0 exp. 4300a theor.

4.297(l)

exp. B

. ^

3-Picoline

13

Gr.

t-zz-K$

4-Picoline

a. This work. last digit. b. Values a from

from

4.358(3)

The numbers Error

in parentheses

in f values

literature.

Ref. 3 and 9 from

c. The experimental

values

Theor.values Ref.

give

the error

in the

is 420%. from

Ref. 4, exp.

values:

7.

are taken

from

solution

spectra.

of the n + ?r* (‘B,) and ?r + ?r* (I&) states (12). We expect this vibronic coupling to exist in picolines, and this might explain the case of 3-picoline, where the smaller reduction of 10a appears in the n * ?r* state which is further from the lowest ?r + a* one. Rydberg Transitions As in pyridine, the highest occupied molecular orbital (MO) of picolines is n type (4). Thus, the Rydberg series converging to the frrst IP are expected to be different from the ones observed in benzene and methylbenzenes (5, IO), where the highest MO is of ?r type. In Figs. Id and e, we see that there is no distinguishable Rydberg structure to the blue of ‘,!L. The only clear Rydberg transitions are seen on top of ‘Blu in 4-picoline and less in 2-picoline. The sharp peaks in 4-picoline can be assigned to the vibrational structure of a Rydberg member with vibrational frequencies R N 5 10 cm-’ and P 1: 770 cm-‘, which are very close to the ground state values (6) 5 13 and 801 cm-’ of the ~CX(CY,) and 12(a,) vibrations. The IP value of the n MO of 4-picoline is reported to be 9.01 eV (13) or 9.56 eV (14). From these we can calculate a quantum defect 6 equal to 0.78 or 0.95, respectively, for the O-O transition. Thus a 3s assignment, which is also symmetry allowed, is well supported. Such a member is not seen, on the other hand, in the onephoton spectrum of toluene or pyridine, but it is seen in the two-photon resonant three-photon ionization of pyridine (15), where the 6a vibration is also active.

WV

21

ABSORPTION SPECTRA OF PICOLINES TABLE IV

Frequency Values and Symmetry of Vibrations in n -, r* Transitions Molecule

Vibration

Vibrational frequency in excited state (cm -'j

Pyridine

2-Picoline

3-Picoline

4-Picoline

a.

Vibrationala frequency in ground state -1 (cm

Corresponding Benzene

Symmetry

Vibration 16b

A 6

60 330

405 886

bl

C

550

605

D

995

1030

A

200

398

a 'I

16a

B

330

884

a '*

10a

C

545

545

a'

D

740

800

a'

12

A

270

400

a"

16a

B

485

534

a'

C

565

922

a 'I

10a

D

800

803

a'

12

A'

295

400

a I'

16a

B'

470

534

a'

C'

420

922

a 'I

A

245

388

8 C

360 465

873 513

D

785

801

10a

a2

6a

a1

12

a1

6a

6a

6a 10a 16a

a2

10a

a2

6a

"1 a1

-

12

Ref. 6.

In 2-picoline we can assign most of the peaks in two progressions of a -6 IO-cm-’ vibrational frequency which can thus be considered as overtones of the 6b vibration (ground state value 633 cm-‘). The O-O transition cannot be distinguished, on the other hand, although it would be expected to lie in the same place with the AA valence transition. We plan to further investigate these spectra with the MPI technique. In 2-picoline the peaks which we assign as Rydberg are not that sharp and the situation becomes worse for 3-picoline. This could be explained as follows. The 3s Rydberg orbital might be a nitrogen-centered one. In this case the methyl substituent could introduce a perturbation resulting in the shortening of its lifetime and the subsequent broadening. CONCLUSIONS

Absolute extinction coefficient values have been presented for the first time for 2-, 3-, and 4-picolines in the region of 4.0-9.5 eV. Their spectra were compared with the ones of toluene and pyridine. The ones which correspond to the benze rr * u* transitions do appear clearly and are in good agreement with theoretical predictions. The n + ?r* transitions are weaker than the ones in pyridine. A 3s Rydberg state with extended vibrational activity is clearly seen in 4-picoline.

22

KOSMIDIS, BOLOF’INOS, AND TSEKERIS ACKNOWLEDGMENT

We thank Dr. J. Philis for useful discussions. RECEIVED:

December 27, 1985 REFERENCES

1. A. B~LO~INOS,J. PHILIS,E. PANTOS,P. TSEKERIS,AND G. ANDRITSOPOULOS, J. Mol. Spectrosc. 94, 55-68 (1982). 2. A. B~LOV~NOS,P. TSEKERIS,J. PHILIS,E. PANTOS,AND G. ANDRITSOPOULOS, J. Mol. Spectrosc. 103, 240-256 (1984). 3. J. H. RUSH AND H. SPONER,J. Chem. Phys. 20,1847-l 862 (1952). 4. R. L. ELLIS,G. KUEHNLENZ,AND H. H. JA&, Theoret. Chim. Acta 26, 131-140 (1972). 5. E. PANTOS,J. PHILIS,AND A. BOLCNIN~~,J. Mol. Spectrosc.72, 36-43 (1978). 6. 0. P. LAMBA, J. S. PARIHAR,H. D. BIST, AND V. S. JAIN, Indian J. Pure Appl. Phys. 21, 236-242 (1983). 7. H. P. STEPHENSON, J. Chem. Phys. 22, 1077-1082 (1954). 8. W. ROEBKE,.I. Phys. Chem. 74,4198-4203 (1970). 9. J. R. PLATT,J. Chem. Phys. 19,263-271 (1951). 10. A. BOLOVINOS,J. PHILIS,E. PANTOS,P. TSEKERIS,AND G. ANDRITSOPOULOS, J. Chem. Phys. 75, 4343-4349 (1981). 11. J. P. JESSON,H. W. KROTO,AND D. A. RAMSAY,J. Chem. Phys. 56,6257-6258 (1972). 12. Y. MCCHIZUKI,K. KAYA, AND M. ITO, J. Chem. Phys. 65,4163-4169 (1976). 13. D. W. TURNER,Adv. Phys. Org. Chem. 4,31-71 (1966). 14. H. BABA,I. OMURA, AND K. HIGASI,Bull. Chem. Sot. Japan 29,521 (1956). IS. R. E. TURNER, V. VAIDA, C. A. MOLINI, J. 0. BERG, AND D. H. PARKER,Chem. Phys. 28, 47-54 (1978).