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Detection of n-π* transitions in pyridine and pyrazine in polyethylene solution by linear dichroism
Voh~me 23, number 2
CHEMICAL PHYSICS L ETFERS
15 November 1973
DETECTION OF II-rr*TRANSITIONS IN BYRIDINE AND PYRAZiNE IN POLYETHYLENE SOLUTION BY LINEAR DICHROISM Bengt NURDl?N
Received I August 1973 Revised manus~ipt received 30 August 1973
Studies
of linear dichroism. using a differcnthl technique, on pyridine nnd pyrazinc, quasioriented in stretched matrices, give evidence for the energy positions of the n-z* transitions. These mofecules are mainly oriented in the s&me way as bcnzcne, i.e., the orient~tian axes in ihe motecular phne are almost degenerate, snd the out-of-
pol~ma
plane pohrized
transitions
appear
as negative
linear dichroism
components.
A new variant of the stretched polymer - high linear dichroism method II-31 has been tried in our laboratory: the sample is injected into an IR cell containing the polymer matrix which has been stretched in adt:ance. By afterwards soaking out the sample a well reproducible zero-spectrum can be obtained and it is thus also possible to use the method for studying transitions in the re$xx where the matrix itself exhibits considerable linear dichroism (LD). Furthermore, since the same film can be used for different samples, this procedure allows for a better control of the orientational degree of the matrix than was clbtained with the previously used “stretch ratio” [4,5] . The film can be calibrated with a suitable standard (benzene) by recdrding the ratio LDl.4 (= 2(AI,--A,)l(d,,+A,)) [5] at a certain wavelength [33. In this form the method has been recently successfully applied in our 1aborato;y to quite small molecules (SO,, CS,, NG,, Br,, CQ, to give support to assignments for some intensive absorption bands. In the
r--
sensitivity
present
work,
on a series
which
of nitrogen
of determining
is the first report heterocycles
the polarization
from
[6],
of weaker
2 Q
-0.
5
‘i, -0
+QOIC
CII
-
0.0 I-
O
; l-
a study
the feasibility
Wovele~th
is demonstrated*. f For 5~description
of the stretched f%n technique applied to larger moI!culcs and a quantitalive evaiuation of dichroic absorption spectra, see refs. [7,8].
I
26 -2+o
transitions
I
1 1 1 300
(nm)
Fig. 1. Pyridine in PE and PP. Absorbance (- - PP, . . . PE) and LD (below, PP, PE). The nwnbers under the bands refer tof, the degree of orientation [S] .
200 ‘,
‘. .’
.’ .’ .,‘..
,,, .,._..
‘....
:
_‘_.
‘-
Volume 23, number 2
CHEMICAL
1
x
---
-_
0 oL7 -
Fig. 2. Pyrazine above. For other matrix was done i.e., f = 0.039 *
PHYSICS LETTERS
wo;
yz -
in PE and PP. Absorbanu: with PP is shown notarions, see fig. 1. Calibration of the PP with benzene: (LDIA),,, = 0.029 f 0.002, 0.003.
The nitrogen atom has a lone pair of electrons, and hence n-n* transitions must occur in the aza compounds. In the solution absorption spectrum of pyridine such transitions cannot be distinctly observed. In the vapour spectrum a weak system of discrete bands, partially submerged under the o! band (in Clar’s notation), has been assigned to the lA,-LB, (n-n*) transition [9, 101 - The symmetry of this transition (B1) implies x-polarization (for notation see fig. l), i.e., a negative
JD sign should
be expected
with the
molecular plane parallel to the stretch direction [1,5]. For stretched poIyethylene (PE), matrix effects other than mechanical orientation are small (vide infra), so orientation is determined by the unsymmetric shape of the sample molecules only. I.e., we assume the z and y axes to be approximately
equivalent
in an
orientational sense. (This is to some extent confirmed by the LD{A values which yield orientational degrees of the same magnitude as that obtained with benzene [3]t .) Th us, identification of tie n-n* transition is
15 November
1973
clear (fig. 1, PE, maximum at 273 nm). The positive bands (252, 257 and 263 nm) are due to yr-polarized transitions (rr-rr*, cf. the 260 nm system of benzene). Analogous assignments can be made for pyrazine (fig. 2, n-n* 320 nm, IT-Z* 250,255,260 and 267 nm). We have also used isotactic polypropylene (PP) as a matrix (figs. 1, 2 middle, PP). In most cases PP gives orientation effects which are equivalent to those in PE. However, preliminary results [ 1 l] indicate that certain molecules without a unique mechanical orientation axis (planar with Dnh symmetry II >, 3 [S]) in PP can exhibit secondary orientation effects, One of the equivalent orientation axes in the plane is preferred, probably as a result of association with the polymer chain; the orientation being determined by the charge distribution in the molecule (dipole and quadrupole moments) or hydrogen bonding. Orientation due to the electric field in the polymer is very small. This has been shown for a series of substituted benzenes by the absence of correlation between dipole moment and degree of orientation [ II] _If the increased size of the negative band in PP as compared to PE (at about 260 and 270 nm for pyridine and pyrazine, respectively) is due to the removal of the degeneracy (in orientational sense) of the z and y axes, it should be possible to distinguish between y and z polarizatioil as well as determining the x polarization. This suggestion (fig. 1, PP) is preliminary and mainly serves to demonstrate the further possibility, which is offered by two matrices with different orientational specificity to make a complete analysis of which transitions are x, y and z polarized. In the molecules considered here, any electric dipole transition must be polarized along one of.the coordinate axes but the theory can easily be expanded to molecules with lower symmetry. The degree Gorizn!ation,f(shown under the LD bands in th2 figures), is calc:dated separately for yz and x polarizations). The discr,spancy between thefvalues fdr high and low energy baMs must depend on a negative component under the high energy band, 7 As will be discussed elsewhere [6 J, the introduction orient:\tiomI
of two
degrees/,, and/, (for the axes acording to fig. 1; y may be the long axis, i.e., fy > J+,) will irnyiy ihat / must be more than twice as large as f,. to give a negat: ye L D for a r.polarized transition. *f= S(LD/A)/[6 - (LD/A)] fromyz-polarized bands,j= -4(LD/I)/[6 - (LD/A)] from x-polarized bands (S. 61.
Volume 23, number 2
CHEMICAL PHYSICS LE-ITERS
since the largest fshould be closest to the correct value. This study of the n-n* system could be compared to the recent reinvestigations of the pyridine spectrum by Ramsay et al. [ 12, 13 3 . Also for pyrazine our results are in agreement with rotational analyses [ 14, IS] , i.e., the n-n’ transition is at 300-310 nm. According to lnnes et al. [15] this region should contain two vibronic components; one allowed along the x axis (in our notation) giving sharp bands and one weaker “forbidden” z-polarized transition. Indeed, one must explain the 50% decrease in LDIA between the high and low energy side of this band system by the existence of a moderately strongyz-polarized band centered at about 310 nm.
References
I b. Davidson,
hf. Gouterman, L.Y. Johansson, R. Lwsson, B. NordBn md hf. Sundbom, Acta Chem. Sund. 26 (1972) 840. B. NordCn and A. David-n, Acta Chem. Sand. 26 (1972) 842. A. Davidsson and B. Nordk, Tetrahedron Letters 30 (1972) 3093. [4] B. Nordin, R. HIkansson and M. Sundbom, Acta Chem. Stand. 26 (1972) 429. IS 1 B. NordEn. Chemic3 Sniota 1 11971) 145. ii A. David&n, B. NordCnand hi. Sundbom. Acta Chem. Stand., to be published. 171 J. Michl, E.W. Thulstrup and J.H. Eggers, J. Phys. Chem. 74 (1970) 3878. 181 E.W. Thulstrup, J. Michl and J-H. Eggers, J. Phys. Chem. 74 (1970)
I thank Professors I. Fischer-Hjalmars and D.A. Ramsay and Drs. M. Sundbom and I. Chabay for interestkg and stimulating discussions.
1.5 November 1973
3868.
191 hf. Kasha, Discussions Faraday Sot. 9 (1950) 14. [lOI L. Goodman, J. Mol. Spectry. 6 (1961) 109. [I11 A. Davidss-on, unpublished results. [12] J.P. Jesson, H.W. Kroto and D.A. Ramsay. J. Chem. Phys. 56 (1972) 6257. [ 13) S. lapar and D.A. Ramsay. J. Chem. Phys., submitted for publication. [14] M. Ito, R. Shimada, T. Kuraishi and W. Mizushima, J. Chern. Phys. 26 (1957) 1506. [IS] K-K. Innes, J.D. Simmonsand S.G. Tilford, J. hfol.. Spectry. 11 (1963) 257.
CHEMICAL PHYSICS L ETFERS
15 November 1973
DETECTION OF II-rr*TRANSITIONS IN BYRIDINE AND PYRAZiNE IN POLYETHYLENE SOLUTION BY LINEAR DICHROISM Bengt NURDl?N
Received I August 1973 Revised manus~ipt received 30 August 1973
Studies
of linear dichroism. using a differcnthl technique, on pyridine nnd pyrazinc, quasioriented in stretched matrices, give evidence for the energy positions of the n-z* transitions. These mofecules are mainly oriented in the s&me way as bcnzcne, i.e., the orient~tian axes in ihe motecular phne are almost degenerate, snd the out-of-
pol~ma
plane pohrized
transitions
appear
as negative
linear dichroism
components.
A new variant of the stretched polymer - high linear dichroism method II-31 has been tried in our laboratory: the sample is injected into an IR cell containing the polymer matrix which has been stretched in adt:ance. By afterwards soaking out the sample a well reproducible zero-spectrum can be obtained and it is thus also possible to use the method for studying transitions in the re$xx where the matrix itself exhibits considerable linear dichroism (LD). Furthermore, since the same film can be used for different samples, this procedure allows for a better control of the orientational degree of the matrix than was clbtained with the previously used “stretch ratio” [4,5] . The film can be calibrated with a suitable standard (benzene) by recdrding the ratio LDl.4 (= 2(AI,--A,)l(d,,+A,)) [5] at a certain wavelength [33. In this form the method has been recently successfully applied in our 1aborato;y to quite small molecules (SO,, CS,, NG,, Br,, CQ, to give support to assignments for some intensive absorption bands. In the
r--
sensitivity
present
work,
on a series
which
of nitrogen
of determining
is the first report heterocycles
the polarization
from
[6],
of weaker
2 Q
-0.
5
‘i, -0
+QOIC
CII
-
0.0 I-
O
; l-
a study
the feasibility
Wovele~th
is demonstrated*. f For 5~description
of the stretched f%n technique applied to larger moI!culcs and a quantitalive evaiuation of dichroic absorption spectra, see refs. [7,8].
I
26 -2+o
transitions
I
1 1 1 300
(nm)
Fig. 1. Pyridine in PE and PP. Absorbance (- - PP, . . . PE) and LD (below, PP, PE). The nwnbers under the bands refer tof, the degree of orientation [S] .
200 ‘,
‘. .’
.’ .’ .,‘..
,,, .,._..
‘....
:
_‘_.
‘-
Volume 23, number 2
CHEMICAL
1
x
---
-_
0 oL7 -
Fig. 2. Pyrazine above. For other matrix was done i.e., f = 0.039 *
PHYSICS LETTERS
wo;
yz -
in PE and PP. Absorbanu: with PP is shown notarions, see fig. 1. Calibration of the PP with benzene: (LDIA),,, = 0.029 f 0.002, 0.003.
The nitrogen atom has a lone pair of electrons, and hence n-n* transitions must occur in the aza compounds. In the solution absorption spectrum of pyridine such transitions cannot be distinctly observed. In the vapour spectrum a weak system of discrete bands, partially submerged under the o! band (in Clar’s notation), has been assigned to the lA,-LB, (n-n*) transition [9, 101 - The symmetry of this transition (B1) implies x-polarization (for notation see fig. l), i.e., a negative
JD sign should
be expected
with the
molecular plane parallel to the stretch direction [1,5]. For stretched poIyethylene (PE), matrix effects other than mechanical orientation are small (vide infra), so orientation is determined by the unsymmetric shape of the sample molecules only. I.e., we assume the z and y axes to be approximately
equivalent
in an
orientational sense. (This is to some extent confirmed by the LD{A values which yield orientational degrees of the same magnitude as that obtained with benzene [3]t .) Th us, identification of tie n-n* transition is
15 November
1973
clear (fig. 1, PE, maximum at 273 nm). The positive bands (252, 257 and 263 nm) are due to yr-polarized transitions (rr-rr*, cf. the 260 nm system of benzene). Analogous assignments can be made for pyrazine (fig. 2, n-n* 320 nm, IT-Z* 250,255,260 and 267 nm). We have also used isotactic polypropylene (PP) as a matrix (figs. 1, 2 middle, PP). In most cases PP gives orientation effects which are equivalent to those in PE. However, preliminary results [ 1 l] indicate that certain molecules without a unique mechanical orientation axis (planar with Dnh symmetry II >, 3 [S]) in PP can exhibit secondary orientation effects, One of the equivalent orientation axes in the plane is preferred, probably as a result of association with the polymer chain; the orientation being determined by the charge distribution in the molecule (dipole and quadrupole moments) or hydrogen bonding. Orientation due to the electric field in the polymer is very small. This has been shown for a series of substituted benzenes by the absence of correlation between dipole moment and degree of orientation [ II] _If the increased size of the negative band in PP as compared to PE (at about 260 and 270 nm for pyridine and pyrazine, respectively) is due to the removal of the degeneracy (in orientational sense) of the z and y axes, it should be possible to distinguish between y and z polarizatioil as well as determining the x polarization. This suggestion (fig. 1, PP) is preliminary and mainly serves to demonstrate the further possibility, which is offered by two matrices with different orientational specificity to make a complete analysis of which transitions are x, y and z polarized. In the molecules considered here, any electric dipole transition must be polarized along one of.the coordinate axes but the theory can easily be expanded to molecules with lower symmetry. The degree Gorizn!ation,f(shown under the LD bands in th2 figures), is calc:dated separately for yz and x polarizations). The discr,spancy between thefvalues fdr high and low energy baMs must depend on a negative component under the high energy band, 7 As will be discussed elsewhere [6 J, the introduction orient:\tiomI
of two
degrees/,, and/, (for the axes acording to fig. 1; y may be the long axis, i.e., fy > J+,) will irnyiy ihat / must be more than twice as large as f,. to give a negat: ye L D for a r.polarized transition. *f= S(LD/A)/[6 - (LD/A)] fromyz-polarized bands,j= -4(LD/I)/[6 - (LD/A)] from x-polarized bands (S. 61.
Volume 23, number 2
CHEMICAL PHYSICS LE-ITERS
since the largest fshould be closest to the correct value. This study of the n-n* system could be compared to the recent reinvestigations of the pyridine spectrum by Ramsay et al. [ 12, 13 3 . Also for pyrazine our results are in agreement with rotational analyses [ 14, IS] , i.e., the n-n’ transition is at 300-310 nm. According to lnnes et al. [15] this region should contain two vibronic components; one allowed along the x axis (in our notation) giving sharp bands and one weaker “forbidden” z-polarized transition. Indeed, one must explain the 50% decrease in LDIA between the high and low energy side of this band system by the existence of a moderately strongyz-polarized band centered at about 310 nm.
References
I b. Davidson,
hf. Gouterman, L.Y. Johansson, R. Lwsson, B. NordBn md hf. Sundbom, Acta Chem. Sund. 26 (1972) 840. B. NordCn and A. David-n, Acta Chem. Sand. 26 (1972) 842. A. Davidsson and B. Nordk, Tetrahedron Letters 30 (1972) 3093. [4] B. Nordin, R. HIkansson and M. Sundbom, Acta Chem. Stand. 26 (1972) 429. IS 1 B. NordEn. Chemic3 Sniota 1 11971) 145. ii A. David&n, B. NordCnand hi. Sundbom. Acta Chem. Stand., to be published. 171 J. Michl, E.W. Thulstrup and J.H. Eggers, J. Phys. Chem. 74 (1970) 3878. 181 E.W. Thulstrup, J. Michl and J-H. Eggers, J. Phys. Chem. 74 (1970)
I thank Professors I. Fischer-Hjalmars and D.A. Ramsay and Drs. M. Sundbom and I. Chabay for interestkg and stimulating discussions.
1.5 November 1973
3868.
191 hf. Kasha, Discussions Faraday Sot. 9 (1950) 14. [lOI L. Goodman, J. Mol. Spectry. 6 (1961) 109. [I11 A. Davidss-on, unpublished results. [12] J.P. Jesson, H.W. Kroto and D.A. Ramsay. J. Chem. Phys. 56 (1972) 6257. [ 13) S. lapar and D.A. Ramsay. J. Chem. Phys., submitted for publication. [14] M. Ito, R. Shimada, T. Kuraishi and W. Mizushima, J. Chern. Phys. 26 (1957) 1506. [IS] K-K. Innes, J.D. Simmonsand S.G. Tilford, J. hfol.. Spectry. 11 (1963) 257.