- Email: [email protected]
“Forbidden” character in the 3200-Å transitions of pyrazine-h4 and -d4 vapors
.IOl’RN.41.
OF
\tOI.ECt’LAR
“Forbidden”
SPECTROSCOPY
Character Pyrazine-h4
11,
“57-261
(l!)fi:j)
in the 3200-A Transitions and -d4 Vapors
of
It. iu sh~nvn that all strong vibronic bands of the 3200-.‘I abs~~rption region 4 I)!.r:izine (1,4diazine) must be assigned to otlc electronic transition of mixed polarizatjion. The allowed bands show polarization I)erpendicul:tr tcj the p1:1nc of the mt Aecule and the somewhat weaker “forbidden” bands shc~w pol:lriz:ltion along the line of the nitrogen atoms. A second allowed electronic transition, pohtrized along the line of the nitrogen atoms. loans the intensity, wllilr an out-of-plane hydrogen-bending motion is active in the “lwrrowing.” ~1:~s~ dependence of the “forbidden” intensity and the R:LI~MII intensit,>, tjt’ the act iw vibrxtion c)H’er uniclue manifestatil,ns of the Herzberg-Tellr,r thclor>- of vit)rrjllic tr:lnsitiolrs.
I’yraxitw-h, absorption bands ill the region 2~JOO-~~~XOO .1 were subjected to clcm tailed vibrollic analysis by Ito and coworkers (I ). They cotlclrIded that tlwsc ba11ds hf.401l~ to two a* - 12electronic trallsitiofla, oiw I-ia~ili~~llai~p,atid tlic otlirr I’en-cr ho&r, bands and they assigned the sharp bands to a11 allonrcl ’ 7’, - I. I ,, tratlsition’ a~1 the broader bauds to a forbiddell ‘Kh - ‘_.I,, t,rallsitiotl. I3oth transitions WIP expected 011 theoretical groullds. Itotatiotlal analysis i 21 POIIfimed the cotrlusiou about the sharp bauds and indicat,ed that caref’ul ha11(1hcmi mcasrwemeltts and isotope studies wol~lti allow a critical check of’ the illterpret:ltioll of Ito a11t1 coworkers. Xow., we are able t,o outlitle definit,ivr cvitlclw tlmt stmtlg haI& of hoth types should be classified together about the suuc origitt, that is, iit ml,// firw fdcctronic trnnsitifm. Ikwxsr of ttle favorable cotiditiolls :itt(l complcte~wss of the obserrations, some ot’ t,lie ar~mwlit,s which I’ollo~ otfcr \lliic !Iw iiinllif’estat,ioiIs of the Hcrzberg-r~cJleJ~ theory 01’ \-ibroilic trausitiolis ill pal? atomic niolcc~~lt~s i-3). ‘l’lw strotlgwt of the broader bauds ot’ pyrazitlc-h, was st,udirtf at \TI.~ high iwolutioii ( C1. lW,utioilal alialysis of estrrnicly sharp “CJaid “(1 bimirliw she\\-cvl
258
INNES,
SIMMONS,
AND
TILFORD
that the transition moment is perpendicular to the axis of largest moment of inertia and parallel to the plane of the molecule. Distortion of the central part of the band (K < 14) and a very strong maximum near the band center showed that the band is of type A of an asymmetric top, that is, the transition moment is parallel to the axis passing through the nitrogen atoms. The new rotational constants obtained from the analysis verified a planar geometrical model proposed earlier for the ground state of the molecule (2). The molecule evidently is nearly planar also in the excited state and none of the in-plane geometrical parameters seems to change by as much as 1% (4). All sharp bands are of type C and all broader bands are of type A. Ito and coworkers have already shown how most of the strong type C bands may be arranged about the strong origin band at 30,875.8 cm-‘. We have found similar arrangements for pyrasine-d4 , about 31,030.2 cm-’ (5). Here, we focus attention on the type A bands. The strongest is at 31,258.4 cm-’ for pyrazine-h4 and at 31,322.0 cm-’ for pyrazine-d4 . The separations from the type 6’ O-O bands are 383 and 292 cm-‘, respectively. Since 383/292 is reasonable as an isotopic ratio, we test below the possibility that the two frequencies are fundamental ones, that is, that the type A and type C bands arise from a common origin. In such a case, the type A bands would be attributed to an antisymmet,ric vibration of species l’b (‘!!‘, X ‘1’b = ‘“T,, , as observed), active in intensity borrowing from a second, strongly allowed electronic transition ‘T, - IA, . Table
1
Pyrazine-hq Deslandres
\V’ V”
Table
for
Typs
A and
Associated
0
30875.8
(C,50)
2 919.5
29956.6
382.6
(A.31
31258.4
29037.1
383.9
(A.20)
CC.01
383.5
917.9 1
C Bands+
1 919.2
0
Type
919.9 30340.5
(C.4)
-1 are those of the sharpest *Frequencies in cm Band types and estimated relative intensities In the text, the differences 918.5 and 585.4 the fundamental frequency JS.
29420.6
(A.01
edges in the bands. a:? in parentheses, cm are assigned to
FORBIDDEN
TRASdITIOh-8 Table
OF PYRAZIK\;II:
2.59
2
pyrazine-dq Deslandres
x
Table
for
Type
A and
Associated
0
31030.2 (C.50)
TO309.1
291.8 1
31322.0 (A.11)
C Bands
1
2
721.1 0
Type
(A.21
720.3 29588.8
291.9 721.0
30601.0 CC.31
(C.0)
292.4 719.8
29881.2
(A .O)
The strongest type A bands at frequencies lower than the type (’ O-O batids are at - 1.50 and -919 cm-’ for pyrazine-h4 and at -721 cm-’ for pyrazinc-d, Again a reasonable isotope effect is seen, in the 91% and 721~cm-’ frequencies. Very sensitive tesbs of these differences are found by completing the Deslatldrw tables as illustrated in Table 1 for pyrazine-h* . There, 30,87;i.8 err-’ is also t,he origin for the type C bauds and each upper and loww state vibrat)ional differelIw is checked. Sote for example, the constant difference between the rows 11’= 0 alld 11’= 1. .\ further check of the analysis is t,he alternation betwee band types 1t and (.’ in each row and column, observed as expected for successive rlliatita of ati antisynnnrtric vibration. For example, tdw O-O band is t.ypr C’, ""T,. - _I!, ;tllc - .4,, ;and the 2-O hand is type (‘, '"T,- -1,,WI~CIT I-Obaiid is t#ypc -4, "'T,, ‘(1’, Z ’ 21,. x “,‘,, x rl’b . 12inally, au entirely aualogous treatmrtit has been givcbn pyrazinc-di , using V” = 721 cm-’ aild V’ = ?!I? cm--‘, as shown in Table 2. ‘I’lw evidence of this paragraph firmly est#ablishrs that tAe t*ype =1 and type (’ bat~cl~ arise from the same electronic transition and that the transition has, ill part, character which would he forbidden eswpt for vibronic interaction. It remains to assign the active freclwucies t#o a normal motion of the molrcl~lt~. The maguitudcs of the frequencies and the isotope effects show t,hat t,he motiotl is a. I~yd~~o~r~~-hc~tdillg one. The band t,ype shows t,hat the species t~ransforlnh like I’~~ . .\wtrtdi~~g to I,ord, I\Iarxt.oll, alld Aliller (8) [see also Sully (;il sllch :I tltwriptiot, a,pplies only to the out-of-plane brllditlg n&c v5 'l'lw GO'( rrcllic*tioli in its freclwiicy on clechronic escitat8iojl is Ilotew~rthy. The assignmel~t just made recluiws that the lowc~r st,ate frequency should l)(b Ijaman, hut not infrared, a&l-e. Lord, AIarston, anti AIiller have discussed tlw difficulty ill deciding from previous studies whether !)I!) CIC’ is a true Ramall f~w~wncy of pyrazine-h4 . Dr. C;. 11. Br~un of ( )ak Ridge Sational J,aboratoriw has kindly recorded for us the Raman spect,ra of small liquid aud solid samples (;,I. of both pyrazinc-hr and pyrazine-dl , using a Vary Model 81 spectrometer I,ilrw comparable in strength with all but the strongest polarized OIPS nwc folllld at 92.5 cn1--1 and 727 cm-‘, respect,irrly. These vallws evidently corrwpo~~tl to those gi\.call nbo\-e and shown ill the tables for tlw Illt.ra\-iok+ spectra of then
200
INNES,
SIMMONS,
AND
TILFORD
vapors. It was possible to study liquid pyrazine-h4 with polarized light and the 925cm-’ line was found to be highly depolarized. This too is consistent with the present interpretation. Xlbrecht suggested recently (8) that it would be worthwhile to test experimentally the rule that those normal modes which are most responsible for “forbidden” intensity in allowed electronic transitions should show greatest activity in Raman scattering, especially as ~wsonance conditions are approached. The record of observations of ~5 of pyrazine seems to offer a first, qualitative check of the rule. We have shown that vg is responsible for “forbidden” 1-O bands about two-fifths as strong as the O-O band in the hydrogen compound and one-fifth as strong as the O-O band in the deuterium compound. Vsing 5461 ~1 for the Raman exciting line, Lord, Marston, and Miller did not observe v6 However, using -13.58 A, which is of course closer to the resonance wavelength of 3215 8, we have found ~6 to exhibit the moderate intensity noted above. It is expected that isotopic substitution should affect the efficiency of vibronic mixing and so the intensities of “forbidden” bands relative to allowed ones (9). I’yrazine again offers a first striking example. The effect of deuteration in reducing the “forbidden” intensity by a factor of order two for the 1-O bands would, if maintained throughout the “forbidden” subsystem, exceed expectation by a factor of one-and-a-half. That the effect is maintained is indicated by the observation of more than three times as many type il bands for pyrazine-h* as for pyrazine-d4 . Other assignments in the vibrational and electronic spectra (5) will be discussed in later publications. Experiments and measurements described above were carried out much as described earlier (Z), except where noted. The pyrazine-d4 was supplied by Merck Sharp & Dohme of Canada. Its infrared spectrum was free of carbon-hydrogen stretching frequencies. The relative intensities of ultraviolet bands were es6 mated from absorbing paths and microphotometer tracings, and all were made consistent with low dispersion recordings obtained with a Gary Model II spectrophotometer. ACRNOWLEDOMENTS We are very grateful for the llelp given by Dr. Begun. The work was supported by the Advanced Research Projects Agency, the Xational Science Foundation and the Petroleum Research Fund of the American Chemical Society. RECEIVED
: February
25, 1963 REFERENCES
1. ill. ITO, R. SHIMADA,T. KURAISHI, AND W. MIZUSHIMA, J. Chew. Whys. 26, 1508 (195T) 2. K. INNES, J. MERRITT, W. TINCHER, AND S. TILFORD, Natzlre 187, 500 (1960); J. MERRITT AND K. INNES, Spectrochiw Acta 16, 945 (1960). S. G. HERZBERGAND E. TELLER, Z. Physik. C’hem. B21, 410 (1933).
FORBIDDEN
TR,ANSITIOSS
OF PYRAZINE:
“Gil
Q. Fi. INSES, International Symposium on Molecular Structure and Spectroscopy, Tok>.~t. 1962, Paper B 316, Science Council of Japan preprints. 5. J. SIMMONS, thesis, Vanderbilt University, 1963. 6. It. LORIJ. -4. MARSTON, AND P. MILLER, Spwlrw&w .l&/ 9, 113 (1957). 7. 1). Scr,r.~.u, Spedrochim. Acla 1’7, 233 ~1961). 8. A. ALBRECHT, J. Chen~. Phys. 34, 1470 (1961). 9. (:. ROBINSON, ir/ “Molecular Physics” (I>. Williams, ed.), p. 253. Academic Press, Ncu York, 1962.
OF
\tOI.ECt’LAR
“Forbidden”
SPECTROSCOPY
Character Pyrazine-h4
11,
“57-261
(l!)fi:j)
in the 3200-A Transitions and -d4 Vapors
of
It. iu sh~nvn that all strong vibronic bands of the 3200-.‘I abs~~rption region 4 I)!.r:izine (1,4diazine) must be assigned to otlc electronic transition of mixed polarizatjion. The allowed bands show polarization I)erpendicul:tr tcj the p1:1nc of the mt Aecule and the somewhat weaker “forbidden” bands shc~w pol:lriz:ltion along the line of the nitrogen atoms. A second allowed electronic transition, pohtrized along the line of the nitrogen atoms. loans the intensity, wllilr an out-of-plane hydrogen-bending motion is active in the “lwrrowing.” ~1:~s~ dependence of the “forbidden” intensity and the R:LI~MII intensit,>, tjt’ the act iw vibrxtion c)H’er uniclue manifestatil,ns of the Herzberg-Tellr,r thclor>- of vit)rrjllic tr:lnsitiolrs.
I’yraxitw-h, absorption bands ill the region 2~JOO-~~~XOO .1 were subjected to clcm tailed vibrollic analysis by Ito and coworkers (I ). They cotlclrIded that tlwsc ba11ds hf.401l~ to two a* - 12electronic trallsitiofla, oiw I-ia~ili~~llai~p,atid tlic otlirr I’en-cr ho&r, bands and they assigned the sharp bands to a11 allonrcl ’ 7’, - I. I ,, tratlsition’ a~1 the broader bauds to a forbiddell ‘Kh - ‘_.I,, t,rallsitiotl. I3oth transitions WIP expected 011 theoretical groullds. Itotatiotlal analysis i 21 POIIfimed the cotrlusiou about the sharp bauds and indicat,ed that caref’ul ha11(1hcmi mcasrwemeltts and isotope studies wol~lti allow a critical check of’ the illterpret:ltioll of Ito a11t1 coworkers. Xow., we are able t,o outlitle definit,ivr cvitlclw tlmt stmtlg haI& of hoth types should be classified together about the suuc origitt, that is, iit ml,// firw fdcctronic trnnsitifm. Ikwxsr of ttle favorable cotiditiolls :itt(l complcte~wss of the obserrations, some ot’ t,lie ar~mwlit,s which I’ollo~ otfcr \lliic !Iw iiinllif’estat,ioiIs of the Hcrzberg-r~cJleJ~ theory 01’ \-ibroilic trausitiolis ill pal? atomic niolcc~~lt~s i-3). ‘l’lw strotlgwt of the broader bauds ot’ pyrazitlc-h, was st,udirtf at \TI.~ high iwolutioii ( C1. lW,utioilal alialysis of estrrnicly sharp “CJaid “(1 bimirliw she\\-cvl
258
INNES,
SIMMONS,
AND
TILFORD
that the transition moment is perpendicular to the axis of largest moment of inertia and parallel to the plane of the molecule. Distortion of the central part of the band (K < 14) and a very strong maximum near the band center showed that the band is of type A of an asymmetric top, that is, the transition moment is parallel to the axis passing through the nitrogen atoms. The new rotational constants obtained from the analysis verified a planar geometrical model proposed earlier for the ground state of the molecule (2). The molecule evidently is nearly planar also in the excited state and none of the in-plane geometrical parameters seems to change by as much as 1% (4). All sharp bands are of type C and all broader bands are of type A. Ito and coworkers have already shown how most of the strong type C bands may be arranged about the strong origin band at 30,875.8 cm-‘. We have found similar arrangements for pyrasine-d4 , about 31,030.2 cm-’ (5). Here, we focus attention on the type A bands. The strongest is at 31,258.4 cm-’ for pyrazine-h4 and at 31,322.0 cm-’ for pyrazine-d4 . The separations from the type 6’ O-O bands are 383 and 292 cm-‘, respectively. Since 383/292 is reasonable as an isotopic ratio, we test below the possibility that the two frequencies are fundamental ones, that is, that the type A and type C bands arise from a common origin. In such a case, the type A bands would be attributed to an antisymmet,ric vibration of species l’b (‘!!‘, X ‘1’b = ‘“T,, , as observed), active in intensity borrowing from a second, strongly allowed electronic transition ‘T, - IA, . Table
1
Pyrazine-hq Deslandres
\V’ V”
Table
for
Typs
A and
Associated
0
30875.8
(C,50)
2 919.5
29956.6
382.6
(A.31
31258.4
29037.1
383.9
(A.20)
CC.01
383.5
917.9 1
C Bands+
1 919.2
0
Type
919.9 30340.5
(C.4)
-1 are those of the sharpest *Frequencies in cm Band types and estimated relative intensities In the text, the differences 918.5 and 585.4 the fundamental frequency JS.
29420.6
(A.01
edges in the bands. a:? in parentheses, cm are assigned to
FORBIDDEN
TRASdITIOh-8 Table
OF PYRAZIK\;II:
2.59
2
pyrazine-dq Deslandres
x
Table
for
Type
A and
Associated
0
31030.2 (C.50)
TO309.1
291.8 1
31322.0 (A.11)
C Bands
1
2
721.1 0
Type
(A.21
720.3 29588.8
291.9 721.0
30601.0 CC.31
(C.0)
292.4 719.8
29881.2
(A .O)
The strongest type A bands at frequencies lower than the type (’ O-O batids are at - 1.50 and -919 cm-’ for pyrazine-h4 and at -721 cm-’ for pyrazinc-d, Again a reasonable isotope effect is seen, in the 91% and 721~cm-’ frequencies. Very sensitive tesbs of these differences are found by completing the Deslatldrw tables as illustrated in Table 1 for pyrazine-h* . There, 30,87;i.8 err-’ is also t,he origin for the type C bauds and each upper and loww state vibrat)ional differelIw is checked. Sote for example, the constant difference between the rows 11’= 0 alld 11’= 1. .\ further check of the analysis is t,he alternation betwee band types 1t and (.’ in each row and column, observed as expected for successive rlliatita of ati antisynnnrtric vibration. For example, tdw O-O band is t.ypr C’, ""T,. - _I!, ;tllc - .4,, ;and the 2-O hand is type (‘, '"T,- -1,,WI~CIT I-Obaiid is t#ypc -4, "'T,, ‘(1’, Z ’ 21,. x “,‘,, x rl’b . 12inally, au entirely aualogous treatmrtit has been givcbn pyrazinc-di , using V” = 721 cm-’ aild V’ = ?!I? cm--‘, as shown in Table 2. ‘I’lw evidence of this paragraph firmly est#ablishrs that tAe t*ype =1 and type (’ bat~cl~ arise from the same electronic transition and that the transition has, ill part, character which would he forbidden eswpt for vibronic interaction. It remains to assign the active freclwucies t#o a normal motion of the molrcl~lt~. The maguitudcs of the frequencies and the isotope effects show t,hat t,he motiotl is a. I~yd~~o~r~~-hc~tdillg one. The band t,ype shows t,hat the species t~ransforlnh like I’~~ . .\wtrtdi~~g to I,ord, I\Iarxt.oll, alld Aliller (8) [see also Sully (;il sllch :I tltwriptiot, a,pplies only to the out-of-plane brllditlg n&c v5 'l'lw GO'( rrcllic*tioli in its freclwiicy on clechronic escitat8iojl is Ilotew~rthy. The assignmel~t just made recluiws that the lowc~r st,ate frequency should l)(b Ijaman, hut not infrared, a&l-e. Lord, AIarston, anti AIiller have discussed tlw difficulty ill deciding from previous studies whether !)I!) CIC’ is a true Ramall f~w~wncy of pyrazine-h4 . Dr. C;. 11. Br~un of ( )ak Ridge Sational J,aboratoriw has kindly recorded for us the Raman spect,ra of small liquid aud solid samples (;,I. of both pyrazinc-hr and pyrazine-dl , using a Vary Model 81 spectrometer I,ilrw comparable in strength with all but the strongest polarized OIPS nwc folllld at 92.5 cn1--1 and 727 cm-‘, respect,irrly. These vallws evidently corrwpo~~tl to those gi\.call nbo\-e and shown ill the tables for tlw Illt.ra\-iok+ spectra of then
200
INNES,
SIMMONS,
AND
TILFORD
vapors. It was possible to study liquid pyrazine-h4 with polarized light and the 925cm-’ line was found to be highly depolarized. This too is consistent with the present interpretation. Xlbrecht suggested recently (8) that it would be worthwhile to test experimentally the rule that those normal modes which are most responsible for “forbidden” intensity in allowed electronic transitions should show greatest activity in Raman scattering, especially as ~wsonance conditions are approached. The record of observations of ~5 of pyrazine seems to offer a first, qualitative check of the rule. We have shown that vg is responsible for “forbidden” 1-O bands about two-fifths as strong as the O-O band in the hydrogen compound and one-fifth as strong as the O-O band in the deuterium compound. Vsing 5461 ~1 for the Raman exciting line, Lord, Marston, and Miller did not observe v6 However, using -13.58 A, which is of course closer to the resonance wavelength of 3215 8, we have found ~6 to exhibit the moderate intensity noted above. It is expected that isotopic substitution should affect the efficiency of vibronic mixing and so the intensities of “forbidden” bands relative to allowed ones (9). I’yrazine again offers a first striking example. The effect of deuteration in reducing the “forbidden” intensity by a factor of order two for the 1-O bands would, if maintained throughout the “forbidden” subsystem, exceed expectation by a factor of one-and-a-half. That the effect is maintained is indicated by the observation of more than three times as many type il bands for pyrazine-h* as for pyrazine-d4 . Other assignments in the vibrational and electronic spectra (5) will be discussed in later publications. Experiments and measurements described above were carried out much as described earlier (Z), except where noted. The pyrazine-d4 was supplied by Merck Sharp & Dohme of Canada. Its infrared spectrum was free of carbon-hydrogen stretching frequencies. The relative intensities of ultraviolet bands were es6 mated from absorbing paths and microphotometer tracings, and all were made consistent with low dispersion recordings obtained with a Gary Model II spectrophotometer. ACRNOWLEDOMENTS We are very grateful for the llelp given by Dr. Begun. The work was supported by the Advanced Research Projects Agency, the Xational Science Foundation and the Petroleum Research Fund of the American Chemical Society. RECEIVED
: February
25, 1963 REFERENCES
1. ill. ITO, R. SHIMADA,T. KURAISHI, AND W. MIZUSHIMA, J. Chew. Whys. 26, 1508 (195T) 2. K. INNES, J. MERRITT, W. TINCHER, AND S. TILFORD, Natzlre 187, 500 (1960); J. MERRITT AND K. INNES, Spectrochiw Acta 16, 945 (1960). S. G. HERZBERGAND E. TELLER, Z. Physik. C’hem. B21, 410 (1933).
FORBIDDEN
TR,ANSITIOSS
OF PYRAZINE:
“Gil
Q. Fi. INSES, International Symposium on Molecular Structure and Spectroscopy, Tok>.~t. 1962, Paper B 316, Science Council of Japan preprints. 5. J. SIMMONS, thesis, Vanderbilt University, 1963. 6. It. LORIJ. -4. MARSTON, AND P. MILLER, Spwlrw&w .l&/ 9, 113 (1957). 7. 1). Scr,r.~.u, Spedrochim. Acla 1’7, 233 ~1961). 8. A. ALBRECHT, J. Chen~. Phys. 34, 1470 (1961). 9. (:. ROBINSON, ir/ “Molecular Physics” (I>. Williams, ed.), p. 253. Academic Press, Ncu York, 1962.