The ã←X˜1A1 and A˜1B1 ←X˜1A1 electronic transitions of pyridazine-d0, -d2, and -d4 vapors

J O U R N A L OF MOLECULAR SPECTROSCOPY 36, 1 1 4 - 1 4 0

(1970)

The ~ ~ )~A~ and ~B~ ~- )~A~ Electronic Transitions of Pyridazine-d0, -d2, and -d4 Vapors ~ K . K . INNES

Department of Chemistry, State University of New York, Binghamton, New York AND

W . C. TINCHER 2 AND EARL :F. PEARSON 3

Department of Chemistry, Vanderbilt University, Nashville, Tennessee High resolution a b s o r p t i o n spectra of 1,2-diazine a n d two d e u t e r a t e d modifications have been measured between 3000 A a n d 5000 A, and analyzed. Two excited electronic states, a singlet and a triplet, have been identified. R o t a tional analyses of the 0-0 b a n d s of the two systems have shown the systems to be 1Bl-lA1 (3700 ~ ) and ~A~-IA1 or 3B1 1A1 (4450 A). I t has been found t h a t the s h a r p rotational s t r u c t u r e of the ~Bl-lA1 0-0 b a n d of pyridazine-d0 indicates a condition on the change of inertial cons t a n t s not identified before for p l a n a r asymmetric rotors, namely, C' - C" = /~' -- B" (where/~ = (A ~- B)/2). Changes in A, B, and C found b y contour analysis prove t h a t the molecule is elongated by an (atomic) average of a b o u t 10% along a direction perpendicular to the s y m m e t r y axis. This considerable change of geometry is reflected in the F r a n c k - C o n d o n contour of v i b r a t i o n a l s t r u c t u r e of the singlet system. Isotope effects are used to d e m o n s t r a t e t h a t the extreme complexity of v i b r a t i o n a l s t r u c t u r e on the high-frequency side of the origin arises from large and extensive Fermi (anharmonic) resonances, linking P6a and the overtone of an a n t i s y m m e t r i c mode t h o u g h t to be ~16b . I t has followed t h a t identification of the active v i b r a t i o n s has h a d to be based m a i n l y on observations of t h e i r less p e r t u r b e d lower-state c o u n t e r p a r t s a t frequencies as far as 2400 cm -~ below t h a t of the origin. A unique feature is the large reduction of ~6~ from 636 to 374 cm -1 (pyridazine-d4) on electronic excitation. T h a t reduction, in a d d i t i o n to the large a n h a r m o n i c t e r m in the p o t e n t i a l energy of the ~B~ s t a t e and i n t e n s i t y stealing by the t o t a l l y - s y m m e t r i c mode, ~ , implies the presence of a second 1BI state, nearby, b u t not recognized directly. 1 Based in p a r t upon the P h . D . Theses of W. C. T i n c h e r (1960) and E. F. P e a r s o n (1968). Esso Fellow, 1959-60. P r e s e n t address: C h e m s t r a n d Research Center, Inc., D u r h a m , N o r t h Carolina. 3 NASA Trainee, 1964-7 and U S P H S Fellow, 1967-8. P r e s e n t address: W e s t e r n K e n t u c k y Univ., Bowling Green, K e n t u c k y . 114

ELECTRONIC SPECTRUM OF 1,2-DIAZINE

H_ c/

115

I \C_H

a

Fla. 1. Model and axis conventions for the pyridazine molecule. Inertial axes a and b are denoted for the hydrogen compound according to the convention I~ < Ib < Ic • For symmetry classification, the z axis coincides with a and the x axis is perpendicular to the plane of the molecule for pyridazine-d0 . In contrast, most of the intensity of the triplet system is found in a single, regular progression in V6a = 639 cm-1. If the triplet state comes from the same electron configuration as the 1B1state characterized here, the molecular singlettriplet splitting is 4161.7 cm-1. I t has been emphasized repeatedly (1) t h a t the sharp ~*-n spectra of azabenzenes offer a highly favorable opportunity for detailed rotational and vibrational analysis. Perhaps the richest in rotational fine structure, but also the most complex in its vibrational structure, is the 3700 ~- system of pyridazine (1,2 diazine) (2). Recent progress with the microwave (3) and infrared and R a m a n (4) spectra has added significantly to the basis for analysis. A further stimulus has been the discovery in pyridazine crystals of a triplet-singlet transition near 4000 A (5). We take as our main purposes in the present paper: the exploitation of rotational fine structure to fix, insofar as possible; the directions of the transition moments for the 3700 A and 4000 A transitions, and the effects of these electronic j u m p s upon the geometry of the molecule; the understanding of the complexities of vibrational structure, at least to the extents of concluding whether more than one electronic transition is needed to account for the vibrational structure in either (or both) regions, and of correlation of active vibrations with the change of geometry inferred from the rotational analysis; a comparison of some of these results with those for pyrimidine (6); and the determination of the separation between the lowest singlet and lowest triplet ~*, n states of the free molecule. I. EXPERIMENTAL Most of our spectra were taken in the second, third, or fourth orders of a 3.4-m o Jarrell-Ash E b e r t spectrograph. The dispersion was 1.2, 0.5, or 0.3 A/ram, respectively, and the m a x i m u m resolving power actually observed was nearly 200 000. The spectra were measured against iron lines using a D a v i d G. M a n n Model 300 C o m p a r a t o r or a G r a n t I n s t r u m e n t s Co. Automatic C o m p a r a t o r or, in the case of the weakest bands, photographic enlargements of the original plates. Pyridazine-d0 was obtained from the Chemical Procurement and Aldrich Chemical Companies and was always used without purification. A pyridazine-d4

116

INNES, TINCHER, AND PEARSON 40240

--I

2200

30876

I

X(Z,)-- 3240

26650

3700

FIG. 2. Low-resolution recording of the u l t r a v i o l e t absorption s p e e t r u m of pyridazine-d0 vapor. Peaks in the p y r a z i n e - i m p u r i t y s p e c t r u m are barely visible a t energies of 30876 em -~ and higher. T h e benzene-like t r a n s i t i o n a t 2500 _~ will not be discussed in the present paper.

sample was supplied by Merck, Sharp, & Dohme of Canada, while our sample of pyridazinc 3,6-d~ was kindly supplied by Drs. H. D. Stidham and J. V. Tucci. Absorbing paths varied from one to sixteen meters when the vapors were at room temperature in a 1-m multiple reflection cell. These conditions were particularly appropriate for fine structure photographs of the origin band of the strong, singlet system near 3700 A. However, for vibrational analysis of this system, it was desirable to record transitions originating in high vibrational states of the electronic ground state ("hot" bands) and this could be best achieved by heating the sample and the two-meter absorption cell to temperatures as high as 200°C, which is near the boiling point. Molecules with initial vibrational energies as high as 1800 cm-1 for pyridazine-d4 and 2400 cm-J for pyridazine-d0 were excited. A dividend of the high temperature experiments was that, by comparison with the room temperature ones, some sorting of "hot" bands was accomplished. Although for constant path all bands decreased in intensity (above background) at higher temperature, those bands arising from the zero level of the ground state showed the greatest percentage decrease. An absorbing path of up to 48 m a t m was necessary in order to observe the weaker system near 4400 /~. This was achieved with a four-meter cell, which could be heated to the boiling point of pyridazine (208°C), and external mirrors, which made it possible to pass the incident beam through the cell twelve times. It was found that, if dissolved air was carefully removed before pyridazine was vaporized into the hot cell, the rate of thermal decomposition could be greatly reduced. The continuous background for the absorption spectra was provided by Hanovia and Osram high pressure Xenon arcs of ratings up to 1600 W. In addition to the high resolution plates, recordings were made of the absorption spectra of pyridazine-d0, -d~ and -d4 between 2200 .~ and 4000 /k using a Cary Model 14 recording spectrophotometer (Fig. 2) and a JarreII-Ash Ebert Spectrometer, Model 82-000. Fig. 3 shows microphotometer tracings of high dispersion photographs of the vibrational peaks of the 4400 .~ system. Representative rotational structure is displayed in Fig. 4.

ELECTRONIC SPECTRUM OF 1,2-DIAZINE

3700A

4100

~

4200

J~

4300

~.

117

16b, 0-0

4400

4500

FIG. 3. The 4400 A system of pyridazine-d0 vapor, at high dispersion. The curve is a composite of microphotometer tracings of plates made for several absorbing paths. Halfwidths of the peaks are in fact less than 2 cnv ~ but have been exaggerated in the figure. Bands degraded to higher frequencies have been marked with dots. Bands degraded to lower frequencies are unmarked and are assigned to "hot" bands of the 3700 A_system. II. ROTATIONAL ANALYSIS AND POLARIZATIONS OF TRANSITIONS

A. The Singlet System (3700 A) Fine structure of the 0-0 band of pyridazine-d0 is displayed in Fig. 4 ( a ) . I t is quite typical, also of pyridazine-d2 and -d4, except t h a t in a few bands the small degradation is reversed and in other bands there is no degradation. Each band shows a narrow and intense central Q branch with weaker rotational features on both sides. The strongest bands are all type C (parallel) bands of a near-oblate asymmetric top. The transition m o m e n t is therefore parallel to the x axis (out of plane), and the excited state is 1B1. The bands show well-resolved QP and OR branch "lines" (see Fig. 4), each "line" consisting of the overlap of rotational lines with the same values of the q u a n t u m number J , in various subbands with different values of K c . I n sucl~ a case, if the change in the rotational constant/~ = 1/~(A -~ B) between the two states of the band is not too large, then the QQbranch features exhibit an exceedingly sharp head, and it can be shown that the position of the QQ head is a very good approximation to the position of the band origin. We have analyzed the QP and QR branch "lines" of eight pyridazine-d0 and two pyridazine-d4 bands (Table I ) , and derived the rotational constants,/~ and /}", and the positions of the band origins (Tables I and I I ) . (Measurements of a pyridazine-d~ band have been published earlier (7).) I t was not found necessary

FIG. 4(a). R o t a t i o n a l fine s t r u c t u r e of pyridazine-d0 : 0-0 b a n d at 3700 7\. T h e other Q-branch peak evident in the figure is assigned a~ 16al 1. (See discussions of assignments in Sections I I and I I I . )

FIG. 4b. R o t a t i o n a l fine s t r u c t u r e of the 16b0~ b a n d of pyridazine-d4

1.05Cm -1 (c)

Cm-l,~ ._ I 22,487.1 FIG. 4c. R o t a t i o n a l contour of the central portion of the 0-0 b a n d a t 4400 A : pyridazine-d0 118

ELECTRONIC

SPECTRUM

119

OF 1,2-DIAZINE

TABLE I ASSIGNED FINE STRUCTURE OF THE PYRIDAZINE-do BANDS (~,. . . . c m -1) *

Vo = 27 021.45 cm-I

6a~ 5 i 2 3 4 5 6 7 8 9 I0 ii 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

R(j) 27 022.00 22.41 22.99 23.38 23.81 24.22 24.62 25.01 25.44 25.78 26.16 26.55 26.90 27.24 27.63 28.01 28.27 28.71 29.07 29.41 29.77 30.16 30.48 30.84 31.17 31.50

p(j)

27 018.98 18.56 18.14 17.73 17.30 16.85 16.46 16.00 15.57 15.14 14.69 14.24 13.77 13.32 12.88 12.42 11.97 11.49 11.03 10.56 10.09

5

R(5)

P(5)

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51

27 031.83 32.16 32.50 32.81 33.12 33.45 33.75 34.05 34.34 34.64 34.98

27 009.62 09.15 08.68 08.19 07.70 07.21 06.71 06.21 05.70 05.15 04.66 04.20 03.81 03.39 02.91 01.97 Ol.55

01.02 00.62 00.18 26 999.58 99.26 98.67 98.37

* The band centers derived and given here are identical with the Q-branch edges within the accuracy of measurement of the latter.

to allow for the effects of centrifugal distortion, even at the highest J values observed. The rotational quantum numbers were assigned by fitting the line positions to a regular series, and taking the position of the QQ head as the approximate origin. The numbering is unambiguous, since the separation of successive rotational lines in a branch is about 0.4 cm -1 and the branches extrapolate to the measured Q-head positions to ~dthin 0.05 cm -1. Recently (6), we have emphasized that the degradation of the Q branch of a type C band is determined by the difference of smallest inertial constants (C' -- C"). In earlier work (8), we have formed combination differences R ( J ) - - P ( J ) and R ( J - - 1) P ( J + 1) from similar fine structure "lines" and have assumed that the resulting A2F may be set equal to 4/~(J + ~/~). This procedure is justified for pyridazine-d0 by the agreement of the constant obtained for the zero level of the ground state

120

INNES, TINCHER, AND PEARSON Table I (Continued) v o = 27 736.10 cm-I

J

R(J)

P(J)

J

R(J)

P(J)

ii 12 13 14 15 16 17 18 19 20 21 22

27 741.05 41.49 41.83 42.20 42.59 42.58 43.32 43.73 44.11 44.47 44.86 45.26

27 731.09 30.68 30.28 29.93 29.48 29.04 28.61 28.20 27.79 27.34 26.94

23 24 25 26 27 28 29 30 31 32 33 34

27 745.62 46.02 46.38 46.76 47.13 47.53 47.90 48.30 48.69 49.05 49.45

27 726.52 26.09 25.65 25.27 24.80 24.38 23.98 23.57 23.14 22.71 22.29 21.91

J

R(J)

P(J)

16 17 18 19 20 21 22 23 24 25 26 27 28 29

27 946.54 47.01 47.43 47.86 48.29 48.66

27 933.02 32.65 32.27 31.87 31.51 31.13 30.74 30.36 29.98 29.60 29.20 28.83 2 8.44 28.07

Vo = 27 939.27 cm-I

J

R(J)

i 2 3 4 5 6 7 8 9 i0 ii 12 13 14 15

27 940.04 40.50 40.93 41.35 41.79 42.21 42.64 43.07 43.50 43.93 44.36 44.79 45.22 45.62 46.08

P(j)

27 936.84 36.46 36.11 35.73 35.34 34.93 34.56 34.17 33.78 33.41

(0.2034 cm -1, see Fig. 5) with the value that may be calculated from the microwave results (0.2035 cm-1). In order to obtain a determination of the change of geometry effected by electronic excitation in Section IV, it is necessary for us to determine separately the changes of Ia and Ib, the two in-plane moments of inertia, from analysis of the 0-0 band, that is, to determine A and B rather than just their average. Earlier efforts with pyridazine and related molecules (7, 8) did not accomplish direct determinations of these changes and, as a result, some ambiguities remained. The changes can be found only through a detailed understanding of the

T~b|e I v o = 26

(Continued) 648.75

cm - 1

0-0

J 1 2 3 4 5 6 7 8 9 i0 ii 12 13 14 15 16 17 18 19 20 21 22 23

~(J)

P(J)

26 650.03 50.36 50.75 51.16 51.56 51.98 52.36 52.73 53.16 53.57 53.96 54,36 54.76 55.17 55.54 55.95 56.36 56.74 57.13 57.52 57.92 58.31

26 648.43 48.00 47.52 47.12 46.73 46,26 45.83 45.44 45.04 44.65 44.23 43.82 43.41 42.98 42.56 42.15 41.73 41.31 40.91 40.47 40.06 39.64 39,21

J 24 25 26 27 28 29 3O 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

~(J)

P(J)

26 658.69 59.09 59.47 59.86 60.23 60.64 61.01 61.42 61.80 62.17 62.58 62.96 63.35 63.72 64.12 64.50 64.85 65.28 65.72 66.08

26 638.79 38.37 37.93 37.51 37. i0 36.68 36.24

66.88

Vo = 27 966.29 cm -l

J 3 4 5 6 7 8 9 i0 ii 12 13 14 15 16 17 18

~(J)

P(J)

J

~(J)

P(J)

27 968.68 69.12 69,56 69.95 70.38 70.68 71.10 71.51 71.9 4 72.36 72.76 73.18 73.56 73.93

27 965.29 64.79 64.36 63,93 63.52 63.07 62.60 62.22 61.83 61.42 60.96 60.61 60.18 59.75 59.35 58.92

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

27 974.36 74.75 75.15 75.54 75.96 76.43 76.81 77.25 77.63 78,04 78.47 78.87 79,26 79,70

27 958.51 58.13 57.72 57.26 56.83 56.38 56.02 55.63 55.25 54.84 54.40 54.07 53.64 53.30 52.92

121

122

INNES, TINCHER, AND PEARSON Table I (Continued) v o = 26 356.89 cm -I

J

R(J)

7 8 9 i0 ii 12 13 14 15 16 17 18 19 20 21 22 23

26 359.87 60.23 60.62 60.98 61.35 61.74 62.11 62.47 62.87 63.25 63.62 63.95 64.28 64.64 65.01 65.35 65.70

P(J)

J

R(J)

P(J)

26 351.02 50.66 50.63 49.91 49.45 49.01 48.57 48.10 47.66 47.18 46.70

24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

26 366.05 66.37 66.69 67.05 67.35 67.68 67.99 68.30

26 346.27 45.80 45.32 44.86 44.39 43.93 43.41 42.86 42.39 41.93 41.59 41.20 40.80 40.37 39.96 39.61 39.12

v o = 26 556.30 cm -I

J

R(J)

P(J)

J

R(J)

P(J)

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

26 563.93 64.31 64.68 65.07 65.45 65.96 66.42 66.74 67.17 67.57 67.94 68.31 68.72 69.08 69.53

26 549.33 48.94 48.53 48.12 47.67 47.29 46.87 46.43 46.10 45.62 45.19 44.72 44.31 43.87 43.46 43.04

33 34 35 36 37 38 39 40 41 42 43 44 45 46

26 569.93 70.29 70.66 71.04 71.48 71.90 72.25 72.62 73.04 73.45 73.92 74.32 74.62 75.00

26 542.65 42.22 41.84 41.41 40.98 40.51 40.10 39.69 39.24 38.81 38.47 38.01

ELECTRONIC SPECTRUM OF 1,2-DIAZINE

123

Table I (Continued) v o = 27 116.91 cm -I 6a~

5

P(5)

P(5)

5

R(5)

P(5)

20 21 22 23 24 25 26 27 28

27 124.07 24.44 24.75 25.05 25.42 25.64 26.09 26.36 26.68

27 109.72 09.23 08.81 08.49 08.06 07.69 07.32 06.96 06.55

29 30 31 32 33 34 35 36

27 127.02 27.32 27.61 27.93 28.29 28.59 28.92

27 i06.15 05.76 05.38 05.00 04.57 04.23 03.81 03.41

Vo = 27 238.21 cm-I 16b

5

R(5)

P(5)

5

R(5)

P(5)

7 8 9 i0 ii 12 13 14 15 16 17 18 19 20 21 22 23 24

27 241.41 41.77 42.13 42.48 42.83 43.18 43.53 43.91 44.27 44.59 44.97 45.32 45.69 46.02 46.36 46.71 47.08

27 235.72 35.36 34.99 34.68 34.36 33.96 33.54 33.23 32.88 32.52 32.21 31.82 31.46 BI. i0 30.74 30.39 30.03 29.67

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

47.44 47.82 48.16 27 248.50 48.86 49.19 49.57 49.93 50.28 50.66 50.99 51.35 51.70 52.07 52.41 52.76

29.30 28.96 28.59 27 228.23 27.90 27.54 27.18 26.82 26.42 26. ii 25.76 25.42 25.05 24.69 24.30 23.9 8 23.62 23.26

way in which an individual "line" of Fig. 4(a) arises: we must attempt contour analysis of a representative "line" such as R ( l l ) . Because the ground state moments of inertia are known precisely for pyridazinc-d0 (see above), the problem is most straightforward for that case. We have only to determine what set of three upper-state inertial constants can cause all

INNES, TINCHER,

124

Table I

AND PEARSON

(Continued)

v o = 25 9 8 3 . 9 0

cm-1

6a~ j

~(j)

9 i0 ii 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

25 988.03 88.37 88.77 89.18 89.60 90.00 90.40 90.79 91.17 91.60 91.99 92.38 92.77 93.16 93.55 93.92 94.32 94.73 95.10 95.47 95.86 96.25 96.63 96.98 97.37

p(j)

25 978.62 78.23 77.82 77.42 76.98 76.57 76.15 75.75 75.34 74.91 74.48 74.05 73.64 73.19 72.76 72.34 71.89 71.50 71.05 70.60 70.18

TABLE

J

R(J)

34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

25 997.75 98.14 98.49 98.85 99.21 99.63 99.98 26 000.35 00.72 01.ii 01.49 01.84 02.25 02.59 02.94 03.33 03.68 04.01 04.36 04.71 05.11 05.49

P(J) 25 969.74 69.33 68.90 68.46 68.01 67.58 67.14 66.69 66.28 65.80 65.36 64.93 64.48 64.07 63.63 63.20 62.70 62.29 61.90 61.64 61.28 60.89 60.42 59.96

II

ROTATIONAL CONSTANTS FOR ASSIGNED BANDS

Assignment

B" (cm-1)

~' (cm-I)

Pyridazine-do 6al ° 6al 1 0-0 6ao 1

0.2032 0.2030 0.2034 0.2031

0.2029 0.2018 0.2030 0.2018

Pyridazine-d4 6ao 1 16bo2

0.1778 0.17770

0.1772 0.17769

ELECTRONIC SPECTRUM, OF 1,2-DIAZINE

125

( +

+ O~

*851

nOrm

I 0

O0

u

u

cr'-'O-'-'C) --'mO

,3

u

(2)-'-'-0

O ~

~

0

~

0--

0

0

0

,SO -i

.75 --

I

100

I

200

I

300

I

400

I

soo

I

Goo

I

7oo

I

~oo

I

900

( j + ½)2

FIG. 5. Test of linear relationship between A~F" and (J -t- ~ ) for the 0-0 band of pyridazine-d0 . transitions, K j ' = 0-11, jr, = 11 and h J = + 1, to fall within an interval of about 0.15 cm -1, the observed line width. The restrictions of the preceding analysis, A B ---- - - 0 . 0 0 0 4 c m - 1 and AC = --0.0004 cm -1, the latter from an estimate of the degradation of the Q-branch, fix C' = 0.1013 = A t B t / ( A t + B ' ) (if the molecule is rigid) and/~t = 0.2025 = ( A t -F B t ) / 2 , which require t h a t A t ~___ B t ~" 0.2025. Thus, it is reasonable to begin the fitting process b y calculating all R (11) transitions using the known lower-state constants (K" = 0.824) and the symmetric rotor ( t = 1.0) upper-state constants just deduced. When this is done it is found t h a t the strong transitions fall within a region of 0.60 cm -~ rather t h a n 0.15 cm -1. Energy derivatives (9) are accordingly calculated so that, b y inspection, one m a y see what adjustments of AA ~, AB t, and AC t are necessary to sharpen the collection of transitions. The corrected values are AA = --0.0037, AB = -F0.0029, and AC = --0.0004 cm -~ (K' = 0.94) for the 0-0 band of pyridazine-d0. I t should be noted t h a t the AK = -F0.12 found here is large b y the standard of our earlier criterion for the appearance of sharp features in t y p e C bands, namely, t h a t I A~I --< 0.1 (8). Indeed, we have discovered here a new cause of sharp structure, namely, t h a t AB = AC. This has the effect of causing all rotational lines with K~ >= ,[/2 or J / 3 to be virtually superimposed. I n contrast, the AK ~ 0

126

INNES, TINCHER, AND PEARSON

I

I

0.2c~

N

N

OO

KC--z, ~"

QI

I

=0,1

I BAND r

FIG. 6. Microphotometer tracings at two absorbing paths of the 6a01 band of the 3700 system of pyridazine-d0 . The upper-right insert shows the observed feature qR (11) enlarged five times and accounted for by rotational transitions of differing K~-vMues. The upperleft insert shows the displacements of ring atoms in this normal vibration.

criterion causes a peak to form fl'om the superposition of very strong K¢ <=J/3 lines, with the lines of high K giving a shading of the feature to higher or lower frequencies depending upon whether A(C -- /~) is positive or negative, respectively. In both cases the behavior of the high K contributions is close to that expected from the symmetric top equation. A microphotometer tracing of the 6a01 band of pyridazine-d0 has been presented in Fig. 6 because it gives a clear example of the criterion AK --~ 0. Here, if we repeat the procedure used in analysis of the R ( l l ) feature of the 0-0 band, we find ALl = --0.0011, AB = --0.0019, AC = --0.0007 cm -1, and AK = --0.015 for the 6a0~ band. In Fig. 6, an insert shows the way in which the peak R(11) arises. Figure 7 confirms that analysis by computer simulation of the complete contour using the rotational constants shown there and the exact asymmetric rotor contour program of Professor Louis Pierce (10). A similar confirmation has been carried out for the 0-0 band. In both cases it has been checked that the

ELECTRONIC SPECTRUM OF 1,2-DIAZINE

i

A" =0.20820 C" =0.10170

p(,j)

i

t

/ / c=ooooc A ~=0.20700

1

B" =0.19880 cm - I

I.I I I I I I I I I I I I I 2 2 2 0 18 16 14 12 I0

127

B' = 0 . 1 9 7 0 0

2

4.

J

I I I I I t I I I I I 6 8 I0 12 14 16

R(J)

FIG. 7. Computer simulation of the band contour of the 6a01 band of pyridazine-d0 . The asymmetric-rotor program of Professor L. Pierce was used to produce this curve. Rotational constants put into the program were those shown. They correspond to a change of asymmetry parameter, AK, of -0.015. Comparison with Fig. 6 indicates good agreement with the P- and R-branch features discussed in the text but also that the observed Q branch would be better fitted by a slightly higher value of Ct, say 0.10120 cm-I. The higher value would bring the Q-branch width close to the observed value, and would remove the weaker of the two maxima. (Each observed band shows only one maximum in the Q branch.) Presumably the resulting negative inertial defect arises from the Fermi resonance of this band with the overtone of an out-of-plane vibration (see Section III, A). c o m b i n a t i o n differences of t h e s i m u l a t e d spectra lead, w i t h i n e x p e r i m e n t a l error, to the same B - v a l u e s as were d e t e r m i n e d experimentally.4

B. The Triplet System (6600 A) T h e poorly resolved r o t a t i o n a l s t r u c t u r e (which m i g h t be more d i s t i n c t a t lower pressures a n d / o r longer a b s o r b i n g p a t h s ) is c o n s i s t e n t with the 3B1 excited It should be noted, however, that these values of "B" are not always exactly the are put into the computer program. From this we must conclude that the

(A + B)/2 that

128

INNES, TINCHER, AND PEARSON

state expected at an energy somewhat below that of the ~B1 state. Figure 4(c) shows the origin band. While the rotational structure is not completely resolved, it does exhibit (as does each band assigned to the triplet-singlet transition) branches depending upon the selection rule, AK~ = 0: the strong and sharp feature in the center of the band can only occur in the spectrum of a near oblate symmetric top molecule for the selection rule AK~ = 0, for reasons already discussed. A difference here is that the selection rules for changes in rotational angular momentum about the top axis, AK = 0, =i=l, must also include AK = + 2 for triplet-singlet transitions. Hougen (11) has shown that not all these branches will appear in the spectra of all molecules. Which branches appear depends upon the symmetry of the molecule and the orientation of the unique symmetry axis to the top axis. Hougen has worked out the transitions possible for a D2h molecule. Following specific instructions given for generalization of his results to C2~ molecules, we find that AKc = 0 can occur for two types of tripletsinglet transitions from a totally symmetric singlet state. These are: 3BI-1A ~ and ~A2-- IA 1 . Therefore, our experimental observation of branches depending upon AK~ = 0 eliminates two of the possible triplet state symmetries, but cannot distinguish between the other two. III. VIBRATIONAL ANALYSIS A . The Singlet System For the few measured bands of pyridazine-d2, as well as for the thousands measured for each of the compounds pyridazine-d0 and -d4, there is observed a sharp edge of the Q-branch similar to those of Fig. 4(a). Hence, it is assumed that all bands are of type C and that all vibrational differences may be assigned to totally symmetric fundamental, combination or overtone levels of the ground or excited electronic states (or to levels in which an odd number of quanta of appropriate antisymmetric modes is involved in the ground and the excited state). Bands become difficult to distinguish above 28 700 cm -1, as is evident even in Fig. 2, but it is not clear whether this is an effect of an underlying continuum or of the extensive Fermi resonances discussed below, or, as seems most likely, of both. Available evidence is fully consistent with the point group C2, for the groundstate pyridazine molecule (3, 4). Thus, there are nine totally symmetric fundamental vibrations for each electronic state (4). Two of the nine are carbonhydrogen stretching modes. Their failure to give rise to observed " h o t " bands just to the high frequency side of the 6a02 band of the triplet system (in Fig. 3) indicates that these modes are of minor interest in the spectrum. Five of the remaining seven are ring modes which range in frequencies from 608 to 1572 cm -1 symmetric-rotor approximation, A~Fexpt./4(J + 1/~) = B = (A + B)/2, breaks down in the fourth or fifth significant figure of the average value of A~F/4(J + 1/~).

ELECTRONIC SPECTRUM OF 1,2-DIAZINE

120

TABLE I I I PYRIDAZINE-d0 AND-d4 PRINCIPAL BAND CENTERS: FREQUENCIES (~vae, cm-1), ASSIGNMENTS, AND ESTIMATED RELATIVE INTENSITIES

do 24 24 24 24 24

259.3 350.9 486.0 528.2 653.5

I(d0) w w w w w

--25 001 25 081.9 --

0.1 0.05 w 0.2 0.1 0.2 1 1 1 5

25 272.9 25 165.9 -25 631,2 25 470.9 25 542.8 25 466.3 25 912.5 25 802.0 26 103.1 26 106.4 26 742.9 26 768.6 27 117.1 27 238.6

--

24 25 25 25 25 25 25 25 25 25

923.5 014.9 085.17 149.3 232.5 301.0 319.0 586.6 678.9 984.0 --

26 26 27 27

648.8 649.8 021.5 182.8

d4

--

3 (d4)

50 6 50 50

Assignments 6a2°151° 11°6a2° 6alO112° 1520 6an° 120

6a1°151° 11°6al° 8al ° 1120 19bi° 1410 6a2° 1510 110 6al ° 16b~ °

origin 16ad 6ao1 16bo2

for t h e g r o u n d electronic state. T h e other two are c a r b o n - h y d r o g e n b e n d i n g modes whose frequencies fall n e a r the middle of this range. I t is clear t h a t , e v e n if seven low-lying, t o t a l l y s y m m e t r i c (al) frequencies c a n be identified, t h e r e will be some a r b i t r a r i n e s s i n assigning t h e m to p a r t i c u l a r n o r m a l modes. W e shall m a k e use of the r e c e n t i n f r a r e d a n d R a m a n a s s i g n m e n t s of S t i d h a m a n d T u c c i (4) to assign m a n y lower-state v i b r a t i o n a l differences, d e t e r m i n e d from measurem e a t s of " h o t " b a n d s i n t h e electronic spectra. 5 T h e n u m b e r i n g n o t a t i o n for the n o r m a l modes is t h a t of (4). A n a s s i g n m e n t i n the electronic s p e c t r u m referenced as k~',, signifies a v'-v" t r a n s i t i o n i n mode k a c c o m p a n y i n g the electronic j u m p , F ' indexes the vth level of k i n t h e u p p e r electronic state, a n d so forth. T h e electronic origin b a n d of t h e system, n e a r 26 700 e m -1, is m o s t conv i n c i n g l y established b y the a p p e a r a n c e of progressions i n k n o w n lower-state 5 However, w~ interchange the Stidham and Tucci assignments of yea(a1) and r~(bl) on the basis of the following strong evidence; the 0-0 and 0-664.9 cm-1 bands of the singlet system each have been analyzed as type C (see Section II). Selection rules then require that the interval between them represent a totally symmetric vibration. Moreover, the other choice, 630 em-1, does not appear as a difference in the electronic spectrum.

130

INNES, TINCHER, AND PEARSON TABLE IV PYRIDAZINE-d0 DESLANDRES TABLES 1B~-IA1 SYSTEM A. P~ (al)

v v~, ' ~

0

1

2

664.8 0

26 648.8(500) 372.7

1

27 021.5(500) 365.4

2

27 386.9(300)

665.0 25 984.0(50) 372.7

664.8

0 1 2 3 4

665.5 25 319.0(10) 372.5

24 653.5(vw)

665.2 26 356.7(40) 365.4

664.8

v_'~':~,v"

3

25 691.5(10) 365.6 665.0

26 722.1(30)

26 057.1(10)

0

1

26 648.8(500) 534.0 27 182.8 (600) 553.3 27 736.1 (250)

[26 518.1 (20)] 476.4 [26 994.5 (10)]

2

26 443.9 (4) 553.6 26 997.5 (80)

differences terminating (crossing) at a common--and intrinsically relatively weak--type-C band assumed to be the 0-0 band. As we shall see, progressions on the high frequency side of the origin are extremely irregular, so much so that most assignments could only be quite tentative there. Accordingly, in listing only rather definite assignments in Table III (see also Table IV), we have included mostly "hot" bands of pyridazine-d0 and -d4. (Separations of the "hot" bands from the origins should be of interest since they represent ground state vibrational frequencies of the free pyridazine-d0 and -d4 molecules, accurate to within 0.2 c m -1.)

The most prominent intervals to the low frequency side of the origin bands are in each case assigned to successive quanta of v6a " (ring elongation) and v15 " (hydrogen bending) or combinations of the two. In analogous transitions of 1,3 and 1,4-diazine, v~ has been found to be associated with strong bands (6, 12), but this is the first ease for which an in-plane hydrogen-bending motion has been dominant. This will have significance for the correlation of the molecular geometry change with the Franck-Condon principle in Section IV, A, 2. Also of significance in this respect is the absence of the 12° band emphasized in Table III. Since, after correction for the Boltzmann factor, the 110 band seems nearly as strong as the bands 631° and 151°, and the 6a2° and 152° bands are easily

E L E C T R O N I C S P E C T R U M OF 1 , 2 - D I A Z I N E

131

5 3 4 " - -

372

4 7 8 - -

4 9 6 - - -

3 8 6 - -

3 7 4 - -

0.--

0 - -

Ev-Eo~

O - do

3,6 d 2

d4

FIG. 8. Effect of d e u t e r a t i o n on observed low-lying v i b r a t i o n a l levels of A1BI pyridazine. Note added in proof: A large correction of the level a t 478 em -1 s h o u l d be noted. I t aCtually lies a t 527 cm -1 w i t h respect to t h e zero level of pyridazine 3,6-d~. One of the a r g u m e n t s t h a t this level is involved in F e r m i resonance (Section I I I A) is eliminated b y the correction.

located, we conclude that the 11° intensity is not mainly Franck-Condon in its origin but is "borrowed" by vibronlc mixing of the excited 1B1 electronic state with another 1B~ state. It is worth noting that this effect of ~ has been found also in the analogous transition of s-tetrazine (13) (though not in that of s-dimethyltetrazine (13a)). In both cases ~1 is the only vibration which is detected to be active in mixing electronic states. For each of the molecules, pyridazine-d0 and -d4, only one prominent band to the low frequency side of the origin has to be associated with a first overtone of a nontotally symmetric mode. These give the lower-state differences 1499 and 1112 cm-1, respectively. According to Stidham and Tucci, each may be assigned to 2~1"1and each is found as a surprisingly strong Raman band (4). Therefore, we assume that in both the Raman and the ultraviolet spectra, 2~1"~is observed on account of intensity gained through anharmonic resonance. It is reasonable that for pyridazine-d0 most of the intensity is gained from vs"(a~) = 1572 cm-1, while, for pyridazine-d4, the source is ~14(al) = 1203 cm-1. Of course, it is conceivable that in one or both cases the supposed overtone is in fact the appropriate a~ fundamental itself. The most prominent intervals to the high frequency side of the origin are emphasized in the level diagram, Fig. 8, which shows a striking comparison among pyridazine-d0, -d~, and -d4 • Each vibrational level shown is totally symmetric. It is clear that the frequency 372 cm-~ of the d0-compound has been perturbed downward more than has the frequency 386 cra-I of pyridazine-d~ since, in the absence of perturbation, the deuterated compound must exhibit the lower frequency. By entirely similar reasoning, one must conclude that the frequency 496 cm-1 of the d4-compound has been perturbed upward more than has the frequency

INNES, TINCHER, AND PEARSON

132

/4v'16 b

724.8":

t/ J

639.86 36.5"

-v~; a +2~'16 b

~

.l

\2,,'6° "~2 v'16 b "~//'6a

o

2u"16 b ~-//" 6a

(~ o FIG. 9. Some vibrational levels affected by Fermi resonance: A1BI and )~IA~ electronic states of pyridazine-d4 . The separation of the ground state levels, unresolved in the vibrational spectrum (~), is exaggerated in our drawing. Observed transitions are shown with very rough estimates of intensities in parentheses. Other vibrational levels thought to lie within the compass of the upper-state triad are ~', 2/1~, , and 3pJ6b • 478 em -1 of pyridazine-d2. In each case, also, the perturbing level must be of s y m m e t r y a l , and the closest observed al level is t h a t shown. I t follows t h a t each pair of levels of Fig. 8 forms a Fermi diad. Evidently the very strong interaction sho~ll, especially b y pyridazine-d0, requires a large term of the type kQ~Q~2 in the ~IB~ potential energy expression. The anomalous potential surface m a y be related to the fact that, whichever level one takes to represent (most closely) a fundamental frequency of the excited state, one must accept a lowerlying, totally symmetric fundamental t h a n has been authenticated for any state of any monocyc]ic aromatic molecule. This point is discussed further in Section ¥ . I t is assumed first t h a t the lower component of a given diad of Fig. 8 is mainly the fundamental, since it gives a band which seems to show intensity greater t h a n or equal to t h a t shown by the band involving the upper component for two of the three isotopic compounds. The rotational constants of Table I I offer weak support for this choice. However, it should be emphasized t h a t our labelling is mainly a matter of convenience since we are assuming the levels to be mixed. Secondly, we note that the ratio of the sum of intensities of the diad components to the intensity of the origin band is at least two. This ratio is, after correction for the Boltzmann factors, greater t h a n any of those of " h o t " bands

ELECTRONIC SPECTRUM OF 1,2-DIAZINE

133

TABLE V VIBRATIONALBANDS IN THE 4400 A SYSTEM OF PYRIDAZINE ~'~o (cm-1)

V-~oo

21 822.9

-664.2

22 22 22 22 22 22

-191.9 - 27.5 00.0 192.8 211.7 445.7 614.4 639.2 829.3 848.6 1052.2 1083.4 1278.5 1471.8 1713.9 1918.8

295.3 459.6 487.1 679.9 698.9 932.8

23 101.6

23 23 23 23 23 23 23 24 24

126.3 316.4 335.7 539.3 570.5 765.6 958.9 201.0 405.9

Intensity 0.1 0.1 0.1 1.0 0.2 0.1 0.2 0.1 1.1 0.2 0.1 0.1 0.0 O.6 0.0 0.3 0.0

Assignment 6al ° 16al ~ 6all (?) origin 16bl1 6boq6bl °

6a0q6al 1 6al 2 6a01 6a0q6bl 1 6aoH6blO6bo 1

1501 6a0q6al ~ 6ao2 6a0~16bl1 6aoq5o1 6ao3

except for 6ai ° and 1510 (Table I I I ) . Since the 1501 b a n d should exhibit a large normal isotope effect which is n o t obvious in Fig. 8 we identify the f u n d a m e n t a l l as ~6~, t h a t is, Q~ = Q ~ . T o w a r d identification of the mode whose p e r t u r b e d • overtone is 534 c m -1 in pyridazine-d0, we note t h a t ~ ! --~ 267 c m - - 1 is v e r y small for a n y except the out-of-plane modes ~16aand p16b • Accordingly we test the identification Q~ = Qi~(v16b" = 369 cm-1), using the detailed measurements for pyridazine-d~ (n'~b = 326 cm-1), and the m e t h o d of analyzing the F e r m i resonance recently outlined (6). T h e resulting assignments are summarized in Fig. 9 where it is shown t h a t an analogous Fermi pair of levels ( b u t with AE only 3.3 cm - I ) has been discovered in the g r o u n d state. T h e F e r m i triad shown in the figure was identified b y prediction using an interaction constant, k', of 114.6 c m -I, determined from the excited-state diad. E a c h c o m p o n e n t of the triad was found within 15 c m - I of the predicted position. Since two other ai levels (~i' and 2~6a) are believed to lie a m o n g Components of the triad, and since the region of the F e r m i t e t r a d involving the 6a03 b a n d is so complex, it has n o t seemed worthwhile to t r y to refine the analysis. I t is interesting to compare u n p e r t u r b e d values of 2/1~ derived for analogous excited electronic states of the three fully-deuterated diazines, namely, 455, 625, and 422 cm -1 for 1,2-, 1,3-, and 1,4-diazine, respectively. Finally, we note t h a t 2 Pl6a was not found to lead to a self-consistent analysis of the t y p e shown in Fig. 9. Assignments of resonating levels of pyridazine-d0 are made in Table I V b y analogy with the detailed t r e a t m e n t of the d4-compound. T h e c o n s t a n t k could

134

INNES, TINCHER, AND PEARSON TABLE VI DESLANDRES T A B L E FOR

8~

0 1 2 3

V6a.

T R I P L E T - S I N G L E T TRANSITION OF PYRIDAZINE

0

22 487.1 (639.2) 23 126.3 (639.3) 23 765.6 (640.0) 24 405.6

1

(664.2)

21 822.9 (1278.7)

(664.0)

23 101.6

not be determined in this case on account of the comparable intensities of the diad components so that all assignments involving triad levels are uncertain. Indeed, because Fig. 8 indicates that k is largest for pyridazine-d0, it is very likely that t the anharmonicity apparent for v6~ in the Deslandres table (IV) should be still larger. Professor I. G. Ross and Dr. R. D. McAlpine have very recently and kindly made available to us a copy of their absorption spectrum of pyridazine-d0 in durene at 4.2°K. The complexity of the spectrum persists under these conditions. In fact, a strong peak which probably is buried in the upper component of the "diad" in the vapor spectrum is found 32 cm -1 above that component in the crystal. It therefore seems likely that more than two vibrational levels are involved in the resonance. If so, a potentially troublesome feature of Fig. 8 may be accounted for in part. The levels of Fig. 8 have been assigned to normal modes which are expected to involve only small contributions from the hydrogen atoms. Yet, in the Fermi diad interpretation, the isotopic shifts imply a strong de~ pendence of the interaction parameter ]c on deuterium substitution. Although this is (just) conceivable, it now seems more likely that a third level is affecting the components of Fig. 8. The strongest sequence band of the pyridazine-d0 system is found 1.0 cm -1 to the high frequency side of the origin band [see Fig. 4(a)]. A corresponding "satellite band" is found with many other strong bands, for example with that of Fig. 6, but only the 1-1 band is included in Table III. Its estimated intensity relative to the origin makes plausible its assignment as 16al ~. We conclude that the antisymmetric mode Vl6aincreases, from 410 cm -~ (4), by only 1.0 cm -1 in the electronic transition. Similarly, for pyridazine-d4, /216a increases by 24.7 cm --1 , from 351 cm -~. The frequency changes are so small that it is expected that not only the forbidden 1-0 and 0-1 bands should be missing but also the 2-0 and 0-2 bands. Indeed, none of these bands was found.

ELECTRONIC SPECTRUM OF 1,2-DIAZINE

135

B. The Triplet System Experimental difficulties preclude a "hot" band analysis of the weaker system comparable to that of Table III. It is therefore indeed fortunate that the vibrational pattern to the high frequency side of the origin is so much simpler in this case. Table V collects the measurements and assignments of all the bands marked with dots in Fig. 3. The only progression-forming mode is V6a and its behavior is t closely harmonic. In further contrast to the excited singlet state, V6a 639.2 cm-1, a value much closer to those found in other azines (1) and in agreement with the value found for the crystal (5). All of these points are brought out in Deslandres Table VI for v~. =

IV. CHANGES OF GEOMETRICAL STRUCTURE EFFECTED BY THE ELECTRONIC TRANSITIONS

A. The Singlet System 1. Evidence of the rotational analysis. Iao of pyridazine-d0 increases by 2 % while Ib0 decreases by 1.5 %, an effect suggestive of atomic displacements in the direction shown in Fig. 6. Because of the large number of nonequivalent bonds, we cannot estimate on this basis alone more than a rough average displacement of all atoms. We therefore defer further detail for Section IV, A, 2. The only further information from the rotational analysis is that the 0-0 band for pyridazine-d4 shows no regular J structure, from which we deduce that neither the condition A~ __~0 nor the condition AC __~ A/~ is satisfied for this case. Regular J structure is observed m • the 6a01band. Unfortunately the ground state rotational constants are not known precisely for pyridazine-d4 so that we can only achieve semiquantitative consistency with the preceding paragraph as follows: For the ground state, one may estimate from the microwave parameters that the axes of small (a) and intermediate (b) inertia will be interchanged when pyridazine is completely deuterated, such that ~ is again near +0.8. If this be so, then the effect of the suggested geometry change will certainly decrease Ia by about 2 % and increase Ib by about 1.5 % which will increase (A -- B) and reduce K to nearly +0.6. Regular J structure could not be expected for such a case. Finally, the effect of one quantum of v6~would substantially reduce the ] AK I from 0.2 to approximately 0.05 and some structure might be expected (8). (Here we have assumed that the effect on Kwill be simi]ar in magnitude to that for pyridazine-d0 for which both the 0-0 and 6a01 bands were analyzed and the respective values of A~ found were -0.015 and +0.12.) The fact that structure is observed in the 6a01 band of pyridazine-d4 lends confidence in the analysis proposed. We emphasize that this qualitative view of the structure change is on the sound basis of changes in all three rotational constants for a transition between zerovibrational levels of the two states. It therefore supersedes our earlier estimate (7) which had to be based on A/} only for the 6a01 bands of the three isotopic species. (All three of these 6a6~ bands were shown in Section III to be heavily

136

INNES, TINCHER, AND PEARSON

perturbed by Fermi resonance so that the/~-values for 6a 1levels now are expected to be incapable of simple interpretations.) 2. Evidence of the vibrational analysis. To a first! approximation, we m a y take the F r a n c k - C o n d o n maximum in the system at v6~ --~ 3. If, in spite of the low ! value of ~6~, we assume that the normal displacements have closely the same form in both electronic states, as well as closely the same form as has been worked out in detail for tetrazine (13), then we have an independent estimate of the magnitude (tbough not the correct choice of two possible directions) of the atomic displacements, and in this case, unlike that of Section IV, A, 1, they are individual atomic displacements. Evidently the best analysis should combine this information from band intensities with that of the change of rotational constants. If we require consistency of all the data, we must conclude that another vibration is of major importance in the F r a n c k - C o n d o n pattern. Otherwise the relatively broad F r a n c k - C o n d o n envelope is hard to reconcile with the very small increase in I~. As was emphasized in Section I I I , the relative importance of upper-state overtones and combinations of vibrational energies greater than a few hundred cm -1 is extremely difficult to assess. Accordingly, we refer to the two very " h o t " bands of the 3700 A system that are identified by arrows in Fig. 3. These represent lower-state vibrational energies of over 2000 cm -1, and it is clear that the two are of comparable importance. Moreover, both are more intense than all but one of the other bands of the region. 6 Fortunately, they are unambiguously assigned to 6a3° and 6a~°151°, as indicated in the figure, from which we m a y conclude that the combination mode 6a2151 has an amplitude that provides at least as close a matching of the effect of the purely electronic change as does 6a~. Now ~15, judged by the example of pyridine (14), is a (mainly) H-bending mode of a form such that (particularly) the displacements of the hydrogen atoms in positions five and four could cancel some of the effect of ,6~ on I~ (by reducing the effect of P6ain increasing I , and in decreasing Ib). 7 However, we emphasize that the large reduction of ~8~ and the large anharmonic constant in its contribution to the potential energy of the upper state (Section I I I , see also Section V) make risky the assumption that the normal coordinates of the excited state are precisely those of the ground state. I t is suggested that the ring-geometry change of the purely electronic transition be accepted as qualitatively that of the upper-left insert of Fig. 6 and that the 6These bands were obtained with a two-meter absorption cell near 200°C so that they are about 700X weaker than the origin band. Correcting for a Boltzmann factor of about 2000, we note that their Franck-Condon factors must be about three times that of the 0-0 band, consistent with our earlier claims concerning the intensities of the 6aol:16b02 and and 6al°:1662° Fermi diads. A third strong "hot" band in Fig. 3 lies at 24486.0 cin-1, midway between the arrows. It could be used to elaborate the preceding rationalization of the very small increase in Ic . However, on account of the ambiguity in the assignment 6al ° 1120 (see Section III, A), we shall not pursue the elaboration.

ELECTRONIC SPECTRUM OF 1,2-DIAZINE

137

lengths of the arrows of the figure be estimated, as in our similar work on pyrimidine (6), to be about 0.1 A. The foregoing intensity analysis assumes the validity of the Born-Oppenheimer approximation. Section V, A proposes the mixing of two ~B1 electronic states. If the mixing is later proven to be strongly dependent upon vibrations, the intensity analysis will need to he checked. Indeed, even our earlier discussion of vibrational assignments to the high frequency side of the 0-0 band could be affected.

B. The Triplet System From earlier sections it will be obvious that we have much less information about the geometry of the triplet than of the excited singlet state. The only striking point in the rotational contour is the degradation to higher energies, that is, that Ic is slightly decreased by the electronic transition. This may imply that the NN bond length is decreased, in contrast to the large increase shown for the ~B~-~A~ transition. The vibrational evidence is simpler than for the singlet system in that there is no evidence of complication! by hydrogen-bending frequencies. Rather, the dominant progression is in ~ with a maximum intensity at 6a0~ or 6a02. This suggests a geometry change somewhat smaller in magnitude than that for the ~B~-IA1 transition, with direction either as shown in the upper-left insert of Fig. 6, or reversed. V. ORBITAL ASSIGNMENTS OF THE EXCITED ELECTRONIC STATES 2

21

In the ground state pyridazine has the electron configuration • • • (al) (be) A~ (1). The lowest excited states are obtained by taking an electron from the lone pair as or be orbital to an antibonding ~r* orbital which may be of species as or bl (see Fig. 10). This gives rise to eight excited states 0

0

6

J

t

t

O

I

O

~ O 0

(al)2(b2) (ae) 3'1B1 (al)2(be)(b~) 3'1Ae (al)(b2)2(ae) 3'1A2 (a~)(b~)2(b~) ~'IB~.

Only transitions to the first and last singlets of these are formally allowed by electric-dipole selection rules. Each would be polarized perpendicular to the plane of the molecule as is characteristic for ~r*-n transitions.

A. Near Degeneracy of 1B~ Electronic States All strong, well-resolved bands of the 3700/t system have been shown to be of type C and to be based on a common origin. Thus, they must be assigned to one of the ~Bl-lAi trar~sitions indicated above. The rotational analysis has shown

138

INNES, TINCHER, AND PEARSON

n b ( b 2)

n (oI)

FIG. 10. Molecular orbitals of pyridazine t h a t the electronic transition effects an appreciable lengthening of the N N bond and some shortening of the C N bonds. The 1B1 state therefore should not have been formed b y promotion to the bl orbital, which would have introduced nodes in the C N bonds (1). We accept the assignment . - - (as)2(b~)(as). I t is perhaps surprising t h a t no bands have been found which can be assigned to the ~B1 upper state of configuration • • • (al) (b2)2(bl). Indeed, this is a general problem, which has been much remarked upon in the literature of ~r*-n transitions of nitrogen heterocyclic compounds (1) We believe t h a t the peculiar features of the vibrational analysis of Section I I I offer the most definite experimental evidence to date of the existence of two singlet ~r*-n! states lying within a few thousand cm -~ of one another. The very low value of ~6a (compared to other diazines) and the very large anharmonic t e r m in the excited-state potential function (compared to t h a t of the ground state) strongly suggest a distortion of the potential surface b y t h a t of another 1B1 state, probably higher, but in any case " n e a r b y . " s There seems no reason to doubt t h a t the second state has the configuration • • • (al) (b~)2(bl). The separation of the two states is probably less t h a n 0.5 eV and this fact should offer a basis of improvement of existing theory (15, 16). A small separation of states is consistent with our observation of intensity gained b y vibronic mixing of two 1B1 states b y ~1 (see Section I I I , A). I t seems likely t h a t some of the unresolved intensity of Fig. 2 belongs to the higher 1BrlA1 transition. I t is well to note t h a t the smaller, but still quite striking Fermi resonance t h a t we have described for an analogous excited state of pyrimidine (6) m a y result also from a (more distant) second ~B1 electronic state. The tentative conclusion for pyrimidine (6) was t h a t the (orbital) order of ~B1 states was inverted compared to t h a t found above for pyridazine. s A behavior analogous to that predicted for two interacting, nondegenerate states of

different symmetries by Hochstrasser and Marzzacco ("Molecular Luminescence," E. C. Lim, Ed., Benjamin, New York, 1969). It should, however, be emphasized that we are not at present assuming knowledge of the mechanism that couples the two electronic states.

ELECTRONIC SPECTRUM OF 1,2-DIAZINE

139

Hochstrasser and Marzz~cco (17) are among those who have suggested that the complexity of the vibrational structure of the absorption spectra of the low temperature crystals of diazabenzenes must arise from near degeneracies of . - n states. In that connection, we emphasize that discrete, strong bands of the corresponding vapor spectrum are, for each molecule, assigned to a single excited state, the second state being inferred from perturbations (6, 12), and therefore not located at all precisely. We conclude that any discrete absorption to those states not observed directly must be several times less probable than absorption to those states that are observed directly. In crystals, the ratio may be changed. However, a precautionary comment should be useful. In a case for which detailed evidence about a second state was Sought from the crystal spectra, namely that of 2,3-diazanaphthalene, it was found mainly in the fact that a band was observed at ~00 + 1672 cm -1 for the ordinary compound but at p00 4- 1685 cm -1 for a deuterated compound (18). I t is true that deuteration cannot increase an unperturbed fundamental vibration and that a possible assignment of the band is to a second electronic transition. Unfortunately, it is also true that the entirely similar behavior of the pyridazine vibrational structure (see Section I I I ) suggests an alternative explanation in terms of Fermi resonance within a single electronic transition. Such an alternative seems to us most likely and should often confuse analyses of spectra of large molecules which possess many totally-symmetric modes.

B. The Excited Triplet State Section II, B has shown only that the excited state of the 4400 A system is 3B1 or ;A~. This does not permit a choice from the four possible orbital configurations listed above but at least confirms the ~*-n character of the state. To obtain best agreement with the calculated value of the triplet-singlet splitting (3900 cm -1) given for ~'1B1 by Goodman (19), we make the tentative assignment 3B1 • " (al) 2(b~)(a2), and obtain the splitting accurately from the separation of the 0-0 bands of our two systems as 4161.7 cm -1. There is no evidence from the vibrational structure of any nearby states (see, however, the crystal spectrum of Hochstrasser and Marzzacco (5)). The major difficulty with our assignment is that it does not account for the possible opposite molecular geometry changes of the triplet-singlet and singlet-singlet systems (see Section IV, B). If the effect of the triplet-singlet absorption is to reduce the N N distance, a more reasonable assignment is one in which a node is removed from that bond b y the orbital promotion, viz., a ~A~ . . . (al)~(b2)(bl) excited state. ACKNOWLEDGMENTS

This work was supported in part by Grant GP-402 and later grants from the National Science Foundation and in part by the Petroleum Research Fund administered by the American Chemical Society. Grateful acknowledgment is hereby made to the donors of said funds. We thank also Mr. G. Blankenship who photographed manyof the bands. Mrs.

140

INNES, TINCtIER, AND PEARSON

E. G. Poynor who measured the spectrum of pyridazine-d4, Drs. Stidham and Tucci for their gift of pyridazine-d~, and Professor Louis Pierce for making available his excellent computer program for producing exact band contours. RECEIVED: J a n u a r y 24, 1970 REFERENCES 1. 2. 3. $. 5. 6. 7. 8. 9. 10.

11. 12. 13.

15. 15. 16. 17. 18. 19.

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