D Johnson, D.A Ramsey and I Ross have accumulated much high resolution data on tills spectrum, and of the spectrum of C415N 2 *

In document Rotational and vibronic effects in molecular electronic spectra (Page 104-109)

State IV presents a choice between an indene-like x,7t* (:r™)

position 7. Despite an unfavourable energy discrepancy, using the first assignment (but the observed energy is that of a broad maximum), the

G. D Johnson, D.A Ramsey and I Ross have accumulated much high resolution data on tills spectrum, and of the spectrum of C415N 2 *

They have not been able to analyse it to their satisfaction, but a brief account of their conclusions is given below.

The interesting problems in the vapour spectrum reside

- +

primarily in the lowest transition, for which the assignment as , by Miller and Hannan, appears to be firm. Johnson et al. find that the bands are double headed, and hence of perpendicular polarization, and the intensity derives from a n state. They are also degraded to the blue, which suggests that the molecule contracts by bending. However,

following on a region of absorption, extending no further than 500 cm" 1 from the origin, there is a clear span of 1000 cm-1 which contains no evidence of progressions in the bending frequencies. The explanation of the blue degradation may therefore be found in terms of large amplitude bending vibrations even in the zero-point level. Our experience with formaldehyde (Sec. 3.3.3) suggests that zero-point amplitudes can be quite large.

Probable locations for the origins of both transitions are established by the observation, in each, of an interval of 241 cm 1

between prominent bands in the low frequency part of the spectrum. This is exactly the difference between vg" and V 7" and these bands are

identified as 7} and 6^. The origins are then at 35463 cm" 1 and

37801 cm 1. In the second transition, there are two corresponding bands to high energy, reasonably identified as 7q and 6q, from which, in that transition, we obtain V ß ’ = 369 (cf. Vß” 504), v y f = 266 (vy" 263). These figures prepare us for sizeable, though not prodigious changes in Vß and V 7 in the first transition.

The structure near the origin in the first transition appears to have too many bands. Pig. 2.5a illustrates the non-overlapped part of this transition and the onset of the second. Prominent intervals

CO 1.0

CJ O

20mm

___ L

2700 2900

Fig. 2.5a: Densitometer tracing of 2800 A system of dicyanoacetylene, adapted from Miller and Hannan [1958].

from the origin are 418, 338, 220 and 85 cm“ 1 . The hot bands below the origin show that vg is considerably more vibronically active than is V7. The problem then, is to identify vg and V7 among the frequencies just listed, and to explain the others, which may be hot bands.

We have measured the pure crystal absorption spectrum of C4N2 at 4 °K, in the hope that it would prove possible to superimpose it in a unique way on the vapour spectrum and thus to identify the bands

originating from the zero-point level of the ground state. In the event the results proved to be quite valuable for this purpose.

2.5.2 Experimental

Dicyanoacetylene was prepared by Dr. Johnson (by thermal

dehydration of acetylene dicarboxamide, N H2.CO.C=C.CO.NH2) . The compound is fairly unstable, especially in moist air; the absorption we sought to observe is weak and it was estimated that crystals of thickness in the range 0 . 1 - 5 mra would be required in order to see it. Sublimed crystals form in thin needles. Conventional single crystal techniques were thus assessed as unpromising, and it was decided to aim for a spectrum of the

polycrystalline solid. Two tapered silica cells were made (by Mr. E. Bellantoni); two flat plates were fused together at one end, and the spacing at the top was 0.1 nra in one cell and 2 mm in the other; inlet and outlet tubes were also provided and the whole cell# was fabricated to the constrained geometry of the cryogenic (cold flow and immersion)

dewars. The cell was connected to a vacuum line, filled by distillation, sealed, and kept, except during handling, in liquid nitrogen.

Our samples decomposed visibly after long irradiation but the absorption spectrum did not change. We did observe however that freshly prepared samples showed a long lived luminescence which deteriorated with age, and exposure to UV radiation. This luminescence could be a

phosphorescence; we found it difficult to record and the spectrum obtained, of seven sharp lines (in cm- 1 ; 23150 (m), 23.139 (ms), 23113

(vvw), 23096 (s), 23086 (vw), 23074 (w), 23028 (m)) on a broad background, is inconclusive. We do not know whether it belongs to C4N2 or a

photolabile impurity. We do, however, have confidence in the absorption spectrum.

Spectra were obtained after searching the polycrystalline sample visually and spectrographically. Fairly clear regions of solid were employed for the experiment; thicknesses ranged from ~ .05- .5 mm.

2.5.3 Results and Discussion

The spectrum obtained consisted of a few sharp lines in front of and interspersed between regions of apparently continuous absorption. All the details, except the first six lines, are so faint, or else so broad, that reproduction is not justified. Table 2.5 lists the features of the spectrum, recorded from two samples which together show all the features observed in the spectrum. Lines tend to fall in pairs, which are bracketed, and the separation of the members is given to the left of the bracket.

The pairing of the stronger lines in the spectrum suggests a splitting of 7T vibrations. Although Davydov splittings are also

8

possible, the estimated f-value of 2 x 10" 5 makes them unlikely. The next column in the table lists the separations of lines from the very weak lowest energy line in the spectrum. If we assume that this line

respresents the crystal-induced origin (and hence it is weak), and superimpose the crystal and vapour spectra (cf. Fig. 2.5a) as follows,

Table 2.5

Crystal absorption spectrum of dicyanoacetylene at 4 °K. Frequencies in cm 1. V V - 33912 Int. 33912 0 vvw sharp ' 34070 158 w \ sharp 54 * 34100 188 vvw sharp ■V 7 1 k 34124 212 w sharp 34305 393 s sharp 16

I

34321 419 s sharp f

~ 34360 448 fairly sharp onset of stronger unsymmetrical

49

47

50

continuous absorption, a ~ 1000 cm” 1 wide 34405 493 1 probable: very broad lines superimposed on the 34454 542 j diffuse absorptior1 35968 2056 w fairly sharp | 158 ^ + V o * 36015 2103 w fairly sharp 212 36179 2267 w sharp 158 [ 212 1 36229 2317 w sharp 36373 2461 w broadish

36545 2633 w broad, overlapping the onset of a second region ~ 800 cm 1 wide 36661 2749 m broad 36707 2795 m broad 37574 3662 s broad 37648 3736 s broad 37734 3822 s broad

4

!

8

338

419

393

220

212

150

85

vapour

3 crystal

0

then the 212 and 419 cm 1 crystal lines closely correspond to the vapour bands at 220 and 418 cm 1. We thus assign these as v-j* and V g ’; the lines at 158 and 393 may be the partners of these degenerate modes, split by the crystal field. The intensity ratio of the two pairs is

qualitatively correct, relative to the hot bands in the vapour spectrum of the second transition. The proposed frequencies of V g ’ and \)y' are also reasonably in line with those in the second transition. The additional bands in the vapour spectrum of the first transition remain hard to assign, even though they now appear to be hot bands.

The next observed sharp band pairs in the crystal spectra, 2056, 2103 cm-1 and 2267, 2317 cm- 1 , may be plausibly assigned to one quantum mode of o+ vibrations (in the second transition; V2U , 2119 era- 1 ;

8

V!n , 2290, V j ’, 2192 cm-1), built upon the V 7 vibronic origins (158 and 212 cm-1). If this assignment is correct, the crystal frequencies of the o modes are ~ 1895 cm 1 (v2 *) and ~ 2107 cm 1 (vj1), which are entirely

o

compatible with the values quoted above for the second transition. Our crystal spectrum still leaves unsolved the bulk of the problem of assigning the vapour spectrum, but it seems to promote one plausible interpretation of some bands in the vapour spectrum, and thus provides a useful basis for the re-examination of this complex system.

CHAPTER 3

VIBRONIC COUPLING IN MOLECULAR

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