and 6-methyl substitutions do little to alter the gross vibrational structure.

In document Rotational and vibronic effects in molecular electronic spectra (Page 78-83)

STUDIES OF CRYSTAL SPECTRA

2- and 6-methyl substitutions do little to alter the gross vibrational structure.

The other unrelated compounds, the results for which are reported at the conclusion of the chapter, are purine and dicyano- acetylene.

2.2 THE 3500 Ä ABSORPTION TRANSITION OF AZULENE-(1,3)-d2 2.2.1 Introduction

Azulene has been studied in this laboratory for twenty years, and the findings are mainly documented in a paper of Hunt and Ross [1962] and in theses of McCoy [1964] and Lacey [1972]. Studies in the

rotational fine structure by McHugh [1967] have bearing on the first chapter of this thesis, but have no relevance to the present discussion.

The first four excited electronic states are all of concern to

O

M polarized and fairly well behaved, though its vibronic activity is referred to in the next chapter. The second transition at 3500 Ä is L polarized and is the focus of interest in Sec. 3.5. It is very

strongly perturbed by the fourth state, the transition to which is also L polarized. The third transition is M polarized; no significant spectral role is attributed to it other than its own absorption. A theory of vibronic coupling between states 2 and 4, advocated by LMR, proposed that the prominent active vibration has a "natural frequency" of ~ 1580 cm-1 (the strong band seen in fluorescence), but that there must be at least two lower frequency modes which are also coupled.

Because the 1580 cm-1 mode effects such strong coupling to state 4, all the modes are coupled together, resulting in a complex pattern of

frequency shifts and intensity interchanges, which LMR suggested could be tuned by placing the molecule in different media in which the energy gap between the perturbing and perturbed state (henceforth denoted AE) was altered.

The essence of the theory of LMR is given in Fig. 2.2a, which depicts the diagonal elements (energy levels) and off-diagonal elements (vibronic coupling constants) of the molecular vibronic Hamiltonian, in a simple scheme based on crude adiabatic wavefunctions, involving two vibrations for which no more than two quanta of each are involved. The scheme is generalizable (and we have done so) at the expense of a rapid increase in the size of the matrix to be diagonalized.

AE + 2vb ____ AE + Va + Vb --- A E< 2 v a _ A E 4- v * _ _b A F 4- v ~ a ZA fc — 2v. - a V !_ _ 2a b %2b b VU 4 Va l - 2v„..a I Vi -b J. Vo -a o-

Fig. 2.2a: Vibronic coupling scheme for two vibrations, from Lacey, McCoy and Ross [1973],

2,2.2 Spectral Results and Interpretation

Azulene-(3., 3)-d2 was prepared by treatment of azulene with freshly prepared D3PO4, along the lines of the synthesis outlined by Bauder and Giinthard [1958], Hass spectrometric analysis (and our subsequent vapour and crystal spectra) showed that the only major impurity present, to the extent of < 10%, was a monodeutero derivative doubtless azulene-l-di.

The spectrophotometric spectra shown in Fig. 2.1 clearly demonstrate that increased deuteration has a remarkable effect upon the frequency and intensity distribution in the strong vibronic region

(~ 3330 to 3370 Ä) at the second transition, which results in a complete transformation in the structure to high energy of these main peaks.

Fig. 2.2b documents the vapour spectra at higher resolution. The upper spectrum is a microdensitometer trace of the whole transition,

photographically recorded using the first order of a 4000 A-blazed

grating, and the lower spectrum records just the origin band group in the seventh order of a grating for 25000 X. The vapour cell employed was 3 m long and was maintained at a temperature of 43 °C. The vapour pressure was regulated by a sidearm containing the sample at 41 °C. Further

details are the same as those given in Sec. 1.3.

In the upper trace, the parent bands have pronounced sequence structure to low energy, and as a result, their positions are identified by steep cliffs, and the frequencies shown are those at this intensity edge. There is abundant evidence of Fermi resonances throughout the spectrum (as in azulene—do)• The detailed spectrum of the origin and the associated sequence structure is sufficiently similar to that seen in the -do and -dg species to permit indexing on the same basis as that of McCoy

[1964].

Fig. 2.2c shows the mixed crystal absorption spectrum in naphthalene, at 4.2 °K, for three polarization directions. The c ’ polarization is the most important, since most of the intensity lies along that direction, and the b and a spectra (mainly crystal-induced, as in azulene-do) are of value in giving a reliable depiction of the

relative intensity of the strong c ’-polarized bands. Lines arising from

t

A brief account of the experimental procedure required for recording such spectra will be given in Sec. 2.4.

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l - % %. $ % % t u

F i g . 2 . 2 b .

Hi gh r e s o l u t i o n v a p o u r s p e c t r u m of

a z u l e n e - ( l , 3 ) - d p .

The u p p e r t r a c e s hows t h e wh o l e o f

t h e 3 5 0 0 A s y s t e m and t h e l o we r s hows t h e o r i g i n band

g r o u p w i t h s e q u e n c e l a b e l l i n g s i m i l a r t o t h a t o f McCoy

( 1 9 6 4 ) .

3m c e l l a t 43° C.

I m p u r i t y l i n e s ( due t o

a z u l e n e - l - d - j ) a r e l a b e l l e d ( i ) .

F r e q u e n c i e s i n c m ~ ^

i n t h e u p p e r t r a c e a r e t o h i g h e n e r g y , and i n t h e l o we r

t o l ow e n e r g y , o f t h e o r i g i n ( O ) .

•o i

In document Rotational and vibronic effects in molecular electronic spectra (Page 78-83)