Electronic Origin Band Research Articles

Rotational contour analysis of the electronic origin band in the emission spectrum of the orthoxylyl radical at 4683 Å

The rotational contour of the 4683 Å emission band of the o-xylyl radical was studied at high resolution. Calculations of the rotational contour of this hybrid band were made in the rigid rotor approximation for various sets of values of the excited state rotational constants and directions of the transition moment μ. Matching of computed and experimental rotational features showed that μ is oriented at +37° or −37° with respect to the b inertial axis. The nature of the excited states of o-xylyl and the methyl-to-ring interaction are discussed with respect to these two possible assignments

Structure and Dipole Moment of the First Electronically Excited State of Pyridine N-Oxide

A study is presented of the lowest-energy singlet—singlet transition of pyridine N-oxide in mixed crystals with p-dichlorobenzene at 4.2°K. Polarization and Stark effect measurements on the electronic orgin of the transition (at 29 599.6 cm−1) show that the transition is π*← π, 1B2← 1A1. The spectrum of d5-pyridine N-oxide is blue shifted by 123 cm−1 and its absorption and emission spectra are quite similar to those of h5. The vibrational analysis of the absorption and fluorescence spectrum, and the Franck—Condon patterns, suggest that the equilibrium geometry is little affected by electronic excitation. Stark effect measurements on the electronic origin band yield a value of Δ μ=0.71 ± 0.03 D (assuming E ≡ 0.60 Eeff) for the change in dipole moment on electronic excitation; where E and Eeff are the applied and effective Stark electric fields corresponding to an external field directed down the a′ crystallographic axis of the p-dichlorobenzene host crystal. This small change is much less than predicted by π-electron theory.

Electronic assignment as1A′(ππ*)-1A′ of the 2980 Å system of 2-aminopyridine by rotational band contour analysis

In 2-aminopyridine the electronic origin band of the 2980 a electronic system, the longest wavelength system which has been observed, is shown to be a type B-type A hybrid band containing 55 ± 10 per cent of type A character. It follows that the electronic transition is 1 A′-1 A′ which implies a π*-π and not a π*-n electron promotion. The degree of type A character is interpreted as indicating a swing of the transition moment by 10 ± 10° from the b axis of pyridine (the in-plane axis perpendicular to C 2). Since it is expected that the first π*-π transition in pyridine has its transition moment polarized along the b axis the amino-group does not seem to perturb the pyridine transition strongly in this respect. The changes of rotational constants from the ground to the excited state in 2-aminopyridine are very similar to those in aniline and probably reflect similar changes in geometry, namely a substantial contraction of the C-NH2 bond, an opening of the internal ring angle adjacent to the substituent and...

The crystal spectra of phenanthrene and phenanthrene-d10near 3400 Å

The weak absorption system of phenanthrene near 3400 Å has been measured in the vapour, pure crystal, and mixed crystal spectra for both phenanthrene-h10and -d10. The assignmentA1—A1is confirmed. Except for a small number of weak vibronic bands the pure electronic and vibrationally induced components have the same polarization, along the symmetry axis, showing that the perturbing vibration is totally symmetrical. The vapour spectra in 50 m paths at 90 °C show absorption envelopes similar to solution spectra, with a small number of sharp features of uncertain assignment. In the pure crystal the pure electronic origin band is split by 59 cm-1, the same for both isotopic species within the experimental limits of ± 5 cm-1. Frequencies, intensities, and polarization ratios are reported for pure crystals, and for mixed crystals in fluorene and biphenyl as hosts. The results are analysed and discussed in relation to previously studied vibrationally induced transitions in benzene and naphthalene.