Ian G. Ross

Electronic states of azabenzenes: A critical review

No group of polyatomic molecules has been more comprehensively chara<*t,erized, with respect to their electronic states, than the six known azasubstituted benzenes (“azines”). Yet there are, in this same context, certain questions of chentral theoretical significance about which there is no sure evidence at all. It’ is the intention of this review to summarize the present experimental information to sketch lightly also the relevant theory, and thereby to exhibit both the strengths and the weaknesses of the present status of this subject. Figure 1 shows the six molecules, with symmetry axes labelled according to Mulliken’s convent,ions (I), conventions which one can only hope will become standard in this field. Figure 1 also specifies the location of the a and b inertial axes. The c axis is always normal to t’he molecular plane. The convention is that I, 5 Ib 5 I,. The identificat’ions given apply only to ground-state molecules containing the most abundant isotopes, ‘H, 14K and l?C. Because the two in-plane moments of inertia , are so close (the rings are never quite regular hexagons) t#he a and b axes can readily change over on isotopic subst’itut’ion. Thus they are known t,o interchange on going from pyrazine-do to pyrazine-& Axis-int’erchange in the course of an electronic transition is also conceivable, t’hough not yet observed. For example, if pyrazine-rlo had an excited stat,e with the geometry of benzene, t’he a axis would become b, and vice versa. As Kasha (9) first pointed out, two kinds of molecular orbitals are significant, in t#he lower excited states: the usual r orbitals, and molecular orbitals derived from radially directed, u-type lone-pair orbitals of the nitrogen at’oms (n orbitals).

The planar vibrations of naphthalene

Abstract A valence force field, including selected interaction constants, has been used to compute the thirty-three planar vibration frequencies of each of naphthalene, naphthalene-α-d4, naphthalene-β-d4 and naphthalene-d8. Starting with benzene-like force constants, the method of steepest descents was used in an iterative calculation which forced agreement with seven observed Ag-type fundamentals of the undeuterated compound, using (in the more successful of two such calculations) the Ag assignments of Luther. The final force field gave encouraging results when used to calculate all the other frequencies, and appears to be particularly suitable for the contentious low frequencies. The only really large and unavoidable disagreement was found in the case of the particular B2u vibration which carries naphthalene between two Kekule structures. The normal modes are depicted; they are strikingly similar to the modes found by Schmid in a calculation which led to considerably different numerical frequencies.

Spectrum of azulene: Part II. The 7000-A and 3500-A absorption systems

Abstract The first two electronic transitions of azulene have been photographed in the vapor, and the first transition in the pure crystal as well (4°K). The pure crystal spectrum is very diffuse; this is attributed to the disordered structure of the crystal. For the first vapor transition, which is also somewhat diffuse, the positions of 76 bands are recorded; most of them can be analyzed in terms of seven upper state vibrational frequencies and one difference interval. The sharper and richer second transition is more complicated; nine upper state frequencies and four difference intervals form the basis of a tentative analysis. It is provisionally concluded that azulene retains C 2 v symmetry in the two excited states. There are substantial geometrical changes, especially in the 7000-A transition. Upper state vibrational frequencies are difficult to correlate with those of the ground state. Attention is drawn to a large decrease in a very low frequency (probably the “butterfly” vibration), an effect observed in other molecules as well. The vapor spectrum and the mixed-crystal spectrum of Sidman and McClure are compared. They are difficult to correlate when, in the dilute mixed crystal, the absorption of the guest molecule (azulene) approaches the onset of absorption by the host (naphthalene). A partial explanation is suggested. There are also anomalies in respect of the role of nontotally symmetric ( b 1 ) vibrations in the two spectra. These, while apparent in the mixed crystal transition at 3500 A, do not appear to be present in the vapor. On the other hand, it is concluded (from consideration of the Franck-Condon principle and rough numerical comparisons of the effectiveness of intensity stealing in other molecules) that the greater part of the intensity of the 3500-A system derives from intensity stealing by totally symmetric ( a 1 ) vibrations. The unusual emission and internal converion properties of azulene are considered to be explicable in terms of the geometrical changes accompanying excitation. Internal conversion, by tunnelling, is then offered as the reason for the diffuseness of the weak long-wavelength system. Diffuseness in other polyatomic spectra is also discussed.

Band contour analyses of the spectra of asymmetric rotor molecules: Part III. The 3200-Å absorption of napthalene

Abstract The rotational contours of the naphthalene absorption bands at 32 458 and 32 521 cm−1 are re-analyzed, in asymmetric, rigid-rotor approximation. They are confirmed to exhibit transition moments in the plane of the molecule, the first along the central carbon—carbon bond and the second along the in-plane direction perpendicular to the first; that is the bands are type-B and type-A, respectively. The parameters that are well determined are the band center and the changes of two inertial constants for type-B and the band center only for type-A.

SPECTRUM OF AZULENE

Abstract The infrared spectrum of azulene has been measured in the vapor, solution, and solid phases, the latter with polarized radiation and single crystals. Attention was principally concentrated on the region 300–2000 cm −1 . The symmetry species of the observed absorption bands have been determined and an almost complete set of symmetry-assigned fundamental vibration frequencies is proposed.

Spectrum of azulene: Part III. Fluorescence intensities in azulene and azulene-d8

Abstract The fluorescence yield of azulene is enhanced by deuteration. An explanation is offered.

Asymmetric rotor energy levels: An improved computational procedure

Abstract The quotient-difference algorithm of Rutishauser has been advantageously applied to the calculation of asymmetric rotor energies.

On the Application of the Molecular Orbital Method to the Spectra of Substituted Aromatic Hydrocarbons

The MO theory of the spectra of substituted hydrocarbons is presented in rather general terms, with careful emphasis on the precautions to be observed in introducing the inevitable approximations. Previous treatments of the problem, notably by Sklar, Herzfeld, and Matsen, are then examined. Satisfactory calculaof energy‐level shifts are considered to require closer attention to the definition and dissection of the perturbed Hamiltonian. The most interesting intensity effects concern the enhancement of weak transitions: here earlier treatments do not satisfy the requirements of orthonormality of the perturbed MOs, and unjustifiably neglect interactions with intense transitions. The consequences of neglecting overlap in these calculations are discussed in an appendix.

Band contours in the electronic spectra of large prolate asymmetric tops

Abstract Type A , B and C band contours are computed (at 300°K), and their sub-band structure described, for molecules of the type of naphthalene and azulene (κ ~ —0.65, rotational levels significantly populated up to J = 120). Band contours of widely different types occur for changes in A and B of up to about ± 5%. For larger changes, significant structure practically disappears. Calculations indicate that the changes of shape in π ∗ ← π electronic transitions in aromatic molecules should mostly fall inside the 5% limit.