I.G Ross

The out-of-plane vibrations of aza-aromatic molecules

Abstract A transferable valence force field has been developed for application to the out-of-plane vibrations of benzene, naphthalene, and their aza- (i.e., nitrogen-) substituted derivatives. The azanaphthalenes were the principal objective of the work. Existing vibrational frequency assignments have been reviewed, rationalized, and in some cases extensively revised. The molecules treated are quinoline and isoquinoline; nine diazanaphthalenes (1,2-; 1,3-; 1,4-; 1,5-; 1,6-; 1,7-; 1,8-; 2,3-; 2,7-) and nine tri- and tetraazanaphthalenes (1,3,5-; 1,3,6-; 1,3,7-; 1,3,8-; 1,4,5-; 1,4,6; 1,3,5,8-; 1,4,5,8-; 2,3,6,7-). In the course of reconsidering the spectra of these molecules it was necessary to review the assignments of the planar frequencies in the range 350–1000 cm −1 . These too are tabulated. The calculations on the azabenzenes (pyridine, etc.) support recent work of Kakiuti et al. (J. Mol. Spectrosc. 61 , 164 (1976)).

Electronic spectra of azanaphthalenes. Vapor absorption (π∗ ← n) of quinoxaline and quinoxaline-d6

Abstract The vapor absorption spectra of quinoxaline-d0 and -d6 have been measured, the former in agreement with published work of Glass, Merritt, and Robertson. Data for the latter are given here. The analysis of Glass et al. is confirmed and pursued in more detail, and a corresponding analysis given for quinoxaline-d6. The principal topics dealt with are: the identification of a new false origin in quinoxaline-d0 which coincides with a progression member at the same frequency, with interesting consequences in respect of band contours; further data bearing on the location of the origin and the identification of the entire absorption as a single allowed transition, 1 B 1 ← 1 A 1 ; sequences; and a quantitative Franck-Condon analysis. Excitation induces large changes in the ring angles, but they are confined to the ring which contains the nitrogen atoms.

Electronic spectrum of 1,5-naphthyridine: Theoretical treatment of vibronic coupling

Abstract The first singlet transition of 1,5-naphthyridine (point group C 2 h ) is predicted to be for-bidden: B g (n, π ∗ )-A g . It may acquire intensity through the seven a u out-of-plane vibrations. The assignments of these vibrations, in 1,5-naphthyridine- d 0 and - d 6 , are reviewed, and their normal coordinates are then empolyed in CNDO S calculations of their effectiveness in inducing intensity from allowed π ∗ -π transitions. Two different B u (π, π ∗ )-A g transitions, differently polarized, are predicted to be important sources of intensity. Using matrix elements derived from the computed intensities induced by the seven vibrations, the vibronic matrix is solved to yield a priori predictions of the vibrational structure expected in the π ∗ -n absorption and fluorescence of 1,5-naphthyridine- d 0 , - d 6 . In absorption, the consequences of the vibronic coupling are predicted to be severe and complex.

CNDO-calculation of second order vibronic coupling in the 1B2u-1A1g transition of benzene

Abstract The intensifies of second-order (two-quantum) vibronic transitions in the forbidden 1 B 2u - 1 A 1g electronic transition of benzene have been calculated by an extension of a method already applied to first-order bands (single quanta of e 2g vibrations). For the unobserved overtones and combination of the planar e 2g vibrations ν 6 and ν 8 the intensifies are predicted to be exceedingly small. For non-planar vibrations the calculated intensifies are much too large, but their relative values correlate encouragingly with experiment. Consideration of isotope effects indicates that the Herzberg—Teller mechanism, rather than Born—Oppenheimer breakdown, is responsible for the observed second-order intensities.

Electronic spectra of azanaphthalenes: Mixed crystal spectra of quinoxaline and quinoxaline-d6

Abstract The following spectra have been measured at 4 K, in three polarizations: 1 B 1 (n,π ∗ ) ← 1 A 1 absorption of quinoxaline in durene, durene-d14, naphthalene and naphthalene-d10, and of quinoxaline-d6 in durene and naphthalene. The polarized phosphorescence of quinoxaline-d6 in durene is also recorded. The analyses permit unusually extensive assignments of excited state vibrational frequencies. Also noted are various crystal effects: isotope shifts, modification of molecular frequencies, prominence of Herzberg-Teller origins, site splittings, effects of perdeuterated hosts, and polarization ratios. An unusual line-splitting seen in durene is analyzed as a resonance between coincident a1 and b1 vibrations of the quinoxaline molecule, coupled through the exciton bands of the host.

Electronic spectrum of 1,5-naphthyridine: Crystal spectra

Abstract Polarized spectra (4 K) of the following systems are recorded in the region 26 000 – 29 000 cm −1 ( π ∗ ← n transition): 1,5-naphthyridine- d 0 in durene, p -xylene, neat crystal and naphthalene; 1,5-naphthyridine- d 6 in durene and in naphthalene. The spectra in naphthalene differ radically from the others, which resemble the vapor spectrum in being built principally upon two main origins interpreted as the true origin 0 and a vibronic origin (6 a u ) 0 1 near 0 + 183 cm −1 (0 + 163 cm −1 , - d 6 ). The favored interpretation of the naphthalene spectrum, which is polarized in the molecular plane, invokes an origin ( L -polarized), but (6 a u ) 0 1 is missing, and there are then three further vibronic origins, each with distinctive polarization at 0 + 309, 0 + 362, 0 + 516 cm −1 (- d 0 ), 0 + 312, 0 + 384, 0 + 492 cm −1 (- d 6 ). The changed appearance of the spectrum in naphthalene is attributed to a diminution, because of an increased spacing between the interacting states, of the strong vibronic coupling which is considered to exist between the n , π ∗ state and two higher π, π ∗ states.

The lowest singlet state of dibenzofuran

Abstract The vapor absorption of dibenzofuran has been reexamined, and the spectrum and lifetime of the vapor fluorescence have been measured. Also measured were the polarized absorption in durene host and the depolarized absorption, fluorescence, and phosphorescence in cyclododecane host at 4 K. The fluoresecent state is that corresponding to the 3000 A absorption in all media, and no evidence was found for a weak transition at 3175 A as reported by Pinkham and Wait. The mixed crystal spectra are heavily perturbed, but from the polarizations observed in durene the 3000 A transition is assigned as 1 B 2 ← 1 A 1 (long axis polarized).

Electronic spectra of azanaphthalenes: Solid state spectra of cinnoline and quinazoline

Abstract The first electronic absorption region ( π ∗ ← n ) of cinnoline has been measured in durene and naphthalene as host crystals at 4 K and of quinazoline likewise in durene, naphthalene, and the pure crystal. The π ∗ ← n region is analyzed as a single transition in both cases in agreement with prior vapor studies, but all the spectra are complicated by the presence of at least two inequivalent sites with site splittings as large as 450 cm−1. In one instance (quinazoline in naphthalene) polarization ratios permit the conclusion that the two observed site spectra correspond to the two alternative placements of the guest molecule in a vacancy in the host lattice. Site and host dependent effects are documented on phonon activity, vibrational frequencies, polarization ratios, and the incidence of vibronic (Herzberg-Teller) activity. The salient properties of the first π ∗ ← π absorptions of both molecules and of the π ∗ → π phosphorescence of quinazoline are briefly noted.

Electronic spectrum of 1,5-naphthyridine: Vapor spectra

Analyses of the vapor spectra of 1,5-naphthyridine-d0 and -d6 are presented. The spectra are characterized by two principal origins, one the true electronic origin, magnetic dipole allowed, 1Bg ← 1Ag, 27 598.5 cm−1 (-d6 27 676.9), and the other a vibronic origin, electricd-dipole allowed, corresponding to the activity of a low-frequency vibration 6au, 183 cm−1 (-d6 163). Extensive sequence structure is evident and the relative intensities of the sequence bands associated with the two origins provide the strongest argument for their assignments. The absence of 6au as a major source of intensity in the hot bands is in agreement with vibronic coupling calculations which propose that in absorption intensity “flows” to the lower-frequency vibrations.

Microcrystals in frozen solutions: The luminescence spectra of pyrazine

Abstract Additional bands observed in the fluorescence and phosphorescence spectra of pyrazine in frozen solutions are due to emission from microcrystals.