Study of singlet-triplet transitions in the ozone molecule using the multiconfigurational self-consistent field theory

The intensities and dipole moments of the lower singlet-triplet transitions ³ A ₂ ← X ¹ A ₁ and ³ B ₁ ← X ¹ A ₁ in the ozone molecule were calculated by the multiconfiguration self-consistent field theory with the quadratic response function. The results of calculations of the intensities of singlet-triplet transitions using different basis sets and complete active spaces were compared. The assignment of the ³ A ₂ ← X ¹ A ₁ transition in the ozone spectrum to the Wulf band is discussed.     

Two-photon absorption spectrum of (2,2)-paracyclophane

The two-photon absorption spectrum of crystalline (2,2)-paracyclophane was measured in the regions of 31200–33800 cm^?1, 34700–36500 cm^?1, and 40800–48600 cm^?1 by monitoring the fluorescence intensity using a tunable dye laser as a two-photon excitation source. Two absorption bands in the region 34700–36500 cm^?1 were assigned to the two even-parity allowed ¹B_1g ← ¹A_1g and ¹B_2g ← ¹A_1g transitions.     

π* ← n electronic absorption spectrum of 2,6-dichloropyrazine

The π* ← n electronic absorption system of 2,6-dichloropyrazine, corresponding to the ¹B_3u ← ¹A_1g transition of pyrazine, has been recorded in the vapour phase and in solution in cyclohexane. A vibrational analysis of this system has been proposed and it is shown that vibronic interaction between two excited states of 2,6-dichloropyrazine exists. Another system is observed in the solution spectrum of this molecule in cyclohexane and it is shown to be a π* ← π transition analogous to the ¹B_2u ← ¹A_1g transition in pyrazine.     

Dependence of impurity zero-phonon band thermal broadening and shift on impurity site and vibron level in organic crystals

Experimental results on the thermal broadening and shifts of impurity zero-phonon bands in some organic mixed crystals are reported and interpreted in terms of the electron-phonon interaction. Attention is focused on the variation of the thermal broadening and line shift parameters with (1) impurity site in a system exhibiting several energetically different impurity sites and with (2) the vibron level in a given final electronic state of the impurity. Experiments were conducted over a temperature range which ensures that hot vibron transitions are negligible. Data from the 5600 A ¹B₁ ← ¹A₁ transition of 2-phenyl-l-monoazaazulene in p-terphenyl show that the variations of the above parameters with impurity site are as large as the variations of the same between different chemical systems. No dependence of the thermal broadening parameters on the impurity vibron level is observed for the above system or the 7000 A ¹B₁ ← ¹A₁ transition of azulene in naphthalene. It is shown that, within the adiabatic (electron-nucleus) and harmonic approximations, the zero-phonon band spectral shape function arising from the electron-phonon interaction should be independent of the final vibron level of the impurity transition.     

π* ← π systems in the electronic absorption spectra of some trisubstituted benzenes

The electronic absorption spectra of 2,3-, 2,4-, 2,5- and 3,4-difluorobenzaldehyde in the UV region in vapour have been recorded on medium quartz and Hilger Large Quartz Spectrographs, and on a Hitachi U-3200 UV—vis spectrophotometer and analyzed. All the molecules investigated have exhibited two π* ← π band systems corresponding to ¹B_2u ← ¹A_1g (λ 2600 Å) and ¹B_1u ← ₁A_1g (λ 2100 Å) systems of benzene.     

An ab initioCI calculation of the triplet–triplet absorption spectrum of naphthalene

The triplet–triplet absorption spectrum of naphthalene (³B_1g ← ³B_2u; ³A_g ← ³B_2u) was calculated using an ab initio method with a STO -3G basis set and moderately large configuration interaction. Eight bands were found in the region 18,000–45,000 cm^?1 and compared with those of the experimental spectrum. The strongest band observed at about 44,000 cm^?1 was assigned to a ³A_g ← ³B_2u tansition. The excited triplet and singlet ground-state electronic charge distributions are compared.     

Triplet—triplet absorption and polarization spectra of phenazine

Employing laser photoselection spectroscopy, the triplet—triplet (T—T) polarization spectrum over the visible portion of the spectrum of phenazine was recorded. The strong T—T absorption band located in the violet portion of the spectrum is differently polarized from the flat band stretching from the blue to the red portion of the spectrum. This indicates the presence of two electronic transitions. The portion located in the violet spectral region is long-axis-polarized and presents the ³B⁻_3g ← ³B⁺_2u transition and the long-wavelength portion to the short-axis-polarized ³A⁻_1g ← ³B⁺_2u transition.     

Infrared and electronic absorption spectra of some trisubstituted benzenes

The electronic absorption spectra of 2,3-, 2,4-, 2,5-, 2,6- and 3,4-difluorobenzonitriles, 3,4-difluoroaniline and 3,4-difluoroanisole in the ultraviolet region in vapour phase have been recorded on medium quartz and Hilger large quartz spectrographs and on a Hitachi model 150-20 UV-VIS ratio recording spectrophotometer. All the molecules investigated have exhibited two π* ← π band systems corresponding to ¹B_2u ← ¹A_1g (λ2600 Å) and ¹B_1u ← ¹A_1g (λ2100 Å) systems of benzene. The infrared absorption spectra of all the molecules studied have also been recorded and analysed. These infrared data have been taken to help analyse the u.v. spectra of the molecules studied.     

The two-photon fluorescence excitation spectrum of dibenzothiophene

The two-photon fluorescence excitation spectra of dibenzothiophene crystal (≈77 K) and solution have been investigated in the spectral region 30000–38000 cm^?1 (≈3350-2600 Å). Two electronic transitions have been found. Both the oriented gas model results applied to the crystal and the circular/linear polarization ratio to the solution claim for an ¹A₁ ← ¹A₁ assignment of the transitions. Not completely resolved vibronic structures in the crystal spectrum were observed for both transitions for which a tentative assignment to A₁ total symmetry is discussed. It has been found that the intensity of the first electronic band system arises not only from a purely electronic (i.e., Franck-Condon) mechanism, but also from its interaction with a higher state through totally symmetric vibrations.     

The two-photon spectrum of toluene vapor

The two-photon excitation spectrum of toluene-h₈ and toluene-d₈ vapor has been recorded under low resolution (1 cm^?1) in the region of the S₁ ← S₀ (¹B₂ ← ¹A₁) transition. Although the electronic transition is formally allowed in two-photon spectroscopy, a large fraction of intensity exists in a subsystem induced by the out-of-phase CC stretching vibration ν₁₄ (b₂). Band contours associated with each of the two assigned tensor components of the transition are identified and partially analyzed by comparison with the two-photon contours of fluorobenzene.     

Multiphoton ionization spectra of two caged amines

The multiphoton ionization spectra of quinuclidine (ABCO) and triethylenediamine (DABCO) have been measured. All of the observed resonances are two-proton transitions to low-lying localized Rydberg states. For ABCO the lowest energy transitions are assigned as ¹A₁ (3s) ← ¹A₁ (n) 4.84 eV, and ¹E (3p_xy) ← ¹A₁ (n), 5.42 eV. In the case of DABCO, orbitals localized on the nitrogens interact and are split into two new orbitals. In terms of the split orbitals the low energy resonances in DABCO are assigned as ¹E′[3p_xy(?)] ← ¹A₁ (n+_, 4.44 eV, and ¹E″ [3p_xy(+)] ← ¹A₁ (n+), 4.94 eV.     

Analysis of polyatomic spectra using tunable laser-induced fluorescence: applications to the NO2 visible band system

By exciting NO₂ with a narrow band, pulsed, dye laser in the region between 5934 and 5940 Å, we have obtained a partial rotational analysis of a K = 0 sub-band of a ²B₂ ← ²A₁ transition. In addition, we find evidence for a ²B₁ ← ²A₁ transition in this region. By measuring the decay of resolved fluorescence features, we have determined the lifetimes of these two states, and find that they are markedly different.     

Electronic states and “Geometric Isomerism” of p-disubstituted cyanoanilines

Computations of CNDO/s CI type show that the two lowest energy ¹Γ

states of p-disubstituted cyanobenzenes are 1¹B₁(ππ^*) and 2¹A₁(ππ^*), in order of increasing energy. If the (CH₃)₂N-group of N,N-dimethyl-p-cyanoaniline (NNDMPCA or dimethylaminobenzonitrile) is twisted out of the plane of the benzenoid ring, the two transitions 1¹B₁ ← 1¹A₁ and 2¹A₁ ← 1¹A₁ undergo an increase of energy and a decrease of intensity. The postulated state in which an electron is promoted from the twisted (90°) (CH₃)₂N-group to a vacant in-plane π_χ^* orbital of the cyano group is predicted to lie ≈35409 cm^?1 above the 1¹B₁(ππ^*) state. This prediction contradicts an essential postulate of the excited state isomerization model which has been used to interpret the anomalous fluorescence behavior of NNDMPCA.     

Ab initio study of low-lying electronic states of the PF2 radical

The equilibrium geometries, excitation energies, force constants, and vibrational frequencies of the low-lying electronic states X²B₁, ²A₁, ²B₂, and ²A₂ of the PF₂ radical have been calculated at the MRSDCI level with a double zeta plus polarization basis set. Our calculated geometry, force constants, and vibrational frequencies for the X²B₁ state are in good agreement with experimental data. The electronic transition moments, oscillator strengths for the ²A₁ → X²B₁ and ²A₂ → X²B₁ transitions, and radiative lifetimes for the ²A₁ and ²A₂ states are calculated based on the MRSDCI wave functions. © 1994 by John Wiley & Sons, Inc.     

New vibronic bands of CH2 observed in a pulsed supersonic jet

Spectroscopic studies of the methylene radical in a supersonic expansion have resulted in the observation of previously unreported CH₂ B?¹ B₁ ← ã¹A₁, hot band transitions. These ã state levels are populated by radiative cascade following multiphoton dissociation of ketene. Multiphoton excitation of ketene is also found to produce a diffuse luminescence with a lifetime longer than 50 μs. The conclusion of Lengel and Zare are singlet methylene is not produced one-photon nitrogen laser photolysis of cold ketene is confirmed.     

The use of the CNDO method in spectroscopy

The Pariser approximation for the two center Coulomb repulsion integrals Γ _rshas been replaced by the Nishimoto-Mataga approximation in the original CNDO/S method. This modification has significantly improved the calculated position of the benzene ¹ B _1u ←¹ A _1g(¹ L _a ) electronic transition in benzenoid compounds. The calculation of transition moments of n — π ^* transitions is also considered. These moments vanish formally in any theory employing the ZDO approximation since integrals of the form 〈2s¦er2p〉 vanish even when the 2s and 2p atomic orbitals are on the same center. In this work the ZDO approximation is abandoned in the evaluation of the electronic transition moment resulting in calculated intensities for n — π ^*, ¹W←¹A, transitions which are in good agreement with experiment.     

The MRSDCI studies of four low-lying electronic states of the BF2 radical

The equilibrium geometries, excitation energies, force constants, and vibrational frequencies for four low-lying electronic states X ²A₁, ²B₁, ²B₂, and ²A₂ of the BF₂ radical have been calculated at the MRSDCI level with a double zeta plus polarization basis set. Our calculated excitation energy for X²A₁ → ²B₁ is in agreement with available experimental data. The electronic transition dipole moments, oscillator strengths for the ²B₁ → X²A₁ and ²B₂ → X²A₁ transitions, radiative lifetimes for the ²B₁ and ²B₂ states, and the spin properties for the X²A₁ state are calculated based on the MRSDCI wave functions, predicting results in reasonable agreement with available experimental data. © 1993 John Wiley & Sons, Inc.     

The intensities of low-energy electronic absorption transitions of some carbonyls and thiocarbonyls

The intensities of low-energy electronic transitions for some carbonyls and thiocarbonyls have been calculated from CNDO wavefunctions.Quite good agreement with experimental results has been obtained, where the latter are available. A satisfactory approximation for calculating intensities employs only one-center integrals. From the calculated trends in oscillator strengths, the absorption of thiophosgene at 4.46 eV can be identified as the π→π* ¹A₁←X?¹A₁ system. Another, very weak, system of thiophosgene at ≈ 3.9 eV is tentatively assigned to an n→π* ¹A₁ ← X?¹ A₁ transition, with the n orbital localized on the chlorine atoms.     

Magnetic circular dichroism studies of aromatic polymers

The magnetic circular dichroism (MCD) spectra of syndiotactic and isotactic polymers which contain aromatic chromophores have been found to be sensitive to configurational and conformational differences. For isotactic polymers it was determined that as the aromatic ring moved farther from the main chain the ration of B terms of the polymers to those of their model compounds reached a minimum but increased significantly when the aromatic ring was separated from the main chain by four atoms. This enhancement of MCD is believed to be caused by the alignment of the more flexible side chains which would allow the interaction of the aromatic rings with neighboring groups and could result in a favorable mixing of the ester electronic transition with the aromatic ¹A_1g?¹B_2u transition. This effect was not felt to any great extent by the syndiotactic polymers because the necessary nearest-neighbor interaction was sterically unfavorable. The ratio of the B terms of isotactic poly(phenyl methacrylate) to its model compound decreased as the polymer coil expanded, whereas it increased for the syndiotactic polymer. This effect reflects the different changes that the side chain interaction and orientations undergo in these polymers during coil expansion. The MCD ratios for iso- and syndiotactic poly(phenylethyl methacrylate) were not so sensitive during coil expansion. The ratio of the dipole strengths of the polymers and model compounds paralleled the MCD results, but the ultraviolet (UV) technique was less sensitive than MCD to subtle conformational differences. Poly(benzyl methacrylate) and benzyl pivalate were unsuitable systems for studying the MCD effect because the B terms of these materials approached zero.     

Excited-state absorption spectroscopy and state ordering in polyenes: 1,3,5,7-Octatetraene

The S_n ← S₁ spectrum of 1,3,5,7-octatetraene has been obtained in cyclohexane. Calculations predict different S_n ← S₁ spectra for the lowest excited ¹Ag^? or ¹B_u⁺ states. The experimental S_n ← S₁ spectrum is consistent with the 2 ¹A_g^? as the lowest excited state. Extension of this technique to smaller polyenes is discussed.