Jens Spanget-Larsen

Electronic states of the phenoxyl radical

The phenoxyl radical and two of its isotopomers were investigated by UV-VIS and IR polarization spectroscopy of molecular samples immobilized in cryogenic argon matrices. Analysis of the combined electronic and infrared linear dichroism data led to determination of absolute transition moment directions and symmetry assignments for four low-lying excited electronic states. The bands observed at 16 000, 25 200, 33 900, and 41 800 cm−1 were assigned to 2A1, 2B1, 2A1, and 2B1 π–π* states, respectively. A very weak transition observed in the near-infrared close to 8900 cm−1 was assigned to an optically forbidden 2B2 n–π* state. The electronic transitions predicted by time dependent density functional theory (TD-UB3LYP/cc-pVTZ) were in good agreement with the observed transitions.

Structure and reactivity of norbornene and syn-sesquinorbornene

Abstract The origin of the observed non-planarity of the syn-sesquinorbornene (syn-tetracyclo-[6.2.1.1 3,6 .0 2,7 ]dodec-2(7)-ene) π system is discussed in terms of simple molecular orbital theory. Calculated out-of-plane bending energies for the double bonds in cyclohexene, bicyclo[2.2.2]oct-2-ene, bicyclo[2.1.1]hex-2-ene, bicyclo[3.2.1]oct-6-ene, and bicyclo(2.2.1]hept-2-ene closely parallel the published cycloaddition rate constants for these compounds, indicating that ease of deformation may be a rate determining factor. The “staggering effect” considered by Houk et al . is discussed and it is demonstrated that this effect is not equivalent with the norbornene “x” factor introduced by Huisgen et al .

On the exceptional reactivity of the norbornene double bond

Abstract The anomalous reactivity of the bicyclo [2.2.1] heptene double bond is interpreted in terms of hyperconjugative effects, leading to an exceptionally low endo out-of-plane deformation potential.

Tetrazete (N4). Can it be prepared and observed

Abstract The nitrogen plasma generated by microwave or electrical discharge in gaseous N 2 was quenched on a cold window (6.2–35 K) and resulting matrix was examined by IR and UV–Vis absorption spectroscopies. In samples prepared with 14 N 2 we observe a weak infrared transition at 936.7 cm −1 . It shifts to 900.0 cm −1 when 15 N 2 is used. Both peaks do not correlate with any other features in the spectra and are best explained as originating from tetrahedral tetrazetes (N 4 ). Their positions are compatible with quantum chemical estimates for 14 N 4 , at 936.0 cm −1 and 15 N 4 , at 904.4 cm −1 .

Infrared absorption and Raman scattering of (Z)-3-hydroxypropenal. A density functional theoretical study

Abstract The molecular and vibrational structure of ( Z )-3-hydroxypropenal (β-hydroxyacrolein, malonaldehyde enol), a key compound in the study of intramolecular H-bonding, was investigated by a series of density functional theoretical (DFT) calculations. The vibrational frequencies obtained with the B3LYP, B3PW91, BLYP, or BPW91 density functionals are generally consistent with the spectral assignments recently suggested by S.F. Tayyari and F. Milani-Nejad [Spectrochim. Acta A 54 (1998) 255], but the DFT results are in much better quantitative agreement with the observed spectra than the MP2 results published by these authors, particularly in the case of B3LYP. Infrared and Raman vibrational transitions predicted with B3LYP/6-31G * and B3LYP/6-311++G ** are reported for the title compound and some isotopomers, and the assignment of the infrared spectra of these compounds is discussed.

Ultraviolet-visible and infrared linear dichroism spectroscopy of 1,8-dihydroxy-9,10-anthraquinone aligned in stretched polyethylene

Abstract Linear dichroism (LD) absorption spectra of the title compound (Chrysazin, Danthron) have been recorded in the UV-VIS and IR regions using uniaxially stretched polyethylene as an anisotropic solvent. The LD results show that only three different transition moment directions are present in the observed ≈ 30 transitions. This indicates that the molecular symmetry of the compound in stretched polyethylene is C2v (in contrast to Cs in the stable crystal form) and allows an unambiguous symmetry classification of the observed electronic and vibrational states. The results lead to reassignment of at least three vibrational modes, including the asymmetric δ(OH) and δ(CO) in-plane bending modes that are of interest in the discussion of intramolecular hydrogen bonding effects.

Transition moment directions in 9,10-anthraquinones. Symmetrical cases

Abstract Linear dichroism (LD) absorption spectra of 9,10-anthraquinone and a number of derivatives of C 2v symmetry have been recorded using specially produced thick stretched polyenthylene sheets as an anisotropic solvent. The spectra were recorded in the infrared region, which provided very precise information on the relevant properties of the solute orientation distribution. Similarly, absorption spectra were recorded in the visible-ultraviolet region which made a separation of the spectrum into contributions from different excited electronic state symmetries possible. This allows a detailed discussion of transition moment directions and the spectral assignment of the electronic transitions.

Electronic states of symmetrically disubstituted s-tetrazines

Abstract Six symmetrically disubstituted derivatives of s-tetrazine have been studied by linear dichroism spectroscopy in the UV-visible region, magnetic circular dichroism spectroscopy, and theoretical calculations of structure and spectra. The electronic absorption spectra, recorded up to 48 000 cm−1, show three major, characteristic transitions: An out-of-plane polarized n π ∗ transition at lowest energy, an in-plane, short axis polarized ππ ∗ transition at higher energies (in some compounds overlapping the first transition), and a third, long axis polarized transition at higher energy (for two compounds above 48 000 cm−1 and not observed in the spectra). The calculated and observed transition moments and energies for these transitions are in good agreement. In addition, several weaker transitions, both of n π ∗ - and ππ ∗ - type are predicted for all compounds and several of these are observed and tentatively assigned. The linear dichroism spectra, recorded on solutions in stretched poly(ethylene) (PE) or, when solubility in PE was low, in poly(vinyl alcohol), demonstrate distinct differences in the degree of alignment of similarly shaped molecules. This is remarkable, since studies of well over 100 aromatic hydrocarbons have shown a fairly simple relation between molecular shape and alignment.

Simple molecular orbital theory interpretation of the photoelectron spectra of the azines ag

Abstract A modified Iterative Extended Huckel Method (MIEHM) is introduced and applied to the photoelectron spectra of benzene, pyridine, pyrazine, pyrimidine, pyridazine, s -triazine and s -tetrazine. Guided by comparison with available ab initio Hartree-Fock calculations, the molecular orbitals are divided into four categories. The correlation between calculated orbital energies and experimental ionization energies is then established separately for each group of orbitals, leading to a consistent interpretation of the first ten bands in the photoelectron spectra. The resulting assignment is in general agreement with the one proposed by Gleiter. Heilbronner and Hornung 4 on the basis of empirical correlation procedures.

The electronic structure of p-terphenyl, 3,6-diphenyl-pyridazine and 3,6-diphenyl-s-tetrazine

Photoelectron spectroscopy, polarized electronic absorption spectroscopy in stretched polymer sheets, and several semi-empirical theoretical methods have been applied in an investigation of the valence electronic structures of the three title compounds. Overall consistency between theory and experiment has been obtained by assuming an effectively planar molecular structure. Large MCD B-terms are predicted for the two lowest π-π* transitions in the two aza-compounds.