Polarization and assignment of the two-photon excitation spectrum of benzene vapor

The two-photon excitation (TPE) of benzene fluorescence in the vapor phase at 60 torr is reported for the total-energy region from 38 086 cm^?1 to 42 441 cm^?1 using both circular and linear polarized light from a nitrogen-pumped dye-laser. The theory of the polarization dependence of the vibronic transitions in benzene is briefly reviewed, and it is seen how transitions involving vibrations of b_1u symmetry are expressly forbidden for this type of TPE experiment in which the two photons are identical. Five vibronic origins with distinctive rotational contours and polarization dependence are identified in the TPE spectrum. The υ₁₄(b_2u) vibronic origin at 1570 cm^?1 (above the electronic origin of the ^IB_2u state) stands out very prominently in the linear polarized spectrum, but nearly disappears in the circular polarized spectrum. This striking polarization dependence indicates a significant contribution of A_2u electronic states to the intermediate states of this TPE vibronic transition. The relatively great strength of the υ₁₄ band may be due to vibronic borrowing by the b_2u mode from the ground electronic state (A_1g).     

The vibration spectrum and the assignment of the lattice modes of sodium formate. II

The far i.r. data given in a previous paper are supplemented with Raman data on single crystals of sodium formate and powders of sodium formate-d₁. Arguments are now given for a better founded assignment of the lattice modes. Erroneous assignment in the previous paper was caused by the coupling of translatory and rotatory modes.     

The electronic spectrum of benzene

Assignment of the benzene spectrum is discussed using a new approach to the semi-empirical π-electron theory. The ordering ¹B_1u <¹E_1u <¹E_2g is shown to be theoretically preferred.     

The circular dichroism spectrum of S(+)-sec-butyl benzene

The absorption and CD spectra of S(+)-sec-butyl benzene are reported in the spectral region 195–180 nm. The CD spectrum contains two bands of opposite sign at 190.5 nm and 187 nm. The results are interpreted to confirm previous results for 1-methylindan in this spectral region. The observed bands are assigned to the state derived from the benzene E_2g state.     

The spectrum of vibration modes in soft opals

Numerous vibrational modes of spherical submicrometer particles in fabricated soft opals are experimentally detected by Brillouin light scattering and theoretically identified by their spherical harmonics by means of single-phonon scattering-cross-section calculations. The particle size polydispersity is reflected in the line shape of the low-frequency modes, whereas lattice vibrations are probably responsible for the observed overdamped transverse mode.     

Angular distribution of the photoelectron spectrum for benzene

The angular distribution of photoelectrons ejected from benzene using 21.22 eV radiation has been measured and the results applied to the analysis of the ionization bands.     

On the assignment of the 13C-NMR spectrum of phenothiazine

The assignment of the ¹³C-nmr spectrum of phenothiazine was made by comparison with the ¹³C-nmr spectrum of 1,9-dideuteriophenothiazine, synthesized via repeated lithiations and subsequent deuterations of phenothiazine.     

The polarised two-photon excitation spectrum of benzene monocrystals at 6 eV

The two-photon excitation spectrum of a benzene single crystal at 4.2 K has been recorded in the region of the second absorption system. The onset of two-photon absorption occurs near 46 500 cm^?1 (quoted as a two-photon frequency). The spectrum has the appearance of a forbidden transition in that the system begins with weak lines which become dominated by an intense continuum at higher energies. The two-photon cross section at 55 000 cm^?1 (the limit reached in this study) is about 200 times greater than at 47 490 cm^?1 although the peak of this strongly allowed system has not yet been reached. The fwhm of the bands near 47 000 cm^?1 is 280 cm^?1, the same as in the one-photon spectrum at these energies. The polarisation ratio is much the same over the entire energy range, and is consistent with the two-photon operators (xz, yz) or (zz). An analysis of all the data available from the one- and two-photon spectra suggests that the transition is ¹B_1u ← ¹A_1g in which the vibronic intensity is derived from the ¹E_1u state in the one-photon and ¹E_1g in the two-photon spectrum.     

Absorption spectrum and assignment of the Chappuis band of ozone

New global diabatic potential energy surfaces of the electronic states 1B1 and 1A2 of ozone and the non-adiabatic coupling surface between them are constructed from electronic structure calculations. These surfaces are used to study the visible photodissociation in the Chappuis band by means of quantum mechanical calculations. The calculated absorption spectrum and its absolute intensity are in good agreement with the experimental results. A vibrational assignment of the diffuse structures in the Chappuis band system is proposed on the basis of the nodal structures of the underlying resonance states.     

The orbital interpretation of the photoelectron spectrum of benzene, 1,3,5-trifluorobenzene and hexafluorobenzene

An interpretation of certain features in the vibrational pattern of the 11.5 to 13.0 eV photoelectron spectrum of benzene in terms of a Jahn-Teller effect is presented. This supports Lindholm's orbital assignment of the ionization processes in this region. A change in the assignment of some of the higher systems in the photoelectron spectra of 1,3,5,-trifluorobenzene and hexafluorobenzene is proposed. Some discussion of Rydberg series found in these molecules is also given.     

On the assignment of the carbon-13 NMR spectrum of bilirubin

An individual assignment of certain peaks in the proton and ¹³C NMR spectra of bilirubin and of bilirubin dimethyl ester is described. The assignment was achieved through the spectra of vinylneoxanthobilirubinic acid and by off-resonance decoupled ¹³C spectra of bilirubin dimethyl ester. The signals due to the endo and exo vinyl groups were separately assigned. This assignment allows a rational interpretation of previously obtained spin-lattice relaxation times of the vinyl carbon peaks. The two vinyl groups are found to differ considerably in their motional correlation time.     

Infrared spectrum and structural assignment of the water trimer anion

The bending vibrational spectrum of the perdeutero isotopomer of the water trimer anion has been measured and compared with spectra calculated using the MP2, CCSD, and Becke3LYP electronic structure methods. Due to its low electron binding energy (approximately 150 meV), only the OD bending region of the IR spectrum of (D2O)3(-) is accessible experimentally, with electron ejection dominating at higher photon energies. The calculated spectrum of the isomer having three water molecules arranged in a chain agrees best with the experimental spectrum. In the chain isomer, the excess electron is bound to the terminal water monomer with two dangling OH groups. This is consistent with the electron binding mechanism established previously for the (H2O)n(-) (n = 2, 4-6) anions.     

Fluorescence spectrum of the benzene radical cation in solid argon

Argon/benzene samples were condensed at 12 K with continuous argon resonance radiation. Laser excitation at 421 nm produced a weak emission with structure at 19770, 19140 and 18290 cm^?1, assigned to the ²A_2u → ²E_1g emission of C₆H₆⁺. Observation of 630 and 1480 cm^?1 intervals for the vibrations v₁₈ (e_2g) and v₆ (e_2g), respectively, supports this assignment.     

Complete assignment of the carbon-13 NMR spectrum of chlorophyl a

The ¹³C NMR spectrum (at natural abundance) of monomeric chlorophyll α in acetone-d₆ has been recorded to re-examine the assignments of the low field (aromatic-olefinic) region of the spectrum. The assignments, made by the examination of the fully coupled spectrum and by the use of long-range selective ¹H decoupling (LSPD) with low-power irradiation, were compared with those of the previous reports. The results of the present work clarify the ambiguities previously encountered in the assignment of the 10a-ester, 7c-propionyl, P-2-phytyl, 2b-vinyl, γ- and β-methine carbon atoms, as well as the β-pyrrolic carbon-6 and α-pyrrolic carbons ?16 and ?17 of chlorophyll α. Reassignment of the three last carbons was found necessary. Knowledge of the chemical shifts of these carbon atoms was considered to be particularly valuable, as it yields relevant information on the delocalized π electron system which is crucial for the function of chlorophyll in photosynthesis.     

Microwave spectrum of methoxyacetic acid: assignment of the hydrogen-bonded rotamer

Microwave spectra of CH₃OCH₂COOH and CH₃OCH₂COOD are reported. One conformation has been assigned. This form of the molecule has a planar HCOCCOOH skeleton with four out-of-plane hydrogens. A weak five-membered intramolecular hydrogen bond is formed between the hydroxyl proton and the ether oxygen thus stabilizing the planar form. Absolute intensity measurements and arguments based on a few reasonable assumptions have been used to show that the assigned rotamer is present at concentrations amounting to between 10 and 30 per cent of the total. Other forms are not identified. Seven vibrationally excited states were assigned and attributed to the three lowest torsional modes. The dipole moment was determined to be μ_a = 4.72±0.04 D, μ_b = 0.15±0.02 D, and μ_total = 4.72±0.04 D.