摘要:

New measurements of the singlet–tripletabsorption spectra of pyrazine and pyrazine-d4at longer pathlengths and higher vapour pressures have been recorded. Particular attention was paid to the hot band region to the red of the electronic origin. The activities of the out-of-plane vibrations, υ4and υ10a, were identified in sequence bands, and their triplet state vibrational frequencies were determined, 295 (276, -d4) and 254 (194, -d4) cm−1, respectively. Model calculations were undertaken to match the observed singlet and triplet absorption and emission spectra for vibronic coupling induced by (i) υ10a(b1g), and (ii) υ4and υ5(b2g). The calculations allowed the ordering and the relative energies of the higher lying triplet states to be determined. The3B2uand3B1uelectronic states are above the ground electronic state by about 3.9 and 4.4 eV, respectively. The ordering is in analogy with the corresponding singlet states but opposite to that predicted by most theoretical calculat

ISSN:0008-4042    

DOI:10.1139/v93-193

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

TheS2–S0absorption, emission, and emission excitation spectra of thiophosgene in a perfluoroalkane solvent have been measured as a function of the analytical concentration,C, of the solute. Strong evidence of the formation of van der Waals dimers at 10−4 M < C < 10−1 M and of higher oligomers atC > 10−1 M has been found. Variations in the absorption spectra atC < 10−1 M have been analyzed successfully in terms of a singleequilibrium, for which the value ofKis (5.0 ± 0.4) × 102 M−1at 295 K. The mole fractions of monomer and dimer have been calculated at eachC, and have been used to construct the absorption spectrum of the dimer. Both the monomer and dimer have similarS0 → S1, andS0 → S2absorption spectra, but the dimer has an oscillator strength that is about twice that of the monomer. The effects of self-aggregation on theS2–S0emission and excitation spectra have been analyzed. Absorption by the dimer in the ultraviolet also leads to emission, but with a different quantum efficency compared with direct excitation of the monomer. The implications of these observations on the interpretation of the spectroscopy, photophysics, and photochemistry of thiophosgene in solution are assessed.

ISSN:0008-4042    

DOI:10.1139/v93-194

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Thea″ vibrational frequencies of all possible H and D isotopomers of propynal (HCCCHO) are shown to allow two solutions for thea″ harmonic potential constants. Equations relating the solutions are given. In theground electronic state one solution can be rejected as chemically unrealistic, but in theexcited electronic state the choice is more difficult. It is shown that a choice can be made on the basis of the observed cross-sequence intensities in theband system.

ISSN:0008-4042    

DOI:10.1139/v93-195

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

General criteria are discussed for the prediction of thermodynamically stable diatomic dications (i.e., cations of + 2 charge). For a diatomic AB2+, the most useful quantity for this purpose is Δ, the difference in energy between the attractive channel with asymptote A + B2+and the repulsive channel with asymptote A+and B+. Excluding noble gas compounds and using this measure, diatomic dications that may be thermodynamically stable include BeF2+, MgO2+, MgF2+, MgCl2+, and SiF2+, as well as several other less likely combinations. The dication BeF2+was selected for further analysis.Abinitiocalculations show that the X2Π state of this ion is thermodynamically stable by 1.08 eV and undergoes an avoided crossing that causes the effective dissociation energy to be 2.07 eV. The excited stateA2∑+is also very low-lying and predicted to be long-lived. TheX2Π–A2∑+band system is electric dipole allowed but very weak in intensity. Possible mechanisms of formation of BeF2+are discussed.

ISSN:0008-4042    

DOI:10.1139/v93-196

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Five substituted derivatives of the main chromophore, 2-phenyl-3,3-dimethyl-3H-indole (1), have been studied over a wide range of acidity by UV absorption and steady state fluorescence spectroscopy. These are as follows: 2-[(p-amino)phenyl]-3,3-dimethyl-3H-indole (2), 2-[(p-dimethylamino)phenyl]-3,3-dimethyl-3H-indole (3), 2-[(p-amino)-phenyl]-3,3-dimethyl-5-carboethoxy-3H-indole (4), 2-[(p-methylamino)phenyl]-3,3-dimethyl-5-carboethoxy-3H-indole (5), and 2-[(p-dimethylamino)phenyl]-3,3-dimethyl-5-carboethoxy-3H-indole (6). The dependence of absorption and fluorescence spectral characteristics upon acidity has allowed us to characterize four species, namely, the anion (A), the neutral molecule (N), the cation (C), and the dication (DC), for molecules2and3. Molecules4,5, and6give rise to an additional anion due to the hydrolysis of the ester group. The relative order of protonations to available sites is discussed. The ring-protonated species have shown the Phcand indole rings to be coplanar in both the ground and first excited singlet states. A low-energy non-emissive twisted intramolecular charge transfer (TICT) state taking its origin in the dimethylamino moiety is suggested from INDO-S calculations to explain the strong fluorescence quenching of these ring-protonated species. Acidity constants for the various prototropic equilibria have been determined experimentally both in the ground and in the excited singlet states. Förster cycle calculations have been used to supplement the various equilibrium constants. Possible applications in organized assemblies are discussed.

ISSN:0008-4042    

DOI:10.1139/v93-197

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Extensive multireference CI calculations using a contracted 6s5p3d1fGaussian basis set on the B and N atoms indicate that theX2∑+,A2Π, andB2∑+states of BN−are bound with respect to the electron attachment BN + e−. The 14∑+, 14Δ, 14∑−, and 14Π states of BN−are resonant states, all autodetaching into neutral BN. The electron affinityX3Π → X2∑+is predicted to be 2.84 eV (with a best estimate of 3.10 eV). Other features studied for BN−include the infrared spectrum of theX2∑+ground state as well as the electronic radiative properties involving the excited statesA2Π andB2∑+. The present study supports aX3Π ground state for BN, witha1∑+lying about 0.03 eV higher. As an alternative to optical spectroscopy on BN, experimental studies of the photodetachment spectrum of BN−are proposed in order to find the ordering and energetic separation of the lowest3Π and1∑+states of BN.

ISSN:0008-4042    

DOI:10.1139/v93-198

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Rotational analyses have been carried out for the 510, 501, and 511bands of thesystem ofcis-glyoxal, where ν5is the totally symmetric C—C=O bending frequency. The vibrational frequenciesandare smaller than the corresponding frequencies fortrans-glyoxal by nearly a factor of two. A vibronically induced band with a different type of rotational structure has been analysed and shown to be ana-type band. The coupling vibration is the out-of-plane C—H wagging vibration ν6(a2) and is similar in form and frequency to the vibronically active ν8(bg) vibration fortrans-glyoxal.

ISSN:0008-4042    

DOI:10.1139/v93-199

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The yttrium monosulfide molecule (YS) has been investigated using the techniques of molecular beam fluorescence spectroscopy and density functional theory. Fluorescence spectra in the region of the electronic orgin of theB2∑+ ← X2∑+system (ν00 = 14 826.07 cm−1) were recorded using a ring dye laser, the experimental resolution being 120 MHz. TheB2Σ+ ← X2Σ+(0,0) band, and a cold band of a hitherto unreported4Π±1/2 ← X2Σ+system (νυ′0 = 14 926.02 cm−1) have been rotationally analysed. (The ± 1/2 notation implies that the state has either4Π1/2or4Π−1/2symmetry.) Improved molecular rotational constants were obtained for the ν = 0 levels of theX2Σ+andB2Σ+states (r0(X) = 2.27191(17) Å,r0(B) = 2.32202(19) Å, γ0(B) = −0.15150(14) cm−1(2σ error bounds)). The magnetic hyperfine and spin rotation parameters determined for theX2Σ+state were found to be in good agreement with previous work. An accurate bond length has been derived for the upper vibrational level of the 14 926.0 cm−1band (rυ′ = 2.49510(16) Å). The ν = 1 level of theB2Σ+state is found to be strongly perturbed by another vibrational level of the4Π±1/2state. The spin-forbidden4Π±1/2 ← X2Σ+transition gains intensity via spin-orbit mixing between the4Π state and theB2Σ+state. Radiative lifetimes of the observed bands were measured by digitizing the fluorescence decay traces obtained upon excitation with a pulsed dye laser. The results of the density functional treatment provide broad confirmation of the experimental measurements. A molecular orbital description of the bonding in the YS molecule is presente

ISSN:0008-4042    

DOI:10.1139/v93-200

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Fluorescence excitation spectra of HgCl produced and cooled in a corona-excited supersonic jet discharge are presented. The dominant feature between ≈ 368 and 267 nm is an extensive vibrationally and isotopically resolved band system which is assigned to theB2Σ+(ν′) ← X2Σ+(ν″ = 0, 1, and 2) valence-to-ion-pair state transition of the radical. Transition wavenumbers for vibrational bands with 75 ≥ ν′ ≥ 21 and 2 ≥ ν″ ≥ 0 were measured, andB-state Dunham parameters were derived which are valid between 75 ≥ ν′ ≥ 0. The equilibrium bondlength for theB-state was determined to be 2.960(2) Å by Franck–Condon calculations.B–XFranck–Condon factors are tabulated which are expected to help in the interpretation of HgCl2photodissociation and HgCl laser spectr

ISSN:0008-4042    

DOI:10.1139/v93-201

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

A new group of bands has been observed in emission in the 500–600 nm region under conditions that generate known systems of GeS. It has been deduced from the appearance and position of the bands, and from their intensity distribution, that these bands constitute the long-wavelength component of theE1Σ+–X1Σ+system of GeS, which gives rise to strong absorption and emission in the ultraviolet.E–XFranck–Condon factors have been estimated.

ISSN:0008-4042    

DOI:10.1139/v93-202

出版商:NRC Research Press    

年代:1993

数据来源: NRC