Photophysics and photochemical dynamics of chlorotoluene molecule

Laser-induced fluorescence spectra of jet-cooled chlorotoluene molecules are reported for the S₁ state. The fluorescence excitation spectrum of m-chlorotoluene shows some low-frequency bands up to 200 cm⁻¹ above the S₁ origin, which are assigned to internal rotational modes of the methyl group. Beyond 300 cm⁻¹ and up to approximately 1500 cm⁻¹ sharp vibrational bands are observed, which are assigned by measurement of the dispersed fluorescence spectrum on excitation of each vibrational band. The vibrational energies of the C---Cl stretching modes for the o-, m- and p-chlorotoluene molecules are 341, 378 and 360 cm⁻¹ respectively in the S₁ state.     

High resolution electronic spectroscopy of p-vinylphenol in the gas phase

A recent controversy regarding the proper assignment of two closely spaced bands in the S₁ ← S₀ electronic transition of trans-p-coumaric acid (pCA) has been addressed by recording their spectra at full rotational resolution. The results show unambiguously that the carrier of these two bands is p-vinylphenol (pVP), a thermal decomposition product of pCA. The two bands belong to two conformers of pVP; trans-pVP at 33,207.3 cm⁻¹ and cis-pVP at 33,211.8 cm⁻¹.     

Electronic spectrum of a photochromic diarylethene derivative in a supersonic free jet. Internal conversion from S2(1B) to S1(2A)

The fluorescence excitation and dispersed fluorescence spectra of the open-ring isomer of 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene have been measured in a supersonic free jet. No vibronic structure has been observed in the excitation spectrum. The intensity of fluorescence gradually increases with the excitation energy in the 25,500–28,700 cm⁻¹ region, indicating that the geometry of the molecule substantially changes upon photoexcitation. The dispersed fluorescence spectrum is anomaly Stokes-shifted with respect to the excitation energy, suggesting that the S₂(1B) state is initially excited followed by rapid internal conversion from the S₂(1B) to S₁(2A) state. The fluorescence is due to the S₁(2A)–S₀(1A) transition. Density functional theory calculations at the B3LYP/6-31G^** level have been carried out to investigate stable conformations responsible for the observed spectra.     

Resonance-enhanced multiphoton ionization (REMPI) spectroscopy of photolytically generated hot CF radicals

In this paper, we investigate the rotationally resolved spectra of hot CF radicals generated after IR multiphoton dissociation (IRMPD) of CFCl₃ or CF₂Cl₂ and subsequent UV photodissociation. It is shown that these conditions are advantageous for the spectroscopy of transitions involving high rotational quantum numbers and hot bands. Thus molecular constants of CF for the first vibrationally excited state of the electronic ground state (A_v=77.1 cm⁻¹, B_v=1.389 cm⁻¹, D_v=6.570×10⁻⁶ cm⁻¹) are determined for the first time or are calculated more accurately. The spectroscopic method used was resonance-enhanced multiphoton ionization (REMPI) spectroscopy.     

S2 → S0 fluorescence in an aromatic thioketone, xanthione

In addition to the red phosphorescence (T₁(³ A₂n, π^*) → S₀) xanthione exhibits in solution an emission with a maximum at ≈ 23 000 cm⁻¹ and φ_f(298°) = 5 × 10⁻³. It is shown that this emission is fluorescence from the second excited singlet state (S₂ (¹A₁ π, π^*) → S₀).     

Quantum-chemical analysis of vibrational intensity distributions in the S0-S1 absorption spectra and raman excitation profiles of azulene

The vibronic, Franck—Condon and Dushinsky activity of the totally modes of azulene in S₀-S₁ transition is investigated by quantum-chemical methods. It is found that the 900 and 1400 cm⁻¹ mode mixing proposed earlier to explain the relative intensities of Raman fundamentals in the region of the S₁ state is not supported by the calculations. The alternative mechanism arising from vibronic activity of the a₁ modes produces signs of the non-Condon parameters in agreement with those found from fitting the Raman excitation profiles; however, the absolute value of this parameter for the 900 cm⁻¹ mode is almost one order of magnitude too small. Thus, although quantum-chemical calculations favour non-Condon effects over Dushinsky effects in shaping the relative intensities of Raman fundamentals, they do not give a conclusive answer. It is argued that such an answer can be obtained from measurements of the 900 + 1400 cm⁻¹ combination band Raman excitation profiles.     

On the vibrational spectra and conformational stability of 1,1-dimethylhydrazine from temperature dependent FT-IR spectra of krypton solutions and ab initio calculations

Variable temperature (−105 to −150 °C) studies of the infrared spectra (3500–400 cm⁻¹) of 1,1-dimethylhydrazine, (CH₃)₂NNH₂, in liquid krypton have been carried out. No convincing spectral evidence could be found for the trans conformer which is expected to be at least 600 cm⁻¹ less stable than the gauche form. The structural parameters, dipole moments, conformational stability, vibrational frequencies, and infrared and Raman intensities have been predicted from MP2/6-31G(d) ab initio calculations. The predicted infrared and Raman spectra are compared to the experimental ones. The adjusted r₀ parameters from MP2/6-311+G(d,p) calculations are compared to those reported from an electron diffraction study. The energy differences between the gauche and trans conformers have been obtained from MP2 ab initio calculations as well as from density functional theory by the B3LYP method calculations from a variety of basis sets. All of these calculations indicate an energy difference of 650–900 cm⁻¹ with the B3LYP calculations predicted the larger values. The potential function governing the conformational interchange has been predicting from both types of calculations and comparisons have been made. The barrier to internal rotation by the independent rotor model of the inner methyl group is predicted to have a value of 1812 cm⁻¹ and that of the outer one of 1662 cm⁻¹ from ab initio MP2/6-31G(d) calculations. These values agree well with the experimentally determined values of 1852±16 and 1558±12 cm⁻¹, respectively, from a fit of the torsional transitions with the coupled rotor model. For the coupled rotor model the predicted V′₃₃ (sin 3τ₀ sin 3τ₁ term) value which ranged from 190 to 232 cm⁻¹ is in reasonable agreement with the experimental value of 268±3 cm⁻¹ but the predicted V₃₃ (cos 3τ₀ cos 3τ₁ term) value of −73 to −139 cm⁻¹ is 25% smaller and of the opposite sign of the experimental value of 333±22 cm⁻¹. These theoretical and spectroscopy results are compared to similar quantities of some corresponding molecules.     

Study of the N2 bΠu state via 1 + 1 multiphoton ionization

Medium-resolution spectra of the N₂ b¹Π_u-X¹Σ_g⁺ band system were recorded by 1 + 1 multiphoton ionization. In the spectra we found different linewidths for transitions to different vibrational levels in the b ¹Π_u state: Δν₀ = 0.50 ± 0.05 cm⁻¹, Δν₁ = 0.28 ± 0.02 cm⁻¹, Δν₂ = 0.65 ± 0.06 cm⁻¹, Δν₃ = 3.2 ± 0.5 cm⁻¹, Δν₄ = 0.60 ± 0.07 cm⁻¹, and Δν₅ = 0.28 ± 0.02 cm⁻¹. From these linewidths, predissociation lifetimes τ_ν were obtained: τ₀ = 16 ± 3 ps, τ₁ > 150 ps, τ₂ = 10 ± 2 ps, τ₃ = 1.6 ± 0.3 ps, τ₄ = 9 ± 2 ps, and τ₅ > 150 ps. Band origins and rotational constants for the b ¹Π_uν = 0 and 1 levels were determined for the ¹⁴N₂ and ¹⁴N¹⁵N molecules.     

Low-lying excited singlet states and S2→S0 luminescence of dipyrido[3,4-b: 2,3-d]phenazine

The absorption, fluorescence and excitation fluorescence spectra dipyrido[3,4-b:2,3-d]-phenazine (DPPZ1) have been measured in non-polar and polar matrices at room temperature, and were taken into account to explain the origin of the relatively weak emission of this molecule in both type of environment. The electronic structure of DPPZ1 was calculated using a modified INDO CI method. The geometry optimization has been performed using the MNDO method. According to the spectra and the results of calculations, the lowest excited singlet state S₁ of DPPZ1 molecule is of n,π^*-type and the next one, S₂ state, is of π,π^*-type. The energy gap ΔE_calc is equal 4770 cm⁻¹. The low efficiency of the emission observed in the hydroxylic solvent can be interpreted in terms of thermal quenching of the π,π^*-type fluorescence. However, experimental results obtained suggest that in nonpolar solvents the emission of the molecule examined is an anomalous S₂→S₀ fluorescence.     

Simulated T ← S spectra of benzaldehydes as a function of the energy gap between the ππ and nπ levels

The T_1,2 ← S₀ spectra of benzaldehydes have been studied as a function of the energy separation between the vibrationless levels. It is shown that the spectra are very complicated in the region of ΔE[T₂⁰(nπ^*)-T₁⁰(ππ^*)] = 250–400 cm⁻¹, reflecting effective vibronic interferences between T₂⁰(0-0) and each of the ν₃₆-ν₃₃ out-of-plane vibrational levels of T₁⁰(ππ^*). The simulated spectra correspond to the observed spectra. In the case of T₁⁰ = ³nπ^* and T₂⁰ = ³ππ^* the spectral change is not so drastic as in the reverse case loc. cit. because the optical intensity generally concentrates in the longest wavelength band, i.e., the origin band of the T₁(nπ^*) ← S₀ transition. The simulation spectra are useful for interpretation of the absorption spectra in similar electronic structure systems of substituted benzaldehydes.     

EPR and IR studies on the local structure of 80MoO3–20B2O3 glass

EPR lineshape simulation studies have been performed on a specimen of 80MoO₃–20B₂O₃ glass in the temperature range of 300–77 K. The values of the obtained spin Hamiltonian parameters are: g=1.940, g=1.974, A=150.0×10⁻⁴ cm⁻¹, A=35.6×10⁻⁴ cm⁻¹ and g=1.935, g=1.975, A=141.9×10⁻⁴ cm⁻¹, A=34.5×10⁻⁴ cm⁻¹ at 300 and 77 K, respectively. The paramagnetic site in the specimen is molybdenyl, MoO³⁺, ion in which the Mo is in a distorted octahedral environment of six oxygen atoms with C_4v symmetry. The 11-parallel and 11-perpendicular line feature of the EPR lineshape shows that two Mo nuclei are magnetically equivalent in the glassy matrix, in the temperature range 300–77 K.     

Vibrational spectra and structure of chlorine substituted cyclopropanes : Part I. cis-1,2,3-Trichlorocyclopropane

The synthesis of cis-1,2,3-trichlorocyclopropane is reported. The infrared spectra have been recorded between 4000 and 400 cm⁻¹ in the polycrystalline solid phase, and between 4000 and 200 cm⁻¹ in the gas phase. The spectrum of a solution in carbon disulphide was measured from 1400 to 400 cm⁻¹. The Raman spectrum has been obtained between 4000 and 100 cm⁻¹ in the solid phase. An assignment of the fundamentals of the title compound is proposed and compared with similar molecules. The spectrum unequivocally proves the C_JV structure of the molecule.     

Spectra and structure of phosphorus-boron compounds : IV. Vibrational spectra of solid trifluorophosphineborane

The Raman spectra of F₃PBH₃ and F₃PBD₃ have been recorded (2500-10 cm⁻¹) of the liquids (−80°C) and solids (−196°C) as well as the infrared spectra (4000-33 cm⁻¹) of the solids. In the spectrum of the solid state many of the ¹⁰B and ¹¹B fundamentals were clearly defined and it was also possible to assign the BH₃ torsional frequency from the infrared and Raman spectra of the solids. A complete vibrational assignment is proposed and a normal coordinate calculation carried out. The force constant of 2.46 mdyn Å⁻¹ for the P-B stretching mode is consistent with the short P-B bond; this constant is compared to the similar quantity for several other phosphorus-boron compounds. All of the E modes for the “free” molecule are shown to be split by the site symmetry which indicates that the molecules occupy C_s or C₁ sites. The large number of observed lattice modes is consistent with two or more molecules per primitive cell. The torsional frequency was observed at 224 cm⁻¹ and 167 cm⁻¹ in hydrogen and deuterium compounds in the solid, respectively. These frequencies gave a periodic barrier of 4.15 kcal mole⁻¹ for F₃PBH₃ and 4.31 kcal mole⁻¹ for F₃PBD₃. CNDO/2 calculations have been carried out for F₃PBH₃ and the isoelectronic F₃SiCH₃ molecule in both the staggered and eclipsed forms and the dipole and barrier origins are discussed.     

The infrared spectrum of gaseous Sb4O6

The vibrational spectrum of Sb₄O₆ in the gas phase has been measured at 1000 K by high-temperature infrared spectroscopy. The four infrared-active absorption bands were observed at ν₇ = 785.0 cm¹, ν₈ = 176.2 cm⁻¹, ν₉ = 292.4 cm⁻¹ and ν₁₀ = 415.6 cm⁻¹. By combining these results with data on the molecular geometry and the infrared-inactive modes, as reported in the literature, the thermodynamic functions of Sb₄O₆ have been calculated.     

An investigation of the structure of butadiene by high resolution infra-red and raman spectroscopy

The rotational Raman spectra of butadiene and butadiene-d₆ are found to consist of discrete lines having small ≈0·4 cm⁻¹) yet almost constant spacings, as would be expected for symmetric or nearly symmetric top molecules. An infra-red absorption band (Type C) of butadiene at 908 cm⁻¹ is observed to have a spacing of about 2·5 cm⁻¹. Both the Raman and infra-red spectra provide evidence for the trans structure of the butadiene molecule. From the rotational constants A″ and ″ the following structural parameters were obtained: C=C---C) = 122·9 ± 0·5° rC---C) = 1·47₆ ± 0·010 Å dy]somewhat shorter than recently determined from electron-diffraction experiments).     

Spectres infrarouges et structure des anions 2-methylallyle, CH3-C(CH2)2- et triméthylèneméthane, C(CH2)3

The infrared spectra of solid samples of C₄H₇K and C₄D₇K have been investigated in the 4000 to 30 cm⁻¹ range. A complete assignment of intramolecular fundamentals of C₄H₇⁻ and C₄D₇⁻ ions and of potassium-allyl vibrations is proposed and the intramolecular force constants are calculated. The C(CH₂)₃²⁻ anion has been identified spectroscopically. Structures of C₃H₅⁻, C₄H₇⁻ and C(CH₃)₃²⁻ are discussed and compared with those optimised by the MINDO/3 method.     

Caractéristiques spectroscopiques des liaisons hydrogène N-H-N très fortes. Études des acides A3(CN)6, (A=H,D; M=Fe,Co) par spectroscopie de vibration

Infrared and Raman spectra of the polycrystalline complex cyanide acids H₃M^III(CN)₆ (M=Fe,Co) and their deutero analogues were investigated at 300 and 90K in the region 4000-100 cm⁻¹. The spectra indicate clearly that the site symmetry of the M(CN)₆³⁻ ion is C_3v for M=Fe and D_3d for M=Co. These conclusions are consistent with an asymmetric N-H·N bond in H₃Fe(CN)₆ and with a symmetric one in H₃Co(CN)₆. The N-H stretching frequencies are assigned as ca. 1100 cm⁻¹ (Fe) and as 560 cm⁻¹ (Co), the shift being related to the difference in the hydrogen bonding strength, 2.665 Å (Fe) and 2.582 Å (Co). The spectroscopic behaviour of these very short N-H·N bonds appears to be similar to that of the strong O-H·O bonds in type A (for M=Co) or type pseudo-A compounds (for M=Fe).     

Vibrational frequencies of CO adsorbed on silica supported Mo atoms from density functional calculations

The interaction of CO with silica supported molybdenum atoms has been studied by means of density functional calculations and cluster models. Experimentally two bands in the IR spectra of adsorbed CO have been observed at 2170 and 1990 cm⁻¹ with vibrational shifts of +27 and −153 cm⁻¹, respectively, with respect to the gas-phase molecule, the peak at +27 cm⁻¹ has been related to the presence of neutral Mo atoms anchored to two oxygen atoms of the SiO₂ substrate. Possible reactive sites at the Mo/SiO₂ interface have been explored as candidates for CO adsorption. Mo atoms in various formal oxidation states, from +II to +VI, have been considered. Both molecular and cluster models of the Mo/SiO₂ interface have been employed. The analysis shows that a neutral Mo(II) atom, proposed to be responsible for the blue-shift of ν(CO), is not likely to be the origin of the IR band at 2170 cm⁻¹. Only Mo atoms in high oxidation states or Mo cations carrying a real positive charge can account for the positive shifts in the CO frequency.     

Raman and infrared spectra, conformational stability, ab initio calculations and vibrational assignments for 2-chloroethyl silane

The infrared (3500–30 cm⁻¹) spectra of gaseous and solid and the Raman (3500–10 cm⁻¹) spectra of liquid with quantitative depolarization ratios and solid 2-chloroethyl silane, ClCH₂CH₂SiH₃, have been recorded. Similar data have been recorded for the Si–d₃ isotopomer. These data indicate that two conformers, trans and gauche, are present in the fluid states but only one conformer, trans, is present in the solid. The mid-infrared spectra of the sample dissolved in liquified xenon as a function of temperature (−55 to −100°C) has been recorded. The enthalpy difference between the conformers has been determined to be 181±12 cm⁻¹ (2.17±0.14 kJ/mol) with the trans rotamer the more stable form. From the isolated Si–H frequencies from the Si–d₂ isotopomer the r_o Si–H distances of 1.484 and 1.483 Å for the trans and 1.481 for the gauche conformers have been obtained. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311+G** from which structural parameters and conformational stabilities have been determined. With all the basis sets the trans form is predicted to be the more stable conformer which is consistent with the experimental results. These results are compared to the corresponding quantities for the carbon analogue.     

Infrared studies of chromones—I Carbonyl and hydroxyl regions

Quantitative IR solution data in carbon tetrachloride and chloroform are recorded for the CO and OH regions of 31 chromones. In the 1580–1700 cm⁻¹ region, 5-hydroxychromones show three main maxima, the two of highest frequency, at 1663 ± 3 cm⁻¹ and 1630 ± 5 cm⁻¹ in CCl₄ (1661 ± 2 cm⁻¹ and 1627 ± 5 cm⁻¹ in CHCl₃), being sufficiently intense as to possess high CO character. Typically, 5-alkoxychromones exhibit two intense maxima in this region, 1663 ± 3 cm⁻¹ and 1613 ± 7 cm⁻¹ in CCl₄ (1657 ± 2 cm⁻¹ and 1608 ± 12 cm⁻¹ in CHCl₃). Diagnostically useful changes in contour and principal peak positions can be seen for substituted and annellated 5-hydroxychromones. In the 2500–3650 cm⁻¹ region, the stretching frequencies of OH groups at the most commonly encountered positions (C-5, C-7, and 2-CH₂OH) in natural chromones, are identified.