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Photoinduced reversible intramolecular hydrogen-atom migration between 9H-fluorene and 1H-fluorene isolated in an Ar matrix is found by FTIR spectroscopy with an aid of DFT calculation. The forward isomerization from 9H-fluorene to 1H-fluorene occurs upon UV irradiation (λ ≥ 295 nm), while the backward isomerization occurs upon λ ≥ 320 nm light irradiation. The less stable isomer, 1H-fluorene, is identified by comparison of the measured IR spectrum with the corresponding simulated spectral pattern obtained at the B3LYP/6-31++G(d,p) level. In addition, an IR spectrum of 9H-fluorene in the T1 state is measured during UV irradiation (λ ≥ 275 nm).

Photoreaction of molecular complex composed of ozone and dimethylamine is investigated by matrix-isolation IR and UV–visible spectroscopies. The UV–visible spectrum of the molecular complex isolated in an Ar matrix shows two broad bands in the region between 400 and 700 nm. Upon red-light irradiation (λ ≥ 680 nm), dimethylamine-N-oxide is produced by intermolecular oxygen-atom transfer from O3 to the lone pair on the nitrogen atom of dimethylamine. Dimethylamine-N-oxide isomerizes to N,N-dimethylhydroxylamine by intramolecular hydrogen-atom migration upon the second light irradiation (λ ≥ 385 nm). The both photoproducts are identified by the DFT calculation at the B3LYP/6-31++G(d,p) level.

Temperature dependence of the time evolution of chemiluminescence intensity from γ -irradiated polytetrafluoroethylene was examined by heating isothermally in the range of 150 and 200 °C. Kinetic analysis was carried out to estimate the rate constants, from which the dissociation energy of the O O bond in the fluoroperoxide group was determined to be 97 ± 4 kJ mol −1 , being consistent with the corresponding value for small fluorocarbon model systems obtained by quantum chemical calculations. This strongly supports the emission mechanism [ CF(OOF) CF 2 → CO CF 2 + OF 2 + hν ] proposed in our previous paper to explain chemiluminescence from the γ -irradiated polytetrafluoroethylene.

Thermal chemiluminescence spectra of polytetrafluoroethylene powder irradiated by γ rays in air at room temperature were measured with a multichannel Fourier-transform chemiluminescence spectrometer. The luminescence appeared immediately after heating the irradiated samples at 160, 180 and 200 °C in dry air and in N 2 and then disappeared within a few minutes, whereas virgin samples showed no luminescence. The lifetime of luminescence decreased as the heating temperature increased, but the total amount of luminescence at each temperature was nearly constant. From this observation an emission mechanism was derived with the aid of ESR and IR spectroscopy.

Photoreaction pathways of salicylic acid have been investigated by the low-temperature matrix-isolation infrared spectroscopy with an aid of the density-functional-theory calculations. Only the most stable E form existed in an argon matrix. The second stable R form and the fourth stable H form were produced by UV irradiation ( λ > 290 nm). The third stable O form was assumed to return immediately to E form even if it was produced. Ketoketene–water complex was yielded from R form by dissociation of the H and OH parts interacting through intramolecular hydrogen bonding upon prolonged UV irradiation.

Infrared (IR) spectra of the enol and keto forms of 4-amino-6-oxopyrimidine (AOP) isolated in a low-temperature Xe matrix were recorded, where the change from the keto to the enol form was found to be induced by UV irradiation ( λ > 270 nm). On the other hand, the hydrated crystal of AOP exhibited a similar IR spectrum to the anhydrous crystal by dehydration, suggesting that the dehydrated and anhydrous crystals are polymorphs. It has been found from the IR spectral analyses that the AOP crystal is dominated by infinite double H-bonded ribbons, which has been supported by quantum chemical calculations.

Chemiluminescence emission spectra of polypropylene pellets pre-irradiated in air with γ rays were measured using a time-dependent multichannel Fourier-transform chemiluminescence spectrometer. Spectra were detected immediately after heating the irradiated polymer at 150 °C, which was not the case for the virgin material, and deconvoluted into emission bands by least-squares fitting using Gaussian curves. The peak intensity of the initial emission band around 490 nm rapidly increased and then decreased with a constant peak wavelength during the thermal oxidation time between 0 and 10 min. We have elucidated the emission mechanism from the dependence of γ -irradiation dose on the peak intensity.

Delayed fluorescence spectra of intact leaves of Green pak choi (Brassica rapa var. chinensis) were measured with a multichannel Fourier-transform chemiluminescence spectrometer, which we developed recently. The intact samples, photoexcited by sunlight without artificial light sources, showed delayed fluorescence around 740 nm with a lifetime of ∼6 s. The observed spectra were deconvoluted into two Gaussian bands: the delayed fluorescence from photosystem II and photosystem I complexes. Their relative intensities depended on the chlorophyll concentration, but their wavelengths were unchanged.

The thermal chemiluminescence spectra of the dicyclohexylamine (DCHA) salts of glycine, alanine, serine, and threonine were measured with a multichannel Fourier-transform chemiluminescence (FT-CL) spectrometer. The zwitterionic amino acids themselves are non-luminescent because their amine groups are protonated, whereas the corresponding DCHA salts show strong emission in the presence of oxygen because their NH 2 groups are free. It was found that the CH 3 and OH groups on the side chain caused red and blue shifts, respectively, suggesting that thermal luminescence spectroscopy using an FT-CL spectrometer can be used to identify the presence of electron donating or withdrawing groups in chemiluminescent molecules.

Infrared spectra of ethanol in low-temperature argon and xenon matrices were measured with an FT-IR spectrometer. The relative intensities of the IR bands for anti and gauche conformers changed according to the Boltzmann distribution law, when the matrix temperature was varied in the ranges between 15 and 30 K in argon and between 15 and 55 K in xenon. The energy difference between the two conformers was determined from the spectral changes to be 29.4 ± 2.1 cm −1 in argon and 20.0 ± 1.2 cm −1 in xenon, where anti is more stable than gauche. The results are discussed in terms of the interaction between an ethanol molecule and noble-gas atoms.

A thermal luminescence spectrum of solid lysine in O 2 with an emission peak at ∼600 nm was measured at 450 K with a Fourier-transform chemiluminescence spectrometer. No emission bands were observed for lysine in N 2 , nor its monohydrochloride in O 2 or N 2 , under the same experimental conditions. This observation led us to the conclusion that the intermolecular interaction between the free NH 2 group on the side chain of lysine and O 2 molecules plays an essential role in this thermal luminescence. This scheme was given support by the finding that the dicyclohexylamine salt of phenylalanine, which has a free NH 2 group, but not phenylalanine itself, showed similar thermal luminescence in O 2 .

Photoreaction of 7-hydroxyquinoline monomer in a low-temperature argon matrix was investigated, and electronic absorption spectra of three unstable species generated by UV-light irradiation were measured. By comparison with the calculated spectral patterns obtained by the time-dependent density-functional-theory calculation, the electronic absorption bands around 460, 360 and 260 nm appearing after UV irradiation were assigned, respectively, to the keto form produced from the enol form of 7-hydroxyquinoline by migration of the hydrogen atom of the OH group to the N atom, quinolinoxyl radical by elimination of a hydrogen atom, and 7-ketene by Wolff rearrangement following elimination of a hydrogen molecule.

Fourier-transform chemiluminescence spectra of H 2 O 2 /gallic acid/K 3 [Fe(CN) 6 ] systems containing organic solvents were measured. Emission bands with peaks around 530 and 700 nm were observed in systems containing solvents with a carbonyl group such as N , N -dimethylformamide, and those with a hydroxyl group such as methanol, respectively. The relative band intensities depended strongly on the concentration of these organic solvents. The emission species are attributed to gallic acid–ferricyanide complexes excited by energy transfer from singlet oxygen dimol, ( 1 O 2 ) 2 . The effects of organic solvents are interpreted in terms of intermolecular interactions of gallic acid–ferricyanide complexes, water molecules and organic solvents.

A Fourier-transform spectrometer equipped with a Savart-plate polarization interferometer was developed for observation of weak chemiluminescence and applied to a measurement of emission spectra in the decomposition of hydrogen peroxide with gallic acid and K3[Fe(CN)6]. The band appearing at ∼580 nm in the chemiluminescence spectrum was assigned to the emission of singlet-oxygen dimol, the peak wavelength being shifted from that observed in the reaction of hydrogen peroxide with sodium hypochlorite, ∼633 nm. The band intensity was increased with the increasing concentration of K3[Fe(CN)6] up to ∼100 mM, and thereafter the peak wavelength was shifted from 580 to 700 nm with a decrease in the intensity.

Photoreaction of 2-nitrophenol has been investigated by low-temperature matrix-isolation IR spectroscopy and density-functional-theory (DFT) calculation. A reaction intermediate was identified as syn - trans 2-hydroxyphenyl nitrite, and the reaction product caused by dissociation of NO from the intermediate as 2-hydroxyphenoxyl radical. Both syn - trans and syn - cis 2-hydroxyphenyl nitrite isomers were produced by recombination of 2-hydroxyphenoxyl radical with NO in annealing at 27 K after the UV irradiation. The conformational change from cis to trans around the O–N axis occurred upon visible-light irradiation. The rate constants of photoreaction were determined by a least-squares fitting of the changes in the IR band intensities.

Matrix-isolation infrared spectroscopy was used to identify a reaction intermediate of 2-nitrophenol with an aid of density-functional-theory (DFT) calculations. The observed FTIR bands of a product from 2-nitrophenol upon irradiation of 248 nm were assigned to an aci- nitro isomer among 2 × 2 possible conformations, cis / trans around the C N + (O − )OH and CN + (O − ) OH bond axes, by comparison with calculated spectral patterns. A reverse reaction from aci- nitro isomer to 2-nitrophenol was observed upon irradiation with λ > 450 nm. The mechanism of this reversible photoreaction is discussed in terms of the vertical transition wavelengths and the oscillator strengths obtained by time-dependent DFT calculation.

Photoisomerization mechanism of 4-amino-3-penten-2-one, a chelated keto-enamine isomer, has been studied by low-temperature matrix-isolation infrared (IR) spectroscopy. Conformations of the reactant and two photoproducts were determined by comparison of the observed IR spectra with the calculated spectral patterns obtained by density-functional-theory (DFT) calculations. It is concluded that two isomers are produced upon UV irradiation: a non-chelated keto-enamine isomer by internal rotation around the CH C bond with breaking the intramolecular hydrogen bond, and an imino-enol isomer by hydrogen-atom migration from the amino group to the carbonyl group.

Isomerization induced by ultraviolet radiation due to intramolecular hydrogen-atom migration in 3,6-diimino-1,4-cyclohexadiene-1,4-diamine (DCD) was studied by matrix-isolation Fourier transform infrared (IR) spectroscopy and density functional theory (DFT) calculation. Two isomers produced by single and double inversion of the hydrogen atom(s) around the C N bond(s) were identified using the wavelength dependence of the IR band-intensity changes. In addition, the IR bands of an intermediate produced by single hydrogen-atom transfer from the amino to the imino group ( C – NH 2 ⋯ NH C ) in the hydrogen bond were measured and identified by a comparison with the calculated spectral patterns. These observations confirm stepwise photoisomerization of DCD.

1,2-Diaminobenzene in an argon matrix was exposed to UV light coming from a superhigh-pressure mercury lamp without optical filters, resulting in production of three isomers around the C N bonds of 3,5-cyclohexadiene-1,2-diimine by elimination of hydrogen atoms. When the matrix sample was subsequently irradiated by a UV photon of lower energy ( λ > 290 nm), an infrared spectrum of a new species produced from trans , trans -3,5-cyclohexadiene-1,2-diimine was observed, in addition to the spectral change due to photoisomerization from cis – trans to cis – cis . The new species was identified as 7,8-diazabicyclo[4.2.0]octa-1,3,5-triene by a comparison with the predicted spectral pattern obtained by density-functional-theory calculation.

Infrared spectrum of 2-hydroxyphenyl, C 6 H 4 -OH, produced from 2-iodophenol in a low-temperature argon matrix upon UV irradiation ( λ >280 nm) was measured with an FT-IR spectrophotometer. This radical was found to be less stable by 120 kJ mol −1 than phenoxyl, C 6 H 5 -O , by a density functional theory calculation. Two final photoproducts were identified as cyclopentadienylidenemethanone and 4-iodo-2,5-cyclohexadienone in analogy with the photoproducts of 2-chloro and 2-bromophenols. A kinetic analysis shows that the latter is produced via 2-hydroxyphenyl by hydrogen migration and iodine recombination while the former is produced by Wolff rearrangement after elimination of hydrogen iodide.

Photo-excited transient species of 2,3-dicyanonaphthalene produced during UV irradiation was studied by low-temperature matrix-isolation Fourier transform infrared (IR) spectroscopy. To identify the transient species, the density-functional-theory method was used for optimization of the geometrical structures and for estimation of the IR spectral patterns of the T 1 and T 2 states. By a comparison of the observed IR spectrum with the predicted spectral patterns, the observed photo-excited transient species was identified as the T 1 state of 2,3-dicyanonaphthalene.

Reversible photoisomerization of 2,3,6,7-tetracyano-1,4,5,8-tetraazanaphthalene (TTN) in low-temperature rare-gas matrices was investigated by electronic absorption and Fourier transform infrared (FTIR) spectroscopies aided bydensity-functional-theory (DFT) calculations. The reactant, TTN, was found to change to 4,5,9,10-tetracyano-1,3,6,8-tetraazacyclodeca-1,2,4,6,7,9-hexaene (TTH) upon ultraviolet-light irradiation, while the product, TTH, returns to the original reactant, TTN, upon visible-light irradiation. The time-dependent DFT calculation led to the conclusion that the forward and backward photoisomerizations occur by way of the S3–S0 transition of TTN and the S1–S0 transition of TTH, respectively.

The chelated enol-type isomer was found to isomerize to a non-chelated enol isomer by UV irradiation between 260 and 300 nm, while backward isomerization was observed by that between 320 and 340 nm although no electronic absorption band was detectable in this region. Wavelength dependence of the isomerization rate constants, measured by monitoring the infrared spectral changes, shows that the isomerization by UV radiation is caused by the ππ* transition for the chelated enol located around 280 nm and by the ππ* and nπ* transitions for the non-chelated enol around 260 and 340 nm, respectively.

Photoisomerization of hexafluoroacetylacetone was investigated by a low-temperature matrix-isolation technique with an aid of the density functional theory (DFT) calculation. A less stable isomer produced by UV irradiation ( λ >260 nm) from the most stable enol was identified by its IR and UV–visible absorption spectra. This isomer was found to be non-chelated enol with cis , trans , and trans conformations around the CO–CH, CHC, and C–OH bonds, respectively, by a comparison with the spectral patterns of seven possible isomers with non-chelated enols derived from DFT simulations.

Infrared spectra of transient species produced from 1,2- and 1,4-dicyanobenzenes in low-temperature argon matrices during UV irradiation were measured by Fourier transform infrared (FTIR) spectroscopy. The observed bands were assigned to the lowest excited triplet, T1, states of the dicyanobenzenes by comparison with the spectral patterns obtained by hybrid density-functional-theory calculations at the B3LYP/6-31++G** level. The CN stretching bands in the S0 state shift to the low-wavenumber side by 172 and 251 cm −1 in the T1 state for 1,2- and 1,4-dicyanobenzenes, respectively. These findings suggest that the quinoide-type structure similar to that in o- and p-benzoquinones contributes to the T1 states.

Infrared spectrum of a spiropyran, 1 ′ ,3 ′ -dihydro-1 ′ ,3 ′ ,3 ′ -trimethyl-6-nitrospiro[2H-1-benzopyran-2,2 ′ -[2H]indole], in a low-temperature argon matrix was measured with a Fourier transformed infrared spectrophotometer. The conformation of this spiropyran was determined by comparison of the observed spectrum with a spectral pattern obtained by the density-functional-theory calculation. This conformation differs from the optimized geometry derived from an HF/6-31G* calculation. The conformations of a merocyanine produced from the spiropyran upon UV irradiation were also determined by the same method.

Photoreaction mechanisms of 2-bromo, 2-bromo-4-chloro and 2,4-dibromophenols in low-temperature argon matrices have been investigated by Fourier transform infrared spectroscopy. Density-functional-theory calculations are performed to assign the observed infrared bands of photoproducts, where a 6-311+G(d,p) basis set is used to optimize geometrical structures. It is concluded that 4-bromo-2,5-cyclohexadienone is mainly produced from 2-bromophenol, while formation of cyclopentadienylidenemethanone⋯HBr complex is a minor pathway, in contrast to the case of 2-chlorophenols. The branching ratio between the two reaction pathways is estimated from the absorbance changes of the infrared bands to be 5.4±0.3.

Torsional isomerization between cis and trans hydroquinone in low-temperature matrices has been studied by FTIR spectroscopy with an aid of DFT calculations. When the temperature of Xe matrix was changed between 16 and 75 K, the cis/trans population ratio changed reversibly in conformance with the Boltzmann distribution law, despite the high torsional barrier, 10.8 kJ mol −1 . The cis–trans enthalpy difference was estimated from the population changes to be 0.19±0.06 kJ mol −1 . The cis–trans isomerization equilibrium at low temperature is accounted for in terms of the matrix-induced tunneling of hydrogen atoms.

CH 3 I in a supersonic jet was isolated in a low-temperature argon matrix. The observed infrared bands were assigned to monomer and dimers by examining dependence of the intensities on the mixing ratio with argon gas and the stagnation pressure of the supersonic jet. Ab initio calculations at the MP2 level predicted that head-to-tail (HT) and head-to-head (HH) structures of CH 3 I dimers were stable. The dimerization energy of HT obtained by a single-point-energy calculation at the MP4(SDTQ) level was 185 cm −1 larger than that of HH. A comparison of the observed and calculated wavenumbers evidences that HT and HH co-exist in the jet.

Photoisomerization of acetyl methyl iminoxy radical produced from NO2 and 2-butyne in low-temperature argon matrices has been studied by density functional theory (DFT) calculation. The infrared bands observed upon 585 nm irradiation are assigned, using the spectral patterns predicted by DFT, to the trans–trans, cis–trans, and cis–cis conformers, where the former and latter conformations refer to the CO–CN and CN bond axes, respectively. Conformational isomerization around the CN bond is found to occur upon 515 nm irradiation. The isomerization mechanism is discussed with the aid of the DFT calculation.

Dewar 4-picoline produced by UV irradiation on 4-picoline in a low-temperature argon matrix was monitored by FT-IR spectroscopy. The observed infrared bands were assigned to an isomer which has a C–C bridging bond with the aid of DFT (density functional theory) calculation, where the 6-31++G ** basis set was used to optimize the geometrical structure. No other isomer bands were observed. A systematic comparison of the scaling factors derived from the observed and calculated wavenumbers for pyridine, 2-picoline, 4-picoline and their corresponding Dewar isomers indicates a linear relationship between the scaling factors and the vibrational wavenumbers.

Infrared spectra of Dewar 2-picoline, produced from 2-picoline in an argon matrix by UV irradiation, were measured. The observed bands were assigned to a single isomer, which has a C–C bridging bond and an NC–CH 3 group, with the aid of DFT calculations, where the 6-31++G ∗∗ basis set was used to optimize the geometrical structure. No other isomer bands were observed. The selectivity of the isomerization to Dewar 2-picoline is discussed in terms of the atomic charge distribution in electronically excited states of 2-picoline.

Matrix isolation infrared spectroscopy has been applied to a study of conformational cooling in a supersonic jet of 1,2-dichloroethane. The conformational temperature is estimated from the intensity ratio of the infrared absorption bands for the gauche and trans conformers to be 210±10 K under a stagnation pressure of 1.0 atm. Dependence of the conformational temperature on the stagnation pressure has been examined below 4 atm, and an empirical equation is proposed. These results differ from a previous report of Felder and Gunthard who reported no conformational cooling in a supersonic jet of this molecule.

The visible light-induced reaction of NO 2 with methylamine (CH 3 NH 2 ) in a cryogenic argon matrix has been investigated using an FTIR spectrometer. The photochemical products were identified as methyleneimine (CH 2 = NH), water and nitric oxide. This photoinduced dehydrogenation reaction, CH 3 NH 2 + NO 2 + hν → CH 2 = NH + H 2 O + NO, is interpreted by the following reaction sequence: (1) photoexcitation of NO 2 , (2) oxygen atom transfer from excited NO 2 to methylamine and (3) isomerization from the methylamine N-oxide to methylhydroxylamine (intramolecular hydrogen atom migration), followed by (4) dissociation into methyleneimine and water. it was confirmed by the infrared spectrum that two of the dissociation products interact with each other in the matrix cage (CH 2 = NH · H 2 O). This reaction mechanism is also supported by the formation of CD 2 = NH · HDO in the CD 3 NH 2 /NO 2 system.

Microwave spectra of the inner-outer and outer-outer conformers of 1,2-dimethylhydrazine were assigned. Then rotational constants and the electron diffraction intensity were used for a joint analysis, from which their CNNC dihedral angles, 88.4 ± 0.7° and 164.2 ± 14°, and abundance ratio at room temperature, 62 ± 8% : 38 ± 8%, respectively, were determined.