Guy Collin - Academia.edu (original) (raw)

Papers by Guy Collin

Journal de Chimie Physique, 1969

Canadian Journal of Chemistry, 1978

A systematic study of the pressure effects on the quantum yields of some products between 0.1 and... more A systematic study of the pressure effects on the quantum yields of some products between 0.1 and 600 Torr (13 and 80 000 N m−2) was carried out in the 7.6 and 8.4 eV photolysis of normal, iso- and cis-2-butenes. The propylene quantum yield (s-C4H9* → C3H6 + CH3) decreased with the increase in the n-butene pressure and a good linearity of S/D (stabilization/decomposition) vs. pressure plot, over a broad pressure region, was observed. It is concluded that hydrogen atoms involved in the s-C4H9* radical formation are produced with a relatively narrow energy distribution. The slope of S/D vs. pressure lines decreased with the increase in photon energy, indicating the trend in the kinetic energy of the H-atoms.In the case of isobutene and cis-2-butene photolysis, the Stern–Volmer plots for allene formation were nonlinear. It is concluded that the formation of two different allene precursors is needed to account for this result. By the use of a simple RRK-type formalism we also conclude t...

Canadian Journal of Chemistry, 1980

The vacuum uv photolysis of gaseous 2-butyne has been studied at 147, 163, and 174 nm. The use of... more The vacuum uv photolysis of gaseous 2-butyne has been studied at 147, 163, and 174 nm. The use of hydrogen iodide as radical scavenger shows the importance of methyl radical formation in the fragmentation of the photoexcited molecule. It is concluded that the principal fragmentation of the photoexcited molecule involves the cleavage of a C—CH3 bond: [Formula: see text] and [Formula: see text]. Unfortunately, it was not possible to scavenge the corresponding C3H3 radicals. From the fast decrease of the transparency of the window it is concluded that an important radical polymerization takes place, even in the presence of oxygen or nitric oxide. Finally, from the pressure effect, it is shown that propyne results from the fragmentation of the vibrationally excited butenyl radical and vinylacetylene arises from the fragmentation of an excited C4H5 radical.

Canadian Journal of Chemistry, 1980

We have studied the isomerization of butenes and methylbutenes by noble gas-sensitized radiolysis... more We have studied the isomerization of butenes and methylbutenes by noble gas-sensitized radiolysis. The isomerization of isobutene to but-2-ene is more efficient in the presence of xenon than in the presence of krypton or argon. The isomerization of but-1-ene into isobutene has a low radiolytic yield and occurs only in the presence of small quantities of dimethylamine. These observations are in agreement with the isomerization of the excited parent ion.In the methylbutene systems, whatever the sensitizing agent, isomerization occurs with a high radiolytic yield. Thus, a chain mechanism is needed to explain the results. The addition of dimethylamine has a reverse effect relative to that observed in but-1-ene. It is concluded that the mechanism invoked for the butene systems is not adequate for explaining the isomerization observed in the methylbutene systems.

Canadian Journal of Chemistry, 1974

The vacuum u.v. photolysis of trimethylethylene (2-methyl-2-butene) was carried out in a static s... more The vacuum u.v. photolysis of trimethylethylene (2-methyl-2-butene) was carried out in a static system using rare gas resonance lamps: xenon (147.0 nm) and krypton (123.6 nm). The main hydrocarbon products were isoprene, 1,3-butadiene, propyne, allène, ethylene, and other minor products. Identification and measurements of the yields of hydrogen atoms, methyl, and ethyl radicals were carried out quantitatively by the use of radical–radical reactions. Because of the high yield of isoprene, the effect of conversion was studied. At a high conversion (i.e. 0.1%) the isoprene quantum yield decreases. Hydrogen atoms add mainly to the secondary carbon of the monomer (≥90%). The Δ(CH3,tert-C5H11) value was calculated to be 1.32 ± 0.14. With the krypton line (10.0 eV) no evidence was found for the participation of ionic reactions in the formation of the measured products except for the formation of 2-methyl-1-butene in a low yield. At this wavelength the ion quantum yield is 0.224 ± 0.005.

Canadian Journal of Chemistry, 1991

We have studied the 184.9 and 147.0 nm photochemistry of gaseous cw-1,3-pentadiene: the main prod... more We have studied the 184.9 and 147.0 nm photochemistry of gaseous cw-1,3-pentadiene: the main products observed at 184.9 nm are trans-1,3-pentadiene and 1,3-cyclopentadiene. The formation of radicals also occurs as shown through the use of DI as a radical scavenger. Cyclopentadiene is the likely product of successive eliminations of two hydrogen atoms from the photoexcited molecule after rearrangement of the pentadienyl radical to the cyclopentenyl structure: Ф0(cyclo-C5H6) ≈ 0.25. Elimination of a methyl radical (Ф0 ≈ 0.50) also occurs, with formation of CH2=CHCH=ĊH* and, in a lower yield, CH2=C=CHĊH2*. However, it is not possible to identify properly the mechanism of the formation of the latter radical. It may be the result of the isomerization of either the first photoexcited molecule or the excited 1,3-butadienyl radicals. A small amount of 1,4-pentadiene formation is also observed (Ф0 ≈ 0.035 ± 0.015). At 147.0 nm, the same characteristics are observed, together with higher frag...

Canadian Journal of Chemistry, 1985

α,α,β-, α,β,γ-, and α,α,γ-trimethallyl radicals have been generated in the 147.0–nm gas phase pho... more α,α,β-, α,β,γ-, and α,α,γ-trimethallyl radicals have been generated in the 147.0–nm gas phase photolysis of 2,3,3-trimethyl-1-butene, 3,4-dimethyl-2-pentene, and 2,4-dimethyl-2-pentene, respectively. Under these conditions, the majority of allyl radicals have an internal energy sufficient for further decomposition: they give rise to the formation of various 1,3-dienes and small amounts of either 1,2- or 2,3-dienes. An internal sigmatropic 1,4-hydrogen atom transfer process is part of the proposed mechanism to explain such products. Moreover, the fragmentation of the trimethyl substituted allyl radicals involves the split of one β(C—C) bond, then one β(C—H), and, to a lesser extent, one central C—CH3 bond.

Canadian Journal of Chemistry, 1976

cis-2-Butene has been photolysed at 8.44, 8.44–9.56, and 10.03 eV in the presence of SF6 and of s... more cis-2-Butene has been photolysed at 8.44, 8.44–9.56, and 10.03 eV in the presence of SF6 and of several other additives. A major isomerization process is observed at 10.03 eV: Φ(trans-butene-2) [Formula: see text]. At 8.44 eV this yield is of the order of unity. A chain mechanism initiated by kinetically excited hydrogen atoms is proposed to explain the observed effects. Although we were unable either to identify the chain carrier with certainty or to eliminate the possibility of an ionic initiation at 10.03 eV, the chain carrier is a derivative of SF6, possibly the SF5 radical.

The aim of this work is to know if the hydrosols produced on a commercial basis are stable over a... more The aim of this work is to know if the hydrosols produced on a commercial basis are stable over at least 12 months at room temperature. The chemical composition of the oils and hydrosols collected from wild plantations were determined by GC-FID and GC/MS analyses. The components obtained from the hydrosols are mainly monoterpene alcohols, aldehydes and ketones and as such the composition of the hydrosols is different or very different from that of the corresponding oils: the oil with the most important fraction of the hydrocarbon compounds, the composition of the two phases is different. Benzyl alcohol, a compound not observed in the oil of S. puberula is by far the main compound (ca. 73%) observed in the corresponding hydrosol. Camphor is the main product observed in both oils (ca. 20%) and hydrosol (ca. 67%) of P. glauca. Menthol and menthone and their isomers (ca. 75%) are both observed in the oil and the hydrosol of M. piperita. These main compounds seem relatively stable in aci...

Canadian Journal of Chemistry, 1978

The gas phase photolysis of n-pentene was carried out in a static system using nitrogen resonance... more The gas phase photolysis of n-pentene was carried out in a static system using nitrogen resonance lines at [Formula: see text] and the bromine line at [Formula: see text] The mechanism for the photolysis was proposed and compared to what was concluded at 8.4 eV (147 nm, the xenon resonance line). The kinetics of the decomposition of the excited C3H5* radicals formed in the primary photochemical process and the C5H11* radicals formed by the addition of hydrogen atoms to the parent molecules were discussed. The investigations were extended to the n-C5D10 photolytic System. The observed decomposition rate constants of the excited pentyl radicals as well as the secondary non-equilibrium isotope effects agree with the data published earlier. It is concluded from these experiments that, at least at 7.6 eV, hot hydrogen atoms are produced.Only a small fraction of the C3H5* radicals décompose and yield aliène. At the same time the combined primary–secondary non-equilibrium isotope effects a...

Journal of Essential Oil Research, 1990

The essential oils of two plants (Hedeoma mandoniana and Minthostachys andina) of the mint family... more The essential oils of two plants (Hedeoma mandoniana and Minthostachys andina) of the mint family from the high valleys and mountains of Bolivia have been analyzed by a combination of GC/MS and retention indices. Both oils were found to contain pulegone as the major compound. The physical and chemical properties of the oils are similar to those of the well-known

Molecules

Over 15 years, with the support of a Canadian funding agency, the Universidad Mayor de San Simón,... more Over 15 years, with the support of a Canadian funding agency, the Universidad Mayor de San Simón, in Bolivia, undertook a large survey of aromatic plants of the South American country. More than a hundred species were studied under various aspects, including the production and characterization of essential oils. As part of this survey, the chemical composition of an essential oil sample obtained from Pentacalia herzogii (Asteraceae) growing wild in the High Valley region of the department of Cochabamba was determined by a combination of GC and GC-MS measurements. α-Pinene was the main constituent of this essential oil (34%), accompanied by limonene (22%) and germacrene D (7.5%) as well as an important fraction of methoxylated monoterpenoids. They were mainly isomers of thymol methyl ether, accounting for 13% of the chromatogram. A new quantitatively important compound (9%) was identified through NMR and chemical synthesis as 4-isopropyl-6-methylbenzo[d][1,3]dioxole, and designated h...

Journal of Photochemistry

The one-photon ionization of perylene (Pe) and tetracene (Tet) in anionic (NaLS) or neutral micel... more The one-photon ionization of perylene (Pe) and tetracene (Tet) in anionic (NaLS) or neutral micelles (Mi) is compared with the photoionization process in homogeneous solutions (tetramethylsilane or methanol). Two striking results are emphasized. (a) The shape of the photoionization efficiency curves in the threshold energy region is found to differ markedly for Pe or Tet in NaLS Mi on the one hand and in neutral Mi or homogeneous solutions on the other. It is suggested that such a difference illustrates the reduced recombination of geminate ion pairs in the former case. {b) The decrease dZ,i of the ionization threshold energies of Pe and Tet in Mi relative to their gas phase ionization potentials amounts to 2.3-2.35 eV for both solutes. This decrease AZ,,+, is interpreted in terms of the parameters which are currently considered to govern the optical ionization of impurity molecules in condensed media. In such a framework the electric field gradient at the Mi-water interface does not seem to contribute significantIy to the observed AZMi value.

Research on Chemical Intermediates

The degradation of the 184.9 nm photoezcited 1-methyl-l-cyclopentene molecule shows the involveme... more The degradation of the 184.9 nm photoezcited 1-methyl-l-cyclopentene molecule shows the involvement of various excited isomers. The most important fragmentation products are 1-and 2-methyl-l,8-cyclopentadiene. These products are probably the result of a one-step elimination process of a hydrogen molecule. This process has also been observed in the case of the 18(.9 nm photochemistry of cyelopentene. On the other hand, the involvement of isomers, although possible, is not so obvious at 147.0 nm. Moreover, in this case, the 1-and 2-methyl-l,8cyclopentadiene formation is the result of hydrogen atom elimination in a two-step process. Cyclopentadiene, ethylene, and various C3 and C4 compounds are formed as well as methyl radicals and hydrogen atorr~. These products are probably formed in successive elimination reactions as this is also observed in aeyelie alkenes.

The Journal of Physical Chemistry

ABSTRACT The infrared multiphoton (IRMP) decomposition of cis-2-pentene and 3-methyl-cis-2-penten... more ABSTRACT The infrared multiphoton (IRMP) decomposition of cis-2-pentene and 3-methyl-cis-2-pentene has been studied in order to compare the fragmentation pattern of the photoexcited molecules with that observed in the vacuum ultraviolet photolysis. The decompositions were studied at pressures from 0.2 to 5 Torr, using a pulsed COâ laser weakly focused to five fluences from about 5 to 20 J/cm² in the focal region. The IRMP decomposition of both molecules can be explained on the basis of a primary dissociation of a ..beta..C-C bond, followed by secondary fragmentation of the allylic moiety and the usual stabilizing processes and radical-radical reactions. The fragmentation patterns are generally similar to those observed in the vacuum UV photolysis, and the present results simply show the possibility that the ground state is involved in the vacuum UV photolysis; the behavior of the latter system itself lends support to this. Differences between the two systems can be explained by the higher initial energy of the molecules excited in the vacuum UVC. Some isomerization was also observed, mostly arising from radical-radical reactions. The formation of 1-pentene, however, indicates that the sigmatropic 1,3 hydrogen atom transfer can take place via the vibrationally excited ground state of cis-2-pentene. Conversely, 2-methyl-1-butene is not observed; its formation requires an internal methylene shift, and its occurrence in the vacuum UV photolysis suggests its direct formation from an electronically excited state.

Http Dx Doi Org 10 1080 10412905 2004 9698763, Nov 28, 2011

ABSTRACT The essential oil of the aerial parts of Baccharis tricuneata var. ruiziana (Asteraceae)... more ABSTRACT The essential oil of the aerial parts of Baccharis tricuneata var. ruiziana (Asteraceae) from Bolivia was analyzed by GC and GC/MS. The oil is characterized by a high percentage of (E)-nerolidol (∼68%) and the presence of several minor constituents composed mainly of monoterpenes (∼16%), followed by sesquiterpene hydrocarbons (∼5%) and oxygenated sesquiterpenes (∼3%).

Res Chem Intermediates, 1990

The 184.9 nm photochemistry of gaseous 3-methylcyclopentene and 3-methyl-I,4-pentadiene have been... more The 184.9 nm photochemistry of gaseous 3-methylcyclopentene and 3-methyl-I,4-pentadiene have been studied. Both photoexcited species decompose mainly through the primary rupture of the C-CH3 bond. Vibrationally excited 3-cyclopentenyl and pentamethylene radicals are formed in the primary decomposition in the former and latter systems respectively. These radicals are connected through isomerization reactions: in the presence of DI, the isomers cyclopentene, and trans-l,3pentadiene and//or vinylcyclopropane are formed in both systems. The quantum yields depend on the pressure and the starting monomer: cyelopentene and cyclopentadiene are the major products from the photolysis of 3-methylcyclopentene § DI mixtures and only minor quantities of the other C5H8 compounds are formed. Cyclopentadiene is the major product of the photolysis of 3-methyl-I,4-pentadiene § 02 mixtures whereas vinylcyclopropane and trans-I,3-pentadiene are the major C5 products of the photolysis of 3-methyl-l,4-pentadiene § DI mixtures.

Journal de Chimie Physique, 1969

Canadian Journal of Chemistry, 1978

A systematic study of the pressure effects on the quantum yields of some products between 0.1 and... more A systematic study of the pressure effects on the quantum yields of some products between 0.1 and 600 Torr (13 and 80 000 N m−2) was carried out in the 7.6 and 8.4 eV photolysis of normal, iso- and cis-2-butenes. The propylene quantum yield (s-C4H9* → C3H6 + CH3) decreased with the increase in the n-butene pressure and a good linearity of S/D (stabilization/decomposition) vs. pressure plot, over a broad pressure region, was observed. It is concluded that hydrogen atoms involved in the s-C4H9* radical formation are produced with a relatively narrow energy distribution. The slope of S/D vs. pressure lines decreased with the increase in photon energy, indicating the trend in the kinetic energy of the H-atoms.In the case of isobutene and cis-2-butene photolysis, the Stern–Volmer plots for allene formation were nonlinear. It is concluded that the formation of two different allene precursors is needed to account for this result. By the use of a simple RRK-type formalism we also conclude t...

Canadian Journal of Chemistry, 1980

The vacuum uv photolysis of gaseous 2-butyne has been studied at 147, 163, and 174 nm. The use of... more The vacuum uv photolysis of gaseous 2-butyne has been studied at 147, 163, and 174 nm. The use of hydrogen iodide as radical scavenger shows the importance of methyl radical formation in the fragmentation of the photoexcited molecule. It is concluded that the principal fragmentation of the photoexcited molecule involves the cleavage of a C—CH3 bond: [Formula: see text] and [Formula: see text]. Unfortunately, it was not possible to scavenge the corresponding C3H3 radicals. From the fast decrease of the transparency of the window it is concluded that an important radical polymerization takes place, even in the presence of oxygen or nitric oxide. Finally, from the pressure effect, it is shown that propyne results from the fragmentation of the vibrationally excited butenyl radical and vinylacetylene arises from the fragmentation of an excited C4H5 radical.

Canadian Journal of Chemistry, 1980

We have studied the isomerization of butenes and methylbutenes by noble gas-sensitized radiolysis... more We have studied the isomerization of butenes and methylbutenes by noble gas-sensitized radiolysis. The isomerization of isobutene to but-2-ene is more efficient in the presence of xenon than in the presence of krypton or argon. The isomerization of but-1-ene into isobutene has a low radiolytic yield and occurs only in the presence of small quantities of dimethylamine. These observations are in agreement with the isomerization of the excited parent ion.In the methylbutene systems, whatever the sensitizing agent, isomerization occurs with a high radiolytic yield. Thus, a chain mechanism is needed to explain the results. The addition of dimethylamine has a reverse effect relative to that observed in but-1-ene. It is concluded that the mechanism invoked for the butene systems is not adequate for explaining the isomerization observed in the methylbutene systems.

Canadian Journal of Chemistry, 1974

The vacuum u.v. photolysis of trimethylethylene (2-methyl-2-butene) was carried out in a static s... more The vacuum u.v. photolysis of trimethylethylene (2-methyl-2-butene) was carried out in a static system using rare gas resonance lamps: xenon (147.0 nm) and krypton (123.6 nm). The main hydrocarbon products were isoprene, 1,3-butadiene, propyne, allène, ethylene, and other minor products. Identification and measurements of the yields of hydrogen atoms, methyl, and ethyl radicals were carried out quantitatively by the use of radical–radical reactions. Because of the high yield of isoprene, the effect of conversion was studied. At a high conversion (i.e. 0.1%) the isoprene quantum yield decreases. Hydrogen atoms add mainly to the secondary carbon of the monomer (≥90%). The Δ(CH3,tert-C5H11) value was calculated to be 1.32 ± 0.14. With the krypton line (10.0 eV) no evidence was found for the participation of ionic reactions in the formation of the measured products except for the formation of 2-methyl-1-butene in a low yield. At this wavelength the ion quantum yield is 0.224 ± 0.005.

Canadian Journal of Chemistry, 1991

We have studied the 184.9 and 147.0 nm photochemistry of gaseous cw-1,3-pentadiene: the main prod... more We have studied the 184.9 and 147.0 nm photochemistry of gaseous cw-1,3-pentadiene: the main products observed at 184.9 nm are trans-1,3-pentadiene and 1,3-cyclopentadiene. The formation of radicals also occurs as shown through the use of DI as a radical scavenger. Cyclopentadiene is the likely product of successive eliminations of two hydrogen atoms from the photoexcited molecule after rearrangement of the pentadienyl radical to the cyclopentenyl structure: Ф0(cyclo-C5H6) ≈ 0.25. Elimination of a methyl radical (Ф0 ≈ 0.50) also occurs, with formation of CH2=CHCH=ĊH* and, in a lower yield, CH2=C=CHĊH2*. However, it is not possible to identify properly the mechanism of the formation of the latter radical. It may be the result of the isomerization of either the first photoexcited molecule or the excited 1,3-butadienyl radicals. A small amount of 1,4-pentadiene formation is also observed (Ф0 ≈ 0.035 ± 0.015). At 147.0 nm, the same characteristics are observed, together with higher frag...

Canadian Journal of Chemistry, 1985

α,α,β-, α,β,γ-, and α,α,γ-trimethallyl radicals have been generated in the 147.0–nm gas phase pho... more α,α,β-, α,β,γ-, and α,α,γ-trimethallyl radicals have been generated in the 147.0–nm gas phase photolysis of 2,3,3-trimethyl-1-butene, 3,4-dimethyl-2-pentene, and 2,4-dimethyl-2-pentene, respectively. Under these conditions, the majority of allyl radicals have an internal energy sufficient for further decomposition: they give rise to the formation of various 1,3-dienes and small amounts of either 1,2- or 2,3-dienes. An internal sigmatropic 1,4-hydrogen atom transfer process is part of the proposed mechanism to explain such products. Moreover, the fragmentation of the trimethyl substituted allyl radicals involves the split of one β(C—C) bond, then one β(C—H), and, to a lesser extent, one central C—CH3 bond.

Canadian Journal of Chemistry, 1976

cis-2-Butene has been photolysed at 8.44, 8.44–9.56, and 10.03 eV in the presence of SF6 and of s... more cis-2-Butene has been photolysed at 8.44, 8.44–9.56, and 10.03 eV in the presence of SF6 and of several other additives. A major isomerization process is observed at 10.03 eV: Φ(trans-butene-2) [Formula: see text]. At 8.44 eV this yield is of the order of unity. A chain mechanism initiated by kinetically excited hydrogen atoms is proposed to explain the observed effects. Although we were unable either to identify the chain carrier with certainty or to eliminate the possibility of an ionic initiation at 10.03 eV, the chain carrier is a derivative of SF6, possibly the SF5 radical.

The aim of this work is to know if the hydrosols produced on a commercial basis are stable over a... more The aim of this work is to know if the hydrosols produced on a commercial basis are stable over at least 12 months at room temperature. The chemical composition of the oils and hydrosols collected from wild plantations were determined by GC-FID and GC/MS analyses. The components obtained from the hydrosols are mainly monoterpene alcohols, aldehydes and ketones and as such the composition of the hydrosols is different or very different from that of the corresponding oils: the oil with the most important fraction of the hydrocarbon compounds, the composition of the two phases is different. Benzyl alcohol, a compound not observed in the oil of S. puberula is by far the main compound (ca. 73%) observed in the corresponding hydrosol. Camphor is the main product observed in both oils (ca. 20%) and hydrosol (ca. 67%) of P. glauca. Menthol and menthone and their isomers (ca. 75%) are both observed in the oil and the hydrosol of M. piperita. These main compounds seem relatively stable in aci...

Canadian Journal of Chemistry, 1978

The gas phase photolysis of n-pentene was carried out in a static system using nitrogen resonance... more The gas phase photolysis of n-pentene was carried out in a static system using nitrogen resonance lines at [Formula: see text] and the bromine line at [Formula: see text] The mechanism for the photolysis was proposed and compared to what was concluded at 8.4 eV (147 nm, the xenon resonance line). The kinetics of the decomposition of the excited C3H5* radicals formed in the primary photochemical process and the C5H11* radicals formed by the addition of hydrogen atoms to the parent molecules were discussed. The investigations were extended to the n-C5D10 photolytic System. The observed decomposition rate constants of the excited pentyl radicals as well as the secondary non-equilibrium isotope effects agree with the data published earlier. It is concluded from these experiments that, at least at 7.6 eV, hot hydrogen atoms are produced.Only a small fraction of the C3H5* radicals décompose and yield aliène. At the same time the combined primary–secondary non-equilibrium isotope effects a...

Journal of Essential Oil Research, 1990

The essential oils of two plants (Hedeoma mandoniana and Minthostachys andina) of the mint family... more The essential oils of two plants (Hedeoma mandoniana and Minthostachys andina) of the mint family from the high valleys and mountains of Bolivia have been analyzed by a combination of GC/MS and retention indices. Both oils were found to contain pulegone as the major compound. The physical and chemical properties of the oils are similar to those of the well-known

Molecules

Over 15 years, with the support of a Canadian funding agency, the Universidad Mayor de San Simón,... more Over 15 years, with the support of a Canadian funding agency, the Universidad Mayor de San Simón, in Bolivia, undertook a large survey of aromatic plants of the South American country. More than a hundred species were studied under various aspects, including the production and characterization of essential oils. As part of this survey, the chemical composition of an essential oil sample obtained from Pentacalia herzogii (Asteraceae) growing wild in the High Valley region of the department of Cochabamba was determined by a combination of GC and GC-MS measurements. α-Pinene was the main constituent of this essential oil (34%), accompanied by limonene (22%) and germacrene D (7.5%) as well as an important fraction of methoxylated monoterpenoids. They were mainly isomers of thymol methyl ether, accounting for 13% of the chromatogram. A new quantitatively important compound (9%) was identified through NMR and chemical synthesis as 4-isopropyl-6-methylbenzo[d][1,3]dioxole, and designated h...

Journal of Photochemistry

The one-photon ionization of perylene (Pe) and tetracene (Tet) in anionic (NaLS) or neutral micel... more The one-photon ionization of perylene (Pe) and tetracene (Tet) in anionic (NaLS) or neutral micelles (Mi) is compared with the photoionization process in homogeneous solutions (tetramethylsilane or methanol). Two striking results are emphasized. (a) The shape of the photoionization efficiency curves in the threshold energy region is found to differ markedly for Pe or Tet in NaLS Mi on the one hand and in neutral Mi or homogeneous solutions on the other. It is suggested that such a difference illustrates the reduced recombination of geminate ion pairs in the former case. {b) The decrease dZ,i of the ionization threshold energies of Pe and Tet in Mi relative to their gas phase ionization potentials amounts to 2.3-2.35 eV for both solutes. This decrease AZ,,+, is interpreted in terms of the parameters which are currently considered to govern the optical ionization of impurity molecules in condensed media. In such a framework the electric field gradient at the Mi-water interface does not seem to contribute significantIy to the observed AZMi value.

Research on Chemical Intermediates

The degradation of the 184.9 nm photoezcited 1-methyl-l-cyclopentene molecule shows the involveme... more The degradation of the 184.9 nm photoezcited 1-methyl-l-cyclopentene molecule shows the involvement of various excited isomers. The most important fragmentation products are 1-and 2-methyl-l,8-cyclopentadiene. These products are probably the result of a one-step elimination process of a hydrogen molecule. This process has also been observed in the case of the 18(.9 nm photochemistry of cyelopentene. On the other hand, the involvement of isomers, although possible, is not so obvious at 147.0 nm. Moreover, in this case, the 1-and 2-methyl-l,8cyclopentadiene formation is the result of hydrogen atom elimination in a two-step process. Cyclopentadiene, ethylene, and various C3 and C4 compounds are formed as well as methyl radicals and hydrogen atorr~. These products are probably formed in successive elimination reactions as this is also observed in aeyelie alkenes.

The Journal of Physical Chemistry

ABSTRACT The infrared multiphoton (IRMP) decomposition of cis-2-pentene and 3-methyl-cis-2-penten... more ABSTRACT The infrared multiphoton (IRMP) decomposition of cis-2-pentene and 3-methyl-cis-2-pentene has been studied in order to compare the fragmentation pattern of the photoexcited molecules with that observed in the vacuum ultraviolet photolysis. The decompositions were studied at pressures from 0.2 to 5 Torr, using a pulsed COâ laser weakly focused to five fluences from about 5 to 20 J/cm² in the focal region. The IRMP decomposition of both molecules can be explained on the basis of a primary dissociation of a ..beta..C-C bond, followed by secondary fragmentation of the allylic moiety and the usual stabilizing processes and radical-radical reactions. The fragmentation patterns are generally similar to those observed in the vacuum UV photolysis, and the present results simply show the possibility that the ground state is involved in the vacuum UV photolysis; the behavior of the latter system itself lends support to this. Differences between the two systems can be explained by the higher initial energy of the molecules excited in the vacuum UVC. Some isomerization was also observed, mostly arising from radical-radical reactions. The formation of 1-pentene, however, indicates that the sigmatropic 1,3 hydrogen atom transfer can take place via the vibrationally excited ground state of cis-2-pentene. Conversely, 2-methyl-1-butene is not observed; its formation requires an internal methylene shift, and its occurrence in the vacuum UV photolysis suggests its direct formation from an electronically excited state.

Http Dx Doi Org 10 1080 10412905 2004 9698763, Nov 28, 2011

ABSTRACT The essential oil of the aerial parts of Baccharis tricuneata var. ruiziana (Asteraceae)... more ABSTRACT The essential oil of the aerial parts of Baccharis tricuneata var. ruiziana (Asteraceae) from Bolivia was analyzed by GC and GC/MS. The oil is characterized by a high percentage of (E)-nerolidol (∼68%) and the presence of several minor constituents composed mainly of monoterpenes (∼16%), followed by sesquiterpene hydrocarbons (∼5%) and oxygenated sesquiterpenes (∼3%).

Res Chem Intermediates, 1990

The 184.9 nm photochemistry of gaseous 3-methylcyclopentene and 3-methyl-I,4-pentadiene have been... more The 184.9 nm photochemistry of gaseous 3-methylcyclopentene and 3-methyl-I,4-pentadiene have been studied. Both photoexcited species decompose mainly through the primary rupture of the C-CH3 bond. Vibrationally excited 3-cyclopentenyl and pentamethylene radicals are formed in the primary decomposition in the former and latter systems respectively. These radicals are connected through isomerization reactions: in the presence of DI, the isomers cyclopentene, and trans-l,3pentadiene and//or vinylcyclopropane are formed in both systems. The quantum yields depend on the pressure and the starting monomer: cyelopentene and cyclopentadiene are the major products from the photolysis of 3-methylcyclopentene § DI mixtures and only minor quantities of the other C5H8 compounds are formed. Cyclopentadiene is the major product of the photolysis of 3-methyl-I,4-pentadiene § 02 mixtures whereas vinylcyclopropane and trans-I,3-pentadiene are the major C5 products of the photolysis of 3-methyl-l,4-pentadiene § DI mixtures.