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  • 1
    Electronic Resource
    Electronic Resource
    Kinetics of intramolecular carbon atom exchange in ketene (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 4081-4093 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Intramolecular carbon atom exchange in highly vibrationally excited ketene was studied by monitoring the carbon monoxide fragments (12CO and 13CO ) from the photodissociation of 12CH213CO and 13CH212CO. Two experimental techniques were employed. In one set of experiments the IR transient absorptions of 12CO and 13CO were measured following pulsed excimer excitation of ketene 13C isotopomers, giving carbon atom exchange yields at 351 and 308 nm in a low pressure gas cell. In the other set of experiments, jet-cooled ketene 13C isotopomers were excited with tunable near-UV radiation, and the CO products were detected by monitoring their VUV laser-induced fluorescence. Carbon atom exchange yields were measured for energies extending from below the triplet decomposition threshold (CH2CO→CH2(X 3B1)+CO(X 1Σ+)) to about 4000 cm−1 above the singlet threshold (CH2CO→CH2(a 1A1)+CO(X 1Σ+)).The exchange yields range from 4 to 19%, and the energy dependence of the yield exhibits pronounced structure, with maxima at the triplet and singlet decomposition thresholds. Kinetic measurements of the appearance of the CO products were also performed. The time constant for the appearance of the exchanged CO (e.g., 13CO from 13CH212CO ) is significantly longer than that for the direct CO fragment (e.g., 12CO from 13CH212CO ). All the experimental observations are consistent with a simple reaction mechanism involving ketene isomerization, 13CH212CO(arrow-right-and-left)12CH213CO, and dissociation, 13CH212CO→13CH2+12CO and 12CH213CO→12CH2+13CO. The isomerization rate constant was determined by analyzing the CO kinetics and the carbon atom exchange yields in terms of the simple isomerization mechanism. A fit of the energy dependence of the isomerization rate constant to the results of tunneling-corrected Rice–Ramsberger–Kassel–Marcus (RRKM) calculations gave the threshold (28360±60 cm−1 ) for the isomerization process.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460764
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  • 2
    Electronic Resource
    Electronic Resource
    Determination of the singlet/triplet branching ratio in the photodissociation of ketene (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1954-1960 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rotational distributions of CO products from the dissociation of ketene at photolysis energies 10 cm−1 below, 56, 110, 200, 325, 425, 1107, 1435, 1720, and 2500 cm−1 above the singlet threshold (30 116.2 cm−1 ), are measured in a supersonic free jet of ketene. The CO(v‘=0) rotational distributions at 56, 110, 200, 325, and 425 cm−1 are bimodal. The peaks at low J's, which are due to CO from the singlet channel, show that the product rotational distribution of CO product from ketene dissociation on the singlet surface is well described by phase space theory (PST). For CO(v‘=0) rotational distributions at higher excess energies (1107, 1435, 1720, and 2500 cm−1 ), the singlet and triplet contributions are not clearly resolved, and the singlet/triplet branching ratios are estimated by assuming that PST accurately predicts the CO rotational distribution from the singlet channel and that the distribution from the triplet channel changes little from that at 10 cm−1 below the singlet threshold. The singlet yield shows a rapid increase in the low excess energy region (0–300 cm−1 ), and a slower increase above. The singlet and triplet rate constants are derived from the directly measured total rate constants using the singlet yields. The triplet rate constant increases monotonically with increasing photolysis energy through the singlet threshold region. The singlet rate constant is accurately established in the threshold region and found to increase much less rapidly than predicted by phase space theory. At 2500 cm−1 excess energy, the CO(v‘=1) rotational distribution is obtained, and the ratio of CO(v‘=1) to CO(v‘=0) products for the singlet channel is measured to be 0.045±0.017. This ratio is close to the variational Rice–Ramsberger–Kassel–Marcus (RRKM) calculation 0.038, and the separate statistical ensembles (SSE) prediction 0.041, but much greater than the PST prediction, 0.016.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459917
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  • 3
    Electronic Resource
    Electronic Resource
    Resonant-enhanced two photon ionization and mass-analyzed threshold ionization spectroscopy of jet-cooled 2-aminopyridines (2AP–NH2,–NHD,–NDH,–ND2) (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 2131-2140 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Resonantly-enhanced two photon ionization and mass-analyzed threshold ionization (MATI) spectra of 2-aminopyridine (2AP–NH2) and its deuterated analogs have been obtained using two-photon (1+1′) excitation process via S1 intermediate states for ionization. Ionization energies of 2AP–NH2 and 2AP–ND2 are both precisely and accurately determined to be 8.1086±0.0005 and 8.1027±0.0005 eV, respectively. Two geometrical isomers, 2AP–NHD or 2AP–NDH, of which H or D is hydrogen-bonded to the nitrogen atom on the pyridine ring, respectively, are spectroscopically well isolated using the hole-burning spectroscopy in the S1 states. Corresponding ionization energies are thus separately determined to be 8.1067±0.0005 or 8.1048±0.0005 eV for 2AP–NHD or 2AP–NDH, respectively. Vibrational bands of 2-aminopyridine ions associated with various aromatic ring-skeletal modes are identified in the MATI spectra and appropriately assigned with the aid of ab initio calculation. All of the ring-skeletal vibrational frequencies observed in this work become slightly higher than those in the S1 states when the molecules are ionized, consistent with the fact that the S1–S0 excitation is due to π*–π transition. According to ab initio calculation, the amino group is in the molecular plane both in the S1 and D0 states, while it is slightly distorted in the ground state of 2-aminopyridine. Inversion modes in 2AP–ND2 and 2AP–NDH in S1 states are split into two bands due to their strong coupling with the other mode, which is most probably due to torsional motion of the amino group. Strong mode couplings are clearly manifested in interferencelike patterns observed in vibrational band structures of MATI spectra taken via those two bands in S1 states as intermediate states. A new spectroscopic scheme, in which MATI signals are used for obtaining mode-resolved spectra for the intermediate state is introduced. The vibrational band at 911 cm−1 from the S1–S0 origin that has been previously assigned as the inversion mode of 2AP–NH2 is found to actually consist of two closely-spaced different modes giving two clearly-resolved different Franck–Condon active modes in corresponding MATI spectra. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1488922
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  • 4
    Electronic Resource
    Electronic Resource
    One-photon mass-analyzed threshold ionization spectroscopy of 1- and 2-iodopropanes in vacuum ultraviolet (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 5568-5576 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: One-photon mass-analyzed threshold ionization (MATI) spectroscopy of 1- and 2-iodopropanes has been studied using coherent vacuum ultraviolet (VUV) radiation generated by four-wave mixing in Kr gas. Accurate ionization energies to the lower and upper spin–orbit states of the molecular ions have been determined. These are 9.1755±0.0005 and 9.6903±0.0017 eV for the lower and upper spin–orbit states, respectively, of 2-iodopropane ion. For 1-iodopropane ion, gauche and trans peaks were resolved in the MATI spectra. Ionization energies to the lower spin–orbit states are 9.2567±0.0005 and 9.2718±0.0005, respectively, for the gauche and trans conformers. The corresponding values are 9.8332±0.0017 and 9.8466±0.0017 for the upper spin–orbit states. The pure ion beam of the gauche-only or that of trans-only could be selectively generated by tuning the VUV wavelength. Dissociation of 1- and 2-iodopropane ions, C3H7I+→C3H7++I, occurring in the ion core of highly excited Rydberg neutrals has been observed. Fragmentation thresholds for these reactions have been determined. This has led to an accurate potential energy diagram for the dissociation of the C3H7I+ system in the threshold region. The heat of formation at 0 K of 2-C3H7+ has been determined, 821.7±3.8 kJ mol−1, together with the proton affinity at 0 K of C3H6, 741.6±3.9 kJ mol−1. Plausible mechanisms for the production of 2-C3H7++I from 1-C3H7I+ have been proposed. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1353548
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  • 5
    Electronic Resource
    Electronic Resource
    Vacuum-ultraviolet mass-analyzed threshold ionization spectra of iodobutane isomers: Conformer-specific ionization and ion-core dissociation followed by ionization (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 2492-2498 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Mass-analyzed threshold ionization (MATI) spectra using coherent vacuum ultraviolet radiation have been obtained for t-butyl iodide, iso-butyl iodide, 2-iodobutane, and 1-iodobutane. The ionization energy to the lower spin–orbit state of t-butyl iodide ion has been determined to be 8.9984±0.0006 eV, while the threshold for fragmentation to t-C4H9+ and I has been estimated to be 9.1762±0.0047 eV. Heat of formation of t-butyl cation, ΔfH0(t-C4H9+), has been re-estimated, 733.7±3.3 kJ mol−1. Peaks due to two different conformers of iso-butyl iodide ion, PH and PC, are clearly resolved in the MATI spectra, enabling the measurement of ionization energies to the lower spin–orbit states of respective conformers, 9.1725±0.0006 and 9.1972±0.0006 eV. Corresponding values for the upper spin–orbit states have been determined from the MATI spectra for the C4H9+ fragments generated by dissociation in the ion core of neutral as 9.7394±0.0024 and 9.7649±0.0023 eV. Only two out of three possible 2-iodobutane conformers have been observed in the MATI spectra with ionization energies to their lower spin–orbit states of 9.0883±0.0006 and 9.0913±0.0006 eV, even though conformer identification was not possible. Similarly, it is likely that four distinct peaks observed in the ionization threshold region of the MATI spectra of 1-iodobutane are the origins for the different conformers. Plausible mechanisms for the ion-core fragmentation of iodobutanes are discussed. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1386786
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  • 6
    Electronic Resource
    Electronic Resource
    Direct femtosecond observation of the transient intermediate in the α-cleavage reaction of (CH3)2CO to 2CH3+CO: Resolving the issue of concertedness (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 477-480 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: When a reaction involving two equivalent bonds has sufficient energy to break both of them, it can proceed by either a concerted or a stepwise mechanism. For Norrish type-I and other reactions, this issue has been controversial since direct time resolution of the individual C–C cleavage events was not possible. Here, for the elementary α-cleavage of acetone, we report on the femtosecond resolution of the intermediates using mass spectrometry. The results show the nonconcertedness of the reaction, provide the times for the primary and secondary C–C breakage, and indicate the role of electronic structure (σ*, antibonding impulse) and the vibrational motions involved. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469614
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  • 7
    Electronic Resource
    Electronic Resource
    Transition state vibrational level thresholds for the dissociation of triplet ketene (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 3202-3219 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rate constants for the unimolecular dissociation of ketene (CH2CO) and deuterated ketene (CD2CO) have been measured at the threshold for the production of CH2 (X˜ 3B1) or CD2 (X˜ 3B1) and CO (X˜ 1Σ+) by photofragmentation in a cold jet. The rate constant increases in a stepwise manner as energy increases. This is in accord with the long-standing premise that the rate of a unimolecular reaction is controlled by flux through quantized transition-state thresholds at each energy level for vibrational motion orthogonal to the reaction coordinate. The first step in rate constant and/or photofragment excitation (PHOFEX) spectrum gives accurate values for the barrier to dissociation above the zero-point energy of the products, 1281±15 cm−1 for CH2CO and 1071±40 cm−1 for CD2CO. The measured rate constants are fit by Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The vibrational frequencies at the transition state obtained from the fits are compared with ab initio results. Vibrational motions at the transition state orthogonal to the reaction coordinate are also revealed in CO product rotational distributions. Calculations using an impulsive model which includes vibrational motions at the transition state reproduce the experimental dependence of the PHOFEX spectra on the CO J state quite well. The small dependence of rate constant on jet temperature (4–30 K) indicates that the Ka quantum number for rotation about its symmetry axis is conserved in the energized ketene molecule. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468631
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  • 8
    Electronic Resource
    Electronic Resource
    Unexpectedly large O37ClO/O35ClO intensity ratios of the fluorescence from the low-energy vibrational levels of OClO (A˜ 2A2) (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 456-459 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The fluorescence intensity ratios of O37ClO/O35ClO excited to the A˜ 2A2 state have been found to be much larger than that expected from the 37Cl/35Cl natural abundance ratio for several low-energy vibrational levels. For instance, the O37ClO/O35ClO ratios are 1.67 and 〉10 for the (0,0,0) and (1,1,0) bands, respectively, which are 5 and 30 times larger than those expected from the natural abundance ratio. These unusual isotope effects are observed in a narrow energy region within ∼1000 cm−1 above the electronic origin. It is likely that the isotopomer-selective, near-resonant spin-orbit coupling of the low-energy vibrational levels of the 2A2 state with those of the 2A1 state, which provides a decay channel for the 2A2 state via spin-orbit coupling, is responsible for the unusual isotope effect. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479325
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  • 9
    Electronic Resource
    Electronic Resource
    Photodissociation dynamics of acetylacetone: The OH product state distribution (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 11850-11855 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Acetylacetone in the supersonic jet, which exists predominantly as an enolic form, is found to give rise to the OH fragment after the π–π* transition induced by the UV absorption. The nascent OH product state distributions are determined using a laser-induced fluorescence technique at the excitation wavelengths of 291 and 266 nm. The OH fragment is vibrationally cold, and its rotational state distribution is peaked at N=3 or 4 at the pump wavelength of 291 or 266 nm, respectively. No fluorescence from the excited acetylacetone has been observed even in the energy region near the origin, suggesting the ultrafast nonradiative processes of the excited state. From the measured OH product state distributions, the upper bound for the dissociation energy of the acetylacetone is estimated to be 90.3 kcal/mol. The ratios of Λ-doublets and spin–orbit states of the OH fragment are also measured. A slight preference of the OH fragment in the 2Π3/2 state over the 2Π1/2 state is observed. The Π−/Π+ ratios, determined by the relative intensity ratios of Q and P (or R) lines, are found to be less than unity, suggesting the preferential cleavage of the C–OH bond on the molecular plane probably due to a relatively strong intramolecular hydrogen bonding of the enolic acetylacetone. The prior calculation reproduces the experiment quite well for the OH rotational state distribution at 291 nm, while it does not for that at 266 nm. This suggests that the transition state in the acetylacetone dissociation, at the low energy near threshold, may be completely loosely defined on the potential energy surface which does not have a barrier to recombination, and it becomes tightened as the energy increases above the reaction threshold. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479126
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  • 10
    Electronic Resource
    Electronic Resource
    Fluorescence excitation spectroscopic study of the jet-cooled acetyl cyanide (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 7185-7191 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Fluorescence excitation spectrum of acetyl cyanide (CH3COCN) in a supersonic jet has been obtained for the 1(n,π*) transition (S1–S0). A spectral origin band is located at 27 511 cm−1, and its fluorescence lifetime is measured to be 3.5±0.3 μs. The S1 state vibrational modes associated with CH3 torsion, CCO bend, CCN bend, and CO wag are found to be optically active. The fluorescence intensity decreases with increasing the energy from the origin up to ∼2000 cm−1, while the fluorescence lifetime changes little over the same energy region. Instead, a broad background signal appears in the high energy region, indicating that the intramolecular vibrational redistribution (IVR) process becomes important in the S1 state as the density of states increases. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478622
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