Browsing by Author "Lee, Yu-Fang"

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Alcohol dimers - how much diagonal OH anharmonicity?
(2014)
Kollipost, Franz
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Papendorf, Kim
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Lee, Yu-Fang
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Lee, Yuan-Pern
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Suhm, Martin A.
The OH bond of methanol, ethanol and t-butyl alcohol becomes more anharmonic upon hydrogen bonding and the infrared intensity ratio between the overtone and the fundamental transition of the bridging OH stretching mode decreases drastically. FTIR spectroscopy of supersonic slit jet expansions allows to quantify these effects for isolated alcohol dimers, enabling a direct comparison to anharmonic vibrational predictions. The diagonal anharmonicity increase amounts to 15-18%, growing with increasing alkyl substitution. The overtone/fundamental IR intensity ratio, which is on the order of 0.1 or more for isolated alcohols, drops to 0.004-0.001 in the hydrogen-bonded OH group, making overtone detection very challenging. Again, alkyl substitution enhances the intensity suppression. Vibrational second order perturbation theory appears to capture these effects in a semiquantitative way. Harmonic quantum chemistry predictions for the hydrogen bond-induced OH stretching frequency shift (the widely used infrared signature of hydrogen bonding) are insufficient, and diagonal anharmonicity corrections from experiment make the agreement between theory and experiment worse. Inclusion of anharmonic cross terms between hydrogen bond modes and the OH stretching mode is thus essential, as is a high level electronic structure theory. The isolated molecule results are compared to matrix isolation data, complementing earlier studies in N-2 and Ar by the more weakly interacting Ne and p-H-2 matrices. Matrix effects on the hydrogen bond donor vibration are quantified.
Detailed mechanism of the CH2I + O-2 reaction: Yield and self-reaction of the simplest Criegee intermediate CH2OO
(2014-09-14)
Ting, Wei-Lun
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Chang, Chun-Hung
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Lee, Yu-Fang
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Matsui, Hiroyuki
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Lee, Yuan-Pern
The application of a new reaction scheme using CH2I + O-2 to generate the simplest Criegee intermediate, CH2OO, has stimulated lively research; the Criegee intermediates are extremely important in atmospheric chemistry. The detailed mechanism of CH2I + O-2 is hence important in understanding kinetics involving CH2OO. We employed ultraviolet absorption to probe simultaneously CH2I2, CH2OO, CH2I, and IO in the reaction system of CH2I + O-2 upon photolysis at 248 nm of a flowing mixture of CH2I2, O-2, and N-2 (or SF6) in the pressure range 7.6-779 Torr to investigate the reaction kinetics. With a detailed mechanism to model the observed temporal profiles of CH2I, CH2OO, and IO, we found that various channels of the reaction CH2I + O-2 and CH2OO + I play important roles; an additional decomposition channel of CH2I + O-2 to form products other than CH2OO or ICH2OO becomes important at pressure less than 60 Torr. The pressure dependence of the derived rate coefficients of various channels of reactions of CH2I + O-2 and CH2OO + I has been determined. We derived a rate coefficient also for the self-reaction of CH2OO as k = (8 +/- 4) x 10(-11) cm(3) molecule(-1) s(-1) at 295 K. The yield of CH2OO from CH2I + O-2 was found to have a pressure dependence on N-2 and O-2 smaller than in previous reports; for air under 1 atm, the yield of similar to 30% is about twice of previous estimates. (C) 2014 AIP Publishing LLC.
Infrared absorption of CH(3)SO(2) observed upon irradiation of a p-H(2) matrix containing CH(3)I and SO(2)
(2011-03-28)
Lee, Yu-Fang
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Lee, Yuan-Pern
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應用化學系
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應用化學系分子科學碩博班
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Department of Applied Chemistry
Irradiation with a mercury lamp at 254 nm of a p-H(2) matrix containing CH(3)I and SO(2) at 3.3 K, followed by annealing of the matrix, produced prominent features at 633.8, 917.5, 1071.1 (1072.2), 1272.5 (1273.0, 1273.6), and 1416.0 cm(-1), attributable to nu(11) (C-S stretching), nu(10) (CH(3) wagging), nu(8) (SO(2) symmetric stretching), nu(7) (SO(2) antisymmetric stretching), and nu(4) (CH(2) scissoring) modes of methylsulfonyl radical (CH(3)SO(2)), respectively; lines listed in parentheses are weaker lines likely associated with species in a different matrix environment. Further irradiation at 365 nm diminishes these features and produced SO(2) and CH(3). Additional features at 1150.1 and 1353.1 (1352.7) cm(-1) are tentatively assigned to the SO(2) symmetric and antisymmetric stretching modes of ISO(2). These assignments are based on comparison of observed vibrational wavenumbers and (18)O- and (34)S-isotopic shifts with those predicted with the B3P86 method. Our results agree with the previous report of transient IR absorption bands of gaseous CH(3)SO(2) at 1280 and 1076 cm(-1). These results demonstrate that the cage effect of solid p-H(2) is diminished so that CH(3) radicals, produced via UV photodissociation of CH(3)I in situ, might react with SO(2) to form CH(3)SO(2) during irradiation and upon annealing. Observation of CH(3)SO(2) but not CH(3)OSO is consistent with the theoretical predictions that only the former reactions proceed via a barrierless path. (C) 2011 American Institute of Physics. [doi:10.1063/1.3567117]
Infrared absorption of CH3O and CD3O radicals isolated in solid para-H-2
(2015-04-01)
Lee, Yu-Fang
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Chou, Wei-Te
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Johnson, Britta A.
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Tabor, Daniel P.
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Sibert, Edwin L., III
Irradiation of a p-H-2 matrix containing methyl nitrite (CH3ONO) at 3.2 K with laser light at 355 nm yielded vibrational fundamental absorption lines at 2773.1/2804.5 (1(+) mixed with background states), 2866.5/2887.5?/2967.07 (1(-) mixed with 4(+) and 2(2)(-)), 1365.7 (2(+)), 1411.3 (2(-)), 1041.8 (3), 2901.3?/ 2917.4/2982.2 (4(-) mixed with background states), 2940.5 (4(+)), 1346.8 (5(+)), 1427.2 (5(-)), 1519.9 (5(+)), 1523.0 (5(-)), 689.3/694.4 (6(+)), 945.9/951.7 (6(-)), 1224.6/1228.9 (6(+)), and 1233.7/1239.1 (6(-)) cm(-1) that are assigned to the methoxy (CH3O) radical; the numbers in parentheses indicate vibrational normal modes, the superscript + and - indicate the spin-orbit components E-2(3/2) and E-2(1/2), respectively, and the "?" marks indicate tentative assignments. Experiments with CD3ONO yielded lines of CD3O at 2012.4? (1(-)), 1005.9 (3(+)), 1037.3 (3(-)), 2168.8 (4(+)), 2174.9 (4(+)), 2202.8 (4(-)), 2212.3 (4(-)), 974.3 (5(+)), 1185.2 (5(+)), 1224.8 (5(-)), 529.6/531.3 (6(+)), 728.3/731.3 (6(-)), 912.4 (6(+)), and 923.2 (6(-)) cm(-1). Many additional lines associated with combination or overtone modes were also observed. The assignments are based on the photolytic behavior and comparisons of observed vibrational wavenumbers and IR intensities with those predicted with a fitted CCSD(T)/cc-pVTZ force field; deviations between experiments and fitted theoretical results are within 2% with an average value (0.4 +/- 0.4)%. These newly observed spectra show vibrational wavenumbers of CH3O consistent with most vibrational energies derived from stimulated-emission-pumping experiments on gaseous CH3O and those of CD3O are consistent with some lines derived from laser-induced fluorescence. Intensities of these lines of CH3O diminished with a rate coefficient (7.6 +/- 0.3) x 10(-3) s(-1) after irradiation, likely due to the tunneling reaction of CH3O with p-H-2 to form CH2OH. (C) 2014 Elsevier Inc. All rights reserved.
Infrared absorption of CH3OSO and CD3OSO radicals produced upon photolysis of CH3OS(O)Cl and CD3OS(O)Cl in p-H-2 matrices
(2012-03-28)
Lee, Yu-Fang
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Kong, Lin-Jun
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Lee, Yuan-Pern
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應用化學系
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應用化學系分子科學碩博班
Irradiation at 239 +/- 20 nm of a p-H-2 matrix containing methoxysulfinyl chloride, CH3OS(O)Cl, at 3.2 K with filtered light from a medium-pressure mercury lamp produced infrared (IR) absorption lines at 3028.4 (attributable to nu(1), CH2 antisymmetric stretching), 2999.5 (nu(2), CH3 antisymmetric stretching), 2950.4 (nu(3), CH3 symmetric stretching), 1465.2 (nu(4), CH2 scissoring), 1452.0 (nu(5), CH3 deformation), 1417.8 (nu(6), CH3 umbrella), 1165.2 (nu(7), CH3 wagging), 1152.1 (nu(8), S=O stretching mixed with CH3 rocking), 1147.8 (nu(9), S=O stretching mixed with CH3 wagging), 989.7 (nu(10), C-O stretching), and 714.5 cm(-1) (nu(11), S=O stretching) modes of syn-CH3OSO. When CD3OS(O)Cl in a p-H-2 matrix was used, lines at 2275.9 (nu(1)), 2251.9 (nu(2)), 2083.3 (nu(3)), 1070.3 (nu(4)), 1056.0 (nu(5)), 1085.5 (nu(6)), 1159.7 (nu(7)), 920.1 (nu(8)), 889.0 (nu(9)), 976.9 (nu(10)), and 688.9 (nu(11)) cm(-1) appeared and are assigned to syn-CD3OSO; the mode numbers correspond to those used for syn-CH3OSO. The assignments are based on the photolytic behavior and a comparison of observed vibrational wavenumbers, infrared intensities, and deuterium isotopic shifts with those predicted with the B3P86/aug-cc-pVTZ method. Our results extend the previously reported four transient IR absorption bands of gaseous syn-CH3OSO near 2991, 2956, 1152, and 994 cm(-1) to 11 lines, including those associated with C-O, O-S, and S=O stretching modes. Vibrational wavenumbers of syn-CD3OSO are new. These results demonstrate the advantage of a diminished cage effect of solid p-H-2 such that the Cl atom, produced via UV photodissociation of CH3OS(O)Cl in situ, might escape from the original cage to yield isolated CH3OSO radicals. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3696894]
Infrared absorption of CH3SO2 observed upon irradiation of a p-H-2 matrix containing CH3I and SO2
(2011-03-28)
Lee, Yu-Fang
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Lee, Yuan-Pern
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應用化學系
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應用化學系分子科學碩博班
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Department of Applied Chemistry
Irradiation with a mercury lamp at 254 nm of a p-H-2 matrix containing CH3I and SO2 at 3.3 K, followed by annealing of the matrix, produced prominent features at 633.8, 917.5, 1071.1 (1072.2), 1272.5 (1273.0, 1273.6), and 1416.0 cm(-1), attributable to nu(11) (C-S stretching), nu(10) (CH3 wagging), nu(8) (SO2 symmetric stretching), nu(7) (SO2 antisymmetric stretching), and nu(4) (CH2 scissoring) modes of methylsulfonyl radical (CH3SO2), respectively; lines listed in parentheses are weaker lines likely associated with species in a different matrix environment. Further irradiation at 365 nm diminishes these features and produced SO2 and CH3. Additional features at 1150.1 and 1353.1 (1352.7) cm(-1) are tentatively assigned to the SO2 symmetric and antisymmetric stretching modes of ISO2. These assignments are based on comparison of observed vibrational wavenumbers and O-18- and S-34-isotopic shifts with those predicted with the B3P86 method. Our results agree with the previous report of transient IR absorption bands of gaseous CH3SO2 at 1280 and 1076 cm(-1). These results demonstrate that the cage effect of solid p-H-2 is diminished so that CH3 radicals, produced via UV photodissociation of CH3I in situ, might react with SO2 to form CH3SO2 during irradiation and upon annealing. Observation of CH3SO2 but not CH3OSO is consistent with the theoretical predictions that only the former reactions proceed via a barrierless path. (C) 2011 American Institute of Physics. [doi:10.1063/1.3567117]
Infrared absorption of iodomethylperoxy (syn-ICH2OO) radical generated upon photolysis of CH2I2 and O-2 in solid para-H-2
(2015-08-18)
Lee, Yu-Fang
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Lee, Yuan-Pern
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應用化學系
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應用化學系分子科學碩博班
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Department of Applied Chemistry
Irradiation of a para-hydrogen (p-H-2) matrix containing diiodomethane (CH2I2) and O-2 at 3.2 K with light at 280 +/- 20 nm, followed by annealing of the matrix at 4.0 K, yielded infrared (IR) absorption lines at 2982.4, 1408.9, 1231.8, 1226.5/1225.6, 1085.6, 917.7, 841.6/841.1, 550.5, and 490.2 cm(-1) that are assigned to the syn-iodomethylperoxy (syn-ICH2OO) radical. Further irradiation of the matrix at 365 nm diminished these features. Experiments with CH2I2 and O-18(2) yielded lines of syn-(ICH2OO)-O-18-O-18 at 1407.3, 1228.1/1227.7, 1217.7/1217.0, 1031.5, 899.9/899.4, 836.7/836.0, and 473.6 cm(-1). The assignments are based on the photolytic behaviour and comparisons of observed vibrational wavenumbers, IR intensities, and O-18-isotopic shifts with those predicted with the B3LYP/aug-cc-pVTZ-pp method. The observation is consistent with a mechanism that, upon photolysis of CH2I2 at 280 nm, CH2I was formed and subsequently reacted with O-2 to yield syn-ICH2OO. Compared with the gaseous reaction CH2I + O-2 -> CH2OO + I at low pressure, observation of ICH2OO in a p-H-2 matrix instead of CH2OO in the gaseous phase indicates that the excess energy of internally excited ICH2OO, produced upon reaction of CH2I + O-2, was rapidly quenched in the matrix so that ICH2OO became readily stabilised without further decomposition to form CH2OO + I.
Infrared absorption of methanol-water clusters (CH3OH)(n)(H2O), n=1-4, recorded with the VUV-ionization/IR-depletion technique
(2017-04-14)
Lee, Yu-Fang
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Kelterer, Anne-Marie
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Matisz, Gergely
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Kunsagi-Mate, Sandor
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Chung, Chao-Yu
We recorded infrared (IR) spectra in the CH- and OH-stretching regions of size-selected clusters of methanol (M) with one water molecule (W), represented as MnW, n = 1-4, in a pulsed supersonic jet using the photoionization/IR-depletion technique. Vacuum ultraviolet emission at 118 nm served as the source of ionization in a time-of-flight mass spectrometer to detect clusters M-n W as protonated forms Mn-1 WH+. The variations in intensities of Mn-1 WH+ were monitored as the wavelength of the IR laser light was tuned across the range 2700-3800 cm(-1). IR spectra of size-selected clusters were obtained on processing of the observed action spectra of the related cluster-ions according to a mechanism that takes into account the production and loss of each cluster due to IR photodissociation. Spectra of methanol-water clusters in the OH region show significant variations as the number of methanol molecules increases, whereas those in the CH region are similar for all clusters. Scaled harmonic vibrational wavenumbers and relative IR intensities predicted with the M06-2X/aug-cc-pVTZ method for the methanol-water clusters are consistent with our experimental results. For dimers, absorption bands of a structure WM with H2O as a hydrogen-bond donor were observed at 3570, 3682, and 3722 cm(-1), whereas weak bands of MW with methanol as a hydrogen-bond donor were observed at 3611 and 3753 cm(-1). For M2W, the free OH band of H2O was observed at 3721 cm(-1), whereas a broad feature was deconvoluted to three bands near 3425, 3472, and 3536 cm(-1), corresponding to the three hydrogen-bonded OH-stretching modes in a cyclic structure. For M3W, the free OH shifted to 3715 cm(-1), and the hydrogen-bonded OH-stretching bands became much broader, with a weak feature near 3179 cm(-1) corresponding to the symmetric OH-stretching mode of a cyclic structure. For M4W, the observed spectrum agrees unsatisfactorily with predictions for the most stable cyclic structure, indicating significant contributions from branched isomers, which is distinctly different from M-5 of which the cyclic form dominates. Published by AIP Publishing.
Infrared spectra of free radicals and protonated species produced in para-hydrogen matrices
(2014)
Bahou, Mohammed
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Das, Prasanta
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Lee, Yu-Fang
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Wu, Yu-Jong
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Lee, Yuan-Pern
The quantum solid para-hydrogen (p-H-2) has emerged as a new host for matrix isolation experiments. Among several unique characteristics, the diminished cage effect enables the possibility of producing free radicals via either photolysis in situ or bimolecular reactions of molecules with atoms or free radicals that are produced in situ from their precursors upon photo-irradiation. Many free radicals that are unlikely to be produced in noble-gas matrices can be produced readily in solid p-H-2. In addition, protonated species can be produced upon electron bombardment of p-H-2 containing a small proportion of the precursor during deposition. The application of this novel technique to generate protonated polycyclic aromatic hydrocarbons (PAH) and their neutral counterparts demonstrates its superiority over other methods. The technique of using p-H-2 as a matrix host has opened up many possibilities for the preparation of free radicals and unstable species and their spectral characterization. Many new areas of applications and fundamental understanding concerning the p-H-2 matrix await further exploration.