Browsing by Author "Camacho, Cristopher"

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Choosing a proper complete active space in calculations for transition metal dimers: ground state of Mn(2) revisited
(2008)
Camacho, Cristopher
;
Yamamoto, Shigeyoshi
;
Witek, Henryk A.
;
應用化學系分子科學碩博班
;
Institute of Molecular science
The potential energy curve of the ground state of Mn(2) has been studied using a systematic sequence of complete active spaces. Deficiencies of the routinely used active space, built from atomic 4s and 3d orbitals, has been identified and discussed. It is shown that an additional sigma(g) orbital, originating from the atomic virtual 4p(z) orbitals, is essential for a proper description of static correlation in the 1 Sigma(+)(g) state of Mn(2). The calculated spectroscopic parameters of the 1 Sigma(+)(g) state agree well with available experimental data. The calculated equilibrium bond lengths are located between 3.24 and 3.50 angstrom, the harmonic vibrational frequencies, between 44 and 72 cm(-1), and the dissociation energies, between 0.05 and 0.09 eV. An urgent need for an accurate gas-phase experimental study of spectroscopic constants of Mn(2) is highlighted.
FORMATION AND IDENTIFICATION OF INTERSTELLAR MOLECULE LINEAR C(5)H FROM PHOTOLYSIS OF METHANE DISPERSED IN SOLID NEON
(2009-08-10)
Wu, Yu-Jong
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Chen, Hui-Fen
;
Camacho, Cristopher
;
Witek, Henryk A.
;
Hsu, Sheng-Chuan
Photolysis of methane dispersed (1/1000) in solid Ne at 3 K with vacuum-ultraviolet light from a synchrotron produced infrared absorption lines of several products, including new lines at 3319.3 and 1955.5 cm(-1). Based on experiments with isotopic labeling and results of quantum-chemical calculations, these lines are assigned to the C-H stretching and C=C stretching modes, respectively, of interstellar molecule linear C(5)H radicals.
FORMATION AND IDENTIFICATION OF INTERSTELLAR MOLECULE LINEAR C5H FROM PHOTOLYSIS OF METHANE DISPERSED IN SOLID NEON
(2009-08-10)
Wu, Yu-Jong
;
Chen, Hui-Fen
;
Camacho, Cristopher
;
Witek, Henryk A.
;
Hsu, Sheng-Chuan
Photolysis of methane dispersed (1/1000) in solid Ne at 3 K with vacuum-ultraviolet light from a synchrotron produced infrared absorption lines of several products, including new lines at 3319.3 and 1955.5 cm(-1). Based on experiments with isotopic labeling and results of quantum-chemical calculations, these lines are assigned to the C-H stretching and C=C stretching modes, respectively, of interstellar molecule linear C5H radicals.
Infrared absorption of methanol clusters (CH(3)OH)(n) with n=2-6 recorded with a time-of-flight mass spectrometer using infrared depletion and vacuum-ultraviolet ionization
(2011-04-14)
Han, Hui-Ling
;
Camacho, Cristopher
;
Witek, Henryk A.
;
Lee, Yuan-Pern
;
應用化學系
We investigated IR spectra in the CH-and OH-stretching regions of size-selected methanol clusters, (CH(3)OH)(n) with n = 2-6, in a pulsed supersonic jet by using the IR-VUV (vacuum-ultraviolet) ionization technique. VUV emission at 118 nm served as the source of ionization in a time-of-flight mass spectrometer. The tunable IR laser emission served as a source of predissociation or excitation before ionization. The variations of intensity of protonated methanol cluster ions (CH(3)OH)(n)H(+) and CH(3)OH(+) and (CH(3)OH)(2)(+) were monitored as the IR laser light was tuned across the range 2650-3750 cm(-1). Careful processing of these action spectra based on photoionization efficiencies and the production and loss of each cluster due to photodissociation yielded IR spectra of the size-selected clusters. Spectra of methanol clusters in the OH region have been extensively investigated; our results are consistent with previous reports, except that the band near 3675 cm(-1) is identified as being associated with the proton acceptor of (CH(3)OH)(2). Spectra in the CH region are new. In the region 2800-3050 cm(-1), bands near 2845, 2956, and 3007 cm(-1) for CH(3)OH split into 2823, 2849, 2934, 2955, 2984, and 3006 cm(-1) for (CH(3)OH)(2) that correspond to proton donor and proton acceptor, indicating that the methanol dimer has a preferred open-chain structure. In contrast, for (CH(3)OH)(3), the splitting diminishes and the bands near 2837, 2954, and 2987 cm(-1) become narrower, indicating a preferred cyclic structure. Anharmonic vibrational wavenumbers predicted for the methanol open-chain dimer and the cyclic trimer with the B3LYP/VPT2/ANO1 level of theory are consistent with experimental results. For the tetramer and pentamer, the spectral pattern similar to that of the trimer but with greater widths was observed, indicating that the most stable structures are also cyclic. (C) 2011 American Institute of Physics. [doi:10.1063/1.3572225]
Infrared absorption of methanol clusters (CH3OH)(n) with n=2-6 recorded with a time-of-flight mass spectrometer using infrared depletion and vacuum-ultraviolet ionization
(2011-04-14)
Han, Hui-Ling
;
Camacho, Cristopher
;
Witek, Henryk A.
;
Lee, Yuan-Pern
;
應用化學系
We investigated IR spectra in the CH-and OH-stretching regions of size-selected methanol clusters, (CH3OH)(n) with n = 2-6, in a pulsed supersonic jet by using the IR-VUV (vacuum-ultraviolet) ionization technique. VUV emission at 118 nm served as the source of ionization in a time-of-flight mass spectrometer. The tunable IR laser emission served as a source of predissociation or excitation before ionization. The variations of intensity of protonated methanol cluster ions (CH3OH)(n)H+ and CH3OH+ and (CH3OH)(2)(+) were monitored as the IR laser light was tuned across the range 2650-3750 cm(-1). Careful processing of these action spectra based on photoionization efficiencies and the production and loss of each cluster due to photodissociation yielded IR spectra of the size-selected clusters. Spectra of methanol clusters in the OH region have been extensively investigated; our results are consistent with previous reports, except that the band near 3675 cm(-1) is identified as being associated with the proton acceptor of (CH3OH)(2). Spectra in the CH region are new. In the region 2800-3050 cm(-1), bands near 2845, 2956, and 3007 cm(-1) for CH3OH split into 2823, 2849, 2934, 2955, 2984, and 3006 cm(-1) for (CH3OH)(2) that correspond to proton donor and proton acceptor, indicating that the methanol dimer has a preferred open-chain structure. In contrast, for (CH3OH)(3), the splitting diminishes and the bands near 2837, 2954, and 2987 cm(-1) become narrower, indicating a preferred cyclic structure. Anharmonic vibrational wavenumbers predicted for the methanol open-chain dimer and the cyclic trimer with the B3LYP/VPT2/ANO1 level of theory are consistent with experimental results. For the tetramer and pentamer, the spectral pattern similar to that of the trimer but with greater widths was observed, indicating that the most stable structures are also cyclic. (C) 2011 American Institute of Physics. [doi:10.1063/1.3572225]
Intruder States in Multireference Perturbation Theory: The Ground State of Manganese Dimer
(2009-02-01)
Camacho, Cristopher
;
Witek, Henryk A.
;
Yamamoto, Shigeyoshi
;
應用化學系
;
應用化學系分子科學碩博班
A detailed analysis of a severe intruder state problem in the multistate multireference perturbation theory (MS-MRPT) calculations on the ground state of manganese dinner is presented. An enormous number of detected intruder states (> 5000) do not permit finding, even an approximate shape of the X(1) Sigma(+)(g) potential energy curve. The intruder states are explicitly demonstrated to originate from quasidegeneracies in the zeroth-order Hamiltonian spectrum. The electronic Configurations responsible for appearance of the quasidegeneracies are identified as single and double excitations from the active orbitals to the external orbitals. It is shown that the quasidegeneracy problem can be completely eliminated using shift techniques despite of its severity. The resultant curves are smooth and continuous. Unfortunately, strong dependence of the spectroscopic parameters of the X(1) Sigma(+)(g) state on the shift parameter is observed. This finding rises serious controversies regarding validity of employing shift techniques for solving the intruder state problem in MS-MRPT. Various alternative approaches of removing intruder states (e.g., modification of the basis set or changing the active space) arc tested. None of these conventional techniques is able to fully avoid the quasidegeneracies. We believe that the MS-MRPT calculations on the three lowest A(g) states of manganese dimer constitute a perfect benchmark case for studying the behavior of MRPT in extreme Situations. (C) 2008 Wiley Periodicals, Inc.
Multireference perturbation theory can predict a false ground state
(2010)
Camacho, Cristopher
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Cimiraglia, Renzo
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Witek, Henryk A.
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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
Prediction of a false ground state with popular variants of multireference perturbation theory (CASPT2 and MRMP) is reported for a remarkably simple chemical system: the Sc(2) molecule.
Reply to the 'Comment on "Multiconfigurational perturbation theory can predict a false ground state"' by J. Soto, F. Avila, J. C. Otero and J. F. Arenas, Phys. Chem. Chem. Phys., 2011
(2011)
Camacho, Cristopher
;
Cimiraglia, Renzo
;
Witek, Henryk A.
;
應用化學系
;
應用化學系分子科學碩博班
The low-lying states of the scandium dimer
(2010-06-28)
Camacho, Cristopher
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Witek, Henryk A.
;
Cimiraglia, Renzo
;
應用化學系
;
應用化學系分子科學碩博班
A systematic investigation of low-lying states of Sc(2) using multireference perturbation theory (NEVPT2 and NEVPT3) indicates that the ground state of this system is 5 Sigma(u) with r(e)= 2.611 angstrom, w(e)= 241.8 cm(-1), and De= 1.78 eV. This state is closely followed by other low-lying states of Sc(2): (3)Sigma(u)-, (5)Delta(u), (3)Pi(g), (1)Pi(g), and (1)Sigma(u)-. Our energy ordering of the (5)Sigma(u)- and (3)Sigma(u)- states confirms the recent MRCI results of Kalemos et al. [J. Chem. Phys. 132, 024309 (2010)] and is at variance with the earlier diffusion Monte Carlo predictions of Matxain et al. [J. Chem. Phys. 128, 194315 (2008)]. An excellent agreement between the second-and third-order NEVPT results and between the computed and experimental values of omega(e) (241.8 versus 238.9 cm(-1)) for the (5)Sigma(u)- state suggests high accuracy of our predictions. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3442374]
The polarisability potential as a steric index
(2010-10-01)
Camacho, Cristopher
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Mata-Segreda, Julio F.
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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
;
Department of Applied Chemistry
The acid-catalysed hydrolysis of carboxylic esters is the reference chemical reaction used for the empirical evaluation of steric effects. In this work, the polarisability potential (Hehre, et al., J. Am. Chem. Soc. 1986, 108, 1711-1712) is identified as quantum-mechanical size of substituent groups. Correlation is found between this quantity and reactivity features of the reference reaction. Copyright (C) 2010 John Wiley & Sons, Ltd.
Theoretical Interpretation of the UV-vis Spectrum of the CS(2)/Cl Complex in the Spectral Region 320-550 nm
(2010-10-21)
Camacho, Cristopher
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Cheng, Chi-Wen
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Witek, Henryk A.
;
Lee, Yuan-Pern
;
應用化學系
Accurate multireference configuration interaction and time-dependent density functional calculations have been performed to interpret the experimental UV-vis spectrum of the CS(2)/Cl complex in the spectral region 320-550 nm. The molecular structure of the complex responsible for the previously observed UV-vis spectrum is recognized as ClSCS, not CICS(2). Two low-lying excited states of ClSCS, responsible for its optical absorption, have been identified and analyzed. Optical excitation of ClSCS leads to the excitation-specific bond elongation that may lead to photofragmentation of the molecule. In addition, experimental conditions for verifying the presence of ClCS(2) are identified and detailed characterization of its optically active excited states with possible photofragmentation pathways is given.
Theoretical Interpretation of the UV-vis Spectrum of the CS2/Cl Complex in the Spectral Region 320-550 nm
(2010-10-21)
Camacho, Cristopher
;
Cheng, Chi-Wen
;
Witek, Henryk A.
;
Lee, Yuan-Pern
;
應用化學系
Accurate multireference configuration interaction and time-dependent density functional calculations have been performed to interpret the experimental UV-vis spectrum of the CS2/Cl complex in the spectral region 320-550 nm. The molecular structure of the complex responsible for the previously observed UV-vis spectrum is recognized as ClSCS, not CICS2. Two low-lying excited states of ClSCS, responsible for its optical absorption, have been identified and analyzed. Optical excitation of ClSCS leads to the excitation-specific bond elongation that may lead to photofragmentation of the molecule. In addition, experimental conditions for verifying the presence of ClCS2 are identified and detailed characterization of its optically active excited states with possible photofragmentation pathways is given.