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1.
Anal Chem ; 93(32): 11268-11274, 2021 08 17.
Artigo em Inglês | MEDLINE | ID: mdl-34347440

RESUMO

Discrimination of isomers in a mixture is a subject of ongoing interest in biology, pharmacology, and forensics. We demonstrate that femtosecond time-resolved mass spectrometry (FTRMS) effectively quantifies mixtures of ortho-, para-, and meta-nitrotoluenes, the first two of which are common explosive degradation products. The key advantage of the FTRMS approach to mixture quantification lies in the ability of the pump-probe laser control scheme to capture distinct fragmentation dynamics of each nitrotoluene cation isomer on femtosecond timescales, thereby allowing for discrimination of the isomers using only the signal of the parent molecular ion at m/z 137. Upon measurement of reference dynamics of each individual isomer, the molar fractions of binary and ternary mixtures can be predicted to within ∼5 and ∼7% accuracy, respectively.


Assuntos
Lasers , Cátions , Isomerismo , Espectrometria de Massas
2.
Phys Chem Chem Phys ; 2021 Jun 07.
Artigo em Inglês | MEDLINE | ID: mdl-34096943

RESUMO

The electronic potential energy surfaces of the nitrobenzene cation obtained from time-dependent density functional theory and coupled cluster calculations are used to predict the most efficient excitation wavelength for femtosecond time-resolved mass spectrometry measurements. Both levels of theory identify a strongly-coupled transition from the ground state of the nitrobenzene cation with a geometry-dependent oscillator strength, reaching a maximum at 90° C-C-N-O dihedral angle with a corresponding energy gap of ∼2 eV. These results are consistent with the experimental observation in the nitrobenzene cation of a coherent superposition of vibrational states: a vibrational wave packet. Time-resolved measurements using a probe wavelength of 650 nm, nearly resonant with the strong transition, result in enhanced ion yield oscillation amplitudes as compared to excitation with the nonresonant 800 nm wavelength. Analogous behavior is found for the closely related molecules 2- and 4-nitrotoluene. These results demonstrate that computational chemistry can predict the best choice of probe wavelength in time-resolved measurements of vibrational coherent states in molecular cations.

3.
Molecules ; 24(3)2019 Jan 31.
Artigo em Inglês | MEDLINE | ID: mdl-30708973

RESUMO

Organic phosphates and phosphonates are present in a number of cellular components that can be damaged by exposure to ionizing radiation. This work reports femtosecond time-resolved mass spectrometry (FTRMS) studies of three organic phosphonate radical cations that model the DNA sugar-phosphate backbone: dimethyl methylphosphonate (DMMP), diethyl methylphosphonate (DEMP), and diisopropyl methylphosphonate (DIMP). Upon ionization, each molecular radical cation exhibits unique oscillatory dynamics in its ion yields resulting from coherent vibrational excitation. DMMP has particularly well-resolved 45 fs ( 732 ± 28 cm - 1 ) oscillations with a weak feature at 610⁻650 cm - 1 , while DIMP exhibits bimodal oscillations with a period of ∼55 fs and two frequency features at 554 ± 28 and 670⁻720 cm - 1 . In contrast, the oscillations in DEMP decay too rapidly for effective resolution. The low- and high-frequency oscillations in DMMP and DIMP are assigned to coherent excitation of the symmetric O⁻P⁻O bend and P⁻C stretch, respectively. The observation of the same ionization-induced coherently excited vibrations in related molecules suggests a possible common excitation pathway in ionized organophosphorus compounds of biological relevance, while the distinct oscillatory dynamics in each molecule points to the potential use of FTRMS to distinguish among fragment ions produced by related molecules.


Assuntos
Cátions/química , Espectrometria de Massas , Organofosfonatos/química , Compostos Organofosforados/química , Espectrometria de Massas/métodos , Modelos Químicos , Modelos Moleculares , Conformação Molecular , Estrutura Molecular
4.
J Phys Chem A ; 123(6): 1140-1152, 2019 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-30668102

RESUMO

2-Nitrotoluene (2-NT) is a good model for both photolabile protecting groups for organic synthesis and the military explosive 2,4,6-trinitrotoluene (TNT). In addition to the direct C-NO2 bond-cleavage reaction that initiates detonation in TNT, 2-NT undergoes an H atom attack reaction common to the photolabile 2-nitrobenzyl group, which forms the aci-nitro tautomer. In this work, femtosecond pump-probe measurements with mass spectrometric detection and density functional theory (DFT) calculations demonstrate that the initially prepared vibrational coherence in the 2-NT radical cation (2-NT+) is preserved following H atom attack. Strong-field adiabatic ionization is used to prepare 2-NT+, which can overcome a modest 0.76 eV energy barrier to H atom attack to form the aci-nitro tautomer as soon as ∼20-60 fs after ionization. Once formed, the aci-nitro tautomer spontaneously loses -OH to form C7H6NO+, which exhibits distinctly faster oscillations in its ion yield (290 fs period) as compared to the 2-NT+ ion (380 fs period). The fast oscillations are attributed to the coherent torsional motion of the aci-nitro tautomer, which has a significantly faster computed torsional frequency (86.9 cm-1) than the 2-NT+ ion (47.9 cm-1). Additional DFT calculations identify reaction pathways leading to the formation of the dissociation products C7H6NO+, C7H7+, and C6H6N+. Collectively, these results reveal a rich picture of coherently and incoherently driven dissociation pathways in 2-NT+.

5.
J Vis Exp ; (138)2018 08 06.
Artigo em Inglês | MEDLINE | ID: mdl-30124651

RESUMO

We present a pump-probe method for preparing vibrational coherences in polyatomic radical cations and probing their ultrafast dynamics. By shifting the wavelength of the strong-field ionizing pump pulse from the commonly used 800 nm into the near-infrared (1200-1600 nm), the contribution of adiabatic electron tunneling to the ionization process increases relative to multiphoton absorption. Adiabatic ionization results in predominant population of the ground electronic state of the ion upon electron removal, which effectively prepares a coherent vibrational state ("wave packet") amenable to subsequent excitation. In our experiments, the coherent vibrational dynamics are probed with a weak-field 800 nm pulse and the time-dependent yields of dissociation products measured in a time-of-flight mass spectrometer. We present the measurements on the molecule dimethyl methylphosphonate (DMMP) to illustrate how using 1500 nm pulses for excitation enhances the amplitude of coherent oscillations in ion yields by a factor of 10 as compared to 800 nm pulses. This protocol may be implemented in existing pump-probe setups through the incorporation of an optical parametric amplifier (OPA) for wavelength conversion.


Assuntos
Cátions/química , Espectrometria de Massas/métodos , Vibração , Elétrons
7.
J Chem Phys ; 148(13): 134305, 2018 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-29626885

RESUMO

Monosubstituted nitrotoluenes serve as important model compounds for nitroaromatic energetic molecules such as trinitrotoluene. This work investigates the ultrafast nuclear dynamics of 3- and 4-nitrotoluene radical cations using femtosecond pump-probe measurements and the results of density functional theory calculations. Strong-field adiabatic ionization of 3- and 4-nitrotoluene using 1500 nm, 18 fs pulses produces radical cations in the ground electronic state with distinct coherent vibrational excitations. In both nitrotoluene isomers, a one-photon excitation with the probe pulse results in NO2 loss to form C7H7+, which exhibits out-of-phase oscillations in yield with the parent molecular ion. The oscillations in 4-nitrotoluene with a period of 470 fs are attributed to the torsional motion of the NO2 group based on theoretical results showing that the dominant relaxation pathway in 4-nitrotoluene radical cations involves the rotation of the NO2 group away from the planar geometry. The distinctly faster oscillation period of 216 fs in 3-nitrotoluene is attributed to an in-plane bending motion of the NO2 and CH3 moieties based on analysis of the normal modes. These results demonstrate that coherent nuclear motions determine the probability of C-NO2 homolysis in the nitrotoluene radical cations upon optical excitation within several hundred femtoseconds of the initial ionization event.

8.
Phys Chem Chem Phys ; 20(7): 4636-4640, 2018 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-29239428

RESUMO

Femtosecond pump-probe measurements of the nerve agent simulant dimethyl methylphosphonate (DMMP) demonstrate the preparation of a robust coherent vibrational state in the corresponding radical cation. The oscillations in the transient ion yields have a period of 45 fs (750 cm-1), which is at least 3 times faster than any previously observed oscillations in polyatomic radical cations. Use of 1200-1600 nm, as opposed to 800 nm, wavelengths for ionization increases the oscillation amplitude by a factor of 5 and doubles the number of visible oscillation periods from 6 to 12, indicating that an adiabatic ionization mechanism significantly enhances preparation of the coherent state. The coherent motion is assigned to a bending mode in DMMP+ with frequency in the range of 742.2-754.7 cm-1 based on the results of DFT calculations. The observation of coherent nuclear dynamics in the dissociation of DMMP+ suggests the potential utility of coherent control schemes for controlling the dissociation of DMMP and related molecules, which has important implications for developing detection and decontamination technologies for organophosphorus chemical warfare agents.

9.
J Phys Chem A ; 121(44): 8414-8424, 2017 Nov 09.
Artigo em Inglês | MEDLINE | ID: mdl-29035556

RESUMO

Using both mass spectrometry with intense femtosecond laser ionization and high-level computational methods, we have explored the structure and fragmentation patterns of dimethyl methylphosphonate (DMMP) cation. Extensive search of the geometries of both neutral and positively charged DMMP yields new isomers that are appreciably lower in total energy than those commonly synthesized using the Michaelis-Arbuzov reaction. The stability of the standard isomer with CH3PO(OCH3)2 topology is found to be due to the presence of high barriers to isomer interconversion that involves several transition states. Our femtosecond laser ionization experiments show that the relative yields of the major dissociation products as a function of peak laser intensity correlate well with the theoretical estimates for the energies of the DMMP+ decay via various channels. In contrast, the peak laser intensities required for observation of minor dissociation products exhibit no correlation with the computed decay energies, which suggests that barrier heights and/or excited electronic states of DMMP+ determine its preferred fragmentation pathways in an intense femtosecond laser field.

10.
Free Radic Res ; 50(7): 756-66, 2016 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-27043476

RESUMO

The products of oxidative damage to double-stranded (ds) DNA initiated by photolytically generated sulfate radical anions SO4(•-) were analyzed using reverse-phase (RP) high-performance liquid chromatography (HPLC). Relative efficiencies of two major pathways were compared: production of 8-oxoguanine (8oxoG) and hydrogen abstraction from the DNA 2-deoxyribose moiety (dR) at C1,' C4,' and C5' positions. The formation of 8oxoG was found to account for 87% of all quantified lesions at low illumination doses. The concentration of 8oxoG quickly reaches a steady state at about one 8oxoG per 100 base pairs due to further oxidation of its products. It was found that another guanine oxidation product identified as 2-amino-5-(2'-alkylamino)-4H-imidazol-4-one (X) was released in significant quantities from its tentative precursor 2-amino-5-[(2'-deoxy-ß-d-erythro-pentofuranosyl)amino]-4H-imidazol-4-one (dIz) upon treatment with primary amines in neutral solutions. The linear dose dependence of X release points to the formation of dIz directly from guanine and not through oxidation of 8oxoG. The damage to dR was found to account for about 13% of the total damage, with majority of lesions (33%) originating from the C4' oxidation. The contribution of C1' oxidation also turned out to be significant (17% of all dR damages) despite of the steric problems associated with the abstraction of the C1'-hydrogen. However, no evidence of base-to-sugar free valence transfer as a possible alternative to direct hydrogen abstraction at C1' was found.


Assuntos
Ânions/química , Dano ao DNA , Guanina/química , Imidazóis/química , Sulfatos/química , Hidrogênio/química , Oxirredução
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