RESUMO
One of the reasons that thermally induced reactions are not considered a crucial mechanism in ultraviolet matrix-assisted laser desorption ionization (UV-MALDI) is the low ion-to-neutral ratios. Large ion-to-neutral ratios (10(-4)) have been used to justify the unimportance of thermally induced reactions in UV-MALDI. Recent experimental measurements have shown that the upper limit of the total ion-to-neutral ratio is approximately 10(-7) at a high laser fluence and less than 10(-7) at a low laser fluence. Therefore, reexamining the possible contributions of thermally induced reactions in MALDI may be worthwhile. In this study, the concept of polar fluid was employed to explain the generation of primary ions in MALDI. A simple model, namely thermal proton transfer, was used to estimate the ion-to-neutral ratios in MALDI. We demonstrated that the theoretical calculations of ion-to-neutral ratios exhibit the same trend and similar orders of magnitude compared with those of experimental measurements. Although thermal proton transfer may not generate all of the ions observed in MALDI, the calculations demonstrated that thermally induced reactions play a crucial role in UV-MALDI.
Assuntos
Prótons , Espectrometria de Massas por Ionização e Dessorção a Laser Assistida por Matriz/métodos , Espectrofotometria Ultravioleta/métodos , TermodinâmicaRESUMO
RATIONALE: In most previous studies, the ratios of desorbed ions and neutrals from matrix-assisted laser desorption/ionization (MALDI) were measured outside the common MALDI conditions. In this work, we measured the ratios under common MALDI conditions. METHODS: Ions were detected using a time-of-flight mass spectrometer in combination with a time-gated ion imaging detector. Mass-resolved desorbed neutral molecules at different angles and velocities were measured using a modified crossed molecular beam apparatus. RESULTS: The upper limit of the ion-to-neutral ratio from pure 2,5-dihydroxybenzoic acid (25DHB) is 4 × 10(-9) at laser fluence 40 J/m(2), it increases to 3 × 10(-7) at laser fluence 250 J/m(2). The ratios of matrix from the mixture of 25DHB and analyte remain in the same order of magnitude as pure 25DHB. However, the ratio of analyte depends strongly on the analyte. Values as large as 10(-3)-10(-4) for bradykinin and as small as <10(-8) for glycine were observed at laser fluence ~100 J/m(2). CONCLUSION: The ion-to-neutral ratios of 25DHB matrix measured in this work are much smaller than some of the values reported in previous work using different methods and/or under different MALDI conditions.
Assuntos
Gentisatos/química , Espectrometria de Massas por Ionização e Dessorção a Laser Assistida por Matriz , Desenho de Equipamento , Íons/química , Espectrometria de Massas por Ionização e Dessorção a Laser Assistida por Matriz/instrumentaçãoRESUMO
By using time-resolved Fourier-transform infrared emission spectroscopy, the fragments of HCN(v = 1, 2) and CO(v = 1-3) are detected in one-photon dissociation of acetyl cyanide (CH(3)COCN) at 308 nm. The S(1)(A(")), (1)(n(O), π(∗) (CO)) state at 308 nm has a radiative lifetime of 0.46 ± 0.01 µs, long enough to allow for Ar collisions that induce internal conversion and enhance the fragment yields. The rate constant of Ar collision-induced internal conversion is estimated to be (1-7) × 10(-12) cm(3) molecule(-1) s(-1). The measurements of O(2) dependence exclude the production possibility of these fragments via intersystem crossing. The high-resolution spectra of HCN and CO are analyzed to determine the ro-vibrational energy deposition of 81 ± 7 and 32 ± 3 kJ∕mol, respectively. With the aid of ab initio calculations, a two-body dissociation on the energetic ground state is favored leading to HCN + CH(2)CO, in which the CH(2)CO moiety may further undergo secondary dissociation to release CO. The production of CO(2) in the reaction with O(2) confirms existence of CH(2) and a secondary reaction product of CO. The HNC fragment is identified but cannot be assigned, as restricted to a poor signal-to-noise ratio. Because of insufficient excitation energy at 308 nm, the CN and CH(3) fragments that dominate the dissociation products at 193 nm are not detected.
RESUMO
Energy transfer of highly vibrationally excited naphthalene in the triplet state in collisions with CHF(3), CF(4), and Kr was studied using a crossed-beam apparatus along with time-sliced velocity map ion imaging techniques. Highly vibrationally excited naphthalene (2.0 eV vibrational energy) was formed via the rapid intersystem crossing of naphthalene initially excited to the S(2) state by 266 nm photons. The shapes of the collisional energy-transfer probability density functions were measured directly from the scattering results of highly vibrationally excited naphthalene. In comparison to Kr atoms, the energy transfer in collisions between CHF(3) and naphthalene shows more forward scatterings, larger cross section for vibrational to translational (V â T) energy transfer, smaller cross section for translational to vibrational and rotational (T â VR) energy transfer, and more energy transferred from vibration to translation, especially in the range -ΔE(d) = -100 to -800 cm(-1). On the other hand, the difference of energy transfer properties between collisional partners Kr and CF(4) is small. The enhancement of the V â T energy transfer in collisions with CHF(3) is attributed to the large attractive interaction between naphthalene and CHF(3) (1-3 kcal/mol).
RESUMO
The energy transfer of highly vibrationally excited isomers of dimethylnaphthalene and 2-ethylnaphthalene in collisions with krypton were investigated using crossed molecular beam/time-of-flight mass spectrometer/time-sliced velocity map ion imaging techniques at a collision energy of approximately 300 cm(-1). Angular-resolved energy-transfer distribution functions were obtained directly from the images of inelastic scattering. The results show that alkyl-substituted naphthalenes transfer more vibrational energy to translational energy than unsubstituted naphthalene. Alkylation enhances the VâT energy transfer in the range -ΔE(d)=-100~-1500 cm(-1) by approximately a factor of 2. However, the maximum values of VâT energy transfer for alkyl-substituted naphthalenes are about 1500~2000 cm(-1), which is similar to that of naphthalene. The lack of rotation-like wide-angle motion of the aromatic ring and no enhancement in very large VâT energy transfer, like supercollisions, indicates that very large VâT energy transfer requires special vibrational motions. This transfer cannot be achieved by the low-frequency vibrational motions of alkyl groups.
RESUMO
The energy transfer between Kr atoms and highly vibrationally excited, rotationally cold phenanthrene and diphenylacetylene in the triplet state was investigated using crossed-beam/time-of-flight mass spectrometer/time-sliced velocity map ion imaging techniques. Compared to the energy transfer between naphthalene and Kr, energy transfer between phenanthrene and Kr shows a larger cross-section for vibrational to translational (V â T) energy transfer, a smaller cross-section for translational to vibrational and rotational (T â VR) energy transfer, and more energy transferred from vibration to translation. These differences are further enlarged in the comparison between naphthalene and diphenylacetylene. In addition, less complex formation and significant increases in the large V â T energy transfer probabilities, termed supercollisions in diphenylacetylene and Kr collisions were observed. The differences in the energy transfer between these highly vibrationally excited molecules are attributed to the low-frequency vibrational modes, especially those vibrations with rotation-like wide-angle motions.
RESUMO
By employing time-resolved Fourier transform infrared emission spectroscopy, the fragments HCl (v=1-3), HBr (v=1), and CO (v=1-3) are detected in one-photon dissociation of 2-bromopropionyl chloride (CH(3)CHBrCOCl) at 248 nm. Ar gas is added to induce internal conversion and to enhance the fragment yields. The time-resolved high-resolution spectra of HCl and CO were analyzed to determine the rovibrational energy deposition of 10.0±0.2 and 7.4±0.6 kcal mol(-1), respectively, while the rotational energy in HBr is evaluated to be 0.9±0.1 kcal mol(-1). The branching ratio of HCl(v>0)/HBr(v>0) is estimated to be 1:0.53. The bond selectivity of halide formation in the photolysis follows the same trend as the halogen atom elimination. The probability of HCl contribution from a hot Cl reaction with the precursor is negligible according to the measurements of HCl amount by adding an active reagent, Br(2), in the system. The HCl elimination channel under Ar addition is verified to be slower by two orders of magnitude than the Cl elimination channel. With the aid of ab initio calculations, the observed fragments are dissociated from the hot ground state CH(3)CHBrCOCl. A two-body dissociation channel is favored leading to either HCl+CH(3)CBrCO or HBr+CH(2)CHCOCl, in which the CH(3)CBrCO moiety may further undergo secondary dissociation to release CO.
Assuntos
Gases/química , Propionatos/química , Raios Ultravioleta , Processos Fotoquímicos , Espectroscopia de Infravermelho com Transformada de FourierRESUMO
In one-photon dissociation of gaseous acetyl chloride at 248 nm, time-resolved Fourier-transform infrared emission spectroscopy is used to detect the fragments of HCl, CO, and CH(2) in the presence of Ar or O(2). The high-resolution spectra of HCl and CO are analyzed to yield the corresponding internal energy deposition of 8.9 +/- 1.1 and 6.2 +/- 0.9 kcal/mol. The presence of the CH(2) fragment is verified by detecting the CO(2) product resulting from the reaction of CH(2) and the added O(2). The probability of the HCl formation via a hot Cl reaction with the precursor is examined to be negligible by performing two experiments, the CH(3)COCl pressure dependence and the measurement of Br(2) with Cl reaction. The HCl elimination channel under the Ar addition is verified to be slowed by 2 orders of magnitude, as compared to the Cl elimination channel. The observed fragments are proposed to dissociate on the hot ground electronic state via collision-induced internal conversion. A two-body dissociation channel is favored leading to HCl and CH(2)CO, followed by secondary dissociation.
Assuntos
Acetatos/química , Monóxido de Carbono/química , Cloretos/química , Ácido Clorídrico/química , Argônio/química , Gases/química , Oxigênio/química , Fotoquímica , Rotação , Espectroscopia de Infravermelho com Transformada de Fourier , Fatores de Tempo , VibraçãoRESUMO
An experimental and theoretical investigation of rotational energy transfers (RET) of CH involving the B (2)Sigma(-) (v=0, 0=N=5, F) state by collisions with Ar is undertaken, using the photolysis-probe technique. Time-resolved laser-induced fluorescence resulting from an initially prepared fine-structure label is dispersed using a step-scan Fourier transform spectrometer. The spin-resolved RET rate constants are evaluated with the simulation of a kinetic model. The quantum-scattering method is used for the calculation of the fine-structure-resolved cross sections and rate constants in the rotationally inelastic collisions. The theoretical values are generally consistent with our experimental findings, both in the order of magnitude and trend of N and DeltaN dependence. The propensity rules obtained from the experiments are essentially obeyed by theoretical calculations, and are also in accordance with those reported by Kind and Stuhl. The RET rate constants obtained for the v=0 level are smaller than those obtained previously for v=1. The discrepancy in the RET behavior may be caused by an anisotropy difference of the interaction potential resulting from vibrational excitation.
Assuntos
Alcinos/química , Argônio/química , Teoria Quântica , Transferência de Energia , Cinética , Modelos Químicos , Fotólise , Rotação , Espectrometria de FluorescênciaRESUMO
With photolysis-probe technique, we have studied vibrational and rotational energy transfers of CH involving the B (2)Sigma(-) (v=1, 0