The Journal of Physical Chemistry A最新文献

Although iron is the most abundant transition metal in the interstellar medium, its interaction with hydrogen─by far the most abundant element─in small gas-phase molecules or complexes is poorly understood. Herein, we study the infrared spectroscopy of cationic iron complexes with one and two dihydrogen ligands, Fe⁺(H₂)_1,2, as well as their deuterated counterparts, Fe⁺(D₂)_1,2, using infrared multiple photon dissociation (IRMPD) spectroscopy. Quantum chemical calculations, including multireference configuration interaction (MRCI) with spin–orbit coupling, are used to simulate the electronic and vibrational contributions to the spectra. Broad electronic transitions are observed in the studied energy range of 2230–4000 cm^–1, which arise from d–d transitions at the metal center between states of quartet spin multiplicity. In the complex, the H–H stretching mode of the H₂ ligand becomes infrared active, and features arising from this mode are assigned with the help of quantum chemical calculations in the spectra of Fe⁺(H₂) and Fe⁺(D₂)₂. In Fe⁺(H₂), we assign a band with local maxima centered at ∼3138 cm^–1 and ∼3219 cm^–1 to the P and R branches of the H–H stretching mode, while the D–D stretch of Fe⁺(D₂)₂ has a band centered at 2448 cm^–1, with P and R branches not resolved. With a D/H wavenumber ratio of 0.726, the D–D stretch of Fe⁺(D₂) and the H–H stretch of Fe⁺(H₂)₂ are expected at 2309 cm^–1 and 3372 cm^–1, respectively. The rovibrational bands in Fe⁺(H₂) and Fe⁺(D₂)₂ exhibit pronounced broadening that cannot be explained by temperature. We assign the broadening to the strong dependence of the H–H and D–D stretching frequencies on the torsional motion of the complex, as shown by the calculations. The extreme redshift of the H–H and D–D stretching frequencies is caused by back-donation from iron d_xz, d_yz atomic orbitals into the σ* orbital of the H₂ molecule, which weakens the H–H bond.

We report the interaction of anhydrous acetonitrile, CH₃CN (ACN), and deuterated methanol, CD₃OD (MeOD), in the condensed crystalline phase by electron impact with 2.3 keV of energy. Theoretical and experimental investigations are focused on fragments and aggregates formed as a result of electron-stimulated ion desorption. Positively charged fragments and aggregates were collected using time-of-flight mass spectrometry (TOF-MS) and temperature-programmed desorption based on quadrupole spectroscopy (TPD). The structures of clusters identified in the TOF spectra were studied by applying density functional theory combined with a global minimum search. Two different deposition methods were used for the formation of the condensed molecular films, bilayer and codeposition, and in a second step, the annealing process was performed. The ionic species released from the surface into the vacuum are highly dependent on the annealing. A discussion of the interaction between the molecules was made. The formation of complex organic species comes from the intermolecular or intramolecular interactions of pure MeOD and ACN molecules. Anhydrous compounds were used, and the background water content was minimized to inhibit caging of the ACN molecules by water molecules.

We report the interaction of anhydrous acetonitrile, CH₃CN (ACN), and deuterated methanol, CD₃OD (MeOD), in the condensed crystalline phase by electron impact with 2.3 keV of energy. Theoretical and experimental investigations are focused on fragments and aggregates formed as a result of electron-stimulated ion desorption. Positively charged fragments and aggregates were collected using time-of-flight mass spectrometry (TOF-MS) and temperature-programmed desorption based on quadrupole spectroscopy (TPD). The structures of clusters identified in the TOF spectra were studied by applying density functional theory combined with a global minimum search. Two different deposition methods were used for the formation of the condensed molecular films, bilayer and codeposition, and in a second step, the annealing process was performed. The ionic species released from the surface into the vacuum are highly dependent on the annealing. A discussion of the interaction between the molecules was made. The formation of complex organic species comes from the intermolecular or intramolecular interactions of pure MeOD and ACN molecules. Anhydrous compounds were used, and the background water content was minimized to inhibit caging of the ACN molecules by water molecules.

The ozonolysis reaction of trans-2-hexenal was studied theoretically on the basis of highly accurate CCSD(T)-F12b/AVTZ energy values obtained in M06–2X/AVTZ preoptimized nuclear configurations. The kinetics was modeled with the help of the master equation solver MESMER. Apart from the expected stable oxidation products 1-butanal (17%) and glyoxal (35%), a secondary ozonide is formed on the glyoxal channel, which is the principal first-generation product (49%). It is further shown that glyoxal is created on two competing pathways, one of which leads to simultaneous production of the ester propylformate (18%). The inclusion of all of these mechanisms explains the experimental findings and identifies for the first time the origin of the experimental carbon deficit.

Information about the intermolecular potential energy surface for the interaction between solute and solvent molecules is required to understand the impact of solvation on reaction mechanisms and dynamics. In this study, we measured vibrational-specific infrared (IR) spectra of 4-aminobenzonitrile–(argon)_n cation clusters, 4ABN⁺–Ar_n (n = 1, 2), in the NH stretching range to elucidate the energetics of the photoionization-induced π → NH migration of Ar. The IR spectra of 4ABN⁺–Ar_n generated by resonant photoionization of neutral π-bonded clusters display the hydrogen-bonded NH₂ stretching vibration (ν_NH2) only when intermolecular vibrational levels are excited. This is the first observation of Ar migration from the aromatic ring toward the NH₂ group upon photoionization in the n = 1 cluster. From the vibrational-level dependence of the IR spectra, the activation barrier heights are determined to be 21–47 (34 ± 13) and <27 cm^–1 for 4ABN⁺–Ar₁ and 4ABN⁺–Ar₂, respectively. The potential energy surfaces and mechanism of the Ar migration are discussed with the help of complementary density functional theory calculations.

The local bond-stretch (LBS) method is presented as a means of obtaining a set of localized, rectilinear vibrational modes. Three variants of the LBS method are considered: pure LBS, projected LBS (pLBS), and orthogonal, projected LBS (opLBS). These variants feature different degrees of localization and different coupling terms in the kinetic energy operator, such that the most localized method (LBS) has the largest number and magnitude of coupling terms, and the least localized (opLBS) has the least coupling terms. The different LBS variants are exemplified in computations on overtone vibrational spectra of water, nitroxyl (chemical formula HNO), formaldehyde, and 1,3-butadiene computed with a vibrational coupled cluster band Lanczos approach. These spectra are calculated using potential energy surfaces (PESs) obtained with the adaptive density-guided approach (ADGA). We observe faster convergence with respect to the coupling level in the PES when using the LBS variants compared to normal coordinates. Among the LBS variants, pLBS and opLBS appear most promising.

The local bond-stretch (LBS) method is presented as a means of obtaining a set of localized, rectilinear vibrational modes. Three variants of the LBS method are considered: pure LBS, projected LBS (pLBS), and orthogonal, projected LBS (opLBS). These variants feature different degrees of localization and different coupling terms in the kinetic energy operator, such that the most localized method (LBS) has the largest number and magnitude of coupling terms, and the least localized (opLBS) has the least coupling terms. The different LBS variants are exemplified in computations on overtone vibrational spectra of water, nitroxyl (chemical formula HNO), formaldehyde, and 1,3-butadiene computed with a vibrational coupled cluster band Lanczos approach. These spectra are calculated using potential energy surfaces (PESs) obtained with the adaptive density-guided approach (ADGA). We observe faster convergence with respect to the coupling level in the PES when using the LBS variants compared to normal coordinates. Among the LBS variants, pLBS and opLBS appear most promising.

Although iron is the most abundant transition metal in the interstellar medium, its interaction with hydrogen─by far the most abundant element─in small gas-phase molecules or complexes is poorly understood. Herein, we study the infrared spectroscopy of cationic iron complexes with one and two dihydrogen ligands, Fe⁺(H₂)_1,2, as well as their deuterated counterparts, Fe⁺(D₂)_1,2, using infrared multiple photon dissociation (IRMPD) spectroscopy. Quantum chemical calculations, including multireference configuration interaction (MRCI) with spin-orbit coupling, are used to simulate the electronic and vibrational contributions to the spectra. Broad electronic transitions are observed in the studied energy range of 2230-4000 cm^-1, which arise from d-d transitions at the metal center between states of quartet spin multiplicity. In the complex, the H-H stretching mode of the H₂ ligand becomes infrared active, and features arising from this mode are assigned with the help of quantum chemical calculations in the spectra of Fe⁺(H₂) and Fe⁺(D₂)₂. In Fe⁺(H₂), we assign a band with local maxima centered at ∼3138 cm^-1 and ∼3219 cm^-1 to the P and R branches of the H-H stretching mode, while the D-D stretch of Fe⁺(D₂)₂ has a band centered at 2448 cm^-1, with P and R branches not resolved. With a D/H wavenumber ratio of 0.726, the D-D stretch of Fe⁺(D₂) and the H-H stretch of Fe⁺(H₂)₂ are expected at 2309 cm^-1 and 3372 cm^-1, respectively. The rovibrational bands in Fe⁺(H₂) and Fe⁺(D₂)₂ exhibit pronounced broadening that cannot be explained by temperature. We assign the broadening to the strong dependence of the H-H and D-D stretching frequencies on the torsional motion of the complex, as shown by the calculations. The extreme redshift of the H-H and D-D stretching frequencies is caused by back-donation from iron d_xz, d_yz atomic orbitals into the σ* orbital of the H₂ molecule, which weakens the H-H bond.

For simulations of strong field ionization using time-dependent configuration with a complex absorbing potential (TDCI-CAP), standard molecular basis set must be augmented by several sets of diffuse functions to support the wave function as it is distorted by the strong field and interacts with the absorbing potential. Various sets of diffuse functions used in previous studies have been extended and evaluated for their ability to model the angular dependence of the strong field ionization. These sets include diffuse s, p, d, and f Gaussian functions with selected even-tempered exponents of the form 0.0001 × 2ⁿ placed on each atom. For single-centered test cases, the largest contribution to the ionization rate is from functions with a maximum in the radial distribution close to the onset of the complex absorbing potential, while functions with smaller exponents also contributed to the rate. For molecules, diffuse functions on adjacent centers overlap strongly, leading to linear dependencies. The transformation to remove these linear dependencies mixes functions of different angular momenta making it difficult to assess the importance of individual s, p, d, and f functions in simulating the rate for molecules. As an alternative, a hierarchy of diffuse basis sets was constructed by starting with a small set and adding one or two functions at a time. These basis sets were evaluated for their ability to reproduce the rate and shape of the angular dependence of strong field ionization. When combined with the aug-cc-pVTZ molecular basis set and an absorbing potential starting at 3.5 times the van der Waals radius for each atom, the most diffuse s, p, d, and f functions need to have exponents of 0.0032, 0.0032, 0.0064, and 0.0064, respectively, or smaller. Strong field ionization from electronegative atoms such as oxygen required additional f functions with tight exponents of 0.0512 and 0.1024. Diffuse basis sets that perform well for the angular dependence of the ionization rate with a static field are equally effective for strong field ionization with a linearly polarized 7-cycle 800 nm pulse.

Benzothiazoles are in widespread use as components of, or precursors to, a variety of consumer and industrial products. This class of compounds encompasses the simplest molecule benzothiazole (BTH) in which a benzene ring is fused to a thiazole ring, as well as a series of derivatives which are commonly functionalized at the C2 position of the thiazole ring. The addition of groups at this position modifies the reactivity in ways that are not well-known. While the reactions of benzothiazoles in water have been the subject of investigation, in part for wastewater treatment applications, much less is known about their atmospheric reactions where gas phase oxidation by the OH radical is expected to dominate. We report here studies of the kinetics, products, and mechanism of reaction of 2-methylbenzothiazole (MeBTH) with OH in the gas phase using a combination of experiments and theory. Comparison to previous studies of the OH oxidation of BTH highlights the impact of substitution of a methyl group at the 2-position on the products and reactivity. Specifically, the rate constant at 298 K and 1 atm pressure for the MeBTH-OH reaction is (3.0 ± 0.4) × 10^-12 cm³ molecule^-1 s^-1 (1σ), about 50% faster than that of BTH. In addition, attack of OH on the -CH₃ group at the 2-position of the thiazole ring to form the aldehyde as the stable product becomes important, accounting for ∼ 33% of the overall reaction. Formation of the phenol-type products from attack on the benzene ring accounts for the remainder, with the experimental relative yields consistent with theoretical predictions based on energies of formation of the prereaction MeBTH···OH complex. The formation of the aldehyde product (2-CHO-BTH) involves a sequence of five distinct stages involving two oxygen molecules and one NO. Both processes involve a spin flip of unpaired electrons, which enables a transition between electronic states that is essential for the reaction to proceed. Using the room temperature rate constant, the estimated lifetimes of MeBTH in air range from about 9 h to 4 days over OH concentrations of 10⁷ - 10⁶ cm^-3. Thus, this reaction represents a significant loss process for MeBTH in air both outdoors and indoors, and exposures and toxicity of both the parent MeBTH and its oxidation products need to be taken into account in assessments of its environmental fates.