Reactivity Consequences of Conformational Isomerism in 1-Propanol

Buffer gas cooled ice chemistry. II. Ice generation and mm-wave detection of molecules desorbed from an ice

This second paper in a series of two describes the chirped-pulse ice apparatus that permits the detection of buffer gas cooled molecules desorbed from an energetically processed ice using broadband mm-wave rotational spectroscopy. Here, we detail the lower ice stage developed to generate ices at 4 K, which can then undergo energetic processing via UV/VUV photons or high-energy electrons and which ultimately enter the gas phase via temperature-programmed desorption (TPD). Over the course of TPD, the lower ice stage is interfaced with a buffer gas cooling cell that allows for sensitive detection via chirped-pulse rotational spectroscopy in the 60-90 GHz regime. In addition to a detailed description of the ice component of this apparatus, we show proof-of-principle experiments demonstrating the detection of H2CO products formed through irradiation of neat methanol ices or 1:1 CO + CH4 mixed ices.

Conformer-Specific Desorption in Propanol Ices Probed by Chirped-Pulse Millimeter-Wave Rotational Spectroscopy

We present a new technique for the detection of molecules desorbed from an ice surface using broad-band millimeter-wave rotational spectroscopy. The approach permits interrogation of molecules that have undergone the slow warmup process of temperature-programmed desorption (TPD), analogous to the warmup phase of icy grains in the interstellar medium as they approach the central protostar. The detection is conformer- and isomer-specific and quantitative, as afforded by chirped-pulse rotational spectroscopy. To achieve this, we combine ice TPD with buffer gas cooling, followed by detection in the millimeter-wave regime. In this report we examine the TPD profiles of n- and i-propanol, the former of which may be in five different conformational isomeric forms, and which display distinct desorption profiles. The limited conformational isomerization and temperature-dependent relative yields of n-propanol conformers observed show that the desorption is highly conformer-specific.

16OSTM10: A new open-shell transition metal conformational energies database to challenge contemporary semiempirical and force field methods

transition metal (OSTM) complexes has been developed. Contemporary composite density functional theory (DFT) (PBEh-3c, B97-3c), semiempirical (PM6, PM7) and the methods of GFNn-xTB/FF family were examined against conventional DFT (PBE-D3(BJ),...

Predicting OH stretching fundamental wavenumbers of alcohols for conformational assignment: different correction patterns for density functional and wave-function-based methods

A model is presented for the prediction of OH stretching fundamental wavenumbers of alcohol conformers in the gas phase by application of a small set of empirical anharmonicity corrections to calculations in the harmonic approximation. In contrast to the popular application of a uniform scaling factor, the local chemical structure of the alcohol is taken into account to greatly improve accuracy. Interestingly, different correction patterns emerge for results of hybrid density functional (B3LYP-D3 and PBE0-D3) and wave-function-based methods (SCS-LMP2, LCCSD(T*)-F12a and CCSD(T)-F12a 1D). This raises questions about electronic structure deficiencies in these methods and differences in anharmonicity between alcohols. After its initial construction on the basis of literature assignments the model is tested with Raman jet spectroscopy of propargyl alcohol, cyclohexanol, borneol, isopinocampheol and 2-methylbutan-2-ol. For propargyl alcohol a spectral splitting attributed to tunneling is resolved. PBE0-D3 is identified as a well performing and broadly affordable electronic structure method for this model. A mean absolute error of 1.3 cm-1 and a maximum absolute error of 3 cm-1 result for 46 conformers of 24 alcohols in a 60 cm-1 range, when a single parameter is adjusted separately for each alcohol substitution class (methanol, primary, secondary, tertiary).

Multichannel dynamics in the OH+ n-butane reaction revealed by crossed-beam slice imaging and quasiclassical trajectory calculations

Multidimensional reactions present various channels that can exhibit very different dynamics and give products of varying subsequent reactivity. Here, we present a combination of experiment and theory to reveal the dynamics of hydrogen abstraction by OH radical at primary and secondary sites in n-butane at a collision energy of 8 kcal/mol. Crossed molecular beam slice imaging experiments unequivocally probe the secondary abstraction channel showing backward angular distributions with mild energy release to product translation, which are accurately captured by trajectory calculations using a specific-reaction-parameter Hamiltonian. Experiments containing both reaction channels indicate a less marked backward character in the angular distribution, whose origin is shown by trajectory calculations to appear as an evolution toward more sideways scattering from the secondary to primary channel. While the two channels have markedly different angular distributions, their energy release is largely comparable, showing ample energy release into the water product. The synergistic combination of crossed-beam imaging and trajectories opens the door to detailed reaction-dynamics studies of chemical reactions with ever-increasing complexity.

Universal crossed beam imaging studies of polyatomic reaction dynamics

Crossed-beam imaging studies of polyatomic reactions show surprising dynamics not anticipated by extrapolation from smaller model systems.

Ab Initio and Quasiclassical Trajectory Study of the O(3P) + 2-Propanol Hydrogen Abstraction Reaction

We present a theoretical study of the hydrogen abstraction reaction from 2-propanol by ground-state oxygen atoms. First, ab initio calculations are used to characterize the stationary points of the potential energy surface. Rotation around the C-C-O-H dihedral affords two conformers in 2-propanol, which gives rise to 13 hydrogen abstraction reaction pathways grouped into three channels, Cα, Cβ, and O, depending on the abstraction site. Reaction at Cα exhibits the lowest barrier and largest exothermicity, followed by reaction at Cβ, and at 2-propanol's oxygen atom. Additional ab initio calculations beyond the stationary points are employed to obtain a grid of energies with which a specific-reaction-parameters (SRP) PM6 semiemipirical Hamiltonian is derived for the title reaction. The SRP-PM6 model captures the energetics of the reaction with higher accuracy than some conventional first-principles methods but is efficient enough to allow for extensive reaction dynamics calculations. Quasiclassical trajectories are subsequently propagated with the SRP-PM6 Hamiltonian to obtain reaction dynamics properties that are compared to experiments. Product translational energy and angular distributions for reaction at Cα with the two conformers of 2-propanol are in good agreement with recent molecular-beam measurements, and they exhibit largely backward scattering with modest energy release to relative translation. Most of the energy is deposited into the organic product, substantiating a reaction mechanism dominated by rebound dynamics.