Evaluation of Infrared Intensities Using Diffusion Monte Carlo.

Approaches for evaluating excited state wave functions and energies using diffusion Monte Carlo (DMC) with guiding functions (guided DMC) are discussed. For this work, the guiding functions are functions of a subset of the 3 - 6 coordinates that are needed to describe the structure of the molecule of interest. The DMC wave functions are used to evaluate intensities using two approaches.

Infrared spectroscopy of the syn-methyl-substituted Criegee intermediate: A combined experimental and theoretical study.

An IR-vacuum ultraviolet (VUV) ion-dip spectroscopy method is utilized to examine the IR spectrum of acetaldehyde oxide (CH3CHOO) in the overtone CH stretch (2νCH) spectral region. IR activation creates a depletion of the ground state population that reduces the VUV photoionization signal on the parent mass channel. IR activation of the more stable and populated syn-CH3CHOO conformer results in rapid unimolecular decay to OH + vinoxy products and makes the most significant contribution to the observed spectrum.

Infrared Spectrum of the Pyrene Anion in the CH Stretching Region.

In this work, we report the infrared spectrum of the pyrene anion, measured using messenger tagging with up to three Ar atoms. We assign the spectrum using density functional theory and vibrational perturbation theory. We discuss our results in the context of computed and experimental spectra from the literature as well as recent observations from astronomical sources, addressing the question of whether polycyclic aromatic hydrocarbon anions could contribute to the strong infrared emission bands at 3.

Probing Ion-Receptor Interactions in Halide Complexes of Octamethyl Calix[4]Pyrrole.

Ion receptors are molecular hosts that bind ionic guests, often with great selectivity. The interplay of solvation and ion binding in anion host-guest complexes in solution governs the binding efficiency and selectivity of such ion receptors. To gain molecular-level insight into the intrinsic binding properties of octamethyl calix[4]pyrrole (omC4P) host molecules with halide guest ions, we performed cryogenic ion vibrational spectroscopy (CIVS) of omC4P in complexes with fluoride, chloride, and bromide ions.

Correlations between the Structures and Spectra of Protonated Water Clusters.

Badger's rule-like correlations between OH stretching frequencies and intensities and the OH bond length are used to develop a spectral mapping procedure for studies of pure and protonated water clusters. This approach utilizes the vibrationally averaged OH bond lengths, which were obtained from diffusion Monte Carlo simulations that were performed using the general potential developed by Yu and Bowman. Good agreement is achieved between the spectra obtained using this approach and previously reported spectra for H(HO) clusters, with = 3, 4, and 5, as well as their perdeuterated analogues.

Characterization of the Oxazolone and Macrocyclic Motifs in the b ( = 2-5) Product Ions from Collision-Induced Dissociation of Protonated Oligoglycine Peptides with Isomer-Selective, Cryogenic Vibrational Spectroscopy.

Collision-induced dissociation (CID) of small, protonated peptides leads to the formation of b-type fragment ions that can occur with several structural motifs driven by different covalent intramolecular bonding arrangements. Here, we characterize the so-called "oxazolone" and "macrocycle" ion structures that occur upon CID of oligoglycine peptides (G) ions ( = 2-6). This is determined by acquiring the vibrational band patterns of the cryogenically cooled, D-tagged ions obtained using isomer-selective, two-color IR-IR photobleaching and analyzing them with predicted (DFT) harmonic spectra for the candidate structures.

Exploring the Origins of the Intensity of the OH Stretch-HOH Bend Combination Band in Water.

While the intensity of the OH stretching fundamental transition is strongly correlated to hydrogen-bond strength, the intensity of the corresponding transition to the state with one quantum of excitation in both the OH stretching and HOH bending vibrations in the same water molecule shows a much weaker sensitivity to the hydrogen-bonding environment. The origins of this difference are explored through analyses of the contributions of terms in the expansion of the dipole moment to the calculated intensity. It is found that the leading contribution to the stretch-bend intensity involves the second derivative of the dipole moment with respect to the OH bond length and HOH angle.

Isotope Effects in the Zundel-Eigen Isomerization of H(HO).

The isomerization pathway between the energetically low-lying Zundel and Eigen isomers of the protonated water hexamer was investigated using high-level calculations including a treatment of zero-point corrections. On the basis of these calculations, the Zundel-Eigen isomerization was found to proceed through a stable intermediate isomer, which consists of a four-membered ring with two single acceptor water molecules. The inclusion of vibrational zero-point energy is shown to be important for accurately establishing the relative energies of the three relevant isomers involved in the Zundel-Eigen isomerization.

High-Resolution Photoelectron Spectroscopy of Vibrationally Excited Vinoxide Anions.

High-resolution photoelectron spectra of vibrationally pre-excited vinoxide anions (CHCHO) are reported using the recently developed IR-cryo-SEVI technique. This method is combined with a newly developed implementation of vibrational perturbation theory that can readily identify relevant anharmonic couplings among nearly degenerate vibrational states. IR-cryo-SEVI spectra are obtained by resonant infrared excitation of vinoxide anions via the fundamental C-O (ν, 1566 cm) or isolated C-H (ν, 2540 cm) stretching vibrations prior to photodetachment.

Observation of Slow Eigen-Zundel Interconversion in H(HO) Clusters upon Isomer-Selective Vibrational Excitation and Buffer Gas Cooling in a Cryogenic Ion Trap.

The formation of isomers when trapping floppy cluster ions in a temperature-controlled ion trap is a generally observed phenomenon. This involves collisional quenching of the ions initially formed at high temperature by buffer gas cooling until their internal energies fall below the barriers in the potential energy surface that separate them. Here we explore the kinetics at play in the case of the two isomers adopted by the H(HO) cluster ion that differ in the proton accommodation motif.

Isotope Effects on Ground and Excited States of Ethyl Cation, H(CH).

The structure and spectra of ethyl cation, H(CH), and its deuterated analogues are investigated using diffusion Monte Carlo (DMC). These calculations all show that the ground state wave function for H(CH) is localized near the minimum energy configuration in which the excess proton is in a bridging configuration, although the amplitude of the vibrational motions of the bridging proton is large. Deuteration of the bridging proton reduces the amplitude of this motion, while deuteration of only the ethylenic hydrogen atoms in H(CD) has little effect on the amplitude of the motion of the bridging proton.

A wave function correction-based approach to the identification of resonances for vibrational perturbation theory.

An approach for identifying resonances in vibrational perturbation theory calculations is introduced. This approach makes use of the corrections to the wave functions that are obtained from non-degenerate perturbation theory calculations to identify spaces of states that must be treated with degenerate perturbation theory. Pairs of states are considered to be in resonance if the magnitude of expansion coefficients in the corrections to the wave functions in the non-degenerate perturbation theory calculation is greater than a specified threshold, χ.

Exploring Expansions of the Potential and Dipole Surfaces Used for Vibrational Perturbation Theory.

A scheme for evaluating expansions of the potential and dipole moment surfaces for vibrational perturbation theory is described. The approach is based on numerical differentiation of the Hessian in the coordinates of interest. It is shown that performing these calculations in internal coordinates generates expansions that are transferable among isotopologues of the molecule of interest.

Character of the OH Bend-Stretch Combination Band in the Vibrational Spectra of the "Magic" Number HO(HO) and DO(DO) Cluster Ions.

The fundamental transitions that contribute to the diffuse OH stretching spectrum of water are known to increase in width and intensity with increasing red shift from the free OH frequency. In contrast, the profile of the higher-energy combination band involving the OH stretching and the intramolecular HOH bending modes displays a qualitatively different spectral shape with a much faster falloff on the lower-energy side. We elucidate the molecular origin of this difference by analyzing the shapes of the combination bands in the IR spectra of cryogenically cooled HO(HO) and DO(DO) clusters.

Fast Near Potential Energy Surfaces Using Machine Learning.

A machine-learning based approach for evaluating potential energies for quantum mechanical studies of properties of the ground and excited vibrational states of small molecules is developed. This approach uses the molecular-orbital-based machine learning (MOB-ML) method to generate electronic energies with the accuracy of CCSD(T) calculations at the same cost as a Hartree-Fock calculation. To further reduce the computational cost of the potential energy evaluations without sacrificing the CCSD(T) level accuracy, GPU-accelerated Neural Network Potential Energy Surfaces (NN-PES) are trained to geometries and energies that are collected from small-scale Diffusion Monte Carlo (DMC) simulations, which are run using energies evaluated using the MOB-ML model.

Electronic and mechanical anharmonicities in the vibrational spectra of the H-bonded, cryogenically cooled X · HOCl (X=Cl, Br, I) complexes: Characterization of the strong anionic H-bond to an acidic OH group.

We report vibrational spectra of the H-tagged, cryogenically cooled X · HOCl (X = Cl, Br, and I) ion-molecule complexes and analyze the resulting band patterns with electronic structure calculations and an anharmonic theoretical treatment of nuclear motions on extended potential energy surfaces. The complexes are formed by "ligand exchange" reactions of X · (HO) clusters with HOCl molecules at low pressure (∼10 mbar) in a radio frequency ion guide. The spectra generally feature many bands in addition to the fundamentals expected at the double harmonic level.

Preparation and Characterization of the Halogen-Bonding Motif in the Isolated Cl·IOH Complex with Cryogenic Ion Vibrational Spectroscopy.

In the presence of a halide ion, hypohalous acids can adopt two binding motifs upon formation of the ion-molecule complexes [XHOY] (X, Y = Cl, Br, I): a hydrogen (HB) bond to the acid OH group and a halogen (XB) bond between the anion and the acid halogen. Here we isolate the X-bonded Cl·IOH ion-molecule complex by collisions of I·(HO) clusters with HOCl vapor and measure its vibrational spectrum by IR photodissociation of the H-tagged complex. Anharmonic analysis of its vibrational band pattern reveals that formation of the XB complex results in dramatic lowering of the HOI bending fundamental frequency and elongation of the O-I bond (by 168 cm and 0.

Vibrational Signatures of HNO Acidity When Complexed with Microhydrated Alkali Metal Ions, M·(HNO)(HO) (M = Li, K, Na, Rb, Cs), at 20 K.

The speciation of strong acids like HNO under conditions of restricted hydration is an important factor in the rates of chemical reactions at the air-water interface. Here, we explore the trade-offs at play when HNO is attached to alkali ions (Li-Cs) with four water molecules in their primary hydration shells. This is achieved by analyzing the vibrational spectra of the M·(HNO)(HO) clusters cooled to about 20 K in a cryogenic photofragmentation mass spectrometer.

Exploring the Origins of Spectral Signatures of Strong Hydrogen Bonding in Protonated Water Clusters.

The effects of anharmonicity on the spectral features of strong ionic hydrogen bonds are explored through reduced dimensional studies of the couplings between the hydrogen bonding OH and the donor-acceptor OO stretching vibrations in protonated water clusters with 2-4 water molecules. Specifically, this study focuses on how the anharmonicities and couplings in these ions are reflected in the vibrational spectra by exploring the intensities of the transitions to states with excitation in both the OH and the OO stretching vibrations and changes in the frequency of the OO stretching vibration when the OH stretching vibration is excited. These questions are addressed through the application of several approximate treatments that are based on an adiabatic separation of the high-frequency OH and low-frequency OO stretching vibrations as well as low-order expansions of the potential and dipole surfaces.

A flexible approach to vibrational perturbation theory using sparse matrix methods.

A sparse linear algebra based implementation of Rayleigh-Schrödinger vibrational perturbation theory is presented. This implementation allows for flexibility in the coordinates used to expand the vibrational Hamiltonian as well as the order to which the perturbation theory is performed. It also provides a powerful tool for investigating the origin of spectral intensity and transition frequencies.

Infrared spectroscopic signature of a hydroperoxyalkyl radical (•QOOH).

Infrared (IR) action spectroscopy is utilized to characterize a prototypical carbon-centered hydroperoxyalkyl radical (•QOOH) transiently formed in the oxidation of volatile organic compounds. The •QOOH radical formed in isobutane oxidation, 2-hydroperoxy-2-methylprop-1-yl, •CH(CH)COOH, is generated in the laboratory by H-atom abstraction from tert-butyl hydroperoxide (TBHP). IR spectral features of jet-cooled and stabilized •QOOH radicals are observed from 2950 to 7050 cm at energies that lie below and above the transition state barrier leading to OH radical and cyclic ether products.