Identification of the dimethylamine-trimethylamine complex in the gas phase

Clusteromics I: Principles, Protocols, and Applications to Sulfuric Acid-Base Cluster Formation

We recently coined the term clusteromics as a holistic approach for obtaining insight into the chemical complexity of atmospheric molecular cluster formation and at the same time providing the foundation for thermochemical databases that can be utilized for developing machine learning models. Here, we present the first paper in the series that applies state-of-the-art computational methods to study multicomponent (SA)0-2(base)0-2 clusters, with SA = sulfuric acid and base = [ammonia (A), methylamine (MA), dimethylamine (DMA), trimethylamine (TMA), and ethylenediamine (EDA)] with all combinations of the five bases. The initial cluster configurations are obtained using the ABCluster program and the number of relevant configurations are reduced based on PM7 and ωB97X-D/6-31++G(d,p) calculations. Thermochemical parameters are calculated based on the ωB97X-D/6-31++G(d,p) cluster structures and vibrational frequencies using the quasi-harmonic approximation. The single-point energies are refined with a high-level DLPNO-CCSD(T0)/aug-cc-pVTZ calculation. Using the calculated thermochemical data, we perform kinetics simulations to evaluate the potential of these small (SA)0-2(base)0-2 clusters to grow into larger cluster sizes. In all cases we find that having more than one type of base molecule present in the cluster will increase the potential for forming larger clusters primarily due to the increased available vapor concentration.

Molecular Understanding of Solvents and Glycitein Interaction during Extraction

Hydrogen bonding interaction plays a crucial role in liquid systems. Methanol, ethanol, and acetone are the most commonly used solvents to extract isoflavones from soybeans. The structural and electronic properties of the molecular clusters of naturally occurring glycitein with solvents were investigated using the density functional theory method employing the B3LYP-D3/cc-pVTZ approach. The influence of the solvent was carried out by using the polarized continuum model (PCM). The geometry optimization, vibrational frequencies, and topological parameters have been assessed at the same level of theory. From the molecular structure and thermodynamic point of view, the most stable structures are formed by the interaction between the carbonyl group of glycitein and MeOH or EtOH. For acetone-glycitein, the strongest interaction is formed by the interaction of the hydroxyl group of glycitein with the carbonyl group of acetone. All the hydrogen bonds in the MeOH/EtOH/acetone-glycitein complexes are closed-shell interactions. This study can help increase the efficiency of extraction.

The infrared study of fluoroform + methyl fluoride mixtures in argon and nitrogen matrices. Evidence of nonlinear blue-shifting complex formation

The FTIR spectra of fluoroform + methyl fluoride mixtures trapped in argon and nitrogen matrices are studied at T~10-30 K. Spectroscopic changes observed in the region of the CH stretching mode of fluoroform are typical for weak blue shifting H - bonds of CH⋯F type. The degeneracy lifting effect found on E - type bands of fluoroform interacted with methyl fluoride suggests the complex formation of a nonlinear form. The experimental results are confirmed by ab initio calculations of fluoroform + methyl fluoride based on the second order Møller-Plesset theory of perturbations utilizing advanced basis set. Nonlinear complexes are stabilized by the basic CH⋯F interaction and additionally by van der Waals-type CD⋯FC contacts between deuterated methyl fluoride and fluoroform.

Hydrogen bond docking preference in furans: OH⋯π vs. OH⋯O

The docking sites of hydrogen bonds in complexes formed between 2,2,2-trifluoroethanol (TFE), furan (Fu), and 2-methyl furan (MF) have been investigated. Using density functional theory (DFT) calculations, gas phase and matrix isolation FTIR spectroscopies, the strengths of OH⋯O and OH⋯π hydrogen bonds in the complexes were compared to find the docking preference. Calculations suggest that the hydrogen bond donor, TFE, is more likely to dock onto the oxygen atom of the aromatic furans ring, and consequently, the OH⋯O type hydrogen bond is relatively stronger than the OH⋯π type. The FTIR spectrum in the OH-stretching fundamental range obtained at room temperatures has been compared with that obtained at extremely low temperatures in the matrix. The fundamental and the red shifts of OH-stretching vibrations were observed in both FTIR spectra, confirming the formation of hydrogen bonded complexes. By assessing the ability of furan and MF to participate in the formation of OH⋯O hydrogen bond, the effect of ring methylation has been highlighted. From the calculated geometric and thermodynamic parameters as well as the frequency shift of the OH-stretching vibrations in complexes, TFE-MF is found to be more stable than TFE-Fu, which suggests that the strength of the OH⋯O hydrogen bond in TFE-MF originates from the high activity of the furan molecule caused by the methylation of the aromatic ring. The present study furthers the knowledge of docking preference in heteroaromatic molecules and is helpful to understand the nature of intermolecular interactions between hydrogen bond donors and acceptors, including both electron-deficient atoms and π cloud.

Environment-dependent conformation investigation of 3-amino-1,2,4-triazole (3-AT): Raman Spectroscopy and density functional theory

In the paper, diverse tautomers of 3-amino-1,2,4-triazole (3AT) in solid and polar solvent have been explored by FT-IR, FT-Raman and 488nm Raman experiments combing with quantum chemical theoretical calculation using PCM solvent model and normal mode analysis. The vibrational spectra prefer the 3-amino-1,2,4-2H-triazole (2H-3AT) dimer in solid, while in a polar solvent 3AT is apt to the 3-amino-1,2,4-2H-triazole (2H-3AT) monomer. The significant wavenumber difference and Raman intensity patterns in solid and different solvents are induced by hydrogen bond perturbation along >NH⋯N≤ hydrogen bonds on five-membered N-heterocyclic ring. The ground state proton transfer reaction mechanism along the five-membered N-heterocyclic ring is supported by intermolecular hydrogen bonding between 3AT and protonic solvent molecules.

Ring-Size Effects on the Stability and Spectral Shifts of Hydrogen Bonded Cyclic Ethers Complexes

In order to investigate ring-size effects on the stability and spectral shifts of hydrogen bonded cyclic ethers complexes, the strength of hydrogen bonds in gas phase complexes formed between 2,2,2-trifluoroethanol (TFE) and selected cyclic ethers were examined using FTIR spectroscopy. TFE was chosen as hydrogen bond donor in these complexes, while trimethylene oxide (TMO), tetrahydrofuran (THF) and tetrahydropyran (THP) were selected as hydrogen bond acceptors. Comparable OH-stretching red shifts were observed in the three kinds of complexes. The difference of red shifts is so small (<7 cm⁻¹) for TFE−TMO/THF/THP complexes that one can conclude that their stabilities and the strength of the hydrogen bonds are nearly similar and do not show any marked dependence with the ring size of the hydrogen bond acceptor. The equilibrium constants for the complexation were determined, and atoms-in-molecules (AIM) and natural bond orbital (NBO) analyses were performed to further investigate the intermolecular interactions. Regardless of the ring size, hydrogen bonds in the complexes showed similar strength, in agreement with the observed OH-stretching red shifts.

The Influence of the Position of the Double Bond and Ring Size on the Stability of Hydrogen Bonded Complexes

To study the influence of the position of the double bond and ring size on the stability of hydrogen bonded complexes, the 1:1 complexes formed between 2,2,2-trifluoroethanol (TFE) and three heterocyclic compounds including 2,3-dihydrofuran (2,3-DHF), 2,5-dihydrofuran (2,5-DHF) and 3,4-dihydropyran (3,4-DHP) were investigated systematically. The formation of hydrogen bonded TFE−2,3-DHF, TFE−2,5-DHF and TFE−3,4-DHP complexes were identified by gas phase FTIR spectroscopy at room temperature, and the OH-stretching fundamental transition of TFE was red shifted upon complexation. The competition between the O atom and π-electrons bonding sites within the complexes was studied, and the O−H···π type hydrogen bond was found to be less stable than the O−H···O in all three cases. The observed red shifts of the OH-stretching fundamental transitions in the complexes were attributed to the formation of O−H···O hydrogen bond. Equilibrium constants of the complexation reactions were determined from measured and calculated OH-stretching fundamental intensities. Both theoretical calculations and experimental results reveal that the hydrogen bond strengths in the complexes follow the sequence: TFE−2,5-DHF > TFE−2,3-DHF ≈ TFE−3,4-DHP, thus the position of the double bond exerts significantly larger influence than ring size on the stability of the selected hydrogen bonded complexes.

Hydrogen bond docking site competition in methyl esters

The OH⋯O hydrogen bonds in the 2,2,2-trifluoroethanol (TFE)-methyl ester complexes in the gas phase have been investigated by FTIR spectroscopy and DFT calculations. Methyl formate (MF), methyl acetate (MA), and methyl trifluoroacetate (MTFA) were chosen as the hydrogen bond acceptors. A dominant inter-molecular hydrogen bond was formed between the OH group of TFE and different docking sites in the methyl esters (carbonyl oxygen or ester oxygen). The competition of the two docking sites decides the structure and spectral properties of the complexes. On the basis of the observed red shifts of the OH-stretching transition with respect to the TFE monomer, the order of the hydrogen bond strength can be sorted as TFE-MA (119cm^-1)>TFE-MF (93cm^-1)>TFE-MTFA (44cm^-1). Combining the experimental infrared spectra with the DFT calculations, the Gibbs free energies of formation were determined to be 1.5, 4.5 and 8.6kJmol^-1 for TFE-MA, TFE-MF and TFE-MTFA, respectively. The hydrogen bonding in the MTFA complex is much weaker than those of the TFE-MA and TFE-MF complexes due to the effect of the CF₃ substitution on MTFA, while the replacement of an H atom with a CH₃ group in methyl ester only slightly increases the hydrogen bond strength. Topological analysis and localized molecular orbital energy decomposition analysis was also applied to compare the interactions in the complexes.

Basis set convergence of the binding energies of strongly hydrogen-bonded atmospheric clusters

We present the first binding energy benchmark set at the CBS limit of strongly hydrogen bonded atmospheric molecular clusters.

Matrix isolation FTIR study of hydrogen-bonded complexes of methanol with heterocyclic organic compounds

Hydrogen bonded complexes of heterocyclic compounds with methanol were studied using matrix isolation FTIR spectroscopy and theoretical calculations.

Weak hydrogen bonding competition between O–H⋯π and O–H⋯Cl

The weak hydrogen bonding competition between O–H⋯π and O–H⋯Cl has been studied using FTIR spectroscopy and theoretical calculations.

Spectroscopy and dynamics of double proton transfer in formic acid dimer

We present the isolated gas phase infrared spectra of formic acid dimer, (HCOOH)2, and its deuterated counterpart formic-d acid, (DCOOH)2, at room temperature. The formic acid dimer spectrum was obtained by spectral subtraction of a spectrum of formic acid vapor recorded at low pressure from that recorded at a higher pressure. The spectra of formic acid vapor contain features from both formic acid monomer and formic acid dimer, but at low and high pressures of formic acid, the equilibrium is pushed towards the monomer and dimer, respectively. A similar approach was used for the formic-d acid dimer. Building on the previous development of the Molecular Mechanics with Proton Transfer (MMPT) force field for simulating proton transfer reactions, molecular dynamics (MD) simulations were carried out to interpret the experimental spectra in the OH-stretching region. Within the framework of MMPT, a combination of symmetric single and double minimum potential energy surfaces (PESs) provides a good description of the double proton transfer PES. In a next step, potential morphing together with electronic structure calculations at the B3LYP and MP2 level of theory was used to align the computed and experimentally observed spectral features in the OH-stretching region. From this analysis, a barrier for double proton transfer between 5 and 7 kcal mol(-1) was derived, which compares with a CCSD(T)/aug-cc-pVTZ calculated barrier of 7.9 kcal mol(-1). Such a combination of experimental and computational techniques for estimating barriers for proton transfer in gas phase systems is generic and holds promise for further improved PESs and energetics of these important systems. Additional MD simulations at the semi-empirical DFTB level of theory agree quite well for the center band position but underestimate the width of the OH-stretching band.

Infrared spectroscopic probing of dimethylamine clusters in an Ar matrix

Amines have many atmospheric sources and their clusters play an important role in aerosol nucleation processes. Clusters of a typical amine, dimethylamine (DMA), of different sizes were measured with matrix isolation IR (infrared) and NIR (near infrared) spectroscopy. The NIR vibrations are more separated and therefore it is easier to distinguish different sizes of clusters in this region. The DMA clusters, up to DMA tetramer, have been optimized using density functional methods, and the geometries, binding energies and thermodynamic properties of DMA clusters were obtained. The computed frequencies and intensities of NH-stretching vibrations in the DMA clusters were used to interpret the experimental spectra. We have identified the fundamental transitions of the bonded NH-stretching vibration and the first overtone transitions of the bonded and free NH-stretching vibration in the DMA clusters. Based on the changes in vibrational intensities during the annealing processes, the growth of clusters was clearly observed. The results of annealing processes indicate that DMA molecules tend to form larger clusters with lower energies under matrix temperatures, which is also supported by the calculated reaction energies of cluster formation.

Accurate thermodynamic properties of gas phase hydrogen bonded complexes

The increase in temperature causes a decrease in equilibrium constant, which makes accurate determination of the enthalpy of complex formation possible.

Hydrogen bonding in alcohol–ethylene oxide and alcohol–ethylene sulfide complexes

The O–H⋯O and O–H⋯S hydrogen bonds are of similar strength in the corresponding alcohol–EO and alcohol–ES complexes.

Gas Phase Detection of the NH-P Hydrogen Bond and Importance of Secondary Interactions

We have observed the NH···P hydrogen bond in a gas phase complex. The bond is identified in the dimethylamine-trimethylphosphine complex by a red shift of the fundamental NH-stretching frequency observed using Fourier transform infrared spectroscopy (FT-IR). On the basis of the measured NH-stretching frequency red shifts, we find that P is a hydrogen bond acceptor atom similar in strength to S. Both are stronger acceptors than O and significantly weaker acceptors than N. The hydrogen bond angle, ∠NHP, is found to be very sensitive to the functional employed in density functional theory (DFT) optimizations of the complex and is a possible parameter to assess the quality of DFT functionals. Natural bonding orbital (NBO) energies and results from the topological methods atoms in molecules (AIM) and noncovalent interactions (NCI) indicate that the sensitivity is caused by the weakness of the hydrogen bond compared to secondary interactions. We find that B3LYP favors the hydrogen bond and M06-2X favors the secondary interactions leading to under- and overestimation, respectively, of the hydrogen bond angle relative to a DF-LCCSD(T)-F12a calculated angle. The remaining functionals tested, B3LYP-D3, B3LYP-D3BJ, CAM-B3LYP, and ωB97X-D, as well as MP2, show comparable contributions from the hydrogen bond and the secondary interactions and are close to DF-LCCSD(T)-F12a results.

Benchmarking ab initio binding energies of hydrogen-bonded molecular clusters based on FTIR spectroscopy

Models of formation and growth of atmospheric aerosols are highly dependent on accurate cluster binding energies. These are most often calculated by ab initio electronic structure methods but remain associated with significant uncertainties. We present a computational benchmarking study of the Gibbs free binding energies in molecular complexes and clusters based on gas phase FTIR spectroscopy. The acetonitrile-HCl molecular complex is identified via its redshifted H-Cl stretching vibrational mode. We determine the Gibbs free binding energy, ΔG°295 K, to between 4.8 and 7.9 kJ mol(-1) and compare this range to predictions from several widely used electronic structure methods, including five density functionals, Møller-Plesset perturbation theory, and five coupled cluster methods up to CCSDT quality, considering also the D3 dispersion correctional scheme. With some exceptions, we find that most electronic structure methods overestimate ΔG°295 K. The effects of vibrational anharmonicity is approximated using scaling factors, reducing ΔG°295 K by ca. 1.8 kJ mol(-1), whereby ΔG°295 K predictions well within the experimental range can be obtained.

The effect of fluorine substitution in alcohol–amine complexes

The effect of fluorine substitution on the hydrogen bond strength in alcohol–amine molecular complexes was investigated, with a combination of vapour phase infrared and near infrared spectroscopy and theoretical calculations.

Resolving the anomalous infrared spectrum of the MeCN–HCl molecular cluster using ab Initio molecular dynamics

Molecular dynamics simulations reveal that the asymmetric peak seen in IR spectra of acetonitrile–HCl molecular complexes are due to high population of complexes with partially broken hydrogen bonds.

The gas-phase reaction of methane sulfonic acid with the hydroxyl radical without and with water vapor

The gas phase reaction between methane sulfonic acid (CH3SO3H; MSA) and the hydroxyl radical (HO), without and with a water molecule, was investigated with DFT-B3LYP and CCSD(T)-F12 methods. For the bare reaction we have found two reaction mechanisms, involving proton coupled electron transfer and hydrogen atom transfer processes that produce CH3SO3 and H2O. We also found a third reaction mechanism involving the double proton transfer process, where the products and reactants are identical. The computed rate constant for the oxidation process is 8.3 × 10(-15) cm(3) s(-1) molecule(-1). CH3SO3H forms two very stable complexes with water with computed binding energies of about 10 kcal mol(-1). The presence of a single water molecule makes the reaction between CH3SO3H and HO much more complex, introducing a new reaction that consists in the interchange of H2O between HO and CH3SO3H. Our kinetic calculations show that 99.5% of the reaction involves this interchange of the water molecule and, consequently, water vapor does not play any role in the oxidation reaction of methane sulfonic acid by the hydroxyl radical.

Fundamental and overtone vibrational spectroscopy, enthalpy of hydrogen bond formation and equilibrium constant determination of the methanol-dimethylamine complex

We have measured gas phase vibrational spectra of the bimolecular complex formed between methanol (MeOH) and dimethylamine (DMA) up to about 9800 cm(-1). In addition to the strong fundamental OH-stretching transition we have also detected the weak second overtone NH-stretching transition. The spectra of the complex are obtained by spectral subtraction of the monomer spectra from spectra recorded for the mixture. For comparison, we also measured the fundamental OH-stretching transition in the bimolecular complex between MeOH and trimethylamine (TMA). The enthalpies of hydrogen bond formation (ΔH) for the MeOH-DMA and MeOH-TMA complexes have been determined by measurements of the fundamental OH-stretching transition in the temperature range from 298 to 358 K. The enthalpy of formation is found to be -35.8 ± 3.9 and -38.2 ± 3.3 kJ mol(-1) for MeOH-DMA and MeOH-TMA, respectively, in the 298 to 358 K region. The equilibrium constant (Kp) for the formation of the MeOH-DMA complex has been determined from the measured and calculated transition intensities of the OH-stretching fundamental transition and the NH-stretching second overtone transition. The transition intensities were calculated using an anharmonic oscillator local mode model with dipole moment and potential energy curves calculated using explicitly correlated coupled cluster methods. The equilibrium constant for formation of the MeOH-DMA complex was determined to be 0.2 ± 0.1 atm(-1), corresponding to a ΔG value of about 4.0 kJ mol(-1).