Novel model for calculating the intermolecular part of the infrared spectrum for molecular complexes

Perspectives on the Lindman Hypothesis and Cellulose Interactions

In the history of cellulose chemistry, hydrogen bonding has been the predominant explanation when discussing intermolecular interactions between cellulose polymers. This is the general consensus in scholarly textbooks and in many research articles, and it applies to several other biomacromolecules' interactions as well. This rather unbalanced description of cellulose has likely impacted the development of materials based on the processing of cellulose-for example, via dissolution in various solvent systems and regeneration into solid materials, such as films and fibers, and even traditional wood fiber handling and papermaking. In this review, we take as a starting point the questioning of the general description of the nature of cellulose and cellulose interactions initiated by Professor Björn Lindman, based on generic physicochemical reasoning about surfactants and polymers. This dispute, which became known as "the Lindman hypothesis", highlights the importance of hydrophobic interactions in cellulose systems and that cellulose is an amphiphilic polymer. This paper elaborates on Björn Lindman's contribution to the subject, which has caused the scientific community to revisit cellulose and reconsider certain phenomena from other perspectives.

Thermodynamic and optical properties of HCOOH(H2O)n and HCOOH(NH3)(H2O)(n-1) clusters at various temperatures and pressures: a computational study

Density functional theory has been used to compute the gas-phase geometries, binding energies, ZPE-corrected binding energies, BSSE-corrected binding energies, binding enthalpies, and binding free energies of HCOOH(H₂O)_n and HCOOH(NH₃)(H₂O)_(n-1) clusters with n = 1-8, 10, 12, 14, 16, 18, and 20. Enthalpies and free energies are calculated for a range of atmospherically relevant temperatures (T) and pressures (P) (from T = 298.15 K, P = 1013.25 hPa to T = 216.65 K, P = 226.32 hPa). The optical properties of those clusters have been studied at the CAM-B3LYP/aug-cc-pVDZ level of theory.

High level ab initio investigation of the catalytic effect of water on formic acid decomposition and isomerization

Formic acid (FA) is a ubiquitous molecule found in the atmosphere, and is relevant to many important processes. The FA molecule generally exists as the trans isomer, which can decompose into H2O and CO (dehydration). It can also exist in the less favorable cis isomer which can decompose into H2 and CO2 (decarboxylation). Our work examines the complexes formed between each isomer of FA with water. We present geometries and vibrational frequencies obtained at the reliable CCSD(T)/aug-cc-pVTZ level of theory for seven FAwater complexes. We utilize the focal point method to determine CCSDT(Q)/CBS plus corrections binding energies of 7.37, 3.36, and 2.02 kcal mol-1 plus 6.07, 3.79, 2.60, and 2.55 kcal mol-1 for the trans-FAwater and cis-FAwater complexes, respectively. Natural bond orbital analysis is used to further decompose the interactions in each complex and gain insight into their relative strengths. Furthermore, we examine the effect that a single water molecule has on the barrier heights to each decomposition pathway by optimizing the transition states and verifying their connectivity with intrinsic reaction coordinate computations as well as utilizing a kinetic model. Water lowers the barrier to dehydration by at most 15.78 kcal mol-1 and the barrier to decarboxylation by up to 15.90 kcal mol-1. Our research also examines for the first time the effect of one water molecule on the interconversion barrier and we find that the barrier from trans to cis is not catalyzed by water due to the strong FA and water interactions. Our results highlight some instances where different binary complexes result in different decomposition pathways and even a case where one binary complex can form the same decomposition products via two distinct mechanisms. Our results provide a reliable benchmark of the FAH2O system as well as provide insight into future studies of similar atmospheric systems.

Infrared spectroscopic characterization of hydrogen-bonded propylene oxide − ethanol and propylene oxide − 2-fluoroethanol complexes isolated in solid neon matrices

The infrared absorption spectra of hydrogen-bonded complexes of propylene oxide with either ethanol or 2-fluoroethanol have been recorded in neon matrices. Mixtures of propylene oxide and ethanol or propylene oxide and 2-fluoroethanol vapors were mixed with an excess of neon gas and deposited onto a KBr substrate at 4.2 K. The results indicate that hydrogen-bonded complexes were formed with propylene oxide as the hydrogen bond acceptor and either ethanol or 2-fluoroethanol as the hydrogen bond donors. The features assigned to the O−H stretch were red-shifted by 175 and 193 cm−1 for the ethanol- and 2-fluoroethanol-containing complexes, respectively. The difference in red shifts can be accounted for due to the greater acidity of 2-fluroethanol. Deuterium isotope experiments were conducted to help confirm the assignment of the O–H stretch for the complexes. As well, structures and infrared spectra were calculated using B3LYP/6-311++G(2d,2p) calculations and were used to compare with the experimental spectra. A “scaling equation” rather than a scaling factor was used and is shown to greatly increase the utility of the calculations when comparing with experimental spectra. An examination of the O–H stretching red shifts for many hydrogen-bound complexes reveals a relationship between the shift and the difference between the acidity of the hydrogen bond donor and the basicity of the hydrogen bond acceptor (the enthalpy of proton transfer). Both hydrogen-bonded complexes and proton-bound complexes appear to have a maximum in the reduced frequency value that corresponds to complexes where the hydrogen/proton are equally shared between the two bases.

AN AB INITIO MOLECULAR DYNAMICS STUDY ON THE SOLVATION OF FORMATE ION AND FORMIC ACID IN WATER

Formate ion and formic acid are linked in water by the equilibrium for the acidic dissociation of formic acid, which as the simplest carboxylic acid is an important model system. In this study, the microscopic details of the solvation around a formate ion and around a formic acid molecule in aqueous solution are explored by ab initio molecular dynamics simulations, at 300, 500, 700, and 900 K. The formate ion exerts a strong influence on the surrounding solvent molecules by hydrogen bonding, which restricts the access of other water molecules. With rising temperature, the hydrogen bonds are disrupted, and the space around formic acid becomes more accessible. Solvation of the formic acid is marked by its partial dissociation to produce a proton, and the hydrogen bond interaction around a formic acid is not as strong as that around a formate ion. The acidic dissociation becomes less favorable as temperature rises, which indicates a lesser catalytic role for the water molecules in the thermal dissociation of formic acid.

Conformational analysis of the chemical shifts for molecules containing diastereotopic methylene protons

Quantum chemistry SCF/GIAO calculations were carried out on a set of compounds containing diastereotopic protons. Five molecules, including recently synthesized 1,3-di(2,3-epoxypropoxy)benzene, containing the chiral or pro-chiral center and the neighboring methylene group, were chosen. The rotational averages (i.e. normalized averages with respect to the rotation about the torsional angle τ with the exponential energy weight at temperature T) calculated individually for each of the methylene protons in 1,3-di(2,3-epoxypropoxy)benzene differ by ca. 0.6 ppm, which is significantly less than the value calculated for the lowest energy conformer. This value turned out to be low enough to guarantee the proper ordering of theoretical chemical shifts, supporting the interpretation of the (1)H NMR spectrum of this important compound. The rotational averages of chemical shifts for methylene protons for a given type of conformer are shown to be essentially equal to the Boltzmann averages (here, the population-weighted averages for the individual conformers representing minima on the E(τ) cross-section). The calculated Boltzmann averages in the representative conformational space may exhibit completely different ordering as compared to the chemical shifts calculated for the lowest-energy conformer. This is especially true in the case of molecules, for which no significant steric effects are present. In this case, only Boltzmann averages account for the experimental pattern of proton signals. In addition, better overall agreement with experiment (lower value of the root-mean-square deviation between calculated and measured chemical shifts) is typically obtained when Boltzmann averages are used.

Toward spectroscopic accuracy for open-shell systems: molecular structure and hyperfine coupling constants of H2CN, H2CP, NH2, and PH2 as test cases

In the present paper, we investigate the molecular structure and hyperfine couplings of a series of σ radicals containing first- and second-row atoms (H(2)CN,H(2)CP,NH(2),PH(2)) for which accurate gas-phase microwave results are available. The presence of α- and, especially, β-hydrogen atoms makes the evaluation of magnetic properties of these radicals particularly challenging. Geometrical parameters have been computed by the coupled-cluster ansatz in conjunction with hierarchical series of basis sets, thus accounting for extrapolation to the complete basis-set limit. Core correlation as well as higher excitations in the electronic-correlation treatment have also been taken into account. An analogous approach has been employed for evaluating hyperfine coupling constants with particular emphasis given to basis-set, correlation, and geometrical effects. The corresponding vibrational corrections, required for a meaningful comparison to experimental data, have also been investigated. The remarkable agreement with experiment confirms the reliability of the present computational approach, already validated for π radicals, thus establishing the way for setting up a benchmark database for magnetic properties.

Properties of liquid water from a systematic refinement of a high-rank multipolar electrostatic potential

We build on previous work [S. Y. Liem and P. L. A. Popelier, J. Chem. Theory Comput. 4, 353 (2008)], where for the first time, a high-rank multipolar electrostatic potential was used in molecular dynamics simulations of liquid water at a wide range of pressures and temperatures, and using a multipolar Ewald summation. Water is represented as a rigid body, with atomic multipole moments defined by quantum chemical topology partitioning its gas phase electron density. The effect of the level of theory on the local structure of liquid water is systematically addressed. Values for Lennard-Jones (LJ) parameters are optimized, for both oxygen and hydrogen atoms, against bulk properties. The best LJ parameters were then used in a set of simulations at 30 different temperatures (1 atm) and another set at 11 different pressures (at 298 K). Inclusion of the hydrogen LJ parameters significantly increases the self-diffusion coefficient. The behavior of bulk properties was studied and the local water structure analyzed by both radial and spatial distribution functions. Comparisons with familiar point-charge potentials, such as TIP3P, TIP4P, TIP5P, and simple point charge, show the benefits of multipole moments.

Microsolvation of the formic acid dimer — (HCOOH)2(H2O)n clusters with n = 1, . . ., 5

Density functional theory and spin-component-scaled Møller–Plesset perturbation theory calculations are used to examine the microsolvation of the formic acid dimer. The lowest energy structures with n water molecules consist of a n-water cluster, not necessarily of lowest energy, with two formic acid molecules attached to its surface by hydrogen bonds. The total number of hydrogen bonds does not correlate directly with relative stability.

Analysis of Nuclear Quantum Effects on Hydrogen Bonding

The impact of nuclear quantum effects on hydrogen bonding is investigated for a series of hydrogen fluoride (HF)n clusters and a partially solvated fluoride anion, F-(H2O). The nuclear quantum effects are included using the path integral formalism in conjunction with the Car-Parrinello molecular dynamics (PICPMD) method and using the second-order vibrational perturbation theory (VPT2) approach. For the HF clusters, a directional change in the impact of nuclear quantum effects on the hydrogen-bonding strength is observed as the clusters evolve toward the condensed phase. Specifically, the inclusion of nuclear quantum effects increases the F-F distances for the (HF)n=2-4 clusters and decreases the F-F distances for the (HF)n>4 clusters. This directional change occurs because the enhanced electrostatic interactions between the HF monomers become more dominant than the zero point energy effects of librational modes as the size of the HF clusters increases. For the F-(H2O) system, the inclusion of nuclear quantum effects decreases the F-O distance and strengthens the hydrogen bonding interaction between the fluoride anion and the water molecule because of enhanced electrostatic interactions. The vibrationally averaged 19F shielding constant for F-(H2O) is significantly lower than the value for the equilibrium geometry, indicating that the electronic density on the fluorine decreases as a result of the quantum delocalization of the shared hydrogen. Deuteration of this system leads to an increase in the vibrationally averaged F-O distance and nuclear magnetic shielding constant because of the smaller degree of quantum delocalization for deuterium.

Hydrogen Bonding between Formic Acid and Water: Complete Stabilization of the Intrinsically Unstable Conformer

We studied hydrogen bonding between formic acid (FA) and water in solid argon and identified the first water complex with the higher-energy conformer cis-FA. In sharp contrast to cis-FA monomer, cis-FA interacting with water is very stable at low temperatures, which was explained by strong O-H...O hydrogen bonding. These benchmark results show that hydrogen bonding can terminate proton tunneling reactions and efficiently stabilize intrinsically unstable conformational structures in complex asymmetrical hydrogen-bonded networks. This general effect occurs when the energy difference between conformers is smaller than the hydrogen bond interaction energy, which opens perspectives in chemistry on intrinsically unstable conformers.

Hydrogen Bonding in Substituted Formic Acid Dimers

The hydrogen-bonded dimers of formic acid derivatives XCOOH (X = H, F, Cl, and CH3) have been investigated using density functional theory (B3LYP) and second-order Møller-Plesset perturbation (MP2) methods, with the geometry optimization carried out using 6-311++G(2d,2p) basis set. The dimerization energies calculated using aug-cc-pVXZ (with X = D and T) basis have been extrapolated to infinite basis set limit using the standard methodology. The results indicate that the fluorine-substituted formic acid dimer is the most stable one in comparison to the others. Topological analysis carried out using Bader's atoms in molecules (AIM) theory shows good correlation of the values of electron density and its Laplacian at the bond critical points (BCP) with the hydrogen bond length in the dimers. Natural bond orbital (NBO) analysis carried out to study the charge transfer from the proton acceptor to the antibonding orbital of the X-H bond in the complexes reveals that most of the dimers are associated with conventional H-bonding except a few, where improper blue-shifting hydrogen bonds are found to be present.

Solvent Effects on the Photodissociation of Formic Acid: A Theoretical Study

Photodissociation of aqueous formic acid has been investigated with the CASSCF, DFT, and MR-CI methods. Solvent effects are considered as a combination of the hydrogen-bonding interaction from explicit H2O molecules and the effects from the bulk surrounding H2O molecules using the polarizable continuum model. It is found that the hydrogen-bonding effect from the explicit water in the complex is the major factor to influence properties of aqueous formic acid, while the bulk surrounding H2O molecules has a noticeable influence on the structures of the complex. The direct C-O bond fission along the S1 pathway is predicted to be an important channel upon photolysis of aqueous formic acid at 200 nm, which is consistent with experimental observation that aqueous formic acid dissociates predominantly into fragments of HCO and OH. The existence of a dark channel upon photolysis of aqueous formic acid at 200 nm is assigned as fast relaxation from the S1 Franck-Condon geometry to the T1/S1 intersection and subsequent S1-->T1 intersystem crossing process. S1-->S0 internal conversion followed by molecular elimination to CO+H2O is the most probable primary process for formation of carbon monoxide, which was observed with considerable yield upon photolysis of aqueous formic acid at 253.7 nm.

Quantum Monte Carlo calculations of the dissociation energy of the water dimer

We report diffusion quantum Monte Carlo (DMC) calculations of the equilibrium dissociation energy D(e) of the water dimer. The dissociation energy measured experimentally, D(0), can be estimated from D(e) by adding a correction for vibrational effects. Using the measured dissociation energy and the modern value of the vibrational energy Mas et al., [J. Chem. Phys. 113, 6687 (2000)] leads to D(e)=5.00+/-0.7 kcal mol(-1), although the result Curtiss et al., [J. Chem. Phys. 71, 2703 (1979)] D(e)=5.44+/-0.7 kcal mol(-1), which uses an earlier estimate of the vibrational energy, has been widely quoted. High-level coupled cluster calculations Klopper et al., [Phys. Chem. Chem. Phys. 2, 2227 (2000)] have yielded D(e)=5.02+/-0.05 kcal mol(-1). In an attempt to shed new light on this old problem, we have performed all-electron DMC calculations on the water monomer and dimer using Slater-Jastrow wave functions with both Hartree-Fock approximation (HF) and B3LYP density functional theory single-particle orbitals. We obtain equilibrium dissociation energies for the dimer of 5.02+/-0.18 kcal mol(-1) (HF orbitals) and 5.21+/-0.18 kcal mol(-1) (B3LYP orbitals), in good agreement with the coupled cluster results.

Impact of Nuclear Quantum Effects on the Molecular Structure of Bihalides and the Hydrogen Fluoride Dimer

The structural impact of nuclear quantum effects is investigated for a set of bihalides, [XHX](-), X = F, Cl, and Br, and the hydrogen fluoride dimer. Structures are calculated with the vibrational self-consistent-field (VSCF) method, the second-order vibrational perturbation theory method (VPT2), and the nuclear-electronic orbital (NEO) approach. In the VSCF and VPT2 methods, the vibrationally averaged geometries are calculated for the Born-Oppenheimer electronic potential energy surface. In the NEO approach, the hydrogen nuclei are treated quantum mechanically on the same level as the electrons, and mixed nuclear-electronic wave functions are calculated variationally with molecular orbital methods. Electron-electron and electron-proton dynamical correlation effects are included in the NEO approach using second-order perturbation theory (NEO-MP2). The nuclear quantum effects are found to alter the distances between the heavy atoms by 0.02-0.05 A for the systems studied. These effects are of similar magnitude as the electron correlation effects. For the bihalides, inclusion of the nuclear quantum effects with the NEO-MP2 or the VSCF method increases the X-X distance. The bihalide X-X distances are similar for both methods and are consistent with two-dimensional grid calculations and experimental values, thereby validating the use of the computationally efficient NEO-MP2 method for these types of systems. For the hydrogen fluoride dimer, inclusion of nuclear quantum effects decreases the F-F distance with the NEO-MP2 method and increases the F-F distance with the VSCF and VPT2 methods. The VPT2 F-F distances for the hydrogen fluoride dimer and the deuterated form are consistent with the experimentally determined values. The NEO-MP2 F-F distance is in excellent agreement with the distance obtained experimentally for a model that removes the large amplitude bending motions. The analysis of these calculations provides insight into the significance of electron-electron and electron-proton correlation, anharmonicity of the vibrational modes, and nonadiabatic effects for hydrogen-bonded systems.

Intermolecular Vibrations of the Water Trimer, a Matrix Isolation Study

Infrared spectra from 25 to 4000 cm(-1) have been recorded of water (H2O, D2O and H218O) matrix isolated in neon, argon, and krypton matrices. Intermolecular absorption bands of different isotopologues of the water trimer and tetramer have been assigned from concentration dependencies and diffusion behavior, using the well-known mid-infrared trimer and tetramer absorption bands as measures of the trimer and tetramer concentrations. The results are compared to ab initio calculations.

Investigation of isotope effects with the nuclear-electronic orbital approach

This paper addresses fundamental issues that arise in the application of the nuclear-electronic orbital (NEO) approach to systems with equivalent quantum nuclei. Our analysis illustrates that Hartree-Fock nuclear wave functions do not provide physically reasonable descriptions of systems comprised of equivalent low-spin fermions or equivalent bosons. The physical basis for this breakdown is that the ionic terms dominate due to the localized nature of the nuclear orbitals. Multi-configurational wave functions that include only covalent terms provide physically reasonable descriptions of these types of systems. The application of the NEO approach to a variety of chemical systems is presented to elucidate the isotope effects on the geometries and electronic wave functions. Deuteration of hydrogen halides, water, ammonia, and hydronium ion decreases the bond length and the magnitude of negative partial atomic charge on the heavy atom. These results are consistent with experimental spectroscopic data. Deuteration at the beta position for formate anion and a series of amines increases the magnitude of negative partial atomic charge on the protonation site for the unprotonated species. This observation is consistent with the experimentally observed increase in basicity upon deuteration at the beta position for carboxylic acids and amines.

Noncovalent Complexes between Dimethyl Ether and Formic Acid-An Ab Initio and Matrix Isolation Study

The complexes formed by noncovalent interactions between formic acid and dimethyl ether are investigated by ab initio methods and characterized by matrix isolation spectroscopy. Six complexes with binding energies between -2.26 and -7.97 kcal mol(-1) (MP2/cc-pVTZ+zero point vibrational energy+basis set superposition erros) are identified. The two strongest bound complexes are, within a range of 0.3 kcal mol(-1), isoenergetic. The binding in these six dimers can be described in terms of OH...O, C=O...H, C-O...H and CH...O interactions. Matrix isolation spectroscopy allowed to characterize the two strongest bound complexes by their infrared spectra.

Matrix isolation infrared and ab initio study of the hydrogen bonding between formic acid and water

The infrared spectra of the formic acid-water complexes isolated in argon matrices are reported. Both supersonic jet expansion and a conventional effusive source followed by trapping in solid argon at 10K are used to obtain the matrices. The experimental IR spectra are compared to the data obtained from high level ab initio (MP2) and DFT (B3LYP) calculations with 6-311++G(d,p) and aug-cc-pVTZ basis sets. The complex formation results in red shifts in the C=O and O-H stretching vibrations and a blue shift in the C-O stretching vibration of formic acid. The O-H stretching modes of water also exhibit pronounced red shifts. Both the MP2 and B3LYP calculations located three minima corresponding to cyclic HCOOH...H2O complexes with two hydrogen bond interactions. The binding energies are -10.3, -5.1, and -3.5 kcal mol(-1), respectively, for the three complexes at the MP2/ aug-cc-pVTZ level, corrected for the basis set superposition error (BSSE) using the Boys-Bernardi counterpoise scheme. Comparison of the calculated frequencies of the three complexes with the matrix IR spectrum reveals that the lowest energy complex is formed. In addition, a complex of formic acid with two water molecules is observed.