1:2 Formic Acid/Acetylene Complexes: Ab Initio and Matrix Isolation Studies of Weakly Interacting Systems

Molecular Tailoring Approach for the Estimation of Intramolecular Hydrogen Bond Energy

Hydrogen bonds (HBs) play a crucial role in many physicochemical and biological processes. Theoretical methods can reliably estimate the intermolecular HB energies. However, the methods for the quantification of intramolecular HB (IHB) energy available in the literature are mostly empirical or indirect and limited only to evaluating the energy of a single HB. During the past decade, the authors have developed a direct procedure for the IHB energy estimation based on the molecular tailoring approach (MTA), a fragmentation method. This MTA-based method can yield a reliable estimate of individual IHB energy in a system containing multiple H-bonds. After explaining and illustrating the methodology of MTA, we present its use for the IHB energy estimation in molecules and clusters. We also discuss the use of this method by other researchers as a standard, state-of-the-art method for estimating IHB energy as well as those of other noncovalent interactions.

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.

A Cyclic Periodic Wave Function Approach for the Study of Infinitely Periodic Solid-State Systems. I. Application to the C-H···π(C≡C) Hydrogen Bonding Systems

The cyclic periodic wave function (CPWF) approach is applied at the AM1 and PM3 semiempirical levels of approximation to infinitely periodic solid-state systems stabilized by weak CH-pi (C-H···π) interactions between repeat units. The reliability of the AM1 and PM3 methods for modeling C-H···π bonding is first demonstrated using two representative dimer systems: the T-shaped ethyne dimer and the T-shaped propyne dimer. The CPWF method is then applied to two different crystal systems that are stabilized by C-H···π interactions: (1) pent-4-ynoic acid solid and (2) a series of three infinite crystal systems-tetrakis(4-ethynylphenyl)methane solid, tetraethynylmethane solid, and tetrabutadiynylmethane solid. A comparison of our results with available data demonstrates that the use of the CPWF approach at the AM1 and PM3 levels of approximation provides a convenient and reliable method for the study of infinitely periodic systems containing very weak C-H···π bonding.

Polymer Soft-Landing Isolation of Acetylene on Polystyrene and Poly(vinylpyridine): A Novel Approach to Probing Hydrogen Bonding in Polymers

Hydrogen-bonded complexes of acetylene (Ac) with the polymers polystyrene (PS), poly(4-vinylpyridine) (P4VP), and poly(2-vinylpyridine) (P2VP) have been characterized for the first time at 16 K in a "polymer soft-landing isolation" experiment which is being pioneered in our research laboratory. In particular, changes in vibrational modes of Ac provide ample evidence for hydrogen-bonded complexes between Ac and the phenyl groups of PS or the pyridyl groups of P4VP and P2VP. With PS, the proton on the top Ac molecule of the classic T-shaped Ac dimer interacts with the π cloud of the benzene (Bz) ring to form a C-H---π interaction, while the π cloud of the lower Ac forms a second C-H---π interaction with a proton on the Bz ring. An analogous (ring)1-(Ac)2 double interaction occurs between an Ac dimer and the pyridine (Pyr) rings on both P2VP and P4VP, yielding a C-H---N and C-H---π interaction. With P4VP and P2VP a second bridged (ring)2-(Ac)2 product is formed, with the Ac dimer forming nearly collinear C-H---N hydrogen bonds to adjacent Pyr rings. On P2VP this bridged product is the only one after extensive annealing. These complexes in which Ac acts as both proton donor and acceptor have not previously been observed in conventional matrix isolation experiments. This study is the second from our laboratory employing this method, which represents a slight modification of the traditional matrix isolation technique.

Estimation of N-H...O=C intramolecular hydrogen bond energy in polypeptides

The previously proposed molecular tailoring approached (MTA) [Deshmukh, M. M.; Gadre, S. R.; Bartolotti, L. J. J. Phys. Chem. A 2006, 110, 12519] for the estimation of intramolecular O-H...O hydrogen bond energy is extended to that for the N-H...O=C bond within polypeptides. The methodology is initially tested on a tetrapeptide containing two types of N-H...O=C hydrogen bonds and is found to distinguish between them. The estimated values are in good agreement with the trends predicted by the geometrical parameters. Furthermore, this methodology is applied to partially as well as fully substituted, capped polyglycines that contain five glycine residues (acetyl-(gly)(5)-NH(2)) to check the effect of substituents on the energetics of hydrogen bonds. The estimated N-H...O=C bond energy values lie in the range of 4-6 kcal/mol. These estimated values are not only in concurrence with the geometric parameters but also able to reflect the subtle effects of substituents for the substituted polypeptides studied in the present work.

Formamide Dimers: A Computational and Matrix Isolation Study

The dimerization of formamide (FMA) has been investigated by matrix isolation spectroscopy, static ab initio calculations, and ab initio molecular dynamics (AIMD) simulations. Comparison of the experimental matrix IR spectra with the ab initio calculations reveals that two types of dimers A and C are predominantly formed, with two and one strong NH...O hydrogen bonds, respectively. This is in accordance with previously published experiments. In addition, there is also experimental evidence for the formation of the thermally labile dimer B after deposition of high concentrations of FMA in solid xenon. The AIMD simulations of the aggregation process show that in all cases dimer C is initially formed, but rearrangement to the more stable doubly hydrogen-bonded structures A or B occurs for a fraction of collisions on the sub-picosecond time scale.

Estimation of intramolecular hydrogen bond energy via molecular tailoring approach

A novel method, based on the molecular tailoring approach for estimating intramolecular hydrogen bond energies, is proposed. Here, as a case study, the O-H...O bond energy is directly estimated by addition/subtraction of the single point individual fragment energies. This method is tested on polyhydroxy molecules at MP2 and B3LYP levels of theory. It is seen to be able to distinguish between weak ( approximately 1 kcal mol(-1)) and moderately strong ( approximately 5 kcal mol(-1)) hydrogen bonds in polyhydroxy molecules.

Furan−Formic Acid Dimers: An ab Initio and Matrix Isolation Study

The dimers formed by formic acid (FA) and furan are investigated by ab initio methods and matrix isolation spectroscopy. Nine complexes with binding energies between -3.91 and -0.82 kcal/mol (MP2/6-311++G(d,p) + ZPE + BSSE) are identified. Another five weaker bound complexes are localized at lower level of theory only. The binding in the furan-FA dimers can be described in terms of OH...O, C=O...H, HO...H, CH...O, OH...pi, and CH...pi interactions. Therefore, the furan-FA complexes are classified in two types: (1) the dimers where the OH hydrogen of formic acid interacts with the furan molecule and (2) the dimers where the main interactions of FA with the furan molecule are via the less acidic CH hydrogen. Duning's and Pople's triple and double basis sets were used to study the dependence of the geometries and energies of the complexes from the basis set. BSSE (basis set superposition error) counterpoise corrections (CP) were included during the geometry optimizations of all dimers at the MP2/6-31G(d,p) level of theory. Matrix isolation spectroscopy allowed us to record the IR spectrum of aggregates between FA and furan. By comparison of the experimental IR spectrum with calculated IR spectra of a variety of complexes, it was possible to identify the most stable furan-FA dimer as the major product of the aggregation.

Computational Study of Noncovalent Complexes between Formamide and Formic Acid

The geometries and binding energies of 1:1, 1:2, and 1:4 formic acid-formamide complexes (FA-FMA) are calculated by quantum chemical procedures. Vibrational spectra and intermolecular distances of the most stable FA-FMA dimers as well as the influence of the basis set superposition error (BSSE) on the geometries and energies of the dimers are also discussed. All FA-FMA dimers are optimized at the B3LYP/cc-pVTZ, the MP2/cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ levels of theory to study the influence of the level of theory on the calculated geometries and energies. CCSD(T)/cc-pVTZ single-point calculations at the MP2/aug-cc-pVTZ-optimized geometries were performed as reference for estimating the quality of lower level calculations. These calculations allow us to qualitatively describe the competition between different types of hydrogen-bonding interactions in FA-FMA complexes. FA-FMA dimers are compared to other formamide complexes and to the FA-FMA crystal structure.

Quantification of Intramolecular Nonbonding Interactions in Organochalcogens

Intramolecular nonbonding interactions between chalcogen atoms in a series of ortho substituted arylselenides (S/O...Se-Y, with Y = -Me, -CN, -Cl, and -F) are quantified using the coupled cluster CCSD(T)/cc-pVDZ level of theory. A homodesmic reaction method as well as an ortho-para approach are employed in evaluating the strength of intramolecular interactions. Comparison of the results obtained using the ab initio MP2 method and pure and hybrid density functional theories are performed with that of the coupled cluster values to assess the quality of different density functionals in evaluating the strength of nonbonding interactions. The interaction energies are found to be higher when the thioformyl group acts as the donor and the Se-F bond acts as the acceptor. In a given series with the same donor atom, the strength of the interaction follows the order Me < CN < Cl < F, exhibiting fairly high sensitivity to the group attached to selenium (Se-Y). Analysis of electron density at the S/O...Se bond critical point within the Atoms in Molecule framework shows a very good correlation with the computed intramolecular interaction energies.

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.