Current Themes in Microwave and Infrared Spectroscopy of Weakly Bound Complexes

Ionic Dynamics and Vibrational Spectral Diffusion of a Protic Alkylammonium Ionic Salt through Intrinsic Cationic N-H Vibrational Probe from FPMD Simulations

We employed density functional theory (DFT)-based molecular dynamics simulations to explore the structure, dynamics, and spectral properties of the protic ionic entity trimethylammonium chloride (TMACl). Structural investigations include calculating the site-site radial distribution functions (RDFs), the distribution of constituent cations and anions in three-dimensional space, and combined distribution functions of the hydrogen-bonded pair RDF versus angle, revealing the structural characteristics of the ionic solvation and the intermolecular interactions within ions. Further, we determined the instantaneous vibrational stretching frequencies of the intrinsic N-H stretch probe modes by applying the time-series wavelet method. The associated ionic dynamics within the protic ionic compound were investigated by calculating the time-evolution of the fluctuating frequencies and the frequency-time correlation functions (FFCFs). The time scale related to the local structural relaxation process and the average hydrogen bond lifetime, ion cage dynamics, and mean squared displacement were investigated. The faster decay component of the FFCFs, depicting the intermolecular motion of intact hydrogen bonds in TMACl, is 0.07 ps for the Perdew-Burke-Ernzerhof (PBE)-based simulation and 0.06 ps for the PBE-D2 representation. The slower time scale of the longer picosecond decay time component of PBE and PBE-D2 representations are 3.13 and 2.87 ps, respectively. These picosecond time scales represent more significant fluctuations of the hydrogen-bonding partners in the ionic entity and hydrogen-bond jump events accompanied by large angular jumps. The longest picosecond time scales represent structural relaxation, including large angular jumps and ion-pair dynamics. Also, ion cage lifetimes correlate with the slowest time scale of the associated dynamics of vibrational spectral diffusion despite the type of DFT functional. This study benchmarks DFT treatments of the exchange-correlation functional with and without the van der Waals (vdW) dispersion correction scheme. The inclusion of vdW interactions to the PBE functional represents a less structured state of the ionic entity and faster dynamics of the molecular motions relative to the one predicted by the PBE system. All the results illustrate the necessity of accurately describing the Coulomb interactions, vdW dispersive interactive forces, and localized hydrogen bonds required to sustain the energetic balance in this ionic salt.

Intermolecular amide and aldehyde interactions: rotational spectroscopy of the complexes of formaldehyde with 2-azetidinone and formamide

The binding topologies and strength of amide–aldehyde interactions were explored by rotational spectroscopy and computations.

Van der Waals interactions of the disulfide bond revealed: A microwave spectroscopic study of the diethyl disulfide-argon complex

The van der Waals complex formed between diethyl disulfide (DEDS) and an argon atom was investigated by pulsed-jet Fourier transform microwave spectroscopy in conjunction with quantum chemical computations. One set of transition lines belonging to the configuration of the global potential energy minimum was measured and assigned. The rotational constants A, B, and C were accurately determined to be 1262.5758(1) MHz, 845.402 12(9) MHz, and 574.006 38(8) MHz, respectively. The distance between the argon atom and the center of mass of the DEDS subunit is 4.075(16) Å. Quantum theory of atoms in molecules and non-covalent interaction analyses reveal that the interactions take place between the argon atom and four sites of the DEDS subunit. Furthermore, the usage of the energy decomposition analysis approach provides further understanding of the characteristics of the van der Waals interactions. Additionally, ab initio calculations and symmetry-adapted perturbation theory analysis of the binary complexes of DEDS with He, Ne, Kr, and Xe atoms were carried out to get further insight into the characteristics of the van der Waal interactions of the disulfide bond.

Weak hydrogen bonds between alkyl halides and amides: The microwave spectroscopic and theoretical study of the difluoromethane⋯formamide complex

The pure rotational spectrum of the complex of difluoromethane with formamide was investigated by means of microwave spectroscopy supplemented with theoretical calculations. The hyperfine structure arising from the ¹⁴N nuclear quadrupole coupling effect was completely resolved. The most stable isomer that displays the C_s symmetry with the ∠HCH angle of difluoromethane being bisected by the ab-plane of formamide was detected. The two moieties in the detected isomer are connected via one N-H⋯F and one bifurcated CH₂⋯O weak hydrogen bonds confirmed by the non-covalent interaction plot and natural bond orbital analyses. The distances of the NH⋯F and CH₂⋯O interactions were determined to be 2.140(14) Å and 2.749(14) Å, respectively. The NH⋯F bond angle was determined to be 150.7°. Symmetry-adapted perturbation theory analysis indicates that the electrostatic component is the largest contributor to the total attractive interaction energy of the difluoromethane⋯formamide complex.

Stretching force constants as descriptors of energy and geometry of F···HF hydrogen bonds

In this work applicability of proton donor group stretching vibration force constants k_s and intermolecular stretching force constants k_σ for evaluations of hydrogen bond strength and geometry are discussed. For a set of 30 complexes with F···HF hydrogen bonds in a wide range 0.5-48 kcal/mol by means of quantum chemical calculations equilibrium geometries, complexation energies, vibrational frequencies and corresponding force constants were calculated (MP2/aug-cc-pVTZ). It is shown, that properties of a hydrogen bond are more strictly correlated with the values of force constants than with vibrational frequencies. Easy-to-use equations for estimations of hydrogen bond energy ∆E and geometry (r_FH, r_FF) based on k_s and k_σ values are proposed.

Structure and non-covalent interactions of 1,3-difluoropropane and its complex with water explored by rotational spectroscopy and quantum chemical calculations

1,3-difluoropropane and its complex with water were characterized by using Fourier transform microwave spectroscopy with the aid of quantum chemical calculations. Rotational spectra of the monomer were extended to the ¹³C isotopologues which lead to a precise structural determination of the two observed conformers. For the 1,3-difluoropropane-water complex, the most stable isomer has been observed and assigned in the supersonic jet expansion. In the observed isomer, water acts as a proton donor and a proton acceptor forming one O-H⋯F weak hydrogen bond and two C-H⋯O weak hydrogen bonds, respectively. The measurements on the three ¹³C, ¹⁸O, and deuterated water allowed determination of the skeleton structure and evaluation of the weak hydrogen bond parameters. Quantum theory of atoms in molecules, non-covalent interaction, and symmetry-adapted perturbation theory analyses were performed to reveal the nature of the intermolecular non-covalent interactions.

The role of secondary interactions on the preferred conformers of the fenchone-ethanol complex

New atomic-level experimental data on the intermolecular non-covalent interactions between a common odorant and a relevant residue at odorant binding sites are reported. The preferred arrangements and binding interactions of fenchone, a common odorant and ethanol, a mimic of serine's side chain, have been unambiguously identified using a combination of high resolution rotational spectroscopy and computational methods. The observed conformers include homochiral (RR) and heterochiral (RS) conformers, with a slight preference for a heterochiral form, and exhibit primary OH-O hydrogen bonds between fenchone and ethanol. Secondary interactions play a key role in determining the relative configurations of fenchone and ethanol, and in shaping quite a flat potential energy surface, with many conformers close in energy and small barriers for interconversion.

Chasing Weak Forces: Hierarchically Assembled Helicates as a Probe for the Evaluation of the Energetics of Weak Interactions

London dispersion forces are the weakest interactions between molecules. Because of this, their influence on chemical processes is often low, but can definitely not be ignored, and even becomes important in cases of molecules with large contact surfaces. Hierarchically assembled dinuclear titanium(IV) helicates represent a rare example in which the direct observation of London dispersion forces is possible in solution even in the presence of strong cohesive solvent effects. Hereby, the dispersion forces do not unlimitedly support the formation of the dimeric complexes. Although they have some favorable enthalpic contribution to the dimerization of the monomeric complex units, large flexible substituents become conformationally restricted by the interactions leading to an entropic disadvantage. The dimeric helicates are entropically destabilized.

The covalent interaction between dihydrogen and gold: A rotational spectroscopic study of H2-AuCl

The pure rotational transitions of H2-AuCl have been measured using a pulsed-jet cavity Fourier transform microwave spectrometer equipped with a laser ablation source. The structure was found to be T-shaped, with the H-H bond interacting with the gold atom. Both 35Cl and 37Cl isotopologues have been measured for both ortho and para states of H2. Rotational constants, quartic centrifugal distortion constants, and nuclear quadrupole coupling constants for gold and chlorine have been determined. The use of the nuclear spin-nuclear spin interaction terms Daa, Dbb, and Dcc for H2 were required to fit the ortho state of hydrogen, as well as a nuclear-spin rotation constant Caa. The values of the nuclear quadrupole coupling constant of gold are χaa=-817.9929(35) MHz, χbb=504.0(27) MHz, and χcc=314.0(27). This is large compared to the eQq of AuCl, 9.63 312(13) MHz, which indicates a strong, covalent interaction between gold and dihydrogen.

Understanding the electron density reorganization upon stacking vs. H-bonding interaction in methyl gallate–caffeine complexes

QTAIM analysis of selected subcomplexes contained in methyl gallate-caffeine crystal indicate: a) significant differences between electron density reorganization involved in H-bonds and stacking interactions; b) cooperative effects are only significant when bond paths associated to homomonomeric hydrogen bonds are found.

Rotational spectrum of the mixed van der Waals triad pyridine-Ar-Ne

We report the Fourier transform microwave spectra of four isotopologues of the hetero triad pyridine-Ar-Ne, formed by the combination of two isotopes of the nitrogen atom (14N and 15N) in pyridine with two isotopes of the rare gas (RG) atoms (20Ne and 22Ne), by using pulsed jet spectroscopy. We detected the conformer denoted [1,1], with the Ne and Ar atoms located one on each side of the ring plane. The [2,0] species, with the two RG atoms on the same side of the ring, was not observed. Ab initio MP2/6-311++G** calculations suggest the rotational spectrum of this species to be complicated by the presence of several almost equivalent minima, separated by very low barriers. Four structural van der Waals parameters, R(Ar), R(Ne), theta(Ar) and theta(Ne), which localize the RG atoms with respect to pyridine, are determined. The 14N nuclear quadrupole coupling constants are obtained for the isotopologues containing this nucleus.

Chirality recognition between neutral molecules in the gas phase

Noncovalent interactions are particularly intriguing when they involve chiral molecules, because the interactions change in a subtle way upon replacing one of the partners by its mirror image. The resulting phenomena involving chirality recognition are relevant in the biosphere, in organic synthesis, and in polymer design. They may be classified according to the permanent or transient chirality of the interacting partners, leading to chirality discrimination, chirality induction, and chirality synchronization processes. For small molecules, high-level quantum chemical calculations for such processes are feasible. To provide reliable connections between theory and experiment, such phenomena are best studied in vacuum isolation at low temperature, using rotational, vibrational, electronic, and photoionization spectroscopy. We review these techniques and the results which have become available in recent years, with special emphasis on dimers of permanently chiral molecules and on the influence of conformational flexibility. Analogies between the microscopic mechanisms and macroscopic phenomena and between intra- and intermolecular cases are drawn.

Infrared spectra and intensities of the H2O and N2 complexes in the range of the nu1- and nu3-bands of water

The IR spectra of complexes of water with nitrogen molecules in the range of the symmetric (nu(1)) and antisymmetric (nu(3)) bands of H(2)O have been studied in helium droplets. The infrared intensities of the nu(3) and nu(1) modes of H(2)O were found to be larger by factors of 1.3 and 2, respectively, in the N(2)-H(2)O complexes. These factors are smaller than those obtained in recent theoretical calculations. The conformation of the N(2)-H(2)O complex was estimated. Spectra and IR intensities of the (N(2))(2)-H(2)O and N(2)-(H(2)O)(2) complexes were also obtained and their structures are discussed.

Transferable Potentials for Phase Equilibria. 9. Explicit Hydrogen Description of Benzene and Five-Membered and Six-Membered Heterocyclic Aromatic Compounds

The explicit hydrogen version of the transferable potentials for phase equilibria (TraPPE-EH) force field is extended to benzene, pyridine, pyrimidine, pyrazine, pyridazine, thiophene, furan, pyrrole, thiazole, oxazole, isoxazole, imidazole, and pyrazole. While the Lennard-Jones parameters for carbon, hydrogen (two types), nitrogen (two types), oxygen, and sulfur are transferable for all 13 compounds, the partial charges are specific for each compound. The benzene dimer energies for sandwich, T-shape, and parallel-displaced configurations obtained for the TraPPE-EH force field compare favorably with high-level electronic structure calculations. Gibbs ensemble Monte Carlo simulations were carried out to compute the single-component vapor-liquid equilibria for benzene, pyridine, three diazenes, and eight five-membered heterocycles. The agreement with experimental data is excellent with the liquid densities and vapor pressures reproduced within 1 and 5%, respectively. The critical temperatures and normal boiling points are predicted with mean deviations of 0.8 and 1.6%, respectively.

High-Accuracy Quantum Mechanical Studies of π−π Interactions in Benzene Dimers

Although supramolecular chemistry and noncovalent interactions are playing an increasingly important role in modern chemical research, a detailed understanding of prototype noncovalent interactions remains lacking. In particular, pi-pi interactions, which are ubiquitous in biological systems, are not fully understood in terms of their strength, geometrical dependence, substituent effects, or fundamental physical nature. However, state-of-the-art quantum chemical methods are beginning to provide answers to these questions. Coupled-cluster theory through perturbative triple excitations in conjunction with large basis sets and extrapolations to the complete basis set limit have provided definitive results for the binding energy of several configurations of the benzene dimer, and benchmark-quality ab initio potential curves are being used to calibrate new density functional and force-field models for pi-pi interactions. Studies of substituted benzene dimers indicate flaws in the conventional wisdom about substituent effects in pi-pi interactions. Three-body and four-body interactions in benzene clusters have also been examined.

Motional narrowing of the rotational spectrum of trifluoropropyne at 6550 cm−1 by intramolecular vibrational energy redistribution

We present the basic principles of dynamic rotational spectroscopy for the highly vibrationally excited symmetric top molecule trifluoropropyne (TFP,CF3CCH). Single molecular eigenstate rotational spectra of TFP were recorded in the region of the first overtone of the nu(1) acetylenic stretching mode at 6550 cm(-1) by infrared-pulsed microwave-Fourier transform microwave triple resonance spectroscopy. The average rotational constant (B) of the highly vibrationally mixed quantum states at 6550 cm(-1) is 2909.33 MHz, a value that is 40 MHz larger than the rotational constant expected for the unperturbed C-H stretch overtone (2869.39 MHz). The average rotational constant and rotational line shape of the molecular eigenstate rotational spectra are compared to the distribution of rotational constants expected for the ensemble of normal-mode vibrational states at 6550 cm(-1) that can interact by intramolecular vibrational energy redistribution (IVR). The normal-mode population distribution at 6550 cm(-1) can be described using a Boltzmann distribution with a microcanonical temperature of 1200 K. At this energy the rotational constant distribution in the normal-mode basis set is peaked at about 2910 MHz with a width of about 230 MHz. The distribution is slightly asymmetric with a tail to the high end. The experimentally measured dynamic rotational spectra are centered at the normal-mode distribution peak; however, the spectral width is significantly narrower (40 MHz) than normal-mode ensemble width (230 MHz). This reduction of the width, along with the Lorentzian shape of the eigenstate rotational spectra when compared to the Gaussian shape of the calculated ensemble distribution, illustrates the narrowing of the spectrum due to IVR exchange. The IVR exchange rate was determined to be 120 ps, about ten times faster than the rate at which energy is redistributed from the v=2 level of the acetylenic stretch.

Design and application of a multicoefficient correlation method for dispersion interactions

A new multicoefficient correlation method (MCCM) is presented for the determination of accurate van der Waals interactions. The method utilizes a novel parametrization strategy that simultaneously fits to very high-level binding, Hartree-Fock and correlation energies of homo- and heteronuclear rare gas dimers of He, Ne, and Ar. The decomposition of the energy into Hartree-Fock and correlation components leads to a more transferable model. The method is applied to the krypton dimer system, rare gas-water interactions, and three-body interactions of rare gas trimers He3, Ne3, and Ar3. For the latter, a very high-level method that corrects the rare-gas two-body interactions to the total binding energy is introduced. A comparison with high-level CCSD(T) calculations using large basis sets demonstrates the MCCM method is transferable to a variety of systems not considered in the parametrization. The method allows dispersion interactions of larger systems to be studied reliably at a fraction of the computational cost, and offers a new tool for applications to rare-gas clusters, and the development of dispersion parameters for molecular simulation force fields and new semiempirical quantum models.

Infrared spectrum and predissociation dynamics of H[sub 2]O[sup +]–Ar

The infrared (IR) spectrum and vibrational predissociation of the proton-bound H(2)O(+)-Ar ionic complex are investigated within an ab initio and quantum dynamical study. For this purpose, a two-dimensional potential energy surface (2D PES) is determined as a function of the HO-H and OH-Ar coordinates. This PES is then employed in a wave-packet calculation to determine spectral properties of the system and to calculate the IR absorption spectrum. The vibrational energy levels and relative IR intensities agree well with the experimental spectrum reported earlier. On the other hand, the predissociation lifetimes in the nanosecond regime derived from the 2D PES are in disagreement with the experimental observations, indicating the importance of the neglected degrees of freedom for a correct description of the dynamics of the complex.