Intermolecular potentials, internal motions, and spectra of van der waals and hydrogen-bonded complexes

Attaching molecular hydrogen to metal cations: perspectives from gas-phase infrared spectroscopy

In this perspective article we describe recent infrared spectroscopic investigations of mass-selected M(+)-H(2) and M(+)-D(2) complexes in the gas-phase, with targets that include Li(+)-H(2), B(+)-H(2), Na(+)-H(2), Mg(+)-H(2), Al(+)-H(2), Cr(+)-D(2), Mn(+)-H(2), Zn(+)-D(2) and Ag(+)-H(2). Interactions between molecular hydrogen and metal cations play a key role in several contexts, including in the storage of molecular hydrogen in zeolites, metal-organic frameworks, and doped carbon nanostructures. Arguably, the clearest view of the interaction between dihydrogen and a metal cation can be obtained by probing M(+)-H(2) complexes in the gas phase, free from the complicating influences of solvents or substrates. Infrared spectra of the complexes in the H-H and D-D stretch regions are obtained by monitoring M(+) photofragments as the excitation wavelength is scanned. The spectra, which feature full rotational resolution, confirm that the M(+)-H(2) complexes share a common T-shaped equilibrium structure, consisting essentially of a perturbed H(2) molecule attached to the metal cation, but that the structural and vibrational parameters vary over a considerable range, depending on the size and electronic structure of the metal cation. Correlations are established between intermolecular bond lengths, dissociation energies, and frequency shifts of the H-H stretch vibrational mode. Ultimately, the M(+)-H(2) and M(+)-D(2) infrared spectra provide a comprehensive set of benchmarks for modelling and understanding the M(+)···H(2) interaction.

Resonances in rotationally inelastic scattering of OH(X2Π) with helium and neon

We present detailed calculations on resonances in rotationally and spin-orbit inelastic scattering of OH (X(2)Π, j = 3/2, F(1), f) radicals with He and Ne atoms. We calculate new ab initio potential energy surfaces for OH-He, and the cross sections derived from these surfaces compare well with the recent crossed beam scattering experiment of Kirste et al. [Phys. Rev. A 82, 042717 (2010)]. We identify both shape and Feshbach resonances in the integral and differential state-to-state scattering cross sections, and we discuss the prospects for experimentally observing scattering resonances using Stark decelerated beams of OH radicals.

Formation of supramolecular structures in organic solvents

The unusual interactions in organic liquids such as methane derivatives, arenes, and alkanes by the infrared (IR) method were revealed. The transformations of molecular shapes, arising from nonclassical hydrogen and dihydrogen bonds, as well as water factor provide the existence of supramolecular structures in organic fluids. The interpretation of the obtained results in terms of the quantum-chemical calculations has been suggested.

An adiabatic model for calculating overtone spectra of dimers such as (H(2)O)(2)

The near-infrared and visible wavelength spectrum of the water dimer is considered to be the major contributor to the so-called water continuum at these wavelengths. However, theoretical models of this spectrum require the simultaneous treatment of both monomer and dimer excitations. A model for treating this problem is proposed which is based upon a Franck-Condon-like separation between the monomer and dimer vibrational motions. In this model, one of the monomers is treated as the chromophore and its absorption is assumed to be given by its, possibly perturbed, vibrational band intensity. The main computational issue is the treatment of separate monomer and dimer motions. Various approaches for obtaining dimer vibration-rotation tunnelling spectra that allow for monomer motion are explored. These approaches include ways of treating the adiabatic separation of dimer vibrational modes from monomer vibrational modes. We classify the adiabatic separation methods under four main approaches: namely fixed-geometry, free-monomer, perturbed-monomer and coupled-monomer methods. The latter being the most computationally expensive as the monomer wave functions are dependent on the dimer coordinates. For each of these approaches, expectation values over the full potential are calculated for the given monomer vibrational wave functions. Various full (named VAP 2pD in the text) and partial (VAP (+p)D) averaging techniques are outlined to calculate the vibrationally averaged, monomer state-dependent, dimer interaction potentials. The computational costs associated with application of these techniques to the water dimer are estimated and the prospects for full calculations based on this approach are assessed.

Density functional theory study on the interaction between keto-9H guanine and aspartic acid

A theoretical study was performed using density functional theory (DFT) to investigate hydrogen bonding interactions in signature complexes formed between keto-9H guanine (Gua) and aspartic acid (Asp) at neutral pH. Optimized geometries, binding energies and the theoretical IR spectra of guanine, aspartic acid and their corresponding complexes (Gua-Asp) were calculated using the B3LYP method and the 6-31+G(d) basis set. Stationary points found to be at local minima on the potential energy surface were verified by second derivative harmonic vibrational frequency calculations at the same level of theory. AIM theory was used to analyze the hydrogen bonding characteristics of these DNA base complex systems. Our results show that the binding motif for the most stable complex is strikingly similar to a Watson-Crick motif observed in the guanine-cytosine base pair. We have found a range of hydrogen bonding interactions between guanine and aspartic acid in the six complexes. This was further verified by theoretical IR spectra of ω(C-H--O-H) cm(-1) stretches for the Gua-Asp complexes. The electron density plot indicates strong hydrogen bonding as shown by the 2p(z) dominant HOMO orbital character.

Dynamic behaviors of interactions: application of normal coordinates of internal vibrations to AIM dual functional analysis

A method to evaluate the dynamic nature of interactions is proposed based on the AIM dual functional analysis. Normal coordinates of internal vibrations (NIV) are employed to generate the perturbed structures necessary for the analysis. H(b)(r(c)) are plotted versus H(b)(r(c))-V(b)(r(c))/2 [= (variant Planck's over 2pi(2)/8m)nabla(2)rho(b)(r(c))] at bond critical points for the purpose. The plots are represented by the polar (R, theta) coordinate. Each plot for an interaction shows a specific curve, which is expressed by (theta(p), kappa(p)): theta(p) corresponds to the tangent line for the plot from the y-direction, and kappa(p) is the curvature. Although (R, theta) values correspond to the static nature of interactions, (theta(p), kappa(p)) values show the dynamic nature. The applicability of NIV is examined exemplified by the charge-transfer interactions as the first step to analyze the dynamic behaviors of interactions with NIV. The (theta(p), kappa(p)) values evaluated with NIV are very close to those obtained by the partial-optimization method (POM), where the distances or angles in question are fixed suitably, if the internal vibrations are substantially located on the interactions in question. The magnitudes of differences in theta(p) and kappa(p) between those evaluated with NIV and POM are < or = 2 degrees and < or = 2 au(-1), respectively, for usual interactions. The treatment is demonstrated to be applicable to a wide range of interactions.

Polar coordinate representation of Hb(rc) versus (h2/8m)nabla2rhob(rc) at BCP in AIM analysis: classification and evaluation of weak to strong interactions

Polar coordinate (R, theta) representation is proposed for the plot of Hb(rc) versus (h2/8m)nabla2rhob(rc) in AIM analysis to classify, evaluate, and understand weak to strong interactions in a unified way and in more detail; Hb(rc) and nabla2rhob(rc) are total electron energy densities and the Laplacian of rhob(rc) at bond critical points (BCPs: rc), respectively, where rhob(rc) are electron densities at rc. Both the x- and y-axes of the plot are expressed in the common unit of energy since Hb(rc) = Gb(rc) + Vb(rc) and (h2/8m)nabla2rhob(rc) = Hb(rc) - Vb(rc)/2 (= Gb(rc) + Vb(rc)/2), where Gb(rc) and Vb(rc) are kinetic energy densities and potential energy densities, respectively. Data employed for the plot are calculated at BCPs for full-optimized structures and optimized structures with the fixed distances (r) of r = r(o) + wa(o), where r(o) are the full-optimized distances, a(o) is the Bohr radius, and w = +/-0.1 and +/-0.2. The plot draws a helical stream starting from near origin (Hb(rc) = (h2/8m)nabla2rhob(rc) = 0) for very weak interactions and turns to the right as interactions become stronger. The helical stream is well described by the polar coordinate representation with (R, theta); R is given in the energy unit, and theta in degrees is measured from the y-axis. The ratio of Vb(rc)/Gb(rc) (= k) controls theta, of which an acceptable range in the plot is 45.0 < theta < 206.6 degrees. Each plot for an interaction gives a curve, which supplies important information. It is expressed by theta(p) and kappa(p); theta(p) corresponds to the tangent line measured from the y-direction, and kappa(p) is the curvature of the plot at w = 0. The polar coordinate (R, theta) representation with (theta(p), kappa(p)) helps us to classify, evaluate, and understand the nature of weak to strong interactions in a unified way.

Beryllium Bonds, Do They Exist?

The complexes between BeX2 (X = H, F, Cl, OH) with different Lewis bases have been investigated through the use of B3LYP, MP2, and CCSD(T) approaches. This theoretical survey showed that these complexes are stabilized through the interaction between the Be atom and the basic center of the base, which are characterized by electron densities at the corresponding bond critical points larger than those found in conventional hydrogen bonds (HBs). Actually, all bonding indices indicate that, although these interactions that we named "beryllium bonds" are in general significantly stronger than HBs, they share many common features. Both interactions have a dominant electrostatic character but also some covalent contributions associated with a non-negligible electron transfer between the interacting subunits. This electron transfer, which in HBs takes place from the HB acceptor lone-pairs toward the σYH* antibonding orbital of the HB donor, in beryllium bonds goes from the lone pairs of the Lewis base toward the empty p orbital of Be and the σBeX* antibonding orbital. Accordingly, a significant distortion of the BeX2 subunit, which in the complex becomes nonlinear, takes place. Concomitantly, a significant red-shifting of the X-Be-X antisymmetric stretching frequencies and a significant lengthening of the X-Be bonds occur. The presence of the beryllium bond results in a significant blue-shifting of the X-Be-X symmetric stretch.

Quantum chemical study and infrared spectroscopy of hydrogen-bonded CHCl3–NH3 in the gas phase

Molecular association of chloroform with ammonia is studied by high-level quantum chemical calculations including correlated MP2 and CCSD(T) calculations with basis sets up to6-311++G(d,p) and counterpoise corrected energies, geometries, and frequencies. The calculations predict an eclipsed hydrogen-bonded complex of C(3v) symmetry (DeltaE(0)=-15.07 kJ mol(-1)) with 225.4 pm intermolecular CHcdots, three dots, centeredN distance. Intermolecular interactions are analysed by Kitaura-Morokuma [Int. J. Quantum Chem. 10, 325 (1976)] interaction energy decomposition. Compared to the monomer, the C-H bond is elongated, and the CH-stretching fundamental shifts to lower wave numbers and has a marked approximately 340-fold increase of its intensity. Based on these predictions, the complex is observed by infrared spectroscopy in the gas phase at room temperature. A subtraction procedure isolates its spectrum, and a dilution series confirms the presence of a 1:1 complex. The CHCl(3)cdots, three dots, centeredNH(3) complex has an experimental -17.5 cm(-1) shift of its CH-stretching vibration, and CDCl(3)cdots, three dots, centeredNH(3) a -12.5 cm(-1) shift of the CD-stretching vibration. After a deperturbation of the CH-stretching/bending mode Fermi resonance system, this indicates a "redshifting" or more appropriately, a "C-H elongating" hydrogen bond in agreement with the ab initio calculations. An estimate of the complex concentration gives the equilibrium constant K(p)=0.024 (p(theta)=10(5) Pa) at 295 K for the dimerization, providing one of the few examples where a hydrogen-bonded gas phase complex at room temperature could be quantitatively studied by infrared spectroscopy.

Infrared spectroscopy of hydrogen-bonded CHCl3-SO2 in the gas phase

A molecular association between chloroform and sulfur dioxide in the gas phase at room temperature was studied by Fourier transform infrared spectroscopy. Since the intensity of the CH-stretching fundamental vibration of monomer chloroform is very weak but much stronger upon complexation, a simple subtraction procedure isolated the CH-stretching vibration spectrum of the complex. The presence of a 1:1 complex was confirmed by two dilution series, where the monomer concentrations were varied. The molecular association manifested itself as a shift of the peak absorbance of the CH-stretching vibration of CHCl3-SO2 by +7 cm(-1) and of the CD-stretching vibration of CDCl3-SO2 by +5 cm(-1) to higher wave numbers compared to monomer chloroform, accompanied by a considerable broadening of the band contour. In agreement with previous ab initio calculations, this indicates a "blueshifting" or more appropriately, a "C-H contracting" hydrogen bond between chloroform and sulfur dioxide. An estimate of the complex concentration was made based on ab initio calculations for the integrated band strength and the measured spectrum. With this estimate, the equilibrium constant Kp (295 K)=0.014 (po=10(5) Pa) for the dimerization was calculated, providing one of the very few cases where the formation of a hydrogen-bonded gas phase complex at room temperature could be quantitatively studied by infrared spectroscopy.

Quantum-chemical study of CHCl3-SO2 association

CHCl(3)-SO(2) association is studied by high-level quantum-chemical calculations of stationary points of the dimer electronic potential-energy hypersurface, including correlated second-order Moller-Plesset and CCSD(T) calculations with basis sets up to 6-311++G(d,p). During geometry optimization, frequency, and energy calculations, a self-written computer code embedding the GAMESS ab initio program suite applies counterpoise correction of the basis set superposition error. A CH...O hydrogen-bonded complex (DeltaE(0)=-8.73 kJmol) with a 2.4 A intermolecular H...O distance and two very weak van der Waals complexes (DeltaE(0)=-3.78 and -2.94 kJmol) are located on the counterpoise-corrected potential-energy surface. The intermolecular interactions are characterized by Kitaura-Morokuma interaction energy decompositions and Mulliken electron population analyses. The unusual hydrogen bond is distinguished by a CH-bond contraction, a pronounced enhancement of the IR intensity and a shift to higher frequency ("blueshift") of the CH-stretching vibration compared to the CHCl(3) monomer. Spectroscopy and association in liquid solution is also discussed; our results provide an alternative explanation for features in the CH-stretching vibration spectrum of chloroform dissolved in liquid sulfur dioxide which have been attributed previously to an intermolecular Fermi resonance.

Can an OH radical form a strong hydrogen bond? A theoretical comparison with H2O

In this study, we apply UCCSD/6-31++G** to investigate the ability of an OH radical acting as a hydrogen bond acceptor with HF, HCl, and H(2)O (HO...HX; X=F, Cl, OH) or as a hydrogen bond donor with H(2)O and H(2)S (OH...XH(2); X=O and S). We also replace OH with H(2)O and make a fair comparison between them. Additionally, the counterpoise method (CP) has been used to examine the effect of basis set superposition error (BSSE). Our results reveal that OH is a stronger hydrogen bond donor but a weaker hydrogen bond acceptor than H(2)O. This conclusion is independent of the correction for BSSE and can be rationalized by the NBO analysis, the results of which indicate that OH radical has a lower n(O) and sigma*(O-H) in energy than that of H(2)O.

Van der Waals Complexes of Small Molecules with Benzenoid Rings: Influence of Multipole Moments on Their Mutual Orientation

Intermolecular interaction between some small molecules (HF, H2O, NH3, and CH4) and certain benzenoid ring systems (benzene, hexafluorobenzene, and 1,3,5-trifluorobenzene) has been investigated in detail at MP2 level of theory using 6-311++G** basis set, and the results are corrected for basis set superposition error (BSSE). Vibrational frequencies were calculated for all the geometries at the same level of theory and basis sets to ensure that the geometries obtained correspond to true minima. In the complexes with benzene, which has a large negative quadrupole moment, the preferred geometry has the electropositive end of the small molecule (HF, H2O, and NH3) pointing toward the ring and the corresponding interaction energies are -3.24, -2.43, and -1.57 kcal/mol, respectively. For the complexes with hexafluorobenzene which has a large positive quadrupole moment, the most stable geometries are those in which the electropositive end of HF, H2O, and NH3 points away from the ring and the corresponding interaction energies are -1.59, -2.73, and -3.14 kcal/mol, respectively. Methane, which has neither a dipole nor a quadrupole moment, is weakly bound and is oriented differently in different systems. 1,3,5-Trifluorobenzene has a negligible quadrupole moment, and the complexes with small molecules are stabilized by cyclic hydrogen bonding. Although the point dipole-quadrupole and point quadrupole-quadrupole interactions present in these complexes account qualitatively for the preferred orientations, distributed multipole moments of the constituent atoms are found to give a quantitative description of the interaction in such complexes.

Dissymmetry effects on the laser spectroscopy of supersonically expanded rare gas/chiral arene heteroclusters

The R2PI-TOF spectra of supersonically expanded rare gas/chiral arene heteroclusters have been rationalized in terms of the distortion of the pi-electron density reflecting the different dipole and quadrupole momenta induced in the rare gas atoms by interaction with the opposite pi-faces of the chiral arene itself.

A mass and time-of-flight spectroscopy study of the formation of clusters in free-jet expansions of normal D2

The mass spectra in the range of 2(D+)-38(D19+) amu of clusters formed in a supersonic free-jet expansion of normal D2 are investigated as functions of source temperature in the range of 95-220 K and of source pressure in the range of 10-120 bars. For some of the small ion fragments, time-of-flight distributions are also measured. For large clusters (n > 200) the intensities of the odd-numbered ion fragments exhibit magic numbers at D9+ and D15+ in accordance with previous experiments and calculations. The even-numbered ion fragments have much smaller intensities and exhibit new magic numbers at D10+ and D14+. For source conditions such that large clusters are formed, the intensities of the various different ion fragments are observed to saturate beyond a certain source pressure. At lower source pressures, where only small clusters are formed, the terminal mole fractions of the neutral dimers are analyzed in the light of available theories which take into account both the thermodynamics and the kinetics of the expansion. At higher source pressures and lower temperatures, where larger clusters are formed, the sizes of the neutral clusters are estimated using scaling laws and are found to be consistent with the mass spectra and measured time-of-flight distributions. By using a variety of techniques it has been possible to obtain reliable conclusions about the neutral cluster sizes for the present free-jet expansion conditions.

On the Gas Phase Hydrogen Bond Complexes between Formic Acid and Hydroperoxyl Radical. A Theoretical Study

We present a systematic study on the gas-phase hydrogen-bonded complexes formed between formic acid and hydroperoxyl radical, which has been carried out by using B3LYP and CCSD(T) theoretical approaches in connection with the 6-311+G(2df,2p) basis set. For all complexes we have employed the AIM theory by Bader and the NBO partition scheme by Weinhold to analyze the bonding features. We have found 17 stationary points, and 11 of them present a cyclic structure. Their computed stabilities vary from 0.3 to 11.3 kcal/mol, depending on several factors, such as involvement in the hydrogen bond interaction, the geometrical constraints, and the possible concurrence of further effects such as resonance-assisted hydrogen bonds or inductive effects. In addition, three stationary points correspond to transition structures involving a double proton-transfer process whose features are also analyzed.

Molecular reorientation of CD(4) in gas-phase mixtures

Spin-lattice relaxation times were measured for the deuterons in CD(4) in pure gas and in mixtures with the following buffer gases: Ar, Kr, Xe, HCl, N(2), CO, CO(2), CF(4), and SF(6). Effective collision cross sections sigma(theta, 2) for the molecular reorientation of CD(4) in collisions with these ten molecules are obtained as a function of temperature. These cross sections are compared with the corresponding cross sections sigma(J) obtained from (1)H spin-rotation relaxation in mixtures of CH(4) with the same set of buffer gases. Various classical reorientation models typically applied in liquids predict different ratios of the reduced correlation times for the reorientation of spherical tops. The Langevin model comes closest to predicting the magnitude of the sigma(theta, 2)/sigma(J) ratio that we obtain for CD(4).

Theoretical Study of the α-Cyclodextrin Dimer

The molecular structure, stabilization energy, and thermodynamic properties of the plausible modes of the interaction for the three possible alpha-cyclodextrin (alpha-CD) dimers (head-to-head, tail-to-tail, and head-to-tail) with a water cluster were obtained using quantum chemical methods for the first time. Nine distinct spatial arrangements were investigated. The head-to-head mode of interaction with water is preferred by more than 10 kcal.mol(-1) (BLYP/6-31G(d,p)//PM3 Gibbs free energy difference value at room temperature) in relation to the next stable structure, with a water dimer structure placed inside each cavity and cyclic water tetramers surrounding each tail end. The inter alpha-CD hydrogen bonds play a major role to stabilize the dimeric structures, with no water tetramer being found between the two alpha-CD subunits for the preferred global minimum structure. Therefore, a theoretical model aimed to describe the behavior of alpha-CD dimer, or their inclusion complexes, in the aqueous media should take into account this preference for binding of the water molecules.

Phenol vs Water Molecule Interacting with Various Molecules: σ-type, π-type, and χ-type Hydrogen Bonds, Interaction Energies, and Their Energy Components

The nature of interactions of phenol with various molecules (Y = HF, HCl, H2O, H2S, NH3, PH3, MeOH, MeSH) is investigated using ab initio calculations. The optimized geometrical parameters and spectra for the global energy minima of the complexes match the available experimental data. The contribution of attractive (electrostatic, inductive, dispersive) and repulsive (exchange) components to the binding energy is analyzed. HF favors sigma O-type H-bonding, while H2O, NH3, and MeOH favor sigma H-type H-bonding, where sigma O-/sigma H-type is the case when a H-bond forms between the phenolic O/H atom and its interacting molecule. On the other hand, HCl, H2S, and PH3 favor pi-type H-bonding, which are slightly favored over sigma O-, sigma H-, sigma H-type bonding, respectively. MeSH favors chi H-type bonding, which has characteristics of both pi and sigma H. The origin of these conformational preferences depending on the type of molecules is elucidated. Finally, phenol-Y complexes are compared with water-Y complexes. In the water-Y complexes where sigma O/sigma H-type involves the H-bond by the water O/H atom, HF and HCl favor sigma O-type, H2O involves both sigma O-/sigma H-type, and H2S, NH3, PH3, MeOH, and MeSH favor sigma H-type bonding. Except for HF, seven other species have larger binding energies with a phenol molecule than a water molecule.

Prediction of the thermophysical properties of pure neon, pure argon, and the binary mixtures neon-argon and argon-krypton by Monte Carlo simulation usingab initiopotentials

Gibbs ensemble Monte Carlo simulations were used to test the ability of intermolecular pair potentials derived ab initio from quantum mechanical principles, enhanced by Axilrod-Teller triple-dipole interactions, to predict the vapor-liquid phase equilibria of pure neon, pure argon, and the binary mixtures neon-argon and argon-krypton. The interaction potentials for Ne-Ne, Ar-Ar, Kr-Kr, and Ne-Ar were taken from literature; for Ar-Kr a different potential has been developed. In all cases the quantum mechanical calculations had been carried out with the coupled-cluster approach [CCSD(T) level of theory] and with correlation consistent basis sets; furthermore an extrapolation scheme had been applied to obtain the basis set limit of the interaction energies. The ab initio pair potentials as well as the thermodynamic data based on them are found to be in excellent agreement with experimental data; the only exception is neon. It is shown, however, that in this case the deviations can be quantitatively explained by quantum effects. The interaction potentials that have been developed permit quantitative predictions of high-pressure phase equilibria of noble-gas mixtures.