Oligomers based on weak hydrogen bond networks: a rotational study of the tetramer of difluoromethane

The Role of Non-Covalent Interactions on Cluster Formation: Pentamer, Hexamers and Heptamer of Difluoromethane

The role of non-covalent interactions (NCIs) has broadened with the inclusion of new types of interactions and a plethora of weak donor/acceptor partners. This work illustrates the potential of chirped-pulse Fourier transform microwave technique, which has revolutionized the field of rotational spectroscopy. In particular, it has been exploited to reveal the role of NCIs' in the molecular self-aggregation of difluoromethane where a pentamer, two hexamers and a heptamer were detected. The development of a new automated assignment program and a sophisticated computational screening protocol was essential for identifying the homoclusters in conditions of spectral congestion. The major role of dispersion forces leads to less directional interactions and more distorted structures than those found in polar clusters, although a detailed analysis demonstrates that the dominant interaction energy is the pairwise interaction. The tetramer cluster is identified as a structural unit in larger clusters, representing the maximum expression of bond between dimers.

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.

Intermolecular Interaction in Methylene Halide (CH2F2, CH2Cl2, CH2Br2 and CH2I2) Dimers

The intermolecular interaction in difluoromethane, dichloromethane, dibromomethane, and diiodomethane dimers has been investigated using high level quantum chemical methods. The potential energy curve of intermolecular interaction along the C⋯C bond distance obtained using the coupled-cluster theory with singles, doubles, and perturbative triples excitations CCSD(T) were compared with values given by the same method, but applying the local (LCCSD(T)) and the explicitly correlated (CCSD(T)-F12) approximations. The accuracy of other theoretical methods-Hartree-Fock (HF), second order Møller-Plesset perturbation (MP2), and dispersion corrected DFT theory-were also presented. In the case of MP2 level, the canonical and the local-correlation cases combined with the density-fitting technique (DF-LMP2)theories were considered, while for the dispersion-corrected DFT, the empirically-corrected BLYP-D and the M06-2Xexchange-correlation functionals were applied. In all cases, the aug-cc-pVTZ basis set was used, and the results were corrected for the basis set superposition error (BSSE) using the counterpoise method. For each molecular system, several dimer geometries were found, and their mutual orientations were compared with the nearest neighbor orientations obtained in recent neutron scattering studies. The nature of the intermolecular interaction energy was discussed.

Non covalent interactions stabilizing the chiral dimer of CH2ClF: a rotational study

We report the rotational spectra of three isotopologues of the dimer of chlorofluoromethane, namely CH235ClF-CH235ClF, CH235ClF-CH237ClF and CH237ClF-CH235ClF. The assigned (most stable) conformer is chiral (C1 symmetry) and displays a network of two C-HCl-C and one C-HF-C weak hydrogen bonds, combined with a ClF halogen bond. The hyperfine structures due to the quadrupolar effects of the two non-equivalent 35Cl (or 37Cl) atoms have been fully resolved, leading to an accurate determination of two sets of diagonal and of some mixed quadrupole coupling constants. Information on the rs positions of the two Cl atoms and on the structural parameters of the hydrogen bonds has been obtained. The dissociation energy of the complex has been estimated as 5.9 kJ mol-1.

Structure, Conformational Equilibria, and Weak Hydrogen Bonding in the CH2 F2 -CF3 CH2 F Dimer

To probe the multiple configurations of a weakly bound intermolecular complex, the difluoromethane and 1,1,1,2-tetrafluoroethane dimer was investigated by using pulsed jet Fourier transform microwave spectroscopy coupled with quantum chemical calculations. Three isomers were detected in the supersonic jet. Spectroscopic assignments, ab initio calculations and quantum theory of atoms in molecules (QTAIM) analyses prove that all the observed isomers are stabilized through a net of three weak C-H⋅⋅⋅F-C interactions. The interaction energies are estimated to be within 12-13 kJ mol^-1 , dominated by electrostatic and dispersion according to Symmetry-Adapted Perturbation Theory (SAPT) analysis. The spectroscopic measurements were also extended to three and two ¹³ C isotopologues in natural abundance for the isomers II and III, respectively, which lead to precisely structural determinations of these two isomers. The comparison of the relative intensity of these isomers measured in the carrier gases of argon and helium evidenced that isomer II is the global minimum, while the intensities measured in helium suggested that the population of the three isomers in the jet to be N_I /N_II /N_III ≈1/11/4.

Rotational spectra and theoretical study of tetramers and trimers of 2-fluoroethanol: dramatic intermolecular compensation for intramolecular instability

Cooperativity and chirality dominate in stabilizing the 2-fluoroethanol tetramer, thanks to a fully bifurcated C–H⋯F H-bond network.

Weak Hydrogen Bond Network: A Rotational Study of 1,1,1,2-Tetrafluoroethane Dimer

1,1,1,2-Tetrafluoroethane dimer was investigated by pulsed jet Fourier transform microwave spectroscopy. One conformer, stabilized through a network of four C-H···F-C interactions, was observed, although several almost isoenergetic configurations were suggested by ab initio calculations. The measurements, extended to four ¹³C species in natural abundance, allowed determination of the carbon skeleton structures and evaluation of the weak hydrogen bond parameters. Information on the dissociation energy is also provided.

CF2H, a Hydrogen Bond Donor

The CF₂H group, a potential surrogate for the OH group, can act as an unusual hydrogen bond donor, as confirmed by crystallographic, spectroscopic, and computational methods. Here, we demonstrate the bioisosterism of the OH and CF₂H groups and the important roles of CF₂-H···O hydrogen bonds in influencing intermolecular interactions and conformational preferences. Experimental evidence, corroborated by theory, reveals the distinctive nature of CF₂H hydrogen bonding interactions relative to their normal OH hydrogen bonding counterparts.

On the competition between weak O-H···F and C-H···F hydrogen bonds, in cooperation with C-H···O contacts, in the difluoromethane - tert-butyl alcohol cluster

The 1:1 complex of tert-butyl alcohol with difluoromethane has been characterized by means of a joint experimental-computational investigation. Its rotational spectrum has been recorded by using a pulsed-jet Fourier-Transform microwave spectrometer. The experimental work has been guided and supported by accurate quantum-chemical calculations. In particular, the computed potential energy landscape pointed out the formation of three stable isomers. However, the very low interconversion barriers explain why only one isomer, showing one O-H···F and two C-H···O weak hydrogen bonds, has been experimentally characterized. The effect of the H → tert-butyl- group substitution has been analyzed from the comparison to the difluoromethane-water adduct.

A Direct Link from the Gas to the Condensed Phase: A Rotational Spectroscopic Study of 2,2,2-Trifluoroethanol Trimers

Rotational spectra of the three most stable conformers (I, II, III) of the ternary 2,2,2-trifluoroethanol (TFE) cluster were measured using Fourier transform microwave spectrometers, and unambiguously assigned with the aid of ab initio calculations. The most stable conformer, I, contains one trans-TFE subunit which is unstable in its isolated gas phase form. The study offers new insights into how the trans conformation is stabilized in TFE clusters of increasing size, and eventually becomes a dominant conformation in the liquid phase. A detailed analysis shows that while O-H⋅⋅⋅O-H and O-H⋅⋅⋅F-C hydrogen bonds are the most significant attractive interactions which stabilize all three conformers, the main driving force for the stability of I over III, which has all gauche-TFE subunits, is the attractive interaction of C-F⋅⋅⋅F-C contact pairs. A new type of three-point F⋅⋅⋅F⋅⋅⋅F attractive contact interaction is also identified.

Wetting Camphor: Multi-Isotopic Substitution Identifies the Complementary Roles of Hydrogen Bonding and Dispersive Forces

Using broadband rotational spectroscopy, we report here on the delicate interplay between hydrogen bonds and dispersive forces when an unprecedentedly large organic molecule (camphor, C10H16O) is microsolvated with up to three molecules of water. Unambiguous assignment was achieved by performing multi H2(18)O isotopic substitution of clustered water molecules. The observation of all possible mono- and multi-H2(18)O insertions in the cluster structure yielded accurate structural information that is not otherwise achievable with single-substitution experiments. The observed clusters exhibit water chains starting with a strong hydrogen bond to the C═O group and terminated by a mainly van der Waals (dispersive) contact to one of the available sites at the monomer moiety. The effect of hydrogen bond cooperativity is noticeable, and the O···O distances between the clustered water subunits decrease with the number of attached water molecules. The results reported here will further contribute to reveal the hydrophobic and hydrophilic interactions in systems of increasing size.

Unusual H-Bond Topology and Bifurcated H-bonds in the 2-Fluoroethanol Trimer

By using a combination of rotational spectroscopy and ab initio calculations, an unusual H-bond topology was revealed for the 2-fluoroethanol trimer. The trimer exhibits a strong heterochiral preference and adopts an open OH⋅⋅⋅OH H-bond topology while utilizing two types of bifurcated H-bonds involving organic fluorine. This is in stark contrast to the cyclic OH⋅⋅⋅OH H-bond topology adopted by trimers of water and other simple alcohols. The strengths of different H-bonds in the trimer were analyzed by using the quantum theory of atoms in molecules. The study showcases a remarkable example of a chirality-induced switch in H-bond topology in a simple transient chiral fluoroalcohol. It provides important insight into the H-bond topologies of small fluoroalcohol aggregates, which are proposed to play a key role in protein folding and in enantioselective reactions and separations where fluoroalcohols serve as a (co)solvent.

Weak hydrogen bonds in adducts between freons: the rotational study of CH2F2–CH2ClF

The network of weak hydrogen bonds in CH₂F₂–CH₂ClF suggests C–H⋯Cl–C to be preferred to C–H⋯F–C.

Halogen-Halogen Links and Internal Dynamics in Adducts of Freons

The pulsed jet Fourier transform microwave spectrum of the 1:1 complex between CF3Cl and CH3F shows that the two moieties are linked by a Cl···F halogen bond (RCl···F = 2.995 Å). The two symmetric tops CF3 and CH3 undergo free or almost free internal rotations, which alter the "rigid" value of the rotational constant A by almost 1 order of magnitude.

Interactions between freons: a rotational study of CH₂F₂-CH₂Cl₂

The rotational spectra of two isotopologues of a 1:1 difluoromethane-dichloromethane complex have been investigated by pulsed-jet Fourier-transform microwave spectroscopy. The assigned (most stable) isomer has C(s) symmetry and it displays a network of two C-H⋅⋅⋅Cl-C and one C-H⋅⋅⋅F-C weak hydrogen bonds, thus suggesting that the former interactions are stronger. The hyperfine structures owing to (35)Cl (or (37)Cl) quadrupolar effects have been fully resolved, thus leading to an accurate determination of the three diagonal (χ(gg); g=a, b, c) and the three mixed quadrupole coupling constants (χ(gg'); g, g'=a, b, c; g≠g'). Information on the structural parameters of the hydrogen bonds has been obtained. The dissociation energy of the complex has been estimated to be 7.6 kJ mol(-1).

Competition between weak hydrogen bonds: C–H⋯Cl is preferred to C–H⋯F in CH2ClF–H2CO, as revealed by rotational spectroscopy

CH₂O is linked to CH₂ClF through a C–H⋯Cl weak hydrogen bond, and it rotates along its symmetry axis with a barrier V₂ ∼ 125 cm⁻¹.