: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Kraitchman has shown that a single isotopic substitution on an atom is sufficient to determine directly the coordinates of that atom with respect to the principal axes of the original molecule. Kraitchman's formulas represent exact solutions of the equations for the equilibrium moments of inertia. However, the effects of the zero‐point vibrations are such that the coordinates obtained by substitution from the ground state moments of inertia I0 are systematically less than r0. These coordinates have here been called r (substitution) or rs, and it is found that rs≃(r0+re)/2, and Is= ∑ imirsi2≃(I0+Ie)/2.In the usual method of solution, the coordinate of one atom is determined from the equation for I0, and therefore the difference I0—Is must be made up by this one coordinate. This introduces a large error in the structures normally determined from ground state constants, and results in variations of 0.01 A in structures determined from different sets of isotopic species. If instead, we obtain the structure on...

A new criterion for the determination of molecular structures from ground state rotational constants

This article reviews the history and the current state of knowledge concerning the ability of the heavy chalcogen atoms S and Se, and to some extent Te, to participate in a H-bond as either proton donor or acceptor. These atoms are nearly as effective proton acceptors as O, and only slightly weaker as donor. They can also participate in chalcogen bonds where they act as electron acceptors from a nucleophile. These bonds rapidly strengthen as the chalcogen atom becomes larger: S < Se < Te, or if they are surrounded by electron-withdrawing substituents, and can exceed that of many types of H-bonds. Experimental and computational evidence indicates that both H-bonds and chalcogen bonds involving S and Se occur widely in chemical and biological systems, and play an active role in structure and function.

Participation of S and Se in hydrogen and chalcogen bonds

There have been growing academic interests in the study of strong organic molecule–graphene [or graphene oxide (GO)] systems, owing to their essential noncovalent nature and the consequent chemomechanical behavior within the interface. A more recent experimental measurement [Chem 2018, 4, 896–910] reported that the melamine–GO interface exhibits a remarkable noncovalent binding strength up to ∼1 nN, even comparable with typical covalent bonds. But the poor understanding on the complex noncovalent nature in particular makes it challenging to unveil the mystery of this high-performance interface. Herein, we carry out first-principles calculations to investigate the atomistic origin of ultrastrong noncovalent interaction between the melamine molecule and the GO sheet, as well as the chemomechanical synergy in interfacial behavior. The anomalous O–H···N hydrogen bonding, formed between the triazine moiety of melamine and the −OH in GO, is found cooperatively enhanced by the pin-like NH2–π interaction, which i...

Superstrong Noncovalent Interface between Melamine and Graphene Oxide

Tetrel-bonding emerges as a promising type of non-covalent interaction showing potential applications in molecular recognition and supramolecular chemistry. However, the molecular level characterization and interpretation of tetrel-bonds are still far from being satisfactory. In the present work, accurate structural and energetic information on the tetrel-bonds formed between CO2 and aliphatic amines is explored by means of high resolution rotational spectroscopy combined with quantum chemical calculations. The rotational spectrum of the isopropylamine–CO2 complex was investigated supersonic expansion. Two most stable isomers were observed, in which the isopropylamine moiety adopts trans and gauche forms, respectively. The relative abundance ratio of the two detected isomers was estimated to be NI/NII ≈ 5/1. The rotational spectra of the four 13C and one 15N mono-substituted isotopologues of the most stable isomer were also recorded and assigned, leading to an accurate determination of the backbone structure. The two moieties in both isomers are connected through a dominant C⋯N tetrel-bond and enhanced by one or two weak C–H⋯O hydrogen bonds. Natural bond orbital and quantum theory of atoms in molecules methods were utilized to quantitatively understand the characteristics of the non-covalent interactions. Symmetry-adapted perturbation theory analysis suggests that the electrostatic and dispersion interactions play a dominant role in stabilizing the titled complex.

Structure and C⋯N tetrel-bonding of the isopropylamine–CO2 complex studied by microwave spectroscopy and theoretical calculations

The rotational spectrum of the 2,2,4,4-tetrafluoro-1,3-dithietanewater complex has been investigated by high resolution rotational spectroscopy. Experimental evidence and quantum theoretical analyses revealed that the two moieties are linked together through a dominant SO chalcogen bond. Two secondary FO interactions contribute to the stability of the complex. The rotational transitions of four isotopologues are split into two component lines due to the internal rotation of the water moiety around its C2 axis. In the HDO isotopologue, a small μc dipole moment component is generated which inverts upon internal rotation of water, allowing the experimental determination of the tunneling splitting (21.46(5) GHz). Such splitting can be reproduced with a one-dimensional flexible model when the barrier to internal rotation of water is 87.4(2) cm-1.

Chalcogen bond and internal dynamics of the 2,2,4,4-tetrafluoro-1,3-dithietanewater complex.

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.

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

The 1:1 benzofuran-formaldehyde complex has been chosen as model system for analyzing π→π* interactions in supramolecular organizations involving heteroaromatic rings and carbonyl groups. A joint "rotational spectroscopy-quantum chemistry" strategy unveiled the dominant role of π→π* interactions in tuning the intermolecular interactions of such adduct. The exploration of the intermolecular potential energy surface led to the identification of 14 low-energy minima, with 4 stacked isomers being more stable than those linked by hydrogen bond or lone-pair→π interactions. All energy minima are separated by loose transition states, thus suggesting an effective relaxation to the global minimum under the experimental conditions. This expectation has been confirmed by the experimental detection of only one species, which was unambiguously assigned owing to the computation of accurate spectroscopic parameters and the characterization of 11 isotopologues. The large number of isotopic species opened the way to the determination of the first semi-experimental equilibrium structure for a molecular complex of such a dimension.

Stacked but not Stuck: Unveiling the Role of π→π* Interactions with the Help of the Benzofuran-Formaldehyde Complex.

In order to explore the -CF3 substitution effect on the complexation of pyridine, we investigated the 2-(trifluoromethyl)pyridine⋯water complex by using pulsed jet Fourier transform microwave spectroscopy complemented with quantum chemical calculations. Experimental assignment and ab initio calculations confirmed that the observed complex is stabilized through N⋯H-O and O⋯H-C hydrogen bonds forming a five-membered ring structure. The bonding distance in N⋯H-O is determined to be 2.027(2) A, whilst that in O⋯H-C interaction is 2.728(2) A. The quantum theory of atoms in molecules analysis indicates that the interaction energy of N⋯H-O hydrogen bond is ∼22 kJ mol-1 and that for O⋯H-C hydrogen bond is ∼5 kJ mol-1. The water molecule lies almost in the plane of the aromatic ring in the complex. The -CF3 substitution to pyridine quenches the tunneling splitting path of the internal motion of water molecule.

Microwave spectroscopy of 2-(trifluoromethyl)pyridine⋯water complex: Molecular structure and hydrogen bond.

Diethyl disulfide was investigated by pulsed jet Fourier transform microwave spectroscopy. The spectroscopic study was complemented by ab initio calculations. The first two most stable conformers predicted at the MP2/6-311++G(d,p) level of theory were observed in the supersonic expansion. Two 13C and one 34S isotopologues for the most stable conformer (GGG) and two 34S isotopes for the second most stable form (GGG′) were also assigned in natural abundance, respectively, allowing a thorough investigation of the molecular structure and conformation. The relative population ratio of the two conformers in the supersonic expansion was estimated to be about 4:1.

Xiaolong Li

Papers

Chalcogen bond and internal dynamics of the 2,2,4,4-tetrafluoro-1,3-dithietanewater complex.

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

Stacked but not Stuck: Unveiling the Role of π→π* Interactions with the Help of the Benzofuran-Formaldehyde Complex.

Microwave spectroscopy of 2-(trifluoromethyl)pyridine⋯water complex: Molecular structure and hydrogen bond.

Disulfide Bond in Diethyl Disulfide: A Rotational Spectroscopic Study