Modulation of π character upon complexation captured by molecular rotation spectra.

Two configurations of the furan-CF3Cl complex have been observed by high-resolution rotational spectroscopy. One is characterized by a dominant Cl lone pair⋯π*aromatic interaction and the other is stabilized by a C-Cl···π-CC- halogen bond. This is the first rotational spectroscopic evidence, to the best of our knowledge, that shows how a complexation with a partner like CF3Cl (the weak lone pair belt of Cl, to be more specific) can modulate both the aromatic π* and diene π characters of a heteroaromatic molecule in the formation of non-covalent interactions. The results emphasize the partner molecules' role in modulating the π electron structure and will not only deepen our understanding on non-covalent interactions but also lead to better designs of heteroaromatic-based drugs and materials.

Conformational Equilibria of 2-Methoxypyridine⋠⋠⋠CO2 : Cooperative and Competitive Tetrel and Weak Hydrogen Bonds.

The rotational spectrum of 2-methoxypyridine⋅⋅⋅CO2 was recorded and analysed employing a cavity-based Fourier transform microwave spectrometer, complemented with quantum chemical calculations which predicted three possible isomers within energies less than 1000 cm-1 . The two most stable isomers were observed in the pulsed jet, which are stabilized by a network of C⋅⋅⋅N/O tetrel and C-H⋅⋅⋅O weak hydrogen bonds. The relative population ratio of the two detected isomers was estimated to be NI /NII ≈2.5. The competition and cooperation of the present non-covalent interactions in both isomers are discussed within the framework of Bader's quantum theory of atoms in molecules and Johnson's non-covalent interaction analyses. The study shows, that when looking for CO2 adsorbents, one might prefer candidates with multiple interactions in one site over candidates with few but strong interactions.

Determination of the "Privileged Structure" of 8-Hydroxyquinoline.

An accurate semi-experimental equilibrium structure of 8-hydroxyquinoline (8-HQ) has been determined combining experiment and theory. The cm-wave rotational spectrum of 8-HQ was recorded in a pulsed supersonic jet using broadband dual-path reflection and narrowband Fabry-Perot-type resonator Fourier-transform microwave spectrometers. Accurate rotational and quartic centrifugal distortion constants and 14 N quadrupole coupling constants are determined. Rotational constants of all 13 C, 18 O and 15 N singly substituted isotopologues in natural abundance and those of a chemically synthesized OD isotopologue were used to obtain geometric parameters for all the heavy atoms and the hydroxyl hydrogen from a number of structure determination models. Theoretical approaches allowed for the determination of a semi-experimental equilibrium structure, reSE in which computed rovibrational and electronic corrections were utilized to convert vibrational ground state constants into equilibrium constants. Despite the molecule having only a horizontal plane of symmetry and possessing 11 individual heavy atoms, microwave spectroscopy has allowed for a reliable and accurate structure determination. A mass dependent, rm2 structure was determined and proved to be equally reliable by comparison with the B3LYP-D3(BJ)/aVTZ equilibrium structure.

Competitive and cooperative n →π* and n →σ* interactions in benzaldehyde-formaldehyde: rotational characterization.

The rotational spectrum of the 1 : 1 benzaldehyde-formaldehyde complex has been investigated by pulsed jet Fourier transform microwave spectroscopy combined with ab initio calculations. The two most stable isomers were observed, with the relative abundance ratio NI/NII≈ 3/1 estimated with intensity measurements. Both observed isomers are stabilized by one dominating O[double bond, length as m-dash]CO tetrel bond (n →π* interaction) and one secondary C-HO hydrogen bond. Natural bond orbital analysis and electron localization function analysis were applied to characterize the nature of the noncovalent interactions in the target complex.

Gas-phase pyrolysis of trans 3-pentenenitrile: competition between direct and isomerization-mediated dissociation.

The flash pyrolysis of trans 3-pentenenitrile (3-PN, CH3-CH[double bond, length as m-dash]CH-CH2-CN) was studied by combining the results of VUV photoionization mass spectra with broadband microwave spectra recorded as a function of the temperature of the pyrolysis tube. The two separated functional groups (vinyl and nitrile) open up isomerization as an initial step in competition with unimolecular dissociation. Primary products were detected by keeping the 3-PN concentration low and limiting reaction times to the traversal time of the gas in the pyrolysis tube (∼100 µs). The reaction is quenched and products are cooled by expansion into vacuum before interrogation over the 8-18 GHz region using chirped-pulse broadband methods. 118 nm VUV photoionization of the same reaction mixture provides a means of detecting all products with ionization potentials below 10.5 eV with minimal fragmentation. These results are combined with a detailed computational investigation of the C5H7N and related potential energy surfaces, leading to a consistent picture of the unimolecular decomposition of 3-PN. Loss of two H-atoms to form a 79 amu product is proven from its microwave transitions to contain trans-Z-2,4-pentadienenitrile, while no pyridine is observed. Methyl loss, HCN loss, and breaking the central C(2)-C(3) bond all occur following isomerization of the position of the double bond, thereby opening up low-energy pathways to these decomposition channels.

Internal rotation arena: Program performances on the low barrier problem of 4-methylacetophenone.

In the rotational spectroscopy community, several popular codes are available to treat multiple internal rotors in a molecule. In terms of the pros and cons of each code, it is often a difficult task to decide which program to apply to a specific internal rotation problem. We faced this issue when dealing with the spectroscopic fingerprint of 4-methylacetophenone (4MAP), recently investigated in the microwave region, which we here extended into the millimeterwave region. The methyl group attached to the phenyl ring in 4MAP undergoes internal rotation with a very low barrier of only 22 cm-1. The acetyl methyl group features a much higher barrier of about 580 cm-1. The performances of a program using the so-called "local" approach in terms of Herschbach's perturbative treatment, SPFIT, as well as three programs XIAM, ERHAM, and ntop, representing "global" fits, were tested. The results aim at helping spectroscopists in the decision on how to tackle their own internal rotation problems.

The Characteristics of Disulfide-Centered Hydrogen Bonds.

The disulfide-centered hydrogen bonds in the three different model systems of diethyl disulfide⋅⋅⋅H2 O/H2 CO/HCONH2 clusters were characterized by high-resolution Fourier transform microwave spectroscopy and quantum chemical computations. The global minimum energy structures for each cluster are experimentally observed and are characterized by one of the three different S-S⋅⋅⋅H-C/N/O disulfide-centered hydrogen bonds and two O⋅⋅⋅H-C hydrogen bonds. Non-covalent interaction and natural bond orbital analyses further confirm the experimental observations. The symmetry-adapted perturbation theory (SAPT) analysis reveals that electrostatic is dominant in diethyl disulfide⋅⋅⋅H2 O/HCONH2 clusters being consistent with normal hydrogen bonds, whilst dispersion takes over in diethyl disulfide⋅⋅⋅H2 CO cluster. Our study gives accurate structural parameters for the disulfide bond involved non-covalent clusters providing important benchmarking data for the theoretical evaluation of more complex systems.

A Journey from Thermally Tunable Synthesis to Spectroscopy of Phenylmethanimine in Gas Phase and Solution.

Phenylmethanimine is an aromatic imine with a twofold relevance in chemistry: organic synthesis and astrochemistry. To tackle both aspects, a multidisciplinary strategy has been exploited and a new, easily accessible synthetic approach to generate stable imine-intermediates in the gas phase and in solution has been introduced. The combination of this formation pathway, based on the thermal decomposition of hydrobenzamide, with a state-of-the-art computational characterization of phenylmethanimine laid the foundation for its first laboratory observation by means of rotational electric resonance spectroscopy. Both E and Z isomers have been accurately characterized, thus providing a reliable basis to guide future astronomical observations. A further characterization has been carried out by nuclear magnetic resonance spectroscopy, showing the feasibility of this synthetic approach in solution. The temperature dependence as well as possible mechanisms of the thermolysis process have been examined.

XeOCS: relatively straightforward?

We report a benchmark-quality equilibrium-like structure of the XeOCS complex, obtained from microwave spectroscopy. The experiments are supported by a wide array of highly accurate calculations, expanding the analysis to the complexes of He, Ne, Ar, Kr, Xe, and Hg with OCS. We investigate the trends in the structures and binding energies of the complexes. The assumption that the structure of the monomers does not change significantly upon forming a weakly bound complex is also tested. An attempt at reproducing the r structure of the XeOCS complex with correlated wavefunction theory is made, highlighting the importance of relativistic effects, large basis sets, and inclusion of diffuse functions in extrapolation recipes.

Reactivity and rotational spectra: the old concept of substitution effects.

The internal rotation of methyl groups and nuclear quadrupole moments of the halogens Cl, Br, I in o-halotoluenes cause complex spectral fine and hyperfine structures in rotational spectra arising from angular momentum coupling. Building on the existing data regarding o-fluorotoluene and o-chlorotoluene, the investigations of o-bromotoluene and o-iodotoluene allow for a complete analysis of the homologous series of o-halogenated toluenes. The trend in the methyl barriers to internal rotation rising with the size of the halogen can be rationalised by repulsion effects as predicted by MP2 calculations. Furthermore, the analysis of the observed quadrupole coupling serves as a quantitative intra-molecular probe, e.g. for the explanation of the relative reaction yields in the nitration of halotoluenes, related to the different π-bond character of the C-X bond depending on the position of substitution.

Local and global approaches to treat the torsional barriers of 4-methylacetophenone using microwave spectroscopy.

The Fourier transform microwave spectrum of 4-methylacetophenone recorded from 8 GHz to 18 GHz under jet-cooled conditions has revealed large tunneling splittings arising from a low barrier to internal rotation of the ring methyl group and small splittings from a high torsional barrier of the acetyl methyl group. The large splittings are especially challenging to model, while the small splittings are difficult to analyze due to the resolution limit of 120 kHz. The combination of two methyl groups undergoing internal rotations caused each rotational transition to split into five torsional species, which were resolved and fitted using a modified version of the XIAM code and the newly developed ntop code to a root-mean-square deviation close to measurement accuracy, providing an estimate of the V3 potential barriers of about 22 cm-1 and 584-588 cm-1 for the ring and the acetyl methyl groups, respectively. The assignment was aided by separately fitting the five torsional species using odd-power order operators. Only one conformer in which all heavy atoms are located on a symmetry plane could be identified in the spectrum, in agreement with results from conformation analysis using quantum chemical calculations.

The unusual symmetry of hexafluoro-o-xylene-A microwave spectroscopy and computational study.

The rotational constants and quartic centrifugal distortion coefficients of hexafluoro-o-xylene were precisely derived from the 8 GHz to 18 GHz gas phase microwave spectrum. In addition, the rotational constants of all singly substituted 13C isotopologues were determined. Instead of the intuitively expected symmetry of C2v, as in o-xylene, calculations with a variety of methods (B3LYP, CAM-B3LYP, ωB97XD, MP2, and coupled-cluster singles, doubles, and perturbative triples) predict a C2 symmetry structure in which the two CF3 groups rotate in opposite directions by about 16°. While the experimental results in this study are not capable of proving this unusual symmetry, they can support future microwave, gas phase electron diffraction or nuclear magnetic resonance studies. From the presented data, a preliminary r0 structure was determined, reproducing the experimental rotational constants with deviations of no more than 15 kHz. Analysis of the interactions between the two CF3 groups using an effective fragment potential approach identifies two major contributions to their interaction, due to exchange-repulsion and electrostatic repulsion, with electrostatic repulsion responsible for the barrier at the C2v geometry.

Tetrel bonds and conformational equilibria in the formamide-CO2 complex: a rotational study.

The rotational spectra of the complex formamide-CO2 have been measured and assigned by pulsed jet Fourier transform microwave spectroscopy. Two isomers of the complex have been detected where a CO tetrel bond dominates the interactions, and either N-HO or C-HO forms a secondary linkage. Bader's quantum theory of atoms in molecules and Johnson's non-covalent interaction analyses were applied to unveil the intermolecular binding sites and energetic properties in the complex. Relative intensity measurements on a set of µa-type transitions allowed estimating the relative population of the observed two isomers as NI/NII ≈ 18/1.

Unveiling the Sulfur-Sulfur Bridge: Accurate Structural and Energetic Characterization of a Homochalcogen Intermolecular Bond.

By combining rotational spectroscopy in supersonic expansion with the capability of state-of-the-art quantum-chemical computations in accurately determining structural and energetic properties, the genuine nature of a sulfur-sulfur chalcogen bond between dimethyl sulfide and sulfur dioxide has been unveiled in a gas-jet environment free from collision, solvent and matrix perturbations. A SAPT analysis pointed out that electrostatic S⋅⋅⋅S interactions play the dominant role in determining the stability of the complex, largely overcoming dispersion and C-H⋅⋅⋅O hydrogen-bond contributions. Indeed, in agreement with the analysis of the quadrupole-coupling constants and of the methyl internal rotation barrier, the NBO and NOCV/CD approaches show a marked charge transfer between the sulfur atoms. Based on the assignment of the rotational spectra for 7 isotopologues, an accurate semi-experimental equilibrium structure for the heavy-atom backbone of the molecular complex has been determined, which is characterized by a S⋅⋅⋅S distance (2.947(3) Å) well below the sum of van der Waals radii.

Chlorine "Equatorial Belt" Activation of CF3Cl by CO2: The C···Cl Tetrel Bond Dominance in CF3Cl-CO2.

Besides its typical halogen donor behavior (exhibiting a Cl σ-hole) in forming Cl···B halogen bonds (B is an electron-rich region), CF3Cl reveals a new interaction site in its complex with CO2 when explored by rotational spectroscopy. Experimental evidence and theoretical analyses point out irrefutably that CF3Cl prefers to link to CO2 through its Cl "equatorial belt" consisting of the lone pairs of the Cl atom, resulting in a C···Cl tetrel bond. In addition, a secondary plausible C···O tetrel bond and a F···O halogen bond might contribute to the relative orientation of the moieties forming the complex. The effects of the Cl "equatorial belt" present in perhalogenated molecules, such as CF3Cl, have been hitherto overlooked in describing the origin of noncovalent interactions. That left a significant void that the present study tries to fill by outlining its importance.

Internal rotation and chlorine nuclear quadrupole coupling in 2-chloro-4-fluorotoluene explored by microwave spectroscopy and quantum chemistry.

2-Chloro-4-fluorotoluene was investigated using a combination of molecular jet Fourier transform microwave spectroscopy in the frequency range from 5 to 21 GHz and quantum chemistry. The molecule experiences an internal rotation of the methyl group, which causes fine splittings of all rotational transitions into doublets with separation on the order of a few tens of kHz. In addition, hyperfine effects originating from the chlorine nuclear quadrupole moment coupling its nuclear spin to the end-over-end rotation of the molecule are observed. The torsional barrier was derived using both the rho and the combined-axis-method, giving a value of 462.5(41) cm-1. Accurate rotational constants and quadrupole coupling constants were determined for the 35Cl and 37Cl isotopologues and compared with Bailey's semi-experimental quantum chemical predictions. The gas phase molecular structure was deduced from the experimental rotational constants supplemented with those calculated by quantum chemistry at various levels of theory. The values of the methyl torsional barrier and chlorine nuclear quadrupole coupling constants were compared with the theoretical predictions and with those of other chlorotoluene derivatives.

The structure and low-barrier methyl torsion of 3-fluorotoluene.

The rotational spectra of 3-fluorotoluene and its seven 13C isotopic species have been recorded at natural abundance in the frequency range from 4 to 26 GHz using a pulsed molecular jet Fourier transform microwave spectrometer. The molecular structure comprising bond lengths and angles as well as parameters describing the methyl torsion were determined with high accuracy. Due to the very low torsional barrier of 17 cm-1, the lowest torsional states of the vibrational ground state exhibited large splittings in the spectrum, which were modeled satisfactorily with a modified version of the program XIAM and the program aixPAM, both developed to treat the methyl internal rotation effects. They were also applied to refit the microwave data of 3,4-difluorotoluene to standard deviations close to measurement accuracy.