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The conformational isomerism of isopropylamine and n-propylamine has been investigated by means of an integrated strategy combining high-level quantum-chemical calculations and high-resolution rotational spectroscopy. The equilibrium structures (and thus equilibrium rotational constants) as well as relative energies of all conformers have been computed using the so-called “cheap” composite scheme, which combines the coupled-cluster methodology with second-order Møller–Plesset perturbation theory for extrapolation to the complete basis set. Methods rooted in the density functional theory have been instead employed for computing spectroscopic parameters and for accounting for vibrational effects. Guided by quantum-chemical predictions, the rotational spectra of isopropylamine and n-propylamine have been investigated between 2 and 400 GHz with Fourier transform microwave and frequency-modulation millimeter/submillimeter spectrometers. Spectral assignments confirmed the presence of several conformers with comparable stability and pointed out possible Coriolis resonance effects between some of them.

A joint experimental-theoretical spectroscopic investigation has focused on a better understanding of the nature of weak, non-covalent interactions in amine-water model systems.

In this study, the design, realization, and characterization of an ultrathin triple-band polarization-insensitive wide-angle metamaterial absorber are reported. The metamaterial absorber comprises a periodic array of modified six-fold symmetric snowflake-shaped resonators with strip spiral line load, which is printed on a dielectric substrate backed by a metal ground plane. It is shown that the absorber exhibits three distinct near-unity absorption peaks, which are distributed across C, X, Ku bands, respectively. Owing to the six-fold symmetry, the absorber is insensitive to the polarization of the incident radiation. In addition, the absorber shows excellent absorption performance over wide oblique incident angles for both transverse electric and transverse magnetic polarizations. Simulated surface current and field distributions at the three absorption peaks are demonstrated to understand the absorption mechanism. Particularly, the absorption modes come from the fundamental and high-order dipole resonances. Furthermore, the experimental verification of the designed absorber is conducted, and the measured results are in reasonable agreement with the simulated ones. The proposed ultrathin (∼0.018λ0, λ0 corresponding to the lowest peak absorption frequency) compact (0.168λ0×0.168λ0 corresponding to the area of a unit cell) absorber enables potential applications such as stealth technology, electromagnetic interference and spectrum identification.

Two isomers of a complex formed between thiazole and two water molecules, thi···(H2O)2, have been identified through Fourier transform microwave spectroscopy between 7.0 and 18.5 GHz. The complex was generated by co-expansion of a gas sample containing trace amounts of thiazole and water in an inert buffer gas. For each isomer, rotational constants, A0, B0 and C0, centrifugal distortion constants, DJ, DJK, d1, d2, and nuclear quadrupole coupling constants, and , have been determined through fitting of a rotational Hamiltonian to the frequencies of observed transitions. The molecular geometry, energy, and components of the dipole moment of each isomer have been calculated using Density Functional Theory (DFT). The experimental results for four isotopologues of isomer I allow accurate determinations of the atomic coordinates of the oxygen atoms by r0 and rs methods. Isomer II has been assigned as the carrier of an observed spectrum on the basis of very good agreement between DFT-calculated results and spectroscopic parameters (including A0, B0 and C0 rotational constants) determined by fitting to measured transition frequencies. Non-covalent interaction (NCI) and natural bond orbital (NBO) analyses reveal that two strong hydrogen bonding interactions are present within each of the identified isomers of thi···(H2O)2. The first of these binds an H2O to the nitrogen of thiazole (OH···N) and the second binds the two water molecules (OH···O). A third, weaker interaction binds the H2O sub-unit to the hydrogen atom that is attached to C2 (for isomer I) or C4 (for isomer II) of the thiazole ring (CH···O).