Non-Covalent Forces in Naphthazarin-Cooperativity or Competition in the Light of Theoretical Approaches.

Non-covalent interactions responsible for molecular features and self-assembly in Naphthazarin C polymorph were investigated on the basis of diverse theoretical approaches: Density Functional Theory (DFT), Diffusion Quantum Monte Carlo (DQMC), Symmetry-Adapted Perturbation Theory (SAPT) and Car-Parrinello Molecular Dynamics (CPMD). The proton reaction paths in the intramolecular hydrogen bridges were studied. Two potential energy minima were found indicating that the proton transfer phenomena occur in the electronic ground state. Diffusion Quantum Monte Carlo (DQMC) and other levels of theory including Coupled Cluster (CC) employment enabled an accurate inspection of Potential Energy Surface (PES) and revealed the energy barrier for the proton transfer. The structure and reactivity evolution associated with the proton transfer were investigated using Harmonic Oscillator Model of Aromaticity - HOMA index, Fukui functions and Atoms In Molecules (AIM) theory. The energy partitioning in the studied dimers was carried out based on Symmetry-Adapted Perturbation Theory (SAPT) indicating that dispersive forces are dominant in the structure stabilization. The CPMD simulations were performed at 60 K and 300 K in vacuo and in the crystalline phase. The temperature influence on the bridged protons dynamics was studied and showed that the proton transfer phenomena were not observed at 60 K, but the frequent events were noticed at 300 K in both studied phases. The spectroscopic signatures derived from the CPMD were computed using Fourier transformation of autocorrelation function of atomic velocity for the whole molecule and bridged protons. The computed gas-phase IR spectra showed two regions with OH absorption that covers frequencies from 2500 cm-1 to 2800 cm-1 at 60 K and from 2350 cm-1 to 3250 cm-1 at 300 K for both bridged protons. In comparison, the solid state computed IR spectra revealed the environmental influence on the vibrational features. For each of them absorption regions were found between 2700-3100 cm-1 and 2400-2850 cm-1 at 60 K and 2300-3300 cm-1 and 2300-3200 cm-1 at 300 K respectively. Therefore, the CPMD study results indicated that there is a cooperation of intramolecular hydrogen bonds in Naphthazarin molecule.

The effects of methyl internal rotation and (14)N quadrupole coupling in the microwave spectra of two conformers of N,N-diethylacetamide.

The gas phase structures and internal dynamics of N,N-diethylacetamide were determined with very high accuracy using a combination of molecular beam Fourier-transform microwave spectroscopy and quantum chemical calculations at high levels. Conformational studies yielded five stable conformers with C1 symmetry. The two most energetically favorable conformers, conformer I and II, could be found in the experimental spectrum. For both conformers, quadrupole hyperfine splittings of the (14)N nucleus and torsional fine splittings due to the internal rotation of the acetyl methyl group occurred in the same order of magnitude and were fully assigned. The rotational constants, centrifugal distortion constants as well as the quadrupole coupling constants of the (14)N nucleus were determined and fitted to experimental accuracy. The V3 potentials were found to be 517.04(13) cm(-1) and 619.48(91) cm(-1) for conformer I and II, respectively, and compared to the V3 potentials found in other acetamides. Highly accurate CCSD(T) and DMC calculations were carried out for calculating the barriers to internal rotation in comparison with the experimentally deduced V3 values.

A touch of lavender: gas-phase structure and dynamics of the monoterpene linalool validated by microwave spectroscopy.

The microwave spectrum of linalool, an acyclic monoterpene, was recorded for the first time in the range from 9 to 16 GHz. The only conformer observed under molecular beam conditions was assigned. Fitting the rotational spectrum with two different programs treating internal rotation yielded the rotational constants A = 1.64674020(46) GHz, B = 0.68219862(16) GHz, C = 0.61875100(20) GHz, and the centrifugal distortion constants. The standard deviation of the fit was close to experimental accuracy. A-E splittings due to the internal rotation of one methyl group could be resolved and the internal rotation barrier was determined to be 400.20(64) cm(-1). The results from microwave spectroscopy were used to validate the molecular geometry obtained from quantum chemical calculations.