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.

The Borderline between Reactivity and Pre-reactivity of Binary Mixtures of Gaseous Carboxylic Acids and Alcohols.

By mixing primary and secondary alcohols with carboxylic acids just before the supersonic expansion within pulsed Fourier transform microwave experiments, only the rotational spectrum of the ester was observed. However, when formic acid was mixed with tertiary alcohols, adducts were formed and their rotational spectra could be easily measured. Quantum mechanical calculations were performed to interpret the experimental evidence.

Noncovalent Interactions and Internal Dynamics in Pyridine-Ammonia: A Combined Quantum-Chemical and Microwave Spectroscopy Study.

The 1:1 complex of ammonia with pyridine is characterized by using state-of-the-art quantum-chemical computations combined with pulsed-jet Fourier-transform microwave spectroscopy. The computed potential energy landscape indicates the formation of a stable σ-type complex, which is confirmed experimentally: analysis of the rotational spectrum shows the presence of only one 1:1 pyridine-ammonia adduct. Each rotational transition is split into several components owing to the internal rotation of NH3 around its C3 axis and to the hyperfine structure of both 14 N quadrupolar nuclei, thus providing unequivocal proof that the two molecules form a σ-type complex involving both a N-H⋅⋅⋅N and a C-H⋅⋅⋅N hydrogen bond. The dissociation energy (BSSE- and ZPE-corrected) is estimated to be 11.5 kJ mol-1 . This work represents the first application of an accurate yet efficient computational scheme, designed for the investigation of small biomolecules, to a molecular cluster.

State-of-the-Art Thermochemical and Kinetic Computations for Astrochemical Complex Organic Molecules: Formamide Formation in Cold Interstellar Clouds as a Case Study.

We describe an integrated computational strategy aimed at providing reliable thermochemical and kinetic information on the formation processes of astrochemical complex organic molecules. The approach involves state-of-the-art quantum-mechanical computations, second-order vibrational perturbation theory, and kinetic models based on capture and transition state theory together with the master equation approach. Notably, tunneling, quantum reflection, and leading anharmonic contributions are accounted for in our model. Formamide has been selected as a case study in view of its interest as a precursor in the abiotic amino acid synthesis. After validation of the level of theory chosen for describing the potential energy surface, we have investigated several pathways of the OH + CH2NH and NH2 + H2CO reaction channels. Our results show that both reaction channels are essentially barrierless (in the sense that all relevant transition states lie below or only marginally above the reactants) and once tunneling is taken into the proper account indicate that the reaction can occur under the low temperature conditions of interstellar environments.

Reassessment of the Thermodynamic, Kinetic, and Spectroscopic Features of Cyanomethanimine Derivatives: A Full Anharmonic Perturbative Treatment.

Herein we report a full thermodynamic and vibrational investigation of C-cyanomethanimine isomers rooted into the Density Functional Theory (DFT) and the second-order vibrational perturbation theory (VPT2). We show that an anharmonic treatment affects dramatically the vibrational behavior of the molecules, especially thanks to the inclusion of interaction terms between the various modes. Furthermore, the equilibrium constant between the isomers, as well as the rate constant, have been obtained at both harmonic and anharmonic levels showing, as expected, slight but non-negligible differences. To support our investigation, dispersion effects have been employed.

Accurate Infrared (IR) Spectra for Molecules Containing the C≡N Moiety by Anharmonic Computations with the Double Hybrid B2PLYP Density Functional.

Herein, we report a comprehensive benchmark of C≡N stretching vibrations computed at harmonic and anharmonic levels with the aim of proposing and validating a reliable computational strategy to get accurate results for this puzzling vibrational mode without any ad hoc scaling factor. Anharmonic calculations employing second-order vibrational perturbation theory provide very good results when performed using the B2PLYP double-hybrid functional, in conjunction with an extended basis set and supplemented by semiempirical dispersion contributions. For larger systems, B2PLYP harmonic frequencies, together with B3LYP anharmonic corrections, offer a very good compromise between accuracy and computational cost without the need of any empirical scaling factor.

Vibronic Coupling Investigation to Compute Phosphorescence Spectra of Pt(II) Complexes.

The present paper reports a comprehensive quantum mechanical investigation on the luminescence properties of several mono- and dinuclear platinum(II) complexes. The electronic structures and geometric parameters are briefly analyzed together with the absorption bands of all complexes. In all cases agreement with experiment is remarkable. Next, emission (phosphorescence) spectra from the first triplet states have been investigated by comparing different computational approaches and taking into account also vibronic effects. Once again, agreement with experiment is good, especially using unrestricted electronic computations coupled to vibronic contributions. Together with the intrinsic interest of the results, the robustness and generality of the approach open the opportunity for computationally oriented chemists to provide accurate results for the screening of large targets which could be of interest in molecular materials design.