Isotopomeric Conformational Changes in the Anisole−Water Complex: New Insights from HR-UV Spectroscopy and Theoretical Studies

Non-covalent interactions in anisole–(CO2)n (n = 1, 2) complexes

Non-covalent interactions are a ubiquitous binding motif and a challenge for theory and experiments.

Spectroscopic Properties of Anisole at the Air-Ice Interface: A Combined Experimental-Computational Approach

A combined experimental and computational approach was used to investigate the spectroscopic properties of anisole in aqueous solutions and at the ice-air interface in the temperature range of 77-298 K. The absorption, diffuse reflectance, and emission spectra of ice samples containing anisole prepared by different techniques, such as slow freezing (frozen aqueous solutions), shock freezing (ice grains), or anisole vapor deposition on ice grains, were measured to evaluate changes in the contaminated ice matrix that occur at different temperatures. It was found that the position of the lowest absorption band of anisole and its tail shift bathochromically by ∼4 nm in frozen samples compared to liquid aqueous solutions. On the other hand, the emission spectra of aqueous anisole solutions were found to fundamentally change upon freezing. While one emission band (∼290 nm) was observed under all circumstances, the second band at ∼350 nm, assigned to an anisole excimer, appeared only at certain temperatures (150-250 K). Its disappearance at lower temperatures is attributed to the formation of crystalline anisole on the ice surface. DFT and ADC(2) calculations were used to interpret the absorption and emission spectra of anisole monomer and dimer associates. Various stable arrangements of the anisole associates were found at the disordered water-air interface in the ground and excited states, but only those with a substantial overlap of the aromatic rings are manifested by the emission band at ∼350 nm.

Reliable vibrational wavenumbers for C=O and N-H stretchings of isolated and hydrogen-bonded nucleic acid bases

The accurate prediction of vibrational wavenumbers for functional groups involved in hydrogen-bonded bridges remains an important challenge for computational spectroscopy. For the specific case of the C=O and N-H stretching modes of nucleobases and their oligomers, the paucity of experimental reference values needs to be compensated by reliable computational data, which require the use of approaches going beyond the standard harmonic oscillator model. Test computations performed for model systems (formamide, acetamide and their cyclic homodimers) in the framework of the second order vibrational perturbation theory (VPT2) confirmed that anharmonic corrections can be safely computed by global hybrid (GHF) or double hybrid (DHF) functionals, whereas the harmonic part is particularly challenging. As a matter of fact, GHFs perform quite poorly and even DHFs, while fully satisfactory for C=O stretchings, face unexpected difficulties when dealing with N-H stretchings. On these grounds, a linear regression for N-H stretchings has been obtained and validated for the heterodimers formed by 4-aminopyrimidine with 6-methyl-4-pyrimidinone (4APM-M4PMN) and by uracil with water. In view of the good performance of this computational model, we have built a training set of B2PLYP-D3/maug-cc-pVTZ harmonic wavenumbers (including linear regression scaling for N-H) for six-different uracil dimers and a validation set including 4APM-M4PMN, one of the most stable hydrogen-bonded adenine homodimers, as well as the adenine-uracil, adenine-thymine, guanine-cytosine and adenine-4-thiouracil heterodimers. Because of the unfavourable scaling of DHF harmonic wavenumbers with the dimensions of the investigated systems, we have optimized a linear regression of B3LYP-D3/N07D harmonic wavenumbers for the training set, which has been next checked against the validation set. This relatively cheap model, which shows very good agreement with experimental data (average errors of about 10 cm(-1)), paves the route toward a reliable analysis of spectroscopic signatures for larger polynucleotides.

To π or not to π – how does methanol dock onto anisole?

Supersonic jet spectroscopy reveals that methanol opts against aromatic docking onto anisole despite a very close competition.

Structure and energetics of the anisole–Arn (n = 1, 2, 3) complexes: high-resolution resonant two-photon and threshold ionization experiments, and quantum chemical calculations

We present a concerted experimental and theoretical study of the anisole⋯Ar_n complexes with n = 1–3.

Microsolvation of 2-thiouracil: molecular structure and spectroscopic parameters of the thiouracil-water complex

State-of-the-art quantum-chemical computations have been employed to accurately determine the equilibrium structure and interaction energy of the 2-thiouracil-water complex, thus extending available reference data for biomolecule solvation patterns. The coupled-cluster level of theory in conjunction with a triple-ζ basis set has been considered together with extrapolation to the basis set limit, performed by employing second-order Møller-Plesset perturbation theory, and inclusion of core-correlation and diffuse-function corrections. On the basis of the comparison of experiment and theory for 2-thiouracil [ Puzzarini et al. Phys. Chem. Chem. Phys. 2013 , 15 , 16965 - 16975 ], structural changes due to water complexation have been pointed out. Molecular and spectroscopic properties of the 2-thiouracil-water complex have then been studied by means of the composite computational approach introduced for the molecular structure evaluation. Among the results achieved, we mention the accurate determination of the molecular dipole moment and of the spectroscopic parameters required for predicting the rotational spectrum.

Dispersion corrected DFT approaches for anharmonic vibrational frequency calculations: nucleobases and their dimers

Computational spectroscopy techniques have become in the last few years an effective means to analyze and assign infrared (IR) spectra of molecular systems of increasing dimensions and in different environments. However, transition from compilation of harmonic data to fully anharmonic simulations of spectra is still underway. The most promising results for large systems have been obtained, in our opinion, by perturbative vibrational approaches based on potential energy surfaces computed by hybrid (especially B3LYP) density functionals and medium size (e.g. SNSD) basis sets. In this framework, we are actively developing a comprehensive and robust computational protocol aimed at quantitative reproduction of the spectra of nucleic acid base complexes and their adsorption on solid supports (organic/inorganic). In this contribution we report the essential results of the first step devoted to isolated monomers and dimers. It is well known that in order to model the vibrational spectra of weakly bound molecular complexes dispersion interactions should be taken into proper account. In this work we have chosen two popular and inexpensive approaches to model dispersion interactions, namely the semi-empirical dispersion correction (D3) and pseudopotential based (DCP) methodologies both in conjunction with the B3LYP functional. These have been used for simulating fully anharmonic IR spectra of nucleobases and their dimers through generalized second order vibrational perturbation theory (GVPT2). We have studied, in particular, isolated adenine, hypoxanthine, uracil, thymine and cytosine, the hydrogen-bonded and stacked adenine and uracil dimers, and the stacked adenine-naphthalene heterodimer. Anharmonic frequencies are compared with standard B3LYP results and experimental findings, while the computed interaction energies and structures of complexes are compared to the best available theoretical estimates.