Francisco P. S. C. Gil

Hydrogen‐Bond Dynamics of CH⋅⋅⋅O Interactions: The Chloroform⋅⋅⋅Acetone Case

Spectroscopic evidence for C-H...O hydrogen bonding in chloroform...acetone [Cl(3)CH...O=C(CH(3))(2)] mixtures was obtained from vibrational inelastic neutron scattering (INS) spectra. Comparison between the INS spectra of pure samples and their binary mixtures reveals the presence of new bands at about 82, 130 and 170 cm(-1). Assignment of the 82 cm(-1) band to the nuO...H anti-translational mode is considered and discussed. In addition, the betaC-H mode of CHCl(3) at 1242 cm(-1) is split in the spectra of the mixtures, and the high-wavenumber component is assigned to the hydrogen-bonded complex. The plot of the integrated intensity of this component shows a maximum for x=0.5, in agreement with the 1:1 stoichiometry of the chloroformacetone complex, with a calculated complexation constant of 0.15 dm(3) mol(-1). Results also show that the complex behaves as an independent entity, that is, despite being weak, such interactions play a key role in supramolecular chemistry.

Structures and vibrational spectra of CH3OCH2CH2OH: the hydrogen-bonded conformers

Ab initio calculations at the MP2/6-31G* and MP2/6-31G*//6-31G* levels have been carried out for the monomer of 2-methoxyethanol (CH3OCH2CH2OH). The MP2/6-31G* results indicate that the two more stable conformers (tGg′ and gGg′) display intramolecular hydrogen bonds directed from the hydroxy H atom to one of the lone pairs of the ether O atom, and that the tGg′ conformer is 6.3 kJ mol–1 more stable than the gGg′ conformer. As the MP2/6-31G* and MP2/6-31G*//6-31G* calculations do not yield results differing by more than a few tenths of a kJ mol–1, it is concluded that the structure-sensitive and the dynamic correlation corrections are far from being additive. While the optimization of geometry for the correlated wavefunction generally leads to increase of bond lengths and reduction of bond angles, on the whole the geometrical parameters undergo similar changes in different conformers. Ab initio harmonic 6-31G* derived force fields were used to perform normal mode analyses for the more stable conformers. The calculated ν(CH) frequencies are found to correlate linearly with some of the ab initio calculated CH bond lengths. An interpretation of the FTIR and Raman spectra for the liquid phase consonant with the structural and vibrational ab initio results is presented. Two spectral features observed both in Raman and in FTIR spectra and associated with ν(OH) in monomeric species are ascribed to conformers, in accord with the theoretical and experimental results. On the whole, both the structural and the vibrational results presented point to a distinction between the hydrogen-bonded G-type conformers (tGg′ and gGg′) and the higher energy T-type conformers (tTg and tTt).

CH…O intramolecular interactions determining conformational stabilities in 2-propoxyethanol

Abstract Conformational energies and structural parameters for relevant conformers of methyl propyl ether ( MPE = CH 3 CH 2 CH 2 OCH 3 ), 1,2-dimethoxyethane ( DME = CH 3 OCH 2 CH 2 OCH 3 ), 2-methoxyethanol ( ME = CH 3 OCH 2 CH 2 OH ) and 2-propoxyethanol ( PE = CH 3 CH 2 CH 2 OCH 2 CH 2 OH ) have been determined at the MP2/6-31G ∗ //321G ∗ level. Three interesting structural features due to the presence of intramolecular CH…O contacts stand out of these results. In the first, the gt conformer of MPE and the gttgg′ and g′ttgg′ conformers of PE are stabilized by about 2 kJ mol −1 relative to the tt and tttgg′ conformers, respectively. In the second, the gg arrangement of the propoxy fragment is stabilized when this fragment is “linked” to the tgg′ fragment of ME giving rise to the ggtgg′ and g′g′tgg′ conformers of PE. In turn, the gt fragment of MPE is not much affected in relative conformational energy by merging with the tgg′ fragment of ME to give rise to the more stable gttgg′ conformer of PE. In the third important structural feature, substitution of the hydroxyl H atom in ME by a methyl group leads to stabilization of those conformers which do not have any intramolecular OH…O contact, namely, the ttt and ttg conformers. In spite of this relative stabilization, the tgg′ conformer is the second more stable conformer of DME very close in energy to the more stable all-trans conformer. In addition, tgg′ and ggg′ keep approximately the same energy difference they had in ME.

Structures and vibrational spectra for the conformers of CH3OCH2CH2OH

Abstract The structures of the relevant conformations of 2-methoxyethanol (CH 3 OCH 2 CH 2 OH, abbreviated C 1 E 1 ) are determined by ab initio MO calculations with different basis sets (STO-3G, STO-3G ∗ , 3-21G, 3-21G ∗ , 4-21G ∗ , 6-31G, 6-31G ∗ , 6-31G ∗∗ , MP2/6-31G ∗ //6-31G ∗ , MP2/6-31G ∗ . Normal coordinate analyses and calculation of vibrational frequencies were performed using the ab initio derived force fields for the 3-21G ∗ and 6-31G ∗ basis sets. A comparison of the performances of the various basis sets is made for the conformational energy differences, the structures of the conformers and the ab initio derived vibrational spectra, leading to the general conclusion that the 3-21G ∗ basis set is the lowest level of calculation which is able to reproduce adequately the conformational energy differences and the changes in the structural parameters for the various conformers as given by the MP2/6-31G ∗ calculations, thus accounting in a reliable way for the interplay between the conformational and intramolecular hydrogen bonding interactions in C 1 E 1 .

Conformers, vibrational spectra and laser-induced rotamerization of CH2ClCOOH

Ab initio molecular orbital (MO) calculations with a 6-31G* basis set have been carried out for CH2ClCOOH. Structures and energies of relevant conformations have been determined by gradient geometry refinement and some conformationally dependent geometry trends discussed. The results agree with reported experimental data and provide a good insight for the intramolecular interactions which determine the relative stability of the various conformers. The normal mode analysis based on the 6-31G* harmonic force field is used to review previous assignments of the vibrational spectra of the various conformers and to interpret previous experimental findings (A. Kulbida and A. Nosov, J. Mol. Struct., 1992, 265, 17) on the laser-induced rotamerization of the chloroacetic acid monomer trapped in an argon matrix.

Solvent effects on 2-methoxyethanol conformers: an ab initio DFT study using the SCI-PCModel

The four stable conformers of 2-methoxyethanol, CH3O‐CH 2‐CH 2‐OH (tgg 0 , ggg 0 , ttg and ttt) are studied by ab initio calculations using the SCRF theory and the SCI-PCModel to assess solvent effects on the structure and vibrational spectra of this compound. Full geometry optimizations were carried out at the B3LYP/6-3111G(3df,2df,2p)//B3LYP/6-31G(d) level, and normal mode calculations were performed within the harmonic approximation using the B3LYP/6-31G(d) derived force fields. The solvent, herein represented by a polarizable continuum with a defined dielectric constant, is found to affect significantly the geometry and dipole moment of the individual solute conformers. In addition, the results seem to suggest that the intramolecular H-bond type of interactions, which occur for tgg 0 and ggg 0 , tend to attenuate the solvent influence. In turn, the conformers which do not exhibit intramolecular H-bond interactions, ttg and ttt, exhibit a stronger solvent influence as revealed mainly by structural and electrostatic changes affecting both the ether O atom and the hydroxyl group. q 1999 Elsevier Science B.V. All rights reserved.

Density functional modelling for the conformers of 2-methoxyethanol

Abstract User-defined density functional methods are applied to the four more stable conformers of 2-methoxyethanol to assess the effects, in the conformational energy differences, structural parameters and charge distributions, of systematically varying the relative contribution of the HF and exchange terms and of the local and non-local electronic correlation terms. It is shown that the structural parameters vary monotonically both with the HF term and the non-local correlation term, for the studied ranges of variation. A user-defined density functional calculation with a minimal basis set was modelled to reproduce the trends, in the above-mentioned molecular properties, of an MP2/6–31G∗ calculation, at a fraction of the CPU time (roughly 1 5 ). When both the HF contribution is greater than 90% and the local and non-local correlation terms are equal, the results show that the energy difference between the hydrogen-bonded conformers and the conformer charge distributions gets close to the MP2 corresponding values. In addition, the MP2 vibrational frequencies are reproduced with a high correlation coefficient.

Characterization and rehabilitation of the “Porta Férrea” stone materials, University of Coimbra, Portugal

The “Porta Férrea” (Iron Gate) is the main access to the “Paço das Escolas” (University Palace) at the University of Coimbra, Portugal. It has undergone several adaptations and reconstructions from an original castle gate until the present portal that was built in 1634. The inscription of “University of Coimbra, Alta and Sofia” in the UNESCO Word Heritage list increased the interest in developing conservation works and to do the characterization of the limestone materials of the “Porta Férrea” portal. To support the conservation works, a research project was developed. The intervention plan, the sampling and testing performed and the conservation developed are presented. The rock material in the portal is the Ançã limestone, showing very high porosity around 30%, creating quite important degradation and conservation problems. The deterioration pattern, analyses performed, and the conservative works are presented. In this study it can be confirmed that Ançã limestone of “Porta Férrea” do not decay according a theoretical predictable pattern in response to polluted environments, as the defined for Portland limestone, assuming erosion rates and forms controlled by a range of micro-environment conditions particularly related with architectural constraints.