Evidence of C−H···O Hydrogen Bonds in Liquid 4-Ethoxybenzaldehyde by NMR and Vibrational Spectroscopies

Raman, FTIR, and NMR (both 13C and 17O) spectroscopies are used in a complementary way in order to study the occurrence of C−H···O intermolecular hydrogen bonds in liquid 4-ethoxybenzaldehyde (4EtOB). Additional information concerning the structure of the possible dimers is obtained through ab initio calculations, at the B3LYP/6-31G* level. The strongest evidences of the presence of C−H···O hydrogen bonds in the liquid phase arise from the temperature and solvent intensity dependence of the two bands observed in the νCO region of the vibrational spectra, as well as from the shift to low magnetic field detected for the carbonyl 17O NMR peak at higher dilutions. Further evidence is gathered from the changes observed in the νC-H vibrational modes, the 1JCH concentration dependence detected in the NMR spectra, and ab initio results. The experimental observations are consistent with the decrease of the C−H bond length upon hydrogen-bonding, as predicted for the nonstandard blue-shifting hydrogen bonds. Ab initio calculations predict several possible structures for the dimeric species, with nearly identical energies. The calculated dimerization energy is within the −5.1 to −6.5 kJ mol-1 range, considering both basis set superposition error and zero-point vibrational energy corrections, in agreement with the obtained experimental ΔH value of −5.7 ± 0.5 kJ mol-1

Structures and Vibrational Spectra of CH,OCH,CH,OH : The

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 (CH,OCH,CH,OH). 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 0 atom, and that the tGg’ conformer is 6.3 kJ mol-’ more stable than the gGg’ conformer. As the MP2/6-31G* and MP2/6-3lG*//6-31G* calculations do not yield results differing by more than a few tenths of a kJ mol-‘, 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 v(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 v(0H) 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)

Hydrogen-Bonded Dimers of CH3OCH2CH2OH: Ab Initio Structures and Multivariate Analysis

Structures and vibrational frequencies of relevant hydrogen-bonded dimers of 2-methoxyethanol are obtained at the 3-21G*//Ah41 level (the 3-21G* basis set included polarization functions deliberately added to second period atoms) and the results analyzed by standard multivariate methods. Dimers built from the same pair of isolated monomers belong to the same homologous series. The most stable dimer is obtained from the most stable conformations of both monomers, has a 10-membered intermolecular ring with a boat-chairboat conformation, and presents a zero dipole moment. In addition, the most stable dimers of the three homologous series have 10-membered rings with relatively symmetric conformations and zero or very low dipole moments. The energies of the monomers in these dimers-monomers with geometries frozen in the dimer-are close to the isolated and relaxed monomers energies. Generally speaking, OH 0 and CH 0 hydrogen bonds are found to contribute appreciably for the stability of the dimers. In particular, for the second most stable dimer, two of the three CH -0 hydrogen bonds are shorter than the intramolecular OH- 0 bond for the isolated monomer. Multivariate analysis of all the results shows that the 0. *H-0 angle is highly correlated with the H-0 bond length and that the COC and OCC angles are also highly correlated for both monomers, hence confirming that each monomer in a dimer keeps essentially the same conformation it has in the isolated monomer. The first and second principal components include all the highly correlated variables and account for 45% of the total variance. 3-D plots of the three most important principal components confirm a strong structural resemblance both for 10-membered ring dimers of homomeric series, Le., formed by the same monomers, and for the more stable dimers in each homologous series. In addition, among the heteromeric dimers with seven membered rings, the ones whose first conformation is G or G’ are less stable and located in a separate group

Encerramento de diastemas com restaurações diretas : caso clínico

Poster apresentado no XXV Congresso da Ordem dos Médicos Dentistas. 10-12 Novembro 2016, Exponor, Matosinhos, PortugalN/

Influencia do PRD (irrigação com secamento parcial do sistema radicular) e RDI (irrigação com déficit hídrico controlado) na qualidade de frutos de Laranjeira 'Pera'.

qUALIDADE DE FRUTO E RAI

Produção de frutos de laranjeira ?Pêra? enxertada em limoeiro ?Cravo? sob diferentes manejos de irrigação no litoral norte da Bahia

Sistemas de Produçã

Interactions of vanadates with carbohydrates in aqueous solutions

The interaction between the vanadate ion (VO3-, i.e. vanadium (V)) and the carbohydrates sucrose, glucose and fructose has been studied in aqueous solutions (pH [approximate]6, 298.15 K) using measurements of diffusion coefficients, electrical conductivity, Raman and multinuclear NMR spectroscopy. With sucrose and glucose, indications of hydrolysis of the anion in the absence of the sugars came from a decrease in the diffusion coefficient with increasing concentration. Significant effects on the diffusion coefficients were observed in the presence of sucrose and glucose, suggesting interactions between the carbohydrates and vanadate ion. Support for this came from electrical conductivity measurements, where there were indications of formation of oligomeric species. These were found to depend on the carbohydrate used: confirmation of oligomer formation came from Raman spectroscopy, where it was possible to identify these species, and see their dependence on the particular carbohydrate used. Information on the interactions between the carbohydrates glucose or sucrose and vanadate came from 51V and 1H NMR spectroscopy, where the dominant species appeared to be a 2:2 complex with glucose, possessing trigonal bipyramidal centres, whereas with sucrose it is suggested that octahedral species are formed. Studies with fructose were complicated by competing oxidation of this carbohydrate and reduction of vanadium (V).http://www.sciencedirect.com/science/article/B6TGS-4CTN53R-3/1/7da9099617b4eef5113ec0db3254629

Análise conjunta de caracteres quantitativos e qualitativos em acessos de umbu-cajazeira.

Pertencente à família Anacardiaceae, a umbu-cajazeira é uma frutífera nativa do Nordeste brasileiro, ainda em fase de domesticação, que apresenta acentuada variabilidade em função das variações morfológicas entre frutos (LIMA et al., 2002; CARVALHO et al., 2008). Ocorre espontaneamente no semiárido brasileiro (SILVA JÚNIOR et al., 2004) e seus frutos apresentam ampla perspectiva de aproveitamento agroindustrial. Porém, como sua exploração acontece apenas de forma extrativista, estes são preferencialmente consumidos in natura ou na forma de suco, picolé e sorvete (LIMA et al., 2002; CARVALHO et al., 2008; RITZINGER et al., 2008).pdf 59