Maribel Beltrán Rico

Fundamental vibrations of furan and deuterated derivatives

Abstract Infrared and Raman spectra of furan, furan-2-5-d2, furan-3-4-d2, and furan-d4, and infrared spectra of furan-2d, furan-3d, and furan-2-3-5-d3 are analyzed. An assignment of fundamental frequencies for all these compounds based on the rotational envelope of the vapor bands and the state of polarization of Raman lines is proposed. The proposed assignment fits conveniently all the isotopic rules. To check this assignment, special use has been made of the complete isotopic rule. Values of some thermodynamic functions have been calculated for furan at several temperatures.

Complete quadratic potential function for out-of-plane vibrations of thiophene

Abstract Out-of-plane force constants have been calculated for the thiophene molecule, using complete deuterium isotopic data and the most general quadratic symmetry potential function. The 12 force constants initially obtained by solving the secular equations of the symmetric derivatives were afterwards refined by a least squares method to fit the 60 observed frequencies of thiophene and all its deuterated derivatives. Normal coordinates and potential energy distributions among the various internal coordinates have been calculated for the four resulting sets of force constants for each symmetry species. From the consideration of the normal modes obtained it was possible to propose one of the sets as the one with physical significance.

Complete analysis of the NMR spectra of cis- and trans-1-methoxy-1,3-butadiene

Abstract Proton magnetic resonance spectra of four solutions of cis- and trans-1-methoxy-1,3-butadiene in TMS have been completely analyzed and the results were extrapolated to infinite dilution. The spectrum of the trans isomer was analyzed using a curve-fitting spectral decomposition technique. Accurate values were obtained for the long-range couplings involving the OMe protons in the trans isomer (4JHCOCH = −0.34 Hz, 5JHCOCCH = −0.29 Hz, and 6JHCOCCCH = ±0.12 Hz). The relative signs of all long-range coupling constants have been determined by selective double resonance, these being in agreement with earlier predictions. The conformation of the butadienyl system in the two isomers is largely the planar s-trans as deduced from the magnitudes and signs of their proton coupling constants. Long-range couplings involving the OMe group indicate that the preferred conformation of the OMe group in the trans isomer is the planar s-cis whereas that of the cis isomer is probably the planar s-trans. Some empirical facts concerning to the substituent effects on the magnetic parameters of these molecules are briefly considered.