Vicente Menéndez

Programmed-temperature retention indices : A survey of calculation methods

Abstract Retention temperatures of various solutes were measured on an OV-105 column under different heating rates. The experimental results agree with those evaluated by means of the equation of Curvers et al. Several methods of assigning programmed temperature retention indices to each solute were tested. Comparison of the results does not allow a clear choice of any of the methods, although a procedure based on an interpolation through the use of cubic splines polynomials is preferred. The sensitivity of the various procedures towards small changes in the input data is reported.

Mixed stationary phases in gas—liquid chromatography : Partition coefficients and retention indices in OV-101-OV-25, OV-101-carbowax 20M and OV-225-SP-2340 mixtures

Abstract Partition coefficients of n-alkanes and other substances were measured at 393 K on mixed stationary phase columns prepared by mixing OV-101 with OV-25, OV-101 with Carbowax 20M and OV-225 with SP-2340. The results are examined with respect to the Purnell—Vargas de Andrade and Perry—Tiley relationships. The solvent—solvent interaction parameter was calculated in the three systems. When the value of this parameter is higher than 2 a clear departure from the linear approximation was observed. Retention indices were also calculated and compared with those predicted by the above relationships.

Coordinate of the reaction methylcarbene-ethylene

Abstract An MO study of the coordinate of the reaction methylcarbene—ethylene, using semi-empirical methods, has been performed. The calculated critical energies for the reaction were: 17 kcal/mole by the EH method, 5 kcal/mole by INDO and 1 kcal/mole by MINDO/2.

Method for the classification and selection of stationary phases in gas chromatography

Abstract A new method of classification is applied to 233 stationary phases used in gas chromatography. Clustering together all those which have similar chromatographic behaviour, 45 are found to be singular in the sense that no other stationary phase exhibits similar characteristics and therefore they may not be replaced by any other. The remaining 188 stationary phases may be reduced to 33 selected liquids which may represent the larger set of 188, in the sense that all selectivity possibilities of the 188 stationary phases are retained in the 33 selected. A further reduction in the number of selected stationary phases will adversely affect the separation capacity of the set. The selected list of 33 stationary phases is not unique, and many of them may be replaced by an equivalent one without any loss in separation capacity.

MINDO/2 calculation of the rearrangement of singlet ethylcarbene

Abstract The minimum energy paths for the isomerization of singlet ethylcarbene to propylene and cyclopropane have been calculated by MINDO/2. The formation of the olefin presents no energy barrier but a critical energy of 1.4 kcal mol -1 was found in the path to cyclopropane formation.

Reduction of the number of characterizing probes in gas chromatography by factor analysis

Determination du comportement chromatographique des phases stationnaires, a partir de 10 solutes types

Indirect determination of specific retention volumes in gas chromatography use of the retention indices of McReynolds' samples

Abstract The specific retention volumes of n -decane on 24 liquid stationary phases of the silicone type were calculated from the retention indices of the samples used by McReynolds in his characterization of gas chromatographic stationary phases. The average error of prediction was 2.24%. The effects of errors on the logarithm of the relative retentions of consecutive n -alkanes and the values of the retention indices were studied, and also its dependence on stationary phase polarity.

Indirect determination of specific retention volumes in gas chromatography : Use of retentions of the McReynolds probes relative to n-decane

Abstract Specific retention volumes of n -decane oon 24 liquid stationary phases of the silicone type have been calculated from retention data relative to n -decane, of n -dodecane and the ten probes used by McReynolds to characterize stationary phases in gas chromatography. The average error of predictionis 1.2%, with only one error above 3% (5%). Relative retentions of consecutive n -alkanes varied from 1.18 to 1.64. The experimental values of the specific retention volumes of n -decane varied between 1.39 and 129.2 ml/g.

Primary processes in the photolysis of diazo-n-propane and diazo-n-butane

The electronic absorption spectra of diazo-n-propane and diazo-n-butane have been recorded in the gas phase and in cyclohexane solution; the molar extinction coefficients are reported. CNDO/S calculations have allowed the assignment of the band at 2.2 × 104 cm–1 as an A″â†�A′ transition and the band at 4.4 × 104 cm–1 as an A′â†�A′. The quantum yield of decomposition upon irradiation in the visible band was unity within experimental error. The quantum yield was pressure and wavelength (within the band) independent. Attempts to detect fluorescence or phosphorescence were unsuccessful, indicating that the emission quantum yields were less than 0.01. Correlation diagrams constructed using diazomethane as a model with the conservation of individual electronic angular momenta explained some of the unusual features of these decomposition reactions and of the electronic spectra.

An algorithm for the determination of the vibrational energy distribution in “hot” molecules. Description and test

Abstract An algorithm for the determination of the vibrational energy of molecules energized above their threshold for unimolecular reaction is presented. Using easily obtainable experimental data the method performs their deconvolution applying well-established unimolecular reaction concepts. Their mathematical foundations are described as well as its implementation in IBM AT compatible TB BASIC. In the case example the vibrational energy distribution is known (can be calculated thermochemically), and therefore can be used to test the method performance. It is shown that using precise unimolecular reaction parameters and well-defined average energies transferred by collision 〈Δ E 〉 the agreement between deconvoluted and thermochemical results is good (i.e. within the uncertainties involved in the thermochemical test calculation).