HM⁺-RG complexes (M = group 2 metal; RG = rare gas): Physical vs. chemical interactions.

Previous work on the HM(+)-He complexes (M = Be-Ra) has been extended to the cases of the heavier rare gas atoms, HM(+)-RG (RG = Ne-Rn). Optimized geometries and harmonic vibrational frequencies have been calculated using MP2 theory and quadruple-ζ quality basis sets. Dissociation energies for the loss of the rare gas atom have been calculated at these optimized geometries using coupled cluster with single and double excitations and perturbative triples, CCSD(T)theory, extrapolating interaction energies to the basis set limit. Comparisons are made between the present data and the previously obtained helium results, as well as to those of the bare HM(+) molecules; furthermore, comparisons are made to the related M(+)-RG and M(2+)-RG complexes. Partial atomic charge analyses have also been undertaken, and these used to test a simple charge-induced dipole model. Molecular orbital diagrams are presented together with contour plots of the natural orbitals from the quadratic configuration with single and double excitations (QCISD) density. The conclusion is that the majority of these complexes are physically bound, with very little sharing of electron density; however, for M = Be, and to a lesser extent M = Mg, some evidence for chemical effects is seen in HM(+)-RG complexes involving RG atoms with the higher atomic numbers.

HM⁺ and HM⁺­He (M = Group 2 metal): chemical or physical interactions?

We investigate the HM(+)­He complexes (M = Group 2 metal) using quantum chemistry. Equilibrium geometries are linear for M = Be and Mg, and bent for M = Ca-Ra; the explanation for this lies in the differing nature of the highest occupied molecular orbitals in the two sets of complexes. The difference primarily occurs as a result of the formation of the H-M(+) bond, and so the HM(+) diatomics are also studied as part of the present work. The position of the He atom in the complexes is largely determined by the form of the electron density. HM(+)…He binding energies are obtained and are surprisingly high for a helium complex. The HBe(+)…He value is almost 3000 cm(-1), which is high enough to suspect contributions from chemical bonding. This is explored by examining the natural orbital density and by population analyses.

Blood glucose response to rate of consumption of digestible carbohydrate.

When the glycemic response to consuming digestible carbohydrate is measured, little or no attention appears to have been paid to the possible effect on this response of the rate at which the food is consumed. We compared glycemic responses when volunteers ate or drank foods containing digestible carbohydrate as rapidly as possible, or in five equal portions over 12 min. Expecting that the response would be greater when the food was consumed rapidly, we found that the responses were equally and randomly distributed between the two rates of eating. At the same time, marked differences were noted in the responses elicited when different individuals consumed the same foods, leading to an investigation of this phenomenon, published elsewhere.

The glycemic response is a personal attribute.

The Glycemic Index (GI) is a measure of the extent of the change in blood glucose content (glycemic response) following consumption of digestible carbohydrate, relative to a standard such as glucose. We have explored whether the reported GIs of foods are a sufficient guide to a person wishing to avoid large glycemic responses and thereby avoid hyperglycemia. For this purpose, volunteers carried out multiple tests of four foods, following overnight fasting, measuring the glycemic response over 2 H. The areas under the blood glucose/time curves (AUCs) were compared. Each food tester displayed individual, characteristic glycemic responses to each food, unrelated to any other tester's response. Wide variations (up to 5-fold) were seen between the average AUCs for the same test by different testers. The absolute magnitudes of the glycemic responses are important for individuals trying to control blood sugar and/or body weight, but using published GI lists as a guide to control the glycemic response is not fully informative. This is because in calculating the GI, individual glycemic responses to glucose are normalized to 100. GI values are, therefore, relative and are not necessarily a reliable guide to the person's actual individual AUC when consuming a food. Without knowledge of the person's characteristic blood glucose responses, reliance only on the GI may be misleading.