On symmetrical O-H-O hydrogen bonds

Several hydrogen bonds with O-H-O bond lengths of about 2.4 Å are now known. Some of these have been examined well enough by neutron diffraction and other means to know with reasonable certainty that they are symmetrical hydrogen bonds, with either a single or a symmetrical double distribution peak. Even neutron diffraction results remain ambiguous as to whether the distribution has a single or double maximum. Several investigators have reported crystallographically symmetrical O-H-O hydrogen bonds with distances of about 2.5 — 2.6 Å. Thermal amplitudes of some of these have been given careful study, and most of these longer bonds appear to be truly, not statistically, symmetrical. It seems likely that the bonds of about 2.4 Å are distributions with single maxima, while the longer, apparently symmetrical hydrogen bonds are cases of double maxima. IR and NMR evidence that there are O-H-O bonds as long as 2.6 Å with single maxima should be carefully checked before they are accepted. From some theoretical calculation of hydrogen distributions in double and single wells it can be shown that cases with double maxima have a strong isotope effect with a shorter O-H-O distance than O-D-O distance, while cases with single maxima should show almost no isotope effect. With present accuracy in structure determinations a distinction between the two cases seems possible with or without the use of neutron diffraction. As an example that bond distances reliable to ~ 0.01 Å are now obtainable in nearly routine fashion, the structure of 3, 4-furandicarboxylic acid was determined to about this accuracy with data obtained in ten days.Plusieurs liaisons d'hydrogène avec des longueurs de liaison II-O-II d'environ 2,4 A sont maintenant connues. Parmi celles-ci, certaines ont été examinées par diffraction neutronique et par d'autres moyens avec une précision suffisante pour reconnaitre qu'il s'agit de liaison d'hydrogène symétriques avec un maximum de densité simple ou double et symétrique. Les résultats mêmes de diffraction neutronique ne peuvent décider si la distribution a un maximum simple ou double. Plusieurs expérimentateurs ont rapporté des liaisons cristallographiquement sym6triques 0-H-O avec des distances d'environ 2,5 — 2,6 Å. Les amplitudes thermiques de quelques-unes d'entre elles ont été étudiées avec soin, et la plupart de ces liaisons longues semblent être vraiment, et non pas statistiquement symétriques. Il semble probable que les liaisons d'environ 2,4 Å sont des distributions avec de simples maxima tandis que les liaisons d'hydrogène plus longues, apparemment symétriques sont des cas de double maxima. Toute évidence obtenue par des études dans l'infrarouge et par résonance magnétique nucléaire qu'il y ait des liaisons O-H-O simples symétriques aussi longues que 2,6 Å devrait être soigneusement vérifiée avant d'être acceptée. Quelques calculs théoriques de distribution d'hydrogène dans des puits de potentiel simples et doubles montrent que les cas des maxima doubles devraient avoir un effet isotopique prononcé avec une distance O-H-O plus courte que O-D-O tandis que les cas des maxima simples n'auraient pas d'effet isotopique appréciable. Avec la précision actuelle dans les déterminations de structure, une distribution entre les deux cas semble possible, avec ou sans l'aide de diffraction neutronique. Afin d'illustrer la possibilité d'obtenir des longueurs de liaison avec une precision de 0,01 Å, et cela avec des techniques de routine, mentionnons l'exemple de la structure de l'acide 3-4 furan-dicarboxylique, déterminée avec cette précision, grace à des données expérimentales, obtenues en 10 jours

The structure of TiCl₂ /

Work performed at the Iowa State College."Date Declassified: July 22, 1948.""AECD-2130."Includes bibliographical references.Mode of access: Internet

Destination segmentation: a recommended two-step approach

[Abstract]Market segmentation has been identified in the tourism marketing literature as an effective tool that can be used by management to meet the needs of a market more efficiently and effectively. It has been widely used by researchers seeking to develop tourism segments. Historically, most segmentation studies have developed tourism profiles at a destination using 1) a visitor questionnaire survey developed from studies in the literature or 2) secondary data. Very little research has taken a stakeholder approach to destination segmentation, despite many authors arguing the importance of utilising a stakeholder approach for destination management and marketing purposes. This research proposes a two-step approach to destination segmentation. It details this approach using one Australian destination. The two-step approach to segmentation begins with firstly understanding how tourism stakeholders at a destination segment their market before surveying tourists for the purposes of identifying segments. In contrast to previous tourist-focused segmentation studies, the two-step approach recommended and detailed in this study considers both tourism stakeholders and tourists. The segments devised from the proposed approach are then compared and contrasted with segments currently utilised by the destination marketing organisation (DMO). Step one involved interviewing 14 tourism stakeholders to determine how they segment the market. Based on these findings, a questionnaire survey was developed and data was collected from 852 tourists. This represented step two of the approach. Three tourism segments were identified through cluster analysis. Only one of these three segments was comparable with the segments defined by the DMO. The other two were not considered in the DMO segmentation. These segments represented over half of all tourists in the sample. Based on the sample in this study, the DMO segments target less than a quarter of the types of tourists visiting the destination. Contributions to theory and practice were identified. Firstly, it was recognised that different tourism stakeholders attract different tourists, not all stakeholders segment their market, and some stakeholders do not adhere to segments targeted by the DMO which may lead to an inconsistent message in the market. Secondly, the two-step approach is a new method incorporating a stakeholder view, which gives a more holistic view and a richer description for the segments obtained when compared with academic and practitioner segmentation approaches. The two-step approach can be utilised at other tourist destinations. The two-step approach to segmentation is capable of assisting tourism marketers to target more of the tourists frequenting the destination. This study suggests that many dollars may be wasted targeting tourists that are not likely to travel to the destination and not targeting those who would. Future research should be conducted at alternative destinations to further the understanding of the recommended two-step approach to segmentation