Recebido em 30/1/04; aceito em 30/6/04; publicado na web em 5/11/04 RARE EARTHS: INDUSTRIAL AND BIOLOGICAL APPLICATIONS. The history of the rare earths is rich in innovation and these elements have been the object of study of a number of scientists. Rare earths are used practically in almost all aspects of life and these applications are due to their outstanding properties, mainly spectroscopic and magnetic. In industry, the applications of rare earths are many, such as in catalysis, phosphors, magnetism, glass and lasers. In biological systems, rare earths are used, for example, as luminescent probes in the investigation of binding sites in proteins, labels in immunoassays and in noninvasive tests.Keywords: rare earths; spectroscopy; medical applications. INTRODUÇÃOAs terras raras, para as quais se utiliza o símbolo Ln, correspondem aos elementos do lantânio (La, Z = 57) ao lutécio (Lu, Z = 71), entre os quais se incluem o ítrio (Y, Z = 39) e o escândio (Sc, Z = 21). Mas, segundo recomendações da IUPAC 1 , usam-se os termos lantanídeos para designar os elementos do La ao Lu e terras raras quando aos lantanídeos são incluídos o Sc e o Y.A expressão terras raras é imprópria para designar estes elementos, que receberam esta denominação porque foram inicialmente conhecidos em forma de seus óxidos, que se assemelham aos materiais conhecidos como terras. Além da expressão "terras" não ser apropriada à denominação de tais elementos, a expressão "raras" também não está de acordo, pois os lantanídeos são mais abundantes (com exceção do promécio que não ocorre na natureza) do que muitos outros elementos. Por exemplo, os elementos túlio (0,5 ppm) e lutécio (0,8 ppm) que são as terras raras menos abundantes na crosta terrestre, são mais abundantes que a prata (0,07 ppm) e o bismuto (0,008 ppm) 2,3 . O primeiro elemento das terras raras descoberto foi o cério, em 1751, pelo mineralogista suíço A. F. Cronstedt, quando obteve um mineral pesado, a cerita. Porém, existem controvérsias quanto a este fato e atribui-se o ano de 1787 como o início da história das terras raras, quando Carl Axel Arrhenius encontrou um mineral escuro, a iterbita (também conhecido como gadolinita), em uma pequena vila, Ytterby, próxima a Estocolmo 2 . Por constituírem uma família que apresenta propriedades físicas e químicas semelhantes, exigindo um trabalho imenso para separá-los com a obtenção de espécies relativamente puras, este grupo de elementos foi pouco explorado durante anos e somente em 1907 é que praticamente todas as terras raras naturais foram conhecidas 4-9 . A industrialização das terras raras teve início com a fabricação de camisas de lampiões. Com o passar do tempo suas propriedades foram tornando-se mais conhecidas e seus compostos passaram a ser mais utilizados, tais como na produção de "mischmetal" para pedras de isqueiro, baterias recarregáveis e aplicações metalúrgicas 5,10 . Com o desenvolvimento tecnológico as terras raras passaram a ganhar novos usos e, hoje em dia, o universo de suas aplicações é muito abrangente, sendo utiliz...
Ion cyclotron resonance spectroscopy has been used to study the general gas-phase reaction of alkoxide ions (RBO-) with alkyl formates (HCOORb) for R = CH3, C&, and i-CbHT. These systems are shown to lead in all cases to the formation of a complex ion, (R,Rb02H)-, and to produce alkoxide displacement (RbO-) only when the electron affinity of RbO . is equal to or larger than that of R,O.. Evidence is presented that the complex ions correspond to alkoxide ions "solvated" by a single neutral molecule of alcohol and presumably held by hydrogen bonding, The "solvated" alkoxide ions can react further with alcohols to establish the intrinsic clustering ability of alcohols by the formation of a preferential "solvated" species. The order of solvating ability is shown to follow the scale of gas-phase acidity, t-C4H90H > i-CaH70H > on cyclotron resonance spectroscopy (icr) has been I successfully used in gas-phase studies of ion-molecule reactions to point out the importance of solvent effects in determining reactivity in solution. For example, the acidity order of simple alcohols in the gas phase is the reverse of the order in so1ution.'P2 Likewise, the acidities of the hydrocarbons toluene and propylene are greater than the acidity of water in the gas phase.3 The basicities of the amines follow thewhereas the solution order does not obey this simple trend. Since equilibria and kinetics are generally complex functions of intrinsic effects and solvation, these studies are particularly relevant for the separation of these factors and the understanding of the correct relationship between structure and reactivity. This paper.reports the application of icr to a new and unique method for the indirect preparation of gasphase alkoxide ions, "solvated" by a single neutral molecule of alcohol. This method is based on the general gas-phase reaction of alkoxide ions with alkyl formates in the pressure range of to 5 X 10-5 Torr, where bimolecular collisions are the only important processes. The "solvated" alkoxide ions thus formed can be further reacted with other neutral molecules to establish the order of preference of neutrals as solvating agents. In this study, the relative order of intrinsic solvating ability of the simple alcohols is determined by icr and related to hydrogen-bonding ability.The trend observed for relative stability of "solvated" species parallels closely the extensive measurements recently carried out by high-pressure mass spectrometry on gas-phase ionic equilibria of C1-solvated by different hydrogen-bonding neutral^.^ Furthermore, the relationship established in ref 5 by Kebarle between gas-phase acidity and the hydrogen-bonding capability of the "solvent" molecule is further explored for the case of the simple alcohols. Experimental SectionAll the experiments were carried out in a Varian V-5900 icr spec-(1) J. I. Brauman and L. K. Blair, J. Amer. Chem. SOC., 90, 6561 (2) J. I. Brauman and L. K. Blair, ibid., 92, 5986 (1970). (3) J. 1. Brauman and L. K. Blair, ibid., 93, 4315 (1971). (4) J. I. Brauman, J. M. Ri...
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