Autor(es):
Jarmelo, Susana 
; Maria, Teresa M. R. ; Leitão, Maria Luísa P. ; Fausto, Rui
Data: 2001
Identificador Persistente: http://hdl.handle.net/10316/10718
Origem: Estudo Geral - Universidade de Coimbra
Detalhes do Documento
The low temperature crystalline and glassy states of methyl -hydroxy-isobutyrate
Descrição
The low temperature crystalline and glassy phases of methyl -hydroxy-isobutyrate (MHib) were identified and characterized structurally by differential scanning calorimetry, IR and Raman spectroscopy and molecular modeling. Within the temperature range 13–171 K, MHib exists as a glassy state, where individual molecules may assume the two conformational states previously observed for this compound isolated in an argon matrix and in the liquid phase [S. Jarmelo and R. Fausto, J. Mol. Struct., 1999, 509, 193]. At ca. 171 K, devitrification occurs and a crystalline phase may then be formed [T(onset)213 K], the enthalpy of crystallization being ca. 5 kJ mol–1. The crystalline phase was found to exhibit conformational selectivity—in this phase all individual molecules assume a conformation analogous to the most stable conformer found for the isolated molecule and in the liquid (the Syn-syn s-cis conformer, where the H–O–C–C, O–C–CO and OC–O–C dihedrals are ca. 0°). Molecular modeling and Raman data are consistent with a structural unit within the crystal where two MHib molecules form a centrosymmetrical hydrogen bonded dimer. The observed temperature of fusion [Tf(peak)] for the crystal is 240 K. http://dx.doi.org/10.1039/b007722o Fundação para a Ciência e Tecnologia, programas PRAXIS QUI/10137/98 e SFRH/157/2000.
The low temperature crystalline and glassy phases of methyl -hydroxy-isobutyrate (MHib) were identified and characterized structurally by differential scanning calorimetry, IR and Raman spectroscopy and molecular modeling. Within the temperature range 13–171 K, MHib exists as a glassy state, where individual molecules may assume the two conformational states previously observed for this compound isolated in an argon matrix and in the liquid phase [S. Jarmelo and R. Fausto, J. Mol. Struct., 1999, 509, 193]. At ca. 171 K, devitrification occurs and a crystalline phase may then be formed [T(onset)213 K], the enthalpy of crystallization being ca. 5 kJ mol–1. The crystalline phase was found to exhibit conformational selectivity—in this phase all individual molecules assume a conformation analogous to the most stable conformer found for the isolated molecule and in the liquid (the Syn-syn s-cis conformer, where the H–O–C–C, O–C–CO and OC–O–C dihedrals are ca. 0°). Molecular modeling and Raman data are consistent with a structural unit within the crystal where two MHib molecules form a centrosymmetrical hydrogen bonded dimer. The observed temperature of fusion [Tf(peak)] for the crystal is 240 K. http://dx.doi.org/10.1039/b007722o Fundação para a Ciência e Tecnologia, programas PRAXIS QUI/10137/98 e SFRH/157/2000.
Tipo de Documento
Artigo
Idioma Inglês
Idioma Inglês
| Financiadores do RCAAP | |||||||
|
|
|
|
|
|
||