Monoclinic-to-orthorhombic phase transition of the hexamethylenetetramine-2-methylbenzoic acid (1/2) cocrystal with temperature-dependent dynamic molecular disorder.
Acta Crystallogr C Struct Chem
; 72(Pt 12): 971-980, 2016 Dec 01.
Article
em En
| MEDLINE
| ID: mdl-27918299
ABSTRACT
As a function of temperature, the hexamethylenetetramine-2-methylbenzoic acid (1/2) cocrystal, C6H12N4·2C8H8O2, undergoes a reversible structural phase transition. The orthorhombic high-temperature phase in the space group Pccn has been studied in the temperature range between 165 and 300â
K. At 164â
K, a t2 phase transition to the monoclinic subgroup P21/c space group occurs; the resulting twinned low-temperature phase was investigated in the temperature range between 164 and 100â
K. The domains in the pseudomerohedral twin are related by a twofold rotation corresponding to the matrix (100/0-10/00-1. Systematic absence violations represent a sensitive criterium for the decision about the correct space-group assignment at each temperature. The fractional volume contributions of the minor twin domain in the low-temperature phase increases in the order 0.259â
(2)â
ââ
0.318â
(2)â
ââ
0.336â
(2)â
ââ
0.341â
(3) as the temperature increases in the order 150â
ââ
160â
ââ
163â
ââ
164â
K. The transformation occurs between the nonpolar point group mmm and the nonpolar point group 2/m, and corresponds to a ferroelastic transition or to a t2 structural phase transition. The asymmetric unit of the low-temperature phase consists of two hexamethylenetetramine molecules and four molecules of 2-methylbenzoic acid; it is smaller by a factor of 2 in the high-temperature phase and contains two half molecules of hexamethylenetetramine, which sit across twofold axes, and two molecules of the organic acid. In both phases, the hexamethylenetetramine residue and two benzoic acid molecules form a three-molecule aggregate; the low-temperature phase contains two of these aggregates in general positions, whereas they are situated on a crystallographic twofold axis in the high-temperature phase. In both phases, one of these three-molecule aggregates is disordered. For this disordered unit, the ratio between the major and minor conformer increases upon cooling from 0.567â
(7)0.433â
(7) at 170â
K via 0.674â
(6)0.326â
(6) and 0.808â
(5)0.192â
(5) at 160â
K to 0.803â
(6)0.197â
(6) and 0.900â
(4)0.100â
(4) at 150â
K, indicating temperature-dependent dynamic molecular disorder. Even upon further cooling to 100â
K, the disorder is retained in principle, albeit with very low site occupancies for the minor conformer.
Buscar no Google
Coleções:
01-internacional
Base de dados:
MEDLINE
Tipo de estudo:
Prognostic_studies
Idioma:
En
Revista:
Acta Crystallogr C Struct Chem
Ano de publicação:
2016
Tipo de documento:
Article
País de afiliação:
Malásia