Different Conformational Ground States of A Pseudosaccharin Ether in the Gaseous Phase and in Solid Krypton

ANDREA GOMEZ-ZAVAGLIA, AGNIESZKA KACZOR, RUI ALMEIDA, MARIA DE LURDES S. CRISTIANO AND RUI FAUSTO.

Opatija, Croacia

Congreso; XXIX European Congress on Molecular Spectroscopy. Opatija, Croatia. Setiembre de 2008.; 2008

Benzisothiazoles are often vital structural units of biologically active systems. In particular, their use as herbicides and antibiotic agents has been extensively described [1]. Benzisothiazoles have also been described as phospholipase inhibitors efficient in the treatment of hepatic diseases [2]. In this work, the conformational space of the pseudosaccharyl ether 3-(allyloxy)-1,2-benzisothiazole 1,1-dioxide (ABID) has been studied by means of infrared spectroscopy and density functional theory (DFT) calculations. Five different low energy conformers (TSk, TC, GSk, GSkf and GC, with relative energies of 0.00, 1.97, 2.00, 3.82 and 6.02 kJ mol-1, respectively) were found on the DFT(B3LYP)/6-311++G(3df,3pd) potential energy surface of the molecule, all of them differing in the conformation of the allyl substituent. According to calculations, in the gaseous phase all conformers are significantly populated TSk:TC:GSk:GSk´:GC = 47%:16%:18% :12%:7%, at 350 K). However, in the cryogenic matrices only the TSk and TC conformers exist due to isomerization from the higher energy gauche forms to the most stable trans isomers during deposition of the matrix (conformational cooling). The observed conformational cooling is in consonance with the low calculated energy barriers for the GSk¨TSk, GSk´¨TSk and GC¨TC isomerization processes. Results from annealing experiments in a krypton matrix doubtlessly show that in this matrix the order of stability of the TSk and TC conformers is reversed, with the more planar TC form becoming the most stable conformer.