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Conformational Landscape, Photochemistry, and Infrared Spectra of Sulfanilamide
journal contribution
posted on 2013-01-31, 00:00 authored by Ana Borba, Andrea Gómez-Zavaglia, Rui FaustoA combined matrix isolation FTIR and theoretical DFT(B3LYP)/6-311++G(3df,3pd)
study of sulfanilamide (SA) was performed. The full conformational
search on the potential energy surface of the compound allowed the
identification of four different minima, all of them bearing the sulfamide
nitrogen atom placed in the perpendicular orientation relatively to
the aromatic ring and differing from each other in the orientation
of the hydrogen atoms connected to the two nitrogen atoms of the molecule.
All conformers were predicted to be significantly populated in the
gas phase (at 100 °C, their relative populations were estimated
as being 1:0.9:0.3:0.2). However, in agreement with the theoretically
calculated low-energy barriers for conformational isomerization, in
the low-temperature matrices, only the most stable conformer could
be observed, with the remaining forms being converted into this form
during matrix deposition (conformational cooling). The unimolecular
photochemistry of matrix-isolated SA (in both argon and xenon) was
also investigated. Upon broadband UV irradiation (λ > 215
nm),
two photofragmentation pathways were observed: the prevalent pathway
(A), leading to extrusion of sulfur dioxide and simultaneous
formation of benzene-1,4-diamine, which then converts to 2,5-cyclohexadiene-1,4-diimine,
and the minor pathway (B), conducting an γ-cleavage
plus [1,3] H-atom migration from the sulfamide group to the aromatic
ring, which leads to formation of iminosulfane dioxide and aniline,
the latter undergoing subsequent phototransformation into cyclohexa-2,5-dien-1-imine.
Finally, the crystalline polymorph of SA resulting from warming (265
K) the amorphous solid obtained from fast cooling of the vapor of
the compound onto the cold (13 K) substrate of the cryostat was identified
spectroscopically, and found to be the γ-crystalline phase,
the one exhibiting in average longer H-bonds and an infrared spectrum
resembling more that of the low temperature SA glass. Full assignment
of the infrared spectra of this crystalline variety as well as of
those of the β-polymorph room temperature crystalline sample
and low temperature amorphous state was undertaken with help of theoretical
results obtained for the crystallographically relevant dimer of SA.
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Keywords
unimolecular photochemistrycompoundconformerenergy surfaceConformational Landscapenitrogen atomsmatrix depositionphotofragmentation pathwayspolymorphmatrix isolation FTIRsulfur dioxideUVgas phaseFull assignmentInfrared Spectrasulfamide nitrogen atomcoolingDFTiminosulfane dioxideorientationhydrogen atomstemperature SA glasssulfamide group