PhD thesis:
Aggregation of aminosulfonic acids and the products of their thermodissociation.
MSc Karolina Haupa
Supervisor: prof. dr hab. Zofia Mielke
The main aim of the presented PhD thesis was the study of the aggregation ability of aminosulfonic
acids and characterization of the molecular complexes formed between their thermodissociation
products. Aminosulfonic acids play an important role in biological processes. Their
thermodissociation products are constituents of the molecular complexes that play significant role in
atmospheric chemistry.
Experimental studies of the sulfonic acids aggregation were performed using ESI-MS and ESI-MS/MS
techniques. Five ω-aminosulfonic acids were studied: sulfamic acid, aminomethanesulfonic acid,
taurine, homotaurine and cysteic acid. The experimental data, obtained for sulfamic and
aminosulfonic acids, were supplemented by theoretical calculations at the DFT/B3LYP/aug-cc-pVDZ
level of theory. The structures of the neutral clusters of both acids ((NH 3·SO3)n, n=1-6;
(H2NCH2SO3H)n, n=1-3) and their anions were optimized in the gas phase and water solution.
Energetic analysis of the optimized clusters was performed. For the most stable structures the charge
distribution was calculated using NBO method.
The ESI-MS experiments performed in negative mode demonstrated that all studied aminosulfonic
acids have the ability to form singly and doubly charged homoassociates involving up to 20
monomeric subunits. Singly charged aggregates contain less than 10 monomers (n=1-10) whereas
doubly charged clusters involve 8 or more subunits (n=8-20). An increase of the length of a carbon
chain between the amino and sulfonic groups as well as the addition of functional group in the side
chain decreases the aggregation ability. The calculations showed that the sulfamic acid aggregates
are stabilized by dipole-dipole interactions between NH3·SO3 subunits and by N-HO hydrogen
bonds. Formation of aggregate leads to shortening of the N-S distance in NH3·SO3 subunit which
decreases with an increase of n. It was also concluded that ESI-MS spectra do not mirror the
structure of sulfamic acid in water solution; clusters observed in ESI-MS spectra are formed during
electrospray ionization processes. Both, theoretical and experimental studies confirmed that doubly
charged clusters can be formed in association process of two smaller singly charged ones.
Thermal decomposition products of sulfamic acids were identified and their thermodissociation
paths were studied using TGA-DTA method and matrix isolation technique coupled with FTIR
spectroscopy. The study of the molecular complexes formed by the aminosulfonic acids
decomposition products (NH3, SO3, H2O) was carried out by help of matrix isolation FTIR technique.
Computational studies of the complexes were performed at DFT/B3LYP, MP2 and DFT/B3LYP-D level
of theory with aug-cc-pVTZ basis set. The geometrical structures of the complexes were optimized,
their vibrational frequencies were calculated and the energetic analysis was carried out. For selected
complexes the NBO charge distribution analysis and topological electron density distribution using
QTAIM were performed. The complexes with different stoichiometry containing the following
molecules: SO3; NH3+SO3; NH3+SO3+H2O; RCOOH+H2O; RCOOH+NH3; RCOOH+SO3; RCOOH+SO3+NH3;
RCOOH+SO3+H2O (R=CH3, C6H5) were studied.
It was shown that sulfamic acid thermodissociates into NH3 and SO3 and aminomethanesulfonic acid
into NH3, SO2 and H2O molecules which allows to use the two acids as a source of molecules relevant
in atmospheric chemistry. Trapping of sufamic acid thermal decomposition products into an argon
matrix led to formation and characterization of the double EDA (electron-donor-acceptor) NH3-SO3NH3 complex. In this complex the sulfur atom plays a role of electron density acceptor from the
nitrogen atoms of both ammonia molecules, the two N-S bonds being nonequivalent. It is the first
adduct, observed experimentally, that involves two EDA bonds. The new, nontraditional way of
trapping acetic acid in an argon matrix enabled formation and characterization of the metastable
acetic acid – water complex. The complex was identified when acetic acid was sublimated, from the
solid sample kept at ca. 200 K, directly into the cryostat chamber where it was diltuted with H2O/Ar
gaseous mixture. In the metastable complex the water molecule is attached to the CO group of acetic
acid forming an O-HO hydrogen bond whereas the most stable cyclic complex involves two O-HO
bonds, the second one between acidic OH group and the water oxygen atom. The study of the
ammonia – sulfur trioxide complexes and acetic acid – water ones demonstrated the strong influence
of the temperature and concentration on the type of the complex which is formed. The structural
and spectral characteristics of the most stable 1:1 complexes formed by acetic and benzoic acid with
water and ammonia were obtained. The complexes have cyclic structure stabilized by two O-HO
(with water) or O-HO and N-HO (with ammonia) hydrogen bonds. The ternary NH3-SO3-H2O
complex was also identified in the matrix and characterized. It was shown that association of acetic
or benzoic acid with SO3-NH3 or SO3-H2O binary complexes to form the most stable ternary RCOOHSO3-NH3 complex or RCOOH-SO3-H2O one (R=CH3, C6H5) leads to the shortening of the N-S distance in
the SO3-NH3 adduct or the S-O distance in the SO3-H2O one. This fact suggest that carboxylic acid
molecule may play the same role as water molecule in aerosol formation process. The study showed
that EDA NH3-SO3 complex and its adducts with carboxylic acids can be potential cloud condensation
nuclei (CCN) in aerosol formation process.