(253w) Study on the Computational Calculations and Experiments for Five-membered Nitrogen Heterocyclic Compounds and Polymers

Study on the Computational
Calculations and Experiments for Five-membered Nitrogen Heterocyclic Compounds
and Polymers

Abstract

Five-membered
nitrogen heterocyclic compounds, as both proton carriers and acceptors, have
been drawing more attentions due to their electrochemical and thermal stability
in high-temperature proton exchange membranes (PEM) research field. The structural and
electronic properties of triazole and tetrazole were studied by the way of computational calculations in this work.
It?s the theoretical foundation of selection of N-containing functional group for
preparing PEM. All optimizations
and vibration frequency calculations were carried out by DFT/B3LYP method. The NBO calculations were also
investigated for their natural charge distributions. According to the
value
of E_HOMO, E_LUMO, PA and pKa, the acidity of
1H-1,2,3,4-tetrazole is the strongest with pKa value of 4.73 due to its highly delocalized
conjugated electron and more uniform electron distribution. It was supposed
that tetrazole might be the ideal PEM monomer which could donate and accept
protons both easily. Thirty stable tetrazole-tetrazolium dimers were obtained by
optimization,
and the two rings of these dimers were connected with the N∙∙∙H?N
hydrogen bonds. The hydrogen bond lengths were closely related with the natural
charge distributions. Six transition state (TS) structures in the proton
transfer reactions among the eight main dimers with lower energy were
investigated. The proton could
transfer freely at the room temperature and the energy barrier of inter-molecular
proton transferring was
smaller as expected, between 3.53 kJ×mol^-1
and 12.66 kJ×mol^-1.
On
the basis of the theory, a novel N-heterocyclic PEM based on tetrazole
functional group was experimentally obtained and its properties were also studied.

Materials:

4-cyanophenol;
sodium azide; dibromozinc; 1,8-bis(triethoxysilyl)octane
(Si-C8);  tetraethyl orthosilicate (TEOS); phosphoric acid; polytetrafluoroethene
(PTFE) membrane.

Experimental:

Synthesis
of 5-[4-[(methyldimethoxylsilyl)propoxyl]phenyl]-1H-tetrazole

Macromonomer 5-[4-[(methyldimethoxylsilyl)propoxyl]phenyl]-1H-tetrazole
was synthesized through Williamson reaction between 5-(4-hydroxyphenyl)-1H-tetrazole
and (3-chloropropyl) trimethoxysilane in an alkaline condition.

Synthesis
of PA doped tetrazole-based
polymer membrane

PA doped
tetrazole-based polymer membrane was prepared through in-situ polymerization
into the porous polytetrafluoroethene (PTFE) membrane after sol-gel process
among macromonomer 5-[4-[(methyldimethoxylsilyl)propoxyl]phenyl]-

1H-tetrazole, tetraethyl
orthosilicate (TEOS) and 1,8-bis(triethoxysilyl)octane in a mixture of methanol and ethanol. Meanwhile, phosphoric
acid was used as both the catalyst to complete the sol-gel reaction and the
acidic dopant was added into the sol

solution.

Characterization
of PA doped tetrazole-based polymer membranes

The
morphology of surface was examined by SEM and the thermal stability was tested
by TG. Proton conductivities were measured by AC impedance. The result
indicated that the polymer was well in-situ polymerized into the PTFE micropores.
The membrane was of good thermal, dimensional and chemical stability, and also good
proton conductivity.

The
proposed synthetic route to inorganic/organic hybrid polymers bearing 5-(4-hydroxyphenyl)-1H-tetrazole

KeyWords: Triazole; 5-(4-hydroxyphenyl)-1H-tetrazole; DFT; proton
exchange membrane; in-situ polymerization