1. Excitation
STRUCTURAL INVESTIGATION OF METHYLPARABEN
Angelica Dibble, John T. YI. Department of Chemistry
ABSTRACT
Methylparaben (Methyl 4-hydroxybenzoate) is mainly used as an
antiseptic in cosmetics, food and medicine. It has an excellent biological
degradation function, and is a highly efficient emulsifier. In this work,
we used theoretical calculations to evoke its unique molecular geometry
and electronic transition moment (TM). The minimum energy
conformational structures only differ in the location of hydrogen atoms
in both carboxylic and hydroxyl groups due to intramolecular forces.
These groups display a coplanar configuration with respect to the
benzene ring in both the ground state and excited state. Determination
of the most stable conformation reveals the highest occupied (HOMO)
and lowest unoccupied (LUMO) molecular orbitals to further deduce
the electronic TM and its orientation. The relevance of these findings
will be discussed.
CONCLUSIONS
• Methyl 4-hydroxybenzoate plays important role as an antiseptic in
cosmetics and pharmaceutical drugs.
• Able to classify the structures into two categories using the functional
groups with respect to the benzene : Methyl-H-out, and Methyl-H-in.
• Methyl-H-out groups found to have lower relative energies than Methyl-
H-in.
• Orientation of the lone pairs on the oxygen atom has an effect on the
structural stability and plays a key role in intramolecular forces.
• Significant structural changes were found in the excited state theoretical
calculation.
• Greatest contribution of the single electronic excitation was HOMO to
LUMO.
Acknowledgment
NSF (HBCU-RIA-1505311)
Split oil commonly contain ingredients (surface active agents) which
have a stronger affinity to bond to the surface being cleaned than that of
the hydrocarbon material (usually oil) being removed. In effect, the
chemistry sneaks under the oil and bonds to the surface being cleaned
thereby displacing the oil as shown in the following illustration.*
*Cleaning Technologies Group Posted on January 31, 2014 by John Fuchs
Emulsification
Skeletal Structure of Methyl 4-hydroxybenzoate
Combination of methyl benzoate and hydroxyl groups in benzene with allocation
of lone-pair electrons
Conformers Relative Energy(cm-1)
HF DFT MP2
Methyl-H-out- Trans 0.0 0.0 0.0
Methyl-H-out- CIS 31.8 28.9 25.5
Methyl-H-in- CIS 485.9 321.8 447.4
Methyl-H-in- Trans 519.2 351.7 476.0
Quantum Mechanical Calculations
Four lowest theoretical structures with its inertial axis
Calculated (MP2, B3LYP and HF / 6-31G+*) relative energies of
Methyl 4-hydroxybenzoate
Ground State Conformers
Methyl-H-out- CIS
Methyl-H-in- CIS Methyl-H-in- Trans
Methyl-H-out- Trans
a
b
Light (hv)
Structural changes in Methyl 4-hydroxybenzoate on S1 excitation
CIS/6-31G+* calculated one-electron molecular orbitals of
Methyl 4-hydroxybenzoate
LUMO
HOMO
OH
O
OH3C