FLUORINATION OF POLYFORMALDEHYDE: STRUCTURAL PATTERN RECOGNITION STUDY USING FIRST-PRINCIPLES

SREEPAD H.R.¹*, RAMEGOWDA M.^2
1Research Centre, Government College (Autonomous), Mandya – 571401, Karnataka State, India
²Research Centre, Government College (Autonomous), Mandya – 571401, Karnataka State, India
* Corresponding Author : hrsreepad@gmail.com

Received : 29-09-2011     Accepted : 03-11-2011     Published : 07-11-2011
Volume : 3     Issue : 3       Pages : 96 - 99
Int J Mach Intell 3.3 (2011):96-99
DOI : http://dx.doi.org/10.9735/0975-2927.3.3.96-99

Conflict of Interest : None declared
Acknowledgements/Funding : HRS is highly thankful to the University Grants Commission, India for granting minor research project [MRP(S)-798/10-11/KAMY022/UGC-SWRO]. HRS is also thankful to Prof. Umesh V. Waghmare of Theoretical Sciences Unit, JNCASR, Bangalore for his guidance

Fluorination of organic compounds is an important task which brings in changes in Physical and Chemical parameters in the organic material. Even the structural pattern of the molecule gets altered due to replacement of Hydrogen atoms by Fluorine atoms. Structural pattern recognition in case of molecules is an important task in the field of bio-engineering. Theoretical structure simulation is also employed in this technologically important field. An attempt has been made to look for the changes that take place due to fluorination in the technologically important polymer Polyformaldehyde. Electronic Density of States and Phonon modes in the material have been computed using First-principles calculations based on Density Functional Theory.

Electron Density of States, First-principles, Phonons, Structural Pattern recognition, Theoretical structure simulation, Density Functional Theory, Fluorination, Polyformaldehyde.

Structural pattern recognition study plays an important role in the world of molecules. Several experimental techniques indirectly work on the basis of pattern recognition by looking at the signatures of molecules [1-3] .
Bio-engineering highly depends on the structural pattern recognition of bio-molecules. Some of the available prominent experimental techniques are X-ray image plating, Fourier Infra-Red Spectroscopy and Raman Spectroscopy [4-6] .
Theoretically also, one can look for structural pattern recognition in organic molecules. Structure simulation is one such technique which is widely used in this field. Fluorination of organic compounds is an important task. Because, it can bring in several changes in physical and chemical properties of molecules [7,8]

Structure simulation can be done using several programs [9] . Using those codes one can simulate different possible structures for a given molecule. Theoretically one can look at various parameters of the simulated structures.
Using those parameters one can compute several other physical and chemical parameters pertaining to those structures. By looking at the computed values, one can decide whether the structure is feasible or not. Hence one can arrive at a definite conclusion regarding the possible structure without any ambiguity. Phonon modes calculated at the gamma point and along different axes give a clear idea about the stability of the structural pattern of the simulated molecule.

First-principles calculations based on Density Functional Theory (DFT) can be effectively used to study the internal structure and properties of the material in detail [10] .
We use plane wave self consistent field (PWSCF) [11] implementation of density functional theory (DFT), with a Local density approximation (LDA) [12] to exchange correlation energy of electrons and ultrasoft pseudopotentials [13] to represent interaction between ionic cores and valence electrons.
Kohn-Sham wave functions were represented with a plane wave basis with an energy cutoff of 40 Ry and charge density cutoff of 240 Ry.
Integration over Brillouin zone was sampled with a Monkhorst-Pack scheme [14] with appropriate k point mesh and occupation numbers were smeared using Methfessel-Paxton scheme [15] with broadening of 0.003 Ry. The structure was relaxed to minimize energy.

Polyformaldehyde is a technologically important polymer. Corrosion resistant conductive Polyformaldehyde compositions with carbon nanotube and carbon nano fibre conductive fillers have been patented [16] .
Completely relaxed structure of the non fluorinated Polyformaldehyde as observed using the XCrySDen [17] is given in [Fig-1] . [18] .
Structural parameters of the simulated structural pattern of Polyformaldehyde are given in [Table-1] , [Table-2] and [Table-3] .
Theoretical structure simulation has been carried out by replacing one Hydrogen atom with Fluorine atom and then by replacing both Hydrogen atoms by Fluorine atoms. The structural parameters of the partially and completely fluorinated Polyformaldehyde are tabulated in [Table-4] and [Table-5] .
Electron Density of States (EDOS) have been computed in case of non fluorinated, partially fluorinated and completely fluorinated Polyformaldehyde using Electronic structure calculation code of Quantum espresso and are shown in [Fig-2] , [Fig-3] and [Fig-4] .
The phonon modes along X-axis have been calculated in case of non fluorinated, partially fluorinated and completely fluorinated Polyformaldehyde and are shown in [Fig-5] , [Fig-6] and [Fig-7] . Phonon modes indicate that the stability of the molecule decreases due to fluorination. This can be attributed to the deformation caused by replacing Hydrogen atoms by Fluorine atoms having larger radius.
Value of Fermi energy and band gap have been obtained from Electron Density of States computation using Electronic structure calculation code of Quantum espresso. The value of Dielectric constant has been computed using the phonon data. The parameters so obtained are tabulated in [Table-6] and [Table-7] .
As it can be seen from the above tables, the fluorination is bringing change in Electronic Density of States and Phonon modes. It is altering values of Fermi energy, Band gap and also the value of average static Dielectric constant. The value of band gap increases due to fluorination whereas, the average value of Static Dielectric constant decreases due to fluorination.

Structural pattern recognition plays an important role in deciding the structural conformation of molecules. Fluorination of Polyformaldehyde alters both Electronic Density of States and Phonon modes. The band gap also gets altered. The upper level of frequency in phonon modes increases due to fluorination. This is essentially because of the change in the structural pattern of the molecule due to fluorination.

HRS is highly thankful to the University Grants Commission, India for granting minor research project [MRP (S) - 798 / 10 - 11 / KAMY022 / UGC - SWRO]. HRS is also thankful to Prof. Umesh V. Waghmare of Theoretical Sciences Unit, JNCASR, Bangalore for his guidance.

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	Fig. 1- Structural pattern of non fluorinated Polyformaldehyde
	Fig. 2- EDOS in non fluorinated Polyformaldehyde
	Fig. 3- EDOS in partially fluorinated Polyformaldehyde
	Fig. 4- EDOS in completely fluorinated Polyformaldehyde
	Fig. 5- Phonon modes in non fluorinated Polyformaldehyde
	Fig. 6- Phonon modes in partially fluorinated Polyformaldehyde
	Fig. 7- Phonon modes in completely fluorinated Polyformaldehyde
	Table 1- Bond lengths in Polyformaldehyde
	Table 2- Bond angles in Polyformaldehyde
	Table 3- Other Structural Parameters in Polyformaldehyde
	Table 4- Structural Parameters in partially fluorinated Polyformaldehyde
	Table 5- Structural Parameters in completely fluorinated Polyformaldehyde