Thermal decomposition and vibrational spectroscopic aspects of pyridinium hexafluorophosphate (C5H5NHPF6)

Article abstract of DOI:10.1016/j.molstruc.2016.06.078

Thermal decomposition and vibrational spectroscopic properties of pyridinium hexafluorophosphate (C₅H₅NHPF₆) have been studied. The structure of the compound is better interpreted as having a cubic space group, based on Raman and infrared vibrational spectroscopy experiments and group theoretical correlation data between site symmetry species and the spectroscopic space group. The ¹³C NMR data shows three significant signals corresponding to the three chemical environments expected on the pyridinium ring i.e. γ, β and α carbons, suggesting that the position of the anion must be symmetrical with respect to the pyridinium ring's C_2v symmetry. The process of thermal decomposition of the compound using TGA methods was found to follow a contracting volume model. The activation energy associated with the thermal decomposition reaction of the compound is 108.5?kJ?mol^?1, while the pre exponential factor is 1.51?×?10⁹?sec^?1.

Full text of DOI:10.1016/j.molstruc.2016.06.078

Accepted Manuscript
Thermal decomposition and vibrational spectroscopic aspects of pyridinium
hexafluorophosphate (C H NHPF )
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5
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M.D.S. Lekgoathi, L.D. Kock
PII:
S0022-2860(16)30665-2
10.1016/j.molstruc.2016.06.078
MOLSTR 22697
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 22 December 2015
Revised Date: 28 June 2016
Accepted Date: 28 June 2016
Please cite this article as: M.D.S. Lekgoathi, L.D. Kock, Thermal decomposition and vibrational
spectroscopic aspects of pyridinium hexafluorophosphate (C H NHPF ), Journal of Molecular Structure
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5
6
(2016), doi: 10.1016/j.molstruc.2016.06.078.
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Graphical Abstract
NMR
Raman, IR
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Thermal decomposition and vibrational spectroscopic aspects of
Pyridinium Hexafluorophosphate (C H NHPF )
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M.D.S. Lekgoathi ,L.D. Kock
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Department of Applied Chemistry, South African Nuclear Energy Corporation SOC
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Limited (Necsa), P.O. Box 582, Pretoria 0001, South Africa.
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Abstract
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Thermal decomposition and vibrational spectroscopic properties of pyridinium
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hexafluorophosphate (C H NHPF ) have been studied. The structure of the compound is
better interpreted as having a cubic space group, based on Raman and infrared vibrational
spectroscopy experiments and group theoretical correlation data between site symmetry
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species and the spectroscopic space group. The C NMR data shows three significant
signals corresponding to the three chemical environments expected on the pyridinium
ring i.e. γ, β and α carbons, suggesting that the position of the anion must be symmetrical
with respect to the pyridinium ring’s C2v symmetry. The process of thermal
decomposition of the compound using TGA methods was found to follow a contracting
volume model. The activation energy associated with the thermal decomposition reaction
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of the compound is 108.5 kJ.mol , while the pre exponential factor is 1.51 x 10 sec .
Key words: Pyridinium hexafluorophosphate, Group theory, Raman, TGA, Kinetics
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Corresponding author: M.D.S Lekgoathi, Tel: +27 12 305 6839 Fax: +27 12 305 6945.
E-mail: mpho.lekgoathi@necsa.co.za
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1.
Introduction
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Pyridinium hexafluorophosphate is a well-known precursor used in the synthesis of metal
hexafluorophosphate salts [1]. These salts find applications as components of inhibitors,
polymerization catalysts and flame retardants, but most importantly, in rechargeable
batteries (lithium, potassium and sodium ion-batteries). The vibrational spectroscopic
properties of pyridinium hexafluorophosphate have been studied before [1,2], but no
mention of its thermal decomposition signature has been made and no correlation of
vibrational spectroscopy experimental data with group theory was made to seek
agreement and structural consistency with the space group for this important compound.
It is possible to misinterpret vibrational spectroscopy data of a compound if the
underlying space group is not verified by correlating Raman and IR experimental data
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with group theory [3]. For example, It has been shown that for the LiPF salt, Raman
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spectroscopy is a much more sensitive probe in situations where order/disorder or phase
transition processes occur [4], a blind spot for pXRD. Phase transitions associated with
order-disorder orientations in the pyridinium ring have also been observed for pyridinium
5 5 6
hexafluorophosphate (C H NHPF ) and related compounds [5]. This is in contrast with
the works of Copeland et al. [6], who could not observe any associated phase transitions
for the same compound, and concluded that there is only one crystal structure for
C H NHPF , being R-3m (166). However, the works of Cook [7], on the vibrational
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spectroscopy of pyridinium salts show that the pyridinium ring itself has a C symmetry,
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v
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and when interpreted from the perspective of C H NHPF6, we should expect the presence
of several triply degenerate phonon modes as part of the irreducible representations of the
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compound, emanating from the octahedral PF
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anion.
The structural data for a particular material has a strong dependence on the local
symmetry of that material. Vibrational spectroscopy has been shown to be a better
technique in detecting local symmetry changes as compared to pXRD which is very good
in determining crystal structure for crystalline materials with well defined long range
order [3]. It is clear that since bond energies are dependent on the bond properties, such
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as bond constants and atom positions, the underlying thermodynamic properties will
depend on the data provided by techniques that accurately reflect a local structure.
The controversies in explaining phase transitions related to space group information in
C H NHPF is analogous to the LiPF case, which has been shown to display a possible
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cubic phase at local symmetry level using Raman spectroscopy, while the long range
cooperative symmetry is a trigonal based structure according to XRD data [3]. The effect
of this on LiPF thermogravimetric data is profound, particularly when it comes to
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activation energy determinations, which are said to be reported with varying magnitudes
[8]. This shows that a change in chemical composition or a possible phase transition,
accompanied by a change in crystal structure of a material, will have an effect on the
thermal properties of a substance.
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In this work, we report the vibrational spectroscopy data of C H NHPF as correlated to
group theoretical works and we show that the space group R-3m (166) as reported in
literature [6] is not an accurate reflection of the compound, but rather an Fm3m (225)
space group is a more representative alternative. We further show the thermal
decomposition kinetics of C H NHPF and its thermal decomposition signature.
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Experimental methods and materials
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2.1 Synthesis of C H NHPF
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A method proposed by Willmann et al. [9] for the synthesis of pyridinium
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hexafluorophosphate was used. Approximately 20 ml of aqueous HPF acid was added to
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a beaker and stirred continuously. About 10ml of pyridine was added drop wise using a
syringe while continuously stirring. The precipitate was filtered using a vacuum system
and washed with water, and then the product was dried in an oven at 90 degrees Celsius
for 6 hours.
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2.2 Raman and infrared spectroscopy
The Ram II module (FT-Raman) mounted onto a vertex 70 FT-IR spectrometer (Bruker)
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was used to record the Raman spectra from 3500 to 0 cm . The sample was in a solid
form. A 1064 nm wavelength excitation radiation was used and a 50 mW power setting
-1
and a wavenumber resolution of 4 cm was used throughout this study. FT-ATR Infrared
reflectance spectra for powder samples were recorded with a Tensor T27 FT-IR
spectrometer fitted with a Harrick Mvp-pro ATR cell. All reflectance spectra were
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recorded between 4000 to 500 cm with 32 scans and a resolution of 4 cm .
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2.3 C NMR analysis
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For the C NMR, a Bruker NMR Spectrometer 500 was used for the analysis. The
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instrument was set for C dipolar dephasing (non-quaternary suppression) at room
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temperature (24.5 °C); C NMR (126 MHz, Deuterated methanol solvent) δ 244.94,
236.46, 224.39, 149.36, 141.47, 128.90, 53.68, 45.99, 33.77.
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2.4 Thermal decomposition
2.4.1 TG
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About 20 mg sample of C H NHPF was analyzed on a modified TGS-2 Perkin Elmer
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TGA instrument. The instrument was enclosed within a dry glovebox and constantly
purged with nitrogen gas. The sample was therefore heated from 30 to 600 °C at 10
°C/min under nitrogen purge gas.
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2.4.2 Thermal decomposition reactor-IR system
An in-house developed thermal decomposition reactor system coupled to a BOMEM MB
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3000 spectrometer was used. The C H NHPF salt was loaded into a Ni sample pan, fitted
into a tube reactor and then purged with a constant flow of laser-grade helium. A Hi-Tech
Elements heater was used to heat the stainless-steel tube reactor to 350 °C. The evolved
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gas was captured at 20-minute intervals with an Infrared gas cell fitted with zinc selenide
windows, and a spectrum was recorded.
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2.4.3 Kinetic studies
Decomposition kinetics studies were carried out using an isothermal method based on TG
data obtained by using a dedicated TG instrument placed in a nitrogen glove box as in
section 2.4.1. Experimentally obtained data was fitted into various solid state kinetic
models tabulated in the ICTAC kinetics committee recommendations by Vyazovkin et al.
[10] and the best line fit using the least squares statistical evaluation technique was
chosen. A confidence interval of 95% (Table 3) was used.
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The following integrated rate equation was used:
ꢃ
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gꢀαꢁ = ꢂ1 − ꢀ1 − αꢁ
ꢄ
ꢅ = ꢆkt
(1)
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In Eq. (1), α is the extent of the reaction as derived from the global mass loss based on
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TG experimental data, k is the rate constant (s ) and t is time (s). The extent of the
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reaction is calculated using Eq. (2).
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ꢇ
ꢈ
ꢉꢇ
ꢉꢇ
ꢊ
α =ꢆ
(2)
ꢇ
ꢈ
ꢋ
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0 t f
Where m is the initial mass, m is the mass at a particular time and m is the final mass.
The dependence of the rate constant on temperature is based on the Arrhenius equation:
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ꢉꢍ
ꢎ
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k = Aexp ꢌ
ꢑ
(3)
ꢏꢐ
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Where E
a
is the activation energy in J.mol , R is the universal gas constant (8.314 J.mol
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-1
.K ) and A is the pre-exponential factor (s ) associated with the rate of the reaction. The
temperature T has units in Kelvin. The value of the rate constant, k at a particular
temperature, was obtained from the slope of the g(α) versus time curve (Eq. 1).
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3.
Results and discussion
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Raman and IR data of C H NHPF in Figure 1a and b showing the vibrational band
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frequencies and their assignments are given in Table 1. The 551 and 795 cm IR active
bands belong to T (ν and ν respectively). The T is the only IR active mode from the
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u
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1u
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octahedral PF anion in the compound; the others are contribution from the pyridinium
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ring. Previously, the vibrational band assignments for the C H NHPF were done by
Mohamed et al [2] and they were in line with the presence of an octahedral PF anion in
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the molecule. The frequencies of these P-F bending and stretching modes are similar to
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+
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other MPF salts (M= Li , Na and K ) as reported by Kock et al. [3]. The normal modes
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of this ionic solid are a combination of those from the octahedral PF structure and the
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ring breathing modes (Table 1) [2,7]. It is therefore important that the space group
assigned to the C H NHPF supports the presence of octahedral irreducible
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representations.
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Table 1: Here
Figure 1: Here
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The 1608 cm vibration splits into two, and when interpreted together with the broad
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nature of the 3326 cm band, vibrations associated with H O can be said to be
superimposed with the N-H and ring vibrations at these frequencies. This could indicate
the presence of crystal water.
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According to Copeland et al [6], the crystallographic space group for the compound
C H NHPF belong to R-3m (166) based on the single crystal x-ray determination. From
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the basis of the space group data [6] and considering the relevant atomic equilibrium in
the site symmetries, the crystallographic space group R-3m (166) can be correlated to a
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spectroscopic space group D 3d [11] Considering the full site symmetry associated with
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this space group, there are two distinct kinds of D3d sites, one accommodating one
nitrogen atom and the other taking one phosphorus atom. The six hydrogens occupy a C2
s
site, while the fluorine atoms occupy a C site. The carbon atoms would split to occupy
two different sites; one C2h site would accommodate three and then a C site would
3v
accommodate two carbon atoms. The total irreducible representations for this scenario
would therefore be given in Equation 4 below.
ꢙꢚꢛꢜꢝ
ꢀꢡꢁ
ꢀꢡꢁ
ꢀꢤꢁ
ꢀꢤꢁ
(4)
Г
=ꢆ3ꢞꢟꢠ + 5ꢢꢠ + 8ꢢꢣ + 5ꢞꢥꢣ + 3ꢞꢟꢣ
ꢒ ꢔ ꢕꢔꢖꢗ
ꢘ
ꢓ
ꢓ
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Therefore eight Raman active modes are expected, noting that five of them are doubly
degenerate. In addition, thirteen IR active modes are expected, noting that eight of them
are doubly degenerate. On the other hand, the A1u term is silent. Considering a three
dimensional solid material, there are 3N-3 number of vibrations expected, and this would
B
translate into 54 vibrations for a 19 atom compound like C
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H
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NHPF
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(with Z = 1 and N
= 19), which is clearly incompatible with the 37 total vibrations predicted by Equation 4.
Based on this explanation, and taking into account that there is no single T1u mode
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predicted by Equation 4, an inherent part of a PF anion octahedral structure [2,3,4], the
space group for C H NHPF may not be interpreted as R-3m (166) based on group theory
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and vibrational spectroscopy data.
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The understanding of the orientation of the PF anion relative to the pyridinium ring
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becomes important in order to better predict a correct crystal structure for C H NHPF .
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Solid state C NMR of pyridinium hexafluorophosphate is shown in Figure 2a. The
NMR spectrum indicates three significant signals corresponding to the three chemical
environments expected on a pyridinium ring i.e. γ (8C and 12C), β (11C and 9C) and α
(10C) carbons as shown in Figure 2b.
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Figure 2a: Here
Figure 2b: Here
A signal at δ = 149.36 ppm, integrating for one carbon, correlates with the γ carbon on
the ring. The signal at δ = 141.47 ppm integrating for two carbons corresponds to the two
α carbons of the pyridinium ring, the two α carbons appear at such a high chemical shift
of δ = 141.47 ppm due to the deshielding effect from the electron withdrawing
hexafluorophosphate ion. Finally the signal at δ = 128.90 ppm integrating for two carbons
is representative of the two β carbons of the pyridinium ring. It should be noted that the
lack of resolution resulting in the broadening of signals is the results of solid state run
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NMR, hence poor splitting. From the NMR data in Figure 2 and Table 2, the presence of
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a pyridinium ring and PF
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anion in the synthesized compound are confirmed.
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Table 2: Here
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The prediction of three chemically different carbon atoms by C NMR means that the
hexafluorophosphate anion must be symmetrical with respect to the pyridinium ring’s C2v
symmetry, with monodentate coordination between the anion’s fluorine atom and the
positive point charge at the nitrogen position of the ring. In this configuration, the most
significant consideration to bear in mind is that the pyridinium ring appears as a positive
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point charge relative to the PF anion, an arrangement analogous to other MPF salts (M=
Li, Na, K). These inorganic MPF salts have been shown to belong to an Fm3m (225)
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space group by kock et al. [3]. Taking this view into account, and therefore assuming an
Fm3m (225) space group for C H NHPF , we obtain the following irreducible
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5
6
representations based on group theory calculations (Equation 5):
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ꢀ
ꢟꢠ
ꢡꢁ
ꢀꢡꢁ
ꢀꢡꢁ
ꢀꢤꢁ
ꢀꢧꢁ
ꢀꢧꢁ
2
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ꢞ
+ ꢢꢠ + ꢦꢥꢠ + 12ꢦꢟꢣ + ꢦꢟꢠ + 3ꢦꢥꢣ
(5)
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5 5 6
Equation 5 predicts 54 vibrations for C H NHPF , noting that 6 of them are Raman active
with subscript (R), 36 are infrared active (I) and 12 of them are silent (S). It is therefore
better to represent the crystal structure of C H NHPF as a cubic Fm3m (225), consistent
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with the assignments of an octahedral PF anion from Mohamed et al [2] for the same
compound.
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Further evidence of a cubic related structure for C
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H
5
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NHPF comes from Raman and
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infrared data (Figure 1). The T band at 111 cm is inherently an IR active mode but
1u
appears in Raman spectra. Its appearance in the Raman spectra of C H NHPF , together
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with A1g appearing in the IR spectra, is a sign of a break in the centrosymmetric geometry
of the molecule and thus points to a distortion in the PF anion. This behavior has been
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observed in other hexafluorophosphate salts such as the LiPF
6
by Kock et al [3] with a
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T band appearing at 99 cm and also at 76 cm for CsPF as observed by Heyns et al
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u
6
[18], which are all cubic compounds within the space group Fm3m (225).
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The space group information has direct influence on the thermodynamic properties of the
material. In order to understand the decomposition profile of C H NHPF the material
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was subjected to a strictly anhydrous environment (section 2.4.2), and thermally
decomposed into its gaseous components. The composition of the thermal decomposition
off-gas products shows presence of crystal water in the pyridinium hexafluorophosphate
sample. This follows from the fact that the thermal decomposition reactor, the sample
powder and the lines were thoroughly purged with pure PF gas until no traces of
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hydrolysis products were observed prior to thermal decomposition of C H NHPF .
5
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However, the IR spectrum of the off-gas products (Figure 3) shows bands at 1416, 993
-1
and 868 cm , in line with the values reported by Yang et al. [12] for POF . Hydrogen
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-1
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fluoride is also observed at 3884 cm as expected. Although PF gas was expected as a
product according to Equation 5, it is known that in the presence of water, POF evolves
3
as a by-product [13]. Following the argument on the basis of the experimental procedure
to eradicate moisture in the system, and considering possible existence of water according
to IR spectrum in Figure 1b, a possible source of water would therefore be in the form of
crystal water, which would be an intricate part of the compound.
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The intense vibrational bands associated with the strong IR absorber, the POF
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gas,
suppresses the observed intensity of pyridinium ring vibrational bands at 1157 and 1298
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cm .
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Figure 3: Here
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∆
3
3
00
01
C H NHPF → C H Nꢆ + HF + PF
(6)
(7)
ꢨ
ꢨ
ꢩ
ꢨ
ꢨ
ꢨꢀꢪꢁ
3
3
3
02
03
04
PFꢨꢀꢪꢁ + H O ꢆ→ POFꢫꢀꢪꢁ + 2HFꢀꢪꢁ
ꢥ
ꢀꢪꢁ
3
3
3
3
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08
The TG profile of C H NHPF is presented in Figure 4. The onset of thermal
5
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decomposition of C H NHPF is 125 °C. Only 0.24% of the substance remains as non
5
5
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volatile material, which could essentially just be impurities (Figure 4).
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3
3
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Figure 4: Here
Several models were evaluated for the thermal decomposition study of pyridinium
hexafluorophosphate (Table 3). Based on the best line fit of 99.98%, the compound
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3
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C
5
H
5
6
NHPF was found to thermally decompose according to the contracting volume
model (R3, three dimensional), giving off HF and pyridine, with the observed POF as a
3
5
byproduct of PF gas (Figure 3).
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3
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3
3
3
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Table 3: Here
Figure 5: Here
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The activation energy (E ) for the thermal decomposition reaction of C H NHPF is
a
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-1
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found to be 108.5 (kJ.mol ), which is of similar magnitude as that of the LiPF reported
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to be 116.0 kJ.mol [8]. Furthermore, compared to the LiPF
6
, there is a higher fraction of
collisions for the C H NHPF as indicated by an increased frequency factor during
5
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6
9
-1
thermal decomposition; a pre exponential factor value (A) of 1.51 x 10 sec is obtained.
This means that the thermal decomposition of C NHPF is highly favoured.
H
5 5
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33
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4.
Conclusion
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36
Thermal decomposition and vibrational spectroscopic properties of pyridinium
hexafluorophosphate have been presented, and based on vibrational spectroscopy, the
results support a space group Fm3m (225) as opposed to the previously proposed R-3m
(166). The conclusions were drawn based on the vibrational spectroscopic experiments
and correlation data between the site symmetry and spectroscopic space group of the
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same compound. The C NMR of the pyridinium hexafluorophosphate indicates a
deshielding effect from the electron withdrawing hexafluorophosphate anion, showing
strong influence of the anion’s fluorine atoms on alpha carbons. Furthermore, the data
indicates three significant signals corresponding to the three chemical environments
expected on a pyridinium ring i.e. γ, β and α carbons, suggesting that the anion must be
symmetrical with respect to the pyridinium ring’s C2v symmetry, with a monodentate
coordination. The thermal decomposition of the compound follows a contracting volume
-1
model, with activation energy of 108.5 kJ.mol and a pre exponential factor of 1.51 x
9
-1
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50
51
10 sec .
ACKNOWLEDGEMENTS
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The South Africa Nuclear Energy Corporation Soc. Ltd and the Fluorochemical
Expansion Initiative (FEI) program of the Department of Science and Technology
(Republic of South Africa) are greatly acknowledged for their funding for the PhD study.
The University of Pretoria is acknowledged for PhD studies. We also acknowledge Benni
Vilakazi for assistance with experiments and Lindani Ngidi for NMR data review.
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6
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salts by
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Hexafluorophosphate and Pyridine, Its preparation and Preparation Process for Lithium
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battery electrolytes containing LiPF , J. Power Sources 161 (2006), 573-579.
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[13] T. Kawamura, S. Okada, J. Yamaki, Decomposition of LiPF -based electrolytes for
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lithium ion cells J. Power Sources, 156 (2006) 547-554.
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the cation substitution on the structural and vibrational properties of Cs
solid solution, J.Chem. Phys. 121 (2004) 3184-3190.
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[15] A.B. Campos, A.Z. Simoes, E. Longo, J.A. Varela, V.M. Longo, A.T. de Figueiredo,
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List of Figure Captions
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1
Figure 1: Raman spectrum of C H NHPF (a) and FTIR-ATR spectrum of
5
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6
C H NHPF (b).
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1
3
Figure 2a: A C NMR spectrum and chemical structure depiction of C
compound
5 5 6
H NHPF
Figure 2b: Graphical representation of the relative positions of the two ions in the
NHPF
1
1
C
5
H
5
6
1
1
1
2
3
4
Figure 3: IR spectrum of gaseous decomposition products of C H NHPF under helium
5 5 6
atmosphere
1
1
5
6
Figure 4: TG curve of C H NHPF sample heated under dry nitrogen from 30 to 600°C
5
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6
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8
Figure 5: A plot of lnK vs 1/T for thermal decomposition of solid C H NHPF
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Table 1: FTIR vibrational spectra of C H NHPF .
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6
-
1
-1
Reference (cm )
Assignment
T Octahedral
This work (cm )
475
[
4]
4
5
5
6
7
8
1
1
3
71,475,478
2
g
[
4]
71, 579, 586
Eg Octahedral
T Octahedral
643
[
2]
[17]
58 , 551
551
1
u
[
2]
[7]
70 ,680
A , Ring vibrations
1
668
[
17]
[16]
45 ,738
Ring breathing mode
A1g Octahedral
735
[
2]
30
795
[
2]
[2]
[7]
490 ,1540 , 1481
B Ring vibrations
1
1490,1540
1608
3326
[
2] [7]
585 ,
A Ring vibrations
1
[
7]
236
A Ring vibrations
1
Observed = this work.

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Table 2: A C NMR data of solid C H NHPF .
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6
Observed shift
*Reported shift
141.97
Ring Carbons
141.47
128.90
149.36
2 α
2 β
1-γ
127.25
146.43
*Mohamed and Padma, (1985).

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Table 3: Comparison of the three closest models that best fit TG experimental data.
Model
R2
Description
Equation
ꢃ
ꢄ
Contracting area
gꢀαꢁ = ꢂ1 − ꢀ1 − αꢁ
ꢅ = ꢆꢇꢈ
ꢅ = ꢆꢇꢈ
ꢃ
ꢉ
R3
D1
Contracting volume gꢀαꢁ = ꢂ1 − ꢀ1 − αꢁ
ꢄ
1-D diffusion
gꢀαꢁ = ꢊꢀαꢁ ꢋ = ꢆꢇꢈ

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Highlights
•
•
•
•
•
Vibrational spectroscopy of C H NHPF correlated to group theory is presented.
5
5
6
The space group for C H NHPF is not R-3m(166) as previously thought.
5
5
6
1
3
The C NMR data supports vibrational spectroscopy space group calculations.
-
6
The PF anion is symmetrical with respect to the pyridinium ring’s C symmetry.
2v
Therefore C H NHPF is better interpreted as having cubic space group Fm3m (225).
5
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6