Hexamethyldisilazane as an acylation generator for perfluorocarboxylic acids in quantitative derivatization of primary phenylalkyl amines confirmed by GC/MS and computations

Article abstract of DOI:10.1021/ac503786j

A novel, selective acylation of primary phenylalkyl amines (PPAAs) using hexamethyldisilazane (HMDS) and perfluorocarboxylic acids (PFCAs) is noted. Couples, like HMDS and trifluoroacetic acid, HMDS and pentafluoropropionic acid, or HMDS and heptafluorobutyric acid trigger PPAAs' quantitative acylation. Processes' selectivity was characterized by applying all couples to derivatize benzyl, 2-phenylethyl, 3-phenylpropyl, 4-phenylbutyl amines, and their relevant substituted versions. Aliphatic amines were unreactive. Identification, quantification, proportionality, and stoichiometry in derivatization processes were determined by gas chromatography/mass spectrometry. Reaction conditions were optimized depending on reagents' molar ratios, solvents, and temperatures applied. The new acylation method, in comparison to the traditional ones, obtained with trifluoroacetic anhydride, heptafluorobutyric anhydride, and N-methyl-bis(trifluoroacetamide), offers numerous advantages. Derivatives, provided by couples, can be directly injected onto the column, avoiding loss of species, saving time, work, and cost in the preparation process. Due to traditional reagents' excess evaporation by nitrogen drying, the loss of trifluoroacylated species proved to be 65% or less. Regarding heptafluorobutyryl species, their losses varied between 25% and 5%. Unified huge responses, obtained with the HMDS and PFCA couples are attributable to their direct injection onto the column and to fragments sourced from the molecular ions and from their self-chemical ionization ([M]_?+, [M+147]⁺, i.e., [M+(CH₃)₂-Si=O-Si-(CH₃)₃]⁺). The reaction mechanism, due to the HMDS symmetrical structure, acting HMDS as acylation generator for PFCAs, was confirmed by density functional theory (DFT) computation.

Full text of DOI:10.1021/ac503786j

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Technical Note
Hexamethyldisilazane as an Acylation Generator for
Perfluorocarboxylic Acids in Quantitative Derivatization of Primary
Phenylalkyl Amines Confirmed by GC-MS and Computations
Borbála Molnár, Antal Csámpai, and Ibolya Molnár-Perl
Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac503786j • Publication Date (Web): 10 Dec 2014
Downloaded from http://pubs.acs.org on December 23, 2014
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Hexamethyldisilazane as an Acylation Generator for
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Perfluorocarboxylic Acids in Quantitative Derivatization of Primary
Phenylalkyl Amines Confirmed by GC-MS and Computations
Borbála Molnár1-3, Antal Csámpai2, Ibolya Molnár-Perl1*
Institute of Chemistry1,2, Departments of Analytical1 and Inorganic2 Chemistry, L. Eötvös University, 1117, Pázmány
Péter sétány 1/A, Budapest, Hungary; Doctoral School of Pharmaceutical Sciences3, Semmelweis University, 1085,
Üllői út 26, Budapest, Hungary
ABSTRACT: A novel, selective acylation of primary phenylalkyl amines (PPAAs) using hexamethyldisilazane (HMDS) &
perfluorocarboxylic acids (PFCAs) is noted. Couples, like HMDS & trifluoroacetic acid (TFA), or HMDS &
pentafluoropropionic acid (PFPA), or HMDS & heptafluorobutyric acid (HFBA) trigger PPAAs’ quantitative acylation.
Processes’ selectivity was characterized by applying all couples to derivatize benzyl, 2-phenylethyl, 3-phenylpropyl, 4-
phenylbutyl amines and their relevant substituted versions. Aliphatic amines were unreactive. Identification,
quantification, proportionality and stoichiometry in derivatization processes were determined by gas chromatography-
mass spectrometry. Reaction conditions were optimized depending on reagents’ molar ratios, solvents and temperatures
applied. The new acylation method, in comparison to the traditional ones - obtained with trifluoroacetic anhydride
(TFAA), heptafluorobutyric anhydride (HFBAA) and N-methyl-bis(trifluoroacetamide) (MBTFA) – offers numerous
advantages. Derivatives, provided by couples, can be directly injected onto the column, avoiding loss of species, saving
time, work and cost in preparation process. Due to traditional reagents’ excess evaporation by nitrogen drying, the loss of
trifluoroacylated species proved to be 65% or less. Regarding heptafluorobutyryl species, their losses varied between 25%
and 5%. Unified huge responses, obtained with the HMDS & PFCA couples are attributable to their direct injection onto
the column and to fragments sourced from the molecular ions and from their self chemical ionization ([M] , [M+147]+ i.e.
[M+(CH3)2-Si=O-Si-(CH3)3]+). The reaction mechanism - due to HMDS symmetrical structure, acting HMDS as acylation
generator for PFCAs - was confirmed by Density Functional Theory (DFT) computation.
Since the pioneer work of Pierce,1 analytical chemists both
in academia and industry have used numerous silylating
reagents, including HMDS, to prepare volatile products
from organics with exchangeable proton.2-7 We preferred
the use of HMDS because of its simplicity, cost affectivity
in quantitation of active protons, being even in hindered
positions.3-6
On course of trialkylsilylation optimization of 2-(3,4,5-
trimethoxyphenyl)ethylamine with the HMDS & TFA
reagent, to our surprise, instead of the expected
ditrimethylsilyl derivative,7 its trifluoroacylated product
was obtained. This phenomenon proved to be a unique
novelty in chemistry, especially in analytical chemistry.
Namely, traditional acylation - one of the most widely
used protocol8 derivatizing amines for GC analysis – are
performed with TFAA,9-11 or HFBAA11-13 or MBTFA14
reagents, applying ethyl acetate (ETAC) as solvent
(reagent/solvent=1/1, v/v).
MS studies, DFT modeling was performed to highlight the
mechanism and stoichiometry of principle. Analytical ad-
vantages of the novel method in comparison to traditional
derivatizations9-14 (TFAA, HFBAA, MBTFA) were defined.
EXPERIMENTAL SECTION
HMDS (99.9), TFA (99.5), PFPA (97), HFBA (99.5), TFAA
(≥ 99), HFBAA (≥ 99), MBTFA (∼98), acetonitrile (ACN, ≥
99.9), ETAC, ≥ 99.9), pyridine (PYR, ≥ 99.9), benzylamine
(BA, ≥ 99.5), 2-phenylethylamine (2-PEA, ≥ 99), o-meth-
oxybenzylamine (OMBA, 98), 3-phenylpropylamine (3-
PPA, ≥ 98), m-methoxybenzylamine (MMBA, 98), p-
methoxybenzylamine (PMBA, 98), 2-(m-methoxy-
phenyl)ethylamine (2-MMPEA, 97), 4-phenylbutylamine
(4-PBA, 98), 2-(p-methoxyphenyl)ethylamine (2-PMPEA ≥
98),
2-(3,4-dimethoxyphenyl)ethylamine
{2-(3,4-
98} and 2-(3,4,5-
{2-(3,4,5-TriM)PEA;
DiM)PEA; homoveratrylamine,
trimethoxyphenyl)ethylamine,
≥
To investigate the characteristic properties of our novel
acylation principle, a many-sided research was underta-
ken, along with computation. Reagent composition, sol-
vent, time and temperature for optimum analytical condi-
tions were optimized and controlled. In parallel with GC-
mescaline, 99} were of highest analytical grade available,
used as received (percent purity in parentheses): all
products of Sigma Aldrich, St Louis, Mo, USA.
Model compounds, 10-12 mg/10 mL, weighed with 0.01
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mg uncertainty, were dissolved in distilled water, neutral-
cases, lead to optimum conditions (80 °C, 20 min).
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ized with hydrochloric acid and further diluted into a
unified stock solution containing in 10-20 µL aliquots
about 0.1-0.2 µg PPAA of each. Model solutions, in
triplicate, were rotary evaporated to dryness at 30–40 °C.
Residues, or freeze dried cactus tissues∗, weighed directly
into the derivatization vessel ( 0.001 mg uncertainty),
were treated with 70 µL HMDS, 30 µL TFA or 30 µL PFPA
or 30 µL HFBA and 100 µL ETAC, heated in oven at 80 °C
for 20 min. Thereafter acylated solutions were transferred
into the autosampler vial and 1 µL (containing 500-1000
pg PPAA derivatives of each) was injected into the GC-MS
system. ∗(Homogenized 2.95 g gave freeze dried 0.260 g.)
Derivatizations with TFAA, HFBAA and MBTFA were
performed as detailed above except dried residues were
treated with 100 µL TFAA & 100 µL ETAC or 100 µL
HFBAA & 100 µL ETAC or 100 µL MBTFA & 100 µL ETAC
and heated in oven at 80 °C for 20 min. Thereafter TFAA
and HFBAA derivatives were evaporated to dryness by
using N2 at ambient temperature. The dry residues were
redissolved in 200 µL ETAC.
Table 1. Efficiency of PPAAs Derivatization with the
HMDS & TFA Couple Depends on the Molar Ratios
([HMDS]/[TFA]) and Solvents Applied: Ethyl Acetate
(ETAC), Acetonitrile (ACN) and Pyridine (PYR), all in
Proportions of (HMDS & TFA)/Solvent = 1/1 (v/v),
Determined by GC-MS.
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Iu/pg x 104* (RSD%); [HMDS]/[TFA]
varied ⇓
PPAAs
0.56/1 -
2.07/1
0.44/1
3.3/1 7.1/1
0.85/1
ETAC**
3.96
ACN PYR
3.19
3.18 2.86 3.92
BA
-
-
(1.53) (4.76) (2.60) (0.77) (2.90)
3.59 4.81 3.41 3.06 4.68
(1.14) (4.28) (2.20) (0.79) (3.30)
3.48 4.99
(3.81) (4.36) (3.93) (4.06) (3.28) (2.46)
3.28 3.88 3.01 3.01 4.09 3.33
(0.22) (3.64) (3.31) (1.22) (3.56) (3.74)
3.75 4.93 4.21 4.01 4.69 4.34
(2.47) (4.81) (1.91) (0.94) (3.12) (0.73)
2.78 3.70 3.25 2.82 3.70 3.73
(1.83) (2.66) (2.76) (3.61) (4.12) (2.55)
3.15 4.72 3.89 3.59 4.60 4.73
(3.78) (3.70) (1.35) (1.86) (2.73) (2.49)
1.76 2.55 2.33 2.29 2.55 2.53
(1.33) (4.81) (1.90) (4.58) (1.77) (3.21)
1.61 2.26 2.09 2.14 2.13 2.19
(3.19) (2.25) (0.84) (2.38) (1.15) (2.85)
2-PEA
4.07 3.82 5.00 0.77
OMBA
The apparatus consisted of a Varian 240 GC-MS/MS
system (Varian, Walnut Creek, CA, USA). The analyses
were carried out using a Varian CP-8400 Autosampler,
and a Septum Programmable Injector (SPI). The column
used was a product of SGE (Victoria, Australia); SGE forte
MMBA
PMBA
2-MMPEA
2-PMPEA
capillary BPX5: 30 m x 0.25 mm; df = 0.25
µm. The
temperatures of the transfer line, ion trap and manifold
were, in order of listing 300 °C, 210 °C and 80 °C,
respectively. Under optimized temperature programs
injections were made at 280 °C, and held at 280 °C for 3
min, then cooled down to 100 °C (100 °C/min); the
column temperature profile was: 100 °C, held for 1.00 min,
then heated up to 145 °C for 10 °C/min, than heated up to
195 °C for 5°C/min with a 1.00 min hold and finally heated
up to 280 °C for 50 °C/min with a 1.80 min hold at 280 °C
(total elution time was 20.00 min. Helium (purity: 6.0,
99.9999%) was used. The column flow rate was 1 mL/min.
The general MS parameters were: Fil/Mul delay: 3.00 min;
electron energy: 70 eV.
2-(3,4-
DiM)PEA
2-(3,4,5-
TriM)PEA
Indications: * = average responses from 500 pg/injections
and 1000 pg/injections of each; in parentheses: relative
standard deviation percentages (RSD%); The bold printed,
blue values are obtained from optimum range
[HMDS]/[PFCA] = 0.56/1 - 2.07/1; ** = peak profiles and
mass spectra are detailed in Supporting Information, Figs.
S1a – S1e, values for the HMDS & TFA couple.
Statistical analysis was performed with Student's two-
tailed t-test, p < 0.05 was considered significant.
The experimental data in Tables 1, 2 clearly demonstrate
that HMDS together with PFCAs behave as an acylation
reagent in proton donor solvents. Therefore, we also
provided computational evidence (DFT modeling at
B3LYP/6-31 G(d)15,16 level: Scheme 1, Supporting Informa-
tion, Tables S1 - S14). The special interaction of HMDS A
with PFCAs B, the formation/transformation intermed-
iaries C - G, result in stable endproducts J - K, including
the acylated amine J. In this process, HMDS, due to its
symmetrical structure, triggers to form cationic interme-
diate, C, which getting stabilized by two electrons’ delo-
calization, exemplifying the Olah theory.17 Ring opening C
with perfluorocarboxylate D provides activated ester E.
Again, E with amine F affords carbinol I and acylated
amine J via the proton transfer between ions G and H. In
the last step I is converted irreversibly into silane species
K by half equivalent of A, necessary for the overall pro-
cess completion (Scheme 1). This exceptional reactivity
RESULTS AND DISCUSSION
Introductory, GC-MS derivatization study showed that
the HMDS & TFA couple acylates PPAAs selectively:
primary aliphatic amines do not react at all.
In order to gain further insight into the selectivity and
stoichiometry of this process, further studies were per-
formed, to determine optimum molar ratio range
applying [HMDS]/[TFA] from 0.44/1 up to 7.1/1. The im-
pact of solvents (ETAC, ACN, PYR) was also investigated
(Table 1).
As optimum molar ratio, the [HMDS]/[PFCA] = 0.85/1 -
reagent proportion of HMDS/PFCA/ETAC = 7/3/10 (v/v/v)
- as solvent ETAC was selected. Bold printed, blue values
represent the optimum reagent composition, the mean of
the optimum range. Varying time (10, 20, 30 min) and
temperature (70 °C, 80 °C, 90 °C) of reactions, in each
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Scheme 1. Acylation Route of PPAAs via the Reaction with the HMDS & PFCA Couples
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of HMDS is also in line both with our data and the lite-
rature8, confirming that reagents - BSA, BSTFA, MSTFA,
MTBSTFA, up to now also HMDS - are silylating, exclu-
sively. Summing up mechanism from stoichiometrical
point of view it is clear that calculated molar ratio of
[HMDS]/[PFCA] = 1.5/1 supports our experimental
proof, being compatible with the wide optimum molar
ratio ranges of [HMDS]/[PFCA] = 0.56/1 - 2.07/1, due to
the huge excess of both parts of the couple (Table 1.:
bold printed blue response values). As to the similar ef-
ficacy of solvents ETAC and ACN, this was also sup-
ported by the reaction mechanism attributable to the
fact that both solvents do have an acidic character in
contrary to PYR’s basic one; PYR is a competitor of
HMDS to react with the proton (H+), which has a crucial
role in the primary generation of cyclic cationic
intermediate C with a hydrogen in a bridged position
(Scheme 1.).
The scope and validity of the novel acylation process
was extended, by replacing the HMDS & TFA both with
the HMDS & PFPA and the HMDS & HFBA couples
(Table 2.). To our delight, the corresponding pentafluo-
ropropionyl and heptaflurobutyryl species were formed,
quantitatively and proportionally, proving the selective
acylation of PPAAs, equally (Table 2., values in lines
TFA, PFPA and HFBA; peak profiles and mass spectra
are detailed in Supporting Information, Figs. S1a - S1e).
As an example, the last vertical column in Table 2.
shows the peak profiles from BA up to 2-PMPEA and the
fragmentation for BA derivatives (spectra 1A, 2A, 3A).
The joint evaluation of the HMDS & PFCA acylated
products’ fragmentation patterns along with their
products’ efficiencies reveals unique properties and
advantages of the new acylation principle.
acylation process, meaning excellent selectivity, reliabi-
lity and reproducibility of derivatives. This phenomenon
could be best characterized with the relative standard
deviation percentages (RSD%) of responses (Table 2.,
values in 9th vertical column): varying between 0.56% (2-
PMPEA) and 2.01% (3-PPA).
c) Contrasting the response values with the counterpart
products - derivatized with TFAA, HFBAA and MBTFA
reagents - it turned out that all three classical acylation
processes resulted in lower responses (regarding as the
basis of comparison, - as 100% - the average values for
the PFCAs’ derivatives in Table 2.). The most commonly
used TFAA derivatives show up with the lowest
responses due to derivatives’ loss under TFAA excess
elimination by N2 gas. Differences in responses,
obtained with the TFAA and the HMDS & PFCA
couples, served for calculation: significant differences
were found, all proved to be at the level p < 0.05.
Exclusive benefit of the HMDS containing reagents can
be attributed to the ions of [M+147]+.
d) Setting the elution profiles and mass spectra of the
HMDS & PFCAs derived species (Supporting Informati-
on, Figs. S1a - S1e) - against those obtained via the tradi-
tional acylation processes (Supporting Information, Figs.
S2a – S2d) - strict correlations of retention times and
considerable differences in favor of the HMDS & PFCA
couples are shown. The novel acylation process, resulted
in fragments - sourced from self chemical ionization of
molecular ions - benefit of HMDS, exclusively.
Practical utility of the method. We can foresee num-
erous useful applications of our principle, which may
include the analysis of amphetamines and cardiotonic
amines. As a proof of principle we measured mescaline
in freeze-dried cactus stem (Lophophora Williamsii),
Figure 1. Supporting the utility of our method, we
directly derivatized mescaline with the HMDS & TFA
reagent, omitting the 24 h Soxhlet extraction before
chromatography.18 Various amounts of freeze dried
tissues were determined (a, 0.454 mg; b, 0.200 mg; c,
0.100 mg), in comparison to mescaline standard (d,
0.800 µg mescaline) and blank (e, reagent only). Via the
novel method comparable mescaline contents were: a,
1.34%; b, 1.41%; c, 1.43%. Reproducibility and reliability
proved to be: a, 4.70 RSD%; b, 2.69 RSD%; c, 3.48 RSD%.
a) Acylated species obtained with the HMDS & PFCA
couples demonstrated unified huge responses (Table 2.),
expressed as integrator units/pg values (Iu/pg) attribu-
table to their molecular ions ([M] ) and/or to their char-
acteristic fragments formed by self chemical ionization
{[M+147]+ i.e. [M+(CH3)2-Si=O-Si-(CH3)3]+, Table 2., Sup-
porting Information, Figs. S1a - S1e}.
b) Responses proved to be independent on the PFCA
part of couples (Table 2., average responses, values in
the 9th vertical column). These concordant response val-
ues call attention to the special properties of the new
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Table 2. Chemical Properties, Structures and Fragmentation Patterns of Primary Phenylalkyl Amines (PPAAs)
Obtained with the HMDS & TFA, HMDS & PFPA and HMDS & HFBA Couples, Applying ETAC Under Optimum
Conditions, Determined as Acylated Derivatives by GC-MS, Based on Their Selective Fragment Ions.
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Iu/pg × 104∗
peak profiles from BA up to 2-PMPEA;
selective fragment
PPAAs
ions, m/z
spectra 1A, 2A, 3A for BA derivatives
tR,
min
[M+
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MW, mol structures
[M]
147]+
TFA 4.95 203
PFPA 4.92 253
HFBA 5.36 303
TFA 5.93 217
PFPA 5.92 267
HFBA 6.40 317
TFA 7.22 233
PFPA 7.07 283
HFBA 7.54 333
TFA 7.70 231
PFPA 7.61 281
HFBA 8.13 331
TFA 7.84 233
PFPA 7.74 283
HFBA 8.28 333
TFA 8.17 233
PFPA 8.06 283
HFBA 8.63 333
TFA 9.21 247
PFPA 9.10 297
HFBA 9.65 347
TFA 9.43 245
PFPA 9.29 295
HPBA 9.87 345
TFA 9.47 247
PFPA 9.36 297
HPBA 9.92 347
TFA 12.79 277
PFPA 12.55 327
HPBA 13.05 377
TFA 15.23 307
PFPA 14.94 357
350
400
450
364
414
3.93
3.98
4.01
4.81
4.73
4.76
4.99
4.90
4.94
2.09
2.13
1.70
(1.46)
{43}
BA
3.97
(1.02)
HMDS &TFA
107.15
2.56
(0.31)
{54}
4.77
(0.85)
2-PEA
121.18
HMDS & PFPA
464
380
430
480
378
428
478
380
430
480
380
430
480
394
444
494
392
442
492
394
444
494
424
474
524
454
504
1.86
(2.13)
{38}
4.94
(0.91)
OMBA
137.18
HMDS & HFBA
2.09
(2.01)
3-PPA
135.21
2.05
3.88
3.85
3.79
4.93
4.94
4.88
3.77
3.74
3.67
3.17
1.89
(4.23)
{49}
3.87
(1.19)
MMBA
137.18
1.73
(3.71)
{35}
4.92
(0.65)
PMBA
137.18
1.75
(3.65)
{47}
3.73
(1.36)
2-MMPEA
151.21
3.16
(1.55)
4-PBA
149.23
3.20
3.10
4.72
4.67
4.71
2.55
2.46
2.48
2.26
2.18
2.21
(3.71)
{47}
4.70
(0.56)
2-PMPEA
151.21
1.10
2-(3,4-
DiM)PEA
181.23
2.50
(1.89)
(
2.86
)
{44}
2-(3,4,5-
TriM)PEA
1.14
2.21
(1.97)
(
4.74
)
{52}
211.26
HPBA 15.39 407
554
2.19
Indications, as in Table 1., as well as [M] = molecular ion; [M+147]+ = [M+(CH3)2-Si=O-Si-(CH3)3]+; in square braces =
{abundant m/z values}; ∗ = average response values obtained from 500 pg or 1000 pg/injections of each; peak profiles and mass
spectra, as example, are in the last vertical column for BA derivatives, for all others are demonstrated in Supporting Information,
in Figs. 1a – S1e; Differences between the TFAA and the HMDS & TFA couples provided responses served for calculation;
significant differences were found, all, at the level p < 0.05.
The mescaline content of the Lophophora Williamsii
cactus stem samples (1.39% in average) falls in the range
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reported by others (0.053% and 4.7%).18
being compatible with the wide optimum molar ratio
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ranges of [HMDS]/[PFCA] = 0.56/1 - 2.07/1, determined by
GC-MS.
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ASSOCIATED CONTENT
Supporting Information
Figs. S1a-1e and S2a-2d show GC-MS elution profiles and
mass spectra of PPAA derivatives Tables S1- S14 give DFT
calculation details. This material is available free of charge
via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding author
*E-mail: perlne@chem.elte.hu
Author Contributions
Figure 1. Peak profiles (a - e) and mass spectra (1A, 1B) of
mescaline from HMDS & TFA direct derivatizations: freeze-
dried cactus (a, 1A; b, c); standard (d, 1B); blank (e)
The manuscript was written through contributions of all
authors; all have given approval to manuscript’s final version.
Notes
CONCLUSION
The authors declare no competing financial interest
A new, selective way of acylation was demonstrated
recognizing the unique potential of HMDS facilitating
PFCA-mediated acylation reactions of PPAAs. Novel
acylation reagents, like HMDS & TFA, HMDS & PFPA and
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Molnár-Perl, I. Talanta 2013, 115, 367–373.
quantitative and selective derivatization of PPAAs.
Aliphatic amines are unreactive. The optimum condition
and analytical advantages of the novel principle are
compiled in Tables 1. and 2. and in Supporting
Information, Figs. S1a - S1e. Comparison of species derived
by the new method with those performed via TFAA,
MBTFA and HFBAA derivatizations (Supporting Figs. S2a
(6) Boldizsár, I.; Füzfai, Zs.; Molnár-Perl, I. J. Chromatogr. A.
2013, 1293, 100-106.
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S2d) – are highlighted: retention properties of
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derivatives obtained via the novel and traditional ways
were identical, however differences in their responses,
proved to be in favor of derivatives, prepared with the
couples. Advantages, indicating the superiority of the new
acylation principle are as follows: the couples provided
species can be directly injected onto the column, avoiding
loss of species, saving time, work and cost of preparation
process. The couples provided derivatives resulted in
concordant high responses, independent on the PFCA
part of the couple (Table 2., average responses, values in
the 9th vertical column). Concordant response values call
attention to the unique properties of the new acylation
process, meaning excellent selectivity, reliability and
reproducibility of derivatives. This phenomenon was
characterized with the RSD% of derivatizations varying
between 0.56% (2-PMPEA) and 2.01% (3-PPA). Statistical
significance between the traditional and new method was
calculated using Student's t-test. It was found to be
significant at p < 0.05. The evidence of mechanism due to
the symmetrical structure of HMDS was confirmed.
Evaluating the mechanism from stoichiometrical point of
view it turned out that calculated molar ratio of
[HMDS]/[PFCA] = 1.5/1 supports our experimental proof,
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