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Carbon Dioxide Chemisorption by Ammonium and Phosphonium

Ionic Liquids: Quantum Chemistry Calculations
Vitaly V. Chaban*

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ABSTRACT: Carbon capture and storage is an important technological endeavor

aiming to improve the ecology by combating global warming. The present work
investigates reaction paths that are responsible for CO2 chemisorption by the
ammonium- and phosphonium-based ionic liquids containing an aprotic heterocyclic
anion 2-cyanopyrrolide. We exemplify that 2 mol of CO2 per 1 mol of the gas scavenger
can be theoretically fixed by such ionic liquids. Both the cation and anion participate in
the chemisorption. The corresponding standard enthalpies and potential energies are
moderately negative. The chemisorption reaction, as revealed by the simulations of
competing pathways, is started by the donation of the proton from the cation to the
anion. The double covalent bond in the cation’s structure emerges. The barriers to all
reactions involving the phosphonium-based cation are relatively small and favor
practical applications of the considered sorbents. The performance of the ammonium-
based cation is less favorable due to the inherent instability of the tetraalkylammonium
ylide. The role of phosphonium ylide in the mechanism of the reaction is carefully characterized. The performance of the aprotic
anion as a CO2 scavenger is unaffected by the chemical identity of the counterion. The essential heights of the identified steric
barriers underline the necessity to simulate the entire structures of the reacting species to obtain a reliable description of
chemisorption. The reported results foster a fundamental understanding of the outstanding CO2 sorption performance of the
quaternary ammonium- and phosphonium-based 2-cyanopyrrolides.

■ INTRODUCTION
The term global warming stands for the designation of an
presents evident drawbacks, such as perpetual degeneration of
the amine solutions due to their working chemical reactions.1
ongoing large-scale trend.1 During the last 12 decades, the Time is necessary for humanity to implement more chemically
average Earth’s temperature increased by 0.08 degrees Celsius advanced solutions on the large scale.
per year. Furthermore, during the last 40 years, the pace of this Room-temperature ionic liquids (RTILs) represent a huge-
change increased more than twice. Although the recorded sized class of organic−inorganic chemical compounds that are
changes may seem insignificant for a non-specialist, they do have liquid under standard conditions.20−26 The liquid state of the
ecological consequences including the thawing of glaciers, prospective CO2 sorbent is a definite advantage since it allows
climate changes, more acidic oceans, and so forth.2−4 Human the gas molecules to spontaneously diffuse to the location where
technologies lead to higher emissions of carbon dioxide (CO2) chemisorption takes place. Furthermore, RTILs exhibit
that may be the cause of environmental problems. The negligible volatility even in their pure state thanks to a strong
technological superstructures are currently being developed cation−anion attraction.
aiming to capture, store, and chemically modify CO2 to obtain An ideal CO2 sorbent would allow reversible chemisorption,
useful and/or benign products.5,6 so that the fixed gas could be extracted and the working liquid
CO2 is a highly thermodynamically stable compound that is a could be regenerated. RTILs contain chemically tunable
final product of organic matter combustion. Its stability is a structures meaning that new compounds can be synthesized
major reason why CO2 scavenging is a challenging chemical on request in a relatively easy way. Fine-tunable physical−
endeavor that will likely never get a straightforward solution.
Nevertheless, novel robust methods for CO2 processing7−15 can
be elaborated and industrially implemented.5 Thoughtful Received: April 29, 2022
application and fine-tuning of the versatile nanoscale pores Revised: June 12, 2022
play a paramount role in the gradual increase of the presently Published: July 14, 2022
available CO2 sorption capacities.16−19 Apart from the up-to-
date breakthrough developments, amine scrubbing is a leading
and well-established technology nowadays, even though it

© 2022 American Chemical Society https://doi.org/10.1021/acs.jpcb.2c02968

5497 J. Phys. Chem. B 2022, 126, 5497−5506
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Figure 1. Proposed mechanisms of the two CO2 chemisorption reactions. (A) Formation of carbamate out of the AHA anion. (B) Formation of the
ylide molecule out of the quaternary cation after its deprotonation. (C) Carboxylation of the ylide molecule. (D) Tetrabutylphosphonium cation and
its α-carbon atom.

chemical properties form a prerequisite to developing task- tetrabutylphosphonium 2-cyanopyrrolide, both the cation and
specific systems and environments. Computer simulations of the anion must reversibly react with CO2. The chemisorption
RTILs and relevant chemical processes constitute a large, vivid, reaction at the anion’s reaction site follows the well-known
and vigorously developing research field with substantial carbamate formation mechanism. In turn, the reaction at the
methodological and engineering advances.27−31 Pádua and co- quaternary cation takes place at its α-carbon atom. It is
workers30 recently developed a highly efficient polarizable force facilitated by the cation’s deprotonation leading to the ylide
field to simulate condensed phases of ionic liquids. In turn, formation (Figure 1).23 While tetrabutylphosphonium ylide is a
Canongia Lopes and co-workers made a considerable impact on stable chemical compound with negative formation energy,
the understanding of versatile aqueous ionic liquid-based tetrabutylammonium ylide is unstable due to the inability of the
systems through the simulation of peculiar non-covalent nitrogen atom to support the valence of five. Nonetheless, the
interactions.28,29 The method of classical molecular simulation ultimate products are stable in all cases of the chemisorption
represents a mighty tool to understand the adsorption of CO2 in reactions.
various prospective sorbents, such as clay. Numerous important The previous experimental works21,23,25 revealed and
descriptors can be derived.32 Cordeiro and co-workers confirmed an outstanding CO2 sorption potential of the aprotic
contributed an advanced description of the ionic liquids’ heterocyclic anions paired with quaternary cations. In particular,
behavior at the interface and linked new insights to the the Brennecke group made a significant impact on this field by
differential capacitance of a possible electrochemical device.31 introducing a separate class of liquid-state compounds. The
Andreeva and co-workers investigated thermodynamics for a RTILs based on the heterocyclic anions exhibit high promise in
large number of ionic liquid families and outlined their the CO2 capturing endeavor. They react stoichiometrically,
suitability to act as non-volatile and non-flammable CO2 whereas the chemisorption is reversible. Brennecke and co-
sorbents.33,34 workers found a solution to the high-viscosity problem of the
The most promising RTILs in the context of CO2 sorption are AHA-based RTILs by adding a certain proportion of
composed of bulky organic cations and chemically reactive tetraglyme.36 The latter is not viscous, but at the same time, it
anions.21,23,25 The aprotic heterocyclic anions (AHAs) is relatively non-volatile representing a rare set of physical−
represent an interesting family of organic structures with a set chemical properties. In combination with the AHA-containing
of potential applications in different fields of chemical RTILs, tetraglyme engenders solutions with potentially
engineering due to their strong affinity to the proton. This important and quite unusual physical−chemical properties.
feature of AHAs can arguably convert certain thermodynami- The phosphonium-based RTILs paired with aprotic anions form
cally forbidden reactions into favorable ones. It is essential to both a carbamate product out of the anion and a carboxyl
couple AHAs with the bulky and asymmetric cations to block all product out of the cation. Therefore, 2 mol of CO2 per 1 mol of
strong electrostatic attractions in the resulting liquid-state the RTIL can be bound at a time.23,37
system. The organic structures with the delocalized excess or The phosphonium ylide is an important intermediate in the
deficient electronic charge sterically prevent RTILs from CO2 chemisorption by the tetraalkylphosphonium-based
crystallizing at the ambient temperature. Higher conformational RTILs. The ylide is a neutral compound that forms reversibly
flexibility of the particles favors their smarter performance as at above 333 K in the case of the chemisorption reaction of
greenhouse gas scavengers. The inability of the cation and the tetraalkylphosphonium-based RTILs with carbon dioxide.23,37
anion to attain a distinct electrostatically driven coordination The presence of CO2 in the system favors the formation of ylide.
pattern liberates their potential to bind CO2. Upon the Compared to the current industrial-scale technology that
development of competitive AHA-based RTILs, there is always employs aqueous monoethanolamine solutions, the finely
an interplay of melting point and shear viscosity that limits our tuned RTILs can substantially decrease the energy demands.
possibilities to tune the physical−chemical properties of the According to the very recent quantification performed by
prospective CO2 scavengers.35 Gohndrone and co-workers,23 the anion’s chemisorption is
In the present theoretical work, we investigate a few of the kinetically more favorable, that is, the major product of CO2
most promising ionic liquids in the context of CO2 capture. chemisorption by the bulky phosphonium-based ionic liquids is
Indeed, in the tetrabutylammonium 2-cyanopyrrolide and a carbamate.
5498 https://doi.org/10.1021/acs.jpcb.2c02968
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We presently divide the chemisorption reactions that coordinate alteration was employed deliberately to record all
presumably occur in the systems of liquid tetrabutylammo- potential barriers on the way of the chemisorption. Particularly,
nium/tetrabutylphosphonium 2-cyanopyrrolide plus gaseous the sterical potential barriers due to the presence of four alkyl
carbon dioxide type into several sub-processes. Each sub-process chains were identified. In total, over 1 000 000 self-consistent
includes an independent chemical reaction, that is, the field calculations (optimizations of the system’s wave function)
formation of the covalent bonds. We scan the reaction paths were carried out. The bonded and non-bonded interactions in
of every stage, identify potential barriers, characterize stationary the systems were described according to the PM7 Hamil-
points or prove their absence, and compute thermodynamic tonian.38 This Hamiltonian includes empirical corrections for all
properties of the chemisorption-related physical and chemical applicable chemical phenomena, such as interatomic dispersion
transformations. The existence of the first-order saddle points forces, hydrogen bonding, and correlation of the valence
along the reaction pathway is confirmed by the presence of a electrons.
single imaginary frequency in the system’s vibrational profile and The self-consistent field convergence criterion upon the
characterized in terms of geometric parameters and electron iterative optimization of the wave function was set to 0.5 × 10−5
density distributions. For the computationally efficient inves- kJ mol−1 for the two successive cycles of computation. The force
tigation, we select a less bulky cation, as compared to the one minimization convergence criterion was set to 0.5 kJ mol−1 nm−1
used experimentally. However, the investigated length of the for the gradient norm. The standard enthalpy is defined as the
alkyl chain is large enough to detect possible sterical hindrances potential energy of a given particle minus the sum of potential
that impact CO2 chemisorption in real-world greenhouse gas energies of the corresponding simple substances. A thermal
capture. correction (in gas-phase approximation) was added to obtain a

■ METHODS AND PROCEDURES

The chemical compositions for which the reaction paths were
finite-temperature property.
The computed reaction paths provide the smooth total
potential energy change upon the internal coordinate change.
investigated include one cation (tetrabutylphosphonium or The ion-molecular configurations corresponding to the
tetrabutylammonium), one AHA (2-cyanopyrrolide), and one potential barriers may be transition states. To ensure that they
CO2 molecule (Figure 2). The simulation of the ionic indeed are such, we re-optimized the corresponding inter-
mediate structures. The vibrational profiles without Raman
frequencies were computed to identify imaginary frequencies
(one per system) that are direct proofs of the successful saddle
point location. The empirical dispersion correction proposed by
Grimme and co-workers39 for ninety-four chemical elements
was applied. The density functional theory calculations using the
Becke-3-Lee-Yang-Parr exchange−correlation hybrid function-
al40,41 were conducted in GAMESS.2021.42 The atom-centered
split-valence triple-zeta basis set including polarization and
diffuse functions, 6-311++G**, was used. For certain compar-
isons, the 6-31G(d) basis set was also used.
Figure 2. Optimized ground-state geometries of the simulated systems. The wave function calculations, geometry optimization
(A) Tetrabutylphosphonium 2-cyanopyrrolide and (B) tetrabutylam- procedures, and reaction coordinate propagation were per-
monium 2-cyanopyrrolide. The eigenfollowing procedure of minimiz- formed in a tightly bound set of the computational chemistry
ing forces acting on every atom was applied before the propagation of programs, in which MOPAC-2016 (URL: http://openmopac.
the reaction paths. The oxygen atoms are red; the nitrogen atoms are net)38 and in-home utilities combine their capabilities and
blue; the carbon atoms are cyan; and the hydrogen atoms are black.
routinely exchange input and output structures of one another.
The in-home software adopts the functions, procedures, and
compounds involves a strong cation−anion attraction that interfaces that are freely available in the ASE and SciPy.org
needs to be reproduced for accurate results. For this reason, our computational chemistry libraries.43,44
systems explicitly include all three essential components The visualization of molecular configurations, automatic
participating in the investigated reactions. The maximum size generation of Z-matrices, visual investigation of the resulting
of the systems is limited by the affordable number of the internal geometries, and preparation of molecular graphics for the paper
variables to avoid the numerical problem upon the analysis of the were carried out in VMD-1.9.1,45 Gabedit-2.5,46 and Avogadro-
sampled process. 1.2.0 semantic analysis platform.47
Each chemical composition was employed for the simulation
of three reaction paths: (1) proton exchange; (2) CO2 reaction
with the α-carbon of the quaternary cation (carboxylation); and
■ RESULTS AND DISCUSSION
Most of the chemical reactions have potential barriers that can
(3) CO2 reaction with 2-cyanopyrrolide (carbamate formation). trivially exceed a dozen of the k × T products. The barriers are
The geometries of all systems were optimized before the stages associated with a necessity to first break the covalent bonds in
of the reactions were simulated. The proposed reaction paths the reactants before forming new chemical bonds in the
were simulated by forcibly moving one of the internal system’s products. The reaction barriers are a specific feature of nature
coordinates toward the desired value. The selected reaction that makes the surrounding world look and behave in its actual
coordinate was adjusted with a spatial step of 0.001 nm, which way. The presence of the potential barriers largely forbids brute-
corresponded to the gradual approach of the prospective force simulations of chemical transformations as we do with
reactants to one another. The partial geometry optimization of physical phenomena in which the changes of the gradients are
the system using the eigenfollowing algorithm was performed at much smoother. The high energy barrier means that the
every reaction step. The very small step of the reaction corresponding process is driven by the so-called rare events in
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the system, whereas its spontaneous modeling requires

extensive, often computationally unaffordable sampling. To
recapitulate, it is currently impossible to model chemical
reactions without imposing a set of suppositions and restraints.
A chemical reaction path can be modeled assuming that a
certain mechanism was previously hypothesized. In turn, it is
possible to model a few anticipated mechanisms and compare
their energetic profiles. By comparison of these profiles, the
corresponding molecular geometries, and locations of the most
essential saddle points, it is then possible to outline more and
less probable paths of the chemical reaction. The reaction path
can be represented by a smoothly moving internal coordinate,
while other geometrical parameters are free to steadily
accommodate the changes following the immediate wave Figure 4. Reaction path for the carboxylation of the α-carbon atom of
function of the alternating system. the tetrabutylphosphonium cation. We hereby assume that no cation−
In the present work, we have the hypotheses that were anion proton transfer has taken place before. The depicted reaction
inspired by the previously published experimental analyses.21,25 coordinate is the distance between the carbon atom of CO2 and one of
While the CO2 chemisorptions by the cation and the anion are the four α-carbon atoms of the tetrabutylphosphonium cation. The
likely rather independent chemical processes, the deprotonation initial reaction coordinate corresponds to the optimized geometry of
of the α-carbon atom of the quaternary cation and grafting of the the system with no restraints in the Z-matrix applied.
CO2 molecule are sequential. No experimental insights are
helpful in deciding which of them takes place first. The phosphorus bond length is 0.173 nm, whereas non-involved
difference in thermodynamic potentials of the sub-reactions is carbon−phosphorus bond lengths range between 0.184 and
also not a decisive factor because both sub-reactions take place 0.186 nm. The other carbon−hydrogen distance in the reacting
in a single macroscopic observation. However, the potential α-methylene group also temporarily elongates, to 0.112 nm,
energy barriers that can be extracted from the reaction path while the activation barrier is being crossed.
simulations can be highly insightful. The sub-reaction with a The geometry of the aprotic anion is notoriously less sensitive
smaller barrier is naturally a more probable pathway of to the deprotonation reaction. The carbon−nitrogen covalent
carboxylation. We simulated the mentioned processes in distances amount to 0.137 nm both in the anionic state and in
different orders and concluded that the deprotonation (Figure the transition state. The fluctuations of these distances with an
3) must occur before the carboxylation (Figure 4). If the CO2 amplitude of 0.001−0.002 nm were observed along the reaction
path. Such fluctuations are much smaller than the above-
recorded perturbations of the tetrabutylphosphonium cation.
The carbon−nitrogen−carbon covalent angle in 2-cyanopyrro-
lide is also insensitive amounting to 106−108° in all
configurations between the starting and transition-state geo-
metries. In the protonated anion, the discussed angle increases
to 108°. The nitrogen−hydrogen covalent bond length in its
unconstrained geometry is 0.100 nm according to the PM7
Hamiltonian. To recapitulate, the cyclic anion gets way less
perturbed during the course of deprotonation than the aprotic
cation.
The barrier for carboxylation without prior deprotonation is
fairly high amounting to 365 kJ mol−1. Our analysis suggests that
such a stage is non-probable even upon significant heating, much
over 333 K. In turn, the barrier after α-carbon deprotonation is
Figure 3. Recorded reaction path for the proton exchange reaction
as small as 13 kJ mol−1, which is in good agreement with the
between the cation and the anion. The depicted reaction coordinate is experimentally proven23 sufficient heating up to 333 K to launch
the distance between the ring nitrogen atom of the 2-cyanopyrrolide the chemical transformation. Furthermore, the shapes of the
anion and the hydrogen atom linked to the α-carbon atom of the computed reaction path curves differ drastically.
tetrabutylphosphonium cation. The initial reaction coordinate Figure 5 shows CO2 attachment to the deprotonated α-
corresponds to the optimized geometry of the system with no restraints carbon atom and its eventual transformation into the negatively
in the Z-matrix applied. charged carboxyl group. This process occurs without any barrier;
see the reaction coordinate of 0.238 nm. However, the approach
molecule approaches the tetrabutylphosphonium cation rather of CO2 to the methyl group of the TBP cation is associated with
than the tetrabutylphosphonium ylide, it gives rise to no stable two distinct barriers, at 0.529 nm (hight of 14 kJ mol−1) and
intermediate. 0.406 nm (height of 17 kJ mol−1). Both of them are steric due to
The investigated chemical reaction leads to significant the collisions of CO2 with the methylene groups of the TBA
changes in the involved molecular geometries. Primarily, the butyl chains. Since the density of the alkyl chains increases with
carbon−phosphorus distance decreases from 0.182 nm in the the decrease of their distance to the center of TBP, the barrier
reactant to 0.164 nm in the ylide. In the meantime, the adjacent height increases proportionally. The overcoming of these steric
three carbon-phosphorus covalent bonds somewhat elongate, to barriers adds to the system’s energies −7 and −10 kJ mol−1,
0.187−0.189 nm. In the transition state, the involved carbon− respectively. Recall that the reaction coordinate step was chosen
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Figure 7 visualizes the atomic configurations that correspond

to the above-identified steric hindrances. Understanding these

Figure 7. Atomic configurations along the reaction path corresponding

Figure 5. Reaction path for the carboxylation of the α-carbon atom of
to the sterical hindrances on the way of the CO2 molecule to the α-
the tetrabutylphosphonium cation. This stage occurs after the proton
carbon atom of the tetrabutylphosphonium cation. (A) Reaction
exchange between the cation and the AHA. The reaction coordinate is
coordinate of 0.529 nm and (B) reaction coordinate of 0.406 nm. The
the distance between the carbon atom of CO2 and the α-carbon atom of
oxygen atoms are red; the nitrogen atoms are blue; the carbon atoms are
the tetrabutylphosphonium cation.
cyan; and the hydrogen atoms are black.

to be 0.001 nm (much smaller than that reported in the other

research) to obtain the precise locations of the states that peculiar geometries is essential to assessing the feasibility of
determine both covalent and non-covalent energetics of the different quaternary cations for the CO2 chemisorption. The
chemisorption reactions. long alkyl chains, such as those selected by Brennecke and co-
The most essential changes during the course of the ylide workers,21,23,25,36 are important to modulate the phase behavior
carboxylation reaction concern CO2. The molecule’s covalent of the resulting RTILs, but they also increase the viscosity. High
angle changes drastically, from 180 to 125°. The carbon−oxygen viscosity complicates the processing of RTILs in the experiment.
bond lengths change from 0.117 in physically bound CO2 to The short chains decrease the viscosity but increase the melting
0.124 nm in chemically bound CO2. The carboxylated cation point of an RTIL. The present work suggests that the barriers
features increase the carbon−phosphorus bond length of the associated with the flexibility of the alkyl groups are significantly
involved chain. The bond in the ylide is 0.164 nm long, whereas high. In this context, the usage of asymmetric tails that help
the bond in the obtained carboxylated zwitterion amounts to eliminate some steric barriers may be an interesting idea. At the
0.181 nm. The carbon−phosphorus distances in the non- reaction coordinate of 0.529 nm, the distance between the
involved alkyl chains range between 0.183 and 0.184 nm, which oxygen atom of CO2 and the hydrogen atom of one of the
is smaller than the same distances in the ylide molecule, 0.187− methylene groups of TBP equals 0.25 nm (Figure 7).
0.189 nm. Overall, the carboxylation supplements higher Furthermore, the simultaneous distance between the carbon
symmetry to the chemical structure (as compared to ylide), atom of CO2 and the nitrogen atom of AHA is 0.32 nm. These
which is also confirmed by a smoothly decreasing potential two collisions are responsible for the observed steric barrier.
energy curve. Similarly, the small distances between the CO2 molecule and
The carbamate formation reaction proceeds even more both ions of the RTIL can be observed at the reaction coordinate
smoothly (Figure 6) than the previously considered reaction of 0.406 nm.
at the α-methylene group at the cation and brings about −102 kJ Deprotonation of the TBA cation is principally different from
mol−1 to the enthalpy. The reaction on the 2-cyanopyrrolide the case of the TBP cation. This phenomenon has an evident
anion does not require activation energy unlike the reaction on chemical explanation since the TBA cation cannot form a
the TBA cation. corresponding ylide. Indeed, the formation of stable compounds
with the nitrogen’s valence of five is impossible. The electron
transfer reaction is thermodynamically unfavorable, with the
standard enthalpy change of +15 kJ mol−1 (Figure 8). In turn,
the barrier of the reaction is 29 kJ mol−1. The fairly insignificant
steric barrier of 2 kJ mol−1 at the reaction coordinate of 0.22 nm
should be noticed. The overcoming of the mentioned steric
barrier brings the system 5 kJ mol−1. The above values are small
enough to be exceeded by the thermal motion energy under
room conditions.
The electron transfer from the cation to the AHA is
thermodynamically forbidden because the ammonium ylide is
an unstable chemical entity. Nonetheless, it was desirable to
characterize the entire reaction of the CO2 capture. Figures 9
and 10 provide the corresponding reaction paths.
Figure 6. Reaction path for the carbamate formation reaction occurring The reaction coordinate of 0.64 nm depicted in Figure 9
at the electron-richest center of the 2-cyanopyrrolide anion in describes the state of the system in which the anion is
tetrabutylphosphonium 2-cyanopyrrolide. The chosen reaction coor- protonated, whereas the TBA cation is a free radical. As a
dinate is the distance between the carbon atom of CO2 and the ring result, the chemisorption barrier is rather small amounting to 19
nitrogen atom of the 2-cyanopyrrolide anion. kJ mol−1. Compare this barrier to the case of the TBP cation in
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As a result, the potential usage of the TBA cation in the CO2

capture is more cumbersome.
The carbamate formation reaction at the electron-rich N-site
of the AHA in the case of the ammonium-based cation (Figure
10) is similar to the above-considered case of the TBA cation.
The enthalpy gain is −74 kJ mol−1. Note the steric barrier of 18
kJ mol−1 observed at the reaction coordinate of 0.362 nm. It
corresponds to the rearrangement of all components of the
system (TBA, AHA, and CO2) in response to the gradual
movement of the CO2 molecule toward the nitrogen atom of
AHA. The overcoming of this barrier results in the system’s
enthalpy gain of −6 kJ mol−1.
Figure 8. Reaction path for the proton exchange between the The key energetic descriptors corresponding to every CO2
tetrabutylammonium cation and 2-cyanopyrrolide anion. The chosen chemisorption reaction stage are summarized for the cases of
reaction coordinate is a distance between the ring nitrogen atom of the both cations in Table 1. The activation barriers are divided into
2-cyanopyrrolide anion and the hydrogen atom linked to the α-carbon
atom of the tetrabutylammonium cation.
Table 1. Energetic Effects ΔH and Activation Barriers ΔA
Observed During the Course of CO2 Chemisorption by
Tetrabutylphosphonium 2-Cyanopyrrolide and
Tetrabutylammonium 2-Cyanopyrrolidea
tetrabutylphosphonium tetrabutylammonium
ΔH, kJ ΔH, kJ
# reaction mol−1 ΔA, kJ mol−1 mol−1 ΔA, kJ mol−1
1 proton transfer −24 +13 +14 +29 (covalent)
(covalent)
2 carboxylation −49 +26 (steric) −63 +37 (steric); +24
(covalent)
3 carbamation −77 +17 (steric) −80 +19 (steric)
4 total −150 −129
a
The reaction coordinates were scanned by using the PM7
Figure 9. Reaction path for the carboxylation reaction taking place at semiempirical Hamiltonian.
the α-carbon atom of the tetrabutylammonium cation. The chosen
reaction coordinate is a distance between the carbon atom of CO2 and
one of the α-carbon atoms of the tetrabutylammonium cation. covalent and steric ones to provide a high-resolution description
of each sub-process. The steric activation barrier represents the
energy, which is required by the system to bring the reactive
species into direct contact with one another. In the dense
(condensed-state) system, such steric barriers can range from a
few kJ mol−1 to a few dozen kJ mol−1. For instance, the approach
of the carbon dioxide molecule to the carbon atom of the ylide
costs 37 kJ mol−1. The cost is associated with diffusing through
obstacles created by the four alkyl chains. In turn, the covalent
barrier concerns the process that involves the electron
rearrangement and formation of new chemical bonds. The
differentiation between these two can be performed by visual
inspection of the reacting system geometry evolution and
through consideration of the reaction coordinate specific value
at the scan point of interest.
Figure 11 provides representative molecular configurations
for some reactions studied herein. In the case of the
Figure 10. Reaction path for the carbamate formation reaction taking
tetrabutylphosphonium cation, the transition state for the
place at the electron-richest nitrogen atom of the 2-cyanopyrrolide
anion in tetrabutylammonium 2-cyanopyrrolide. The chosen reaction hydrogen transfer is depicted. In the case of the tetrabuty-
coordinate is the distance between the ring nitrogen atom of the 2- lammonium cation, the distance at which CO2 starts deforming,
cyanopyrrolide anion and the carbon atom of CO2. 0.249 nm, to become the carboxyl group is exemplified. Note
that in our simulations, proton transfer occurs at the first stage,
while carboxylation occurs at the second stage. In real chemical
reactions, we anticipate these sub-processes to take place more
which the analogous barrier is 13 kJ mol−1. Furthermore, the or less simultaneously.
chemisorption reaction energy gain equals −68 kJ mol−1, While the PM7-based potential energy surface provides a
whereas the carboxylation of the TBP cation brings just −22 kJ decent means to qualitatively evaluate the possibilities of the
mol−1. To recapitulate, the carboxylation of the α-carbon atom hypothesized reaction pathways and feasibility of the energy
of TBA is thermodynamically allowed but this reaction does not landscape, the identified noteworthy patterns must be verified at
have a very convenient intermediate as the phosphonium ylide. a higher level of theory. Hybrid density functional theory Becke-
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Figure 11. Noteworthy atomic configurations detected in the course of

the simulated chemical reactions. (A) Proton exchange between the
tetrabutyphosphonium cation and the 2-cyanopyrrolide anion. (B)
Carboxylation of the deprotonated α-carbon atom of the tetrabuty-
lammonium cation. The depicted black arrows display the key
interatomic distances. The oxygen atoms are red; the nitrogen atoms
are blue; the phosphorus atom is orange; the carbon atoms are cyan;
and the hydrogen atoms are black.

3-Lee-Yang-Parr exchange−correlation functional together with

a comprehensive atom-centered split-valence triple-zeta polar-
ized basis set, 6-311++G**, was used to confirm energetics and
locations of the relevant transition states. The stationary point
geometries were computed through the unconstrained geometry
optimization up to the local energy minimum/maximum. The
initial geometries for the saddle point search were taken from the Figure 12. Potential energy change in the course of the CO2 capture by
reaction path calculations by picking up a few highest-energy the phosphonium-based cation according to the B3LYP/6-311++G**
molecular conformations. Upon search along the reaction level of theory calculations: (a) proton transfer from the α-carbon atom
coordinate, the force constants were updated at every step. of the phosphonium-based cation to the electron-rich nitrogen atom of
the 2-cyanopyrrolide anion; (b) carboxylation of the α-carbon atom of
Each saddle point state exhibits a single imaginary vibrational the phosphonium-based cation by the CO2 molecule. The reaction
frequency, whereas each local minimum state exhibits real coordinates are hydrogen(cation)−nitrogen(anion) distance upon
vibrational frequencies only. The alkyl chains of the cations were proton transfer and carbon(CO2)−α-carbon distance upon carbox-
shortened in the transition state search studies to overcome ylation of the cation. The insets in both graphs depict representative
technical problems due to a large number of degrees of freedom atomic configurations of the reacting species and emerging products.
and vibrations at similar frequencies. Therefore, ethyltrimethyl-
phosphonium and ethyltrimethylammonium cations were
hereby modeled instead of tetrabutylphosphonium and To understand the role of the diffuse and polarization basis
tetrabutylammonium cations, respectively. The methylene functions, we reoptimized the transition state geometry by using
group of the ethyl chain was used to study the relevant chemical a less comprehensive basis set, 6-31G(d). The resulting
reactions. parameters differ by 0.001 nm in terms of interatomic distances
The deprotonation of the TBA reaction’s transition state and by 10 cm−1 in terms of the imaginary frequency. Thus, the
(Figure 12) exhibits an imaginary frequency of i1200 cm−1. This basis set containing no diffuse functions and no polarization
vibration corresponds to the stretching of the carbon (cation)− functions for the hydrogen atoms provides nearly the same
hydrogen (cation) covalent bond. The carbon−hydrogen transition state as the above-discussed high-precision calcu-
distance in the transition state amounts to 0.143 nm. In turn, lation.
the nitrogen−hydrogen distance equals 0.129 nm. Therefore, The carboxylation of the phosphonium ylide proceeds
the proton locates somewhat closer to the anion than to the without a transition state (Figure 12) formation. This
cation in the corresponding energy maximum. Upon proton observation is in line with conventional chemical wisdom
exchange, the distance from the phosphorus atom to the because ylides are relatively unstable structures that tend to
deprotonating α-carbon atom of the phosphonium-based cation actively participate in compound reactions. The no-barrier
decreases down to 0.175 nm. This observation is in line with an carboxylation reaction was also suggested by PM7 (Figure 5).
emergence of the unpaired electron in the course of the proton The energy gain due to carboxylation is −71 kJ mol−1.
exchange reaction. Compare the length of the phosphorus− Therefore, the CO 2 chemisorption in the case of a
carbon covalent bond to its equilibrium value in the unperturbed phosphonium-based ionic liquid appears, in total, thermody-
state, 0.183 nm. The height of the barrier associated with the namically favorable, −23 kJ mol−1.
deprotonation of the phosphonium-based cation, that is, the The ammonium-based ylide is an impossible compound due
formation of the phosphonium-based ylide, amounts to 48 kJ to nitrogen’s inability to afford the valence of five. However, the
mol−1. The ylide formation is a thermodynamically unfavorable ammonium-based ylide can still be a reaction intermediate
process according to density functional theory unlike suggested under favorable conditions assuming the emergence of stable
by a less precise method used previously PM7. However, both products and the existence of affordable reaction barriers. The
methods agree qualitatively on the geometry of the transition goal of this work is to understand the energetic profile associated
state and the identified potential barrier of the reaction. In the with the CO2 chemisorption by the ammonium-based cation.
cases of discrepancies, we opt to stick to the results of the Figure 13 reports the relaxed potential energy scans for the
B3LYP/6-311++G** calculations as they are more reliable. deprotonation of the ammonium-based cation and its
5503 https://doi.org/10.1021/acs.jpcb.2c02968
J. Phys. Chem. B 2022, 126, 5497−5506
The Journal of Physical Chemistry B pubs.acs.org/JPCB Article

tetraalkylammonium cation increases to 0.155 nm, whereas the

nitrogen−carbon distance in the unperturbed tetraalkylammo-
nium cation amounts to 0.150 nm. Unlike the phosphorus−
carbon bond in the tetraalkylphosphonium cation discussed
above, the nitrogen−carbon distance does not shrink. There-
fore, it would be chemically incorrect to term this intermediate
as ylide, that is, a compound in which a double covalent bond
exists. In realistic reaction conditions, carboxylation and
deprotonation take place more or less simultaneously. Thus,
the energy loss originating from the deprotonation is at least
partially compensated by the energy gain due to CO 2
attachment.
The carboxylation of the deprotonated ammonium-based
cation takes place smoothly in terms of the energy profile. At the
carboxyl carbon−α-carbon covalent distance of 0.160 nm, the
carboxylation terminates. The further approach of the
mentioned atoms leads to the potential energy increase due to
electron−electron repulsion. The attachment of the CO2
molecule as a carboxyl group brings −152 kJ mol−1. The
resulting energy effect amounts to −63 kJ mol−1. This is even
more favorable than the case of the tetraalkylphosphonium
cation, −23 kJ mol−1. Note, however, that the reaction barrier
observed in the case of ammonium-based cation is also
significantly higher and it will require a more essential activation
Figure 13. Potential energy change in the course of the CO2 capture by energy to be supplied in the course of the reaction.
the ammonium-based cation according to the B3LYP/6-311++G**
level of theory calculations: (a) proton transfer from the α-carbon atom
of the ammonium-based cation to the electron-rich nitrogen atom of
the 2-cyanopyrrolide anion; (b) carboxylation of the α-carbon atom of
■ CONCLUSIONS AND FINAL REMARKS
To recapitulate, the present work rationalizes carbon dioxide
the ammonium-based cation by the CO2 molecule. The reaction chemisorption by the quaternary ammonium and phosphonium
coordinates are hydrogen(cation)−nitrogen(anion) distance upon cations coupled with the aprotic heterocyclic anion 2-
proton transfer and carbon(CO2)−α-carbon distance upon carbox- cyanopyrrolide. In each simulated system, we considered three
ylation of the cation. The insets in both graphs depict representative chemical processes that are involved in the CO2 capture and
atomic configurations of the reacting species and emerging products. identified their key features. It was found that the chemisorption
reaction is initiated by the donation of the proton from the
subsequent carboxylation. No transition state corresponds to the cation to the anion. This process is thermodynamically
formation of the ammonium-based ylide. The potential energy unfavorable in the case of both cations; however, the formation
smoothly grows upon the proton detachment. This process of the phosphonium-based ylide is associated with a much
results in an energy loss of +89 kJ mol−1. Compare the above smaller potential barrier. The chemisorption event is expected to
value to the deprotonation cost of +39 kJ mol−1 and the fail if the CO2 molecule approaches the α-carbon atom of the
deprotonation barrier of +48 kJ mol−1 recorded in the case of the cation before its deprotonation. This conclusion was drawn from
phosphonium-based cation. Naturally, the formation of the the analysis of the reaction barriers upon the simulation of the
ammonium-based ylide appears to be significantly more costly as competitive processes.
compared to the phosphonium-based one. The absence of the The sterical hindrances introduced by the saturated alkyl
potential barrier suggests that any coexistence of the protonated chains of the quaternary ammonium- and phosphonium-based
AHA and the deprotonated ammonium-based cation is cations play a significant role in the CO2 chemisorption. The
impossible. Such a hypothetical system must immediately role and the scale of the hindrances are revealed in the reaction
return to its initial state upon constraint removal. path diagrams. The hindrances are more essential upon
Both physical and chemical sorptions of CO2 may lead to carboxylation than upon deprotonation. In turn, the CO2
essential technological advances. The chemical sorption is capture according to the carbamate formation mechanism is
normally more energetically favorable, and therefore, the also influenced by the alkyl chains of the cations. This feature
capacities of such sorbents are, on average, higher. In turn, might have been expected since the cation−anion attraction is,
physical adsorption occurs thanks to the coupling of the by default, stronger than cation−CO2 and anion−CO2 non-
molecules’ electric moments and London forces. The former is bonded (dispersion and electrostatic) attractions.
somewhat stronger than the latter. Physical adsorption is always The chemisorption of CO2 by the tetrabutylphosphonium
a reversible process being a positive feature in the context of its cation involves the formation of the tetrabutylphosphonium
technological implementation. Chemisorption can be either ylide. Subsequently, the CO2 molecule gets grafted. Thanks to
reversible or irreversible depending on the kinetic and the double carbon−carbon bond, carboxylation takes place with
thermodynamic stabilities of the corresponding products. The essentially no barrier. In turn, the tetrabutylammonium ylide is a
goal of chemical engineering is to find a proper balance between metastable sub-product. It was possible to detect the energy cost
a high sorbent capacity and a low sorbent loss during the course of it. Such a compound is highly reactive due to the inability of
of CO2 scavenging. nitrogen to afford the valence of five. As a result of its chemical
Upon proton exchange, the covalent distance from the activity, the ylide reacts with the CO2 molecule with no
nitrogen atom to the deprotonating α-carbon atom of the associated potential barrier and forms the carboxyl group linked
5504 https://doi.org/10.1021/acs.jpcb.2c02968
J. Phys. Chem. B 2022, 126, 5497−5506
The Journal of Physical Chemistry B pubs.acs.org/JPCB Article

to the α-carbon atom of the ammonium-based cation. The 2-

cyanopyrrolide participates in the carbamate formation reaction,
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Corresponding Author
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