Competition between electron-donor and electron-acceptor substituents in nitrotoluene isomers: A photoelectron spectroscopy and ab initio investigation

Article abstract of DOI:10.1039/c3ra45705b

We present an investigation of the close relationship between chemical structure, physical properties and reactivity of the three nitrotoluene isomers: a joint experimental and theoretical study, based on X-ray photoelectron spectroscopy (XPS) measurements and ab initio calculations, addressing the complex interplay between the competing electron-donor and electron-acceptor effects of the nitro- and methyl-substituents on the chemical properties of the nitrotoluene isomers. As the main results of the investigation we: (i) point out that accurate ab initio calculations play a key role in the complete assignment of photoemission measurements, as well as in the estimate of proton affinities in the case of all the eligible sites; (ii) revisit, at a more quantitative level, textbook models based on inductive and resonant effects of different substituents of the aromatic ring, as well as on the hyper-conjugative connection of the methyl group to the π-conjugated system; (iii) provide an accurate analysis of correlation patterns between calculated proton affinities and core-ionization energies, which represent a powerful tool, capable of predicting site-specific reactivities of polysubstituted molecules in the case of electrophilic aromatic substitution reactions.

Full text of DOI:10.1039/c3ra45705b

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PAPER
Competition between electron-donor and
electron-acceptor substituents in nitrotoluene
isomers: a photoelectron spectroscopy and ab
initio investigation†
Cite this: RSC Adv., 2014, 4, 5272
a
b
c
Flaminia Rondino, Daniele Catone, Giuseppe Mattioli, Aldo Amore Bonapasta,c
Paola Bolognesi,d Anna Rita Casavola,d Marcello Coreno,d Patrick O'Keeffed
and Lorenzo Avaldid
*
We present an investigation of the close relationship between chemical structure, physical properties and
reactivity of the three nitrotoluene isomers: a joint experimental and theoretical study, based on X-ray
photoelectron spectroscopy (XPS) measurements and ab initio calculations, addressing the complex
interplay between the competing electron-donor and electron-acceptor effects of the nitro- and
methyl-substituents on the chemical properties of the nitrotoluene isomers. As the main results of the
investigation we: (i) point out that accurate ab initio calculations play a key role in the complete
assignment of photoemission measurements, as well as in the estimate of proton affinities in the case of
all the eligible sites; (ii) revisit, at a more quantitative level, textbook models based on inductive and
resonant effects of different substituents of the aromatic ring, as well as on the hyper-conjugative
connection of the methyl group to the p-conjugated system; (iii) provide an accurate analysis of
correlation patterns between calculated proton affinities and core-ionization energies, which represent a
powerful tool, capable of predicting site-specific reactivities of polysubstituted molecules in the case of
electrophilic aromatic substitution reactions.
Received 10th October 2013
Accepted 12th December 2013
DOI: 10.1039/c3ra45705b
www.rsc.org/advances
properties.4 Early theoretical investigations of structure–prop-
erties correlations in substituted benzenes were focused on the
1 Introduction
displacements of the ground-state valence electronic density at
given molecular sites.5 Core-ionization energies (IEs), measured
by X-ray photoelectron spectroscopy (XPS), represent a more
direct probe of the ability of aromatic molecules to accept
positive charge at a specic site, as they are related in a strict
quantitative way to the screening of photoinduced core holes by
means of the displacement of the valence electronic density in a
positively charged ionized state.6,7 The addition of H+ ions to the
same molecules by means of proton transfer processes, in
which aromatic compounds act as Brønsted bases, introduces
an alternative class of positively charged intermediates. The
proton affinity (PA) of different molecular sites, regulating the
formation of different protonated species, represents the key
quantity of such processes.8 PA estimates are pivotal in under-
standing concepts and processes such as proton-coupled elec-
tron transfer reactions, acidity and alkalinity of organic
molecules, and kinetics of EAS reactions. Reliable information
of PAs is therefore intrinsically valuable.
Chemical moieties substituting H atoms of benzene and larger
aromatic rings are generally known to have signicant effects
on the chemical properties of the aromatic compounds,
through their ability to either withdraw or donate electrons to
the p-conjugated system.1 Substituents already present on the
aromatic ring behave, for example, as activating/deactivating as
well as positional selectivity factors in electrophilic and nucle-
ophilic aromatic substitution (EAS and NAS, respectively) reac-
tions.2,3 These observations triggered the birth of an area of
study of the relationships between structure and aromatic
a
`
ENEA Centro Ricerche di Frascati, Unita Tecnica Sviluppo di Applicazioni delle
Radiazioni, Via E. Fermi 45, I-00044 Frascati, RM, Italy
bIstituto di Struttura della Materia del CNR, Via del Fosso del Cavaliere 100, I-00133
Roma, Italy
cIstituto di Struttura della Materia del CNR, Via Salaria Km 29.300, I-00015
Monterotondo Stazione, RM, Italy. E-mail: giuseppe.mattioli@ism.cnr.it
dIstituto di Metodologie Inorganiche e dei Plasmi del CNR, Via Salaria Km 29.300,
I-00015 Monterotondo Stazione, RM, Italy
A fruitful relationship has been established between the
above two classes of positively charged aromatic molecules:
several studies have reported patterns of linear correlations
between protonation enthalpies and IEs, and deviations from
† Electronic supplementary information (ESI) available: Valence spectra and
molecular orbitals of the nitrotoluene isomers; N(1s) and O(1s) XPS;
comparison of theoretical IE values in the case of toluene; atom coordinates
and total energies of nitrotoluene isomers. See DOI: 10.1039/c3ra45705b
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these linear patterns have been used to identify preferential ring. In order to provide a full account of the nitrotoluene
sites of electrophilic attack to the aromatic ring. The occurrence investigation, these additional measurements and calculations
of such a linear correlation has been long established in the have all been reported in the ESI.†
case of N(1s) and O(1s) XPS measurements.9–20 It has been
shown recently that the same holds in the case of C(1s).21–23
However, there are practical limitations to the experimental 2 Experimental and computational
evaluation of PA; protonation enthalpy values can be measured
only for the most favorable protonation sites of each mole-
details
cule.8,23 On the other hand, IEs can, in principle, be measured XPS spectra have been measured at the gas phase photoemis-
for all the atoms of the molecule, even if accidentally degenerate sion beamline of the ELETTRA synchrotron radiation source
lines, especially in the case of polysubstituted benzene rings, (Trieste, Italy)30 using a commercial 6-channels, 150 mm
can be grouped into unresolved broad peaks. These experi- hemispherical electron energy VG analyzer, and the same setup
mental limitations can be overcome by performing accurate which has been described previously.6 The three nitrotoluene
theoretical ab initio estimates of PA values and XPS lines, isomers were obtained from Sigma-Aldrich with a minimum
univocally assigned to all the eligible sites, which can be used to purity of 99% and used without any further purication. The
draw correlations patterns between PA and XPS data.
ortho- and meta-nitrotoluene isomers are liquids under stan-
In this scenario, nitroaromatic compounds have been dard conditions, with a vapour pressure sufficiently high for the
selected in order to shed light on different facets of the above present experiment. The para-nitrotoluene isomer forms yellow
structure–properties relationship. They are relatively rare in crystals and requires moderate heating of the gas inlet system to
nature, but widely used in the chemical industry for the 313 K to obtain the same target density, avoiding the decom-
production of dyes, resins, pesticides, herbicides, explosives, position of the sample. The XPS spectra were calibrated using a
and other useful materials.24–26 In particular, we focus here on mixture of the molecule under investigation and of a calibration
the three nitrotoluene isomers, both because of their usage as gas with well-known XPS lines in the same IE range, that is, CO2
starting reactants or intermediates in a large number of tech- (C(1s) at 297.7 eV).31
nologically relevant synthesis processes, and because of the
Theoretical values of vertical XPS C(1s) IE have been calcu-
concurrent presence of competing substituents on the aromatic lated by means of ab initio density functional theory (DFT)
ring. More specically, we present the rst in-depth experi- methods in a supercell approach, as implemented in the
mental and theoretical investigation of C(1s) IEs and PAs of Quantum-ESPRESSO package.32 Equilibrium geometries have
such compounds, aimed at elucidating how their electronic and been found by fully relaxing all the molecules accommodated in
3
˚
chemical properties are inuenced by the presence of the two large cubic supercells (25 A ) to minimize the occurrence of
competing electron-donor methyl and electron-attracting nitro spurious interactions between periodically replicated images.
groups.1 The different relative positions on the benzene ring of Total energies have been calculated by using norm-conserving
the CH3 and NO2 moieties play a crucial role in the electronic Troullier–Martins atomic pseudopotentials,33 a plane-wave basis
and chemical properties of the nitrotoluene compounds, thus set, and the B3LYP hybrid exchange–correlation functional.34,35
offering a signicant benchmark for testing whether IE and PA Satisfactorily converged results have been achieved by using
estimates can be used to foresee and elucidate chemical cutoffs of 80 Ry on the plane waves and of 320 Ry on the elec-
quantities such as reaction rates in EAS processes.2 The results tronic density, respectively, as well as the G point for the k-point
obtained allow quantitative predictions of the activation of sampling of the Brillouin zone. H(1s), C(2s) and (2p), N(2s) and
different molecular sites, e.g., in the case of further nitration of (2p), and O(2s) and (2p) electrons have been treated as valence
several CH3- and NO2-substituted rings.27 Moreover, a detailed electrons. All of the inner shells are embedded in the pseudo-
comparison between experimental measurements and theoret- potentials. C(1s) XPS chemical shis have been estimated by
ical calculations, including the screening effects of core holes, total energy differences between “standard” and “core-hole”
provides a deeper understanding of the electronic properties of calculations.6,36 In the latter case, an excited state C pseudopo-
substituted aromatic compounds, e.g., the study on a quanti- tential containing a 1s core hole has been used in place of the
tative basis of the positional selectivity induced by the conju- regular pseudopotential. A different calculation has been
gation of the methyl group to the benzene ring,4,28,29 in performed for each nonequivalent C atom. The energy differ-
agreement with the textbook hyper-conjugation model.1
ences between the standard and core-hole calculations have
Further effects of the competition of CH3 and NO2 groups on been compared with the corresponding difference obtained for
the electronic properties of nitrotoluene isomers have been the C atom of a CO2 molecule, accommodated in every supercell
investigated by performing experimental photoelectron and used as a reference for both the theoretical and experi-
measurements of the O(1s) and N(1s) shells, and in the spectral mental chemical shis. Despite the fact that the above theoret-
region of valence electrons, and by comparing the results with ical framework is not oen used to investigate the properties of
the properties of molecular orbitals, as described by ab initio isolated molecules, it provides very accurate predictions of C(1s)
calculations. Such investigations provide direct information on XPS IEs: differences between experimental measurements and
molecular orbital changes due to the displacement of conju- theoretical estimates not exceeding the value of about ꢀ0.1 eV
gated electrons, as well as additional proofs of the activation/ have been reported in the case of halogenated pyrimidines,
deactivation effect of aromatic substituents on the benzene where all the XPS lines could be assigned to given atoms.6
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Moreover, this kind of calculation is easily scalable with the benzene ring, can be accounted for at a more quantitative
comparable accuracy to larger molecules and molecule–surface level. From this point of view, a close relationship between
interacting systems.37
collected experimental measurements and two different kinds
Theoretical calculations of PA have been performed by using of theoretical estimates, the former related to core shis, the
the Gaussian 03 package.38 Optimized geometries have been latter concerning PA of unsubstituted C sites of the benzene
obtained by means of DFT calculations, performed using the ring, will be discussed in the following.
B3LYP functional and a localized 6-311G++(d,p) basis set. Stable
molecular structures have been obtained, in full agreement with
the above plane-wave based calculations. Moreover, proton
3.1 Fine analysis of C(1s) ionization energy values
affinities have been estimated by calculating protonation The C(1s) core-level spectra of the ortho-, meta- and para-nitro-
enthalpy values. Such values have been calculated using a toluene isomers measured at a photon energy of 382 eV are
renement of the well established procedure proposed by shown in Fig. 1 together with the results of the theoretical
8,39,40
´
Maksic et al.
In this theoretical approach, PAs are calcu- predictions. The spectra have been measured with an overall
lated employing the general equation.
energy resolution of 0.32 eV, mainly determined by the energy
resolution of the electron analyzer. Carbon dioxide, whose
binding energy, vibrational structure and natural width are well
known,44 has been mixed with the molecule under investigation
to calibrate the binding energy scale as well as to estimate the
energy resolution of the present spectra. In the XPS measure-
ments, the core ionization of each inequivalent C atom
produces a distinct prole in the C(1s) spectrum.45 With respect
to the benzene ring where all the C atoms are equivalent, the
inequivalent C sites of the three isomers of the nitrotoluene
molecules result in bands shied by different amounts, due to
the position and effect of the concurrent methyl and nitro
groups, and composed by different peaks related to the vibra-
tional structure which characterize the different core-ionized
states. In o- and m-nitrotoluene all of the seven C atoms are
formally inequivalent, while in the case of the p-isomer there are
only ve inequivalent C atoms: the methyl C0, the C1, C4, C2/C6
and C3/C5 atoms (see Fig. 1 and Table 1). The present XPS
spectra are measured about 95 eV above the ionization
threshold. In such conditions, the expected post collisional
interaction (PCI) energy shi of a typical C(1s) state of lifetime G
z 0.10 eV is less than 0.013 eV,46,47 i.e., negligible with respect to
the present energy resolution. However the lineshape, being
affected by an undened number of overlapping and unre-
solved vibrational progressions, may be different from one state
to another. A tting procedure of the spectra would require a
complete series of 7, 7, and 5 bands in the ortho-, meta- and
para-nitrotoluene cases, respectively. Independent of the line-
shapes, which can be represented by simple asymmetric
Gaussian functions,6 as well as by more accurate convolution of
theoretically calculated vibrational contributions to the ionized
PA(Ba) ¼ (DEel)a + (DZPEn)a
(1)
where (DEel)a ¼ [E(B) ꢁ E(BH)a+] and (DZPEn)a ¼ [ZPE(B) ꢁ
ZPE(BH)a+] are the electronic and the zero-point vibrational
energy (ZPE) contributions to the PA, respectively. Here, B and
BH+ denote the molecule and its positively charged conjugate
acid, respectively, and a indicates the site of proton attack. The
method has been widely applied with a surprising success to a
large variety of polysubstituted benzenes and naphthalenes by
using a MP2(fc)/6-31G**//HF/6-31G* + ZPE (HF/6-31G*) setup,
which indicates that the ZPE was estimated at the HF/6-31G*
level multiplied by a common empirical factor 0.89.40 The
results are generally in good agreement with measured values;
however, qualitatively wrong PA values have been calculated in
the case of this setup when applied to strongly deactivating
groups such as NO2, NO and CN.41 For these reason, the PAs
presented in this work have been obtained by using the G3B3
approach.42 G3B3 is an accurate, composite method for the
calculation of the energies of molecules containing rst and
second row elements. It is based on the G3 method43 which uses
geometries optimized at the MP2(FU)/6-31G* level and scaled
HF/6-31G* zero point energies, followed by a series of single
point energy calculations as well as spin orbit and high level
corrections. The G3B3 method is identical except that it is based
on B3LYP/6-31G* geometries and scaled B3LYP/6-31G* zero
point energies.
3 Results and discussion
As introduced above, C(1s) core level spectra, assisted by state,23 a procedure which leaves all the tting parameters as
parallel theoretical estimates, give access to a deeper under- free variables would result in strongly correlated parameters,
standing of electronic properties as well as of chemical prop- leading to a representation of the spectrum which is far from
erties, e.g., reactivity in EAS reactions, in the case of being “unique”. Hence, no assignment of the C(1s) IE values
poly-substituted benzene rings such as the nitrotoluene can be achieved on the grounds of the measurements alone. It
isomers. The presence of competing electron-donor and requires an accurate theoretical analysis which takes into
-attractor groups such as CH3 and NO2, respectively, together account the chemical shis, as reported in Table 1. However
with highly accurate experimental and theoretical estimates of some qualitative observations can be done by looking at the XPS
C(1s) core shis, represents a fruitful chance to reach the heart spectra in Fig. 1. They show two broad asymmetrical bands, the
of well-established text-book models. These models, describing low-energy one (A in Fig. 1) with a distinct shoulder (B) in the
the inductive and resonant effects of benzene substituents, the case of the o-nitrotoluene. All the peaks are shied to higher IE
activation/deactivation of the chemical reactivity of a given with respect to those of toluene, due to the well known, strong
molecular position, the hyperconjugation of the CH3 group with electron attracting ability of the NO2 group. In particular, the
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Table 1 XPS C(1s) and PA values. IE values corresponding to the main
features of XPS spectra (labeled A–C in Fig. 1) are compared with
calculated C(1s) ionization energy values related to the three nitro-
toluene isomers, reported together with proton affinity values
Measured
Peak
Calculated
IE (eV)
IE (eV)
290.8
Atom
PA (eV)
o-Nitrotoluene
A
C6
C4
C5
C3
C0
C1
C2
290.61
290.66
290.76
290.78
290.82
291.15
291.75
ꢁ7.38
ꢁ7.42
ꢁ7.30
ꢁ7.25
—
B
C
291.2
291.8
—
—
m-Nitrotoluene
A
290.8
C6
C2
C4
C5
C1
C0
C3
290.68
290.71
290.75
290.79
290.97
291.08
291.81
ꢁ7.38
ꢁ7.35
ꢁ7.42
ꢁ7.30
—
—
—
C
291.8
290.8
p-Nitrotoluene
A
C2,6
C3,5
C1
C0
C4
290.75
290.93
291.08
291.20
291.79
ꢁ7.35
ꢁ7.17
—
—
—
C
291.7
Calculated XPS C(1s) values are represented by numbered
bars below the spectra in Fig. 1, while the corresponding
Gaussian convolutions by the dashed lines above the spectra.
An overall good agreement between the shapes of the experi-
mental and theoretical spectra is observed. We exploit this close
agreement between theoretical and experimental data to
attempt a rst analysis of the present results on the grounds of
calculated IE values. We employ, at a more quantitative level, an
elementary model based on the interplay between the stronger,
isotropic inductive (I) effects and the weaker, anisotropic reso-
nant (R) effects, both contributing to the screening of photo-
induced C(1s) core holes. The former are related to the
electronegativity of the substituting groups and affect both the
whole molecule, with global charge density withdrawal from, or
donation to the aromatic system, and every single atom, the
nearer the atom to the substituting group, the stronger the
effect. The latter are conventionally ascribed to the extension of
p-conjugation to the substituents, and are generally interpreted
by using resonance structures in which positive charges are
anisotropically shared between different sites of the p-conju-
gated network of bonds. The I–R model has been oen used to
illustrate the properties of substituents in aromatic compounds
Fig. 1 C(1s) XPS spectra of o-nitrotoluene (a), m-nitrotoluene (b), and
p-nitrotoluene (c). Experimental measurements (open circles) have
been fitted with Gaussian lineshapes (full black lines). The spectra have
been measured at a photon energy of 382 eV with an overall resolution
of 0.32 eV. The labels A–C indicate the main features of the spectra,
also reported in Table 1 and discussed in the text. Dashed lines
represent convolutions of Gaussian peaks centered on the 0–6 bars
(0–4 in the case of para-nitrotoluene), corresponding to the theo-
retical estimates of C(1s) XPS lines for the C0–C6 carbon atoms, also
reported in Table 1.
shis are signicantly pronounced (1.5 eV) for C atoms directly because it provides an intuitive and powerful tool to understand
bonded to the nitro-group. The bands at higher energies, cen- and predict the results of relevant classes of chemical processes
tred around 291.8 eV (C), are therefore easily attributed to the such as the EAS reactions.1,2 This simple, two-contribution
ionization of the C2, C3, and C4 positions of the o-, m-, and model has been applied in the present investigation to a
p-nitrotoluene isomers, respectively.
different kind of positively charged system, in order to provide a
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P
ne analysis of the screening of a core hole, by using IEs as the expression DðIEÞ ¼ niai þ mibi where the ai and bi parame-
i
leading quantities. Moreover, the I–R model is generally
ters represent the effects of ipso, ortho, meta and para substi-
extended to non-conjugated groups bonded to aromatic rings,
tutions of the ni methyl and mi nitro groups on the IE values,
e.g., the CH3 moiety investigated in the present work, through
respectively. This expression is well t by the calculated values
the hyper-conjugation effect.4,23,28,29 This effect is described as
shown in Fig. 2. The best t ai and bi parameters have been
the formation of a p-like overlap between the true aromatic
reported in Table 2. First of all, as mentioned above, the large
conjugated system and an almost vertical sp3 s orbital
belonging to a non-conjugated substituent, inducing a partial
electron-withdrawing behavior of the NO2 group dominates the
charge density distribution of nitrobenzene and nitrotoluenes;
delocalization of the s charge density on the aromatic ring
a generalized and consistent blue shi of all the C(1s) IEs of
(a detailed description of the hyper-conjugative effect involving
nitrotoluenes with respect to those of benzene is indeed
the CH3 moiety is provided in Section 3.2). This, in turn, allows
found, which is responsible for a signicant deactivation of
one to analyze in a unied framework the effects of the CH3 and
NO2 groups on the C(1s) IEs of the nitrotoluene isomers. We
Table 2 XPS C(1s) and PA additivity parameters. Additivity parameters
for the effect of CH3 and NO2 substitutions on the C(1s) vertical
ionization energies and on enthalpies of protonation of C sites. All the
values are in eV
anticipate here the existence of a close relationship between two
different kinds of positively charged state, i.e., the reaction
intermediate of EAS (s complex) and the ionized molecule
carrying a C(1s) core hole; this relationship will be discussed in
more detail in Section 3.3.
The calculated IE values of benzene, toluene, nitrobenzene,
and the three nitrotoluene isomers, together with I and R effects
of the CH3 and NO2 groups, which can both stabilize (ꢁ) and
destabilize (+) a positive charge, have been displayed in Fig. 2 to
favor the cross-comparison of data. As previously shown in the
case of CH3- and F-polysubstituted benzene rings,22,23 the effects
of multiple substituents on the C(1s) IEs can be effectively
described in terms of an additivity model. In detail, each energy
shi D(IE) with respect to the benzene value (290.37 eV) is
considered to be the sum of independent effects of the indi-
vidual substituents.48 In the case of toluene, nitrobenzene, and
the three nitrotoluene isomers, this corresponds to a simple
Ionization
energies
Enthalpies of
IE std errora protonation
EP std errora
CH3 ipso
0.15
0.04
0.04
0.04
0.04
0.04
0.03
0.03
0.04
—
—
ortho ꢁ0.21
ꢁ0.31
ꢁ0.19
ꢁ0.37
—
0.72
0.71
0.74
0.02
0.02
0.03
—
0.02
0.02
0.03
meta ꢁ0.11
para ꢁ0.18
NO2 ipso
1.59
0.60
0.52
0.55
ortho
meta
para
a
Standard errors (SE) have been calculated as SE ¼ (95% condence
interval)/2a where the a parameter depends on the number of degrees
of freedom of the t; a ¼ 2.1199 (a ¼ 2.2281) in the case ionization
energies (enthalpies of protonation).
Fig. 2 Theoretical estimates of vertical C(1s) XPS lines (black values) and of protonation enthalpies (magenta values) of benzene, toluene,
nitrobenzene, and of the three nitrotoluene isomers, all reported as negative or positive shift with respect to the benzene values. Positive weak (+)
and strong (++), as well as negative (ꢁ) inductive (I) and resonant (R) effects on the IE and PA, induced by the CH3 (blue) and NO2 (red) groups, are
reported. See the text for further details.
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nitro-aromatic derivatives in the case of EAS reactions. In the C(1s) core hole. This has been investigated by analysing differ-
case of the CH3 group, a further (weak) electron-attractive ence electron density maps, obtained by subtracting the valence
inductive effect is expected, due to the fact that the methyl electronic density of a ground state calculation from the valence
moiety is more electronegative than the H atom, as conrmed electronic density of the corresponding calculation of a core
by the fact that the ipso coefficient only has a positive value. hole ionized state. The resulting density difference maps,
Despite such a weak inductive contribution, the ortho, meta and displayed in Fig. 3, show the amounts of charge density owing
para coefficients of the CH3 group are all negative, thus leading from red-colored negative zones to blue-colored positive zones
to an average IE shi of ꢁ0.1 eV estimated in the case of the in order to screen the core hole. Three different screening
unsubstituted C atoms of toluene with respect to the benzene patterns have been identied, which elucidate the relationship
value. This negative shi supports the idea that the CH3 group between the sp3 methyl carbon and the sp2 ring: (i) the Rꢁ effect
is able to extend the p-conjugation of benzene through a hyper- connected to the textbook hyper-conjugative mechanism is fully
conjugation mechanism and promotes a better stabilization of acknowledged by comparing the screening patterns of a C(1s)
core holes. The resonant behavior of CH3 and NO2 substituting core hole induced in the C2 and C3 positions of the p-nitro-
groups induces opposite effects on the benzene ring, as shown toluene (the upper row in Fig. 3). In the case of the C2 position,
by a comparison between ortho–para and meta coefficients in the CH3 group (enclosed in a blue ellipse) is more involved in
Table 2. The CH3 group stabilizes a core hole in ortho–para, in the screening of the core hole, in contrast to the C3 position,
agreement with the stronger negative deviations of the corre- where a visibly lesser contribution of the methyl group to the
sponding coefficients; the NO2 group displays an opposite screening is added to the R+ contribution of the NO2 group
behavior and destabilize a core hole in ortho–para, in agreement (enclosed in a red ellipse), which is able to drain part of the
with the stronger positive deviations of the corresponding charge density from the neighboring p-type red zone. (ii) When
coefficients. It may be also worth noticing that, coeteris paribus, the core hole is located in the C0 positions (i.e., the methyl
such positive deviations are more pronounced in the case of C carbon, see the middle row in Fig. 3), valence charge is
atoms holding ortho positions than in the case of those in para displaced to screen both the almost spherical blue zone around
with respect to the NO2 group. This is in agreement with a quite the C0 position and a different p-shaped blue zone centered on
long range effect of the isotropic contribution of the nitro group the C1 position (see the blue arrows in Fig. 3). The rst of these
to the core holes (blue I+ labels in Fig. 2), which has to be blue zones reects the expected behavior for a C(1s) core hole,
summed to its R+ contribution.
as already pointed out by Bolognesi et al.,6 while the second
Deeper insight can be provided by performing further anal- zone was not observed in the previous study. The p-shaped blue
ysis beyond the additivity model. In the case of o-nitrotoluene, region on the C1 atom indicates that its pz orbital is involved in
the twisting of NO2 induced by the ortho-proximity of the methyl the formation of the ionized state and has to be screened by a
group leads to an attenuation of the p-conjugation of the nitro corresponding amount of valence charge. Apart from the role of
group, which, in turn, seems to suppress its R+ effect while not the neighboring C1 position, negligible contributions to the
affecting the isotropic inductive contribution. IE values lower screening come from the aromatic ring, indicating that the
than the corresponding additive coefficients have indeed been relationship between the sp3 core hole and the sp2 system is not
estimated in the case of the C3 and C5 positions of the reciprocal with the relationship discussed in (i). (iii) In the case
o-nitrotoluene (0.41 and 0.39 eV, respectively, in comparison of the C1 maps (the lower row in Fig. 3), the screening pattern of
with the expected 0.49 and 0.44 eV values). A direct comparison the almost spherical blue zone around the C1 position is
of the strength of opposite R+ and Rꢁ effects can be attempted dominated by the isotropic I+ effect of the neighboring methyl
in the case of the m-nitrotoluene, where competing effects fall group, not displaying any Rꢁ contribution. An expected intra-
exactly on the same C2, C4 and C6 positions. Quite low IE shis ring p-type contribution to the screening of the C1 core hole,
(0.34, 0.38 and 0.31 eV, respectively, to be compared with the comes instead from the C2/C6, and C4 positions (red arrows in
corresponding additive coefficients, 0.39, 0.42 and 0.34 eV) Fig. 3), as already discussed elsewhere.6
indicate a stronger Rꢁ effect of the methyl group; an opposite
The above screening patterns provide a guide to the
behavior, if compared with the stronger I+ effect of the nitro analysis of the corresponding C0 and C1 IE values, even if
group, consistent with a predominance of the CH3 group as a small differences between IE values do not produce appre-
director in EAS reactions.2
ciable variations in the charge density maps corresponding
to different nitrotoluene isomers. Regarding the calculated
IE of C0 positions, all the nitrotoluenes values are quite close
to that of toluene (within 0.2 eV, see Fig. 2) indicating that the
strong electron-attracting behavior of the nitro group is only
3.2 Connection between the methyl group and the aromatic
ring: about and beyond the hyperconjugation model
Further calculations have been performed in the framework of slightly propagated to the CH3 moiety through its connection
the plane wave/pseudopotential method introduced above to with the aromatic system. A higher IE value has been calcu-
shed more light on the connection of the methyl group to the lated for the p-nitrotoluene isomer (291.20 eV), where the
benzene ring, as well as to its relationships with the hyper- resonant contribution of the nitro group can affect the
conjugation mechanism. In detail, we have considered the displacement of positive charge in the C1 position (Fig. 3). A
displacement of the electronic charge density in the toluene similar contribution is not expected in the case of m-nitro-
molecule and nitrotoluene isomers caused by the formation of a toluene (291.08 eV), where the C1 atom holds a meta position
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Fig. 3 Difference density maps (top and side view) of “standard” versus “core-hole” calculations (see the text): (upper row) screening of a core
hole induced in the C2 and C3 positions of p-nitrotoluene. (Middle row) Screening of a core hole induced in the C0 position of toluene,
o-nitrotoluene, m-nitrotoluene and p-nitrotoluene. (Lower row) Screening of a core hole induced in the C1 position of toluene, o-nitrotoluene,
m-nitrotoluene and p-nitrotoluene. Blue and red zones represent positive and negative isosurfaces, respectively, of electronic density sampled
at 0.002 electrons per a.u.3 (0.005 electrons per a.u.3) in the case of the upper (middle and lower) row: an amount of valence charge density flows
from red to blue regions to screen the core hole.
with respect to the nitro group, and in the case of o-nitro-
toluene (290.82 eV), due to the twisting of the NO2 group as
well as to more complex interference patterns between the
two sterically close substituents. All three C1 positions fulll
instead the additivity model, in agreement with the methyl
ipso coefficient reported in Table 2. They are affected, in
contrast to the C0 positions, by the strong NO2 electron-
withdrawing effect; all of the corresponding nitrotoluene IE
values are therefore blue-shied by about 0.5–0.6 eV with
respect to that of toluene.
3.3 Correlation between C(1s) ionization energy and proton
affinity in nitrotoluene isomers
Protonation enthalpy values calculated at the G3B3 level for the
three nitrotoluene isomers are shown in Table 1 and in Fig. 2,
where they are also accompanied by the corresponding
benzene, toluene and nitrobenzene values calculated by using
the same method. As noted above, PA values can be calculated
for all the possible protonated intermediates of all the investi-
gated molecules, as opposed to measured values, which are
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available for the most stable intermediates only. The same all we note that the strong hyper-conjugative effect of the methyl
additive model introduced in Section 3.1 in the case of IE shis substituent plays a leading role in determining the position of
can be applied to PA shis; data points have been t with a data points scattered (R ¼ 0.7603) both along and across the t
similar degree of accuracy. The additivity coefficients for line, that is, in turn, the chemical properties of nitrotoluene
protonation of ortho, meta and para positions with respect to the compounds. A clear partition between the more activated C2,4,6
methyl and nitro substituents are reported in Table 2. Proton- atoms, holding ortho and para positions with respect to the CH3
ation enthalpies affected by the position of the NO2 group are group, where it can display its Rꢁ effect, and the more deacti-
characterized by the expected order (meta < ortho, para), in vated C3,5 atoms in meta has been marked by a dashed red line
contrast to the MP2 values mentioned above.41 IE values are now in Fig. 4(b). On the contrary, a substantially meaningless
correlated with the present PA values, thus allowing for a close distribution of data points is obtained if we group them
connection between core-ionized and protonated positively according only to their ortho–para or meta position with respect
charged states involving the same C atom. The present G3B3 to the NO2 group. Nevertheless, the NO2 group acts as a ne
investigation has been extended to previous MP2 results related modulator of the position of data points. This main achieve-
to the mesitylene and to the o-, m-, p-xylene compounds,23 in ment can be enriched and supported by further considerations:
order to extend the IE and PA range and, in turn, to achieve a (i) if we compare the activated C4 and C6 positions of o-nitro-
better tting of data. All of the correlation data are shown in toluene (blue circles) with those of the m- and p-isomers (C2, C4,
Fig. 4 (a). All data points have been used to obtain the least- C6 red circles and C2,6 green circle, respectively), the blue
squares t represented by a red line. Such data points are circles correspond to the most activated (i.e. closer to the le-
slightly scattered around this line (correlation coefficient low end of the distribution) positions of all nitrotoluenes, in
R ¼ 0.9917 and rms ¼ 0.07 eV), as reported in previous contri- agreement with the partial suppression of the conjugation of
butions,21–23 ranging from the strongly activated mesitylene the NO2 group to the ring, due to its twisting. (ii) The C4 posi-
ring, placed at the low-le end of the line, to the strongly tions of o- and m-nitrotoluene are the most easily protonated. In
deactivated nitrobenzene ring, placed at the high-right end of the case of the ortho isomer (blue circles), this is in agreement
the line. Positive and negative deviations from the t line can be with a stronger Rꢁ effect of the CH3 group on the C4 para than
analyzed to gain the deepest insight possible into the on the C6 ortho position (see Table 2); in the case of the meta
competing properties of the CH3 and NO2 groups in the nitro- isomer (red circles), the large positive shi of the protonation
toluene isomers as activating and deactivating factors, as well as enthalpy in the C6 position is not in agreement with its lowest
positional selectivity directors, in EAS reactions.
IE value, compared with those corresponding to the positions
In this regard, we focus now on the inset (b) of Fig. 4, C2 and C4. We suggest that the formation of weak H bonds
showing in more detail the nitrotoluenes correlations. First of between the methylene H atoms of the s-complex formed in C2
Fig. 4 (a) Protonation enthalpy of benzene and of different sites of several CH3- and NO2-substituted benzene rings, plotted against the
calculated vertical C(1s) ionization energy of the C protonation site. The red line is the least-squares fit of the calculated data points. Data not
reported in Fig. 2 and Table 1 are taken from Myrseth et al.23 (b) Correlation data corresponding to nitrotoluene isomers only are shown in the
inset of figure, where the positions of C atoms on the ring are also indicated. A dashed line, orthogonal to the solid line, separates data points
corresponding to the C2,4,6 positions from those corresponding to the C3,5 positions.
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Table 3 Rates of nitration of nitrotoluenes and distribution of dinitro-products. Measured rate ratios of nitrotoluenes (kNT), with respect to
nitrobenzene (kNB), and measured distribution of nitro- and X,Y-dinitro-products related to the competitive nitration of toluene, nitrobenzene
and nitrotoluenes in CH3NO2 with NO2PF6 at 298 K (data taken from Olah, et al.2,27). In the cases of nitrotoluene isomers, the C position
undergoing nitration is also reported to avoid ambiguity
Species
kNT/kNB
Distribution of nitrotoluene products (%)
Toluene
—
2-NT
68
3-NT
2
4-NT
30
Distribution of di-nitrobenzene products (%)
Nitrobenzene
1
1,2-Di-NB
10
1,3-Di-NB
89
1,4-Di-NB
1
Distribution of di-nitrotoluene products (%)
2,3-Di-NT
1.0 (C3)
42.1 (C2)
—
2,4-Di-NT
59.0 (C4)
—
2,5-Di-NT
0.1 (C5)
18.6 (C6)
—
2,6-Di-NT
39.9 (C6)
—
—
3,4-Di-NT
—
35.8 (C4)
0.2 (C3)
3,5-Di-NT
—
3.5 (C5)
—
o-Nitrotoluene
m-Nitrotoluene
p-Nitrotoluene
384
91
147
99.8 (C2)
and C4 and the neighboring NO2 group, which do not occur in products. The p-nitrotoluene case is straightforward, as
the case of C6, is responsible for the lowering of the C2 and C4 mentioned above. In the case of o-nitrotoluene, the slight
values, which deviate from the distribution expected in terms of predominance of the 2,4-di-nitro product is in agreement with
the additive model.
the higher PA of the C4 position. In the case of m-nitrotoluene,
We test such an apparently sensitive theoretical machine on a the low nitration rate of the C6 position parallels its lowest PA,
well characterized chemical process such as the nitration by possibly for the same sterical reason suggested in the previous
NO2PF6 of the toluene, nitrobenzene, and of the three nitro- paragraph. Regarding the C2 and C4 positions, we note that the
toluene isomers,2,27 whose previously measured parameters are slight preference for the nitration of the C2 position in a NO2PF6/
reported in Table 3. Let us consider rst the nitration of toluene CH3NO2 solution is inverted in the case of a similar competitive
and nitrobenzene. In both cases the positional selectivity of the process in a NO2PF6/H2SO4 solution,27 thus justifying the
reaction is quite low and the expected, close to the statistically apparent disagreement between calculated and measured data.
determined, distribution of ortho and para products in the case
of toluene and meta (with a 10% population of ortho) in the case
of nitrobenzene is obtained. The 10% population of ortho-di-
nitrobenzene obtained in the latter case reects the lower reso-
4 Conclusion
nant effect of NO2, in agreement with the very small difference We have performed a joint experimental and theoretical inves-
between the ortho and meta protonation enthalpy values. Con- tigation of the three nitrotoluene isomers, aimed at unraveling
cerning the nitrotoluene isomers, let us compare now the the complex interplay between the competing effect of the nitro-
measured reaction rates and product distributions reported in and methyl-substituents on the chemical properties of such a
Table 3 with the theoretical correlation data shown in Fig. 4(b). If synthetically important class of aromatic compounds. We have
we consider the whole distribution, we note that there is a clear pointed out that: (i) accurate ab initio calculations are an
partition in Fig. 4(b) between C positions which are quantita- essential complement to experimental investigation, in order to
tively nitrated and C positions which are not, the largest sepa- achieve the complete assignment of photoemission measure-
ration being between the C2,6 and C3,5 positions of the ments in the case of congested XPS spectra, as well as to esti-
p-nitrotoluene, in agreement with the most unbalanced distri- mate protonation enthalpies in the case of all the eligible sites
bution of di-nitro products (99.8% vs. 0.2% nitration of such of the molecule; (ii) textbook models based on inductive and
positions). This separation between active and non-active posi- resonant effect of different substituents of the aromatic ring, as
tions have been marked by a dashed line in Fig. 4(b). Active well as on the hyper-conjugative connection of the methyl group
positions are distributed along the t line in agreement with the with the p-conjugated system, can be revisited, at a more
kNT/kNB values reported in Table 3, which indicate a faster quantitative level, by using core-ionization energies of inequi-
nitration of o-nitrotoluene, closer to the le-low end of the valent atoms as leading quantities; such an analysis can
distribution, with respect to the meta and para isomers. If we account indeed for minimal oscillations of the core-ionization
consider each of the nitrotoluene isomers alone, we can correlate energies related to the weak, opposite resonant effects of the
the distribution of di-nitro products with the PA of the sites, CH3 and NO2 groups. (iii) The analysis of correlation patterns
which is more closely related to the formation energy of the between ab initio calculated proton affinities and core-ioniza-
different s complexes, leading to the nal distribution of tion energies represent a powerful tool, which is able to predict
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site-specic reactivities of polysubstituted molecules in the case 23 V. Myrseth, L. J. Sæthre, K. J. Børve and T. D. Thomas, J. Org.
of electrophilic aromatic substitution reactions.
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Acknowledgements
This work was partially supported by the MIUR PRIN 26 A. M. Tafesh and J. Weiguny, Chem. Rev., 1996, 96, 2035–
2009W2W4YF and 2009SLKFEX, and by the Italy-Slovenia Joint
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