Phenylnaphthalenes: Sublimation equilibrium, conjugation, and aromatic interactions

Article abstract of DOI:10.1021/jp2111378

In this work, the interplay between structure and energetics in some representative phenylnaphthalenes is discussed from an experimental and theoretical perspective. For the compounds studied, the standard molar enthalpies, entropies and Gibbs energies of

Full text of DOI:10.1021/jp2111378

Article
pubs.acs.org/JPCB
Phenylnaphthalenes: Sublimation Equilibrium, Conjugation, and
Aromatic Interactions
†
†
‡
§
∥
̈
Carlos F. R. A. C. Lima, Marisa A. A. Rocha, Bernd Schroder, Lígia R. Gomes, John N. Low,
,
†
and Luís M. N. B. F. Santos*
†
Centro de Investigaca
̧
o
̃
em Química, Departamento de Química e Bioquímica, Faculdade de Cien
̂
cias da Universidade do Porto,
P-4169-007 Porto, Portugal
§
CIAGEB-Faculdade de Ciencias da Saude, Escola Superior de Saude da UFP, Universidade Fernando Pessoa, P-4200-150 Porto,
̂
́
́
Portugal
‡
CICECO, Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro, Portugal
∥
Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, AB24 3UE, Scotland
*
S Supporting Information
ABSTRACT: In this work, the interplay between structure
and energetics in some representative phenylnaphthalenes is
discussed from an experimental and theoretical perspective.
For the compounds studied, the standard molar enthalpies,
entropies and Gibbs energies of sublimation, at T = 298.15 K,
were determined by the measurement of the vapor pressures as
a function of T, using a Knudsen/quartz crystal effusion
apparatus. The standard molar enthalpies of formation in the
crystalline state were determined by static bomb combustion
calorimetry. From these results, the standard molar enthalpies
of formation in the gaseous phase were derived and, altogether
with computational chemistry at the B3LYP/6-311++G(d,p)
and MP2/cc-pVDZ levels of theory, used to deduce the relative molecular stabilities in various phenylnaphthalenes. X-ray
crystallographic structures were obtained for some selected compounds in order to provide structural insights, and relate them to
energetics. The thermodynamic quantities for sublimation suggest that molecular symmetry and torsional freedom are major
factors affecting entropic differentiation in these molecules, and that cohesive forces are significantly influenced by molecular
surface area. The global results obtained support the lack of significant conjugation between aromatic moieties in the α position
of naphthalene but indicate the existence of significant electron delocalization when the aromatic groups are in the β position.
Evidence for the existence of a quasi T-shaped intramolecular aromatic interaction between the two outer phenyl rings in 1,8-
di([1,1′-biphenyl]-4-yl)naphthalene was found, and the enthalpy of this interaction quantified on pure experimental grounds as
−1
−
(11.9 ± 4.8) kJ·mol , in excellent agreement with the literature CCSD(T) theoretical results for the benzene dimer.
INTRODUCTION
The relative structural simplicity of phenylnaphthalenes can
be explored in order to obtain some fundamental knowledge on
the physical−chemical properties of these systems, both in bulk
and gas phases. In this work, a systematic study of some
phenylnaphthalenes was carried out in order to isolate and
characterize the factors that lead to structural and energetic
differentiation. These factors are basic physical−chemical
phenomena, like enthalpic or entropic effects, electronic
delocalization, specific inter and intramolecular interactions,
which, although present in many molecular systems, are often
superimposed by a myriad of other factors, complicating the
task of properly isolating them from the globally intertwined
ensemble of phenomena.
■
Phenylnaphthalenes are organic compounds consisting of a
variable number of phenyl substituents bonded to a unit of
naphthalene, as depicted in Figure 1.
Phenylnaphthalenes and related compounds encounter a
wide number of applications. They are of interest for the fields
of polymer chemistry, nanotechnology, and molecular
1
−3
biology.
Substituted phenylnaphthalenes can be used as
4,5
enzyme inhibitors and antibacterial compounds.
Received: November 18, 2011
Revised: January 31, 2012
Published: February 22, 2012
Figure 1. Schematic molecular structure of phenylnaphthalenes,
showing the α and β positions of naphthalene.
©
2012 American Chemical Society
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The Journal of Physical Chemistry B
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Figure 2 presents the structures and the abbreviations that
will be used throughout this paper for the eight phenyl-
Aromatic interactions are also a hot topic in supramolecular
chemistry.
9
,15,16
This work is a combined structural/energetic study of this
family of compounds, comprising the following: X-ray
crystallography for the determination of crystal phase
structures; Knudsen/quartz crystal effusion for vapor pressure
measurement as a function of T, and consequent derivation of
the thermodynamic quantities of sublimation; combustion
calorimetry for evaluation of the enthalpies of formation of the
crystalline compounds, and consequent derivation of the
enthalpies of formation in the gas phase in combination with
the sublimation data; UV−vis spectroscopy for exploring the
electronic properties of some relevant phenylnaphthalenes; a
computational study, using B3LYP and MP2 methods, as a
support of the experimental results and assessment of gas phase
geometries and energetics.
A series of homodesmotic reaction schemes is used in order
to detect and quantify, on pure experimental grounds, some
important electronic effects that can influence energetics in
some of the molecules under study.
EXPERIMENTAL SECTION
■
Synthesis, Purification, and Characterization of the
Studied Compounds. 1,2,3,4-Tetraphenylnaphthalene was
commercially obtained from Sigma-Aldrich, washed with
boiling methanol and sublimed under reduced pressure. All
the other compounds, excluding 1,4-diphenylnaphthalene, were
synthesized and purified as described elsewhere.
,4-Diphenylnaphthalene. A 40 mL solution of K CO (28
Figure 2. Schematic representation of the phenylnaphthalenes studied
in this work and the adopted abbreviations: 1-phenylnaphthalene (1-
PhN); 2-phenylnaphthalene (2-PhN); 1-([1,1′-biphenyl]-4-yl)-
naphthalene (1-bPhN); 2-([1,1′-biphenyl]-4-yl)naphthalene (2-
bPhN); 1,8-diphenylnaphthalene (1,8-diPhN); 1,4-diphenylnaphtha-
lene (1,4-diPhN); 1,8-di([1,1′-biphenyl]-4-yl)naphthalene (1,8-
dibPhN); 1,2,3,4-tetraphenylnaphthalene (1,2,3,4-TPhN).
17,18
1
2
3
mmol) in water was added to a solution of 1,4-dibromonaph-
thalene (7 mmol), phenylboronic acid (25 mmol) and
palladium acetate (2 mol %) in 40 mL of DMF. The resultant
solution was stirred at 90 °C for 9 h. The crude product was
extracted with ethyl acetate. The organic phase was washed
with water and NaOH(aq) 1 M, and evaporated yielding a
yellowish solid (yield (%) = 66). The compound was washed
with boiling methanol and sublimed under reduced pressure to
yield pure white crystals of 1,4-diPhN (mp = 134.8−135.7 °C).
naphthalenes studied. Three groups of structural isomers can be
distinguished as follows: A, 1-phenylnaphthalene (1-PhN) and
2
-phenylnaphthalene (2-PhN); B, 1-([1,1′-biphenyl]-4-yl)-
naphthalene (1-bPhN), 2-([1,1′-biphenyl]-4-yl)naphthalene
2-bPhN), 1,8-diphenylnaphthalene (1,8-diPhN) and 1,4-
(
diphenylnaphthalene (1,4-diPhN); C, 1,8-di([1,1′-biphenyl]-4-
yl)naphthalene (1,8-dibPhN) and 1,2,3,4-tetraphenylnaphtha-
lene (1,2,3,4-TPhN). This subdivision is useful because more
direct comparisons can be made between isomers, and chemical
stability can only be meaningfully compared within the same
potential energy surface (PES).
1
H NMR (400 MHz, CDCl , 300 K, TMS): 8.01 (2H, dd, H
3
5
13
and H , J
= 6.5, Jmeta = 3.3), 7.61−7.44 (14 H, m);
NMR (100.6 MHz, CDCl , 300 K): 141.7, 140.8, 132.8, 131.1,
C
8
ortho
3
1
29.2, 128.2, 127.4, 127.3, 126.8.
The presence of aromatic moieties can induce an extensive
conjugation along the π systems, which is associated with
charge delocalization and electronic correlation, and may lead
to significant molecular stabilization. Additionally, the existence
of adjacent phenyl substituents (like in 1,8-diPhN), opens the
possibility for considerable intramolecular steric repulsions
and/or aromatic interactions. Aromatic interactions are non-
covalent intra- and/or intermolecular interactions involving at
least one aromatic system. The relatively large polarizability of
aromatics and the quadrupole moment created by the interplay
between the σ and π frameworks of an aromatic group can lead
to the establishment of significant dispersive van der Waals and
The purity of the compounds was verified by gas−liquid
chromatography, using an HP 4890 apparatus equipped with a
HP-5 column, cross-linked, 5% diphenyl and 95% dimethylpo-
lysiloxane, showing a %(m/m) purity greater than 99.8% for all
the compounds.
Structural Characterization. Crystals suitable for X-ray
diffraction of 1-bPhN were grown from evaporation of a
CH Cl /isooctane (1:1) solution. Crystals of 1,8-dibPhN and
2
2
1
,8-diPhN were grown from evaporation of a CH Cl solution.
2 2
Crystals of 1,4-diPhN were grown from evaporation of a
CH Cl /acetone (1:1) solution. The intensity data was
2
2
collected in a Bruker-Nonius CCD diffractometer. Data
collection, cell refinement and data reduction were made with
the software package of the diffractometer: APEX2 and
6
−11
electrostatic interactions, respectively.
Three types of
structures are generally observed for the interaction between
two aromatic groups: T-shaped (T), parallel-displaced (PD),
19
SAINT. Absorption correction was performed with SA-
8
,9,11
19
and stacked (S).
The T structure is in its essence a C−
DABS. The structure was solved and refined using the
8,11,12
20
21
H···π interaction.
Aromatic interactions are cohesive
following software: OSCAIL and SHELXL97. H atoms were
treated as riding atoms with C−H(aromatic), 0.95 Å. The
crystal of 1,4-diphenylnaphthalene was a merohedral twin with
a twin component ratio of 66/34. Molecular graphics were
forces present in the bulk phases of aromatic compounds,
and they have an irrefutable importance in molecular
recognition processes and in defining the conformation of
9
,13,14
22
23
molecules and macromolecules, like proteins or DNA.
made with ORTEP and PLATON. The complete set of
3
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Article
structural parameters in CIF format is available as an Electronic
Supplementary Publication from the Cambridge Crystallo-
graphic Data Centre, CCDC: 698100 (1-bPhN), 668280 (1,8-
dibPhN), and 802922 (1,4-diPhN). Information about crystal
data, data acquisition conditions and refinement parameters for
these compounds is given as Supporting Information. Despite
all efforts, crystals suitable for X-ray diffraction for 2-PhN and
(MP2) and the correlation consistent basis set cc-pVDZ, and
density functional theory (DFT) with the hybrid exchange
correlation functional (B3LYP) and the 6-311++G(d,p) basis
37
set. The spin-component-scaled MP2 approach (SCS-MP2)
was also used for the calculation of the ground-state electronic
energies. Some of the phenylnaphthalenes studied are prone to
establish intramolecular dispersive interactions. For that reason
were chosen a method that is virtually blind to these
interactions (B3LYP) and one that can describe them
satisfactorily (MP2), for then the presence of such interactions
in one molecule will probably lead to a significant discrepancy
between the B3LYP and MP2 results.
2
-bPhN could not be obtained.
UV−Vis Spectroscopy. UV−vis spectra for 1-PhN (1.835
−5
−3
−5
−3
×
(
10 mol·dm ), 2-PhN (1.995 × 10 mol·dm ), 1-bPhN
5
−
−3
−5
1.576 × 10 mol·dm ), and 2-bPhN (1.379 × 10
3
−
mol·dm ), in the region of (190−1090) nm, in CH Cl
2
2
were measured with an Agilent 8453 diode array UV−vis
spectrometer, at T = 298.1 K; the molar concentrations are
given in parentheses. A quartz cell with a path length of 10.00
mm was used. Temperature control was achieved by means of a
Julabo F25 HP refrigerated circulator.
RESULTS
■
X-ray Crystallographic Structures. The X-ray structure
determination for 1-bPhN, 1,8-dibPhN, 1,4-diPhN, and 1,8-
diPhN confirmed the expected molecular structures. A
literature search revealed that the crystalline structures for
Combustion Calorimetry. 1-([1,1′-Biphenyl]-4-yl)-
2
8
29
naphthalene. The standard molar enthalpy of combustion,
1
,8-diPhN and 1,2,3,4-TPhN have been previously
o
m
Δ H , at T = 298.15 K, was measured in an isoperibol static
c
determined. The geometrical parameters and supramolecular
structure obtained in this work, corresponding to low
temperature data acquisition of 1,8-diPhN, compares well
with the structural data published by Tsuji et al. at room
temperature and thus it will not be discussed here. The
molecular structures for 1,4-diPhN, 1,8-dibPhN, and 1-bPhN
are presented in Figures 3−5.
bomb combustion calorimeter with a twin valve bomb of
3
internal volume of 0.290 dm , formerly used at the National
Physical Laboratory, Teddington, U.K. The measurements were
performed mainly as previously described, although a few
changes in technique, due to different auxiliary equipment, were
28
2
4,25
applied.
Other Compounds. The standard molar enthalpies of
0
m
combustion, Δ H , at T = 298.15 K, were measured in an
c
26
isoperibol mini-bomb combustion calorimeter. The mini-
bomb is made of stainless steel with 0.46 cm wall thickness and
3
1
8.2 cm of internal volume. The internal fittings located on the
head of the mini-bomb (electrodes, crucible support and sheet)
are all made of platinum.
27
The program LABTERMO was used to compute the
corrected adiabatic temperature change, ΔT . The densities, in
ad
−3
g·cm , of the compounds were taken from the crystallographic
data presented in this work and in the literature: 1-bPhN, 1.25;
,4-diPhN, 1.26; 1,8-diPhN, 1.14; 1,8-dibPhN, 1.28; 1,2,3,4-
TPhN, 1.19; 2-PhN, 1.25 (considered the same as for 1-
28
1
29
bPhN); 2-bPhN, 1.25 (considered the same as for 1-bPhN).
The values of (∂u/∂p) at T = 298.15 K were assumed to be 0.2
T
−
1
−1 30,31
J·g ·MPa .
The corresponding energetic correction
usually leads to negligible errors in the final combustion
results. Standard state corrections were calculated for the initial
32
and final states by the procedures given by Hubbard et al. and
33
by Good and Scott. The relative atomic masses used were
34
those recommended by the IUPAC Commission in 2005.
Knudsen/Quartz Crystal Effusion. The vapor pressures of
the studied compounds were measured as a function of
temperature by the combined Knudsen/quartz crystal effusion
Figure 3. View of 1,4-diPhN with our numbering scheme. Displace-
ment ellipsoids are drawn at the 30% probability level. Molecule 1 with
C1X labels and molecule 2 with C2X labels.
35
method recently developed in our laboratory. This technique
is based on the simultaneous gravimetric and quartz crystal
microbalance mass loss detection, enabling the use of a
temperature-step methodology, and having the advantages of
smaller sample sizes and effusion times, and the possibility of
achieving temperatures up to 650 K. In a typical Knudsen
effusion experiment, the system is kept at high vacuum,
enabling free effusion of the vapor from the cell, while the oven
is kept at a fixed temperature, T.
Compound 1,4-diPhN crystallizes with two molecules on the
asymmetric unit whose carbon atoms are labeled as C1X and
C2X for molecule 1 and molecule 2, respectively. The main
bond lengths for the compounds are within the expected range
for aromatics. The distances found for C−C bonds of the
phenyl rings show values within the typical range of 1.384(13)
Å, corresponding to the mean value for the Car−Car bond in
38
Computational Details. All theoretical calculations were
aromatic compounds. Nevertheless, a significant elongation of
the C9−C10 bond length on the naphthalene ring in 1,8-
dibPhN (1.439(3) Å) and 1,4-diPhN (1.442(7) and 1.436(7)
Å) is observed when compared with the corresponding value
36
performed using the Gaussian 03 software package. The full
geometry optimizations were performed using the Moller−
Plesset perturbation theory with a second order perturbation
3
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The supramolecular structures of 1-bPhN and 1,8-dibPhN
are stabilized by three C−H···π intermolecular interactions,
whereas in 1,4-diPhN there are no significant C−H···π or π···π
interactions. The geometric parameters for the interactions are
summarized in Table 3 and depicted in Figures 6−8.
In 1-bPhN the center of gravity of the C1−C2−C3−C4−
C9−C10 aromatic ring of the naphthalene moiety acts as
acceptor for two short C−H···π interactions from which the
donor H atoms are the ortho hydrogens attached to the outer
biphenyl ring. These link symmetry related molecules into
chains running parallel to the a-axis (Figure 6). A third and
longer C(7)−H(7)···π(141) interaction involving the C7−H7
donor of the naphthalene ring and the center of gravity of the
C141−C146 phenyl ring links the previous chains to form
sheets lying parallel to the ab plane (Figure 6). Similarly, in 1,8-
dibPhN, it is the C1−C2−C3−C4−C9−C10 aromatic ring of
the naphthalene moiety located at (x, y, z) that is the acceptor
of two C−H···π interactions: one involving the C4−H4 donor
of the naphthalene ring at (0.5 − x, −0.5 + y, 0.5 − z), linking
the molecules in antiparallel chains, and one involving the
C15−H15 acceptor located at (x, 1 + y, z) that links the two
molecules into a chain along the b-axis by a unit translation
Figure 4. View of 1,8-dibPhN with our numbering scheme.
Displacement ellipsoids are drawn at the 30% probability level.
(
Figure 7). These chains are then linked by a third C(844)−
H(844)···π(141) interaction to form a sheet which lies parallel
to the bc plane (Figure 8). It is interesting to note that the D−
A···A angular values (Table 3) for the shorter C−H···π
interactions in 1,8-dibPhN show that they are practically
coaxial, favoring the electrostatic dipole−quadrupole contribu-
8
−11
tion for this type of interaction.
Combustion Calorimetry. The detailed results for
combustion calorimetry of the compounds studied are provided
as Supporting Information. The products of combustion in the
experiments consist of a gaseous phase and an aqueous mixture
for which the thermodynamic properties are known. The values
0
m
of Δ U refer to the combustion reactions of the studied
c
compounds, represented by the general equation:
C H (cr) + (n + m/4)O (g)
n m
2
→
nCO (g) + (m/2)H O(l)
(1)
2
2
Table 4 lists the derived standard molar energies of
0
combustion, Δ U (cr), the standard molar enthalpies of
c
m
0
m
combustion, Δ H (cr), and the standard molar enthalpies of
c
0
formation, Δ H (cr), of the crystalline solids. In accordance
f
m
Figure 5. View of 1-bPhN with our numbering scheme. Displacement
ellipsoids are drawn at the 30% probability level.
with normal thermochemical practice, the uncertainties
0
m
0
m
assigned to Δ H (cr) and Δ H (cr) are twice the overall
c
f
39
standard deviation of the mean and include the uncertainties in
calibration and in the auxiliary quantities used. To derive
for naphthalene at 135 K (1.424(2)) Å. For compound 1-
bPhN, no significant differences in the geometrical parameters
of the naphthalene ring is observed when compared with
naphthalene in spite of the 1,1′-biphenyl substitution. Table 1
shows the dihedral angles between the mean planes of the
phenyl rings and the naphthalene ring for the characterized
compounds, and for 1,8-diPhN and 1,2,3,4-TPhN. Com-
pounds with phenyl rings inserted in ortho or peri positions of
the naphthalene moiety show higher dihedral angles. This
variation lies within the 11° to 23° range reflecting the expected
higher steric hindrance of ortho/peri substitution.
0
m
0
m
Δ H (cr) from Δ H (cr), the standard molar enthalpies of
f
c
formation of H O(l) and CO (g) at T = 298.15 K, −(285.830
2
2
−1
−1
±
0.042) kJ mol and −(393.51 ± 0.13) kJ mol , respectively,
40
were used.
Heat Capacity Measurements and Estimations. The
28
29
o
values of C (cr), at T = 298.15 K, were measured for 1,4-
p,m
diPhN and 1,2,3,4-TPhN using a high-precision heat capacity
4
1,42
drop calorimeter, developed by Wadso
̈
at the Thermo-
chemistry Laboratory of Lund, Sweden, afterward transferred to
Porto, Portugal, in where it was modernized with regards to the
temperature sensors and electronics, and enabling the use of
Table 2 shows the dihedral angles between the mean planes
of each of the phenyl rings for 1-bPhN and 1,8-dibPhN.
43
computer data acquisition in a user-friendly environment. For
3
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Article
Table 1. Dihedral Angles between the Mean Planes of the Phenyl Rings and the Naphthalene Ring for the Characterized
28
29
Compounds and for 1,8-diPhN and 1,2,3,4-TPhN
dihedral angles/deg
a
a
a
a
a
C1
C2
C3
C4
C8
1
1
1
-bPhN
56.52(11)
60.64(9)
53.1(2)
−
−
−
−
−
−
−
−
,8-dibPhN
,4-diPhN
−
−
−
−
73.80(9)
50.4(2)
54.2(2)
−
−
−
5
2.2(2)
1
1
,8-diPhN
66.48(10)
66.50(7)
−
67.10(10)
,2,3,4-TPhN
68.81(8)
65.17(8)
71.48(8)
−
−
b
b
7
6.04(8)
62.55(8)
62.55(8)
76.04(8)
a
b
Attachment atom position. There are 1.5 molecules in the asymmetric unit. One of the molecules sits on a 2-fold crystallographic axis, which
bisects the naphthalene moiety lengthwise.
Table 2. Dihedral Angles between the Mean Planes of Each
of the Phenyl Rings for 1-bPhN and 1,8-dibPhN
dihedral angles/deg
a
a
C14
C84
1
1
-bPhN
28.23(12)
42.34(9)
,8-dibPhN
30.05(11)
a
Attachment atom position.
Table 3. Intermolecular Interactions, Distances, and Angles
a
(Å and deg)
H···A
D−A···A
/°
compound
D−H···A
/Å
D···A/Å
i
1-bPhN
C(7)−H(7)···π(141)
C(142)−H(142)···π(1)
C(146)−H(146)···π(1)
2.94
2.79
2.71
2.75
2.87
2.64
3.708(3)
3.599(3)
3.565(3)
3.691(3)
3.679(3)
3.587(3)
139
143
150
172
144
174
ii
iii
iv
1
,8-dibPhN C(4)−H(4)···π(1)
C(15)−H(15)···π(1)
C(844)−
v
vi
H(844)···π(141)
Figure 6. View of the supramolecular structure of 1-bPhN, showing
the C−H···π intermolecular interactions. Hydrogen atoms not
involved in the motifs are not included.
a
π(141) is the centroid of the C141−C146 ring, π(1) is the centroid
of the C1−C2−C3−C4−C9−C10 ring of naphthalene. Symmetry
1
1
1
1
codes: (i) x, −1 + y, z; (ii) −x, / + y, / − z; (iii) 1 − x, / + y, /
−
+
2
2
2
2
1
1
1
1
1
z; (iv) / − x, / + y, / − z; (v) x, −1 + y, z; (vi) − / − x, − /
2
2
2
2
2
relatively small magnitude and lies within the uncertainties
3
y, / − z.
2
g
o
derived for the Δ C
values. For the other compounds
cr p,m
the other compounds, C po ,m(cr), at T = 298.15 K, was estimated
by an additive method based on the equation:
o
C
(g) was calculated according to an equation analogous to
p,m
o
C
(compound, cr)
o
p,m
=
C
(naphthalene, cr) + n(Ph)Co
p,m p,m
(Ph increment, cr)
(2)
where Co (naphthalene, cr) = 165.7 J·K ·mol , n(Ph) is the
−1
−1
p,m
o
number of phenyl substituents in the compound, and C (Ph
increment, cr) is the mean value for the increment in C (cr)
p,m
o
p,m
per phenyl ring, estimated by a least-squares method, using the
o
p,m
experimental values of C (cr) for: naphthalene (165.7), 1,4-
diPhN, 1,2,3,4-TPhN, biphenyl (198.4), o-terphenyl (274.8), p-
terphenyl (278.7), p-quaterphenyl (362.5), and 1,3,5-triphe-
−1
−1
nylbenzene (361.0); all values in parentheses are in J·K ·mol
44
and were taken from Domalski. The theoretical values of
o
p,m
C
(g), at T = 298.15 K, for 1-bPhN, 1,4-diPhN, and 1,8-
diPhN were computed at the B3LYP/6-311++G(d,p) level of
theory using the scaling factor of 0.9688 for the fundamental
frequencies calculations. Correction for hindered rotation of
the phenyl groups was not considered since it is generally of
Figure 7. View of the supramolecular structure of 1,8-dibPhN,
showing the C−H···π intermolecular interactions. Hydrogen atoms
not involved in the motifs are not included.
45
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⟨
T⟩, were derived using the integrated form of the Clausius−
Clapeyron equation:
ln p = a − b/T
(3)
g
cr
where a is a constant and b = Δ H (⟨T⟩)/R. Figure 9 presents
the plots of ln (p/Pa) = f (T /K ) for the compounds
m
−
1
−1
studied, obtained in the Knudsen/quartz crystal effusion
35
apparatus, recently developed in our laboratory.
The detailed experimental results are given as Supporting
g
Information. The molar enthalpy of sublimation, Δ H , at the
cr
m
mean temperature, ⟨T⟩, was derived from the parameter b of
the Clausius−Clapeyron equation, and the molar entropy of
sublimation at p(⟨T⟩) and at ⟨T⟩, Δ S (⟨T⟩, p(⟨T⟩)),
g
cr
m
Figure 8. View of the supramolecular structure of 1,8-dibPhN,
showing the sheets that lie parallel to the bc plane. Hydrogen atoms
not involved in the motifs are not included.
is calculated as:
g
g
Δ S (⟨T⟩, p(⟨T⟩)) = Δ H (⟨T⟩)/⟨T⟩
(4)
cr m
cr m
0
5
g
0
Table 4. Derived standard (p = 10 Pa) Molar Energies of
The standard molar enthalpy of sublimation, Δ H , at T =
298.15 K is determined by eq 5:
cr
m
0
Combustion, Δ U (cr), Standard Molar Enthalpies of
c
m
0
m
Combustion, Δ H (cr), and Standard Molar Enthalpies of
c
g
o
0
Δ H (298.15 K)
cr
Formation, Δ H (cr), in the Crystalline State, at T = 298.15
m
g
f
m
g
o
K, for the Compounds Studied
=
Δ H (⟨T⟩) + (298.15 − ⟨T⟩)Δ C
cr cr p,m
m
(5)
(6)
0
0
0
Δ U (cr)/kJ
Δ H (cr)/kJ
Δ H (cr)/kJ
c
m
c
m
f
m
−
1
−1
−1
where:
mol
mol
mol
g
o
= C (g) − Co (cr)
o
2
1
2
1
1
1
1
-PhN
−8143.9 ± 2.2
−11145.9 ± 3.0
−11132.2 ± 2.9
−11156.7 ± 3.2
−11177.4 ± 3.2
−17171.1 ± 4.8
−17204.1 ± 4.6
−8151.3 ± 2.2
−11155.8 ± 3.0
−11142.2 ± 2.9
−11166.6 ± 3.2
−11187.4 ± 3.2
−17186.0 ± 4.8
−17219.0 ± 4.6
140.2 ± 3.0
212.0 ± 4.2
198.3 ± 4.1
222.7 ± 4.3
243.5 ± 4.3
376.7 ± 6.5
409.7 ± 6.4
Δ C
cr p,m
p,m p,m
-bPhN
-bPhN
The values for the heat capacities, Co (cr) and C po ,m(g), are
those presented in Table 5. The standard molar entropy of
sublimation, Δ S , at T = 298.15 K, was calculated according
to eq 7:
p,m
,4-diPhN
,8-diPhN
,8-dibPhN
,2,3,4-TPhN
g
cr
0
m
g
0
g
g
0
Δ S (298.15 K) = Δ S (⟨T⟩, p(⟨T⟩)) + Δ C
cr m
cr m
ln(298.15/⟨T⟩) − R ln{p /p(⟨T⟩)}
7)
cr p,m
o
eq 2, using the following calculated values of C (g) (at the
p,m
0
o
p,m
same level of theory) for the estimation of C (Ph increment,
(
g) by a least-squares method: naphthalene (134.2), 1-PhN
o
5
where p = 10 Pa.
(
217.6), 2-PhN (217.8), 1-bPhN (300.8), 2-bPhN (301.2), 1,4-
diPhN (300.5), and 1,8-diPhN (300.9); all values are in
DISCUSSION
−1
−1
■
J·K ·mol . For the least-squares methods the average errors
Data Compilation. A compilation of all relevant
thermodynamic data, at T = 298.15 K, for the phenyl-
naphthalenes studied in this work and naphthalene is given in
of 1.9 (for C po ,m(cr)) and of 0.1 (for C (g)) between the
o
p,m
experimental/theoretical and estimated heat capacities were
obtained, corresponding respectively to approximately 0.6%
and 0.05% deviation. The uncertainties in the estimated and
theoretically calculated heat capacities were assumed to be of
Table 6. The standard molar enthalpies of formation in the gas
0
m
phase, Δ H (g), at T = 298.15 K, for the compounds studied,
f
were calculated according to eq 8:
−
1
−1
o
−1
−1
0
±
C
2.0 J·K ·mol for C (cr) and of ±5.0 J·K ·mol for
Δ H (g, 298.15 K)
p,m
f
m
o
(g). The global heat capacities results of the compounds
0
g
0
p,m
=
Δ H (cr, 298.15 K) + Δ H (298.15 K)
cr
(8)
f
m
m
studied are presented in Table 5.
Sublimation Thermodynamics. The data presented in
Table 6 clearly indicates the existence of a significant
differentiation in the enthalpies and entropies of sublimation
among the studied phenylnaphthalenes.
Knudsen/Quartz Crystal Effusion. For some of the
g
0
compounds studied, the standard molar enthalpies, Δ H ,
cr
m
g
cr
0
m
and entropies, Δ S , of sublimation at the mean temperature,
In these compounds the entropy of sublimation can be
approximately dissected into the following terms:
Table 5. Heat Capacities, C po ,m(cr) and C po ,m(g), at T = 298.15
K, for the Compounds Studied
g
0
1
. The increase in Δ S due to the presence of additional
cr
m
o
p,m
(cr)/J·K−1·mol−1
o
p,m
(g)/J·K−1·mol−1
C
C
phenyl substituents.
a
2
.
Molecular symmetry: higher symmetry leads to higher
S (cr) (more symmetry increases the number of atomic
1
1
1
1
1
-bPhN
328.3 ± 2.0
324.0 ± 1.4
300.8 ± 5.0
300.5 ± 5.0
300.9 ± 5.0
467.9 ± 5.0
467.9 ± 5.0
0
,4-diPhN
,8-diPhN
,8-dibPhN
,2,3,4-TPhN
a
permutations by molecular rotation that are compatible
with a given macrostate, thus increasing the number of
ways into which the crystal lattice can be realizedthis is
valid since a unique set of coordinates within the lattice
can be attributed to each atom, and thus each atom in the
328.3 ± 2.0
a
a
a
490.9 ± 2.0
491.6 ± 0.9
a
Estimated values (see text).
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Article
−1
−1
Figure 9. Plots of ln(p/Pa) = f (T /K ) for the compounds studied, obtained in the Knudsen/quartz crystal effusion apparatus.
Table 6. Compilation of Relevant Thermodynamic Data, at
T = 298.15 K, for the Phenylnaphthalenes Studied in This
Work and Naphthalene
in gaseous 1-bPhN and 1,4-diPhN are rather alike. Figure 10
illustrates the reasoning presented above.
g
0
−1
Δ S /J·K−1.mol−1 Δ H (g)/kJ mol
g
0
0
−1
compound
Δ H /kJ mol
cr
m
cr
m
f
m
4
6
46
17
17
47
naphthalene
72.70 ± 0.04
168.1 ± 0.1
−
150.6 ± 1.5
252.3 ± 3.4
248.0 ± 3.1
350.9 ± 4.3
338.5 ± 4.3
355.2 ± 4.3
369.9 ± 4.3
555.2 ± 6.6
563.9 ± 6.4
a
1
2
1
2
1
1
1
1
-PhN
−
1
7
-PhN
107.8 ± 0.6
138.9 ± 0.8
140.2 ± 1.3
132.5 ± 0.6
126.4 ± 0.5
178.5 ± 1.1
154.2 ± 0.8
211.5 ± 1.8
245.0 ± 2.1
228.0 ± 3.6
237.4 ± 1.6
230.9 ± 1.4
280.2 ± 2.7
246.9 ± 2.1
-bPhN
1
7
-bPhN
,4-diPhN
,8-diPhN
,8-dibPhN
,2,3,4-TPhN
a
Estimated value (see details in the text).
crystal is distinguishable from all the others), and/or
0
lower S (g) (the Pauli exclusion principle does not allow
microscopically indistinguishable rotational states in a
g
0
48−60
symmetric molecule), therefore decreasing Δ S .
cr
m
3
.
The internal rotational freedom of the phenyl
substituents in the gas phase: higher barrier heights
being associated with a decrease of S (g), and thus a
0
g
cr
0
decrease in Δ S .
m
The molecules of 1-bPhN and 2-bPhN belong to the C1
Figure 10. Entropic diagram showing the possible causes for the
g
0
differentiation in Δ S observed among the three isomers: 1-bPhN,
symmetry point group, having associated a symmetry number
of 1, σsym = 1. On the contrary, the X-ray structures of 1,8-
diPhN and 1,4-diPhN present a molecular geometry consistent
cr m
1
,4-diPhN, and 1,8-diPhN.
Nevertheless, the 2-bPhN isomer does not seem to fit this
with the C point group, for which σsym = 2. This nonthermal
g
0
2
rationalization in Δ S . The molecule has no symmetry (σ
=
g
cr
0
0
m
cr
m
sym
entropic contribution should decrease Δ S by about R ln(2)
1
), and at a first glance, the phenyl rotors in the gas phase are
−
1
−1
0
=
5.8 J·K ·mol , either by increasing S (cr), decreasing S (g),
6
not more hindered than the ones in the 1-bPhN isomer.
0−64
or a combination of both to varying extents.
Because of
g
cr
0
m
Notwithstanding, the lower than expected Δ S of 2-bPhN can
the presence of one adjacent phenyl neighbor, the barrier
height associated with hindered phenyl rotation in 1,8-diPhN
should be significantly higher than in the other isomers and
also be related with hindered rotation of the phenyl
substituents. Because of the β-phenyl relation, this isomer is
more likely to present an enhanced electron delocalization (as
it will be shown in a following section); a fact that is associated
with higher ring coplanarity and should increase the energetic
penalty associated with internal phenyl rotation, thus lowering
0
g
0
contribute to a decrease in S (g), and hence in Δ S . For the
cr
m
0
g
0
m
decrease in S (g) to be directly reflected in Δ S , one must
cr
assume that in the solid phase the respective vibrational modes
are hindered to the same extent in all isomers, having very high
rotational barriers due to the constraints imposed by the crystal
lattice. It can also be assumed that the phenyl internal rotations
0
S (g). This in turn suggests a more planar molecular geometry
for 2-bPhN, which can lead to some positional degeneracy in
0
the crystal lattice, thus increasing S (cr).
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Relative to the 1,8-dibPhN and 1,2,3,4-TPhN isomers, the
value of c (constant multiplying the factor R·ln(σsym)) is very
g
0
large difference in Δ S can be mainly attributed to the
similar to the one observed in our previous work concerning
cr
m
60
different degree of hindered rotation of the phenyl substituents
in the gaseous phase. The X-ray structures of both compounds
are consistent with σsym = 2, and thus there should be no
polyphenylbenzenes (c = 1.4). As referred to that paper, this
result suggests that symmetry leads to entropic differentiation
both in crystal and gas phases and that its influence should be
more correctly described in terms of continuous symmetry
g
cr
0
m
significant differentiation in Δ S due to molecular symmetry.
5
0,59,60
In the 1,2,3,4-TPhN isomer the two β-phenyl substituents are
surrounded by two adjacent phenyl neighbors, being therefore
highly hindered relative to internal rotation, whereas the
torsional profiles of the two outer phenyl substituents in 1,8-
dibPhN should have a considerably lower rotational barrier and
resemble the one for biphenyl. As demonstrated by the X-ray
structure of 1,8-dibPhN, the outer phenyls are significantly
further away from each other than the phenyl substituents
directly bonded to naphthalene. In this way, differentiation in
numbers.
By nature, the intermolecular interactions in these com-
pounds are mostly of the van der Waals type. Hence, the
cohesive energy of the solid is expected to depend substantially
on the available free molecular surface for interaction, with a
higher intermolecular surface increasing the likelihood of short
atomic contacts, thus contributing to stronger intermolecular
interactions. In fact, for the C H isomers the enthalpy of
22
16
sublimation increases in the order: 1,8-diPhN < 1,4-diPhN < 1-
bPhN ≈ 2-bPhN, which nicely follows the increase in molecular
surface area. 1,8-diPhN has two blocked aromatic π faces, which
leads to a decrease of the cohesive energy. The biphen-4-yl
isomers are the ones with larger surface areas, in agreement
0
S (g) should mainly arise due to the significantly different
internal rotations associated with the two highly hindered β-
phenyl substituents in 1,2,3,4-TPhN, and the two less hindered
outer phenyl rings in 1,8-dibPhN.
g
cr
0
m
The Δ S data obtained in this work was fitted to a
g
cr
0
m
with their greater Δ H in this series. The 1,4-diPhN isomer is
semiempirical additive model analogous to the one proposed
an intermediate case, since both phenyl substituents are bonded
to the α position of naphthalene, which reduces slightly the
volume extension of the aromatic π clouds relative to 1-bPhN
and 2-bPhN. A similar trend is observed for the C H isomers
g
0
60
recently for estimation of Δ S in polyphenylbenzenes. It
cr
m
was found that the previous model describes quite well the
presented set of data for Δ S (including naphthalene and
seven phenylnaphthalenes studied), by the following equation:
g
cr
0
m
3
4
24
(
1,8-dibPhN and 1,2,3,4-TPhN). In 1,2,3,4-TPhN only two π
g
0
faces from a total out of eight are arising from the four phenyl
groups, which hence are free to interact intermolecularly,
whereas in 1,8-dibPhN, there are four unblocked π faces, and
an increased extension of electron density due to the biphen-4-
yl relation of the phenyl substituents. Moreover, as more
phenyl substituents cluster around naphthalene, the less prone
the naphthyl moiety is to participate in significant van der
Waals intermolecular contacts. This explains the significantly
Δ S = a + bn(Ph) − cR ln(σsym
cr m
)
+
dn(α/β‐phenyl/biphenyl) + en(peri‐α
‐
phenyl)
(9)
where a, b, c, d, and e are fitting parameters; σsym is the external
symmetry number of the molecule, which corresponds to the
number of unique orientations of the rigid molecule that only
interchange identical atoms; n(Ph) is the total number of
phenyl substituents; n(α/β-phenyl/biphenyl) is the number of
α and β phenyl substituents with zero phenyl neighbors (e.g.,
the two phenyls in 1,4-diPhN or the phenyl in 2-PhN), plus
those that are bonded to another phenyl ring (e.g., the outer
phenyl of 1-bPhN or 2-bPhN); n(peri-α-phenyl) is the number
of α phenyl substituents with one adjacent peri phenyl neighbor
g
0
higher Δ H for the 1,8-dibPhN isomer. Linear correlations
cr
m
g
cr
0
m
were obtained by plotting Δ H as a function of the number of
phenyl substituents in naphthalene for the following series: A,
naphthalene, 2-PhN, and 2-bPhN (R2 = 0.9995); B,
2
naphthalene, 1,8-diPhN, and 1,8-dibPhN (R = 0.9999). This
indicates the existence of a simple additive pattern in
intermolecular interactions in this class of compounds as the
number of structurally similar phenyl rings is incremented.
Similar findings, respecting enthalpies of sublimation, were
observed in our previous work concerning the related series of
(
e.g., the two phenyls in 1,8-diPhN). Symmetry numbers, σsym,
were taken from the X-ray crystallographic structures and/or
from the MP2/cc-pVDZ optimized geometries as: naphthalene
60
3
9
polyphenylbenzenes.
Gas Phase Energetics. Estimation of Δ H (g) for 1-
(
4), 2-PhN (1), 1-bPhN (1), 2-bPhN (1), 1,4-diPhN (2), 1,8-
0
m
29
f
diPhN (2), 1,8-dibPhN (2), 1,2,3,4-TPhN (2). The
parameters d and e take into account the respective
contributions from hindered rotation relative to the most
hindered rotor, the β-phenyl in 1,2,3,4-TPhN. Applying a least-
squares method, the values for the fitting parameters were
Phenylnaphthalene. The experimental determination of
0
m
Δ H (g) of 1-phenylnaphthalene was not possible, since the
f
purification of this compound to the required standards was not
successfully achieved. Considering that in α-phenylnaphtha-
lenes conjugation between the naphthyl and phenyl moieties is
−1
−1
−1
−1
found to be: a = 187.1 J·K ·mol , b = 9.7 J·K ·mol , c = 1.5,
1
−1
−1
−1
−1
very weak (support for this fact comes from UV−vis and H
d = 18.3 J·K ·mol , and e = 14.9 J·K ·mol , giving an
1
8,65,66
g
0
m
NMR spectroscopy
), estimation by an additive method
average error between the experimental and estimated Δ S
cr
−1
−1
was applied. The X-ray structures and optimized gas phase
geometries at B3LYP/6-311++G(d,p) and MP2/cc-pVDZ
levels of theory for the α-phenylnaphthalenes studied indicate
a phenyl-naphthyl dihedral angle near 60°, far away from
coplanarity, in all the cases. Therefore, the two homodesmotic
gas phase reactions presented in Figure 11 should be nearly
athermal, in accordance with what is expected if the energetic
factors are additive. Our computational results support this
values of 2.4 J·K ·mol (approximately 1% deviation). The
compound 2-bPhN is clearly an outsider, and therefore was not
considered in the fitting. For 2-bPhN to be part of the
−
1
−1
g
0
m
correlation, a contribution of ca. −13 J·K ·mol for its Δ S
cr
should be considered. Curiously, this value is very near −R
ln(4), the contribution that is to be expected if 2-bPhN
presents a structural degeneracy in the crystal lattice consistent
with a planar linear molecule, for which σsym = 4. Unfortunately,
suitable crystals of 2-bPhN for X-ray diffraction could not be
obtained, in order to confirm this hypothesis. Interestingly, the
−
1
observation, giving values of Δ E (g, 0 K), in kJ·mol , of 0.3
r
el
(HR1) and 0.0 (HR2) at the B3LYP/6-311++G(d,p), and of
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Article
0
Table 7. Experimental Results for Δ H , at T = 298.15 K,
sep
m
for the Homodesmotic Reaction Scheme Presented in Figure
12
0
−1
compound
n
ΔsepH (g)/kJ mol
m
1
2
1
2
1
1
1
1
-PhN
1
1
2
2
2
2
4
4
−2.6 ± 3.9
1.7 ± 2.0
-PhN
-bPhN
−2.1 ± 2.5
10.3 ± 2.5
−6.4 ± 2.8
−21.1 ± 2.7
−8.2 ± 3.2
−16.9 ± 2.9
-bPhN
,4-diPhN
,8-diPhN
,8-dibPhN
,2,3,4-TPhN
Using this approach, some direct comparisons can be made
between all the compounds studied, with positive/negative
0
Δ H values indicating that the total interaction between all
Figure 11. Homodesmotic gas phase reactions used for the estimation
sep
m
0
of Δ H (g) for 1-PhN. These reactions should be practically athermal,
phenyl groups in naphthalene is stabilizing/destabilizing
relative to separated biphenyl and naphthalene.
These results clearly indicate significant energetic differ-
entiation among the compounds studied.
f
m
as supported by B3LYP/6-311++G(d,p) (DFT) and SCS-MP2/cc-
−1
pVDZ (MP2) calculations. Values of Δ E (g, 0 K) in kJ·mol .
r
el
−
0.4 (HR1) and 1.1 (HR2) at the SCS-MP2/cc-pVDZ levels
of theory.
Hence, Δ H (1-PhN, g) = (252.3 ± 3.4) kJ·mol was
Conjugation in β-Phenylnaphthalenes Leads to Signifi-
cant Energetic Stabilization. As referred before, the results
obtained in this and other works point to a lack of conjugation
between naphthalene and phenyl substituents in the α position.
The main cause for this phenomenon may be related with the
significant steric interaction between the ortho hydrogen of the
phenyl rings and the adjacent peri hydrogen of naphthalene,
which prevents the achievement of more planar geometries
(smaller angle Φ(Ph−naphthyl)), and thus a significant overlap
of electron density that would lead to enhanced electron
delocalization. Table 8 presents the values for the ring−ring
0
m
−1
f
derived, as the resulting mean value from HR1 (252.9 ± 2.3)
and HR2 (251.8 ± 4.4), assuming Δ H (g) = 0.0 kJ·mol
for both reactions. The experimental values, in kJ·mol , of
o
m
−1
HR
−1
0
m
47
0
m
Δ H (benzene, g) = (82.9 ± 0.9), and Δ H (biphenyl, g) =
f
f
6
7
(
182.0 ± 0.7) were used.
0
m
General Overview. The values of Δ H (cr) for all the
f
compounds presented in Table 6, except for 1-PhN, were
determined by combustion calorimetry. In this way, for the
calculation of the uncertainties in Δ H (g) for all the
forthcoming reactions, the uncertainties related to the
0
m
r
Table 8. Ring−Ring Dihedral Angles, Φ(ring−ring), for All
the Ring−Ring Bonds for the Compounds Considered in
This Work, Derived from X-ray Crystallography and
Theoretical Calculations at the MP2/cc-pVDZ Level of
Theory
calibration of the calorimeters and the formation of CO (g)
2
and H O(l) in the course of the corresponding combustion
2
reactions can be ignored since these contributions cancel out in
the homodesmotic reactions used.
Before proceeding with the analysis of gas phase energetics it
is worth noting the significant similarity that was observed
between the X-ray and optimized MP2/cc-pVDZ molecular
structures, for the cases where both of them could be obtained.
Moreover, due to the relatively low molecular flexibility of these
systems, one should not expect to observe noteworthy
deviations between the equilibrium structures adopted in the
crystal and gas phases. These observations suggest that the X-
ray structures can be used in order to give some support to gas
phase energetics in the discussion that follows.
Φ(ring−ring)/deg
compound
X-ray
MP2
1
2
1
2
1
1
1
1
-PhN
−
−
58
42
-PhN
a
-bPhN
57; 28
57; 42
42; 42
58; 58
56; 56
−
-bPhN
−
,4-diPhN
,8-diPhN
53; 52
66; 67
a
,8-dibPhN
74; 61/30; 42
74; 71/64; 66
b
,2,3,4-TPhN
−
Figure 12 presents a general homodesmotic separation
reaction scheme that can be used to compare gas phase
energetics among the phenylnaphthalenes studied, and Table 7
a
Left values: Ph−naphthyl torsions/right values: Ph−Ph torsions.
Left values: α-phenyl substituents/right values: β-phenyl substituents.
b
0
m
summarizes the experimental results for Δ H at T = 298.15
sep
dihedral angles, Φ(ring−ring), obtained in this work by X-ray
crystallography and theoretical calculations at the MP2/cc-
pVDZ level of theory, with respect to all the ring−ring bonds in
the compounds studied. The Ph−naphthyl dihedral angles for
the α-phenylnaphthalenes are of about 60°, while β-phenyl-
naphthalenes and biphenyl derivatives present substantially
lower ring−ring dihedral angles, for instance Φ(Ph-Ph) = (44.4
1.2)° in gaseous biphenyl.
From Table 6 one can see that in the gas phase 1-bPhN is
slightly more stable than 1,4-diPhN. The difference in Δ H (g)
for these two isomers can be related to the different molecular
K.
68
±
Figure 12. General gas phase separation reaction for the phenyl-
naphthalenes studied.
0
m
f
3
565
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Figure 13. Gas phase isomerization reactions for α- and β-phenylnaphthalenes, and respective UV−vis spectra, recorded in CH Cl , at T = 298.1 K.
2
2
Enthalpic stabilization is associated with a bathochromic shift. Experimental (in bold), B3LYP/6-311++G(d,p) (DFT), and SCS-MP2/cc-pVDZ
−1
(
MP2) values in kJ·mol are presented.
environment of the phenyl substituents. While in 1-bPhN one
phenyl ring has a biphenyl-like relation, for which Φ(Ph-Ph) =
28° (X-ray), in 1,4-diPhN both phenyl substituents are bonded
to the α position of naphthalene, with Φ(Ph-naphthyl) = 52°
(
X-ray). This is a good indication that a main factor that
energetically distinguishes these two isomers in the gas phase is
the degree of planarity of the phenyl rings.
The β-phenylnaphthalenes studied, 2-PhN and 2-bPhN, are
significantly more stable than their corresponding α isomers,
with the stabilization being more pronounced in 2-bPhN.
Figure 13 presents the experimental gas phase isomerization
0
enthalpies, Δ H (g), at T = 298.15 K, and the theoretically
isom
m
calculated electronic isomerization energies, ΔisomEel(g), at T =
K (ZPE and thermal correction contributions were neglected
0
since they are usually of little importance in an isomerization
reaction), alongside with their respective UV−vis spectra.
Figure 13 shows that a significant bathochromic shift in the
UV−vis spectra is observed on going from the α to the β
isomers. It can also be observed that on going from 2-PhN to 2-
bPhN the λmax varies significantly, whereas from 1-PhN to 1-
bPhN it is virtually unchanged. As can be seen in Table 8, β-
phenylnaphthalenes tend to present lower values of Φ(Ph-
naphthyl), which is consistent with greater conjugation and
stability. The bathochromic shift is more pronounced for 2-
Figure 14. Homodesmotic gas phase reactions used for the evaluation
of the intramolecular aromatic interactions in 1,8-diPhN and 1,8-
dibPhN. Experimental (in bold), B3LYP/6-311++G(d,p) (DFT), and
−1
SCS-MP2/cc-pVDZ (MP2) values in kJ·mol are reported.
homodesmotic reactions planned with the intention of
evaluating experimentally and computationally the intra-
molecular interaction between the adjacent phenyl substituents
in 1,8-diPhN and 1,8-dibPhN. The experimental results for the
0
m
standard gas phase enthalpies of reaction, Δ H (g), at T =
bPhN, being nicely accompanied by a more negative
r
0
m
2
98.15 K, and electronic energies of reaction, Δ E (g), at T = 0
Δ
isom
H (g). These results constitute a good indication that
r el
K, are presented (ZPE and thermal corrections were neglected
since they are usually of little importance in a homodesmotic
reaction).
enthalpic stability in phenylnaphthalenes is related to the extent
of conjugation between the phenyl and naphthyl moieties in
these molecules. Nevertheless, the theoretical results are not
able to distinguish between the two isomerization reactions.
The fact that both the enthalpic stabilization and bathochromic
shift are considerably more pronounced in 2-bPhN suggests
that a significant electron delocalization, not totally accounted
for by the theoretical results, exists in this molecule and extends
through the whole π system. In reality, the orbital overlap
between the various rings in 2-bPhN appears to be more
extensive than considered by the computational calculations. In
this way, a geometry more closer to planarity than the one
obtained computationally may be expected for this molecule.
Intramolecular T-Shaped Aromatic Interaction in 1,8-
Di([1,1′-biphenyl]-4-yl)naphthalene. Figure 14 presents two
A repulsive interaction is clearly observed between the two
phenyl substituents in 1,8-diPhN. Because of their close
proximity favorable intramolecular aromatic interactions could
be established. However, the two aromatic rings are too close,
and the repulsive short-range contribution dominates the
interaction at this distance. In the crystal structure of 1,8-
28
diPhN one can observe that the distance between the
centroids of the two Ph rings is of 3.51 Å, which matches the
theoretical calculated equilibrium distance of the parallel
8
−12
displaced benzene dimer,
but the distance between the
two carbon atoms directly bonded to naphthalene is only 2.98
Å. The enthalpic destabilization then probably arises from this
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close proximity at the base of the rings, and from the slight
geometric deformation of the molecule, which is more
perceived in the increase of the angle between the phenyl
rings and naphthalene and in the increase of the Ph−naphthyl
dihedral angles (see Table 8).
Figure 15 illustrates the parallel displaced orientation that the
two phenyl rings adopt relative to each other in the crystal
Figure 16. Intramolecular T-shaped (CH···π) aromatic interaction
between the outer phenyl rings in 1,8-dibPhN: X-ray crystallographic
structure (left) and schematic representation (right). The two C−
H···π interaction distances (in Å) and the two Ph−Ph dihedral angles
are shown.
presented in Figure 14, which gives ΔH (CH···π) = −(11.9 ±
m
−1
4
.8) kJ·mol , in gaseous 1,8-dibPhN. This value is in perfect
agreement with recent high-level ab initio calculations at the
CCSD(T) level of theory at the basis set limit, for T-shaped
Figure 15. Schematic picture showing the parallel displaced
8
−12
benzene dimers.
The true equilibrium geometry for this
orientation of the phenyl substituents in the crystal phase of 1,8-
28
diPhN.
intramolecular interaction in the crystal phase of 1,8-dibPhN is
presented in Figure 17. Strictly speaking, this is not a typical T-
28
phase of 1,8-diPhN. Nevertheless, the fact that the DFT result
is significantly more positive than the experimental and MP2
data indicates that attractive dispersive contributions are
significant for the total interaction energy, which is typical of
8
−12
a parallel displaced aromatic interaction.
It is well
recognized that most DFT methods, including B3LYP, are
not able to properly describe the attractive components of van
8
der Waals interactions.
Since 1,4-diPhN can practically be viewed as a combination
of two noninteracting aromatic rings, the 1,4-diPhN → 1,8-
diPhN isomerization reaction is also a good way of evaluating
the intramolecular interaction in 1,8-diPhN. The values of
Figure 17. Schematic picture showing the relative orientation of the
two outer phenyl rings in the crystal phase of 1,8-dibPhN.
0
−1
Δ H (g) for this reaction, in kJ·mol , are 14.7 ± 2.3
r
m
(
experimental), 29.9 (B3LYP), and 11.5 (SCS-MP2). As
shaped structure, but the two interacting C−H bonds of one
ring still adopt a favorable orientation toward the π cloud of the
other aromatic ring. The molecular geometry in the gas phase
should closely resemble the one adopted in the crystal;
however, the absence of an intermolecular potential may allow
the establishment of a more perpendicular T-shaped structure
for this interaction in the gas phase. Owing to the nature of the
molecules shown in Figure 14, the existence of the aromatic
interaction between the outer phenyls in 1,8-dibPhN is the
expected, these results are very similar to the ones obtained
for the first homodesmotic reaction presented in Figure 14.
There is no reason for the interaction between the two
phenyl substituents directly bonded to naphthalene in 1,8-
dibPhN to be less repulsive than in 1,8-diPhN. Their respective
crystallographic structures show that the geometric parameters
relative to this pair of rings (centroid−centroid distance, bond
lengths and angles, angles and dihedral angles relative to
naphthalene) are very similar for both molecules. Hence, the
0
most obvious explanation to the difference observed in
significantly lower Δ H (g) observed for the second
r
m
0
m
Δ H (g). The outer rings in this molecule are relatively far
homodesmotic reaction presented in Figure 14 is probably
due to an attractive aromatic intramolecular interaction
between the two outer phenyl rings in 1,8-dibPhN. The X-
ray crystal structure of 1,8-dibPhN is consistent with this
finding and suggests an interaction geometry that approaches
the T-shaped structure of the benzene dimer. As shown in
Figure 16, the two outer phenyl rings in this molecule can
establish two C−H···π intramolecular contacts. This explains
why the two Ph−Ph dihedral angles, as shown in Table 8, are
significantly different in this molecule. One ring tends to
become perpendicular relative to the other, increasing Φ(Ph−
Ph), in order to maximize the two C−H···π contacts. The
intramolecular distances are shown in Figure 16 and are within
r
away from each other, and thus it seems logical that they adopt
a structure that more resembles the T-shaped one in order to
maximize the intramolecular interaction. A geometry more
closer to parallel displaced would increase the distance of short
contacts between the rings (the X-ray structure indicates a
centroid-centroid distance of 5.2 Å) and probably dilute the
interaction to a marginal value, reducing the difference in
0
Δ
H
r
(g) for the two reactions. According to recent theoretical
m
studies, the calculated Potential Energy Surface (PES) for the
parallel displaced benzene dimer points to an interaction
energy, ΔE , at these distances of ≈2−3 kJ·mol ,
below 11.9 kJ·mol as indicated by our experimental results.
1,2,3,4-Tetraphenylnaphthalene. The experimental results
indicate that the four adjacent phenyl substituents in the
molecule of 1,2,3,4-TPhN do not lead to significant repulsive
−1
8,10
well
int
−1
8
−12,69
the typical values for this type of interaction.
The aromatic interaction enthalpy can then be easily
0
m
calculated as the difference in Δ H (g) for the two reactions
r
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Article
−1
Figure 18. Homodesmotic gas phase reaction schemes used for the evaluation of energetics in 1,2,3,4-TPhN. Experimental values in kJ·mol .
steric interactions. The first homodesmotic reaction scheme in
Figure 18 indicates that intramolecular repulsions are much
more prevalent when the phenyl groups have a peri relation,
like in 1,8-diPhN. In this situation, the phenyl rings are very
close to each other and overlap considerably more electron
density than the phenyl substituents in 1,2,3,4-TPhN. This
scheme also indicates that, within experimental error, the
energetics of 1,2,3,4-TPhN can be approximated by the additive
contribution of one naphthalene and four α-phenyl sub-
substituents in benzene. As noted in a previous work, the
presence of o-phenyl rings in benzene do not impose significant
60
destabilization in polyphenylbenzenes. Despite being rela-
tively close to each other (prone to steric hindrance), the
contribution of dispersive interactions between the rings,
typical of aromatic π···π interactions, prevents a noteworthy
destabilization of these molecules. This partial cancellation of
effects thus allows the rings in 1,2,3,4-TPhN to be treated like
virtually noninteracting α-phenyls.
0
stituents. This can also be observed by plotting Δ H (g) as a
f
m
function of the number of Ph substituents for the following
series: naphthalene, 1-PhN, 1,4-diPhN, and 1,2,3,4-TPhN. This
CONCLUSION
■
The relationships between structure and energetics in a series
of relatively simple phenylnaphthalenes has been explored,
experimentally and theoretically, in order to properly character-
ize and quantify the relevant fundamental chemical aspects that
lead to enthalpic and entropic differentiation among these
compounds. It was found that the differences in the entropy of
sublimation can be rationalized in terms of molecular symmetry
and hindered internal rotation of the phenyl substituents.
2
yields a straight line with R = 0.99996, thus supporting the
validity of the group additivity assumption in these compounds.
When the two α-phenyl and two β-phenyl relations in
1,2,3,4-TPhN are considered in a homodesmotic scheme (last
reaction in Figure 18), an enthalpic destabilization is observed.
In 1,2,3,4-TPhN, each β-phenyl substituent is surrounded by
two phenyl neighbors, and thus they are forced to adopt a more
perpendicular geometry relative to the naphthalene plane, a fact
that is proved by their high Ph-naphthyl dihedral angles (see
Table 8). This departure from planarity shall disrupts the
conjugation between the β-phenyl and naphthyl moieties that
was observed in 2-PhN, and is probably the main source of
destabilization in 1,2,3,4-TPhN. Hence, with respect to
energetics, the β-phenyls in this molecule can be regarded as
α-phenyls. The combination of these findings precludes the
existence of strong repulsive interactions in the molecule of
0
Higher molecular symmetry increases S (cr) and/or decreases
0
g
0
S (g), thus decreasing Δ S , while more hindered rotations
cr
m
0
g
0
decrease S (g), and consequently Δ S . The trend in the
cr
m
enthalpies of sublimation can be understood by considering the
surface area of the molecules, showing that as the surface area,
and hence the likelihood to establish short atomic contacts,
increases the cohesive energy of the crystalline solid becomes
stronger.
With respect to molecular energetics, it was found that
1,2,3,4-TPhN. Moreover, a significant deformation of the
conjugation in α-phenylnaphthalenes is practically absent,
0
m
naphthalene ring is not observed in the crystal structure of this
leading to a scenario of group transferability in Δ H (g) for
f
2
9
0
m
compound. Nevertheless, because Δ H (g) is slightly positive
these compounds. On the other hand, a significant enthalpic
stabilization, due to enhanced electron delocalization, was
observed in β-phenylnaphthalenes, a phenomenon that is
particularly pronounced in 2-bPhN. The global results suggest
that enthalpic stability in these systems is strongly related with
r
for the homodesmotic reaction presented in Figure 18 that
includes 1,4-diPhN one cannot discard the existence of a small
repulsive contribution in 1,2,3,4-TPhN. In principle, the phenyl
rings in 1,2,3,4-TPhN can be compared with o-phenyl
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the dihedral angles adopted by the various phenyl substituents,
with lower dihedrals being associated with increased molecular
stability. While the intramolecular interaction between the two
phenyl substituents in 1,8-diPhN is strongly repulsive, a
stabilizing aromatic interaction between the two outer phenyl
rings in 1,8-dibPhN was found, and quantified on pure
experimental grounds as −(11.9 ± 4.8) kJ·mol . The X-ray
crystallographic structure of 1,8-dibPhN corroborates this
finding and indicates that the rings adopt a quasi T-shaped
structure, establishing two C−H···π intramolecular contacts.
Intramolecular repulsion was found to be of little importance in
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■
*
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Corresponding Author
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351 220402520.
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ACKNOWLEDGMENTS
■
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Thanks are due to Fundaca
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