Influence of molecular geometry on the formation, architecture and dynamics of H-bonded supramolecular associates in 1-phenyl alcohols

Article abstract of DOI:10.1016/j.molliq.2021.115349

Combination of calorimetric, dielectric, infrared, diffraction studies and quantum DFT computations was used to analyze the impact of the molecular architecture of a set of four 1-phenyl alcohols (1-phenylethanol, 1-phenyl-1-propanol, 1-phenyl-1-butanol and 2-methyl-1-phenyl-1-propanol) on their glass transition temperature, molecular dynamics, relaxation processes, hydrogen-bonding pattern and intermolecular association. We showed that all these alcohols form H-bonded supramolecular nanoassociates even at room temperature, despite containing a steric hindrance in the form of the phenyl ring in the most disfavored position. However, the concentration and the size of the H-bonded structures as well as the mutual arrangement of molecules in these clusters are tremendously affected by the molecular architecture. In linear-shaped 1-phenyl alcohols, i.e., 1-phenylethanol, 1-phenyl-1-propanol, 1-phenyl-1-butanol, the intermolecular O-H···O bonds organize themselves into chain-like patterns. Moreover, these alcohols are characterized by similar strength of intermolecular H bonds at each temperature and similar glass transition temperature. In turn, the globular molecular shape of 2-methyl-1-phenyl-1-propanol leads to a weakening of H bonds in this system, an increase in the glass transition temperature and the formation of supramolecular clusters in which O-H···O connections imply ring-like organization of molecules. Finally, these studies clearly show that unlike the glass transition temperature, the molecular dynamics of the 1-phenyl alcohols in the liquid state is not only affected by the molecular architecture and hydrogen bond strength but also by the morphology of the associates composed of H-linked molecules.

Full text of DOI:10.1016/j.molliq.2021.115349

Journal of Molecular Liquids 326 (2021) 115349
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Influence of molecular geometry on the formation, architecture and
dynamics of H-bonded supramolecular associates in 1-phenyl alcohols
a,
a,b
b,c
d
d
Andrzej Nowok ⁎, Karolina Jurkiewicz , Mateusz Dulski , Hubert Hellwig , Jan G. Małecki ,
a,b
a,b
a,b
Katarzyna Grzybowska , Joanna Grelska , Sebastian Pawlus
a
Institute of Physics, University of Silesia in Katowice, 75 Pułku Piechoty 1, 41-500 Chorzów, Poland
b
Silesian Center for Education and Interdisciplinary Research, 75 Pułku Piechoty 1A, 41-500 Chorzów, Poland
c
Institute of Materials Engineering, University of Silesia in Katowice, 75 Pułku Piechoty 1A, 41-500 Chorzów, Poland
d
Institute of Chemistry, University of Silesia in Katowice, Szkolna 9, 40-003 Katowice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 June 2020
Received in revised form 29 December 2020
Accepted 7 January 2021
Combination of calorimetric, dielectric, infrared, diffraction studies and quantum DFT computations was
used to analyze the impact of the molecular architecture of a set of four 1-phenyl alcohols (1-phenylethanol,
1-phenyl-1-propanol, 1-phenyl-1-butanol and 2-methyl-1-phenyl-1-propanol) on their glass transition tem-
perature, molecular dynamics, relaxation processes, hydrogen-bonding pattern and intermolecular association.
We showed that all these alcohols form H-bonded supramolecular nanoassociates even at room temperature,
despite containing a steric hindrance in the form of the phenyl ring in the most disfavored position. However,
the concentration and the size of the H-bonded structures as well as the mutual arrangement of molecules in
these clusters are tremendously affected by the molecular architecture. In linear-shaped 1-phenyl alcohols,
i.e., 1-phenylethanol, 1-phenyl-1-propanol, 1-phenyl-1-butanol, the intermolecular O-H···O bonds organize
themselves into chain-like patterns. Moreover, these alcohols are characterized by similar strength of intermo-
lecular H bonds at each temperature and similar glass transition temperature. In turn, the globular molecular
shape of 2-methyl-1-phenyl-1-propanol leads to a weakening of H bonds in this system, an increase in the
glass transition temperature and the formation of supramolecular clusters in which O-H···O connections
imply ring-like organization of molecules. Finally, these studies clearly show that unlike the glass transition
temperature, the molecular dynamics of the 1-phenyl alcohols in the liquid state is not only affected by the
molecular architecture and hydrogen bond strength but also by the morphology of the associates composed
of H-linked molecules.
Available online 12 January 2021
Keywords:
H-bonded supramolecular assembly
Broadband dielectric spectroscopy
Molecular dynamics
Phenyl alcohols
Structural relaxation
Debye process
©
2021 Elsevier B.V. All rights reserved.
1
. Introduction
mechanical or vibrational spectroscopy [8,13–15]. It should be empha-
sized that a unique feature of the dielectric loss spectra of many alcohols
is the Debye process, slower than the structural one. According to the
transient chain model, the Debye relaxation is strictly connected with
reorientational motions of the whole supramolecular structures or at-
tachment and detachment processes of molecules to and from the asso-
ciates [8,16]. Therefore, in the case of many monohydroxy alcohols, it is
possible to follow the molecular dynamics of supramolecular associates
in the broad time range by dielectric spectroscopy.
Among many factors affecting the formation, architecture and dy-
namics of H-bonded self-assemblies (e.g., temperature, pressure, high
electric field, concentration, solvent), the molecular structure seems to
have a decisive impact [17–21]. Systematic studies on geometric iso-
mers of octanol revealed the generation of the ring- or chain-like supra-
molecular structures in these liquids, depending on, among others, the
architecture of molecules [22–24]. It was stated that isomers sterically
hindered by an alkyl substituent tend to form dimers or higher-
membered rings. In turn, isomers with less-hindered hydroxyl group
Hydrogen bonds are one of the most important inter- and intramo-
lecular interactions which play a crucial role in countless biological and
chemical processes and have a decisive impact on the physicochemical
properties of a wide range of substances [1–6]. These interactions are
also responsible for self-assembly in liquids, leading to the formation
of supramolecular aggregates [7–9]. Although such a phenomenon
was observed for some van der Waals systems and ionic liquids, self-
assembly through H bonding is of particular interest [10–12]. This be-
havior was observed for a great number of monohydroxy alcohols,
which due to structural simplicity and high vitrification ability are be-
lieved to be model systems for the studies on the self-assembly phe-
nomena, carried out by means of various techniques, such as
broadband dielectric spectroscopy, dynamic and static light scattering,
⁎
Corresponding author.
E-mail address: andrzej.nowok@smcebi.edu.pl (A. Nowok).
https://doi.org/10.1016/j.molliq.2021.115349
167-7322/© 2021 Elsevier B.V. All rights reserved.
0

A. Nowok, K. Jurkiewicz, M. Dulski et al.
Journal of Molecular Liquids 326 (2021) 115349
agglomerate in chain-like associates more likely [22]. Moreover, it
was found that linear alcohols (such as n-octanol, n-butanol) orga-
nize themselves into chain-like associates, whereas alcohols with
molecules of globular shape (such as tert-butanol) tend to form
ring-like aggregates [25–28]. The phenyl ring is another component
of the molecular structure influencing the dynamics of alcohols.
This moiety plays a role of a steric hindrance so that the formation
of H-bonded supramolecular clusters may be hampered [29,30].
Such a situation leads to the disappearance of the Debye relaxation
process as a separate relaxation peak from dielectric spectra. How-
ever, pressure evolution of the steepness index for 1-phenyl-2-buta-
nol, determined for structural relaxation, shows the characteristic
abrupt increase in the value of this parameter, but only after exceed-
ing a critical pressure - similarly to the behavior of monohydroxy al-
cohols with a well visible Debye process (e.g., 2-ethyl-1-hexanol)
transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and
density functional theory (DFT) computations.
2. Materials and methods
2.1. Materials
The analyzed set of chemical compounds consists of four mono-
hydroxy alcohols: (S)-1-phenylethanol, 1-phenyl-1-propanol, 1-
phenyl-1-butanol and 2-methyl-1-phenyl-1-propanol, labeled fur-
ther as 1P1E, 1P1P, 1P1B and 2M1P1P, respectively. Their chemical
structures are shown in Fig. 1a. The two first liquids were purchased
from Sigma-Aldrich. 1P1B and 2M1P1P were synthesized by Gri-
gnard reaction of benzaldehyde with a proper organomagnesium
reagent in dry diethyl ether, that is propylmagnesium bromide
and 2-methylpropylmagnesium bromide, respectively. The Gri-
gnard reagents were prepared by reaction of corresponding alkyl
halides with magnesium, started by the addition of a small amount
of elemental iodine. Grignard reactions were followed by standard
acidic (HCl) aqueous workup, extraction, and drying of organic
[
17,31]. Such a pressure behavior of the steepness index was ascribed
to modifications of the morphology of the H-bonded associates [17].
Moreover, most recent studies on the series of phenyl-substituted
propanols, based on the conjunction of broadband dielectric spec-
troscopy and photon correlation spectroscopy, have indicated that
self-assembly through H bonding is possible even for the most steri-
cally disfavored 1-phenyl-1-propanol [32]. It was shown that the di-
electric spectra registered for these alcohols above the glass
phases by anhydrous MgSO
in Kugelrohr apparatus, purified using column chromatography
(SiO , DCM-hexane mixture as eluent) and again vacuum distilled
4
. Raw products were vacuum distilled
2
transition temperature, T
the structural α-relaxation and the low-frequency Debye process
32]. The architecture of H-bonded supramolecular structures in liq-
g
, can be interpreted as a superposition of
to ensure high purity, dryness and lack of silica gel dust contamina-
tion. In order to eliminate traces of water from the liquids, each sub-
stance was additionally lyophilized prior to measurements. Apart
from 1P1E, all other substances were racemic mixtures of both R
and S isomers.
All the analyzed alcohols contain the phenyl ring located at the first
carbon atom of the alkyl chain (Fig. 1a). Therefore, this chemical moiety
plays the role of a potential steric hindrance for the linkage of molecules
into H-bonded associates. Moreover, each alcohol is of the secondary
order. The difference between them lies in the number of carbon
atoms in the alkyl chain, varying from 2 for 1P1E, through 3 for 1P1P,
up to 4 for 1P1B and 2M1P1P. In order to investigate the influence of
the molecular architecture on the molecular dynamics and morphology
of supramolecular structures, two isomeric alcohols with four carbon
[
uid phenyl-substituted propanols is, however, complex. While 2-
phenyl-1-propanol and 3-phenyl-1-propanol form rather chain-like
aggregates, there is a temperature-dependent balance between
ring- and chain-like configurations of H bonds in the associates in
the case of 1-phenyl-1-propanol [32]. Apart from the main relaxation
process being a superposition of the structural α-relaxation and the
low-frequency Debye mode, also secondary relaxation processes,
characterized by the Arrhenius temperature dependence, were de-
tected for 1-phenyl-1-propanol [33].
Based on infrared (IR) spectroscopy studies supported by
quantum calculations, 1-phenylethanol was also found to form
aggregates, such as dimers, trimers and multimers. However, the
degree of association and architecture of the H-bonded supramo-
lecular structures were solvent-dependent [34]. Like other chiral
compounds, 1-phenylethanol can adopt various conformations,
differentiated in terms of energy and, consequently, stability
3
alkyl atoms and differentiated only in terms of the position of -CH moi-
ety were probed: 1P1B and 2M1P1P. The alkyl chain is branched in the
latter compound, making the hydroxyl group more sterically screened,
and contributing to the change in the molecular shape to a nearly glob-
ular one (Fig. 1b). In the case of 2M1P1P, the ratio of the long axis of the
molecule to the short one takes the value of ~1.4, whereas it grows up to
~2.3 for 1P1B. Therefore, we will call 2M1P1P alcohol as globular one
while the others, 1P1E, 1P1P and 1P1B, as linear ones. It should also be
emphasized that 2M1P1P is the only possible branched isomer among
the 1-phenyl alcohols of secondary order containing four carbon
atoms in the alkyl chain.
[
34]. Although its predominant, most stable conformer is stabi-
…
…
lized by weak intramolecular O-H π and C-H O hydrogen
bonds, the molecules in the liquid state are involved into the for-
mation of two-coordinated intermolecular H bonding networks,
leading to the creation of multimers [34]. Additionally, molecules
…
of 1-phenylethanol may also form intermolecular O-H π bonds,
typical for all phenyl alcohols in the liquid state and visible in
−
1
the IR spectra as a weak band at 3567 cm
[34]. Despite numer-
2.2. Differential scanning calorimetry (DSC)
ous studies, there are still many deficiencies in the description of
how the molecular structure and the steric hindrance influence
the formation and the architecture of the H-bonded supramolecu-
lar clusters in phenyl alcohols.
The calorimetric measurements of the 1-phenyl alcohols were per-
formed with the use of a Mettler-Toledo DSC apparatus equipped with
a liquid nitrogen cooling accessory and a HSS8 ceramic sensor (heat
flux sensor with 120 thermocouples). Each liquid was poured into a
sealed 40 μl aluminium pan. The thermograms were collected during
heating runs with a heating rate equal to 10 K/min at the temperature
range of 173–298 K. All scans were carried out in the atmosphere of ni-
trogen with a flow of 60 mL/min. Glass transition temperature for each
compound was determined from these measurements as an onset of the
proper thermal effect.
In this article, we study a set of four monohydroxy 1-phenyl alco-
hols, i.e., 1-phenylethanol, 1-phenyl-1-propanol, 1-phenyl-1-butanol
and 2-methyl-1-phenyl-1-propanol, as model H-bonded sterically hin-
dered glass-formers. We analyze how the increase in the length of
alkyl chain and the change in the molecular geometry from linear to
globular affect their ability to self-assembly through H bonding, molec-
ular dynamics in supercooled liquid and glassy states and glass transi-
tion temperature. Furthermore, we discuss the impact of steric
hindrance and molecular architecture on the strength of hydrogen
bonds and the architecture of the H-bonded supramolecular structures.
The studies are based on broadband dielectric (BDS) and Fourier
Using a stochastic temperature-modulated differential scanning
calorimetry (TMDSC) technique implemented by Mettler-Toledo
TOPEM®, the dynamic behaviors of the investigated phenyl alcohols
have been analyzed in the frequency range from 4 mHz to 20 mHz in
2

A. Nowok, K. Jurkiewicz, M. Dulski et al.
Journal of Molecular Liquids 326 (2021) 115349
Fig. 1. (a) Overview of the chemical structures for the studied monohydroxy 1-phenyl alcohols. The structural moiety common for all compounds is marked with a blue loop.
b) Comparison of 3D models of 1P1B and 2M1P1P molecules.
(
one single measurement at a heating rate of 0.5 K/min. In the experi-
ment, a temperature amplitude of the pulses of 0.5 K was selected
with a switching time range with minimum and maximum values of
2.5. Fourier transform infrared spectroscopy (FTIR)
Fourier transform infrared measurements were carried out using the
Thermo Scientific IS50 spectrometer equipped with a standard source
and DTGS Peltier-cooled detector. A small amount of alcohol was put
1
5 and 30 s, respectively. The calorimetric structural relaxation times,
= 1/2πf, have been determined from the temperature dependences
of the real part of the complex heat capacity, c ’(T), obtained at different
frequencies in the glass transition region for all samples. The glass tran-
sition temperature, T , was determined for each frequency as the tem-
perature of the half step height of c ’(T).
τ
α
p
2
between CaF glasses with 1 μm distance to preserve the sample thick-
ness. The spectra were measured in an absorbance mode in the
−
1
g
400–4000 cm
range. The temperature measurements were per-
p
formed during cooling using the Linkam heating/cooling stage in the
broad temperature range 293–113 K. The spectra were recorded by
the accumulation of 32 scans with a spectral resolution of 4 cm− . The
obtained data in post-processing mode were subjected to the baseline,
water and carbon dioxide corrections. Finally, in order to separate the
α/β and γ/δ bands, the band fitting analysis using Voigt function (con-
volution of Gaussian-Couchy-Lorentz distributions) was carried out in
Grams 9.2. Software Package. During the analysis, the minimum num-
ber of components was taken into account to preserve the physical as-
pect and avoid the problem of random peaks, which might be equated
with the unreal situation referred to the studied system. Detailed infor-
mation on the selection of the minimal number of components was in-
cluded in Supporting Information. The hydrogen stretching bands were
refined with three functions because three separate inputs stemming
from α/β OHs, γ OHs and δ OHs were reported on the basis of the differ-
ence IR spectra [8]. The applied procedure allowed achieving the correct
peak position, intensity, area (integrated intensity), area ratios, and full
width at half maximum (FWHM). Additionally, the correctness of the
fitting procedure was checked by the calculation of the 2-nd derivative.
1
2
.3. Broadband dielectric spectroscopy (BDS)
Molecular dynamics at ambient pressure of the analyzed set of
-phenyl alcohols was analyzed by means of broadband dielectric spec-
1
troscopy. Each liquid was poured between two parallel plates (of 10 mm
diameter, made of stainless steel and distanced with two 100 μm thick
glass fibres) of a capacitor, sealed with a Teflon ring. The dielectric mea-
surements were performed over a wide frequency range from 10 to
1
Quatro Cryosystem. Prior to measurements, each sample was vitrified:
in the case of 1P1E, 1P1P and 2M1P1E by cooling with a rate of 10 K/
min in the setup or by rapid cooling in liquid nitrogen in the case of
−1
6
0 Hz using a Novocontrol Alpha BDS spectrometer equipped with a
1
P1B due to its higher crystallization tendency. The temperature was
stabilized by means of nitrogen gas for 2 min before each measurement
with a precision better than 0.2 K. The temperature range was different
for each compound, depending on its glass transition temperature. The
dielectric spectra were collected during heating with a step of ΔT = 5 K
g
below T or ΔT = 2 K in the vicinity and above the glass transition.
2.6. Refractometry
2.4. X-ray diffraction (XRD)
The temperature-dependent refractometric measurements of the
analyzed set of 1-phenyl alcohols in the liquid phase were carried out
with a resolution of 0.0001 by means of the Mettler Toledo refractome-
ter RM40 in the temperature range of 288–353 K. The temperature was
controlled by Peltier thermostat with a precision better than 0.1 K.
X-ray diffraction measurements for the studied alcohols were per-
formed at 293 K and then at 143 K, after cooling with a rate of ~2 K/
min, on a Rigaku-Denki D/MAX RAPID II-R diffractometer equipped
with a rotating Ag anode, an incident beam (002) graphite monochro-
mator, and an image plate in the Debye−Scherrer geometry. The tem-
perature was controlled using an Oxford Cryostream Plus and
Compact Cooler. Samples were measured in glass capillaries with a di-
ameter of 1.5 mm. The collected two-dimensional diffraction patterns
were converted into one-dimensional intensity data versus the scatter-
ing vector, Q = 4π(sinθ)/λ, where 2θ is the scattering angle and the
wavelength of the incident beam, λ, is equal to 0.56 Å.
2.7. Density functional theory (DFT) calculations
The theoretical calculations were performed with the use of the den-
sity functional theory in the Gaussian09 program on B3PW91/6-311 g+
+ level (functional and basis set implemented in the Gaussian09 pack-
age) [35,36]. The B3PW91 functional was chosen because it is recom-
mended to describe the electronic properties of organic molecules
3

A. Nowok, K. Jurkiewicz, M. Dulski et al.
Journal of Molecular Liquids 326 (2021) 115349
[37,38]. The molecular geometry of the singlet ground state of the stud-
ied compounds was optimized in the gas phase. Vibrational frequency
calculations were performed for each compound to ensure that the op-
timized geometries represent the local minima of the potential energy
surface and that only positive eigenvalues were found.
3. Results and discussion
3.1. Calorimetric studies
All analyzed 1-phenyl alcohols are liquid at room temperature and
can be vitrified with ease. No crystallization process was detected for
any compound during cooling with a rate of 10 K/min. The subsequent
heating scans, also performed with a rate of 10 K/min, revealed the oc-
currence of the characteristic small thermal effect connected with the
glass transition, which takes place in the vicinity of 198 K for 1P1E,
1
P1P, 1P1B and 206 K for 2M1P1P (Fig. 2). Noteworthy is that despite
differences in molar mass, the linear alcohols are characterized by basi-
cally the same glass transition temperature. Change in the molecular
shape to a globular one by the branching of the alkyl chain results in
Fig. 3. X-ray diffraction patterns of 1P1E, 1P1P, 1P1B and 1P2M1P in low scattering vector
range, at 293 and 143 K.
an increase in T
g
, which is well visible when comparing the isomeric
1
P1B and 2M1P1P. Further heating of 1P1E and 1P1B leads to the cold
XRD data analysis and the obtained values of the structural parameters),
reflecting the difference in the geometry of these two molecules. At
crystallization from the supercooled liquid state, which is evident on
the heating thermograms as an exothermic peak with the onset at 234
and 235 K, respectively (Fig. 2). An additional small exothermic peak
was detected for 1P1B at ~255 K, suggesting polymorphism of this com-
pound. However, further investigations are required to confirm this as-
sumption. The crystal phases of 1P1E and 1P1B are characterized by the
melting points at 255 and 289 K, respectively (Fig. 2). On the other hand,
no crystallization process was detected for 1P1P and 2M1P1P on
heating. In order to characterize the factors responsible for such calori-
metric properties of the 1-phenyl alcohols, the XRD patterns were
analyzed.
1
g
43 K, that is below the T , the maximum of the principal peak shifts to-
wards greater Q for each sample, indicating shortening of the nearest in-
termolecular distances due to the temperature drop, which is a typical
thermal effect. Moreover, the 1P1B crystallizes at 143 K what can be rec-
ognized as the appearance of sharp Bragg peaks in the diffraction pat-
tern. In turn, one may observe a transformation of the MP shape for
2
M1P1P. Such a change may result from the modification of the mutual
arrangement of molecules on the local scale at low temperature,
e.g., from the chain-like to ring-like architecture. Besides, the intensity
of the MP slightly decreases with temperature for 1P1E, 1P1P and
2
M1P1P (for 1P1B crystallization precludes the analysis). This is a
3.2. X-ray diffraction studies
non-Debye-Waller behavior, which may support the conception of the
temperature-dependent reorganization of the intermolecular structure
of these alcohols due to the increase in H bond strength, the ongoing
association process and/or formation of a greater number of multimer
H-bonded structures. On the other hand, the conformational transfor-
mation of molecules, already predicted for 1P1E [34], may also trigger
such changes. However, the diffraction data collected for high-Q values
The range of diffraction data up to around 2.5 Å−1, presented in
Fig. 3, is contributed mainly from the intermolecular correlations in
the studied alcohols. Two broad maxima are observed in this region.
−
1
The main diffraction peak (MP), appearing at QMP ≈ 1.45 Å
93 K, is caused by the short-range intermolecular order and corre-
sponds to an average nearest-neighbor intermolecular distance,
MP = 2π/QMP, of around 4.3 Å. This length is slightly shifted towards
shorter intermolecular distances for 1P1B, and longer distances for
M1P1P (see Supporting Information for a detailed description of the
at
2
−1
up to 20 Å and the atomic pair distribution functions calculated based
on the data (see Fig. S7 and Fig. S8 in Supporting Information, respec-
tively) do not exhibit any significant temperature-dependent differ-
ences in the intramolecular structure of these alcohols. Therefore, this
scenario is unlikely in this case.
d
2
−1
The diffraction data in the low Q range around 0.6 Å exhibits a
characteristic pre-peak (PP), differing in the intensity between the stud-
ied alcohols (Fig. 3). The appearance of such a characteristic feature is a
signature of the spatial organization of molecules on the medium-range
length scale that goes beyond the usual short-range liquid order. In var-
ious alcohols, the PP is usually assigned to an ordering of the H-bonded
supramolecular associates on the length scale of nanometers [8]. The PP
integrated intensity is dependent on the degree of the medium-range
order (the mean size, homogeneity and number of the organized supra-
molecular structures). As can be seen, the PP is the lowest for 1P1E and
the most intense for 1P1B and 2M1P1P. From the full width at half max-
imum, FWHM, of the MP and PP the average coherence lengths, Lmp
=
2
π/FWHMmp and Lpp=2π/FWHMpp, respectively, may be determined.
These parameters are related to the static length scales of the
short- and medium-range intermolecular order, respectively. At room
temperature, Lmp and Lpp for 1P1E are around 8 and 16 Å, respectively,
(
see Table S1 in Supporting Information), suggesting that the supramo-
lecular associates are rather small, mainly dimeric, trimeric, with the in-
Fig. 2. DSC thermograms of the studied 1-phenyl alcohols collected on heating with a rate
of 10 K/min.
termolecular order limited to around 1–2 nm, as suggested in reference
4

A. Nowok, K. Jurkiewicz, M. Dulski et al.
Journal of Molecular Liquids 326 (2021) 115349
[
34]. Lmp of 1P1P and 1P1B is similar to that of 1P1E. However, these
three samples have a different ability to organize on the medium-
range scale. At 293 K the range of the intermolecular order Lpp extends
to around 16, 22 and 31 Å, for 1P1E, 1P1P and 1P1B, respectively.
Thus, the XRD data confirm the formation of nanosized supramolecular
associates in the studied set of phenyl alcohols. For linear alcohols, the
longer the alkyl chain, the higher the tendency to intermolecular order-
ing in the medium-range scale, reflected in the intensities of the pre-
peaks and the values of the Lpp. The diffraction pre-peaks get sharper
g
for all the studied alcohols at a temperature near T , reflecting the for-
mation of greater associates and extension of the medium-range order
upon temperature drop. Isomeric 1P1B and 2M1P1P form the biggest
supramolecular clusters with the highest degree of the nanoscale
order (Lpp ≈ 37 and 31 Å at 293 K, respectively). However, they slightly
differ in the average medium-range order distance related to the
pre-peak position (dpp ≈ 10.3 and 9.7 Å at 293 K for 1P1B and
2
M1P1P, respectively). To shed more light on this issue, broadband di-
Fig. 4. Temperature dependence of Kirkwood-Fröhlich factor for 1P1E, 1P1P and 2M1P1P.
electric measurements were performed.
3
.3. Dielectric results
It should be emphasized that the obtained results for 1P1P are in good
agreement with previous studies on this compound [32]. In turn, the
Kirkwood-Fröhlich factor of the globular 2M1P1P decreases when ap-
proaching the glass transition, taking the values lower than 1. It
means that, in contrast to 1P1E and 1P1P, ring-like H-bonded associates
are preferable in 2M1P1P when lowering the temperature. This out-
come is in good agreement with XRD data, which indicated differences
in the morphology of the supramolecular clusters between the linear
Broadband dielectric spectroscopy is a powerful tool that provides
information on the molecular dynamics in the supercooled liquid and
glassy states and gives an insight into orientational cross-correlations
of dipole moments between neighboring molecules in the liquid
phase. For this purpose, the analysis of temperature evolution of the
k
Kirkwood-Fröhlich factor, g , is used, calculated according to the for-
9
kBε0MTðεs−ε∞Þð2εsþε∞Þ
mula: g ¼
N
2
, where k
B
is the Boltzmann's constant,
k
2
ρNAμ εsðε∞þ2Þ
(
1P1E, 1P1P and 1P1B) and globular (2M1P1P) alcohols. Interestingly,
A
- Avogadro number, M - molar mass, ε
0
- permittivity of a vacuum,
for each analyzed alcohol, the values of the Kirkwood-Fröhlich factor
tend to ~1 with increase in T. Similar behavior was previously reported
for other monohydroxy alcohols [8].
Representative frequency-dependent dielectric loss spectra of the ti-
tled phenyl monohydroxy alcohols, ε”(f), registered both above and
below the glass transition, are presented in Fig. 5. No relaxation process
ρ - density, and μ is molecular dipole moment [39]. For the calculations,
the values of μ were taken directly from the performed DFT computa-
tions (see Table 1).
The temperature evolution of density, ρ(T), was calculated
according to the Lorentz-Lorenz equation [40,41], based on the mo-
lecular polarizability, α, obtained from DFT computations and a lin-
ear extrapolation of temperature measurements of refractive index,
n(T) (see Fig. S3 in Supplamentary Information). The n(T) was also
g
can be detected at temperatures lower than T in the case of 1P1E. On
the contrary, one secondary relaxation of a low magnitude is visible
for the other 1-phenyl alcohols. The maxima of this process, shifting to-
wards higher frequencies when raising the temperature, are visible
used for calculation of the permittivity values at infinite frequencies,
2
ε
∞
(ε
∞
≈ n ), whereas the temperature evolution of the static values
much below T
relaxation peaks remain visible also in the vicinity of T
g
for 1P1P and 1P1B. In the case of 2M1P1P, the secondary
. However, they
of dielectric permittivity, ε
spectra of the real part of the complex dielectric permittivity, ε’. Cal-
culations and further analysis of the thermal evolution of g were
omitted in the case of 1P1B because of its high tendency to undergo
crystallization and melting point close to room temperature. The re-
sults for the other alcohols are shown in Fig. 4.
The temperature evolution of g
comparing the linear alcohols with the globular one. In the case of the
linear 1P1E and 1P1P, the values of g are higher than 1 in the whole
temperature range and increase when approaching T . Taking into ac-
count the previously discussed X-ray diffraction studies showing the
presence of supramolecular H-bonded clusters in the phenyl alcohols
even at room temperature, these findings indicate that the linear 1P1E
and 1P1P tend to form chain-like associates and, what is more, the
share of cross-correlations of dipole moments involved in the chain-
like organization of H bonds increases when lowering the temperature.
s
, was derived directly from the dielectric
g
become gradually less marked with temperature increase due to over-
lapping with the main relaxation. Surprisingly, each alcohol is charac-
terized by only a single dominating relaxation peak above T (labeled
g
k
as the main relaxation in Fig. 5), much more pronounced in magnitude
and of greater sensitivity to temperature changes than the secondary
one. Temperature rise, followed by an acceleration in motions of mole-
cules and the whole H-bonded clusters in the supercooled liquid, may
promote the formation of crystallization nuclei leading to the rapid
cold crystallization process. Such a phenomenon was observed in the
case of 1P1E and 1P1B, what can be seen in the dielectric loss spectra
as a significant drop in the amplitude of the main relaxation process
k
is fundamentally different when
k
g
(
see Fig. 5a and d). The cold crystallization was not observed for 1P1P
and 2M1P1P, which is in good agreement with previous works
reporting hindrance of crystallization for various phenyl derivatives of
propanol [32].
Molecules of all the analyzed 1-phenyl alcohols organize themselves
into supramolecular clusters. Therefore, the main relaxation peak
should be treated, according to [32], as a combination of the structural
α- and Debye processes, connected with the dynamics of single mole-
cules and their aggregates, respectively. Besides, it was previously
shown that phenyl alcohols could be characterized by a small separation
between the Debye and α-relaxation processes so that they are often
merged [32]. To verify if such a scenario happens for the alcohols stud-
ied here, the dielectric loss spectra registered above glass transition
temperature were refined with the Fourier transform of the
Table 1
Glass transition temperature determined from calorimetry (Tg (DSC)), steepness index
(m100s), dipole moment (μ) and polarizability (α) of the analyzed 1-phenyl alcohols.
α·10− [C m /J]
40
2
2
Substance
T
g(DSC) [K]
m
100s
μ [D]
1P1E
198(1)
198(1)
198(1)
206(1)
87(2)
73(2)
66(2)
67(2)
1.71
1.66
1.65
1.69
14.43
16.46
18.46
18.26
1
1
2
P1P
P1B
M1P1P
5

A. Nowok, K. Jurkiewicz, M. Dulski et al.
Journal of Molecular Liquids 326 (2021) 115349
Fig. 5. Selected frequency-dependent dielectric loss spectra of 1P1E (a), 1P1P (b), 1P1B (c) and 2M1P1P (d).
Kohlraush-Williams-Watts (KWW) function [42]. As shown on the ex-
emplary ε”(f) spectrum gathered at 205 K for 1P1E (Fig. 6), the main re-
laxation peak is described with the KWW function only in the very
vicinity of the maximum (Fig. 6). Similar results were also obtained
for the other alcohols investigated herein. Based on the temperature de-
pendence of the βKWW factor one should notice that the values of this
parameter are smaller than 1 in the whole temperature range for all an-
alyzed monohydroxy 1-phenyl alcohols (see inset in Fig. 6). In the vicin-
smaller contribution to the overall relaxation dominated by the struc-
tural α-process. Such a scenario is possible when we consider lower
concentration, smaller size and/or different architecture of the supra-
molecular H-bonded associates resulted from the increase in the length
of the aliphatic chain and/or its branching. This is highly likely in the
case of 2M1P1P which tends to organize itself in ring-like clusters, for
ity of T
g
, the broadening is considerable for the isomeric 1P1B and
2
M1P1P, however, it decreases significantly for the 1P1E and 1P1P.
The values of βKWW increase with temperature raise (see inset in
Fig. 6), similarly to other previously reported phenyl alcohols, such as
2
-phenyl-1-propanol [32]. In the case of 1P1P, the βKWW exceed 0.9 at
T/T = 1.14. On the one hand, the narrowing of the main relaxation
g
peak while temperature raising may indicate that the time-
temperature superposition is violated. However, it may also suggest
that the peak is a combination of two processes with different temper-
ature dependencies: the low-frequency Debye process and the struc-
tural α-relaxation with a maximum located close to the maximum of
the Debye mode. It should be stressed that this conception is supported
by the previous studies of 1-phenyl-1-propanol by Böhmer et al. [32], as
well as our XRD results, thermal evolution of the Kirkwood-Fröhlich co-
efficient and the fact that the main relaxation peak is poorly described
with a single narrow KWW function for each analyzed alcohol.
The significant broadening of the relaxation peak in the case of
isomeric 1P1B and 2M1P1P suggests a total disappearance of the
Debye process or its much smaller amplitude, leading to a noticeably
Fig. 6. Dielectric loss spectrum registered for 1P1E at 205 K and refined with KWW
function. The inset shows the temperature dependencies of the βKWW parameter for all
1-phenyl alcohols.
6

A. Nowok, K. Jurkiewicz, M. Dulski et al.
Journal of Molecular Liquids 326 (2021) 115349
which the overall dipole moment is close to 0 D. However, the KWW
function does not describe well the high-frequency slope of the relaxa-
tion process even for this alcohol (see Fig. S6 in Supplementary Informa-
tion). Therefore, it cannot be excluded that also chain-like associates are
formed by 2M1P1P, being in the thermal equilibrium with the ring-like
aggregates. Consequently, the Debye process still may be present in this
case, but its amplitude and, consequently, its contribution to the main
relaxation peak are much lower in comparison with 1P1E and 1P1P.
On the other hand, it can be related only to the contribution of the sec-
ondary relaxation process.
To better understand the unobvious dynamic properties of the set of
1
-phenyl alcohols and to obtain detailed information on temperature
behavior of the relaxation processes, the peaks were described with
fitting functions in the frequency domain. Since the relaxation peaks
of secondary processes are symmetrical in shape (see Fig. 5), a descrip-
tion with the Cole-Cole fit function (with fixed shape parameter β = 1
from the Havriliak-Negami equation [43]) was applied. Such a proce-
dure allowed determining the relaxation times, τMax,secondary. As shown
in Fig. 8, the logarithm of τMax,secondary increases linearly with tempera-
ture inverse for the secondary process of each analyzed alcohol. There-
fore, the Arrhenius law was used for parametrization of the temperature
À
Á
Ea
dependences, given by the formula: τMax,secondary ¼ τ∞ exp
, where E
a
RT
is the activation energy and R is the gas constant. As shown in Fig. 8, the
β-relaxation times of 2M1P1P are much longer (up to 4 decades) in
comparison with those of 1P1B and 1P1P. Also the activation energy
Fig. 7. Selected dielectric loss spectra of 1P1E, 1P1P and 1P1B with added fitting functions
of the Debye and α-processes.
(
33(1) kJ/mol) is much higher for this compound. On the contrary, the
temperature dependences of β-relaxation times of 1P1B and 1P1P are
similar to each other, being characterized by the same value of activa-
tion energy with respect to inaccuracy ~25 kJ/mol. However, the relax-
ation times are slightly shorter, by about 0.25 decade, for 1P1B.
Secondary relaxation processes can be of intra- or intermolecular char-
acter. The first ones are connected with rotational motions of molecular
fragments. The second ones, associated with limited reorientation of
whole molecules, have been named as Johari-Goldstein (JG) type. In
order to determine the origin of a secondary relaxation we decided to
check the empirical relation proposed by Kudlik et al., which links the
the case of 1P1E, are highly unlikely to be observed, what is in good
agreement with the obtained results (only α-relaxation process is
registered).
In contrast to the secondary relaxation, the main relaxation peaks of
1P1E, 1P1P and 1P1B are asymmetrically broadened and, what is more,
contain a considerable kink in the high-frequency slope (Fig. 7). Consid-
ering positive cross-corelation between neighboring molecules and
values of βKWW parameter close to 1, such a feature suggests that the
activation energy of this process, E
a
, with the glass transition tempera-
spectra registered above T are a combination of two overlapping pro-
g
Ea
ture:
¼ 24 [44–46]. Although some deviations were observed, this
cesses. Therefore, refinement with a sum of the Cole-Davidson (ob-
RTg
served for structural relaxations of numerous mono- and polyhydroxy
alcohols such as glycerin, 1-phenyl-2-butanol, 2-phenyl-1-butanol
[31,48]) and the Debye functions was applied for the exemplary ε”(f)
spectra of 1P1E, 1P1P and 1P1B. As shown in Fig. 7, the time separation
between the maxima of the Debye and structural α-relaxation is less
than 1 decade for each alcohol. Such a small value is in good agreement
with previous studies on 1P1P based on the combination of PCS and BDS
techniques. It should also be emphasized that the time separation be-
tween the maxima of the Debye and the main relaxation peaks is basi-
cally negligible (is less than 0.1 decade). Therefore, one can suppose
that the temperature dependence of the relaxation time determined di-
rectly from the main relaxation peak will correspond to the temperature
evolution of the Debye process. Such an assumption allow us to avoid
possible mistakes, particularly at higher temperatures, connected with
a model-dependent fitting procedure due to the too small time-scale
separation of the relaxation processes.
relation is fulfilled for the vast majority of compounds if the analyzed
secondary relaxation process is of JG type [47]. Taking the T
g
and E
a
de-
Ea
termined previously for the analyzed set of 1-phenyl alcohols, the
RTg
ratio takes the value of 15.2(7), 14.6(7) and 19.9(7) for the 1P1P,
P1B and 2M1P1P, respectively. Although the determined ratio for the
1
latter compound deviates from 24, the value is comparable with those
obtained for β-relaxations of JG type for 5-methyl-2-hexanol or 2-
ethyl-1-hexanol [47]. Therefore, one can assume that the secondary re-
laxation process in 2M1P1P belongs to this group. On the other hand,
the determined values for 1P1P and 1P1B correspond well to those cal-
culated for γ-relaxations of non-JG type for poly[(phenyl glycidyl
ether)-co-formaldehyde] (PPGE) or benzoin isobutylether (BIBE) [47],
being far lower than 24. The obtained results indicate that the registered
secondary relaxations for 1P1P and 1P1B are of intramolecular charac-
ter, connected with rotational motions of a substituent. Since all ana-
lyzed compounds possess a phenyl ring and no γ-relaxation was
observed in the case of 1P1E, rotational motions of the aromatic substit-
uent can be excluded from the possible sources of this process. Small
amplitude and short relaxation times even at very low temperatures
Therefore, in order to obtain more precise information on a temper-
ature behavior of the main dielectric process for each compound, the re-
laxation peaks were described with the Havriliak-Negami (HN)
function only in the very vicinity of the maximum:
(
such as 118 K) indicate that it may originate from reorientational mo-
⁎
Δε
⁎
, where ε is complex permittivity, Δε - dielec-
−α
ε ðωÞ ¼ ε∞ þ
β
1
tions of the ends of alkyl chains, that is C and C groups of 1P1P
H
2 5
H
3 7
½1þðiωτHNÞ
ꢀ
and 1P1B, respectively. Rotation of these moieties, e.g., around the C1-
C2 bond, should be fast even at low temperatures and may cause
small changes in the value of dipole moment vector, making the rota-
tion possible to be registered via dielectric studies. What is more, rota-
tric strength, ε∞ - the high-frequency limit of permittivity, ω - angular
frequency, τ - the relaxation time, α and β are the shape parameters
[43]. For each analyzed compound the α parameter of HN function
took the value of 1 in the whole temperature range, transforming the
fit function to the Cole-Davidson form. As shown in Fig. 8, the obtained
temperature dependence of relaxation times, τMax,main in the liquid
3
tional motions of a methyl CH group around the C1-C2 bond, leading
to changes in conformations between staggered and opposite ones, in
7

A. Nowok, K. Jurkiewicz, M. Dulski et al.
Journal of Molecular Liquids 326 (2021) 115349
Fig. 8. (a) Relaxation map of the analyzed set of 1-phenyl alcohols. (b) Comparison of the relaxation times obtained from dielectric and calorimetric studies of 1P1E and 1P1P.
ꢂ
phase has a non-linear, super-Arrhenius character for each analyzed al-
d log 10τMax,main
dðTref =TÞ
ꢂ
ꢂ
ꢂ
[
52]: m ¼
. It should be mentioned that although the re-
cohol, which can be refined with the Vogel-Fulcher-Tamman-Hesse
T¼Tref
ꢀ
ꢁ
DT
equation: τMax,main ¼ τ∞ exp
infinitely high temperature, D
, where τ
∞
is a relaxation time at an
laxation times were taken from the thermal evolution of the main relax-
ation peaks instead of the Debye ones, a negligible time-separation
between them allow conducting such an analysis. The reference tem-
perature was defined here as a temperature at which the relaxation
time is equal to 100 s. Hence, in order to determine the steepness
index for each alcohol, the main relaxation times were extrapolated
by about two decades. As presented in Table 1, the estimated values of
the steepness index decrease with an increase in the number of carbon
atoms in the alkyl chain of the monohydroxy alcohol from 87(2) for
T−T0
T
- a material constant, and T
0
denotes to
the ideal glass temperature [49–51]. The results from dielectric mea-
surements were also superimposed on the relaxation times obtained
from TMDSC. This technique gives a direct insight into the thermal evo-
g
lution of structural α-process in the vicinity of T . As shown in Fig. 8, the
TMDSC and dielectric data are in very good agreement (with respect to
the inaccuracy stemming from extrapolation) in the case of 2M1P1P. On
the other hand, there is a slight discrepancy between the dielectric and
calorimetric results for the linear 1P1E, 1P1P and 1P1B (see Fig. 8b).
However, the difference is small and does not exceed 0.8 decade for
given isothermal conditions. It should be emphasized that a similar
finding was obtained previously for 1P1P [32]. These results confirm
that the main relaxation peak of 2M1P1P is related to the structural α-
process, whereas the Debye mode has a decisive contribution to the
main relaxation process in the case of the other studied phenyl alcohols.
Its occurrence for the linear alcohols is closely related to their self-
organization in chain-like clusters of non-zero dipole moment. The ag-
gregates are small in each case, which leads to a small time-separation
and, as a result, difficulty in identification of the Debye peak in the di-
electric loss spectra.
1
P1E, through 73(2) for 1P1P, to 66(2) for 1P1B and 67(2) for the iso-
meric 2M1P1P. This tendency seems to reflect the behavior of the dif-
fraction pre-peak feature: the higher the values of the pre-peak
intensity and width, the lower the steepness index. Taking into account
that the steepness index can also be treated as an indicator of H-bonded
supramolecular clusters [31], two scenarios must be taken into account:
the size of the suprastructures becomes larger, or their concentration
rises as the number of carbon atoms in the alkyl chain of the phenyl al-
cohol increases. In order to explain the self-assembly phenomenon
thoroughly and to characterize the related structural features of the
set of 1-phenyl alcohols, particularly the hydrogen-bonding pattern,
further studies by means of infrared spectroscopy were performed.
Moreover, there is a high degree of similarity between 1P1E, 1P1P
and 1P1B in terms of both temperature dependences of α- and Debye
relaxation times (Fig. 8). Taking into account basically the same values
3.4. Infrared spectroscopy studies
of T
g
, this result may suggest that increase in the number of carbon
In the case of IR measurements the main emphasis was put on the
−1
atoms in the alkyl chain does not significantly affect the molecular dy-
namics of the substance, at least when the alkyl chain is short. The relax-
ation times of 2M1P1P (determined form both TMDSC and BDS) differ
significantly from the values for the other alcohols, being always slower
at each measured temperature. Such an image clearly shows that addi-
tion of a side substituent to the alkyl chain (in this case a methyl group),
accompanied by a change in the shape of a molecule from linear to more
globular, significantly affects the glass transition temperature and the
molecular dynamics in the supercooled liquid.
To get a deeper insight into the self-organization phenomenon, the
steepness index, m, was calculated for the analyzed alcohols. This param-
eter describes the sensitivity of the relaxation times (and thus the mo-
lecular dynamics) to temperature changes when approaching the
reference temperature, Tref, and it is defined as a slope of the tangent
analysis of the high wavenumber 3700–2600 cm spectral region be-
cause of the origin of bands in this range linked to the stretching
ν(OH) modes, as well as the characteristic asymmetric and symmetric
stretching vibrations of the CH
x
groups (x = 1,2,3). A very similar
band arrangement with small differences appearing in the 3800-
−
1
2800 cm range results from the variation in the alkyl chain length
and branching. As shown in Fig. 9, the IR spectra of each phenyl alcohol
measured at 298 K in the liquid phase are characterized by two OH
−
1
bands centered at 3553 and 3347 cm for 1P1E (Fig. 9a), 3547 and
−
1
−1
3334 cm for 1P1P (Fig. 9b), 3553 and 3353 cm for 1P1B (Fig. 9c),
−1
3548 and 3409 cm
for 2M1P1P (Fig. 9d). According to previous
reports, the band with lower intensity located at higher wavenumber
stems from stretching vibrations of non-bonded or weakly bonded α
OH groups of monomeric molecules in the liquids or β OH groups of
terminal molecules of supramolecular structures, in which they play a
to the log10τMax, main = f(Tref/T) curve at the temperature equal to Tref
8

A. Nowok, K. Jurkiewicz, M. Dulski et al.
Journal of Molecular Liquids 326 (2021) 115349
role of only proton acceptors [8]. Hence, the term of α/β OH groups is
connected with hydroxyl units, which do not play the role of proton do-
…
nors for the interactions via O-H O scheme. However, one should keep
in mind that such hydroxyl groups can still interact with other mole-
cules in the liquid phase. Since these intermolecular contacts are
weak, the α/β-band was reported for numerous compounds to appear
−1
at similar wavenumbers (that is above 3600 cm ) compared to the
stretching modes of non-bonded OH groups of isolated alcohol mole-
cules in the gas phase or CCl
4
solutions [8,53,54]. For example, the α/
solution is centered at similar
β-band of liquid tert-butanol and its CCl
4
−1
wavenumber (~ 3625 cm ) [53,54]. In the case of phenyl alcohols, the
OH stretching band originating from isolated molecules in dilution or
−
1
gas phase is also located above 3600 cm , e.g., 3647, 3631 and
−
1
3
1
616 cm
4
for the supersonic jet, Ar matrix and CCl solutions of
-phenylethanol, respectively [34,55]. However, contrary to tert-
butanol or other aliphatic alcohols, the less-intense blue-shifted OH
−
1
−1
band is located much below 3600 cm (~ 3550 cm ) for the non-
diluted phenyl alcohols in liquid phase [34,55]. Therefore, it was
assigned to the stretching modes of the hydroxyl groups involved into
intermolecular OH···π interactions [34]. Nevertheless, it is still not
clear whether this band is an image of an additional molecular ordering
or is it a result of random interactions of the phenyl ring with α or β
hydroxyl units of the monomers or terminal molecules of H-bonded
oligomers, respectively. Therefore, in the case of the studied phenyl
alcohols we will label this blue-shifted band of lower intensity as
α/β-band connected with OH···π interactions.
The secondred-shiftedband, much stronger in intensity, results from
the occurrence of the H-bonded dimers or multimer structures and is
associated with stretching modes of OH groups that are simultaneously
acceptors and donors of the proton in hydrogen bonds (δ OHs) or play a
role of only proton donor in the H-bonded clusters (γ OHs) [8]. The α/
−1
β-bandislocatedatsimilarwavenumber(~3550cm )atroomtemper-
ature for all analyzed phenyl alcohols,what suggests a similarstrengthof
intermolecular OH···π interactions, independently from the molecular
architecture. This statement seems to be reinforced by previous studies
on phenyl-substituted butanols, accordingto whichtheα/β-band is cen-
−1
teredat3559cm for1-phenyl-2-butanoland4-phenyl-1-butanol, and
−1
3
560cm for4-phenyl-2-butanol[55].Moreover, theoccurrenceofthis
band, resulting from weakly or non-H-bonded OH groups, points that
the association degree of the analyzed 1-phenyl alcohols is less than
1
3
00% at room temperature. The other bands visible in the range of
−1
200–2800 cm are connected with stretching vibrations of aromatic
, CH and CH moieties (Fig. 9).
CH and aliphatic CH
3
2
Lowering the temperature from 278 K down to 113 K with a cooling
rate of 20 K/min leads to a gradual change in the position and intensity
of the bands connected with the stretching ν(OH) modes. For each alco-
hol, the intensity of the high-wavenumber α/β-band connected with
the occurrence of α/β hydroxyl groups was found decreasing with tem-
perature decline (Fig. 9). This band still occurs and is visible in the IR
g
spectra as a shoulder even at temperatures below T (Fig. 9). On the
other hand, the amplitude of the second ν(OH) stretching band cen-
tered at lower wavenumber and resulting mainly from H-bonded
multimer structures is gradually increasing with temperature decrease
for all 1-phenyl alcohols (Fig. 9). The obtained results clearly suggest re-
arrangement of molecules within the H-bonded network when ap-
g
proaching T , a decrease in the population of the weak intermolecular
…
OH···π and OH CH interactions, a reduction of the concentration of
monomers and, as a consequence, the formation of a greater number
of multimer H-bonded structures. This conclusion is supported by the
temperature evolution of the Kirkwood-Fröhlich coefficient calculated
from dielectric studies, which also suggested the gradual ordering of
the molecules through H-bonds with temperature decrease. The
temperature-dependent behavior of 1P1B is a little bit different from
Fig. 9. Infrared spectra of 1P1E (a), 1P1P (b), 1P1B (c) and 2M1P1P (d) in the 3650–2600
cm range, measured on cooling with 20 K/min from 293 K down to 113 K.
−1
9

A. Nowok, K. Jurkiewicz, M. Dulski et al.
Journal of Molecular Liquids 326 (2021) 115349
and similar for the studied compounds at each temperature. Taking
into account the previously discussed calorimetric and dielectric results,
showing that the linear alcohols are characterized by almost the same
g
value of T whereas it is about 8 K higher for the globular 2M1P1P, it
demonstrates that the strength of hydrogen bonds have a decisive im-
pact on the value of the glass transition temperature. Apart from gradu-
ally decreasing thermal energy, the appearance of stronger H bonds
when approaching T contributes to the slowdown of the molecular dy-
g
namics of the systems. However, considering the similar temperature
red-shift of the ν(OH) stretching band for all 1-phenyl alcohols
−1
(
0.55–0.59 cm /K) and different temperature dependences of the re-
laxation times (see Fig. 8a) one should conclude that there must also
be another factor affecting the molecular dynamics of the studied phe-
nyl alcohols in the supercooled liquid state.
As pointed before, the degree of association of molecules via H bonds
is less than 100% for each alcohol at both room and low temperatures.
Nevertheless, distinct changes are observed while cooling. Moreover,
some differences are observed when comparing the IR spectra for
these compounds with each other. To enable such a comparison, the
IR spectra were normalized so that the intensity of the γ/δ-band is
equal to 1. As shown, the α/β-band has the highest intensity for the
globular 2M1P1P at room temperature whereas the linear 1P1E and
1P1B are characterized by the lowest one (Fig. 11a). It indicates that
the structure of the phenyl alcohol molecule affects not only the type
of the formed supramolecular associates (ring- or chain-like) but also
the degree of association, which is the lowest in the case of 2M1P1P. It
seems that branching of the alkyl chain in the very vicinity of the hy-
droxyl group plays the role of a steric hindrance for the organization
of molecules in supramolecular H-bonded clusters. Cooling of the alco-
hols down to the temperature close to the glass transition (e.g., 203 K)
leads to a considerable increase in the degree of association, which is
visible as a decrease in the intensity of the α/β-band (Fig. 11b). At this
temperature region, the differences between the alcohols become less
pronounced, but 2M1P1P is still the least associated. Further cooling
down to 113 K leads to an only slight increase in the ordering of mole-
cules (compare Fig. 11b and c), which is due to freezing of molecular
motions in the glassy state.
Fig. 10. Temperature dependence of the ν(OH) stretching band position. The glass
transition temperatures (T
g
) of 1P1E, 1P1P and 2M1P1P and onset of crystallization
process (T ) of 1P1B are marked with arrows.
c
the other 1-phenyl alcohols. For this compound, the gradual ordering of
molecules in liquid with lowering the temperature results in a rapid
crystallization process taking place in the vicinity of 223 K even when
cooling with a rate of 20 K/min. This phenomenon is visible in the IR
spectra as a substantial red-shift of the γ/δ-band (Fig. 9c). The less in-
tense α/β-band, connected with the occurrence of weakly bonded OH
groups, vanishes after the crystallization process is completed what sug-
gests that all hydrogen groups are involved in the formation of the 3D
network of hydrogen bonding in the crystal lattice.
To provide a more detailed description of the supramolecular associ-
ation at different thermal conditions, the position of the most intense
γ/δ-band was plotted versus temperature (Fig. 10). As a result, a linear
shift of the OH band position towards lower wavenumbers is observed
−
1
when approaching the glass transition: 3347 → 3289 cm
(1P1E),
(2M1P1P). The
change in the OH band position is similar for all alcohols and ranges
−
1
−1
3
334 → 3289 cm
(1P1P) and 3409 → 3320 cm
−1
from 0.55 to 0.59 cm /K. The red-shift of the γ/δ-band indicates the
formation of stronger H bonds due to the shortening of the distance be-
tween acceptor and donor atoms while cooling. The formation of H
bonds is usually correlated with the transfer of the proton acceptor
from the lone pair orbital or π electron system to σ∗(X–H) of the
donor (hyperconjugation concept). As a result, the appearance of the
H bonding scheme in the molecular system leads to a weakening and
slight elongating of the H–X bond. At the same time, the hydrogen
bonding Y⋯H–X (e.g. O⋯H\\O) distance becomes shorter due to stron-
ger molecular interaction [56]. The decrease in the length of the whole
O⋯H\\O moieties and thus increase in the strength of hydrogen
bonds also takes place during cooling when less thermal energy is deliv-
ered to the system in the wake of a reduction in molecular mobility. As a
consequence, the main γ/δ OH stretching band observed on the infrared
The temperature-induced changes in the degree of association can
be analyzed more precisely in terms of the thermal evolution of the
area under the α/β-band, I
I
(α/β), with respect to the area under the
(α/β)/I (γ/δ) ratio decreases
γ/δ band, I (γ/δ). As shown in Fig. 12 the I
I
I
I
with temperature for all alcohols. Nevertheless, the temperature depen-
dences are different for each compound. Due to the crystallization pro-
cess, 1P1B was excluded from this analysis. The greatest changes while
cooling are observed for liquid 2M1P1P, and the kink visible at ~200 K is
related to the glass transition. Similar behavior is observed for 1P1P.
However, in this case the changes in the liquid phase are smaller. In
the case of 1P1E the values of the I
manner with temperature. Taking into account the small variation of I
(α/β)/I (γ/δ) for 1P1E (and thus of the degree of association) between
298 K and the temperature close to T , accompanied by significant
I I
(α/β)/I (γ/δ) ratio change in a linear
I
I
g
spectrum shifts towards lower wavenumbers. In the vicinity of T
g
a kink
changes in the Kirkwood-Fröhlich factor, one should conclude that the
systems become more ordered and hydrogen bonds become more ho-
mogenous in terms of strength at low temperatures. This statement is
reinforced by the decrease of the band full width at half of the maximum
(FWHM) of the γ/δ-band when approaching glass transition (Table 2).
A similar effect is observed for the other alcohols. 1P1B is character-
ized by smaller FWHM at room temperature (the most homogenous
strength of H bonds) and also by the lowest intensity of the α/β-band
among all studied alcohols, which means that this compound is ordered
the most in terms of H bonding pattern at 293 K. This results in its par-
ticularly high tendency to crystallize while cooling. The similar low in-
tensity of α/β-band (high degree of association) at room temperature
was recognized for 1P1E, which also crystallizes. However, its tendency
to undergo this process is far much lower compared to 1P1B. Based on
is observed (Fig. 10), after which the ν(γ/δ) position red-shifts signifi-
−
1
−1
cantly slower (about 0.2 cm /K) towards 3279 cm
3
for 1P1E,
−1
−1
286 cm for 1P1P and 3312 cm for 2M1P1P at 113 K.
Furthermore, the intensity of the γ/δ band does not undergo signifi-
cant changes below T . This behavior may be justified by freezing of the
g
molecular mobility in glass or occurrence of an equilibrium state in the
systems. Fig. 10. also shows that hydrogen bonds are of similar strength
for the linear alcohols (1P1E, 1P1P and 1P1B), whereas the more globu-
lar 2M1P1P is characterized by weaker H bonds in the whole tempera-
ture range. On the other hand, the position of the α/β band, similar for
all alcohols at room temperature, does not change significantly while
cooling (Fig. 9). It means that the strength of the intermolecular
…
OH···π and OH CH interactions is almost temperature-independent
10

A. Nowok, K. Jurkiewicz, M. Dulski et al.
Journal of Molecular Liquids 326 (2021) 115349
Fig. 11. FTIR spectra of the analyzed 1-phenyl alcohols in the non-crystalline state, in the 3100–3650 cm−1 frequency range at 298 K (a), 203 K (b) and 113 K (c). The spectra were
normalized to the intensity of γ/δ-band.
in the liquid phase depends on the thermal evolution of H bond
strength, degree of association as well as the morphology of supramo-
lecular structures.
4
. Conclusions
The influence of the molecular architecture of a set of four 1-phenyl
alcohols (1-phenylethanol, 1-phenyl-1-propanol, 1-phenyl-1-butanol
and 2-methyl-1-phenyl-1-propanol) on hydrogen-bonding pattern,
molecular dynamics, glass transition temperature and relaxation
processes was analyzed. It was found that all studied alcohols form H-
bonded supramolecular clusters, despite containing a phenyl ring in
the most disfavored position. Moreover, the conjunction of calorimetric
and IR studies allowed us to determine that the molecular architecture
and the strength of intermolecular hydrogen bonds have a decisive im-
pact on the glass transition temperature. 1P1E, 1P1P and 1P1B, charac-
terized by a more linear shape of molecules, have a similar strength of
hydrogen bonds at each temperature and comparable glass transition
I I
Fig. 12. Thermal evolution of the I (α/β)/I (γ/δ) ratio for 1P1E, 1P1P and 2M1P1P.
g
temperature. The T was determined to be 198 K for these three alco-
hols. The change in the shape of the alcohol molecules from linear to
globular (by the introduction of an additional methyl side group to the
alkyl chain), as it is in the case of 2M1P1P, leads to a significant weaken-
this observation, one can conclude that the length of the alkyl chain
plays a crucial role in crystallization process and the 1-phenyl alcohols
with an even number of carbon atoms (1P1B and 1P1E) are more
prone to undergo this phase transition.
g
ing of HBs and an increase in T to 206 K. Molecular architecture also af-
fects the vitrification abilities of the 1-phenyl alcohols. However, the
length of the alkyl chain plays a crucial role in this case. Similarly to
other phenyl derivatives of propanol studied in literature before, two
propanol derivatives, 1P1P and 2M1P1P, were found to be good glass-
formers, whereas 1P1E and 1P1B showed a tendency to undergo
crystallization.
Temperature-dependent dielectric studies of the 1-phenyl alcohols
enabled us to observe relaxation processes both above and below the
glass transition temperature. Only single secondary relaxation was de-
tected in the glass phase of 1P1P, 1P1B and 2M1P1P. This process was
found to be of intramolecular origin for the first two compounds, con-
The performed IR measurements show that the architecture of mol-
ecule plays a crucial role not only in the molecular architecture of the
clusters (ring- or chain-like) but also affects the degree of association,
strength of intermolecular H bonds and crystallization tendency. Addi-
tional branching of the alkyl chain in the vicinity of OH group contrib-
utes to a significant reduction of the degree of association and
hydrogen bond weakening, while the length of the alkyl chain has a de-
cisive impact on the crystallization tendency. Moreover, IR measure-
ments revealed that the glass transition temperature is controlled
mainly by the strength of H bonds, whereas the molecular dynamics
2 5 3 7
nected most probably with rotational motions of C H and C H moie-
ties, respectively. The activation energies of this relaxation are similar
for both alcohols, taking the value close to 25 kJ/mol. However, the re-
laxation times are slightly shorter by about 0.25 decade in the case of
Table 2
Comparison of FWHM of the γ/δ-band of the studied 1-phenyl alcohols for 298 and 203 K.
Compound
FWHM at 298 K [cm−1]
FWHM at 203 K [cm−1
]
1
P1B. Although a single relaxation process seems to occur in the
1
1
1
2
P1E
P1P
P1B
M1P1P
243(4)
254(4)
232(4)
250(4)
230(4)
223(4)
–
supercooled liquid phase, further refinement with the KWW function
brought us to the conclusion that the relaxation peaks are a combination
of the Debye and structural α-processes. This statement was reinforced
by X-ray diffraction studies, which showed for each 1-phenyl alcohol
218(4)
11

A. Nowok, K. Jurkiewicz, M. Dulski et al.
Journal of Molecular Liquids 326 (2021) 115349
effectiveness of hydrogen bonds, and appearance of debye process, J. Phys. Chem.
C 124 (2020) 2672–2679, https://doi.org/10.1021/acs.jpcc.9b09803.
the occurrence of a pre-peak characteristic for the presence of supramo-
lecular structures where molecules order on the nanoscale. Further
studies by means of broadband dielectric spectroscopy allowed us to
conclude that the molecular dynamics of the alcohols in the liquid
state is not only affected by the intramolecular architecture and hydro-
gen bond strength but also depends on the morphology (concentration,
size and architecture) of the H-bonded associates. Here, a decrease in
the steepness index with an increase in the number of carbon atoms in
the alkyl chain of the linear monohydroxy alcohol was observed. This
tendency is in good agreement with the changes in the diffraction
pre-peak intensity and FWHM. The pre-peak is the least intense and
the widest for 1P1E, while for 1P1B it is sharper and has higher intensity,
indicating that the isomeric compounds organize into greater supramo-
lecular clusters with a higher degree of the nanoscale order. However,
the architecture of the molecules, particularly the location of the steric
hindrance, affects the mutual arrangement of molecules in the clusters
and the structure of H bond network significantly. The linear 1-phenyl
alcohols (1P1E, 1P1P, 1P1B) form preferably chain-like H bond connec-
tions, whereas globular-shaped 2M1P1P – ring-like structures. Temper-
ature analysis of the Kirkwood-Fröhlich factor points out that both types
of supramolecular structures are in equilibrium at higher temperatures.
However, at lower temperatures, linear molecules tend to form bigger
and more ordered chain-like structures while globular molecules prefer
to arrange into ring-like associates.
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Declaration of Competing Interest
There are no conflicts to declare.
Acknowledgments
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K.J. is grateful for the financial support from the Foundation for Pol-
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2021.115349.
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