# Temperature Evolution of Cluster Structures in Ethanol

Authors: P. Golub; I. Doroshenko; V. Pogorelov; V. Sablinskas; V. Balevicius; J. Ceponkus
Journal: Dataset Papers in Physics
 (2013)

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## Abstract

The dependence of FTIR spectrum of pure ethanol on the temperature was investigated. The measurements were performed for frozen (the minimum temperature −180°C) and liquid ethanol (the maximum temperature 40°C). All changes in IR spectrum of ethanol during gradual warming were detected and analyzed. On the bases of preset observations, the conclusions concerning the evolution of cluster structures in ethanol during transition from solid (frozen) state to liquid state were made.


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## Body

## 1. Introduction

The alcohols belong to the specific kind of species since their molecules can form hydrogen bonds and arrange in different structures named clusters. Numerous works were devoted to the investigation of these substances. Special attention was paid to the investigation of monohydric alcohols among which methanol [1–5] and ethanol [6–9] were the most popular as the simplest ones. The main competitive structures are ring and chainlike clusters. In the first type of clusters, all hydroxyl groups of alcohol molecules are H-bonded with neighboring molecule until the conditionally last molecule would bond with the first one. In chainlike structures, the last molecule is not bonded with the first one, thus leaving one hydroxyl group free. But the situation is still not obvious, and different authors postulate different sets of clusters that exist in liquid phase on the basis of different experimental and theoretical techniques [1–11]. Also the evolution of cluster structures during the change of ambient conditions has not been followed yet. In the first stage of such type of investigations, we decided to choose infrared spectroscopy as the experimental tool since any changes in clusters sizes and types may be clearly detected by IR spectrum. Ethanol was chosen as the object of investigation.


## 2. Methodology

Registration of the presented spectra was performed in the laboratory of Fourier transform infrared absorption spectroscopy in the Faculty of Physics of Vilnius University, Lithuania. All spectra were recorded using Bruker’s FTIR-spectrometer VERTEX 70 equipped with LINKAM cryostat (model FTIR 600). The spectra were recorded in the spectral range from 750 to 4000 cm−1 and in temperature range from −180 to 40°C. Liquid-N2-cooled mercury cadmium telluride (MCT) was used as a detector. Spectral resolution was set to 1 cm−1, and in order to increase signal-to-noise ratio, each spectrum was taken as an average of  128 scans. Liquid ethanol with purity greater than 99.9 from Fluka was used as received.In Figure1, the registered FTIR spectra of ethanol at different temperatures are shown. The brief interpretation of these bands according to [12–16] is the following: the band at 805 cm−1 is the combination of wagging vibrations of C–H bonds in groups CH2 and CH3; 1489 cm−1, the combination of scissoring vibrations of C–H bonds in groups CH2 and CH3; 1459 cm−1, the asymmetric (in relation to the plane C–C–O) bending vibration of C–H bonds in CH3 group; 1380 cm−1, the symmetric bending vibrations of C–H bonds in CH3 group; and 1276 cm−1, the twisting vibration of C–H bonds in CH2 group; for the band 1091 cm−1, the main contribution stems from stretch vibration of C–O bond and lesser from stretch vibration of  C–C bond realized in antiphase to the previous vibration [12]; the band at 1051 cm−1 is much similar to the previous band but with larger contribution from bending vibration of  O–H group; the band at 882 cm−1 is the in-phase stretch vibrations of C–O and C–C groups. All the bands considered do not undergo any significant change during warming. The band at 1429 cm−1 in frozen ethanol is shifted to 1415 cm−1 at 20°C and to 1409 cm−1 at 40°C. According to [12, 16], this band is attributed to the bending vibration of O–H group and such shift may be due to the reorganization of cluster structures. The band at 1333 cm−1 decreases significantly with temperature increasing. This band may be attributed to the combined twisting and wagging vibrations of C–H bonds in CH2 group and bending vibration of O–H bond. Bands at 1156 cm−1 and 1126 cm−1 registered in frozen ethanol disappear at the room temperature (or become too weak to be distinguished with the present experimental set-up). Presumably, these bands are attributed to the rocking vibrations of CH2 and CH3 groups.FTIR spectra of ethanol at different temperatures: (a) 40°C, (b) 20°C, (c) −80°C, (d) −120°C, and (e) −180°C.
(a)
(b)
(c)
(d)
(e)The spectral bands in the region 2600–3800 cm−1 are more sensitive to the temperature. The intense inhomogeneously broadened spectral band 3242 cm−1 in frozen ethanol (t=-180°C) gets blue-shifted with temperature increasing, and at room temperature, it is suited at the 3354 cm−1. This band is attributable to the stretching vibrations of hydrogen-bonded hydroxyl groups, and such shift indicates a significant rebuilding of molecular structures with the temperature changing. Formation of the larger clusters in alcohols was accompanied by decreasing of wavenumber of this band, and it can be concluded that under low temperatures, the alcohol preferentially exists as large (hexamers or even bigger) clusters. Upon temperature increase, the smaller clusters become dominant, and at the room conditions, the alcohol preferentially forms tetramers and may be even trimers. Also the appearance of a new band at the frequency 3675 cm−1 at the temperature 20°C should be admitted. This band is attributed to the stretch vibration of free hydroxyl group, and its appearance indicates the formation of the chainlike clusters. The size of these chainlike clusters generally should be larger than tetramers since in general the positions of the bands attributable to the vibrations of hydrogen-bonded hydroxyl groups in chainlike hexamers and pentamers are similar to the positions of the corresponding spectral bands in ring tetramers and trimers. Under low temperatures, the band that corresponds to the stretch vibration of free O–H group cannot be observed; thus at these conditions, all clusters should have ring form.The spectral bands attributed to stretch vibrations of C–H bonds in CH2 and CH3 groups are located in 2800–3000 cm−1 region and do not undergo significant change. In detail, this region is described in [17] including the accounting of existence gauche and trans-conformers of ethanol molecule. The brief interpretation of these bands is the following: the peak at 2975 cm−1 corresponds to the asymmetric stretch vibrations of C–H bonds in CH3 group, the peak at 2927 cm−1 corresponds to the asymmetric stretch vibrations of  C–H bonds in CH2 group, the peak at 2894 cm−1 corresponds to the symmetric stretch vibrations of C–H bonds in CH3 group, and the peak at 2870 cm−1 corresponds to the symmetric stretch vibrations of C–H bonds in CH2 group. These two bands undergo temperature changes only at low temperatures, but they merge into one inhomogeneous band at room and higher temperatures.


## 3. Dataset Description

The dataset associated with this Dataset Paper consists of 5 items which are described as follows.1
FTIR spectrum of ethanol at temperature 40°C.2
FTIR spectrum of ethanol at temperature 20°C.3
FTIR spectrum of ethanol at temperature −80°C.4
FTIR spectrum of ethanol at temperature −120°C.5
FTIR spectrum of ethanol at temperature −180°C.


## 4. Concluding Remarks

The IR spectrum of ethanol undergoes pronounced changes with temperature increasing. The region of low and medium wavenumbers shows changes associated only with bands contribution to those made by O–H bending vibration. This fact serves as evidence for the rebuilding of the cluster structures in ethanol upon the transition from frozen to liquid state. This assumption is testified by significant shifting of the band in the high wavenumber range attributed to the stretching vibrations of hydrogen-bonded hydroxyl groups. After measuring the rate of this shift, it can be suggested that in the frozen state ethanol exists as large ring clusters (preferentially hexamers). In the liquid state, the smaller ring clusters (tetramers and trimers) would be dominant with inclusion of a fraction of chainlike clusters (presumably hexamers). The last can be proved by emerging of the band 3675 cm−1 at the room temperature that attributed to the stretching vibrations of free hydroxyl groups.



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*Source: 473294-2013-06-02.xml*