Raman high-pressure study of butane isomers up to 40 GPa

Raman spectroscopy studies on n and i-butane were performed at pressures of up to 40 GPa at ambient temperatures using the DAC technique. Normal butane undergoes two phase transitions at 1.9(5) GPa and 2.9(5) GPa and isobutane at 2.7(5) GPa and 3.5(5) GPa. These phase transitions were identified based on observations of the splitting Raman modes and the appearance or disappearance of particular Raman peaks. Our results demonstrate the complex, high-pressure behavior of butane isomers.Raman spectroscopy studies on n and i-butane were performed at pressures of up to 40 GPa at ambient temperatures using the DAC technique. Normal butane undergoes two phase transitions at 1.9(5) GPa and 2.9(5) GPa and isobutane at 2.7(5) GPa and 3.5(5) GPa. These phase transitions were identified based on observations of the splitting Raman modes and the appearance or disappearance of particular Raman peaks. Our results demonstrate the complex, high-pressure behavior of butane isomers.


I. INTRODUCTION
Butane is a gaseous saturated hydrocarbon, the fourth member of the alkane homologous series, with a molecular formulae C 4 H 10 and two structural isomers -normal butane (unbranched) and isobutane (or 2-methylpropane).Due to their different structure, these two isomers are totally different as concerns chemical and physical properties as well as their different occurrence and application in industry.
Despite the number of differences, both butanes experience high levels of petrochemical demand and are present not only in the natural gas, but are found in various geological settings, 1 Earth's atmosphere 2,3 and also potentially could be found on outer planets and their satellites. 4,57][8] High-pressure examinations of butanes would also clarify the problem of petroleum systems in the Earth's interior, their generation and retention in the geological structures.
Several spectroscopical studies of butanes were conducted in the middle of previous century, 9-12 including a deep and careful analysis of conformational isomers and the difference between isobutane and butane vibrational bands, but no high-pressure studies were performed, except the work 13 where butane was studied up to 5 GPa.Surprisingly, butane is the last light alkane to be examined with the use of high-pressure techniques, while methane, [14][15][16] ethane 17 and propane 13,18 have been studied extensively.0][21] The increasing interest in hydrocarbon occurence in the universe and origin of this group of compounds makes this study beneficial.Here, we present a paper which focuses on Raman spectroscopy study of butanes at ambient temperatures and under pressures of up to 40 GPa with some modifications of the previous assignments.
symmetric BX-90-type diamond anvil cells (DAC) equipped with synthetic, CVD-type IIa diamonds with a culet size of 250 µm.The rhenium gasket was compressed to a thickness of 35 µm.With the help of laser techniques, a hole of 110 µm in diameter was drilled in the center to form a cylindrical sample chamber.The Raman spectra were obtained by exciting with a He-Ne laser (632.8 nm excitation) and acquired using a LabRam spectrometer with 2cm -1 spectral resolution.For pressure determination, a ruby chip approximately 5 µm in diameter was placed into the pressure chamber near the center of the hole, and the change in the pressure was determined using the shift in the ruby luminescence line.The uncertainties in the Raman peak positions were ±1 cm −1 .A decision was taken to measure the pressure in the DAC before and after the collection of the Raman spectra in order to determine the hysteresis value in the cell.As a result, the hysteresis value does not exceed the error in the pressure measurement using the ruby luminescence line (±0.5 GPa).Raman spectra were collected both for compression and decompression experiments to reveal the differences in the butanes' phase behavior.To avoid the wrong interpretation of results, Raman spectra were collected in three different places in the pressure chamber.

III. RESULTS AND DISCUSSION
In Raman spectra of hydro-carbons the most useful information is located at the 3200-2800 and 1700-300 cm -1 regions.Raman spectra of n-alkanes have presumably the same representation due to the methyl (-CH 3 ) and methylene (-CH 2 -) bands -on the contrary, Raman spectra of n-alkanes and isoalkanes show a strong difference because of the branched character of the latter.Another major difference between normal and isobutane which could be seen on spectra is that n-butane is the simplest hydrocarbon in which rotational isomerism occurs.Normal butane introduces an additional possibility that the carbon skeleton could lead to the existence of more than one configuration due to rotation around the C-C bonds.The problem of the influence of molecular conformation on spectral properties of organic compounds (especially normal paraffins) has earned a certain interest among scientists, 22,23 moreover, vibrational spectroscopy provides the broadest data on conformational isomers' behaviour.It is common knowledge that hydrostatic pressure affects the changes of the intra and intermolecular forces.Furthermore, the conformational changes in the high-pressure region plays a certain role in the investigation of different phenomena observed in the science of high-molecular compounds and polymers, especially in the biological field. 22,23Butane has three conformers with a difference in the C-C-C-C torsion angle -one trans (or anti) and two gauche (mirror images).As stated in some studies, several lines of gauche forms in the Raman spectra of liquid alkanes disappear during the solidification because of the reversion to the trans form in the crystal statewhich could be useful for study of lipid bilayer systems. 24Of beneficial importance is the fact that the spectral investigations indicate that each configuration of n-butane has its own set of vibrational frequencies.
Combining all of the experimental and calculated data it could be concluded that the butane molecule possesses 36 distinct vibrational modes, but no experimental studies have been done in the high-pressure region.Moreover, spectroscopical study of butane's high-pressure behaviour has never been performed, furthermore the interpretation of solid butane's spectra could be a challenging task due to weaker lines and scattering background.As could be seen from the Raman work (Figure 1) no single configuration of n-butane will fit the spectra, since many more lines are observed than are to be expected.
Theoretically, isobutane also posesses 36 distinct vibrational modes, but according to the recorded spectra, there are notable differences in n-butane in all of the 3 regions (Figure 2), mentioned previously.This difference could be explained by symmetry of the i-butane molecule which leads to the degeneration of several vibrations.For example, in the region of C-H stretching vibrations (2800-3300 cm -1 ), vibrations of iso-butane are more distinct because of -CH 3 fragment domination in the molecule (the same assessment applies to CH 3 bending region).The vibrations from the region of 1100-1300 cm -1 of isobutane are more intense and release in the triplet while n-butane gives only singlet in this diapason -this is due to the complicated interaction of tert-butyl fragments in isobutane.Tertiary wagging of isobutane (v 15 ) is another striking difference of these compounds.Isobutane has a significantly smaller number of vibrations on the spectra that normal butane because of the absence FIG. 1. Raman spectra of normal butane obtained at ambient temperature and at high pressures upon compression from 1.4(5) to 40.2(5) GPa.For clarity, the spectra have been divided into three regions: (a) 300-1300 cm -1 (C-C stretching and methyl rocking region), (b) 1400-1600 cm -1 (methyl bending region), and (c) 2800-3300 cm -1 (C-H stretching region).The 1250-1350 cm -1 and 2100-2700 cm -1 regions are dominated by diamond first-second-order peaks and have not been shown.As it could be seen from the spectra, the specific vibrations of gauche-form have totally disappeared starting from the 3.6(5) GPa, which shows the phase transition and total solidification of n-butane.At pressures between 1.9(5) and 2.4(5) GPa we can see a considerable change of spectrum appearance, especially in the 2800-3300 cm -1 region, where all the C-H stretching vibrations became blurred in comparison with 1.4(5) GPa.This phase transition was also discovered in this paper 13 using XRD technique at 1.8 GPa.Another phase transition has appeared in the region between 2.4(5) and 2.9(5) GPa, where we can see double splitting of v 11t mode, dramatic increase of v 10 and v 9 intensity, the disappearance of v 14g and v 32g modes, while in the region of CH 3 bending region, a number of peaks became distinct.The decompression spectra of n-butane also show good correspondence with the compression experiment (see supplementary material S1).
of its conformational isomers.][27] From the vibrational assignments of the simpler molecules from the alkane homologous series such as methane, ethane and propane and from a comparative examination of the spectra of a large number of other organic molecules, certain general frequency regions have come to be recognized.
In this study, we classify the Raman modes of butanes into several regions.

A. C-C-C deformation (longitudinal acoustic mode < 650 cm -1 )
This region of spectra is quite weak in the case of n-alkanes both in the liquid and solid phases however, while performing the measurement, it was possible to record several vibrations.According to Ref. 11 this region is dominated by two peaks of C-C-C bending, one for trans configuration (v 11 430.1 cm -1 ) and one for gauche (319.5 cm -1 v 17 ) however, we observed only trans-configuration peak, which is splitting to two bands after the second phase transition (Fig. 3).As for the isobutane, it is expected that this molecule has two deformational vibrations -symmetrical and unsymmetrical (v 8 and v 19 respectively), but during the current study only v 19 was found on the spectra.

B. C-C skeletal stretching region and CH/CH 2 /CH 3 rocking region (600-1350 cm -1 )
In the region in which the skeletal stretching vibrations of normal paraffins are expected (approximately between 700 to 1100 cm -1 ) n-butane has two bands of appreciable intensity -v 10 (v 15 for gauche) and v 9 (v 13 for gauche).Relatively strong v 15 and v 13 Raman mode in the liquid state suggests that gauche rotamer is present only in the liquid state.The disappearance of these modes with the phase transition and increase in intensity of v 10 and v 9 mode suggest that n-butane in the condensed AIP Advances 8, 115104 (2018) FIG. 2. Raman spectra of isobutane obtained at ambient temperature and at high pressures upon compression from 1.6(5) to 37.6(5) GPa.For clarity the spectra have been divided into three regions: (a) 300-1300 cm -1 (C-C stretching and methyl rocking region), (b) 1400-1600 cm -1 (methyl bending region), and (c) 2800-3300 cm -1 (C-H stretching region).The 1250-1350 cm -1 and 2100-2700 cm -1 regions are dominated by diamond first-second-order peaks and have not been shown.The spectra of isobutane demonstrate several phase transitions -at pressure between 1.6(5) and 2.7(5) GPa the region of C-H stretching became less sharp, the same as in the region of methyl bending.Another phase transition was noted in the interval from 2.7(5) to 3.5 (5) GPa where the spectrum changes dramatically -methyl bending vibrations became sharp and distinct, while CH 3 -rocking modes v 6 \v 16 change its structure from triplet to doublet.With pressure increase no substantial changes were registered.As it could be seen from the spectra, the specific vibrations of gauche-form totally disappear starting from 3.6(5) GPa, which shows the phase transition and total solidification of i-butane.The decompression spectra of isobutane also correspond well with the compression experiment (See the supplementary material S2).
phase (like all n-alkanes) prefers to be in trans conformation.Moreover, based on the following study 27 in the whole region (until the first-order diamond peak) seven fundamentals are allowed in the Raman spectrum including two further types of vibration, C-C stretching vibrations and motions involving changes of HCC angles.One of the most representative vibration is C-C stretching mode v 10 , located at the value of 840 cm -1 .At the liquid state the same vibration of the gauche-configuration could also be observed (∼830cm -1 , v 15 ) which disappears with the phase transition (Figure 4).At the value of 3.6 GPa, this peak splits to the doublet, but with the pressure increase only the singlet was presented on the spectra.Another C-C stretching vibration v 9 (v 13 for gauche) is located at ∼1060 cm -1 and has weaker intensity than v 10 .The C-C stretching of isobutane is represented by v 7 and v 18 non-degenerate skeletal vibrations with no substantial changes during the pressure increase (Figure 5).
When moving to the methyl rocking frequencies, two observations could be noted directly: 1.While examining n-butane and higher n-paraffins, the terminal CH 3 groups are strongly separated so that very minor interaction between them is to be expected that makes symmetric and antisymmetric vibrations indistinguishable, and very little separation in their frequencies will occur.2. Secondly, the studies on propane 6,12,18 show that the difference to be expected between in-plane and out-of-plane CH 3 rocking frequencies is small (∼20cm -1 ).
The band which was noticed on the spectra around ∼800 cm -1 (Figure 6) could possibly be assigned to the CH 2 -rocking mode. 7Another viewpoint on this vibration is that it is caused by skeletal vibrations of non-planar forms and not to CH 2 rocking vibrations. 10H 3 -rocking modes of gauche-conformation are presented in the low-pressure region only (Figure 7) of the spectra which indicates the solidification of n-butane, on the contrary, the CH 3rocking of trans-conformation is presented all over the spectra with no substantial changes with pressure increase.CH 3 -rocking of isobutane possesses many more frequencies than n-butane due to complex CH 3 -interactions in the tert-butyl group (Figure 8).CH 2 -twisting modes (Figure 9) of n-butane are located closely to a diamond first-order peak and strating from 14 GPa, the intense diamond band is overlapping in this mode, however in the low-pressure region we could see a disappearance of the gauche vibrational mode, while the trans mode could be distinguished until the overlapping of the diamond mode.

C. Methyl bending region (1350-1600 cm -1 )
Butane's methyl bending mode region lays between 1400-1600 cm -1 .Because of the complex interaction between CH 3 asymmetric and symmetric deformational modes and CH 2 scissoring, it FIG.8.The Raman modes associated with the CH 3 wagging\rocking mode (v 6+16 ) of iso-butane as a function of pressure.could be difficult to refer each band to a particular mode.In the case of n-butane, this interval is characterized by intense asymmetric CH 3 deformational mode centered at ∼1460 cm -1 .After total solidification of n-butane the intensities of the bands in this region were decreased.Isobutane, on the contrary, here has a relatively high number of vibrational frequencies which are due to three -CH 3 fragments in the molecule, which is why isobutane's spectra are rich with various CH 3 vibrations and their combinational modes (Figures 10-13).The last vibration which should be considered is the tertiary wagging (Figure 14).The double degenerate CH-rocking mode of isobutane is a subject of doubt among alkane spectroscopical studies -this mode has been attributed to 900 cm -110 or to 1200 cm -1 and 1095 cm -1 , 12 but none of them were FIG.14.The Raman modes associated with the CH wagging mode v 15 iso-butane as a function of pressure.found on the spectra obtained.Based on the previous investigation of propane, 18 the CH 2 wagging mode was found at the value of ∼1330 cm -1 , the same mode of isobutane (v 15 ) mode could possibly give a singlet at the value of ∼1340 cm -1 which is splitting to the doublet at a pressure of 6.0 (5) GPa and then returning to the singlet type of peak starting from 11.9 (5) GPa.

D. C-H stretching region
The complex vibrations involving principally the stretching of C-H bonds give rise to several Raman shifts in the diapason from 2800 to 3200 cm-1 .However, these butane vibrations are not that distinguishable (Figure 15), nevertheless, in terms of this study it was possible to find four types of stretching -methyl symmetric (v 2 )\asymmetric(v 20 +v 1 ) and methylene symmetric\asymmetric(v 13 +v 21 ).Unfortunately, strong interaction of modes and various overtones and combinational modes make these frequencies difficult to resolve.
The Raman vibrations in this region (Figure 16) have a striking contrast with n-butane's spectra first of all because of the absence of methylene vibrations in the case of isobutane, instead of a -CH 2 bond on the spectra of iso-butane, the tertiary CH bond appears.
The phase diagrams outlined for n-butane is shown in Fig.   FIG.17. Phase diagrams of n-butane: the boiling point at 0.1 MPa and 273 K (green circle), 28 melting point at 0.1 MPa and 135 K (orange diamond), 29 and the critical point 3.79 MPa and 425.1 K (yellow circle) 30 as well as freezing point 1.60 GPa and 295 K (orange triangle) + 2.50, 2.60, and 3.44 GPa (orange circles) obtained from optical observation of n-butane melting in the DAC − spectroscopic pressure calibration and a thermocouple temperature measurement (orange circles) and diffractometric determination of n-butane (blue squares). 13Violet circles are the phase transitions, determined during current investigation.For the reference Equations of State for the thermodynamic properties of fluid phase n-Butane and Isobutane up to 575 K/69 MPa and up to 575 K/35 MPa respectively the following work is recommended. 3730]37 Unfortunately, isobutane's phase behavior was studied no so extensively as n-butane's that's why the phase diagram in the high-pressure region wasn't developed.

IV. CONCLUSION
In summary, high pressure Raman spectroscopic studies on normal and isobutane were conducted in the range of up to 40 GPa.It was found that n-butane undergoes two phase transitions at 1.9 (5) GPa and 2.9 (5) GPa as well as i-butane undergoes the same number of phase transitions at 2.7 (5) GPa and 3.5 (5) GPa respectively.The Raman behavior of butane isomers was reported, discussed and compared to other alkane spectroscopical studies (Table I).The table includes only the data on phase transitions which are occurring at room temperature, while the data on high-pressure and high temperature studies is presented in the review of Kolesnikov et.al. 36 As could be seen, the phase transitions affect the behavior of the vibrational mode in the same way as in lighter or higher alkanes [31][32][33][34][35]37 (Table I). Forall of the alkanes mentioned in the table the following statement could be made -Raman bands gradually shifted to higher frequencies with increasing pressure because of reduction in bond length and atomic distances under higher pressure.Starting from butanes after the first phase transition all of the modes attributed to the gauche conformations totally or partially disappeared with the pressure increasement.The best indicators for phase transitions are the modes from C-C stretching and C-H stretching region, first of all due to they high intensity as well as to simplicity and evidence of the changes with the phase transitions.
It was found that isomeric structure in the case of butanes does not affect the number or character of phase transitions.Moreover, it could be concluded, that no correlation was observed between the length of the hydrocarbon chain and the quantity of phase transitions.Isobutane's and normal butane's spectra were compared at a whole range of pressures, the difference between these two compounds has been saved during the pressure increase.All of these modes became more distinct during the phase transition.

iso-Butane
This study: -1) liquid-solid 2.7 (5) GPa ) mode in the liquid state suggests that large numbers of gauche rotamers are present in the liquid state but with the phase transition it disappears, while C-Cstr of trans mode increases in its intensity (1130 cm -1 ) and another mode appears at 1060 cm -1 .
1) liquid-solid ∼1.2-1.5 GPa 1) Above 10 kbar, with increasing pressure, significant changes occur in the environment around the CH 3 group rather than around the CH 2 groups.The same tendency of mode change like it could be seen for all of the light alkanes -with the liquid-solid transition all of the peaks become sharper, more intense and distinguishable.
2) solid-solid ∼ 7.5 GPa 19 2) solid-solid transition ∼3 GPa 35 2) Definitive conversion of gauche to trans conformation in the solid phase.Disappearance of 197 cm -1 (lattice mode).Disappearance of 278 and 295 cm-1 methyl torsional modes.Disappearance of 367, 399, and 568 cm -1 and the appearance of a new mode at 1144 cm -1 .An increase in intensity of the 1148 cm -1 mode with a decrease in intensity of the 1090 cm -1 mode.CH 2 and CH 3 stretching modes remain the same.

FIG. 3 .
FIG. 3. The Raman modes of n-butane (left) and i-butane (right) associated with the deformational C-C-C bending as a function of pressure.

FIG. 5 .
FIG.5.The Raman modes associated with the C-C stretching of iso-butane v 7 (left) and v 18 (right) as a function of pressure.

FIG. 6 .
FIG.6.The Raman modes associated with the CH 2 -rocking mode (v 25 ) as a function of pressure.

FIG. 9 .
FIG. 9.The Raman modes associated with the CH 2 twisting mode (v 23 trans and v 11 gauche) of n-butane as a function of pressure.

FIG. 10 .
FIG.10.The Raman modes associated with the CH 3 symmetrical deformation mode (v 6\32 ) of n-butane (left) and (v 14 ) of iso-butane (right) as a function of pressure.

FIG. 15 .
FIG. 15.CH 2 and CH 3 stretching Raman modes of n-butane as a function of pressure.

17
FIG.16.CH 2 and CH 3 stretching Raman modes of iso-butane as a function of pressure.

8 , 1 (
115104 (2018) TABLE I. Brief characteristic of known phase transitions among the alkane hydrocarbons.-solid (phase I, fcc) 1.7 GPa; 1) liquid-solid (phase I, fcc) 1.7 GPa; 1) Phase I-Phase A ∼5 GPa Each of these phase transitions could be clearly observed by the change of C-H symmetric stretching mode, v 1 (both for Raman and IR) or v 3 asymmetric stretching mode, for example, the A-B phase transition could be seen by splitting of the C-H symmetric stretching mode, v 1 , in the Raman and IR spectra.2) phase I -phase A (rhombohedral structure or tetragonal structure) 5.2 GPa; phase A -phase B (cubic or hcp) 10-18 GPa; 2) phase I -phase A (rhombohedral structure or tetragonal structure) 5.2 GPa; 2) Phase A -Phase B ∼10 GPa 3) pre-B phase with structure close to phase B but rotation of molecules close to the phase A ∼12 GPa 3) phase A -phase B (cubic or hcp) 10-18 GPa; 3) Phase A-Phase B 19 GPa 4) phase B -phase HP 1 (cubic or hcp) 25 GPa; 4) phase B -phase HP cubic or hcp) 25 GPa; 4) Phase SC (simple cubic) -Phace cHP 69-94 GPa 16 strong C-C stretching (1090 cm-1 its intensity and disappears above 9.1 GPa.In the region of Skeletal C-C Stretching Region all of the gauche-conformers modes are disappering.Change in the slope in the CH a considerable mode broadening as well as changes in their intensity ratio could be observed. 3) solid-solid transition ∼ 12.3 GPa.32 AIP Advances 8, 115104 (2018)