FTIR Spectroscopic Studies on the Binary Solutions of 1 – Propanol with Xylene Isomers

Introduction

Hydrogen bond is a strong form of intermolecular interaction which is a very important intermolecular force in chemical, physical and biological processes that is being an active research area ^{1 – 8}. The H-bond is a bond between electron – deficient hydrogen and a region of high electron density. The H-bond is of the X – H…Y type, where X and Y are electronegative elements and Y possesses one or two electron pairs.  Based on the shift observed in the X – H stretching frequency, it is broadly classified as classical (red shifting) and the non – classical(red and blue shift in mostly the C – H stretching frequency) H – bonds. The relatively weaker non – conventional H – bonds also play a key role in stabilization of biological systems ^9–13.

1 – Propanol (PRO) is a well known common solvent used in chemical, pharmaceutical, cosmetic industry and biochemical research ^{14 – 18}. Fang et  al. ¹⁹ studied the overtone absorption spectrum of PRO by intracavity photoacoustic and FTIR spectroscopy in gas phase. The associative nature of PRO with water has been investigated by Max et al. ²⁰. Its electrical conductivity has been reported at different temperature ²¹ . The degree of associative properties of n-alcohols, including PRO, in carbon tetrachloride solutions by hydrogen bond formation has been examined by Wilson ²². The experimental and theoretical study on liquid PRO  in the MIR region has been carried out by Michniewicz et al. ²³ probe  the self-association through a detailed numerical analysis of the vibrational spectra. They have confirmed that the cyclic polymeric structures of  PRO are more stable.

Xylene is an aromatic compound that has  many industrial applications which exist as three different isomers namely o-xylene, m-xylene, and p-xylene. They are used as materials for polyester fibers or films,  materials for thermally stable aramid fibers or alkyd resins and plasticizers ^24-26. Hobza et al. ²⁷ have studied the various complexes of benzene, hydrogen cyanide, chloroform and given the initiation to study the possibility of C−H…π bond.  The viscosity, ultrasonic velocity and refractive indices of PRO and xylene isomers at two different temperatures has been reported by Gahlyan et al. ²⁸ and they studied the intermolecular interaction between PRO and xylene binary solutions. The objective of this work is to pay attention on the study of FTIR spectroscopic behaviour  and heteromolecular interactions in the solutions of these important molecules.

Materials and methods

Propanol (PRO) of anhydrous grade and 99.7% purity is purchased from Sigma Aldrich, USA. O-xylene (OXY), M-xylene (MXY) and P-xylene (PXY) of HPLC grade were obtained from Sisco Research Laboratories Pvt. Ltd., Mumbai, India. These chemicals were used without any further purification.

The FTIR spectra of pure OXY/MXY/PXY, Pure PRO and their binary solutions (SS1 = 0.2 PRO + 0.8 OXY/MXY/PXY, SS2 =0.4+0.6, SS3 = 0.6 + 0.4 and SS4 = 0.8 + 0.2) were recorded using Perkin Elmer FTIR spectrophotometer of resolution 1 cm⁻¹ in the 4000 to 400 cm⁻¹ region at room temperature.

Results and discussion

Figs. 1 – 3 present the FTIR spectra of pure 1-propanol (PRO), three isomers of xylene and their binary solutions. The two O – H  stretching frequencies can be observed (Fig. 1 and Table 1), one is the sharp band at 3671.7 cm^-1 and the other is a broad absorption centering around 3352.1 cm^-1. The former is assigned to free O – H of monomer and the latter is assigned to the bonded/self – associated O – H  stretching vibration of multimeric PRO units. The appearance of bonded O – H  at 3352.1 cm^-1, is close to the value of cyclic tetramer [23] and it could be interpreted as the domination of the cyclic tetramer structures in the pure PRO. This conclusion is also supported by the finding of Sum et al.[29] who studied clusters of various alcohols including PRO theoretically and Isao Akiyama et al.[30] who have done large angle X-ray studies on liquid propanol. The band due to the asymmetric and symmetric stretching vibrations of CH₃ of PRO occurred at 2972.8 cm^-1 and 2874.3 cm^-1 respectively. The CH₂  symmetric vibration is observed at 2941.1 cm^-1.The assignment of the infrared spectra of 1-propanol in the liquid state is on the basis of comparison of the spectra given in the earlier work[23].In the spectrum of (Fig. 1) pure O-xylene(OXY), 2940.6,2881.8 and 3109.8 cm^-1 are assigned the CH₃ asymmetric, symmetric and  C – H symmetric stretching bands, respectively. The observed doublet of 3067.5cm^-1 and 3020.6 cm^-1 are assigned to  C – H asymmetric vibration.

Figure 1: FTIR spectrum of (a) Pure PRO, (b) Pure OXY, (c) PRO 0.2+ OXY 0.8, d) PRO 0.4+ OXY 0.6, e)  PRO 0.6+ OXY 0.4 and (f) PRO 0.8+ OXY 0.2 binary solutions. Click here to View figure

Table 1: Vibrational bands of PRO, OXY and their binary solutions.

Vibrational Bandsa	PRO	Solutions (PRO + OXY)
		0.2 + 0.8 SS1	0.4 + 0.6 SS2	0.6 + 0.4 SS3	0.8 + 0.2 SS4
Free ν(O – H)	3671.7	3671.7	3671.4	3671.6	3671.5
Bonded ν(O – H)	3352.1	3344.7	3353.2	3344.8	3345.3
νas (CH3)	2972.0	2972.8	2972.1	2972.1	2971.0
νas (CH2)	2941.5	2941.1	2941.3	2941.1	2940.7
νs (CH2)	2881.4	2874.3	2880.5	2880.5	2880.5
ν(C – O)	1066.0	1065.0	1065.8	1066.0	1066.0
ν(C – C – O)	1056.9	1053.0	1052.6	1053.6	1056.1
Vibrational Bandsa	OXY	Solutions (PRO + OXY)
		0.2+0.8	0.4+0.6	0.6+0.4	0.8+0.2
νs (C – H)	3109.8	3095.0	3108.2	3108.9	3109.1
νas (C – H)	3067.5 3020.6	3066.8 3020.9	3066.1 3018.2	3067.3 3018.6	3069.6 3018.7
νas (CH3)	2940.6	2924.4	2926.9	2927.0
νs (CH3)	2881.8	2894.5	2891.8	2891.6	2899.4

The frequency of bonded PRO O – H band is red shifted to 3347.7 cm^-1 in sample SS1 and the trend continued in SS3(3344.8 cm^-1) and SS4(3345.3 cm^-1) solutions, whereas in SS2, the O – H band is blue shifted to 3353.2cm^-1. The red/blue shifting of bonded O – H band may be reported as the breaking of multimers/dissociation of self-associated PRO molecules through the classical H-bond interaction (PRO) O – H ⋯ O – C(PRO), in to monomers/open multimers (blue shift) and the interaction of freed O – H  with the π electrons(red shift) in the OXY. The interaction consists of higher energy will be the prominent. The free O – H is red shifted in all the binary solutions from the corresponding value in its pure form. This revealed us the formation of PRO+MXY structures that are H-bonded through the (PRO) O – H ⋯ π(OXY) hetero interaction. The red shift trend is observed also in CH₂  asymmetric and symmetric vibrational mode, confirmed the intermolecular H-bond between PRO and OXY through the non-classical H-bonding interaction (PRO) O – H ⋯ π(OXY). The ν(C – O) and ν(C – C – O) of PRO have absorbed at 1066.0 cm^-1 and 1056.9 cm^-1 in its pure form. Very feeble red shiftsin SS1,SS2, SS4 and null shift in SS3 have been observed in ν(C – O). A significant red shift has also been noticed in ν(C – C – O)  band of all the solutions.

In the PRO+MXY system, the v( O – H )  band (Fig. 2 and Table2) of self-associated  O – H bond absorbed at 3352.1 cm^-1 in pure propanol has suffered appreciable blue shifts in all the sample solutions except in SS4. The blue shift of v( O – H ) band of (PRO) O – H ⋯ O – C (PRO)  homo associated propanol structures is an indication of breaking/dissociation in the classical O – H ⋯ O – C  bond ³¹ which holds the alcohol molecules together. Moreover, the magnitude of this blue shift is very strongin SS1 which has M-xylene in high proportion whereas in the other two solutions SS2 and SS3 the magnitude of blue shift is only about 9 cm^-1 approximately. Thus, with the increasing concentration of alcohol from SS1 to SS2, the degree of dissociation is reduced and the band experiences no shift in SS4. The formation of (PRO) O – H ⋯π(MXY) and (MXY)aromatic/methyl  C – H ⋯ O – H (PRO) H-bonds can also influence the shift of v( O – H )  band but the overruling effect of classical O – H ⋯ O – C hydrogen bond has resulted in the blue shift. In the alcohol concentrated SS4 solution with the more numbers of MXY molecules, the dissociation of PRO multimer structures would have not happened and thus the O – H stretching vibration has been absorbed at the same position observed in pure PRO. On the other hand, the v( O – H ) band of free O – H of PRO monomer has been shifted from 3671.7 cm^-1 (pure PRO) to 3671.9, 3671.3, 3671.8 and 3671.7 cm^-1, respectively in SS1, SS2, SS3 and SS4 solutions. Since the blue shift of monomer band cannot be considered as the effect of dissociation, the feeble blue shift noticed in SS1 and SS3 solutions can be ascribed as the result of (MXY) C – H ⋯ O – H (PRO)  H-bond formation only. The development of (PRO) O – H ⋯π(MXY) hydrogen bond at the H-bonding site of PRO monomer could not be discussed in these SS1 and SS3 solutions since this classical bond formation would have resulted in a appreciable red shift whereas in SS2 the feeble red shift can be understood as the combined effect of both this classical and non-classical H-bonds and in SS4 solution the band absorbs at the same frequency as in pure PRO similar to behavior of bonded v( O – H ) band. Thus, in SS4 solution, the influence of MXY at O – H  H-bonding position is nearly negligible.  The formation of  (MXY)aromatic/methyl  C – H ⋯ O – H (PRO) H-bonds is again confirmed from the appreciable blue /red shifts of v_s (C – H), v_as (C – H), v_as (CH₃), v_s (CH₃) bands of MXY and these shifts of v_s (C-H), v_as (C-H) bands can also result from the dissociation of  dimers interacting with (MXY) C – H ⋯π(MXY) homoassociating H-bonds ³² in the presence of polar PRO molecules. The occurrence of other possible (PRO) C – H ⋯π(MXY) non-classical hydrogen bonds in this system can be confirmed from both the red/blue shifts of methyl and methylene groups of PRO. A significant red shift has been observed in all the solutions of ν(C – O) and ν(C – C – O ) except in ν(C – O)  band of SS3 of and ν(C – C – O) band of SS4 of stretching bands. The values in both the bands in the alcohol rich solution SS4  approach theoriginal values in their pure form indicate the occurrence of the dominant O – H ⋯O interaction.

Figure 2: FTIR spectrum of (a) Pure PRO, (b) Pure MXY, (c) PRO 0.2+ MXY 0.8, d) PRO 0.4+ MXY 0.6, e)  PRO 0.6+ MXY 0.4 and (f) PRO 0.8+ MXY 0.2 binary solutions. Click here to View figure

Table 2: Vibrational bands of PRO, MXY and their binary solutions.

Vibrational Bandsa	PRO	Solutions (PRO + PXY)
		0.2 + 0.8	0.4 + 0.6	0.6 + 0.4	0.8 + 0.2
Free ν(O – H)	3671.1	3671.9	3671.3	3671.8	3671.7
Bonded ν(O – H)	3352.1	3461.2	3361.0	3361.7	3352.1
νas (CH3)	2972.0	2971.6	2975.2	2972.3	2972.8
νas (CH2)	2941.5	2945.6	2944.5	2943.6	2940.5
νs (CH2)	2881.4	2870.9	2879.8	2883.0	2882.8
ν(C – O)	1066.0	1064.3	1064.4	1068.7	1065.7
ν(C-C-O)	1056.9	1045.0	1054.3	1054.1	1057.2
Vibrational Bandsa	OXY	Solutions (PRO + PXY)
		0.2+0.8	0.4+0.6	0.6+0.4	0.8+0.2
νs (C – H)	3106.8	3108.7	3107.9	3113.0
νas (C – H)	3027.6	3020.7	3028.7	3033.8	3039.4
νas (CH3)	2923.4	2924.8	2926.8	2927.0	2928.9
νs (CH3)	2867.5	2880.9	2892.8	2893.2	2892.0

Fig. 3 and Table3 presentthe vibrational spectrum and band assignment of pure PRO, pure PXY and PRO-PXY binary solutions, respectively. The peaks observed at 3095.9 cm^-1in the pure PXY is assigned to C – H symmetric mode whereas the asymmetric mode of C – H stretching vibration appeared as a doublet at 3048.6 cm^-1 and 3021.0 cm^-1. Further more, the observed peaks at 2924.4 cm^-1 and 2872.0 cm^-1 are assigned to CH₃ asymmetric and symmetric modes of stretching vibration. The band assignment of xylene isomers are based on data given in[33 – 35].The monomer/free O – H stretching band in the pure PRO is unaltered in SS1 and have red shifted in all other solution. The bonded O – H red shifted to 3344.7cm^-1and 3344.0 cm^-1in SS1 and SS4 respectively while blue shift have observed in the other two solutions. The shifting of lower frequencies in the monomer peak of O – H indicated that the interaction with the xylene molecule through H-bonding. It appears more likely that the polymers might have broken in to lower orders and interacted with the xylene molecules. There are the possibilities of either one interactionto take place or both might have occurred simultaneously, hence the red and blue shifts in the solutions.

The substitution order of CH₃ in the isomers of xylene didn’t influence the position of C – H and CH₃ stretching bands. The CH₃ asymmetric band of PRO has blue shifted in SS1 and SS2 but red shifted in the remaining solutions. The mixed trend is also followed in  CH₂ asymmetric band, which has suffered blue shift in the first three sample solutions and red shifted in SS4. The ν_s (CH₂ ) band is downshifted in all their binary solutions from the corresponding value in its pure form. The red and blue shifts of  CH₂ and CH₃ stretching bands might be the consequences of the hetero molecular interactions between propanol and xylene molecules which could be (PRO) O – H ⋯π(PXY)  and (PRO) C – H ⋯π(PXY) .

Figure 3:  FTIR spectrum of (a) Pure PRO, (b) Pure PXY, (c) PRO 0.2+ PXY 0.8, d) PRO 0.4 + PXY 0.6, e)  PRO 0.6+ PXY 0.4 and (f) PRO 0.8+ PXY 0.2 binary solutions. Click here to View figure

Table 3: Vibrational bands of PRO, PXY and their binary solutions.

Vibrational Bandsa	PRO	Solutions (PRO + PXY)
		0.2 + 0.8	0.4 + 0.6	0.6 + 0.4	0.8 + 0.2
	3671.7	3669.4	3671.0	3670.5	3671.3
	3352.1	3344.7	3354.1	3352.0	3344.0
	2972.0	2977.0	2972.3	2971.8	2971.8
	2941.5	2946.0	2942.7	2944.2	2940.4
	2881.4	2873.1	2880.3	2881.2	2880.8
	1066.0	1064.0	1064.2	1066.6	1066.5
ν(C-C-O)	1056.9	1047.4	1053.0	1055.5	1056.4
Vibrational Bandsa	OXY	Solutions (PRO + PXY)
		0.2+0.8	0.4+0.6	0.6+0.4	0.8+0.2
	3095.9	3094.6	3095.4	3098.5	3103.3
	3048.6 3021.0	3047.2 3021.7	3047.6 3023.2	3047.9 3022.8	3050.2 3021.2
	2924.4	2923.5	2926.9	2928.6
	2872.0	2892.0	2892.7	2893.4	2898.7

 

The vibrational bands of CH₃ asymmetric and symmetric modes of PXY have  acquired a strong blue shifts in all the binary solutions except v_as (CH₃)  in SS1 which is red shifted(2923.5 cm^-1,2892.0 cm^-1 in SS1, 2926.9 cm^-1,2892.7 cm^-1 in SS2, 2928.6 cm^-1and 2893.4 cm^-1 in SS3). The v_s (CH₃) shifted to 2898.7 cm^-1 and v_as (CH₃) band is not observed in SS4. In the doublet of v_as (C – H) the higher wave number has suffered red shifts in all the sample solution except SS4 while the lower wave number is blue shifted in all the solutions. Mixed shift is obtained in C – H symmetric band. The magnitude of the blue shift in v_s (CH₃) is high and hence these shifts suggest that the (PXY aromatic or methyl) C – H …O – C(PRO) has also been formed in this solution. The same trend of shifts in PRO-MXY of ν(C – O) and ν(C – C – O) is continued in PRO-PXY sample solutions, which indicates that the behavior of xylene isomers in the environment of PRO are the same.These shifts in the PRO and OXY/MXY/PXY binary solutions confirm the presence of (PRO)H – O ⋯π(OXY/MXY/PXY), (PRO methyl)C – H ⋯π(OXY/MXY/PXY) and (OXY/MXY/PXY aromatic/methyl) C – H … O – H (PRO) hetero interactions but with different strengths.

Conclusions

The FITR spectroscopic studies carried out on neat PRO, xylene isomers and the binary solutions of isomers reveal the following.

The pure PRO is a mixture of monomer, chain- or cyclic dimers, trimers, tetramers etc.

The cyclic tetramer structures are dominant in the pure PRO.

The MXY is a mixture of monomer and dimer but the dimer structures are unstable.

(PRO) C – O⋯H(OXY/MXY/PXY aromatic C – H or methyl , (PRO methyl and/ or  methylene)H ⋯π(OXY/MXY/PXY) and (PRO ) O – H ⋯π(OXY/MXY/PXY) H – bonds have been formed in all the solutions of PRO with each of the xylene isomers.

Conflict of interest

The authors declare that we have no conflict of interest.

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