Study of Vibrational Modes in 3-Methyl-2-Hydroxypyridine: Spectroscopic Analysis

Swasti Saxena1,a) and Ankit K. Srivastava2,b)

1Applied Physics Department, Sardar Vallabhbhai National Institute of Technology, Surat, Gujarat, India – 395007.

2Department of Physics – Humanities & Science, Indrashil University, Ahmedabad, Gujarat, India – 380054.

a)swastisaxenaa@gmail.comb)Corresponding author: pushpankit@gmail.com

Abstract. In this present article, a systematic study of the different vibrational modes in 3-methyl-2-hydroxypyridine molecule using various spectroscopic techniques has been presented. The resonance enhanced multiphoton ionization (REMPI) spectroscopic technique has been used for the study of the molecule in gaseous form while RAMAN and IR spectroscopic techniques have been used for the study of the molecule in condensed phase. The experimentally observed vibrational as well as torsional bands in the REMPI spectrum are in good agreement with the low frequency bands observed from RAMAN and IR spectroscopy. The mixing of various kinds of vibrational modes observed in the REMPI spectrum.

INTRODUCTION

The barrier to internal rotation about a single bond is an important tool to monitor the conformation of the selected molecules and also determines the structural as well as functional characteristics of carbohydrates, nucleic acids, proteins. Methyl groups are deeply involved in pharmalogical field [1] as their presence can easily change not only the effectiveness and nature but also the inhibition property of molecule [2]. The effect of the presence of methyl group can be easily understood by studying DNA and RNA. Though presence of a methyl group on pyrimidine ring signifies the only structural difference in DNA and RNA (thymine and uracil), but there are many other differences between these two nucleic acids which exhibit due to the presence of methyl group in the structure of DNA. The presence of methyl group leads to some biological processes such as replication, transcription, molecular recognition, and cell metabolism [3 – 5]. The internal rotation of methyl group in nitrogenous heterocyclic molecules has its importance as these molecules are one of the building block in many biomolecules. To understand the photophysical and photochemical properties of these molecules in isolation system like proteins, the study of methyl internal rotation is focused as it controls the rate of intramolecular vibrational (IVR) energy redistribution. The nitrogenous heterocyclic molecules have lone pairs in their molecular orbital. In presence of lone pairs, methyl group experiences a different electronic environment and the potential barrier. The study of internal rotation of methyl group in aromatic and heterocyclic molecules have important implications in understanding non-covalent forces governing the conformational preference in polyatomic molecules and in molecular dynamics. Hence it has important implications in chemical and biological processes. From the spectroscopic study of the nitrogen heterocyclic molecules, the change in torsional behavior of different substitutions as well as the change in periodicity of methyl torsional potential can be determined [6 – 8]. The theoretical investigation of these molecules was carried out using ab initio calculation [9]. In many nitrogenous heterocyclic molecules the n π* state is in close proximity to the ππ* state of the molecule. The location of the lowest energy nπ* singlet state relative to that of origin band is crucial for the interpretation of the photo-physical properties of the nitrogenous heterocyclic molecules as well as for the aromatic carbonyl compounds. The fluorescence yield due to these close states to the

origin band is found to be very less. Hence, it is difficult to study those states close to origin using laser induced fluorescence spectroscopy.

EXPERIMENTAL DETAILS

The molecule 3-methyl-2-hydroxypyridine was purchased from Sigma-Aldrich chemical company and used without further purification. The excitation from ground electronic state (S0) to excited electronic state was carried out using Resonance enhanced multi-photon ionization technique in the gaseous phase. The details for the experiment are provided in our previous article [6]. However, the RAMAN and FT-IR spectroscopic techniques have also used for the study of the band positions in condensed phase. The bands obtained in REMPI are in good agreement with the bands observed with RAMAN and FT-IR.

RESULTS AND DISCUSSION

Raman and IR Spectroscopy

The spectroscopic study of the molecule 3-methyl-2-hydroxypyridine has been previously discussed in our previous article [6]. All the obtained vibrational as well as torsional bands from REMPI spectroscopy were well assigned. The REMPI spectrum confirms the vibronic coupling between the nπ* and the ππ* states due to which some of the low intense bands observed near the origin band of the molecule. The FT-IR and RAMAN spectroscopic techniques have been used to study the molecule in condensed phase. The FT-IR in the far IR region and RAMAN spectrum of 3-methyl-2-hydroxypyridine is shown in Fig. 1. These experiments were performed in condensed phase of the molecule. Some of the calculated low frequency bands are in close proximity with the experimentally obtained bands in FT-IR and RAMAN spectrum of the molecule. The band position observed from the REMPI spectrum of the molecule along with the FT-IR and RAMAN spectrum is listed in Table 1.

(a) (b)

Figure 1. (a) RAMAN spectrum and (b) IR spectra in far IR region.

The observed frequencies from the REMPI spectrum of 3-methyl-2-hydroxypyridine in gaseous phase as listed in Table 1 are in good agreement with the observed from RAMAN and FT-IR spectroscopic techniques in condensed phase. All of the bands were observed due to the torsional motion of methyl group [6]. The low frequency bands at 278, 521 and 553 cm-1 were observed due to the strong vibronic coupling of the ππ* and nπ* transition states. The band at 396 cm-1 is observed due to the mixing of stretching and bending of the ring frame while the band at 501 cm-1 is due to the mixing bending as well as the torsional motion of the ring frame.

TABLE 1. Band position in far IR region and Raman spectra along with the experimentally observed frequencies.REMPI Obs. Frequency (cm-1)[4] IR (cm-1) RAMAN (cm-1)

131 135.01 135.34144 144.66 145.86233 237.24 236.75278 271.26 271.26294 295.10 294.40383 379.97 378.13396 391.54 395.81426 420.47 420.63472 476.40 470.28501 503.41 500.58521 520.77 522.88553 557.42 550.65593 586.35 589.36

CONCLUSION

The frequencies observed from the REMPI spectroscopic technique is in agreement with the obtained bands from RAMAN and FT-IR spectroscopy. The low frequency bands observed due to the mixing of different kind of vibrational modes such as stretching, bending, wagging motion. The bands at 278, 521 and 553 cm-1 confirm the mixing of the two transition states (ππ* and nπ*). However, the other observed bands are because of the different kind of motion of the molecule. The methyl group torsional motion is responsible for this type of vibronic coupling between the two states.

ACKNOWLEDGMENTS

Authors would like to thank Department of Science and Technology, India for financial support and SAIF, Indian Institute of Technology Bombay, India for the RAMAN and FT-IR. Authors would also thank to Prof. T. Kundu from Indian Institute of Technology Bombay, India and Dr. R. K. Sinha from Manipal University, India for their valuable suggestions.

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