Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
DFT, FT-Raman and FT-IR investigations of 5-o-tolyl-2-pentene
Graphical abstract
Highlights
► The FT-IR and FT-Raman spectra of 5-o-tolyl-2-pentene (OTP). ► Geometric isomers of OTP. ► DFT/B3LYP level is able to provide satisfactory results for predicting vibrational wavenumbers. ► The trans isomer is supposed to be the most stable form of OTP molecule.
Introduction
OTP has been part of many different scientific studies. For example, there are spectroscopic studies and cycloisomerization of OTP absorbed on many different zeolites which are applied in drying of process air, CO2 removal from natural gas, CO removal from reforming gas, air separation, catalytic cracking, and catalytic synthesis and reforming [1], [2], [3], [4].
Density functional theory (DFT) has big popularity as a cost effective general procedure for studying the physical properties of molecules. Unlike Hartree Fock theory, DFT recovers electron correlation in the self-consistent Kohn–Sham procedure through the functions of electron density, so it is a cost effective and reliable method [5], [6], [7], [8], [9]. The DFT/B3LYP model exhibits good performance on electron affinities, excellent performance on bond energies and reasonably good performance on vibrational frequencies and geometries of organic compounds [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15].
Even though, OTP has wide applications in zeolite science, there is no detailed information present in literature about its molecular structure and spectroscopic properties. A detailed quantum chemical study will aid in making definitive assignments to the fundamental normal modes of OTP and in clarifying the obtained experimental data for this molecule. Furthermore, the presented data as theoretically and experimentally may be helpful in context of the further studies of OTP in example for some catalyst purposes. The commercial product as OTP is sold as a mixture of cis and trans isomers; therefore, the aim of the study is also to investigate the most stable isomer of OTP. In this work, FT-IR and FT-Raman spectra with the vibrational assignments of OTP have been reported. The vibrational frequencies and structural parameters of OTP are also calculated with PED data for the most stable isomer at B3LYP level of theory using the 6-31G(d) and 6-31++G(d,p) basis sets. The results of the theoretical and spectroscopic studies are reported here.
Section snippets
Experimental
A commercial sample of OTP was purchased (Aldrich, 98%) and used without further purification. FT-MIR spectrum of OTP was recorded in the region of 4000–400 cm−1 with Perkin–Elmer FT-IR 2000 spectrometer at a resolution of 4 cm−1. FT-FIR spectrum was recorded with Bruker Optics IFS66 v/s FTIR spectrometer with the resolution of 2 cm−1 in the spectral region of 400–10 cm−1. The Raman spectrum was obtained using a Bruker Senterra Dispersive Raman microscope spectrometer with 532 nm excitation from a 3B
Calculations
Many possible conformers could be proposed for OTP (Fig. 1), but here the discussion was confined for trans and cis isomers of the title molecule. They are considered as cis and trans positions according to the same and opposing directions of the substituent groups in surrounding double bond between C9 and C10 atoms of OTP (Fig. 1). For the calculations, two forms of OTP were first optimized by B3LYP with 6-31G(d) and 6-31++G(d,p) basis sets in the gas phase. Trans isomer was found more stable
Geometrical structures
To study the vibrational frequencies, it is essential to examine the geometry of any compound as small changes in geometry can potentially cause substantial changes in frequencies. Gibbs free energies, relative stability, equilibrium constant and mole percent of the optimized geometries in the gas phase of two conformations of OTP with B3LYP/6-31G(d) and B3LYP/6-31++G(d,p) are given in Table 1. Regarding the calculated free energies for gas phase, trans isomer is more stable than cis by 2.42
Conclusion
The experimental and theoretical vibrational spectroscopic investigations of OTP are successfully performed by using FT-IR, FT-Raman and quantum chemical calculations. Results of gas phase energy calculations indicate that trans form is the most stable conformer of OTP. Any differences observed between the experimental and calculated wavenumbers could be due to the fact that the calculations have been performed for single molecule in the gaseous state contrary to the experimental values in the
Acknowledgements
The authors are very grateful to Anadolu University for all the experimental measurements.
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