Molecular geometries, vibrational spectra and electronic properties of biphenyl nematic liquid crystals: a quantum chemical analysis

Abstract

The present work reports the vibrational spectroscopic signature, thermodynamics, and natural bond orbital analysis (NBO) of 4-Cyano-4-pentylbiphenyl (5CB) and 4-Cyano-4'-hexyl diphenyl (6CB) liquid crystals. The optimization of both compounds (5CB and 6CB) has been performed at DFT/B3LYP/6-31++G(d, p) level of theory. Most of the vibrational modes have wavenumbers in the expected range. The comparison of vibrational frequencies of both compounds has been made. Subsequently, the different thermodynamical parameters and electronic properties have been calculated and discussed in detail. The NBO analysis has been performed to check the stability of the electronic structures. The UV visible, HOMO- LUMO, MEP, and charge analysis has been performed. Moreover, the strong electronic transition from HOMO to LUMO (97% of MO contribution) has been predicted for 5CB and 6CB at 289 nm with oscillator strengths of 0.7242 and 0.7286, respectively. The MEP plots of both 5CB and 6CB are almost the same and show that strong electrophilic substitution would take place near the N atom attached to the CN group. The energy gap of 5CB and 6CB compounds is found to be 4.65eV. Molecules with large energy gaps are less polarizable and are termed as hard molecules, having low chemical reactivity and high kinetic stability.

GRAPHICAL ABSTRACT

KEYWORDS:

• DFT
• IR
• Raman
• NBO

Acknowledgements

The authors A. K. Vishwkarma and T. Yadav are grateful to UGC India to provide financial support in form of UGC-JRF (Award Number: 1499/ (CSIR-UGCNET DEC.2018) and UGC-DSKPDF (Award Number: No.F.4-2/2006 (BSR)/PH/19-20/0069) respectively. A. Pathak acknowledges financial support from the IoE grant of Banaras Hindu University (R/Dev/D/IoE/Incentive/2021-22/32439) and financial support through the Core Research Grant of SERB, New Delhi (CRG/2021/000907). A. Pathak also acknowledges associateship of IUCAA, Pune.

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Additional information

Funding

This work was supported by University grants commission (UGC), New Delhi, India: [Grant Number 1499/(CSIR-UGCNET DEC.2018, India)]; UGC- DS Kothari Postdoc Fellowship: [Grant Number No.F.4-2/2006 (BSR)/PH/19-20/0069]; Institute of Eminance grant: [Grant Number (R/Dev/D/IoE/Incentive/2021-22/32439ï1/4‰].