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A quantum computation perspective on density functional theory analysis of charge transfer and nonlinear optical behavior in L-Valine methyl ester hydrochloride

R. RAJA1, RUBARANI P. GANGADHARAN2,* , R. KUMUTHA1

Affiliation

  1. Department of Physics, S.A. Engineering College (Autonomous), Chennai, India
  2. Department of Physics, Rajalakshmi Engineering College (Autonomous), Chennai, India

Abstract

L-Valine methyl ester hydrochloride (LVMEH), a promising semi-organic nonlinear optical (NLO) material, was systematically investigated using density functional theory (DFT) to elucidate its electronic structure, charge-transfer mechanism, and nonlinear optical behaviour. Geometry optimization performed at the B3LYP/6-31G(d,p) level confirmed the structural stability of the molecule in its ground state. Natural Bond Orbital (NBO) analysis revealed strong donor–acceptor interactions, with the LP(1)N2 → σ(Cl23–H24)* interaction exhibiting the highest stabilization energy (185.93 kcal mol⁻¹), indicating efficient intramolecular charge transfer responsible for enhanced molecular stability. Frontier molecular orbital analysis yielded HOMO and LUMO energies of −6.4357 eV and −0.7251 eV, respectively, corresponding to an energy gap of 5.7106 eV, which suggests an appropriate balance between chemical stability and electronic reactivity for optical applications. Molecular electrostatic potential (MEP) mapping identified oxygen and chlorine atoms as the principal electron-rich regions, while protonated hydrogen atoms constitute the electron-deficient sites governing intermolecular interactions. Time-dependent DFT calculations successfully reproduced the experimental UV–Vis absorption profile, demonstrating that the intense absorption band near 250 nm arises predominantly from HOMO → LUMO and HOMO−1 → LUMO transitions, whereas higher-energy absorptions originate from deeper occupied orbitals. The calculated first-order hyperpolarizability (β₀ = 3.136 × 10⁻³⁰ esu) is approximately 4.3 times greater than that of urea, confirming the strong nonlinear optical response of LVMEH. The theoretical findings are consistent with previously reported experimental optical and second-harmonic generation (SHG) studies, providing a molecular-level understanding of its enhanced NLO performance. These results establish LVMEH as a promising candidate for second-order nonlinear optical, photonic, and optoelectronic applications.

Keywords

DFT, NBO, NLO, MEP, Hyperpolarizability.

Citation

R. RAJA, RUBARANI P. GANGADHARAN, R. KUMUTHA, A quantum computation perspective on density functional theory analysis of charge transfer and nonlinear optical behavior in L-Valine methyl ester hydrochloride, Optoelectronics and Advanced Materials - Rapid Communications, 20, 9-10, September-October 2026, pp.461-470 (2026).

Submitted at: May 11, 2026

Accepted at: Oct. 5, 2026