Investigation on the Structure, Spectral and Third-Order Nonlinear Optical Analysis of a New Organic Crystal: 4-Methylbenzylammonium Hydrogen Succinate

4-Methylbenzylammonium hydrogen succinate (4MLBAHS) was synthesized and successfully grown via a slow solvent evaporation process. The structure was solved and the space group identified as \({\text{P}}\overline{1 }\). In the 4MLBAHS molecule, 4-methylbenzylammonium and hydrogen succinate are linked by O–H···O and N–H···O bonds. Functional group vibrations were revealed through vibrational analysis. UV–Vis–NIR analysis was used to determine the transparency bandwidth range from 260 nm to 1100 nm. ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy established the carbon–hydrogen network in 4MLBAHS. In ¹H NMR, the chemical shift for the NH₂ group of 4-methylbenzylamine was revealed at 1.42 ppm. This was shifted to 8.094 ppm in 4MLBAHS, owing to the origination of the N–H···O bond between the 4-methylbenzylammonium cation and hydrogen succinate anion. The third-order nonlinear optical characteristics were analysed using Z-scan analysis. χ³ was established as 3.0879 × 10⁻⁵ esu. The presence of hydrogen bond interactions in the 4MLBAHS crystal enhanced the χ³ value.