**1. Introduction**

Plasticizers are polymer additives that are commonly used in packaging materials to increase plasticity [1]. Phthalic acid esters (PAEs) are the most commonly used plasticizers, which can enter the body with people's breath, diet, and even skin contact. When PAEs accumulate to a certain extent in the body for a long time, they can be harmful to human health, and will cause feminization of men, increase the risk of breast cancer in women, and leads to deformity and cancer [2]. In recent years, the frequent occurrence of excessive PAEs in food has attracted widespread concern and great importance from the government and society. In 2011, Taiwan's Food and Drug Administration found high concentrations of Di(2-ethyl)hexyl phthalate in a batch of probiotic ingredients [3]. Since then, PAEs have become known to the public. In 2012, the Plasticizer incident of Chinese Baijiu caused a sensation [4]. The incidents of artificially added PAEs in milk tea in 2017 and excessive PAEs in Ladue Blue Joe walnut oil in 2019 are even more controversial [5]. Therefore, it is very necessary to effectively monitor PAEs in food to protect people's health.

In recent years, Raman spectroscopy has been widely used in the field of food safety [6–8] and gradually applied to the detection of PAEs due to its characteristics of no sample pretreatment, fast detection speed, and response to molecular fingerprint information. Wu et al. [9] prepared homogeneous AuNPs films for the detection of Di (2-ethyl)hexyl phthalate in sorghum wine. Zhou et al. [10] formed Ag@Fe3O4@PEI nanoparticles, then modified them with cyclodextrin (β-CD). Finally, 1.3 mg/kg of BBP in white wine was able to detect using this substrate. Cao et al. [11] prepared Au-Ag-S nanostructured substrates using a one-pot method and used them for the detection of Di(2-ethyl)hexyl phthalate in juice. Wang et al. [12] and Wu et al. [13] used 2D silver plate and AuNPs as enhanced

**Citation:** Sun, T.; Wang, Y.; Li, M.; Hu, D. Raman Spectroscopic Study of Five Typical Plasticizers Based on DFT and HF Theoretical Calculation. *Foods* **2023**, *12*, 2888. https:// doi.org/10.3390/foods12152888

Academic Editor: Ana Teresa Sanches-Silva

Received: 30 June 2023 Revised: 22 July 2023 Accepted: 24 July 2023 Published: 29 July 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

substrates to detect PAEs in edible oils. However, few of these studies incorporate structural theory, whereas identifying PAEs requires the incorporation of structural theory. Structure theory includes the semi-empirical method, density functional theory (DFT), and ab Initio [14,15]. It can help to understand the experimental results [16–18]. Ji et al. [19] assigned the Raman vibrations to the eight Raman peaks of the Dimethyl phthalate by DFT 6-31+G(d) calculations. Liu et al. [20] simulated the theoretical Raman spectra of di(2-ethyl)hexyl phthalate, dibutyl phthalate, and diethyl phthalate using DFT 3-21G; then, the theoretical Raman spectra with the corresponding experimental Raman spectra were compared and analyzed. Qiu et al. [21] calculated the Raman spectra of dimethyl phthalate, dibutyl phthalate, Di-n-octyl phthalate and their derivatives in the gaseous environment using DFT B3LYP 6-31g(d), which contributed to the studies of PAEs. Xu et al. [22] used DFT 6-31G(d) to calculate the theoretical spectra of di(2-ethyl)hexyl phthalate, dibutyl phthalate, and butyl benzyl phthalate. The theoretical Raman spectra were consistent with the experimental spectra, and the Raman vibrations were assigned to Raman peaks. Zuo et al. [23] used molecular dynamics simulations and DFT to reveal inter-molecular interactions of phthalic acid esters.

There is no comparison of theoretical Raman spectra of PAEs calculated by different theoretical methods and basis sets in the existing studies. The explanation of basis sets can be found in the second paragraph of Section 2.3. The theoretical Raman spectra calculated by some theoretical methods will have many spurious peaks, which are not found in the experimental Raman spectra; this will interfere with the analysis of the experimental data and cause errors. In addition, existing studies have only assigned the Raman vibrations to individual Raman peaks, but not to all major Raman peaks. In this research, the theoretical Raman spectra of five typical PAEs were simulated by different theoretical methods and basis sets, and were compared with the experimental Raman spectra in order to obtain the most applicable theoretical method and basis set for PAEs, which can effectively reduce the influence of spurious peaks on the analysis of PAEs detection in Food or other products. Also, all Raman peaks of the five PAEs were assigned according to the theoretical and experimental Raman spectra.
