A Study on the Electric Field Degradation of Common Pollutant Gases in Archive Rooms Based on Density Functional Theory

Abstract

According to the “Technical Specification for Air Quality Testing in Archives Repositories,” air pollutants in archives can be categorized into exogenous and endogenous pollutants. Common exogenous pollutants include sulfur dioxide (SO₂), nitrogen dioxide (NO₂), ozone (O₃), and hydrogen sulfide (H₂S), while endogenous pollutants mainly consist of formaldehyde (HCHO) and acetic acid (CH₃COOH). This study combines external electric field technology with density functional theory (DFT) and the B3LYP method to theoretically analyze the spectral characteristics and degradation mechanisms of these six pollutant gases. Molecular models of the six gases were constructed using Gaussian software. The configurations of five pollutant gas molecules (SO₂, NO₂, O₃, H₂S, and HCHO) were optimized using the B3LYP/6-31G(d) basis set, while the configuration of acetic acid was optimized using the B3LYP/3-21G basis set, yielding their stable structures and spectral information. The study found that characteristic peaks in the spectra shifted under the influence of an electric field. Additionally, by scanning the potential energy surfaces of selected molecular bonds under varying electric field strengths along specific directions, the required external electric field strengths for the degradation of the six common pollutant gases in archives were determined as follows: 0.1050 a.u. for SO₂, 0.0975 a.u. for NO₂, 0.0925 a.u. for O₃, 0.1000 a.u. for H₂S, 0.1500 a.u. for HCHO, and 0.0705 a.u. for CH₃COOH. The results clarify the degradation thresholds of these six pollutant gases under an external electric field. The findings indicate that acetic acid (0.0705 a.u.) and ozone (0.0925 a.u.) are highly sensitive to electric fields, while formaldehyde requires the strongest electric field (0.1500 a.u.) for degradation. These results provide a reference and theoretical foundation for electric field-assisted degradation technology targeting pollutant gases in archives.

1. Introduction

Air quality is directly related to human survival and health. According to the newly released Technical Specifications for Air Quality Testing in Archives [1], air pollutants in archives can be divided into exogenous pollutants and endogenous pollutants [2]. Exogenous pollutants mainly include sulfur dioxide, nitrogen dioxide, ozone, hydrogen sulfide, and particulate matter, while endogenous pollutants mainly come from carrier materials, building materials, construction equipment, insect and mildew prevention drugs in the archives, and the main pollutants include formaldehyde, acetic acid, volatile organic compounds, the total number of colonies and radon. These pollutants not only pose a threat to human health but also cause serious damage to the preservation of archives [3,4,5]. Exogenous pollutants are oxidizing and corrosive, which can erode the material of the archive carrier, leading to oxidation and acidification reactions in paper archives, and accelerating the aging of archives [6]. Endogenous pollutants, such as acetic acid (also known as acetic acid), are more common in the air of archive warehouses, especially in film warehouses and traditional paper archive warehouses. The presence of acetic acid will aggravate the aging and decomposition of archival materials, which seriously threatens the long-term preservation of archives. Therefore, ensuring the monitoring and management of air quality in the archives warehouse is crucial for the protection and storage of archives.

The degradation of polluting gases remains a critical research focus. Industrial emissions, such as those from municipal waste incinerators, require complex multi-stage treatments to ensure the effective elimination of hazardous molecular species and particulates [7]. To enhance purification efficiency, recent studies have systematically reviewed advanced technologies, including photocatalytic nanotechnology for VOC removal [8] and emerging single-atom catalysts for gas detection and purification [9], with particular emphasis on stability and enhancement mechanisms. Complementing these theoretical overviews, mechanistic models have been developed to simulate specific processes like the UV degradation of H2S and NOx [10], while experimental works have demonstrated the efficacy of heterojunction materials, such as g-C3N4/CeO₂ composites, in degrading automobile exhaust under various light conditions [11]. Beyond degradation, novel approaches like “zeolitic ice” are also being explored for the purification and safe storage of critical gases, offering new pathways toward net-zero emissions [12].

Currently, external electric fields play an important role in the research on pollutant degradation. External electric fields can induce changes in molecular structure and properties [13,14]. Han Boyuan et al. employed density functional theory (DFT) with the B3LYP/6-31G++(2d, p) basis set to deeply explore the influence of an external electric field ranging from 0 V·nm⁻¹ to 15.42 V·nm⁻¹ on the molecular structure and dissociation properties of dimethyl sulfone [15]. The structural properties, UV spectrum, and dissociation mechanism of trichlorotrifluoroethane (CFC-113A) under external electric fields were investigated via density functional theory, identifying the C-Cl bond as the primary cleavage site and the Cl atom as the electrophilic reaction center [16].

It should be noted that the threshold electric fields obtained in this work are derived from idealized single-molecule calculations and correspond to theoretical critical field strengths for barrierless bond dissociation along selected coordinates. When converted into macroscopic units, these values are far higher than the dielectric breakdown strength of air. Therefore, they should not be directly interpreted as practical operating electric fields for archive air-purification systems. Instead, they are more appropriately understood as comparative indicators of the intrinsic sensitivity of different pollutant molecules to external electric fields. In realistic applications, air breakdown, discharge processes, and field nonuniformity must be considered. Moreover, excessively strong electric fields may adversely affect archive materials, including paper, inks, and polymeric components, through local heating, electrical damage, or secondary chemical reactions.

In the present study, the critical electric field strength is operationally defined as the field at which the potential energy barrier along the selected bond elongation coordinate disappears in the calculated potential energy scan. This criterion is used as a theoretical indicator of field-induced barrierless bond dissociation tendency along specific molecular coordinates, rather than as a complete mechanistic proof of molecular degradation. A more rigorous mechanistic characterization, including transition-state searches, intrinsic reaction coordinate analyses, and identification of final dissociation products, would be valuable in future work.

There is no literature report on the systematic study of the electric field degradation of common polluting gases in archives. In this paper, the infrared, Raman spectra and potential energy surfaces of six common polluting gases (sulfur dioxide, nitrogen dioxide, ozone, hydrogen sulfide, formaldehyde, acetic acid) commonly found in archives under the external electric field were studied by adding an external electric field, using density functional theory (DFT) and B3LYP methods, and the electric field strength required to degrade these gases was obtained, which will provide a theoretical basis for the spectral research and degradation of common polluting gases in archives.

The electric-field-induced weakening or dissociation of chemical bonds has been reported previously. Therefore, the novelty of the present work does not lie in the discovery of this general phenomenon itself, but in the systematic comparative investigation of six typical gaseous pollutants relevant to archive environments within a unified DFT framework. By combining structural optimization, vibrational spectral analysis, and field-dependent potential energy scans, this study reveals molecule-dependent differences in bond distortion, spectral response, and barrier-lowering behavior under external electric fields, thereby providing theoretical insight into the relative electric-field susceptibilities of these representative pollutants.

2. Theory

In this paper, Gaussian09 [17] quantum chemical calculation software was used to model and calculate six common polluting gas molecules in archives. Under the action of an external electric field, the Hamiltonian quantity H of the molecular system is expressed as:

$$H=H_0+H_{int}=H_0-\mu ·F$$

(1)

where H₀ is the Hamiltonian amount in the absence of an external electric field, and H_int is the Hamiltonian amount of interaction between the external electric field F and the molecular dipole moment $\mu$ [18]. Based on the density functional theory, the B3LYP/6-31G(d) group was used to study sulfur dioxide, nitrogen dioxide, ozone, hydrogen sulfide, and formaldehyde molecules, and the B3LYP/3-21G(d) group was used to study acetic acid molecules. In experiments, this intensity and higher-intensity electric fields can be achieved with laser fields [19,20].

3. Calculation Results and Discussion

3.1. Optimization of Molecular Configurations

To validate the computational methodology, six molecular models were established in Gaussian, and the calculated bond lengths obtained with different basis sets were compared with available experimental values from the literature. The corresponding RMSEs were evaluated to determine the most appropriate basis set for each molecule. Based on this comparison, B3LYP/6-31G(d) was selected for SO₂, NO₂, O₃, H₂S, and HCHO, while B3LYP/3-21G was selected for CH₃COOH because it gave the smallest RMSE. As summarized in

Table 1. Calculated and experimental bond lengths of the studied molecules.

Molecule	Molecular Bond	Calculated (Å)	Experimental (Å)	Percentage Error/%
SO2	3S=2O	1.46366	1.46364	0.00137
NO2	3N=2O	1.20314	1.20313	0.00083
O3	1O=2O	1.26445	1.28081	1.27737
H2S	1H-3S	1.34968	1.34962	0.00445
HCHO	1C=4O	1.20660	1.20670	0.00829
CH3COOH	4O-5H	0.97571	0.97568	0.00307

, the calculated bond lengths at the selected levels of theory showed good agreement with the experimental data, supporting the reliability of the chosen DFT approach for describing the equilibrium geometries of the studied molecules. The optimized structures are shown in Figure 1. The optimized bond lengths are as follows: SO₂ (S–O: 1.5 Å), NO₂ (N–O: 1.2 Å), O₃ (O–O: 1.3 Å), H₂S (H–S: 1.3–1.4 Å), HCHO (C–O: 1.2 Å), and CH₃COOH (C–C: 1.5 Å). Subsequent analyses of the vibrational spectra and potential energy surfaces under external electric fields were then performed using the selected level of theory for each molecule. The directions of the applied electric fields are indicated by the arrows in Figure 1.

The direction of the applied electric field for each molecule was selected along the molecular axis or bond-related direction most relevant to the investigated structural distortion and bond dissociation process, as indicated by the arrows in Figure 1. Since electric-field effects are orientation-dependent, the reported threshold electric fields correspond to the specific molecular orientations considered in this work and should be regarded as orientation-specific theoretical values.

3.2. Spectroscopic Study of Six Pollutant Molecules

3.2.1. Infrared Spectroscopy

Infrared spectroscopy reflects the characteristic absorption of infrared light by chemical bonds and functional groups within molecules, thereby revealing structural information about the molecules. The infrared spectra of six pollutant molecules under different external electric fields are shown in Figure 2. Nine characteristic peaks were selected and labeled as V1, V2, V3, V4, V5, V6, V7, V8, and V9 for subsequent studies. The corresponding frequencies of these nine peaks in the absence of an external electric field are presented in Table 1. As can be seen from the figure, V1 corresponds to the stretching vibration of the S=O bond in sulfur dioxide molecules, V2 corresponds to the stretching vibration of the N=O bond in nitrogen dioxide molecules, and V3 corresponds to the asymmetric stretching vibration of the O-O bond in ozone molecules, V4 corresponds to the symmetric stretching vibration of the O-O bond in ozone molecules, V5 and V6 correspond to the stretching vibrations of the H-S bond in hydrogen sulfide molecules, V7 and V8 correspond to the bending vibration of the C-H bond and the stretching vibration of the C=O bond in formaldehyde molecules, and V9 corresponds to the stretching vibration of the H-O bond in acetic acid molecules. Under an applied external electric field, V1, V2, V3, V5, V6, V7, V8, and V9 all exhibit a redshift phenomenon, indicating that the interaction between the external electric field and the dipole moment alters the potential energy distribution of the molecules, resulting in a decrease in the vibrational frequencies of the corresponding modes. Conversely, V4 shows a blueshift phenomenon, and the intensity of this characteristic peak increases significantly under the external electric field, suggesting an increase in the vibrational frequency of the corresponding mode and the possible presence of the Stark effect [21,22].

3.2.2. Raman Spectroscopy

As another important branch of molecular vibration spectroscopy, Raman spectroscopy and infrared spectroscopy exhibit complementarity in detecting molecular vibrational modes. The Raman spectra of six pollutant molecules under different external electric fields are shown in Figure 3 and

Table 2. Initial frequencies corresponding to the nine characteristic peaks.

Characteristic Peak	Frequency (cm−1)	Characteristic Peak	Frequency (cm−1)	Characteristic Peak	Frequency (cm−1)
V1	1140	V2	1404	V3	1241
V4	1266	V5	2697	V6	2717
V7	1563	V8	1849	V9	3470

. Six characteristic peaks were selected, labeled as V10, V11, V12, V13, V14, and V15, respectively, for subsequent studies. The frequencies corresponding to these six characteristic peaks in the absence of an external electric field are listed in

Table 3. Initial frequencies corresponding to the six characteristic peaks.

Characteristic Peak	Frequency (cm−1)	Characteristic Peak	Frequency (cm−1)
V10	502	V11	749
V12	735	V13	1250
V14	2967	V15	3079

. As shown in the figure, V10 corresponds to the bending vibration of the S=O bond in sulfur dioxide molecules, V11 corresponds to the bending vibration of the N=O bond in nitrogen dioxide molecules, V12 corresponds to the bending vibration of the O-O bond in ozone molecules, V13 corresponds to the bending vibration of the H-S bond in hydrogen sulfide molecules, V14 corresponds to the asymmetric stretching vibration of the C-H bond in formaldehyde molecules, and V15 corresponds to the stretching vibration of the C-H bond in acetic acid molecules. Under an external electric field, V10, V11, V12, V13, and V15 all undergo a blue shift, while V14 undergoes a red shift. This indicates that the external electric field affects the molecular polarizability, thereby altering the molecular vibrational frequencies.

3.3. Study on the Potential Energy Surfaces of Six Pollutant Molecules

3.3.1. Molecular Potential Energy Surface Under External Electric Field

Structure optimization of sulfur dioxide, nitrogen dioxide, ozone, hydrogen sulfide, and formaldehyde molecules under different external electric fields was performed using the B3LYP method with the 6-31G(d) basis set. For chemical interpretation, the key point of the present analysis is the field-dependent evolution of the potential barrier along the selected coordinate, rather than the absolute value of the electronic energy itself. Potential energy surface scans were carried out for the 3S–2O bond, 3N–2O bond, 1O–2O bond, 2H–3S bond, and 1C–4O bond using the same B3LYP method and 6-31G(d) basis set, with the results shown in Figure 4a–e. In these calculations, rigid single-point scans were performed by incrementally elongating the selected bond while keeping the remaining geometric parameters fixed under each applied electric field. Structure optimization of acetic acid molecules under different external electric fields was conducted using the B3LYP method with the 3-21G basis set, and a potential energy surface scan for the 1C-2C bond was performed using the same method and basis set, with the result presented in Figure 4f. As shown in the figure, in the absence of an external electric field, the stable structures exhibit bond lengths of approximately 1.5 Å for the 3S-2O bond in sulfur dioxide, 1.2 Å for the 3N-2O bond in nitrogen dioxide, 1.3 Å for the 1O-2O bond in ozone, 1.3–1.4 Å for the 2H-3S bond in hydrogen sulfide, 1.2 Å for the 1C-4O bond in formaldehyde, and 1.5 Å for the 1C-2C bond in acetic acid. The molecular potential energy initially decreases and then increases. As the external electric field is gradually increased, the upward trend in molecular potential energy is reduced in all cases.

It should be noted that rigid scans do not include full structural relaxation along the dissociation coordinate. Therefore, the corresponding barrier changes and threshold electric fields should be regarded as approximate theoretical values, mainly suitable for comparative analysis of different molecules within a consistent computational framework. The mechanistic interpretation in the present work is mainly based on field-induced structural distortion, vibrational spectral variation, and potential barrier lowering. These results consistently suggest a bond-weakening tendency under the applied electric field along the selected molecular coordinates. However, more detailed electronic-structure analyses, such as charge redistribution, dipole moment evolution, frontier-orbital variation, and electron density difference mapping, were not included in the present study. Therefore, the current discussion provides a qualitative mechanistic interpretation rather than a complete electronic-level description of the field-molecule interaction. Such analyses would be valuable for future work.

3.3.2. Field-Dependent Potential Barrier Evolution and Dissociation Tendency

In this work, the threshold electric field is defined as the critical field strength at which the potential energy barrier along the scanned bond dissociation coordinate disappears. This criterion is used as a theoretical indicator that bond cleavage becomes barrierless along the selected coordinate under the applied electric field, rather than as a complete kinetic proof of molecular degradation.

As shown in Figure 5d, when the applied electric field strength reaches 0.10 a.u., the potential energy profile of the hydrogen sulfide molecule shows almost no increasing region, indicating that the potential barrier along the scanned H–S bond dissociation coordinate has essentially vanished. To investigate the behavior of the remaining molecules, the applied electric field was further increased and the corresponding potential energy surfaces were calculated. The results are presented in Figure 5. It can be seen that when the electric field strengths reach 0.105 a.u., 0.0975 a.u., 0.0925 a.u., and 0.15 a.u., respectively, the potential barriers along the scanned bond coordinates of sulfur dioxide, nitrogen dioxide, ozone, and formaldehyde disappear, indicating that bond dissociation becomes barrierless along the selected coordinates.

Figure 6 shows the polynomial fitting results for the potential barrier of the acetic acid molecule under different electric field intensities. The fitting coefficient R² is 0.99969, indicating good agreement with the fitted curve. According to the fitting result, when the electric field intensity reaches approximately 0.0705 a.u., the potential barrier approaches zero, suggesting a strong tendency toward field-induced C–C bond cleavage along the scanned coordinate. For CH3COOH, the threshold electric field was estimated from a polynomial fit of the calculated barrier heights at discrete field strengths. Since this fitting is an empirical numerical approximation, the obtained value is interpreted here as an approximate comparative indicator rather than an exact physical transition point.

Therefore, the above electric field strengths are regarded in this work as theoretical threshold values for barrierless bond dissociation along the selected scanning coordinates.

Molecular dissociation energy refers to the energy required to decompose a molecule into its constituent atoms. Molecules with high dissociation energies exhibit greater stability and lower reactivity. As shown in Figure 5d, when the applied electric field strength reaches 0.10 a.u., the potential energy of the hydrogen sulfide molecule shows almost no increasing trend, the potential barrier disappears, and molecular degradation is achieved. To investigate the degradation conditions for the remaining molecules, the applied electric field was further enhanced, and the molecular potential energy surfaces were calculated. The results are presented in Figure 5. It can be concluded that when the electric field strengths reach 0.105 a.u., 0.0975 a.u., 0.0925 a.u., and 0.15 a.u., respectively, the potential energies of sulfur dioxide, nitrogen dioxide, ozone, and formaldehyde molecules show almost no increasing trend, the potential barriers disappear, and molecular degradation is achieved.

Figure 6 shows the polynomial fitting results for the potential barrier of acetic acid molecules under different electric field intensities. The fitting coefficient R² is 0.99969, indicating that the relationship between the potential barrier and electric field intensity is consistent with the curve described. It can be calculated that when the electric field intensity reaches approximately 0.0705 a.u., the potential barrier will disappear, which means the acetic acid molecule will completely dissociate due to the rupture of the C–C bond.

The present results should be regarded as a theoretical reference for understanding the intrinsic electric-field response of typical archive pollutant molecules, rather than as directly applicable operating conditions for archive air-purification systems. Their practical significance lies mainly in supporting the future evaluation of field-assisted or plasma-assisted purification strategies in combination with realistic engineering and material-safety considerations.

4. Summary

This study employs DFT and the B3LYP method to optimize the molecular structures, obtaining their stable configurations. Simultaneously, under a specific applied electric field direction, potential energy surface scans were performed on selected molecular bonds under different electric field intensities. The required applied electric field strengths for the degradation of six common archival pollutants—sulfur dioxide, nitrogen dioxide, ozone, hydrogen sulfide, formaldehyde, and acetic acid—were determined as 0.1050 a.u., 0.0975 a.u., 0.0925 a.u., 0.1000 a.u., 0.1500 a.u., and 0.0705 a.u., respectively. The results clarify the electric field degradation thresholds for these six archival pollutants under an external electric field. The findings indicate that acetic acid (0.0705 a.u.) and ozone (0.0925 a.u.) exhibit higher sensitivity to the electric field, while formaldehyde requires the strongest electric field for degradation (0.1500 a.u.). Future work could combine experimental validation to explore the quantitative relationship between electric field intensity and degradation efficiency, providing a reference for the development of air purification equipment in archives.

It should be noted that the present study considers isolated molecules in the gas phase, which represents an idealized model. In realistic archive environments, humidity and complex air compositions may introduce intermolecular interactions and cluster formation. In particular, water molecules may affect the electric-field response of pollutant molecules through hydrogen bonding, polarization, and screening effects, which could modify their structural stability, vibrational features, and dissociation thresholds. Therefore, the present results mainly reflect the intrinsic behavior of individual molecules under external electric fields and should be regarded as a theoretical reference. More realistic models, including hydrated clusters and multicomponent systems, will be considered in future work.