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. 2025 Apr 21;10(17):17777–17782. doi: 10.1021/acsomega.5c00265

Investigating the Photolysis Mechanism of Propylene Oxide Using DFT and TD-DFT

Manussada Ratanasak 1, Yuta Hori 1,*, Mitsuo Shoji 1, Yasuteru Shigeta 1,*
PMCID: PMC12059913  PMID: 40352537

Abstract

graphic file with name ao5c00265_0008.jpg

Propylene oxide (PO) is a chiral molecule initially identified in the Sagittarius B2 star-forming region through telescopic observations under interstellar conditions. The possibility of homochirality of PO by photolysis is interesting for elucidating the cosmic origin of life on Earth. Existing research shows that photolysis of PO in the gas phase at 185 nm yields propanal and acetone, respectively. This study investigates the detailed photolysis mechanisms of PO using density functional theory (DFT) and time-dependent DFT calculations based on previously proposed mechanisms (J. Org. Chem. 1977, 42, 1252–1254). Our findings suggest that PO photolysis can progress through diradical pathways, as previously proposed. Potential energy diagrams for the excited states indicated that both pathways are more favorable under 185 nm irradiation than in their ground states. In particular, the potential energy diagrams show that the reaction can proceed toward the product through the excited states, even though the activation energies in the ground state are quite high for these reactions to occur. The potential energy surfaces along the intrinsic reaction coordinate partially support the preferential formation of propanal. This study expands our understanding of PO photolysis mechanisms and offers insights into processes that could affect the homochirality of PO.

1. Introduction

The discovery of numerous complex interstellar species has significantly advanced our understanding of the chemical processes in the interstellar medium (ISM).1 Quantum chemical calculation is one of the powerful methods for studying these processes and allows for the exploration of formation and decomposition pathways of complex organic molecules (COMs) in both the gas phase and on grain and ice surfaces. This calculation has been used to investigate possible formation pathways for various COMs,2 including formamide,3,4 formaldehyde,5 urea,6 acetone,7 and acetic acid.8

In 2012, Occhiogrosso et al.9 modeled the formation of ethylene oxide (EO)10 in space, successfully replicating its observed abundance. Subsequently, in 2016, propylene oxide (PO), another COM with an epoxy group, was identified in the Sagittarius B2 star-forming region within the ISM using radio telescopes.11 PO is of particular interest as a chiral molecule owing to the potential role in the generation of homochirality. The origin of homochirality has been a fundamental mystery in the investigation of the origin of life on Earth.12 However, enantiomeric excess (ee) of PO has not been observed in space due to measurement limitations and incomplete polarization state data.

The cosmic origin of homochirality has been extensively studies.13 One of the mechanisms is the preferential synthesis or destruction of a single enantiomer through exposure to ultraviolet circularly polarized light (UV-CPL),13−15 which induces asymmetry in amino acids. Therefore, understanding the formation and photolysis processes of PO provides astrophysical and chemical insights into the possibility of the cosmic origin of homochirality.

Several studies have investigated the formation of PO in space using theoretical approaches.16−19 In our previous study,20 we explored PO formation pathways and photoabsorption properties using quantum chemical calculations to discuss the possibility of ee generation. The results suggested that photolysis of PO under UV-CPL irradiation could contribute to chiral symmetry breaking. However, the detailed photolysis mechanism of PO was not addressed.

Previous experimental studies21 have shown that the photolysis of PO in the gas phase at 185 nm UV right irradiation produces propanal and acetone yields, respectively. This finding suggests that the photolysis of PO upon irradiation may play a significant role in chiral symmetry breaking. Figure 1 depicts the proposed mechanisms for the photolysis of PO via diradical pathways, leading to the formation of either propanal or acetone. According to this proposed mechanism, photolysis of PO through path A yields propanal, while path B leads to acetone production. Despite these observations, the detailed reaction mechanism of PO photolysis remains unclear. A more comprehensive understanding of this mechanism would offer valuable astrophysical and chemical insights into the processes contributing to the generation of homochirality in PO.

Figure 1.

Figure 1

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Proposed photolysis mechanism of PO via paths A and B.21

In this study, we investigated the intricate photolysis mechanism of PO based on the previously proposed model illustrated in Figure 1. Our calculations offer a theoretical explanation for the observed preference toward producing the propanal product. Additionally, we performed simulations of ultraviolet–visible (UV–vis) spectra through time-dependent density functional theory (TD-DFT) calculations, which were subsequently compared with experimental UV–vis spectra.17 These results provide essential insights into the photolysis process of PO, especially in space.

2. Computational Details

DFT calculations were performed for full geometry optimizations using the Gaussian 16 Rev. C.01 program package,22 employing the unrestricted B3LYP23 functional and the aug-cc-pVTZ basis set. Subsequently, 20 excited states were calculated, and UV–vis spectra were obtained through TD-DFT24 calculations at the CAM-B3LYP/daug-cc-pVQZ level of theory.25 According to the benchmark studies on CD simulations using TD-DFT calculations, the M06-2X, CAM-B3LYP, and ωB97X-D functionals are feasible.26 Moreover, rotatory strengths require doubly augmented basis sets of at least triple-ζ quality to reach a similar degree of convergence with the CAM-B3LYP functional.27 In our previous work,20 we used the B3LYP/daug-cc-pVTZ level of theory for the geometrical optimization and the CAM-B3LYP/daug-cc-pVQZ level of theory for the CD spectral simulation of PO, and we confirmed the obtained CD spectrum were consistent with the experimental observations.28 Therefore, in the present study, we used the same DFT functional as our previous work. Reaction energy profiles were obtained from the ground (S0) state energies using the same level of theory employed for UV–vis spectra calculations. The geometries in the first excited (S1) state were optimized at the B3LYP/daug-cc-pVTZ level of theory. Subsequently, single-point calculations were performed using CAM-B3LYP/daug-cc-pVQZ. The optimized stationary and transition state (TS) structures were validated through vibrational frequency calculations, confirming zero and one imaginary frequency, respectively. Moreover, the TS structures were verified by intrinsic reaction coordinate (IRC)29 calculations to ensure the connectivity to the correct intermediates.

3. Results and Discussion

3.1. UV–vis Absorption Spectra of PO

We initially conducted TD-DFT calculations to simulate the UV–vis spectra, aiming to characterize the photochemical properties of PO and compare them with experimental data. Figure 2 shows the calculated UV–vis absorption spectra of PO derived from excitation energies (Eg) and intensities in the nine lowest singlet excited (S1–S9) states. The Eg and oscillator strengths in these states are listed in Table S1 of the Supporting Information. This analysis provided absorption information for PO within the energy range of 173.7–147.7 nm. The findings revealed that the wavelength of 174 nm in the first excited (S1) state closely matched the experimental spectra at 185 nm21. Moreover, our results are also in good agreement with previously reported excitation energies.30,31 Therefore, we focus on the photolysis of PO in the S1 state.

Figure 2.

Figure 2

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Calculated UV–vis absorption spectra of PO using CAM-B3LYP/daug-cc-pVQZ level of theory.

We analyzed the photochemical properties of PO by computing electron density difference maps (EDDMs) between the S0 and S1 states (Figure S1). These EDDMs revealed that electron density is mainly concentrated on the epoxide ring of PO. This concentration implies that the photochemical reaction is expected to take place at the epoxide ring, where alterations in the electronic structure can enhance chemical reactivity.

3.2. Calculated Potential Energies for the Photolysis of PO

We performed DFT calculations to elucidate the detailed photolysis mechanism of PO. The reaction pathways were explored based on the proposed paths A and B, as illustrated in Figure 1. Figure 3 shows the calculated relative energy profiles for paths A (black line) and B (blue line) at the S0 and S1 states.

Figure 3.

Figure 3

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Relative energy profiles of paths A and B of the photolysis reaction of PO at the ground (S0) state and the first excited (S1) state. Paths A and B are indicated in black and blue, respectively.

According to the profile for path A at the S0 state, as indicated by the solid black line in Figure 3, the reaction initiated with PO in the S0 state, RA(S0). The optimized structures of the reactant (R), transition states (TS), and product (P) for paths A and B in the S0 state are illustrated in Figure 4. In path A, the O–C2 bond breaks, and the C1–H1 bond length increases from 1.08 Å to 1.16 Å through a concerted TS, TSA(S0), resulting in the formation of propanal, PA(S0). Thus, in this process, PA(S0) is produced through the simultaneous cleavage of the O–C2 bond and the transfer of H1 to C2. This process in the S0 state has an activation energy (Ea) of 2.49 eV and is an exothermic reaction with an energy release of 0.92 eV. On the other hand, for path B at the S0 state, represented by the solid blue line in Figure 3, the reaction initiates with PO in the S0 state, RB(S0). Subsequently, the O–C1 bond breaks, and the C2–H2 bond length increases from 1.09 Å to 1.16 Å. Simultaneously, H2 migrates to C1 via a concerted transition state, TSB(S0), with an Ea of 3.18 eV. This reaction results in the formation of acetone, PB(S0), through an exothermic reaction with an energy release of 1.27 eV. The calculated spin densities indicate that TSA and TSB contain diradical species, which aligns with the findings of the previous study,21 as illustrated in Figure 1. Therefore, our calculations confirm that these reactions proceed via diradical pathways.

Figure 4.

Figure 4

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Optimized structures of reactant (R), transition states (TS), and product (P) for paths A and B in the ground (S0) state. Distances (Å) are indicated in black.

Next, we investigated the photolysis of PO at the S1 state. The relative energy of PO at the S1 state was approximately 7.03 eV compared to that at the S0 state. Path A at the S1 state is represented by the dashed black line in Figure 3. In the energy profile, the excited state of PO, RA(S1), leads to the TS species at the S1 state, TSA(S1), with an exothermic energy release of 3.31 eV. Path B at the S1 state is represented by the dashed blue line in Figure 3. In the energy profile, the excited state of PO, RA(S1), leads to the TS species at the S1 state, TSB(S1), with an exothermic energy release of 2.92 eV. Therefore, the reactions from R and TS for both paths A and B at the S1 state are energetically feasible. The optimized structures of R, TS, and P for paths A and B in the S1 state are illustrated in Figure 5. The O–C1 bond (1.50 Å) and the O–C2 bond (1.45 Å) of PO in the S1 state are longer compared to the O–C bond distances (1.43 Å) in the S0 state. In path A, the O–C2 bond breaks, and the C1–H1 bond length increases from 1.10 to 1.14 Å via a concerted transition state, TSA(S1). This process leads to the formation of propanal, PA(S1), which occurs through the simultaneous cleavage of the O–C2 bond and the transfer of H1 to C2. Furthermore, we observed that the O–C1 bond in PA(S1) (1.30 Å) is longer than the corresponding O–C bond distance in PA(S0) (1.20 Å). For path B in the S1 state, the O–C1 bond breaks, while the C2–H2 bond length slightly increases from 1.10 to 1.11 Å. Simultaneously, H2 migrates to C1 via a concerted transition state, TSB(S1). This reaction results in the formation of acetone, PB(S1). Similarly, we observed that the O–C1 bond in PB(S1) (1.31 Å) is longer than the corresponding O–C bond distance in PB(S0) (1.21 Å).

Figure 5.

Figure 5

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Optimized structures of reactant (R), transition states (TS), and product (P) for paths A and B in the first excited state (S1). Distances (Å) are indicated in black.

Although the reactions from R and TS for both paths A and B at the S1 state are energetically feasible, the products, PA(S1) and PB(S1), are energetically unstable and anticipated to revert to the ground state products, PA(S0) and PB(S0). Consequently, our calculations suggest that the photolysis of PO may proceed through the following sequence: R(S0) → R(S1) → TS(S1) → P(S0).

Furthermore, we computed the EDDMs between the S0 and S1 states for paths A and B, as shown in Figure S1. The EDDMs revealed that the electron density predominantly resides on the epoxide ring in both pathways. This observation implies that the reaction is expected to occur at the epoxide ring, as modifications in the electronic structure enhance the chemical reactivity at this site.

Previous experiments on the photolysis of PO in the gas phase at 185 nm identified propanal as the primary product with a relative yield exceeding 18; the yield of acetone was normalized to a ratio of 1.21 These findings indicate selectivity in the photolysis reaction. To further investigate this selectivity, we calculated the potential energy surface (PES) from the TS to R at the S1 state.

For path A, IRC and the corresponding energies at the S1 state were determined by performing 37 backward steps from TSA(S1) to RA(S1) using the CAM-B3LYP/daug-cc-pVQZ method. The relative PES with respect to TSA(S1) for path A is illustrated in Figure 6. Based on the PES, we found a local energy minimum, denoted as IA(S1), between RA(S1) and TSA(S1). The energy barrier of the transition from IA(S1) to TSA(S1) was 0.15 eV.

Figure 6.

Figure 6

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Calculated relative PES with respect to TSA(S1) at the S1 state along the IRC from RA(S1) to TSA(S1).

Additionally, for path B, the PES at the S1 state was calculated along the IRC. The IRC and the corresponding energies at the S1 state were determined by performing 51 backward steps from TSB(S1) to RB(S1) using the same computational method as Path A. The obtained relative PES is shown in Figure 7. A local energy minimum, denoted as IB(S1), was found between RB(S1) and TSB(S1). The energy barrier for the transition from IB(S1) to TSB(S1) was calculated to be 0.50 eV.

Figure 7.

Figure 7

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Calculated relative PES with respect to TSB(S1) at the S1 state along the IRC from RB(S1) to TSB(S1).

Based on the shape of PES in the ground state, if the transition from the excited state to the ground state occurs before reaching the TS, the reaction is more likely to return to R than to proceed toward P. Comparing the energy barriers between local energy minima and TSs in the S1 state for Paths A and B reveals that the barrier for Path B is higher than that for Path A. Therefore, these PES analyses along the IRC could explain the preferential formation of the propanal product (Path A) in the photolysis reaction.

Further, we investigated pathways involving the triplet states at both the ground (T0) state and the first excited (T1) state. Nevertheless, the pathways are unsuitable for the photolysis mechanism of PO as it is energetically unfavorable (Figures S2–S5). Therefore, the photolysis mechanism of the PO is expected to predominantly occur via the lower-energy singlet state rather than the higher-energy triplet state.

4. Conclusions

In this study, we investigated the photolysis mechanism of PO using DFT and TD-DFT calculations. Our UV–vis spectra calculations revealed a peak at 174 nm in the S1 state, aligning well with experimental spectra that exhibited a peak at 185 nm21.

Our findings reveal that the photolysis of PO can proceed via diradical pathways, which aligns with the findings of previous studies.21 Specifically, path A, leading to the formation of propanal, exhibits lower activation energies in both the S0 and S1 states compared to path B, which results in acetone formation. Potential energy diagrams for the S1 state indicated that both paths A and B are more likely to occur upon irradiation at 185 nm than in the S0 state. The preference for propanal formation is partially supported by potential energy surfaces along the IRC. This study expands our understanding of the photolysis mechanisms in PO and provides valuable insights into processes relevant to the emergence of homochirality in PO. We hope that our study will provide essential insights into the photolysis process of PO in space and serve as a valuable reference for future research on molecules in space, particularly in relation to potential energy surfaces.

Acknowledgments

This work was supported by Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research [Grant Number 24K17108]; MEXT Promotion of Development of a Joint Usage/Research System Project: Coalition of Universities for Research Excellence Program (CURE) [Grant Number JPMXP1323015474]; and the Multidisciplinary Cooperative Research Program at the Center for Computational Sciences, University of Tsukuba. Some of the computations were performed using computer facilities at the Research Center for Computational Science, Okazaki, Japan (Project: 24-IMS-C103).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c00265.

  • The details excitation energies and oscillator strengths, the computed electron density difference maps, photolysis of PO in the triplet state, and atomic coordinate of the optimized structures (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao5c00265_si_001.pdf (548.2KB, pdf)

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Supplementary Materials

ao5c00265_si_001.pdf (548.2KB, pdf)

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