Hydroxyl Radical-Initiated Reaction of Nerol: A Pathway to Secondary Pollutants in an Indoor Environment

Abstract

Nerol ((Z)-3,7-dimethylocta-2,6-dien-1-ol), (C₁₀H₁₈O), is a monoterpene alcohol that belongs to the family of BVOCs emitted naturally by means of vegetation and is found in various medicinal plants. This species attracted attention in the field of atmospheric chemistry due to its unique structural, chemical and environmental properties. In this work, OH-addition and H-abstraction reactions of Nerol by OH radical have been investigated using M06-2X, CBS-QB3 and CCSD(T) with 6-311++G(d,p) basis set. The OH addition at the C=C double bond of Nerol was shown to be the most favorable, with a small relative energy barrier of −6.86 kcal/mol and H-abstraction at the CH₂ group exhibits a relative energy barrier of 0.08 kcal/mol at CCSD(T)/6-311++G(d,p) level of theory. The obtained overall rate coefficient at 298 K is 9.68 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ using canonical variational transition state theory with small curvature tunnelling method (CVT/SCT), which is in good agreement with the experimental rate coefficient determined by Mahecha et al. (k_OH = (1.60 ± 0.2) × 10⁻¹⁰) at 296 ± 2 K. The obtained rate coefficient exhibits negative temperature dependence, and the atmospheric lifetime of Nerol is about 18 min. The predicted oxidation pathways reveal the formation of key products such as formaldehyde, glycolaldehyde and 6-Methyl-hept-5-en-2-ol, which is also observed in previous experimental studies, indicating good agreement between theoretical and experimental findings. This study constitutes the first theoretical study and its dependence on temperature exploration, offering detailed insights into the degradation pathways and environmental impact of Nerol initiated by hydroxyl radicals.

1. Introduction

Biogenic volatile organic compounds (BVOCs) emitted by plants are the key components in the atmosphere and play a significant role in the formation of secondary organic aerosols (SOAs) [1]. BVOCs have significant effects not only within vegetation and organisms but also in the atmosphere at different spatiotemporal scales, mainly during heat waves and in densely populated areas [2,3]. Among BVOCs, the emission of terpenes and their Terpene Oxidation Products (TOPs) in the indoor air environment impacts atmospheric chemistry. Volatile biogenic terpenes in the indoor environment are crucial, and several studies report the indoor OH radical concentration order [4,5,6,7] as 10⁵ molecules cm⁻³, whereas the outdoor OH radical concentration is typically about 10⁶ molecules cm⁻³ in forested areas, as reported in earlier field studies [8,9]. The major atmospheric pollutants in the indoor environment are primarily emitted by humans, who act as a potent and mobile source of various chemicals. Several studies have shown that these TOPs are found with high levels in closed environments, particularly from cleaning products, air fresheners, perfumes and essential oils [10,11,12]. Once emitted into the atmosphere, terpenes are mainly removed by reacting with main atmospheric oxidants (OH, NO₃, O₃, Cl), leading to the formation of secondary species that are sometimes more toxic and harmful, including formaldehyde, ozone, carbon dioxide and SOAs, whose effects on human health and climate are well-known. The atmospheric degradation of terpenes has important implications in both indoor and outdoor environments. These TOPs reactions influence indoor air quality, and indoor SOA contributes significantly to the total human exposure burden as people spend ~90% of their time indoors [13,14,15].

Nerol ((Z)-3,7-dimethylocta-2,6-dien-1-ol) ((C₁₀H₁₈O)) is a monoterpene alcohol that belongs to the family of BVOC, which is emitted naturally by means of vegetation and found in various medicinal plants like Lippia spp. and Melissa officinalis L. [16,17,18]. Nerol is commonly used in essential oils and fragrance, flavour in soaps, detergents, shampoos, perfumes and in household cleaning products [19,20]. Nerol has widespread applications in cosmetics, pharmaceutical and medicinal uses, such as anti-inflammatory, anti-microbial and antioxidant, and it has potential anti-cancer effects [21,22]. Although Nerol has numerous applications, its degradation mechanism with OH radicals in the atmosphere are of significant interest due to its impact on air quality, which remains insufficiently explored, to the best of our knowledge. Due to its limited attention, herein, in this study, the first theoretical insight into the mechanistic and kinetic exploration of Nerol with the OH radical has been explored. Figure 1 depicts the chemical structure of Nerol. Nerol is a monoterpene alcohol that consists of two double bonds at C8=C4 and C5=C6, one hydroxyl group at C11 and methyl groups at C4 and C6. Nerol is presumed to undergo both H-abstraction and OH addition reactions with atmospheric OH radicals. In addition to these initial reaction pathways, it can subsequently react with O₂, NO and HO₂ radicals, leading to the formation of peroxy and alkoxy radical intermediates. To the best of our knowledge, there is no theoretical study of the reaction mechanism of Nerol with the OH radical. With respect to the available data on the kinetics of the reaction of Nerol with OH radical, experimental studies indicated that the reaction proceeds rapidly, with the rate coefficient typically in the range of 10⁻¹¹ to 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at 298 K under atmospheric conditions [18,23]. Using relative rate technique, G.L. Mahecha et al. [18] determined the rate coefficient of Nerol with OH radical 1.6 ± 0.2 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at 298 K and there is only one experimental study on gas phase kinetics of Nerol with O₃ by Alossaily et al. [23] over the temperature range of 298–353 K at atmospheric pressure using absolute and relative rate method by rigid atmospheric simulation chamber coupled to a proton transfer reaction mass spectrometer (PTR-ToF-MS). They found the rate coefficient of Nerol with ozone to be 8.89 ± 0.90 × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹ at 298 K. In addition to atmospheric oxidants, no study has been reported on Nerol with Cl and NO₃ radicals.

Hence, this study investigates the atmospheric degradation of Nerol by OH radicals, emphasizing the significance of its transformation products and its estimated lifetime under typical atmospheric OH radical concentrations. Theoretical analysis is employed to assess the extent to which the molecular structure of Nerol facilitates its degradation, with particular focus on its potential role in secondary organic aerosols (SOA) formation and implications with respect to the function of temperature. Hence, a deeper understanding of its reaction pathways is essential for evaluating its role in secondary pollutant formation and for improving models of indoor and atmospheric chemistry.

2. Computational Details

Quantum chemical computations were performed to predict the reaction pathways. The minimum energy structures and transition states were located on the potential energy surface (PES) at the M06-2X/6-311++G(d,p) level of theory [24,25,26]. To ensure that each transition state smoothly connects the correct reactant and product minima, intrinsic reaction coordinate analysis (IRC) [27,28] calculations were carried out at the same level of theory. All the reactive species exhibited only real (positive) vibrational frequencies, confirming their nature as minima on the PES, except for the transition states, which exhibited a single imaginary frequency. To obtain more accurate energies, single-point calculations were performed at the CCSD(T) [29,30] level in conjunction with the 6-311++G(d,p) level of theory and the CBS-QB3 [31] level of theory. M06-2X is more efficient and reliable for TS geometries and vibrational analyses. CBS-QB3 gives thermochemical accuracy, whereas CCSD(T) is used for benchmark, single-point energetics for key stationary points. All the electronic structure calculations were performed using Gaussian 16 [32] and MOLPRO 2015 [33] program packages.

The rate coefficient of all the initial H-abstraction and OH-addition reaction of Nerol with OH radical are calculated using canonical variational transition state theory (CVT) with small curvature tunnelling method (CVT/SCT) with the temperature range of 278 to 350 K. Temperature (T) is given by the following relation from canonical variational transition state theory (CVT) [34,35,36]:

$$k^{CVT}(T)={\min }_s{k}^{GT}(T,s)$$

(1)

where

$$k^{GT}(T,s)={\displaystyle \frac{\sigma k_{B}T}{h}}{\displaystyle \frac{Q^{GT}(T,s)}{{\phi }^R(T)}}e{\displaystyle \frac{-v_{MEP(s)}}{k_{B}T}}$$

(2)

where $k^{GT}(T,s)$ is the Generalized Transition State (GTS) theory rate coefficient dividing the surface s, $\sigma$ is the symmetry factor to illustrate the possibility of more than one symmetry-related reaction path, $k_B$ is Boltzmann’s coefficient, h is Planck’s coefficient, ${\phi }^R(T)$ is the reactant classical partition function per unit volume, $Q^{GT}(T,s)$ is the classical partition function of a GTS and $v_{MEP(s)}$ is the classical potential energy at point s on the minimum energy path. The kinetics of all reaction pathways were calculated using the GAUSSRATE 2017B program [37], which is an interface between the GAUSSIAN 16 [32] and POLYRATE 2017 [38] programs.

3. Results and Discussion

3.1. Abstraction and Addition Pathways of Nerol with OH Radical

Nerol undergoes both H-abstraction and OH-addition mechanisms with the OH radical due to the presence of unsaturated alcohol (C=C) double bonds. The reaction scheme of H-abstraction and OH radical addition sites is shown in Scheme 1. The structures of all the species involved in H-abstraction and OH addition of Nerol with OH radical optimized at M06-2X/6-311++G(d,p) are given in the Supporting Information Figure S1. The unsaturated double bonds at C8=C4 at the terminal carbon (α-site) and C5=C6 of Nerol allow an OH-addition mechanism. Remaining OH, C-H, CH₂ and CH₃ groups of Nerol undergo H-abstraction reaction with OH radical. The relative energy profile of H-abstraction and OH-addition mechanism of Nerol with OH radical calculated at CCSD(T)/6−311++G(d,p)//M06−2X/6−311++G(d,p) is shown in Figure 2. Single-point energy is calculated at the CBS-QB3 and CCSD(T)/6-311++G(d,p) level of theory and the thermodynamic parameters enthalpy (ΔH) and Gibbs free energy (ΔG) calculated at M06−2X/6−311++G(d,p) are tabulated in

Table 1. The relative energy, enthalpy and Gibbs free energy (in kcal/mol) for the reaction of Nerol with OH radical (H-abstraction and OH-addition) pathways calculated at M06-2X/6-311++G(d,p), CCSD(T) and CBS-QB3 level of theory.

REACTION PATHWAYS		M06-2X			CCSD(T)	CBS-QB3
		ΔE	ΔH	ΔG	ΔE	ΔE
PATHWAY 1	R	0	0	0	0	0
	RC1	−10.81	−8.84	−0.34	−8.31	−6.32
	TS1	1.13	0.11	8.54	3.83	2.22
	IC1	−19.71	−18.96	−12.38	−17.22	−16.95
	I1+H2O	−13.40	−14.27	−15.03	−11.79	−14.13
PATHWAY 2	TS2	−2.09	−2.73	5.93	0.08	−2.59
	IC2	−45.52	−44.43	−38.00	−41.75	−44.01
	I2+H2O	−37.14	−37.87	−38.59	−33.78	−39.57
PATHWAY 3	TS3	1.21	−0.36	8.59	3.43	−0.93
	IC3	−16.19	−14.51	−8.32	−12.83	−12.92
	I3+H2O	−8.17	−8.44	−10.24	−6.49	−9.42
PATHWAY 4	TS4	0.90	−0.15	8.05	2.35	−0.90
	IC4	−37.16	−35.68	−27.51	−34.88	−36.25
	I4+H2O	−28.66	−29.27	−29.88	−27.16	−31.98
PATHWAY 5	TS5	−0.66	−1.56	8.42	1.15	−2.38
	IC5	−31.72	−32.09	−28.16	−29.02	−35.98
	I5+H2O	−30.47	−31.60	−32.65	−25.22	−33.70
PATHWAY 6	TS6	−1.24	−2.29	6.71	0.61	−1.95
	IC6	−38.94	−38.38	−33.21	−36.30	−38.58
	I6+H2O	−32.97	−33.77	−35.35	−30.82	−36.06
PATHWAY 7	TS7	1.95	0.53	8.84	3.69	−0.28
	IC7	−16.54	−15.46	−9.79	−13.48	−13.55
	I7+H2O	−9.85	−10.08	−11.81	−8.07	−10.89
PATHWAY 8	TS8	1.96	0.96	9.09	3.17	−0.25
	IC8	−34.57	−33.77	−26.79	−31.61	−33.08
	I8+H2O	−28.63	−29.14	−30.33	−27.30	−31.73
PATHWAY 9	TS9	3.62	2.18	10.42	5.52	2.10
	IC9	−33.65	−32.89	−26.59	−31.39	−33.26
	I9+H2O	−28.76	−29.16	−29.68	−27.08	−31.54
PATHWAY 10	TS10	−6.61	−4.91	4.83	−4.45	−5.93
	P1	−39.04	−35.28	−25.56	−33.90	−33.99
PATHWAY 11	TS11	−8.04	−5.83	4.76	−6.86	−6.92
	P2	−39.00	−35.28	−24.22	−34.26	−33.61
PATHWAY 12	TS12	−5.17	−3.54	6.13	−2.81	−3.37
	P3	−35.64	−32.08	−22.85	−31.11	−30.46
PATHWAY 13	TS13	−5.99	−4.41	5.12	−4.35	−4.91
	P4	−34.80	−31.74	−21.54	−30.80	−31.96

. Harmonic vibrational frequencies of reactant (R), pre-reactive complex (RC), transition states (TS), intermediate complexes (IC), intermediates (I) and products (P) of Nerol with OH radical at the M06-2X/6-311++G(d,p) level of theory for H-abstraction and OH-addition pathways are given in Supplementary Information Table S1.

At the entrance channel, the reaction of Nerol with OH radical reactant (R) proceeds to form the pre-reactive complex (RC) with an intermolecular hydrogen bond O30-H31•••O1 in the form of barrierless reaction with a relative energy of −8.31 kcal/mol at CCSD(T)/6−311++G(d,p)//M06−2X/6−311++G(d,p). The H-abstraction and OH-addition mechanism of Nerol with OH radical undergoes the following steps for the formation of intermediates (I1 to I9) and the products (P1 to P4). (i) The reactant (R) leads to the formation of pre-reactive complex (RC), (ii) RC undergoes to form transition state (TS1–TS13), leading to the formation of intermediate complex of Nerol along with water molecule and products, and (iii) further intermediate complexes are decomposed into intermediates and H₂O as shown in Figure S1. The H-abstraction pathways of Nerol with OH radical via OH, CH, CH₂ and CH₃ groups undergo transition state (TS1–TS9) from the pre-reactive complex (RC) with relative energy of 0.08–5.52 kcal/mol at CCSD(T)/6−311++G(d,p)//M06−2X/6−311++G(d,p). The obtained relative energy values are comparable with the CBS-QB3 level of theory depicted in Table 1. Similarly, at the CBS-QB3 level of theory, all nine H-atom abstraction pathways exhibit the relative energy of −0.25 to 2.22 kcal/mol for TS1–TS9. From the overall abstraction pathways of Nerol with OH radical, we found the H-abstraction from allylic -CH₂ group at terminal carbon α-site is more dominant than all other H-abstraction pathways with small relative energy barrier of TS2 0.08 kcal/mol CCSD(T)/6−311++G(d,p)//M06−2X/6−311++G(d,p), whereas in CBS-QB3 it shows a relative energy barrier of −2.59 kcal/mol. Even though the OH radical abstracts the H-atom of Nerol from different positions -OH, -CH, -CH₂ and -CH₃ groups through transition states TS1–TS9 from different C-atoms, TS2 is found to be more favorable with a small barrier. This kind of stable formation may be due to the presence of hydrogen bonding at the terminal carbon. From the optimized structures of Figure S1, it is well known that H31-O30•••H27 in TS2 has a hydrogen bond length of 1.58 Å, whereas the C11-H27 bond breaks with a bond length of 1.14 Å, leading to the formation of intermediate complex IC2 along with a water molecule. Further, the intermediate complex breaks into intermediate I2 and H₂O. Remaining H-abstraction pathways from -OH, -CH₂, -CH and -CH₃ groups initiated via RC undergo transition state TS1, TS3–TS9, leading to the formation of intermediate complexes along with a water molecule. All the abstraction reaction mechanisms are exothermic and spontaneous in nature.

As we discussed earlier, H-abstraction of Nerol by the OH radical dominates at the allylic CH₂ at the terminal carbon α-site. In the same manner, the reactive π bonds of Nerol undergo electrophilic addition of hydroxyl radical OH at its two double bonds, C6=C5 and C4=C8. OH radical addition at two double-bond positions of Nerol takes from the reactive complex RC through a transition state (TS10–TS13) with a relative energy barrier ranging from −4.35 to −6.86 kcal/mol at CCSD(T)/6−311++G(d,p)//M06−2X/6−311++G(d,p). As shown in Figure 2 and Scheme 1, addition leads to the formation of products P1 to P4. Among these four addition pathways C4=C8, the addition of OH radical at C4 is the most dominant pathway with a relative energy barrier of −6.86 kcal/mol via transition state TS11 leading to the formation of 3,7-Dimethyl-oct-6-ene-1,3-diol (P2) at CCSD(T)/6−311++G(d,p)//M06−2X/6−311++G(d,p), whereas at CBS-QB3, it exhibits −6.92 kcal/mol. The addition of the OH radical at the C4 position favors the predominant pathway due to less steric hindrance at this terminal carbon α-site. These kind of products acts as a precursor in atmospheric chemistry. From the PES Figure 2, the reported relative energy barrier for transition states TS10, TS12 and TS13 is less dominant than that of TS11. All four addition reaction pathways are exothermic and spontaneous in nature with respect to the thermodynamic parameters tabulated in Table 1. Hence, from the above discussion, we found OH-addition pathways are more dominant due to the presence of double bonds than H-abstraction pathways.

3.2. Competing Pathways of RO₂ and Their Reactions with NO and HO₂

The intermediates (I1 to I9) obtained via H-abstraction pathways of Nerol may undergo reaction with O₂ to form a peroxy radical intermediate [39], which later may react with NO and HO₂ to form possible intermediates and products as depicted in Scheme 2. The PES of peroxy radical intermediates and products are given in Figure 3, calculated at the M06−2X/6−311++G(d,p) level of theory, and the values of H-abstraction secondary reaction pathways are depicted in Table S2. The optimized structures of secondary abstraction pathways with O₂, NO and HO₂ are given in Figure S2. Initially, all the intermediates (I1 to I9) obtained from H-abstraction pathways undergo reaction with O₂ to form a peroxy radical. The formation of peroxy radical RO₂ and its interaction with NO proceed in the form of a barrierless reaction. These reactions are highly exothermic in nature. The reactions of NO with peroxy radicals are mainly responsible for atmospheric reactions, especially in polluted air [40,41]. From the Scheme 2, we found that the intermediate I1, which reacts with O₂, leads to an intermediate I10. The intermediate I10, which reacts with NO, leads to the formation of I11 3,7-Dimethyl-octa-2,6-dien-1-yl-peroxide radical $(\mathrm{R}\stackrel{•}{\mathrm{O}}{\mathrm{O}+\mathrm{NO}}_2)$ through a transition state TS15 with a relative energy barrier of 42.46 kcal/mol. The reaction is endothermic and non-spontaneous with reaction enthalpy ΔH and Gibbs free energy ΔG as 17.70 and 17.88 kcal/mol, respectively, at the M06-2X level of theory (Table S2). Hence, from the PES Figure 3, we found that all the VOC intermediates reaction products with NO exhibit exothermic reaction in nature except intermediate I10, which reveals an endothermic nature with the release of NO₂.

According to studies from the literature, we know VOCs undergo more competing pathways with other atmospheric oxidants [40,42,43] like NO and HO₂. A schematic representation of the peroxy radical intermediate with HO₂ is clearly depicted in Scheme 2. HO₂ is an important intermediate species, and it plays a crucial role in oxidation reactions and radical propagation [44]. Hence, in this study, the peroxy intermediates interact with HO₂ through H-atom abstraction by O₂. Each individual peroxy intermediate interacts with HO₂ via transition state (TS14–TS23) with a relative energy barrier varying from −0.74 to 24 kcal/mol, respectively, at the M06-2X level of theory, leading to product (P5 to P13), as shown in PES Figure 3 and to the formation of hydroperoxide (ROOH) product, along with O₂. From the thermodynamic parameters, we found that all these reactions are exothermic and spontaneous in nature, and the values are tabulated in Table S2.

This study aims to offer a comprehensive insight into the full range of reaction pathways and mechanisms involved in Nerol with the OH radical. This is the first theoretical study to explore the entire reaction pathways of products (P1 to P4) of Nerol by OH-addition pathways, as shown in Scheme 3, and the thermodynamic parameters calculated at M06-2X/6-311++g(d,p) level of theory. The PES of all the secondary reactions of addition pathways is depicted in Figure 4. These products have been obtained through the addition of the OH radical to Nerol at the double-bond position. As a subsequent reaction, these products (P1 to P4) react with O₂ in the form of a barrierless reaction. Product P1 reacts with O₂ to lead to an intermediate (I28) 3-Hydroperoxy-3-methyl-oct-6-ene-1,2-diol radical with relative enthalpy and Gibbs free energy as −34.17 and −28.31 kcal/mol, respectively (

Table 2. The relative energy, enthalpy and Gibbs free energy (in kcal/mol) for the reaction of secondary reactions of OH-addition pathways with O₂, NO, calculated at M06-2X/6-311++G(d,p) level of theory.

Reaction Pathways	ΔE	ΔH	ΔG
P1+O2	0	0	0
I28	−37.62	−34.17	−28.31
I28+NO	0	0	0
I29	−60.79	−58.86	−59.46
P2+O2	0	0	0
I30	−33.13	−29.17	−24.39
I30+NO	0	0	0
I31	−67.74	−65.68	−64.56
P3+O2	0	0	0
I32	−36.49	−32.86	−26.28
I32+NO	0	0	0
I33	−63.83	−62.00	−61.25
P4+O2	0	0	0
I34	−36.74	−32.90	−28.54
I34+NO	0	0	0
I35	−66.72	−64.97	−64.92
I31	0	0	0
TS24	3.47	2.74	2.69
P14+I36	1.52	−0.03	−2.78
I36+O2	0	0	0
TS25	26.92	25.68	29.45
P15+HO2	8.57	6.78	−1.68
I31	0	0	0
TS26	14.60	13.13	12.54
I37+I38	4.34	1.96	−1.41
I38+O2	0	0	0
TS27	−3.57	−5.75	−2.18
P16+HO2	−39.62	−38.05	−34.47

). The addition of O₂ takes place at the terminal carbon C4 as shown in Figure S2. In the same manner, the remaining three products, P2 to P4, lead to an intermediate 2-Hydroperoxy-3,7-dimethyl-oct-6-ene-1,3-diol radical (I30), 7-Hydroperoxy-3,7-dimethyl-oct-6-ene-1,6-diol radical (I32) and 6-Hydroperoxy-3,7-dimethyl-octane-1,7-diol radical (I34), respectively (Scheme 3). The PES of the obtained intermediates is given in Figure 4. All these reactions are exothermic in nature.

NO plays a significant role in radical chemistry, mainly in atmospheric reactions. When a peroxy radical intermediate reacts with NO, it undergoes rapid decomposition of $\mathrm{R}\stackrel{•}{\mathrm{O}}$, along with NO₂. These kinds of intermediates are more unstable and play a crucial role in ozone formation in polluted environments [40]. As per our reaction, Scheme 3, all four peroxy intermediates undergo reaction with NO through a barrierless reaction, as shown in PES Figure 4. These reactions are highly exothermic and spontaneous in nature with high reaction enthalpy and Gibbs free energy as tabulated in Table 2. As we know from the above discussion, P2 is the most favorable reaction site in which the addition of OH occurs at the double bond of Nerol with the terminal carbon atom C4. Hence, P2 reacts with O₂ in the form of a barrierless reaction leading to an intermediate I30 with reaction enthalpy and Gibbs free energy as −29.17 and −24.39 kcal/mol, respectively, at M06-2X/6-311++g(d,p) level of theory. Further, the intermediate I30 reacts with NO, leading to an intermediate (I31) 3,7-Dimethyl-oct-6-ene-1,2,3-triol radical with the release of NO₂. This reaction is more unstable and occurs quickly due to high exothermic values of enthalpy and Gibbs free energy of −6.68 and −64.56 kcal/mol, respectively, at the M06-2X/6-311++g(d,p) level of theory. The obtained results are consistent with a previous experimental study conducted by Mahecha et al. [18], published in 2025. They obtained the reaction products in the presence of NOx, identified by solid-phase microextraction coupled to gas chromatography–mass spectrometry (SPME-GC–MS). They reported the yields of formaldehyde and acetone, which were found to be (0.06 ± 0.01) and (0.5 ± 0.1), respectively.

3.3. Mechanistic Study: Formation of Formaldehyde, Glycoaldehyde and 6-Methyl-hept-5-en-2-ol from Intermediate I31

It is worth noting and interesting to study the end products of the reaction mechanism of Nerol with OH radical in the presence of atmospheric oxidants like O₂, NO and HO₂ to know their atmospheric implications at the tropospheric level. To the best of our knowledge, there is no theoretical study on the investigation of Nerol oxidation products. Therefore, in order to investigate the entire mechanistic study for the most favorable reaction site of Nerol, we made an attempt to study the full decomposition mechanism. Scheme 4 shows the possible reaction pathways of intermediate I31, which is the most important intermediate compound expected under polluted conditions, with O₂ and their thermodynamic values calculated at M06-2X/6-311++g(d,p) level of theory are tabulated in Table 2. In order to assess the atmospheric implications of Nerol, we made an attempt to explore the decomposition pathways of most of the favorable reactions. As an initial step of decomposition reaction, I31 undergoes cleavage of P14 (6-Methyl-hept-5-en-2-ol) and I36 (CH₂OH-CHO) glycoaldehyde through a transition state TS24 with a relative energy barrier of 3.47 kcal/mol at M06-2X/6-311++g(d,p) level of theory. In transition state TS24, the C8=C4 double bond undergoes cleavage with the bond length of 2.08 Å to give their respective product (P14) and the intermediate (I36). This reaction is exothermic and spontaneous in nature with reaction enthalpy and Gibbs free energy of −0.03 and −2.78 kcal/mol, respectively, as shown in PES Figure 4. The formation of 6-methyl-hept-5-en-2-ol (P14) clearly supports the findings of a previous experimental study on the reaction of Nerol with OH radicals, as reported by Mahecha et al. [18]. Further, CH₂OH-CHO glycoaldehyde (I36) undergoes reaction with O₂ through a transition state TS25 in which the C2-H4 bond elongates with the bond length of 1.59 Å and the H4 atom is abstracted with O9 of the oxygen atom O₂. The reaction occurs with a relative energy barrier of 26.92 kcal/mol through a transition state, TS25, at M06-2X/6-311++g(d,p) level of theory, leading to P15 (CH₂OH-C=O) along with HO₂.

As a next step, intermediate I31 undergoes bond cleavage of C11-C8 to give an intermediate I37 (2,6-Dimethyl-hept-5-ene-1,2-diol radical) and I38 (CH₂OH). This reaction occurs through a transition state, TS26, in which C11-C8 bond cleavage takes place with the bond length of 2.09 Å with a relative energy barrier of 14.60 kcal/mol, as shown in PES Figure 4. Finally, the last major reaction pathway of intermediate I38 reacts with O₂ present in the atmosphere, in which the H atom is abstracted with O₂, leading to stable product P16 formaldehyde (HCHO) along with HO₂. The formation of formaldehyde occurs through a transition state TS27, in which the O1-H5 bond elongates with a bond length of 1.03 Å and the H5 atom is abstracted with O6 with a bond length of 1.43 Å, with a relative energy barrier of −3.57 kcal/mol. The obtained product is highly exothermic and spontaneous in nature with reaction enthalpy and Gibbs free energy as −38.05, −34.47 kcal/mol, respectively, at M06-2X/6-311++g(d,p) level of theory. The predicted oxidation pathway of formaldehyde acts as a key product, indicating a good agreement between theoretical and experimental findings. Among the possible entrance channel pathways, OH addition to the C=C double bond of Nerol is by far the dominant process, more than H-abstraction pathways.

3.4. Rate Coefficient, Branching Ratio of Nerol with OH Radical

The rate coefficients were determined theoretically using canonical variational transition state theory with small curvature tunnelling method (CVT/SCT) with the temperature range of 278–350 K, 1 atmospheric pressure, using M06-2X/6-311++g(d,p) level of theory, and their values are tabulated in

Table 3. Rate coefficient of H-atom abstraction and OH addition reaction pathways of Nerol calculated at CVT/SCT method at M06-2X/6-311++G(d,p) level of theory (cm³molecule⁻¹s⁻¹).

T (K)	kI1 (10−13)	kI2	kI3 (10−13)	kI4	kI5 (10−12)	kI6 (10−12)	kI7 (10−13)	kI8 (10−14)	kI9 (10−14)	kII10	kI11	kI12 (10−11)	kI13	Overall
278	4.49	5.19 × 10−11	2.08	1.03 × 10−12	1.53	7.25	1.94	5.98	1.35	2.11 × 10−10	1.11 × 10−9	9.01	2.00 × 10−10	1.67 × 10−9
288	4.64	4.46 × 10−11	2.15	1.07 × 10−12	1.42	6.73	2.09	6.36	1.51	1.47 × 10−10	8.31 × 10−10	7.37	1.54 × 10−10	1.26 × 10−9
298	4.80	3.88 × 10−11	2.23	1.12 × 10−12	1.32	6.29	2.24	7.16	1.69	1.05 × 10−10	6.34 × 10−10	6.12	1.20 × 10−10	9.68 × 10−10
308	4.9	3.41 × 10−11	2.30	1.16 × 10−12	1.24	5.92	1.46	7.54	1.88	7.63 × 10−11	4.93 × 10−10	2.52	9.53 × 10−11	7.33 × 10−10
318	5.14	3.02 × 10−11	2.38	4.75 × 10−13	1.17	5.61	1.02	7.93	2.09	5.69 × 10−11	3.90 × 10−10	2.13	7.70 × 10−11	5.84 × 10−10
328	5.31	2.70 × 10−11	2.46	4.80 × 10−13	1.11	5.33	1.09	8.32	2.31	4.32 × 10−11	3.14 × 10−10	1.83	6.31 × 10−11	4.74 × 10−10
338	5.49	2.44 × 10−11	2.55	4.86 × 10−13	1.06	5.10	1.17	8.73	2.55	3.34 × 10−11	2.56 × 10−10	1.58	5.24 × 10−11	3.90 × 10−10
348	3.48	9.75 × 10−12	2.64	4.92 × 10−13	1.02	4.90	1.25	9.14	2.81	2.63 × 10−11	2.12 × 10−10	1.38	4.41 × 10−11	3.13 × 10−10
350	3.50	9.57 × 10−12	2.66	4.94 × 10−13	1.01	4.86	1.26	9.23	2.86	2.51 × 10−11	2.04 × 10−10	1.35	4.27 × 10−11	3.02 × 10−10

. Based on the obtained rate values, we found that OH addition is more favorable than H-abstraction. Tunnelling factors are included for both H-abstraction and OH-addition pathways. The formation of product P2 is more kinetically favorable with a rate coefficient of 6.34 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at 298 K. Hence, the overall rate coefficient of Nerol with OH radical for the most favorable pathway P2 exhibits 9.68 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. The rate coefficient shows negative temperature dependence; as the temperature increases, the rate coefficient decreases. This might be due to the formation of pre-reactive complexes in radical reactions and is well agreed with our previous theoretical studies and the literature [11,45]. Similarly, the reactions of α-pinene, myrcene and limonene with OH radicals also exhibit strong negative temperature dependence, consistent with OH addition dominating their entrance channels [13,14,15]. The three-parameter Arrhenius expression for all the pathways is given in the Supplementary Information, Table S3. As shown in Table 3, the rate coefficients obtained for H-abstraction pathways exhibit a (10⁻¹¹ to 10⁻¹⁴) order of magnitude for the whole temperature range of 278–350 K. The obtained rate coefficient results of Nerol with OH are comparable with previous experimental study determined by Mahecha et al. [18], where a rate coefficient of 1.60 ± 0.2 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ was obtained at 298 K. Similarly, among the H-abstraction pathways, we found pathway 2, H-abstraction at terminal carbon is the most dominant pathway with small energy barrier, which is thermodynamically and kinetically favorable with 3.88 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ at 298 K. The overall three-parameter Arrhenius expression is given below in Equation (3).

k = 4.14 × 10⁻¹³ Exp (2308.6/T)

(3)

The contribution of each reaction channel is calculated theoretically using Equation (4) given below:

$$\mathrm{Branching}\mathrm{ratio}={\displaystyle \frac{\mathrm{k}}{{\mathrm{k}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}}}\times 100$$

(4)

Table 4. Branching ratios of H-atom abstraction and OH addition reaction pathways of Nerol at M06-2X/6-311++G(d,p) level of theory.

T (K)	kI1	kI2	kI3	kI4	kI5	kI6	kI7	kI8	kI9	kI10	kI11	kI12	kI13	Overall
278	0.03	3.10	0.01	0.06	0.09	0.43	0.01	0.00	0.00	12.61	66.30	5.38	11.97	100.00
288	0.04	3.54	0.02	0.09	0.11	0.53	0.02	0.01	0.00	11.64	65.97	5.85	12.19	100.00
298	0.05	4.00	0.02	0.12	0.14	0.65	0.02	0.01	0.00	10.79	65.48	6.33	12.39	100.00
308	0.07	4.65	0.03	0.16	0.17	0.81	0.02	0.01	0.00	10.40	67.25	3.43	13.00	100.00
318	0.09	5.18	0.04	0.08	0.20	0.96	0.02	0.01	0.00	9.74	66.83	3.65	13.20	100.00
328	0.11	5.71	0.05	0.10	0.23	1.13	0.02	0.02	0.00	9.13	66.31	3.86	13.33	100.00
338	0.14	6.25	0.07	0.12	0.27	1.31	0.03	0.02	0.01	8.58	65.68	4.06	13.46	100.00
348	0.11	3.11	0.08	0.16	0.32	1.56	0.04	0.03	0.01	8.39	67.68	4.42	14.08	100.00
350	0.12	3.17	0.09	0.16	0.33	1.61	0.04	0.03	0.01	8.31	67.54	4.46	14.12	100.00

depicts branching ratios of H-atom abstraction and OH addition reaction pathways of Nerol with the OH at M06-2X/6-311++G(d,p) level of theory over the temperature range of 278–350 K. The branching ratios of all reaction channels were evaluated, revealing that Pathway 11 is the dominant contributor, with a major contribution of 65.48% at 298 K. The addition pathways at the double bonds of Nerol contribute more significantly than the whole H-atom abstraction pathways.

4. Evaluating the Role of Nerol in Atmospheric Chemistry and Conclusions

This study represents the first theoretical investigation into the OH-initiated atmospheric degradation of Nerol. Our computational results elucidate the primary reaction pathways, key intermediates and products contributing to the formation of secondary organic aerosol (SOA). The calculated energetics and rate coefficients indicate that the OH-addition mechanism is more kinetically and thermodynamically favorable than the H-abstraction pathways of Nerol under atmospheric conditions. These results give valuable insights into monoterpenoid degradation in atmospheric chemistry. As we know, terpenes are significant contributors to global VOCs emissions, suggesting that monoterpene alone accounts for about 10.9% of total global VOCs emissions [3]. The atmospheric lifetime of Nerol with OH radical is ~18 min at the local scale by using the OH radical concentration [46] of 1 × 10⁶ molecule cm⁻³ using the relation given below in Equation (4).

$${\tau }_{OH}={\displaystyle \frac{1}{k_{OH}\left[OH\right]}}$$

(5)

Indeed, emissions of terpenes increase with global warming, thereby enhancing the formation of greenhouse gases (GHGs) and secondary organic aerosols (SOAs), which further contribute to climate change. However, regarding terpene emissions, the atmospheric lifetime of Nerol due to its reaction with OH radicals is found to be very short. This indicates that Nerol has a limited global impact, as it is rapidly removed from the atmosphere with an impact at the local scale.