Kinetics and mechanisms of gas phase reactions of hexenols with ozone

Abstract

An absolute kinetic study is reported for the reactions of O₃ with a series of C6 hexenols, (Z)-2-hexen-1-ol, (Z)-3-hexen-1-ol, (Z)-4-hexen-1-ol, (E)-2-hexen-1-ol, (E)-3-hexen-1-ol, and (E)-4-hexen-1-ol. At 298 K and atmospheric pressure, the rate constants (in units of 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹) were measured to be 7.44 ± 1.03, 5.47 ± 0.71, 7.09 ± 0.91, 16.6 ± 2.2, 6.19 ± 0.72 and 10.5 ± 1.4, respectively. To gain a deeper insight into the reactivity and mechanism, theoretical calculations were also performed for the title reactions with the methods of density functional theory (DFT) and transition-state theory (TST). The geometries, energies, and harmonic vibrational frequencies of each stationary point were obtained at the BH&HLYP/6-31+G(d,p) level of theory. The calculated rate constants are in good agreement with the experimental data, and the reactivity of hexenols with O₃ shows a strong dependence on their chemical structure based on the theoretical results. Finally, lifetimes of the C6 hexenols, with respect to their reactions with some important atmospheric oxidants such as O₃, OH and NO₃ radicals, have also been discussed in the article.

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1. Introduction

The atmospheric chemistry of biogenic volatile organic compounds (BVOCs) has a direct and distinct impact on air quality, ozone production, and secondary organic aerosol (SOA) formation.^1,2 In addition to the known BVOCs, e.g. isoprene and monoterpenes, a number of oxygenated BVOCs as vegetative emissions have also been widely observed.^3–5 The oxygenated BVOCs primarily include C5 and C6 aldehydes, alcohols, and esters and are referred to as “green leaf volatiles” released by the plants in response to wounding.^6,7 The large emissions of C6 hexenols have been observed as a group of the most significant green leaf volatiles. For example, (Z)-3-hexen-1-ol have been observed from a diversity of plants including grass, clover, grape, onion, and peach et al.⁸ In some places, the emission of (E)-3-hexen-1-ol can account for about 0.3% of the total VOC flux.⁹ The emitted (Z)-2-hexen-1-ol were substantially increased from uncut clover to clipped clover.¹⁰ To understand the atmospheric effect of these C6 hexenols in detail, it is very important to know their reaction information such as atmospheric degradation pathways and the reaction rate constants.

After emissions, the unsaturated hexenols can be scavenged by reaction with OH radicals, NO₃ radicals and O₃. Their lifetimes are greatly influenced by the presence of the C=C double bond, to which OH radical, O₃, and NO₃ add.¹¹ Several groups have measured the rate constants of some C6 hexenols with OH.^11–16 However, O₃-initiated oxidations of the hexenols have been seldom studied which may also be important due to the high O₃ concentration in the atmosphere. Of all the six hexenols included in this study, (Z)-3-hexen-1-ol has been studied previously, and only very recently have the (E)-2-hexen-1-ol and (E)-3-hexen-1-ol been studied, which were (5.98 ± 0.73) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ for the reaction of O₃ with (E)-2-hexen-1-ol, and (5.83 ± 0.86) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ for the reaction of O₃ with (E)-3-hexen-1-ol.¹⁷ The rate constant for the reaction of O₃ with (Z)-3-hexen-1-ol measured by Atkinson et al.¹² at 296 ± 2 K and Gibilisco et al.¹⁷ at 298 K were (6.4 ± 1.7) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ and (6.04 ± 0.95) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ respectively. However, Grosjean et al.¹⁸ measured a value of (1.05 ± 0.07) × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹ at ambient temperature for (Z)-3-hexen-1-ol with ozone, which is 60% higher than the value obtained by Atkinson et al. and Gibilisco et al. Previous studies remain controversial, and further experimental work is needed to assess the reactivity of the hexenols with O₃.

In this work, gas-phase reactions of a series of C6 hexenols (E- and Z-geometry isomers) with O₃ were investigated by both experimental and theoretical methods.

O₃ + (E)-2-hexen-1-ol → products k₁ (R1)

O₃ + (E)-3-hexen-1-ol → products k₂ (R2)

O₃ + (E)-4-hexen-1-ol → products k₃ (R3)

O₃ + (Z)-2-hexen-1-ol → products k₄ (R4)

O₃ + (Z)-3-hexen-1-ol → products k₅ (R5)

O₃ + (Z)-4-hexen-1-ol → products k₆ (R6)

A series of experiments were performed in a smog chamber using absolute method to measure their overall rate constants. The determined rate constants of the hexenols have been compared to those of the parent hexene (C₆H₁₂).^19,20 Theoretically, the mechanisms and reaction rate constants of O₃-initiated reactions of hexenols were investigated using a combined density functional theory (DFT) and transition-state theory (TST) method. The rate constants obtained from theoretical calculations were compared with our experimental results. Besides, the effect of the substituents and their position on the reactivity of hexenols towards O₃ was discussed. Finally, the relative importance of their ozonolysis reactions and atmospheric implications were evaluated.

2. Experimental and theoretical methods

2.1. Experimental

The hexenols including (E)-2-hexen-1-ol (97%), (E)-3-hexen-1-ol (97%), (E)-4-hexen-1-ol (97%), (Z)-2-hexen-1-ol (94%), (Z)-3-hexen-1-ol (98%) and (Z)-4-hexen-1-ol (97%) with their purities in brackets were purchased from Alfa Aesar Chemical Company and used without further purification in the experiments. Cyclohexane (≥99.5%), which was used as OH radical scavenger during the reactions, was purchased from Sinopharm chemical reagent Co., Ltd.

Experiments were conducted in a 150 L Teflon chamber, which has been previously introduced.¹⁶ Clean compressed air generated from a zero air generator (AADCO 737, pure air generator) was used as bath gas. Cyclohexane with concentration of about 1.2 × 10¹⁵ molecule cm⁻³ was added as the OH radical scavenger. Ozone was generated via electrical discharge by flowing oxygen through an ozone generator (COM-AD-01, ANSEROS, Germany) into the chamber and measured by an ozone analyzer (Model 49i, Thermo Electron Corporation). In a typical experiment, exact volumes of liquid hexenols and cyclohexane were injected into a 3-way tube by microsyringe and flushed into the chamber by bath gas, warming when necessary. All the bath gas flowed into the chamber through a flow accumulator (D08-8C, Beijing Sevenstar Electronics Co.,Ltd) which could record the total volume of the gas into the reactor. So, the initial concentrations of the hexenols could be calculated from the added amount of liquid organics and the total volume of the reactor.²¹ Sufficient time was allowed to make the concentration inside the reactor reach steady state. GC/FID measurement experiments revealed that wall loss of the studied hexenols in the typical time of O₃ experiments could be negligible (see Fig. S1 in the ESI†). With ozone added, the reactor was flapped and squeezed up and down from the outside to make them mix quickly. Then ozone concentration was measured for about 10 to 20 min with 10 s time intervals. In different experiments, the initial O₃ concentrations used were in the range of (1.46–3.80) × 10¹² molecule cm⁻³, and the concentrations of the hexenols were in the range of (0.22–1.27) × 10¹⁴ molecule cm⁻³. All experiments were carried out at 298 K and atmosphere pressure.

The O₃ reaction rate constants of the unsaturated alcohols were measured using absolute method described previously.^21,22 In brief, the initial concentrations of hexenols are in great excess than O₃ in our experiment so that the reactions could be treated as pseudo-first-order reactions, and the following equation could be obtained:

where k′ indicates the pseudo-first-order rate constant, and k_wall is the O₃ wall loss rate in the absence of the hexenols. When plotting k′ versus [hexenol]₀, the rate constant for the reaction of O₃ with the hexenol, k_i, could be obtained from the slope of the fitted straight line, with the intercept being k_wall.

2.2. Theoretical calculations

The reaction mechanism of O₃ with the six hexenols was investigated using the DFT method which has been widely employed in the context of the ozonolysis reactions. Leonardo et al. have employed BH&HLYP density functional to study the mechanisms and the rate constants of gas-phase ozonolysis of the unsaturated alcohol (geraniol-trans) which is in good agreement with the experimental studies.²³ Zhao and Truhlar demonstrated that BH&HLYP functional gave a better performance in describing unbonded interactions especially for hydrogen bonding.²⁴ In this work, the BH&HLYP functional was employed to obtain the geometries and energies of each stationary point structures with the 6-31+G(d,p) basis set. The harmonic vibrational frequencies were calculated and used to characterize the nature of the stationary points. Local minima were found to have all positive frequencies, and only a single imaginary frequency was found for transition states (TSs). Additionally, the intrinsic reaction coordinate (IRC) calculations were performed to examine the relationships between the computed transition states and the corresponding reactants and products. All calculations were carried out using the Gaussian 09 program package.²⁵ On the basis of the quantum-chemical results, the rate constants of the bimolecular reactions between the hexenols and O₃ were evaluated using the conventional transition state theory (TST) which was carried out in the TheRate program.^26,27

3. Results and discussion

3.1. Kinetic measurements

In all experiments, the concentration of O₃ was monitored with time, and the logarithms of the ratios of [O₃]₀ to [O₃]_t in the presence of hexenols were plotted for different reaction time. Fig. 1 shows an example of the typical pseudo-first-order plots for different concentrations of (E)-2-hexen-1-ol. Straight lines were also obtained for other hexenols in the experiments, which are shown in Fig. S2–S6 in the ESI.† The pseudo-first-order rate constant, k′, can be yielded from the slope of the plots in Fig. 1. The final rate constant for each hexenol, k_i, was then obtained by fitting all available pseudo-first-order rate data, k′, versus their corresponding [hexenol]₀ according to eqn (a). The final rate constants, which could be obtained from Fig. 2, are:

k_{(Z)-2-hexen-1-ol} = (7.44 ± 1.03) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹

k_{(Z)-3-hexen-1-ol} = (5.47 ± 0.71) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹

k_{(Z)-4-hexen-1-ol} = (7.09 ± 0.91) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹

k_{(E)-2-hexen-1-ol} = (1.66 ± 0.22) × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹

k_{(E)-3-hexen-1-ol} = (6.19 ± 0.72) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹

k_{(E)-4-hexen-1-ol} = (1.05 ± 0.14) × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹

Fig. 1 Pseudo-first-order plots for O₃ reactions with different concentrations of (E)-2-hexen-1-ol (in molecule cm⁻³).

Fig. 2 Plots of k′ versus initial hexenol concentrations at room temperature. The lines represent linear least-squares fitting. (In order to express more clearly, the plots of (Z)-4-hexen-1-ol and (E)-3-hexen-1-ol were shifted downward by 0.003 and 0.004, respectively.)

The quoted errors for the final rate constants included 2σ from the least-squares analysis and an estimated systematic error of about 10%.

The rate constant measured here for the reaction of O₃ with (Z)-3-hexen-1-ol at 298 K is in reasonable agreement with those reported by Atkinson et al. and Gibilisco et al., which were (6.4 ± 1.7) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ and (6.04 ± 0.95) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ respectively.^12,17 Our experimental data of k_{(E)-3-hexen-1-ol} = (6.19 ± 0.72) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ also agrees well with the value of Gibilisco et al., (5.83 ± 0.86) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹, while the experimental result of k_{(E)-2-hexen-1-ol} = (1.66 ± 0.22) × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹ is much larger than the result of Gibilisco et al., (5.98 ± 0.73) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹.¹⁷ The rate constants measured by Gibilisco et al. for the reactions of O₃ with (Z)-3-hexen-1-ol, (E)-2-hexen-1-ol and (E)-3-hexen-1-ol are almost the same, which implies that the position of the C=C bond and the spatial arrangement of the atoms in these compounds have no influence on their reactivity. However, our experimental results demonstrate that the reactivity of hexenols towards O₃ depends strongly on their chemical structure especially for (E)-2-hexen-1-ol whose reactivity is much higher than the other hexenols.

The rate constants for the hexenols and their parent alkenes with O₃ are summarized in Table 1. It shows that the rate constants for the reactions of C6 hexenols with O₃ are all smaller than their corresponding alkenes except for the (E)-2-hexen-1-ol. This is because the ozonolysis reaction of unsaturated VOCs was considered to proceed mainly by the electrophilic cycloaddition of ozone to the carbon–carbon double bond.^28,29 The reactivity of the unsaturated hexenols towards O₃ would be decreased as the –OH group lowered the electron density on the C=C bond due to its electron-withdrawing effect. Pfrang et al. has also found the activity-lowing effect of –OH group on the C=C bond in their theoretical study.³⁰ While the reason for the different reactivity of the C6 hexenols with O₃, particularly for the much higher reactivity of (E)-2-hexen-1-ol, will be discussed below.

Table 1 Experimental and calculated rate constants (in units of cm³ molecule⁻¹ s⁻¹) for reactions of hexenols with O₃ at 298 K and atmospheric pressure, and the rate constants of their alkene structural homologues with O₃

Hexenols	Experimental		Theoretical	Alkene structural homologue
	kO3 × 10^17^a	kO3 × 10^17^b	k^TST × 10^17^a	Name	kO3 × 10^17
a This work.b Previous study.
(Z)-2-Hexen-1-ol	7.44 ± 1.03	—	10.4	(Z)-2-Hexene	10.9 ± 0.04 (ref. 19)
(E)-2-Hexen-1-ol	16.6 ± 2.2	5.98 ± 0.73 (ref. 17)	28.6	(E)-2-Hexene	15.5 ± 0.06 (ref. 19)
(Z)-3-Hexen-1-ol	5.47 ± 0.71	6.4 ± 1.7 (ref. 12)	4.40	(Z)-3-Hexene	14.4 ± 1.7 (ref. 20)
		6.04 ± 0.95 (ref. 17)
		10.5 ± 0.7 (ref. 18)
(E)-3-Hexen-1-ol	6.19 ± 0.72	5.83 ± 0.86 (ref. 17)	5.08	(E)-3-Hexene	15.7 ± 2.5 (ref. 20)
(Z)-4-Hexen-1-ol	7.09 ± 0.91	—	8.38	(Z)-2-Hexene	10.9 ± 0.04 (ref. 19)
(E)-4-Hexen-1-ol	10.5 ± 1.4	—	8.78	(E)-2-Hexene	15.5 ± 0.06 (ref. 19)

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3.2. Reaction mechanism and TST calculations

The ozonolysis reaction of the unsaturated alcohols in the atmosphere is initiated by a 1,3-dipolar cycloaddition reaction of O₃ to their C=C bond, forming a primary ozonide (POZ).³¹ The structures of the corresponding hexenol reactants have been calculated and are shown in Fig. S7 (ESI).† The structures of pre-reactive complex (RC), the transition state (TS) for cycloaddition, and the corresponding POZ have been optimized at the BH&HLYP/6-31+G(d,p) level of theory and are presented in Fig. 3 and 4. It can be seen that the reactions are initiated by the formation of van der Waals complexes with the C–O distances of the RCs between 2.90 and 3.57 Å. Specially, the formation of (E)-2-hexen-1-ol RC occurs by two extra hydrogen bonds between the H atom of hydroxyl from (E)-2-hexen-1-ol and the O atom of O₃ which are stronger than van der Waals interactions. The H–O bonds of (E)-2-hexen-1-ol RC are much shorter than the C–O distances with the distance of 2.24 Å and 2.96 Å respectively. For the structures of the transition states, the C–O distances are between 2.29 and 2.42 Å which are shorter than those of the corresponding RCs and longer than those of the corresponding POZs. Similarly, a special case is the structure of (E)-2-hexen-1-ol TS. In addition to the interaction between C atom of C=C and the O atom of O₃, the hydrogen bond interaction is more obvious for (E)-2-hexen-1-ol TS of which the distance of H–O bond is only 2.21 Å. Thus we can infer that the structure of (E)-2-hexen-1-ol RC and TS are more stable than other RCs and TSs.

Fig. 3 Optimized geometries of reactant complexes, the transition states and the primary ozonides of the reactions of the (E)-geometry of the C6 hexenols with O₃ calculated at the BH&HLYP/6-31+G(d,p) level of theory (bond lengths in angstroms).

Fig. 4 Optimized geometries of reactant complexes, the transition states and the primary ozonides of the reactions of the (Z)-geometry of the C6 hexenols with O₃ calculated at the BH&HLYP/6-31+G(d,p) level of theory (bond lengths in angstroms).

The relative energies and the corresponding potential energy surface for the reactions of (E)- and (Z)-hexenol with O₃ are calculated at the BH&HLYP/6-31+G(d,p) level of theory and given in Fig. 5, S8 and S9 (ESI)† respectively. The addition reactions are highly exothermic, with the reaction energies of −73.72 kcal mol⁻¹ for (E)-2-hexen-1-ol, −71.79 kcal mol⁻¹ for (Z)-2-hexen-1-ol, −72.24 kcal mol⁻¹ for (E)-3-hexen-1-ol, −72.78 kcal mol⁻¹ for (Z)-3-hexen-1-ol, −72.30 kcal mol⁻¹ for (E)-4-hexen-1-ol, and −71.58 kcal mol⁻¹ for (Z)-4-hexen-1-ol. The results are in good agreement with those reported by Leonardo et al. which are −71.80 and −73.97 kcal mol⁻¹ for the similar reaction of geraniol-trans with O₃.²³ The binding energies and the reaction barrier heights for the formation of each POZ can also been found in Fig. 5, S8 and S9 (ESI).† In terms of energies, it can be seen that the (E)-2-hexen-1-ol RC and TS have the lower energy and are more stable than other RCs and TSs due to the effect of hydrogen bond interaction which is consistent with the above inference.

Fig. 5 Schematic potential energy surface for the reactions of (E)-2-hexen-1-ol and (Z)-2-hexen-1-ol with O₃ at the BH&HLYP/6-31+G(d,p) level of theory (in kcal mol⁻¹).

The reaction from C6 hexenol reactants to POZ is predicted to be the rate-controlling step of the O₃ oxidation process. A combined quantum-chemical and the TST approach has been employed to determine the rate constants. The reaction starts to form a pre-reactive complex (RC) and the reaction process is depicted by the eqn (b)

It is assumed that the RC is in equilibrium with the reactants and its concentration does not change with time under steady conditions, then the reaction rates are calculated according to eqn (c)

where the K_eq is the equilibrium constant of the pre-reactive complex, k₂ is the rate constant of unimolecular reaction between the pre-reactive complex and the POZ product, σ is the symmetry factor, k_B and h are the Boltzmann and Planck constants, respectively, Q^≠ is the partition function of the transition state, Q_R is the partition function of the reactants (R), and ΔE^≠ is the barrier height of the reaction.

The rate constants at room temperature (298 K) have been collected for the reactions of the six C6 hexenols with ozone. The calculated rate constants at the BH&HLYP/6-31+G(d,p) level of theory agree well with our experimental data (see Table 1).

3.3. Reactivity trends

As can be seen from Table 1, both the experimental and theoretical results show that substitution of the –OH group at the parent hexene (C₆H₁₂) lower the reactivity towards O₃ in most cases. This is due to that the electron withdrawing –OH group lowers the electron density on the C=C and thus the reactivity for the ozone electrophilic addition reactions. (E)-2-Hexen-1-ol with the –OH on carbon atom adjacent to the C=C is more reactive than other hexenols in the O₃ reactions. This difference in the reactivity might reflect the effect of the position of the hydroxyl substituent during the addition step. There exists stronger hydrogen bond interaction between –OH group and O₃ when O₃ getting closer to the C=C bond for that the –OH group (α-OH) on the carbon atom adjacent to the C=C is very close to the O atom of O₃ (see Fig. 3). It makes the pre-reactive reactant more stable which can significantly influence the energy barrier of transition state that is lowering the reaction energy barrier. Therefore, the ozonolysis of (E)-2-hexen-1-ol is more reactive than the other hexenols. While for (Z)-2-hexen-1-ol, although there is also an α-OH, the –OH group does not have so obvious influence on the reactivity of (Z)-2-hexen-1-ol towards ozone. This is because the H atom of –OH is far away from O₃ when O₃ is closer to the C=C bond (see Fig. 4) and the hydrogen bond interaction between –OH group and O₃ can be ignored. As a result, in the (E)-2-hexen-1-ol + O₃ reaction, we found the reaction involves formation of a strong pre-reactive complex of 2.68 kcal mol⁻¹ stability, while in the case of (Z)-2-hexen-1-ol + O₃ reaction, the pre-reactive complex is less stable of 0.68 kcal mol⁻¹ stability (see Fig. 5).

Moreover, the (E)-/(Z)-configuration can also affect the reactivity towards O₃. It can be seen from Table 1, the (E)-/(Z)-geometry of the compound shows different reactivity towards O₃ for the studied hexenols and their alkene structural homologues. Interestingly, both the experimental and theoretical results indicate that (E)-geometry isomers tend to react faster towards O₃ than (Z)-geometry. The reason for this difference is unclear and further investigations are needed. It can be confirmed that the rate of the reactions of C6 hexenols + O₃ show a strong dependence on the structure of the unsaturated alcohols including their (E)-/(Z)-isomers. Both the nature of the substituent and its position affect the reactivity of the C6 hexenols toward O₃.

3.4. Atmospheric implications

The atmospheric lifetimes of the hexenols can be estimated based on the measured rate constants, using the expression, τ = 1/(k[X]), where [X] is the typical atmospheric concentration of the major atmospheric oxidants, e.g. O₃, OH or NO₃ radicals, and k is the corresponding rate constant. The results are presented in Table 2. As can been seen from Table 2, the removal of the hexenols by ozone, OH radicals and NO₃ radicals are of comparable magnitude, with the lifetimes of a few hours, indicating that these hexenols are easily removed close to their emission sources and also that the three oxidant species compete as the main loss processes for the studied hexenols. It is worth mentioning that, the ozone concentration used in Table 2 is only 7 × 10¹¹ molecule cm⁻³ (about 28 ppb) from remote areas. In some urban areas, the concentration of O₃ could reach about 4.7 × 10¹² molecule cm⁻³.³² Under these conditions, the reaction with ozone will be even more important.

Table 2 Atmospheric lifetimes, τ (in hours), for studied hexenols

Hexenols	τO3 ^a	τOH ^a^,^b	τNO3 ^a^,^c
a Typical atmospheric concentrations were used: [O3] = 7 × 10^11 molecule cm^−3,^33 [OH] = 1.0 × 10^6 molecule cm^−3,^34 [NO3] = 5 × 10^8 molecule cm^−3.^35b kOH+hexenol: Gai et al.^16c kNO3+hexenol: Pfrang et al.^36
(Z)-2-Hexen-1-ol	5.3	3.2	3.6
(Z)-3-Hexen-1-ol	7.2	2.8	2.1
(Z)-4-Hexen-1-ol	5.6	3.5	1.9
(E)-2-Hexen-1-ol	2.4	3.4	4.3
(E)-3-Hexen-1-ol	6.4	3.0	1.2
(E)-4-Hexen-1-ol	3.8	3.9	—

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4. Conclusion

In this work, we have investigated the gas-phase reactions of six different C6 hexenols including their (E)-/(Z)-isomers with ozone by experimental and theoretical methods. The rate constants of the oxidation of these compounds with O₃ were measured at (7.44 ± 1.03) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ for (Z)-2-hexen-1-ol, (5.47 ± 0.71) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ for (Z)-3-hexen-1-ol, (7.09 ± 0.91) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ for (Z)-4-hexen-1-ol, (16.6 ± 2.2) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ for (E)-2-hexen-1-ol, (6.19 ± 0.72) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ for (E)-3-hexen-1-ol and (10.5 ± 1.4) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ for (E)-4-hexen-1-ol using absolute method in an environmental chamber at 298 K and atmospheric pressure. The substitution of –OH group would lower the reactivity of hexenols with O₃ when compared with their parent alkenes. (E)-2-Hexen-1-ol is more reactive than other C6 hexenols in the O₃ reactions of which the phenomenon has been confirmed and explained by our theoretical calculation results. In addition to the weak van der Waals interactions, there exists a stronger hydrogen bond interaction when O₃ adding on the C=C in (E)-2-hexen-1-ol. Both the experimental and theoretical results indicate that (E)-geometry isomers tend to react faster towards O₃ than (Z)-geometry. Both the nature of the substituent and its position play a fundamental role in the reactivity of the C6 hexenols toward O₃. Comparison of atmospheric lifetimes for C6 hexenols due to their reactions with O₃, OH and NO₃ radicals shows that the removal of C6 hexenols by ozone can be competitive with the major recognized sinks by OH radicals. The importance for reactions of these unsaturated alcohols with O₃ especially with high O₃ levels both day and night should be considered in the atmospheric model in the near future.

Acknowledgements

We would like to thank financial support from the National Natural Science Foundation of China (91544228, 21307137, 41575125, 41375127, 41375147, and 41575126) and the Presidential Foundation of Hefei Institutes of Physical Science, Chinese Academy of Sciences (Grant No. YZJJ201508).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17107a

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