Early Chemistry of Nicotine Degradation in Heat-Not-Burn Smoking Devices and Conventional Cigarettes: Implications for Users and Second- and Third-Hand Smokers

Abstract

Nicotine exposure results in health risks not only for smokers but also for second- and third-hand smokers. Unraveling nicotine’s degradation mechanism and the harmful chemicals that are produced under different conditions is vital to assess exposure risks. We performed a theoretical study to describe the early chemistry of nicotine degradation by investigating two important reactions that nicotine can undergo: hydrogen abstraction by hydroxyl radicals and unimolecular dissociation. The former contributes to the control of the degradation mechanism below 800 K due to a non-Arrhenius kinetics, which implies an enhancement of reactivity as temperature decreases. The latter becomes important at higher temperatures due to its larger activation energy. This change in the degradation mechanism is expected to affect the composition of vapors inhaled by smokers and room occupants. Conventional cigarettes, which operate at temperatures higher than 1000 K, are more prone to yield harmful pyridinyl radicals via nicotine dissociation, while nicotine in electronic cigarettes and vaporizers, with operating temperatures below 600 K, will be more likely degraded by hydroxyl radicals, resulting in a vapor with a different composition. Although low-temperature nicotine delivery devices have been claimed to be less harmful due to their nonburning operating conditions, the non-Arrhenius kinetics that we observed for the degradation mechanism below 873 K suggests that nicotine degradation may be more rapidly initiated as temperature is reduced, indicating that these devices may be more harmful than it is commonly assumed.

Rapid Gas-Phase Autoxidation of Nicotine in the Atmosphere

“…Overall rate coefficients from C1, C3, and C6 are 3.79 × 10 −11 , 3.10 × 10 −11 , and 1.28 × 10 −11 cm 3 molecule −1 s −1 , respectively. Our predictions here are somewhat different from those by Chavarrio Canãs et al, 19 who included torsional anharmonicity in their variational TST calculations and obtained an overall rate coefficient of 5.76 × 10 −11 cm 3 molecule −1 s −1 (1.93 × 10 −11 , 3.41 × 10 −11 , and 4.18 × 10 −12 cm 3 molecule −1 s −1 from C1, C3, and C6). The differences here are still within the uncertainties.…”

“…Figure 1 shows the relative energies for intermediates, transition states, and products and the rate coefficients for these product channels (complete list in Table S1). The , 19 and that of −13.4 kJ/mol for HAA-TS1 at the M06-2X/6-311+G(d,p) level by Borduas et al 12 Weak PRCs, with energies ∼5 and ∼9 kJ/mol below nicotine + OH, are also identified to have weak hydrogen bonds between C1−H and OH as in previous studies. 12,19 The overall rate coefficient at 298 K, 8.37 × 10 −11 cm 3 molecule −1 s −1 , agrees excellently with the experimental value of (8.38 ± 0.28) × 10 −11 cm 3 molecule −1 s −1 by Borduas et al 12 The dominance of HAA channels is obvious.…”

“…The , 19 and that of −13.4 kJ/mol for HAA-TS1 at the M06-2X/6-311+G(d,p) level by Borduas et al 12 Weak PRCs, with energies ∼5 and ∼9 kJ/mol below nicotine + OH, are also identified to have weak hydrogen bonds between C1−H and OH as in previous studies. 12,19 The overall rate coefficient at 298 K, 8.37 × 10 −11 cm 3 molecule −1 s −1 , agrees excellently with the experimental value of (8.38 ± 0.28) × 10 −11 cm 3 molecule −1 s −1 by Borduas et al 12 The dominance of HAA channels is obvious. Among HAAs, the ones from C1, C3, and C6 have the lowest barrier heights because of the activation by the electron-withdrawing amine-N atom, and HAA from C1 is further assisted by the aromatic group to form a benzyl radical.…”

“…Previous studies found multiple conformers for nicotine with respect to the chiral C1 center, the internal rotation around the C1–C1′ bond, and the orientation of the methyl group (axial vs equatorial). ,− However, the two lowest energy conformers for enantiomer S account for >99.9% of nicotine, and therefore the current study includes only these two conformers. Rotation around the C1–C1′ bond has a barrier of ∼25 kJ/mol (see the energy profiles in Figure S1 for a relaxed scan of dihedral angle), suggesting equilibrium distribution between the two conformers in the reactions.…”

“…HNCO should be formed from secondary processes because its formation continues even after all nicotine is consumed . Theoretical studies by Borduas et al and Chavarrio Cañas et al suggested that the reaction of nicotine and OH at room temperature proceeds mainly by hydrogen-atom abstraction from the amine-N-activated sites, while no further study is available after the initial step. Chamber studies also found rapid gas-phase oxidation of nicotine and formation of secondary organic aerosols in the presence of ozone although the rate coefficient was not measured. , However, the mechanism and kinetics of nicotine degradation remain largely unknown, making it difficult to evaluate the effect of exposure to potentially harmful degradation intermediates.…”