Cold-Surface Photochemistry of Primary and Tertiary Alkyl Nitrites

Abstract

Reflection-absorption infrared spectroscopy (RAIRS) is used to explore the photochemistry of primary and tertiary alkyl nitrites deposited on a gold surface. The primary alkyl nitrites examined for this study were n-butyl, isobutyl, and isopentyl nitrite. These compounds showed qualitatively similar spectra to those observed in previous condensed-phase measurements. The photolysis of the primary nitrites involved the initial formation of an alkoxy radical and NO, followed by production of nitroxyl (HNO) and an aldehydic species. In addition, the formation of nitrous oxide, identified from its distinctive transition near 2230 cm(-1), was observed to form from the self-reaction of nitroxyl. The reaction rates for cis and trans conformer decay, as tracked through their intense N═O stretching modes, were found to be significantly different, potentially due to a structural bias that favors HNO formation for the initial trans conformer photoproducts over recombination. Tert-butyl nitrite demonstrates only the trans conformer in the RAIRS spectra prior to photolysis; however, recombination of the initial NO and RO(•) photoproducts was observed to produce the cis conformer in the photolyzed samples. The primary photoproducts from tert-butyl nitrite can also react to form acetone and nitrosomethane, but the absence of HNO prohibits the formation of N(2)O that was observed for the primary alkyl nitrites. Additionally, the RAIRS spectrum of isobutyl nitrite co-deposited with water was measured to examine the photolysis of this species on a water-ice surface. No change in the identity of the photoproducts was observed in this experiment, and minimal frequency shifting (1-3 cm(-1)) of the vibrational modes occurred. In addition to being a known atmospheric source of NO and various aldehydes, our results point to cold surface processing of alkyl nitrites as a potential environmental source of nitrous oxide.

Cold-Surface Photochemistry of Selected Organic Nitrates

“…At 2230 cm –1 , an absorption assigned to N 2 O is again found to grow as the thin films are photolyzed. Notably, this photolysis behavior mimics that found previously for the corresponding alkyl nitrite (RONO, where R = i -But), where an identical band in the RAIR spectra due to N 2 O also developed upon irradiation (λ > 230 nm). A positive band at 1708 cm –1 also develops during photolysis, indicating the formation of an aldehyde or ketone .…”

“…The former peak is readily identifiable as the N–O stretching vibration of nitric oxide, NO . Notably, the latter band at 2230 cm –1 has previously been found to arise due to a photolysis product in low temperature RAIR spectra of primary and tertiary alkyl nitrites (RONO), where it was shown to result from the asymmetric stretch of nitrous oxide, N 2 O …”

“…Together, these emerging bands are found to correspond well with RAIR vibrational wavenumbers previously reported for propanone deposited to water ice films at 95 K. 41 Similar to what was observed in the RAIR spectra during the photolysis of methyl nitrate (Figure 2), a further weak band at 2230 cm −1 was found to appear upon photolysis of the thin films, identifiable as the asymmetric stretching vibration of nitrous oxide. 39,42 The presence of N 2 O within the thin films after photolysis was further probed using TPD measurements, as shown in Figure 5. Here, the initial desorption of a volatile species with m/z = 44, 28, and 16 can be seen, consistent with the loss of N 2 O from the films.…”

“…Through the use of cavity ring down spectroscopy, the authors demonstrated that the alkyl nitrite was formed via the secondary reactions: NO 2 + italich ν ( λ < 420 nm ) → NO + normalO ( P 3 ) normalR ′ CH 2 normalO • + NO → normalR ′ CH 2 ONO The intermediate formation of alkyl nitrites via reactions – provides a plausible explanation for the apparent generation of nitrous oxide during the photolysis of the alkyl nitrates, as evidenced by both RAIR and TPD spectra obtained in the current study. Notably, the low temperature photolysis of primary and tertiary alkyl nitrites codeposited with water yields an identical RAIR feature at 2230 cm –1 , which begins to grow immediately upon irradiation . In the gas, − matrix isolated, − and solid phase, photolysis of alkyl nitrites initially leads to the formation of an aldehyde or ketone, along with the nitroxyl molecule, HNO: normalR ′ CH 2 ONO + italich ν → normalR ′ CH 2 normalO • + NO normalR ′ CH 2 normalO • + NO → normalR ′ CHO + HNO Nitroxyl, formed via reaction , may th...…”

Loss of NO(g) to painted surfaces and its re‐emission with indoor illumination

“…Other aqueous reservoir compounds, 28 such as alkyl nitrates or RONO, could exist on the painted surface and be in equilibrium with the gas‐phase. RONO has a weak RO‐NO bond 31 and can also give rise to NO(g) through photolysis. …”