Cold-Surface Photochemistry of Primary and Tertiary Alkyl Nitrites

Article pubs.acs.org/JPCA

Cold-Surface Photochemistry of Primary and Tertiary Alkyl Nitrites Ryan P. McLaughlin,*,†,§ Daniel O’Sullivan,‡,§ and John R. Sodeau*,§ §

Department of Chemistry, Center for Research in Atmospheric Chemistry, University College Cork, Cork, Ireland Department of Chemistry, Seattle University, 901 12th Avenue Seattle, Washington 98122, United States ‡ School of Earth and Environment, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, United Kingdom †

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 N2O 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.

1. INTRODUCTION Reactive nitrogen oxides and alkoxy radicals are well-known as key species in the chemistry of the atmosphere.1−4 Alkyl nitrites (RONO, where R is an alkyl moiety) have been demonstrated to be photochemical sources for these components and, through the release of these compounds, a contributor to air pollution and photochemical smog formation. Alkyl nitrites have also been identified as photochemical sources of OH radical in the atmosphere and are intermediates in the photochemical decay of alkyl nitrate compounds.2 Understandably then, the photochemical behavior of alkyl nitrites has been widely studied during the past few decades using a variety of methods. While a substantial amount has been learned about these molecules as a result of such efforts, the potential role of ice surfaces in the release of reactive photoproducts has yet to be investigated. Past research has demonstrated that heterogeneous transformations facilitated by surfaces exposed to the atmosphere, such as bare soil, airborne particles, and ice, can play an important role in the chemical cycling of reactive oxides within the troposphere.5−7 In this work, the alkyl nitrite photoproducts that appear on neat surfaces, as well as a water−ice surface, are examined in order to more thoroughly understand the potential environmental impact of these compounds. © 2012 American Chemical Society

Past photochemical studies of alkyl nitrites have examined a wide variety of alkyl groups, including methyl, ethyl, and tertbutyl nitrite. Cyclic alkyl nitrites and longer chain primary alkyl nitrites have also been examined. These studies have involved vapor-phase experiments8−25 as well as condensed-phase and low-temperature matrix studies.26−33 The surface photochemistry of adsorbed tert-butyl nitrite,34−36 isobutyl nitrite,35,36 and methyl nitrite37 has been examined as well. In general, the absorption spectra for these compounds present two main transitions, a strong ππ* transition in the 220−300 nm region and a weaker nπ* band in the 300−400 nm region, the latter of which demonstrates a vibrational progression due to excitation of the NO stretching mode. The body of work on alkyl nitrites identifies the primary photodissociation channel for the two UV bands as breaking the relatively weak (∼172 kJ/mol) RO−NO bond.29 Following the initial photolysis event, the alkoxy radical and NO photoproducts can undergo further reactive steps to form a variety of nitroso products. For example, upon photolysis with a broad-band source, methyl30,32 and ethyl nitrite31 have Received: April 4, 2012 Revised: May 15, 2012 Published: May 21, 2012 6759

dx.doi.org/10.1021/jp303228p | J. Phys. Chem. A 2012, 116, 6759−6770

The Journal of Physical Chemistry A

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in the description, but distinguishing between the different conformers at this level in experimental vibrational spectra can be challenging due to overlapping spectral transitions. Mátyus et al., however, used differences in rates of photolysis for cis- and trans-CONO conformers of n-propyl and isopropyl to identify transitions associated with various conformers of these compounds.28 This study focuses on the surface photochemistry of alkyl nitrites using reflection−absorption infrared spectroscopy (RAIRS). The RAIRS spectra of neat n-butyl, isobutyl, isopentyl, and tert-butyl nitrite as well as the spectra of the photolyzed samples are reported. The RAIRS spectrum of isobutyl nitrite on a water−ice surface was also obtained to examine potential differences in photochemistry brought about through interactions with the surface. In general, the photochemical behavior observed in our experiments is consistent with previous matrix measurements of alkyl nitrites; however, the formation of N2O in all of the primary nitrite samples has also been observed, both neat and in the sample co-deposited with water. As suggested in this work, the likely source of N2O from alkyl nitrite photolysis is the self-reaction of nitroxyl, which produces water as well as nitrous oxide. It has been shown that some bacterial and fungal denitrification processes, well-known sources of environmental N2O, may involve a nitroxyl intermediate.47,48 The mechanism suggested in this work, therefore, involves the use of photochemical energy rather than biological processing to initiate HNO intermediacy and, ultimately, N2O production. The results presented here suggest that given large enough sample densities, alkyl nitrites can provide a photochemical source of nitrous oxide in addition to NO, HNO, and various aldehydic species. Relative rates of decomposition for the alkyl nitrites as well as relative rates of formation for the photoproducts produced are examined, including an exploration of the conformationally dependent photodecomposition rates observed for the primary alkyl nitrites.

been observed to produce an aldehyde (CH2O and CH3CH2O, respectively) and the nitroxyl radical, HNO. Similarly, larger primary alkyl nitrites have also been shown to produce an aldehydic species and HNO, as well as their 1:1 complex, following photolysis of the parent nitrite.28,29 Further photolysis of methyl and ethyl nitrite in low-temperature matrixes has been shown to produce other products, such as nitrosomethanol and nitrosoethanol.32 Additional photoproducts have been observed for the secondary and tertiary alkyl nitrites, with photolysis of tert-butyl nitrite33,34 and isopropyl nitrite28 producing acetone and nitrosomethane in low-temperature matrixes. These studies also reported isomerization occurring for tert-butyl and isopropyl nitrite through recombination of the initial photoproducts. Several groups have also examined the occurrence of rotational conformers in these compounds using both experimental and computational methods.10,27,28,35−46 Microwave spectroscopy experiments have demonstrated three conformers for vaporphase ethyl nitrite,39 while matrix isolation experiments showed a similar number for isopropyl nitrite, and up to eight conformers were observed for n-butyl nitrite.28 Generally speaking, however, primary and secondary alkyl nitrites exist as a mixture of cis and trans conformers, where these designations refer to the orientation around the CONO dihedral angle. Figure 1 illustrates

2. EXPERIMENTAL SECTION A detailed description of the experimental apparatus has been given elsewhere; therefore, only a brief summary is presented in this paper.49 The thin-film samples used in the RAIRS measurements reported here were deposited via glass tubes onto a gold foil substrate within an ultrahigh vacuum (UHV) chamber, shown in Figure 2. Chamber pressures were maintained at

Figure 1. Representative cis and trans structures of the primary alkyl nitrite compounds, n-butyl, isobutyl, and isopentyl nitrite, optimized at the B3LYP/6-31G* level.

the lowest-energy structures calculated for the cis and trans conformers of several alkyl nitrites. Due to an intramolecular interaction between the terminal oxygen and the α-carbon hydrogen atom(s), the cis-CONO conformers are generally predicted to be more stable than the trans versions.45 However, as branching on the α-carbon increases, the relative proportion of conformers tends to favor the trans structures. This pattern culminates with the tertiary alkyl nitrites existing overwhelmingly as the trans form.45 Further categorization of the conformers can be made by including a CCON (or comparative) dihedral angle

Figure 2. Schematic of the RAIRS experiment.

10−8 mbar during the measurements using a rotary-backed oil diffusion pump system. The gold foil substrate within the chamber was placed in thermal contact with a liquid nitrogen reservoir to cool the surface and could be warmed using resistive 6760



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Figure 3. RAIRS spectra of neat n-butyl nitrite, both unphotolyzed (A) and photolyzed (B) for 40 min.

with previous IR and Raman measurements in the vapor and condensed phases.45 Generally speaking for these compounds, strong C−H stretching modes are located in the 3000− 2800 cm−1 region, NO stretches are found between 1650 and 1590 cm−1, CH2 and CH3 deformations, rocks, wags, and C−C stretches are located between 1500 and 1000 cm−1, C−O stretches are found near 1000 cm−1, various O−N stretches are located between 900 and 750 cm−1 and C−C−C, C−O−N, and O−N−O bending modes are located below 600 cm−1. As mentioned in the Introduction, alkyl nitrite structures are generally categorized in terms of a cis- or trans-CONO dihedral angle. When the CCON dihedral is also considered, there are potentially several versions of these two general classes of conformer that can be labeled, such as cis−trans, where the second term refers to the CCON dihedral orientation. For larger alkyl nitrites, spectral congestion can make assignment of these specific conformers difficult; however, the general groupings of cis- and trans-CONO structures are still readily observable in the less dense NO stretching region. Specifically, the trans conformers of alkyl nitrites produce a more intense and higherfrequency NO stretch than the cis conformers due to an interaction between the terminal oxygen of the cis form and its α-carbon hydrogen atoms. As a result, conformers with a transCONO dihedral present a NO stretch of around 1640 cm−1, while the cis NO stretch occurs approximately 40 cm−1 to the red (see Figures 3−5). Previous calculations have suggested that the trans−gauche and cis−trans structures are the lowest-energy conformers for n-butyl and isobutyl nitrite and that several low-lying rotational conformers also exist.45,46 The asymmetric appearance of the trans NO stretching transitions in the RAIRS spectra of these three molecules indicates that there are likely multiple versions of structures with a generally transCONO dihedral in our thin films. The cis conformer region measured for n-butyl nitrite also clearly shows a second vibrational transition, suggesting that more than one of this type of cis conformer is present. These results are in contrast to those for

heating (up to 500 K) to clear the sample between experiments. Substrate temperatures were monitored using a chromel/alumel thermocouple spot-welded to the gold foil. RAIRS spectra of the samples were measured using a Bruker Vertex 70 fourier transform infrared (FTIR) instrument, with the optical output passing through the chamber via vacuum-compatible KBr windows and detected with a liquid-nitrogen-cooled HgCdTe detector. Spectra were collected with 4 cm−1 resolution. Estimating the number of monolayers from the exposure time and pressure (10−6 Torr·s = 1 Langmuir ≈ 0.5 monolayers) leads to an average of approximately 100 monolayers for the experiments performed in this study.50 n-Butyl nitrite (Sigma-Aldrich, 95%), isobutyl nitrite (SigmaAldrich, 95%), isopentyl nitrite (Sigma-Aldrich, 96%), and tertbutyl nitrite (Sigma-Aldrich, 90%) were obtained commercially and used without further purification. Thin films of alkyl nitrites were produced through effusive deposition of the samples onto the low-temperature gold foil substrate through glass dosing lines. The amount of material deposited was controlled via manual or piezoelectric pulse valves. Samples of alkyl nitrites co-deposited with water to generate a mixed ice were produced by simultaneously opening two dosing lines, one containing the alkyl nitrite and one containing water vapor. Photolysis experiments on the prepared surfaces were performed using a broad-band xenon arc lamp (ORIEL, model 60010) with the samples exposed through a quartz window in the side of the chamber. Structural calculations for the alkyl nitrites in this study were performed with the Spartan software package (Wave function, Inc.) using density functional theory (B3LYP) and a 6-31G* basis set.51,52

3. RESULTS 3.A. Neat Primary Alkyl Nitrites. The cold surface RAIRS spectra of neat n-butyl, isobutyl, and isopentyl nitrite are shown in Figures 3−5, respectively. The spectra of these primary alkyl nitrites are qualitatively similar to one another and consistent 6761



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Figure 4. RAIRS spectra of neat isobutyl nitrite, both unphotolyzed (A) and photolyzed (B) for 60 min.

Figure 5. RAIRS spectra of neat isopentyl nitrite, both unphotolyzed (A) and photolyzed (B) for 60 min.

and 1603 cm−1 for n-butyl, isobutyl, and isopentyl nitrites, respectively. The RAIRS measurements of n-butyl nitrite place the trans NO stretch at 1639 cm−1 and the two cis stretches at 1599 and 1587 cm−1, while the trans NO stretch for isobutyl nitrite is located at 1641 cm−1, and the cis NO stretch is at 1592 cm−1. For isopentyl nitrite, the RAIRS transitions for

tert-butyl nitrite (see below), which is sterically limited to the trans structure prior to photolysis. As shown in Table 1, IR measurements of neat liquid alkyl nitrite samples identified the NO stretches of trans-n-butyl, isobutyl, and isopentyl nitrites at 1648, 1649, and 1647 cm−1 respectively. The cis NO stretches are located at 1607, 1606, 6762



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Table 1. Vibrational Mode Frequencies (in cm−1) and Assignments for the NO Stretching Modes of Primary and Tertiary Alkyl Nitrites molecule n-butyl nitritea

isobutyl nitritea isopentyl nitriteb tert-butyl nitritea a

conformer

vapor IR

int

neat IR

int

neat Raman

int

neat RAIRS

int

B3LYP/6-31G*

trans−gauche cis−gauche cis−trans trans−gauche cis−trans trans−gauche cis−trans trans

1670

vs

1647

vs

1648

s

1619 1671 1617 1674 1619 1652

m vs m vs s vs

1605 1649 1604 1647 1603 1628

m vs m vs s vs

1607 1649 1606 − − 1626

m s m − − m

1639 1599 1587 1641 1592 1639 1598 1620

vs s s vs s vs s Vs

1767 1698 1694 1768 1696 1764 1692 1747

Reference 45. bThis work.

the NO stretch are located at 1639 and 1598 cm−1 for the trans and cis structures, respectively. The trans NO stretches are therefore shifted approximately 10 cm−1 to the red in the thinfilm spectra as compared to the liquid-phase samples, while the cis NO stretches are shifted 15−20 cm−1 to the red. Interestingly, there is a blue shift observed for the O−N stretch of the alkyl nitrite group for all three primary nitrites. For isobutyl nitrite, the O−N stretch region shows strong modes at 838 (with a shoulder at 850 cm−1) and 805 cm−1. Corresponding modes for neat IR liquid measurements of this compound are located at 834, 824, and 791 cm−1.45 For n-butyl nitrite, the O−N stretches are located at 841, 832, and 801 cm−1 in the RAIRS spectrum and at 836, 819, and 786 cm−1 in the liquid phase.45 Similarly, for isopentyl nitrite, the RAIRS transitions most likely associated with the O−N modes are located at 882, 864, and 817 cm−1, shifted to the blue from 876, 860, and 799 cm−1 in the neat liquid IR sample. Upon photolysis of the thin-film samples, transitions associated with the intact alkyl nitrite are observed to decrease in intensity (most clearly tracked by examining the disappearance of intensity from the two NO stretching transitions), while a number of new transitions appear. The RAIRS spectra of the photolyzed samples are shown in Figures 3−5 for n-butyl, isobutyl, and isopentyl nitrite, respectively. The new transitions located between 1730 and 1700 cm−1 and between 2830 and 2700 cm−1 for n-butyl, isobutyl, and isopentyl nitrite have been assigned to vibrational modes of the corresponding aldehydes (n-butanal, isobutanal, and isopentanal) that form from the alkyl fragment (see discussion). Formation of aldehydes following the photolysis of alkyl nitrites has been observed previously and helped inform this assignment.28,30,32 The formation of a sharp transition at 2230 cm−1 was also observed following photolysis of each of the three primary alkyl nitrites. The frequency and shape of this transition was nearly identical in each of the primary alkyl nitrites, and this mode was ultimately assigned to the ν3 asymmetric stretch of nitrous oxide, N2O. Given that this transition was induced from the photolysis of an alkyl nitrite parent compound, the number of likely candidates for this mode was relatively small. Previous RAIRS measurements observed a transition for NO+ in this same region; however, this transition was weaker and more broad than the transition observed in this study.49 The formation of a peak at 1285 cm−1 in the photolyzed sample of isobutyl nitrite is also consistent with the ν1 symmetric stretch of N2O and helped verify the assignment. Further, pyrolysis experiments of primary and secondary alkyl nitrites have previously indicated N2O as a potential product in the dissociation mechanism of these compounds, arising from the reaction of HNO with itself.53,54

Given this body of evidence, it was clear that nitrous oxide was being formed in the thin films of these alkyl nitrites. In order to provide additional evidence that N2O and an aldehydic species were being formed in the photolyzed alkyl nitrite samples, however, the RAIRS spectrum of a thin film of isobutanal doped with pure N2O was obtained (illustrated in Figure 6) and compared with the photolyzed isobutyl nitrite spectrum. As shown in Figure 6, the transitions observed for isobutanal and the new peaks present in the 1700 cm−1 region of the photolyzed isobutyl nitrite sample correspond well in frequency, although the intensity of the blue-edge transition is stronger in the isobutanal sample. Further, the 2230 cm−1 transition in the N2O-doped isobutanal corresponds in shape and frequency to the same transition in the photolyzed isobutyl nitrite as does the ν1 mode at 1283 cm−1. Lastly, given the similarity in photolysis results for the three primary alkyl nitrites examined here, it is reasonable to conclude that N2O and the corresponding aldehydic species are occurring in the RAIRS spectra of n-butyl and isopentyl nitrite as well. The rates associated with the photolysis of n-butyl, isobutyl, and isopentyl nitrite are shown in Figures 7−9. These plots show the integrated intensity of the two NO transition regions, trans and cis, for each of the compounds, as well as the formation of the aldehydic species and N2O, plotted as a function of photolysis time. As demonstrated in these figures, the photolysis rates for the trans conformers were faster than the rates of the corresponding cis conformers. Further, the trans structures of the primary alkyl nitrites examined here each photolyze with different first-order rates, with n-butyl nitrite > isopentyl nitrite > isobutyl nitrite. The cis conformers were more similar in their decomposition rates than the trans conformers but followed the same general order with n-butyl nitrite > isopentyl nitrite ≥ isobutyl nitrite. The rate of appearance of the aldehydic species, fit to an exponential rise, follows a relative order that is the reverse of that observed for the decomposition of the alkyl nitrites, with formation of isobutanal > isopentanal > n-butanal. Lastly, the rates of formation for N2O are consistent with the order observed for the aldehydes, with N2O formed from isobutyl nitrite > N2O from isopentyl nitrite > N2O from n-butyl nitrite. 3.B. Neat Tert-Butyl Nitrite. The RAIRS spectrum of neat tert-butyl nitrite before and after photolysis is shown in Figure 10. Previous IR and Raman measurements have shown that neat tert-butyl nitrite is sterically limited to a single conformer, the trans form, in both the liquid and vapor phases.10,34,45 The same behavior was observed in the RAIRS measurements of the tert-butyl nitrite thin films. Again, the presence of conformers is most clearly identified in the NO stretching region, and as shown in Figure 10, this region presents only one stretching 6763



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Figure 6. RAIRS spectra of neat unphotolyzed (A) and photolyzed (B) isobutyl nitrite compared with that of isobutanal doped with nitrous oxide (C).

Figure 7. Rates of decay for the cis and trans conformers of n-butyl nitrite are shown on the left. These rates were determined by examining the decrease in NO stretch intensity for these two conformers over time. The rates of formation of n-butanal and nitrous oxide are shown on the right. The aldehydic species was tracked by following the increase in the CO stretch region, while the rate of nitrous oxide formation was examined by following the increase in the ν3 asymmetric stretch.

mode at 1620 cm−1, indicative of the trans conformer. This frequency is red-shifted by approximately 8 cm−1 compared to the neat liquid sample IR frequencies, similar to what was observed for the primary alkyl nitrite trans NO stretches. Furthermore, the transitions associated with the O−N stretch of

tert-butyl nitrite at 829 and 786 cm−1 demonstrate a blue shift from the neat IR measurements of 16 and 21 cm −1 , respectively.45 The C−O stretching modes measured here at 956 and 1190 cm−1, however, are much closer to the neat IR values of 952 and 1189 cm−1. 6764



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Figure 8. Rates of decay for the cis and trans conformers of isobutyl nitrite are shown on the left. These rates were determined by examining the decrease in NO stretch intensity for these two conformers over time. The rates of formation of isobutanal and nitrous oxide are shown on the right. The aldehydic species was tracked by following the increase in the CO stretch region, while the rate of nitrous oxide formation was examined by following the increase in the ν3 asymmetric stretch.

irradiation of the thin film of tert-butyl nitrite, several new transitions appear in the RAIRS spectrum, consistent with this photodissociation pathway. These new transitions occur at 3009, 1712, 1700, 1586, 1548, 1413, 1352, 1226, 942, 919, 744, and 670 cm−1. The peak observed to form at 1548 cm−1, just lower than the energy of the trans NO stretch, has been assigned to a conformer (or conformers) with a cis-CONO dihedral angle, indicating that isomerization between the trans and cis forms of tert-butyl nitrite is occurring. The new vibrational transitions observed at 670, 744, 919, and 942 cm−1 are also consistent with the presence of cis conformers in the photolyzed sample. The transitions appearing at 1712, 1700, 1413, 1352, and 1226 cm−1 following photolysis are all consistent with the formation of acetone. The characteristic CO stretch of acetone is split into two transitions, with the higher-frequency 1712 cm−1 peak demonstrating a more intense transition than the lower 1700 cm−1 mode. The appearance of acetone in the photolysis of tert-butyl nitrite has been shown to be accompanied by the formation of nitrosomethane, CH3NO.33,34 Relatively weak vibrational transitions located at 1548, 1413, and 1352 cm−1 are consistent with the presence of the nitrosomethane monomer; however, the peaks at 1413 and 1354 cm−1 are also associated with the presence of acetone; therefore, the observation of nitrosomethane is a tentative one. The rates of decomposition and formation for the species involved in the photolysis of tert-butyl nitrite are shown in Figure 11. As illustrated in this figure, isomerization between the cis and trans conformers occurs with exponential decay of the trans conformer and an exponential rise in the less stable cis form. Furthermore, the acetone produced from the photolysis products, as tracked by the formation of the strong carbonyl stretch, appears at a slower rate than the disappearance of the trans conformer. 3.C. Isobutyl Nitrite Co-deposited with H2O. In order to explore the effect of water−ice on the vibrational frequencies and

Figure 9. Rates of decay for the cis and trans conformers of isopentyl nitrite are shown along with the rates of isopentanal and nitrous oxide formation. The NO stretch intensities for isopentyl nitrite were followed to determine the initial photolysis rates of the cis and trans conformers, while increases in the CO stretch intensity and the increase in the ν3 asymmetric stretch intensity were used to determine the rates of formation of isopentanal and nitrous oxide, respectively.

As with the primary alkyl nitrites, the initial photoproducts for tert-butyl nitrite photolysis are an alkoxy radical and NO.34 Upon 6765



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Figure 10. RAIRS spectra of neat tert-butyl nitrite, both unphotolyzed (A) and photolyzed (B) for 75 min.

significant shifts in frequency as compared to the neat sample. All vibrational modes match within 1−3 cm−1, with most having only a 1−2 cm−1 difference. Following photolysis of the sample, the same transitions appear as were observed in the neat isobutyl RAIRS spectrum of isobutyl nitrite. The resolvable isobutanal transitions in the co-deposited sample occur at 2878, 1738, 1723, and 1708 cm−1 with the ν3 asymmetric stretch of nitrous oxide located at 2231 cm−1 and the ν1 symmetric stretch located at 1284 cm−1. All of these vibrational transitions are within 1−3 cm−1 of those observed in the photolyzed neat sample. The rates of decomposition and formation for isobutyl nitrite on the water−ice surface are shown in Figure 13. The rate of photodecomposition of the trans conformer for isobutyl nitrite was similar to that observed for the neat sample. The rate of decay for the cis conformer NO stretch was more difficult to fit due to the relatively weak intensity of the starting transition; however, the disappearance of this conformer is slower than the trans form, as was also observed with the neat samples. Interestingly, the rate of formation of N2O is slower on the water−ice surface than that in the neat isobutyl nitrite, and the formation of isobutanal appears to occur at a faster rate than that of the trans NO decay.

4. DISCUSSION 4.A. Primary Alkyl Nitrites. Initial photolysis of an alkyl nitrite compound involves breaking the relatively weak O−NO bond to produce NO and the corresponding alkoxy radical fragment. The photoproducts of primary alkyl nitrites can then undergo hydrogen removal to form an aldehyde and nitroxyl, HNO, as shown below

Figure 11. Rates of decay for the trans conformer of tert-butyl nitrite are shown along with the rates of cis−tert-butyl nitrite and acetone formation. The NO stretch intensities for cis- and trans-tert-butyl nitrite were followed to determine formation and decay of these species, respectively, while the CO stretch of acetone was used to track the formation of this species.

photochemistry of alkyl nitrites, a thin film of isobutyl nitrite co-deposited with water was examined. The RAIRS spectrum of the co-deposited sample is shown in Figure 12 along with the neat spectrum of isobutyl nitrite. The vibrational frequencies of isobutyl nitrite when co-deposited with water do not show

RCH 2ONO + hν → RCH 2O + NO

(1)

RCH 2O + NO → RCHO + HNO

(2)

Following the formation of photoproducts in reaction 2, the most probable scenario for the appearance of nitrous oxide is the selfreaction of HNO53,54 2HNO → N2O + H 2O 6766

(3)



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Figure 12. RAIRS spectra of isobutyl nitrite on a water−ice surface, both unphotolyzed (A) and photolyzed (B) for 50 min.

1568 cm−1.2 These transitions could not be conclusively identified in our RAIRS measurements; however, the appearance of the aldehydic species and nitrous oxide suggests that this pathway is indeed occurring on our thin-film samples. Futhermore, while, to our knowledge, reaction 3 has not been observed in matrix photolysis experiments performed on primary alkyl nitrites, the higher density of species in the thin films studied here could reasonably lead to this reaction occurring in measurable amounts. In addition, previous RAIRS measurements involving thin films of ammonium nitrate and nitric acid also demonstrated the photochemical formation of N2O and water from the reaction between NH2 and NO2.55 Water is the other product associated with reaction 3, and although an increase in the intensity of the broad 3000 cm−1 region absorption (see Figures 3−5) consistent with its formation is observed, the increase may originate from (or be partly due to) the deposition of water vapor present in the chamber rather than the product of the proposed HNO reaction. The relative ordering of the cis and trans rates of decomposition for the primary alkyl nitrites suggests a steric dependence, with the least bulky alkyl group associated with the fastest decay. However, the opposite ordering was observed for the formation of the aldehydic species, suggesting that the branching near the α-carbon, where hydrogen abstraction to form the aldehyde likely occurs, is deterministic in this rate. The ordering of the rates for N2O formation follows that of the aldehydic rates. This result is consistent with the above rate mechanism for the formation of this species from the selfreaction of HNO, which would appear to progress at the same rate as that of the aldehyde in reaction 2. Different rates of decomposition were also observed for the cis and trans conformers of the primary alkyl nitrites. In general, the rate of decomposition is dependent on the absorption cross sections of the different conformers as well as their steric arrangement, particularly around the CONO dihedral angle. Given the broad-band source used for these measurements, it is likely that the molecular configuration of each conformer, rather than the absorption cross section, is the most relevant property in

Figure 13. Rates of decay for the cis and trans conformers of isobutyl nitrite co-deposited with water are shown along with the rates of isobutanal and nitrous oxide formation. The NO stretch intensities for isobutyl nitrite were followed to determine the initial photolysis rates of the cis and trans conformers, while increases in the CO stretch intensity and the increase in the ν3 asymmetric stretch intensity were used to determine the rates of formation of isobutanal and nitrous oxide.

Nitroxyl has been observed to form from the photolysis of isopropyl and n-propyl nitrites in matrix studies and was identified in these studies through its transitions at 3010 and 6767



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examining the difference in rates. As described above, the trans conformers decay at a faster rate than the cis conformers for the primary alkyl nitrites. The decrease in NO stretch intensity of the trans conformer over time fits well to an exponential decay, suggesting first-order kinetics for this step. Our observation of first-order decay is consistent with the kinetics scheme outlined above. Because the observed decay for the cis conformer spans just over one half-life, first-order behavior does not fit as well as for the trans conformer. However, a slower rate of decay for the cis conformer was observed. This slower rate may be due to more efficient structural caging of the NO photoproduct as compared to the trans structures, potentially brought about by the orientation of the −ONO group. Such caging would reasonably prevent the secondary step shown in reaction 2 from occurring and favor recombination of the photoproducts. Along this same line, comparison of the decay rates for the cis and trans conformers for each of the three primary alkyl nitrites to the rate of formation of the corresponding aldehyde (via reaction 2) shows that the cis decay occurs slower than the formation of the aldehydic species while the trans form is faster than the rate of aldehydic formation. This rate difference suggests that the trans form is likely responsible for producing the initial photoproducts that proceed to form the subsequent aldehydic, nitroxyl radical, and nitrous oxide species. A more detailed structural comparison of the trans and cis conformers lends a possible explanation for the increased rate of trans decay and its likely role as the dominant source of nitroxyl and aldehydic products. Turner used microwave spectroscopic measurements of deuterated ethyl nitrite to demonstrate that the hydrogen abstracted in the gas-phase photolysis of this compound to form HNO arises exclusively from the α-carbon.39 The trans−gauche and cis−trans structures of isobutyl nitrite are shown in Figure 14. While one can expect that somewhat different structures would be present in the thin films, the similarity in relative frequencies and intensities in the RAIRS spectra compared to those in the previous IR and Raman measurements suggests that a general comparison between the gas-phase structures and those on the surface is justified.45 The trans− gauche structure has been calculated to represent the lowestenergy gas-phase trans-CONO conformer of the primary nitrites, while cis−trans and cis−gauche structures are lower in energy than the trans−gauche. The trans−gauche structure calculated using density functional theory (B3LYP/6-31G*) locates the nitrogen atom as nearly eclipsing the α-carbon hydrogen atom at a distance of 2.310 Å.45 Due to the orientation of the terminal oxygen atom, the cis structures appear to be more hindered with respect to hydrogen transfer. Indeed, the cis−trans structure shows the nitrogen located between the two α-carbon hydrogen atoms, with a nearest distance of 2.670 Å. Immediately following photolysis then, the trans structures generate NO and alkoxy radicals already aligned in an appropriate orientation for the formation of HNO via intramolecular hydrogen abstraction. Potentially because of this, a faster rate was observed for the trans disappearance over the cis (reaction 3 would be favored over recombination in this picture, opposite to that for the cis conformers) along with a rate of formation of the aldehydic product inconsistent with cis decay. Hydrogen abstraction from a neighboring alkyl nitrite compound could also conceivably occur in the condensed phase; however, our results on the reaction rates of the cis and trans conformers appear to support the gasphase work, indicating that hydrogen transfer occurs predominantly via an intramolecular process.

Figure 14. Favorable arrangement of the trans−gauche structure of isobutyl nitrite (A) for hydrogen abstraction as compared to the cis− trans structure (B). Dotted lines are present to indicate the relative orientation of the atoms.

It is interesting to point out that matrix photolysis work by Mátyus et al.28 observed a faster rate of decay for the trans conformer of isopropyl nitrite but showed that the cis conformer of n-propyl nitrite decomposed more quickly than its trans conformer. In fact, they observed no decrease in the intensity of the trans NO stretch for n-propyl nitrite for the first 40 min of photolysis in one experiment. This result may be due to the particular experimental conditions of their work but suggests an interesting future question for consideration. The sample of isobutyl nitrite co-deposited with water showed the same photochemical behavior as the neat isobutyl nitrite in that formation of isobutanal and nitrous oxide were observed to occur with little change in frequency. However, the rate of isobutanal formation was found to be faster than that observed for the neat sample of isobutyl nitrite, perhaps facilitated by the presence of water on the surface. In addition, the rate of formation of isobutanal was also faster than decay of both cis and trans conformers of isobutyl nitrite, which does not agree with the rates observed for the neat sample. The quality of the kinetics data is not high for this compound, however. Further experimentation with other alkyl nitrites co-deposited with water may provide a clearer picture of the kinetics on these surfaces. Lastly, it should be noted that the monolayer kinetics of alkyl nitrite samples are quite possibly different than the multilayer rates examined in this study; however, multilayer kinetics are likely more representative of photochemical processes in real ice and snow surfaces where trace species can become quickly embedded. 4.B. Tert-Butyl Nitrite. Similar to previous photolysis studies of tert-butyl nitrite, the formation of acetone and nitrosomethane was observed in the thin-film photolysis of this compound.33,34 The mechanism identified by Barnes et al. for the formation of the these two photoproducts is (CH3)3 CONO(trans) + hν → (CH3)3 CO* + NO

(4)

(CH3)3 CO* + NO → (CH3)3 CONO(cis / trans)

(5)

(CH3)3 CO* → (CH3)2 CO + CH3•

+ NO → CH3NO

CH3•

(6) (7)

This proposed mechanism arose from an observation by McMillan et al. that while acetone was produced immediately 6768



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following photolysis in their matrix samples, nitrosomethane was formed after an induction period.56 The most likely nitrosomethane transition in our tert-butyl nitrite spectra appeared with a rate slower than that for acetone formation, which is consistent with this previously observed behavior. The proposed mechanism occurring in thin films outlined here is made more credible if this nitrosomethane assignment is sound. The formation of the sterically hindered cis conformer from the trans conformer was also observed, most clearly tracked by monitoring the intensity of the transitions in the NO stretching region. The isomerization between the favored trans form and the less stable cis form of this compound, shown in reaction 5, has also been observed previously.33,34 Isomerization most likely occurs through the recombination of the tert-butoxy radical with a NO of an adjacent molecule, aligned in the appropriate geometry to form a cis (or near-cis) CONO dihedral. Reformation of the trans configuration could also occur, of course. Jenniskens et al. examined the photolysis of tert-butyl nitrite on Ag(1,1,1) as a function of layer depth.34 They demonstrated that for coverages of greater than five monolayers, the outer layers of tert-butyl nitrite are able to undergo dissociation to form photoproducts, but the inner layers experience strong enough caging to prevent full dissociation and undergo isomerization instead. The Ar matrix measurements of Barnes et al. demonstrated a similar result in that isomerization is increasingly favored as the amount of tert-butyl nitrite to argon is increased.33 As suggested initially by Barnes et al., the presence of isomerization in the photolysis of tert-butyl nitrite lends evidence to the formation of acetone and nitrosomethane via a mechanism involving the initial breakage of the O−N bond rather than an intramolecular rearrangement. Lastly, there was no evidence for the formation of N2O in the photolysis of tert-butyl nitrite. This observation is understandable because tert-butyl nitrite lacks an α-carbon hydrogen atom for the NO photoproduct to remove, either through an intramolecular or intermolecular process. Rather, the NO formed from the initial photolysis event associates with a methyl group from the alkoxy radical to form nitrosomethane and acetone, as described above.

responsible for the subsequent formation of nitroxyl radical and aldehydic species. There was no difference in photoproduct formation upon co-depositing isobutyl nitrite with water. Previous experiments that identified the α-carbon hydrogen atoms as those responsible for HNO formation via hydrogen abstraction, coupled with a consideration of the structural relationships, suggests that the initial photoproducts of the primary alkyl nitrite trans conformer should be more efficient in forming HNO, while the cis structure may be more likely to undergo recombination. The more rapid disappearance of the trans conformer in the primary alkyl nitrites is consistent with this picture, as is the rate of aldehyde formation. The nitroxyl formed as a secondary product is proposed to react with another HNO to produce the observed nitrous oxide. However, nitrous oxide was not observed to form in the photolysis of tert-butyl nitrite because there are no α-carbon hydrogen atoms present to form the HNO precursor. Rather, nitrosomethane is produced along with acetone from the parent alkoxy radical of tert-butyl nitrite. While nitrous oxide is stable in the troposphere (and therefore not thought of as a NOy species in the lower atmosphere), the majority of NOx in the stratosphere originates from the dissociation of this long-lived gas. In addition to its role as a greenhouse gas in the troposphere, then, N2O can play a significant role in stratospheric ozone chemistry.1 This work shows then that, in addition to being an atmospheric source of NO, alkoxy radicals, aldehydic species, and a likely source of HNO, alkyl nitrites may also be an environmental source for N2O, given sufficiently high concentrations. In a forthcoming publication, the formation of N2O from various alkyl nitrate compounds deposited on water− ice is reported as well as the role that alkyl nitrites, such as those examined in this study, have in this process.

5. CONCLUSION The RAIRS spectra of primary and tertiary alkyl nitrite thin films deposited onto a gold surface have been measured. RAIRS spectra of the neat thin films demonstrate qualitatively similar spectra to other condensed-phase IR measurements, demonstrating a mixture of cis and trans conformers in the primary alkyl nitrites and only the trans form for tert-butyl nitrite. Due to interactions with surrounding species, the vibrational frequencies associated with the NO transitions of all of the alkyl nitrites observed in the RAIRS measurements were red-shifted relative to the neat liquid IR spectra, while the O−N stretching frequencies were blue-shifted. In general, the cis conformer NO stretch frequencies of the primary alkyl nitrites were red-shifted to a greater extent than the trans conformers, suggesting a stronger interaction relative to the liquid phase than that experienced by the trans NO group. The RAIRS spectrum of isobutyl nitrite was also examined on a thin film of water ice. No significant frequency shifts in the co-deposited sample were observed as compared to the neat isobutyl nitrite RAIRS frequencies. Changes in the spectra brought about from the broad-band photolysis of the samples were all consistent with the initial formation of NO and alkoxy radical photoproducts. Secondary processes for the primary alkyl nitrites are then most likely

The authors declare no competing financial interest.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (206) 296 5943. Fax: (206) 296 5786 (R.P.M.); E-mail: [email protected]. Phone: +353 (0)21 4902680. Fax: +353 (0)21 4274097 (L.R.S.). Notes

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ACKNOWLEDGMENTS R.P.M. acknowledges the Fulbright Program, Council for the International Exchange of Scholars, the Fulbright Commission in Ireland, and the Department of Chemistry, University College Cork, for their support.

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REFERENCES

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Cold-Surface Photochemistry of Primary and Tertiary Alkyl Nitrites

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