What was the diffusion rate of ethyl formate

Article views PDF downloads Cited by 0. Figures 6. Numerical simulation of the effect of diluents on NO x formation in methane and methyl formate fuels in counter flow diffusion flame[J].

Previous Article Next Article. Research article. Numerical simulation of the effect of diluents on NO x formation in methane and methyl formate fuels in counter flow diffusion flame. Download PDF. The increasing global demand for energy, the need to reduce green house gasses, and the depletion of fossil fuel resources have led for the need for renewable fuel sources such as biodiesel fuels.

In the diesel engines, biodiesel fuels can also be used directly without comprehensive engine changes. Biodiesel relates to a diesel fuel that is based on vegetable oil or animal fat consisting of longchain of methyl, ethyl, or propyl esters.

Methyl ester fuel burns more efficiently and has lower emissions of particulate matter, unburnt hydrocarbon, and carbon monoxide than fossil fuels.

However, combustion of methyl ester fuel results in increased nitrogen oxides NO x emissions relative to fossil fuels. This study is concerned with characterizing the formation of NO x in the combustion of methyl formate under a counter diffusion flame.

This was found to be true when examining the amount of other metrics viz. Related Papers:. Renew Sus energ Rev Egypt J Petrol Report for National Renewable Energy Laboratory. Proceedings of the European Combustion Meeting. Combust Flame J Clean Energ Tech 1. J Sust Res Engin 1. J Clean Energ Tech 6. Renew Energ For a temperature of 80 K and a source size of 4 , the column density of the a -type lines translates into a column density of 1.

Again, the uncertainty on the level of the baseline and the partial dust absorption at 1. In addition, we note that our model at 3 mm reproduces quite well both the a - and b -type lines with the same temperature and column density see Appendix A , online material , while Nummelin et al.

We believe that this discrepancy results from the fact that they did not properly take into account the line blending, which is large in Sgr B2 N and should affect the weak b -type lines the most, and that they underestimated the line opacities of the strong a -type lines that our model predicts to be on the order of in the 1. Using the BIMA interferometer at This translates into a column density of 1.

However, our model predicts an opacity of 0. This is still about a factor 2 times lower than our estimate and suggests that, like in the case of formic acid, half of the single-dish flux may actually come from a region more extended than the size of our model and may be filtered out by the interferometer.

This conclusion is further supported by the flux ratio of 1. As a result, the methyl formate column density of the compact sources listed in Table 5 may be overestimated by up to a factor 2.

Most lines of ethanol are optically thin at 3 mm , except for three lines that are marginally optically thick. As a result, the source size is not well contrained and we fixed it to 3. This translates into a column density of 2. However, Nummelin et al. With this older version, we determined column densities of 2. The column densities given in Table 5 were obtained with the latest JPL entry for ethanol Pearson et al. The high-energy lines K detected by Friedel et al.

Only the 4 1,4 -3 0. Our LTE model is also too weak for this transition compared to the spectrum obtained with the 30 m telescope. However, it fits well the low-energy transitions at Therefore, it is unclear whether the BIMA missing flux of the 4 1,4 -3 0.

Our model of dimethyl ether predicts line opacities up to 2. The size of the emitting region is thus reasonably well constrained for this molecule. The discrepancy most likely comes from the beam filling factor of unity assumed by Nummelin et al. Our LTE model indeed predicts line optical depths up to 9 in the 1. We discuss these ratios and the implications for the interstellar chemistry in Sect. The n indicates the normal isomer with the carbon atoms forming a chain, in contrast to the iso isomer which has a branched structure.

This isomer has been studied to a lesser extent. However, its rotational spectrum is currently under investigation in Cologne. Again, the anti -conformer is the lower energy form, is strongly prolate, and has a large a -dipole moment component of 3. The gauche -conformer is 1. The energy difference has been estimated at room temperature and at K from relative intensities in the ground state spectra. Since excited vibrational states have not been taken into consideration the error in the energy difference may well be slightly larger than mentioned above.

Moreover, only newly determined rotational and centrifugal distortion parameters were given for the gauche -conformer. Therefore, new sets of rotational and centrifugal distortion parameters were determined for both conformers in the present study.

Two b -type transitions from Wodarczak et al. The methyl internal rotation is unlikely to be resolved in astronomical observations. The quadrupole splitting may be resolvable for some low energy transitions, but these will be generally too weak. Therefore, only the unsplit frequencies were used from these two studies. There were comparatively few transitions reported in Hirota , and their uncertainties were fairly large.

Trial fits with these transitions omitted from the fits caused essentially no change in the values and in the uncertainties of the spectroscopic parameters. Therefore, these transitions were omitted from the final fits. Two transitions, 36 1,36 0,35 of the anti -conformer and 31 5,27 5,26 of the gauche -conformer, had residuals between observed and calculated frequencies larger than four times the experimental uncertainties.

Therefore, these transitions were omitted from the data sets. The total number of transitions is larger by 62 because of unresolved asymmetry splitting. The corresponding numbers of different transition frequencies for the gauche -conformer are 4, , and Unresolved asymmetry splitting causes the total number of transitions to be larger by The final line lists for both conformers are given in Tables 6 and 7 online material.

The asymmetry parameter is In such cases it is advisable to avoid using Watson's A -reduction and use the S -reduction instead. In the case of the gauche -conformer one finds. In this case both reductions may be used.

In the present work the S -reduction was used throughout for consistency reason. The sextic distortion parameter H K of the anti -conformer was initially estimated to be smaller than D K by the same factor that that parameter is smaller than A. This is certainly only a crude estimate. Trial fits with H K released suggested its value to be slightly larger than this estimate. But since the uncertainty was more than a third of its value and since the difference was smaller than the uncertainty, H K was finally fixed to the estimated value.

The final spectroscopic parameters are given in Table 8. Overall, the transition frequencies have been reproduced within experimental uncertainties as the dimensionless rms errors are 0. The values for the individual data sets do not differ very much from these values. Moreover, this is reasonably close to 1. Table 8: Spectroscopic parameters a MHz of n -propyl cyanide. The gauche -conformer is considerably more asymmetric than the anti -conformer.

Therefore, it is probably not surprising that the distortion parameters describing the asymmetry splitting the off-diagonal d i and the h i are not only larger in magnitude for the former conformer, but also more of these terms are required in the fits. In addition, two octic centrifugal distortion parameters L were needed in the fit of the gauche -conformer resulting in an overall much larger parameter set and thus a much slower converging Hamiltonian compared with the anti -conformer.

A similar situation occured in the recent investigation of ethyl formate Medvedev et al. The partition function of n -propyl cyanide is 5. In the course of the analysis, the two conformers again have been treated separately on occasion to evaluate if the abundance of either conformer is lower than would be expected under LTE conditions.

To identify n -propyl cyanide, we used the same method as for ethyl formate see Sect. In our spectral survey, transitions of the anti -conformer and transitions of the gauche -conformer are predicted above the threshold of 20 mK defined in Sect.

They are listed in Tables 9 and 10 online material , respectively, which are presented in the same way as Tables 1 and 2. Again, as can be seen in these tables, most of the n -propyl cyanide lines covered by our survey of Sgr B2 N are heavily blended with lines of other molecules and therefore cannot be identified in this source. Only 50 of the transitions of the anti -conformer are relatively free of contamination from other molecules, known or still unidentified according to our modeling.

The 50 detected transitions correspond to 12 observed features that are shown in Fig. We identified the n -propyl cyanide lines and the blends affecting them with the LTE model of this molecule and the LTE model including all molecules see Sect. The parameters of our best-fit LTE model of n -propyl cyanide are listed in Table 12 , and the model is overlaid in red on the spectrum observed toward Sgr B2 N in Fig.

For the frequency range corresponding to each detected n -propyl cyanide feature, we list in Table 11 the integrated intensities of the observed spectrum Col. Columns 1 to 7 of Table 11 are the same as in Table 9. As we did for ethyl formate, we show in Fig. Figure 4 b displays the corresponding diagram after removing the expected contribution from contaminating molecules see Sect. This figure is less helpful than in the case of ethyl formate because all features containing several transitions 6 out of 12 have transitions with different energy levels and cannot be shown in a population diagram.

Therefore, this diagram does not help much for the determination of the temperature. Feature 3, which is a blend of transitions with upper energy levels from 61 to K, is however reasonably well fitted by our K model see panel 2 of Fig.

This is further confirmed by the high temperatures measured in our survey for chemically related molecules see Sect. Our model for the anti -conformer of n -propyl cyanide consists of two components with different velocities. The need for a second component mainly comes from the shape of features 2, 9, and Its velocity is consistent with the velocity of the second component we find for many other, more abundant molecules in our survey toward Sgr B2 N.

It was shown interferometrically that this second velocity component is a physically distinct source located to the North of the main hot core in Sgr B2 N see, e. Our data are consistent with a second component about half as strong in n -propyl cyanide as the first component Table Finally, since all detected transitions are optically thin and the two regions emitting in n -propyl cyanide are most likely compact given their high temperature, column density and source size are degenerated.

This is approximately the size of the region emitting in the chemically related molecule ethyl cyanide that we measured with the IRAM Plateau de Bure interferometer see Table 5 of Belloche et al.

From this analysis, we conclude that our best-fit model for the anti -conformer of n -propyl cyanide is fully consistent with our 30 m data of Sgr B2 N. This is, to our knowledge, the first clear detection of this molecule in space. No feature of the gauche -conformer of n -propyl cyanide is clearly detected in our spectral survey of Sgr B2 N.

If we consider this feature as a detection, it implies a column density a factor 2 smaller than for the model of the anti -conformer, which may suggest a non-thermal distribution of the molecules in the two conformers. However, we first have to evaluate the uncertainty on the ratio of the anti - and gauche -conformer populations coming from the uncertainty on their energy difference 25 cm -1 , see Sect.

For cm -1 , the anti to gauche ratio is 0. This is not enough to explain the factor 2 mentioned above, but it can have a significant contribution. Above all, the uncertainty on the baseline level at Figure 4: Population diagram of the anti -conformer of n -propyl cyanide presented in the same way as for ethyl formate in Fig.

Panel a shows the population diagram derived from the measured integrated intensities while panel b presents the population diagram after correction for the expected contribution from contaminating molecules. Features 1, 2, 3, 6, 9, and 10 are blends of several transitions with different energy levels and were therefore omitted. Using the same source size, linewidth, and temperature as for Srg B2 N see Table 12 , we find a column density upper limit of 6 10 15 cm -2 in the LTE approximation for both conformers.

The column density of n -propyl cyanide is thus at least a factor 2 lower toward Sgr B2 M than toward Sgr B2 N , which is again consistent with the results of, e.

Table Parameters of our best-fit LTE model of n -propyl cyanide with two velocity components. The parameters of our current best fit models of these two molecules are listed in Table Our models use also constraints from the weaker isotopologues containing 13 C see, e. The source size is constrained by the optically thick transitions, once the temperature has been fitted.

For ethyl cyanide, we used in addition the constraints on the source size derived from our high angular resolution observations with the IRAM Plateau de Bure interferometer see Table 5 of Belloche et al. The first two velocity components detected in methyl cyanide and ethyl cyanide correspond to the two hot cores embedded in Sgr B2 N see, e. They are also seen in n -propyl cyanide. In addition, methyl cyanide and ethyl cyanide show a third component that may arise from the blueshifted lobe of an outflow see the cyanoacetylene emission in Figs.

The redshifted counterpart is blended with the northern component in the single-dish beam see Fig. The third velocity component is too faint to be detected in n -propyl cyanide. They found that the emission consists of several components hot core, warm envelope, diffuse and hot envelope , and mentioned that their modeling toward Sgr B2 N is uncertain because of the large opacities. However, their Fig. Therefore, the emission of the optically thin 13 C isotopologues should be dominated by the compact hot cores which gives us some confidence within a factor of 2 in the column densities listed in Table Friedel et al.

For a source size of 2. This is in very good agreement with our result see Table Table Parameters of our best-fit LTE models of methyl cyanide, ethyl cyanide, vinyl cyanide, and aminoacetonitrile, and column density upper limit for cyanomethylidyne. Assuming a temperature of K and optically thin emission, Liu et al. For a source size of , this translates into a column density of 2.

As a result, Liu et al. This is also confirmed by the reasonable agreement between the 30 m and Plateau de Bure Interferometer fluxes published by Belloche et al. The compactness of the source of ethyl cyanide emission most likely explains the discrepancy with the column density found by Nummelin et al.

While they find an order of magnitude difference between the column densities of the a - and b -type lines, we successfully reproduce the ethyl cyanide emission in our 3 mm survey with a single model for the two types of lines, the former being optically thick while the latter are optically thin.

Our model with a small source size predicts line opacities on the order of for the a -type lines in the 1. Hence, we believe that the column density derived by Nummelin et al. On the other hand, since our model predicts opacities 1 for the b -type lines at 1. For a source size of , their column density of the b -type lines translates into a column density of 9. As in Sect. Our current model, which suffers from the same problems, also over-predicts intensities for the lines detected in our partial 1.

Garrod et al. Surface formation was assumed to occur primarily by the addition of functional-group radicals derived from molecular ices or from other molecules formed in this way. The network also includes destruction mechanisms for all complex species, consisting of neutral-neutral reactions on the grain surfaces, ion-molecule reactions with simple ions in the gas phase, and cosmic ray-induced photodissociation both in the gas phase and on the grains.

To this network we have added appropriate formation and destruction mechanisms for ethyl formate, ethyl and n -propyl cyanide, and also the recently identified aminoacetonitrile NH 2 CH 2 CN, Belloche et al. In addition, surface hydrogenation routes have been added to allow for the full hydrogenation of the carbon chains C 3 and C 4 , which was not previously considered, as well as the associated hydrogenated species and their destruction channels. The techniques used to construct the new reaction set are presented in detail by Garrod et al.

Their T 2 warm-up profile is assumed, in which the hot-core temperature has a t 2 dependence on the time, t , elapsed in the warm-up phase. Dust and gas temperatures are assumed to be well coupled, hence we let.

The warm-up timescale is representative of the time required for a parcel of gas to achieve a temperature of K, as the hot core forms; it therefore does not relate directly to the current infall timescale. This model traces the evolution of the chemistry up to a temperature of K, associated with the central hot-core region. However, these time-dependent results may also be considered to represent differing spatial extents from the hot-core center, with the innermost regions being the most evolved and achieving the highest temperatures.

As such, the time-dependent abundance profiles presented below also indicate a snapshot of the chemistry through the hot core. Since we are interested mainly in specific features of the model, we choose not to fix the ice composition prior to the warm-up phase, but use the unadulterated composition computed in the collapse-phase.

Other details of the model may be found in Garrod et al. The available evidence, however, suggests there is no barrier. This change makes HCO radicals somewhat more abundant on the grains, tending to increase the final abundances of species such as methyl formate, which is consistent with our observational results. Table 14 shows the full set of surface reactions employed in the current model to form methyl cyanide, ethyl cyanide, n -propyl cyanide, aminoacetonitrile, and ethylformate, as well as a selection of significant cosmic ray-induced photodissociation processes that may occur on grain surfaces.

The same CR-induced processes are assumed also to occur in the gas phase, at the same rates. A cosmic-ray ionization rate of 10 s -1 is assumed. Table Surface reactions and cosmic-ray induced surface photodissociation processes related to the formation of cyanides, and ethyl formate.

The new reactions allow each cyanide to be constructed by sequential formation of its carbon backbone by the addition of CH 2 , CH 3 , or yet larger hydrocarbon radicals; however, photodissociation also allows the break-down of these structures. The resultant radicals may further react with another functional-group radical, to extend the backbone, or with a hydrogen atom, to terminate this sequence. Different routes will dominate according to the relative mobilities of competing radicals, and their availabilities.

Hence, the net direction of inter-conversion between cyanides may change with temperature, or as the abundances of molecular precursors vary. These latter species are formed directly by cosmic ray-induced photodissociation of methyl formate or ethanol on the grains; hence, methyl formate need not be the only precursor for ethyl formate, nor the most important one. Alternatively, addition of an oxygen atom to C 2 H 5 is unlikely to be important, due to the relative scarcity of atomic oxygen, which is mainly bound in the ice mantles as H 2 O; however, this route cannot be entirely ruled out.

When the grain surface-produced molecules evaporate, they are subject to gas-phase destruction mechanisms. Ion-molecule and dissociative recombination reaction rates are of a similar order for all new species; see Garrod et al. Table Peak gas-phase abundances from each model, with corresponding model temperatures, as well as source sizes, rotation temperatures, and gas-phase abundances derived from the observations of the main source in Sgr B2 N.

We consider first the results of the basic model described above called hereafter Basic model , using an intermediate warm-up timescale of 2 10 5 yr. This timescale was found by Garrod et al. Model abundances are converted to values per mean particle with a mean molecular weight, , of 2.

Also listed are the observed rotational temperatures and abundances Cols. The latter were derived from the column densities given in Tables 4 , 5 , 12 , and 13 , assuming an H 2 column density of 1. Given that the dust properties are uncertain by a factor 2 at least and that the contribution of the vibrationally or torsionally excited states of some molecules studied here e.

Ethyl formate is clearly formed most significantly at late times see Fig. Grain-surface methyl formate is, in fact, the primary source of precursor radicals via photodissociation for the formation of ethyl formate. When methyl formate evaporates, and ethanol is left as the dominant source of precursor radicals, ethyl formate production becomes dependent on the addition of HCO to C 2 H 5 O.

The post-evaporation gas-phase abundance of ethyl formate relative to methyl formate and formic acid appears to match observational abundances and rotational temperatures reasonably well. The gas-phase methyl formate peak abundance is also relatively close to the observed abundance within a factor 5 , and the model temperature at this peak is in very good agreement with the observed rotational temperature see Table 5.

The Basic model uses the same binding energies for methyl formate and dimethyl ether as were employed by Garrod et al. These values cause relatively early evaporation of those species, resulting in significant destruction in the gas-phase, and low fractional abundances in comparison to observed values in the case of methyl formate.

The binding energies of those molecules were obtained by simple interpolation of measured values obtained for other species. Laboratory data for methyl formate and dimethyl ether evaporation from appropriate ice surfaces are not currently available. Figure 5: a Basic model, showing methyl formate, ethyl formate, formic acid, and related species.

Solid lines indicate gas-phase species; dotted lines of the same color indicate the same species on the grain surfaces. Figure 6: a Basic model, showing cyanides. For species comprising at least one -OH functional group, binding-energy estimates take account of hydrogen-bonding interactions with the ice surface. Such species may act as both hydrogen-bond donors and acceptors, raising their binding strengths.

However, both methyl formate and dimethyl ether have at least one unbonded electron pair attached to a strongly electro-negative atom oxygen , allowing them to be hydrogen-bond acceptors. This may give them a somewhat stronger bond to the predominantly water-ice surface than has been assumed. Here, the binding energy of methyl formate is raised beyond that of the Basic model, such that it falls approximately half way between its old value and that of ethanol, its most closely-matched counterpart with a single, fully hydrogen-bonding functional group.

The binding energy of dimethyl ether is similarly raised by K. Augmentation of methyl formate binding energy allows it to remain on grains for longer, reducing the time available for gas-phase destruction, before the majority of other species evaporate, damping the effect of ion-molecule destruction pathways see Fig. This allows gas-phase methyl formate fractional abundances to remain high for longer, although the resulting peak-abundance temperature is somewhat greater, at K. Dimethyl ether does have a viable gas-phase formation mechanism, and is largely produced in the gas phase, due to the large abundance of methanol ; hence, the peak abundance is not strongly affected by the augmentation of its binding energy.

Its gas-phase abundance in the model is consistent with the observed value within a factor 2, see Table The peak-abundance temperature of the model is somewhat higher than that derived observationally. A slightly lower grain-surface methanol abundance would remedy this, as post-evaporation gas-phase methanol abundances should diminish more rapidly, reducing the rate of dimethyl ether formation. A slower warm-up subsequent to methanol evaporation would also produce a similar effect. Nevertheless, the observed rotational temperature of dimethyl ether seems consistent with gas-phase formation.

Surface formation rates of ethyl formate, methyl formate and ethanol are not strictly dependent on methanol abundance in the ices, but rather on the rate of formation of its photodissociation products, CH 3 O, CH 2 OH, and CH 3. These rates are not well constrained; however, they seem appropriate for this model. A lower grain-surface methanol abundance, as suggested above, would therefore necessitate slightly greater methanol photodissociation rates, in order to achieve appropriate abundances for methyl formate and other surface-formed species.

Gas-phase and grain-surface ethyl formate abundances are largely unaffected by the changes in methyl formate binding energy. Both the gas-phase and grain-surface abundances of formic acid are strongly dependent on gas-phase processes see Garrod et al. As a result, there appears to be no simple correlation with ethyl or methyl formate abundances. However, the low rotational temperature reported in Sect. In Sect 3. As discussed in Sect. In order to understand the behavior of the cyanide network, the different grain-surface formation mechanisms, and combinations, were isolated by artificially de-activating particular reaction routes.

In fact, all combinations that include either the hydrogenation of the cyanopolyyne HC 3 N and of vinyl cyanide, C 2 H 3 CN, or the addition of large, pre-formed hydrocarbons directly to the CN radical, produce wildly inaccurate ratios. In some such cases, n -propyl cyanide is the most abundant of all, often with methyl cyanide abundances deeply depressed.

The only combination in which the correct proportion is reproduced is that in which only the sequential addition of grain-surface CH 2 and CH 3 functional groups is allowed see Fig. We label this model, combined with the augmented binding energies of methyl formate and dimethyl ether, as the Select model. In this scheme, formation of the larger cyanides begins with cosmic ray-induced photodissociation of a smaller grain-surface alkyl cyanide molecule resulting in the ejection of a hydrogen atom , or with the accretion of CH 2 CN which may be formed in the gas-phase following the evaporation of HCN.

A methyl-group radical is then added to produce a larger alkyl cyanide molecule. Methyl cyanide itself is mainly formed on the grains by addition of CH 3 and CN radicals, but it may also be formed by gas-phase processes fuelled by the evaporation of HCN. Methyl cyanide evaporates from the dust grains around 90 K, producing its greatest gas-phase abundance; however, the subsequent evaporation of all molecular material from the grains promotes rapid gas-phase formation, maintaining methyl cyanide abundances for longer, and providing qualitative agreement with the large rotational temperature derived from the observational data.

The abundance obtained for aminoacetonitrile is in reasonable agreement with that obtained observationally within a factor 8 , suggesting that the addition of NH or NH 2 to CH 2 CN on grain surfaces, similar to the suggested mechanism for ethyl cyanide, is a plausible route to its formation. There may therefore be some degree of correlation between these two species, which should be investigated in future. The removal of the other formation routes for aminoacetonitrile, comprising the addition of grain-surface CN to either CH 2 NH or CH 2 NH 2 , makes little difference to the results, mainly due to limited availability of the latter two radicals.

Vinyl cyanide, C 2 H 3 CN, a potential precursor of ethyl cyanide and n -propyl cyanide, is formed predominantly in the gas-phase in both the Basic and Select models. This occurs through the reaction of CN with ethylene C 2 H 4 , which has been shown experimentally to be rapid over a range of temperatures Carty et al.

The resultant gas-phase vinyl cyanide then accretes onto the grains until greater temperatures are achieved.

This effect is also in qualititative agreement with its relatively high rotational temperature. Both models show good agreement with the observational abundance of this molecule, but the Select model produces an excellent match see Table These seem a fair match to the observed values of Sects. Consideration of only the low temperature formic acid peak in the models further improves its ratio with ethyl formate abundances.

Both the alternative routes - the grain-surface hydrogenation of gas phase-formed HC 3 N and C 2 H 3 CN, or the direct grain-surface addition of pre-formed large hydrocarbon radicals like C 2 H 5 or C 3 H 7 to a CN radical - appear to be very much too fast, resulting in excessive quantities of the two largest alkyl cyanides. To achieve the appropriate ratios, those two formation routes must be artificially disabled within the model.

Why should these mechanisms be less efficient in reality than they would appear from the model? The evaporation, and subsequent reaction, of HCN from the grains is a primary cause of gas-phase formation for each of these molecules. However, the agreement between observed and modeled abundances of vinyl cyanide is very good. Indeed, the Select model shows excellent agreement, providing further justification for the omission of its hydrogenation reactions.

Alternatively, surface hydrogenation of HC 3 N and C 2 H 3 CN, once they have accreted onto the grains, may be less efficient than has been assumed here. Importantly, activation energies are required for hydrogenation of both these species, whose values are poorly constrained. The fact that it is these very reactions that must be disabled suggests strongly that their activation energies should be significantly higher than has been assumed here.

In the case of the addition of large hydrocarbon radicals to CN, the over-dominance of these channels is probably due to the incompleteness of the hydrocarbon chemistry as a whole, particularly on the grains. Whilst up to 10 carbon atoms in a chain are considered in this model, the hydrogenation states of the larger chains are typically limited to 4 hydrogen atoms.

Crucially, hydrogenation is the only type of reaction included in the network for most hydrocarbons, aside from the newly-added CN addition reactions.

The hydrocarbon reaction set was largely devised with cold dark clouds in mind, where hydrogenation dominates. By including only a single new reaction addition to CN for any particular hydrocarbon, that reaction can easily become the dominant channel. The completion of the hydrocarbon network to include reactions with all major reactants would be beneficial, although this is not a trivial task.

The small hydrocarbons CH 2 and CH 3 , on the other hand, as well as CN itself, have a much more comprehensive reaction network, making sequential addition and its apparent degree of efficiency more credible.

Ethyl formate and aminoacetonitrile also seem to be well reproduced with a similar addition scheme to that of the alkyl cyanides. Ethyl formate abundance may be dependent on ethanol as well as methyl formate, depending on the specific conditions. The Select model reproduces well the abundance ratios for alkyl cyanides, but their absolute abundances are an order of magnitude lower than observational values.

This also results in a poor match to abundance ratios relative to methyl formate and other methanol-related species. In fact, the chemistries of the cyanides and the methanol-related species do not strongly influence one another in the model. The overall abundances of each category of molecule are mainly influenced by different, independent parameters: the formation rate of the products of methanol photodissociation i.

Similarly, the modeled abundance of aminoacetonitrile relative to the alkyl cyanides is very high. The formation rate of this molecule is strongly dependent on the product of the abundance of NH 3 in the ices, and its rate of photodissociation.