Many meteoritic and interplanetary dust particle (IDP) samples contain bulk enhancements and hotspots rich in N-15. Similarly low C(14)N/C(15)N ratios have been observed in numerous comets, An almost constant enrichment factor in comets from disti'nct formation zones in the nebular disk (i.e. both Jupiter Family and Oort Cloud comets), strongly suggests that this fractionation is primordial and was set in the protsolar cloud core. Deuterium enrichment is observed in both meteorites and IDP

Charnley, S. B.

Cordiner, M. A.

Milam, S. N.

Wirstrom, E. S.

NASA Technical Reports Server

43rd Lunar and Planetary Science Conference (2012) 2457.pdf 
SPIN-STATE-DEPENDENT ION-MOLECULE CHEMISTRY AS THE ORIGIN OF 15N AND D ISOTOPIC 
ANOMALIES IN PRIMITIVE MATTER. E. S. Wirstrom', S. B. Charnley" M. A. Cordiner', S. N. Milam', 
'Astrochemistry Laboratory, Code 691.0, NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 
20771, USA (email: eva.s.wirstrom@nasa.gov). 
Introduction: Many meteoritic and IDP samples 
contain bulk enhancements and hotspots rich in 15N (e.g. 
[1,2,3]). Similarly low C14N/C15N ratios have been ob-
sen'ed in numerous comets [4], An almost constant en-
richment factor in comets from disti'nct formation zones 
in the nebular disk (i.e. both Jupiter Family and Oort 
Clo~d comets), strongly suggests that this fractionation 
is primordial and was set in the protsolar cloud core [5J. 
Deuterium enrichment is observed in both meteorites 
and IDPs [6,2, 7J. 
Interstellar Chemistry: Ion-molecule reactions at 
low (~IO K) temperatures can lead to an enhanced D 
and 15N fractionation in interstellar molecules [8, 9J. 
Atcm-molecular ion reactions of the type 
15N + 14N2H+ ~14N + 15N14NH+ are important in en-
riching molecules in heavy nitrogen and, as in: the Cfse of 
deuterium, significant CO depletion permits much larger 
isotopic effects [IOJ. Helium ion attack on 14N15N re-
leases the 15N nuclei, as well as 15N+ ions which may 
react with H2 to form 15NH+ and subsequently ammo-
nia. Condensation of these gaseous products on dust 
grains leads to ices with bulk enhancements in ammonia 
of +800 %" and peak monolayer enhancements of a fac-
tor of seven (i.e. 6'5N = +6000 %" [II]. CO depletion 
and a negligible ratefor the reaction N + CN --+ N, + C 
pre'lents recycling of N atoms back into N, and leads 
to two distinct 15N fractionation pathways in dense in-
terstellar gas [12]: a slow one to ammonia (_10' years) 
and more rapid one to HCN and other nitriles (~lO' 
years). 
Isotopic Carriers and Correlations: The major 
goals for theories of isotopic fractionation in priLlitive 
materials based on interstellar chemistry are the ex-
planations of isotopic correlations, or lack thereof, the 
highly variable enrichment in specific molecular func-
tional groups, and the identification of candidate inter-
stellar fractionation environments and precursor molecules 
[13, 14, IS, 16]. In principle, the nature of the functional 
groups which display isotopic enhancements should be 
rel<ted to the interstellar molecules from which they de-
rive. For example, we may expect that isotopic anoma-
lies measured in carbonyl, nitrile, amine, aliphatic and 
aromatic functional groups [17, 18, 13,2,7, 19J should 
be related to the isotope ratios in potential interstellar 
pregenitors such as CO, HCN, NU3 , CH, & C,H6 , and 
PARs. 
The most pressing problem, however, concerns the 
fact that, while 15N and D hotspots do seem to corre-
late in some samples (e.g. [18]), they clearly do aot in 
others (e.g. [2, 7]), even down to the level of molecular 
subgroups in amino acids [16J. Yet, as has been noted 
by several authors [20, 21], the interstellar environments 
most conducive to producing enormous 15NP4N ratios, 
should also produce concomitantly large molecular DIH 
ratios (e.g. in ammonia or HCN). Thus, one would 
expect 15N and D hotspots to always be spatially corre-
lated, contrary to what is seen. This presents a serious 
challenge for interstellar-type ion-molecule chemistry. 
Spin-State-Dependent Isotopic Chemistry: The 
reaction rates associated with 15N and D fractionation 
vary strongly in the temperature range ~5-40 K [15]. 
Although a temperature regime may exist where modestly-
deuterated molecules with minimal 15N enrichment could 
exist, the converse - large 15N enrichments and no 
deuteration- , as seen in primitive matter, cannot occur. 
However, careful consideration of interstellar chemistry 
suggests a possible resolution of this problem. Some 
interstellar molecules can exist in either of two distinct 
states depending upon the relative alignment of the spins 
of their H nuclei (parallel or antiparallel), giving rise to 
ortlw and para forms. For molecular hydrogen, the o-H, 
ground state (J = 1) is 170 K above the p-H, ground 
state (J = 0); this internal energy difference can have a 
profound effect on ion-molecule chemistry at low tem-
peratures, especially deuteration [22, 23]. 
The key reaction for making molecular clouds molec-
ular is the formation of H, molecules on dust grains 
which are probably ejected into the gas upon forma-
tion with an olp abundance ratio (aPR) of 3: 1. At low 
temperatures, ion-molecule spin-exchange reactions in-
volving H+ and Ht will tend to convert most o-H, to 
p-H2 • The time-scale for this conversion depends on the 
ionization rate (, but is generally shorter than, or com-
parable to, the estimated lifetime of molecular clouds 
[24J. If present, a high abundance of 0-H, molecules 
acts as a 'poison' for deuterium fractionation because 
the reaction 
0-H,D+ + 0-H2 --+ p-Ht + HD 
can proceed rapidly, even at 10 K, and suppress inter-
stellar deuteration [24]. 
Conversely, the internal energy of o-H, is necessary 
to overcome the small barrier in the initiating reaction 
for ammonia formation, and hence 15NH3 fractionation: 
N+ + o-H, --+ NH+ + H 
Early estimates concluded that an H, aPR in excess of 
~ I 0-4 would be sufficient to account for the observed 
dense cloud ammonia abundances [25]. However, a re-
cent re-assessment of the aPR dependence in the orig-
inal experimental data indicates that the rate coefficient 
of this process has been overestimated by almost three 
orders of magnitude at low temperatues [26]. Thus, if 
https://ntrs.nasa.gov/search.jsp?R=20120013455 2019-08-30T21:52:40+00:00Z
43rd Lunar and Planetary Science Conference (2012) 
Dashed overlay: ~ OP"I • 1 e-5 
10' 10' 
lime (yrs) 
Figure 1: Time evolution of 15N enrichment in a cloud at 
physical paramets as given in the frame. The 14N/15N abun-
dance ratio is set to 400. Solid and broken curves are for H2 
OPR values of 3 and 1 x 10-5 respectively. Values for gas-
phase nitriles are not plotted after l05yr because the molecu-
lar abundances are negligible (see [12]). 
the H, aPR is comparatively large, deuteration is inhib-
ited but 15N fractionation can proceed. This leads to the 
expectation that, at low-temperatures, the H, aPR could 
mejiate a large range of D- 15N fractionation ratios in 
interstellar molecules. 
Model: To demonstrate the effect of the H, aPR on 
interstellar 15N fractionation, we have incorporated the 
resalts of Dislaire et al. [26] into the chemical model 
of Rodgers & Charnley [12]. We consider aPR values 
of 3 and I x 10-5 , the latter consistent with observed 
limits [27]. Fig. I shows that a low aPR does sup-
press 15N fractionation in ammonia although the ni-
triles remain enriched. On his time-scale (t ~ 105 yr) 
the gaseous molecules will become increasingly en-
riched in D as CO freezes out on dust. The fact that 
N+ + H, is less efficient when aPR ~ lx 10-5 means 
that at later times (t ~106 yr) the exchange reaction 
15N+ + 14N2 ~14N+ + 15N14N operates to enrich 15N 
in !'h, N,H+ and NH3 , although their abundances are 
quite low (cf. [II]). 
Summary: A more sophisticated treatment of low-
temperature ion-molecule chemistry, involving spin-state 
dependence in molecular reactants, has the potential to 
provide a natural explanation of the distribution of iso-
topic anomalies exhibited in primitive matter. Compre-
hensive theoretical models which account for these pro-
cesses in D, l5N and l3C fractionation can be tested by 
comparison with future ALMA observations of multiply-
fractionated molecules in analogues ofthe presolar cloud 
core, and allow us to quantify the interstellar contti-
bution to (functional group) fractionation measured in 
2457.pdf 
primitive materials. 
Acknowledgment This work was supported by NASA's 
Origins of Solar Systems Program. 
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Spin-State-Dependent Ion-Molecule Chemistry as the Origin of N-15 and D Isotopic Anomalies in Primitive Matter.

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