1996MNRAS.281..137P
Mon. Not. R. Astron. Soc. 281,137-144 (1996)
Novel pathways to CN - within interstellar clouds and circumstellar
envelopes: implications for IS and CS chemistry
Simon Petrie*
School of Chemistry, University College, University of New South Wales, ADFA, Canberra, ACT 2600, Australia
Accepted 1996 January 18. Received 1996 January 18; in original form 1995 November 6
ABSTRACT
Prospects for detection of the diatomic negative ion CN- within dense interstellar
clouds and/or circumstellar envelopes are reassessed in the light of new pathways to
small negative ions in such environments. Formation of CN- by charge transfer
from PAH- to CN, and by dissociative attachment of free electrons to MgCN and
MgNC, is discussed. These pathways are expected to lead to much higher CNabundances than can be formed by those reactions which have been included in
previous models of interstellar negative-ion chemistry. Investigation of electron
attachment to Mg(CN) is assisted byab initio calculations, at the G2level of theory,
upon the structure and total energy of singlet and triplet states of MgCN- and
MgNC - . The model of CN- chemistry which we present here shows that interstellar
abundances of PAH- are probably too low to produce detectable CN-, but the
efficient occurrence of dissociative attachment to MgNC and MgCN within the
circumstellar envelope IRC + 10216 may produce CN- at an abundance of
",2 x 10- 10 n(Hz), which should permit the detection of CN- via its rotational
spectrum.
Key words: molecular processes - circumstellar matter - ISM: abundances - ISM:
clouds - ISM: molecules.
1 INTRODUCTION
Formation of small negative ions within dense interstellar
(IS) clouds has been discussed previously by several authors
(Dalgarno & McCray 1973; Sarre 1980; Herbst 1981). It is
generally considered, however, that the role of small negative ions within gas-phase IS cloud chemistry is minimal, in
contrast to the major contribution made by positive-ion
chemistry. Negative-ion formation within IS clouds involves
attachment of an electron to a neutral species: in its simplest
form, this is a radiative process
X+e--+X-+hv.
(1)
Reaction (1) is of negligible efficiency for small species X
(e.g., 1-3 atoms) having few rotational and vibrational
degrees of freedom available for energy dispersal, owing
to the very short lifetime against autodetachment
(X-)* --+X + e
*E-mail: [email protected]
(2)
of the activated complex (X-)*. The lifetime of this complex increases steeply with increasing molecular complexity:
Herbst (1981) contends that occurrence of radiative attachment (RA) to high-electron-affinity species as small as C 3N
and C4H may be significant, and RA to polycyclic aromatic
hydrocarbon (PAH) molecules
PAH+e--+PAH- +hv
(3)
may well be efficient for several PAHs under both dense
and diffuse IS cloud conditions (Omont 1986; Lepp & Dalgarno 1988). Radiative attachment to the fullerenes C60 and
C70 has also been discussed by Millar (1992). Recent laboratory studies have, however, indicated that attachment of
low-energy (T::;; 300 K) electrons to anthracene, C 14H lO
(Canosa et al. 1994) and to C60 (Smith, Spanel & Mark 1993;
laffke et al. 1994) is impeded by activation energy barriers,
implying that RA to these species under IS cloud conditions
will be very inefficient. Similar studies on C70 (Spanel &
Smith 1994) suggest the absence of a barrier to one mode of
electron attachment. Clearly, the topic of RA to molecules
at the very low temperatures characteristic of IS clouds is
one in which much remains to be explored.
©1996 RAS
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1996MNRAS.281..137P
138 S. Petrie
The cyanogen radical CN is abundant within IS clouds for example, n(CN)=3 x 10- 8 n(Hz) within TMC-l (Irvine,
Goldsmith & Hjalmarson 1987) - and larger species containing the CN group comprise an important subset of the
species detected within dense IS clouds and circumstellar
(CS) envelopes. Cyanogen also has a very high electron
affinity, EA(CN)=3.82±0.02 eV (Berkowitz, Chupka &
Walter 1969), probably the highest of any atomic or
diatomic species. Furthermore, CN- is a polar species,
,u(CN-)~0.6 Debye (Botschwina 1985; Peterson & Woods
1989), which should be detectable by virtue of its rotational
transitions: this contrasts with the situation for an atomic
negative ion, whose identification requires detection of
electronic absorption or emission features. The cyanide ion
is therefore an ideal test species to investigate the factors
affecting the formation of small negative ions within IS
clouds and CS envelopes. We note that, while no negative
ions have yet been detected within any IS or CS environments, the Giotto probe has detected several negative ions,
having molecular masses in the ranges 7-19, 22-65 and
85-110 amu, as components of the inner coma of comet
P/Halley (Chaizy et al. 1991): CN- has been implicated as a
likely constituent of the 22-65 amu 'group' in this source.
In the present work, we provide a reassessment of pathways to CN - formation under typical IS and CS conditions,
and discuss the prospects for detection of this ion within
objects such as TMC-l and IRC + 10216.
predicts very low abundances for CN- as well as for the
other small negative ions which are included. At 'early'
time (3.16 x 105 yr), which is generally held to mark the
peak
abundance
of many
molecular
species,
n(CN-) = 1.28 x 10- 15 n(Hz) is obtained. This is far too low
an abundance to permit the detection of CN - with current
techniques - especially since, with ,u ~ 0.6 Debye (Botschwina 1985; Peterson & Woods 1989), CN- is not strongly
polar.
2.2 Radiative attachment to CN
The inclusion of reaction (10) as a source for CN- is not
expected to increase n (CN -) significantly. This reaction has
not been studied experimentally, but is likely to proceed at
a rate similar to that of RA to an atomic species. In the
UMIST model (Millar et al. 1991), RA rate coefficients of
k ~ 3 x 10 -15 em3 molecule -I s -I are estimated for C, 0 and
S. If a value of klO~ 1 x 10- 14 em3 molecule-I S-I is used for
reaction (10), and the UMIST early-time abundances are
used for other species in the above reactions, then the relative rates of CN- formation by the UMIST reactions and by
reaction (10) are (r4+r5+r6+r7):rlO~2. A significantly
higher rate coefficient for reaction (10) would, of course,
favour the occurrence of this reaction, but the value of
1 x 10- 14 cm3 molecule-I S-I which we have suggested
appears realistic. A more detailed mechanism for RA,
CN+e-+(CN-)*
(12)
2 DISCUSSION
(CN-)* -+CN + e
(13)
2.1 Existing models of CN - chemistry
(CN-)* -+CN- +hv,
(14)
Of the complex kinetic models of dense cloud chemical
evolution, only the UMIST ratefile of Millar et al. (1991)
has included a treatment of negative-ion formation and
destruction. This model, which includes only the simple
negative ions H-, C-, 0-, S-, OH- and CN-, incorporates CN- formation via the charge-transfer reactions
illustrates the competition between autodetachment (13)
and radiative stabilization (14) of the activated complex
(CN-)*. For a polyatomic species, a high density of vibrational states at the energy of the activated complex allows
dispersal of the excess energy of (CN -) *, greatly extending
its lifetime against autodetachment and increasing the probability for radiative stabilization. The CN - ion possesses
only one vibrational model, with a calculated frequency
v=2052±6 cm- I (Botschwina 1985). When anharmonicity
is taken into consideration, this yields a density of states
N ~6.0 x 10- 4 (cm-I)-I at the energy of the activated complex. Use of this density-of-states value, in the expression
proposed by Herbst (1981) as an approximate measure of
the RA rate coefficient, yields klo~ 1.4 x 10- 17 cm3 molecule-I S-I at T=10K. This value, of a similar order of
magnitude to those employed by Millar et al. (1991) for RA
to C, 0 and S atoms, serves to illustrate that vibrational
dispersal of the energy of (CN-)* is unable to promote
efficient RA to the CN radical.
The above discussion indicates that RA is incapable of
generating detectable CN- within IS environments.
0- +CN-+CN- +0
(4)
OH- +CN-+CN- +OH
(5)
and the proton-transfer reactions
0- + HCN -+CN- + OH
(6)
OH- + HCN -+CN- + HzO.
(7)
Of these processes, (7) predominates as a CN - source by
virtue of the higher abundances calculated for the reactants
involved. Removal of CN - in the UMIST model is achieved
by the rapid associative detachment reactions
CN- +H-+HCN +e
(8)
CN- + CH 3 -+CH3CN + e.
(9)
This simple model, which neglects CN- formation by radiative attachment
CN +e-+CN- +hv,
(10)
The inclusion of reactions (4) and (5) as sources of CN - in
the UMIST model (Millar et al. 1991) is of interest, even
though these reactions produce only very small CN- abundances. Charge transfer
(11)
(15)
as well as its removal by mutual neutralization
CN- +M+-+M+CN,
2.3 Charge transfer to CN
© 1996 RAS, MNRAS 281,137-144
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1996MNRAS.281..137P
CN- within interstellar clouds
is intrinsically more efficient than radiative stabilization
(10): a common feature of charge-transfer reactions involving negative ions and neutral molecules is that they occur at
high rates, k - kc (where kc is the collision rate coefficient)
and without apparent activation energy barriers when exothermic (Kebade & Chowdhury 1987). The very low yield
of CN- from reactions (4) and (5) in the UMIST model
is a consequence of the very low abundances of the
reactants 0- and OH- [n(0-)=1.7 x 10- 17 n(H2);
n(OH-)=6.8 x 10- 14 n(H2)] in this model (Millar et al.
1991).
A high electron affinity [EA(CN) =3.82 eV] ensures that
reaction (15) is exothermic for many negative ions Y-. If
suitably abundant reactant ions Y- can be identified, therefore, reaction (15) may be significant at typical dense IS
cloud temperatures.
The prime candidates for reactants capable of initiating
reaction (15) are PAH- ions, and perhaps fullerene ions
such as C6ii. We have noted in Section 1 that recent studies
indicate the presence of activation energy barriers to electron attachment to anthracene and C60 : Canosa et al. (1994)
have suggested that the rate coefficients used in previous
models of interstellar PAH chemistry (e.g. Omont 1986;
Lepp & Dalgarno 1988) for RA to PAHs may be too high by
several orders of magnitude. However, the apparent 'lack of
an activation barrier to s-wave electron capture by ~O,
manifested by a negative temperature dependence for this
process (Spanel & Smith 1994), illustrates the dangers of
overgeneralizing from a small collection of measurements.
It appears that RA to some PAHs and fullerenes will be very
inefficient at IS cloud temperatures: it remains, however,
also very likely that some PAHs will readily attach electrons
under IS conditions. Further studies are needed on this
matter. Here we assume that a total PAH- abundance of
(1-5) x 10- 9 n (H2) is reasonable, this value being somewhat
lower than the most optimistic estimates based on efficient
RA to all PAHs (Omont 1986; Lepp & Dalgarno 1988).
The composition of neutral, as well as negatively charged,
PAHs within IS environments is still the subject of much
conjecture: while PAHs as a class of compounds are widely
held to be the carriers of the diffuse IS bands (Leger &
d'Hendecourt 1985; van der Zwet & Allamandola 1985;
Cossart-Magos & Leach 1990), no particular PAH has yet
been conclusively identified as an IS molecule. Similarly,
fullerenes such as Coo have also been proposed as probable
large IS molecules (see, e.g., Hare & Kroto 1992; Webster
1992; Petrie, Javahery & Bohme 1993), and, while the formation of fullerene negative ions within dense clouds has
been discussed by Millar (1992), evidence for the presence
of fullerenes in IS environments is rather tenuous (Webster
1993; Foing & Ehrenfreund 1994). Nevertheless, an impression of the reactivity of interstellar PAHs and fullerenes can
be gained from the general features of the chemistry of
these compounds under laboratory conditions. With regard
to PAH- reactivity, the most important parameter is the
EA of the corresponding PAH molecule. We list representative values of EA (PAH) in Table 1.
The exothermicity of charge transfer
139
Table 1. Electron affinities of polycyclic aromatic hydrocarbons
(PAHs).
PAHa
Naphthalene
Formula
EA:
Expt b
Theor c
CIOHS
0.15 d
0.07
0.26
CU#12
0.28 d
Phenanthrene
C14H IO
0.31d
0.27
Chrysene
CIsHI2
0.42d
0.51
0.61
Triphenylene
C20H 12
0.49 d
Benzo[c]phenanthrene
CIsHI2
O.54 d
0.38
Anthracene
Cl~1O
0.57 e
0.65
Pyrene
CIsHlO
0.58 d
0.66
Dibenz[a.h]anthracene
C22H 14
O.68 d
0.65
Dibenz[a,j]anthracene
Benzo[c]pyrene
C22H I4
0.69 d
0.58
Benz[a]anthracene
CIsHI2
0.70 d
0.64
Benz[a]pyrene
CzoHI2
O.7g e
0.93
Perylene
C2oH12
0.97 e
Tetracene
C1SH 12
1.04 e
Pentacene
C22H 14
1.35 e
Buckminsterfullerene
C60
1.06
>2.271
-2.7K
Notes.
·PAH molecules, in order of increasing (experimental) EA. bEx_
perimental electron affinity in eV. cTheoretical electron affinity in
eV, from molecular orbital calculations (Dewar, Hashmall & Venier 1968). dFrom Becker & Chen (1966). eFrom Crocker, Wang &
Kebarle (1993). fFrom Sunderlin et al. (1991). "From Yang et al.
(1987).
for all of the PAHs listed; charge transfer from C6ii is also
exothermic, but considerably less so. These values suggest
that all charge-transfer reactions of odd-electron PAH - ions
to CN are probably exothermic, with the potential to be
efficient under IS cloud conditions. Consideration should
also be given to charge transfer to CN from even-electron
PAH- ions, since such species may also account for a significant fraction of PAH- ions in IS clouds. However, very
few thermochemical data exist on such species. The tabulation of Lias et al. (1988) indicates only that charge transfer
from the fluorenide ion
(17)
is exothermic by -2.0 eV.
The product channel (16) is not the only possible outcome of PAH-/CN reactions. Other channels which may
compete with charge transfer from odd-electron PAH - ions
are radiative association
PAH- +CN-+PAH.CN- +hv
(18)
and hydrogen-atom transfer
CnH';;- + CN -+CnH';;-_1 + HCN,
(19)
(16)
while associative detachment is a probable competing process for both odd- and even-electron PAH- ions:
is given by -MlI6=EA(CN)-EA(PAH). The values in
Table 1 show that charge transfer is exothermic by ~ 2.4 e V
(20)
© 1996 RAS, MNRAS 281, 137-144
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1996MNRAS.281..137P
140 S. Petrie
No such reactions involving CN have yet been studied
experimentally. There are grounds for expecting that reactions (18) and (19) are unlikely to dominate over (16) when
the latter is considerably exothermic, although competition
from channel (20) may be important in limiting the efficiency of charge transfer (16).
Association (channel 18}. Model calculations suggest that
radiative association reactions involving many-atom species
are usually efficient under IS conditions (Dunbar 1990;
Herbst & Dunbar 1991). The principal requirements for
efficient radiative association are that the collision complex
potential energy well is sufficiently deep, and the coupling
between internal modes of energy dispersal is sufficiently
strong, to promote a long-lived collision complex. These
conditions may well not be met in the case of reactions of
type (18), since PAHs possess (for their size) comparatively
few low-energy bending or stretching vibrational modes
available for energy dispersal. The product channel for
reaction (18) presumably involves a collison complex featuring the interaction of CN- with the 1t cloud above one face
ofthe PAH. Short-range repulsive forces between the negative charge on CN- and the 1t cloud of the PAH will limit
the collision complex well depth, and are hence likely to
disfavour adduct formation in the presence of a competing
exothermic product channel (16). Furthermore, and regardless of the nature of the collision complex, the coupling
between the C-C stretching vibrational modes (which are
associated with the PAH's 1t electron cloud) and the
C-H modes (which are not) is likely to be weak, further
limiting the accessible degrees of freedom for complex
stabilization.
Hydrogen transfer (channel 19). Grounds can also be
established for expecting that reaction (19) is inefficient,
especially in comparison to the exothermic transfer of an
electron (16). Hydrogen atom transfer necessitates an interaction between CN and a C-H bond of PAH -, but CN
should be preferentially drawn (by ion-induced dipole interactions) towards the 1t cloud of PAH - , limiting the accessibility of product channel (19) for which an 'edge-on' attack
of PAH - by CN appears most favourable. Experimental
evidence for the inefficiency of hydrogen atom abstraction
reactions of polyatomic, aromatic ions with radicals has
been reported recently, in a selected-ion flow tube (SIFT)
study involving the reactions of C6H5+ ,C6~+ and C6Hi with
atomic hydrogen: for each of these ions, hydrogen atom
abstraction is the sole exothermic bimolecular product
channel
C6H,! +H-+C6H'!_1 +Hz
(21)
but does not occur detectably: kZI < 1 X 10- 11 em3 molecule-I S-I at 300 K (Petrie, Javahery & Bohme 1992).
Associative detachment (channel 20). The thermochemistry of reaction (20) is known only for the reaction of the
naphthalenide ion CloH 7- , an even-electron PAH - ion.
Associative detachment involving CN is exothermic in this
instance by -5.4 eV (Lias et al. 1988): this is much more
exothermic than the competing charge-transfer channel
(16) is expected to be, suggesting that associative detachment should dominate in this instance. Since associative
detachment of CN to other even-electron PAH- ions is
likely to be similarly exothermic, we expect that reaction
(20) should dominate over reaction (16) for all even-electron PAH - ions. The situation regarding associative
detachment to odd-electron PAH - is less certain: while
almost certainly exothermic, it involves formation of a
neutral radical and hence is likely to be less exothermic than
channel (20) involving even-electron PAH-. Conversely,
product channel (16) is almost certainly more exothermic
for most odd-electron PAH - than for even-electron PAH - ,
and so we anticipate that channel (16) is more likely to
compete with channel (20) in the reactions of odd-electron
PAH- with CN. Experimental investigation of this issue
would be very useful.
For odd-electron PAH- ions, one further factor likely to
affect the efficiency of the possible product channels (16),
(18) or (19) is that of electron spin conservation. If spin is
conserved in these reactions, and if it is assumed that reactions (16), (18) and (19) are exothermic for generation of
ground-state products but endothermic for all product
channels involving excited electronic states, then a selection
factor of less than unity must be imposed on these product
channels upon the grounds of spin conservation. It should
be noted, however, that positive ion/radical reactions are
often not constrained by spin conservation (Ferguson 1983;
Federer et al. 1986), and the same may well be the case in
the reactions of negative ions. It should also be borne in
mind that associative detachment (20) of CN will always
conserve spin, and so should dominate for those spin states
which correspond to excited states of the products of channels (16), (18) and (19).
In the model which we present below, we shall consider
that charge-transfer reactions (16) conserve spin. However,
since the thermochemistry of even-electron PAH- ions is
not well established [and since associative detachment (20)
is likely to dominate in the reactions of these ions with CN],
we shall limit our model to the inclusion only of odd-electron PAH - ions.
2.4 Proton transfer from H(eN)
By analogy with the consideration of charge transfer from
PAH - to CN, it is of interest also to consider the prospects
for proton transfer from HCN or HNC to PAH-:
CnH';;- + HCN-+CnHm+1+CN-
(22)
CnH,;;- + HNC-+CnHm+1+ CN-.
(23)
Proton transfer is generally rapid when exothermic (Bartmess & McIver 1979). The exothermicity of reaction (22)
or (23) depends upon the relative gas-phase acidities
of CnHm+1 and H(CN): -MlZZ=Mlacid(CnHm+I)Mlacid(HCN),
and
-Ml23=Mlacid(CnHm+I)MlaciiHNC), where Mlacid(HCN) = 1438 kJ mol-I (Lias et
al. 1988) and Mlacid(HNC) -1377 kJ mol-I. Few thermochemical data exist on the gas-phase acidities of PAHs:
measurements on derivatized PAHs (Lias et al. 1988) suggest that reaction (22) may be exothermic only for small
PAH- ions, while reaction (23) may be exothermic for
much larger PAH - ions. Given the lack of thermochemical
data on such processes, we will not include reactions (22) or
(23) in our model of CN- chemistry: however, consideration of such processes may be worthwhile if gas-phase acidities of representative PAHs are determined.
© 1996 RAS, MNRAS 281, 137-144
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1996MNRAS.281..137P
CN- within interstellar clouds
2.S Dissociative attachment to ZCN
141
Dissociative attachment (DA) to CN-containing species
micity of DA is not an absolute guarantee of its efficiency.
For example, the DA reactions
ZCN+e--+CN-+Z
Clz + e --+ CI + CI- ,
(27)
CH3Br + e --+ CH3+ Br-
(28)
(24)
has not been considered in previous models of IS chemistry.
Reaction (24) is exothermic if the bond strength D(Z-CN)
is less than 3.82 eV, the electron affinity of CN. In Table 2,
we list the bond strengths of known interstellar or circumstellar CN-containing species. The table shows that covalent
bonds from CN to C, H or N are too strong for DA to such
species to be exothermic, and so none of the dense-cloud
species in Table 2 can act as sources of CN -. Of the CS
metal cyanides, which have recently been detected within
IRC + 10216 (Kawaguchi et al. 1993; Turner, Steimle &
Meerts 1994; Ziurys et al. 1995), the Na-CN bond strength
is also too high to permit DA: however, the comparatively
low bond strength calculated (Gardner et al. 1993; Petrie
1996a) for both isomers of Mg(CN) indicates that DA is
exothermic for MgCN and for MgNC:
MgCN +e--+Mg+CN-,
(25)
MgNC + e --+ Mg + CN- .
(26)
Given the comparatively high abundance of MgNC within
IRC + 10216 (Kawaguchi et al. 1993), these processes may
be of considerable importance as potential precursors to
CN-.
It is necessary to estimate the efficiency of reactions (25)
and (26). Very few studies of DA at subthermal energies
have been reported (Dunning 1995). Room-temperature
studies, involving low-energy DA to haloalkanes (Underwood-Lemons, Gergel & Moore 1995), to HI (Smith &
Adams 1987), to XeF2 (Sides & Tieman 1976), and to
various 'strong acids' such as HN0 3 and H 2S04 (Adams et
al. 1986), suggest that exothermic DA often approaches
collisional efficiency at low energies. However, the exother-
Table 2. Bond strengths D (Z-CN) for interstellar and circumstellar CN-containing species.
are exothermic by 1.1 and 0.3 e V respectively (Lias et al.
1988), yet are inhibited by activation energy barriers (Christodoulides, Schumacher & Schindler 1975; Datskos, Christophorou & Carter 1992; Underwood-Lemons et al. 1995)
which would render them very inefficient at low temperatures. The efficiency of DA involving MgCN or MgNC can
only be assessed through experimental investigation: in the
absence of such a study, we shall assume here the most
'optimistic' case, in which reactions (25) and (26) are unimpeded by activation barriers.
An additional consideration which has not previously
been addressed in studies of low-energy DA is the influence
of spin conservation. The reaction of doublet Mg(CN)
(either isomer) with an electron will occur upon the triplet
potential energy surface on 75 per cent of collisions, and
upon the singlet surface the remaining 25 per cent. To
investigate the thermochemistry of the [Mg(CN)-)* activated complexes, we have performed Gaussian-2 (G2) ab
initio calculations according to the method of Curtiss et al.
(1991) upon the singlet and triplet negative ions formed by
electron attachment to MgCN and MgNC. The results of
these calculations are summarized in Table 3.
Comparison of the results for the lowest-lying singlet and
triplet states of Mg(CN) - indicates that formation of triplet
Mg( CN) - from Mg(CN) + e is endothermic for both isomers, and thus presumably involves approach upon a repulsive surface. While such a surface is not necessarily an
impediment to DA, formation of triplet (Mg + CN-) from
Mg(CN) + e is also substantially endothermic, so that DA
upon the triplet surface is possible only upon spin flipping.
In contrast, DA upon the singlet surface is exothermic and
fully spin-allowed. As mentioned above, we assume here
that exothermic DA to MgCN and MgNC (i.e., upon the
singlet surface) does not involve any barriers, and hence is
efficient at low temperatures.
Reference
2.6 Loss processes for CN-
HCN
5.36 eV
Lias et al. 1988 b
HNC
4.68 eV
Lias et al. 1988 b
In keeping with previous treatments of negative-ion IS
chemistry (Dalgarno & McCray 1973; Herbst 1981; Millar
et al. 1991), we consider the two principal loss processes for
CN- to be the associative detachment reaction (8) and
mutual neutralization (11). Experimental studies suggest
that both of these classes of reaction are highly efficient
(Howard, Fehsenfeld & McFarland 1974; Smith, Church &
Miller 1978), with typical values (at 300 K) of k8 - 1 X 10- 9
cm3 molecule-I S-I and kl1 -(4-10) x 10- 8 cm3 molecule-I
S-I. Mutual neutralization reaction rate coefficients are
expected, on theoretical grounds (Hickman 1979), to
possess a temperature dependence of _T- O.5, so that at IS
cloud temperatures of - 10 K, values of kl1 - (2-6) X 10- 7
em3molecule -I s -I are anticipated. If n (H) _ 5 x 10-5 n (Hz)
and n(M+)-3 x 10- 8 n(Hz) within a 'typical' dense IS
cloud, it can be inferred that reactions (8) and (11) will be
broadly comparable in importance as loss processes for
CN-.
ZCN
D(Z-CN) "
- 6.0eV
CHaCN
Lias et al. 1988 b
CH3CN
5.23 eV
Lias et al. 1988 b
CCCN
5.23 eV
Francisco & Richardson 1994 c
HCCCN
6.37 eV
Francisco & Richardson 1994 c
CH2CHCN
5.34 eV
Lias et al. 1988 b
H2NCN
5.34 eV
Lias et al. 1988 b
Na(CN)
4.50eV
Petrie 1996b c
MgCN
3.26 eV
Gardner et al. 1993 c
MgNC
3.33 eV
Petrie 1996a c
Notes.
"Bond dissociation energy Dg in eV. bExperimental value.
<Theoretical value, obtained from the G2 ab initio method;
estimated uncertainty of ±O.1 eV.
© 1996 RAS, MNRAS 281,137-144
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1996MNRAS.281..137P
142 S. Petrie
Table 3. Geometries and total energies determined for
Mg(CN)-.
Geometries:
Species'
j'
r(Mg-C)b
IMgNCIMgCN"
2.285
3MgNC3MgCN"
2.174
Eo (Hartrees) C
IMgNC-
Bb
products
kj
C
f(A)
d
n(B) •
r(Mg-N)b
10
e
CN
CN- +hv
1(-14)
0.9
3(-8)
1.196
2.126
16
PAir
CN
PAH + CN'"
5(-10)
0.031
3(-8)
1.191
25
e
MgCN
Mg+CN'"
2.5(-8)
0.9 0; 7(-10)g,/l
1.194
26
e
MgNC
Mg+CN'"
2.5(-8)
0.9
0; 1.5(-8) g,i
8
CN'"
H
H(CN) +e
1(-9)
7
5(-5)
11
M+
CN'"
M+CN
5(-7)
1
2.051
1.179
Erel d
tlll'lf,o e
-292.41879
-117.8
90.9
IMgCN"
-292.41369
-104.4
104.3
Mg + eN"
-292.373931
0.0
MgNC + e
-292.35009 h
62.6
271.3
MgCN + e
-292.34777 i
68.7
277.4
3MgCN"
-292.33079
113.2
321.9
-292.33057
113.8
322.5
3MgNC-
Ab
r(C-N)b
Energies:
Species
Table 4. Mechanism of CN - formation and destruction in IS clouds
and CS envelopes.
208.7g
Notes.
"All the structures listed are linear (LMgXY = 180°).
bBond length in A, obtained from the optimized geometry
at the MP2(full)/6-31G* level of theory. CTotal energy
(including ZPE) in hartree, obtained according to the G2
procedure of Curtiss et al. (1991). dG2 energy (in kJ
mol-I) expressed relative to Mg + CN-. 'G2 enthalpy of
formation (zero-K value) of the molecular species indicated. 'From Curtiss et al. (1991). gExperimental value:
LlliJ.o(Mg)+MIJ(CW)=220.5 kJ mol- 1 (Lias et al.
1988). hFrom Petrie (1996a). 'From Gardner et al.
(1993).
2.7 A general model of eN - chemistry
The reactions discussed in Sections 2.1-2.6 can be incorporated in a model of cyanide-ion chemistry, which is
detailed in Table 4. This model does not include those
reactions (4)-(7) which feature in the UMIST model
(Millar et al. 1991): these reactions are excluded here
because they appear not to lead to significant abundances of
CN- and because the abundances of the reactant ions 0and OH- are arguably very uncertain. In contrast, the
radiative attachment reaction (10) has been included despite its believed unimportance as a pathway to CN- because
the abundances of the reactants involved are well established in various models of cloud chemistry. It should be
recalled, from Section 2.2, that reaction (10) is of comparable efficiency to the UMIST reaction set (4)-(7) as a pathway to CN- within IS clouds.
Some comments on the abundances and rate coefficients
chosen for Table 4 are required. We assume
n(I-)=3 x 10- 8 n(H2) as a total abundance of negativecharge-carrying species 1-, and abundances of free electrons, PAH - and M + are expressed relative to this. The
odd-electron PAH- abundance of 0.03 n(I-) reflects the
recent indications that PAH- formation may be comparatively inefficient: free electrons are therefore regarded as
the predominant negative charge carriers. The rate coeffi-
Notes.
"Reaction number, as identified in text. bReactants. CEstimated rate
coefficient in units of cm3 molecule- 1 S-I. dEstimated relative
abundance of reactant A, expressed as a fraction f of the total
abundance n (I -) of negative-charge-carrying species. 'Estimated
abundance of reactant B, expressed relative to n (H2) = 1. The notation a( -b) represents a x 1O- b• 'Estimated fractional abundance
of odd-electron PAH - ions. gAbundance within dark clouds (e.g.,
TMC-1) and within circumstellar envelopes (e.g., IRC + 10216)
respectively. hCircumstellar abundance taken from Ziurys et al.
(1995). 'Circumstellar abundance taken from Kawaguchi et al.
(1993).
cients for reactions (16), (25) and (26) assume that only 1/4
of such collisions lead to product formation upon the singlet
potential energy surface: CN is weakly polar, while MgCN
and MgNC are calculated to have large dipole moments and
so their collision rate coefficients with e are expected to be
high (kc -1O- 7 cm3 molecule-I S-I) at IS or CS temperatures. Finally, we assume for the sake of simplicity that,
excepting MgCN and MgNC, the abundances of reactants
within CS envelopes are identical to those within IS
clouds.
A pseudo-steady-state expression for the CN- abundance is obtained from this model:
n(CN-)=
j(e)[klo1l(CN) +k25n(MgCN) +k26n(MgNC)]
+k 1d(PAH-)n(CN)
where all parameters are as defined in Table 4. Two sets of
conditions can be distinguished. In the first, which we call
the 'IS model', MgCN and MgNC are absent, and so CNarises only via reactions (10) and (16). This appears to
mirror the situation in a dense cloud such as TMC-1, in
which no magnesium- (or other metal) containing species
have been detected: in this scenario, the steady-state abundance found is n(CN-)=2 x 10- 13 n(H2)' This abundance
is approximately two orders of magnitude greater than that
expected if CN- arises only via reaction (10) and/or via the
UMIST reaction set (4)-(7). Clearly, the CN- formed via
the IS model arises almost exclusively by charge transfer
from PAH- ions. However, the calculated CN- abundance
is still almost certainly too low to permit its radioastronomical detection within IS clouds.
The second scenario, which we term the 'CS model',
includes MgCN and MgNC at their observed abund© 1996 RAS, MNRAS 281,137-144
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1996MNRAS.281..137P
CN- within interstellar clouds
ances
within
IRC + 10216.
In
this
model,
n(CN-)=1.7 x 10- 10 n(Hz) is achieved if it is assumed that
DA occurs with spin conservation [Le., only 1/4 of
Mg(CN) + e collisions lead to DA upon the singlet surface].
Even with this restriction, dissociative attachment to
Mg(CN) is clearly the dominant pathway to CN-, and the
calculated abundance should permit detection of CNwithin a source such as IRC + 10216. A higher abundance
of CN- [up to ",7 X 10- 10 n(H2 )] would be expected if the
DA reactions (25) and (26) occur without spin conservation,
which is feasible given the tendency for violation of spin
conservation in several classes of ion/molecule reactions
(Ferguson 1983; Federer et al. 1986). Radioastronomical
detection of CN - would require determination of the spectroscopic constants of this ion to high accuracy. High-level
ab initio calculations on CN - have been performed by
Botschwina (1985) and by Peterson & Woods (1989), but as
yet no experimental study has supplemented the theoretical
results. Such a study would be of great benefit in investigating the prospects for production of CN-, and of other small
negative ions, within IS clouds and CS envelopes.
2.8 Implications for IS and CS chemistry
The prospects for chemical evolution via the reactions of
CN- are poor, since this species is expected to be depleted
rapidly by reactions (8 and 11) which do not lead to a
significant increase in molecular complexity. This does not
mitigate the importance of CN- as a possible 'indicator'
species: it should be noted that, even though negatively
charged species (including free electrons) must be present
within dense IS clouds and CS envelopes, no such species
have yet been detected. The CN - ion constitutes a (potentially) readily identifiable negative-charge carrier, and its
detection, perhaps within a source such as IRC + 10216,
would serve a useful role in expanding our understanding of
IS or CS chemical processes. A full understanding of the
processes of CN- formation and removal would permit an
estimation of free electron abundances (and, hence, of the
degree of ionization) within various sources based on their
observed CN- abundances.
Some comments on the formation of other small negatively charged species, by reactions analogous to (16), (25)
and (26), are in order. The halogens F, CI and Br all have
large ( > 3.3 eV) electron affinities, and so charge transfer
from PAH - is a viable route to F -, Cl- and Be, and is
likely to predominate over electron attachment by analogy
with the factors which have been discussed for CN- production. Other species such as H, C, 0, Si and S have lower
electron affinities (0.7 <EA <2.1 eV) but are expected to
be present in IS clouds at substantially higher abundances
than the halogen atoms; charge transfer from some PAHions may also represent a route to H-, C-, etc. However,
none of the atomic negative ions which might arise by these
pathways are easily detectable within IS clouds, since they
lack rotational spectra. Of diatomic species, OH
(EA = 1.828 eV; Miller 1991) is the most abundant IS
species having a significant electron affinity: however, as
with CN, charge transfer from PAH- to OH is probably
insufficient to produce OH- in detectable quantities.
Identification of a larger species having a very weak Z-OH
bond might allow the formation of OH- by dissociative
143
attachment, but no such species has yet been observed. We
conclude, therefore, that CN - remains the most favourable
diatomic negative ion for IS or CS detection.
3 CONCLUSIONS
Radiative electron attachment has traditionally been
regarded as the principal route to negative ions within IS
and CS environments. We have suggested here that two
chemical pathways - namely, charge transfer from PAHions to CN, and dissociative electron attachment to
Mg(CN) - offer substantially more effective pathways to
CN- than does radiative attachment. Simple model calculations indicate that, if dissociative attachment to Mg(CN) is
efficient, this process can lead to detectable CN- abundances within CS envelopes such as IRC + 10216. The significance of CN - as a constituent of IS clouds or CS
envelopes is more as an 'indicator' of physical or chemical
conditions - for example, the degree of ionization - than as
a pathway to other IS species. Neutralization of CN- is
expected to yield principally CN, HCN and (possibly) HNC,
all of which are formed by several other pathways also. In
any event, the detection of CN- will rely on the determination of the rotational spectrum for this species.
ACKNOWLEDGMENTS
The author thanks Professor David Smith for supplying
preprints, and Professor Eric Magnusson for helpful
advice.
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