•Interstellar and Circumstellar
Chemistries: The Role of
Neutral-Neutral Reactions
ERIC HERBST
DEPARTMENTS OF PHYSICS AND
ASTRONOMY
THE OHIO STATE UNIVERSITY
dense (giant) molecular clouds
organic molecules
H
core
4 -3
n = 10 cm
T = 10 K
2
PDR’s
embedded
stars
hot
ionized
gas
HII region
protoplanetary disk
studied in millimeter-wave and IR
GAS PHASE INTERSTELLAR/CIRCUMSTELLAR MOLECULES - HIGH RESOLUTION (9/02)
_____________________________________________________________________________________________
H2
KCl
HNC
C3S
C5
C6H
HC4CN
HCO
NH3
CH3
H3O+
CH
AlCl
CH4
CH3OH
AlF
HCO+
H2CO
SiH4
CH3SH
NH
PN
HOC+
H2CS
CH2NH
C2H4
OH
SiN
HN2+
HCCH
H2C3(lin)
CH3CN
C2
SiO
HNO
HCNH+
c-C3H2
CH3NC
C7H, C6H2
C8H
HCOOCH3
CH3COOH
CH3C2CN
H2C6(lin)
C6H2
H2COHCHO
C2H5OH
(CH3)2O
CH+
CN
SiS
HCS+
H2CN
CH2CN
HC2CHO
C2H5CN
CO
CO+
SO+
C3
C2O
CO2
C2S
C3H(lin)
c-C3H
NH2CN
CH3C4H
CH2CO
NH2CHO
HC3NH+
HCCN
HCOOH
C4H2
H2C4(lin)
HNCO
SiC3
HOCO+
C4H
HNCS
C2CN
C3O
NaCN
HCCNC
HNCCC
C4Si
H2COH+
CSi
+
CP
H3
CS
HF
NO
CH2
NH2
SiC2
SiCN
SO2
NS
SO
HCl
NaCl
H2O
H2S
C2H
HCN
OCS
MgNC
MgCN
N2O
HC2CN
C5H
C5N
CH3NH2
CH2CHOH
CH3CCH
CH3CHO
CH2CHCN
c-CH2OCH2
c-CH2SCH2
HC6CN
(CH2OH)2
(CH3)2CO
CH3C4CN?
NH2CH2COOH?
HC8CN
c-C6H6
HC10CN
+ ISOTOPOMERS
Dust particles contain 1% of interstellar matter.
POTENTIAL ENERGY OF REACTION
activation energy
typical neutral reactions
radical-radical reactions
A+B
ion-molecule reactions
k(T) = A(T) exp(-Ea /kT)
C+ D
Cosmic rays produce
ions
Radical-Neutral Reactions
Radicals: C, CN, CCH
1) Inverse T dependence
2) Large rate coefficients by
10-50 K: k  10(-10) cm3 s-1
(diffusion)
FORMATION OF GASEOUS
WATER
H2 + COSMIC RAYS  H2+ + e
Elemental
abundances:
C,O,N
=
10(-4);
C<O
Elemental abundances: C,O,N = 10(-4); C<O
H2+ + H2  H3+ + H
H3+ + O  OH+ + H2
OHn+ + H2  OHn+1+ + H
H3O+ + e  H2O + H; OH + 2H, etc
FORMATION OF O2 ,N2 CO
OH + O  O2 + H
OH + N  NO + H
NO + N  N2 + O
CH + O  CO + H
CO, N2 + He+  C+, N+ +…
Precursor to ammonia, hydrocarbons
ORGANIC SYNTHESIS CONT.
SOME SYNTHETIC REAC TION CLASSES:
A. CARBON INSERTION
C+ + CH 4 -----> C2H3+ + H
------> C2H2+ + H2
B. CONDENSATION
C2H2+ + C2H2 -----> C4H3+ + H
C. ATOM IC INSERTION
N + C3H3+
-----> HC3NH+ + H
D. RADIATIVE ASSOCIATION
CH3+ + H2O -----> CH3OH2+ + h
E. NEUTRAL-NEUTRAL
C + C2H2

C3H + H
NEUTRAL-NEUTRAL RX (CONT)
CN + C2H2  HCCCN + H
CCH + C2H2  C4H2 + H
CCH + HCN  HCCCN + H
YES
YES
NO
O + CCH  CO + CH
k  1.2 10(-11) cm3 s-1
MAYBE (Ea = 250K?)
GAS-PHASE MODELS
A+ + B  C+ + D
k1
C+ + D  PRODUCTS
k2
d[C+]/dt = k1[A+][B] – k2[C+][D]
Constraints: initial concentrations, elemental
abundances, density, charge neutrality
Steady-state solution: d[C+]/dt = 0
exists for constant density but takes very long
(107 yr) to be achieved.
CURRENT GAS-PHASE MODEL NETWORKS
4,000 reactions; 10-20% "studied";
400 species through 13 atoms in size
elements: H, He, N, O, C, S, Si, Fe, Na, Mg, P, Cl
elemental abundances: “low metal”
photodestruction: external, internal (via cosmic rays)
Successes for quiescent cores:
(1)Reproduces 80% of abundances
including ions, radicals, isomers
(2)Predicts strong deuterium fractionation
CURRENT APPROACH TO
NEUTRAL-NEUTRAL RX
DO NOT EXTRAPOLATE NEW LOW
TEMPERATURE RESULTS UNLESS IN
FAMILIES (e.g. C + HCCCN NO)
 MAINTAIN PAST ESTIMATES UNLESS
CONSENSUS THAT THEY ARE INCORRECT.
(E.G. O + Cn, CnH) INCLUDES LOW RATE
CONSTANT.
 ESTIMATES IMPROVED WITH IAN; WORK IN
PROGRESS

IRC10216: An AGB (Old) Star
LTE
Molecules and
dust here
C>O
N,T similar to
cloud
CO, C2H2,
HCN
UV radiation
+ cosmic rays
Actual Distributions
GROWTH OF MOLECULES
Occurs via neutral and ionic (+ and -)
reactions. Modified network necessary to
account for acetylenic chemistry.
Photochemistry important in the
production of radicals such as CN
and CCH
C2H2 + h  CCH + H
GROWTH OF MOLECULES. II
CCH + C2nH2  C2n+2H2 + H
CN + C2nH2  HC2n+1N + H
C2nH reactions with hydrocarbons,
HCN(?), HNC(?) as well as
cyanoacetylenes.
Benzene formed via ion-molecule rx.
CYANOPOLYYNES in IRC
Radius 
CRL618: A Protoplanetary
Nebula
Detection of
benzene
empty
250 K
Dense
Thin shell
Photons, X-rays from central star!
100 x normal ionization rate
SUMMARY
Thanks to Ian Smith and others
(Bertrand Rowe, Ian Sims, David Clary
etc.), we now know that neutral-neutral
reactions are competitive in both
oxygen-rich and carbon-rich chemistries
in interstellar and circumstellar
sources!!!!!
SYNTHESIS OF BENZENE
C2H2+ + C2H2  C4H3+ + H
C4H3+ + C2H2  c-C6H5+ + h
c-C6H5+ + H2  c-C6H7+ + h
c-C6H7+ + e  c-C6H6 + H
INITIAL ABUNDANCES w.r.t. H2





CO
C2H2
HCN
CH4
NH3





6(-4)
5(-5)
8(-6)
2(-6)
2(-6)
ACTUAL CLOUD CORES
Model
TMC-1
L134N
NSM/+grains
80%
80%
NNM
50%
70%
NOTES: (1) C=0.42 x O
(2) N-N reactions involving C and O
most important.
(3) Larger molecules more strongly
affected.
TYPES OF SURFACE REACTIONS
REACTANTS: MAINLY MOBILE
ATOMS AND RADICALS
A +
B 
H +
AB
H  H2
association
X  XH (X = O, C, N, CO,
etc.)
WHICH CONVERTS
H +
O  OH  H2O
C  CH  CH2  CH3  CH4
N  NH  NH2  NH3
CO  HCO  H2CO  H3CO  CH3OH
X + Y  XY
??????????
H + HX  H2 + X abstraction