Table 2: Dominant reactions, branching ratios, and rate coefficients for the C3H2 ortho/para problem.
        Standard models
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Default and sets of models 1-6		   Alternative models
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Trial1a         Trial2b

Reactions		 BRc $\alpha$d $\beta$d SMe   $\alpha$ $\alpha$
FORMATION      
C3H+ + p-H2 $\to$ p-C3H3+ k1 3.30(-13) 1.0 $\bullet$      
o-C3H3+ + e $\to$ o-C3H2 + H 1.0 k2 3.15(-7) 0.5 $\bullet$      
  $\to$ p-C3H2 + H 0.0 k2 0 0        
  $\to$ C3H + ...... f 3.15(-7) 0.5        
p-C3H3+ + e $\to$ o-C3H2 + H 0.5 k2 1.58(-7) 0.5 $\bullet$      
  $\to$ p-C3H2 + H 0.5 k2 1.58(-7) 0.5 $\bullet$      
  $\to$ C3H + ..... f 3.15(-7) 0.5        
o-C3H2 + H3+ $\to$ o-C3H3+ + H2 0.67 k3 5.16(-9) 0.5 $\bullet$   5.16(-9) 1/5 $\times$ 5.16(-9)
  $\to$ p-C3H3+ + H2 0.33 k3 2.54(-9) 0.5 $\bullet$   2.54(-9) 1/5 $\times$ 2.54(-9)
o-C3H2 + H3O+ $\to$ o-C3H3+ + H2O 0.67 k3' 2.48(-9) 0.5     2.48(-9) 1/5 $\times$ 2.48(-9)
  $\to$ p-C3H3+ + H2O 0.33 k3' 1.22(-9) 0.5     1.22(-9) 1/5 $\times$ 1.22(-9)
o-C3H2 + HCO+ $\to$ o-C3H3+ + CO 0.67 k3'' 2.14(-9) 0.5     2.14(-9) 1/5 $\times$ 2.14(-9)
  $\to$ p-C3H3+ + CO 0.33 k3'' 1.06(-9) 0.5     1.06(-9) 1/5 $\times$ 1.06(-9)
DESTRUCTION      
p-C3H2 + H3+ $\to$ o-C3H3+ + H2 0.0 k3 0 0 $\bullet$   0 0
  $\to$ p-C3H3+ + H2 1.0 k3 7.70(-9) 0.5 $\bullet$   7.70(-9) 1/5 $\times$ 7.70(-9)
p-C3H2 + H3O+ $\to$ o-C3H3+ + H2O 0.0 k3' 0 0     0 0
  $\to$ p-C3H3+ + H2O 1.0 k3' 3.70(-9) 0.5     3.70(-9) 1/5 $\times$ 3.70(-9)
p-C3H2 + HCO+ $\to$ o-C3H3+ + CO 0.0 k3'' 0 0     0 0
  $\to$ p-C3H3+ + CO 1.0 k3'' 3.20(-9) 0.5     3.20(-9) 1/5 $\times$ 3.20(-9)
${o,\ p}$-C3H3+ + C $\to$ products k4 1.00(-9) 0 $\bullet$      
${o,\ p}$-C3H3+ + O $\to$ products k4' 4.50(-11) 0        
${o,\ p}$-C3H3+ + S $\to$ products k4'' 1.00(-9) 0        
${o,\ p}$-C3H3+ + Si $\to$ products k4''' 2.00(-10) 0        
${o,\ p}$-C3H2 + C+ $\to$ products k5 4.20(-9) 0.5        
${o,\ p}$-C3H2 + S+ $\to$ products k5' 3.16(-9) 0.5        
${o,\ p}$-C3H2 + Si+ $\to$ products k5'' 3.28(-9) 0.5        
INTERCONVERSION BY H+      
o-C3H2 + H+ $\to$ p-C3H2 + H+ k6   0.5     1/4 $\times$ 1.30(-8) 1/4 $\times$ 1.30(-8)
p-C3H2 + H+ $\to$ o-C3H2 + H+ k-6   0.5     3/4 $\times$ 1.30(-8) 3/4 $\times$ 1.30(-8)
INTERCONVERSION BY HX+      
o-C3H2 + H3+ $\to$ p-C3H2 + H3+ k7   0.5     1/4 $\times$ 7.70(-9) 1/4 $\times$ 7.70(-9)
p-C3H2 + H3+ $\to$ o-C3H2 + H3+ k-7   0.5     3/4 $\times$ 7.70(-9) 3/4 $\times$ 7.70(-9)
o-C3H2 + H3O+ $\to$ p-C3H2 + H3O+ k'7   0.5     1/4 $\times$ 3.70(-9) 1/4 $\times$ 3.70(-9)
p-C3H2 + H3O+ $\to$ o-C3H2 + H3O+ k'-7   0.5     3/4 $\times$ 3.70(-9) 3/4 $\times$ 3.70(-9)
o-C3H2 + HCO+ $\to$ p-C3H2 + HCO+ k''7   0.5     1/4 $\times$ 3.20(-9) 1/4 $\times$ 3.20(-9)
p-C3H2 + HCO+ $\to$ o-C3H2 + HCO+ k''-7   0.5     3/4 $\times$ 3.20(-9) 3/4 $\times$ 3.20(-9)
Note: a(-b) = a $\times$ 10-b.
a The interconversion rates are obtained as discussed in Sect. 2.2.
b Competitive protonation rates are suppressed by factors of 5.
c Branching ratio between ortho and para modifications obtained by angular momentum algebra (Oka 2004).
d k = $\alpha$ (T/300)$^{-\beta}$ exp (-$\gamma/T$); $\alpha$ is in units of cm3 s-1.
e SM (Simple Model): the reactions marked with $\bullet$ are used in the model of MFK; with the generic forms HX+ and Y. See Sect. 3.3.
f In our standard approach, the branching fraction to form C3H is set equal to that for C3H2 except for models where it is a free parameter.

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