## 3 The SET Synthesis of Adenine from Hydrogen Cyanide (HCN)  
Although adenine can be synthesized under laboratory conditions from HCN, e.g., [11; 12; 13], it might be interesting to the possibilities for a SET synthesis under prebiotic conditions when HCN and COS came streaming out together from an undersea vent. One notes then that the molecular vibrations of HCN as given by [PERSON] (table 42 and Figure 61 in reference [10]) are: v1 = 2089 cm\({}^{-1}\), v2 = 712 cm\({}^{-1}\) and v3 = 3312 cm\({}^{-1}\). We reproduce here in Figure 4 (the data from Figure 61 of Reference [10]). As said above, the v3 vibration of COS is 2079 cm\({}^{-1}\) and that will give a good possibility of resonance between the two vibrations now described (v3 of COS and v1 of HCN). Figure 4: Vibrational pattern of hydrogen cyanide (HCN). Note that v1 implies an increase of the C-N distance. Figure 3: Ribose (\(\beta\)-D-ribofuranose). Numbering code: first carbon on the left of the oxygen atom is C1. Last position in the five-ring is C4. The carbon in the side branch is C5. Therefore, the more energy that is transferred from COS to HCN, the larger will be the C-N distance in HCN. Because of this, the weaker will be the 2p-2p interaction in the HCN molecule; Scheme 2. In the Scheme 2. is shown excitation of the HCN molecule: \(\Delta\) E is the energy of one vibration quantum, given from the catalyst COS; n is the number of such quanta to reach reaction. Then the triple bond between C and N is destroyed and the resulting \"free\" p orbitals are designated by a point. On the other hand, there will appear \"ferer\" 2p orbitals ready to join with a 2p orbital of a neighboring molecule, whereby a single bond will be created between the two molecules. In Scheme 3 we illustrate this event with four of the five HCN molecules that are supposed to build the adenine molecule. The bond between N and C in the HCN molecule, activated by resonance with COS, is depicted as a double bond, what might possibly be an overstatement. Anyway, the appearance of a C-N single bond between two molecules makes it imperative that the two linear molecules attach in an angle approaching 120 degrees (weakly indicated in Scheme 3). In Scheme 4 the gap that results between the first and the fourth HCN molecule is illustrated. This gap will be filled by the carbon atom from the fifth HCN molecule. It is difficult to tell in what form the HCN molecule molecule appears, but most probably it is the isomeric form HNC, with two half-used p-orbitals available on the carbon atom (Scheme 4). The important point is that the carbon atom will find itself surrounded by three neighbouring atoms, more than willing to form new bonds with the intruder. This results, for one sake, in a weakening of the original CN triple bond so that the nitrogen atom is expelled and can bind to the carbon atom of the starting HCN unit. The two H atoms, that are set free in this maneuver, add to the N atom forming the top NH\({}_{2}\) group. Secondly, the result of the penetration of the last HCN molecule (or possibly CN\({}^{-}\) ion or CNH) is, of course, that three new bonds are established what gives a closed structure; (Scheme 4 and Figure 5). It is interesting to note that [PERSON] and [PERSON] [13] find, by CBS-QB3 calculations, that the formation in gas phase of oligomers (HCN)\({}_{n}\) where n = 2 up to n = 5 (adenine) requires very high activation energies. For each step of that reaction \"a few hundred kJ\" are used [13] and consequently a catalyst is strongly needed. These authors used a proton as a catalyst, that strongly decreased the activation energy. Nevertheless only \"some oligomerization stages\" were predicted [13] to occur in an atmosphere like that around the moon Titan (of the planet Saturn). No formation of adenine could, however, be foreseen. Near the surface of the planet Earth things might have been better. From hot volcano eruptions lots of hydrogen cyanide streamed out, but also lots of warm, already vibrationally excited carbonyl sulfide (COS).