H-S distances

For cis-l(N2H2) we did not find a hydrogen bond. By contrast, cis-2(N2H2) exhibits a weak one, whose energy is about 5 kJ/mol, as we would have expected in view of its relatively short H S distance when compared with cis-l(N2H2). 

Figure 17. Intramolecular hydrogen bridges in the [p-N2H2 MS2(thiolate) 2] cores of [p-N2H2 M(S ) 2] complexes (N—H S distances in pm were retrieved from [p-N2H2 Fe(NHS4) 2]).

Ab Initio Calculations. To gain some insight about the photodissociation process of HS, we show in Figure 27 the ab initio potential energy curves calculated by Bruna and Hirsch [144] as a function of the H-S distance [r(H-S)] in atomic units (Bohr radius = Og). The HS( E ) and HS( A) potential curves are repulsive and correlate asymptotically to the formation... 

P2j Z = 2 D = 1.57 R = 0.048 for 931 intensities. The base exists in the thioxo form, with C-8=S and N-7 protonated. The 8-thio substituent causes the base to assume the syn (—102.6°) orientation. The o-ribosyl group is 2T3 (174.8 °, 44.1 °). The exocyclic, C-4 -C-5 bond orientation is trans (—173.2°). This does not favor intramolecular hydrogen-bonding of 0-5 to N-3 of the syn base. The C=S distance is 166.8 pm. The S atom is involved in a weak, acceptor hydrogen-bond to a water molecule, S H-O(w) = 361 pm. The bases are stacked head-to-tail, with overlap of the C=S bonds and the purine ring, in contrast to the known, related structure l-/ -D-ribofuranosyl-2-thioxo-3ff-benzimidazole,197 where similar head-to-tail stacking of the bases involves overlap of the base rings only. 

After many incorrect assignments, structures 178 and 179 were eventually proposed on the basis of the IR spectrum of the former and on an X-ray structure for the latter. Static structure 178 is correct, but 179 showed equal S-S distances. Also, the H NMR spectrum of 179 showed equivalent methyls and equivalent methines. Moreover, the two S-S distances in the parent l,6,6aA4-trithiapentalene 175 are equal, and the SSS angle is 178°. It was therefore proposed that these molecules are symmetric. 

To the lowest order, we may neglect all correlations and take w) eZpE/fp in Eq. (262) in this case the velocity field term in (273) behaves like exp( —Kr)/r for large distances, while the inhomogeneous term on the l.h.s. 

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