1,2,5-Thiadiazole 1-oxides. 3. An experimental and theoretical investigation of the inversion barrier

Abstract

inosine (Ino) and 5'-GMP complexes such as [(NH3)2Pt(InoH_1)]+ and [(NH3)2Pt(GMPH_i)]"; there were attributed to polynuclear complex formation.12•13 Furthermore, there is clear evidence that none of the species formed during the reaction is a Pt complex with two 5'-GMP molecules bound via N7 of the guanine ring to the Pt atom from both 'H and l95Pt NMR data of the m-[Pt(NH3)2(5'-GMP)2] complex. The latter exhibits a H8 proton resonance at 3.931 ppm and a 195Pt resonance of-2451.3 ppm which is considerably shifted to high field from the 195Pt resonances of species IG/IIG and IIIG (see Table III). Further, the resonances for the HI' protons in the c«-[Pt(NH3)2(5'-GMP)2] complex are at 2.143 ppm, 0.020 ppm to high field of the corresponding resonance in free 5'-GMP (2.163 ppm), while in species IG, IIG, and IIIG the HI' signals are at ca. 2.28 ppm which is to low field of the corresponding free signal.From the structures of species IG/IIG given in Figure 9 in which the Pt atom is bound to N7 of the guanine ring, one would expect the 195Pt chemical shift of species IG/IIG to be given approximately by the mean of the 195Pt chemical shifts of cis-[Pt(NH3)2Cl2] and m-[Pt(NH3)2(5'-GMP)2]; this has a value of-2306.4 ppm which is in excellent agreement with the measured value of -2302.0 ppm.Comparison of the Kinetics of the Reactions of 5'-AMP and 5'-GMP with Excess c/s-[Pt(NH3)2Cl2] in the Presence of KC1. A comparison of the kinetics of the reactions of 5'-AMP and 5'-GMP with excess m-[Pt(NH3)2Cl2] in the presence of KC1 at 80 °C leads to the following observations. Tables I and II show the relationship klG ~klA > k3A = k4A > k2A for the second-order association rate constants, which implies that the dissociation of a Cl atom from ris-[Pt(NH3)2Cl2] is not rate limiting. Considering only the 5'-AMP reaction with m-[Pt(NH3)2Cl2], we observe that the association rate constant for binding to N7 (k]A) is larger than that for binding to N1 (k2A), a finding which correlates inversely with the smaller pA of N7 compared to N1 of the adenine ring.25 However, once a Pt atom is bound to either N1 or N7, the association rate constants (k3A and k4A) for binding to the remaining site are equal. This implies an electronic redistribution upon binding of a Pt atom to either N1 or N7, such that the reactivity toward cri-[Pt(NH3)2Cl2] of the remaining available site is less than that of N7 but greater than that of N1 in free 5'-AMP. Considering the schemes for the reaction of 5'-GMP with excess dr-[Pt(NH3)2Cl2], it is clear that species IG and IIG could either be formed simultaneously (as in the case of species IA and 11A in the 5'-AMP reaction) or in sequence. Given our proposed structures for species IG and IIG (Figure 9), the latter seems unlikely. Moreover, it is likely that the association rate constants for the formation of the two rotamers, IG and IIG, would be equal and given by k]G/2. Assuming this assumption is correct, we note the relationship k1A > klG/2 > k2A for the second-order association rate constants for the binding of ...

Analogy between sulfuryl and phosphino groups: the aromaticity of thiophene-oxide

“…While for the activation energy of 2,5-bis(2,3,3,4,4-pentamethylpentan-2-yl)thiophene-oxide 14.8 kcal mol -1 has been measured [26], for DMSO 36-42 kcal mol -1 barrier has been reported [26]. Quantum chemical calculations are in accordance with this trend: while for thiophene-oxide 19.6 kcal mol -1 [30] and 11.2 kcal mol -1 [29] have been obtained (at the HF/4-31G* and MP2/6-31G* levels, respectively), the corresponding gap was 51.4 kcal mol -1 (HF/4-31G*) for DMSO [30], 43.2 kcal mol -1 (HF/4-31G*) for H 2 SO [30], and 40.0 kcal mol -1 (MP2/6-31G*) for divinyl-sulphoxide [29]. Despite the apparent decrease of the barrier, and the reported equalization of the bond lengths [30], the extent of aromaticity in planar thiopheneoxides has not yet been investigated in detail.…”

“…The three coordinate sulfur atom in the ring is pyramidal [26,[29][30][31][32][33], thus its lone pair is unable to overlap with the butadiene moiety. This structure makes thiophene-oxide potentially similar to phosphole (Fig.…”

Generation and Decay of Aryl Sulfinyl and Sulfenyl Radicals: A Transient Absorption and Computational Study

“…86 Given the results at the various basis sets, we believe 1.49 Å is the best estimate of the ROHF value of the S-O bond length. Experimental and computed bond lengths of typical sulfoxides are 1.48-1.49 Å.…”

Crystallographic study and molecular orbital calculations of 1,2,5-thiadiazole 1,1-dioxide derivatives

“…The structural details in Tables 5-7 allow comparisons among these compounds and with reported studies on related compounds, such as thiadiazines, 24-28 a thiadiazoline, 29 a thiadiazole 1-dioxide 30 and a thiadiazole, 31 and suggest relationships with their experimental chemical behavior that will be discussed below.…”