Carbon skeletal rearrangement and hydrogen migration in thiophene

Abstract

Abstract-The mass spectra of %-labelled thiophenes were studied and the label-distributions for the C,H, @, HCS @, C,H2S @ and [M -CH,] @ ions are interpreted in terms of a carbon skeletal rearrangement. From a comparison of the results of 13C-labelling and D-labelling, a concurrent hydrogen migration process is demonstrated. The production of HCS @ ions is preceded by partial carbon scrambling and hydrogen migration is of minor importance. In contrast, hydrogen scrambling predominates over carbon scrambling in the loss of acetylene.

Intramolecular hydrogen transfer in mass spectra. I. Rearrangements in aliphatic hydrocarbons and aromatic compounds

“…In the case of the thiophenes, however, both carbon and hydrogen scrambling are observed. Thus studies of deuterium-labeled thiophenes such as thiophene-2,5-d2 (17) indicated that considerable 16 // w xs// 17 D but not complete hydrogen randomization precedes the formation of such ions as CHS+ and C2H2S4", 225,228,229 while studies of 13C-labeled thiophenes229,230 indicate that substantial carbon scrambling occurs prior to formation of the major fragment ions, although the exact interpretation of the extent of scrambling differs somewhat in the papers cited. Substituent scrambling occurs in phenylated thiophene,226 and a recent study231 has shown that this occurs essentially independently of any ring-carbon scrambling in 3-phenylthiophene-2-13C (18) and two bromo derivatives.…”

Positive and Negative Photoion Spectroscopy Study of Monochlorothiophenes

“…Besides the parent cation C 4 H 3 SCl + (m/z = 118), two daughter sulfo-chloro cations (C 2 HSCl + and CSCl + ) are detected, and their PIE curves are shown in Figure 2b, c, e, and f. The IP a value of 2-Cl-Th was previously determined to be 8.694 eV, 10 which is slightly higher than the present value 8.58 ( 0.04 eV (shown with an arrow in Figure 2a). The present experimental IP a of 3-Cl-Th, 8.72 ( 0.04 eV (see Figure 2d), is a little higher than that of 2-Cl-Th.…”

“…In the previous study of the photoionization dissociation of thiophene, , the corresponding cation C 2 H 2 S + produced near the threshold was hypothesized to be the linear thiolketen CH 2 CS + . Moreover, the concerted dissociation process (i.e., by the concerted bond breakings of C3–C4 and C5–S) rather than through the rearrangement of C 4 H 4 S + to so-called Ladenburg , or linear structure was proposed. , Therefore, we conjecture that it should be in a similar way to produce C 2 HSCl + (1). In Figure , the stepwise dynamic processes from the parent cations to C 2 HSCl + (1) + C 2 H 2 are plotted on the basis of the B3LYP/6-311+G(3df,3pd) calculations.…”

“…The photochemistry of thiophene and its derivatives received considerable attention due to the possible photoinduced rearrangements of the neutral 3À8 and the positive ion. 9À12 On the structure of thiophene cation C 4 H 4 S + , the structural rearrangement from the planar structure of the neutral to the Ladenburg conformation 9,10 is queried by a series of studies. 11,12 In the more recent work, Rennie et al 13 reexamined the dissociation dynamics of thiophene cation, C 4 H 4 S + f C 2 H 2 S + + C 2 H 2 , and concluded that this was a concerted dissociation process (i.e., by the concerted bond breakings of CÀC and CÀS) rather than through the rearrangement of C 4 H 4 S + to so-called Ladenburg 9,10 or linear structure.…”

“…In this work, the photoionization and photoion-pair dissociation dynamics of monochlorothiophenes (2-chlorothiophene, noted as 2-Cl-Th and 3-chlorothiophene, noted as 3-Cl-Th) is investigated by means of the positive and negative photoion mass spectrometry. The photochemistry of thiophene and its derivatives received considerable attention due to the possible photoinduced rearrangements of the neutral − and the positive ion. − On the structure of thiophene cation C 4 H 4 S + , the structural rearrangement from the planar structure of the neutral to the Ladenburg conformation , is queried by a series of studies. , In the more recent work, Rennie et al reexamined the dissociation dynamics of thiophene cation, C 4 H 4 S + → C 2 H 2 S + + C 2 H 2 , and concluded that this was a concerted dissociation process (i.e., by the concerted bond breakings of C–C and C–S) rather than through the rearrangement of C 4 H 4 S + to so-called Ladenburg , or linear structure . Monochlorothiophenes as the substituted thiophene are suitable for clarifying the above arguments.…”