Butadiene radical cation

A case where both effects are important is the ring opening of the cyclobutene radical cation l +.13 This reaction was studied experimentally by Bally and coworkers, who found that, in contrast to the thermal reaction, only the trans-1,3-butadiene radical cation, trans-2 +, is formed in a matrix isolation experiment.14,15 This surprising result can be explained by considering the electronic states of the species involved in the reaction, which are shown in Fig. 1. 

Fig. 9 Reaction pathways for the cycloaddition of the 1,3-butadiene radical cation with ethane. Results from QCISD(T)/6-31G //QCISD/6-31G calculations in plain text,42a results from UCCSD(T)/DZP//UMP2/DZP in italies.42b,c Numbers in brackets indicate the number of additional minima in the pathway.

Early El studies by King suggested that 1,3-butadiene radical cations suffer isomerization and complete hydrogen scrambling prior to loss of H and C2H2. Later, Gross, Nibbering and coworkers- -" showed by field ionization kinetic (FIK) measurements that... 

Oxgaard, J., Wiest, O. (2002). Rehybridized 1,3-Butadiene Radical Cations How Far WUl a Radical Go To Maintain Conjugation Journal of Physical Chemistry A, 106, 3961-391A. 

To illustrate the latter point, consider the butadiene radical cation (BD+ ). On the basis of Hiickel theory (or any single-determinant Hartree-Fock model) one would expect this cation to show two closely spaced absorption bands of very similar intensity, due to 7i i -> ji2 and ji2 —> JI3 excitation (denoted by subscripts a and v in Figure 28), which are associated with transition moments /xa and /xv of similar magnitude and orientation. Using the approximation fiwm) —3 eV288 the expected spacing amounts to about 0.7 eV. 

The prototype hole-catalyzed Diels-Alder reaction between the butadiene radical cation and ethylene has also been studied by Bauld [53]. He finds strongly exothermic formation of a l-hexene-3,6-diyl radical cation intermediate without activation energy followed by a weakly activated (activation energy 2.3 kcal mol ) closure of the second C-C bond to form the cyclohexene radical cation, The reaction shows no overall activation energy relative to the... 

Reaction pathways for the addition of ethylene to butadiene radical cation involving H-shifts have been investigated at the coupled cluster UCCSD(T)/DZP//UMP2(fc)/DZP-b ZPE level of theory.Several rearrangement reactions have been found to occur below the energy limit of separated ethylene and butadiene radical cation. The cyclopentenyl cation ( 5117)+ in the gas phase may originate from various pathways. 

In this context it is also of interest that the parent cyclobutene radical cation can be observed at 77 K and, upon warming to 110 K, undergoes stereoselective ring opening to transoid butadiene radical cation [270]. A cyclopropylcarbindiyl radical cation has been proposed as an intermediate for this transformation [271]. 

In subsequent argon matrix isolation studies, similar bands were found when hexatriene or cyclohexadiene are ionized , and eventually, five of the six possible rotamers of hexatriene radical cation were identified by selective, wavelength-specific in ter conversions . Similar results were later obtained for octatetraene , where six of the twenty possible rotamers are formed on ionization in argon (Figure 31) which could be interconverted and identified by selective photolysis . Interestingly, in the case of the butadiene radical cation the s-cis rotamer could not be detected, even if the diene radical cation was formed from the cyclobutene radical cation . In contrast, in a recent resonance Raman study, some weak bands were detected and assigned to the s-cA-butadiene radical cation which might have escaped detection in the earlier ESR and EA experiments . ... 

Butadiene Radical Cations. Cycloaddition reactions between the radical cations of... 

The interconversion of butadiene radical cations and ionized cyclobutene represents a model case for a formal pericyclic process. Much work has been invested to study not only the distinguishability of these isomers and their derivatives by mass spectrometry, but also to check the role of orbital symmetry in the ionic species. Hass has addressed the latter problem in depth in a review on pericyclic reactions in radical cations in both the gas and condensed phases and no further survey on the papers mentioned there will be given here. The topic pertains also to the ring-opening of ionized benzocyclobutene to ionized ortho-quinodimethane (cf Section V) and various otha- phenyl-, methyl- and carboxy-substituted derivatives. In this context, we restrict ourselves hwe mentioning that an upper limit of 7 kcalmol only has been detemined by CE mass spectrometry for the activation barrier of the cycloreversion of the parent cyclobutene radical cations. The energy requirement for the cycloreversion of ionized 1- and 3-substituted cyclobutenes were found, by experiment, to be markedly different. Obviously, dissociation of the (in a sense bis-allylic) strained C—C bond is much more facile when the substituent is at C-3,... 

A homolytic cleavage in a [1,4] shift results in the production of a butadiene radical cation, whose HOMO is W2 having C2 symmetry. Therefore, under thermal conditions, the [1,4] sigmatropic shift occurs via a suprafacial pathway with inversion at the migrating center (Figure 3.19). 

The 2 + 1 and 4 + 1 cyclizations are considered to involve a concerted but nonsynchronous process in solution (Bellville et al., 1983). However, in the gas phase the addition of butadiene radical cation to butadiene proceeds via a stepwise mechanism involving [16] as an intermediate (Groenewald and Gross, 1984a,b). The addition of CH2=CH—CH=CH2 to... 

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