Rotational Mode Specificity in the F^– + CH₃I(v = 0, JK) S_N2 and Proton-Transfer Reactions

Abstract

Quasiclassical trajectory computations are performed for the F – + CH 3 I( v = 0, JK ) → I – + CH 3 F (S N 2) and HF + CH 2 I – (proton-transfer) reactions considering initial rotational states characterized by J = {0, 2, 4, 6, 8, 12, and 16} and K = {0 and J } in the 1–30 kcal/mol collision energy ( E coll ) range. Tumbling rotation ( K = 0) counteracts orientation effects, thereby hindering the S N 2 reactivity by about 15% for J = 16 in the 1–15 kcal/mol E coll range and has a negligible effect on proton transfer. Spinning about the C–I bond ( K = J ), which is 21 times faster than tumbling, makes the reactions more direct, inhibiting the S N 2 reactivity by 25% in some cases, whereas significantly enhancing the proton-transfer channel by a factor of 2 at E coll = 15 kcal/mol due to the fact that the spinning-induced centrifugal force hinders complex formation by breaking H-bonds and activates C–H bond cleavage, thereby promoting proton abstraction on the expense of substitution. At higher E coll , as the reactions become more direct, the rotational effects are diminishing.

Stretching-mode specificity in the Cl + CH3D(v1-I, v1-II, and v4 = 1; |jK〉) reactions: dependency on the initial |jK〉 selectivity

“…Owing to the REMPI-spectral overlap, the product image actually constitutes the contributions from both the CH 2 D(0 0 ) and CH 2 D(6 1 ) products. Using a previously developed successive subtraction scheme 3 —a partition method that does not invoke fitting the total P ( u ) distribution (the black curve) with presumed multiple functions but rather bases on the apparent differences among the three stretch-excited distributions for a given | jK 〉-state, the deduced P ( u ) for the (6 1 , 0) s pair is depicted in red in each panel. The above noted (6 1 , 0) s -propensity of v 1 -II > v 1 -I > v 4 from the raw images (Fig.…”

“…[19][20][21][22] Several theoretical studies have also addressed the rotational-mode specificity in a few polyatomic reactions. 15,[23][24][25][26][27][28][29][30][31][32][33][34] Depending on the system, the rotational excitation of the reagent can either promote or hinder the overall reactivity. Even the type of the rotational motion matters, for instance, the spinning (K = AEj) or the tumbling (K = 0) motion.…”

Reaction mechanism conversion induced by the contest of nucleophile and leaving group

“…Bimolecular nucleophilic substitution (S N 2) reactions play a vital role in chemistry and biochemistry; thus, many experimental [1][2][3] and theoretical [4][5][6] studies have focused on the dynamics of the typical S N 2 reactions, X À + CH 3 Y -CH 3 X + Y À (X = F, Cl and Y = F, Cl, Br, I). 7,8 Compared to the S N 2 reactions with halogen ions as the reactant, the multiple reaction pathways and more complex atomic-level mechanisms were observed in the simulations [9][10][11] and experiments 12 for the hydroxyl ion with methyl halide reactions, which is quite interesting since F À and OH À are isoelectronic.…”

Rotational Mode-Specificity in the Cl + C2H6 → HCl + C2H5 Reaction

“…Thus, the K / K a K c quantum number/labels, which can adopt values in the [0, ±1, ..., ± J ] interval, necessarily introduce rotational mode-specificity as their different values correspond to different rotational modes (states) of the reactant molecule. Accordingly, rotational mode-specific studies for the H 2 O + + H 2 /D 2 , , H/F/Cl + H 2 O, − F/Cl/OH + CH 4 , − H/Cl/O + CHD 3 , − and the F – + CH 3 F/CH 3 Cl/CH 3 I , reactions involving asymmetric, spherical, and symmetric top polyatomic reactants have been carried out.…”

“…In the former case, both the spinning ( K = J ) and the tumbling ( K = 0) rotation of the CH 3 F reactant had a similar inhibiting effect, whereas for the latter reaction, the tumbling rotation was found to be less effective in hindering reactivity . Very recently, the F – + CH 3 I reaction was also studied and showed considerable rotational hindrance in the case of the S N 2 channel for both tumbling and spinning excitations, whereas the proton-transfer reaction was noticeably promoted with increasing J by exciting the spinning reactant rotational mode; however, it was left unaffected by tumbling excitation …”