“…The potential energy surface for a gas-phase reaction between an ion and a neutral molecule is generally expected to show the initial formation of an ion−molecule complex. , Thus the minimum energy pathway involving back-side nucleophilic attack at carbon (leading to an S N 2 reaction with inversion of configuration) is typically found to proceed through a loose C 3 v ion−molecule complex 1 followed by a D 3 h transition structure 2 . , Let us now consider possible structures for complexes on the reaction pathways associated with front-side attack. …”
Section: Resultsmentioning
confidence: 99%
“…According to the FMO treatment, the major interaction during an S N 2 reaction is that between the highest occupied MO (HOMO) of the incoming nucleophile Nu - and the lowest unoccupied MO (LUMO) of the substrate (Figure ). The LUMO is mainly an out-of-phase combination (σ* C - X ) of the carbon atomic φ C orbital and the atomic φ X orbital of the leaving atom. − 2 Trends in the G2(+) overall barriers of reaction 3 with retention of configuration (Δ H ⧧ ovr (R)) and with inversion of configuration (Δ H ⧧ ovr (I)) for the set of halogens. G2(+) Δ H ⧧ ovr (R) and Δ H ⧧ ovr (I) values are listed in Table . 3 Trends in the G2(+) central barriers of reaction 3 with retention of configuration (Δ H ⧧ cent (R)) and with inversion of configuration (Δ H ⧧ cent (I)) for the set of halogens.…”
Section: Resultsmentioning
confidence: 99%
“…It is clear from the very large number of calculations already carried out on S N 2 reactions at carbon ,,− that the computational data are very sensitive to the level of theory employed. For this reason, in our earlier study of identity S N 2 halide-exchange with inversion of configuration at carbon, we used a level of theory, specifically a modification 23 of G2 theory, that describes the energetics at a higher level than has been used in previous comparative studies.…”
Section: Methodsmentioning
confidence: 99%
“…Bimolecular nucleophilic substitution (S N 2) at saturated carbon holds a central role within organic chemistry, and consequently the stereochemistry of the reaction has been intensively investigated, both experimentally − and theoretically. ,− Both experimental and theoretical 8-12 studies show that the preferred stereochemical route in the gas phase is one of back-side attack with inversion of configuration, as opposed to front-side attack with retention of configuration, precisely as is observed in solution. While retention of configuration has been found in nucleophilic substitution reactions at saturated silicon, early reports on the possibility of retention at saturated carbon 6 were not confirmed by further experiments …”
High-level ab initio molecular orbital calculations at the G2(+) level of theory have been carried out on the identity front-side nucleophilic substitution reactions with retention of configuration, X -+ CH 3 X, for X ) F, Cl, Br, and I. Overall gas-phase barrier heights do not show a strong variation with halogen atom and are calculated to be 184.5 (X ) F), 193.8 (X ) Cl), 178.9 (X ) Br), and 171.4 kJ mol -1 (X ) I), substantially higher than the corresponding barriers for back-side attack (-8.0 for X ) F, 11.5 for X ) Cl, 5.8 for X ) Br, and 6.5 kJ mol -1 for X ) I). The difference between the overall barrier for back-side attack and front-side attack is smallest for X ) I (164.9 kJ mol -1 ). Central barrier heights for front-side attack decrease in the following order: 241.0 (X ) F), 237.8 (X ) Cl), 220.0 (X ) Br), and 207.4 kJ mol -1 (X ) I). The minimum energy pathways for both back-side and front-side S N 2 reactions are found to involve the same ion-molecule complex (X -‚‚‚H 3 CX), with the front-side pathway becoming feasible at higher energies. Indeed, our results suggest that the chloride exchange in CH 3 Cl, which has been found in gas-phase experiments at high energies, may be the first example of a front-side S N 2 reaction with retention of configuration at saturated carbon. Analysis of our computational data in terms of frontier orbital theory suggests that elongation of the bond between the central atom and X could be a significant factor in decreasing the unfavorable nature of the front-side S N 2 reaction with retention of configuration in going from X ) F to X ) I. Ion-molecule complexes CH 3 -X‚‚‚X -, which may be pre-reaction complexes in direct collinear halophilic attack, were found for X ) Br and I but not for X ) F and Cl. The calculated complexation energies (∆H comp ) for halophilic complexes are considerably smaller (7.3 and 19.4 kJ mol -1 for X ) Br and I, respectively) than those for the corresponding pre-reaction complexes for S N 2 attack at carbon (41.1 and 36.0 kJ mol -1 for X ) Br and I, respectively). Nucleophilic substitution reactions at the halogen atom in CH 3 X (X ) F-I) (halophilic reactions) are highly endothermic and appear to represent an unlikely mechanistic pathway for identity halide exchange.
“…The potential energy surface for a gas-phase reaction between an ion and a neutral molecule is generally expected to show the initial formation of an ion−molecule complex. , Thus the minimum energy pathway involving back-side nucleophilic attack at carbon (leading to an S N 2 reaction with inversion of configuration) is typically found to proceed through a loose C 3 v ion−molecule complex 1 followed by a D 3 h transition structure 2 . , Let us now consider possible structures for complexes on the reaction pathways associated with front-side attack. …”
Section: Resultsmentioning
confidence: 99%
“…According to the FMO treatment, the major interaction during an S N 2 reaction is that between the highest occupied MO (HOMO) of the incoming nucleophile Nu - and the lowest unoccupied MO (LUMO) of the substrate (Figure ). The LUMO is mainly an out-of-phase combination (σ* C - X ) of the carbon atomic φ C orbital and the atomic φ X orbital of the leaving atom. − 2 Trends in the G2(+) overall barriers of reaction 3 with retention of configuration (Δ H ⧧ ovr (R)) and with inversion of configuration (Δ H ⧧ ovr (I)) for the set of halogens. G2(+) Δ H ⧧ ovr (R) and Δ H ⧧ ovr (I) values are listed in Table . 3 Trends in the G2(+) central barriers of reaction 3 with retention of configuration (Δ H ⧧ cent (R)) and with inversion of configuration (Δ H ⧧ cent (I)) for the set of halogens.…”
Section: Resultsmentioning
confidence: 99%
“…It is clear from the very large number of calculations already carried out on S N 2 reactions at carbon ,,− that the computational data are very sensitive to the level of theory employed. For this reason, in our earlier study of identity S N 2 halide-exchange with inversion of configuration at carbon, we used a level of theory, specifically a modification 23 of G2 theory, that describes the energetics at a higher level than has been used in previous comparative studies.…”
Section: Methodsmentioning
confidence: 99%
“…Bimolecular nucleophilic substitution (S N 2) at saturated carbon holds a central role within organic chemistry, and consequently the stereochemistry of the reaction has been intensively investigated, both experimentally − and theoretically. ,− Both experimental and theoretical 8-12 studies show that the preferred stereochemical route in the gas phase is one of back-side attack with inversion of configuration, as opposed to front-side attack with retention of configuration, precisely as is observed in solution. While retention of configuration has been found in nucleophilic substitution reactions at saturated silicon, early reports on the possibility of retention at saturated carbon 6 were not confirmed by further experiments …”
High-level ab initio molecular orbital calculations at the G2(+) level of theory have been carried out on the identity front-side nucleophilic substitution reactions with retention of configuration, X -+ CH 3 X, for X ) F, Cl, Br, and I. Overall gas-phase barrier heights do not show a strong variation with halogen atom and are calculated to be 184.5 (X ) F), 193.8 (X ) Cl), 178.9 (X ) Br), and 171.4 kJ mol -1 (X ) I), substantially higher than the corresponding barriers for back-side attack (-8.0 for X ) F, 11.5 for X ) Cl, 5.8 for X ) Br, and 6.5 kJ mol -1 for X ) I). The difference between the overall barrier for back-side attack and front-side attack is smallest for X ) I (164.9 kJ mol -1 ). Central barrier heights for front-side attack decrease in the following order: 241.0 (X ) F), 237.8 (X ) Cl), 220.0 (X ) Br), and 207.4 kJ mol -1 (X ) I). The minimum energy pathways for both back-side and front-side S N 2 reactions are found to involve the same ion-molecule complex (X -‚‚‚H 3 CX), with the front-side pathway becoming feasible at higher energies. Indeed, our results suggest that the chloride exchange in CH 3 Cl, which has been found in gas-phase experiments at high energies, may be the first example of a front-side S N 2 reaction with retention of configuration at saturated carbon. Analysis of our computational data in terms of frontier orbital theory suggests that elongation of the bond between the central atom and X could be a significant factor in decreasing the unfavorable nature of the front-side S N 2 reaction with retention of configuration in going from X ) F to X ) I. Ion-molecule complexes CH 3 -X‚‚‚X -, which may be pre-reaction complexes in direct collinear halophilic attack, were found for X ) Br and I but not for X ) F and Cl. The calculated complexation energies (∆H comp ) for halophilic complexes are considerably smaller (7.3 and 19.4 kJ mol -1 for X ) Br and I, respectively) than those for the corresponding pre-reaction complexes for S N 2 attack at carbon (41.1 and 36.0 kJ mol -1 for X ) Br and I, respectively). Nucleophilic substitution reactions at the halogen atom in CH 3 X (X ) F-I) (halophilic reactions) are highly endothermic and appear to represent an unlikely mechanistic pathway for identity halide exchange.
“…Bimolecular nucleophilic substitution (S N 2) reactions at saturated carbon in the gas phase have attracted considerable interest, both experimentally , and theoretically, ,− because of their fundamental nature. Yet despite this great interest, it is remarkable that considerable uncertainty still exists regarding the barrier heights for these reactions.…”
High-level ab initio molecular orbital calculations at the
G2(+) level of theory have been carried out for
the six non-identity nucleophilic substitution reactions,
Y- + CH3X → YCH3 +
X-, for Y, X = F, Cl, Br, and I.
Central barrier heights
(ΔH
⧧
cent) for reaction in the
exothermic direction vary from 0.8 kJ mol-1
for Y = F, X = I
up to 39.5 kJ mol-1 for Y = Cl, X = Br
(at 0 K), and are in most cases significantly lower than those for the
set
of identity SN2 reactions X- +
CH3X → XCH3 + X- (X =
F−I). Overall barriers
(ΔH
⧧
ovr) for reaction in
the
exothermic direction are all negative (varying from −68.9 kJ
mol-1 for Y = F, X = I to −2.3 kJ
mol-1 for Y = Br,
X = I), in contrast to the overall barriers for the identity
reactions where only the value for X = F is negative.
Complexation enthalpies (ΔH
comp) of the
ion−molecule complexes Y-···CH3X
vary from 30.4 kJ mol-1 for Y =
F,
X = I to 69.6 kJ mol-1 for Y = I, X = F
(at 298 K), in good agreement with experimental and earlier
computational
studies. Complexation enthalpies in the reaction series
Y- + CH3X (Y = F−I, X = F, Cl, Br, I)
are found to
exhibit good linear correlations with halogen electronegativity.
Both the central barriers and the overall barriers
show good linear correlations with reaction exothermicity, indicating a
rate−equilibrium relationship in the Y-
+
CH3X reaction set. The data for the central barriers
show good agreement with the predictions of the Marcus
equation,
though modifications of the Marcus equation that consider overall
barriers are found to be less satisfactory. Further
interesting features of the non-identity reaction set are the good
correlations between the central barriers and the
geometric looseness (%L
⧧), geometric
asymmetry (%AS), charge asymmetry (Δq(X−Y)), and bond
asymmetry (ΔWBI)
of the transition structures.
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