S. Okovytyy
298
&
&
2
1X
+
+
+
+
'
'
Interesting from methodological point of view investigations have been performed
by Glad and Jensen [33]. They have calculated deuterium α-secondary kinetic isotope effects (SKIE) as an example of oxirane interaction with nucleophiles and
compared them with optimized transition state geometries (Table 10.1).
In the row of Н
−
…NH 3 when nucleophilicity decreases transition states becomes
to have more pronounced late character. All calculated at MP2/6-31 + G(d) level
values of SKIE lie between 0.91 and 1.06 in absolute value, typical for an S N 2 reaction [34]. Analysis of changes of SKIE as function of broken C 1 –O bond length
confirms relationship between TS geometry and KIE. For second-row nucleophiles
and hydride ion a difference in SKIE of 0.02 (a typical experimental uncertainty)
corresponds to geometry changes at the TS of ~ 0.03 Å in bond length (R CO ) and
~ 3
0
difference in torsional angle (Θ HCCO ), i.e. isotope effects are quite sensitive to
the TS geometry.
To discuss the early/late and loose/tight features of the TSs, degrees of forming
and breaking of bonds (n
≠
) have been calculated according to (Eq. 10.1)
(10.1)
where r
0
—bond length in oxirane, r
≠
—corresponding parameters in transition state.
Analysis of two-dimensional More O’Ferrall-Jencks diagram, drawn as function
of degrees of formation of С–Nu bond on (1 − n
≠
C1–O
) value, has shown that the main
determining factor for the SKIE is the loose/tight feature of the TS rather than the
early/late position, i.e. the lower KIE for the more product-like TSs is primarily due
to the increased tightness.
0
n
exp (
)/0.6 ,
r r
≠
≠
=
-
Table 10.1 Geometrical parameters and values of SKIE calculated at MP2/6-31 + G(d) level of
theory [33]
Nu
С 1 –О
C 1 –Nu
С 2 –О
C 1 C 2 O
NuC 1 C 2
SKIE
H
−
1.654
2.148
1.436
70.2
106.7
1.058
NH 2
−
1.708
2.307
1.423
73.1
113.5
1.011
OH
−
1.753
2.129
1.420
75.4
112.1
0.981
F
−
1.831
1.922
1.413
79.7
111.7
0.940
SH
−
1.861
2.463
1.402
81.3
110.9
1.049
Cl
−
1.988
2.212
1.388
88.2
113.6
0.999
NH 3
2.075
1.763
1.361
93.2
114.8
0.916
298
&
&
2
1X
+
+
+
+
'
'
Interesting from methodological point of view investigations have been performed
by Glad and Jensen [33]. They have calculated deuterium α-secondary kinetic isotope effects (SKIE) as an example of oxirane interaction with nucleophiles and
compared them with optimized transition state geometries (Table 10.1).
In the row of Н
−
…NH 3 when nucleophilicity decreases transition states becomes
to have more pronounced late character. All calculated at MP2/6-31 + G(d) level
values of SKIE lie between 0.91 and 1.06 in absolute value, typical for an S N 2 reaction [34]. Analysis of changes of SKIE as function of broken C 1 –O bond length
confirms relationship between TS geometry and KIE. For second-row nucleophiles
and hydride ion a difference in SKIE of 0.02 (a typical experimental uncertainty)
corresponds to geometry changes at the TS of ~ 0.03 Å in bond length (R CO ) and
~ 3
0
difference in torsional angle (Θ HCCO ), i.e. isotope effects are quite sensitive to
the TS geometry.
To discuss the early/late and loose/tight features of the TSs, degrees of forming
and breaking of bonds (n
≠
) have been calculated according to (Eq. 10.1)
(10.1)
where r
0
—bond length in oxirane, r
≠
—corresponding parameters in transition state.
Analysis of two-dimensional More O’Ferrall-Jencks diagram, drawn as function
of degrees of formation of С–Nu bond on (1 − n
≠
C1–O
) value, has shown that the main
determining factor for the SKIE is the loose/tight feature of the TS rather than the
early/late position, i.e. the lower KIE for the more product-like TSs is primarily due
to the increased tightness.
0
n
exp (
)/0.6 ,
r r
≠
≠
=
-
Table 10.1 Geometrical parameters and values of SKIE calculated at MP2/6-31 + G(d) level of
theory [33]
Nu
С 1 –О
C 1 –Nu
С 2 –О
C 1 C 2 O
NuC 1 C 2
SKIE
H
−
1.654
2.148
1.436
70.2
106.7
1.058
NH 2
−
1.708
2.307
1.423
73.1
113.5
1.011
OH
−
1.753
2.129
1.420
75.4
112.1
0.981
F
−
1.831
1.922
1.413
79.7
111.7
0.940
SH
−
1.861
2.463
1.402
81.3
110.9
1.049
Cl
−
1.988
2.212
1.388
88.2
113.6
0.999
NH 3
2.075
1.763
1.361
93.2
114.8
0.916