Deuterium Kinetic Isotope Effects
J. Phys. Chem. A, Vol. 108, No. 45, 2004 9891
In conclusion, measured inverse deuterium KIEs are consis-
tent with the previous results on the water-solvated SN2
reactions. KIEs from differently labeled solvents suggest that
the transition states have structures analogous to that for
-
F (H2O) + CH3X. Site-specific deuteration effects have also
been observed for ion-molecule association kinetics of hy-
2
5
droxide and alkoxide ions. Detailed analysis of the KIEs as
well as product branching will be discussed in a future
2
6
publication.
Acknowledgment. This work was supported by a grant
(CHE-0349937) from the National Science Foundation. On the
occasion of his landmark birthday, we are pleased to dedicate
this paper to Tomas Baer, a great scientist, colleague, and friend.
References and Notes
(
77.
1) Laerdahl, J. K.; Uggerud, E. Int. J. Mass Spectrom. 2002, 214,
2
(
2) (a) Viggiano, A. A.; Arnold, S. T.; Morris, R. A. Int. ReV. Phys.
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ReV. 1998, 17, 409.
(
(
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Figure 4. Kinetic isotope effects (k
methyl group in methanol plotted against the reaction rate constant
). The hydrogen site pertinent to the isotope effect of concern is
H D
/k ) for the deuteration of the
1
997, 101, 4598.
5) O’Hair, R. A. J.; Davico, G. E.; Hacaloglu, J.; Dang, T. T.; DePuy,
C. H.; Bierbaum, V. M. J. Am. Chem. Soc. 1994, 116, 3609.
(k
D
(
shown in bold font.
(
6) Tachikawa, H. J. Phys. Chem. A 2001, 105, 1260.
cussion. It has been suggested computationally,12 however, that
(7) Poirier, R. A.; Wang, Y.; Westaway, K. C. J. Am. Chem. Soc. 1994,
-
i-C3H7OH‚‚‚F has a weaker hydrogen-bonding interaction than
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(8) Hu, W.-P.; Truhlar, D. G. J. Am. Chem. Soc. 1994, 116, 7797.
9) Most product channels (analogous to eqs 1a and 1b) are exothermic
-
CH3OH‚‚‚F , as indicated by the smaller decrease in the O-H
(
stretching frequency (see above), the longer ROH‚‚‚F bond
length (1.374 Å vs 1.339 Å), the lower H‚‚‚F stretching
-
1
for these reactions. The reaction enthalpies (in kcal mol ) are -8.6
-
-
-
(
Br + CH3F + CH3OH), -23.1 (Br (CH3OH) + CH3F), -14.4 (I +
-
1
-1
-
-
frequency (339 cm vs 374 cm ), and the more localized
charge on F (-0.884 vs -0.878). Clearly the hydrogen-bonding
interaction is a complex issue; direct vibrational and structural
data on the SN2 transition state would be required to understand
the KIE trend fully.
CH3F + CH3OH), -26.3 (I (CH3OH) + CH3F), -11.4 (I + CH3F +
-
-
i-C3H7OH), -24.5 (I (i-C3H7OH) + CH3F), +0.8 (I + CH3F + HF), and
-
-
14.1 (I (HF) + CH3F).
(
10) (a) Van Doren, J. M.; Barlow, S. E.; DePuy, C. H.; Bierbaum, V.
M. Int. J. Mass Spectrom. Ion Processes 1987, 81, 85. (b) Bierbaum, V.
M. Encycl. Mass Spectrom. 2003, 1, 98.
(
(
(
11) Kawaguchi, K.; Hirota, E. J. Chem. Phys. 1987, 87, 6838.
12) Bogdanov, B.; McMahon, T. B. J. Phys. Chem. A 2000, 104, 7871.
13) Baer, T.; Hase, W. L. Unimolecular Reaction Dynamics; Oxford
KIE at Methyl Group in Methanol. Figure 4 shows the
KIE for the deuteration of the methyl group in the solvent
methanol. The KIE values (kCH /kCD ) are similar to those
3
3
University Press: New York, 1996.
(14) Kato, S.; Davico, G. E.; Lee, H. S.; DePuy, C. H.; Bierbaum, V.
M. Int. J. Mass Spectrom. 2001, 210/211, 223.
observed for the deuteration of the methyl halide (kCH X/kCD X).
3
3
The origin of the inverse KIE is an intriguing issue for
examination. Apparently, more remote from the SN2 center
(
(
15) Su, T.; Chesnavich, W. J. J. Chem. Phys. 1982, 76, 5183.
16) CRC Handbook of Chemistry and Physics, 75th ed.; Lide, D. R.,
(
Figure 1B), the terminal CH3 group may produce an isotope
Ed.; CRC Press: Boca Raton, FL, 1994.
(17) Robinson, P. J.; Holbrook, K. A. Unimolecular Reactions; Wiley:
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18) Davico, G. E.; Bierbaum, V. M. J. Am. Chem. Soc. 2000, 122,
740.
(19) Herzberg, G. Molecular Spectra and Molecular Structure. III.
-
effect through a mechanism analogous to that for F (H2O) +
8
-
CH3Cl. At the transition state for F (H2O) + CH3Cl (Figure
(
1
A), theory predicts that the terminal O-H(5) bending motion
1
couples with the internal rotation of the CH3Cl methyl group,
Electronic Spectra and Electronic Structure of Polyatomic Molecules; Van
Nostrand Reinhold: New York, 1966.
giving rise to low-frequency fundamental modes ν18 (D2O-
8
sensitive) and ν19 (CD3X-sensitive). These low-frequency
(
(
20) Combariza, J. E.; Kestner, N. R. J. Phys. Chem. 1994, 98, 3513.
21) Herzberg, G. Molecular Spectra and Molecular Structure. I. Spectra
of Diatomic Molecules; Van Nostrand Reinhold: New York, 1950.
(22) Unless otherwise specified, all thermochemical values have been
taken or derived from the NIST Chemistry Webbook, NIST Standard
Reference Database Number 69, March 2003 release and references therein.
23) Bogdanov, B.; Peschke, M.; Tonner, D. S.; Szulejko, J. E.;
McMahon, T. B. Int. J. Mass Spectrom. 1999, 185/186/187, 707.
24) Weis, P.; Kemper, P. R.; Bowers, M. T.; Xantheas, S. S. J. Am.
Chem. Soc. 1999, 121, 3531.
25) Kato, S.; Dang, T. T.; Barlow, S. E.; DePuy, C. H.; Bierbaum, V.
-
1
modes (<100 cm ) are nonexistent in the reactants and thus
8
contribute to the inverse ηvib,low observed. If the terminal methyl
-
group in F (HOCH3) similarly interacts with the CH3X methyl
internal rotation at the SN2 transition state (Figure 1B), then
we can speculate that ν18-type vibrational modes could account
(
-
for the KIE observed for F (methanol) + CH3X upon the
(
deuteration of CH3OH to CD3OH (or CH3OD to CD3OD). High-
level theoretical predictions would be challenging for the subtle
KIE involving the larger and floppier SN2 transition states with
a fluorine atom and are beyond the scope of this paper.
(
M. Int. J. Mass Spectrom. 2000, 195/196, 625.
(26) Davico, G. E.; et al., to be submitted for publication.