Rate constants for hydrogen abstraction from alkoxides by a perfluoroalkyl radical. An oxyanion accelerated process

Article abstract of DOI:10.1039/b405074f

A combination of laser flash photolysis and competitive kinetic methods has been used to measure the absolute bimolecular rate constants for hydrogen atom abstraction in water from a series of fluorinated alkoxides and aldehyde hydrates by the perfluoroalkyl radical, CF₂CF₂OCF ₂CF₂SO₃^-Na⁺. The bimolecular rate constants observed for the β-fluorinated alkoxides were in the 10⁵ M^-1 s^-1 range, such rates representing enhancements (relative to the respective alcohols) of between 100 and almost 1000-fold, depending on the reactivity of the alkoxide. Likewise, the monobasic sodium salts of chloral and fluoral hydrate exhibit similar rate enhancements, relative to their respective hydrates.

Full text of DOI:10.1039/b405074f

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Rate constants for hydrogen abstraction from alkoxides by a
perfluoroalkyl radical. An oxyanion accelerated process†
Joseph A. Cradlebaugh,a Li Zhang,a G. Robert Shelton,a Grzegorz Litwinienko,b
Bruce E. Smart,‡c Keith U. Ingoldb and William R. Dolbier, Jr.*a
a Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA
b National Research Council, 100 Sussex Drive, Ottawa, ON, Canada K1A OR6
c DuPont Central Research and Development, Experimental Station, Wilmington,
DE 19880, USA
Received 5th April 2004, Accepted 26th May 2004
First published as an Advance Article on the web 24th June 2004
A combination of laser flash photolysis and competitive kinetic methods has been used to measure the absolute
bimolecular rate constants for hydrogen atom abstraction in water from a series of fluorinated alkoxides and
aldehyde hydrates by the perfluoroalkyl radical, CF2CF2OCF2CF2SO3Ϫ Naϩ. The bimolecular rate constants
ؒ
observed for the β-fluorinated alkoxides were in the 105 MϪ1 sϪ1 range, such rates representing enhancements
(relative to the respective alcohols) of between 100 and almost 1000-fold, depending on the reactivity of the alkoxide.
Likewise, the monobasic sodium salts of chloral and fluoral hydrate exhibit similar rate enhancements, relative to
their respective hydrates.
Table 1 Correlation of C–H homolytic reactivity with alcohol bond
dissociation energies
Introduction
Soon after the discovery by Evans of the huge rate accelerations
exhibited during anionic oxy-Cope rearrangements,1,2 it was
hypothesized that such accelerations were related to the bond-
weakening effect of the anionic alkoxy group on the adjacent
C3–C4 bond.3 The basic conclusions derived from Goddard’s
GVB theory work have recently been confirmed and elaborated
upon by Houk4 and Baumann5 using DFT methodology.
Houk, for example, found that the C–H BDE of methoxide ion
was ∼23 kcal less than that of methanol.4 Such weakening of
the α-C–H bonds of alkoxides should provide acceleration to
other reactions that involve the breaking of such bonds, and
such has been found to be the case,6 examples being alkoxy-
accelerated [1.3]-sigmatropic rearrangements,7–10 the Ireland
Claisen rearrangement of allyl ester enolates,11,12 and even
1,5-hydrogen or alkyl shifts.13,14
kH(per H)/
BDE’s (calcd)/
kcal molϪ1
Alcohol
104 MϪ1 sϪ1
krel
CH3OH
CH3CH2OH
(CH3)2CHOH
0.6 (0.2)
6 (2)
48 (16)
(1)
10
80
93
91
89
compounds, the relative rates of H-abstraction by NaϩϪO3-
CF2CF2OCF2CF2 correlate well with their relative C–H
ؒ
BDE’s, as shown for the series of alcohols in water below.16,17
The bond dissociation energies of methanol, ethanol, and
isopropanol in Table 1 were estimated by first optimizing the
geometries of the alcohols and their corresponding α-hydroxy
radicals at the B3LYP/6-31G(d) level and then calculating their
single point energies at the B3LYP/6-311ϩG(2df,2p) level.
We considered it an intriguing prospect to probe the quanti-
tative impact of the bond-weakening effects of an alkoxide
substituent on the rate constants of hydrogen abstraction
reactions. To our knowledge, the only related previous work
is that of Bunnett in which he found that methoxide ion was a
good hydrogen atom donor to aryl radicals,18,19 methoxide, for
example, being about 45 times better than methanol in donating
a hydrogen atom to the p-nitrophenyl radical.20
In recent years we have been successful in developing reliable
competition methods for determining the absolute rate con-
stants for abstraction of hydrogen from organic substrates by
perfluorinated radicals.15–17 As exemplified below, isopropanol
is fairly reactive towards perfluoroalkyl radicals in both the
non-polar solvent 1,3-bis-trifluoromethylbenzene (BTB) and in
water, with this hydrogen abstraction reaction being modestly
faster in water than in BTB.
In order for us to carry out a kinetic study of hydrogen
abstraction from alkoxides, it would, of course, be necessary for
the alkoxide substrate to be present in the competition mixture
as a totally homogeneous solute, something that would be
impossible in both the BTB and the aqueous medium, the latter
because the basicities of the alkoxides would preclude their
existence in water. However, as a part of our kinetic studies
on H-atom abstraction by fluorinated radicals from alcohols
in water, we have also determined the rate constants for
H-abstraction from trifluoroethanol and hexafluoroiso-
propanol. Because of the polar and electrostatic influence
of the fluorine substituents, these two compounds were,
predictably, orders of magnitude less reactive than their
hydrocarbon counterparts, as seen in Table 2.16,17
Although electrostatic and polar effects can have a dramatic
effect on the C–H abstraction rates from organic compounds by
a fluorinated radical, when working with a series of similar
† Electronic supplementary information (ESI) available: Tables of kin-
etic data and plots of kinetic data. See http://www.rsc.org/suppdata/ob/
b4/b405074f
However, these alcohols have one property that that was
extremely propitious in view of our interest in the kinetics of
H-atom abstraction from alkoxides – they are much more acidic
‡ DuPont Contribution No. 8521.
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 2 0 8 3 – 2 0 8 6
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Table 2 Contrast of rate constants for hydrogen abstraction from
fluorinated and nonfluorinated alcohols
Table 3 Rate constants for hydrogen abstraction from alkoxides and
alcohols
Alcohol
kH/103 MϪ1 sϪ1
krel
Alkoxide/Alcohol Hydrogen Donor
kH/103 MϪ1 sϪ1
krel
CH3CH2OH
CF3CH2OH
(CH3)2CHOH
(CF3)2CHOH
12 (4)
0.08 (0.02)
48 (16)
CF3CH2OϪNaϩ (TFEO)
CF3CH2OH
77 (20)
150
123
0.08 (0.02)
108 (30)
0.39 (0.12)
155 (50)
1.5 (0.5)
962
277
100
(CF3)2CHOϪNaϩ (HFIPO)
(CF3)2CHOH
0.39 (0.12)
(CF3)(CH3)CHOϪNaϩ (TFIPO)
(CF3)(CH3)CHOH
than their fluorine-free counterparts, having pKa’s of 12.4 and
9.3, respectively.21 Their conjugate bases, the alkoxides, would
therefore be relatively weak, stable and homogeneously soluble
in water.
being involved. Combining this measured kH/kD value (4.64)
with the value of the primary isotope effect (kH/kD = 5.88) for
H- vs. D-abstraction from HFIPO,24 and using the rate constant
of 7.7 × 104 MϪ1 sϪ1 for TFEO, yields a rate constant (kH) for
HFIPO of 9.8 × 104 MϪ1 sϪ1
.
Results and discussion
Secondly, a “reverse” competition experiment was carried
out, using deuterated HFIPO [(CF3)2CDOϪNaϩ] and undeuter-
ated THF. This gave a kH/kD value of 1.94, which when com-
Relative rate constants for H-atom abstraction by
NaϩϪO3CF2CF2OCF2CF2 from sodium trifluoroethoxide
ؒ
(TFEO), sodium hexafluoroisopropoxide (HFIPO), and
bined with the kH value for THF (3.3 × 104 MϪ1 Ϫ1) and the
s
sodium
trifluoroisopropoxide,
(CF3)(CH3)CHOϪNaϩ
(TFIPO),22 were first determined by competition kinetics
involving H-transfer from the alkoxide versus D-transfer from
perdeuterio tetrahydrofuran, THF-d8. The general procedure
has been described. These relative rate constants were converted
to absolute values using the rate constant, kD = 4.2 × 103 MϪ1
HFIPO isotope effect (kH/kD = 5.88) provides a value for kH
of HFIPO of 10 × 104 MϪ1 sϪ1. There is therefore a self consist-
ency in the values of kH for HFIPO that were obtained from the
three different competition studies (11, 9.8 and 10 × 104 MϪ1
s
s
Ϫ1), and we feel confident that their average, viz. 10.8 × 104 MϪ1
Ϫ1, can be regarded as a reliable value for the kH of HFIPO.
Aldehyde hydrates and their monosodium salts are also of
s
Ϫ1, previously determined by combining a laser flash photolysis
(LFP) direct measurement of the rate constant for H-atom
abstraction from non-deuterated THF with competition
experiments involving THF and THF-d8, all in water. The
results obtained (see Table 3 below) demonstrate that all three
alkoxides are not only very much more reactive than their
respective parent alcohols, but that they are also considerably
more reactive than the corresponding non-fluorinated alcohols.
This high reactivity undoubtedly derives from a combination of
the bond-weakening effect of the alkoxide function and the
enhanced nucleophilicity of the alkoxide C–H bond towards
the highly electrophilic perfluoro radical.
interest and relevant to the present work because they can be
regarded as α-hydroxy-substituted alcohols and alkoxides.
Again, in order to study such compounds in water, one must
have aldehyde hydrates that (a) exist virtually 100% in the
hydrate form when in water, and (b) are acidic enough that their
monobasic forms are stable in water. Chloral and fluoral
hydrate meet these requirements nicely, the presence of the
three β-chlorines or fluorines insuring that the hydrates are
exclusively present in water, and that they are sufficiently acidic
to cleanly form their monosodium salts upon treatment with
one equivalent of NaH. In fact the pKa’s of chloral and fluoral
hydrate have been estimated to be 10.1 and 10.2, respectively.25
Because of the inductive effect of the three β-halogens, these
hydrates, like the fluorinated alcohols, should be poor H-atom
LFP measurements of the rate constants for H-atom abstrac-
tion by NaϩϪO3CF2CF2OCF2CF2 from TFEO and HFIPO
ؒ
met with mixed success. For TFEO the LFP value for kH was
11 × 104 MϪ1 Ϫ1, which is in reasonable agreement with the
s
donors to NaϩϪO3CF2CF2OCF2CF2 . This is borne out by the
ؒ
7.7 × 104 MϪ1 sϪ1 obtained by competition, as described above,
particularly when it is remembered that the estimated reliability
of these LFP-derived rate constants is ca. 30% and that
the competitive method for obtaining absolute rate constants
relies, ultimately, on an LFP measurement.16 However for
HFIPO the observed LFP kH was 50 × 104 MϪ1 sϪ1, which is 4.5
times greater than the competition value of 11 × 104 MϪ1 sϪ1. It
seemed probable that this large difference was due to some
unidentified problem23 in the LFP experiment because the
competition experiment conformed to our strict standards
with respect to conversion, yield, lack of side products, etc.
Nevertheless, additional competition experiments designed to
validate the competition method in this system were carried out
as described below.
data in Table 4, which shows that chloral hydrate is slightly
more reactive than fluoral hydrate, which is slightly more
reactive than trifluoroethanol, results which are consistent with
stabilization of the carbon-centered radicals formed from the
hydrates by the “extra” α-hydroxy group. More significantly, the
monosodium salt of fluoral hydrate exhibits the largest
enhancement, relative to its non-alkoxy counterpart (krel
=
1315), as well as the largest rate constant that we have yet
observed for H transfer from carbon to the fluorinated radical,
5
NaϩϪO3CF2CF2OCF2CF2 : kH = 1.7 × 10 MϪ1 sϪ1
.
ؒ
Conclusions
First a direct kinetic competition between the two alkoxides
using sodium 2-deuteriohexafluoroisopropoxide in competition
with sodium 2,2,2-trifluoroethoxide (TFEO), with no THF
In conclusion, an alkoxide functionality has long been known
to provide marked acceleration to sigmatropic processes
wherein a bond to the carbon bearing the alkoxy group is
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 2 0 8 3 – 2 0 8 6
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Table 4 Rate constants of hydrates and hydrate monoanions
via vacuum pump overnight to obtain chloral hydrate, sodium
salt (2.20 g, 97% yield): 1H NMR (300 MHz, D2O) δ 4.19
(s, 1H). For fluoral hydrate sodium salt, 1H NMR (D2O) δ 5.28
(q, 1H). 19F NMR (D2O) δ Ϫ85.58 (d, 3F).
Hydrate/hydrate monoanion
CCl3CH(OH)2
kH/103 MϪ1 sϪ1
krel
0.83 (0.25)
157 (50)
General procedure for 19F kinetic experiments
189
The kinetic experiments were run in NMR tubes containing
a sealed capillary tube of C6D6 and CFCl3 as the internal
standard. The NMR tubes were capped with natural rubber
septa, and sealed with Teflon tape before any chemicals were
added. Using a micro-syringe, 15 µL (6.04 × 10Ϫ6 moles) of
IRfSO3Na solution were added to each tube. Exact amounts of
stock salt solutions were added to each NMR tube using micro-
syringes. Equal amounts of THF-d8 were added to each tube,
along with varying amounts of the hydrogen donor. Deionized
water was added to each NMR tube so that the total volume
of reaction mixture was 565 µL. The samples were degassed
3 times using the freeze–pump–thaw method. Subsequent to the
initial 19F NMR spectra taken, the samples were irradiated for
16 hours in the RPR-204 Rayonet photochemical reactor. 19F
NMR spectra were again taken, and all peaks of significance
were integrated. The product ratios were obtained from the
integration of the CF2H (d, –138.3 ppm) and CF2D (s, Ϫ139.0
ppm) peaks. The conversion and yield were obtained by
integration of the CFCl3 peak in the starting material and the
integration of the product peaks.
CF3CH(OH)2
CF3CH2OH
0.13 (0.04)
171 (50)
1315
0.08 (0.02)
broken. Absolute rate data now demonstrate that an α-alkoxide
also dramatically enhances homolytic hydrogen atom abstrac-
tions, in particular, by highly electrophilic perfluoro radicals.
Thus, β-fluorinated alkoxides exhibit bimolecular rate con-
stants for H-abstraction by a fluorinated radical in the 105 MϪ1
sϪ1 range, such rates representing enhancements (relative to
the respective alcohols) of between 100 and almost 1000-fold,
depending on the reactivity of the alkoxide. Likewise, the
monobasic sodium salts of chloral and fluoral hydrate exhibit
similar rate enhancements, relative to their respective hydrates.
Tables of kinetic data and plots of kinetic data are available
in the ESI.†
Experimental
All reagents used were commercially available, and were
purchased from CIL, Aldrich, Fisher, or Lancaster. 1,1,1-
Trifluoroacetaldehyde hydrate was purchased from SynQuest
Laboratories. All reagents were used without further
purification.
Kinetic measurements by time-resolved laser flash photolysis.
The apparatus and procedures have been described in detail
elsewhere.26,27 The radicals, CF2CF2OCF2CF2SO3 Na, were
ؒ
generated “instantaneously” by 308 nm LFP of aqueous
solution of the precursor iodide16 at ambient temperature.
NMR spectra and kinetic 19F NMR measurements were
performed at 282 MHz using a Varian VXR-300 spectrometer.
All chemical shifts reported are in ppm downfield from the
internal standard, CFCl3.
ؒ
Since the products of the CF2CF2OCF2CF2SO3 Na reaction
with H atom donors, TFEO, HFIPO, and TFIPO were not
easily observed, the rate constants of the process, kgl, were
obtained by measurements of the rate of adduct formation
ؒ
of CF2CF2OCF2CF2SO3 Na with 1.7
0.4 mM sodium
Typical procedure for preparation of alkoxide sodium salts.
Hexafluoroisopropoxide, sodium salt
4-(1-propenyl)benzoate (CH CH᎐CHC H CO Na) as
a
᎐
3
6
4
2
spectroscopic probe monitored at 320 nm.
NaH (0.534 g, 22 × 10Ϫ3 mol) was suspended in 30 cm3 of
diethyl ether. The suspension was stirred and cooled to ice bath
temperature. Hexafluoroisopropanol (3.1 g, 31 × 10Ϫ3 mol) was
added to 15 cm3 of diethyl ether and then added dropwise via
an addition funnel to the NaH suspension. The solution was
allowed to stir for 3 hours during which time it reached
room temperature. The ether was removed by rotovap and the
resulting solid dried via vacuum pump overnight to obtain
Probe addition rate constant. The rate constant for addition,
kadd (k ), to the spectroscopic probe, CH CH᎐CHC H CO Na,
᎐
gl
3
6
4
2
was taken to be 3.95 × 107 MϪ1 s
.
Ϫ1 16 In any set of experiments,
the probe’s concentration was kept constant and the grow-in of
the absorption at 320 nm was monitored.
Laser flash photolysis probe experiments. The procedure has
been described in detail previously.28,29 1.5 cm3 of aqueous solu-
tions (0.027 M) of IRfSO3Na, 3, in quartz cuvettes (8 × 8 mm)
sealed with rubber septa were deaerated by flushing with N2
during 20 minutes, then the various amounts (50–400 µL) of
deaerated THF or isopropanol and the volume of the sample
was made up (when necessary) to 2.0 cm3. After addition of 100
µL of a stock solution (32.9 to 35.7 mM) of 2, the mixture was
vortexed for 20 seconds and purged with nitrogen for a further
2–5 minutes. The growths of the optical density at 320 nm
following each of 6 to 9 pulses of a 308 nm laser were recorded
for each concentration of H-atom donor. These growth traces
of the radical were analyzed by least-squares fitting on the basis
of pseudo-first-order kinetics to obtain experimental rate
1
hexafluoroisopropoxide, sodium salt (2.66 g, 98.2% yield): H
NMR (300 MHz, D2O) δ 4.42 (hept, JHF = 6.9 Hz, 1H); 19F
NMR (282 MHz, D2O) δ Ϫ76.58 (d, JFH = 6.5 Hz, 6H).
Typical procedure for the preparation of monobasic sodium salts
of hydrates. Chloral hydrate, sodium salt
NaH (0.290 g, 12 mmol) was suspended in 30 cm3 of diethyl
ether. The suspension was stirred and cooled to ice bath tem-
perature at which time chloral hydrate (2.0 g, 12 mmol) in 15
cm3 of diethyl ether was added dropwise. The solution was
allowed to stir for 3 hours, reaching room temperature. The
ether was then removed by rotovap and the resulting solid dried
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 2 0 8 3 – 2 0 8 6
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Table 5 Experimental LFP rate constants
3 M. L. Steigerwald and W. A. Goddard III, J. Am. Chem. Soc., 1979,
101, 1997.
4 H. Y. Yoo, K. N. Houk, J. K. Lee and M. A. Scialdone, J. Am. Chem.
Soc., 1998, 120, 205.
Hydrogen Donor
kgl/104 MϪ1 sϪ1
5 H. Baumann and P. Chen, Helv. Chim. Acta, 2001, 84, 124.
6 S. R. Wilson, Org. React., 1993, 43, 93.
7 T. Cohen, M. Bhupathy and J. R. Matz, J. Am. Chem. Soc., 1983,
105, 520.
8 R. L. Danheiser, C. Martinez-Davila and H. Sard, Tetrahedron,
1981, 37, 3943.
9 S.-H. Kim, S. Y. Cho and J. K. Cha, Tetrahedron Lett., 2001, 42,
8769.
CF3CH2ONa
CF3CH2ONa
(CF3)2HCONa
(CF3)2HCONa
11.3 0.1
10.0 0.1
49.2 2.8
51.5 3.4
constants, kexp(320nm). The experimental rate constant is the sum
of the rate constants for all competitive processes.29
10 N. J. Harris and J. J. Gajewski, J. Am. Chem. Soc., 1994, 116,
6121.
Preparation of samples for laser flash photolysis. The
procedure of the “probe method” was described in detail
previously.29 1.5 cm3 of aqueous solutions (0.027 M) of IRf-
SO3Na in quartz cuvettes (8 × 8 mm) sealed with rubber septa
were deaerated by flushing with N2 during 20 minutes, then the
various amounts (50–300 µL) of deaerated aqueous solution of
H-atom donors were added with microliter syringe, and the
mixtures were vortexed during 20 seconds. The measurements
of the growth of the optical density at 320 nm during 6 to 9
pulses of 308 nm laser were recorded for each concentration of
H-atom donor. The transient growth traces of the radical were
analyzed by least-squares fitting, on the basis of pseudo-first-
11 R. E. Ireland, P. Wipf and J. D. Armstrong III, J. Org. Chem., 1991,
56, 650.
12 F. E. Ziegler, Chem. Rev., 1988, 88, 1423.
13 L. A. Paquette, G. D. Crouse and A. K. Sharma, J. Am. Chem. Soc.,
1980, 102, 3972.
14 P. J. Battye and D. W. Jones, Chem. Commun., 1984, 990.
15 A. B. Shtarev, F. Tian, W. R. Dolbier Jr. and B. E. Smart, J. Am.
Chem. Soc., 1999, 121, 7335.
16 L. Zhang, J. Cradlebaugh, G. Litwinienko, B. E. Smart, K. U. Ingold
and W. R. Dolbier Jr., Org. Biomol. Chem., 2004, 2, 689.
17 L. Zhang, The Addition and Hydrogen Abstraction Reactions of
a Fluorinated Radical in Aqueous Solution, Ph. D. Dissertation,
University of Florida, Gainesville, 2001.
18 J. F. Bunnett, Acc. Chem. Res., 1992, 25, 2.
19 See also: S. E. Vaillard, A. Postigo and R. A. Rossi, J. Org. Chem.,
2004, 69, 2037.
20 W. J. Boyle Jr. and J. F. Bunnett, J. Am. Chem. Soc., 1974, 96, 1418.
21 C. M. Timperley and W. E. White, J. Fluorine Chem., 2003, 123, 65.
22 There is some uncertainty about the pKa of trifluoroisopropanol,
but one would expect it to be slightly less acidic than trifluoro-
ethanol.21
23 Both H-bonding and, particularly, ion pairing (with the metal
counterion) are known to cause large variations in alkoxide-
accelerated rate constants.10 Indeed kinetic studies on alkoxide-
accelerated sigmatropic processes10 have shown that anything that
influences the degree of free charge on the alkoxide will significantly
influence the degree of acceleration provided by the alkoxide.
24 J. Cradlebaugh, L. Zhang, A. B. Shtarev, B. E. Smart and
W. R. Dolbier Jr., Org. Biomol. Chem., 2004, DOI: 10.1039/
b405075d (following paper).
25 J. L. Kurz and M. A. Stein, J. Phys. Chem., 1976, 80, 154.
26 L. Zhang, W. R. Dolbier Jr., B. Sheeller and K. U. Ingold, J. Am.
Chem. Soc., 2002, 124, 6362.
27 S. Kazanis, A. Azarani and L. J. Johnston, J. Phys. Chem., 1991, 95,
4430.
28 D. V. Avila, K. U. Ingold, J. Lusztyk, W. R. Dolbier Jr., H. Q. Pan
and M. Muir, J. Am. Chem. Soc., 1994, 116, 99.
29 H. Paul, R. D. Small Jr. and J. C. Scaiano, J. Am. Chem. Soc., 1978,
100, 4520.
30 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr., T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi,
V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,
P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador,
J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels,
M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul,
S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-
Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W.
Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and
J. A. Pople, Gaussian 03, Revision B.04, Gaussian, Inc., Pittsburgh
PA, 2003.
order kinetics, to obtain experimental rate constants, kexp(320nm)
.
Kinetics measurements using the “spectroscopic probe
method”. The observed experimental rate constant is a sum of
the rate constants of competitive processes:
kexp(320 nm) = ko ϩ kprobe[probe] ϩ kgl [H-donor]
thus, if [probe] = constant, the kexp(320
= kЈ ϩ kgl [H-donor].
nm)
Indeed, values of (kexp(320
Ϫ kprobe[probe]) plotted vs.
nm)
[H-donor] gave straight line fits with R2 greater than 0.96 in
almost all cases (see ESI†) and the absolute second order rate
constants kgl for RfSO3Ϫ reaction with the H-atom donors were
ؒ
obtained. The rate constants kgl for the alkoxide donors in
aqueous solutions are listed in Table 5.
Computational
The geometries of methanol, propanol, isopropanol, and their
corresponding α-hydroxy radicals were optimized at the
B3LYP/6-31G(d) level. Single point energies were then calcu-
lated at the B3LYP/6-311ϩG(2df,2p) level. The restricted
B3LYP was used for closed-shell systems and the unrestricted
method for open shell systems. All the calculations were
performed by using the Gaussian-03 suite of programs.30
An accepted value from experiment for the C–H dissociation
is D298 = 94
G-311GϩG(3df,2p)//MP2/6-31G*) by the late John Pople
predicted D298 = 96.2 kcal molϪ1 31
2 kcal molϪ1 and calculations (QCISD(T)/
.
Acknowledgements
Support of this research in part by the National Science
Foundation is acknowledged with thanks.
References and notes
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