Time-resolved infrared studies of triplet 1,3-cyclopentanediyl

Article abstract of DOI:10.1002/1099-1395(200006)13:6<309::AID-POC242>3.0.CO;2-A

Triplet-sensitized photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene (1) in argon- or oxygen- saturated acetonitrile-d₃ solutions results in the formation of bicyclo[2.1.0]pentane (3), a ring closure product arising from an intermediate 1,3-cyclopentanediyl triplet biradical (2). Time-resolved infrared (TRIR) spectroscopy was used to monitor the kinetics of bicyclopentane 3 production. This analysis provides a measurement of the triplet biradical lifetime and an estimate of the bimolecular reaction rate between biradical 2 and oxygen, both in good agreement with previous investigations. Our studies also indicate that certain IR bands due to 3 in the C-H stretching region overlap with corresponding bands in biradical 2. This interpretation is supported by computational investigations. Copyright

Full text of DOI:10.1002/1099-1395(200006)13:6<309::AID-POC242>3.0.CO;2-A

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2000; 13: 309–312
Time-resolved infrared studies of triplet 1,3-
cyclopentanediyl
Brett M. Showalter,1 Timothy C. Bentz,2 Lev R. Ryzhkov,2* Christopher M. Hadad3* and
John P. Toscano1*
1Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA
2Department of Chemistry, Towson University, 8000 York Road, Towson, Maryland 21252, USA
3Department of Chemistry, Ohio State University, 100 West 18th Avenue, Columbus, Ohio 42310, USA
Received 29 October 1999; revised 7 January 2000; accepted 10 January 2000
ABSTRACT: Triplet-sensitized photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene (1) in argon- or oxygen- saturated
epoc
acetonitrile-d3 solutions results in the formation of bicyclo[2.1.0]pentane (3), a ring closure product arising from an
intermediate 1,3-cyclopentanediyl triplet biradical (2). Time-resolved infrared (TRIR) spectroscopy was used to
monitor the kinetics of bicyclopentane 3 production. This analysis provides a measurement of the triplet biradical
lifetime and an estimate of the bimolecular reaction rate between biradical 2 and oxygen, both in good agreement with
previous investigations. Our studies also indicate that certain IR bands due to 3 in the C–H stretching region overlap
with corresponding bands in biradical 2. This interpretation is supported by computational investigations. Copyright
2000 John Wiley & Sons, Ltd.
KEYWORDS: Time-resolved infrared spectroscopy; 1,3-cyclopentanediyl; triplet biradical
INTRODUCTION
of 2 by low-temperature matrix EPR and the first report
of oxygen trapping of 2,7 the lifetime of this fundamental
triplet biradical has been measured by various experi-
mental methods. In solution, these include quantitative
oxygen trapping experiments (351 Æ 57 ns)8,9 and
photoacoustic calorimetry (258 Æ 14 ns9 and 316 Æ 80
ns10), while in the gas phase time-resolved coherent anti-
Stokes Raman spectroscopy has been used (195 ns).11 In
addition, photoacoustic calorimetry experiments have
Triplet1 and singlet2 hydrocarbon biradicals, molecules
possessing two weakly interacting unpaired electrons, are
an important class of organic free radicals. Their
chemistry has received increasing attention owing to
their potential use as building blocks of high-spin organic
molecules with unique magnetic properties.3 Recently,
they have also been used to analyze substituent effects on
zero-field splitting parameters4 and on intersystem
crossing rate constants.5
While direct detection of biradicals in solution is
possible for derivatives possessing chromophoric
groups,2a,5 characterization of prototypical hydrocarbon
triplet biradicals such as 1,3-cyclopentanediyl (2) and its
alkyl-substituted derivatives has been accomplished
largely through indirect means. Since the pioneering
experiments of Buchwalter and Closs6 on direct detection
provided a measurement of the bimolecular rate constant
1
for the reaction of 2 with oxygen (ko = 5.3 Â 109 l mol
2
s
1).10 Oxygen also catalyzes intersystem crossing of 2 to
the singlet state; however, experiments suggest that this
process is negligible relative to chemical trapping.9
The lifetimes of triplet biradicals are usually governed
by intersystem crossing rates, which depend on numerous
factors including the effects of orbital geometry and
singlet–triplet energy gaps on spin–orbit coupling.12,13
Thus, the relatively long lifetime of 2 is the result of slow
intersystem crossing, a direct consequence of limited
spin–orbit coupling. The latter is mainly due to the time-
averaged planarity of the biradical and the resulting near-
parallel orientation of the orbitals bearing the unpaired
electrons. Confirmation of this structural characteristic is
available from low-temperature matrix EPR experi-
ments6 and quantum chemical calculations.14
*Correspondence to: J. P. Toscano, Department of Chemistry, Johns
Hopkins University, 3400 North Charles Street, Baltimore, Maryland
21218, USA, or L. R. Ryzhkov, Department of Chemistry, Towson
University, 8000 York Road, Towson, Maryland 21252, USA, or C. M.
Hadad, Department of Chemistry, Ohio State University, 100 West
18th Avenue, Columbus, Ohio 42310, USA.
Contract/grant sponsor: Camille and Henry Dreyfus New Faculty
Awards Program.
In an attempt to provide the first experimental insight
into the solution-phase structure of 2, we undertook a
time-resolved infrared (TRIR) spectroscopic study. As
described here, we were unable to detect IR bands due to
Contract/grant sponsor: Petroleum Research Fund.
Contract/grant sponsor: NSF.
Contract/grant sponsor: Research Corporation.
Contract/grant sponsor: Office of Research Services of Towson
University.
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 309–312

310
B. M. SHOWALTER ET AL.
Scheme 1
Figure 1. TRIR difference spectrum averaged over 7±9 ms
following triplet-sensitized (45 mM anisole) photolysis
(266 nm, 10 ns, 0.4 mJ) of diazene 1 (87 mM) in argon-
saturated acetonitrile-d3
and the production of a series of signals that agree very
well with the known IR spectrum15 of bicyclopentane 3
(Table 1). We also examined the spectral region 1550–
1150 cm 1, but only depletion bands due to 1 are
1
observed at 1494, 1446, 1282, 1254, and 1194 cm
.
2 cleanly. We have, however, analyzed the kinetics of
bicyclo[2.1.0]pentane (3) formation despite the fact that
certain IR bands for 3 in the C–H stretching region
overlap with corresponding bands in biradical 2. This
analysis is supported by computational investigations.
Comparable results were obtained with 4,4'-dimethoxy-
benzophenone (ET = 70 kcal mol 1),16 4-methoxyaceto-
1 16
phenone (ET = 72 kcal mol
)
and anisole (ET = 81
1 16
kcal mol
)
as the triplet sensitizer consistent with
efficient quenching by 1 (ET = 62 kcal mol 1)17. The rate
constant for diazene 1 quenching of a 4-methoxyaceto-
phenone triplet excited-state IR band18 was determined to
be approximately 1 Â 109 l mol 1 s
.
1
RESULTS AND DISCUSSION
Given the known photochemistry of 1 (Scheme 1), we
initially expected that each positive IR band due to 3 in
Fig. 1 would display the same, single-exponential
kinetics defined by the lifetime of biradical 2 under the
conditions of our TRIR experiments. As shown in Fig. 2,
this initial expectation was not realized. The growth
1
Typical TRIR data observed from 3100 to 2700 cm
following triplet-sensitized photolysis of diazene 1 in
argon-saturated acetonitrile-d3 are shown in Fig. 1. Data
are obtained in the form of difference spectra. We
observe depletion of the 3004 cm 1 band due to diazene 1
1
kinetics for peaks at 3050, 2924, and 2856 cm are not
identical. We have analyzed the kinetic data in Fig. 2 by
considering the possibility that the observed kinetics may
be complicated by overlapping bands due to biradical 2
and product 3.
Table 1. Comparison of experimental and calculated IR
frequencies in the C±H stretching region for bicyclo[2.1.0]-
pentane (3) and triplet 1,3-cyclopentanediyl (2)
Idealized kinetic traces for the growth and decay of
biradical 2 and for the growth of bicyclopentane 3 are
shown in Fig. 3. Here, the rate of production of biradical
2 is given as 2 Â 107 s 1, the present time response of our
spectrometer, since previous studies9–11 have indicated
that 2 is formed within 25 ns of the laser pulse. Its rate of
decay is, of course, required to equal the rate of growth of
3. For an overlapping band, different relative contribu-
tions of the two kinetic traces in Fig. 3 will result in
different observed kinetics. A greater intensity contribu-
tion from biradical 2 will result in what appears (given
the limited signal-to-noise ratio of our data) to be a faster
initial growth rate. Substantial signal averaging was
required to obtain the kinetic traces in Fig. 2. Each trace
is the result of averaging approximately 6500 laser shots.
Thus, we have assumed that the IR band observed at
2924 cm 1 is due exclusively to bicyclopentane 3. Its rate
of growth can be fit [Fig. 2(b)] to a single exponential
function [kobsd = (3.6 Æ 0.4) Â 106 s 1] that corresponds
to a biradical lifetime of 278 Æ 30 ns, in very good
agreement with previous work.9–11 Kinetics observed at
3 TRIR
observed
3 Thin film
observed (cm
3 Calculated 2 Calculated
1 c 1 d
1 a
1 b
(cm
)
)
(cm
)
(cm )
3050
3056 [cp CH2 ( )] 3058 (29)
3045 (bh CH) 3042 (4)
3064 (3)
3061 (36)
2968
2924
2974 [cp CH2 (‡)] 3036 (25)
2961 [cb CH2 ( )] 2976 (50)
2939 (bh CH)
2929 [cb CH2 (‡)] 2958 (2)
2900 (sh) 2910 [cb CH2 ( )] 2916 (70)
2856
2861 [cb CH2 (‡)] 2908 (53)
2964 (25)
2898 (32)
2895 (39)
2824 (50)
2816 (20)
2772 (56)
2748 (27)
aThis work. sh = shoulder.
bRef. 15. Assignments: bh = bridgehead; cp = cyclopropyl; cb = cy-
clobutyl; (‡) = in-phase; ( ) = out-of-phase.
cB3LYP/6–311 ‡ G(2d,p) scaled by 0.96. Calculated intensities are
given in parentheses.
dUB3LYP/6–311 ‡ G(2d,p) scaled by 0.96 (s2 = 2.01). Calculated
intensities are given in parentheses.
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 309–312

TIME-RESOLVED IR STUDIES OF TRIPLET 1,3-CYCLOPENTANEDIYL
311
Figure 3. Idealized kinetic traces for (a) the growth and
decay of biradical 2 and (b) the growth of bicyclopentane 3
UB3LYP/6–311 ‡ G(2d,p) calculations on triplet biradi-
cal 2 indicate that overlap with 3 is likely for our
experimentally observed 3050 and 2856 cm 1 bands, but
not for bands observed at 2968 and 2924 cm 1. (We were
1
not able to collect adequate kinetic data at 2968 cm
owing to the low signal intensity.) These calculations also
suggest that an unperturbed biradical band should be
detectable in the region 2850–2750 cm 1. We did not,
however, detect any IR bands in this region (Fig. 1).
As expected (Scheme 1), we find that the rate of
growth of the 2924 cm 1 band is affected by oxygen with
1
kobsd ꢀ2 Â 107 s in oxygen-saturated acetonitrile-d3.
This observed rate constant (which is now limited by the
time response of our spectrometer) can be analyzed
according to the pseudo-first-order equation kobsd = k0
‡ kO [O2], where kO is the second-order rate constant for
2
2
the reaction of biradical 2 with oxygen and k0 is the rate
of biradical decay (and bicyclopentane 3 growth) in the
absence of oxygen. (The concentration of oxygen in
saturated acetonitrile solutions is 9.1 mM.20) This analy-
Figure 2. Kinetic traces observed at (a) 3050, (b) 2924 and
1
(c) 2856 cm
following triplet-sensitized (50 mM 4,4' -
dimethoxybenzophenone) photolysis (355 nm, 5 ns, 3 mJ) of
diazene 1 (110 mM) in argon- or oxygen-saturated aceto-
nitrile-d3. The dotted curves are experimental data; the solid
curves are calculated best ®ts as described in the text
sis leads to a lower limit for the oxygen quenching rate
1
constant kO ꢀ2 Â 109 l mol
s
1, again in good
2
agreement with previous work.10
3050 and 2856 cm 1 were fitted manually [Fig. 2(a) and
CONCLUSIONS
(c)] using the traces in Fig. 3 with the decay of 2 and
1
production of 3 fixed at 3.6 Â 106
s
(from the
fit corre-
Kinetic analysis of the rate of growth of bicyclopentane 3
IR bands observed following triplet-sensitized photolysis
of diazene 1 has provided a direct measurement of the
lifetime of biradical 2, 278 Æ 30 ns, in good agreement
with previous studies. In addition, this analysis, with the
support of computational investigations, has indicated
1
1
2924 cm
band analysis). The 3050 cm
sponds to a biradical:bicyclopentane intensity ratio of
1:1; the 2856 cm fit corresponds to a ratio of 2:3.
1
Additional support for overlapping biradical 2 and
bicyclopentane 3 IR bands comes from computational
studies. As shown in Table 1, experimental IR frequen-
cies of 3 in the C–H stretching region are reasonably
reproduced by B3LYP/6–311 ‡ G(2d,p) vibrational fre-
quency calculations (scaled by 0.96).19 (A table of all
calculated frequencies for both 2 and 3 at the HF, MP2,
and B3LYP levels of theory is included as Supporting
Information; calculated vibrational frequencies at each
level are consistent with each other.) Analogous
that biradical 2 likely possesses observable IR bands near
1
3050 and 2856 cm
.
EXPERIMENTAL
Materials. Unless noted otherwise, materials were
obtained from Aldrich Chemical and used without further
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 309–312

312
B. M. SHOWALTER ET AL.
purification. Acetonitrile-d3 was used as received from
Cambridge Isotope Laboratories. 2,3-Diazabicyl-
co[2.2.1]hept-2-ene (1) was prepared according to the
literature procedure.21
Towson University for financial support of this research.
C.M.H. thanks the NSF for financial support and the Ohio
Supercomputer Center for the use of their computing
facilities.
Time-resolved IR methods. We conducted TRIR experi-
ments following the method of Hamaguchi and co-
workers22 as described previously.23 Briefly, the broad-
band output of an MoSi2 IR source (JASCO) is crossed
with excitation pulses from an Nd:YAG laser. Changes
in IR intensity are monitored by an MCT photovoltaic
IR detector (Kolmar Technologies, KMPV11-1-J1),
amplified, and digitized with a Tektronix TDS520A
oscilloscope. The experiment is conducted in the
dispersive mode with a JASCO TRIR-1000 spectro-
meter. TRIR difference spectra were collected at
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Acknowledgements
J.P.T. gratefully acknowledges the Camille and Henry
Dreyfus New Faculty Awards Program, the donors of the
Petroleum Research Fund, administered by the American
Chemical Society, and an NSF Faculty Early Career
Development Award for generous support of this
research. J.P.T. and L.R.R. thank the donors of the
Petroleum Research Fund, administered by the American
Chemical Society for an ACS–PRF Summer Faculty
Research Fellowship and the NSF for a Research
Opportunity Award. L.R.R. acknowledges the Research
Corporation and the Office of Research Services of
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 309–312