Ultraviolet absorption and fluorescence studies of 1-methyl-2(1H)-pyridinimine and derivative in polar and non-polar solvents

Article abstract of DOI:10.1071/CH00053

The ultraviolet (u.v.) and fluorescence spectra and relative fluorescence intensities of 1-methyl-2(1H)-pyridinimine (1-MIP) and 1,2-dimethylpyridinimine (2-MIP) were measured in several solvents at room temperature. The solvents' effect on the absorption and fluorescence spectra of MIP indicates that the ππ* transition shifts to higher energies (shorter wavelengths) are due to solvents' interactions and the formation of the pyridinium cation. One major difference in the u.v. and fluorescence spectra is the small relative intensity in non-polar solvents. This was attributed to the proximity effect of ππ* and nπ* states which can lead to vibronic interactions, and to distortions of the excited-state potential surfaces. In polar solvents, however, these two states are shifted in energy due to solvent-solute interactions, which decrease the energy of the potential surface.

Full text of DOI:10.1071/CH00053

C S I R O P U B L I S H I N G
Australian Journal
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Volume 53, 2000
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Aust. J. Chem., 2000, 53, 951–957
Ultraviolet Absorption and Fluorescence Studies of
-Methyl-2(1H)-Pyridinimine and Derivative in
1
Polar and Non-Polar Solvents
Hassimi Traore,A,B Michael Saunders and Scott Blasiman
A
A
A
Department of Chemistry, University of Wisconsin–Whitewater, Whitewater, WI 53190.
Author to whom correspondence should be addressed (e-mail: traoreh@mail.uww.edu).
B
Dedicated to Dr John Zdysiewicz on the occasion of his retirement
The ultraviolet (u.v.) and fluorescence spectra and relative fluorescence intensities of 1-methyl-2(1H)-pyridinimine
1-MIP) and 1,2-dimethylpyridinimine (2-MIP) were measured in several solvents at room temperature. The solvents’
effect on the absorption and fluorescence spectra of MIP indicates that the ππ* transition shifts to higher energies
shorter wavelengths) are due to solvents’ interactions and the formation of the pyridinium cation. One major
(
(
difference in the u.v. and fluorescence spectra is the small relative intensity in non-polar solvents. This was
attributed to the proximity effect of ππ* and nπ* states which can lead to vibronic interactions, and to distortions
of the excited-state potential surfaces. In polar solvents, however, these two states are shifted in energy due to
solvent–solute interactions, which decrease the energy of the potential surface.
Keywords: Ultraviolet absorption; 1-methyl-2(1H)-pyridinimine; 1,2-dimethyl-pyridinimine; fluorescence
quantum yield; polar and non-polar solvents; proximity effects; Franck–Condon factor.
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Introduction
Results
Ultraviolet and fluorescence studies of nitrogen-containing
heterocycles are of central importance in a wide range of
The u.v. absorption spectra of 1-MIP dissolved in hexane and
water are illustrated in Figure 1. In hexane, the first
prominent absorption is centered at about 347 nm and
extends to approximately 420 nm. Expansion of the baseline
revealed no other features beyond 410 nm. There is a second,
stronger absorption with a maximum at about 251 nm. The
structure at wavelengths less than 220 nm is due to
instrumental artifacts either deriving from the solvent or
from the spectrometer cutoff. The long-wavelength
absorption band manifests clear vibronic features at 400,
380, 355, 345, and 325 nm that give a hint of a Franck–
Condon progression in the excited state. The relative energy
spacing of these peaks are collected in Table 1 along with
suggested assignments.
1
–4
biological processes and biochemical systems.
These
species can have very complicated spectroscopy due to the
presence of the ring nitrogen and the associated lone-pair
electrons. The resultant photochemical mechanisms can also
be quite rich and varied. Perturbations induced by solvent
species exacerbate the difficulty of investigating the
properties of relevant heterocycles. Strong hydrogen-
bonding interactions, such as those present in the aqueous
environment of biological systems, can induce spectral
changes that make interpretation of condensed-phase
spectral data difficult. In addition, the most interesting
heterocycles are generally quite large, engendering a great
deal of spectroscopic congestion in ground- and excited-
state studies.
The goal of the current work is to synthesize and
investigate spectroscopically a stable imino heterocycle that
can serve as a model compound for the purine or
pyridinimine deoxyribonucleic acid (DNA) base species. We
have attempted to carefully characterize the spectroscopy of
the imino compound and a derivative, in a variety of
solvents, to learn more about the relevant electronic
structures. Clearly, a great deal of insight into proton-transfer
processes can result from such studies. Spectroscopy of
imino compounds is also of general interest because of the
important role they play in the metabolism of living cells,
and also in industrial applications.4
Table 1. Vibronic spacings of 1-MIP in hexane
Peak ν(cm–1
)
∆νcm
–1 A
)
∆∆ν(cm
–1 B
)
IRC
Assign-
ment
No.
Origin 25119
0.00
1
2
3
4
26497
27894
28960
17986
1378
2775
3841
5488
1378
1397
1066
1070
1406 CH, ΝΗ
1082 CH out-of-plane
A
B
C
Spacing from Peak No. 1.
Spacing between peaks.
From ref. 14.
Manuscript received 20 April 2000
© CSIRO 2000
10.1071/CH00053
0004-9425/00/110951

9
52
H. Traore et al.
Fig. 2. U.v. absorption spectra of 2-MIP in hexane (A) and in water
Fig. 1. U.v. absorption spectra of 1-MIP in hexane (A) and in water
B). A significant blue-shift is observed for the long-wavelength
feature in going from hexane to water. Note that while the long-
wavelength absorption in hexane shows some vibronic bands, the
corresponding feature in water is quite smooth.
(B). The spectra are similar to those recorded for 1-MIP (Fig. 1)
(
except that a red-shift is observed for the long-wavelength
absorption band.
The absorption spectroscopy of 1-MIP was investigated in
a number of other polar solvents such as DMSO and methanol.
In all cases, a spectrum very similar to that collected for 1-
MIP in water was observed. Representative results for 1-MIP
in DMSO and methanol are contained in Figure 3. Absorption
by the solvent precludes detection of features below about
The absorption spectrum of 1-MIP in water, Fig. 1, is very
different from that recorded in hexane. The shorter
wavelength feature is still present and is blue-shifted to about
231 nm while the longer wavelength feature has also
undergone a significant blue-shift, placing the band
maximum at approximately 300 nm. In addition, the
absorption band is smooth and does not exhibit any
vibrational structure.
260 nm in the DMSO spectrum. For purposes of comparison,
the absorption spectra of the iodide salts of 1-MIP and 2-MIP
dissolved in water are shown in Figure 4. The peak of the
long-wavelength band for the 1-MIP salt is at 300 nm and for
the 2-MIP salt it is at 314 nm.
Beer’s law plots for 1-MIP and 2-MIP in hexane (at a
wavelength of 300 nm) were generated and the results are
shown in Figure 5. 1-MIP Was found to have a molar
A similar set of spectra for 2-MIP are contained in
Figure 2. The appearance of the 2-MIP spectrum in hexane is
similar to the corresponding spectrum of 1-MIP except that
the long wavelength absorption band exhibits a red-shift.
The band maximum is now at approximately 370 nm and
extends to approximately 442 nm. A similar set of vibronic
features characterizes the 2-MIP absorption profile and the
relative energy spacing are collected in Table 2. The water
spectrum is also analogous to 1-MIP although the band
maximum appears to be at a slightly longer wavelength,
about 312 nm. The short-wavelength band peaks at
approximately the same wavelength, 235 nm, as in 1-MIP.
Table 2. Vibronic spacings of 2-MIP in hexane
–1 A
–1 B
IRC
Peak
No.
ν(cm–1) ∆ν(cm
)
⌬⌬ν(cm
)
Assign-
ment
Origin 23474 0.00
1
2
3
4
24480 1536
25793 2849
26969 4025
27972 5028
1536
1313
1176
1003
1884 CH
1014 CH out-of-plane
A
B
C
Spacing from Peak No. 1.
Spacing between peaks.
From ref. 14.

Absorption and Fluorescence Studies of 1-Methyl-2(1H)-Pyridinimine
953
Fig. 4. U.v. absorption spectra of the iodide salts of 1-MIP (A) and
-MIP (B) dissolved in water. Note the similarities with the associated
2
spectra of these compounds in polar solvents.
Table 3. Fluorescence bands of 1-MIP in hexane
Peak ν(cm–1
No.
)
∆ν(cm
–1 A
)
∆∆ν(cm
–1 B
)
IRC
Assign-
ment
Origin 23474
0.00
2
2124
1350
1350
1342 CH, CH rock
3
NC , NC str
Fig. 3. U.v. absorption spectra of 1-MIP in DMSO (A) and in
2
6
methanol (B).
2
3
4
20747
19455
17986
2727
4019
5488
1377
1292
1469
1477 CH3
absorption coefficient of ε = 500 ± 20 M–1 cm–1 while the
value for 2-MIP was markedly larger, ε = 1760 ± 70 M
cm . Beer’s law plots for the two compounds dissolved in
water (at a wavelength of 300 nm) are contained in Figure 6.
The absorption coefficients were seen to increase for both
A
B
C
–
1
Spacing from Peak No. 1.
Spacing between peaks.
From ref. 14.
–
1
–1
compounds with a measured value of ε = 5500 ± 140 M
spacing of the observed peaks and tentative band
assignments are reported in Table 3. In contrast, the
fluorescence signal from 2-MIP under similar conditions is
barely detectable. As the 10 × expansion shows, the resultant
poor signal-to-noise precludes the identification of
vibrational band structure in the 2-MIP emission spectrum.
The two compounds were also investigated in water and
the results are contained in Figure 8. For both 1-MIP and 2-
MIP, a very strong fluorescence signal is detected subsequent
to excitation at 300 nm. Comparison of the peak-emission
signal with the Raman band or the scattered excitation light
illustrates the healthy fluorescence quantum yield, especially
when compared with the corresponding results of Figure 7 in
hexane. However, no structure is observed on the emission
band profiles for either compound. The 2-MIP fluorescence
cm for 1-MIP and ε = 7200 ± 120 M–1 cm for 2-MIP. The
absorption was recorded at the band maximum in all cases in
the preceding analysis.
–
1
–1
The fluorescence spectroscopy of the imino compounds
in a number of solvents was also investigated in a similar
fashion. In Figure 7, the fluorescence spectra for 1-MIP and
2
-MIP in hexane with an excitation wavelength of 360 nm are
depicted. The strong sharp peaks at approximately 360 nm
correspond to a Raman transition in the hexane solvent,
which appears again at 720 nm due to second-order
diffraction from the fluorimeter grating. The fluorescence
spectrum of 1-MIP is highly structured, as is the absorption
spectrum, and exhibits some type of vibrational progression
active in the ground electronic state. The relative energy

9
54
H. Traore et al.
Fig. 6. Beer’s law plots for 1-MIP (A) and 2-MIP (B) in water at 300
Fig. 5. Beer’s law plots for 1-MIP (A) and 2-MIP (B) in hexane at
nm. The measured values for the molar absorption coeeficients are
3
00 nm. The measured values for the molar absorption coeeficients
–1
–1
–1
–1
ε = 5500 ± 140 M cm for 1-MIP and ε = 7200 ± 120 M cm for
–
1
–1
–1
–1
are ε = 505 ± 19 M cm for 1-MIP and ε = 1760 ± 70 M cm for
-MIP.
2
-MIP.
2
is slightly red-shifted from that of 1-MIP, in correspondence
with the red-shift observed in the absorption spectra. In order
to more fully characterize the effect of solvent polarity on the
imino compound emission spectra, the fluorescence of 1-MIP
in water, methanol and acetonitrile under similar conditions
is compared in Figure 9. A clear trend is observed with the
fluorescence signal increasing in the relative order; MeOH <
H O < CH CN.
Our observations appear to be consistent with the long-
wavelength absorption feature in MIP arising from a
transition to a ππ* electronic state. The long wavelength and
–
1
the relatively low oscillator strengths (ε = 500 ± 20 M
–
1
–1
–1
cm for 1-MIP and ε = 1760 ± 70 M cm for 2-MIP)
measured in hexane indicate that the state may have
considerable nπ* character. In fact, it is expected that these
electronic states are relatively close in energy in 1- and 2-MIP.
Interactions between the two states can lead to a number of
experimentally observable effects. For instance, strong
mixing between the ππ* and nπ* states can lead to
perturbations in the electronic spectrum. As a result, strong
2
3
Discussion
In azabenzenes, such as pyridine, the lowest singlet state
5
is generally of nπ* character. However, methylation of the
ring can change the ordering of the nπ* and ππ* states,
activity in out-of-plane ring modes may be activated and
6
9,10
making the latter the lowest singlet state. In addition, the
observed in the absorption spectrum.
Such an effect may
electronic structure of MIP may more strongly resemble a
conjugated polyene than the aromatic structure of pyridine,7
favoring the ππ* state as the lowest energy singlet. Indeed,
the red-shift observed for the long-wavelength 2-MIP
absorption maximum relative to the 1-MIP band maximum in
hexane is consistent with the behavior expected for a ππ*
electronic state, due to the donation of electron density to the
π-bonding system by the methyl substituent.8
rationalize the observed vibronic features in the absorption
profile of 1-MIP in hexane (Fig. 1A).
It must be noted that the observation of a similar structure
in the 2-MIP spectrum (Fig. 2A) is at odds with the above
interpretation. Methyl substitution will shift the ππ* to lower
energies while the nπ* state is shifted to higher energies,
increasing the energy gap between the two states. As a result,
one might expect that the intensity of any out-of-plane

Absorption and Fluorescence Studies of 1-Methyl-2(1H)-Pyridinimine
955
measured spectral changes may also be attributed to the
formation of the ionic species (1-methyl-2-aminopyridinium
hydroxide) due to the strong basicity of 1-MIP (pKa
=
1
3.02).11 In studies of the absorption spectra of
1
2
aminopyridines, Seeger and Anderson noted that when 1-
MIP is dissolved in water it forms a colorless, basic solution.
The absorption spectra of 1-MIP in a mixture of 72:28%
dioxane–water12 indicates that an equilibrium is established.
The equilibrium is shifted towards the protonated form upon
further addition of water, and any color gradually disappears
and the solution becomes more basic.
Fig. 7. Fluorescence spectra of 1-MIP (A) and 2-MIP (B) in hexane
with an excitation wavelength of 360 nm.
modes induced by state mixing would decrease, in contrast
to the observation of band structure in the 2-MIP data. In
addition, the emission spectrum of 1-MIP exhibits clear
structure that is very similar to that measured in the
absorption spectrum. Structure in the emission spectrum is
due to Franck–Condon activity induced by the electronic
transition rather than any type of state mixing. Thus, this
suggests that the vibrational bands seen in the 1- and 2-MIP
absorption spectra are due to progressions activated by the
ππ* transition. Carbon–carbon or carbon–nitrogen
stretching modes, or perhaps ring distortions, may be
responsible for the vibronic features.
The similarities between the 1-MIP and 2-MIP absorption
spectra in hexane solvent provides no clear clues as to the
role of cis/trans conformers. One might expect that, if the
electronic transitions of the two conformers were
significantly different, it might be possible to detect
overlapping features in the 1-MIP spectrum. Such features
would be absent in the 2-MIP spectrum which should only
have a contribution from the trans conformer. The similar
vibronic structure measured for the two species belies this
facile interpretation.
Seeger and Anderson12 also proposed that the absorption
band at 350 nm is due to the neutral form while the band at
3
03 nm corresponds to the cationic form. The latter band is
similar to that observed for 2-aminopyridine which has a
broad feature at 299 nm characteristic of the pyridine ring
structure.12 More importantly, the absorption spectra
measured for the iodide salts (Fig. 4) in water is virtually
indistinguishable to those obtained when the pure
compounds are dissolved in water. Also, the Beer’s law plots
show that the aqueous 300 nm feature has a much larger
–1
–1
absorption coefficient, ε = 5500 ± 140 M cm for 1-MIP
and ε = 7200 ± 120 M–1 cm for 2-MIP, than that obtained in
hexane, suggesting that a different chromophore is
responsible. These observations support the view that
formation of the protonated species in aqueous solutions,
due to the strong basicity of MIP, is responsible for the
spectroscopic changes.
–1
A large blue-shift is observed in the absorption spectra of
MIP when it is dissolved in polar solvents. A blue-shift
indicates that the compound is more strongly stabilized in the
electronic ground state than in the excited state. The
It is interesting to note that similar spectra can also be
obtained in other polar solvents such as methanol, DMSO or
acetonitrile (Fig. 9), each of which manifest the same
featureless absorption band with a peak in the vicinity of 300
nm. Significant concentrations of the cationic species are not
expected to form in these solvents. For instance, the pK of
a
methanol is approximately 17 and the equilibrium constant
for protonated 1-MIP would be of the order of 10– , too small
to rationalize the relatively strong absorption band. We have
also observed evidence for the 300 nm band in hexane using
sensitive excitation spectroscopy, which is possibly due to a
finite amount of water present in the sample. Strong solvent–
solute interactions in these polar media may engender
changes in the electronic structure of the chromophore that
are similar to the cationic form. It is more likely, however,
that our sample of ‘pure’ compound readily absorbs moisture
from the atmosphere so that the hydroxide is always present
and the cationic species can be stabilized in polar
environments. It is especially intriguing to consider studies
of 1-MIP complexed with various solvent molecules in the
environment of a supersonic expansion.
18
Fig. 8. Fluorescence spectra of 1-MIP (A) and 2-MIP (B) in water
with an excitation wavelength of 300 nm.

9
56
H. Traore et al.
radiatiative processes in the excited-state species. Also, in
the expected trans configuration, the imino methyl group
may be much less hindered than the ring-substituted methyl
group. The latter must contend with steric interactions with
the imino lone-pair (in the trans configuration) or the imino
hydrogen (in the cis configuration) which may yield
significant barriers to rotation and, consequently, a higher
frequency torsional motion. In any case, the smaller quantum
yield for 2-MIP has important ramifications for the jet studies
(
vide infra).
Conclusion
The solvents’effect on the absorption and fluorescence
spectra of MIP indicates that the ππ* transition shifts to
higher energies (shorter wavelengths) in polar solvents due
to solvent interactions and the formation of the pyridinium
cation. The fluorescence spectra are characterized by a
relatively small intensity in hexane compared with polar
solvents, such as water. Another major difference is
illustrated by the smooth absorption band obtained in water
while in hexane the spectrum exhibits evidence for Franck–
Condon activity in various vibrational modes. We have
recorded the fluorescence spectra of 1-MIP in a number of
polar solvents (methanol, acetonitrile, and DMSO), and have
observed a similar blue-shift. The absorption spectrum of
MIP in hydrocarbon solvents can be understood as being the
result of a ππ* state that is close in energy to a nπ* state. The
close proximity can lead to vibronic interactions and to
distortions of the excited state potential surfaces, as has been
Fig. 9. Fluorescence spectra of 1-MIP in acetonitrile
A), water (B) and methanol (C) with the same
excitation wavelength, 230 nm, and similar
concentrations.
(
It is clear that the fluorescent yield of both 1-MIP and 2-
MIP is considerably enhanced in polar solvents. This is most
noticeable in the series of spectra presented in Figure 9. The
fluorescence quantum yield is seen to increase as the polarity
9
of the solvent is increased; MeOH (µ = 1.70 D) < H O (µ =
observed for many similar N-heterocyclic compounds. In
2
1
.85 D) < CH CN (µ = 3.92 D). Interactions between nearby
polar solvents, however, the states are shifted in energy due
to solvent–solute interactions which decreases the surface
distortion. Thus, an increase in the fluorescence yield results
from a decrease in the rate of non-radiative processes.
3
ππ* and nπ* states can lead to a marked reduction in
fluorescence quantum yield in non-polar solvents due to an
enhancement in the rate of non-radiative processes such as
internal conversion. In polar solvents, the energy gap
between the two electronic states is increased, diminishing
state mixing, and resulting in a much increased quantum
yield for fluorescence. Such a phenomenon has been termed
Experimental
Synthesis of Methyl-2(1H)-pyridinimine (1-MIP) and 1,2-Dimethyl
pyridinimine (2-MIP)
9
The preparation of the iminopyridine compounds, specifically 1-
methyl-2(1H)-pyridinimine, utilized a modified version of Taylor and
coworkers’ procedure13 in the following manner. A mixture of 2-
aminopyridine (Aldrich Chemical Co.) and methyl iodide was placed in
a 150 ml round-bottomed flask, and warmed on a steam bath until an
exothermic reaction commenced. The reaction was then allowed to
continue for about 3 h. After the reaction was complete, the mixture was
cooled and dried in vacuum. The mixture was recrystallized three times
from absolute ethanol (Midwest Grain Products Co.) to give the 1-
methyl-2(1H)-pyridinium iodide salt. The salt was then dissolved in a
one equivalent solution of sodium hydroxide and stirred vigorously for
about 15 min. The extraction of the free base with ether was followed
by drying over anhydrous sodium sulfate followed by filtration. A
rotary evaporator was then used to remove residual solvent and yield the
the proximity effect and our results are in accordance with
this mechanism.
The quantum yield for 1-MIP in hexane, while relatively
small, is still sufficient to measure emission spectra and
would seem to indicate that this molecule is amenable to
supersonic jet studies. While we did not attempt to
quantitatively measure the quantum yield, Fujimoto and
5
11
coworkers have reported a value of Φ = 3.7 ± 0.9 × 10– .
F
We have observed that the corresponding yield for 2-MIP in
hexane is much reduced. Based on the measured differences
in absorption coefficients we estimate that the quantum yield
of 2-MIP is about 60 times less than that of 1-MIP. In contrast,
1
-MIP product, which was a viscous yellow–brown liquid. Yields were
Fujimoto et al.11 report a value of Φ = 3.6 ± 0.9 × 10 for
–5
F
typically 15% to 20%.
2
-MIP, a number that is essentially identical to the yield
The nuclear magnetic resonance (n.m.r.) spectrum of 1-MIP in
determined for 1-MIP.
CDCl was recorded with a Bruker AC300 NMR. The major peaks
3
1
observed in the H n.m.r were: δ 7.98, t, J 11.9 Hz, 1H; 7.72, t, J 8.7 Hz,
The source of the apparent discrepancy in the observed
quantum yield for 2-MIP is not clear. It is reasonable to
suppose that 2-MIP has a smaller yield for fluorescence. The
low frequency modes due to the presence of the second
methyl rotor in 2-MIP can accelerate the rate of non-
1
H; 6.97, d, J 11.9 Hz, 1H; 6.74, d, J 11.9 Hz, 1H; 3.68, s, CH .
3
The mass spectrum was recorded with a commercial magnetic mass
spectrometer as follows: a 10 ng/µl sample of 1-MIP was prepared and
then injected into a gas chromatograph (Model 5890, Hewlett Packard)
which is connected to VG Trio-1 mass spectrometer (Fison Instr.) for

Absorption and Fluorescence Studies of 1-Methyl-2(1H)-Pyridinimine
957
mass analysis. The result of the measured mass spectrum gave a parent
mass of 108.0000 amu. The calculated mass of 1-MIP is 108.0680 amu,
providing further confirmation that the desired compound was
successfully synthesized.
Acknowledgments
The authors thank Dr Akira Fujimoto, Dr Timothy R.
Proctor and Dr Mark Young for their feedback. The authors
are also grateful for support from the University of
Wisconsin-Whitewater.
The related compound, 1,2-dimethyl pyridinimine (2-MIP), was also
synthesized. The synthetic route was similar except that the starting
material was changed to 2-methylaminopyridine. The 2-MIP was
produced in similar yields and with similar purity to 1-MIP. The 2-MIP
compound is expected to exist only in the trans conformation due to a
destabilizing steric interaction between the two methyl groups in the cis
configuration. This compound was synthesized in order to provide an
illustrative contrast to the spectroscopic results obtained for 1-MIP,
which has accessible cis and trans conformations. We conducted a
References
1
Cantor, C. R., and Schimmel, P. R., in ‘Biophysical Chemistry’
Vol. 2, Ch. 7–8 (Freeman: San Francisco 1980.)
Weinryb, I., and Steiner, R. F., in ‘Excited States and Nucleic Acid’
2
(
Eds R. F. Steiner and I. Weinryb) Ch. 5 (Plenum Press: New York
14
1971.)
semi-empirical molecular orbital calculation with the AM1 basis
3
1
5
See for example, ‘Excited States of Biological Molecules’ (Ed. J.
B. Birks) (Wiley–Interscience: London 1976); Albert, A., in
using MOPAC (version 6.0) implemented on a Tektronix CAChe
system. The heats of formation for the cis and trans conformers of 1-
MIP were calculated to be 54.1 kcal/mole and 55.8 kcal/mole,
respectively. These results indicate, albeit with some degree of
crudeness, that both 1-MIP isomers should be produced in relatively
equal amounts in the synthesis since the thermodynamic difference is
negligible.
‘Heterocyclic Chemistry’ 2nd Edn., p. 3 (The Athlone Press:
University of London 1968.)
4
5
Herzberg, G., in ‘Molecular Spectra and Molecular Structure of
Polyatomic Molecules’ (Van Nostrand Reinold: New York 1966.)
Innes, K. K., Byrne, J. P., and Ross, J. G., J. Mol. Spec., 1967, 22,
1
25.
6
7
Testa, A. C., J. Photochem., 1983, 22, 41.
Nuzuka, K., Fujimoto, A., and Ito, H., Bull. Chem. Soc. Jpn, 1993,
66, 287.
Hager, J. W., Demmer, D. R., and Wallace, S. C., J. Phys. Chem.,
1987, 91, 1375.
Felker, P. M., and Zewail, A. H., Chem. Phys. Lett., 1983, 94, 448.
Weissstuch, A., and Testa, A. C., J. Phys. Chem., 1968, 72, 1982.
Inuzuka, K., Fujimoto, A., and Ito, H., Bull. Chem. Soc. Jpn, 1993,
66, 287.
Anderson, L. C., and Seeger, N. V., J. Am. Chem. Soc., 1949, 71,
340.
Taylor, E. C., and Kan, R. O., J. Am. Chem. Soc., 1963, 85, 776.
Dewar, M. J. S., Zoebisch, E. G., Healy, E. F., and Stewart, J. J. P.,
J. Am. Chem. Soc., 1985, 107, 3902.
Absorption and Fluorescence Methods
All of the absorption spectra were measured with the same standard
8
1
cm quartz cuvette at room temperature with a Hewlett Packard 8452A
diode array spectrophotometer. The standard solvents used were either
high-performance liquid chromatographic (HPLC) or
9
0
1
1
1
spectrophotometric grade (e.g. Aldrich). Water was obtained from a
commercial water purification system (Model D4641, Barnstead
Thermodyne) which filtered particulates and trace organics and also
has an ion exchange column. The spectra of the pure solvent was first
recorded in order to serve as a background and the spectra of the
dissolved samples were then referenced to this background. The
fluorescence spectra were measured in various solvents with a SPF-
1
2
1
1
3
4
5
00CTM spectrofluorimeter (SLM Aminco Co.) at room temperature
1
5
in the same cuvette.
Stewart, J. P., J. Comput. Aided Mol. Des., 1990, 4, 1.