The interaction of Cinnamic acids with 60Co gamma radiation

Article abstract of DOI:10.1080/15421400600786330

In the interest of expanding our knowledge of the interaction of cinnamic acid and its derivatives with ionizing radiation, (E)-cinnamic acid and several of its chlorinated and brominated derivatives were exposed to ⁶⁰Co gamma radiation. Dimer yields were estimated from integration of the proton NMR signals of the irradiated material. (E)-Cinnamic acid itself is relatively unreactive. The largest yield of dimer was obtained with p-bromocinnamic acid where exposure to a dose of 116 megarads led to a 24% yield.

Full text of DOI:10.1080/15421400600786330

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The Interaction of Cinnamic
Acids with 60Co Gamma
Radiation
Jeffrey M. Njus a , Chantana Sae-Lim a & Daniel J.
Sandman a
a Center for Advanced Materials, Department of
Chemistry , University of Massachusetts Lowell ,
Lowell, Massachusetts, USA
Published online: 21 Dec 2006.
To cite this article: Jeffrey M. Njus , Chantana Sae-Lim & Daniel J. Sandman (2006)
The Interaction of Cinnamic Acids with 60Co Gamma Radiation, Molecular Crystals and
Liquid Crystals, 456:1, 55-61, DOI: 10.1080/15421400600786330
To link to this article: http://dx.doi.org/10.1080/15421400600786330
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Mol. Cryst. Liq. Cryst., Vol. 456, pp. 55–61, 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400600786330
The Interaction of Cinnamic Acids with 60Co
Gamma Radiation
Jeffrey M. Njus
Chantana Sae-Lim
Daniel J. Sandman
Center for Advanced Materials, Department of Chemistry, University
of Massachusetts Lowell, Lowell, Massachusetts, USA
In the interest of expanding our knowledge of the interaction of cinnamic acid and
its derivatives with ionizing radiation, (E)-cinnamic acid and several of its chlori-
nated and brominated derivatives were exposed to 60Co gamma radiation. Dimer
yields were estimated from integration of the proton NMR signals of the irradiated
material. (E)-Cinnamic acid itself is relatively unreactive. The largest yield of
dimer was obtained with p-bromocinnamic acid where exposure to a dose of 116
megarads led to a 24% yield.
Keywords: cinnamic acid dimerization; gamma radiation; heavy atom effect
INTRODUCTION
In the investigation of solid-state reactions, our current level of under-
standing behooves the researcher to study each reaction with all avail-
able probes: heat, ultraviolet light, ionizing radiation, and pressure.
The structural basis of the photodimerization of cinnamic acids with
ultraviolet light [1] is well known. Attempts to study the dimerization
process thermally are made difficult by the sublimation that occurs
This work was supported in part by the Petroleum Research Fund under Grant
40263-AC7. Ms. Mary Montesalvo of the Radiation Laboratory of the University of Mas-
sachusetts Lowell facilitated access to the 60Co source. Chantana Sae-Lim thanks the
government of Thailand for a fellowship. The authors thank Professor B.M. Foxman
(Brandeis University) for useful discussions in the course of this work. He also facilitated
access to the Gammacell at Brandeis University when D.J.S. was employed at GTE
Laboratories Incorporated.
Address correspondence to Daniel J. Sandman, Center for Advanced Materials,
Department of Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts
01854-5046, USA. E-mail: Daniel_Sandman@uml.edu
55

56
J. M. Njus et al.
on heating. Cinnamic acid has typically been regarded as relatively
unreactive to ionizing radiation [2,3]. A relative lack of reactivity is
not surprising because aromatic compounds are much more resistant
to radiation than alkanes and alkenes [4]. Yet, ionizing radiation pro-
duces higher excited states as well as ionization. The higher excited
states might rapidly decay to the lowest singlet state by Kasha’s Rule
and that state should lead to dimerization. Such was the situation when
it was reported [5] that electron beam irradiation (1 MeV, 0.5 mA) of
(E)-cinnamic acid with a 500 Mrad dose gave truxillic acid in 58% yield.
In the interest of learning more about the reactivity of (E)-cinnamic
acids with ionizing radiation, we have exposed (E)-cinnamic acid and
several of its chloro and bromo derivatives to 60Co gamma radiation.
This line of inquiry uses a heavy atom strategy. In the interaction of
c-radiation with matter, the photoelectric effect and Compton scatter-
ing are major processes. Both of these processes are more probable for
materials with high atomic numbers [6]. Hence, it would be expected
that molecular structures containing heavy atoms that are involved
in the electronic structure would be more reactive to c-radiation than
similar molecular structures lacking a heavy atom, other things (e.g.,
crystal structure in the solid state) being equal. Earlier, the role of
heavy metal atoms in the reaction of metal propynoates with 60Co
c-radiation [7,8] was discussed.
EXPERIMENTAL SECTION
Cinnamic acid and its derivatives were used as received from Aldrich
Chemical Company. Their crystalline phase was established by X-ray
powder diffraction in earlier studies. Truxillic and truxinic acids were
prepared by uv irradiation [9] as previously described. Proton NMR
spectra were recorded on Bruker 500 or 250 MHz spectrometers. The
compounds were irradiated in screw cap vials in the presence of air
at ambient temperature. The dose rate was 0.76 Mrad=hr. The irra-
diated solids were dissolved in an appropriate solvent and their
NMR spectra were recorded. The product yield was obtained from
the relative areas of the olefinic signals of the cinnamic acid and the
aliphatic protons of the dimer. Obtaining the yields of the irradiation
1
experiments by integration of the H NMR spectra is clearly a more
accurate method than solvent extraction [3] of the irradiated solid.
RESULTS AND DISCUSSION
The products of dimerization of the cinnamic acids are illustrated in
Figure 1. The compounds that were exposed to c-radiation are listed

Cinnamic Acids and Gamma Radiation
57
FIGURE 1 Cinnamic acid, its dimerization to truxillic and truxinic acids and
the reverse processes.
in Table 1 along with the radiation dose and the product yield. The
radiation dose was a minimum of 50 Mrad as this is the dose used to
polymerize diacetylene monomers to completion when possible. Inte-
gration of the proton resonances of the vinylic protons of the reactant
and the aliphatic protons of the dimer was used to obtain the yields.
With respect to Table 1, the largest yield of dimer was obtained with
TABLE 1 Cinnamic Acids and their Dimers, the Dose of c-Radiation they
Received, and the Product Yield
Starting compound
Dose of 60Co, Mrad
Yield of product
E-Cinnamic acid
50
50
50
0.8–5.6%
8.1%
7.1%
0.0%
20%
4-Chlorocinnamic acid
4-Bromocinnamic acid
2,4-Dichlorocinnamic acid
2,4-Dichlorocinnamic acid
4-Bromocinnamic acid
Truxillic acid
50
116
116
50
50
50
24%
0.03%
0.4%
1.3%
4,40-Dichloro-b-truxinic acid
4,40-Dibromo-b-truxinic acid

58
J. M. Njus et al.
FIGURE 2 Expanded region of the proton NMR spectrum (DMSO-d6 solution)
of p-bromocinnamic acid irradiated with 116 Mrad 60Co gamma radiation. The
two multiplets located in the 3.5–4.5 ppm range are the methine protons of
the truxinic acid. The doublets near 6.6 and 7.6 ppm are alkene protons of
p-bromocinnamic acid.
4-bromocinnamic acid with a dose of 116 Mrad, and the yields of
dimers were typically higher with cinnamic acids with halogen sub-
stituents than without. The 1H NMR spectrum of the products of
c-radiation of 4-bromocinnamic acid is displayed in Figure 2.
Since the yield of truxillic acid decreased [5] at electron beam doses
greater than 500 Mrad, we chose to inquire if the dimer was reactive to
radiation. As shown in Table 1, the largest amount of cinnamic acid
was obtained with the Br-substituted truxinic acid, 1.3% yield. This
invited the question as to whether the dimers were stable under UV
irradiation. Under conditions (254-nm light) [9] where 4-bromocin-
namic acid was converted to dimer in 90% yield, the Br-substituted
truxinic acid converted to monomer in 1.5% yield.
While (E)-cinnamic acid has the a-crystal structure and the haloge-
nated cinnamic acids have the b-structure [1], it appears that the
major reason for the results in Table 1 is the heavy atom effect. While
it is also apparent that both electron beams [5] and 60Co c-radiation
can convert cinnamic acid to its dimer, the doses involved are very
large. Ultraviolet irradiation and sunlight remain the methods of
choice to carry out this dimerization for synthetic purposes.

Cinnamic Acids and Gamma Radiation
59
It was suggested [5] that the interaction of electron beams with cin-
namic acid to give truxillic acid proceeds via the same lowest excited
state as the photochemical dimerization. While this suggestion is
plausible, as discussed above, it may well be oversimplified. The poten-
tial of ionizing radiation to produce excited states that are electric
dipole forbidden is at the heart of this suggestion. In addition, it is
appropriate to note that the excited state leading to dimerization
may be more complex than the lowest singlet of monomeric (E)-
cinnamic acid. The fluorescence of solid cinnamic acids [9,10] is clearly
not molecular and may be excimeric.
While the presence of halogens in the cinnamic acid molecular
structure enhances reactivity toward c-radiation, it is not the only
example of this type. The molecular structures given in Figure 3 pro-
vide several additional examples. While N-propargylcarbazole is rela-
tively unreactive to c-radiation, N-(3-bromo-2-propynyl)carbazole [11]
reacts. The term ‘‘relatively unreactive’’ means that, after exposure to
radiation, the solid retained its original color and its melting point is
not changed. Ultimately, everything is reactive to ionizing radiation.
Again, 10-undecyanamide [12] is relatively unreactive to c-radiation,
11-bromo-10-undecynamide [13] reacts. Vacuum sublimed crystals of
FIGURE 3 Molecular structures and names of solid compounds that were
exposed to 60Co c-radiation.

60
J. M. Njus et al.
the semicarbazone of propiolaldehyde and propiolamide [14] are rela-
tively unreactive, but crystals of these compounds obtained from
chlorinated solvents show reactivity that was traced to the presence
of solvent in crystalline defects. The presence of short intermolecular
contacts between alkyne carbons in the crystal structures of 10-
undecyanamide [12] and the semicarbazone of propiolaldehyde [14]
is not sufficient to render these compounds reactive to c-radiation.
They are also melt-stable. Certain aromatic acetylenes, e.g., 1,4-
diethynylbenzene [15], are reactive to c-radiation, but p-ethynylben-
zoic acid, with a linear chain crystal structure [16], is unreactive to
both c-radiation and UV light.
While the above examples show the effectiveness of halogen atoms
in imparting solid state reactivity to otherwise unreactive molecular
structures, the presence of a heavy atom in a molecular structure is
not sufficient to impart significant reactivity. The mercury compounds
[17] in Figure 3 are relatively unreactive to c-radiation.
While (E)-cinnamic acid is relatively unreactive to radiation, its
molecular structure contains an a,b unsaturated aromatic acid that
undergoes a 2 þ 2 cycloaddition. a,b Unsaturated acids such as sorbic
and muconic give some dimer and also oligomer on exposure [18] to
c-radiation. Acenaphthene is an a,b unsaturated aromatic compound
that undergoes a 2 þ 2 cycloaddition [19] under the influence of
c-radiation.
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