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Fullerenes, Nanotubes and Carbon
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Determination of the Integrated Molar
Absorptivity and Molar Extinction
Coefficient of Hydrogenated Fullerenes
Franco Cataldo
Hernandez
c d
a b
, Susana Iglesias-Groth
& Arturo Manchado
c d
, D. Anibal Garcia-
c d e
a
Istituto Nazionale di Astrofisica – Osservatorio Astrofisico di
Catania, Catania, Italy
b
Actinium Chemical Research, Rome, Italy
c
Instituto de Astrofisica de Canarias, La Laguna, Tenerife, Spain
d
Departamento de Astrofısica, Universidad de La Laguna (ULL), La
Laguna, Spain
e
Consejo Superior de Investigaciones Científicas, Madrid, Spain
Version of record first published: 25 Oct 2012.
To cite this article: Franco Cataldo, Susana Iglesias-Groth, D. Anibal Garcia-Hernandez & Arturo
Manchado (2013): Determination of the Integrated Molar Absorptivity and Molar Extinction Coefficient
of Hydrogenated Fullerenes, Fullerenes, Nanotubes and Carbon Nanostructures, 21:5, 417-428
To link to this article: http://dx.doi.org/10.1080/1536383X.2011.629756
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Fullerenes, Nanotubes, and Carbon Nanostructures, 21: 417–428, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1536-383X print / 1536-4046 online
DOI: 10.1080/1536383X.2011.629756
Determination of the Integrated Molar
Absorptivity and Molar Extinction Coefficient
of Hydrogenated Fullerenes
FRANCO CATALDO1,2 , SUSANA IGLESIAS-GROTH3,4 ,
D. ANIBAL GARCIA-HERNANDEZ3,4
AND ARTURO MANCHADO3,4,5
Downloaded by [Franco Cataldo] at 09:07 25 October 2012
1
Istituto Nazionale di Astrofisica – Osservatorio Astrofisico di Catania,
Catania, Italy
2
Actinium Chemical Research, Rome, Italy
3
Instituto de Astrofisica de Canarias, La Laguna, Tenerife, Spain
4
Departamento de Astrofısica, Universidad de La Laguna (ULL),
La Laguna, Spain
5
Consejo Superior de Investigaciones Científicas, Madrid, Spain
The integrated molar absorptivity () and the molar extinction coefficient (ε) of each
infrared transition of the hydrogenated fullerenes (known as fulleranes) C60 H36 , C70 H38
and a mixture of fulleranes generally referred as 77% of C60 Hx and 22% C70 Hy with
x ≈ y > 30 were determined. These data are useful for the search, identification and
quantitative determination of fulleranes in space after the recent discovery that their
parent molecules C60 and C70 are present in certain planetary nebulae and in the interstellar medium. It is shown that the C-H stretching band of fulleranes C60 H36 , C70 H38
and their mixture at about 2905 and 2820 cm−1 are the most useful for the identification
of these molecules because their and ε values are unique in terms of strength overcoming by far the typical and ε values of reference molecules like adamantane and
docosane as well as typical ε literature data for aliphatic molecules.
Keywords Fulleranes, C60 H36 and C70 H38 , infrared spectroscopy, integrated molar
absorptivity, molar extinction coefficient
1. Introduction
Immediately after the discovery of fullerenes (1), their possible existence around late-type
carbon-rich stars was postulated (2,3). Particularly attractive was the thesis of the fullerene
formation around the R Coronae Borealis (RCB) class of stars (4) characterized by a very
low hydrogen content and high helium and carbon enrichment, conditions that are analogous to those employed in the laboratory synthesis of fullerenes (5). The first faint hint
about the possible existence of fullerenes in the interstellar medium was suggested from the
identification of C60 + transitions with some diffuse interstellar bands (6), but the definitive
certitude about the fact that fullerenes are naturally occurring was reached only last year
when the infrared transitions of C60 and C70 were detected as neutral molecules around
the young planetary nebula Tc1 (7). The detection of fullerenes was then reported in a
Address correspondence to Franco Cataldo, Osservatorio Astrofisico di Catania, Via S. Sofia 78,
95132 Catania, Italy. E-mail: franco.cataldo@fastwebnet.it
417
Downloaded by [Franco Cataldo] at 09:07 25 October 2012
418
F. Cataldo et al.
series of H-containing planetary nebulae challenging our knowledge of fullerene formation
mechanism (8). The C60 infrared signature was also reported in two RCB stars having a
certain degree of hydrogen depletion but not enough to justify the presence of fullerenes
unless it is admitted that in this case fullerenes were formed by the shock decomposition
of hydrogenated amorphous carbon (9). Furthermore, C60 was detected in several planetary
nebulae of the Magellanic clouds (10), in the protoplanetary nebula IRAS 01005 + 7910
(11) and in reflection nebulae (12).
In order to permit both the qualitative and quantitative determination of C60 and C70
fullerenes, we have measured the molar extinction coefficient (ε) and the molar absorptivity
() of the main infrared transitions of these molecules. Also, we have studied the dependence of these parameters and the band position as function of temperature (13,14). These
results were of key importance for an accurate determination of fullerene abundances in
space (10).
Since fullerenes are easily reactive with hydrogen, yielding products having variable degrees of hydrogenation and collectively known as fulleranes (15), it is highly
probable that the next astrochemistry discovery will regard these hydrogenated derivatives of fullerenes. Fullerenes are easily hydrogenated when exposed to hydrogen and the
hydrogenation is reversible since the resulting fulleranes release hydrogen under thermal
or photochemical treatment, yielding back the pristine fullerene and amorphous carbon
(16,17). This consideration prompted us to measure the molar extinction coefficient (ε) and
the molar absorptivity () of a series of fulleranes at room temperature.
2. Experimental
2.1. Materials and equipment
Fullerenes C60 and C70 were high purity grades (99.95% and 99.0+%, respectively)
from MTR Ltd. (Cleveland, OH, USA). They were converted into C60 H36 and C70 H38 ,
respectively, according to the hydrogenation procedure already detailed elsewhere (15–17).
A mixture of fulleranes produced by dry hydrogenation under pressure of both C60 and
C70 were obtained from ABCR GmbH (Karlsruhe, Germany) and fully characterized as
reported previously (18). The fullerane mixture consisted of 77% of C60 Hx and 22% C70 Hy
with x ≈ y > 30.
As reference materials for the measurement of the ε and values of the C-H stretching
bands, use was made of a straight chain paraffin n-docosane C22 H46 (obtained from Merck,
Darmstadt, Germany) and of a cage compound adamantane C10 H16 obtained from SigmaAldrich (St. Louis, MO, USA). Potassium bromide (infrared spectroscopy grade) was
obtained from Sigma-Aldrich (USA9). The infrared spectra were recorded on a ThermoFischer FT-IR spectrometer model Nicolet IR-300 at a resolution of 1 cm−1 . The variable
temperature cell from Specac model P/N 21525 equipped with KBr windows was used to
record the spectra under vacuum at room temperature. The cell was evacuated with a Buchi
vacuum pump model V-710 equipped with four diaphragm heads and a three-stage vacuum
creation process which delivers 3.1 m3 /h and an absolute vacuum of 2 mbar.
2.2. Sample preparation and FT-IR spectrum registration
C60 H36 and C70 H38 were used in KBr pellet preparation as soon as they were synthesized
and isolated after solvent evaporation under vacuum. As general procedure, 2–4 mg of
sample under study was accurately weighted and then quickly mixed with 300 mg of
Molar Absorptivity of Hydrogenated Fullerenes
419
KBr (accurately weighted) in an agate mortar. The grounded mixture was transferred in
a KBr pellet die designed for 13 mm diameter pellet. The KBr pellets were produced in a
Silfradent press equipped with a pressure meter; the KBr pellets were pressed at a pressure
between 4.0 and 5.0 tonns/cm2 . The thickness of the KBr pellets immediately after the
extraction from the die was measured with a Somet digital micrometer having a sensitivity
of 0.01 mm. Typically, thickness around 1.0 mm was achieved. A background was collected
with the FT-IR spectrometer on the empty and evacuated Specac cell. Afterwards the cell
was opened and the KBr pellet mounted on the sample holder inside the cell, which was
then evacuated for the recording of the sample spectra normally at a temperature between
30◦ C and 55◦ C.
3. Results and Discussion
Downloaded by [Franco Cataldo] at 09:07 25 October 2012
3.1. General aspects on the FT-IR spectra of fulleranes
The hydrogenated fullerenes and their importance in the possible explanation for certain
unidentified infrared astronomical emission were first explored by Webster (19). In particular, a mixed population of fulleranes with different degrees of hydrogenation can explain
the so-called 21 micron feature observed in a few proto planetary nebulae (20). The extreme
fragility of the carrier of the 21 um feature observed in proto planetary nebulae (i.e., it is
rapidly destroyed by UV photons (21)) is quite consistent with the easy dehydrogenation
of fulleranes under the action of UV photons or thermal processing, giving back fullerenes
and amorphous carbon (16). Webster also extended his reasoning to the possible contribution of fulleranes to the diffuse interstellar bands (22) and the interstellar light extinction
curve explanation (23).
Recently we discovered that the UV spectra of the fulleranes match the peak and the
shape of the “bump” at 220 nm observed in the interstellar light extinction curve (16).
This implies that fulleranes, which are indeed a form of hydrogenated carbon, may be key
contributors to the interstellar light extinction feature at 220 nm. Electric dipole emission
by fulleranes may also contribute to the anomalous microwave emission detected in several
molecular clouds and star forming regions (24,25).
The infrared spectra of fulleranes are dominated by very strong absorption bands due
to the C-H stretching between 2905 and 2820 cm−1 as shown in Figure 1. In comparison to
the C-H stretching transitions, all the other transitions in the infrared spectrum of fulleranes
appear quite negligible. Therefore, it can be anticipated that the bands at about 2905 and
2820 cm−1 , being the strongest bands of fulleranes, will be the right bands for searching
these molecules in the adapt circumstellar and interstellar environments. The detail of the
C-H stretching band of fulleranes is shown in Figure 2 where the band of the fulleranes mixture (77% of C60 Hx and 22% C70 Hy with x ≈ y > 30) is compared with the similar band of
pure C60 H36 and C70 H38 . It is evident that the band profile is similar although not identical
for all three samples of fullerane. Furthermore, in this spectral region it is possible to distinguish quite easily the fullerane C-H stretching at 2905 and 2820 cm−1 from the aromatic
C-H stretching of PAHs occurring at shorter wavelengths at about 3030 cm−1 (26,27).
The infrared spectra of the hydrogenated fullerenes and their perdeuterated derivatives
were already discussed previously (15–18) and will not be rediscussed here. Instead the
focus will be on the molar extinction coefficient (ε) and the integrated molar absorptivity
() of these molecules.
Particularly important was the discovery that the position of C-H stretching band of
fulleranes in the infrared spectrum is not particularly sensitive to temperature in the range
420
F. Cataldo et al.
0.60
0.55
2830
2903
0.50
0.45
2850
0.35
0.30
0.25
486
816
665
1007
1272
1219
1179
865
0.10
1623
0.15
1484
1448
0.20
1701
Downloaded by [Franco Cataldo] at 09:07 25 October 2012
Absorbance
0.40
0.05
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm−1)
Figure 1. FT-IR spectrum in KBr of fulleranes mixture 77% of C60 Hx and 22% C70 Hy with x ≈ y >
30. Note the strong C-H stretching bands at 2903, 2850 and 2830 cm−1 .
from 93 K to 523 K (18,28) with shifts of the order to 3 cm−1 maximum. Furthermore, the
other fulleranes bands comprised those of C60 H18 (not discussed in this work) are not particularly sensitive to temperature changes (28). Deuterated fulleranes are slightly more stable
to photolysis and thermal decomposition than the hydrogenated isomer (15–17), and this
may cause a deuterium enrichment of the surface of hydrogenated/deuterated fullerenes in
space due to this isotope effect.
3.2. About the molar extinction coefficient of fulleranes
If I0 is the intensity (radiant power) of the monochromatic radiation entering a sample and I
the intensity transmitted by the sample, then the logarithm of the ratio I0 /I is the absorbance
A (26):
A = Log[I0 /I] = εbc
(1)
According to the Lambert and Beer law, the absorbance is directly proportional to the thickness b (or optical path length) and the concentration c of the absorbing molecules in the
infrared beam. As shown in equation (1), the proportionality constant linking absorbance
with the optical path length and with concentration is ε, known as molar absorptivity
or more commonly known in the literature as molar extinction coefficient whose typical
physical dimensions are L•cm−1 •mol−1 (26).
Molar Absorptivity of Hydrogenated Fullerenes
Abs
2826.2
0.6
2848.4
2909.4
65-37 C60Hx+C70Hy
421
0.4
0.2
2798.4
0.2
0.0
78-37 C70H38
0.6
2821.4
2900.7
0.8
Abs
Downloaded by [Franco Cataldo] at 09:07 25 October 2012
2825.1
Abs
0.4
2842.0
2903.8
0.0
77-37 C60H36
0.6
0.4
0.2
3100
3000
2900
2800
Wavenumbers (cm−1)
2700
2600
Figure 2. Comparison of the C-H stretching region of the FT-IR spectra of fulleranes mixture 77%
of C60 Hx and 22% C70 Hy with x ≈ y > 30 (top), fullerane C60 H36 (middle) and fullerane C70 H38
(bottom) (color figure available online).
The determination of the molar extinction coefficient at a given wavenumber ελ was
made through the equation using the absorbance Aλ read at a given wavenumber:
ελ = Aλ (bc)−1
(2)
The value of (bc)−1 was then determined for both C60 and C70 using the experimental
parameters reported in the experimental section and the KBr density of 2.753 g/cm3 .
From the infrared spectrum of C60 , the absorbance values Aλ have been determined
using the EZ Omnic (version 6A) software of the infrared spectrometer. In Table 1 we
summarize the results of such measurements and report the molar extinction coefficient
values ελ calculated according to equation (2).
From the data in Table 1 it is immediately evident that the ελ of the ν C-H of all three
fulleranes samples considered in this work are extremely high exceeding by far the ελ of the
ν C-H of the reference straight chain paraffin docosane and the cage compound adamantane.
The ε2905 is 182.2 L•cm−1 •mol−1 for C60 H36 and 148.8 L•cm−1 •mol−1 for C70 H38 and
167.2 L•cm−1 •mol−1 for the fullerane mixture C60 Hx + C70 Hy ; this is in contrast with
the value of ε2905 = 35.0 L•cm−1 •mol−1 for the small cage compound adamantine, which
contains hydrogenated tertiary carbon atoms as fulleranes and the paraffin docosane, is
characterized by hydrogenated primary and secondary carbon atoms and whose ε2960 =
23.0 L•cm−1 •mol−1 and ε2917 = 84.4 L•cm−1 •mol−1 . The last two ελ values are completely
422
F. Cataldo et al.
Table 1
Molar extinction coefficients of fulleranes, adamantane and docosane
(ε values in L cm−1 mol−1 )
cm−1
Downloaded by [Franco Cataldo] at 09:07 25 October 2012
wavenumber
2960
2917
2905
2844
2820
1690
1633
1472
1457
1445
1369
1350
1234
1220
1125
1098
1076
1068
1015
1026
967
950
890
795
717
440
μm
ελ
wavelength C60 H36
3.38
3.43
3.44
3.52
3.55
5.92
6.12
6.79
6.86
6.92
7.30
7.41
8.10
8.20
8.89
9.11
9.29
9.36
9.85
9.75
10.3
10.5
11.2
12.6
13.9
22.7
ελ
C70 H38
ελ
ελ
ελ
C60 Hx +C70 Hy Adamantane Docosane
23.0
84.4
182.2
148.4
167.2
134.1
138.0
154.4
23.7
35.0
16.2
67.7
25.7
78.1
11.5
23.9
26.7
20.2
8.2
18.0
31.6
22.2
5.1
25.8
10.9
9.1
3.4
16.1
11.6
5.5
1.4
7.3
9.8
57.7
1.8
in line with literature data since Nakanishi (27) quotes an ε2960 ≈ 70 L•cm−1 •mol−1 for
methyl groups and ε2917 ≈ 75 L•cm−1 •mol−1 for methylene groups.
The other fullerane band due to C-H stretching occurs at 2820 cm−1 (see Table 1)
and again is characterized by a considerably high ε2820 value of 134–138 L•cm−1 •mol−1
for both C60 H36 and C70 H38 and 154.4 L•cm−1 •mol−1 for the fullerane mixture.
These high ε2820 values of fulleranes are again in contrast with the “normal” ε2844 =
16.2 and 67.7 L•cm−1 •mol−1 , respectively, for adamantane and docosane. Nakanishi
(27) quotes ε2850 ≈ 30–45 L•cm−1 •mol−1 while for aromatic hydrocarbons the ε3030 ≤
60 L•cm−1 •mol−1 (27). From these data emerges the peculiarity of the fulleranes bands at
2905 and 2844 cm−1 not only for their position and their insensitiveness to a broad range of
temperatures but also for their unique intensity. A closer compound to fulleranes is dodecahedrane (C20 H20 ) an hydrogenated hydrocarbon cage compound which exhibits three C-H
stretching bands at 2900, 2901 and 2950 cm−1 whose molar extinction coefficient is still
unknown (29).
Molar Absorptivity of Hydrogenated Fullerenes
423
3.3. The integrated band intensities of fulleranes at ambient temperature
Downloaded by [Franco Cataldo] at 09:07 25 October 2012
The peak height measurements, which correspond to the absorbance A as shown by equation (1), are sensitive to changes in the resolution of the spectrometers used and can vary
from instrument to instrument (26). The integrated band intensity corresponds to the measurement of the total intensity of the band intended as the measurement of the area below
a given absorption band. Such measurement is much less sensitive to instrumental resolution than the peak height measurement. The EZ Omnic software (version 6A) of our FTIR
spectrometer is able to measure either the height of a given absorption band or the total
area below the same band, in both cases subtracting automatically from the baseline. Thus,
the integrated intensity has been determined in the infrared spectra of fulleranes at ambient
temperature.
The integrated intensity can be expressed as the integrated absorptivity defined as
follows (26):
= ∫ ελ dλ
(3)
where λ is the wavenumber and ελ is the absorptivity measured with a spectrometer with
unlimited resolution, integrated over the whole band. In practice, by substituting equation
(2) into equation (3) we get (26):
= (bc)−1 ∫ Aλ dλ
(4)
If the optical path length b is measured in cm and the concentration is measured in mol/cm3 ,
and the spectrum is plotted using the wavenumber scale in cm−1 versus the absorbance Aλ ,
then the integrated absorbance is expressed in cm•mol−1 or 10−5 Km•mol−1 (26).
In these units, is related to the square of the dipole moment change with respect to
the normal coordinates (∂μ/∂Q)2 by the relationship (26):
= [πN/3C2 (Ln10)] (∂μ/∂Q)2 = 304.75 (∂μ/∂Q)2 cm · mol−1
(5)
with N the Avogadro’s number, C the velocity of light in cm/sec and (∂μ/∂Q) in (esu
cm)/(cm g1/2 ).
With these premises, the integrated molar absorptivity of the infrared absorption bands
of fulleranes were determined at ambient temperature (shown in Table 2 with the relative
intensity). The uncertainty of the data reported in Table 2 and pertaining the values of
C60 are in the range of ±10%.
In the measurement of the integrated molar absorptivity of the C-H stretching band
of fulleranes, we proceeded in two different ways. In the first way, the two stretching
bands, respectively, at 2905 and 2820 cm−1 were integrated separately; in the second
case, the entire group of bands in the C-H stretching region was integrated together.
Therefore, in Table 2 the columns with the symbol refer to the first case of separate
integration of the stretching bands while the symbol ν C-H refers instead to the integration over the entire set of bands in the C-H stretching region. The 2905 = 55.4 km/mol
for C60 H36 , 45.8 km/mol for C70 H38 and 42.7 km/mol for the mixture of fulleranes.
As expected, the 2905 = 19.0 km/mol for adamantane, thus considerably lower than
the analogous band of fulleranes. Similarly, also the 2960 = 4.2 km/mol and 2917 =
23.7 km/mol for docosane. Also the infrared band at lower frequency, that is, at 2820 cm−1 ,
is intense for fulleranes 2820 = 109.0 km/mol for C60 H36 , 81.5 km/mol for C70 H38 and
424
μm
λ
3.38
3.43
3.44
3.52
3.55
5.92
6.12
6.72
6.79
6.86
6.92
7.30
7.41
7.87
8.10
8.20
8.49
8.89
9.11
9.29
cm−1
ν
2960
2917
2905
2844
2820
1690
1633
1489
1472
1457
1445
1369
1350
1270
1234
1220
1178
1125
1098
1076
1.2
26.6
4.9
2.6
25.5
5.3
109.0
55.4
C60 H36
309.4
←
39.3
14.5
81.5
45.8
C70 H38
257.4
←
2.0
1.2
0.8
18.2
93.9
26.2
42.7
C60 Hx +C70 Hy
232.2
←
1.6
1.9
4.1
19.0
3.2
Adamantane
41, 4
←
Table 2
Molar absorptivity of fulleranes, adamantane and docosane ( values in km mol−1 )
ν C-H
ν C-H
ν C-H
ν C-H
Downloaded by [Franco Cataldo] at 09:07 25 October 2012
0.5
1.6
18.7
13.2
4.2
23.7
Docosane
←
ν C-H
143.1
425
1068
1015
1026
967
950
890
795
727
717
692
467
440
9.36
9.85
9.75
10.3
10.5
11.2
12.6
13.8
13.9
14.5
21.4
22.7
6.4
3.3
16.0
3.7
2.1
9.3
1.0
0.8
1.4
n.d.
Downloaded by [Franco Cataldo] at 09:07 25 October 2012
7.9
0.9
0.3
426
F. Cataldo et al.
93.9 km/mol for the fullerane mixture. The analogous band of adamantane shows a 2844 =
3.2 km/mol, extremely low, and for docosane we have measured 2844 = 13.2 km/mol.
Thus, as expected, the high intensity of the C-H stretching bands of fulleranes is reflected
also in the molar absorptivity values.
The integration of the entire C-H stretching region yields for C60 H36 a ν C-H =
309.4 km/mol and lower values but with the same order of magnitude found for C70 H38
(257.4 km/mol) and the fullerane mixture (232.2 km/mol). For comparison,
the integration
of the C-H stretching region in the adamantane spectrum yields a ν C-H = 41.4 km/mol,
one order of magnitude lower.
Downloaded by [Franco Cataldo] at 09:07 25 October 2012
4. Conclusions
Fullerenes were found in different environments in space, and it is probable that further
research will show how ubiquitous these molecules are in space. Fullerenes are incredibly stable molecules that can survive for a very long time in harsh circumstellar and
interstellar environments, resistant to the action of high energy photons irradiation and to
cosmic rays (30). Since fullerenes are easily reactive with hydrogen and are transformed
into the hydrogenated derivatives known as fulleranes (15), it is reasonable to anticipate
that fulleranes will be the next fullerene derivatives to be found in space (19–21). For
these reasons, we have already synthesized a series of different fulleranes hydrogenated
and perdeuterated and studied their infrared transitions (15–18,28). In the present work,
we have shown that the infrared spectrum of all fulleranes, namely C60 H36 , C70 H38 and a
mixture of fulleranes generally referred as 77% of C60 Hx and 22% C70 Hy with x ≈ y > 30,
display strong transition bands in the C-H stretching region of the spectrum. First, these
transitions are located at 2905 and 2820 cm−1 , far away from the aromatic C-H stretching
expected at about 3030 cm−1 so they are easily distinguished. Furthermore, the intensity
of the C-H transitions of fulleranes were found to be unique both in terms of molar extinction coefficient ελ and molar absorptivity in comparison to those of reference molecules
such as adamantane and docosane and literature data. To the best of our knowledge, there
are no other molecules with such high values of ελ and as those measured in fulleranes
in the present work. The knowledge of these parameters will permit us to determine the
abundance of fulleranes in space.
Acknowledgments
This research work has been supported by grant AYA2007-64748 Expte. NG-014-10 of the
Spanish Ministerio de Ciencia e Innovacion.
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