Recent Advances in IR and UV/VIS Spectroscopic Characterization of the C76 and C84 Isomers of D2 Symmetry

Jovanović, Tamara; Koruga, Đuro; Jovančićević, Branimir

doi:https://doi.org/10.1155/2014/701312

Journal of Nanomaterials

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Research Article | Open Access

Volume 2014 | Article ID 701312 | https://doi.org/10.1155/2014/701312

Recent Advances in IR and UV/VIS Spectroscopic Characterization of the C₇₆ and C₈₄ Isomers of D₂ Symmetry

Tamara Jovanović,¹Đuro Koruga,¹and Branimir Jovančićević²

Academic Editor: William W. Yu

Received23 Jul 2014

Revised13 Nov 2014

Accepted13 Nov 2014

Published15 Dec 2014

Abstract

The stable isomers of the higher fullerenes C₇₆ and C₈₄ with D₂ symmetry as well as the basic fullerenes C₆₀ and C₇₀ were isolated from carbon soot and characterized by the new and advanced methods, techniques, and processes. The validity of several semiempirical, ab initio, and DFT theoretical calculations in predicting the general pattern of IR absorption and the vibrational frequencies, as well as the molecular electronic structure of the C₇₆ and C₈₄ isomers of D₂ symmetry, is confirmed, based on recent experimental results. An excellent correlation was found between the previously reported theoretical data and the recently obtained experimental results for these molecules over the relevant spectral range for the identification of fullerenes. These results indicate that there are no errors in the calculations in the significant spectral regions, the assumptions that were based on previous comparisons with partial experimental results. Isolated fullerenes are important for their applications in electronic and optical devices, solar cells, optical limiting, sensors, polymers, nanophotonic materials, diagnostic and therapeutic agents, health and environment protection, and so forth.

1. Introduction

There is only one stable isomer of the higher fullerene C₇₆, as well as of the basic fullerenes, characterized by D₂, icosahedral (Ih) and symmetry, isolated pentagons, and an electronic closed shall structure [1]. From the 24 possible isomers of C₈₄ obeying the isolated pentagon rule [2, 3], the two most stable, most abundant, and almost isoenergetic structures are those with D₂ and symmetry [4–12], found in a 2 : 1 ratio [13–15].

Some of the numerous possible vibrational modes of the higher fullerenes C₇₆ [16, 17], C₈₄ [5, 6], and its D₂(IV) [18–20] and (II) isomers [19, 21] were detailed in previous studies.

In this research, a series of unique, new, and dominant IR absorption maxima of the isolated C₇₆ and the most abundant, stable C₈₄ isomer of D₂ symmetry is registered and confirmed in the spectral region relevant for the identification of fullerenes, from ca. 450 to 1650 cm⁻¹. The general pattern of the obtained spectra and all the observed absorption bands of the chromatographically isolated samples of the stable C₇₆ and C₈₄ isomers with D₂ symmetry from the several different original, advanced separation processes [22–28] are in excellent agreement with the semiempirical, ab initio, and density functional theory (DFT) theoretical calculations for these molecules [5–12, 29, 30] over the mentioned relevant region. It should be noted that some low frequencies can exist below the observational limit of 400 cm⁻¹.

These results have not been previously reported. Some discrepancies between the previous experimental data [16–19] and the aforementioned theoretical calculations [5–12, 29, 30] appeared in the significant spectral regions.

Quantum chemical force field for electrons theoretical (QCFF/PI) calculations yielded a set of the vibrational frequencies for the stable C₇₆ isomer of D₂ symmetry [29]. The IR vibrational properties of C₇₆ were also studied using the high-level ab initio B3LYP DFT with the TZVP basis set [30].

The IR vibrational spectra of the two most abundant and most stable, major C₈₄ isomers, the D₂:22 and :23 isomers, were previously theoretically studied using the semiempirical PM3, AM1, MNDO [5–8], QCFF/PI [9], and tight-binding (TB) potential calculations [10]. Vibrational properties of the D₂, , and C₂ isomers of C₈₄ were determined by ab initio Hartree Fock (HF) calculations with the STO-3G, 3-21G, and D95V basis sets [11]. The IR vibrational properties of the two major C₈₄ isomers were also studied using the B3LYP DFT with the basis sets as large as 6-31G [12].

These calculations were already shown to be successful in predicting the overall absorption pattern and the vibrational frequencies, as well as the molecular structures of the basic fullerenes C₆₀ and C₇₀ [31–35]. Excellent agreement with the experimental data was observed [23, 24, 36–38].

IR spectra of the chromatographically isolated C₇₆-D₂ and C₈₄-D₂:22 isomers were recorded over the entire relevant region from 400 to 2000 cm⁻¹, by the KBr disk technique. Characterization of the obtained C₇₆ and C₈₄ fractions from the previous separation processes was performed using different IR techniques, in different spectral regions [5, 6, 16–21].

In the previous articles, the UV/VIS absorption of the higher fullerenes C₇₆ and C₈₄ and its D₂(IV) and (II) isomers was recorded in different spectral regions. Their solutions in different solvents, of different concentrations, were used [14, 39–47].

In this study, the UV/VIS absorption of the chromatographically purified C₇₆ and C₈₄ isomers of D₂ symmetry is registered over the entire relevant region from 200 to 900 nm. A series of their unique, new, and dominant UV absorption maxima are registered and confirmed in the most significant spectral region from 200 to 400 nm, where fullerenes intensively absorb. Solutions of these fullerenes in hexane of determined concentrations were used. It should be mentioned that the region from 200 to 300 nm has not been previously presented for C₈₄ and its isomers under any experimental conditions. Complete appearance of their electronic absorption spectra and all the observed absorption bands [22–28] correlates well with the previous semiempirical QCFF/PI, TB, and DFT theoretical predictions of the molecular electronic structure and the optical absorption of these molecules that behave as electron deficient arenes [48–51]. Their overall absorption also correlates well with the experimentally obtained photoemission spectra (PES) of C₇₆ and C₈₄ [49–51].

It is important to mention that fullerenes C₆₀ and C₇₀ were recently found in space around various astrophysical objects [52], such as certain planetary [53] and protoplanetary nebulae [54] and in other space environments ranging from postasymptotic giant branch stars [55], to young stellar objects [56], to reflection nebulae [57], and to certain R-Coronae Borealis stars [58]. It is expected that also higher fullerenes, such as C₇₆ and C₈₄ and their stable isomers with D₂ symmetry, can be found in space.

The obtained original spectra of the isolated stable C₇₆ and C₈₄ isomers, measured at room temperature in this study, as well as their comparison with the recent spectra of C₇₆ and C₈₄ (mixture of isomers) at temperatures between −180°C and +250°C [52], are very significant for better understanding of IR and UV/VIS optical absorption properties of these higher fullerenes and for their identification either in natural resources in space and on Earth or in artificially synthesized carbon soots.

2. Experimental Methods

In the first phase of this research, C₆₀, C₇₀, and the higher fullerenes, mainly C₇₆ and C₈₄, were Soxhlet extracted with a series of different and previously unapplied solvents or combinations of solvents from the samples of carbon soot, produced by electric arc (MER Corporation, Tucson, AZ, USA). Solvents used were -heptane, toluene, chlorobenzene, -xylene, xylenes, and pyridine, as well as the successive use of toluene and chlorobenzene, and -xylene and pyridine. The extractions were performed until the complete disappearance of color in the Soxhlet extraction thimble. The yields and the compositions of all the extracts were determined by the spectroscopic and chromatographic methods. The procedures for increases of fullerenes yields, as well as for additional selective extraction of higher order fullerenes, were found [22–28, 36–38].

In the second phase, C₆₀, C₇₀, and the higher fullerenes C₇₆ and C₈₄ from the obtained soot extracts were chromatographically separated on the activated Al₂O₃ columns by the new and improved methods [22–28]. The elution was performed continuously with the several different original, defined gradients of solvents: from pure hexane or 5% toluene in hexane to pure toluene, at ambient conditions. The main advancement, in comparison to previous methods under pressure [13–19, 39–45], is the isolation of the purified stable isomers of the higher fullerenes C₇₆ and C₈₄ (the only stable C₇₆-D₂ isomer and the most abundant, stable C₈₄ isomer of D₂ symmetry, the C₈₄-D₂:22 isomer), successively after the basic fullerenes, in one phase of each of the processes, under atmospheric pressure and smaller flow of 1.5 mL/min, in increased milligrams yields. The other advantages of the developed methods are the use of significantly smaller amounts of the initial materials, including fullerene extracts (10 mg), finely granulated Al₂O₃ (50 g), activated for 2 h at 105°C, and eluent (1.5 to 1.75 L) per chromatographic separation, as well as less expensive laboratory equipment. The entire material and energy expense and the time spent on the purification processes were decreased. The environment pollution was also decreased, using smaller amounts of less toxic solvents [22–28].

Before separation, each sample of the extract (ca. 10 mg) was dissolved in hexane and toluene (few mL), dispersed onto silica (1 g), which adsorbed the solvent producing gelatinous mass, and finally put onto top of the new alumina column.

Purification of higher fullerenes under pressure, on a preparative scale, either by flash chromatography or by HPLC, generally required larger amounts of the initial materials and repeated chromatographies, and the fullerenes were obtained in smaller yields [14, 39–43].

Characterization of the chromatographically purified fullerene fractions, as well as of the obtained fullerenes soot extracts, was performed using determined techniques of IR and UV/VIS spectroscopy that have not been presented previously for the higher fullerenes [22–28, 36–38].

In this paper, the IR spectra of the chromatographically isolated samples of the C₇₆ and C₈₄ isomers of D₂ symmetry were measured by a Nicolet FT-IR 6700 spectrometer Thermo Scientific, by the KBr disk technique, at room temperature, 23°C, over the entire relevant region from 400 to 2000 cm⁻¹.

The UV/VIS spectra of the chromatographically purified samples of the C₇₆ and C₈₄ isomers of D₂ symmetry were recorded on GBC Cintra 40 spectrophotometer, in the region from 200 to 900 nm. Diluted solutions of fullerenes in hexane, concentrations to mol/dm⁻³, were used.

The UV/VIS spectra of the chromatographically purified samples of the C₇₆ and C₈₄ isomers of D₂ symmetry were also recorded on a Perkin-Elmer Lambda 5 spectrophotometer, from 200 to 900 nm, using both diluted solutions of fullerenes in hexane, concentrations to mol/dm⁻³, and much diluted solutions of fullerenes in hexane to complete discoloring, for comparison.

In the previous articles, the IR and UV/VIS absorption of the obtained samples of the higher fullerenes C₇₆, C₈₄, and its D₂(IV) and (II) isomers were recorded in different spectral regions, using different techniques [5, 6, 14, 16–21, 39–47].

3. Results and Discussion

The main advancement in spectroscopic characterization of the higher fullerenes C₇₆ and C₈₄ in this research [22–28], in comparison to previously obtained experimental results [5, 6, 16–21], is the observation of the unique, new, and the main, dominant absorption maxima of the isolated stable C₇₆ and C₈₄ isomers of D₂ symmetry in the spectral regions where they intensively absorb, in excellent agreement with the semiempirical, ab initio, and DFT theoretical calculations for these molecules [5–12, 29, 30].

The achieved agreement between our experimental results [22–27] and the aforementioned theoretical predictions [29, 30] is better in comparison to previous characterizations of C₇₆ samples from other separation processes, by other IR techniques [16, 17].

Whereas there is a good correlation between our experimental results [22–27] and the theoretical predictions [29, 30], in the previous experimentally obtained IR spectra of the C₇₆ samples [16, 17], some discrepancies of the general pattern and vibrational frequencies with the theoretical predictions [29, 30] appear in the central significant part of the spectrum, from ca. 800 to 1200 cm⁻¹.

In the first, partial IR spectrum of C₇₆ [16], some disagreements of absorption bands with the theoretical calculations [29, 30] occur in the mentioned region, as well as from ca. 1500 to 1600 cm⁻¹, of up to 40–60 cm⁻¹ [16, 29].

The next IR measurement [17] was not in agreement with the mentioned first published spectrum of C₇₆, suggesting that the previous measurement was carried out with an impure sample [16, 17]. In this study, the absorption bands were not registered from ca. 800 to 1000 cm⁻¹. In the remaining part of the spectrum [17], a larger number of C₇₆ features were observed and also some discrepancies with the theoretical calculations [29, 30] in the region around 1020 to 1030 cm⁻¹, as large as 26 cm⁻¹ [17, 29]. The observed IR features of the higher fullerene C₇₆ from this separation process were tentatively rather assigned to a subset of fundamental vibrations, although there was presumption that some of these features were weaker overtone or combination bands [17], which our results also confirm.

There is a general agreement between the IR absorption bands of C₇₆ observed previously and those observed in this work. In several cases, smaller shifts are observed. However, new, characteristic, and the main, dominant absorption bands not reported in previous works [16, 17] were registered in the central significant part of the region relevant for the identification of fullerenes, from ca. 800 to 1200 cm⁻¹, as well as in the region from ca. 1530 to 1740 cm⁻¹.

In Table 1 are reported the IR absorption bands of the chromatographically purified C₇₆ samples as measured in this work at 23°C in comparison with the recent data at different temperatures, as well as with the theoretical calculations by the QCFF/PI method [29].

The original experimentally obtained IR spectrum of the chromatographically isolated C₇₆-D₂ sample is presented in Figure 1 (Table 1, first IR).

In this paper, the main, most intense, sharp, well resolved absorption maxima of the higher fullerene C₇₆ were observed at 967, 1082, and 1187 cm⁻¹. Weak neighboring features at 1025, 1057, 1122, and 1162 cm⁻¹ correspond to C₇₆. In the recent work [52], a group of four close C₇₆ intense absorption bands were found at 1175, 1102, and 1086 and the most intense at 1030 cm⁻¹, with the neighboring feature at 961 cm⁻¹, appearing in the form of one strong, broad absorption band. Some of these bands were reported with weak intensity in previous works [16, 17], such as features at 1031 and 1175 cm⁻¹ [17].

Characteristic, sharp bands unique to the higher fullerenes C₇₆ were observed in the first relevant part of our spectrum at 892 and 823 cm⁻¹, with the neighboring absorption at 789 cm⁻¹. Pronounced C₇₆ absorption bands also appear at 705 cm⁻¹, with the shoulders at 729 and 743 cm⁻¹, at 603 cm⁻¹ next to absorption at 646 cm⁻¹, and at 533 cm⁻¹. The three most intense absorption bands in the first relevant part of the recent spectrum [52] were observed at 709 and 654 and the main band at 526 cm⁻¹. Their neighboring features appear at 743, 608, and 550 cm⁻¹. Several other neighboring bands are listed in Table 1. Corresponding bands from the previous report were found with some shifts at 693, 628, and 538 cm⁻¹ [17].

In the second relevant part of our spectrum, the most intense, strong absorption band appears at 1461 cm⁻¹ and the next pronounced intense band at 1386 cm⁻¹, with the neighboring feature at 1398 cm⁻¹. Intense band was found at 1438 cm⁻¹ in the recent spectrum, followed by the features at 1393 and 1383 cm⁻¹ [52]. It appears at 1438 cm⁻¹, with the neighboring bands at 1461 and 1466 cm⁻¹ in the previous report [17]. Sharp C₇₆ feature observed at 1312 cm⁻¹ in our spectrum, with the next bands at 1276 and 1248 cm⁻¹, was found at 1315 cm⁻¹ recently at 45°C. The neighboring band was observed at 1288 cm⁻¹ at −178°C [52]. Band at 1493 cm⁻¹ was found at 1498 cm⁻¹ in the recent spectrum [52] and at 1513 cm⁻¹ in the previous work [17]. Pronounced bands were also observed in this work at 1735 and 1631 cm⁻¹, with the shoulders at 1580 and 1681 cm⁻¹. The most intense maximum in the recent spectrum was registered at 1688 cm⁻¹, with the next intense band at 1721 cm⁻¹ and the neighboring band at 1598 cm⁻¹, in addition to the band at 1580 cm⁻¹ [52] which was detected only with Raman spectroscopy in the previous work [17].

Concerning the dependence of the C₇₆ infrared bands with temperature, some smaller band shifts were observed in our spectrum at 23°C in comparison to recent spectra at various temperatures [52]. There is observed a remarkable change of the intensity of certain characteristic infrared bands, depending on temperature.

No important and evident band shifts as a function of temperatures were observed in the mentioned recent spectra as a function of temperature. Their main characteristics and the overall absorptions are similar. However, the intensity of certain infrared bands of C₇₆ is remarkably sensitive to temperature. For example, the band at 1317 cm⁻¹ is sharp at −178°C [52], and still in our spectrum at room temperature, registered at 1312 cm⁻¹, and becomes less intense at +45°C and weak at +250°C. Another example is the group of bands comprised between 1288 and 1032 cm⁻¹ which are well defined and separated at −178°C [52] and also in our spectrum seen at 1276 and 1248 cm⁻¹, but appear much less intense at +250°C. The IR spectrum of C₇₈ that was also presented in the recent work has different properties [52].

It is important to mention that the general pattern of our C₇₆-D₂ spectrum and all of the experimentally observed IR absorption bands over the spectral region relevant for the identification of fullerenes are in excellent agreement with the theoretical calculations for the only stable C₇₆ isomer of D₂ symmetry, by the semiempirical QCFF/PI method [29], as well as by the most recent ab initio DFT [30].

The overall configuration of absorption of our experimentally obtained FT-IR(KBr) spectra of the chromatographically isolated samples of the neutral solid C₇₆ [22–28] correlates well with the next obtained, most recent infrared multiphoton electron detachment (IR-MPED) spectrum of the unsolvated gas-phase dianion , as well as with the adequate most recent B3LYP/TZVP DFT calculations [30].

The obtained generally good correlation between our recent results for the neutral C₇₆ [22–28] and the most recent IR spectrum of [30] provides the significant experimental evidence that the dianionic molecule retains its overall symmetry (i.e., D₂ point group) with ¹A₁ ground state with respect to the neutral cage. This statement was previously based on comparison of the experimental IR spectrum with the DFT calculations [30]. Spectral shifts of characteristic tangential modes, as well as some changes of their intensity [30], observed relative to the neutral C₇₆ cage [22–28, 30], were shown to originate from the excess charge density on the fullerene cage that leads to some specific changes of bond lengths [30].

The presented results in this study indicate that the previous semiempirical QCFF/PI [29], as well as the recent ab initio DFT theoretical calculations [30], provides an overall excellent prediction of the IR spectrum and vibrational frequencies of the higher fullerene C₇₆. A one to one assignment is achieved over the entire relevant spectral region for fullerenes. Only in a few cases is the accuracy not enough to permit a one to one assignment, as when two IR bands are separated by a small frequency interval. Their assignment can be supported by considering the calculated frequencies by DFT [30] in addition to the QCFF/PI frequencies [29] and conversely.

On the basis of the previous theoretical calculations for the higher fullerene C₈₄, two most abundant stable D₂:22 and :23 isomers [5–12], a series of characteristic absorption bands is predicted to occur around 780 cm⁻¹ (from ca. 700 to 840 cm⁻¹), followed by the bands around 630 and 475 cm⁻¹. Some minor bands should also appear between ca. 585 and 520 cm⁻¹, in the first relevant part of the spectrum (ca. 450 to 850 cm⁻¹). Pronounced and dominant, most intense absorption bands are predicted to occur in the second relevant part (ca. 1050 to 1650 cm⁻¹), around 1600 cm⁻¹ and a group between ca. 1125 and 1390 cm⁻¹, including the main band. A cluster of minor bands should appear at higher wave numbers than the main band.

The two calculated D₂-C₈₄ and -C₈₄ spectra resemble each other, the difference being that the D₂ isomer shows more IR active lines due to splitting of the lines from the higher symmetry isomer. However, these splitting are much too small and they could not be resolved in the first published IR spectrum of C₈₄, between 500 and 2000 cm⁻¹ [5], or in the previous infrared resonance enhanced multiphoton ionization (IR-REMPI) spectrum of C₈₄, presented between 450 and 1600 cm⁻¹ [6]. This spectrum consisted of only three absorption bands at 748, 632, and 475 cm⁻¹ in the first part, followed by a series of partially unresolved peaks, ranging from ca. 1050 to 1600 cm⁻¹.

Our experimentally obtained IR spectra of the chromatographically purified samples of the most abundant, stable C₈₄ isomer of D₂ symmetry, C₈₄-D₂:22 [23–28], have much better resolution of absorption bands, in comparison to the first published IR absorption spectrum of C₈₄ [5] and to the previous IR-REMPI spectrum [6], as well as to the IR spectra of the C₈₄- samples, presented in the regions from 484 to 1631 cm⁻¹ [19], as well as from 50 to 500 cm⁻¹ and 500 to 800 cm⁻¹ [21]. They agree and look more similar to the theoretically calculated spectrum of the D₂:22 isomer [5–12], which shows more IR active lines.

The achieved agreement between our experimental results [23–28] and the aforementioned theoretical predictions [5–12] is better in comparison to previous characterizations of C₈₄ samples (partially separated isomers), from other separation processes, by other IR techniques [18, 19].

Whereas there is a good correlation between our experimental results [23–28] and the theoretical calculations [5–12], in the previous experimental IR spectrum of the main chromatographically purified fraction of the higher fullerene C₈₄ (partially separated isomers, presented from 400 to 1650 cm⁻¹) [18], as well as in the next IR spectrum of the obtained C₈₄-D₂ fraction from another separation process (presented from 484 to 1631 cm⁻¹) [19], some discrepancies of the general pattern and vibrational frequencies with the theoretical predictions for the most abundant, stable C₈₄ isomer of D₂ symmetry (the C₈₄-D₂:22 isomer) [5–12] appeared in the significant spectral regions from ca. 450 to 850 cm⁻¹, as well as from ca. 1050 cm⁻¹ to 1650 cm⁻¹. The main disagreement [5–12] is the appearance of the three strong, dominant absorption bands in the first relevant part of the spectrum at 792 and 794 and the most intense 648 cm⁻¹ [18, 19], unlike our results.

There is a general agreement between most of the vibration modes reported in the previous spectrum of the C₈₄-D₂ fraction [19], as well as in another study [18] and those observed in this work. However, significant changes of relative intensities of certain absorption bands were observed. Characteristic and the main, dominant absorption bands were registered in the second relevant part of the spectrum.

In Table 2, the IR absorption bands of the chromatographically purified C₈₄-D₂:22 samples are reported as measured in this work at 23°C in comparison with the recent data for the C₈₄ sample (mixture of isomers) at different temperatures [52], as well as with the theoretical calculations by the QCFF/PI method [9].

The original experimentally obtained IR spectrum of the chromatographically isolated C₈₄-D₂:22 sample in this research is presented in Figure 2 (Table 2, first IR).

There is a considerable change of intensity of IR bands in our spectrum of C₈₄-D₂:22, in comparison to IR spectra of the C₈₄ sample (mixture of isomers) at different temperatures. The main characteristics of the reported spectra [52] are similar. However, the intensity of certain infrared bands of C₈₄ is changing significantly with temperature.

The most intense, dominant absorption bands in our spectrum appear in the second relevant part. The first group is present between absorption band at 1122 cm⁻¹, with the neighboring features at 1107 and 1095 cm⁻¹, and the most intense maximum in the spectrum at 1385 cm⁻¹, with a neighboring absorption at 1398 cm⁻¹. Pronounced bands also appear around 1600 cm⁻¹, between the next intense maxima at 1456 cm⁻¹, with a neighbor very weak absorption at 1433 cm⁻¹, and at 1731 cm⁻¹. A cluster of minor bands appears from 1494 to 1558 cm⁻¹ at the wave numbers higher than the main band.

The group of bands with the main maximum at 1433 cm⁻¹ in the second relevant part of the recent spectra of C₈₄ is more intense than the following band at 1384 cm⁻¹ at −170°C. However, at +50°C and +250°C, the two groups of bands, although less defined, have approximately the same intensity. Similarly, in the mentioned previous spectrum of C₈₄ (partially separated isomers) [18], as well as in the IR spectrum of the obtained C₈₄-D₂ fraction at room temperature [19], the two main, most intense bands in the second part of approximately the same intensity appear at 1432 and 1383 cm⁻¹. Relatively broad feature at 1730 cm⁻¹ was recorded in the IR spectrum of C₈₄ at −170°C and +250°C, but not so evident at 50°C [52].

In the first relevant part of our spectrum, a series of characteristic sharp absorption maxima are observed around 780 cm⁻¹, at 699, 711, 746, 779, 826, and 843 cm⁻¹, with the neighbor very weak absorption at 884 cm⁻¹. The absorption bands around 635 cm⁻¹ (from 574 to 657 cm⁻¹) and 473 cm⁻¹ (from 402 to 540 cm⁻¹) follow them. Weak feature at 647 cm⁻¹ is hardly observable. A group of minor bands is registered from 516 to 574 cm⁻¹.

The main bands in the first part of the recent spectra of C₈₄ at different temperatures [52] appear at 470 and 647 and the most intense at 797 cm⁻¹. The two distinct bands at 670 and 648 cm⁻¹ observed at −170°C appear much less intense at +50°C and +250°C with the band at 670 cm⁻¹ reduced to a weak feature. The most intense bands in the previous spectrum of C₈₄ (partially separated isomers [18]), as well as in the IR spectrum of the obtained C₈₄-D₂ fraction, appear at 648, 792, and 797 cm⁻¹ [19].

There are no pronounced absorption bands in our spectrum of C₈₄-D₂:22 between ca. 850 and 1050 cm⁻¹. Several weak features were observed. In the case of C₈₄ sample, the FT-IR spectra [52] show a dominant, broad band centered at 1077 cm⁻¹ and 1031 cm⁻¹, which were already reported in the literature [5–12, 18, 19], but not as intense as detected recently [52].

The general pattern of our spectrum, its fine structure with more splitter lines, and all of the experimentally observed IR absorption bands, in the entire spectral region relevant for the identification of fullerenes, are in excellent agreement with the aforementioned semiempirical [5–10], as well as with the ab initio HF [11], and DFT theoretical calculations [12] for the most abundant, stable C₈₄ isomer of D₂ symmetry.

The presented results in this study indicate that the aforementioned semiempirical [5–10], ab initio, and DFT calculations [11, 12] provide an overall excellent prediction of the IR spectrum and vibrational frequencies of the most abundant stable isomer of the higher fullerenes C₈₄ with D₂ symmetry. A one to one assignment is achieved over the entire relevant spectral region for fullerenes. Only in a few cases is the accuracy not enough to permit a one to one assignment, as when two IR bands are separated by a small frequency interval. Their assignment can be supported by considering the frequencies obtained by the ab initio and DFT calculations [11, 12] in addition to the frequencies obtained by the semiempirical PM3, AM1, MNDO, and QCFF/PI, as well as by the TB potential calculations [5–10], and conversely.

These results remove the need for the assumptions of possible errors of the theoretical calculations for the C₇₆ and C₈₄ isomers with D₂ symmetry [5–12, 29, 30] in the significant spectral regions, based on the previous comparisons with partial experimental results [16–19]. They provide the evidence of their validity over the entire relevant region.

In this study also, the unique UV/VIS absorption maxima of the chromatographically purified C₇₆ and C₈₄ isomers of D₂ symmetry are registered over the entire region from 200 to 900 nm, including the most significant region from 200 to 400 nm where fullerenes have allowed transitions and intensively absorb.

The experimentally obtained UV/VIS spectrum of the chromatographically isolated C₇₆-D₂ sample is presented in Figure 3. Dominant UV absorption maxima are present at 256 and 329 nm. Their relative intensities are decreased in comparison to the spectra of the previous chromatographically purified C₆₀ and C₇₀ fractions [23, 24, 36–38]. The third dominant, most intense band appears as a shoulder at ca. 210 nm. The absorption is prolonged to the region below 200 nm, which is characteristic for C₇₆. Pronounced C₇₆ shoulder is registered at 275 nm, as well as its shoulders at ca. 230, 285, 350, and 378 nm. In the visible part, a weaker C₇₆ band appears at 405 nm. Absorption is prolonged to 900 nm.

In the UV/VIS spectrum of the purified C₇₆ sample from much diluted hexane solution to complete discoloring, we also recorded for comparison a series of more splitter absorption maxima which are registered at 229 and 285 nm, with the neighbors at ca. 256 and 275 nm, as well as at 328, 350, 378, and 405 nm. This spectrum has shown some differences and some similarities compared to the mentioned spectrum of C₇₆ measured from more concentrated solution in hexane. With the change of solution concentration, such as significant dilution, the appearance of several new close absorption maxima or the fine structure may occur.

The experimentally obtained UV/VIS absorption spectrum of the chromatographically isolated sample of the most abundant, stable isomer of the higher fullerene C₈₄ is presented in Figure 4. The most intense maximum is present at 239 nm, with a shoulder at 230 nm. The next intense absorption maximum appears at 272, with the neighboring bands at 251, 261, and 287 nm. They are followed by the bands at 333, 305, 318, 357, and 381 nm.

Complete configuration of absorption and all the observed absorption bands in the spectra of the purified C₇₆-D₂ and C₈₄-D₂:22 samples is in good correlation with the semiempirical QCFF/PI, TB, and DFT theoretical predictions for these molecules [48–51], which behave as electron-deficient arenes. It also correlates well with the previously obtained PES of C₇₆ and C₈₄ [49–51].

In the previous work of Jinno et al. [39, 40], the UV/VIS spectra of the chromatographically purified C₇₆ and C₈₄ fractions were recorded from the mixture of acetonitrile and toluene (55 : 45 or 50 : 50, v/v) in the region from 300 to 600 nm. Kikuchi et al. [41, 42] presented the optical absorption of the purified higher fullerenes C₇₆ and C₈₄ dissolved in benzene, in the range of 500 to 1100 nm. Ettl et al. [16] and Diederich and Whetten [43] reported the UV/VIS spectra of the molecule C₇₆, as well as that of other higher fullerenes from dichloromethane solutions, in different regions. In the previous paper by Diederich et al. [14], the UV/VIS spectrum of C₇₆ recorded from very diluted solution in hexane was presented in the region from 200 to 800 nm. Locations of C₈₄ absorption bands, measured from 280 to 912 nm, from dichloromethane solution were only mentioned in this paper. Dennis et al. reported the UV/VIS/NIR absorption spectra of the isolated fractions of the D₂(IV) and (II) isomers, as well as of other six minor isomers of the higher fullerene C₈₄, from 400 to 2000 nm [44, 45]. Xenogiannopoulou et al. presented the absorption spectra of C₈₄ and its D₂(IV) and (II) isomers dissolved in toluene in the range of ca. 300–350 nm to 1100 nm [46, 47].

All the presented results indicate the achieved progress in the spectroscopic characterization and chromatographic separation of the C₇₆ and C₈₄ isomers of D₂ symmetry, as well as of the basic fullerenes, due to the application of the new and advanced experimental methods and processes.

Identification of fullerenes in the chromatographically purified fractions, as well as in the obtained extracts, was performed using determined IR and UV/VIS techniques that have not been presented for the higher fullerenes before.

The results of UV/VIS analysis are in agreement with the results of IR analysis. Characteristic properties, the unique and new absorption bands, and changes of relative intensities and locations of absorption maxima are observed, showing isolation and separation of the basic and the higher fullerenes in the similar, regular way within the several different original, advanced separation processes [22–28].

The obtained original IR and electronic absorption spectra of the isolated C₇₆-D₂ and C₈₄-D₂:22 isomers, in the spectral regions relevant for the identification of fullerenes, where they intensively absorb, are in excellent agreement with the several theoretical predictions for these molecules, which has not been previously presented.

For the first time, the validity of semiempirical, ab initio, and DFT calculations in predicting the general pattern of IR absorption and the vibrational frequencies, as well as the molecular electronic structure of the stable C₇₆ and C₈₄ isomers of D₂ symmetry, is confirmed over the entire relevant spectral range, based on comparison with our recent experimental results.

4. Conclusion

The obtained excellent correlation between the experimentally observed general pattern of IR absorption and vibrational frequencies of the isolated stable isomers of the higher fullerenes C₇₆ and C₈₄ isomers with D₂ symmetry [22–28] and the theoretical predictions [5–12, 29, 30] for these molecules is presented in this paper. These results provide the first significant experimental evidence of validity of the aforementioned semiempirical, ab initio, and DFT calculations for the C₇₆-D₂ [29, 30] and the C₈₄-D₂:22 [5–12] isomers over the entire relevant spectral region, from ca. 450 to 1650 cm⁻¹.

The experimentally obtained electronic absorption of the isolated C₇₆ and C₈₄ isomers of D₂ symmetry over the relevant region from 200 to 900 nm, including the most significant region from 200 to 400 nm [22–28], is also in very good correlation with the previous semiempirical QCFF/PI, TB, and DFT theoretical predictions for these molecules [48–51].

It is important to mention also that the obtained generally good correlation between the overall configuration of absorption in our recent experimental IR spectra of the neutral solid C₇₆ [22–28] and the next obtained, most recent IR-MPED spectrum of the unsolvated gas phase [30], as well as with the adequate most recent B3LYP/TZVP DFT calculations [30], provides the significant experimental evidence that the dianionic molecule retains its symmetry (i.e., D₂ point group) with ¹A₁ ground state with respect to the neutral cage.

These results are of great importance for further possible and even more sophisticated calculations of vibrational properties and molecular electronic structure of fullerenes and other molecules.

The presented original spectra in this study, as well as their comparison with the recent spectra of C₇₆ and C₈₄ (mixture of isomers) at different temperatures [52], will significantly contribute to better understanding of the IR and UV/VIS optical absorption properties of the higher fullerenes C₇₆ and C₈₄, and their stable isomers with D₂ symmetry, as well as of fullerenes generally. They will enable easier identification of C₇₆, C₈₄, and its most abundant isomer, as well as of C₆₀ and C₇₀, either in artificially synthesized carbon soot or in natural resources in space and on Earth.

Isolated fullerenes and their derivatives are important for the applications in electronic and optical devices, superconductors, semiconductors, solar cells, optical limiting, sensors, polymers, nanophotonic materials, lenses with optical absorption properties closer to human eye light sensitivity, diagnostic and therapeutic agents, encapsulation of metal atoms and radio isotopes, targeted drug and gene delivery, free radicals scavengers, health and environment protection, DNA cleavage, antibacterial and antiviral agents, water purification, storage of hydrogen, high energetic batteries, lubricants, synthesis of diamond, catalysts, and so forth.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors are grateful to the Ministry of Education, Science, and Technological Development of the Republic of Serbia and to the University of Belgrade for financial support of this research (Project III 45009).

Supplementary Materials

The IR spectra of the three chromatographically purified samples of the C₇₆-D₂ isomer, as well as of the chromatographically purified samples of the C₈₄-D₂:22 isomer obtained in this research from the three different original advanced separation processes are presented in the Supplementary Material.

Figures 1, 2, 3 and 4 are included and presented in the article. Figures 1-2, 1-3, 2- 2 and 2-3 are additional, supplementary material for the explanation of absorption bands in Tables 1 and 2. Figure 1 presented in the article is equivalent with Figure 1-1 and Figure 2 presented in the article is equivalent with Figure 2-1 in Supplementary Material.

Figure 1-1. The IR spectrum of the first chromatographically purified C₇₆-D₂ sample.

Figure 1-2. The IR spectrum of the second chromatographically purified C₇₆-D₂ sample.

Figure 1-3. The IR spectrum of the third chromatographically purified C₇₆-D₂ sample.

Figure 2-1. The IR spectrum of the first chromatographically purified C₈₄-D₂:22 sample.

Figure 2-2. The IR spectrum of the second chromatographically purified C₈₄-D₂:22 sample.

Figure 2-3. The IR spectrum of the third chromatographically purified C₈₄-D₂:22 sample.

Supplementary Material

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