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Raman spectroscopy as tool for the characterization of thio-polyaromatic hydrocarbons in organic minerals
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Spectrochimica Acta Part A 68 (2007) 1065–1069
Raman spectroscopy as tool for the characterization of thio-polyaromatic hydrocarbons in organic minerals Otakar Frank a,b,∗ , Jan Jehliˇcka a , Howell G.M. Edwards c a
Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University in Prague, Albertov 6, 128 43 Prague 2, Czech Republic b J. Heyrovsk´ y Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejˇskova 3, 182 23 Prague 8, Czech Republic c Chemical and Forensic Sciences, University of Bradford, Bradford BD7 1DP, UK Received 30 October 2006; received in revised form 15 December 2006; accepted 17 December 2006
Abstract Benzothiophene and dibenzothiophene have been studied by Raman microspectroscopy using a 785 nm excitation wavelength. The spectra obtained have been compared with the previously measured spectra of idrialite, a complex natural mineral composed entirely of cata-condensed polyaromatic hydrocarbons (PAHs), usually containing a thiophenic or aliphatic five-membered ring. For comparison, the Raman spectra of 2,3benzofluorene crystals have been obtained for the first time. Some of the bands in the idrialite spectra are attributed to specific vibrational modes of thiophene or fluorene-type PAHs, especially in the region bellow 1000 cm−1 . These modes at 495, 705 and 750 cm−1 along with C–H or C–H2 stretching modes around 3000 cm−1 can be then used to distinguish such groups of PAHs in complicated organic mineral mixtures like idrialite. © 2007 Elsevier B.V. All rights reserved. Keywords: Raman spectroscopy; Idrialite; Polyaromatic hydrocarbon; Fullerene; Thiophene
1. Introduction Thiophene containing polyaromatic hydrocarbons (thioPAHs) have already been identified as constituent species in the products of alteration of organic remnants in geological materials. They can be found in different concentrations in crude oils, marine or non-marine sediments or soils [1,2]. In these environments, they occur in variously substituted forms and in complicated mixtures and their characterization using solid-state spectroscopic techniques in such blends would be very complicated so chromatography and mass spectroscopy have been the main analytical techniques adopted for their characterization. However, polyaromatic hydrocarbons may also appear rarely in accumulated crystalline forms, e.g. idrialite (approximately C22 H14 ), curtisite (similar to idrialite), pendletonite (equivalent to karpatiite, coronene, C24 H12 ), kratochvilite (fluorene, C13 H10 ) and ravatite (phenanthrene, C14 H10 ). The occurrence of these organic minerals is often related to the high temperature alteration of organic precursors. Investigation of these minerals ∗ Corresponding author at: J. Heyrovsk´ y Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejˇskova 3, 182 23 Prague 8, Czech Republic. Tel.: +420 266 053 804; fax: +420 286 582 307. E-mail address: [email protected] (O. Frank).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2006.12.033
can therefore extend our knowledge about such processes and also provide new data for spectral database construction related to life-signature exploration both terrestrially and extraterrestrially. However, for this purpose, a non-destructive analytical method is highly desirable to study minerals in the frame of the complex rock samples and in association with their inorganic matrices without the need of previous isolation and extraction. Hitherto, only ravatite [3] and idrialite [4] have been studied by means of Raman spectroscopy; whilst the assignment of the Raman spectra of ravatite (pure phenanthrene) is pretty straightforward [3], idrialite (and also curtisite) represents more complex mixtures of chain-type PAHs with molecular weights ranging from 216 to 372 amu [5]. Typical combinations of benzonaphthothiophenes (C16 H10 S, 234 amu), benzophenanthrothiophenes or dinaphthothiophenes (C20 H12 S, 284 amu) or naphthophenanthrothiophenes (C24 H14 S, 334 amu) are found in these crystalline organic minerals (Fig. 1A). Simoneit et al. [6] have also reported the presence of higher-mass thio-PAHs similar to those occurring in idrialite in hydrothermal petroleum and tars from the Guyamas Basin, Escanaba Trough and Middle Valley hydrothermal systems wherein the temperature estimated for these systems ranges from 300 to 350 ◦ C [6]. Fullerene precursors provide another geochemically interesting molecular system to be investigated for the presence of
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Fig. 1. (A) Six principal molecules identified in idrialite, their masses and approximate concentrations (summing up for 83% in idrialite). Modified after [5]. (B) Structures of PAHs, which have been compared and investigated in this study.
with calculated spectra for anthracene, pyrene and perylene [12], and for benzo(a)pyrene and benzo(e)pyrene [13] have also been reported. Fourier transform surface enhanced Raman (FTSER) spectra of 18 PAHs, including dibenzothiophene, have been reported [14]. However, in spite of the very low detection limits (of the order of 10−8 to 10−9 mol) claimed for the PAHs investigated, this approach is not deemed applicable to the study of complex rock samples because of the need for the previous isolation and extraction of the material from the inorganic matrices. A detailed vibrational study of benzothiophene [15] and dibenzothiophene [16] have also been reported. It is clear from the above overview that: (1) the particular PAHs present in idrialite have not hitherto been studied by Raman spectroscopy. (2) The most studied PAH of the same structural series as those in idrialite, i.e. with angular annelation (cata-condenzation) of benzene rings, was chrysene [9]. (3) The thus-far studied PAHs of equal or at least similar molecular weights as those present in idrialite belong to the linear polyacenes up to pentacene in [9] or to peri-condensed PAHs such as perylene, benz(a)- and benz(e)pyrene [9,12–14]. Structures of these molecules are shown in Fig. 1B. Therefore, the goal in this research project was the analysis of all existing data on vibrational properties of PAHs which in combination with our own measurements will produce a compilation of reliable spectral data for the determination of PAHs in geological materials. In the first step we have chosen thio-PAHs, namely benzothiophene (BT) and dibenzothiophene (DBT) for detailed studies, and also report preliminarily work on 2,3-benzofluorene (BF), which is in fact the parent molecular structure for the PAHs in idrialite [5]. We have obtained new and comparable spectra of these molecules that can be used as discriminative fingerprint and has allowed us to make further molecular band assignements in the Raman spectra of idrialite and related organic minerals. 2. Materials and methods
thiophene moieties. Sarobe et al. [7] reported the sulfurmediated cyclotrimerization of 4,5-dihydrobenz[l]acephenanthrylene to trinaphthodecacyclene (C60 H30 ), which would be expected to give fullerene (C60 ) upon dehydrogenation. ThioPAH (C40 H20 S) isomers are typical byproducts of this reaction. The same type of reaction yields, for example, decacyclene from acenaphthene [8] with diacenaphthothiophene as a byproduct. Thus, the identification of such molecules in the geological record may indicate process routes that can lead to fullerene generation in the host rock. Generally, there are relatively few detailed studies reported of the vibrational spectra of higher-molecular mass PAHs, which is detrimental to the provision of complete and accurate band assignments for the Raman spectra of minerals like idrialite. For unsubstituted molecules larger than naphthalene, Maddams and Royaud [9] presented FT-Raman spectra of 10 PAHs (anthracene, tetracene, pentacene, rubrene, phenanthrene, chrysene, triphenylene, pyrene, perylene and fluoranthene). These studies focused more on the simple analytical spectroscopic characterizational aspects of the PAHs as a whole. Detailed vibrational assignments have been reported for crystalline phenanthrene [10], fluorene [11], comparison of observed
Micro-Raman analyses were performed on a multichannel Renishaw In Via Reflex spectrometer coupled with a Peltiercooled CCD detector. Excitation was provided by the 785 nm line of a diode laser. The samples were scanned from 40 to 3700 cm−1 wavenumber shift at a spectral resolution of 2 cm−1 . Calibration has been carried out using the Si–Si stretching mode at 520.2 cm−1 . The scanning parameter for each Raman spectrum was taken as 15 s and 40 (for BT and DBT) or 100 (for BF) scans were accumulated for each experimental run to provide better signal-to-noise ratios. Multiple spot analyses were carried out on different regions of the same sample to check for spectral reproducibility. Chemicals were used as purchased and no special purification procedures were effected, namely, benzothiophene (Merck, 99.5% purity), dibenzothiophene (Aldrich, 99.5% purity) and 2,3-benzofluorene (Aldrich, 99.5% purity). 3. Results and discussion The measured spectra of benzothiophene and dibenzothiophene crystals correspond to those reported in [15] and [14,16],
O. Frank et al. / Spectrochimica Acta Part A 68 (2007) 1065–1069
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Fig. 2. Raman spectra of benzothiophene, dibenzothiophene and idrialite crystals (from bottom to top) in the region 200–1800 cm−1 .
for BT and DBT, respectively. As an example, the Raman spectrum of dibenzothiophene crystal, taken from the (1 0 2) plane, is presented in Figs. 2 and 3. The strongest molecular bands at 215, 284, 407, 495, 701, 1023, 1132, 1232, 1309, 1317, 1556 and 1597 cm−1 of the A1 symmetry class, as assigned by [16], correspond to the previously reported data, with a deviation in wavenumber of approximately up to 4 cm−1 . The CH stretching A1 fundamentals were assigned to peaks at 2993, 3023, 3053 and
possibly 3076 cm−1 . The peaks at 2618 and 2628 cm−1 are to be assigned, but the former may be an overtone of the 1309 cm−1 A1 fundamental. The lattice vibrations at 82, 103 and 136 cm−1 correspond to data presented in [16]. Fig. 4 shows a comparison chart of all major Raman bands of 13 unsubstituted PAHs, BT and DBT with idrialite. The chart comprises members of all basic groups of relevant unsubstituted PAHs, namely polyacenes (anthracene, tetracene and pentacene), peri-condensed (pyrene, perylene), peri-condensed with a five-membered ring (fluoranthene), cata-condensed (chrysene), cata-condensed with a five-membered ring (fluorene, 2,3-benzofluorene), naphthalene. The peak intensities are differentiated by grey scale to strong, medium and weak. Where possible, solution data were taken. The strongest intensity idrialite peaks, shown by vertical lines, generally occur in the wavenumber ranges where also the strongest bands of the component PAHs appear. 3.1. Region bellow 1000 cm−1
Fig. 3. Raman spectra of benzothiophene, dibenzothiophene and idrialite crystals (from bottom to top) in the region 2700–3150 cm−1 .
A careful comparison of the measured spectra with those previously published for other PAHs [9–16] shows distinct features of thio-PAHs occur especially in the region bellow 1000 cm−1 . For unsubstituted PAHs, in general, most of the significant vibrations in the region 200–1000 cm−1 are assignable to the CCC in-plane bending modes, indicated as ␣(CCC) in the text, and in the region 600–1000 cm−1 the C–H out-of-plane bending modes are designated as ␥(HCCC). However, the latter modes are usually only of medium to weak intensity. The strongest band in this region for PAHs appears at 750 ± 10 cm−1 , and belongs to the ␣(CCC) mode. It is noteworthy that this band is the most intense in the spectra of fluorene at 745 cm−1 [11] and 2,3benzofluorene at 752 cm−1 , and occurs at 750 cm−1 in idrialite, which corresponds to the fact that benzonaphthofluorene is the
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Fig. 4. A comparison chart showing position of major peaks in various PAHs (squares) and idrialite (vertical lines). The colour intensity scheme is the same for PAHs and for idrialite. Data measured and merged with those from [9–14]. See text.
prime constituent of idrialite [5]. In thio-PAHs, this most intense band appears at 701 cm−1 for DBT and 706 cm−1 for BT. From all other previously measured Raman spectra of PAHs, only the ␣(CCC) or ␥(HCCC) modes of phenanthrene falls into this region at 713 and 710 cm−1 , respectively [10]. In the microRaman spectrum of idrialite, both the 700 and 750 cm−1 bands are present and these are among the most intense in the spectrum, clearly indicating the presence of both carbon-only (containing a five-membered, alicyclic ring) and thio-PAHs in idrialite. Furthermore, the 498 cm−1 band in idrialite, appearing as medium to medium-weak in intensity, can be mainly attributed to the in-plane deformation mode of the thiophene ring, ␣(CSC), a medium intensity band occurring at 495 cm−1 in DBT and at 492 cm−1 in BT. As can be seen from Fig. 4, only weak intensity bands of higher polyacenes and pyrene fall into this region. 3.2. Region 1000–1300 cm−1 This region is dominated by a Raman band at 1020 ± 5 cm−1 in the spectra of idrialite and also for all observed PAHs except the peri-condensed congeners such as pyrene or perylene. It originates from the ring C–H in-plane bending modes of the aromatic systems, further indicated as (HCC) in the text, combined with the C–C stretching modes. It appears at 1023 cm−1 in DBT, 1015 cm−1 in BT and 1020 cm−1 in 2,3-benzofluorene. 3.3. Region 1300–1700 cm−1 Generally, for PAHs, this region is dominated by the C–C stretching modes in aromatic system combined with the (HCC) modes. In idrialite, these are represented by medium to strong intensity bands at 1375, 1393, 1579 and 1617 cm−1 . It is quite difficult to assign these bands conclusively to a contribution of a particular group of PAHs, because all of them have at least one member with a medium to strong intensity band at the wavenumber positions of the bands in idrialite.
3.4. Region around 3000 cm−1 In the PAHs and also the thio-PAHs lacking a five-membered alicyclic ring, this region is dominated by C–H stretching mode for an aromatic systems. In DBT, the strongest band in this region appears as a broad feature centred at 3053 cm−1 as observed also in idrialite. For the fluorene-type PAHs, additional intense bands around 2900 cm−1 are ascribed to CH2 stretching vibrations of the aliphatic five-membered ring. In fluorene, the symmetric stretching vibration of the CH2 group has been assigned to the peak at 2905 cm−1 and the asymmetric stretch at 2920 cm−1 [17]. 4. Conclusions Micro-Raman spectroscopy is demonstrated to be a very promising tool for the identification of various PAHs in complicated molecular assemblages of natural organic minerals such as idrialite. We have shown that in particular the wavenumber region bellow 1000 cm−1 can be used for distinguishing of various groups of PAHs in such spectra. The C–S–C in-plane deformation on the thiophene ring is the most intense band in the wavenumber region around 500 cm−1 in thio-PAHs, clearly corresponding to the medium-weak intensity band in idrialite at this wavenumber position. Furthermore, the CCC in-plane bending mode in thio-PAHs occurring around 700 cm−1 and in fluorene-type PAHs around 750 cm−1 exactly matches the positions of the strongest bands in idrialite in this region. However, further spectral vibrational data from higher-mass PAHs are needed in order to satisfactorily elucidate bands in other complicated natural mixtures of polyaromatic hydrocarbons to facilitate a better understanding of the thermal alteration processes that occur in relevant terrestrial geological systems. Such data will also be relevant to the enlargement of the Raman spectroscopic database for the spectral recognition of organic markers in geological environments which has been given impetus recently by the inclusion of remote miniaturised Raman
O. Frank et al. / Spectrochimica Acta Part A 68 (2007) 1065–1069
instrumentation as part of the ExoMars life-detection planetary mission. Acknowledgements We record our appreciation to the Grant Agency of the Czech Republic project No. 205-06-P348 and Renishaw plc. for funding support for this project. References [1] B.P. Tissot, D.H. Welte, Petroleum Formation and Occurence, SpringerVerlag, Berlin, 1984. [2] J.S. Sinninghe Damste, J.W. De Leeuw, Org. Geochem. 16 (1990) 1077. [3] L. Nasdala, I.V. Pekov, Eur. J. Mineral. 5 (1993) 699. [4] J. Jehliˇcka, H.G.M. Edwards, S.E. Jorge Villar, O. Frank, J. Raman Spectrosc. 37 (2006) 771.
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[5] S.A. Wise, R.M. Cambell, W.R. West, M.L. Lee, K.D. Bartle, Chem. Geol. 54 (1986) 339. [6] B.R.T. Simoneit, J.C. Fetzer, Org. Geochem. 24 (1996) 1065. [7] M. Sarobe, R.H. Fokkens, T.J. Cleij, L.W. Jenneskens, N.M.M. Nibbering, W. Stas, C. Versluis, Chem. Phys. Lett. 313 (1999) 31. [8] M. Sarobe, Polycyclic Aromatic Hydrocarbons under High Temperature Conditions and Fullerene Formation Processes, Ph.D. Thesis, Utrecht University, Utrecht, 1998. [9] W.F. Maddams, I.A.M. Royaud, Spectrochim. Acta A 46 (1990) 309. [10] J. Godec, L. Colombo, J. Chem. Phys. 65 (1976) 4693. [11] A. Bree, R. Zwarich, J. Chem. Phys. 51 (1969) 912. [12] H. Shinohara, Y. Yamakita, K. Ohno, J. Mol. Struct. 442 (1998) 221. [13] H.P. Chiang, B. Mou, K.P. Li, P. Chiang, D. Wang, S.J. Lin, W.S. Tse, J. Raman Spectrosc. 32 (2001) 45. [14] S.D. Stewart, P.M. Fredericks, J. Raman Spectrosc. 26 (1995) 629. [15] T.D. Klots, W.B. Collier, Spectrochim. Acta A 51 (1995) 1273. [16] A. Bree, R. Zwarich, Spectrochim. Acta A 27 (1971) 599. [17] K. Witt, Spectrochim. Acta A 24 (1968) 1115.
Spectrochimica Acta Part A 68 (2007) 1065–1069
Raman spectroscopy as tool for the characterization of thio-polyaromatic hydrocarbons in organic minerals Otakar Frank a,b,∗ , Jan Jehliˇcka a , Howell G.M. Edwards c a
Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University in Prague, Albertov 6, 128 43 Prague 2, Czech Republic b J. Heyrovsk´ y Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejˇskova 3, 182 23 Prague 8, Czech Republic c Chemical and Forensic Sciences, University of Bradford, Bradford BD7 1DP, UK Received 30 October 2006; received in revised form 15 December 2006; accepted 17 December 2006
Abstract Benzothiophene and dibenzothiophene have been studied by Raman microspectroscopy using a 785 nm excitation wavelength. The spectra obtained have been compared with the previously measured spectra of idrialite, a complex natural mineral composed entirely of cata-condensed polyaromatic hydrocarbons (PAHs), usually containing a thiophenic or aliphatic five-membered ring. For comparison, the Raman spectra of 2,3benzofluorene crystals have been obtained for the first time. Some of the bands in the idrialite spectra are attributed to specific vibrational modes of thiophene or fluorene-type PAHs, especially in the region bellow 1000 cm−1 . These modes at 495, 705 and 750 cm−1 along with C–H or C–H2 stretching modes around 3000 cm−1 can be then used to distinguish such groups of PAHs in complicated organic mineral mixtures like idrialite. © 2007 Elsevier B.V. All rights reserved. Keywords: Raman spectroscopy; Idrialite; Polyaromatic hydrocarbon; Fullerene; Thiophene
1. Introduction Thiophene containing polyaromatic hydrocarbons (thioPAHs) have already been identified as constituent species in the products of alteration of organic remnants in geological materials. They can be found in different concentrations in crude oils, marine or non-marine sediments or soils [1,2]. In these environments, they occur in variously substituted forms and in complicated mixtures and their characterization using solid-state spectroscopic techniques in such blends would be very complicated so chromatography and mass spectroscopy have been the main analytical techniques adopted for their characterization. However, polyaromatic hydrocarbons may also appear rarely in accumulated crystalline forms, e.g. idrialite (approximately C22 H14 ), curtisite (similar to idrialite), pendletonite (equivalent to karpatiite, coronene, C24 H12 ), kratochvilite (fluorene, C13 H10 ) and ravatite (phenanthrene, C14 H10 ). The occurrence of these organic minerals is often related to the high temperature alteration of organic precursors. Investigation of these minerals ∗ Corresponding author at: J. Heyrovsk´ y Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejˇskova 3, 182 23 Prague 8, Czech Republic. Tel.: +420 266 053 804; fax: +420 286 582 307. E-mail address: [email protected] (O. Frank).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2006.12.033
can therefore extend our knowledge about such processes and also provide new data for spectral database construction related to life-signature exploration both terrestrially and extraterrestrially. However, for this purpose, a non-destructive analytical method is highly desirable to study minerals in the frame of the complex rock samples and in association with their inorganic matrices without the need of previous isolation and extraction. Hitherto, only ravatite [3] and idrialite [4] have been studied by means of Raman spectroscopy; whilst the assignment of the Raman spectra of ravatite (pure phenanthrene) is pretty straightforward [3], idrialite (and also curtisite) represents more complex mixtures of chain-type PAHs with molecular weights ranging from 216 to 372 amu [5]. Typical combinations of benzonaphthothiophenes (C16 H10 S, 234 amu), benzophenanthrothiophenes or dinaphthothiophenes (C20 H12 S, 284 amu) or naphthophenanthrothiophenes (C24 H14 S, 334 amu) are found in these crystalline organic minerals (Fig. 1A). Simoneit et al. [6] have also reported the presence of higher-mass thio-PAHs similar to those occurring in idrialite in hydrothermal petroleum and tars from the Guyamas Basin, Escanaba Trough and Middle Valley hydrothermal systems wherein the temperature estimated for these systems ranges from 300 to 350 ◦ C [6]. Fullerene precursors provide another geochemically interesting molecular system to be investigated for the presence of
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Fig. 1. (A) Six principal molecules identified in idrialite, their masses and approximate concentrations (summing up for 83% in idrialite). Modified after [5]. (B) Structures of PAHs, which have been compared and investigated in this study.
with calculated spectra for anthracene, pyrene and perylene [12], and for benzo(a)pyrene and benzo(e)pyrene [13] have also been reported. Fourier transform surface enhanced Raman (FTSER) spectra of 18 PAHs, including dibenzothiophene, have been reported [14]. However, in spite of the very low detection limits (of the order of 10−8 to 10−9 mol) claimed for the PAHs investigated, this approach is not deemed applicable to the study of complex rock samples because of the need for the previous isolation and extraction of the material from the inorganic matrices. A detailed vibrational study of benzothiophene [15] and dibenzothiophene [16] have also been reported. It is clear from the above overview that: (1) the particular PAHs present in idrialite have not hitherto been studied by Raman spectroscopy. (2) The most studied PAH of the same structural series as those in idrialite, i.e. with angular annelation (cata-condenzation) of benzene rings, was chrysene [9]. (3) The thus-far studied PAHs of equal or at least similar molecular weights as those present in idrialite belong to the linear polyacenes up to pentacene in [9] or to peri-condensed PAHs such as perylene, benz(a)- and benz(e)pyrene [9,12–14]. Structures of these molecules are shown in Fig. 1B. Therefore, the goal in this research project was the analysis of all existing data on vibrational properties of PAHs which in combination with our own measurements will produce a compilation of reliable spectral data for the determination of PAHs in geological materials. In the first step we have chosen thio-PAHs, namely benzothiophene (BT) and dibenzothiophene (DBT) for detailed studies, and also report preliminarily work on 2,3-benzofluorene (BF), which is in fact the parent molecular structure for the PAHs in idrialite [5]. We have obtained new and comparable spectra of these molecules that can be used as discriminative fingerprint and has allowed us to make further molecular band assignements in the Raman spectra of idrialite and related organic minerals. 2. Materials and methods
thiophene moieties. Sarobe et al. [7] reported the sulfurmediated cyclotrimerization of 4,5-dihydrobenz[l]acephenanthrylene to trinaphthodecacyclene (C60 H30 ), which would be expected to give fullerene (C60 ) upon dehydrogenation. ThioPAH (C40 H20 S) isomers are typical byproducts of this reaction. The same type of reaction yields, for example, decacyclene from acenaphthene [8] with diacenaphthothiophene as a byproduct. Thus, the identification of such molecules in the geological record may indicate process routes that can lead to fullerene generation in the host rock. Generally, there are relatively few detailed studies reported of the vibrational spectra of higher-molecular mass PAHs, which is detrimental to the provision of complete and accurate band assignments for the Raman spectra of minerals like idrialite. For unsubstituted molecules larger than naphthalene, Maddams and Royaud [9] presented FT-Raman spectra of 10 PAHs (anthracene, tetracene, pentacene, rubrene, phenanthrene, chrysene, triphenylene, pyrene, perylene and fluoranthene). These studies focused more on the simple analytical spectroscopic characterizational aspects of the PAHs as a whole. Detailed vibrational assignments have been reported for crystalline phenanthrene [10], fluorene [11], comparison of observed
Micro-Raman analyses were performed on a multichannel Renishaw In Via Reflex spectrometer coupled with a Peltiercooled CCD detector. Excitation was provided by the 785 nm line of a diode laser. The samples were scanned from 40 to 3700 cm−1 wavenumber shift at a spectral resolution of 2 cm−1 . Calibration has been carried out using the Si–Si stretching mode at 520.2 cm−1 . The scanning parameter for each Raman spectrum was taken as 15 s and 40 (for BT and DBT) or 100 (for BF) scans were accumulated for each experimental run to provide better signal-to-noise ratios. Multiple spot analyses were carried out on different regions of the same sample to check for spectral reproducibility. Chemicals were used as purchased and no special purification procedures were effected, namely, benzothiophene (Merck, 99.5% purity), dibenzothiophene (Aldrich, 99.5% purity) and 2,3-benzofluorene (Aldrich, 99.5% purity). 3. Results and discussion The measured spectra of benzothiophene and dibenzothiophene crystals correspond to those reported in [15] and [14,16],
O. Frank et al. / Spectrochimica Acta Part A 68 (2007) 1065–1069
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Fig. 2. Raman spectra of benzothiophene, dibenzothiophene and idrialite crystals (from bottom to top) in the region 200–1800 cm−1 .
for BT and DBT, respectively. As an example, the Raman spectrum of dibenzothiophene crystal, taken from the (1 0 2) plane, is presented in Figs. 2 and 3. The strongest molecular bands at 215, 284, 407, 495, 701, 1023, 1132, 1232, 1309, 1317, 1556 and 1597 cm−1 of the A1 symmetry class, as assigned by [16], correspond to the previously reported data, with a deviation in wavenumber of approximately up to 4 cm−1 . The CH stretching A1 fundamentals were assigned to peaks at 2993, 3023, 3053 and
possibly 3076 cm−1 . The peaks at 2618 and 2628 cm−1 are to be assigned, but the former may be an overtone of the 1309 cm−1 A1 fundamental. The lattice vibrations at 82, 103 and 136 cm−1 correspond to data presented in [16]. Fig. 4 shows a comparison chart of all major Raman bands of 13 unsubstituted PAHs, BT and DBT with idrialite. The chart comprises members of all basic groups of relevant unsubstituted PAHs, namely polyacenes (anthracene, tetracene and pentacene), peri-condensed (pyrene, perylene), peri-condensed with a five-membered ring (fluoranthene), cata-condensed (chrysene), cata-condensed with a five-membered ring (fluorene, 2,3-benzofluorene), naphthalene. The peak intensities are differentiated by grey scale to strong, medium and weak. Where possible, solution data were taken. The strongest intensity idrialite peaks, shown by vertical lines, generally occur in the wavenumber ranges where also the strongest bands of the component PAHs appear. 3.1. Region bellow 1000 cm−1
Fig. 3. Raman spectra of benzothiophene, dibenzothiophene and idrialite crystals (from bottom to top) in the region 2700–3150 cm−1 .
A careful comparison of the measured spectra with those previously published for other PAHs [9–16] shows distinct features of thio-PAHs occur especially in the region bellow 1000 cm−1 . For unsubstituted PAHs, in general, most of the significant vibrations in the region 200–1000 cm−1 are assignable to the CCC in-plane bending modes, indicated as ␣(CCC) in the text, and in the region 600–1000 cm−1 the C–H out-of-plane bending modes are designated as ␥(HCCC). However, the latter modes are usually only of medium to weak intensity. The strongest band in this region for PAHs appears at 750 ± 10 cm−1 , and belongs to the ␣(CCC) mode. It is noteworthy that this band is the most intense in the spectra of fluorene at 745 cm−1 [11] and 2,3benzofluorene at 752 cm−1 , and occurs at 750 cm−1 in idrialite, which corresponds to the fact that benzonaphthofluorene is the
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Fig. 4. A comparison chart showing position of major peaks in various PAHs (squares) and idrialite (vertical lines). The colour intensity scheme is the same for PAHs and for idrialite. Data measured and merged with those from [9–14]. See text.
prime constituent of idrialite [5]. In thio-PAHs, this most intense band appears at 701 cm−1 for DBT and 706 cm−1 for BT. From all other previously measured Raman spectra of PAHs, only the ␣(CCC) or ␥(HCCC) modes of phenanthrene falls into this region at 713 and 710 cm−1 , respectively [10]. In the microRaman spectrum of idrialite, both the 700 and 750 cm−1 bands are present and these are among the most intense in the spectrum, clearly indicating the presence of both carbon-only (containing a five-membered, alicyclic ring) and thio-PAHs in idrialite. Furthermore, the 498 cm−1 band in idrialite, appearing as medium to medium-weak in intensity, can be mainly attributed to the in-plane deformation mode of the thiophene ring, ␣(CSC), a medium intensity band occurring at 495 cm−1 in DBT and at 492 cm−1 in BT. As can be seen from Fig. 4, only weak intensity bands of higher polyacenes and pyrene fall into this region. 3.2. Region 1000–1300 cm−1 This region is dominated by a Raman band at 1020 ± 5 cm−1 in the spectra of idrialite and also for all observed PAHs except the peri-condensed congeners such as pyrene or perylene. It originates from the ring C–H in-plane bending modes of the aromatic systems, further indicated as (HCC) in the text, combined with the C–C stretching modes. It appears at 1023 cm−1 in DBT, 1015 cm−1 in BT and 1020 cm−1 in 2,3-benzofluorene. 3.3. Region 1300–1700 cm−1 Generally, for PAHs, this region is dominated by the C–C stretching modes in aromatic system combined with the (HCC) modes. In idrialite, these are represented by medium to strong intensity bands at 1375, 1393, 1579 and 1617 cm−1 . It is quite difficult to assign these bands conclusively to a contribution of a particular group of PAHs, because all of them have at least one member with a medium to strong intensity band at the wavenumber positions of the bands in idrialite.
3.4. Region around 3000 cm−1 In the PAHs and also the thio-PAHs lacking a five-membered alicyclic ring, this region is dominated by C–H stretching mode for an aromatic systems. In DBT, the strongest band in this region appears as a broad feature centred at 3053 cm−1 as observed also in idrialite. For the fluorene-type PAHs, additional intense bands around 2900 cm−1 are ascribed to CH2 stretching vibrations of the aliphatic five-membered ring. In fluorene, the symmetric stretching vibration of the CH2 group has been assigned to the peak at 2905 cm−1 and the asymmetric stretch at 2920 cm−1 [17]. 4. Conclusions Micro-Raman spectroscopy is demonstrated to be a very promising tool for the identification of various PAHs in complicated molecular assemblages of natural organic minerals such as idrialite. We have shown that in particular the wavenumber region bellow 1000 cm−1 can be used for distinguishing of various groups of PAHs in such spectra. The C–S–C in-plane deformation on the thiophene ring is the most intense band in the wavenumber region around 500 cm−1 in thio-PAHs, clearly corresponding to the medium-weak intensity band in idrialite at this wavenumber position. Furthermore, the CCC in-plane bending mode in thio-PAHs occurring around 700 cm−1 and in fluorene-type PAHs around 750 cm−1 exactly matches the positions of the strongest bands in idrialite in this region. However, further spectral vibrational data from higher-mass PAHs are needed in order to satisfactorily elucidate bands in other complicated natural mixtures of polyaromatic hydrocarbons to facilitate a better understanding of the thermal alteration processes that occur in relevant terrestrial geological systems. Such data will also be relevant to the enlargement of the Raman spectroscopic database for the spectral recognition of organic markers in geological environments which has been given impetus recently by the inclusion of remote miniaturised Raman
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