pyrene congeners 1H and 13 C NMR assignments of the three dicyclopenta-fused

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1 Tetrahedron 6 (006) 0 8 H and C NMR assignments of the three dicyclopenta-fused pyrene congeners María José Otero-Lobato, a Cornelis A. van Walree, a Remco W. A. Havenith, b Leonardus W. Jenneskens, a, * Patrick W. Fowler c and Erich Steiner d a Debye Institute, Organic Chemistry and Catalysis, Utrecht University, Padualaan 8, 8 CH Utrecht, The Netherlands b Debye Institute, Theoretical Chemistry Group, Utrecht University, Padualaan 8, 8 CH Utrecht, The Netherlands y c Department of Chemistry, University of Sheffield, Sheffield S 7HF, UK d School of Biosciences, University of Exeter, Stocker Road, Exeter EX QD, UK Received January 006; accepted 9 March 006 Available online April 006 Abstract Complete H and C NMR assignments of the (di-)cyclopenta-fused pyrene congeners, cyclopenta[cd]- (), dicyclopenta[cd,fg]- (), dicyclopenta[cd,jk]- () and dicyclopenta[cd,mn]pyrene (), respectively, are achieved using two-dimensional (D) NMR spectroscopy. The experimental C chemical shift assignments are compared with computed ab initio CTOCD-PZ/6-G** C chemical shifts; a satisfactory agreement is found. Substituent-induced chemical shifts in the pyrene core induced by annelation of cyclopenta moieties are discussed. Effects of dicyclopenta topology on electronic structure are illustrated for. Ó 006 Elsevier Ltd. All rights reserved.. Introduction The (di-)cyclopenta-fused congeners of pyrene (), viz. cyclopenta[cd]- (), dicyclopenta[cd,fg]- (), dicyclopenta- [cd,jk]- () and dicyclopenta[cd,mn]pyrene (),, belong to an important sub-class of the polycyclic aromatic hydrocarbons (PAHs) known as cyclopenta-fused PAHs (CP-PAHs) (Fig. ). The independent synthesis of CP-PAHs has been instrumental for their identification as significant constituents of the nonpolar fraction of combustion exhausts derived from organic matter, viz. fossil fuels. 6 Compounds have also been put forward as undesired side-products that are formed during thermal treatment of pyrene-contaminated soil. 7 Since CP-PAHs like may exhibit mutagenic and carcinogenic properties, 8 they represent a potential human health concern. 9 Only recently the bacterial mutagenicity response of was assayed employing the standard Amesassay (Salmonella typhimurium strain TA98 with and without exogenous metabolic activation (S9-mix)). CP-PAHs were found to be highly active metabolic-dependent mutagens. Interestingly, congeners and were found to act as direct mutagens, that is, they are potent mutagens even in the absence of exogenous metabolic activation. 6 From a fundamental perspective the nonalternant CP-PAHs also possess unusual physico-chemical properties, such as high electron affinities (low one-electron reduction potentials), 7 0 characteristic upfield-shifted H NMR Figure. Pyrene (), cyclopenta[cd]- (), dicyclopenta[cd, fg]- (), dicyclopenta[cd,jk]- () and dicyclopenta[cd,mn]pyrene (). A generalized carbon atom numbering scheme is used to facilitate the comparison of related positions in compounds. * Corresponding author. Tel.: + 0 8; fax: + 0 ; l.w.jenneskens@chem.uu.nl y Affiliated to Organic Chemistry and Catalysis /$ - see front matter Ó 006 Elsevier Ltd. All rights reserved. doi:0.06/j.tet

2 M. J. Otero-Lobato et al. / Tetrahedron 6 (006) 0 8 chemical shifts,,, and photophysical properties, viz. UV vis spectra that are strongly modulated by the number and topology of the annelated cyclopenta moieties,0 and anomalous fluorescence. As a consequence, CP-PAHs represent interesting probe molecules in the study of the energy and magnetic criteria of aromaticity in p-conjugated polycyclic systems. For it has been shown both experimentally and theoretically that the number and topology of the annelated cyclopenta moieties markedly affects the global and local (distinct rings) aromatic character of. In another connection, have been proposed as potential intermediates in the formation of fullerenes under flame conditions.,6 8 The complete H and C NMR assignments of, so far unreported, are the subject of this paper. Although the H and C NMR assignments of cyclopenta[cd]pyrene (, C 8 H 0 ) have been reported previously, 9,0 the assignment of some of its quaternary carbon atoms (C and C7, see Fig. ) remains ambiguous. Hence, is also reinvestigated. Furthermore, a comparison of the experimental C assignments with computed ab initio CTOCD-PZ/6-G** C chemical shifts are carried out. Hence, conclusions can be drawn as to the effect of cyclopenta annelation to a pyrene perimeter on the H, and C chemical shifts (substituent-induced chemical shifts, SCS; Dd with as the reference compound for, respectively) can be drawn.. Results and discussion The H and C NMR assignments of were achieved using two-dimensional (D) NMR techniques. Nuclear Overhauser Effect Spectroscopy (NOESY) was used for the assignment of proton signals by evaluation of through-space dipolar interactions. Heteronuclear Chemical Shift Correlation (HETCOR) was applied to identify those carbon atoms bearing hydrogen. Long-Range (LR)-HETCOR reflecting one to three bond C H couplings ( J C H J C H ) was used to make a definitive assignment of the quaternary carbon atoms. Since not all expected J C H and/or J C H couplings could be observed in a single LR-HETCOR experiment, different experiments employing different long-range C H coupling constants (range 0 Hz), were executed in order to visualize all the different correlations. In some cases, long-range C H interactions were still not discernible, presumably because their n J C H values deviate from the common long-range coupling constants used in LR-HETCOR experiments (, 6, 8 or 0 Hz). Note that J C H values normally fall in the range of 0 Hz while J C H and J C H values are typically less than and Hz, respectively... Experimental NMR spectra Figure. D C NMR spectrum (solvent CDCl ) of cyclopenta[cd]pyrene (, see also Table ).... Cyclopenta[cd]pyrene (). H and C NMR assignments of (C s symmetry) were first reported by Jans et al. 9 However in a more recent study, 0 in which partially C- labelled was used, the original assignment of the quaternary carbon atoms C, C7, C, C6 and C7 was questioned (Figs. and ). This prompted us to also take into consideration. Since all H NMR resonances of have been assigned unambiguously (Table ), 9,0, the carbon atoms of this molecule that bear hydrogen were readily identified using HETCOR. The C resonances at d.8 and 0. show a cross peak with d 7.7 (H) and 8.8 (H8), respectively, and thus originate from C and C8, respectively. Following similar reasoning the C resonances at d 8., 7.8, 6.9, 6.9, 6.7, 6.8,. and.68 correspond to C0, C, C9, C, C, C6, C and C, respectively, (see Fig. ). These results are all in line with those previously reported. Next, various LR-HETCOR experiments, in which different long-range n J C H coupling constants were used, were performed (Table and see Section.) in order to assign the eight quaternary carbon atoms of. Since with LR- HETCOR two- ( J C H ) and three-bond ( J C H ) long-range couplings between quaternary carbon atoms and distant hydrogens are most likely to be observed only these types of contributions are considered. The hydrogens H6, H and H (Fig. ) show a cross peak with the C resonance at d 9.07, which can only correspond to either C or C8. The C resonance at d.8 shows cross-peaks with H and H, implying that this resonance originates either from C or C8. H and H also show a cross peak with the C resonance at d 7., which may then correspond to C, C or C8. To distinguish between these three carbon atoms, the proton-decoupled D C NMR spectrum of (Fig. ) is carefully examined. The C resonance at d 7. possesses the lowest intensity. This suggests that it belongs to a carbon atom located within the pyrene core, viz. C8. Owing to its increased distance from the hydrogens of the pyrene perimeter, its relaxation will be slower, leading to a reduced intensity. If the assignment of C8 (d 7.) is correct, it implies that the C resonances at d 9.07 and.8 correspond to C and C, respectively. The crosspeaks found between the C signal at d 0.87 and H and H, indicate that this C resonance must originate either from C or C. The d.90 signal can correspond only to C7, since it exhibits only cross-peaks with H6 and H9. In analogy, the C resonance at d. shows interactions with H6,H8,H0 and H. This means that it can arise only from C6. With the identification of C6, the C resonance at d 0.8 can be assigned to C due to its crosspeaks with H0 and H. The available assignments now permit the identification of C at d By elimination, the C resonance at d 0.69 belongs to C7. The complete H and the C assignments of are summarized in Table.... Dicyclopenta[cd, fg]pyrene (). Since dicyclopenta[cd,fg]pyrene (, C 0 H 0 ) possesses C v symmetry, its

3 M. J. Otero-Lobato et al. / Tetrahedron 6 (006) 0 8 Table. Cross-peaks observed in the HETCOR and LR-HETCOR spectra of cyclopenta[cd]pyrene () a,b Nucleus d ( C) ppm H8 H6 H0 H H H H9 H H H 8.8 (d, 7.70 Hz) 8.6 (s) 8.8 (d, 7.60 Hz) 8. (m) 8.08 (m) 8.0 (m) 8.00 (m) 7.98 (m) 7.7 (d,.0 Hz) 7.7 (d,.0 Hz) C b,c,d C6. a d c c,d C.68 HT C. HT b C6 6.8 HT c,d C 6.7 c HT C 6.9 a HT C9 6.9 HT C8 7. d d C 7.8 a,d HT C0 8. b HT d C 0.8 b d C8 0. HT a,d c C 0.87 d b C7.90 a,c b,d C.8 HT a C.8 d c,d C 9.07 a d a a See Figure for the structure and atom numbering of. InFigure the experimental D C NMR spectrum of is shown. Quaternary carbon atoms are typeset in boldface and the multiplicity of the H chemical shifts is indicated between parentheses. b HETCOR cross-peaks ( J C H coupling constant 60 Hz, H relaxation delay s) are indicated as HT. Independent LR-HETCOR experiments gave the crosspeaks indicated as observed with long-range coupling constants of Hz (a), 6 Hz (b), 8 Hz (c) and 0 Hz (d) (relaxation delay s). H and C NMR spectrum contains five and ten distinct resonances, respectively, (D CNMR,Fig. ). In its H NMR spectrum, the singlet at d 7. (H) is readily assigned to hydrogen H. In addition, the vicinal hydrogen pairs H and H, and, H and H, can be distinguished on the basis of their different coupling constants, as the hydrogens of the externally fused cyclopenta-rings (H and H) possess a characteristic coupling constant of ca. Hz. The H assignments of are completed by an NOESY experiment. The presence of cross-peaks between the doublet at d 7.6 (H, 7.60 Hz) and 7.0 (H,.0 Hz) reveals that the former corresponds to H and the latter to H, respectively. Consequently, the doublets at d 6.9 (H,.0 Hz) and 7.7 (H, 7.60 Hz) belong to H and H, respectively. The unequivocal H assignments of now allow the identification of all the carbon atoms bearing hydrogen (HETCOR; see Fig. for an illustrative example). The C resonances positioned at d 7., 7.88, 7.07, Figure. Experimental D C NMR spectrum of dicyclopenta[cd, fg]- pyrene (, see also Table ). and. belong to C,C,C, C and C, respectively, (Table ). Independent LR-HETCOR experiments optimized for different long-range n J C H coupling constants (Table and see Section.) enabled the assignment of all the quaternary carbon atoms of (see Fig. for an illustrative example). Only n J C H long-range interactions with n¼ or between hydrogens and the quaternary carbon atoms are taken into consideration (vide supra). Hydrogen H shows two crosspeaks with the C resonances at d 0.6 and., which may then correspond to either C ( J C H ), C or C8 ( J C H ). Hydrogen H shows two cross-peaks with the C resonances at d 0.6 and 0.9, which may thus arise from C ( J C H )orc7 ( J C H ). Since the C(d 0.6) resonance was also found to interact with hydrogen H, it can only correspond to C. Consequently, the d 0.9 C signal belongs to C7. Hydrogen H shows cross-peaks at d 0.9 (C7) and 0., which may originate from C ( J C H ) and C ( J C H ). Since C (d 0.6) has already been assigned, the C resonance at d 0. has to correspond to C. Thus, the C signal at d. belongs to C8. By elimination, the C resonance at d 7.9 then corresponds to C. This is confirmed by the observation of cross-peaks at d 7.9 with H ( J C H ) and H ( J C H ). The complete H and C NMR assignments of are summarized in Table.... Dicyclopenta[cd, jk]pyrene (). In the case of dicyclopenta[cd,jk]pyrene (, C 0 H 0 ), which possesses C h symmetry, five and ten distinct resonances, respectively, are found in the H and C NMR spectra (Fig. ). The total H assignment was accomplished by an NOESYexperiment. The cross peak between the singlet at d 7.0 (H), which corresponds to H, and the doublet at d 6.6 (H,. Hz) indicates that the latter arises from H (cf. the characteristic coupling constant of ca. Hz for the vicinal olefinic hydrogens in the annelated five-membered rings).

4 M. J. Otero-Lobato et al. / Tetrahedron 6 (006) 0 8 H CHCl H H H H C C C C C 8 7 H CHCl H H H H C7 C8 C C C Figure. HETCOR ( J C C 60 Hz, top) and LR-HETCOR ( J C C 60 Hz and n J C H 8 Hz, bottom) spectra of dicyclopenta[cd,fg]pyrene (). Another interaction is found between the doublet at d 7.7 (H, 7.69 Hz) and the doublet at d 6.7 (H,. Hz). Thus, H corresponds to d 7.7 and H to 6.7. Since all H NMR signals of have been now identified, those of its carbons atoms bearing a hydrogen can be assigned using HETCOR: d.9 corresponds to C, d.6 to C, 9.6 to C,.7 to C and.0 to C (Fig. and Table ). The LR-HETCOR experiments with different long-range C H coupling constants were used (Table and see Section.) in order to assign the quaternary carbon atoms of. Hydrogen H shows two cross-peaks with the C resonances at d 0.66 and.07, which can arise from C, C8 ( J C H ) and C ( J C H ). H exhibits interactions with the C resonances at d.00 and., which may then correspond to C and C7 ( J C H )orc ( J C H ). The C resonance at d.00 gives another cross peak with H. Thus, we assign d.00 to C ( J C H ), as C and C7 would present a four- ( J C H ) and a five-bond ( J C H ) coupling with H, respectively, which is unlikely to be observed under the applied experimental conditions. Since the cross-peaks for H with d 0.66 and.07 can now only arise from C and C8, this leaves d. for C7. By elimination, it is now possible to relate C signal at d 0. to C. These assignments are supported by the behaviour of H, which exhibits two cross-peaks at d. (C7), and 0.66, which can arise from the interaction with C8 ( J C H )or C ( J C H ). The final assignment for C and C8 is achieved by comparing their intensities in the D C NMR spectrum of (Fig. ). The intensity of the C signal at d.07 is stronger than that at d This strongly

5 M. J. Otero-Lobato et al. / Tetrahedron 6 (006) 0 8 Table. Cross-peaks observed in the HETCOR and LR-HETCOR spectra of dicyclopenta[cd, fg]pyrene () a,b Nucleus d ( C) ppm H H H H H 7.7 (d, 7.60 Hz) 7.6 (d, 7.60 Hz) 7. (s) 7.0 (d,.0 Hz) C7 0.9 a,b a C. c HT b a C 7.0 b b HT C 7.07 HT b C 7.88 b HT C 0.6 a,b a,b,c C8. a,b,c C 7. HT a C 7.9 c c C 0. a b,c 6.9 (d,.0 Hz) a See Figure for the structure and atom numbering of. InFigure the D C NMR spectrum and in Figure a HETCOR (top) and an LR- HETCOR (bottom) D spectra of are shown. Quaternary carbon atoms are typeset in boldface and the multiplicity of the H chemical shifts is indicated between parentheses. b HETCOR cross-peaks are indicated as HT. LR-HETCOR experiments gave the cross-peaks shown as observed with long-range coupling constants of 6 Hz (a), 8 Hz (b) and 0 Hz (c). Table. Cross-peaks observed in the HETCOR and LR-HETCOR spectra of dicyclopenta[cd,jk]pyrene () a,b Nucleus d ( C) ppm H H H H H 7.7 (d, 7.69 Hz) 7.0 (s) 7.7 (d, 7.69 Hz) 6.7 (d,. Hz) C7. b,c,d b,c C.0 b,c HT C.7 a,b HT C 9.6 HT a,b,c c,d C c,d b C.6 HT C.07 b,c C.9 c HT a C 0. C.00 a,b a 6.6 (d,. Hz) a See Figure for the structure and atom numbering of.infigure the D C NMR spectrum of is shown. Quaternary carbon atoms are typeset in boldface and the multiplicity of the H chemical shifts are indicated between parentheses. b HETCOR cross-peaks are indicated as HT. LR-HETCOR experiments gave the cross-peaks shown as observed with long-range coupling constants of Hz (a), 6 Hz (b), 8 Hz (c) and 0 Hz (d) Figure. D C NMR spectrum (solvent CDCl ) of dicyclopenta[cd,jk]- pyrene (, see also Table ). indicates that the C resonances at d.07 and 0.66 must originate from C and from C8, respectively, since the latter carbon atom is more distant from the hydrogen containing pyrene perimeter. The complete H and C NMR assignments of are compiled in Table.... Dicyclopenta[cd,mn]pyrene (). Dicyclopenta [cd,mn]pyrene (, C 0 H 0 ) possesses C v symmetry. This may give six and twelve distinct signals in the H and C NMR spectra (Fig. 6). Several of the H resonances of are readily assigned. The triplet at d 8.08 (H, 7.70 Hz) has to correspond to hydrogen H9. The singlets at d 8. (H) and 8.7 (H) have to originate from H and H6, respectively, and the doublet at d 8. (H, 7.70 Hz) belongs to H8. The doublets at d 7. (H,.0 Hz) and 7.6 (H,.0 Hz) corresponds to H and H or vice versa (cf. the characteristic value of ca. Hz for an olefinic hydrogens in a five-membered ring). An NOESY spectrum reveals cross-peaks between the singlet at d 8.7 (H6) and the doublet at d 7.6, on one hand, and the singlet at d 8. (H) and the doublet at Figure 6. D C NMR spectrum (solvent CDCl ) of dicyclopenta[cd,mn]- pyrene (, see also Table ). d 7., on the other hand. Thus, the doublets at d 7. and 7.6 originate from H and H. As the complete HNMR assignments of have been achieved, its carbon atoms bearing hydrogen were identified using HETCOR (Table ). The results show that the C resonances located at d.0,.76, 7.0, 6.,.80 and 8.78 correspond to C, C8, C9, C, C6 and C, respectively. The LR-HETCOR experiments with different long-range n J C H coupling constants allow the assignments of the quaternary C atoms of (Table and see Section.). For H9 two cross-peaks are observed with the C resonances at d 0.0 and 7.96, which may correspond to C7 and C6. Note, however, that for C6 the interaction with H9 has to be a long-range J C H coupling. Hydrogen H6 correlates with the C signals positioned at d 7.96,. and 8.6; possible assignments for these cross-peaks are C7 and C ( J C H )orc8 and C6 ( J C H ). The C resonances at d. and 8.6 can only correspond to C and C8, since the C signal at d 7.96 already has been assigned to either C7 or C6. H correlates with both the d. and

6 M. J. Otero-Lobato et al. / Tetrahedron 6 (006) 0 8 Table. Cross-peaks observed in the HETCOR and LR-HETCOR spectra of dicyclopenta[cd,mn]pyrene () a,b Nucleus C (ppm) H8 H6 H H9 H H 8. (d, 7.70 Hz) 8.7 (s) 8. (s) 8.08 (t, 7.70 Hz) 7. (d,.0 Hz) C7.7 C 8.78 HT C6 0.0 b C6.80 b HT C 6. HT C9 7.0 a HT C c b C8. a a a C8.76 HT b C.0 HT c C.6 b c C 8.6 b b 7.6 (d,.0 Hz) a See Figure for the structure and atom numbering of.infigure 6 the D C NMR spectrum of is shown. Quaternary carbon atoms are typeset in boldface and the multiplicity of the H chemical shifts are indicated between parentheses. b HETCOR cross-peaks are indicated as HT. LR-HETCOR experiments gave the cross-peaks shown as observed with long-range coupling constants of 0 Hz (a) Hz (b) and 0 Hz (c)..6 signals. Consequently, these signals can be assigned to C,C and C8. Since d. and 8.6 belongs to either C or C8, the C resonance at d.6 has to originate from C. Note that in the D C NMR spectrum of, the resonance at d 8.6 is slightly more intense than that at d. (Fig. 6). Hence, the resonance at d 8.6 most likely corresponds to C, since the more distant a carbon atom is from a molecular perimeter, the lower the intensity of the signal will be. This leaves d. for C8. In addition, the C signal at d 7.96 is more intense than that positioned at d 0.0; this allows their assignment to C7 and C6, respectively. By elimination, the C resonance at d.7 then belongs to C7. The complete H and C assignments of are summarized in Table... Computed C chemical shifts In a previous paper, we have tentatively assigned the H NMR resonances of CP-PAHs using computed H chemical shifts obtained with the PZ (paramagnetic zero) variant of the all electron ab initio continuous transformation of origin of current density (CTOCD) method with the 6-G** basis as implemented in the Exeter version of SYSMO 6 using 6-G optimized geometries (see Section.). Although the computed H NMR chemical shifts of deviate by ca. 0. ppm from their experimental values, they satisfactorily reflect the trends found in the experimental H NMR spectra (see also Ref. ). These observations prompted us to compute the C NMR chemical shifts of using the same CTOCD-PZ/6-G**//6-G approach (Table for and Table 6 for ). For C NMR chemical shifts, which span a considerably wider range in parts per Table. CTOCD-PZ/6-G**//6-G computed ab initio absolute carbon nuclear shielding constants (s av in parts per million) of dicyclopenta[cd, fg]- (), dicyclopenta[cd,jk]- () and dicyclopenta[cd,mn]pyrene () (see Fig. for a generalized atom numbering scheme) a Compound Nucleus s out s in s av d calcd d exp Dd b C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C a s out is the component of the absolute shielding out-of-plane of the ring, s in is the mean shielding in-plane and s av the overall mean value. The corresponding d values (expressed in parts per million) were obtained by the rule s av 0 6 +d¼8.6 (see Section.). 8 Quaternary carbon atoms are typeset in boldface. b Dd¼d calcd d exp.

7 6 M. J. Otero-Lobato et al. / Tetrahedron 6 (006) 0 8 Table 6. CTOCD-PZ/6-G**//6-G computed ab initio absolute carbon nuclear shielding constants (s av in parts per million) of pyrene () and cyclopenta[cd]pyrene (, See Fig. for a generalized atom numbering scheme) a Compound Nucleus s out s in s av d calcd d exp Dd b C C C C C C C C C C C C C C C C C C C C C C C a s out is the component of the absolute shielding out-of-plane of the ring, s in is the mean shielding in-plane and s av the overall mean value. The corresponding d values (expressed in parts per million) were obtained by the rule s av 0 6 +d¼8.6 (see Section.). 8 Quaternary carbon atoms are typeset in boldface. b Dd¼d calcd d exp. million, the deviations between the computed and experimental values, which appear to be the most significant for the carbon atoms of the cyclopenta moieties (range.. ppm), are still sufficiently small (for all carbon atoms: range ppm, see Dd values in Tables and 6) to render the computations a material aid for C assignments at least in p-conjugated polycyclics. This is to an extent a fortunate consequence of the combination of basis set (6-G**) and the CTOCD-PZ method, as the results for C shifts in the 6-G** basis can be closer to experiment than accurate Hartree Fock limiting results. 7 For the computed C chemical shifts can thus be used to validate the experimental LR-HETCOR C assignments (Tables and 6). As shown by the data, the assignments in which the intensity of the C signals (cf. compounds, and, vide supra) was used are confirmed by the computed CTOCD-PZ/6-G**//6-G C chemical shifts. A comparison of the absolute carbon nuclear shield constants (s av )of and their out-of-plane (s out ) and mean in-plane (s in ) components reveal that all carbon sites possess a large diamagnetic, s out component. The in-plane principal components approximately cancel each other leaving a relatively small, usually paramagnetic, mean in-plane s in value (Table ). Similar behaviour has been found for other CP- PAHs. 8.. Substituent-induced C chemical shifts: Dd To gain insight into the extent to which mono- and dicyclopenta annelation influences the C NMR chemical shifts of related carbon positions Substituent-Induced Chemical Shifts (SCS; C Dd in parts per million) were calculated using either the experimental or computed ab initio CTOCD- PZ/6-G**/6-G C chemical shifts. For the calculation of SCS Dd exp and Dd calcd values for CP-PAHs, the C chemical shifts of cyclopenta[cd]pyrene (), were taken as a reference (Dd exp () vsdd calcd (), Table 7). The Dd exp () and Dd calcd () values consistently show that the fivemembered ring ipso carbon atoms of and behave differently from those of (Table 7). The ipso carbon atom C (Fig. ) of and are deshielded by ca. ppm (Dd exp () and Dd calcd ()), whereas the other ipso carbon C is Table 7. Computed CTOCD-PZ/6-G**//6-G ab initio (Dd calcd ) and experimental (Dd exp ) C substituent-induced chemical shifts (Dd (); relative to cyclopenta[cd]pyrene ()) for dicyclopenta[cd, fg]- (), dicyclopenta[cd, jk]- () and dicyclopenta[cd,mn]pyrene (, see Fig. and also Tables and 6) Nucleus Dd () Dd () Dd () Calcd Exp Calcd Exp Calcd Exp C C C C C C6 C a C a C a C a C7 C8 a C8 a C a C a C8 C a C a C a C a.90. C9 C a C a C a C a C0 C a C a C a C a a C8 a C8 C C a C a C8 a C8 a C7 a C7 a C C a C a C a C a C6 a C6 a C C a C a C C8 a C8 a C C a C a C6 C7 a C7 a C7 a C7 a.0. C C a Positions equivalent by molecular symmetry; quaternary carbon atoms are typeset in boldface.

8 M. J. Otero-Lobato et al. / Tetrahedron 6 (006) slightly shielded in (Dd exp (),. ppm) but deshielded in (Dd exp (),. ppm). The corresponding Dd calcd () values are ca. ppm for C in both and. In contrast, the related Dd exp () values for are 0.0 (C) and 0. (C) (for comparison: Dd calcd (),.0 (C) and 0.00 (C)). This confirms that the topology of the two annelated cyclopenta moieties affects the electronic structure of (see also Ref. 9). This is further substantiated by the Dd exp () and Dd calcd () values found for some of the more distant carbon atoms (e.g., C8, C0,C,C7 and C8) in the series (Table 7 and Fig. ), in which all the carbon atoms show a different response.. Conclusions Complete H and C NMR assignments for the nonalternant (di-) cyclopenta-fused pyrene congeners are reported. The experimental assignments are corroborated by chemical shift calculations using the all electron ab initio CTOCD- PZ/6-G** method. A comparison of the SCS values, Dd exp () and Dd calcd (), shows that for with as a reference the number and topology of the annelated cyclopenta moieties markedly affects their electronic structure.. Experimental CP-PAHs were synthesized using Flash Vacuum Thermolysis (FVT). The corresponding (-choroethenyl)- substituted PAH precursors were prepared as described previously. Pure CP-PAHs were isolated from the pyrolysates by re-crystallization from n-hexane. Caution: CP-PAHs have to be handled according to the NIH guidelines for carcinogens (see also Ref. 6)... NMR spectroscopy H and C NMR spectra were recorded at C (Varian Unity Inova Spectrometer operating at 00. and 7.7 MHz, respectively). The two-dimensional (D) NMR experiments were also performed on the Varian Unity Inova Spectrometer at C. In all the experiments, saturated CDCl solutions of the CP-PAH were used. H and C NMR chemical shifts are reported in parts per million using residual CHCl (d 7.6 ppm) and CDCl (d ppm), respectively, as an internal standard. Prior to the Long-Range-Heteronuclear Chemical Shift Correlation LR-HETCOR experiments, the proton-coupled C H C spectra of were collected to establish the magnitude of J C H coupling constants. The H decay time (T ) was determined in order to optimise the relaxation delay in the D NMR experiments. Nuclear Overhauser Effect Spectroscopy (NOESY): the relaxation delay was set approximately at T for each compound, mixing time 0.7 s, acquisition time. s, D width 700 Hz and number of increments 6. The recording time of the NOESY spectra was in the range 0 h. HETCOR and LR-HETCOR: in the HETCOR and LR-HETCOR experiments, a J C H ¼ 60 Hz coupling constant was used. The relaxation delay was set as s for HETCOR and as either s or s for LR-HETCOR in order to obtain the two- ( J C H ) and three-bond ( J C H ) C H coupling constants. HETCOR and LR-HETCOR experiments were recorded with a proton and carbon sweep width of and Hz, respectively. The number of increments was. Recording times were ca. h for the HETCOR spectra and 60 7 h for the LR-HETCOR spectra... Computations The absolute carbon nuclear shielding constants (s av in parts per million), their out-of-plane (s out in parts per million) and mean in-plane (s in in parts per million) components of CP- PAH were computed using the PZ (paramagnetic zero) variant 8 of the all electron ab initio continuous transformation of origin of current density (CTOCD) method as implemented in the Exeter version of SYSMO, 6 all within the 6-G** basis. The s av values (s av ¼/(s out +s in )) were converted into d values by the rule s av 0 6 +d¼ The molecular geometries of were optimized at the restricted Hartree Fock level in the 6-G basis using the GAMESS-UK program 9 and were taken from a previous study. 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