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Nuclear Magnetic Resonance Spectroscopy. Ring-Current Effects on Carbon-13 Chemical Shifts (Dihydropyrenes/Bridged Annulenes) RICHARD DU VERNET and V

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Nuclear Magnetic Resonance Spectroscopy. Ring-Current Effects on Carbon-13 Chemical Shifts (Dihydropyrenes/Bridged Annulenes) RICHARD DU VERNET and V Proc. Nat. Acad. Sci. USA Vol. 71, No. 8, pp. 2961-2964, August 1974 Nuclear Magnetic Resonance Spectroscopy. Ring-Current Effects on Carbon-13 Chemical Shifts (dihydropyrenes/bridged annulenes) RICHARD DU VERNET AND V. BOEKELHEIDE Department of Chemistry, University of Oregon, Eugene, Oregon 97403 Contributed by V. Boekelheide, April 22, 1974 ABSTRACT A comparison of the '-C nuclear magnetic hybridization, and charge. Although in these studies compari- resonance chemical shifts of some 15,16-dialkyl-15,16- son was made to reference models, the models available were dihydropyrenes with the corresponding 15,16-dialkyl- 2,7,15,16-tetrahydropyrenes provides a measure of the not ideal. For example, comparison of 5 with either 6 or 7 as effect of ring current on carbon-13 chemical shifts in reference models involves changes in geometry. One particular which other effects may be expected to be negligibly effect of change in the peripheral geometry is to alter the 7r- small. The general conclusion is that the absolute magni- orbital interaction of the C-1 and C-6 carbons (12-14) and the tude of the ring-current effect is the same for carbon-13 as should be for protons when they occupy the same position in space chemical shift of the bridging methano carbon sensi- relative to the aromatic 7r-electron cloud. tive to these changes (15, 16). Early in the investigation of nuclear magnetic resonance the (CH2)1 concept of a ring current was introduced to explain the ob- served chemical shifts of protons in the vicinity of aromatic rings (1). In the case of protons this ring current effect is 4 5 6 7 usually large compared to the other factors affecting the value of the chemical shift, and this is dramatically evident for the In their study on 1 ,6-methano [10]annulene Jones, Mlasu- simple and bridged annulenes (2-4). However, for heavier mune, and their colleagues (10) suggested that the bridged elements the ring-current effect is less significant and is nor- [14]annulenes would be especially suitable for analyzing the mally wholly overshadowed by other kinds of influence. ring-current contributions to carbon-13 chemical shifts. Thus, attempts to evaluate the ring-current effects on 19F- or These molecules have atoms placed within the cavity of the '3C-chemical shifts have commonly been frustrated by lack of aromatic 7-electron cloud and it is already known that the suitable models in which these other factors would be nullified ring-current contribution to the chemical shift of protons or unimportant. For example, Jones and Grant have invoked a llaced internally to the 7r-electron cloud is exceptionally large p)aramagnetic ring-current to explain the unusually low field (2-4). In a study directed toward this end Gfinther, Vogel, signals of the quaternary carbons in the carbon magnetic and their colleagues (11) measured the '3C-nuclear magnetic resonance spectrum of biphenylene, 1 (5, 6) and have proposed resonance (NMR) spectrum of the bridged [14]annulene 8 that the upfield shifts of the internal quaternary carbons of and estimated its ring-current contribution by comparison to pyrene, 2 (7), and acepleiadylene, 3 (8), are due to the dia- that of the dihydro derivative 9, as a model. Their conclusion magnetic ring current of the 4n + 2 r-electron peripheral sys- was that the ring-current contribution to the chemical shift of tems in these molecules. However, without suitable models carbons a and b is approximately 8.9 and 6.9 parts per million these shifts are within the range that can be rationalized by (ppm), respectively. Although these values are probably quite other factors such as electronegativity or ring strain. good, there is some change in geometry between 8 and 9 so, even though small, some contribution to the chemical shift differences is due to factors other than ring current. 0 0 1 2 3 In other studies on [12]paracyclophane, 4 (9) and 1,6- 8 9 methano [10 Jannulene, 5 (10, 11), the general conclusion In the course of our investigations of the physical and chem- reached was that the ring-current effect on carbon-13 chemical ical properties of the dihydropyrenes (17), we prepared a series shifts is small compared to other factors such as geometry, of compounds ranging from the parent molecule, trans-15,16- dihydropyrene, through various dialkyl derivatives, struc- Abbreviations: NMAIR, nuclear magnetic resonance; ppm, parts tures 10 a-c. Also at hand were the corresponding 2,7-dihydro structures 11 a-c. These molecules seem ideally per million; h, the shift in parts per million from the signal of the derivatives, reference compound, tetramethylsilane. suited for analyzing the ring-current effect on carbon-13 2961 Downloaded by guest on October 1, 2021 2962 Chemistry: Du Vernet and Boekelheide Proc. Nat. Acad. Sci. USA 71 (1974) -.-603C) [ppmj FIGA. 1. Schematic representation of the "C NMR spectra of compounds 10 a-c and 11 a-c. Spectra of approximately 0.1 M solutions in deuterochloroform were taken at ambient temperature (16). The chemical shifts are given in ppm downfield of tetramethylsilane (TMS) (8 CDCd, = a TMS + 76.9 ppm) and are accurate to within 0.1 ppm. These spectra were obtained using a Varian XL-100-15 spectrometer operating at 25.2 MHz in the Fourier transform mode. The deuterium resonance of the deuterochloroform solvent served as an internal lock. Chemical shift measurements were made from proton noise-decoupled spectra. Peak assignments were achieved through the use of on- and off-resonance decoupling (19). chemicals shifts and so we undertook such a study. carbon in the same region permits a comparison of the effect of the same ring current for two different nuclei. This point H H is of some interest since predictions from the theory of nuclear magnetic resonance suggest that the magnitude of the ring- current effect for different nuclei should be the same if the nuclei are located at the same position in space relative to the aromatic ir-electron cloud (18). The carbon-13 spectra for compounds 10 a-c and 11 a-c are shown schematically in Fig. 1 with each signal from the car- H' H bons in the aromatic system being compared directly to the a, 4=-CH3 signal from the corresponding carbon in the localized model. b, R=-CHXCH% The upfield shift (Aa = bdelocalied - biloalized) of the internal c, R -CH2CH2CH3 carbons in 10 a-c, i.e., the ring current effect, is readily ap- First of all, the aromatic molecules, 10 a-c, are bridged parent, the magnitude diminishing with increasing distance [14]annulenes in which the carbons under consideration are from the aromatic ir-electron cloud. The consistency of struc- internal substituents lying within the shielding region of the tures 11 a-c as good localized models is evident from the simi- aromatic r-electron cloud. Further, examination of molecular lar upfield shifts exhibited by a given internal carbon (E, F, models indicates clearly that the comparison molecules, and G) throughout the series, especially so for the diethyl and 11 a-c, lacking an aromatic ring current, are otherwise es- di-n-propyl derivatives. Thus, one has confidence that the sentially unchanged with regard to the geometry of the inter- changes in geometry and electronic effects for the internal nal framework. Thus, to a very close approximation all of the atoms are minimal in going from the aromatic compounds to factors affecting chemical shift, other than ring current, are their reference models, and so the upfield shifts can be attrib- the same for the two series of compounds. Secondly, the vary- uted unambiguously to the influence of ring current. ing distances of the atoms of the internal alkyl groups from To compare carbon-13 and proton ring-current effects we the plane of delocalization of the aromatic 7r-electron cloud give a schematic summary of the proton chemical shifts in allows a mapping of the ring-current effect over a range of Fig. 2. These data from Figs. 1 and 2 are then combined to positions in space. Finally, the presence of both hydrogen and provide a graphical comparison of the two effects in Fig. 3. Downloaded by guest on October 1, 2021 Proc. Nat. Acad. Sci. USA 71 (1974) 18C-Chemical Shifts 2963 6 (IH) [ppml FIG. 2. Schematic representation of the 'H NMR spectra of compounds 10 a-c and 11 a-c and the parent system trans-1,16-dihydro- pyrene. * 2,7-trans-15,16-tetrahydropyrene has not been synthesized; the chemical shift given for He is an estimate. All spectra were taken in deuterochloroform and chemical shifts are relative to tetramethylsilane. In Fig. 3 the Ab values for the internal carbons and protons are the center of the molecule. The solid line in Fig. 3 represents a plotted against the out-of-plane distance (Z) of these atoms theoretical plot of the Johnson and Bovev calculations for from the mean plane of delocalization. The values of Z are benzene (22, 23) converted from ring radii to Angstroms. If derived from measurements from Dreiding models using the p does not vary, the chemical shift due to ring current (Ad) conformation of the alkyl groups shown in Fig. 4. Both space- should reach a maximum finite value when Z = 0. Although filling models and data from x-ray crystallographic analysis these calculations for benzene are not directly comparable to (20) indicate that this should be the favored conformation. the results obtained for the dihydropyrenes, the shape of the Furthermore, for the case of the di-n-propyl derivative, 10 c, theoretical curve should be similar.
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