Carbon-13Chemical ...

1 carbon -13 Chemical-Shift Tensors in Polycyclic Aromatic Compounds. H. Barich, Anita M. Orendt, Ronald J. Pugmire, and David M. Grant*Departments of Chemistry and Chemical and Fuels Engineering, UniVersity of Utah, Salt Lake City, UT, 84112 ReceiVed: May 25, 2000 The principal values of the13C chemical-shift tensors of natural abundance biphenylene were measured atroom temperature with the FIREMAT experiment. Of 18 crystallographically distinct positions (three sets ofsix congruent carbons each), the three primary bands have been resolved into seven single peaks and fourdegenerate peaks (two double, one triple, and one quadruple). Hence, eleven different chemical-shift tensorsare reported. An interpretation of the data is made by comparison to carbon chemical-shift tensors in othermolecules with similar chemical environments. Experimental and theoretical values based on a model of theasymmetric unit of the crystal unit cell are in good measurement of13C chemical-shift tensors of polycyclicaromatic hydrocarbons has received considerable attention inrecent to measuring chemical-shift tensorsin aromatic microcrystalline powders include the complexityof the spectrum due to the spinning sideband patterns, therelatively close isotropic chemical shifts of the various carbonsin the molecule (typically from 120 to 140 ppm), and theextensive overlap of molecularly equivalent peaks from crys-tallographically inequivalent positions.

2 The complexityof thesespectra may be reduced by employing a two-dimensional (2D)magic angle turning (MAT) experiment that isolates individualsideband patterns associated with different isotropic chemicalshifts from the composite spectrum. Recent advances in suchmethods have made possible the isolation of several dozensideband patterns from a complicated to thedevelopment of the FIREMAT experiment (FIREMAT standsforfive- replicatedMAT), the spectrum of biphenylene wouldhave been too complicated to obtain well-resolved spectra,largely due to coincidental degeneracy of nearly overlappingisotropic shifts that arise from similar molecular positions foundin different crystallographic positions. The FIREMAT experi-ment separates individual spinning sideband patterns by iden-tifying them with their respective isotropic chemical shifts. Anisotropic chemical shift spectrum is extracted from the pseudo-2D data.

3 Hence, a sideband pattern can be extracted for eachresolvable isotropic chemical shift. At first glance, one wouldexpect to find only three peaks in the biphenylene crystal system,one for each molecular position; however, the crystal systemhas eighteen inequivalent carbon positions. The fact that thethree molecular positions of biphenylene are further resolvedin the crystal system (due to magnetic inequivalencies in thecrystallographic space group) demonstrates the potential ad-vantages of solid-state are six biphenylene molecules present in a total of fourasymmetric units in theP21/a unit cell,8and they exhibit twodifferent biphenylene structures present in a 1:2 ratio. The minorstructure labeled P (with two molecules per unit cell) is planarwhile the major structure (with four molecules per unit cell)labeled D, deviates very slightly from planarity. Thus, there isone entire biphenylene molecule D and half of a biphenylenemolecule P in each asymmetric unit, resulting in eighteeninequivalent NMR carbon environments in the crystal.

4 Theeighteen lines group into three sets of six closely positionedcongruent carbon bands with some lines resolved in each this work, the FIREMAT experiment7isolated 11 sidebandpatterns in the biphenylene crystal system, one for each resolvedisotropic chemical shift in the spectrum (Figure 1).Biphenylene is an interesting model compound in the studyof13C chemical-shift tensors due to its strained detailed relationship between ring strain and thechemical-shift tensor principal values, while incompletelyunderstood, is of fundamental interest. In a pragmatic sensebiphenylene is a possible component in carbonaceous materialssuch as coal and , the chemical-shift tensors ofbiphenylene make an important contribution to the study ofcomplex carbonaceous materials important to industry and has also been used as a model compound in thestudy of antiaromaticity,13-15as this fused aromatic moleculeis the dibenzo derivative of cyclobutadiene, a classic exampleof antiaromaticity that is difficult to study due to thermalinstability.

5 The four-membered ring of biphenylene contains four -electrons, thus satisfying Hu ckel s rule16for an antiaromaticring. Hu ckel s rule predicts that amonocyclicpolyene with (4n+2) electrons will be more stable than the correspondingacyclic analogue because of a degeneracy in the lowest energystate. Conversely, if there are only 4n -electrons in themonocyclic polyene, the molecule will be less stable than theacyclic analogue. Such molecules are called antiaromatic. Theantiaromaticity of the central ring of biphenylene has beendetermined through a variety of methods. Among them isSchleyer s Nucleus Independent Chemical Shift (NICS) scheme,17which has been applied to numerous NICS scheme is a criterion for determining aromatic and antiaromaticcharacter in which one evaluates the isotropic chemical shift atthe nonweighted geometric center of a ring by calculating thechemical shielding at that location and changing its sign.

6 Thechange in sign aligns thedirectionof the NICS and theexperimental chemical shift scales, although they still havedifferent zero points. The NICS value is then interpreted asaromatic (negative) or antiaromatic (positive) depending uponthe sign of the NICS shift. While the central four-member ringof biphenylene has antiaromatic character, the two six-membered8290J. Phys. Chem. A2000,104, CCC: $ 2000 American Chemical SocietyPublished on Web 08/10/2000rings are markedly aromatic in character, which contributes tothe stability of the with previously determined13C chemical-shifttensors of some chemical environments similar to biphenyleneis made to better understand the chemical-shift tensors instrained molecules. Comparison of measured chemical-shifttensors to those calculated at several levels of theory was alsomade to rationalize the assignment of the chemical-shift tensorsto individual crystallographic positions of congruent was purchased from Aldrich andused without further purification.

7 The crystalline form wasverified by comparing an X-ray diffraction spectrum to asimulated spectrum based on the literature crystal NMR experiments were carried out on a CMX-200 NMRspectrometer with a13C Larmor frequency of MHz. The1H T1of the sample was determined by saturation recovery tobe approximately 69 s at a1H frequency of MHz. Toenhance signal intensity, cross polarization21was used with anoptimal contact time of ms. Proton decoupling was carriedout at a field strength of approximately kHz. A FIREMAT experiment7was carried out on the sample. With use of a flip-back pulse, the optimum recycle delay was 60 s. A total of 384scans were collected in each of the eight evolution points sothat the entire experiment time was slightly more than 2 the FIREMAT experiment, the sample spinning speed was480 Hz. The spectral width in the acquisition and evolutiondimensions were 23 040 and 3840 Hz, respectively.

8 Data weretransferred to a Sun computer for processing and spectralanalysis. TIGER processing6of the pseudo-2D data generatedan isotropic guide FID that was then fit as the sum of elevenmodel FIDs. The resulting model was used to extract the FIDsfor each of the resolved spinning sideband patterns that werethen fit to yield principal values for the chemical-shift simulate the spectra of crystallographicallyinequivalent carbons in the experimental portion of this work,a faithful model is needed that captures the degeneraciescommon to congruent carbons in the crystallographic asym-metric unit. The smallest practical model is one in which theasymmetric unit is augmented with the remainder of thefragmented biphenylene molecule, thus forming a two-moleculeasymmetric unit model (AUM). The level of theory used tocalculate the chemical-shift tensors for the AUM was selectedby performing a series of calculations on a single molecule spectra for natural abundance biphenylene.

9 All spectra were derived from one FIREMAT dataset. See text for numbers listed on the right side of the figure indicate the approximate relative intensity of each Chemical-Shift TensorsJ. Phys. Chem. A, Vol. 104, No. 35, 20008291In the single molecule model (SMM), a partial optimizationwas carried out at the B3PW91 level of theory22with Dunning scc-pVDZ basis set23on molecule D from the asymmetric the carbon nuclei in their crystallographic positions,only the proton positions were optimized with Gaussian tensors were calculated for the SMM at theRHF,25MP2,26,27B3 LYP,28,29and B3PW91 levels of shielding calculation on the AUM was performed at theB3PW91 level of theory. All shielding calculations employedthe GIAO shielding calculation was carried outon the SMM at the B3PW91 level of theory with the cc-pVTZbasis set. All other shielding calculations employed the cc-pVDZbasis principal values from the B3PW91/cc-pVTZcalculation were compared against those from the B3PW91/cc-pVDZ to demonstrate the correspondence between thecalculated shieldings with each of these basis sets .

10 For each ofthe four cc-pVDZ calculations on the SMM, the calculatedprincipal shielding values were plotted against the experimentalshift values. The linear regression performed on each plot wasthen used to correlate the calculated chemical shieldings tochemical AUM was partially optimized in the same fashion asSMM above. A single chemical shift calculation was carriedout on AUM at the B3PW91/cc-pVDZ level of theory basedon the chemical-shift tensor results of the SMM for both thechoice of level of theory and the choice of basis set. Calculatedchemical shieldings were correlated to the chemical shift scalein the same fashion as and DiscussionFigure 1 contains the FIREMAT experiment spectra. Thepossibility that some of the lines were due to polymorphs ofbiphenylene was ruled out by comparing the X-ray diffractionto a simulation of a diffraction spectrum based on the crystalstructure.