5153
J. Phys. Chem. 1992, 96,5753-5759
The Relationship between Partial Bond Fixation Induced by Through-Bond and/or Through-Space Perturbations in Nonpianar Benzene Derivatives and 'H Spin-Spin Coupling Constants J. Simon Craw,? Noel S. Hush,*it*tSever Stemhell,f and Charles W. Tanseys Department of Theoretical Chemistry, Department of Biochemistry, and Department of Organic Chemistry, University of Sydney, NS W 2006. Australia (Received: November 1 , 1991; In Final Form: March 12, 1992)
4Ju,-F~c-,,=4J0,3,the spinspin coupling constant over four formal bonds between methyl protons and a proton bonded to
an sp carbon, previously established*" as a probe for bond order, was determined for 6-methyl-1,4-dihydro-1,4methanonaphthalene (1) and for a number of reference compounds 2-5. Selfconsistent field (SCF) calculationsat the 3-21G level for 1-4 predict partial bond fixation in the benzene ring for all molecules in the sense predicted by interpretation of ~ ~ benzene ~~ moiety ?r bond orders were calculated previously-reported photoelectron spectra of closely-related m o I e c ~ l e s . 3The from pseudominimal basis sets obtained by contraction of the double-{ basis. Two types of bond order, one obtained with a nonorthogonal and the other with a Lowdin-orthogonalized pseudominimal basis, are reported. It is found that, as for planar compounds, there is a linear relationshipbetween either type of bond order and 'Joe. However, the correlation coefficients are considerably smaller, owing partly to the small size of the sample and partly to the fact that the range of values is much smaller than in comparisons for planar molecules,*" so that the importance of contributions of terms other than the Fermi contact term and deviations from the McConnell 'average energy" approximationMmay be significant. The partial bond fmtion is attributed to the effects of u through-bond coupling to the attached framework, and, in the case of 1 and 2, additional coupling to the a-linked ?r bonding orbital of the ethylenic group. This constitutes independent NMR evidence for orbital interaction between a-linked ?r orbitals.
1. Introduction
CHART I
There is much current interest in through-bond and throughspace perturbations of molecular electronic structure. In particular, the prediction of Hoffman et al.'S2 of significant perturbation of the x electron structures of conjugated and aromatic molecules by interaction with u or u* "relay" orbitals of appropriate symmetry has been borne out for a large number of systems and through-space and/or through-bond coupling of u systems separated by alkyl bridges is well established. Much of the evidence comes from photoelectron spectroscopy: early work includes, inter alia, study of coupling of nitrogen "lone pairs" in cations of diaza aromatic molecule^^-^ and of benzene moieties in cations of cyclophanes? produced by photoionization. (For surveys, see Gleiter6 and Heilbronner and Maier.') Depending on circumstances, either through-bond or through-space coupling may dominate, and the effects can be either cooperative or opposing. An extensive review of more recent work can be found in ref 8. It is of interest to enquire whether effects arising from through-bond and/or through-space perturbations can be detected by measurement of other physical properties. These perturbations, in favorable circumstances, may cause signifcant changes in bond orders and bond lengths in derivatives of molecules such as benzene, leading to partial bond fixation. One quantity, whose relation to x bond orders in aromatic molecules has been extensively studied, is the orthobenzylic (4JMe,H) spinspin coupling constant determined by NMR ~pectroscopy.~'~ This work describes the results of such measurements for a number of nonplanar hydrocarbon derivatives of benzene and of theoretical calculations of partial bond fixation, bond orders and related properties, and the nature of the correlations between experimental and theoretical quantities. Sections 2 and 3 describe preparation of compounds and experimental methods and results, respectively. Section 4 describes the quantum chemical methods of calculation used and lists results for the four molecules studied in detail. Section 5 discusses the correlation of theory and ex-
1
3
2
4
5
SCHEME I -ON0
I
H p i PdiC
n = 1 (3) n = 2 (4)
'Department of Theoretical Chemistrv. *Department of Biochemistry. 1 Department of Organic Chemistry.
periment, and in the Conclusion (section 6 ) , the extent to which spinspin coupling constants may be useful as an additional probe
0022-365419212096-5753%03.00/0 .. .., . 0 1992 American Chemical Society I
- I
-
~
5754 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
Craw et al. protons. The results obtained were as follows: I. chemical shifts (Hz at 400 MHz): coupling constant (Hz):
lib
iA
R M S error = 0.04 12'
3
-
Figure 1. Carbon atom numbering scheme for compounds 1-4.
TABLE I: 'H-HCoupling Constants for Bonds a, b, and c, Compounds 1-7
compd 1
2 3 4
5 6
I
4JoB (Hz) a b -0.78 -0.60 -0.65 -0.79 -0.74 -0.79 -0.75 -0.75 -0.70 -0.68 -0.71 -0.68 -0.67 -0.66
3Jvic(Hz), C
a/b
1.00 1.03
reference this work this work this work this work this work
7.07 7.08 7.29 7.15 7.65 7.79 7.76
0.77
1.04 1.02
14 14
0.83 0.94
of through-bond and through-space perturbations is considered. 2. Preparation of Compounds The compounds prepared (1-5) are shown in Chart I. Compounds 1,2, and 5 were prepared by the method shown in Scheme I. In each case, 2-amino-5-methylbenzoic acid was treated with isoamyl nitrite to generate the benzyne intermediate which was trapped by the appropriate diene to give the desired adduct. The reaction of anthracene and cyclopentadiene with methylbenzyne gave a single product, but the reaction between benzyne with 1,3-cyclohexadiene gave a number of products, an observation in accord with previous results.I8 Compounds 3 and 4 were prepared by catalytic hydrogenation of 1 and 2, respectively. All compounds gave the spectral data expected from their structure, and the previously unreported compounds 2 and 4 were shown to have the expected molecular composition. The carbon atom numbering scheme is shown in Figure 1. 3. Experimental Results Melting points were determined on a Reichert micro melting point apparatus and are uncorrected. The IH NMR spectra were acquired on Bruker WM, AMX 400, or AMX 600 spectrometers, using 5-mm sample tubes, in the solvent stated, at concentrations of about 5% w/v. Spectra were acquired at 300 K. Tetramethylsilane (TMS) was used as internal standard. Infrared spectra were recorded on a Digilab FTS 20/80 Fourier transform spectrometer from solutions in the solvent indicated. Abbreviations used in the description of the infrared spectra are s, strong; m, medium; w, weak. Mass spectra were measured on an AEI MS902 (modified) mass spectrometer at 70 eV. Peaks are described in terms of mass/charge ratio (m/z) and intensity relative to the base peak. Ultraviolet spectra were recorded on an Hitachi 150-20 spectrophotometer. High-performance liquid chromatography was carried out on a Waters instrument equipped with refractive index and ultraviolet detectors, using a Whatman Partisil 10 column, 22-mm i.d. X 50 cm at flow rates of 13.5 mL/min. The values for the orthobenzylic coupling constants were obtained from samples which had been degassed. The spectra were acquired under conditions of high digital resolution (0.03 Hz), achieved by the use of small spectral widths and large numbers of data points. The free induction decays were also processed to enhance resolution. The data were weighted exponentially until no further improvement in resolution was seen but before distortion effects set in. The coupling constant and chemical shift data for the aromatic and methyl region of compounds (1-4) were obtained using the PANIC (closely related to LAOCOON) program, with irradiation of the signals due to H1 and H4, the benzylic bridge
theoretical number of transitions = 104 2. chemical shifts (Hz at 400 MHz): coupling constants (Hz): R M S error = 0.04
theoretical number of transitions = 104 3. chemical shifts (Hz at 400 MHz): coupling constants (Hz): R M S error = 0.09
theoretical number of transitions = 104 4. chemical shifts (Hz at 600 MHz): coupling constants (Hz): R M S error = 0.06
number of transitions = 112 assigned transitions = 61 The data relevant to the purpose of this paper are summarized in Table I. The C-C bonds 5-6 and 6-7 are referred to as a and b respectively; we are principally concerned here with 4J0B,the 'H spinspin coupling constants between protons at position 6 and protons bonded to carbon at positions 5 and 7 in bonds a and b, respectively. For comparison, previously-determined valuesI4 for 4-methylindane (6) and 4-methyltetralin (7) are also included in the table.
m m 7
6
6-Methyl-1,4-dibydro-l,4methonaphttmlene (1) was prepared by a previously described m e t h ~ d 'in~ 56% . ~ ~ yield. IR u, (CCb) 3070, w; 2986, s; 2937, s; 2869, m; 1467, m; 1305, m; 1226, w; 1112, W;1010 CIII-', W. & (CHCl3) 240 (log e, 3-15), 275 (2.94), 282 nm (2.90). IH NMR (400 MHz, CDCI,): 6 (2.25, m, 1 H, H5), 2.28 (s, 3 H, CH, (6)), 2.31 (m, 1 H, H59, 2.81 (m, 2 H, H1 and H4), 6.73 (dm, J = 7.20 Hz, 1 H, H7), 6.76 (m, 2 H, H2 and H3), 7.06 (m, 1 H, H5), 7.09 (d, J = 7.20 Hz, 1 H, H8). Mass spectrum ( m / z ) : 156 (M', 96), 14 (loo), 128 (23), 115 (31), 77 (16), 66 (20), 63 (12), 51 (13). 6-Methyl-1,2,3,4-tetrahydro-1,4-methanonaphthalene (3) was prepared by a literature method2' in 89% yield. IR umaX(CC14) 3008, w; 2968, s; 2921, m; 2872, m; 1483, w; 1449, w; 1112, w; 819 cm-I, w. A,, (CHCl,): 270 (log e, 3.22), 276 nm (3.20). 'H NMR (400 MHz, CDC13): 6 1.15 (m, 2 H, H3 and H4), 1.49 (m, 1 H, H9), 1.71 (m, 1 H, H99, 1.87 (m, 2 H, H2' and H3'), 2.30 (s, 3 H, CH3 (6)), 3.33 (bs, 2 H, H1 and H4), 6.88 (dd, J = 7.07, 2.24 Hz, 1 H, H7), 7.00 (d, J = 2.24 Hz, 1 H, H5), 7.05 (d, J = 7.07 Hz, 1 H, H8). Mass spectrum ( m / z ) : 158 (M', 38), 143 (18), 130 (loo), 127 (27), 115 (27), 77 (6). CMethyl-l,4-ethano-1,4dihydronaphtbalene(2). 2-Amino- 5methylbenzoic acid (755 mg, 5 mmol) and 1,3-cyclohexadiene (400 mg, 0.48 mL, 5 mmol) were dissolved in acetone (30 mL), and the mixture was added dropwise to a refluxing solution of isoamyl nitrite (644 mg, 0.74 mL, 5.5 mmol) in dichloromethane (60 mL) over 3 h. Following the addition, the mixture was refluxed for a further hour. The mixture was allowed to cool, and the low-boiling solvents were removed under reduced pressure. Water was added and the mixture steam distilled until no trace
Through-Bond and/or Through-Space Perturbations of oily droplets was visible in the distillate. Petroleum ether (20 mL) was added, and the organic phase was extracted and washed with water (2 X 50 mL) and brine (50 mL) and the solvent dried over magnesium sulfate. The brown oil was purified by flash chromatography and the resultant product purified by HPLC to yield 6-methyl-1,Cethano-l,4-dihydronaphthalene(2) (65 mg, 0.4 mmol, 8%). C13H14requires 170.1095, found 107.1098. IR Y,,, (CCI,): 3049, w; 2957, s; 2871, m; 1489, m; 1217, m; 1108, s; 1025 cm-l, s. ,A, (CHCI,): 256 (log c, 3.37), 276 (3.20), 435 nm (2.73). IH NMR (400 MHz, CDC13): b 1.33 and 6 1.52 (m and m, 2 H and 2 H, CH2 groups), 2.30, s, 3 H CH3 (6)), 3.84 (m, 2 H, H1 and H4), 6.45 (m, 2 H, H2 and H3), 6.88 (ddq, J = 7.33, 1.74, -0.69 Hz, 1 H, H7), 6.99 (ddq, J = 1.74,0.65,-0).75 Hz, 1 H, HS), 7.04 (dm, J = 7.33 Hz, 1 H, H8). Mass spectrum (mfz): 170 (M', 13), 155 (S), 143 ( l l ) , 142 (loo), 141 (34), 128 (S), 115 (13). dMethyl-l,2,3,4-tetrahydro-1,4-ethanonaphthalene(4). 6Methyl-1,4-ethano-l,4dihydronaphthalene (2) (35 mg, 0.2 m o l ) was dissolved in ethanol (20 mL), and Pd/C catalyst (lo%, 20 mg)was added. The mixture was stirred under 1 atm of hydrogen for 2 h and then filtered, and the solvent was removed under reduced pressure to yield 6-methyl-l,4-ethano- 1,2,3,4-tetrahydronaphthalene (4)(27 mg, 0.16 mmol,76%). C13H16 requires 172.1252, found 172.1270. IR,v (CCb): 3009, w; 2943, s; 2866, m; 1702, w; 1620, w; 1491, w; 1451, w; 1135 cm-', w. A, (CHC13): 259 (log t, 2-96),266 (3.03), 273 (2.99), 420 nm (1.62). 'H NMR (600 MHz, CD2C12): 6 1.33 (m, 4 H) and 1.52 (m, 4 H) (CHI groups), 2.32 (s, 3 H, CH3 (6)), 2.92 (m, 2 H, H1 and H4), 6.95 (m, 1 H, HS), 6.96 (m, 1 H, H7), 7.00 (m, 1 H, H8). Mass spectrum (m/z): 172 (M', 52), 144 (53), 143 (73), 129 (31), 128 (24), 115 (4), 32 (24), 28 (100). 2-Methyltriptycew (5) was prepared by a previously described method.22 A- 272 (log c, 3.70) nm, 280 (3.74). 'H NMR (400 MHz, CDC13): 6 2.24 (s, 3 H, CH3 (2)), 5.36 (s, 1 H, H13 or H14), 5.39 (s, 1 H, H14 or H13), 6.78 (dd, J = 7.65, 1.55 Hz, 1 H, H3), 6.98 (m, 4 H, H6, H7, H10, H l l ) , 7.21 (bs, 1 H, Hl), 7.25 (d, J = 7.65 Hz, 1 H, H4), 7.36 (m, 4 H, H5, H8, H9, H12). Mass spectrum (mfz): 269 (22), 268 (93), 267 (42), 254 (20), 253 (loo), 252 (58), 127 (16), 126 (25). 4. Theoretical Methods and Results Molecular orbital calculations were carried out for the molecules 1-4 for which (as discussed in Section 5) there is independent evidence of modification of the benzene A electron structure by through-bond and/or through-space perturbation. These were fully optimized at the ab initio self-consistent field (SCF) level, with the HONDO suite of programs,23utilizing the split valence 3-21G basis set of Binkley et al.24 The tolerance on the gradient is that all Cartesian components should be smaller than 0.0005 au. This provides bond lengths (angles) converged to the third (first) decimal place, and an estimate of the fourth (second) decimal place. In all cases the total energy between the penultimate and ultimate optimization steps changed by less than 10" Hartree. These criteria are believed to be sufficient to enable the difference between the average bond length and the actual bond lengths in the phenyl ring, to be deduced. It is assumed that the absolute error in the actual bond lengths will cancel with the corresponding error in the average, to produce a difference accurately determined in the third decimal place. Before defining what is meant by bond order, and in particular u and A bond order, it should be noted that, strictly speaking, there is no U / A separability in these systems, as the molecules lack any symmetry. After the optimizations it was found that the benzene ring itself is predicted to be slightly nonplanar ( l o at most). The effects of perturbation of the benzene 77 system are seen in the mixing, increasingly extensive as the A orbital energy is lowered, with the u orbitals of the attached framework and for 1 and 2 of the 77 orbital of the ethylenic group. This mixing was then minimized by rotating the molecule so as to position the benzene ring as closely as possible in the xy plane. A single point calculation was then performed to obtain the eigenvectors with the minimal possible 6/77 mixing in the relevant bonding molecular
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5755 TABLE II: Bod Distances and Angles for Compounds 1-4'
compound 1
3
2
4
Bonds, A RSJO R5,6 R6,2 R6,1
4 . 8
Rs,9 R9JO R1.9 R4.10
R7.11b
RBJh R1,2 R4.3 R2.3
1.364 1.406 1.518 1.378 1.402 1.364 1.401 1.537 1.537 1.564 1.564 1.545 1.545 1.320
1.373 1.396 1.518 1.382 1.393 1.374 1.392 1.519 1.518 1.566 1.566 1.523 1.522 1.317
1.369 1.40 1 1.518 1.382 1.397 1.369 1.398 1.522 1.522 1.555 1.555 1.562 1.562 1.570
1.377 1.394 1.518 1.384 1.390 1.377 1.392 1.510 1.510 1.550 1.549 1.549 1.549 1.561
Angles, deg 810.9,' 89,10.4 e10.4,3 e9.1.2
~lO.ll,lln 89,l . I I b e4.3.2
e1.2.3
~l,llb.lla 84.1 la,l I b
106.4 106.4 105.1 105.2 98.7 98.7 107.9 107.9 93.0 93.0
113.0 113.0 107.8 107.8 106.0 106.0 114.4 114.5 109.0 109.0
106.9 106.8 105.8 105.9 100.1 100.0 103.2 103.1 94.3 94.3
113.6 113.5 107.7 107.7 108.3 108.4 109.6 109.5 109.6 109.6
"Calculated at the SCF 3-21G level. TABLE IIk Total Bond Orders for Compouods 1-4a
compound bond
1
2
3
4
5-10 5-6 6-7 7-8 8-9 9-10
1.524 1.317 1.479 1.339 1.518 1.284
1.474 1.354 1.442 1.376 1.468 1.320
1.496 1.343 1.453 1.365 1.489 1.292
1.462 1.366 1.430 1.389 1.456 1.318
'Calculated at the SCF 3-21G level. orbitals, i.e. those which closely resemble the azuand elgorbitals of benzene. Bond orders, in the traditional sense, are uniquely defined only for a minimal basis set.25 To obtain a set of 77 bond orders for the benzene ring, the split valence basis set was first contracted down to a minimal basis set size (Le. five basis functions on the carbons and one on the hydrogens), using the method described by Ahlrichs et al.26,27The A bond orders were then obtained as the relevant ij(th) elements of the density matrix in the minimal basis. Two tYpes of SCF A bond order qpp!between centers p and p' are reported here. Firstly in the nonorthogonal minimal basis, qJSCF),,, and secondly in a Lowdin orthogonalized2*minimal basis set, q&(SCF),. Total bond orders are calculated using the scheme suggested by Villar and D u p u i ~ . ~ ~ The calculated geometries for molecules 1 4 are shown in Table I1 with the atom numbering scheme given in Figure 1.30 The predicted partial bond fmtion in the benzene ring is best illustrated by obtaining the difference of each bond length from the average value. The average is 1.386 A for 1,3,and 4 and almost exactly the same (1.385 A) for 2. The results are given in Figure 2. Partial bond fixation is predicted for all four molecules. For 1, the average absolute deviation from the average is 0.017 A; this is reduced by 30% in the related structure 3 in which the vinyl group is absent. For structure 2, average absolute deviation is smaller than for 1, 0.008 A, and the related structure 4 shows a comparable decrease (24%) accompanying saturation of the vinyl group. Calculated total bond orders are shown in Table 111. In Tables IV and V, the A bond orders qp (SCF),, and V ~ ~ ( S C F ) ~ obtained using the contracted nonortgogonal basis set and the Lowdin-orthogonalized basis sets, respectively, are shown.
5756 The Journal of Physical Chemistry, Vol. 96, No. 14, I992
Craw et al.
TABLE V bond 5-10 5-6 6-7 7-8 8-9 9-10
2
1
T
Bond orders n(SCF),, for C o m d 1-4" 1 0.706 0.597 0.698 0.592 0.707 0.583
2 0.686 0.618 0.679 0.636 0.685 0.607
4 0.678 0.633 0.671 0.644 0.677 0.616
3 0.683 0.61 1 0.686 0.618 0.688 0.602
"Calculated at the SCF 3-21G level, with subsequent contraction and Uwdin orthogonalization (Lo) of basis to a pseudominimal basis set (see text). 1/43
3
4
Figure 2. Partial bond fixation, calculated at the SCF 3-21G level, in benzene rings of compounds 1-4. The differences from the average bond length are shown; the average bond length is identical for 1, 3, and 4 (1.386 A) and marginally smaller (by 0.001 A) in 2. TABLE Yy: r Bond Orders dSCF), for Compollnds 1-4" compound bond 1 2 3 4 5-10 5-6 6-7 7-8 8-9 9-10
0.5 13 0.402 0.510 0.385 0.514 0.419
0.490 0.426 0.488 0.424 0.49 1 0.443
0.492 0.4 19 0.495 0.409 0.496 0.439
0.482 0.440 0.480 0.433 0.482 0.449
"Calculated at the SCF 3-21G level, with a subsequent contraction of the basis set to a nonorthogonal (no) pseudominimal basis set (see text). 5. Discussion (i) -Spectrascopl 'c Evidence for OrbitalI n t e " Photoelectron spectra have been interpreted to give quantitative accounts of energy level shifts in systems with potential through-bond and/or through-space long-range orbital interactions. Two molecules so studied are the unmethylated analogues of 1 and 3, i.e. benzonorbornadiene (8) and benzonorbornene (9), respecti~ely.~~A further pair of substituted benzenes with
0
9
identical symmetries, which have also been studied by photoelectron spectroscopy?z are hexahydrodimethanoanthracene(10) and octahydrodimethanoanthracene(11).
10
11
The three bonding r orbitals of benzene are the most strongly bound, Phl (laz,), and the degenerate pair (Pb, Ph,) (le$, illustrated in Figure 3. In benzene itself, the first ionization potential I , corresponds to removal of an electron from the degenerate HOMO pair (Ph,, Ph,). Calculation of Z133at the Koopmans theorem/SCF using experimental geometry and a contraction of Dunning's ( 9 ~ , 5 p ; 4 s basis ) ~ ~ set contracted to a (4s,2p;2s) set yielded the value of 9.24 eV, in good agreement with experiment?6 9.3 eV, and an independent calculation by von Niessen et The deeplying ( 1a2,(r))-I state is calculated to lie 4.5 eV below this; according to the assignments of Koch and
Figure 3. The bonding 7r orbitals of benzene. Left to right: Ph1(lazu in D6* symmetry), the most strongly bound; Ph, and Ph,, the degenerate lel, pair.
n
Y t d, =S
Figure 4. Schematic diagram of the HOMO of 10, showing coupling of Phl and Ph, orbitals of benzene both by u through-bond coupling to framework u orbitals and vinyl 7r orbital. Mixing is qualitatively similar in 11, but smaller owing to absence of coupling to vinyl r orbital. (from ref 32). Note finite r amplitude at carbon 5 (and 8) consistent with decrease of 5-10 bond length and increase of 5-6 (a) bond length.
Otto38and B e r k ~ w i t zthe ~ ~experimental difference is smaller, 3.1 eV. A u-ionized state eg(u))-I is predicted to lie 0.3 eV above the deepest x-l state; a s ~ i g n m e n tof~ ~the , ~experimental ~ spectrum is in reasonable agreement, placing the u-l state 0.8 eV above the deepest d state. The substituted benzenes 8-11 have C, symmetry. In this symmetry, the u orbitals Ph, and Phl and the 3eg(u) orbital all belong to the same irreducible representation, a'; thus in the substituted molecules admixture of these orbitals is anticipated. As a result, the HOMO level is a modified Ph,, and the out-ofphase Phl component contributes negative amplitude to all six ring centers, This is the primary reason for the pattern of partial bond fuation calculated for 1 4 in Figure 2. The mechanism of this orbital mixing is through-bond and/or through-space coupling with the u orbtials (and for 1 and 2 the ethylenic orbital) of the linked system.38 This is shown schematically for the HOMO of 10 in Figure 4 from an earlier c a l ~ u l a t i o n .For ~ ~ the corresponding HOMO of the molecule 11, in which the ethylene moiety is absent, the structure is qualitatively similar but with smaller admixture of Phl into Ph,. Qualitatively similar patterns were also calculated for the HOMO of the benzene moiety in 8 and 9 and 1 4 (the last four exhibiting asymmetry with respect to reflection in the plane of symmetry present in 6-9 owing to the presence of the methyl group). An experimental measure of the effects of orbital interaction is provided by the shifts of photoionization potential. For 9-11 the first 2 or 3 experimentalpotentials are as shown in Table VI. For the HOMO level, u through-bond interaction results in twice the energy destabilization for the longer alkyl framework of 11 relative to 9. The additional coupling to the ethylene system increases the destabilization by 50% for the larger benzeneethylene separation in 10 and by 100% with the shorter separation in 8, where through-spaceinteraction is anticipated to dominate.
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5151
Through-Bond and/or Through-Space Perturbations TABLE VI: Experimental Ioniutioll Potentialsa 11
9
-1.1013*x-0.0902 1
8
10
I](Ph,)-1 8.45 (4.12) 8.33 (4.24) 8.32 (-0.25) 8.22 (4.35) IZ(Ph,)-l 8.95 (4.14) 8.83 (4.26) 9.03 (4.06) 8.79 (4.30) IdT)-l
9.356
.
2 3
+ 0
I
K
-
-0.5
9.19 -0.6
‘8, 9: ref 31. 10, 11: ref 32, with assignments indicating largest
benzene (Ph,, Ph,) or ethylene ( T ) orbital components. Values in parentheses indicate the lowering of I. with respect to corresponding data for o-xylene, which provides an approximate measure of decrease due to coupling of benzene T system with u system of 9 and 11, and with u and x systems of 8 and 10. bThe photoelectron spectrum shows bands at 9.35 and 9.5 eV. Reasons for assigning the former rather than the latter to the ionization are discussed in ref 32.
The additional effect of A coupling 0.1 1 and 0.13 eV, respectively) is in good agreement with calculation (0.0932 and 0.13 eV (this work), respectively). We conclude from this that the prediction of the SCF calculations of significant perturbations to the benzene A structure by both saturated linkages and linkages containing an ethylene group is verified by photoelectron measurements. This strongly suggests that prediction of partial bond fixation due to these perturbations is reliable. (i) Interprecstionof tbe Cwpliag Coostanb. The interpretation of H-H nuclear spin-spin coupling constants has been pursued intensively. Most theoretical approaches are based on the second-order perturbation approach of Ramsey;40in particular, it is assumed that the H-H’ coupling constants can be accounted for adequately by considering only the Fermi contact term. In all such approaches, where only the Fermi contact term41-43is considered, the coupling constant JHHT can be written as
I
-1.1
0.2
0.25
0.3
0.35
0.45
0.4
0.5
0.55
0.6
0.65
I 0.7
bond ordera
Figure 5. Dependence of ‘Joe coupling constant of a and b bonds of compounds 1-4 on square of r bond order ?(SCF),, (see text for definition). -1.4777*x+0.2280
-
-0.5
-0.6
where AH is given by AH = (16Afih/3)7H4hZ(H)
(2)
where fi is the Bohr magneton, */H is the magnetogyric ratio of the proton, and A2(H) is the 1s electron density at proton H. The constant AH, is similarly defined. The term F H H ~is the spincoupling function. It is the form assumed for this function (and possible modificationsof the constants AH) that distinguishes the several themetical approaches to the interpretation of the coupling constants. The approach which has been most widely applied makes the “average energy assumption” introduced by McConnelLU In this approximation, the energy denominators in the second-order perturbation expression are replaced by a constant AE, so that the spin-coupling function F H H t can then be expressed as where q h h , is the HH’ bond order. The A electron contribution to JHH,,J-HH,, is assumed to be the dominant term. This can be related“ to experimental ESR hyperfine coupling constants uH, UH‘ JrHH,
= h(&!?)-’UHUHr(~ppt)2
(4)
where qpptis now the A electron bond order associated with the p~ atomic orbitals p and p‘ on the carbon centers attached to the coupled protons. Owing to the different signs of the hyperfine coupling constants, the spinspin coupling constants are predicted to be negative, as observed. In the independent electron Huckel approach, qpp‘is given by the Coulson-Longuet-Higgins formula4s qPp4Huckel)
= Cn,cipcip, i
(5)
in which the sum is over occupied molecular orbitals i with coefficients cipand cig, respectively, of PA atomic orbitals p and p’. In the SCF formulation, it becomes
where SPpr is the p r p r overlap integral. In the independent-
..-
0.4
0.41
0.5
0.55
0.65
0.6
0.7
0.15
0.8
0.05
0.9
bond order’
Figure 6. Dependence of 4Joe coupling constant of a and b bonds of compounds 1-4 on square of r bond order q(SCF), (see text for definition).
electron approximation, it is also possible to approximate JwHt without introducing the average energy assumption, in terms of the mutual atom-atom polarizability rpgbetween centers p and P’?~ However, such a correlation is not possible at the SCF level. In a detailed study,” the correlation between 4JoBand V(SCF)~ for a wide range of methyl-substitutedhydrocarbon molecules has been made. Over the range of 0-3 in bond order, the correlation is found to be
4J0B(Hz) = -0.056
- 2.457?(SCF)’
(7)
with correlation coefficient 9 = 0.9844. A similar correlation of coupling constant with q itself yields
4JoB(Hz) = -0.315 - 2.112q(SCF) (8) with correlation coefficient 9 = 0.9708. This latter correlation has no theoretical foundation, but is useful as an empirical relationship. The above relationships have been obtained for planar molecules. As mentioned above, there is strictly speaking no u / r separability in the nonplanar molecules considered here. However, the bond orders q(SCF), and p(SCF), defined in Section 4 can be employed to investigate the relationship between 4J0Band squared bond order for the a and b bonds of the molecules 1 4 . This is shown in Figures 5 and 6. It is evident that for both comparisons there is a trend in the expected direction, with quite small ordinate intercepts for 7 = 0. The least-squares fits are respectively 4JoB(Hz) = -0.412 - 1.514q(SCF),,; (9) 4J0B
(Hz) = -0.252 - 1.133?(SCF)L;
(10)
57%
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
1.65
1
1.55
:.2
1
1.35
1.36
1.37
1.38 1.39 bond length
1.4
1.41
1
L.42
Figure 7. Total bond order-bond length (A) relationship for compounds 1-4.
with correlation coefficients 3 = 0.650and 0.662,respectively. Relationships with similar correlation coefficients obtain for the plots of coupling constant against the bond order itself. As,over this small range of bond order, the bond order-bond length relationship is linear, the experimental coupling constants are in overall agreement with the calculated trends of partial bond fixation in the benzene ring, thus providing NMR evidence in addition to the photoelectron evidence for operation of throughbond and/or through-space orbital interaction perturbations in these molecules. Owing to the small size.of the sample, it is not possible to discuss the scatter in Figure 5 and 6 in any detail. However, there is one anomalous feature: this is the value of the coupling constant for the a bonds in the molecules with saturated linkages, and which show the largest deviations from the anticipated trend. The resulting differences between coupling constants for the a and b bonds is significantly smaller here (0.05and 0) than is predicted by the calculations. The decrease of the first ionization potential of 3 and 4 when the terminal bridging bond 2-3 becomes unsaturated to yield 1 and 2, respectively, can be estimated from the calculated shifts of the HOMO levels. Using the Koopmans a ~ s u m p t i o n this ,~~ is -0.13 and -0.05 eV respectively. These values point to a decrease of coupling between ring and vinyl T electrons when the u bridge has the structure of 2 instead of that of 1, consistent with the conclusions from relative magnitudes of the coupling constants. 6. Conclusion
The interpretation of proton spin-spin coupling constants in aromatic molecules linked to nonplanar frameworks encounters difficulties owing to the lack of u / a separability. In addition, since the concept of bond order is not uniquely defined when (as in this work) other than minimal basis sets are used in the SCF calculations, contractions of the sets used down to minimal basis size is necessary in order to obtain bond orders which can be used to test for correlations with coupling constants analogous to those well established at the minimal basis set level for planar hydrocarbon molecules. Two methods of achieving this are described. For the molecules of the type examined in this preliminary study, the evidence for through-bond and or through-space perturbation of the benzene T system, previously obtained from photoelectron spectroscopy, is confinned by general trends of ‘Joe with calculated bond order and, by implication, confirms the predicted partial bond fmtion in the benzene ring induced by this interaction. However, further work is required in order to account for some anomalously large quantitative deviations from the average correlations. It is not likely that changes in lateral strain are responsible for these, as it has been shownI4that ‘JOB is quite insensitive to this. Out of plane distortions of the benzene ring (which are in any case very small for the molecules discussed) will also have negligible influence, as ‘Joe is remarkably insensitive even to drastic distortions of this type.” The adequacy of McConnell’s ”average
Craw et al. energy” assumption and the importance of electron correlation effects with respect to the SCF predictions of bond fixation for nonplanar systems of the type considered here are topics worthy of future study. In addition, the adequacy of the assumption of attributing variations in spin-spin coupling constants to those of the Fermi contact term alone may be questioned where such comparatively small changes are in question. Estimates of the contributions of the remaining terms ignored in such correlati~ns~~ (spin-dipole, paramagnetic spin-orbit, and diamagnetic spin-orbit) would also be valuable; however, these cannot be made for nonplanar molecules of this size until reliable calculationshave been carried out for smaller, planar systems.
Acknowledgment. We are grateful for support from the Australian Research Council and for several helpful discussions with Dr. G. B. Bacskay.
NO. 1, 4897-73-8; 2, 141319-46-2; 3, 16499-70-0; 4, 141319-47-3; 5, 19399-53-2; 2-amino-5-methylbenic acid, 2941-78-8; anthracene, 120- 12-7; 1,3-cyclopentadiene, 542-92-7; 1,3cyclohexadiene, 592-57-4. References and Notes (1) Hoffmann, R.; Imamura, A.; Hehre, W. J. J . Am. Chem. SOC.1968, 90, 1499. ( 2 ) Hoffmann, R. Acc. Chem. Res. 1971, 4, 1, and references therein, (3) Gleiter, R.; Heilbronner, E.; Homung, V. Helu. Chem. Acra 1972,55, 255. (4) Hush, N. S.;Cheung, A. S.;Hilton, P. R. J . Electron Spcrrsc. Relar. Phenom. 1975, 7, 385. (5) Gleiter, R. Tetrahedron h r r . 1969,4453. Heilbronner, E.; Maier, J. P.; Helu. Chim. Acta 1974, 57, 151. (6) Gleiter, R. Angew. Chem., Inr. Ed. Engl. 1974, 13, 69. (7) Heilbronner, E.; Maier, J. P. In Electron Spectroscopy; Brundle, C. R., Baker, A. D., as. Academic: ; New York, 1977; Vol. 1, p 205. (8) Paddon-Row, M. N. Angew. Chem., Inr. Ed. Engl. 1983, 22, 245. (9) Collins, M. J.; Hatton, P. M.; Sternhell, S.;Tansey, C. W. Magn. Reson. Chem. 1987, 25, 824. (10) Barfield, M.; Fallick, C. J.; Hata, K.; Sternhell, S. A.; Westerman, P. W. J . Am. Chem. Soc. 1983, 105, 2178. (1 1) (a) Barfield, M.; Collins, M. J.; Gready, J. E.; Sternhell, S.;Tansey, C. W. J. Am. Chem. SOC.1989, 111,4285. (b) Collins, M. J.; Sternhell, S., unpublished results. (12) Barfield, M.; Collins, M. J.; Gready, J. E.; Hatton, P. M.; Stemhell, S.; Tansey, C. W. Pure Appl. Chem. 1990, 62,463. (13) Collins, M. J.; Stemhell, S.;Tansey, C. W. Aust. J . Chem. 1990,43, 1541. (14) Collins, M. J.; Gready, J. E.; Sternhell, S.;Tansey, C. W. Ausr. J. Chem. 1990, 43, 1547. (15) Sternhell, S.;Tansey, C. W. Ausr. J . Chem. 1990, 43, 1577. (16) Gready, J. E.; Hata, K.; Sternhell, S.;Tansey, C. W. Ausr. J . Chem. 1990,43,593. (17) Gready, J. E.; Hambley, T. W.; Kakiuchi, K.; Kobiro, K.; Sternhell, S.;Tansey, C. W.; Tobe, Y. J. Am. Chem. Soc. 1990, 112, 7537. (18) Crews, P.; Beard, J. J . Org. Chem. 1973, 38, 522. (19) Friedman, L.;Logullo, F. M. J . Org. Chem. 1969, 34, 3089. (20) Tanida, H.; Muneyuki, R.; Tsuji, T. Bull. Chem. Soc. Jpn. 1964,37, 40. (21) Inamoto, N.; Masuda, S.; Tori, K.; Aono, K.; Tanida, H. Can. J . Chem. 1967,45, 1185. (22) Friedman, L.;Logullo, F. M. J. Am. Chem. Soc. 1963, 85, 1549. (23) Dupuis, M.; Rys, J.; King, H. F. J . Chem. Phys. 1976, 65, 111. (24) Binkley, J. S.;Pople. J. A,; Hehre, W. J. J . Am. Chem. SOC.1980, 102, 939. (25) Gready, J. E. J. Compur. Chem. 1984, 5 , 411. (26) Heinzmann, R.; Ahlrichs, R. Theor. Chim. Acra 1976, 42, 33. (27) Ehrardt, C.; Ahlrichs, R. Theor. Chim. Acra 1985, 68, 231. (28) Lowdin. P.-0. Adu. Quanr. Chem. 1970, 5, 185. (29) Villar, H. 0.;Dupuis, M. Chem. Phys. Lerr. 1987, 142, 59. (30) Professor M. N. Paddon-Row (University of New South Wales) has kindly informed us of his unpublished calculations on 1 and 3, which also indicate partial bond fixation in the benzene ring in these molecules. (31) (a) Haselbach, E.; Rossi, M. Helu. Chim. Acra 1976, 59, 278. (b) Domelsmith, L.N.; Mollere, P. D.; Houk, K. N.; Hahn, R. C.; Johnson, R. P. J . Am. Chem. Soc. 1978, 100, 2959. (32) Hush, N. S.; Willett, G. D.; Paddon-Row, M. N.; Patney, H. K.; Peel, L. B. J. Chem. Soc., Perkin Trans. 2 1986, 827 and references therein. (33) Kilwyne, D. A. L.; Nordholm, S.; Hush, N. S. Chem. Phys. 1986, 107. 255.
(34)-Koopmans, T. Physika 1934, 1, 104. (35) Dunning, T. H., Jr. J . Chem. Phys. 1970,53, 2823. (36)’Bieri, G.; Asbrink, L. J. Electron Specrrosc. 1980, 20, 149 and references therein. (37) von Niessen, W.; Cederbaum, L. S.; Kraemer, W. P. J. Chem. Phys. 1976, 65, 1378.
J. Phys. Chem. 1992, 96, 5159-5165 (38) Koch, E. E.; Otto, A. Chem. Phys. Lett. 1972, 12,416. (39) Berkowitz, J. Photoabsorption, photoionization and photoelectron spectroscopy; Academic Press: New York, 1981. (40) Ramsey, N. F. Phys. Rev. 1953, 91, 303.
(41) Barfield, M.; Grant, D. M. Adv. Magn. Reson. 1965, I, 149.
5759
(42) Murrell, J. N. Prog. Nucl. Ma@. Reson. Spectrosc. 1971, 6, 1. (43) Barfield, M.; Spear,R. J.; Sternhell, S. Chem. Rev. 1976, 76, 593. (44) McConnell, H. M. J . Mol. Spectrosc. 1957, 1, 11. (45) Coulson, C. A.; Longuet-Higgins, H. C. h o c . R . SOC.London Ser. A 1947, 191, 39; 1948, 193,441.
Chlral Recognltlon and Molecular Interaction in Cellulose Derivatives Kenji Itoh; Tomiki Ikeda,*" Shigeo Tazuke,*and Tohru Shibatal Research Laboratory of Resources Utilization, Tokyo Institute of Technology. 4259, Nagatsuta, Midori-ku, Yokohama 227, Japan, and Research Center, Daicel Chemical Industries, Ltd., 1239, Shinzaike, Aboshi- ku, Himeji 671 - 12, Japan (Received: November 12, 1991; In Final Form: February 25, 1992) Nineteen 1,3-bis(aryl)propane derivatives and six model compounds with a single chromophore were used to explore the chiral discrimination ability of cellulose derivatives, in particular cellulose triphenylcarbamate (CTC), in connection with the mode of interaction of theseprobe molecules with adsorbents. The interaction was evaluated by conformational analysis of the probe molecules and circular dichroism. The 1,3-bis(aryl)propaneswith 9-anthryl moieties possessed a highly limited conformation owing to the bulky substituent and hardly changed their shape on CTC as evidenced by CD spectra; thus, they may be regarded as "rigid" substrates. However, these "rigid" substrates were resolved quite effectively. On the other hand, those with 2-naphthyl moieties possessed a number of stable conformationsand could change their shape in diastereomeric complexes on CTC; thus, they may be assumed as "soft" substrates, against which the chiral discrimination was inefficient. The present study revealed that the optical resolution may be interpreted at least partly in terms of the "rigid" and "soft" concept.
Introduction Molecules in vivo are optically active whenever chiral atoms are involved and much study has been performed on stereochemistry of biological molecules. Stereochemistry is closely related to optical resolution of racemates. Many methods of optical resolution have been developed since the first discovery of 'spontaneous resolution by crystallization" by Pasteur for tartaric acid: resolution via diastereomer formation, enzymatic optical activation, kinetic resolution, chromatographicresolution, and asymmetric synthesis.' Among these methods, the chromatographic resolution may be the most convenient and time-saving method to resolve racemates. Diastereomeric complexes are formed on optically active adsorbents and their different stabilities lead to different rates of elution of the two relevant enantiomers.* Resolution of mandelic acid,3 metal complexes? and biphenyl derivativess on Sephadex, cellulose, and starch as adsorbents was reported, but in these early works there were problems in terms of versatility and efficiency of resolution. Recent progress in high-performance liquid chromatography (HPLC) has enabled diastereomeric complexes with energy difference as small as 10-20 cal/mol to be separated with high efficiency. In the 1980s, many chiral stationary phases were examined for their chiral discrimination ability: (1) chiral polymers; (2) chiral polymers adsorbed onto silica gels; (3) chiral small molecules adsorbed onto silica gels.6 The ability of chiral discrimination of the stationary phases composed of (3) can be anticipated on the basis of the chiral recognition ability of the chiral small molecules on the adsorbents. On the other hand, the ability of chiral discrimination of the polymers depends strongly on the second or higher-order structures, thereby a higher ability may be expected for the polymer stationary phases.6 Polymers so far reported as potential chiral adsorbents are proteins, cellulose and other polysaccharide derivatives, optically active polyacrylamides, and poly(triphenylmethy1 methacrylate): Bovine serum albumin adsorbed onto silica gels was found to exhibit an effective resolution against amino acids in buffer solutions as eluent and many medicinal compounds were suocessfully resolved on this c o l ~ m n . ~Polypeptides are known to show Author to whom correspondence should be addressed. Tokyo Institute of Technology. 'Deceased July 11, 1989. 1 Daicel Chemical Industries, Ltd.
higher-order structures like a-helix, and the correlation between this higher-order structure and the chiral discrimination ability was investigated for poly(N-benzyl-L-glutamine)grafted onto polystyrene beads.6 It was found that the helix structure was essential for the chiral recognition. Hesse and Hagel reported that "microcrystalline cellulose triacetate (MCCTA)" prepared by acetylation of cellulose under heterogeneous condition showed a remarkable resolution, particularly against aromatic racemates, and they ascribed this high ability of chiral recognition to the microcrystalline ~ t r u c t u r e . ~ Thus, reprecipitated MCCTA from organic solvents (CTA 11) reportedly lost partially or completely the original However, it was found later that the crystallinity of CTA I1 was not directly related to the ability of chiral recognition.I0 CTA I1 supported on macroporous silica gels was successfully used in chromatographic resolution.'*I2 The successful example of CTA stimulated the exploration of other cellulose derivatives as chiral adsorbents. Cellulose triesters such as benzoate, phenylcarbamate (CTC), methylcarbamate, and cinnamate were examined and were found to exhibit an excellent ability of chiral recogniti~n.'*'~ Among these cellulose triesters, CTC generally shows a higher ability of chiral discrimination against a wider range of r a c e m a t e ~ . ' ~ J Okamoto ~J~ et al. investigated extensively the ability of chiral recognition of CTC with various substituents on phenyl rings.'s Substitution at the para position of the phenyl ring with methyl, ethyl, and halogen resulted in a higher ability of resolution while substitution with methoxy and nitro groups led to a lower ability. Owing to the carbamate group ( - N H C ( 4 ) O - ) , the ability to form hydrogen bonding with substrates is expected to be much enhanced in CTC and this hydrogen bond formation has been considered as an origin of the higher ability of chiral recognition of CTC over a wider range of substrates. Although the chromatographic resolution of racemates on cellulosic adsorbents may be one of the best methods to obtain optically pure substrates in view of versatility, high resolution ability, and wide applicability, the mechanism of resolution on cellulosic adsorbents still remains obscure.I6 Aromatic racemates are resolved better than nonaromatic racemates. Furthermore, as a general tendency, molecules with a rigid structure seem to be resolved more effectively, although there are many exceptions like cyclic compounds; cyclization of acyclic molecules often leads to a loss of separation.I6
QQ22-3654I92 12O96-5759SO3.OO ._ ..,I O . 0 1992 American Chemical Society I
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