2 Physical Methods Part (iii) Theoretical Organic Chemistry and ESCA ~~~~~ -By D. T. CLARK Department of Chemistry University of Durham 1 Introduction In such rapidly expanding fields it is inevitable and indeed desirable that in a report of this size there has to be a certain amount of selection. As far as theoreti- cal organic chemistry is concerned reasonably complete coverage of develop- ments in applications of non-empirical treatments has been made together with particularly important semi-empirical treatments. The trend clearly evident in last year’s report of detailed studies of systems of real chemical importance rather than yet another calculation of the barrier to rotation in ethane has been main- tained. This is typified for example in studies of the protonation of benzene and the investigation of bridged us.classical ions in the ethylenebenzenium-phenyl-ethyl cation system. A fairly complete coverage is also given of theoretical and experimental aspects of ESCA applied to organic chemistry. This exciting field is continuing to develop rapidly and applications reported range from the study of non-classical ions to structural isomerisms in co-polymers. 2 Theoretical Organic Chemistry Barriers to Rotation.-Neutral Molecules. The successful interpretation of barriers to internal rotation about carbon-carbon single bonds within the Hartree-Fock formalism suggests strongly that correlation energy effects are small. It is nice to have this confirmed and Clementi and Popkie2 have investi- gated this in some detail for ethane.Using a very large basis set GTO (C 12s 7p Id; H 6s 2p) CGTO (C 6s 3p Id; H 3s 2p) approaching the Hartree-Fock limit the computed barrier to rotation is in good agreement with experiment. Electron correlation taken into account by making use of Wigner’s formula relating the electron density distribution to the correlation energy is closely similar for the eclipsid and staggered conformers and hence makes little contribution to the barrier. ’ Cf.Ann. Reports (B) 1971 68 43. E. Clementi and H. Popkie J. Chem. Phys. 1972,57,4870. 40 Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA The barrier to rotation about the central bond in buta-1,3-diene has been investigated3 in order to study the importance of electron correlation in rota- tion about bonds with some double-bond character.The basis set consisted of GTO (7s 3p for C 3s for H) CGTO (2s lp for C 1s for H) and part of the correla- tion energy may be taken into account by a second-order perturbation treatment. Rigid rotation was assumed and the results are shown in Figure 1. O 0O 45 90O 135" I goo Torsional nngle Figure 1 Potential energy for the internal rotation about the central C-C bond of buta-1,3-diene. Curve a corresponds to the SCF calculations curve b to the SCF plus correlation results. The zero reference values are -154.4643 and -154.7309 a.u.for curves a and b respectively (s-trans conformation) (Reproduced by permission from Chem. Phys. Letters 1972 13 249) It is clear that correlation effects modify the potential energy curve to quite a small extent except in the vicinity of the second minimum corresponding to torsion angles of 145"and 157" in the SCF and perturbation treatments respec- tively being 2.60 and 1.68 kcal mol-' above the trans energy respectively U.Pincelli B. Cadioli and B. Levy Chem. Phys. Letters 1972 13,249. D.T. Clark (observed 2.3 kcal mol- I). This all suggests that rotation about essential single bonds is quite adequately dealt with within the Hartree-Fock formalism. In an extensive series of studies4 Pople and co-workers have examined internal rotation in a large number of systems of considerable importance to organic chemists uiz. C-C C-N C-0 N-N N-0 and 0-0 bonds.The molecules studied were ethane methylamine methanol hydrazine hydroxyl- amine and hydrogen peroxide and -each of the monomethyl and monofluoro derivatives. Calculated energies were then analysed in terms of a Fourier-type expansion of the potential function uiz. for rotation in ethane methylamine and methanol internal rotation is described by a simple three-fold potential V(cp) = +v3(1-cos 3cp) where V is a three-fold barrier and cp a dihedral angle. For hydrazine hydroxyl- amine and hydrogen peroxide the potential functions are more complicated and to a reasonable approximation may be written as V(cp)= $V,(l -cos cp) + $V2(1 -cos 2cp) + +V3(1 -cos 3cp) and finally for asymmetric molecules such as substituted hydrazines etc. addi-tional terms are needed to reflect the lack of symmetry about cp = 180°,i.e.~(cp)= $~,(l-cos cp) + +V2(1 -cos 2q) + +V3(I -cos 3cp) + V sin cp + V sin 2cp The analysis of barriers in terms of one-fold (Vl)? two-fold (V2),and three-fold (V,) components facilitates the interpretation of the results. Standard geometries were assumed and calculations were carried out at the STO 4.31 G level. The great value of such systematic investigation is the quantitative interpretation of data and the recognition of fundamental factors influencing conformational preferences. The following generalizations are apparent from this study. Three principal effects are discernible. The first effect is some form of bond-bond repulsion which is sufficiently peaked to lead to a negative V component in the Fourier expansion.The magni- tudes of V for the parent molecules is given in Table 1. The decrease in V3 for Table 1 Magnitudes of V3/kcal mol- for molecules X-Y" X Y CH3 NH2 OH CH 3.26(9) 2.13(6) 1.12(3) NH2 2.13(6) 1.27(4) 0.84(2) OH 1.12(3) 0.84(2) 0.22(1) Numbers in parentheses are bond-bond interacticns the sequence CH3-CH, CH3-NH, CH,-OH in the approximate ratio 3 :2 1 is well known. The results in the table show that this is a general effect i.e. there is a decrease in V3 for all the sequences X-CH, X-NH2 X-OH L. Radom W. J. Hehre and J. A. Pople J. Amer. Chem. SOC., 1972 94 2371. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA (X = CH, NH, or OH) and the values are seen to be approximately propor- tional to the number of bond-bond interactions.This suggests it is not neces- sary to invoke any V components involving lone-pair interactions to explain the data. A consequence of this bond-bond interaction is a preference for staggered conformations in molecules with a methyl group at one end. The second effect is the stabilizing influence of back-donation from lone-pair orbitals at one end of the molecule into antibonding a-orbitals at the other. Such an effect is greatest if the effective axis of the lone-pair orbital is coplanar with the bond into which electrons are being transferred. It is also strongly accentuated if this bond is polar and electron-withdrawing. The lone-pair back-donation effect leads to large V terms in the total potential with important stereochemical consequences.This is particularly the case for fluorine-substi- tuted molecules (such as FCH,-OH). If there are lone-pair orbitals at both ends of the molecule (as in NH,-NH, NH,OH and HOOH) their axes tend to be perpendicular to each other so that such back-donation can occur more effectively. The third effect is the interaction of local dipoles at the two ends of the mole- cule. This leads to lower energies for conformations in which the dipole compo- nents perpendicular to the bond are anti-parallel to each other and is reflected in ' a V term which may also be an important factor determining the equilibrium conformation. The V term may also be influenced by steric interactions. In a further paper5 internal rotation in some twenty organic molecules of the type X-Y X-CH,-Y X-NH-Y X-0-Y and X-CO-Y (X and Y are CH, NH, OH or CHO) have been investigated again with STO 4.31 G basis sets.Where experimental data are available the mean absolute deviation for rota- tional barriers is an impressive 1.2 kcal mol-'. For most of the molecules considered the conformation with lowest energy is predicted to be one with single bonds in the trans arrangement where possible in good agreement with experi- mental data. In molecules where an appropriate choice is available lower energies are achieved if methyl groups are arranged cis to neighbouring carbonyl groups. This effect has been recognized in experimental structures and is well accounted for by the theory. It may be interpreted as a consequence of the dipole-induced-dipole interaction between the highly polar carbonyl group and the polarizable methyl group.Pople and co-workers have also investigated6 barriers to rotation in an exten- sive series of monosubstituted benzenes Ph-X (STO 3G basis set). Where available the results are in good agreement with experiment. Of particular interest are the results for the series X = CH, CH,CH, CH,F CHF, and CH=CH . The corresponding barriers are (partially flexible rotation) 0.0 2.2 0.25 0.18 and 4.42 kcal mol- '. For ethylbenzene the most stable conformer is predicted to be orthogonal suggesting a preference due to C-C hyperconjuga-tion (rather than C-H). The calculated barriers for benzyl and benzal fluorides L. Radom W.A. Lathan W. J. Hehre and J. A. Pople Austral. J. Chem. 1972 25 1601. ' W. J. Hehre L. Radom and J. A. Pople J. Amer. Chem. SOC.,1972 94 1496. D. T. Clark are similar and very small the preferred conformers having the C-F and C-H bonds in the plane of the ring respectively. For styrene the barrier is somewhat larger than that calculated for rotation about the 2-3 bond in buta-1,3-diene. An investigation has been made’ of component energy terms for barriers to rotation in the series ethane propane propene and acetaldehyde. Two interest- ing conclusions may be drawn from this work computed barriers to rotation are fairly insensitive within wide limits to the basis set (c$ ref. 1); on the other hand the signs and magnitudes of component terms are very sensitive to basis set and hence caution is necessary in their interpretation.The calculations reveal evi- dence for hyperconjugative contributions to the attractively dominated barriers in propene and acetaldehyde. The particular merit of carefully parametrized semi-empirical calculations foremost amongst which are Dewar’s MIND0 schemes is that some sort of allowance can be made for electron correlation effects. Such effects are likely to be important in for example rotation about a double bond and for this reason non-empirical calculations tend to overestimate such barriers. This fact in itself is useful since information can be obtained on the importance of electron correlation in such processes. On the other hand a parametrized treatment may well give very accurate values for barriers to rotation about double bonds and allow trends to be rationalized and predictions to be made but without giving any real insight into the relative importance of the many contributing factors.As an example of the value of careful semi-empirical investigations however we may cite the work of Dewar and Kohn’ on barriers to rotation in cumulenes and ethylene derivatives and a representative series of results is given in Table 2. Particularly worthy of note are the calculated reductions of barrier heights in the series ethylene methylenecyclopropane and cyclopropylidenecyclopropane amounting to -5 kcal mol- per cyclopropane ring. Introduction of a cyclo- propene residue has a much more dramatic effect lowering the barrier by 23 kcal mol- ’.The calculated barrier for calicene is 26.8 kcal mol-’ the low value being explicable in terms of a low-energy zwitterionic structure (as revealed by charge distribution). On this basis substituents capable of stabilizing positive and negative charges in the three-membered and five-membered ring respec- tively might be expected to lower the barrier (cf:l-formyl-5,6-di-n-propylcalicene 18-19.4 kcal mol-I). This rosy picture of the interpretation of barriers to rotation by MIND0/2 calculations must be tempered however. Often results of incomplete investiga- tions have been reported. Thus for buta-1,3-diene the calculated barrierg to rotation about the 2-3 bond obtained from calculations on the cis and trans conformers is 2.2 kcal mol- l in excellent agreement with the experimental results.Unfortunately both cis-and trans-butadiene are calculated (MIND0/2) to be less stable than a conformer of C2symmetry with the double bonds ’ A. Liberles B. O’Leary J. E. Eilers and D. R. Whitman J. Amer. Chem. SOC.,1972 94 6894. M. J. S. Dewar and M. C. Kohn J. Amer. Chem. SOC.,1972,94,2699. J. W. McIkr jun. and A. Komornicki J. Amer. Chem. SOC.,1972 94 2625. -/ Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA Table 2 Molecule Rotational barrier kcal mol-' calcd (obsd) Ethylene 53.46 (65.0) Allene 36.73 Butatriene 31.60 (30.0) Pentatetraene 24.84 Hexapentaene 22.05 (20.0) Propene 53.5 cis-But-2-ene 50.1 52.5 (62.8) lsobutene 51.2 47.5 (43.4) -v 46.1 51.1 D= 48.1 w 43.4 D-30.6 35.4 45.6 26.8 approximately at right angles.Barriers to rotation in amides are grossly under- estimated' by MINDO/2. It is strange that these and other deficiencies of MIND0/2 (H,O linear NH planar) have not received better publicity to avoid the possibility of completely erroneous theoretical studies for which the method is not well suited. One of the more novel conformational processes investigated" is that involv- ing linearly hydrogen-bonded peptides when the plane of one peptide unit is rotated around the hydrogen bond from 0 to 90 degrees. The system studied consisting of the formamide dimer is a prototype for a process of fundamental importance with regard to the interpretation of the role attributed to hydrogen lo H.Berthod and A. Pullman Chem. Phys. Letters 1972 14 217. D. T. Clark bonding in the stabilization of protein conformations. The interesting result emerges that there is virtually no energy change involved in this rotation. Carbonium ions. The electronic structures of carbonium ions (or more correctly carbocations' ') still continue to excite considerable interestI2 theoretically at the non-empirical level. Conformations and stabilities of substituted ethyl propyl and butyl cations have been investigated at the STO 3G level. A separation of substituent effects into hyperconjugative and inductive effects is possible assum- ing that the latter are conformationally independent. Representative data are given in Table 3.The results suggest that whereas for X = H the two conformers Table 3 Rotational barriers (kcal mol- ')forprimary carbonium ions R+ Barrier X Y (B-A) 2.52 XH X H CH3 CCH 0.45 H 0 CN -1.95 OH -7.67 F -9.31 H CH3 -2.68 CH3 0.01 F 4.63 F -0.01 F 7.82 CH3 -11.10 3.73 2.52 2.42 F 2.11 OH 0.91 BH CN 0.87 4.30 3.97 3.73 3.61 3.38 2.92 A B A and B have approximately the same energy P-substitution is accompanied by a definite conformational preference. This can be associated with the number of electrons in the formally vacant 2p orbital at the positive carbon in A and is largely a measure of the relative hyperconjugative abilities of CH and other C-X " G.A. Olah J. Amer. Chem. SOC., i972 94 808. l2 L. Radom J. A. Pople and P. von R. Schleyer J. Amer. Chem. SOC.,1972,94 5935. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA bonds.' In particular C-C is more effective than C-H hyperconjugation. Both conformations are stabilized or destabilized relative to the unsubstituted ethyl cation by an inductive type of effect. Substituents in 7 and 6 positions modify the hyperconjugative interaction with the positive carbon and also have appreciable inductive stabilizing and destabilizing components (falling off by -5 for each interposed CH group). Carbanions. Barriers to rotation (and inversion) in simple carbanions have received little attention compared with studies of carbonium ions.An interest- ing comparison has now been drawnI3 for P-substituted ethyl cations and anions. At both a qualitative and a quantitative level the role of hyperconjugative inter- actions is substantial and leads to an important generalization for anions which should be compared with that discussed in the previous paragraph for cations if X (Table 3) is more electronegative than H then the anion favours conformer A and if X is less electronegative than H then the anion favours conformer B. As simple examples Table 4gives the calculated barriers (STO 3G) for the Table 4 Calculated barriers [E(B) -E(A)J/kcal mol- for two XCH,-CH species Cation Radical Anion (BH2)CH2-CH2 FCH2-CH2 + 10.4 -8.4 +0.3 0.0 -6.2 +9.2 extremes represented by X = BH and F.The striking reversal in conformational preferences in going from cation to anion is clearly evident as is the small con- formational preference suggested for the radicals. For the anions there will undoubtedly be distortion from a planar to a pyra- midal arrangement at the charge centre and although this will modify the quantitative results discussed above the conclusion should not be invalidated by this factor. These factors have been taken into account in a study of the system X-CH; (X = H CH, and CH;) by Wolfe and Csizmadia and co-~orkers'~ as part of a general study of the stereochemical consequences of adjacent lone pairs. The results for rotation inversion are displayed in Figures 2 and 3. The principal features of note are that the inversion barriers are in the order X = CH > H > CH, whereas the rotation barriers are in the order X = CH < CH; the barrier to rotation in ethyl anion being comparable with that in ethane.The barrier to double inversion in the ethylene dianion is almost twice that of a single inversion and the energy maximum corresponds to a structure having two planar CH groups at right angles to each other. Analysis of the total and component energy curves suggests that lone-pair-lone-pair interactions behave as l3 R. Hoffman L. Radom J. A. Pople P. von R. Schleyer W. J. Hehre and L. Salem J. Amer. Chem. SOC.,1972 94 6223. l4 S. Wolfe L. M. Tel J. H. Liang and I. G. Csizmadia J. Amer. Chem. SOC.,1972 94 1361 D.T. Clark ROTATION ALONG THE C-C BOND (0) y= 0' 124 24r (r 1W 240 360.1'1'1. I ' 1 I ' 17845 -77.44 --77.46 -CI w -77.48-a Figure 2 Lejt-hand side the total energies of ethane (l),ethyl carbanion (3),and ethylene dicarbanion (4) as a function of rotation about the C-C bond (8). In the direction of decreasing energy the curves are as follows (4) at the ethylenic C-C bond length and pyramidal angles 105 and 110" (top two curves); (4) at the optimized C-C bond length (1.60A) and pyramidal angles 105 and 110"; (3);(1). Right-hand side the total energy of (4) and irs components as a function of rotation about the C-C bond at the optimized C-C bond length (1.60A) and pyramidal angles (105"). The scale of the curve for total energy has been expanded fourfold with respect to those of the components (Reproduced by permission from J.Amer. Chem. SOC. 1972,94 1361) though they are invariant with dihedral angle. This contrasts with many previous discussions. The relative importance of gauche effects associated with polar bonds and lone pairs is inferred to be polar-bond-polar-bond > polar-bond-lone-pair > lone-pair-lone-pair. Electronic Structure of Molecules Radicals and Ions.-Substituent Efects. A distinct advantage of a non-empirical as opposed to semi-empirical study of substituent effects is that the approximations and limitations of basis set etc. may be closely controlled for the former and hence trends within series and dif- ferences between members may be interpreted with a considerable degree of confidence.The effect of substituents on the benzene ring is of prime interest Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA PYRAMIDAL ANGLE (9) BOND ANGLE (0) Figure 3 Inversion curves of methyl carbanion (2)(upper curve) ethyl carbanion (3)(lower curve) and the curves for single inversion and double inversion of (4) ; the latter two cross-sections were taken near the absolute minimum of the hypersurface of (4) (Reproduced by permission from J. Amer. Chem. Soc. 1972,94 1361) and in a detailed and fascinating study Pople and co-workers have studied6 35 monosubstituted benzenes with a minimal STO 3G basis set. Although the results are too numerous to study individually a few representative examples illustrate the utility of such detailed studies (phenol nitrosobenzene and trifuoromethyl- benzene).Phenol is predicted to be planar with a barrier to rotation (C-0 bond) of 5.2 kcal mol- comparable with that calculated for nitrosobenzene. The calculated n-electron distributions are given in Figure 4 together with the total oand 7t charges donated by the substituent to the ring (for comparison for H substituent q = -0.063; qn = 0). OH is thus seen to be cr withdrawing and n donating whereas CF is both cr and 7t withdrawing. The order of cr withdrawal is OH > NO > CF,. D.T. Clark H 0 0’ N4 10.975 1.039 0.984 40 +0.185 +0.110 +0.021 4 -0.102 +0.037 +0.011 Figure 4 For NO and CF (nwithdrawing) the n distribution follows the traditionally expected order rn > o,p whereas for OH (a good donor) the reverse applies.Overall the results are gratifyingly in agreement with most organic chemists’ expectations. Neutral Molecules. Considerable emphasis has been put on the fact that one of the most readily calculable properties of a molecule is its geometry. This applies equally to non-empirical and specifically parametrized semi-empirical (e.g. MINDO) treatments although from the literature the uninitiated might get the erroneous impression that empirical calculations of the Extended Huckel Theory (EHT) variety are adequate for this task. The author can do no better to correct this viewpoint than suggest a perusal of the paper by Bloemer and Bruner.15 The clear prediction evident from this work is that as far as EHT is concerned benzene does not exist ! (three acetylenes are much more stable).This should be sufficient to induce a healthy scepticism of ‘detailed’ potential energy surfaces calculated with EHT. The computational simplicity of calculations employing the FSGO model has previously been remarked upon.’ With very small basis sets giving in absolute terms very poor energies (typically -85 %) of the Hartree-Fock limit a detailed study by Frost and Nelson16 has shown that none the less calculated geometries for three- and four-carbon hydrocarbons are in excellent agreement with experi- ment and the claim is made that the simple FSGO method appears to be the most practical ab initio model at the present time for doing extensive geometry minimizations.In continuation of the extensive work on simple molecules using the FSGO approach Christoffersen and co-workers have investigated methylamine dimethylamine hydrazine methylimine di-imide pyridine pyrazine and pyrrole. In general a good qualitative account is given of molecular geometries barriers to rotation and ordering of both core and valence energy levels by comparison with more sophisticated calculations. In a further paper18 a series of simple oxygenated organic systems have been studied. Two of the more unusual systems which have been investigated non-empirically and which are of special interest are CH,PH as the simplest hypothetical example Is W. L. Bloemer and B. L. Bruner Chem. Phys. LeKterS 1972 17 452. l6 J. L. Nelson and A.A. Frost J. Amer. Chem. SOC.,1972,94 3727. ” D. W. Gensin and R. E. Christoffersen J. Amer. Chem. Sac. 1972 94. 6904. B. V. Cheney and R. E. Christoffersen J. Chem. Phys. 1972 56 3503. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA 5 1 of a phosphorus ylidelg and glycylglycine20 as the simplest example of a system containing a peptide bond. Extensive studies have been reported of aspects of the electronic structure of strained hydrocarbons ; namely bicycle[ 1,1,0]-butane (1),21,22 bicyclo[l,l,l]pentane (2),21-23 and tri~yclo[l,l,l,O~*~]pentane (3).22 H&H HwH (1) (2) (3) Notable features for (1H3) which one might hopefully shed some light on in a detailed calculation are (i) for (1) the central bond exhibits both olefinic and non-olefinic properties the bridgehead protons are acidic with a large 3C-H coupling constant ;(ii) (2)has the shortest non-bonded carbon-carbon distance on record and a surprisingly large long-range spin-coupling constant between bridgehead protons (18 Hz) ;(iii)(3) represents the first member of the propellanes.[1,1,1]Propellane and its electronic structure is obviously of considerable interest more particularly since one might expect the molecule to have an inordinately large strain energy. (Hoffman and St~hrer~~ have outlined qualitatively aspects of the structure of propellanes in general.) For bicyclobutane the three sets of calculations are in broad general In the more extensive study of Newton and Schulmanz2 considerable insight into the electronic structures of all three strained systems is obtained by transforming the usual (delocalized) MO's to a set of maximally localized ones according to the Edmiston and R~edenberg~~ procedure (which minimizes the total exchange energy).Such a model provides a conceptually easier way of looking at the bonding as far as organic chemists are concerned. Straight analysis of the delocalized MO's does not provide a simple picture of the nature of the central C-C bond since there is significant mixing of CC and CH symmetry orbitals. Employing the localized orbital description however the picture becomes clearer. The hybridizations involved in each 'localized' bond are given in Table 5 together for comparison with data for ethane and cyclopropane with equivalent basis sets.Essentially the same pattern emerges from INDO calculations. The striking feature evident from these data is the negligible s character in the hybrids comprising the central bond ( -96 % p in character) and the orientation of these hybrids is such as to give a bond bent by 30.8" (i.e.only 4"more bent than for cyclopropane in this basis set). This is shown more clearly by a density contour map (Figure 5). The bicyclobutane 'side' bonds are formed from carbon bridge- ~ head and methylene orbitals which are hybridized s~ and s~~.' ~. respectively '' I. Absar and J. R. Van Wazer J. Amer. Chem. SOC.,1972 94,2382. 2o J. A. Ryan and J. L. Whitten J. Amer. Chem. SOC.,1972 94 2396. 2' D. R. Whitman and J. F. Chiang J.Amer. Chem. SOC.,1972,94 1126. '' M. D. Newton and J. M. Schulman J. Amer.*Chern.SOC.,1972 94 767. 773. '' J. M. Lehn Chem. Phys. Letters 1972 15 450. 24 W. D. Stohrer and R. Hoffman J. Amer. Chem. SOC.,1972 94 779. 25 C. Edmuston and K. Riedenberg J. Chem. Phys. 1965 43 597. D. T.Clark Table 5 Hybridization Molecule Bond (minimal basis set GTO) Ethane cc SP4.01 SP3.O2 Cyclopropane CC sP5.69 CH sP2.27 Bicyclobutane C,-C3 sP2.43 sp2 .9 7-sp 5.10 c,-c* C1-Hbrhd sP1.58 C H endo sP2.23 C2H exo sP2.26 the latter being close to the cyclopropane value sp5.7. The side bonds are also bent outward by 33" from their C-C bond vectors and are rotated downward into the region between the two cyclopropane planes. The out-of-plane distor- tion of the side bond arises mainly from the bridgehead hybrids (-10% out of plane) while the methylene hybrids are within 2" of planarity.The bridgehead Figure 5 Total electron density (a.u.) of bicyclobutane (Reproduced by permission from J. Amer. Chem. SOC.,1972,94,767) CH bonds are constructed from SP'.~hybrids (high % s character) in contrast to the methylene hybrids sp2.* and sp2.3 for endo and exo protons respectively (cf:cyclopropane s~~.~). This is consistent with the acidic nature of the bridge- head protons. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA 53 With the extended basis set (STO 4.31G) some aspects of the energetics of the ring system have been studied. By setting up a series of isodesmic reactions e.g.for cyclopropane for bicyclobutane C4H + C2H6 + 2C3H6 and knowing the calculated enthalpies of reaction the zero point energy for each species and the experimental enthalpies of formation for methane and ethane strain energies for bicyclobutane and cyclopropane may be calculated. The values are 70 and 32 kcal mol- l respectively which compare well with the corresponding experimental values (63 and 25 kcal mol- I). These results are based on experimental geometries and it is of some interest to investigate the geometry theoretically. The molecule is too large for complete optimization. With the minimal basis CGTO basis set trends in C-C bond lengths in the series ethane cyclopropane bicyclobutane are reproduced (but in absolute terms are a little too large).Steric crowding in 1,3-disubstituted bicyclobutanes might be expected to distort the H-C-1-C-3 angles. The calculations show that this angle is soft and changes of as much as 15" require only 5 kcal mol-',an amount which could be partially recovered by conjugative interaction in the case of unsaturated 1,3-derivatives. Distortion of the dihedral angle is somewhat more costly but the angle is still also quite soft changes of lo" requiring only -5 kcal mol- *. By contrast stretching the C-1-C-2 and C-3-C-4 bonds by 0.2A whilst maintaining equilibrium values for the other C-C bonds and dihedral angle as might represent the early stages of the thermal conversion into butadiene is calculated to be relatively expensive costing 27 kcal mol- I.The energetics of breaking the side and central bonds to obtain 1,2- and 1,3-diradicals respectively have also been investigated. The interesting results that emerge are that for 1,3-diradical with square-planar geometry with the same C-C side-bond lengths a lower limit of 19 kcal mol- ' in energy above the ground state is calculated whereas for a l12-diradical obtained by expanding the C-1 -C-2 bond until the C-1-C-3-C-2 bond angle reaches 110" (keeping the dihedral angle and other C-C bond Iengths fixed and allowing the CCH moiety to become planar) is much higher in energy -34 kcal mol- Thus the l12-diradical appears to be less stable than the 1,3. In the case of [l,l,l]propellane (D3hsymmetry) partial geometry optimization [STD 4.31G basis set] gives normal side-bond lengths of 1.53 A and a distance between the bridgehead carbons of 1.60 A.The analogous bridgehead distance in bicyclopentane is calculated to be 1.885 A,in very good agreement with experi- mental estimates. Transformation to localized orbitals reveals negative overlap in the region between the bridgehead carbon for both molecules i.e. there is no evidence for a central bond in these molecules (cf Figure 6 and Figure 5 for bi- cyclobutane). The hybridizations for the relevant localized orbitals (for the minimal CGTO basis sets) are given in Table 6. D. T. Clark Figure 6 Total electron densities (a.u.) of [l,l,l]propellane(above) and bicycle[ 1,1,1]-pentane (below) (Reproduced by permission from J.Amer. Chem. Soc. 1972,94,773) Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA Table 6 Molecule Bond Hybridization [1,1,1]Propellane C-1-C-2 side sP1.33 sP4.31 C-1-C-3 central SP4. Bicycle[l,l,l]pentane C-1-C-2 C-2-H-1 3P3.46 sP3.64sP2.66 C-1 -H sP2.22 C-2-H sP2.62 If the numbers in Tables 5 and 6 look a little strange in terms of the concept of localized hybrid orbitals familiar to organic chemists it should be emphasized that the localization transformation employed leads to hybrid orbitals which are not constrained to be orthonormal. Comparisons with the data in Table 5 are instructive. An interesting feature of the electronic structure of bicyclopentane is the unusually large long-range coupling constant between the two bridgehead protons (18 Hz).This is successfully reproduced from an INDO finite perturba- tion method. Strain energies can again be calculated from thermodynamic data and appro- priate isodesmic reactions and are 6M4 kcal mol- ' for bicycle[ l,l,l]pentane and 105-110 kcal mol- ' for [l,l,l]propellane depending on the isodesmic processes chosen. In terms of strain energy per framework bond cyclopropane (3 bonds) bicyclobutane (5) bicyclopentane (6) and [ l,l,l]propellane (7) the calculated values range from 11 kcal mol- per bond for the former through 14 11 to 15 kcal mol-' per bond. Viewed in this way the strain energy for [l,l,l]propellane which is larger than the bond energy for a typical C-C bond looks more reasonable. The inclusion of d functions to an STO 6.31G basis set for carbon has been shown to improve the agreement between theory and experiment for the relative energies of cyclic uersus open-chain hydrocarbons.26 With this basis set strained cyclic molecules such as cyclopropene and cyclopropane are preferentially stabilized with respect to their open-chain analogues by the addition of d func-tions.It should be emphasized however that these serve merely to polarize the valence s,p basis set and as shown above the gross features of the electronic structures of strained species are well accounted for within a framework of valence s-and p-orbitals. Dewar and by revised parametrization have extended MIND0/2 to include organofluorine compounds. Calculated heats of formation molecular geometries and dipole moments for a wide variety of saturated and unsaturated compounds are overall in impressive agreement with experiment.Within the limitations of the method this extension should prove particularly valuable for predicting the properties of organofluorine systems. Non-empirical investigations have almost exclusively been directed to investi- gations of ground-state properties although increasing attention is being paid P. C. Hariharan and J. A. Pople Chem. Phys. Letters 1972 16 217. '' M. J. S. Dewar and D. H. Lo J. Amer. Chem. SOC.,1972,94 5296. 56 D.T. Clark to the discussion of excited states. A topic of considerable fundamental interest is the nature of the lowest excited (m*) singlet state of ethylene.Surprisingly at the ub initio level the picture is confused. Early work has suggested that in the planar geometry the TIT* state is a diffuse Rydberg-like state.28 The experi- mental evidence however militates against such a diffuse picture for the state and the suggestion has been made that this apparent discrepancy may be due to neglect of electron correlation effects.29 Three sets of workers have now investi- gated this problem but with no great measure of agreement between them.29-31 Excited states of ap unsaturated ketones32 and of keten33 have been investi- gated and a comprehensive study of substituent effects on n -,TC*transitions in simple molecules has been reported. 34 Pople and co-~orkers~~ have now extended their studies at the STO 4G and 4.31G level to the calculation of excitation energies and accompanying geometry changes for m*transitions in simple molecules.The theoretical results are in good agreement with experiment. Radicals. A valuable survey of the electronic properties of diradicals has been given by Salem and Rowlands. 36 Considerable computational effort has been expended in trying to resolve one of the principal physical problems underlying carbene chemistry namely the magnitude of the singlet-triplet separation in methylene.37-39 Although the electronic structures of the 'A and 3B1states of CH have been investigated extensively within the Hartree-Fock formalism over the past few years with some considerable success (e.g.that the 3B state should be bent rather than linear as had originally been inferred from spectroscopic investigations) the question of the exact singlet-triplet separation has not been settled either theoretically or experimentally.(We might expect that calculations within the HF formalism i.e.neglecting electron correlation would overestimate the splitting.) The best theoretical estimate including the effect of electron correla- tion now is 11.0 2 kcal mol- for the singlet-triplet energy differen~e.~~ The effect of inclusion of d polarization functions is quite substantial as far as this energy gap is concerned and a discussion has been given of this effect on addition and insertion reactions and a rationalization put forward for the unreactivity towards insertion into carbon-carbon single bonds3* 28 T.H. Dunning W. J. Hunt and W. A. Goddard Chem. Phys. Letters 1969 4 147. 29 H. Bash Mol. Phys. 1972 23 683. 30 B. Levy and J. Ridard Chem. Phys. Letters 1972 15 49. 31 C. F. Bender T. H. Dunning H. F. Schaeffer W. A. Goddard and W. J. Hunt Chem. Phys. Letters 1972 15 171. 32 A. Devaquet J. Amer. Chem. SOC.,1972 94 5160. 33 J. E. Del Bene J. Amer. Chem. SOC.,1972 94 3717. 34 R. Ditchfield J. Del Bene and J. A. Pople J. Amer. Chem. SOC.,1972 94 703. 35 R. Ditchfield J. Del Bene and J. A. Pople J. Amer. Chem. SOC.,1972 94 4806. 3h L. Salem and C. Rowland Angew. Chem. Internat. Edn. 1972 11 92. 37 P. J. Hay W. J. Hunt and W. A. Goddard Chem. Phys. Letters 1972 13 30. 38 C. F. Bender H. F. Schaeffer D. R. Franceschetti and L. C. Allen J. Amer.Chem. SOC.,1972 94 6888. 3y S. Y. Chu A. K. Q. Siu and E. F. Hayes J. Amer. Chem. SOC.,1972 94 2969. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA Carbonium Ions. In discussing computations of rotational barriers for substi- tuted ethyl propyl and butyl cations mention was made of Pople's extensive ~tudies.'~.'~ In general these calculations also give a good account of the relative stabilities of simple alkyl carbonium ions and in this connection it is of interest to compare the results with Dewar's reparametrized MIND0/2 studies.40 In the latter heats of formation have been computed directly which are in good agreement with available experimental results. Some representative results for isomeric species are given in Table 7.Detailed studies of geometries and relative Table 7 Re/. energieslkcal mol-Ab initio12 Species STO 3G MIND0/240 CH3CH2CH2CH2+ 0 0 CH3CH2CH+CH -20 -26 CH3CH2CH,+(CH313Cf 0 -37 0 -38 (CH312CH+ -20 -25 energies for isomeric C3H7+species have been carried out by both Pople41 and Dewar4' and their co-workers and form an interesting comparison between non- and semi-empirical calculations. In an extensive series of geometry optimizations at the STO 3G level followed by further calculations with the more flexible STO 4.31G basis set Pople and co-workers studied C,H,+ cations the two conformers of methyl-staggered 1-propyl cation (4)and (5) the eclipsed conformer of 1-propyl cation (6),corner- face- and edge-protonated cyclopropane (7t-(10) H-bridged propyl cation (1 l) and the 2-propyl cation (12).The results are given in Table 8. By comparison \ Methyl-staggered I-propyl cation (4). Methyl-staggered 1-propyl cation (5). 40 N. Bodar M. J. S. Dewar and D. H. Lo J. Amer. Chem. SOC.,1972,94 5303. 41 L. Radom J. A. Pople V. Buss and P. von R. Schleyer J. Amer. Chem. SOC.,1972 94 311. D.T. Clark /c2\ Corner-protonated cyclopropane (8) Edge-protonated cyclopropane (9). Hl H3 2-Propyl cation (1 2). Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA Table 8 Rel. energieslkcal mol- Non-empirical STO 4.31G41 MIND0/240 2-Propyl(12) 0 0 Methyl-eclipsed (6) 16.9 24.5 Corner-protonated(7) 17.3 Methyl-staggered (4) 17.4 Corner-protonated(8) 17.4 3.5 H-bridged (11) 18.2 Methyl-staggered (5) 19.4 Edge-prot ona ted (9) 27.1 -3.7 Face-protonated(10) 139.6 56 Dewar and co-workers investigated 1- and 2-propyl cations and corner- face- and edge-protonated cyclopropane; their results are also given in Table 8.The more complete studies of Pople and co-workers reveal an interesting feature of the electronic structure of the 'classical' 1-propyl cation which in conformers (4H6) shows strong distortion towards a partially bridged species looking somewhat like a distorted corner-protonated cyclopropane. The two sets of results are in striking disagreement! The MINDO calcula- tions predict edge-protonated cyclopropane as the most stable species whereas for the non-empirical calculations the chemically more reasonable result that the isopropyl cation is the lowest energy species is obtained.Both calculations agree however on the high-energy status accorded to face-protonated cyclo- propane. Cross-sections through potential energy surfaces show in the case of the MINDO investigation that the only potential minima are edge-protonated cyclopropane and the 2-propyl cation whereas in the non-empirical investiga- tion the two potential minima correspond to the 2-propyl cation and the methyl-eclipsed 1-propyl cation (6),which is so distorted as to be best considered as distorted corner-protonated cyclopropane. Unfortunately the potential energy surface is relatively flat [only 0.5 kcal mol-* separates (6) from (7) (8) and (4)] and solvent effects may have significant effects.In terms of detailed interpretation of the experimental results therefore neither study can be regarded as definitive. The semi-empirical calculations however undoubtedly consider- ably overestimate the stability of species of high degree of connectivity (bridged us. open ions) and this is characteristic of such calculations not only for ions. For example MIND0/2 incorrectly predicts4' that C is an equilateral triangle whereas even a relatively crude ab initio calculation correctly predicts it to be linear. Pople and co-workers have reported44 detailed calculations including polarization functions (d functions on carbon p functions on hydrogen) in the basis sets of the structures of the cations CH3+ CH5+ CZH3+ and C2H5+.A 42 M.J. S. Dewar Topics Curreni Chem. 1971 23 I. 43 D. H. Liskow C. F. Bender and H. F. Schaeffer J. Chem. Phys. 1972,57,4509. 44 P. C. Hariharan W. A. Lathan and J. A. Pople Chern. Phys. Lerrers 1972 14 385. D. T. Clark systematic procedure for improving basis sets and refining equilibrium geometries has been developed. It is found that the addition of polarization functions has a greater stabilizing effect on non-classical forms of C2H,+ and C2H,+. In the case of C,H,+ the energy for bridge-protonated ethylene is now computed to be slightly lower (0.9 kcal mol-’) than for the classical ethyl cation. Potential Energy Surfaces for Organic Reactions.--~nntroduction. The detailed understanding of the dynamics and stereochemistry of organic reactions requires a knowledge of many-dimensional potential energy (PE) surfaces.Leaving aside theoretical difficulties (e.g. the inability of the HF model to describe bond break- ing) the sheer scale of computations on all but the very simplest systems dictates that some form of compromise be made. These may be put into two categories. In the first type the dimension of the surface is reduced by eliminating certain degrees of freedom which can reasonably be assumed to remain constant throughout the course of the reaction. The second type involves consideration of most if not all of the degrees of freedom of the system but seeks only to locate certain chemically interesting points on the PE surface perhaps followed by limited investigations of interconnections between such points.Both approaches have their merits and representatives of each type are given below. The location of a transition state on a PE surface can in principle be accomp- lished by brute force (suitably tempered by chemical and theoretical intuition). In an elegant series of investigations however M~Iver~,~’ has presented a logical sequence for locating and identifying transition states for systems of many degrees of freedom by focusing attention on the gradient of the PE function. The tech- nique has then been applied with some success to the electrocyclic transformation of cyclobutene to butadiene studied by means of MIND0/2 calculation^.^ The question of symmetric transition states for cycloaddition reactions has also been in~estigated.~’ Thus considering (e.g.2 + 2 2 + 4 2 + 6 4 + 6) cycloadditions (represented by A + B) the question often arises is the transition state symmetric? By considering the form of the force constant matrix at the transition state the interesting prediction may be made that the likelihood of the transition state being non-symmetric (i.e. corresponding to different extents of bond-making of the two new 0 bonds) should increase as n and m increase. MIND0/2 calcula- tions on representative systems4’ indicate that the likelihood at this level of approximation is translated into a high probability and suggest that in general transition states for cycloaddition reactions are very likely to be non-symmetric. 45 J. W. McIver J.Amer. Chem. SOC.,1972 94 4782. Physical Methods-Part (iii) Theoreticul Organic Chemistry und ESCA 61 General Reviews. A general review has been presented of the formulation of the Woodward-Hoffman rules in terms of an extended valence bond The general formalism for the analysis of electrocyclic reactions using localized orbitals has also been discus~ed.~' In an important and lengthy paper G~ddard~~ has examined the results of recent ab initio calculations (by the generalized valence bond method) of reaction co-ordinates and has noted that for a reaction to have a low activation energy certain phase relationships must occur between the orbitals of the reactants and the orbitals of the products. This is developed into a generalized scheme denoted as the orbital phase continuity principle (OPCP),which has the distinct advantage compared with simple orbital symmetry arguments that it does not depend on molecular symmetry and can therefore be applied easily to reactions involving no symmetry.A further advantage accrues from the fact that OPCP is based on generalized valence bond (GVB) SCF theory and hence overcomes the principal deficiency of the Hartree-Fock model of not properly describing bond breaking. A general discussion reveals a pattern of results fully compatible with predic- tions based on simple orbital symmetry considerations for electrocyclic sigma- tropic group-transfer and elimination reactions. Some interesting exceptions are predicted however for addition reactions involving open-shell molecules [e.g.NH('A) and O,('A,)] which for reaction with ethylene e.g. 0-0 -* II 02(lA,) + >=( HzC-CH are favourable according to OPCP but forbidden in the WH approach (for C, geometry). This original approach clearly has great potential and is worthy of very close investigation by organic chemists. Concerted Cycloadditions. The qualitative predictions based on simple orbital symmetry arguments continue to be the subject of quantitative treatments. The formally symmetry-forbidden ,2 + ,2 decomposition of cyclobutane into two ethylenes has been investigated in some detail by Salem and Wright.49 A minimal basis set of Slater orbitals was employed and limited configuration interaction included to accommodate some of the change in correlation energy.Some 50 points were investigated in which the variables were R R, and B:A section 46 W. J. van der Hart J. J. C. Mulder and L. J. Oosterhoff J. Amer. Chem. SOC.,1972 94 5724. 47 J. Langlet and J. P. Malrieu J. Amer. Chem. SOC.,1972 94 7254. 48 W. A. Goddard J. Amer. Chem. SOC.,1972 94,973. J. S. Wright and L. Salem J. Amer. Chem. SOC., 1972 94 322. 62 D. T. Clark 2.55 2.35 2.15 ul 1.95 1.75 1.55 83 1 1.55 1SO 1.45 1.40 1.35 Rl/A Figure 7 Ab initio potentiul surface for the rectangulur decomposition of C,H (Reproduced by permission from J. Amer. Chem. SOC.,1972,94 322) of the PE surface is given in Figure 7. Cyclobutane (p = 45") is in the bottom left-hand corner (energy reference -155.839 a.u.) whilst the region in the top right-hand corner represents two ethylene units (0 = 0).The configuration and state correlations for the proposed reaction co-ordinate are shown in Figure 8. For the reaction C,H +2C,H along this rectangular decomposition path the calculated activation barrier is 156 kcal mol- ' thus giving a quantitative estimate of the forbidden nature of such a transformation. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA 63 -154.50 C,H,** -154.70 -154.90 -155.10 -155.30 -155.50 -155.70 -155.90 Reaction co-ordinate T.S. Figure 8 Conjguration and state correlation diagram from the ab initio calculation (Reproduced by permission from J. Amer. Chem. Soc. 1972,94,322) Electrocyclic Reactions.Following the investigation of the cyclobutene-buta- diene system reported last year5' a more extensive study has now been published5 in which the effects ofring torsion upon the reaction mechanism for the thermally induced transformation have been studied. The main conclusions of previous work require very little modification and the main effect of ring torsion upon the mechanism is to decrease the C-C distance R at which CH rotation becomes favoured relative to the corresponding value for a constrained reaction path in which torsion is not allowed. The calculations also clearly show that formation of trans-butadiene the ultimate product of the reaction involves the cis-isomer as intermediate rather than direct conversion as a result of simultaneous CH rotation and ring torsion.Zsornerizations. Further details have appeared of Salem's on the geometrical isomerism of cyclopropane reported last year. 50 K. Hsu R. J. Buenker and S. D. Peyerimhoff J. Amer. Chem. SOC.,1971 93 21 17. 51 K. Hsu R. J. Buenker and S. D. Peyerimhoff J. Amer. Chem. Soc. 1972,94 5639. 52 J. A. Horsley Y. Jean C. Moser L. Salem R. M. Stevens and J. S. Wright J. Amer. Chem. SOC.,1972 94 279. 64 D.T. Clark The isomerization of methyl cyanide to the isocyanide has been the subject of investigation at both the non-empirical level (with an extended basis set)53 and by the MIND0/2 method.54 Interestingly enough the latter predicts that the reaction should proceed via a stable intermediate resembling a 7t complex H \\ C-H H' I NrC Dewar and Kohn5 rightly point out that such an intermediate would be unique since no case has yet been reported of a stable neutral metal-free organic 7c complex.Unfortunately the more detailed non-empirical study5 33 does not show this behaviour and casts doubt again on the behaviour of the MINDO/2 schemes as far as bridged species are concerned. Radical Reactions. The PE surfaces for hydrogen abstraction and exchange in the H + CH and for addition of the NH; radical to ethylene,58 have been investigated. Extensive studies have also been presented of the electronic structure of the ground and lower excited states of formaldehyde and their dissociation into both radical and molecular produ~ts.~~,~' Nucleophilic Substitution.Further details of Dedieu and Veillard's work on prototype potential energy surfaces for S,2 reactions have been published.6 ' Carbonium Ion Rearrangements. In investigating aspects of potential energy surfaces of carbonium ions the most logical approach is to investigate chemically interesting points in some detail and then consider interconversion between species. For example for the surface of the C4H7+ system obvious points of interest are represented by cyclobutyl cyclopropylcarbinyl and homoallyl cations. For the latter two species Hehre and Hiberty have now investigated62 geometries (at the STO 3G level) and relative energies (at the STO 4.31G level). For each of the four possible homoallyl cations (1 3)+ 16) possessing planes of (13) (14) (15) (16) (17) 53 D.H. Liskow C. F. Bender and H. F. Schaeffer J. Amer. Chem. SOC., 1972,94 5 178. 54 M. J. S. Dewar and M. C. Kohn J. Amer. Chem. SOC.,1972,94 2704. 55 D. H. Liskow C. F. Bender and H. F. Schaeffer J. Chem. Phys. 1972,57,4509. 56 S. Ehrenson and M. D. Newton Chem. Phys. Letters 1972 13 24. 57 K. Morokuma and R. E. Davis J. Amer. Chem. SOC.,1972 94 1060. '* S. Shih R. J. Buenker S. D. Peyerimhoff and C. J. Michejda J. Amer. Chem. Soc. 1972 94 7620. 59 D. M. Hayes and K. Morokuma Chem. Phys. Letters 1972 12 539. 6o W. H. Fink J. Amer. Chem. Soc. 1972 94 1073 1078. 61 A. Dedieu and A. Veillard J. Amer. Chem. SOC.,1972,94 6730. 62 W. J. Hehre and P. C. Hiberty J. Amer. Chem. SOC.,1972 94 5917.Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA 65 symmetry the transformation to the bisected cyclopropylcarbinyl(17) was found to be a downhill process (Table 9). This suggests that the observation of solvolysis products derived from homoallyl cation arises from a modification of this PE surface owing to solvent effects. Table 9 Energy data (kcal mol- ')formolecules (I 3)-( 17) Molmdr~ 4.31G STO 3G Bisected cyclopropylcarbinyl (17) 0 0 trans staggered homoallyl(l5) 20.0 33.4 cis staggered homoallyl (13) 21.4 35.0 trans perpendicular homoallyl (16) 22.3 32.7 cis perpendicular homoallyl(l4) 23.2 34.4 Illustrative of the sort of problems now amenable to detailed theoretical study are investigations of the ethylenebenzenium cation and of protonated ben~ene.~ The long-standing problem of the representation in terms of rapidly equilibrat- ing classical /?-phenylethyl cation or non-classical bridged ion has largely been settled by the work of Olah.64 However it is nice to confirm the much greater stability of the bridged ion by direct calculation.Four conformers of the classical ion have been investigated and the results are shown in Table 10 together with the computed (partially optimized) geometry for ethylenebenzenium ion. Table 10 Energy &tcr (kcal mol- ')for molecules (18F(22) and the computed geometry ofthe ethylenebenzenium ion Molecule STO 3G Bridged ethylenebenzenium (18) 0 Orthogonal perpendicular ethylenebenzenium (20) 35.4 Orthogonal staggered ethylenebenzenium (19) 42.3 Planar perpendicular ethylenebenzenium (22) 46.5 Planar staggered ethylenebenzenium (2 1) 48.8 r(C,C2) = 1.598 A r(C,C,) = 1.431 A H2 H2 r(C2C2')= 1.460A HI,'c ',HI r(C,C4) = (1.400 A) r(C4C5)= (1.400 A) <-,c2 r(C2H,) = (1.080A) r(C3H3)= (1.080 A) H3 ,Cl /H3 r(C4H4)= (1.080A) r(C5Hs)= (1.080A) c3 c3 LC,C,C,' = 54.4" LC3C1C3'= 115.8" I I LC,C,C4 = 122.1" LC3C4C5= (120.0") /c4.34 LC4C,C4' = (120.0") H4 C H LH12C2C2"Z= 158.2" LH,C,H2 = 115.7" I LH,C,C = 118.3" LH4C4C = (120.0") H5 LHsCsC4 = (120.0") b3 W. J. Hehre J. Amer. Chem. SOC. 1972 94 5919. 64 G. A. Olah and R. D. Porter J. Amer. Chem. SOC.,1972,94 6877. D.T. Clark The theoretical of the protonation of benzene provides striking con- firmation for the model of electrophilic substitution built up by painstaking experimental investigations.The lowest energy structure corresponds to the proton bonded to a ring carbon which assumes approximately tetrahedral geometry. This is found to be -20 kcal lower in energy than a proton-bridging carbon-carbon bond whilst edge- and face-protonated structures are very much higher in energy. Preliminary calculations indicate that the bridged protonated species is a saddle point for proton migration. 3 ESCA Introduction.-The growing importance of ESCA as a spectroscopic technique is evidenced by meetings devoted exclusively to this and by the introduction of a new Useful reviews of the literature in addition to that reported last year are given in ref.69. An interesting comparative review of applications of n.q.r. n.m.r. and ESCA to organonitrogen systems has been given.70 A useful introduction to ESCA has been given by N~rdling.~’ An introduction to electron spectroscopy has also appeared in Chemical Society Reviews.72 65 W. J. Hehre and J. A. Pople J. Amer. Chem. SOC. 1972 94 6901. 66 ‘Electron Spectroscopy’ Proceedings of International Conference Asilomar Sept. 1971 ed. D. A. Shirley North-Holland Amsterdam 1972. 67 Chem. SOC. Faraday Division Discussion Meeting Brighton September 1972. 68 Journal uJ Electron Spectroscopy and Related Phenomena Elsevier Oct. 1972 Vol. 1. :9 D. M. Hercules Analyt. Chem. 1972 44 106R. ’H. G. Fitzky D. Wendisch and R. Holm Angew. Chern. Internat. Edn. 1972 11 979.” C. Nordling Angew. Chem. Internat. Edn. 1972 11 83. ’’ C. R. Brundle A. D. Baker and M. Thompson Chem. SOC. Reu. 1972 1 355. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA 67 Instrumentation.-With the majority of commercial instruments employing an unmonochromatized X-ray source (either MgKor ,,or AlKor ,,,) the best attain- able resolution is more often than not limited by the inherent width of the exciting radiation (-0.8 eV for MgKor,,, -1.0eV for AlKcr,,,). Chemical shifts are frequently of the same order of magnitude and although under carefully controlled conditions lineshapes and peak widths may be sufficiently accurately defined to allow deconvolution of unresolved peaks an improvement in resolu- tion would clearly extend the scope of the technique.The development of X-ray monochromators has drastically improved the situation and there are indications that the inherent widths of many core levels of interest to organic chemists may be sufficiently small to allow subtle variations in electronic environment which are unobservable with present instrumentation to be detected by ESCA. (The inherent width of the Cls level in some molecules may be <0.1 eV which is less than +5 of the width normally observed with most commercial instruments). Most of the design innovations in ESCA spectrometers are attributable to the father of the technique Kai Siegbahn who has now publi~hed’~,’~ design details for new instruments incorporating monochromators and employing either disper- sion compensation or slit filtering.The importance of such developments cannot be overstressed since they will provide the same sort of impetus to ESCA that development of n.m.r. spectrometers operating at higher field strength had to the development of n.m.r. in its infancy. Theory.-The most fundamental measurements made in ESCA are shifts in core levels. The interpretation of such data provides a fertile area of research for the theoretician and considerable computational effort has been expended in under- standing ‘chemical’ shifts. In theory the binding energy of an electron in a core level of an atom in a molecule may be calculated as the energy difference between the neutral molecule and the hole state. In practice such calculations are normally carried out within the Hartree-Fock model which neglects both relativistic and correlation-energy changes.Although for core levels changes in the latter two terms are known to be small the question of how small is small has remained open in the case of molecules. In a detailed study of the ionized states of the CH molecule with a basis set approaching the Hartree-Fock limit Clementi and Popkie have demon~trated’~ that for the 1s hole state the correlation energy is exactly the same as for the neutral molecule. Correlation energy therefore makes no contribution to the binding energy of the 1s electron which is given within experimental error as the energy difference at the Hartree-Fock limit. The diffi- culty and expense of carrying out such calculations has provided the impetus for searching for computationally simpler models.For limited series of closely related molecules Koopmans’ Theorem gives an adequate interpretation of chemical shifts of core levels. However this depends upon the fact that for 73 K. Siegbahn D. Hammon H. Fellner-Feldegg and E. F. Barnett Science 1972 176 245. 74 K. Siegbahn Proceedings Third International Conference on Atomic Physics Boulder Colorado Aug. 1972. 75 E. Clementi and H. Popkie J. Amer. Chem. Soc. 1972,94 4057. 68 D.T. Clark related molecules with similar valence electron distributions relaxation energies (ignored in Koopmans’ Theorem) tend to be closely similar. If this is not the case then Koopmans’ Theorem cannot be expected to apply.In the case of sydnones for+ example the unusual valence electron distribution results in different relaxa- tion energies for different atoms and hence Koopmans’ Theorem does not give a quantitative description of shifts for such a system.76 The incorporation of relaxation into chemical shift calculations is most readily accomplished with the Equivalent Core Thermodynamic model of Jolly and Hendri~kson.~~ More often than not the necessary thermodynamic data are not available although the heats of reaction necessary for computing shifts corre- spond to simple isodesmic reactions for which theoretical heats of formation may be calculated quite accurately with relatively modest basis sets. Clark and Adams have inve~tigated~~ this approach in some detail and Cls binding energies for a variety of organic molecules encompassing wide variations in valence electron reorganization or relaxation accompanying photoionization of core levels with conspicuous success.Calculations employing MIND0/2 are also qualitatively successf~l.~ Non-empirical calculations are feasible on only a minute propor- tion of molecules of interest to the organic chemist and hence considerable effort has been devoted to the development of reliable computationally inexpensive models in which only the valence electrons are considered. Two distinct but related approaches have been developed. The charge potential model in which binding energies are related to the charge distribution in a molecule as was originally developed by Siegbahn and co-workers” and may be related to Koopmans’ Theorem (i.e.in a zero differential overlap treatment by expanding the expression for the Fock eigenvalues and grouping together terms independent of the local electronic environment).This model has great conceptual appeal to the average chemist relating as it does charge distribution and binding energies. Charge distributions defined in terms of electron populations on atoms are however somewhat arbitrary since the electron distribution in a molecule is a continuous function. As an alternative (and more correct approach) therefore Schwartz” has developed the potential at an atom model the main drawback of which is lack of conceptual simplicity as far as the average chemist is concerned.Most discussions of chemical shifts of binding energies for larger molecules have 76 M. Barber S. J. Broadbent J. A. Connor M. F. Guest I. H. Hillier and H. J. Puxley J.C.S. Perkin II 1972 1517. ” W. L. Jolly and D. N. Hendrickson J. Amer. Chem. Soc. 1970,92 1863. 78 D. T. Clark and D. B. Adams J.C.S. Furuduy II 1972,68 1819. 79 D. C. Frost F. G. Herring C. A. McDowell and I. S. Woolsey Chem. Phys. Letters 1972 13 391. K. Siegbahn C. Nordling G. Johansson J. Hedman P. F. Heden K. Hamrin U. Gelius T. Bergmark L. 0.Werme R. Manne and Y. Baer ‘ESCA Applied to Free Molecules’ North-Holland Amsterdam 1969. 81 M. E. Schwartz Chem. Phys. Letlers 1970 6 631; 7 78. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA 69 centred around these two models or variants of them.82-87 In two important papers Shirley and co-workers have shown how within valence electron only treatments the effect of relaxation energies may be incorporated into the charge potential model," and they have demonstrated the common features in the potential at an atom and equivalent core models.89 The majority of ESCA investigations on organic molecules refer to measure- ments on the condensed phase.If the sample is in electrical contact with the spectrometer then the Fermi level is a convenient energy reference. On the other hand theoretical calculations refer to isolated molecules with the vacuum level as energy reference. (Measurements on solids and theoretical calculations can be related by means of an appropriate Born cycle.) The relaxation or reorganization energy the neglect of which is implicit in Koopman's Theorem as discussed above relates to isolated molecules and the question then arises will the relaxation of valence electrons accompanying photoionization of a core level be different if the molecule is studied in the condensed phase? An interesting discussion of this consideration which has largely been neglected up to now has been given by Shirley."' Chemical shift data and their relationship to electronic structure form a large part of the information derived from ESCA experiments on molecules radicals and ions.For paramagnetic species in addition the phenomenon of multiplet splitting can give direct information (complementary in many ways to that obtainable from e.s.r.) on the spin distribution of unpaired electron~.~~ As an example of this type of information derived from ESCA measurements Table 11 gives data pertaining to the Ols,Nls and Cls core levels of di-t-butyl nitroxide radicalg2 and for comparison data for NO.If there are k unpaired electrons (coupled to spin S) in the valence shell of a paramagnetic species then for an s hole state (e.g. 1s in Ols Nls) the multiplet splitting is given by where.&= fraction of unpaired valence electrons on atom i and 82 F. 0.Ellison and L. L. Larcon Chem. Phys. Lerters 1972 13 399. 83 K. Ishida H. Kato H. Nakatsuyi and T. Yonezawa Bull. Chem. SOC.Japan 1972 45 1574. 84 R. Rein A. Hartrnan and S. Nir IsraelJ. Chem. 1972 10 93.85 A. Imamarnura H. Fiyita and C. Nagata J. Amer. Chem. SOC.,1972 94 6287. 86 M. E. Schwartz and J. D. Switalski J. Amer. Chem. SOC.,1972 94 6298. M. E. Schwartz J. Amer. Chem. Sac. 1972 94 6899. D. W. Davis and D. A. Shirley Chem. Phys. Letters 1972 15 185. 89 D. A. Shirley Chem. Phys. Letters 1972 15 325. 'O D. A. Shirley Chem. Phys. Letters 1972 16 220. " Cf. ref 80. 92 D. W. Davis and D. A. Shirley J. Chem. Phys. 1972 56 669. 70 D. T. Clark Table 11 Binding energies and splittings of Is core levels (eV)92 Binding Linewidth Measured Case" energy (FWHM) splitting AE -NO(' n) 41 1.5(5)* 1.412( 16) -NOP n) 410.1(5) 0.93(2) No(lnj 543.6(5j 543.1(5) 0.9 l(2) 0.53q21) NG( n) dtb NO('n) 406.9(5) 1.1 3(4) 0.539(42) dtb NO(jIl) 406.4(5) dtb NO(' n) 536.7(5) 0.88(3) 0.448(26) dtb N?k3ITl-536.2(5) MethTC ' 290.3i5j 1.16(5) Tertiary C 29 1.4( 5) "The atom losing a Is electron is underlined.Assumed final-state symmetry is denoted parenthetically and 'dtb' means 'di-t-butyl'. I Standard deviation in the last digit is given parenthetically. Abso-lute values of binding energies are accurate to only 0.5 eV. i.e. a one-centre exchange integral between the core and valence orbitals on atom i. Hartree-Fock calculation^^^ on the four final states that can be formed by removing a single Is electron from NO(2x,) are in excellent agreement with the experimental measurements and indicate that there is a much larger unpaired spin density on nitrogen. By comparison the splittings for the Nls and 01s core levels of di-t-butyl nitroxide radical indicate that the unpaired spin density on oxygen is similar to that in NO but the nitrogen atom loses spin density (to the alkyl groups).As an added bonus the observation that the absolute binding energies for both the Nls and 01score levels are reduced on going from NO to the organic radical is indicative of increased electron density about these atoms. Clearly with improvements in resolution the investigation of multiplet effects in organic free radicals will be valuable. Chemical shift data and multiplet splittings give information regarding the distribution of valence electrons in molecules etc. Further detailed information can also sometimes be obtained from the observation of satellite peaks to the low kinetic energy side of the primary photoionization peaks associated with double ionization (shake-off) and excitation (shake-up) processes accompanying photoionization.The sudden perturbation of the atomic potential at the moment of departure of a core electron in the initial photoionization process can bring about the emission or excitation of a valence electron and in the sudden approxi- mation the probability of exciting an electron from the orbital nlj of the neutral atom to the n'lj orbital of the ion is given by pn,lj+nlj =NiJ'+~~j+~,~jd~ l2 Since this probability involves the overlap of the two orbitals the selection rules governing the shake-up excitation are of the monopole type i.e. AJ =AL = AS =AMJ =AML =AMS =O 93 P.S. Bagus and H. F. Schaeffer J. Chem. Phys. 1971 55 1174. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA The first observation and interpretation of such phenomena was for the inert gases but several organic molecules have now been investigated which demons- trate the interesting information contained therein. In an important paper,94 Hillier and co-workers have shown how the positions and intensities of satellites in simple molecules may be calculated using relatively crude (INDO) wave-functions. A discussion of shake-up satellites for CO COz,and C,Oz has been given together with a valuable general discussion rationalizing the failure to observe shake-up satellites in many simple molecules (e.g. benzene thiophen pyrrole and furan).An interesting example provided by 3-methyl~ydnone,’~ is shown in Figure 9. A satellite is clearly evident close to the Nls peaks (incom- pletely) resolved corresponding to N-1 and N-2 respectively in order of decreas- ing binding energy. Detailed calculations show that the satellite arises from shake-up of an electron from the highest occupied n orbital with large amplitude at N-2 to the lowest x* orbital also with large amplitude at N-2. 3-Met hylsydnone I I 1 I 1 407 405 403 401 399 Figure 9 Nls spectrum of 3-methylsydnone showing the presence of a satellite (binding energy 405.8 eV) Substituent Eflects. Systematic studies ofsubstituent effects in benzene pyridine pyrazine pyrimidine and pyradazine and their perchloro- and perfluoro- derivatives have been reported by Clark and co-worker~.~~ The results may be y4 L.J. Aarons M. F. Guest and I. H. Hillier J.C.S. Furuduy ff 1972 68,1866. D. T. Clark R. D. Chambers D. Kilcast and W. K. R. Musgrave J.C.S. Furuduy ZZ 1972 68 309. 72 D.T. Clark discussed quantitatively in terms of the charge potential model and all-valence- electron SCFMO calculations. The close analogy between organic chemists' intuitive ideas concerning charge distribution and chemical shifts is nicely demonstrated by the data. A detailed study has been made by Shirley and co- workers of fluorinated benzenes." The results may be discussed quantitatively employing the potential at an atom model using CND0/2 wavefunctions. Fluorobenzene has been the subject9' of a detailed non-empirical LCAO MOSCF treatment which confirms that in order of decreasing binding energy the assignment of Cls levels previously made98 is Substituent effects in alkyl iodides have been studied99 and by comparison with data corresponding to the 5p ionization potentials obtained from U.V.photo- electron spectroscopy it can be established that chemical shifts are due to varia- tions in electron distribution along the carbon-iodine bond. Nitrogen Is core binding energies have been reported'00 for salts (primarily hydroiodides) of pyridine monoprotonated pyrazine 1,2- 1,4- and 1,Sdiaza- naphthalenes 1,2- and l75-diarninonaphthalenes,1,lO-phenanthroline and 2,2'-bipyridyl. The binding energy differences between the protonated and unprotonated nitrogens are indicative of substantial electron withdrawal from the aromatic rings.Charge Distributions Direct from ESCA Measurements.-The direct determina- tion of charge distributions for complex molecules from molecular core binding energies presents an interesting challenge. The most obvious way of accomplish-ing this would be by inversion of the charge potential model i.e. knowing the constants Eo and kj [equation (l)] for each core level of the constituent atoms of the molecule and its geometry having measured the binding energies (EJ,set up a series of simultaneous equations and solve for the charges qi. The feasibility of this approach has been spectacularly demonstrated by several groups of workers.96,101-103 The most detailed investigation has included the computa- tion of charge distributions in bicyclic aromatic hydrocarbons and their perfluoro- analogues.'o1 The parameters kiand EP for Cls and Fls can be established by studying series of closely related molecules for which theoretically calculated charge distributions (usually CND0/2) are available.In the particular case of 96 D. W. Davis D. A. Shirley and T. D. Thomas J. Amer. Chem. SOC.,1972 94 6565; J. Chem. Phys. 1972,56 671. 97 D. T. Clark D. Kilcast D. B. Adams and I. Scanlan J. Electron Spectroscopy 1972 1 153. 98 D. T. Clark D. Kilcast and W. K. R.Musgrave Chem. Comm. 1971 576. 99 J. A. Hashmall B. E. Mills D. A. Shirley and A. Streitweiser J. Amer. Chem. Soc. 1972 94,4445. loo L. E.Cox J. J. Jack and D. M. Hercules J. Amer. Chem. Soc. 1972 94 6575. '0' D. T. Clark D. B. Adams and D. Kilcast Chem. Phys. Letters 1972 13 439. lo* D. T. Clark W. J. Feast D. Kilcast D. B. Adams and W. E. Preston J. Fluorine Chem. 1972,2 199. lo3 G. D. Stucky D. A. Mathews J. Hedman M. Klasson and C. Nordling J. Amer. Chem. Soc. 1972 94 8009. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA molecules containing hydrogen a problem arises since there are no energy levels characteristic of the hydrogen 1s orbitals. This can be overcome by defining pseudo Ei-EP and ki values for hydrogen which reproduce the computed charge distributions in reference molecules such as methane and benzene. The experimental charge distributions obtained in this way are in excellent agree- ment with direct theoretical calculations for C F and H atoms.In a study of experimental charge distributions in fluorobenzenes Shirley and Davis96 circumvented this particular problem by requiring that the charge distribution on each hydrogen be the same and then made use of the extra equation express- ing the overall neutrality of the molecules. Clark and co-workers have shown how for measurements on solid samples this extra equation can be used to investigate sample charging."' As a representative example of the state of the art,"' Figures 10 and 11 show the measured Cls levels of tetradecafluoro- tricyclo[6,2,2,0'*7]dodeca-2,6,9-triene (23) which together with the Fls levels allows the direct determination of the experimental charge distribution as shown.The very close agreement with theoretically calculated (in this case CND0/2 since the parameters ki and EP have been established for such charge -0.14) (-0.15) '(-0.18) -0.16 -0.19 Figure I0 Experimental and CND0/2 charge distributions in compound (23). CND0/2 SCF-MO charges in parentheses (Reproduced by permission from J. Fluorine Chem. 1972,2 199) D. T. Clark Binding energy feV Figure 11 The Cls spectrum and its deconuolution for compound (23) (Reproduced by permission from J. Fluorine Chem. 1972,2 199) distributions) charge distributions is quite striking. For this size of molecule it is probably true to say that it is easier to obtain charge distributions by experi- ment than by direct calculation.Nordling and co-workers have in~estigated"~ charge distributions for tetra- cyanoethylene tetracyanoethylene oxide tetracyanopropane cyclopropane and ethylene oxide. A qualitative discussion of charge delocalization in 1,3,5-trithian oxides has also been given.lo4 Although not of direct interest to the material presented in this report mention might also be made of an interesting investigation of the valency of iron in ferredoxins.lo5 Structural Studies.-Halogenocarbun Chemistry. The large shifts induced in core levels on replacing hydrogen by fluorine or chlorine make ESCA studies of halo- genocarbons particularly attractive with currently available instrumentation. The interesting structural features in this field also pose problems for many conventional spectroscopic techniques.With detailed investigations of simple systems and development of quantitative models application of ESCA to struc- tural problems in this field is beginning to have a considerable impact. Thus the structure of allylpentachlorocyclopentadiene obtained by reaction of a solution of hexachlorocyclopentadiene in diethyl ether which has been successively treated at -20 "Cwith two molecular proportions of LiAlH and allyl bromide could be d e orfas shown in Figure 12. Previous attempts at unambiguously determining the structure of the product by conventional techniques had not allowed an unambiguous assignment between d and e. The Cls spectra of the precursor and allyl pentachlorocyclopentadiene' O6 shown in Figure 13 allow a ready distinction to be made between d and e,fon the basis of the presence or absence of high binding energy peak due to a CCI group.The structure may '04 H. Iwamura M. Fukunaga and K. Kushida J.C.S. Chem. Comm. 1972 450. D. Leibfritz Angew. Chem. Internal. Edn. 1972 11 232. lo6 D. T. Clark W. J. Feast M. Foster and D. Kilcast Nature 1972 236 107. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA a b C d e f Figure 12 Reaction pathway and possible chemical structures (Reproduced by permission from Nature 1972,236 107) 289 J i 241 249 287 2k5 ' 2k7 2k5 2b Binding energyjeV Figure 13 Cls spectra of hexachlorocyclopentadieneand allylpentachlorocyclopentadiene (Reproduced by permission from Nature 1972,236 107) D.T.Clark thus be unambiguously determined on the basis of measurements requiring -20 min of instrument time and -0.1 pl of sample. By comparison the structure can also be settled by n.m.r. (3sCl 37Cl) measurements but the sample size is now -1 ml and the time taken -5 h. The advantage of ESCA is clearly demonstrated. A more complex example is provided by the determinationlo7 of the site of nucleophilic substitution'in perfluoroindene. Reaction of perfluoroindene with sodium borohydride in diglyme gave a mixture ofdihydro products in a 4 1ratio. 'H and 19F n.m.r. measurements together with i.r. studies established that the product was a mixture of 1,1,3,4,5,6,7- and 1,1,2,4,5,6,7-heptafluoroindenes(24) and (25).However it was not possible by conventional spectroscopic techniques to identify the major isomer. The isomers were not separable on available g.1.c. packings and the quantity of sample available was -0.1 gm. From theoretical 293.0 291.0 284.0 283.0 28t.O 293.0 291.0 284.0 287.0 283.0 Figure 14 Compirter-simulated theoretical Cls spectra for the individual components and mixtures of(24) and (25). (a) (24);(b) (25);(c)(24):(25),4 :1 ;(d) (25):(24) 4 :1 (Reproduced by permission from Nature 1972 239 47) lo' D. B. Adams D. T. Clark W. J. Feast D. Kilcast W. K. R. Musgrave and W. E. Preston Nature J 972 239 47. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA (CND0/2) SCFMO calculations of charge distributions spectra for the Cls levels for 4 1 mixtures of (24):(25)and (25):(24)may be computed as shown in Figure 14.Comparison in terms of peak intensities and absolute binding energies with the measured Cls levels for the mixture Figure 15 allows un- ambiguous assignment of major component as (25). I 29j.O 29 1 .o 26.0 28i.o 28i.O Figure 15 Experimental Cls spectrum of the mixture of (24) and (25) hydrohepta-fluoroindene. (Binding energies in eV) (Reproduced by permission from Nature 1972,239,47) Organonitrogen Compounds. In addition to the papers previously mentioned on six-membered ring nitrogen heterocycle^,^^ protonated nitrogen bases,’” and ~ydnones,’~ interesting applications of ESCA have been made to the electronic structure of ‘hexanitrosobenzene’108 and the nature of protonated 1,8-bis- (dimethy1amino)naphthalene (‘proton sp~nge’).’’~ The Cls Nls and 01s ‘On J.Bus Rec. Trav. chim. 1972 91 552. ‘09 E. Haselbach A. Henriksson F. Jachimowicz and J. Wirz Helv. Chim. Acta 1972 55. 1757. D. T. Clark N& ,k-O O4 I> Cls Nls 01s 405.0 533.4 287.5 285.6 I I I I 290 285 405 400 535 530 -Electron binding energy/eV Figure 16 Electron spectrum of benzotri~[c]-2-0~yfurazan, excited with MgKcr-radiation (Reproduced by permission from Rec. Trau. chim. 1972,91 552) spectra measured for ‘hexanitrosobenzene’ Figure 16,show that the hexanitroso structure is ruled out and support the formulation as benzotris[c]-2-oxyfurazan which had previously been suggested on the basis of X-ray diffraction and i.r.investigations. The doublet nature of the Nls region of protonated 1,8-bis- (dimethylamino)naphthalene,Figure 17 indicates that the system possesses an unsymmetrical N-H. .N hydrogen bridge. Carbonium Ions. More details have appeared of ESCA investigations of nor- bornyl cation and related systems by Olah and co-workers.”’ The difficulty in generating and maintaining uncontaminated surface layers containing reactive species is evidenced by the poorly resolved spectra which are none the less sufficiently resolved to demonstrate the distinction between classical and non- classical ions. The time scale involved as far as ESCA measurements are con- cerned is extremely short (the lifetimes of hole states are typically in the range ‘‘O G.A. Olah G. D. Mateescu and J. L. Riemenschneider J. Amer. Chem. SOC.,1972 94. 2529. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA I I 4 1 eV 403 40 1 399 Figure 17 (Reproduced by permission from Helv. Chirn. Acta 1972,55 1757) 10-14-10-s). ESCA is thus in principle capable of distinguishing between non-classical and rapidly equilibrating classical ion formulation of norbornyl cation (26),(27a and b). Olah and co-workers have investigated norbornyl cation and related model ions (28H30) with varying degrees of charge localization. D. T. Clark The Cls spectra show two distinct peaks for (28H30) with the peak at higher binding energy in each case being attributed to the carbon atom bearing substan- tial positive charge (cJ Table 12 and comparison with t-butyl cation) Figure 18.Table 12 Binding energy digerences of curbonium ion centres from neighbouring carbon atoms Ion AEb(Ct-C) Approximate rel. C+/C intensity (CH3)$+ (29) (30) (28) 3.9 +_ 0.2 4.2 f0.2 3.7 * 0.2 4.3 & 0.5 113 115 117 114 (26) 1.7 f0.2 215 Cls spectra for 2-methylnorbornyl cation (30) show a smaller separation indicative of some CT delocalization in the bicyclo[2,2,l]heptyl system. In striking contrast the Cls spectrum of norbornyl cation shows a single broad line with a pronounced shoulder on the higher binding energy side (corresponding to C-2 and C-6). The separation of 1.7eV clearly shows the extensive charge delocaliza- tion and thus confirms the I3C n.m.r.and Raman spectroscopy data that the norbornyl cation is best formulated as a methylene-bridged five-co-ordinated 'non-classical' ion. Charge delocalization in acyl cations has also been investigated" for the hexafluoroantimonates. Spectra for CH,CO+ and C,H,CO+ are shown in Figure 19. The shifts in Cls binding energies between CO and CH (or C6H5) are substantial 6.0 eV (5.1 eV) (c$ 2.6 eV for CH,CHO). The larger shift and higher absolute binding energy for the CO carbon 1s level in CH,CO+ compared with C,H,CO provides strong evidence for the much greater charge delocaliza- + tion in the latter. Polymers. The application of ESCA to problems of structure and bonding in polymers is a rapidly expanding field. The great advantages of the technique in being able to study in principle the core and valence levels of any element regard- less of nuclear properties such as magnetic or electric quadrupole moments coupled with the low sample requirements and the ability to study involatile insoluble solids is nowhere more apposite than in the study of polymers.As part of a systematic study the general philosophy of applying ESCA to polymer chemistry has been outlined.l12 A detailed study has been made of nitroso rubbers. * Theoretical calculations employing CND0/2 charge distributions and the charge potential model show l1 that for saturated systems the factors determining shifts are sufficiently short range for quantitative *I1 G. D. Mat-escu J. L. Riemenschneider J. J. Svoboda and G. A. Olah J. Amer.Chem. SOC.,1972 94 7 19 1. 'Iz D. T. Clark D. Kilcast W. J. Feast and W. K. R. Musgrave J. Polymer Sci. Purr A-I Polymer Chem. 1972 10 1637. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA 8 1 CIS 202 I\ +CH ’\ 0 206.4 c I 1 I 290 285 280 e Binding energy Figure 18 Carbon Is electron spectrum of norbornyl cation (lower truce) and 2-methyl- norbornyl cation (upper trace) (Reproduced by permission from J. Amer. Chem. SOC. 1972,94,2529) treatments of ESCA data for polymers to be feasible. For the nitroso rubbers investigated (copolymers of CF,NO and CF,=CFX X = F H or C1) individual core levels have linewidths little different than that for comparable monomers. The ESCA data together with theoretical calculations on model systems incor- porating all short-range effects give a wealth of data which in increasing level of complexity may be listed as :element maps demonstration of alternating 1,l-copolymer nature of the polymers and in the case of X = H and C1 information D.T.Clark CIS 698 1872 0 290 285 280 eV Binding energy Figure 19 Carbon 1s electron spectra of methyloxocarbenium and phenyloxocarbenium hexa$uoroantimonate (Reproduced by permission from J. Amer. Chem. Soc. 1972,94 7191) l3 C. R. Ginnard and W. M. Riggs Analyr. Chem. 1972,44 1310. J. M. Andre and J. Delhalle Chem. Phys. Letters 1972 17 145. '15 M. H. Wood M. Barber I. H. Hillier and J. M. Thomas J. Chem. Phys. 1972 56 1788. ' l6 M. M. Millard Analyt.Chem. 1972,44 828. Physical Methods-Part (iii) Theoretical Organic Chemistry and ESCA on structural isomerism. The effect of fluorine substitution on molecular core binding energies has been investigated for homopolymers of vinyl fluoride vinylidene fluoride and trifluoro- and tetrafluoro-ethylenes.' l3 The results have been discussed in terms of CND0/2 and EHT charge distributions' l4 and Pauling electronegativities.' An experimental and theoretical study has been reported of the valence bond structure of polyethylene' ' which complements the study reported last year for PTFE. As an interesting example of the scope of ESCA as a technique an investiga- tion has been reported of surface oxidized wool fibre.'