3 Theoretical Chemistry By G.KLOPMAN Chemistry Department Case Western Reserve University, Cleveland Ohio44106 1 New Methods and Modern Trends The scope and applications of theoretical chemistry continue to expand as the quantum mechanical methods become better and more efficient. Thanks to improv-ing computational techniques the a6 initio procedures can now deal with fairly large molecules. The use of the STO/nG formalism has become common practice even for relatively complex systems and the generalized valence bond method is progres-sively applied to larger systems. A new entry the PNO (pair natural orbital)’ with configuration interaction CI or coupled electron pair approximation CEPA is claimed to reaver ca. 80% of the correlation energy. The method has already been used to calculate the properties of a fairly large number of small molecules with consis tent success.2 The literature was particularly rich last year in new formalisms designed to improve the scope and reduce the computational time of ab initio techniques.Thus several authors suggested procedures for the calculation of valence-shell molecular orbitals in a pseudopotential formed by the nucleus and care Others proposed to simplify the ab initio formalism by approximatingsome of the integrals in a minimal STO basis The SAM0 (simulated ab initio MO)’ method makes use of the transferability of Fock matrix elements between closely related groups in different molecules. The procedure was applied to the calculation of the conformation of cyclohexane with moderate success.An interesting and rare attempt was also made to develop a non-empirical MO theory of the electronic structure of molecular crystals.’ The semi-empirical field has been equally active with the development of several new formalisms. Animproved IOC (inclusionof overlap charges) omega techniqueg provided better agreement for some properties of aromatic derivatives. lo The R.Alrichs H. Lischka V. Staemmler and W. KutzeInigg J. Chem. Phys. 1975,62,1225. 2 R.Alrichs F. Driessler,H.Lischka V.Staemmler,and W.Kutzelnigg J. &m. Phys. 1975,62,1235. J. N.Murrell and I. G. Vincent J.J.C.S.Furaday II 1975,71,890. P.Coffey,C. S. Ewig and J. R.Van Wazer J. Amer. Chem. SOC.,1975,97 1656. M. J. Zerner J. am. Fhys. 1975,62,27SS. P.Volkmer H.J.Kohler D. Mopper and F. Birnstock Chern. Phys. ,!hers 1975,31,566. ’J. E. Eilers B. O’Leary B.J. Duke A. Liberies and D. R.Whitman J. Amer. Chem. Soc. 1975 97 1319. S. F. O’Shea and D. P.Santry new. aim. Acra 1975,37 1. S. C. Sharma A. K. Srivastava and B. Krishna J.C.S. Faraday 11 1975,71,168. lo S. C. Sharma A. K. Srivastava,and B. Krishna J.C.S. Faruday 11 1975,71,172. 37 38 G. Klopman extended Huckel method was improved by Anderson“ and now provides better bond distances and force constants in diatomic molecules. The CND0/2 method was ‘simplified’ by Gayoso12 (CND0/2-S) who claims that in spite of his simplifica- tions the new formalism gives equivalent or better results than the original method. The one-centre integrals of the INDO formalism were reparametrized from atomic promotional energies by Figeys et all3 The resulting method yielded improved heats of formation for various molecules formed of second-row atoms.An interesting parametrization of the CNDO formalism for Pt was suggested by Sakaki et all4 Both the electronic spectra of PtC1:- and the electronic structure of PtR molecules where R is ethylene or acetylene were calculated with credible success. This may open the door to possible future usage of CNDO-type techniques in the study of inorganic complexes. The new semi-empirical method whose development last year had the strongest impact was undoubtedly MIND0/3. The new MINDO method” differs from its predecessor by the fact that new orbital exponents are used for the calculation of the overlap integrals used in the determination of the resonance integral.The method has immediately been applied to the calculation of the properties of a wide range of molecules including atoms of the second and third row.16 The numerous applications of the successive MINDO methods were also reviewed by Dewar.” The claims of MIND0/3 to be an inexpensive and reliable theoretical technique for investigating energetic and structural features of molecules were challenged by Pople” on the basis of energetic considerations and by Hehre” on the basis of structural data. These criticisms were answered by Dewar,” who pointed out that the reported deficiencies of MIND0/3 were all implied in the unusually large error obtained for the heats of formation of only three compounds C& C(CH3)4 and CH,C=CCH,.Actually such criticisms should really not be restricted to one particular semi- empirical method. It should be well understood that limitations are implied in every method including the ab initio ones and it is only if we understand the limitations of our theoretical tools that we can use them properly. In this respect cross- comparison of various methods is often beneficial and this has been done in a few instances. For example Combs and Holloman2’ compared the ability of various semi-empirical methods to predict internal barriers of rotation in small organic molecules but they found no trends. The predicted barrier can be too small or too large but among the tested methods (CND0/2 INDO and MINDO/2’) INDO was found the most reliable.The failure of the CND0/2 method to predict the conformation of systems where the twisted bond is delocalized was confirmed by Sieiro et ~1.~~ The method was also 11 A. B. Anderson J. Chem. Phys. 1975,62 1187. l2 J. Gayoso Compt. rend. 1975,280 C,105. 13 H. P. Figeys P. Geerlings and C. Van Alsenoy Bull. Soc.chim. beiges 1975,84 145. l4 S. Sakaki H. Kato andT. Kawamura Bull. Chem. Soc. Japan 1975,48 195. Is R. C. Bingham M. J. S. Dewar and D. H. Lo,J. Amer. Chem. Soc. 1975,97,1285. l6 R. C. Bingham M. J. S. Dewar and D. H. Lo,J. Amer. Chem. SOC., 1975,97,1294. 1’ M. J. S. Dewar Gem. In Britain 1975 11 97. J. A. Pople J. Amer. Chem. Soc. 1975 97 5306. l9 W. J. Hehre J. Amer. Chem. SOC.,1975,97 5308.2O M. J. S. Dewar J. Amer. Chem. SOC. 1975,97 6591. 21 L. L. Combs and M. Holloman J. Phys. Chem. 1975,79 512. 22 C. Sieiro P. Gonzalez-Dim and Y. G. Smeyers J. Mol. Structure 1975 24 345. Theoretical Chemistry 39 found to fail in the case of some localizedbonds particularly when the two atoms that constitute the bond possess lone pairs.23 Both CNDO/2 and INDO predicted the correct conformation of the central CO bond in dirneth~xymethane,~~ but EHT and MIND0/2 produced the wrong confor-mation. Heats of Formation.-Only a few of the semi-empirical methods (e-g. MIND0/3) are capable of providing heats of formation that are directly comparable with experimental values. Yet several empirical procedures exist or have been proposed recently for achieving such a purpose.Benson and Luria assigned a partial positive charge to each hydrogen of a molecule and a neutralizing charge to the corresponding carbon atoms. The resulting electrostatic energy provided a good estimate (better than 0.5 kcal) of the heat of formation of alkanes,25 dkenes,26 and free radicals.27 Bond energies of saturated molecules were also found satisfactory in Sanderson's empirical proce- dure which is based on electronegativities and atomic radii.28 The calculationof strain energies was realized by MaksiE et from a considera- tion of the angular strain due to the bending of local hybrid orbitals. Resonance energies of benzenoid hydrocarbons equivalent to the best obtainable by SCF procedures were obtained simply by counting resonance An interesting paper by Kruszewski and Krygowski3' proposes an extension of the Huckel 4n +2 rule to non-alternant hydrocarbons.The proposed rule apparently permits predictions as to substituent effects on the stability of the hydrocarbons. 2 Electronic Structures of Organic Molecules Ab initio methods are making increasing inroads into organic chemistry. After the successful demonstration of the usefulness of the ST0/3G programs in organic problems the Generalized Valence Bond programs are beginning to be used. Goddard and his collaborators have used their GVB program to study the ground and low-lying states of dia~omethane,~' and f~rrnarnide.~~ f~rmaldehyde,~~ The calculations proved valuable; the excitation energies and dipole moments are in excellent agreement with experiment.The allyl radical was also studied by the GVB35method and a resonance energy of 11.4 kcaI was found in good agreement with the thermochemical estimate of 11.6kcal. The allyl radical the pentadienyl radical and several ions were studied by Hinchcliff e using a high-quality Gaussian program. 36 23 A. Veillard Chem. Phys. Letlers 1975,33,15. 24 C.Ivaroska and T. Bleha J. Mol Saucture 1975,24,249. 25 S. W.Benson and M. Luria J. Amr. Chem. Soc. 1975,97,704. 26 S.W.Benson and M. Luria J. Amer. am.SOC.,1975,97,3337. 27 S.W.Benson and M. Luria 3. Amer. Chem. Sm.,1975,97,3342. R.T.Sanderson,J. Amer. Chem. Soc. 1975,97,1367. 29 Z. B. MaksiC K. KovakviE and M. Eckert-Maksie TetrahedronLetters 1975 101.3o R.Swinborne-Sheldrake,W. C. Herndon and I. Gutman Tetrahedron Letters 1975,755. 31 J. Kruszewski and T. M. Krygowski Canad. J. Chem. 1975,53,945. 32 S.P.Walch and W. A. Goddard J Amer. Chem. Soc. 1975,97,5319. 33 L. B.Harding and W. A. Goddard J. Amer. Chem. Soc.,1975,97,6293. 34 L. B.Harding and W. A. Goddard J. Amer. Chem. Soc. 1975,97,6300. 35 G. Levin and W. A. Goddard J. Amer. am. Sac. 1975,97 1649. 36 A.Hinchdiffe J. Mol. Strucmre 1975,27,329. 40 G.Klopman It was found3’ that the mean C-C bond length at 0 K is longer in GH6 than in C2D6by 0.0015 A. The C,& molecule as well as the C2H4 and C2H2 systems were also studied by the CEPA-PNO method by Alrichs et aZ.38 A very extensive CI calc~lation~~ of the ground and excited states of trans-butadiene produced values of 7.05 and 8.06 eV for the llBu and 2’B states respectively compared with the observed values of 5.9 and 7.1eV.The use of simulated ab initio molecular orbital (SAMO)was suggested as a valid alternative to full ab initio calculation for large organic molecules. In this method some of the Fock matrix elements are transferred from pattern molecules. The method was found successful in calculations for aromatic hydrocarbon^^^ but required modifications when applied to ionic RX species.41 Ab initio calculations on large molecules could also benefit from the inclusion of ‘molecular fragments’. This is the approach chosen by Spangler et al. to investigate the electronic structure of ethyl chlorophyllide as well as the free and Mg-bound porphin and ~hlorin.~~ An extensive study of 0,S N and P heterocycles was undertaken by Palmer Findlay and their co-w~rkers.~~ Their LCGO calculations provided interesting results concerning the aromaticity and electronic structure of these heterocycles.Excellent agreement has been between the calculated and the experi- mental relative energies of the C4H4 and C4H,systems. The ST0/3G method was also used to calculate the electronic structure of several carbonylnitrenes XC(0)N45 and ethylenedione O=C=C=0.46 Both classes of compounds are believed to be ground-state triplets. However in contrast to previous studies the ethylenedione is now found to be unstable with respect to ‘bent’ dissociation into two COs. 3 Molecular Conformational Analysis Although attempts are being made to isolate the essential features that determine whether one or another conformation is the more most current contribu- tions in this area simply perform a geometry search for the most stable conformation.An increasingly popular method is to use a relatively simple procedure such as CNDO or STO/3G to obtain a rough description of the molecule and follow up by a more complete procedure such as STOInG where n >3. 37 L. S. Bartell S. Fitzwater and W. J. Hehre J. Chem. Phys. 1975,63 3042. R. Alrichs H. Lischka B. Zurawski and W. Kutzelnigg,J. Chem. Phys. 1975,63,4685. 39 R. P. Hosteny T. H. Dunning jun. R. R. Gilman A. Pipano and I. Shavitt J. Chem. Phys. 1975,62 4764. 40 J. E. Eilers B. O’Leary A.Liberles and D. R. Whitman J. Amer. Chem. SOC.,1975,97,5979. 41 B. J. Duke M. Pickering,B. O’Leary and J. E. Eilers J.C.S. Faraday 11 1975,71. 1401. 42 D. Spangler R. McKinney R. E. Christoffersen G. M. Maggiora and L. L. Shipman Chem. Phys. Letters 1975,36 427. 43 M. H. Palmer R. H. Findlay W. Moyes and A. J. Gaskell J.C.S. Perkin 11,1975,841;M. H. Palmer and R. H. Findlay ibid.,pp. 974 1223. 44 J. S. Binkley J. A. Pople and W. J. Hehre Chem. Phys. Letters 1975 36 1. 45 J. F. Harrison and G. Shalhoub J. Amer. Chem. SOC.,1975,97 4172. 46 R. C. Haddon D. Poppinger and L. Radom J. Amer. Chem. SOC.,1975,97 1645. 47 J. E. Eilers and A. Liberles J. Amer. Chem. SOC.,1975,97 4183. 48 G.M. Gimarc Accounts Chem. Res. 1974,7 384. Theoretical Chemistry 41 Among the systems recently studied by CND0/2 are some phosphorus deriva- tives such as the phosphoranyl radicals,49 found to have a trigonal-bipyramidal ge~metry,~' some di-t-butylph~sphines,~' and the strongly pyramidal aminophos- phine.52 Other systems include some hindered a-diazo-ketones XCH2COCHN2,53 di- nitrogen heterocycle^,^^ halogenocyclohexanes,55 and the cyclopropylmethyl ati ion,'^ where it was found that the rotation of the methylene group around the three-membered ring requires an activation energy of 31kcal mol-'.A somewhat modified CNDO procedure was also used to calculate the geometry and electronic structure of a series of highly fluorinated alkanes ketones and aldehyde^.'^ The INDO method was used to optimize the geometry in the special cases of poly(viny1 borane~)~~ systems.An interesting comparison6' was and s~ccinimidyl~~ made between the optimal geometries of several bridged aromatic compounds (ArXAr) calculated by EHT CND0/2 MIND0/2 and PCILO. The results are substantially different depending on which method was chosen. EHT and CND0/2 judged on the basis of experimental values give the most satisfactory results. The geometry of a number of molecules was investigated with the ST0/3G method. It was found that peroxyacetic acid6' exists in a planar cis-configuration. Similarly hexachlorobenzene to no one's surprise was also found planar as shown by the 84-orbital basis set calculation of Pedersen and Carlson.62 In contrast the hydroxymethyl radical was to be non-planar.The out-of-plane angle of the CH2 group was calculated to be 25" but with a barrier of only 0.4 kcal mol-'. The unexpected observation that cyclohexa-l,4-diene may exist in a planar rather than a boat conformation received support from an ab initio cal~ulation~~ showing the planar conformation to be ca. 7 kcal mol-' more stable than the bent one. Other ST0/3G calculations include65 a systematic study of the influence of substituents on the electronic structure and geometry of a series of carbonyl derivatives. ST0/3G as well as semi-empirical methods was used to calculate the conformation of phenethylamine66 and of p -propi~lactone~~ in an effort to under- stand their biological activity. Radom and Stiles6* reported the intriguing observation that the rotational barrier in ethane increases when one of the hydrogen atoms is replaced by a fluorine atom 4y Y.I. Gorlov and V. V. Penkovsky Chem. Phys. Letters 1975,3525. J. M. F. van Dijk J. F. M. Penning and H. M. Buck J. Amer. Chem. SOC. 1975,97,4836. 51 M. Corosine and F. Crasnier J. Mol. Strucfure 1975,27 105. 52 M. Barthelat R. Mathis J.-F. Labarre and F. Mathis Compt. rend. 1975,280 C 645. 53 S. Sorriso F. Stefani A. Flamini and E. Semprini J.C.S. Furuduy ZI 1975,71,682. 54 Z. Latajka H. Ratajczak W. J. Orville-Thomas and E. Ratajnak J. Mul. Strucfure 1975 28 323. 55 V. M. Grimmer D. Heidrich H.-J. Kohler and M. Scholz,2.phys. Chem. (Leipzig) 1975,255,1084. 56 E. Yurtsever and J. Moreshead Chem. Phys. Letters 1975,36 365. ST V. P.Zhukov and V. A. Gubanov J. Mol. Structure 1975,28,247. 58 N. L. Allinger and J. H. Siefert J. Amer. Chem. SOC.,1975,97 752. 5y T. Koenig and K. A. Wielesek Tetrahedron Letters 1975,2007. 6o V. A. Zubkov T. M. Birshtein and I. S. Milevskaya J. Mol. Structure 1975,27 139. L. M. Hjelmeland and G. H. hew Chem. Phys. Letters 1975,32 309. 62 L. G. Pedersen and G. L. Carlson J. Chem. Phys. 1975,62,2009. 63 Tae-Kyj Ha Chem. Phys. Letters 1975 30 379. 64 G. Ahligren B. Akermark and J.-E. Backvall Tetrahedron Letrers 1975 3501. 65 J. E. Del Bene G. T. Worth F. T. Marchese and M. E. Conred Theor. Chim. Acfu 1975,36 95. 66 M. Martin R. Carbo C. Petronogolo and J. Tomasi J. Amer. Chem. Soc. 1975,97 1338. 67 L. M. Boggiz 0.M. Sorarrain J. A. Frones and M.C. Villani,Zphys.Chern.(Leipzig),1975,255,44. 68 L. Radom and P. J. Stiles Tetrahedron Letters 1975 789. 42 G.Klopman but decreases with additional geminal fluorine substitutions. Jordan6' came to the no less intriguing conclusion that the geometry of pyridinium chloride is such that the N,H and C1 atoms are callinear. The structure where the N-H and H-Cl bonds are each 1.4 A is 17 kcal mol-' more stable than the separated pyridine and Ha and 148 kcal mol-' more stable than the infinitely separated pyridinium cation and chloride anion. The question of whether the elusive cyclobutadiene would exist as a square or a rectangular ground state has been discussed. Borden7' suggested that cyclo- butadiene could indeed be a square singlet but Dewar and Kollmar71 found it to be 13.1 kcal mol-' less stable than square triplet.Haddon and Williarn~,'~ on the other hand favoured a ground-state rectangular singlet. The latter authors pointed out that the observed single band ascribed to a low-temperature matrix isolated form of cyclobutadiene could be assigned to a transition from the rectangular 'A form rather than as has been suggested from a square form (see also Chapter 10 p. 240). 4 Bonded Pairs of Molecules Hydrogen Bonding.-In an attempt to analyse the mechanism of hydrogen bonding Morokuma and collaborators devised an energy decomposition which coupled with an ab initio technique was applied to the study of the water dimer as well as to the formaldehyde-water cyclopropanone-water,74 and a~rolein-water~~ pairs.Hydrogen-bonded carbonyl-water pairs were also investigated by Paoloni et uZ.,~~who showed that correlations exist between the stretching frequency of the carbonyl group the shift of the OH absorption band the amount of charge transfer and the dipole-moment increment. An extensive of the various possible hydrogen-bonded formamide-water complexes showed that the formation of a hydrogen bond leads to increased conjugation of the NCO fragment. The formamide-water system was also studied by Del Bene,7'a along with a series of other and di-substit~ted~' carbonyl compounds. In this thorough investigation equilibrium geometries and energies as well as singlet n-m* ionization energies were determined by a minimal ST0/3G procedure." It was found that the dimers usually assume open-chain trans struc-tures (1) and that n +?r* excitation is probably accompanied by rupture of the hydrogen bond.The stereochemistry of intramolecular hydrogen-bonding presents a special chal- lenge because of the delicate balance that exists between the tautomeric forms. This 69 F. Jordan J. Amer. Chem. Soc.,1975,97 3330. 70 W.T.Borden J. Amer. Chem. SOC.,1975,97,5968. 71 M. J. S. Dewar and H. W. Kollmar J. Amer. Chem. SOC.,1975,97,2933. 72 R.C.Haddon and G. R. J. Williams J. Amer. Chem.Soc. 1975,97,6582. 73 H. Umeyama K. Kitaura and K. Morokuma Chem. Phys. Lefters 1975,36 11. 74 S.Yamabe and K. Morokuma J. Amer. Chem. Soc. 1975,97,4458. 75 S.Iwata and K. Morokuma J. Amer. Chem. SOC.,1975,97 966. 76 L.Paoloni A.Patti and F. Mangano J. Mol. Structure 1975,27 123. 77 T.Ottersen and H. H. Jensen J. Mol. Structure 1975,26 355,375; T.Ottersen ibid. p. 365. 78 (a)J. E. Del Bene J. Chem. Phys. 1975,62,1961; (b)J. E.Del Bene ibid. p. 1314. 79 J. E.Del Bene J. Chem. Phys. 1975,63,4666. J. E. Del Bene J. Chem. Phys. 1975,62,666. Theoretical Chemistry is illustrated by the case of malonaldehyde,s'~s2 where repeated calculations failed to resolve the question of whether the C (2) or the C2"(3)configuration is preferred. The case of salicylic acid is somewhat easier to deal with because of the inequality of the two oxygens that share the proton. Here CND0/2 predictss3 the correct position of the proton in both the ground and the excited state of the molecule.Ion Pairsand Charge-transfer Complexes.-Both the Li' and Na' formaldehyde ion pairs have been in~estigated~~ by ab initif techniques. metal-oxygen distances were found to be very short (LiO = 1.46 A NaO = 1.9 A) and curiously enough the oxygen bond with Na was found to be strongly polar whereas that with Li was mostly covalent. The sodium complex and one of the two stable Li' complexes with formaldehyde were found to be such that the metal lies above the oxygen atoms. In contrast the Li' was found to be collinear with the C=O bond in the Li'-formic acid complex.s5 Indeed both this and the pyrazine-lithium complexes were found to be planar. Although semi-empirical methods do not always reproduce the ab initio they can often be used advantageously in problems of ionic association.86 For example extended HMO was shown to provide satisfactory association con-stants for equilibria between nitrobenzene radical anion" and allene8' with various univalent cations.CND0/2 calculations have been used by Fukui and his collaboratorssg to investigate ion-molecule complexes such as N&+CH4 and H30+CH4.They found that the systems are stabilized by some unusual type of hydrogen- bonding. In the unusual system consisting of a fluoride ion and an allyl fation the ion pair was foundg0 to be unsymmetrical ,with the fluoride ion lying 1.8 A above the central carbon of the allyl plane. Other molecular orbital studies of donor-acceptor complexes include the benzoquinone-benzene,91 benzene-carbonyl cyanide," and tetracyanoethylene- waterg3 complexes.G. Karlstrom H. Wennerstrom B. Jonsson S. Forstn J. Almlof and B. ROOS J. Amer. Chem. SOC. 1975,97,4188. 82 A. D. Isaacson and K. Morokuma J. Amer. aem. Soc. 1975,97,4453. 83 J. Catalan and J. 1. Fernandez-AIonso,J. Mol. Structure 1975,27,59. 84 F. Bernardi and G.F. Pedulli J.C.S.Perkin 11,1975,194; F. Bernardi G.F. Pedulli M. Suerra and H. B. Schlegel Gazzetta 1975,105,711. 85 B. M. Rode B. Breuss and P. Schuster Chem. Phys. Letters 1975,32,34. 86 S.Miertus and 0.Kysel Chem. Phys. Letters 1975,35 531. 87 T. M. Krygowski M. Lipsztajn and P. Radzilkowski J. Mol. Structure 1975,28 163. 88 E. V.Borisvo,V. L. Lebediv A. A. Bagatur'yants T. V. Fokina A. M. Taber and I. V. Kalechits Russ. J. Phys. Chem. 1975,48 1374.89 K. Tanaka T. Yamabe H. Kata and K. Fukui Bull. Chem. Soc. Japan 1975,48 1740. * F. Bernardi N. D. Epiotis and R. L. Yates J. Amer. Chem. Soc.,1975,97 1334. 91 0.Mo M. Yanez and J. I. Fernandez-Alonso,J. Phys. Chem. 1975,79,137. 92 W. A. Lathan G. R. Pack and K. Morokuma J. Amer. Gem. Soc. 1975,97,6624. 93 W. A. Lathan and K. Morokuma J. Amer. Chem. Soc. 1975,97,3615. 44 G.Klopman Dimem.-Ethylene dimers were investigated by unrestricted MO LCAOg4 and ~alence-bond~’ methods. The neutral dimer is unstable but the positively charged one was found to be stable by 0.6-0.7 eV with respeCt to dissociation. Two acetonitrile dimers were found to be more stable than the isolated m01ecules.~~ Of the two the skewed Tconfiguration (4) was favoured over the antiparallel one (5).The relative stability of the latter species is worth noting in view of the fact that its formation would occur along a symmetry ‘forbidden’ path. H3C-C-N NEC-CH < 0 N H3C-CSN Ill C I CH3 (4) Catalysis.-Catalytic activity and the role of the catalyst have been discussed in terms of frontier orbital~,~’ and a formalism was proposed to estimate the effect of a catalyst on the frontier The theory was tested on Lewis-acid-catalysed Diels-Alder and Meerwein-Ponndorf reactions. 5 Chemical Dynamics Reaction Intermediates.-Carbocations. The geometry of protonated alkanes has been re-examined by the MINDO/3 method.99 As in other recent calculations a C’ geometry was found for the pentaco-ordinated carbon atom.This was also true in an ‘a6 initio’ study’00of the protonation of cyclobutane where both edge and corner protonations were found to be equally favourable. In this latter study it was found that the protonation energy of cyclobutane is significantly lower than that of cyclopropane. This result supports the fact that cyclopropyl carbonium ions are often postulated as reaction intermediates whereas cyclobutyl carbonium ions are rarely encountered. Other interesting contributions in this area include the study of the protonation of cresols,’o’ of azulene,’** and an attempt to correlate the 13C chemical shifts of phenyl-substituted onium ions’03 with CNDO calculated charge densities. The controversy between the supporters of edge and corner protonation con- tinues and is reaching other cationic species.It has been found’04 that the bridged form (6)of the chlorovinyl cation is 11kcal mol-’ less stable than a slightly bent open form (7). An EHT calculation of the site of protonation of methylcyclopropane 94 J. Almlof A. Lund and K. Thuomas Chem. Phys. Letters 1975,32 190. 95 P. E. S. Wormer and A. Van der Avoid J. Chem. Phys. 1975,62 3326. 96 L. Paoloni and S. Hauser Bull. SOC.chim. belges 1975,84 219. K. Fukui and S. Inagaki J. Amer. Chem. SOC. 1975,97,4445. 98 A. Imamura and T. Hirano J. Amer. Chem. SOC.,1975,97,4192. 99 P. K. Bischof and M. J. S. Dewar J. Amer. Chem. SOC.,1975,97,2278. loo T. Pakkanen and J. L. Whitten J. Amer. Chem. Soc. 1975,97,6337. Io1 M. M. Bursey R.S. Greenberg and L. G. Pedersen Chem. Phys. Letters 1975 36,470. lo2 E. Stilla J. Bertran and J. I. Fernindez-Alonso J.C.S. Perkin 11 1975 366. Io3 G. A. Olah P. W. Westerman and D. A. Forsith J. Amer. Chem. Soc. 1975,97,.3419. lo4 V. Lucchini G. Modean and I. G. Csizmadia Gazzetta 1975 105 675. Theoretical Chemistry 45 revealed that two reaction pathways are equally favourable lo5 formation of a corner-protonated methylcyclopropane and hydride abstraction from the methyl group giving H2 and the cyclopropyl carbinyl cation (8). Pl ,c1 (6) (7) (8) The study of trico-ordinated carbenium ions has mostly centred around the determination of the stabilizing effect of adjacent substituents. Thus the influence of cy cloprop yl,'06 phenyl,'06,' O7 and aminophenylloS groups was investigated by ST0/3G methods.The influence of heteroatoms was also investigated and it was found that the stabilizing effect of an NH group on an adjacent C' is practically nil1'* compared with that of a BH2 for example. A comparison was also made between cH2SHand 6H,0H.Io9 It was found here that the effect of the sulphur was very different from that of the oxygen. Thus whereas SH releases electrons to the carbenium centre both from its CT-and 7r-orbitals OH behaves as a 7r-donor but a cT-acceptor. Furthermore geometrical isomerization of CH2+SH proceeds by rptation about the C-S bond with a barrier of ca. 36 kcal mol-' whereas that of CHzOHproceeds by linear inversion at oxygen. Curbunions. Very little new information has been gathered on the electronic structure of carbanions.The problem is difficult because of the importance of correlation energy in these electron-rich states. Furthermore the orbital exponents used for the neutral molecules are unsatisfactory to describe the expanded atomic orbitals of negatively charged species. In spite of these difficulties the geometry and electronic structure of the methyl ethyl vinyl and ethynyl anions have been satisfactorily calculated.110 Sulphur d -Orbitals. Evidence continues to accumulate against a strong participation of the sulphur d-orbitals in 7r-bonding. Thus it was found that the sulphur atom strongly stabilizes the adjacent carbanion centre in -CH,SH even though its d-orbitals were not included in the basis set."' Including the d-orbitals lowers both the energy of -CH,SH and that of CH,SH thus showing no effect on the proton affinity of the carbanion.A similar conclusion was reached from a comparative study112 of -CH,SH and -CH20H. Ab initio113 and CND0114studies show a modest d-orbital contribution to the bonding in the episulphones (9). *05 A. H. Andrist J.C.S. Chem. Comm. 1975,446. IM J. F. Wolf P. G. March R. W. Taft and W. J. Hehre J. Amer. Chem. SOC.,1975,97,2902. lo7 J. D. Dill P. von R. Schleyer and J. A. Pople Tetrahedron Letters 1975 2857. Io8 P. Kollman J. McKelvey and P. Gund J. Amer. Chem. SOC., 1975,97 1633. lo9 F. Bernardi I. G. Csizmadia H. B. Schlegel and S. Wolfe Canad.J. Chem. 1975,53 1144. 110J. E. Williams jun. and A. Streitwieser jun.J. Amer. Chem. SOC.,1975,97 2639. A. Streitwieser jun. and J. E. Williams jun. J. Amer. Chem. Soc. 1975,97 191. 11* F. Bernardi I. G. Csizmadia A. Mangini H. B. Schlegel M. H. Whangbo and S. Wolfe J. Amer. Chem. SOC.,1975,97,2209. 113 M. M. Rohmer and B. Roos J. Amer. Chem. SOC.,1975,97,2025. 114 C. Muller A. Schweig and H. Vermeer J. Amer. Chern.SOC.,1975,97 982. G.Klopman O\ S/O /\ H,C-CH2 (9) Oxidation Reactions. The reaction of singlet molecular oxygen with olefins was investigated by Dewar Thiel and their collaborators using MINDO/3 potential energy surfaces. They found the most favourable path to be that yielding the corresponding peroxirans (lo) with an activation energy of ca. 11.4 kcal m~l-'."~ The rearrangement of the peroxirans into 1,2-dioxetans (1l),which are the normal products of such reactions is postulated on the fact that 1,2-dioxetan is ? 0-0I I 0-0 02('AR)+ H,C=CH2 -+ ;'% --* ,$)< -* /?T"+ I1 ., H2C'-CH2 H2C-CH H2C-CH2 H2C-CH2 (10) (1 1) 49.4 kcal mol-' more stable than the corresponding peroxiran.However the activa- tion energy required to achieve this process was given as 34.1 kcalmol-' which seems relatively high. Indeed the peroxiran could have also reacted either with another singlet molecular oxygen to give an oxiran and ozone (exothermicity 36.2 kcal mol-' activation energy 29.2 kcal mol-') or even better with another alkene to give two oxiran molecules (exothermicity 98.1 kcal mol-' activation energy 8.6 kcal mo1-')."6 Although oxirans have been suggested as the products of such reactions it is not clear from these results why it is the 1,2-dioxetans that are the usually observed product.The structure of the zwikerion H2C=O+-O- postulated as an intermediate in the ozonolysis of alkenes was studied by Wadt and G~ddard."~ The results indicate that the molecule would rather exist as a biradical(l2) and would rearrange into the singlet biradical(l4) by way of the cyclic dioxiran (13) found to be more stable than (14) by 23 kcal mol-'. An ST0/3G study''8 of the electrolytic oxidation of formic acid showed the cyclic structure (15) to be preferred to the open-chain structure. (15) However it is now widely recognized that Gaussian methods tend to overestimate the stabilityof small rings and caution should thus be exercised when considering the above results.M. J. S. Dewar and W. Thiel J. Amer. Chem. SOC.,1975,97,3978. 116 M. J. S. Dewar A. C. Griffin W. Thiel and I. J. Turchi J. Amer. Chem. SOC.,1975,97 4439. 11' W. R. Wadt and W. A. Goddard J. Amer. Chem. SOC.,1975,97 3004. 118 W. F. Maier and M. T. Reetz J. Amer. Chem. Soc. 1975,97,3687. Theoretical Chemistry Potential Energy Surfaces.-Inversion Barriers. The inversion barriers about the nitrogen of a set of mines amides and heterocycles were calculated by the CNDO meth~d."~It was estimated that the contribution of correlation energy to such inversion barriers is very small; in ammonia it was found to be less than 10%of the total.12o The influence of the electronegativity of the atom X about which inversion occurs in XH has been examined by Levin.'" It was found that the barrier increases drastically as the electronegativity of X decreases.This was related to the decrease in the splitting between the highest occupied and lowest unoccupied orbitals in the C3h transition state. Internal Rotation Barriers. Internal rotational barriers were examined for several compounds. In ethane the barrier was assigned to the required orthogonality of the CH bonds on opposite ends of the The barrier to internal rotation in thioacetaldehyde was also in~estigated'~~ and a comparison was made between the results obtained from PCILO and CND0/2. The PCILO calculations yielded a satisfactory value (1.06 kcal mol-') but the CND0/2 calculations resulted in the wrong conformation.Valence Tautomers. The tautomerism of pyridinealdoximes and acetamidopyridines was investigated using the CND0/2 method. It was found that the lactim forms are generally more stable than the lactam forms and that the tautomers are more stable when their substituents appear in a ~yn-configuration.~~~ The potential surface for the valence tautomerism of 4-thioformyl1,2-dithiole-3-thione (16) and other thiocarbonyl derivatives was examined by the Huckel method. (16) Isomerization. The rules of conservation of symmetry proposed by Woodward and Hoffman continue to stimulate the interest of theoreticians eager to find the most favourable rearrangement reaction paths. Although most of the work in this area is done by quantum mechanical methods some inroads are also made by empirical force-field procedures such as that recently proposed for the study of the stereoisomerization pathway of several trimesityl derivatives.126 119 B.E. Ley A. E. Foti and S. M. Rithstein J. Amer. Chem. SOC.,1975,W 2030. N. C. Dutta and M. Karplus Chem. Phys. Letters 1975,31,455. 121 C. C. Levin J. Amer. Chem. Soc.,1975,W,5649. 122 P. A.Christiansen and W. E. Palke Chem. Phys. Letters 1975,31,462. 123 T.Weller D. Klopper and H.-J. Kohler Chem. Phys. Letters 1975,36,475. lZ4 J. S.Kwiatkowski W. Herzog (Boniewicz) and B. Czaodrowska J. Mol. Structure 1975,26,333. n5 G. Calzaferri and R. Gleiter J.C.S. Perkin ZZ,1975 559. lZ6 M. R. Kates J. D. Androse P. Finocchiam-D. Gust and K.Mislow J. Amer. Chem. Soc.,1975,97 1772. 48 G.Klopman Among the recently published quantum mechanical calculations of potential- energy surfaces for isomerization reactions we noted the ab initio (SCF-CI) calcula-tion of fhe cyanide-isocyanide rearrangement. 127 The activation barrier was found to be between 35 and 40 kcal mo1-' while the HNC isomer is calculated to be less stable than HCN by 9-15 kcal mol-'. The rearrangement of methylcarbene to ethylene was examined by an a6 initio procedure using a double-zeta basis. The reaction was found to proceed smoothly along an 'allowed' pathway.'** The thermal rearrangement of cyclic organic molecules was investigated by Dewar and Kirschner using MIND0/3. They found that the forbidden thermal rearrange- ment of Dewar benzene to benzene'29 requires an activation energy of 114 kcal mol-' and that of bicyclo[2,1 ,O]pent-2-ene to cy~lopentadiene'~~ 111.7 kcal mol-l.These results thus generally agree with the predictions of the conservation of symmetry rules in that they find high activation energies for forbidden processes. The situation is not as clear in the rearrangement of bicyclobutane to b~tadiene.'~' In this case it was found that the activation energy for the path proceeding to an unsymmetrical biradicaloid transition state is only 2.7 kcal mol-' above that of the 'allowed' reaction intermediate. The rearrangement of benzvalene to benzene132 is even more interesting because the activation energy needed to reach the biradicaloid transition state was found to be only 21.5 kcal mol-' making it a very favourable pathway.The 'allowedness' of the reaction was further emphasized by the fact that no HOMO-LUMO crossing exists when going from reactants to products. The thermal 173-sigmatropic rearrangements of bicyclo[3,2,0]heptanes to norbor- nenes were investigated by Gavezzotti and Simonetta in an attempt to define the relative ease of various reaction paths.133 Bimoleculur Reactions. The concerted elimination of HCI from ethyl chloride was investigated by Hibert~'~~ with a singledeterminant ab initio MO. His conclusions are that the reaction proceeds as is usually believed uia a planar four-membered transition state. However he found that cis-elimination is more favourable than trans-elimination.This was rationalized on the basis that one of the chlorine's lone pairs takes a prominent part in the proton-abstraction process. The halide exchange reaction Br- +RBr where R is an alkyl group was investi- gated by Abraham et ~1.'~'with a force-field method. Three transition-state models were investigated a stiff model a flexible model with one bondbending mode and a plastic model with several degrees of freedom. Both the flexible and plastic model reproduced experimental data satisfactorily. On the basis of their results the authors postulated that steric effects alone are responsible for the observed differ- ences in reactivity. 1z7 P. K. Pearson H. F. Schaefer and U. Wahlgren J. Chem. Phys. 1975,62 350. lZ8 J. A. Altmann I.G. Csizmadia and K. Yates J. Amer. Chem. Suc. 1975,97 5217. 129 M. J. S.Dewar and S. Kirschner J.C.S. Chem. Comm. 1975,463. l3O M. J. S.Dewar and S. Kirschner J.C.S. Chem. Comm. 1975,461. 131 M. J. S. Dewar and S. Kirschner J. Amer. Chem. Soc. 1975 97 2931. I32 M. J. S. Dewar and S. Kirschner J. Amer. Chem. SOC.,1975,97 2932. 133 A. Gavezzotti and M. Simonetta Tetrahedron 1975,31 1611. 134 P. C. Hiberty J. Amer. Chem. SOC.,1975,97 5975. 135 M. H. Abraham P. L. Grellier and M. J. Hogarth J.C.S. Perkin ZI,1975 1365. Theoretical Chemistry 49 Reactivity Control.-Reactiility Indices. The once popular 'static' reactivity indices e.g. free valence index localization energy etc. are seldom used nowadays in view of the success of the 'dynamic' reactivity indices such as HOMO and LUMO energies and orbital coefficients.The advantage of the latter indices over the former ones is that they provide a much better picture of the reaction paths by taking both reagents into consideration. The failure of a single or 'universal' index to account for the reactivity of a set of molecules or the various positions of a molecule has thus been recognized. This was lately re-emphasized for heterocyclic where it was shown that no single index could account for the observed electrophilic substitutions. One of the recent developments in this field consists of the determination of the electrostatic potentials that exist around a molecule. This allows one to visualize the possible path to be followed by an approaching reagent usually a proton.Such calculations have recently been made for the dimethylphosphate anion,'37 where it was found that the anionic oxygens are surrounded by circular zones of nearly constant negative potentials while smaller potential wells occur around the ester oxygens. The diprotonation of adenine and some of its derivatives was also studied by such a te~hnique.'~' It was found that the N-1 and N-7 are the most favourable sites for double protonation of adenine itself while the N-7 methyl derivative was preferentially diprotonated at N-3 and N-9. Gas-phase acidities and basicities have also been calculated by more conventional methods i.e. differences in stability. These include a CNDO/2 calculation of the basicityof a series of ph~sphines'~~ and an ab initio study of the basicity and acidity of a series of alcohols alkyl amines and alkyne~.'~' In the latter cases it was found that the bulkier alkyl groups increase both acidities and basicities and that these groups can function on demand as electron donors and acceptors.ControE of Reaction Path. The search for new and better descriptions of the orbital control of reaction paths continues to stir the interest of theoreticians. Several new concepts have emerged lately. For example Desl~ngchamps'~~ presented a new stereoelectronic theory for the study of the most favourable cleavage path of the tetrahedral intermediate of carboxylic derivatives and Liotta14' proposed to study nucleophilic and electrophilic reactions by an orbital-distribution technique.Salem and his co-workers introduced a new classification of photochemical reactions based on the consideration of avoided surface crossing^.'^^ These can be found by drawing state-correlation diagrams between the reactants low-lying excited states and the primary product. The application of the theory to new systems has also been fruitful by helping to uncover new physical and chemical propertie~.'~~ Epiotis Yates and Bernardi discovered that subjacent orbital control can be 136 S. Clementi A. R. Katritzky and H. 0.Tarhan Tetrahedron Lerrers 1975 1395. 137 H. Berthod and A. Pullman Chem. Phys. Letters 1975,32,233. 138 R. J. Bartlett and H. Weinstein Chem. Phys. Letters 1975,30,441. 139 M. Graffeuil J.-F. Labarre M. F. Lappert C. Leibovici and 0.Stelzer J.Chim. phys. 1975,72,799. 140 L. Radom Austral. J. Chem. 1975,28 1. 141 P. Deslongchamps Tetrahedron,1975,31 2463. 142 C. L. Liotta Tetrahedron Letters 1975 519 523. 143 L. Salem C. Leforestier G. Segal and R. Wetmore J. Amer. Chem. SOC. 1975,97,479. 144 W. G. Dauben L. Salem and N. J. Turro Accounts Chem. Res. 1975,8,41. G.Klopman important in sigmatropic reaction^.'^'" The same authors also called attention to the importance of non-bonded attractions in the determination of the conformation of rn01ecules~~~~ and the stereochemistry of displacement reactions.’45o Jorgensen and Borden presented extended Huckel calculations explaining Hoge-veen’s observation of the drastic difference of Diels-Alder reactivity between (17) and (18).They found that the low reactivity of (17) is due to the unfavourable interaction between the orbitals of the bridge and the newly formed double bond compared with what it was before reaction.’46 The Walsh orbitals of the cyclopropyl groups in dispiro[2,2,2,2]deca-4,9-diene (19) were similarly shown to interact through conjugation with neighbouring double bonds.’47 This was found to be the case irrespective of whether the vinylcyclopropanes are in a syn or anti periplanar conformation. The analysis of the correspondence between the Woodward and Hoffman rules for pericyclic reactions and the Hiickel-Mobius rules of aromaticity was discussed by Day.148 Other discussions of orbital correlation diagrams were presented by ma ha^^,'^^ for X+R reactions and in terms of valence bonds by Silver and Karpl~s.’~~ Considerable interest has been shown lately in attempts to express the laws of conservation of symmetry by means of mathematical and graphical methods.In the former category Stanton and McIver’” and Hale~i’~~ described new ways by which group theory can be used in the study of the distortion of molecular orbitals produced by their reaction. In the latter group Ras~af”~ has attempted to describe the Woodward-Hoffman rules in terms of parity rules and Cvetkovic Gutman and Trinaj~tic”~ presented a graphical method for correlating the non-bonding molecu- lar orbitals of reactants and products. In st related vein Sinanoglu’” presented a geometric description of reaction mechanisms along with a set of rules for possible use in computer-assisted design of organic syntheses.Computer-assisted graph analysis was also used by Gund ef ~1.’’~to analyse the mechanism of diamantene formation from various pentacyclotetradecanes. In this work however the search (a) N.D. Epiotis R. L. Yates and F. Bernardi J. Amer. Chm Soc. 1975,97,4198; (b)R. L. Yates N. D. Epiotis andF. Bernardi ibid.,p. 6615;(c)N.D. Epiotis,R. L.Yates andF. Bernardi,ibid.,p. 5961. 146 W. L.Jorgensen and W.T.Bordem Tetrahedron Lerrers 1975,223. 14’ P. Asmus M.Klessinger L. U. Meyer and A. de Meijere TetrahedronLetters 1975,381. 148 A. C. Day J. Amer. Chem. Soc. 1975,97,2431. 14q B. €3. Mahan Accounts Chem. Res. 1975,8,55. lSo D. M.Silver and M.Karplus J. Amer.Chem. Soc. 1975,97,2645. l5I R. E. Stanton and J. W. McIver J. Amer. Chem. Soc. 1975,97,3632. 152 E. A. Halevi Helv. aim. Acru 1975,58 2136. 153 A. Rassat Tetrahedron Letters 1975,4081. 154 D. Cvetkovic,I. Gutman and N. Trinajstic 1.Mol. Smture 1975,uI 289. 0.Sinanoglu,3. Amer. Chem. Soc. 1975 37 2309. lSc T. M.Gund P. von R. SchIeyer,P. H.Gund and W.T.Wipke J. Amer. Chm. Soc. 1975,97,743. Theoretical Chemistry 51 was not guided by quantum mechanical considerations but rather by an empirical formfield program. 6 Calculation of Physical Properties of Large Organic Molecules Ionization Potentiah-The determination of new experimental ionization poten- tials by photoelectron spectroscopy and ESCA techniques continues to stimulate the interest of theoreticians.In most recently treated cases however the molecules were rather large and could be treated only by semi-empirical methods. Thus Bieri and Heilbr~nner'~~ attempted to resolve the order of orbitals in Dewar-benzene in an effort to determine the relative importance of through-bond and through-space interaction between the two double bonds. They found CND0/2 inadequate to explain the experimental order of orbital energy but both ST0/3G and SPINDO gave adequate results. On these bases they concluded that the b state lies below the a1state and that the through-bond and through-space effects acciden- tally cancel in this molecule. In a similar study on a different system it was found that ab initio and INDO methods as well as CND0/2 provide a reasonable picture of the photoelectronic spectra of 1,6-methano[ 1OIann~lene.'~~ On the basis of these calculations it was concluded that the system possesses only weak aromaticity.Other interesting CNDO/2 calculations of photoelectronic spectra include those of a series of substituted benzothiazole~'~~ and of a group of nucleic acid bases,'60 where the results are also correlated with the ability of the bases to associate with riboflavine. Electronic Spectra.-Electronic spectra calculations were amongst the first to be satisfactorily performed for organic molecules yet no method seems to have achieved a scope wide enough to establish predominance. Indeed new parametriza- tions of the basic molecular orbital framework are being proposed in the literature at a greater rate than in any other area of theoretical chemistry.For example Hayashi and Nakajima16' parametrized the local core matrix elements of a CND0/2 framework to calculate the spectra of a few unsaturated hydrocarbons. The most consistent parametrization of the CNDO formalism for the evaluation of U.V. transitions was suggested a couple of years ago by Jaffe and collaborators as the CNDO/S and has now been modified by its authors16* in such a way as to allow the direct calculation of triplet states. The resulting procedure was used to calculate spectroscopic data of various substituted and heterocyclic aromatic derivatives. Two new parametrizations CNDO/S2 and CNDO/S3 involving a change in the orbital exponent of the conjugated carbon atoms have been used to calculate the excited states of alkyl-substituted aromatics and polycyclic aromatic derivative^.'^^ lS7 G.Bieri and E. Heilbronner Tetrahedron Letters 1975 581. lS8 G.L. Grunewald,I. M. Uwaydah R. E. Christofferson and D. Spangler,TetruhedronLetters,1975,933. Is9 G. Salmona R. Faure and E.-J. Vincent Compt. rend. 1975,280 C 605. 160 N. S. Hush and A. S. Cheung Chem. Phys. Letters 1975,34 11. T. Hayashi and T. Nakajima Bull. Chem. Soc.Japan 1975,48,980. 16* H. M. Chang H. H. Jaffe and C. A. Masmanides J. Phys. Chem. 1975,79,1109 1118. 163 N. 0.Lipari and C. B. Duke J. Chem. Phys. 1975,63,1748; C. B. Duke N. 0.Lipari W. R. Salaneck and L. B. Schein ibid. p. 1758; N. 0.Lipari and C. B. Duke ibid. p. 1768. 52 G.Klopman Electronic spectra have also been calculated for fluorophenols and anilines (CNDO/S),lWnaphthalene (CNDO/S CI),16' stilbene (IND0),'66and a series of di-and tri-atomic linear radicals with degenerate ground states (IND0/CI).l6' Limited configuration interaction Pariser-Parr calculations (SCF-MO CI) have also been used to calculate the spectra of benzoquinolines168 and a large series of nitrogen heterocyclic aromatic molecules.169 Application to biochemical problems is illus- trated by the calculation,of the excitation energy of various isomers of refinall7' in an attempt to understand the process of vision. An interesting attempt to correlate luminescence and SCF-CI calculations was presented by Fratev and Tadjer.171 A rare attempt to calculate the U.V.spectra of a fairly large organic molecule tetracyanoquinoline by an ab initio technique (4 12 primitive Gaussians) was presented by J0han~en.l'~ The ability of various semi-empirical (CNDO type) methods to predict the dipole moments of molecules containing second-row atoms was tested by Gordon Richards and K0~th.I~~ They found that none of the methods was capable of reproducing magnitudes or even qualitative trends. Vibrational Spectra.-The calculation of the force constants necessary to evalu- ate i.r. frequencies can be done with any quantum mechanical method provid- ing that the geometry predicted by this method is sufficiently realistic. Both the CNDO and MIND0 formalisms fulfil this criterion and have been successfully used to calculate the harmonic force constants of GH, G&,and CJ&,174 and HCN.17' A study of the effect of deuterium substitution on the i.r.spectrum of silaethylene H,Si=CH, has been re~0rted.l~~ N.M.R. Specga.-Olah and his co-workers have related the I3Cchemical shifts of phenyl-substituted onium ions to the CND0/2 calculated total \ charge of the electron-deficient atoms. lo3 On the basis of a comparison between the experimental and MIND0/2 calculated 13 C chemical shielding anisotropy of acetic acid it was postulated that the molecule exists as a dime^.^^^ Coupling constants have also been calculated usually by the finite perturbation method in INDO wavefunctions. Thus l3C-I3Ccoupling constants were calculated for various conformations of a series of aliphatic and bicyclic and compared with the corresponding experimentally determined values.1e4 J. S. Yadav P. C. Mishra and D. K. Rai Chem. Phys. Letters 1975,31 129. H. Meyer K. W. Shulte and A. Schweig Chem. Phys. Letters 1975,31 187. 166 D. A. Luippold Chem. Phys. Letters 1975 35 131. 167 P. Carsky J. Kuhn and R. Zahradnik,J. Mol. Spectroscopy 1975,55 120. H. Yamaguchi T. Ikeda and H. Mametsuka Bull. Chem. Soc. Japan 1975,48,1118. 169 R. W. Wagner P. Hochmann and M. A. El-Bayoumi J. Mol. Spectroscopy 1975,54,167 170 B. Honig A. Warshel and M. Karplus Accounts Chem. Res. 1975,8 92. 171 F. Fratev and A. Tadjer J. Mol. Structure 1975 27 185. 172 H. Johansen International J. Quantum Chem. 1975,9,459. 173 M. S. Gordon B. Richards and M.Korth J. Mol. Structure 1975,28,255. 174 K. Kozmuza and P. Pulay Theor. Chim. Acta 1975,37,67. 17s T. Miyazaki M. Ikeda and M. Shibata Bull. Gem. SOC. Japan 1975,48 1138. 176 H. B. Schlegel and S. Wolfe J.C.S. Chem. Comm. 1975 246. I. Ando and A. Nishioka Bull. Chem. Soc. Japan 1975,48,841. l78 M. Barfield I. Burfit and D. Doddrell J. Amer. Chem. SOC.,1975,97,2631.