Molecular Orbital Perturbation Research Articles

Fragment interaction analysis in the framework of ab initio UHF MO computations: Part II. Conformational preferences in substituted ethyl radicals

The small conformational preference in the radicals YCH 2CH 2, with Y = CH 3, NH 2, OH and F has been rationalized using a quantitative perturbational molecular orbital (PMO) analysis performed in the framework of an ab initio unrestricted Hartree—Fock molecular orbital (UHF MO) treatment. The results show that this problem can be satisfactorily rationalized considering all types of π-interactions, while a description based only on the interactions associated with the singly occupied molecular orbital (SOMO) is incomplete.

(He 1) photoelectron spectra of vinyl- and (1-dimethylaminovinyl)pyridines

The (He 1) photoelectron spectra of α-, β-, γ-vinyl, α-, β-, and γ-(1-dimethylvinyl)-pyridines, 1-dimethyl- and 1-diethylaminostyrenes were obtained and interpreted within the framework of the molecular orbital perturbation theory. In both pyridine derivative series, there is a regular increase in the ionization energy of the 1a2, πC=C and nen orbitals and decrease in the ionization energy of the 2b1 orbitals in the order α < β < γ (inversion of orbitals 1a2 and 2b1 is found for γ-vinylpyridine). The splitting of the energy levels of the heterocycle in dimethylaminovinylpyridines is less than in the corresponding vinyl derivatives, which indicates a weakening of the interaction between the aromatic (or heteroaromatic) ring and the enamine fragment extruding from the ring plane. The ionization energy of the unshared electron pair of the nitrogen atom of the pyridine ring for all the compounds except for α-(1-dimethylaminovinyl)pyridine (which displays an ortho effect) is close to that for pyridine. The photoelectron spectral data are compared with the MO energies calculated by the MINDO/3 method.

Carcinogenesis by polycyclic aromatic hydrocarbons: a multilinear regression on new type PMO indexes

A new MCS model of chemical carcinogenicity of polycyclic aromatic hydrocarbons is presented. M stands for metabolism at the M region, C for carbocation formation, and S for size and solubility. Two perturbational molecular orbitals (MO) indices are introduced and discussed together with the size criterion. Thereby, perturbational molecular orbital method is extended similar to the omega-method in order to differentiate between carbocations and radicals. A three-variable linear regression on carcinogenic potency yields a multiple correlation coefficient r = 0.961 for a representative sample of 26 polycyclic aromatic hydrocarbons. The deviations are within the confidence limits of the experimental Iball index. Predictions are given for some hydrocabons whose carcinogenic potencies are yet unknown.

Novel 2,4-disilathietane ring system from [2 + 2]cycloaddition of 1,1-dimethyl-1-silaethylene,Me 2SiCH 2, to dimethylsilanthione, Me 2SiS. Perturbation molecular orbital (PMO) study on reactivity of intermediates with a double-bonded silicon atom

Copyrolysis of 1,1-dimethyl-1-silacyclobutane (I) with both hexamethylcyclotrisilthiane (II) and tetramethylcyclodisilthiane (III) at 560°C involves 1,1-dimethyl-1-silaethylene, Me 2SiCH 2 (IV), and dimethylsilanthione, Me 2SiS (V), intermediates and yields the following cycloaddition products: the new 2,2,4,4-tetramethyl-2,4-disilathietane (VI), 1,1,3,3-tetramethyl-1,3-disilacyclobutane (VII), and III. Six-membered cyclocarbosilthianes, 1,1,3,3,5,5-hexamethyl-2-thia-1,3,5-trisilacyclohexane (VIII) and 1,1,3,3,5,5-hexamethyl-2,4-dithia-1,3,5-trisilacyclohexane (IX) have also been derived by inserting IV and V into the SiS bond of VI. Copyrolysis of I with thietane (X) also results in four- and six-membered cyclocarbosilthianes, the major product being VI. This is discussed in terms of dimethylsilanthione formation via [2 + 2]cycloaddition of IV to thioformaldehyde (XI) followed by [4 → 2 + 2]cyclodecomposition of the 2-silathietane intermediate. A perturbation molecular orbital study of [2 + 2]cycloaddition involving intermediates IV, V, and XI has shown that IV reacts more readily with V and XI than it cyclodimerizes. Dimerization of V is the most prominent reaction.

Donor-Acceptor Reactions between Alcohols and M(I) Cations in the Gas Phase

From a re-examination of published thermodynamic data for the reactions of alkanols with Li+, Al+, Mn+, Co+, Ni+ and Cu+, correlations were established between the apparent softness of the Lewis acid in the gas phase and the Kamlet-Taft basicity parameters for the alcohols. Certain deductions from the molecular orbital perturbation theory as developed by Klopman were used to interpret these observed trends in M(I) acceptor behavior toward the basic oxygen in the alcohols. The intrinsic softness for each of the six cations appears to be determined by an unequal mix of frontier and charge-controlled contributions with the latter being the minor component for the acidic species. Of the various semiempirical scales for representing donor basicity and dipolarity, the Kamlet-Taft (1976-1981) multiple parameter model in equation 1 has been more successful for the analysis of medium effects upon equilibria and reactions in nonaqueous solutions than single parameter scales. For this LFE function, P = P0 + s(7r* + db) + bp + aa (1) the solvent-dependent property P is related to the dipolarity and polarizability parameters (r* and 6), to the hydrogen bond acceptor basicity (0), and to the hydrogen bond donor acidity (a) of the medium. In a preceding study (Kolling, 1982) it was demonstrated that the HBA basicity number (0) can be used as a measure of base strength for Lewis bases reacting in donoracceptor equilibria involving organometallic species in nonaqueous solutions. As a part of the rationale for such applications to ROH ligands as hard bases, it was observed that 0-values correlate well with gas phase proton affinities and NO+ affinities for the alcohols and water (Kolling, 1982). In this report possible relationships between the / numbers and the thermodynamic quantities for gas phase reactions between the alkanols and singly charged cations (M+) are examined. The data base for this investigation comes from a series of papers by Ralph H. Staley et al. (1982) with This content downloaded from 207.46.13.158 on Tue, 15 Nov 2016 03:51:48 UTC All use subject to http://about.jstor.org/terms VOLUME 87, NUMBERS 3-4 109 the results derived from pulsed-laser volatilization methods for producing the metal cations and the evaluation of the energetics of the processes by ion cyclotron resonance spectroscopy. For the generalized metal-ligand reaction in equation 2, data for the standard free energies and bond dissociation energies are available for six cations ranging from the relatively hard (Li+) to soft (Cu+) Lewis acids.

Quantitative orbital analysis of ab initio SCF-MO computations. Part 3. Torsional isomerisms in but-2-enes

We describe the results obtained from quantitative orbital analysis, using ab initio SCF-MO computations, of the torsional isomerism of but-2-ene. The analysis is based on the use of σ fragment localized MOs and π fragment canonical MOs and the energy effects associated with their interactions are estimated using the quantitative Perturbational Molecular Orbital (PMO) and Total Energy approaches. It is found that conformational preferences in both cis- and trans-but-2-ene are determined by the π non-bonded interactions under matrix element control, while the trend in the relative stability of the two stable geometric isomers, tee > cee, is determined mainly by the difference in the steric repulsions between the σCH bond MOs of the two methyl groups.

Theoretical analysis of K‐region and bay‐region indicators of carcinogenicity

AbstractThe correlation of the K‐region carcinogenicity indices of Pullman and of Mainster and Memory with bay‐region carbocation delocalization energy (BCDE) is displayed and shown to be maintained at the perturbational molecular orbital (PMO) level. PMO algebraic relations are examined to reveal the underlying relationships. Some correlation between BCDE and K‐region indices exists simply because carbocation formation occurs preferentially on carbons which are bonded to positions α (adjacent) to ring fusion centers. Sampling a K region is synonymous with sampling such α carbons. However PMO analysis shows that a very strong correlation exists between BCDE and indices for a certain type of K region (called “primitive”). It is further shown that high BCDE is favored by a straight‐line chain of benzene rings as one moves away from the bay region. Kinks in the chain will lower the BCDE, and the closer the kink is to the bay region, the greater is its effect. Effects of methylation upon BCDE are shown to correlate with K‐region indices if the methylation occurs at a carbon which is in the opposite class as the carbon to which the bay‐region carbocation is attached.

A theoretical approach to substituent effects. A comparison of the isoelectronic BH, CH3, and NH groups and their interaction with substituents in disubstituted benzenes

AbstractAb initio molecular orbital theory with the STO‐3G basis set is used to examine both charge and energy interactions in a series of meta‐ and para‐substituted phenylborate anions and toluenes. Comparison of the results is made with data for substituted anilinium cations. It is concluded that whereas NH is a powerful σ acceptor, with essentially no π interaction, BH is primarily a π donor, and, to a slight extent only, a π donor. CH3 is indicated to be both a weak σ and π donor. Energies of interaction of BH and NH with a series of substituents are an order of magnitude larger than corresponding values for CH3. Interaction energies for BH are of opposite sign to those for NH. The results may be understood qualitatively using perturbation molecular orbital (PMO) theory.

A theoretical study of the CSH4 and CPH5 hypersurfaces. Geometries, tautomerization, and dissociation of sulfonium and phosphonium ylides

Calculations have been performed at four basis set levels (STO-3G, STO-3G*, 4-31G, 4-31G*) on the model ylides methylenesulfurane (CH2SH2) and methylenephosphorane (CH2PH3), their stable tautomers (CH3SH and CH3PH2), their dissociation products (SH2, PH3 and CH2), and the protonated species CH3SH2+ and CH3PH3+. At each basis set level all geometries have been optimized fully, using the FORCE method. The conformational behaviour of the ylides as a function of C—X bond-stretching, C—X torsion, and CH2 (or SH2) bending has been examined in some detail. The experimental properties (i.e., geometries, relative stabilities, proton affinities, rotation–inversion behaviour) of sulfonium and phosphonium ylides are reproduced well by the model calculations with the 4-31G* basis set, which contains d-type functions on both carbon and sulfur (or phosphorus). All other basis sets are deficient in different ways and for different reasons, which are discussed in detail. The principal result of this work is the conclusion that d-type functions are essential for a proper description of the hypervalent species CH2SH2 and CH2PH3, but not for the normal-valent species SH2, PH3, CH3SH, and CH3PH2. The conclusion concerning hypervalent species reverses our earlier views. The role of the d-type functions is to concentrate charge into the C—X region of the ylides, and thus to stabilize the system. Evidence for this effect has been obtained from quantitative perturbational molecular orbital (PMO) analyses of interactions associated with the carbon lone pair, as well as comparisons of electron density plots with and without the d AO's. A second conclusion is that the imposition of various geometrical constraints such as assumed C—X, C—H, or X—H bond lengths, and HCH or XHn bond angles, as was necessary for computational reasons in all previous work on such systems, has major and previously unrecognized consequences. For example, the assumption that the CH2 moiety is planar in CH2SH2 leads to very similar geometries with and without d AO's, although only in the latter case does such a geometry at carbon correspond to the true energy minimum; in the absence of d AO's the C—S bond length is maintained by a symmetry-enforced barrier to dissociation. These and other consequences of geometrical constraints at carbon, sulfur, or phosphorus are analyzed in detail.

Pyrite, marcasite, and arsenopyrite type minerals: Crystal chemical and structural principles

Quantitative molecular orbital (MO) calculations and qualitative perturbational MO arguments are used to interpret the spectra and structure of transition metal dichalcogenides and related compounds. Competition between pyrite (FeS2), marcasite (FeS2) and loellingite (FeAs2) structure types is explained in terms of the number of electrons occupying a set of MO's obtained from the mixing of dianion (A 2) orbitals and metal (M) dσ orbitals. Direct metal-metal d orbital interaction is argued to be small. Attention is focused upon the M - A - M angles which differ substantially among the three structure types as a consequence of varying numbers of electrons in orbitals which closely resemble the pπ* orbitals of the dianions. Variations in M - A and A - A distances can also be understood in terms of the occupations of this set of MO's. Disulfide valence region photo-emission spectra are interpreted in terms of calculations on MS6 and S6 molecular clusters. M3d orbitals are found to progressively approach the S3p orbitals with increasing atomic number of M from Fe to Ni. For CuS2 comparison of calculation and experiment supports an approximate electron configuration of Cu+1 S 2 −1 .

ChemInform Abstract: CYCLODIMERIZATION OF VINYLPYRIDINES

When isomeric 2- and 4-vinylpyridines, as well as 2-methyl-5-vinylpyridine, are heated in polyphosphoric or acetic acid, they undergo dimerization, which proceeds via a 1,4-cycloaddition scheme to give pyridyl-substituted 5,6,7,8-tetrahydroquinclines or isoquinolines. Quantum-chemical calculations with the use of the concepts of molecular orbital perturbation theory make it possible to predict the regiospecificity of the reaction. The regiospecific cross cycloaddition of 4-vinylpyridine to 2-methyl-5-vinylpyridine was proposed theoretically and proven experimentally.

Cyclopenta-polycyclic aromatic hydrocarbons: potential carcinogens and mutagens.

Cyclopenta-polycyclic aromatic hydrocarbons (cyclopenta-PAHs) are a group of compounds that have been detected as environmental pollutants. Perturbation molecular orbital (PMO) calculations on their presumed ultimate carcinogenic metabolites, the cyclopenta-PAH expoxides, predict that they may have a greater biological hazard than the classic PAHs.

Reactivity–selectivity relationships. Part 12. Intermediates formed in the solvolysis of substituted benzyl derivatives. The importance of HOMO–LUMO interactions in determining selectivity

The intermediates undergoing nucleophilic attack during the solvolysis of a series of substituted benzyl derivatives in aqueous ethanol are examined in the presence of m-chloroaniline. Conclusions are based on the measurement of substrate selectivity towards the competing nucleophiles, ethanol and m-chloroaniline and the application of perturbation molecular orbital (PMO) theory. The results suggest that p-chlorobenzyl, benzyl, and p-methyl benzyl chlorides undergo nucleophilic attack on intimate ion pairs while p-methoxybenzyl chloride undergoes attack on solvent separated ion pairs. These conclusions are in accord with a previous study on benzyl derivatives utilizing ethanol and water as competing nucleophiles. The importance of HOMO–LUMO interactions in determining substrate selectivity in nucleophilic substitution reactions, using two dissimilar nucleophiles, is confirmed.

ChemInform Abstract: QUANTITATIVE PERTURBATIONAL MOLECULAR ORBITAL CALCULATIONS. PART 2. RADICALS AND HETEROCYCLES

Chemischer InformationsdienstVolume 10, Issue 20 Physical Organic Chemistry ChemInform Abstract: QUANTITATIVE PERTURBATIONAL MOLECULAR ORBITAL CALCULATIONS. PART 2. RADICALS AND HETEROCYCLES C. F. COOPER, C. F. COOPERSearch for more papers by this author C. F. COOPER, C. F. COOPERSearch for more papers by this author First published: May 15, 1979 https://doi.org/10.1002/chin.197920084AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume10, Issue20May 15, 1979 RelatedInformation

On the magnitudes and origins of the “anomeric effects”, “exo-anomeric effects”, “reverse anomeric effects”, and CX and CY bond -lengths in XCH 2YH molecules

Ab-initio molecular-orbital calculations have been performed at the STO-3G level on the series of molecules XCH 2YH in which X  H 2N, CH 3, OH, F, and Cl, and Y  O and S, and on the series of fluoromethanes CH nF 4-n. The CX and CY bond lengths in the gauche and antiperiplanar conformations of XCH 2YH have been optimized. In all cases, the experimental trends in the bond lengths from conformation to conformation and from molecule to molecule, and in the torsional behavior as a function of X and Y have been reproduced. Thus, the “anomeric effects” are found to be Cl > F > OH, as a function of X, and O > S, as a function of Y. Molecules in which X  CH 3 and NH 2 exhibit “reverse anomeric effects”; in methanediol, the “anomeric effect” and the “exo-anomeric effect” are equal. The results have been analyzed by a quantitative, perturbational molecular-orbital (PMO) treatment that calculates orbital interactions between XCH 2 and YH in the cases of the “anomeric” and “exo-anomeric effects”, and between X and CH 2YH in the case of the “reverse anomeric effect”, by using fragments and fragment orbitals generated from the ab-initio wavefunction. Attention has been focused, especially, upon the stabilizing interactions between the lone pair of Y and antibonding orbitals of XCH 2, and between the highest-lying orbital of X and antibonding orbitals of CH 2YH. The justification for this choice is that these orbitals make a dominant contribution to the highest occupied molecular orbital (HOMO) of XCH 2YH, and the stereochemical behavior of the HOMO parallels that of the total energy. In all cases, the trends in these stabilizing orbital interactions parallel the trends in the “anomeric” and “reverse anomeric effects”, suggesting that, within the framework of the PMO model, such interactions may be regarded as the “origin” of these effects. Although both σ* and π* antibonding orbitals have to be taken into account to achieve quantitative agreement between the calculated orbital interactions and the total energy-differences, only those interactions associated wit σ* vary significantly as X and Y are varied, as suggested originally by Lucken and by Altona. In the series of fluoromethanes, a linear relationships exist between the stabilizing orbital interactions and the experimental CF bond energies, and also between stabilizing orbital interactions and the number of double bond-no bond resonance structure. This result provides quantitative support for a description of these various effects in terms of the classical concept of double bond-no bond resonance. Finally, the in the CX and CY bond lengths in XCH 2YH and XCH 2X molecules have found to be the result of the combination of coulombic effects and the principle of maximization of overlap between fragments.

ChemInform Abstract: QUANTITATIVE PERTURBATIONAL MOLECULAR ORBITAL CALCULATIONS FOR CONJUGATED HYDROCARBONS

Chemischer InformationsdienstVolume 9, Issue 31 Physical Organic Chemistry ChemInform Abstract: QUANTITATIVE PERTURBATIONAL MOLECULAR ORBITAL CALCULATIONS FOR CONJUGATED HYDROCARBONS C. F. COOPER, C. F. COOPERSearch for more papers by this author C. F. COOPER, C. F. COOPERSearch for more papers by this author First published: August 1, 1978 https://doi.org/10.1002/chin.197831090AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume9, Issue31August 1, 1978 RelatedInformation

ChemInform Abstract: A SIMPLE MOLECULAR ORBITAL PERTURBATION TREATMENT OF THE SWITCHING OF BOND ORDER PATTERNS WITHIN MOLECULAR SYSTEMS

Chemischer InformationsdienstVolume 9, Issue 31 Physical Organic Chemistry ChemInform Abstract: A SIMPLE MOLECULAR ORBITAL PERTURBATION TREATMENT OF THE SWITCHING OF BOND ORDER PATTERNS WITHIN MOLECULAR SYSTEMS M. GODFREY, Search for more papers by this author M. GODFREY, Search for more papers by this author First published: August 1, 1978 https://doi.org/10.1002/chin.197831093Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume9, Issue31August 1, 1978 RelatedInformation

A simple molecular orbital perturbation treatment of the switching of bond order patterns within molecular systems

A simple molecular orbital perturbation method for determining the effects of disturbances in molecular systems is applied to predict the conditions required for small perturbations of the energy to produce major changes in individual bond orders. The results are used to explain several important physical and chemical observations concerning quantitative linear correlations which are puzzling when considered in terms of commonly used theoretical models. Their implications for simple resonance theory are also discussed. It is concluded that effects of mutual conjugation are not the principal source of differences between the various empirical scales of substituent effects on properties of benzene and its derivatives.

Molecular orbitals from group orbitals. IV. Quantitative perturbational molecular orbital analysis of the methyl rotational barriers in (CH3)2X molecules. Effect of the fragmentation mode upon the results of the analysis

A quantitative perturbational molecular orbital (PMO) analysis has been performed on ab initio SCF-MO wavefunctions associated with the rotation of the methyl groups in a series of (CH3)2X molecules (X = CH2, O, S, C=O, C=CH2). Two fragmentation modes have been investigated: Method a, in which the system is dissected into X and (CH3)2; and Method b, in which the system is dissected into CH3X and CH3. Both fragmentation modes reproduce the principal property of these molecules, viz., that the more crowded SS conformation is preferred. However, whether this conformational preference is controlled by two-electron stabilizing effects or four-electron destabilizing effects is found to depend upon both the mode of fragmentation and the nature of the substituent X. The quantitative results are supplemented by a detailed qualitative description of the nature of the group orbitals associated with the two fragmentation modes and the various types of orbital interactions. It is shown that orbital energy differences control the qualitative discussion of Method a, and overlap effects control that of Method b. Although the final result, i.e., the preference for the SS conformation, and the behaviour of individual orbital interactions are anticipated correctly by the qualitative arguments, these are unable to assess the relative contributions of the stabilizing and destabilizing interactions.

ChemInform Abstract: A TOPOLOGICAL HUECKEL MODEL FOR ORGANOMETALLIC COMPLEXES. PART III. THE PERTURBATION MOLECULAR‐ORBITAL METHOD

Chemischer InformationsdienstVolume 8, Issue 17 Physical Organic Chemistry ChemInform Abstract: A TOPOLOGICAL HUECKEL MODEL FOR ORGANOMETALLIC COMPLEXES. PART III. THE PERTURBATION MOLECULAR-ORBITAL METHOD D. M. P. MINGOS, Search for more papers by this author D. M. P. MINGOS, Search for more papers by this author First published: April 26, 1977 https://doi.org/10.1002/chin.197717077AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume8, Issue17April 26, 1977 RelatedInformation