The effects of substituents on the stability of 3‐substituted(X) bicyclo[1.1.1]pent‐1‐yl cations (3) and 4‐substituted(X) bicyclo[2.2.1]hept‐1‐yl cations (4), for a set of substituents (X = H, NO2, CN, NC, CF3, CHO, COOH , F, Cl, HO, NH2, CH3, SiH3, Si(CH3)3, Li, O−, and NH3+) covering a wide range of electronic substituent effects were calculated using the DFT theoretical model at the B3LYP/6‐311 + G(2d,p) and B3LYP/6‐31 + G (d) levels of theory, respectively. Linear regression analysis was employed to explore the relationship between the calculated relative hydride affinities (ΔE, kcal/mol) of the appropriate isodesmic reactions for 3/4 and polar field/group electronegativity substituent constants (σF and σχ, respectively). The analysis reveals that the ΔE values for both systems are best described by a combination of both substituent constants. The result highlights the importance of the σχ dependency of charge delocalization in these systems. Copyright © 2012 John Wiley & Sons, Ltd.

O-17 NMR shieldings of 3-substituted(X)bicyclo[1.1.1]pentan-1-ols (1, Y = OH), 4-substituted(X)bicyclo[2.2.2]octan-1-ols (2, Y = OH), 4-substituted(X)-bicyclo[2.2.1]heptan-1-ols (3, Y = OH), 4-substituted(X)-cuban-1-ols (4, Y = OH) and exo- and endo-6-substituted(X)exo-bicyclo[2.2.1]heptan-2-ols (5 and 6, Y = OH, respectively), as well as their conjugate bases (1-6, Y = O-), for a set of substituents (X = H, NO2, CN, NC, CF3, COOH, F, Cl, OH, NH2, CH3, SiMe3, Li, O-, and NH3+) covering a wide range of electronic substituent effects were calculated using the DFT-GIAO theoretical model at the B3LYP/6-311 + G(2d, p) level of theory. By means of natural bond orbital (NBO) analysis various molecular parameters were obtained from the optimized geometries. Linear regression analysis was employed to explore the relationship between the calculated O-17 SCS and polar field and group electronegativity substituent constants (sigma(F) and sigma(chi), respectively) and also the NBO derivedmolecular parameters (oxygen natural charge, Q(n), occupation numbers of the oxygen lone pairs, n(o), and occupancy of the C-O antibonding orbital, sigma* (CO)(occup)). In the case of the alcohols (1-6, Y = OH) the O-17 SCS appear to be governed predominantly by the sigma(chi) effect of the substituent. Furthermore, the key determining NBO parameters appear to be n(o) and sigma* (CO)(occup). Unlike the alcohols, the calculated O-17 SCS of the conjugate bases (1-6, Y = O-), except for system 1, do not respond systematically to the electronic effects of the substituents. An analysis of the SCS of 1(Y - O-) raises a significant conundrum with respect to their origin. Copyright (C) 2010 John Wiley & Sons, Ltd.

The effects of substituents on the stability of 4-substituted(X) cub-1-yl cations (2), as well as the benchmark 4-substituted(X) bicyclo[2.2.2]oct-1-yl cation systems (7), for a set of substituents (X = H, NO2, CN, NC, CF3, COOH, F, Cl, HO, NH2, CH3, SiH3, Si(CH3)(3), Li, O-, and NH3+) covering a wide range of electronic substituent effects were calculated using the DFT theoretical model at the B3LYP/6-311 + G(2d, p) level of theory. Linear regression analysis was employed to explore the relationship between the calculated relative hydride affinities (Delta E, kcal/mol) of the appropriate isodesmic reactions for 2/7 and polar field/group electronegativity substituent constants (sigma(F) and sigma(chi), respectively). The analysis reveals that the DE values of both systems are best described by a combination of both substituent constants. This highlights the distinction between through-space and through-bond electronic influences characterized by sigma(F) and sigma(chi), respectively. Copyright (C) 2010 John Wiley & Sons, Ltd.

F-19 NMR shieldings of 4-substituted (X) cub-1-yl fluorides (4) for a set of substituents (X=H, NO2, CN, NC, CF3, COOH, F, Cl, HO, NH2, CH3, Si(CH3)(3), Li, O- and NH3) covering a wide range of electronic substituent effects were calculated using the DFT-GIAO theoretical model. The level of theory, B3LYP/6-311+G(2d,p), provided F-19 substituent chemical shifts (SCS) in good agreement with experimental values where known. By means of NBO analysis, various molecular parameters were obtained from the optimized geometries. Linear regression analysis was employed to explore the relationship between the calculated F-19 SCS and polar field, resonance and group electronegativity substituent constants (sigma(F), sigma(R) and sigma(x), respectively) and also the NBO derived molecular parameters (fluorine natural charges (Q(n)), electron occupancies on fluorine of lone pairs (n(F)) and occupation number of the C-F antibonding orbital (sigma(CF)*)). The key determining parameters appear to be n(F) and sigma(CF)*(occup). Both factors are a function of the electrostatic field influence of the substituent (sigma(F) effect) but are counteractive in their influence on the shifts. No evidence for a significant resonance effect influence on the shifts could be identified. Copyright (C) 2009 John Wiley & Sons, Ltd.

The acid-catalyzed addition of CH3(18)OH to 2-methylene-adamantanes bearing a chlorine atom in the 4-equatorial (1e) or 4-axial (1a) position has been investigated in the gas phase, at 760 Torr, in the 40-120 degrees C temperature range. Two different experimental approaches were employed: (1) by adding neutral CH3(18)OH to the 2-methyl-4-Cl-adamant-2-yl cation, generated by protonation of the corresponding 2-methylene-4-Cl-adamantane (the extracomplex reaction) and (2) by reaction of 2-methylene-4-Cl-adamantane with CH3(18)OH2+, generated by methylation of H2(18)O (the intracomplex reaction). The crucial role of the nature of the noncovalent intermediates involved along the reaction coordinates emerges from the difference between the results obtained in the extracomplex and intracomplex reactions for both substrates investigated. The kinetic and stereochemical results indicate that the 4-Cl substituent plays a different role depending on its equatorial or axial orientation. Examination of the experimental results in the light of MP2/6-31G* theoretical calculations provides important information about the intrinsic factors governing the facial diastereoselectivity of trigonal carbocations. The effects due to differential face solvation phenomena emerge from the comparison of the present gas-phase results with those obtained from strictly related studies in solution.

An extensive series of mixtures of exo- and endo-6-substituted(X)-exo-2-fluorobicyclo[2.2.1]-heptanes (4 and 5, respectively) were synthesized and characterized by C-13 NMR and their F-19 chemical shifts measured. Additionally, the latter parameters for a more limited series of 4(eq)- and 4(ax)-substituted (X) 2(eq)-fluoroadamantanes (9 and 10, respectively) were also obtained. The corollary from correlations of the F-19 substituent chemical shifts(SCS) of 4 and 5 versus the corresponding results for the known 4-substituted(X) bicyclo[2.2.1]hept-1-yl fluorides( 3) is that electronegativity effects (sigma(chi) effect) underly the SCS of 4 and 5. Differences between the SCS of 4 and 5 as well as 9 and 10 indicate that there is a stereoelectronic component to the polar effect significantly determining the F-19 SCS of 4 and 9. F-19 NMR shieldings of 3, 4, 5, 3-substituted(X) adamant-1-yl fluorides (8), 9 and 10 for a common set of substituents (X = H, NO2, CN, NC, CF3, COOH, F, Cl, HO, NH2, CH3, Si(CH3)(3) and Li) were calculated using the DFT-GIAO theoretical model. The level of theory, B3LYP/6-31+G*, was chosen based on trial calculations which gave good agreement with experimental values where known. By means of NBO analysis various molecular parameters were obtained from the optimized geometries. Linear regression analysis was employed to explore the relationship between the calculated F-19 SCS and polar field and group electronegativity substituent constants(sigma(F) and sigma(chi), respectively) and also the NBO derived molecular parameters( fluorine natural charges(Q(n)), electron occupancies on fluorine of lone pairs(n(F)), and occupation number of the C-F antibonding orbital(sigma(CF)*)). The key determining parameters appear to be n(F) and sigma(CF)*(occup).

A limited series of 4(eq)-substituted (X) 2-methyleneadamantanes (6, Y=CH2, X=F, Cl, Br, 1, and SnMe3) has been synthesized and diastereoselectivities for their hydrochlorination (HCl/CH2Cl2) have been determined. Diastereoselectivities for the fluorination (DAST/CH2Cl2) of secondary alcohol mixtures, obtained from the hydride reduction of the precursor ketones (6,Y=O) to the alkenes, have also been measured. A comparison of this selectivity data for nucleophilic trapping of 4eq-substituted (X) 2-adamantyl cations (4, R=H and Me) with the corresponding information for 5-substituted (X) 2-adamantyl cations (1, R=H and Me) has revealed important distinctions between the two series. In particular, whereas extended hyperconjugative effects appear to be the predominant electronic effect governing facial selectivity in the 5,2-series, electrostatic influences prevail in the 4,2-disposition. Copyright (C) 2007 John Wiley & Sons, Ltd.

Secondary 5-X-adamant-2-yl cations I-X (X = F, Si(CH3)(3)) have been generated in the gas phase (total pressure = 760 Torr) from protonation-induced defluorination of epimeric 2-F-5-X-adamantanes 1(X) and their kinetic diastereoselectivity toward (CH3OH)-O-18 investigated in the 40-160 degrees C range. The experimental results indicate that the facial selectivity of I-X is insensitive to the composition of the starting 1(X) epimers as well as to the presence and the concentration of a powerful base (N(C2H5)(3)). This kinetic picture, supported by B3LYP/6-31G* calculations, is consistent with a single stable pyramidalized structure for I-X, that is, (Z)-5-F-adamant-2-yl (I-F(Z)) and (E)-5-Si(CH3)(3)-adamant-2-yl cations (I-Si(E)). The temperature dependence of the I-X diastereoselectivity lends support to the intermediacy of noncovalent adducts [(IXCH3OH)-C-.-O-18], characterized by a specific C2-(H+...O18)(H)CH3 hydrogen bonding interaction. Their conversion to the covalently bonded O-methylated (Z)- (IIXZ) and (E)-5-X-adamantan-2-ols (IIXE; X = F, Si(CH3)(3)) is governed by activation parameters, whose magnitude depends on the specific I-X face accommodating (CH3OH)-O-18. The gas-phase diastereoselectivity of I-X toward (CH3OH)-O-18 is compared to that exhibited in related gas-phase and solution processes. The emerging picture indicates that the factors determining the diastereoselectivity of I-X toward simple nucleophiles in the gaseous and condensed media are completely different.

π-Facial selectivity data for the reduction and methylation of some 4ax-substituted (X) 2-adamantanones (3, Y = O) as well as the nucleophilic trapping of secondary and tertiary 4ax-substituted (X)-2-adamantyl cations (4; R = H and CH3, respectively) and the 4-methylene-2-adamantyl radical (8) are presented. The pronounced anti-face selectivities observed for (3, Y = O and 4, R = CH3) emphasize the importance of the steric factor as expected for systems with a strong steric bias. However, the dominant syn-face capture of 4 (R = H) was completely unexpected and highlights a subtle interplay between steric and electronic effects. Finally, the very high anti-face stereoselectivity for the trapping of (8) with the trimethylstannyl anion (Me3Sn−) is rationalized in terms of an electrostatic effect overwhelming the steric factor. Copyright © 2007 John Wiley & Sons, Ltd.

Experimental gas-phase acidities are reported for a series of 3-substituted (X) bicyclo [1.1.1]pent-1-yl carboxylic acids (1, Y = COOH). A comparison with available calculated data (MP2/6-311++G**// B3LYP/6-311+G**) reveals good agreement. The relative substituent effects are shown to be adequately described by a much lower level of theory (B3LYP/6-31+G*). Various correlations are presented which clearly point to polar field effects as being the origin of the relative acidities.

Hydrogen bond enthalpies for the interaction of 4-fluorophenol with syn-2,4-difluoroadamantane (5) and, for comparison, 2-fluoroadamantane (6) and 1,3-difluoroadamantane (7) have been determined by Fourier transform infrared spectrometry. Among a series of seven fluoroalkanes (1-7) it is found that syn-2,4-difluoroadamantane is the best hydrogen bond acceptor. This high hydrogen bond acceptor strength is explained, using electrostatic and 'atoms in molecules' properties and from density functional theory and ab initio calculations on hydrogen fluoride complexes, by the formation of an F... H... F three-centre hydrogen bond. In this system, the three-centre hydrogen bonding is energetically (mainly on the Gibbs energy scale) superior to the two-centre hydrogen bonding. Copyright (C) 2004 John Wiley Sons, Ltd.

The diastereofacial selectivity of 2-methyl-5-X-adamant-2-yl cations IX (X = CN, Cl, Br, CH3O, COOCH3, C6H5, CH3, and (CH3)3Sn) toward methanol has been investigated in the gas phase at 750 Torr and in the 40-120 degrees C temperature range and compared with that of IF (X = F) and ISi (X = (CH3)3Si) measured previously under similar conditions. Detailed analysis of the energy surface of the IMe (X = CH3) ion reveals that the activation barrier of its syn addition to methanol is significantly lower than that of the anti attack. In the 40-100 degrees C range, such a difference is strongly reduced by adverse entropic factors which are large enough to invert the IMe diastereoselectivity from syn to anti at T > 69 degrees C. The behavior of IMe diverges markedly from that of IF and ISi. Large adverse entropic factors account for the predominant syn diastereoselectivity observed in the reaction with IF (X = F), notwithstanding the anti enthalpy barrier is lower than the syn one. Adverse entropy plays a minor role in the reaction with ISi (X = (CH3)3Si) which instead exhibits a preferred anti diastereoselectivity governed by the activation enthalpies. Depending on the electronic properties of X, the kinetic behavior of the other IX ions obeys one of the above models. The gas-phase diastereoselectivity of IX ions responds to a subtle interplay between the sigma-hyperconjugative/electrostatic effects of the X substituent and the activation entropy terms. sigma-Hyperconjugation/field effects determine the pyramidal structure and the relative stability of the syn and anti conformers of IX as well as the relative stability of their addition transition structures and their position along the reaction coordinate. The diastereoselectivity of IX in the gas phase is compared with that measured in solution and with theoretical predictions.

The secondary alpha-deuterium kinetic isotope effect (alpha-kie) for the solvolysis of (Z)-5-trimethylstannyl 2-adamantyl p-bromobenzenesulfonate in 97% w/w aqueous 2,2,2-trifluoroethanol (97T) at 25 degrees C has been measured (k(H)/k(D) = 1.33). The alpha-kie is abnormally high compared to the value of 1.23 for the corresponding limiting S(N)1 solvolysis of 2-adamantyl p-bromobenzenesulfonate, which proceeds via an extended ion-pair mechanism. A novel mechanism for the solvolysis of the tin compound is proposed that accommodates not only the high alpha-kie but also the absence of internal return.

The 19F NMR shieldings for several remotely substituted rigid polycyclic alkyl fluorides with common sets of substituents covering a wide range of electronic effects were calculated using the DFT‐GIAO theoretical model. The level of theory, B3LYP/6–311+G(2d,p), was chosen based on trial calculations which gave good agreement with experimental values where known. The optimized geometries were used to obtain various molecular parameters (fluorine natural charges, electron occupancies on fluorine of lone pairs and of the CF bond, and hybridization states) by means of natural bond orbital (NBO) analysis which could help in understanding electronic transmission mechanisms underlying 19F substituent chemical shifts (SCS) in these systems. Linear regression analysis was employed to explore the relationship between the calculated 19F SCS and polar substituent constants and also the NBO derived molecular parameters. The 19F SCS are best described by an electronegativity parameter. The most pertinent molecular parameters appear to be the occupation number of the NBO p‐type fluorine lone pair and the occupation number of the CF antibonding orbital. This trend suggests that in these types of rigid saturated systems hyperconjugative interactions play a key role in determining the 19F SCS. Electrostatic field effects appear to be relatively unimportant. Copyright © 2003 John Wiley & Sons, Ltd.

High-resolution electron momentum spectroscopy (EMS) has been used to determine the character of the two outermost π-orbitals of norbornadiene. Definitive evidence, from comparisons of measured and calculated momentum distributions, for the dominance of the through-space interaction is presented. This through-space bond dominance is consistent with previous hypotheses based on molecular orbital considerations [Acc. Chem. Res. 4 (1971) 1; J. Am. Chem. Soc. 92 (1970) 706; J. Am. Chem. Soc. 112 (1990) 1710].

Momentum Distributions (MDs), obtained using high-resolution electron momentum spectroscopy (HREMS), are reported for norbornadiene's 18 valence orbitals. Corresponding theoretical results, using generalized gradient approximation density functional theory (DFT) together with TZVP, DZVP, and DZVP2 basis functions and a plane wave impulse approximation to describe the ionization process, are also detailed. This work represents the first comprehensive HREMS/DFT investigation into the complete valence electronic structure of norbornadiene (NBD), with significant results being obtained. In particular, an exacting comparison between our experimental and theoretical MDs enables us to define the “optimum” basis for NBD, from those we studied. This “optimum” basis is then used to extract a wide range of NBD's important molecular property information, which are subsequently compared with the results of independent measurements and calculations. Agreement between our results and those from independent measurements was generally very good, highlighting the utility of HREMS in a priori basis set evaluation.

The intrinsic factors governing the diastereofacial selectivity of 2-methyl-5-X-2-adamantyl cations (X = F (I(F)), Si(CH(3))(3) (I(Si))) toward a representative nucleophile, i.e., methanol, have been investigated in the gas phase at 750 Torr and in the 20-80 degrees C temperature range. The kinetic results indicate that CH(3)OH addition to I(F) proceeds through tight transition structures (TS(F)(syn) and TS(F)(anti)) characterized by advanced C-O bonding. The same interactions are much less pronounced in the comparatively loose transition structures involved in the CH(3)OH addition to I(Si) (TS(Si)(syn) and TS(Si)(anti)). The experimental evidence indicates that the activation barriers for the anti addition to I(F) and I(Si) are invariably lower than those for the syn attack. Large adverse entropic factors account for the preferred syn diastereoselectivity observed in the reaction with I(F). Entropy plays a minor role in the much looser transition structures involved in the reaction with I(Si), which instead exhibits a preferred anti diastereoselectivity. Comparison of the above gas-phase results with related theoretical and solution data suggests that the diastereofacial selectivity of I(F) and I(Si) measured in solution arises in part from the differential solvation of the two faces of the pyramidalized ions.

A series of 2,5(or 1,4)-dihaloadamantanes (4 and 5, X = Y = halogens) and 9,10-dihalotriptycenes (7, X = Y = halogens) as well as two 5-halo (X) adamantan-2-ones (6, Y = O, X = Br and I) have been treated with Me(3)SnLi in THF in the absence and presence of tert-butylamine (TBA) and dicyclohexylphosphine (DCHP). The product distributions of these reactions have been established by (13)C and (119)Sn NMR spectroscopy, vapor-phase chromatographic analyses, and GC/MS. The former compounds (4 and 5) appear to react exclusively by a free-radical chain process (S(RN)1 mechanism) to yield tin substitution products. By contrast, the triptycenes react predominantly by a polar mechanism initiated by the formation of a carbanion. In the case of the halo ketones (6, Y = O, X = Br and I), a mechanistic divergence of the reaction was unexpectedly encountered. Whereas the bromo ketone provides the substitution product (6, Y = O, X = SnMe(3)) in good yield (ca. 75%), apparently by a radical pathway, the iodo ketone yields a fragmentation product (ca. 95% yield) by a polar mechanism. This mechanistic switch highlights the importance of the electronegativity of the leaving group as well as substituent-induced electron delocalization as molecular factors governing the competition between radical and polar pathways.

We report on the application of electron momentum spectroscopy to the chemically interesting molecules [1.1.1]propellane, cubane and norbornadiene. Detailed binding energy spectra and orbital momentum distributions (MDs) were measured. This work, particululy for the larger molecules C8H8 (cubane) and C7H8 (norbornadiene), was made feasible by the development at Flinders of an (e,2e) monochromator, so that our coincidence energy resolution improved from typically 1.4 eV (FWHM) to about 0.5 eV (FWHM). In conjunction with these experiments we utilise the UniChem computational chemistry codes and the Flinders-developed AMOLD program to calculate theoretical MDs for each orbital of each species. A critical comparison between the experimental and theoretical MDs enabled us to determine, for a given molecule, the optimum basis set from those we studied. The determination of this basis set then allowed us to make further use of the UniChem package to derive each molecule's molecular properties and, in particular, address open questions as to the nature of the bonding in [1.1.1]propellane, cubane and norbornadiene.

A study of the electronic structure of the complete valence shell of cubane is reported. Results from our many-body Green's function calculation, to the third-order algebraic diagrammatic construction (ADC(3)) level, for the binding energies and spectroscopic factors of the respective valence orbitals of cubane are presented. Binding-energy spectra were measured in the energy regime 6-35 eV over a range of different target electron momenta, so that momentum distributions (MDs) could be determined for each orbital. The corresponding theoretical MDs were calculated using a plane wave impulse approximation (PWIA) model for the reaction mechanism and density functional theory (DFT) for the wave function. Seven basis sets, at the local density approximation (LDA) level and, additionally, incorporating nonlocal correlation functional corrections, were studied. The sensitivity of the level of agreement between the experimental and theoretical MDs to the nonlocal corrections is considered. A critical comparison between the experimental and theoretical MDs allows us to determine the "optimum" wave function for cubane from the basis sets we studied. This wave function is then used to derive cubane's chemically interesting molecular properties. A summary of these results and a comparison of them with those of other workers is presented with the level of agreement typically being good.